The effect of polarity on the optical and microstructural properties of GaN is presented. A sample with adjacent domains of Ga- and N-polarity material was grown by varying the nucleation layer. This allows a unique opportunity to study the two different polarities under controlled conditions. We found that the N-polarity material has a much lower dislocation density than the Ga-polarity material. The N-polarity material contains voids that are not present in the Ga-polarity region. The surface roughness of the N-polarity material appears to be caused by Ga-polarity inversion domains which lead the growth. A cathodoluminescence study showed that the N-polarity material is much brighter than the Ga-polarity material, suggesting a higher donor concentration, probably due to increased impurity incorporation in the [0001] growth direction or possibly due to an increase in intrinsic point defects. There is also evidence that the N-polarity region contains two types of material, one is flat and can be etched, the other has an inclined facet and does not etch.

GaN1-yAsy epitaxial alloys on the N-rich side with high As content were grown by metalorganic vapor phase epitaxy. They had specular surfaces and the single-phase epitaxial nature was confirmed by X-ray diffraction. The As incorporation increased through both a decrease in the growth temperature and V/III ratio. These trends were similar to that found in other III-V alloy systems which exhibit a large miscibility gap and the anion incorporation was considered to have been limited kinetically under the conditions of the low V/III ratio. The range of achieved As content was extended up to y=0.067, which is a composition well within the miscibility gap. The As-content dependence of the band gap energy was determined by optical absorption measurements and large bowing parameter of 16.8 ± 0.9 eV was determined.

GaN films doped with rare earth (RE) elements have attracted considerable attention due to the unique optical luminescent properties of the RE intra 4ƒ(n)-shell electron transitions which lead to sharp blue (Tm), green (Er) and red (Eu) emissions. This paper presents an overview of investigations of GaN films implanted with each of these ions using a combination of electron-beam and optical techniques. The ion implantations were performed under a wide range of conditions, covering variations in fluences, energies and temperatures and followed by different high-temperature annealing steps. The resulting RE:GaN films were analysed using an electron probe micro-analyser modified to allow cathodoluminescence (CL) spectroscopy. Elemental microanalysis data obtained by wavelength dispersive X-ray analysis (WDX) is correlated with simultaneously collected room temperature CL spectra.

WDX allows the quantification of the RE elemental concentrations in GaN down to ∼ 0.03 at. % in very thin layers (∼ 100 nm deep). Furthermore, by varying the incident electron beam energy, details concerning the depth profile of RE implants can be determined. The effects of both implantation conditions and rapid thermal annealing on the depth profile and on the luminescence properties are reported. CL measurements performed on annealed samples reveal sharp visible and near IR emission lines characteristic of the RE3+ intra-4ƒ(n) atomic shell transitions.

Heterostructure field effect transistors (HFETs) based on AlGaN / GaN structures have shown good performance as high power and high frequency devices. Theoretical simulations of transport in short channel HFETs show that there are several areas where considerable improvements in mobility, etc. can be made if thin composite structures (as considered in this work) can be utilized. We examine transport in a metal / AlGaN / InN / GaN composite structure. The InN region is very thin (∼ 15 Å) and is introduced to improve the low field transport without significantly impacting the device breakdown properties. Our model is capable of examining any other type of composite structure as well. Our simulation method consists of charge control solution of the heterostructure wave functions, followed by Monte Carlo simulation of scattering and free flight events. We present comparison of results on i) metal / AlGaN / GaN structure, and ii) a metal / AlGaN / GaN structure with a thin InN channel of the order of a few mono-layers. We find that mobility in the channel can improve considerably with very little effect on the mobility - charge product. Indeed, the charge density induced in the thin InN channel region is ∼ 1013cm∼2. While the peak velocities in a metal / AlGaN / InN / GaN structure exhibit an increase of nearly 30% over the values in metal / AlGaN / GaN structures, the low field mobilities are also increased. Low-field mobilities of ∼ 2500 cm2 (V · s)∼ are predicted along with high sheet charges for low interface disorder for the structure with a thin InN layer. For higher degree of interface disorder (∼ 70Å), we have found good agreement with experimental Hall mobility data for similar structures. At higher electric fields, we find that most electron population transfers to higher valleys or other sub-bands that lie in AlGaN or GaN. This ensures that high field breakdown of low band gap InN layer is also suppressed.

Thin films of InN were grown on (0001) Sapphire by MOCVD. The effect of growth conditions and buffer layer on the film morphology was studied. Growth temperature and TMI flow rate were important factors in the growth of InN. The use of a low temperature AlN buffer layer was also found to improve the morphology and crystal quality of the films. Thin (<40Å) AlN buffer layers produced the best results while polycrystalline InN was obtained when the buffer layer thickness exceeded 60Å. Delamination of the InN films was observed to occur at growth temperature, which limited the thickness of the films to less than 300 nm. A room temperature mobility of 792 cm2/Vs and an electron concentration of 2.1×1019 cm-3 were measured in an approximately 200 nm thick InN layer grown on sapphire.

Electron microscope phase images, produced by direct reconstruction of the scattered electron wave from a focal series of high-resolution images, were used to determine the nature of defects formed in GaN:Mg crystals. We studied bulk crystals grown from dilute solutions of atomic nitrogen in liquid gallium at high pressure and thin films grown by the MOCVD method. All the crystals were grown with Ga-polarity. In both types of samples the majority of defects were three dimensional Mg-rich hexagonal pyramids with bases on the (0001) plane and six walls on {1123} planes seen in cross-section as triangulars. Some other defects appear in cross-section as trapezoidal (rectangular) defects as a result of presence of truncated pyramids. Both type of defects have hollow centers. They are decorated by Mg on all six side walls and a base. The GaN which grows inside on the defect walls shows polarity inversion. It is shown that change of polarity starts from the defect tip and propagates to the base, and that the stacking sequence changes from ab in the matrix to bc inside the defect. Exchange of the Ga sublattice with the N sublattice within the defect leads to 0.6±0.2Å displacement between Ga sublattices outside and inside the defects. It is proposed that lateral overgrowth of the cavities formed within the defect takes place to restore matrix polarity on the defect base.

InN epitaxial layers are grown on sapphire substrates. The investigated samples have electron concentration in a range of 1.8 ×1018 − 1.1 ×1019 cm-3. Optical reflection and transmission measurements are performed. The plasma edge energy position in the spectra is constant in a measurement temperature range of 5 – 300 K. The reflection and transmission spectra are calculated on the basis of the LO phonon-electron coupling scheme and non-parabolic conduction band structure. From this analysis we find that the observed absorption edge is attributed to valence band to conduction band transition rather than the valence band to defect (impurity) band transition, and the intrinsic bandgap energy of 0.64 (±0.03) eV. This bandgap energy increases by 40 – 50 meV as the temperature decreases from 295 to 10 K.

The energy of coincidence grain boundaries in wurtzite structure was investigated with the Stillinger-Weber potential after modification for GaN. A boundary can have two interfaces with a different period corresponding to the edge and diagonal of the unit cell of coincidence site lattices for rotations around [0001] in the range 0–60°. Their energy depends mainly on the atomic structure of the dislocation cores used for the boundary plane reconstruction. As a function of the structure units in the period, the energy of the edge and diagonal shows two minima which are observed for the edge boundaries.

In this work, the evolution of morphology and defect structure in GaN nucleation layers on the a-plane of sapphire are investigated using TEM. The growth temperature and pressure were varied from 560 to 1100°C and from 20 to 600 torr, respectively. Whereas the highest growth temperature leads to a continuous layer, a 2D growth mode is not attained when the chamber pressure is varied from 20 to 600 torr at 1028°C. At the highest pressures (>300 t orr), a large distribution is obtained for the island sizes.

AlGaN/GaN-based ultraviolet (UV) Schottky barrier photodetectors (PDs) with and without the LT GaN cap layer were both fabricated. It was found that we could achieve a lower leakage current from sample A. With incident light wavelength of 320 nm and a –1 V reverse bias, the measured responsivity was around 0.03 A/W and 0.015 A/W for samples with and without the LT GaN cap layer, respectively. The response speed of the sample A was also found to be faster.

Characterizations of transparent Schottky barrier GaN and AlGaN UV detectors in the vacuum UV (VUV) and soft X-ray (SX) region using synchrotron radiation are described. In the GaN UV detectors, the responsivity achieved about 0.05 A/W at 95 eV (13 nm). Thus, their device performance is shown between 3.4 and 100 eV (10 and 360 nm). Furthermore, the high responsivity spectra were realized by using AlGaN Schottky UV detectors consisting of Al0.5Ga0.5N on AlN epitaxial layer.

We report the effects of thermal annealing and growth interruption on the formation of GaN self-assembled quantum dots (QDs) grown by low-pressure metalorganic chemical vapor deposition (MOCVD). For the uncapped QD structures, atomic force microscopy (AFM) analysis shows that the density of the QDs rapidly increases and then decreases with increasing the annealing time. This is due to the combined effect of the QD formation process and the ripening process. The size of the QDs increases with increasing the annealing time. We have also investigated the photoluminescence (PL) property of the AlN capped QD structures with different growth interruption times. The PL from the QDs showed a red-shift with increasing the growth interruption time. In contrast, the PL from the wetting layer (WL) shows a blue-shift. These results indicate that the formation of the QDs is proceeding at the expense of the WL, which is consistent with the AFM analysis of the thermal annealing effect. These results show that the growth interruption plays an important role in the formation of the QDs.

A variety of different crucible materials have been suggested and/or employed for the sublimation-recondensation growth of AlN single crystals above 2000 C. Representative materials all have melting points well above 2300 C, a reasonable degree of chemical compatibility with AlN, relatively low vapor pressures, and relatively small thermal expansion coefficients. We analyze the current state of knowledge on crucible materials such as C, W, Re, W-Re alloys, BN, HfN, HfC, NbC, TaC, Ta2C, TaN, ZrC and ZrN with respect to published bulk AlN growth conditions. Crucible materials pyrolytic graphite, pyrolytic BN, and W have integrated thermal contraction values (upon cooling from growth temperatures) that are less than that of AlN; the other materials have larger values. The lowest vapor pressure materials in a nitrogen atmosphere are W, TaC, and Re; thus they are expected to yield higher purity crystals than the other candidates. The materials C, BN, Hf, and ZrN are expected to contribute to higher impurity levels in the AlN crystals.

Density functional theory is used to investigate Eu, Er and Tm rare earth (RE) impurities in GaAs, GaN and AlN. The most stable site is when the RE is located at a group III substitutional site but in GaN and GaAs these defects do not then possess any gap levels, unlike AlN. RE-VN defects in GaN are shown to possess levels which could act as traps for excitons. The interaction of oxygen with substitutional REs is also considered.

We present a self-consistent approach to examine current flow in a general metal-polar heterostructure junction. The approach is applied to examine properties of two classes of junctions that are important in devices: (i) GaN/AlGaN/high-κ insulator structures for potential applications in very small gate devices to suppress gate tunneling current; (ii) GaN/AlGaN/LiNbO3 junctions for both n-type and p-type semiconductors with practical application for low source resistance regions. The physical parameters used for high-κ dielectrics and polarization charges reflect values typically found in ferroelectric materials. Our studies indicate that tailoring of junction properties is possible if polar oxides as thin as ∼ 20Å can be achieved.

GaN films have been grown homoepitaxially by MOCVD on MBE-grown GaN template layers, using both porous and nonporous SiC substrates. The effect of the porous SiC substrates on dislocations in the MBE and MOCVD GaN layers has been studied using TEM and x-ray characterization. A reduction in dislocation density from ≥1×1010 cm-2 in the MBE template to 2.5×109 cm-2 at the top of the MOCVD film is found, with similar final values in the MOCVD films for both porous and nonporous substrates. We discuss various mechanisms by which dislocation density is reduced in the MOCVD layers.

Zirconium diboride (ZrB2) is a promising lattice-matched substrate for GaN-based materials. A key issue to realize high-quality heteroepitaxial growth is preparation of the substrate surface. The ZrB2 surface was studied by x-ray photoemission spectroscopy (XPS) and reflection high-energy electron diffraction (RHEED). XPS results indicated the presence of both ZrO2 and ZrB2 on the as-received substrate surface. Thermal cleaning at 1000°C in ultra-high vacuum, Ar+ ion sputtering, and wet chemical treatments were examined as surface preparation methods. After treatment with HF acid, the O peak intensity was much reduced. The combination of HF treatment and thermal cleaning resulted in sharp and intense RHEED from the ZrB2 surface. GaN grown on the surface by molecular-beam epitaxy exhibited intense photoluminescence, suggesting that this treatment is effective to obtain high-quality GaN on ZrB2 substrates.

Growth of GaN bulk crystals under ammonothermal conditions has been developed. The experiments were performed in ammono-basic solutions in high nickel content autoclaves for up to 3 weeks. Nutrients were crystalline GaN made from vapor phase transport growth. Single crystal clusters of GaN on the order of 500 μm – 1 mm long were obtained. These crystals were spontaneously nucleated on the walls of the autoclave. Transport growth on polycrystalline GaN seeds and single crystal HVPE seeds was also achieved. GaN has a high solubility in ammono-basic solutions, on the order of several weight percent. The ammonothermal crystals were characterized by photoluminescence (PL), X-ray diffraction and SEM.

ZnO is a highly efficient photon emitter, has optical and piezoelectric properties that are attractive for a variety of applications. Due to its stacking order and close lattice to GaN, it is also considered as a substrate material for GaN epitaxy. In the past the poor preparation of ZnO surface has been a major handicap to GaN epitaxy. However, proper treatment we developed recently can make both faces of ZnO smooth with atomic level terraces. Epitaxy of GaN on O-face and Zn-face ZnO by reactive molecular beam epitaxy was performed. We used low-temperature RF growth of GaN buffer layer on ZnO surface to protect it from both ammonia and Ga. No Ga2ZnO4, an oxide with the spinel structure formed due to reaction of ZnO with Ga, was found, in contrast to earlier reports. The low-temperature photoluminescence (PL) indicates that both faces of ZnO can provide a high quality GaN with high radiative efficiency. In previous research it has been reported that O-face ZnO is slightly better for GaN epitaxy. Our new finding demonstrates that high-quality GaN epilayers can be grown on Zn face of ZnO.

Characterization by reciprocal space mapping of x-ray diffraction (XRD) intensity was carried out for epitaxial layers of GaN-based laser structures on two GaN substrates: GaN substrate and GaN template on sapphire substrate. The difference between these two substrates was shown clearly. The distribution of XRD intensity of the epitaxial layers on GaN substrate was smaller than that of the epitaxial layers on GaN template on sapphire substrate. In the lasers with the epitaxial structure on GaN substrate, the light output power was as high as 200 mW under continuous-wave operation at room temperature. Excellent noise characteristics with relative intensity noise of -132 dB/Hz were also obtained at a low light output power of 3 mW without any high-frequency modulation. These results support that GaN substrates are promising for realizing GaN-based lasers with high performance.