JOURNAL OF ELECTRONIC MATERIALS
	ABSTRACTS Volume 24, Number 10, October 1995 This Month Featuring: Regular Issue Papers. View October 1995 Table of Contents.

REGULAR ISSUE PAPERS

KEY WORDS Contactless electroreflectance (CER), heterostructure, molecular beam epitaxy, step quantum well (QW)

Using contactless electroreflectance at 300 and 77K, we have studied the inter-subband transitions from a GaAlAs/InGaAs/GaAs/GaAlAs step quantum well structure (small well inside a large well) consisting of two layers A (InxGa1-xAs) and B (GaAs) with widths LA and LB, respectively, bounded by two thick barrier regions of Ga1-yAlyAs. By comparison of the observed spectral features with an envelope function calculation, including the effects of strain, we have been able to characterize the potential profile of the structure, i.e., LA, LB, x, and y. There is very good agreement between experiment and the intended materials parameters. Such configurations are of considerable importance since (a) they form the basis for pseudomorphic high electron mobility transistors, and (b) also have applications in optoelectronics due to their large Stark shifts.

Structural Characterization of Oxide Layers Thermally Grown on 3C-SiC Films
Q. WAHAB, L. HULTMAN, M. WILLANDER, and J.-E. SUNDGREN
Department of Physics, Linköping Univerisity, S-581 83 Linköping, Sweden.

KEY WORDS Auger electron spectroscopy (AES), oxidation, silicon carbide, transmission electron microscopy (TEM), wide bandgap semiconductors

The oxidation of 3C-SiC films deposited on off-oriented Si(001) substrates by reactive magnetron sputtering has been studied. The oxidation was carried out using dry conditions at a temperature of 1200°C. The composition of the oxide layer was investigated by Auger electron spectroscopy (AES). The oxide layer was found to contain no C except for the region very close to the interface, and the stoichiometry was found to be close to that of SiO2. Cross-sectional transmission electron microscopy (XTEM) showed the oxide layer to be completely amorphous, dense, and homogeneous with a uniform thickness. High-resolution XTEM imaging showed an atomically sharp SiO2/SiC interface.

Ion-Beam Mixed Ultra-Thin Cobalt Silicide (CoSi2) Films by Cobalt Sputtering and Rapid Thermal Annealing
S. KAL,¹ I. KASKO,² and H. RYSSEL²
1--Department of Electronics & Elect. Comm. Engineering, Indian Institute of Technology, Kharagpur, 721302 India. 2--Lehrstuhl fur Electronische Bauelemente, Universitat Erlangen-Nurnberg, Cauerstrasse 6, D-91058, Erlangen, Germany.

KEY WORDS Cobalt silicide, ion-beam mixing, rapid thermal anneal

The influence of ion-beam mixing on ultra-thin cobalt silicide (CoSi2) formation was investigated by characterizing the ion-beam mixed and unmixed CoSi2 films. A Ge⁺ ion-implantation through the Co film prior to silicidation causes an interface mixing of the cobalt film with the silicon substrate and results in improved silicide-to-silicon interface roughness. Rapid thermal annealing was used to form Ge⁺ ion mixed and unmixed thin CoSi2 layer from 10 nm sputter deposited Co film. The silicide films were characterized by secondary neutral mass spectroscopy, x-ray diffraction, tunneling electron microscopy (TEM), Rutherford backscattering, and sheet resistance measurements. The experimental results indicate that the final rapid thermal annealing temperature should not exceed 800°C for thin (<50 nm) CoSi2 preparation. A comparison of the plan-view and cross-section TEM micrographs of the ion-beam mixed and unmixed CoSi2 films reveals that Ge⁺ ion mixing (45 keV, 1 x 10¹⁵ cm^-2) produces homogeneous silicide with smooth silicide-to-silicon interface.

Electron-Concentration Dependence of Absorption and Refraction in n-In0.53Ga0.47As Near the Band-Edge
D. HAHN,¹ O. JASCHINSKI,¹ H.-H. WEHMANN,¹ A. SCHLACHETZKI,¹ and M. VON ORTENBERG²
1--Institut für Halbleitertechnik, Technische Universität Braunschweig, Hans-Sommer-strasse 66, D-38106 Braunschweig, Germany. 2--Institut für Physik, Lehrstuhl Magnetotransport, Humboldt Universität Berlin, Invalidenstrasse 110, D-10115 Berlin, Germany.

KEY WORDS Absorption coefficient, band-gap shift, electron-concentration dependence, n-In0.53Ga0.47As, refractive index

The optical constants of InGaAs were determined as a function of electron concentration in the range from 10¹⁵ to 2 x 10¹⁹ cm^-3 by reflectance- and transmission-spectroscopy. A pronounced shift of the fundamental absorption edge toward shorter wavelengths with increasing doping concentration was found. The experimental results can be satisfactorily explained by band-filling and band-gap shrinkage.

Optical Properties of Annealed, Single GaAs Quantum Wells: Cap Doping and Mask Width Dependence
A.C. CROOK, D.V. FORBES, and C.M. HERZINGER
University of Illinos at Urbana, Engineering Research Center for Compound Semiconductor Microelectronics and Department of Electrical and Computer Engineering, 1406 West Green St., Urbana, IL 61801.

KEY WORDS Disordering, transmission, vacancy, waveguide

Band edge absorption measurements are used to characterize the degree of Al-Ga intermixing (blue-shifting) and the linear optical loss of waveguides formed in five annealed and encapsulated single GaAs quantum well laser heterostructure wafers which differed only by the amount of Zn doping in the GaAs cap layer. In addition to the transmission measurements, secondary ion mass spectroscopy data was used to verify the diffusion of Zn before and after annealing. High zinc doping in the cap is observed to cause quantum well disordering below the encapsulant (Si3N4) and is attributed to impurity induced layer disordering. Moderate doping in the cap results in selective area intermixing via controlled gallium vacancy production. A stripe width dependence is also observed, which suggests a role of lateral diffusion of species which affect the intermixing. For an undoped (n^-) cap, the degree of intermixing is heavily dependent on the arsenic overpressure used during the anneal and is independent of the nitride stripe width suggestive of a volumetric Fermi level dependent production of vacancies within the cap.

Ultra-Shallow Raised p+-n Junctions Formed by Diffusion from Selectively Deposited In-situ Doped Si0.7Ge0.
DOUGLAS T. GRIDER,¹ MEHMET C. ÖZTüRK,² STANTON P. ASHBURN,² JIMMIE J. WORTMAN,² GARI HARRIS,³ and DENNIS MAHER³
1--Present Address: Texas Instruments, Inc., Semiconductor Process adn Design Center, 13536 North Central Expressway, MS 461, Dallas, TX 75243. 2--North Carolina State University, Department of Electrical and Computer Engineering, Box 7911, Raleigh, NC 27695-7911. 3--North Carolina State University, Department of Materials Science and Engineering, Box 7916, NC 27695-7916.

KEY WORDS Diborane, germanium, metal-oxide-silicon-field-effect transistors (MOSFETs), rapid thermal chemical vapor deposition (RTCVD), rapid thermal process (RTP), shallow junction, silicon

In this paper, a novel raised p⁺-n junction formation technique is presented. The technique makes use of in-situ doped, selectively deposited Si0.7Ge0.3 as a solid diffusion source. In this study, the films were deposited in a tungsten halogen lamp heated cold-walled rapid thermal processor using SiCl2H2, GeH4, and B2H6. The microstructure of the Si0.7Ge0.3 layer resembles that of a heavily defected epitaxial layer with a high density of misfit dislocations, micro-twins, and stacking faults. Conventional furnace annealing or rapid thermal annealing were used to drive the boron from the in-situ doped Si0.7Ge0.3 source into silicon to form ultra-shallow p⁺-n junctions. Segregation at the Si0.7Ge0.3/Si interface was observed resulting in an approximately 3:1 boron concentration discontinuity at the interface. Junction profiles as shallow as a few hundred angstroms were formed at a background concentration of 10¹⁷ cm^-3.

Experimental Determination of Tie-Lines in the Hg-Cd-Te System
HAO-CHIEH LIU and R.F. BREBRICK
Marquette University, Material Science and Engineering Program, College of Engineering, Milwaukee, WI 53233.

KEY WORDS Mercury-cadmium-telluride, microstructure, phase diagram, tie-lines

Starting with powdered Hg1-xCdxTe, several tie-lines at 500 and 560°C were established using an energy dispersive spectrometer on a scanning electron microscope for the quantitative analysis. After holding at 500 or 560°C for time periods based upon the powder size and the published interdiffusion constant, then water quenching to room temperature, the primary grains were found to be uniform in composition and covered with a 5-6 u layer of HgTe or low x Hg1-xCdxTe. The primary grain and overall compositions establish directions for tie-lines that are in good agreement with published experimental and theoretical results.

Conduction Band Offset of Strained InGaP by Quantum Well Capacitance-Voltage Profiling
S.H. PARK, M. MARKARIAN, P.K.L. YU, and P.M. ASBECK
Department of Electrical and Computer Engineering, University of California, San Diego, CA 93106.

KEY WORDS Capacitance-voltage (C-V) profiling, conduction band offset, GaInP, strained quantum well

The conduction band alignment of compressively strained In1-xGaxP relative to lattice matched InGaP/GaAs has been determined by capacitance-voltage profiling. A modified version of Kroemer's capacitance-voltage profiling method is developed wherein a quantum well is profiled instead of a single heterojunction. A one-dimensional Poisson-Schrodinger solver was used to fit the reconstructed carrier profiles corresponding to a value of [[Delta]]Ec at varying temperatures. Schottky barrier diode structures containing a single strained InGaP quantum well were grown by low pressure metalorganic chemical vapor deposition. The two strained compositions studied contained 35 and 31% gallium. Conduction band offsets of 101 and 131 meV were found for the 35 and 31% samples, respectively, with an estimated accuracy of +/-5 meV. These results agreed closely with values predicted by empirical calculations.

High Performance AlAs/GaxIn1-xAs Resonant Tunneling Diodes by Metalorganic Chemical Vapor Deposition
J.C. YEN,¹ B.P. KELLER,¹ S.P. DENBAARS,² and U.K. MISHRA¹
1--Department of Electrical and Computer, University of California, Santa Barbara, CA 93106. 2--Department of Materials Engineering, University of California, Santa Barbara, CA 93106.

KEY WORDS AlAs/GaInAs heterostructure, metalorganic chemical vapor deposition (MOCVD), resonant tunneling diode

We report on AlAs/GaxIn1-xAs (x = 0.47) quantum well heterostructures grown by metalorganic chemical vapor deposition (MOCVD) on InP substrates. Heterostructure quality was evaluated by high resolution x-ray diffraction for various growth conditions. Double barrier quantum well heterostructures were grown and processed into resonant tunneling diodes (RTDs). Room temperature electrical measurements of the RTDs yielded maximum peak to valley current ratios of 7.7 with peak current density of 96 kA/cm² and 11.3 with peak current density of 12 kA/cm², for devices grown by atmospheric and low pressure MOCVD, respectively.

Investigations on Indium Phosphide Grown by Chemical Beam Epitaxy
R.T.H. RONGEN, M.R. LEYS, P.J. VAN HALL, C.M. VAN ES, H. VONK, and J.H. WOLTER
COBRA Interuniversity Research Institute, Eindhoven University of Technology, Department of Physics, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.

KEY WORDS Chemical beam epitaxy (CBE), deep-doner centers, Hall measurements, indium phosphide (InP), photoluminescence

In this paper, we present a systematic study of the properties of indium phosphide (InP) layers grown by chemical beam epitaxy (CBE). Trimethylindium (TMIn) and phosphine (PH3) are used as source materials. The relation between the phosphine cracker temperature and the cracking efficiency has been studied by mass spectroscopy during growth. The growth rate and morphology of the layers have been studied by varying the TMIn and phosphine flow rates as well as the substrate temperature. We have found that, under a wide range of growth conditions, the deposition rate is only determined by and proportional to the TMIn flow rate. This is in agreement with literature. Additionally, we observe that the growth rate decreases below a certain phosphine to TMIn flow rate (V/III) ratio and becomes phosphine flow limited. From investigations of the growth rate as a function of temperature, it is concluded that the desorption of indium species from InP starts at a temperature slightly below 540°C. For this desorption process, we have found an activation energy of (217 +/- 20) kJ/mol. Further characterization of the InP layers has been carried out by photoluminescence and Hall measurements. From both methods, the optimum growth conditions have been established. Under these conditions, we reproducibly obtain InP layers showing linewidths of the donor-bound exciton transition at 5K around 0.25 meV and a mobility at 77K of about 7.0.10⁴ cm²/Vs. From the analysis of the mobility in the temperature range from 20 to 300K, we conclude that, additionally to shallow donors and acceptors, deep-donor centers with an activation energy of about 150 meV are present in all layers.

Low Temperature Epitaxial Growth of Si0.5Ge0.5 Alloy Layer on Si (100) by Ion Beam Assisted Deposition
S.W. PARK, J.Y. SHIM, and H.K. BAIK
Refining and Thin Film Materials Laboratory, Department of Metallurgical Engineering, Yonsei University, 134 Shinchon-dong, Seodaemun-ku, 120-749, Seoul, Korea.

KEY WORDS Epitaxial growth, ion beam assisted deposition (IBAD), Si0.5Ge0.5 alloy

The first results were reported on low temperature epitaxial growth of Si0.5Ge0.5 alloy layer on Si (100) by ion beam assisted deposition. Nucleation and the growth of Si0.5Ge0.5 alloy layer had been investigated by atomic force microscopy and reflection high energy electron diffraction analysis. The Si0.5Ge0.5 alloy layer nucleated on Si (100) via Stranski-Krastanov (SK) mode. The Ar ion bombardment improved crystallinity and prolonged layer-by-layer stage of the SK mode. The epitaxial temperature was 200°C lower than 550-600°C in molecular beam epitaxy. In order to explain the mechanism of low temperature epitaxial growth EAr (energy transferred to growing film by bombarding Ar ion, eV/atom) value was experimentally calculated. In conclusion, the ion bombardment induced dissociation of three-dimensional islands and enhanced the surface diffusion. The variation of tetragonal strain and its effect on electron mobility were taken into consideration. Electron mobility increased with tetragonal strain as a result of band split.

Temperature-Dependent Minority-Carrier Lifetime Measurements of Red AlGaAs Light Emitting Diodes
F.M. STERANKA, D.C. DEFEVERE, M.D. CAMRAS, S.L. RUDAZ, D.K. MC ELFRESH, L.W. COOK, W.L. SNYDER, and M.G. CRAFORD
Hewlett-Packard, Optoelectronics Division, 370 W. Trimble Road, San Jose, CA 95131.

KEY WORDS AlGaAs, double heterostructure light emitting diode (LED), minority carrier lifetime

Electroluminescent decay and internal quantum efficiency measurements are made as a function of temperature on two double heterostructure AlGaAs light emitting diodes (LEDs) that emit in the visible (red) portion of the spectrum. The electroluminescent lifetimes increase by more than a factor of ten and the internal quantum efficiency falls by a factor of three as the temperature is raised from 90 to 400K. By analyzing the data with a model that accounts for the transfer with increasing temperature of the minority-carrier electrons from the direct-gap to the indirect-gap minima in the p-type active layer of these near-crossover LEDs, values for the radiative and nonradiative lifetimes as a function of temperature are obtained. A fit to the radiative-lifetime data results in an estimate of 1.3 x 10^-10 cm³s^-1 for the room-temperature radiative recombination coefficient of Al0.39Ga0.61As, which is very similar to values reported for GaAs. The nonradiative lifetimes are found to be nearly independent of temperature from 220 to 400K and provide upper limits of 940 and 1250 cms^-1 for the interface recombination velocities of the two samples. These values are roughly an order of magnitude lower than any previously reported values for high-Al-content (x > 0.3) AlxGa1-xAs heterostructures.

Sensitivity Analysis of Ion Implanted Silicon Wafers after Rapid Thermal Annealing
YOUN TAE KIM,¹ CHI HOON JUN,¹ JONG-TAE BAEK,¹ HYUNG JOUN YOO,¹ and SANG-KOO CHUNG²
1--Semiconductor Technology Division, Electronics and Telecommunications Research Institute, Taejon 305-600, Korea. 2--Department of Electronics Engineering, Ajou University, Suwon 442-749, Korea.

KEY WORDS Activation, ion implantation, rapid thermal annealing (RTA), sensitivity, sheet resistance

In this study, we have investigated sensitivities of the ion implanted silicon wafers processed by rapid thermal annealing (RTA), which can reveal the variation of sheet resistance as a function of annealing temperature as well as implantation parameters. All the wafers were sequentially implanted by the arsenic or phosphorous implantations at 40, 80, and 100 keV with the dose level of 10¹⁴ to 2 x 10¹⁶ ions/cm². Rapid thermal annealing was carried out for 10 s by the infrared irradiation at a temperature between 850 and 1150°C in the nitrogen ambient. The activated wafer was characterized by the measurements of the sheet resistance and its uniformity mapping. The values of sensitivities are determined from the curve fitting of the experimental data to the fitting equation of correlation between the sheet resistance and process variables. From the sensitivity values and the deviation of sheet resistance, the optimum process conditions minimizing the effects of straggle in process parameters are obtained. As a result, a strong dependence of the sensitivity on the process variables, especially annealing temperatures and dose levels is also found. From the sensitivity analysis of the 10 s RTA process, the optimum values for the implant dose and annealing temperature are found to be in the range of 10¹⁵ ions/cm² and 1050-1100°C, respectively. The sensitivity analysis of sheet resistance will provide valuable data for accurate activation process, offering a guideline for dose monitoring and calibration of ion implantation process.

Plastic Constraint of Large Aspect Ratio Solder Joints
JOHN P. RANIERI, FREDERICK S. LAUTEN, and DONALD H. AVERY
Division of Engineering, Brown University, Providence, RI 02912.

KEY WORDS Aspect ratio solder joints, Sn-Pb solder, solder joint tensile stress

The aspect ratio (joint area/joint thickness) of thin (0.001-0.006 in.) surface mount solder (60S-40Pb) joints plays an important role in determining the mechanical properties and fracture behavior of the joints. This study demonstrates that plastic constraint of a large aspect ratio 60Sn-40Pb solder joint can develop triaxial (hydrostatic) stresses several times greater than the average tensile strength of the bulk solder material. A four to sixfold increase in average joint stress and up to a tenfold increase in peak stress was measured on joints with aspect ratios ranging from 400 to 1000. Although a direct relationship of the aspect ratio to the average tensile stress is shown, as the Friction Hill model predicts, the observed stress increase is not nearly as high but proportional to the classical prediction. This is attributed to the existence of internal defects (oxide particles and micro-voids) and transverse grain boundaries which fail producing internal free surfaces. Thus, the actual aspect ratio is thickness/d², where d equals the distance between internal surfaces. The fracture of these constrained joints was brittle, with the separation occurring between a tin-rich copper tin intermetallic at the interface and the solder matrix. Voids within the solder joint are shown to relieve the plastic constraint and lower the average tensile stress of the joint. The Friction Hill model may play an important role in explaining the small percentage of atypical solder joint failures which sometimes occur on electronic assemblies. In particular, the sudden failure of a thin joint in a strain controlled environment may be attributed to the development of a large hydrostatic stress component. Therefore, a flaw free, plastically constrained joint which develops a high stress state will be a high risk candidate for failure.

Development of a Solder Material Process to Relieve the Plastic Constraint Associated with Thin Joints
FREDERICK S. LAUTEN, JOHN P. RANIERI, and DONALD H. AVERY
Division of Engineering, Brown University, Providence, RI 02912.

KEY WORDS Brittle fracture, Sn-Pb solder, triaxial stress

High aspect ratio (large diameter/thickness) solder joints which are plastically constrained develop large hydrostatic stresses (Friction Hill) greatly in excess of their yield strength. Because the local high triaxial stresses arising from the Friction Hill prevent homogeneous yielding and, in a strain controlled system, will localize plastic deformation within the regions near free surfaces, abrupt brittle fracture through an intermetallic or along an interface can occur. In such situations, the service life of the joint during fatigue situations such as thermal cycling will be greatly reduced. The prevention of triaxial stress build up within such a strain controlled environment which can occur in, for example, leadless chip carrier solder joints requires a distribution of internal free surfaces within the joint. The solder system developed in this study is a thin porous metal film with a regular distribution of pores. The solder material is formed from the usual components, tin and lead. Small lead or tin particles are coated with a thin film of the other component, mixed with flux paste, and the temperature is raised to just above the eutectic temperature. Solid state diffusion occurs across the lead-tin interface until its composition reaches the melting point. The particles then are interconnected by a thin near eutectic liquid film. Additional metal from the solid particle dissolves into the liquid increasing its position and, thus its melting point. Diffusion into the liquid continues until it solidifies isothermally. This forms an interconnecting network of solder "mini-elements" with a dense pore structure.

The Role of Cu-Sn Intermetallics in Wettability Degradation
H.L. REYNOLDS and J.W. MORRIS, JR.
Center for Advanced Materials, Lawrence Berkeley National Laboratory, and Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720.

KEY WORDS Cu-Sn intermetallics, oxidation, solder, wettability

Wettability of pretinned Cu decreases after long aging times. This work provides insight into the role Cu-Sn intermetallics play in wettability degradation. This study investigates the effects of aging in air and argon at 170°C on Cu coupons which were pretinned with 75Sn-25Pb solder. Coating was applied using an electroplating technique. The coating thickness was controlled between 3 to 30 um and the specimens were aged for 0, 2, 24 h, and two weeks. Wetting balance tests were used to evaluate the wettability of the test specimens. Microstructural development was evaluated using x-ray diffraction, energy dispersive x-ray, and Auger spectroscopy, as well as optical and scanning electron microscopy. Results indicate that Cu-Sn intermetallics protected from oxidation do not contribute to a decrease in wettability. Oxidized intermetallics, however, significantly decrease the wettability of aged pretinned samples. The extent of degradation is determined by the type of oxide formed on the surface of the intermetallic. This study shows that a predominantly Cu oxide forms on Cu3Sn, while a Sn oxide forms on Cu6Sn5. No evidence of internal oxidation was found.

Effects of Annealing in O2 and N2 on the Electrical Properties of Tantalum Oxide Thin Films Prepared by Electron Cyclotron Resonance Plasma Enhanced Chemical Vapor Deposition
IL KIM,¹ JONG-SEOK KIM,¹ OH-SEUNG KWON,¹ SUNG-TAE AHN,² JOHN S. CHUN,¹ and WON-JONG LEE¹
1--Department of Materials Science and Engineering, Korea Advanced Institute Science and Technology, Taejon, 305-701, Republic of Korea. 2--Samsung Electronics Co., Ltd, San-24, Nongseo-lee, Kiheung-eup, Yongin-gun, Kyungki-do, 440-600, Republic of Korea.

KEY WORDS Annealing, electrical properties, tantalum oxide, thin films

The tantalum oxide thin films with a thickness of 14 nm were deposited at 95°C by electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR PECVD), and annealed at various temperatures (700~850°C) in O2 and N2 ambients. The microstructure and composition of the tantalum oxide thin films and the growth of interfacial silicon oxide layer were investigated and were related to the electrical characteristics of the film. Annealing in an O2 ambient led to a high dielectric constant ([[epsilon]] r(Ta2O5) = 24) as well as a small leakage current (Ebd = 2.3 MV/cm), which were due to the improved stoichiometry and the decreased impurity carbon content. Annealing in an N2 ambient resulted in poor and nonuniform leakage current characteristics. The as-deposited tantalum oxide films were crystallized into d-Ta2O5 after annealing at above 750°C regardless of the ambient. The leakage current of the film abruptly increased after annealing at 850°C probably because of the stress caused by thermal expansion or contraction.

Development of rf Sputtered, Cu-Doped ZnTe for Use as a Contact Interface Layer to p-CdTe
T.A. GESSERT, A.R. MASON, R.C. REEDY, R. MATSON, T.J. COUTTS, and P. SHELDON
National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401.

KEY WORDS Contact interface layer, Cu-doped ZnTe, rf-magnetron sputtering

Cu-doped ZnTe films deposited by rf-magnetron sputtering have been analyzed with the intention to use this material as a contact interface in CdS/CdTe thin-film photovoltaic solar-cell devices. It is observed that unless careful attention is made to the pre-deposition conditioning of the ZnTe target, the electrical resistivity of thin films (~70 nm) will be significantly higher than that measured on thicker films (~1.0 um). It is determined that N contamination of the target during substrate loading is likely responsible for the increased film resistivity. The effect of film composition on the electrical properties is further studied by analyzing films sputtered from targets containing various Cu concentrations. It is determined that, for targets fabricated from stoichiometric ZnTe and metallic Cu, the extent of Zn deficiency in the film is dependent on both sputtering conditions and the amount of metallic Cu in the target. It is observed that the carrier concentration of the film reaches a maximum value of ~3 x 10²⁰ cm^-3 when the concentrations of Te and (Zn+Cu) are nearly equal. For the conditions used, this optimum film stoichiometry results when the concentration of metallic Cu in the target is ~6 at.%.

Thermodynamics and Kinetics of Hydrogen Evolution in Hydrogenated Amorphous Silicon Films
NAGARAJAN SRIDHAR,^1,2 D.D.L. CHUNG,^1,2 and W. A. ANDERSON³ and J. COLEMAN⁴
1--Center for Electronic and Electro-Optic Materials, State University of New York at Buffalo, NY 14260-4400. 2--Also with Department of Mechanical and Aerospace Engineering. 3--Also with Department of Electrical and Computer Engineering. 4--Plasma Physics Corporation, P.O. Box 548, Locust Valley, NY 11650.

KEY WORDS Differential scanning calorimetry, hydrogenated amorphous films, Si films, thermodynamics

The changes in enthalpy and entropy due to hydrogen evolution in hydrogenated amorphous silicon films were measured by differential scanning calorimetry (DSC). Hydrogen evolution was associated with an endothermic DSC peak, as supported by thermogravimetric analysis and evolved gas analysis. The enthalpy and entropy changes of hydrogen evolution increased with heating rate and hydrogen content, because the evolution involved not only Si-H bond breaking, but also defect formation (such as Si-Si bond breaking), which was enhanced by a high flow of evolving hydrogen. In contrast, the activation energy of hydrogen evolution was controlled by the doping rather than the hydrogen content, because doping affected the Si-H bonding, which in turn affected the state before hydrogen evolution. Crystallization, which occurred at temperatures higher than hydrogen evolution, was delayed for the amorphous silicon film in a higher disordered state after hydrogen evolution, suggesting that hydrogen content influenced the crystallization process.

Accurate Measurement of Capture Cross Sections in Deep Level Transient Spectroscopy: Application to EL2 in GaAs
D.C. LOOK,¹ Z.-Q. FANG,¹ and J.R. SIZELOVE²
1--University Research Center, Wright State University, Dayton, OH 45435. 2--Solid State Electronics Directorate, Wright Laboratory, WL/ELRA, Wright-Patterson Air Force Base, OH 45433.

KEY WORDS Capture cross section, deep level transient spectroscopy (DLTS), EL2, GaAs

A rigorous formulation of capacitance changes during trap filling processes is presented and used to accurately determine the electron capture cross section of EL2 in GaAs at a particular temperature, 377K, in this case. The value, [[sigma]]n (377K) = 2.7 x 10^-16 cm², is compared with that predicted from the emission dependence.

The Effect of Soldering Process Variables on the Microstructure and Mechanical Properties of Eutectic Sn-Ag/Cu Solder Joints
WENGE YANG, LAWRENCE E. FELTON, and ROBERT W. MESSLER, JR.
Center for Integrated Electronics and Electronics Manufacturing and Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180-3590.

KEY WORDS Creep, microhardness, shear strength, Sn-Ag, solder, solder joint microstructure

Fundamental understanding of the relationship among process, microstructure, and mechanical properties is essential to solder alloy design, soldering process development, and joint reliability prediction and optimization. This research focused on the process-structure-property relationship in eutectic Sn-Ag/Cu solder joints. As a Pb-free alternative, eutectic Sn-Ag solder offers enhanced mechanical properties, good wettability on Cu and Cu alloys, and the potential for a broader range of application compared to eutectic Sn-Pb solder. The relationship between soldering process parameters (soldering temperature, reflow time, and cooling rate) and joint microstructure was studied systematically. Microhardness, tensile shear strength, and shear creep strength were measured and the relationship between the joint microstructures and mechanical properties was determined. Based on these results, low soldering temperatures, fast cooling rates, and short reflow times are suggested for producing joints with the best shear strength, ductility, and creep resistance.

Stress Relaxation Behavior of Eutectic Tin-Lead Solder
E.W. HARE¹ and R.G. STANG²
1--Materials and Process Engineer, Alliant Techsystems, Inc., Mukilteo, WA 98275. 2--Associate Professor, Department of Materials Science and Engineering, University of Washington, Seattle, WA 98195.

KEY WORDS Creep, eutectic tin-lead, solder, stress relaxation

The creep behavior of eutectic tin-lead solder was investigated using stress relaxation techniques. Stress relaxation experiments were performed on cast tensile specimens of commercial eutectic tin-lead solder, SN63. The sample casting conditions were controlled to produce microstructures similar to those found in typical solder joints on electronic assemblies. The stress relaxation data was analyzed to extract constitutive relations for creep. The strain rate during relaxation was found to follow two power law expressions, one with n = 3.2 at low stress levels and the other with n = 6.2 at higher stress levels. The apparent activation energy for creep and the power law exponent are discussed with relation to the published data for this alloy.

Effects of Barrier Layer and Processing Conditions on Thin Film Cu Microstructure
E.M. ZIELINSKI, R.P. VINCI, and J.C. BRAVMAN
Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305-2205.

KEY WORDS Copper, microstructure, texture, thin films

The crystallographic texture and grain size of sputtered Cu films were characterized as a function of deposition temperature, barrier layer material, and vacuum conditions. For Cu deposited in a HV chamber, (111) Cu texture was found to weaken with increasing deposition temperatures on W, amorphous C and Ta barrier layers, each deposited at 30°C. Conversely, under identical Cu deposition conditions, texture was found to strengthen with increasing deposition temperature on Ta deposited at 100°C. Median Cu grain size varied parabolically with deposition temperature on all barrier layers and was slightly higher on the 100°C Ta at a given Cu deposition temperature, relative to the other underlayers. For depositions in an UHV chamber, Cu texture was found to strengthen with increasing Cu deposition temperature, independent of Ta deposition temperature. Median Cu grain size, however, was still higher on 100°C Ta than on 30°C Ta. The observed differences between the two different chambers suggest that the trend of weak texture at elevated deposition temperatures may be related to contamination. Characterization of the Ta underlayers revealed that the strengthened texture of Cu films deposited on 100°C Ta is likely related to textural inheritance.

Intermetallic Compound Layer Growth By Solid State Reactions Between 58Bi-42Sn Solder and Copper
P.T. VIANCO, A.C. KILGO, and R. GRANT
Center for Solder Science and Technology, Sandia National Laboratories, Albuquerque, NM 87185.

KEY WORDS Bi-Sn solder, copper/tin-bismuth diffusion, intermetallic compound formation, intermetallic growth kinetics

Solid state intermetallic compound layer growth was examined following thermal aging of the 58Bi-42Sn/Cu couple for a temperature range of 55 to 120°C and time periods of from 1 to 400 days. The intermetallic compound layer was comprised of sublayers that included the traditional Cu6Sn5 stoichiometry as well as one or more complex Cu-Sn-Bi chemistries. The number of sublayers increased with aging temperature and time. Time-dependent layer thickness computations based upon the empirical expression, Atⁿ + B, revealed a time exponent, n, that decreased with increasing temperature from a maximum of 0.551 at 70°C to 0.417 at 120°C. The apparent activation energy for growth (at 100 days) was 55 +/- 7 kJ/mol. The Bi-Sn/Cu data, together with that from the other solder/copper systems, suggested that at a given homologous temperature, the quantity of Sn in the solder field determines the intermetallic compound layer thickness as a function of time.

Low Temperature Plasma Enhanced Chemical Vapor Deposition of Silicon Oxide Films Using Disilane and Nitrous Oxide
JUHO SONG, G.S. LEE, and P.K. AJMERA
Solid State Laboratory, Department of Electrical and Computer Engineering, Louisiana State University, Baton Rouge, LA 70803-5901.

KEY WORDS Disilane, low temperature, nitrous oxide, plasma enhanced chemical vapor deposition (PECVD), silicon oxide

Silicon oxide films have been deposited between room temperature and 300°C using disilane and nitrous oxide by plasma enhanced chemical vapor deposition. Film deposition was investigated as a function of the gas flow ratio of nitrous oxide to disilane, the substrate temperature, the total gas flow rate, the radio frequency discharge power, and the process pressure. The stoichiometric SiO2 films were obtained when the gas ratio of nitrous oxide to disilane was in the range of 50-150. The deposition was found to be nearly temperature independent indicating the mass transport limited regime.

Selective Silicon Epitaxy by Photo-Chemical Vapor Deposition at a Very Low-Temperature of 160°C
AKIRA YAMADA, TAKAYUKI OSHIMA, MAKOTO KONAGAI, and KIYOSHI TAKAHASHI
Department of Electrical and Electronic Engineering, Tokyo Institute of Technology, O-okayama 2-12-1, Meguro ku, Tokyo 152, Japan.

KEY WORDS Heavily doped silicon, photo-chemical vapor deposition (CVD), selective silicon epitaxy

We have grown epitaxial Si films by the photo-chemical vapor deposition (photo-CVD) technique with SiH4 and H2 at a very low-temperature of 160°C. Epitaxial films were grown on silicon substrates, while amorphous-like films were deposited on glass substrates. Furthermore, it was found from the atomic hydrogen etching which was produced by photo-dissociation of hydrogen that the etching rate of amorphous silicon was much higher than that of crystal silicon. By using these selectively, we have demonstrated selective epitaxial growth of silicon by the photo-CVD technique followed by the atomic hydrogen photo-etching. Furthermore, heavily phosphorus-doped silicon films (>1 x 10²¹ cm^-3) were also selectively grown by this novel technique.