Low-temperature electrical conductivity and optical absorption edge of ZnO films prepared by chemical vapour deposition

Abstract

The optical and electrical properties of c-axis-oriented ZnO films prepared by chemical vapour deposition (CVD) in oxygen atmosphere containing water vapour are investigated. The minimum dc resistivity gives 2.20 12 cm at a film formation temperature of 550 "C. The temperature dependence of electrical conductivity for the films indicates the band conduction obeying Arrhenius plots for temperatures between 170 and 297 K, which involves thermionic and thermal field emissions over grain boundaries in the films. The donor levels, Ed, produced by interstitial Zn atoms are between 0.02 and 0.20 eV. It is found that the variable-range hopping conduction occurs in the films at temperatures lower than about 170 K. The films are transparent in the visible range and have sharp ultraviolet absorption edges at 380 nm wavelength. The optical band gap energy of the films lies in a range of 3.19 to 3.23 eV. From the analysis of the Urbach tail at the absorption edge the width of the tail of localized states E, = 0.09 to 0.10 eV is obtained. It is concluded that the localized levels are situated in a range E , extending from near the edge of the conduction band into the band gap.Les propriete optiques et electriques des couches ZnO orientees a axe c et preparees par CVD dans I'atmosphere oxygtne contenant la vapeur d'eau sont ttudiees. La resistivitk a C.C. minimale de 2.20 R cm est donnee pour une temperature de 550 "C de la formation de couche. La dependence de temperature de la conductivitt: klectrique des couches indique la conduction due a bande qui obei aux traces Arrhenius de 170 a 290 K de temperature, et qui implique les emissions thermoionique et au champ thermique a travers des frontieres de borne. Les niveaux de donneur, Ed, produit par les atomes Zn interstitiels sont entre 0.02 et 0.20 eV. I1 see trouve que la conduction due a "variable-range hopping" a lieu dans les couches aux temperatures inferieures a 170 K. Les couches sont transparentes dans le domaine visible et ont les bords tranchants d'absorption ultraviolette a une longeure d'onde 380 nm. L'interstice de bande optique dknergie des couches a une portee de 3.19 a 3.23 eV. A partir de I'analyse du bout d'Urbach au bord d'absorption, on obtient une largeur du bout pour les &tats localists E, = 0.09 a 0.10 eV. On conclut que les niveaux localists se situent dans l'etendue de ceux-ci due a la bande aux impuretes i tout au-dessous du bord de la bande a conduction.') 11 17 Kitakaname, Hiratsuka-shi, 259-12 Kanagawa, Japan. 35 physica (a) 148/2 486 Y. NATSUME, H. SAKATA, and T. HIRAYAMARecently we obtained polycrystalline and c-axis-oriented ZnO films prepared by atmospheric pressure CVD [12, 131. The films were transparent in the visible region and exhibited sharp absorption in the UV region, and variable-range hopping conduction [14] as a result of electron tunnelling between localized states occurred at temperatures lower than about 170 K. The variable-range hopping conduction of electrons has already been reported for films of amorphous H,WO, [15] as well, but...

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