B.H. Li

X-ray photoelectron spectroscopy measurement of n-ZnO/p-NiO heterostructure valence-band offset

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.

ZnO light-emitting devices with a lifetime of 6.8 hours

Lithium-nitrogen doped p-type Mg0.25Zn0.75O films have been realized, and p-Mg0.25Zn0.75O/n-ZnO single-heterostructured light-emitting devices (LEDs) have been constructed. Obvious emission at around 392 nm has been observed from the LEDs under the injection of continuous current, which can be attributed to the near-band-edge emission of ZnO. The LED can work continuously for 6.8 h under a continuous current of 20 mA, revealing the good reliability of the LED. The results reported in this letter reveal that reliable ZnO-based LEDs can be realized, thus high-performance ZnO-based LEDs may be promised in the future.

Biaxial stress-dependent optical band gap, crystalline, and electronic structure in wurtzite ZnO: Experimental and ab initio study

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 sm...

Valence-band offset of epitaxial ZnO∕MgO (111) heterojunction determined by x-ray photoelectron spectroscopy

The valence-band offset of ZnO∕MgO (111) heterojunction has been directly measured by x-ray photoelectron spectroscopy. Excluding the strain effect, the valence-band offset is determined to be 0.87±0.20eV, and the conduction-band offset ΔEC is deduced to be −3.59±0.20eV, indicating that ZnO∕MgO heterojunction has a type-I band alignment. The conduction-band and valence-band offset of MgO∕ZnO is used to interpret the origination of p-type conduction in undoped MgxZn1−xO alloy and deeper acceptor level in undoped and N-doped p-type MgxZn1−xO alloy than in ZnO.

Investigation on the formation mechanism of p-type Li-N dual-doped ZnO

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).

Valence band offset of ZnO/Zn0.85Mg0.15O heterojunction measured by x-ray photoelectron spectroscopy

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.

Ferromagnetic FeSe: Structural, electrical, and magnetic properties

FeSe thin films were grown on GaAs (001) substrates using low-pressure metalorganic chemical vapor deposition. X-ray diffraction analysis showed that FeSe thin films were in tetragonal structure with (002) orientation. It was found that the FeSe thin films were ferromagnetic above room temperature, revealing a maximum saturation magnetization about 590emu∕cc along the in-plane magnetic easy axis. The Hall measurement indicated that the as grown FeSe thin films was of p-type conduction with hole concentration of as high as 1020∼1021cm−3. The magnetic circular dichroism spectrum was employed to study the electronic structure.FeSe thin films were grown on GaAs (001) substrates using low-pressure metalorganic chemical vapor deposition. X-ray diffraction analysis showed that FeSe thin films were in tetragonal structure with (002) orientation. It was found that the FeSe thin films were ferromagnetic above room temperature, revealing a maximum saturation magnetization about 590emu∕cc along the in-plane magnetic easy axis. The Hall measurement indicated that the as grown FeSe thin films was of p-type conduction with hole concentration of as high as 1020∼1021cm−3. The magnetic circular dichroism spectrum was employed to study the electronic structure.

A reproducible route to p-ZnO films and their application in light-emitting devices

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.

Evidence of cation vacancy induced room temperature ferromagnetism in Li-N codoped ZnO thin films

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.

Two-carrier transport and ferromagnetism in FeSe thin films

α- and β‐FeSe thin films were grown by metal organic chemical vapor deposition. Compared to the other parameters, the growth temperature shows decisive influence on the phase transition of the FeSe samples. In temperature-dependent electrical measurements, n-type to p-type reversion was observed for both the α- or and β‐FeSe samples. Furthermore, the p-type character of the films becomes more and more obvious with increasing the Se∕Fe atomic ratio in the samples. Ferromagnetism was observed in the α‐FeSe films although which is not supported by calculation on density of states. The ferromagnetic character shows significant dependence on Se∕Fe atomic ratio in the films and was attributed to the Fe vacancies or Fe clusters in the α‐FeSe thin films. The magnetic domain and hysteresis loop of the β‐FeSe thin films are also studied.