Lattice defects in As-grown and irradiated GaAs, AlGaAs and InGaGs

• Main Author: Irvine, A. C.
• Published: University of Sussex 1993
• Subjects: 530.41; Semiconductors
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.334991

The alloying of two or more semiconducting compounds is a powerful means of tailoring the properties of semiconductor devices, since material parameters generally display a continuous shift with composition. For the same reason, observing the characteristics of a defect across a range of alloys is a useful way of gaining infonnation on the defect's microscopic structure. In this work, Schottky diodes are employed to study lattice defects, both as-grown and created by irradiation, in the alloys AlxGal-~ (O~x~O.27) and InxGal_~ (O~x~O.18), grown by the Metal~rganic Chemical Vapour-phase Deposition (MOCVD) and Molecular Beam Epitaxy (MBE) techniques respectively. The Deep-Level Transient Spectroscopy (DLTS) method is used in conjunction with measurements, detennined fram capacitance-voltage characteristics, of the immobile space-charge concentration, Nt, as a function of distance fram the Schottky contact. InxGal_~ is lattice mismatched in growth on GaAs substrates and, as growth proceeds, the InxGal_~ in-plane lattice parameter increases towards its bulk value through the introduction and glide of dislocations at the InxGal_~~s interface. Nt profiles measured in relaxed InO.04sGaO.955As on GaAs structures show a substantial degree of carrier depletion near this interface, and similar features are observed in an Ino.lgGaO.8~-GaAs structure and in a GaAs layer deposited on a part-relaxed InO.2GaO.8As sandwich layer. A GaAs epilayer grown on GaAs as a control, on the other hand, shows no evidence of carrier trapping. It is clear that this depletion is evidence of defects related to the relaxation process. DLTS studies reveal the presence, confinned by photocapacitance quenching experiments, of the technologically important EL2 defect in the relaxed InxGa1-xAs layers, though not in the relaxed GaAs sample This represents the first time that EL2 has been observed in InxGal_~ grown by molecular-beam epitaxy (MBE). A reason for the non-observation of EL2 in the GaAs layer is put forward on the basis of strain relief considerations, and a model for EL2 fonmation in these samples is suggested. Profiling of the EL2 concentration in InO.045CaO.955As by DLTS reveals a sharply peaked distribution and indicates the absence of EL2 within ~O.4pm of the InO.045Cao.955As-GaAs interface. Consequently, it is proposed that EL2 fonmation occurs near the free surface during growth, and depends on the nucleation of dislocation half-loops. An existing strain-thickness relation is adapted on this basis to produce a model for EL2 fonmation. Several qualitative aspects of the experimental data are explained in this way. Proton irradiations were perfonned on InxGal_~s, CaAs and AlxCal_~ samples at 85K, and it was seen that, within experimental error, the introduction rates of the main DLTS-detected electron traps were independent of alloy camposition, though a marked variation in E3 emission energy with camposition was noted. More significantly, the N+ recovery stages observed below 300K in GaAs were shifted to lower temperature in AlxCal_~s. It is suggested that this may be explained simply by the small size of an aluminium atom relative to gallium. The complex implications of this observation for defect annealing in compound alloys are discussed, and a qualitative model is presented.