John E. Halpin

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.

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.

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.

Characterization of SiGe thin films using a laboratory X-ray instrument

This article reports the characterization of thin SiGe/Si(100) epilayers using reciprocal space maps measured by a laboratory X-ray instrument and a high-resolution X-ray diffraction study of partially relaxed SiGe/Si thin films.

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.

Tensile strained Ge membranes

Germanium membranes of 50-1000 nm thickness have been fabricated by a combination of epitaxial growth on a Si substrate and simple etching processes. The biaxial tensile strain in these membranes has been measured by high-resolution X-ray diffraction and by ultrasonic vibrational spectroscopy. The later technique also shows that membranes have a Q-factor of ~3000 at low temperature. The stain in these membranes is extremely isotropic and the surface is observed to be very smooth, with an rms roughness of below 2 nm. The process of membrane fabrication also serves to remove the misfit dislocation network that originally forms at the Si/Ge interface, with benefits for the mechanical, optical and electrical properties of the crystalline membranes.

Reverse terrace graded Si 1−x Ge x /Ge/Si(001) virtual substrates grown using RP-CVD on on-axis and 6° off-axis substrates for future developments in III-V materials integration

Reverse terrace graded (RTG) Si1-xGex/Ge relaxed buffer layers grown on Si(001) substrates that are oriented on-axis and off-axis (6° toward the [110] direction) show similar levels of strain, surface roughness and threading dislocation density (TDD) to Ge and Si1-xGex epilayers of similar thickness and Ge content. For identical growth conditions and times, a reduced growth rate is observed for buffer layers on off-axis substrates; which leads to thinner layers. The findings in this article contradict previous reports for relaxed Ge buffer layers grown on 6° off-axis Si(001) substrates [1].

Weak anti-localization behavior of high mobility 2D hole gas in a strained Ge QW heterostructure

We have measured the low temperature and low field MR of a high mobility Ge 2DHG. The resulting MR curves demonstrate WL-like behavior at temperatures below 2K with WAL-like behavior appearing between 3K and 12K. Evidence of WAL has not been previously observed in Ge. We believe this transition to be the result of a summation of WL and WAL effects in the main conduction channel and parallel conduction channel(s).This is a promising result for Ge as a possible channel for future spin-FETs.