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MgZnO thin films were deposited on c-plane sapphire and fused quartz substrates with the same growth parameters by rf-reactive magnetron sputtering. Both the MgZnO thin films on the two substrates are single hexagonal phase and show similar Mg content. However, the absorption edge of MgZnO thin film on sapphire substrate shows a blue shift compared with that on fused quartz substrate. Similar shift also appears in the photoresponse of the detectors based on them. These phenomena were attributed to the more Mg atoms in grain boundary caused by the smaller grain size in MgZnO film on fused quartz substrate.

Wurtzite MgxZn1−xO thin films were grown on sapphire substrates by low-pressure metal-organic chemical vapor deposition. The as-grown films show clear exciton absorption at room temperature until the composition x = 0.25. A representative metal–semiconductor–metal structured photodetectors were fabricated from Mg0.06Zn0.94O film showed a peak responsivity of about 14.62 A/W at 340 nm, and the ultraviolet-visible rejection ratio (R340 nm/R400 nm) was more than two orders of magnitude at 3 V bias. The photodetector showed fast photoresponse with a rise time of 20 ns and a fall time of 400 ns.

Cubic MgZnO thin films were prepared on fused quartz substrate by radio frequency (rf) reaction magnetron co-sputtering technique, and composition modulation of cubic MgZnO films could be accomplished by changing rf power on Zn metal target. During growth process crystal quality of our MgZnO films can be improved by increasing the ratio of O 2 /Ar. Unlike reported growth of (1 1 1) plane cubic MgZnO films, our cubic MgZnO films show (2 0 0) plane growth properties. This phenomenon was assigned to a preferred selection growth characteristic of MgZnO on non-crystalline quartz substrate. Band gaps of cubic MgZnO films calculated by empirical formula are different from that obtained in the absorption spectrum. It is suggested that parts of Mg do not contribute to the band gaps of Mg x Zn 1− x O thin films.

The polar and non-polar ZnO thin films were fabricated on cubic MgO (1 1 1) and (0 0 1) substrates by plasma-assisted molecular beam epitaxy. Based on X-ray diffraction analysis, the ZnO thin films grown on MgO (1 1 1) and (1 0 0) substrates exhibit the polar c-plane and non-polar m-plane orientation, respectively. Comparing with the c-plane ZnO film, the non-polar m-plane ZnO film shows cross-hatched stripes-like morphology, lower surface roughness and slower growth rate. However, low-temperature photoluminescence measurement indicates the m-plane ZnO film has a stronger 3.31 eV emission, which is considered to be related to stacking faults. Meanwhile, stronger band tails absorbance of the m-plane ZnO film is observed in optical absorption spectrum.

The full regions of the electronic and optical properties of in-plane biaxial strained thin film ZnO are studied using pseudopotential plane-wave method. The fundamental band gap at the Γ point increase linearly with the increase of tensile strains, but decreased with the compressive ones. The strains affected the local tetrahedral symmetry, and so the splitting of crystal field energy. The band dispersion relation of the valence band maximum changes with the strains, which means the residual strains have effects on the effective hole mass, thus the transportation properties of the p -type ZnO. The changes tendency of optical properties up to full regions under strains have been shown and discussed.

We report the growth of cubic Mg x Zn 1− x O alloy thin films on quartz by electron beam evaporation. It can be found that all the samples have sharp absorption edges by the absorption measurements. X-ray diffraction measurements indicate the Mg x Zn 1− x O films are cubic phase with preferred orientation along the (1 1 1) direction. Energy dispersive spectrometry (EDS) demonstrates that the Mg concentration in Mg x Zn 1− x O films is much higher than the ceramic target used, and the composition can be tuned in a small scope by varying the substrate temperature and the beam electric current. The reasons of this phenomenon are also discussed.

Photoluminescence of CdSe thin films under different growth conditions was studied in this paper, and typical double emission peaks were observed at room temperature. Temperature and excitation intensity-dependent photoluminescence studies in combination with X-ray diffraction measurements revealed that the two emission peaks originate from cubic- and hexagonal-phased CdSe, respectively, which indicates that the crystalline phases of CdSe can be controlled by changing the growth conditions in metalorganic chemical vapor deposition process.

The undoped ZnO thin films grown on Si (1 1 1) substrates by plasma-assisted molecular beam epitaxy (P-MBE) were reported. The effects of growth temperature, the thickness of low temperature ZnO buffer layer and Zn/O ratio on the quality of high-temperature-overgrown ZnO main layer were studied by atomic force microscopy (AFM), X-ray diffraction (XRD) and room-temperature photoluminescence (PL) spectra. These results showed that it was difficult to directly obtain high quality ZnO films on Si substrate. By introducing a thin ZnO buffer layer at 350 °C, c -axis preferred orientation ZnO films with improved optical properties were obtained at 750 °C. However, the thickness of ZnO buffer layer and Zn/O ratio in the ZnO main layer greatly influenced the quality of high-temperature-overgrown ZnO main layer.

Fe-based cadmium sulfide alloy thin films have been grown on c-plane sapphire substrates by a low-pressure metalorganic chemical vapor deposition technique at different growth temperatures. From X-ray diffraction and absorption spectra of the samples, the evolutions with growth temperature show an inflexion at the growth temperature of 300 degrees C. This was attributed to the phase transformation from zinc-blende to wurtzite. With increasing growth temperature from 270 degrees C to 360 degrees C, Fe concentration in the films increases monotonously. The electronic states of Cd1-xFexS were investigated by X-ray photoelectron spectroscopy. Magnetic measurement shows Van Vleck paramagnetism of the Cd1-xFexS thin film in the temperature region below 7 K. (C) 2006 Elsevier Ltd. All rights reserved.

In this work, ZnS thin films with single-phase zinc blende structure were fabricated on silica glass substrates by plasma-assisted metalorganic chemical vapor deposition (PA-MOCVD). Dimethyl zinc and hydrogen sulfide were used as the precursors. The films were characterized using X-ray diffraction and photoluminescence spectroscopy. The optimal temperature for ZnS growth, 350 °C, was confirmed via a series of growth at different temperatures. In order to obtain stoichiometric ZnS films using PA-MOCVD, larger VI / II gas flow ratios was needed than the one used without plasma assistance. The reason for this phenomenon is discussed.

In this paper, Mg 0.12 Zn 0.88 O/ZnO heterostructures were fabricated on c -plane sapphire (Al 2 O 3 ) substrates by plasma-assisted molecular beam epitaxy (P-MBE). The quality of the Mg 0.12 Zn 0.88 O alloy thin film was characterized by X-ray diffraction (XRD) and reflection high-energy electron diffraction (RHEED). Optical properties of the Mg 0.12 Zn 0.88 O/ZnO heterostructure were studied by absorption and photoluminescence (PL) spectra. At room temperature (RT), Mg 0.12 Zn 0.88 O/ZnO heterostructures show two absorption edges originating from ZnO and Mg 0.12 Zn 0.88 O layers, respectively. In PL spectra, two ultraviolet emission bands related to the ZnO layer and the Mg 0.12 Zn 0.88 O layer were observed. The emission band from Mg 0.12 Zn 0.88 O layer dominates at moderately lower temperature, and the luminescence of ZnO becomes gradually important with increasing temperature. This is suggested to exist as a potential barrier in the interface and to restrict the relaxation of the carriers from the Mg 0.12 Zn 0.88 O layer to ZnO layer. As the thickness of ZnO layer decreases, the emission from the Mg 0.12 Zn 0.88 O layer becomes weaker and weaker. When the ZnO thickness is up to 2 nm, only the luminescence of the ZnO layer is observed at RT. The quenching of the emission corresponding to the MgZnO layer indicates the existence of a strong injection process in the samples with thinner ZnO layer.

Epitaxial growth of cubic Mg(0.33)Zn(0).(67)O films on MgO (1 0 0) substrate by metalorganic chemical vapor deposition (MOCVD) was reported. X-ray diffraction (XRD) omega-scans and phi-scans demonstrate the films exhibit single (2 0 0) orientation and highly uniform in-plane orientation with four-fold symmetry. The epitaxial relationship between the layer and substrate is MgZnO (1 0 0)//MgO (1 00). Smooth surface with the rms roughness of 1.8 nm in 5 x 5 mu m area is obtained. Atomic force microscopy (AFM) reveals formation of islands attributed to the strain in the epitaxial MgZnO films. These results demonstrate the high quality single crystal Mg(0.33)Zn(0.67)O films and their potential in high-performance deep ultraviolet optoelectronic devices. (C) 2010 Elsevier B.V. All rights reserved.

A Mg0.48Zn0.52O thin film was deposited on a sapphire substrate by metal-organic chemical vapor deposition, and the thin film exhibits a single cubic phase with high crystal quality from X-ray diffraction measurements. A metal–semiconductor–metal (MSM) structured photodetector was fabricated on the film. The device exhibits a peak response of 268 nm with a cutoff wavelength at 283 nm. Rise and decay times of 10 ns and 150 ns, respectively, are obtained with a load resistance of 50 Ω . The pulse response for the device is limited by the RC time constant.

Absatract Mn-doped ZnO nanocrystallite thin films with different Mn contents were synthesized by sol–gel method. Thermal annealing processes in N2 ambient at 500 °C were employed to improve the film quality. Physical properties were investigated using XRD, PL and EPR. XRD spectra show all the films are hexagonal wurtzite structures and diffraction peaks shift to the big angle with increasing Mn content, which indicates that Mn has entered the ZnO lattice and substituted the Zn site. Small quantity of manganese oxide phase also presents in the spectra. Room-temperature photoluminescence spectra show the ultraviolet emission shift to low-energy side with the increasing Mn content, and the defect-related emissions were passivated by manganese oxide. EPR pattern indicates that Mn2+ exist in ZnMnO system.

Optical properties of ZnO thin films fabricated on GaAs substrate by molecular beam epitaxy method were investigated. The emissions from acceptor-bound exciton and free electron acceptor were enhanced after annealing at 550 °C for 1 h. In the photoluminescence spectra of the annealed sample, donor–acceptor pair emission was confirmed by changing excitation density. Hall measurement indicated that the conductivity of the ZnO thin film converses from n-type to high resistant after thermal diffusion by annealing at 550 °C.

High Mg content mixed-phase Zn0.38Mg0.62O was deposited on a-face sapphire by plasma-assisted molecular beam epitaxy, based on which a metal-semiconductor-metal solar-blind ultraviolet (UV) photodetector was fabricated. The dark current is only 0.25 pA at 5 V, which is much lower than that of the reported mixed-phase ZnMgO photodetectors. More interestingly, different from the other mixed-phase ZnMgO photodetectors containing two photoresponse bands, this device shows only one response peak and its −3 dB cut-off wavelength is around 275 nm. At 10 V, the peak responsivity is as high as 1.664 A/W at 260 nm, corresponding to an internal gain of ∼8. The internal gain is mainly ascribed to the interface states at the grain boundaries acting as trapping centers of photogenerated holes. In view of the advantages of mixed-phase ZnMgO photodetectors over single-phase ZnMgO photodetectors, including easy fabrication, high responsivity, and low dark current, our findings are anticipated to pave a new way for the dev...

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.

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.

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.

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.

Cd1−xFexS thin films with different Fe contents were grown on c-plane sapphire by low-pressure metal organic chemical vapor deposition. The resistivity of the thin films was found to increase with the addition of more Fe contents into the Cd1−xFexS thin films by elevating the growth temperature, which was attributed to the ionization of holes from trivalent Fe ions. High density Fe doping eventually reversed the conductivity of Cd1−xFexS thin film from n to p type. With increasing Fe content, the band-to-band transition at 2.5eV was suppressed while the emission from the donor-acceptor pairs at 2.0–2.4eV finally dominated the photoluminescence spectra.

A wurtzite N-doped MgZnO film with 20at.% Mg (MgZnO:N) was grown by plasma-assisted molecular beam epitaxy on c-plane sapphire using radical NO as oxygen source and nitrogen dopant. The as-grown MgZnO:N film behaves n-type conduction at room temperature, but transforms into p-type conduction after annealed for 1h at 600°C in an O2 flow. The p-type MgZnO:N has a hole concentration of 6.1×1017cm−3 and a mobility of 6.42cm2∕Vs. X-ray photoelectron spectroscopy measurement indicates that substitution of N for O site is in forms of N atom (N)O and N molecule (N2)O for the as-grown MgZnO:N, but almost only in a form of (N)O for the annealed MgZnO:N. The mechanism of the conduction-type transition induced by annealing is discussed in the present work.