P-doped ZnO Thin Films Research Articles

Effect of oxygen pressure performance of PZO thin films deposited by pulsed laser deposition at low temperature

The P doped ZnO (PZO) thin films were deposited by pulsed laser deposition via controlling oxygen pressure at low temperature (200 °C). The structural, surface morphological, electrical and optical properties of the thin films are systematically and detailedly studied. The surface morphologies show that the grain size of PZO thin films increases with oxygen pressure. The change mechanism of morphology is proposed. The increase of mobility is associated with the development of grain size. Thus, the carrier mobility can be improved by grain size control. The average optical transmittance in the visible region (380–780 nm) is above 90%. The estimated optical band gaps of the thin films decrease from 3.49 to 3.39 eV with the oxygen pressure increasing from 0 to 1 Pa, due to the Burstein–Moss effect and band gap renormalization. The highest figure of merit observed in this present study is 8.3 × 10−3 Ω−1, while the thin films are deposited at 0.01 Pa. The qualities of PZO thin films are advantages for the application in solar cells.

Electron doping of ALD-grown ZnO thin films through Al and P substitutions

Several series of Al- and P-doped ZnO thin films have been deposited with atomic layer deposition at different temperatures using diethyl zinc, trimethyl aluminum, trimethyl phosphate, and H2O as the precursors. The optimal deposition temperature range is found at 160–220 °C where the film growth is stable and well-crystallized films with low resistivity are obtained. The effect of the dopant content on the charge carrier density of the films is evaluated based on optical reflectivity, Seebeck coefficient, and electrical resistivity data for the films. Both dopants are found to be effective in controlling the carrier density of ZnO; systematic increase in carrier density is demonstrated for Al-doping up to 2 at.% and for P-doping up to 5 at.%. The conductivity of the as-deposited films remains n-type.

P-Doped <i>p</i>-Type ZnΟ Films Deposited by Sputtering and Diffusing

P-doped ZnO thin films were prepared on different Si substrates by RF magnetron sputtering in Ar and O2mixed atmosphere. The P-doped ZnO films were changed from n-type to p-type by phosphorus diffusing from the n-Si substrates with higher phosphorus concentration into the ZnO films during depositing. The crystal structure of the ZnO films was examined by X-ray diffraction and confirmed to belong to wurtzite and highly c-axis oriented primarily perpendicular to the substrate. The Hall effect measurement results show that the corresponding hole concentration and risistivity of the P-doped ZnO film was 8.982×1017cm-3and 1.489 Ω•cm respectively. This reveals that the P-doped ZnO thin film is really p-type behavior.

Properties of phosphorus-doped zinc oxide films grown by pulsed laser deposition

Electrical and chemical bonding properties of P-doped ZnO thin films grown by pulsed laser deposition on sapphire substrates were systematically characterized utilizing the Hall effect and x-ray photoelectron spectroscopy (XPS) measurements. Oxygen growth pressure and postannealing processing play a great role in the properties of these films. Increasing oxygen growth pressure from 5 to 20 Pa enhanced the resistivity of P-doped ZnO films by three orders of magnitude. P-doped ZnO films grown at 700 °C under 20 Pa O2 exhibited p-type conductivity with hole concentration of 5×1017 cm−3 and hole mobility of 0.3 cm2/V s. Rapid thermal annealing processing decreased the electron density in the P-doped ZnO films. XPS binding energies of P 2s and 2p peaks showed formation of P–O bonds which increased with oxygen pressure in the films. This indicates formation of defect complexes of P dopants occupying zinc sites PZn and zinc vacancies VZn in the P-doped ZnO films.

Electrical and optical properties of P-doped ZnO thin films by annealing temperatures in Nitrogen ambient

The effects of post-annealing temperature on the optical and electrical properties of P-doped ZnO thin films, grown on sapphire substrate, have been investigated when the annealing is performed under nitrogen ambient. Analysis of the XRD shows that regardless of the post-annealing temperature, the P-doped ZnO thin films have grown the (002) peak. The full width of half maximum decreases from 0.194 to 0.181° as the annealing temperature increases from 700 to 900 °C. This phenomenon means that the increase of annealing temperature causes enhancement of the thin film’s crystalline properties. The results of Hall effect measurements indicate that the P-doped ZnO thin films, annealed at 750 and 800 °C exhibit p-type behavior, with hole concentrations of 5.71 × 1017 cm−3 and 1.20 × 1018 cm−3, and hole mobilities of 0.12 cm2/Vs and 0.08 cm2/Vs, respectively. The low-temperature (10 K) photoluminescence results reveal that the peaks related to the neutral-acceptor exciton (A0X) at 3.355 eV, free electrons to neutral acceptor (FA) at 3.305 eV and donor acceptor pair (DAP) at 3.260 and 3.170 eV are observed in the films showing p-type behavior with the acceptors. Because P atoms replace O atoms to produce acceptors from P-doped ZnO thin films by the thermal activation process at the appropriate annealing temperature with nitrogen ambient, the p-type ZnO thin films can be fabricated in this way.

Determination of electrical types in the P-doped ZnO thin films by the control of ambient gas flow

Phosphorus (P)-doped ZnO thin films with amphoteric doping behavior were grown on c-sapphire substrates by radio frequency magnetron sputtering with various argon/oxygen gas ratios. Control of the electrical types in the P-doped ZnO films was achieved by varying the gas ratio without post-annealing. The P-doped ZnO films grown at a argon/oxygen ratio of 3/1 showed p-type conductivity with a hole concentration and hole mobility of 1.5 × 10 17 cm −3 and 2.5 cm 2/V s, respectively. X-ray diffraction showed that the ZnO (0 0 0 2) peak shifted to lower angle due to the positioning of P 3− ions with a larger ionic radius in the O 2− sites. This indicates that a p-type mechanism was due to the substitutional P O. The low-temperature photoluminescence of the p-type ZnO films showed p-type related neutral acceptor-bound exciton emission. The p-ZnO/ n-Si heterojunction light emitting diode showed typical rectification behavior, which confirmed the p-type characteristics of the ZnO films in the as-deposited status, despite the deep-level related electroluminescence emission.

Local electronic structure of phosphorus-doped ZnO films investigated by X-ray absorption near-edge spectroscopy

Using X-ray absorption near-edge structure spectroscopy (XANES), we investigate the local electronic structure of phosphorus (P) and its chemical valence in laser-ablated n-type (as-grown), and p-type (annealed) P-doped ZnO thin films. Both the P L 1- and P L 2,3-edge XANES spectra reveal that the valence state of P is 3− (P3−) in the p-type as well as in the n-type P-doped ZnO. However, the peak intensity is stronger in the former than that in the latter, suggesting that P replaces O (O2− sites with the P3−) after rapid thermal annealing. The Zn and O K-edges XANES spectra consistently demonstrate that, in the p-type state, P ions substitutionally occupy O sites in the ZnO lattice.

Electrical and optical properties of phosphorus-doped p-type ZnO films grown by metalorganic chemical vapor deposition

P-doped p-type ZnO thin films have been grown by metalorganic chemical vapor deposition. By modulating the P evaporating temperature, p-type conductivity can be controlled due to the different P content incorporated into the ZnO films. The P-doped p-type ZnO thin films are of high optical quality, as indicated by low-temperature photoluminescence. P-related acceptor state with an energy level of 163 meV is identified from free-to-neutral-acceptor transitions. In addition, x-ray photoelectron spectroscopy confirms that only one chemical bonding state of P exists in the P-doped ZnO thin films.

Study on anomalous n-type conduction of P-doped ZnO using P2O5 dopant source

The unexpected n-type conduction observed in P-doped ZnO thin films fabricated from rf magnetron sputtering, was studied systematically through a combined approach of experiment and computer modeling. The carrier stability was predicted from first-principles density functional theory and chemical thermodynamic calculations. It demonstrated that, under oxygen-poor growth condition and low temperature, the stable doping defect PO−1 may have negative effect on n-type conduction and, under oxygen-poor growth condition and high temperature, the stable doping defect may contribute significantly to the n-type conduction. Furthermore, under oxygen-rich growth condition, the stable doping defect PZn1 may help to maintain the n-type conduction at high oxygen partial pressures. Our model predictions are in good agreement with experimental observations in anomalous conduction of P2O5-doped ZnO thin films and provide scientific explanation. This research not only revealed increased fundamental understanding on electronic behaviors but also provided a fabrication strategy for P-doped n-type ZnO.