Numerous defects are generated in the heteroepitaxy of GaN, with threading dislocations (TDs) being the most prevalent. A novel method of reducing the defect density has been the Epitaxial Lateral Overgrowth (ELO) technology, where parts of the highly dislocated starting GaN is masked with a dielectric mask, after which growth is restarted. At the beginning of the second step, deposition only occurs within the openings with no deposition observed on the mask. This is referred as Selective Area Epitaxy (SAE). The TDs are prevented from propagating into the overlayer by the dielectric mask, whereas GaN grown above the opening (coherent growth) keeps the same TDs density as the template, for at least during the early stages of the growth.
Currently, two main ELO technologies exist: the simpler one involves a single growth step after stripe opening. In this one-step-ELO (1S-ELO), growth in the opening remains in registry with the GaN template underneath (coherent part), whereas GaN over the mask extends laterally (wings). This leads to two grades, namely highly dislocated GaN above the openings, and low dislocation density GaN over the masks. With this technique, devices have to be fabricated on the wings. Therefore, conversely, in the two-step-ELO (2S-EL0) process, the growth conditions of the first step are monitored to obtain triangular stripes. Inside these stripes, the threading dislocations arising from the templates are bent by 90° when they encounter the inclined lateral facet. In the second step, the growth conditions are modified to achieve full coalescence. In this two-step-ELO, only the coalescence boundaries are defective.
In depth characterisation of these ELO GaN layers reveals that the intermediate stages of the process induce an inhomogeneous impurity incorporation and stress distribution. However, the ELO technology produces high quality GaN, with TDs densities in the mid 106cn-2, linewidths of the low temperature photoluminescence (PL) near band gap recombination peaks below 1 meV, and deep electron traps concentration below 1014cm-3 (compared to mid 1015cm-3 in standard GaN). To further reduce the TDs density, multiple step ELO have also been implented.
For applications such as read/write laser light sources of digital versatile disks, higher power and longer operation lifetimes are required, thus necessitating the production of better quality material. Several options are also currently available to pave the way towards self supported high quality GaN. These technologies involve growing thick GaN layers (possibly on MOVPE ELO GaN) and then separating the layers from the substrate. HVPE has proven to be a reliable method to grow GaN with growth rates ranging from 30 to 100 pm/hour. In thick layers (several hundred μm), the mecanisms used for the reduction of dislocations become more efficient. Separation from the starting substrate is currently achieved by either laser lift off, chemically or by strain induced.
AlxGa1-xN material system, whose bandgap lies in the 3.42-6.2 eV range, is extremely interesting for visible and solar blind UV photodetector applications. This paper describes the device performances of AlxGa1-xN UV Schottky barrier photodetectors for visible-blind applications grown on c-oriented sapphire, with a detailed balance with the basic materials properties. Conventional low temperature grown AlN or GaN were used in all applications. High quality Schottky barrier photodiodes made of Epitaxial Lateral Overgrown (ELOG) GaN are also presented. All Schottky barrier devices show a fast time response, a high UV-visible rejection factor, and high absolute values of above bandgap responsivities. A new application of AlGaN UV Schottky barrier photodetectors to monitor the biological action of the solar UV radiation, as well as the device performance of high quality GaN and AlGaN Metal Semiconductor Metal with cutoff wavelengths as short as 310 nm, are described in detail.
Nitride semiconductor alloys have merged as the most promising materials for short wave lengths light emitting diodes and laser diodes (LDs). The GaInN multiquantum wells structure was used as the active part of LDs and have presently proven to work at room temperature in cw mode for 10,000 hrs. These achievements would have not been possible without the emergence of new approaches in heteroepitaxy of GaN leading to layers exhibiting lower dislocation densities than those grown using conventional heteroepitaxy. Metal Organic Vapor Phase Epitaxy (MOVPE) has demonstrated its ability to fabricate structures for optoelectronics GaN based devices. Several nitrogen sources have been tested, but, so far NH3 remains the best nitrogen precursor despite the stringent requirement of high V/III ratio in the vapor phase. With the epitaxial lateral overgrowth (ELOG), high quality GaN layers have been obtained. This ELOG technology can be applied either by HVPE or MOVPE, on sapphire or 6H-SiC substrates. The dislocations densities in the overgrowth region are orders of magnitude lower than in the standard heteroepitaxial GaN layers.
In the recent years, the depletion of the stratospheric ozone layer has alerted the scientific community about the risks of a solar ultraviolet (UV) radiation overexposure. Biological research has confirmed the very important role of the UV-B (320 - 280 nm) and UV-A (400 - 320 nm) bands on the Earth biosystem. AlxGa1-xN semiconductor alloys, with a bandgap tunable between 3.4 eV and 6.2 eV, are the most suitable materials for the fabrication of solar UV detectors. In this paper we describe the fabrication and characteristics of AlGaN photoconductive and Schottky barrier photodetectors, with Al mole fractions up to 35%. Photoconductive detectors show very high gains, that decrease with increasing incident optical power. They present persistent photoconductivity effects, and a significant below-the-gap response. The physics of this behavior is discussed. On the other hand, AlGaN Schottky barrier photodetectors show a very fast response that is independent of the optical power, and their UV/visible rejection ratio is rather high. As the Al content increases, the evolution of the responsivity and cut-off wavelength is presented. Al0.22Ga0.78N Schottky barriers are very good candidates to monitor the UV-B band. The prospective applications of AlGaN photodiodes to determine the biological action of the solar UV radiation are also discussed.
Deep donor levels are observed in Al(x)Ga(1-x)As for x of greater than 0.22 and GaAs under hydrostatic pressure (for p of more than 2GPa). Persistent photoconduction (PPC) is the most striking feature of this deep donor, the DX center. Upon illumination at low temperature, the free-electrons concentration increases and remains at this new value even after the light is off. Basically the DX centers are photoionized and one (or several) electrons per center are transferred to the conduction band. The bistable character of the donor which involves two electronic configurations is studied by Moessbauer spectroscopy (MS). Electronic wavefunctions, near-neighbor geometries and lattice vibrational properties can be probed. Moessbauer spectroscopy is used to observe the Sn DX center in Al(x)Ga(1-x)As near x