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An ultraviolet photodetector was fabricated on MgZnO thin film grown by metal-organic chemical vapor deposition. The peak response of the device centers at 238 nm and cutoff wavelength is 253 nm. The peak responsivity is 129 mA/W at 15 V bias, and the UV/visible reject ratio is 4 orders of magnitude. Internal gain is due to the hole trapping at interface that brings low response speed. Native defects at the Au/MgZnO interface degrade the barrier effect, which caused large dark current and high visible response.

Room temperature ferromagnetism (RTFM) was observed in Li-N codoped ZnO thin films [ZnO:(Li, N)] fabricated by plasma-assisted molecular beam epitaxy, and p-type ZnO:(Li, N) shows the strongest RTFM. Positron annihilation spectroscopy and low temperature photoluminescence measurements indicate that the RTFM in ZnO:(Li, N) is attributed to the defect complex related to VZn, such as VZn and Lii-NO-VZn complex, well supported by first-principles calculations. The incorporation of NO can stabilize and enhance the RTFM of ZnO:(Li, N) by combining with Lii to form Lii-NO complex, which restrains the compensation of Lii for VZn and makes the ZnO:(Li, N) conduct in p-type.

Nitrogen-doped ZnO thin films were grown on c-plane sapphire (Al2O3) substrates via plasma-assisted molecular beam epitaxy using plasma activated nitric oxide (NO) as the oxygen source and dopant. X-ray diffraction measurements indicate that a small NO flux benefits the crystal quality of the thin films. Hall effect measurements indicate that the electron density of the ZnO films decreases gradually with decreasing NO flux, and the conduction reverses to p-type at a certain flux. Optical emission spectra indicate that the N atom content in the NO plasma increases with decreasing NO flux, and the origin of this is discussed. X-ray photoelectron spectroscopy measurements demonstrate that the number of N atom occupied O sites in the ZnO lattice increases correspondingly.

Nitrogen-doped p-type zinc oxide (p-ZnO:N) thin films were fabricated on a-/c-plane sapphire (a-/c-Al2O3) by plasma-assisted molecular beam epitaxy. Hall-effect measurements show that the p-type ZnO:N on c-Al2O3 degenerated into n-type after a preservation time; however, the one grown on a-Al2O3 showed good stability. The conversion of conductivity in the one grown on c-Al2O3 ascribed to the faster disappearance of NO and the growing N2(O), which is demonstrated by x-ray photoelectron spectroscopy (XPS). Compressive stress, caused by lattice misfit, was revealed by Raman spectra and optical absorption spectra, and it was regarded as the root of the instability in ZnO:N.

Doping efficiency and optical and electrical properties of the N-doped ZnO films grown on c-sapphire were investigated. An anomalous Raman mode at 275 cm−1 was observed in the films grown at high Zn temperature, which implied that the films have a large number of defects induced by high N concentration. The intensity ratio of the donor-acceptor pair (DAP)/donor-bound exciton (DX) increases with increasing Zn temperature; this was attributed to the increase of N concentration. The films exhibit a conversion from n-type to p-type conductivity with increasing Zn temperature, and reproducible p-type conductivity was obtained at the Zn temperature of 255 °C. This study offers a simple and effective route to enhance the N solubility in ZnO films and confirms that the anomalous Raman mode at 275 cm−1 was related to substitution of N for O site (NO) and not related to substitution of N2 for O site (N2)O.

Surface defect-related electroluminescence (EL) was realized from a single ZnO microwire-based metal–semiconductor–metal structure on a glass substrate. ZnO microwires were successfully fabricated using a simple chemical vapour deposition approach. Schottky contacts were detected between Au electrodes and the ZnO microwire. The EL spectrum showed a broad emission band covering the visible range from 400 to 700 nm. The possible EL emission mechanism is discussed in detail in this paper.

Electrically pumped ultraviolet lasing has been realized in a Au/MgO/MgZnO/n-ZnO structure. The lasing is located at around 330 nm. Notably the threshold of the lasing is about 30.0 mA, over one order of magnitude smaller than the corresponding values reported before.

Metal-semiconductor-metal structured ultraviolet (UV) photodetector has been fabricated from zinc oxide films. The responsivity of the photodetector can reach 26 000 A/W at 8 V bias, which is the highest value ever reported for a semiconductor ultraviolet photodetector. The origin of the high responsivity has been attributed to the carrier-trapping process occurred in the metal-semiconductor interface, which has been confirmed by the asymmetric barrier height at the two sides of the metal-semiconductor interdigital electrodes. The results reported in this paper provide a way to high responsivity UV photodetectors, which thus may address a step toward future applications of UV photodetectors.

Lithium and nitrogen dual-doped ZnO films [ZnO:(Li, N)] with Li concentrations of 0%–11.2% were grown on sapphire by plasma-assisted molecular beam epitaxy, and a stable p-type ZnO:(Li, N) film was obtained by doping 6.1% of Li. The p-type conductivity of ZnO:(Li, N) is attributed to the formation of the Lii–NO complex, which depresses the compensation of Lii donor for LiZn acceptor and the generation of (N2)O donors. It is demonstrated that the Lii–NO complex can form an impurity band above the valance band maximum, resulting in a decrease in the ionization energy of the acceptor and an improvement in the conductivity and stability of the p-type ZnO:(Li, N).

Titanium dioxide (TiO2) thin films were prepared by an atomic layer deposition technique and a metal–insulator–semiconductor–insulator–metal structured ultraviolet photodetector was fabricated from the TiO2 thin films. Meanwhile, a metal–semiconductor–metal structured photodetector was also fabricated under the same condition for comparison. By measuring their photoresponse properties, it was found that the existence of an insulation layer is effective in improving the photodetector’s responsivity. The mechanism for the improvement has been attributed to the carrier multiplication occurring in the insulation layer under a high electric field. (Some figures in this article are in colour only in the electronic version)

Zinc oxide (ZnO) films have been prepared on sapphire substrates by molecular beam epitaxy. It is found that the electron concentration of the films decreases, while the mobility increases with increasing the film thickness. Temperature-dependent Hall measurement reveals the existence of a degenerate layer at the ZnO/sapphire interface, which will increase the electron concentration and decrease the mobility in the ZnO film. By using a two-layer conduction model, the electron concentration and mobility of the film excluding the influence of the degenerate layer have been determined. A fitting to the corrected electron concentration of the ZnO film yields an activation energy of about 31 meV for the residual donors.

Stable p-type Sb-doped ZnO (ZnO:Sb) was fabricated reproducibly by sintering mixture of ZnO and Sb2O3 powders under 5 GPa at temperatures of 1100–1450 °C. The best p-type ZnO:Sb with resistivity of 1.6×10−2 Ω cm, carrier concentration of 3.3×1020 cm−3, and mobility of 12.1 cm/V s was obtained by doping 4.6 at. % Sb and sintering at 1450 °C. The p-type conduction is due to complex acceptor formed by one substitutional Sb at Zn site and two Zn vacancies. The acceptor level was measured to be 113 meV. Effect of pressure on formation and electrical properties of the p-type ZnO:Sb is discussed.

Temperature-dependent photoluminescence of MgZnO alloy film has been studied, and it is found that the emission intensity increases significantly at a certain temperature range and then decreases when increasing temperature further. The anomalous increase is resulted from the localized excitons in MgZnO alloy, as revealed by the enhanced second-order longitudinal optical phonon in the Raman spectrum of the MgZnO film. A schematic model was suggested to depict the carrier transportation process in the MgZnO film considering the existence of localized exciton states. The results reported in this paper indicate that localized excitons in MgZnO alloy can result in greatly enhanced emission efficiency, which is eagerly wanted for the application of ZnO-based materials in high-efficiency light-emitting devices.

P-Mg0.2Zn0.8O/n-ZnO heterojunction ultraviolet photodiode was fabricated on a sapphire substrate by plasma-assisted molecular beam epitaxy. The current–voltage measurement indicates that the heterojunction has a weak rectifying behaviour with a turn-on voltage of ~5 V. The spectral response measurement shows that the photodiode has a peak responsivity at around 340 nm, and it has a wide detection range in the ultraviolet region from 400 to 320 nm. The response in the long and short wavelength region is due to the contribution of the n-ZnO and p-MgZnO layers, respectively. The ultraviolet–visible rejection ratio (R340 nm/R500 nm) of two orders of magnitude was obtained at a reverse bias of 8 V.

In this study, we report the growth of MgxNi1?xO thin films on quartz substrates by electron beam evaporation. The absorption edge shows a blue shift from 340?nm to 260?nm with increase in the Mg content from 0.2 to 0.8. A metal?semiconductor?metal structured photodetector is fabricated from the Mg0.2Ni0.8O film. At a bias of 5?V, the dark current of the photodetector is about 70?nA. The maximum responsivity is about 147.3??A?W?1 at 320?nm. In addition, the ultraviolet (UV) (320?nm) to visible (400?nm) rejection ratio is nearly two orders of magnitude. Based on these results, it is proposed that MgxNi1?xO is a potential candidate for application in UV photodetectors.

The phase stability of cubic Mg0.55Zn0.45O thin film grown by metal-organic chemical vapor deposition was studied through continuous thermal annealing. The crystal quality and surface smoothness were greatly improved after a continuous thermal annealing at 750 °C. It is attributed to the reducing of interstitial Zn by thermal annealing. However, phase segregation occurred when the sample was annealed at a higher temperature (850 °C), which is identified from both x-ray diffraction patterns and optical transmission spectra.

Valence-band offset (VBO) of n-ZnO/p-NiO heterojunction has been investigated by x-ray photoelectron spectroscopy. Core levels of Zn 2p and Ni 2p were used to align the VBO of n-ZnO/p-NiO heterojunction. It was found that n-ZnO/p-NiO heterojunction has a type-II band alignment and its VBO is determined to be 2.60±0.20 eV, and conduction-band offset is deduced to be 2.93±0.20 eV. The experimental VBO value is in good agreement with the calculated value based on the electron affinity of ZnO and NiO.

Cubic MgxZn1?xO thin films (x = 0.70) were grown on quartz substrates by radio frequency magnetron sputtering and a metal?semiconductor?metal structured photodetector was fabricated based on the film. The photodetector exhibited a dominant response peak at about 225?nm with a cutoff wavelength at about 230?nm, which lies in the solar-blind spectrum range (220?280?nm). A shoulder also appeared in the response spectrum at about 265?nm besides the dominant peak, which resulted from the phase separation in the Mg0.70Zn0.30O thin films as revealed by transmission electron microscopy. The photodetector showed very low dark current (2?pA at 3?V bias).

A series of MgxZn1−xO thin films has been prepared by metalorganic chemical vapor deposition and metal-semiconductor-metal structured ultraviolet photodetectors are fabricated from these films. The cutoff wavelengths of the photodetectors can cover the whole solar-blind spectrum range (220–280 nm) by varying Mg content in the MgxZn1−xO thin films. As a representative, the photodetector fabricated from Mg0.52Zn0.48O shows an ultraviolet/visible rejection ratio of about four orders of magnitude, and the dark current is 15 pA at 10 V bias. These results demonstrate that high-performance photodetectors operating in the whole solar-blind spectrum range can be realized in MgxZn1−xO films.

ZnMgO films are prepared by RF magnetron sputtering using a composite target and the Mg composition of the samples can be controlled easily even at a high growth temperature. The metal?semiconductor?metal photodetector based on the wurtzite Zn0.6Mg0.4O film exhibits a very low dark current (5?pA at |Vbias| = 3?V) and a high UV/visible rejection ratio (more than three orders of magnitude). The peak responsivity of the photodetector is at around 270?nm and a very sharp cutoff wavelength is at a wavelength of about 295?nm corresponding to the absorption edge of the Zn0.6Mg0.4O film.

We report temperature dependent photoluminescence studies on phosphorus doped ZnO nanowires. The shape of the spectra is very similar to those of phosphorus doped ZnO films. The photoluminescence spectrum at 10K is dominated by neutral acceptor bound exciton (AX0) emissions. The acceptor binding energy determined also agrees with the corresponding value in phosphorus doped films. Studies on the AX0 intensity show two quenching channels, associated with the thermal dissociations of AX0 to a free exciton and of shallow residual donors. The residual donors revealed provide a clue for the difficulty in p doping of ZnO.

In this work, MgxZn1−xO (MZO) thin films were grown on quartz by rf magnetron sputtering technology. It was found that MZO films possess preferred c-axis orientation and exhibit hexagonal wurtzite structure up to a Mg composition of 44.26 mol%. Furthermore, the band gap determined by absorption spectra was smaller than the theoretical calculation for the as-grown MZO thin film. The band gap blueshifted initially and then redshifted with increasing the annealing temperature of the MZO films. The reason for the shift was attributed to the displacement, effusion of Mg atoms in the films and phase separation at different annealing temperatures.

Zn0.76Mg0.24O p-n photodiode was fabricated on (000l) Al2O3 substrate by plasma-assisted molecular beam epitaxy. Ni∕Au and In metals deposited using vacuum evaporation were used as p-type and n-type contacts, respectively. Current-voltage measurements on the device showed weak rectifying behavior. The photodetectors exhibited a peak responsivity at around 325nm. The ultraviolet-visible rejection ratio (R325nm∕R400nm) of four orders of magnitude was obtained at 6V bias. The photodetector showed fast photoresponse with a rise time of 10ns and fall time of 150ns. In addition, the thermally limited detectivity was calculated as 1.8×1010cmHz1∕2∕W at 325nm, which corresponds to a noise equivalent power of 8.4×10−12W∕Hz1∕2 at room temperature.

FeSe film was prepared on GaAs (001) substrate by low pressure metal-organic chemical vapor deposition. The x-ray diffraction measurement indicated that the sample was preferentially oriented with tetragonal structure. The structure relationship between FeSe epilayer and GaAs (001) substrate has been studied. The critical behavior in the temperature-dependent resistivity at ∼290K is close to the Curie temperature, which confirmed that the transformation from ferromagnetism to paramagnetism could be responsible for the critical behavior.

The optical and electrical properties of FeSe thin films are studied by both optical absorption and Hall measurements, which suggest that ferromagnetic FeSe is a metal instead of a semiconductor. No absorption gap is observed in the whole spectrum range from far infrared to ultraviolet. Temperature dependent transport measurement indicates that FeSe has a resistivity about 10−3Ωcm. It is also found that there is a transition from n-type conductivity at low temperatures to p-type conductivity at higher temperatures in FeSe, which is attributed to the two-carrier transport nature and the thermal activation of localized carriers in the thin film.