Elucidation of factors obstructing quality improvement of MOVPE-grown InN

A systematic study of the crystallographic and electrical/optical properties of MOVPE-grown InN was performed, and the factors that restrict the quality of MOVPE InN were elucidated. The quality of grown InN is highly dependant on the thermal decomposition of NH₃ as a nitrogen source. At a lower growth temperature (~550 °C) a shortage of active nitrogen, due to a lower decomposition rate of NH₃, causes the formation of N vacancies in the grown InN. With increasing growth temperature, a more stoichiometric crystal is grown and the electrical/optical properties improve. At temperatures above 600 °C, however, deterioration occurs at the N-face of In-polar InN near the substrate interface. This deterioration results in the formation of a porous layer during high temperature (~650 °C) growth. There are a few evidences that show that the hydrogen produced by NH₃ decomposition causes this degradation. Thus, improving the quality of MOVPE-grown InN by changing the growth temperature can be difficult. However, a short growth time at a high growth rate and a relatively high temperature is one effective way to solve this dilemma, and one can achieve carrier concentrations as low as 4×10¹⁸ cm⁻³ by growth at 650 °C for 30 min.     

High-resolution imaging of 1:1 [0 0 0 1] ordered a-plane Al0.3Ga0.7N

We report the observation of ordering in Al_0.3Ga_0.7N as part of an epitaxial lateral overgrowth (ELO) of GaN carried out using (1 1 2¯ 2) GaN templates grown by metal-organic chemical vapor deposition on m-plane sapphire. Transmission electron microscopy showed that the crystalline quality of the ELO GaN was greatly improved when the ELO SiO₂ mask was patterned along the [1 1 2¯ 0]_sapphire direction. The ELO GaN wings had an inclined columnar shape with smooth (0 0 0 1) and (1 1 2¯ 0) facets. Layers of 1:1 [0 0 0 1] ordered a-plane Al_0.3Ga_0.7N were observed on the a-plane GaN facets by high-resolution transmission electron microscopy and high-angle annular-dark-field scanning transmission electron microscopy. However, no ordering was observed for c-plane Al_0.3Ga_0.7N layers grown at the same time on the c-plane GaN facets.     

Unsupported Molybdenum and Cobalt-Molybdenum Nitrides for Thiophene Hydrodesulfurization

The catalytic activity and the structure of unsupported Mo and CoMo nitrided catalysts were investigated. It was found that the structure and catalytic activity of the nitrided catalysts are influenced by the conditions of nitridation. Molybdenum oxynitrides are more active in hydrodesulfurization (HDS) of thiophene than MoS₂. The addition of cobalt to nitrided Mo improves its HDS activity, however, sulfided CoMo catalyst is still more active than the nitrided one. Synergy between Co and Mo for the nitrided unsupported CoMo catalyst exists at lower degree than for the sulfided form of CoMo.     

Die Kristallstruktur des Komplexnitrides Mo(Ta,Mo)2N2

In the ternary system Ta/Mo/N a complex nitride of formula Mo(Ta,Mo)₂N₂ was observed at a nitrogen pressure of 360 bar and a temperature of 1,600°C. The crystal structure was determined from X-ray powder diagrams. The tetragonal unit cell, space groupI4/mmm-D _4h ¹⁷ , lattice parametersa=0.3051 nm,c=1.2530 nm contains ten atoms with an arrangement of the metal atoms corresponding to the MoSi₂-Type structure.

     

Physicochemical Modelling of Solid/Liquid Interfacial Phenomena

Summary. The proposed modelling of solid/liquid interfacial phenomena consists in relating the main experimental parameter – contact angle θ for metals or alloys on different substrates to combinations of the interatomic interaction parameters of the phases in contact. Physically, these interatomic interaction parameters are analogous to the electronic density distribution in the first and second coordination spheres of the crystallographic lattice of the component, to the bond length, and to the electrochemical factors. The contact angle θ of some alloys on AlN substrates in the case of non-reactive or reactive wetting is calculated.     

Expansion ratio dependence of lattice vibration of GaN using ab initio molecular dynamics calculations

Temperature dependence of Raman shift wavenumbers are important for the measurements of heat distribution of high-power GaN devices. In this study, calculated results of vibrational modes of ground state structure and some expanded structures of wurtzite type gallium nitride were presented using ab initio molecular dynamic (AIMD) simulations. Frequency analyses have been done for these calculated results and compared with the experimental results of temperature dependence of Raman spectra. Good agreement was achieved between the calculated and the experimental results quantitatively. AIMD would be a useful tool for the prediction of vibrating analysis for III-N systems.     

Multiple-step annealing for 50% enhanced p-conductivity of GaN

In this article, multiple-step rapid thermal annealing (RTA) processes for the activation of Mg doped GaN are compared with conventional single-step RTA processes. The investigated multiple-step processes consist of a low temperature annealing step at temperatures between 350°C and 700°C with dwell times up to 5 min and a short time high temperature step. With optimized process parameters, and multiple-step processes, we achieved p-type free carrier concentrations up to 1–2×10¹⁸ cm⁻³. The best achieved conductivity, so far, lies at 1.2 Ω⁻¹ cm⁻¹. This is a 50% improvement compared to conventional single-step process at 800°C, 10 min.     

In situ optical monitoring of AlGaN thickness and composition during MOVPE growth of AlGaN/GaN microwave HFETs

Real-time spectral reflectometry has been implemented to monitor the MOVPE growth of AlGaN/GaN microwave HFET structures. The aim is to monitor and control the thickness and composition of the thin AlGaN layer during growth. In order to extract useful information from the in situ spectra the optical constants of AlGaN as a function of alloy composition are required at the growth temperature (1050°C). As the first step to obtaining the high temperature optical constants, a room temperature spectroscopic ellipsometry study (energy range 1.65–4.95 eV) has been carried out on thin AlGaN films of various thickness (30 and 100 nm) and aluminium content (0.15 and 0.25). The multilayer model of each sample from the ellipsometry study is used to generate a reflectance spectrum which is compared with the in situ spectral reflectometry spectrum of the same sample acquired at room temperature to verify the technique. Further work is in progress to model the bandgap and optical constants of GaN and AlGaN at growth temperature.     

Growth of AlN films on SiC substrates by RF-MBE and RF-MEE

Epitaxial AlN films have been grown on SiC substrate by molecular beam epitaxy (MBE) and migration-enhanced epitaxy (MEE) using radio frequency (RF) plasma-excited nitrogen. In the RF-MBE growth, the growth rates have been found to be almost constant and the crystal quality improved with increasing the substrate temperature up to 850°C. Further increases of substrate temperature decreased the growth rate and degraded the crystal quality. Using the optimum substrate temperature of 850°C and optimizing the shutter open time, smooth AlN films with atomic force microscope roughness as low as 0.2 nm have been grown by RF-MEE growth.     

Industrial production of GaN and InGaN-light emitting diodes on SiC-substrates

Blue light emitting diodes (LEDs) based on GaN or InGaN have a large market, e.g. for communication, industry and automotive applications. Since 1998, OSRAM Opto Semiconductors has been producing blue LEDs on a large scale, concentrating on the automotive market. LEDs in the wavelength range 450–480 nm are grown by metalorganic vapour phase epitaxy (MOVPE) on SiC. This substrate material offers many advantages from the epitaxial and device processing points of view. To fulfil the strong consumer recommendations not only do the epitaxial processes have to be developed, but also improvements in chip technology and package design help to stabilise electrical and optical properties on these high level demands. For example, low forward voltages, high light output powers at 20 mA, low reverse currents, long term stability and high electrostatic discharge robustness have to be guaranteed in a temperature range of −55–+85°C and at a maximum humidity of 85%. Electrical and optical parameters were tested on every produced chip not only to remove LEDs before packaging, which do not meet the specifications, but to control and optimise the applied technological steps. As an example we demonstrate a testing method for controlling the process and optimising the p-contact.Optimisation of n- and p-contact as well as the improvement of epitaxy helped to increase the light output power from 1 mW in 1998 to more than 6 mW at present when mounted in a 5 mm radial lamp. All electrical parameters of these high brightness LEDs could also be improved or at least kept in the demanded specification range.