Vicinal Sapphire Substrates Research Articles

Morphological dependent Indium incorporation in InGaN/GaN multiple quantum wells structure grown on 4° misoriented sapphire substrate

The epitaxial layers of InGaN/GaN MQWs structure were grown on both planar and vicinal sapphire substrates by metal organic chemical vapor deposition. By comparing the epitaxial layers grown on planar substrate, the sample grown on 4° misoriented from c-plane toward <101̄0> m-plane substrate exhibited many variations both on surface morphology and optical properties according to the scanning electronic microscopy and cathodoluminescence (CL) spectroscopy results. Many huge steps were observed in the misoriented sample and a large amount of V-shape defects located around the boundary of the steps. Atoms force microscopy images show that the steps were inclined and deep grooves were formed at the boundary of the adjacent steps. Phase separation was observed in the CL spectra. CL mapping results also indicated that the deep grooves could effectively influence the localization of Indium atoms and form an In-rich region.

Influence of vicinal sapphire substrate on the properties of N-polar GaN films grown by metal-organic chemical vapor deposition

The influence of vicinal sapphire substrates on the growth of N-polar GaN films by metal-organic chemical vapor deposition is investigated. Smooth GaN films without hexagonal surface feature are obtained on vicinal substrate. Transmission electron microscope results reveal that basal-plane stacking faults are formed in GaN on vicinal substrate, leading to a reduction in threading dislocation density. Furthermore, it has been found that there is a weaker yellow luminescence in GaN on vicinal substrate than that on (0001) substrate, which might be explained by the different trends of the carbon impurity incorporation.

Effects of crystallinity on laser-induced voltage effect from Zn0.9Co0.1O thin film

Co-doped ZnO epilayer films were grown by pulsed laser deposition (PLD) on vicinal cut silicon and sapphire substrates. Changes in deposition time were observed as a moderate effect on the quality of the films, and the influence of the thickness on thermoelectric signals from Zn0.9Co0.1O thin films were discussed. The effect of one of the main deposition parameters, the deposition time, on the crystallinity and electron mobility properties of the Zn0.9Co0.1O thin films grown on sapphire was investigated by means of X-ray diffraction (XRD) and laser-induced voltage (LIV) effect. It shown that the XRD rocking curve full-width half-maximun (FWHM) decreased as time increasing, and the LIV signals were observed along the tilting angle of the substrate orientation when the pulsed KrF excimer laser of 248 nm were irradiated on the films. When the films illuminated in pulse lasers, the highest signals occurred in the films with best crystalline quality, and the signals were higher in the films grown on sapphire than those on silicon substrates. It suggested that the electrical resistivity and electron mobility have close relations with not only the crystallinity but also with the interface of the thin films.

TEM study of GaN films on vicinal sapphire (0001) substrates by MOCVD

Quality properties and internal defects of unintentionally doped GaN films grown on 0.3 vicinal sapphire (0001) substrates by MOCVD are investigated by TEM. The results show that plenty of dislocations in the GaN films prepared on vicinal sapphire substrates are annihilated in the areas with a distance of 0.8 m away from substrates, and that dislocations gather in the GaN films. Based on these phenomena, a mechanism for dislocation annihilation in the GaN film prepared on vicinal substrate is proposed, which is capable of explaining the fact that vicinal substrates are able to improve the qualities of GaN films.

Two-dimensional electron gas transport anisotropy in N-polar GaN/AlGaN heterostructures

Magnetic-field dependent Hall-effect measurements and mobility spectrum analysis were employed to study anisotropic transport in N-polar GaN/Al0.3Ga0.7N heterostructures grown on vicinal sapphire substrates. The significant anisotropy in the mobility in the parallel and perpendicular directions to the miscut direction was accompanied by a slight anisotropy in charge density. A single electron species was found in the direction parallel to the steps resulting from growth on the vicinal substrates; while in the perpendicular direction two distinct electrons peaks were evident at T≤150 K. The lower average mobility in the perpendicular direction is attributed to interface roughness scattering.

Effects of the inclination direction of vicinal m-plane sapphire substrates on the crystal quality of m-plane GaN film

Herein is a discussion of how the structural and optical properties of m-plane GaN ( m-GaN) films are affected by the inclination direction of vicinal m-plane sapphire substrate. The m-GaN films were grown on three different types of substrates inclined toward the a-axis direction, the c-axis direction, and with no inclination. We found that m-GaN film grown on an m-plane sapphire inclined in the a-axis direction showed the highest quality with a smooth surface and a low stacking fault density. A model is proposed that shows how the surface step of a substrate can reduce the generation of stacking fault in m-GaN film.

Investigation of GaN‐based light‐emitting diodes grown on vicinal sapphire substrates

AbstractGaN‐based light‐emitting diodes (LEDs) grown on c‐plane vicinal sapphire substrates are fabricated and characterized. All the electrical characteristics demonstrate that the LED with a 0.2° tilt sapphire substrate exhibits the lowest defect density while that with a 1.0° tilt sapphire exhibits the highest one. At 2 mA, the enhanced output power of 23.3% indicates the substantial reduction of defect‐related nonradiative recombination centers in MQW for the LED device with a 0.2°‐tilt sapphire. At 60 mA, the improved value is up to 45.7%. This is primarily caused by the presence of indium quantum dots (QDs) in MQW which provides increased quantum efficiency (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Surface Plasmon Resonance on the Sapphire Single Crystal Substrates with 3D Nano-Textured Surface

Surface plasmon resonance (SPR) bio-sensors comprise an attenuated total reflection (ATR) system known as the Kretschmann geometry. The sensing surface comprises flat Au thin films deposited on a high index flat glass substrate (LAL-10, n = 1.72). This paper describes the effects of the 3D nano-textured Au thin films on the sensing properties are discussed. The nano-textured Au thin films were fabricated by the deposition of the uniform Au thin films on the (0001) vicinal sapphire substrates. The vicinal substrates comprise periodic atomic step lines and terraces. The step height is chiefly half lattice spacing of the sapphire (0.22 nm). The terrace width is governed by the off-cut angles. The stepped surface structure were reconstructed by the annealing at 900 to 1400oC (1 to 3 h) in air. The annealed stepped surfaces comprised stepped heights of 5 to 12 nm with the terrace width of 2.7 to 140 nm for the off-cut angles at 5 degrees. The laser diodes (635 nm) were used for the optical light source. It is found that the detection limit of the SPR bio-sensors using the flat Au thin films on the (0001) sapphire substrates is 1.6 ng/ml for the tumor marker (AFP, α-fetoprotein) by antibody/antigen reaction, which is almost the same to the performance of the flat Au thin films on the conventional high index glass substrates as expected by the simulation Winspal 2. The SPR sensitivities using the nano-textured Au thin films with the step height of 12 nm and the terrace width of 140 nm were one order of magnitude smaller than those of Au thin films on the flat (0001) sapphire, 20 ng/ml, probably due to the scattering of the surface plasmon and/or evanescent wave. However, well-designed nano-textured Au thin films will enhance the SPR sensitivity. The sapphire substrates could integrate optical devices and will provide novel optical-bio MEMS.

Effects of annealing atmosphere on thermoelectric signals from ZnO films

Zinc oxide epilayer films were grown on vicinal cut sapphire substrates by pulsed laser deposition with in situ annealing oxygen pressures varied from 0 to 10 × 10 3 Pa. The best crystalline quality was obtained for ZnO layer with annealing oxygen pressure of 6 × 10 3 Pa. Laser induced thermoelectric voltage (LITV) were observed along the tilting angle orientation of the substrate when the pulsed KrF excimer laser of 248 nm were irradiated on the films. The largest LITV signal was measured for the film grown at 6 × 10 3 Pa annealing oxygen pressure. According to the measured LITV signals, Seebeck anisotropy was evaluated and was found to range from 3 to 12 μV/K for ZnO films annealed at different oxygen pressures from 2 to 10 × 10 3 Pa. It is suggested that oxygen ambient plays an important role in the electronic properties of the ZnO films.

N-Polar InAlN/AlN/GaN MIS-HEMTs

N-polar metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs) were fabricated from a GaN/AlN/InAlN/GaN heterostructure grown by metalorganic chemical vapor deposition on a vicinal sapphire substrate, using Si3N4 as the gate insulator. Hall measurements in van der Pauw geometry on the heterostructure showed a sheet charge density and a mobility of 2.15 × 1013 cm-2 and 1135 cm2·V-1·s-1, respectively. Resistance measurements revealed anisotropic conductivity with respect to the surface steps induced by the substrate misorientation, and the sheet resistance of the 2-D electron gas was as low as 226 Ω/□ in the parallel direction. MIS-HEMTs with a gate length of 0.7 μm and a source-drain spacing of 2.2 μm had a peak drain current of 1.47 A/mm and an on-resistance of 1.45 Ω·mm. At a drain bias of 8 V, the current- and power-gain cutoff frequencies were 14 and 25 GHz, respectively.

Growth and characterization of In-polar and N-polar InAlN by metal organic chemical vapor deposition

In x Al 1 − x N layers, with 0.09≤x≤0.23, were grown on GaN on both the In-polar and N-polar orientations by metal organic chemical vapor deposition. The impact of growth conditions, including temperature and the group-III flow rates, on the surface morphology and indium mole fraction was investigated. In-polar layers had a smooth surface morphology characterized by mounds which decreased in size with increasing supersaturation during growth. Smooth N-polar InAlN was achieved through the use of vicinal sapphire substrates with misorientation angles of 3°–5° toward ⟨101¯0⟩GaN, and a trend of an increase in step bunching with decreasing supersaturation was observed for N-polar InAlN layers. The indium incorporation increased with decreasing growth temperature and increasing growth rate for both In-polar and N-polar layers. The indium incorporation was similar for both orientations on samples which were coloaded in our reactor.

Performance investigation of GaN-based light-emitting diodes with tiny misorientation of sapphire substrates

GaN-based light-emitting diodes (LEDs) grown on c-plane vicinal sapphire substrates are fabricated and characterized. Based on the material quality and electrical properties, the LED with a 0.2 degrees tilt sapphire substrate (device A) exhibits the lowest defect density and high performance, while the LED with a 1.0 degrees tilt sapphire (device D) exhibits the highest one. At 2 mA, the extremely enhanced output power of 23.3% indicates of the reduction of defect-related nonradiative recombination centers in active layers for the device A. At 60 mA, the improved value is up to 45.7%. This is primarily caused by the formation of indium quantum dots in MQW which provides an increased quantum efficiency.

Electrostatic Discharge Performance of GaN-Based Light Emitting Diodes with Naturally Textured p-GaN Layers Grown on Vicinal Sapphires

The electrostatic discharge (ESD) characteristics of GaN-based light emitting diodes (LEDs) with textured p-GaN layers grown on c-axis vicinal sapphire substrates are studied and demonstrated. Based on the machine model, the device grown on a 0.35° tilt sapphire shows the highest ESD tolerance, whereas the one grown on a 0.2° tilt sapphire exhibits the poorest tolerance. This phenomenon is primarily influenced by the presence of maximum capacitance (C m ) values induced by a parasitic capacitance effect at the p-GaN/indium tin oxide interface rather than the difference in dislocation densities between LEDs.

The effects of vicinal sapphire substrates on the properties of AlGaN/GaN heterostructures

AlGaN/GaN heterostructures on vicinal sapphire substrates and just-oriented sapphire substrates (0001) are grown by the metalorganic chemical vapor deposition method. Samples are studied by high-resolution x-ray diffraction, atomic force microscopy, capacitance–voltage measurement and the Van der Pauw Hall-effect technique. The investigation reveals that better crystal quality and surface morphology of the sample are obtained on the vicinal substrate. Furthermore, the electrical properties are also improved when the sample is grown on the vicinal substrate. This is due to the fact that the use of vicinal substrate can promote the step-flow mode of crystal growth, so many macro-steps are formed during crystal growth, which causes a reduction of threading dislocations in the crystal and an improvement in the electrical properties of the AlGaN/GaN heterostructure.

Characterizations of GaN films and AlGaN/GaN heterostructures on vicinal sapphire (0 0 0 1) substrates grown by MOCVD

GaN films and AlGaN/GaN heterostructures grown on vicinal sapphire (0 0 0 1) substrates by metalorganic chemical vapor deposition (MOCVD) are investigated. It is found that surface morphologies of GaN films depend on the vicinal angle, however, they are not sensitive to the inclination directions of the substrate. The optimized vicinal angle for obtaining excellent surface morphology is around 0.5°. This conclusion is also confirmed by characterizing the electrical property of two-dimensional electron gas (2DEG) in the AlGaN/GaN heterostructure.

Carrier localization degree of In0.2Ga0.8N/GaN multiple quantum wells grown on vicinal sapphire substrates

In this work, we have grown In0.2Ga0.8N/GaN multiple quantum well (MQWs) epitaxial structure on vicinal sapphire substrates by low pressure metal-organic chemical vapor deposition and investigated the relationship between carrier localization degree and vicinal angles of sapphire substrates. The optical analysis confirmed that the In0.2Ga0.8N/GaN MQWs grown on 0.2°-off sapphire substrate exhibited the smallest carrier localization degree and more ordered In0.2Ga0.8N/GaN MQW structure. In addition, mechanisms for carrier localization in In0.2Ga0.8N/GaN MQWs grown on vicinal substrate were discussed based on the results obtained from the power and temperature dependent photoluminescence measurements. The Raman spectrum showing the in-plane compressive stress of the GaN epitaxial structures grown on vicinal sapphire substrates revealed the relation between the dislocation density and the carrier localization degree in MQWs. From transmission electron microscopy images, the threading dislocation density (TDD) of In0.2Ga0.8N/GaN MQWs grown on 0.2° vicinal sapphire substrate at the bottom of n-GaN layer was about 9.4×108 cm−2 and reduced to 3.0×108 cm−2 at the top of n-GaN layer. We also obtained the TDD of 5.6×107 cm−2 in the MQW region and only 1.0×107 cm−2 in the p-GaN region. Based on the results mentioned above, 0.2°-off substrate can offer In0.2Ga0.8N/GaN MQW blue light-emitting diode structures with benefits, such as high crystal quality, low defects, and small carrier localization degree.

Comparison of surface morphologies in GaN films grown by rf-MBE and MOCVD on vicinal sapphire (0 0 0 1) substrates

Surface morphologies of GaN films grown on vicinal sapphire (0 0 0 1) substrates by molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) are investigated using atomic force microscopy (AFM). It is found that surface morphologies of GaN films depend not only on the vicinal angle and the inclination direction of the substrate, but also on the growth technique. However, the phenomenon concerning the surface step bunching during the growth is common and independent of the growth technique. Our experimental results give important hints to the precise control of the surface morphology in order to achieve high-quality device structures.

Enhancing the Emission Efficiency of In0.2Ga0.8N ∕ GaN MQW Blue LED by Using Appropriately Misoriented Sapphire Substrates

In this paper, we report the fabrication of an multiple-quantum-well (MQW) structure on vicinal sapphire substrates with a very small offset angle of 0–1° by low-pressure metallorganic chemical vapor deposition. Our study demonstrates that the quality of the MQW structure is very sensitive to the offset angle of the vicinal substrate. High-resolution X-ray diffraction analyses demonstrated high-order satellite peaks and clear fringes between them for all MQW structures fabricated, from which the interface roughness (IRN) was estimated. The IRN of the MQW structure fabricated on 0.2° off sapphire substrate was determined as 1.36% of the quantum well layer period. Besides, reciprocal lattice mapping was employed to examine the strain status of the MQW. The lattice relaxation of the same specimen mentioned above was estimated to be . It is therefore manifested that an MQW structure with abrupt interfaces and good layer periodicity was grown. From it, a shortest radiative lifetime of and a lowest fluctuation of in the emission energy of micro-photoluminescence mapping were achieved. In addition, superior material qualities of the whole film fabricated on 0.2° off substrate were recognized by cross-sectional transmission electron microscopy. Based on the results mentioned above, a high-quality MQW blue light, emitting diode (LED) has been fabricated on 0.2° off substrate, which demonstrated a strong room-temperature electroluminescence emission at the wavelength of and with the full width at half maximum of only . The same device also showed an output power of and an external quantum efficiency of 19.2%. Both these characteristics are improved drastically compared with the devices fabricated on the substrates with other offset angles. Conclusively, the use of an appropriately misoriented sapphire substrate is suggested to be effective for elevating the emission efficiency of MQW blue LED fabricated thereon.

M ‐plane InGaN/GaN light emitting diodes fabricated by MOCVD regrowth on c ‐plane patterned templates

AbstractIn this work we demonstrate a light emitting diode (LED) with m ‐plane quantum wells fabricated on a (000$ \bar 1 $) template. N‐polar, n‐type GaN was grown by MOCVD on vicinal sapphire substrates. Stripes, measuring 500 nm wide, 500 nm tall and spaced 2 μm apart, were etched parallel to the 〈11$ \bar 2 $0〉 direction leading to sidewalls that are approximately {10$ \bar 1 $0}. Sputtered AlN was used as a regrowth mask on the c ‐plane surfaces. An active region consisting of 5 InGaN quantum wells and GaN barriers followed by p‐type was grown. The regrowth occurred mostly on the exposed m ‐plane sidewalls, leading to lateral growth in the 〈10$ \bar 1 $0〉 direction. The LED was processed using conventional methods. A thick metal contact was used to connect the p‐regions together. Current vs. voltage measurements showed good rectifying behavior with a turn on of about 6 volts. On‐wafer electroluminescence measurements revealed a peak wavelength of 422 nm. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Rf‐MBE growth and characterizations of AlGaN/GaN HEMTs on vicinal sapphire (0001) substrates

AbstractWe report the rf‐MBE growth and characterizations of AlGaN/GaN heterostructures and their HEMTs on vicinal sapphire (0001) substrates. Vicinal angle dependences of the property (surface morphologies and 2DEG mobility) in AlGaN/GaN heterostructures are investigated. It is found that the surface morphology and the 2DEG mobility is greatly improved by using vicinal substrates. Furthermore, the gate‐leakage current in the AlGaN/GaN‐based HEMTs, which is important for the device operation, is suppressed due to the improvement of the surface morphology and the reduction of the dislocation density in the heterostrucutes. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)