Chuing-Liang Lin

The aspect ratio effects on the performances of GaN-based light-emitting diodes with nanopatterned sapphire substrates

The nanopatterned sapphire substrates (NPSSs) with aspect ratio that varied from 2.00 to 2.50 were fabricated by nanoimprint lithography. We could improve the epitaxial film quality and enhance the light extraction efficiency by NPSS technique. In this work, the aspect ratio effects on the performances of GaN-based light-emitting diodes (LEDs) with NPSS were investigated. The light output enhancement of GaN-based LEDs with NPSS was increased from 11% to 27% as the aspect ratio of the NPSS increases from 2.00 to 2.50. Owing to the same improvement of crystalline quality by using various aspect ratios of NPSS, these results indicated that the aspect ratio of the NPSS is strongly related to the light extraction efficiency.

GaN-Based Light-Emitting Diodes Grown on Photonic Crystal-Patterned Sapphire Substrates by Nanosphere Lithography

GaN-based light-emitting diodes (LEDs) grown on photonic crystal-patterned sapphire substrates (PCPSS) have been demonstrated. PCPSS was fabricated by nanosphere lithography, and the photonic crystal structure was the hexagonal-lattice pattern. The forward voltages of PCPSS and patterned sapphire substrates (PSS) LEDs were smaller than that of conventional sapphire substrates (CSS) LED, and it infers the epitaxial film quality of PCPSS and PSS LEDs has been slightly improved. The luminance intensity of PCPSS LED was 1.63 and 1.51 times higher than those of CSS and PSS LED at 20 mA injection current. The enhancement in the luminance intensity of PCPSS LED is attributed to the photonic crystal structure.

Effects of passivation and extraction surface trap density on the 1/f noise of HgCdTe photoconductive detector

Experimental results are presented for noise voltage, responsivity, and specific detectivity (D*) for the long wavelength infrared (IR) HgCdTe photoconductive detectors. Hg/sub 0.8/Cd/sub 0.2/Te photoconductive detectors passivated with ZnS/photo-enhanced native oxide have an improved noise spectral density and D* than the detectors passivated with only ZnS. The low frequency 1/f noise charges were measured for a Hg/sub 0.8/Cd/sub 0.2/Te photo detector, as a function of bias at 77 K, and the effective surface trap density determined from the 1/f noise charges measured at 1 Hz. It was found that the surface effective trap densities of stacked passivated sample and the sample passivated only with ZnS are close to 4/spl times/10/sup 17/ and 9/spl times/10/sup 17/ cm/sup -2/ eV/sup -1/ under 0.4 V bias and 1 /spl mu/s integration time, respectively. We found that the numerical values of noise are strongly dependent upon surface passivation properties. It can be seen clearly that an HgCdTe photo detector with a stacked ZnS/photo-enhanced native oxide passivation is better than the HgCdTe photo detector passivated with a single ZnS layer.

Electrical properties of the stacked ZnS/photo-enhanced native oxide passivation for long wavelength HgCdTe photodiodes

A new surface treatment method, i.e., stacked ZnS/photo-enhanced native oxide, is proposed for HgCdTe passivation. The photo native oxide layer was deposited by direct photo chemical vapor deposition (DPCVD) using a deuterium (D/sub 2/) lamp as the optical source. By using this method, we found that there is no accumulation of Hg in the oxide/HgCdTe interface regions. Since the photo chemical vapor native oxidation is a dry oxidation method deposited at a low temperature, it can effectively suppress the Hg enhancement and the Cd depletion effects and thus obtain a high quality interface. The electrical properties of a Au-ZnS/photo-enhanced native oxide/HgCdTe metal-insulator-semiconductor (MIS) diodes were characterized by capacitance-voltage (C-V) measurement. It was found that the flat band voltage of such a MIS diode is close to 0.2 V with an effective fixed oxide charge lower than 1/spl times/10/sup 10/ cm/sup -2/.

Photo-enhanced native oxidation process for Hg/sub 0.8/Cd/sub 0.2/Te photoconductors

Proposes an easy and reproducible vapor-phase photo surface treatment method to improve the device performance of the Hg/sub 0.8/Cd/sub 0.2/Te photoconductive detector. We explore the effect of surface passivation on the electrical and optical properties of the HgCdTe photoconductor. Experimental results, including surface mobility, surface carrier concentration, metal-insulator-semiconductor leakage current, 1/f noise voltage spectrum, the 1/f knee frequency, responsivity R/sub /spl lambda//, and specific detectivity D* for stacked photo surface treatment and ZnS or CdTe passivation layers are presented. These data are all directly related to the quality of the interface between the passivation layer and the HgCdTe substrate. We found that, by inserting a photo native oxide layer, we can shift the 1/f knee frequency, reduce the noise power spectrum, and achieve a lower surface recombination velocity S. A higher D* can also be achieved. It was also found that HgCdTe photoconductors passivated with stacked layers show improved interface properties compared to the photoconductors passivated only with a single ZnS or CdTe layer.

HIGH CONDUCTION-BAND OFFSET OF ALINASSB/INGAAS MULTIPLE QUANTUM WELLS GROWN BY METALORGANIC VAPOR PHASE EPITAXY

Unstrained Al0.66In0.34As0.85Sb0.15/In0.53Ga0.47As multiple-quantum-well (MQW) structures have been grown by metalorganic vapor phase epitaxy. Low-temperature photoluminescence was performed for these MQW structures. We compared the experimental data with the theoretical calculations. The conduction-band offset ratio of AlInAsSb/InGaAs heterojunction was set as an adjustable parameter in the theoretical model. We estimated the conduction-band offset ratio to be 0.90±0.05 for the Al0.66In0.34As0.85Sb0.15/In0.53Ga0.47As heterojunction.

Efficiency Improvement of GaN-Based LEDs With

High-performance nitride-based light-emitting diodes (LEDs) grown with SiO2 microrod array have been demonstrated. The light output power of LEDs with SiO2 microrod array was 9.03% higher than conventional LEDs at the injection current of 20 mA. The improvement contributed to the enhancement of the light extraction efficiency, and epitaxial GaN film quality improved by direct heteroepitaxial lateral overgrowth with SiO2 microrod array. The light output power could be further enhanced by about 18.36% as compared with the conventional LEDs when adopting the textured sidewall surface which use buffered oxide etch to remove SiO2 microrod arrays and use NaOH to etch the sidewall again into an inverted pyramid shape. After the texturing process, the LEDs show higher electroluminescence intensity and broader far-field pattern. Furthermore, the LEDs with SiO2 microrod array and additional wet-etching process will not affect the electrical property.

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We introduce a method to enhance the luminescence of GaN-based LEDs by combining the direct heteroepitaxy laterally overgrowth (DHELO) technique with selective wet etching process. The epitaxial overgrowth of GaN layers on sapphire substrate with SiO2 micro-rods array exhibited a reduced dislocation density and improved the crystal quality. The EL intensity of LEDs with SiO2 micro-rods array was 6.5% higher than conventional LEDs at 20 mA. The selective wet etching process was then used to texture the LED sidewalls into inverted pyramid shape. Finally, the EL intensity could be further enhanced about 12.5% as compared with LEDs with SiO2 micro-rods array when adopting the textured sidewalls. 2011 Published by Elsevier B.V.

Microrod Array and Textured Sidewalls

The temperature dependence of barrier height and energy bandgap in Au/n-GaSb Schottky diode was first determined in a wide temperature range (156–316 K) by photoresponse measurement. While the energy bandgap of n-GaSb decreases from 0.79 eV to 0.73 eV with an increase in temperature (from 156 K to 316 K), the barrier height of Au/n-GaSb also decreases from 0.69 eV to 0.57 eV. Though sulfide treatment was taken to modify the metal-semiconductor interface, Fermi level at the interface was almost pinned (0.1–0.16 eV) above the valence band edge. Similar to semiempirical Varshnis law, using our experimental fitting, we obtained the temperature expressions of the barrier height and the energy bandgap of Au/n-GaSb Schottky diode.

Enhanced luminescence of GaN-based light-emitting diodes by selective wet etching of GaN/sapphire interface using direct heteroepitaxy laterally overgrowth technique

The wafer bonding process and chemical mechanical polishing (CMP) technique are used to remove the sapphire substrate for fabricating the thin-GaN light emitting diodes (LEDs). The stress effects in GaN epilayers on very thin sapphire substrates are analyzed by Raman spectroscopy and photoluminescence (PL). By reducing the thickness of the sapphire substrate from 450 µm to the range between 1.1 and 20.3 µm, the compressive stress in GaN epilayers will be almost released. By analyzing the main scattering signal of E2 (high) mode, the sample with thinnest sapphire of 1.1 µm exhibits 248.3 MPa stress relaxation which is obtained from the Raman shift Δω of 1.54 cm-1. By analyzing the variation of the biaxial stress with sapphire thickness, the relationship between the Raman wavenumbers ω (in cm-1) and residual sapphire thicknesses d (in µm) can be expressed by ω=569.51-1.92×exp (-d/5.81) (cm-1). Finally, an almost linear relationship between residual sapphire thickness and energy bandgap of GaN films was also observed during the CMP process.