Scanning Kelvin probe microscopy (SKPM) and conductive atomic force microscopy (C-AFM) are used to image surfaces of GaN grown by molecular beam epitaxy (MBE). Numerical simulations are used to assist in the interpretation of SKPM images. Detailed analysis of the same area using both techniques allows imaging of surface potential variations arising from the presence of negatively charged dislocations and dislocation-related current leakage paths. Correlations between the charge state of dislocations, conductivity of leakage current paths, and possibly dislocation type can thereby be established. Approximately 25% of the leakage paths appear to be spatially correlated with negatively charged dislocation features. This is approximately the level of correlation expected due to spatial overlap of randomly distributed, distinct features of the size observed, suggesting that the negatively charged dislocations are distinct from those responsible for localized leakage paths found in GaN. The effects of charged dislocation networks on the local potential profile is modeled and discussed.

Lattice location studies of radioactive 169Yb ions, implanted at an energy of 60 keV into 2H-AlN at the on-line isotope separator ISOLDE at CERN, were performed using the emission channeling technique. The measurements, which yield a substitutional Al lattice site for the implanted ions, were recorded for annealing temperatures ranging from 293 K to 1273 K. After complete decay of 169Yb to 169Tm cathodoluminescence measurements were performed in the range 12 K – 300 K. The samples show a strong visible luminescence at 460 - 470 nm at room temperature, which is attributed to the 1D2−3F4 intra-4f electron transition of Tm3+. At 12 K the luminescence is dominated by transitions starting from the 1H6 multiplet. Time resolved as well as temperature dependent cathodoluminescence measurements are presented and discussed.

The lattice location as well as the time resolved cathodoluminescence measurements suggest that there is only one pronounced site of the implanted ions in the AlN lattice and that this is the substitutional aluminium site.

Gold, palladium, platinum or nickel ohmic contacts on Mg doped p-type AlxGa1-xN with x = 0.4 and x = 0.45 have been examined. The Au contact provided the lowest contact resistivity with pc = 1.8 (± 1.1) x 10−3 Ωcm2, but only following annealing at 850°C. For the Pd, Au, and Pt contacts annealed at greater than 700°C, a rapid degradation in the current-voltage curves was observed upon testing. The degradation was induced by exposure to sub-bandgap light and was reversed with a mild anneal at 500°C. Possible mechanisms for the degradation are discussed.

We have studied thermal stability of Nb and NbN contacts to GaAs and GaN by x-ray diffraction and SIMS, and demonstrated their excellent behaviour under high temperature annealing. GaAs/Nb and GaAs/NbN contacts are stable up to 800°C and 900°C, respectively while GaN/NbN and GaN/Nb/NbN remain stable up to 1000°C.

We report on high Al-content AlGaN-based deep UV emitter structures grown over single crystal, slightly off c-axis (5.8 degrees) bulk AlN substrates. AlN/AlGaN multiple quantum well (MQW) structures with up to 50% of Al in the well material were grown by using low-pressure MOCVD and characterized by using X-ray, AFM, SEM and photoluminescence techniques. Two light sources, one at 213 nm wavelength for selective excitation of quantum well layers and another one at 193 nm to excite both wells and barriers, were exploited. A weak temperature dependence (from 8 K to 300 K) of the luminescence intensity and the absence of blue-shift of the luminescence peak with increasing excitation intensity pointed to a low density of localized states, in a good agreement with the X-ray data, which indicated very high quality of these MQW structures.

The most striking result was observation of stimulated emission at wavelength as short as 258 nm in Al0.5Ga0.5N/AlN MQWs grown on bulk AlN single crystals.

Photoluminescence (PL) from GaN epilayers is found to be sensitive to the ambient atmosphere and length of UV exposure. We studied the effect of UV illumination in different ambients including air, oxygen, nitrogen and hydrogen gases on room-temperature PL of GaN grown on sapphire by molecular beam epitaxy. In some samples the PL intensity increased markedly in vacuum as compared to excitation in air, whereas in others it decreased appreciably. While air and oxygen showed strong reversible variation of the PL intensity as compared to vacuum, nitrogen and hydrogen atmospheres led to a very small change. In some samples we observed a shift of the yellow luminescence band with change of ambient, in others no shift was detected. PL intensity also changed during UV irradiation when the sample was in air ambient. Possible reasons for our observations are discussed.

InN films were grown on Si (111) substrates by radio-frequency plasma-excited molecular beam epitaxy. InN films highly oriented to the c-axis were obtained by optimizing growth conditions in the direct growth on Si. Growth of single crystalline InN films was realized on Si substrates with substrate nitridation for 3 min. On the other hands, when the substrate nitridation was lasted over 30 min, obtained InN films were polycrystalline due to the amorphous SiNx layer formed on a substrate surface. We also studied the local atomic structure in the single crystalline InN film using extended X-ray absorption fine structure measurements.

In this paper we report sapphire nitridation and GaN film growth by using ionized nitrogen clusters as a nitrogen source. The clusters, typically 2000–3000 molecules/charge, are generated by a prototype GCIB source. The clusters are accelerated to 10–25 kV and disintegrate upon impact with the substrate surface where they react with Ga atoms to form GaN. The efficiency of this novel nitrogen source was tested by studying the nitridation of (0001) sapphire substrates at relatively low temperatures of 200 - 400°C. The effect of exposure of the substrate to the nitrogen cluster-ion beam was examined by XPS, RHEED and AFM. It was found that the amount of retained surface nitrogen increases nonlinearly with increasing beam energy. There exists a threshold energy, ∼ 20 kV, above which nitrogen retention is significantly enhanced.

GaN films were grown with such nitrogen clusters heteroepitaxially on sapphire/AlN-buffer (MBE grown) and homoepitaxially on thick GaN on sapphire (HVPE grown).TEM cross-section images indicate that the heteroepitaxial GaN films had defect density similar to that of MBE and MOCVD grown films. The homoepitaxially-grown GaN films were found to replicate the GaN templates and show strong cathodoluminescence (CL) emission at 363 nm with FWHM of 9 nm. Furthermore, the spectra show no evidence of yellow band emission.

Electron transport properties and DC device characteristics have been examined in the AlGaN/GaN heterostructure field-effect transistors (HFETs) with back-doping design that makes it possible to obtain high two-dimensional electron gas (2DEG) densities even for the devices with thin AlGaN barrier layers. In the back-doping design, an asymmetric double-heterostructure is employed, and donor atoms are doped not only in the surface-side AlGaN layer but also in the underlying AlGaN layer. In this structure, electrons are efficiently supplied also from the back-doped AlGaN barrier layer to the GaN channel and merged into a single 2DEG layer, with the help of the negative polarization charges at the heterointerface between the GaN channel and the underlying AlGaN barrier layer. By using back-doping design, very high 2DEG densities around 3×1013 cm−2 has been achieved in the Al0.3Ga0.7N/GaN HFET whose barrier layer (Al0.3Ga0.7N) is designed to be as thin as 120 Å. An HFET with the gate-length of 1.5 μm has exhibited a high current density of 1.2 A/mm and a high transconductance of 200 mS/mm, which is ascribed to high 2DEG densities and thin barrier layers in these devices. HFETs with the back-doping design are thus promising for high-power applications.

Electron Beam Induced Current (EBIC) is a Scanning Electron Microscope (SEM)-based technique that can provide information on the electrical properties of semiconductor materials and devices. This work focuses on the design and implemenation of an EBIC system in a dedicated Scanning Transmission Electron Microscope (STEM). The STEM-EBIC technique was used in the characterization of an Indium Gallium Nitride (InGaN) quantum well Light Emitting Diode (LED). The conventional “H-bar” Transmission Electron Microscopy (TEM) sample preparation method using Focused Ion Beam Micromachining (FIBM) was adapted to create an electron-transparent membrane approximately 300 nm thick on the sample while preserving the electrical activity of the device. A STEM-EBIC sample holder with two insulated electrical feedthroughs making contact to the thinned LED was designed and custom made for these experiments. The simultaneous collection of Z-contrast images, EBIC images, and In and Al elemental images allowed for the determination of the p-n junction location, AlGaN and GaN barrier layers, and the thin InGaN quantum well layer within the device. The relative position of the p-n junction with respect to the thin InGaN quantum well was found to be (19 ± 3) nm from the center of the InGaN quantum well.

Samples of both AlGaN and GaN terminated HEMT structures were studied using x-ray photo spectroscopy (XPS). It was found that the XPS spectra of both AlGaN and native oxide surfaces were shifted by a surface charge of 0.5 to 1.0 eV. The samples were then oxidized using an UV-ozone treatment for 25 minutes at room temperature. The ozone oxide XPS spectra of the AlGaN terminated surface was found to have the same 0.5 to 1.0 eV shift while the ozone oxide XPS spectra of the GaN terminated surface was found to have a 5.0 eV shift, indicating that the native GaN oxide is more insulating. Processed HEMT devices using both surface terminations were given the same UV-ozone treatment followed by a 10nm MBE grown Sc2O3 film for passivation. The GaN terminated HEMT structures showed on average a 10% increase in channel current (from gate-lag measurements, pulse mode) over the AlGaN terminated HEMT structures.

AlGaN/GaN heterojunction field effect transistors (HFET) on sapphire substrates have demonstrated ability as power devices operating with high current densities and high breakdown voltages. Additionally, AlGaN/GaN HFET devices have a very low on-state resistance. This makes these devices ideal for automotive applications such as switching relays, DC-DC converters, and power inverters. By 2006, switching devices using GaN-based FETs are anticipated to be employed in luxury automobiles and transitioned to the mass market by 2009.

In this presentation, data from AlGaN/GaN HFET's grown in an Emcore D180 MOCVD system will be presented. Typical production-scale material results (on 2” sapphire substrates) for these wafers were: μ ∼ 1000 cm2/Vs, Ns = 1.0×1013 cm−2, and Rs ∼ 450 Ω/square with <3% variation across the wafer. These wafers were then processed into devices using Pt/Au gate contacts with 2 μm gate length, 200 μm gate width, and a source to drain spacing of 13 μm. A total of 1000 FETs were combined in parallel for an effective gate width of 20 cm for high current operation (10A). These devices have a lower on-state resistance (<0.01 Ω-cm2) and higher Schottky breakdown voltages (400 V) than the theoretical limit of Si MOSFET devices. These devices demonstrate suitability for insertion in automotive electrical harnesses.

We present a study of the electrical and optical characteristics of 280 nm emission deep ultraviolet light emitting diodes (LED) at room and cryogenic temperatures. At low bias the defect assisted carrier tunneling primarily determines the current conduction. The room-temperature spectral performance and optical power are limited mostly by pronounced deep level defect assisted radiative and non-radiative recombination as well as poor electron confinement in the active region. At temperatures below 100 K the electroluminescence peak intensity increases by more than one order of magnitude due to suppression of non-radiative recombination channels indicating that with a proper device design and improved material quality, milliwatt power 280 nm LED are viable.

Recent experimental and theoretical studies highlighted the importance of the growing surface structure on the final morphology of GaN. Actually, optimum morphology is achieved by growth in presence of a Ga bilayer adsorbed on the GaN surface. The threshold fluxes limiting the region of the Ga bilayer adsorption have been measured as a function of the GaN substrate temperature, giving rise to a Ga adsorption phase diagram. The Ga flux limiting the regions in the adsorption phase diagram exhibit a linear behavior in an Arrhenius plot. However, both energy activation (about 5 eV) and prefactor (in the 1025 range) are surprinsingly high. These questions were adressed by studying the adsorption/desorption of Ga adatoms and small islands (consisting of 2 and 3 Ga adatoms) on the Ga bilayer surface employing first principle density functional theory calculations. We find a desorption barrier of 2.1 eV and a binding energy between two Ga atoms of approximately 0.3 eV. Using these numbers we derived a simple growth model (based on rate equations). An analysis of the experimental data with the model revealed the origin of the large difference in the activation energies and the unusually large prefactor. We find that the nucleation of the droplets cannot be described by a simple Arrhenius behavior (as commonly assumed to fit experimental data) but that the nucleation energy is temperature dependent.

The emergence of III-nitride technology and fabrication of high quality GaN based devices is possible due to the advances in the heteroepitaxial growth of III-N thin-films on lattice-mismatched substrates. Typically, the substrate of choice is either SiC or sapphire. We have adopted 100mm Si as our substrate of choice; uniform substrates of high quality are inexpensive and plentiful due to decades of use in the microelectronics industry. Growth of device quality GaN on Si is challenged by the ∼17% lattice mismatch and an additional thermal expansion coefficient (TEC) mismatch of ∼56%. In order to accommodate this strain and TEC mismatch between Si and GaN, a novel transition layer was designed, grown and successfully optimized, ® obviating the need for either a PENDEO based overgrowth process or a SiC interlayer-based process. This growth technique (SIGANTIC®) does not require any wafer conditioning prior to growth and thus reduces the process complexity and maintains the cost effectiveness of the GaN on Si strategy. We will report on this manufacturable 100mm MOCVD heteroepitaxial process that consistently produces device quality AlGaN/GaN heterostructures with two dimensional electron gas (2DEG) mobilities typically around 1400 cm2/Vs at room temperature. Structural and electrical properties as determined by optical reflectance, atomic force microscopy, capacitance-voltage and van der Pauw Hall measurements, which are measured across the 100mm wafer, will be presented. Device results will be mentioned to show continuous wave (CW) RF operation at 2 GHz with competitive power output, gain and power added efficiency (PAE).

We have investigated the formation of inversion domain boundaries in Al0.13Ga0.87N layers grown on sapphire substrates by metalorganic chemical vapor deposition using transmission electron microscopy. By increasing the Mg source flow rate, the reduction of dislocation density occurred up to the Mg source flow rate of 0.103 μmol/min. While the vertical type inversion domain boundaries (IDBs) were observed in the Al0.13Ga0.87N layers grown with the low Mg source flow rate, the IDBs in the Al0.13Ga0.87N layers grown with the high Mg source flow rate have horizontally multifaceted shapes. The change of polarity by the IDBs of horizontal type also resulted in the 180°rotation of pyramidal defects within the same AlGaN layer. Therefore, We found that the Mg source flow rate affects significantly the dislocation density, the type of IDBs, and the shape of pyramidal defects in AlGaN layers.

We present the details of epitaxial growth interface structure of single wurtzite AlN thin films on (111) Si substrates by laser-molecular-beam-epitaxy. High quality AlN thin films with atomically sharp interfaces can be obtained by Laser-MBE at a substrate temperature of 650 ±10°C. X-ray diffraction and high resolution transmission electron microscopy was used to study the details of epitaxial growth of AlN on Si(111) substrate. The orientation-relationship of AlN on Si(111) was studied from Si <110> and <112> zone axis and determined to be AlN [2110]|Si[110] and AlN [0110]|Si[211]. The atomic structure of the interface was studied by high-resolution transmission electron microscopy and Fourier filtered image of cross-sectional AlN/Si(111) samples from both Si<110> and <112> zone axis. The results revealed the domain matching epitaxy of 4:5 ratio between the interplanar distances of Si(110) and AlN [2110]. We also present similarities and differences between the growth mechanism of AlN/Si(111) and GaN/Si(111) heterostructures.

Present hydride vapor phase epitaxial growth of GaN on Al2O3 can produce material of very high quality, especially in regions of the crystal far from the substrate/epilayer interface. In the present study, we characterize a 248-μm-thick epilayer, which had been separated from its Al2O3 substrate and etched on top and bottom to produce flat surfaces. Temperature-dependent Hall-effect data have been fitted to give the following parameters: mobility μ(300) = 1320 cm2/V-s; μ(peak) = 12,000 cm2/V-s; carrier concentration n(300) = 6.27 × 1015 cm−3; donor concentration ND = 7.8 × 1015 cm−3; acceptor concentration NA = 1.3 × 1015 cm−3; and effective donor activation energy ED = 28.1 meV. These mobilities are the highest ever reported in GaN, and the acceptor concentration, the lowest. Positron annihilation measurements give a Ga vacancy concentration very close to NA, showing that the dominant acceptors are likely native defects. Secondary ion mass spectroscopic measurements show that ND is probably composed of the common donors O and Si, with [O] > [S1]. Transmission electron microscopy measurements yield threading dislocation densities of about 1 × 107 cm−2 on the bottom (N) face, and < 5 × 105 cm−2 on the top (Ga) face. Photoluminescence (PL) spectra show a strong donor-bound exciton (D°X) line at 3.47225 eV, and a weaker one at 3.47305 eV; each has a linewidth of about 0.4 meV. In the two-electron satellite region, a strong line appears at 3.44686 eV, and a weaker one at 3.44792 eV. If the two strong lines represent the same donor, then ED,n=1 – ED,n=2 = 25.4 meV for that donor, and the ground-state activation energy (EC – ED,n=1) is (4/3)25.4 = 33.9 meV in a hydrogenic model, and 32.7 meV in a somewhat modified model. The measured Hall-effect donor energy, 28.1 meV, is smaller than the PL donor energy, as is nearly always found in semiconductors. We show that the difference in the Hall and PL donor energies can be explained by donor-band conduction via overlapping donor excited states, and the effects of non-overlapping excited states which should be included in the n vs. T data analysis (charge balance equation).

Halide vapor transport epitaxy (HVTE) is demonstrated for growth of AlN layers with thickness up to 50 μm at deposition rates up to 60 μm/h. The HVTE process uses an aluminum chloride amine adduct as the aluminum source of both aluminum and nitrogen. This new technique eliminates the main difficulties of the conventional hydride VPE growth, where aluminum oxidation and the strong reactivity of aluminum chloride with fused silica create the potential for oxygen contamination. This study shows the effect of temperature, gas flow velocities, and reactor pressure on the growth rate and layer quality. It is found that the growth rate and the layer quality strongly depend on the gas ratio. The species of carrier gas, the flow rates and partial pressures can be used as tools to optimize growth rate and to avoid any etching effects resulting from reverse chemical reactions. The crystalline layer quality as determined by x-ray rocking curve measurement shows FWHM of 300–900 and 500–1300 arc-sec for (002) and (102) planes, respectively.

PACS: 81.05.Ea; 81.15.Kk; 68.55.Jk.

C-plane oriented thin films of gallium nitride (GaN) were grown on both amorphous quartz substrates and single crystalline c-sapphire substrates at sub-atmospheric pressures by exposing molten gallium thin films to electron cyclotron resonance (ECR) microwave generated nitrogen plasma. Gallium nitride crystals nucleated from molten gallium and self-aligned with respect to each other due to the mobility of nitrogenated gallium and formed textured film directly on amorphous substrates. Scanning electron microscopy (SEM) images and X-ray Diffraction (XRD) spectra confirmed the orientation among crystals. Micro-Raman spectra exhibited a FWHM of 3 cm−1. Self-assembled, nanocrystalline GaN thin films were obtained when spin-coated gallium thin films (< 1 μm) on quartz substrates were nitrided.