R. A. Kubiak

Evidence for quantum confinement in the photoluminescence of porous Si and SiGe

We have used anodization techniques to process porous surface regions in p-type Czochralski Si and in p-type Si0.85Ge0.15 epitaxial layers grown by molecular beam epitaxy. The SiGe layers were unrelaxed before processing. We have observed strong near-infrared and visible light emission from both systems. Analysis of the radiative and nonradiative recombination processes indicate that the emission is consistent with the decay of excitons localized in structures of one or zero dimensions.

On the practical applications of MBE surface phase diagrams

Abstract This paper reports detailed surface phase diagrams for (100) GaAs and (100)InAs. In the case of GaAs, growth rates from 0.07 to 3.0 μm/h, As 4 : Ga flux ratios from 0.25:1 to 100:1 and growth temperatures from 300°C to 800°C were used, covering the whole range of growth conditions of practical use in MBE. Results on the nucleation of the (100) InAs/GaAs heterojunction are also presented. The correlation between material properties, surface reconstruction and growth conditions is discussed for both GaAs and homo- and heteroepitaxial InAs. The emphasis of the paper is on the practical application of the results as an aid to optimising (and reproducing) MBE growth conditions with reasonable efficiency.

Scattering mechanisms affecting hole transport in remote‐doped Si/SiGe heterostructures

Boron modulation‐doped Si/SiGe heterojunctions have been grown by molecular beam epitaxy. The two‐dimensional hole gas formed along the heterojunction, just inside the alloy, has a sheet density in the range 2–5×1011 cm−2 and a typical mobility at 5 K of 2000 cm2 V−1 s−1. An explanation for the magnitude of the mobility is sought by considering likely scattering mechanisms, namely those due to remote impurities, interface roughness, alloy disorder, and interface impurities. A self‐consistent model is used to determine the sheet density in terms of structural and energy parameters and dopant concentrations in the heterostructure. It is shown that the presence of negatively charged impurities at the heterojunction provides the basis for a consistent interpretation of the experimental results.

Concentration of atomic hydrogen diffused into silicon in the temperature range 900–1300 °C

Boron-doped Czochralski silicon samples with [B]~1017 cm−3 have been heated at various temperatures in the range 800–1300 °C in an atmosphere of hydrogen and then quenched. The concentration of [H-B] pairs was measured by infrared localized vibrational mode spectroscopy. It was concluded that the solubility of atomic hydrogen is greater than [Hs] = 5.6 × 1018 exp( − 0.95 eV/kT)cm−3 at the temperatures investigated.

Elemental boron doping behavior in silicon molecular beam epitaxy

Boron-doped Si epilayers were grown by molecular beam epitaxy (MBE) using an elemental boron source, at levels up to 2×1020 cm−3, to elucidate profile control and electrical activation over the growth temperature range 450–900 °C. Precipitation and surface segregation effects were observed at doping levels of 2×1020 cm−3 for growth temperatures above 600 °C. At growth temperatures below 600 °C, excellent profile control was achieved with complete electrical activation at concentrations of 2×1020 cm−3, corresponding to the optimal MBE growth conditions for a range of Si/SixGe1−x heterostructures.

X-ray diffraction and reflectivity characterization of SiGe superlattice structures

The authors demonstrate the effectiveness of both X-ray diffraction and X-ray reflectivity in the structural characterization of semiconductor structures. By combining information from both techniques the abruptness of the interfaces for Si1-xGex structures, with x=0.1-0.57, may be determined. For superlattice structures with x<0.3 both types of interface were found to have a root mean square (RMS) roughness of 0.5+or-0.3 nm. For a Si/Si0.45Ge0.55 superlattice structure the interfaces are found to have differing roughnesses. For the SiGe-on-Si interface the RMS roughness is found to be 0.5+or-0.2 nm; however, the Si-on-SiGe interface has a larger value of roughness, 1.0+or-0.3 nm. This roughness at the Si-on-SiGe interface is found to be dependent on the Ge content of the layer and it is shown by transmission electron microscopy analysis to be long ranged (about 70 nm) and wavy at the interface.

Temperature dependence of incorporation processes during heavy boron doping in silicon molecular beam epitaxy

Boron doped layers were grown by silicon molecular beam epitaxy to establish incorporation processes at temperatures between 900 and 450 °C. For temperatures exceeding 650 °C a surface accumulated phase of boron was formed when doping levels exceeded solid solubility limits. The properties of this surface phase were used to determine solubility limits for boron in silicon. Above 750 °C, the measured equilibrium solubility limit was in the 1019‐cm−3 range in good agreement with previously published annealing data and showing a gradual decrease with decreasing temperature. Below 650 °C, the processes leading to the formation of the surface phase were kinetically limited, manifested by a sharp increase in boron solubility limit, with completely activated levels above 1 × 1020 cm−3 realized. At intermediate growth temperatures the degree of dopant activation was found to be dependent on growth rate. The stability of fully activated highly‐doped boron layers, grown at low temperatures, to ex situ annealing is ...

Very high two‐dimensional hole gas mobilities in strained silicon germanium

We report on the growth by solid source MBE and characterization of remote doped Si/SiGe/Si two‐dimensional hole gas structures. It has been found that by reducing the Ge composition to ≤13% and limiting the thickness of the alloy layer, growth temperatures can be increased up to 950 °C for these structures while maintaining good structural integrity and planar interfaces. Record mobilities of 19 820 cm2 V−1 s−1 at 7 K were obtained in normal structures. Our calculations suggest that alloy scattering is not important in these structures and that interface roughness and interface charge scattering limit the low temperature mobilities.

SIMS response functions for MBE grown delta layers in silicon

Abstract SIMS response functions and depth resolution parameters have been measured using O 2 + primary ions at normal incidence and 45°, for a range of silicon MBE grown epilayers containing ultrathin buried layers or “deltas” doped with B. Growth and decay slopes and differential shifts are shown to be species dependent. For B, the depth resolution is superior at normal incidence, and has a linear energy dependence for both normal incidence and 45°. The differential shift is dependent on the depth of a feature with respect to the final crater depth, and is due to erosion rate changes across the pre-equilibrium region, species dependent probe-induced mass transport, and the effect of the residual altered layer on crater depth measurement.

Growth studies on Si0.8Ge0.2 channel two-dimensional hole gases

We report a study of the influences of MBE conditions on the low‐temperature mobilities of Si/Si0.8Ge0.2 2DHG structures. A significant dependence of 2DHG mobility on growth temperature is observed with the maximum mobility of 3640 cm2 V−1 s−1 at 5.4 K being achieved at the relatively high‐growth temperature of 640 °C. This dependence is associated with a reduction in interface charge density. Studies on lower mobility samples show that Cu contamination can be reduced both by growth interruptions and by modifications to the Ge source; this reduction produces improvements in the low‐temperature mobilities. We suggest that interface charge deriving from residual metal contamination is currently limiting the 4‐K mobility.