J. M. Erie

Generation of nitrogen acceptors in ZnO using pulse thermal processing

Bipolar doping in wide bandgap semiconductors is difficult to achieve under equilibrium conditions because of the spontaneous formation of compensating defects and unfavorable energetics for dopant substitution. In this work, we explored the use of rapid pulse thermal processing for activating nitrogen dopants into acceptor states in ZnO. Low-temperature photoluminescence spectra revealed both acceptor-bound exciton (AX0) and donor-acceptor pair emissions, which present direct evidence for acceptors generated after pulse thermal processing of nitrogen-doped ZnO. This work suggests that pulse thermal processing is potentially an effective method for p-type doping of ZnO.

Ultrafast carrier relaxation and diffusion dynamics in ZnO

We report on measurements and calculations of the ultrafast exciton relaxation dynamics in ZnO. Time-resolved differential reflectivity measurements of bulk ZnO were performed as a function of excitation wavelength. Bi-exponential decays of the A and B exciton states are observed with a fast (~2-5 ps scale) and a slower (~50-100 ps scale) component, which depend strongly on excitation wavelength. Theoretical calculations based on a multi-state, coupled rate equation model were directly compared with the experiments to account for the rapid scattering between the A and B valence bands. Results show that the inter-valence band scattering is most likely not responsible for the fast initial relaxation. Instead our results show that carrier diffusion can play an important role in explaining the initial fast relaxation.

P-type ZnO thin films via phosphorus doping

ZnO is a wide bandgap semiconductor that exhibits properties that are near-ideal for light-emitting diodes, but presents materials challenges that must be overcome in order to achieve highly efficient light emission. The most significant issue with ZnO is p-type doping. Related materials issues include understanding electron-hole transport across pn junctions, as well as understanding and minimizing leakage current paths within the bulk and on the surface. In this paper, the formation and properties of phosphorus-doped Zn1-xMgxO films, ZnO-based pn homojunctions and heterojunctions is discussed. The behavior of phosphorus within the ZnO and ZnMgO matrices will be described. Comparisons with other acceptor dopants will be made. Discussion will include stability of transport properties, stabilization of surfaces, and device characteristics.

Effect of argon annealing of phosphorus-doped ZnO and (Zn,Mg)O thin-films grown pulsed laser deposition

The transport and annealing properties of phosphorus-doped (Zn,Mg)O thin films grown via pulsed laser deposition (PLD) are studied. The electron carrier concentration for (Zn,Mg)O:P films decreases with increasing deposition and Ar annealing temperature. All the films exhibit good crystallinity with c-axis orientation. This result indicates the importance of activation of the P dopant in (Zn,Mg)O:P films. The as-deposited ZnO:P film properties show less dependence on the deposition growth temperatures. The resistivity of the (Zn,Mg)O:P films is significantly higher than the ZnO:P films grown under similar conditions, indicating separation of the conduction band edge relative to the defect donor state. The annealed ZnO:P films are n-type with resistivity dependent on annealing temperature.

Properties of phosphorus-doped ZnO and (Zn,Mg)O thin films via pulsed laser deposition

The room temperature transport and optical properties of phosphorus-doped ZnO and (Zn,Mg)O thin films are studied. Pulsed laser deposition (PLD) has been employed to grow epitaxial and polycrystalline layers on c-plane (0001) sapphire substrate. The ZnO:P film properties show a strong dependence on the deposition ambient at different growth temperatures. The resistivity of the samples deposited in O3/O2 mixture is two orders of magnitude higher than the films grown in oxygen and O2/Ar/H2 mixture. The photoluminescence (PL) spectra of the as-deposited films are composed of both the near band-edge and broadband visible emission, which peak at 3.29 and 1.87 eV, respectively. It has been shown that growing in the O2/Ar/H2 mixture ambient significantly increases the band edge emission while inhibiting the visible emission. The opposite effect on the PL emissions is shown for the films grown in pure oxygen and O3/O2 mixture. There is an inverse correlation between the intensity of the visible broadband emission and the carrier density. The enhanced UV emission in the films grown in O2/Ar/H2 mixture may result from hydrogen passivation of the deep level emission centers. For the P-doped (Zn,Mg)O grown at 500°C, increasing the oxygen partial pressure from 20 to 200 mTorr yields a carrier type conversion from n-type to p-type without post-annealing. The films grown at 150 mTorr oxygen partial pressure are p-type and exhibit a hole concentration of 2.7 x 1016 cm-3, a mobility of 8.2 cm2/Vs and a resistivity of 35 Ω-cm. All the films exhibit good crystallinity with c-axis orientation. These results indicate the importance of oxidation conditions in realizing p-type (Zn,Mg)O:P films.