Hydrogen-related 3.8 eV UV luminescence in <b><i>α</i></b>-Ga₂O₃

Temperature-dependent photoluminescence was used to investigate the impact of H on the optical properties of α-Ga2O3 films grown by halide vapor phase epitaxy. An additional UV luminescence line centered at 3.8 eV is observed at low temperatures, which strongly correlates with the concentration of H in the films. This luminescence line is assigned to donor–acceptor pair recombination involving an H-related shallow donor and H-decorated Ga vacancy (VGa-nH) as the acceptor, where n = 1, 2, 3. Previous reports have already suggested the impact of H on the electrical properties of Ga2O3, and the present study shows its clear impact on the optical properties of α-Ga2O3. </jats:p

Low temperature growth and optical properties of alpha-Ga2O3 deposited on sapphire by plasma enhanced atomic layer deposition

Plasma enhanced atomic layer deposition was used to deposit thin films of Ga2O3 on to c-plane sapphire substrates using triethylgallium and O2 plasma. The influence of substrate temperature and plasma processing parameters on the resultant crystallinity and optical properties of the Ga2O3 films were investigated. The deposition temperature was found to have a significant effect on the film crystallinity. At temperatures below 200°C amorphous Ga2O3 films were deposited. Between 250°C and 350°C the films became predominantly α-Ga2O3. Above 350°C the deposited films showed a mixture of α-Ga2O3 and ε-Ga2O3 phases. Plasma power and O2 flow rate were observed to have less influence over the resultant phases present in the films. However, both parameters could be tuned to alter the strain of the film. Ultraviolet transmittance measurements on the Ga2O3 films showed that the bandgaps ranges from 5.0 eV to 5.2 eV with the largest bandgap of 5.2 eV occurring for the α-Ga2O3 phase deposited at 250°C

Dislocations at coalescence boundaries in heteroepitaxial GaN/sapphire studied after the epitaxial layer has completely coalesced.

We have performed cross-sectional scanning capacitance microscopy (SCM), cathodoluminescence (CL) microscopy in the scanning electron microscope (SEM) and transmission electron microscopy (TEM) all on the same few-micron region of a GaN/sapphire sample. To achieve this, it was necessary to develop a process flow which allowed the same features viewed in a cleaved cross-section to be traced from one microscope to the next and to adapt the focused ion beam preparation of the TEM lamella to allow preparation of a site-specific sample on a pre-cleaved cross-section. Growth of our GaN/sapphire samples involved coalescence of three-dimensional islands to form a continuous film. Highly doped marker layers were included in the sample so that coalescence boundaries formed late in the film growth process could be identified in SCM and CL. Using TEM, we then identified one or more dislocations associated with each of several such late-coalescing boundaries. In contrast, previous studies have addressed coalescence boundaries formed earlier in the growth process and have shown that early-stage island coalescence does not lead to dislocation formation.ER

Cross-shaped markers for the preparation of site-specific transmission electron microscopy lamellae using focused ion beam techniques.

We describe the use of a cross-shaped platinum marker deposited using electron-beam-induced deposition (EBID) in a focused ion beam - scanning electron microscope (FIB-SEM) system to facilitate site-specific preparation of a TEM foil containing a trench defect in an InGaN/GaN multiple quantum well structure. The defect feature is less than 100 nm wide at the surface. The marker is deposited prior to the deposition of a protective platinum strap (also by EBID) with the centre of the cross indicating the location of the feature of interest, while the arms of the square cross make an acute angle of 45° with the strap's long axis. During the ion-beam thinning process, the marker may be viewed in cross-section from both sides of the sample alternately, and the coming together of the features relating to the arms of the cross indicates increasing proximity to the feature of interest. Although this approach does allow increased precision in locating the region of interest during thinning, it also increases the time required to complete the sample preparation. Hence, this method is particularly well suited to directly correlated multi-microscopy investigations in previously characterised material where high yield and the precise location are more important than preparation time. In addition to TEM lamella preparation, this method could equally be useful for preparing site-specific atom probe tomography (APT) samples

Sequential plan-view imaging of sub-surface structures in the transmission electron microscope

Transmission electron microscopy (TEM) is a central technique for the characterisation of materials at the atomic scale. However, it requires the sample to be thin enough to be electron transparent, imposing strict limitations when studying thick structures in plan-view. Here we present a method for sequential plan-view TEM that allows one to image complex structures at various depths. The approach consists of performing an iterative series of front-side ion milling followed by TEM imaging. We show it is possible to image how the sample properties vary with depth up to several microns below the surface, with no degradation of the sample and imaging conditions throughout the experiment. We apply this approach to 3D cavities in mesoporous GaN distributed Bragg reflectors, demonstrating the ability to characterise the morphology of the pores, local crystal features and chemical composition through the multilayer structure. The same workflow can be applied to a variety of complex micron-scale systems which are by nature too thick for standard TEM analysis, and can also be adapted for profiling samples in cross-section

Sequential plan-view imaging of sub-surface structures in the transmission electron microscope

Transmission electron microscopy (TEM) is a central technique for the characterisation of materials at the atomic scale. However, it requires the sample to be thin enough to be electron transparent, imposing strict limitations when studying thick structures in plan-view. Here we present a method for sequential plan-view TEM that allows one to image complex structures at various depths. The approach consists of performing an iterative series of front-side ion milling followed by TEM imaging. We show it is possible to image how the sample properties vary with depth up to several microns below the surface, with no degradation of the sample and imaging conditions throughout the experiment. We apply this approach to 3D cavities in mesoporous GaN distributed Bragg reflectors, demonstrating the ability to characterise the morphology of the pores, local crystal features and chemical composition through the multilayer structure. The same workflow can be applied to a variety of complex micron-scale systems which are by nature too thick for standard TEM analysis, and can also be adapted for profiling samples in cross-section

Atomic layer deposited alpha-Ga2O3 solar-blind photodetectors

Low temperature atomic layer deposition was used to deposit α-Ga2O3 films, which were subsequently annealed at various temperatures and atmospheres. The α-Ga2O3 phase is stable up to 400 °C, which is also the temperature that yields the most intense and sharpest reflection by x-ray diffraction. Upon annealing at 450 °C and above, the material gradually turns into the more thermodynamically stable ε or β phase. The suitability of the materials for solar-blind photodetector applications has been demonstrated with the best responsivity achieved being 1.2 A W−1 under 240 nm illumination and 10 V bias, for the sample annealed at 400 °C in argon. It is worth noting however that the device performance strongly depends on the annealing conditions, with the device annealed in forming gas behaving poorly. Given that the tested devices have similar microstructure, the discrepancies in device performance are attributed to hydrogen impurities