Sylwester Porowski

Equilibrium pressure of N2 over GaN and high pressure solution growth of GaN

Abstract Studies of the equilibrium pressure of N 2 over Ga-GaN mixtures at pressures of up to 60 kbar and temperatures of up to 2300°C were carried out. The experimentally measured equilibrium pressures of N 2 are considerably lower than those predicted by the ideal gas law. At 1700°C, for instance, the equilibrium pressure equals only 20 kbar instead of 650 kbar as calculated for an ideal gas. Due to the above mentioned behaviour of N 2 it was possible to obtain equilibrium conditions in a gas pressure chamber up to a temperature of 1700°C. At high temperature, the solubility of GaN in Ga steeply increases, reaching molar 2% at 1500°C. Owing to this high solubility we obtained GaN crystals and epitaxial layers with growth rates of about 10 μm/h at 16 kbar and 1500°C. By varying the temperature and pressure of nitrogen, layers with a resistivity ranging from 10 -4 Ω cm up to higher than 10 8 Ω cm have been obtained.

Lattice parameters of gallium nitride

Lattice parameters of gallium nitride were measured using high‐resolution x‐ray diffraction. The following samples were examined: (i) single crystals grown at pressure of about 15 kbar, (ii) homoepitaxial layers, (iii) heteroepitaxial layers (wurtzite structure) on silicon carbide, on sapphire, and on gallium arsenide, (iv) cubic gallium nitride layers on gallium arsenide. The differences between the samples are discussed in terms of their concentrations of free electrons and structural defects.

Bulk and homoepitaxial GaN-growth and characterisation

Gallium nitride single crystals in the form of hexagonal platelets with dimensions exceeding 1 cm are currently grown from the solution in liquid gallium, at nitrogen pressure of 12-20 kbar and at temperatures of 1400-1700°C. The crystals grown without an intentional doping are always highly conductive with typical free electron concentration 3-5 x 10 19 cm -3 and resistivity of 10 -3 10 -2 Ω cm. The electron concentration in GaN crystals can be strongly reduced by the introduction of Mg into the growth solution. The resistivity of the crystals grown from the solutions containing Mg can be as high as 10 6 Ω cm. Physical properties for the both low- and high-resistivity GaN are compared in the paper. The results of both mechanical and mechano-chemical polishing of the (0001) surfaces of GaN are presented. In particular, it is shown that the atomically flat GaN surfaces can be achieved by mechano-chemical polishing. Homoepitaxial layers of GaN and its ternaries have been grown on the pressure-grown GaN crystal by both MOCVD and MBE. The results of optical, electrical, structural and microscopic characterization are briefly reviewed.

Luminescence and reflectivity in the exciton region of homoepitaxial GaN layers grown on GaN substrates

Abstract 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.

Exciton region reflectance of homoepitaxial GaN layers

Reflection measurements in the exciton region of GaN homoepitaxial layers grown by metalorganic chemical vapor deposition (MOCVD) on GaN single crystals are reported. At low temperature (4.2 K) three free exciton lines have been found with energies; EA=3.4776 eV, EB=3.4827 eV, and EC=3.502 eV. The spin‐orbit parameter Δso=19.7±1.5 meV and the crystal field parameter Δcr=9.3±0.3 meV have been obtained. From temperature dependence of exciton spectra the energy gap dependence has been found: E(T)=E(0)−λ/[exp(β/T)−1] (λ=0.121 eV, β=316 K).

High pressure thermodynamics of GaN

The thermodynamics of a Ga-N2 system under high N2 pressure was investigated. On the basis of the experimentally measured equilibrium pressure of N2 over GaN, the standard Gibbs free energy, the heat and entropy of formation of GaN, and the equilibrium constant were calculated at temperatures of up to 1700°C and pressures of up to 20 kbar. A kinetic model and calculations of the decomposition reaction rate of GaN at high pressure are presented.

The microstructure of gallium nitride monocrystals grown at high pressure

Abstract This work shows the results of X-ray diffractometric measurements performed on gallium nitride (wurtzite structure, (00.1) oriented plates) bulk crystals grown using the high-pressure (12–15 kbar)-high-temperature (about 1800 K) method. The examinations included: rocking-curve analysis, reciprocal lattice mapping, topography and measurements of lattice parameters. Monocrystals of size up to about 1 mm exhibit a very high crystallographic perfection (rocking curves of 20–30 arc sec). Bigger crystals possess a mosaic structure (0.1–1 mm crystallites separated by 1–3 arc min angle boundaries) visualised by X-ray topography. A model of the creation of those low-angle boundaries is proposed. It is based on the following observations: (i) the mosaic crystals are dome-shaped; (ii) the concave side is a “nitrogen-terminating” one, which grows faster; (iii) this side possesses smaller lattice parameters with respect to the “gallium-terminating” side. We have related the decrease of the lattice parameters to the gallium precipitation (observed in electron microscopy) beneath the “nitrogen-terminating” side. The difference between the lattice parameters on the two sides of the crystal causes its bending, which is then relaxed by emission of the low-angle boundaries.

Growth and properties of single crystalline GaN substrates and homoepitaxial layers

Abstract It is shown that high quality crystals of III–V nitrides can be grown at high gas pressure (up to 20 kbar) and high temperature (up to 2000 K). From the analysis of the thermodynamic properties of AlN, GaN and InN, which are briefly summarized in the paper, it follows that the best conditions for crystal growth at available pressures and temperature conditions can be achieved for GaN. The crystallization of AlN is less efficient due to the relatively low solubility of AlN in liquid Al. The 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. Recently, high quality, transparent and colorless, 5 mm GaN platelets have been grown from solution in liquid gallium at N 2 pressure up to 20 kbar. The mechanisms of nucleation and growth of GaN crystals are discussed on the basis of the experimental results. Structural, electrical and optical properties of these crystals are reported in the paper. The crystals have been used as substrates for the homoepitaxial growth of GaN by MOCVD. Both n- and p-type layers have been obtained by doping with Si and Mg. At low temperature (4.2 K) very strong and narrow PL lines at 3.4666 eV (FWHM = 1.0 meV) assigned to an exciton bound to a neutral acceptor and at 3.4711 eV (FWHM = 1.3 meV) and 3.4719 eV (FWHM = 1.3 meV) due to excitons bound to two neutral donors have been found for undoped GaN layers.

Lattice constants, thermal expansion and compressibility of gallium nitride

High-resolution X-ray diffraction measurements can be performed at variable temperatures and pressures. The usefulness of such experiments is shown when taking gallium nitride, which is a wide-band semiconductor, as an example. The GaN samples were grown at high pressures (bulk crystals) and as epitaxial layers on silicon carbide and sapphire. The X-ray examinations were done at temperatures of 293-750 K and at pressures of up to 8 kbar. The results served for an evaluation of the basic physical properties of gallium nitride; namely lattice constants, thermal expansion and compressibility. The comparison of monocrystals with epitaxial layers grown on highly mismatched substrates provided important information about the influence of the substrate on the crystallographic perfection of the layers.

Decay of stimulated and spontaneous emission in highly excited homoepitaxial GaN

The high-density effects in the recombination of electron–hole plasma in photoexcited homoepitaxial GaN epilayers were studied by means of transient photoluminescence at room temperature. Owing to the “backward” and “lateral” photoluminescence measurement geometries employed, the influence of stimulated transitions on the decay of degenerate nonthermalized plasma was revealed. The lateral stimulated emission was demonstrated to cause a remarkable increase in the recombination rate on the early stage of the luminescence transient. A delayed enhancement of the stimulated emission due to the cooling of plasma from the initial temperature of 1100 K was observed. After completion of the thermalization process and exhaustion of the stimulated emission, the spontaneous-luminescence decay exhibited an exponential slope that relates to the nonradiative recombination of the carriers. The homoepitaxially grown GaN layer featured a luminescence decay time of 445 ps that implies a room-temperature free-carrier lifetim...