Identification of Deep-Level States in Electronic Materials by Optically Stimulated Deep-Level Impedance Spectroscopy

Optically s t imulated deep level impedance spectroscopy (OS-DLZS) is suggested for analysis of electronic transi t ions involving deep-level states in semiconductors with large bandgaps. The technique is based on interpretat ion of both the real and imaginary components of the impedance response measured over a continuous range of electrical frequencies under sub-bandgap il lumination. The applicat ion of OS-DLZS is i l lustrated for a ZnS:Mn electroluminescent panel and for ZnO varistors. The lower frequency range of the impedance spectrum is shown to be more sensitive to electronic transitions caused by monochromat ic sub-bandgap i l lumination than is the capacitance measured at high frequency. Optically s t imulated deep level impedance spectroscopy (OS-DLZS) is suggested here for characterization of electronic transi t ions in large bandgap semiconductor devices. This technique involves analysis of both real and imaginary components of the impedance response over a cont inuous range of appl ied frequency under the influence of sub-bandgap illumination. The response of deep-level states is dr iven by an electrical per turbat ion ( imposed as a s inusoidal variat ion of the appl ied potential) and an oPtical perturbat ion. The appl ied potential bias can also be changed to alter the steady-state occupancy of deep-level states in the space charge region. In contrast to most spect roscopic techniques, OS-DLZS can be regarded as encompassing two frequency domains, one that is electrical and one that is optical. The potent ial uti l i ty of employing a broad electrical frequency range is consistent with exper imenta l observation that surface states have the largest influence on the impedance response at low frequehcies (1,2) and that the space charge capaci tance is obtained most easily from high frequency measurements . The appl icat ion of sub-bandgap optical exci tat ion of deep-level states was suggested by the body of work descr ibing electrochemical photocapacitance spect roscopy (EPS) (3-6). The emphasis on interpretat ion of both the real and imaginary components of the impedance was driven by the results of a mathematical model (7-10) that t reated the influence of deep-level states on the impedance response of a semiconductor by solving the equations which govern the physics of the system, e.g., Poisson 's equation, conservation equations for electrons and holes, and homogeneous and heterogeneous rate expressions for generation and recombination. The model ing work suggested that, through use of monochromat ic subbandgap optical excitation, the influence of even low concentrat ions of deep-level electronic states could be seen on the real part of the impedance measured at low electrical frequencies. The object ive of this work was to demonstra te the feasibi l i ty of the OS-DLZS method. The exper imenta l data presented here are pre l iminary in that the optical spectrum is represented by only a few selected optical energy levels. Future exper imenta l work will entail use of smaller increments of optical wavelength and use of various constant temperatures to freeze out selected states, e.g., thermally s t imulated deep level impedance spectroscopy (TSDLZS). The mathemat ical model (7, 8) will be refined to allow extract ion of physical parameters by comparison and regression to exper imenta l data.