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The available data indicate that solubility of nitrogen in the liquid transition metals at high temperatures attains very interesting, high values even for relatively low pressure of N2 gas. For instance, at pN2 = 10 MPa (app. 100 bar) and 1600 °C, the N solubility in Fe-35%Cr solution is as high as 10 at%, what is significantly above N-solubility in liquid gallium at the typical conditions of high pressure solution growth of GaN (pN2 = 1.0 GPa, T = 1500 °C). Therefore, for pressures required for GaN stability at T > 1300 °C (much higher than 100 bar) one can expect N solubility in Fe and its mixtures with Co and Cr on the level of 10–20 wt% which is extremely promising for growth of GaN from liquid solution. The results of some “proof – of – concept” experiments exploring Fe-Ga-N system in both high pressure gas systems of high volume (1.0 GPa, 1600 °C, 1–5 dcm3) used previously for crystal growth of GaN at the Institute of High Pressure Physics PAS and in the toroid high pressure (8.0 GPa, 2500 °C) systems for bulk growth of diamond at the Institute of Superhard Materials in Kyiv are shown and discussed. The results of studies of: melting of Fe and Fe-Ga alloys in contact with nitrogen by DTA, dissolution of nitrogen from gas and solid (GaN) sources as well as physical properties of GaN crystals grown in Fe-Ga-N system are reported.

In this paper we initiate further discussion on phase diagrams and structural transitions in tetrahedrally bonded semiconductors with particular emphasis on GaN. Comprehensive experimental data and the most important theoretical predictions are comprised in an effort to bring up a generalized universal scheme that explain melting that may or may not be accompanied by the change of the coordination number. The scheme considers the “anomalous” melting temperature dependence on pressure for GaN. Principal possibility of GaN melting into low density semiconducting liquid is demonstrated and challenges on the way to experimental observation of such liquid phase are disputed. Prospects of future experiments that can provide reliable information on the nature of atomic bonding for different GaN phases are laid out.

Advanced Substrates consist of a thin GaN layer bonded to a carrier wafer. The layer is separated from starting material by Smart Cut™ technology. GaN on sapphire Advanced Substrates were successfully used as seeds for HVPE-GaN growth. Unintentionally doped and silicon-doped thick GaN layers were crystallized. The results were compared to HVPE-GaN grown on standard MOCVD-GaN/sapphire templates. Free-standing HVPE-GaN with high free carrier concentration was obtained. A laser diode was built on the n-type doped HVPE-GaN grown on an Advanced Substrate.

Investigation of source of stress in HVPE-GaN grown in c-direction on native seeds is reported. The study was based on experimental results. Ammonothermally grown GaN seed was used for growth of a thick HVPE-GaN layer in the c-direction. Deterioration of quality of the HVPE-GaN layer appeared. Numerical simulations were used for determining and examining radial, tangential, and axial stress distributions in the grown GaN crystal and the seed. The maximum equivalent stress (von Mises stress) was calculated and analyzed. It was shown that growth in non-polar and semi-polar directions, which appears on the edges of the crystal during crystallization process, can generate a very large tensile stress in GaN growing in the c-direction. This stress is much larger than the one generated due to a lattice mismatch between the ammonothermal seed and the deposited HVPE layer.

The growth of multiquantum well (MQW) AlGaN/GaN structures was carried out by plasma assisted molecular beam epitaxy simultaneously on semipolar ( 2 0 2 ¯ 1 ) , m -plane ( 1 0 1 ¯ 0 ) and c -plane (0 0 0 1) GaN substrates under nitrogen-rich (N-rich) conditions in order to investigate the growth mechanisms on different crystal surfaces. A smooth surface with atomic steps was found only for the semipolar ( 2 0 2 ¯ 1 ) structure as opposite to m -plane and c -plane surfaces which were rough and covered by islands. Transmission electron microscopy (TEM) studies of the semipolar MQW structure showed uniform AlGaN layers with sharp interfaces and no extended defect formation, which proves good structural quality. At higher resolution, on the surface and at the interfaces of the semipolar MQWs, {10-11} and {10-10} nano-facets were found. In contrast to the semipolar case, the TEM studies of the m -plane structure showed no clear periodicity and very uneven distribution of Al. The AlN was formed on island top surfaces and AlGaN grew only on island slopes that were tilted from the m -plane. The growth rate was slightly smaller for the semipolar structure and much smaller for the m -plane one, as compared to the c -plane MQW structure. This can be attributed to Ga losses from semipolar and m -plane surfaces, in contrast to c -plane where no Ga losses were observed. The mechanisms for the Ga losses under N-rich conditions are discussed.

In this work Ca 3 N 2 was investigated as a potential flux for crystallization of GaN. Melting temperature of the potential flux at high N 2 pressure evaluated by thermal analysis as 1380 °C is in good agreement with the theoretical prediction. It is shown that Ca 3 N 2 present in the liquid gallium in small amount (1 at%) dramatically accelerates synthesis of GaN from its constituents. On the other hand, it does not influence significantly the rate of GaN crystallization from solution in gallium in temperature gradient for both unseeded and seeded configurations. However the habit and color of the spontaneously grown GaN crystals change drastically. For 10 mol% Ca 3 N 2 content in the liquid Ga it was found that the GaN thick layer and GaN crystals (identified by micro-Raman scattering measurements) were grown on the substrate. For growth from molten Ca 3 N 2 (100%) with GaN source, the most important observations were (i) GaN source material was completely dissolved in the molten Ca 3 N 2 flux and (ii) after experiment, GaN crystals were found on the sapphire substrate.

The interaction of Mn thin films on atomically flat GaN( 000 1 ) has been investigated by Scanning Tunneling Microscopy (STM) and Time of Flight Secondary Ion Mass Spectroscopy (ToF-SIMS). GaN surfaces prepared by repeated sputtering/annealing cycles show various reconstructions but essentially the 6×6 one. A high density of small islands (height 0.6 nm, diameter 5 nm) nucleates upon deposition of 0.3 ML Mn. Upon annealing at 575 ∘C, these islands coalesce into larger islands (height 4 nm, diameter 50 nm) between which the GaN surface is visible. Atomically resolved STM images between the islands show the (3×3) reconstruction of GaN. Annealing the sample further to 675 ∘C leads to the restoration of a bare GaN surface. ToF-SIMS reveals that Mn diffused into the bulk of the sample.

Results of high-pressure directional growth of GaN on foreign substrates: SiC, sapphire and GaN/sapphire MOCVD templates are presented. The role of nitrogen pressure and supersaturation in the growth process is discussed. The conditions for stable growth of the nitride are determined. The results of the crystallization process are compared with those obtained for directional growth on pressure grown GaN crystals.

The results of high-pressure directional growth of GaN on GaN/sapphire templates are presented. The GaN growth rate as a function of the temperature gradient and time is determined. Changes of the growth rate with time are explained. The results of the crystallization process are briefly compared with those obtained for directional growth on pressure-grown GaN crystals.

The (0 0 0 1 ) (N-polar) (1 × 1) surface of bulk gallium nitride crystals has been investigated by means of angle-resolved photoemission spectroscopy. The surface was prepared by repeated cycles of Ar+ ion sputtering and annealing. The electronic band structure was explored along the Γ–A and Γ–K–M directions of the Brillouin zone, by measuring normal and off-normal photoemission spectra. A similar set of data was also collected for the same surface subjected to Ga deposition under MBE conditions. The identification of bulk and surface related spectral features was based on analysis of the acquired experimental data and supported by comparison with the results of available band structure calculations. The features characteristic of both (1 × 1) N-terminated and (1 × 1) Ga-adlayer terminated surface configurations were revealed for the surface under investigation.

The great advantage of the growth of GaN from high temperature (∼1500°C) gallium solutions under high nitrogen pressure (∼10–15 kbar) is that the crystals grown by this method are almost free of dislocations (density of 10–100 cm−2). Typically, the crystals grown under pressure during 150–200 h are relatively thin (about 0.1 mm) hexagonal platelets of 10–14 mm in their lateral size, which indicates strong anisotropy of such growth. The evolution of the crystal morphological forms in time and as a function of temperature distribution allows the discovery of qualitative relations between supersaturation and the nucleation rates on particular growth surfaces. It is of special importance for growth in directions parallel to the c-axis, which is very sensitive to the changes at the crystallization front at the higher supersaturations needed to increase the thickness of the final crystal. This often leads to the growth instabilities which can be avoided by special control of the conditions at the particular growing surface that is achieved by the application of directional methods of crystallization. The possible mechanisms of hindering the lateral growth of the platelets after 150–200 h can be related to changes of the mass transport mechanisms in the solution due to the presence of the crystals. Modeling of the mass transport phenomena in liquid gallium, based on the experimental temperature distribution data, shows that the convectional flows are again quite sensitive to small changes in the temperature gradients in the system. Such changes can be induced by a set of GaN crystals appearing in the solution. Therefore, to increase the lateral size of the crystals, an additional modification of temperature distribution during the crystallization process is necessary.

Experimental photoemission results on GaN (0 0 0 1 ) and CdTe(1 0 0) surfaces on a function of samarium adsorbate coverage are presented. Energy distribution curves were recorded at photon energies around the Sm 4 d →4 f threshold (135–136 eV) to reveal the Sm contribution to the electronic structure and enhance the sensitivity. The evolution of core-level Ga 3d and Cd 4d spectra was also studied. The interaction of Sm with both GaN and CdTe leads to reactive interface formation. In the case of CdTe, it results in creation of an interfacial Sm–CdTe compound, which inhibits further diffusion of Sm into the bulk. The interaction of Sm with GaN leads to the release of Ga atoms on the surface and segregation of Ga clusters.

GaN single crystals obtained by the high nitrogen pressure solution method without an intentional seeding, show strong growth anisotropy which results in their platelet shape. The attempts to enhance the growth into (0 0 0 1) directions by the increase of the integral supercooling in the solution, often lead to the growth instabilities on both Ga-polar and N-polar (0 0 0 1) surfaces. This can be avoided only by the precise control of the growth conditions at the crystallization front on the particular surface. The results of the seeded growth on both Ga- and N-polar (0 0 0 1) surfaces in configuration enabling such a control is reported. It is shown that dominating mechanisms of the unstable growth such as the cellular growth or edge nucleation can be suppressed. Differences in nucleation and growth in dependence on surface polarity are discussed.

We present the results of photoemission studies on (0 0 0 1 ) (N-polar) surfaces of bulk gallium nitride crystals. The procedures that yield clean and well-ordered (0 0 0 1 ) (1×1) surfaces were developed and tested. The surface crystallinity was assessed by low-energy electron diffraction. Angle-resolved photoemission spectroscopy with synchrotron radiation was used to determine the valence-band electronic structure of GaN along the Γ – Δ – A direction of the Brillouin zone. The band structure derived from our experimental data is compared with the results of recent band structure calculations.

GaN is the key material for blue and ultraviolet optoelectronics. It is a strongly bonded wurztite structure semiconductor with the direct energy gap 3.5 eV. Due to strong bonding, the diffusion processes require high temperatures, above 1300 K. However at this temperature range at ambient pressure, GaN becomes unstable and dissociates into Ga and N2. Therefore high pressure of N2 is required to study the diffusion and other annealing related processes. We studied annealing of bulk GaN nitride single crystals grown under high pressure and also annealing of homo- and heteroepitaxial GaN layers grown by MOCVD technique. Annealing at temperatures above 1300 K influences strongly the structural and optical properties of GaN crystals and layers. At this temperature diffusion of the Mg and Zn acceptors have been observed. In spite of very interesting experimental observations the understanding of microscopic mechanisms of these processes is limited.

Recent magnetoluminescence results obtained for homoepitaxial GaN layers are presented. The neutral impurity-bound excitons and donor–acceptor pair emission lines have been studied in magnetic fields up to 27 T. Low-temperature luminescence spectra have been measured with the magnetic field parallel and perpendicular to the hexagonal c-axis of the GaN layers. Experimental results allowed us to evaluate diamagnetic shifts, effective g-factors of electrons and holes involved in neutral donor and neutral acceptor complexes as well as the electron–hole exchange constant for close donor–acceptor pairs. Both the fine structure of the neutral acceptor-bound exciton emission and the specific properties of donor–acceptor pair spectra observed in magnetoluminescence experiments are tentatively attributed to the internal structure of the acceptor state.

N 2 adsorption on liquid Ga surface has been analyzed using quantum mechanical calculations in density functional theory (DFT) formulation. It has been shown that N 2 molecule undergoes the dissociation during the adsorption. The energy barrier is not higher than 5.1 eV which is considerably smaller than N 2 dissociation energy equal to 9.76 eV. These results indicate the single N atom configuration of nitrogen solution in liquid Ga.

High resistivity 104 -106 Ω cm (300 K) GaN single crystals were obtained by solution growth under high Ν2 pressure from melted Ga with 0.1-0.5at.% of Mg. Properties of these crystals are compared with properties of conductive crystals grown by a similar method from pure Ga melt. In • particular, it is shown that Mg-doped GaN crystals have better structural quality in terms of FWHM of X-ray rocking curve and low angle boundaries. Temperature dependence of electrical resistivity suggests hopping mechanism of conductivity. It is also shown that strain free GaN homoepitaxial layers can be grown on the Mg-doped GaN substrates. PACS numbers: 71.55.Eq, 72.80.Ey

In this paper, thermodynamical properties of AIN, GaN and InN are considered. It is shown that significant differences in melting conditions, thermal stability and solubilities in liquid group III metals lead to different possibilities of growing crystals from high temperature solutions, at N 2 pressure up to 20 kbar. It is shown that the best conditions for crystal growth at available pressure and temperature conditions can be achieved for GaN. High quality 6–10 mm single crystals of GaN have been grown at high N 2 pressure in 60–150 h processes. The mechanisms of nucleation and growth of GaN crystals are discussed on the basis of the experimental results. The crystallization of AlN is less efficient due to relatively low solubility of AlN in liquid Al. Possibility for the growth of InN crystals is strongly limited since this compound loses its stability at T > 600°C, even at 2 GPa N 2 pressure. The crystals of GaN grown at high pressure are the first crystals of this material used for homoepitaxial layer deposition. Both MOCVD and MBE methods have been successfully applied. Structural, electrical and optical properties of both GaN single crystals and homoepitaxial layers are reviewed.

A comprehensive study of the nitrogen K edge and gallium M{sub 2,3} edge in gallium nitride is presented. Results of two different experimental techniques, x-ray absorption by total photocurrent measurements and glancing-angle x-ray reflectivity, are compared with each other. First-principles calculations of the (polarization averaged) dielectric response {epsilon}{sub 2}({omega}) contributions from the relevant core-level to conduction-band transitions and derived spectral functions are used to interpret the data. These calculations are based on the local density approximation (LDA) and use a muffin-tin orbital basis for the band structure and matrix elements. The angular dependence of the x-ray reflectivity is studied and shown to be in good agreement with the theoretical predictions based on Fresnel theory and the magnitude of the calculated x-ray optical response functions. The main peaks in the calculated and measured spectra are compared with those in the relevant partial density of conduction-band states. Assignments are made to particular band transitions and corrections to the LDA are discussed. From the analysis of the N K and Ga M{sub 2,3} edges the latter are found to be essentially constant up to {approximately}10 eV above the conduction-band minimum. The differences in spectral shape found between the various measurements were shown to more » be a result of polarization dependence. Since the c axis in all the measurements was normal to the sample surface, p-polarized radiation at glancing angles corresponds to {bold E}{parallel}{bold c} while s polarization corresponds to {bold E}{perpendicular}{bold c} at all incident angles. Thus, this polarization dependence is a result of the intrinsic anisotropy of the wurtzite structure. Spectra on powders which provide an average of both polarizations as well as separate measurements of reflectivity with s polarization and p polarization were used. {copyright} {ital 1997} {ital The American Physical Society} « less

An attempt to identify the polarity of (0001) polar surface of GaN bulk single crystals grown by high nitrogen pressure solution method has been made using Auger electron spectroscopy (AES). AES concentration depth profiles of the top layer in (0001) direction starting from both (0001) faces of the sample have been measured. Distinct difference in the Ga concentration at the sample surface of both faces has been observed. The dependence of Ga concentration on depth is also different for both faces of the sample.

In this work we report results of photoluminescence (PL) and reflectivity measurements in the exciton region of GaN homoepitaxial layers grown by metalorganic chemical vapour deposition on GaN substrates. At low temperature (4.2K), very narrow (FWHM = 1.0meV) PL lines related to excitons bound to neutral acceptor (3.4666eV) and neutral donor (3.4719eV) were observed. The energies of free excitons from reflectivity and PL measurements were found to be: E A = 3.4780eV, E B = 3.4835eV and E C = 3.502eV.

In this paper, thermodynamical properties of AIN, GaN and InN, melting, thermal stability and solubility in liquid Al, Ga and In at N 2 pressures up to 20 kbar are considered. It is shown that significant differences in the thermodynamical properties of AlN, GaN and InN are caused mainly by different bonding energy in the solid phase. These differences lead to different results in the crystal growth of AlN, GaN and InN from the solutions in liquid Al, Ga and In, at high nitrogen pressure. High quality, 1-mm single crystals of GaN can be grown in a 5–24 h processes. The crystallization of AlN is less efficient due to the relatively low solubility of AlN in liquid Al, in the experimentally accessible temperature range. Possibility for the growth of InN crystals is strongly limited since this compound loses its stability at T > 600 °C, even at 20 kbar N 2 pressure. The mechanisms of nucleation and growth of GaN crystals is discussed on the basis of the experimental results. The quality of the 1-mm and 1-cm GaN single crystals is compared and discussed in terms of growth stability, which is the necessary condition for obtaining high quality, large single crystals of GaN. The physical properties of pressure grown crystals are reviewed briefly.

A1N has been synthesized at nitrogen pressure highly exceeding the equilibrium one. The combustion of bulk A1 leading to high degree conversion into A1N have been observed at pressures of 0.1–6.5 kbar. Small, single crystals of A1N were grown from the solution in Al, at N2 pressure of 8 kbar, in temperature range of 1600–1800°C.

We report a synthesis of new wide band gap semiconductor – Ga 0.25 Mg 0.25 N 0.5 by means of high-pressure, high-temperature method. Stoichiometry of the monocrystalline samples was determined using an energy dispersive X-ray (EDX) microprobe. We characterized the material obtained by X-ray diffraction revealing, that it crystallizes in the hexagonal structure with unit cell of P3 symmetry and a =3.277A, c =16.065A (almost three time larger than c -lattice constant of GaN). The measurements of the absorption edge showed that the energy gap of this material is substantially reduced in comparison with GaN and is close to 3.0 eV at room temperature. Measurements of thermal expansion coefficient of this material indicates the appearance of large anisotropy.

MnAs has been grown by means of MBE on the GaN(000 (1) over bar)-(1x1) surface. Two options of initiating the crystal growth were applied: (a) a regular MBE procedure (manganese and arsenic were delivered simultaneously) and (b) subsequent deposition of manganese and arsenic layers. It was shown that spontaneous formation of MnAs dots with the surface density of 1x10(11) cm(-2) and 2.5x10(11) cm(-2), respectively (as observed by atomic force microscopy), occurred for the layer thickness higher than 5 ML. Electronic structure of the MnAs/GaN systems was studied by resonant photoemission spectroscopy. That led to determination of the Mn 3d-related contribution to the total density of states distribution of MnAs. It has been proven that the electronic structures of the MnAs dots grown by the two procedures differ markedly. One corresponds to metallic, ferromagnetic NiAs-type MnAs, the other is similar to that reported for half-metallic zinc-blende MnAs. Both systems behave superparamagnetically (as revealed by magnetization measurements), but with both the blocking temperatures and the intradot Curie temperatures substantially different. The intradot Curie temperature is about 260 K for the former system while markedly higher than room temperature for the latter one. Relations between growth process, electronic structure, and other properties of the studied systems are discussed. Possible mechanisms of half-metallic MnAs formation on GaN are considered.

Corrigendum to “Growth mechanisms in semipolar (2021) and nonpolar m plane (1010) AlGaN/GaN structures grown by PAMBE under N-rich conditions” [Cryst. Growth 377C (2013) 184–191] M. Sawicka , C. Cheze , H. Turski , J. Smalc-Koziorowska , M. Kryśko , S. Kret , T. Remmele , M. Albrecht , G. Cywinski , I. Grzegory , C. Skierbiszewski a,b a Institute of High Pressure Physics, Polish Academy of Sciences, Sokolowska 29/37, 01-142 Warsaw, Poland b TopGaN Sp. z o.o., Sokolowska 29/37, 01-142 Warsaw, Poland c Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, 02-668 Warsaw, Poland d Leibniz Institute of Crystal Growth, Max-Born Strasse 2, Berlin 12489, Germany

Until recently, the heat capacity of GaN had only been measured for polycrystalline powder samples. Semiempirical as well as first-principles calculations have appeared within the past few years. We present in this article measurements of the heat capacity of hexagonal single crystals of GaN in the $20--1400\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ temperature range. We find that our data deviate significantly from the literature values for polycrystalline materials. The dependence of the heat capacity on the isotopic mass has also been investigated recently for monatomic crystals such as diamond, silicon, and germanium. Multiatomic crystals are expected to exhibit a different dependence of these heat capacities on the masses of each of the isotopes present. These effects have not been investigated in the past to our knowledge. We also present first-principles calculations of the dependence of the heat capacities of GaN, as a canonical binary material, on each of the Ga and N masses. We show that they are indeed different, as expected from the fact that the Ga mass affects mainly the acoustic, that of N the optic phonons. It is hoped that these calculations will encourage experimental measurements of the dependence of the heat capacity on isotopic masses in binary and more complex semiconductors.

Nitrides of group-III metals, namely GaN, AlN, BN, InΝ, are characterized by very short, strong atomic bonds and relatively large ionicity. Because of that, they have many physical properties different from other Ill-V semiconductors. In particular, it concerns the magnitude of energy gaps and their pressure coefficients. In this paper we present the results of the pressure variation of the direct energy gap, dEg/dp, measured for GaN and InN as well as those calculated for the four mentioned above nitrides. We find that first principle calculations reproduce experimental results very well. Comparison with other Ill—V compounds shows that nitrides possess much smaller dE g/dp values and in addition it appears that neither the Phillips ionicity nor the bond length can be used as a single parameter describing the discussed behavior.

International audience; We investigated the relation between structural properties and carrier recombination processes in InGaN∕GaNmultiple quantum well(MQW) structures with quantum well widths of 3 and 9nm, grown by metal-organic chemical-vapor deposition on bulk GaN crystals. Quantum barriers of the samples are heavily n-type doped in order to effectively screen the large polarization-induced electric fields which commonly occur in hexagonal InGaN∕GaN quantum structures. High thermal stability in these structures, reflected by strong photoluminescence(PL) even above 400K, is attributed to a combination of low-dislocation densities and potential profile fluctuations in the InGaN∕GaNquantum wells. The role of potential profile fluctuations is further investigated by time-resolved photoluminescence and cathodoluminescence(CL) mapping. Comparison of both samples shows that the sample with 3‐nm-wide QWs exhibits (i) a larger width of the PL peak in the temperature range of 8–420K, (ii) a higher amplitude of potential profile fluctuations as measured by CL mapping, and (iii) higher radiative and nonradiative PL recombination times. At the same time a much weaker drop of PL intensity with temperature is recorded on the sample with 9‐nm-wide QWs. Our results show that, contrary to intuitive expectation, a decrease of the potential profile fluctuation amplitude can be helpful in enhancing the radiative recombination efficiency, particularly at high temperatures.

Nonpolar multiple quantum wells (MQWs) have been grown by plasma assisted molecular beam epitaxy on bulk GaN crystals oriented along the (112¯0) direction. The photoluminescence intensity of the nonpolar MQWs was significantly higher than that found for the polar samples, both at low (10 K) and room temperature. This is a consequence of the lack of built-in electric field in samples grown along the (112¯0) direction. Clearly resolved spectra of the excitons have been observed in the studied MQWs. Studies of these excitonic structures, by means of polarization and temperature measurements enabled us to assign the observed lines to free and bound excitons in GaN quantum wells.

MnAs dots have been grown by means of MBE on the GaN(0001)‐(1×1) surface. The growth processes were initiated by two different procedures: a regular MBE or subsequent deposition of manganese and arsenic layers. Electronic structures of the MnAs/GaN systems were studied by means of resonant photoemission spectroscopy. The total density of states distribution of MnAs was determined and the Mn 3d — related contribution revealed. It has been proven that the applied growth procedures led to two different electronic structures of the MnAs dots.