H.Z. Wu

Low-temperature epitaxy of ZnO films on Si(001) and silica by reactive e-beam evaporation

Abstract Low-temperature epitaxy of ZnO films was carried out on both Si(0xa00xa01) and silica substrates by reactive e-beam evaporation. Growth temperature was varied between 100°C and 450°C. The optimal growth temperatures were between 200°C and 300°C for Si(0xa00xa01) substrates and 300°C and 350°C for silica, respectively. Structural and optical characteristics grown on both of Si and silica were compared by measurements of X-ray diffraction, Raman scattering and photoluminescence excitation (PLE) spectroscopy. X-ray diffraction showed the ZnO films grown both on Si(0xa00xa01) and silica substrates were all highly c -axis-oriented and their linewidth of (0xa00xa02) diffraction peak is significantly smaller than that measured from ZnO films deposited by magnetron sputtering. However, Raman scattering demonstrated the presence of excess zinc in the ZnO/silica structure. PLE measurements exhibited the sharp band-absorption edge and exciton absorption in the ZnO films on Si(0xa00xa01) and poor optical properties of ZnO on silica. PLE also revealed that the absorption characteristic near the band edge remarkably changed with the variation of oxygen content in the ZnO lattice while X-ray diffraction showed that the crystalline structures of ZnO films grown under different O 2 pressure almost remained unchanged. The underlying mechanism of low-temperature epitaxy of high-quality ZnO films and its applications in electronic and optoelectronic devices are discussed.

Optical studies of ZnO quantum dots grown on Si(001)

Abstract Zinc oxide quantum dot (QD) structures were grown on Si(0xa00xa01) substrates by reactive electron beam evaporation at low substrate temperature. The samples were then annealed at various temperatures. Photoluminescence excitation characterization revealed the red shifts of the optical absorption peaks related to the QD transitions after the annealing of the samples, i.e. 340xa0nm for the as-grown QD sample, and 363xa0nm for the annealed sample. Effective mass approximation is used to interpret the observed optical transitions of the ZnO quantum dots. The quantum dot sizes grown by this method are estimated to be between 2.6 and 3.0xa0nm in diameters. Calculation also shows that the quantum-confinement effect of the ZnO QDs becomes very weak when the size of ZnO quantum dots is larger than 8.0xa0nm.

Temperature-dependent optical properties of hexagonal and cubic MgxZn1-xO thin-film alloys

Both hexagonal and cubic films were grown on c-plane (0001) sapphire substrate at low temperature. X-ray diffraction measurements show that the cubic MgxZn1−xO films grow along the [111] direction while the hexagonal ZnO films grow along [0001]. The temperature-dependent optical properties of MgxZn1−xO films were measured by ultraviolet optical transmission with temperature variation from 10 to 300xa0K and analysed by theoretically fitting the optical absorption spectra. For MgxZn1−xO with , only stable hexagonal phase was observed and the optical absorption edge red shifts with temperature increase monotonically. For MgxZn1−xO with , the crystal structure is cubic at 300xa0K. However, as measurement temperature decreases from 300 to 10xa0K an abnormality of the optical absorption is observed, which is attributed to the possible phase transition from cubic to hexagonal structure. The underlying physical mechanism for the crystal phase transition is attributed to the interaction of stress with stacking faults in the cubic MgxZn1−xO.

Ba0.9Sr0.1TiO3-based Bragg reflectors fabricated from one single chemical solution

Periodic ferroelectric multilayers consisting of alternating stack of the dense and porous Ba0.9Sr0.1TiO3 layers have been fabricated by spin-coating and annealing sol-gel techniques using one single precursor. With 16 periods, the Ba0.9Sr0.1TiO3 multilayers exhibit excellent performance as dielectric mirrors: symmetric peak reflectivities of above 95% and flattopped stop bands of about 75nm. The reflectance peak position is tunable through varying the spinning rate in the spin-coating process. The lead-free Ba0.9Sr0.1TiO3 multilayers show high stability.

Observation of negative thermo-optical coefficient in cubic MgZnO thin films

We report on the investigation of thermo-optical coefficient (dn∕dT) in cubic MgZnO thin films through temperature-dependent (10–295K) optical transmission measurements. As a result of potential fluctuation, we observe an anomalous thermal behavior of the band gap in the ternary MgZnO alloy, which limits the observation of negative dn∕dT up to 200K. The negative dn∕dT is evidenced by the phase shift from the prominent Fabry-Perot interference fringes and modeled by taking into account the thermal and electronic background contribution, as well as the Wannier exciton effect. It is found that the large thermal expansion in cubic MgZnO plays a key role in the observed negative dn∕dT.

Band alignment determination of ZnO/PbSe heterostructure interfaces by synchrotron radiation photoelectron spectroscopy

Band offsets at ZnO/PbSe heterostructure interfaces are determined by synchrotron radiation photoelectron spectroscopy. A type-I band alignment with the valence band offset (VBO) ?EV?=?0.73?eV and conduction band offset (CBO) ?EC?=?2.36?eV is concluded for ZnO/PbSe film heterojunctions. The approach is extended to ZnO/PbSe nanocrystal (NC) heterojunctions, which reveals that band alignment can be adjusted via varying the dot size and a type-II alignment is formed when the dot size is ?5?nm. The small conduction band offset of the ZnO/PbSe film heterojunction and the tunable band alignment of the ZnO/PbSe NCs heterojunction with different crystal sizes shall benefit the design and fabrication of improved optoelectronic devices.

Strain relaxation in graded SiGe grown by ultra-high vacuum chemical vapor deposition (UHVCVD)

Abstract We have used ultra-high vacuum chemical vapor deposition (UHVCVD) system to grow step-graded SiGe/Si structures with relatively high growth temperature (780°C). By using the phenomenon of Ge segregation to the growing surface during epitaxial growth we realized almost a linear Ge content variation in the buffer layer. X-ray diffraction and Raman spectroscopy measurements show that the upper layer is totally relaxed and the relaxation ratio is equal to 1. However, the measured results show that the density of dislocation in the compositionally graded structure is much lower than that in single-step buffer structures.

Band alignment at the interface of PbTe/SnTe heterojunction determined by X-ray photoelectron spectroscopy

We report the determination of band alignment of PbTe/SnTe (111) heterojunction interfaces using X-ray photoelectron spectroscopy (XPS). Multiple core levels of Pb and Sn were utilized to determine the valence band offset (VBO) of the heterojunction. The XPS result shows a type-III band alignment with the VBO of and the conduction band offset (CBO) of . The experimental determination of the band alignment of the PbTe/SnTe heterojunction shall benefit the improvement of PbTe/SnTe-related optoelectronic and electronic devices.

Observation of thermal-misfit strain relaxation in a PbTe semiconductor grown on Cd0.96Zn0.04Te(1?1?1)

The thermal-misfit strain relaxation in epitaxial PbTe grown on Cd0.96Zn0.04Te(1u20091u20091) substrates has been studied by the combined characterization of atomic force microscopy and high-resolution transmission electron microscopy. It is shown that the strain relaxes by the movement of dislocations in the 1u20091u20090–{1u20090u20090} primary glide system and leaves straight slip steps on the surface. The thermal-misfit strain relaxation is greatly affected by the growth temperature and post-growth cooling rate. Post-growth treatment with a slower cooling rate improves the crystalline quality of PbTe grown on Cd0.96Zn0.04Te(1u20091u20091). Because of the easy dislocation glide down to the interface in the 1u20091u20090–{1u20090u20090} primary glide system, PbTe/Cd0.96Zn0.04Te(1u20091u20091) heterostructures exhibit good crystal and optical qualities, which indicate that PbTe/CdZnTe heterostructures may become a promising candidate for the fabrication of PbTe-based mid-infrared optoelectronic devices.