a-Si:H Schottky gate on n-GaInAs

The metal Schottky contact leads to low barrier heights on small-gap (<1 eV) semiconductors. This is the case of the n-type GaInAs material matched to InP where this barrier does not exceed 0.3 eV. We have found an original method to improve this result considerably by using a deposition of an amorphous semiconductor a-Si or a-Si: H. A Pt metal acts as the Schottky contact on the amorphous layer. The device behaves like a heterostructure of a high-gap (amorphous layer: Eg?1.8 eV) on a small-gap (GaInAs:0.75 eV) material. The Schottky-barrier height (0.8 eV) is greater than the GaInAs bandgap (0.75 eV). The reverse current is very low: 20 nA at 1 V reverse voltage for a 0.6 mm diode diameter. An FET using a-Si: H as a gate realised on a GaInAs layer shows a good electrical characteristic.     

Improved reliability of large-sized a-Si thin-film-transistor by back channel treatment in H2

A hydrogen plasma treatment on the back-channel region of large-sized amorphous silicon thin film transistor (a-Si TFT) with high RF power and optimal process time of 20 s is proposed in this work to effectively reduce off current (I_off) and threshold voltage (V_th) shift under high and low electrical-field stresses. The channel width (W) of large-sized a-Si TFT is ranged from 1000 to 10,000 μm, which are comparable to the realistic TFTs used in the gate driver on array (GOA) of display. It is experimentally found that the mechanism of V_th shift (ΔV_th) after high electrical stress is dominated by the defect generation in a-Si layer rather than charge trapping in the gate insulator (GI) layer, which is different from the observation in previous literatures. It could be due to the effects of back-channel treatment (BCT). In addition, after low electrical stresses, the mechanism of ΔV_th is dominated by defect generation in a-Si layer, which is consistent with previous reports.     

Bandgap engineering of amorphous semiconductors for solar cell applications

Different bandgap engineering approaches are discussed with respect to their application in thin-film solar cells based on hydrogenated amorphous silicon (a-Si: H) and its alloys with Ge or C. After a survey of the different approaches reported so far in the literature, the main emphasis will lie on the application of a-Si: H/a-Si_1-xC_x:H multilayers for use in the p-doped window layer, or a graded bandgap a-Si _1-xC_x:Hintrinsic layer with varying Ge content x. But also the possibility of using an a-Si: H/a-Si_1-xC_x:H multilayer as the i-layer will be addressed. Intrinsic films of both multilayers and graded bandgap material have been deposited in different series, varying electronic well and barrier widths in the first case, and the amount, form or local position of a change of the bandgap in the second case. Physical properties of both classes of materials have been investigated thoroughly. the results are discussed with respect to an application in amorphous thin-film solar cells, also taking into account results from the literature about the implementation of these heterostructures in devices. Conclusions for the future perspectives of these approaches are drawn.     

Negative capacitance peculiarities in a-Si:H/c-Si rectifier structure

Nontrivial negative capacitance (NC) effect, observed in a-Si:H/c-Si heterostructure devices, is discussed emphasizing the theoretical interpretation of experimental data. To explain NC effect, we have performed dark current voltage (I-V) and admittance measurements (C-V, G-V, C-f and G-f). The calculated values of series resistance (R_s) and barrier height (Φ_Bo) have the values from 100 to 114.7 Ω and 0.94 to 0.83 eV, respectively. Also, below 50% helium dilution rate, diode ideality factor (n) becomes bigger than 2, because tunneling at junction interface plays a major role. The measured room temperature (294 K) dark I-V result has been used during the fitting process for suggested capacitance model (Eq. (18)). The measured NC values exhibit strongly voltage depended behavior. This unexpected behavior is attributed to the presence of inductively coupled space charge region which might possibly be stemmed from the helium diluted a-Si:H material. It is seen that the measured NC values are well fitted with suggested capacitance model (Eq. (18)). Application of suggested correction formula on to experimental C-V data yields satisfactory results. It is shown that the calculated inductance values of the investigated device range from 10 to 42 μH and after correction, NC values are no longer observed in the C_d-V data.     

Role of dual SiOx: H based buffer at the p/i interface on the performance of single junction microcrystalline solar cells

In p-i-n structure a-Si solar cell a buffer layer with proper characteristics plays important role in improving the p/i interface of the cell, reducing mismatch of band gaps and number of recombination centres. However for p-i-n structure microcrystalline ( µc-Si: H) cell which has much less light induced degradation than a-Si:H cell, not much work has been done on development of proper buffer layer and its application to µc-Si:H cell. In this paper we have reported the development of two intrinsic oxide based microcrystalline layer having different characteristics for use as buffer layers at the p/i interface of µc-Si:H cell. Previously SiO_x:H buffer layer has been used at the p/i interface which showed positive effects. To explore the possibility of improving the performance of p-i-n structure µc-Si:H cell further we have thought it interesting to use two buffer layers with different characteristics at the p/i interface. The two buffer layers have been characterized in detail and applied at the p/i interface of the µc-Si:H cell with positive effects on all the PV parameters mainly improves the open circuit voltage (V_oc) and enhances short circuit current (I_sc). The maximum initial efficiency obtained is 8.97% with dual buffer which is 6.7% higher than that obtained by using conventional single buffer layer at the p/i interface. Stabilized efficiency of the cell with dual buffer is found to be ~9.5% higher than that with single buffer after 600 h of light soakings.     

薄有源层非晶硅薄膜晶体管特性的研究

本文研究了薄ａ－Ｓｉ：Ｈ有源层结构的ａ－Ｓｉ：Ｈ ＴＦＴ的特性，实验结果表明，当ａ－Ｓｉ：Ｈ层的厚度小于一个临界值时，ａ－Ｓｉ：Ｈ厚度的变化对ａ－Ｓｉ：Ｈ ＴＦＴ静态特性的影响明显增大，本文中详细分析了有源层背面空间电荷层对ａ－Ｓｉ：Ｈ ＴＦＴ特性的影响，从表面有效空间电荷层的概念出发，从理论上分析了有源层厚度与阈值电压的关系，计算的临界有源层厚度为１３０ｎｍ，这与实验结果基本一致。     

高效硅基异质结太阳能电池的模拟

a-Si:H/c-Si异质结太阳能电池的基本参数,如层厚度、掺杂浓度、a-Si:H/c-Si界面缺陷、功函数等是影响载流子传输特性和电池效率的关键因素。在本文中,利用AFORS-HET程序,研究了这些参数与a-Si:H/c-Si电池的性能的关联性。最后,具有TCO/n-a-Si:H/i-a-Si:H/p-c-Si/p -a-Si:H/Ag结构的太阳能电池的最优化性能被获得,其光电转换效率为27.07%（VOC: 749 mV, JSC: 42.86 mA/cm2, FF: 84.33%）。深入地了解异质结电池的输运特性,对进一步提高电池的效率有很大的帮助,同时对实际太阳能电池的制造也能提供有益的指导。     

Influence of deposition pressure on p-type a-Si:H window layer doped by trimethylboron for a-Si:H superstrate solar cell in plasma enhanced chemical vapor deposition

Wide bandgap (E_g) p-type window layer is very important for silicon based thin film solar cell to obtain high performance, especially high open-circuit voltage (V_OC). In this work, the influence of the deposition pressure on the properties of p-type a-Si:H window layer doped by trimethylboron (TMB) in plasma enhanced chemical vapor deposition (PECVD) was investigated systematically by transmission, Raman, and Fourier transform infrared (FTIR) spectroscopies. As a result, high performance hydrogenated amorphous silicon (a-Si:H) p–i–n superstrate solar cell with V_OC up to 927 mV was successfully achieved on Asahi Type-U SnO₂:F coated glass. In this case, excellent wide bandgap p-type a-Si:H window layer was fabricated under a mild deposition condition, including a low hydrogen dilution ratio (H₂/SiH₄) of 20, a relatively high deposition temperature of 220 °C, which was also adopted for the i-layer and n-layer deposition, and a moderate deposition pressure of about 160 Pa. We think it is the compromise between wide E_g and good microstructure quality of the p-layer that brings about the good solar cell performance. Such p-type window layer will be very helpful for the fabrication of a-Si:H solar cell, especially of the cell finished in a single PECVD chamber, due to its mild deposition condition.     

Evaluation of the steady-state photocarrier grating technique with respect to A-Si: H and its application to A-Si1–X GeX: H alloys

The steady-state photocarrier grating technique is analysed with respect to deviations from local charge neutrality. We present a generalized expression for the normalized photoresponse β as a function of the grating period as well as of the external electric field; the transport is characterized in the sense of bipolar and ambipolar, diffusion and drift displacement, respectively. Experimental data for undoped a-Si: H, especially the variation of the β coefficient with the external electric field, are discussed and fitted by the generalized equation. the effective dielectric relaxation time, which takes the trapped photogenerated majority carriers into account, is found to be responsible for ambipolar transport in undoped a-Si: H in the low electric field regime. Further, the μτ-products of electrons and holes in a-Si_1-x Ge_x: H alloys are deduced experimentally. the low-gap alloys are an appropriate candidate for photovoltaic applications.     

a-Si:H太阳能电池i层中氧、硼杂质对其稳定性影响

基于pin结构的a－Si：H太阳能电池中空间电荷效应，讨论了i层中氧、硼杂质对其性能的影响，结果表明，氧硼杂质存在都改变a－Si：H太阳能电池内部电场分布，不利于光生载流子收集，但i层轻度因掺杂可抑制a－Si：H太阳能电池光诱导性能衰退，提高电池稳定性。而i层氧含量和助长了a－Si：H太阳能电地光诱导性能衰退的发生，高于1020cm－3的氧含量是有害的。     

An a-Si:H vacuum-compatible photoresist process for fabricating device structures in HgCdTe

A process for transferring patterns into HgCdTe epilayers using a hydrogenated amorphous silicon (a-Si:H) photomask has been demonstrated. a-Si:H films were grown using plasma enhanced chemical vapor deposition (PECVD). A latent image of a projected mask pattern was created at the a-Si:H surface by ultraviolet enhanced oxidation in the load lock of the PECVD vacuum chamber. This image was transformed into a mask by hydrogen plasma removal of the unexposed areas. A hydrogen plasma etch selectivity value greater than 500:1 for oxide and a-Si:H allows patterns as thick as 700 nm to be generated. a-Si:H masks were used to create arrays of mesas in planar HgCdTe epilayers by etching in an electron cyclotron resonance (ECR) plasma reactor. Etch selectivity between a-Si:H and HgCdTe during an ECR hydrogen plasma etch was measured to be greater than 18:1. R_oA values > 10³ were obtained for mid-wavelength infrared diodes made from HgCdTe heterojunctions using a-Si:H masks.     

用光电灵敏度法研究a-Si:H中的电荷放大效应

测量了缝电极和梳形电极a-Si:H样品的光伏安特性和光电灵敏度,提出了由光电灵敏度计算电荷放大增益的方法。由此法测出的a-Si:H的电荷放大增益,在105V/cm电场下,高达4.3103。本文从能态图讨论了a-Si:H中电荷放大效应的产生过程。由测量的增益值计算了电子迁移率与寿命之积。     

INVESTIGATION OF CHARGE INTENSIFICATION EFFECT IN a-Si:H BY MEANS OF PHOTOELECTRIC SENSITIVITY METHOD

The photocurrent-voltage characteristics and photoelectric sensitivity of a-Si:H samples with slit and comb electrodes are measured. A method for calculating the charge intensifying gain from the photoelectric sensitivity is proposed. The obtained charge intensifying gain of a-Si:H under an electric field of 105 V/cm through this method is as high as 4.3×103. The generation process of the charge intensification effect in a-Si:H is discussed on the basis of the energy level diagram. And the product of electron's mobility and its lifetime is calculated from the measured values of the gains.     

Performance of bifacial HIT solar cells on n-type silicon substrates

The performance of amorphous silicon(a-Si:H) /crystalline silicon(c-Si) heterojunction is studied,and the effects of the emitter layer thickness,doping concentration,intrinsic layer thickness,back heavily-doped n layer,interface state and band offset on the optical and electrical performance of bifacial heterojunction with intrinsic thin-layer(HIT) solar cells on ntype silicon substrates are discussed.It is found that the HIT solar cells on n-type substrates can obtain a higher conversion efficiency than th...     

Cesium carbonate-doped 1,4,5,8-naphthalene-tetracarboxylic-dianhydride used as efficient electron transport material in polymer solar cells

The use of appropriate charge carrier transport materials in organic solar cells strongly influences the device performance. In this work, we focused on the molecular electron transport material 1,4,5,8-naphthalene-tetracarboxylic-dianhydride (NTCDA) doped by cesium carbonate (Cs₂CO₃). We first investigated the electrical properties of such n-type doped material as a function of the doping concentration before using it as electron transport layer (ETL) in polymer solar cells. The doped transparent ETL reduces the series resistance leading to an increased open circuit voltage. A power conversion efficiency of 3.8% was finally achieved in a device with a blend of poly(3-hexylthiophene-2,5-diyl):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) as the active layer and a 5 nm-thick NTCDA:Cs₂CO₃ film with a molar ratio of 30% as ETL.     

An a-Si:H/a-Si,Ge:H bulk barrier phototransistor with a-SiC:Hbarrier enhancement layer for high-gain IR optical detector

The design and fabrication of a high-gain amorphous silicon/amorphous silicon germanium (a-Si:H/a-Si,Ge:H) bulk barrier phototransistor for infrared light detection applications are reported. The a-Si,Ge:H material featured a lower energy gap and is suitable for the absorption of longer wave light, but it also leads to a low breakdown voltage and high dark current. An additional a-SiC:H thin-film layer was used at the collector/base interface in the conventional amorphous bulk barrier phototransistor to enhance the function of the bulk barrier and obtain high optical gain     

Device analysis for a-Si:H thin-film transistors with organicpassivation layer

Two dimensional device analysis has been performed to explain the experimental drain current-gate voltage (I_D-V_GS) characteristics of hydrogenated amorphous silicon thin-film transistors with various passivation layers. The shift of the I_D-V_GS curve in the negative direction and the increase of S-factor (the inverse of subthreshold slope in logarithmic I_D-V_GS curve) can be explained well by introducing positive fixed charges and defect states in the back interface region. It was found that the positive fixed charge and the defect density of a-Si:H TFT with an organic passivation layer are higher than those of conventional a-Si:H TFT with a silicon-nitride (SiN_x) passivation layer. The simulation shows that the front and back interfaces interact and this explains why the passivation affects the device performance such as V_th and S-factor in a-Si:H TFTs     

Characterisation of the Al2O3 films deposited by ultrasonic spray pyrolysis and atomic layer deposition methods for passivation of 4H–SiC devices

Al₂O₃ films were deposited using atomic layer deposition (ALD) and ultrasonic spray pyrolysis (USP) methods on p- and n-type Si substrates, n-type 4H–SiC substrates and 4H–SiC diodes for passivation studies. UV exposure in N₂ atmosphere and 5% HF treatment were used as two separate surface preparation procedures prior to Al₂O₃ deposition. The films deposited with USP technique contain a large amount of fixed negative charge and are vulnerable to water incorporation into the material. The Al₂O₃ film prepared by ALD method shows much better uniformity and less negative charge. Decrease of the leakage current in the 4H–SiC diodes is observed after Al₂O₃ passivation using both methods.     

Modification of GaAs Schottky barriers using a-Si:H interfaciallayers

The electrical characteristics of metal/a-Si:H/n-GaAs diode structures were studied in order to investigate the role of the a-Si:H and the claim of no barrier at the GaAs/a-Si:H interface. Diodes were fabricated using a-Si:H layers between 30 and 1920 Å thick, with Al and Mg metallization, and the current-voltage and capacitance-voltage characteristics were examined. Rectifying Schottky barriers were formed at Al/a-Si:H junctions, while good ohmic contacts were formed at Mg/a-Si:H junctions, enabling effects due to the metal/a-Si:H and a-Si:H/GaAs interface to be isolated. A dramatic increase in the forward turn-on voltage was observed as the thickness of the a-Si:H layer increased. The diode behavior can be explained by considering three effects in series: (1) an a-Si:H/GaAs barrier of about 0.6 eV, consistent with Fermi-level pinning in GaAs; (2) a metal/a-Si:H barrier, dependent on the metallization; and (3) space-charge-limited current (SCLC) in the bulk a-Si:H. The SCLC effectively gives rise to a voltage-dependent resistance and causes the increased turn-on voltages