Synthesis,Crystal Structure and Lithium Motion of Li8SeN2 and Li8TeN2

The compounds Li₈EN₂ with E = Se, Te were obtained in form of orange microcrystalline powders from reactions of Li₂E with Li₃N. Single crystal growth of Li₈SeN₂ additionally succeeded from excess lithium. The crystal structures were refined using single‐crystal X‐ray diffraction as well as X‐ray and neutron powder diffraction data (I4₁md, No. 109, Z = 4, Se: a = 7.048(1) Å, c = 9.995(1) Å, Te: a = 7.217(1) Å, c = 10.284(1) Å). Both compounds crystallize as isotypes with an anionic substructure motif known from cubic Laves phases and lithium distributed over four crystallographic sites in the void space of the anionic framework. Neutron powder diffraction pattern recorded in the temperature range from 3 K to 300 K and X‐ray diffraction patterns using synchrotron radiation taken from 300 K to 1000 K reveal the structural stability of both compounds in the studied temperature range until decomposition. Motional processes of lithium atoms in the title compounds were revealed by temperature dependent NMR spectroscopic investigations. Those are indicated by significant changes of the ⁷Li NMR signals. Lithium motion starts for Li₈SeN₂ above 150 K whereas it is already present in Li₈TeN₂ at this temperature. Quantum mechanical calculations of NMR spectroscopic parameters reveal clearly different environments of the lithium atoms determined by the electric field gradient, which are sensitive to the anisotropy of charge distribution at the nuclear sites. With respect to an increasing coordination number according to 2 + 1, 3, 3 + 1, and 4 for Li(3), Li(4), Li(2), and Li(1), respectively, the values of the electric field gradients decrease. Different environments of lithium predicted by quantum mechanical calculations are confirmed by ⁷Li NMR frequency sweep experiments at low temperatures.     

The effect of annealing on the surface morphology of strained and unstrained InxAl1−xN thin films

In_xAl_1−xN is a particularly useful group-III nitride alloy because by adjusting its composition it can be lattice matched to GaN. Such lattice-matched layers may find application in distributed Bragg reflectors (DBRs) and high electron mobility transistors (HEMTs). However, compared with other semiconducting nitride alloys, In_xAl_1-xN has not been researched extensively. In this study, thin In_xAl_1−xN epilayers were grown by metal-organic vapour phase epitaxy (MOVPE) on GaN and Al_yGa_1−yN layers. Samples were subjected to annealing at their growth temperature of 790 °C for varying lengths of time, or alternatively to a temperature ramp to 1000 °C. Their subsequent surface morphologies were analysed by atomic force microscopy (AFM). For both unstrained In_xAl_1−xN epilayers grown on GaN and compressively strained epilayers grown on Al_yGa_1−yN, surface features and fissures were seen to develop as a consequence of thermal treatment, resulting in surface roughening. It is possible that these features are caused by the loss of In-rich material formed on spinodal decomposition. Additionally, trends seen in the strained In_xAl_1−xN layers may suggest that the presence of biaxial strain stabilises the alloy by suppressing the spinode and shifting it to higher indium compositions.     

Improvement of crystal quality in ammonothermal growth of bulk GaN

Several key improvements in crystal quality of bulk GaN grown by the ammonothermal method are presented. Full width at half maximum of (0 0 2) X-ray rocking curve was reduced to 53 and 62 arcsec for Ga-side and N-side, respectively. Transparent bulk GaN crystal was also demonstrated. Oxygen and sodium concentrations were reduced to mid-10¹⁸ and mid-10¹⁵ cm⁻³, respectively. We are currently searching for a growth condition that produces transparent bulk GaN with high structural quality and low impurities. Small-sized, semi-transparent GaN wafers were fabricated by slicing the grown bulk GaN crystals, which demonstrate the high feasibility of ammonothermal growth for production of GaN wafers.     

Enhanced growth rates and reduced parasitic deposition by the substitution of Cl2 for HCl in GaN HVPE

The main limitation in the application of hydride vapor phase epitaxy for the large scale production of thick free-standing GaN substrates is the so-called parasitic deposition, which limits the growth time and wafer thickness by blocking the gallium precursor inlet. By utilizing Cl₂ instead of the usual HCl gas for the production of the gallium chlorine precursor, we found a rapid increase in growth rate from ∼80 to ∼400 μm/h for an equally large flow of 25 sccm. This allowed us to grow, without any additional optimization, 1.2 mm thick high quality GaN wafers, which spontaneously lifted off from their 0.3° mis-oriented GaN on sapphire HCl-based HVPE templates. These layers exhibited clear transparencies, indicating a high purity, dislocation densities in the order of 10⁶ cm⁻², and narrow rocking curve XRD FWHMs of 54 and 166 arcsec in for the 0002 and 101−5 directions, respectively.     

Thermogravimetric analysis and microstructural observations on the formation of GaN from the reaction between Ga2O3 and NH3

Thermogravimetric analysis (TGA) and microstructural observations were carried to investigate the nitridation mechanism of β-Ga₂O₃ powder to GaN under an NH₃/Ar atmosphere. Non-isothermal TGA showed that nitridation of β-Ga₂O₃ starts at ∼650 °C, followed by decomposition of GaN at ∼1100 °C. Isothermal TGA showed that nitridation follows linear kinetics in the temperature range 800–1000 °C. At an early stage of nitridation, small GaN particles (∼5 nm) are deposited on the β-Ga₂O₃ crystal surface and they increase with time. We proposed a mechanism for the nitridation of Ga₂O₃ by NH₃ whereby nitridation of β-Ga₂O₃ proceeds via the intermediate vapor species Ga₂O(g).     

GaN single crystals of different habit grown from solution at near atmospheric pressure

Near atmospheric pressure solution growth is one of the many developing methods for growing bulk GaN from solution. Apart from other approaches, this method holds certain advantages, such as relatively low growth pressure and temperature, and the ability to grow high quality GaN crystals with different orientations by varying the solvent composition. GaN whiskers of millimeter scale size with exceptional mechanical and optical properties were grown from solution. Crystals of near isotropic shape were also grown from solution by manipulating additives in the basic solvent.     

Extended defects in bulk GaN and III-nitrides grown on this substrate

A short review of the structural perfection of high-pressure grown bulk crystals is given. As-grown undoped and Mg-doped crystals are described. The dependence of defect arrangement and quality of the surface on growth polarity is described. A high perfection of homoepitaxial layers grown on these substrates is shown. However, growth of thick layers by HVPE may lead to the formation of differently arranged dislocations and the formation of low angle grain boundaries associated with cracks. It is shown that the introduction of dopant or growth of mismatched layers on undoped high-pressure substrates may lead to the formation of additional defects.     

The K2S2O8–KOH photoetching system for GaN

A recently developed photoetching system for n-type GaN, a KOH solution containing the strong oxidizing agent potassium peroxydisulphate (K₂S₂O₈), was studied in detail. By careful selection of the etching parameters, such as the ratio of components and the hydrodynamics, two distinct modes were defined: defect-selective etching (denoted by KSO-D) and polishing (KSO-P). Both photoetching methods can be used under open-circuit (electroless) conditions. Well-defined dislocation-related etch whiskers are formed during KSO-D etching. All types of dislocations are revealed, and this was confirmed by cross-sectional TEM examination of the etched samples. Extended electrically active defects are also clearly revealed. The known relationship between etch rate and carrier concentration for photoetching of GaN in KOH solutions was confirmed for KSO-D etch using Raman measurements. It is shown that during KSO-P etching diffusion is the rate-limiting step, i.e. this etch is suitable for polishing of GaN. Some constraints of the KSO etching system for GaN are discussed and peculiar etch features, so far not understood, are described.     

Thick homoepitaxial GaN with low carrier concentration for high blocking voltage

High voltage GaN Schottky diodes require a thick blocking layer with an exceptionally low carrier concentration. To this aim, a metal organic chemical vapor deposition process was developed to create a (14 μm) thick stress-free homoepitaxial GaN film. Low temperature photoluminescence measurements are consistent with low donor background and low concentration of deep compensating centers. Capacitance–voltage measurements performed at 30 °C verified a low level of about 2×10¹⁵ cm⁻³ of n-type free carriers (unintentional doping), which enabled a breakdown voltage of about 500 V. A secondary ion mass spectrometry depth profile confirms the low concentration of background impurities and X-ray diffraction extracted a low dislocation density in the film. These results indicate that thick GaN films can be deposited with free carrier concentrations sufficiently low to enable high voltage rectifiers for power switching applications.     

Differences in nucleation and properties of InN islands formed using two different deposition procedures

The initial stages of metalorganic chemical vapor deposition of InN have been investigated using two different growth procedures: growth of InN over a GaN buffer layer in one continuous run and growth of InN on a pre-deposited GaN template. While the growth conditions and material quality of the GaN underlying layers are nominally the same, characterization by AFM, X-ray diffraction and PL spectroscopy reveals significantly different material properties of InN islands formed using the two procedures and suggests a different path of evolution during the initial stages of growth. In particular, InN islands grown on a pre-deposited GaN template seem to nucleate directly on the GaN template and are 5 times larger in volume and 2 times lower in surface density as compared with InN growth in one continuous run with the GaN underlying layer. Our studies suggest that the Ga incorporation into the InN during the growth on a GaN buffer layer in one continuous run plays a significant role in altering InN growth mechanisms and material properties.