B.F. Usher

The nature of nitrogen related point defects in common forms of InN

The role of point defects related to the presence of excess nitrogen is elucidated for InN thin films grown by different techniques. Elastic recoil detection analysis has shown the presence of excess nitrogen in state-of-the-art InN films. Using x-ray photoelectron spectroscopy and x-ray diffraction it is shown that two distinct forms of point defects can be distinguished; one of these appears to be an interstitial form of nitrogen, common in some forms of polycrystalline InN. The other is associated with a combined biaxial and hydrostatic strain observed for molecular beam epitaxy (MBE) and chemical vapor deposition (CVD) grown films, and may be a mixture of the nitrogen-on-metal antisite defect and lower densities of indium vacancies and interstitial nitrogen. The high density of defects present in all the InN samples examined suggests that stoichiometry related point defects dominate the electrical and optical properties of the material. The difference in the type of point defect observed for polycryst...

Deviation of the AlGaAs lattice constant from Vegard's law

A structure consisting of a multiple quantum well, 10×[GaAs/AlxGa1-xAs], followed by two single layers, AlAs and AlxGa1-xAs, grown on a GaAs substrate by molecular beam epitaxy is studied by x-ray diffraction. Poissons ratio of AlAs is deduced from the x-ray diffraction data based on two assumptions for the relationship between the AlGaAs lattice constant and its composition. For the first, Vegards law is assumed and Poissons ratio is found to be 0.255±0.004, which is much lower than expected ({>}0.31 for III-V semiconductor materials). However, higher values of 0.322±0.004 and 0.328±0.004 are obtained when assuming nonlinear relationships proposed in two previous studies. These high values are in excellent agreement with those obtained by independent measurements. Our results show the deviation from Vegards law in describing the relationship between the lattice constant of AlGaAs and its composition.

Evidence of the Effect of Nucleus Spatial Distributions on Thin Film Growth Kinetics

Experimental results are presented for the deposition of gold onto NaCl surfaces. These reveal significant inadequacies in some detailed aspects of current thin film nucleation and growth theories. The usual rate equations used in existing theories can be modified to allow for two important consequences of the adatom spatial distributions which occur as a result of adatom diffusion and subsequent capture by stable nuclei. Firstly, a non-uniform adatom density will result in a higher nucleation rate than would a uniform distribution of the same total adatom population. Secondly, non-random nucleation leading to a relative scarcity of near neighbour nuclei will reduce the rate of coalescence. These two effects, together with a modified expression for surface coverage which allows for coalescence, are incorporated in the rate equations and computed solutions are compared with the original solutions and with the experimental observations.

Formation of misfit dislocations in strained-layer GaAs/InxGa1−xAs/GaAs heterostructures during postfabrication thermal processing

It is demonstrated that relaxation of GaAs/InxGa1−xAs/GaAs strained-layer heterostructures can be brought about by postfabrication thermal processing. Misfit dislocations are introduced into the structure during thermal processing, even though the thickness of the strained layer is well below the critical value predicted by the Matthews–Blakeslee model. The misfit dislocations are observed to be of both 60° mixed type and 90° pure edge type. As no relaxation occurs at the lower temperatures encountered during fabrication by molecular-beam epitaxy, it can be inferred that the critical condition for the formation of misfit dislocations is not only a function of strained-layer thickness and composition, but also of temperature. This observation cannot be accounted for by differential thermal expansion or diffusion across the strained-layer interfaces, but the temperature-dependent Peierls force may offer an explanation. The high temperature required to produce relaxation of these structures suggests that the...

Investigation of oval defects in InGaAs/GaAs strained-layer heterostructures using cathodoluminescence and wavelength dispersive spectroscopy

Oval defects in In0.04Ga0.96As/GaAs strained-layer heterostructures have been investigated using cathodoluminescence (CL) and wavelength dispersive x-ray spectroscopy (WDS). WDS studies showed that the particulates seen at the center of oval defects are indium rich and gallium depleted. A luminescent halo was seen around the indium rich particulates in the CL mode. When the halo was studied further, it was shown that the peak obtained from CL spectroscopy due to the luminescent halo shifts to lower wavelengths as the beam is moved from the center of the oval defect to the edge of the halo region, indicating a decreasing gradient in indium concentration.

Edge-type misfit dislocations produced by thermal processing of pre-relaxed InxGa1−xAs/GaAs heterostructures

Dislocation structures are presented for GaAs/InxGa1−xAs/GaAs heterostructures before and after thermal processing. Cathodoluminescence has allowed nondestructive examination of bulk specimens, while transmission electron microscopy has been used to establish the details of the dislocation configurations. In each as-grown specimen the thickness of the InxGa1−xAs layer was above its critical value, so 60° misfit dislocations were already present. It is shown that new pure edge, i.e., 90°, dislocations are formed at the interfaces by thermal processing at 1040 K. Their Burgers vectors are a/2〈101〉 perpendicular to their 〈010〉 directions. Although individual 90° misfit dislocations are more effective relievers of strain than 60° ones, the self-energy for an array of such dislocations is higher and hence 60° misfit dislocations form first. A model of the formation of 90° edge misfit dislocations is proposed in which the climb of vacancy-producing jogs on pre-existing 60° dislocations leaves a trailing disloca...

Thickness and composition determination of MBE-grown strained multiple quantum well structures by x-ray diffraction

We present a new approach to determine the thickness and composition of individual layers in molecular beam epitaxy (MBE) grown multiple quantum well (MQW) structures by double crystal X-ray diffraction. The mean perpendicular lattice constant (d(perpendicular)SL0) and the period (P) of the MQW structures are calculated from the zeroth satellite peak position and the satellite peak spacings respectively. The thickness and composition of the individual layers are found by combining the growth times of the individual layers with d(perpendicular)SL0 and P. MQW structures of the form GaAs/XGaAs (X equals group III elements) grown on GaAs substrates are used to derive the theory for this approach and two MQW samples in the GaAs/InxGa1-xAs system are studied as an example of this approach. The results for both samples are checked by dynamical simulation. The simulated x-ray rocking curves for both samples have excellent agreement with the corresponding experimental x-ray rocking curves. This approach may be applied to other MQWs structures with two layers where one is a binary and the other is a ternary which includes the binary, such as AlAs/AlxGa1-xAs. It is therefore a technique not only for characterizing device structures containing MQWs but also for calibrating group III fluxes in MBE systems.

Poisson's ratio of GaAs

Most epitaxial layer growth involves mismatch between layer and growth substrate, even in the case of nearly lattice matched systems such as GaAs/AlAs. The accurate determination of layer thicknesses and stoichiometry in single and multi-layer heterostructures by x-ray diffraction requires knowledge of the elastic properties of the layers so that accurate account can be taken of strain effects. A novel technique has been developed which allows Poissons ratio to be determined by measuring a semiconductor sample in both the pseudomorphic and completely relaxed states. The relationship between the two strain states determines the materials Poisson ratio and this has been found to be 0.320/spl plusmn/0.001 for GaAs.

High Temperature Growth of the Dilute Nitride GaAsN using a Nitrogen ECR Plasma Source

The dilute nitride GaAsN has been grown by MBE using an ECR nitrogen plasma source. This has allowed growth at a substrate temperature of 600 C, which in combination with an ion trap, has produced higher quality as-grown material. Layer chemistry has been assessed by SIMS, XRD and optical quality measured using photoluminescence.

Experimental Observation of X-Ray Diffraction from a Thin Crystalline Film at a 90° Bragg Reflection

The first experimental observation of diffraction from a thin surface layer at a 90° Bragg reflection is reported. A thin (>1 μm) InGaAs film deposited on a GaAs(800) substrate was studied near the 90° Bragg position. Slight, less than 0.1%, difference in the lattice spacing between the layer and the substrate, has allowed, for the first time, a direct and exclusive observation of the diffraction profile from a thin layer as if it was a “free-standing” thin crystal.