High Nitrogen Pressure Solution Method Research Articles

The Origin of the Yellow Luminescence Band in Be-Doped Bulk GaN

Photoluminescence (PL) from Be-doped bulk GaN crystals grown by the High Nitrogen Pressure Solution method was studied and compared with PL from GaN:Be layers on sapphire grown by molecular beam epitaxy and metalorganic chemical vapor deposition techniques. The yellow luminescence band in the latter is caused by the isolated BeGa acceptor (the YLBe band), while the broad yellow band in bulk GaN:Be crystals is a superposition of the YLBe band and another band, most likely the CN-related YL1 band. The attribution of the yellow band in bulk GaN:Be crystals to the BeGaON complex (a deep donor) is questioned.

Negative Magnetoresistivity in Highly Doped n-Type GaN.

This paper presents low-temperature measurements of magnetoresistivity in heavily doped n-type GaN grown by basic GaN growth technologies: molecular beam epitaxy, metal-organic vapor phase epitaxy, halide vapor phase epitaxy and ammonothermal. Additionally, GaN crystallized by High Nitrogen Pressure Solution method was also examined. It was found that all the samples under study exhibited negative magnetoresistivity at a low temperature (10 K < T < 50 K) and for some samples this effect was observed up to 100 K. This negative magnetoresistivity effect is analyzed in the frame of the weak localization phenomena in the case of three-dimensional electron gas in a highly doped semiconductor. This analysis allows for determining the phasing coherence time τφ for heavily doped n-type GaN. The obtained τφ value is proportional to T−1.34, indicating that the electron–electron interaction is the main dephasing mechanism for the free carriers.

Systematic study of shockley-read-hall and radiative recombination in GaN on Al2O3, freestanding GaN, and GaN on Si

Here we study and correlate structural, electrical, and optical properties of three GaN samples: GaN grown by metalorganic chemical vapor deposition on sapphire (GaN/Al2O3), freestanding GaN crystals grown by the high nitrogen pressure solution method (HNPS GaN), and GaN grown by hydride vapor phase epitaxy on silicon (GaN/Si). Defect and impurity densities and carrier concentrations are quantified by x-ray diffraction, secondary mass ion spectroscopy, and Hall effect studies, respectively. Power-dependent photoluminescence measurements reveal GaN near-band-edge emissions from all samples having mixtures of free exciton and band-to-band transitions. Only the defect luminescence in the GaN/Si sample remains unsaturated, in contrast to those from the HNPS GaN and GaN/Al2O3 samples. Carrier lifetimes, extracted from time-resolved photoluminescence measurements, and internal quantum efficiencies, extracted from temperature-dependent photoluminescence measurements, are used to extract radiative and nonradiative lifetimes. Shockley–Read–Hall (A) and radiative recombination coefficients (B) are then calculated accordingly. Overall, the A coefficient is observed to be highly sensitive to the point defect density rather than dislocation density, as evidenced by three orders of magnitude reduction in threading dislocation density reducing the A coefficient by one order of magnitude only. The B coefficient, while comparable in the higher quality and lowly doped GaN/Al2O3 and HNPS GaN samples, was severely degraded in the GaN/Si sample due to high threading dislocation density and doping concentration.

Photo-EPR study of compensated defects in Be-doped GaN substrates

Time-dependent photo-induced electron paramagnetic resonance measurements were performed on GaN crystals grown by the high nitrogen pressure solution method. A resonance was induced in Be-doped samples by illuminating with photon energies greater than 2.7 eV. The EPR was thermally stable at T=4K and was quenched with photon energies between 0.5 and 2.7 eV. Optical cross sections for EPR excitation and quenching processes were extracted from time-dependent measurements using a model for defect charge state populations. A fit to the cross sections was performed using a lineshape for photon absorption by deep centers in GaN. The threshold for photo-excitation of a compensated acceptor (A−+hν→A0+eCB−) was 2.5 eV followed by a lattice relaxation of 0.3 eV. The threshold for the complimentary process (A0+hν→A−+hVB+) was 0.75 eV with a relaxation energy of 0.3 eV. These values are consistent with CN being the EPR-active acceptor. Thermal quenching is attributed to delocalization of carriers from a beryllium-related electron trap, which are then captured by neutral CN.

Influence of crystallization front direction on the Mg-related impurity centers incorporation in bulk GaN:Mg grown by HNPS method

We studied the incorporation of Mg-related impurity centers in GaN crystals depending on the direction of the crystallization front. Two series of GaN crystals – (i) undoped and (ii) Mg-doped – were grown by High Nitrogen Pressure Solution (HNPS) method under otherwise identical conditions. Each series contained four samples with (101¯0), (112¯0), (202¯1¯) and (202¯1) orientations. The low-temperature photoluminescence (PL) spectroscopy was used for characterization of the obtained crystals. The observed differences in the PL spectra of GaN:Mg crystals suggested that Mg incorporation in GaN grown by HNPS method depends considerably on the orientation of crystallization front. The concentration of Mg impurity incorporated into the GaN crystals subsequently increases for the following sequence of planes: (101¯0), (112¯0), (202¯1¯) and (202¯1). For (101¯0), (112¯0) and (202¯1¯) planes the blue band is related only to ON – MgGa donor-acceptor pair (DAP) transitions, while for (202¯1) plane the incorporation of Mg-H complexes occurs additionally to the formation ON – MgGa DAP.

Electron paramagnetic resonance studies of bulk Mg‐doped GaN grown by high nitrogen pressure solution method

AbstractAn acceptor‐related impurity was studied in bulk free standing GaN using electron paramagnetic resonance spectroscopy. Both undoped and Mg‐doped substrates grown by the high nitrogen pressure solution method were examined, but only the doped sample revealed the presence of an EPR‐detected center. Comparison with earlier work on p ‐type GaN films suggests that the defect is an acceptor, heavily compensated during growth. Photo‐EPR studies revealed a photo‐ionization threshold of 2.8 eV, which is interpreted as excitation of an electron from the acceptor to the conduction band. Based on the concentration and photo‐threshold, the EPR center detected in the HNPS doped samples is tentatively identified as Be, which SIMS measurements indicate is a trace impurity in the doped GaN sample. (© 2015 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Determination of an acceptor level in bulk GaN grown by high nitrogen pressure solution method

A point defect in acceptor‐doped bulk free standing GaN grown by the high nitrogen pressure solution method (HNPS) was investigated using electron paramagnetic resonance (EPR) spectroscopy. A nearly isotropic spectrum with g‐value of 1.984 is observed after illumination with 2.8 eV light and is stable after removal of the light. Comparison with earlier work on p‐type GaN films and SIMS data on the HNPS substrates suggests that the defect is related to an acceptor, heavily compensated during growth by oxygen. Time‐dependent photo‐EPR data are well modeled as a single defect‐to‐band transition where the defect level is 0.67 eV above the valence band edge. Although Mg‐ and Be‐doped samples were studied, only those samples containing Be produced the EPR signal. This observation, coupled with the similarity of the defect level measured to that predicted from the theory of others, suggests that the defect is related to a Be impurity.

Growth of GaN:Mg crystals by high nitrogen pressure solution method in multi-feed–seed configuration

Crystallization of GaN by High Nitrogen Pressure Solution method in multi-feed–seed (MFS) configuration without intentional doping [1] results in: (1) Growth of strongly n-type crystals with free electron concentration increasing with growth temperature in ranges of 2–6×1019cm−3 and 1350–1430°C, (2) stable growth on Ga-polar surface and unstable growth on N-polar surface, crystals slightly brown, (3) improvement of (0001) crystallographic planes curvature (flattening) with respect to bowing of these planes in the seed crystals.The addition of magnesium into the growth solution causes strong compensation of free electrons in the crystals. Therefore, highly resistive GaN crystals can be grown. In this work, the crystallization of Mg doped GaN on flat ∼1in. seeds (substrates) grown by HVPE in MFS configuration has been studied. It is shown that: (1) Highly resistive GaN:Mg crystals with resistivity higher than 107Ωcm were grown, (2) the growth is stable on N-polar surfaces of the seeds whereas it is unstable on the Ga-polar surfaces, which is opposite to the HNPS growth of the n-type crystals. The GaN:Mg crystals are fully transparent with no visible color, (3) shape of (0001) crystallographic planes improves (flattens) with respect to bowing of these planes in the seed crystals (HVPE substrates).

High nitrogen pressure solution (HNPS) growth of GaN on 2 inch free standing GaN substrates

Recent results of High Nitrogen Pressure Solution (HNPS) growth of GaN crystals deposited on and separated from 2 inch, and smaller, GaN substrates grown by Hydride Vapor Phase Epitaxy (HVPE) have been presented. The influence of the c-plane bowing in the initial substrate on quality, rate and mode of growth by HNPS method has been analyzed in details.

Seeded growth of GaN at high N 2 pressure on (0 0 0 1) polar surfaces of GaN single crystalline substrates

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.

High pressure growth of bulk GaN from solutions in gallium

In this paper, the growth of GaN single crystals from solutions of atomicnitrogen in liquid gallium under high N2 pressure is described. GaN singlecrystals obtained by the high nitrogen pressure solution method, without anintentional seeding, show strong growth anisotropy, which results in theirplatelet shape. The attempts to enhance the growth into (0001) directions bythe increase of the integral supercooling in the solution often lead togrowth instabilities on both N-polar and Ga-polar (0001) surfaces. This can beavoided only by the precise control of the growth conditions at thecrystallization front on the particular surface.The results of the seeded growth in directions parallel and perpendicular tothe c-axis of GaN are discussed. In particular, it is shown that dominatingmechanisms of the unstable growth on (0001) polar surfaces such as thecellular growth or edge nucleation can be suppressed and the crystal can begrown in a much more stable way.Physical properties most relevant for understanding the growth of GaN crystalsare reviewed. The most important feature of GaN crystals grown by the HNPSmethod is that they are almost free of dislocations and therefore used assubstrates give a possibility to grow perfect epitaxial structures.

GaN substrates for molecular beam epitaxy growth of homoepitaxial structures

The high nitrogen pressure solution (HNPS) method allows the growth of GaN single crystals of very high structural quality, with a dislocation density as low as 10–100 cm−2. The crystals are hexagonal platelets with a (0001) surface area of 1 cm2 and a thickness of 0.1 mm. Both highly conductive and non-conductive GaN crystals can be grown by the HNPS method. The conductive crystals are grown without intentional doping, whereas the non-conductive are intentionally magnesium-doped during crystallization. It will be shown that atomically flat epi-ready N-polar surfaces can be obtained by mechanical and mechano-chemical polishing with the use of alkaline water solutions. The opposite Ga-polar surface is chemically inert to room temperature wet etching, and has to be treated by reactive ion etching to remove subsurface damage introduced by mechanical polishing. The crystals were used for the epitaxy of GaN by molecular beam epitaxy (MBE) with both N2-plasma and NH3 nitrogen sources. In most MBE studies, the N-polar surfaces were used. Both methods allow 2D-growth of near dislocation-free GaN homoepitaxial layers and multi-quantum well (MQW) structures.

Polarity identification of GaN bulk single crystals (0001) surface by Auger electron spectroscopy

AbstractAn 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 eoncentration on depth is also different for both faces of the sample.