Electroabsorption of CdS and CdSe Single Crystals in the Exciton Region

Abstract

Electroabsorption spectra in the exciton region were measured on very thin CdS and CdSe single crystals at 77 O K in the field range 1 x lo3 to 2.4 x 104 V/cm for both polarizations of incident light with respect to the c-axis. The electroabsorption spectra exhibit much more details than electroreflection spectra know so far. The variation of peak positions and peak heights with electric field strength is different for different parts of the spectrum. Some of the peaks shift to lower and others to higher energies with increasing field strength. The spectra can be interpreted in terms of a second order Stark shift, field induced line broadening and Franz-Keldysh type interband oscillations.An sehr diinnen CdS-und CdSe-Einkristallen wurde die Elektroabsorption im Bereich der Exzitonenabsorption bei 77 O K im Feldstiirkebereich von 1 x 109 bis 2.4 x 104 V/cm fur Polarisation des Lichtes parallel und senkrecht zur hexagonalen Achse untersucht. Die Elektroabsorptionsspektren zeigen eine betrgchtlich grodere Feinstruktur als die bishen bekannten Elektroreflexionsspektren. Die Variation der Lage und der Hohe der verschiedenen Extrema mit der elektrischen Feldstiirke ist in verschiedenen Bereichen des Spektrums unterschiedlich. Einige der Extrema verschieben sich mit anwachsender Feldstiirke zu kleineren, andere zu hoheren Energien. Die Spektren lassen sich auf der Grundlage des quadratischen Stark-Effektes, einer feldinduzierten Linienverbreiterung und von Interbandoszillationen veratehen.

Excitonic electroabsorption spectra and Franz-Keldysh effect ofIn0.53Ga0.47AsJaeger1, Weiser2 1998Phys. Rev. B271130View full textAdd to dashboardCite

“…29͒ a weak redshift is observed at low field but field broadening dominates the electroabsorption spectra, which again show no oscillations above the gap. 10 To our knowledge no Franz-Keldysh oscillations are reported for samples with pronounced excitons. This is not surprising because large exciton binding energy is related to a larger reduced mass which is unfavorable for FranzKeldysh oscillations by two reasons: ͑i͒ the field required for a certain value of ប⌰ increases quadratically with the mass and ͑ii͒ the smaller mobility resulting from a larger mass reduces the mean free path for coherent acceleration.…”

“…In high mobility semiconductors the exciton binding energy was too small to avoid strong mixing of exciton and band states by the relatively large fields near the surface while in the case of larger binding energy only an excitonic response was observed and Franz-Keldysh oscillations did not develop. [10][11][12] Modern epitaxy improved the experimental situation by growing thin single crystals of high quality. If grown on a transparent substrate field-induced changes of the transmission can be studied which depends little on a surface field and employs the relatively homogeneous field inside a sample.…”

Thermo‐absorption of CdS and CdSe in the exciton absorption region

“…A s according to (2) and (3) the A, B ground state exciton lines of CdS and CdSe exhibit Lorentzian line shape in the temperature range considered in this note, the half-widths can be determined from the energetic positions of the maxima and minima of the A1 /I spectrum of these lines, using the formulae reported in (5). The results are compiled in Table 1.…”

“…shaped with an area of 2x3 mm and a thickness of (3 This might be accounted for by a larger contribution of broadening to AI/I than for the A, B, n = 1 lines. Carrying out &-I analysis on the basis of the formulae given in (5) and taking for dEex/dT the value reported in (2) it is found that for CdS at 94 OK for the C, n = 1 line dr/dT = 2x10 eV/deg. This value is about five times that for the A, B, n = 1 lines.…”

Excitonic thermoreflectance of CdS

“…Therefore, a more precise description of thermo-modulated spectra should also include the influence of the temperature on the position and lineshape of excitonic lines. The importance of this fact was pointed out by Lange and Gutsche [3] for the case of electroabsorption, but their expressions for the relative changes in the imaginary part of the complex dielectric function A&, near an isolated Lorentzian line can be directly used in thermoabsorption studies. I n a similar manner, expressions for the changes A&, in the real part of the complex dielectric function can be derived, thus providing a basis for a qualitive interpretation of the experimental thermoreflectance data.…”