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Introduction It is predicted that the continuous improvement in the performance of conventional, top-down integrated circuits will end in about ten years [1, 2]. This has spurred a great deal of research into novel devices. In the near term, the most likely device to replace the conventional planar metal-oxide-semiconductor field effect transistor (MOSFET) is a nonplanar variation of the same device.[3-6] Such a nonplanar structure increases the device drive current by increasing the surface (channel) area of the transistors, but do not improve overall system performance as dramatically. Interconnect is usually the factor limiting the performance of digital circuits that use such highly scaled devices. Copper and low permittivity insulators have helped reduce interconnect delays, but as devices scale below gate lengths of 50 nm, even these improvements become insufficient.[7] Interconnect limitations are even more obvious at the system level where chips running at 3 GHz are being put on boards running an order of magnitude slower. Many of the nanodevices that have been proposed as a long term replacement to the MOSFET display interesting physics, but have a limited current output or have poor yields that would necessitate a “fat-tree” architecture[8] which further exacerbates interconnect-related on-chip delays. Thus, there is no obvious succession path once reducing the size of the planar MOSFET is complete.

A much more powerful solution to these scaling limitations would be to move from integrated circuits to integrated systems: chips that have many levels of different types of devices on a single substrate. For example, such a system could have numerous layers of transistors interspersed with interconnect. GaAs devices might be used for RF communication, while silicon devices are used for logic. Optical devices such as lasers and LEDs could be used for off-chip and cross-chip communication and magnetic devices for remote power and nonvolatile storage. All of these devices would be integrated on an insulating, low-loss substrate, eliminating the problems associated with propagating high-speed signals across lossy substrates such as silicon. By monolithically integrating different materials and devices into a single component, system speed would increase substantially as “bus” distances fall from centimeters to microns. The obvious problem with this approach is that there is no known way to form single crystal layers of lattice-mismatched, chemically incompatible materials on an arbitrary substrate. Yet, defect free material is essential for high mobilit y and long lifetime devices to be built at yields comparable to, or better than, today’s structures. Furthermore, to allow many device layers with embedded interconnect, materials must be deposited at low temperature without excessive post deposition heating. While this appears to be a difficult problem, there is an important mitigating factor. If their size and location can be controlled, one only needs islands of high quality single crystals that are the size of the device. As devices scale to tens of nm, this becomes an increasingly attractive approach. We therefore propose a radical new approach to the manufacture of information systems: the directed self-assembly of single crystal semiconductor nanoparticles for building advanced devices. As an initial demonstration, we will focus on single crystal silicon devices. Even a silicon-only technology would permit three dimensional circuits which has been predicted to reduce the critical interconnect lengths in an integrated circuit by as much as 75%.[9], Our performance estimates suggest that, due to the lack of parasitic effects from the substrate, the switching speed of nanoparticle devices should be hundreds of GHz. Our recent work has shown that high concentrations of extremely monodisperse (standard deviation ~3% of diameter), non agglomerated, silicon nanoparticles can be grown with well controlled diameters from 10 nm to 150 nm in a properly operated high density plasma.[10] Furthermore, we have made single crystal particles as large as 80 nm in diameter in the diamond-cubic structure with bulk Si lattice

Figure 1 – Schematic of an integrated structure showing nanoparticles of different materials (colors) localized electrostatically.

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spacing.[11] We have shown that nanoparticles as small as 40 nm can be localized using electrostatics to do bottom-up assembly. We have demonstrated that electrical contact can be made to silicon nanoparticles using conventional deposition and etch processes and that the current follows normal Schottky behavior at low voltage and space charge limited behavior at higher voltage. Finally we have done 2D and 3D simulations of typical transistors that could be made from such particles and found that excellent device performance can be expected. In this proposal we will review these recent enabling advances and suggest how one could build self assembled systems from crystalline nanoparticle devices (Figure 1).

An overview of the proposed NIRT and some of the enabling results are given in Figure 2. The following sections will describe the three research areas that make up the technical section of this NIRT: nanoparticle synthesis, nanoparticle assembly, and nanoparticle devices. The three areas clearly have strongly overlapping interests. Investigators from the three areas will work closely together. On the other hand, none of the three areas depend completely on the success of the other two. For example, devices can be built from both amorphous and crystalline nanoparticles, both randomly deposited and electrostatically localized. Similarly, the production of single -crystal silicon nanoparticles and their electrostatic localization are revolutionary advances in their own right without the fabrication of the specific devices we will consider here. We expect that the availability of these processes will spur collaborations with a variety of device researchers outside the NIRT. The characterization of these materials is not a separate research area but is an integral component of each of the three areas described here. For each of the three technical areas we will discuss our objectives, the recent advances that enable this work, and the work that we plan to carry out under the NIRT. Following the technical portion of the proposal, we will review the management and education plan for the NIRT and go on to discuss the broader impacts of the proposal.

Synthesis Can make large single crystal or highly monodisperse silicon particles

PI

Pla

nned

Wor

k

C

urre

nt S

tatu

s

Improve process to make monodisperse single crystal silicon nanoparticles

Kortshagen

Assembly Demonstrated patterns of charge with 100 nm resolution. Captured 40 nm particles using electrostatic forces.

Localize 30 nm aerosol particles with charge patterns and external bias. Passivate particles.

Jacobs

Devices Fabricated metal-semiconductor-metal devices. Modeled transistors

Demonstrate nanoparticle transistors, both MOSFETs and SBTs. Compare with model.

Campbell

Carter – Support all efforts with electron microscopy (TEM, SEM, XEDS and EELS). Investigate crystallinity, morphology, contact structures and chemistry, and details of electrostatic self assembly results including local orientation and distribution of nanoparticles.

Metal

SiO2 Metal

Figure 2 – Schematic showing the three technical sections of the proposal. For each section, the present state of the art, the planned work, and the PI responsible are given.

200 nmRIGID SUPPORT

charged area

3

Nanoparticle Synthesis Objectives: This work will center on the use of high density, pure and dilute, silane plasmas for nanoparticle synthesis. We have three objectives: 1) Devise a plasma process that reproducibly produces monodisperse, non-agglomerated, single -crystal silicon nanoparticles with a diameter that can be selected between 30 and 100 nm. 2) Explore and understand the growth process of nanoparticles in inductively coupled plasmas. 3) Relate the growth parameters to the crystallinity, size, shape and surface characteristics of the particles. Background: Gas-phase processes appear attractive to synthesis nanoparticles for electronic applications due to their high degree of purity and their compatibility with current semiconductor manufacturing technology. A variety of gas-phase processes has been proposed to synthesize nanoparticles such as inert gas condensation [12] , flame synthesis [13], furnace flow reactors [14] and thermal plasma particle synthesis [15]. While these processes are often rather efficient in the synthesis of particles, they also often produce nanoparticles with broad size distributions that require post-synthesis size selection [16, 17]. Particle agglomeration is another frequent problem. Low-pressure nonthermal plasmas, on the other hand, can be tailored to avoid most of these problems during the synthesis process. Several groups studying the formation of silicon particles in silane plasmas have pointed out that often highly monodisperse, non-agglomerated particles can be achieved [18-21]. The monodisperse is usually related to the fact that nanoparticles, once formed in the plasma, scavenge the precursors for the formation of new nuclei so that after an initial nucleation and growth phase particles grow due to surface deposition. Agglomeration of particles is prevented by their negative charge once they reach a size of about 10 nm [22]. This negative charge also leads to the trapping of the particles in the plasma by the ambipolar electric field [23, 24]. Until now, no successful synthesis attempts of single -crystal nanoparticles in the 20—100 nm range have been reported. Small crystallites of 2-5 nm in size have been observed in numerous studies [25-28]. However, during the following growth phase these small crystallites are assumed to rapidly coagulate since most of particles in this size range are uncharged [19, 29]. Consequently, larger particles with sizes of tens of nanometers have been reported to be amorphous or polycrystalline.

Enabling Work: Over the past year, we have made rapid progress on using inductively coupled high-density plasmas for the synthesis of nanoparticles. Inductively coupled plasmas (ICP), a commonly used type of high-density plasma, are known to produce plasma densities about 1 to 2 orders of magnitude higher than those of capacitively coupled plasmas [30] that were used in most of the particle studies mentioned above. Our rationale for using this alternative plasma type was that the higher plasma density and higher gas temperature might favor crystalline particle formation.

Our initial studies were performed in a modified Gaseous Electronics Conference (GEC) reference cell

Figure 3 - Left: Monodisperse Si particles synthesized in a SiH4 ICP at 1.3 Pa, 200 W and 10 s plasma-on time. Right: On axis bright-field TEM image of a crystalline Si nanoparticle; the beam is parallel to [001]. Inset is the SAD pattern. The diffuse region surrounding the particle is associated with the amorphous surface layer.

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reactor that uses a planar 5-turn induction coil to produce a plasma of about 1000 cm-3 at RF powers of 200 W. Pure silane was used in this discharge at pressures of 10-20 mtorr. We were able to demonstrate that highly monodisperse, non-agglomerated nanoparticles can be achieved in this arrangement[10], as seen in Fig.3. The standard deviations σ observed were as low as 5% of the average particle diameter Dp. In terms of a log-normal size distribution this corresponds to a geometric standard deviation exp (σ/Dp) of 1.05. This is impressively more monodisperse than the size distribution obtained in a neutral aerosol undergoing Brownian free molecular coagulation for which the geometric mean standard deviation always approaches a limit of ~1.355 [31]. The particle size in this system could sensitively be determined by the plasma on time and ranged from about 15 nm after 1 second plasma duration to about 80 nm after 30 seconds of plasma duration. The narrow size distributions suggest that the presence of particles that are trapped in the plasma reduces the concentration of growth species what prevents further nucleation of small particles. Unfortunately, we were not able to observe single -crystal particles in this system. Particles were either amorphous, or poly-crystalline if strong hydrogen dilution (more on 95%) of silane was used; this observation can probably be attributed to hydrogen-induced crystallization [32].

In a second step we turned to a tube style reactor for running an inductively coupled silane plasma. The rationale behind the design of this system was to reduce the plasma volume to about 100 cm3 in order to achieve a higher power density in the plasma. The use of a large background pressure of helium and argon as buffer gases enabled us to run the process at total pressures of 500-800 mtorr for the purpose of achieving a better RF coupling efficiency due to the more resistive plasma [33]. After plasma-on times of 60-90 seconds we were able to produce single -crystal silicon particles, as shown in Figures 4 and 5. While exclusively single -crystal particles could be achieved at high RF power and high total gas pressures, particles were found to be le ss monodisperse than in the low-pressure discharge that was previously used. Chemical analysis using X-ray Energy Dispersive Spectroscopy (XEDS) confirmed that all particles were silicon. The thin amorphous surface layer is almost certainly a native oxide formed from air exposure. These two images were recorded on the FEI CM200 variable Cs microscope at the Institut fuer Festkoerperforschung in Jülich. One advantage of using this instrument is that the spherical aberration (Cs) can be set very close to zero allowing the image to be formed with very little contrast from the supporting carbon film. Selected area diffraction (SAD) patterns determined that all particles observed were diamond-structure silicon. High-pressure phases of silicon reported in other studies[25] were not observed. Scanning electron microscope (SEM) and transmission electron microscopy (TEM) studies at various tilt angles suggest that the particles produced are cubic, sometimes with surface steps. To our knowledge, these preliminary studies are the first that have produced large (several tens of nm) highly perfect silicon single -crystals.

Figure 4 - SE SEM micrographs of a single Si nanoparticle at different angles of tilt. As the tilt is varied the different projected shape of the nanoparticle is made clear. In the far left, the apparent cubic shape can be seen, while the hexagonal shape can be discerned in the final image on the right. Steps on the surface of the nanoparticle can be seen at intermediate tilts.

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Proposed Research: In order to achieve a better understanding of the nanoparticle growth process detailed plasma characterization is required. For the understanding of the formation of single -crystalline particles, a study of the gas temperature in the plasma is of utmost importance. Experimental studies of the gas temperature will focus on measuring the rotational temperature of the Fulcher band (d3Πu – a3Σg

+ transition) of molecular hydrogen [34]. We expect that a certain amount of H2 will form due to recombination of hydrogen released in silane dissociation or we may add hydrogen as diagnostic trace gas. Only a moderate spectral resolution is required for this method due to the large energy spacing of rotational levels in H2. Rotational temperatures have been shown to be in good agreement with gas temperatures found from Doppler broadening of atomic hydrogen lines [34]. Laser scattering studies will be performed in order to map the regions of particle trapping in the plasma, as we have done in previous studies [10]. In addition, we will perform Langmuir probe studies of the plasma density and electron temperature profiles in plasmas without silane addition in order to avoid probe contamination by silicon films. Since the silane partial pressure will typically be only about 1% of the total pressure, we do not expect that silane addition will drastically change these plasma properties. These experimental studies will be complemented by modeling efforts. We have developed accurate models for inductive plasmas [35-37] and the most comprehensive model for nanoparticle growth in silane plasmas [38, 39]. We will combine these two models to the first of its kind model describing particle nucleation and growth in inductive plasmas. This model will provide valuable guidance to tailor the particle synthesis process. In addition to the studies of the plasma and particle growth process, we will attempt to gain information about the particles themselves. Both in-situ and ex-situ particle characterization will be done. In our preliminary studies we have observed an orange glowing sheath around the plasma under high-power and high pressure conditions. Since this sheath also shows strong scattering of laser radiation directed through it, we assume that this sheath is formed by nanoparticles that emit thermal radiation. We will study this radiation in detail and derive the nanoparticle temperature from it. This will allow us to better understand the mechanisms of single -crystal particle production in the ICP and to optimize the process. Of particular interest will be the production of monodisperse single-crystal particles. Unlike the amorphous particles that are highly monodisperse, the single -crystal particles appear to show several discrete populations. Individual particles and when present, particle clusters, will be analyzed in several different ways. Particles will be collected on carbon film TEM grids for rapid statistical assessment of the size and shape. Other particles will be collected on TEM grids with thin metal films similar to those being used to build the transistors in this study. Nanoparticle Assembly

Goals: This section of this NIRT program has two goals. The first is to develop a parallel tool to position nanoparticle building blocks on surfaces. The strategy for this goal is based on directed self-assembly using a patterned surface with areas (receptors) that interact electrostatically with nanoparticle based

Figure 5 - HRTEM images showing the crystalline structure of the particles and the thin (1 to 1.6 nm thick) amorphous layer on its surface.

6

device components. The secondary goal is to investigate the effects of thermal treatments and various types of surface passivation on the electrical performance of the nanoparticle device. We will use electrostatics to inject nanoparticles into a reactor where thermal annealing, oxidation, and/or chemical vapor deposition (CVD) can be performed before depositing the particles on the substrate.

Background: The ability to assemble nanoparticles in two and three dimensions will enable the fabrication of a whole range of quantum-effect-based devices. The use of nanoparticles as building blocks requires novel assembling strategies. Most actively studied approaches include: (i) single particle manipulation [40-45], (ii) random particle deposition [41, 46-48], and (iii) parallel particle assembly based on self-assembly [49-54]. Strategies that use single particle manipulation and random particle deposition are considered straightforward methods to fabricate and explore new device architectures. One common example is to use a modified scanning probe microscope to position individual nanoparticles [40, 41, 44, 55]. Although such strategies allow testing new device prototypes at an early stage, they have seemingly insurmountable obstacles in yield and speed for the manufacture of nano devices. Random particle deposition will be used in this work to build the initial prototype devices, but also is unsuitable for manufacturing. Fabrication strategies that rely on mechanisms of self-assembly (reviews: [56-58]), however, may overcome these difficulties. Self-assembly is well known in chemistry and biology. It can handle extremely small objects and is massive ly parallel. In self-assembly, components (molecules, meso- or macroscopic objects) come together to form more complex structures without the intervention of humans or their agents (e.g., robots, assembly devices, manufacturing systems) [59-61]. Self-assembly occurs due to forces between the objects themselves. We and others have begun to use self-assembly and directed assembly to assemble components and nanoparticles onto substrates. Most actively investigated areas, currently, use protein recognition [62-65], DNA hybridization [51, 52, 66, 67], hydrophobicity/hydrophilicity [61, 68] , and magnetic interactions [54, 69, 70]. We have investigated the use of surface tension [71] and electrostatic interactions [72] to drive the self-assembly process. In the area of electrostatics, we have developed a method to pattern charge with 100 nm resolution, which provides the basis for the proposed research [72]. The advantage of using electrostatic forces compared to other strategies is that it is long-range and non-material specific (any particle can be trapped). In the proposal we will use such high-resolution charge patterns to accomplish parallel directed assembly of nanoparticles. Enabling Work: (I) Serial Charge Patterning. We developed an atomic force microscope (AFM) based instrument to: (a) pattern charges in dielectric thin films, (b) visualize the charge patterns with 100 nm resolution, and (c) measure the surface potential and the electrostatic field distribution on conducting and insulating surfaces [73-75]. In the proposed research we will use this instrument as a tool to generate and image charge patterns and to measure the electrostatic field distribution generated by externally biased electrodes. Figure 6 illustrates the principle and two representative charge patterns that we generated and visualized using our instrument [75]. (II) Parallel Charge Patterning. We have invented a printing process to pattern charges with ~100 nm resolution [72]. The process uses a flexible, micropatterned electrode to pattern an electret thin film in a parallel process by injecting and trapping charges over areas of ~ 1 cm2 in less than 20 s. The process and two representative images of a charge pattern are illustrated in Figure 7. (III) Nanoxerographic Printing. We have demonstrated the directed self-assembly of

Figure 6 – Principle of serial charge patterning (left) and recorded image of a fabricated charge pattern (right).

7

nanoparticles onto charged based receptors; we call this method Nanoxerography. We have demonstrated the directed self-assembly of nanoparticles from a powder, gas phase (aerosol), and liquid phase (suspension) onto high resolution charge [76]. Figure 8 shows a representative image of graphitized carbon nanoparticles that were assembled onto positively charged 200 nm wide lines.

(IV) Particle Extraction and Charging. Minnesota has a long history of developing aerosol technologies. Most pertinent to this proposal are two key discoveries. The first is the aerodynamic lens. First demonstrated by Liu[77, 78] , the lens uses aerodynamic drag forces to focus the particles contained in a diffuse aerosol into a narrow (~ 1mm) beam (Figure 9). The particles can then easily be extracted from the particle generation system using a critical orifice. This allows particles to be injected into a high vacuum region when they can be charged using an electron gun. The saturation charge has been found to be given by[79]:

)50/(8.21 21pp DnmDnmZ ++= −

Thus, a saturated 100 nm particle has about 190 charges, and a 50 nm particle has about 70 charges. Compared to the dipole forces or the singly charged particles that have been used for localization to date, the large net charge on these particles dramatically increases the electrostatic force. A number of particle detection systems have been built and demonstrated using this effect, most notably the particle beam mass spectrometer which several groups have used to measure concentrations and size distributions for particles between 5 and 300 nm in diameter.[80-82] Proposed Research: The first goal of this research is to further develop Nanoxerography (Figure 8) to enable to position nanoparticle building blocks on surfaces. To accomplish this goal we propose to address the following questions and strategies. Directed assembly from the gas phase. For a first demonstration, we generated a particle cloud inside a glass column using a fan to mix a powder of nanoparticles with the surrounding gas [76]. This experiment allowed us to demonstrate that a directed assembly from the gas phase is possible. The next step is to demonstrate the directed assembly of nanoparticles that are synthesized in a low-pressure plasma. A system that generates silicon nanoparticles of uniform size and shape has been installed in the Microtechnology Laboratory and is up and running. [83-85] To trap a single 100 nm-sized particle at a charged surface site, it will be necessary to have about the same amount of charges on both the particle and the charged surface area. The charge patterns we have fabricated thus far have a charge density of 100 elementary charges per surface area of 100 nm x 100 nm [72], which is in very good agreement with the

RIGID SUPPORT

charged area

200 nm

Figure 8 - Nanoxerography: Directed self-assembly of nanoparticles on patterned surface. charges.

Figure 7 - Principle of parallel charge patterning (left) and recorded images of different sized charge patterns (right). The charge patterns were recorded using Kelvin probe force microscopy. (published in Science)

8

saturated charge of 70 to 190 elementary charges per particle as discussed above. We believe that a close match will be necessary to neutralize the charged sites to prevent further particle assembly. We will also investigate the optimal velocity for nanoparticles to approach the endstation. A voltage offset applied to the substrate holder (Figure 10) will be used to slow the particles as they approach the substrate surface to enhance capture and prevent particle bounce.

Development of an assembly module that attaches to the existing plasma system generating silicon nanoparticles. We propose to develop an assembly module that is compatible with the plasma systems described in the previous section. The primary purpose of the proposed assembly module is to study the assembly of nanoparticles onto charged based receptors at different kinetic energies. The kinetic energy of the particles can be changed using a DC voltage offset that is applied to the sample holder. A glass tube will be used to enable a uniform electrostatic field distribution. The proposed assembly module will contain a Faraday cup to detect the charge on the particles, an electrometer that is attached to the sample to count the particles that assemble on the surface, and an integrated microscope for fast visual inspection. Although individual nanoparticles are difficult to visualize by optical means, surfaces with micrometer-sized areas that get covered with nanoparticles exhibit sufficient optical contrast. A direct visual feedback simplifies the optimization of the assembly parameters.

Processing of nanoparticles. Although the goal of the nanoparticle synthesis section is to produce perfectly crystalline particles, some additional processing may be required. The entire system consists of three chambers: particle synthesis, a high vacuum chamber for particle charging, and a third chamber to process nanoparticles in the aerosol (Figure 11). A photograph this system currently being assembled is shown in Figure 12. The electrostatic charge on the particles will be used to accelerate them through a small orifice into the processing chamber. Gas flow and particle dynamics calculations indicate that particles with a saturated charge require, an injection voltage of approximately 30 volts for very small particles and about 50 volts for particles up to 100 nm. We may also be able to use this effect to further screen the particle size and/or shape by balancing the viscous drag force on the particle against the

Figure 9 – Gas flow contours (a), and trajectories for three sizes of particles in a aerodynamic lens for a Reynolds number of 100. Perfect focusing occurs in (c). By combining several orifice diameters, a large range of particle Stokes numbers (sizes) can be focused.

High Voltage to slow down particles

100e

q,v,m

+

Electrometer that counts incoming

particles -

Glass

GND

Particle Assembly Module

Ekin

Figure 10 – Particle Assembly Module: After the processing the particles are injected into the particle assembly module. A DC voltage offset is used to slow down the particles, the electrometer allows to count the particles that assemble on the surface.

9

electrostatic attraction. The individual nanoparticles will be characterized using SEM and TEM both before and after processing. In this second chamber we will look at several effects. The first is nanoparticle annealing, particularly annealing in hydrogen. Hydrogen has been reported as being effective in lowering the temperature required for grain growth during the deposition[86, 87] and annealing[88, 89] of amorphous silicon. The mechanism for this effect is H insertion into strained Si-Si bonds as discussed in Sriraman.[90] Thus, even particles with poor crystallinity as-synthesized may be made crystalline after annealing. This may be important if, during the synthesis step, one has to trade off crystallinity against other desirable attributes such as size uniformity or particle production rates. The second area of investigation for this chamber is the deposition/growth of surface passivation layers. If the particles have a high concentration of surface states, these states will increase the off-state leakage and may reduce the carrier mobility. The effect is particularly pronounced in surface effect devices such as the MOSFET. For that reason we will investigate several surface passivation processes. The simplest will be thermal oxidation. The oxide thickness, which will be inferred from high-resolution transmission electron microscopy, will generally be in the 1.5 to 2.5 nm range. We may also coat some particles with high permittivity films since we have a great deal of experience in this type of deposition. This may be done in the gas phase during transport to the end station, or it may be done after the particles have reached the endstation. The latter would be particularly appropriate for low temperature processes. These coatings will be characterized using TEM in both cross-section and plan-view modes. Nanoparticle Devices

Objective: The goal of this work is to design, build, and characterize nanoparticle transistors, optimizing the device performance and using these devices to electrically characterize the material and the contacts. Both randomly deposited and electrostatically assembled devices will be studied. The primary goal of the

-Vinj

Electron Gun

Gas Inlet

Particle Processing Chamber High

Vacuum

Figure 11 – Particles from the synthesis chamber (not shown) enter a high vacuum chamber (left) are charged to saturation with an electron gun and accelerated through an orifice into the particle processing chamber.

+ + + +

Plasma Synthesis Chamber

Aerodynamic Lens

Electron Gun

Particle Processing Chamber

End Station Gun

Figure 12 – Photograph of nanoparticle synthesis, charging, and assembly system that is currently being constructed. The end station for electrostatic self-assembly is not yet built.

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project is to build devices from single crystal nanopartic les, however, these devices will also be used to characterize the material properties of the nanoparticles that are not perfectly crystalline.

Background: Many groups have investigated the optical properties of semiconductor nanoparticles[91-100], however the use of these nanoparticles in electronic devices is a relatively new field. Several groups have investigated nanoparticle precipitation in thin films. DiMaria and colleagues studied charge storage in nonstoichiometric SiO2.[101] This work was followed by Wahl[102] , who was able to distinguish true nanoparticle charge storage from interface effects. Flagan and Atwater carried out pioneering work in the use of silicon nanoparticles embedded in a SiO2 matrix.[103] The deposition conditions were adjusted to deposit silicon nanoparticles embedded in SiO2 to be used as the gate electrode of MOSFET memory devices.[104] While this has been seminal work, it has focused on many-particle devices. While the formation of individual semiconductor nanoparticles for ultimate use in electronics has been studied [105-110], very little is published concerning actual devices. Scanning probes have been used to manipulate the position of nanoparticles and contact them.[110] Flagan used this technique to make two contact measurements of silicon nanoparticles.[111] In some of the most impressive electrical results to date, Erokhin et. al. have made ~3 nm CdS/CuS heteroparticles in a Langmuir trough and observed negative differential resistance.[112] The construction of transistors, however, is much more difficult since it requires at least three contacts. Furthermore, proximal probes cannot be used to manufacture nanoparticle devices that integrate many transistors.

Enabling Work: One of the key unknowns for building nanoparticle transistors is the formation of contacts. To avoid the need for using extremely high performance lithography in early demonstrations, nanoparticle topography was used to form self-aligned contacts. This was tested by depositing highly monodisperse amorphous silicon nanoparticles on Ti. Next a conformal film of SiO2 was deposited in a high-density plasma. Photoresist was spun on and etched back in an oxygen plasma until the oxide was exposed on top of the particles, but nowhere else. The plasma chemistry was then switched to an oxide etch and ~30 nm holes were opened up to the particles. Finally nickel was evaporated, patterned by photolithography, and annealed to form a silicide. The result is shown in Figure 13. The electrical performance of the metal-semiconductor-metal (MSM) device is shown in Figure 14. For voltages on the nickel less than 2 V, the device displays conventional Schottky emission. A barrier height of 0.2 eV is extracted from the temperature dependent current-voltage characteristics, in good agreement with the nickel work function. At high bias the current follows space charge limited behavior[113] for injection into an amorphous semiconductor:

12

111

112

)1( +

++−

++

+

=m

mmm

t

meffSi d

Vmm

mNmqNJ εµ

where NSi is the silicon conduction band density of states, Nt is the trap density in the particle, q is the elementary charge, ε is the permittivity of silicon, d is the diameter of the particle, and V is the applied voltage. m is given by T/Tt, where T is the temperature and Tt is a parameter that describes the energy distribution of the traps, which are assumed to decay exponentially from the band edge (Ec) as

−=t

c

t

tt kT

EEkTNN exp

Figure 13 – NiSi contact to a 100 nm amorphous Si particle

Metal

SiO2 Metal

Particle

11

Figure 14 shows our data for an array of roughly 2000 100 nm particles connected in parallel. We find a Tt of ~1100 K and a trap density of 1018 cm-

3. The fact the we can observe this current and that is follows the expected behavior for amorphous silicon clearly shows that we can make contact to nanoparticles using conventional optical lithography, along with etch and metallization processes. Furthermore, the use of nanoparticle topography for forming contacts appears to be a robust process since it works for thousands of contacts in parallel.

Another question is how best to make nanoparticle transistors. A geometry in which the current flow is out of the plane of the substrate (vertical flow) is attractive for several reasons, notably that it allows one to build very short gate length devices since the device length is determined by the deposited thickness, not by lithography. To begin with a manufacturable device, we have chosen transistors made from 30 to 100 nm Si particles. To enable this work we have modeled both metal-oxide-semiconductor field effect transistors (MOSFETs) and Schottky barrier transistors (SBTs). Nanoparticle MOSFETs are similar in principle to FINFETs, which have gathered considerable attention recently[114]. They are superior in several ways, however. They have four current carrying surfaces instead of two, and they do not rely on a substrate for their crystallinity, allowing 3D integration. In a nonplanar geometry, particularly one that lacks a contact to the body of the transistor, MOSFET performance is affected by this floating substrate potential. SBTs are immune from there so-called body effects [115-117]. Furthermore, they can be scaled to extremely small dimensions since a metal-semiconductor junction rather than a dopant junction is used to create a potential barrier for carrier injection into the channel. Finally, the geometry of the device would strongly encourage bulk conduction which, combined with the undoped particle, produces high mobilities. The combination of a vertical flow geometry with an SBT device has been recognized by others recently.[118] DESSIS, a commercial 2D/3D hydrodynamic drift/diffusion simulator offered by Integrated Systems Engineering, was used to model an SBT. The results (Figure 15) show good agreement with the measurements given in Figure 14 when the gate voltage is held at zero. The map of the electron density shows the high electron concentration (n~4x1017 cm-3) carrying most of the current through the bulk of the particle. The current can exceed 10 µA for a single crystal silicon device, which is quite suitable for high performance applications. Many particles can easily be connected in parallel in a single transistor when more current is desired. On/off current ratios exceed 104 for a properly designed device.

Voltage (V)

2.0 3.0 4.0 5.0 6.0

Cur

rent

(A)

10-9

10-8

10-7

10-6

50 C, m=2.92675 C, m=2.712100 C, m=2.570125 C, m=2.496

Tt=958 K

1000/T(K)2.4 2.6 2.8 3.0 3.2

m

2.2

2.4

2.6

2.8

3.0

Figure 14 – Current voltage characteristic for ~1000 parallel nanoparticles. Insert shows m factor vs 1/T.

Figure 15 – Charge density due to electron injection for 50 nm particle, VDS=VGS=1 V. Family of curves for such a device.

Vds (V)

I ds (

µA

)

Vgs = 1.0

Vgs = 0.8

Vgs = 0.4

Vgs = 0.2

0 0

5

10

.2 .8 1 .4 .6

Source

Source

Gate

Drain

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We can now roughly estimate the switching performance of a nanoparticle transistor. Let us assume that the ultimate form of this device uses 30 nm cubic particles. The capacitance of a nanoparticle of diameter D is given by 2π k ε o D or about 10 to 20 aF for a 30 nm particle, depending on the surrounding media. The current carrying capability of this device is about 10 µAmp (Figure 15). The intrinsic speed of such a device then is of order 1 V * 10-17 F / 10-5 Amp = 1 psec. This performance can be understood through a simple charge calculation. Space charge effects limit the charge density in the nanoparticle transistor to approximately 5x1017 cm-3 (see Figure 15). Multiplying by the volume of the particle, this corresponds to approximately 13 electrons. This number of electrons can be supplied with 10 µA of current in much less than 1 psec. (Note that the SBT does not have a S/D junction capacitance per se. Of course, this simple calculation ignores effects such as gate and wire charging. The oxide capacitance for a 2.0 nm layer of SiO2 is about 1.5 µF/cm2. Multiplying by the gate area gives a gate capacitance of about 15 aF. This suggests that, including gate charging, transistors should be capable of ring oscillator (fan in / fan out = 1) performance of up to 2.5 psec or ~400 GHz if we ignore wire capacitance. Furthermore, it is likely that a 30 nm nanoparticle will approach quantum dot behavior[119] which exhibits diamond shaped conduction due to charge discretization[120]. Because nanoparticle devices can formed on low loss substrates, however, they have a much better prospect of approaching these intrinsic speed limits than devices built on silicon wafers which have significant parasitic capacitances and resistances.

Proposed Research: Thus far we have built MSM structures from amorphous nanoparticles and simulated the operation of amorphous and crystalline MOSFETs and amorphous and crystalline SBTs. In this NIRT we focus on the construction of crystalline SBTs and MOSFETs, scale them to very small dimensions, and compare their operation to simulation results. Some amorphous devices will also be built to electrically characterize the material quality. In the 30 to 100 nm particle regime, bulk behavior is expected. Since the fabrication process will be easier for larger particles, we expect our initial demonstrations to be near 100 nm, but will scale the devices quickly as we gain experience. Quantum devices with diameters less than 30 nm will be pursued in follow-on projects that will spin out of the NIRT. Figure 16 – Fabrication sequence for electrostatically assembled (left)

and randomly deposited nanoparticle transistors.

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The basic transistor fabrication sequence will be an extension of our very successful MSM process. Particles will be deposited on the drain metal. This lower metal will be unpatterned for random particle deposition (right side of Figure 16) or patterned for self assembled devices (left side of Figure 16). The lower insulating layer will then be deposited and planarized at a thickness greater than the diameter of the particle. Next the lower insulator is etched down. The gate / upper insulating stack is then deposited and polished back, exposing the gate metal over the top of the particle. Heavily doped (N+) polysilicon will be our initial gate material. The gate is then etched back on the sides of the particle using a simple isotropic etch. An insulator to fill the hole left by this etch is again deposited and polished back to the surface of the particle. The source electrode is then deposited, patterned and etched, and contacts to the gate and drain are opened up. N+ poly may also be used as our top and bottom electrodes for building MOS devices since this provides a simple dopant source. Alternately, for building MOSFETs, the metal contacts could be implanted and the source and drain diffused from the silicide. This process has been studied in the literature for forming very shallow Si junctions and is known as silicide as a diffusion source (SADS). [121, 122] Optical lithography will be used for device with many particles in parallel. We will also use a new Raith 150 direct write e-beam lithography system, capable of 30 nm resolution, to pattern single particle devices.

The fabrication process does not require a critical alignment and should be usable for particles down to ~10 nm. The baseline process will use a thin (~1.5 nm thick) layer of thermal oxide on the particle. We have had considerable experience depositing high permittivity films on silicon however, [123-125] and will also investigate their use on nanoparticle devices. It would be reasonable to expect that the increased gate coupling provided by these films will improve both SBT and MOSFET performance. The finished SBT is shown in cross section in Figure 17. As described in the previous section, we will explore different patterns of surface charge to localize the particle to fabricate the transistor at predetermined locations. In the illustrated example, we will pattern a charge onto the gate dielectric to generate a stray field. To pattern the charge, we will use the parallel printing process [72]. The stray field pulls a charged nanoparticle into the center of the hole. We will explore different electrode layouts. In the illustrated example we will apply an external potential between the gate and source electrode to generate a stray field. This stray field pulls a charged nanoparticle into the hole in the gate dielectric. After a successful assembly of the nanoparticle,

the completion of the fabrication would involve steps identical to those used in the random deposition process: deposition of the gate (B2 only) and upper insulating layer; polishing to expose the surface of the particle (C); and deposition of the drain contact (D). The fabrication process for these devices will be done using a new Raith 150 electron beam lithography system installed in January, 2002, at the University of Minnesota. One interesting question is the effect of particle orientation on device behavior. This is an issue with many nonplanar structures.[126] In our work to date we find that crystalline nanoparticles are highly faceted and that the predominant facet is (100), with a minority of (111) facets. We believe that this distribution is not inherent in the crystalline nanoparticle creation process, and will optimize the process to produce (100) facets exclusively. The (111) facets will induce an undesired variability in the materials properties. The electron velocity for (111) Si is identical (100) Si at low and high fields. At moderate fields it is about 10% less in (111) Si[127]. The silicide contact reactions, however, may be different on the two faces. Our experience thus far is that nanoparticles always come to rest on a facet. When the

Figure 17 – Cross section schematic of nanoparticle transistor

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particle creation and annealing process is refined to provide only (100) facets, the source and drain contacts will be made on (100) faces. The axial orientation of the particle is unimportant since the current flow is perpendicular to the facet on which the particle rests. The orientation of individual nanoparticles which are less than 100nm on the side can only be determined directly by TEM. However, the shape of the particle observed in the SEM can give a good indication of the orientation when correlated with the TEM results. The formation of silicon nanoparticle contacts may be different than the same reaction at a planar metal silicon interface. Furthermore, the nanoparticle may have a thin surface passivation layer that will need to be chemically reduced for the lower contact. Ti is well known to reduce SiO2. This effect is evident in our MSM results. The contact formation process will be studied both electrically and using both imaging and chemical analysis in the TEM. Methods to reduce the resistance of this contact will be developed. One possibility is to deposit hydrogen passivated nanoparticles on the metal and form the gate insulator after bottom contact formation. Since the gate oxide is only a few nanometers, this approach may ultimately be workable with only modest substrate temperatures, particularly if one uses a high permittivity gate insulators. We typically deposit these layers at ~250 oC. The SBT will require the development of Schottky barriers of the correct height for the top contact. NiSi2, which was used for our MSM structures, appears to have an excellent barrier height for forming n-channel devices (~0.2 eV). The most favorable Schottky contacts for p-type conduction is PtSi.[128] TEM has been extensively used for studying conventional thin film reactions and large-area contact layers but not yet for interactions with nanoparticles which are necessarily three-dimensional objects. Testing of these devices will be done using existing systems including HP 4156 Parameter Analyzers and an HP 4194 Impedance Analyzer, using conventional low frequence wafer probing equipment. High freqency measurements will also be done to extact fT and fmax using an HP 8510 Network Analyzer capable of 26 GHz measurements on a Cascade microwave probe stand. Simulation of both SBTs and MOSFETs is planned to design transistors and to understand measured device characteristics. The devices will be modeled using the Integrated System Engineering package, DESSIS. This is a device simulator for one, two and three-dimensional semiconductor devices. It can be used in either hydrodynamic drift-diffusion mode or Monte Carlo modes. We will validate default models for various Schottky barriers before applying them to the nanoparticle structures. We will use the code to predict optimal transistor structures and help direct the device demonstration work.

Project Management We have assembled a Nanoscale Interdisciplinary Research Team that includes four faculty from three different academic departments at the University of Minnesota. In the proposed budget we request support for four graduate students and one postdoctoral research associate. Student lines will be assigned on a project basis rather than having them reserved by individual faculty members. All of the students will be co-advised by two members of the team. This will provide many interdisciplinary opportunities for the students. In addition, we plan to leverage funding from the University’s Nano IGERT to support additional graduate research associates. The Nano IGERT program provides funding for the first two years that the student is in the U of MN Graduate School Three students are currently on IGERT support and will be available to work on this project. Finally, some of the NIRT PIs have obtained seed program funding from the University of Minnesota MRSEC. The MRSEC will provide materials, supplies, and expertise on the materials aspects of this work, which allow us to direct most of the NIRT resources toward assembly and device demonstrations and for electron microscopy characterization of both the nanoparticles and device structures.

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The proposed research activities nanoparticle synthesis, nanoparticle assembly, and nanoparticle devices are not separate projects, but parts of one project that interrelate with each other on a continuous basis. Professor Campbell, the P.I., will have responsibility for project management. Campbell has considerable experience as the P.I. of multi-department, multi-investigator projects including three projects currently funded by Sematech and the Semiconductor Research Corporation. In these projects, faculty and students have interacted on essentially a daily basis; meetings of the entire group are held every two weeks. We have learned that such regular meetings are essential for nurturing the interdisciplinary spirit of our research and for making progress toward meeting project objectives. We plan to continue such an approach in the proposed research. Education and training are central aspects of our research, and are discussed in the next section. As further evidence of administrative competence, Professor Campbell also directs the University’s Microtechnology Lab, with an annual budget of approximately 1.6 M$.

Education And Outreach Uwe Kortshagen will head education and outreach for the proposed NIRT. It will take advantage of a number of programs already in existence to leverage NIRT funds. At the undergraduate level, our goal will be to increase the flow of graduate students into this research area. REU: The first is the NSF sponsored Research Education for Undergraduates (REU), which has been renewed for a second five years in the ECE Department (Ernie, PI). The project sponsors juniors and seniors who may be interested in pursuing a graduate program in engineering for ten weeks during the summer. Several of the NIRT faculty members have had experience working with REU students. SENIOR HONORS: The NIRT will also work to include senior honors students each year as part of the research program. Our past experience with these students is that they overwhelmingly go to graduate school and may fit well into our IGERT.

At the graduate level, the NIRT will benefit from collaboration with the NSF-IGERT program for “Nanoparticle Science and Engineering” (PI: Kortshagen). The IGERT provides both an educational and an outreach infrastructure. At the core of the innovative educational approach of the IGERT is the development of a new free-standing graduate minor in “Nanoparticle Science and Engineering.” Through this new M.S. and Ph.D. level minor program, students from any science and engineering departments will have access to three newly developed interdisciplinary core courses covering all aspects of nanoparticle research. The minor was first offered during the Fall of 2002. All NIRT PIs are members of the Graduate Faculty of the minor program and will be involved in teaching the new core courses. Campbell and Carter are currently co-teaching Introduction to Nanoparticle Science and Engineering. Kortshagen is Director of Graduate Studies of the minor and will teach the course “Nanoparticle Science and Engineering Laboratory” in Spring 2003.

NIRT outreach will also leverage IGERT programs. Companies such as 3M, General Dynamics Information Systems, and TSI have committed to provide summer internship opportunities for students. The NIRT will offer an international exchange through the IGERT’s formal student exchange agreement with a large federally funded special research project in Germany, the Sonderforschungsbereich 445 (SFB 445) “Nano-Partikel aus der Gasphase: Entstehung, Struktur und Eigenschaften” (Nanoparticles from the gas phase: creation, structure, and properties) that involves 11 faculty from 6 different departments. In addition to the internantional exchange program a unique collaboration with Florida A&M University (FAMU) with the aim to recruit minority students into nanoparticle research at the U of MN has been put in place using the IGERT program. This collaboration awards scholarships to FAMU undergraduates that provide financial support during the junior and senior year and funds for research summer internships for two summers. The NIRT outreach will use this collaboration to recruit minority students.

As another part of our outreach activities, we will apply for supplements through the NSF-Research Experience for Teachers (RET) program. Some of the NIRT faculty have already used the RET program to involve high school teachers in research projects. These activities have resulted in ongoing interactions with those teachers that include classroom visits of faculty in high school physics courses. The NIRT will

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also leverage the outstanding Minnesota MRSEC outreach program since half of the NIRT participants are also MRSEC participants. This will involve the Faculty-Student Team Research Fellowship Program, which is designed to provide unique research experiences for faculty and students from four-year and tribal colleges. It has proven exceptionally effective in attracting Native American students. The NIRT will provide these participants opportunities in the area of nanoparticle device research.

Broader Impacts Statement

This proposal will have the following broad impacts as defined by the National Science Board: Advance Discovery and Understanding While Promoting Teaching, Training and Learning

• The NIRT support is an excellent fit with existing programs. The MRSEC (one year seed grants) provides very limited support that is restricted to materials aspects of the project. The IGERT (two-year fellowships) support is limited to student support with little funds for materials and supplies. The NIRT would form the missing core of the funding picture, providing longer-term support to students now completing IGERT fellowships and supporting the processing and device work critical to the success of the project. By “graduating” students from the IGERT or MRSEC to the NIRT, the NIRT funding could be stretched to support as many as seven graduate students participating in different parts of this work over the four-year NIRT. These students will be some of the seminal researchers in the area of crystalline nanoparticle materials and devices.

• The nanoparticle IGERT has already developed a three part introductory course on nanoparticle synthesis, characterization, and use in device structures. Approximately 40 students enrolled for the first offering this semester. Funding this NIRT would ensure that the material in this course is codified into a new text or monograph to help strengthen this research area.

• Professor Campbell has authored the leading textbook on Microelectronic Fabrication. Currently in its second edition, the book has gone through eight printings, sold approximately 20,000 copies, and has been used worldwide at leading schools. NIRT funding would ensure the inclusion of material on nanoparticle and other nonplanar devices in the third edition.

• Professor Carter has co-authored (with Dave Williams) the leading textbook on Transmission Electron Microscopy. This text has gone through three printings, sold over 8,000 copies, and is used worldwide at leading schools. Again, NIRT funding would ensure the inclusion of material on nanoparticle and other nonplanar devices in the second edition.

• Approximately 70 students take the Microfabrication course each year at the University of Minnesota (the course is offered twice per year). A draft of the new material for both the monograph and the nonplanar devices chapter will be distributed to these students during the second year of the project to provide valuable feedback before publication in the textbook and to help introduce the material before a new edition of the book is available. A copy of this material will be made available on the project web site (see below) for other schools that are using the book.

• In addition to the graduate student support, we plan to create research opportunities for undergraduates. In the past this has included senior honors projects, REU students, and internally funded undergraduates

• We anticipate that the students involved in the project will be presenting this work at professional meetings. Our budget includes this travel component. In addition, NIRT partic ipants will present at the annual IGERT summer symposium on nanoparticle research.

Enhance Infrastructure for Research and Education

• This project will support the final components needed to build our nanoparticle synthesis and nanoparticle assembly system. This unique tool will allow us to create nanoparticles, extract them

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from the plasma, anneal, oxidize, coat, charge, and localize them. The instrument will be a world-wide unique tool for research in non-planar device manufacturing.

Broad Dissemination to Enhance Scientific and Technological Understanding

• As part of our mission, we will disseminate the results of our work, not only in conventional publications and technical conferences, but also through multidisciplinary presentations and through interactive web-based discussions of nanoparticle technology. We will set up a new web page with links to other nanoparticle sites to make it easy to get information on these technologies. This will link to the existing IGERT web site, providing content and strengthening the appeal of both.

Related previous NSF Support (in last 5 Years) S. A. Campbell Studies of Particle Nucleation, Growth and Transport in Advanced Semiconductor Processing NSF (GOALI, CTS-9909563); $440,000; 9/1/99-8/31/03 McMurry, PI, with co-PIs Campbell, Girshick & Kortshagen

We have studied silicon nanoparticles created in a silane oxidation process and mapped out the regions of particle creation and incorporation. The morphology of the film agreed well with the presence of particles in the reactor as measured by particle beam mass spectrometry (PBMS). We have also used high-resolution electron microscopy to image embedded particles. The particle-laden films had an apparant negative differential resistance due to the charging of these silicon islands. The permittivity of the particle -laden films scaled with the PBMS current. Charge retention scaled with particle density. The GEC reference cell reactor was modified to operate as an inductively coupled plasma (ICP). Time-resolved measurements showed that large concentrations of particles were formed in Ar:SiH4 (100:5) plasmas. Most of the particles were confined by the space charge electric field and were negatively charged as indicated by the sharp negative current pulse detected by the PBMS after the plasma was turned off. At 10 mtorr particles were sampled on TEM grids. Particles were highly monodisperse and non-agglomerated.[129, 130] We also established a direct relation between the particle size and the duration of plasma operation.

We have also developed computational models of nucleation and growth of particles during low-pressure (~1 torr) chemical vapor deposition of SiO2 films from silane and oxygen for both thermal and plasma environments. We developed a detailed silicon oxide clustering mechanism, assuming that the dominant clustering species were the highest concentrations species, namely SiO, SiO2, HSiOOH and SiH2O. This was coupled to a moment-type solution of the aerosol general equation, which modeled single particle growth by heterogeneous chemical reactions at partic le surfaces, coagulation, and particle transport by convection, Brownian diffusion and thermophoresis. Simulations were conducted for both time-dependent, zero-dimensional systems[130] and for a one-dimensional stagnation-point flow system.[131] We found excellent agreement between the model and our experimental results (Fig. 18). To the best of our knowledge, this is the first time that such calculations have shown this level of agreement with experimental results. More recently we have developed a one-dimensional model for clustering in inductively-coupled silane-oxygen plasmas including electron-impact dissociation, electron attachment, charge exchange, neutralization, ion-molecule reactions, etc.[132]

The work performed under the GOALI grant has to date been reported in 6 journal papers .[133, 134] [129, 131, 135, 136] Three additional journal papers have been submitted and are presently under

0.5

1

1.5

200 300 400 500 600 700 800 900 Substrate temperature (degrees C)

Particle free

Particle formation

(snow region)

Dirty wafer

Clean wafer

— Mod

Figure 18 - Predictions (lines) and experimental results (points) for gas-phase nucleation onset and on-wafer deposition.

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review.[10, 11, 137] In addition, this work has generated twelve conference publications, two PhD theses, and one MS report. Five students have been involved in this activity.

C. Barry Carter Internal Interfaces In Compound Semiconductors NSF/DMR-9522253-002 This program (funding $300,000, 16 Aug. ’95 - 16 Aug. ’98) partially supported two students, Dov Cohen (Ph.D. ’99) and Jeff Farrer. 8 journal papers were published, 2 more in print; 15 papers published in conference proceedings.

The part of this program that is particularly relevant to the present proposal is the use of HRTEM to image internal defects and defects and their quantitative analysis. We grew GaP epilayers on lattice-matched Si substrates with different orientations using MBE. We formed different antiphase boundaries, twin boundaries and both intrinsic and extrinsic stacking-faults in the films grown on Si substrates. We also examined low-angle interfaces in polycrystalline GaP boules. ANTIPHASE BOUNDARIES (APBs) are planar defects across which the crystal polarity undergoes inversion; the Ga and P sublattices are interchanged upon crossing the APB. The inversion of crystallographic polarity across an APB was confirmed using the asymmetrical interference contrast in CBED patterns. We found many APBs faceting along well-defined planes ({110} and {100}). {110} facets have equal combinations of Ga and P antisite bonds, while the {100} facets contain entirely either Ga or P antisite bonds. Since the lengths of Ga-Ga and P-P antisite bonds are different, a rigid-body lattice translation, R, is present which causes SF (or α) fringes in the TEM The magnitude of R was determined quantitatively by comparing the fringes with computer simulations. Combinations of the different stoichiometric and antisite bond are possible along any interface in GaP. DOUBLE RIBBONS, observed for the first time in GaP, consist of both intrinsic and extrinsic stacking faults separated and bounded by three parallel Shockley partial dislocations. They have been observed previously in Ge, Si, GaAs and FCC metals. By measuring the separation of the dislocations, the intrinsic and extrinsic stacking-fault energy (SFE) was calculated from elasticity theory. The SFE is a measure of a crystal’s tendency to twin during growth and is an important parameter in computer modeling.

HRTEM is the ideal method for investigating the atomic structure of twin boundaries in GaP. Microtwins were observed to facet primarily along the {111} planes, and to a lesser extent {112}/{112} and {111}/{115} planes. The observation of so many {112}/{112} facets is unusual and may be related to the interface bonding. Second-order (? =9),{221} boundaries were also found in areas where two ? =3 boundaries intersected. Quantitative HRTEM for the interpretation of atomic structure requires a detailed comparison of the experimental image with atomic models [64-66]. We worked in collaboration with Livermore (LLNL) to developed computer routines to extract quantitative information and made a preliminary application of this analysis to HRTEM images of twin boundaries in GaP.

U. Kortshagen

NSF ECS-9731568, award $169,148; 07/01/98-06/30/03 Contaminant Particle Formation in Radio Frequency Silane Plasmas U. Kortshagen PI: Co-PI: S. L. Girshick

Research under this grant has focused on the development of a model for particle nucleation and growth in silane plasmas. The model we have developed couples a detailed plasma-chemistry model to describe nucleation, a plasma-aerosol model to describe particle coagulation and surface growth, and a global plasma model to describe the temporal evolution of the plasma parameters during particle growth.

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Our plasma chemistry model is the most comprehensive model available for silane plasmas. It includes more than 300 species – including neutral as well as negatively and positively charged silicon hydride clusters with up to 10 Si atoms – and 5000 reactions and is solved for a 0-dimensional system using a modified version of SENKIN [138]. The thermochemical data of all species were estimated using a group additivity scheme [139]. Ab initio calculations of electron affinities were performed in order to determine the thermochemical properties of negative ion [140]. Diffusion of neutral and positive clusters is accounted for by approximating it as an irreversible reaction, while it is neglected for negative clusters since these are confined in the plasma due to the space charge field.

Our plasma aerosol model is based on the solution of the Aerosol General Dynamic Equation using sectional approach [141]. It determines the particle size distribution over six orders of magnitude of particle volume divided logarithmically into 180 sections [22]. Coagulation between aerosol in various sections as well as surface growth is considered taking into account the charge distribution of particles within each section. This charge distribution is obtained from a comprehensive model that includes electron and ion attachment to the particles, secondary electron emission, and UV photo detachment [22].

Over the course of the development of this model, we have achieved the following novel results:

1. We identified the positive ion density in the plasma as an important threshold density that the concentration of primary particles must reach for coagulation of primary particles to occur. If the primary particle density remains below this threshold, most primary particles are negatively charged and coagulation is thus prevented [22].

2. Our nucleation model demonstrated that anion-neutral reactions are the fastest reactions leading to cluster formation in silane plasmas. These reaction proceed through two pathways of either silyl (mono-radical) anions or silylene (di-radicals) anions reacting with silane molecules [39].

3. Recently, we were able to solve the longstanding open question about the temperature dependence of the particle formation. Many studies reported that an increase of the gas temperature delays p nucleation [142]. Explanations found in the literature including temperature dependence of the electron attachment rate [143], collisional quenching of vibrationally excited species [144], and the reduced gas density at higher temperatures [145] (at constant pressure) did not yield a conclusive explanation. Our model, including all the above effects, showed that the Tgas

3/2 -dependence of the diffusion rate of particles and clusters can explain the observed delay of particle growth [38] , see Fig 19.

To date this work has resulted in 3 journal publications [22, 39, 146], 5 invited conference talks, and 12 contributed papers. Upendra Bhandarkar (M.S. 2000) will graduate with the Ph.D. in Dec. 2002.

Figure 19: Left: measured laser induced particle evaporative emission intensity at different gas temperatures [142]. Right: Computation of the same signal. Dotted lines are with diffusion coefficient kept at 20°C, while solid curves use actual temperature for the diffusion coefficient, demonstrating that diffusion is the main influence for the delay in nucleation.

p pp pp p

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