Optical signatures of nitrogen acceptors in ZnO

We report on the optical properties of nitrogen acceptor-doped ZnO epilayers in the medium and high doping regimes using temperature and excitation power-dependent, as well as time-resolved photoluminescence experiments. The epilayers were doped with ammonia during homoepitaxial growth on ZnO single-crystal substrates with different surface polarities. Significant differences in the optical characteristics of the epilayers are observed between growth on nonpolar $a$-plane, polar $c$-plane Zn-face substrates and polar $c$-plane O-face substrates, which demonstrates different incorporation of the nitrogen acceptor depending on the substrate polarity. The incorporation of nitrogen into the ZnO films ranges between 10${}^{19}$ and 10${}^{21}$ cm${}^{-3}$ as determined by secondary ion mass spectrometry. Within this doping range the samples change from lightly compensated to highly doped compensated. We discuss the unique photoluminescence features of nitrogen-doped ZnO epilayers within the concept of shallow donor-acceptor-pair recombinations and at the highest doping level by the appearance of potential fluctuations.

Figure 1

Nitrogen concentrations of $a$-plane grown ZnO:N epilayers with constant dopant flow for substrate temperatures between 200 °C and 600 °C.

Figure 2

PL of ZnO:N epilayers on O-face $c$-plane substrates with increasing nitrogen concentration measured at $T$ $=$ 4 K.

Figure 3

PL of ZnO:N epilayers on Zn-face $c$-plane substrates with increasing nitrogen concentration measured at $T$ $=$ 4 K.

Figure 4

Ratio of the DAP to donor bound exciton luminescence ($DX$) for $c$-plane Zn-face and O-face epilayers at $T$ $=$ 4 K.

Figure 5

Temperature dependence of the DAP transition of a highly nitrogen-doped Zn-face epilayer.

Figure 6

Power-dependent PL measurements of a Zn-face epilayer with a nitrogen content of 6 × 10${}^{19}$ cm${}^{-3}$ at $T$ $=$ 4 K. Inset: Shift of the zero-phonon line positions of the DAP recombination with increasing excitation power density.

Figure 7

PL transients of the DAP transition of N-doped ZnO samples grown on Zn-face substrates with different N concentrations at 10 K. (Inset) 1/$e$ decay time for the closest donor-acceptor pairs as function of N concentration.

Figure 8

Excitation power-dependent PL measurements using pulsed fs excitation measured at 10 K. (a) N concentration 2 × 10${}^{19}$; (inset) decay times as function of excitation power; (b) N concentration 4 × 10${}^{20}$.

Figure 9

Energy shift of the DAP transition calculated within the model of fluctuating potentials for two hole concentrations as a function of the total impurity concentration.