氢稀释比对氢化硅薄膜两相结构和电学性能的影响

引用本文

鲁媛媛, 李贺军, 杨冠军. 氢稀释比对氢化硅薄膜两相结构和电学性能的影响. 无机材料学报, 2015, 30(5): 474-478
LU Yuan-Yuan, LI He-Jun, YANG Guan-Jun. Influence of Hydrogen Dilution on Two-phase Structure and Electrical Properties of Hydrogenated Silicon Thin Films. Journal of lnorganic Materials,2015, 30(5): 474-478 复制到剪切板

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氢稀释比对氢化硅薄膜两相结构和电学性能的影响

鲁媛媛, 李贺军, 杨冠军

西北工业大学材料科学与工程学院, 西安710072

李贺军, 教授. E-mail: lihejun@nwpu.edu.cn

作者简介: 鲁媛媛(1985-), 女, 博士研究生. E-mail: luyuanyuan85@163.com

基金:国家自然科学基金(51221001)

摘要

通过改变氢气对硅烷的稀释比 R, 采用等离子体增强化学气相沉积(PECVD)方法制备出具有非晶/微晶相变过渡区的氢化硅薄膜, 并研究了所得硅膜在不同沉积阶段的微观结构和形貌、晶化效果和电学性能。研究结果表明, 当 R=10时, 样品呈典型的非晶特性; 随着氢稀释比的增大, 薄膜表现出两相结构, 且衬底表面处的非晶过渡层逐渐减薄, 也即非晶向微晶的转变提前。但XRD结果显示, 硅膜的晶化率和平均晶粒尺寸随着 R的增加呈先增后减的趋势, 在 R=28.6时达到最大值。另外, 暗电导率和载流子浓度表现出了与晶化率一样的变化趋势, 显示出硅膜的电学性能与微观结构的高度正相关性。

关键词: 氢稀释比; 硅膜; 两相结构; 电学性能

中图分类号:TQ174 文献标志码:A 文章编号:1000-324X(2015)05-0474-05

Influence of Hydrogen Dilution on Two-phase Structure and Electrical Properties of Hydrogenated Silicon Thin Films

LU Yuan-Yuan, LI He-Jun, YANG Guan-Jun

School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China

Fund:National Natural Science Foundation of China (51221001)

Abstract

Hydrogenated silicon thin films with two-phase structure were prepared by plasma enhanced chemical vapor deposition (PECVD) at different hydrogen dilution ratios ( R) and their microstructure and electrical properties were investigated. The results indicated that the film was amorphous when R was 10. As R increased, the film presented two-phase structure, and the thickness of the amorphous layer tended to thin and the transition to crystalline from amorphous started earlier. From XRD results, both crystallinity and average grain size of the films increased firstly and then decreased with increase of R, and at maximum values when R was 28.6. The change rule of dark conductivity and carrier density agreed with the change rule of crystallinity and average grain size, which showed a close positive relationship between electrical properties and the microstructure.

Keyword: hydrogen dilution; silicon thin film; two-phase structure; electrical property

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非晶硅薄膜因其制备原料低廉和能源消耗低而成为极具潜力的光电能量转换材料^{[1, 2, 3]}。但经长时间光照后光电导率和暗电导率的下降, 也即光致衰减效应^[4]又制约了它在光伏电池中的应用。总体上来说, 非晶硅薄膜的光致亚稳变化与非晶硅的无序网络结构有关。无序固体材料处于非平衡态, 在外界条件扰动下易发生结构的亚稳变化。这一特性提示, 要改善非晶硅稳定性必须改善非晶硅的整体无序网络结构。而从热力学角度来看, 无序网络结构的改善最终将导致结构的晶化^[5]。

在众多的制备方法中, 可在低温状态下大面积沉积非晶硅薄膜的等离子体增强化学气相沉积法(PECVD)^[6]较为常用。采用PECVD制备非晶硅薄膜时, 用氢气稀释硅烷可以显著改善材料的稳定性, 甚至还可形成微晶硅(μ c-Si: H)材料^[7]。关于氢稀释导致硅膜从非晶到微晶的转变有三种模型^{[8, 9, 10]}试图解释, 其中表面扩散模型较为合理地解释在高氢稀释条件下微晶硅薄膜的沉积机理。氢可以钝化非晶硅薄膜中大量存在的悬挂键, 降低薄膜的缺陷密度, 减少带隙中的非辐射复合中心^[11], 从而显著提高薄膜的稳定性。

事实上, 微晶硅薄膜具有典型的两相结构^[12], 从衬底到薄膜表面的纵向结构是不均匀的。其结构在沉积初期为非晶相, 当经过一定的沉积时间后才转变为晶相。这就使得微晶硅薄膜的光电性质密切依赖于其生长过程。要获得较高的稳定效率, 须使微晶硅膜在纵向上具有相近的晶粒尺寸和晶相比。氢稀释作为改善非晶硅结构和形成微晶硅的关键参量, 会显著地影响薄膜的两相结构和生长过程, 进而影响薄膜的光电特性^[13]。然而, 对于不同氢稀释比条件下微晶硅薄膜的界面状态和相变过程的直观分析尚未见报道。

本研究于不同氢稀释比条件下制备出一系列硅基薄膜, 并对其沉积初期的相变过程和后期生长阶段的截面形貌进行直观分析, 试图为制备高稳定效率的微晶硅薄膜提供实验支持。

1 实验方法

实验以纯度为99.999%的氢气(H₂)和硅烷(SiH₄)为反应气体, 经过13.56 MHz射频激发形成等离子态并发生化学反应, 从而在单腔PECVD设备中于单晶硅衬底上制得不同氢稀释比条件下的微晶硅薄膜。氢稀释比R即氢气流量和硅烷流量的比值, 反映了氢气对硅烷的稀释程度, 实验选取氢稀释比R变化范围为10~66.6。沉积室内较高氢气浓度有助于薄膜晶化, 所以实验选取R≥ 10的一系列氢稀释条件。各条件下制得硅膜的厚度均为1 µ m, 以避免厚度对电学性能的影响。射频功率、基片温度和工作压力分别保持在100 W、200℃和40 Pa。

采用JEM-3010型高分辨透射电子显微镜(HRTEM)和选区电子衍射(SAED)研究了不同氢稀释比条件下硅膜在不同沉积阶段的微观结构和形貌。采用XRD-7000型X射线衍射仪(XRD)检测硅膜的结晶状态。作为半导体材料重要性能指标, 采用HMS-7077型半导体光电测试系统测试暗电导率和载流子浓度, 以表征硅膜的电学性能。

2 结果与讨论

2.1 HRTEM截面观察

微晶硅是典型的两相结构, 既有非晶相又有晶相。在微晶硅的沉积过程中, 靠近衬底表面存在晶粒孵化层^[14](该层为非晶相), 只有沉积到一定阶段, 微晶晶粒才会出现, 且其尺寸和晶相比随着膜厚的增加而逐渐增大。其模型图如图1所示。

本研究采用HRTEM对薄膜沉积初期(如图1中A区域所示)和晶态生长阶段(如图1中B区域所示)的微观结构进行了分析, 其结果分别如图2、3所示。其中图2为不同氢稀释条件下各硅膜在沉积初期衬底附近的截面形貌, 着重观察界面处的非晶孵化层和非晶相向晶相转变的过程; 图3为发生相变后晶态沉积阶段的微观结构和电子衍射图谱, 用来对比分析薄膜内部的晶粒尺寸和晶相比。

由图2可知, 当氢稀释比R< 28.6时, 在距离衬底近40 nm的观察范围内没有发现晶粒存在; 随着氢稀释比的增大, 在观察范围内逐渐出现微晶颗粒, 且非晶过渡层的厚度也随之减小; 在氢稀释比增大到66.6时, 非晶过渡层的厚度仅为10 nm。高氢稀释比可使硅膜结构由非晶态向晶态的转变提前, 非晶层厚度减小。这与美国联合太阳能公司^[15]提出的“ 在沉积过程中, 需要将氢稀释递减从而形成氢稀释分布, 以保持微晶硅膜内在纵向具有相近晶粒尺寸和晶相比” 的说法相一致。

当薄膜发生晶化以后, 其结构即为非晶相和晶相共存的状态, 如图3所示。R=10时, 薄膜内部几乎观察不到晶粒存在, 因此SAED衍射花样显示为典型的非晶晕环, 说明此氢稀释比不足以使非晶硅膜在沉积过程中发生晶化。当氢稀释比增大到20(图3(b)), 一些孤立的、仅有几纳米尺寸的晶粒清晰地分布于薄膜中; 且其SAED花样由一系列具有不同半径的圆环组成, 并有一些分散的、较为明亮的斑点分布于圆环上, 此为多晶结构的典型特征, 从而揭示出硅膜结构有明显的从非晶相向晶相转变的趋势。进一步增大R值至28.6(图3(c))、40(图3(d))和66.6(图3(e)), 可以发现晶粒尺寸和晶化面积均有增加。另外, 从其衍射花样还可以观察到, 分布于圆环上的斑点变得更加明亮, 也说明了高氢稀释比有效地提高了硅膜的晶化程度。从图中可以看出, 尽管这三种条件下晶化区域与所观察总区域的面积比均大于90%, 但R值为40和66.6时的晶粒尺寸较28.6有所减小。在R=28.6条件下制得的硅膜中, 可在接近表面区域发现大量如图3(c)所示的尺寸达几十纳米的晶粒。对于这一现象, 本研究将结合XRD结果作进一步分析。

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	图1 微晶硅薄膜的结构模型Fig. 1 Structure model of micro-crystalline silicon thin film

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	图2 不同氢稀释条件下各硅膜在沉积初期衬底附近的截面形貌Fig. 2 Interface structure of silicon thin films obtained at different hydrogen dilutions (a) R=10; (b) R=20; (c) R=28.6; (d) R=40; (e) R=66.6

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	图3 不同氢稀释条件下晶态沉积阶段的微观结构和电子衍射花样Fig. 3 TEM images and SAED patterns of silicon thin films obtained at different hydrogen dilutions (a) R=10; (b) R=20; (c) R=28.6; (d) R=40; (e) R=66.6

2.2 XRD分析

不同氢稀释比R条件下所得硅膜的XRD结果见图4(a)。由图可见, 在R=10时, 仅出现一个宽泛的隆起而没有尖锐的衍射峰, 表明薄膜为非晶

相; 当R增至20时, 在2θ 角为 28.442° 的位置检测出Si(111)衍射峰, 此时硅膜出现晶化现象。随着氢稀释比的增加, 不仅(111)衍射峰明显增强, 且在2θ 角为47.302° 和 56.121° 的位置分别出现了相对较弱的(220)和(311)两个衍射峰, 说明材料中晶相成分的增加。在三个衍射峰中, Si(111)峰最强, 这是因为该取向的自由能最低^[16]。当氢稀释比大于28.6, Si(111)峰的强度又开始逐渐减弱。

根据主衍射峰可由公式(1)和(2)^[13]分别计算出其结晶度X_c和平均晶粒尺寸D。

X_c= I_c/ (I_c+kI_a) (1)

式中: I_c为晶体部分的衍射强度; I_a为非晶体部分的衍射强度; k为相对衍射线系数, 理论上k≈ 1。

D = kλ / (β cosθ ) (2)

式中: k为Scherrer常数, 取值0.89; λ 为单色入射X射线波长; β 为衍射峰半高宽, 在计算过程中需转化为弧度; θ 为布拉格衍射角。计算结果如图4(b)所示: 随着氢稀释比的增加, 硅膜的结晶度显著增加, 当R> 28.6时趋于平缓; 而平均晶粒尺寸则呈先增后减的趋势。

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	图4 不同氢稀释比条件下硅膜的XRD图谱(a)、晶化率和晶粒尺寸(b)Fig. 4 XRD results (a), crystallinity and grain size (b) of silicon films deposited at different R values

由表面扩散模型可知, 氢原子从高氢稀释的等离子体中流向衬底表面可饱和薄膜硅表面的硅悬挂键^[17], 同时释放一定的能量, 这两个作用使得从等离子体中到达生长表面的粒子的扩散系数增加。具有较高扩散系数的粒子和离子容易在沉积表面找到能量较低的位置, 这些低能量的位置通常是在晶粒表面, 所以在高氢稀释比条件下容易形成微晶硅。因此, 氢稀释比的增加使得硅膜在沉积过程中晶化提前、非晶层变薄。然而, 由HRTEM观察结果可知, 硅膜在转变为晶态沉积以后, 晶粒尺寸并没有随着氢稀释比的增加而增大, 而是在R=28.6时薄膜内部出现较大晶粒, 因此此时硅膜的平均晶粒尺寸最大。这可能是因为: 本实验条件下氢稀释比的增大是通过固定SiH₄流量、加大H₂流量而实现的, 在过高的氢稀释比条件下, 便会有过多的沉积粒子和离子流向衬底表面, 轰击较早到达表面的粒子和离子, 使其没有足够的时间和空间扩散到晶粒表面, 从而影响了晶粒尺寸的增大。

2.3 电学性能

图5是不同氢稀释比条件下制得硅膜的暗电导率和载流子浓度。可以看到, 随着氢稀释比的增加, 暗电导率和载流子浓度均先增后减, 在氢稀释比为28.6时达到最大值。硅基薄膜的暗电导率和载流子浓度与其微观结构密切相关^[18]。硅膜的晶化程度越高, 晶粒尺寸越大, 膜内缺陷越少, 载流子在迁移过程中消失的几率越小, 载流子浓度就越高。因此, 暗电导率和载流子浓度的变化趋势与硅膜晶化率和晶粒尺寸的变化趋势一致。

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	图5 不同氢稀释比条件下硅膜的暗电导率和载流子浓度Fig. 5 Dark conductivity and carrier concentration of silicon films deposited with different R values

3 结论

本研究采用PECVD法制备出一系列具有不同氢稀释比的氢化硅膜, 并通过XRD、HRTEM和半导体光电测试系统等检测手段对硅膜的微观结构和电学性能进行了研究, 得出以下结论: 1)氢稀释导致硅膜从非晶硅转变为微晶硅, 且微晶硅薄膜具有典型的两相结构。2)氢稀释比的增加可以使得硅膜在沉积过程中由非晶相向晶相的转变提前、非晶过渡层变薄, 但过高的氢稀释比又会影响晶粒尺寸的增长。3)硅膜的晶化度和平均晶粒尺寸越大, 暗电导率和载流子浓度也越大, 说明其电学性能与整体结构状态有明显正相关性。

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2011

1.545

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Surface Engineering. 2011, 27(6):414-423

Laser ablation modelling of aluminium, silver and crystalline silicon for applications in photovoltaic technologies.

COLINA M , MOLPECERES C , MORALES M

Laser material processing is being extensively used in photovoltaic applications for both the fabrication of thin film modules and the enhancement of the crystalline silicon solar cells. The two temperature model for thermal diffusion was numerically solved in this paper. Laser pulses of 1064, 532 or 248 nm with duration of 35, 26 or 10 ns were considered as the thermal source leading to the material ablation. Considering high irradiance levels (10 8 –10 9 W cm −2 ), a total absorption of the energy during the ablation process was assumed in the model. The materials analysed in the simulation were aluminium (Al) and silver (Ag), which are commonly used as metallic electrodes in photovoltaic devices. Moreover, thermal diffusion was also simulated for crystalline silicon (c-Si). A similar trend of temperature as a function of depth and time was found for both metals and c-Si regardless of the employed wavelength. For each material, the ablation depth dependence on laser pulse parameters was determined by means of an ablation criterion. Thus, after the laser pulse, the maximum depth for which the total energy stored in the material is equal to the vaporisation enthalpy was considered as the ablation depth. For all cases, the ablation depth increased with the laser pulse fluence and did not exhibit a clear correlation with the radiation wavelength. Finally, the experimental validation of the simulation results was carried out and the ability of the model with the initial hypothesis of total energy absorption to closely fit experimental results was confirmed.

... 非晶硅薄膜因其制备原料低廉和能源消耗低而成为极具潜力的光电能量转换材料^[1,2,3] ...

2011

2.112

0.0

Applied Surface Science. 2011, 257(9):3898-3903

Properties of a-Si: H films deposited by RF magnetron sputtering at 95 ◦C.

GIRGINOUDI D , TSIARAPAS C , GEORGOULAS N.

In this work we have investigated the dependence of optical and electrical properties of RF sputtered undoped a-Si:H films and B or P doped a-Si:H films on hydrogen flow rate ( F H ). Low deposition temperature of 95 °C was used, a process compatible with low-cost plastic substrates. FTIR spectroscopy and ESR measurements were used for the investigation of Si–H x bonding configurations, and concentrations of hydrogen and dangling bonds. We found that there is a strong correlation between the total hydrogen concentration, the dangling bonds density and the optoelectronic properties of the films. The best photosensitivity value was found to be 1.4 × 10 4 for the undoped films. The dark conductivity ( σ D ) of the doped layers varied from 5.9 × 10 −8 to 6.5 × 10 −6 (Ω cm) −1 for different ratios F Ar / F H . These variations are attributed to both the different B and P concentrations in the films (according to SIMS measurements) and the enhanced disorder of the films introduced by the large number of inactive impurities. The B doping efficiency is lower compared to the P one. A small photovoltaic effect is also observed in n–i–p solar cells fabricated on polyimide (PI) substrates having ITO as antireflective coating, with an efficiency of 1.54%.

... 非晶硅薄膜因其制备原料低廉和能源消耗低而成为极具潜力的光电能量转换材料^[1,2,3] ...

2014

0.0

Thin Solid Films. 2014, 562:430-434

Polycrystalline silicon thin-film solar cells prepared by layered laser crystallization with 540 mV open circuit voltage.

PLENTZ J , ANDR#cod#x000c4; G , GAWLIK A

Leibniz Institute of Photonic Technology, Albert-Einstein-Str. 9, 07745 Jena, Germany. Tel.: + 49 3641 206421; fax: + 49 3641 206499.

Polycrystalline silicon thin film solar cells on a glass substrate are investigated. The solar cell layer structure was generated by a two-step process in which first a 100–600 nm thin seed layer is formed by diode laser crystallization of electron beam evaporated amorphous silicon. In a second step this layer is epitaxially thickened to 2–3.5 μm by layered laser crystallization. In this process further amorphous silicon is deposited and in situ repeatedly is irradiated by excimer laser pulses. The polycrystalline layer consists of grains several hundreds of microns long and several tens of microns wide and it contains a p + –p–n + doping profile. After deposition a rapid thermal annealing and hydrogen passivation steps follow. The back and front contacts are prepared after mesa structuring. The influence of the seed layer thickness on the solar cell performance was investigated. In addition, the absorber contamination due to the background pressure during absorber deposition and its influence on the short circuit current density was investigated. The best parameters reached for various solar cells are 540 mV open circuit voltage, 20.3 mA/cm 2 short circuit current density (without light trapping), 75% fill factor, and 5.2% efficiency.

... 非晶硅薄膜因其制备原料低廉和能源消耗低而成为极具潜力的光电能量转换材料^[1,2,3] ...

2011

1.604

0.0

Thin Solid Films. 2011, 519:6896-6898

Comparison between silicon thin films with and without incorporating crystalline silicon nano-particles into the film.

KOGA K , MATSUNAGA T , NAKAMURA W M

We have deposited Si thin films using a multi-hollow discharge plasma CVD method to compare properties of the films with and without incorporating crystalline Si nanoparticles into the films. After the deposition of the films, we have evaluated crystallization of the films by irradiating laser. We have found that a laser power at which crystalline Si nanoparticles embedded a-Si:H films start to be crystallized is lower than that for a-Si:H films without the nanoparticles. The incorporation of the nanoparticles has no effect on the defect density of the films. These results suggest incorporation of crystalline Si nanoparticles into the films play a role of crystallization of Si films during the deposition.

... 但经长时间光照后光电导率和暗电导率的下降, 也即光致衰减效应^[4]又制约了它在光伏电池中的应用 ...

2001

3.794

0.0

Applied Physics Letters.. 2001, 78:2509-2511

Gap states of hydrogenated amorphous silicon near and above the threshold of microcrystallinity with subtle boron compensation.

SHENG S , LIAO X , KONG G.

The effects of hydrogen dilution, subtle boron compensation, and light-soaking on the gap states of hydrogenated amorphous siliconfilms (a- Si:H ) near and above the threshold of microcrystallinity have been investigated in detail by the constant photocurrent method and the improved phase-shift analysis of modulated photocurrent technique. It is shown that high hydrogen dilution near the threshold of microcrystallinity leads to a more ordered network structure and to the redistribution of gap states; it gives rise to a small peak at about 0.55 eV and a shoulder at about 1.2 eV below the conduction band edge, which are associated with the formation of microcrystallites embedded in the amorphous silicon host matrix. A concurrent subtle boron compensation is demonstrated to prevent excessive formation of microcrystallinity, and to help promote the growth of the ordered regions and reduce the density of gap defect states, particularly those associated with microcrystallites. Hydrogen-diluted and appropriately boron-compensated a- Si:H films deposited near the threshold of microcrystallinity show the lowest density of the defects in both the annealed and light-soaked states, and hence, the highest performance and stability.

... 而从热力学角度来看, 无序网络结构的改善最终将导致结构的晶化^[5] ...

2011

2.112

0.0

Applied Surface Science. 2011, 257:7993-7996

Structural, optical and electrical properties of amorphous silicon thin films prepared by sputtering with different targets.

QIN Y , FENG T , LI Z Q

Amorphous silicon (a-Si) films were prepared by sputtering method with polycrystalline and monocrystalline silicon targets. Structural, optical and electrical properties of the a-Si films have been systematically studied. The deposition power is from 100 to 200 W. Compared with the a-Si films deposited by using monocrystalline silicon target, the a-Si films prepared with polycrystalline silicon target exhibit better growth property, similar optical band gap, and own the highest mobility of 1.658 cm 2 /Vs, which make a good match with the optimal window of optical band gap for a-Si solar cells. The results indicated that the polycrystalline silicon target is superior to the monocrystalline silicon target when used to prepare a-Si films as the intrinsic layer in a-Si solar cells.

... 在众多的制备方法中, 可在低温状态下大面积沉积非晶硅薄膜的等离子体增强化学气相沉积法(PECVD)^[6]较为常用 ...

2008

0.537

1.106

Journal of Inorganic Materials. 2008, 23(2):369-372

Fabrication of polycrystalline silicon films with columnar grains structure on quartz.

ZHANG LI-WEI , ZHOU LING-LI , LI RUI

N/I silicon thin films were deposited on amorphous silicon thin film coated quartz substrate by radio frequency plasma enhanced chemical vapor deposition(RF-PECVD)system at low temperature, and subsequently annealed by two-step rapid thermal processing (RTP). Through Raman scattering, X-Ray diffraction (XRD), scanning electron microscope(SEM) and transmittance electron microscope (TEM) measurement, the crystallization and morphologies of the sample were investigated. The results show that the crystallinity of the N/I silicon thin films reaches about 94% after being annealed. The cross sectional morphology of the N/I silicon thin films is of columnar grains and the average grain size of the sample is about 30nm while the biggest grain cluster is about 1.5 μ m in landscape orientation.

... c-Si: H)材料^[7] ...

1999

1.604

0.0

Thin Solid Films. 1999, 337:1-6

Growth mechanism of microcrystalline silicon obtained from reactive plasmas.

MATSUDA A.

Three models proposed for the growth mechanism of hydrogenated microcrystalline silicon films (μc-Si:H) from reactive (silane and hydrogen mixture) plasmas are reviewed. The `etching model' is discussed using experimentally obtained relationship between radical generation rate in plasmas and growth rate of films. The `chemical annealing model' is investigated through the growth of films using a layer-by-layer method with and without cathode shutter. Substrate-temperature dependence of crystallinity of the resulting films and initial growth behavior of silicon films on atomically flat GaAs substrate clearly support the ‘surface diffusion model’.

... 关于氢稀释导致硅膜从非晶到微晶的转变有三种模型^[8,9,10]试图解释, 其中表面扩散模型较为合理地解释在高氢稀释条件下微晶硅薄膜的沉积机理 ...

1999

2.21

0.0

Journal of Applied Physics. 1999, 85:1172-1177

In situ observation of hydrogenated amorphous silicon surfaces in electron cyclotron resonance hydrogen plasma annealing.

NOZAWA R , TAKEDA H , ITO M

The surface reactions of hydrogenated amorphous silicon ( a -Si:H) films exposed to electron cyclotron resonance (ECR) H 2 plasma, which is called ECR H 2 plasmaannealing, has been investigated by in situ polarization modulation infrared reflection absorption spectroscopy (PM-IR-RAS). The surfaces of a -Si:H films were exposed to ECR H 2 plasmas at various substrate temperatures. To distinguish the roles of ionic species and atomic hydrogens, permanent magnets were employed to shut out charged species incident on the substrate. The surface reactions of a -Si:H films exposed to only atomic hydrogens were observed by in situ PM-IR-RAS and the results were compared with those exposed to ECR H 2 plasmas. It was found that atomic hydrogens reacted with the Si–H 2 and ( SiH 2 ) n bonds in the vicinity of the surfaces and did not etch the bulk of the films at room temperature. Moreover, it was suggested that atomic hydrogens penetrated into the bulk of a -Si:H film and promoted the Si-network modification as the substrate temperature increased. On the other hand, it was found that ionic hydrogens with the energy of 50 eV had energy enough to etch a -Si:H films.

... 关于氢稀释导致硅膜从非晶到微晶的转变有三种模型^[8,9,10]试图解释, 其中表面扩散模型较为合理地解释在高氢稀释条件下微晶硅薄膜的沉积机理 ...

2010

0.0

Energy Procedia. 2010, 2:235-241

Formation of thin-film crystalline silicon on glass observed by in situ XRD.

WESTRAA J M , VAVRUNKOVA V , #cod#x00160;UTTA P

Thin-film poly-crystalline silicon (poly c-Si) on glass obtained by crystallization of an amorphous silicon (a-Si) film is a promising material for low cost, high efficiency solar cells. Our approach to obtain this material is to crystallize a-Si films on glass by solid phase crystallization (SPC). As the grain size of SPC poly c-Si films will be smaller than that of multi-crystalline wafers, lower solar cell efficiencies are expected for this technology. Despite the smaller grain size, a 2-micron-thick polycrystalline silicon solar cell with light trapping was shown to have a conversion efficiency of more than 10% [1]. Obtainable efficiencies up to 15% are expected for solar cells made using SPC of a-Si:H films. Expanding thermal plasma chemical vapor deposition (ETP-CVD) was used to prepare hydrogenated a-Si films; this technique is chosen because the deposition rates are much higher than with plasma enhanced CVD. A-Si:H films with different hydrogen contents were annealed using temperatures ranging from 500 °C to 700 °C. The evaluation of the films after annealing treatments revealed that the hydrogen content and bonding configuration did not influence the structural properties of the crystallized films significantly. The average crystallite size in the fully crystallized films was between 100 and 150 nm. Full crystallization of 1 micrometer thick films was achieved within 20 minutes for annealing at 625 °C and 650 °C. During annealing at 600 °C crystallization is much slower, and no crystallization is observed at 500 °C. The relation between the annealing temperature and the rate with which the films are fully crystallized is of great importance to develop a solar cell technology, to limit the thermal budget and processing time.

... 关于氢稀释导致硅膜从非晶到微晶的转变有三种模型^[8,9,10]试图解释, 其中表面扩散模型较为合理地解释在高氢稀释条件下微晶硅薄膜的沉积机理 ...

2014

4.63

0.0

Solar Energy Materials and Solar Cells. 2014, 122(0):31-39

On the effects of hydrogenation of thin film polycrystalline silicon: a key factor to improve hetero junction solar cells.

QIU Y , KUNZ O , FEJFAR A

The hydrogen plasma passivation of thin film polycrystalline silicon (pc-Si) was investigated in conjunction with plasma texturing process to make efficient heterojunction solar cells. The pc-Si layers were first treated using direct and remote hydrogen plasma technologies. The heterojunction solar cells were then fabricated by subsequent deposition of i / n + a-Si:H. Hydrogenation at high temperature (610 °C) results in enhanced dissolution and diffusion of hydrogen in pc-Si by a factor of about 3 and 4, respectively, in comparison with those at low temperature (420 °C). The hydrogen atoms in the pc-Si layer mainly bond to the silicon dangling bonds and form complexes with dopant atoms. In addition, platelets defects are generated by the hydrogen plasma in the sub-surface region of pc-Si hydrogenated at 420 °C and cause higher saturation current in the space charge region whilst they form in the region deeper than 1 μm at 610 °C. Removal of the platelets using SF 6 /N 2 O plasma post-texturing after low-temperature hydrogenation not only enhances the short circuit current but also improves the open circuit voltage and the fill factor simultaneously. Combining plasma pre-texturing with high-temperature hydrogenation, the best 2 µm-thick pc-Si heterojunction solar cell reaches an efficiency of 8.54%.

... 氢可以钝化非晶硅薄膜中大量存在的悬挂键, 降低薄膜的缺陷密度, 减少带隙中的非辐射复合中心^[11], 从而显著提高薄膜的稳定性 ...

2014

1.522

0.0

Physica E. 2014, 61:95-100

Spectroscopic studies on nanocrystalline silicon thin films prepared from H₂-diluted SiH₄-plasma in inductively coupled low pressure RF PECVD.

CHAKRABORTY M , BANERJEE A , DAS D.

A comprehensive analysis on the evolution of the microstructure as well as optical constants and dielectric functions of intrinsic hydrogenated nano-crystalline silicon thin films prepared by highly H 2 diluted SiH 4 plasma in a planar inductively coupled RF plasma chemical vapor deposition (ICP-CVD) reactor has been performed by spectroscopic ellipsometry. Films are assumed to have a three-layer structure, with a thin incubation layer at substrate/bulk interface, the bulk layer and a thin growth zone and surface roughness layer. Individual composition and the thickness of each layer have been estimated from the simulation of the ellipsometry data using Bruggeman effective medium approximation (BEMA). The ellipsometry results are correlated with atomic force microscopy and micro-Raman data of these films. The effect of the flow rate of SiH 4 and the key role of hydrogen dilution on growth dynamics, optical constants and dielectric functions of highly crystalline nanosilicon films is discussed elaborately. The bulk crystalline volume fraction of the deposited films varies considerably (~67–84%) with the change in flow rate of SiH 4 . With increasing SiH 4 flow rate the overall bulk crystallinity reduces; however the ultra-nanocrystalline component ( X unc ) enhances substantially that helps reducing the porosity and surface roughness.

... 事实上, 微晶硅薄膜具有典型的两相结构^[12], 从衬底到薄膜表面的纵向结构是不均匀的 ...

2014

0.0

0.2679

... 氢稀释作为改善非晶硅结构和形成微晶硅的关键参量, 会显著地影响薄膜的两相结构和生长过程, 进而影响薄膜的光电特性^[13] ...

... 根据主衍射峰可由公式(1)和(2)^[13]分别计算出其结晶度X_c和平均晶粒尺寸D ...

2010

4.63

0.0

Solar Energy Materials and Solar Cells. 2010, 94:1860-1863

Nanocrystalline p-layer for a-Si: H p-i-n solar cells and photodiodes.

VYGRANENKO Y , FATHI E , SAZONOV A

We report on structural, electronic, and optical properties of boron-doped, hydrogenated nanocrystalline silicon (nc-Si:H) thin films deposited by plasma-enhanced chemical vapor deposition (PECVD) at a substrate temperature of 150 °C. Film properties were studied as a function of trimethylboron-to-silane ratio and film thickness. The absorption loss of 25% at a wavelength of 400 nm was measured for the 20 nm thick films on glass and glass/ZnO:Al substrates. By employing the p + nc-Si:H as a window layer, complete p–i–n structures were fabricated and characterized. Low leakage current and enhanced sensitivity in the UV/blue range were achieved by incorporating an a-SiC:H buffer between the p - and i - layers.

... 在微晶硅的沉积过程中, 靠近衬底表面存在晶粒孵化层^[14](该层为非晶相), 只有沉积到一定阶段, 微晶晶粒才会出现, 且其尺寸和晶相比随着膜厚的增加而逐渐增大 ...

2006

0.0

... 这与美国联合太阳能公司^[15]提出的#cod#x0201c ...

2008

1.604

0.0

Thin Solid Films. 2008, 516(12):3940-3947

Nano-crystalline Si formation in the a-Si/Al system on polyimide and silicon dioxide substrates.

ALFORD T L , SHETTY P K , THEODORE N D

Polysilicon (poly-Si) films fabricated on flexible substrates are of considerable interest because of their potential application in flexible displays. In this study, an 800 nm layer of amorphous silicon (a-Si), followed by a 20 nm layer of aluminum (Al), were deposited on polyimide/silicon and silicon dioxide/silicon (SiO 2 /Si) substrates. Samples on polyimide were rapid thermal annealed at 900 °C for 20 s, while those on SiO 2 /Si were vacuum annealed at temperatures between 200 and 600 °C for 1 h. Film properties were analyzed using Rutherford backscattering spectrometry, cross-section transmission electron microscopy, and X-ray diffraction. Silicon films containing nanocrystallites and pores were obtained, with a pore formation activation energy ( E A ) of 0.59 eV. A short-range self-diffusion model is proposed for the formation of Si crystallites in cases where the solid-solubility limit for Si dissolution into Al has not been reached.

... 在三个衍射峰中, Si(111)峰最强, 这是因为该取向的自由能最低^[16] ...

2013

2.112

0.0

Applied Surface Science. 2013, 270:737-740

The study of the substrate temperature depended growth properties of microcrystalline silicon films deposited by VHF-PECVD method.

CHEN YONG-SHENG , CHEN XI-PING , HAO XIU-LI

In this paper, we have measured the temperature depended growth properties of hydrogenated microcrystalline silicon (μc-Si:H) films, prepared by very high frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) from SiH 4 and H 2 gas mixtures. And, a 1D plasma model coupled with a well-mixed reactor model is used to simulate the growth process, in which concentrations of gas phase species, the crystalline orientation, the hydrogen content and the deposition rate are calculated. It suggests that the increasing surface fraction of the dangling bonds with the increase of substrate temperatures is responsible for the increase in the grain sizes. At the same time, the observed variations of the X-ray-diffraction intensities and the deposition rates of the films with temperature result from the differences in the growth rates of the facets.

... 由表面扩散模型可知, 氢原子从高氢稀释的等离子体中流向衬底表面可饱和薄膜硅表面的硅悬挂键^[17], 同时释放一定的能量, 这两个作用使得从等离子体中到达生长表面的粒子的扩散系数增加 ...

2011

1.597

0.0

Journal of non-crystalline solids. 2011, 357:1114-1117

DAVID R A, et. al. Distribution of hydrogen in low temperature passivated amorphous silicon (a-Si: H) films from neutron reflectivity.

BRYAN D V , BARRY O B

Hydrogen plays a critical role in the passivation of dangling bonds in hydrogenated amorphous silicon (a-Si:H) to enable acceptable semiconducting characteristics during operation in devices. Low temperature processing enables fabrication of high performance transistors on flexible substrates such as plastic or stainless steel foils, but also leads to a decrease in the stability of the electronic performance. Generation of defects at the a-Si:H/insulator (hydrogenated silicon nitride, SiN:H) during electrical use due to localized heating will lead to decreased performance unless the dangling bonds are passivated in-situ by residual hydrogen. For this reason, the distribution of hydrogen within a-Si:H may be critical to understanding their aging phenomena. Here the distribution of hydrogen within both a-Si:H and SiN:H layers is probed with sub-nanometer resolution using neutron reflectivity. The hydrogen concentration within the bulk of the a-Si:H (11 ± 2 at.%) and SiN:H (18 ± 3 at.%) agree well with previous reports, but the increased resolution of the neutron measurement is able to identify an approximate three fold increase in the concentration within 2 nm of the semiconductor–insulator interface. This enhanced hydrogen content may act in the short-term as a sink to passivate any dangling bonds formed during operation.

... 硅基薄膜的暗电导率和载流子浓度与其微观结构密切相关^[18] ...