ALBERT

Notes: Excess As is incorporated in GaAs grown at low substrate temperatures by molecular beam epitaxy. Excess As is distributed in the epilayer as defects and the material exhibits considerable strain. When annealed to moderate temperatures, the strain is seen to disappear and the excess As is now in the form of semimetallic clusters. It has been proposed that these As clusters form buried Schottky barriers with the GaAs matrix and are surrounded by spherical depletion regions. In this article, we examine the effects of doping on the material properties and compare our results to the buried Schottky barrier mode. Si-doped GaAs epilayers grown at 250 °C, with doping densities between 5×1017 and 5×1018 cm−3, were annealed to temperatures between 700 and 1000 °C for 30 s. Be-doped GaAs epilayers grown at 250 °C, with doping densities between 5×1017 and 5×1019 cm−3, were annealed to temperatures between 700 and 900 °C for 30 s. Using extensive Hall measurements and transmission electron microscopy, we observe that the As precipitates deplete the surrounding GaAs matrix. © 1995 American Institute of Physics.

Notes: We have systematically investigated the microstructure and interface structure of ε1-Cu3Ge films on n-type (001)GaAs substrates using high-resolution transmission electron microscopy (HRTEM). The copper-germanium alloy forms ohmic contacts on n-type GaAs over a wide range of Ge concentrations from 5 to 40 at. % with a minimum contact resistivity of 6.5×10−7 Ω cm2 on (001) GaAs (doping concentration ∼1.0×1017 cm−3 in the Ge concentration range 25–30 at. %). The cross-sectional HRTEM results show that a low-resistivity ε1-Cu3Ge phase is formed at 25 at. % Ge concentration, and above this concentration excess Ge precipitates out between the ε1-Cu3Ge grains and the GaAs substrate. Ge grows epitaxially with the GaAs substrate, but it is not present as a continuous interfacial layer. The interface between ε1-Cu3Ge and GaAs is quite sharp, with no secondary phases. The secondary ion mass spectrometry results indicate interdiffusion between Ge and Ga, which results in highly doped regions by the incorporation of Ge atoms into the GaAs on the Ga sites. The current transport by the tunneling of the carriers through this doped region provides the low-resistance ohmic behavior of the contact. From correlations between the microstructure and the properties of the heterostructure we deduce the optimum concentration of Ge to be 30 at. % for formation of low-resistance ohmic contacts.

Notes: To investigate the effect of growth area on interface dislocation density in strained-layer epitaxy, we have fabricated 2-μm-high mesas of varying lateral dimensions and geometry in (001) GaAs substrates with dislocation densities of 1.5×105, 104, and 102 cm−2. 3500-, 7000-, and 8250-A(ring)-thick In0.05Ga0.95As layers, corresponding to 5, 10, and 11 times the experimental critical layer thickness as measured for large-area samples, were then deposited by molecular-beam epitaxy. For the 3500-A(ring) layers, the linear interface dislocation density, defined as the inverse of the average dislocation spacing, was reduced from greater than 5000 to less than 800 cm−1 for mesas as large as 100 μm. A pronounced difference in the linear interface dislocation densities along the two interface 〈110〉 directions indicates that α dislocations nucleate about twice as much as β dislocations. For samples grown on the highest dislocation density substrates, the linear interface-dislocation density was found to vary linearly with mesa width and to extrapolate to a zero linear interface-dislocation density for a mesa width of zero. This behavior excludes dislocation multiplication or the nucleation of surface half-loops as operative nucleation sources for misfit dislocations in these layers. Only nucleation sources that scale with area (termed fixed sources) are active. In specimens with lower substrate dislocation densities, the density of interface dislocations still varies linearly with mesa size, but the slope becomes independent of substrate dislocation density, indicating that surface inhomogeneities now act as the dominant source for misfit dislocations.Thus, in 3500-A(ring)-thick overlayers, substrate dislocations and substrate inhomogeneities are the active fixed nucleation sources. Since only fixed nucleation sources are active, a single strained layer will dramatically reduce the threading dislocation density in the epilayer. For the 7000-A(ring) layers, we observe a superlinear increase in linear interface-dislocation density with mesa size for mesas greater than 200 μm, indicating that dislocation multiplication occurs in large mesas. For mesas less than 200 μm in width, linear interface-dislocation density decreases linearly with mesa size, but extrapolates to a nonzero linear interface-dislocation density for a mesa size of zero. This nonzero extrapolation suggests an additional active source which generates a dislocation density that cannot be decreased to zero by decreasing the mesa size. Cathodoluminescence (CL) images using radiative recombination indicate that the additional source is nucleation from the mesa edges. Despite a doubling in epilayer thickness from 3500 to 7000 A(ring), the linear interface-dislocation density for mesas 100 μm in width is still very low, approximately 1500 cm−1. The 8250-A(ring) layers possess interface-dislocation densities too high to be accurately determined with CL. However, increases in CL intensity as mesa width is reduced indicate that the interface-dislocation density is decreasing and that growth on small areas produces higher-quality layers than growth on large areas. Our investigations show that different sources for misfit dislocations become active at different epilayer strain levels. The critical thickness depends on which type of nucleation source becomes activated first; therefore, different critical thicknesses can be observed depending on which kind of source is present in a specimen.

Notes: The defect structure of lattice-mismatched 1-μm InxGa1−xAs (x≈0.12, misfit Δa/a≈8.5×10−3) epilayers on GaAs was studied with scanning cathodoluminescence (CL), transmission electron microscopy (TEM), high-voltage electron microscopy, and scanning electron microscopy. CL shows that nonradiative recombination lines exist in the GaAs buffer layer as far as 4000 A(ring) from the interface. The density of these defects is independent of substrate dislocation density. Plan-view TEM analysis indicates that the majority of these dislocations in the buffer layer are sessile edge half-loops. Cross-sectional TEM shows that loops also extend into the InGaAs epilayer, but the majority of the loops are located on the buffer layer (substrate) side of the interface. A model is proposed to explain sessile edge dislocation formation in the buffer layer. A comparison of CL and high-voltage electron microscopy images from the same interface area reveals that the dark nonradiative recombination lines seen in scanning luminescence images in this high misfit system do not correspond to the normal, isolated misfit dislocation. The nonradiative recombination line spacing is 3 μm, whereas the interface dislocation spacing is 400–1000 A(ring). It is shown that the nonradiative recombination lines observed in CL of the interface correspond to specific groups of dislocations with different TEM contrast behavior. The dark nonradiative recombination lines also correlate with asymmetric surface ridges, suggesting that they introduce preferred nucleation sites, and that these effects are different for the two 〈110〉 directions.

Notes: The photoluminescence (PL) from the surface region of GaAs passivated by photowashing or coating with Na2S⋅9H2O is shown to be sensitive to the gas ambient. Both water vapor and oxygen must be present in order to obtain a large PL signal. The effects are activated by the measuring laser light.

Notes: We have studied the formation of As precipitates in doped GaAs structures that were grown by molecular beam epitaxy at low substrate temperatures and subsequently annealed. We find that the As precipitates form preferentially on the n side of such fabricated GaAs pn junctions. As the coarsening process proceeds, there is a gradual increase in the amount of As in precipitates in the n-GaAs region and a decrease in the p-GaAs region; the depletion region between the pn junction becomes free of As precipitates. These observations can be understood qualitatively based on the charge states of the As interstitial and using thermodynamic arguments in which the crystal attempts to minimize the chemical potential during the anneal. The presence of the excess As results in a stable Be profile even to anneals of 950 °C. Finally, a temperature cycling technique to grow arbitrarily thick GaAs epilayers containing As precipitates was demonstrated.

Notes: The effect of thermal annealing on the interface quality in undoped, AlAs/GaAs multiple quantum well (MQW) structures grown at a low substrate temperature (310 °C) by molecular beam epitaxy has been investigated using chemical lattice imaging and high resolution x-ray diffraction. The low-temperature-grown MQW is of high crystalline quality comparable to the standard-temperature-grown MQW. However, significant interface roughening and intermixing occurs at the quantum well heterointerface when the structures are annealed beyond 700 °C. The effective activation energy for interdiffusion is estimated as 0.24±0.07 eV. The structural properties observed here suggest that the excess arsenic associated with the low-temperature growth substantially enhances the diffusion of column III vacancies across an interface, which leads directly to intermixing of Al and Ga. © 1995 American Institute of Physics.

Notes: We report a photoreflectance study of surface photovoltage (VS) effects on the determination of Fermi level pinning (VF) on (100) n-GaAs in air and with W-metal coverage (in situ) as a function of temperature (77 K〈T〈450 K) and light intensity (I). The dependence of VS on T and I can be explained by a modification the theory of M. Hecht [Phys. Rev. B 41, 7918 (1990)] yielding a value of VF=0.73±0.02 V. The effect of metal coverage is to reduce the influence of VS.

Notes: Carbon tetrabromide (CBr4) and bromoform (CHBr3) have been studied as carbon doping sources for GaAs grown by gas source molecular beam epitaxy (GSMBE) with elemental Ga and thermally cracked AsH3. Hole concentrations in excess of 1×1020 cm−3 have been measured by Hall effect in both CBr4- and CHBr3-doped GaAs, which agrees closely with the atomic C concentration from secondary-ion mass spectrometry, indicating complete electrical activity of the incorporated carbon. The GaAs growth rate is unaffected by the CBr4 and CHBr3 fluxes over the range of dopant flow investigated. The efficiencies of carbon incorporation from CBr4 and CHBr3 are, respectively, 750 and 25 times that of trimethylgallium (TMG), which is commonly employed as a carbon doping source in metalorganic MBE (MOMBE). The sensitivity of carbon incorporation to varying substrate temperature and V/III ratio has been observed to be significantly reduced with CBr4 and CHBr3 from that obtained under similar growth conditions with TMG in MOMBE.

Notes: We have measured the photoreflectance spectra of a strained layer (001) In0.21Ga0.79As/GaAs single quantum well as a function of temperature in the range 10 K〈T〈500 K. The details of the lineshape of the fundamental conduction to heavy-hole feature (11H) demonstrates its excitonic nature even up to 500 K. From the temperature dependence of the 11H linewidth we have obtained important information about the quality of the material and interface. The variation of the 11H energy gap with temperature agrees with that of bulk material. Comparison of the energies of 11H and higher lying transitions with an envelope function calculation yields a conduction band offset parameter Qc=0.65±0.07.