节点文献
氢化非晶硅薄膜制备及其微结构和光电性能研究
Research on the Preparation, Microstructure and Optoelectronic Properties of Hydrogenated Amorphous Silicon Thin Films
【作者】 廖乃镘；
【导师】 李伟；
【作者基本信息】 电子科技大学 ， 光学工程， 2009， 博士
【摘要】 氢化非晶硅（a-Si：H）薄膜具有光吸收率较高、电阻温度系数较大、与Si半导体IC工艺兼容等特点，在微测辐射热计、太阳能电池、医疗仪器等领域具有广泛的应用前景。然而，a-Si：H薄膜导电性及电学性能稳定性较差的问题至今没有得到根本解决。因而，当前及今后的研究重点主要围绕高品质、高稳定性a-Si：H薄膜制备技术及性能优化而开展。本文采用等离子体增强化学气相沉积（PECVD）法制备磷（P）掺杂a-Si：H薄膜，借助多种现代分析测试方法，研究了基片温度、硅烷气体温度、N掺杂等对a-Si：H薄膜微结构、光学、电学等特性的影响；研究了电子束辐照过程中辐照剂量和入射电子初始能量对P重掺杂a-Si：H薄膜结构和性能的影响；对薄膜微结构和性能之间的关系进行了深入分析。本文取得的重要结论和创新性研究成果如下：（1）当硅烷（SiH4）气体温度从室温升高到160℃，P掺杂a-Si：H薄膜非晶网络结构的有序程度逐步得到改善，薄膜中未成对电子自旋密度降低，薄膜暗电导率得到大幅度提高。当SiH4气体温度为160℃时，薄膜中以SiH键为主，暗电导率提高了2个数量级。尽管此时薄膜的TCR绝对值减小了约1．6％／℃，但仍然可以达到|TCR |≈2．0％／℃，表明加热硅烷气体可以制备出质量较优的P掺杂a-Si：H薄膜。经130℃长时间保温后，加热SiH4气体制备a-Si：H薄膜的电阻变化率ΔR／R与不加热相比小许多，表明加热SiH4气体温度可使a-Si：H薄膜的电学稳定性得到改善。（2）通过喇曼（Raman）光谱技术对a-Si：H薄膜纵向有序性的差异进行了深入研究，发现从a-Si：H薄膜内部到表面，薄膜非晶网络的短程和中程有序程度逐步提高。热处理可使薄膜内部的非晶网络结构短程和中程有序程度得到提高，但只能使薄膜表面非晶网络的中程有序程度得到提高。（3）采用傅里叶变换红外光谱（FTIR）分析技术，深入研究了a-Si：H薄膜中H的键合方式及其演变过程，并讨论了其与薄膜性能的关系。当薄膜中H含量cH＜16 at．％时，以SiH键为主；当cH＞16 at．％时，则以聚集H原子为主。随着聚集H原子的增加，薄膜非晶网络有序程度降低，暗电导率随之降低。由于二氢硅化物（SiH2）和多氢硅化物（SiHn）比单氢硅化物（SiH）更容易在热作用下分解，因而，以SiH键为主的a-Si：H薄膜的热稳定性比SiH2或（SiHn）键含量较多的薄膜的热稳定性好。（4）当N元素掺杂浓度较低时，P掺杂a-Si：H薄膜中Si-N键很少，薄膜结构和电学性能变化很小。继续提高N元素掺杂浓度，薄膜中H含量减少，薄膜表面颗粒尺寸变大，非晶网络有序性明显降低，光学带隙明显变宽，电学性能恶化。（5）采用椭圆偏振（Ellipsometry）技术深入研究了a-Si：H薄膜的微结构和光学性能。椭偏反射法与椭偏透射法测得a-Si：H薄膜的微结构和光学参数值相当，表明透射法也可用于准确测量a-Si：H薄膜的微结构和光学参数。（6）电子束辐照P重掺杂a-Si：H薄膜容易引起结构损伤和Si-H键断裂。然而，辐照引起的薄膜结构损伤和电学性能衰退最终趋于饱和，这是由于电子束辐照过程中存在退火作用。对辐照a-Si：H薄膜进行纵向分析后发现，薄膜表面电学性能衰退比内部更明显，且薄膜表面的短程和中程有序程度明显低于其内部，结构损伤和性能衰退主要集中在薄膜表面层。采用较低能量的入射电子进行辐照时，a-Si：H薄膜暗电导率衰退程度更大，非晶网络短程和中程有序程度更低。
【Abstract】 Hydrogenated amorphous silicon （a-Si:H） thin films have attracted much more attention for use in uncooled microbolometers, solar cells and medical apparatus, etc., due to its enhanced optical absorption, high temperature of resistance and compatibility with semiconductor technology. However, the problems on the poor conductivity and stability of a-Si:H films are yet not ultimately resolved. Thus, the studies at present or in the future are to be carried out on preparing device-quality and high stability a-Si:H films.In this dissertation, Phosphor-doped （P-doped） a-Si:H films were deposited by plasma-enhanced chemical vapor deposition （PECVD）. The effects of substrate temperature, silane temperature （before glow-discharge） and nitrogen doping, on the microstructure and photoelectronic properties of a-Si:H films, have been investigated by means of many modern characterization methods. Furthermore, electron irradiation effects on the properties of heavily P-doped a-Si:H films have been studied by prolonging irradiation time or irradiating with electrons of different energies. Then the relationship between the microstructure and properties of a-Si:H films are revealed. The main results in the dissertation are shown as follows:（1） There is an improvement in amorphous network order and an increase in dark conductivity of P-doped a-Si:H films with increasing silane temperature （Tg） before glow discharge. However, the spin density of unpaired electrons and the temperature coefficient of resistance （TCR） of the films decrease. For the films deposited at Tg=160℃, the isolated silicon-hydrogen （SiH） bonding configuration is predominant, and the dark conductivity increases by two orders of magnitude compared with those deposited at Tg=RT （room temperature）. Although the TCR decreases by about 1.6 %/℃at Tg=160℃, it can be obtained above 2.0 %/℃. These results indicate that a-Si:H films with better quality can be prepared at higher silane temperatures. After holding at 130℃for some time, the resistance variation,ΔR/R, of the films deposited at higher temperature is much lower than that of the films deposited at Tg=RT, indicating that a-Si:H films deposited at higher silane temperatures behavior better in electronic stability.（2） The ordering evolution of the amorphous network in a-Si:H thin films was investigated by Raman spectroscopy. There exists a gradual ordering of the amorphous network on the short and intermediate scales towards the surface of a-Si:H thin films. Annealing taken on the films leads to an improved ordering of amorphous network on the short and intermediate scales in the interior region, but the network in the surface region becomes more ordered only on the intermediate scale.（3） Hydrogen bonding configurations and their evolution in a-Si:H films were intensively studied by Fourier transform infrared （FTIR） spectroscopy. The relationship between hydrogen bonding configurations and the properties of the films were discussed. It is found that the SiH dominates in a-Si:H films for hydrogen content ch<16 at.%, but the polysilanes dominate in the films for cH>16 at.%. With the increase of polysilanes amount in a-Si:H films, the amorphous network becomes more disordered and the dark conductivity decreases. Since the dihydrides （SiH2） and the polysilanes （SiHn） are more easily dissociated than SiH, a-Si:H films dominated by SiH behavior better in thermal stability than those films containing more SiH2 or SiHn.（4） When nitrogen （N） concentration in a-Si:H films is relatively low, the microstructure and electronic properties of heavily P-doped films change little. With the continuous increase of N concentration in a-Si:H films, hydrogen content decreases and amorphous network becomes more disordered. Meanwhile, the surface morphology of the films gets worse and the optical bandgap widens. Furthermore, the electronic properties of the films deteriorate significantly for N content cN>1.0 at.%.（5） The microstructure and optical constants of a-Si:H films have been intensively studied by Ellipsometry. The results obtained from the transmittance spectra agree well with those measured by the reflecttance spectra, indicating that the microstructure and optical properties of a-Si:H films can also be accurately determined from transmittance spectra.（6） Electron irradiation induces the breaking of Si-H bonds and structural damage in heavily P-doped a-Si:H films. However, the structural damage and the degradation in dark conductivity of the films come to saturations after irradiation for some hours. This is because there is an irradiation-induced annealing effect during electron irradiation. Depth profile studies on irradiated P-doped a-Si:H films reveal that, the degradation in dark conductivity is much distinct in the near surface, and the film surfaces become more disordered as compared with their interior regions. These indicate that the created defects and structural damage are concentrated in the near surface of a-Si:H films. When heavily P-doped a-Si:H films are irradiated with lower electron energies, the degradation in dark conductivity of the films is greater and the amorphous network becomes more disordered.