Huiyong Deng

Growth and Raman spectra of GaSb quantum dots in GaAs matrices by liquid phase epitaxy

The self-assembled type-II GaSb quantum dots (QDs) are successfully grown on semi-insulting GaAs (100) matrix by liquid phase epitaxy technique. The topography of QDs with high growth temperature is characterized by atomic force microscopy (AFM). The cap layer, which is needed for the device fabrication, is obtained for only some tens of nanometers. The non-resonant Raman spectra are applied to investigate the GaSb-like optical phonons localized in the QDs and to confirm convincingly the existence of GaSb QDs.

GROWTH AND MICROSTRUCTURES OF ULTRATHIN Bi2Te3 NANOPLATES BY MODIFIED HOT WALL EPITAXY

Ultrathin Bi2Te3 nanoplates have been grown on an oxidized silicon substrate by a modified hot wall epitaxy (HWE) method, in which a quartz plate with holes was employed. The microstructures and optical properties of Bi2Te3 nanoplates were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscope (AFM) and micro-Raman spectroscopy. The results show that ultrathin Bi2Te3 nanoplates with the thickness of about six quintuple layers (QLs) are obtained, which is difficult for the traditional HWE technique. The Raman vibration mode from the nanoplates exhibits an obviously red shift with decreasing thickness. The thickness variation of one nanoplate was obtained by the Raman map derived from the vibration frequency of mode and is in good agreement with the AFM result, which indicates that Raman map is an effective method to characterize the thickness difference of ultrathin Bi2Te3 nanoplates.

High-Quality Bi2Te3 Single Crystalline Films on Flexible Substrates and Bendable Photodetectors*

Recently, great efforts have been made in the fabrication of arbitrary warped devices to satisfy the requirement of wearable and lightweight electronic products. Direct growth of high crystalline quality films on flexible substrates is the most desirable method to fabricate flexible devices owing to the advantage of simple and compatible preparation technology with current semiconductor devices, while it is a very challenging work, and usually amorphous, polycrystalline or discontinuous single crystalline films are achieved. Here we demonstrate the direct growth of high-quality Bi2Te3 single crystalline films on flexible polyimide substrates by the modified hot wall epitaxy technique. Experimental results reveal that adjacent crystallites are coherently coalesced to form a continuous film, although amounts of disoriented crystallites are generated due to fast growth rate. By inserting a quartz filter into the growth tube, the number density of disoriented crystallites is effectively reduced owing to the improved spiral interaction. Furthermore, flexible Bi2 Te3 photoconductors are fabricated and exhibit strong near-infrared photoconductive response under different degrees of bending, which also confirms the obtained flexible films suitable for electronic applications.

Bulk photovoltaic effect at infrared wavelength in strained Bi2Te3 films

As a prominent three-dimensional (3-D) topological insulator, traditional thermoelectric material Bi2Te3 has re-attracted greater interest in recent years. Herein, we demonstrate for the first time that c-axis oriented strained Bi2Te3 films exhibit the bulk photovoltaic effect (BPVE) at infrared wavelengths, which was only found in wide band-gap ferroelectric materials before. Moreover, further experiments show that the bulk photovoltaic effect probably comes from the flexoelectric effect which was induced by the stress gradient in strained Bi2Te3 films. And we anticipate that the results are generalizable to other layer-structured or two-dimensional (2-D) materials, e.g., Bi2Se3 and MoS2.

Anomalous thermoelectricity in strained Bi2Te3 films.

Bi2Te3-based alloys have been intensively used for thermoelectric coolers and generators due to their high Seebeck coefficient S. So far, efforts to improve the S have been made mostly on changing the structures and components. Herein, we demonstrate an anomalous thermoelectricity in strained Bi2Te3 films, i.e., the value of S is obviously changed after reversing the direction of temperature gradient. Further theoretical and experimental analysis shows that it originates from the coupling of thermoelectric and flexoelectric effects caused by a stress gradient. Our finding provides a new avenue to adjust the S of Bi2Te3-based thermoelectric materials through flexoelectric polarization.

Microstructural characterization of Bi2Te3 thin films prepared by hot wall epitaxy

The influence of growth conditions on the microstructures of Bi2Te3 films grown on (111) and (100)-oriented GaAs substrates by hot wall epitaxy is investigated using X-ray diffraction, scan electron microscopy, energy dispersive spectrum, high resolution transmission electron microscopy and micro-Raman spectroscopy. It is found that high quality Bi2Te3 thin films with c-axis oriented are prepared when the temperatures of the Bi2Te3 source and (111) GaAs substrate are 505°C and 375°C respectively. The low substrate temperature and the crystal symmetry mismatch between the (100) GaAs substrate and Bi2Te3 epitaxial film make the crystalline grains mis-oriented, which are responsible for the degradation of the crystal quality of Bi2Te3 films. In addition, the low substrate temperature could lead to the non-stoichiometry.

Microstructural characterization of Bi2Te3thin films prepared by hot wall epitaxy

The influence of growth conditions on the microstructures of Bi2Te3 films grown on (111) and (100)-oriented GaAs substrates by hot wall epitaxy is investigated using X-ray diffraction, scan electron microscopy, energy dispersive spectrum, high resolution transmission electron microscopy and micro-Raman spectroscopy. It is found that high quality Bi2Te3 thin films with c-axis oriented are prepared when the temperatures of the Bi2Te3 source and (111) GaAs substrate are 505°C and 375°C respectively. The low substrate temperature and the crystal symmetry mismatch between the (100) GaAs substrate and Bi2Te3 epitaxial film make the crystalline grains mis-oriented, which are responsible for the degradation of the crystal quality of Bi2Te3 films. In addition, the low substrate temperature could lead to the non-stoichiometry.

Microstructural characterization of Bi 2 Te 3 thin films prepared by hot wall epitaxy

The influence of growth conditions on the microstructures of Bi2Te3 films grown on (111) and (100)-oriented GaAs substrates by hot wall epitaxy is investigated using X-ray diffraction, scan electron microscopy, energy dispersive spectrum, high resolution transmission electron microscopy and micro-Raman spectroscopy. It is found that high quality Bi2Te3 thin films with c-axis oriented are prepared when the temperatures of the Bi2Te3 source and (111) GaAs substrate are 505°C and 375°C respectively. The low substrate temperature and the crystal symmetry mismatch between the (100) GaAs substrate and Bi2Te3 epitaxial film make the crystalline grains mis-oriented, which are responsible for the degradation of the crystal quality of Bi2Te3 films. In addition, the low substrate temperature could lead to the non-stoichiometry.

Temperature dependence of photoluminescence property in BaIn2O4

The temperature-dependent photoluminescence (PL) spectra of BaIn2O4, prepared by coprecipitation, are measured and discussed. Aside from the reported 3.02-eV violet emission, the 1.81-eV yellow emission involved with oxygen vacancy is also observed at room temperature wherein the deep donor level is at 1.2 eV. With the temperature increasing, the peak energies for both emissions show a red shift. Moreover, the yellow emission intensity decreases while the violet emission intensity increases. The temperature dependence of the yellow emission intensity fits very well into the one-step quenching process equation, indicating a fitted activation energy at 19.2 meV.

Electronic transitions and hybrid resonance in InAsSb films by reflectance spectra

Electronic properties of InAs1−xSbx films with x up to 0.09 have been investigated by reflectance spectra in 1.5–5 eV energy range at room temperature. The real and imaginary parts of the dielectric function were derived by Kramers–Kronig analysis on the reflectance spectra, which show satisfactory agreement with the spectroscopic ellipsometry data. The E1′ and E1′+Δ1′ peaks are attributed to electronic interband transitions at the E1 and E1+Δ1 critical points, respectively. The prominent E2′ peaks, which exhibit high reflectivity and large blueshift, are found to be contributed by hybrid resonance due to the cooperative behavior of both E2-state electrons and plasmons.