A. Ballestad

Effect of the starting surface on the morphology of MBE-grown GaAs

Abstract In this paper, we study the homoepitaxial growth of GaAs by molecular beam epitaxy on substrates that have different pre-growth roughness due to the method of removing the native oxide. The evolution of the surface roughness of 1 μm thick GaAs films grown at 553°C was monitored in real time using ultraviolet light scattering, and compared with ex situ atomic force microscopy measurements of the power spectral density (PSD) of the surface morphology. The PSD at a spatial frequency of 2 μm −1 , is approximately three orders of magnitude larger for films grown on thermally cleaned substrates than for films grown on substrates cleaned with atomic hydrogen. No mounding indicative of unstable growth was observed in the films cleaned with atomic hydrogen.

Predicting GaAs surface shapes during MBE regrowth on patterned substrates

We have developed a coupled equations continuum model that explains the complex surface shapes observed in epitaxial regrowth on micron scale gratings. This model describes the dependence of the surface morphology on film thickness and growth temperature in terms of a few simple atomic scale processes including adatom diffusion, step-edge attachment and detachment, and a net downhill migration of surface adatoms. The continuum model reduces to the linear part of the Kardar-ParisiZhang equation with a flux dependent smoothing coefficient in the long wavelength limit.

Smoothing of textured GaAs surfaces during molecular beam epitaxy growth

The surface morphology of homoepitaxial GaAs layers grown by molecular beam epitaxy on random and periodically textured substrates has been measured by atomic force microscopy and elastic light scattering. The random texture was obtained by thermal evaporation of the surface oxide and the periodic texture consisted of one-dimensional grating patterns fabricated by holographic lithography. The time evolution of the surface morphology was simulated numerically with a nonlinear growth equation that includes deposition noise and anisotropy in the surface diffusion. The surface of the random substrate develops shallow mounds as the large amplitude initial texture smooths out, an effect that has previously been attributed to unstable growth.

In Situ Etch Rate Measurements by Alpha-Particle Energy Loss

When alpha-particles pass through thin films they lose an amount of energy proportional to the film thickness with the proportionality constant depending on the film composition. Thus, by measuring this energy loss one can determine the film thickness. We have applied this technique to measurements of the etch rate of various III-V semiconductor layers grown by molecular beam epitaxy. Prior to film growth, GaAs substrates were recoil-implanted with the alpha-emitting 224 Ra isotope by exposure to a 5µCi source of 228 Th. The implanted isotope decays with a half-life of 3.7 days, which allows measurements to be done for up to about two weeks after implantation. Following growth, the samples were etched in an electron cyclotron resonance etcher using a Cl 2 /BCl 3 /Ar gas mixture. As the film is etched the energy of the alpha-particles emitted from the surface increases. By introducing a high resolution Si detector into the etcher we are able to measure changes in the alpha-emission spectrum without removing the sample from the etcher. Thickness changes with an uncertainty of 5–10nm are obtained in 5 minute measurements at the end of each etch step. Some of the samples were also measured by SEM, yielding results in good agreement with values obtained by the alpha-particle measurements. As an example of an application of the technique we will describe measurements of the temperature dependence of the etch rate of GaAs in the 15–150 °C temperature range using optical bandgap thermometry to determine the substrate temperature. In a second example, we explore the application of the technique to etch rate of short pitch (250–500nm) grating. In this case the shape of the alpha-spectrum is sensitive to the profile of the etched trenches.

Surfactant enhanced growth of GaNAs and InGaNAs using a Bi flux

InGaNAs containing a dilute amount of nitrogen is a promising new material for fabricating optoelectronic devices in the 1.3 - 1.55 /spl mu/m wavelength range on GaAs substrates, in particular as the active material for vertical cavity surface emitting lasers. Recent edge-emitting InGaNAs lasers have demonstrated an improvement in both threshold currents and characteristic temperatures over those of an InP based reference laser. A flux of bismuth was applied during the growth of InGaNAs quantum wells and bulk GaNAs layers by elemental source MBE.