Low resistivity single crystal aluminum nitride-carbon (AIN:C) films were grown by metal organic chemical vapor deposition (MOCVD). The growth system used ammonia (NH3), trimethylaluminum (TMA), hydrogen (H2), and propane (C3H8) precursors. Films produced with high partial pressure of propane during growth exhibited high conductivity. Van der Paw measurements indicated that the resistivity of the as grown films changed dramatically from 108 ohm-cm for unintentionally doped samples to less than .2 ohm-cm for partial pressures of propane greater than 0.5×10−3 torr. Reflection electron diffraction (RHEED) measurements performed "in situ" just after film growth indicated that the material is single crystal up to a propane partial pressure of 2.5×10−3 torr. P-n junctions of n-type 6H-SiC and p-type AIN:C were fabricated, blue emission (centered at 490nm) was observed from the heterojunction under forward bias.

Ternary alloys; GaAsN (N<3%) were grown by plasma-assisted metalorganic chemical vapor deposition using triethylgallium, AsH3, and plasma-cracked NH3 or N2 as the precursors. More N atoms were incorporated into the alloys from N2 than NH3 at constant N/As ratios. Both photoluminescence peaks and optical absorption edges were redshifted from GaAs bandgap with increasing the N content, indicating the GaAsN alloys have narrower bandgaps than GaAs.

GaN/GaAs double-hetero structures were grown by exposing GaAs surfaces to N-radical flux to replace surface As atoms by N atoms, and by growing GaAs on the thin GaN layers. When the GaN thickness exceeded one-monolayer, the GaN/GaAs interfaces and the GaAs cap layers deteriorated drastically. The one-monolayer-thick GaN embedded in GaAs attracts electrons and shows intense photoluminescence, whereas the GaN cluster is non-radiative, probably because of the defects caused by the large lattice-mismatch between GaN and GaAs.

We report on the growth and characterization of three dimensional nanoscale structures of GaN. GaN dots were grown by metal organic chemical vapor deposition (MOCVD) on 6H-SiC substrates. The actual size of the dots measured by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) ranged from ∼20 nm to more than 2 μm. The average dot density ranged from 107 to 109 cm−2. The single crystal structure of the dots was verified by reflectance high energy electron diffraction (HEED) and TEM. Cathodoluminescence (CL) and photoluminescence (PL) of the dots were studied at various temperatures and excitation levels. The PL and CL edge peak for the GaN dots exhibited a blue shift as compared with edge peak position for continuous GaN layers grown on SiC.

GaN was grown by supersonic jet epitaxy(SSJE), seeding triethylgallium in helium carrier gas. Activated nitrogen was supplied by a microwave plasma source. Single crystalline GaN films were deposited on the Si-face 6H-SiC and the c-plane sapphire substrates at 600–670°C. A cubic SiC buffer layer was grown onSi(111) at 800°C by SSJE using dichlorosilane, acetylene, and a high quality GaN crystal was grown on this template at 630°C. The materials high quality was proved by hard rectifying characteristics of a diode with an N-GaN/β-SiC/P-Si(111) structure.

The current approach of depositing a low temperature then annealed AlN or GaN buffer for the growth of GaN on sapphire results in a high dislocation density. These dislocations thread through the GaN layer to the surface. Reducing their density either by growing thicker films or using a strained layer superlattice is ineffective. Two new approaches for AlN/GaN buffer layer growth for GaN on sapphire have been employed: Atomic Layer Epitaxy (ALE) and Molecular Stream Epitaxy (MSE). ALE is distinguished by organo-metallic/ammonia separation while MSE is distinguished by cyclic annealing of the growing film. Both ALE and MSE enhance two dimensional growth of single crystal GaN on sapphire. The structural quality of epitaxial GaN grown on these buffer layers was studied by transmission electron microscopy (TEM) and x-ray diffraction (XRD). The initial result for the ALE buffer shows an improved quality GaN film with lower defect densities. The MSE grown buffer layer closely resembles that of conventionally grown MOCVD buffer layers observed by others, with dislocations threading through the GaN epilayer. The effects of these buffer layers on the structural and optical properties of GaN grown on sapphire will be presented.

The epitaxial layers of AIN and GaN were grown on Si and Sapphire substrate at a relatively low temperature of around 500 °C using the process of reactive ion beam assisted deposition. The optimum ion beam energy for epitaxial growth of AIN and GaN films was found to be about 50 eV. Characterization of the epitaxial layers was carried out by GID (Grazing-Incidence x-ray Diffraction) and high resolution TEM observation. The orientational relations between epitaxial layer and substrate were determined through these analysis. Very thin amorphous layers were observed at the interfaces of bom AIN and GaN films grown on Si(111) substrate, whereas the films grown on Sapphire substrate has no amorphous layer. The amorphous layer may act as a buffer layer enabling the growth of the epitaxial layers of AIN and GaN by relaxing the misfit strain in the early growing stage.

Highly oriented aluminum nitride (0001) films have been grown on Si(001) and Si (111) substrates at temperatures between 550° C and 775° C with dual supersonic molecular beam sources. Triethylaluminum (TEA;[(C2H5)3Al]) and ammonia (NH3) were used as precursors. Hydrogen, helium, and nitrogen were used as seeding gases for the precursors, providing a wide range of possible kinetic energies for the supersonic beams due to the disparate masses of the seed gases. Growth rates of AIN were found to depend strongly on the substrate orientation and the kinetic energy of the incident precursor; a significant increase in growth rate is seen when seeding in hydrogen or helium as opposed to nitrogen. Growth rates were 2–3 times greater on Si(001) than on Si(111). Structural characterization of the films was done by reflection high energy electron diffraction (RHEED) and x-ray diffraction (XRD). X-ray rocking curve (XRC) full-width half-maxima (FWHM) were seen as small as 2.5°. Rutherford back scattering (RBS) was used to determine the thickness of the films and their chemical composition. Films were shown to be nitrogen rich, deviating from perfect stoichiometry by 10%–20%. Surface analysis was performed by Auger electron spectroscopy (AES).

AIN thin films have been grown epitaxially on Si(111) and Al2O3(0001) substrates by pulsed laser deposition. These films were characterized by FTIR and UV-Visible, x-ray diffraction, high resolution transmission electron and scanning electron microscopy, and electrical resistivity. The films deposited on silicon and sapphire at 750-800°C and laser energy density of ∼ 2 to 3J/cm2 are epitaxial with an orientational relationship of AIN[0001]║ Si[111], AIN[2 110]║Si[011] and AlN[0001]║Al2O3[0001], AIN[1 2 1 0]║ Al2O3[0110] and AIN[1010] ║ Al2O3[2110]. The both AIN/Si and AIN/Al2O3 interfaces were found to be quite sharp without any indication of interfacial reactions. The absorption edge measured by UV-Visible spectroscopy for the epitaxial AIN film grown on sapphire was sharp and the band gap was found to be 6.1eV. The electrical resistivity of the films was about 5-6×l013Ω-cm with a breakdown field of 5×106V/cm. We also found that the films deposited at higher laser energy densities ≥10J/cm2 and lower temperatures ≤650°C were nitrogen deficient and containing free metallic aluminum which degrade the microstructural, electrical and optical properties of the AIN films

Gallium nitride films have been deposited on Si(100) and Al2O3(0001) substrates using triethylgallium and ammonia seeded into highly expanded helium gas streams. A two step deposition process that reproducibly results in continuous crystalline GaN films has been developed. The microstructure and composition of the resultant films were characterized by scanning electron microscopy, reflection high energy electron diffraction and Auger electron spectroscopy and film character was correlated to deposition conditions.

Alternative precursors to group-in nitrides have been studied based on two schemes: 1) The reaction Me3M (M=Al,Ga) with t-BuNH2 and 2) the decomposition of NH3 based adducts. Polycrystalline growth of AIN has been demonstrated by both routes. Decomposition of the adduct Me3Al:NH3 has been used to prepare epitaxial AIN on (0001) sapphire with an X-ray FWHM (full width at half maximum) of 16 arcrnin at a growth temperature of 1050C. Similar growth using analogous gallium precursors always resulted in gallium droplets. We have attributed this difference in chemical reactivities to the lower electronegativity of gallium compounds, thus leading to dissociation rather than sequential methane loss to form the nitride.

Oriented GaN has been successfully grown at low substrate temperatures (∼480°C) on a- and r-planes of sapphire, using the pulsed laser deposition process. We have examined the effects of several deposition parameters on film growth, including substrate temperature (∼50–500°C), ambient pressure (1×10−3 – 10 torr of NH3), and target material (Ga or GaN). The film deposition rate was typically ∼3–4 μm/hr. Film characterization was performed using x-ray diffraction (XRD), optical microscopy, x-ray photoelectron spectrometry (XPS), and atomic force microscopy (AFM). In the case of the Ga metal target, a plasma (∼500V) between the target and substrate was necessary to promote formation of the GaN phase. The ammonia ambient enhanced the nitrogen content in the films compared to vacuum deposition. In general, the GaN target yielded better quality films (smaller rocking curve widths and smoother film morphology) compared to the Ga metal target. These results suggest that pulsed laser deposition is a promising approach to fabricating high quality films of this potentially important semiconducting material.

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.

Samples of representative AlxGayIn1−x-yN compositions have been studied with secondary ion mass spectrometry (SIMS). First, ionized species of common interest (H, B, C, O, Mg, Si, and Cd) were implanted into the Ill-nitride samples to provide calibrated standards. Depth profiles and conversion factors for quantification of dopants were then obtained using O2+ or Cs+bombardment and positive or negative SIMS to measure B+ and Mg+; H−, B−, C−, O−, and Si−; and CdCs+. In addition calibration curves for quantification of stoichiometry were prepared using MCs+ ions (NCs+, AlCs−, GaCs+, InCs+) for which the ion yields are relatively independent of the matrix composition; and using atomic, dimer, and trimer ions (Al, Ga, In, Al2, Ga2, In2, Al3, Ga3) which are very sensitive to matrix composition. The empirical calibration curves show small non-linearities. Dopant concentrations can be quantified with great sensitivity (detection limits usually below 1 ppm), accuracy (usually better than 10%), and precision (better than 25%). Matrix stoichiometry can be quantified with an accuracy of about 1–3%.

The effects of hydrogen passivation in MOCVD grown Mg doped p-type GaN were studied using low temperature (5K) photoluminescence (PL) and secondary-ion-mass spectroscopy (SIMS). GaN films with different Mg doping level were annealed at 700° C in N2 ambient with different annealing times. The SIMS results indicate that the hydrogen concentration increases with increasing Mg doping level in the as-grown Mg:GaN film. After 20 minutes of annealing, most of the hydrogen escapes from the film. The 3.455 eV PL peak before annealing and the 3.446 eV peak after annealing found in the Mg doped samples were attributed to the exciton bound to the Mg-H complex and to the Mg acceptor, respectively. The shift of the bound exciton peak to higher energy (3.465 eV) in the lightly doped sample is due to an effective n-type compensation associated with an annealing-induced increase in the nitrogen vacancies. In heavily doped Mg:GaN, the decreases in the integrated PL intensity after 700° C annealing may be associated with the hydrogen depassivation of nonradiative recombination centers in the film. The increase of PL intensity in the lightly doped sample after annealing is attributed to the reduction of defects by the annealing process.

X-ray photoelectron spectroscopy is used to determine the band-offset at the SiC/AIN (0001) interface. First, the valence band spectra are determined for bulk materials and analyzed with the help of calculated densities of states. Core levels are then measured across the interface for a thin film of 2H-AIN on 6H-SiC and allow us to extract a band offset of 1.4 ±0.3 eV. The analysis of the discrepancies between measured peak positions and densities of states obtained within the local density approximation provides information on self-energy corrections in good agreement with independent calculations of the latter.

The surface structure of organometallic vapor phase epitaxy (OMVPE) grown α-GaN films was investigated using optical and scanning force microscopy (SFM). Optical microscopy shows that the surface is decorated with several different types of faceted features that have lateral dimensions of 10 to 75 μm and occur with a density of approximately 104/cm2. SFM images show that on the flat regions of the surface, single diatomic layer steps, 2.6 Å high, are straight, evenly spaced (at 500 to 1500 Å intervals), and oriented along á101ñ directions. The SFM images also show that the regular step patterns are often interrupted by faceted growth hillocks, 0.8 to 5 μm in diameter and 120 to 400 Å high, that occur with a density of 106/cm2. An open-core screw dislocation with a Burgers vector of 5.2 Å occurs at the center of each hillock and is a source for spiral steps. Other dislocations are also observed to intersect the flat regions of the surface and create a step, but these have smaller Burgers vectors, do not form spirals, and do not have open cores. Based on these observations, we conclude that thick OMVPE GaN films grow by a combination of the layer-by-layer and spiral growth mechanisms.

The microstructural study of wide-band gap semiconductor AlN thin films grown on (0001) and (1012) sapphire and (111), (100) Si was carried out using plan-view and cross-sectional high-resolution electron microscopy and x-ray diffraction. The films were grown by MOCVD from TMAl + NH3 + N2 gas mixture. Epitaxial relationship for AIN grown on (0001)α 01-Al2O3 was determined to be the following: (0001)AIN ║ (0001)sap with the 30° in-plane rotation - [0110]AIN ║ [1210]sap. We report also TEM observation of the following epitaxial relationship of the AlN/(1012)α-Al2O3 heterostructure: (1120)AIN ║ (1012)sap; [0001]AIN ║ [1011]sap and [1100] AIN ║ [1210]sap. These epitaxial relationships were determined to be controlled by the bonding of Al and O ions at the interface. The study of interfaces and the defects present in the film was also carried out. Main type of defects were established to be inverted domain boundaries, misfit and threading dislocations - in the films on (0001) sapphire, and stacking faults of high density in the films on (1012) sapphire. The epitaxial AIN films on (0001) sapphire contained dislocation density about 1010 cm−2 and exhibited device quality electrical characteristics. The films on both orientations of Si were found to be highly <0001> textured polycrystalline.

By a combination of conventional, HREM and CBED TEM experiments we have studied wurtzite GaN layers grown by Metal-Organic Chemical Vapour Deposition (MOCVD) on (0001)Al2O3. We experimentally determine the structure of the macroscopic hexagonal pyramids that are visible at the surface of the layers when no optimised buffer is introduced. These pyramids look like hexagonal volcanoes with one hexagonal microscopic chimney (up to 75nm wide) at their core. The crystal inside the chimney is a pure GaN crystal with a polarity opposed to the one of the neighbouring material : the GaN layers grown on (0001)Al2O3 are everywhere Ga-terminated except in the chimneys where they are N-terminated. Some of the N-terminated chimneys grow faster and form macroscopic hexagonal pyramids. Chimneys bounded by Inversion Domains Boundaries (IDBs) originate from steps at the surface of the substrate and may be suppressed by an adapted buffer layer.

The elastic constants of the Group-III nitrides, c-BN, AlN and GaN were calculated from first-principles using the full-potential linear muffin-tin orbital method and local density approximation. The relation between the elatic constants in zincblende and wurtzite is studied by means of a tensor coordinate transformation approach. The latter combined with a correction for the internal displacement of the rotated tetrahedra is found to provide good results for the Ch11Ch12 and Ch44 but not for Ch13 and Ch33. These two require explicit calculations involving distortions along the c-axis. The calculations also provide information on the transverse optical phonons. By deriving Keating model parameters we show that BN is much stiffer against bond-angle distortions than the other nitrides.

We report the dielectric functions of various GaN samples as measured by spectroscopic ellipsometry. Structure related to the A and B excitons is resolved at room temperature, in principle allowing strain to be assessed. However, the data indicate that dead-layer and dispersion effects are present, preventing a simple interpretation. We discuss various complications including the Edn/dE contribution to dispersion, which is important for laser action. Our data appear to indicate that the spin-orbit splitting of GaN is about 15 meV, somewhat larger than the currently accepted value of about 11 meV.