Jian-Xing Xu

Self‐Assembled Quantum Dot Structures in a Hexagonal Nanowire for Quantum Photonics

Two types of quantum nanostructures based on self-assembled GaAs quantumdots embedded into GaAs/AlGaAs hexagonal nanowire systems are reported, opening a new avenue to the fabrication of highly efficient single-photon sources, as well as the design of novel quantum optics experiments and robust quantum optoelectronic devices operating at higher temperature, which are required for practical quantum photonics applications.

Single InAs quantum dot coupled to different “environments” in one wafer for quantum photonics

Self assembled small InAs quantum dots (SQDs) were formed in various densities and environments using gradient InAs deposition on a non-rotating GaAs substrate. Two SQD environments (SQD I and SQD II) were characterized. SQD I featured SQDs surrounded by large QDs, and SQD II featured individual SQDs in the wetting layer (WL). Micro-photoluminescence of single QDs embedded in a cavity under various excitation powers and electric fields gave insight into carrier transport processes. Potential fluctuations of the WL in SQD II, induced by charge redistribution, show promise for charge-tunable QD devices; SQD I shows higher luminescence intensity as a single-photon source.

Strain-driven synthesis of self-catalyzed branched GaAs nanowires

We report the strain-driven synthesis of self-catalyzed branched GaAs nanowires (NWs). Decoration of facets with branches is achieved as NWs elongate or with the insertion of InAs. The hemisphere tip shaped branches on the backbone implies identical Vapor-Liquid-Solid growth mechanism. We present the homogeneous gallium-droplets (GDs) nucleation on the GaAs {110} side facets in the form of GaAs quantum-rings, specifying the role of GDs in branching. Structural characterization revealed strain defects at the crotch between the backbones and branches of the NWs. The evolution mechanism of self-catalyzed branched NWs is discussed and finally nano-trees with hyper-branches are demonstrated.

Small linewidth and short lifetime of emission from GaAs/AlAs core/shell quantum dots on the facet of GaAs nanowire

Inspired by the novel quantum dot (QD) structure, GaAs/AlAs core/shell QD on the facet of GaAs/AlGaAs nanowire (NW), and its nice single-photon emission properties in Heiss et als work (2013 Nature Mater. 12 439), a mechanism of wavefunction resonance in QD, induced by the AlAs shell surrounding QD, is proposed and proved by Schrequation simulation. The thin AlAs shell between GaAs NW and QD, acting as a tunnelling barrier, determines QD emission linewidth. A 3 nm thick AlAs shell gives a linewidth of 35 µeV. QD emission efficiency and lifetime are studied by rate equations. The parallel decay channel along continuous GaAs NW band, in rate ofR, reduces QD emission lifetime. For � R = 1 × 10 9 s −1 and 3 nm thick AlAs shell, the lifetime is 680 ps. The AlAs shell thickness shows opposite influences on QD spectral linewidth and emission efficiency, and thus must be traded off. (Some figures may appear in colour only in the online journal)

Proper In deposition amount for on-demand epitaxy of InAs/GaAs single quantum dots*

The test-QD in-situ annealing method could surmount the critical nucleation condition of InAs/GaAs single quantum dots (SQDs) to raise the growth repeatability. Here, through many growth tests on rotating substrates, we develop a proper In deposition amount (θ) for SQD growth, according to the measured critical θ for test QD nucleation (θ c). The proper ratio θ/θ c, with a large tolerance of the variation of the real substrate temperature (T sub), is 0.964−0.971 at the edge and > 0.989 but < 0.996 in the center of a 1/4-piece semi-insulating wafer, and around 0.9709 but < 0.9714 in the center of a 1/4-piece N+ wafer as shown in the evolution of QD size and density as θ/θ c varies. Bright SQDs with spectral lines at 905 nm–935 nm nucleate at the edge and correlate with individual 7 nm–8 nm-height QDs in atomic force microscopy, among dense 1 nm–5 nm-height small QDs with a strong spectral profile around 860 nm–880 nm. The higher T sub in the center forms diluter, taller and uniform QDs, and very dilute SQDs for a proper θ/θ c: only one 7-nm-height SQD in 25 μm2. On a 2-inch (1 inch = 2.54 cm) semi-insulating wafer, by using θ/θ c = 0.961, SQDs nucleate in a circle in 22% of the whole area. More SQDs will form in the broad high-T sub region in the center by using a proper θ/θ c.

Two-dimensional electron gas characteristics of InP-based high electron mobility transistor terahertz detector*

The samples of two-dimensional electron gas (2DEG) are grown by molecular beam epitaxy (MBE). In the sample preparation process, the In content and spacer layer thickness are changed and two kinds of methods, i.e., contrast body doping and δ-doping are used. The samples are analyzed by the Hall measurements at 300 K and 77 K. The 2DEG channel structures with mobilities as high as (300 K) and (77 K) are obtained, and the values of carrier concentration (N c) are 3.465×1012/cm2 and 2.502×1012/cm2, respectively. The THz response rates of InP-based high electron mobility transistor (HEMT) structures with different gate lengths at 300 K and 77 K temperatures are calculated based on the shallow water wave instability theory. The results provide a reference for the research and preparation of InP-based HEMT THz detectors.

Optimization of wide band mesa-type enhanced terahertz photoconductive antenna at 1550 nm

A mesa-type enhanced InGaAs/InAlAs multilayer heterostructure (MLHS) terahertz photoconductive antenna (PCA) at 1550 nm is demonstrated on an InP substrate. The InGaAs/InAlAs superlattice multilayer heterostructures are grown and studied with different temperatures and thickness ratios of InGaAs/InAlAs. The PCAs with different gap sizes and pad sizes are fabricated and characterized. The PCAs are evaluated as THz emitters in a THz time domain spectrometer and we measure the optimized THz bandwidth in excess of 2 THz.