Structural and optical characterization of Zn doped CdSe films

https://doi.org/10.1016/j.apsusc.2004.03.252Get rights and content

Abstract

Undoped CdSe and CdSe:Zn thin films have been grown on silicon substrate by using pulsed laser deposition technique. The electrical, structural and optical properties have been investigated. The films grow crystalline and highly oriented. Electrical measurements show that they are n-type doped. The reflectivity and photoluminescence are consistent and point out that the undoped CdSe film present excitonic features at low temperature, differently from CdSe:Zn films, whose spectral features are related to band–band transition. The luminescence efficiency of CdSe:Zn persists up to room temperature, whereas the luminescence of undoped CdSe is scarcely visible above 250 K.

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]:σd(T)=σ1expEa1kBT2expEa2kBTwhere 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)

References (14)

  • S Permogorov et al.

    J. Lumin.

    (1992)
  • G Perna et al.

    Solid State Commun.

    (2000)
  • G Perna et al.

    Appl. Surf. Sci.

    (2003)
  • S Antohe et al.

    J. Cryst. Growth

    (2002)
  • K.C Sharma et al.

    Jpn. J. Appl. Phys.

    (1992)
  • P.P Hankare et al.

    Semicond. Sci. Technol.

    (2004)
  • E.O Kane

    Phys. Rev.

    (1969)
There are more references available in the full text version of this article.

Cited by (0)

View full text