Thermoreflectance of CdB near the Fundamental AbsorDtion Edge BY M. ILIEV, I. AMENOV, andS. STOYANOV The temperature-modulated spectra of absorption and reflectance in Cd9 were measured at room temperature by Berglund ( l ) and Lange and Henrion (2). However, because of thermal broadening the structure near the fundamental absorption edge cannot be definitely resolved. Thus, it is of considerable interest to study temperature-modulated spectra at low temperatures. In the present paper we report the results of thermoreflectance studies for CdS single crystals at low temperature.In this case the thermal broadening is so reduced and the peak height so increased that the peaks which appear near the fundamental absorption edge can be eaeily identified with the excitonic transitions.
Recently many publications have been devoted to the reflectance spectra near the band edge of ZnTe /1 to 3/. The data reported differ remarkably. In this letter we focus our attention upon one of the possible origins of the discrepancies. The data have been obtained at a constant temperature. Undoped and P-doped ZnTe crystals, grown by the Bridgman technique, were used. The samples with dimensions n 2x2x(0.15 to 0.2)mmJ were mechanically polished with a 0.25 p m particle abrasive, etched with a H SO -K C r 0 -H 0 mixture, and finally rinsed with deionized water.
4 7 2A conventional thermomodulation technique, described in /4/, was used. The 0 monochromatic light beam was reflected from the c r y s t a l surface at an angle of 30 to fall onto a multiplier. The spectral resolution of the optical system was X I 2 meV.
Systematic investigations yielded the following results:i) The observed thermoreflectance (TR) structure is not typical for the classical TR behaviour near a discrete exciton state /5/. The spectrum consists of two negative peaks and a relative narrow positive peak, nearly a s high a s the negative ones.ii) The high-energy negative peak i s more intensive than the lm-energy negative one. Both negative peaks a r e asymmetric to different sides (Fig. 1). The positive peak is nearly symmetric.iii) The general features of the TR spectra a r e similar for the different samples investigated. However, the intensity, the width, and the energy position of the TR peaks change considerably from sample to sample, e.g. the observed energy separation between the negative peaks varies from 4 to 9 meV, the width of the peaks changes 2 to 3 times. iv) All these quantities depend essentially on the mechanical and chemical treatment of the crystal surface (see Fig. l), too. After a given treatment, however, the TR spectra are very well reproducible.
Diffuse reflectance, diffuse thermoreflectance (DTR), and specular thermoreflectance (TR) of the ferromagnetic semiconductor CdCr2Se4 are measured in the region of photon energies from 1.2 to 3.3 eV, between 95 and 320 °K. The TR spectra clearly show that a magnetic‐order‐induced band splitting appears below the Curie‐Weiss temperature θc ≈200 °K. The results are consistent with the theory of spin‐polarized bands and explain unambigously the anomalous behaviour of the absorption edge in the ferromagnetic semiconductors. A simple model is proposed for the optical band‐to‐band transitions in CdCr2Se4.
The influence of the magnetic ordering on the intraionic d‐d transitions of Fe3+ of pure and Gadiluted YIG is investigated by means of diffuse thermoreflectance (DTR). It is shown that the DTR peak intensity passes through a maximum at the Curie point. The relative decrease of the peaks corresponding to the tetrahedral transition in the Ga‐diluted YIG gives evidences for the dominant distribution of the Ga3+ ions in the tetrahedral sites.
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