Band tail state related photoluminescence and photoresponse of ZnMgO solid solution nanostructured films

Abstract

A series of Zn_{1- x} Mg _x O thin films with the composition range x = 0.00-0.40 has been prepared by sol-gel spin coating on Si substrates with a post-deposition thermal treatment in the temperature range of 400-650 °C. The morphology of the films was investigated by scanning electron microscopy and atomic force microscopy while their light emission properties were studied by photoluminescence spectroscopy under excitation at 325 nm. It was found that annealing at 500 °C leads to the production of macroscopically homogeneous wurtzite phase films, while thermal treatment at higher or lower temperature results in the degradation of the morphology, or in the formation of ZnO particles embedded into the ZnMgO matrix, respectively. Local compositional fluctuations leading to the formation of deep band tails in the gap were deduced from photoluminescence spectra. A model for the band tail distribution in the bandgap is proposed as a function of the alloy composition. Thin films were also prepared by aerosol spray pyrolysis deposition using the same sol-gel precursors for the purpose of comparison. The prepared films were tested for photodetector applications.

Keywords: ZnMgO semiconductor alloy; aerosol spray pyrolysis deposition; energy band tails; photodetector; photoluminescence; photosensitivity; spin coating; thin films.

Figure 1

a) SEM images of ZnMgO films deposited on p-Si substrates by spin coating (left column) and aerosol deposition (right column) methods. b) Graphical representations of the AFM profiles for ZnMgO films.

Figure 2

a) SEM image of a ZnMgO film prepared by aerosol spray pyrolysis. b) SEM image of a ZnMgO film prepared by spin coating and annealed at 500 °C. c) SEM image of a ZnMgO film prepared by spin coating and annealed at 650 °C.

Figure 3

Elemental composition of a ZnO (a) and a Zn_0.6Mg_0.4O (b) film determined by EDAX analysis.

Figure 4

PL spectra of Zn_1−xMg_xO films deposited by spin coating with x values of 0.00 (1); 0.05 (2); 0.15 (3); and 0.40 (4), annealed at 500 °C and measured at a) 300 K and b) 20 K.

Figure 5

PL spectra of Zn_1−xMg_xO films deposited by spin coating with x values of 0.00 (1); 0.10 (2); and 0.25 (3), annealed at 400 °C and measured at a) 300 K and b) 20 K. For comparison, the spectrum of a bulk ZnO single crystal is shown by curve (4).

Figure 6

a) XRD pattern of a Zn_0.8Mg_0.2O film deposited by spin coating on a Si substrate and annealed at 500 °C. b) XRD pattern of a Zn_0.8Mg_0.2O film deposited by spin coating on a glass substrate and annealed at 400 °C.

Figure 7

A model for the band tails distribution at 20 K in Zn_1−xMg_xO films with the x value composition of a) 0.10; b) 0.15; c) 0.25 and d) 0.40.

Figure 8

PL spectra of Zn_0.85Mg_0.15O films by spin coating (curve 1) and aerosol spray pyrolysis (curve 2) measured a) at room temperature and b) at 20 K with a linear intensity axis.

Figure 9

Current–voltage characteristics in dark and under UV illumination for a p-Si/n-Zn_0.9Mg_0.1O heterostructure plotted on linear (a), semi-logarithmic (b) and double logarithmic coordinates (c).

Figure 10

Current–voltage characteristics in dark and under UV illumination for a p-Si/n-Zn_0.6Mg_0.4O heterostructure plotted on linear (a), semi-logarithmic (b) and double logarithmic coordinates (c).