Improvement Of Crystalline Quality Research Articles

Study on GaN-based light emitting diodes grown on 4-in. Si (111) substrate

GaN-based light emitting diodes (LEDs) grown on 4-in. Si (111) substrate by metal–organic chemical vapor deposition have been systematically characterized. The significantly smooth surface of the sample has been confirmed by an atomic force microscope. In X-ray diffraction measurement, two kinds of fringe peaks were observed, which are believed to originate from the strained-layer superlattice (SLS) and multiple quantum wells. Moreover, from cross-sectional scanning electron microscope images of the sample, it is found that the interfaces of SLS are smooth. The 100-pair AlN/GaN SLS can be employed to modulate the strain between the GaN layer and substrate, resulting in the improvement of GaN crystalline quality. The full width at half maximum of ω-scan of the GaN (0002) diffraction is around 630ʺ. In addition, the device properties have been investigated in detail, and the maximum light output power can reach 2.02mW with a high saturation injection current of 320mA.

Progress on HgCdTe MBE for the application of IRFPAs

The progress on molecularbeam epitaxial growth of HgCdTe focusing on the requirements by the 3rd generation of infrared focal plane arrays are described. As large format HgCdTe IRFPAs demanded, the efforts on HgCdTe MBE on large area alternative substrates were presented, such as the improvement of crystalline quality, surface morphology, as well as Cd composition umiformity. The device fabrication on 512×512 MW and SW IRFPAs well demonstrated the performance of HgCdTe epi-layer. The studies of HgCdTe MBE on lattice-match substrates also were described, which were essential for the fabrication of LW and APD HgCdTe detectors. After growth condition optimized, the uniformity of epi-layer was greatly improved. On 10 mm HgCdTe/ZnCdTe( x =0.22), a typical value of FWHM of (422) X-ray diffraction was found to be less than 25 arcsec, as well as the best results of EPD reached 3×10 4 cm - 2 .

Crystalline quality improvement of GaN coalesced using nano‐PSS with void‐embedded nanostructure by MOVPE

AbstractThe nanoscale patterned sapphire substrates (NPSS) were prepared combining nanoimprint lithography and dry/wet etching technology. The NPSS was patterned with 300 nm diameter, 330 nm depth, 550 nm periodicity hexagonal close packed pillars. Coalescence of GaN has been achieved on the NPSS using MOVPE. The growth mode and conditions were discussed in detail, which was different from that of GaN epitaxial on planar sapphire. The AFM images showed the GaN on NPSS had a smoother surface, and the XRD results showed that the crystal quality had been improved by the NPSS. The threading dislocation density (TDD) suppression was observed in transmission electron microscopy. Depth‐resolved CL measurements were used to quantify the redshift in terms of bandgap renormalization and strain effects. It was supposed that the compressive stress in the GaN layers was changed gradually towards the growth surface. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

AlGaN/GaN High Electron Mobility Transistors with Multi‐MgxNy/GaN Buffer

We report the fabrication of AlGaN/GaN high electron mobility transistors with multi‐MgxNy/GaN buffer. Compared with conventional HEMT devices with a low‐temperature GaN buffer, smaller gate and source‐drain leakage current could be achieved with this new buffer design. Consequently, the electron mobility was larger for the proposed device due to the reduction of defect density and the corresponding improvement of crystalline quality as result of using the multi‐MgxNy/GaN buffer.

Fabrication and characterization of ZnS:(Cu,Al) thin film phosphors on glass substrates by pulsed laser deposition

We fabricated ZnS:(Cu,Al) thin film phosphors on glass substrates by pulsed laser deposition (PLD) using a KrF 248nm laser in a H2S gas flow. Firstly, we investigated effects of annealing temperature, stress and crystallinity on photoluminescence properties of powder samples. For thin films, photoluminescence intensity increased with increasing the substrate temperature up to 700°C and then decreased. The former increase is attributed to improvement of crystalline quality and strain, while the latter degradation is attributed to reduction of the Cu acceptors. Intense photoluminescence is observed under an ambient light when excited by 325nm monochromated light from a fluorescence spectrophotometer. It was confirmed that the PLD process stabilized the wurtzite structure, which is not stable in bulk samples at the thermal equilibrium.

High growth rate deposition of phosphorus-doped homoepitaxial (001) diamond films for deep-ultraviolet light emitting device

We have characterized P-doped diamond films grown on vicinal (001) high-pressure/high-temperature-synthesized substrates with high growth rate (more than 1μm/h) using a conventional microwave-plasma chemical vapor deposition apparatus with a quartz-tube chamber, and have fabricated deep-ultraviolet (DUV) light-emitting diodes composed of P-doped (n), undoped (i) and B-doped (p) layers. The P-doped diamond films grown at 1160°C had the clear cathodoluminescence (CL) peaks at the wavelength of 232.5 and 239nm which were attributed to the radiative recombination of P-donor bound excitons associated with no phonon emission and a transverse optical phonon emission, respectively. A sharp emission peak was also observed at the wavelength of 235nm, which was attributed to radiative recombination of a free exciton. These CL features demonstrated rather high quality of the homoepitaxially grown layers. The p–i–n junction whose P-doped layer was grown at 1160°C had a clear diode characteristic with the rectification ratio of about 103 at ±20V. However, some broad CL peaks were additionally observed in the UV and visible regions, indicating that further improvement of crystalline quality of the P-doped diamond film is required for observing monochromatic DUV light emissions.

EFFECTS OF Au ON THE GROWTH OF ZnO NANOSTRUCTURES ON Si BY MOCVD

The effects of Au on the growth of ZnO nanostructures on Si by metal organic chemical vapor deposition (MOCVD) at a relatively low temperature (450°C) were investigated. The experimental results showed that Au nanoparticles played a critical role during the growth of the ZnO nanostructures and affected their morphology and optical properties. It was found that Au nanoparticles particularly affected the nucleation of ZnO nanostructures during the growth process and the Au -assisted growth mechanism of ZnO nanostructures should be ascribed to the vapor–solid (VS) mechanism. The formation of a nanoneedle may be attributed to a more reactive interface between Au and ZnO , which leads to more zinc gaseous species absorbed near the interface. Different nucleation sites on ZnO nuclei resulted in the disorder of ZnO nanoneedles. Moreover, the crystalline quality of nano- ZnO was improved due to the presence of Au , according to the smaller full width at half maximum (FWHM) of the low-temperature exciton emission. We confirmed that ZnO nanoneedles showed better crystalline quality than ZnO nanorods through the HRTEM images and the SAED patterns. The reason for the improvement of the crystalline quality of nano- ZnO may be due to the less lattice mismatch.

Effect of swift heavy ion irradiation on lead sulfide quantum dots embedded in polyvinyl alcohol

The present study compares structural and optical modifications of lead sulfide (PbS) quantum dots under swift heavy ion (SHI) irradiation. PbS quantum dots are prepared following an inexpensive chemical route using polyvinyl alcohol as the dielectric host matrix. SHI irradiation of the samples is carried out with 100 MeV Si7+ ion beam with fluences in the range from 1×1011 to 3×1013 ions cm−2 and their structural and optical properties, before and after irradiation, are compared by X-ray diffraction (XRD), photoluminescence (PL) and UV–Vis spectroscopy. The structural parameters of PbS quantum dots are studied by X-ray line profile analysis using the Williamson–Hall plot. The values of average crystallite sizes are found to vary from 8 to 18 nm. XRD studies confirmed the formation of the cubic nanocrystalline PbS. Improvement of crystalline quality for lower fluences is exhibited by an increase in the X-ray intensities of the films. The UV–Vis absorption spectra reveal blue shift relative to the bulk material. Size enhancement of PbS quantum dots after irradiation has been indicated in XRD line profiles of the samples which has also been supported by optical absorption spectra. PL studies of all the samples are carried out with excitation wavelength 350 nm. A broad PL emission with peak centered at 473 nm is observed in pristine as well as all the irradiated samples. PL study also shows that PL intensity increases with ion fluences.

Characterization of SnO2 obtained from the thermal oxidation of vacuum evaporated Sn thin films

Tin oxide has been prepared by thermal oxidation of evaporated tin thin films onto pyrex glass substrates. Films oxidation was achieved in air at a temperature of 600°C with varied duration from 20min to 3h. Structural, optical and electrical properties of the films were characterized by means of X-ray diffraction, UV–vis spectroscopy and electrical resistivity measurements respectively. The X-ray analysis revealed the transformation of Sn into SnO2 with preferential orientation along (101) plans. No intermediate phases such as SnO and Sn3O4 were evidenced. It was also found that the SnO2 crystallites orientation changed with the annealing time due to the strain energy effect. Both band gap energy and electrical resistivity decrease with annealing time due to the crystalline quality improvement and films densification. We have noticed that oxidation at 600°C for 3h leads to transparent and conductive films with suitable properties for photovoltaic applications.

Structural and optical properties of Cerium doped Calcium Fluoride nanoparticles prepared by copercipitation

Calcium fluoride nanoparticles with special amount of cerium ion (%1) were synthesized by coprecipitation. Fourier transforms infrared spectroscopy (FTIR), X-ray diffraction (XRD), absorption and photoluminescence (PL) from CaF2Ce nanoparticles has been investigated. FTIR spectra reveal that there are some amounts of OH, NO3 and other organic functional groups on the particle surfaces before the post annealing process. X-ray diffraction results indicate an improvement in crystalline quality with annealing. Intensity of fluorescent bands as well as the absorption edge of the prepared nanoparticles is also very relevant to the annealing temperature.

Structural characterization and magnetic properties of perpendicularly magnetized MnAl films grown by molecular-beam epitaxy

Perpendicularly magnetized MnAlx thin films with different Al contents have been epitaxied on GaAs (001) substrates by a molecular-beam epitaxy system. Crystalline quality of MnAlx films is closely related to Al content, and magnetic properties of MnAlx films are improved as crystalline quality of MnAlx fims increases. MnAl0.9 film shows the best crystalline quality and magnetic property among all samples. So we grew MnAl0.9 films at different growth temperatures to further optimize growth conditions. With increasing temperature, the chemical order parameter increases and the full width at half maximum of the τ(002) peak decreases, which reveal the improvement of crystalline quality. Higher perpendicular magnetization, coercivity and magnetic anisotropy are found as growth temperature increases. The best crystalline quality and perpendicularly magnetized properties are found at 350℃; the coercivity of 8.3 kOe, saturation magnetization of 265 emu/cm3, Mr/Ms of 0.933 and perpendicular magnetic anisotropy constant of 7.74 Merg/cm3 are achieved. These tunable perpendicularly magnetized properties and good compatibility associated with semiconductor materials make the noble-metal-free and rare-earth-free MnAl films attractive in the application of spintronic devices.

Enhancement of near-band edge photoluminescence of ZnO film buffered with TiN

ZnO films were deposited on Si substrate by RF-sputtering using titanium nitride (TiN) as buffer layer that was deposited at different thicknesses: 160 and 2290nm. Despite the lattice mismatch of up to 6.35% between ZnO and TiN, the ZnO films deposited on TiN buffer layers show enhanced near-band-edge photoluminescence (PL) emission at room temperature which is two times higher of magnitude than those grown directly on Si. The PL enhancement intensity, provided by TiN buffer introduction, is attributed to the improvement of ZnO crystalline quality and stoichiometry. The use of a good electrical conductor which has high thermal stability like TiN as buffer layer for the blue emission enhancement of ZnO would make it promising for optoelectronic applications.

Structural and optical properties of pulsed laser deposited ZnO thin films

Nanocrystalline ZnO thin films were grown by means of pulsed laser deposition. The ablation process was carried out at relatively low background oxygen gas pressure (10 Pa) and by varying the substrate temperature up to 600 °C. Information on the structural and morphological properties of the deposited thin films have been obtained by means of X-ray photoelectron, Raman spectroscopies, X-ray diffraction (XRD) and atomic force microscopy (AFM). The results showed that all the deposited films are sub-stoichiometric in oxygen and with a hexagonal wurtzite crystalline structure, characterized by features of some tens of nanometers in size. An improvement of the films' crystalline quality was observed for the deposition temperature of 300 °C while the further increase of the deposition temperature up to 600 °C induces a worsening of the material's structural properties with the development of a large amount of nanoparticle's clusters. The analysis of the XRD patterns shows a growth crystallographic preferential direction as a function of the deposition temperature, in agreement with the appearance of the only E2 optical phonon mode in the Raman spectra. Such findings are compatible with the changes observed in the photoluminescent (PL) optical response and was related to the modification of the ZnO thin film structural quality.

Enhanced band-gap emission in ZnO Nanocaves by two-step thermal oxidation Zn film

ZnO Nanocaves are grown on p-type silicon substrate by two-step thermal oxidation Zn films. The samples are characterized by X-ray diffraction, scanning electron microcopy and photoluminescence. We observe that the band-gap emission can be great improvement while the defects emission is suppressed to noise level after the ZnO Nanocaves annealed in nitrogen ambient at 600°C for 1h. The improvement of crystalline quality, Au nanoparticles and ZnO Nanocaves layer should responsible for the giant enhancement of band-gap emission and the disappearance of defects emission. The results indicate that the ZnO Nanocaves have great potential applications in optoelectronics such as laser emitting devices, laser diodes and spintronics.

Effect of deposition temperature on the properties of Al-doped ZnO films prepared by pulsed DC magnetron sputtering for transparent electrodes in thin-film solar cells

A simple but scalable approach to the production of surface-textured Al-doped ZnO(AZO) films for low-cost transparent electrode applications in thin-film solar cells is introduced in this study by combining pulsed dc magnetron sputtering (PDMS) with wet etching in sequence. First, structural, electrical, and optical properties of the AZO films prepared by a PDMS were investigated as functions of deposition temperature to obtain transparent electrode films that can be used as indium-free alternative to ITO electrodes. Increase in the deposition temperature to 230°C accompanied the improvement in crystalline quality and doping efficiency, which enabled the lowest electrical resistivity of 4.16×10−4Ωcm with the carrier concentration of 1.65×1021cm−3 and Hall mobility of 11.3cm2/Vs. The wet etching of the films in a diluted HCl solution resulted in surface roughening via the formation of crater-like structures without significant degradation in the electrical properties, which is responsible for the enhanced light scattering capability required for anti-reflective electrodes in thin film solar cells.

Growth of a-plane GaN on r-plane sapphire by self-patterned nanoscale epitaxial lateral overgrowth

We report on the use of a novel technique to grow the nonpolar a-plane GaN on r-plane sapphire by metal–organic chemical vapor deposition. A thin InGaN interlayer was deposited on the substrate followed by a low temperature (LT) GaN buffer layer. A stripe-like template was obtained by annealing the LT GaN/InGaN layers at 1100°C for 2min. This special template facilitated the nanoscale epitaxial lateral overgrowth of a-plane GaN. Scanning electron microscopy shows that the surface morphology was rather flat for a 1μm-thick sample. The improvement in crystalline quality was also demonstrated by high-resolution x-ray diffraction, room temperature Raman spectroscopy and photoluminescence measurements. Compared with the traditional epitaxial lateral overgrowth technique, our technique greatly simplified the template preparing process and the crystalline quality of a-plane GaN was improved.

High resolution X-ray diffraction study of InAs layers grown with and without bismuth flow on GaAs substrates by metalorganic vapor phase epitaxy

InAs layers were grown with and without bismuth flow by atmospheric pressure metalorganic vapor phase epitaxy on exactly oriented, 2° and 10° misoriented (100) GaAs substrates. Structural analysis was carried out using high resolution X-ray diffraction. Without bismuth flow, only InAs layers grown on 10° misoriented substrates exhibit a mosaic structure. Layers grown on exactly oriented and 2° misoriented substrates show large full widths at half maxima of their diffraction rocking curves. Growing InAs under bismuth flow leads to the reduction of this full width indicating a clear improvement of their structural quality. Particularly for samples grown on 10° misoriented substrates, a complete disappearance of the mosaic structure was obtained. The crystalline quality improvement is attributed to the contribution of Bi nanodots in relieving strain.

Optical Characteristic Improvement of Neodymium-Doped Lanthanum Fluoride Thin Films Grown by Pulsed Laser Deposition for Vacuum Ultraviolet Application

Neodymium-doped lanthanum fluoride (Nd3+:LaF3) thin films were successfully grown on MgF2(001) substrates by pulsed laser deposition. Photoluminescence spectra revealed a dominant peak at 173 nm with a decay time of 7.8 ns, which is similar to the results obtained from a bulk Nd3+:LaF3 crystal. Improvements in crystalline quality and vacuum ultraviolet (VUV) luminescence quantum efficiency were achieved by substrate heating, with optimum results being obtained at 400 °C. These results would open up possibilities in the development of a light-emitting device operating in the VUV region.

Optical Characteristic Improvement of Neodymium-Doped Lanthanum Fluoride Thin Films Grown by Pulsed Laser Deposition for Vacuum Ultraviolet Application

Neodymium-doped lanthanum fluoride (Nd3+:LaF3) thin films were successfully grown on MgF2(001) substrates by pulsed laser deposition. Photoluminescence spectra revealed a dominant peak at 173 nm with a decay time of 7.8 ns, which is similar to the results obtained from a bulk Nd3+:LaF3 crystal. Improvements in crystalline quality and vacuum ultraviolet (VUV) luminescence quantum efficiency were achieved by substrate heating, with optimum results being obtained at 400 °C. These results would open up possibilities in the development of a light-emitting device operating in the VUV region.

High quality GaN epilayers grown on carbon nanotube patterned sapphire substrate by metal–organic vapor phase epitaxy

Aligned carbon nanotube films were utilized to pattern a c-plane sapphire substrate. High quality GaN was obtained by epitaxy growth on carbon nanotube patterned sapphire substrates (CPSS) by a metal–organic vapor phase epitaxy system. Results of X-ray diffraction and transmission electron microscopy (TEM) confirmed the improvement of crystalline quality of GaN by introduction of the carbon nanotube film. A large number of voids with sizes of tens to hundreds of nanometers were observed on carbon nanotubes by scanning electron microscopy (SEM) and TEM. The dislocation annihilation around voids was believed to be helpful for the reduction of dislocation density. Performance of light-emitting diodes manufactured on CPSS was also clearly improved, as 15% enhancement of luminous intensity at 20 mA, and 50% delay of saturation current from 80 mA to 120 mA.