Abstract : Betavoltaic power cells utilize beta-emitting radioisotopes and semiconductor devices to produce long-lived power in a variety of small for factors. Qynergy is developing betavoltaic power cells based on silicon carbide junction converters and 85Kr and 147Pm isotopes for a variety of applications.

A SiC p-i-n junction betavoltaic was fabricated, and electrical power output under irradiation from an 8.5 GBq P-33 source was monitored over a period of four half-lives of the radioisotope. The open-circuit voltage (V-OC) of the device was 2.04 +/- 0.02 V, and the peak power (P-out) was 0.58 +/- 0.02 mu W (2.1 +/- 0.2 mu W/cm(2)) at 1.60 V. The conversion efficiency (eta(conv)) was 4.5%+/- 0.3% and the normalized power output indicates no device degradation over more than 3 months (four half-lives of the source).

Quantum mechanical CASTEP software calculations were performed using nickel and rhenium atom deposition onto cleaved surfaces (active, hydrogenated, and oxygenated). These calculations were performed without metal atoms and with metal atoms at selected positions (origin, a- and b-axis) in the unit cell. Binding energies for each of the metal atoms (nickel and rhenium) were calculated. Additionally, calculated energy bands with associated density-of-states and partial density-of-states were examined regarding the population of s, p, and d bonding characteristics. Nickel atom deposition onto silicon-rich surfaces tended to bond to the silicon atoms as well as the underlying carbon atoms. However, rhenium atom deposition showed bonding only to the silicon atoms. This was observed experimentally and is reported herein. Experimentally, the rhenium deposition surface is extremely smooth and has only Ohmic characteristics and low resistance.

Alternate substrate technology holds promise for the growth of high-quality lattice-mismatched epitaxial films. Unfortunately, the technology has been plagued by difficulties in reproducibility of results. Some of this problem resides in a lack of characterization of the thin, twist bonded layer used as the template for subsequent epitaxial growth. In this work, grazing-incidence diffraction (GID) and micro-Raman spectroscopy were used to characterize the alternative substrate prior to growth. The 14- and 50-nm-thin GaAs layers were bonded to (100) GaAs substrates and, subsequently, exposed by standard thinning and etching techniques. The crystalline quality of the thin bonded substrates was studied by GID. The full widths at half maximum (FWHM) of the 004 peaks were used to monitor the optimum bonding condition. The measured FWHM varied from 29 to 601 arc s with smooth surfaces exhibiting the lowest values. The effects of bond pressures on template layers were studied for a series of 50 nm alternative substrates prepared using pressures ranging from 75 to 25 psi with a constant bonding temperature of 450 °C. All transferred template layers prepared within this pressure range showed poor quality (FWHM ranged from 324 to 601 arc s) when compared to bulk GaAs of 15 arc s. Micro-Raman measurements were also carried out on these samples. A transverse optical (TO) phonon line is seen and the intensity ratios of the TO to longitudinal optical phonon were much higher than that of GaAs substrate, confirming that the bonding process is causing damage to the thin template layer.

High quality homoepitaxial 6H-SiC films have been grown by solid-source molecular beam epitaxy (MBE) using C60 and Si effusion cells. Scanning electron micrographs show terraced surfaces indicative of step-flow growth. Cross-sectional transmission electron microscopy results demonstrate extremely good epitaxial growth with no hint of dislocations, double-positioning boundaries, or 3C inclusions. We believe this is the first report of homoepitaxy of 6H-SiC using C60 and the first instance of silicon carbide (SiC) epitaxy using a Si effusion cell in the evaporation rather than the sublimation mode. This combination of solid-source MBE and determination of appropriate growth conditions have led to superior homoepitaxial growth of 6H-SiC.

A resonant-cavity-enhanced p-i-n photodetector has been designed and analyzed to operate at a wavelength of 360 nm based on the AlxGa1-xN material system. The novel approach has been adopted of using epitaxial AlN/AlxGa1-xN quarter-wave stacks as the distributed Bragg reflector that serves as the front mirror. An AlxGa1-xN absorptive filter layer is incorporated to suppress all but one resonant mode to ensure single, narrow-band operation. This device structure is projected to achieve wavelength selective, high speed, and high quantum efficiency operation in the ultraviolet. MOCVD-grown 6 1/2-pair AlN/AlxGa1-x

We present an optically-detected time-of-flight technique with femtosecond resolution that monitors the change in the electroabsorption due to charge transport in a p-i-n diode, and show how it may be used to determine the electron transit time, velocity overshoot, and velocity-field characteristic in GaN at room temperature. In a GaN homojunction p-i-n diode, the transit time drops with increasing electric field E in the intermediate field regime (50 - 100 kV/cm), and the electron velocity possesses a weak, quasi-linear dependence on E attributed to polar optical phonon scattering. In the high field regime the transit time and the electron velocity gradually become independent of E. The peak electron velocity of 1.9 X 107 cm/s, corresponding to a transit time of approximately 2.5 ps across the 0.53 micrometers depletion region, is attained at approximately 225 kV/cm. The experimental results are in qualitative agreement with theoretical steady-state velocity-field characteristics found in the literature. A measurement of the high field (approximately 300 kV/cm) transient electron velocity overshoot was also performed using a semi-transparent p-contact AlGaN/GaN heterojunction p-i-n diode. The peak electron velocity of 6.25 X 107 cm/s attained within the first 200 fs decays within 1 ps to a steady-state velocity of 3.2 X 107 cm/s in this improved device.

The influence of p-dopants (Mg and Be) on the structure of GaN has been studied using Transmission Electron Microscopy (TEM). Bulk GaN: Mg and GaN: Be crystals grown by a high pressure and high temperature process and GaN: Mg grown by metal-organic chemical-vapor deposition (MOCVD) have been studied. A structural dependence on growth polarity was observed in the bulk crystals. Spontaneous ordering in bulk GaN: Mg on c-plane (formation of Mg-rich planar defects with characteristics of inversion domains) was observed for growth in the N to Ga polar direction (N polarity). On the opposite side of the crystal (growth in the Ga to N polar direction) Mg-rich pyramidal defects empty inside (pinholes) were observed. Both these defects were also observed in MOCVD grown crystals. Pyramidal defects were also observed in the bulk GaN: Be crystals.

The optical properties of undoped and modulation-doped AlGaN/GaN single heterostructures (SHs) grown by metalorganic chemical vapor deposition are investigated at low temperature using photoluminescence measurements. The formation of a two-dimensional electron gas at the heterojunction is verified by temperature-dependent Hall mobility and 300 K capacitance-voltage measurements. Radiative recombination is observed between the electrons in two-dimensional quantum states at the heterointerface and the holes in the flat-band region or bound to residual acceptors both in undoped and modulation-doped AlGaN/GaN SHs. These peaks disappear when the top AlGaN layer is removed by reactive ion etching. In addition, the photoluminescence results under different laser excitation intensity and lattice temperature are also described for undoped and modulation-doped AlGaN/GaN SHs with various Al compositions and growth interrupt times.

Heterojunction bipolar transistors based on aluminium gallium nitride/gallium nitride (AlGaN/GaN) structures have been fabricated and characterised. The devices were grown by metal organic chemical vapour deposition on e-plane sapphire substrates. The Npn structure consists of an n-GaN layer followed by an n(+)-GaN subcollector contact, an unintentionally doped GaN collector, p-GaN base, and an N-Al(0.1)G(0.9)N emitter with n(+)-GaN contact. Devices yielded good transistor performance with a DC current gain as high as 100 at room temperature.

A detailed study of photoluminescence (PL) of GaN(1 nm)/Al0.2Ga0.8N(3.3 nm) twenty periods superlattice grown via metal-organic chemical vapor deposition is presented. The dependence of the PL emission energy, linewidth, and intensity on temperature, in the low temperature regime, is consistent with recombination mechanisms involving bandtail states attributed to a small degree of interfacial disorder. The activation energy of the nonradiative centers in our superlattice agrees well with the value we derive for the width of the tail-state distribution. Moreover, we find that the average phonon energy of the phonons that control the interband PL energy at high temperatures is larger for the superlattice than for a high-quality GaN film. This observation is consistent with model calculations predicting the phonon mode properties of GaN–AlN-based wurtzite heterostructures.

The radiative recombination of carriers in a two-dimensional electron gas (2DEG) in undoped and modulation-doped AlxGa1−xN/GaN heterostructures grown by metalorganic chemical vapor deposition is investigated at low temperature using photoluminescence measurements. Temperature-dependent Hall effect and Shubnikov–de Haas measurements, verify the formation of a high-quality 2DEG. Radiative recombination is observed between the 2DEG in quantum states at the hetero-interface and the holes in the flat-band region or bound to residual acceptors both in undoped and modulation-doped AlxGa1−xN/GaN heterostructures. The luminescence peaks related to the 2DEG disappear when the top AlGaN layer is removed by reactive ion etching. In addition, the effect of the growth interruption time, laser excitation intensity, and doping conditions upon the photoluminescence is also described.

We have studied the effect of ultrasonic agitation on the surface of n-type gallium nitride (GaN) layers grown on sapphire that were subjected to a selective photoelectrochemical etch. Solutions of various concentrations of KOH were used along with an ultraviolet lamp to oxidize the exposed GaN surface, resulting in an anisotropic etch. Smooth surfaces with root-mean-square (rms) roughness of similar to 4 nm were obtained for a narrow range of etching conditions. It was found that this window could be extended by using etch conditions which produced "whisker" growth. Subsequent ultrasonic agitation was then used to remove these whiskers and obtain smoother surfaces with the best rms roughness values being similar to 0.9 nm. (C) 1999 The Electrochemical Society. S1099-0062(99)09-032-X. All rights reserved.

GaN epitaxial layers with different crystalline quality grown on sapphire substrates by metalorganic chemical vapor deposition are investigated using time-resolved photoluminescence at 300 K. It is found that the time-dependent photoluminescence of low-quality GaN decays faster than that of the high-quality GaN films. The time constants for the dual-exponential decay of the photoluminescence are calculated to be 50 and 250 ps for high-quality undoped GaN and 30 ps for low-quality undoped GaN. For high-quality Si-doped GaN, time constants of 150 and 740 ps are extracted while corresponding time constants of 40 and 200 ps are measured for low-quality Si-doped GaN. We believe that the time constant of 740 ps measured for our high-quality Si-doped GaN is the longest ever reported for thin GaN/sapphire films.