JOURNAL OF ELECTRONIC MATERIALS
	ABSTRACTS Volume 25, Number 7, July 1996 This Month Featuring: Regular Issue Papers; Letters Section; Erratum; and 1996 Electronic Materials Conference Program. View July 1996 Table of Contents.

REGULAR ISSUE PAPERS

KEY WORDS Photoluminescence, RTCVD, SiGe

Relaxed Si_1-xGe_x layers grown by rapid thermal chemical vapor deposition (RTCVD) have been characterized by photoluminescence (PL) spectroscopy. The structures consist of a Si_1-xGe_x capping layer with a 0.32 and 0.52 Ge concentration, grown on a compositionally graded Si_1-xGe_x buffer layer. The effect of the composition grading rate on the layer quality has been intensively studied. Well-resolved near band edge luminescence (excitonic lines with no-phonon and phonon replica similar as in bulk SiGe alloys) coming from the relaxed alloy capping layer and dislocation-related bands (D1, D2, D3, D4 lines) in the graded buffer layer have been measured. The electronic quality of this relaxed capping layer, controlled by the design of the compositionally graded buffer layer, has been determined by the excitonic photoluminescence. A detailed analysis of the energy of the D4 dislocation band demonstrates that the main misfit dislocations remain confined in the first steps of the graded buffer layer. Si_1-xGe_x layers grown on these pseudo-substrates either under compressive or tensile strain and the well-defined PL results obtained are discussed on the bases of strain symmetrization and of high quality of the layers. This points out the possibility of using such high quality relaxed Si_1-xGe_x layers as substrates for the integration of new devices associated with Si technology.

Effect of Composition on Deep Levels in Heteroepitaxial Ge_xSi_1-x Layers and Evidence for Dominant Intrinsic Recombination-Generation in Relaxed Ge Layers on Si
P.N. GRILLOT,¹ S.A. RINGEL,¹ and E.A. FITZGERALD²
1--Electronic Materials and Devices Laboratory, Department of Electrical Engineering, The Ohio State University, Columbus, OH 43210. 2--Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139.

KEY WORDS Electron traps, Ge photodetectors, GeSi, hole traps, recombination-generation centers

Electron traps, hole traps, and the dominant recombination-generation (R-G) centers have been investigated with deep level transient spectroscopy and current-voltage/temperature measurements in heteroepitaxial Ge_xSi_1-x alloys with x ranging from 0.15 to 1, grown on graded Ge_ySi_1-y/Si substrates. For all samples with compositions x < 0.85, which retain the Si-like conduction band structure, we detect a dominant electron trap and R-G center whose activation energy is DE=0.5 eV, independent of composition. This energy agrees with that of electron traps previously reported for plastically deformed (PD) Si, suggesting a connection to the Si-like band structure. This 0.5 eV level dominates the reverse leakage current over a wide range of growth and annealing conditions for the 30% Ge samples, indicating that the electronic state at DE=0.5 eV is a very efficient R-G center, as would be expected from its midgap position. Alternatively, for strain relaxed, pure Ge (x=1), we detect electron traps at EC-0.42 eV and EC-0.28 eV, in agreement with the literature on PD Ge and Ge bicrystals. These energies are significantly different from those observed for x < 0.85, and we conclude that these changes in activation energy are due to changes in the conduction band structure for high Ge content. Moreover, in contrast with the Si-like samples (x < 0.85), the reverse leakage current in the relaxed Ge cap layer is not controlled by deep levels, but is rather dictated by intrinsic, band-to-band generation due to the reduced bandgap of Ge as compared to Si-like alloys. Only for reverse bias magnitudes which incorporate a significant portion of the graded buffer within the depletion region do R-G centers dominate the reverse leakage current. These results confirm the high quality of the strain-relaxed, pure Ge cap region which was grown on a Ge_ySi_1-y/Si step graded heterostructure (where y was increased from 0 to 1) by ultra high vacuum chemical vapor deposition. Finally, we report for the first time, what is apparently the dislocation kink site state at EC-0.37 eV, in a Ge_xSi_1-x alloy.

Degradation of Silicon Dioxide During Selective Silicon Epitaxy in a Dichlorosilane Environment
C.C. HOBBS, J.J. WORTMAN, and M.C. ÖZTÜRK
North Carolina State University, Department of Electrical and Computer Engineering, Box 7911, Raleigh, NC 27695-7911.

KEY WORDS Degradation, dichlorosilane, epitaxy, Si, SiO2

The electrical degradation of dry thermal SiO₂ upon exposure to selective silicon epitaxy using dichlorosilane has been investigated. Capacitors were fabricated with thermal gate oxides (120 to 440Å thick) grown on p-type silicon (100) substrates. Prior to the gate electrode formation, the oxides were exposed to hydrogen and dichlorosilane + hydrogen anneals. Leakage current and electric field breakdowns were measured to evaluate the effects of these anneals on the SiO₂ degradation. The SiO₂ degradation occurring because of dichlorosilane exposure was studied as a function of the temperature and time. While dichlorosilane exposure at temperatures above 850°C was found to cause high leakage current and breakdowns at low electric fields for silicon dioxide films thinner than 440Å, little effect was observed as a result of hydrogen and chlorine exposures. The degradation mechanism was attributed to pinhole etching via volatile SiO formation along defects present in the as-grown SiO₂.

Strain Accommodation in Mismatched Layers by Molecular Beam Epitaxy: Introduction of a New Compliant Substrate Technology
C. CARTER-COMAN,¹ A.S. BROWN,¹ N.M. JOKERST,¹ D.E. DAWSON,¹ R. BICKNELL-TASSIUS,² Z.C. FENG,¹ K.C. RAJKUMAR,¹ and G. DAGNALL¹
1--School of Electrical and Computer Engineering, Georgia Institute of Technology, 791 Atlantic Dr. NW MC0269, Atlanta, GA 30332-0269. 2--EOEML, Georgia Tech Research Institute, Georgia Institute of Technology, 791 Atlantic Dr. NW MC0269, Atlanta, GA 30332-0269.

KEY WORDS Compliant substrates, growth kinetics, molecular beam epitaxy (MBE), strain

Compliant substrates allow a new approach to the growth of strained epitaxial layers, in which part of the strain is accommodated in the substrate. In this article, compliant substrates are discussed and a new compliant substrate technology based on bonded thin film substrates is introduced. This technology has several advantages over previously published methods, including the ability to pattern both the top and bottom of the material. A new concept enabled by this compliant substrate technology, strain-modulated epitaxy, will be introduced. Using this technique, the properties of the semiconductor material can be controlled laterally across a substrate. Results of two experiments are presented in which low composition In_xGa_1-xAs was grown by molecular beam epitaxy on GaAs compliant substrates at thicknesses both greater than and less than the conventional critical thickness. It was found that for t > t_c, there was an inhibition of defect production in the epitaxial films grown on the compliant substrates as compared to those grown on conventional reference substrates. For t < t_c, photoluminescence and x-ray diffraction show the compliant substrates to be of excellent quality and uniformity as compared to conventional substrates.

Gas-Source Molecular Beam Epitaxy and Characterization of InGaAs/InGaAsP Quantum Well Structures on InP
W.G. BI and C.W. TU
Department of Electrical and Computer Engineering, University of California, San Diego, La Jolla, CA 92093-0407.

KEY WORDS Gas-source molecular beam epitaxy (GSMBE), InGaAs/InGaAsP QWs, interfaces

In this paper, we report the effect of using a group-V residual source evacuation (RSE) time on the interfaces of InGaAs/InGaAsP quantum wells (QWs) grown by gas-source molecular beam epitaxy. High-resolution x-ray rocking curve and low-temperature photoluminescence (PL) were used to characterize the material quality. By optimizing the RSE time, a PL line width at 15K as narrow as 6.6 meV is observed from a 2 nm wide single QW, which is as good as or better than what has been reported for this material system. Very sharp and distinct satellite peaks as well as Pendellosung fringes are observed in the x-ray rocking curves of In_xGa_1-xAs/In_xGa_1-xAs_yP_1-y multiple QWs, indicating good crystalline quality, lateral uniformity, and vertical periodicity. Theoretical considerations of the PL linewidths of In_xGa_1-xAs/In_xGa_1-xAs_yP_1-y single QWs show that for QW structures grown with the optimized RSE time, the PL linewidth is mainly due to alloy scattering, whereas the contribution from interface roughness is small, indicating a good interface control.

Electrothermal Investigation of the Switching Effect in p-Type TlInSe₂, TlInTe₂, and TlGaTe₂ Chain Chalcogenide Semiconductors
B. ABAY, B. GÜRBULAK, M. YILDIRIM, H. EFEOGLU, S. TÜZEMEN, and Y.K. Yogurtcu
Department of Physics, Faculty of Arts and Sciences, Atatürk University, Erzurum, Turkey.

KEY WORDS Switching effect, TlGaTe2, TlInSe2, TlInTe2

Large p-type TlInSe₂, TlInTe₂, and TlGaTe₂ single crystals have been grown by the Bridgman-Stockbarger method, starting from stoichiometric melts. The first observations of the switching process in p-type TlGaTe₂ single crystal are reported. Current-voltage (I-V) characteristics of symmetrical In/p-TlInSe₂/In, In/p-TlInTe₂/In, and In/p-TlGaTe₂/In structures exhibit two distinct regions: an ohmic region at low current densities and nonlinear regions (S-shape) having negative differential resistance (NDR) at moderate and higher current densities. An electrothermal model was used to explain the nonlinear behavior. The nonlinear behavior of the I-V curves was studied at different ambient temperatures in the 100-340K region; the sample temperature and the threshold voltage of the NDR region were examined as a function of the current density and the ambient temperature, respectively. The electrothermal model is a satisfactory explanation.

A Novel Synthetic Route to Ether-Free Metal Alkyl Precursors
T.J. WHITAKER,¹ R.W. FREER,¹ T. MARTIN,¹ A.C. JONES,² and S.A. RUSHWORTH²
1--DRA Malvern, St. Andrews Road, Great Malvern, Worcestershire, WR14 3PS, UK. 2--Epichem Ltd., Power Road, Bromborough, Wirral, Merseyside, L62 3QF, UK.

KEY WORDS AlGaAs, chemical beam epitaxy (CBE), metal alkyl precursors

A new synthetic route to metal alkyl precursors has been developed, eliminating ether solvents which have been shown to result in unintentional oxygen incorporation in epitaxial AlGaAs layers. The synthesis is performed in a trialkylamine solvent and the need for an additional adduct purification step is eliminated. The products of triisopropylgallium synthesis solvated by either ethyldimethylamine or triethylamine have been assessed as potential growth precursors for chemical beam epitaxy by HNMR spectroscopy, vapor pressure measurements and GaAs growth rate measurements.

Improved Quality of Al_xGa_1-xAs Grown on Se-Doped Al_xGa_1-xAs Substrate-Layers by Metalorganic Chemical Vapor Deposition
KUN-JING LEE,^1,2 Z.C. HUANG,^1,3 and J.C. CHEN¹
1--Department of Computer Sciences and Electrical Engineering, University of Maryland, Baltimore County, Baltimore, MD 21228. 2--Department of Electrical Engineering, University of Maryland, College Park, College Park, MD 20742. 3--Current address: Hughes STX Corporation, 7701 Greenbelt Road, Suite 400, Greenbelt, MD 20770.

KEY WORDS AlGaAs, metalorganic chemical vapor deposition (MOCVD), regrowth, Se doping

Al_xGa_1-xAs epilayers were grown directly on different Al_xGa_1-xAs substrate-layers by metalorganic chemical vapor deposition (MOCVD). The quality of Al_xGa_1-xAs layers was significantly improved when Se-doped Al_xGa_1-xAs substrate-layers were used. Al_0.13Ga_0.87As epilayers with excellent morphology, optical, and crystal quality were grown on Se-doped Al_0.26Ga_0.74As. The full width at half maximum of the bound excitor peak as low as 4.51 meV was measured by low-temperature (14.9K) photoluminescence. The improvement is attributed to a Se passivation effect at the surface of Se-doped Al_xGa_1-xAs substrate-layers. Results suggest that Se will reduce and delay the formation of native oxides.

Characteristics of Island Growth of Hg_1-xCd_xTe on (100) CdTe Substrates by Chemical Vapor Transport
YU-RU GE and HERIBERT WIEDEMEIER
Department of Chemistry, Rensselaer Polytechnic Institute, Troy, NY 12180-3590.

KEY WORDS Chemical vapor transport (CVT), HgCdTe, island growth

The characteristics of island growth of Hg_1-xCd_xTe (MCT) by chemical vapor transport on (100) CdTe substrates have been studied, for the first time, by optical microscopy, by infrared transmission spectroscopy, and by chemical etching. Nonuniformities of the growth thickness, of the growth rate, and of the composition of islands are observed. Based on the experimental results, the origin and the inhibition of island growth of MCT on (100) CdTe substrates are discussed.

Transient Behavior of Hg_1-xCd_xTe Films Deposited on (100) CdTe Substrates by Chemical Vapor Transport
HERIBERT WIEDEMEIER and YU-RU GE
Department of Chemistry, Rensselaer Polytechnic Institute, Troy, NY 12180-3590.

KEY WORDS Chemical vapor transport (CVT), epitaxial growth, HgCdTe, mass flux, transient behavior

The growth history of Hg_1-xCd_xTe films deposited on (100) CdTe substrates by chemical vapor transport (CVT) has been studied, for the first time, by using a transient growth technique. The observed morphological evolution of Hg_1-xCd_xTe films deposited at 545°C shows a transition behavior from three-dimensional (3D) islands to two-dimensional (2D) layer growth. The experimental results indicate that the so-called critical time needed for the above morphological transition is about 1h under present experimental conditions. Based on the chemical bonding properties of Hg_1-xCd_xTe, and on the behavior of the morphological transition, the Stranski-Krastanov growth mode is suggested for the epitaxial growth system. The time dependence of the growth thickness, of the growth rate (R100) along the [100] direction, and of the surface composition all reveal a transient behavior. These are related to the nature of the Hg_1-xCd_xTe/(100)CdTe heterojunction and to the surface reactions. Comparison of the growth rates and of the total mass deposited as a function of time shows the relationship between epitaxial growth and mass flux of the Hg_1-xCd_xTe-HgI₂ chemical vapor transport system.

Growth and Characterization of GaSb Bulk Crystals with Low Acceptor Concentration
A.N. DANILEWSKY,¹ S. LAUER,¹ J. MEINHARDT,¹ K.W. BENZ,¹ B. KAUFMANN,² R. HOFMANN,² and A. DÖRNEN²
1--Kristallographisches Institut, Albert-Ludwigs-Universität, D-79104 Freiburg, Germany. 2--4. Physikalisches Institut, Universität, D-70550 Stuttgart, Germany.

KEY WORDS Bi-solution, bulk crystal growth, gallium antimonide, GaSb, Hall measurements, intrinsic acceptor, photoluminescence, traveling heater method

GaSb bulk single crystals with low acceptor concentration were grown from a bismuth solution by the traveling heater method. The result is isoelectronic doping by Bi which gives a variation of the opto-electronic properties as a function of grown length as well as a pronounced microscopic segregation. Photoluminescence spectra at 4K show a decrease of the natural acceptor during growth, which is confirmed by Hall measurements. The electrical properties of this isoelectronic doped GaSb are hole concentrations and mobilities of N_A-N_D=1.7 X 10¹⁶ cm^-3 and µ=870 cm²/Vs at room temperature and N_A-N_D=1 X 10¹⁶ cm^-3 and µ=4900 cm²/Vs at 77K, respectively. The lowest p-type carrier concentration measured at 300K is N_A-N_D=3.3 X 10¹⁵ cm^-3.

Surface Condition of Si Implanted GaAs Revealed by the Noncontact Laser/Microwave Method
MASAYA ICHIMURA,¹HIDEAKI YOSHIDA,² and AKIRA USAMI^2,3
1--Center for Cooperative Research. 2--Department of Electrical and Computer Engineering, Nagoya Institute of Technology, Gokiso, Showa, Nagoya 466, Japan. 3--deceased.

KEY WORDS GaAs, high-low junction, ion implantation, microwave probe, surface recombination

The surface recombination of GaAs which has a heavily doped surface layer formed by Si implantation and subsequent annealing has been investigated using the noncontact laser/microwave evaluation method. The experimental results of the samples implanted with doses ranging from 1.0 X 10¹¹ to 3.9 X 10¹² cm^-2 at an energy of 100 keV indicate that the effective surface recombination velocity decreases with dosage because of the heavily doped layer formed after the annealing. On the other hand, the results of the samples implanted with a dose of 3.9 X 10¹² cm^-2 at energies raging from 50 to 180 keV indicate that the effective surface recombination velocity increases with energy. This is mainly due to the decrease in the peak carrier concentration in the heavily doped layer.

Copper Migration in CdTe Heterojunction Solar Cells
H.C. CHOU,¹ A. ROHATGI,¹ N.M. JOKERST,¹ E.W. THOMAS,² and S. KAMRA¹
1--School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332. 2--School of Physics, Georgia Institute of Technology, Atlanta, GA 30332.

KEY WORDS CdTe, Cu migration, heterojunction, solar cells

CdTe solar cells were fabricated by depositing a Au/Cu contact with Cu thickness in the range of 50 to 150Å on polycrystalline CdTe/CdS/SnO₂/glass structures. The increase in Cu thickness improves ohmic contact and reduces series resistance (R_s), but the excess Cu tends to diffuse into CdTe and lower shunt resistance (R_sh) and cell performance. Light I-V and secondary ion mass spectroscopy (SIMS) measurements were performed to understand the correlations between the Cu contact thickness, the extent of Cu incorporation in the CdTe cells, and its impact on the cell performance. The CdTe/CdS/SnO₂/glass, CdTe/CdS/GaAs, and CdTe/GaAs structures were prepared in an attempt to achieve CdTe films with different degrees of crystallinity and grain size. A large grain polycrystalline CdTe thin film solar cell was obtained for the first time by selective etching the GaAs substrate coupled with the film transfer onto a glass substrate. SIMS measurement showed that poor crystallinity and smaller grain size of the CdTe film promotes Cu diffusion and decreases the cell performance. Therefore, grain boundaries are the main conduits for Cu migration and larger CdTe grain size or alternate method of contact formation can mitigate the adverse effect of Cu and improve the cell performance.

Solder Flow on Narrow Copper Strips
F.M. HOSKING, F.G. YOST, E.A. HOLM, and J.R. MICHAEL
Sandia National Laboratories, MS/0340, Albuquerque, NM 87185.

KEY WORDS Capillary flow, printed wiring board, solderability

Various solderability tests have been developed over the years to quantify the wetting behavior of solder on metallic surfaces. None offer an exact measure of capillary flow normally associated with conventional plated-through-hole and surface mount soldering. With shrinking package designs, increasing reliability requirements, and the emergence of new soldering technologies, there is a growing need to better understand and predict the flow of solder on printed wiring board (PWB) surfaces. Sandia National Laboratories has developed a capillary flow solderability test, through a joint effort with the National Center for Manufacturing Sciences, that considers this fundamental wetting issue for surface mount technology. The test geometry consists of a metal strip (width, ) connected to a circular metal pad (radius, r_c). Solder flow from the pad onto the strip depends on the geometric relationship between and r_c. Test methodology, experimental results, and validation of a flow model are presented in this paper.

Zinc Diffusion in Tellurium Doped Gallium Antimonide
G.J. CONIBEER,¹ A.F.W. WILLOUGHBY,¹ C.M. HARDINGHAM,² and V.K.M. SHARMA³
1--Engineering Materials, University of Southampton, Southampton SO17 1BJ, UK. 2--EEV Ltd, Chelmsford CM1 2QU, UK. 3--Department of Materials, Imperial College, Prince Consort Road, London SW7 2BP, UK.

KEY WORDS Diffusion, dissociative, gallium antimonide, zinc, substitutional-interstitial

Zinc diffusion into tellurium doped gallium antimonide, GaSb, has been carried out as a function of time, temperature, and antimony over-pressure. Total zinc profiles as well as carrier concentration profiles have been measured. Results favor a substitutional-interstitial vacancy (Frank-Turnbull) or kick-out (Gösele-Morehead) mechanism, although there is insufficient evidence to conclusively distinguish between them. There is also an inverse dependence of the diffusivity on antimony over-pressure, this is discussed in terms of zinc diffusion superimposed on gallium vacancy diffusion. Tellurium doping seems to have little effect on the diffusion because of its low level in comparison to that of zinc. Furthermore, at high zinc concentrations, the profiles indicate an additional component associated with a non-electrically active zinc species which has a small, strongly temperature dependent diffusion coefficient.

Interfacial Reactions During Soldering with Lead-Tin Eutectic and Lead (Pb)-Free, Tin-Rich Solders
S.K. KANG, R.S. RAI, and S. PURUSHOTHAMAN
IBM Research T. J. Watson Research Center, P. O. Box 218, Yorktown Heights, NY 10598.

KEY WORDS Dissolution kinetics, interfacial reactions, intermetallics, lead (Pb)-free solders, Ni barrier layer, soldering, tin (Sn)-rich solders

The use of lead (Pb)-containing solders for the interconnections of microelectronic subsystem assembly and packaging has become an environmental issue. Extensive research and development activities for replacing Pb-containing solders with environmentally safe Pb-free solders are in progress in electronic industries, universities, and national laboratories. One key technical issue recognized with the Pb-free, Sn-rich solders is a need to develop a good barrier metallurgy to control the interfacial reactions, namely, dissolution of the base metal(s) and concurrent formation of intermetallics at the soldering interfaces. In this study, the interfacial reactions of Cu and Ni metallizaton with several Pb-free and Pb-containing solders are investigated. The dissolution kinetics of the base metal(s) as well as the growth kinetics of the intermetallics are discussed.

High Thermoelectric Figures of Merit in PbTe Quantum Wells
T.C. HARMAN, D.L. SPEARS, and M.J. MANFRA
Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, MA 02173-9108.

KEY WORDS Molecular beam epitaxy (MBE), multiple quantum wells (MQWs), PbTe, Pb1-xEuxTe/PbTe, thermoelectric figure of merit

High-quality Pb_1-xEu_xTe/PbTe multiple quantum wells (MQWs) have been grown by molecular beam epitaxy. The measured 300K thermoelectric properties have been compared with that of the best bulk PbTe. This experimental investigation is the first detailed study of MQW structures designed to improve ZT of thermoelectric materials and has resulted in a breakthrough in the decades-long ZT1 barrier for a room-temperature thermoelectric material. A value of Z_2DT >1.2 has been achieved for these PbTe quantum wells.

LETTERS SECTION

KEY WORDS Bi-45%Sn-0.33%Ag solder, lead-free, solder

Wave soldering with low solid fluxes at temperatures as low as 175°C on test boards with a Cu/Imidazole surface finish has been shown to be feasible using a Pb-free, Bi-45%Sn-0.33%Ag solder that melts at temperatures of ~140-145°C. Other surface finishes such as Pd/Ni, Au/Ni, and Bi exhibit unacceptable soldering at temperatures below 210°C. Intermediate in performance are Sn surface finishes, which exhibit acceptable soldering at 190°C, but not at 175°C. Acceptable joints wave soldered on Cu/Im finishes passed class I/II inspection criterion and exhibited pull strengths in excess of the ultimate strength of the component leads.

ERRATUM

1. N.A. Hill and K.B. Whaley, Phys. Rev. Lett. 76, 3039 (1996).
2. C. Delerue, M. Lannoo and G. Allan, Phys. Rev. Lett. 76, 3038 (1996).
*Journal of Electronic Materials Vol. 25, pp. 269-285 (1996).

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