Impurity diffusivity in thin films of GaAs is affected by native defect concentrations which were grown into the film, and which enter the film from both the substrate and the external ambient. Variations in measured diffusivity are related to native defects whose concentrations gradually relax to equilibrium values via different kinetic pathways. The time required for native defects to equilibrate depends upon the experimental design which in turn determines which region of the phase diagram that an annealing experiment begins and ends in. The Gibbs phase rule is applied to several commonly used experimental designs to show why the equilibrium native defect concentrations are generally not defined solely by temperature. The necessary conditions which define an equilibrium state, and the sufficient conditions which determine whether that equilibrium state can be approximated within a short time, are explicitly discussed. Experiments begun far from equilibrium are often associated with unusually high or low time-dependent diffusivities, but the results are often irreproducible as the native defect concentrations drift for extended periods of time. Experiments begun close to equilibrium are generally associated with reproducible diffusivities because the native defect concentrations reach constant values in a time which is short compared with the anneal period. Some diffusion results can be adequately described by equilibrium models, and the so-called ‘Fermi-level effect’ model is shown to apply as a special case of solid-vapor equilibrium. Near-equilibrium, results from several groups provide strong evidence that a Ga vacancy, with a charge of −1, controls group III element marker diffusion in n-type, intrinsic, and p-type GaAs at T > 800 °C. By relating an anneal ambient to a region of the ternary phase diagram, it becomes clear why the group II-related diffusion and other variables are extremely sensitive to the experimental design. Pinning of the Fermi energy at the surface during vapor phase epitaxy appears to explain why nonequilibrium concentrations of dopants and charged point defects can be grown into GaAs and affect diffusion in post-growth anneals. The relatively large random measurement noise often accompanying interdiffusion appears to be largely associated with the presence of Al in GaAs. We conclude that this behavior is associated with a residual contaminant, most likely oxygen. It also appears likely that small amounts of an oxidant have been present in many closed ampoule anneals and affected the reported interdiffusion results. We conclude that poorly understood metallurgical reactions are probably responsible for the enormous range of interdiffusion observed in device structures annealed under SiO 2 and Si 3 N 4 glass caps.

Point defects and diffusion in thin films of GaAs

Compositional disordering of InGaAs/GaAs superlattices using a low‐temperature‐grown GaAs cap layer (LT‐GaAs) by molecular beam epitaxy has been studied. The disordering of the superlattice was verified by photoluminescence and double‐crystal x‐ray rocking curve measurements. The Ga‐vacancy‐enhanced interdiffusion due to the presence of LT‐GaAs was found to be the disordering mechanism. Diffusion equations and the Schrodinger’s equation were solved numerically to obtain the composition profile and the transition energies in the disordered quantum well, respectively. The simulated energy shifts for samples under different annealing conditions agreed very well with the experimental results. The calculated effective diffusivity for the In–Ga interdiffusion has an activation energy of 1.63 eV, which is smaller than the activation energy 1.93 eV, for intrinsic interdiffusion. The diffusivity for the enhanced In–Ga interdiffusion due to the presence of LT‐GaAs is about two orders of magnitude larger than the intrinsic In–Ga diffusivity.

Compositional disordering of InGaAs/GaAs heterostructures by low-temperature-grown GaAs layers

Impurity-induced disordering seems to be a general phenomenon affecting all III–V materials. This paper reviews this phenomenon. Before the available data on impurity-induced disordering is discussed, the diffusion mechanisms for the dopants are briefly reviewed, and for silicon-doped material a more detailed discussion is provided because of its technological importance. Moreover, there has been no critical review of the recent progress in silicon diffusion mechanisms. In zinc-diffused multi-quantum wells and superlattices the mechanism for the enhancement of the interdiffusion seems to be primarily by the fast diffusion of group III interstitials whose concentration has been significantly increased over their equilibrium value by the diffusion of the dopant. For silicon impurity-induced disorder the situation is less clear. However, it is certain that the effect of the position of the Fermi level on the native defect concentrations plays a significant role in the disordering mechanism. The study of the other dopants which cause diffusion-induced disordering is less advanced. For n-type dopants there is an argument which suggests that they all have a common cause, namely the increase in the group III triply charged vacancy which is caused by the increase in the Fermi energy. This will be explored in this review. A brief discussion of ion implantation-induced disordering is also provided. This essentially provides a bibliography of the past work in this field. It is clear, for at least one dopant, that there needs to be some damage to the crystal before the enhancement of the interdiffusion occurs. However, for other dopants too much damage reduces the enhancement of the intermixing.

Impurity-induced disordering in III–V multi-quantum wells and superlattices

Several diffusion runs of Mn in GaAs are performed in sealed quartz ampoules with four different Mn‐containing sources: (a) solid crystal granules of Mn, (b) Mn3As, (c) MnAs, and (d) Mn thin films coated on GaAs substrates. Among these, only MnAs results in a smooth GaAs surface and uniform doping distributions. For the others interactions between the source materials and GaAs substrates give rise to poor surface morphologies and inhomogeneous distributions of new‐phase (Mn,Ga) crystals. For diffusion at 800 °C, surface p‐type carrier concentrations as high as 1020/cm3 are obtained. Diffusion profiles determined by C‐V techniques resemble those obtained for Zn diffusions. A substitutional‐interstitial mechanism is suggested as the primary diffusion mechanism for Mn in GaAs. Data are also presented showing that layer disordering in AlGaAs‐GaAs superlattices can be induced by Mn impurities.

Diffusion of manganese in GaAs and its effect on layer disordering in AlxGa1−xAs‐GaAs superlattices

Data are presented showing that the interdiffusion of Ga and Al (AlGaAs-GaAs) and of Ga and In (pseudomorphic InGaAs-GaAs) at a heterojunction can be significantly enhanced or suppressed by controlling group III vacancies and interstitials. High resistivity low-temperature GaAs which contains excess As is shown to accelerate the interdiffusion process. The emission wavelength from monolayer-thick quantum wells can remain relatively unchanged even after annealing for 200 h at 850° C.

Enhanced/suppressed interdiffusion of lattice-matched and pseudomorphic III–V heterostructures by controlling Ga vacancies

The interdiffusion of In and Ga at an InGaAs-GaAs interface subjected to different annealing temperatures, times, and environments is demonstrated. The interdiffusion coefficients and activation energies are determined by correlating the shift in the photoluminescence peaks with the calculated quantum well transition energies based on an error function composition profile. The results indicate that a higher In composition InxGa1-xGaAs single quantum well (SQW) leads to a higher interdiffusion coefficient of In and Ga in an As overpressure annealing condition. Also, As overpressure increases the interdiffusion, whereas Ga overpressure reduces the interdiffusion. The thermal activation energies for different In composition InGaAs-GaAs SQW’s (x = 0.057, 0.10, 0.15) range from 3.3 to 2.6 eV for an As overpressure environment and from 3 to 2.23 eV for the Ga overpressure situation. With respect to impurity induced disordering by Zn using a Ga or As overpressure significantly effects the depth of the Zn diffusion front but significant mixing does occur in either case when the impurity front reaches the quantum well.

Y. L. Hwang

Papers

Enhanced/suppressed interdiffusion of InGaAs-GaAs-AlGaAs strained layers by controlling impurities and Gallium vacancies