This article investigates the effects of different kinds of trap states on the degradation of polycrystalline silicon (poly-Si) thin-film transistors (TFTs) under alternating current (ac) gate pulse bias stress. P-type TFTs with different ON-state and subthreshold characteristics are fabricated, and thus, different distributions of donor-like tail states and deep states are achieved. The decrease of ON-state current is the dominant degradation phenomena for all the TFTs due to trap state generation occurring during the pulse rising edges. TFTs with higher donor-like trap states show severe degradation. It is attributed to the higher transient lateral electric field and hole concentration due to the higher donor-like deep state density, while acceptor-like trap states do not contribute to the degradation of p-type poly-Si TFTs under ac gate bias stress.

Objective Microbolometers offer advantages such as wide working range,fast response time,and simple device structure.The working principle is that infrared radiation incidents on the absorption layer,generating thermal energy which is then transferred to the thermosensitive material.The thermosensitive material changes its electrical resistance upon absorbing the heat and thus produces a change in output signal detected by the readout circuit.The thermosensitive layer has a significant impact on the performance of microbolometer.Currently,the main industrial materials for the thermosensitive layer are vanadium oxide and amorphous silicon,with a general temperature coefficient of resistance(TCR)of-2％/K.Compared to vanadium oxide and amorphous silicon,manganese cobalt nickel oxide(MCNO)with high TCR coefficients has great potential in uncooled infrared detector applications.Unlike most MCNO thin films prepared by magnetron sputtering of a stoichiometric compound,this paper reports the synthesis of MCNO by combining magnetron sputtering and electron beam evaporation.MnO2 and Co2O3 were co-sputtered initially,followed by e-beam evaporation of Ni.Multilayers of MnO2-Co2O3 oxides and Ni were thus deposited alternatively and subjected to in-situ and/or post-annealing to promote interdiffusion and the formation of MCNO compound.The objective was to investigate the feasibility of the new fabrication method based on solid state synthesis.The prepared thin films showed good crystallinity and negative temperature coefficient of resistance,indicating that the proposed method could be applied to further fine-tuning the compositions of MCNO thin films in the future. Methods Thin films were prepared using a magnetron sputtering-electron beam evaporation hybrid method using a laboratory-developed high vacuum coating unit,where an e-gun and two magnetron sputtering targets coexisted in the same chamber.The sample stage was controlled by a stepper motor for 180-degree rotation,thus enabling alternative up-facing magnetron co-sputtering and down-facing electron beam evaporation in a single fabrication process.Sapphire substrate was cleaned with acetone,ethanol,and deionized water sequentially,dried up with blowing N2 gas,and pre-heated in the vacuum chamber at 450 ℃ for one hour prior to the deposition.To study the effect of post-annealing on the performances of MCNO thin films,the as-deposited thin film samples were first annealed in-situ in the vacuum chamber for an hour,then annealed in a tube furnace in the air at 750 ℃,850 ℃,and 950 ℃ respectively.The surface morphology of the resultant MCNO thin films were characterized using scanning electron microscopy(SEM).The crystal phase structure was characterized using X-ray diffraction analysis.The resistance at variable temperature from 220 K to 300 K was measured using a two-probe setup on a UV-THz full-spectrum photoelectric test probe stage.The transmittance and absorbance of the thin films were examined using Fourier-transform infrared spectroscopy(FTIR)in the 2.5-25 μm range. Results and Discussions XRD characterization showed that the thin films exhibited polycrystalline structures with different preferential orientations as the annealing temperature increased.The primary crystal orientation obtained was(111),and the peak intensity increased with increasing annealing temperature.SEM characterization showed that the grain size of the thin film increased with increasing annealing temperature,consistent with the XRD results.FTIR characterization showed that the absorbance of the thin films increased toward the long wavelength in the 6-8.5 μm range and reached 1.50 for the sample annealed at 850 ℃.XPS quantification showed that the ratio of Mn4+/Mn3+in the film increased with increasing annealing temperature.All thin films exhibited ohmic characteristic curves measured with four-point probe method at room temperature.It was shown that the samples annealed at 750,850,and 950 ℃ respectively exhibited characteristic negative temperature coefficients,with calculated TCR of-1.95％/K,-4.20％/K,and-4.14％/K accordingly.The high TCR could be attributed mainly to the fabrication method,as solid phase synthesis of MCNO thin films via layer-by-layer deposition could result in metastable states in the thin films,thereby achieving better TCR and absorbance unobtainable in systems of thermal equilibrium. Conclusions This paper explored a magnetron sputtering-electron beam evaporation hybrid method based on solid phase synthesis to fabricate MCNO thin films.The impact of post-annealing on the properties of the thin films was investigated.Thin films prepared with optimized processing parameters showed high absorbance and excellent negative temperature coefficient of resistance.This study demonstrated the feasibility of fabricating MCNO films using the proposed method,offering a new approach for further development of MCNO thin films with optimal composition-property relationship.

Sequential three-layers of Pd/Pt/Au were deposited on p-GaN by magnetron sputtering under high vacuum (HV, -1.33 x 10-4 Pa) and ultra-high vacuum (UHV, -1.33 x 10-7 Pa) background conditions, respectively, for investigating the electrical contact properties. Linear I-V curves are observed in the samples deposited under the UHV conditions, whereas nonlinear I-V characteristics are obtained in the samples deposited under the HV vacuum. The depth profiles of X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) of the as-deposited samples were probed in detail. It is found that the amounts of O and OH as well as Pd oxide of the sample deposited under the HV background condition are larger than those of the sample deposited under the UHV conditions due to the higher residual gases such as water in the HV chamber. The oxide layer leads to an extra barrier, influencing the electrical characteristics of Pd/Pt/Au/p-GaN contact. This study demonstrates that metal deposition under the UHV environmental conditions can reduce and even prevent formation of oxide on p-GaN surface, and hence favor making a good ohmic contact on p-GaN.

Degradation phenomena featured with positive shift of the on-state transfer curve are reported for the amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) under negative bias illumination stress (NBIS). Such a positive shift is absent when the gate bias or the illumination is independently applied. With the assistance of TCAD simulation, the positive shift of the transfer curve is attributed to the generation of acceptor-like trap states, which is proposed to be due to oxygen interstitials produced as a consequence of electron generation by the illumination, acceleration under the effect of negative gate bias, and breaking weakly bonded oxygen. The proposed degradation mechanism is consistent with the low frequency noise characteristics and the degradation behavior under bipolar gate bias stress of the TFTs after NBIS. The whole degradation phenomena for the a-IGZO TFT under the NBIS are then consistently explained.

The Li7M1-xAlB12O24: xEu2+ (M = Ca、Sr、Ba) phosphors were prepared by the conventional high temperature solid-state reaction. The as-prepared phosphors had novel and attractive ultraviolet emissions at 386 nm (M = Ca), 388 nm (M = Sr), 391 nm (M = Ba) under the excitation of 289 nm, 293 nm, and 304 nm, respectively. The concentration quenching process between Eu2+ ions is determined and the critical quenching concentration of Eu2+ in Li7Ca1-xAlB12O24: xEu2+ phosphor is x = 0.11. The corresponding concentration quenching mechanism is verified as dipole-quadrupole (d-q) interaction. The critical transfer distance of Li7Ca0.89AlB12O24:0.11Eu2+ was determined to be 1.88 nm, Further evaluation on the absorbance spectra indicated that the energy transfer between Eu2+ was little affected by the matrix crystal field. The fluorescence lifetime of Li7Ca0.89AlB12O24: 0.11Eu2+ was 768.5 ns and the quantum efficiency was 55.91%. Therefore, the as prepared Li7M1-xAlB12O24: xEu2+ (M = Ca、Sr、Ba) phosphors exhibited a rare near ultraviolet emission with a potential applications on UV LED and anti-counterfeiting ink.

In an interconnected high-vacuum illustration system, the surfaces of p-InGaN/GaN heterostructures grown with integrated metalorganic chemical vapor deposition were treated in O2 gas for different times at room temperature and then transferred to another chamber for fabricating ohmic contacts via sputtering Pd/Pt/Au multi-layers. X-ray photoelectron spectroscopy measurements unveil that the proportions of Ga–O of the treated samples significantly increase compared with that of the as-grown sample, indicating that a thin layer of GaOx forms on the p-InGaN/GaN surface after exposing to oxygen or air atmospheres. Meanwhile, the samples exposed to O2 or air were found to have much higher specific contact resistance, i.e., higher by two orders of magnitude than that of the as-grown sample. The specific contact resistance of the as-grown sample was derived as 9.3 × 10−5 Ω cm2 using the circular transmission line measurement. Furthermore, the Schottky barrier height of the samples was determined from the measured I–V curves with the thermionic field emission model and was revealed to be closely related to surface treatments. Despite the fact that the insulating layer of GaOx was as thin as 1.2–1.4 Å for the samples exposed to oxygen at room temperature, it can act as an extra barrier layer causing significant increase in the specific contact resistance via blocking the tunneling of carriers. Therefore, the effective removal of p-InGaN surface oxide plays a vital role in preparing good ohmic contacts.

Spin injection and detection in bulk GaN were investigated by performing magnetotransport measurements at low temperatures.A non-local four-terminal lateral spin valve device was fabricated with Co/GaN Schottky contacts.The spin injec-tion efficiency of 21％was achieved at 1.7 K.It was confirmed that the thin Schottky barrier formed between the heavily n-doped GaN and Co was conducive to the direct spin tunneling,by reducing the spin scattering relaxation through the inter-face states.

Plasmonic metallic nanostructures could concentrate optical fields into nanoscale volumes and support efficient light scattering and absorption, which therefore stimulates the continuing development of advanced plasmonic-assisted semiconductor photodetectors. In this work, by fabricating Al nanoparticle (NP) arrays in AlGaN surface using the AAO template transferring method, significant broadband ultraviolet (UV) photoresponse enhancement was demonstrated on AlGaN/GaN heterojunction photodetectors. By deliberately designing the close-packed Al NP arrays, the broadband UV plasmonic resonance with large optical field absorption and strong interface field enhancement are enabled, hence, the highest responsivity exceeding 8.1 A W-1 and maximum external quantum efficiency of 3500% was obtained at the resonance wavelength 292 nm, revealing more than 80 times the excellent enhancement in responsivity. Specifically, owing to coupling among NPs at the Al/AlGaN interface, the smaller size Al NP array exhibits an excellent photoresponse enhancement encompassing the entire UV band compared to the relatively larger size Al NP array. In addition, different photoresponse enhancements depending on the applied bias were observed. The Al NPs detector also demonstrates a fast photoresponse with a rise time of around 60 ms and a relatively long fall time of 1.42 s. This work could be of great significance for gaining a low and efficient approach to achieve plasmonic-empowered heterojunction broadband UV detectors.

In growing InGaN/GaN multiple quantum wells (MQWs) with the technique of metal-organic chemical vapor deposition (MOCVD) the introduction of an appropriate amount of H-2 into the N-2 carrier gas for the growth of the GaN barrier layers can effectively improve the crystalline quality of the well/barrier interface and therefore enhance the luminescence efficiency of the quantum wells. In this work, we carried out detailed photoluminescence (PL) spectroscopic measurements on the luminescence properties of InGaN/GaN MQWs in the device structure for blue-light laser diodes, and the effects of H-2 in the carrier gas for GaN barrier growth on the MQWs, including the improved interface quality, the enhanced luminescence and the underlying mechanisms, have been investigated. The PL spectra of the InGaN/GaN MQWs acquired at room temperature reveal that the introduction of 2. 5% H-2 in the N-2 carrier gas leads to increased emission efficiency by 75% , blue-shifted peak energy by 17 meV, and narrowed full width at half maximum (FWHM) by 10 meV. With the PL spectra measured at varied excitation powers, the quantum-confined Stark effect (QCSE) and band-filling effect on the emission performance of the MQWs have been distinguished, and the QCSE effect is found to dominantly determine the emission energy and width, which can be effectively reduced by the introduction of H-2. Upon the complete screening of the QCSE effect, the peak energy of the MQWs emission is located at 2. 75 eV. The dependence of the PL spectra on temperature indicates that the introduced H-2 in the carrier gas can also reduce the carrier localization effect and narrow the energy fluctuation of the well potential, which leads to the narrowed PL spectral width in the samples grown with the mixture of H-2/N-2 carrier gas. The variation of the PL intensity with respect to temperature reveals that the physical nature of the nonradiative recombination centers at the interface is not influenced by the introduction of H-2, but the amount of these centers is greatly reduced, which accounts for the improved emission efficiency. The results of time-resolved PL measurements exhibit that the introduced H-2 in the carrier gas has no impact on the nonradiative recombination lifetime, but causes a shorter radiative recombination lifetime, which further confirms the influences of H-2 introduction on both QCSE screening and nonradiative recombination centers. The in-depth analyses of the PL results have revealed that the introduction of H-2 in the N-2 carrier gas for GaN barrier growth can significantly improve the crystalline quality of InGaN/GaN MQWs and therefore enhance the light emission performance. This work has demonstrated PL spectroscopy as a powerful technique in characterizing the optical properties of semiconductor quantum structures, and the spectral findings could provide helpful insight into the growth of InGaN/GaN MQWs.

Carriers in atomically thin two-dimensional (2D) semiconductors like WS2 monolayers are prone to interact with their surroundings, which may have a significant impact on both the electric and optical properties of 2D semiconductors. In this work, abnormal dependence of photoluminescence (PL) intensity on excitation power densities is observed in mechanically exfoliated WS2 monolayers on SiO2/Si substrates, which is manifested as a sudden intensity drop of exciton and trion emissions at certain power density followed by much slowed intensity increase. As-exfoliated and wetted WS2 monolayers exhibit similar abnormal variation of PL intensity but different power densities that trigger the intensity drop. The sudden drop of PL intensity is ascribed to the modification to the contact status of the monolayer with the substrate, caused by the interplay of the trapped air or water with the monolayer and the substrate upon laser irradiation, which therefore alters the carries transport rate from the monolayer to the substrate. A rate equation model has been developed to delicately deal with the carrier recombination and transport processes in a steady state, and the variation dependence of exciton, trion and free charge populations on the cross-plane transport rate is demonstrated, which consistently explains the abnormal PL intensity dependence on the excitation power density. The important role of carrier transport in the PL performance has been verified in this work, and further understanding of the carrier interactions in atomical thin 2D semiconductors has been achieved.

Based on interconnected ion sputtering and cathodoluminescence spectroscopy, an optical spectroscopic strategy has been developed to profile depth-dependent polarization effects in an AlGaN multiple-quantum-well structure. Two emission bands at about 258 nm and 315 nm have been identified to originate from the quantum wells and Ga-rich domains, respectively, and their depth-dependent spectral characteristics reveal that the spontaneous polarization dominates the piezoelectric polarization in the quantum wells, and the emission wavelengths vary with respect to the etching depth due to interactions of the spontaneous polarization field with the piezoelectric polarization field and the surface charge induced electric field.

CdSe/CdS dot‐in‐rod (DiR) nanocrystals are an advantageous heterostructured semiconductor system with a quasi‐type‐II electronic band structure, which enables a rich diversity of optical manipulation of carrier excitations and interactions. Herein, the selective excitation of respective CdSe quantum dots (QDs) and CdS nanorods of the CdSe/CdS DiR nanostructure is reported, which results in the distinct presence of different types of carriers at the nanorod surface. The presence of only electrons at the surface upon CdSe QD excitation leads to photophysical interactions with ambient air, which is manifested as reversible photoluminescence (PL) of the CdSe QDs. The excitation of CdS nanorods produces holes at the surface, which induces photooxidation of the CdS surface and irreversible change in PL intensity and spectral lineshape. Benefitting from unambiguously relating carrier types to photophysical and photochemical interactions, this research also verifies that trap states at the CdS surface do not act as nonradiative recombination centers, which can be removed by photochemical reactions. Herein, distinct roles of electrons and holes in the photophysical and photochemical interactions with ambient air are identified, and a more precise understanding of the interacting mechanisms is achieved.

Effects of low-temperature annealing in nitrogen atmosphere on elevated-metal metal-oxide (EMMO) thin-film transistors (TFTs) are investigated and reported in this article. Compared with the short-circuit behavior of EMMO TFTs after annealing in nitrogen atmosphere at 300 °C, it is found that TFTs received annealing in nitrogen atmosphere at 200 °C prior to that at 300 °C not only remain the transfer characteristics, but also exhibit improved on-state current and subthreshold characteristics. Both the results of X-ray photoelectron spectroscopy (XPS) and persistent photoconductivity (PPC) confirmed the reduction of oxygen vacancies in the a-IGZO after annealing in nitrogen atmosphere at 200 °C, indicating that the passivation of oxygen vacancy with nitrogen is dominant rather than the generation of oxygen vacancy. Furthermore, the thermal stability improvement could not be realized when the annealing in nitrogen atmosphere at 200 °C was performed before the source/drain formation annealing at 400 °C, which emphasizes the importance of annealing procedures during device fabrication.

近几年,Ⅲ-Ⅴ族半导体GaN由于其宽直接带隙,在高温、高功率器件方面得到了广泛研究。但是,目前GaN器件的性能依然受到了p型欧姆接触性能不良的限制,在长期使用过程或高温环境中激光器等器件性能退化严重。因此,获得性能优异的p-GaN接触仍然是一个巨大的挑战。虽然Pd基的金属体系已然在p-GaN获得了欧姆接触,但是Pd与GaN接触之后的微观结构及其高温特性尚不为人知。本文针对常用于p型GaN接触的第一层金属Pd材料,讨论了Pd/p-GaN接触界面的特性和退化机制。通过四探针测试仪、X射线光电子能谱（XPS）和原子力显微镜（AFM）实验测试和分析对比,发现Pd/p-GaN界面受到氧气和温度影响的退化过程。高温退火在界面处促成Ga-Pd合金相生成利于形成良好的接触,但是在有氧参与的情况下,金属的氧化反应超越其他因素成为主导,致使界面和性能发生明显的退化。温度越高退化越严重,甚至表面形貌状态完全改变,由平滑的原子台阶形貌转化呈现出树枝状晶粒状态。因此,保持Pd与p-GaN界面清洁、控制界面的氧成分不仅是形成合金态获得良好接触的关键,而且也关系着器件的长期稳定和可靠,是防止器件性能衰减和退化要害所在。

This letter reports the influence of material quality and device processing on the performance of AlGaN-based Schottky barrier deep ultraviolet photodetectors grown on Si substrates. The thermal annealing can significantly improve Schottky barrier height and wet chemical etching can effectively remove etching damage. Meanwhile, the decrease of threading dislocation density and the pit size, especially the later, can substantially suppress reverse leakage. As a result, the reverse leakage current density of the as-fabricated deep UV photodetector was reduced down to 3×10-8 A/cm2. Furthermore, the responsivity of the deep UV photodetectors was greatly improved by reducing the point defect concentration.

We propose a method of oxygen plasma treatment to realize normally-off p-GaN/AlGaN/GaN high-electron-mobility transistors. The fabricated device features an oxide surface passivation layer and a high-resistivity GaN cap layer at the access region, both transformed from p-GaN by the oxygen plasma treatment technique. With optimized treatment conditions, a low sheet resistance of 682 Ω/□ has been successfully obtained and the fabricated device shows high performance with a positive threshold voltage of +1.02 V, a maximum drain current of 301 mA mm−1, a high on/off ratio of ∼108, a breakdown voltage of 660 V, and low current collapse.

Ultraviolet (UV) photodetectors have demonstrated wide applications in both civil and military fields such as pollution monitoring and missile warning. ZnO-based UV photodetectors have attracted tremendous attention due to the advantages of low cost and high chemical and thermal stability. In this work, Ga-doped ZnO (GZO) films were grown with the technique of RF magnetron sputtering and metal–semiconductor–metal (MSM) UV detectors were fabricated with the GZO films as the active detection layers. The as-sputtered films were further treated by the means of thermal annealing such that reduced oxygen vacancy and improved crystallization were achieved for the films. The effect of patterned Si substrates on the GZO UV detectors performance was revealed for the first time. With optimized annealing temperature and delicately designed substrate patterns, the responsibility of GZO MSM photodetectors has been greatly enhanced (e.g. about 2.5-folds higher than ones fabricated on non-patterned Si substrate). Our work on the GZO material and the structural design may pave a new way towards developing low-cost but high-performance UV detectors.

Broadband ultraviolet (BUV) photodetectors responding to the multiband spectrum can effectively reduce false alarm rates and improve the accuracy and versatility of detection systems in various situations. A high-responsivity BUV photodetector based on vertical Ga2O3/GaN nanowire array is proposed and demonstrated. Ga2O3/GaN nanowires are obtained by partially thermally oxidizing GaN nanowires grown by molecular beam epitaxy and used to combine with a monolayer graphene film to form graphene/Ga2O3/GaN heterojunction. Moreover, the oxidation mechanism of GaN nanowires is further investigated by the developed thermal oxidation model. The fabricated devices exhibit excellent performance with a broadband spectral response of exceeding 550 A W-1 at -5 V and a fast-response speed in the millisecond range, which can be attributed to the optical properties of vertical nanowire array structure and the internal gain mechanism of graphene/Ga2O3/GaN heterojunction.

GaInP alloy could be the most trusted key material for fabricating super-high-efficiency single- and multi-junction solar cells, especially for space applications. The storage and transfer of optical excitation energy in this key alloy is thus a key subject of the energy conversion from optical to electrical. In this article we present a study of the subject through investigating photoluminescence (PL) degradation in the GaInP epilayer at 4 K under the continuous optical excitations of ultraviolet (UV) 325 nm, visible 488.0 and 514.5 nm lasers. It is found that the decline of PL intensity with the irradiation time may be represented by I(t)/I-0 = (1 + t tau(-1))(-1) + C, where I-0 is the luminescence intensity at the beginning of irradiation, tau a time constant, and C a background. Moreover, the PL degradation degree reduces with increasing the excitation wavelength. In addition, some red shift of the PL peak is observed accompanying with the intensity decline under the UV laser excitation. These PL signatures indicate that the localized carriers within the local atomic ordering domains play a major role in the storage and transfer of the excitation energy via photon recycling processes. (C) 2019 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.

We present a study of recombination dynamics and lifetimes of minority carriers in a GaInP/GaAs heterostructure single-junction solar cell by using a variable-temperature time-resolved photoluminescence (TRPL) technique. It was found that the minority carriers, i.e., electrons, in the p-type GaInP base layer have super long lifetimes of similar to 220 mu s under the excitation of nanosecond laser pulses. On the basis of a newly developed model for time-resolved luminescence of a localized-state ensemble [e.g., Z. C. Su and S. J. Xu, Sci. Rep. 7, 13 (2017)], it is revealed that as much as 38% of photons emitted by localized carriers may be involved in the photon recycling in the GaInP base layer. The effective photon recycling is believed to be an important factor that causes the super long lifetime of the minority carriers, and can help reduce the radiative recombination loss of photogenerated carriers in heterostructure based solar cells.