Fabrication and characterization of transparent conductive ZnO:Al thin films prepared by direct current magnetron sputtering with highly conductive ZnO(ZnAl2O4) ceramic target

Using a highly conductive ZnO(ZnAl₂O₄) ceramic target, c-axis-oriented transparent conductive ZnO:Al₂O₃ (ZAO) thin films were prepared on glass sheet substrates by direct current planar magnetron sputtering. The structural, electrical and optical properties of the films (deposited at different temperatures and annealed at 400°C in vacuum) were characterized with several techniques. The experimental results show that the electrical resistivity of films deposited at 320°C is 2.67×10⁻⁴ Ω cm and can be further reduced to as low as 1.5×10⁻⁴ Ω cm by annealing at 400°C for 2 h in a vacuum pressure of 10⁻⁵ Torr. ZAO thin films deposited at room temperature have flaky crystallites with an average grain size of ∼100 nm; however those deposited at 320°C have tetrahedron grains with an average grain size of ∼150 nm. By increasing the deposition temperature or the post-deposition vacuum annealing, the carrier concentration of ZAO thin films increases, and the absorption edge in the transmission spectra shifts toward the shorter wavelength side (blue shift).     

Ultra-thin polycrystalline Si layers on glass prepared by aluminum-induced layer exchange

In this work, we present studies of ultra-thin polycrystalline silicon layers (5–100 nm) prepared by the aluminum-induced layer exchange process. Here, a substrate/Al/oxide/amorphous Si layer stack is annealed at temperatures below the eutectic temperature of the Al/Si system of 577 °C, leading to a layer exchange and the crystallization of the amorphous Si. We have studied the process dynamics and grain growth, as well as structural properties of the obtained polycrystalline Si thin films. Furthermore, we derive a theoretical estimate of the grain density and examine characteristic thermal activation energies of the process. The structural properties have been investigated by Raman spectroscopy. A good crystalline quality down to a layer thickness of 10 nm has been observed.     

Growth chemistry of nanocrystalline silicon and germanium films

We report on the growth of nanocrystalline Si:H and Ge:H films. The films were grown using plasma deposition and hot wire chemical growth techniques. Conditions such as pressure, temperature and hydrogen dilution were systematically varied. It is shown that excessive hydrogen dilution during growth leads to smaller grains in nanocrystalline Si and Ge. Films with very large grains (56 nm) could be obtained using hot wire growth techniques under appropriate conditions of growth. From the data, it is concluded that the natural growth direction for the films is 〈2 2 0〉, and that excessive bonded hydrogen leads to smaller grains.     

Amorphous to crystalline transition and optoelectronic properties of nanocrystalline indium tin oxide (ITO) films sputtered with high rf power at room temperature

ITO thin films were deposited on quartz substrates by the rf sputtering technique using various rf power keeping the substrates at room temperature. The influence of rf power on the structural, electrical, optical and morphological properties was studied by varying the rf power in the range 50–350 W. X-ray diffraction results show an amorphous – crystalline transition with nano grains. At a power of 250 W, the ITO film showed preferential orientation along (4 0 0) peak. It is observed from the optical transmission studies that the optical band gap increased from 3.57 to 3.69 eV when the rf power was increased from 50 to 250 W. The resistivity value is minimum and grain size is maximum for the ITO film deposited at 250 W. The X-ray photoelectron spectroscopy (XPS), Energy dispersive X-ray (EDX) and Atomic force microscopy AFM results confirm that the ITO films are stoichiometric and the surface contained nano-sized grains distributed uniformly all over the surface. It can be concluded that the ITO film deposited at room temperature with 250 W rf power, can provide the required optical and electrical properties useful for developing optoelectronic devices at lower temperatures.     

High quality nanocrystalline silicon thin film fabricated by inductively coupled plasma chemical vapor deposition at 350 °C

High quality nanocrystalline silicon (nc-Si) film was deposited by inductively coupled plasma chemical vapor deposition (ICP-CVD) without substrate RF bias at 350 °C. The nc-Si with a dense crystalline structure of the columnar type grew from the bottom to the top of the nc-Si film. A troublesome incubation layer did not exist at the bottom of the fabricated nc-Si film. A grain size of 40 nm was measured by using a SEM image. When a RF bias of 100 and 200 W was applied to the substrate to induce ion bombardment on the substrate, the crystalline structure and grains were not observed and a-Si deposition became dominant. The transition from nc-Si deposition into a-Si deposition can be attributed to ion bombardment which prevents nucleation and crystal growth at the surface of deposition. This shows that the reduction of ion bombardment can be a key factor to fabricate high quality nc-Si film. By using ICP-CVD with no substrate RF bias, ion bombardment can be reduced, while the density of plasma is kept high, so that high quality nc-Si can be fabricated due to the enhancement of crystalline growth on the surface.     

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.     

Structural investigations of nitrided Nb2O5 and Nb2O5–SiO2 sol–gel derived films

Sol–gel derived xNb₂O₅–(100 ? x)SiO₂ films (where x = 100, 80, 60, 50, 40, 20, 0 mol%) were nitrided at various temperatures (800 °C, 900 °C, 1000 °C, 1100 °C and 1200 °C). The structural transformations occurring in the films as a result of ammonolysis were studied using X-ray diffraction (XRD), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The XRD results have shown that the temperatures below 1100 °C were too low to obtain a pure NbN phase in the samples. The AFM observations indicate that the formation of the NbN phase and the size of NbN grains are related to the silica content in the layer. NbN grains become more regular and larger as the niobium content increases. The maximum grain size of about 100 nm was observed for x = 100. Preparation of the Nb₂O₅–SiO₂ sol–gel derived layers and the subsequent nitridation is a promising method of inducing crystalline NbN in amorphous matrices. It follows from the XPS results that a small amount of Nb₂O₅ remains in the films after nitridation at 1200 °C and that nitrogen reacted not only with Nb₂O₅ but also with SiO₂.     

Spectral response of amorphous–nano-crystalline silicon thin films

A representative set of amorphous–nano-crystalline Si thin films was deposited by radio-frequency plasma enhanced chemical vapor deposition using silane highly diluted by hydrogen. By Raman spectroscopy it was found that the variation of silane to hydrogen ratio resulted in films with crystal fraction between 0 and 55 vol.% and individual crystal sizes between 2 and 20 nm with bi-modal, broad size distribution. High resolution transmission microscopy, done on certain number of samples, confirmed the nano-meter size of crystallites and bi-modal size distribution. The optical properties measured by Fourier transform photocurrent spectroscopy and photo thermal deflection spectroscopy correspond to the material with structure between amorphous and crystalline. The spectral distribution of relative quantum efficiency of photovoltaic solar cell made from this material shows ‘blue shift’ with increase of crystal to amorphous fraction. This result is discussed as a possible consequence of quantum effects accompanied with actual size and size distribution of crystals.     

The comparisons of growth mechanisms for Si thin films with different deposition rates in CVD process by the description of the roughness evolution

The roughness evolutions of micro-crystalline silicon thin films (μc-Si:H) with different growth rates prepared by chemical vapor depositions have been investigated by atomic force microscopy. The growth exponent β was measured as 0.8 ± 0.03, 1.1 ± 0.07 and 0.75 ± 0.02 for three sets of samples prepared by PECVD with and without hydrogen dilution ratio modulation and by HWCVD, respectively, and does not correlated with the deposition rate in a set. However, the root-mean-square roughness and lateral correlation length decrease with increasing the deposition rate for both PECVD and HWCVD process. We suggested that the nonstationary growth with large β is correlated with the shadowing effect. The influence of the deposition rate on the surface roughness could be related to the diminishing of the shadowing effect by surface species diffusion with higher mobility on an H-covered surface. The initial surface and nucleation condition play an important role in the surface roughness evolution.     

Electrodeposited and selenized CIGS thin films for solar cells

Cu(In,Ga)Se₂ polycrystalline thin films were deposited adopting the potentiostatic electrochemical method on Mo/soda lime glass substrate. All the as-deposited Cu(In,Ga)Se₂ thin films were annealed in a selenium atmosphere at 550 °C for 1 h to improve the film crystalline properties. The selenized CIGS thin films were characterized by energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and scanning electron microscopy (SEM).The results indicate Cu(In,Ga)Se₂ thin films have single chalcopyrite structure and the grain size varies from 0.8 to 2.5 μm.     

Characteristics of ITO films fabricated on glass substrates by high intensity pulsed ion beam method

Transparent conductive oxides such as indium tin oxide (ITO) are interesting materials due to their wide-band gaps, high visible light transmittance, high infrared reflectance, high electrical conductivity, hardness and chemical inertness. ITO films were fabricated on soda lime glass substrates by using high-intensity pulsed ion beam (HIPIB) technique. The as-deposited films comprised of partially crystallized In₂O₃ and after annealing at 500 °C for 1 h the film changed to polycrystalline phase. After annealing carrier concentration and Hall mobility increased while specific resistance and sheet resistance decreased quickly; and this trend was also observed when film thickness increased up to 300 nm for the post-annealed samples. Further increase in thickness of the film changed the electrical properties slightly. Atomic force microscopy (AFM) revealed that roughness decreased after 500 °C annealing for 1 h in air, except for the film of 65 nm thick. The thickness of the film which relates to the carrier concentration and mobility, degree of crystallization, size of the grain, and connections among grains in film are main factors to determine film’s electrical properties.     

ESR study of the hydrogenated nanocrystalline silicon thin films

We studied the stability and light-induced paramagnetic centers in hydrogenated nanocrystalline silicon thin films (nc-Si:H) by electron-spin-resonance (ESR) and photothermal-deflection-spectroscopy (PDS). There is no measurable change in defect density upon illumination with white light with a light intensity of 300 mW cm^?2 for 300 h. At low temperatures, upon illumination with sub-bandgap light, a light-induced ESR signal appears. This signal is similar to that in hydrogenated micro-crystalline silicon (μc-Si:H).     

Characteristics of p-type nanocrystalline silicon thin films developed for window layer of solar cells

Different rf-power and chamber pressures have been used to deposit boron doped hydrogenated silicon films by the PECVD method. The optoelectronic and structural properties of the silicon films have been investigated. With the increase of power and pressure the crystallinity of the films increases while the absorption decreases. As a very thin p-layer is needed in p–i–n thin film solar cells the variation of properties with film thickness has been studied. The fraction of crystallinity and thus dark conductivity vary also with the thickness of the film. Conductivity as high as 2.46 S cm⁻¹ has been achieved for 400 Å thin film while for 3000 Å thick film it is 21 S cm⁻¹. Characterization of these films by XRD, Raman Spectroscopy, TEM and SEM indicate that the grain size, crystalline volume fraction as well as the surface morphology of p-layers depend on the deposition conditions as well as on the thickness of the film. Optical band gap varies from 2.19 eV to 2.63 eV. The thin p-type crystalline silicon film with high conductivity and wide band gap prepared under high power and pressure is suitable for application as window layer for Silicon thin film solar cells.     

Deposition of device grade poly-Si films on glass substrate directly at 450 °C and fabrication of bottom-gate poly-Si TFTs

We have observed the progressive stages of nucleation and growth of the poly-Si films with the source gasses of Si₂H₆ and F₂ on glass substrates directly at 450 °C and found that nuclei density and size are controllable effectively via governing process pressures. We introduced a nuclei governed layer of approximately less than 2 nm and it brought about 33% increases in grain diameter. Finally, we fabricated n- and p-channel bottom-gate TFTs whose field effect mobility was higher than 50 cm²/V s. However, the devices with the nuclei governed layers faced degradation due to the propagation of fluorine into gate oxide. Therefore, it needs further studies.     

Comparative study of electroluminescence from annealed amorphous SiC single layer and amorphous Si/SiC multilayers

Si quantum dots (Si QDs) films were prepared by annealing amorphous SiC single layer and amorphous Si/SiC multilayers fabricated in plasma enhanced chemical vapor deposition system. The microstructures were characterized by Raman spectroscopy as well as Fourier transforms infrared spectroscopy for both samples. It was found that Si QDs with average size of 2.7 nm were formed after annealing and the electroluminescence (EL) band centered at 650 nm can be observed at room temperature. The EL intensity from the Si/SiC multilayers was obviously improved by one order of magnitude and the corresponding EL band width was reduced compared with that from SiC single layer film. The improved electroluminescence behavior can be attributed to the formation of the denser Si QDs, good size distribution and the strong confinement effect of carriers in multilayerd structures.     

Electrochromic properties of nickel oxide and mixed Co/Ni oxide films prepared via sol–gel route

The Ni oxide and mixed Co/Ni oxide films were prepared by sol–gel dip coating method at optimum conditions. The XRD analysis reveals the pure and Co mixed nickel oxide films to be in amorphous state. The field emission SEM images reveal nanopore like structure for Ni oxide film and well defined grains with pores for Ni oxide films containing 5 wt.% of Co. Electrochromic properties have been studied using cyclic voltammetric (CV) and in situ spectro-electrochemical techniques. The pure and cobalt mixed (5 wt.%) Ni oxide films exhibit anodic/cathodic diffusion coefficient of 4.93 ± 0.14/3.74 ± 0.10 × 10^?10 cm²/s and 10.00 ± 0.24/7.60 ± 0.20 × 10^?10 cm²/s respectively after 300 cycles. The cobalt mixed (5 wt.%) Ni oxide films exhibit the bleached/coloured state transmission of 90.42/7.21% with a photopic constrast ratio of 12.54 and the colouration and bleaching time were 5.9 and 2.4 s respectively. The addition of cobalt beyond 5% leads to poor transparency and inhibited electrochromic switching character.     

Effect of P incorporation on aggregation of nanocrystallites in amorphous and nanocrystalline mixed-phase silicon thin films

We report the effects of P incorporation on the nanometer-scale structural and electrical properties of amorphous and nanocrystalline mixed-phase Si:H films. In the intrinsic and weakly P-doped (3 × 10¹⁸ at/cm³) films, the nanocrystallites aggregate to cone-shaped structures. Conductive atomic force microscopy images showed high current flows through the nanocrystalline cones and a distinct two-phase structure in the micrometer range. Adding PH₃ into the processing gas moved the amorphous/nanocrystalline transition to a higher hydrogen dilution ratio required for achieving a similar Raman crystallinity. In a heavily P-doped (2 × 10²¹ at/cm³) film, the nanocrystalline aggregation disappeared, where isolated grains of nanometer sizes were distributed throughout the amorphous matrix. The heavily doped mixed-phase film with 5–10% crystal volume fraction showed a dramatic increase in conductivity. We offer an explanation for the nanocrystalline cone formation based on atomic hydrogen enhanced surface diffusion model, and propose that the coverage of P-related radicals on the existing nanocrystalline surface during film growth and the P segregation in grain boundaries are responsible for preventing new nucleation on the surface of the existing nanocrystallites, resulting in nanocrystallites dispersed throughout the amorphous matrix.     

A simple quality factor for characterization of thin silicon films

Four series of intrinsic thin Si films were prepared by plasma enhanced chemical vapor deposition at standard and high growth rate conditions. We suggest a simple ‘μc-Si:H layer quality factor’ based on the ratio of subgap optical absorption coefficient values: α(1.4 eV)/α(1 eV). This ratio minimizes the light scattering effects for rough films and can serve as a reliable detection of the amorphous/microcrystalline structure transition and also as a figure of merit for the microcrystalline layer. The quality factor is evaluated for series of our samples with well known structure and also compared with samples from other laboratories with different deposition and measurement techniques.     

Insights into effects of annealing on microstructure from SnO2 thin films prepared by pulsed delivery

Tin dioxide thin films were prepared by pulsed laser deposition techniques on clean glass substrates, and the thin films were then annealed for 30 min from 50 to 550 °C with a step of 50 °C, respectively. The influence of the annealing temperature on the microstructural and morphological properties of the tin dioxide thin films was investigated using X-ray diffraction, scanning electron microscopy, transmission electron microscopy and selected area electron diffraction. The experimental results showed that the amorphous microstructure almost transformed into a polycrystalline tin dioxide phase exhibiting a preferred orientation related to the (1 1 0), (1 0 1) and (2 1 1) crystal planes with increased temperatures. The thin film annealed at 200 °C demonstrated the best crystalline properties, viz. optimum growth conditions. However, the thin film annealed at 100 °C revealed the minimum average root-mean-square roughness of 20.6 nm with average grain size of 26.6 nm. These findings indicate that the annealing temperature is very important parameter to determining the thin film quality, which involves the phase formation, microstructure and preferred orientation of the thin films.     

Micro-Raman mapping on layers for crystalline silicon thin-film solar cells

Micro-Raman mappings have been used for characterization of our layers system developed for thin-film silicon solar cells. For the cubic SiC barrier layer a preferential orientation of the grains in 〈1 1 1〉 direction normal to the substrate was revealed. A high density of stacking faults resulted in the splitting of transversal optical (TO)-phonon modes, usually only observed in several non-cubic SiC polytypes. Within the silicon layers, which were obtained by zone melting recrystallization (ZMR) and subsequent epitaxial growth, a high residual stress of about 625 MPa was measured near the boundary towards the SiC layer. Outside of this boundary no residual stress could be detected, in spite of commonly found twin boundaries. Thus the main origin of residual stress in the silicon layers is due to the different expansion coefficients of the respective layers, while grain boundaries have no dominant effect.