R. L. Forrest

High-quality strain-relaxed SiGe films grown with low temperature Si buffer

High-quality strain-relaxed SiGe templates with a low threading dislocation density and smooth surface are critical for device performance. In this work, SiGe films on low temperature Si buffer layers were grown by solid-source molecular beam epitaxy and characterized by atomic force microscope, double-axis x-ray diffraction, photoluminescence spectroscopy, and Raman spectroscopy. Effects of the growth temperature and the thickness of the low temperature Si buffer were studied. It was demonstrated that when using proper growth conditions for the low temperature Si buffer the Si buffer became tensily strained and gave rise to the compliant effect. The lattice mismatch between the SiGe and the Si buffer layer was reduced. A 500 nm Si0.7Ge0.3 film with a low threading dislocation density as well as smooth surface was obtained by this method.

Compliant effect of low-temperature Si buffer for SiGe growth

Relaxed SiGe attracted much interest due to the applications for strained Si/SiGe high electron mobility transistor, metal-oxide-semiconductor field-effect transistor, heterojunction bipolar transistor, and other devices. High-quality relaxed SiGe templates, especially those with a low threading dislocation density and smooth surface, are critical for device performance. In this work, SiGe films on low-temperature Si buffer layers were grown by solid-source molecular-beam epitaxy and characterized by atomic force microscope, double-axis x-ray diffraction, and photoluminescence spectroscopy. It was demonstrated that, with the proper growth temperature and Si buffer thickness, the low-temperature Si buffer became tensily strained and reduced the lattice mismatch between the SiGe and the Si buffer layer. This performance is similar to that of the compliant substrate: a thin substrate that shares the mismatch strain in heteroepitaxy. Due to the smaller mismatch, misfit dislocation and threading dislocation de...

Fitting dynamical x-ray diffraction data over the World Wide Web.

The first implementation of fitting X-ray Bragg diffraction profiles from strained multilayer crystals at a remote web-based X-ray software server is presented. The algorithms and the software solutions involved in the process are described. The suggested technology can be applied to a wide range of scientific research and has the potential to promote remote collaborations across scientific communities.

Lateral composition modulation in InAs'GaSb superlattices

We report the analysis of lateral composition modulation in (InAs)m/(GaSb)m superlattices by x-ray diffraction. Vertical and lateral satellite peaks for a 140 period structure were observed. The lateral modulation wavelength, average superlattice composition, and vertical superlattice wavelength were determined. The lateral modulation was observed only along one in-plane direction resulting in quantum wire-like structures along the [110] direction. The unconventional structure of the lateral composition modulation, in which the stacking of the layers leads to a doubling of the vertical superlattice period, is discussed.We report the analysis of lateral composition modulation in (InAs)m/(GaSb)m superlattices by x-ray diffraction. Vertical and lateral satellite peaks for a 140 period structure were observed. The lateral modulation wavelength, average superlattice composition, and vertical superlattice wavelength were determined. The lateral modulation was observed only along one in-plane direction resulting in quantum wire-like structures along the [110] direction. The unconventional structure of the lateral composition modulation, in which the stacking of the layers leads to a doubling of the vertical superlattice period, is discussed.

Determination of the order parameter of CuPt-B ordered GaInP2 films by x-ray diffraction

We present quantitative characterization of atomic ordering in semiconductor alloy films by x-ray diffractometry. In particular, we show that the order parameter of CuPt-B ordered GaInP2 films can be determined without measuring the fundamental reflections or examining structural details of the ordered domains. Our method is based on the fact that the ordering peak is modulated by statistical displacements of atom planes, which is a function of the degree of ordering. Therefore, by comparing two or more ordering peaks in an x-ray spectrum, the order parameter of an ordered film can be extracted solely for those regions that are, in fact, ordered. The method can straightforwardly be extended to other ordered alloys.

Vacuum and flow field results from the wake shield facility flight experiment

The Wake Shield Facility (WSF) created to characterize and utilize the ultra‐vacuum of space has corroborated the formation of a wake region behind an orbiting vehicle. In its maiden flight on the remote manipulator arm of STS‐60 in February, 1994, the WSF generated the highest vacuum ever measured in low earth orbit space. In addition, details of wake formation and flow field characterization have been obtained with the added benefit of the characterization of the contaminating effects of the Orbiter on the low earth vacuum environment.

X-Ray Diffraction Analysis of Ordering in Epitaxial III–V Alloys

The X-ray diffraction investigation of atomic ordering and lateral composition modulation is discussed. An overview of X-ray diffraction theory is given so that the reader can appreciate the results, potential, and challenges of the technique. Quantitative analysis of long-and short-range order parameters, lateral composition modulation, and ordered domain structure, including experimental results, are discussed.

Single and Double Variant Cupt-B Ordered GalnAs

Nominally lattice-matched GaInAs layers grown by metal organic vapor phase epitaxy on InP substrates have been studied using high-resolution x-ray diffraction (HRXRD) to determine the growth conditions under which ordering is introduced. HRXRD provides an independent means to quantify the order parameter of semiconductor heterostructures as well as the ordering on different {111} planes, i.e., double variant ordering. This independent means to determine ordering provides for a better understanding of the effects of ordering on the electronic and optical properties. Double variant ordering was observed for epitaxial layers grown on exact (001) InP substrates, with an order parameter of about 0.1 in both variants. For substrates that were miscut by 6 degrees, single variant ordering was detected. In these cases, an order parameter as high as 0.66 was measured for certain growth conditions. The layers grown on vicinal substrates are all of high crystalline quality, those on (001) substrates exhibit some mosaic spread.

Fe/Au multilayer growth fronts: Comparison of X-ray diffuse scattering results with atomic-force microscopy profiles

Abstract The interfacial roughness morphology of sputtered Fe/Au multilayers was derived by combining the results from X-ray specular and non-specular (diffuse) scattering. From the non-specular intensity collected by a two-dimensional image plate we determined the in-plane height–height correlation function 〈 H i ( R ) H i (0)〉 and the growth correlation length as a function of feature size. From these parameters we determined both the static and dynamic roughness exponents α and z , which agree with the Kardar–Parisi–Zhang (KPZ) model of surface growth. The height difference function derived from the diffuse scattering is compared with several model functions that incorporate the asymptotic behavior of 〈 H ( R ) 2 〉 for the cases of R →0 and R →∞. We show that only one particular function is appropriate. The evolution of the interfacial slope is determined from the roughness exponents, and it is shown that it decreases as a function of time. This asymptotic smoothing is analyzed using the noiseless and stochastic KPZ equations. The noiseless KPZ equation predicts that the growth front is composed of paraboloids, which increase in lateral size and coalesce. This result is supported by an Atomic-force Microscopy measurement of the surface of one of the Fe/Au Multilayers.

Propagation of THz acoustic wave packets in GaN at room temperature

We use femtosecond laser pulses to generate coherent longitudinal acoustic phonons at frequencies of 1–1.4 THz and study their propagation in GaN-based structures at room temperature. Two InGaN-GaN multiple-quantum-well (MQW) structures separated by a 2.3 μm-thick GaN spacer are used to simultaneously generate phonon wave packets with a central frequency determined by the period of the MQW and detect them after passing through the spacer. The measurements provide lower bounds for phonon lifetimes in GaN, which are still significantly lower than those from first principles predictions. The material Q-factor at 1 THz is found to be at least as high as 900. The measurements also demonstrate a partial specular reflection from the free surface of GaN at 1.4 THz. This work shows the potential of laser-based methods for THz range phonon spectroscopy and the promise for extending the viable frequency range of GaN-based acousto-electronic devices.