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Spectral Cathodoluminescence in Semiconductors
Cathodoluminescence emission is the emission of light from a specimen when bombarded with an electron beam in a specially equipped electron microscope. The spectral content of the light emitted from this specimen contains information on the impurities in semiconductors. Spectral CL(Cathodoluminescence) imaging allows for imaging of lateral distribution of impurities. Optically inactive centres may inhibit the CL emission and affect the intensity of the detected CL signal. CL measured as a function of electron beam energy can provide insight into the depth distribution of the relevant centres. At a given beam energy, the generation of CL as a function of depth can be modeled using Monte Carlo techniquies, and it provides information on the density of the impurity at a given distance from the film surface when used in conjunction with the measured spectra. CL kinetics is obtained by measuring CL intensities as a function of electron beam irradiation time. This irradiation in resistive specimens creates a strong electric field, and the electromigration of charged defects and impurities. Cathodoluminescence offers a very powerful capability of studying the evolution of the CL spectra as a function of the excitation current density over a very wide range of currents.
The cathodoluminescence spectra needs to be corrected for the response of the detection and measurement systems, deconvoluted and typically curve fitted using Gaussian curve fitting techniques. The Gaussian peak parameters(energy, FWHM and intensity) can then be carefully monitored as a function of experiment parameters, such as beam energy, temperature and irradiation dose. Once the CL peak positions and halfwidths are known, spectral selected high spatial resolution maps can be measured to provide information on the distribution and relative concentration of the luminescent defects. The CL maps can be compared with the high sensitivity wavelength dispersive spectrometry(WDS) X-ray maps acquired from the same regions using binary logic false colouring overlay techniques to detect regions of high correlation or anticorrelation. These composite CL and WDS maps can then be used to assign CL emission peaks to specific extrinsic impurities.
SL spectroscopy, with its mapping capabilities, can be used to access the defect concentration and/or compensation in a variety of approaches. Detailed empirical studies of the edge emission in a series of carefully calibrated specimens have been used earlier as a benchmark for shallow impurity content and/or compensation in other materials. The properties of the edge emission reflecting the impurity content and compensation include the relative intensities of various spectral features and their line shapes. In order to make an accurate assessment of the line shape, the obtained spectra need to be corrected for the system's response and employ peak-fitting procedures in conjunction with existing theories of the line-shape to arrive at the quantities of interest. The calibrated edge emission can be productively used to determine the impurity concentration over a wide range of densities, likely beteen high 1018cm-3 and 1014-1015cm-3. Importantly, the edge emission can be used in a mapping mode and in the "spot" mode, thus revealing non-uniformities of electrical properties.
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