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Electrical properties of carbon-doped halide vapor phase epitaxy-GaN are presented and discussed. Crystals of the highest structural quality and with different carbon concentrations are investigated. Resistivity and Hall measurements as a function of temperature are analyzed in detail. It is found that the concentration of free holes systematically decreases with the increase of carbon concentration. Such behavior results from the fact that the compensation of the carbon acceptor level increases with the carbon concentration. It is accepted that carbon is amphoteric impurity in GaN, creating an acceptor as well as a donor state, which leads to self-compensation. The analysis of existing electron paramagnetic resonance results is extremely important. It enabled us to determine the compensation ratio as a function of carbon concentration. A combination of electron paramagnetic resonance, secondary ion mass spectrometry, and Hall data allowed us to conclude that the acceptor level (CN) exhibits rather significant temperature shift equal to 0.35 meV/K.

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In this work, we present measurements of the dynamics of photoexcited carriers in GaInN/GaN quantum wells (QWs) grown on ammonothermal GaN, especially thermalization and recombination rates. Emission properties were measured by time-resolved photoluminescence (PL) and electroluminescence spectroscopy. Due to the use of high quality homoepitaxial material, we were able to obtain very valuable data on carrier thermalization. The temperature dependence of the QW energy observed in PL shows characteristic S-shape with a step of about 10 me V. Such a behavior (related to thermalization and localization at potential fluctuations) is often reported for QWs; but in our samples, the effect is smaller than in heteroepitaxial InGaN/GaN QWs due to lower potential fluctuation in our material. Absorption properties were studied by photocurrent spectroscopy measurements. A comparison of emission and absorption spectra revealed a shift in energy of about 60 me V. Contrary to PL, the QW energy observed in absorption decreases monotonically with temperature, which can be described by a Bose-like dependence E(T) = E(0) - (lambda)/(exp((theta)/T) - 1), with parameters (lambda) = (0.11 (+/-) 0.01) eV, (theta) =(355 (+/-) 20)K, or by a Varshni dependence with coefficients (alpha) = (10 (+/-) 3) x 10(-4) eV/K and (beta) = (1500 (+/-) 500) K. Taking into account absorption and emission, the fluctuation amplitude (according to Eliseev theory) was (sigma) = 14 me V. The time resolved PL revealed that in a short period (<1 ns) after excitation, the PL peaks were broadened because of the thermal distribution of carriers. We interpreted this distribution in terms of quasi-temperature (T(q)) of the carriers. The initial T(q) was of the order of 500K. The thermalization led to a fast decrease of T(q). The obtained cooling time in the QW was (tau)(c) = 0.3 ns, which was faster than the observed recombination time (tau)(r) = 2.2ns (at 4K)

Photoluminescence (PL) from GaN substrates fabricated by the ammonothermal growth method was studied in a wide range of temperatures and excitation intensities, both with steady-state and time-resolved PL techniques. Three defect-related PL bands were detected: the ultraviolet luminescence band with the zero-phonon line at 3.27 eV, the Zn-related BL1 band with a maximum at 2.9 eV, and the yellow luminescence band (labeled YL2) with a maximum at 2.3 eV. The YL2 band belongs to an unknown defect and is different from the CN-related YL1 band. Its maximum blueshifts by 0.06 eV with increasing excitation intensity and redshifts by more than 0.1 eV with a time delay after a laser pulse. The YL2 band is preliminarily attributed to a defect complex containing the gallium vacancy.

The defect structure of HVPE-GaN crystals is examined using synchrotron white-beam X-ray topography (SWXRT) and topography results are interpreted and discussed in comparison to reciprocal lattice point broadening from high resolution X-ray diffraction (HRXRD) measurements. Two as-received commercial HVPE-GaN wafers from two different vendors and one HVPE-GaN which was grown on an ammonothermal GaN-seed are investigated in this study. To our knowledge SWXRT large area back-reflection analysis of HVPE-GaN grown on an ammonothermal GaN seed has been performed for the first time. From large-area topography the formation of a cellular defect network is identified for the commercial HVPE-GaN. Large differences in the crystal lattice misorientation deformation (mosaicity) are determined for the different samples by transmission section topography. For the HVPEGaN grown on an ammonothermal GaN-seed a very low defect density was ascertained. From the contrasts of the topography threading screw-type dislocations and threading mixed-type dislocations were identified. The X-ray topography analysis shows clearly and for the first time that the nature of the defect structure and the low density of ammonothermal GaN seeds can be transferred by HVPE growth of GaN. For demanding GaN-based (opto-)electronic applications such as laser diodes or transistors for high power electronics there is a need for material with low defect densities, since threading dislocations act as centers of non-radiative recombination, increasing the leakage current, reducing the room temperature mobility and so limiting the efficiency, performance and lifetime of devices. Freestanding GaN with low defect density is the substrate material of choice for the realization of such GaN-based device structures with superior properties. Different approaches for crystal growth of bulk GaN like ammonothermal growth, hydride vapor-phase epitaxy (HVPE) and high-and low-pressure solution growth were followed in recent years.1 The ammonothermal growth and HVPE seem to be the most promising methods to produce GaN substrates in sufficient number, size and material quality and substrates prepared with these growth methods have become available commercially in recent years. In the ammonothermal growth method, GaN crystalizes from a solution of Ga in supercritical ammonia with the addition of a mineralizer. For seed material, the ammonothermal growth method uses native GaN crystals. Benefits of the ammonothermal growth method are very low dislocation densities as low as 5 × 10 3 cm −2 and large curvature radii of the crystal planes (>100 m).2 Drawbacks of GaN ammonothermal growth method are the incorporation of impurities and the low growth rate (0.1 mm/day), even when the method can potentially produce hundreds of crystals in a single batch. In comparison, in HVPE the GaN crystallization takes place from the vapor phase by the reaction of ammonia with gallium chloride at temperatures of about 1050• C. HVPE uses GaN seed layers (templates) deposited e.g. by metalorganic v...