In this paper we report that, in addition to dislocations, two other types of defects are observed in high quality GaN thin films. These defects have a filamentary nature, are oriented along the <0001> direction. and may not be easily distinguished from the pure dislocations. Using a combination of conventional electron microscopy with convergent beam electron diffraction techniques we show that one of these types of dislocations consist of nanopipes, which are coreless dislocations with Burgers vectors <0001>. The other type of observed defects consist of inversion domains with [0001 ] orientation within the [0001] matrix. The origin of the inversion domains and nanopipes is discussed.

Angle resolved near edge X-ray absorption measurements (NEXAFS) are used to access the existence and concentration of the two allotropic phases, cubic (β-GaN) and hexagonal (α-GaN), that coexist in a mixed-phase GaN sample grown by ECR-MBE. The resonance intensities in the NEXAFS spectra from a pure cubic GaN sample are independent of the angle of incidence ϑ, while they have a characteristic angular dependence on cos2ϑ for the hexagonal material, i.e. I=A±Bcos2ϑ. From the values of A and B the bond orientations with respect to the surface normal are calculated. Furthermore, the NEXAFS resonances in a pure α- or β-GaN sample appear at characteristic energies which are independent of the angle of incidence ϑ. Contrary to that the NEXAFS resonances in the spectra from a mixed-phase GaN show a characteristic ϑ-dependent shift. On the basis of this finding, a method is proposed and applied for the determination of the fractions of the co-existing polytypes in a mixed-phase sample.

Formation of vertical hollow nano-tubes in GaN grown on different substrates using different growth methods is described. These defects are shown to be of several different types, some related to threading dislocations, but others originating at tubular inversion domains, or crystal inhomogeneities. Kinetic mechanism based on slow growth rate on polar {0111} surfaces is proposed to explain the origin of these defects.

Inversion domain boundaries (IDBs) in GaN grown on sapphire (0001) were studied by a combination of high resolution transmission electron microscopy, multiple dark field imaging, and convergent beam diffraction. Films grown by molecular beam epitaxy (MBE), metalorganic vapor deposition (MOCVD), and hydride vapor phase epitaxy (HVPE) were investigated and all found to contain IDBs. Inversion domains (IDs) that extended from the surface to the interface were found to be columnar with facets on the {10–10} and {11–20} planes. Other domains ended within the film that formed IDBs on the (0001) and {1–102} planes. The domains were found to grow in clusters and connect at points along the boundary.

We have investigated Al0.12Ga0.88N layers with and without a 1 µm GaN buffer, grown on the c-face of α – Al2O3 substrate with an intermediate AlN nucleation layer grown by LP-MOVPE. We used spatially resolved cathodoluminescence spectroscopy at a temperature of 8K to investigate the strain and the homogeneity of composition that can be determined from the energy of the luminescence peak. The larger thermal expansion coefficient of the sapphire in comparison to the nitrides leads to a biaxial compressive strain of the upper GaN layer when cooling down from growth temperature. For AlGaN layers directly grown on the nucleation layer this cannot be confirmed. The layer stays relaxed and fluctuations in the aluminium composition of 0.4% can be observed. When growing an intermediate GaN buffer, the AlGaN layer gets tensilely strained. This strain is of elastic nature and microcracks can be observed preferentially at the edges due to the smaller lattice constant of AlGaN in comparison to GaN. Even detaching of the AlGaN layers grown on the buffer can be observed. In the regions without cracks the layers are quite homogeneous. A deformation potential of (19±4)eV was estimated for Al0.12Ga0.88N.

By monitoring RHEED reconstruction patterns during gas source molecular beam epitaxy growth, the optimization of the growth for cubic GaN was carried out successfully. Cubic GaN epilayer having a X-ray diffraction width of 16min and a low temperature photoluminescence emission width of 19meV was obtained on a 3C-SiC substrate by adjusting the effective III/V ratio in-situ during the growth, which can be inferred from the surface reconstruction transitions. It was found that the surface reconstructions of cubic GaN surfaces are good indices for the optimization of growth parameters.

We have investigated lattice structures of GaN and InGaN/GaN single-heterostructures (SH) and double-heterostructures (DH) by the reciprocal space mapping using X-ray diffraction technique from (0002) plane. For a single GaN layers, the transition of the film structure from grains with relatively independent orientation at about 0.3 μm followed by the coalescence at about 1 μm, to a uniform film with mosaic structure at 1.4 μm was clearly observed. For DH structure, ω-mode FWHMs both from InGaN (In composition approximately 7 %) and capping-GaN layers increased with increasing film thickness from 20 nm to 110 nm indicating the increased mosaic structure in the thick InGaN. We also observed the dislocation related to this increasing mosaic structure.

To investigate of the initial stage of GaN growth on Si, 0.2 Ga monolayers (ML) on Si (111) was nitrided and then the nitrided surfaces were observed by scanning tunneling microscopy (STM). An aggregation of islands whose longest edges had a direction rotated 15 ° from Si [110] direction was observed. The shape of islands looked like a pentagon. Surface roughness was estimated for several nitrided conditions. It was found that surface roughness becomes larger as the nitridation process proceeds.

(0001) aluminium nitride thin films were grown epitaxially on (0001) Sapphire substrates by MOCVD at 1200° C and PLD at 800° C. Both films have the same epitaxial growth relationship: (0001)AlN//(0001)Sap, and the same in-plane relationship which shows a 30° rotation between A1N and Sapphire: [ 12 10]AlN//[0 110]Sap and [10 10]AlN //[ 2110]Sap. The full width at half maximum (FWHM) of x-ray rocking curve of the MOCVD A1N film was 0.16° and PLD A1N film was 0.2°. Films grown by both MOCVD and PLD showed high crystalline quality. HRTEM images showed that these films are single crystalline with very low density of defects.

Dislocations in the film parallel to the film / substrate interface were observed in both A1N films. Atomic force microscopy images showed that the MOCVD films have flatter and larger terraces than the PLD films. The PLD technique for A1N growth needs to be improved further. But both films have a surface roughness of approximately 100nm.

X-ray absorption measurements at the N and O-K-edges are used to study the local microstructure in cubic and hexagonal GaN films grown by ECR-MBE and HVPE. A distortion in the local microstructure is identified in the 1st nearest neighbor (nn) shell, consisting of Ga atoms, in both the cubic and hexagonal samples. Two N-Ga distances are identified, R1 and R2, where R1 is the expected distance of 1.95Å while R2=R1+0.25Å. The same distortion is detected in the next nn shell containing Ga atoms, where the two distances are 3.7Å and 4.1Å. All the reported distance variations are larger than the error-bar. The nitrogen 2nd nn neighbor is found at the expected distance of 3.12Å while N deficiency is not detected. Finally, the O-Ga distance is found equal to 1.60Å and therefore it can be proposed that the oxygen atom occupies interstitial positions.

The growth of Group III-Nitrides on Si substrates offers the possibility of combining optoelectronics with Si technology. We have been studying the growth of Group III-Nitrides on clean and oxidised Si surfaces with a view to local area epitaxy. X-ray data indicates that alloys of (AlGa)N and (InGa)N of controlled composition can be grown on Si using a plasma enhanced Molecular Beam Epitaxy method over the entire composition range from InN to AlN. Films grown on uncleaned, oxidised surfaces of Si are polycrystalline/amorphous in contrast to growth on chemically cleaned Si substrates which show the usual columnar structure common in Group III-Nitrides. XPS studies indicate that there is little tendency for spinodal decomposition, but the In peaks in (InGa)N alloys show that more than one chemical environment is present. The composition of the alloys deduced from electron probe microanalysis studies agree well with those from X-ray measurements, assuming Vegard’s law is valid for both alloy systems.

We describe a study of the hexagonal growth hillocks commonly present in gallium nitride films. The MOVPE-grown epilayers of the present work exhibit a predominantly smooth morphology but small groups of hexagonal hillocks were found to populate the surface, particularly at the sample edges.

Scanning electron (SE) micrographs were taken of several groups of hillocks. At the maximum beam energy of 25 keV, two types of hexagonal hillock are visible. Hillocks in the first group are terminated by an apex (ie. they are pyramidal in form), while the other, flat-topped, hillocks terminate on (0001)-facets. As one lowers the electron beam energy, thereby reducing beam penetration, some of the flat-topped hillocks disappear from the image. From this we tentatively deduce that these hillocks are buried. The result of further investigations, using an atomic force microscope, are consistent with the presence of sub-surface features.

The relationship between the luminescence and morphological properties of a pyramidal hillock is studied via cathodoluminescence imaging. The band-edge emission originates from the full hexagonal structure, except for the central region, where only the defect-related yellow luminescence is apparent. We suggest this might be explained by defects associated with inversion domain boundaries at the hillock centre.

X-ray rocking curves are frequently used to assess the structural quality of GaN thin films. In order to understand the information given by the line shape, we need to know the primary mechanism by which the curves are broadened. The GaN films used in this study were grown by low pressure metalorganic chemical vapor deposition (MOCVD) on (00•1) sapphire substrates. GaN films with both broad and very narrow (open detector linewidth of 40 arcseconds for the (00•2) GaN reflection) rocking curves are examined in this work. Reciprocal space maps of both symmetric and asymmetric reciprocal lattice points are used to determine that the cause of the broadening of GaN rocking curves is a limited in-plane coherence length.

The width of double axis X-ray rocking curves of epitaxial GaN layers is shown to be critically dependent on the width of the detector aperture. We show that triple axis diffraction measurements using a crystal analyser before the detector enables the instrument function to be defined and the tilt and dilation distributions separated. All GaN samples examined showed a mosaic structure of misoriented sub-grains with little dilation within the mosaic blocks. In reciprocal space maps this was revealed as a wide distribution of intensity in a direction perpendicular to the reciprocal lattice vector.

We demonstrate the use of triple axis diffraction measurements, including Φ scans (in which the sample is rotated about an axis perpendicular to its surface) to assess the crystal perfection of wurtzite GaN layers on sapphire grown using different pre-nitridation growth treatments by or-ganometallic vapor phase epitaxy. The Φ scans determine the in-plane misorientation angles between the crystallites and hence provide information on the edge dislocation density. Using glancing incidence (1014) and (1015) reflections, we determined that the misorientation among the GaN crystallites decreases with increasing layer thickness and that the pre-nitridation conditions control the initial level of misorientation. Triple axis ω and ω-2θ scans around the (0002) reflection did not show a systematic trend with increasing layer thickness. However, layers grown without a pre-nitridation step tended to exhibit higher values of both mosaic spread and strain. The appropriate asymmetric reflections for GaN-based Φ scan measurements are determined using structure factor calculations, which are presented here.

We report on photoluminescence experiments on hexagonal GaN epitaxial films grown on 6H-SiC and sapphire substrates by organo-metallic and hydide vapor phase epitaxy. At low temperatures we observe free and neutral donor bound exciton transitions which allow to establish properties of the free excitons and localisation energies of the bound excitons involving different shallow donors. From temperature dependent luminescence experiments thermal activation energies are determined which measure the exciton localization and donor binding energies. The localization energies of the excitons scale with the respective donor binding energies (Haynes rule). The inter-impurity transitions of neutral donors are observed in fourier transform infra-red absorption. Three shallow donors with binding energies of 34.7 meV, 55 meV and 58 meV can be seen. We present evidence for the chemical nature of the shallow impurities.

We report on the OMVPE growth and characterization of AlGalnN and its heterostructures, including measurements of electrical properties (Hall), optical properties (photo- and cathodo- luminescence), structural characteristics (x-ray diffraction and TEM); and also the emission of InGaN/AlGaN heterostructures subject to optical and electrical pumping.

Micro-infrared reflectivity and micro-Raman scattering have been used to determine free electron density and residual strain distribution in bulk GaN crystals. As-grown samples exhibit significant variation of electron concentration and strain along their surfaces. Increase of electron concentration correlates with the growth direction. The observed inhomogeneities in the properties of bulk GaN crystals can be eliminated by mechanical polishing of the crystal and removing the surface layer or growth figures formed during the sample cooling, which follows its high-pressure, high-temperature growth. Properties of that surface layer can differ from the bulk due to different growth mechanisms and impurity distribution. We suggest that oxygen is a likely candidate for the donor impurity supplying high amount of electrons to the conduction band of GaN.

The electronic properties of heteroepitaxial GaN were investigated for unintentionally doped, n-type films grown by hydride vapor phase epitaxy on sapphire substrates. The GaN layers were characterized by variable temperature Hall-effect measurement, capacitance-voltage (C-V) measurements, and deep level transient spectroscopy (DLTS). The measurements were performed on as-grown, 13 μn thick films and repeated after thinning by mechanical polishing to 7 μm and 1.2 μm. The room temperature electron concentrations as determined by the Hall-effect measurements were found to increase from ∼1017 cm−3 (13 μm) to ∼1020 cm−3 (1.2 μm) with decreasing film thickness. However, the C-V and DLTS measurements revealed that the ionized, effective donor and deep level concentrations, respectively, remained unchanged in regions close to the top surface of the films. These findings are consistent with the presence of a thin, highly conductive near interface layer which acts as a parasitic, parallel conduction path. Possible sources of such a shunt near the GaN/sapphire interface include oxygen contamination from the sapphire substrate or a structurally highly defective, 300 nm thick interface layer.

Persistent photoconductivity has been observed in n-type Si-doped GaN grown on sapphire substrates by metalorganic vapor phase epitaxy. The effect has been seen both in films which are of electrically high quality, based on low temperature photoluminescence (PL) and Hall analysis, and in films which either appear to be compensated or which exhibit strong donor-acceptor pair recombination. The photoconductivity has been observed to persist for several days at room temperature (300K). Modeling of the resistance recovery by a stretched exponential treatment may suggest a distribution of deep level defect centers in the Si-doped GaN films.