ABSTRACT
Experimentally, the optical properties determined by the electronic band structure (UV-visible spectral region, typically 5.5 to 1.5 eV) have been measured using several methods, such as spectral ellipsometry [12], reflectivity [13], angle-resolved photoemission [14], and Faraday rotation [15]. For the bandgap energy, Eg = EG6-EG8, at room temperature (RT) values ranging from 1.49 to 1.53 eV have been reported [16]. It seems that the actual generally accepted RT value of Eg is 1.512 ± 0.003 eV [17]. The value at liquid-helium temperature is well established as 1.606 eV
[1,2,17]. According to Ref. [17], the temperature dependence of the bandgap energy in the range 4-500 K is best fitted by the expression proposed by Pässler in Ref. [18], although the most popular empirical relation proposed by Varshni [19],2gg g( )= (0)– ( + )TE T E Taq , (1.1)still can be used, with the parameters ag = 0.372 meV/K and q= 110 K. A value of (6.5 ± 0.2) × 10-6 eV ∙ bar-1 has been obtained for the pressure coefficient, dEg/dp [9].The spin-orbit splitting energy is DSO ≈ 0.95 eV [14]. Higher energy features in the spectrum of the complex dielectric function, corresponding to the van Hove critical points lie at E1 ≈ 3.3 eV, E1 + DSO ≈ 4 eV and E2 ≈ 5.5 eV [13]. Some further band structure parameters (average of the values that were either calculated or measured experimentally, taken mostly from Refs. [1,2]) are listed in Table 1.1, where P denotes the dipole moment matrix element between the Γ8 and Γ6 bands. This inter-band transition is dipole-allowed and determines the optical absorption of CdTe in the visible range. Table 1.1 Some band structure parameters at T = 300 K Parameter Eg (eV) me mlh mhh P (eV ∙ cm)Value 1.512 0.10 0.12 0.5 7 × 10-8
The exciton effect near Eg is weaker than in other II-VI semiconductors (such as CdS or ZnO) but still quite significant. The exciton Bohr radius is aex ≈ 7.3 nm [20] and the binding energy is Rex ≈ 10.1 meV [17]. Free exciton photoluminescence (PL) line and its phonon replicas have been observed [16,17,21] up to room temperature. Moreover, it was shown that even at higher temperatures, T > 300 K, discrete excitonic transitions still strongly influence the absorption edge [18] and its shape deviates from the simple dependence, g g-,( ) ( )E Ewa w ww (1.2)expected for direct transitions within the non-interacting electronhole model [22]. It was fitted using a combination of exponential
and Lorentzian functions [17] (resembling the Elliott formula valid for excitonic absorption at zero temperature [22]).
