Using molecular beam epitaxy, we have successfully grown device quality GaAs/AlGaAs on (100)-oriented Ge and Si substrates. Modulation doped field effect transistors have been fabricated from these layers which exhibit room-temperature transconductances as high as 160 and 170 mS/mm for layers on Ge and Si, respectively, and showed no looping in either case. At 77 K, these values rose to 345 and 275 mS/mm for Ge and Si, respectively. Analysis by transmission electron microscopy of layers grown on Ge showed that the antiphase disorder was contained within the 250-Å-thick initial layer which was grown at a 0.1-μ/h growth rate at a substrate temperature of 500 °C. For both the layers grown on Si and Ge specular surface morphologies were obtained. The photoluminescence of GaAs/AlGaAs quantum wells grown on Si and Ge was similar in intensity to the same quantum well structures grown on GaAs. In quantum wells grown on Ge, the luminescence was dominated by a donor-acceptor recombination at 1.479 eV, which appears to be Ge0Ga -Ge0As. These results show that high-quality GaAs/AlGaAs is obtainable on nonpolar substrates, which has important implications for the monolithic integration of III-V’s with Si.
Transmission electron microscopy was applied to cross-sectioned samples to study surface morphology, sample polarity and defect distribution in bulk GaN samples doped with Mg. These crystals were grown from a Ga melt under high hydrostatic pressure of Nitrogen. It was shown that the types of defects and their distribution along the c-axis depends strongly on sample polarity. Based on this finding the growth rate along the c-axis for the two polar directions was compared and shown to be approximately ten times larger for Ga polarity than for N-polarity. In the part of the crystals with Ga polarity pyramidal defects with a base consisting of high energy stacking faults were found. The parts of the crystals grown with Npolarity were either defect free or contained regularly spaced stacking faults. Growth of these regularly spaced cubic monolayers is polarity dependent; this structure was formed only for the growth with N polarity and only for the crystals doped with Mg. Formation of this superstructure is similar to the polytypoid structure formed in AlN crystals rich in oxygen. It is also likely that oxygen can decorate the cubic monolayers and compensate Mg. This newly observed structure may shed light on the difficulties of p-doping in GaN:Mg.
A study of Ga and In outdiffusion into Cd1−xMnxTe (0≥ x ≥ .70) epitaxial layers grown on (111)GaAs and (001)lnSb was carried out. The layers were grown by pulsed laser evaporation and epitaxy on substrates held at temperatures below 310 °C. The structural quality of the layers was examined using x-ray diffraction and transmission electron microscopy. A tendency toward precipitation of Ga at the near surface region of (11 1)CdMnTe grown on (11 1)GaAs, usually less than 300 nm wide, has been observed with secondary ion mass spectroscopy. Similar results were observed for the migration behaviour of In in (001)CdTe grown on (001)InSb. The ion imaging revealed that both In and Ga accumulate near the surface at localized spots, up to about 10 pm in diameter. The concentration of the spots is in the range of 104 - 106 cm−2. The Ga- and In-rich channels sometimes extend over the whole sample. Annealing at temperatures as low as 400 °C for 2 h significantly increases the concentration of the Ga spots and the average concentration of Ga in the films to above 1016 cm−3. Low-temperature photoluminescence data obtained for annealed samples do not indicate any structural deterioration typical for heavily doped Bridgman grown samples. A sharp neutral-donor bound-exciton transition (D°,X) is observed for samples with 0 < x < .10.
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