Stephen Rhead

An extremely high room temperature mobility of two-dimensional holes in a strained Ge quantum well heterostructure grown by reduced pressure chemical vapor deposition

An extremely high room temperature two-dimensional hole gas (2DHG) drift mobility of 4230 cm2 V−1 s−1 in a compressively strained Ge quantum well (QW) heterostructure grown by an industrial type RP-CVD technique on a Si(001) substrate is reported. The low-temperature Hall mobility and carrier density of this structure, measured at 333 mK, are 777000 cm2 V−1 s−1 and 1.9 × 1011 cm−2, respectively. These hole mobilities are the highest not only among the group-IV Si based semiconductors, but also among p-type III–V and II–VI ones. The obtained room temperature mobility is substantially higher than those reported so far for the Ge QW heterostructures and reveals a huge potential for further application of strained Ge QW in a wide variety of electronic and spintronic devices.

Observation of Rashba zero-field spin splitting in a strained germanium 2D hole gas

We report the observation, through Shubnikov-de Haas oscillations in the magnetoresistance, of spin splitting caused by the Rashba spin-orbit interaction in a strained Ge quantum well epitaxially grown on a standard Si(001) substrate. The Shubnikov-de Haas oscillations display a beating pattern due to the spin split Landau levels. The spin-orbit parameter and Rashba spin-splitting energy are found to be 1.0 × 10−28  eVm3 and 1.4 meV, respectively. This energy is comparable to 2D electron gases in III-V semiconductors, but substantially larger than in Si, and illustrates the suitability of Ge for modulated hole spin transport devices.

Weak antilocalization of high mobility holes in a strained Germanium quantum well heterostructure

We present the observation of weak antilocalization due to the Rashba spin-orbit interaction, through magnetoresistance measurements performed at low temperatures and low magnetic fields on a high mobility (777,000 cm(2) V(-1) s(-1)) p-Ge/SiGe quantum well heterostructure. The measured magnetoresistance over a temperature range of 0.44 to 11.2 K shows an apparent transition from weak localization to weak antilocalization. The temperature dependence of the zero field conductance correction is indicative of weak localization using the simplest model, despite the clear existence of weak antilocalization. The Rashba interaction present in this material, and the absence of the un-tuneable Dresselhaus interaction, indicates that Ge quantum well heterostructures are highly suitable for semiconductor spintronic applications, particularly the proposed spin field effect transistor.

Ge/SiGe quantum confined Stark effect electro-absorption modulation with low voltage swing at λ = 1550 nm

Low-voltage swing (≤1.0 V) high-contrast ratio (6 dB) electro-absorption modulation covering 1460 to 1560 nm wavelength has been demonstrated using Ge/SiGe quantum confined Stark effect (QCSE) diodes grown on a silicon substrate. The heterolayers for the devices were designed using an 8-band k.p Poisson-Schrödinger solver which demonstrated excellent agreement with the experimental results. Modelling and experimental results demonstrate that by changing the quantum well width of the device, low power Ge/SiGe QCSE modulators can be designed to cover the S- and C-telecommunications bands.

Electrical isolation of dislocations in Ge layers on Si(001) substrates through CMOS-compatible suspended structures

Abstract Suspended crystalline Ge semiconductor structures are created on a Si(001) substrate by a combination of epitaxial growth and simple patterning from the front surface using anisotropic underetching. Geometric definition of the surface Ge layer gives access to a range of crystalline planes that have different etch resistance. The structures are aligned to avoid etch-resistive planes in making the suspended regions and to take advantage of these planes to retain the underlying Si to support the structures. The technique is demonstrated by forming suspended microwires, spiderwebs and van der Pauw cross structures. We finally report on the low-temperature electrical isolation of the undoped Ge layers. This novel isolation method increases the Ge resistivity to 280 Ω cm at 10 K, over two orders of magnitude above that of a bulk Ge on Si(001) layer, by removing material containing the underlying misfit dislocation network that otherwise provides the main source of electrical conduction.

Tensile strain mapping in flat germanium membranes

The membranes have the potential to be excellent growth and integration platforms: compared to bulk Ge epitaxially grown on Si (001) they are perfectly flat and XRD and PV-TEM confirm the misfit dislocation network has been removed. The strain profile across the membrane is symmetrical and the membrane is slightly more tensile strained than the bulk material. The difference in strain across the membrane is too small to create a large variation in optical device performance across the entire membrane. Coupled with the smoother surface and absence of misfit dislocation network compared to the bulk material, the membranes are both excellent strain tuning platforms for optical applications and, more generally, for growth of subsequent active layers.

High quality single crystal Ge nano-membranes for opto-electronic integrated circuitry

A thin, flat, and single crystal germanium membrane would be an ideal platform on which to mount sensors or integrate photonic and electronic devices, using standard silicon processing technology. We present a fabrication technique compatible with integrated-circuit wafer scale processing to produce membranes of thickness between 60 nm and 800 nm, with large areas of up to 3.5 mm2. We show how the optical properties change with thickness, including appearance of Fabry-Perot type interference in thin membranes. The membranes have low Q-factors, which allow the platforms to counteract distortion during agitation and movement. Finally, we report on the physical characteristics showing sub-nm roughness and a homogenous strain profile throughout the freestanding layer, making the single crystal Ge membrane an excellent platform for further epitaxial growth or deposition of materials.

Laser-vibrometric ultrasonic characterization of resonant modes and quality factors of Ge membranes

Abstract The vibrations of a single-crystal germanium (Ge) membrane are studied in air and vacuum using laser vibrometry, in order to determine mechanical properties such as Q-factors, tensile stress, anisotropy, and robustness to shock. Resonance modes up to 3:2 are identified, giving a residual stress measurement of 0.22 GPa, consistent with the value obtained from x-ray relaxation studies. The membrane is found to be isotropic, with Q-factors ranging from around 40 at atmospheric pressure to over 3200 at mbar, significantly lower than those found in polycrystalline Ge micromechanical devices. The robustness to shock is explained through the high resonance mode frequencies and the dissipation mechanism into the substrate, which is a direct consequence of having a high quality film with low residual tensile stress, giving the potential for such films to be used in optoelectronic devices.

Growth of complex SiGe/Ge superlattices by reduced pressure chemical vapour deposition at low temperature

In this work the growth of complex n-type, high Ge content superlattice structures by reduced pressure chemical vapor deposition is presented. The structures feature 50 repeats of a 14 layer period, which includes a main quantum well that is between 13 and 21 nm wide. The total epitaxy thickness is approximately 8 μm. Diffusion and segregation in the structures was minimized by using a low growth temperature. Materials characterization shows the structures to be of good crystalline quality, with the thickness of all layers close to the design, abrupt interfaces, and uniformity throughout the structures. High angle annular dark field scanning transmission electron microscopy is shown to be an ideal technique for measuring layer thickness and interface quality in these structures.

Mapping the Strain State of 3C-SiC/Si (001) Suspended Structures Using μ-XRD

The residual strain has been mapped across suspended 3C-SiC membranes and wires using synchrotron based micro X-ray diffraction (μ-XRD). Residual tensile strain is observed to relax slightly upon suspension in both sets of structures. Similar maps were acquired by calculating the residual strain from the shift in 3C-SiC Raman peaks. Comparable trends in strain relaxation are observed by both methods, although the sensitivity of μ-XRD is higher using our measurement conditions. While Raman shift provides a fast and convenient method for mapping strain variations, it cannot give direct measurements of the lattice parameters that can be achieved with μ-XRD, making these techniques excellent complimentary methods of mapping residual strain in 3C-SiC.