Band Edge Emission Intensity Research Articles

Surface Degradation of GaN after Thermal Processes

The effects of thermal process on the surface degradation of GaN were investigated. The GaN was grown on Si(111) substrate and thermally treated. When the temperature of thermal processes increased, the grain boundary expanded and pits generated as a result of the decomposition. The band-edge emission intensity in the photoluminescence spectra was increased by thermal processes. The deep-level emission centered at about 2.2 eV suppressed after thermal processes. We estimate that they were caused by the increase of band-edge emission intensity or the surface decomposition associated with impurity-related recombination in GaN. The full-width at half-maximum of (0002) X-ray rocking curve did not change significantly before and after the thermal processes. This indicated that the surface decomposition of our thermal processes did not affect the crystal quality. A side effect of decomposition owing to the high-temperature process should be considered in the fabrication of GaN-based electronic devices.

THE STUDY OF Al0.29Ga0.71N-BASED SCHOTTKY PHOTODIODES GROWN ON SILICON BY PLASMA-ASSISTED MOLECULAR BEAM EPITAXY

In this paper, the growth and characterization of epitaxial Al 0.29 Ga 0.71 N grown on Si (111) by RF-plasma assisted molecular beam epitaxy (MBE) are described. The Al mole fraction was derived from the HR-XRD symmetric rocking curve (RC) ω/2θ scans of (0002) plane as x = 0.29. PL spectrum of sample has shown sharp and intense band edge emission of GaN without the existence of yellow emission band, showing that it is comparable in crystal quality of the sample when compared with previous reports. From the Raman measurement of as-grown Al 0.29 Ga 0.71 N layer on GaN / AlN / Si sample. We found that the dominant E 2 (high) phonon mode of GaN appears at 572.7 cm-1. The E 2 (high) mode of AlN appears at 656.7 cm-1 and deviates from the standard value of 655 cm-1 for unstrained AlN . Finally, AlGaN Schottky photodiode have been fabricated and analyzed by mean of electrical characterization, using current–voltage (I–V) measurement to evaluate the performance of this device.

Improved performance of semi-polar (11-22) GaN-based light-emitting diodes grown on SiNx interlayer

We report on the effectiveness of the in-situ SiNx nanomask in reducing defects in semipolar (11-22) GaN films grown on m-plane sapphire. The properties of the semipolar InGaN/GaN double quantum well (DQW) LEDs were improved with a high-quality (11-22) GaN epilayer grown on the SiNx interlayer. High resolution X-ray diffraction analysis revealed that there was a great reduction in the full width at half maximum of both on-axis and off-axis planes on SiNx interlayer. The room temperature cathodoluminescence (CL) band-edge emission intensity of (11-22) GaN grown on the SiNx interlayer was approximately 4 times higher than that of GaN without the SiNx interlayer, which suggests reduction in the nonradiative recombination centers. The optical power of LEDs with the SiNx interlayer was 200% and 270% higher at injection currents of 20mA and 100mA, respectively, compared to the reference LEDs.

High brightness nonpolar a-plane (11–20) GaN light-emitting diodes

We report on high brightness nonpolar a-plane InGaN/GaN LEDs using patterned lateral overgrowth (PLOG) epitaxy. High crystal-quality and smooth surfaces for a-plane GaN (a-GaN) films were achieved using PLOG with an array of hexagonal SiO2 patterns. The XRC FWHMs of as-grown PLOG a-GaN films were found to be 414 and 317 arcsec (450 and 455 arcsec for planar a-GaN films) along the c-axis and m-axis directions, respectively. Plan-view CL clearly reveals the periodic hexagonal patterns with higher band edge emission intensity, implying that the luminescence properties of a-GaN films lying above the SiO2 mask are improved. The light output powers of a-InGaN/GaN PLOG LEDs were measured to be 7.5 mW and 20 mW at drive currents of 20 mA and 100 mA, respectively. A negligible blue-shift was observed in the peak emission wavelength with increasing drive current up to 100 mA, indicating that there are no strong internal fields in nonpolar a-InGaN/GaN LEDs. We believe that nonpolar a-plane InGaN/GaN LEDs hold promise for efficient nitride emitters if the growth conditions are further optimized.

Control and characterization of structural and optical properties of ZnO thin films fabricated by thermal oxidation Zn metallic films

The physical properties of ZnO thin films fabricated by controlling thermal oxidation zinc metallic films process have been investigated. Comparative characterization of crystallographical, optical or spectroscopic properties of the samples was performed. The as-oxidized sample by rapid thermal process showed level of crystallinity degraded, higher optical transmittance in the visible and near-infrared region, and lower intensity of the near band edge emission compared to those prepared by conventional thermal oxidation. The overall results suggested that with in-depth understanding of the oxidation mechanism, rapid oxidation process could be employed as an approach to fabricate amorphous transparent oxide thin films from low-melting-point metals, which might have potential advantages in microelectronic and optoelectronic applications.

Surface plasmon enhanced bandgap emission of electrochemically grown ZnO nanorods using Au nanoparticles

Electrochemical deposition of ZnO nanorods having a diameter of 80–150nm and length ~2μm has been carried out. Au particles were sputtered on the ZnO nanorods for different sputtering times (from 0 to 100s). The Photoluminescence spectra of bare ZnO nanorods showed a weak bandgap emission at around 375nm and a broad defect-related emission band centered at ~596nm. After the Au sputtering, the defect-related emission disappeared for all the samples. Moreover, the band edge emission intensity was enhanced with Au sputtering time 50s. The enhancement factor reached a maximum value for the Au sputtering time of 50s The enhancement in band edge emission is due to the transfer of electrons from defect states to the Au nanoparticles that cause not only an increase of resonant electron density, but also creates energetic electrons in the higher energy states. These resonant electrons can escape from the surface of the Au nanoparticles to conduction band of ZnO nanorods leading to the suppression of defect related emission intensity.

Enhanced electroluminescence of a-plane InGaN light emitting diodes grown on oxide-patterned r-plane sapphire substrates

We report on the new fabrication method of a-plane InGaN light emitting diodes (LEDs) using the epitaxy on patterned insulator on sapphire substrate (EPISS). Cathodoluminescence spectrum of the fully coalesced a-plane GaN template showed that band edge emission intensity of the wing region was four times higher than that of the window region. Threading dislocations and basal stacking faults densities in wing region were ~1×10⁷ cm⁻² and ~5☓10⁴ cm⁻¹, respectively. Blue-emitting (443.4 nm) a-plane InGaN LED employing EPISS showed the optical power of 3.1 mW and the EL FWHM of 25.2 nm at the injection current of 20 mA.

Photoluminescence and microstructural properties of SiO 2–ZnO–B 2O 3 system containing TiO 2 and V 2O 5

Phase analysis, thermal behavior, microstructure and photoluminescence properties of six specimens in a ZnO–SiO 2–B 2O 3 ternary glass system were investigated by means of DTA, XRD, SEM and PL spectroscopy to reveal the effect of introducing different TiO 2 and V 2O 5 contents to the base glass composition. Both chemicals improved crystallization through reducing the surface energy as well as disrupting the large scale order in the glass network. The optical basicity of every individual metal oxide included in the samples was presented as an estimation of the number of non bridging oxygens (NBOs). Facilitated precipitation of willemite and ZnO due to introducing TiO 2 and V 2O 5 to the base glass compensated for the increase of NBOs and ultimately resulted in the enhancement of width and intensity of the near band edge emission.

The Fabrication of Ag Islands on AlN/GaN/AlN/Si(111) by Using Thermal Evaporator and Thermal Annealing Methods

In this paper, we studied growth of AlN/GaN/AlN on Si (111) by using plasma assisted molecular beam epitaxy (PA-MBE) system. The structural and optical characteristics of the sample have been investigated by using high resolution X-ray diffraction (HR-XRD), Raman spectroscopy and photoluminescence (PL). PL spectrum of the sample has shown sharp and intense band edge emission of GaN with the absence of yellow emission band, indicating good crystal quality of the sample. The silver (Ag) metal contact was then deposited on the sample followed by thermal treatment at 500°C and 700°C, respectively. Treated sample at 700°C showed good spherical Ag islands on sample compared to the treated sample at 500°C. The effect of Ag islands on the electrical characteristics of sample was also examined by usingI-Vmeasurement. The results showed that the treated sample at 700°C has decreased the photo-current of Schottky diode.

Improvements in (112̅2) semipolar GaN crystal quality by graded superlattices

We report on the use of graded superlattices (SLs) for defect reduction in semipolar (112̅2) GaN films, grown by metal-organic chemical vapor deposition. High-resolution x-ray diffraction analysis revealed that there was a great reduction in the full width at half maximum, both on-axis and off-axis, with the SLs. Atomic force microscopy images revealed a significant decrease in slate features which was associated with the basal-plane stacking faults. The transmission electron microscopy images showed that the threading dislocation was greatly reduced after the graded superlattices. Room temperature photoluminescence measurement revealed that the band-edge emission intensity increased with the insertion of the SLs, which suggested reduction in the nonradiative recombination centers.

Effect of 100 MeV Ni9+ ion irradiation on MOCVD grown n-GaN

Metal-organic chemical vapor deposition (MOCVD) grown n-type Gallium nitride (GaN) has been irradiated with 100MeV Ni9+ ions at room temperature. Atomic force microscopy (AFM) images show the nano-clusters' formation upon irradiation and the irradiated GaN surface roughness increases with the increasing ion fluences. High-resolution X-ray diffraction (HR-XRD) analysis reveals the formation of Ga2O3 due to the interface mixing of GaN/Al2O3 upon irradiation. FWHM values of GaN (0002) increases due to the lattice disorder. Photoluminescence studies show reduced band edge emission and yellow luminescence (YL) intensity with the increasing ion fluences. Change in the band gap energy between 3.38 and 3.04eV was measured by UV-visible optical absorption spectrum on increasing the ion fluences.

Defect Reduction in Semipolar (11-22) GaN Grown on m-Sapphire Using Epitaxial Lateral Overgrowth

We investigated the optical and the crystal qualities of epitaxial lateral overgrown (ELO) semipolar (11-22) GaN produced using three growth steps: growth of the seed layer for the [11-20] a-direction, the formation of a semipolar (11-22) plane, and the lateral coalescence step with a [11-22] direction. Fully-coalesced ELO-GaN had a good surface morphology and a very low defect density of ∼5.5 × 105 cm−2, which was smaller than that of conventional c-plane ELO-GaN. In spite of the generation of some crystallographic tilts, the crystal properties were significantly enhanced as a result of the ELO process. The cathodeluminescence intensity of band edge emission in the low defect region was about 3.1 times higher than in the highly defective region, indicating that the optical properties of semipolar GaN could be significantly enhanced by reducing defects.

Disorder driven optical processes in nanocrystalline ZnO

A systematic study on the modification of optical properties in mechanically milled ZnO powder has been reported here. The average grain size of the powder becomes ∼20 nm within 4 h of milling. Fluctuations of average grain size have been noticed at the initial stage of milling (within 15 min). Changes in grain morphology with milling have also been noticed in scanning electron micrographs of the samples. Room temperature optical absorption data shows a systematic red shift of absorption band edge (∼3.25 eV). The band tail parameter (extracted from the optical absorption just below the band edge) follows a simple exponential relation with the inverse of the average grain size. Significant increase of the band tail parameter has been noticed at low grain size regime. It has been analyzed that high values of band tail parameter is a representative of V Zn– V O type divacancy clusters. Room temperature photoluminescence spectra show decrease (except for 120 min milling) of band edge emission intensity with increase of milling time. Subsequent decrease of sub-band edge emission is, however, less prominent. The variation of PL intensity ratio (intensity at band edge peak with that at 2.3 eV) follows simple exponential decrease with the increase of band tail parameter. This indeed shows that band edge emission in ZnO is related with the overall disorder in the system, not grain size induced only.

Optimized ICP etching process for fabrication of oblique GaN sidewall and its application in LED

Inductively coupled plasma (ICP) etching of GaN is systemically investigated by changing ICP power/RF bias power, operating pressure, and Cl2/BCl3 gas mixing ratio. The hexagonal etch pits related to screw dislocation existing along GaN epitaxial layer were observed on the etched GaN surface after ICP etching. The intensity of band-edge emission is significantly reduced from the etched n-GaN surface, which reveals that plasma-induced damage are generated after ICP etching. The oblique sidewall is transferred into GaN using a combination of Cl2/BCl3 plasma chemistry and hard mask SiO2. By adjusting ICP etching process parameters, oblique sidewalls with various oblique angles can be formed, allowing for conformal metal lines coverage across the mesa structures, which can play an important role in the interconnection of multiple microchips for light emitting diodes (LEDs) fabrication.

Highly c-axis oriented GaN films grown on free-standing diamond substrates for high-power devices

GaN films with highly c-axis preferred orientation are deposited on free-standing thick diamond films by low temperature electron cyclotron resonance plasma enhanced metal organic chemical vapor deposition (ECR-PEMOCVD). The TMGa and N 2 are applied as precursors of Ga and N, respectively. The quality of as-grown GaN films are systematically investigated as a function of deposition temperature by means of X-ray diffraction (XRD) analysis, Hall Effect measurement (HL), room temperature photoluminescence (PL) and atomic force microscopy (AFM). The results show that the dense and uniformed GaN films with highly c-axis preferred orientation are successfully achieved on free-standing diamond substrates under optimized deposition temperature of 400 °C, and the room temperature PL spectra of the optimized GaN film show a intense ultraviolet near band edge emission and a weak yellow luminescence. The obtained GaN/diamond structure has great potential for the development of high-power semiconductor devices due to its excellent heat dissipation nature.

Post-Annealing Effects on Properties of ZnO Nanorods Grown on Au Seed Layers

ZnO nanorods were grown by hydrothermal method. Two kinds of seed layers, Au film and island seed layers were prepared to investigate the effect of seed layer on ZnO nanorods. The ZnO nanorod on Au island seed layer has more unifom diameter and higher density compared to that of ZnO nanorod on Au film seed layer. The ZnO nanorods on Au island seed layer were annealed at various temperatures ranging from 300 to <TEX>$850^{\circ}C$</TEX>. The pinholes at the surface of the ZnO nanorods is formed as the annealing temperature is increased. It is noted that the pyramid structure on the surface of ZnO nanorod is observed at <TEX>$850^{\circ}C$</TEX>. The intensity of ZnO (002) diffraction peak in X-ray diffraction pattern and intensity of near band edge emission (NBE) peak in photoluminescence (PL) are increased as the ZnO nanorods were annealed at the temperature of <TEX>$300^{\circ}C$</TEX>.

Investigation of yellow luminescence intensity of N-polar unintentionally doped GaN**

This paper reports that the yellow luminescence intensity of N-polar GaN Epi-layers is much lower than that of Ga-polar ones due to the inverse polarity, and reduces drastically in the N-polar unintentionally-doped GaN after etching in KOH solution. The ratio of yellow luminescence intensity to band-edge emission intensity decreases sharply with the etching time. The full width at half maximum of x-ray diffraction of (10–12) plane falls sharply after etching, and the surface morphology characterized by scanning electron microscope shows a rough surface that changes with the etching time. The mechanism for the generation of the yellow luminescence are explained in details.

Characterization of transparent PVA/amino acid complex capped ZnO nano composite films

Zinc oxide (ZnO) nanocrystals capped with the amino acid complex, L-histidine tartrate, having an average size of about 10 nm were prepared by wet chemical route at room temperature and blended with polyvinyl alcohol (PVA) in various weight percentages by solution mixing. The structural and optical properties of the capped zinc oxide nano crystals were investigated by X-ray Diffraction, UV-visible NIR spectroscopy and photoluminescence (PL)studies. Highly transparent nano composite thin films of the capped ZnO in PVA were prepared by spin coating technique. The UV-visible absorption spectra of these films show that the λ onset of absorption of capped ZnO films exhibits red shift and the PL spectra of films show an increase in intensity with increase in the weight percentage of ZnO added. The characteristic green–yellow emission of the ZnO quantum dots was found to be quenched. It is inferred that surface modification stabilizes the quantum dots in the polymer and enhances the intensity of band edge emission.

Help nanorods “stand” on microsubstrate to form hierarchical ZnO nanorod-nanosheet architectures

ZnO nanorods were helped to “stand” vertically on microsubstrates by an interesting seed-mediated approach. Taking ZnO nanosheets as the microsubstrates, ZnO nanorods can grow vertically, not lying horizontally, on the facets with the aid of a seed layer precoating to form hierarchical ZnO nanorod-nanosheet architectures. The diameter as well as the length of the standing nanorods can be controlled effectively by adjusting the growth time and ammonia amount in the growth solution. The precoated seed layer has been found to be the key factor in determining the resultant morphology. Room-temperature photoluminescence studies revealed that the near band-edge (NBE) emission intensity of the hierarchical architectures was increased greatly compared with that of bare ZnO nanosheets. These special hierarchical architectures make them appealing for future microdevice applications.

Deposition of Zinc Oxide Thin Films Using a Surface Reaction on Platinum Nanoparticles

ABSTRACTA new chemical vapor deposition method for the growth of ZnO films using the reaction between dimethylzinc (DMZn) and thermally excited H2O produced by a Pt-catalyzed H2–O2 reaction was investigated. The thermally excited H2O molecules formed by the exothermic reaction of H2 and O2 on the catalyst were ejected from a fine nozzle into the reaction zone and allowed to collide with DMZn ejected from another fine nozzle. The ZnO films were grown directly on a-plane (11-20) sapphire substrates at substrate temperatures of 773-873 K with no buffer layer. X-ray diffraction patterns exhibited intense (0002) and (0004) peaks from the ZnO(0001) index plane. The smallest full width at half maximum (FWHM) value of the ω- rocking curve of ZnO(0002) was less than 0.1º. The largest Hall mobility and the smallest residual carrier concentration of the ZnO films were 169 cm2V−1s−1 and 1.7×1017 cm−3, respectively. Photoluminescence (PL) spectra at room temperature exhibited a band edge emission at 3.29 eV, with a FWHM of 104 meV. Green luminescence from deeper levels was generally about 1.5% of the band edge emission intensity. PL spectra at 5 K showed a strong emission peak at 3.3603 eV, attributed to the neutral donor-bound exciton Dºx. The FWHM was as low as 1.0 meV. Free exciton emissions also appeared at 3.3757 eV (FXA, n=1) and 3.4221 eV (FXA, n=2).