Wei-Hsun Lin

Room-temperature operation type-II GaSb/GaAs quantum-dot infrared light-emitting diode

A GaSb/GaAs quantum-dot light-emitting diode (QD LED) with a single GaSb QD layer is investigated in this paper. The room-temperature photoluminescence peak blueshift with increasing excitation power densities suggests a type-II alignment of the GaSb/GaAs heterostructures. Significant electroluminescence (EL) is observed for the device under forward biases, which suggests that pronounced dipole transitions occur at the GaSb/GaAs interfaces. With increasing forward biases, the observed EL peak blueshift confirms that the origin of luminescence is from the type-II GaSb/GaAs QD structures. A model is established to explain the operation mechanisms of the type-II QD LED.

Growth and ferroelectricity of epitaxial‐like BaTiO3 films on single‐crystal MgO, SrTiO3, and silicon substrates synthesized by pulsed laser deposition

The epitaxial‐like BaTiO3 (BTO) films with good ferroelectricity are obtained both on (001)SrTiO3 (STO) single crystal and on CeO2 buffered silicon substrate by pulsed laser deposition. The deposition parameters need to be stringently controlled in order to grow BTO films with good crystallinity. The BTO films grown on YBa2Cu3O7−x (YBCO)/CeO2/STO substrates are epitaxial, as confirmed by rocking curve, φ scan, and wide‐angle x‐ray‐diffraction techniques. The alignment of a and b axes of BTO films on YBCO/CeO2/Si substrate is, however, not as perfect as BTO film on YBCO/STO substrate. The BTO/YBCO/CeO2/Si films are only (00l) textured. The ferroelectric property measurement, using the YBCO layer as the base electrode material, shows that the remanent polarization Pr and coercive field Ec of the BTO/YBCO/CeO2/Si films (Pr=3.6 μC/cm2, Ec=11.1 kV/cm) are, however, as good as those of the BTO/YBCO/STO films (Pr=4.0 μC/cm2, Ec=12.5 kV/cm).

800 meV localization energy in GaSb/GaAs/Al0.3Ga0.7As quantum dots

The localization energies, capture cross sections, and storage times of holes in GaSb quantum dots (QDs) are measured for three GaSb/GaAs QD ensembles with different QD sizes. The structural properties, such as height and diameter, are determined by atomic force microscopy, while the electronic properties are measured using deep-level transient spectroscopy. The various QDs exhibit varying hole localization energies corresponding to their size. The maximum localization energy of 800 (±50) meV is achieved by using additional Al0.3Ga0.7As barriers. Based on an extrapolation, alternative material systems are proposed to further increase the localization energy and carrier storage time of QDs.

Room-Temperature Electro-Luminescence of Type-II GaSb/GaAs Quantum Rings

The influence of Sb/background As flux ratios on GaSb nano-structures is investigated in this letter. With decreasing Sb/background As flux ratios under high Sb irradiation during the post soaking procedure, ring formation, photoluminescence (PL) intensity enhancement, and PL peak red shift are observed. With further reduced Sb flux and Sb/background As ratios, the observed more intense PL intensities of the quantum-ring (QR) samples compared with quantum dots suggest that more electron-hole wave function overlapping is obtained. The observation of room-temperature electro-luminescence of a QR PIN diode has revealed the potential of the nano-structure in light-emitting device application.

Type-II GaSb/GaAs coupled quantum rings: Room-temperature luminescence enhancement and recombination lifetime elongation for device applications

Type-II GaSb/GaAs coupled quantum rings have exhibited two-order-of-magnitude luminescence enhancement and ten-times elongation of recombination lifetime at room temperature as compared with regular rings. The longer lifetime suggests that a significant amount of electrons are confined in coupled rings rather than simply leaking away. These phenomena indicate that type-II nanostructures can be potentially utilized for room-temperature luminescence and carrier storage applications.

Influence of as on the Morphologies and Optical Characteristics of GaSb/GaAs Quantum Dots

The influence of As atoms on the morphologies of GaSb quantum dots (QDs) is investigated. Without any special treatment, GaSb quantum rings (QRs) are observed in the embedded GaSb layer even when the uncapped layer reveals QD like morphologies. With intentional As supply after the uncapped GaSb QD deposition, a QD to QR transition is observed. The phenomenon suggests that insufficient Sb atoms on the GaSb QDs would lead to the QD to QR transition as in the case of embedded GaSb layers. With extended Sb soaking time following GaSb deposition, QD structures could be well maintained for the embedded GaSb layers. A light-emitting diode operated at room temperature is fabricated based on the GaSb/GaAs QD structure. Identical peak positions in photoluminescence and electroluminescence (EL) spectra of the device show that type-II GaSb QDs are responsible for the observed EL.

Compact microdisk cavity laser with type-II GaSb/GaAs quantum dots

Microdisk lasers with active region made of type-II GaSb/GaAs quantum dots on the GaAs substrate have been demonstrated. A microdisk cavity with diameter of 3.9 μm was fabricated from a 225-nm-thick GaAs layer filled with GaSb quantum dots. Lasing at wavelengths near 1000 nm at 150 K was achieved for this microdisk. A high threshold characteristic temperature of 77 K was also observed. It is found that the lasing wavelength matches closely with the first-order whispering-gallery mode of the cavity as obtained from the finite-element method simulation.

Voltage-tunable two-color quantum-dot infrared photodetectors

A two-terminal quantum-dot infrared photodetector with stacked five-period InAs/GaAs and InGaAs-capped InAs/GaAs quantum-dot (QD) structures is investigated. The device has exhibited distinct responses at mid-wavelength and long-wavelength infrared regions under positive and negative biases, respectively. The results suggest that the QD confinement states near the anode side are completely filled, such that selective responses at different wavelength ranges would be observed for the stacked structure under different voltage polarities. Also observed are the similar absorption ratios of the device under different incident light polarizations at the two response regions.

InGaAs-Capped InAs–GaAs Quantum-Dot Infrared Photodetectors Operating in the Long-Wavelength Infrared Range

A ten-period InAs-GaAs quantum-dot infrared photodetector (QDIP) with 8-nm In0.15Ga0.85 As capping layer grown after quantum-dot (QD) deposition is investigated. With reduced InAs QD coverage down to 2.0 mono-layers, responses at 10.4 and 8.4 mum are observed for the device under positive and negative biases, respectively. The phenomenon is attributed to the large Stark effect resulted from the asymmetric band diagrams of the device under different voltage polarities. The demonstration of long-wavelength infrared detections with the simple structures of the InGaAs-capped QDIP is advantageous for the development of multicolor QDIP focal-plane arrays.

Enhanced Normal-Incident Absorption of Quantum-Dot Infrared Photodetectors With Smaller Quantum Dots

Ten-period InAs-GaAs quantum-dot (QD) infrared photodetectors grown under different In adatom supply procedures are investigated. Two In adatom supply procedures of In shutter 1) always opened and 2) periodically opened/closed are adopted in this letter. Larger QD sizes in both height and diameter and more uniform size distribution are observed for samples grown under an In shutter periodically opened/closed condition. The device with QDs grown under the In shutter always opened condition has revealed shorter detection wavelengths and enhanced normal incident absorption. The phenomenon shows that beside the increase of energy difference between confinement states, smaller QD sizes would also enhance the normal incident absorption predicted for the theoretically zero-dimensional QD structures.