D. R. Leadley

Reverse graded relaxed buffers for high Ge content SiGe virtual substrates

An innovative approach is proposed for epitaxial growth of high Ge content, relaxed Si1−xGex buffer layers on a Si(001) substrate. The advantages of the technique are demonstrated by growing such structures via chemical vapor deposition and their characterization. Relaxed Ge is first grown on the substrate followed by the reverse grading approach to reach a final buffer composition of 0.78. The optimized buffer structure is only 2.8μm thick and demonstrates a low surface threading dislocation density of 4×106cm−2, with a surface roughness of 2.6nm. The buffers demonstrate a relaxation of up to 107%.

Reverse graded SiGe/Ge/Si buffers for high-composition virtual substrates

The effect of compositional grading rate on reverse linear graded silicon germanium virtual substrates, grown by reduced pressure chemical vapor deposition, is investigated. For a Si(001)/Ge/RLG/Si0.22Ge0.78 buffer of 2.4 μm total thickness the threading dislocation density (TDD) within the top, fully relaxed, Si0.22Ge0.78 layer is 4×106 cm−2, with a surface roughness of 3 nm. For a thicker buffer, where the grading rate is reduced, a lower TDD of 3×106 cm−2 and a surface roughness of 2 nm can be achieved. The characteristics of reverse graded Si0.22Ge0.78 virtual substrates are shown to be comparable to, or exceed, conventional buffer techniques, leading to thinner high-quality high Ge composition SiGe virtual substrates.

Ohmic contacts to n-type germanium with low specific contact resistivity

A low temperature nickel process has been developed that produces Ohmic contacts to n-type germanium with specific contact resistivities down to (2.3 ± 1.8) × 10−7 Ω-cm2 for anneal temperatures of 340 °C. The low contact resistivity is attributed to the low resistivity NiGe phase which was identified using electron diffraction in a transmission electron microscope. Electrical results indicate that the linear Ohmic behaviour of the contact is attributed to quantum mechanical tunnelling through the Schottky barrier formed between the NiGe alloy and the heavily doped n-Ge.

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.

Modulation of the absorption coefficient at 1.3 μm in Ge/SiGe multiple quantum well heterostructures on silicon

We report modulation of the absorption coefficient at 1.3 μm in Ge/SiGe multiple quantum well heterostructures on silicon via the quantum-confined Stark effect. Strain engineering was exploited to increase the direct optical bandgap in the Ge quantum wells. We grew 9 nm-thick Ge quantum wells on a relaxed Si0.22Ge0.78 buffer and a contrast in the absorption coefficient of a factor of greater than 3.2 was achieved in the spectral range 1290-1315 nm.

Modelling the inhomogeneous SiC Schottky interface

For the first time, the I-V-T dataset of a Schottky diode has been accurately modelled, parameterised, and fully fit, incorporating the effects of interface inhomogeneity, patch pinch-off and resistance, and ideality factors that are both heavily temperature and voltage dependent. A Ni/SiC Schottky diode is characterised at 2 K intervals from 20 to 320 K, which, at room temperature, displays low ideality factors (n   8), voltage dependent ideality factors and evidence of the so-called “thermionic field emission effect” within a T0-plot, suggest significant inhomogeneity. Two models are used, each derived from Tungs original interactive parallel conduction treatment of barrier height inhomogeneity that can reproduce these commonly seen effects in single temperature I-V traces. The first model incorporates patch pinch-off effects and produces accurate and reliable fits above around 150 K, and at current densities lower than 10−5 A cm−2. Outside this region, we show that resistive effects within a given patch are responsible for the excessive ideality factors, and a second simplified model incorporating these resistive effects as well as pinch-off accurately reproduces the entire temperature range. Analysis of these fitting parameters reduces confidence in those fits above 230 K, and questions are raised about the physical interpretation of the fitting parameters. Despite this, both methods used are shown to be useful tools for accurately reproducing I-V-T data over a large temperature range.

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.

Investigation of Strain Engineering in FinFETs Comprising Experimental Analysis and Numerical Simulations

This study combines direct measurements of strain, electrical mobility measurements, and a rigorous modeling approach to provide insights about strain-induced mobility enhancement in FinFETs and guidelines for device optimization. Good agreement between simulated and measured mobility is obtained using strain components measured directly at device level by a novel holographic technique. A large vertical compressive strain is observed in metal gate FinFETs, and the simulations show that this helps recover the electron mobility disadvantage of the (110) FinFET lateral interfaces with respect to (100) interfaces, with no degradation of the hole mobility. The model is then used to systematically explore the impact of stress components in the fin width, height, and length directions on the mobility of both n- and p-type FinFETs and to identify optimal stress configurations. Finally, self-consistent Monte Carlo simulations are used to investigate how the most favorable stress configurations can improve the on current of nanoscale MOSFETs.

CRITICAL COLLAPSE OF THE EXCHANGE-ENHANCED SPIN SPLITTING IN TWO-DIMENSIONAL SYSTEMS

The critical filling factor v_c where Shubnikov-de Haas oscillations become spin split is investigated for a set of GaAs-GaAlAs heterojunctions. Finite temperature magnetoresistance measurements are used to extract the value of v_c at zero temperature. The critically point is where the disorder potential has the same magnitude as the exchange energy, leading to the empirical relationship v_c = g* n t h / 2 m_0. This is valid for all the samples studied, where the density n and single particle lifetime t both vary by more than an order of magnitude and g* the exchange enhanced g-factor has a weak dependence on density. For each sample the spin gap energy shows a linear increase with magnetic field. Experiments in tilted magnetic field show the spin gap is the sum of the bare Zeeman energy and an exchange term. This explains why measurements of the enhanced g-factor from activation energy studies in perpendicular field and the coincidence method in tilted fields have previously disagreed.