$n$- to $p$-type conductivity transition and band-gap renormalization in ZnO:(Cu+Te) codoped films

The natural conductivity of as-grown ZnO is $n$-type. It has been challenging to produce stable $p$-type material, which has delayed possible technological applications. In this work, the conductivity transformation from $n$- to $p$-type ZnO films deposited by sputtering was followed as a function of copper and tellurium concentrations and of the substrate temperature during growth (room temperature, $150{}^{\circ }\mathrm{C}, 250{}^{\circ }\mathrm{C}$, and $350{}^{\circ }\mathrm{C}$). The nominal codopant concentrations ranged from 1 to 12 at %. For the minimum concentration, compensation effects yielded highly resistive ZnO. Nonetheless, by tailoring the concentration of Cu and Te, it was possible to vary the resistivity $({10}^3–{10}^{-2}\mathrm{\Omega }\mathrm{cm})$, mobility $(\sim {10}^{-2}–{10}^{\circ }\mathrm{c}{\mathrm{m}}^2/\mathrm{V}\mathrm{s})$, and free-hole density $({10}^{15}–{10}^{20}\mathrm{c}{\mathrm{m}}^{-3})$ of $p$-type ZnO grown at $250{}^{\circ }\mathrm{C}$. Besides modifying the electrical properties, codoping changed the host band structure significantly, producing a band-gap renormalization from 3.2 eV (UV) to 1.8 eV (red). This control over the band gap is advantageous for applications where controllable photon absorption or emission are sought. Experimentally, films were stable for a period of at least six months. Band-gap engineered $p$-type ZnO:(Cu+Te) films open the possibility for the fabrication of all-ZnO optoelectronic devices such as homojunction solar cells and/or light-emitting diodes.

Figure 1

(a) X-ray diffraction patterns of films grown at RT (black) and $250{}^{\circ }\mathrm{C}$ (red) for the indicated Cu and Te concentrations in the sputtering target. The patterns at large diffraction angles were amplified by the indicated factors. The labels of the diffraction peaks correspond to hexagonal (wurtzite) ZnO. (b) and (c) show the crystallite size and lattice parameters as a function of the dopant nominal concentration in the target, respectively. The dashed lines in (c) indicate the single-crystal values for the lattice parameters from Ref. [3].

Figure 2

Surface morphology of films grown at $250{}^{\circ }\mathrm{C}$ with (a) 1 at %, (b) 2 at %, (c) 3 at %, (d) 5 at %, (e) 7 at %, and (f) 12 at % 2Cu:Te. The scale bar corresponds to 100 nm. The insets show the appearance to the eye of each film under white-light illumination.

Figure 3

Diffraction patterns of undoped ZnO films (red dashed lines) and films with 3 at % 2Cu:Te (dark gray continuous lines) for the indicated substrate temperatures. The vertical dashed lines correspond to the position of the diffraction peaks in the ZnO powder diffraction data file (PDF no. 05-0664).

Figure 4

Elemental chemical composition of the films determined by wavelength-dispersive spectroscopy as a function of the atomic concentration of 2Cu:Te in the sputtering target. Data are shown for films grown at (a) room temperature, (b) $150{}^{\circ }\mathrm{C}$, (c) $250{}^{\circ }\mathrm{C}$, and (d) $350{}^{\circ }\mathrm{C}$.

Figure 5

Photographs of undoped and codoped films. The dopant atomic percentage, substrate temperature $\left(T_s\right)$, and thickness in nanometers (d) for each film are indicated. The photographer and the camera can be observed in the specular reflection.

Figure 6

Transmittance spectra of the ZnO:(Cu+Te) films grown at (a) RT, (b) $250{}^{\circ }\mathrm{C}$, and (c) $350{}^{\circ }\mathrm{C}$. The blue shaded areas depict the visible spectral region; the vertical dashed lines correspond to the band gap of undoped ZnO film. The black horizontal arrows in the top panels indicate the shift of the transmittance edge (with respect to ZnO) for the 12 at %-doped samples.

Figure 7

Plots of ${\left(ah\nu \right)}^2$ vs energy $\left(h\nu \right)$ for the films grown at (a) room temperature and (b) $250{}^{\circ }\mathrm{C}$ showing the band-gap renormalization of the ZnO:(Cu+Te) films. The intersection of the extrapolated lines with the energy axis correspond to the value of the band gap for each film according to the model of direct transitions between parabolic bands. It is noticed that all values of $E_g$ for the codoped samples lay within the visible region of the spectrum.

Figure 8

Resistivity, mobility, and free-carrier concentration of undoped and ZnO:(Cu+Te) films deposited at $T_s=250{}^{\circ }\mathrm{C}$ as a function of the dopant nominal concentration in the target. Data for [2Cu:Te] = 1 at % could not be obtained because the sample was highly resistive due to compensation effects. Next to the data points for each concentration appears a section of the corresponding photograph in Fig. 5. The thick horizontal arrow highlights the evolution from full transparency to complete opacity in the visible when going from undoped to 12 at % films.

Figure 9

(top) ZnO hexagonal unit cell depicting a projection to the basal plane (top right) of the atoms whose $c$-axis fractional coordinates are indicated. (bottom) Two-dimensional projection of the ZnO structure describing randomly located ${\mathrm{Cu}}_{\mathrm{Zn}}, {\mathrm{Cu}}_{\mathrm{i}}, {\mathrm{Te}}_{\mathrm{O}}$, and ${\mathrm{Te}}_{\mathrm{i}}$ impurities. ${\mathrm{Cu}}_{\mathrm{Zn}} {\mathrm{Te}}_{\mathrm{i}}$ and the complexes ${\mathrm{Cu}}_{\mathrm{Zn}}+{\mathrm{Te}}_i$ and ${\mathrm{Cu}}_{\mathrm{Zn}}+{\mathrm{Te}}_{\mathrm{O}}$ may act as acceptors.