R-plane Sapphire Substrates Research Articles

Substrate Dependence of the Proton Transport and Oriented Structure in Oligo[(1,2-propanediamine)-alt-(oxalic acid)] Thin Films

Abstract Three oligo[(1,2-propanediamine)-alt-(oxalic acid)] thin films were prepared on SiO2, R-plane sapphire, and MgO(100) substrates, and their proton conductivities were investigated. Conductivity depended on the substrate and varied from 4 × 10−3 to 1 × 10−4 S cm−1 under the same conditions (80% relative humidity and 298 K). The thin films between the R-plane (1102) sapphire and MgO(100) substrates showed different molecular oriented structures. The thin film on the sapphire substrate, which shows higher proton conductivity, has a structure oriented parallel to the substrate. The molecular orientation in the thin film could be correlated with the proton-transport properties.

Characteristics of a-plane GaN films grown on optimized silicon-dioxide-patterned r-plane sapphire substrates

We report on the characteristics of a-plane GaN films directly grown on optimized silicon-dioxide-patterned r-plane sapphire substrates. Various shapes and sizes of silicon dioxide patterns were considered with the aim of achieving fully coalescent a-plane GaN films with a smooth surface and high crystalline quality. The omega full widths at half maximum of the (11–20) X-ray rocking curve values of optimized a-plane GaN films with regular hexagonal patterns of 1μm window width and 6μm mask width were measured to be 597arc sec along the c-axis direction and 457arc sec along the m-axis direction. Atomic force microscopy images revealed a significant reduction in the density of submicron pits in the mask region. Plan-view and cross-sectional transmission electron microscopy images showed that basal stacking faults and threading dislocation densities were reduced from ~5.7×105cm−1 and ~1×109cm−2 in the window region to ~1.8×105cm−1 and ~2.1×108cm−2 in the mask region, respectively.

Study on the relationships between Raman shifts and temperature range for a-plane GaN using temperature-dependent Raman scattering

In this paper, Raman shifts of a-plane GaN layers grown on r-plane sapphire substrates by low-pressure metal—organic chemical vapor deposition (LPMOCVD) are investigated. We compare the crystal qualities and study the relationships between Raman shift and temperature for conventional a-plane GaN epilayer and insertion AlN/AlGaN superlattice layers for a-plane GaN epilayer using temperature-dependent Raman scattering in a temperature range from 83 K to 503 K. The temperature-dependences of GaN phonon modes (A1 (TO), E2 (high), and E1 (TO)) and the linewidths of E2 (high) phonon peak are studied. The results indicate that there exist two mechanisms between phonon peaks in the whole temperature range, and the relationship can be fitted to the pseudo-Voigt function. From analytic results we find a critical temperature existing in the relationship, which can characterize the anharmonic effects of a-plane GaN in different temperature ranges. In the range of higher temperature, the relationship exhibits an approximately linear behavior, which is consistent with the analyzed results theoretically.

Super rapid response of humidity sensor based on MOCVD grown ZnO nanotips array

This work deals with the fabrication of a-plane ZnO nanotip based humidity sensor on r-plane sapphire substrate by metal organic chemical vapor deposition (MOCVD). The humidity sensing property has been studied for this sensor by impedance measurements. The impedance of the humidity sensor decreases by about six orders of magnitude (from 2.8×108Ω to 4.3×102Ω at 1V, 100Hz) with increase in relative humidity (RH) (from 19% to 95%). The response and recovery times were found to be 3 and 12s, respectively. It shows super fast response and recovery time with good selectivity to humidity. Considering different sensor parameters investigated in the present study, it is observed that MOCVD grown ZnO nanotips respond promptly to the changes in RH due to quantum size effect as is evident from the blue shift of its band gap; and high atomic density on the a-plane of ZnO. The response has been explained on the basis of the adsorption of water molecules on a-plane ZnO and proton transport mechanism.

LOW THIRD-ORDER INTERMODULATION DISTORTION IN Ba<sub>0.6</sub>Sr<sub>0.4</sub>TiO<sub>3</sub> THIN FILM INTERDIGITAL CAPACITORS

A new method for the theoretical analysis of the thirdorder intermodulation distortion (IM3) in barium strontium titanate (BST) thin film interdigital capacitors (IDCs) on r-plane sapphire substrates is presented. Two circuit topologies - the dual and series dual BST varactor circuit - are proposed and their theoretical models along with simulated and measured results are presented. Low IM3 is demonstrated and experimentally verified. By proper selective biasing, very low nulls are observed in both dual and series dual BST varactor circuit topologies which indicate minimum distortion. The measured first nulls are achieved at ±13V and ±20V for the dual and series dual topologies respectively. These results demonstrate the potential of incorporating these highly linear BST varactors in silicon on sapphire (SoS) applications.

分子束外延生长的极性与非极性BeZnO薄膜的比较研究

BeZnO films were grown on different crystallographic planes( c-,a-and r-planes) of sapphire substrates using plasma-assisted molecular beam epitaxy( P-MBE). High quality BeZnO films were achieved using a multi-layer buffer design with full widths at half maximum( FWHMs) of rocking curves up to 600 arcsec. Polar BeZnO films were obtained on the a- and c-plane sapphire substrates,while the nonpolar ones were obtained on the r-plane sapphire substrate. The Raman spectroscopy confirmed the Be dopants in the ZnO were at the same level in three samples. The BeZnO sample grown on the r-sapphire substrate were found to have largest grains and higher FWHM, while the ones grown on a- and c-sapphire subatrates had the similar fine grains and lower FWHM. However,the photoluminescence( PL) spectra indicated the non-polar BeZnO sample had significantly stronger ultraviolet emission and weaker green emission than polar samples.

Structural, Morphological, Optical and Electrical Properties of Zn<sub>(1-x)</sub>Cd<sub>x</sub>O Solid Solution Grown on <i>a</i>- and <i>r</i>-Plane Sapphire Substrate by MOCV

Zn(1-x)CdxO films have been grown on (a-plane) and (r-plane) sapphire substrate by metal organic chemical vapor deposition. A maximum cadmium incorporation of 8.5% and 11.2% has been respectively determined for films deposited on a- and r-plane sapphire. The optical transmission spectra and energy band-gap equation established by Makino et al. were used to estimate the cadmium mole fraction of the solid solutions. Structural, morphological and optical properties of these films were examined using high resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM) and room and low temperature photoluminescence (Pl) as Cd incorporation and employed substrate. X-ray diffraction study revealed that all films had wurtzite phase but solid solution grown on a-plane sapphire are polycrystalline with a preferred orientation along the [0001] direction and a-plane film are epitaxially grown on r-plane sapphire. AFM image show significant differences between morphologies depending on orientation sapphire substrate but no significant differences on surface roughness have been found. The near band-edge photoluminescence emission shifts gradually to lower energies as Cd is incorporated and reaches 2.916 eV for the highest Cd content (11.2%) at low temperature (20 K). The room temperature hall mobility decreases with the Cd incorporation but it is larger for Zn(1-x)CdxO grown on r-plane sapphire.

Enhancement of Optical Polarization Anisotropy of a-Plane InGaN/GaN Multiple Quantum Well Structure from Violet to Blue-Green Light

A nonpolar a-plane (1120) InGaN/GaN epitaxial layer was grown on r-plane (1012) sapphire substrates by metal–organic chemical vapor deposition (MOCVD). In this work, a set of step-stage multiple quantum wells (MQWs) is inserted between underlying GaN and overlying high indium-content MQWs to investigate its influence on the optical properties of the active region. The step-stage MQWs were deposited by varying growth temperature at fixed precursor flow rate. Optical properties were investigated by the measurement of temperature-dependent photoluminescence (TD-PL). The optical polarization ratio, activation energy, and the smile-like curve in full width at half maximum (FWHM) of PL were analyzed in detail.

Annealing Conditions of Bi2Sr2CaCu2O8+x/CeO2/r-plane Sapphire by MOD Method

Abstract We have been studying Bi2Sr2CaCu2O8+x (Bi-2212) thin films on CeO2 buffered r-plane sapphire substrates by a metal organic decomposition (MOD) method. The deposited Bi-2212 thin films showed the c-axis oriented, and the in-plane orientation of the films showed 4-fold symmetry. However, we had a problem that Bi-2212 thin films were dissipation by thermal decomposition when the thin films were annealed above 840 °C in air atmosphere. In this study, we investigate the annealing conditions of Bi- 2212 thin films. We found TC of Bi-2212 thin films were dependence on the annealing condition such as a gas atmosphere and annealing times. The TC of Bi-2212 thin film annealed at 840 °C for 60 min in nitrogen atmosphere was 68.5 K. On the other hand, in oxygen atmosphere, TC was 65 K at the same annealing condition.

Epitaxial growth of Sb-doped nonpolar a-plane ZnO thin films on r-plane sapphire substrates by RF magnetron sputtering

► Sb-doped nonpolar a -plane ZnO layers were epitaxially grown on sapphire substrates. ► Crystallinity and electrical properties were studied upon growth condition and doping concentration. ► The out-of-plane lattice spacing of ZnO films reduces monotonically with increasing Sb doping level. ► The p-type conductivity of ZnO:Sb film is closely correlated with annealing condition and Sb doping level. In this study, the epitaxial growth of Sb-doped nonpolar a -plane ( 1 1 2 ¯ 0 ) ZnO thin films on r -plane ( 1 1 ¯ 0 2 ) sapphire substrates was performed by radio-frequency magnetron sputtering. The influence of the sputter deposition conditions and Sb doping concentration on the microstructural and electrical properties of Sb-doped ZnO epitaxial films was investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) and the Hall-effect measurement. The measurement of the XRD phi-scan indicated that the epitaxial relationship between the ZnO:Sb layer and sapphire substrate was 1 1 2 ¯ 0 ZnO / / ( 1 1 ¯ 0 2 ) Al 2 O 3 and 1 1 ¯ 0 0 ZnO / / 1 1 2 ¯ 0 Al 2 O 3 . The out-of-plane a -axis lattice parameter of ZnO films was reduced monotonically with the increasing Sb doping level. The cross-sectional transmission electron microscopy (XTEM) observation confirmed the absence of any significant antimony oxide phase segregation across the thickness of the Sb-doped ZnO epitaxial film. However, the epitaxial quality of the films deteriorated as the level of Sb dopant increased. The electrical properties of ZnO:Sb film are closely correlated with post-annealing conditions and Sb doping concentrations.

Realization of p-type non-polar a-plane ZnO films via doping of Na acceptor

p-Type non-polar ZnO films have been realized via doping of Na acceptor on r-plane sapphire substrates by plasma-assisted molecular beam epitaxy. The film is (112¯0) oriented (a-plane) as identified by the X-ray diffraction pattern. Structural properties are anisotropic and surfaces of films show stripes running along the ZnO [0001] direction. Secondary ion mass spectroscopy confirmed that Na has been doped into ZnO films. The chemical states of Na were analyzed by x-ray photoelectron spectroscopy. Deep level emission is dominant in room-temperature photoluminescence, which is mainly due to oxygen vacancy. With an effective incorporation of Na, Na-doped non-polar ZnO film exhibits p-type conductivity with a hole concentration of 1.81×1015cm−3, a Hall mobility of 0.402cm2V−1s−1, and a resistivity of 8575Ωcm, respectively. The origin of weak p-type behavior is most likely a certain amount of oxygen vacancies existed in the film, which partly compensates Na acceptor.

MOCVD growth of vertically aligned InGaN nanowires

In this work, we report the growth of vertically aligned bulk InGaN nanowires (NWs) on r-plane sapphire substrate by metal organic chemical vapor deposition (MOCVD). Through the optimization process of growth conditions, such as growth temperature and pressure, we obtained high density InGaN NWs consisting of one (0001) polar- and two equivalent {1101} semi-polar planes. We have shown the highest InGaN NWs wire density of 8×108cm−2,with an average diameter of 300nm and a length of 2μm. From results of photoluminescence (PL) at 30K and 300K, we observed the intense and broad emission peak from InGaN NWs at around 595nm, and confirmed that the luminescence could be tuned from 580nm to 660nm by controlling the indium flow (TMIn) rate. Our results indicate that MOCVD-grown InGaN NWs can be effective absorbers of the blue–green range of solar spectrum and may be one of the good candidates for high efficiency photovoltaic devices targeting at blue–green photons.

Structure and optical properties of a-plane ZnO/Zn0.9Mg0.1O multiple quantum wells grown on r-plane sapphire substrates by pulsed laser deposition

A series of 10-period ZnO/Zn0.9Mg0.1O multiple quantum wells (MQWs) with well widths varying from 2.2 to 5.6 nm have been grown on r-plane sapphire substrates by pulsed laser deposition. A good periodic structure with clear interfaces was observed by transmission electron microscopy. In a-plane ZnO/Zn0.9Mg0.1O MQWs, the luminescence was dominated by localized exciton emissions at low temperatures, while the free exciton (FE) transition was dominating emissions at temperatures above 100 K. The thermal quenching behavior of exciton emission has been analyzed. A rate equation assuming two nonradiative recombination channels is used to describe the quenching of the transitions observed. Moreover, the FE emission energy in the MQWs shows a systematic blueshift with decreasing well width, which is consistent with a quantum confinement effect.

Growth and Characterization of an a-Plane InxGa1−xN on a r-Plane Sapphire

The non-polar a-plane (112̄0) InxGa1−xN alloys with different indium compositions (0.074 ≤ x ≤ 0.555) were grown on r-plane (101̄2) sapphire substrates by metalorganic chemical vapor deposition, and the indium compositions x are estimated from x-ray diffraction measurements. The in-plane orientation of the InxGa1−xN with respect to the r-plane substrate is confirmed to be [1̄100]sapphire‖ [112̄0]InxGa1−xN and [1̄101]sapphire‖ [0001]InxGa1−xN. The effects of substrate temperature, reactor pressure and trimethylindium input flow on the indium incorporation and growth rate are investigated. The morphology of the a-plane InxGa1−xN is found to be significantly improved with the decreasing indium composition x and growth rate. Moreover, the in-plane anisotropic structural characteristics are revealed by high resolution x-ray diffraction employing azimuthal dependence, and the degree of anisotropy decreases with the increase of indium composition.

Study of a-Plane GaN Epitaxial Lateral Overgrowth Using Carbonized Photoresist Mask on r-Plane Sapphire

Epitaxial lateral overgrowth (ELO) a-plane GaN samples were successfully grown on masked sapphire (11̄02) substrates using an in-situ carbonized photoresist (PR) mask by a metalorganic chemical vapor deposition (MOCVD) method. The PR masks for the ELO process were prepared using conventional lithography in the form of a stripe with an opening of 4 µm and a period of 12 µm. The stripe-patterned PR was annealed at 1100 °C in a H2 atmosphere. The stripes were aligned parallel to the <11̄00>GaN direction. The ELO process of GaN was strongly dependent on the direction of the stripes. Overall, the PR masks on the r-plane sapphire substrate were carbonized during the heating step before the main growth, so that the carbonized PR mask acted as an ELO mask. The study results confirmed the promising potential of the ELO process using an in-situ carbonized PR mask on r-plane sapphire.

Defect states of a-plane GaN grown on r-plane sapphire by controlled integration of silica nano-spheres

An undoped a-plane GaN epi-layer was grown on r-plane sapphire substrate with silica nano-spheres by metal organic chemical vapor deposition. Defect states in the GaN epi-layer with Pt Schottky diode and Ti/Au Ohmic contact were characterized by using a deep level transient spectroscopy (DLTS) measurement. According to the results of DLTS spectra for a-plane GaN epi-layer with silica nano-spheres, the defect state with activation energy of 0.56eV and capture cross section of 9.72×10–16cm−2 originated in non-interacting point defect appeared dominantly. A dislocation related defect also appeared with small intensity. It shows that the silica nano-sphere layer integrated in the valley of the buffer layers on the r-plane sapphire substrate can improve the electrical property by the reduction of defect states in the GaN epi-layer.

Effect of substrate orientation on terahertz optical transmission through VO_2 thin films and application to functional antireflection coatings

We report studies of the terahertz (THz) transmission through vanadium dioxide (VO2) thin films grown on c-, m-, and r-plane sapphire substrates. Our results revealed THz amplitude modulation as large as 84% for VO2 films grown on r-plane sapphire substrates upon crossing the metal&#x2013;insulator phase transition temperature. Complex optical conductivity and refractive indices were determined for all investigated samples in the metallic state. Results are consistent with electrical resistivity measurements and described based on the Drude model. The real and imaginary parts of the optical conductivity and refractive index are obtained, and associations with variations in the grain morphology and crystal quality are described. We show that VO2 thin films can be used as tunable broadband THz frequency antireflecting coatings.

Effects of the V/III Ratio of a Low-Temperature GaN Buffer Layer on the Structural and Optical Properties of a-GaN Films Grown on r-Plane Sapphire Substrates by MOCVD

We investigate the effects of the V/III ratio of a low-temperature GaN buffer layer on the growth of the overlaying nonpolar a-plane GaN film grown on r-plane sapphire by metal-organic chemical-vapor deposition (MOCVD). With other experimental conditions keeping fixed, the low-temperature GaN buffer layers are grown under various V/III ratios of 1000, 3000, 6000 and 9000, respectively. The characteristics of the a-plane GaN films are analyzed by scanning electron microscopy, high resolution x-ray diffraction, Raman spectrum, and low temperature photo-luminescence. The results show that the V/III ratio of the buffer layer has significant effects on the crystal quality of the a-plane GaN film, and a V/III ratio of 6000 is found to be the most suitable condition to achieve pit-free flat GaN surface.

Solar-Blind Photoresistors Based on $\hbox{Mg}_{0.48} \hbox{Zn}_{0.52}\hbox{O}$ Thin Films Grown on $r\hbox{-Al}_{2} \hbox{O}_{3}$ Substrates by Radio-Frequency Plasma-Assisted Molecular Beam Epitaxy

Solar-blind photoresistors based on Mg0.48Zn0.52O thin films were fabricated on r-plane sapphire substrates by radio-frequency plasma-assisted molecular beam epitaxy. High-quality a-plane-orientation single-phase wurtzite Mg0.48Zn0.52O thin films are demonstrated by X-ray diffraction and absorption spectra. The MgZnO photoresistors exhibit a large dark/photoresistance ratio up to 1.7 × 104 with the light intensity of 0.61 mW/cm2 at 260 nm. The spectral response shows a sharp response peak only in the solar-blind region with maximum responsivity of 1.5 × 10-3 Ω-1· W-1.

Characteristics of Polarized Light Emission in $a$-Plane GaN-Based Multiple Quantum Wells

In this paper, we investigated polarized light emission properties on a series of a-plane GaN/AlGaN multiple quantum wells grown on r-plane sapphire substrates with various well widths by using the polarization-dependent photoluminescence measurement. To clarify reasons of polarization properties in light emission, we applied the 6 × 6 k·p model to simulate the E-K dispersion relationship and wave functions to obtain optical transitions of different polarized emissions. According to our results, the sub-bands of |Y>;-like states are raised toward the top of the valence sub-band level with increasing the well width. And the optical matrix element of y-polarized light emission will dominate the optical transition, leading to the increase of degree of polarization in the thicker well.