Search

ZnO nanowires were grown on Si (100) substrates with and without Au catalyst by chemical vapor deposition employing the vapor–liquid–solid (VLS) and vapor–solid (VS) mechanisms, respectively. The diameters of the resulting nanowires were in the range 80–150 nm with typical length about 10 μm. The near-band-edge (NBE) emission of ZnO nanowires grown with and without catalyst was observed at 382 nm and 386 nm, respectively. The intensity of the NBE emission of ZnO nanowires grown without the catalyst was higher than that of the green luminescence. By sharp contrast, the intensity of the NBE emission of ZnO nanowires grown with catalyst was lower than that of green luminescence. The X-ray diffraction (XRD) spectrum of the ZnO nanowires grown without catalyst exhibited a peak intensity of c-plane 5 times higher than that of m-plane and 10 times higher than that of a-plane. However, the XRD spectrum of the ZnO nanowires grown with catalyst exhibited a peak intensity of the c-plane about 1.5 times higher than that of the m-plane and 4 times higher than that of a-plane intensity. Thus, the ZnO nanowires grown without catalyst have a preferential orientation along the c-axis direction. Our results show that the catalyst strongly effects optical and structural properties of the ZnO nanowires.

Blue InGaN micro-size light emitting diodes (LEDs) with diameters from 3 to 70 μm have been fabricated. An ion implantation technique and a 12 μm electro-ridge were used to simplify the fabrication processes. The 3–70 μm LEDs exhibiting a large emission of photon blue shift (40–240 meV) were observed in electro-luminescence (EL) spectra. The dependence of the blue shift on size is studied. The characteristics of the micro-size LEDs are also investigated numerically with the use of an advanced physical model of semiconductor devices (APSYS). The experimental measurements and simulation results are in close agreement for maximum blue shift. Base on the simulation, the difference between blue shift caused by band filling effect and red shift caused by lateral carrier confinement are approximately the same. Hence, the maximum blue shift increases when the size of micro-size LED increases because of decreased red shift resulted from thermal effect.

Nitrogen ions were implanted into melt-grown ZnO (0001) substrates and subsequently annealed at 800 °C under an oxygen ambient. The photoluminescence spectrum of N + -implanted ZnO excited by a He–Cd laser exhibited donor–acceptor pair (DAP) transition emission at 385 nm with a full width at half maximum of 30 nm at 10 K. The DAP emission is associated with the acceptor energy of nitrogen in ZnO, calculated to be 170 meV. Defect-related red emission at about 610 nm observed in N + -implanted ZnO was due to the residual damage from the implantation step because it was also observed in Ar + -implanted ZnO but not in un-implanted ZnO annealed at 800 °C under the same oxygen ambient.

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.

Photoluminescence (PL) of N-doped epitaxial ZnO and N+-implanted bulk ZnO are studied. The N-doped epitaxial ZnO films were grown on GaN templates by plasma-assisted molecular beam epitaxy with a flux of N2 gas was added to the O2 gas flow in the RF plasma source. The N+-implanted bulk ZnO was fabricated using nitrogen ions implanted into a melt-grown ZnO (0001) substrate with subsequent annealing at 800 °C under oxygen ambient. PL spectra of N-doped epitaxial ZnO and N+-implanted bulk ZnO excited by a He-Cd laser exhibit donor-acceptor pair (DAP) transition emission at 385 nm with a full width at half maximum of 30 nm at 10 K. The DAP emission is associated with the acceptor energy of nitrogen in ZnO, 170 meV. Defect-related red luminescence at about 610 nm observed in N+-implanted ZnO is attributed to the damage due to implantation because it is also observed in P+- or Ar+-implanted bulk ZnO but is not observed in N-doped epitaxial ZnO. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

We have observed a fixed wavelength emission at 178.7 nm in sodium vapour under 578.7 nm two-photon resonance excitation. The proposed non-linear wave mixing scheme is described by ω178.7 nm = 2ωL + ω465.7 nm; where ω178.7 is the 178.7 nm photon frequency, ωL is the laser-photon frequency, and ω465.7 is the 465.7 nm photon frequency. This 465.7 nm emission comes from another six-wave mixing process involving two hyper-electronic Raman scattering photons. The excitation spectrum of the 178.7 nm emission has a typical multiwave mixing pattern with a competing effect appearing at higher temperatures under two-photon resonance excitation. Numerical analysis indicates that this vacuum ultraviolet emission has a poor phase-match condition that will depress the emission intensity to a certain extent. This makes the observation more difficult compared with other reported four-wave mixing generated emissions. Fortunately, on the one hand, it is enhanced by quasi-auto-ionization resonance when the 3s–5s transition is coupled to the sodium continuum by a 330.2 nm photon. On the other hand, its wavelength sits so close to the sodium Cooper minimum that weak absorption will not suppress this vacuum ultraviolet emission further.

Coupling of a InGaN/GaN multi-quantum well (MQW) and semitransparent metal layer is shown to result in dramatic enhancement of spontaneous emission rate by the surface plasmon effect in the optical spectral range. A five-pairs 18.5nm InGaN/GaN MQW is positioned 175nm, form various thickness (t=5~50nm) silver layer. And periodic patterns (p=0.25~0.8μm) are defined in the top semitransparent metal layer by e-beam lithography, which are grating structures can be incorporated into the metal film to excite surface plasmon between the interference of the metal film and semiconductor. We have experimentally measured photoluminescence intensity and peak position of spontaneous emission of the fabricated structures and compared with the unprocessed samples, whilst still ensuring that most of the emission takes place into the surface plasmon (SP) mode. And the implication of these results for extracting light by reducing total internal reflection (TIR) from light emission diode is discussed.

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.

In this study, ZnO:Al(AZO) Ni/AZO and NiO x /AZO films were deposited on p-type GaN films followed by thermal annealing to form Ohmic contacts. After thermal annealing, the resistivities reduced from 5×10 -3 to 4.4×10 -4 Ω-cm, 2.6×10 -3 Ω-cm, and 1.1×10 -3 Ω-cm for AZO, Ni/AZO, and NiO x /AZO films, respectively. The Ohmic characteristic could be highly improved after inserting Ni and NiO x between AZO and p-GaN. Both the Ni/AZO and NiO x /AZO contacts exhibit Ohmic characteristic after annealed at 800°C in N 2 ambient. The light transmittance of Ni/AZO and NiO x /AZO films were higher than 80% in the range of 380-700nm after the 800°C -annealing treatment. In addition, we fabricated InGaN/GaN MQW LEDs with a dimension of 1×1mm 2 using the transparent Ni/AZO and NiO x /AZO Ohmic contact as a current spreading layer for p-GaN in order to increase the light extrication efficiency. For the LED with Ni/AZO contact, the light output approach to saturation when the injection current was about 400mA. But the light output still doesn't approach to saturation when the injection current was 500mA for the LED with NiO x /AZO contact. This may be due to that the resistivity of Ni/AZO was higher than that of NiO x /AZO and exhibit more heavy current clouding effect. The increasing of resistivity may be due to the interdiffusion of Ni into AZO. Comparing to GaN LED with Ni/Au ohmic contact, the light output intensity of LEDs with Ni/AZO and NiO x /AZO contacts was increased by 41% and 60% at 350mA, respectively.

Vertical-cavity surface-emitting lasers with variant compressively strained InGaAlAs quantum wells have been investigated. The valence band structures, optical gain spectra, and threshold properties of InGaAlAs/AlGaAs quantum wells are compared and analyzed. The simulation results indicate that the characteristics of InGaAlAs quantum wells can be improved by increasing the amount of compressive strain in quantum well. Furthermore, the properties of VCSELs with these compressively strained InGaAlAs quantum wells are studied numerically. The results of numerical calculations show that the threshold current and maximum output power can be enhanced by using higher compressively strained InGaAlAs quantum well. However, when the compressive strain is larger than about 1.5%, further improvement of the laser performance becomes minimal. The effects of the position and aperture size of the oxide-confinement layers on the laser performance are also investigated. Variation of the oxide layer design is shown to affect the current distribution which makes the temperature in the active region different. It is the main reason for the power roll-off in the VCSEL devices.

Homoepitaxial and heteroepitaxial ZnO films were grown by plasma-assisted molecular beam epitaxy (P-MBE). Homoepitaxial ZnO layers were grown on an O-face melt-grown ZnO (0001) substrate. Heteroepitaxial ZnO layers were grown on an epitaxial GaN template predeposited by metalorganic chemical vapor deposition on a c-plane sapphire substrate. There exists a residual strain in the heteroepitaxial ZnO, which is e = -0.25%. Low-intensity excitation PL spectra of ZnO epilayers excited by a He-Cd laser exhibit only bound-exciton emission with phonon replicas. The quality of ZnO epilayers is better than that of ZnO substrate. However, under high-intensity excitation by a N2 laser, the emission due to exciton-exciton collisions dominates the PL spectrum from heteroepitaxial ZnO layer but is not observed from homoepitaxial ZnO layer.

With 150KeV Mg+ ion implantation, the optical and structural characteristics of GaN films were studied. Post-implant annealing up to 1000°C was performed in N2 ambient with a rapid thermal annealing (RTA) system, without an encapsulation layer. We observed a green band photoluminescence from Mg-implanted GaN. This green band photoluminescence should be associated with Mg induced defect-clustering in GaN. We also use the x-ray diffraction method to study the correlation between structure defects and implantation. We observed an extra shoulder peak at the small angle side of the GaN[0004] diffraction peak. The origin of this shoulder may be attributed to implanted magnesium induced GaN lattice strain.

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.

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.

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.

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.

The authors have prepared bulk p-AlInGaN layers and light-emitting diodes (LEDs) with p-AlInGaN surface layers by metal organic chemical vapor deposition. They found that the surfaces of the LEDs with p-AlInGaN layers are rough with high density of hexagonal pits. They also found that the pit width and the pit density depend on the growth temperature of the p-AlInGaN layer. Furthermore, it is found that a 62% enhancement in output intensity can be achieved from the LED with an 820°C p-AlInGaN cap layer without increasing the LED operation voltage.

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.

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.