Bulk aluminum nitride boules have been grown at driving rates of 0.9mm/h by the self-seeded sublimation-recondensation technique. Up to 15mm diameter substrates cut from those boules present large single crystal grains that have been analyzed using different techniques. X-ray double crystal diffraction shows a full-width-at-half-maximum of around 100 arcsec and X-ray topography reveals extensive areas with a density of dislocations less than 104 cm−2. These substrates have been prepared by chemical mechanical polishing techniques to obtain a surface roughness of 1.4-1.6nm.

Surface segregation and phase separation are investigated as processes limiting the indium incorporation in InGaN grown by ammonia Molecular Beam Epitaxy (MBE) and Metal- Organic Vapor Phase Epitaxy (MOVPE). It is shown that a significant concentration of indium on the growing surface may prevent the adsorption of ammonia via site blocking mechanism and result in appearance of In droplets instead of InGaN growth. Another conclusion is that the composition fluctuations in InGaN are related to coexistence of strained and relaxed InGaN islands rather than to the phase separation as commonly assumed.

A new in-situ technique to reduce threading dislocation density (TDD) within sub- micron growth is demonstrated by using metal-organic vapor-phase epitaxy (MOVPE). We achieved drastic reduction of TDD of AlGaN buffer on SiC substrate by inserting highly-Si- incorporated AlGaN/undoped AlGaN superlattice (SL). TDD of AlGaN was decreased from 2×1010 to 7×107 cm−2 by inserting the SL with the total growth thickness of 0.8νm. Si incorporation in AlGaN SL was estimated to be 1.2×1020 cm−3. This technique is exactly in- situ process without complicated fabrication processes, and the surface is kept flat throughout the total growth. This method is especially useful on SiC wafer in order to prevent cracks with thin growth layer. We confirmed the similar effects for GaN and AlGaN buffer on sapphire substrates.

The optical properties of Mn-doped GaN were investigated. The films were grown by metalorganic vapor phase epitaxy using tricarbonyl (methylcyclopentadienyl) manganese as the dopant source. Two characteristic bands were observed in the absorption spectra of Mn-doped epilayers. The low energy band had a threshold at 1.4 ± 0.05 eV with a maximum at 1.5 ± 0.02 eV, with a full width half maximum of 245 ± 10 meV at 296 K. A second higher energy band was observed as a shoulder to the band edge absorption with a threshold energy of 2.06 eV at room temperature. Using photoluminescence spectroscopy, a new broad band was observed in the infrared spectra of GaN:Mn at 1.27 ± 0.02 eV with a full width half maximum of 0.26 ± 0.01 eV at 20K. From analysis of optical absorption and emission spectra Mn forms a deep acceptor level with optical transitions at 1.4 and 2.06 eV. The deep level nature of Mn indicates that it is a potential dopant for semi-insulating GaN.

The large piezoelectric constants of GaN suggest possible application of GaN-based materials in piezoelectric sensors, among other areas. GaN's wide band gap implies that these sensors will fare well over a broad temperature range and/or in a harsh environment.

In this work, films of gallium nitride approximately 0.75 micron thick and grown by MOCVD were subject to an externally-imposed radial stress condition. Deposition was performed in a commercial MOCVD reactor (CVD, Inc.) at 1323K using trimethylgallium and ammonia as the chemical precursors. The substrate was one-inch diameter silicon (111). After deposition, titanium dots were deposited in various locations, including the wafer center, by evaporation. Stress was applied to the film/substrate system using a modified micrometer head (Mitutoyo) mounted to an Ionic Systems Basic Stressgauge (model 30285). Stress levels were calculated based on the magnitude of the imposed deflection as read from the micrometer head display, and the piezoelectric response at any particular dot with respect to the center dot was measured by measuring the voltage difference using a digital multimeter (Keithley 175). The micrometer head impinged on the center dot and served as one electrical contact point.

Effective piezoelectric coefficients were measured as a function of imposed radial stress. Applied stresses in the range of 1 to 5 GPa resulted in effective piezoelectric coefficients ranging from –0.6 to –2.0 × 10-5 C/m2

There is currently a high degree of interest in understanding the diverse mechanisms that determine the growth morphology of epitaxial films. To understand and control over these mechanisms, it is essential to know how the growth mechanisms are correlated with morphologies. In GaN MOVPE processes, growth temperature has a remarkable effect on the film morphologies and properties. In present work, the temperature dependency of surface morphology of GaN epilayers grown by low-pressure metal-organic vapor phase epitaxy (LP-MOVPE) was studied using atomic force microscopy. It was demonstrated that dislocations strongly influence the growth mechanisms and the morphology of the films. Three growth modes were identified at different growth temperature ranged from 1030°C to 1100°C, which are, (1) spiral growth dominated by dislocation with screw type, (2) monolayer step flow, and (3) nucleation-assisted step flow growth. The significant effect of dislocation on the surface morphologies and growth mechanisms may be attributed to the high dislocation density and higher activation energy for a step overcoming the dislocation pinning in GaN. Because few dislocations will be introduced under monolayer step flow growth mode, we can infer the useful information about GaN initial growth.

InGaN alloys with (0001) or (000) polarities are grown by plasma-assisted molecular beam epitaxy. Scanning tunneling microscopy images, interpreted using first-principles theoretical cal- culations, show that there is strong indium surface segregation on InGaN for both (0001) and (000) polarities. The dependence on growth temperature and group III/V ratio of indium incorporation in InGaN is reported, and a model based on indium surface segregation is proposed to ex- plain the observations.

Short wavelength Vertical Cavity Surface Emitting Laser based on the group III nitrides, GaN, AlN, InN, and their ternary alloys is reported. Optical properties such as band gap and index of refraction of the nitride binary compound were calculated based on the fitting of the experimental data and the first principle calculations. The ternary alloy optical properties were determined in the same manner but based on the binary compound data. The active region containing InGaN strained multiple quantum wells is formed between two Distributed Bragg Reflectors. GaInN/AlN material systems are shown to be the most suitable for highly reflective Bragg mirrors with minimized number of layers. LiGaO2 substrate is proposed for GaN growth due to its small lattice mismatch with GaN (about 0.9%) and its ability to provide a good thermal matching between the two materials. We report VCSEL calculations on threshold current and emission spectra.

To facilitate an understanding of defect production in gallium nitride during lateral epitaxial overgrowth, computer models have been developed to simulate the complete mechanical stress and strain fields. The virtual process included the deposition of a GaN seed layer on a sapphire substrate followed by a silicon dioxide stencil mask through which, and over which, the GaN product layer evolved and then cooled down to room temperature. Lattice mismatch and thermal strain were continuously assessed. Shear stresses on different crystallographic planes were analyzed to predict dislocation generation.

Stress induced piezoelectric fields in III-nitride semiconductors induce charge at the surface that is comparable to the free carrier concentration intended by doping. These fields, coupled with mixed Ga-polarity and N-polarity within the film, are detrimental to device performance due to an additional scattering caused by a reversal of the electric field normal to the surface. Consequently, investigation of the polarization effects is important and timely as nitride semiconductor devices are being contemplated for system applications. By imaging the surface potential by electric force microscopy (EFM), both a qualitative and a quantitative measure of polarization induced charge distribution across the surface could be obtained. In this vein, we undertook an EFM study in a series of samples. EFM images of as-grown and progressively etched samples not only led to the identification and imaging of inversion domains, but also showed the evolution of different polarities. The polarization of the surface was calculated and was found to agree reasonably well with the values found experimentally. The role of polarization was addressed by extraction of the second harmonic term from EFM signal.

A high-power metal semiconductor field-effect transistor (MESFET) for operating at a very large-current using GaN is reported for the first time. GaN was grown by metalorganic chemical vapor deposition (MOCVD). Sapphire substrates were used for GaN growth. A GaN MESFET with a large size was fabricated. Multi-finger gates were used for large-current operation. The total gate width was 8 cm and the gate length was 2 νm. The electrode materials of the source and the drain were Al/Ti/Au and the schottky electrodes were Pt/Au. The gate, source, and drain were isolated using SiO2. An FET structure was fabricated using a dry-etching technique. Multi-electrode structures were also fabricated using SiO2 for isolating the source, drain, and gate electrodes, respectively. The FET was operated at a current of over 5 A. The breakdown voltage was over 500 V. The transconductance (gm) was about 12 mS/mm. The pinch-off voltage was about -8 V. We confirmed that this GaN MESFET can also be operated at a current of 10 A.

Using free standing layer of InGaAsN we have succeeded in measuring the optical absorption in very broad spectral range (0.8-2.5eV). This gave us an insight into the conduction band density of states for the energies higher than the energy gap of this compound. By combining Hall effect measurements with determination of plasma edge frequency in infrared reflectivity for differently doped samples we were able to deduce the density of states, conduction band electrons effective mass and dispersion relation. In particular it turned out that both i) experimentally measured dispersion relation of the conduction band shows extremely high degree of nonparabolicity and consistently ii) the effective mass of electrons is few times larger than that corresponding to GaAs of the same electron concentration. So far the obtained experimental results are in line with recently proposed band anticrossing model of the electronic structure of III-N-V alloys.

Luminescence studies of amorphous AlN doped with Cu, Mn, or Cr were performed at 300 K. Thin films of Cu, Mn, and Cr doped amorphous AlN, ∼200 nm thick, were grown on p-doped silicon (111) substrates using RF magnetron sputtering in a nitrogen atmosphere. Cathodoluminescence (CL) showed that pure Cu doped amorphous AlN has strong emission in the blue (∼420 nm) and Mn and Cr doped films luminesce in the red (∼690 nm). Cr+3 emission is more intense than Mn+4 because chromium does not suffer from incomplete charge compensation in the III-V semiconductor. Luminescence studies of crystalline and amorphous AlN:Mn thin films showed a red shift in the emission peak by almost 100 nm and is believed to be caused by the different crystal field of the amorphous host compared to the crystalline host material. Secondary ion mass spectrometry (SIMS) depth profiling was conducted to confirm the presence of the Cu and Cr in the films and to show the amount of dopant in relation to the Si substrate.

In this work we studied a set of nominally undoped epitaxial InxGa1−xN wurtzite films grown on (0001) sapphire substrates. In order to separate the contribution of the strain and indium content in the phonon mode frequency, indium mole fraction was determined using a strain insensitive method, Rutherford backscattering spectrometry (RBS). Strain was evaluated by comparing the lattice constants measured by X-ray diffraction (XRD) with the relaxed lattice parameters given by Vegard's law. Samples with comparable indium content, but under different states of strain were used as reference. This allowed the behaviour of different Raman shift modes for both, strain and composition to be independently established. We also assess the potentiality of Raman spectroscopy for the evaluation of crystalline quality by comparing the results obtained with the ones provided by other well-established methods such as XRD and RBS.

Defects and stress are the most serious issues for growth of AlGaN. Low-temperature deposited (LT -) AlN interlayer between AlGaN and GaN is found to reduce tensile stress during growth, and at the same time suppress the propagation of dislocations having screw components, by which UV-photodetector showing very-low-dark current has been successfully fabricated. However, additional pure-edge dislocations are generated at the LT -interlayer, which resulted in the poor emission property. In addition to the LT -interlayer, lateral growth at the trenched structure was used, thereby achieving crack-free AlGaN and reduction of the density of all types of dislocations in the AlGaN layer. UV light emitting diodes having AlGaN/GaN multi-quantum well active layer was fabricated on the low dislocation density AlGaN. The LED shows strong and sharp UV-emission from GaN-wells.

This work investigates dislocation etch pits in epitaxial lateral overgrowth (ELO) GaN by wet chemical etching. A mixture of H2SO4 and H3PO4 was used as a dislocation etchant, and SEM and AFM were employed to observe the surface topography. For the as-grown sample, SEM images present the flat, smooth surface without any pits or hillocks. After the chemical etching, hexagonal shaped etch pits were observed at the edge of ELO GaN. AFM observation of etched ELO GaN displayed high densities of etch pits clustered in the “window” region and the coalescent line of two growing fronts. In contrast, the overgrowth region was nearly free of etch pits. Moreover, we observed that different sizes of etch pits dominated in “window” region and coalescent region. This implied different types dislocations dominated in these regions.

A systematic study of photoluminescence (PL) and time-resolved PL spectra from In0.15Ga0.85N/In0.015Ga0.985N quantum wells (QWs) with different Si doping concentration in the barriers has been carried out. As the Si doping concentration increases, the PL emission intensity was increased and the PL peak energy was blueshifted. The energy separation between the spontaneous emission (SPE) and stimulated emission (SE) peaks decrease with increasing Si doping concentration. We also observed that the slow decay time τ2 in the QWs decreases with increasing Si doping concentration, from ∼ 130 ns for [Si] = 2 × 1018 cm−3 to ∼ 30 ns for [Si] = 1 × 1019 cm−3. The PL emission peak shifts to lower energies with delay time after a pulsed excitation and this shift decreases with increasing Si doping concentration. The increased recombination rate, the decrease of peak shift with delay time, and the reduced separation between the SPE and SE peaks with increasing Si doping concentration are attributed to the screening of piezoelectric field by carriers originated from Si doped barriers.

The Daresbury synchrotron radiation source (SRS) provides bright, tunable x-rays for scattering and absorption probes of local structure. Scanning confocal microscopy and luminescence decay measurements employ the SRS in alternative ways, as a tunable luminescence excitation engine and as a source of weak, 160 ps pulses with a large pulse-topulse separation, respectively. This report first describes local atomic structure studies of InGaN epilayers by extended x-ray absorption fine structure (EXAFS). In addition, we report photoluminescence (PL) imaging, PL microspectroscopy and photoluminescence decay studies of various nitride samples, including tailored InGaN quantum wells and discs.

Spectroscopic Ellipsometry from the mid-infrared (mid-ir) to the vacuum-ultraviolet (vuv) spectral range (350 cm−1 … 8.8 eV) is used to study the optical properties of hexagonal MOVPE-grown Al1−xInxN films for 0.11 ≤ × ≤ 0.21. The AlInN E1(TO) phonon shows a onemode behavior in contrast to recent theoretical predictions [H. Grille, Ch. Schnittler, and F. Bechstedt, Phys. Rev. B 61, 6091 (2000)]. Approximately 120 nm thick Al1−xInxN films grown on slightly compressively strained hexagonal GaN buffer layers reveal the influence of in-plane strain on the E1(TO) phonon mode frequencies. Al1−xInxN deposited directly on [0001] sapphire substrate possesses E1(TO) mode frequency which indicate fully relaxed film growth. For highquality Al0.890In0.110N one A1(LO) phonon mode was observed. Furthermore, we present the complex dielectric function of hexagonal Al0.872In0.128N from the mid-ir to vuv spectral range.

Highly spatial resolved cross-sectional cathodoluminescence (CL) has been used to study the difference in the defect-formation and growth-mechanism between Ga-polar and N-polar GaN epitaxial layers. These epilayers were grown on sapphire substrates by low pressure MOCVD. Their polarities were controlled by the sapphire nitridation and the trimethylaluminum (TMAl) pre-flow just before the conventional two-step growth, and were confirmed with the coaxial impact collision ion scattering spectroscopy (CAICISS). For the Ga-polar GaN epilayers, cross-sectional CL images show distinct two layers, the upper layer with dark lines along c-direction and the layer close to the film-substrate interface with pyramidal dark regions. These two layers correspond to quasi 2D and 3D growth mode respectively. Whereas, the crosssectional CL image taken on the N-polar epilayers is predominated with small dark spots which were randomly distributed throughout the whole GaN epilayer, illustrating the predominant island growth mode in N-polar films.