The charge deep-level transient spectroscopy (Q-DLTS) experiments on undoped hydrogenated amorphous silicon (a-Si:H) demonstrate that during light soaking the states in the upper part of the gap disappear, while additional states around and below midgap are created. Since no direct correlation is observed in light-induced changes of the three groups of states that we identify from the Q-DLTS signal, we believe that we deal with three different types of defects. Positively charged states above midgap are related to a complex formed by a hydrogen molecule and a dangling bond. Negatively charged states below midgap are attributed to floating bonds. Various trends in the evolution of dark conductivity due to light soaking indicate that the kinetics of light-induced changes of the three gap-state components depend on their initial energy distributions and on the spectrum and intensity of light during exposure.

Pulsed laser deposition (PLD) was used to grow microcrystalline thin films of germanium (Ge) and Ge-carbon (Ge,C) alloys on fused quartz and silicon substrates at substrate temperatures 25°C ≤ Ts ≤ 325°C. The films were analyzed structurally with x-ray diffraction (XRD), optically, electrically with four-point probe measurements, and chemically with x-ray photoelectron spectroscopy (XPS). XRD results displayed a varying degree of crystallinity, with the most crystalline films obtained at Ts > 150°C. The resistivity of the Ge films decreased with increasing temperature, displaying a significant decrease for the films deposited at Ts ≥ 230°C. The growth conditions for Ge films served as a starting point for low-temperature deposition of Ge,C alloys with up to 5% C. The effects of Ts and carbon concentration on film properties are discussed.

Al-doped ZnO films were deposited on glass in an in-line system by reactive mid-frequency (MF) magnetron sputtering. The influence of substrate position on the film properties as well as the relation between static and dynamic deposition are studied. All films showed low resistivity (<4x10-4 Ωcm) and excellent transparency (> 80 % in the visible region). The resistivity ρ for substrate positions above the sputter craters (race tracks) is up to a factor of two higher than on other positions where the smallest ρ is 1.9 x 10-4 Ωcm. Major differences in statically deposited films as a function of the position on the substrate are found for the structural film properties as characterized by x-ray diffraction (XRD) and etching behaviour. The different surface textures obtained after etching are directly related to variations in the short-circuit current densities of amorphous silicon p-i-n solar cells prepared on these etched ZnO:Al films.

Hydrogenated amorphous silicon photodiode arrays form the basis of monolithic three-dimensional integrated circuit sensor technology. In these arrays, the intrinsic a-Si:H layer covers the entire area to maximize light collection. One technique by which the pixel diode is defined, is to pattern the bottom contact layer independently of the intrinsic layer. One of the most important characteristics of any diode array, however, is that the dark-state reverse bias leakage currents must be as low as possible to minimize diode noise. This study examines the leakage currents associated with the pixelated array. These structures are unique in that the edge of the diode is defined by the local electric field between diodes, rather than the physical surface of an a-Si:H film. The effect of the diode edge has been found to induce a field-dependent component to the reverse bias leakage current. For example, diodes with 5500Å i-layer, have a junction leakage of 14 to 20 pA/cm2, at 5.0 x 104 V/cm, while the pixel edge-dependent current component can be as high as 30 pA/cm2. In addition, it will be shown that the i-layer thickness and junction doping plays a key role in determining the behavior of the leakage currents.

An interpretation of post-transit photocurrents in a time-of-flight experiment in terms of the underlying density of localized gap states in the sample is presented for the case of hydrogenated amorphous silicon cells prepared by the expanding thermal plasma technique. It is pointed out that part of the observed current is not generated by re-emission of trapped photo-generated charge and should, therefore, not be used for density-of-states calculations.

Electron spin resonance and conductivity measurements were used to study adsorption and oxidation effects on microcrystalline silicon with different structure compositions ranging from porous, highly crystalline to compact, mixed phase amorphous/crystalline. We found a correlation between active surface area and the magnitude of observed meta-stable and irreversible effects.

Post-transit time-of-flight spectroscopy has been used to study the density of states distribution in hot-wire CVD microcrystalline silicon pin solar cell structures. For an absorber layer Raman scattering intensity ratio ICRS of 0.4 or less, behaviour consistent with multiple-trapping carrier transport is observed and may be interpreted in terms of a conduction-band tail of some 18 meV slope plus a broad defect bump of order 1017 cm-3 centered at 0.55 eV relative to the mobility edge. As ICRS is increased beyond 0.4, the temperature-dependence of the photocurrent transient becomes inconsistent with multiple-trapping and above 0.6 the decays are almost temperature-independent. By comparing data taken at 300 K, it may be inferred from the multiple-trapping model that localised states between 0.35 and 0.5 eV are associated with the presence of columns or clusters of nanocrystals and those deeper than 0.5 eV with the amorphous tissue. Results are compared with previous work on coplanar and sandwich structures.

This paper presents an alternate strategy to reduce the mechanical stress issues pertinent to the process integration of molybdenum/hydrogenated amorphous silicon (Mo/a-Si:H) Schottky diodes and thin film transistors (TFTs), used as X-ray sensor pixels for medical imaging. The previous approach was to minimize the intrinsic stress in the Mo layer through appropriate process conditions and film thickness, but over narrow process latitude and with a compromise in X-ray sensitivity. Alternatively, the mechanical stress in Mo can be reduced by reducing and/or avoiding the extrinsic stress exerted on Mo by the underlying films through a different masking sequence in the fabrication. This modified process allows for a more flexible design of the Mo layer for enhanced X-ray sensitivity, while maintaining the mechanical integrity of the various layers. Also, the performance of the Schottky diode is improved, in terms of its forward current. The pixel shows good linearity in the X-ray response over the range of 40 ∼ 100 kVp.

Thin films of hydrogenated amorphous silicon containing nanocrystalline silicon inclusions (a/nc-Si:H) have been synthesized in an RF capacitively coupled PECVD system using a mixture of hydrogen diluted silane and helium, under deposition conditions at the edge of powder formation within the plasma. High resolution TEM confirms the presence of nanocrystallites as small as 2 nm in these films. Measurements of the optical absorption spectrum using CPM and PDS indicates a broadening of the Urbach slope in the a/nc-Si:H, compared to a-Si:H films, but no appreciable increase in midgap absorption. Despite the deposition conditions for the a/nc-Si:H being very different from those associated with producing optimal quality a-Si:H, the dark conductivity and photoconductivity values, and the sensitivity to light-induced defect creation in the a/nc-Si:H films are comparable to those in a-Si:H.

Metal-induced crystallization (MIC) of amorphous Si is gaining increased interest because of its potential use for low-temperature fabrication of integrated circuits. In this work, the MIC technique was used to make Si nanocrystals and the effects of stress on the crystallization were studied. Amorphous Si films were deposited onto the Si substrate with thermal oxides on top by low-pressure chemical vapor deposition (LPCVD) and then patterned into nanoscale pillars by electron beam lithography and reactive ion etching. A conformal low-temperature oxide (LTO) layer was deposited to cover the pillars, followed by an anisotropic etch back to form a spacer, leaving only the top surface of the pillars exposed to the 5 nm Ni sputtering deposition afterwards. An HF dip was used to partially remove the LTO spacers on the pillars, leading to different LTO thicknesses on different samples. These samples were then annealed to crystallize the amorphous Si pillars, forming Si nanocrystals. Transmission electron microscope (TEM) observations after anneal found a clear dependence of the crystallization rate on the pillar size as well as the LTO thickness. The crystallization rate was lower for pillars with thicker LTO spacers, while for the same LTO thickness the crystallization rate was lower for pillars with narrower width. A model based on the stress in the pillars is proposed to explain this dependence. This model suggests some methods to control the nickel-induced crystallization process and achieve higher quality Si nanocrystals.

We have established a new thermal CVD technique, Reactive Thermal CVD, for polycrystalline silicon (poly-Si) and silicon germanium (poly-SiGe) thin films aiming at thin film transistors (TFTs) applications, in which a low substrate temperature of 450°C enables us to use glass substrates. This technique achieved high crystallinity at very early stage of the film growth, resulting no amorphous incubation layer on the substrate surface. We fabricated bottom and top gate n-and p-channel TFTs with these of 200 nm thick films on SiO2/Si wafers and glass substrates, respectively: the high field effect mobilities as high as 55 cm2/Vs and 25 cm2/Vs were achieved in the bottom-gate and top-gate TFTs, respectively. Here, we discuss the technical requirements in the low-temperature CVD technique for the large-area poly-Si thin films and how they can be achieved in the reactive thermal CVD.

We have successfully obtained large lateral grains with well-controlled grain boundary. The proposed excimer laser annealing (ELA) method produces 2-dimensionally controlled grain growth because the temperature gradient is induced in two directions. Along the channel direction, the floating active structure produces large thermal gradient due to very low thermal conductivity of the air-gap. Along the perpendicular direction to the channel, the surface tension effect also produces thermal gradient. The proposed ELA method can control the grain boundary perpendicular and parallel to current path with only one laser irradiation.

Material characterization of metal induced lateral crystallization (MILC) process of amorphous silicon (a-Si) has been performed by using the spreading resistance probe (SRP) measurements. It was found that carrier mobility in boron ion implanted layer, formed in MILC region, is up to 65 % in comparison with mobility in boron ion implanted layer, formed in single crystalline silicon. It was also observed in this work that prolongation of MILC process from 1 hour to 2 hours had induced the increasing of mobility from 24 cm2/Vs to 34 cm2/Vs.

We report on a detailed study on gap-state distribution in thin amorphous silicon layers (a-Si:H) with film thicknesses between 5 nm and 20 nm on c-Si wafers performed by UV excited photoelectron spectroscopy (UV-PES). We measured how the work function, the gap state density, the position of the Fermi-level and the Urbach-energy depend on the layer thickness and the doping level of the ultra thin a-Si:H(n) layers. It was found, that for phosphorous doping the position of the Fermi level saturates at EF–EV=1.47 eV. This is achieved at a gas phase concentration of 10000 ppm PH3 in the SiH4/H2 mixture which was used for the PECVD deposition process. The variation of the doping level from 0 to 20000 ppm PH3 addition results in an increase of the Urbach energy from 65 meV to 101 meV and in an increase of the gap state density at midgap (EV-Ei= 0.86eV) from 3·1018 to 2·1019 cm-3eV-1.

This work deals with the structural properties of nanocrystalline (nc) silicon films for solar cell applications, grown using a new PECVD process based on an arc discharge plasma characterized by low ion energies, called LEPECVD (Low energy PECVD). This process permits to increase the intensity of the plasma discharge in the growth region and thus to achieve higher growth rates while avoiding ion-induced surface damage of films. The structural properties of the LEPECVD grown films were studied as a function of the deposition parameters (substrate temperature, growth rate, hydrogen dilution) by Raman Spectroscopy, SEM, and HRTEM analysis. The results of this work allowed us to identify the process requirements suitable for the growth of nc-grains in an amorphous matrix.

In the last few years, tremendous progress has been made in the field of Hot Wire Chemical Vapor Deposition (HWCVD): (1) It has been shown that there are no fundamental limitations in HWCVD with respect to substrate area. Using a periodic configuration of multiple short wires, good uniformity (± 7.5 %) has been demonstrated by Anelva over an area of 96 cm × 40 cm. (2) High quality microcrystalline Si can be produced. Solar cells in the n-i-p configuration are currently better than those made by PECVD. At Jülich, the efficiency of such cells is 9.4 %, and Utrecht has recently made the first HWCVD multibandgap triple junction solar cells, (3) HWCVD offers the potential of ultra high deposition rates. At NREL, a-Si:H has been deposited at rates in excess of 12 nm/s, and at Utrecht University μc-Si:H rates are in excess of 1 nm/s. (4) Thin film transistors (TFTs) with mobilities in excess of 1 cm2/Vs with an a-Si:H channel have been shown to be stable and μc-Si:H TFTs have been made with mobilities in excess of 40 cm2/Vs. (5) The efficient production of atomic H in HWCVD is beneficial in passivation processes, but it can also be applied in efficient etching processes. (6) Alloys of Si with various functions can be made, such as SiNx for antireflection and passivation coatings. Remarkably, all of the above results have been achieved without detailed knowledge about the primary reactions at the filament, the gas phase reactions, and the reactions with the growing film. The choice of filament material and its operation temperature have a large influence on the production of various reaction species and thus, on the structure of the resulting film. HWCVD is basically an ion-free deposition technique, which is an advantage for many kinds of thin films. HWCVD has also proven its feasibility in polymer deposition and nanotube formation.

In the fabrication of thin film transistors (TFTs), little attention has been paid to the polycrystalline silicon thin films prepared at low temperatures where the glass substrates are adopted so far. Since the film quality is not sufficient to achieve high mobility, e.g., over 50 cm2/Vs in spite of high benefit in their industrial fabrication. We have fabricated bottom gate TFTs with poly-Si and poly- Si1-xGex thin films deposited at 450°C by newly developed low-temperature LPCVD technique and characterized electrical characteristics of the TFTs: disilane and a small amount of either germanium tetrafluoride or fluorine were used as material gases and helium as carrier gas. Thermal annealing for dopant activation and atomic hydrogen treatment for defect passivation were carried out. We found that the defect elimination process is important for improving TFT performance significantly. Finally the mobility of p-channel and n-channel TFTs have attained 36.3-54.4 cm2/Vs and 57 cm2/Vs, respectively.

Surface diffusion of a growth species is needed to give the observed smooth surface of hydrogenated amorphous silicon (a-Si:H). But what diffuses, the weakly bound SiH3 radical on the hydrogenated surface, or the bound SiH3 at a growth site. Diffusion is complicated by the change in the surface termination of a-Si:H as temperature rises. We use total energy pseudopotential calculations on a variety of periodic Si:H surface configurations to show that it is the weakly bound SiH3 that diffuses. We provide an overall energy scheme of the bound states and transport levels of SiH3 on a-Si:H surfaces.

This paper reports about the interface of silicon nitride (SiNx) formed on Si(100) prepared by combination of catalytic-nitridation and catalytic-vapor deposition method in a catalytic chemical vapor deposition system. It is found that flat interface of SiNx/Si(100) is formed by inserting nitridation layer before growing the SiNx films.

We investigate the temperature dependence of an additional 1H NMR signal that appears in aSi:H at T = 7 K only after light soaking. This “doublet” signal is attributed to a pair of hydrogen atoms approximately 2.3 Å apart. We present the first lineshape of this doublet, and we examine the spin-lattice relaxation from 6.5 K to 20 K at a magnetic field of about 3.5 T. At low temperatures, the spin-lattice relaxation time of these paired hydrogen sites has a strong dependence on temperature, which is probably the result of some form of local motion.