Understanding the cubic AlN growth plane from first principles

The structure of clean and reconstructed cubic AlN (0 0 1) surfaces has been investigated from first principles. In the nitrogen rich part of the phase diagram, a nitrogen termination has been found to be favored. Under aluminum-rich conditions, a contracted adlayer of Al-atoms on top of an Al-termination is favored.     

Possibility of AlN growth using Li–Al–N solvent

The possibility of AlN growth using Li–Al–N solvent was investigated. Based on theoretical prediction, we selected Li₃N as a suitable nitrogen source for AlN growth. First, vapor phase epitaxy using Li₃N and Al as source materials was performed to confirm the following reaction on the growth surface: Li₃N+Al=AlN+3Li. The results suggest that the reaction proceeds to form AlN on the substrate under appropriate conditions. Next, AlN growth using Li–Al–N solvent was carried out. The Li–Al–N solvent was prepared by annealing of mixtures composed of Li₃N and Al. The results imply that AlN was formed under an Al-rich condition. Moreover, it was found that Li was swept out from AlN grains during growth. The results suggest that AlN growth using Li–Al–N solvent might be a key technology to obtain an AlN crystal boule.     

Synthesis of Ti-Cr-N thin films by reactive ion-beam mixing of Ti/Cr interfaces

Titanium/chromium interfaces produced by titanium thin film deposition on chromium substrates have been reactively mixed using ion-beams of 2 and 3 keV at room temperature (RT). The ion-beam mixing (IBM) has been analysed using X-ray photoelectron spectroscopy (XPS), angle-resolved X-ray photoelectron spectroscopy (ARXPS) and Monte Carlo TRIDYN simulations. Below ∼2 × 10¹⁶ ions/cm², the IBM kinetic is characterised by a strong decrease of the Ti concentration along with a fast nitrogen incorporation, that can be explained mainly by Ti sputtering and nitrogen implantation. Above that ion dose, the Ti/Cr ratio can be varied in a broad range maintaining nearly constant the nitrogen concentration (∼50%), as a consequence of sputtering, nitruration and mixing effects taking place simultaneously. ARXPS results show that the composition of the ternary films formed by reactive IBM is rather uniform in the near surface region. The comparison of experimental results with those obtained from TRIDYN, that uses pure ballistic mechanisms, suggests that the chemical reaction with the nitrogen partial pressure and processes driven by residual defects are the rate-controlling mechanisms during the reactive IBM of Ti/Cr interfaces.     

Intermetallic and metal-rich phases in the system Li-Ba-In-N

Three new intermetallic phases, BaLi_2.1In_1.9, BaLi_1.12In_0.98, and BaLi_1.06In_1.16 and two subnitrides Li₃₅In₄₅Ba₃₉N₉ and LiIn₂Ba₃N_0.83 have been synthesized and their crystal structures have been determined. According to single crystal X-ray diffraction data BaLi_2.1In_1.9 and BaLi_1.12In_0.98 crystallize with hexagonal symmetry (BaLi_2.1In_1.9: P6₃/mmc, a=10.410(2), c=8.364(2) Å, Z=6, V=785.0(2) Å³) and BaLi_1.12In_0.98: P6/mmm, a=17.469(1), c=10.6409(7) Å, Z=30, V=2813.5(8) Å³), while BaLi_1.06In_1.16 has a rhombohedral structure (R-3c, a=18.894(3), c=85.289(17) Å, Z=276, V=26368(8) Å³). BaLi_2.1In_1.9 is isostructural with the known phase BaLi₄. The phase BaLi_1.12In_0.98 is structurally related to Na₈K₂₃Cd₁₂In₄₈, while BaLi_1.06In_1.16 is isostructural with Li_33.3Ba_13.1Ca₃. A sample containing structurally similar BaLi_1.12In_0.98 and BaLi_1.02In_1.16 was also investigated by transmission electron microscopy. Li₃₅In₄₅Ba₃₉N₉ and LiIn₂Ba₃N_0.83 crystallize with tetragonal (I-42m, a=15.299(2), c=30.682(6) Å, Z=2, V=7182(2) Å³) and cubic (Fd-3m, a=14.913(2) Å, Z=8, V=3316.7(7) Å³) symmetry, respectively. While the first-mentioned subnitride belongs to the Li₈₀Ba₃₉N₉ structure type, the second extends the structural family of Ba₆In_4.78N_2.72. The structural features of the new compounds are discussed in comparison to the known phases and the results of total energy calculations.     

Vacancy defects in bulk ammonothermal GaN crystals

We have applied positron annihilation spectroscopy to study in-grown vacancy defects in bulk GaN crystals grown by the ammonothermal method. We observe a high concentration of Ga vacancy related defects in n-type samples in spite of the low growth temperature, suggesting that oxygen impurities promote the formation of vacancies also through other mechanisms than a mere reduction of thermodynamical formation enthalpy. On the other hand, no positron trapping at vacancy defects is observed in Mg-doped p-type samples, as expected when the Fermi level is close to the valence band and intrinsic defects are dominantly positively charged. Annealing of the samples at temperatures well above the growth temperature is found to change significantly the defect structure of the material.     

Optical probing of low-pressure solution grown GaN crystal properties

The structural and optical properties of self-nucleated crystals grown by a near atmospheric pressure solution growth method are presented. High-resolution room temperature Raman scattering studies demonstrate that stress-free crystals with low free-electron background have been produced. Low and room temperature photoluminescence experiments confirm the presence of shallow donors and an unknown shallow acceptor. Large relative intensity variations of the emission bands assigned to recombination process involving donors and acceptor, resulting from significant changes in the incorporation and/or activation of defect associated with each recombination channel, reflect major changes in the intrinsic material properties.     

Ternary and quaternary layered nitride halides, Ca2N(X,X′) (X,X′=Cl, Br, I): Evolution of structure with composition

The quaternary systems Ca-N-Cl-Br and Ca-N-Br-I have been investigated resulting in the synthesis of a number of new layered nitride mixed halides. The evolution of structure with composition has been investigated by powder X-ray and powder neutron diffraction techniques. A continuous solid solution exists between Ca₂NCl and Ca₂NBr with intermediate compounds adopting the same anti- α-NaFeO₂ structure (rhombohedral space group ) as the ternary end members. A phase transition occurs in the Ca₂NBr_1−yI_y system between y=0.7 and y=0.8 corresponding to a switch from cubic close packing to hexagonal close packing of metal-nitrogen layers and corresponding adoption of the anti-β-RbScO₂ (filled anti-CdI₂) structure (hexagonal space group P6₃/mmc). While nitride and halide anions occupy distinct crystallographic sites, there is no ordering of halides in quaternary materials irrespective of stoichiometry or structure type. All the nitride halides show temperature independent paramagnetic behaviour between 2 and 300 K.     

Präkeramische Polyazane über die Sol‐Gel‐Route im Ammonosystem und aus molekularen Einkomponentenvorläufern — ein Leistungsvergleich

Preceramic Polyazanes via Sol Gel Route in the Ammono System and via Molecular Single Source Precursors — a Comparison of Performance All steps of the sol gel process in the oxo system can be transferred analogously to the ammono system. For this purpose, element alkyl amides of titanium, zirconium, tantalum and boron, have been co‐ammonolised with silicon alkyl amides in aprotic solvents. Pyrolysis of the resulting polyazanes yields multinary nitridic ceramic powders. For the model systems Si(NHMe)₄/Ti(NMe₂)₄, Si(NHMe)₄/Zr(NMe₂)₄, Si(NHMe)₄/Ta(NMe₂)₅, and Si(NHMe)₄/B(NMe₂)₃ the corresponding polymeric polyboro‐, polytitano‐, polyzirkono‐ and polytantalosilazanes were synthesised. Pyrolysis in flowing ammonia at 1000°C results in ternary amorphous silicon nitrides; further heating in nitrogen leads to partly crystalline composites. The process of pyrolysis as well as the morphology and composition of the final pyrolysis products were analysed by means of NMR, MAS‐NMR, FT‐IR, DTA‐TG‐MS, XRD, SEM, EDX, and elemental analyses. In order to verify this new and convenient access to multinary nitride ceramics, single source precursors were synthesised and, via polymer intermediates, processed to ceramic powders. A specially developed reaction sequence is generally applicable to syntheses of single source precursors for various ternary metal silicon nitrides. Dilithiated silazanes of the type Si(NMe₂)₂(NLiR)₂, with R = t‐Butyl, SiMe₃, CH₃, synthesised and structurally characterised for the first time, are good silicon synthones. While forming nitrido bridges, those compounds react with e.g. two‐ or four valent transition metal chlorides, to silicon transition metal adducts. Actually, we have analysed the reaction of Si(NMe₂)₂(NLiR)₂ and TiCl₄, ZrCl₄, TaCl₅, CrCl₃, MnCl₂, and ZnCl₂, respectively. The adducts as formed were crosslinked with ammonia and pyrolysed. In the case of the chlorides of the 4.—6. group metals, amorphous ceramics were obtained. Treatment at higher temperatures results in composites of transition metal nitride and an amorphous matrix. MnSiN₂ and a new hexagonal modification of ZnSiN₂ were obtained in the systems Mn/ Si/ N and Zn/ Si/ N. Surprisingly, during the synthesis of ZnSiN₂, ZnCN₂ was obtained as a side product.     

Enhanced two-dimensional growth of MOVPE InN films on sapphire (0 0 0 1) substrates

MOVPE growth of InN on sapphire substrates is compared using two different designs of horizontal reactor. The major difference between the two designs is a variation in the reactant-gas flow-spacing between the substrate and the ceiling of the quartz chamber: 33 mm for the Type A reactor and 14 mm for Type B. Compared with the Type A reactor, the Type B reactor brings about InN films with a larger grain size. This is especially true when InN is grown at 600°C using the Type B reactor, in which case the two-dimensional (2D) growth of InN is found to be extremely enhanced. An investigation of the NH₃/TMIn molar ratio dependence of the surface morphology of grown InN films using the two reactors suggests that the enhanced 2D growth is attributed to the decrease in the effective NH₃/TMIn ratio in the growth atmosphere. Even using the Type A reactor, a film with enhanced 2D growth can be obtained when the NH₃/TMIn ratio is considerably low (1.8×10⁴). The enhanced 2D growth results in a smaller XRC-FWHM (full-width at half maximum of the X-ray rocking curve) (1500 arcsec), than that for a 3D-grown film (5000 arcsec).     

Room-temperature luminescence study on the effect of Mg activation annealing on p-GaN layers grown by MOCVD

An Mg-doped p-GaN layer was grown by the metalorganic chemical vapor deposition method. The dissociation extent of hydrogen-passivated Mg acceptors in the p-GaN layer through Mg activation annealing was estimated by using room-temperature cathodoluminescence (CL) spectroscopy. The CL measurement revealed that the CL spectra intensities tend to increase with increasing the activation annealing temperature. The sample annealed at 925 °C showed the most intense emission and the narrowest width among the emission peaks. Consequently, it was the most excellent dissociation extent of Mg–H complexes caused by the Mg activation annealing. The hole concentration under this optimum condition was 1.3×10¹⁷ cm⁻³ at room temperature. The photoluminescence (PL) measurement showed a 2.8 eV band having characteristically a broad peak in heavily Mg-doped GaN at room temperature. By analyzing the PL results, we learned that this band was associated with the deep donor–acceptor pair (DAP) emission rather than with the emission caused by the transition from the conduction band to deep acceptor level. The four emission peaks in the resolved 2.8 eV band were emitted by transiting from deep donor levels of 0.14, 0.26, 0.40, and 0.62 eV below the conduction band to the shallow Mg acceptor level of 0.22 eV above the valence band.