SiO2 Passivation Research Articles

Characteristics of GaN-Based LEDs With Hybrid Microhole Arrays and SiO2Microspheres/Nanoparticles Structures

The Characteristics of GaN-based LEDs with hybrid microhole arrays and SiO2 microspheres (MSs)/ nanoparticles (NPs) are comprehensively studied. The SiO2 MSs/NPs antireflection coating, deposited by a rapid convection deposition, acts as a passivation layer of GaN-based LEDs. Since the critical angle could be enlarged by antireflection coating, Fresnel reflection could be reduced. In addition, due to the roughened surface of SiO2 MSs/NPs antireflection coating, the scattering effect could also be increased. Thus, the light extraction efficiency could be further enhanced. As compared with a conventional LED with a planar aluminum-doped zinc oxide current spreading layer (Device A), the studied device with the proposed hybrid structure and a sputtered SiO2 passivation layer (Device E) causes a suppressed leakage current and % enhancements on light output power, external quantum efficiency, and wall-plug efficiency performance.

Toward realization of small-size dual-band long-wavelength infrared photodetectors based on InAs/GaSb/AlSb type-II superlattices

In this study, we demonstrate 12×12µm2 high-performance, dual-band, long-wavelength infrared (LWIR) photodetectors based on InAs/GaSb/AlSb type-II superlattices. The structure consists of two back-to-back heterojunction photodiodes with 2µm-thick p-doped absorption regions. High quality dry etching combined with SiO2 passivation results in a surface resistivity value of 7.9×105Ωcm for the longer (red) channel and little degradation of the electrical performance. The device reaches dark current density values of 4.5×10−4A/cm2 for the longer (red) and 1.3×10−4A/cm2 for the shorter (blue) LWIR channels at quantum efficiency saturation. It has 50% cut-off wavelengths of 8.3 and 11.2µm for the blue and red channel, respectively, at 77K in back-side illumination configuration and exhibits quantum efficiencies of 37% and 29%, respectively. This results in specific detectivity values of 2.5×1011cm·Hz1/2/W and 1.3×1011cm·Hz1/2/W at 77K.

Reduction of ∙Si≡Si3 defect density at the Si/SiO2 interface by sol-gel SiO2 thin film passivation

The effect of SiO2 thin film passivation by sol-gel processes with various acid catalysts, ethanol (EtOH): tetraethyl orthosilicate (TEOS) volume ratios and baking temperatures on the carrier lifetime of the Si surface and Si/SiO2 interface properties was studied. SiO2 thin film passivation prepared by H2SO4 catalyst with a EtOH:TEOS volume ratio of 10:1 at a baking temperature of 50°C greatly increased the carrier lifetime on the various Si wafers regardless of dopant type, wafer resistivity and crystal orientation. For example, the carrier lifetime increased from 28 to 316μs before and after the sol-gel SiO2 passivation on the Si (100) surface with resistivity higher than 1000Ωcm. It was confirmed that the increase of carrier lifetime resulted from the significant decrease in the defect density related to the ∙Si≡Si3 dangling bond at the Si/SiO2 interface. In the current study, proper sol-gel SiO2 film passivation increased the carrier lifetime on the Si surface by controlling the formation of ∙Si≡Si3 defects at the Si/SiO2 interface.

Recovery of Bias-Stress Ionized Igzo/SiO2 Interface States Via Cryogenic Relaxation

Indium gallium zinc oxide (IGZO) has been considered a potential replacement for hydrogenated amorphous silicon in TFT applications due to process compatibility and an order of magnitude improvement in electron channel mobility. However the mechanisms responsible for instability under bias-stress remain an active research topic. Silicon dioxide serving as the gate dielectric and back-channel passivation layer in bottom-gate IGZO TFTs results in high quality interfaces. Applied bias-stress conditions demonstrate a voltage shift that can be attributed to ionization and/or alteration of oxygen-related defects at these IGZO/SiO2interfaces. Bottom-gate IGZO TFTs with SiO2 gate dielectric and passivation layers were fabricated and tested under rigorous positive bias-stress (PBS) and negative bias-stress (NBS) conditions. The devices demonstrate good stability under PBS (t > 104 s), with minor distortion in the subthreshold region and a slight characteristic left-shift from an initial pre-stress state. This PBS-shift is attributed to a change in the energy distribution of defect states at the front-channel interface. During NBS the devices exhibited a significant left-shift (ΔV ~ 1-2 V), which is attributed to the transformation of neutral oxygen vacancies to ionized donor states at the back-channel interface. This transformation appears to improve the electrical homogeneity of the back-channel interface, inferred by a suppression of DIBL-like behavior. The NBS-shift was found to be reversible over long recovery times at room temperature. The recovery time was dramatically reduced when samples were subjected to cryogenic temperature (77 K), which represents an accelerated return to the pre-stress condition. TCAD simulation provides additional support to the interpretation of bias-induced stress on IGZO TFTs.

필드 플레이트가 설계된 다이아몬드 쇼트키 장벽 다이오드

Power semiconductor devices required the low on-resistance and high breakdown voltage. Wide band-gap materials opened a new technology of the power devices which promised a thin drift layer at an identical breakdown voltage. The diamond had the wide band-gap of 5.5 eV which induced the low power loss, high breakdown capability, low intrinsic carrier generation, and high operation temperature. We investigated the p-type pseudo-vertical diamond Schottky barrier diodes using a numerical simulation. The impact ionization rate was material to calculating the breakdown voltage. We revised the impact ionization rate of the diamond for adjusting the parallel-plane breakdown field at 10 MV/cm. Effects of the field plate on the breakdown voltage was also analyzed. A conventional diamond Schottky barrier diode without field plate exhibited the high forward current of 0.52 A/㎜ and low on-resistance of 1.71 Ω-㎜ at the forward voltage of 2 V. The simulated breakdown field of the conventional device was 13.3 MV/㎝. The breakdown voltage of the conventional device and proposed devices with the SiO₂ passivation layer, anode field plate (AFP), and cathode field plate (CFP) was 680, 810, 810, and 1020 V, respectively. The AFP cannot alleviate the concentration of the electric field at the cathode edge. The CFP increased the breakdown voltage with evidences of the electric field and potential. However, we should consider the dielectric breakdown because the ideal breakdown field of the diamond is higher than that of the SiO₂, which is widely used as the passivation layer. The real breakdown voltage of the device with CFP decreased from 1020 to 565 V due to the dielectric breakdown.

Dark current mechanisms and spectral response of SiO2-passivated photodiodes based on InAs/GaSb superlattice

The electrical performance of SiO2 passivated type-II superlattice double heterostructure photodiodes is demonstrated. Compared to unpassivated samples, a dark current of 4.009×10−5A/cm2 and the corresponding differential resistance-area of 165.82Ω·cm2 at zero bias voltage are achieved at 77K. The temperature-dependence and bias-dependence of the dark current are studied experimentally. According to fitting results, the existence of the SiO2 passivated layer alters the band bending at the interface, which influence the generation-recombination and trap-assisted tunneling current and then it is presented that the total dark current is reduced by introducing the SiO2 passivation technology. At 77K, the corresponding spectral responsivity and detectivity of the samples are measured at a bias voltage of −50mV and the 100% cutoff wavelength is determined to be 10.6μm, which is fitted well with the simulation results of temperature dependence of experimental data.

Manipulating the Band Bending of InGaN/GaN Quantum Dots in Nanowires by Surface Passivation

InGaN/GaN quantum dots (QD) in nanowires exhibit excellent optical properties and are promising candidates for nanoscale optoelectronic devices. However, a large amount of surface states would cause low quantum efficiency more severely than bulk materials, through not only nonradiative recombination centers but also upward band bending. Therefore, it is necessary to control the band bending effect in order to improve the quantum efficiency of QDs. In this work, quantitative measurements are carried out by ultraviolet photoelectron spectroscopy (UPS) to describe the band bending effect in InGaN/GaN QD in nanowires coated with three different dielectric layers including SiNx, Al2O3, and SiO2. Furthermore, their passivation mechanisms are investigated by photoluminescence (PL), time-resolved PL. Contrary to SiO2 passivation, the SiNx and Al2O3 passivation nanowires demonstrate notable improvements in emission intensity. Most essentially, all experimental findings are consilience with the physical model that ...

Direct observation of trapped charges under field-plate in p-GaN gate AlGaN/GaN high electron mobility transistors by electric field-induced optical second-harmonic generation

Trapped charges underneath the field-plate (FP) in a p-gallium nitride (GaN) gate AlGaN/ GaN high electron mobility transistor device were visualized by using electric field-induced optical second-harmonic generation imaging. Second-harmonic (SH) signals in the off-state of the device with FP indicated that the electric field decreased at the p-GaN gate edge and concentrated at the FP edge. Nevertheless, SH signals originating from trapped charges were slightly observed at the p-GaN gate edge and were not observed at the FP edge in the on-state. Compared with the device without FP, reduction of trapped charges at the p-GaN gate edge of the device with FP is attributed to attenuation of the electric field with the aid of the FP. Negligible trapped charges at the FP edge is owing to lower trap density of the SiO2/AlGaN interface at the FP edge compared with that of the SiO2/p-GaN sidewall interface at the p-GaN gate edge and attenuated electric field by the thickness of the SiO2 passivation layer on the AlGaN surface.

Surface Plasmon Enhanced Light Trapping in Metal/Silicon Nanobowl Arrays for Thin Film Photovoltaics

Enhancing the light absorption in thin film silicon solar cells with nanophotonic and plasmonic structures is important for the realization of high efficiency solar cells with significant cost reduction. In this work, we investigate periodic arrays of conformal metal/silicon nanobowl arrays (MSNBs) for light trapping applications in silicon solar cells. They exhibited excellent light-harvesting ability across a wide range of wavelengths up to infrared regimes. The optimized structure (MSNBsH) covered by SiO2 passivation layer and hemisphere Ag back reflection layer has a maximal short-circuit density (Jsc) 25.5 mA/cm2, which is about 88.8% higher than flat structure counterpart, and the light-conversion efficiency (η) is increased two times from 6.3% to 12.6%. The double-side textures offer a promising approach to high efficiency ultrathin silicon solar cells.

SiO2 passivation layer grown by liquid phase deposition for silicon solar cell application

Surface passivation is one of the primary requirements for high efficient silicon solar cells. Though the current existed passivation techniques are effective, expensive equipments are required. In this paper, a comprehensive understanding of the SiO2 passivation layer grown by liquid phase deposition (LPD) was presented, which was cost-effective and very simple. It was found that the post-annealing process could significantly enhance the passivation effect of the LPD SiO2 film. Besides, it was revealed that both chemical passivation and field-effect passivation mechanisms played important roles in outstanding passivation effect of the LPD SiO2 film through analyzing the minority carrier lifetime and the surface recombination velocity of n-type and p-type silicon wafers. Although the deposition parameters had little influence on the passivation effect, they affected the deposition rate. Therefore, appropriate deposition parameters should be carefully chosen based on the compromise of the deposition rate and fabrication cost. By utilizing the LPD SiO2 film as surface passivation layer, a 19.5%-efficient silicon solar cell on a large-scale wafer (156 mm × 156 mm) was fabricated.

Development of gel polymer electrolytes for application in quantum dot-sensitized solar cells

A methylcellulose–polysulfide gel polymer electrolyte has been prepared for application in quantum dot-sensitized solar cells (QDSSCs) having the configuration FTO/TiO2/CdS/ZnS/SiO2/electrolyte/Pt(cathode). The electrolyte with the composition of 30.66 wt.% methylcellulose, 67.44 wt.% Na2S, and 1.90 wt.% sulfur exhibits the highest conductivity of 0.183 S cm−1 with the lowest activation energy of 6.14 kJ mol−1. CdS quantum dot sensitizers have been deposited on TiO2 film via the successive ionic layer absorption and reaction (SILAR) method. The QDSSC fabricated using the highest conducting electrolyte and CdS QD prepared with five SILAR cycles exhibits a power conversion efficiency (PCE) of 0.78%. After deposition of zinc sulfide (ZnS) and silicon dioxide SiO2 passivation layers, the PCE of the QDSSC with photoanode arrangement of TiO2/CdS(5)/ZnS(2)/SiO2 increased to 1.42%, an improvement in performance by 82%.

Research on Black Silicon Based on Surface Photo Voltage

The solar cell research has experienced more than half a century. The black silicon solar cells have the advantages of low cost, low reflectivity. Due to the very large surface state density of Black silicon solar cells, the surface passivation effect directly affects its efficiency. Since the Surface photo voltage technology is very sensitive to surface states, three kinds of black silicon solar cells: no surface passivation and SiO2 passivation were tested and analyzed by this technology. The electrical characteristics of the solar cells were tested. The passivation effects were studied using surface photo voltage technology. We found that the recombination in the bulk and on the surface dominated at shorter times and at longer times respectively from the transient photo voltage results. Surface photo voltage can reveal the generation and recombination mechanisms of light-induce charge carriers, it provides the theoretical guidance to improve the efficiency of the solar cells.

Influence of ITO layer application on electrical parameters of silicon solar cells with screen printed front electrode

Purpose The purpose of this paper was investigation and comparison of electrical and optical properties of crystalline silicon solar cells with ITO or TiO2 coating. The ITO, similar to TiO2, is very well transparent in the visible part of optical radiation; however, its low resistivity (lower that 10-3 Ohm/cm) makes it possible to use simultaneously as a transparent electrode for collection of photo-generated electrical charge carriers. This might also invoke increasing the distance between screen-printed metal fingers at the front of the solar cell that would increase of the cell’s active area. Performed optical investigation showed that applied ITO thin film fulfill standard requirements according to antireflection properties when it was deposited on the surface of silicon solar cell. Design/methodology/approach Two sets of samples were prepared for comparison. In the first one, the ITO thin film was deposited directly on the crystalline silicon substrate with highly doped emitter region. In the second case, the TCO film was deposited on the same type of silicon substrate but with additional ultrathin SiO2 passivation. The fingers lines of 80 μm width were then screen-printed on the ITO layer with two different spaces between fingers for each set. The influence of application of the ITO electrode and the type of metal electrodes patterns on the electrical performance of the prepared solar cells was investigated through optical and electrical measurements. Findings The electrical parameters such as short-circuit current (Jsc), open circuit voltage (Voc), fill factor (FF) and conversion efficiency were determined on a basis of I-V characteristics. Short-circuit current density (Jsc) was equal to 32 mA/cm2 for a solar cell with a typical antireflection layer and 31.5 mA/cm2 for the cell with ITO layer, respectively. Additionally, electroluminescence of prepared cells was measured and analysed. Originality/value The influence of the properties of ITO electrode on the electrical performance of crystalline silicon solar cells was investigated through complex optical, electrical and electroluminescence measurements.

Metal-Semiconductor-Metal Near-Ultraviolet (~380nm) Photodetectors by Selective Area Growth of ZnO Nanorods and SiO2 Passivation.

Metal-semiconductor-metal near-ultraviolet (NUV) photodetectors (PDs) based on zinc oxide (ZnO) nanorods (NRs), operating at λ ~ 380 nm, were fabricated using conventional photolithography and hydrothermal synthesis processes. The vertically aligned ZnO NRs were selectively grown in the channel area of PDs. The performance of ZnO NR-based NUV PDs was optimized by varying the solution concentration and active channel width (Wch). For the fabricated samples, their electrical and photoresponse properties were investigated under the dark state and the illumination at wavelength of ~380 nm, respectively. For the device (Wch = 30 μm) with ZnO NRs at 25 mM, the highest photocurrent of 0.63 mA was obtained with the on/off ratio of 1720 at the bias of 5 V. The silicon dioxide passivation was also carried out to improve the photoresponse properties of PDs. The passivated devices exhibited faster rise and reset times rather than those of the unpassivated devices.

Enhanced Visible Transmittance of Thermochromic VO₂ Thin Films by SiO₂ Passivation Layer and Their Optical Characterization.

This paper presents the preparation of high-quality vanadium dioxide (VO2) thermochromic thin films with enhanced visible transmittance (Tvis) via radio frequency (RF) sputtering and plasma enhanced chemical vapor deposition (PECVD). VO2 thin films with high Tvis and excellent optical switching efficiency (Eos) were successfully prepared by employing SiO2 as a passivation layer. After SiO2 deposition, the roughness of the films was decreased 2-fold and a denser structure was formed. These morphological changes corresponded to the results of optical characterization including the haze, reflectance and absorption spectra. In spite of SiO2 coating, the phase transition temperature (Tc) of the prepared films was not affected. Compared with pristine VO2, the total layer thickness after SiO2 coating was 160 nm, which is an increase of 80 nm. Despite the thickness change, the VO2 thin films showed a higher Tvis value (λ 650 nm, 58%) compared with the pristine samples (λ 650 nm, 43%). This enhancement of Tvis while maintaining high Eos is meaningful for VO2-based smart window applications.

Improved optical and electrical performances of GaN-based light emitting diodes with nano truncated cone SiO2 passivation layer

GaN-based light emitting diodes (LEDs) with nano truncated cone SiO2 passivation layer via laser interference lithography and inductively coupled plasma dry etching technology were fabricated. The sidewall profile of the nano truncated cones was optimized by adjusting the etching process to obtain high fill factor. The light output power and electroluminescence intensity of the LEDs with textured SiO2 surface show better results than the conventional LEDs without SiO2 layer. At an injection current of 20 mA, the enhancement of the light output power is about 64 %. Moreover, the reverse leakage current, under a reverse bias of −5 V, is obviously reduced in contrast to the conventional LEDs. The method of texturing the passivation layers is proved to be an alternative method to improve both the optical and electrical performances of GaN-based LEDs.

Intrinsic Photoconductive Switches Based on Semi-Insulator 4H-SiC

A high-purity semi-insulator 4H-SiC intrinsic photoconductive switch is presented. The photoconductive semiconductor switch device is fabricated as lateral structures with the electric contact on the same side. The effect of the SiO2 passivation layer has been investigated on the breakdown voltage. The minimum ON-state resistance is 16 $\Omega $ , and the breakdown voltage is 11 kV. A new phenomenon that two steps exist on the rising edge of the photocurrent is observed, and a model of resistor and capacitor in parallel is built to explain it.

Physical understanding of trends in current collapse with atomic layer deposited dielectrics in AlGaN/GaN MOS heterojunction FETs

Many passivation dielectrics are pursued for suppressing current collapse due to trapping/detrapping of access-region surface traps in AlGaN/GaN based metal oxide semiconductor heterojuction field effect transistors (MOS-HFETs). The suppression of current collapse can potentially be achieved either by reducing the interaction of surface traps with the gate via surface leakage current reduction, or by eliminating surface traps that can interact with the gate. But, the latter is undesirable since a high density of surface donor traps is required to sustain a high 2D electron gas density at the AlGaN/GaN heterointerface and provide a low ON-resistance. This presents a practical trade-off wherein a passivation dielectric with the optimal surface trap characteristics and minimal surface leakage is to be chosen. In this work, we compare MOS-HFETs fabricated with popular ALD gate/passivation dielectrics like SiO2, Al2O3, HfO2 and HfAlO along with an additional thick plasma-enhanced chemical vapor deposition SiO2 passivation. It is found that after annealing in N2 at 700 °C, the stack containing ALD HfAlO provides a combination of low surface leakage and a high density of shallow donor traps. Physics-based TCAD simulations confirm that this combination of properties helps quick de-trapping and minimal current collapse along with a low ON resistance.

Normally OFF GaN-on-Si MIS-HEMTs Fabricated With LPCVD-SiNx Passivation and High-Temperature Gate Recess

Low-current-collapse normally OFF GaN-on-Si MIS high-electron-mobility transistors (MIS-HEMTs) are fabricated with low-pressure chemical-vapor-deposited SiN x (LPCVD-SiN x ) passivation and high-temperature low-damage gate-recess technique. The high-thermal-stability LPCVD-SiN x enables a passivation-prior-to-ohmic process strategy and effectively suppresses deep states at the passivation/HEMT interface. The fabricated MIS-HEMTs feature a high $V_{\rm {TH}} $ of +0.85 V at the drain current of 1 $\mu $ A/mm and a remarkable ON/OFF current ratio of 1010 while reduced dynamic ON-resistance as compared to plasma-enhanced chemical-vapor-deposited SiO2 passivation. High field-effect channel mobility of 180 cm $^{2}/ \text{V} \cdot \text{s} $ is achieved, leading to a high maximum drain current density of 663 mA/mm.

Enhanced stability of black phosphorus field-effect transistors with SiO2 passivation

Few-layer black phosphorus (BP) has attracted much attention due to its high mobility and suitable band gap for potential applic5ations in optoelectronics and flexible devices. However, its instability under ambient conditions limits its practical applications. Our investigations indicate that by passivation of the mechanically exfoliated BP flakes with a SiO2 layer, the fabricated BP field-effect transistors (FETs) exhibit greatly enhanced environmental stability. Compared to the unpassivated BP devices, which show a fast drop of on/off current ratio by a factor of 10 after one week of ambient exposure, the SiO2-passivated BP devices display a high retained on/off current ratio of over 600 after one week of exposure, just a little lower than the initial value of 810. Our investigations provide an effective route to passivate the few-layer BPs for enhancement of their environmental stability.