Spectroscopic Characterization For The Bandedge Electronic Structure Of Hg_1-xCd_xTe And Cu₂ZnSnS₄ Materials

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.