Darryl Barlett

The molecular beam epitaxy growth of InGaAs on GaAs(100) studied by in situ scanning tunneling microscopy and reflection high‐energy electron diffraction

In situ real space images and reflection high energy electron diffraction (RHEED) data for the initial stages of molecular beam epitaxy growth of highly strained InxGa1−xAs (0.3≤x≤0.6) on GaAs(100) are reported. RHEED patterns, specular intensity oscillations, and surface lattice constant data are recorded simultaneously during growth. From the RHEED analysis we divide the film evolution into two stages, corresponding to a largely unrelaxed and relaxed surface lattice constant. Scanning tunneling microscopy images of the surface of the film as it appeared during growth reveal that the initial phase has a rippled morphology. With further growth the film evolves to a distinctive three‐dimensional island microstructure. Both of these characteristic morphologies are interpreted as manifestations of strain in the growing film.

CCD‐based reflection high‐energy electron diffraction detection and analysis system

A CCD‐based, computer controlled RHEED detection and analysis system that utilizes an on‐chip integration technique and on‐board data manipulation is described. The system is capable of in situ time‐resolved measurements of specular and integral‐order intensity oscillations, their phase differences, streak linewidths, and epitaxial layer lattice constants. The digital RHEED techniques are described in the context of Co/Au bilayer, GaAs/GaAs, and InxGa1−xAs/GaAs MBE growth. The system is compared to other RHEED detection devices.

Real-time strain monitoring in thin film growth: cubic boron nitride on Si (100)

We demonstrate the application of real-time film-stress monitoring and control using a multi-beam optical sensor. In situ measurements on wide-bandgap boron nitride films grown by ECR-assisted sputtering reveal a critical stress beyond which defects are injected into the silicon substrate. This is marked by a rapid onset of wafer curvature. The method should be particularly useful for monitoring stress build-up in other wide-bandgap nitride films where no appropriate lattice-matched substrates are presently available.

An alternate route to giant magnetoresistance in MBE‐grown Co–Cu superlattices (invited)

Co–Cu superlattices grown by MBE in the (111) orientation show weak or nonexistent interlayer exchange coupling, yet several groups have observed large high‐field magnetoresistance signals in excess of 30%. In the present work, we address some of the questions relating to GMR and the interlayer coupling by studying samples with atomically abrupt interfaces, as probed by real‐time RHEED techniques, HRTEM, and spin‐echo NMR. We propose that the lowered dimensionality of the structure leads to an enhancement of the scattering of conduction electrons from paramagnetic interfaces obeying a Langevin‐like saturation at very high fields, well beyond the switching field of the Co layers. Scaling between the GMR and thermopower measurements suggests that a spin‐dependent density of states at the Co–Cu interfaces is responsible for the observed magnetotransport behavior in these samples, rather than antiferromagnetically coupled Co layers.

Interface anisotropy in cobalt‐based epitaxial superlattices

We have measured the magnetic anisotropy in a series of Co‐Au and Co‐Cu superlattices prepared by molecular‐beam epitaxy. Significant epitaxial strains give rise to a magnetoelastic contribution and a large crossover thickness (∼19 A) for perpendicular easy magnetization. The results are discussed in the context of a careful analysis of the interfacial strains and coherence determined by in situ. time‐resolved reflection high‐energy electron diffraction techniques and x‐ray scattering.

Growth related interference effects in band edge thermometry of semiconductors

During epitaxial growth on GaAs and InP substrates two effects were observed that can interfere with correct substrate temperature measurements when using band edge thermometry. The first effect was due to specular reflection from a hot effusion cell opposite the optical detector. The worst case observed was a false temperature drop of ∼30°C upon starting growth. This effect was eliminated by moving the detector to a port that doesn’t allow specular reflection from any cell. The second effect is due to the development of constructive and destructive interference fringes while growing heterostructures. False temperature oscillations as large as ±5°C were seen when growing a thick AlGaAs∕GaAs heterostructure on GaAs. The solution to reducing the false TS oscillations is the use of a third derivative of the spectra, making the location of the band edge much less susceptible to interference effects. This technique reduced the false oscillations to ±1.5°C. However, the second interference effect also opens up ...

Controlling stress in cubic boron nitride coatings

Using the method of ion-assisted plasma deposition under conditions of reduced-bias growth, we have been able to achieve thick coatings of cubic boron nitride on silicon. A key aspect of our technique is to monitor the accumulation of compressive stress in the film and to reduce its effects by appropriate choice of deposition parameters and strain-relieving buffer layers. For this purpose, we employ a battery of real-time in situ characterization tools, which are capable of accurately tracking important film growth parameters, such as nucleation density, surface roughness, crystallographic structure and texture, and residual film stress. We use a combination of electron (reflection high-energy electron diffraction, RHEED) and laser beam (multi-beam optical stress sensor, MOSS) probes to provide this information during growth. Our results demonstrate that we can successfully produce superhard cubic boron nitride coatings up to 2 μm in thickness.

A scanning tunneling microscopy study of epitaxial Ge growth

We report new findings on Ge homoepitaxy on the (110) face using molecular beam epitaxy techniques. Our in-situ scanning tunneling microscopy study reveals that low temperature growth (T ∼ 300°C) followed by annealing at T ∼ 500°C produces superior surface morphology with long terraces (> 1 μm). High temperature growth (T > 500°C), on the other hand, promotes the formation of facets and pits. A roughening transition occurs for growth temperatures < 300°C, where long and regularly spaced ridges form along the [110] direction.

Real-Time Rheed Studies of Epitaxial Co-Cr Superlattices

The epitaxial growth1,2 of a bcc form of cobalt, on (110) GaAs, has stimulated a great deal of interest in metastable phases of the magnetic transition metal elements. An issue of general importance in this kind of system concerns the accommodation of lattice mismatch, including possible differences in symmetry, at the heterostructure interfaces. The resulting interface structure is known to play a crucial role in the magnetic properties of such materials, especially with regard to anisotropy3, interlayer coupling,4,5 and giant magnetoresistance effects6.