B. J. Pong

Enhanced light output of GaN-based power LEDs with transparent Al-doped ZnO current spreading layer

In this study, Al-doped ZnO (AZO) Ni-AZO and NiO/sub x/--AZO films were deposited on p-type GaN films. The depositions were followed by thermal annealing to form Ohmic contacts. The p-GaN-AZO contacts exhibited a non-Ohmic electrical characteristic. However, electrical characteristic could be greatly improved by insertion of Ni or NiO/sub x/ between AZO film and p-GaN layer. In case of 1/spl times/1 mm/sup 2/ ultraviolet light-emitting diodes (LEDs) with Ni-AZO contacts, the light output approached saturation point when the injection current was about 400 mA. However, the saturation point was as high as 500 mA for the LEDs with NiO/sub x/--AZO contacts. The lower saturation point could be due to the fact that the resistivity of Ni-AZO films was higher than that of NiO/sub x/--AZO films, thus leading to a severe current crowding effect. The increased resistivity of the Ni-AZO films could be attributed to the interdiffusion between Ni and AZO films. When compared to the LEDs with Ni-Au Ohmic contacts, the light output of the LEDs with Ni-AZO and NiO/sub x/--AZO contacts was higher by 38.2% and 60.6% at 350 mA, respectively.

Structural defects and microstrain in GaN induced by Mg ion implantation

The optical and structural characteristics of GaN films implanted with Mg and Be ions, grown by low-pressure metalorganic chemical vapor deposition were studied. The low temperature (20 K) photoluminescence (PL) spectra of annealed Mg implanted GaN show a 356 nm near band edge emission, a 378 nm donor-acceptor (D-A) transition with phonon replicas, and a 528 nm green band deep level emission. The origin of the 528 nm green band emission and the 378 nm D-A emission might be attributed, respectively, to the Mg implantation induced clustering defect and the vacancy defect in GaN film. Observations of in-plane and out-of-plane x-ray diffraction spectra for as-grown undoped, Mg implanted and rapid thermal annealed GaN suggest that ion implantation induced anisotropic strain may be responsible for the observed PL emission characteristics.

PHOTOLUMINESCENCE SPECTROSCOPY OF MG-DOPED GAN

We have grown Mg-doped GaN films by metalorganic chemical vapor deposition with various CP2Mg flow rates. After 750 °C postgrowth annealing, p-type GaN films with carrier concentrations and mobilities about 2×1017/cm3 and 10 cm2/V s, respectively, have been achieved. A dominant photoluminescence (PL) line around 2.9 eV was observed at room temperature. By studying the dependence of PL on excitation density at 20 K, the emission line around 2.95 eV can be attributed to a donor-to-acceptor pair transition rather than to a conduction band-to-impurity transition involving the Mg-related deep level. We suggest that the DAP transition line is caused by a Mg related deep level at about 510 meV above the valence band. It is much deeper than the acceptor level at 250 meV commonly produced by the Mg dopants.

Stress relaxation in GaN by transfer bonding on Si substrates

The stress state of GaN epilayers transferred onto Si substrates through a Au–Si bonding process was studied by micro-Raman scattering and photoluminescence techniques. By increasing the Au bonding thickness from 1to40μm, the high compressive stress state in GaN layer was relieved. A 10μm Au bonding layer thickness is shown to possess the maximum compressive stress relief and also the deformation potential of the quantum well was found to be ∼85meV. A nonlinear parabolic relation between luminescent bandgap and the biaxial stress of the transferred GaN epilayer in the compressive region was observed.

Improvement of near-ultraviolet nitride-based light emitting diodes with mesh indium tin oxide contact layers

The authors have demonstrated nitride-based near-ultraviolet (NUV) light emitting diodes (LEDs) with mesh indium tin oxide (ITO) contact layer. With 20mA injection current, it was found that forward voltages were 3.94 and 4.05V while the output powers were 7.54 and 9.02mW for the planar ITO LED and mesh ITO LED, respectively. The larger LED output power should be attributed partially to the reduced absorption of ITO in the NUV region and partially to the better current spreading.

Residual strain in ZnO nanowires grown by catalyst-free chemical vapor deposition on GaN/sapphire (0001)

ZnO nanowires were grown on 2-μm-thick GaN templates by chemical vapor deposition without employing any metal catalysts. The GaN template was deposited by metal-organic chemical vapor deposition on a c-plane sapphire substrate. The diameters of the resulting nanowires were in the range of 40–250nm depending on growth time. The ZnO nanowires were vertically well aligned with uniform length, diameter, and distribution density as revealed by electron microscopy. X-ray diffraction spectra showed that ZnO grew in single c-axis orientation with the c axis normal to the GaN basal plane, indicating a heteroepitaxial relationship of (0002)ZnO‖(0002)GaN. The lattice constant of the c axis of the ZnO nanowires with diameter of 40nm was 5.211A, which is larger than that of bulk ZnO (5.207A). The ZnO nanowires exhibit a residual tensile strain along the c axis, which decreases with increasing diameter.

Si diffusion in p-GaN

The characteristics of p-type Mg-doped GaN films diffused with Si are studied. N-type conductivity is achieved, and the carrier mobility of diffused GaN is 90–150 cm2 V−1 s−1, higher than that of p-GaN but less than that of epitaxially grown n-GaN. The Mg acceptor states could become deep compensating defects, and the compensation ratio NA/ND is 0.3, 0.45, 0.6, and 0.75 for 800, 900, 1000, and 1100 °C diffused GaN, respectively. The carrier transport may be dominated by electron hopping through these deep compensating centers or through diffusion. The results of temperature-dependent carrier concentration indicate that thermal annealing may induce defects at the surface, leading to an additional activation energy Ed∼10 meV in the 200–500 K region in diffused GaN.

Nitride-based light-emitting diodes with p-AlInGaN surface layers prepared at various temperatures

We have prepared bulk p-AlInGaN layers and light-emitting diodes (LEDs) with p-AlInGaN surface layers by metal-organic chemical vapor deposition. By properly control the TMAl and TMIn flow rates, we could match the lattice constant of p-AlInGaN to that of GaN. It was found that surface of the LED with p-AlInGaN layer was rough with a high density of hexagonal pits. Although the forward voltage of the LED with p-AlInGaN layer was slightly larger, it was found that we can enhance the output power by 54% by using p-AlInGaN surface layer.

GaN-based stacked micro-optics system

A prototype of a GaN-based stacked micro-optics system is demonstrated. The system consists of a GaN microlens, GaN membrane gratings, six spacers, a spatial filter, and a 980 nm VCSEL. The laser beam is collimated by the GaN microlens and diffracted by the GaN membrane grating. The systems can be used in blue-violet-UV micro-optics systems.

Microstructure of InN quantum dots grown on AlN buffer layers by metal organic vapor phase epitaxy

InN quantum dots (QDs) were grown over 2in. Si (1 1 1) wafers with a 300nm thick AlN buffer layer by atmospheric-pressure metal organic vapor phase epitaxy. When the growth temperature increased from 450to625°C, the corresponding InN QDs height increased from 16to108nm while the density of the InN QDs decreased from 1.6×109cm−2to3.3×108cm−2. Transmission electron microscopy showed the presence of a 2nm thick wetting layer between the AlN buffer layer and InN QDs. The growth mechanism was determined to be the Stranski–Krastanov mode. The presence of misfit dislocations in the QDs indicated that residual strain was introduced during InN QDs formation. From x-ray diffraction analysis, when the height of the InN QDs increased from 16to62nm, the residual strain in InN QDs reduced from 0.45% to 0.22%. The residual strain remained at 0.22% for larger heights most likely due to plastic relaxation in the QDs. The critical height of the InN QDs for releasing the strain was determined to be 62nm.