Ambipolar microcrystalline silicon thin-film transistors

Hydrogenated microcrystalline silicon (µc-Si:H) has recently received significant attention as a promising material for thin-film transistors (TFTs) in large area electronics due to its high electron and hole charge carrier mobilities. We report on ambipolar TFTs based on microcrystalline silicon prepared by plasma-enhanced chemical vapor deposition at temperature of 160 °C with high electron and hole charge carrier mobilities of 40 cm²/Vs and 10 cm²/Vs, respectively. The ambipolar microcrystalline silicon TFTs provide a simple route in realizing large area integrated circuits at low cost. The electrical characteristics of the ambipolar microcrystalline silicon TFTs will be described and the first results on ambipolar inverters will be presented. The influence of the ambipolar TFT characteristics on the performance of the inverter will be also discussed.     

Metastable defects in hydrogenated amorphous silicon

The electronic structure of hydrogenated amorphous silicon (a-Si:H) is in a state of metastable equilibrium and can change upon application of external stimuli. We study the effect of thermal quenching and light soaking in lithium-doped a-Si:H, on its conductivity and thermopower. We present evidence showing that the metastable state obtained after fast quenching is different than that obtained after light exposure. Experiments on chalcogenides show that they are not affected by thermal quenching although they change upon light soaking. This is in contrast with lithium doped a-Si:H in which both effects are observed. Our experiments suggest that hydrogen present in a-Si:H plays an important role by controlling heterogeneities and potential fluctuations in a-Si:H. Light soaking appears to enhance these potential flucutations, whereas fast cooling seems to have little effect on them.     

Stress induced crystallization of hydrogenated amorphous silicon

Hydrogenated amorphous silicon films were grown on to thermally oxidized silicon wafers by Radio Frequency magnetron sputtering, and SiN_x and Al₂O₃ capping layers were used to control the residual thermal stress. After annealing, a comparison of the silicon films with and without capping layers indicates that tensile stress induced by the capping layer enhances the crystallinity of the annealed amorphous silicon film. The stress is due to the mismatch between the coefficients of thermal expansion for the capping layer and amorphous silicon film. These results highlight the potential of thermal stress as a means to alter the crystallization in thin film architectures and suggest that even larger effects can be obtained with suitable choices of capping layer chemistry.     

Raman study of ion-implanted hydrogenated amorphous silicon

The effect of silicon and hydrogen ion implantations on the structural properties of hydrogenated amorphous silicon films was studied by means of Raman spectroscopy, with the aim of revealing the influence of hydrogen atoms inserted into the silicon matrix on its short-range order. To separate the implantation-induced increase in the structural disorder from the effect of the implanted hydrogen, the implantation doses of silicon and hydrogen ions were selected to create closely similar numbers of host-atom displacements. The results obtained suggest that the presence of hydrogen in amorphous silicon reduces the structural disorder related to variations in the silicon bond length, but affect the bond-angle deviations to a lesser extent.     

Photoconductivity studies of n-type hydrogenated amorphous silicon and microcrystalline silicon

Photoconductivity techniques serve as useful tools for the characterization of amorphous and microcrystalline silicon. From the link between the majority carrier mobility–lifetime product from steady-state photoconductivity and the position of the Fermi level, useful insight can be gained when comparing sample properties. The temperature dependence of the minority carrier mobility–lifetime product implies that the band-tail region of the density-of-states (DOS) is steeper in microcrystalline silicon than in amorphous silicon. Transient and modulated photoconductivity determine the DOS in the upper half of the band gap, for which we find an exponential variation. We indicate that the Fermi level or quasi-Fermi level impose limitations on the DOS extraction from the measured data. In samples in which the Fermi level is shifted towards the conduction band, the DOS calculation then yields values that are too low.     

Effect of TiO2 nanopatterns on the performance of hydrogenated amorphous silicon thin-film solar cells

We investigate how TiO₂ nanopatterns formed onto ZnO:Al (AZO) films affect the performance of hydrogenated amorphous silicon (a-Si:H) solar cells. Scanning electron microscopy results show that the dome-shaped TiO₂ nanopatterns (300 nm in diameter) having a period of 500 nm are formed onto AZO films and vary from 60 to 180 nm in height. Haze factor increases with an increase in the height of the nanopatterns in the wavelength region below 530 nm. Short circuit current density also increases with an increase in the height of the nanopatterns. As the nanopatterns increases in height, the fill factor of the cells slightly increases, reaches maximum (0.64) at 100 nm, and then decreases. Measurements show that a-Si:H solar cells fabricated with 100 nm-high TiO₂ nanopatterns exhibit the highest conversion efficiency (6.34%) among the solar cells with the nanopatterns and flat AZO sample.     

Potential fluctuations and metastabilities in hydrogenated amorphous silicon

The inhomogeneities present in hydrogenated amorphous silicon (a-Si : H) give rise to potential fluctuations. They influence the electronic properties of a-Si : H in a profound manner. Nevertheless, very few theoretical and experimental studies acknowledge their presence, because of the inherent difficulties in dealing with them. We find that the width of the potential fluctuations obtained from transport measurements is much smaller than that obtained by optical methods. This may be because the former (transport) depends on long-range potential fluctuations, whereas the latter is sensitive to short-range ones. External perturbations, e.g. light soaking and thermal quenching, are found to have different effects on the long-range potential fluctuations. Light soaking (the Staebler–Wronski effect) seems to increase them, whereas thermal quenching leaves them unchanged.     

Investigation on schottky diodes on amorphous hydrogenated silicon

Schottky barrier diodes with near-ideal characteristics have been fabricated on amorphous hydrogenated silicon prepared by decomposition of a mixture of 10% silane and 90% hydrogen. The interface properties are found to be stable up to heat treatment of 300°C. From a detailed investigation of dark and photovoltaic properties it is concluded that the density of states in the mobility gap is sufficiently small so that there is no significant carrier recombination in the space charge region.     

Shallow defect states in hydrogenated amorphous silicon oxide

The theoretical cluster-Bethe-lattice method is used in this study to investigate the shallow defect states in hydrogenated amorphous silicon oxide. The electronic density of states (DOS) for the SiO₂ Bethe lattice of various Si–O–Si angles, non-bridging oxygen Si–O^√, peroxyl radical Si–O–O^√, threefold coordinated O₃ and Si–H bonds are calculated. The variation of the Si–O–Si bond angle causes the bandgap fluctuation and induces tail states near the conduction band minimum. The Si–O^√ and Si–O–O^√ bonds introduce shallow defect states in the energy gap near the top of the valence band. The Si–H bond induces a defect state, in the energy gap near the conduction band minimum, in a-SiO_x with high oxygen concentration, but not low oxygen concentration. The O₃ bond itself does not induce defect state in the energy gap. The O₃⁺D⁻ complex, formed by the O₃ and threefold coordinated silicon, induces shallow state in the energy gap near the conduction band minimum. This defect state can explain the energy shift of photoluminescence of a-SiO_x:H under annealing.     

High-performance amorphous indium-gallium-zinc oxide thin-film transistors with polymer gate dielectric

The fabrication of amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs) with a spin-coated polymer gate dielectric on a glass substrate is reported. The interface state density at the poly(4-vinylphenol)/a-IGZO interface is only around 4.05 × 10¹¹ cm^− 2. The TFTs' threshold voltage, subthreshold swing, on-off current ratio, and carrier mobility are 2.6 V, 1.3 V/decade, 1 × 10⁵, and 21.8 cm²/V s, respectively. These characteristics indicate that the TFTs are suitable for use as nonvolatile memory devices and in flexible electronic applications.     

Photovoltaic characterization of amorphous hydrogenated and chlorinated silicon films

Hydrogenated and chlorinated silicon films were used to deposit Schottky barrier solar cells. Photovoltaic characterization, together with the results of electronic transport measurements, led to the conclusion that the presence of chlorine is detrimental to the properties of this kind of device.     

Pyrolytic transformation from polydihydrosilane to hydrogenated amorphous silicon film

The fabrication of thin film silicon devices based on solution processes rather than on conventional vacuum processes is of substantial interest since cost reductions may result. Using a solution process, we coated substrates with polydihydrosilane solution and studied the pyrolytic transformation of the material into hydrogenated amorphous silicon (a-Si:H). From thermal gravimetry and differential thermal analysis data a significant reduction in weight of the material and a construction of SiSi bonds are concluded for the pyrolysis temperature T_p = 270 to 360 °C. The appearance of amorphous silicon phonon bands in Raman spectra for films prepared at T_p ≥ 330 °C suggests the construction of a three-dimensional amorphous silicon network. Films prepared at T_p ≥ 360 °C exhibit a hydrogen content near 10 at.% and an optical gap near 1.6 eV similar to device-grade vacuum processed a-Si:H. However, the infrared microstructure factor, the spin density, and the photosensitivity require significant improvements.     

Stress in hydrogenated amorphous silicon determined by X-ray diffraction

The residual strain in an a-Si:H layer has been directly determined with X-ray diffraction techniques from variations in the diffraction angle of the first amorphous peak using CuK radiation. The layer was deposited by HW-CVD on glass substrates at a growth temperature of 300 °C, and is known from previous studies to be highly disordered. It was found to have an average compressive stress of 750 MPa, using the c-Si lattice parameter as a reference, and typical values of the elastic constants for a-Si:H, increasing strongly towards the surface.     

Broadband wavelength conversion in hydrogenated amorphous silicon waveguide with silicon nitride layer

Broadband wavelength conversion based on degenerate four-wave mixing is theoretically investigated in a hydrogenated amorphous silicon (a-Si:H) waveguide with silicon nitride inter-cladding layer (a-Si:HN). We have found that enhancement of the non-linear effect of a-Si:H waveguide nitride intermediate layer facilitates broadband wavelength conversion. Conversion bandwidth of 490 nm and conversion efficiency of 11.4 dB were achieved in a numerical simulation of a 4 mm-long a-Si:HN waveguide under 1.55 μm continuous wave pumping. This broadband continuous-wave wavelength converter has potential applications in photonic networks, a type of readily manufactured low-cost highly integrated optical circuits.     

Operation model with carrier-density dependent mobility for amorphous In-Ga-Zn-O thin-film transistors

An operation model for an amorphous In-Ga-Zn-O (a-IGZO) based thin film transistor (TFT) is studied. The model is not based on the exponential tail states employed in hydrogenated amorphous Si (a-Si:H) TFT, but on a power function of the carrier density which is observed in the TFT and Hall mobilities of a-IGZO. A 2D numerical simulator employing this model reproduced current-voltage characteristics under on operation of coplanar homojunction a-IGZO TFTs. Although the mathematical expression of the mobility is similar to the field effect mobility of a-Si:H TFT, the present model explains the temperature dependence of the on characteristics of a-IGZO TFT.     

Thickness dependence of mobility of pentacene planar bottom-contact organic thin-film transistors

We have fabricated planar bottom-contact organic thin-film transistors as a function of the thickness of the pentacene active layer. The highest mobility of the planar bottom-contact transistors is 0.47 cm²/Vs with only a 7 nm pentacene active layer. Our planar bottom-contact transistors show much higher mobility than conventional bottom-contact counterparts and even higher than the reported mobility values of top-contact counterparts for each thickness in the range from 2.5 to 10 nm. We find that spike at the edges of source and drain electrodes seriously deteriorates device performance.     

Large-grain polycrystalline silicon thin films obtained by low-temperature stepwise annealing of hydrogenated amorphous silicon

Large grained polycrystalline silicon thin films have been prepared by low-temperature solid phase crystallisation of sputter-deposited hydrogenated amorphous silicon (a-Si:H), with relatively short processing times, and a considerably low thermal budget. Various a-Si:H samples, deposited under different conditions and with varying hydrogen concentrations and hydrogen bonding configurations, were simultaneously annealed. Only a particular set of deposition conditions led to crystallisation. The a-Si:H thin film which was successfully crystallised was prepared in an argon-hydrogen mixture, in which the last few minutes of film deposition occurred in a hydrogen-rich atmosphere. For that film, the hydrogen concentration profile resulted in a much higher hydrogen content on the sample surface than in the bulk, and H-Si bonds were predominantly of the weak type. Crystallisation was accomplished by low-temperature stepwise annealing from 200°C to 600°C at 100°C steps, with samples being cooled down to room-temperature between each annealing step. This resulted in large grained (> 10 μm range) polycrystalline silicon after the 600°C annealing step for a 1.1 μm thick sample. Fourier transform infrared (FTIR) spectroscopy, elastic recoil detection analysis (ERDA) and scanning electron microscopy (SEM) techniques were used to analyse samples before and after crystallisation.