ALBERT

Notes: A detailed x-ray double-crystal-diffractometry investigation of GaAs on Si tilted towards [110] direction has been carried out to understand the structure, misorientation, and residual stress present in this system. The structure of the GaAs epilayer is approximately triclinic, with the in-plane lattice parameters a and b being in most cases slightly larger than, and that along the growth direction c less than, the bulk GaAs. For vicinal angles between 0° and 5°, a tilt angle of up to 0.12° between GaAs and Si lattice is observed, inclined mostly in the same direction as the vicinal angle, but also having a small component in a perpendicular direction. A residual tensile stress of the order of 109 dyn/cm2 exists in the epilayer, which can be accounted for by the difference in the thermal expansion coefficients of GaAs and Si. The strain due to the lattice parameter mismatch between the two materials is found to be negligibly small.

Notes: Two-dimensional growth of GaAs on (100) silicon has been achieved by combining laser-assisted growth and modulation molecular-beam epitaxy techniques. The misfit dislocations were shown to form a regular cross-grid with satellite reflections characteristic of a two-dimensional homoepitaxial growth as also revealed by the perfection of the moiré patterns of the GaAs/Si interface. The few antiphase domains generated at the interface were observed to annihilate very fast. The top surface of GaAs was free of antiphase domains. Planar defects such as stacking faults and microtwins are also eliminated.

Notes: Room-temperature electrotransmittance has been used in order to investigate the interband excitonic transitions in a 250-A(ring)-thick In0.53Ga0.47As/In0.52Al0.48As single-quantum-well system as a function of an externally applied electric field. Parity forbidden transitions, involving conduction-band states with quantum numbers up to n=5, which become more pronounced at high electric fields were observed. The ground-state and the forbidden transitions showed a significant red shift due to the quantum confined Stark effect. A comparison with previously reported results on thinner InGaAs/InAlAs quantum wells indicated that the wide-well sample exhibits the largest shift, as expected from theory. Despite the appreciable Stark shift, the rather large, field-induced linewidth broadening and the relatively low electric field at which the ground-state exciton is ionized poses limitations on using this wide-quantum-well system for electro-optic applications.

Notes: A systematic study of the growth of high-quality films of GaAs on Si substrates has been performed for applications in devices, particularly in optoelectronic devices for cointegration in optical interconnects. The effort for optimized active layers was approached through the separate optimization of substrate preparation, growth time parameters, and postgrowth treatment. In particular, the study of growth involved the investigation of the effect of silicon substrate orientation, post-growth treatment, as well as multilayer and, especially, silicon buffer layers. For quantification of film quality, a number of characterization methods were used both in situ: reflected high-energy electron diffraction (RHEED); and ex situ: optical, electrical [current versus voltage (I-V), capacitance versus voltage (C-V), deep-level transient spectroscopy (DLTS), Hall], transmission electron microscopy (TEM), scanning electron microscopy (SEM), electron channeling patterns, x-ray double-crystal diffractometry (DDX). Schottky diodes, p-n heterojunctions, and metal-semiconductor-metal photoconductors/photodetectors (MSM PC/PDs), field-effect transistors, and high electron mobility transistors were fabricated on these films. The most crucial parameter for device operation and film uniformity is the complete absence of antiphase boundaries which increase leakage, degrade mobilities, and seem to result in interface two-dimensional electron gas in substrates misoriented toward 〈110〉.Absolutely smooth GaAs morphology is obtained using a molecular-beam epitaxy grown Si buffer layer and controlling the orientation of the GaAs film so that the [110] direction is parallel to the 〈110〉 misorientation direction of the vicinal (001) substrates. This can be ensured by an As4 prelayer grown at 350 °C. A double 2×1 domain Si surface seems to be preferable, as it allows the choice of such a GaAs orientation. GaAs growth is then 2D from the very early stages of growth, following the homogeneous nucleation of 3D GaAs islands, resulting in the complete elimination of planar faults. A perfectly regular displacement-type moiré pattern in the GaAs/Si interface is then observed. GaAs buffers on Si with an MBE Si buffer exhibit high resistivity, probably due to growth on contamination-free surfaces. The lowest ever reported 1 μm DDX full width at half-maximum of 255 arcsec was observed for such a GaAs/Si/Si layer. Nevertheless, accurate TEM dislocation counts indicate a dislocation density in the low 108 cm−2 range. In addition, a saturation in DDX FWHM values appears for an epilayer thickness of about 2 μm. This may be related to values being limited by wafer bowing or it may indeed reflect a limit in film quality. Post-growth rapid thermal annealing results in redistribution of dislocations in a nonuniform way with most congregating in small areas of high dislocation density, leaving large areas with low dislocation density.It is concluded that by either increasing the GaAs epilayer thickness or the sample temperature one produces a residual compressive stress that forces the threading dislocations to slip, thus reducing their density by reactions that become moreprobable with proximity. The residual dislocation density of about 108 cm−2 is attributed partly to threading dislocation generation during the early stages of epitaxy and only partly to generation from tensile thermal stress during cooling. Schottky diodes on GaAs/Si break down at the same or similar voltages as on homoepitaxial material. MSM PC/PDs have comparable dark dc leakage currents, somewhat lower dc photoresponse, and comparable rise and fall times, and metal-semiconductor field-effect transistors (1.5 μm gate length) fabricated on GaAs/Si/Si show a maximum extrinsic transconductance of 230 mS/mm, actually somewhat higher than for homoepitaxial devices. Thus, device results allow us to claim that we have achieved a technology that leads to heteroepitaxial GaAs/Si films which compare in performance to homoepitaxial GaAs/GaAs within about 10% for applications in most devices. The use of an MBE Si buffer layer, in addition to improving the quality of the GaAs layer, results in a reduction of a processing temperature by at least 100 °C. This reduction, along with the elimination of the step-doubling processing step, makes GaAs film growth compatible to unmetallized fully processed complementary metal-oxide-semiconductor (CMOS) Si wafers.

Notes: The valence-band splitting due to strain in molecular-beam epitaxially grown GaAs on Si has been observed by photoreflectance. The strain has been obtained from the valence-band splitting and was found to be in agreement with results obtained by x-ray rocking curve measurements, photoluminescence, and Raman spectroscopy. The temperature dependence of the strain has also been measured and found to be in agreement with thermal expansion effects.

Notes: GaAs field-effect transistors (FETs) exposed to 40 α/sec for about 60 sec in the gate region revealed burnout from under the gate to both the drain and source. To explain this result, we show by using a 2D numerical FET simulation that a single event, particularly normal to the gate, has all the harmful electrical and thermal transients as that of a reverse gate-voltage pulse or positive drain-voltage pulse. The latter two are well known to initiate burnout failure mechanisms in GaAs FETs, depending on duty cycle and peak power applied. The onset of burnout due to a succeeding single event may be further aided by the ionization-enhanced outdiffusion of deep-level traps to the active channel during a series of single events. The experimental data, principally from SEM analysis and degradation of I-V characteristics, seem to support the thermal runaway burnout mechanism proposed in this paper. Other mechanisms previously suggested are either ruled out or deemed inadequate. A high-fluence radiation-hardened structural FET design is suggested.

Notes: The effects of He ion irradiation have been investigated in high electron mobility transistors. Heterojunctions have been also used to monitor the two-dimensional electron-gas degradation. Devices with different layer structures have been employed for the better understanding of failure mechanism sources. Finally, introduction of a charge control model allowed the determination of buffer layer degradation. © 1995 American Institute of Physics.

Notes: The structural properties of GaAs grown by molecular-beam epitaxy at low temperatures have been investigated by scanning electron microscopy, transmission electron microscopy, and high-resolution x-ray double-crystal rocking curves as a function of arsenic overpressure during growth. It was found that surface smoothness and excess arsenic incorporation both depend strongly on growth temperature and on As/Ga flux ratio. For each growth temperature there is a "window'' in the flux ratio which results in smooth surfaces. As-grown layers have an increased lattice constant in the growth direction. This relative lattice expansion increases with flux ratio at a constant growth temperature and eventually saturates. Transmission electron micrographs have revealed the presence of arsenic precipitates in material annealed at 600 °C. Increasing the As4 pressure during growth results in increases in precipitate diameter by almost 50% while their density and shape remain constant. Based on these observations a model has been developed to explain the lattice expansion dependence on arsenic overpressure. © 1996 American Institute of Physics.

Notes: Electrical transport in a modulation doped heterostructure of In0.53Ga0.47As/In0.52Al0.48As grown on Si by molecular beam epitaxy has been measured. Quantum Hall effect and Subnikov–De Haas oscillations were observed indicating the two-dimensional character of electron transport. A mobility of 20 000 cm2/V s was measured at 6 K for an electron sheet concentration of 1.7×1012 cm−2. Transmission electron microscopy observations indicated a significant surface roughness and high defect density of the InGaAs/InAlAs layers to be present due to the growth on silicon. In addition, fine-scale composition modulation present in the In0.53Ga0.47As/In0.52Al0.48As may further limit transport properties.

Notes: The effectiveness of In0.10Ga0.90As/GaAs strained-layer superlattices (SLSs) as barriers for the threading dislocation propagation, in molecular-beam-epitaxy GaAs-on-Si structures with Si buffer layers, has been investigated. It is shown that the interaction of threading dislocations with the strain field of SLSs is effective in limiting their propagation. The interaction is stronger as the total thickness of In0.10Ga0.90As (i.e., SLS periods) is increased. SLSs with thinner individual layers resulted in a lower dislocation density and a better structural quality at the GaAs/Si interface. © 1994 American Institute of Physics.