Overview

Analyzing Semiconductor Devices Using Modulation Spectroscopy

Fred H. Pollak, Hao Qiang, Dong Yan, Yichun Yin, and Wojciech Krystek

Improvements in the quality and yield of semiconductor devices will rely on charac­terization methods that are informative, non­destructive, convenient, easy to use, and inexpensive. Ideally, one would like to per­form the characterization procedure at room temperature on entire wafers, possibly even before the structure is removed from the growth chamber. Because of their simplicity and proven ability, the contact/ess electro­modu lation methods of photoreflectance and contact/ess electroreflectance are, ideally suited for this purpose. Modulation spec­troscopy has already been applied to examine such devices as heterojunction bipolar tran­sistors, pseudomorphic high-electron-mobil­ity transistors, quantum-well lasers, verti­cal cavity surface-emitting lasers, muItiple­quantum-well infrared detectors, superlat­tice optical mirrors, resonant tunneling structures, solar cells, and meta/-oxide-semi­conductor configurations.

INTRODUCTION

In the past ten years there have been considerable advances in semiconductor device fabrication due to thin-film growth by techniques such as molecu­lar-beam epitaxy (MBE), organometallic chemical vapor deposition (OMCVD), and gas-phase MBE, as well as various processing procedures. Developmental efforts are driving device design toward more compact structures. This trend places ever increasing demands on de­vice parameters and, hence, evaluation procedures in order to upgrade perfor­mance and yield.

The devices produced by modern growth/ processing methods can be char­acterized by a variety of optical, electri­cal, and structural measurements, in­cluding photoluminescence (PL), elec­troluminescence (EL), photolumine­scence excitation spectroscopy (PLE), absorption spectroscopy, photoconduc­tivity (PC), spectral ellipsometry, modu­lation spectroscopy, Raman and reso­nance Raman scattering, resisti vity, Hall effect, current voltage and capacitance voltage, cyclotron resonance, transmis­sion electron microscopy, secondary ion mass spectroscopy, etc. Many of the above methods are specialized, destruc­tive, and difficult. Also, a number of these techniques generally require cryo­genic temperatures.

For applied work, an optical charac­terization technique should be as simple,

1994 September • JOM

inexpensive, compact, rapid, and infor­mative as possible. Other valuable as­pects are the ability to perform measure­ments in a contactless manner at (or even above) room temperature on wa­fer-sized samples. Because ofits simplic­ity and proven ability, modulation spec­troscopy (particularly contactless modes) is now a major tool for the study and characterization of bulk/ thin-film semi­conductorsl -4 in addition to semiconduc­tor microstructures (quantum wells, superlattices, quantum dots, etc.)1-3 and interfaces (heterojunctions, semiconduc­tor / vacuum, semiconductor / metal) .1-3 Furthermore, this optical method is also useful for the evaluation of process-in­duced damage.I-3 Recent works also have clearly demonstrated its considerable po­tential and growing importance in the evaluation of important device pa­rameters for structures such as het­erojunction bipolar transistors (HBTs), pseudomorphic high-electron-mobility transistors (p-HEMTs), quantum-well lasers, vertical-cavity surface-emitting lasers (VCSEL), multiple quantum well (MQW) infrared detectors, superlattice (SL) optical mirrors, resonant tunneling structures, solar cells, metal-oxide-semi­conductor (MOS) configurations, etc.1.5•6

Modulation spectroscopy is an analog method for taking the derivative of the optical spectrum (reflectance or trans­mittance) of a material by modifying in some manner the measurement condi­tions.1-4 This optical technique is based on a general principle of experimental physics. Instead of measuring the opti­cal reflectance (or transmittance) of the material, the derivative with respect to some parameter is evaluated. This can easily be accomplished by varying some property of the sample or measuring system in a periodic fashion and meas­uring the corresponding normalized change in the reflectance (transmittance).

The periodic variations of the meas­urement conditions give rise to sharp, differential-like spectra in the region of interband (intersubband) transitions. Therefore, modulation spectroscopy emphasizes relevant spectral features and suppresses uninteresting back­ground effects. Because the changes in the optical spectra are typically small, in some cases one part in 106, phase-sensi­tive detection or some other signal proc­essing procedure is required. Since

modulation spectroscopy is a normal­ized technique, not every photon need be collected as long as there are enough to produce a good signal-to-noise ratio. Thus, this experimental method does not have stringent conditions on the morphology of the sample surface.

A particularly useful form of modu­lation spectroscopy is electromodulation (electric field modulated reflectivity [transmission]) since it is sensitive to surface/interface electric fields and can be performed in contactless modes that require no special mounting of the sample.1-4 Therefore, these modes can be employed on wafer-sized material with­out altering the sample. The most widely used form of electromodulation (EM) is photoreflectance (PR).l-4

The sensitivity of EM methods such as PR to surface / interface electric fields has become very important for device char­acterization. For sufficiently high built­in electric fields the EM spectrum can display an oscillatory behavior above the bandgap called Franz-Keldysh oscil­lations (FKOS).I-4 The period of these FKOs is a direct measure of the built-in electric field.

Because EM is an alternating current (a. c.) technique there is also important information in other modulation vari­ables such as phase, modulation fre­quency, modulation amplitude, etc., in addition to the sharp, derivative-like spectral features. Thus, PR also can be used as a contactless optical impedance spectroscopy.l,7

It has been demonstrated that modu­lation spectroscopy measurements can be performed with a lateral spatial reso­lution of about 10 /lm, thus making it possible to perform topographical scans with this resolution.s

BASIC TECHNIQUES

Shown in Figure 1 is a schematic draw­ing ofthe PR system developed at Brook-1yn College. l -4 In PR, modulation of the built-in electric field of the sample is caused by photo-excited electron-hole pairs created by the pump source, which is chopped/ modulated at frequency Oml

typically about 100-200 Hz. Most PR experiments have utilized a mechanical chopper with maximum Om - 5 kHz. Higher modulation frequencies up to about 1 MHz can be achieved by replac­ing the mechanical chopper by an

55

acousto-optic modulator! or using an internally modulated laser diode (no chopper required).5 The photon energy of the pump beam must be larger than the lowest energy gap of the material.

The entire data acquisition procedure can be computer controlled. Multiple scans over a given photon energy range can be programmed until a desired sig­nal-to-noise level has been attained. The computer also can be used for data analy­sis, such as lineshape fits, and compari­son of results with the data base.

Another con tactless method of EM is contactlesselectroreflectance(CER).9The CER method utilizes a condenser-like system consisting of a thin, transparent, conductive coating (indium-tin-oxide or 5-6 nm of a metal such as gold or nickel ona transparent substrate (glass, quartz, etc.) that serves as one electrode. A sec­ond electrode consisting of a metal strip is separated from the first electrode by insulating spacers. The sample (-0.5 mm thick) is placed between these two ca­pacitor plates. Thus, there is no pump beam required in CER.

An example of a contact mode of EM would be the metal-insulator (oxide)­semiconductor configuration which, consists of the semiconductor, about 20 nm of an insulator such as A120 y and a semi-transparent metal (about 5 nm of

/ LAMP ~-::.

/ I <'

SERVO

DETECTOR

DC SIGNAL

AC SIGNAL

nickel or gold).! Modulating (a.c.) and bias (direct current [d.c.]) voltages are applied between the front semitranspar­ent metal and a contact on the back of the sample. To employ this mode the sample must be conducting.

APPLICATIONS Heterojunction Bipolar Transistors

Heterojunction bipolar transistors have been under development for the past ten years due to their advantages for high-speed operation over other semi­conductor devices. DevelopmentofHBT technology has been hampered by the extreme sensitivity of device perfor­mance to small variations in the epi­taxial structures. The general consensus is that the most critical region for deter­mining overall device performance is the base-emitter region. Small differences in the placement of the base-emitter p-n junction relative to the heterojunction can have considerable impact on the char­acteristics of the device.

It has been demonstrated that PR can be an effective, nondestructive screen­ing method for HBT structures.lO,n For example, certain features in the PR spec­tra at 300 Kfrom GaAs/ Gal1_xAlxAs HBT structures have been correlated with ac­tual device performance,lo and hence PR can be used as an effective screening

\

" \ VARIABLE', \ SAMPLE

NEUTRAL "a.,., \\~ DENSITY "

FILTER ,,/;

.\'y./ .. " ;/ ./''\) ~

;' FILTER / /

/ II II II ON /......J LJ LJ L.OFF

REFERENCE (Om) / LOCKIN

COMPUTER

/

!JLASER (OR OTHER SECONDARY LIGHT SOURCE)

Figure 1 . Schematic representation of the Brooklyn College photoreflectance apparatus.

56

tool. From an analysis of certain spectral features it has been possible to evaluate the built-in d,c, electric fields (Fd) in the Ga1_xAlxAs emitter, as well as in the n­GaAs collector region. The behavior of Fde(GaAIAs) has been found to have a direct relation to actual device perfor­mance (i.e., d,c, current gain).

Shown in the two curves in Figure 2 are the PR spectrum at 300 K for MBE and OMCVD fabricated HBT samples,lO The bandgap of GaAs is 1.42 eV while the bandgaps of the Ga1_xAlxAs portions of the two samples are 1.830 eV and 1,670 eV, which corresponds to x = 0,28 and 0,17, respectively. The oscillatory features above the bandgaps of the GaAs and GaAIAs are the FKOs. From the period of these FKOs it is possible to directly evaluate Fde in the collector-base and emitter-base regions, respectively, by this nondestructive optical approach.

The most important aspects of Figure 2 are the FKOs of the GaAlAs bandgap. From these features one can evaluate F de in the emitter-base p-n junction, The elec­tric fields as deduced from the GaAlAs FKOs [F de<GaAIAs) 1 were compared with device parameters of fabricated HBT MBE samples. Below electric field val­ues of about 2 x 105 V / cm high current gains were obtained.

Shown in Figure 3 is F de< GaAIAs) as a function of d,c. current gain at 1 mAo Note that there is a sudden drop when Fde(GaAlAs) > 2 X 105 V /cm, caused by the redistribution of the beryllium dopant in the p-region in these MBE samples. When the redistribution moves the p-n junction into the emitter, there is an increase in the electric field in this region (i.e., the value of Fde becomes greater), The movement of the beryl­lium has been verified by secondary ion mass spectroscopy (SIMS), When the p­n junction and theGaAs/ GaAIAs hetero­junction are not coincident, carrier re­combination occurs, reducing the cur­rent and the performance of fabricated HBTs, These observations have made it possible to use PR as a screening tech­nique to eliminate wafers with unwanted characteristics before the costly fabrica­tion step,lO

As mentioned above most modula­tion spectroscopy investigations have used the sharp, derivative-like optical structures produced by this technique, including FKOs, However, since PR is an a.c' technique there is also important information in other modulation vari­ables such as phase, modulation fre­quency, amplitud~, etc., in addition to the sharp, derivative-like spectral fea­tures. Photoreflectance is the optical re­sponse of the material to the modulating a,c, electric field and hence can be em­ployed as a contactless optical imped­ance spectroscopy. Krystek etal. have reported the contactless determination of the time constants of the equivalent

JOM • September 1994

circuits of both the GaAs collector and GaAlAs emitter portions of a GaA1As/ GaAs HBT structure (similar to the MBE sample in Figure 2) from the Qm depen­denceof the in-phase and quadrature PR signals from these spectrally separated regions.7 Measurements were made in the frequency range 2 Hz < Q m < 100 kHz using an acousto-optic modulator to modulate the 670 nm line of a laser di­ode. An analysis of the collector time constant, which is equal to the product of the space charge resistance (R,) and capacitance (Csc)' revealed that the re­combination mechanism in the collector base region is dominated by the hole current and is due to midgap trap states. These authors also have investigated the effects of increased pump beam inten­sity. They observed a dramatic decrease in the R"Cse time constant due to the lowering of the barrier height caused by the photovoltaic effect. This experiment demonstrates a new area of application for PR for the characterization of these devices.

Photoreflectance at 300 K also has been used to study an InP /InxGal_xAs HBT structure with a carbon-doped base grown by gas source molecular beam epitaxy.I2 From the FKOs associated with both the InGaAs and InP signals it was possible to determine F de in the n-InGaAs collector (30 kV / cm) and n-InP emitter (10.0 kV / cm) regions. These field values were compared with numerical simula­tions based upon the intended structure. The PR data showed a much lower do­nor concentration in both the collector and emitter regions. These PR results were subsequently confirmed by (de­structive) capacitance-voltage (C -V) and SIMS determinations of the n-doping levels. Extracting the energy gap of the InGaAs alloy gives E = 0.737 eV, show­ing that lattice-matc~ing was achieved during growth.

lnAlAs/lnGaAs,BlnGaP/GaAs,14 and InGaAs/GaAs15 HBT structures also have been characterized by similar PR studies.

2.0 Sample I Gaa.nAla.2sAs FKO ~

1.5 1.8 2.1 2.4 Energy (eV)

Figure 2. Photoreflectance spectra at 300 K from GaAs/GaAIAs HBT samples. Curve (a): grown byMBE. Curve (b):grownbyOMCVD.

1994 September • JOM

60 u « DO E [J[]

~ 40 0 '(ij <D C 0

~ 20 ~

(.) 0 Do (.)

0 0 0 On

2 3 4

Fdc (GaAIAs) [105 VlcmJ

F,ure 3. Current gain at 1 mA as a function of Fdc(GaAIAs) for actual GaAs/GaAIAs HBT devices fabricated from MBE-grown samples such as those in Figure 2.

High-Electron-Mobility Transistor Structures

High-electron -mo bili ty transistors (HEMTs) offer superior transistor noise performance compared to other solid­state amplifiers. HEMTs have gained widespread acceptance as the device of choice for the majority of high-per­formance receivers at frequencies up to 100 GHz. They are also proving to be an excellent choice for power amplifiers. One type of HEMT in particular, the pseudomorphicGal_yAlyAs/InxGal_xAs/ GaAs modulation doped structure, has demonstrated outstanding power performance and is likely to become the preferred transistor for many power applications at frequencies ranging from 10GHz t0100GHz. The InGaAs channel of the p-HEMT) is beneficial in many respects. This material produces an ex­cellent conduction band profile, form­ing an InGaAs quantum well that pro­vides good confinement of the electrons, a necessity for high-effiCiency power devices. The large GaA1As /InGaAs con­duction band discontinuity allows high two-dimensional electron gas densities (N,) in the InGaAs channel (of width L), and hence high current-handling capabil­ity for power HEMTs.

Photorefiectance or CER at 300 K has been used to characterize Gal_"AlyAs/ InxGal_xAs/GaAs PHEMTs with dIffer­ent materials parameters. I 6-19 One such samplel7 tested had nominal y = 0.19, x = 0.2, and Lw = 10 nm (Figure 4). Based on the above parameters there are two con­fined electron and two confined heavy­holes states, with the Fermi level (EF) occurring between the first and second electron levels. Thus, four intersubband transitions (i.e., 11 H, 12H, 21 H, and 22H) are possible at room temperature. The notation mnH denotes an intersubband transition between the mth conduction and nth heavy-hole-like valence sub­bands.

Several features from the InGaAs por­tion of the sample in addition to signals from the GaAlAs and GaAs regions have been observed. The spectralfeatures from the InGaAs channel can be explained on the basis of the derivative of a broad-

ened step-like two-dimensional density of states, due to the screening of the excitons by the two-dimensional elec­tron gas (2DEG), and a Fermi level filling factor. From the details of the lineshape fit, it has been possible to extract the Fermi level position and hence Ns' These values for the electron sheet density are in good agreement with Hall measure­ments on the same samples. In addition, several intersubband transition energies in the InGaAs channel can be obtained. The GaAs and GaAIAs Signals exhibit FKOs from which it is possible to obtain information about the electric fields in these regions. Also the GaAlAs signal yields the Al composition (y). Thus the entire potential profiles in such struc­tures can be determined in a con tactless manner at room temperature.

Displayed by the solid line in Figure 5 is the CER spectrum originating in the InGaAs section of the sample, except for the feature denoted Eo(GaAs). This reso­nance corresponds to the direct bandgap of GaAs and originates in the GaAs buffer / substrate. The trace of Figure 5 consists of four peaks, labeled mnH, riding on a background, which is due to the thermal tail of the Fermi distribution function. The dashed line in Figure 8 is a fit to the lineshape function mentioned above. The obtained energies of the mnH transitions are denoted by arrows. By comparing these experimental energies with a theoretical self-consistent calcu­lation it is possible to evaluate several important materials parameters such as Lw' indium composition, and built-in electric field. The fit to the background yields EF and hence N,.

It should be noted that at room tem­perature, transitions to the first conduc­tion level (i.e., 11H and 12H), which lies below Ep are only seen because of the significant thermal broadening of the Fermi distribution. As the temperature is lowered these resonances are no longer observed. 19

n·GaAIAs InGaAs -----Lw-- GaAs

_,,,,,,,r-- - - EF , Two·Dimensional

Electron Gas

Ev

Figure 4. Schematic representation of the potential profile of a GaAIAs/lnGaAs/GaAs HEMT structure.

57

Quantum Well Lasers

Quantum well (QW) laser structures employ complex sequences of thin lay­ers of single-crystal materials, where a few light-emitting QWs are embedded in waveguide regions and enclosed by p- and n-doped contact layers. Analysis of the active region, buried under much thicker layers, is difficult after the se­quence of layers is completed. Com­monly, PL and EL are used to analyze the positions of energy levels and qual­ity of the wells. During their lifetime, however, excited electrons and holes may migrate to defects or to wider portions of the wells and the emission spectrum may not be representative of the sample. More direct access to the density of states by absorption is difficult since the ab­sorption of light by a few wells buried under thick conducting layers may be too weak to be resolved. Thus, PC mea­surements are often substituted.

Low-temperature (19 K) electroab­sorption (EA) measurements of InGaAs / InGaAsP QW lasers grown lattice­matched on InP substrates and tuned to room-temperature emission at 1.55 /lm have been performed.6.2o Signals were observed from both the InGaAs / InGaAsP QW region and the InGaAsP waveguide portions. The narrow line­width of the spectra makes it possible to accurately determine the transition ener­gies in the QW. Comparison with a theo­retical model yields information about the well width. Both the QW and

300 K

Experimental

- - - - - - Lineshape Fit

llH 12H

1.2

waveguide signals can be used to evalu­ate the built-in electric fields in the struc­ture. The EA results were compared with PL and PC measurements.

A comprehensive study comparing lasing frequency with room tempera­ture PR and PL has been performed for 24 samples of Gao.91Alo.09As (12 nm)/ Gao6Alo.4As(40 nm)/Gao.2Alo.sAs QW la­sers.21 The PL wavelength was system­atically shorter than the lasing wave­length (-800 nm), typically by about S-15 nm. However, it was found that the wavelength of the fundamentalllH exci­tonic PR signal was within only ±1.5-3 nm of the lasing wavelength. About half the PR wavelengths were shorter /longer than the corresponding lasing wave­length, in contrast to the PL data. Thus PR could be used to characterize the properties of these laser structures.

Moneger et al. have measured the electromodulation spectrum from an InGaAs VCSEL test structure at 300 K using CER.22 They have observed fea­tures from the InGaAs QW region at 1.216 eV (1020 nm) and 1.313 eV (944 nm), corresponding to the IIH and 22H transitions. Comparison of these ener­gies with an envelope function calcula­tion, including strain, yields values of 8 nm for the well width and 21 % for the In composition, in good agreement with the growth conditions. In-plane broad­area lasers were fabricated to study the quantum well gain spectrum from this material. The lasing spectrum at 1020 nl!l shows a high correlation to the CER

21H 22H

1.3

Energy (eV)

Eo (GaAs)

~

1.4

Figure 5. The solid line is the experimental room-temperature CER spectrum from the InGaAs region of a GaAIAs/ lnGaAs/GaAs HEMT sample. The dashed line is a least-squares fit to an appropriate lineshape function. The obtained values of the intersubband energies mnH are designated by arrows.

58

spectrum. This indicates thatnondestruc­tiveCERcan be used to study the energy levels of quantum wells placed in a reso­nant cavity.

Other Device Structures

Since the fabrication of reasonably high detectivity GaAs/Ga1_xAlxAs MQW in­frared OR) detector structures capable of overcoming some of the shortcomings of HgCdTe and extrinsic detector tech­nologies, there has been considerable interest in the understanding of the physical properties of these materials. Contactless PR at room temperature has been employed to assess the composi­tion, uniformity, and design of such GaAs/GaAIAs based 10 /lm MQW IR detector structures.23 The dominant fea­tures in the spectra were used to assess well width, composition and peak wave­lengths for the MQW detector structures.

Piezoreflectance (PZR) at 300 K has been used to investigate GaAs/GaAlAs double-barrier resonant tunneling struc­tures.24 This technique probed the Single quantum well buried below the -0.5 /lm­thick n+-GaAs surface layers required for ohmic contacts. The positions of sa­lient PZR spectral features denoted subtle variations of well width and barrier height and width; lineshapes indicated interface roughness.

The electric field response of a GaAs / GaAIAs SL mirror was investigated us­ing Schottky barrier electroreflectance (ER). A strong enhancement in ER re­sponse is predicted near the minimum in the mirror spectrum. This enhance­ment is important in electro-optic reflec­tance modulators.25

Photo reflectance and ER (with applied d.c. bias) at room temperature have been employed to characterize the bandgaps, materials quality, internal p-n junction fields and net carrier concentration in complete InP and GaAs solar cell struc­tures.26 An important aspect of the study was the determination of the carrier concentration since unlike the more com­monly used C-V method, the ER ap­proach is not dependent on the total area of the solar cell. The study involved five organometallic vapor phase epitaxial (OMVPE) n+pp+ InP and three OMCVD p+nn+ GaAs samples.

Low field ER in the MOS configura­tion has been used to determine the sur­face properties of thermally oxidized sili­con (300 K)27 and HgCdTe passivated by Photox™-SiO2 (77 K),zs including the determination of the flatband condition.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the support of NSF grant no. DMR-9120363, Army Research Office contract no. DAAL-03-92-G-0189 and the New York State Sci­ence and Technology Foundation through its Centers for Advanced Technology pro­gram.

JOM • September 1994

References 1. F.H. Pollak and H. Shen, "Modulation Spectroscopy of Semiconductors: Bulk/ThinFilm,Microstructures,Surfacesl Interfaces and Devices," Materials Science and Engineering, RIO (]993), pp. 275-374. 2. O.J. Glembocki and B.V. Shanabrook, "Photoreflectance Spectroscopy of Microstructures," Semiconductors and Sem;­metals, 67, ed. D.G. Seiler and c.L. Littler (New York: Aca­demic, 1992), pp. 222-292. 3. F.H. Pollak and O.J. Glembocki, "Modulation Spectros­copy of Semiconductor Microstructures: AnOverview," Proc. Soc. Photo-Optical Instrllm. Engineers, 946 (SPIE, Bellingham, 1988), pp. 2-35. 4. F.H. Pollak, "Modulation Spectroscopy:' Encyclopedia of Materials Characterization: Sllrfaces. Interfaces and Thill Films, ed. C. Evans, R Brundle, and S. Wilson (Butterworth­Heinemann, Boston, 1992), pp. 385-400. 5. F.H. Pollak et aI., "Better Device Yields with Modulation Spectroscopy:' Photonics Spectra Magazine, 27 (8) (] 993), pp. 78-84. 6. H. Qiang et aI., "Modulation Spectroscopy Characteriza­tion of Semicond uctor Device Structures," to be published in Asia-Pacific Engineering Journal. Part A: Electrical Engineering (1994). 7. W. Krystek et aI., "Contaclless HBT Equivalent Circuit Analysis Using the Modulation Frequency Dependence of Photoreflectance" (Paper to be presented at the 21st Interna­tional Symposium on Compound Semiconductors, San Di­ego, CA, September 1994 and published in the proceedings). 8. V.T. Boccio and F.H. Pollak, private communication. 9. X. Yin and F.H. Pollak, "Novel Contaclless Mode of Electrorefleetance," Appl. Phys. l.etl. , 59 (] 991), pp. 2305-2307. 10. X. Yin e t a I. , "Characterization of GaAs/GaAlAs Heterojunction Bipolar Transistor Structures Using Photo­reflectance," Appl. Phys. Lett., 56 (]990), pp. 1278-1280; also, "Photoreflectance of GaAs/GaAIAs Heterojunction Bipolar Transistor Structures," Proc. Soc. PhotO-Optical Illstrum. Engi­neers, 1286 (SPIE, Bellingham, 1990), pp. 404-413. 11. N. Bottka et aI., "Qualification of OMVPE AlGaAs/GaAs HBTStructures Using Nondestructive Photoreflectance Spec­troscopy:' J. Cryst. Growth, 107 (]991), pp. 893-897. 12. D. Yan et al., "Photoreflectance Characterization of an InP I lnGaAs Heterojunction Bipolar Transistor Structure," Appl. Phys. Lett ., 61 (1992), pp. 206&-2068. 13. K.T. Hsu et aI., "Photoreflectance Characterization of an lnAlAs/ lnG.As Heterostructure Bipolar Transistor," Appl. Phys. l.ett., 64 (1994), pp. 1974-1976. 14. L.W. Yang et a l., "Material and Device Characterization of High Quality InGaP IGa As HBT Structures Using Photoreflectance, Capacitance- and Current-Voltage Meas-

Prepare

TMS/A

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urements" (Paper presented at the 1993 Electronic Materials Conference, Santa Barbara, CA, June 1993). 15. A. Badakhshan et aI., "Photoreflectance of Semiconduc­tors above the Band Gap," Nallosiructure Physics and Fabrica­tion, eds. M.A. Reed and W.P. Kirk (New York: Academic, 1989), pp. 485-493. 16. Y. Yin et aI., "Two-Dimensional Electron Gas Effects in the Electromodulation Spectra of a Pseudomorphic GaAIAsl InGaAs/GaAs Modulation-Doped Quantum Well Struc­ture," App/. Phys. Lett., 61 (1992), pp. 1579-1581; also, "Electromodulation Spectroscopy of a Pseudomorphic GaAIAs/ lnGaAs/ GaAs Modulation-Doped Quantum Well Structure: Two-Dimensional Electron Gas Effects." Proc. Soc. Photo-Opticallnstrum. Engineers, 675 (SPIE, Bellingham, 1992), pp. 498-509. 17. Y. Yin et aI., "Room Temperature Photoreflectance Char­acterizationofPseudomorphicGaAIAs/ lnGaAs/GaAsHigh Electron Mobility Transistor Structures Including the Two­Dimensional Electron Gas Density/' Semicond. Sci. Technol., 8 (1993), pp. 1599-1604. 18. A. Dimoulas et aI., "Degenerate Electron Gas Effects in the Modulation Spectroscopy of Pseudomorphic AI, ,,Gao.,,As/ln,\,Gaos5As/GaAs High Eleetron Mobility Transistor Structures," App/. Phys. Lett ., 63 (1993), pp. 1417-1419. 19. G. Gumbs et aI., "Many-Body Effects in the Eleetro­modulation Spectra of Modulation-doped Quantum Wells: Theory and Experiment," Phys. Rev., 848 (1993), pp. 18328-18331. 20. K. Satzke et aI., "Electroabsorption Studies on InGaAs/ InGaAsP Quantum-Well Laser Structures." J. Appl. Phys., 69 (1991 ), pp. 7703-7710. 21. V.T. Boccio and F.H. Pollak, private communication. 22. S. Moneger et aI., "Contactless Eleetrorefleetance ofThree InGaAs Quantum Wells Placed in a GaAsl AlGaAs Reso­nant Cavity" (Paper to be presented at the 21st International Symposium on Compound Semiconductors, San Diego, CA, September 1994 and to be published in the proceedings). 23. P.A. Dafesh, "Characterization of GaAs/GaAIAs Mul­tiple Quantum Well Infrared Detector Structures Using Photoreflectance." I. Appl. Phys., 71 (1992), pp. 5154-5160. 24. RL. Tober et al., "Piezorefiectance Characterization of Resonant Tunneling and Modulation-Doped Heterostruc­tures." J. Electron. Mat., 18 (1989), pp. 379-384. 25. I.J. Fritz, P.L. Gourley, and T.J. Drummond, "Electric Field Response of a Semiconductor Superiattice Optical Mir­ror from Electroreflectance Spectra," Appl. Phys . Lett., 55 (1989), pp. 1324-1326. 26. RG. Rodrigueset aI., "Photoreflectance Characterization of InP and GaAs Solar Cells," Proceedings of the Twenty-Third IEEE Photovoltaic Specialists Conference (New York: IEEE, 1993), pp. 681-685.

27. K. Misawa, A. Moritani, and J. Nakai, "Electroreflectance of Si-MOS," Ipn. I. Appl. Phys .. 15 (]976), pp. 1309-1316. 28. A. Ksendzov et aI., "Electroreflectance Study of HgCdTe in the Metal-Insulator-Semiconductor Configuration at 77 K," Appl. Phys.l.ett., 49 (1986), pp. 648-650.

ABOUT THE AUTHORS •••• _

Fred H. Pollak earned his Ph.D. in physics at the University of Chicago in 1965. He is cur­rently Distinguished Professor of physics and director of the Semiconductor Institute at Brooklyn College of the City University of New York (CUNY). Dr. Pollak is also a member of TMS.

Hao Qiang earned his Ph.D. in experimental condensed matter physics at CUNY in 1993. He is currently a research associate at the Semiconductor Institute of Brooklyn College of CUNY.

Dong Van earned his Ph.D. in physics at CUNY in 1992. He is currently a research associate at the Semiconductor Institute of Brooklyn College of CUNY.

Vichun Yi earned his Ph.D. in physics at CUNY in 1994. He is currently a PC/LAN specialist at Debevoise and Plimpton.

Wojciech Krystek earned his B.S. in physics at Warsaw University in Warsaw, Poland, in 1990. He is currently a research assistant and Ph.D. candidate at the Semiconductor Insti­tute of Brooklyn College of CUNY.

For more information, contact F.H. Pollak, clo Physics Department, 2900 Bedford Avenue, Brooklyn College, Brooklyn, New York 11210.

Attend the

Student Forum

In addition\ you will have the opportunity for informal discussion

with practicing materials professionals during the IIEngineering Round Tables."

opportunity for a rewarding career in the materials field. Making the most of that opportunity will depend on you and your ability

Production engineers, research and development engineers\ operations managers\ scientific staff\ and corporate

executives from a variety of organizations will be on hand to answer your questions. to adapt to a changing world.

By attending the Materials Week '94 Student Forum on Careers in Materials Science and Engineering, you will not only learn the nuances of landing your first job\ you will

also gain vital inSight from a program of distinguished speakers on how you can help lead the materials profession into the 21 st century.

TMS Minerals· Metals' Materials

Lay the foundation for your professional future by attending the Materials Week '94 Student Forum on Careers in

Materials Science and Engineering.

Participation in the Forum is free, but you must preregister.

Please contact: • Attn: Materials Week '94 Student Forum

420 Commonwealth Drive Warrendale, PA 15086 U.S.A.

Telephone: (412) 776-9000\ ext. 213 Fax: (412) 776-3770