Study for amorphous silicon etching process using dielectric barrier discharge

Characteristics of amorphous silicon (a-Si) etching using atmospheric pressure plasma discharge had been studied. Dielectric barrier discharge (DBD) plasma with nitrogen gas was employed for the study. The active chemical agent for etching was generated by mixing a small quantity of sulfur hexafluoride (SF₆) gas into the plasma. The two distinguishable plasma zones are generated with the specially designed DBD plasma generator. The one is the main discharge zone generated between the two parallel plate electrodes. And the other one is downstream plasma zone extracted from the main discharge zone through the holes perforated on the bottom electrode. A test specimen was etched located at the plasma zone and moved the zone several times for etching on a temperature controlled stage. The etch rate of a-Si and the selectivity to silicon nitride (SiNx) were improved by addition of hydrogen (H₂) or methane (CH₄) gas into the plasma. However, when the specimen temperature was lower than 100 °C with H₂ or CH₄ gas added plasma condition, a-Si layer was not etched at all, but in the range of 100-140 °C of specimen temperature, the a-Si layer started to be etched while the influence of the specimen temperature on etching of a-Si was ignorable in that temperature range. At the optimized condition, the a-Si etch rate was up to 3000 A/min in the downstream plasma zone with the 3 mm of the distance between the surface of the specimen and the bottom side of the DBD plasma generator module. And the etch rate ratio between a-Si and SiNx was more than 100:1.     

Boron induced recrystallization of amorphous silicon film by a rapid thermal process

Rapid thermal process (RTP) is to induce boron-doped amorphous silicon into a high degree of crystallization of polycrystalline silicon in 5 min. In addition to the short time characteristic, it also provides a relatively lower temperature route to prepare high percentage of polycrystalline silicon in comparison with solid phase crystallization method. Before RTP, boron is homogeneously doped into the amorphous silicon film by ion implantation technology. After rapid thermal processing, the grain size of the polycrystalline silicon was found about at 0.1-0.5 μm. The degree crystallization of silicon is reached up to 99.1% with a good hole mobility of 138.6 cm²/V s.     

Study of polymorphous silicon as thermo-sensing film for infrared detectors

In this work we have deposited and characterized pm-Si:H thin films obtained by plasma deposition. Our aim is to use pm-Si:H as thermo-sensing element for infrared (IR) detectors based on un-cooled microbolometers. We have studied the electrical characteristics of pm-Si:H that are figures of merit important for IR detection, as activation energy, thermal coefficient of resistance (TCR), room temperature conductivity (σ_RT) and responsivity under IR radiation. The influence of the substrate temperature (200 °C and 300 °C) on the pm-Si:H characteristics has been also studied. Our results shown that pm-Si:H is an excellent candidate to be used as thermo-sensing film for microbolometers, due to its large activation energy and TCR, with an improved σ_RT.     

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.     

Synthesis of amorphous Si nanowires from solid Si sources using microwave irradiation

Si nanowires were synthesized from Si wafers and from thin Si films deposited on various substrates by microwave irradiation. The power and time were key determinants of the diameter and morphology of the synthesized Si nanowires. The nanowires had an amorphous structure due to the extremely high heating rate. Carbon coating of the Si nanowires was easily achieved by introducing acetylene after synthesizing the nanowires. Carbon-coated Si nanowires are potential candidates for use as the anode material in next generation Li-ion batteries.     

Thermodynamic properties of amorphous silicon via tight binding simulations

An atomic-scale structure of amorphous silicon, generated by reverse Monte Carlo, has been used as a starting configuration for finite temperature molecular dynamics simulations performed by an orthogonal tight binding Hamiltonian. Structural, dynamic, elastic and electronic structure properties have been investigated in the range of temperatures up to and above the melting transition. The amorphous silicon structure undergoes a melting transition at a temperature sensibly smaller than that of the crystalline structure. Above this temperature, the structure has the same properties of an under-cooled liquid and it has a metallic behavior.     

Band gap profiles of intrinsic amorphous silicon germanium films and their application to amorphous silicon germanium heterojunction solar cells

Intrinsic amorphous silicon germanium (i-a-SiGe:H) films with V, U and VU shape band gap profiles for amorphous silicon germanium (a-SiGe:H) heterojunction solar cells were fabricated. The band gap profiles of i-a-SiGe:H were prepared by varying the GeH₄ and H₂ flow rates during the deposition process. The use of i-a-SiGe:H with band gap profile in an absorber layer for a-SiGe:H heterojunction solar cells was investigated. The solar cell using a VU shape band gap profile shows a higher efficiency compared to other shapes. The highest efficiency obtained for an a-SiGe:H heterojunction solar cell using the VU shape band gap profile technique was 9.4% (V_oc = 0.79 V, J_sc = 19.0 mA/cm² and FF = 0.63).     

Hot-wire deposition of amorphous and microcrystalline silicon using different gas excitations by a coiled filament

Microcrystalline silicon (μc-Si:H) and amorphous silicon (a-Si:H) films were deposited using a hot-wire CVD (HWCVD) system that employs a coiled filament. Process gasses, H₂ and Si₂H₆, could be directed into the deposition chamber via different gas inlets, either through a coiled filament for efficient dissociation or into the chamber away from the filament, but near the substrates. We found that at low deposition pressure (e.g. 20 mTorr) the structure of the films depends on the way gases are introduced into the hot-wire chamber. However, at higher pressure (e.g. 50 mTorr), Raman measurement shows similar results for films deposited with different gas inlets.     

Correlation of structural and optoelectronic properties of thin film silicon prepared at the transition from microcrystalline to amorphous growth

Photovoltaic properties of 4 µm thick microcrystalline silicon p–i–n solar cells have been studied, over a range of crystallinity determined using Raman spectroscopy. Low-crystallinity material (below 10%) appears to absorb disproportionately strongly in the infrared, possibly due to increased light scattering or to relaxation of the crystal momentum selection rule. A minimum in solar cell efficiency is observed under AM1.5 illumination when V_OC ≈ 580 mV, with blue response most strongly affected. This is consistent with a reduction in electron mobility to a value below that of amorphous silicon for low-crystallinity material, in agreement with time-of-flight measurements.     

High-rate deposition of amorphous silicon films using hot-wire CVD with a coil-shaped filament

To reduce the manufacturing cost of amorphous silicon (a-Si:H)-based photovoltaic devices, it is important to deposit high-quality a-Si:H and related materials at a high deposition rate. To this end, we designed and constructed a hot-wire deposition chamber with a coiled filament design and with multiple gas inlets. The process gas could be directed into the chamber through the filament coil and have maximum exposure to the high-temperature filament surface. Using such a chamber design, we deposited a-Si:H films at high deposition rates up to 800 Å s⁻¹ and dense, low-void a-Si:H at rates up to 240 Å s⁻¹.     

Improving the stability of amorphous silicon ultraviolet sensors

In this paper we present an improved structure of an amorphous silicon/amorphous silicon carbide ultraviolet sensor, previously presented in literature, whose overall performances have been enhanced by growing a very thin layer of chromium silicide film on the top of the sensor. The sensor is a n-type amorphous silicon/intrinsic amorphous silicon/p-type amorphous silicon carbide stacked structure deposited on a glass substrate. The substrate is covered with a chromium film that acts as back metal contact. The top metal contact is a grid shaped chromium/aluminum/chromium metal stack that allows the incident radiation to reach the active p-type layer.The responses of two sets of sensors fabricated with and without the alloy film under ultraviolet radiation have been studied. The role of the very thin chromium silicide layer is to increase the conductivity of the top surface without attenuating the UV radiation absorbed in the device active layer. The increased top-surface conductivity ensures a better collection of the photogenerated carriers and hides the resistivity variation of the underlying p-doped layer under ultraviolet light caused by dopant activation, leading to a stable and linear behavior. Comparing the photocurrent values obtained on sensors with different area and distance between the grid electrodes, we found that the presence of the chromium silicide film extends the charge collection length by a factor of 10, allowing a better device photoresponse.     

Suppression of crystalline growth in silicon films deposited from hydrogen diluted silane using a layer-by-layer approach

Si:H films with a thickness of approximately 450 nm have been deposited with rf-PECVD using a plasma of silane diluted with hydrogen. The aim was to grow heterogeneous films without an amorphous to microcrystalline phase transition. A layer-by-layer scheme was applied in which thin interlayers deposited from pure silane are included with the intention to interrupt the crystalline growth that is characteristic to the deposition with hydrogen dilution of silane. Raman spectroscopy and TEM imaging have confirmed that the application of the amorphous interlayers results in a decrease of the crystalline fraction of the layer-by-layer films compared to films grown with continuous hydrogen dilution. Absorption coefficient spectra of the films before and after light soaking have been investigated.     

Cracking behavior of evaporated amorphous silicon films

The residual stress in amorphous silicon films deposited by evaporation is investigated with different substrate temperatures. The stress measured from all the films studied in this paper is tensile. The level of stress decreases from 580 MPa to 120 MPa with increasing substrate temperature from 60 °C to 350 °C. When the film becomes thicker, strain increases and cracks are formed for stress relaxation. 10 µm thick amorphous Si films are deposited at 350 °C without cracks. This cracking behavior is theoretically studied and confirmed by experiment.     

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.     

A direct correlation between film structure and solar cell efficiency for HWCVD amorphous silicon germanium alloys

The film structure and H bonding of high deposition rate a-SiGe:H i-layers, deposited by HWCVD and containing ~ 40 at.% Ge, have been investigated using deposition conditions which replicate those used in n-i-p solar cell devices. Increasing the germane source gas depletion in HWCVD causes not only a decrease in solar cell efficiency from 8.64% to less than 7.0%, but also an increase in both the i-layer H preferential attachment ratio (PA) and the film microstructure fraction (R?). Measurements of the XRD medium range order over a wide range of germane depletion indicate that this order is already optimum for the HWCVD i-layers, suggesting that energetic bombardment of a-SiGe:H films may not always be necessary to achieve well ordered films. Preliminary structural comparisons are also made between HWCVD and PECVD device layers.     

Electrodeposition of amorphous silicon anode for lithium ion batteries

Amorphous silicon has been successfully electrodeposited on copper using a SiCl₄ based organic electrolyte under galvanostatic conditions. The electrodeposited silicon films were characterized for their composition, morphology and structural characteristics using glancing angle X-ray diffraction (GAXRD), scanning electron microscopy (SEM), and Raman spectroscopy. GAXRD and Raman analyses clearly confirm the amorphous state of the deposited silicon film. The deposited films were tested for possible application as anodes for Li-ion battery. The results indicate that this binder free amorphous silicon anode exhibits a reversible capacity of ∼1300 mAh g⁻¹ with a columbic efficiency of >99.5% up to 100 cycles. Impedance measurements at the end of each charge cycle show a non-variable charge transfer resistance which contributes to the excellent cyclability over 100 cycles observed for the films. This approach of developing thin amorphous silicon films directly on copper eliminates the use of binders and conducting additives, rendering the process simple, facile and easily amenable for large scale manufacturing.     

非晶硅平板探测器不一致性校正技术

针对非晶硅平板探测器不一致性偏大,导致探测器灵敏度出现非线性的问题,提出了基于探测器像元灵敏度均值的校正方法。此方法基于线性拟合校正的思想,在一定管电压下,改变管电流采集一组探测器基准图像,利用这组图像与其相应均值进行拟合,再把拟合系数作为校正因子对试件成像进行校正。该方法通过缩小校正的输入范围,满足线性系统成立的条件,使非线性问题线性化,实现了各像元点对点的线性校正。实验结果表明,与常规线性校正相比,该方法不仅提高了系统成像的动态范围,而且随着标准差的降低,信噪比相应地提高了近1.3倍。     

Effect of thermal coupling on the electronic properties of hydrogenated amorphous silicon thin films deposited by electron cyclotron resonance

In photovoltaic devices, rather thin intrinsic layers of good quality materials are required and high deposition rates are a key point for a cost-effective mass production. In a previous study we have shown that good quality amorphous silicon (a-Si:H) films can be deposited by matrix distributed electron cyclotron resonance (MDECR) plasma CVD at very high deposition rates (∼ 2.5 nm/s). However, only thick films (> 1 μm) exhibited good transport properties. A very poor thermal coupling between the substrate holder and the substrate is the main reason for such a behaviour. We present here experimental data which support this conclusion as well as the improved transport and defect-related properties of new very thin a-Si:H samples (thickness around 0.3 μm) deposited at a higher temperature than the previous ones.     

A hybrid tandem solar cell based on hydrogenated amorphous silicon and dye-sensitized TiO2 film

Hydrogenated amorphous silicon film (a-Si:H) as top cell is introduced to dye-sensitized titanium dioxide nanocrystalline solar cell (DSSC) as bottom cell to assemble a hybrid tandem solar cell. The hybrid tandem solar cell fabricated with the thicknesses a-Si:H layer of 235 nm, ZnO/Pt interlayer of 100 nm and DSSC layer of 8.5 μm achieves a photo-to-electric energy conversion efficiency of 8.31%, a short circuit current density of 10.61 mA·cm^− 2 and an open-circuit voltage of 1.45 V under a simulated solar light irradiation of 100 mW·cm^− 2.     

Status report: solar cell related research and development using amorphous and microcrystalline silicon deposited by HW(Cat)CVD

This article reviews the research and development of a-Si:H and μc-Si:H based solar cells by using hot wire chemical vapor deposition (HWCVD). The groups involved and the present status of conversion efficiencies attained are listed and will be discussed for different cell structures realized entirely or partly using this method. There are three main advantages of HWCVD: a quite simple set up, higher useable deposition rates and higher stability of HW-a-Si:H. It will be discussed how these advantages can be exploited to make HWCVD an alternative to plasma enhanced chemical vapor deposition.