Optical Processes in Semiconductors

In steady-state PL, the shape of the spectrum is determined by the level of excitation intensity as the defect-related PL often saturates at power densities on the order of to 10 Wcm, and the overall PL spectrum may be skewed in favor of the excitonic emission at higher excitation densities. Similarly, focusing the laser beam and using small monochromator slit widths would also skew the PL in favor of excitonic transitions. In such a case, the chromatic dispersion of the lenses used to collect the PL, as well as the different effective sizes of the emission spots for the ultraviolet (UV) and visible emission attributed in particular to photon recycling process [24], may lead to a noticeable artificial enhancement of the UV (near band edge) over the visible part in the PL spectrum (mainly defect related). Qualitative terms such as very intense PL attesting to the high quality of the material are omnipresent in the literature on ZnO. In contrast to the wide use of PL measurements, relatively little effort has been made to estimate the absolute value of the PL intensity or its quantum efficiency (QE) for a quantitative analysis. 

The time-resolved PL requires additional instrumentation for capturing the evolution of intensity, such as a fast CCD or a streak camera for detecting very fast transients. However, a simple digital oscilloscope in combination with a pulsed laser may be very useful for measuring the defect-related PL decays in ZnO, which by their very nature are slow and are typically in the range from a few nanoseconds to milliseconds. 

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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Pankove J I 1971 Optical Processes in Semiconductors (New York Dover)... 

G. Pankove, Optical Processes in Semiconductors, Englewood Cliffs (USA) Prentice-Hall, 1971. 

D. S. Chemla, Ultrafast Transient Nonlinear Optical Processes in Semiconductors M. Sheik-Bahae and . W. Van Stryland, Optical Nonlinearities in the Transparency Region of Bulk Semiconductors... 

Oshe, E. K., and Rosenfeld, I. L. (1978). Itogi Nauki Tekh. Korroz. Zashch. Korroz. 7, 111. Pankove, J. I. (1971). Optical Processes in Semiconductors. Prentice-Hall, Englewood Cliffs, New Jersey. 

Two-volume set 17.90 Mathematical Physics, Donald H. Menzel. (60056-4) 8.00 Optical Processes in Semiconductors, Jacques I. Pankove. 

Pankove IL (1971) Optical Processes in semiconductors. Prentice Hall, Englewood Cliffs, UK Knox RS, Dexter KL (1965) Exdtons, New York... 

Pankove, J.L (1975) Chapter 3, Optical Processes in Semiconductors, Dover Publications, New York, p. 38. 

J.I. Pankove, Optical Processes in Semiconductors (Electronic Engineering Series, ed. by N. Holonyak, Jr., Prentice Hall, Englewood Cliff, NJ, 1971)... 

Pankove, J.I., Optical Process in Semiconductors, Dover, New York, 1971. 

Pankove, J.J. 1971. Optical processes in semiconductors. New York Dover Publication. 

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