Jifang He

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.

Coherent versus incoherent light scattering from a quantum dot

We analyze the light scattered by a single InAs quantum dot interacting with a resonant continuous-wave laser. High resolution spectra reveal clear distinctions between coherent and incoherent scattering, with the laser intensity spanning over four orders of magnitude. We find that the fraction of coherently scattered photons can approach unity under sufficiently weak or detuned excitation, ruling out pure dephasing as a relevant decoherence mechanism. We show how spectral diffusion shapes spectra, correlation functions, and phase-coherence, concealing the ideal radiatively-broadened two-level system described by Mollow.

In situ accurate control of 2D-3D transition parameters for growth of low-density InAs/GaAs self-assembled quantum dots

A method to improve the growth repeatability of low-density InAs/GaAs self-assembled quantum dots by molecular beam epitaxy is reported. A sacrificed InAs layer was deposited firstly to determine in situ the accurate parameters of two- to three-dimensional transitions by observation of reflection high-energy electron diffraction patterns, and then the InAs layer annealed immediately before the growth of the low-density InAs quantum dots (QDs). It is confirmed by micro-photoluminescence that control repeatability of low-density QD growth is improved averagely to about 80% which is much higher than that of the QD samples without using a sacrificed InAs layer.

Optimization of the GaAs-on-Si Substrate for Microelectromechanical Systems (MEMS) Sensor Application

Resonant Tunneling Diodes (RTD) and High Electron Mobility Transistor (HEMT) based on GaAs, as the piezoresistive sensing element, exhibit extremely high sensitivity in the MEMS sensors based on GaAs. To further expand their applications to the fields of MEMS sensors based on Si, we have studied the optimization of the GaAs epitaxy layers on Si wafers. Matching superlattice and strain superlattice were used, and the surface defect density can be improved by two orders of magnitude. Combing with the Raman spectrum, the residual stress was characterized, and it can be concluded from the experimental results that the residual stress can be reduced by 50%, in comparison with the original substrate. This method gives us a solution to optimize the epitaxy GaAs layers on Si substrate, which will also optimize our future process of integration RTD and HEMT based on GaAs on Si substrate for the MEMS sensor applications.

An effective reflectance method for designing broadband antireflection films coupled with solar cells

The solar spectrum covers a broad wavelength range, which requires that antireflection coating (ARC) is effective over a relatively wide wavelength range for more incident light coming into the cell. In this paper, we present two methods to measure the composite reflection of SiO2/ZnS double-layer ARC in the wavelength ranges of 300–870 nm (dual-junction) and 300–1850 nm (triple-junction), under the solar spectrum AM0. In order to give sufficient consideration to the ARC coupled with the window layer and the dispersion effect of the refractive index of each layer, we use multidimensional matrix data for reliable simulation. A comparison between the results obtained from the weighted-average reflectance (WAR) method commonly used and that from the effective-average reflectance (EAR) method introduced here shows that the optimized ARC through minimizing the effective-average reflectance is convenient and available.

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.

Influence of growth temperatures on the quality of InGaAs/GaAs quantum well structure grown on Ge substrate by molecular beam epitaxy

Molecular beam epitaxy growth of an InxGa1−xAs/GaAs quantum well (QW) structure (x equals to 0.17 or 0.3) on offcut (100) Ge substrate has been investigated. The samples were characterized by atomic force microscopy, photoluminescence (PL), and high resolution transmission electron microscopy. High temperature annealing of the Ge substrate is necessary to grow GaAs buffer layer without anti-phase domains. During the subsequent growth of the GaAs buffer layer and an InxGa1−xAs/GaAs QW structure, temperature plays a key role. The mechanism by which temperature influences the material quality is discussed. High quality InxGa1−xAs/GaAs QW structure samples on Ge substrate with high PL intensity, narrow PL linewidth and flat surface morphology have been achieved by optimizing growth temperatures. Our results show promising device applications for III—V compound semiconductor materials grown on Ge substrates.

Molecular beam epitaxy growth of GaAs on an offcut Ge substrate

Molecular beam epitaxy growth of GaAs on an offcut Ge (100) substrate has been systemically investigated. A high quality GaAs/Ge interface and GaAs film on Ge have been achieved. High temperature annealing before GaAs deposition is found to be indispensable to avoid anti-phase domains. The quality of the GaAs film is found to strongly depend on the GaAs/Ge interface and the beginning of GaAs deposition. The reason why both high temperature annealing and GaAs growth temperature can affect epitaxial GaAs film quality is discussed. High quality In0.17Ga0.83As/GaAs strained quantum wells have also been achieved on a Ge substrate. Samples show flat surface morphology and narrow photoluminescence line width compared with the same structure sample grown on a GaAs substrate. These results indicate a large application potential for III—V compound semiconductor optoelectronic devices on Ge substrates.

Elastic and inelastic light scattering from a quantum dot

We spectrally resolve the light scattered by a single InAs semiconductor quantum dot and analyze the relative contribution of elastic and inelastic scattering processes.