Modulation of the electronic properties of GaN films by surface acoustic waves

We report on the interaction between photogenerated electron-hole pairs and surface acoustic waves (SAW) in GaN films grown on sapphire substrates. The spatial separation of photogenerated carriers by the piezoelectric field of the SAW is evidenced by the quenching of the photoluminescence (PL) intensity. The quenching levels in GaN are significantly smaller than those measured in GaAs under similar conditions. The latter is attributed to the lower exciton ionization efficiency and carrier separation probabilities mediated by the piezoelectric effect. The PL spectra also evidence energy shifts and broadenings of the electronic transitions, which are attributed to the band gap modulation by the SAW strain [email protected]

Transport Phenomena of Inversion Layers in High Magnetic Fields

The discovery of the quantum Hall effect in 1980 has demonstrated the importance of two- dimensional electronic systems for application and fundamental research. In this paper a review of some transport phenomena in such systems in high magnetic fields is given

Thermohydrodynamics in Quantum Hall Systems

A theory of thermohydrodynamics in two-dimensional electron systems in quantizing magnetic fields is developed including a nonlinear transport regime. Spatio-temporal variations of the electron temperature and the chemical potential in the local equilibrium are described by the equations of conservation with the number and thermal-energy flux densities. A model of these flux densities due to hopping and drift processes is introduced for a random potential varying slowly compared to both the magnetic length and the phase coherence length. The flux measured in the standard transport experiment is derived and is used to define a transport component of the flux density. The equations of conservation can be written in terms of the transport component only. As an illustration, the theory is applied to the Ettingshausen effect, in which a one-dimensional spatial variation of the electron temperature is produced perpendicular to the current.Comment: 10 pages, 1 figur

Controlled incorporation of mid-to-high Z transition metals in CVD diamond

We report on a general method to fabricate transition metal related defects in diamond. Controlled incorporation of Mo and W in synthetic CVD diamond was achieved by adding volatile metal precursors to the diamond chemical vapor deposition (CVD) growth process. Effects of deposition temperature, grain structure and precursor exposure on the doping level were systematically studied, and doping levels of up to 0.25 at.% have been achieved. The metal atoms are uniformly distributed throughout the diamond grains without any indication of inclusion formation. These results are discussed in context of the kinetically controlled growth process of CVD diamond

Technology and thermal stability of ALGAN/GAN HFETs

New approaches towards high reliable thermal stable ohmic and Schottky contacts for GaN/AlGaN-HFETs have been developed and implemented in device structures. The contact technology has been systematically optimized towards i) good electrical properties, ii) superior contact morphology and contour definition and Hi) high reliability and thermal stability. HFETs employing source/drain contacts based on WSiN diffusion barriers sandwiched between an Ti/Al/Ti/Au internal layer system and an overlayer metal as well as WSiN/Au and Ir/Au gate contacts have demonstrated long term stability at 400°C, Even after temperature storage tests at 500°C no significant device degradation could be detected. Ir/Au Schottky gates have found to be stable at the rather high level of Schottky barrier height of about 1.1 eV resulting in dramatically reduced leakage currents as compared to standard Pt/Ti/Au gate contacts to GaN-HFETs

Compositional and structural analysis of AlSb (As) tunneling barriers in InAs/AlSb (As) / GaSb resonant interband-tunneling structures

InAs/AlSb/GaSb/AlSb/InAs interband-tunneling structures have been analyzed with respect to the composition and structural quality of the AlSb tunneling barriers. The addition of AlAs monolayers at the interfaces between the AlSb barriers and the InAs and GaSb layers was found to result in the expected reduction in the valley current density of the resonant interband-tunneling diode. Vibrational mode Raman spectroscopy showed that the introduction of AlAs monolayers led to the formation of pseudoternary Al(SbAs) barriers, which cause the observed reduction of the valley current density. Ellipsometric measurements indicate that the structural quality of both types of barrier layers, with and without AlAs monolayers added to the interfaces, is inferior to that of thick bulklike AlSb layers. The observation of Raman scattering from a coupled hole plasmon-phonon mode indicates the formation of a hole gas in the GaSb quantum well at the center of the tunneling structure

Size-controllable and homogeneous platinum nanoparticles on diamond using wet chemically assisted electrodeposition

Platinum particles deposited via electrochemical methods are frequently inhomogeneous and have poor stability on diamond electrode surface. These problems are solved in this study by applying a two-step deposition route. In the first step wet chemically synthesized Pt nanoparticles are seeded and these seeds distribute homogeneously on diamond surface with sizes around 15 nm in diameters. Electrochemical deposition at a constant potential is then applied to overgrow Pt seeds. The sizes of Pt nanoparticles are controlled by the consumed charge during electrochemical deposition process. The homogeneity, sizes variation, activity, and stability of these Pt nanoparticles on diamond surface are investigated with scanning electron microscope (SEM) and electrochemical techniques. The simulation of electrochemical overgrowth of Pt seeds as well as electrochemical deposition of Pt nanoparticles on diamond is performed, revealing different nucleation processes. The proposed method is promising and useful for depositing other various catalysts on different electrodes

Piezoelectric field and confinement effects on the dielectric function spectrum of InGaN/GaN quantum wells

In(0.13)Ga(0.87)N/GaN double heterostructures and quantum wells (QWs) have been studied by room-temperature photoluminescence (PL) and spectroscopic ellipsometry (SE). PL revealed the presence of strong piezoelectric fields, which strongly influence the luminescence properties for InGaN QW widths in the 3-12 nm range. The pseudodielectric function spectrum derived from the SE measurements were analyzed using a multilayer approach, describing the dielectric function of the individual layers by a parametric oscillator model. The fundamental band-gap resonance in the InGaN dielectric-function spectrum was found to broaden for an InGaN layer width of 12 nm, as compared to bulk-like InGaN layers, due to piezoelectric field effects. For a much narrower QW width of 1.7 nm, however, quantum confinement was found to dominate over piezoelectric-field effects, resulting in a much sharper band-gap resonance shifted to higher energies and an increased oscillator strength