Photoconducting a-Si:H films grown at high deposition rates by pulsing a VHF 100 MHz discharge

In a novel experiment hydrogenated amorphous silicon films were deposited by modulating the very high frequency (VHF) (100 MHz) discharges, at low frequency (2 Hz) with a nonzero low power level, using pure as well as 25% hydrogen and 25% helium diluted silane as the source gases. During these studies deposition rate is found to depend on the dwell time as in the case of RF pulsed plasma CVD reported earlier by the authors. The films were characterised for optical bandgap, dark and photoconductivity, hydrogen content, microstructure factor, Urbach energy and defect density. The results indicate that, unlike the RF pulsed plasma case, there is an order of magnitude improvement in the photoconductivity of the material due to pulsing the VHF discharges. Urbach energy and defect density studies also indicate an improvement in the film quality. The improvements are more pronounced in diluted silane deposited films. Controlled ion bombardment (of high flux and lower energy) and the resulting ion bombardment induced preparation of the growth surface in the VHF discharges are believed to be the main factors contributing to the observed results. Thus, a more favourable sheath characteristics as obtained during pulsed VHF discharge conditions over RF (13.56 MHz). Silane discharges holds the key to obtain high growth rate deposition of a-Si:H films of acceptable opto-electronic quality     

热丝辅助MW ECR CVD法高速沉积优质氢化非晶硅薄膜

我们用热丝辅助MW ECRCVD系统,在热丝温度分别为0、1350、1400、1450、1500、1600和1700℃时制备出a-SiH薄膜.通过膜厚测定,红外光谱分析光、暗电导测量等手段,分析了其沉积速率、光敏性及光学带隙的变化规律.结果表明沉积速率和薄膜质量均得到明显的提高,沉积速率超过3nm/s,光暗电导之比提高到6×105.找到最佳辅助热丝温度为1450℃.通过对带隙值的分析,发现当带隙值在1.6～1.7范围内时,薄膜几乎都具有105以上的光暗电导之比.     

Quantitative hydrogen measurements in PECVD and HWCVD a-Si:H using FTIR spectroscopy

A series of hydrogenated amorphous silicon films have been deposited using plasma enhanced chemical vapour deposition (PECVD) and hot-wire chemical vapour deposition (HWCVD) techniques. The total concentration of bonded hydrogen in the films was varied between 3% and 18% as determined by hydrogen effusion measurements. Fourier transform infra-red (FTIR) spectra of the PECVD and HWCVD samples exhibit strong absorption peaks that correspond to Si–H bend and stretch modes, and Si–H₂ stretch modes. A quantitative fit of the FTIR peak areas to the hydrogen effusion concentrations reveals that there is reasonable agreement between the required proportionality constants in PECVD and HWCVD material for the Si–H bend and stretch modes. The uncertainty error for the FTIR proportionality constants is consistently greater for the HWCVD data set, however, which may indicate that the effective dynamical charge of the Si–H dipoles is perturbed in the HWCVD material by bonded impurities that are sourced from the tungsten wire.     

Hydrogen distribution, nanostructures and optical properties of high deposition rate hot-wire CVD a-Si:H

Nuclear magnetic resonance shows that under certain growth conditions hot-wire CVD a-Si:H grown at high rates contains a large amount of nanovoids. It was found that such nanovoids are filled with H₂ gas and the nanovoids are elongated with the along axis in the growth direction. Measured on the same samples photoluminescence (PL) showed that the tail-to-tail PL peak at 80 K was red-shifted significantly from 1.36 to 1.05 eV when the growth rate increases from 10 to 55 Å/s. These studies indicate that the nanostructure is important to the behavior of PL in a-Si:H.     

Comparison of a-Si TFTs fabricated by Cat-CVD and PECVD methods

We investigate the characteristics of amorphous silicon thin film transistors (a-Si TFTs) fabricated by plasma-enhanced chemical vapor deposition (PECVD) and catalytic CVD (Cat-CVD), and their stability under bias and temperature (BT) accelerated stress. The Cat-CVD a-Si TFTs have off-leak current as small as 10^− 14 A, and a smaller threshold voltage shift under the BT stress. The superiority in off-leak current and stability is observed in the Cat-CVD a-Si TFTs fabricated at both 320 °C and 180 °C. The high performance and stability of the Cat-CVD a-Si TFTs will enable to use low-cost glass substrates and result in a cost reduction of TFT fabrication.     

High Rate Growth of a-Si and μc-Si Thin Films with Facing-target PECVD

The first investigation on high rate growth of uniform a-Si and μc-Si thin films with facingtarget plasma enhanced chemical vapour deposition (FTPECVD) has been presented here. It has been shown that by employing FTPECVD both a-Si and μc-Si thin films can be fabricated with rates of up to 10μm/h and l μm/h, respectively, around 10 times higher than those of the conventional PECVD, and the correspondent gas utility is about 20 times higher. As indicated by Raman spectroscopy measurement, the crystallinity of the materials is as high as 85%. A columnar structure in the films has been revealed by TEM analysis, a reflection of epitaxial growth in the deposition process.     

ECR-CVD方法生长a-SiNx:H薄膜的研究

使用微波电子回旋共振等离子体化学气相沉积(ECR-CVD)方法室温生长了非晶氢化的氮化硅薄膜,通过改变前驱气体(SiH4 80%Ar和NH3)的流量比,研究了薄膜的生长速率、等离子体的发射光谱和薄膜的红外特性.结果表明:随着NH3流量的增加,氮化硅薄膜的生长速率呈下降趋势,这主要是由于等离子体中的气相前驱成分之一硅基团浓度的不断下降所导致的;随着NH3流量的增加,薄膜中键合了较多的具有较高电负性的N原子是Si-N和Si-H伸缩振动发生蓝移的主要原因.红外光谱的定量计算表明所制备的氮化硅薄膜具有相对较低的H浓度,约15%左右.文中对氮化硅薄膜的生长机制也进行了讨论.     

Resonant Raman scattering of a-SiNx:H

Micro-Raman measurements were carried out to investigate the microstructure of amorphous silicon–nitrogen alloy (a-SiN_x:H) samples with different N contents prepared by plasma enhanced chemical vapor deposition (PECVD). Resonant Raman effect was discovered by using 647.1- and 514.5-nm excitation wavelengths. The frequency of TO mode downshifts with increasing photon energy without varying its width, while LO mode expands to a great extent. The frequency-dependent shift of TO band is explained by heterogeneous structure model and quantum confinement model, and the width expansion of LO mode may be related to the overlapping of LA and LO bands.     

Structural changes studies of a-Si:H films deposited by PECVD under different hydrogen dilutions using various experimental techniques

Plasma enhanced chemical vapour deposition (PECVD) has been used to prepare hydrogenated amorphous silicon (a-Si:H) thin films at different hydrogen dilution of silane source gas. The films were deposited on Corning glass 1737 substrate and on (100) oriented c-Si wafers and characterized by XRD diffraction, micro-Raman and FTIR spectrometry. Experimental data show evolution from amorphous to nanocrystalline silicon and contain the medium-range order (MRO) with varying hydrogen dilution during deposition. From X-ray diffraction and Raman analysis, it is found that the presence of crystalline phase depends on the kind of substrate and on the dilution scale.     

A comparison of grain nucleation and grain growth during crystallization of HWCVD and PECVD a-Si:H films

From TEM, XRD and Raman measurements, we compare the crystallization kinetics when HWCVD and PECVD a-Si:H films, containing different initial film hydrogen contents (C_H), are crystallized by annealing at 600 °C. For the HWCVD films, the nucleation rate increases, and the incubation time and the full width at half maximum (FWHM) of the XRD (111) peak decrease with decreasing film C_H. However, the crystallization kinetics of HWCVD and PECVD films of similar initial film C_H are quite different, suggesting that other factors beside the initial film hydrogen content affect the crystallization process. Even though the bonded hydrogen evolves very early from the film during annealing, we suggest that the initial spatial distribution of hydrogen plays a critical role in the crystallization kinetics, and we propose a preliminary model to describe this process.     

Micropore density in a-Si:H and a-Si:H:Cl films

a-Si:H films deposited from various silane-containing gas mixtures have been studied regarding their porosity. Micropore densities of 2.O × 10²cm^-2, 5.0 × 10⁴cm^-2 and 5.0 × 10⁴cm^-2 were determined for a-Si:H:C1, a-Si:H(H₂) and a-Si:H(Ar) films, respectively. It is suggested that these values correlate with the structural properties of the films, so that a-Si:H:Cl films seem to be the most uniform on the microstructural scale.     

a-Si∶H及其多层膜光致发光某些性质的研究

本文研究了 a-Si∶H 及 a-Si∶H/a-SiN_x∶H 多层膜光致发光的某些性质。实验研究表明,a-Si∶H 及其多层膜的光致发光峰值能量强烈地依赖于沉积偏压、a-Si∶H 层厚度和内应力,并对这些结果进行了讨论。     

Effect of silane flow rate on structural,electrical and optical properties of silicon thin films grown by VHF PECVD technique

Hydrogenated silicon thin films deposited by VHF PECVD process for various silane flow rates have been investigated. The silane flow rate was varied from 5 sccm to 30 sccm, maintaining all other parameters constant. The electrical, structural and optical properties of these films were systematically studied as a function of silane flow rate. These films were characterized by Raman spectroscopy, Scanning Electron Microscopy (SEM), atomic force microscopy (AFM), Fourier transform infrared (FTIR) spectroscopy and UV–visible (UV–Vis) spectroscopy. Different crystalline volume fraction (22%–60%) and band gap (∼1.58 eV–∼1.96 eV) were achieved for silicon thin films by varying the silane concentration. A transition from amorphous to nanocrystalline silicon has been confirmed by Raman and FTIR analysis. The film grown at this transition region shows the high conductivity in the order of 10⁻⁴ Ω⁻¹ cm⁻¹.     

Solid-phase crystallization kinetics and grain structure during thermal annealing of a-Si:H grown by chemical vapor deposition

Solid-phase crystallization kinetics are examined during thermal annealing of as-deposited hydrogenated amorphous silicon (a-Si:H) thin films deposited by hot-wire chemical vapor deposition (HWCVD) and plasma-enhanced chemical vapor deposition (PECVD). The influence of deposition temperature of HWCVD material on crystallization is also considered. Real-time observation of the crystallization process using in situ transmission electron microscope heating allowed tracking of the crystalline volume fraction and grain number density by image-processing methods. Beyond an initial incubation period, roughly constant grain nucleation rate and growth velocity are observed. Extrapolation from early stages of crystallization allows estimation of the final average grain sizes. PECVD material shows a much lower nucleation rate than does HWCVD material under the same annealing conditions, whereas the grain growth velocities are comparable, leading to dramatically larger grain sizes in PECVD material. X-ray diffraction line widths from PECVD material are broader compared to HWCVD material. The diffraction line broadening is primarily determined by intragranular defect structure, rather than grain size. Low-temperature preannealing reduced the final XRD line widths of HWCVD material, indicating an influence on defect structure or density. Lattice contraction during crystallization of HWCVD material is observed to be independent of the initial hydrogen content.     

DBDCVD法制备a-Si:H的性能研究

本文用介质阻挡放电化学气相沉积(DBDCVD)在室温下进行了非晶氢硅薄膜制备.通过硅烷氢气流量比、DBD放电电压等工艺条件的调整,在玻璃上沉积了系列样品.研究表明,DBDCVD法可以在室温下快速制备非晶氢硅薄膜,最大沉积速率可达0.34nm/s,由于DBDCVD的高能量和室温沉积的特点,薄膜中硅-氢键以SiH2为主.随硅烷反应气体浓度的变化,薄膜的光学带隙可在1.92eV～2.18eV之间调整.     

High rate deposition of ta-C:H using an electron cyclotron wave resonance plasma source

A compact electron cyclotron wave resonance (ECWR) source has been developed for the high rate deposition of hydrogenated tetrahedral amorphous carbon (ta-C:H). The ECWR provides growth rates of up to 1.5 nm/s over a 4-inch diameter and an independent control of the deposition rate and ion energy. The ta-C:H was deposited using acetylene as the source gas and was characterized as having an sp³ content of up to 77%, plasmon energy of 27 eV, refractive index of 2.45, hydrogen content of about 30%, optical gap of up to 2.1 eV and RMS surface roughness of 0.04 nm.     

Hot-wire deposition of a-Si:H thin films on wafer substrates studied by real-time spectroscopic ellipsometry and infrared spectroscopy

Film growth of hydrogenated amorphous silicon (a-Si:H) by hot-wire chemical vapor deposition was studied simultaneously and in real-time by spectroscopic ellipsometry and attenuated total reflection infrared spectroscopy. The a-Si:H films were deposited on native oxide-covered GaAs(100) and Si(100) substrates at temperatures ranging from 70 to 350 °C. A temperature dependent initial growth phase is revealed by the evolution of the surface roughness and the surface and bulk SiH_x absorption peaks. It is discussed that the films show a distinct nucleation behavior by the formation of islands on the surface that subsequently coalesce followed by bulk a-Si:H growth. Insight into a temperature-activated smoothening mechanism and the creation of a hydrogen-rich interface layer is presented.     

Mass spectrometry of a silane glow discharge during plasma deposition of a-Si: H films

We undertook a mass spectrometric investigation of the ionic and neutral species present during the deposition of a-Si: H using an r.f. glow discharge in silane (mixed with helium or hydrogen). A correlation between the neutral composition of the plasma and the nature of the IR vibrational modes in the deposited film is proposed. The ionic species extracted from the silane discharge are not characteristic of the direct ionization of SiH₄. The predominance of the SiH₃⁺ ion is attributed to the ion-molecule reaction $\text{SiH}{}_2{}^{\mathrm{+}}\text{+ SiH}{}_4\text{→ SiH}{}_3{}^{\mathrm{+}}\text{+ SiH}{}_3$ Secondary ions Si₂H_n⁺ (n = 1?7) are also observed. Mass spectrometry of the ionic species resulting from the interaction of a hydrogen plasma with the a-Si: film suggests that atomic hydrogen plays an active role during the growth of the film.     

Characterization of intrinsic a-Si:H films prepared by inductively coupled plasma chemical vapor deposition for solar cell applications

The hydrogenated amorphous silicon (a-Si:H) films, which can be used as the passivation or absorption layer of solar cells, were prepared by inductively coupled plasma chemical vapor deposition (ICP-CVD) and their characteristics were studied. Deposition process of a-Si:H films was performed by varying the parameters, gas ratio (H2/SiH4), radio frequency (RF) power and substrate temperature, while a working pressure was fixed at 70 m Torr. Their characteristics were studied by measuring thickness, optical bandgap (eV), photosensitivity, bond structure and surface roughness. When the RF power and substrate temperature were 300 watt and 200 degrees C, respectively, optical bandgap and photosensitivity, similar to the intrinsic a-Si:H film, were obtained. The Si-H stretching mode at 2000 cm(-1), which means a good quality of films, was found at all conditions. Although the RF power increased up to 400 watt, average of surface roughness got better, compared to a-Si:H films deposited by the conventional PECVD method. These results show the potential for developing the solar cells using ICP-CVD, which have the relatively less damage of plasma.     

Comparison of surface passivation of crystalline silicon by a-Si:H with and without atomic hydrogen treatment using hot-wire chemical vapor deposition

We compared surface passivation of c-Si by a-Si:H with and without atomic hydrogen treatment prior to a-Si:H deposition. The atomic hydrogen is produced by hot-wire chemical vapor deposition (HWCVD). For this purpose, we deposited a-Si:H layers onto both sides of n-type FZ c-Si wafers and measured the minority carrier effective lifetime and implied V_OC for different H treatment times ranging from 5 s to 30 s prior to a-Si:H deposition. We found that increasing hydrogen treatment times led to lower effective lifetimes and implied V_OC values for the used conditions. The treatments have been performed in a new virgin chamber to exclude Si deposition from the chamber walls. Our results show that a short atomic hydrogen pretreatment is already detrimental for the passivation quality which might be due to the creation of defects in the c-Si. AFM measurements do not show any change in the surface roughness of the different samples.