Photoconductivity and transient response of Al:ZnO:Al planar structures fabricated via a thermal oxidation process
Introduction
Bulk and thin-film ZnO materials have attracted a great deal of attention, primarily due to their potential applications as gas sensors, and as electronic and light-emitting nano-devices. In particular, ZnO films and nanowires are especially promising for the realization of nano-scale light emitting devices in the blue–ultraviolet spectral range, and as hydrogen gas sensors for future hydrogen-based technologies [1]. In particular, ZnO-based ultraviolet (UV) photodetectors have attracted a great deal of attention for their light detection properties, with several studies concerning photoconductivity [2], [3]. Applications for UV photodetectors include air-quality monitoring, missile warning systems and gas detection [4], [5], [6], [7].
Zinc oxide-based UV photodetectors have been fabricated in a variety of manners in an effort to optimize performance. Previously, ZnO nanowires and thin films have been synthesized using relatively costly chemical vapor deposition processes [8], [9]. Lower-cost methods such as radio frequency (rf) and direct current (dc) magnetron sputtering have also been employed. A technique for low-cost growth has been demonstrated where zinc-metal films are grown and subsequently thermally oxidized to produce polycrystalline ZnO films [3], [10], [11]. Highly resistive films with polycrystalline structure have resulted, when starting with approximately 200 nm-thick Zn-metallic films [12], [13], [14]. These ZnO films have been utilized as a base for ZnO ultraviolet photodetectors, with various metals applied as contacts for photodetectors having a metal–semiconductor–metal (MSM) planar structure [6], [11], [15], [16].
Persistent photoconductivity has been reported for ZnO-based UV photodetectors [17]. This phenomenon occurs when electromagnetic radiation is removed from exposed photoconductors after the point of photocurrent saturation. Rather than immediately returning to a base level of photocurrent, typical relaxation times for decreasing photocurrent are observed to range from hours to several days [17]. Often, persistent photoconductivity is attributed to the rates of various excitation–recombination mechanisms. A two-step photocurrent relaxation, consisting on an initial sharp drop in photoconductivity followed by a slow decline, appears throughout the previous literature [17], [18], [19], [20].
We have grown ZnO films on sapphire substrates via thermal oxidation of Zn-metal films. Photodetectors with ohmic Al contacts were fabricated having a MSM planar structure. We report on the morphological and optical properties of the base films, as well as UV photoresponse and persistent photoconductivity of the fabricated photodetectors. Furthermore, we present a phenomenological model for the persistent photoconductivity based on a two-step relaxation.
Section snippets
Experimental details
In this study, zinc films were deposited on c-plane sapphire substrates via direct current sputter deposition without reactive gas. Sapphire substrates were chosen because of their UV transparency and thermal properties conducive to high temperature annealing. Metallic 1″ diameter 99.99% purity zinc targets were obtained commercially and mounted on a water-cooled stage. Before deposition, substrates were cleaned via immersion in acetone, ultrasonically cleaned in methanol/acetone and rinsed in
Structure and morphology
The evolution of the surface morphology with increasing annealing temperature is shown in the AFM images presented in Fig. 1(a–b). The as-grown zinc film demonstrates a high surface roughness and approximately 100 nm diameter protrusions (not shown). Fig. 1(a–b) shows the surface morphology of resulting ZnO films annealed at 300 °C and 600 °C. There is very little change in the underlying characteristics between zinc-metallic films and ZnO films annealed at 300 °C; however, surface roughness is
Kohlrausch stretched-exponential function analysis
In previous literature, the traditional approach to describe transient behavior has been to use a Kohlrausch stretched exponential function analysis, which is used to describe decays in the presence of an energy transfer in disordered systems [20], [26], [27]. The Kohlrausch function is a convenient and relatively flexible fitting function, consisting of a “stretching” parameter γ that distinguishes it from a classical exponential decay. The Kohlrausch function is as follows:where A
Conclusions
In summary, we have investigated the photoconductivity of polycrystalline zinc oxide (ZnO) films grown on c-plane sapphire substrates. Zinc-metal films where grown on sapphire substrates via dc-sputter deposition at room temperature with subsequent thermal annealing in air at 300 °C, 600 °C, 900 °C, and 1200 °C. Photoluminescence spectra indicate four emission bands: excitonic ultraviolet, blue, and deep-level green and yellow emissions. The ratio of deep-level green emission to UV excitonic
Acknowledgments
The National Science Foundation's Division of Materials Research (DMR #1104600) provided funding for this project through the Research at Undergraduate Institutions (RUI) program. PL data where acquired in the laboratory of Dr. Michael Reshchikov and XRD patterns were acquired in the lab of Dr. Everett Carpenter at Virginia Commonwealth University in Richmond, VA.
References (30)
- et al.
Sens. Actuators B Chem.
(2002) - et al.
Mater. Lett.
(1998) - et al.
J. Cryst. Growth
(2002) - et al.
Surf. Coat.
(2005) - et al.
J. Cryst. Growth
(2003) - et al.
J. Cryst. Growth
(2005) - et al.
Sensors Actuators A. Phys.
(2007) - et al.
Mater. Sci. Semicond. Process.
(2002) - et al.
Chem. Phys.
(2005) - et al.
J. Appl. Phys.
(2005)
Proc. SPIE
J. Appl. Phys.
J. Appl. Phys.
J. Vac. Sci. Technol.
Proc. SPIE
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