E. H. C. Parker

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.

Extremely high room-temperature two-dimensional hole gas mobility in Ge/Si0.33Ge0.67/Si(001) p-type modulation-doped heterostructures

To extract the room-temperature drift mobility and sheet carrier density of two-dimensional hole gas (2DHG) that form in Ge strained channels of various thicknesses in Ge/Si0.33Ge0.67/Si(001) p-type modulation-doped heterostructures, the magnetic field dependences of the magnetoresistance and Hall resistance at temperature of 295 K were measured and the technique of maximum entropy mobility spectrum analysis was applied. This technique allows a unique determination of mobility and sheet carrier density of each group of carriers present in parallel conducting multilayers semiconductor heterostructures. Extremely high room-temperature drift mobility (at sheet carrier density) of 2DHG 2940 cm2 V–1 s–1 (5.11×1011 cm–2) was obtained in a sample with a 20 nm thick Ge strained channel.

Effective mobilities in pseudomorphic Si/SiGe/Si p-channel metal-oxide-semiconductor field-effect transistors with thin silicon capping layers

The room-temperature effective mobilities of pseudomorphic Si/Si0.64Ge0.36/Si p-metal-oxidesemiconductor field effect transistors are reported. The peak mobility in the buried SiGe channel increases with silicon cap thickness. It is argued that SiO2/Si interface roughness is a major source of scattering in these devices, which is attenuated for thicker silicon caps. It is also suggested that segregated Ge in the silicon cap interferes with the oxidation process, leading to increased SiO2/Si interface roughness in the case of thin silicon caps.

RBS ANALYSIS OF MBE GROWN SiGe/(001)Si HETEROSTRUCTURES WITH THIN HIGH Ge CONTENT SiGe CHANNELS FOR HMOS TRANSISTORS

The growth of Si1-xGex quantum wells with high Ge composition (x>0.5) on Si(001) substrates by MBE is of great interest both for HMOS device applications and fundamental research. One of the possibilities to obtain a high Ge content Si1-xGex channel for mobile carriers, while retaining its strain, is to grow a relaxed Si1-yGey buffer layer on an underlying Si substrate. Such a buffer is termed a virtual substrate (VS), which is a constituent of SiGe metamorphic heterostructures. The effect of annealing on the structure of the Si(001)/VS/Si0.7Ge0.3/Si0.2Ge0.8/Si0.7Ge0.3 heterostructures was studied by grazing angle of incidence RBS. The thickness of the Si0.2Ge0.8 channel is inhomogeneous, which makes the analysis of the data by traditional means very hard. We have developed a model whereby the influence of the thickness inhomogeneity of each layer in the apparent energy resolution as a function of depth can be calculated. Automatic fits to the data were performed, and the roughness parameters, that is, the standard deviation of the thickness inhomogeneity of the relevant layers, were obtained, together with the thickness and stoichiometry of each layer. The results are compared with TEM and high resolution XRD experiments.

Potential enhanced Sb and As doping in Si molecular beam epitaxy

Potential enhanced doping (PED) is a method of enhancing the incorporation efficiency of certain low sticking coefficient dopants in Si molecular beam epitaxy, and obtaining precise control over profiles. The efficacy of PED is demonstrated for Sb, As (and Ga) doping, using elemental and III‐V compound coevaporation sources. Enhanced doping by PED depends on increased incorporaton efficiency from the adsorbed dopant adlayer, and does not significantly affect adsorption/desorption processes.

Spin transport in germanium at room temperature

Spin-dependent transport is investigated in a Ni/Ge/AlGaAs junction with an electrodeposited Ni contact. Spin-polarised electrons are excited by optical spin orientation and are subsequently used to measure the spin dependent conductance at the Ni/Ge Schottky interface. We successfully demonstrate electron spin transport and electrical extraction from the Ge layer at room temperature.

Hole density dependence of effective mass, mobility and transport time in strained Ge channel modulation-doped heterostructures

We performed systematic low-temperature (T = 350 mK–15 K) magnetotransport measurements on the two-dimensional hole gas with various sheet carrier densities Ps = (0.57–2.1)×1012 cm–2 formed in the strained Ge channel modulation-doped (MOD) SiGe heterostructures grown on Si substrates. It was found that the effective hole mass deduced by temperature dependent Shubnikov–de Hass oscillations increased monotonically from (0.087±0.05)m0 to (0.19±0.01)m0 with the increase of Ps, showing large band nonparabolicity in strained Ge. In contrast to this result, the increase of the mobility with increasing Ps (up to 29 000 cm2/V s) was observed, suggesting that Coulomb scattering played a dominant role in the transport of the Ge channel at low temperatures. In addition, the Dingle ratio of the transport time to the quantum lifetime was found to increase with increasing Ps, which was attributed to the increase of remote impurity scattering with the increase of the doping concentration in MOD SiGe layers.

Ultra-high hole mobility exceeding one million in a strained germanium quantum well

In this paper, we report a Hall mobility of one million in a germanium two-dimensional hole gas. The extremely high hole mobility of 1.1 × 106 cm2 V−1 s−1 at a carrier sheet density of 3 × 1011 cm−2 was observed at 12 K. This mobility is nearly an order of magnitude higher than any previously reported. From the structural analysis of the material and mobility modeling based on the relaxation time approximation, we attribute this result to the combination of a high purity Ge channel and a very low background impurity level that is achieved from the reduced-pressure chemical vapor deposition growth method.

High conductance Ge p-channel heterostructures realized by hybrid epitaxial growth

Strained Ge p-channel heterostructures have been realized by hybrid-epitaxial growth. Strain-tuning Si0.4Ge0.6 virtual substrates were grown by ultra-high vacuum chemical vapour deposition and active layers were deposited by solid-source molecular beam epitaxy at low temperature. Following ex situ annealing, Hall effect measurements revealed a hole mobility of 1900 cm2 V−1 s−1 at 300 K (27 000 cm2 V−1 s−1 at 10 K), with a density of 1.8 × 1012 cm−2, giving a conductance in excess of current Ge heterostructures. Using a maximum-entropy mobility-spectrum analysis, 1.0 × 1012 cm−2 of these holes were found to have a mobility of 2700 cm2 V−1 s−1 at 300 K.