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A range of experimental techniques has been used to measure point defect concentrations in GaAs layers grown at low temperatures (250 °C) by molecular-beam epitaxy (LT-GaAs). The effects of doping on these concentrations has been investigated by studying samples containing shallow acceptors (Be) or shallow donors (Si) in concentrations of ∼1019 cm−3. Material grown under As-rich conditions and doped with Be was completely compensated and the simultaneous detection of As0Ga by near-band-edge infrared absorption and As+Ga by electron paramagnetic resonance confirmed that the Fermi level was near the midgap position and that compensation was partly related to AsGa defects. There was no evidence for the incorporation of VGa in this layer from positron annihilation measurements. For LT-GaAs grown under As-rich conditions and doped with Si, more than 80% of the donors were compensated and the detection of SiGa–VGa pairs by infrared localized vibrational mode (LVM) spectroscopy indicated that compensating VGa defects were at least partly responsible. The presence of vacancy defects was confirmed by positron annihilation measurements. Increasing the Si doping level suppressed the incorporation of AsGa. Exposure of the Be-doped layer to a radio-frequency hydrogen plasma, generated a LVM at 1997 cm−1 and it is proposed that this line is a stretch mode of a AsGa–H–VAs defect complex. For the Si-doped layer, two stretch modes at 1764 and 1773 cm−1 and a wag mode at 779 cm−1 relating to a H-defect complex were detected and we argue that the complex could be a passivated As antisite. The detection of characteristic hydrogen-native defect LVMs may provide a new method for the identification of intrinsic defects.

High-resolution triple-axis x-ray diffractometry has been used to examine the structural properties of a δ-doped superlattice of sixty periods, each consisting of half a monolayer of Si and 500 Å of GaAs, grown by molecular beam epitaxy (MBE) at 400 °C under an arsenic flux. The measurements indicated that the superlattice was of high structural quality. Using dynamical simulation, it was demonstrated that the period variation was equal to 3%, while the silicon spreading was no greater than 2 monolayers. It was possible to extract this information because of the high-resolution diffractometer which produced the theoretical line shape and wide dynamic range. Using a simple model, it was deduced that virtually all Si atoms were located on Ga lattice sites.

We have realized two dimensional electron gases (2DEGs) in tensile strained silicon (Si) channels between strain relaxed silicon germanium (Si0.70Ge0.3o) barriers grown on Si(100) substrates by Gas Source Molecular Beam Epitaxy (GSMBE). Disilane (Si2Hs), germane (GeH4), and arsine (AsHa) are used as the source gases. Compositionally graded buffer layers with a linear gradient of 30% Ge/1 gm relax the strain of the Si0.7oGe0.30 barrier layers by an amount greater than 95% as determined from X-ray diffraction (XRD) rocking curves. Dislocation densities in the vicinity of the active strained Si channels are below 107 cm -2 as determined from transmission electron microscopy (TEM) measurements. These structures have low n-type background impurity concentrations ( < 1016 cm -3) and the SioJoGe0.ao barriers can be successfully doped with a unity activation ratio in the 1017 to 102o cm 3 range. At present, we obtain 300 K (0.4 K) electron mobilities and sheet densities in our 2DEGs of 103 (5.3 x 104 ) cm2/Vs and 3 x 1012 (5.2 x 1011 ) cm -2, respectively. A discussion of the requirements for growing these structures by GSMBE and the modifications needed to improve the transport properties of the 2DEGs is presented.

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Arsenic-rich InAs/lnAsr-,Sbx strained layer superlattices (SLSs) grown on GaAs substrates by molecular beam epitaxy (MBE) are studied for their potential application as infrared emitters. The long-wavelength emission (4-1 1 pm) is highly sensitive to superlattice design parameters and is accounted for by a large type4 band offset, greater than in previously studied antimony-rich InSb/lnAs,_,Sb, SLSs. High internal PL efficiencies (>lo%) and intense luminescence emission were observed at these long wavelengths despite large dislocation densities. Initial unoptimized InAs/lnAsr_,Sb, SLS light emitting diodes gave ss200 nW of A = 5 pm emission at 300 K.In(AsSb) strained layer superlattices (SLSs) offer infrared detector and emitter applications beyond IO p m , Although mid-infrared lead salt lasers already exist they suffer from thermal conductivity, doping and metallurgical problems which have restricted their operation to temperatures below 200 K [l]. In(AsSb) offers superior metallurgical stability and compatibility with existing II-V technology compared to other infrared materials like cadmium mercury telluride. To date, because of the narrow bandgaps of the alloy constituents, InSb/InAsl-,Sb, SLSs composed of Sb-rich alloys have mainly been studied for long-wavelengh (510 pm) applications, notably longwavelength photodiodes [2]. The 10 pm response of the latter arises from the type-I1 band offset which gives a superlattice structure with an effective bandgap lower than either of the alloy constituents.The lack of lattice matched substrates, however, leads to defect densities of the order of (1-3) x IO9 cm-* (largely independent of alloy composition) in the active region of the device [3]. These give rise to Shockley-Read generation-recombination centres which limit the detectivity and emission efficiencies of Sb-rich alloy optoelectronic devices. In contrast, Walukiewicz [4] has shown that InAs is unique among the m-Vs in that pointlike lattice defects produce electronic states above the conduction band edge as opposed to in the forbidden gap.