Planck's constant from a CdS photoconductivity cell

Abstract

How a light-sensitive semiconductor can be used to obtain an estimate of Planck's constant .

Preparation of semiconducting materials in the laboratory: Production of CdS thin films and estimation of their band gap energy

“…Even though this procedure is not very accurate, the results thus obtained are satisfactory for our purposes, i.e., kg from Figures 5a, b, c, d are found to be 519, 490, 515, and 523 nm (note that the thicker film (d), deposited under high vacuum, requires a larger absorbance scale and that it yields an undulation due to light interference patterns). By using eq 1, the Eg's calculated are 2.39,2.53,2.41, and 2.37 eV, which can be compared to the reported value of 2.42 eV (16,19). In order to obtain more accurate values of Eg, the nature of the optical transitions in the SC (i.e., direct or indirect) would need to be elucidated by calculating the functions (ahv )m, (a/tv)2and (ahv)2/3 where a = absorption coefficient, and plotting them versus hv for each data point (23,35); however, this procedure requires as input the a values, which, in turn, require knowledge of the thickness of each film deposited.…”

“…Nevertheless, the much simpler procedure used above provides a good and practical qualitative understanding of the theoretical concept of Eg, as well as a simple and semiquantitative technique for its evaluation. Two alternative procedures for evaluating Flg in commercially available CdS optoelectronic devices have been reported in this Journal (19).…”

“…Electrons can be excited from the VB to the CB by the absorption of incident radiant energy only if this energy is equal to or larger than Eg. Thus, absorption of light occurs when the energy of a quantum (hv) equals Eg (1,19):…”