The enhanced high temperature gate metallizations consisting of sputtered TiWSi or TiWN were investigated in order to attain high temperature stability at temperatures in excess of 250°C. The TiWN/Au system resulted in a sheet resistance of only 11.5 mΩ/□ while TiWSi/Au resulted in 75.0 mΩ/□. The HEMTs and FETs processed with additional stable ohmic contacts of epitaxial Ge/Pd structures exhibited a stable transconductance of 160 -180 mS/mm at temperatures of 300°C. Thermal analysis indicated the peak junction temperature increase with an input power of 200mW to be less than 18°C at substrate temperature of 60°C.

MBE grown GaAs/Al0.25Ga0.75As/In0.85Ga0.85 As structures were subjected to SiNx capped rapid thermal annealing and their electrical and material properties were characterized by Hall measurements and photoluminescence (PL). Low temperature (5°K) PL spectra from undoped structures annealed up to 900°C indicated negligible intermixing at the AIGaAs/lnGaAs interface. For planar doped structures (Nd≈5×1012/cm2) the Hall mobility began to decrease at anneal temperatures as low as 800°C with significant degradation observed for annealing temperatures at 850°C. This data is supported by PL spectra which indicate no significant change for samples annealed at 800°C. For the samples annealed at ≥ 850°C a large increase in the full width at half maximum of the transitions from the electron sub-bands of the InGaAs quantum well were observed, suggesting that the change in electrical characteristics is primarily due to diffusion of the Si doping pulse. In contrast. Hall measurement of uniformly doped structures reveal only small decreases in mobility and no significant change in sheet concentration for anneal temperatures up to 900°C and doping levels up to 2.5×1018/cm3. PL spectra reveal no structural changes.

High-resolution electron-beam lithography has been used to fabricate GaAs MESFETs with gate-lengths ranging from 1 μm down to 30 nm. Devices were fabricated on two MESFET epitaxial layers; one with undoped GaAs-buffer layer while the other had a p- -GaAs-buffer layer. The DC characteristics including transconductance, output conductance, threshold voltage, and subthreshold current of these devices have been measured. Devices on both epitaxial layers exhibited significant short-channel effects. A negative threshold voltage shift and an increase in the subthreshold current were observed. These effects become prominent as the device aspect ratio (gate length/channel thickness) falls below 5. It is shown however that the effects were considerably suppressed in the layer with p- -GaAs buffer due to better confinement of electrons in the channel.

We report on the growth and fabrication of p-doped long wavelength GaAs/AlxGa1−x As quantum well infrared photodetectors (QWIP) grown by organometallic vapor phase epitaxy. The operation of these devices is based on the photocurrent induced through valence band intersubband absorption by holes and, unlike n-doped QWIPs, can utilize normal incidence illumination. Carbon and zinc were used as the p-type dopants in a low-pressure (30 Torr) vertical-geometry reactor. The Zn-doped QWIP consisted of fifty periods of 48 nm-thick undoped Al0.36Ga0.64As barriers and nominally 4 nm-thick doped GaAs quantum wells. Using normal incidence, a quantum efficiency of η = 2.5% and a detectivity of at 77K were obtained for a peak wavelength λp = 6.8 μm and a cutoff wavelength λ∫ =7.6 μm. The C-doped QWIP had 54 nm-thick Al0.31Ga0.69As barriers and exhibited a normal incidence These initial studies indicate the superiority of carbon to zinc as the p-type dopant for these structures. The detectivity of the C-doped QWIPs is about four times less than n-doped QWIPs for the same λp but have the advantage of utilizing normal incidence illumination.

Transport properties of a pseudomorphic In .20Ga.80As/Al.23Ga.77As HEMT structure have been measured by Hall and SdH techniques. Two samples of identical structures but different doping levels were compared. Low temperature mobility measurements as a function of concentration shows a sharp peak at a Hall concentration of 1.9.1012/cm2. This concentration coincides with the onset of second subband occupancy, indicating that the decrease in mobility is due to intersubband scattering. In spite of the low Al content (23%) large PPC was observed in the highly doped sample only, showing a direct correlation between the PPC and doping concentration of the barrier layer.

An MOCVD grown InP based optothyristor has been fabricated and tested for high power pulsed switching applications. To increase the power handling capability, the thyristor structure has a 250 μm thick Fe doped semi-insulating (SI) InP sandwiched between two pn junctions of a conventional thyristor. The turn-on of the thyristor is controlled by optical illumination on the SI-InP which creates a high concentration of electron and hole pairs. More than 1,100 V device hold-off voltage has been observed and over 66 A switched current has been realized with a di/dt rating of 1.38×1010 A/s. The switched current as a function of switch voltage and of optical illumination power has also been studied. Comparison with the switching characteristics of a bulk SI-InP photoconductive switch clearly indicates the advantage of this optothyristor in terms of power handling capability.

GaAs has been encapsulated with sputter or electron-beam deposited, thin films of Si or SiGe and annealed in open tube oxygen ambients. The presence of oxygen increases the diffusivity of both Si or Ge in the substrate. Forming gas anneals reduce the diffusivity by orders of magnitude. The diffusivities are greater for sputtered films compared to electron-beam deposited material of the same thickness. And the diffusivity is greater for thicker films and for Ge-rich films whether sputtered or electron-beam deposited.

The extrinsic device is known to degrade the performance of heterostructure field-effect transistors (HFET's) through the introduction of a parasitic source resistance (Rs). To date, however, there has been no recognition of the fact that carrier velocity saturation (vsat) can occur in both the extrinsic source and drain, setting the ultimate limit on maximum drain current (I,D,max) and on the useful VGS swing in HFET's. In this study, we demonstrate the mechanisms through which vsat in the extrinsic device limits device performance, using AlGaAs/n+-InGaAs Metal-Insulator-Doped-channel FET's (MIDFET's) as a vehicle. These devices show that gm falls at a lower VGSthan does fT, by as much as 1 V. This reveals that there are two mechanisms at work. The approach of vsat in the extrinsic source first causes the small-signal source resistance (Ts)to rise rapidly, leading gm to decline but leaving fT unaffected. As the carrier velocity in the extrinsic device approaches Vsat more closely, there is an actual decline of the carrier velocity in the intrinsic device. This process degrades velocity-related figures of merit such as and fT.

A series of Graded-Index Waveguide Separate-Confinement Heterostructure Quantum Well (GRINSCH-QW) laser diodes were grown by MBE at the systematically varied substrate temperatures. The threshold current of laser diodes were found to depend strongly on the growth temperature. The structure and electrical characteristics of the laser diodes were studied by double-crystal x-ray diffraction, I-V-T, C-V and deep level transient spectroscopy (DLTS). The interface recombination is found to be the dominant carrier transport process in the high threshold current laser diodes and is closely related to the presence of the high concentration of deep traps and interface states. In the low threshold current laser diodes, diffusion process is found to be the dominant carrier transport process.

An empirical tight-binding Koster-Slater approach is used to determine the electronic properties of ultrathin“quantum wells”in semiconducting host materials of the zincblende or diamond structure. The“quantum well”is viewed as a giant two-dimensional isoelectronic impurity, and treated in a perturbational Green's function approach. We present results on the AlAs/GaAs and on the InP/InAs systems.

Valence and conduction band discontinuities in AlAs-GaAs heterostruc-tures have been tuned through fabrication of epitaxial Ge layers at the interface. The local interface dipole associated with the Ge layer can be added to, or subtracted from the natural band offsets depending on the growth sequence. Comparison with earlier results for AlAs-Si-GaAs heterostructures, shows that the observed dipole is consistent in direction and order of magnitude for Ge and Si interface layers. The dipole initially increases with interface layer thickness more rapidly for Ge than for Si, however the total maximum dipole achievable at the interface is identical (0.4eV), within experimental uncertainty, for the two group IV elements.

We present a study of the effects of the active cavity layer thickness variation on the operating characteristics of normally-on low-voltage high performance asymmetric Fabry-Perot modulators. For a modulator consisting of 25.5 periods of a multiple-quantum-well active region (100Å GaAs / 45Å (GaAs/AlAs) short period superlattices) with 5 pairs and 20.5 pairs of top and bottom quarter-wave stacks respectively, assuming only layer thickness variation in the active cavity caused by Ga flux nonuniformity, the shift of the Fabry-Perot mode wavelength is ∼5.8 times that of the QW heavy-hole exciton. This affects the relative distance between the wavelengths of the quantum well exciton and the Fabry-Perot resonance, and hence the performance of the modulators. Also, the tolerable percentage change of the Fabry-perot mode wavelength should be less than 0.13% in order that such modulator arrays have at least 10:1 contrast ratios at a fixed optimum operating wavelength. This defines the epitaxial growth tolerance for obtaining the uniformity of the operating wavelength of an array and the precision with which we can obtain a desired wavelength, its reproducibility, and its uniformity across a wafer.

We report the realization of all-optical photonic switches using strained InGaAs/GaAs multiple quantum well based inverted cavity asymmetric Fabry-Perot reflection modulators monolithically integrated with GaAs/AlGaAs based heterojunction phototransistors. The photonic switches show both bistable and non-bistable switching behavior with a contrast ratio of 12:1 and optical gain of 2 to 4 dB. The design and growth considerations for such an integrated structure are also discussed.

In a thick strained GaAs/InGaAs MQW we recently reported the unusual observation of the exciton linewidth initially narrowing upon application of a reverse bias, before the usually observed broadening set in with further increase in the bias. The phenomena suggested the existence of a spatially varying electric field in the MQW region arising from a depletion of the net charge density with increasing reverse bias. Here we provide an explanation for the unusual observation arrived at through a systematic examination of the sample behavior using electro-transmission, electro-photoluminescence, capacitancevoltage profiling and transmission electron microscopy. We conclude that shallow levels cannot account for the observation and that the presence of strain induced point defect related deep levels (either n of p type) offers a consistent explanation. This is the first clear manifestation of the influence of deep levels on the free exciton electroabsorption behavior and has practical implications for MQW based electroabsorptive / electrorefractive light modulators.

Electrical and structural investigation of AlAs/GaAs/AlAs resonant tunneling structures with pseudomorphic strained Ga1−xInxAs (x=0, 0.05, 0.1, 0.15, and 0.2) emitter spacer layer are presented. As indium composition increased, the peak current density, peak voltage, and peak to valley ratio increased. For a theoretical understanding of these increases, a self-consistent simulation was employed. In the simulation, we treated the 2-dimensional electrons confined in the low energy bandgap GalnAs emitter spacer well as pseudo-3-dimensional electrons, distributed continuously down to the emitter launching energy. In the simulation, we used the bottom energy of the pseudo-3-dimensional electrons to be ⅔δEc below the emitter conduction band edge. Using the above values, an excellent agreement of peak current density and peak voltage between the experiment and the simulation was achieved. Also, for structural identification, standard double crystal x-ray rocking curve technique has been used. From the interference analysis of the x-ray results, we could obtain the indium composition times thickness product.

We have studied the lattice-mismatch-induced defects both at the interface and in the bulk (GaAs buffer layer ) of Ga0.92In0.08As/GaAs heterojunctions by means of C-V characteristics and constant capacitance deep level transient spectroscopy (CC-DLTS). The Ga0.92In0.08As (n+)/GaAs (P) samples, with the thickness of GalnAs layer at 0.1μm, 0.25μm, 0.5μm, and 1.0μm, were grown by MBE. The depth profiles of bulk hole traps in the GaAs buffer layer were measured by the DLTS technique as a function of the in-plane lattice mismatch. The concentration of lattice-mismatch-induced traps decreased exponentially with distance away from the interface. The depth distributions of the mismatch-induced bulk traps appeared to be affected by the pre-existing bulk traps and the dopant impurities in the GaAs buffer layer. Energy distribution of the interface states was obtained by the CC-DLTS and C-V measurements independently. All heterojunctions showed a minimum in the interface state density at about EV+0.83eV. The U-shaped energy distribution with donor- and acceptor-like states is interpreted by the disorder-induced-gap-states model.

Theoretical studies of microscopic properties of localized defects, impurities, and complexes in semiconductors have greatly progressed in the past decade. Theory has advanced beyond “point” defects to include lattice relaxations and distortions, interactions between defects, complex formation, and even some extended structures. Vibrational frequencies, hyperfine parameters, and other measurable quantities are being calculated from first principles. Both the one-effective-particle density-functional approach and the all-electron Hartree-Fock methods are able to predict a variety of microscopic properties of defects at or near the ab initio level. However, despite the progress achieved, theoretical descriptions only approximate the real world. In this paper, an overview is given of the way these calculations are done and of the main approximations involved. Although some of them will be eliminated by progress in computer technology, other problems such as electron correlation or excited states are likely to require new thinking.

Hydrogen implantation at 80K is shown to introduce As-H centers in GaAs and InAs, and P-H centers in InP. These As (P)-H centers anneal between 150 and 300K. By analogy to suggested defect assignments in electron-irradiated GaAs, it is suggested that the sites for the As (P)-H centers are metastable defects such as close vacancy-interstitial pairs. In contrast to GaAs, where absorption by Ga-H centers increases upon loss of As-H, no absorption attributable to In-H was observed in either InAs or InP. Hydrogen in an As vacancy is the suggested model for Ga-H centers. If such vacancies are thermally unstable in In-based compounds, it would explain an absence of In-H bonds and may reduce the capability for ion implantation to produce electrical isolation in In-based compound semiconductors.

Hydrogen implantation into either n-GaAs:Si or p-GaAs:Zn crystals leads to a neutralization of charge carriers, primarily through induced crystal damage. The resulting decrease in carriers is useful in opto-electronics applications for confining electrical current or optical radiation. Exposure of the same crystal materials to a hydrogen plasma also leads to reduction of the charge carriers, but through a passivation mechanism involving hydrogen-dopant atom complexes. This technique may be useful for opto-electronics device processing but further details converning the physical mechanisms need to be established. Here we present the results of investigations aimed at correlating the changes in carrier concentration as determined by infrared reflectivity measurements with atomic depth profiles obtained from secondary ion mass spectrometry (SIMS). Differences between the use of hydrogen or deuterium plasmas are also reported.

Hydrogen passivation and thermal reactivation of Si donors and Be acceptors were investigated in In0.52Al0.48As grown by molecular beam epitaxy. The semiconducting alloy was passivated by exposure to monatomic hydrogen or deuterium from a remote microwave plasma. The passivation was achieved by exposing the samples to monatomic hydrogen at temperatures between 200 and 250 °C for 1h. The electrical activity of the dopants was monitored by spreading resistance and C-V measurements. The samples were homogeneously doped to concentrations of 1.5×1016 or 6×1017 Si / cm3 and 6×1017 Be / cm3. Both dopants were passivated by more than two orders of magnitude through the epitaxial layers. An additional annealing step (440°C, 5 min) resulted in a complete reactivation of the passivated dopants. In addition to the electrical measurements, secondary ion mass spectroscopy showed that for both the Be- and the Si-doped layers the hydrogen profiles were essentially identical to the dopant profiles throughout the epilayers. This behaviour suggests that hydrogen migration is a dopant-trapping-limited process in n-and p-type In0.52Al0.48As.