Persistent Photoconductivity in Bulk Strontium Titanate
- PDF / 185,011 Bytes
- 5 Pages / 432 x 648 pts Page_size
- 16 Downloads / 260 Views
Persistent Photoconductivity in Bulk Strontium Titanate Matthew D. McCluskey,1 Caleb D. Corolewski,1 Violet M. Poole,1 and Marianne C. Tarun1 1
Washington State University, Pullman, WA 99164-2814, U.S.A.
ABSTRACT Strontium titanate (SrTiO3) has novel properties, including a large temperature-dependent dielectric constant, and can be doped to make it metallic or even superconducting. The origin of conductivity observed at the SrTiO3/LaAlO3 interface is a topic of intense debate. In the present work, bulk single crystal SrTiO3 samples were heated at 1200°C, with the goal of producing cation vacancies. These thermally treated samples exhibited persistent photoconductivity (PPC) at room temperature. Upon exposure to sub-band-gap light (>2.9 eV), the free-electron density increases by over two orders of magnitude. This enhanced conductivity persists in the dark, at room temperature, for several days with essentially no decay. Light excites an electron from the vacancy to the conduction band, where it remains, due to a large recapture barrier. These observations highlight the importance of defects in determining the electrical properties of oxides and may point toward novel applications. INTRODUCTION Oxide semiconductors and insulators are used in a broad range of devices and are actively investigated for applications in energy production and storage, optoelectronics, and transparent electronics. The range of properties exhibited by oxide materials provides numerous possibilities for novel electronic devices [1]. Oxides show physical phenomena including ferroelectricity [2], high-temperature superconductivity [3], and colossal magnetoresistance [4]. These materials have a complex phase behavior that is often highly sensitive to temperature, doping, or the application of stress. Heterointerfaces may produce unexpected properties. Famously, the interface between the insulating oxides strontium titanate (SrTiO3) and lanthanum aluminate (LaAlO3) results in a highly conductive layer [5,6]. The physical mechanisms behind this conductivity are a subject of intense debate. Our initial studies investigated hydrogen in SrTiO3. In as-grown SrTiO3 bulk samples, hydrogen forms an O-H bond with a host oxygen atom, resulting in a bond-stretching vibration near 3500 cm-1 [7]. In samples annealed in hydrogen, Tarun and McCluskey [8] found this peak (HI) along with new peaks at 3355 and 3384 cm-1 (HII). The corresponding O-D peaks showed the expected isotope shift to lower vibrational frequency. Samples annealed in a hydrogendeuterium mixture showed new HII peaks. These peaks corresponded to HD centers, showing that the HII center consists of two hydrogen atoms. The microscopic structures of these hydrogen complexes are not known. Theoretical studies have suggested that the HI line arises from an isolated hydrogen interstitial [9] while HII represents a pair of hydrogen interstitials that are attracted to each other via lattice strain [10]. Tarun and McCluskey [11] attributed HII to a strontium vacancy that is fully passivated by two
87
Data Loading...
