-
[show abstract]
[hide abstract]
ABSTRACT: By employing an insulating zinc oxide (i-ZnO) as an electron accelerating layer, and an n-type ZnO as an active layer, ultraviolet (UV) emissions at 385 nm caused by the excitation of the n-ZnO layer by the accelerated electrons from the i-ZnO layer have been realized. By replacing the active layer with larger bandgap Mg0.39Zn0.61O and properly optimizing the structure, shorter wavelength emissions at around 328 nm have been obtained. Considering that the p-type doping of wide bandgap semiconductors is still a challenging issue, the results reported in this Letter may provide a promising alternative route to UV emissions.
Optics Letters 05/2012; 37(9):1568-70. · 3.40 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Owing to the low doping concentration of nitrogen and strong compensation of intrinsic donors, the attainment of highly conductive
p-type ZnO films remains one of the largest challenges for the application of ZnO. An approach has been proposed to increase
the doping concentration of nitrogen in ZnO by exposing the ZnO:N films in the ambient of nitrogen plasma periodically in
this paper. Hall measurements and photoluminescence spectroscopy indicate that this approach is effective in improving the
hole concentration in ZnO films. Under the optimized conditions, a p-type ZnO film with a hole concentration of 1.68×1018 cm−3 has been achieved.
Journal of Materials Science 04/2012; 45(15):4093-4096. · 2.02 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: By proper controlling the carrier generation and multiplication processes, an Au/MgO/Mg0.52Zn0.48O/MgxZn1−xO/n-ZnO structure has been designed and fabricated. In this structure, holes are multiplied via an impact ionization process in the MgO layer and injected into the Mg0.52Zn0.48O layer, and electrons are injected into the Mg0.52Zn0.48O layer from the n-ZnO layer through a composition-gradient MgxZn1−xO bridging layer. With the injection of electrons and holes, a deep ultraviolet emission at around 276 nm, coming from the Mg0.52Zn0.48O active layer, has been observed. The results reported in this letter may provide a promising route to high performance deep ultraviolet light-emitting devices.
Applied Physics Letters 09/2011; 99(10):101110-101110-3. · 3.84 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: 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.
Journal of Applied Physics 05/2011; 109(9):093518-093518-5. · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Although great efforts have been made, reproducible p-type doping is still one of the largest hurdles that hinders the optoelectronic applications of ZnO. In this Letter, a reproducible route to p-type ZnO films employing lithium-nitrogen as a dual-acceptor dopant has been demonstrated, and p-i-n structured light-emitting devices (LEDs) have been constructed. Obvious purple emissions have been observed from the LEDs, confirming the applicability of the p-type ZnO films in optoelectronic devices. The results reported in this Letter provide a reproducible route to p-type ZnO films, and thus may lay a solid ground for future optoelectronic applications of ZnO.
Optics Letters 02/2011; 36(4):499-501. · 3.40 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: 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.
Applied Physics Letters 12/2010; 97(25):251102-251102-3. · 3.84 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: 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).
Applied Physics Letters 11/2010; 97(22):222101-222101-3. · 3.84 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: ZnMgO nanowalls were prepared by plasma-assisted molecular beam epitaxy without a catalyst on c-Al2O3 substrate. The obtained nanowalls have preferred orientation along c axis. The nanowalls are about 10 to 20 nm in thickness and about 50 nm in height. Only Zn, Mg, O and Al signals are detected in the nanowalls from the energy dispersive spectroscopy (EDS). The Mg content is about 3% in ZnMgO nanowalls. The room temperature photoluminescence (PL) spectra shows the emission peak of the ZnMgO nanowalls at 3.346 eV. The origin of the ultraviolet emission is discussed with the help of temperature-dependent PL spectra. The ultraviolet emission band is free exiton recombination observed in the low temperature PL spectra (at 81 K). We also observe the free-to-acceptor (FA) emission of the ZnMgO nanowalls. The acceptor binding energy obtained from photoluminescence studies is about 123 meV. The results show that Mg doping leads to an increase of the acceptor binding energy. The possible growth mechanism of the ZnMgO nanowall networks was discussed.
Journal of Nanoscience and Nanotechnology 03/2010; 10(3):1681-4. · 1.56 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: By employing a relatively low growth temperature and oxygen-rich conditions, single-crystalline cubic MgZnO films were prepared. A solar-blind deep ultraviolet (DUV) photodetector was finished on the MgZnO film. The maximum responsivity of the photodetector is 396 mA/W at 10 V bias, which is almost three orders of magnitude larger than the highest value ever reported in MgZnO-based solar-blind photodetectors. The dark current density is 1.5×10−11 A/cm2, comparable with the smallest value ever reported in solar-blind photodetectors. The improved performance reveals that the single-crystalline cubic MgZnO films have great potential applications in DUV optoelectronic devices.
Applied Physics Letters 09/2009; 95(13):131113-131113-3. · 3.84 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: 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.
Journal of Physics D Applied Physics 09/2009; 42(19):195403. · 2.54 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: 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.
Applied Physics Letters 07/2009; 95(2):022101-022101-3. · 3.84 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: 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.
Journal of Applied Physics 06/2009; · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We report on the fabrication of an n-Mg0.12Zn0.88O/p-GaN heterojunction light-emitting diode with an MgO dielectric interlayer by plasma-assisted molecular beam epitaxy. The current−voltage curve of the heterojunction diode showed obvious rectifying characteristics with a threshold voltage of about 8 V. Under forward bias, an ultraviolet electroluminescence (EL) emission located at about 374 nm coming from the Mg0.12Zn0.88O layer was observed at room temperature. This is one of the shortest EL emissions observed in ZnO-based pn junctions to the best of our knowledge. The origin of the EL emission was elucidated in terms of the carrier transportation process modulated by the MgO interlayer in the heterojunction.
02/2009;
-
R. Deng,
B. Yao,
Y. F. Li,
Y. M. Zhao, B. H. Li,
C. X. Shan,
Z. Z. Zhang,
D. X. Zhao,
J. Y. Zhang,
D. Z. Shen,
X. W. Fan
[show abstract]
[hide abstract]
ABSTRACT: 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.
Applied Physics Letters 01/2009; 94(2):022108-022108-3. · 3.84 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: 10 nm sized hexagonal CdS nanoparticles were decorated on the surface of well-aligned ZnO nanowall through a facile hydrothermal approach. The effects of CdS-cap layer on the optical and photoelectrical properties of ZnO nanowalls have been studied. It was found the CdS acted not only as a passivation layer to suppress the detrimental surface states of ZnO nanowalls, which reduced the deep-level emissions, but also as an interfacial carrier transport layer to reduce the probability of carrier recombination. Due to this effect a significant enhancement in ultraviolet photoconductivity of ZnO nanowalls was observed by the surface modification with CdS nanoparticles.
Applied Physics Letters 12/2008; 93(23):233115-233115-3. · 3.84 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Undoped n-ZnO films have been deposited onto p-GaN to form a pn heterojunction, and the current−voltage curve of the heterojunction shows obvious rectifying behaviors. A photodetector is fabricated from the heterojunction. Under back-illumination conditions, the GaN layer on one hand acts as a p-type counterpart for the n-ZnO layer, on the other hand as a “filter” that is transparent to the illumination light with wavelength longer than 360 nm. Because of the GaN “filter”, the photodetector shows a narrow band-pass response of only 17 nm in width. The results reported in this paper may provide a facile route to photodetectors with high spectrum selectivity.
11/2008;
-
Y. F. Li,
B. Yao,
Y. M. Lu,
Y. Q. Gai,
C. X. Cong,
Z. Z. Zhang,
D. X. Zhao,
J. Y. Zhang, B. H. Li,
D. Z. Shen,
X. W. Fan,
Z. K. Tang
[show abstract]
[hide abstract]
ABSTRACT: The relationship between band gap and biaxial stress in wurtzite ZnO thin films has been investigated by side-inclination x-ray diffraction technique and optical absorbance spectrum as well as ab initio calculation. The experimental result shows that differing from other semiconductor thin films with hexagonal structure, such as GaN, the band gap of ZnO thin films increases with the increase in biaxial tensile stress. For explaining the difference, ab initio calculation is performed to simulate the relationship between band gap and biaxial stress of wurtzite ZnO and GaN. The calculated result indicates that the band gap of ZnO increases under biaxial tensile stress but GaN is opposite, supporting our experimental result. The band offset calculation shows that the conduction-band minimum (CBM) and the valence-band maximum (VBM) of ZnO and GaN offset to low energy under biaxial tensile stress. The VBM offset of ZnO is larger than the CBM, responsible for the increase in band gap. The VBM offset of GaN is smaller than the CBM, responsible for the decrease in band gap. The difference between ZnO and GaN is attributed to the strong p-d coupling in valence band of ZnO, but strong p-p coupling in valence band of GaN.
Journal of Applied Physics 11/2008; · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: X-ray photoelectron spectroscopy was used to measure the valence band offset at the ZnO/Zn0.85Mg0.15O heterojunction grown by plasma-assisted molecular beam epitaxy. The valence band offset (ΔEV) is determined to be 0.13 eV. According to the experimental band gap of 3.68 eV for the Zn0.85Mg0.15O, the conduction band offset (ΔEC) in this system was calculated to be 0.18 eV. The ΔEc:ΔEv in ZnO/Zn0.85Mg0.15O heterojunction was estimated to be 3:2.
Applied Physics Letters 08/2008; 93(8):082108-082108-3. · 3.84 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: This work reports on the fabrication and characteristics of n-ZnO/p-GaN and n-ZnO/n-MgZnO/n-ZnO/p-GaN heterojunction light emitting diodes (LEDs). Both devices exhibited diode-like rectifying current–voltage characteristics. Room temperature electroluminescence (EL) spectra for both LEDs consisted of dominant emission at 375 nm and two weaker bands centred at 415 nm and 525 nm, which were attributed to ZnO excitonic transition and defect-related emissions from GaN and ZnO, respectively. Moreover, it was demonstrated that the single heterojunction required a higher injection current to obtain an excitonic EL than that for the n-ZnO/n-MgZnO/n-ZnO/p-GaN LEDs. This indicated that the insertion of the MgZnO layer confined the injection carriers and thus increased the intensity of excitonic emission in the ZnO active region.
Journal of Physics D Applied Physics 07/2008; 41(15):155103. · 2.54 Impact Factor
-
Z. G. Ju,
Y. M. Lu,
J. Y. Zhang,
X. J. Wu,
K. W. Liu,
B. S. Li,
D. X. Zhao,
Z. Z. Zhang, B. H. Li,
B. Yao,
D. Z. Shen
[show abstract]
[hide abstract]
ABSTRACT: Diluted magnetic semiconductors Cd1−xFexSe were grown by metalorganic chemical vapor deposition. Their structure was studied by X-ray diffraction and selected area electron diffraction. The effect of Fe doping on the morphology and photoluminescence of CdSe was investigated. The photoluminescence study indicated that the crystalline structure of the Cd1−xFexSe films changed from a mixture of cubic and hexagonal phase to a single hexagonal phase with increasing Fe content.
07/2008;
