Effect of nanocrystalline inclusions on the photosensitivity of amorphous hydrogenated silicon films

Photosensitivity of amorphous hydrogenated silicon films containing inclusions of Si nanocrystals, along with spectral characteristics of photoconductivity, were studied. A correlation between photosensitivity and features of the Raman spectra was established. The highest photosensitivity is observed in films with medium-range order formed to the maximum extent.     

Amorphous chromium silicide formation in hydrogenated amorphous silicon

The interaction between thin films of hydrogenated amorphous silicon and sputter-deposited chromium has been studied. Following deposition of the chromium films at room temperature, the films were annealed over a range of times and temperatures below 350°C. It was found that an amorphous silicide was formed only a few nanometers thick with the square of thickness proportional to the annealing time. The activation energy for the process was 0.55±0.05 eV. The formation process of the silicide was very reproducible with the value of density derived from the thickness and Cr surface density being close to the value for crystalline CrSi₂ for all films formed at temperatures ≤300°C. The specific resistivity of the amorphous CrSi₂ was ≈600 μΩ·cm and independent of annealing temperature.     

Photoconductivity of two-phase hydrogenated silicon films

Electrical, photoelectric, and optical properties of hydrogenated amorphous silicon films with various ratios between the nanocrystalline and amorphous phases in the structure of the material have been studied. On passing from an amorphous to a nanocrystalline structure, the room-temperature conductivity of the films increases by more than five orders of magnitude. With increasing fraction of the nanocrystalline component in the film structure, the steady-state photoconductivity varies nonmonotonically and is determined by the variation in the carrier mobility and lifetime. Introduction of a small fraction of nanocrystals into the amorphous matrix leads to a decrease in the absorption in the defect-related part of the spectrum and, accordingly, to a lower concentration of dangling bonds, which are the main recombination centers in amorphous hydrogenated silicon. At the same time, the photoconductivity in these films becomes lower, which may be due to appearance of new centers that are related to nanocrystals and reduce the lifetime of nonequilibrium carriers.     

Photoconductivity of nanostructured hydrogenated silicon films

The photoconductivity of nanostructured hydrogenated silicon films prepared by different techniques was studied in relation to the Fermi level position, the density of defects, and the type of Si-H bonding. The influence of Si⁺ ion implantation on the photoconductivity and other parameters of a-Si:H films was determined.     

Microstructure evolution of hydrogenated silicon thin films

This paper describes the microstructure evolution of hydrogenated silicon films containing various amounts of hydrogen. Microcrystalline silicon films were produced when the hydrogen content of the films was adjusted by using the diluted hydrogen and hydrogen atom treatment methods. Polycrystalline silicon films having grain sizes in the micrometre range were deposited at low temperatures (250°C) by electron cyclotron resonance chemical vapour deposition with the hydrogen dilution method. The micro crystalline and polycrystalline films were characterized by NMR, FTIR, Raman, X-ray and optical spectroscopy and electrical measurements. The results suggest the possibility of even larger grain silicon films suitable for high-performance solar cells which avoid the fundamental difficulties of amorphous Si:H solar cells.     

Amorphous boron films with enhanced electrical conductivity

The results of studying the electrical properties of nanostructured films of amorphous boron a-B are compared with the results obtained for bulk samples of the same material and also for the crystals of certain quasi-amorphous borides with complex icosahedral structure. Models accounting for the enhanced electrical conductivity of amorphous boron films are suggested.     

Diffusion of chromium in thin hydrogenated amorphous silicon films

The diffusion of a chromium bottom contact has been studied through thin 10-nm amorphous silicon film. The concentration of the diffused impurity has been analyzed by an X-ray photon spectroscopy technique and the diffusion coefficient was estimated. Diffusion annealing was carried out in vacuum (10^?6 mTorr), the temperature was kept at 400°C, and the annealing time was varied from 0 to 300 min. The authors propose that diffusion of chromium in thin hydrogenated amorphous film is limited by silicide formation at the metal-silicon interface.     

Conductivity and structure of Er-doped amorphous hydrogenated silicon films

A columnar structure of a-Si:H(Er) film serving as a working layer in electroluminescent structures has been demonstrated. The diameter of columns is in the range of 60–100 Å. In a structure of this kind, the conductivity depends on the direction of current. In the planar configuration, room-temperature transport occurs through hopping via localized states near the conduction band edge, within the band tail. In the sandwich configuration, the conduction occurs along the column boundaries, where the conductivity is higher, via hopping conduction at the Fermi level.     

Photoinduced conductivity change in erbium-doped amorphous hydrogenated silicon films

Changes in the dark conductivity of erbium-doped amorphous hydrogenated silicon (a-Si:H(Er)) films after their preliminary illumination at room temperature have been studied. The effect of a compensating boron impurity on the photoinduced change in the conductivity of a-Si:H(Er) films is analyzed. It is established that the magnitude and the sign of the change in conductivity depend on the duration of illumination and position of the Fermi level in the mobility gap. Possible mechanisms leading to a photoinduced change in the conductivity of a-Si:H(Er) films are discussed.     

Crystallization of amorphous hydrogenated silicon films deposited under various conditions

The possibility of using the magnetron-assisted silane decomposition technique for the deposition of a-Si:H films as the basic materials for the production of polysilicon is analyzed. It is shown how specific features of the film structure affect the crystallization process.     

Thermodynamics and kinetics of hydrogen evolution in hydrogenated amorphous silicon films

Plasma Physics Corporation, P.O. Box 548, Locust Valley, NY 11650 The changes in enthalpy and entropy due to hydrogen evolution in hydrogenated amorphous silicon films were measured by differential scanning calorimetry (DSC). Hydrogen evolution was associated with an endothermic DSC peak, as supported by thermogravimetric analysis and evolved gas analysis. The en-thalpy and entropy changes of hydrogen evolution increased with heating rate and hydrogen content, because the evolution involved not only Si-H bond breaking, but also defect formation (such as Si-Si bond breaking), which was enhanced by a high flow of evolving hydrogen. In contrast, the activation energy of hydrogen evolution was controlled by the doping rather than the hydrogen content, because doping affected the Si-H bonding, which in turn affected the state before hydrogen evolution. Crystallization, which occurred at tempera-tures higher than hydrogen evolution, was delayed for the amorphous silicon film in a higher disordered state after hydrogen evolution, suggesting that hydrogen content influenced the crystallization process.     

Paramagnetic structural defects and conductivity in hydrogenated nanocrystalline carbon-doped silicon films

The behavior of paramagnetic defects and dark conductivity in heterogeneous samples of hydrogenated nanocrystalline carbon-containing silicon (nc-SiC:H) prepared by the photo-CVD method is studied. It is shown that, with increasing carbon content in nc-SiC:H, a phase transition from a nanocrystalline to an amorphous structure occurs, producing a reduction in paramagnetic defect density and a significant decrease in the dark conductivity.     

Special features of photoelectric properties of nanostructured films of hydrogenated silicon

Nanostructured Si films differing in hydrogen content, in the forms of Si-H bonds, and in certain characteristics of Si inclusions in an amorphous matrix (volume fraction, size, and structure) were studied. The behavior common to all the studied films, i.e., an increase in the defect density and nonmonotonic enhancement of photoconductivity at the “red wing” of the spectral characteristic compared to a-Si:H, was assessed. At the same time, there are films with either enhanced or reduced photoconductivity compared to a-Si:H.     

1/f noise in amorphous silicon and hydrogenated amorphous silicon thin films

Excess noise measurements have been carried out on either sputtered a-Si or sputtered a-Si:H thin films, at 300 K and in the absence of light. The dependence of excess noise amplitude on hydrogen partial pressure in the sputtering chamber during the film growth has been studied. Investigations of 1/f noise in B-doped a-Si:H thin films have also been carried out. The results of this study show that1/f noise can be correlated to both the hydrogen content in a-Si:H films and to the doping of the films. The noise is explained in terms of fluctuations in the number of gap states due to thermally activated configurational changes involving hydrogenated bonds and dangling bonds.     

Effect of boron dopant on the photoconductivity of microcrystalline hydrogenated silicon films

The effect of boron dopant of microcrystalline hydrogenated silicon films on the temperature dependence of photoconductivity and photoresponse time was studied. The measurements were carried out in the temperature range of 130–450 K under irradiation with 1.4-eV photons. It is established that the steady-state photoconductivity and photoresponse time increase with doping level. A model of nonequilibrium charge carrier recombination, which takes into account the mixed-phase structure of microcrystalline silicon, is suggested. The model satisfactory explains the results obtained.     

Optical characterization of hydrogenated amorphous silicon thin films deposited at high rate

The aim of this paper is to provide a better understanding of hydrogenated amorphous silicon thin films (a-Si:H) in relation to their optical properties: refractive index, optical gap, absorption coefficient, thickness and surface roughness. The transmission spectrum of the films, deposited with various rf discharge power densities by an optimized plasma enhanced chemical vapor deposition (PECVD) method, at a high rate (>10 Å/sec), was measured over a range in wavelength from 500 to 1100 nm. An approximate model is utilized to describe the surface roughness. In this model, the surface roughness is modeled as a mixed layer of 50 percent of a-Si:H and 50 percent of air and the optical constant of the rough layer is derived using the Bruggemann effective medium approximation (EMA). The gradient iteration method of numerical analysis is used to solve the nonlinear equations in the study. Our results show that the potential underestimation of refractive index and resulting overestimation of film thickness can be overcome by considering the reflection of the rough surface. The method is carried out on the transmission data and the influence of rf discharge power density on the properties of the film is discussed in detail.     

Fabrication of silicon field-emission arrays using masks of amorphous hydrogenated carbon films

A fabrication process of silicon field-emission arrays is reported, in which thin films of amorphous hydrogenated carbon (a-C:H) are employed as masks in a two-step plasma-etching process, using pure SF₆ and a mixture of SF₆ and O₂. In comparison with processes that involve SiO₂ masks, the use of a-C:H improved the selectivity of the plasma etching, particularly in the case of pure SF₆. An estimation of Utsumi's figure of merit showed that a significant enhancement in electric field can be achieved, as a result of the sharp tips fabricated through this process.     

Sputtered hydrogenated amorphous silicon

Hydrogenated a-Si, whose properties can be modified by impurity doping, can be produced either by decomposition of silane or by reactive sputtering of Si in an argon-hydrogen plasma. This article reviews advances made during the past few years in the preparation and characterization of films produced by the second method. The basic deposition conditions are summarized. The conclusions of analytical studies (photo-emission, infrared spectroscopy, and volumetric measurements of evolved gases), regarding the amount of hydrogen and its bonding configuration in the network, are outlined. The optical, carrier transport, photoconductivity and photoluminescence properties as a function of hydrogen content and doping are described. Electron drift mobilities, deduced from steady state and transient photoconductivity are presented. The transport and recombination properties are discussed with existing models of amorphous semiconductors, and are found to be consistent with atomic relaxations, i.e. polaronic effects.     

Specific features of photoelectric properties of layered films of amorphous hydrogenated silicon

Temperature dependences of photoconductivity of layered and conventional undoped films of amorphous hydrogenated silicon have been studied within a wide range of temperatures (130–420 K) and illumination intensities (0.1–60 mW cm⁻²). It is established that a higher photosensitivity of layered films compared with conventional films is governed by a low dark conductivity of layered films as a consequence of a deeper position of the equilibrium Fermi level in the band gap and the absence of temperature quenching of photoconductivity in these films. It is shown that these specific features of electrical and photoelectric properties of layered films can be attributed to a low concentration of silicon dangling bonds in comparison with the concentration of oxygen-related acceptor centers, which feature a larger capture coefficient for holes.     

Study of the electronic properties of hydrogenated amorphous silicon films by femtosecond spectroscopy

Experimental results on the electron relaxation time and diffusion coefficient in hydrogenated amorphous silicon films that exhibit intrinsic and electronic conductivity at room temperature are reported. It is found that, for these two types of films, the relaxation times are 1 ns and 465 ps and the diffusion coefficients are 0.54 and 0.83 cm² s^?1. It is established that, as the pulse intensity is increased, the decay time of the induced-grating signal shortens.