IR selective sensitivity of amorphous/crystalline silicon heterostructure

An IR detector based on a-Sin-type) /c-Si(p-type) heterostructure is reported to have selective sensitivity in the wavelength ranged for 1000–1100 nm with a maximum at 1080 nm. The energy band diagram of the heterostructure is considered taking into account the existence of the interfacial layer. The analysis of the photogeneration and carrier transport processes in the a-Si(n-type) / c-Si(p-type) heterostructure has been carried out. It is shown that the interfacial layer and defect on the a-Si/c-Si boundary control the spectral photosensitivity characteristic.     

Characterization of thermal, optical and carrier transport properties of porous silicon using the photoacoustic technique

In this work, the porous silicon layer was prepared by the electrochemical anodization etching process on n-type and p-type silicon wafers. The formation of the porous layer has been identified by photoluminescence and SEM measurements. The optical absorption, energy gap, carrier transport and thermal properties of n-type and p-type porous silicon layers were investigated by analyzing the experimental data from photoacoustic measurements. The values of thermal diffusivity, energy gap and carrier transport properties have been found to be porosity-dependent. The energy band gap of n-type and p-type porous silicon layers was higher than the energy band gap obtained for silicon substrate (1.11 eV). In the range of porosity (50-76%) of the studies, our results found that the optical band-gap energy of p-type porous silicon (1.80-2.00 eV) was higher than that of the n-type porous silicon layer (1.70-1.86 eV). The thermal diffusivity value of the n-type porous layer was found to be higher than that of the p-type and both were observed to increase linearly with increasing layer porosity.     

n-ZnO/i-MgO/p-GaN异质结发光二极管

用等离子体辅助分子束外延的方法生长了n-ZnO/i-MgO/p-GaN异质结发光二极管。I-V测量表明其具有典型的二极管整流特性。电致发光峰位于382nm,通过与n型ZnO和p型GaN的光致发光谱比较,其发光峰位与线形都与ZnO的自由激子发射一致,表明该电致发光来自于ZnO的自由激子发射。通过Anderson模型比较了n-ZnO/i-MgO/p-GaN和n-ZnO/p-GaN异质结的能带示意图,证明了由于MgO层的插入抑制了ZnO向GaN层中的电子注入,且有利于空穴向ZnO层注入,从而实现了ZnO层中的电注入发光。     

Hydrogen sensitive gas sensor based on porous silicon/TiO2−x structure

Porous silicon (PS) layer was formed by electrochemical anodization on a p-type Si surface. Thereafter, n-type TiO_2−x thin film was deposited onto the PS surface by electron-beam evaporation. Pt catalytic layer and Au electrical contacts for further measurements were deposited onto the PS/TiO_2−x structure by ion-beam sputtering. Current–voltage characteristic, sensitivity to different concentration of hydrogen and resistance change of obtained structures versus time were examined. Results of measurements have shown that the current–voltage characteristics of structures are similar to that of diode. High sensitivity to hydrogen of obtained structures was also detected. Note that all measurements were carried out at room temperature.     

Evidence for p-type doping of InN

The first evidence of successful p-type doping of InN is presented. It is shown that InN:Mg films consist of a p-type bulk region with a thin n-type inversion layer at the surface that prevents electrical contact to the bulk. Capacitance-voltage measurements indicate a net concentration of ionized acceptors below the -type surface. Irradiation with 2 MeV He+ ions is used to convert the bulk of InN:Mg from p to n-type, at which point photoluminescence is recovered. The conversion is well explained by a model assuming two parallel conducting layers (the surface and the bulk) in the films.     

Improved color rendering of phosphor-converted white light-emitting diodes with dual-blue active layers and n-type AlGaN layer

An InGaN/GaN blue light-emitting diode (LED) structure and an InGaN/GaN blue-violet LED structure were grown sequentially on the same sapphire substrate by metal-organic chemical vapor deposition. It was found that the insertion of an n-type AlGaN layer below the dual blue-emitting active layers showed better spectral stability at the different driving current relative to the traditional p-type AlGaN electron-blocking layer. In addition, color rendering index of a Y3Al5O12:Ce3+ phosphor-converted white LED based on a dual blue-emitting chip with n-type AlGaN reached 91 at 20 mA, and Commission Internationale de L'Eclairage coordinates almost remained at the same point from 5 to 60 mA.     

Suppression of electron and hole overflow in GaN-based near-ultraviolet laser diodes

In order to suppress the electron leakage to p-type region of near-ultraviolet GaN/In_xGa_(1-x )N/GaN multiple-quantumwell(MQW) laser diode(LD), the Al composition of inserted p-type AlxGa_(1-x)N electron blocking layer(EBL) is optimized in an effective way, but which could only partially enhance the performance of LD. Here, due to the relatively shallow GaN/In_(0.04)Ga_(0.96)N/GaN quantum well, the hole leakage to n-type region is considered in the ultraviolet LD. To reduce the hole leakage, a 10-nm n-type Al_xGa_(1-x)N hole blocking layer(HBL) is inserted between n-type waveguide and the first quantum barrier, and the effect of Al composition of Al_xGa_(1-x)N HBL on LD performance is studied. Numerical simulations by the LASTIP reveal that when an appropriate Al composition of Al_xGa_(1-x)N HBL is chosen, both electron leakage and hole leakage can be reduced dramatically, leading to a lower threshold current and higher output power of LD.     

SERS-active substrates based on n-type porous silicon

Porous silicon (PS) prepared from n-type Si crystal is proposed as a new material for the fabrication of sensitive substrates for surface-enhanced Raman scattering (SERS). The formation procedure for nanostructured silver films on the surface of PS was optimized. Maximum of SERS enhancement for rhodamine 6G probing molecule is observed for samples obtained by the immersion plating from the water solution of AgNO₃ with the 10 mM concentration during 5 min. The dependence of morphological parameters of PS and corresponding silvered surfaces on the anodization current density has been studied. It is shown that the most SERS activities possess substrates produced from PS with lower porosity. The optimum of the PS layer thickness for high Raman signal is about 5 μm. The detection limit for rhodamine 6G adsorbed on Ag-coated PS from the 100 pM solution is established to be comparable with that for p-type PS-based substrates. Thus, the n-type porous silicon is suitable material for the preparation of sensitive SERS-active substrates.     

SiC1-xGex/SiC 异质结光电二极管特性的研究

使用二维器件模拟软件Medici, 对SiC_1-xGe_x/SiC异质结的光电特性进行了模拟.设计了N型重掺杂SiC层的厚度为1 μm, P型轻掺杂SiC_1-xGe_x层厚为0.4 μm, 二者之间形成突变异质结.在反向偏压3 V、光强度为 0.23 W/cm²的条件下, p-n+ SiC_0.8Ge_0.2/SiC和p-n+ SiC_0.7Ge_0.3/SiC敏感波长λ分别可以达到0.64 μm和0.7 μm, 光电流分别为7.765×10^－7 A/μm和7.438×10^－7 A/μm; 为了进一步提高SiC_1-xGe_x/SiC 异质结的光电流, 我们把p-n+两层结构改进为p-i-n三层结构.在同样的偏压、光照条件下, p-i-n SiC_0.8Ge_0.2/SiC和p-i-n SiC_0.7Ge_0.3/SiC的光电流分别达到1.6734×10－6 A/μm和1.844×10－6 A/μm.     

Electroluminescence from ZnO nanowires with a p-ZnO film/n-ZnO nanowire homojunction

ZnO homojunction light-emitting diodes based on ZnO nanowires were fabricated on Si(100) substrates. An N–In codoped p-type ZnO film grown by ultrasonic spray pyrolysis and an unintentionally doped n-type ZnO nanowire quasi-array grown by an easy low-temperature hydrothermal method were employed to form the homojunction diode. Under a forward bias larger than 8 V, electroluminescence, which was composed of an ultraviolet peak centered at 387 nm and a green band around 540 nm, was observed. The electroluminescence emission was contributed by the ZnO nanowires. The results reported here suggest that ZnO-based ultraviolet light-emitting devices could be realized at low cost. PACS 42.72.Bj; 73.40.Lq; 85.60.Jb     

Electronic structure of transparent oxides with the Tran-Blaha modified Becke-Johnson potential

We present electronic band structures of transparent oxides calculated using the Tran-Blaha modified Becke-Johnson (TB-mBJ) potential. We studied the basic n-type conducting binary oxides In(2)O(3), ZnO, CdO and SnO(2) along with the p-type conducting ternary oxides delafossite CuXO(2) (X=Al, Ga, In) and spinel ZnX(2)O(4) (X=Co, Rh, Ir). The results are presented for calculated band gaps and effective electron masses. We discuss the improvements in the band gap determination using TB-mBJ compared to the standard generalized gradient approximation (GGA) in density functional theory (DFT) and also compare the electronic band structure with available results from the quasiparticle GW method. It is shown that the calculated band gaps compare well with the experimental and GW results, although the electron effective mass is generally overestimated.     

White light electroluminescence from poly(9-vinylcarbazole) (PVK)/ZnO NRs/poly(2-methoxy-5-(2-ethylhexyloxy)-1, 4-phenylenevinylene) (MEH-PPV) dual heterojunctions

In this paper, we fabricated a p-PVK/n-ZnO nanorods (NRs)/p-MEH-PPV dual heterojunctions white light-emitting diode. Relative to previously reported p–n heterojunction structure including ZnO NRs and polymer, the device exhibits a low turn-on voltage of 7 V. An obviously broad electroluminescence emission band, originated from the overlap of PVK emission and ZnO defects emissions, was observed extending from 360 up to 700 nm. The influence of the two introduced p-type polymer layers on the device characteristic is discussed. With its hole conductivity, the p-PVK layer cannot only improve the holes tunnel into ZnO NRs layer, but also lower the barrier between ITO and the valance band of ZnO NRs. On the other hand, p-MEH-PPV could be regarded as block layer for the injection of electrons from the Al electrode. Both of two p-type polymers dramatically improve the injection balance of carriers, leading to a low turn-on voltage. Meanwhile, the carrier transport mechanism of the device under different forward bias region was discussed on the basis of current–voltage curve.     

用电化学沉积法制备ZnO/Cu2O异质p-n结

由于P型ZnO的制备仍然存在一定的困难,限制了ZnO在光电方面的应用,尤其是在发光二极管和激光器的实际应用,目前利用P型的透明半导体氧化物与n型ZnO制备异质p-n结,成为新的研究热点。选择P型导电Cu2O与ZnO制备出异质p-n结。Cu2O是一种典型的P型半导体材料,禁带宽度为2．1eV,可见光范围的吸收系数较高。首次利用电化学沉积的方法制备了ZnO／Cu2O异质p-n结,研究了电沉积ZnO,Cu2O的生长机制和ZnO／Cu2O异质结的结构、光学和电学特性。     

Adsorption-controlled transition of the electrical properties realized in Hematite（alpha-Fe2O3） nanorods ethanol sensing

Alpha-Fe2O3 nanorods are synthesized through a hydrothermal method with no surfactant introduced and ethanol sensors are fabricated from these nanorods.The device can respond to ethanol vapour in a concentration range from 1 to 1500 parts per million and shows both p-type and n-type responding characteristics during the investigation of the ethanol sensing.The sensor displays a p-type characteristic when the ethanol concentration is low and converted into an n-type characteristic as the concentration exceeds a certain value.Such a phenomenon is attributed to the chemisorbed oxygen,which leads to different modifications of the energy band at the surface,namely,depletion layer or inversion layer.     

反对称n-AlGaN层对GaN基双蓝光波长发光二极管性能的影响

采用数值分析方法对在InGaN/GaN混合多量子阱活性层和n-GaN之间引入n-AlGaN层的GaN基双蓝光波长发光二极管进行模拟分析.结果发现,与传统的具有p-AlGaN电子阻挡层的双蓝光波长发光二极管相比,这种反对称n-AlGaN层能有效改善电子和空穴在混合多量子阱活性层中的分布均匀性及减少电子溢出,实现电子空穴在各个量子阱中的平衡辐射,从而减弱了双蓝光波长发光二极管的效率衰减.此外,通过改变Al组分可以提高双蓝光波长发光二极管发射光谱的稳定性:当Al组分为0.16时,双蓝光波长发光二极管的光谱在小电流下比较稳定,而Al组分为0.12时,光谱在大电流下比较稳定.     

Hybrid solar cells with conducting polymers and vertically aligned silicon nanowire arrays: The effect of silicon conductivity

Organic/inorganic hybrid solar cells, based on vertically aligned n-type silicon nanowires (n-Si NWs) and p-type conducting polymers (PEDOT:PSS), were investigated as a function of Si conductivity. The n-Si NWs were easily prepared from the n-Si wafer by employing a silver nanodot-mediated micro-electrochemical redox reaction. This investigation shows that the photocurrent-to-voltage characteristics of the n-Si NW/PEDOT:PSS cells clearly exhibit a stable rectifying diode behavior. The increase in current density and fill factor using high conductive silicon is attributed to an improved charge transport towards the electrodes achieved by lowering the device's series resistance. Our results also show that the surface area of the nanowire that can form heterojunction domains significantly influences the device performance.     

Photoluminescence and electroluminescence properties of ZnO films on p-type silicon wafers

A simplified n-ZnO/p-Si heterojunction has been prepared by growing n-type ZnO rods on p-type silicon wafer through the chemical vapour deposition method. The reflectance spectrum of the sample shows an independent absorption peak at 384 nm, which may be originated from the bound states at the junction. In the photoluminescence spectrum a new emission band is shown at 393 nm, besides the bandedge emission at 380 nm. The electroluminescence spectrum of the n-ZnO/p-Si heterojunction shows a stable yellow luminescence band centred at 560 nm,which can be attributed to the emission from trapped states. Another kind of discrete ZnO rod has also been prepared on such silicon wafer and is encapsulated with carbonated polystyrene for electroluminescence detection. This composite structure shows a weak ultraviolet electroluminescence band at 395 nm and a yellow electroluminescence band. These data prove that surface modification which blocks the transverse movement of carriers between neighbouring nanorods plays important roles in the ultraviolet emission of ZnO nanorods. These findings are vital for future display device design.     

Low-voltage blue light emission from n-ZnO/p-GaN heterojunction formed by RF magnetron sputtering method

High quality n-ZnO/p-GaN heterojunction was fabricated by growing highly crystalline ZnO epitaxial films on commercial p-type GaN substrates via radio frequency (RF) magnetron sputtering. Low-voltage blue light emitting diode with a turn-on voltage of ∼2.5 V from the n-ZnO/p-GaN heterojunction was demonstrated. The diode gives a bright blue light emission located at ∼460 nm and a low threshold voltage of 2.7 V for emission. Based on the results of the photoluminescence (PL) and electroluminescence (EL) spectra, the origins of the EL emissions were studied in the light of energy band diagrams of ZnO–GaN heterojunction, and may attribute to the radiative recombination of the holes in p-GaN and the electrons injected from n-ZnO, which almost happened on the side of p-GaN layer. These results may have important implications for developing short wavelength optoelectronic devices.     

Structural comparison of n-type and p-type LaAlO3/SrTiO3 interfaces

Using a surface x-ray diffraction technique, we investigated the atomic structure of two types of interfaces between LaAlO3 and SrTiO3, that is, p-type (SrO/AlO2) and n-type (TiO2/LaO) interfaces. Our results demonstrate that the SrTiO3 in the sample with the n-type interface has a large polarized region, while that with the p-type interface has a limited polarized region. In addition, atomic intermixing was observed to extend deeper into the SrTiO3 substrate at the n-type interface compared to the p type. These differences result in distinct degrees of band bending, which likely contributes to the striking contrast in electrical conductivity between the two types of interfaces.     

Polar discontinuity doping of the LaVO_{3}/SrTiO_{3} interface

We have investigated the transport properties of LaVO_{3}/SrTiO_{3} Mott-insulator-band-insulator heterointerfaces for various configurations. The (001)-oriented n-type VO_{2}/LaO/TiO_{2} polar discontinuity is conducting, exhibiting a LaVO3 thickness-dependent metal-insulator transition and low temperature anomalous Hall effect. The (001) p-type VO_{2}/SrO/TiO_{2} interface, formed by inserting a single layer of bulk metallic SrVO3 or SrO, drives the interface insulating. The (110) heterointerface is also insulating, indicating interface conduction arising from electronic reconstructions.