Transparent Contact Layer Research Articles

Effects of Temperature on Niobium-Doped MgZnO Films Grown Using Radio-Frequency Magnetron Sputtering

Niobium-doped MgxZn1−xO (Nb-MZO) mixed oxide films with high transmittance were successfully deposited on sapphire substrates through radio-frequency (RF) magnetron sputtering using a 4-in ZnO/MgO/NbOx (75/20/5 wt%) target. In this study, the films were analyzed using Hall measurements, X-ray diffraction (XRD), transparent performance measurements, and X-ray photoelectron spectroscopy (XPS). XRD results showed two peaks: MgO2 (002)-wurtzite and MgxZn1−xO (111)-cubic peaks. The Nb-MZO films exhibited high transparency, with transmittance exceeding 90% in the visible region and a sharp absorption edge in the ultraviolet (UV) region, implying that the MgO content in the Nb-MZO layer increased the bandgaps. These results indicate that Nb-MZO films are ideal for use as transparent contact layers in near-UV light-emitting diodes. Hall measurements and XPS results successfully demonstrated the p-type conductivity of the Nb-MZO films because of the composition of the N-Nb binding caused by annealing in nitrogen atmosphere.

Anion-controlled passivation effect of the atomic layer deposited ZnO films by F substitution to O-related defects on the electronic band structure for transparent contact layer of solar cell applications

Anion-controlled chemistry in ZnO films is essential to obtain stable charge–carrier transport and to prevent degradation in the performance of the transparent contact layer in solar cells through passivation of O-related defects by F substitution. Therefore, in this work, the passivation effect of F doping in ZnO was confirmed by analysis for the chemical state of O in ZnO films using XPS surface analysis in the O 1s region. Furthermore, we investigated the effect of F doping in a ZnO matrix on the electronic structure of the resulting films as a function of electron concentration by optical band gap shift (Burstein–Moss theory) and near-band-edge transition (photoluminescence with Stokes shift theory) measurements in ZnO films with different F doping concentrations. The band gap narrowing effect was not significant in the F-doped ZnO films due to valence band perturbation from anion doping rather than conduction band perturbation. Finally, the band structure could be approximated by the relationship between experimental analysis and the formation of valence and conduction bands in ZnO orbitals.

Recent progress and future prospects of AlGaN-based high-efficiency deep-ultraviolet light-emitting diodes

In this paper, recent advances in AlGaN-based deep-ultraviolet (DUV) light-emitting diodes (LEDs) are demonstrated. 220–350-nm-band DUV LEDs have been realized by developing crystal growth techniques for wide-bandgap AlN and AlGaN semiconductors. Significant increases in internal quantum efficiency (IQE) have been achieved for AlGaN DUV emissions by developing low-threading-dislocation-density (TDD) AlN buffer layers grown on sapphire substrates. The electron injection efficiency (EIE) of the LEDs was also significantly increased by introducing a multiquantum barrier (MQB). We also discuss light extraction efficiency (LEE), which is the most important parameter for achieving high-efficiency DUV LEDs. We succeeded in improving LEE by developing a transparent p-AlGaN contact layer. The maximum external quantum efficiency (EQE) obtained was 7% for a 279 nm DUV LED. EQE could be increased by up to several tens of percent through the improvement of LEE by utilizing transparent contact layers and photonic nanostructures in the near future.

Effects of rapid thermal annealing on properties of Ga-doped MgxZn1−xO films and Ga-doped MgxZn1−xO/AlGaN heterojunction diodes

This study investigated the thermal annealing effects of Ga-doped MgxZn1−xO (GMZO) films and GMZO/AlGaN heterojunction diodes. GMZO films were deposited using a radio-frequency magnetron sputtering system with a 4-in. ZnO/MgO/Ga2O3 target. In addition, the Hall results, X-ray diffraction, transparent performance, and X-ray photoelectron spectroscopy (XPS) spectra were measured. The as-grown GMZO film deposited in this study exhibited a high transparency with transmittances over 95% in the visible region (360–700 nm) and a sharp absorption edge in the UV region (275–350 nm). The phenomenon of phase separation in the GMZO films was investigated based on the XPS spectra, revealing that an increase in the O-Zn signal accompanied a decline in the O-Ga signal after the thermal annealing. Moreover, the current-voltage (I-V) characteristics of the GMZO/AlGaN n-p junction diodes were examined at different annealing temperatures. The light emission derived from the forward-biased junction and near-ultraviolet (near-UV) light emission was evident at all p-n junctions. The n-GMZO/p-AlGaN diode annealed at 800 °C exhibited a brighter near-UV emission compared with the other diodes. In addition, the spectrum of diode annealed at 800 °C exhibited a broad peak at 474 nm (2.62 eV) and a tail of the emission spectrum extending to 850 nm. Based on these findings, the GMZO films are suitable for forming transparent contact layers in optoelectronic devices, and the n-GMZO/p-AlGaN junction diode is a feasible alternative in near-UV light emission devices.

Transparent back contacts for P3HT:PCBM bulk heterojunction solar cells

A new combination of layers functioning as a transparent contact is proposed and tested in real solar cells. The contacts consist of TiO2 layers and thin metal layers (Ag, Cu) and are deposited by magnetron sputtering. The optical transmission and electrical conductivity of the transparent contact layers (TCL) are measured. The TCLs are applied as back contacts in bulk heterojunction polymer solar cells deposited on ITO covered glass and consisting of the following layers: ITO/PEDOT:PSS/P3HT:PCBM/back contact. The organic layers are deposited by spin-coating. For comparison, the same bulk heterojunction polymer solar cells are prepared with a sputtered Ag back contact. The first results show a dependence of the current-voltage parameters of the studied solar cells on the thickness of the different component layers of the transparent back contacts. There is a balance that has to be observed between the electrical characteristics of the contacts and their optical transparency. Future plans involve their inclusion as intermediate contacts in tandem organic solar cells.

Thermal stability of post-growth-annealed Ga-doped MgZnO films grown by the RF sputtering method

ABSTRACTA radio-frequency magnetron sputtering technique and subsequent rapid thermal annealing (RTA) at 600, 700, 800, and 900 °C were implemented to grow high-quality Ga-doped MgxZn1-xO (GMZO) epi-layers. The GMZO films were deposited using a radio-frequency magnetron sputtering system and a 4 inch ZnO/MgO/Ga2O3 (75/20/5 wt %) target. The Hall results, X-ray diffraction (XRD), and transmittance were determined and are reported in this paper. The Hall results indicated that the increase in mobility was likely caused by the improved crystallization in the GMZO films after thermal annealing. The XRD results revealed that MgxZn1-xO (111) and MgO2 (200) peaks were obtained in the GMZO films. The absorption edges of the as-grown and annealed GMZO films shifted toward the short wavelength of 373 nm at a transmittance of 90%. According to these results, GMZO films are feasible for forming transparent contact layers for near-ultraviolet light-emitting diodes.

Optimisation of surface treatments of TiO2:Nb transparent conductive coatings by a post-hot-wire annealing in a reducing H2 atmosphere

Transparent and electrically conductive niobium-doped TiO2 thin films have been deposited on glass surfaces by d.c.-pulsed reactive magnetron sputtering from a composite Ti:Nb target, using oxygen as reactive gas. A rapid 1min annealing at 500°C in an atomic hydrogen rich atmosphere, obtained by flowing H2 on a Ta filament resistively heated to 1750°C in vacuum (hot-wire), proved to be very efficient in enhancing the electrical properties of these ~100nm thick TiO2:Nb thin films. Dark conductivity (σd) and its activation energy were measured as a function of (inverse) temperature and the value of σd at room temperature was used to assess the effect of the H2 annealing on the transport properties. A 5-order of magnitude increase in electrical conductivity was observed for optimised treatment conditions at a hydrogen pressure of 10Pa. A maximum value of σd in the range of ~1.4×103S/cm was attained for optimised conditions, where a level of ~6at.% of H doping was measured close to the film surface. X-ray photoelectron spectroscopy, elastic recoil detection analysis, Rutherford backscattering and Raman spectroscopies were used to access information of composition and film structure for the explanation of the strong enhancement of the film's electrical conductivity and band-gap widening to 3.45eV following hot-wire treatments. These thin films can be used as transparent conductive oxide contact layers for photovoltaic applications.

Realization of high‐efficiency deep‐UV LEDs using transparent p‐AlGaN contact layer

AbstractWe demonstrated high‐efficiency deep‐ultraviolet light‐emitting diodes (DUV‐LEDs) by improving light‐extraction efficiency (LEE) by using a transparent p‐AlGaN contact layer and a highly‐reflective p‐type electrode. We fabricated DUV‐LEDs with emission wavelengths between 265‐288 nm using p‐AlGaN contact layers with compositional wavelength at around 275 nm. The reflectivity of p‐type electrode was increased to be approximately 70% by introducing Ni(1nm)/Al layers. The external quantum efficiency (EQE) of the 287 nm LED was increased from 2% to 5.5% by replacing conventional p‐GaN contact layer by transparent p‐AlGaN contact layer. The increase of LEE was estimated to be by approximately more than 1.7 times. The use of transparent p‐AlGaN contact layer is considered to be basic and quite important technique in order to obtain high LEE in DUV LEDs. The EQE of DUV‐LED would be much increased by optimizing the device structure and by combining with other approaches to improve LEE. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Linear facing target sputtering of the epitaxial Ga-doped ZnO transparent contact layer on GaN-based light-emitting diodes

In this study, we reported on the plasma damage-free sputtering of epitaxial Ga-doped ZnO (GZO) films on the p-GaN layer for use as a transparent contact layer (TCL) for GaN-based light-emitting diodes (LEDs) using linear facing target sputtering (LFTS). Effective confinement of high-density plasma between faced GZO targets and the substrate position located outside of the plasma region led to the deposition of the epitaxial GZO TCL with a low sheet resistance of 25.7 Ω/s and a high transmittance of 84.6% on a p-GaN layer without severe plasma damage, which was found using the conventional dc sputtering process. The low turn-on voltage of the GaN-based LEDs with an LFTS-grown GZO TCL layer that was grown at a longer target-to-substrate distance (TSD) indicates that the plasma damage of the GaN-LED could be effectively reduced by adjusting the TSD during the LFTS process.

The effects of thickness on the electrical, optical, structural and morphological properties of Al and Ga co-doped ZnO films grown by linear facing target sputtering

We investigated the effects of thickness on the electrical, optical, structural, and morphological properties of Al and Ga co-doped ZnO films (AGZO) grown by linear facing target sputtering (LFTS) for use as a transparent contact layer (TCL) in GaN-light emitting diodes (LEDs). Below a critical thickness of 200 nm, the resistivity and optical transmittance of the AGZO films were significantly affected by the thickness of the AGZO films. However, above a thickness of 200 nm, the AGZO films had similar resistivities and optical transmittances due to the stable columnar structure, which developed at a thickness of 200 nm. Due to the change of the growth mode with increasing thickness, the microstructure and surface morphology were also affected by the film thickness. Based on the figure of merit values, we determined that the optimized thickness of the LFTS-grown AGZO film was 200 nm, which was applied in a GaN-LED as a TCL. Successful operation of GaN-LEDs with an optimized AGZO film without plasma damage indicates that the LFTS-grown AGZO film is promising plasma damage-free TCL for use in GaN-LEDs.

SILAR‐Based Application of Various Nanopillars on GaN‐Based LED to Enhance Light‐Extraction Efficiency

We reported the various nanopillars on GaN‐based LED to enhance light‐extraction efficiency prepared by successive ionic layer adsorption and reaction method (SILAR). Indium tin oxide (ITO) with thickness of 1 μm as transparent contact layer was grown to improve the electrical characteristics of the LEDs, including series resistance and operating voltage. SILAR‐deposition ZnO nanoparticles on SiO2 were used as etching nanomasks. Multiple nanopillars were simultaneously formed on overall surfaces of ITO p‐ and n‐GaN by ICP etching. The proposed GaN‐based LEDs with nanopillars increase light output power by 7%–20.3% (at 20 mA) over that of regular GaN‐based LEDs. The difference in light output power can be attributed to differences in materials and shapes of nanopillars, resulting in a reduction in Fresnel reflection by the roughened surface of GaN‐based LEDs.

Mepitel® One: a wound contact layer with Safetac® technology

This article examines the use of Mepitel One, a wound contact layer that promotes wound healing by using the clinical benefits of Safetac technology. A wide range of clinical evidence shows that dressings with Safetac prevent trauma, minimise wound pain and associated psychological stress, while facilitating undisturbed wound healing. Mepitel One has been developed as a highly transparent wound contact layer with Safetac on the wound contact surface alone; this makes it easy to apply without sticking to gloves, and allows it to be used with a choice of secondary dressings. The dressing can stay in place for up to 14 days and allows examination of the wound without removal. The clinical evidence reviewed here shows that Mepitel One is a versatile option for use on a wide range of wound types to promote undisturbed healing and minimise pain at dressing changes.

Improvement of GaN light-emitting diodes with surface-treated Al-doped ZnO transparent Ohmic contacts by holographic photonic crystal

This letter presents a holographic photonic crystal (H-PhC) Al-doped ZnO (AZO) transparent Ohmic contact layer on p-GaN to increase the light output of GaN-based LEDs without destroying the p-GaN. The operating voltage of the PhC LEDs at 20 mA was almost the same as that of the typical planar AZO LEDs. While the resultant PhC LED devices exhibited significant improvements in light extraction, up to 1.22 times that of planar AZO LEDs without PhC integration. Temperature dependence of the integrated photoluminescence intensity indicates that this improvement can be attributed to the increased extraction efficiency due to the surface modification. These results demonstrate that the surface-treated AZO layer by H-PhCs is suitable for fabricating high-brightness GaN-based LEDs.

Enhancement of Light Extraction Efficiency of InGaN Light-Emitting Diodes with an Air-Hole-Array Structure

The truncated-conical air-hole (TAH) array structure of an InGaN light-emitting diode (LED) was fabricated on the mesa-edge region to increase the light extraction efficiency. The fabrication consisted of a dry process and a crystallographic wet etching process on the AlN buffer layer to form a truncated-conical air-hole array pattern. The light output power of the TAH-LED structure has a 55% enhancement compared with the conventional LED structure at 20 mA operation current. At 20 mA operation current, the forward voltage and peak electroluminescence wavelength of the TAH-LED were measured to be 3.09 V and 455.6 nm, respectively, similar to those of the conventional LED structure because the truncated-conical air-hole array pattern was fabricated around the mesa-edge region without affecting the current injection area with a top transparent contact layer.

Enhancement of Light Extraction Efficiency of InGaN Light-Emitting Diodes with an Air-Hole-Array Structure

The truncated-conical air-hole (TAH) array structure of an InGaN light-emitting diode (LED) was fabricated on the mesa-edge region to increase the light extraction efficiency. The fabrication consisted of a dry process and a crystallographic wet etching process on the AlN buffer layer to form a truncated-conical air-hole array pattern. The light output power of the TAH-LED structure has a 55% enhancement compared with the conventional LED structure at 20 mA operation current. At 20 mA operation current, the forward voltage and peak electroluminescence wavelength of the TAH-LED were measured to be 3.09 V and 455.6 nm, respectively, similar to those of the conventional LED structure because the truncated-conical air-hole array pattern was fabricated around the mesa-edge region without affecting the current injection area with a top transparent contact layer.

Enhanced Output Power of GaN-Based Resonance Cavity Light-Emitting Diodes With Optimized ITO Design

We fabricated and measured GaN-based resonant cavity light-emitting diodes with a 30 nm thick Indium tin oxide (ITO) thin film as a transparent contact layer. Four different ITO structures on p-type GaN samples were deposited by sputter and e-gun, and the corresponding device performance was compared. Each of these four samples has been annealed by its optimal parameters. The ITO thin film deposited by sputter demonstrated better electrical characteristics, surface morphology, specific contact resistance, and the overall device light output compared to those of the e-gun samples. Between the two sputtered ITO types, the hybrid type shows higher roll-over current density of 14 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , and the output power is increased from 15 to 39 μ W. From statistical data of the 2-D light intensity under the same current, we saw the lateral current spreading of the pure crystalline ITO by sputter is worst. The hybrid type, which combines the crystalline and amorphous ITO, has the best overall performance when we consider all the electrical, optical, and metrology measurements. From these results, we believe the 30 nm thick hybrid ITO thin film has the best potential to be applied in light emitting devices such as light-emitting diodes, laser diodes, etc.

Enhancement in Light Extraction of GaN-Based Light-Emitting Diodes With High Reflectivity Electrodes

In GaN-based light-emitting diodes (LEDs), using gold-based metals to serve as electrodes, light absorption is a problem that severely restricts the light extraction in LEDs. This study demonstrates the GaN-based LEDs with high reflectivity metals (Ag/Pt) onto an n-type GaN surface and transparent contact layer (indium-tin-oxide), to serve as the n-type electrode and the p-type electrode, respectively. By replacing Cr/Au with Ag/Pt to serve as electrodes of LEDs, the light output power of the LEDs was increased by 15.7%, thereby significantly reducing the manufacturing cost of LEDs.

Effect of hot-filament annealing in a hydrogen atmosphere on the electrical and structural properties of Nb-doped TiO2 sputtered thin films

In this work Nb-doped TiO2 thin films were deposited by d.c.-pulsed reactive magnetron sputtering at 500°C from a composite target with weight fractions of 96% Ti and 4% Nb, using oxygen as reactive gas. In order to enhance the conductive properties, the as-deposited samples were treated in vacuum with atomic hydrogen at a substrate temperature of 500°C. The atomic hydrogen flow was generated by a hot filament, inside a high-vacuum chemical vapour deposition reactor, at a temperature of 1750°C. In order to optimise the hydrogen hot-wire treatments, the H2 pressure was varied between 1.3 and 67Pa, the treatment time was monitored between 1 and 5min and the hot-filament current was changed between 12 and 17A. Dark conductivity was measured as a function of temperature and its value at room temperature was extrapolated and used to assess the effect of the hydrogen annealing on the charge transport properties. A two-order of magnitude increase in dark conductivity was typically observed for optimised hydrogen treatments (10Pa), when varying the hydrogen pressure, resulting in a minimum resistivity of ~3×10−3Ωcm at room temperature. The maximum amount of atomic H incorporation in oxygen vacancies was determined to be ~5.7at.%. Carrier mobility and resistivity were also investigated using Hall effect measurements. Correlations between structural and electrical properties and the hydrogen treatment conditions are discussed. The purpose of these films is to provide a transparent and conductive front contact layer for a-Si based photovoltaics, with a refractive index that better matches that of single and tandem solar cell structures. This can be achieved by an appropriate incorporation of a very small amount of cationic doping (Nb5+) into the titanium dioxide lattice.

Pulsed laser deposition of ITO/AZO transparent contact layers for GaN LED applications

In this study, indium-tin oxide (ITO)/Al-doped zinc oxide (AZO) composite films were fabricated by pulsed laser deposition and used as transparent contact layers (TCLs) in GaN-based blue light emitting diodes (LEDs). The ITO/AZO TCLs were composed of the thin ITO (50 nm) films and AZO films with various thicknesses from 200 to 1000 nm. Conventional LED with ITO (200 nm) TCL prepared by E-beam evaporation was fabricated and characterized for comparison. From the transmittance spectra, the ITO/AZO films exhibited high transparency above 90% at wavelength of 465 nm. The sheet resistance of ITO/AZO TCL decreased as the AZO thickness increased, which could be attributed to the increase in a carrier concentration, leading to a decrease in the forward bias of LED. The LEDs with ITO/AZO composite TCLs showed better light extraction as compared to LED with ITO TCL in compliance with simulation. When an injection current of 20 mA was applied, the output power for LEDs fabricated with ITO/AZO TCLs had 45%, 63%, and 71% enhancement as compared with those fabricated using ITO (200 nm) TCL for the AZO thicknesses of 200, 460, and 1000 nm, respectively.

Improved Light Extraction Efficiency by Photonic Crystal Arrays on Transparent Contact Layer Using Focused Ion Beams

Improved Light Extraction Efficiency by Photonic Crystal Arrays on Transparent Contact Layer Using Focused Ion Beams