Ming Chun Xu

Synthesis, growth, and characterization of Nd-doped SrGdGa3O7 crystal

A disordered Nd:SrGdGa3O7 (Nd:SGGM) laser crystal was grown by the Czochralski method. The space group and effective segregation coefficient of Nd3+ were determined to be P4¯21m and 1.36, respectively. Thermal properties, including the average linear thermal expansion coefficient, thermal diffusivity, specific heat, and thermal conductivity were measured. It was found that the thermal conductivity increases with increasing temperature, indicating glasslike behavior. Sellmeier’s equations were fitted by measuring the refractive indices in the range of 253–2325 nm. The polarization absorption and emission spectra were measured at room temperature, and Judd–Ofelt analysis was carried out to calculate the fluorescence branching ratios from the upper F43/2 state and the fluorescence lifetime. The stimulated emission cross section for the F43/2→I411/2 transition was calculated to be 2.00×10−20 cm2. Finally, a diode-pumped laser experiment at 1.06 μm is described. Thermal, optical, and laser properties have show...

Shape transition of InAs quantum dots on GaAs(001)

InAs quantum dots (QD’s) grown by molecular-beam epitaxy on GaAs(001) substrates were studied by atomically resolved in situ (without breaking a ultrahigh vacuum) scanning tunneling microscopy. A detailed sequence of shapes, appearing during the growth process, is evaluated: The smallest QD is terminated by {137}A bounding facets with a rhombic base. With increasing size, {1¯3¯5¯}B and {1¯1¯2¯}B facets appear in succession besides {137}A, making the QD more rounded. At a critical diameter of the base, the QD undergoes a shape transition from flat to steep by the formation of steeper {110} and {111} facets at the sides and gradually achieves a squared base. For all QD’s the {137}A facets remain on top. Almost all shapes that have been observed for InAs QD’s on GaAs(001) in earlier publications are covered.

Growth and characterization of SrMoO3 thin films

Highly conductive SrMoO3 thin films with good crystallinity and smooth surface were grown on SrTiO3 (0 0 1) substrates by pulsed laser deposition. The effects of substrate temperature and oxygen pressure on the structure, surface morphology, and electrical properties were studied. In the range of substrate temperatures from 560 degreesC to 640 degreesC and oxygen pressure from 10 (-3) to 10 (-4) Pa studied in our experiments, high-quality SrMoO3 thin films were produced. Beyond the top temperature or oxygen pressure limit, SrMoO4 appears as an impurity phase. High-resolution transmission electron microscopy (HRTEM) study shows that the SrMoO3 film has high-quality crystallinity and an epitaxial nature. The root-mean-square surface roughness of the film deposited at 2.5 x 10 (-4) Pa is 3.9 Angstrom. The films exhibit metallic conduction, which results from the delocalized electrons from Mo. X-ray photoelectron spectroscopy (XPS) measurements characterized its core level spectra and the valence band

Mature InAs quantum dots on the GaAs(114)A surface

InAs quantum dots (QDs), grown by molecular-beam epitaxy on GaAs(114)A surfaces, were studied in situ by atomically resolved scanning tunneling microscopy. At their mature stage, the QDs present a complicated but regular shape being bound by flat {110}, (111)A, and {2 5 11}A facets, and a steep part composed of rather variable combinations of {110}, (111)A, {111}B, and {2 5 11} surfaces. The QD shape can be derived from mature InAs QDs on GaAs(001).

Growth and characterization of Nd-doped Ca0.28Ba0.72Nb2O6 crystal

We report the growth of a Nd-doped calcium barium niobate (Nd:CBN-28) single crystal using the Czochralski method. The point group and space group of Nd:CBN-28 are determined to be tetragonal 4mm and P4bm, respectively, and the effective segregation coefficients of Nd, Ca, Ba, and Nb are 0.988, 1.029, 0.992, and 0.985, respectively. The density of the material is 5.321 g cm−3 at 19.6 °C, in good agreement with the theoretical value. Thermal expansion measurements reveal that during the heating cycle, the c-axis of Nd:CBN-28 undergoes both thermal contraction and thermal expansion, while at the same time there is only expansion along the a-axis. Specific heat measurements reveal that an anomaly appears at about 220 °C due to a phase transition. The thermal diffusivity increases slowly with increasing temperature. The thermal conductivity of the crystal has been calculated from these results, and the phonon mean free paths la and lc along the a- and c-axes of Nd:CBN-28 crystal are estimated. The transmissio...

InAs quantum dots on the GaAs(5 2 11)B surface

We report on InAs quantum dots grown by molecular-beam epitaxy on the stable GaAs(5 2 11¯)B surface discovered recently. Atomically resolved scanning tunneling microscopy images acquired in situ reveal compact quantum dots terminated by (101), (011), and (111)B facets and an unresolved vicinal (001) region. A flat base of (315)B orientation extends in front of the (101) and (111)B facets. The quantum dots exhibit a very narrow size distribution attributed to the well ordered substrate and to high nucleation efficiency.

Transition metal decorated graphene-like zinc oxide monolayer: A first-principles investigation

Transition metal (TM) atoms have been extensively employed to decorate the two-dimensional materials, endowing them with promising physical properties. Here, we have studied the adsorption of TM atoms (V, Cr, Mn, Fe, and Co) on graphene-like zinc oxide monolayer (g-ZnO) and the substitution of Zn by TM using first-principles calculations to search for the most likely configurations when TM atoms are deposited on g-ZnO. We found that when a V atom is initially placed on the top of Zn atom, V will squeeze out Zn from the two-dimensional plane then substitute it, which is a no barrier substitution process. For heavier elements (Cr to Co), although the substitution configurations are more stable than the adsorption ones, there is an energy barrier for the adsorption-substitution transition with the height of tens to hundreds meV. Therefore, Cr to Co prefers to be adsorbed on the hollow site or the top of oxygen, which is further verified by the molecular dynamics simulations. The decoration of TM is revealed to be a promising approach in terms of tuning the work function of g-ZnO in a large energy range.

Thermal stability of cubic GaN film grown by molecular-beam epitaxy on GaAs(001)

The thermal stability of cubic-phase GaN (c-GaN) film grown by molecular-beam epitaxy was investigated by Raman scattering spectroscopy and X-ray scattering. The results of Raman scattering shows that, after annealing at 1000degreesC, the intensity of transverse (TO) and longitudinal (LO) optical peaks from cubic phase obviously decreases while the intensity of TOb peak from the boundary effect slightly decreases, but the transformation of the hexagonal phase (alpha-GaN) can not be detected due to a little of alpha-GaN inclusion. X-ray reflectivity measurements indicate that there is a high-electron-density layer between the substrate and the GaN film, and it becomes uniform and much thinner after high-temperature annealing, counting for the Raman results of the intensity change of the TOb peak. The results of high-angle X-ray diffraction and X-ray reciprocal space mapping revealed that the relative content of alpha-GaN obviously increases after annealing at 1000degreesC, and (10 (1) over bar1) is the most stable diffraction lattice of the alpha-GaN hexagonal phase

InAs quantum dots on GaAs((1)over-bar(1)over-bar(2)over-bar)B

InAs quantum dots (QDs) were prepared by molecular beam epitaxy on GaAs(1¯1¯2¯)B substrates. Shape and size distribution of the QDs were investigated using in situ scanning tunneling microscopy as function of preparation temperature between 435 and 550°C. The wetting layer is not flat but undulated in submicrometer scale in a similar way as the bare substrate. The atomic structure of the wetting layer is the same as found for the flat base of InAs QDs grown on GaAs(1¯1¯3¯)B substrates. The shape of the QDs is given by {110}, (1¯1¯1¯)B, and {1¯4¯3¯}B bounding facets and a round vicinal (001¯) region. Unexpectedly, the number density increases and the size distribution sharpens, when the growth temperature is increased from 435to470°C, which is attributed to lattice defects incorporated into the QDs during growth at 435°C.

Ultrathin Pb film growth on Cu(111) studied by photoemission

The valence bands and the Pb 5d, Cu 3p core levels of Pb films evaporated on Cu(111) were measured by synchrotron radiation photoemission and characterized by low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES). The variation of the surface state at the center of the surface Brillouin zone (SBZ) of Cu(111) with Pb coverage shows that the submonolayer Pb grows on Cu(111) at room temperature (RT) as two-dimensional (2D) islands. With the Pb coverage increasing, the Pb 5d(5/2) core level shifts to higher binding energy monotonically. While the Cu 3p(3/2) core level is shifted toward higher binding energy by about 120 meV due to the deposition of 1.0 ML Pb. At low Pb coverage, subsequent annealing at 200 degreesC gives rise to Pb-Cu surface alloy formation in the first layer of Cu(111). The Pb 5d core level is shifted toward Fermi level by 20 similar to 30 meV due to the surface alloying. An assumption about electron charge transfer from Cu to Pb was adopted to interpret the observed core level shifts