Figure 1

(a) (Color online) Room-temperature photoconductivity (solid) and electronic absorption (dotted) spectra of a $c\text{-ZnO}$ film ($\sim 100\text{nm}$ thick, 0.1 V applied). Inset: schematic of the experimental configuration used to measure photoconductivity. (b) Room-temperature photoconductivity spectrum (solid) of an $a{\text{-Zn}}_{0.96}{\text{Co}}_{0.04}\text{O}$ film ($\sim 500\text{nm}$ thick, 20 V applied), and expanded view of the $d\text{−}d$ and ${\text{ML}}_{\text{CB}}\text{CT}$ region (solid, x10). Room-temperature electronic absorption spectrum (dotted) of an $a{\text{-Zn}}_{0.90}{\text{Co}}_{0.10}\text{O}$ film ($\sim 500\text{nm}$ thick). All photoconductivity spectra were measured using chopped excitation.Reuse & Permissions

Figure 2

(a) (Color online) Photoconductivity spectra of the $a{\text{-Zn}}_{0.96}{\text{Co}}_{0.04}\text{O}$ film from Fig. 1, measured at 293, 175, 148, 130, 100, and 48 K (20 V applied) with chopped excitation. The data have been normalized at 2.877 eV $\left(\sim 430\text{nm}\right)$. Inset: Photoconductivity in the ${{}^4\text{A}}_2\to {{}^4\text{T}}_1\left(\text{P}\right)$ $d\text{−}d$ region plotted on an expanded scale. (b) Fits (dotted lines) of the ${\text{Zn}}_{0.96}{\text{Co}}_{0.04}\text{O}$ $d\text{−}d$ photoconductivity, measured using chopped (◻) or continuous (△, ○) illumination, using Eqs. (2, 3). Together, the data indicate spontaneous ionization from the ${{}^4\text{T}}_1\left(\text{P}\right)$ level, and thermally assisted ionization from the same level with an energy barrier of $E_{\text{a}}=43\pm 11\text{meV}$ at low temperatures.Reuse & Permissions

Figure 3

(a) (Color online) Room-temperature electronic absorption (dotted) and cw photoconductivity (solid) spectra of $a{\text{-Zn}}_{1-x}{\text{Co}}_x\text{O}$ films measured at $0°$ (thick red) and $90°$ (thin gray) polarization angles relative to the $c$ axis of ZnO. (b) Relative room-temperature cw photoconductivity (solid squares) and electronic absorption (open squares) intensities, integrated between 1.75 and 1.96 eV, as a function of polarization angle relative to the $c$ axis of ZnO. (c) 20 K electronic absorption (dotted) and 27 K cw photoconductivity (solid) spectra measured at $0°$ (thick red) and $90°$ (thin gray) angles relative to the $c$ axis of ZnO. The $\le 8.1\text{nm}$ spectral resolution of the low-temperature photoconductivity measurements is indicated. (d) Relative 27 K cw photoconductivity (filled circles) and 20 K electronic absorption (open circles) intensities, integrated between 1.81 and 1.96 eV, as a function of polarization angle relative to the $c$ axis of ZnO. The dotted lines in (b) and (d) plot a ${\text{sin}}^2\left(\theta \right)$ function. For all absorption data shown, $x=0.10$, and for all photoconductivity data shown, $x=0.04$.Reuse & Permissions

Figure 4

(a) (Color online) Tanabe-Sugano $d\text{−}d$ energy level diagram for tetrahedral ${\text{Co}}^{2+}$ (left), neglecting trigonal and spin-orbit contributions ($10Dq/B\sim 5.0$ in ${\text{Zn}}_{1-x}{\text{Co}}_x\text{O}$). The $d\text{−}d$ excited-state energies in ${\text{Zn}}_{1-x}{\text{Co}}_x\text{O}$ are plotted relative to the CT and band-to-band transition energies of the same material (right). (b) Schematic depiction of ${\text{Co}}^{2+/+}$ and ${\text{Co}}^{2+/3+}$ photoionization and ZnO band-to-band transitions. (c) Schematic depiction of electronic coupling between nearly degenerate ${{}^4\text{T}}_1\left(\text{P}\right)$ and ${\text{ML}}_{\text{CB}}\text{CT}$ potential energy surfaces, which yields both direct and thermally assisted photoconductivity following ${{}^4\text{T}}_1\left(\text{P}\right)$ excitation. An energy barrier of $E_{\text{a}}\approx 43\text{meV}$ restricts ionization to a tunneling mechanism at low temperatures.Reuse & Permissions