We have demonstrated great improvement in the smoothness and defect density of GaAs films on Si by lowering the arsenic overpressure during growth of the initial layer (the first 500 Å) of GaAs. We have studied the morphology and defect density of GaAs on Si films in which the initial layers were grown under either low As4 overpressure (7As4:1Ga, beam equivalent pressure) or high As4 overpressure (15As4:1Ga) conditions, with a constant gallium flux. In the early stages of growth there is a significant change in island morphology depending on the As4 overpressure. There is dramatic improvement of surface smoothness and crystal quality with reduced arsenic overpressure for 500-Å-thick layers both immediately after growth at 350 °C and after heating to 580 °C. Diodes fabricated in 3.5-μm-thick films grown on initial layers that were grown under low arsenic overpressure have a very sharp reverse breakdown at voltages as high as 45 V, whereas diodes fabricated in films grown on initial layers that were grown under high arsenic overpressure have a soft reverse breakdown at about 5 V. This demonstrates a significant reduction in the density of electrically active defects in the thick GaAs films with decreasing arsenic overpressure conditions during growth of the initial 500 Å of GaAs on Si. The improvement in film quality for low As4 overpressures is discussed in terms of the observed changes in island morphology.

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High quality strain-free heteroepitaxial GaAs-on-Si has been produced by annealing chemically separated GaAs epitaxial layers grown by molecular beam epitaxy directly on silicon substrates. A process sequence has been developed which retains the GaAs layer in place during chemical separation and post-processing, thus maintaining a monolithic fabrication sequence. Using low temperature photoluminescence, it is shown that the majority of the residual strain is eliminated by chemical separation. Subsequent rapid thermal annealing is found to remove the remaining strain and significantly improve material quality. The presented process sequence forms the basis for monolithic integration of high quality strain-free (Al,Ga)As electrical and optical devices with silicon circuitry.

Low-threshold GaAs/AlGaAs lasers have, for the first time, been grown selectively on 10 μm stripe openings patterned in oxide on Si substrates. Lateral current confinement provided by side facets reduces edge leakage, and results in threshold currents as low as 75 mA for a 10 μm by 210 μm device, a nearly two-fold improvement over comparable etched ridge waveguide lasers. Spectrum measurements show single longitudinal mode emission near 850 nm. This adaptation of selective heteroepitaxial growth for lateral current confinement of AlGaAs/GaAs lasers on Si substrates, adopted from similar work on GaAs substrates, offers potential for significant threshold current reductions of lasers integrable with Si.

Capacitance transient spectroscopy has been applied to identify deep levels associated with heteroepitaxial GaAs grown on silicon. Results from p+n diode test structures reveal creation of the electron trap EL2 and a high density of hole states in the bandgap. This is the first reported observation of hole traps in MBE GaAs on Si. The activation behavior of the electron and hole signal peaks fits the signature of two different charge states associated with EL2, a native point defect complex seen in MOCVD, VPE, and bulk-grown GaAs, but not usually observed in MBE grown GaAs. Interstingly, the spectra seen show many similarities with earlier deep level transient spectroscopy (DLTS) observations on plastically deformed GaAs.