Longxing Su

Formation behavior of BexZn1−xO alloys grown by plasma-assisted molecular beam epitaxy

We report the phase formation behavior of BexZn1−xO alloys grown by plasma-assisted molecular beam epitaxy. We find the alloy with low- and high-Be contents could be obtained by alloying BeO into ZnO films. X-ray diffraction measurements shows the c lattice constant value shrinks, and room temperature absorption shows the energy band-gap widens after Be incorporated. However, the alloy with intermediate Be composition are unstable and segregated into low- and high-Be contents BeZnO alloys. We demonstrate the phase segregation of BexZn1−xO alloys with intermediate Be composition resulted from large internal strain induced by large lattice mismatch between BeO and ZnO.

Stabilization of p-type dopant nitrogen in BeZnO ternary alloy epitaxial thin films

We demonstrate the growth of BexZn1−xO alloy thin films on c-sapphire substrates by plasma-assisted molecular beam epitaxy. The formation of BexZn1−xO is very sensitive to the Be content in the alloy. Be atoms occupy the Zn lattice sites at low Be content, but move partly to the interstitial sites with increasing Be content. We also investigated the thermal stability of N, one of the most frequently used p-type dopants, in the BexZn1−xO films. Secondary ion mass spectrometry shows that Be element plays a crucial role in stabilizing N in the BexZn1−xO host, which is favourable in improving the solid solubility and the thermal stability of acceptor dopants in ZnO-based wide-gap semiconductors.

The modulation of grain boundary barrier in ZnMgO/ZnO heterostructure by surface polar liquid

Modulation of grain boundary barrier in ZnO layer by polar liquid, was investigated in ZnMgO/ZnO heterostructures grown by plasma-assisted molecular beam epitaxy. Traditionally, surface adsorbates can only affect the surface atoms or surface electronic states. However, it was found that the electronic conduction property of ZnO far from the surface could be tailored obviously by the polar liquid adsorbed on the ZnMgO surface. Physically, this phenomenon is supposed to be caused by the electrostatical couple between the liquid polarity and the grain boundary barrier in the ZnO layer through crystal polarization field.

Enhanced Exciton Binding Energy of ZnO by Long-Distance Perturbation of Doped Be Atoms.

The excitonic effect in semiconductors is sensitive to dopants. Origins of dopant-induced large variation in the exciton binding energy (E(b)) is not well understood and has never been systematically studied. We choose ZnO as a typical high-E(b) material, which is very promising in low-threshold lasing. To the best of our knowledge, its shortest wavelength electroluminescence lasing was realized by ZnO/BeZnO multiple quantum wells (MQWs). However, this exciting result is shadowed by a controversial E(b) enhancement claimed. In this Letter, we reveal that the claimed E(b) is sensible if we take Be-induced E(b) variation into account. Detailed first-principle investigation of the interaction between dopant atoms and the lattice shows that the enhancement mainly comes from the long-distance perturbation of doped Be atoms rather than the local effect of doping atoms. This is a joint work of experiment and calculation, which from the angle of methology paves the way for understanding and predicting the E(b) variation induced by doping.

Grain boundary barrier modification due to coupling effect of crystal polar field and water molecular dipole in ZnO-based structures

Surface water molecules induced grain boundaries (GBs) barrier modification was investigated in ZnO and ZnMgO/ZnO films. Tunable electronic transport properties of the samples by water were characterized via a field effect transistor (FET) device structure. The FETs fabricated from polar C-plane ZnO and ZnMgO/ZnO films that have lots of GBs exhibited obvious double Schottky-like current-voltage property, whereas that fabricated from nonpolar M-plane samples with GBs and ZnO bulk single-crystal had no obvious conduction modulation effects. Physically, these hallmark properties are supposed to be caused by the electrostatical coupling effect of crystal polar field and molecular dipole on GBs barrier.

Comparative study on beryllium and magnesium as a co-doping element for ZnO:N

Stable nitrogen doping is an important issue in p-type ZnO research for device applications. In this paper, beryllium and magnesium are systematically compared as a dopant in ZnO to reveal their nitrogen-stabilizing ability. Secondary ion mass spectrum shows that Be and Mg can both enhance the stability of nitrogen in ZnO while Be has a better performance. Zn 2p and O 1s electron binding energies change in both MgZnO and BeZnO thin films. Donor-acceptor luminescence is observed in the BeZnO samples. We conclude that Be is a better co-doping element than Mg for p-type ZnO:N.

Resonant Raman scattering study of BexZn1−xO thin films grown on sapphire by molecular beam epitaxy

Resonance Raman spectra of BexZn1−xO alloy materials were studied using 325 nm Laser. The research showed that the Raman spectra of BexZn1−xO alloys presents a dual-mode vibration. Compare BexZn1−xO alloy with ZnO single crystal, the A1 (LO) phonon vibration mode of BexZn1−xO alloy moved to the larger wave number direction. The position of A1 (LO) phonon vibration modes of Be0.08Zn0.92O and Be0.12Zn0.88O was 580 cm−1 and 582 cm−1, respectively. In addition, the temperature-dependent Raman spectroscopy was employed for Be0.12Zn0.88O, and the phonon mode frequency shift with temperature was studied in detail. Finally, the stability of the polar and nonpolar BexZn1−xO alloy materials was studied using resonance Raman spectroscopy. The results showed that the A1 (LO) phonon mode frequency of polar BexZn1−xO alloy remained in the same position, while the nonpolar BexZn1−xO alloys moved nearly 3.5 cm−1 to larger direction after being placed in the air for two years. The reason may be that the stability of the nonpolar BexZn1−xO alloy is relatively poor upon interaction with molecule such as H2O, O2 in the air.