Y Gu

ZnO based oxide system with continuous bandgap modulation from 3.7 to 4.9 eV

ZnO based oxide system Zn1−x−yBexMgyO has been prepared by pulsed laser deposition. By incorporating different amounts of beryllium and magnesium into ZnO, the bandgap of ZnBeMgO has been modulated from 3.7 to 4.9 eV continuously. The crystal quality of ZnBeMgO film has been improved significantly comparing with that of either ZnMgO or BeZnO. These ZnBeMgO films are promising for fabricating high-efficiency optoelectronic devices such as solar-blind UV detectors.

Suppression of Structural Phase Transition in VO2 by Epitaxial Strain in Vicinity of Metal-insulator Transition

Mechanism of metal-insulator transition (MIT) in strained VO2 thin films is very complicated and incompletely understood despite three scenarios with potential explanations including electronic correlation (Mott mechanism), structural transformation (Peierls theory) and collaborative Mott-Peierls transition. Herein, we have decoupled coactions of structural and electronic phase transitions across the MIT by implementing epitaxial strain on 13-nm-thick (001)-VO2 films in comparison to thicker films. The structural evolution during MIT characterized by temperature-dependent synchrotron radiation high-resolution X-ray diffraction reciprocal space mapping and Raman spectroscopy suggested that the structural phase transition in the temperature range of vicinity of the MIT is suppressed by epitaxial strain. Furthermore, temperature-dependent Ultraviolet Photoelectron Spectroscopy (UPS) revealed the changes in electron occupancy near the Fermi energy EF of V 3d orbital, implying that the electronic transition triggers the MIT in the strained films. Thus the MIT in the bi-axially strained VO2 thin films should be only driven by electronic transition without assistance of structural phase transition. Density functional theoretical calculations further confirmed that the tetragonal phase across the MIT can be both in insulating and metallic states in the strained (001)-VO2/TiO2 thin films. This work offers a better understanding of the mechanism of MIT in the strained VO2 films.

InP-based InAs/InGaAs quantum wells with type-I emission beyond 3 μm

This work reports on InAs/In0.53Ga0.47As strain compensated quantum well structures on InP-based metamorphic buffer to generate the type-I emission of beyond 3 μm. The metamorphic buffer is composed of InxAl1−xAs graded layer and In0.8Ga0.2As virtual substrate layer. Atomic force microscope, transmission electron microscope and x-ray diffraction measurements show the moderate surface and structural properties. A photoluminescence signal up to 3.05 μm has been achieved at 300 K, which is one of the longest wavelengths from the interband emission of InP-based antimony-free structure. It is promising to employ this quantum well structure on metamorphic buffer for the laser demonstration with emission around 3 μm.

InP-based type-I quantum well lasers up to 2.9 μm at 230 K in pulsed mode on a metamorphic buffer

This work reports on up to 2.9 μm lasing at 230 K of InP-based type-I quantum well lasers. This record long wavelength lasing is achieved by applying InP-based Sb-free structures with eight periods of strain-compensated InAs quantum wells grown on metamorphic In0.8Al0.2As template layers. The continuous-wave threshold current density is 797 A/cm2 and the idealized extrapolated threshold current density for infinite cavity length is as low as 58 A/cm2 per quantum well at 120 K. This scheme is a promising pathway for extending the wavelength range of type-I quantum well lasers on InP substrates.

Insulator-quantum Hall conductor transition in high electron density gated InGaAs/InAlAs quantum wells

We study the insulator-quantum Hall conductor transition in two high-density gated InGaAs/InAlAs quantum well samples. We observe a well-defined critical magnetic field and verify this marks a genuine phase transition by investigating the scaling behavior of the longitudinal resistivity with field and temperature at fixed electron density. Consistent with prevailing experimental results the critical field decreases with increasing electron density in one sample (QW0710). In the other sample (QW0715), with higher delta doping density, however, we unexpectedly find that the critical field increases with increasing electron density. This unexpected behavior may be the result of the system entering the classical percolation regime.

Magnetoresistance in high-density two-dimensional electron gas confined in InAlAs/InGaAs quantum well

We study the magnetoresistance of a high-density two-dimensional electron gas confined in an InAlAs/InGaAs quantum well and observe a parabolic negative magnetoresistivity at moderate field. This negative magnetoresistivity is induced by electron-electron interactions. The interaction correction to the Drude conductance is extracted from the negative magnetoresistivity. However, due to inhomogeneity in the electron density, there is a contribution of positive magnetoresistivity, which induces the larger extracted correction than anticipated.

Metamorphic InAs1-xBix/In0.83Al0.17As quantum well structures on InP for mid-infrared emission

This work reports on InP-based metamorphic quantum well structures with bismuth incorporation for mid-infrared applications. InAs1-xBix quantum well structures have been grown on InP-based metamorphic In0.83Al0.17As buffers and photoluminescence beyond 3.1 μm has been achieved at 300 K, which is longer than the referenced InAs quantum well. X-ray diffraction, cross-sectional transmission electron microscopy, and energy dispersive X-ray spectroscopy measurements reveal clear interfaces of InAsBi quantum well with low bismuth, while more defects and bismuth inhomogeneity were observed as more bismuth was incorporated.

Bismuth for tailoring and modification of InP-based detector and laser structures in 2–3 µm band

The effects of bismuth on the performances of InP-based detectors and multiple triangular quantum wells in 2-3 μm band have been investigated. The cut-off wavelength of the InGaAsBi detector is tailored to 2.1 μm by the bismuth incorporation in the absorption layer, but the detector still shows an encouraging dark current due to decreased lattice mismatch to InP substrate. The material quality of the InAs/InGaAs triangular quantum wells has been significantly improved by introducing bismuth as a surfactant during growth. The moderate bismuth reduces the surface roughness, improves the heterostructure interfaces and enhances the photoluminescence intensity obviously, whereas deterioration occurs in the case of excessive bismuth flux. Bismuth shows a promising potential to improve InP-based detector and laser structures both by incorporating into the alloys and acting as a surfactant.