Thermal conductivity of hydrogenated amorphous silicon

The thermal conductivity of amorphous silicon thin films is measured in one dimension steady state condition. The experimental method is based on heating the sample front surface and monitoring the temperatures at its two sides. The experiments were carried out in conditions ensuring one-direction heat flow from top to bottom throughout the sample thickness. Sputtered a-Si:H films prepared with different conditions are used in order to investigate the dependence of thermal conductivity on material properties (i.e. hydrogen content and microstructure). The results show that, firstly, amorphous silicon is a very bad thermal conductor material. Secondly, the disorder in the film network plays an important role in thermal conduction. The highly disordered film exhibits the lowest thermal conductivity.     

The effect of illumination on dark conductivity and photoconductivity of hydrogenated amorphous silicon layered films

It is established that both the amplitude and temperature dependence of dark conductivity and photoconductivity of preilluminated high-sensitivity layered films of amorphous hydrogenated silicon (a-Si:H) prepared by cyclic deposition with layer-by-layer annealing in hydrogen plasma depend on illumination temperature. The relaxation kinetics of the dark conductivity of these films after illumination is shut off is found to be nonmonotonic. The observed effects can be explained by fast and slow changes in the distribution of energy state density below the midgap during and after illumination.     

氢化非晶硅薄膜的热导率研究

采用铂电极为加热电阻,研究了厚度为300—370nm等离子体化学气相沉积(PECVD)工艺制备的氢化非晶硅(a-Si：H)薄膜的热导率随衬底温度的变化规律.用光谱式椭偏仪拟合测量薄膜的厚度,得到了沉积速率随衬底温度变化规律,傅里叶红外(FTIR)表征了在KBr晶片衬底上制备的a-Si：H薄膜的红外光谱特性,SiH原子团键合模的震动对热量的吸收降低了薄膜热导率.从动力学角度分析了薄膜热导率随平均温度升高而增大的原因,并比较了声子传播和自由电子移动在a-Si：H薄膜热导率变化上的作用差异. 关键词： 非晶硅 热导率 薄膜 热能     

Thermoelectric power,photo and dark conductivity of doped amorphous silicon

Summary Conductivity, photoconductivity and thermopower measurements have been made as functions of temperature on a series of p- and n-type a-Si: H films prepared by a reactive, evaporation method. The results have been fitted with a model including two conduction paths: in the high-temperature range through extended band states and in the low-temperature range as hopping in an impurity band, whose density of states increases with increasing dopant content. A shift of the Fermi level up to 0.4 eV towards the mobility edges for both types of dopants has been found. Moreover, phosphorus doping radiply increases the photoconductivity values by about two orders of magnitude, while boron incorporation causes considerable reduction. This work was supported in part by GNSM-CNR, MPI and CNR ?Progetto Finalizzato Energetica?.     

Nanoindentation of hydrogenated amorphous silicon

Nanoindentation was carried out on thin films of hydrogenated amorphous silicon (a-Si:H) prepared by plasma-enhanced chemical vapor deposition. The composite values of elastic (Young's) modulus, E _c, and hardness, H _c, of the film/substrate system were evaluated from the load–displacement curves using the Oliver–Pharr approach. The film-only parameters were obtained employing the extrapolation of the depth profiles of E _c and H _c. Scanning probe microscopy was employed to image the nanoindenter impressions and to estimate the effect of film roughness and material pile-up on the testing results. It was established that the elastic modulus of thin a-Si:H films is in the range 117–131 GPa, which is lower than for crystalline silicon. In contrast, the values of hardness are in the range 12.2–12.7 GPa, which is comparable to crystalline silicon and higher than for hydrogen-free amorphous silicon. It is suggested that the plastic deformation of a-Si:H proceeds through plastic flow and it is the presence of hydrogen in the amorphous matrix that leads to a higher hardness.     

Effect of preparation condition on conductivity activation energy and downward conductivity kink of undoped hydrogenated amorphous silicon

Temperature dependence of conductivity has been measured for a series of undoped a-Si:H films prepared by glow discharge decomposition of silane. Conductivity activation energy and the existence of downward conductivity kinks around 430 K were observed to depend on preparation conditions such as substrate temperature, annealing temperature and discharge configuration. The undoped a-Si:H films whose Fermi levels are well-above the midgap, always show downward conductivity kinks. Variation of conductivity activation energies among undoped a-Si:H films can be interpreted by the transfer of electrons between islands and tissue.     

Oxidation studies of hydrogenated amorphous silicon

Hydrogenated amorphous silicon surfaces, atomically clean and subsequently oxidized to up to 20 Å oxide thickness, were studied using AES and UPS. The oxidation was made in O₂ in the pressure range 10^?9 Torr to 5 atm and at 23 and 300°C. The oxidation rate at 23°C was found to be the same as that of crystalline silicon while at 300°C it was appreciably faster. Changes in the d $\text{N(E)}\text{dE}$ AES Si LVV line shape near 80 eV upon oxidation could be correlated to changes in the silicon-oxygen bonding level observed in UPS. The detailed line shape of the AES Si LVV transition indicates that at 300°C a more homogeneous oxide is produced than at 23°C.     

Persistent photoconductivity in hydrogenated amorphous silicon

The persistent photoconductivity (PPC) has been observed in undoped, phosphorus-doped, and boron-doped hydrogenated amorphous silicon (a-Si: H) films. As the annealing temperature is raised for these films the decay of residual conductivity is accelerated and the PPC disappears almost completely after annealing at 500°C. These experimental results rule out the mechanism requiring the phosphorus-boron complexes or the deep defects. The PPC is found to be related with the sample inhomogeneity from the experimental observation that the decay of residual conductivity is closely correlated with the microstructure. A model is proposed to explain the PPC in a-Si: H films.     

Nitrogen doping in hydrogenated amorphous silicon

A direct evidence of substitutional doping in ion beam deposited amorphous hydrogenated silicon by nitrogen is presented. From the analysis of infrared (IR) absorption spectra and Si-2p core level shape, measured with X-ray photoelectron spectroscopy (XPS), the preferential tendency of nitrogen to go in for three-fold coordination at higher concentration and tetrahedral bonding at lower concentration (⩽4 at %) is established. XPS technique has been used for the first time to deduce the upper limit for substitutional solid solubility of the impurity.     

Extended-state mobility in hydrogenated amorphous silicon

We use the results of time-of-flight experiments in conjunction with recent conclusions about the behavior of the density of localized states below the conduction-band mobility edge to calculate the mobility of electrons moving in extended states in a-Si:H. We find that the extended-state mobility is considerably larger than previous estimates, which were based on the assumption that the exponential behavior responsible for dispersive transport extends all the way to the mobility edge. Using a recent estimate for the density of localized states, we find that the extended state mobility in a-Si:H is about 500 cm²/V-s, a value consistent with the results deduced from high-level injection experiments on p-i-n structures.     

Radiative recombination in hydrogenated amorphous silicon

Hydrogenated amorphous silicon exhibits efficient optical transitions across a gap larger than that of crystalline Si. Hydrogen passivates the dangling bonds and endows the material with a reduced number of non-radiative recombination centers. A gap widening has been observed in other hydrogenated semiconductors.Research reported herein was supported by the Department of Energy, Division of Solar Technology, under Contract No. EY-76-C-03-1286 and by RCA Laboratories, Princeton, NJ 08540.     

Quantum well model of hydrogenated amorphous silicon

A model of barrier-separated regions is proposed that leads to quantization and spatial correlation of carriers near the band gap of hydrogenated amorphous silicon. The size of these regions, which consist of pure Si bounded by potentials emanating from Si-H bonds, is estimated from a classical percolation picture. Near band gap localized states lie in these quantum well regions and are about 0.3 eV more widely separated than in unbounded Si, thereby accounting for a wide variety of hitherto uncorrelated experimental results. Dopants enhance conductivity by providing conduction paths through the barriers. Spatially coincident pairing of conduction with valence band localized states is speculated to be relevant to other amorphous semiconductors as well.     

Ultrashort laser-pulse annealing of hydrogenated amorphous silicon

Crystallization of hydrogenated amorphous silicon (a-SI:H) has been initiated using ultrashort laser-pulse train annealing. Optical microscopy, infrared absorption, Raman spectroscopy and photoluminescence measurements show that in our experiment the crystallized layer is localized on the surface and is non-epitaxial. The depth of the crystalline layer and its surface morphology are discussed. A sharp luminescence band at 0.970 eV with fine structure is found after laser annealing and is identified as a recombination center similar to irradiation induced defects in crystalline Si.     

Dynamics of hydrogen in hydrogenated amorphous silicon

The problem of hydrogen diffusion in hydrogenated amorphous silicon (a-Si:H) is studied semiclassically. It is found that the local hydrogen concentration fluctuations-induced extra potential wells, if intense enough, lead to the localized electronic states in a-Si:H. These localized states are metastable. The trapping of electrons and holes in these states leads to the electrical degradation of the material. These states also act as recombination centers for photo-generated carriers (electrons and holes) which in turn may excite a hydrogen atom from a nearby Si-H bond and breaks the weak (strained) Si-Si bond thereby apparently enhancing the hydrogen diffusion and increasing the light-induced dangling bonds.     

Photoconductivity and dark conductivity studies of (MgO-ZnO) mixed system

The photoconductivity and dark conductivity of (20% MgO-80% ZnO) mixed system in polystyrene binder layer have been studied and compared with (50% MgO-50% ZnO) and ZnO samples. For (20% MgO-80% ZnO) and (50% MgO-50% ZnO) samples, the dark current is found to be space charge limited at higher voltages. The photocurrent shows non-Ohmic behaviour at lower voltages and tends to saturation at higher voltages. For 100% ZnO system at higher voltages the photocurrent does not show any saturation effect and the dark current becomes linear. The change in photocurrent versus intensity curve from superlinear to sublinear indicates the presence of sensitizing centres near to the valence band and the existence of an exponential trap distribution in (20% MgO-80% ZnO) and (50% MgO-50% ZnO) samples. For 100% ZnO system, the photocurrent versus intensity curve changes from sublinear to linear indicating the presence of exponential trap distribution. Photocurrent decays more rapidly with increasing percentage of MgO in the mixed system. The X-ray diffraction pattern of these samples do not show formation of solid solutions.     

Alternating space-charge-limited currents in hydrogenated amorphous silicon

Space-charge-limited currents in n⁺-i-n⁺ sandwich samples of hydrogenated amorphous silicon have been investigated under alternating current (AC) conditions. When the voltage sweeping rate exceeds the release rate of carriers from deep traps, the AC current-voltage characteristics differ markedly from the direct current (DC) characteristics. This is attributed to the remaining space charge in the samples which acts as a potential barrier and reduces the current. A simple model is proposed to describe the time-dependence of this injection-induced barrier.     

Metastability and microstructural changes in hydrogenated amorphous silicon

The effects of prolonged illumination and subsequent thermal treatment, on the valence band density of states in hydrogenated amorphous silicon have been investigated. Significant reversible changes are observed upon light soaking and annealing sequences. A microscopic model consistent with the experimental observations is proposed for the light-induced generation and thermal recovery of defects in a-Si:H.     

Room-temperature electroluminescence from erbium-doped amorphous hydrogenated silicon

We have studied electroluminescence (EL) in the amorphous silicon-based erbium-doped structures under reverse bias in the temperature range 77–300 K. The intensity of electroluminescence at the wavelength of 1.54 μm exhibits a maximum near the room temperature. The excitation of erbium ions occurs by an Auger process which involves the capture of conduction electrons by neutral dangling bonds (D⁰-centers) located close to erbium ions. The stationary current through the structure is kept by a reverse process of thermally activated tunnel emission of electrons from negatively charged dangling-bond defects (D⁻-centers) to the conduction band of the amorphous matrix. A theoretical model proposed explains consistently all of our experimental data.