Guo-Wei Zha

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 Grown at the Junction of Branched Gold- Free GaAs Nanowire

We report a new type of single InAs quantum dot (QD) embedded at the junction of gold-free branched GaAs/AlGaAs nanowire (NW) grown on silicon substrate. The photoluminescence intensity of such QD is ~20 times stronger than that from randomly distributed QD grown on the facet of straight NW. Sharp excitonic emission is observed at 4.2 K with a line width of 101 μeV and a vanishing two-photon emission probability of g(2)(0) = 0.031(2). This new nanostructure may open new ways for designing novel quantum optoelectronic devices.

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)

Enhanced tunneling in the GaAs p+?n+?junction by embedding InAs quantum dots

GaAs p+?n+?junctions with and without a layer of InAs quantum dots (QDs) embedded at the interface are discussed in this article. The current density versus voltage (I?V) characteristics show that the junctions without QDs are weak degenerate due to the Beryllium(Be) atoms diffusion of nominal p++-GaAs; the junctions with QDs generate enhanced tunneling current at forward bias, because the QDs layer reduces the Be diffusion and enables a two-step tunneling process. At room temperature, the current density of the sample with QDs is enhanced to 122 A cm?2?at a forward bias of?+0.32?V, which is about 2 orders of magnitude higher than the reference sample without QDs.

Coupling and single-photon purity of a quantum dot-cavity system studied using hydrostatic pressure

We propose an approach to tune the emission of a single semiconductor quantum dot (QD) to couple with a planar cavity using hydrostatic pressure without inducing temperature variation during the process of measurement. Based on this approach, we studied the influence of cavity mode on the single-photon purity of an InAs/GaAs QD. Our measurement demonstrates that the single-photon purity degrades when the QD emission resonates with the cavity mode. This negative influence of the planar cavity is mainly caused by the cavity feeding effect.

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.

Effect of tunable dot charging on photoresponse spectra of GaAs p-i-n diode with InAs quantum dots

Quantum dot (QD)-embedded photodiodes have demonstrated great potential for use as detectors. A modulation of QD charging opens intriguing possibilities for adaptive sensing with bias-tunable detector characteristics. Here, we report on a p-i-n GaAs photodiode with InAs QDs whose charging is tunable due to unintentional Be diffusion and trap-assisted tunneling of holes, from bias- and temperature (T)-dependent photocurrent spectroscopy. For the sub-bandgap spectra, the T-dependent relative intensities “QD-s/WL” and “WL/GaAs” (WL: wetting layer) indicate dominant tunneling under −0.9 V (trap-assisted tunneling from the top QDs) and dominant thermal escape under −0.2 ∼ 0.5 V (from the bottom QDs since the top ones are charged and inactive for optical absorption) from the QD s-state, dominant tunneling from WL, and enhanced QD charging at >190 K (related to trap level ionization). For the above-bandgap spectra, the degradation of the spectral profile (especially near the GaAs bandedge) as the bias and T tune...