R. S. Balmer

Structural and electrical characterization of AuPdAlTi ohmic contacts to AlGaN∕GaN with varying Ti content

AuPdAlTi∕AlGaN∕GaN ohmic contact structures with varying Ti:Al ratios have been investigated. The relationship between Ti:Al ratio, interfacial microstructure, and contact resistance is examined. Rapid thermal annealing temperatures of 850°C or higher are required to produce an ohmic contact with annealing at 950°C producing the lowest contact resistance in the majority of samples. Samples annealed at 950°C have been analyzed using complementary transmission electron microscopy and electrical characterization techniques. A thin Ti-nitride region is found to form at the contact/semiconductor interface in all samples.u2003Ti-nitride inclusions through the AlGaN∕GaN layer are also observed, surrounded by an Al∕Au rich metallurgical barrier layer, with the size of the inclusions increasing with Ti content. The size of these inclusions does not have any clear effect on the electrical characteristics of the contacts at room temperature, but samples with fewer inclusions show superior electrical characteristics at h...

Optical investigation of exciton localization in AlxGa1−xN

The optical properties of AlxGa1−xN epilayers with x ranging from 0.08 to 0.52 have been studied by photoluminescence (PL). The temperature dependent PL of the AlxGa1−xN epilayers shows a classical “S-shape” behavior. This behavior is attributed to exciton localization due to compositional fluctuations in the AlxGa1−xN layers. The localization parameter σ extracted from temperature dependent PL, which gives an estimate of degree of localization, is found to increase with Al composition, up to a value of 52meV at the highest Al composition studied. Several phonon replicas are observed at the lower energy side of the main excitonic emission peak in these epilayers at low temperature. In all cases, the Huang-Rhys parameter has been estimated. The Huang-Rhys parameter is found to increase with x indicating that the degree of localization again increases with x. In addition, the Huang-Rhys parameter is found to increase with higher order phonon replicas.

Analysis of thin AlN carrier exclusion layers in AlGaN/GaN microwave heterojunction field-effect transistors

We present a study of the effect of the growth of a thin AlN exclusion layer between the AlGaN barrier layer and GaN buffer layer in microwave heterojunction field-effect transistor structures. A dramatic improvement in carrier drift mobility is observed and we present evidence from electronic structure calculations and capacitance–voltage experiments that this improvement is associated with reduced alloy scattering. However, no significant benefit is seen at low carrier concentrations. Reduced electron trapping in the AlGaN is an additional benefit.

Monte Carlo simulations of AlGaN/GaN heterojunction field-effect transistors (HFETs)

Self-consistent Monte Carlo simulations are reported for AlGaN/GaN HFETs. Hot-carrier scattering rates are determined by fitting experimental ionization coefficients and the doping character of the GaN is obtained from substrate bias measurements. Preliminary simulations for a simple model of the AlGaN surface are described and results are found to be consistent with experimental data. The high-frequency response of short-gate-length transistors is found to be sensitive to the charge state of the free AlGaN surface and it is proposed that current-slump phenomena may also be related to deep levels at this surface. Breakdown calculations show interesting two-dimensional effects close to the drain contact.

Structural and electrical characterization of AuPtAlTi Ohmic contacts to AlGaN∕GaN with varying annealing temperature and Al content

The effect of varying annealing temperature and Al layer thickness on the structural and electrical characteristics of AuPtAlTi/AlGaN/GaN ohmic contact structures has been systematically investigated. The relationship between annealing temperature, Al content, interfacial microstructure, surface planarity and contact resistance is nexamined. In particular, the presence of a detrimental low temperature Pt-Al reaction is identified. This is implicated in both the requirement for a higher Al:Ti ratio than is required for related AuPdAlTi contact schemes and through the degraded temperature dependent resistance behaviour of the annealed AuPtAlTi contacts.

Composition measurement in strained AlGaN epitaxial layers using x-ray diffraction

An x-ray diffraction technique is described which, by careful choice of the x-ray reflection used, minimizes errors in composition measurements resulting from strain and uncertainties in the elastic constants of a material. The method is applied to the AlGaN system, which shows a wide range of values for Poisson’s ratio in the literature and significant variation in strain state due to the high dislocation content and large thermal expansion mismatch with the substrate. It is demonstrated that accurate composition measurements of partially relaxed AlxGa1−xN layers (x 20nm can be made from a single measurement.

Z-contrast imaging of AlN exclusion layers in GaN field-effect transistors

The structural characteristics of AlN exclusion layers used to enhance the electron mobility in GaN-based field-effect transistor structures are investigated using scanning transmission electron microscopy. It is shown that a peak in electron mobility is achieved for an AlN exclusion layer with a nominal thickness of 2.3 nm, although significant compositional grading at the interfaces appears to be present. For longer growth times (i.e., 30 s), a transition to three-dimensional growth occurs, roughening the GaN/AlN/AlGaN interfaces. This roughening is likely to be associated with scattering of carriers in the two-dimensional electron gas (2DEG) consistent with an observed increase in 2DEG sheet resistivity.

Nanoscale characterisation of electronic and spintronic nitrides and arsenides

The limits of applicability of the nanoscale spatial resolution analysis techniques of EFTEM, CBED and dark field imaging as applied to ohmic contacts to AlGaN/GaN and Mn distribution within Ga1-xMnxAs epilayers are considered. EFTEM can be limited by acquisition times necessitating the post processing of images to compensate for sample drift. Complementary technique of assessment are required to address problems of peak overlaps in energy loss spectra or signal to noise problems for low elemental concentrations. The use of 002 dark field imaging to appraise Ga1-xMnxAs epilayers is demonstrated.