1997 EMC: Friday Morning Sessions, Part 1

The following sessions are among those that will be held during the 39th Electronic Materials Conference (EMC) on Friday morning June 27, at Colorado State University, Fort Collins, Colorado. To view the other Friday morning sessions as well as other programming planned for the meeting, go to the EMC Calendar of Events.

8:20 am, Invited

Self-Organization of CdSe Quantum Dots on (100)ZnSe/GaAs Surfaces with MOMBE: M. Arita, I. Suemune, K. Uesugi, and A. Avramescu, Research Institute for Electronic Science, Hokkaido University, Kita-12, Nishi-6, Kita-ku, Sapporo 060 Japan

Semiconductor quantum dots have ideally delta-function density of states (DOS) and can be regarded as artificial atoms. Such dot structures have capability of low threshold lasing with excitonic molecules in addition to the peculiar DOS. It is also a possibility that nonradiative recombination and defect formation are reduced by the carrier localization in the dot states and the reduced capture by the nonradiative centers. There have been active works on III-V quantum dot formation, but optical properties in II-VI quantum dots will be more interesting considering the exciton binding energies larger than conventional III-V semiconductors. In this paper, we report the direct observation of II-VI quantum dots formation on (100) ZnSe/GaAs surfaces with atomic force microscopy (AFM). Metalorganic molecular beam epitaxy (MOMBE) was employed for the growth. GaAs(100) surfaces were cleaned with tris-dimethylamino-arsenic (TDMAAs) irradiated on the surfaces at 550°C, and (2x4) (or sometimes c(4x4)) As-stabilized surfaces were prepared. After purge with a Zn precursor (diisopropyl-zinc, DiPZn), ZnSe buffer layers were subsequently grown at 350°C on the GaAs surfaces using DiPZn and ditertiarybutyl-selenide (DtBSe) with the VI/II ratio of 0.5 to 4. After 40-second growth interruption, CdSe was deposited with the nominal thickness of 1.5-5 ML using dimethyl-cadmium (DMCd) as a Cd precursor. All MO precursors were supplied without precracking. The samples were kept at the growth temperature for 18 minutes after the growth and then taken out after cooling for AFM observation. Fairly uniformly-sized quantum dots were observed. Below the CdSe nominal thickness of 4ML, circular shaped dots with the diameter below 100nm were observed. The diameters were not much dependent on the deposited thickness, but the density of the dots increased with the CdSe thickness up to ~1x10⁹cm^-2. Above the CdSe nominal thickness of 5ML, the dots were faceted and showed a rectangular shape. The roof-shaped structure confirms that these islands tend to grow with (311)A crystal facets in the <110> direction. A large mismatch (~7%) of lattice constants between ZnSe and CdSe results in the Stranski-Krastanow growth mode. But the deposition of thick CdSe will result in coalescence of the formed quantum dots. The integrated nominal CdSe thickness that contributed to the dot formation was increased to ~1.5ML for the deposited CdSe thickness of up to 4ML but was decreased in the coalescence region. Detailed dot properties will be discussed during the conference.

9:00 am

Growth and Characterization of Self-Assembled CdSe Quantum Dots: F. Flack and N. Samarth, Department of Physics, The Pennsylvania State University, University Park, PA 16802; V. Nikitin, P.A. Crowell, J. Shi, D.D. Awschalom, Department of Physics, University of California, Santa Barbara, CA 93106

Recent successes with the in-situ fabrication of self-assembled quantum dots from epitaxially grown III-V and group IV semiconductors have prompted us to examine similar growth processes in the wide-gap II-VI semiconductors. We present here a detailed and comprehensive study of the growth and characterization of self-assembled CdSe quantum dots formed during the strained layer epitaxy of (cubic) CdSe (Eg = 1.75 eV) on ZnSe (Eg = 2.8 eV) with a lattice mismatch of ~7%. As a result of the deep confining potentials for electrons and holes in these quantum structures, samples exhibit efficient PL up to room temperature and are hence worth exploring as media for the active region in visible light emitting diodes. Further, the observation of 0D states in II-VI nanostructures opens up exciting vistas for studying static and dynamic spin dependent phenomena in "quantum spin dots" by coupling 0D states with magnetic ions. Careful exploitation of Stranski-Krastonow island growth of CdSe on (100) ZnSe yields self-assembled quantum dot regions in which excitons are laterally confined. These quantum dots are characterized using a variety of structural probes, including reflection high energy electron diffraction, transmission electron microscopy and atomic force microscopy. Furthermore, the electronic structure is probed using both far-field and near-field photoluminescence (PL). In particular, PL spectroscopy using a near-field scanning optical microscope as well as the measurement of far-field PL from sub-micron apertures in masked samples shows resolution-limited spectral structure that is consistent with a 0D density-of-states. Both structural and optical studies consistently show that these CdSe nanostructures are structurally quite different from self-assembled III-V quantum dots, with mean island diameters in the range 10 - 25 nm and mean island heights rarely exceeding 3 nm (~10 monolayers). These small island heights account for the absence of large red-shifts in the optical spectra of CdSe quantum dots. The striking difference in island geometry between the III-V and II-VI nanostructures is attributed to lower surface mobilities and larger interdiffusion in the latter materials. This work was supported by grants ONR N00014-94-1-0297 and -0225, and NSF DMR 92-07567, 95-00460, 94-13708, and STC DMR 91-20007.

9:20 am

Intrinsic Waveguide and RT Lasing Via Nanoscale CdSe Islands in a (Zn,Mg)(S,Se) Matrix: I. Krestnikov, S. Ivanov, M. Maximov, N. Ledenstov, S. Sorokin, P. Kop'ev, A.F. Iofffe Physico-Technical Institute, Politechnicheskaya 26, St. Petersburg, 194021, Russia

We have engineered and fabricated wide bandgap structures providing entirely exciton-induced waveguiding and lasing up to 300K. By stacking sheets of submonolayer (SML) CdSe insertions in a ZnSe (ZnMgSSe) matrix we demonstrate a dramatic increase in the exciton oscillator strength directly revealed in the optical reflectance (OR) spectra. An exciton radiative lifetime derived from the fitting of the OR curve is 5 ps for the 20-period structure. The corresponding resonant enhancement of the refractive index calculated according to the Kramers equation is thus 0.15 and provides a complete confinement of the lightwave with a photon energy corresponding to the low energy side of the 0-phonon exciton emission.In a II-VI bulk and in quantum wells, excitons with high values of k-vector dominating at high temperatures and excitation densities are shown to be radiatively non-active due to the k-conversation law and can convert to photons only via the interaction with an additional particle (phonon). To realize the 0-phonon lasing, resonant to the exciton waveguiding region, one needs to provide an efficient k selection rule breaking mechanism. In our case this is realized by localizing the excitons on CdSe islands formed during the SML deposition. HREM studies of the II-VI structures with ultrathin CdSe insertions directly reveal spontaneous formation of 2D islands having a characteristic lateral size of about 4 nm.In the structure without thick wider bandgap cladding layers the lasing occurs at an energy in the very vicinity of the heavy-hole exciton resonance, directly in the region of exciton-induced refractive index enhancement [ =460 nm (300K); P_th(300K)/P_th(10K)~10].

9:40 am, Student Paper

Carrier Dynamics in Unstrained Zn_0.93Cd_0.07Se/ZnSe Quantum Wells with Different Well Widths: G. Vaschenko, D. Patel, C.S. Menoni, Department of Electrical Engineering, Colorado State University, Fort Collins, CO 80523; Y. Qiu, H. Temkin, Department of Electrical Engineering, Texas Tech University, Lubbock, TX 79409; W.A. Bonner, Crystallod Inc., 25 4th Street, Sommerville, NJ 08876

The carrier lifetime of unstrained Zn_0.93Cd_0.07Se/ZnSe multiple quantum wells is investigated as a function of well width, excitation density and temperature. These experiments revealed the interplay between excitonic and electron-hole recombination at different excitation conditions and for different well widths. The Zn_0.93Cd_0.07Se/ZnSe quantum wells were grown by Metal Organic Molecular Beam Epitaxy on InGaAs substrates and consisted of 5, 10, 21, and 43 Angstrom Zn_0.93Cd_0.07Se wells separated by 186 Angstrom ZnSe barriers. The quantum wells were sandwiched by a 300 Angstrom ZnSe buffer layer and a 300 Angstrom thick cap layer. The well composition was determined using high resolution x-ray diffraction. The lifetime was determined from the decay of the photoluminescence signal acquired using a synchroscan streak camera system having a temporal resolution of 5ps and a spectral resolution of 1 nm. The photoluminescence was excited with a frequency doubled Ti:Sapphire laser producing 150fs pulses centered at 410 nm. We used a wide range of excitation powers, resulting in an excess carrier generation ranging from 3 x 10¹⁰cm^-2 to 2 x 10¹⁴ cm^-2. The measurements were performed at temperatures between 11K and 200K. At low excitation densities (10¹⁰ - 10¹¹ cm^-2) the emission from the 5 and 10 Angstrom wells is excitonic. Both wells show the strongest emission and the longest single exponential decay, of ~50ps. This lifetime is mainly influenced by impurity recombination. With increased well width the lifetime decreases to ~10ps and becomes non-single exponential. The same behavior occurs when the excitation density reaches values of ~ 1 x 10¹² cm^-2 at which the lifetime of the 5 and 10 Angstrom wells is as short as 20ps. The variation of the lifetime with well width and excitation density is the result of the competition between exciton and electron-hole recombination. With increased well widths the exciton binding energy is reduced and therefore excitons can more easily dissociate. Increased excitation density also leads to exciton dissociation as many body effects between the excitons and the photogenerated electron-hole plasma become important. Above carrier densities of 10¹³ cm^-2 the lifetime is that of the electron-hole plasma. The lifetime increase occurs as impurities which control the recombination become saturated with increased excitation density. Implications of these findings on the design of low threshold ZnSe-based lasers will be discussed. This work is supported by the National Science Foundation, grants ECS-9408321 and ECS-9502888.

10:00 am, Break

10:20 am

ZnSe Heteroepitaxy on GaAs(110) Substrate: M.W. Cho, K.W. Koh, D.M. Bagnall, Z. Zhu and T. Yao, Institute for Materials Research, Tohoku University, Aoba-ku, Sendai 980, Japan The instability of the interface between ZnSe-based alloys and GaAs(100) substrates due to heterovalency is crucial to the life of ZnSe-based blue-green laser diodes. The failure mechanism in II-VI devices is based on the nucleation and propagation of degradation defects originating from the preexisting or grown-in defects such as stacking faults. To realize reliable II-VI blue laser diodes, the heterovalent interface problems which caused the generation of defects should be suppressed. From this point of view, the growth on GaAs(110) substrate, on which charge neutrality is automatically preserved, seems to be more promising than the (001) growth. In this work, the crystallinity of ZnSe epilayers has been characterized by ion-beam channeling analyses and the observation of the etch pits density (EPD). The ZnSe epilayers were grown on the semi-insulating GaAs(110) misoriented 6 ^o off and GaAs(001) substrate by Molecular Beam Epitaxy. Although the (111) facets are generally formed on the (110) surface, facet-free growth of ZnSe(110) was successfully achieved on the misoriented substrate, at a substrate temperature 220°C and beam pressures ensuring a Se-rich condition. A streaky (lx1) pattern was observed throughout the growth by RHEED, which indicate 2D growth for the beginning. A 30 mol% NaOH solution was used to measure the EPD, this showed the triangle-type etch pits formed by selective etching in the faulted region. The interface disorder of these ZnSe heteroepitaxial layers was analyzed by disorder-sensitive 2MeV He ion channeling. From the EPD measurement, a very low density of etch pits in the range of low 10⁴/cm² was observed in the (110) system, while the ZnSe on GaAs(100) showed typical EPD of 10⁴-10⁵/cm² with and -10⁸/cm² without GaAs buffer layer, respectively. The ZnSe on GaAs(l00) system exhibited a large interface disorder peak in channeling backscattering spectra, whereas the ZnSe on GaAs(110) system gave smooth and featureless spectra. The dramatically reduced EPD and the disorder-less interface strongly suggest that the (110) orientation is more suitable than (100) for practical blue laser diode.

10:40 am

Electrical Characterization of MBE-ZnSe/GaAs Heterointerface and Regrown ZnSe/ZnSe Homointerface: T. Sawada, Y. Yamagata, K. Fujiwara, K. Imai, I. Tsubono and K. Suzuki, Department of Applied Electronics, Hokkaido Institute of Technology, 7-15 Maeda, Teine-ku, Sapporo 006, Japan

Recent works using TEM have clearly demonstrated that elimination of stacking faults near ZnSe-based/GaAs heterointerfaces is crucially important for achieving long-lived blue/green LDs. However, electrical properties of such heterointerfaces have not been well understood. In the present study, interface properties of MBE-ZnSe/GaAs heterointerfaces prepared by a combined (HF+Se)-pretreatment of GaAs surface are characterized by I-V, C-V, DLTS measurements and their theoretical analyses considering interface states. Air-exposed and regrown ZnSe/ZnSe homointerfaces are also investigated for the first time. After standard chemical etching of GaAs(100) surface, the wafer was dipped into HF solution as an ex-situ pretreatment. The in-situ 2nd pretreatment was performed by irradiating Se-beam onto the thermally cleaned GaAs. The (HF+Se)-pretreated sample showed a streaky RHEED pattern from the beginning of the growth, indicating a quasi 2-dimensional nucleation of ZnSe, while chemically-etched sample experienced a 3-dimensional nucleation in the initial stage of the growth. Measured C-V curves of Au/undoped-ZnSe/p-GaAs "MIS" samples showed that interface state density, N_ss(E), below the midgap of GaAs is considerably reduced with minimum density of l.4xl0¹¹cm^-2eV^-1. The result indicates that combination of (HF+Se) pretreatments is highly effective to control MBE-ZnSe/GaAs heterointerface. Excess voltage drop across n-ZnSe/n⁺-GaAs heterointerface in I-V characteristics is also reduced more than one third by the present pretreatment, and I-V curves can be theoretically fitted by using the previously proposed model considering interface charge and measured N_ss(E) distribution. C-V curves of Au/n-ZnSe/n⁺-GaAs Schottky diodes with regrowth homointerface are characterized by a large frequency dispersion, and they sometimes showed capacitance decrease within the forward bias region. Besides, the current level for the regrowth sample lowered more than four orders of magnitude. These characteristics can be explained by the formation of interface states and resultant potential barrier at the regrowth homointerface. DLTS analysis indicates that a relatively high potential barrier, being about 0.7 eV, is formed at the homointerface even when the regrowth is undertaken after a slight etching of the first grown epilayer. The observed C-V behavior can be quantitatively explained by time-lag of carrier now across the potential barriers and N_ss(E) distributions.

11:00 am, Student Paper

Etch Pit Density Characterization of ZnCdMgSe Grown on InP Substrates: L. Zeng, B.X. Yang, B. Shewareged, M.C. Tamargo, Department of Chemistry, City College-CUNY, New York, NY 10031; J.Z. Wan and Fred. H. Pollak, Physics Department, Brooklyn College-CUNY, New York, NY 11210; E. Snoeks, L. Zhao, Phillips Research, Briarcliff Manor, NY 10510

Wide bandgap Zn_xCd_yMg_l-x-y Se is a new quaternary materials system that can be used in the design and fabrication of blue-green semiconductor lasers. By growing these quaternary layers on InP substrates, entirely lattice-matched heterostructures can be obtained. We have grown high quality lattice- matched ZnCdSe, ZnCdMgSe and their quantum well structures by molecular beam epitaxy (MBE). Optically pumped lasing of this structure at room temperature has been also observed. TEM studies have shown that the material has grown-in defects, mainly stacking faults and dislocations, originating from the II-VI/III-V heterointerface. For device applications, it is important to reduce the defect density in the material and find methods to measure the defect density. Chemical etching is a useful technique, which reveals the defect density and distribution in a large area. The key to this technique is to find an etching solution which can selectively etch the defects. Atomic force microscopy (AFM) and plan-view transmission electron microscopy (TEM) have been used to confirm the performance of the etching solution. In this study we developed a chemical etching technique suitable for Zn_xCd_yMg_l-x-ySe epitaxial layers. We found that a solution of hydrobromic acid and acetic acid, which has been previously used to reveal the defects of InP layers, also worked well for ZnCdMgSe layers. The etching behavior of this solution was investigated in detail. The accuracy or the etch pit density (EPD) method for revealing stacking faults and dislocations was verified by plan-view TEM, Surface scanning and profiling by AFM were used for the first time to characterize the morphology of the surface of the etched samples as a function of etch time. The pit depth became constant after a depth corresponding to roughly the II-VI layer thickness. The AFM and TEM results showed that the etch solution is very selective. We have grown ZnCdMgSe layers on near-lattice-matched InGaAs and ZnCdSe buffer layers, with defect densities as low as 10⁶-10⁷cm^-2..

11:20 am

P-Type ZnSe Grown by MOMBE with Insertion of ZnTe:Li Submonolayers: J. Hirose, I. Suemune, K. Uesugi, M. Hoshiyama, and T. Numai, Research Institute for Electronic Science, Hokkaido University, Kita-12, Nishi-6, Kita-ku Sapporo 060, Japan

p-Type conductivity in II-VI wide bandgap semiconductors such as ZnSe has been realized by nitrogen (N) plasma doping in molecular-beam epitaxy (MBE). Metalorganic vapor phase epitaxy (MOVPE) or MOMBE is regarded to be another practical method for realizing blue/blue-green semiconductor lasers. Although p-type conductivity has been reported with these methods, the reproducibility remains as a serious problem. In this paper we report the reproducible p-type conductivity in ZnSe grown by MOMBE with periodic insertion of submonolayers (ML) of ZnTe:Li. MOMBE was used for the growth of ZnSe and ZnTe on (001) GaAs surfaces. The growth temperature is 350°C, and diisopropyl-zinc (DiPZn), ditertiarybutyl-selenide (DtBSe), and ditertiarybutyl-telluride (DtBTe) were supplied on the substrate surfaces without precracking for the growth of ZnSe and ZnTe. It is well-known that high p-type doping up to 1x10²⁰cm^-3 is possible in ZnTe with N plasma doping in MBE. We have tried to dope N in ZnTe with MOMBE using N plasma excited by ECR. However the net acceptor concentration, N_A-N_D, observed remained low ~lxl0¹⁵cm^-3, most probably due to hydrogenation of N acceptors. On the other hand, Li doping showed the hole concentration of ~2x10¹⁸ cm^-3 with MOMBE and will be free from hydrogenation. This result on p-ZnTe was applied to p-type doping in ZnSe. Since ZnTe and ZnSe have large lattice mismatch of 7.1%, 1/3 monolayer(ML) ZnTe:Li was inserted in every 10 nm-thick ZnSe to prevent misfit dislocations. Growth was interrupted 40s before and after growth of 1/3 ML ZnTe:Li layers. The grown ZnSe films showed reproducible p-type conductivity. N_A-N_D, was measured by the C-V method with surface contacts. The built-in voltage showed a reasonable value for the Au/ZnSe Schottky contact and was around 1V. With the increase of Li doping, N_A-N_D increased up to 2xl0¹⁷cm^-3~5xl0¹⁸cm ^-3 but was decreased for the higher Li doping. The decrease of N_A-N_D for high Li doping will be due to the self-compensation by the increase of Li-interstitial donor. The photoluminescence(PL) from the grown samples at 14K showed broad peak centered around 2.5eV for low Li doping. If we follow a simple Kronig-Penny model, this peak can arise from 1ML-thick ZnSe islands with the size of 8ML. The PL peak was blue-shifted for the higher Li doping, the mechanism of which will be discussed during the conference.

11:40 am

High Resolution X-Ray Diffraction Study of (ZnMg)(Sse) Cladding Layers in ZnSe-Based II-VI Lasers: L. Zhao, B. Greenberg, J. Petruzzello, M.D. Pashley, R. Van Roijen, and D.B. Young, Phillips Research, 345 Scarborough Road, Briarcliff Manor, NY 10510

ZnSe-based II-VI lasers have important applications in high density optical recording and full color displays. Since cladding layers in a (ZnCd)Se/Zn(SSe)/ (ZnMg)(SSe) separate confinement heterostructure (SCH) laser typically make up about 90% of the SCH thickness, control and characterization of their composition and lattice mismatch are critical for a pseudomorphic structure to be grown. Quite often, the two (ZnMg)(SSe) cladding layers have different compositions and lattice mismatches. It is difficult and important to determine how the composition and lattice mismatch have changed during growth for improved composition control subsequently. A non-destructive x-ray diffraction technique is described that employs two x-ray rocking curves (the 004 and 224) to obtain this depth and composition information. SCH samples were grown by MBE. Two cladding layer diffraction peaks were often present in 004 rocking curves, indicating that the cladding layers in the same SCH had different compositions and mismatches. Computer simulations have shown that in 004 rocking curves the peak height of the upper (near surface) layer is only about 2% greater than that of an equally thick lower layer, a difference not significant enough to reliably determine which peak was from the upper layer. Depth information was obtained by combining 004 and 224 rocking curve data. We have observed that in 224 rocking curves the ratio of peak height of the upper layer to lower layer peak height is about 30% greater than this ratio when using the 004 reflection. This difference is directly attributable to differing x-ray absorption paths for these two reflections and can be used to determine the specific order of the two cladding layers. For our samples the upper cladding layer had a strong tendency to have a larger lattice constant (up to 0.04%) due to a lower S and/or higher Mg concentration. Most cladding layer 004 peaks were narrow and sharp, about 30-40", indicating that the composition change was not continuous but abrupt. One possible reason for this abrupt composition change is N-doping during the upper layer growth. This work was supported by the Defense Advanced Research Projects Agency (DARPA).

8:20 am

Scanning Tunneling Microscopy Observation of Atomic Structure of GaAs(001) Surface Grown by Metalorganic Vapor Phase Epitaxy: L. Li, B.K. Han, S. Gan, H. Qi and F.F. Hicks, Chemical Engineering Department, University of California, Los Angeles, CA 90095

We present the first atomically resolved scanning tunneling micrographs of GaAs (001) surfaces prepared by metalorganic vapor phase epitaxy (MOVPE). The apparatus consists of a sample server around which a loadlock, two vacuum chambers, and the MOVPE reactor are radially disposed. The vacuum chambers are equipped with various surface science analytical instruments, including a Park Scientific Instruments scanning tunneling microscope/atomic force microscope. This set-up allows films to be deposited in an MOVPE reactor and then transferred to an ultra high vacuum system without air exposure. Gallium arsenide films were grown at 650°C using tertiarybutylarsine (TBAs) and triethylgallium (TEGa) with a reactor pressure of 20 Torr. The TEGa partial pressure was 6.5x10^-4 Torr, and the V/III ratio was 50.0. In the UHV chamber, the sample was slowly heated to 400°C to desorb the excess arsenic. After sufficient annealing, a c(4x4) LEED pattern was observed. Subsequent heating to 500, 550 and 620°C produced c(2x8), (1x6) and c(8x2) LEED patterns, respectively. After recording the LEED patterns, the GaAs (001) crystal was transferred to the STM chamber where the micrographs were obtained. The formation of pits and islands one step high and one step low on the c(2x8)/(2x4) surface are evident. Remarkably, the atoms making up the As dimers in the (2x4) unit cell are clearly resolved. In the case of the (1x6) surface, zigzagging chains of As dimers along the [] direction are observed. The faint features between these rows are most likely Ga dimers. Note that pits one atomic step lower are also formed on the (1x6) surface. However, on the c(8x2)/(4x2) surface no pits are observed. Instead, Ga clusters appear as white features in the images. In a high-resolution image, double rows of white spots are most likely due to second-layer As atoms.

8:40 am

Near-Field Optical Spectroscopy of II-VI Semiconductor Quantum Dots: V. Nikitin, P.A. Crowell, J.A. Gupta and D.D. Awschalom, Department of Physics, University of California, Santa Barbara, CA 93106; F. Flack and N. Samarth, Department of Physics, Pennsylvania State University, University Park, PA 16802

We report on the spatio-temporal spectroscopy of two classes of MBE-grown ZnSe/CdSe quantum structures exhibiting 0D carrier confinement. In the first class of structures, defect-free Stranski-Krastonow (S-K) growth of CdSe on ZnSe produces self-assembled CdSe quantum dots (QD's) whose presence is directly verified by plan-view TEM and AFM measurements. Time-resolved optical spectroscopy reveals a long (~750 ps) temperature-independent (5-180K) radiative lifetime of photoexcited carriers, consistent with the 0D nature of the quantum confinement and indicating a higher degree of localization than found in III-V QD's. A small number of QD's can be optically probed by utilizing the high spatial resolution (~100nm) of a low-temperature near-field scanning optical microscope, which reveals a number of sharp (~70meV) resolution-limited peaks within the broad photoluminescence spectrum observed in the far field. Near-field images provide direct mapping of the spectral features to spatial locations and show that these features originate from localized (~1µm) centers whose spatial distribution varies with the detection energy. In the second class of structures, 0D carrier confinement is obtained by employing strain-induced local modification of a quantum well (QW) potential. This is achieved by exploiting the strain fields propagating from the CdSe QD's grown in the vicinity of a (Zn,Cd)Se QW. Three peaks in the luminescence spectra originating from the intrinsic QW, strain-modulated QD regions in the QW, and S-K CdSe QDs can be easily distinguished. The temperature dependence of the lifetime reveals the additional confinement in the strain-modulated QW, resulting in enhanced lifetimes and thermal activation energies. Moreover, the 0D confinement of these states is directly confirmed by the observation of ultranarrow luminescence peaks in the near-field as opposed to wide (6 meV FWHM) inhomogeneously broadened luminescence from the intrinsic QW. Finally, various schemes for the incorporation of magnetic Mn²⁺ ions during MBE growth have been successful in both types of structures. The observed polarized luminescence and Zeeman splitting in the presence of modest magnetic fields confirm the presence of exchange between 0D excitonic spin states and the local moments. Such "quantum spin dots" hence provide attractive templates for systematic studies of spin-dependent phenomena in 0D structures.

9:00 am, Student Paper

Spatially Resolved Near Field Luminescence Spectroscopy on Localized GaInAsP-Heterostructures: J. Barenz, Omicron Vakuumphysik, Idsteiner Strasse 78, D-65232 Taunusstein; P. Anger, A. Eska, O. Hollricher and O. Marti, Department of Experimental Physics, University of Ulm, D-89069 Ulm/Donau; M. Wachter, R. Butendeich, U. Schöffel and H. Heinecke, Department of Semiconductor Physics, University of Ulm, D-89069 Ulm/Donau

Localized GaInAsP structures can be fabricated by surface selective growth (SSG) in metalorganic molecular beam epitaxy (MOMBE) with high perfection for applications like low dimensional structures (LDS) or lateral integration of photonic devices. In planar selective area epitaxy (SAE) there is a lateral transition from growth to non growth area. At this point different crystal facets occur dependent on the growth parameter. The growth on these facets is determined by the adsorption flux geometry as well as the growth kinetics at the various index planes. The understanding of these growth mechanisms is consequently a prerequisite for high quality localized low dimensional structures. PL measurements were carried out on InP ridges which were grown on [100] substrate with a [100] top layer. At the edges of the ridge vertically [01-1] respectively [0-11] facets are formed. Additional [11-1] and [1-11] facets can occur just adjacent the top layer and the side facets. The extension of these facets are in the submicron regime. These ridges were overgrown by five 10nm GaInAsP-quantum wells ( =1032nm), each separated BV 30 nm InP barriers. Due to interfacet diffusion of the reactant during the growth a vertical superlattice (VSL) is formed. Spatially resolved photoluminescence measurements (µ-PL) were done with a resolution of about 5-10 mm, which is not sufficient to allocate the luminescence coming from the side facets of the ridges. Furthermore no topographic information can be obtained simultaneously, to compare both the optical image and the topography. To improve the spatial resolution and to achieve topographical information a room temperature reflection SNOM has been built up. Stray light of the optical shear force detection scheme was avoided by an all electrical detection mechanism. Luminescence measurements were taken both from a [100] surface scan and from the interface of the cleaved structure which allows for free access to the horizontal and vertical superlattice. To avoid spatial broadening by charge carrier diffusion the sample was illuminated through the tip and the luminescence light was collected through the same aperture. In the center of the ridge the luminescence has a center wavelength of 1032 nm. 7 µm in front of the [01-1] side facet the center wavelength shifts towards 1016 nm for about 1 mm and then moves towards lower energy to 1065 nm. Simultaneously the width of the luminescence increases from 75 nm to 90 nm. The shift of the center wavelength and the spectral broadening could be effected by a shift in the material composition and increasing material inhomogeneity nearby the side facets. About 20 mm in front of the [01-1] facet the intensity starts to modulate. 2 µm in front of the side facets the intensity has a maximum. The simultaneous recorded topography shows also a modulation of about 70 nm due to anisotropic surface diffusion at the crystal facet during growth. Both can be correlated but show a shift to each other. At the growth/non-growth transition a recombination channel occurs at the [0-11] facet at 1115 nm which can not be found at the [01-1] facet. To localize this recombination channel we investigated the cleaved surface of these structures. With the additional topography information we are able to show that this second recombination channel is originated in the vertical superlattice. Here a resolution better than 1/2 was achieved.

9:20 am, Student Paper

Scanning Force Microscopy Study of GaAs Films Grown on Offcut Ge Substrates: Q. XU and J.W.P. Hsu, Department of Physics, University of Virginia, Charlottesville, VA 22901; S.M. Ting and E.A. Fitzgerald, Department of Material Science and Engineering, MIT, Cambridge, MA 02139; R.M. Sieg and S.A. Ringel, Department of Electrical Engineering, Ohio State University, Columbus, OH 43210

The surface morphology of GaAs films grown on Ge substrates is studied using a scanning force microscope (SFM). The Ge substrates are offcut 6° towards [110] direction to minimize single steps on the substrate. Our study includes films with and without Ge buffer layers, with different growth temperatures, with different prelayers (Ga as opposed to As), and of different GaAs thicknesses (1000Å and 1µm). We find that the surfaces of several samples are very rough with root mean square roughness (s_rms) > 300Å. These surfaces are dominated by faceted domains of average lateral size from 0.6 to 0.8 µm. Transmission electron microscopy study indicates that these samples contain large anti-phase domains which reach the surface. All these samples are grown without annealing the Ge buffer. GaAs films grown on annealed Ge buffer layers are significantly smoother, with s_rms <6Å. SFM images show uniformly distributed elongated mounds on these surfaces. The 1000Å thick samples show small pits with density in the order of 10⁸/cm². The 1 mm thick samples show large scale cross-hatch patterns on the surface. We find Ga or As prelayers make little difference for samples with annealed Ge buffer layers. Samples without Ge buffer layers generally have surface roughnesses in between the smooth and rough samples just described. All the samples we have studied show direction-preferential surface features related
to the Ge substrate offcut direction. We conclude that the high temperature annealing of the Ge buffer layer is crucial for growing good quality GaAs films, i.e. smooth surfaces with cross-hatch patterns. This might be due to the reduction of single steps on Ge surface during the Ge buffer annealing.

9:40 am

Nanostructures Characterization Using Kelvin Force Microscopy and Near Field Photovoltage Spectroscopy: M.C. Hanna, NREL, 1617 Cole Boulevard, Golden, CO 80401; Y. Rosenwaks, Department of Physical Electronics, Faculty of Engineering, Tel-Aviv University, Ramat-Aviv 69978, Israel

In recent years scanning probe microscopy techniques such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM) have been used to measure electronic properties of quantum structures. Recently the AFM has been used to measure the contact potential difference (CPD) between the microscope tip and a semiconductor sample, a technique which is commonly called Kelvin force microscopy (KFM). In this work we present a series of KFM measurements conducted on coherent InP islands formed by the Stranski-Krastanov (SK) growth mode on AlGaAs and Ga_0.48In_0.52P (GaInP₂) substrates. InP island formation on AlGaAs and GaInP₂ is interesting and important because high quality strain-induced dots can be produced using these islands in GaAs/AlGaAs quantum wells, which is the prototype QW system. The correlation between the islands topography and potential distribution is demonstrated and discussed. In addition we present a new method for characterizing semiconductor nanostructures. This technique, which we hereafter call near field photovoltage spectroscopy (NFPVS), combines the measurement of contact potential difference (CPD) with near field optical excitation. The key feature of the technique is that the excited semiconductor sample is in the optical near field region of a tip that measures the electric force between the tip and the sample; in such a case the lateral resolution is determined by the diameter of the aperture at the end of the tip and is not limited by diffraction effects.

10:00 am, Break

10:20 am

AFM Nano-Oxidation Process for Atomically Flat Nb Surface: K. Matsumoto, Jun-ichi Shirakashi, Tatsuro Maeda Electrotechnical Laboratory, MITI, 1-1-4, Umezono, Tsukuba-shi, Ibaraki-kenn, 305, Japan

An atomically flat surface of the niobium (Nb) metal was fabricated and was used for the AFM nano-oxidation process. Owing to the atomic-order flatness of the Nb surface, the reproducibility of the nano-meter wide oxidized Nb line was greatly improved and the continuous 20nmx500nm oxidized Nb line could be formed. AFM nano-oxidation process is the useful tool to fabricate the nano-meter wide oxidized metal line which works as an energy barrier for the electron. Using the oxidized Nb line or the oxidized titanium (Ti) line as tunneling junctions, we demonstrated the room temperature operation of the single electron transistor. In the previous work, the metal such as Nb was deposited on the silicon dioxide (SiO₂). As the SiO₂ is amorphous, the metal deposited on the SiO₂ becomes also amorphous. Therefore, the surface of the metal on the SiO₂ has the roughness of the order of 0.5-1nm. Due to this roughness of the metal surface, AFM nano-oxidation process had the problems that the oxidized line could not have the continuous line and the reproducibility of the nano-meter wide oxidized line was not good enough for the fabrication of the complicated devices. An atomically flat surface of -Al₂O₃ was used for the substrate of the metal. By heating up the (0001) -Al₂O₃ wafer up to 1000-1100°C, the surface of -Al₂O₃ becomes atomically flat with the terrace size of 100nm and the step height of 0.2nm. The step height of 0.2nm corresponds to 1/6 of the unit cell length of -Al₂O₃ crystal. On this atomically flat Al₂O₃ surface, Nb metal was deposited using the electron beam evaporator at the high vacuum of 10^-8 Torr. -Al₂O₃ substrate was heated up to 500C during the Nb deposition. The deposited Nb surface was measured by AFM. Nb surface shows the terrace size of 100nm and the step height of 0.2nm which reflects the structure of -Al₂O₃ substrate. The roughness of the Nb surface is less than 0.1nm. Therefore, the Nb surface can be called as "an atomically flat surface". Using the AFM cantilever as an ultra-small chemical cathode, the atomically flat Nb surface was oxidized, and by scanning the cantilever, the continuous nano-meter wide oxidized line was made. The oxidized line shows the height of 0.8nm, the width of 20nm, and the length of 500nm. Owing to the atomically flat Nb surface, the reproducibility of the oxidized line was improved drastically.

10:40 am

Surface Modification of Heavily Carbon-Doped P-Type GaAs (p = 1.5x10²¹ cm^-3) by Atomic Force Microscope (AFM) Nano-Oxidation Process: J.-I. Shirakashi and K. Matsumoto, Electrotechnical Laboratory (ETL), 1-1-4 Umezono, Tsukuba-shi, Ibaraki 305, Japan; M. Konagai, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152, Japan

Nanometer-sized surface modification of heavily carbon-doped p-type GaAs with extremely high hole concentration of 1.5x10²¹ cm^-3 was successfully performed by atomic force microscope (AFM) nano-oxidation process. As is well known, the AFM under the appropriate bias conditions in ambient humidity can be used to selectively oxidize the surface of electronic materials such as metals and semiconductors. So, the AFM-based surface modification technique would be a powerful tool for fabricating nanometer-sized structures if one can fabricate and control the structures precisely. Moreover, the heavily carbon-doped p-type GaAs is also promising for the application of novel device structures because the carrier concentration is comparable to that of the normal metals. The epitaxial growth of the carbon-doped p-type GaAs layers was performed by metalorganic molecular beam epitaxy (MOMBE) using trimethylgallium (TMG) and solid arsenic (As₄). In this way, the heavy doping of carbon into GaAs could be possible, and the hole concentration becomes higher with decreasing the growth temperature. The electrical activation of carbon still remains ~100% in the epitaxial layers with such high carrier concentrations. The selective surface oxidation of the carbon-doped GaAs was achieved using negatively biased conductive tip with the range -3 ~ -25V. The oxidation shown here was done under 20-25% ambient humidity. By changing the applied bias voltage and the scanning speed of the cantilever, the obtained size of the modified structures was precisely controlled ranging from about 20 nm to 40 nm in width and about 1.3 nm to 4.7 nm in height. As previously reported in the cases of Ti and Nb, the mechanism of oxidation on the carbon-doped GaAs may appear to be tip-induced anodization, and the modified structures may show the insulating material properties. In other words, the modified structures may act as barrier material for electron. This means that GaAs-based devices with semiconductor(S)-insulator(l) system could be realized using the AFM nano-oxidation process.

11:00 am, Student Paper

Creation of Bulk, Sub-µm Semiconductor Devices Using Scanning Probe Microscopy: S. Richter^1,2, V. Lyakhovitskaya,² S.R. Cohen,³ Y. Manassen¹ and D. Cahen,² Departments of Chemical Physics¹, Materials and Interfaces² and Chemical Services Unit³, Weizmann Institute of Science 76100, Israel

We demonstrate a novel method to fabricate sub-µm sized diodes and transistor structures by inducing thermally assisted electromigration of mobile dopants inside the semiconductors CuInSe₂ (CIS) and Silicon doped with Li (Si:Li). We fabricate the devices in situ by application of an electric field to the crystals, using a modified Atomic Force Microscope (AFM), followed by characterization of the structures utilizing a nano spreading resistance method to prove a change in the lateral conductivity of the device. Previously we showed that application of strong electric fields at ambient temperatures to originally electrically homogeneous crystals of CIS or of Si:Li will lead to the creation of a non-equilibrium hemispherical doping profile, which is stable after removing the field. The Electron Beam Induced Current (EBIC) technique was used to demonstrate that 10-100 mm sized diode and transistor- like structures were created in this way. It was shown that the mechanism that leads to this phenomenon is thermally assisted electromigration of mobile Cu⁺ or Li⁺ ions under the influence of the applied electric field. However, in the above-mentioned method the minimum junction size was ~10 µm. In addition, the ability of the EBIC technique to detect structures smaller than 5 mm in CIS is limited. We show that miniaturization and characterization of the junctions can be achieved by introducing the conducting AFM as construction and analysis tool of the devices. Using the AFM to generate thermally assisted electromigration, we generated hemispherical p-n-p⁺ junctions ranging in size from 1 mm down to 150 nm in diameter. Characterization of the structures was done in situ in the AFM, by means of topography, friction, and current (resistance) modes, as well as by spectroscopic techniques such as voltage vs. current (I/V) and light - induced I/V (photocurrent) measurements. These measurements indicate that we succeeded to induce bulk dopant diffusion, which generates sub-mm devices.

11:20 am, Student Paper

Nanostructure Fabrication Using Metastable Neon Atom Lithography and Chemically Assisted Reactive Ion Beam Etching: S.J. Rehse¹, A.D. Glueck¹, A.B. Goulakov², S.A. Lee¹ and C.S. Menoni², ¹Department of Physics and ²Optoelectronic Computing System Center - Department of Electrical Engineering, Colorado Stte University, Fort Collins, CO 80523; K.S. Johnson and M. Prentiss, Department of Physics, Harvard University, Cambridge, MA 02138; D.C. Ralph, Department of Physics, Cornell University, Ithaca, NY 14853

We demonstrate the fabrication of 50 nm pillars in GaAs with well defined edges and an aspect ratio of 2:1 by using contamination resist lithography and chemically assisted reactive ion beam etching (CAIBE). The contamination resist pattern was formed by a beam of metastable neon atoms (Ne*) which, upon collision with the substrate, transferred their excess energy to the Neovac SY diffusion pump oil vapors (alkyldiphenylether) present as dilute contaminants in the vacuum chamber. This energy transfer led to the formation of a contamination resist pattern mainly composed of carbon atoms, as determined from Auger measurements. A beam of metastable neon atoms was produced by a dc discharge in a supersonic nozzle source. The Ne* atoms passed through a 1 µm diameter skimmer hole and entered the deposition chamber where they collided with the diffusion pump oil molecules. The metastable flux was measured to be ~4x10¹¹cm^-2 s^-2 at the substrate. The [100] GaAs substrates were mounted on a micrometer controlled stage and were exposed to the Ne* beam through physical masks mounted on the substrate surface. The mask consisted of an e-beam patterned, 50nm thick, 100 mm x 100 mm Si₃N₄ membrane with nanoscale perforations mounted on Si. Exposure times ranged from 6 to 24 hrs. Typical resist height for a 24 hour exposure was 40±5 nm. The contamination resist patterns were transferred to the GaAs substrate using chemically assisted reactive ion beam etching (CAIBE). CAIBE was performed using an Ar ion beam with chlorine as the reactive gas. Typical etching conditions were an Ar ion beam voltage of 200V and current density of 0.20mA/cm². Under these conditions, the etch rate selectivity was determined to be 11, a factor of ~4 larger than that determined from the sputtering rates, showing the enhancement of the chemical process. The GaAs nanostructures were obtained by etching the samples for 2 min at an ion beam voltage of 200V. The 50 nm features were measured to be ~100 nm tall, giving an aspect ratio of 2:1. The nanofeatures have well defined edges, indicating that even smaller features should be possible with the present technique. Undercutting of the sidewall was observed, and this is mainly attributed to the divergence of the ion beam. A main advantage of atom lithography is that the small de Broglie wavelength, 20 pm for the Ne* atoms used in this experiment, significantly reduces diffraction effects when compared to optical lithography. The high anisotropy and good selectivity obtained in GaAs show the potential of this lithographic process for obtaining nanostructures with high aspect ratios. This work is supported by the National Science Foundation grants PHY-9312572 and EEC-9015128 and the Colorado Advanced Technology Institute grant 0594.75.0738.

11:40 am

Resist-less Patterning of GaAs for Selective Growth: K. Shiralagi, R. Tsui and H. Goronkin, Phoenix Corporate Research Laboratories, Motorola, Inc., 2100 East Elliot Road, M/S-EL308, Tempe, AZ 85284

Conventionally, photoresist is used to pattern semiconductors in all device and circuit fabrication processes. We have discovered a new way of patterning wafers without using photoresist. In this resist-less SRP (Shiralagi Resist-less Patterning) technique, the semiconductor surface itself is optically modified to act as a mask that can be used for subsequent processing. The absence of photoresist in certain semiconductor processing flows brings about many advantages such as cleanliness, fewer process steps, elimination of certain chemicals, reduction in cycle time, enhancement in yield, and improved compatibility with cluster tools, to name a few. Typically, selective growth on GaAs is performed by first patterning a wafer with an oxide or nitride mask and subsequently growing on the exposed areas of the patterned wafer. The patterning procedure involves many process steps and any contamination left on the mask acts as a nucleating site for atoms during selective growth and impedes the selectivity. The growth of polycrystalline material on the mask due to poor selectivity further affects growth conditions which can then drift out of the selective growth process window in such applications as InAs growth. Utilizing SRP, the GaAs surface oxide is modified in one step, eliminating all the cumbersome process steps required with the use of resists. In RP, the GaAs surface oxide that typically consists of GaAsO₃ is modified to Ga₂O₃ in the masking regions. The difference in desorption temperature between the two kinds of oxides is utilized to grow material such as InAs with 100% selectivity. This kind of process also enables in-situ removal of the oxide mask for certain applications requiring re-growth. Since it is an optical technique, SRP can be effectively used in place of conventional lithography for large wafers with complex features of sub-mm dimensions, making it a process compatible with VLSI circuit fabrication. Further details on the technique and its use in a number of applications will be presented.

8:20 am, Student Paper

Flexible Two Dimensional Metallic Photonic Bandgap Structure: Sandhya Gupta^1,2, Gary Tuttle^1,2, Mihail Sigalas^{c, d} and Kai-Ming Ho^3,4, ¹Department of Electrical and Computer Engineering; ²Microelectronics Research Center; ³Ames Laboratory, U.S. Department of Energy, ⁴Department of Physics and Astronomy, Iowa State University, Ames IA 50011

A new two-dimensional photonic bandgap structure has been fabricated using layers of periodic square metal grids embedded in a dielectric material. The metal grid pattern of 0.5 mm thickness has a lattice constant of 32 µm with a line width of 5 µm and is separated from the next layer by 19 µm thick dielectric. Spin-on polyimide of _r =2.9 serves as the dielectric material. The complete structure is a very flexible sheet of polyimide encapsulating two layers of metal grid pattern and has a total thickness of 30 µm. FTIR (Fourier Transform InfraRed) spectroscopy has been used for the measurements. This structure shows a high pass transmission characteristic of cutoff frequency 2.6 THz and attenuation of 20 dB before the cutoff. The cutoff frequency can be adjusted by varying the separation between the metal grids. The measured transmission cutoff is in good agreement with the theoretical simulation result. This structure us very compact, flexible and can be used as a high pass filter operating in a few THz range.

8:40 am, Student Paper

Wafer Bonding for Fabrication of Three-Dimensional Photonic Bandgap Crystals: Shi-di Cheng^1,2, Gary Tuttle^1,2, Rana Biswas^2,3 and Kai-Ming Ho^3,4, ¹Department of Electrical and Computer Engineering, ²Microelectronics Research Center, ³Ames Laboratory, U.S. Department of Energy, ⁴Department of Physics and Astronomy, Iowa State University, Ames IA 50011

Recently photonic bandgap crystals have attracted significant attention theoretically and experimentally due to their potential uses in many different frequency regimes. We have devised techniques for use in fabricating three-dimensional photonic bandgap crystals of micron scale operating in the optical wavelength regime. The method uses alternating steps of wafer fusion boding, selective substrate etching, and pattern etching to sequentially build up photonic bandgap structures in a layer-by-layer fashion. To enhance bonding we have incorporated a thin InAs "bonding" layers to improve fusion between GaAs wafers. With the InAs layers, strong bonding between patterned substrates has been achieved at bonding temperatures of 650C. Examination of the fused structure by SEM, show smooth and uniform surfaces and good adhesion at the interfaces. Using this technique, fabrication of photonic crystals with optical bandgaps in 10 um range are feasible.

9:00 am

Epitaxial Liftoff of 10µm Films of Crystalline Magnetic Garnets: M. Levy, R.M. Osgood, Jr., Department of Applied Physics, Columbia University, 500 W. 120th Street, New York, NY 10027; H. Bakhru, A. Kumar, Department of Physics, State University of New York at Albany, Albany, NY 12222

The integration of magnetic garnet waveguides into silicon, gallium arsenide or indium phosphide-based photonic integrated circuits (PICs) is of considerable importance. Optical isolators and circulators, which are essential component in the laser-source emitter circuit-end, rely on the non-reciprocal properties of magnetic garnets for their operation. Bismuth-substituted yttrium iron garnet (BYIG) and other rare-earth magnetic garnet films, used in the fabrication of these devices, are normally grown by liquid-phase of rf-sputter epitaxy on gadolinium gallium garnet substrates (GGG). No epitaxial growth of magnetic garnets has been realized on semiconductor substrates. Therefore the integration of these and other such thin oxides must rely on techniques such as flip-chip bonding or epitaxial liftoff. We report here the first implementation of epitaxial liftoff in magnetic garnets. Deep ion-implantation is used to create a buried sacrificial layer in single-crystal YIG. Helium ions at 3.8 MeV of energy are implanted several microns below the top surface with little residual damage to the near-surface region. The damage generated by the implantation induces a large differential etch rate between the latter and the sacrificial layer. The-micron-thick films have been lifted off from the original GGG substrates by etching in phosphoric acid for 20 hours. This technique differs from the standard epitaxial liftoff in III-V materials in the use of ion-implantation rather than MBE growth for a faster-etching sacrificial layer. This work has also addressed the elimination of stress-induced microfractures to maintain the integrity of the thin crystalline oxide layer. The detached films have been bonded to glass slides using cyanoacrylate adhesives or van der Waals bonding. Millimeter-size pieces of excellent quality have been transferred to host substrates. The facets remain smooth even after several hours of etching. Study of magnetic properties of the detached epilayers by Faraday contrast show no degradation in coercivity due to pinning of the domains. Possible applications to magnetic micro-mechanical systems are also being pursued. This work supported under MURI/DARPA contract #F49620-96-1-0111.

9:20 am

Si/InGaAs 20 GHz High Speed, Low Dark Current, =1.55µm PIN Photodetectors: B.F. Levine, S. Hiu, B.J. Tseng, C.A. King, L.A. Gruezke, R.W. Johnson and D.R. Zolnowski, Bell Laboratories, Lucent Technologies, 700 Mountain Ave., Murray Hill, NJ 07974; A.R. Hawkins and J.E. Bowers, Electrical and Computer Engineering Dept., University of California, Santa Barbara, Santa Barbara, CA 93106

The advantages of directly bonding Si and III-V wafers to fabricate devices which maximize the advantages of each material are well known. We demonstrate here the first planar high performance l=1.55 mm telecommunication PIN photodetectors on a Si substrate. The fused Si/InGaAs interface is in the center of the active device region, and therefore these detectors can be used to study in detail, the interface characteristics which are important for both PIN and avalanche photo-diodes (APDs). Our detectors show high internal quantum efficiency (h 100 %), high speed (RC limited frequency response of 21 GHz), record low dark current (Id < 100 pA at a bias of 4 V), a long generation recombination lifetime of 400 ns, and a forward bias ideality factor of n=1.1-1.2. In addition, capacitance-voltage measurements and absolute photo-current noise measurements show no evidence of charge trapping, recombination centers or a conduction bandgap discontinuity at the heterointerface. In conclusion, the Si/InGaAs interface is nearly ideal and thus, high performance PINs and APDs can be expected.

9:40 am

Wafer Bonding for Photovoltaic Applications: P.R. Sharps¹, M.L. Timmons¹, J.S. Hills¹, J. Gray², and Lt. D. Keener³, ¹Research Triangle Institute, P.O. Box 12194, Research Triangle Park, NC 27707; ²Purdue University, 1258 Electrical Engineering, W. Lafayette, IN 47907; ³United States Air Force Phillips AFB, 5801 Manzana St. Bldg. 30117, Kirtland, NM 87117-6503

The bonding of materials with different lattice constants provides a means for increasing the efficiency for photovoltaic devices for space applications. Lattice-matched, monolithic GaInP2/GaAs two junction and GaInP2/GaAs/Ge three junction space qualified cells have been developed and are currently being manufactured. In order to have significant increases in the conversion efficiencies, more junctions are needed. Wafer bonding is a means for integrating epitaxial junctions grown on different substrates in one device. In particular, we are interested in bonding GaInP2/GaAs dual junction cell grown on GaAs to a GaInAsP/GaInAs2 dual junction grown on InP, to create a four-junction photovoltaic device. For use in the four junction device, the bonding must meet optical, electrical and mechanical criteria. The bond must have minimal optical absorption, allowing the appropriate wavelengths of light to reach the GaInAsP/GaInAs2 junctions. The electrical resistivity of the bond must be low enough to keep electrical losses at a minimum. Finally, the bond must be robust enough to survive launch as well as the temperature cycling encountered in space photo-voltaic panel. We have used a bonding technique to join GaAs and InP in a such a way that all three criteria are met. The bonding technique has a direct application in the four-junction device. The bonding temperature, pressure, and metals will be described, and the optical, electrical, and mechanical data from the bond will be presented and discussed. An analysis of the processes occurring during the bonding will be given. The four-junction device structure will be described.