Mid-gap electronic states in ${\text{Zn}}_{1-x}{\text{Mn}}_x\text{O}$

Electronic absorption, magnetic circular dichroism (MCD), photoconductivity, and valence-band x-ray photoelectron (XPS) spectroscopic measurements were performed on epitaxial ${\text{Zn}}_{1-x}{\text{Mn}}_x\text{O}$ films to investigate the origin of the mid-gap band that appears upon introduction of ${\text{Mn}}^{2+}$ into the ZnO lattice. Absorption and MCD spectroscopies reveal ${\text{Mn}}^{2+}$-related intensity at energies below the first excitonic transition of ZnO, tailing well into the visible energy region, with an onset at $\sim 2.2\text{eV}$. Photoconductivity measurements show that excitation into this visible band generates mobile charge carriers, consistent with assignment as a ${\text{Mn}}^{2+/3+}$ photoionization transition. XPS measurements reveal the presence of occupied ${\text{Mn}}^{2+}$ levels just above the valence-band edge, supporting this assignment. MCD measurements additionally show a change in sign and large increase in magnitude of the excitonic Zeeman splitting in ${\text{Zn}}_{1-x}{\text{Mn}}_x\text{O}$ relative to ZnO, suggesting that $sp\text{−}d$ exchange in ${\text{Zn}}_{1-x}{\text{Mn}}_x\text{O}$ is not as qualitatively different from those in other II-VI diluted magnetic semiconductors as has been suggested. The singular electronic structure feature of ${\text{Zn}}_{1-x}{\text{Mn}}_x\text{O}$ is the presence of this ${\text{Mn}}^{2+/3+}$ ionization level within the gap, and the influence of this level on other physical properties of ${\text{Zn}}_{1-x}{\text{Mn}}_x\text{O}$ is discussed.

Figure 1

TEM of $c{\text{-Zn}}_{0.95}{\text{Mn}}_{0.05}\text{O}$ on $c{\text{-Al}}_2{\text{O}}_3$ (a) Low magnification micrograph showing a smooth film deposited on $c{\text{-Al}}_2{\text{O}}_3$. (b) High-resolution micrograph of $c{\text{-Zn}}_{0.95}{\text{Mn}}_{0.05}\text{O}$ film and $c{\text{-Al}}_2{\text{O}}_3$ substrate interface.

Figure 2

(Color online) (a) Room-temperature electronic absorption spectra of $c\text{-ZnO}$ and $c{\text{-Zn}}_{1-x}{\text{Mn}}_x\text{O}$ thin films, normalized for film thickness ($\cdots$ $\text{ZnO}$, — ${\text{Zn}}_{0.957}{\text{Mn}}_{0.043}\text{O}$, $–––$ ${\text{Zn}}_{0.993}{\text{Mn}}_{0.007}\text{O}$). Inset: ${\text{ML}}_{\text{CB}}\text{CT}$ intensity vs ${\text{Mn}}^{2+}$ concentration. (b) Energies of the first ZnO band-to-band absorption feature plotted vs $x$ for the films shown in (a) (◼ CT-intersect and ◻ band-maximum methods), and for various films from the literature [● (Ref. 29) CT-intersect; △ (Ref. 42) band-maximum]. $\cdots$ linear best fit, $E_{\text{ZnO}}=3.28+1.29x\left(\text{eV}\right)$. – – linear best fit, $E_{\text{ZnO}}=3.40+2.73x\left(\text{eV}\right)$.

Figure 3

(Color online) (a) Room-temperature electronic absorption spectrum and (b) variable-temperature photoconductivity action spectra (293, 225, 175, 125, and 92 K) of a $c{\text{-Zn}}_{0.975}{\text{Mn}}_{0.025}\text{O}$ thin film. The photoconductivity data have been normalized at 2.987 eV. (c) The ratio of photoconductivity (PC) to dark conductivity plotted vs temperature, measured at the various energies designated by the symbols in (b). The dotted lines are guides to the eye.

Figure 4

(Color online) (a) Room-temperature valence band XPS spectra of ${\text{Zn}}_{0.98}{\text{Al}}_{0.02}\text{O}$ $\left(\cdots \right)$ and ${\text{Zn}}_{0.95}{\text{Mn}}_{0.05}\text{O}$ (—) films. (b) Data from (a) plotted on an expanded scale. (c) Calculated density of states (DOS) for a ${\text{Zn}}_{83}{\text{MnO}}_{84}$ cluster ($\sim 1.5\text{nm}$ dia.), adapted from Ref. 19. The DOS are defined as the number of one-electron orbitals within a bin size of 0.2 eV. The shaded regions indicate the ${\text{Mn}}^{2+}$ partial contributions $\left(\times 10\right)$. Spin-up and spin-down densities are plotted as positive and negative values, respectively. The calculated energies do not match experiment because of the small cluster size considered in Ref. 19, but compare well with ${\text{Zn}}_{1-x}{\text{Mn}}_x\text{O}$ nanocrystal energies and extrapolate well to bulk energies. (d) Room-temperature photoconductivity spectra of ZnO and ${\text{Zn}}_{0.975}{\text{Mn}}_{0.025}\text{O}$ films, plotted on the same energy scale as (b).

Figure 5

(Color online) (a) 7 K electronic absorption spectrum and (b) 5 K, 1–5 T MCD spectra of a $c\text{-ZnO}$ film. Inset: Magnetic field dependence of the leading-edge excitonic MCD intensity measured at 5 K (△), plotted vs ${\mu }_{\text{B}}H/2kT$. The dotted line is a linear fit to the data. (c) 6 K electronic absorption spectrum and (d) 6 K, 1–5 T MCD spectra of a ${\text{Zn}}_{0.993}{\text{Mn}}_{0.007}\text{O}$ film. Inset: Magnetic field dependence of the leading-edge excitonic MCD intensity measured at 6 K (△) and 12 K (▲), plotted vs ${\mu }_{\text{B}}H/2kT$. The dotted line shows the $S=5/2$ Brillouin function calculated at the same two temperatures. (e) 6 K electronic absorption spectrum and (f) 6 K, 1–5 T MCD spectra of a ${\text{Zn}}_{0.957}{\text{Mn}}_{0.043}\text{O}$ film. Inset: Magnetic field dependence of the leading-edge excitonic MCD intensity measured at 6 K (△) and 12 K (▲), and of the mid-gap ${\text{ML}}_{\text{CB}}\text{CT}$ intensity measured at 6 K (○) and 12 K (●) at the energies marked on the spectra, plotted vs ${\mu }_{\text{B}}H/2kT$. The dotted line is the calculated $S=5/2$ Brillouin function (scale on right ordinate).

Figure 6

(Color online) Magnetic field dependence of the leading edge excitonic MCD intensity feature of $c\text{-ZnO}$ $(+)$ and $c{\text{-Zn}}_{0.993}{\text{Mn}}_{0.007}\text{O}$ (◻) thin films, measured at 5 and 6 K, respectively, and plotted as $\Delta {\text{A}}^{\prime }/{\text{A}}_0$ vs ${\mu }_{\text{B}}H/2kT$. The dotted lines show the $S=5/2$ Brillouin function (${\text{Zn}}_{0.993}{\text{Mn}}_{0.007}\text{O}$ film) and a linear function (ZnO film).

Figure 7

(Color online) (a) Energy level scheme for ${\text{Zn}}_{1-x}{\text{Mn}}_x\text{O}$, showing the CT and band-to-band transitions. The empirically predicted energies of the ${\text{Mn}}^{2+}d\text{−}d$ transitions are included for reference. (b) Schematic one-electron density of states for ${\text{Zn}}_{1-x}{\text{Mn}}_x\text{O}$, depicting the approximate transitions from (a) in a one-electron scheme. The split-off ${\text{Mn}}^{2+}$-derived band yields partial hole localization at ${\text{Mn}}^{2+}$ with a binding energy of $E_{\text{b}}$ following ${\text{ML}}_{\text{CB}}\text{CT}$ excitation, analogous to a Zhang-Rice band.

Figure 8

(Color online) Hole wave function in the first electronic excited state of ${\text{Zn}}_{1-x}{\text{Mn}}_x\text{O}$, calculated assuming a hydrogenic $1s$ orbital with an effective Bohr radius of $r_{\text{B}}=0.30\text{nm}$. (a) Wave function amplitude plotted vs distance from ${\text{Mn}}^{2+}$. (b) Wave function contour at radius $r_{\text{B}}=0.30\text{nm}$ from ${\text{Mn}}^{2+}$, superimposed onto the ${\text{Zn}}_{1-x}{\text{Mn}}_x\text{O}$ lattice. For comparison, the average ZnO bond length is 0.19 nm and the Zn-Zn nearest neighbor distance is 0.32 nm.