1997 EMC: Friday Morning Sessions, Part 2

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, Student Paper

Strong Absorption GeSi on Si Materials for 1.3µm Photodetection: L.M. Giovane, D.R. Lim, L.C. Kimerling, and E.A. Fitzgerald, Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave., Rm 13-4154, Cambridge, MA 02139

The silicon-germanium material system is a promising choice for 1.3 mm and 1.54 mm photodetectors in order to bridge the gap between silicon microelectronics and silica fiber optic systems. Previous efforts to design and fabricate a 1.3 mm GeSi photodetector have encountered the challenges presented by the 4% lattice mismatch between silicon and germanium and the indirect bandgap of all GeSi alloys. Although the range of bandgaps for GeSi alloys includes 0.95 eV (1.3 mm) and 0.80 eV (1.54 mm), many previous designs have been unable to utilize fully the entire range of GeSi compositions and strains because they have relied on coherently strained GeSi films grown epitaxially on silicon substrates. This approach limits the germanium fraction and thickness of the film. Although many of these designs have been successful in detecting 1.3 mm light, they are plagued by low responsivities because of the weak absorption of GeSi alloys near the band-edge. In this paper we discuss the use of strained-layer superlattices grown on high quality relaxed GeSi buffers. Graded buffers have been demonstrated as ideal virtual substrates for achieving a complete range of compositions and strains for GeSi alloys. In order to fully utilize this technology, we have calculated absorption spectra as a function of composition and strain. The calculation takes into account the change in bandgap due to composition and strain as well as the lifting of band degeneracy due to strain-splitting of the band extrema. This calculation has allowed us to determine the appropriate strain and composition for achieving high absorption GeSi materials for 1.3 mm photodetection. We have confirmed the calculation with photocurrent spectroscopy on strained GeSi layers grown on relaxed buffers. We have also designed and grown a Ge/Geo_0.5Si_0.5 strained layer superlattice on a high quality Geo_0.75Si_0.25 relaxed buffer. Both layers of the superlattice are strained to fit the lattice parameter of the relaxed buffer. Thus the high germanium content layers are under compression, while the layers with low germanium content are under tension. Although the thickness of the alternating layers is limited by the critical thickness, there is no limit to the number of layers in the superlattice due to the strain balance. Optical absorption measurements yield an absorption coefficient, a, at 1.3 mm of 6x10³cm^-l. The optical absorption results indicate that the absorption of the SiGe strained layer superlattice is comparable to direct gap materials. Our preliminary results, using beam propagation modeling to evaluate various detector and detector-waveguide geometries for optical coupling of light into the photodetector, indicate that strong absorption materials (a>10³cm^-l) are needed for efficient collection of light.

8:40 am, Student Paper

Effect of Grains and Grain Boundaries in Polycrystalline Silicon Waveguides: L. Liao, D.R. Lim, J.S. Foresi, X. Duan, A.M. Agarwal and L.C. Kimerling, Department of Materials Science and Engineering, MIT, 13-4126, 77 Massachusetts Avenue, Cambridge, MA 02139

Polycrystalline silicon (polySi) on oxide waveguides are of interest as a means of integrating microphotonics with silicon VLSI processing technology. The high index contrast of the polySi/SiO₂ system, as well as the compatibility of the material system with CMOS technology makes polySi an excellent candidate for use in silicon optical interconnect technology. However, such waveguides show relatively high losses of over 20 dB/cm, much larger than the 1 dB/cm which is necessary for optical interconnect technology. In this paper we present loss measurement data at a wavelength of 1.54 mm and 1.3 mm along with Transmission Electron Microscopy (TEM) data, and explain the dependence of the absorption loss on the size, structure and quality of grains and grain boundaries. Measurements in 4 mm wide strip waveguides showed that the losses in waveguides with heights of 1 mm exceeded those in guides with heights of 0.2 mm by approximately 10 dB/cm. In spite of the larger grains in the 1 mm polySi waveguides, these higher losses can be attributed to the greater overlap between the optical mode and the lossy polySi core. This hypothesis is supported by simulations showing that the ratio of the losses of the two waveguides (0.78) is the same as the ratio of the power in the cores of the respective waveguides. Furthermore, the transmission loss of fine grained polySi waveguides was more than that of coarse grained waveguides. These results, combined with previous findings showing reduction of loss by hydrogenation, indicate that one of the main mechanisms for loss in polySi waveguides is absorption at the grain boundaries. Cross sectional TEM pictures have shown that there are substantial amorphous regions at the grain boundaries as well as close to the silicon-oxide interface. We have found that an extra high temperature anneal of 1100°C crystallizes these amorphous regions and reduces the waveguide transmission losses from 20 dB/cm to 11 dB/cm, without a hydrogenation step. This transmission loss of 11 dB/cm is the lowest ever measured for a polySi on oxide waveguide. In order to study the possibility of a multilevel polySi interconnect technology, two-level polySi waveguides have been fabricated. The second level of polySi was deposited on a non-planarized layer of cladding separating it from the first layer of polySi waveguides. In spite of the significant bending in the second level waveguides, the absorption losses were comparable to those seen in similar planar waveguides indicating that bending loss is minimal. We believe that with proper engineering of grain boundaries, and the structure within the grain, the losses of the polySi waveguides can be reduced to a value that make it an attractive option for optical interconnects.

9:00 am, Student Paper

Si:Er Ion Implanted LED's for Microphotonics Applications: E. Ouellette, X. Duan, S. Ahn, M. Morse, J. Michel, L.C. Kimerling, Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave., Rm 13-4084, Cambridge, MA 02139

The electronic transition from the 4f shell of the Er³⁺ ion in Si shows a very sharp luminescence at =1.54 mm. The process compatibility of Si:Er LEDs with Si VLSI and low loss of 1.54 mm light in Si allow Si:Er LEDs to integrate seamlessly with Si and polySi waveguides and SiGe detectors, leading to affordable and reliable Si monolithic optoelectronic interconnects. We fabricated and tested Si:Er LEDs compatible with standard Si CMOS process technology. The Er ion implantation was done with 320 keV and 400 keV energies. Independently, we changed the erbium concentration from 1x10¹⁷ cm^-3 to 1x10¹⁸ cm^-3 and oxygen concentration from 2x10¹⁸ cm^-3 to 5x10¹⁸ cm^-3 at the same peak depths. Next we modified the oxygen concentration depth to compensate for oxygen out-diffusion during subsequent annealing. Samples with concentrations below 1x10¹⁸ cm^-3 Er and 5x10¹⁸ cm^-3 oxygen showed no defects in Transmission Electron Microscopy (TEM). X-Ray Diffraction (XRD) analysis showed no erbium nor oxygen precipitates even though the concentrations were above their solubility limits. In contrast, previously measured 4.5 meV samples showed significant secondary defects at the Er-peak depth. Defects are undesirable for Si:Er LEDs because they degrade carrier lifetimes and result in non-radiative recombination pathways due to extra states in the bandgap. Photoluminescence (PL) intensity significantly increased with increasing amounts of erbium and oxygen at optimal oxygen depth, slightly deeper than Er-peak depth. Electroluminescence intensity for forward-biased LEDs was in direct proportion with the PL, further verifying the Si matrix is defect-free. Current devices use a planar, top-only contact scheme to facilitate use with waveguides. Using Spreading Resistance Profiling (SRP) we measure donor concentration, and we demonstrate that PL intensity is proportional to the integrated donors. We are currently characterizing the donors associated with erbium and oxygen using Deep Level Transient Spectroscopy (DLTS) and will present a model that describes our observations. Reverse-bias studies were also performed to increase the light intensity and decrease temperature quenching. Results on waveguide LEDs incorporating all our previous knowledge including novel geometries to enhance intensities will be presented and their performance evaluated.

9:20 am, Student Paper

Photoluminescence Studies of Erbium-Doped GaAs Using a New Pyrazole and Pyridine-Based Erbium Source: T.D. Culp¹, J.G. Cederberg¹, D. Pfeiffer², C.H. Winter², K.L. Bray¹, and T.F. Kuech¹, ¹University of Wisconsin-Madison, Dept. of Chemical Engineering, Madison, WI 53706; ²Wayne State University, Dept. of Chemistry, Detroit, MI 48202

The photoluminescence (PL) properties of MOCVD GaAs:Er grown using a new source, tris(3,5-di-tert-butylpyrazolato)bis(4-tert-butylpyridine)erbium(III), were investigated. The ⁴I_13/2 ⁴ I_15/2 Er³⁺ emission consisted of a series of at least 13 sharp peaks centered around 1.54 µm. The spectrum is dominated by seven lines which have been previously associated with the Er_Ga-(O_AS)₂ center. In those studies, oxygen was intentionally co-doped using O₂ gas. No such intentional dopant was used in the present study. The main Er-related emission peak at 1538.3 nm had a linewidth of less than 0.8 cm^-1, which is a significant reduction from the linewidths observed in GaAs:Er grown with cyclopentadienyl (Cp) based sources, (t-butylCP)₃Er or (IpropylCP)₂CpEr. The latter samples exhibited a relatively broad emission with a main peak linewidth of ~40 cm^-1. Furthermore, in the samples grown with the pyrazole and pyridine-based erbium source, the PL lifetime of the main peak increased by over 50% to 1.42 ms. Differences in the thermal quenching of both the intensity and lifetime have been measured and will be discussed. Electrical characterization of the samples was also performed. The Er-doped layers were semi-insulating with carrier concentrations of ~10¹³ cm^-3, indicating the presence of compensating deep states within the bandgap. Si-co-doped samples exhibited carrier compensation as determined from CV measurements. Further growth studies were performed to optimize Er incorporation. Compensation due to Er increases with decreased growth temperature and reduced V/III ratio. Preliminary deep level transient spectroscopy (DLTS) studies suggest that the deep states responsible for this compensation are two electron traps about 0.55 and 0.63 eV below the conduction band edge. In contrast, GaAs:Er grown with the cyclopentadienyl-based precursors exhibited a hole trap at about 0.92 eV below the conduction band edge. A further complication is that co-doping with Si appears to inhibit the Er³⁺, luminescence. These effects will be discussed with the aid of further DLTS, PL, and secondary ion mass spectroscopy (SIMS) measurements.

9:40 am, Late News

10:00 am, Break

10:20 am

Observations of Delocalized Discrete Electron State Levels Above and Below the X-Conduction Minimum in Polycrystalline Si Using Optoelectronic Modulation Spectroscopy(OEMS): J.G. Swanson and S-A Hossain, King's College London, Strand, London WC2R 2LS, UK; T Liu, Jinan University, Guangzhou, China

Opto-Electronic Modulation Spectroscopy has been used to explore the electronic structure of polycrystalline silicon thin films and has revealed a set of discrete energy levels associated with photon induced transitions in the range 0.7eV to 2.3eV. In OEMS an electrical test device is irradiated with photons having a small superimposed low frequency energy modulation. The OEMS response is the magnitude and phase of the modulation of an electrical parameter due to the photon modulation. The mean energy of the photons is scanned to produce magnitude and phase spectra. In these experiments the test devices were polycrystalline silicon field effect transistors and the observed electrical parameter was the channel current when the biases were chosen to maintain a parallel conduction channel. The 100nm thick poly-Si layer was formed on a silica(APCVD) coated Si substrate by LPCVD and thermally recrystallised at 625°C. The gate insulator was a silica film, 150nm thick formed by low temperature LPCVD. The gate layer was of p⁺ polySi. Rich spectra were observed that consisted of prominent pairs of magnitude peaks. The corresponding phase spectra showed that each pair consisted of an in-phase peak followed by a higher energy peak with reversed sign. The null energy between the peaks was well defined to an accuracy +/- 2meV. The energies were independent of gate and drain bias and were seen to have a weak temperature dependence. Pairs were observed with the following null energies: 0.865, 0.980, 1.135, 1.300, 1.535, 1.730, 1.930 and 2.23OeV. Each of these features in the spectra is consistent with a discrete electron trap level interacting with the G conduction band minimum, the energy of the direct transition reveals the energy depth and allows the set of states to be indexed. The justification for this interpretation will be made in terms of a peaked optical cross-section arising from the delocalised character of the states, this is in contrast with OEMS responses from GaAs that are consistent with states that are localised.

10:40 am

A SiGe:H and a-SiGeC:H Used as Black-Matrix on Arrays for TFT-LCD's: Yoshimine Kato, D/751B, 10-208, IBM TJ Watson Center, P.O. Box 218, Route 134, Yorktown Heights, NY 10598; Y. Kaida, Y. Miyoshi, and M. Atsumi, Display Technology, IBM Japan, 1623-14 Shimotsuruma, Yamato-shi, 242 Japan

A study was made of sputtered hydrogenated-amorphous (a) -SiGe:H and a-SiGeC:H thin films, which were successfully applied to inorganic black-matrix (BM) on thin film transistor (TFT) arrays for liquid crystal displays (LCDs). BM on TFT arrays is useful for increasing the aperture ratio and reducing the surface reflectivity of LCD panels. It is interesting to note that the sheet resistances of a-SiGe:H and a-SiGeC:H films can be varied widely by changing the hydrogen concentration of the film. The deposition and annealing temperatures, and the carbon concentration also vary the sheet resistance by up to about two orders of magnitude. To obtain crosstalk-free and high contrast ratio (about 150) LCD picture quality, it was found that the sheet resistance of the BM needs to be more than 10¹⁵ W/. with an optical density (OD) of more than 2.3. The thickness of the BM should be less than about 1 mm to eliminate the edge reverse-tilt. The deposition condition was optimized and the thin-film characteristics were studied by TEM, SIMS, FR-IR, electron spin resonance (ESR), Rutherford back scattering (RBS), and forward recoil elastic scattering (FRES). The a-SiGe:H and a-SiGeC:H films were deposited on TFT arrays by reactive dc magnetron sputtering, using Si and Ge (and C) mosaic targets with a mixture of Ar and H₂ gas. The hydrogen concentration of the film was raised by increasing the H₂/Ar gas flow ratio. a-SiGe:H and a-SiGeC:H films with sheet resistances of more than 10¹⁵ W/ and ODs of about 2.3 can be obtained with a growth condition of H₂/Ar = 6. The H₂/Ar flow plays an important role in determining the film resistivity and the OD, which are related to the H content of the films. For example, the H content of the film was measured at about 10-11 at% after annealing at 180°C when a sample was grown at H₂/Ar=6. The H, Si, and Ge contents were measured by FRES and RBS as Si:Ge~l:l. In most cases, the sheet resistances of a-SiGe:H BM films dropped after two hours' annealing, which was performed at temperatures of 230°C and 180°C for samples grown at 200°C and 150°C, respectively. This seems to have been due to the desorption of hydrogen from an a-SiGe:H film according to the results of FT-IR. In contrast, the BM sheet resistances tended to increase after two hours' annealing at 180°C for samples grown at room temperature. From the results of ESR and FT-IR, it is suspected that excess H atoms in the film terminate dangling bonds more after the annealing, and suppress hopping conduction. Furthermore, adding a certain amount of carbon to a-SiGe:H film increased the resistivity of the film after annealing. According to the ESR and FT-IR results, it appears that a-SiC is formed and terminates the a-Si dangling bonds. Finally, a-SiGeC:H film sheet resistances as high as 7x10¹⁵ ­ 7x10¹⁶ W/ were achieved after annealing at 180°C. The C content was measured to be about 2-3 at% by nuclear reaction analysis using the 12C(d,p)13C reaction.

11:00 am

Thin Film Transistors of Microcrystalline Silicon Deposited at 290°C: Yu Chen and S. Wagner, Department of Electrical Engineering, Princeton University, Princeton, NJ 08544

Microcrystalline silicon (mc-Si) is expected to provide thin-film transistors (TFTS) with higher mobility than amorphous silicon deposited at the same temperature. Another advantage of mc-Si is that it may support both n and p channel TFTs. However, the structure of mc-Si changes with film thickness, from amorphous near the substrate to increasingly coarse grains. Correspondingly, the electron conductivities of doped and undoped mc-Si films rise with film thickness. To explore the electron transport in the very top layer of mc-Si films, we made and evaluated top-gate TFTs on films with a range of thicknesses. The TFTs have the structure (1) 100 nm Cr and 40 nm n⁺ a-Si:H source/drain contact layers (2) 50 to 250 nm thick mc-Si (3) 400 nm of SiN gate insulator, and (4) 100 nm Al gate electrode. The channel length is 2 to 12 mm. The mc-Si is deposited by plasma-enhanced CVD at 290°C substrate temperature at a gas flow of 10sccm SiF₄, 1 sccm SiH₄ and 50 sccm H₂. The SiN also is deposited by PECVD. The characteristics of a first series of transistors clearly reflect a change in the surface properties as the mc-Si film thickness rises from 50 to 250 nm. The OFF current I_OFF rises from 10^-12 to 10^-9 A per mm gate width, and the saturated electron mobility m_e rises from 0.05 to 1.2 cm²/Vs. Thus the data already show the dependence on film thickness, although the low m_e reflect an immature mc-Si top gate TFT technology. We are improving our transistors with the goal of providing a profile of electron density (from I_OFF) and electron mobility throughout the film.

11:20 am

Crystallographic Defects in Thermally Oxidized Wafer Bonded Silicon on Insulator (SOI) Substrates: L.F. Giles and Y. Kunii, NTT Basic Research Laboratories, 3-1 Morinosato Wakamiya, Atsugi, Kanagawa, 243-01, Japan

Silicon-on-insulator (SOI) materials produced by the wafer bonding (WB) process offer the advantages of very flat Si/SiO₂ interfaces and good crystalline quality. This feature will be useful for the fabrication of ultra-thin SOI quantum devices which require extremely flat Si/SiO₂ interfaces. Since the most common WB SOI substrates have a silicon overlayer thickness larger than a few hundred nanometers, a common route to produce ultra thin silicon overlayers (< 10 nm) is to thermally oxidize the top silicon layer down to the required thickness. However, this method presents some drawbacks since it has been previously reported that the thermal oxidation step deteriorates the crystalline quality of the top silicon layer. Indeed, it has been recently reported that in SOI substrates fabricated by the oxygen implantation process (SIMOX) Oxidation Induced Stacking Faults (OISFs) may be introduced during the oxidation step. The aim of this work is to probe the crystalline quality of oxidized and non oxidized WB SOI substrates in order to determine the effect of the thermal oxidation process on the formation of crystallographic defects. For this purpose WB SOI substrates produced by high dose H⁺ implantation and the bonding/splitting process have been investigated bv means of plan view transmission electron microscopy (PVTEM) , cross-section transmission electron microscopy (XTEM) and chemical defect etching. It has been found that the etch pit density in the top silicon layer varies from 2x10⁴ cm^-2 to 20x10⁴ cm^-2. Furthermore, PVTEM and XTEM analyses have shown the existence of small dislocation loops and SiO_x precipitates (< 50 nm) in the top silicon layer. The crystalline quality of the SOI wafers submitted to thermal oxidations in 100% oxygen ambients was also assessed. Defect etching results have shown that oxidation induced stacking faults (OISFs) with a density of approximately 10³ cm^-2 are formed in the silicon overlayer. Several aspects concerning the nucleation growth and retrogrowth of OISFs in SOI wafers will be reported. Further more, we will discuss methods to optimise the thermal oxidation process in order to reduce and eliminate the formation of OISFs.

11:40 am, Late News

8:20 am, Invited

InGaN-based Blue Semiconductor Laser Diodes: S. Nakamura, Department of Research and Development, Nichia Chemical Industries, Ltd., 491 Oka, Kaminaka, Anan, Tokushima 774, Japan

Major developments in wide-gap III-V nitride semiconductors have recently led to the commercial production of high-brightness blue/green light-emitting diodes (LEDs) and to the demonstration of room-temperature (RT) violet laser light emission in InGaN/GaN/AlGaN-based heterostructures under pulsed currents and continuous-wave (CW) operation. Recombination of localized excitons was proposed as an emission mechanism for the spontaneous emission of the InGaN quantum-well-structure LEDs and LDs. The radiative recombination of the spontaneous and stimulated emission of the InGaN MQW LEDs and LDs was attributed to excitons (or carriers) localized at deep traps (250 meV) which originated from the In-rich region in the InGaN wells acting as quantum dots. The fundamental properties of semiconductor lasers are specified by the optical gain. However, experimental data regarding the optical gain of RT CW-operated III-V nitride-based LDs have not been reported. Recently, RT CW operation of the InGaN MQW LDs with a lifetime of 35 hours has been achieved. Using these RT CW-operated LDs, it is interesting to measure the characteristics of the LDs in detail especially those of the emission mechanism. Here, we report the optical gain and the emission characteristics of InGaN single-quantum-well (SQW) LEDs and MQW LDs. Photocurrent spectra of the InGaN SQW LEDs and MQW LDs were measured at RT. The Stokes shifts of the energy difference between the absorption and the emission energy of the blue/green InGaN SQW LEDs and MQW LDs were 290, 570 and 190 meV, respectively. Both spontaneous and stimulated emission (416 nm) of the LDs originated from this deep localized energy state which is equivalent to a quantum dot-like state. The emission spectra of the LDs under RT CW operation showed periodic subband emissions with an energy separation of 2-5 meV which were different from a longitudinal mode. When the temperature or the operating current of the LDs was varied, a mode hopping of the emission wavelength between these periodic subband emissions was observed. The carrier lifetime and the threshold carrier density were estimated to be 2-10 ns and 1 x 10²⁰/cm3, respectively. For the measurement of the gain spectra of the LDs, the Hakki-Paoli technique was used. The differential gain coefficient, the transparent carrier density, the threshold gain and the intrinsic loss were estimated to be 2.7x10^-17 cm², l.8x10¹⁹ cm^-3, 2200 cm^-1 and 33 cm^-1, respectively, from the measurement of the gain spectra.

9:00 am, Student Paper

Characterization of Metal Contacts on N-type GaN: A.C. Schmitz, A.T. Ping and I. Adesida, Center for Compound Semiconductor Microelectronics and Department of Electrical and Computer Engineering, University of Illinois, Urbana-Champaign, IL 61801; M.A. Khan and Q. Chen, APA Optics, Inc., 2950 NE 84th Lane, Blaine, MN 55449

The III-V nitrides have recently attracted considerable attention for their application in short wavelength optoelectronic devices. In addition, the nitrides are also appealing because their chemical/thermal stability and wide energy gaps make these materials suitable for high power and high temperature electronic devices. Electronic devices such as heterostructure field effect transistors (HFET) [1] and heterostructure bipolar transistors (HBT) [2] have been demonstrated to operate at temperatures of up to 300°C. However, in order to realize the potential of the III-V nitrides for transistor applications, high quality Schottky and ohmic contacts are required. To date, no one comprehensive work has encompassed a wide range of contact metals. Comparing contact characteristics from different authors can be difficult due to varying surface preparation, GaN quality and carrier concentration, deposition method, etc. It is thus necessary to characterize metal contacts on identical GaN in order to compare their characteristics directly. In this work, contacts of numerous metals (W, Zr, Sc, V, Al, Nb, Hf, Mo, Cr, Ti, Ag, Cu, Au, Pd, Ni, and Pt) on n-GaN have been formed and characterized. Six of the metals studied (Ag, Cu, Au, Pd, Ni, and Pt) displayed strong rectifying (Schottky) characteristics. The Schottky barrier heights for these metals have been determined using current-voltage (I-V), current-voltage-temperature (I-V-T), and capacitance-voltage (C-V) measurement techniques. The ideality factor and effective Richardson constant were also determined for contacts of each of the rectifying metals. Barrier height on n-GaN shows dependence upon the metal work function, probably due to the ionic nature of GaN. The contacts formed with the other metals displayed either weak Schottky characteristics or ohmic behavior (linear). Schottky parameters were not extracted for metal contacts displaying only weak Schottky. The contact resistance was determined for metal contacts which displayed linear current-voltage relations.

9:20 am, Student Paper

Study of Specific Contact Resistance, Mechanical Integrity, and Thermal Stability of Ti/Al and Ta/Al Ohmic Contacts to N-type GaN: B.P. Luther, Department of Electrical Engineering, Penn State University, 121 E.E. East, University Park, PA 16802; S.E. Mohney, Department of Materials Science and Engineering, Penn State University, University Park, PA 16802; R.F. Karlicek, Jr., EMCORE Corp., Somerset, NJ 08873

Ohmic contacts with Ti/Al layers (typically 25nm Ti / 100nm Al) are widely used on n-type GaN. We have previously reported that Ti/Al contacts, annealed between 400 and 600°C, become ohmic only after Al diffuses through the Ti and reaches the GaN surface. This study suggested that the mechanism for ohmic contact formation after annealing at moderate temperatures could involve Ti reducing any oxide contamination present at the original GaN surface, followed by the formation of a low work function Al-Ti intermetallic at the contact/GaN interface. We now report results for Ta/Al contacts on n-type GaN and the thermal stability of Ti/Al and Ta/Al contacts aged between 300 and 600°C. Ti/Al and Ta/Al (35nm/115nm) contacts on Si-doped GaN (7x10¹⁷cm^-3) reached minimum specific contact resistances of 8x10^-6Wcm² and 3x10⁵Wcm², respectively, after being annealed in Ar for 15sec at 600°C. Ta/Al ohmic contacts on n-GaN have displayed better mechanical integrity than Ti/Al contacts. Ti/Al contacts are easily scraped away by tungsten probes during electrical testing, exposing the underlying GaN. Ta/Al contacts, however, are not scratched by tungsten probes during routine testing. The higher apparent contact resistance of the Ta/Al contacts appears to be due to higher contact metal sheet resistance, which increased by a factor of 6 after annealing. The use of a low resistivity capping layer to decrease the overall metal sheet resistance is now under investigation. Ti/Al and Ta/Al contacts to n-GaN were aged to test their long term thermal stability. Samples were encapsulated in quartz tubes in a nitrogen atmosphere. Aging at 300°C for 15 days and 400°C for 9 days caused no measurable increase in the contact resistivity of either Ti/Al or Ta/Al contacts. This would be expected for contacts which do not react with GaN at these temperatures. After aging at 500°C for 6 days both contacts showed an increase in specific contact resistance of approximately 25%. Aging at 600°C for 5 days resulted in drastically increased contact resistance for both contacts. The metal contact layers exhibited increased sheet resistance and surface roughness and a phase segregated microstructure after aging at 600°C. These results will be discussed along with the findings from an ongoing investigation of the extent of metallurgical reaction between GaN and the contact metals after aging.

9:40 am

The Effects of Reactive Ion Etching-Induced Damage on the Characteristics of Metal/n-GaN Ohmic Contacts: A.T. Ping and I. Adesida, Microelectronics Laboratory and Department of Electrical and Computer Engineering, University of Illinois, Urbana-Champaign, IL 61801; M.A. Khan and Q. Chen, APA Optics, Inc., 2950 NE 84th Lane, Blaine, MN 55449

Dry etching techniques are a necessity for reliable pattern definition in the III-V nitrides due to the chemical stability of these materials against wet chemical solutions. Reports thus far have concentrated on characterizing the etch rates, etch profiles, and changes in surface stoichiometry and morphology of various etching techniques. However, little work has been done to determine how the optical and electrical properties of the material will be affected as a result of dry etching. Hall measurements have showed that the sheet resistance of InN, InGaN, and InAlN increases after etching in Ar under both electron cyclotron resonance and conventional RIE. Schottky diodes fabricated on etched surfaces exhibited severe degradation in the barrier height even under low bias conditions (-150 V). An area that requires study is the effect dry etching has on the resistance of ohmic contacts. This is especially important in the fabrication of devices where the epilayer has been grown on sapphire. Dry etching is required to access layers such as the base and collector in heterostructure bipolar transistors or the bottom n⁺-GaN layer in laser diodes, in order for ohmic contacts to be formed. Fan et al. have shown that reactive ion etching the GaN surface prior to Ti/Al/Ni/Au metallization improves the contact resistance. However, little work has been done to systematically characterize the effects of dry etch-induced damage on ohmic contacts. In this paper, we will present the electrical characteristics of n-type GaN surfaces etched with reactive ion etching. Reactive ion etching was performed in SiCl₄ and Ar plasmas. Transmission line structures were used with Al and Ti/Al metals to study the contact resistance, specific contact resistance, and sheet resistance of etched samples as a function of DC plasma self-bias voltage and etch time. Etching with SiCl₄ was found to improve the ohmic contact characteristics under all conditions investigated. The damage induced by an Ar plasma was found to degrade the contact resistance. Auger electron spectroscopy and x-ray photoelectron spectroscopy were used to investigate the state of etched surfaces. Rapid thermal annealing was also used to investigate the thermal stability of induced defects.

10:00 am, Break

10:20 am

Dopant-Selective Photoelectrochemical Etching of GaN: C. Youtsey, I. Adesida, Microelectronics Laboratory and Department of Electrical and Computer Engineering, University of Illinois, Urbana-Champaign, IL 61801; G. Bulman, CREE Research, Inc., Durham, NC 27713

As GaN-based electronic and optoelectronic devices are currently making rapid progress towards commercial realization, there is a growing need for effective processing methods within this material system. Controlled wet and dry etching techniques are essential for many aspects of device fabrication. Good progress has been made using dry etching methods with a variety of gas chemistries, but the unusual chemical stability of the group III nitrides have rendered them resistant to conventional wet etching approaches. In particular, no etchants have been reported for GaN that provide etch rates greater than several tens of Å/min. However, Minsky et al. have recently demonstrated the photoenhanced etching of unintentionally doped GaN layers using a HeCd laser (325 nm) with HCl and KOH solutions. Etch rates of up to several thousand Å/min were obtained using a laser power of 4.5 mW. We have conducted experiments using broad-area Hg lamp illumination (~5-10 mW/cm² at 365 nm) and KOH solutions to etch unintentionally-doped (n~1x10¹⁶ cm^-3), n+ GaN (n~1x10¹⁸ cm^-3), and p-type GaN (p~1xl0¹⁷ cm^-3) samples. Etch rates of 200-250 Å/min resulted for the n-type samples, while no observable etching took place for the p-type material. In this work we describe a dopant-selective photoelectrochemical etch process for GaN homostructures using Hg lamp illumination and KOH solution. The photoenhanced etching is believed to occur through the photogeneration of electrons and holes which enhance the oxidation and reduction reactions in an electrochemical cell. Dopant-selective photoelectrochemical etching has previously been described for GaAs. Etch rates of the semiconductor are dependent upon the confinement of the photogenerated holes at the semiconductor/electrolyte interface. The different "surface band bending" occurring in n-type and p-type materials gives rise to the greatly differing etch characteristics of these materials. A highly selective, dopant-dependent etch process holds much interest for etching of layered semiconductor structures used in GaN-based devices. We will discuss the degree of selectivity that is achievable and present our results for the etching of GaN homostructures.

10:40 am, Student Paper

Absorption Coefficient, Excitonic Structure and Band Gap of Gallium Nitride at Room Temperature Using Optical Transmission Measurements: J.F. Muth, I.K. Shmagin and R.M. Kolbas, Electrical and Computer Engineering Department, North Carolina State University, Raleigh, NC 27695-7911; H.C. Casey, Department of Electrical and Computer Engineering, Duke University, NC; P. Fini, S. Keller and S.P. DenBaars, Electrical and Computer and Material Departments, University of California, Santa Barbara, CA 93106

The absorption coefficient and index of refraction of a 0.4 µm sample of gallium nitride were determined from transmission and reflectance measurements at room temperature. The spectral data was obtained from 200 nm to 3.3 mm with a Cary 5 spectrophotometer. Photoluminescence data was taken with a frequency tripled Ti:Sapphire laser as the excitation source. The sample, grown by atmospheric pressure MOCVD, was very uniform in thickness and of high quality. The sample was n-type with a carrier concentration of 1xl0¹⁷ cm^-3 and a mobility of 450 cm²/V-sec. Near the energy gap the absorption spectrum shows a strong well defined exciton peak from the A and B excitons. A change in slope reveals the C exciton and an absorption peak due to phonons is observed. The energy gap and exciton binding energy were determined by analysis of the absorption coefficient above the band edge using Elliott's theory of optical absorption. This theory includes the Coulomb interaction of electrons and holes that are produced and the subsequent formation of excitons. Elliott's theory provides a reliable way of determining the band gap that is not influenced by the exciton absorption at the absorption edge or deep level impurities. Photoluminescence data was taken and compared with absorption data through the Van Roosbroeck - Shockley relation to investigate the thermal equilibrium recombination rate and radiative lifetime of electron-hole pairs. The binding energy of the excitons was found to be 21+/-1 meV, the band gap 3.43+/-0.01 eV.

11:00 am, Student Paper

Effect of Threading Dislocations on the Electron Mobility in GaN: N.B. Weimann and L.F. Eastman, School of Electrical Engineering, Cornell University, Ithaca, NY 14853

Planar GaN devices to date suffer from low effective device mobility. We propose a model that explains the observed low transverse electron mobilities in GaN by scattering of electrons at charged dislocation lines. Epitaxial growth of GaN on Sapphire and 4H-SiC results in a lattice mismatch of 16% and 3.5%, respectively. TEM investigations of hexagonal GaN films grown by MOCVD on sapphire have shown dislocations densities in the range of 10¹⁰cm^-2. These dislocations are shown to be threading from the substrate-epi interface perpendicular to the sample surface. Photoluminescence (PL) analysis shows an emission band at 2.2eV with a FWHM of .4eV. By DLTS, the states involved in the transition have been identified as a shallow donor level and an acceptor-like deep trap level at 1eV above the valence band. A point defect such as a Nitrogen vacancy V_N would lead to a sharp luminescence peak in the PL spectrum. The observed finite PL band width can be explained by overlapping wavefunctions at adjacent trap sites. If the acceptor-like trap level is identified with acceptor states at dangling bonds at the edge dislocation, overlap of the localized wavefunctions on neighbouring trap sites is possible. The spatial distribution of the yellow luminescence of samples showing columnar growth has been correlated to the morphology of the samples. The fraction of filled traps along a dislocation line was first calculated for threading dislocations in Germanium. Following the minimum energy approach developed by Read, the fraction of filled traps in GaN approaches unity for n-doping densities of 10¹⁸cm^-3. In this model, the fraction of filled traps is independent of the dislocation density. It is proposed that the charged dislocation lines act as Coulomb scattering centers for the remaining free electrons moving in a direction perpendicular to the dislocation lines. The resulting transverse mobility component has been calculated using Pödör's approach for scattering at a charged line. The combined mobility due to scattering at charged dislocation lines and ionized impurities calculated using the Conwell-Weisskopf formula shows a maximum transverse mobility at a doping density of 10¹⁸cm^-3 for a dislocation density of 10¹⁰cm^-2. Dislocation scattering occurs only for carrier motion perpendicular to the dislocation line, i.e. parallel to the substrate surface. Vertical devices with current flow parallel to the dislocation lines (LED's¹, lasers and vertical transistors) are only affected by trapping of the free carriers, but not by scattering at charged dislocation lines.

11:20 am, Late News

11:40 am, Late News

8:20 am, Student Paper

The Structural Stability of InAsSb/InAs Strained-Layer Superlattices: S.C. Theiring, M.R. Pillai, B.W. Wessels and S.A. Barnett, 2225 N. Campus Drive, Department of Materials Science and Engineering and Materials Research Center, Northwestern University, Evanston, IL 60208

Bi-axially compressed InAs_xSb_1-x/InAs strained-layer superlattices with alloy compositions ranging from 0.35 < x < 0.85 have been grown. The superlattices were prepared by metalorganic vapor phase epitaxy (MOVPE), and deposited on InAs (111)A substrates. (111)-oriented superlattices were grown in order to test recent theoretical predictions on band structure and orientation. The morphological and structural stability of these superlattice films has been determined as a function of composition and deposition rate. The quality of the superlattice structures was assessed by double crystal x-ray diffraction. Computer simulations of the x-ray patterns were compared with the experimental results. The simulations took into account degradation of the x-ray superlattice structure resulting from interface roughness, compositional broadening at the interfaces, and d-spacing fluctuations. The deposition rates studied ranged between 3 and 60 nm/min., while total thicknesses were 50 to 200 nm. The x-ray diffraction scan showed up to eight orders of satellite peaks for an InAs_xSb_1-x/InAs superlattice with x=0.82, and a period of 170Å. Good agreement between the computer simulation and the experimental results was achieved, and indicated that the rms interface roughnesses of the higher quality structures were between 2 and 8 Å, and that the interfaces extended over 30 to 45 Å, suggesting interface segregation. Both the interface roughness and the interfacial broadness of the superlattice structures increased with increasing strain, as determined by x-ray diffraction analysis. High quality structures with multiple orders of superlattice peaks were only possible with a ternary layer of x > 0.7. The surface roughness, as determined by atomic force microscopy, increased with increasing antimony concentration, which is consistent with x-ray diffraction measurements.

8:40 am

Metal-Organic Chemical Vapor Deposition Growth of InAsSb/InAsP Strain Layer Superlattices for use in Infrared Emitters: A.A. Allerman, R.M. Biefeld, S.R. Kurtz and K.C. Baucom, Sandia National Laboratory, Dept. 1113, P.O. Box 5800, MS 0601, Albuquerque, NM, 87185-0601

We are developing mid-infrared (3-6 mm) laser and light emitting diodes for use in chemical sensor systems. Previously we have reported electrically injected diode lasers which operated to 210K and emitted at 3.8-3.9 mm using a strained InAsSb/InAs superlattice active region. We have been developing InAsSb/InAsP SLS's to achieve higher temperature device operation. The greater separation in the light hole and heavy hole states in these SLS's should reduce non-radiative Auger recombination. InAsSb/InAs SLS structures were grown at 500C and 200 torr in a conventional horizontal system using TMIn, TESb, AsH₃ and PH₃. A purge time of 20 seconds was used between layers. The highly crystalline quality of a SLS lattice matched to InAs is confirmed by XRD where sharp satellite peaks are observed. Low temperature photoluminescence was observed from 3.2 to 4.4 mm by changing the InAsSb layer thickness between 45 to 105 Å and the Sb mole fraction between 0.14 and 0.21. Room temperature photoluminescence was observed to 5.0 mm. Single and double heterostructure devices have been fabricated using InAsSb/InAsP SLS's as the active region with AlAsSb cladding layers. Surface morphology was improved by first growing a lattice matched InAsSb/InAsP SLS buffer. Stimulated emission was observed at 3.86 mm (240K) with optical pumping. Broad band, room temperature LED emission was observed from both conventional p-n junction structures and devices using a p-GaAsSb/n-InAs semi-metal interface for electron injection. This work was supported by the US DOE under Contract DE-AC04-94AL85000.

9:00 am

Mid-Infrared Lasers and LED's with MOCVD-Grown, InAsSb/InAsP Strained Layer Superlattice Active Regions: S.R. Kurtz, A.A. Allerman, and R.M. Biefeld, Sandia National Laboratory, Dept. 1113, P.O. Box 5800, MS 0601, Albuquerque, NM, 87185-0601

With the prospect of reduced Auger rates in compressively strained heterostructures, midwave infrared (3-5 mm) lasers with strained InAsSb active regions are attracting much interest. In this work, we report the properties of the first InAsSb/InAsP strained-layer superlattice (SLS) materials and devices. The InAsSb/InAsP SLS is an "MOCVD variant" of the most promising type I, strained InAsSb heterostructures for Auger supression, and a miscibility gap, reported for InAlAsSb quaternaries, has not been encountered for InAsP. Compared with other compressively strained InAsSb devices, initial tests on InAsSb/InAsP SLS lasers and LEDs show state-of-the-art performance. Band structure calculations for InAsSb/InAsP SLSs indicate that large light-heavy hole splittings ( 70 meV) can be achieved to suppress Auger recombination. InAsSb/InAsP SLSs and devices are grown by MOCVD. X-ray and optical characterization of the SLSs indicate very high crystalline quality for the MOCVD-grown material. Photoluminescence and magneto-photoluminescence characterization of these SLSs are consistent with previous studies of type I, InAsSb/InGaAs and InAsSb/InAs heterostructures. These studies support predictions of large light-heavy hole splittings in InAsSb/InAsP SLSs. Excellent performance was observed for SLS LEDs and optially pumped SLS lasers. A semi-metal injected, broadband LED emitted at 4 mm with 80 mW of power at 300K, 200 mA average current. The laser displayed 3.86 mm emission at 240K, the maximum operating temperature of the laser, and a characteristic temperature of 33K. We report the lowest threshold power, highest characteristic temperature, and highest operating temperature for InAsSb lasers at 3.9 mm, obtained either with pulsed injection or pulsed optical pumping. Studies in progress of injection InAsSb/InAsP SLS lasers will also be discussed. This work was supported by the US DOE under contract DE-AC04-94AL85000.

9:20 am

Structural and Electrical Characterization of AlAsSb-Barriers Grown by Molecular Beam Epitaxy at Low Substrate Temperatures: H.-R. Blank, S. Mathis, H. Kroemer and J. Speck, University of California at Santa Barbara, CA 93106

In the field of III-V semiconductors, AlAs_xSb_1-x represents a high-bandgap material that can be grown lattice-matched to various technologically important substrates, such as GaAs, InP or InAs. In contrast to dual-cation compounds such as Al_xGa_1-xAs or In_xGa_1-xAs, the growth of layers in the dual-anion system As_xSb_1-x is complicated by non-unity sticking coefficients of Sb and As and by a pronounced exchange of Sb by As on the growth surface, where both effects depend strongly on the substrate temperature T_s. In the present work we studied structural and electrical properties of AlAs_xSb_1-x layers grown by molecular beam epitaxy (MBE) at low substrate temperatures on top of InAs on GaAs (100) substrates. Keeping the MBE-growth only slightly group-V stablethe total group-V flux was kept constant for all growthswe were able to establish epitaxial growth for AlAs_xSb_1-x for at least 100nm at substrate temperatures as low as 275C. The As/Sb-flux ratio was varied over a wide range in different growths. The actual composition in the AlAs_xSb_1-x-compound was determined by X-ray measurements for all samples. Even a very small fraction of As₂-flux (3%) resulted in a significantly higher incorporation of As in the compound (x=0.19). Interestly, the As-incorporation above about 3% of the total group-V flux was only weakly enhanced by increasing the As-flux, e.g., a 50% As₂-flux resulted only in a 21% incorporation of As into the low temperature grown (LTG) layer. Using transmission electron microscopy (TEM) the single crystal quality of LTG-AlAs_xSb_1-x was verified and the formation of precipitates was observed. We believe that the precipitates consist mostly of Sb and that the As is mostly incorporated in the regular lattice. We also studied the electrical transport in InAs QW's with AlSb bottom barriers and LTG-AlAs_xSb_1-x top barriers for different x, as well as the transport through InAs/AlAs_xSb_1-x/InAs double heterojunctions. To obtain the transport properties in the QW's we performed temperature dependent Hall-measurements. Compared to samples in which the AlAs_xSb_1-x-top barriers were grown at conventional substrate temperatures (T_s=500C), we measured an increase in the electron concentration n in the QW's by about one order of magnitude when the top barrier was grown at low substrate temperature. The weak temperature dependence of n indicates the formation of deep donors in the LTG-layer. The vertical transport through the InAs/AlAs_xSb_1-x/InAs double heterojunction was strongly affected by adding As to the AlAs_xSb_1-x compound: the more As was added to the LTG-layer the more the current was reduced through the heterojunction, e.g., in a AlAs_0.21Sb_0.79-layer the current through the junction was reduced by about three orders of magnitude compared to AlSb-barriers, indicating the transition from a staggered to a straddled band line up. The current through the heterojunction was only little affected by the low temperature growth, although the specific resistance of the AlAs_xSb_1-x-layer jumped up from about 500Wcm when grown at normal T_s to about 3kWcm for the LTG-compound. Initial annealing studies of LTG-AlAs_xSb_1-x up to 500C only exhibited very little effect on the transport and the structural properties of the LTG-layer.

9:40 am

InAs/Ga(In)Sb Heterostructure for Type-II Quantum Well and Quantum Cascade Lasers: D. Zhang, C.-H. Lin and S.S. Pei, Space Vacuum Epitaxy Center, University of Houston, Houston, TX 77094-5507; J.R. Harper and M.B. Weimer, Department of Physics, Texas A&M University, College Station, TX 77843-4242

Recently, we demonstrated the first type-II quantum cascade (QC) laser based on InAs/Ga(In)Sb quantum wells. The type-II QC lasers combine the advantages of the cascade design of the InGaAs/InAlAs intersubband QC laser and the interband transitions of the conventional diode lasers. Stimulated emission at 4 mm was observed at temperatures above 170 K. Room temperature cw type-II quantum cascade light emitting diodes (LEDs) with emitting wavelengths up to 4.2 mm and 140 mW output power have also been demonstrated. For the more conventional type-II QW lasers, a peak output power of 270 mW at ambient temperature and over 2.2 W at 200 K have also been realized when pumped with optical pulses. The performance of these mid-IR light sources clearly demonstrated the advantages of InAs/Ga(In)Sb type-II QW design. However, the electron and hole wavefunctions in the type-II QWs are primarily confined in different layers. The strength of the optical oscillator depends strongly on the overlap of these wavefunctions and the quality of the InAs/Ga(In)Sb interfaces. We used double crystal X-ray diffractometry (DCXRD), reflection high energy electron diffraction (RHEED), and cross sectional scanning tunneling microscopy (XSTM) to study the quality of the epitaxy layers and interfaces. In particular, the XSTM can probe not only the cross contamination between InAs and GaSb layers and the associated interface roughness, but can also simultaneously acquire filled-and empty-state images. Additionally, scanning tunneling spectroscopy can directly probe the overlap of electron and hole wavefunctions associated with adjoining layers. The impact of MBE growth conditions and the material and interface quality on the performance of type-II quantum well and quantum cascade lasers will be discussed.

10:00 am, Break

10:20 am

Molecular Beam Epitaxy of AlGaAsSb/InGaAsSb/GaSb Mid-Infrared High-Power Separate-Confinement Quantum-Well Lasers: H. Lee, D.Z. Garbuzov, R.U. Martinelli, R.J. Menna and J.C. Connolly, David Sarnoff Research Center, CN 5300, Princeton, NJ 08543-5300

In this work, we will present the MBE growth of AlGaAsSb/InGaAsSb quantum-well lasers emitting at 2 mm. A record-low pulsed threshold current density of 115A/cm² has been achieved for broad-area separate-confinement single-quantum-well AlGaAsSb/InGaAsSb lasers emitting at 2 mm. The high CW differential efficiency of 53% of these lasers results in record-high CW output power of 1.9 W at 15C and quasi-CW output power of 4 W at 10C from 200-mm-aperture lasers. The separate-confinement single-or multiple-quantum-well lasers, grown on n-type Te-doped (100) GaSb substrates, comprise ~1% compressively strained In_0.14Ga_0.86AsSb wells, lattice-matched Al_0.25Ga_0.75As_0.02 Sb_0.98 barriers and waveguides, and lattice-matched Al_0.9Ga_0.1As_0.07Sb _0.93 cladding layers. Lattice-matched grading regions between the n-GaSb buffer and p-GaSb cap layer and Al_0.9Ga_0.1As_0.07Sb _0.93 cladding layers are implemented using GaSb/AlAs_0.08Sb_0.92 superlattices. The grading regions, which eliminate the sharp heterojunctions, are crucial to the laser performance. We shall discuss the details of the MBE growth and the performance of these high-power lasers.

10:40 am

Kinetic Surface Phase Diagrams for the MBE Growth of GaSb, InAs, and AlSb: D.H. Tomich, J.K. Patterson, T.W. Haas, M.L. Seaford, K.G. Eyink, and W.V. Lampert, Materials Directorate/ Wright Laboratory, Wright Patterson AFB, OH 45433-7750; C.W. Tu, Department of Electrical and Computer Engineering, University of California, San Diego, La Jolla, CA 92093-0407

Kinetic surface phase diagrams can be useful tools in the quest for optimum MBE growth conditions. In this work, we report the kinetic surface phase diagrams for experimental parameters appropriate to the growth of epitaxial films of Gasb, InAs, and AlSb. These materials are of interest as components of resonant tunneling diodes and as layers in strained layer superlattices such as the InGaSb/InAs system of interest for mid range IR lasers and for long wavelength IR detectors. Surface phase diagrams were generated using data taken in a Varian Gen II Modular solid source MBE machine using a valved As cracker and a non-valved Sb cracker source and were taken for growth rates from 0.5 to 1 monolayer/second. The results for both Sb containing films were similar and showed a near horizontal line demarcating the metal rich from the Sb rich surface reconstructions. For AlSb, the data covered a substrate temperature range of 450-700° C and Sb BEP's from le-7 Torr to le-6 Torr. For GaSb the data covered substrate temperatures from 400 to 550°C and a similar range of Sb BEP's. These parameters cover the ranges over which the (1x3) Sb rich reconstruction in AlSb converts to the (4x2) Al rich structure and for the (1x3) Sb rich reconstruction for GaSb to the (4x2) Al rich structure. The kinetic surface phase diagrams show that the most important parameter governing the surface reconstruction over the growth conditions of most interest is the Sb BEP and not the substate temperature. The case of InAs is different. Over a range of substrate temperatures from 450 to 530° C and for As BEPs from 1e-6 Torr to 1.4e-5 Torr, the line dividing the (2x4) As rich phase from the (4x2) In rich phase shows a near parabolic shape. This indicates that substrate temperatures are at least as important as the As BEP in determining surface reconstructions. Plotted on these various kinetic surface phase diagrams will be the mobilities of a number of films grown in various regions of these diagrams. Other experimental parameters such as haze, x-ray rocking curve line widths, and surface morphologies as determined by AFM will be presented as measures of film quality. For InAs films these data show that the best results were obtained for films grown near the transition from the (2x4) As reconstruction to the (4x2) In reconstruction. For the Sb films, growths were best performed in the Sb rich (1x3) reconstructions but other trends are not as clear.

11:00 am

Monolayer Control in Molecular Beam Epitaxy of (In,Ga,Al)As/(In,Ga,Al)Sb Heterostructures: B.R. Bennett, B.V. Shanabrook and M.E. Twigg, Naval Research Laboratory, Code 6874, 4555 Overlook Ave., Washington, DC 20375-5347

A variety of important device structures such as IR lasers and detectors, field-effect transistors, and resonant-tunneling transistors include heterojunctions between arsenides and antimonides. During growth of these structures, intermixing may occur at the interfaces, resulting in degraded device performance. In this study, we explore the effect of growth parameters on the properties of III-V As/Sb heterostructures. Samples were grown by solid-source molecular beam epitaxy (MBE) on GaAs(001) substrates. Cracked sources were used for the group IV elements, allowing comparisons of As₂/As₄ and Sb₂/Sb₄. Samples were characterized by x-ray diffraction, reflection high-energy electron diffraction, transmission electron microscopy, and Raman spectroscopy. One set of heterostructures was grown with the substitution of As monolayers (MLs) for Sb MLs after growth of an appropriate strain-relaxed buffer layer. For example, a 1.0 mm buffer layer of GaSb was grown on GaAs, followed by 40 periods of: (14s GaSb/3 s Sb/2 s Ga/7 s As/2 s Ga) with a cation growth of 0.50 ML/s. The resulting nominal structure is a superlattice (SL): 40 x (8 ML GaSb/1 ML GaAs). X-ray diffraction measurements reveal satellite peaks, confirming the existence of an SL, for growth temperatures of 400-450C and As₂. At temperatures of 480-520C, however, the SL peaks disappear, suggesting severe intermixing. When As₂ is replaced by As₄, SL peaks are observed at 480C. Raman spectroscopy measurements of GaSb:As are in agreement with the x-ray results, revealing a strong GaAs-like planar vibrational mode from samples grown at 400C. At 500C, Raman scattering suggests the formation of a GaAsSb alloy. Structures of As monolayers in AlSb and InSb were also investigated as a function of growth temperature. In the case of AlSb:As, x-ray SL peaks were observed in the entire growth temperature range of 400-580C. For InSb:As, SL peaks are present for growth at 325C but not at higher temperatures. The second set of samples was the inverse of the first: monolayers of Sb were substituted for every ninth ML of As in GaAs, AlAs, and InAs. X-ray SL satellite peaks are observed for all three structures at a growth temperature of 400C. The peak positions are consistent with coherent structures and 0.8-1.1 ML Sb per period. As growth temperature is increased, the satellite peaks remain but the amount of Sb incorporation decreases. These results are consistent with evaporation of Sb from the surface at the higher temperatures. The findings of this study have important implications for interface formation in several device heterostructures. For example, short-period InAs/GaSb SLs are being investigated for use as infrared detectors. The formation of GaAs-like interface bonds requires exposing a GaSb surface to an As flux. Our results suggest that substantial intermixing may occur at growth temperatures above 450C.

11:20 am

Morphology and Defect Structure of GaSb Islands on GaAs Grown by Metalorganic Vapour Phase Epitaxy: J.-H. Kim, T.-Y. Seong, Department of Materials Science and Engineering, Kwangju Institute of Science and Technology (K-JIST), Kwangju 506-712, Korea; N.J. Mason and P.J. Walker, Clarendon Laboratory, Department of Physics, University of Oxford, Oxford, OX1 3PA, UK

Transmission electron microscopy (TEM), high resolution electron microscopy (HREM), and transmission electron diffraction (TED) have been used to investigate the morphology and defect behaviour of GaSb islands grown onto (001) GaAs at 520, 540 and 560C by metalorganic vapour phase epitaxy. TEM results showed that GaSb islands experience a morphological transition as the growth temperature increases. For growth at 520C, the islands are longer along the [110] direction; at 540C, they are nearly square, and at 560C, they are longer along the [-110] direction. Possible mechanisms are proposed to describe such a transition. TEM and HREM examination showed that lattice misfit relaxation mechanisms depend on the growth temperature. For the layer grown at 520C, the misfit was accommodated by 90 dislocations; for the layer grown at 540C, the misfit was relieved mostly by 90 dislocations with some of 60 dislocations, and for the layer grown at 560C, the misfit was accommodated by 60 dislocations which caused a local tilt of GaSb islands with respect to the GaAs substrate. The density of threading dislocations was also found to be dependent on the growth temperature. Mechanisms are proposed to explain these phenomena.

11:40 am

Formation of InSb Quantum Dots in a GaSb Matrix: A.F. Tsatsul'Nikov, N.N. Ledentsov, M.V. Maksimov, B.B. Volovik, B.Ya. Mel'tser, P.V. Nekludov, S.V. Shaposhnikov, A.A. Suvorova, N.A. Bert, P.S. Kop'ev, A.F. Ioffe Physico-Technical Institute, Polytekhnicheskaya 26, 194021 St.-Petersburg, Russia; M. Grundmann, D. Bimberg, Institut für Festkörperphysik, Technische Universität Berlin, Hardenbergstr. 36, D-10623, Berlin, Germany

There is a strong interest in application of self-organized quantum dots (QDs) to long wavelength lasers in view of the predicted suppression of Auger recombination in OD systems. Indeed, recently it was shown that the growth of InSb on a GaSb surface occurs in a Stranski-Krastanow growth mode with an initial formation of an InSb wetting layer and, subsequently, with a formation of 3D macroscopic clusters with a lateral size of about 4000Å. In this work we show that it is possible to fabricate coherent nanoscale InSb islands (quantum dots) in the InSb-GaSb system and study their luminescence properties. Deposition of ~1.7 - 2.8 monolayers (MLs) of InSb on GaSb surface results in a 2D-3D growth mode transition. Substrate temperature during deposition was T_s=420C. The dots were covered with GaSb immediately after formation. QDs have an average lateral size of about 300 Å, as it follows from the TEM image. Formation of InSb QDs (1.7 ML) results in a photoluminescence line at ~0.75 eV. This line is related to radiative recombination of nonequilibrium carriers through the ground state of the QDs. Increasing of InSb layer thickness to 3 ML induces a long wavelength shift of the QD line to ~0.73 eV and an increase in its width. This PL behavior we attribute to an increase in the average QD size and to a larger size dispersion.