Phosphorus-doped ZnO Thin Films Research Articles

Unfolding the conductivity reversal n- to p-type in phosphorus-doped ZnO thin films by spin-on dopant (SOD) process

Phosphorus doping induced p-type doping in ZnO thin films based on spin-on dopant (SOD) process is reported in this article. Owing to the reduced dependence on the conventional amenities for diffusion/ion-implantation doping, the SOD process provides a simple and cheap doping method. The effect of SOD process temperature on conductivity ZnO thin films is investigated by altering the temperature from 700°C to 1000°C. Systematic field emission scanning electron microscopy analysis demonstrates the impact of doping temperature on the morphological properties of SOD. The x-ray diffraction measurements reveal that the p-type ZnO thin films had (002) preferred crystal orientation. At the same time, x-ray photoelectron spectroscopy validated the formation of the PZn–2VZn complex, which was responsible for the acceptor behaviour of films. Moreover, the photoluminescence spectra tracked down that the origin of 3.35 and 3.31 eV emission peaks is due to the acceptor bound exciton and free-electron to acceptor level transitions, respectively. Finally, an elevated hole concentration of 2.09 × 1016 cm−3 is achieved with a resistivity of 1.14 Ω-cm at 800°C doping temperature. However, the film doped at 900°C and 1000°C showed n-type behaviour due to the generation of high concentration donor defects. Here, we successfully demonstrate that the SOD process has great potential to produce high-quality p-type ZnO thin films suitable for optoelectronic devices applications.

Phosphorus doping of ZnO using spin‐on dopant process: A better choice than costly and destructive ion-implantation technique

Radio frequency sputtered ZnO thin films doped with phosphorus (ZnO:P) have been prepared employing spin-on dopant process. In the SOD process, the dopant film has been spin-coated on a silicon substrate and positioned close to the as-deposited undoped ZnO film at high temperature to perform the phosphorus doping. The high-resolution X-ray diffraction measurement reveals that the prepared ZnO:P films are good in crystalline quality which improves further by annealing. It is found that the full-width half-maximum corresponding to (002) peak of SOD processed thin films is much narrower than previously reported ion-implanted thin films, indicating the better crystalline quality of SOD processed phosphorus-doped ZnO thin films. The X-ray photoelectron spectroscopy measurement signifies that the P2O5 decomposes into two phosphorus atoms behaving like an acceptor dopant and five oxygen atoms which may fill in oxygen vacancies at high-temperature annealing. The photoluminescence spectra discover the acceptor bound exciton peak at 3.35 eV and free electron to acceptor level transitions at 3.31 eV. The calculated acceptor binding energy is 127 meV for the phosphorus dopant which works as a shallow acceptor level. It is found that the phosphorus-doped ZnO thin films prepared using the SOD process have much superior structural and optical properties in comparison to previously reported ion-implanted film. This study demonstrates that the SOD process is much superior than the ion-implantation process to produce high-quality ZnO:P thin films for very stable p-type conduction.

Enhancing Acceptor-Based Optical Behavior in Phosphorus-Doped ZnO Thin Films Using Boron as Compensating Species

The modulating nature of doping in oxide-based semiconductors has always been an area of interest as it resulted in numerous technological developments. On working with ZnO, the principal challenge faced in its realistic utilization as an optoelectronic material lies in its default n-type of nature due to the presence of native defects. Thus, achieving p-type behavior has been a tedious job, and considerable efforts have been made over the past couple of decades. The incorporation of monodopants has yielded p-type ZnO of unstable reliability, thus spurring research on codoping technology. In present study, we examined the effects of boron implantation time on the structural and optical properties of phosphorus-doped ZnO thin films, with the objective of realizing a material ideal for optoelectronic applications. Furthermore, we investigated the effects of annealing temperature on the behavior of codoped samples. Field emission gun scanning electron microscopy, high-resolution X-ray diffraction, X-ray phot...

Structural, electrical, photoluminescence and optical properties of n–type conducting, phosphorus-doped ZnO thin films prepared by pulsed laser deposition

High-quality transparent conductive phosphorus-doped zinc oxide (PZO) thin films were fabricated on glass substrates by pulsed laser deposition (PLD) at different substrate temperatures. X-ray patterns indicated that (002) preferential growth was observed and P doping did not cause structural degradation of wurtzite ZnO. Hall effect results indicated that 350°C was the optimum substrate temperature to get PZO thin films with the lowest resistivity (7.35×10−4Ωcm). Photoluminescence spectra showed the UV luminescence peak resulting from the band-edge exciton transition observed for PZO thin films. UV–visible transmission spectra showed that PZO thin films had high transparence (about 85%). In addition, the influence of substrate temperature on bandgap shift in PZO thin films was systematically studied.

Oxygen partial pressure dependent electrical conductivity type conversion of phosphorus-doped ZnO thin films

In this study, the oxygen partial pressure dependent physical properties of phosphorous-doped ZnO thin films were investigated. All thin films, grown on Al2O3(0 0 0 1) substrates using pulsed laser deposition, exhibited (0 0 2) orientation regardless of the oxygen partial pressure. However, as the oxygen partial pressure increased, the degree of crystallinity and the concentration of oxygen vacancies in the films decreased. All the thin-film samples showed n-type characteristics except for a sample grown at 100 mTorr, which exhibited p-type characteristics. The optical band gap energy also changed with the oxygen partial pressure. The feasible microscopic mechanism of conductivity conversion is explained in terms of the lattice constant, crystallinity, and the relative roles of the substituted phosphorous in the Zn-site and/or oxygen vacancies depending on the oxygen partial pressure.

The effects of ZnO buffer layers on the properties of phosphorus doped ZnO thin films grown on sapphire by pulsed laser deposition

The properties of phosphorus doped ZnO thin films grown on sapphire by pulsed laser deposition were examined, specifically focusing on the effects of undoped ZnO buffer layers. In particular, buffer layers were grown under different conditions; the transport properties of as-deposited and rapid thermal annealed ZnO : P films were then examined. As-deposited films showed n-type conductivity. After rapid thermal annealing, the film on buffer layer grown at a low temperature showed the conversion of carrier type to p-type for specific growth conditions while the films deposited on buffer layer grown at a high temperature remained n-type regardless of growth condition. The films deposited on buffer layer grown at a low temperature showed higher resistivity and more significant change of the transport properties upon rapid thermal annealing. These results suggest that more dopants are incorporated in films with higher defect density. This is consistent with high resolution x-ray diffraction results for phosphorus doped ZnO films on different buffer layers. In addition, the microstructure of phosphorus doped ZnO films is substantially affected by the buffer layer.

Low temperature electron transport in phosphorus-doped ZnO films grown on Si substrates

Low temperature magneto-transport properties and electron dephasing mechanisms of phosphorus-doped ZnO thin films grown on (111) Si substrates with Lu2O3 buffer layers using pulsed laser deposition were investigated in detail by quantum interference and weak localization theories under magnetic fields up to 10T. The dephasing length follows the temperature dependence with an index p≈1.6 at higher temperatures indicating electron–electron interaction, yet becomes saturated at lower temperatures. Consistent with photoluminescence measurements and the multi-band simulation of the electron concentration, such behavior was associated with the dislocation densities obtained from x-ray diffraction and mobility fittings, where charged edge dislocations acting as inelastic Coulomb scattering centers were affirmed responsible for electron dephasing. Owing to the temperature independence of the dislocation density, the phosphorus-doped ZnO film maintained a Hall mobility of 4.5cm2V−1s−1 at 4K.

Study of Structural and Electrical Properties of Phosphorus-Doped p-Type ZnO Thin Films

Phosphorus-doped ZnO thin films were fabricated by pulsed laser deposition (PLD) with different phosphorus pentoxide (P2O5) concentrations. The as-deposited films were annealed in nitrogen gas at 800 °C for 5 min. Hall effect measurement showed that all the as-deposited films were n-type semiconductors, and confirmed that after annealing treatment, a particular sample could be transformed from an n-type semiconductor into a p-type one. On the basis of the results of X-ray diffraction analysis and X-ray photoelectron spectroscopy, the mechanism of such transformation was discussed.

Effect of Different Substrate Temperature on Phosphorus-Doped ZnO Thin Films Prepared by PLD on Sapphire Substrates

Phosphorus-doped ZnO (ZnO:P) thin films are deposited on a c-plane sapphire in oxygen at 350°C, 450°C, 550°C and 650°C, respectively, by pulsed laser deposition (PLD), then all the ZnO:P samples are annealed at 650°C in oxygen with a pressure of 1 × 105 Pa. X-ray diffraction measurements indicate that the crystalline quality of the ZnO:P thin films is improved with the increasing substrate temperature from 350°C to 550°C. With a further increase of the deposition temperature, the crystalline quality of the ZnO:P sample is degraded. The measurements of low-temperature photoluminescence spectra demonstrate that the samples deposited at the substrate temperatures of 350°C and 450°C show a strong acceptor-bound exciton (A°X) emission. The electrical properties of ZnO:P films strongly depend on the deposition temperature. The ZnO:P samples deposited at 350°C and 450°C exhibit p-type conductivity. The p-type ZnO:P film deposited at 450°C shows a resistivity of 1.846 Ω·cm and a relatively high hole concentration of 5.100 × 1017 cm−3 at room temperature.

Electrical characterisation of phosphorus-doped ZnO thin films grown by pulsed laser deposition

Abstract Phosphorus-doped ZnO films were grown by pulsed laser deposition using a ZnO:P 2 O 5 -doped target as the phosphorus source with the aim of producing p-type ZnO material. ZnO:P layers (with phosphorus concentrations of between 0.01 to 1 wt%) were grown on a pure ZnO buffer layer. The electrical properties of the films were characterised from temperature dependent Hall-effect measurements. The samples typically showed weak n-type conduction in the dark, with a resistivity of 70 Ω cm, a Hall mobility of μ n ∼0.5 cm 2 V −1 s −1 and a carrier concentration of n∼3×10 17 cm −3 at room temperature. After exposure to an incandescent light source, the samples underwent a change in conduction from n- to p-type, with an increase in mobility and decrease in concentration for temperatures below 300 K.

Effect of annealing temperature and ambient gas on phosphorus doped p-type ZnO

The authors report on the thermal activation of phosphorus doped p-type ZnO thin films grown by radio frequency magnetron sputtering. The p-type ZnO was produced by activating phosphorus doped ZnO thin films in N2, Ar, or O2 ambients. The hole concentration of the p-type ZnO, prepared in an O2 ambient, showed a lower hole concentration compared to samples annealed in N2 and Ar ambients. The activation energies of the phosphorus dopant in the p-type ZnO under different ambient gases indicate that phosphorus atoms replace oxygen atoms in the ZnO to form PO which acts as an acceptor.

The Effect of Ar ∕ O2 Sputtering Gas on the Phosphorus-Doped p -Type ZnO Thin Films

The influence of gas on phosphorus-doped ZnO thin films deposited by radio frequency magnetron sputtering was studied. As the partial pressure of increased in the sputtering gas, the structure of changed from nanorods to a smooth thin film. The electron concentration of the thin film was also decreased due to the reduction in oxygen vacancies that act as donors. Photoluminescence spectra of thin films also showed a reduction in the intensity of the deep level emission peaks, due to the reduction in native defects related to the oxygen vacancies as the partial pressure was increased. The thin films grown under an gas ratio of showed -type characteristics after a rapid thermal annealing (RTA) activation process. The -type showed a resistivity of , a mobility of , with a hole concentration of .

P-Type conduction in phosphorus-doped ZnO thin films by MOCVD and thermal activation of the dopant

Phosphorus-doped p-type ZnO thin films have been realized by metalorganic chemical vapor deposition (MOCVD). The conduction type of ZnO films is greatly dependent on the growth temperature. ZnO films have the lowest resistivity of 11.3 Ωcm and the highest hole concentration of 8.84 × 10 18 cm −3 at 420 °C. When the growth temperature is higher than 440 °C, p-type ZnO films cannot be achieved. All the films exhibited p-type conduction after annealing, and the electrical properties were improved comparing with the as-grown samples. Secondary ion mass spectroscopy (SIMS) test proved that phosphorus (P) has been incorporated into ZnO.

Growth of phosphorus doped ZnO thin films by pulsed laser deposition

Undoped and phosphorus doped (1 and 3 mol%) ZnO (P-doped ZnO) films were grown on sapphire substrates by a pulsed laser deposition technique and then the phosphorus doping effect was investigated. Results of X-ray diffraction and scanning electron microscopy revealed that the phosphorus doping did not noticeably affect the microstructure and crystallinity of the undoped ZnO film. In the as-grown state, both of the P-doped ZnO films showed n-type conductivity rather with slightly more electron concentrations than that of the undoped ZnO film. For the 3 mol% P-doped ZnO film a significant decrease in the carrier concentration was observed at particular annealing temperatures (≥800 °C), still showing n-type conductivity. In contrast, in the case of 1 mol% P-doping, the variation in the carrier concentration was not significant under any annealing temperature. Our results suggest that a careful adjustment both of annealing temperature and of phosphorus doping content is required to control the electrical properties including the carrier concentration, resistivity and mobility in ZnO films. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The near band-edge emission and photoconductivity response of phosphorus-doped ZnO thin films grown by pulsed laser deposition

The photo-response of phosphorus-doped ZnO thin films is reported. In particular, room temperature photoluminescence and ultraviolet photoconductivity are measured for epitaxial films grown by pulsed laser deposition. For the as-deposited films, the near band-edge emission in P-doped films is reduced in intensity and shifts to higher energy relative to the undoped materials. Annealing in 100 Torr of oxygen decreases the near band-edge photoluminescence intensity for both the undoped and P-doped material. However, an enhancement in the visible orange-red emission due to defect states in the gap is observed with oxygen annealing, which correlates with a decrease in conductivity. The visible luminescence is consistent with radiative recombination involving deep states. Ultraviolet photoconductivity is observed for the annealed films, and is most notable for the phosphorus-doped materials. These results are consistent with the phosphorus introducing a deep level in the gap that is heavily compensated by a defect-related shallow donor in the as-deposited films. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Shallow donor formation in phosphorus-doped ZnO thin films

ZnO is a direct band gap semiconductor that is attractive for a number of applications, including high-temperature electronics, transparent electronics, and blue to UV optoelectronics. The most significant impediment to the exploitation of ZnO is the difficulty in achieving effective p-type carrier doping. Attractive acceptor-state candidates for ZnO are the group-V elements substituted on the oxygen site. In this paper, the doping behavior of phosphorus in as-deposited ZnO crystalline thin films grown by pulsed-laser deposition is reported. The experimental work reported here indicates that phosphorus doping significantly increases the electron carrier density in as-deposited films, making the material heavily n-type. These results indicate the formation of a shallow donor level associated with the phosphorus impurity, resulting from either substitution on the Zn site or formation of a compensating complex. Based on X-ray photoelectron spectroscopy measurements, the phosphorus-dopant ions in the films occupy multiple sites yielding oxidation states of -3 and +5. This would explain the compensated n-type behavior for group-V doping in the II-VI host material.

Specific contact resistance of Ti/Al/Pt/Au ohmic contacts to phosphorus-doped ZnO thin films

The carrier concentration dependence of Ti/Al/Pt/Au ohmic contact resistance on P-doped n-type ZnO thin films is reported. Ti (200 Å)/Al (800 Å)/Pt (400 Å)/Au (800 Å) was deposited by electron-beam evaporation on ZnO thin films grown by pulsed laser deposition on (0001) sapphire substrates using a ZnO:P0.02 source. Postgrowth annealing from 30 to 600 °C resulted in carrier concentrations of 7.5×1015 cm−3–1.5×1020 cm−3 in the ZnO. After metal deposition, the specific contact resistances were measured at temperatures in the range 30–100 °C prior to alloying annealing at 200 °C and at 30–200 °C after this anneal. The lowest specific contact resistance of 8.7×10−7 Ω cm2 for nonalloyed ohmic contacts was achieved in the sample with carrier concentration of 1.5×1020 cm−3 when measured at 30 °C. In the annealed samples, minimum specific contact resistances of 3.9×10−7 Ω cm2 and 2.2×10−8 Ω cm2 were obtained in samples with carrier concentrations of 6.0×1019 cm−3 measured at 30 °C and 2.4×1018 cm−3 measured at 200 °C, respectively. Auger electron spectroscopy detected Ti–O interfacial reaction and intermixing between Al and Pt at 200 °C.

Transport properties of phosphorus-doped ZnO thin films

The doping behavior of phosphorus in ZnO thin films grown by pulsed laser deposition is examined. The transport properties of epitaxial ZnO films doped with 1–5 at. % P were characterized via room temperature Hall measurements. As-deposited films doped with phosphorus are highly conductive and n type. The origin of the shallow donor level appears to be either substitution of P on the Zn site or formation of a donor complex. Annealing these phosphorus-doped films significantly reduces the carrier density, transforming the transport from highly conducting to semi-insulating. These results indicate that the phosphorus-related donor defect is relatively unstable, and suggests the formation of a deep level upon annealing. The latter is consistent with phosphorus substitution on the O site yielding a deep level in the gap.

Realization of p-type ZnO thin films via phosphorus doping and thermal activation of the dopant

A p-type ZnO was prepared on a sapphire substrate using P2O5 as a phosphorus dopant. As-grown n-type ZnO films doped with phosphorus showed electron concentrations of 1016–1017/cm3 and these films were converted to p-type ZnO by a thermal annealing process at a temperature above 800 °C under a N2 ambient. The electrical properties of the p-type ZnO showed a hole concentration of 1.0×1017–1.7×1019/cm3, a mobility of 0.53–3.51 cm2/V s, and a low resistivity of 0.59–4.4 Ω cm. The phosphorus-doped ZnO thin films showed a strong photoluminescence peak at 3.35 eV at 10 K, which is closely related to neutral acceptor bound excitons of the p-type ZnO. This thermal activation process was very reproducible and effective in producing phosphorus-doped p-type ZnO thin films, and the produced p-type ZnO was very stable.