Burstein shift of the fundamental absorption edge of tin telluride

Solid State Communications Vol. 4, pp.423-424, 1966. Pergainon Press Ltd. Printed in Great Britain.

BURSTEIN SHIFI’ OF THE FW~DAMENTALABSORPTION EDGE OF TIN TELLURIDE R. B. Schoolar, H. R. Riedl, and J. R. Dtxon U. S. Naval Ordnance Laboratory, Silver Spring, Maryland. (Received 15 June 1966 by E. Burstein)

A Burstein shift of the fundamental absorption edge of SnTe has been observed. The band gap is estimated to be less than 0.3 eV at 300°K.

PREVIOUSLY, a steep absorption edge at 0.5 eV was observed in the infrared absorption spectrum of a single-crystal film of tin telluride. 1 The edge was attributed to the fundamental absorption corresponding to direct transitions across the forbidden energy seperating the conduction and valence bands. It was suggested however, that the edge was shifted by a significant amount from the energy of the band gap because of the high carrier concentration of the film (~1 x 1020 holes/cm3 ) producing a large Burstein effect. 2

upon the good agreement between the experimental data and the calculated curves A and B. Curyes A and B were calculated for the two different carrier concentrations using classical free6 and values for the carrier dispersion susceptibility mass,theory optical dielectric constant, and carrier scattering time which have been reported previously. ~ The calculated curves depart from the data at the onset of the valencecondition, interband absorptions. The dashed lines in the figure represent the interband absorptions, obtained by subtracting the calculated free-carrier component from the total absorption.

Recently, single crystal films of lower hole concentrations have become available. ~ The absorption spectrum for one such concentration is compared to the earlier absorption data in Fig. 1. Both curves are room-temperature results calculated from transmittance and reflectance measurements made at normal incidence. The effects of multiple beam inference and the film backings were taken into account in the analysis, as described previously.’ The absorption at the lower photon energies is due to free carriers. This assertion is based

The significance of the new data is that (a) it confirms the earlier assumption’ that these interband transitions are initiated near the Fermi level (since the edge shifts with hole concentration), and (b) it provides a basis for a better estimate of the band gap. From the onset of interband absorption observed in the lower concentration data, the band edge gap is estimated to be less than 0.3 eV.

References 1. BYLANDER E. G., DIXON J. R., RIEDL H. R. and SCHOOLAR R. B., Plivs. Rev. ~ (1965). 2. BURSTEINE., Phvs. Rev. 93, 632 (1954). 3. RIEDL H. R., SCHOOLAR R. B. and HOUSTON B., (to be published). 423

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424

FUNDAMENTAL ABSORPTION EDGE OF SuTe

Vol. 4, No. 9

4. SCHOOLAR It. B. and DIXON J. R., Phys. Rev. 137, A667 (1965). 5. DIXON J. R. and RIEDL H. R., Phys.. Rev.

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A873 (1965).

6. RIEDL H. It., DIXON J. R. and SCHOOLAR R. B., Solid State Commun. 3, 323 (1965); RIEDL H. R. and SCHOOLAR R. B., Bull. Am. Phys. Soc. 11, 348 (1966~

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The absorption coefficients at room temperature for single crystal SnTe films having two different carrier concentrations. Curves A and B are calculated free-carrier absorptions. The dashed curves represent the interband con-

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tributions to the total absorptions.

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PHOTON ENERGY (eV)

Un de place du Burstein de absorption fundamental de SnTe a étê thserv~e.Labande interdite estime ètre <0. 3 eV ~.

300°K.