Photoconductivity and photoluminescence of a-Si:H at low temperature

We have studied the temperature dependence of the photoluminescence steady state and transient photoconductivity of a-Si:H down to 4 K. Below 50 K we observe photoconductivity with $\text{ημτ ? 10}{}^{\mathrm{?11}}\text{cm}{}^2\text{/}\text{V}$, which is independent of temperature and varies only little with the defect density in the films. We propose that this conduction arises predominantly from the drift of the photoexcited electrons and holes during thermalization in the extended states prior to the localization in states below the mobility edge. An important implication of the present data is that, even at helium temperatures, a considerable part of the carriers recombines in a non-geminate process.     

Distribution of hydrogen in low temperature passivated amorphous silicon (a-Si:H) films from neutron reflectivity

Hydrogen plays a critical role in the passivation of dangling bonds in hydrogenated amorphous silicon (a-Si:H) to enable acceptable semiconducting characteristics during operation in devices. Low temperature processing enables fabrication of high performance transistors on flexible substrates such as plastic or stainless steel foils, but also leads to a decrease in the stability of the electronic performance. Generation of defects at the a-Si:H/insulator (hydrogenated silicon nitride, SiN:H) during electrical use due to localized heating will lead to decreased performance unless the dangling bonds are passivated in-situ by residual hydrogen. For this reason, the distribution of hydrogen within a-Si:H may be critical to understanding their aging phenomena. Here the distribution of hydrogen within both a-Si:H and SiN:H layers is probed with sub-nanometer resolution using neutron reflectivity. The hydrogen concentration within the bulk of the a-Si:H (11 ± 2 at.%) and SiN:H (18 ± 3 at.%) agree well with previous reports, but the increased resolution of the neutron measurement is able to identify an approximate three fold increase in the concentration within 2 nm of the semiconductor-insulator interface. This enhanced hydrogen content may act in the short-term as a sink to passivate any dangling bonds formed during operation.     

Effects of deposition temperature,sample thickness and doping on the microstructures of a-Si:H

SEM and TEM were used for studying the growing morphology of a-Si:H prepared by the DC glow discharge. The preparation conditions being controlled as variables were deposition temperature, sample thickness and dopant type. From the micrographs of SEM and TEM, a morphological model for the growth of a-Si:H film is proposed.     

The nature of the defect states above midgap and their role in the light-induced metastability of a-Si:H

Defect states of a-Si:H above midgap, with which the electrons interact through trapping and thermal release and make an important contribution to the imaginary (Y) term of the modulated photocurrent, are determined. Analysis of the ‘transition’ region between the low- and high-frequency regimes enables the densities and capture coefficients of defect states to be probed. A reasonable fit to the data is obtained by assuming three defect states, and it is suggested that the defects with the higher capture probability are hydrogen-related centers that are removed during the initial stage of light degradation.     

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.     

p/i界面掺碳缓冲层沉积时间对非晶硅太阳电池性能的影响

本文研究了pin型非晶硅(a-Si)太阳电池p/i界面掺碳缓冲层(C-buffer layer)沉积时间对电池效率和稳定性的影响.研究发现,随着掺碳缓冲层沉积时间的增加,太阳电池的初始效率有所增加,当沉积时间增加到约60s时,电池的初始效率达最大值,而后随着沉积时间的继续增加,电池效率下降.而在太阳电池的稳定性方面,当缓冲层沉积时间小于50s时,随着沉积时间的增加,电池衰退率增大;大于50s后,电池的衰退率又随沉积时间的增大而减小.     

Large grain μc-Si:H films deposited at low temperature: Growth process and electronic properties

We have studied structural and electronic properties of μc-Si:H films deposited from SiH₄ + H₂ and SiH₄ + H₂ + Ar gas mixtures. The use of Ar containing gas mixtures for depositions allows us to increase deposition rate by a factor of two and to obtain films with an important fraction of large grains in comparison with SiH₄ + H₂ gas mixtures. Electronic properties of fully crystallized films become more intrinsic with the increase of large grain fraction. Deposition of highly p- and n-doped μc-Si:H layers from the dopant/SiH₄ + H₂ gas mixture at a temperature of 175 °C is possible without any remarkable changes in crystallinity in comparison with undoped films deposited with the same discharge conditions.     

P／I界面处理对a-Si：H柔性太阳能电池性能的影响

采用等离子体辅助化学汽相沉积(PECVD)技术制备本征非晶硅薄膜,对p/i界面进行处理.在此基础上,制备P型微晶硅(μc-Si:H)薄膜与柔性太阳能电池.对P型硅薄膜及太阳能电池的性能进行研究.结果表明:对p/i界面采用H等离子体处理,再引入一定厚度的成核层,可以成功得到高电导率的P型微晶硅窗口层,提高柔性太阳能电池的光伏特性.其中的成核层,不仅促进微晶相P层的生长,还可以起到界面缓冲层的作用.     

Properties of a-Si:H prepared by the photochemical decomposition of Si2H6

High quality a-Si:H films have been prepared by the direct photolysis of disilane at a substrate temperature below 350 °C. The growth rate is independent of substrate temperature for both undoped and phosphorus doped films, while it is thermally activated and dramatically enhanced by boron doping. The hydrogen content decreases from 7 to 2 at.%, as deposition temperature is varied from 200 to 300 °C. The photoconductivity as high as 3.7 × 10^?4 Ω^?1cm^?1 (AM1 100 mW/cm²) has been obtained and no light soak degradation was observed.     

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.     

Effect of NH3 on the growth characterization of TiN films at low temperature

Titanium nitride (TiN) films were obtained by the atmospheric pressure chemical vapor deposition method of the TiCl₄–N₂–H₂ system with various flow rates of NH₃ at 600°C. The growth characteristics, morphology and microstructure of the TiN films deposited were analyzed by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Without NH₃ addition, no TiN was deposited at 600°C as shown in the X-ray diffraction curve. However, by adding NH₃ into the TiCl₄–N₂–H₂ system, the crystalline TiN was obtained. The growth rate of TiN films increased with the increase of the NH₃ flow rate. The lattice constant of TiN films decreased with the increase of the NH₃ flow rate. At a low NH₃ flow rate, the TiN (2 2 0) with the highest texture coefficient was found. At a high NH₃ flow rate, the texture coefficient of TiN (2 0 0) increased with the increase of the NH₃ flow rate. In morphology observation, thicker plate-like TiN was obtained when the NH₃ flow rate was increased. When the flow rate of NH₃ was 15 sccm, Moiré fringes were observed in the TiN film as determined by TEM analysis. The intrinsic strain was found in the TiN film as deposited with 60 sccm NH₃.     

Formation and characterization of GaAs/As superlattice grown by molecular beam epitaxy at low substrate temperature

High-resolution X-ray diffractometer and transmission electron microscope (TEM) are used to characterize the redistribution of As precipitates in Si δ-doped GaAs grown by molecular beam epitaxy at low substrate temperature (230°C). The superlattice satellite peaks are observed for samples annealed at 700-800°C for 10 min, which is attributed to the formation of a GaAs/As superlattice during the annealing period. The degree of As precipitates confined on the δ-doped planes is revealed on the intensity of satellite peaks in the X-ray rocking curves, as confirmed by the TEM observations. The lattice expansion and contraction of the annealed low-temperature epitaxial layers can be easily observed from the asymmetry of the satellite peaks.     

Lifetime of dominant radicals for the deposition of a-Si:H from SiH4 and Si2H6 glow discharges

Deposition kinetics of a-Si:H from Si₂H₆ as well as SiH₄ glow discharge has been investigated using a triode reactor with a variable electrode-substrate distance. The results of deposition rate and OES (optical emission spectroscopy) indicate that dominant radicals as a film precursor are different between SiH₄ and Si₂H₆ plasma and the radical in Si₂H₆ has a much shorter lifetime than that in SiH₄.     

Preparation and crystallographic characterization of crystalline modifications of 3,4:9,10‐perylenetetracarboxylic dianhydride at room temperature

3,4:9,10‐Perylenetetracarboxylic dianhydride (PTCDA) powder obtained from Sigma‐Aldrich was purified by three cycles of temperature gradient sublimation in an evacuated quartz glass tube providing variable conditions. By means of X‐ray diffraction (XRD) analysis, a crystallographic characterization of the powders obtained during the sublimation process and selected single crystals was carried out evidencing the presence of a major α‐ and a minor β‐PTCDA component in all samples. Lattice parameters of high precision were obtained for α‐PTCDA (a = 3.73283(4) Å, b = 12.0328(6) Å, c = 17.3998(4) Å, β = 98.689(2)°, V = 772.57(4) ų and d₁₀₂ = 3.219(3) Å) by whole‐X‐ray‐powder‐pattern‐fitting of the experimental XRD pattern using the Le Bail approach. Employing a (001) KCl wafer as substrate for the material purification results in formation of single phase α‐PTCDA accompanied by a decrease in the size of the crystallites which is attributed to the presumed lower temperature of the KCl wafers. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Excitation energy dependence of the photoluminescence band at 2.7 and 4.3 eV in silica glass at low temperature

In this paper, we report the excitation energy dependence of the 2.7 and 4.3 eV photoluminescence (PL) bands in oxygen deficient silica glass at low temperature (20 K). The increase or decrease of the PL intensity at low temperatures is different for different exciting light wavelengths. The PL intensity tended to decrease with low temperatures when the excitation was near the upper and lower end of the excited level. The peak energy of the excitation spectrum increases with cooling. These results indicate that the change in excitation level with cooling is associated with the low-temperature dependence of light emission. Thermal motion is suppressed, when the sample temperature is lowered, and the energy-width of the excited level decreases, i.e., the light emission probability decreases (the emission intensity decreases), when near the upper and lower end of the excitation level. These phenomena were observed in the low-temperature dependence of the 4.3 eV emission intensity.