Photoluminescence of InGaN/GaN multiple quantum wells grown by mass transport

We report on studies of an In_0.12Ga_0.88N/GaN structure with three 35 Å thick quantum wells (QWs) grown by metalorganic vapor phase epitaxy with employment of mass transport. The mass-transport regions demonstrate a threading dislocation density less than 10⁷ cm⁻². The photoluminescence (PL) spectrum is dominated by a 40 meV—narrow line centered at 2.97 eV at 2 K. This emission has a typical PL decay time of about 5 ns at 2 K within the PL contour. An additional line with longer decay time (about 200 ns) is observed at an energy about 2.85 eV. The position of this line shifts towards higher energies with increasing excitation power. The data are consistent with a model, where the PL originates from at least two nonequivalent QWs, which could be realized due to a potential gradient across the layers.     

Investigation of Ni/Au-contacts on p-GaN annealed in different atmospheres

We investigated the effect of different annealing atmospheres on contact behaviour of Ni/Au contacts on moderately doped p-GaN layers. We used the annealing gases N₂, O₂, Ar, and forming gas (N₂/H₂) at varying annealing temperatures from 350°C to 650°C in steps of 50°C. The p-GaN samples were either metalorganic chemical vapor deposition or molecular beam epitaxy grown. Contact characterization was done after each annealing step by using the circular transmission line model. Specific contact resistances were determined to be in the low 10⁻⁴ Ω cm² range for oxidized contacts. Accompanying chemical analysis using depth resolved Auger electron spectroscopy revealed that NiO was formed and Au diffused towards the interface, whereas annealing in forming gas prevented oxidation and did not lead to Ohmic behaviour.     

Hexagonal AlN films grown on nominal and off-axis Si(0 0 1) substrates

Nucleation and growth of wurtzite AlN layers on nominal and off-axis Si(0 0 1) substrates by plasma-assisted molecular beam epitaxy is reported. The nucleation and the growth dynamics have been studied in situ by reflection high-energy electron diffraction. For the films grown on the nominal Si(0 0 1) surface, cross-sectional transmission electron microscopy and X-ray diffraction investigations revealed a two-domain film structure (AlN¹ and AlN²) with an epitaxial orientation relationship of [0 0 0 1]_AlN || [0 0 1]_Si and AlN¹ || AlN² || [1 1 0]_Si. The epitaxial growth of single crystalline wurtzite AlN thin films has been achieved on off-axis Si(0 0 1) substrates with an epitaxial orientation relationship of [0 0 0 1]_AlN parallel to the surface normal and 0 1 1 0_AlN || [1 1 0]_Si.     

Solid State Metathesis Reactions in Various Applications

Solid state metathesis reactions have been studied in fused silica tubes, by differential thermal analysis, and by X‐ray powder diffraction. A selection of reactions between metal (La, Nb, and Ni) chlorides and lithium nitride or lithium acetylide were investigated to get more insight into reaction pathways and intermediate reaction stages that may be adopted on course of the formation of metal nitrides or carbides. Intermediate compounds are considered to be important because they can control the reactivity of a system. Such compounds were traced by changing the molar ratios of reaction partners away from the salt‐balanced binary metal nitride or carbide target compositions. New preparative perspectives are discovered when metal chlorides were reacted with lithium nitridoborate or lithium cyanamide. Due to their reductive nature towards several d‐block metal chlorides, (BN₂)^3‐ and (CN₂)^2‐ react to form metals or metal nitrides plus X‐ray amorphous BN, and probably C₃N₄. With lanthanum chloride they can react to form nitridoborates and nitridocarbonates. The metathesis reaction between lithium cyanamide and cyanuric chloride (C₃N₃Cl₃) instead of metal chloride was studied for the synthesis of C₃N₄.     

Well width study of InGaN multiple quantum wells for blue–green emitter

InGaN/GaN multiple quantum well structures emitting in the blue/green wavelength region were grown by metal organic vapor phase epitaxy. By reducing the quantum well growth time the influence of the quantum well thicknesses between 3.8 and 1.1 nm on the indium incorporation and the distribution of indium in the quantum wells in growth direction were investigated. X-ray diffraction measurements show that the average indium mole fraction in the quantum wells decreases with reducing quantum well width due to a delay in the indium incorporation at the barrier/well interface. Quantitative analysis reveals a segregation length of about 2 nm as a measure of the graded region in growth direction. Cathodoluminescence imaging reveals that the spatial variation of the wavelength is reduced with decreasing quantum well thickness down to 1.7 nm. Reducing the width of the quantum well further results in an increase of the spatial wavelength variation.     

Defect and stress relaxation in HVPE-GaN films using high temperature reactively sputtered AlN buffer

The influence of high temperature buffer layers on the structural characteristics of GaN grown by hydride vapour phase epitaxy on sapphire was investigated. Strain relaxation as well as mismatch-induced defect reduction in thick GaN layers grown on AlN buffer was microscopically identified using cathodoluminescence and micro-Raman spectroscopy in cross-section of the films. The results were correlated with photoluminescence and Hall-effect data of layers with different thicknesses. These relaxation processes were suggested to account for the specific defect distribution in the buffers revealed by high-resolution X-ray diffraction and transmission electron microscopy.     

Thermally induced stress in GaN layers with regard to film coalescence

Thermally induced plane stress in GaN layers of different thicknesses, grown by metalorganic vapour phase epitaxy on sapphire, is investigated. Thin layers, characterized by isolated grains, are found to be stress-free. With increasing layer thickness, however, grains start to coalesce and stress can build up when the samples are cooled down following growth. As soon as the coalescence process is completed and a compact film has been formed, a maximum stress level is reached which does not further increase for still thicker layers. Therefore, it is proposed that grain edges enable non-compact films to elastically relieve in-plane stress.     

Magneto-photoluminescence of AlGaN/GaN quantum wells

The magneto-luminescence of GaN/AlGaN quantum wells in fields up to 52 T shows a field dependence that is strongly dependent on the well width. Strong redshifts are seen for the narrowest wells that are attributed to a Zeeman splitting. This is unexpected, since in bulk GaN epilayers the electron and hole g-factors of the lowest valence band cancel each other almost exactly. Therefore, we attribute this splitting to a reordering of the valence band due to the different band offsets caused by the strain and the aluminium component in the AlGaN barriers. The field dependence also gives information on the size of the exciton that has been converted into values for the exciton binding energy, and these values agree reasonably well with a theory that includes the presence of the electric field.     

Nanoparticles of superconducting γ-Mo2N and δ-MoN

We have been able to prepare nanoparticles (∼4 nm diameter) of cubic γ-Mo₂N by a simple procedure involving the reaction of MoCl₅ with urea at 873 K. The nanoparticles show a superconducting transition around 6.5 K. The γ-Mo₂N nanoparticles are readily transformed to nanoparticles of δ-MoN with a slightly larger diameter on heating in a NH₃ atmosphere at 573 K. Phase-pure δ-MoN obtained by this means shows a superconducting transition around 5 K.