T. Pinnington

Relationship between surface morphology and strain relaxation during growth of InGaAs strained layers

The relationship between the surface morphology and strain relaxation is explored in strained InxGa1−xAs layers grown on GaAs by molecular beam epitaxy. In situ light scattering, detected simultaneously along [110] and [110], reveals an asymmetric surface roughening which is consistent with ex situ scanning force microscopy. Transmission electron microscopy shows that strain relaxation by misfit dislocation formation occurs before the surface roughening is detected, for In0.18Ga0.82As films grown at 490 °C.

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.

Cross‐sectional imaging of doped layers in epitaxial gallium arsenide films by scanning tunneling microscopy

Cross sections of epitaxial GaAs/GaAlAs layers have been imaged by scanning tunneling microscopy in air, and individual doped layers with different conductivity type have been resolved for the first time in air. In constant current images acquired with a positive tip bias, the n regions appear to be higher than the p regions, by ∼5 A depending on the junction bias due to an electronic contrast mechanism. A new imaging method is described which decouples the electronic contrast from the topography. In this method, one obtains an image by repetitively interrupting the feedback loop and sampling the tunneling current at a different tip–sample voltage, selected to yield high conductivity‐type contrast.

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.

Effect of growth conditions on surface roughening of relaxed InGaAs on GaAs

We report elastic light scattering measurements of the surface morphology of strained InxGa1–xAs on GaAs, grown by molecular beam epitaxy at different growth temperatures and In contents. During strain relaxation through formation of interfacial misfit dislocations, the surface of the film roughens in response to inhomogeneous surface strains produced by the interfacial misfit dislocations. The time dependence of this roughening is modeled by an Edwards–Wilkinson equation in which the deposition flux noise is neglected and the inhomogeneous surface stress is the only driving term.

SURFACE ROUGHNESS AND RESONANT SCATTERING EFFECTS IN SOFT X-RAY SPECKLE FROM RANDOM SEMICONDUCTOR INTERFACES

We report the use of coherent soft X-ray scattering, or speckle, to study the morphology of random surfaces on single crystal semiconductor substrates. The experiments were carried out with photon energies in the 266–290 eV range. The effect of the magnitude of the roughness on the scattering was observed by measuring speckle patterns at various angles of incidence from two different surfaces with InP islands in the nanometer size range and in the micron range. The effect of element-specific resonant scattering was explored with a PMMA-coated textured silicon sample by tuning the wavelength to the carbon K-edge. Two dimensional numerical simulations of the coherent scattering have been carried out in the Fraunhofer approximation, using AFM data for the sample surface morphology. Good agreement with the observed speckle patterns was obtained, taking into account the 20 μm lateral coherence length of the source.

In situ detection of misfit dislocations by light scattering

Abstract The in situ detection of strain relaxation from misfit dislocation formation using diffuse light scattering was studied during molecular beam epitaxy (MBE) growth of InGaAs GaAs (0 0 1) . The technique can be implemented for either MBE or metal-organic vapor-phase-epitaxy growth and is sensitive to atomic-scale roughness with correlation lengths in the range 0.4–30 μm, providing information complementary to reflection high energy electron diffraction. An increase in scattering intensity associated with roughening from dislocations is detected above a certain thickness. However, ex situ characterization of quenched samples by transmission electron microscopy, atomic force microscopy (AFM) and X-ray diffraction showed that this onset was associated with roughening from preferential growth associated with misfit dislocations and not to the initial onset of dislocation formation. Information from AFM scans, ex situ optical and soft-X-ray light scattering are compared and show that the detection of lines of steps signaling the first misfit dislocations from strain relaxation was lost in the background signal from the initial roughness of the GaAs substrate buffer surface used in our studies.

Coherent soft x-ray scattering from InP islands on a semiconductor substrate

Coherent soft x-ray scattering experiments from a semiconductor sample consisting of InP islands on a smooth semiconductor substrate are described. The soft x-ray scattering was performed with 266 eV photons produced by an undulator source. Using a position sensitive detector, we are able to detect diffusely scattered x rays in the vicinity of the specular reflection, with an in-plane momentum transfer of up to 6 μm−1. Using Huygens–Fresnel theory and atomic force microscope images of the surface structure, we simulated the scattering assuming a finite lateral coherence length for the incident radiation. The lateral coherence length of the incident beam was found to be 20 μm from a fit to the observed diffraction pattern from a pinhole. The effect of changes in the surface morphology on the speckle pattern was simulated to explore the potential of coherent soft x-ray scattering for the study of surface structure dynamics.

Light Scattering Study of the Evolution of the Surface Morphology During Growth of Ingaas on GaAs

In-situ elastic light scattering is used to measure the evolution of the surface morphology of In{sub x}Ga{sub 1{minus}x}As films during molecular beam epitaxy growth on GaAs substrates. The in-situ measurements are compared with ex-situ measurements o the surface morphology on quenched films by optical scatterometry and atomic force microscopy (AFM). The AFM results are in good agreement with the rms roughness obtained from light scattering and both techniques detect the onset of misfit dislocation formation in this system.