Search

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.

The anisotropy of atomic bonds formed by acceptor dopants with nitrogen in bulk wurtzite GaN crystals was studied by means of linearly polarized synchrotron radiation used in measurements of X-ray-absorption spectra for the K-edgeof Mg and Zn dopants. These spectra correspond to i) a single acceptor N bond along the c-axis and ii) three bonds realized with N atoms occupying the ab-plane perpendicular to the c-axis. The Zn dopant formed resonant spectra similar to that characteristic for Ga cations. In the case of the Mg dopant, similarity to Ga cations was observed for triple bonds in the ab-plane, only. Practically no resonant structure for spectra detected along the c-axis was observed. The absorption spectra were compared with ab initio calculations using the full-potential linear muffin-tin-orbital method. These calculations were also used for determination of the bond length for Mg–N and Zn–N in wurtzite GaN crystals and show that introducing dopants causes an increase of the lengths of the bonds formed by both dopants. Extended X-ray-absorption fine-structure measurements performed for bulk GaN:Zn confirmed the prediction of the theory in the case of the Zn–N bond. Finally, it is suggested that the anisotropy in the length of the Mg–N bonds, related to their larger strength in the case of bonds in the ab-plane, can explain preferential formation of a superlattice consisting of Mg-rich layers arranged in ab-planes of several bulk GaN:Mg crystals observed by transmission electron microscopy. Within the sensitivity of the method used, no parasitic metallic clusters or oxide compounds formed by the considered acceptors in GaN crystals were found.

The N-side of GaN single crystals with off-angle orientations of 0°, 2°, and 4° towards the [100] direction was used as a substrate for homo-epitaxial MOCVD growth. The highest misorientation resulted in a reduction of the density of grown hillocks by almost two orders of magnitude as compared with homo-epitaxial films grown on the exact (000) surface. The features still found on the 4° misoriented sample after growth can be explained by a model involving the interaction of steps, introduced by the misorientation and the hexagonal hillocks during the growth process.

Transmission electron microscopy has been used to study defects formed in Mg-doped GaN crystals. Three types of crystals have been studied: bulk crystals grown by a high pressure and high temperature process with Mg added to the Ga solution and two types of crystals grown by metal-organic chemical vapor deposition (MOCVD) where Mg was either delta-doped or continuously doped. Spontaneous ordering was observed in bulk crystals. The ordering consists of Mg rich planar defects on basal planes separated by 10.4 nm and occurs only for growth in the N to Ga polar direction (000 N polarity). These planar defects exhibit the characteristics of stacking faults with a shift vector of a 1/3 [1 00] +c/2 but some other features identify these defects as inversion domains. Different type of defects were formed on the opposite site of the crystal (Ga to N polar direction), where the growth rate is also an order of magnitude faster compared to the growth with N-polarity. These defects are three-dimensional: pyramidal and rectangular, empty inside with Mg segregation on internal surfaces. The same types of defects seen for the two growth polarities in the bulk crystals were also observed in the MOCVD grown GaN samples with Mg delta doping, but were not observed in the crystals where Mg was added continuously.

The paper reports on the changes of lattice parameters by free electrons (via deformation potential of the conduction band minimum) and by point defects (via size effect) for gallium nitride. The measurements were performed on single crystals grown at about 15 kbar and 1800 K and on homoepitaxial layers using high-resolution X-ray diffraction methods at temperatures 40–770 K. It was found that a high free-electron concentration increase both, the lattice parameters and thermal expansion. It was observed that most of the single crystals of gallium nitride possess different lattice parameters on their two sides and this difference increases at high temperature. The experimental results of X-ray diffraction, electron microscopy and Hall measurements served for creating a model of generation of low-angle boundaries in the pressure-grown single crystals.

This paper describes TEM characterization of bulk GaN crystals grown at 1500–1800Kin the form of plates from a solution of atomic nitrogen in liquid gallium under high nitrogen pressure (up to 20 kbars). The x-ray rocking curves for these crystals were in the range of 20–30 arc-sec. The plate thickness along thec axis was about 100 times smaller than the nonpolar growth directions. A substantial difference in material quality was observed on the opposite sides of the plates normal to thec direction. On one side the surface was atomically flat, while on the other side the surface was rough, with pyramidal features up to 100 nm high. The polarity of the crystals was determined using convergent-beam electron diffraction. The results showed that, regarding the long bond between Ga and N along the c-axis, Ga atoms were found to be closer to the flat side of the crystal, while N atoms were found to be closer to the rough side. Near the rough side, within 1/10 to 1/4 of the plate thickness, there was a high density of planar defects (stacking faults and dislocation loops decorated by Ga/void precipitates). A model explaining the defect formation is proposed.

We present the results of the high magnetic field studies of properties of two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructures grown over high-pressure bulk GaN, sapphire, and insulating SiC substrates. The experimental results include the low field Hall measurements, cyclotron resonance measurements, and cryogenic temperature Quantum Hall Effect studies as well as room-temperature characteristics of High Electron Mobility Transistors fabricated on all these substrates. The room temperature high field measurements allow us to clearly separate the contributions of a parasitic parallel conduction from 2DEG conduction in all investigated heterostructures.The magnetotransport measurements are performed in the magnetic fields up to 30 Tesla for temperatures between 50mK-300K. This high magnetic field in combination with very high mobilities (over 60.000 cm2/Vs) in the sample on the bulk GaN substrates allow us to observe features related both to cyclotron resonance and spin splitting. The temperature dependence of this splitting determines the spin and cyclotron resonance energy gaps and, in combination with cyclotron resonance and tilted field experiments, allows us to determine the complete energy structure of 2DEG conduction band. We also present the first experimental results showing so called “the exchange enhancement” of the energy gaps between spin Landau levels.

GaN semiconductor is characterized by strong bonding and high vapor pressure of nitrogen. The strong bonds limit an efficiency of annealing procedures required for variety of semiconductor technologies, for example, post-implantation annealing, or doping by diffussion. Maximum temperatures employed up to now (at ambient pressure) in GaN annealing have not exceeded about 1100°C. A desired increase of annealing temperatures would cause a decomposition of GaN unless an elevated pressure of N2 is supplied. In this work, we report onapplication of high pressure annealing procedures (temperatures up to 1550°C and pressures up to 16 kbar) which enabled us to study variation of the properties of epitaxial films and bulk crystal of GaN. In particular, we discuss the following results obtained using high pressure annealing: i) structural quality improvement and increase of the thermal strain in as grown epitaxial layers of GaN/A12O3, The annealing at above 1300°C resulted in the decrease of the X-ray rocking curve width from about 700 arc sec. down to 470 arc sec., ii) drastic increase of bandedge (bound exciton) photoluminescence intensity iii) enhancement in removal of implant damage, iv) increase of diffusivity of Zn and Mg atoms (introduced by implantation and/or diffusion from external source). For Zn in epitaxial layers of GaN/A12O3 a diffusion starts at 1200–1250°C, v) enhancement of the blue-photoluminescence intensity in Zn and Mg implanted GaN. The performed experiments give an evidence of the importance of the defect (dislocations) in diffusion of Zn and Mg in the GaN semiconductor.

Photoluminescence connected with excitons and their phonon replicas in undoped homoepitaxial MOCVD grown GaN layers have been studied in the temperature range 2 - 100 K. It is shown that the coupling between LO phonons and neutral acceptor bound excitons (ABE) is much stronger than the coupling between LO phonons and neutral donor bound excitons (DBE). In spite that emission due the DBE no-phonon is one order of magnitude stronger than the ABE one, the predominant feature of the first LO phonon replica of the excitonic structures is related to the ABE. It is argued that this fact is connected with delocalization of the acceptor wavefunction in the k-space which leads to a higher number of interacting LO phonons in the first replica. On the other hand, the second LO phonon replica of the excitonic structures is predominantly connected with the DBE. In the case of two LO phonons interacting with bound excitons the k - conservation has not so direct influence. In addition, the temperature dependence of LO phonon replicas and their kinetics in ps regime are also reported.

Bulk single crystals of GaN were used for epitaxial growth of GaN films by molecular beam epitaxy. Low temperature photoluminescence yields much higher intensity emission in the near bandedge region for epitaxial films with respect to the situation in bulk crystals. Character of this luminescence changes also. Dominant band-to-band transitions in the bulk crystals are exchanged by bound exciton and/or donor-acceptor pair transitions observed in the epitaxial layers. We will compare the obtained results with the available data on the homoepitaxial samples grown by metalorganic chemical vapor deposition method and discuss the importance of establishing the basic information on energetic positions of excitonic transitions in stress free samples.

GaN plays a key role in the exploration of the properties of group-Ill nitrides. As grown GaN often shows a high electron concentration, e.g. 1019 cm−3, of as yet unidentified origin. Applying large hydrostatic pressure we studied the behavior of these donors and a frequently observed strong luminescence band at 3.42 eV. We find a drop of the electron concentration to 3×1017 cm−3 at 27 GPa and derive a binding energy of 126 meV for the neutral singlet donor level at this pressure. Such a pressure behavior of a donor is consistent with the model of strongly localized defects. Within the framework of a bandstructure calculation we predict the neutral level of this donor at 0.40 ± 0.10 eV above the conduction band edge at ambient pressure.

The microstructure and characteristic defects of heteroepitaxial GaN films grown on sapphire using molecular beam epitaxy (MBE) and metal-organic-chemical-vapor-deposition (MOCVD) methods and of homoepitaxial GaN grown on bulk substrates are described based on transmission electron microscopy (TEM), x-ray diffraction, and cathodoluminescence (CL) studies. The difference in arrangement of dislocations along grain boundaries and die influence of buffer layers on the quality of epitaxial films is described. The structural quality of GaN epilayers is compared to diat of bulk GaN crystals grown from dilute solution of atomic nitrogen in liquid gallium. The full width at half maximum (FWHM) of the x-ray rocking curves for these crystals was in the range of 20–30 arc sec, whereas for the heteroepitaxially grown GaN the FWHM was in the range of 5–20 arc min. Homoepitaxial MBE grown films had FWHMs of about 40 arc sec. The best film quality was obtained for homoepitaxial films grown using MOCVD; these samples were almost free from extended defects. For the bulk GaN crystals a substantial difference in crystal perfection was observed for the opposite sides of the plates shaped normal to the c direction. On one side the surface was almost atomically flat, and the underlying material was free of any extended structural defects, while the other side was rough, with a high density of planar defects. This difference was related to the polarity of the crystal. A large difference in crystal stoichiometry was also observed within different sublayers of the crystals. Based on convergent beam electron diffraction and cathodoluminescence, it is proposed that GaN antisite defects are related to the yellow luminescence observed in these crystals.