A.J. Pidduck

The characteristics of strain-modulated surface undulations formed upon epitaxial Si1−xGex alloy layers on Si

Abstract When strained, continuous Si 1- x Ge x alloy layers are prepared by epitaxial growth on Si substrates, the growth surface can become nonplanar. In the present work, combined transmission electron microscope and atomic force microscope studies are employed to reveal the detailed nature of the surface ripples and undulations which form. Under specific ranges of growth conditions, crystallographically aligned, interlocking ripple arrays are produced. TEM contrast studies demonstrate that well-defined, oscillatory strain variations accompany these ripple structures, the presence of which is shown to be associated with partial elastic strain-relief and lowering of the energy of the strained-layer system. It is also demonstrated that the upper surface of a Si cap deposited on such a layer rapidly becomes planar with increasing thickness, this being likely to result from the reduction in surface energy so achieved.

Growth morphology evolution and dislocation introduction in the InGaAsGaAs heteroepitaxial system

Abstract The present work examines in detail heteroepitaxial In x Ga 1 − x As alloy layers on GaAs by use of complementary transmission electron microscopy and atomic force microscopy. The characteristics of the low In x -value, smooth growth regime are established in terms of surface step configurations. Progressively increasing irregularities in step fronts and monolayer island formation with increasing In concentration in the alloy are linked with the transition to undulating growth as an x -value of 0.25 is approached. The evolution of high x -value roughened layers is studied and the structure changes with increasing film thickness and In content are determined. The manner in which final ripple arrays evolve from isolated islands is described and the stress interaction between islands is highlighted. The magnitude of the periodic elastic stress field which accompanies the formation of the ripple structures is microscopically measured and is shown to yield essentially complete misfit relief within the ripple crests. The increased stress present at ripple troughs is shown to lead to misfit defect source behaviour, which is expected to be of wide-ranging importance for defect generation in strained, undulating epitaxial films in general.

Real-time laser-light scattering studies of surface topography development during GaAs MBE growth

Abstract In-situ laser light scattering is shown to yield valuable real-time information on the development of surface micro-topography during GaAs MBE growth. In-vacuo oxide desorption results in a large increase in scattered light intensity. Ex-situ atomic force microscopy has indicated the formation of a high density (>10 9 cm -2 ) of surface pits, attributed to localized GaAs consumption. Subsequent GaAs homoepitaxial growth (at 600°C) results in the development of an anisotropic ridged topography (ridge axes parallel to [110] with ridge dimensions increasing with epilayer thickness (peak-valley ≤ 12 nm for a 2.0 μm thick epilayer). Post growth in-vacuo annealing generates relatively flat surfaces, dominated by misorientation steps. For a 1 μm thick epilayer, annealing has a long time constant ( > 30 min at 600°C).

Surface topography changes during the growth of GaAs by molecular beam epitaxy

Changes in surface roughness taking place during (001) GaAs molecular beam epitaxy growth have been studied in situ using laser light scattering and ex situ using atomic force microscopy (AFM). Substantial increases in light scattering are found to occur firstly during oxide thermal desorption, associated with surface pit formation, and secondly during continued layer growth, due to the buildup of atomic step arrays. Monolayer height GaAs steps are readily resolved using AFM in air.

The formation of dislocations and their in-situ detection during silicon vapour phase epitaxy at reduced temperature

Abstract In (001) silicon vapour phase homoepitaxy from silane at 850°C, we show that carbon-containing impurity particles, formed during oxide removal at 900°C prior to epitaxy, can originate epilayer dislocations during initial growth. The process is studied by in-situ laser light scattering (LLS) and optical and electron microscopies, and conditions for reproducibly minimizing interface carbon and layer defects are discussed. LLS is a practical real-time monitor of layer quality.

Atomic force microscopy studies of substrate cleaning using tris(dimethylamino)arsenic and tris(dimethylamino) antimony and investigations of surface decomposition mechanisms

Atomic force microscopy studies have been performed on substrates cleaned in a chemical beam epitaxy reactor using tris(dimethylamino)arsenic (TDMAAs) or tris(dimethylamino)antimony (TDMASb) at substrate temperatures well below those required for thermal desorption of the native oxide. For the first time, we report surface oxide removal from InAs(001) at 370°C using TDMAAs, and from InSb(001) and GaSb(001) at 405°C and 495°C respectively, using TDMASb. Atomically flat surfaces are achieved which are suitable for epitaxial growth without a buffer layer. Preliminary experiments indicate InSb growth may be achieved using uncracked TDMASb with trimethylindium. Decomposition studies of TDMAAs on GaAs using modulated beam mass spectrometry show an important change in the decomposition efficiency of TDMAAs above 480°C.

Rippled surface topography observed on silicon molecular beam epitaxial and vapour phase epitaxial layers

Abstract A microscopically rippled texture can develop on the surface of Si(001) homoepitaxial layers grown by molecular beam epitaxy (MBE) and vapour phase epitaxy (VPE), and does not appear to depend on layer impurities or defects. From its relationship with substrate misorientation, we conclude that the texture is a result of a step-flow growth, during which 〈110〉 step edges are stable in MBE, but 〈100〉 steps form in VPE.

The measurement of surface boron on silicon wafers annealed in vacuum and gas ambients

Abstract Thermal desorption of surface oxide from silicon wafers has been studied in ultrahigh vacuum and in hydrogen by secondary-ion mass spectrometry. Heating in 132 Pa hydrogen virtually eliminates residual boron contamination which creates a p-type surface layer on vacuum-annealed wafers. Lower pressures of hydrogen and substitution of nitrogen have proved less efficient in reducing surface boron concentration. In-situ ellipsometry indicates differences in reaction kinetics for oxide desorption in vacuum and in hydrogen.

Using a hydrogen ambient to eliminate interfacial boron spikes in reduced temperature silicon epitaxy

We show that the concentration of boron at the interface between a silicon epitaxial layer and an n‐type Si substrate can be reduced to 109–1010 atoms cm−2, near the detection limit of secondary‐ion mass spectrometry, when passivating oxide is desorbed from the substrate surface in a hydrogen ambient. Interfacial boron contamination is found to be much higher when the substrate surface oxide is desorbed in ultrahigh vacuum prior to growth, consistent with previous studies of Si molecular beam epitaxy. The hydrogen ambient is effective in removing boron only during the decomposition of the surface oxide.

Si/SiGe n-type inverted modulation doping using ion implantation

Abstract Inverted modulated-doped Si/Si 1− x Ge x wafers have been prepared using ex-situ ion implantation of a virtual substrate for the doping followed by cleaning and the regrowth of the silicon quantum well in the growth system. This fabrication scheme attempts to circumvent the main problem in the growth of inverted modulation doped structures by chemical vapour deposition, which is the surface segregation and diffusion of n-type dopant causing high dopant densities in all subsequent layers after the introduction of n-type dopant to the growth chamber. Magneto-transport results will be shown for modulation-doped field effect transistor (MODFET) samples which have 1.7 K mobilities up to 73 000 cm 2 /V s for carrier concentrations of 4.25×10 11 cm −2 . Clear Shubnikov-de Haas oscillations and quantum Hall effect plateaux are visible demonstrating the quality of the samples produced using the technique.