The electroluminescence, photoluminescence and cathodoluminescence of GaInN/GaN multiple quantum well light emitting diode dies are analyzed at variable low temperature. Three dies of nominally identical structure but strongly different RT performance from 510 nm to 525 nm have been studied. The electroluminescence peak energy exhibits a blue shift from RT to 158 K followed by a red shift for lower temperature. In the same low-temperature range, a secondary emission peak appears near 390 nm (3.18 eV) that resembles a donor-acceptor pair transition from GaN. Depth profiling spectroscopy of this transition at 77 K reveals its location either in the unintentionally doped quantum barriers or within the n-GaN layer, rather than the commonly believed Mg doped p-type GaN layers. The external quantum efficiency of each die increases as the temperature is lowered. A maximum is reached for all near 158 K while for lower temperature as low as 7.7 K, the efficiency continuously drops. The pronounced efficiency maximum is tentatively assigned to a combination of temperature dependent mobility and shift of the actual pn-junction location.

The amount of residual carbon in the GaAsN film deposited by chemical beam epitaxy (CBE) is decreased by the flow rate modulation growth method (FM-CBE). The number of carbon atoms remained in the grown film increases as the growth temperature decreases. At low temperatures below 440°C, the carbon atoms are mainly originated from the nitrogen source gas mono-methylhydrazine. However, increasing the substrate temperature during the growth causes the deterioration of film qualities and the high growth temperature is not the solution for reducing the impurities. On the other hand, by intermitting the supply of gallium source triethylgallium while the arsenic and nitrogen sources are continuously supplied, the carbon concentration drastically decreases as compared with that grown by the conventional CBE growth. The results of temperature programmed desorption and ab initio calculations suggest that the desorption of adsorbates that contain C atom, such as, NHCH3, is enhanced by the FM-CBE growth, resulting in the decrease of the residual C concentration.

GaInN/GaN quantum wells (QWs) grown on different crystal facets have been investigated by field-dependent photoluminescence and electroluminescence experiments. Externally applied voltage changes the total field strength and direction of the electric fields inside the quantum wells, consisting of piezoelectric and built-in fields. Electroluminescence with increasing current results in a peak shift due to screening of the field by the injected carriers. By modeling the peak shifts of the photoluminescence (PL) and electroluminescence (EL) signals we found strong piezoelectric fields for a {0001} sample and nearly vanishing fields for a sample grown on the {1-101} plane of GaN.

The surface properties of n-type AlxGa1−xN (x=0-0.5) exposed to inductively couple plasma were studied systematically using metal contact measurements and X-ray photoelectron spectroscopy (XPS). Cl2/BCl3 and Ar plasma treatment considerably increased the surface conductivity of AlxGa1−xN (x<0.3) and thus improved the characteristics of Ti/Al-based ohmic contacts, but degraded the surface and contact properties of AlxGa1−xN (x≥0.3). XPS measurements revealed a significant increase in oxygen incorporation and energy shifts of the core-level spectra in plasma-treated samples. In contrast to an upward shift of the surface Fermi level in plasma-treated GaN, the surface Fermi level moved away from the conduction band in Al0.5Ga0.5N upon plasma treatment. These findings suggest that the nature of plasma damage in AlGaN appears to be a function of the Al mole fraction. Plasma-induced defects in high-Al AlGaN may act as deep-level compensation centers, degrading the surface electrical properties and contact characteristics.

Removing artificial oxide layer is important for fabrication process of semiconductor devices. We employed photoluminescence for optical, reflection high energy electron diffraction, atomic force microscope, and Nomarski microscope for surface analysis. We found that it is more difficult to remove the oxide layer made by O-plasma such as in O-plasma asher than that of native one. Our result reached that HF etching is effective for removal of the artificial oxide layer without changing surface morphology. In addition, (NH4)2Sx treatment after HF etching reduces donor bound exciton drastically which is dominant luminescence near the band edge at low temperature.

In the work reported here the Hartree-Fock (HF), restricted open shell HF (ROHF), and multiconfiguration self-consistent field (CI/CASSCF/MCSCF) methods are used to predict electronic properties of several artificial molecules of InAsN and indium nitride whose structure and composition have been derived from those of the corresponding symmetry elements of the zincblende and wurtzite bulk lattices. Both quantum-confined and “vacuum” clusters (whose geometry has been optimized without any spatial constraints applied to the atomic positions) were studied focusing on the electronic energy level structure, direct optical transition energy (OTE), and charge and spin distributions. The obtained results indicate that inclusion of “impurity” atoms (such as As atoms) may enhance stability of both vacuum and confined pyramidal In-N molecules and provide for manipulations of the OTE in a wide range of its values. The CI/CASSCF/MCSCF OTEs of the studied wurtzite-based clusters may also vary in a wide range, from 1.7440 eV for the smallest pre-designed prismatic molecule In6N6 to 6.9780 eV for its almost perfect prismatic “vacuum” counterpart. These evaluations closely correlate with experimental data available in literature.

Epitaxial growth of ZrO2 has been achieved on MOCVD grown GaN template by oxides MBE using a reactive H2O2 oxygen source. Using the low temperature buffer followed by high temperature in-situ annealing and high-temperature growth, monoclinic, (100)-oriented ZrO2 thin film with decent structural quality and smooth surface morphology was achieved. The full width at half maximum of ZrO2 (100) rocking curve is 0.4 arc degree and the rms roughness for a 5μm by 5 μm AFM scan is 4.1Å. The use of epitaxially grown ZrO2 in the AlGaN/GaN HFET structure as a gate dielectric layer have resulted in the increase of the saturation-current density and pinch-off voltage as well as showed near symmetrical gate-drain I-V behavior.

3-D branching GaN nanowires have been grown using the intermediate and Ga-rich growth regimes of plasma assisted molecular beam epitaxy. Evidence that the growth is due to an auto-catalytic VLS process is obtained through SEM images showing droplet termination heads, the composition of which is essentially pure Ga. TEM analysis revealed a defect free crystal structure, even in the trunk to branch junction. Cathodoluminescence from the trunk of the branching nanowires produced a strong luminescence feature at 3.44 eV, while a slight decrease in energy to 3.1 eV was observed at the interface between the nanowire and epilayer or kink site. No yellow luminescence was detected, further suggesting a defect free growth. Preliminary I-V measurements give mixed results, suggesting intrinsic n-type nanowires.

An AlGaN/GaN photo-hetero-field-effect transistor (photo-HFET) with a p-type GaN gate was fabricated and its properties were compared with those of a HFET without a p-type GaN layer. The photo-HFET with a p-GaN gate exhibited a high signal-to-noise ratio of five orders of magnitude and a dark current density of 10 nA/mm at a drain-source bias of 5 V. In contrast, when the photo-HFET was irradiated with 365 nm (490 μW/cm2) UV light, a photocurrent of over 1 mA/mm was achieved. The responsivity of the device was over 1 × 105 A/W.

Cathodoluminescence spectra of GaInN quantum well structures show a strong change of intensity and a emission energy shift during electron beam irradiation. Measurements on samples with quantum well positions varying in depth show, that the spontaneous field, witch is usually screened by surface charges, penetrates the sample in a timescale of minutes and reduces the quantum confined Stark effect.

Low defect density a-plane GaN films were successfully grown by sidewall epitaxial lateral overgrowth (SELO) technique. Control of V/III ratio during the growth of GaN by metalorganic vapor phase epitaxy (MOVPE) was found to be very important to achieve a complete overgrowth on the SiO2 mask regions and atomically flat surface. The threading dislocation and stacking fault densities in the overgrown regions were lower than 106 cm−2 and 103 cm−1, respectively. The root mean square roughness was 0.09 nm. We also fabricated and characterized a-plane-GaN-based-light-emitting diodes (LEDs) using SELO technique. The light output power of the blue-green LED steeply increased with the decrease of threading dislocation density from 1010 cm−2 to 108 cm−2 and tended to saturate at lower dislocation densities.

Deep levels in Si doped AlxGa1−xN samples, with Al mole fraction in the range of x = 0 to 0.30, grown by radio-frequency plasma activated molecular beam epitaxy on sapphire substrates were characterized by deep level transient spectroscopy (DLTS). DLTS measurements show two significant electron traps, P1 and P2, in AlGaN at all aluminum mole fractions. The electron trap, P2, appears to be a superposition of traps A and B , both of which are observed in GaN grown by various growth techniques and are thought to be related to VGa-shallow donor complexes. Trap P1 is related to line defects and N-related point defects. Both of these traps are distributed throughout the bulk of the epitaxial layer. An additional trap P0 which was observed in Al0.20Ga0.80N and Al0.30Ga0.70N is of unknown origin, but like P1 and P2, it exhibits dislocation-related capture kinetics. The activation energy measured from the conduction band of the defects is found to increase with Al mole content, a behavior consistent with other III-V semiconductors.

We report the properties of GaN islands selectively grown by MOCVD on (0001) GaN/sapphire templates and on bare (0001) sapphire substrates. The approach allows us to grow GaN crystals with control over size and density through optical (micro-scale) and e-beam (nano-scale) lithography, and control over shape through MOCVD growth parameters. We have obtained complete pyramidal and prismatic hexagonal GaN islands. The growth characteristics are found to be very different from what we obtain for epitaxial layers, an effect which is attributed to micro/nanoloading and surface diffusion of source materials on the hard mask. From the dependence of growth rate of the pyramids with pitch we evaluate Ga surface diffusion length of ∼ 10 μm over SiO2 mask. We present a growth model based on diffusion and discuss the results of modeling with experimental data. Using micro-Raman spectroscopy the stress in pyramidal and prismatic islands was evaluated to be 20 MPa and 85 MPa, respectively. This implies that the selectively grown islands are relatively more relaxed compared to similar thickness of uniform layers. Micro catholdoluminescence (CL) shows a narrow line width and a better quality of GaN nano islands grown on e-beam lithography patterned templates. The GaN islands, of varying shapes, were overgrown with InGaN multiple quantum wells, which showed a few times stronger micro-CL emission than from GaN.

Variable magnetic field Hall effect, photoluminescence (PL) and capacitance-voltage (CV) analysis have been used to study InN layers grown by plasma assisted molecular beam epitaxy. All three techniques reveal evidence of a buried p-type layer beneath a surface electron accumulation layer in heavily Mg-doped samples. The use of lattice-matched Yttria-stablized Zirconia substrates also provides evidence of a p-type layer.

The dc and rf performance of AlGaN/GaN High Electron Mobility Transistors (HEMTs) grown by Molecular Beam Epitaxy on Si-on-poly (SopSiC) substrates is reported. The HEMT structure incorporated a 7 period GaN/AlN superlattice between the AlGaN barrier and GaN channel for improved carrier confinement. The knee voltage of devices with 2 μm gate-drain spacing was 2.12 V and increased to 3 V at 8 μm spacing. The maximum frequency of oscillation, fMAX, was ∼40 GHz for devices with 0.5 μm gate length and 2 μm gate-drain spacing. Parameter extraction from the measured rf characteristics showed a maximum intrinsic transconductance of 143 mS.mm−1.

Layers of GaN deposited on Si substrates have been studied using Thermal Admittance Spectroscopy (TAS) and Optical Admittance Spectroscopy (OAS). Transparent front-side contacts were used to facilitate the optical measurements. Six specimens were cut from the same location in two different wafers, and three samples were randomly chosen from other growths. A shallow level at EC − 0.051 eV was found in all the specimens using TAS. In some specimens this peak was asymmetric, indicating more than one level near this energy. Deeper levels were also seen in the high-temperature portion of the spectra, but were poorly resolved in most specimens. Illuminating the specimen with UV light at 25 K resulted in the thermal position of the peak shifting to a lower temperature, and hence, the calculated energy, of the peak shifting lower. The amplitude of the peak also decreased. Transient OAS measurements revealed the interesting phenomenon of negative persistent photo conductance at room temperature in some of the specimens when the illumination photon energy was less than the bandgap. The negative response time was very short. At lower temperatures, below 100 K, the negative response diminished, but the response time was still short. At wavelengths above the bandgap energy, normal transient response was seen, with a longer time constant.

Electroreflectance (ER) spectra of InGaN/AlGaN/GaN p-n- heterostructures with multiple quantum wells (MQW) are studied. Structures with MQW InGaN/GaN were grown for blue LEDs by MOCVD technology and “flip-chip” mounted. The ER spectral maxima correspond to the high energy side of electroluminescence spectral line. The ER spectra caused by Franz-Keldysh effect are approximated by Aspnes theory. The ER spectra in a range 400 ÷ 800 nm have interference bands caused by the change of refraction index in the structure.

Intersubband transitions in the spectral range of 1.37-2.90 °Cm is observed in molecular beam epitaxy grown Si-doped GaN/AlN multiple quantum wells using a Fourier-transform spectroscopy technique. A blue shift in the peak position of the intersubband transition is observed as the well width is decreased. A sample with a well width in the order of 2.4 nm exhibited the presence of three bound states in the GaN well. The bound state energy levels are calculated using a transfer matrix method. An electrochemical capacitance voltage technique is used to obtain the three dimensional carrier concentrations in these samples which further enable the calculation of the Fermi energy level position. Devices fabricated from these GaN/AlN quantum wells are found to operate in the photovoltaic mode.

In this paper, we report the growth and fabrication of non-polar A-plane AlGaN multiple quantum well based ultraviolet light emitting diodes (UV-LEDs). The LEDs were grown on R-plane sapphire substrates using molecular beam epitaxy (MBE). The Current-voltage characteristics of the fabricated devices demonstrated rectifying behavior with a series resistance of 38 ohms. An electro-luminescence emission at 338 nm was obtained.

GaN and AlN thin films were implanted with gadolinium (Gd)atoms and characterized using deep ultra-violet (UV) photoluminescence(PL). The Gd-implanted samples were annealed at temperatures up to 1178K in a flowing N2 gas to facilitate recovery of implantation-related damage. Using the output at 195 nm from a quadrupled Ti:sapphire laser, narrow PL emission was observed at 318 nm from the Gd- implanted AlN thin films. This emission is characteristic of the lowest energy 4f transition of the trivalent Gd ion. A boarder emission band, also centered at 318 nm, was observed under excitation at 266 nm. No PL emission was observed from the Gd-implanted GaN thin films at either the bandedge or due to a 4f transition. The dependence of the UV emission on AlN sample temperature was systematically studied. The peak PL emission intensity decreased by less that a factor of 3 over the temperature range of 10 K to 300 K. Decay transients of the UV emission were measured indicating that the lifetime of this emission is very fast.