JOURNAL OF ELECTRONIC MATERIALS
	ABSTRACTS Volume 25, Number 2, February 1996 This Month Featuring: Proceedings of the symposium on II-VI Heterostructures for Short Wavelength Emitters from the 1995 Electronic Materials Conference, Charlottesville, Virginia, June 21-23. View February 1996 Table of Contents.

SPECIAL ISSUE PAPERS

Maria C. Tamargo
City University of New York

Guest Editor

New Mechanism of Excitonic Enhanced Optical Gain for Wide-Gap Quantum Wells
T. UENOYAMA
Central Research Laboratories, Matsushita Electric Industrial Co., Ltd., 3-4 Hikaridai, Seika-cho Souraku-gun Kyoto 619-02, Japan.

KEY WORDS Exciton, localized state, optical gain

New mechanism of optical gain in quantum wells are proposed using excitonic effects. Exciton in wide-gap semiconductors plays an important role in optical phenomena since it has a large binding energy and could be stable at room temperature. However, its bound state is constructed by the electron-hole Coulomb interaction and should be related to the electron and hole distributions when the ground state has many electron and holes. We have evaluated the current-current correlation function, i.e. conductivity, treating the mechanism of optical gain and exciton on equal footing. It is shown that the recombination of the exciton does not yield optical gain directly but that excitonic effects enhance an oscillator strength of the coupled transition. Taking into account a localized level in the energy gap, the optical gain in terms of the population inversion between the localized level and one of the band edge subband states is produced with the very small carrier concentration. Simultaneously, the excitonic absorption occurs due to the band edge electron-hole interaction. It is found that the former optical gain is enhanced extremely by the latter excitonic effect through the coupling between the two transitions. This enhanced optical gain might show a possibility of very low threshold current density for wide-gap laser diodes.

Carrier Dynamics Study of Cd_xZn_1-xSe/ZnSe (x = 0.2) Multiple Quantum Wells
L. WANG, J.H. SIMMONS, M.H. JEON, R.M. PARK, and C.J. STANTON
University of Florida, Gainesville, FL 32611.

KEY WORDS Biexciton, carrier dynamics, CdZnSe/ ZnSe, exciton, multiple quantum wells, optical degradation

In this paper, nonlinear optical properties of Cd_xZn_1-xSe/ZnSe (x = 0.2) multiple quantum wells were studied by low temperature steady-state and transient photoluminescence at high excitation densities. The biexciton transition was observed on the low energy side of the exciton transition. Based on the characteristics of stimulated emission observed in similar structures, we suggest the biexciton transition as the mechanism for stimulated emission. Optical degradation was also studied by room temperature photoluminescence using femtosecond laser pulses as the excitation source. The results confirm the formation of nonradiative recombination centers with a saturating degradation effect after about 10 min of exposure.

Optical Properties and Thermal Transport of Carriers in (Zn, Cd)Se-ZnSe Heterostructures
L. AIGOUY, B. GIL, O. BRIOT, T. CLOITRE, N. BRIOT, R.L. AULOMBARD, and M. AVEROUS
Groupe d'Etudes des Semiconducteurs, Case Courrier 074, Université de Montpellier II, place Eugéne Bataillon, 34095 Montpellier, Cedex 05, France.

KEY WORDS Diffusion, Franz-Keldysh, oscillations, GRIN-SCH, photoluminescence, photoreflectance, thermal escape

We report a detailed optical study of ZnSe-based graded index separate confinement heterostructures. These structures were grown by metalorganic vapor phase epitaxy and are composed of either one or two Zn_0.79Cd_0.21Se central well(s) embedded between two ZnCdSe barriers which cadmium composition varies linearly from 5% near the wells to 0% at the end of the barriers. 2K photoreflectance and reflectivity experiments allow the observation of excitonic transitions involving the third electron and heavy hole confined states. The temperature dependence of the photoluminescence lines under in-well resonant excitation conditions (E_exc=2.661 eV), shows that the thermal quenching of the photoluminescence line is ruled by nonradiative recombinations on defects localized at the heterointerfaces at low temperature and by the thermal escape of the minority carriers at higher temperatures. Under above-barrier excitation conditions (E_exc=3.814 eV), the temperature dependence of the photoluminescence line from the well shows a strong influence of the mechanism of diffusion of the carriers from the barriers to the well.

Formation of a Thin III-VI Compound Interfacial Layer at ZnTe/ZnSe Heterojunction and Its Effect on Energy Band Discontinuity
T. YOSHIDA, T. NAGATAKE, M. KOBAYASHI, and A. YOSHIKAWA
Department of Electrical and Electronics Engineering, Faculty of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263, Japan.

KEY WORDS III-VI compound, defect zinc-blende structure, GaSe, Ga2Se3, layered structure, reduction of energy band discontinuity, relaxation of lattice mismatch

Interfacial layers were inserted at the interface of ZnSe and ZnTe in order to reduce both (1) the effect of strain and (2) the valence band discontinuity. The interfacial layer adapted in this study is the III-VI compound (Ga,Se). The layered structure GaSe is favorable for the present work, because it can be a buffer layer to relax the lattice mismatch at the interface. All layers including ZnTe, (Ga,Se) and ZnSe were grown on (100) GaAs substrate by conventional molecular beam epitaxy. The crystal structure of the (Ga,Se) on ZnSe was investigated. The growth of the layered structure GaSe layer on (100) ZnSe was very difficult, though the defect zinc-blende structure Ga₂Se₃ layer could be easily grown. The defect zinc-blende structure Ga₂Se₃ was inserted at the interface of ZnSe and ZnTe so that the valence band discontinuity could be modified. The discontinuity was decreased to about 0.1 eV when the thickness of the interfacial layer was about 8Å. The current-voltage characteristics were measured for the sample with Ga₂Se₃ interfacial layer. The structure with Ga₂Se₃ exhibited the ohmic property. These results suggest that the valence band discontinuity between ZnTe and ZnSe can be reduced by introducing the Ga₂Se₃ interfacial layer.

Metal-Semiconductor Contacts to n-ZnS_0.07Se_0.93
ALBERT WANG and WAYNE A. ANDERSON
Department of Electrical & Computer Engineering, Center for Electronic & Electro-Optic Materials, State University of New York at Buffalo, Bonner Hall, Amherst, NY 14260.

KEY WORDS II-VI semiconductors, Schottky contacts, stability

A systematic study was conducted on electrical properties of metal-semiconductor contacts to n-ZnS_0.07Se_0.93 with metals of different metal work functions. A clear relation was found between the Schottky barrier height and the metal work function although the Schottky contact theory could not predict it accurately. Thermionic-emission was found to be the dominant current transport mechanism. A significant Schottky barrier height improvement was achieved by means of a cryogenic process technique for forming the Au/n-ZnS_0.07Se_0.93 contact. A thermal stability study was also conducted on Pd and Au/n-ZnS_0.07Se_0.93 contacts. Thermal annealing tests showed a stable electrical property up to T = 250°C for the Pd contact and T = 150°C for the Au contact, however, a dramatic degradation was observed after higher temperature annealing and this is attributed to the interaction between metals and ZnSSe after annealing as supported by Auger electron spectroscopy.

Role of Cadmium in Enhancing Optical Properties and Chlorine Doping of Photo-Assisted OMVPE-Grown ZnSe
M.R. GOKHALE, K.X. BAO, P.D. HEALEY, F.C. JAIN, and J.E. AYERS
Electrical and Systems Engineering Department, The University of Connecticut, Storrs, CT 06269-3157.

KEY WORDS Co-doping, n-type doping, organometallic epitaxy, photoluminescence, zinc selenide

The influence of cadmium doping in enhancing: (i)near-band edge (NBE) to deep-level emissions (DLE) photoluminescence intensity ratio, and (ii) chlorine incorporation in HCl doped ZnSe films, has been studied. The epitaxial ZnSe was grown using low-temperature photo-assisted organometallic vapor phase epitaxy (OMVPE) using DMSe, DMZn, and DMCd alkyl sources. More intense near band-edge emission is observed when the growth temperature is reduced from 400 to 360°C. The deep-level emissions show a threshold-like dependence on UV light intensity for a given temperature. Cadmium doping is found to improve the NBE/DLE photoluminescence intensity ratio, and this improvement is more pronounced at higher growth temperature. Properties of n-type ZnSe films co-doped with cadmium and chlorine, using HCl and DMCd as dopant sources have also been investigated. Samples co-doped with Cd showed higher electron concentrations and this effect was more pronounced at lower UV intensities. By the use of Cd co-doping, we have achieved the highest electron concentrations (2.4 x 10¹⁸ cm^-3) yet reported for HCl doping of OMVPE-grown ZnSe.

Structural Study of Degraded ZnMgSSe Blue Light Emitters
K. NAKANO, S. TOMIYA, M. UKITA, H. YOSHIDA, S. ITOH, E. MORITA, M. IKEDA, and A. ISHIBASHI
Sony Corporation Research Center, 174 Fujitsuka-cho, Hodogaya-ku, Yokohama 240, Japan.

KEY WORDS Blue laser, degradation, dislocation, stacking fault, ZnMgSSe

Using electroluminescence (EL) topography and transmission electron microscopy (TEM), we investigated the nonluminescent regions which form while current is being injected into ZnMgSSe/ZnSSe/ZnCdSe-based blue light emitters. Small dark spots were observed just after turn-on and spread out forming rough nonluminescent triangles in the <100> directions in the EL image of the active region. TEM studies showed that the small dark spots are pre-existing stacking faults originating at the substrate/epitaxial layer interface. The nonluminescent triangles were found to be a dense region of dislocation dipoles and dislocation loops. Each dipole was aligned along two <110> directions in the {111} planes. The Burgers vectors were of the type a/2<011> inclined at 45° to the (001) junction plane.

Effects of GaAs Buffer Layer and Lattice-Matching on Deep Levels in Zn(S)Se/GaAs Heterostructures
MITSURU FUNATO, HIROAKI KITANI, SHIZUO FUJITA, and SHIGEO FUJITA
Department of Electronic Science and Engineering, Kyoto University, Kyoto 606-01, Japan.

KEY WORDS DLTS, GaAs buffer layer, lattice-matching, ZnSe, Zn(S)Se/GaAs heterostructure

The effects of GaAs buffer layer and lattice-matching on the nature of deep levels involved in Zn(S)Se/GaAs heterostructures are investigated by means of deep-level transient spectroscopy (DLTS). The heterojunction diodes (HDs) where n-Zn(S)Se is grown on p⁺-GaAs by metalorganic vapor phase epitaxy are used as a test structure. The DLTS measurement reveals that when ZnSe is directly grown on a GaAs substrate, there exist five electron traps A-E at activation energies of 0.20, 0.23, 0.25, 0.37, and 0.53 eV, respectively. Either GaAs buffer layer and lattice-matching may reduce the incorporation of traps C, D, and E, implying that these traps are ascribed to surface treatment of GaAs substrate and to lattice relaxation. Concentration of trap B, which is the most dominant level, is proportional to the donor concentration. However, in the ZnSSe/GaAs sub. HD, another trap level, instead of trap B, locates at almost the same position as that of trap B, and it shows anomalous behavior that the DLTS peak amplitude changes drastically as changing the rate windows. This is explained by the defect generation through the interaction between sulfide and a GaAs substrate surface. For the trap A, the concentration is a function of donor concentration and lattice mismatch, and the origin is attributed to a complex of donor induced defects and dislocations.

Growth of P-Type ZnSe by Metalorganic Molecular Beam Epitaxy Using Metal Zn and Dimethylselenide
JUN SUDA, MASANORI TSUKA, DAISUKE HONDA, MITSURU FUNATO, YOICHI KAWAKAMI, SHIZUO FUJITA, and SHIGEO FUJITA
Department of Electronic Science and Engineering, Kyoto University, Kyoto 606-01, Japan.

KEY WORDS Deep level transient spectroscopy (DLTS), dimethylselenide (DMSe), metalorganic molecular beam epitaxy (MOMBE), nitrogen doping, photoluminescence (PL), ZnSe

This paper describes metalorganic molecular beam epitaxy (MOMBE) of p-type ZnSe using metal zinc, pre-cracked metalorganic dimethylselenide, and microwave-excited nitrogen plasma as sources. Optical, structural, and electrical properties of the p-type ZnSe layers have been investigated. At present, maximum net acceptor concentration N_a-N_d is 3 x 10¹⁷ cm^-3 without any post-growth annealing. This is the highest acceptor concentration ever reported for MOMBE-grown p-type ZnSe doped with nitrogen plasma, but photoluminescence and deep level transient spectroscopy suggest that acceptors are highly compensated and the reduction of compensating defects is a key to further increase the acceptor concentration.

DX Centers in II-VI Semiconductors and Heterojunctions
TINEKE THIO,¹ J.W. BENNETT,¹ D.J. CHADI,¹ R.A. LINKE,¹ and M.C. TAMARGO²
1--NEC Research Institute, 4 Independence Way, Princeton NJ 08540. 2--Center for Analysis of Structures and Interfaces, City College of New York/CUNY, Convent Ave. @ 138th St., New York NY 10031.

KEY WORDS II-VI semiconductor, DX center, Ga-doped ZnSe, GaAs-ZnSe heterojunction, persistent photoconductivity

Measurements of the photoconductivity and Hall effect in Ga-doped ZnSe indicate that Ga donors form DX states in ZnSe. When the photocarriers remain in the ZnSe:Ga layer, the photoconductivity is persistent up to T_a = 100K, due to a barrier to recapture the photocarriers, E_c 0.3eV. Under certain growth conditions, there is a large conduction band offset at the heterojunction with the GaAs substrate. The photocarriers are trapped at the interface, causing an enhancement of the annealing temperature to T_a 350K. We discuss the implications of these results to device applications.

Nondestructive Analysis of Structural Defects in Wide Bandgap II-VI Heterostructures
M.S. GOORSKY,¹ S.E. LINDO,¹ S. GUHA,^2,3 and G.M. HAUGEN²
1--University of California, Los Angeles, Department of Materials Science and Engineering, Los Angeles, CA 90095-1595. 2--3M Corporate Research Laboratories, 3M Center, 201-1N-35, St. Paul, MN 55144-1000. 3--Present address: IBM T.J. Watson Research Center, Yorktown Heights, NY 10598.

KEY WORDS II-VI compounds, diffuse scattering, reciprocal space map (RSM), stacking faults, triple axis diffraction (TAD)

X-ray scattering measurements of wide bandgap II-VI heterostructures show that stacking faults (which nucleate defects that are responsible for optical degradation of light emitting diodes and lasers) introduce significant levels of diffuse scattering near Bragg reflections of both the epitaxial layers and the GaAs substrate. We employed triple axis x-ray diffraction techniques to investigate stacking fault diffuse scattering and used cathodoluminescence and transmission electron microscopy to independently measure the stacking fault density. For comparison, double axis scans from the same samples were largely incapable of detecting the presence of these defects. Measurements taken at different azimuthal positions exhibit different levels of diffuse scattering and the diffuse scattering intensity is related to the stacking fault intensity in each <110> direction, which suggests that this technique can provide a non-destructive assessment of defects present in these systems. For some samples, the ZnSe buffer layer exhibited a tilt with respect to the substrate along a <110> direction; this tilt was greater than the tilt which would be attributed to growing a strained layer on the slightly miscut substrates which were used here.

Observation of [100] and [010] Dark Line Defects in Optically Degraded ZnSSe-Based LEDs by Transmission Electron Microscopy
L. SALAMANCA-RIBA and L.H. KUO
Department of Materials and Nuclear Engineering, University of Maryland, College Park, MD 20742-2115.

KEY WORDS Dark line defects, mechanism for photodegradation, photodegradation, Zn1-xCdxSe quantum well

We have used transmission electron microscopy to study the [100] and [010] dark line defects (DLDs) produced after photodegradation of a ZnSSe-based/GaAs heterostructure. Our results show that the DLDs are networks of elongated dislocation loops or half-loops that originate in the quantum well region during device operation. Our results also show that after photodegradation the grown-in or pre-existing Frank-type stacking faults become tangles of dislocations. In contrast, the Shockley-type stacking faults remained unchanged for the photodegradation conditions studied indicating that they are more resistant to photodegradation than the Frank-type stacking faults. Our results suggest that the Frank-type stacking faults are the sources of the DLDs. The mechanism for degradation probably starts by the emission of very small clusters of vacancies from the Frank-type faults. Upon further illumination the dislocation loops bounding the vacancies grow by gliding on {111} planes and become hairpin-like dislocation loops.

Effect of GaAs Surface Pretreatment on Electrical Properties of MBE-ZnSe/GaAs Substrate Interfaces
TAKAYUKI SAWADA, YUJI YAMAGATA, KAZUAKI IMAI, and KAZUHIKO SUZUKI
Department of Applied Electronics, Hokkaido Institute of Technology, 7-15 Maeda, Teine-ku, Sapporo 006, Japan.

KEY WORDS C-V, DLTS, MBE, interface states, I-V, surface treatment, ZnSe/GaAs

Interface properties of MBE-grown ZnSe/GaAs substrate systems formed on variously pretreated GaAs surfaces, which include standard chemically etched (5H₂SO₄:1H₂O₂:1H₂O), (NH₄)₂S_x-, NH₄I-, and HF-pretreated surfaces, are investigated by capacitance-voltage (C-V) and deep level transient spectroscopy (DLTS) measurements. An HF-pretreated and annealed ZnSe/p-GaAs sample showed marked reduction of interface state density, N_ss, with N_{ss, min} below 4 x 10¹¹ cm^-2eV^-1 near E_C - E_FS = 1.0 eV. The value is about one order of magnitude smaller than that of the standard chemically etched interface, and comparable to (NH₄)₂S_x- pretreated interface. Nevertheless, C-V characteristics of ZnSe/n-GaAs samples, which were measured for the first time, indicate that interface Fermi level, E_FS, is not completely unpinned due to the interface states located above the midgap. A consistent result was obtained by DLTS method in determining E_FS position. The influence of N_ss distribution on vertical current conduction is also analyzed. It is found that U-shaped interface states with N_ss(E) > 1 x 10¹³ cm^-2eV^-1 above the midgap may cause an excess voltage drop larger than a few volts at the interface.

Observation of the Quantum Confined Stark Effect in ZnSe/ZnCdSe Single Quantum Well Systems
S.W. SHORT,¹ S.H. XIN,¹ A. YIN,¹ H. LUO,^1,2 M. DOBROWOLSKA,¹ and J.K. FURDYNA¹
1--Department of Physics, University of Notre Dame, Notre Dame, IN 46556. 2--Currently at the Department of Physics, State University of New York at Buffalo, Buffalo, NY 14260.

KEY WORDS II-VI semiconductors, electric field, photoluminescence (PL), quantum confined Stark effect (QCSE)

We report the observation of the quantum-confined Stark effect (QCSE) in ZnSe/ZnCdSe single quantum wells grown by molecular beam epitaxy, using photoluminescence. In our experiments the electric field was applied via a reverse-biased Schottky barrier contact. To our knowledge, this is the first observation of the QCSE in any wide gap II-VI semiconductor heterostructure. Significant red shifts, typically 10-15 meV, are detected before quenching. An associated reduction in the transition intensity, consistent with the QCSE, is clearly observed. The dependence of these results will be discussed as a function of quantum well depth and thickness. Complete quenching of the luminescence is observed with applied voltages as low as 5 V. In addition, at lowest voltages, we also detect small blue shifts (up to 4 meV), which we attribute to the interaction between the externally applied electric field and the built-in field of the structure.

MBE Growth of Lattice-Matched ZnCdMgSe Quaternaries and ZnCdMgSe/ZnCdSe Quantum Wells on InP Substrates
MARIA C. TAMARGO,^1,2 ABDULLAH CAVUS,^1,2 LINFEI ZENG,^1,2 NING DAI,^1,2 NEIL BAMBHA,³ A. GRAY,³ FRED SEMENDY,³ WOCJIECH KRYSTEK,^2,4 and FRED H. POLLAK^2,4
1--Center for Analysis of Structures and Interfaces (CASI) and Department of Chemistry, City College-CUNY, New York, NY 10031. 2--NY State Center for Advanced Technology on Ultrafast Photonic Materials and Applications, City University of New York (CUNY), New York, NY 10031. 3--IR Optical Technology OFS, Army Research Laboratory, Fort Belvoir, VA 22060. 4--Department of Physics, Brooklyn College-CUNY, Brooklyn, NY 11210.

KEY WORDS II-VI compounds, II-VI/III-V heteroepitaxy, blue emitters, molecular beam epitaxy, wide bandgap semiconductors, (Zn,Cd,Mg)Se

We report the growth and characterization of a new wide bandgap II-VI alloy, Zn_xCd_yMg_1-x-ySe, grown lattice-matched to InP. High quality quaternary layers with bandgaps ranging from 2.4 to 3.1 eV were grown by molecular beam epitaxy. The bandgaps and lattice constants were measured using photoluminescence and single crystal -2 scans. Quantum well structures with quaternary barriers and ZnCdSe wells were also grown, entirely lattice matched to InP. Their photoluminescence properties suggest that these materials are suitable for the design of visible semiconductor lasers spanning the blue, green, and yellow regions of the visible range. The absence of strain in these heterostructures is expected to improve the reliability of the materials in device applications.

Study on Stacking Faults and Microtwins in Wide Bandgap II-VI Semiconductor Heterostructures Grown on GaAs
G.C. HUA,^1,2 D.C. GRILLO,¹ T.B. NG,¹ C.C. CHU,¹ J. HAN,¹ R.L. GUNSHOR,¹ and A.V. NURMIKKO³
1--School of Electrical Engineering, Purdue University, West Lafayette, IN 47907. 2--Present address: Siltec Corporation, 1351 Tandem Avenue, N.E., Salem, OR 97303. 3--Brown University, Division of Engineering and Department of Physics, Providence, RI 02912.

KEY WORDS Microtwins, stacking faults, transmission electron microscopy (TEM)

ZnMgSSe and ZnSSe layers grown on GaAs substrates with GaAs buffer layers by molecular beam epitaxy have been examined by transmission electron microscopy (TEM). The depth level at which paired triangular stacking faults are nucleated in the ZnMgSSe/GaAs heterostructure has been investigated by using the plan-view TEM technique. It has been found that in the ZnMgSSe/GaAs heterostructure the nucleation of the paired stacking faults occurs within a range of depth which starts at the II-VI/GaAs interface and ends at a level that is above the interface by about 120 nm. The dominant type of defects in ZnSSe layers, which have the single triangular shape, has been identified to be microtwins by high resolution TEM.

REGULAR ISSUE PAPERS

KEY WORDS Light emission, photoluminescence, porous Si, Si nanostructures

The electronic properties of silicon nanostructures are calculated using a time-dependent algorithm within the tight-binding approximation. The algorithm includes the electron-hole Coulomb interaction directly without resort to perturbative correction, allowing accurate calculation of excited state properties. The densities of states, fundamental band gaps, photoluminescence energies, and band edge eigenfunctions and k-distributions are calculated for nanostructures up to 100Å in diameter. The effects of size, geometry, surface termination, and surface reconstruction on the electronic properties are investigated. We show that a model in which the primary photoluminescence peak is due to exciton recombination across the fundamental gap, while the secondary infrared peak is due to recombination of a conduction band electron with a hole in a deep surface trap is consistent with recent observations for both silicon nanocrystals and porous silicon. We infer the geometry of the luminescent region in porous silicon by comparing our calculated results with experimental data on porous silicon samples.

Elastic Strain and Enhanced Light Emission in Dry Etched Si/Si_1-xGe_x Quantum Dots
Y.S. TANG¹ C.M. SOTOMAYOR TORRES,¹ S. NILSSON,² B. DIETRICH,² W. KISSINGER,² T.E. WHALL,³ E.H.C. PARKER,³ W.-X. NI,⁴ G.V. HANSSON,⁴ H. PRESTING,⁵ and H. KIBBEL⁵
1--Nanoelectronics Research Centre, Department of Electronics & Electrical Engineering, University of Glasgow, Glasgow G12 8QQ, UK. 2--Institut für Halbleiterphysik GmbH, D-15204 Frankfurt (Oder), Germany. 3--Department of Physics, University of Warwick, Coventry CV4 7AL, UK. 4--Department of Physics, Linköping University, S-581 83 Linköping, Sweden. 5--Daimler Benz AG, Research Center, D-89081 Ulm, Germany.

KEY WORDS Elastic strain, luminescence, Raman scattering, Si-Si1-xGex quantum dots

Quantum dots of 50 ~ 60 nm diameter fabricated from both Si/Si_1-x Ge_x (x = 0.1 ~ 0.3) strained layer superlattices and a strain symmetried Si₉/Ge₆ superlattice were investigated by a combination of Raman scattering, photoluminescence, and electroluminescence spectroscopy. It was found that, in addition to an enhanced luminescence intensity of the dots by over two orders of magnitude and improved luminescence quenching temperature, all of the nanostructure dots have residual built-in elastic strains, which are of the order of ~50% of the values in corresponding pseudomorphic heterostructures. This result suggests a possible mechanism for explaining the huge enhancement of the optical efficiency in our luminescence measurements.

Effect of Current Reversal on the Failure Mechanism of Al-Cu-Si Narrow Interconnects
CHOONG-UN KIM, S.H. KANG, and J.W. MORRIS, JR.
Department of Materials Science, University of California, Berkeley, and Center for Advanced Materials, Lawrence Berkeley Laboratory, Berkeley, CA 94720.

KEY WORDS Al-Cu-Si, electromigration, interconnects

The work reported here concerns the effect of a brief exposure to a reversed current on the electromigration failure of narrow Al-Cu thin-film conducting lines. While the precise mechanism by which Cu retards electromigration in Al-Cu alloys is not fully understood, the consistent observation that electromigration failure is preceded by the sweeping of Cu from the failure site can be used to improve electromigration resistance by stabilizing the distribution of Cu. One way of doing this is to expose the Al-Cu line to a reverse current for some period of time. The present work shows that this method is particularly effective in thin lines with "quasi-bamboo" microstructures. It has the effect of building a reservoir of Cu at the upstream ends of the polygranular segments that are the preferred failure sites, and significantly increases both the mean time to failure, and the time to first failure of a distribution of lines. It can be inferred from these results that Al-Cu lines that conduct alternating current should be exceptionally resistant to electromigration failure.

Optical Properties of Ge_1-yC_y Alloys
B.A. ORNER,¹ A. KHAN,¹ D. HITS,¹ F. CHEN,¹ K. ROE,¹ J. PICKETT,¹ X. SHAO,¹ R.G. WILSON,² P.R. BERGER,¹ and J. KOLODZEY¹
1--Electrical Engineering Department, 140 Evans Hall, University of Delaware, Newark, DE 19716. 2--Hughes Research Lab, 3011 Malibu Canyon Road, Malibu, CA 90265.

KEY WORDS Carbon, Ge1-yCy, germanium, group IV alloys, molecular beam epitaxy (MBE), Optical absorption, photoluminescence

The Ge_1-yC_y semiconductor alloy system offers promise as a material for use in heterostructure devices based on Si as well as other materials. We have grown Ge_1-yC_y alloys by solid source molecular beam epitaxy on Si substrates. Layer thicknesses ranged from 0.01 to 3 µm, and Auger electron spectroscopy and secondary ion mass spectrometry indicated C fractions up to 3 at. %. Optical absorption in the near-infrared region indicated a shift in the energy bandgap from that of Ge which was attributed to the effects of alloying. The dependence of the bandgap on composition was consistent with linear interpolations of the Ge and C conduction band minimums. We observed a fundamental absorption edge characteristic of an indirect bandgap material. Photoluminescence spectra at 11K of thick, relaxed layers indicated single broad peaks near the expected bandgap energy.

Quantitative Scanning Capacitance Microscopy Analysis of Two-Dimensional Dopant Concentrations at Nanoscale Dimensions
A. ERICKSON,^1,2 L. SADWICK,¹ G. NEUBAUER,³ J. KOPANSKI,⁴ D. ADDERTON,⁵ and M. ROGERS⁵
1--Department of Electrical Engineering, University of Utah, Salt Lake City, UT 84102. 2--Permanent address: Digital Instruments, Inc., 520 E. Montecito St., Santa Barbara, CA 93103-3252. 3--Department of Materials Technology, Intel Corporation, Santa Clara, CA 95052. 4--Semiconductor Research Group, National Institute of Standards and Technology, Gaithersburg, MD 20899. 5--Digital Instruments, Santa Barbara, CA 93103.

KEY WORDS Atomic force microscopy, capacitance-voltage theory, scanning capacitance microscopy, two-dimensional dopant profiles

We have applied the scanning capacitance microscopy (SCM) technique of two-dimensional (2-D) semiconductor dopant profiling to implanted silicon cross sections. This has permitted the first direct comparison of SCM profiling scans to secondary ion mass spectroscopy (SIMS) depth profiles. The results compare favorably in depth and several readily identifiable features of the SIMS profiles such as peak concentration and junction depth are apparent in the SCM scans at corresponding depths. The application of dopant profiling to two dimensions is possible by calibrating the SCM levels with the one-dimensional (1-D) SIMS data. Furthermore, we have subsequently simulated the SCM results with an analytic expression readily derivable from 1-D capacitance vs voltage capacitance-voltage theory. This result represents a significant breakthrough in the quantitative measurement of 2-D doping profiles.

(111)B-oriented AlAs/GaAs/AlAs Double Barrier Resonant Tunneling Devices Grown in a Gas Source Molecular Beam Epitaxy System
L. CONG, F. WILLIAMSON, and M.I. NATHAN
Department of Electrical Engineering, University of Minnesota, Minneapolis, MN 55455.

KEY WORDS (111)B-oriented AlAs/GaAs/AlAs double barrier resonant tunneling, gas source molecular beam epitaxy (GSMBE)

We have studied the growth of (111)B-oriented AlAs/GaAs/AlAs double barrier resonant tunneling structures in a gas source molecular beam epitaxy system. We investigate the current peak-to-valley ratios of the resonant tunneling structures grown on (111)B GaAs substrates under the various growth conditions, such as III/V flux ratios, substrate temperature, growth interruption at the heterointerfaces, buffer or contact layers, etc. We demonstrate that, in contrast to previous reports, high quality heterostructures can be grown in a gas source system if certain III/V flux ratio, substrate temperature, and misoriented substrate are used. We show that the As/Ga flux ratio plays the key role for the growth on the misoriented (111)B GaAs substrate, and growth at extremely high temperatures is not beneficial to the negative differential resistance. We also show that, although inserting a growth interruption in the buffer layer is believed to be helpful to the surface morphology, it is detrimental to the current peak-to-valley ratio.

Effect of Growth Temperature on Performance of AlGaAs/InGaAs/GaAs QW Laser Diodes
F. BUGGE,¹ G. ERBERT,¹ M. PROCOP,² I. RECHENBERG,¹ U. ZEIMER,¹ and M. WEYERS¹
1--Ferdinand-Braun-Institut für Höchstfrequenztechnik, D-12489 Berlin, Germany. 2--Bundesanstalt für Materialforschung und -prüfung, D-12205 Berlin, Germany.

KEY WORDS InGaAs laser diodes, metalorganic vapor phase epitaxy (MOVPE), strained quantum wells

Growth and characterization results are presented for high-power laser diodes with AlGaAs cladding and waveguide layers and strained In_1-xGa_xAs quantum wells with 0.09 < x < 0.25 grown by metalorganic vapor phase epitaxy at different temperatures. Photoluminescence at 300 and 10K, Auger spectroscopy, and high-resolution x-ray measurements are discussed. Broad area laser diodes have been fabricated with different cavity length and threshold current densities, absorption coefficients, internal efficiencies, and degradation rates have been measured.

Characterization of Surface Roughness Anisotropy on Mismatched InAlAs/InP Heterostructures
M.T. SINN,¹ J.A. del ALAMO,¹ B.R. BENNETT,² K. HABERMAN,^3,4 and F.G. CELII³
1--Massachusetts Institute of Technology, Cambridge, MA 02139. 2--Naval Research Laboratory, Washington, DC 20375-5347. 3--Texas Instruments, Inc., Dallas, TX 75265. 4--Current address: Rice University, Houston, TX.

KEY WORDS Heterostructures, InAlAs/InP, surface roughness

We have studied surface roughness on mismatched In_0.65Al_0.35As epilayers of various thicknesses on (001) InP. The sample set spans the entire range from coherently strained to completely relaxed epilayers. As characterization tools, we have used atomic force microscopy (AFM), laser light scattering (LLS), and variable azimuthal angle ellipsometry (VAAE). AFM reveals that the surfaces are covered by densely packed ellipsoidal islands elongated along the [] direction. The island size increases with layer thickness. Island anisotropy and the root mean square of the surface roughness increase with increasing thickness but decrease upon full lattice relaxation. LLS intensity displays a prominent azimuthal dependence that correlates well with the two-dimensional power spectrum of the surface topography, as predicted by theory. VAAE reveals a sinusoidal dependence of the ellipsometric parameter on azimuthal angle. The amplitude of correlates well with the short wavelength anisotropy of the surface power spectrum. Our work suggests that LLS and VAAE are fast, nondestructive, sensitive techniques for characterization of surface roughness in mismatched III-V heterostructures.

On the Thermally Stimulated Conductivity with Small Recapture Rate and Temperature Dependent Capture Cross Sections
S. DORENDRAJIT SINGH, E. DWIJAMANI SINGH, R.K. GARTIA, and P.S. MAZUMDAR
Department of Physics, Manipur University, Canchipur, Imphal-795003 Manipur, India.

KEY WORDS Capture cross sections, conductivity, thermally stimulated current

Thermally stimulated conductivity (TSC) peaks corresponding to small recapture rate and temperature dependent capture cross sections are investigated. The shape of the TSC peak is studied by computing the shape factor µ_g (x) not only at the fractional intensity x = 1/2 as done conventionally but also at x = 2/3 and 4/5. It is found that for a particular value of x, µ_g (x) depends on u_m = E/kT_m (E = activation energy, k = Boltzmann's constant, T_m = peak temperature). A set of expressions based on the shape of the peak has been derived, which in principle can be used to evaluate the activation energy of such TSC peaks. The applicability of those expressions has been tested by considering some numerically computed and experimental TSC peaks.