Thin Solid Films, 206 (1991) 59-63 59

Concentration transient analysis of antimony surface segregation during Si( 00 1) molecular beam epitaxy

L. C. Markert and J. E. Greene Department of Materials Science, Coordinated Science Laboratory, 1101 W. Springfield Avenue, University of Illinois, Urbana. IL 61801 (U.S.A.)

W.-X. Ni, G. V. Hansson and J.-E. Sundgren Department of Physics, Linkdbing University, S-581 83 Link@ing (Sweden)

Abstract

Antimony surface segregation during Si(OO1) molecular beam epitaxy (MBE) was investigated at temperatures T, = 515-800 “C using concentration transient analysis (CTA). The dopant surface coverage 0, bulk fraction y, and incorporation probability 0

during MBE were determined from secondary-ion mass spectrometry depth profiles of modulation-doped films. Programmed T, changes during growth were used to trap the surface-segregated dopant overlayer, producing concentration spikes whose

integrated area corresponds to 8. Thermal antimony doping by coevaporation was found to result in segregation strongly

dependent on T, with QSb values up to 0.9 monolayers (ML): in films doped with Sb+ ions accelerated by 100 V, 8,, was less

than or equal to 4 x 1O-3 ML. Surface segregation of coevaporated antimony was kinetically limited for the film growth

conditions in these experiments.

1. Introduction

Surface segregation during silicon molecular beam epitaxy (MBE) severely limits control over the dopant depth distributions and bulk concentrations. Thermal coevaporation of dopants such as aluminum [ 1,2], gallium [ 1, 31, indium [4], and antimony [ 5,6], can result in significant surface coverages 8,. The dopant overlayer then acts’as a reservoir which continues to dope the silicon film after the incident dopant flux is terminated, broadening depth profiles by up to several hundred nanometers [2,4]. Surface segregation, com- bined with thermal desorption from the dopant over- layer, leads to low dopant incorporation probabilities a, which range, under typical silicon MBE growth conditions, from approximately 10e3 to less than lo-’ for coevaporated antimony [6, 71, gallium [ 1, 31, indium [8], and arsenic [9, lo]. Changes in the film growth temperature T, alter cd exponentially, making control over the steady-state dopant concentration difficult. High dopant surface coverages during growth can also interfere with epitaxy, resulting in high extended defect densities which reduce carrier mobilities and limit the maximum useful dopant concentration C,,,,, obtainable by coevaporation. In the case of thermal antimony doping [6, 111, for example, C,,, < 10” cm-3.

Approaches to solving the above problems generally involve the use of accelerated ions. One technique shown to increase gd for antimony [ 12, 131, arsenic [ 131, indium [8], and aluminum [ 141 in silicon MBE is recoil-

implantation of dopants from the surface-segregated overlayer by applying a negative substrate bias to at- tract and accelerate Si+ ions produced in the electron- beam evaporator. However, this method inherently requires substantial dopant surface coverage, therefore segregation-induced profile broadening remains a prob- lem [8]. A more direct approach is to ionize and accelerate the dopant flux itself. Abrupt depth profiles and increases in od by many orders of magnitude have been obtained using arsenic [9], antimony [7, 151, and indium [ 161 ions with dopant ion energies Ed between 50 and 1000 eV. Substitutional dopant incorporation over many decades of concentration, even exceeding the solid-solubility limit [ 171, has been reported. There was no evidence of residual ion damage in Si( 001) films grown at T, 3 650 “C with Ed < 200 eV and examined using transmission electron microscopy [ 171, tempera- ture-dependent Hall effect measurements [ 17, IS], and photoluminescence [ 19,201.

In order to calculate doping profiles accurately, mod- els of dopant incorporation during silicon MBE [7,21] must account for surface segregation, which requires knowledge of both dopant surface coverage e,(T,) and bulk fraction yd(TS). In this paper, we present initial experimental results, obtained using concentration tran- sient analysis (CTA) to determine 8, and yd simulta- neously, from an investigation of antimony segregation kinetics during Si(OO1) MBE. CTA is a new technique which uses concentration depth profiles, measured by secondary-ion mass spectrometry (SIMS), to determine

0040-6090/91/$3.50 0 1991 - Elsevier Sequoia, Lausanne

Osb from concentration spikes incorporated during tem- perature-programmed modulation-doped film growth. The segregation ratio rSb = QSb/ySb for thermal anti- mony doping was found to range from 4 x IO2 to 3 x 10’ in silicon films grown at T, = 5 15 -800 “C with a deposition rate Rsi = 0.8- 1.9 urn h-‘, whereas rsh was less than 60 in films doped with Sb+ ions accelerated by an applied potential Vs, = 100 V.

2. Experimental procedure

All silicon layers were deposited onto boron-doped Si(OO1) wafers, with resistivities of approximately 10 R cm, in a Vacuum Generators V-80 MBE system with a base pressure of 5 x lo-” Torr. The growth system, substrate preparation, and film growth condi- tions are described in detail elsewhere [7, 221, and only a few essential details will be discussed here. After degreasing the substrates, an oxide etch-regrowth pro- cedure was used to obtain a passive oxide layer approx- imately 1 nm thick. Immediately before film growth, the substrates were thermally degassed in situ at 550 “C for 5 min, then heated to 900 “C for 2 min to desorb the protective oxide, resulting in clean surfaces with sharp 2 x 1 reflection high energy electron diffraction (RHEED) patterns.

The silicon flux, provided by a magnetically focused electron-beam evaporator, was monitored using a quartz crystal oscillator and a nude ion gauge. Anti- mony dopant fluxes were obtained either from a con- ventional effusion cell or from a single-grid hot-cathode low-energy ion source [7], with an electrostatic mirror deflector above the ion source to remove neutral species from the beam. The measured distributions of Sbl: species from the ion source were approximately 23% Sb[ , 53% Sb: , and 23% Sb,+, resulting in an average of 2.2 atoms per ionized cluster. Ion current densities were measured at the sample position using a translatable Faraday cup.

Temperature-programmed modulation-doped film growth, producing antimony-doped layers separated by undoped buffer layers, was accomplished using silicon fluxes Jsi = 1.1 x 1015-2.7 x lOi cme2 ss’ (R,, = 0.8- 1.9 urn h-‘). The growth temperature was constant at T, values between 5 15 and 800 “C for each doped layer and, for ion doping, the acceleration voltage Vsb was 100 V. At the end of each doping period, T, was immediately reduced to less than 500 “C within less than or equal to 10 s to minimize antimony desorption. After the sample cooled to ambient temperature, less than 150 “C, the surface-segregated antimony atoms were trapped under an amorphous silicon (cc-Si) layer with a thickness of approximately 25 nm. The tempera- ture was then raised to TspE = 675 “C for 1 min to

1

Depth -

I

-Time I?.,,”

Fig. I. Schematic depth profile of a doped layer grown for CTA. The

bulk fraction ;I~ is obtained from the measured steady-state bulk

concentration. Surface-segregated dopant atoms (coverage Hd) are

trapped under an amorphous silicon layer which is deposited at

ambient temperature and then crystallized by SPE at r,,,.

accomplish solid phase epitaxy (SPE) of the z-Si layer, as verified by the presence of a well ordered (2 x 1) RHEED pattern. Finally, a 100 nm thick undoped buffer layer was grown to complete the modulation doping cycle. Post-deposition cross-sectional transmis- sion electron microscopy analyses confirmed that SPE and subsequently deposited layers were epitaxial.

Figure 1 shows a schematic depth profile of a modu- lation-doped layer and the corresponding time-temper- ature sequence. The concentration spike formed by trapped surface-segregated dopant atoms has a depth profile which broadens, owing to ion-beam mixing, during SIMS analysis [23]. Integrating the concentra- tion transient gives 8,, provided that desorption is negligible or can be accounted for using the results of adsorption-desorption measurements [24]. The steady- state bulk concentration, from which yd is determined, is subtracted from the transient to account for overlap with the depth profile of the doped layer.

In situ AES measurements provided estimates of 8,, by referencing the antimony MNN (454 eV) to silicon LVV (92 eV) peak intensity ratio to that obtained for 1 monolayer (ML) of antimony on an Si( 111) 4 x

J- 3 Sb surface. The AES es,, results were in reasonable agreement with the CTA results. Dopant depth profiles were determined by SIMS, referenced to an antimony- ion-implanted silicon standard, using a Cameca IMS 3F ion microprobe operated with a + 12.5 kV, 0.25 uA Cs+

L. C. Markert et ul. 1 Sh surface segregation during Si(OOl) MBE 61

primary-ion beam. Depending on the secondary-ion polarity, the sample bias with respect to the grounded

detector and the primary-ion impact energy at the sample were, respectively, - 4.5 kV and 17 keV for nega- tive secondary ions and +4.5 kV and 8 keV for positive

secondary ion+. Detection of SbSi- secondary ions pro- vided the highest antimony sensitivity, while detection of SbCs+ secondary ions provided the best depth reso- lution owing to the lower primary-ion impact energy.

3. Results and discussion

Figure 2 shows the SIMS depth profile (solid line) through a multilayer film, modulation-doped with a thermal antimony flux Jsb = 3.5 x lo’* cm-‘sP’, to- gether with the profile expected (dashed line) assuming no surface segregation and cSb = 1. Comparison of the solid and dashed profiles shows that antimony segre- gates during the entire doping period, forming a do- pant-depleted region and a surface-segregated dopant concentration spike on the substrate and surface side respectively of each doped layer. As the growth temper- ature increases, the time required to achieve steady- state doping decreases, thus the widths of the dopant-depleted regions decrease. The steady-state do- pant fractions ySb are 1.3 x 10-j, 2.0 x 10P5, and 2.9 x 10e6 at T, = 515, 615, and 675 “C respectively. Integration under the concentration spikes in Fig. 2

Sb-Doped MSE Si(OO1) Js,= 2.2~1O'~c1ri%' JSb=3,5X10%"+S-'

102'

h 675 TSP.3 ;

Depth, x (pm) RP ,161

Fig. 2. SIMS SbCs’ depth profile through a thermal antimony

modulation-doped MBE Si(OO1) film with doped layers grown at

rs = 515, 615 and 675 “C. The dashed line shows the profile expected

assuming no surface segregation and gSb = 1.

yields antimony surface coverages dSb = 0.57, 0.79, and 0.84 ML for the same growth temperatures.

The above results show, in agreement with previously reported MBE Si(OOl):Sb doping experiments [7], that ySb decreases rapidly with increasing T,. Dopant incor- poration probabilities, using the relationship gSb = ySb/ (Jsb/Jsi) and the data in Fig. 2, are 0.84, 1.2 x lo-*, and 1.8 x 10-j at Ts=515, 615, and 675 “C respec- tively. The sensitivity of dopant incorporation kinetics to surface segregation and desorption is graphically demonstrated by the fact that gSb decreases by nearly three orders of magnitude while 8,, increases by a factor of 1.5 for a change in T, of 160 “C.

Osb and cSb values obtained from analyses of concen- tration transients and steady-state regions from modu- lation-doped profiles, including those shown in Fig. 2, are plotted in Figs. 3(a) and 3(b) as a function of growth temperatures between 515 and 800 “C. I%_,, ini- tially increases with increasing T, as the rate of dopant diffusion toward the surface, driven by the segregation

potential, increases. However, at temperatures above 675 “C, rapidly increasing antimony desorption from the surface accumulation layer becomes dominant, and QSb decreases. Segregation and desorption act in concert to decrease us,, over the entire temperature range, re- sulting in a drop of nearly five orders of magnitude as T, increases from 515 to 800 “C. The segregation ratio

$3 2*, I I I , I 1

IO500 600 700 800 Growth Temperature, Ts (“C) IP,,,O

Fig. 3. (a) Antimony surface coverage Osb, (b) incorporation

probability dSbr and (c) segregation ratio rSh as a function of

growth temperature Ts for MBE Si(OO1) films doped with thermal

antimony.

62 L. C. Morkert et al. / Sh sur]ticr segregution during Si(OO1) MBE

Sb+-Doped MBE Si(OO1) Js,= 1.1x1d5cm~2s~ JSb= 6.6x10'%m-%-

‘1 0.2

565

v-

.6 0.t Depth, x

615 T,("C) i

Fig. 4. SIMS SbSi- depth profile through an accelerated Sb’ ion

( Vsh = 100 V) modulation-doped MBE Si(OOl) film with doped layers

grown at Ts = 565, 675 and 615 C.

rsb = 8,,/~,, is also shown as a function of T, in Fig.

3(c). rSb increases throughout the T, range, in agree- ment with theoretical calculations [7], which predict a transition temperature between kinetically limited and equilibrium segregation of T* > 800 “C.

Figure 4 shows the SIMS depth profile through an ion-doped multilayer film grown for CTA with VSb= 100 V and T, values similar to those used for the thermal doping results shown in Fig. 2. Even though JSb/JSI was reduced by a factor of approximately 20 compared with the thermally doped case, ySb ranged from 8 x 10d5 to 6 x 10m5 for T, between 565 and 675 “C, yielding gSb values between unity and 0.8. Thus, at Ts = 675 ‘C, crs,, was nearly three orders of magni- tude higher for 100 V accelerated-ion doping than for coevaporative doping. In addition, no dopant-depleted regions were observed in the ion-doped profiles. The abruptness of the trailing (substrate) edge in ion-doped layers was, in all cases, limited by the SIMS depth resolution of approximately 27 nm per concentration decade for negative secondary ion detection and ap- proximately 15 nm per concentration decade for posi- tive secondary ion detection, the difference being due to the primary ion impact energy.

Some antimony segregation is expected even in the ion-doping experiments, however, since the incident ion flux distribution includes approximately 53% Sb; and 23% Sb,‘, with an average energy per accelerated anti- mony ion of only 50 and 25 eV respectively. The mean penetration depth of these species is extremely shallow, which should give rise to a T, dependence in cSb at

temperatures well below values (approximately 850 “C) at which the antimony bulk diffusion rate becomes comparable with the silicon growth rate 171.

The small transients on the surface side of the doped layer profiles in Fig. 4 are close to the measurement resolution for these ySb levels, and provide an upper limit on antimony surface coverage during growth un- der these conditions. 6,, was found to be less than or equal to 4 x 10M3 ML, even at 675 “C. Thus antimony segregation is essentially insignificant during antimony ion doping under typical MBE silicon growth condi- tions, with us,, < 60.

4. Conclusions

We have demonstrated that concentration transient analysis is a useful technique for investigating dopant surface segregation in MBE silicon films. Strong segre-

gation of coevaporated antimony results in Osb between 0.5 and 0.9 ML and rSb increasing from 4 x lo2 to 3 x 10’ for T, = 515 to 800 “C. In contrast, segregation is insignificant, 8,, d 4 x lo-’ ML and rSb I 60, in anti- mony-ion-doped films grown with V,, = 100 V.

Acknowledgments

The authors gratefully acknowledge the financial sup- port of the Semiconductor Research Corporation, the Joint Services Electronics Program, IBM, the NASA Space Vacuum Epitaxy Center, and the Swedish Na- tional Science Research Council (NFR) during the course of this research. The SIMS measurements were carried out in the Center for Microanalysis of Materials which is supported by the Department of Energy under Contract DEFG02-9 1 ER45439.

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