【作者】 王涵；

【导师】 越方禹；

【作者基本信息】 华东师范大学 ， 微电子学与固体电子学， 2018， 硕士

【摘要】 半导体材料/器件的带边电子结构、特别是与缺陷相关的杂质能级结构是决定器件光电性能的关键物理参数。基于傅里叶变换红外（FTIR）光谱仪的透射/吸收谱、反射谱、光致发光（PL）谱和光电流（PC）谱具有非破坏性,是研究半导体材料电子能带结构的常用表征方法,它们各具特色且又互为补充或互相印证。本文利用上述光谱方法对用于研制红外探测器的掺杂碲镉汞(Hg_1-xCd_xTe)和用于太阳能电池吸收层的铜锌锡硫Cu₂ZnSnS₄（CZTS）等两类材料体系的光学带隙及其杂质/缺陷进行了系统研究和分析。主要研究结果如下:1.碲镉汞材料:对两组P型导电的碲镉汞材料,分别是本征汞空位掺杂液相外延（LPE）和非本征砷/As元素掺杂分子束外延（MBE）的碲镉汞薄膜进行了吸收/透射谱和PL谱测试,研究了光学带隙随温度变化的依赖关系。通过对比分析吸收边确定的禁带宽度Eg与PL谱确定的禁带宽度Eg^PL,发现存在明显的斯托克斯Stokes位移、且其符号随温度升高由正变负的现象。结合材料制备和关键物理参数模型,数值计算了材料体系的费米能级E_F与温度的演化关系,理论结果验证了实验现象、且可用于预测材料体系的吸收边发生非线性移动的温度转折点,对相关光电器件设计和研制具有一定的实际意义。研究结果还表明,用吸收谱和PL谱表征碲镉汞光学参数时:i)在极低温（如10 K以下）下,吸收谱获得的碲镉汞光学带隙Eg值比较可靠;ii)在高温（如77 K以上）下,PL谱测定的Eg值更接近实际值;iii)数值模型计算结果获得的Stokes位移随温度的演变规律,与PL谱一样可用于估算碲镉汞样品中浅杂质能级位置。2.铜锌锡硫材料:禁带宽度Eg是半导体材料的重要物理参数,精确测定它对研制高效光电器件、如太阳能电池等具有重要意义。利用变条件反射谱、透射谱、PC谱和PL谱,对基于钠钙玻璃为衬底、利用射频磁控溅射法制备的CZTS光伏电池进行了光学带隙和杂质态的分析和研究。在变温度、变激发功率以及变外加偏压等外扰条件下,四种光学表征手段获得了:i)室温下,基于反射谱获得的CZTS薄膜和器件的禁带宽度分别是1.29eV、1.33 eV;4 K到300 K,透射谱测得的CZTS薄膜禁带宽度稳定在1.34 eV;对于CZTS器件,4 K到300 K,PL谱测得的对应带尾态到杂质能级跃迁能量是1.13 eV到1.16 eV;对于CZTS器件,零偏压下,4 K到300 K,PC谱测得的禁带宽度从1.392 eV下降到1.370 eV。这表明,运用不同的光谱方法测定的禁带宽度Eg值存在略微差别,主要是由于不同的光谱方法与CZTS样品内部结构产生不同的物理作用;ii)借助变偏压下的PC谱手段,研究了CZTS光伏器件的禁带宽度（一阶导）随温度、辐照光强等的变化,发现了偏压调控CZTS器件光学带隙的规律,为研究CZTS器件结区附近的非平衡载流子复合机制提供了新的思路和解释。更多还原

【Abstract】 The band edge electronic structures of semiconductor materials or devices,e.g.,mainly including the optical bandgap（Eg）and the impurity levels,are key physical parameters,which essentially influence the photoelectric properties of devices.Transmission/absorption spectra,reflection spectra,photoluminescence（PL）spectra and photocurrent（PC）spectra based on the Fourier transform infrared（FTIR）spectrometer are commonly used for characterizing semiconductor materials’electronic structure.These optical spectroscopic techniques are non-destructive for material characterization,except for the PC spectra that is exclusively for opto-electronic devices,which can provide some complimentary or verified information in bandage electronic energy levels.In this paper,we applied these spectral techniques to research two kinds of semiconductor material systems including Hg_1-xCd_xTe（MCT）that is used in infrared detectors and Cu₂ZnSnS₄（CZTS）that is used as the absorption layer of solar cells.Systematic research and analysis on semiconductor materials’bandgap and impurity/defect levels are implemented.Obtained results were listed as following:1.Hg_1-xCd_xTe:Absorption spectra and PL spectra were applied to comparatively study the two kinds of P-type doped MCT material due to the intrinsic cation vacancy(V_Hg)prepared by liquid phase epitaxial（LPE）process and due to the extrinsic arsenic（As）element doping by molecular beam epitaxial（MBE）process.We mainly focused on materials’bandgap in dependence of temperature by transmission/absorption spectra and PL spectra.Comparing the Eg measured by the“intrinsic”absorption edge of the absorption spectra with the Eg^PL measured by PL spectra,we found that the Stokes shift(the difference between Eg and Eg^PL)appeared more evidently and even the sign evolved from positive to negative with the increase of temperature.By calculating the Fermi level（E_F）as a function of temperature and the other key parameters including the carrier density and the conductivity of the MCT material system by referring to the existent physical parameter model,we found that the results not only confirmed the experimental phenomenon,but also predicted the critical temperature point where the nonlinear shift of the absorption edge occurred.This can provide practical significance for the design of the related photoelectric devices.The results of characterizing MCT samples by absorption and PL spectra also disclose that:i)it is appropriate to use absorption spectra to measure the band gap of doped MCT（Eg）at low temperature（10K or lower）;ii)PL spectra at high temperature（77K or higher）is suitable to measure the Eg,which is closer to the actual value;iii)the Stokes shift dependence on temperature can be employed to determine the shallow impurity level in semiconductor material,as done by PL spectra.2.Cu₂ZnSnS₄:For the photovoltaic material,e.g.CZTS,which is a promising solar cell material for application,the band gap Eg is an important physical parameter that should be determined accurately.We utilized transmission/absorption spectra,reflection spectra,PL spectra and PC spectra under different experimental conditions to systematically research the Eg and the impurity levels in CZTS photovoltaic materials and devices prepared by radio frequency magnetron sputtering,of which the substrate is the soda-lime glass.Under the experimental conditions of variable temperature,excitation power and/or external bias,we acquired by four spectroscopic characterizing techniques:i)The measured Eg are 1.29 eV and 1.33 eV for CZTS film and device,respectively,by reflection spectra at room temperature,the measured Eg stabilize the value of 1.34 eV for CZTS film by transmission spectra from 4 K to 300 K,the measured value related to transition from the conduction band tail to the impurity level is 1.13 eV to 1.16 eV from 4 K to 300 K for CZTS device by PL spectra,the measured Eg drops to 1.37 eV from 1.39 eV from 4 K to 300 K for CZTS device by PC spectra under no bias voltage.As above,the Eg values determined by different spectroscopies showed a little discrepancy,which is originated from different physical mechanisms of spectral techniques;ii)we applied the first-derivative curves of PC spectra under variable bias to study Eg of CZTS photovoltaic devices in dependence of temperature and irradiation intensity,and obtained the law to control the CZTS devices’bandgap by the external bias,which provided a new view and explanation for non-equilibrium carriers recombination mechanism nearby the P-N junction of CZTS devices.更多还原