Similarities and differences in the field-induced doping effect of n-type and of intrinsic a-Si : H

Field-induced doping was studied in doped and undoped (intrinsic) a-Si:H samples. It was found that field-induced doping occurs in n-type and intrinsic materials. Capacitance and current measurements on doped and undoped Schottky-diode samples show changes in the capacitance and conductivity after heat treatments at 400 K while keeping the specimen under high electric field. These changes depend on the field direction and can be reversed by a new heat treatment with a zero bias. Field-induced doping effects are attributed mainly to changes in the density of shallow states in n-type materials but in the intrinsic material it is mainly due to increases in the number of deep states.     

Defect-state engineering in a-Si:H: An effective tool for studying processes during light-induced degradation

Charge deep-level transient spectroscopy (Q-DLTS) has proved to be a powerful tool for investigating the defect-state distribution in hydrogenated amorphous silicon (a-Si:H) in both annealed and light-soaked states. In this article, we report on Q-DLTS experiments that were designed to further investigate our recently published model for the Staebler–Wronki effect (SWE). By subjecting an a-Si:H based metal/oxide/semiconductor structure to negative (positive) bias voltage during the annealing above the equilibration temperature a programmed p-type (n-type) defect-state distribution can be established in the undoped a-Si:H layer. In this way defect-state engineering in a-Si:H is achieved that enables us to study the role of different components of the defect-state distribution in the light-induced degradation process. Additional possibility to control the role of a particular defect-state component in the degradation process is to apply a bias voltage also during light soaking. The observed changes in the programmed defect-state distributions due to light soaking under various conditions give additional information on the SWE and can be interpreted with our model for the SWE.     

Influence of proton implantation on the properties of CuInSe2 single crystals (II)

Copper deficient p-type conducting CuInSe₂ single crystals were implanted with 40 keV protons in the fluence range from 2.5 · 10¹⁴ to 1.5 · 10¹⁶ cm⁻². Over the whole fluence range the implanted layers were n-type conducting which is ascribed to passivation of the acceptors due to copper vacancies and formation of donors by hydrogen atoms located at interstitial positions. The thermal stability of the conductivity changes due to proton implantation is limited to temperatures below 100 °C.     

Electrical Properties of CuInSe2 Single Crystals Implanted with Xenon

Both p-type and n-type CuInSe₂ single crystals were implanted with 40 keV ¹³⁰Xe⁺ ions up to doses of 5 · 10¹⁶ cm⁻². Implanted layers on p-type substrates showed an initial increase of the resistivity followed by a continuous decrease of the resistivity at higher doses. Implanted layers on n-type substrates gave also an increase of the resistivity at lower doses, but at higher doses a conductivity type conversion from n-type to p-type took place, followed by a decrease of the resistivity. To explain the experimental results it is supposed that the increase of the resistivity is mainly due to charge carrier scattering at extended defects while the decrease of the resistivity at high doses is due to the predominant creation of intrinsic acceptor states during implantation.     

Femtosecond transmission studies of n-type and p-type hydrogenated amorphous silicon pumped in the exponential band tails

The results of transient transmission studies utilizing femtosecond laser pulses on n-type and p-type hydrogenated amorphous silicon are presented. In these studies, both the pump and probe photon energies are tuned through the exponential band tail region. The responses of the different materials are quite different, allowing the technique to clearly distinguish between them. It is shown that while recombination is dominant in intrinsic a-Si:H, trapping becomes important in the doped material. A model of the experimental results gives additional insight into the density of states in the band tail region.     

New approach to capacitance spectroscopy for interface characterization of a-Si:H/c-Si heterojunctions

A new technique for characterization of interface defects in a-Si:H/c-Si heterostructure solar cells from capacitance spectroscopy measurements under illumination at forward bias close to open-circuit voltage is described. The proposed method allows to significantly increase the sensitivity to interface defects compared to conventional capacitance measurements at zero or small negative bias. Results of numerical modelling as well as experimental data obtained on n-type a-Si:H/p-type c-Si heterojunctions are presented. The sensitivity of the proposed method to interface states and the influence of various parameters like band mismatch, density of interface defects, recombination velocity at the back contact are discussed.     

基于第一性原理的二维g-AlN材料p型掺杂研究

半导体材料通过掺杂实现n型和p型载流子导电在半导体器件领域具有重要意义,理论上可通过计算电荷转移能级和缺陷形成能来探索半导体材料的n型和p型掺杂效率。基于第一性原理,结合二维带电缺陷计算方法,类石墨烯氮化铝(graphene-like AlN, g-AlN)中的四种(Be_Al,Mg_Al,Ca_Al,Sr_Al)潜在p型掺杂缺陷的结构、磁学、电学和缺陷形成能及转移能级被系统地计算。结果表明,所有缺陷体系均表现为深受主能级特性,很难为二维g-AlN提供p型载流子,它们反而会捕获g-AlN中的空穴,从而严重影响二维g-AlN材料的空穴导电率。Be_Al在整个电子化学势范围内具有最小的形成能,因此更加容易掺入到g-AlN中,影响g-AlN材料的p型掺杂效率。     

New dopant precursors for n-type and p-type GaN

Tetraethylsilane (TeESi) and bis(ethylcyclopentadienyl)Mg (ECp₂Mg) were employed as Si and Mg dopant precursors for MOVPE growth of n-type and p-type GaN films, respectively. In Si doping, the electron concentration was observed to increase with the increase of the TeESi flow rate. The temperature dependence of the Hall mobility showed good agreement with n-type GaN films grown using different dopant precursors (SiH₄, GeH₄, Si₂H₆). The donor activation energy was estimated to be 27 meV, which is almost the same as the literature values. In Mg doping, we also found that the Mg concentration increases as the ECp₂Mg flow rate increases. All of Mg-doped samples in this study showed p-type conduction after annealing. The acceptor activation energy was estimated to be 170 meV, which was close to the reported values.     

Photo-induced changes of a.c. conduction in amorphous chalcogenides and hydrogenated silicon

Photo-induced changes of a.c. conductance have been observed for both amorphous Se and hydrogenated Si (a-Si:H). After cessation of the illumination, no metastable change of a.c. conductance for Se and a decrease in the a.c. conductance for a-Si:H which could be related to the Staebler-Wronski effect have been found. These are discussed in terms of the negative-U defects.     

Transport properties of hydrogenated amorphous silicon deposited by dc glow discharge decomposition

Conductivity and thermoelectric power measurements have been made as a function of temperature on a series of hydrogenated amorphous silicon samples. The samples were prepared by the dc glow discharge decomposition of silane and silane phosphine mixtures. The activation energy for conduction varied with the substrate temperature and discharge condition for undoped specimens. The difference in the activation energy for conduction as well as the dependence of photoconductivity and optical gap on the activation energy for conduction among undoped specimens can be explained by introducing centers acting as donors or by change transfer between the island and hydrogen rich interfacial region. The kinks in the log σ versus inverse temperature curves always appear at about 430 K for the undoped specimens prepared at 300°C, while they are absent for low substrate temperature specimens. The downward kinks with increasing temperature can be explained by a two-phase material model. A revised two-channel conduction path model including material heterogeneity is applied to interpret the conductivity and thermopower versus inverse temperature curves of doped a-Si:H films, and to determine the position of phosphorus donor levels. The levels are found to lie at about 0.47 eV below E_c, the mobility edge at the conduction band.     

Optical properties of hydrogenated amorphous silicon films doped with fluorinated gases

Doped amorphous silicon films were prepared by plasma-enhanced chemical vapour deposition of silane and hydrogen mixtures, using phosphorus pentafluoride (PF₅) and boron trifluoride (BF₃) as dopant precursors. The films were studied by UV-vis spectroscopy and their photo and dark conductivity were measured, the latter as a function of temperature. The optical gap of the n-type samples, doped with PF₅, diminished as the concentration of this gas in the plasma was increased. However, the optical gap of p-type samples, doped with BF₃, did not show any appreciable optical gap decrease as the concentration of BF₃ was varied from 0.04% to 4.7%. The dark conductivity of the p-type films at these extremes of the doping range were 7.6 × 10⁻¹⁰ and 3.5 × 10⁻¹ Ω⁻¹ cm⁻¹, respectively.     

Improving the performance of amorphous and crystalline silicon heterojunction solar cells by monitoring surface passivation

The influence of thermal annealing on the crystalline silicon surface passivating properties of selected amorphous silicon containing layer stacks (including intrinsic and doped films), as well as the correlation with silicon heterojunction solar cell performance has been investigated. All samples have been isochronally annealed for 1 h in an N₂ ambient at temperatures between 150 °C and 300 °C in incremental steps of 15 °C. For intrinsic films and intrinsic/n-type stacks, an improvement in passivation quality is observed up to 255 °C and 270 °C, respectively, and a deterioration at higher temperatures. For intrinsic/n-type a-Si:H layer stacks, a maximum minority carrier lifetime of 13.3 ms at an injection level of 10¹⁵ cm^? 3 has been measured. In contrast, for intrinsic/p-type a-Si:H layer stacks, a deterioration in passivation is observed upon annealing over the whole temperature range. Comparing the lifetime values and trends for the different layer stacks to the performance of the corresponding cells, it is inferred that the intrinsic/p-layer stack is limiting device performance. Furthermore, thermal annealing of p-type layers should be avoided entirely. We therefore propose an adapted processing sequence, leading to a substantial improvement in efficiency to 16.7%, well above the efficiency of 15.8% obtained with the ‘standard’ processing sequence.     

Preparation and characterization of p-type hydrogenated amorphous silicon oxide film and its application to solar cell

Thin film wide band gap p-type hydrogenated amorphous silicon (a-Si) oxide (p-a-SiOx:H) materials were prepared at 175 °C substrate temperature in a radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) and applied to the window layer of a-Si solar cell. We used nitrous oxide (N₂O), hydrogen (H₂), silane (SiH₄), and diborane (B₂H₆) as source gases. Optical band gap of the 1% diborane doped films is in the range of 1.71 eV to 2.0 eV for films with increased oxygen content. Dark conductivity of these films is in the range of 8.7 × 10^− 5 S/cm to 5.1 × 10^− 7 S/cm. The fall in conductivity, that is nearly two orders of magnitude, for about 0.3 eV increase in the optical gap can be understood with the help of Arrhenius relation of conductivity and activation energy, and may not be significantly dependant on defects associated to oxygen incorporation. Defect density, estimated from spectroscopic ellipsometry data, is found to decrease for samples with higher oxygen content and wider optical gap. Few of these p-type samples were used to fabricate p-i-n type solar cells. Measured photo voltaic parameters of one of the cells are as follows, open circuit voltage (V_oc) = 800 mV, short circuit current density (J_sc) = 16.3 mA/cm², fill-factor (FF) = 72%, and photovoltaic conversion efficiency (η) = 9.4%, which may be due to improved band gap matching between p-a-SiOx:H and intrinsic layer. J_sc, FF and V_oc of the cell can further be improved at optimized cell structure and with intrinsic layer having a lower number of defects.     

Heat treatment of amorphous silicon p-i-n solar cells with high-pressure H2O vapor

We report improvement in characteristics of hydrogenated amorphous silicon (a-Si:H ) p-i-n structured solar cells by high-pressure H₂O vapor heat treatment. a-Si:H p-i-n solar cells were formed on glass substrates coated with textured SnO₂ layer. P-, i-, and n-type a-Si:H layers were subsequently formed by plasma enhanced chemical vapor deposition. Finally an indium-tin-oxide layer was coated on the n-type a-Si:H surface. Heat treatment at 210 °C with 2 × 10⁵ Pa H₂O vapor for 1 h was applied to the a-Si:H p-i-n solar cells. Electrical characteristics were measured when samples were kept in dark and illuminated with light of AM 1.5 at 100 mW/cm². The heat treatment with H₂O vapor increased fill factor (FF) and the conversion efficiency from 0.54 and 7.7% (initial) to 0.57 and 8.4%, respectively. Marked improvement in solar cell characteristics was also observed in the case of a poor a-Si:H p-i-n solar cell. FF and the conversion efficiency were increased from 0.29 and 3.2% (initial) to 0.56 and 7.7%, respectively.     

On the role of hydrogen in a-Si

Samples of a-Si sputtered in a hydrogen enriched argon atmosphere were prepared under various hydrogen partial pressures and deposition temperatures and successfully doped with lithium. A decrease of the electrical conductivity and a shift of the absorption edge to the higher energy were observed and ascribed to the diminishing number of the localized states in the band gap due to the presence of hydrogen.     

单晶金刚石p型和n型掺杂的研究

超宽禁带半导体材料金刚石在热导率、载流子迁移率和击穿场强等方面表现出优异的性质,在功率电子学领域具有广阔的应用前景。实现p型和n型导电是制备金刚石半导体器件的基础要求,其中p型金刚石的发展较为成熟,主流的掺杂元素是硼,但在高掺杂时存在空穴迁移率迅速下降的问题;n型金刚石目前主流的掺杂元素是磷,还存在杂质能级深、电离能较大的问题,以及掺杂之后金刚石晶体中的缺陷造成载流子浓度和迁移率都比较低,电阻率难以达到器件的要求。因此制备高质量的p型和n型金刚石成为研究者关注的焦点。本文主要介绍金刚石独特的物理性质,概述化学气相沉积法和离子注入法实现金刚石掺杂的基本原理和参数指标,进而回顾两种方法进行单晶金刚石薄膜p型和n型掺杂的研究进展,系统总结了其面临的问题并对未来方向进行了展望。     

Effect of statistical fermi level shift on the Meyer-Neldel rule of a-Si:H conductivity

Effect of the statistical Fermi level shift on the Meyer-Neldel rule of a-Si:H conductivity was studied using the temperature dependence of MOSFET conductivity. A model is presented in which the temperature dependence of surface band bending in a MOSFET should compensate for the statistical Fermi level shift of bulk a-Si:H. This compensation is verified by the SiO₂ thickness dependence of the MOSFET conductivity. The pre-exponential factor obtained by analysis of the temperature dependence of MOSFET conductivity does not show the Meyer-Neldel rule. It is therefore concluded that the Meyer-Neldel rule observed in bulk a-Si:H conductivity is predominantly caused by the statistical Fermi level shift.     

Electrical properties of AgGaSe2 epitaxial layers

Epitaxial layers of AgGaSe₂ with thicknesses in the range from 50 to 70 nm were deposited onto (111)A-oriented semi-insulating GaAs substrates by flash-evaporation in the substrate temperature range T_S = 825 … 900 K. Films grown at T_S ≦ 850 K are n-type conducting whereas p-type conductivity was observed at T_S ≧ 875 K. In the p-type samples two acceptor states with ionization energies of 60 and 410 meV were found from an analysis of the electrical measurements.     

Improvement of amorphous silicon n-i-p solar cells by incorporating double-layer hydrogenated nanocrystalline silicon structure

We develop a double-layer p-type hydrogenated nanocrystalline silicon (p-nc-Si:H) structure consisting of a low hydrogen diluted i/p buffer layer and a high hydrogen diluted p-layer to improve the hydrogenated amorphous silicon (a-Si:H) n-i-p solar cells. The electrical, optical and structural properties of p-nc-Si:H films with different hydrogen dilution ratio (R_H) are investigated. High conductivity, low activation energy and wide band gap are achieved for the thin films. Raman spectroscopy and high-resolution transmission electron microscopy (HRTEM) analyses indicate that the thin films contain nanocrystallites with grain size around 3-5 nm embedded in the amorphous silicon matrix. By inserting a p-nc-Si:H buffer layer at the i/p interface, the overall performance of the solar cell is improved significantly compared to the bufferless cell. The improvement is correlated with the reduction of the density of defect states at the i/p interface.     

Influence of oxygen/argon ratio on structural,electrical and optical properties of Ag-doped ZnO thin films

Ag-doped ZnO (ZnO:Ag) thin films were deposited on quartz substrates by radio frequency magnetron sputtering technique. The influence of oxygen/argon ratio on structural, electrical and optical properties of ZnO:Ag films has been investigated. ZnO:Ag films gradually transform from n-type into p-type conductivity with increasing oxygen/argon ratio. X-ray photoelectron spectroscopy measurement indicates that Ag substitutes Zn site (Ag_Zn) in the ZnO:Ag films, acting as acceptor, and being responsible for the formation of p-type conductivity. The presence of p-type ZnO:Ag under O-rich condition is attributed to the depression of the donor defects and low formation energy of Ag_Zn acceptor. The I–V curve of the p-ZnO:Ag/n-ZnO homojunction shows a rectification characteristic with a turn-on voltage of ∼7 V.