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This study presents a straightforward strategy that embeds a graphene interlayer between InGaN-based light-emitting diodes (InGaN LEDs) and patterned sapphire substrate (PSS substrate) for substantial improving device performances of wavelength stability and heat dissipation. The InGaN LEDs on the graphene interlayer/PSS substrate (Gr-LED) have lower piezoelectric fields of 1.16 MV/cm than 1.60 MV/cm for InGaN LEDs on the PSS substrate (Ref-LED). The low piezoelectric field diminishes the screen of the polarization field resulting in a stable electroluminescence peak wavelength. At 100 mA driving current, the chip temperature of Gr-LED shows a decrease in around 24.4 °C relative to Ref-LED. The thermal resistances measured in a vacuum environment are 351 and 128 °C/W for Ref-LED and Gr-LED, respectively. The low thermal resistance of Gr-LED is believed to be due to a low misfit dislocation density of the aluminum nitride nucleation layer that increases the vertical direction of heat transport to PSS. This work demonstrates that the graphene/PSS substrate is a promising substrate for high-power InGaN LEDs.

Displacement in the Raman shift of conventional Raman spectroscopy usually includes an increase in the number of material layers or a strain variation. To distinguish them, polarization-resolved Raman spectroscopy provides an additional degree of freedom to resolve it. In this work, strained graphene induced by different substrates on G band and 2D band phonon modes is studied by polarization-resolved Raman spectroscopy and verified by numerical analysis using Raman tensors and optical Jones calculus. For the strained graphene case, polarization properties of scattered light are obviously different from that in the case of unstrained graphene. Deformation of phonon polarization is seen to originate from different strengths of substrate coupling. This research provides an efficient quantitative method to explore any distortion of strain anisotropy in two-dimensional materials, which is important for the development of straintronics in the future.

This study investigates that high-quality GaN thin films can be grown on a few-layer graphene (FLG)/sapphire substrate by embedding a hybrid AlN buffer layer (BL). The hybrid AlN BL is constructed by low-temperature AlN nucleation layer (LT-AlN NL) and high-temperature AlN BL grown respectively by sputtering and metal organic chemical vapor deposition (MOCVD). The high density of edge-type threading dislocation (TD) in the GaN sample without hybrid AlN BL provide current leakage paths, resulting in a symmetric and temperature-independent I-V characteristic curve for a Ni-based Schottky contact. The excellent adhesion and uniform coverage of LT-AlN NL on the FLG layer by sputtering can overcome the nucleation issue and prevent the thermal etching effect of graphene during MOCVD epitaxial process. The edge-type TD density and carbon concentration of the GaN thin films grown on the hybrid AlN BL/FLG/sapphire substrate can be reduced significantly, resulting in a lower intensity of blue, green, and orange luminescences on a 17-K photoluminescence spectrum. The Ni-based Schottky contact with a barrier height of 0.69 eV and leakage current density of 4.38 × 10−6 A/cm2 is obtained, which demonstrates that a high-quality GaN thin films can be grown onto an FLG substrate by embedding a hybrid AlN BL.

Hydrogen is a great candidate fuel to replace fossil fuels in the future since it has high energy density, zero-emission, and renewability. Here, we report our design with p-type Ag2O and n-type Zn(O,S) loaded on mesoporous silica to form SiO2/Ag2O/Zn(O,S) with the nano p–n heterojunction to improve the efficiency of photocatalytic hydrogen evolution reaction (HER). The photocatalysts were systematically characterized to identify their properties. Through the optimization of the Zn(O,S)-loaded amount and position of p-Ag2O, the highest hydrogen production rate of 9200 μmol·g–1·cat·h–1 was achieved by SiO2/Ag2O/Zn(O,S)-0.6 catalyst with corresponding apparent quantum yield of 10.3%, which was about 2.7 times higher than pure Zn(O,S). By placing n-Zn(O,S) of diodes outward was proposed for the electron-rich part to enhance the reduction reaction, while placing p-Ag2O inward for the hole-rich part to modulate the electron concentration and establish the built-in electrical nanofield. The relative mechanism f...

Sb anion-substituted gallium nitride films were fabricated by radio frequency reactive sputtering with single Sb-containing cermet targets with different Sb contents under Ar/N2 atmosphere. n-type GaN films with electron concentration of (1.40 &plusmn, 0.1) &times, 1017 cm&minus, 3 inverted to p-type Sb-GaN with hole concentration of (5.50 &plusmn, 0.3) &times, 3. The bandgap energy of Sb anion-added Sb-GaN films decreased from 3.20 to 2.72 eV with increasing Sb concentration. The formation of p-type Sb-GaN is attributed to the formation of Ga vacancy at higher Sb concentration. The coexistence of Sb at the Ga cation site and N anion site is an interesting and important result, as GaNSb had been well developed for highly mismatched alloys. The hetero-junction with p-type Sb-GaN/n-Si diodes was all formed by radio frequency (RF) reactive sputtering technology. The electrical characteristics of Sb-GaN diode devices were investigated from &minus, 20 to 20 V at room temperature (RT).

Carbon-doped GaN (GaN:C) Schottky diodes are prepared by controlling the destruction status of the graphene interlayer (GI) on the substrate. The GI without a sputtered AlN capping layer (CL) was destroyed because of ammonia precursor etching behavior in a high-temperature epitaxy. The damaged GI, like nanographite as a solid-state carbon doping source, incorporated the epitaxial growth of the GaN layer. The secondary ion mass spectroscopy depth profile indicated that the carbon content in the GaN layer can be tuned further by optimizing the sputtering temperature of AlN CL because of the better capping ability of high crystalline quality AlN CL on GI being achieved at higher temperature. The edge-type threading dislocation density and carbon concentration of the GaN:C layer with an embedded 550 °C-grown AlN CL on a GI substrate can be significantly reduced to 2.28 × 10

This study grew nitrogen doped ultrananocrystalline diamond (N-UNCD) conductive layer on InGaN-based light emitting diodes (LEDs) using the concave nanopattern (CNP) enhanced nucleation. The low nucleation density of the N-UNCD on bare LEDs (Br-LED) resulted in an island morphology. The nucleation density of the N-UNCD increased significantly from 1.8 × 108 cm−2 for Br-LED to 3.6 × 109 cm−2 for LEDs with a CNP density of 6.1 × 109 cm−2 (CNP-LED). The N-UNCD preferred to nucleate inside the CNP during the initial growth stage. The N-UNCD islands then merged and grew laterally to form a continuous thin film within a thickness of 300 nm. The N-UNCD/CNP-LED exhibited a stable electroluminescence peak wavelength of ~447.1 nm in the injection current range of 10–100 mA. The decrease in the compressive stress due to removing the p-GaN layer (i.e. the CNP structure) resulted in a quantum confined Stark effect (QCSE) mitigation in the multi-quantum wells (MQWs). In addition, the lower turn on voltage caused a lower electric field in the MQWs and diminished the screen of the QCSE. The N-UNCD prepared by the proposed nucleation technique demonstrated a promising conductive layer for InGaN-based LEDs.

This study presents GaN thin films grown on nanoscale-patterned sapphire substrates (NPSSs) with different aspect ratios (ARs) using a homemade metal-organic chemical vapor deposition system. The anodic aluminum oxide (AAO) technique is used to prepare the dry etching mask. The cross-sectional view of the scanning electron microscope image shows that voids exist between the interface of the GaN thin film and the high-AR (i.e. ∼2) NPSS. In contrast, patterns on the low-AR (∼0.7) NPSS are filled full of GaN. The formation of voids on the high-AR NPSS is believed to be due to the enhancement of the lateral growth in the initial growth stage, and the quick-merging GaN thin film blocks the precursors from continuing to supply the bottom of the pattern. The atomic force microscopy images of GaN on bare sapphire show a layer-by-layer surface morphology, which becomes a step-flow surface morphology for GaN on a high-AR NPSS. The edge-type threading dislocation density can be reduced from 7.1 × 10 8 cm −2 for GaN on bare sapphire to 4.9 × 10 8 cm −2 for GaN on a high-AR NPSS. In addition, the carrier mobility increases from 85 cm 2 /Vs for GaN on bare sapphire to 199 cm 2 /Vs for GaN on a high-AR NPSS. However, the increased screw-type threading dislocation density for GaN on a low-AR NPSS is due to the competition of lateral growth on the flat-top patterns and vertical growth on the bottom of the patterns that causes the material quality of the GaN thin film to degenerate. Thus, the experimental results indicate that the AR of the particular patterning of a NPSS plays a crucial role in achieving GaN thin film with a high crystalline quality.

This study presents a low temperature annealing process to improve the photoresponse of the ZnO nanorod (ZnO NR)/nanodiamond film (NDF)/glass substrate bilayer structure of an ultraviolet (UV) photodetector. The wurtzite structure of ZnO NRs with a diameter/length of 160 nm/2.7 μm was grown by the hydrothermal method on a 700-nm-thick NDF layer synthesized by a microwave plasma chemical vapor deposition system. A photoluminescence spectra showing a near-band-edge emission at 3.28 eV with a shoulder peak at 3.20 eV for the Zn interstitial was observed for the 200 °C annealed sample. The Raman spectra of samples showed an improved 1332 cm − 1 sp 3 diamond quality and reduced the non-diamond components while increasing the annealing temperature. A high photoresponse of 249.6 can be achieved by a 150 °C annealing process, which is about 6 times better than the as-grown sample. The low dark current of 1.76 × 10 − 5 A for the 150 °C annealing sample was believed to be due to the improvement of the interface quality between the NDF and ZnO layer. However, the Zn interstitial generated in the 200 °C annealed sample disrupted the carrier's trapping process, resulting in a lower photocurrent. In addition, this type of defect also caused the interface quality of the ZnO NR/NDF bilayer structure to degrade and increase the dark current. The experimental results indicated that the annealing temperature plays an important role in improving the photoresponse of ZnO NR/NDF bilayer structured UV photodetector.

The promising electronic and optical properties of InGaN alloy, such as high two-dimensional electron gas (2DEG) electron mobility, wide tunability of photon emission energy from 0.7 to 3.4 eV, have givenrisetonumbersofapplications inthepastdecade. Theseinclude highpowerandhighelectronmobilitytransistor(HEMT),visiblelight emitting devices, tandem solar cell, and solid state lighting. 1‐7 To realize, a method that capable of growing device-quality III-V nitrides is essential. Among all growth methods, metal-organic vapor-phase epitaxy (MOVPE) has proved itself to be the most reliable and major tool to produce these devices with mass-production feasibility. However, difficulties arise for this method in preparing high In content InxGa1−xN film (x > 30%) with acceptable optical and electrical properties. This stems mainly from the intrinsic properties of material itself, such as thermal instability of InN, wide miscibility gap, and large thermal- and lattice-mismatch between InGaN layer and underlying GaN base layer, and poor cracking efficiency of NH3 .T hese lead to generations of considerable amounts of nitrogen vacancies, pits, In-rich clusters and/or structure defects in the film to deteriorate its material properties. 8‐17 Inourpreviouswork,wehaveemployedaso-calledthetwo-heater MOVPE horizontal reactor, consisting of a pair of paralleled ceiling and substrate heaters, 18 to grow InGaN films. We demonstrated its ability in preparing high In-content thick InGaN films with emission wavelength extended into infrared color region, and more importantly exhibiting good optical-quality and high spectral uniformity. For in

A self-corrected sensing method is proposed and studied using a surface plasmon resonance scheme with metal–dielectric–metal (MDM) structure. The surface plasmon wave (SPW) modes supported by the MDM structure couple further to the SPW at sensing interface to form two kinds of hybridized SPWs. One of them is highly sensitive to the change in the optical constant of analyte solution, whereas the other is barely influenced by the change and can then be a reference signal to remove any drift. By using an intensity interrogation, the entirely different change in the intensity of reflected light at the corresponding resonance angles of the two hybridized SPWs were demonstrated both numerically and experimentally.

In the present study, the structural and nanomechanical characteristics of InN thin films are investigated by means of X-ray diffraction (XRD) and nanoindentation techniques. The InN thin films were deposited on the GaN/(0 0 0 1) sapphire substrates by using the metal-organic chemical-vapor deposition (MOCVD) system. The XRD results indicated that InN thin films are pure hexagonal phase with the (0 0 2)-oriented characteristics. The “pop-ins” phenomena observed in the load–displacement curve during loading are due primarily to the activities of dislocation nucleation and propagation. Based on this scenario, an energetic estimation of dislocation nucleation is made. In addition, the hardness and Young’s modulus of InN thin film are obtained by a Berkovich nanoindenter operated with the continuous contact stiffness measurements (CSM) option. The obtained values of the hardness and Young’s modulus are 4.2 ± 0.1 GPa and 152.5 ± 3.9 GPa, respectively.

The emission properties of an organic layer embedded in a metal–organic–metal (MOM) structure were investigated. A partially radiative odd-SPW as well as a non-radiative even-SPW modes are supported by hybridization of the SPW modes on the opposite organic/metal interface in the structure. Because of the competition by this radiative SPW, the population of excitons that recombine to form non-radiative SPW should be reduced. This may account for why the photoluminescence intensity of the MOM sample is higher than that of an organic–metal sample even though the MOM sample has an additional metal layer that should intuitively act as a filter.

The effects of Mg-containing precursor flow rate on the nanomechanical characteristics of the Mg-doped AlGaN thin films are investigated by using nanoindentation technique. The Mg-doped AlGaN thin films were deposited on sapphire substrates by metal–organic chemical–vapor deposition (MOCVD) with the various flow rates of 100, 200 and 600 sccm of bis-(cyclopentadienyl)-magnesium (Cp 2 Mg) precursor. Nanoindentation results revealed apparent discontinuities (so-called multiple “pop-ins”) in the load–displacement curves, while no discontinuity was observed in the unloading segment of the load–displacement curves. The hardness and Young’s modulus of Mg-doped AlGaN thin films measured by a Berkovich nanoindenter operated with the continuous contact stiffness measurements (CSM) method indicated that both mechanical properties increased with increasing the Mg concentration with the hardness and the Young’s modulus being increased from 20.62 ± 0.53 to 24.17 ± 0.72 GPa and from 317.48 ± 7.82 to 364.63 ± 17.62 GPa, respectively, as the Mg concentration raised from 100 to 600 sccm.

Good-optical-quality, thick In x Ga 1− x N films with high In content were grown using a homemade two-heater metal-organic chemical vapor deposition system. By varying the growth temperature, it was found that the In composition of the InGaN epilayer varied from 18 to 59% as the substrate temperature decreased from 750 to 625 °C. Our results show that the optical properties in terms of the emission peak wavelength and linewidth are uniformly distributed throughout the entire 2 in. wafer for the x =0.40 InGaN sample. The resultant mean peak wavelength and FWHM are 808±6 nm and 229±18 meV, respectively, at 18 K. In addition, for the InGaN film grown at 625 °C, a noticeable decrease in the In composition occurred when the ceiling temperature was >800 °C, indicative of the occurrence of parasitic reactions in the gas phase.

This paper reports the conversion efficiency of thin-film a-Si:H solar cells grown on a 304 stainless steel (SS) substrate with varying molybdenum (Mo) buffer layer thickness. A conversion efficiency of 4.12% was achieved for thin-film a-Si:H solar cells grown on a 2-μm-thick Mo film/304BA SS substrate. The current density–voltage ( J – V ) curve shows that the shunt resistance ( R sh )/series resistance ( R s ) were 82.5/58.1 Ω and 290.6/16.7 Ω for cells grown on the raw 304BA SS substrate and the 2-μm-thick Mo film/304BA SS substrate, respectively. The results from the secondary ion mass spectroscopy (SIMS) indicated that a Mo film thickness exceeding 2-μm could effectively prevent iron (Fe) impurity diffusion from the SS substrate into the thin-film a-Si:H solar cells. Fe impurities in a Si thin film forms deep-level defects that degenerates the junction interface in each layer of the cell structure thereby decreasing the R sh and increasing the R s of the solar cell. A high open voltage ( V oc ) and short current density ( J sc ) can be achieved by adding a Mo buffer layer thereby improving the conversion efficiency of the solar cell.

This study reports the electrochemical mechanical polishing (ECMP) technique for 430 stainless steel (SS) substrate to improve the conversion efficiency of hydrogenated amorphous silicon (a-Si:H) thin-film solar cells. The surface roughness of the untreated 430 SS substrate was ∼38 nm and decreased to ∼23 nm after ECMP processing of the 430 SS substrate. The average diffuse reflection rate decreased from 4.2% of the untreated 430 SS substrate to 1.1% for the ECMP processed 430 SS substrate. The analysis of chemical composition of passive layer on the ECMP processed 430 SS substrate was carried out by the X-ray photoelectron spectroscopic. The passive layer of ECMP processed 430 SS substrate is composed of Cr 2 O 3 and Fe 2 O 3 . The SIMS result indicated that the diffusion of iron (Fe) and chromium (Cr) impurities from the 430 SS substrate into the a-Si:H solar cell can be suppressed by the passive layer which was formed by the ECMP process. In particular, the Cr impurity was effectively blocked from diffusing into a-Si:H layer of the ECMP processed 430 SS substrate. The decreased series resistance and increased shunt resistance for the a-Si:H solar cells on the ECMP processed 430 SS substrate, due to a reduction in the Cr and Fe deep-level defect density, resulted in the cell conversion efficiency increasing to 5.4%. The ECMP process provides an available surface treatment technique for achieving high conversion efficiency of a-Si:H thin-film solar cells on a 430 SS substrate.

We investigated the effect of La additive on the improvement of light reflectance and thermal stability of Ag contacts on p-GaN. A high reflectance of over 90% at 460 nm wavelength and low specific contact resistivity of 5.5 × 10−5 Ω cm2 were obtained from La-containing Ag contacts annealed at 300 °C for 1 min, which also show better thermal stability than Ag contacts after annealing in air ambient. The experimental results reveal that the addition of La could effectively slow down Ag migration in 〈1 1 1〉 direction during annealing, and thus suppress the Ag agglomeration at elevated temperature, leading to a good ohmic contact with improved high reflectance and thermal stability.

InN quantum dots (QDs) were grown on 1 μm thick GaN/(0001) sapphire substrates by low pressure metal organic chemical vapor deposition. A single crystalline 10-nm thick GaN capping layer was achieved on the InN QDs by the flow-rate modulation epitaxy method at 650 °C. The (002) ω/2θ scans of the X-ray diffraction measurements show that the reduction of the lattice constant with a capping thickness indicate that the GaN capping process exerts a compressive strain on the InN QDs. The residual strain was reduced from 0.245% to − 0.245% as the GaN cap thickness increases from 0 to 20 nm. In addition, the analysis of the photoluminescence peak energy estimated that the free electron concentration (i.e. density of indium (In) vacancy) decreased from 1.62 × 1018 cm− 3 to 1.24 × 1018 cm− 3. The suggestion here is that the increase of the compressive strain on InN QDs due to the increased GaN capping layer thickness provides the high driving force for the interdiffusion of the In atom and gallium (Ga) atoms between the interface of InN QDs and the GaN capping layer. Thus, we believe that more Ga atoms can diffuse from the GaN capping layer and substitute the high density of In vacancy in the InN QDs, resulting in a decrease of the free electron concentration in the InN QDs with the increase in the GaN capping layer thickness.

This paper reported a structural and optical study of single InN/GaN dots with varying GaN cap thickness. The surface morphology of the 10-nm thick GaN capping layer shows a truncated pyramidal shape and changed to a dome shape when the thickness of the capping layer increases to 20-nm. The increase in compressive strain of InN dots with the increase in the cap thickness was analyzed by high resolution x-ray diffraction (HRXRD). A redshift was observed in the photoluminescence (PL) peak energy of the InN dots with increasing GaN capping thickness. The redshift of the PL peak energy of the 20-nm thick GaN capping layer sample was believed to be due to the GaN capping avoids InN decomposition and decreases the surface defect density. In addition, the maximum PL intensity of the 20-nm thick GaN capping layer sample was ∼2.5 times higher than that of the uncapped sample.

A hybrid patterned sapphire substrate (hybrid-PSS) was prepared using an anodic aluminum oxide etching mask to transfer nanopatterns onto a conventional patterned sapphire substrate with microscale patterns (bare-PSS). The threading dislocation (TD) suppression of light-emitting diodes (LEDs) grown on a hybrid-PSS (HP-LED) exhibits a smaller reverse leakage current compared with that of LEDs grown on a bare-PSS (BP-LED). The strain-free GaN buffer layer and fully strained InGaN active layer were evidenced by cross-sectional Raman spectra and reciprocal space mapping of the X-ray diffraction intensity for both samples. The calculated piezoelectric fields for both samples are close, implying that the quantum-confined Stark effect was not a dominant mechanism influencing the electroluminescence (EL) peak wavelength under a high injection current. The bandgap shrinkage effect of the InGaN well layer was considered to explain the large red-shifted EL peak wavelength under high injection currents. The estimated LED chip temperatures rise from room temperature to 150 °C and 75 °C for BP-LED and HP-LED, respectively, at a 600-mA injection current. This smaller temperature rise of the LED chip is attributed to the increased contact area between the sapphire and the LED structural layer because of the embedded nanopattern. Although the chip generates more heat at high injection currents, the accumulated heat can be removed to outside the chip effectively. The high diffuse reflection (DR) rate of hybrid-PSS increases the escape probability of photons, resulting in an increase in the viewing angle of the LEDs from 130° to 145°. The efficiency droop was reduced from 46% to 35%, effects which can be attributed to the elimination of TDs and strain relaxation by embedded nanopatterns. In addition, the light output power of HP-LED at 360-mA injection currents exhibits a ∼ 22.3% enhancement, demonstrating that hybrid-PSSs are beneficial to apply in high-power LEDs.

The effects of Mg-containing precursor flow rate on the characteristics of the Mg-doped GaN (GaN:Mg) were systematically studied in this study. The GaN:Mg films were deposited on sapphire substrates by metal-organic chemical-vapor deposition (MOCVD) with various flow rates of 25, 50, 75 and 100 sccm of bis-(cyclopentadienyl)-magnesium (Cp2Mg) precursor. The structural, electrical and nanomechanical properties of GaN:Mg thin films were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), Hall measurement and nanoindentation techniques, respectively. Results indicated that GaN:Mg films obtained with 25 sccm Cp2Mg possess the highest hole concentration of 3.1 � 10 17 cm � 3 . Moreover, the hardness and Young’s modulus of GaN:Mg films measured by a Berkovich nanoindenter operated with the continuous contact stiffness measurements (CSM) option showed positive dependence with increasing flow rate of Cp2Mg precursor, presumably due to the solution hardening effect of Mg-doping. 2012 Elsevier B.V. All rights reserved.

Mg-doped p-GaN films with various Mg flow rates were grown on a sapphire substrate by metal–organic chemical-vapor deposition (MOCVD) at a very low reactor pressure of 50 mbar. The surface roughness of Rrms decreased from 23.4 to 0.72 nm with a decrease in the Mg flow rate from 118 nmol min−1 to 30 nmol min−1. In addition, a high Mg activation efficiency of ∼5% was achieved by an optimized Mg flow rate. The photoluminescence (PL) and (0 0 2) X-ray diffraction (XRD) measurements indicated that the density of MgGa–VN complexes and screw-type dislocations in the p-GaN films were reduced under a low Mg flow rate. Fitting the variable-temperature Hall data indicated that the acceptor activation energy and the compensation ratio were ∼151 meV and ∼10%, respectively. It is therefore believed that the decrease in the compensation effect, by decreasing the density of the compensating center such as MgGa–VN complex and/or screw dislocation, plays an important role in improving the Mg acceptor activation efficiency in the low pressure growth of p-GaN films.

The textured 304BA stainless steel (SS) substrates were fabricated by the lift-off and etching processes, respectively. The total reflection (TR) rate and the diffuse reflection (DR) rate of incidence on the textured 304BA SS substrate were studied. The experimental results showed that the DR rate increased linearly with the increase in the total effective area of the stripe textured 304BA SS substrate. In addition, the Ag films coating/ridged-stripe textured 304BA SS substrate achieved a DR rate of up to ∼97%, a 28-fold increase compared with the untreated 304BA SS substrate. It is believed that the drastically increased DR rate of the Ag films coating/ridged-stripe textured 304BA SS substrate is due to not only the increase in total effective area but also the decrease in the opening angle of the ridged textured substrate which generates a more random distribution of light scattering.

ZnO thin films grown on Si(1 1 1) substrates by using atomic layer deposition (ALD) were annealed at the temperatures ranging from 300 to 500 °C. The X-ray diffraction (XRD) results show that the annealed ZnO thin films are highly (0 0 2)-oriented, indicating a well ordered microstructure. The film surface examined by the atomic force microscopy (AFM), however, indicated that the roughness increases with increasing annealing temperature. The photoluminescence (PL) spectrum showed that the intensity of UV emission was strongest for films annealed at 500 °C. The mechanical properties of the resultant ZnO thin films investigated by nanoindentation reveal that the hardness decreases from 9.2 GPa to 7.2 GPa for films annealed at 300 °C and 500 °C, respectively. On the other hand, the Young's modulus for the former is 168.6 GPa as compared to a value of 139.5 GPa for the latter. Moreover, the relationship between the hardness and film grain size appear to follow closely with the Hall–Petch equation.

This study investigated the effects of hydrogen (H 2 ) treatment on metal contacts to Mg-doped p-GaN films by Hall-effect measurement, current–voltage ( I – V ) analyzer and X-ray photoemission spectra (XPS). The interfacial oxide layer on the p-GaN surface was found to be the main reason for causing the nonlinear I – V behavior of the untreated p-GaN films. The increased nitrogen vacancy (V N ) density due to increased GaN decomposition rate at high-temperature hydrogen treatment is believed to form high density surface states on the surface of p-GaN films. Compared to untreated p-GaN films, the surface Fermi level determined by the Ga 2p core-level peak on 1000 °C H 2 -treated p-GaN films lies about ∼2.1 eV closer to the conduction band edge (i.e., the surface inverted to n-type behavior). The reduction in barrier height due to the high surface state density pinned the surface Fermi level close to the conduction band edge, and allowed the electrons to easily flow over the barrier from the metal into the p-GaN films. Thus, a good ohmic contact was achieved on the p-GaN films by the surface inversion method.

High density InN/GaN nanodots were grown by pulsed mode (PM) metal–organic chemical vapor deposition (MOCVD). InN nanodots density of up to ∼5×10 10 cm −2 at a growth temperature of 550 °C was achieved. The high diffusion activation energy of 2.65 eV due to high NH 3 flow rate generated more reactive nitrogen adatoms on the growth surface, and is believed to be the main reason for the growth of high density InN nanodots. In addition, an anomalous temperature dependence of the PL peak energy was observed for high density InN nanodots. The high carrier concentration, due to high In vacancy ( V In ) in the InN nanodots, thermally agitated to the conduction band. As the measurement temperature increased, the increase of Fermi energy resulted in blue-shifted PL peak energy. From the Arrhenius plot of integrated PL intensity, the thermal activation energy for the PM grown InN nanodots was estimated to be E a ∼51 meV, indicating strong localization of carriers in the high density InN nanodots.

This study presents the electrical properties of graphene that directly is contact on two types of p-type GaN thin films. The diameter of several hundred nanometer V-pits were formed on the p-GaN thin films by adjusting the NH3 flow rate during the metal organic chemical vapor deposition epitaxial process. The single-layer graphene with a high transmittance of 97% in the visible range was transferred on p-GaN thin films to form an Ohmic contact. The V-pits provide more carrier transport paths that promote the carrier tunneling into p-GaN thin films, resulting in a better Ohmic contact performance. In addition, the increased current value was attributed to the presence of V-pits on the p-GaN thin films.

The total reflection (TR) rate and the diffuse reflection (DR) rate of a textured substrate are important indexes when increasing the light trapping efficiency of a solar cell. In this study, we found that the smaller the interval or the larger the diameter of a concave periodically textured 430BA stainless steel (430BA SS) substrate the more the DR rate of light was increased. In addition, a 300 nm thick Ag film was deposited on the surface of the periodically textured 430BA SS substrates by e-beam evaporation. Experimental results showed that the DR rate of Ag film coated/textured 430BA SS substrates increased 2-fold and 14-fold, compared with textured 430BA SS and raw 430BA SS substrates, respectively. Moreover, the TR rate increased to more than 90% for all of the Ag film coated/textured 430BA SS substrates. It is our belief that the increase in the TR and DR rates was due to the high reflectance of the thick and continuous Ag film coating on a textured 430BA SS substrate.

Current conduction behavior on GaN V-defect was studied comprehensively using conductive atomic force microscopy. Experimental results indicate that the forward current in the V-defect region is at least three-order higher than that at surrounding area. On the other hand, a snowflake-like leakage current pattern was observed in the V-defect owing to the ease of current breakdown at the crest lines and perimeters. Further static current–voltage measurement suggests that the current flow is governed by Schottky emission and Fowler–Nordheim tunneling for V-defect region and surrounding area, respectively.

Polycrystalline diamond films were deposited on p-type (1 0 0) silicon substrate using a methane/hydrogen gas mixture in a microwave plasma-assisted chemical vapor deposition system. After the back-etched process, the Al contacts were evaporated on both sides of a 150 m thick isolated diamond film for consecutive high-voltage measurements. It was found that the current–voltage ( I–V) characteristics of the Al/diamond/Al structure exhibited two Schottky barrier diodes in a back-to-back configuration. Since the top diamond surface possessed better diamond quality than the bottom surface, the top Schottky diode with a breakdown voltage of 897 V and a lower breakdown voltage of −515 V for the bottom Schottky diode was observed for the first I–V measurement. However, the breakdown voltage was decreased by 37 and 140 V for the top and bottom Schottky diodes after the consecutive sixth repeated measurements. It was found that the oxygenated phenomenon was more prominent; in addition, the quality of the isolated diamond film was also degraded after the consecutive high-voltage measurements. It was indicated that decrease of the breakdown voltage was due to the oxidation layer and the non-diamond components on both surfaces of the isolated diamond film. © 2001 Elsevier Science B.V. All rights reserved.

We have studied optical properties of V-shaped pits on Al0.16Ga0.84N. The microphotoluminescence spectrum from the pit center shows a broader and stronger emission at 350 nm than the near-band-edge emission at 336 nm from nonpit regions. The results indicated specific defect levels associated with the V-shaped pits. Furthermore, after using atomic force microscopy to probe the surface electrical potential with a conductive tip, the pit’s potential was ∼0.2 V lower than its surrounding region. A simplified energy diagram is tentatively proposed to interpret our observation.

The flexible solar cells fabricated on a stainless steel substrate are being widely used for the building of integrated photovoltaics (BIPVs) in recent years. Because stainless steel has many advantages, such as low cost, high extension, ease of preparing etc. It was believed that the wide application of BIPVs especially rooftop applications, would be the biggest market for flexible PV technology (Kang et al. 2006, Otte et al. 2006, Chau et al. 2010, Fung et al. 2008). Until now, one of the main challenges of the BIPVs remains how to improve the conversion efficiency. Since, the path length of the photovoltaic effect is considerable shorter in a thin film solar cell resulting in reduced efficiency. Many researchers have focused on light trapping, and have adopted a different TCO technology, such as LP-CVD, PVD, to increase the path length of the incoming light, and improve the photovoltaic conversion efficiency of thin film solar cells (Selvan et al. 2006, Llopis et al. 2005, Soderstrom et al. 2008, Muller et al. 2004). Moreover, light trapping provides some significant advantages including, reduction of the cell thickness, reduced processing time and reduced cost, improved cell efficiency and the improved stability of amorphous Si (a-Si:H). The idea of trapping light inside a semiconductor by total internal reflection was reported by John in 1965 (John 1965). It also indicated that the effective absorption of a textured semiconductor film could be enhanced by as much as a factor of 60 over a plane-parallel film (Yablonovitch and Cody 1982). It should be mentioned that a major limitation to thin film solar cell efficiency is the long absorption length of the long wavelength photons and the low thickness of the absorber layer. The absorption length of amorphous silicon (a-Si:H) with a bandgap of 1.6 eV, for red and infrared solar photons, exceeds 1 μm and 100 μm, respectively (Ferlanto et al. 2002, Zhou and Biswas 2008). However, for a-Si:H the hole diffusion length is ~300-400 nm, which limits the solar cell absorber layer thickness to less than the hole diffusion length (Curtin et al., 2009). This makes it exceedingly difficult to harvest these photons since the absorber thickness of a p-i-n single junction solar cell is limited to only a few hundred nanometers for efficient carrier collection. In addition, the low-cost approach of thin-film silicon solar cells is very sensitive to film thickness, since the throughput increases with the decrease in layer thickness. Thus, sophisticated light trapping is an essential requirement for the design of thin-film solar cells (Rech et al., 2002). Enhanced light-trapping in thin film solar cells is typically achieved by a textured metal backreflector that scatters light within the absorbing layer and increases the optical path length of the solar photons. In our recent researches [Lee et al., 2009], various processing

The InN quantum dots (QDs) grown on a GaN buffer layer have been widely studied for their potential applications, such as solar cells, infrared light-emitting diodes and biosensors etc.. However, the InN growth temperature was restricted due to the issue of low dissociation temperature. Leading In (indium) adatoms are less mobile in finding the energetically favored site, resulting in high density structure defects that generally exist in the conventional metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy growth of InN QDs. The flow rate modulation epitaxy (FME) technique was proposed to overcome the In adatoms migration issue and was proven to be a suitable method for preparing high optical quality InN QDs. Nevertheless, the low density of InN QDs grown by the FME technique is a drawback for most optoelectronic devices based on the QDs’ active layer which prefer high density QDs. In other words, the growth technique for high density QDs plays an important role. It was well known that the adatom kinetic processes, such as adsorption, desorption, and diffusion at surfaces, are considered to be key parameters controlling the dot density, material quality and surface morphology. Recently, the pulsed-mode (PM) MOCVD method was reported as a useful approach for preparing high density InN QDs on GaN buffer layers. However, most of these reports could only achieve one benefit, either density or optical quality. Therefore, in this study we proposed a new method to grow high density InN QDs while maintaining the optical quality. The two-step growth method consists of one growth cycle for step-1 and two growth cycles in step-2. The duration of one growth cycle is 50 sec, including a 10 s TMIn flow process, a 20 s NH3 process, and two 10 s purge processes intervened in between. The two-step growth included step-1 for high density InN nucleation at 575 oC, and then, the reactor temperature was raised to 650 oC in 5 min increments to decompose the poor material quality InN islands. The aim of the step-1 growth was only to create high density nucleation sites on the GaN buffer layer. The reactor temperature was then reduced to 600oC to enable the growth of InN QDs with high density and high optical property in step-2 of the process. The experimental result show that the high density 1.5×1010 cm− 2 truncated hexagonal pyramidal InN QDs can be grown by the two-step growth method. The InN nucleation, InN decomposition and In adatom diffusion in the step-1, annealing and the step-2 process, respectively, determined the density, shape and size distribution of the InN QDs. The bimodal size distribution of the two-step growth InN QDs can be attributed to the high mobility of the incoming adatoms or/and existing islands, which favors coalescence process. The double-peak feature in the photoluminescence (PL) is related to the ground-state emission from the InN QDs with bimodal size-distribution. The low residual carrier concentration (1.8×1018 cm− 3), large confinement energy (342 meV) and intense PL emission for 6.1 nm high InN QDs indicated that a two-step growth is suitable for QD device applications.

This paper presents the electrical properties of Al-doped ZnO (AZO) films directly grown on two types of p-type GaN thin films. The low-pressure p-GaN thin films (LP-p-GaN) exhibited structural properties of high-density edge-type threading dislocations (TDs) and compensated defects (i.e., nitrogen vacancy). Compared with high-pressure p-GaN thin films (HP-p-GaN), X-ray photoemission spectroscopy of Ga 3d core levels indicated that the surface Fermi-level shifted toward the higher binding-energy side by approximately 0.7 eV. The high-density edge-type TDs and compensated defects enabled surface Fermi-level shifting above the intrinsic Fermi-level, causing the surface of LP-p-GaN thin films to invert to n-type semiconductor. A highly nonlinear increase in leakage current regarding reverse-bias voltage was observed for AZO/LP-p-GaN. The theoretical fits for the reverse-bias voltage region indicated that the field-assisted thermal ionization of carriers from defect associated traps, which is known as the Poole-Frenkel effect, dominated the I-V behavior of AZO/LP-p-GaN. The fitting result estimated the trap energy level at 0.62 eV below the conduction band edge. In addition, the optical band gap increased from 3.50 eV for as-deposited AZO films to 3.62 eV for 300 °C annealed AZO films because of the increased carrier concentration. The increasing Fermi-level of the 300 °C annealed AZO films enabled the carrier transport to move across the interface into the LP-p-GaN thin films without any thermal activated energy. Thus, the Ohmic behavior of AZO contact can be achieved directly on the low-pressure p-GaN films at room temperature.

A surface plasmon resonance (SPR) scheme was developed using a metal/dielectric/metal (MDM) structure. The surface plasma wave (SPW) dispersion curves within the MDM structure can be controlled by tuning the dielectric layer thickness. Therefore, the properties of the SPWs on the sensing surface of the SPR sensor can be tuned by coupling the tunable odd SPWs in the MDM structure to the sensing SPWs. The proposed tuning procedure was based on the tendency for anticrossing formation to occur between two nonparallel dispersion curves, and the reduction in the angular dip widths of the SPR sensors using the proposed procedure was demonstrated both numerically and experimentally.

The properties of surface plasmon-polaritons (SPPs) in symmetrical metal-dielectric-metal (MDM) structures were theoretically investigated and experimentally demonstrated. These SPPs exhibit several intriguing properties that are not observed in single-interface SPPs: they can directly couple to radiation outside the MDM structure; their frequency may fall within the plasmon gap; their group velocities can be positive, negative, or even zero, and their density of states can be tuned. All of these properties are favorable in devices that exploit the Purcell effect of SPPs to improve performance. Analytical descriptions of the dispersion curves in some important regions are presented.

Photoluminescence (PL) measurements were employed to investigate the emission properties of uncapped InN nanodot samples grown at temperature from 550 to 725 °C. Our results indicate that once the In droplets are formed, the optical properties of InN dots deteriorate markedly. As for those droplet-free samples grown at temperatures ≥600 °C, good luminescence properties are obtained. The corresponding 20 K PL peak energy is found to be almost constant at approximately 0.77 eV with a slow increase in its linewidth from 71 to 74 meV. A strong temperature-induced energy blue shift at approximately 20 to 280 K is considered to be partially connected to the band-filling effects of thermally stimulated surface electrons.

We report Raman measurements on InN islands grown on GaN by metalorganic chemical vapor deposition. The Raman frequency of the InN E2 mode is found to decrease exponentially with the island’s aspect ratio, indicating a size dependent strain relaxation during the island formation. Our results suggest that most of the strain at the InN–GaN interface have been released plastically during the initial stage of island formations. After that, the residual strain of only −3.5×10−3 is further relaxed elastically via surface islanding. The experimental data are in agreement with the strain relaxation predicted from a simplified model analysis as well as three-dimensional finite-element simulations.

InN nanodots grown on GaN by metal organic chemical vapor deposition using conventional growth mode as well as flow-rate modulation epitaxy at various growth temperatures (550–730°C) were investigated. We found that different precursor injection schemes together with the effect of growth temperatures greatly influenced the surface morphology of InN nanodots and their photoluminescence (PL) properties. The sample grown at around 650°C showed the highest growth rate. For samples grown at higher temperatures, the residual carrier concentration was reduced and the PL efficiency was improved. Furthermore, we found that the growth of InN nanodots is still sustainable even at a temperature higher than 700°C while retaining their optical quality.

Structural and photoluminescence (PL) properties of InN dots grown on GaN by metal organic vapor phase epitaxy using the flow-rate modulation technique, and their dependence on growth conditions, were investigated. An ammonia (NH3) background flow was intentionally supplied during indium deposition periods to control the kinetics of adatoms and hence the morphology of InN dots. Samples prepared under lower NH3 background flows generally exhibit narrower and more intense PL signals peaked at lower emission energies. The authors point out that the NH3 background flow is an important parameter that controls not only the nucleation process but also the emission property of InN dots.

Photoluminescence (PL) properties of InN dots embedded in GaN were investigated. We observed a systematic blueshift in the emission energy as the average dot height was reduced. The widely size-tunable emission energy can be ascribed to the size quantization effect. Temperature-dependent PL measurements show that the emission peak energies of the dots are insensitive to temperature, as compared with that of bulk film, indicating the localization of carriers in the dots. A reduced quenching of the PL from the InN dots was also observed, implying superior emission properties for the embedded InN dot structures.