Wednesday Afternoon Sessions (June 26) TMS Logo

About the 1996 Electronic Materials Conference: Wednesday Afternoon Sessions (June 26)

June 26-28, 1996 · 38TH ELECTRONIC MATERIALS CONFERENCE · Santa Barbara, California

Session G: Epitaxy for Devices

Chairman: Ray Tsui, Motorola, 2100 E. Elliot Rd., M/S - EL508, Tempe, AZ 85284. Co Chairman: Yung-Chung Kao, Texas Instruments, PO Box 655936, MS 147, Dallas, TX 75265

1:30PM, G1

"1.3 um Strain-Compensated MQW InAsP/InGaP/InP Heterostructure Lasers with Characteristic Temperature Up to 140K:" A. OUGAZZADEN, N. Bouadma, C. Kazmierski, G. Patriarche, J. Landreau, M. Kamoun, France Telecom, CNET/PAB, BP107, 92225 Bagneux, Cedex France

High temperature operation of 1.3 um lasers without Peltier cooler is a key issue for low cost local area networks. We already reported the first InAsP/InGaP strained compensated structure with low threshold current density together with high characteristic temperature [1]. Due to the high strain in the wells and barriers, this structure presented lateral thickness undulation with a degradation of crystalline quality and luminescence intensity. A spectacular improvement of material quality has been achieved when a thin InP layer is introduced between wells and barriers.

In the present work the InP layer role has been clarified by transmission electron microscopy (TEM). In fact, the InP layer planarizes the surface after each strained layer and prevents the enhancement of undulations as the number of periods increases. We also demonstrate that the amount of global strain influences the formation of thickness modulation. To obtain flat layers all along the structure a minimal global strain is hence necessary in addition to interfacial InP layer.

We have grown ten InAsP compressive wells ([[epsilon]] = 1.7%) embedded between InGaP tensile barriers ([[epsilon]] = -1.5%). Furthermore the optical confinement layers with intermediate gap energy were avoided in order to prevent carriers escape and hence achieve high To. Broad area lasers were processed with this structure. A characteristic temperature higher than that of previously reported (117 K [1]) has been measured to be 140 K between 20 and 60deg.C.

To realize a buried ridge stripe (BRS) laser a standard technological process has been used. However, as the refractive index of strained InGaP layers is comparable to that of InP, this leads to a low confinement factor of buried active layer and hence degrades the lasing conditions. A first optimization of wells and barriers thickness as well as the width of the buried stripe to increase the confinement factor has been performed. The preliminary results are very promising, with a threshold current of 17 mA and output power of 70 mW at room temperature for 260 um cavity length and a differential quantum efficiency ([[eta]]i) of 28 % per facet, with as cleaved facets. The characteristic temperature To has been measured at around 70 K.

Further details of materials growth and device characteristics will be reported.


[1] A. Ougazzaden, A. Mircea and C. Kazmierski, Electron. Lett., 11th May 1995, vol. 31, No. 10, pp. 803-804.
1:50PM, G2+

"Spontaneous Lateral Carrier Confinement in Graded in GaAs/GaAs 1.3 Micron LED Structures:" MAYANK T. BULSARA, Vicky Yang, AnnaLena Thilderkvist, Karl Hausler*, Karl Eberl*, Eugene Fitzgerald, Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139; *Max-Planck-Institut fur Festkorperforschung, Heisenbergstr. 1, D-70569 Stuttgart, Germany

The fabrication of long wavelength InGaAs optoelectronics on GaAs substrates would allow an inexpensive alternative to InGaAsP/InP technology. For this study, graded InxGa1-xAs lattice mismatched layers were used to relax the InxGa1-xAs alloy on the GaAs substrate. Such InGaAs layers were then used to fabricate 1.3 um LED structures. One design uses a single quantum well in the relaxed InGaAs layer to achieve an emission wavelength of 1.3 um. The structures were grown with MBE on GaAs (001) substrates with varying substrate temperatures and graded buffer profiles. For structures grown at low temperatures (350deg.C), cross-sectional transmission electron microscopy (X-TEM), cathodoluminescence (CL), photoluminescence (PL), and x-ray diffraction data show that the graded buffers were not completely relaxed and the residual strain increases the effective bandgap of the quantum well, eliminating any confinement characteristics in the quantum well. However, the structures grown at higher temperatures of 580deg.C exhibited 4K PL at approximately 1.3 um. X-TEM and plan view TEM of these structures showed long wavelength contrast modulations on the order of 175 nm and short wavelength modulations on the order of 10 nm. The origins of the contrast modulations are not clear as no ordering and no compositional changes could be observed. By using spatially resolved 4K CL we are able to show that 1.3 um emission is not distributed uniformly across the sample. CL shows that the 1.3 um emission is originating from linear wire-like structures which appear to be correlated with the cross-hatch pattern generated in the relaxed InGaAs buffer. In addition, slower grading (%In/um) of the graded buffer results in a more evenly distributed quantum well emission. This behavior suggests a connection between the strain fields associated with dislocations in the graded buffer and the quantum well emission. Atomic force microscopy (AFM) shows that the surface undulations were shallow, thus eliminating the possibility of quantum wire formation due to a surface topology effect on composition during growth. We conclude that strain fields from the dislocations in the graded buffer cause a spatial variation of the confinement of carriers in the quantum well.

2:10PM, G3+

"Improving Laser Characteristics in MBE-Grown AlInGaAs Multi-Quantum Well Lasers on GaAs by Rapid Thermal Annealing:" JACK KO, M.J. Mondry, D.B. Young, S.Y. Hu, L.A. Coldren, A.C. Gossard, Electrical and Computer Engineering Department, University of California, Santa Barbara, CA 93106

By using post-growth rapid thermal annealing (RTA) of strained AlInGaAs double-quantum-well (DQW) lasers, threshold current densities have been reduced to state-of-the-art levels. A record low threshold current density of 118 A/cm2 per well has been achieved in 835 nm strained Al0.15In0.25Ga0.6As DQW lasers grown by solid source molecular beam epitaxy (MBE) on a (100) n+ GaAs substrate. By incorporating aluminum into strained InGaAs QW structures, the emission wavelength can be decreased while still maintaining a desired strain level for potential improvements in device properties. If the wavelength is reduced below ~860 nm, these sources can then be used with monolithic GaAs photoreceivers. Although excellent broad area lasers have been achieved in this system by metal-organic chemical vapor epitaxy, laser thresholds have remained relatively high in MBE grown material. RTA has previously been shown to enhance the photoluminescence (PL) efficiency as well as laser performance in both InGaAs/GaAs and GaAs/AlGaAs quantum well lasers. In this work, we show for the first time that RTA can significantly improve laser performance in strained AlInGaAs multi-QW lasers grown by MBE.

The structure was grown at a substrate temperature of 600deg.C except for the QWs and the barrier which were grown at 585deg.C. The QWs are 5-nm thick and the barrier is a 10-nm-thick Al0.2Ga0.8As layer. Si and Be were used for n-and p-type doping, respectively. The post-growth RTA was done in an AG Associates Heat-Pulse 410 system with forming gas ambient. To avoid decomposition and dopant out-diffusion, a Zn-doped GaAs wafer was used as a proximity cap during the annealing process. Broad-area (50 um) stripe lasers were fabricated to evaluate the effects of RTA on laser characteristics. We observe that threshold current densities can be reduced by up to a factor of 5 with post-growth RTA. In addition, we see a 75% reduction in the transparency current density as well as an improvement in the internal quantum efficiency, from 65% to 72%, in the RTA treated samples. The lasing wavelength blue shifts by 3nm and 5nm at RTA temperatures of 900deg.C and 950deg.C, respectively. Low temperature PL suggests that the improvements in laser characteristics may be attributed to a reduction of nonradiative recombination centers in both the active region and the separate-confinement-heterostructure (SCH). The results are compared to those from similar laser structures employing GaAs QWs.

2:30PM, G4

"MOVPE Growth of Strained InGaAs/GaAs Quantum Wells for High Power Long Wavelength Laser Diodes:" F. BUGGE, U. Zeimer, G. Beister, G. Erbert, S. Gramlick, M. Weyers, Ferdinand-Braun-Institut für Höchstfrequenztechnik Berlin, Rudower Chaussee 5, D-12489 Berlin, Germany

For pumping of Pr+-doped fiber amplifiers or the replacement of Nd:YAG lasers, laser diodes with wavelengths of 1017 nm and 1065 nm respectively are of interest. Such wavelengths can be reached by the use of GaAs based lasers with strained InxGa1-xAs quantum wells (QWs). However, the high strain in such structures causes problems even before the onset of lattice relaxation. The incorporation in indium during MOVPE growth of strained InxGa1-xAs-QWs was investigated in dependence of growth temperature, V/III-ratio and total pressure. The In incorporation into strained layers is lower than into relaxed ones and is found to be limited to x ~ 0.3. Further increase of the trimethyl indium supply results in a reduced In content in the QW, which was estimated by high resolution x-ray diffractometry, photoluminescence and secondary ion mass spectroscopy. Investigations by transmission electron microscopy and cathodoluminescence show the formation of In-cluster, loops and finally dislocations with increasing TMIn supply in quantum wells which are below the critical thickness according to the double kink model.

AlGaAs/InGaAs/GaAs broad area laser diodes and with different active zones emitting between 900 nm and 1050 nm were processed and characterized. For typical structures with [[lambda]] = 980 nm threshold current densities of 50-60 A/cm2 (at infinite cavity length), absorption losses between 1-2 cm-1 and wavelength deviations of +/- 1.8 nm over a 2" wafer are obtained. Laser diodes with 50 um stripe width show cw output powers up to 1.5 W at 2.3 A driving current and a far field angle of 23deg..

By using GaAsP spacer layers the strain of the active layer can be compensated and a lasing wavelength of 1050 nm is achieved. The impact of such strain compensating layers on the In incorporation as well as the laser performance will be discussed.

2:50PM, G5

"Polycrystalline MBE-Grown GaAs for Solar Cells:" D.J. FRIEDMAN, S.R. Kurtz, R. Matson, M. Al-Jassim, B. Keyes, D. Niles, J. M. Olson, National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401

In the development of low-cost solar cells, much work has been done with a-Si and II-VI materials while GaAs has received less attention. In this paper, we present the initial results of an investigation of the growth of GaAs on inexpensive Mo-coated glass substrates via MBE (molecular-beam epitaxy - although "epitaxy" is a misnomer in this case). While MBE systems may be too expensive for low-cost growth of large-area devices such as solar cells, such a growth system provides a highly controlled environment and in-situ diagnostics which are useful in the early, research phase of the development of the material. In this case, an MBE system is essentially a sophisticated evaporator; a later phase of development would be to transfer the growth to a less expensive evaporator system.

Samples were grown via MBE using all solid sources. A standard film structure consisting nominally of 0.2 um of InAs to provide ohmic contact to the Mo back contact, followed by 5 um of GaAs, was used for all the films reported here. Films were nominally undoped (i.e. no p/n junction). The Ga flux was kept at a beam-equivalent pressure of 1.4x10-6 torr to provide a nominal growth rate of 2.5 um/hour. Growth temperature and As flux were varied from sample to sample.

The spectral response of the samples was measured using an electrolyte junction to the front surface of the sample. For the initial materials development, we use the intensity of the photocurrent as the basic measure of photovoltaic materials quality; for this abstract, all reported photocurrents are at hv = 1.6 eV, and are relative to the photocurrent measured for high-quality epitaxially grown GaAs material. Samples were examined with additional analytical techniques including cross-sectional scanning electron microscopy (X-SEM), photoluminescence, and Nomarski microscopy, with the aim of correlating the morphological and electrical properties of the material to the growth conditions.

The As flux proved to have a dramatic effect on the photocurrent. Samples were grown with an As/Ga beam-equivalent-pressure (V/III BEP) ratio of 3, 6, and 14 at a constant growth temperature. The relative photocurrents for the 14, 3, and 6 V/III-BEP-ratio samples respectively were 4x10-5, 6x10-4, and 3x10-2. A comparison of this material with the much worse material grown at 14 V/III BEP by X-SEM shows a marked difference in the grain structure in the two materials. The 14-V/III sample shows a columnar grain structure, with grain sizes/column diameters on the order of 0.1-0.3 um. In contrast, the V/III=6 sample shows an evolution of the grain size away from the substrate surface, with grain sizes increasing to 1-2 um near the surface of the sample. The much larger grain sizes, and hence the lesser effect of recombination at grain boundaries, for the V/III=6 sample is presumably responsible for its much better photocurrent.

Sample growth temperature Tg was also found to have a strong effect on the photocurrent of the resulting material, with photocurrents varying by as much as an order of magnitude for 50deg.C difference in Tg. For a constant V/III=6, as Tg is raised the relative photocurrent of the resulting material rises to 20%, (which should be adequate for initial device fabrication) and then falls again as Tg is raised further. These results will be presented, and the correlation with the lifetime measurements and other analytical probes of the material will be discussed. 3:30PM, G6

"Investigation of Compositional Grading for Molecular Beam Epitaxy of Highly Mismatched InGaAs on InP:" S.M. LORD, Department of Electrical Engineering, Bucknell University, Lewisburg, PA 17837; R. Kochhar, W-Y. Hwang, M. Micovic, T.S. Mayer, D.L. Miller, Electronic Materials and Processing Research Laboratory, The Pennsylvania State University, University Park, PA 16802

Highly sensitive near-infrared (1.5-2.5 um) InGaAs photodetectors are required for a variety of commercial applications including industrial process control, wind shear detection, and blood glucose monitoring. Unfortunately, the InGaAs composition needed for detection at these wavelengths leads to a large lattice mismatch between the active layers and the InP substrate (e.g. 1.4% for detection at 2.2 um). The growth of such highly mismatched materials by Molecular Beam Epitaxy (MBE) has been studied previously for SiGe on Si, InGaAs on GaAs, and InGaAs on InP. In these studies, linearly graded buffer layers, which were implemented by increasing the Ge or In composition in a series of small steps (<= 100 Å), were effective in minimizing the formation of misfit dislocations and propagation of threading dislocations. Moreover, the results showed improved material quality as the grading rate decreased while the thickness of the buffer was increased by including additional steps of the same size. To minimize the complexity of the buffer layer growth and to facilitate the commercialization of the process, we investigated the effect of varying the buffer thickness by increasing the size of a constant number of steps.

To determine the effect of step size on device performance, 2.2 um p-i-n photodetectors that consisted of a 3000 Å lattice matched n+-In0.53Ga0.47As buffer followed by a compositionally graded region, a 500 Å n+-In0.74Ga0.26As layer, a 1 um undoped In0.74Ga0.26As active region, and a 3000 Å p+-cap layer were grown and characterized. The compositionally graded regions were implemented using 50 equal-sized steps, which ranged from 100 Å to 400 Å each, for total thicknesses between 0.5 um and 2.0 um. For each thickness, the graded region was grown with substrate temperatures of 300deg.C, 350deg.C, and 400deg.C. The active layers were grown at 400deg.C.

For all growth temperatures investigated, the photodetectors with 2.0 um buffers exhibited the worst electrical characteristics despite the low grading rate. This is evidenced by higher dark current densities and lower zero-bias shunt impedance-area products. These results suggest that the 400 Å steps used for the 2.0 um buffer are too large for effective linear grading. However, the 100 Å steps used for the 0.5 um buffers correspond to a high grading rate, which appears to be less effective at minimizing misfits and threading dislocations. Thus, samples with 200 Å steps used for the 1.0 um buffer offer the best compromise of the conditions tested. For such samples, measured dark current densities of 2.0 mA/cm2 at a bias of -1 V are comparable to state-of-the-art devices grown by VPE. This work demonstrates that both the grading rate and the step size of MBE-grown compositionally graded buffers are important for high quality material. 3:50PM, G7

"Pseudomorphic In-Graded Carbon Doped GaAs Base Heterojunction Bipolar Transistors by Metal Organic Chemical Vapor Deposition:" N. PAN, M.A. Knowles, N. Cheong, D.P. Vu, D. Hill, Kopin Corp., 695 Myles Standish Blvd., Taunton, MA 02780; D. Pierson, P. Zampardi, S. Fitzsimmons, Rockwell Science Center, 1049 Camino Dos Rios, Thousand Oaks, CA 91360; Li-Jen Chou, K.C. Hsieh, Center for Compound Semiconductor Microelectronics, Department of Electrical Engineering, 1406 W. Green St., University of Illinois, Urbana, IL 61801

The base transit time is one of the key factors limiting the high frequency performance of GaAs based heterojunction bipolar transistors (HBT). The base transit time is limited by the diffusion of electrons across a heavily p-doped base layer. A reduction in the base transit time can be achieved with the application of an electric field across the base to improve the transport of electrons. A compositionally graded base layer provides an electric field across the base layer which is expected to increase the current gain and the current gain cutoff frequency of HBT devices.

In this work, a pseudomorphic In-graded composition C-doped GaAs base HBT was demonstrated. An enhancement of the current gain of up to 50% was observed as the indium composition was increased from 0 to 5%. The HBT layer structures were characterized using Hall-effect measurements, double crystal X-ray diffraction, secondary ion mass spectrometry (SIMS), and transmission electron micrograph (TEM). A maximum indium composition of 5% was chosen to avoid exceeding the critical layer thickness.

The structures were grown using metalorganic chemical vapor deposition (MOCVD). The structure consisted of heavily doped GaAs sub-collector, a lightly doped GaAs collector, a 500 Å C-doped GaAs base layer (4 x 1019 cm-3), a thin moderately doped Al0.25Ga0.75As emitter, and an InGaAs contact layer. The indium composition of the base layer was graded from a high indium (3 or 5%) composition at the collector junction to 0% indium at the emitter junction. The standard structure with 0% grading and a measured C-doping density (SIMS) of 4 x 1019 cm-3 showed a dc current gain (1 kA/cm2) of 112 and a measured Vbe (1.8 A/cm2) of 1.119 V. The structure with a 3% indium grading and a measured C-doping density of 4 x 1019 cm-3 showed a dc current gain of 124 and a measured Vbe of 1.116 V. The structure with a 5% In-grading and an measured C-doping density of 2.8 x 1019 cm-3 showed a dc current gain of 159 and a measured Vbe of 1.108 V. An increase in the dc current gain was observed with increasing indium composition. The slight decrease in the C-doping density at 5% indium grading was due to the suppression of C-doping during the growth of InGaAs using trimethylindium. The stability of Vbe as the indium composition was varied up to 5% indicated that the quality of the base/emitter junction was maintained. RF performance of small area devices will follow to confirm a decrease in the base transit time.

4:10PM, G8

"Effects of Layer Design on the Performance of InAs/AlSb/GaSb Resonant Interband Tunneling Diodes on GaAs Substrates:" KUMAR SHIRALAGI, Jun Shen, Ray Tsui, Phoenix Corporate Research Laboratories, Motorola, Inc. 2100 East Elliot Rd., M/S-EL508, Tempe, AZ 85284

Resonant interband tunneling diodes (RITDs) in the InAs/AlSb/GaSb material system have attracted much recent attention due to the high peak-to-valley current ratios (PVCRs) at room temperature. Novel devices and circuits based on these diodes (e.g., resonant interband tunneling FETs, SRAMs and XNORs) have been demonstrated. The epitaxial growth of these devices, however, poses a number of challenges. One is the lack of a lattice matched semi-insulating substrate, and much of the growth is carried out on GaAs. The large lattice mismatch (~7%) means that the buffer layer thickness has important consequences on the dislocation density and hence the electrical characteristics of the RITDs. Another challenge is that the atomic species on both sublattices are switched across the InAs/AlSb interface. This creates different interfacial configurations that also affect device performance.

We report the room temperature characteristics of RITDs grown by CBE on GaAs substrates, as a function of the InAs buffer layer thickness. The PVCR improved from 1 to 12 as the thickness is increased from 0 to 500 nm and the dislocation density decreased to about 108 cm-2. No significant improvement is seen beyond 500 nm. Dislocation-free RITDs grown on lattice-matched InAs substrates show PVCRs of ~15. These results suggest that mechanisms other than dislocations to be limiting the PVCRs beyond a certain buffer layer thickness. The roles of several possible mechanisms will be discussed.

The InAs/AlSb interfaces in these structures can be either InSb-like or AlAs-like, depending on the source-switching sequences during growth. While InSb-like interfaces have been reported in the literature to give superior electronic transport properties in InAs/AlSb FETs, the influence of interface type on RITD performance remains to be clarified. We have grown and characterized RITDs with both types of interfaces as well as with [AlSb]m[AlAs]n superlattice barriers (m = 2 or 3, n = 1) in place of AlSb barriers. Differences in the PVCRs resulting from these layer designs have been observed and will be presented.

4:30PM, G9+

"Selective Area Growth As An Enabling Technology For High-Uniformity Single-Level Metal HEMTs:" KÜRSAD KIZILOGLU, Bernd P. Keller, Prashant M. Chavarkar, Xudong Cao, Steven P. DenBaars, Umesh K. Mishra, University of California, Department of Electrical and Computer Engineering, Santa Barbara, CA 93106

The need for a reproducible, uniform, and reliable FET technology for various high frequency and low noise applications including receivers, transmitters, and OEICs is clear. The rapid developments in materials technologies have made it possible to investigate the fabrication of FETs, where processing and growth are continuously interleaved throughout the fabrication process. In this work, we report on the fabrication of a HEMT, where selective growth by MOCVD is employed twice to arrive at the final device: once to achieve low resistance ohmic contacts, and second time to achieve a stable epitaxial surface passivation. Since low frequency noise (such as 1/f noise) in III-V semiconductors arises from variations in the occupancy of the bulk and surface trap centers and the surface recombination velocity, we expect to have better low frequency noise performance by passivating the surface of our device.

The process starts with a standard AlInAs/GaInAs HEMT structure grown by MBE, lattice-matched on semi-insulating InP substrate. The structure is capped by 50 Å of undoped GaInAs, and has ns = 2.8.1012 cm-2, u = 8300 cm2/V.s. The first processing step is the selective regrowth of n++ GaInAs as source and drain contacts for the HEMTs. A thin layer of SiO2 is patterned and used as a mask for this regrowth, which is done by MOCVD at 620deg.C using the precursors TMGa, TMIn, and TBA. Disilane is used as the source for n-type Si doping. The regrowth is ended by grading to InAs to enable low contact resistance to the device, and is done at atmospheric pressure to maximize dopant incorporation - although selectivity is reduced compared to low pressure regrowth. By avoiding HF treatment and associated deleterious effects of fluorine passivation of Si dopant atoms in AlInAs (as reported by other researchers), we achieve temperature stable contacts with a contact resistance (Rc) of 0.1 [[Omega]]-mm.

Our next process step is electron beam lithography to prepare the device for the succeeding passivation overgrowth. We first pattern the E-beam resist and then evaporate and liftoff SiO2 for the following dual purpose: (i) to protect the contact regrowths by masking them, and (ii) to define the gate footprint. Once we have the SiO2 mask in place, we etch the sample to remove the GaInAs cap and load it back into the MOCVD reactor. We then grow a lightly p-doped (p ~ 5.1016 - 1017 cm-3) layer of InP on the sample. This overgrowth step serves two purposes: (i) it passivates and removes the surface away from the 2DEG in the channel creating a "buried-channel device", and (ii) it creates an "effective recess" for the gate. After the InP overgrowth, we remove the SiO2 mask and do a second E-Beam alignment for metalizing the gate, source, and drain regions all together. The metalization is done by evaporating and lifting off Ti/Pt/Au. We end the process with mesa etching for isolation.

Initial measurements of the devices yields gm = 150 mS/mm, IDSS = 160 mA/mm with fT = 20 GHz, and fmax = 50 GHz. Since we passivated the device surface and effectively buried the channel, we also get very low phase noise performance: -40 dBc/Hz at 100 Hz, and -98 dBc/Hz at 10 KHz offset. We have thus successfully demonstrated the feasibility of a HEMT with low resistance regrown contacts and epitaxially passivated surface with no recess etching, single level of metalization, and low phase noise.

4:50PM, G10

"GaAs p/n Junction with an Epitaxial p-Layer and a Si-Implanted n-Layer: A Path to High Performance HBTs:" H.Q. HOU, A.G. Baca, J.C. Zolper, B.E. Hammons, Sandia National Laboratories, MS 0603, Albuquerque, NM 87185

Epitaxial growth on semi-insulating GaAs substrates with selective area doping formed by ion implantation is being developed to realize a novel AlGaAs/GaAs heterojunction bipolar transistor (HBT) structure. In addition to growth simplicity (no regrowth is required), this approach offers effective modulation of the doping level in the lateral and vertical directions, therefore, a significant reduction in base-collector capacitance by reducing this junction area. Furthermore, a larger tolerance for the base layer ohmic contact formation can be accommodated since this region can be offset from the active junction region, and on-chip electrical interconnect routing can be done directly on a semi-insulating substrate. The critical step in demonstrating this novel structure is the realization of high quality p/n junctions between Si-implanted GaAs and epitaxial p-type GaAs.

As a first step to applying this technology to HBTs, we have fabricated p/n junction diodes with the metallurgical junction at the epitaxial/implanted interface. Undoped semi-insulating GaAs substrates were implanted with Si up to a depth of 0.7 um using single ionized Si-ions at an energy of 40 keV and double charged Si-ions at an energy of 300 keV. The implants were activated at 850deg.C for 15 s in a SiC coated graphite susceptor. We achieved a donor concentration of ~3x1016 cm-3 near the surface (collector) and ~5x1017 cm-3 near 0.5 um (subcollector). Epitaxial growth was carried out in a low-pressure metalorganic vapor phase epitaxy (MOVPE) reactor. Mirror-like surfaces on GaAs/Al0.3Ga0.7As structures, which is very sensitive to growth conditions, have been obtained by varying the initial pre-growth treatment to the sample and including an in situ anneal of 750deg.C for 20 min followed by a slight in situ etchback with CCl4. X-ray diffraction measurements indicate that GaAs/Al0.3Ga0.7As structures grown on the Si-implanted samples and epi-ready substrates have comparable structural quality.

P-type GaAs (1x1019 cm-3) layers were grown on both the Si-implanted and epi-ready substrates and processed into 50x50 um2 mesa diodes. The unimplanted samples included epitaxial n-type layers with approximately the same doping as the implanted layers. Excellent I-V characteristics were obtained for both implant and all epitaxial diodes as evident by the sharp forward turn-on characteristics and extremely low reverse leakage currents (~ 50 to 100 pA at 1 V). With this success on the base/collector diode we are in the process of fabricating complete HBTs using selective area implants to form the collector and thereby to minimize the base/collection junction area. This work was supported by the DOE under contract No. DE-AC04-94AL85000.

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