Solvothermal processes can be described as the reaction or the decomposition of precursor(s) in presence of a solvent in a close system at a temperature higher than the solvent boiling point. Consequently pressure parameter is involved. Such processes are mainly managed by two sets of parameters: (i) thermodynamical parameters and (ii) chemical parameters.

Solvothermal processes concern either supercritical or subcritical conditions, -homogeneous or heterogeneous systems, and thermodynamical parameters (P and T) play an important role. The main interest of solvothermal reactions is the improvement of the chemical reactivity, and in such a case the nature of the reagents and the physico-chemical properties of the solvent are important.

For the last 20 years, solvothermal processes have been developed in different research areas, such as:

- synthesis of new compounds (including in particular hybride systems),

- crystal growth of functional materials,

- preparation of nanocrystallites with well defined size and morphology,

- deposition of thin films,

- sintering in mild temperature conditions.

In particular, solvothermal reactions involve nitrides and derived materials either for preparing new compositions or shaping functional nitrides (single crystals, nanoparticles…).

In a first approach the synthesis of nitrides using solvothermal reactions will be described versus the nature of the precursors and the composition of the solvent. The nitriding process will be analyzed versus the chemical species, in particular N3−, N3−…Different illustrations will be described (transition metal nitrides, light elements-based nitrides…).

The second part will be devoted to the development of solvothermal processes for the preparation of single crystals or nanocrystallites.

As a conclusion, the potential developments of solvothermal nitriding processes either in basic research or industrial applications will be discussed.

Wide-bandgap III-nitride-based avalanche photodiodes (APDs) are important for photodetectors operating in UV spectral region. For the growth of GaN-based heteroepitaxial layers on lattice-mismatched substrates such as sapphire and SiC, a high density of defects is introduced, thereby causing device failure by premature microplasma breakdown before the electric field reaches the level of the bulk avalanche breakdown field, which has hampered the development of III-nitride based APDs. In this study, we investigate the growth and characterization of GaN and AlGaN-based APDs on free-standing bulk GaN substrates. Epitaxial layers of GaN and AlxGa1−xN p-i-n ultraviolet avalanche photodiodes were grown by metalorganic chemical vapor deposition (MOCVD). Improved crystalline and structural quality of epitaxial layers was achieved by employing optimum growth parameters on low-dislocation-density bulk substrates in order to minimize the defect density in epitaxially grown materials. GaN and AlGaN APDs were fabricated into 30μm- and 50μm-diameter circular mesas and the electrical and optoelectronic characteristics were measured. APD epitaxial structure and device design, material growth optimization, material characterizations, device fabrication, and device performance characteristics are reported.

Schottky-barrier photovoltaic devices are fabricated by selective metal deposition on p-GaN. A 1.25 V open-circuit voltage is observed for the best device. Devices were optimized by annealing in forming gas at temperatures ranging from 550°C to 700°C. Annealing time and forming gas flow rate are used to control the metal-semiconductor Schottky barrier formation. Optimum fabrication parameters are achieved based on photovoltaic response from the devices under UV illumination. Barrier heights (0.47 eV - 0.49 eV) were used as basis to compare the device response. The Schottky-barrier height is very sensitive to processing conditions, for example a 2.5% increase in barrier height is observed when Schottky contact annealing temperature is changed from 600 °C to 650 °C. Under UV illumination, the open-circuit voltage and short-circuit current increase with increasing annealing temperature while the series resistance decreases under such conditions.

We present variational calculations of donor binding energy in rectangular wurtzite aluminium gallium nitride / gallium nitride quantum wires. We explicitly take into account the effect of spontaneous and piezoelectric polarization on the energy levels of donors in quantum wires. Wurtzite structure nitride semiconductors have spontaneous polarization even in the absence of externally applied electric fields. They also have large piezoelectric polarization when grown as pseudomorphic layers. The magnitude of both polarization components is of the order of 1013 electrons per cm2, and has a non-trivial effect on the potential profile seen by electrons. Due to the large built-in electric fields resulting from the polarization discontinuities at heterojunctions, the binding energies of donors is a strong function of the position inside the quantum wire. The potential profile in the 0001 direction can vary by as much as 1.5eV due to polarization effects for vertical dimensions of the quantum wire up to 20 angstroms. The probability density of electrons tends to concentrate near the minimum of the conduction band profile in the 0001 direction. Donors located close to this minimum tend to have a larger concentration of electron density compared to those located closer to the maximum. As a consequence, the binding energy of the former are higher compared to the latter. We use Lorentzian variational wavefunctions to calculate the binding energy as a function of donor position. The confinement potential enhances the binding by a factor of about 3 compared to donors in bulk nitride semiconductors, from about 30 meV to about 90 meV. The variation of binding energy with position is calculated to be more than 50% for typical compositions of the quantum wire regions. Our calculations will be useful for understanding device applications involving n-type doped nitride semiconductor quantum wires.

In the context of ferromagnetic spin-coupling in dilute magnetic semiconductors, we present optical investigations on Mg co-doped GaMnN and Fe doped GaN. A complex luminescence feature occurring in Mg co-doped GaMnN around 1 eV was previously attributed to the internal 4T2(F)—4T1(F) transition of Mn4+ involved in different complexes. Selective excitation studies indicate the presence of at least three different complexes. Photoluminescence excitation spectra suggest that the internal Mn3+ transition may represent an excitation mechanism. Magneto photoluminescence spectra indicate equal g values for the ground and excited state. Low temperature infrared absorption spectra of Fe doped GaN allow to unambiguously establish the electronic structure of the Fe2+ center in GaN. Our results suggest that the Fe2+(5T2) state is stabilized against Jahn-Teller coupling by the reduced site-symmetry of the hexagonal lattice.

Aluminum nitride (AlN) is a promising material for a number of applications due to its temperature and chemical stability. Furthermore, AlN maintains its piezoelectric properties at higher temperatures than more commonly used materials, such as Lead Zirconate Titanate (PZT) [1, 2], making AlN attractive for high temperature micro and nano-electromechanical (MEMs and NEMs) applications including, but not limited to, high temperature sensors and actuators, micro- channels for fuel cell applications, and micromechanical resonators.

This work presents a novel AlN micro-channel fabrication technique using Metal Organic Vapor Phase Epitaxy (MOVPE). AlN easily nucleates on dielectric surfaces due to the large sticking coefficient and short diffusion length of the aluminum species resulting in a high quality polycrystalline growth on typical mask materials, such as silicon dioxide and silicon nitride [3,4]. The fabrication process introduced involves partially masking a substrate with a silicon dioxide striped pattern and then growing AlN via MOVPE simultaneously on the dielectric mask and exposed substrate. A buffered oxide etch is then used to remove the underlying silicon dioxide and leave a free standing AlN micro-channel. The width of the channel has been varied from 5 ìm to 110 ìm and the height of the air gap from 130 nm to 800 nm indicating the stability of the structure. Furthermore, this versatile process has been performed on (111) silicon, c-plane sapphire, and gallium nitride epilayers on sapphire substrates. Reflection High Energy Electron Diffraction (RHEED), Atomic Force Microscopy (AFM), and Raman measurements have been taken on channels grown on each substrate and indicate that the substrate is influencing the growth of the AlN micro-channels on the SiO2 sacrificial layer.

The crystal structures of α-Si(NCN)2 and Si2N2(NCN) undergo phase transformations upon compression. A new modification of HP-Si(NCN)2 exhibits all Si atoms in octahedral coordination to N and may be achievable below 30 GPa. In Si2N2(NCN) we find partial increase of coordination for Si, C, and N atoms at about 60 GPa. In both cases, the carbodiimide moiety is the source for the internal reaction that lead to compactification of the structure at elevated pressures. We present 29Si NMR chemical shifts that may help to identify the new coordination environments.

With an external laser excitation, the electroluminescence (EL) of GaInN/GaN green light emitting diodes (LEDs) grown on sapphire by metal organic vapor phase epitaxy has been investigated. The EL was found significantly enhanced under this bias and the difference can not merely be attributed to additional photoluminescence (PL). Under 325 nm photon bias, the EL enhancement starts at its highest value and decreases along with an increase of the LED current. Under 408 nm and 488 nm bias, it increases first to a value smaller than that of the 325 nm bias and then decreases with a much lower rate along the current increase. The EL enhancement is attributed to the more efficient carrier injection into quantum wells (QWs) resulting from the screening of the QW polarization by photon bias. Therefore, an enhanced balance of majority and minority carriers was obtained resulting in a better radiative recombination rate. Meanwhile, the current-voltage characteristics show a negative current first and then a voltage reduction as forward voltage increases. The reverse photocurrent indicates carrier loss due to the solar cell effect in our LED device while at high current region the carrier loss is attributed to another effect controlled by the external electrical field. At the balance point of those two effects, the EL enhancement is the highest. These findings clarify the transition from highly efficient radiative recombination at low current density to the region of efficiency droop at high current densities.

Using the full potential linearized augmented plane wave (FP-LAPW) method, we investigate the bulk structural and electronic properties in the scandium-, yttrium-, and copper-nitrides over a wide range of nitrogen concentrations. The N atom was gradually incorporated into metal matrix with and without metal vacancies. The ground state properties like densities of states (DOS) and formation energies are determined for each calculated alloy. We have found that the semi-conducting state in copper nitride have a tinny compositional margin. Any deviation of the ideal stoichiometry will produce a metallic character. What is more, the stabilities of the conductive phases are very close to the stability of the semi conducting phase, with a little margin favorable to the conducting phases. The calculations of scandium- and yttrium nitrides show, that for very low nitrogen incorporations, the hexagonal and fcc phases may coexist. However, for high nitrogen concentration the cubic phases are favored. For non-stoichiometric nitrogen content, the materials behave as metal, whereas at stoichiometric composition the DOS becomes zero at Fermi level (EF), confirming in this way the semiconductor character of these nitrides.

Transmission electron microscopy (TEM) study was performed to investigate the interface region of AlN/6H-SiC. Thick AlN layers were grown on a 3.5° off-axis 6H-SiC substrate at a temperature of 1790 °C for 100 hours by sublimation-recondensation method. The energy dispersive x-ray spectroscopy (EDXS) analysis indicated considerable amount of aluminum and nitrogen present in the substrate and Si and C present in AlN. Lattice images of cross-sectional TEM samples show a faceted interface with step growth.

High quality homoepitaxial growth of m-plane GaN films on freestanding m-plane HVPE GaN substrates has been performed using metalorganic chemical vapor deposition. For this a large growth space was studied. Large areas of no-nucleation along with presence of high density of defects were observed when layers were grown under growth conditions for c-plane GaN. It is believed that these structural defects were in large part due to the low lateral growth rates as well as unequal lateral growth rates in a- and c- crystallographic directions. To achieve high quality, fully coalesced epitaxial layers, growth conditions were optimized with respect to growth temperature, V/III ratios and reactor pressure. Higher growth temperatures led to smoother surfaces due to increased surface diffusion of adatoms. Overall, growth at higher temperature and lower V/III ratio decreased the surface roughness and resulted in better optical properties as observed by photoluminescence. Although optimization resulted in highly smooth layers, some macroscopic defects were still observed on the epi-surface as a result of contamination and subsurface damage remaining on bulk substrates possibly due to polishing. Addition of a step involving annealing of the bulk substrate under H2: N2 environment, prior to growth, drastically reduced such macroscopic defects.

We synthesized HoxEr1−xN (x =0.1, 0.3, 0.5, 0.7, 0.9) by the carbothermic reduction method. The magnetic entropy change, ΔS, which is an indicator of performance of magnetocaloric effect, MCE, was obtained from data sets of magnetization M (H, T) measured at various temperature, T, and magnetic field, H, through the Maxwell equations. The Thus obtained ΔS vs. T curves have peaks at temperatures depending on x in a range of 8–18 K. ΔS values of HoxEr1−xN expressed in terms of J K−1 m−3 were higher than those of intermetallic compounds of rare earth and transition metals previously reported. These ΔS peak plots are along a convex curve, which may indicate an interaction between the two components.

Precipitate accommodated nitriding is proposed as a new, non-traditional surface treatment of aluminum alloys. In the case of nitriding the aluminum-copper alloys, Al2Cu plays an important role as a nucleation site to form AlN by the two step reactions of Al2Cu + 2N → 2AlN + Cu and Cu + 2Al → Al2Cu and as a helper to preserve the fast nitrogen diffusion path. In this precipitate accommodation, AlN islands form on Al2Cu in coherent, and, gradually cover the matrix surface by the AlN layer. This Al2Cu accommodation in nucleation reduces the pre-sputtering time and shortens the incubation time prior to formation of AlN layer. Furthermore, the interface between Al2Cu and AlN plays a nitrogen diffusion path across the nitrided layer. This Al2Cu accommodation in growth of AlN accelerates the growth rate of AlN layer in practice. The formation rate of AlN approaches to 7 μm/ks in the present nitriding. This reduction of processing time is much favored for industrial manufacturing of automotive parts and electrical components in practice. Low friction and wear is also attained in dry sliding condition by this AlN-layered specimen: μ = 0.13 and Ws = 1.5 × 10−5 mm3N−1m−1. Since Al2Cu is common precipitate to most of aluminum alloys, this AlN-layer formation is also favored to protective coating for automotive parts and electric components.

The contact properties of ZrN on p-type Si obtained by magnetron reactive sputtering were investigated. Schottky diodes characteristics were evaluated by current-voltage (I-V) and capacitance voltage (C-V) measurements. The barrier heights obtained by I-V and C-V are in the range of 0.55-0.63V and 0.88-0.91V, respectively.

Epitaxial aluminum nitride (AlN) thin films were successfully prepared on the (0001) sapphire substrate by chemical vapor deposition (CVD) using aluminum iodide (AlI3) and ammonia (NH3) under atmospheric pressure at 750 ºC. The crystallographic relationship between AlN thin films and Al2O3 substrate is in the following; AlN(0001)//Al2O3(0001) and AlN[1010]//Al2O3[1120]. Lattice parameters of AlN thin film measured by X-ray diffraction revealed that c=0.498 and a=0.311 nm, respectively. Residual stress estimated by modified sin2ψ method was 0.38 GPa in compressive stress. Cross-sectional TEM observation revealed that an interlayer lies between the AlN films and the sapphire substrate. It was suggested that relaxation of residual stress caused by the mismatching of lattice parameter and thermal expansion coefficient was brought about by the interlayer.

The phase boundary between β-Si3N4 and γ-Si3N4 is investigated at high-pressure and high-temperature using first-principles lattice dynamics calculations within the quasi-harmonic approximation. We find a positive slope of the phase boundary. It turns out that the thermal expansion of the spinel-type γ-phase is larger than that of the phenacite-type β-phase. On the other side, pressure affects more the volume of β-Si3N4 than of γ-Si3N4, reflected in the higher bulk modulus of γ-Si3N4. The origin of the different temperature behavior of these phases, consequently, goes along with a larger volume dependence of the zero point energy in γ-Si3N4 in comparison to β-Si3N4.

Investigation of the properties of MnxGa1-xN semiconductor alloy is performed on the basis of LCAO electron band structure of this semiconductor. The authors model this alloy on the basis of Mn substitutions on Ga sites and it is identified that the ternary semiconductor MnxGa1-xN has two tetrahedral binary constituents – GaN and MnN. It is found that the sp3 hybrid orbital of the N atom attracts an electron from the 3d orbital of the Mn atom and thus the Mn-N bonding becomes of sp3 type. In this way the total electron spin of the 3d orbital of the Mn atom becomes 3/2, which determines that the energy levels 4F, 4D, 4P and 4G belonging to this orbital to be occupied. LCAO electron band structure of wurtzite MnxGa1-xN for points Γυc1 and Γυv15 (υ = 1, 2, 3, 4, 5) is calculated by previously developed method and the positions of the levels 4F, 4D, 4P and 4G in this structure are determined. It is found dependence of the total electron spin of the Mn atom on the position of the Fermi level in the electron band diagram of MnxGa1-xN. Also it is found that the total spin depends on electron inter-band transitions as well. The optical properties of the wurtzite MnxGa1-xN are determined as well.

Heteroepitaxial two-dimensional (2D) and three-dimensional (3D) MOVPE growth of nitride layers on sapphire substrates using low temperature (LT) AlN nucleation layer are discussed. It is shown by Atomic Force Microscopy and Transmission Electron Microscopy that the 2D or 3D growth mode of successive AlGaN layers depends predominantly on the growth conditions of the underneath LT AlN nucleation layer.

Modifications of growth conditions of LT AlN nucleation layer in respect of the growth in the conventional way, leads to a drastic change in properties of the successively grown at high temperature AlGaN layer. Main modification is connected with a drastic reduction of the growth rate of LT AlN nucleation layer at higher rector pressure. In this case the growth at high temperature starts evenly on the whole surface, retaining atomic flatness, and occurs in the 2D mode. Therefore, it is possible to grow even a very thin, continuous AlGaN layers directly on top of LT nucleation layer.

The 2D growth mode leads to significant change in extended defect distribution. Edge dislocation density being close to 1010 cm−2, results in the effective relaxation of the internal stress. On the other hand mix dislocations density are close to 108 cm−2. All observed mix dislocations occur in pairs with dislocations with opposite sign. This suggests that all mix dislocations originate from dislocation half-loops. Both types of dislocations, edge and half loops can be effectively reduce in-situ, during the growth using annealing in the presence of silane. This process and precise control of the growth conditions leads to decreasing of mix dislocations density to 2·106 cm−2 and edge dislocations density to 5·108 cm−2.

The newly developed two-dimensional growth mode of nitride layer opens up a new way to reduce strain, decrease density of dislocations and improvement of crystallographic properties.