Electronic structures and optical gains of bulk GaN and GaN/AlGaN quantum wells (QWs) are theoretically investigated for the wurtzite and the zincblende structures, using the k-p theory. It is found that the lower crystal symmetry, that is the wurtzite, is preferable for the lower threshold carrier density in the bulk. Although the QW structure leads to symmetry lowering only in the zincblende, we can not find a significant benefit of the zincblende QWs. As for the reduction of the threshold carrier density, biaxial strains are more effective in the zincblende. However, the threshold carrier density is still higher than in the wurtzite. It is proposed that the uniaxial strain in the c-plane of the wurtzite is more useful for reducing it.

We report on the first scanned-angle x-ray photoelectron diffraction measurements on GaN(0001) in the wurtzite structure, as grown on sapphire substrates using MOCVD. These as-grown samples reveal forward scattering peaks in agreement with a theoretical calculation using a single scattering cluster calculation. The surface contamination by O and C does not exhibit any clear structure. From the combination of experiment and theoretical calculation and from a simple intensity ratio argument the surface termination for these samples could be determined to be N. The data also indicate that C is on average closer to the GaN surface than O.

Samples of AlN grown on silicon [111] substrates were examined using electron energy loss spectroscopy (EELS) and selected area diffraction (SAD) with high-resolution transmission electron microscopy (TEM) to determine the source of out-of-plane tilts and in-plane rotations of the AlN crystallites at the Si interface. SAD results indicate that the interfacial crystallites are sheared along vertical planes, with random, intercrystalline rotation. The interfacial phenomena are believed to be the result of Si-Al-N interaction. Analytical experiments show no evidence of silicon nitride formation, witnessed by nitrogen-K peak shape, up to the Si interface. No evidence of substrate-epilayer interdiffusion was observed. Chemical interaction within one monolayer of the interface is therefore suspected as the cause of the epilayer tilts and rotations.

Secondary ion mass spectrometry (SIMS) and Rutherford Backscattering Spectrometry (RBS) techniques were used to determine InxGa1-xN and AlxGa1-xN compositions. While RBS is generally considered a quantitative technique for compositional analysis, SIMS has not been. We have applied a new analytical technique, which reduces the matrix effect in SIMS analysis, to accurately determine stoichiometry. The composition of InxGa1-xN (AlxGa1-xN) in the multiple layers and quantum well of the LED can be measured by SIMS, but is inaccessible to RBS.

SIMS analysis was applied to the characterization of GaN, AlGaN/GaN and InGaN/GaN grown by MOCVD. Such characterization enables the control of purity and doping, and the determination of growth rate and alloy composition. The analysis can be performed on finished optoelectronic and electronic devices and this makes SIMS technique a powerful tool for failure analysis, reverse engineering, and concurrent engineering.

High resolution electron microscopy has been applied to study the structure of epitaxial GaN layers grown by MOVPE on SiC and sapphire substrates. Defects in GaN were systematically studied for undoped, and Si- and Mg-doped samples. For both substrates, the Si-doping was found to decrease the dislocation density at the layer surface, while Mg-doping increased it. The density of nanopipes increased with both types of doping. Cracking of GaN layers was observed for SiC substrates. Crack formation was not detected in layers grown on sapphire. Mechanisms of defect generation are discussed in relation to the initial growth stages, the effect of doping, and the type of substrate.

We correlate structure analyzed by transmission electron microscopy with photo- and cathodoluminescence studies of GaN/Al2O3(0001) and GaN/SiC(0001) and show that an additional UV line at 364nm/3.4eV can be connected to the occurrence of stacking faults. We explain the occurrence of this line by a model that is based on the concept of excitons bound to stacking faults that form a quantum well of cubic material in the wurtzite lattice of the layer material. The model is in reasonable agreement with the experimental observations.

The paper describes the influence of strain on the optical quality of GaN films grown by MBE on c-plane sapphire. The photoluminescence (PL) line width of the donor-bound exciton can be designed to be as narrow as 1.2 meV by actively utilizing hydrostatic and biaxial stress components. Unstrained p-type Mg-doped GaN films exhibit comparably narrow near band edge transitions. A sharp PL line at 3.261 eV in some of our films is identified as the donor bound exciton of the cubic phase. The formation of these cubic inclusions can be stimulated by a high III/V flux ratio at the growth temperature of T = 725°C. The PL spectrum of an InGaN multi quantum well structure is significantly broadened compared with the spectra of single quantum well structures. Combination of PL and TEM indicates that this effect relates to a progressive increase of the quantum well widths and their spacing along the growth direction. It is argued that strain affects the growth rate and the incorporation of Indium into the quantum well structures.

AlxGa1-xN films were grown on c-plane sapphire by plasma induced molecular beam epitaxy with 0 ≤ x ≤ 1. The composition and purity of the AlxGa1-xN layers was determined by elastic recoil detection analysis with a relative error of 5% for the Al content. Both X-ray diffraction and atomic force microscopy indicate only a slight decrease in epitaxial quality of the AlxGa1-xN films with increasing Al content up to x = 0.65. X-ray diffraction is used to separate the effects of thermally induced biaxial compressive stress and the alloy composition on the shift of interplanar spacings by measuring both lattice constants. The deviation of the c/a ratio from that of fully relaxed films is a quantitative measure of the biaxial compressive stress leading to a distortion of the unit cell. Values up to 0.5 GPa were observed. By the method proposed, the determination of alloy composition can be corrected for this effect. The results obtained by this method are in very good agreement with the elastic recoil detection measurements substantiating the validity of Vegard's law. These results, compared with optical measurements, indicate that the bowing parameter of the optical bandgap is 1.3 eV within the experimental error.

Lattice parameters of semiconductors depend on the concentration of free electrons via the deformation potentials of the occupied minima of the conduction bands. In the presented work we examined the lattice parameters of variously doped GaN samples (epitaxial layers on sapphire and on SiC, bulk crystals grown at high hydrostatic pressure and homoepitaxial layers). The following dopants were used: Si, Mg and O. The measurements were performed using high resolution X-ray diffractometry. The results indicate that free electrons expand the lattice what confirms a negative value of the deformation potential of the Γ minimum of the conduction band. However, for Mg-doping (acceptor) we observed the lattice expansion as well. This violates the Vegard's law, as Mg ions are smaller than Ga ions.

The polarity of GaN films and their AlN buffer layer grown on (0001)si 6H-SiC by an electron cyclotron resonance plasma enhanced molecular beam epitaxy has been investigated by convergent beam electron diffraction (CBED) and high resolution electron microscopy (HREM). The experimental results are in good agreement with the simulations and allow to determine that the free surfaces of the GaN and AlN layers are Ga and Al-terminated respectively. Moreover, (1210) prismatic planar defects observed in the AlN layers have been identified as stacking faults and observations in different areas of the specimens have shown that the layers are unipolar.

Nanometric inversion domains in GaN/Al2O3 layers have been investigated using HREM. They were found to be limited by {1010} planes and to cross the entire epitaxial layer. It has been possible, using extensive image simulation and matching to discriminate between possible atomic models for the boundary plane. It is shown that the inversion domain boundaries correspond to a Holt type model containing wrong bonds (Ga-Ga, N-N), and in that plane, each atom exhibits two such bonds. This probably can explain the small size of the domains (5–20 nm).

The wide gap materials SiC, GaN and to a lesser extent diamond are attracting great interest for high power/high temperature electronics. There are a host of device processing challenges presented by these materials because of their physical and chemical stability, including difficulty in achieving stable, low contact resistances, especially for one conductivity type, absence of convenient wet etch recipes, generally slow dry etch rates, the high temperatures needed for implant activation, control of suitable gate dielectrics and the lack of cheap, large diameter conducting and semi-insulating substrates. The relatively deep ionization levels of some of the common dopants (Mg in GaN; B, Al in SiC; P in diamond) means that carrier densities may be low at room temperature even if the impurity is electrically active - this problem will be reduced at elevated temperature, and thus contact resistances will be greatly improved provided the metallization is stable and reliable. Some recent work with CoSix on SiC and W-alloys on GaN show promise for improved ohmic contacts. The issue of unintentional hydrogen passivation of dopants will also be covered - this leads to strong increases in resistivity of p-SiC and GaN, but to large decreases in resistivity of diamond. Recent work on development of wet etches has found recipes for AlN (KOH), while photochemical etching of SiC and GaN has been reported. In the latter cases p-type materials is not etched, which can be a major liability in some devices. The dry etch results obtained with various novel reactors, including ICP, ECR and LE4 will be compared - the high ion densities in the former techniques produce the highest etch rates for strongly-bonded materials, but can lead to preferential loss of N from the nitrides and therefore to a highly conducting surface. This is potentially a major problem for fabrication of dry etched, recessed gate FET structures.

A photoelectrochemical etching process for n-type GaN using KOH solution and broad-area Hg arc lamp illumination is described. Etch rates as high as 320 nm/min are obtained. The etch rate is investigated as a function of light intensity for stirred and unstirred solutions. Preliminary results on etching of Al0.1Ga0.9N layers are reported.

Fabrication of group-Ill nitride electronic and photonic devices relies heavily on the ability to pattern features with anisotropie profiles, smooth surface morphologies, etch rates often exceeding 0.5 μm/min, and a low degree of plasma-induced damage. Patterning these materials has been especially difficult due to their high bond energies and their relatively inert chemical nature as compared to other compound semiconductors. However, high-density plasma etching has been an effective patterning technique due to ion fluxes which are 2 to 4 orders of magnitude higher than conventional RIE systems. GaN etch rates as high as -1.3 μm/min have been reported in ECR generated ICI plasmas at -150 V dc-bias. In this study, we report high-density GaN etch results for ECR- and ICP-generated plasmas as a function of Cl2- and BCl3-based plasma chemistries.

To fabricate GaN-based optoelectronic devices successfully, a reproducible etch process with high etch rates, vertical etch profile, and damage-free surface is required. In this study, GaN etching was performed using planar inductively coupled plasmas and the effects of process parameters such as inductive power, bias voltage, pressures and, gas combination on the characteristics of the plasmas were studied using Langmuir probe, optical emission spectroscope (OES), and quadrupole mass spectrometer (QMS). Gas combinations of Cl2/Ar and Cl2/HBr were used to etch GaN. GaN etch rates increased with the increase of chlorine radical and ion energy (bias voltage), and the etch rates close to 4000 Å/min could be obtained without substrate heating over 100 °C. The addition of HBr and Ar (more than 20%) generally reduced GaN etch rates. In our experimental condition, it appears that the chemical reaction between Cl and Ga in GaN affects more significantly to GaN etching compared to physical sputtering, and it was partially confirmed by the data measured by Langmuir probe and OES. Angle resolved XP S data showed the variation of surface Ga/N ratio depending on the process parameters, which influence the formation of low resistance ohmic contact.

Dry etching characteristics of GaN using reactive ion beam etching (RIBE) were studied. Etching profile, etching rate and etching selectivity to a photoresist (PR) mask were investigated as a function of various etching parameters. Characteristics of chemically assisted reactive ion beam etching (CARIBE) and RIBE were compared at varied mixtures of CH4 and Cl2. A highly anisotropie etching profile with a smooth surface was obtained for tilted RIBE with Ch at room temperature. Etching selectivity to a PR was dramatically improved in RIBE and CARIBE when a volume fraction of CH4 to the mixture of CH4 and Ch was larger than 0.83.

Sub-micron periodic gratings with pitch ∼3,000Å were formed in GaN and InGaN using holographic lithography and room temperature ECR BCl3/N2 dry etching at moderate microwave (500W) and rf (100W) powers. The process produces uniform gratings without the need for elevated sample temperatures during the etch step.

During gate mesa plasma etching of InN/InAlN field effect transistors the apparent conductivity in the channel can be either increased through three different mechanisms. If hydrogen is part of the plasma chemistry, hydrogen passivation of the shallow donors in the InAlN can occur, we find diffusion depths for 2H of ≥ 0.5 micron in 30 mins at 200°C. The hydrogen remains in the material until temperatures ≥ 700°C Energetic ion bombardment in SF6/O2 or BCl/Ar plasmas also compensates the doping in the InAlN by creation of deep acceptor states. Finally the conductivity of the immediate InAlN surface can be increased by preferential loss of N during BCl3 plasma etching, leading to poor rectifying contact characteristics when the gate metal is deposited on this etched surface. Careful control of plasma chemistry, ion energy and stoichiometry of the etched surface are necessary for acceptable pinch-off characteristics.

Inductively coupled plasma etching of GaN, AlN, InN, InGaN and InAlN was investigated in CH4/H2/Ar plasmas as a function of dc bias, and ICP power. The etch rates were generally quite low, as is common for III-nitrides in CH4 based chemistries. The etch rates increased with increasing dc bias. At low rf power (150W), the etch rates increased with increasing ICP power, while at 350W rf power, a peak was found between 500 and 750 W ICP power. The etched surfaces were found to be smooth, while selectivities of etch were ≤ 6 for InN over GaN, AlN, InGaN and InAlN under all conditions.