Photocapacitance

Several methods not based on DLTS have also been described. White et al. (1976) present a two-light source, scanned photocapacitance technique that yields a spectrum of the deep states in the depletion region of a junction. The method is fast and sensitive, but most useful as a survey technique because knowledge of the dependence of the photoionization cross section on photon energy is required to obtain accurate trap depths. 

Fig. 4. Energy below the conduction band of levels reported in the literature for GaP. States are arranged from top to bottom chronologically, then by author. At the left is an indication of the method of sample growth or preparation liquid phase epitaxy (LPE), liquid encapsulated Czochralski (LEC), irradiated with 1-MeV electrons (1-MeV e), and vapor phase epitaxy (VPE). Next to this the experimental method is listed photoluminescence (PL), photoluminescence decay time (PLD), junction photocurrent (PCUR), photocapacitance (PCAP), transient capacitance (TCAP), thermally stimulated current (TSC), transient junction dark current (TC), deep level transient spectroscopy (DLTS), photoconductivity (PC), and optical absorption (OA).

Fig. 9. Energy above the valence band of levels reported in the literature for GaAs. Arrangement and notations are as in Figs. 4,5, and 8. DSDP indicates double source differentiated photocapacitance.

A probable role of an excited metastable state for the oxygen center of GaAs has already been mentioned in connection with our discussion of the configuration coordinate model (Section 10b). This was suggested by Vincent and Bois (1978) to explain a slow decrease of photocapacitance observed after... 

Fig 18. Comparision between the spectral dependence of the photoneutralization cross section (photoluminescence excitation) and with field (dashed line, taken in a junction by photocapacitance). [After Monemar and Samuel-son (1978, Fig. 3), with the photocapacitance data provided by . H. Henry, and data taken at 190°K.]... 

Fig. 87. Photocapacitance spectrum for p-Zn3P2 measured at 5kHz and V - V, = 0.3 V in acetonitrile/0.1 M tetrabutylammonium perchlorate. The inset shows the assignment based on the spectrum.

It is clear, from the foregoing, that luminescence studies will be immeasurably aided by a simultaneous study of the sub-bandgap photocurrent and of the photocapacitance behaviour. 

The optical absorption arising from the defect transitions is weak because of the low defect densities and in a thin film cannot be measured by optical transmission. The techniques of PDS, CPM and photoemission yield, described in Section 3.3, have sufficient sensitivity. Photocapacitance, which measures the light-induced change in the depletion layer capacitance, is similarly sensitive to weak absorption (Johnson and Biegelsen 1985). PDS measures the heat absorbed in the sample and detects all of the possible optical transitions. At room temperature virtually all the recombination is non-radiative and generates heat by phonon emission. CPM detects photocarriers and so is primarily sensitive to the optical transitions which excite electrons to... 

Fig. 4.23. Photocapacitance data for n-type a-Si H corresponding to the excitation of an electron from the doubly occupied defect to the conduction band. The solid line is the calculated fit with the parameters shown (Johnson and Biegelsen 1985).

R. Haak and D. Tench, Electrochemical photocapacitance spectroscopy method for characterization of deep levels and interface states in semiconductor materials, J. Electrochem. Soc. 131 (1984) 275-283. 

Hagfeldt, A., Bjorksten, U., Gratzel, M. Photocapacitance of nanocrystalline oxide semiconductor films band-edge movement in mesoporous Ti02 electrodes during UV illumination. J. Phys. Chem. 100, 8045-8048 (1996)... 

The migration of Fe during low-temperature aimealing was studied by using photocapacitance techniques. The resultant depth profiles revealed the occurrence of Fe out-diffusion, but no precipitation in the bulk, at up to 470K. The Fej diffusion... 

Photocapacitance measurements yield information on the deep acceptor level in CdS as indicated in the band diagram figure 23. 

The basic instnimentation required for acquiring photoluminescence excitation (PLE) spectrum ofa given PL band is nearly the same as that for a PLsetup. However, the excitation source must be a tunable source such as a tunable laser or a broadband lamp dispersed by a monochromator. The wavelength of the excitation source is varied, and the PL spectrum or simply the intensity of a particular transition (such as the peak PL intensity) is recorded at various excitation wavelengths to obtain the excitation spectrum. The PLE spectrum is similar to the absorption spectrum with the only difference that in the case of absorption spectrum several different transitions may contribute and complicate the spectral analysis. Photoionization of a defect is an inverse process to the luminescence, and in n-type ZnO such a process involves the transition of an electron from an acceptor-like level to the conduction band or to the excited state of the defect. Note that the photoionization spectra measured by PLE, absorption, photocapacitance, and photoconductivity methods should have more or less similar features because the mechanism of the photoexcitation is the same for all these approaches. 

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