Doping properties of microcrystalline silicon prepared by mercury sensitized photochemical vapor deposition

Conductivity of photo-CVD microcrystalline silicon (c-Si:H) in wide range of dopant gas concentration (10^–53/SiH₄, B₂H₆/SiH₄<10^–2) is investigated. As compared with a-Si:H, the conductivity of the film is improved more than two orders of magnitude by microcrystallization for a wide range of dopant concentration at a deposition temperature of as low as 150°C. This indicates the suitability of photo-CVD for low temperature processing. A conductivity minimum is found at a doping ratio of about B₂H₆/SiH₄=1×10^–5.     

ArF laser CVD of hydrogenated amorphous silicon: The role of buffer gases

ArF laser-induced CVD has been employed to generate hydrogenated amorphous silicon (a-Si:H) from Si₂H₆ gas dilute with He, Ar, or H₂. The formation of amorphous films or powder is found to depend critically on the kind of buffer gas, the stationary total and partial gas pressures, and the substrate temperature. These dependences have been investigated in the 1–5 Torr pressure and 100–400 °C temperature ranges. They are semiquantitatively discussed in terms of ArF laser photolysis of disilane, gas heating by heat flow from the substrate and laser irradiation, diffusion, and gas phase polymerization. Furthermore, photo ionization has been observed but found irrelevant for the a-Si:H layer properties. The photo and dark conductivities ( _ph, _d) of the semiconductor layers are determined by the substrate temperature. The _ph values range between 10^–7 and 10^{–4 –1} cm^–1 and the _d values between 10^–11 and 10^{–8 –1} cm^–1. The maximum ratio _ph/ _d amounts to 4×10⁴. The layers are further characterized by their optical band gap and activation energy. The layer properties are compared to literature values of amorphous films prepared by various photo, HOMO, and plasma CVD methods.     

Evolution of microcrystalline growth pattern by ultraviolet spectroscopic ellipsometry on Si:H films prepared by Hot-Wire CVD

Ultra-violet spectroscopic ellipsometry has been applied successfully to determine the evolution of μc-Si:H film structure, including incubation layer, bulk layer and the growth zone and surface over-layer, their individual thickness and composition. In view of the availability of a significantly high atomic H density in H₂-diluted SiH₄ ensemble in Hot-Wire CVD, microcrystallization seems to be achieved easily at a low substrate temperature. Atomic H induced etching at the growing network has been identified as instrumental in controlling the microcrystallization in Si:H. Sharp elimination of the incubation layer as well as surface roughness and significant improvement in the overall crystallinity was obtained on increase in H₂ dilution to SiH₄. A H₂ dilution limited sharp transition from an amorphous dominated (a+μc)-Si mixed phase to a virtually amorphous free μc-Si phase has been identified. However, enhanced atomic H reactivity at the growth zone beyond the attainment of the amorphous free bulk and the surface layer induces porosity in the network and a gradual deviation from crystallinity.     

非晶/微晶相变域硅薄膜及其太阳能电池

采用甚高频等离子体增强化学气相沉积（VHF-PECVD）法，成功制备出从非晶到微晶过渡区 域的硅薄膜. 样品的微结构、光电特性及光致变化的测量结果表明这些处于相变域的硅薄膜 兼具非晶硅优良的光电性质和微晶硅的稳定性. 用这种两相结构的材料作为本征层制备了p- i-n太阳能电池，并测量了其稳定性. 结果在AM15(100mW/cm²) 的光强下曝光 800—5000min后，开路电压略有升高，转换效率仅衰退了29%. 关键词： 相变域硅薄膜 光电特性 太阳能电池     

Dependence of glow discharge Si:H:Cl film morphology on diluent gas and SiCl4 partial pressure

TEM, THEED and field-assisted silver ion exchange have been employed to study the glow discharge (GD) SiHCl films deposited from SiH₄-SiCl₄ gas mixtures. The character of the THEED patterns shows that the films are rather amorphous, and that their structure does not alter with the change in the gas mixture. The honeycomb-like morphology of the films is strongly affected by the type of gas in which SiH₄ is diluted (Ar or H₂). An increase in the SiCl₄ partial pressure leads to the uniformity and to the decrease of the island dimension only for the films deposited from SiH₄(H₂)-SiCl₄. A possible correlation between the film morphology and the micropore density is proposed.     

P/i interfacial engineering in semi-transparent silicon thin film solar cells for fabrication at a low temperature of 150 °C

In this study, we aimed to develop semitransparent solar cells (STSCs) using hydrogenated amorphous silicon (a-Si:H) at a low temperature of 150 °C to support the fabrication of flexible solar modules, applicable in building-integrated photovoltaics (BIPV). To compensate for the presumable loss of device performance at such a low processing temperature, careful control of the p/i interface is proposed. We fabricated buffer layers with hydrogen (H₂)/silane (SiH₄) gas flow ratios (R) ranging from 4 to 16 (R₄–R₁₆) to investigate their characteristics and incorporate them at the p/i interface by considering energy band matching. By employing this buffer, the power conversion efficiency (PCE) of a STSC was improved from 4.83% to 5.57% which is the best record in a-Si:H STSCs processed at a low temperature of 150 °C. This p/i interfacial buffer can support the realization of flexible a-Si:H-based BIPV systems using plastic- or polymer-based substrates.     

Dynamics and correlated performance of a photo-triggered discharge-pumped HF laser using SF6 with hydrogen or ethane

3 , has been performed in Ne/SF₆/H₂ and Ne/SF₆/C₂H₆ mixtures. Parameters involved have been the storage line capacitance and the circuit inductance, the capacitors charging voltage, the RH-molecule type and partial pressure, and the X-ray dose for the preionization. High laser performance has been achieved with C₂H₆: an output energy up to 3 J corresponding to a specific energy of 9.6 J/l at an efficiency of 4.7%, which strengthens the advantage of the photo-triggering technique to energize high-power HF lasers. However the optimum performance achieved with H₂, 5.75 J/l and 3.5%, are lower. It is shown, through a time-resolved study of the electrical discharge and spatial dynamics correlated to laser power and energy measurements, that discharge instabilities are responsible for the poor laser performance of the mixture with H₂. These instabilities, which lead to arc development, are characteristics of the discharge in Ne/SF₆. It is demonstrated for the first time that addition of a heavy hydrocarbon, such as C₂H₆, to that mixture induces the discharge stabilization so that the laser emission arises in a homogeneous active medium. This effect allows us to achieve better laser performance than with H₂. Received: 17 March 1998/Revised version: 13 July 1998     

Study of structural and electronic environments of hydrogenated amorphous silicon carbonitride (a-SiCN:H) films deposited by hot wire chemical vapor deposition

Hydrogenated amorphous silicon carbon nitride (a-SiCN:H) thin films were deposited by hot wire chemical vapor deposition (HWCVD) using SiH₄, CH₄, NH₃ and H₂ as precursors. The effects of the H₂ dilution on structural and chemical bonding of a-SiCN:H has been investigated by Raman and X-ray photoelectron spectroscopy (XPS). Increasing the H₂ flow rate in the precursor gas more carbon is introduced into the a-SiCN:H network resulting in decrease of silicon content in the film from 41 at.% to 28.8 at.% and sp² carbon cluster increases when H₂ flow rate is increased from 0 to 20 sccm.     

Si<Subscript>x</Subscript>Ge<Subscript>1-x</Subscript> ultrahigh-vacuum chemical vapor deposition on Si(111)(7×7) from GeH<Subscript>4</Subscript>/Si<Subscript>2</Subscript>H<Subscript>6</Subscript> mixtures

We report the results of a study on ultrahigh-vacuum chemical vapor deposition of Si_xGe_1-x layers on Si(111)(7×7) with GeH₄ and Si₂H₆ mixtures. Using combined scanning tunneling microscopy and X-ray photoelectron spectroscopy, structural properties, the growth kinetics and the composition of the deposited alloys are analyzed as a function of the growth temperature for two different GeH₄:Si₂H₆ mixture ratios. The mutual influence of the precursors is shown by comparing the structures formed during deposition and the sticking coefficients of Si₂H₆ and GeH₄ with results obtained from exposure of Si(111) to the pure gases. Received: 28 July 2002 / Accepted: 2 October 2002 / Published online: 5 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-731/502-5452, E-mail: hubert.rauscher@chemie.uni-ulm.de     

Temperature dependence of the surface reactivity of SiH3 radicals and the surface silicon hydride composition during amorphous silicon growth

For hydrogenated amorphous silicon (a-Si:H) film growth governed by SiH₃ plasma radicals, the surface reaction probability β of SiH₃ and the silicon hydride (-SiH_x) composition of the a-Si:H surface have been investigated by time-resolved cavity ringdown and attenuated total reflection infrared spectroscopy, respectively. The surface hydride composition is found to change with substrate temperature from -SiH₃-rich at low temperatures to SiH-rich at higher temperatures. The surface reaction probability β, ranging from 0.20 to over 0.40 and with a mean value of β=0.30±0.03, does not show any indication of temperature dependence and is therefore not affected by the change in surface hydride composition. It is discussed that these observations can be explained by a-Si:H film growth that is governed by H abstraction from the surface by SiH₃ in an Eley-Rideal mechanism followed by the adsorption of SiH₃ at the dangling bond created.     

a-Si:H中本征缺陷引起的赝隙态

本文取原子集团模型Si₈H₁₈,Si₁₇及从连续无规网络中挖取的集团模型Si₂₉和Si₄₇,用CNDO LCAO-MO方法计算其电子结构,探讨了a-Si:H中由弱键、弯键和带电组态等本征缺陷引起的赝隙态分布。结果表明,当弱键拉伸时,两个弱键态移动并收缩至带隙中央;过剩电荷使弱键能级移至价带顶或导带底附近;弯键态主要出现在价带顶附近。当弯键曲率较大时,弯键态上移至带隙中央以下的区域。结构拓扑无序导致 关键词：     

Real-time observation of the diffusive motion of 2×2 adlayer formed on Ag(100) nanoparticle facet by nitric acid adsorption

1-x-y Ge_xC_y have been grown on Si(100) substrates by rapid thermal chemical vapor deposition (RTCVD) using C₂H₄ as C source. The composition and microstructure of Si_1-x-yGe_xC_y films were characterized by Auger electron spectroscopy, Raman spectra and Fourier-transform infrared spectroscopy. The results show that lower temperature and higher SiH₄/C₂H₄ flow ratio are helpful in forming the substitutional C and improving the crystal quality. A possible mechanism for C incorporation in Si_1-x-yGe_xC_y layers grown by RTCVD using C₂H₄ is proposed. Received: 26 April 1998 / Accepted: 28 September 1998 / Published online: 24 February 1999     

On a presence of SimHn clusters in a-Si:H/c-Si structures

Dominant aim of the paper was to verify the existence of the Si_mH_n clusters in a-Si:H layers. Thin layers were deposited by plasma-enhanced chemical vapor deposition (PECVD) on both glass and crystalline silicon substrates. Their IR and structural properties were investigated by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction at grazing incidence angle (XRDGI). We have found that the layer probably consists of larger structurally ordered parts corresponding to Si_mH_n clusters and separated groups of (Si-H_x)_N. The ordered parts could be identified as some of Si_mH_n clusters ranging from (10, 16) to (84, 64) represented by corresponding vibration frequencies in three following IR regions: 600-750, 830-900 and 2080-2180 cm⁻¹. XRDGI measurement indicates that diffraction maximum at around 2Θ = 28° can be attributed to an existing Si_mH_n cluster.     

a-Si:H/a-SiNx:H超晶格的掺杂效应

采用带有可转动掩板的沉积系统,合成出一类新的a-Si:H/掺杂a-SiN_x:H超晶格。样品中各子层厚度及a-SiN_x:H子层中N/Si比固定,仅改变掺杂浓度。结果发现:此类超晶格中的费密能级可以通过a-SiN_x:H层中的掺杂来控制,即a-Si:H/a-SiN_x:H超晶格可以从n型转变为p型,依赖于a-SiN_x:H子层中B的掺杂比。然而,a-SiN_x:H子层中P的掺杂对a-Si:H/a-SiN_x:H超晶格传输特性影响并不大。 关键词：     

Pressure-shock-controlled pulsed molecular beams

A method of controlling the duration of pulses of intense molecular beams is suggested. The idea of the method is the shortening of an initial molecular beam pulse by producing a pressure shock in front of a solid surface through which the beam passes. Experiments on shortening H₂, He, SF₆, SF₆/H₂(1/10), and SF₆/He(1/10) molecular beam pulses are reported. The parameters of the beams incident on, and transmitted through, the surface are studied. The gas density in the initial beam and in the pressure shock before the surface is estimated. The intensity and duration of shortened molecular pulses are found as a function of the initial intensity, angle of incidence, and the diameter of a hole on the surface through which the beam passes. It is established that the duration of the shortened beam decreases greatly with increasing incident intensity and decreasing hole diameter. It is shown that intense pulsed H₂, He, SF₆, SF₆/H₂(1/10), and SF₆/He(1/10) molecular beams with a pulse duration of ≤10–15 μs and an extent of ≤1–2 cm can be generated with the method suggested.     

Process characterization and mechanism for laser-induced chemical vapor deposition of a-Si : H from SiH4

The dependence of a-SiH film deposition by laser-induced decomposition of SiH₄ on the different process variables is studied. The gas phase temperature in the beam center, produced by CO₂ laser irradiation in parallel configuration, is estimated using a simple energy balance model. The surface temperature is measured with high accuracy employing a Ni sensor (250°–400 °C). The deposition rate and film properties such as the hydrogen content and the optical energy gap are determined as a function of these parameters. The production of H₂ (10%), Si₂H₆ (2%), and Si₃H₈ in the gas phase during laser irradiation is proved by a mass spectrometric analysis. The chemical reaction processes induced in the gas phase and at the surface are discussed. A mechanism explaining the main features of the complicated chemistry involved is developed.     

500-Hz two-photon Ramsey fringes with a SF6 beam: towards a new frequency standard in the 30-THz spectral region

A two-photon Ramsey-fringe experiment with a supersonic beam of SF₆ has been performed with an interzone distance of up to 50 cm. Using a He-seeded beam with 50% of SF₆, the two-photon transition P(4)E⁰ in the 2ν₃ band reveals its magnetic hyperfine structure and the periodicity of the fringes is 500 Hz. The strength of the central fringe of the main hyperfine component corresponds to a flux of 10¹⁰–10¹¹useful molecules/s, which is very promising for a new frequency standard in the 30-THz spectral region. Received: 27 April 2001 / Revised version: 18 June 2001 / Published online: 18 July 2001     

Low cost wavelength filter of SiGe photodetector with a-Si:H capped layer

The strained Si_0.8Ge_0.2 film has been prepared onto Si substrate by using an ultrahigh-vacuum chemical vapor deposition system. A low cost wavelength filter of photodetector has been demonstrated for the first time. This filter was simply carried out by just inserting a 60 nm thick a-Si:H capped layer onto Si_0.8Ge_0.2 thin film. The room-temperature photoluminescence shows that the sample with Si_0.8Ge_0.2 layer has a tendency to shift wavelength into longer regime than that of Si substrate. The full width at half maximum (FWHM) was 185 nm for Si_0.8Ge_0.2 photodetector without a-Si:H capped. By inserting a 60 nm thick a-Si:H capped layer, the FWHM was narrowed into 97 nm. This demonstrates that the a-Si:H capped layer has an ability acted as wavelength filter in our study.     

Influence of ignition condition on the growth of silicon thin films using plasma enhanced chemical vapour deposition

The influences of the plasma ignition condition in plasma enhanced chemical vapour deposition (PECVD) on the interfaces and the microstructures of hydrogenated microcrystalline Si (μc-Si:H) thin films are investigated. The plasma ignition condition is modified by varying the ratio of SiH₄ to H₂ (R_H). For plasma ignited with a constant gas ratio, the time-resolved optical emission spectroscopy presents a low value of the emission intensity ratio of Hα to SiH^* (I_Hα/I_SiH^*) at the initial stage, which leads to a thick amorphous incubation layer. For the ignition condition with a profiling R_H, the higher I_Hα/I_SiH^* values are realized. By optimizing the R_H modulation, a uniform crystallinity along the growth direction and a denser μ c-Si:H film can be obtained. However, an excessively high I_Hα/I_SiH^* may damage the interface properties, which is indicated by capacitance-voltage (C-V) measurements. Well controlling the ignition condition is critically important for the applications of Si thin films.     

Importance of pre-ionisation for the non-chain discharge-pumped HF laser

The importance of pre-ionisation for the non-chain discharge-pumped HF laser is studied through experiments on an X-ray photo-triggered laser using mixtures of Ne, SF₆, and ethane. The discharge dynamic in Ne/SF₆ mixtures or pure SF₆, as well as the stabilisation effect induced by C₂H₆ and consequences for the laser performance, are investigated for pre-ionisation electron density values, n_eo, ranging from 10⁶ cm^-3 up to 10⁹ cm^-3, as well as for the so-called discharge self-breakdown mode. Without ethane, the minimum n_eo value which is needed to complete 100% homogeneous charge deposition in the plasma is a very sharply increasing function of the SF₆ pressure. This hinders performance optimisation when the molecule used to react with F-atoms, for instance H₂, has no effect on the discharge dynamic. The minimum ethane partial pressure that is needed to stabilise the discharge depends on n_eo, the pumping pulse duration, the deposited electric charge, and the SF₆ pressure. Discharges in Ne/SF₆ can be much more efficiently stabilised by addition of a small amount of ethane than by an increase of n_eo. A pre-ionisation density as low as 10⁶ cm^-3 is sufficient to achieve the maximum laser energy value, but total suppression of the pre-ionisation has a detrimental effect on the active medium homogeneity. Received: 30 May 2000 / Revised version: 9 October 2000 / Published online: 9 February 2001