后退火增强氢化非晶硅钝化效果的研究

在本征氢化非晶硅(a-Si:H(i))/晶体硅(c-Si)/a-Si:H(i)异质结构上溅射ITO时, 发现后退火可大幅增加ITO/a-Si:H(i)/c-Si/a-Si:H(i)的少子寿命(从1.7 ms到4 ms). 这一增强效应可能的三个原因是: ITO/a-Si:H(i)界面场效应作用、退火形成的表面反应层影响以及退火对a-Si:H(i)材料本身的优化, 但本文研究结果表明少子寿命增强效应与ITO和表面反应层无关; 对不同沉积温度制备的a-Si:H(i)/c-Si/a-Si:H(i)异质结后退火的研究表明: 较低的沉积温度(<175 ℃)后退火增强效应显著, 而较高的沉积温度(>200 ℃)后退火增强效应不明显, 可以确定“低温长高温后退火”是获得高质量钝化效果的一种有效方式; 采用傅里叶红外吸收谱(FTIR)研究不同沉积温度退火前后a-Si:H(i)材料本身的化学键构造, 发现退火后异质结少子寿命大幅提升是由于a-Si:H(i)材料本身的结构优化造成的, 其深层次的本质是通过材料的生长温度和退火温度的优化匹配来控制包括H含量、H键合情况以及Si原子无序性程度等微观因素主导作用的一种竞争性平衡, 对这一平衡点的最佳控制是少子寿命大幅提升的本质原因.     

Aluminium-induced crystallization of amorphous silicon films deposited by DC magnetron sputtering on glasses

Amorphous silicon (a-Si) and hydrogenated amorphous silicon (a-Si:H) films were deposited by DC magnetron sputtering technique with argon and hydrogen plasma mixture on Al deposited by thermal evaporation on glass substrates. The a-Si/Al and a-Si:H/Al thin films were annealed at different temperatures ranging from 250 to 550 °C during 4 h in vacuum-sealed bulb. The effects of annealing temperature on optical, structural and morphological properties of as-grown as well as the vacuum-annealed a-Si/Al and a-Si:H/Al thin films are presented in this contribution. The averaged transmittance of a-Si:H/Al film increases upon increasing the annealing temperature. XRD measurements clearly evidence that crystallization is initiated at 450 °C. The number and intensity of diffraction peaks appearing in the diffraction patterns are more important in a-Si:H/Al than that in a-Si/Al layers. Results show that a-Si:H films deposited on Al/glass crystallize above 450 °C and present better crystallization than the a-Si layers. The presence of hydrogen induces an improvement of structural properties of poly-Si prepared by aluminium-induced crystallization (AIC).     

The effect of illumination on dark conductivity and photoconductivity of hydrogenated amorphous silicon layered films

It is established that both the amplitude and temperature dependence of dark conductivity and photoconductivity of preilluminated high-sensitivity layered films of amorphous hydrogenated silicon (a-Si:H) prepared by cyclic deposition with layer-by-layer annealing in hydrogen plasma depend on illumination temperature. The relaxation kinetics of the dark conductivity of these films after illumination is shut off is found to be nonmonotonic. The observed effects can be explained by fast and slow changes in the distribution of energy state density below the midgap during and after illumination.     

Specificities of light-induced relaxation of metastable defects in amorphous hydrogenated silicon

The thermal relaxation kinetics of light-induced metastable defects in a-Si:H was studied prior to and after partial relaxation in the dark and in a dim light. The film lighting was found to change the relaxation rate of the defects and their distribution in relaxation time. This was demonstrated to be due to the concurrent light-induced relaxation and formation of the defects.     

Light-induced metastable state in doping-modulated amorphous silicon superlattices

Photo-induced decrease in conductance after long light exposure has been observed for hydrogenated amorphous silicon (a-Si:H) npnp . doping-modulated superlattices showing large persistent photoconductivity (PPC). The initial increase in PPC is taken over by the decrease even to the negative PPC after long illumination. This light-induced, metastable conductance can be recovered completely by 160°C annealing, which is independent of exposure time. In particular, the metastable defects created in the p-layers are found to be annealed out at 100°C. The subband gap light exposure on the a-Si:H npnp. multilayers showing n-type conduction gives rise to the increase of conductance. On the contrary, the conductance of the multilayers having p-channel conduction decreases by IR exposure. These results strongly suggest the creation and annealing of dangling bonds by light-soaking and annealing in doping-modulated superlattices.     

Structural evolution of a-Si:H/SiO2 multilayers upon step by step thermal annealing

a-Si:H/SiO_2 multilayers were prepared by alternatively changing plasma enhanced chemical vapour deposition of a-Si:H layers and in situ plasma oxidation process. Subsequently, as-grown samples were annealed at temperatures from 350℃ to 1100℃ in N_2 ambient with an increment of 100℃. The evolution of bonding configurations and structures with annealing treatments was systematically investigated by Fourier-transform infrared spectroscopy. The peak position of Si－O stretching vibration of SiO_2 layers shift to 1087cm^{-1} after annealing at 1100℃, which demonstrates that the SiO_2 films fabricated by plasma oxidation after high temperature annealing can have similar properties to the thermal grown ones. A Si－O vibration from interfacial SiO_x was identified: the value x was found to increase as increasing the annealing temperature, which is ascribed to the cooperation of hydrogen effusion and reordering of the oxygen bond in SiO_x networks. The H-related bonds were observed in the form of H－Si－O_3 and H－Si－Si_{3-n}O_n (n=1－2) configurations, which are supposed to be present in SiO_2 and interfacial SiO_x layers, respectively. The H atoms bonded in different bonding configurations effuse at different temperatures due to their different desorption energies.     

Structural changes in a-Si:H during annealing

Annealing experiments on undoped glow-discharge a-Si:H films deposited both from undiluted SiH₄ and from a SiH₄/H₂ mixture, show that annealing after deposition is a more effective way to improve the structure than an increase in deposition temperature, at least in cases when considerable numbers of (SiH₂)_n polymers are created during deposition.     

Polycrystalline silicon films prepared by metal-induced crystallisation using pre- and post-deposited aluminium on amorphous silicon

We report on the successful fabrication of polycrystalline silicon films by aluminium-induced crystallisation (AIC) of Radio frequency (rf) plasma-enhanced chemical vapour deposited (PECVD) a-Si films. The effects of annealing at different temperatures (300 and 400°C), below the eutectic temperature of the Si–Al binary system, on the crystallisation process have been studied. This work emphasises the important role of the position of the Al layer with respect to the Si layer on the crystallisation process. The properties of the crystallised films were characterised using X-ray diffraction, Raman spectroscopy, ellipsometry, field-emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). With an increase in the annealing temperature, it was found that the degree of crystallisation of annealed a-Si/Al and Al/a-Si films increased. The results showed that the arrangement where the Al was on top of the a-Si had a more prominent effect on crystallisation enhancement than when Al was below the a-Si. The interfacial layer between the Al and a-Si film is crucial because it influences the layer-exchange process during annealing. The oxide layer formed between the Al and the a-Si layers greatly retards the crystallisation process in the case of the Al/Si arrangement. Our investigations suggest that polycrystalline Si films formed by AIC can be used as a seed layer in solar cell fabrication.     

Metastability and microstructural changes in hydrogenated amorphous silicon

The effects of prolonged illumination and subsequent thermal treatment, on the valence band density of states in hydrogenated amorphous silicon have been investigated. Significant reversible changes are observed upon light soaking and annealing sequences. A microscopic model consistent with the experimental observations is proposed for the light-induced generation and thermal recovery of defects in a-Si:H.     

Light induced microstructure transformation in a-Si：H films

A series of a-Si:H films are deposited by hot wire assisted microwave electron cyclotron resonant chemical vapour deposition (HW-MWECR-CVD), subsequently exposed under simulated illumination for three hours. This paper studies the microstructure change during illumination by Fourier Transformation Infrared (FTIR) spectra. There are two typical transformation tendencies of microstructure after illumination. It proposes a model of light induced structural change based on the experimental results. It is found that all samples follow the same mechanism during illumination, and intrinsic structure of samples affect the total H content.     

Transport and stability studies on high band gap a-Si:H films prepared by argon dilution

Device quality hydrogenated amorphous silicon films (a-Si:H) are deposited at a high deposition rate (4–5 Å/s) using a mixture of argon and hydrogen-diluted silane. The films exhibit good opto-electronic properties and show less degradation upon light soaking. Light-induced changes in conductivity could be annealed at much lower temperature. The presence of Ar^* and atomic hydrogen in plasma replaces the weak Si-Si bonds, which are responsible for light-induced degradation by strong Si-Si bonds. This results in the improved stability of the films.     

Hydrogen related effects in a-Si:H studied by photothermal deflection spectroscopy

A study of thermal annealing of a-Si:H samples between 300 and 600°C has been carried out. At increasing annealing temperatures, the sub-gap absorption measured by PDS increases showing two inflections, centered at 375 and 550°C. The hydrogen content measured by thermal desorption spectroscopy evolves in the same temperature range, whereas the evolution of the hydrogen content deduced from the IR transmission spectra differs, decreasing sooner and vanishing already at about 450°C.     

Effects of gas temperature on optical and transport properties of a-Si:H films deposited by PECVD

Effects of silane temperature (T _g) before glow-discharge on the optical and transport properties of hydrogenated amorphous silicon (a-Si:H) thin films were investigated. The optical measurements show that the refractive index increases with increasing T _g. The transport characterizations show that when T _g increases, the dark conductivity increases. However, the temperature coefficient of resistance decreases. In addition, after holding at 130°C for 20 h, the resistance variation, ΔR/R, of the films deposited at T _g = room temperature (10.8%) is much larger than those deposited at silane temperatures of 80°C (3%) and 160°C (2%). This can be attributed to different rates of defect creation in a-Si:H films caused by various T _g.     

Structural and optical properties of Si/SiO2 multi-quantum wells

Structural and optical properties of Si/SiO₂ multi-quantum wells (MQW) were investigated by means of Raman scattering and photoluminescence (PL) spectroscopy. The MQW structures were fabricated on a quartz substrate by remote plasma enhanced chemical vapour deposition (RPECVD) of alternating amorphous Si and SiO₂ layers. After layer deposition the samples were subjected to heat treatments, i.e. rapid thermal annealing (RTA) and furnace annealing. Distinct PL signatures of confined carriers evidenced formation of Si-nanocrystals (nc-Si) in annealed samples. Analyses of Raman spectra also show presence of nc-Si phase along with amorphous-Si (a-Si) phase in the samples. The strong influence of the annealing parameters on the formation of nc-Si phase suggests broad possibilities in engineering MQW with various optical properties. Interestingly, conversion of the a-Si phase to the nc-Si phase saturates after certain time of furnace annealing. On the other hand, thinner Si layers showed a disproportionately lower crystalline volume fraction. From the obtained results we could assume that an interface strain prevents full crystallization of the Si layers and that the strain is larger for thinner Si layers. The anomalous dependence of nc-Si Raman scattering peak position on deposited layer thickness observed in our experiments also supports the above assumption.     

退火对Ge诱导晶化多晶Si薄膜结晶特性的影响

本文采用磁控溅射法, 衬底温度500 ℃下在硅衬底上分别制备具有Ge填埋层的a-Si/Ge 薄膜和a-Si薄膜, 并进行后续退火, 采用Raman光谱、X射线衍射、原子力显微镜及场发射扫描电镜等对所制薄膜样品进行结构表征. 结果表明, Ge有诱导非晶硅晶化的作用, 并得出以下重要结论: 衬底温度为500 ℃时生长的a-Si/Ge薄膜, 经600 ℃退火5 h Ge诱导非晶硅薄膜的晶化率为44%, 在相同的退火时间下退火温度提高到700 ℃, 晶化率达54%. 相同条件下, 无Ge填埋层的a-Si薄膜经800 ℃退火5 h薄膜实现晶化, 晶化率为46%. 通过Ge填埋层诱导晶化可使在相同的条件下生长的非晶硅晶薄膜的晶化温度降低约200 ℃. Ge诱导晶化多晶Si薄膜在Si(200)方向具有高度择优取向, 且在此方向对应的晶粒尺寸约为76 nm. 通过Ge诱导晶化制备多晶Si薄膜有望成为制备高质量多晶Si薄膜的一条有效途径.     

中频孪生磁控溅射WO3薄膜及变色性能研究

采用先进的中频孪生非平衡磁控溅射技术,以金属钨为靶材,制备非晶态WO3电致变色薄膜。用X射线衍射(XRD)、X射线光电子能谱(XPS)、紫外分光光度计等测试手段分析薄膜的结构、表面形貌、成分以及透射光谱特性。研究了氧气流量比及热处理温度对WO3薄膜变色性能的影响。结果表明,中频孪生非平衡磁控溅射技术是制备WO3变色薄膜的一种有效方法;室温条件下沉积获得的原始态薄膜为非晶态WO3;提高氧气流量比和适当热处理温度能有效改善薄膜的电致变色性能。实验中在较高氧气流量比,200℃热处理条件下制备的薄膜在380~780 nm的可见光范围内着色态和褪色态平均透光率差值高达50%以上,表现出较好的电致变色性能。     

非晶半导体光致发光的温度依赖关系

研究了a-Si:H,a-Si:H,F,a-Si:H,Cl以及a-Si:H,Cl,O等非晶硅薄膜光致发光的温度依赖关系。测量了各种样品的无辐射跃迁激活能ε,并试图寻求激活能ε与样品的制备条件和结构之间的关系;同时从发光强度的温度相关性确定了带尾态密度分布参数T0,以及T0与发光光谱形状之间的关系。并将实验结果与瞬态光电导的数据进行了比较。     

退火温度对原子层沉积法制备ZnMgO薄膜结构和光学性能的影响

针对目前关于退火温度对原子层沉积法(ALD)制备ZnMgO薄膜晶体结构和光学性质影响鲜有报道的现象,进行了相应的实验研究分析。采用ALD在石英衬底上制备ZnMgO合金薄膜,对制得的样品在空气中进行不同温度的退火处理。利用X射线多晶衍射仪(XRD)、光致发光谱(PL)和紫外可见(UV-Vis)吸收光谱测试,系统的分析了不同退火温度对ALD法制备ZnMgO薄膜晶体结构和光学性能的影响。XRD测试结果表明：退火温度为600 ℃时,薄膜的晶体质量得到改善,且(100)衍射峰的强度明显增强。结合PL和UV-Vis吸收光谱的测试分析得出：退火温度为600 ℃时,能明显促进薄膜中Mg组分的增加使薄膜的禁带宽度进一步增大。从而说明适当温度的退火处理可有效的改善ZnMgO薄膜的晶体质量及光学特性。     

零偏退火与反偏退火对含氢的Au／n—Si肖特基势垒的控制

＜１１１＞晶向的掺磷的ｎ型硅外延片经等离子进氢后连同未经等离子氢处理的对比片一起淀积金，制得Ａｕ／ｎ－Ｓｉ肖特基势垒，实验结果表明：氢能使Ａｕ／ｎ－Ｓｉ的肖特基势垒高度下降０．１３ｅＶ；含氢的肖特基势垒的高度可以被零偏退火与反偏退火所控制，即零偏退火使含氢的肖特基势垒的高度降低，而反偏退火使含氢的肖特基势垒的高度升高；而且零偏退火与反偏退火对肖特基势垒高的这种控制作用至少在三个循环过程中是可逆的。     

零偏退火与反偏退火对含氢的Au/n-Si肖特基势垒的控制

〈１１１〉晶向的掺磷的ｎ型硅外延片经等离子进氢后连同未经等离子氢处理的对比片一起淀积金，制得Ａｕ／ｎ－Ｓｉ肖特基势垒。实验结果表明：氢能使Ａｕ／ｎ－Ｓｉ的肖特基势垒高度下降０．１３ｅＶ；含氢的肖特基势垒的高度可以被零偏退火与反偏退火所控制，即零偏退火使含氢的肖特基势垒的高度降低，而反偏退火使含氢的肖特基势垒的高度升高；而且零偏退火与反偏退火对肖特基势垒高的这种控制作用至少在三个循环过程中是可逆的。在反偏退火以后，含氢的肖特基势垒的高度升高的数值不仅与退火时所应用的偏置电压有关，而且与退火温度也有关。