Structural and optical characterization of Zn doped CdSe films
Introduction
Thin films of II–VI semiconductors are interesting for photovoltaic, photodetection and optoelectronic application [1], [2]. In particular, considerable attention has been devoted to the possibility of tailoring the optical and electrical properties of these materials. This purpose has been mainly achieved by means of two different processes: (i) the doping with different dopant density, which causes the broadening of intragap impurity bands and the formation of band tails and band gap renormalization [3], [4]; (ii) the fabrication of ternary and quaternary alloys, whose band gap can be modulated by controlling the relative concentrations of two elements forming the alloy (for example, CdSxSe1−x, ZnSxSe1−x, CdxZn1−xSe, etc.) [5].
In the last years, we have successfully grown CdSxSe1−x [6], CdSe:In [7] and ZnSe [8] films by means of pulsed laser deposition (PLD) technique, which has become a widespread deposition method for its advantages as the simplicity, low cost and possibility of grow stoichiometric thin films highly oriented and with good optical properties. Our next goal is the deposition of the CdxZn1−xSe alloy, which permits to modulate the band gap between 1.75 eV (CdSe) and 2.70 eV (ZnSe) at room temperature. The first step of our aim is the deposition of CdxZn1−xSe for low density of Zn.
Therefore, in this work we mainly discuss experimental data of CdSe:Zn films with a Zn density of about 1% with respect to Cd and compare them with the data of undoped CdSe films. The CdSe:Zn films show a crystalline quality, as deduced from X-ray diffraction and Raman measurements, and a photoluminescence (PL) efficiency related to band–band emission and persisting up to room temperature.
Section snippets
Experimental methods
Undoped and zinc-doped CdSe thin films have been deposited by laser ablating stoichiometric home-made targets, obtained by properly mixing and cold pressing high purity (99.999%) powders of CdSe and metallic zinc powder. The films were deposited on a silicon substrate. For the doped film we fixed the Zn powder weight to 1% of the CdSe powder weight. A pulsed Nd:Yag laser operating at 532 nm was used as the laser source, with a pulse duration of about 10 ns and a repetition rate of 10 Hz. The laser
Electrical measurements
The temperature dependence of the dark conductivity σd from CdSe:Zn and CdSe films is shown in Fig. 1 (dots). It is well fitted (continuous line) to the expression [9]:where kB is the Boltzmann constant. The first term in Eq. (1) refers to the conductivity of the conduction band mechanism, whereas the second term concerns the carriers hopping conduction through localized states due to structural and/or chemical disorder in the band gap. Ea1 and Ea2 are the
Conclusion
Zn doped CdSe thin films have been deposited using the pulsed laser ablation technique, starting from home-made doped CdSe pellet containing 1% Zn weight fraction. Although the structural quality of the CdSe:Zn films is lower than that of undoped CdSe, as shown by XRD and Raman spectra, crystalline films were obtained in both cases with the (0 0 2) preferential orientation of the grains. The reflectivity and PL measurements of undoped CdSe are characterized by narrow (less than 10 meV of FWHM)
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