Site-selective photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy have been carried out at 6K on the ∼1540 nm 4I15/2 → 4I15/2 emissions of Er3+ in Er-implanted GaN. The PLE spectra exhibit several broad, below-gap, defect- or impurity-related absorption bands which excite three distinct site-selective Er + PL spectra. The near-band edge spectral position and lineshape of the PLE spectrum of one of the site-selective PL bands suggest that this Er site forms a trap level within the band gap and an exciton bound at this trap is involved in the excitation mechanism. In addition, high resolution PLE spectra obtained with a tunable laser in the 810 nm spectral range reveal a set of sharp PLE peaks due to the 4I15/2 → 4I9/2 internal Er+ ƒ-band absorption superimposed on the broad defect PLE band. The site-selective PL spectrum excited by the sharp line ∼810 nm Er+ intra-ƒ shell PLE bands is characteristic of a fourth distinct Er+ site. The simple structure of the site-selective PL and PLE spectra associated with direct intra-ƒ shell absorption suggests that the optically active Er site responsible for these spectra is of high symmetry in wurtzite GaN and that it could be attributed to a single Er atom on a Ga site.

Epitaxial layers and light-emitting i-n-structures with the active region of GaN doped simultaneously with zinc and oxygen have been grown. Effective up to 60% blue polarized luminescence has been observed. Investigated properties of grown layers and structures are presented. A discussion of issues related to GaN utilization based on its found properties is also included.

Gallium nitride crystals were grown from a Ga-based melt at an ambient gas pressure not exceeding 2 atm. Growth temperature was about 1000°C. The crystals were 2H-GaN and had a (0001) plane orientation. Crystal size varied from 0.05×0.05×0.01 mm3 to 2×2×0.05 mm3. Lateral growth rate of the crystals ranged from 0.05 to 1 mm/hr. Normal growth rate was about 0.01 mm/hr. Depending on growth conditions, the crystals have a platelet or dendrite shape. Crystals were characterized by optical and electron microscopy, Auger electron spectroscopy, x-ray differential diffraction, photoluminescence and optical absorption. Lattice parameters and band gap value of the grown crystals were determined at 300 K.

Bulk, polycrystalline gallium nitride and indium nitride were crystallized at sub-atmospheric pressures by saturating the pure metals with nitrogen from a microwave electron cyclotron resonance source. Saturation of Ga/In melts with nitrogen led only to the crystallization of gallium nitride. The polycrystalline samples were wurtzitic. The gallium nitride was well faceted, with narrow Raman lineshapes, and showed near-band-edge and yellow band photo-luminescence at both 4K and 300K. The indium nitride was formed in smaller amounts, was less well faceted, and showed no photoluminescence.

We investigated the Czochralski growth of LiGaO2 single crystal for use as a substrates for the epitaxial growth of hexagonal GaN. Crossed lines were observed in mechano-chemically polished {001} wafers sliced from a (001) axis boule. Chemical etching revealed that there exists a difference in chemical stability between two domains separated by a crossed line. Since LiGaO2 single crystal is polar along the c-axis, the formation of multi-domain structure is due to the polarity inversion of the c-axis, that is, polar twinned defects. We found that the GaN thin film on the (001) substrate with a multi-domain structure peeled off and that this is closely related with multi-polarity of substrate.

Auger electron spectroscopy was used to examine the nitridation behavior of GaAs, sapphire and lithium aluminate (LAO) substrates exposed to an RF nitrogen plasma. No evidence of nitridation was found for the sapphire and LAO substrates. GaAs substrates did show evidence of nitridation which led to smooth InN surface morphology without the need for a low temperature buffer. Comparable InN films were obtained on sapphire and LAO substrates when a low temperature AlN buffer was used. Hall measurements indicate background carrier concentrations are relatively insensitive to substrate type, though mobilities decreased as surface morphology was improved.

Microstructures of GaN films grown on the LiGaO2 by metalorganic chemical vapor deposition (MOCVD) have been characterized by transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM). TEM and HRTEM results show that high quality single-crystal wurtzite GaN films have been deposited on the LiGaO2 and that the GaN film and the LiGaO2 have the following orientation relationship: (2110)(0002)GaN ̂ (002)LiGaO2 ^ 5–8°. A higher density of threading dislocations and stacking faults have been observed near the GáN/LiGaO2 interface, even though the lattice mismatch of GaN to LiGaO2 is only ∼1%. Threading dislocations with burgers vector b=<0001> and b=a/3<1120> are predominant in the GaN films. Also the GaN films contain some columnar inversion domain boundaries (IDBs). Both TEM and HRTEM results reveal that there is an unexpected amorphous or nano-crystalline inter-layer between the GaN and the LiGaO2 with a thickness of 50–100 nm.

The problem of residual strain in GaN epilayers is currently the attention of many studies, since it affects the optical and electrical properties of the epilayers. In order to discuss the origin of this residual strain, we have grown a series of GaN epilayers onto AlN buffer layers, sapphire (0001) being used as substrate. The buffer layer is usually deposited in an amorphous state and is recrystallized by a thermal annealing. Here we have made a systematic study of the buffer recrystallization by changing the annealing temperature and the annealing time. The surface morphology is probed using Atomic Force Microscopy (AFM). The lattice parameter c is carried out from accurate x-ray diffraction measurements. The GaN layers were studied by low temperature photoluminescence and reflectivity. The amount of residual strain is calibrated from the position of the A exciton and the optical quality of the layers is assessed from the photoluminescence linewidths. The longer the annealing time the better the strain relaxation in AlN buffer layers and the higher the lattice mismatch with GaN overlayers.

A model of the exciton bound to stacking faults (SF) in GaN is suggested. It is shown that SFs are potential wells (depth ∼ 120 meV) for electrons and potential barriers (∼ 60 meV) for holes. The binding energy of the exciton at stacking faults is estimated as 30 − 60 meV. The 364 nm line in GaN photoluminescence is attributed to excitons at stacking faults.

Epitaxial GaN films are normally grown on substrates, such as sapphire, that are not an exact lattice match for GaN. Thus during the early stages of film growth, defects may be introduced in the film. These defects can lead to islanding and form voids or other defects just above the interface. These defects produce nonuniformity in the films and affect the quality of the final film. SIMS depth profiling is a widely used to characterize GaN films. The normal SIMS depth profiles provide chemical and depth information but do not provide any lateral information. We show that image depth profiling with SIMS is a technique that can be used to identify the defects and also chemically identify other interface features with lateral dimensions down to 1 μm.

A multilayer buffer layer approach to GaN growth has been developed in which the thermal desorption and mass transport of low temperature buffer layer are minimized by deposition of successive layers at increased temperatures. High quality GaN with featureless surface morphology has been grown on (0001) sapphire substrate by metalorganic chemical vapor deposition using this multilayer buffer layer approach. The lateral growth and coalescence of truncated 3D islands (TTIs) nucleated on low temperature buffer layers at the initial stage of overlayer growth is affected by the thickness of the final buffer layer on which nucleation of TTIs takes place. The effect of the thickness of this buffer layer on the quality of GaN is studied by using scanning electron microscopy, van der Pauw geometry Hall measurements and cathodoluminescence and an optimum value of 400Å is obtained.

The influence of plasma and thermal treatments on the structure and composition of sapphire (00–1) surfaces have been studied by hemispherically recorded x-ray photoelectron spectroscopy in view of substrate preparation for the epitaxy of GaN. Producing well-ordered surfaces, O2 plasma based treatments are found to efficiently remove surface contamination. AlN films with good short-range order are obtained by a simple high temperature nitridation step in the MOCVD reactor.

Basic mechanical properties, such as hardness and fracture toughness are examined for GaN and related materials. The mechanical properties are explored by indentation with a diamond (Vickers) pyramid under less than 20N load. Testing on bulk single crystal GaN indicated hardness and fracture toughness similar to GaP, but with much greater values than GaAs. Data for compound semiconductors are compared with values of a number of substrate materials. The following materials are examined in bulk form: AlN, Al2O3 (sapphire), Al2O3-TiC (ALTIC), GaAs, GaN, GaP, Ge, Si, SiC, ZnS, and ZnSe.

We have studied the growth of GaN on (0001) sapphire and (111) spinel substrates by LP-MOVPE and compared the mosaic structure and cathodoluminescence for the heteroepitaxial films of GaN grown on these substrates.

We report on resonant Raman scattering in Al0.15Ga0.85N/GaN single quantum wells (QWs) and AlxGa1-xN/GaN/lnyGa1-yN heterostructures. By choosing appropriate excitation conditions we could probe selectively the GaN quantum well or the Al0.15Ga0.85N barrier of Al0.15Ga0.85N/GaN single quantum wells. For the InxGa1-xN material system a linear frequency shift of the E2- and A1(LO) phonon mode to lower frequencies was found with increasing In content. The shift was determined to -0.79cm-1 per % In content for the A,(LO) phonon frequency. Resonant excitation of AlxGa1-xN/GaN/InyGa1-YN heterostructures enabled us to detect phonon signals from the InxGa1-xN layer in the heterostructure and to determine its In content.

Raman, photoluminescence, and Hall measurements are reported for Mg doped GaN films grown by molecular beam epitaxy. The compressive and tensile stress determined by the Raman shift of the phonon lines is due to the growth conditions rather than the presence of Mg in the film. The photoluminescence peak of near band-to-band transitions is also shifted to larger (smaller) energies by the compressive (tensile) stress. The study of the longitudinal optical phonon of the Ai branch shows that its Raman line shape is affected not only by phonon-plasmon interactions but by the crystalline quality of the film, as well.

We present results of high-pressure Raman-scattering experiments on bulk GaN and GaN grown on GaAs. We determined the Grüneisen parameters of both the cubic TO and LO phonon modes and the hexagonal A1, E1 and E2 modes. Our measurements reveal that the Grüneisen parameters for the GaAs substrate are about 30% smaller than those of bulk GaAs. This is a consequence of the lower compressibility of GaN compared to GaAs, which results in a pressure-induced biaxial strain on the substrate. From the pressure behavior of the GaAs modes and by comparing with our results for bulk GaN we obtained information about the biaxial strain in the GaN epitaxial layer.

Large-bandgap materials can support very high electric fields (>lMV/cm) without breaking down. The possibility then exists for an electron in a conduction band to become quasi-localized in Wannier-Stark states, a possibility that depends on the scattering rate being less than the Bloch oscillation frequency. If this condition is met the scattering rate itself is affected and the description of transport must be changed from the usual model in which the electron is assumed to be virtually free. Here, we examine the feasibility of obtaining this condition in GaN using a simple three-dimensional tight-binding model for the bandstructure and taking the dominant scattering mechanism to be the polar and non-polar interaction with optical phonons and short-wavelength acoustic phonons.

Optical gain processes in thin GaN and AlGaN are compared by means of gain spectroscopy using the stripe length method and high-excitation photoluminescence, both performed at various densities and temperatures. We find that inelastic excitonic scattering processes and biexciton decay are important at low temperatures and low excitation densities Both materials are similar in that increasing the excitation density results in gain spectra dominated by the electron-hole plasma and phonon-assisted band-to-band recombination. These also prevail at high temperatures.

Group theory states that the spectroscopy of excitons in GaN epilayers grown with biaxial stress on M Plane (10–10)-oriented substrates is strongly anisotropie in the growth plane, in contrast to the situation if growth occurs on conventional C plane substrates. In addition, we predict existence of nine radiative transitions instead of five, three for each family of A, B, and C excitons. We calculate the in-plane anisotropy and show that, due to the crossed configuration of the wurtzite crystal field and the built-in strain one, the A exciton is significantly coupled to the electromagnetic field in π polarization (E//c).