Plane Sapphire Substrates Research Articles

Layered MoS2 grown on c ‐sapphire by pulsed laser deposition

Layered growth of molybdenum disulphide (MoS2) was successfully achieved by pulsed laser deposition (PLD) method on c ‐plane sapphire substrate. Growth of monolayer to a few monolayer MoS2, dependent on the pulsed number of excimer laser in PLD is demonstrated, indicating the promising controllability of layer growth. Among the samples with various pulse number deposition, the frequency difference (A1g–E12g) in Raman analysis of the 70 pulse sample is estimated as 20.11 cm–1, suggesting a monolayer MoS2 was obtained. Two‐dimensional (2D) layer growth of MoS2 is confirmed by the streaky reflection high energy electron diffraction (RHEED) patterns during growth and the cross‐sectional view of transmission electron microscopy (TEM). The in‐plane relationship, (0006) sapphire//(0002)MoS2and sapphire//MoS2 is determined. The results imply that PLD is suitable for layered MoS2 growth. Additionally, the oxide states of Mo 3d core level spectra of PLD grown MoS2, analysed by X‐ray photoelectron spectroscopy (XPS), can be effectively reduced by adopting a post sulfurization process. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

Investigation of light output performance for gallium nitride-based light-emitting diodes grown on different shapes of patterned sapphire substrate

GaN-based blue light-emitting diodes (LEDs) on various patterned sapphire substrates (PSSs) are investigated in detail. Hemispherical and triangular pyramidal PSSs have been applied to improve the performance of LEDs compared with conventional LEDs grown on planar sapphire substrate. The structural, electrical, and optical properties of these LEDs are investigated. The leakage current is related to the crystalline quality of epitaxial GaN films, and it is improved by using the PSS technique. The light output power and emission efficiency of the LED grown on triangular pyramidal PSS with optimized fill factor show the best performance in all the samples, which indicates that the pattern structure and fill factor of the PSS are related to the capability of light extraction.

Implementation of a High-Performance GaN-Based Light-Emitting Diode Grown on a Nanocomb-Shaped Patterned Sapphire Substrate

A GaN-based light-emitting diode (LED) grown on a nanocomb-shaped patterned sapphire substrate (PSS) is fabricated and studied. Nanocomb-shaped patterns are transferred on a sapphire substrate using a well-ordered anodized aluminum oxide (AAO) thin film as a mask for the inductively coupled plasma etching process. This well-ordered AAO thin film with a high aspect ratio is grown on a sapphire substrate by an oxalic acid-based electrochemical system and a three-step anodization. The strain state generated during epitaxial growth could be effectively alleviated by the use of nanocomb-shaped PSS. The treading dislocation density could be reduced. Thus, the enhanced crystalline quality is obtained. In addition, due to the presence of photonic crystal-like air buffer layer, part of reflected photons upward the top side could be scattered by this layer. Therefore, more photons could be extracted outside. Experimentally, at 20 mA, as compared with a conventional LED grown on a planar sapphire substrate, the studied LED grown on a nanocomb-shaped PSS shows 53.8% and 43.7% enhancements in light output power and external quantum efficiency as well as a reduced leakage current.

Improved carrier mobility of InAlN/GaN heterostructure grown on cone-shaped patterned sapphire substrate

InAlN/GaN heterostructures were grown on both patterned and planar sapphire substrates by metal–organic chemical vapor deposition. Hall measurements showed that both samples possessed similar two-dimensional electron gas densities, whereas the electron mobility of the heterostructure grown on a patterned substrate was evidently superior. The enhanced mobility was attributed to the reduced threading dislocation density and improved channel-barrier interface morphology. Furthermore, Raman measurement results showed that there was less residual compressive strain in the GaN grown on the pattered sapphire substrate, which could be the reason for the morphology improvement. Consequently, the patterned sapphire substrate is very promising for further improving the performance of high-electron-mobility transistors.

Characterization of a-plane InGaN light-emitting diodes with a SiNx interlayer grown on a patterned sapphire substrate by metal–organic chemical vapor deposition

Nonpolar, a-plane InGaN light-emitting diodes (LEDs) were grown on a hemispherical r-plane patterned sapphire substrate (HPSS) by the in situ deposition of SiNx masks and on a planar sapphire substrate by metalorganic chemical deposition (MOCVD). The use of the SiNx interlayer together with HPSS resulted in a reduced density of defects, such as basal plane stacking faults (BSFs) and partial dislocations (PDs) in the GaN epilayers, as compared with the use of GaN on a conventional planar substrate. The optical power of the InGaN LEDs with the SiNx layer and HPSS was approximately 2.5- and 10-fold higher than those of LEDs on SiNx and on a planar substrate at a current of 100 mA, respectively.

Self-regulated in-plane polarity of [11¯00]-oriented GaN domains coalesced from twins grown on a SiO2-patterned m-plane sapphire substrate

In-plane polarity of [11¯00]-oriented GaN domains coalesced from twins grown on a SiO2-patterned m-plane sapphire substrate was observed to be self-regulated in such a way that basal faces of coalesced domains were mainly found to have the (0001¯) polarity only. This self-regulation behavior of in-plane polarity was explained by a computational simulation of plan-view surface morphology evolution during coalescence of twins. Based on a computational simulation, asymmetrically suppressed growth rates of twins near a SiO2 pattern were proposed to be responsible for the survival of the slower growing (0001¯) basal faces instead of the faster growing (0001) basal faces during coalescence of twins.

MBE growth and characterization of ZnTe epilayers on m‐plane sapphire substrates

AbstractZnTe epilayers were grown on transparent (10‐10) oriented (m ‐plane) sapphire substrates by molecular beam epitaxy (MBE). Pole figure imaging was used to study the domain distribution within the layer. (211)‐oriented ZnTe domains were formed on m ‐plane sapphire. The presence of only one kind of (211) ZnTe domain formed on the 2°‐tilted m ‐plane sapphire substrates was confirmed. Thus, single domain (211) ZnTe epilayers can be grown on the m ‐plane sapphire using MBE. Although differences in the crystal structure and lattice mismatch are large, precise control of the substrate surface lattice arrangement result in the formation of high‐quality epitaxial layers. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A surface flattening mechanism of a heteroepitaxial film consisting of faceted non-flat top twins: [11¯3¯]-oriented GaN films grown on m-plane sapphire substrates

We carried out experiments and computational simulations in order to answer a yet unanswered question about a surface flattening mechanism of a [11¯03¯]-oriented GaN film consisting of faceted non-flat top twins. Our results revealed that an overgrowth of one variant of twins over the other, which was manifested only at a thickness larger than a few microns due to a slight asymmetric crystallographic tilt (1.0° ± 0.4°) of twins, played a key role in a surface flattening mechanism. In addition, we experimentally demonstrated that GaN grown on a SiO2-patterned m-plane sapphire substrate had no asymmetric tilt and that no surface flattening occurred.

Epitaxial lateral overgrowth of thick semipolar {11$ \bar 2 $2} GaN by hydride vapor phase epitaxy

AbstractA thick semipolar {11–22} GaN layer was grown on m ‐plane sapphire substrates by hydride vapor phase epitaxy (HVPE). We succeeded in using epitaxial lateral overgrowth (ELO) with a striped SiO2 mask to suppress crack generation, improve the flatness, and reduce the dark spot density in the semipolar {11–22} GaN layer. The ELO was performed at a growth rate of over 100 µm/h, and it was found that ELO using a SiO2 mask could be used to achieve high‐growth rate HVPE. Large hillocks were eliminated by using ELO. The dark spot density of the 2 mm {11–22} GaN layer grown by ELO was 2.7 × 107cm–2, which was one digit lower than that of a conventional GaN layer. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Correlation between luminescence and defects in nonpolar and semipolar InGaN/GaN quantum wells on planar and patterned sapphire substrates

The optical and structural properties of nonpolar and semipolar InGaN/GaN quantum wells (QWs) grown on planar sapphire substrates and patterned sapphire substrates (PSSs) were investigated in order to understand the effect of defects on the luminescence. By introducing the PSS technique, surface morphological features were varied in number and size, and the defect density was significantly reduced by the PSS patterns, which resulted in the formation of low-defect regions. The photoluminescence (PL) analyses of nonpolar and semipolar QWs samples revealed that the semipolar \((11\bar 22)\) QWs, which had better optical performance and higher In incorporation, are more suitable for application to long-wavelength emitters in the green-red spectral range and beyond. Moreover, particularly in the semipolar QWs, we could observe that the optical properties were enhanced by the decrease in defect density. It was also noted that In incorporation was increased in the high-defect regions and in the regions where arrowhead-like features existed.

Enhancement of InGaN-Based Light Emitting Diodes Performance Grown on Cone-Shaped Pattern Sapphire Substrates

To enhance light extraction effciency, high-quality InGaN-based light emitting diodes (LED) was grown on cone-shaped patterned sapphire (CPSS) by using metal organic chemical vapor deposition (MOCVD). From the transmission electron microscopy (TEM) observation, the CPSS was confirmed to be an efficient way to reduce the threading dislocation density in the GaN epilayer. A sharp and high intensity Photoluminescence (PL) for LED on CPSS at 457 nm compared to LED on unpattern planar sapphire substrates (USS) indicates that the crystalline quality was improved significantly and the internal reflection on the cones of the substrate was enhanced. The output power of the LED on CPSS is higher than that of LED on USS. The achieved improvement of the output power is not only due to the improvement of the internal quantum efficiency upon decreasing the dislocation density, but also due to the enhancement of the extraction efficiency using the CPSS.

The Influence of Superlattice on the Internal Quantum Efficiency of LED Structures with InGaN / GaN Quantum Wells

epitaxy. The structures were grown on the planar and profiled sapphire substrates with the (0001) orientation. Two groups of samples were investigated. The LED-structures of the group QB were grown on the planar sapphire substrates and were composed of a low-temperature nucleation layer, a 2 µm-thick buffer i-GaN layer, a 2 µm-thick n-GaN layer, an active region consisting of 5 InGaN quantum wells with the thickness 2.5–3 nm and GaN barrier layers with the thickness 3–15 nm (an InN content in the InхGa1–хN solid solution was 10–15%), and a 140 nm-thick p-GaN layer. In addition, in the buffer region of some samples of the group QB, a short-period InGaN / GaN superlattice with the 1 nmthick layers was grown. The samples of the LQB group grown on the planar and profiled sapphire substrates consisted of a low-temperature nucleation layer, a 4 µm-thick buffer i-GaN layer, a 2 µm-thick n-GaN layer, an active region consisting of 10 InGaN quantum wells with the thickness 3 nm and GaN barrier layers with the thickness 15 nm, and a 140 nm-thick p-GaN layer. In the samples of the LQB group, the thickness of the last barrier GaN layer, located between the active region and the p-GaN layer, varied in the range 5–15 nm. The grown samples were studied by the methods of electro- and photoluminescence. For the electroluminescence (EL) measurements, indium contacts were fused into the samples. The measurements were carried out at room temperature at low currents (up to 5mA) in a pulsed mode. The external quantum efficiency (EQE) was estimated from the slope of the photodetector current–current through the sample dependence. Photoluminescence (PL) spectra were measured in the temperature range 10–300 K using an optical cryostat. A pulsed YAG-laser with an average power of 35 mW (1 kHz, pulse duration – 10 ns, wavelength – 355 nm) was used as an excitation source. To determine the internal quantum efficiency (IQE), an integrated PL intensity was measured as a function of the excitation radiation intensity [3, 4].

Structural and optical properties of single-phase ZnO1−S alloy films epitaxially grown by pulsed laser deposition

• We grew epitaxial ZnO 1- x S x ( x ≤ 0.18) films by PLD with a ZnS ceramic target and O 2 . • Lattice parameters ( c , a ) and E g of single-phase ZnO 1−x S x alloys were determined. • C and a expand from 5.204 to 5.366 Å and 3.255 to 3.329 Å with increasing S content. • The optical bandgap shrinks from 3.27 to 2.92 eV with a bowing parameter of 2.91 eV. • In-plane perfectly matched ZnOS/MgZnO heterostructures with max. barrier are proposed. We report on a detailed investigation of the structural and optical properties of single crystalline ZnO 1 − x S x thin films, placing emphasis on the elucidation of the correlation of the band gap and lattice parameters, particularly the lattice constant a , with the S content in the alloy films. High-quality ZnO 1 − x S x thin films with different S concentrations X s (0 ⩽ X s ⩽ 0.18) were grown epitaxially on c -plane sapphire substrates by pulsed laser deposition using a ZnS ceramic target with varying O 2 partial pressures. X-ray diffraction studies revealed that all grown ZnO 1 − x S x thin films have a single-phase wurtzite structure. With increasing X s value from 0 to 0.18, both lattice constants c and a expand monotonically from 5.204 to 5.366 Å and from 3.255 to 3.329 Å, respectively, while the optical band gap shrinks from 3.27 to 2.92 eV with a bowing parameter of 2.91 eV. Based on these information, ZnOS/MgZnO heterostructures that have a perfect in-plane lattice match and a maximum barrier height can be proposed, which might eventually lead to new optoelectronic devices with superior performance.

Phase separation and electronic structure of ZnS0.3O0.7 alloy thin film with and without (Ag, Li) co-doping

Abstract ZnS 0.3 O 0.7 alloy thin film with and without Ag and Li co-doping are grown by pulsed laser deposition on c -plane sapphire substrate. The films are phase separated in S-rich and S-poor regions. Two and four different phases are observed to form in (Ag, Li) 0.05 :Zn 0.95 S 0.3 O 0.7 and ZnS 0.3 O 0.7 films respectively. Different phases and their relative volume fractions have been identified by electron diffraction pattern. The band gap corresponding to each phase is identified by low loss region of high resolution electron energy loss spectra. Band bowing parameter upon S doping is found to be 4.12 eV which closely match with Wien2k based density functional theory calculation utilizing mBJLDA exchange correlation potential. Oxygen positions have been replaced by sulphur in the lattice as confirmed by S L 3,2 electron energy loss near edge absorption spectra. High resolution electron energy loss spectroscopy has been used to collect core level spectra of various dopants in order to identify their locations in the lattice. Experimental Ag M 5,4 extended energy loss fine structure and Li K electron energy loss near edge structure in (Ag, Li):ZnS 0.3 O 0.7 alloy have been compared with calculated spectra using FEFF code, suggesting that Ag and Li have taken up both the substitution and interstitial positions in the lattice. All the samples are resistive with resistance in the range of a few mega-ohms.

Improved Performance of (112̄2) Semipolar InGaN/GaN Light-Emitting Diodes Grown Using a Hemispherically Patterned SiO2 Mask

In this paper, we report on the improved performance of (112̄2) semipolar InGaN/GaN light-emitting diodes (LEDs) grown using a hemispherically patterned SiO2 mask on an m-plane sapphire substrate (HP-SiO2), in comparison with a planar m-plane sapphire substrate and a hemispherically patterned m-plane sapphire substrate (HPSS), by metalorganic chemical vapor deposition. The full widths at half maximum of X-ray rocking curves for the on- and off-axes planes of the GaN layers on HP-SiO2 were the narrowest of the three samples. Cross-sectional transmission electron microscopy images showed larger low defect areas of GaN layers on HP-SiO2 than on HPSS. The electroluminescence results showed that the optical powers of LEDs on HPSS and HP-SiO2 increased by approximately 2.7 and 6 times, respectively, over that of m-planar sapphire at 100 mA. Our results suggest that the introduction of HP-SiO2 is very effective to improve the crystal quality as well as the light extraction efficiency of semipolar InGaN LEDs.

Influence of cone-shaped pattern sapphire substrates on dislocation density and stress-strain of GaN epifilms by MOCVD

GaN epifilms are grown on the 2.5 μm×1.6 μm×0.5 μm cone-shaped pattern sapphire substrates (CPSS) and unpattern planar sapphire substrates (USS) by MOCVD. High resolution X-ray diffraction (HRXRD) measurements show that the edge dislocation density of GaN epifilms on CPSS is less than on USS. From the transmission electron microscopy (TEM) observation, the CPSS can efficiently reduce the threading dislocation in GaN epilayer. The Raman scattering spectroscopy show that the CPSS can effectively decrease the residual stress in GaN epilayer. A strong and sharp PL band edge emission was observed for the GaN grown on CPSS compared to USS. The above result indicates the quality of GaN epilayers on CPSS is advantage to GaN epilayer on USS.

GaN-Based Light Emitting Diodes with Hybrid Micro-Nano Patterned Sapphire Substrate

GaN-based light emitting diodes (LEDs) have been fabricated with hybrid micro-nano patterned sapphire substrate (MNPSS). Random nano pattern distributed on the spacing and inclined surface of the micro pattern was realized utilizing standard photolithography, nickel etching mask and inductively coupled plasma (ICP) etching techniques. Both surface morphology and X-ray diffraction (XRD) results showed that MNPSS and conventional MPSS had similar effect on the crystalline quality of the epilayers. Furthermore, light output powers (LOP) of LEDs with MNPSS was increased considerably compared with conventional planar sapphire substrate (CPSS) and was even higher than that of LEDs employing micro patterned sapphire substrate (MPSS), indicating more superior performance of GaN-based LEDs was obtained by employing the hybrid micro-nano pattern. © 2013 The Electrochemical Society. [DOI: 10.1149/2.007311ssl] All rights reserved.

Structural and optical properties of non-polar ZnO/Zn0.81Mg0.19O multiple quantum wells grown on r $r$ -plane sapphire substrates by plasma-assisted molecular beam epitaxy

Structural and optical properties of non-polar ZnO/Zn0.81Mg0.19O multiple quantum wells grown on r $r$ -plane sapphire substrates by plasma-assisted molecular beam epitaxy

Growth of epitaxial c ‐plane ZnO film on a ‐plane sapphire by radio frequency reactive magnetron sputtering

AbstractIn this Letter, we report the successful growth of high quality c ‐plane oriented epitaxial ZnO films on a ‐plane sapphire substrates by using radio frequency reactive magnetron sputtering. The effect of substrate temperature on the structural and optical properties has been investigated. X‐ray diffraction (XRD) studies reveal that the ZnO film is grown epitaxially on a ‐plane sapphire substrate, and the film quality is improv‐ ed as the substrate temperature is increased. Photoluminescence (PL) results manifest that screw dislocations can exert great influence on the optical properties. It is found that the line width of the near‐band‐edge emission of PL decreases linearly with increase in screw density. In addition, a simple and effective method is proposed to assess the defect density in epitaxial ZnO films by performing PL measurement. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth, doping, and characterization of ZnO nanowire arrays

Zinc oxide (ZnO) nanowire (NW) arrays were grown by chemical vapor deposition using the carbothermal reduction of ZnO powder at different pressures from 0.13 to 1.0 atm on basal plane sapphire substrates. The ZnO NWs were oriented in their [0001] direction. Lower growth pressures led to generally longer and smaller diameter wires. A model relating the length and diameter of the NWs was used to interpret the growth mechanism of these ZnO NWs as a function of pressure as the combination of adatom diffusion along the NW sidewalls and direct impingement growth on the NW tip. Al-doped ZnO NWs were synthesized by introducing Al power into the source material, resulting in an Al mole fraction up to 1.8 at. % in the NWs and a concurrent reduction in NW resistivity. Raman spectroscopy revealed slight lattice distortion to the ZnO crystal lattice, while room temperature photoluminescence showed an increase in the near band edge emission concurrently with a reduction in the green emission. The near band edge emission was also blue shifted in a manner consistent with the Burstein–Moss effect in degenerated semiconductor materials.