Wekun Ge

Photovoltage and photoreflectance spectroscopy of InAs/GaAs self-organized quantum dots

We present a detailed study of the interband excitonic transitions of InAs/GaAs self-organized quantum dots (QDs) based on photovoltage (PV) photoreflectance (PR) and photoluminescence (PL) spectroscopy. At room temperature, the interband absorption transitions of QDs have been observed by using PV spectrum, which clearly exhibits four well-resolved excitonic absorption peaks. The absorption line shape is Gaussian-like. Furthermore, the corresponding excitonic transitions are also observed in PR experiment at 77 K. The first derivative of a Gaussian profile can fit the experimental data well

A voltage-controlled tunable two-color infrared photodetector using GaAs/AlAs/GaAlAs and GaAs/GaAlAs stacked multiquantum wells

A voltage‐controlled tunable two‐color infrared detector with photovoltaic (PV) and photoconductive (PC) dual‐mode operation at 3–5 μm and 8–14 μm using GaAs/AlAs/AlGaAs double barrier quantum wells (DBQWs) and bound‐to‐continuum GaAs/AlGaAs quantum wells is demonstrated. The photoresponse peak of the photovoltaic GaAs/AlAs/GaAlAs DBQWs is at 5.3 μm, and that of the photoconductive GaAs/GaAlAs quantum wells is at 9.0 μm. When the two‐color detector is under a zero bias, the spectral response at 5.3 μm is close to saturate and the peak detectivity at 80 K can reach 1.0×1011 cmHz1/2/W, while the spectral photoresponsivity at 9.0 μm is absolutely zero completely. When the external voltage of the two‐color detector is changed to 2.0 V, the spectral photoresponsivity at 5.3 μm becomes zero while the spectral photoresponsivity at 9.0 μm increases comparable to that at 5.3 μm under zero bias, and the peak detectivity (9.0 μm) at 80 K can reach 1.5×1010 cmHz1/2/W. Strictly speaking, this is a real bias‐controlled...

Dynamic dc voltage band observed within each current branch in the transition from static to dynamic electric-field domain formation in a doped GaAs/AlAs superlattice

A dynamic dc voltage band was found emerging from each sawtooth-like branch of the current-voltage characteristics of a doped GaAs/AlAs superlattice in the transition process from static to dynamic electric-field domain formation caused by increasing the sample temperature. As the temperature increases, these dynamic dc voltage bands expand within each sawtooth-like branch, squeeze out the static regions, and join up together to turn the whole plateau into dynamic electric-field domain formation. These results are well explained by a general analysis of stability of the sequential tunneling current in superlattices

Resistivity minima and Kondo effect in ferromagnetic GaMnAs films

The temperature dependence of the resistivity of ferromagnetic GaMnAs, as-grown or low-temperature-annealed samples is measured from 2to290K. A resistivity minimum is observed with a corresponding temperature TM around 10K for each sample. Below TM, the resistivity exhibits logarithmic temperature dependence, as αln(T), and α is independent of the external magnetic field up to 9T. Such behavior is explained in terms of the Kondo effect arising from the presence of Mn interstitials in the GaMnAs samples. In addition, a well-defined T-squared dependence of resistivity is found in the temperature range between TM and the Curie temperature (TC), which is attributed to single magnon scattering.

Effect of growth interruption on the optical properties of InAs/GaAs quantum dots

The effect of growth interruption (GI) on the optical properties of InAs/GaAs quantum dots was investigated by cw and time-resolved photoluminescence (PL). It is found that this effect depends very much on the growth conditions, in particular, the growth rate. In the case of low growth rate, we have found that the GI may introduce either red-shift or blue-shift in PL with increase of the interruption time, depending on the InAs thickness. The observed red shift in our 1.7 monolayer (ML) sample is attributed to the evolution of the InAs islands during the growth interruption. While the blue-shift in the 3 ML sample is suggested to be mainly caused by the strain effect. In addition, nearly zero shift was observed for the sample with thickness around 2.5 ML.

Alloy states in dilute GaAs1−xNx alloys (x<1%)

A set of GaAs1-xNx samples with small nitrogen composition (x<1%) were investigated by continuous-wave photoluminescence (PL), pulse-wave excitation PL, and time-resolved PL. In the PL spectra, an extra transition located at the higher-energy side of the commonly reported N-related emissions was observed. By measuring the PL dependence on temperature and excitation power, the PL peak was identified as a transition of alloy band edge-related recombination in GaAsN. The PL dynamics further confirms its intrinsic nature as being associated with the band edge rather than N-related bound states

Carrier capture into InAs/GaAs quantum dots detected by a simple degenerate pump–probe technique

We demonstrate that the carrier capture and relaxation processes in InAs/GaAs quantum dots can be detected by a simple degenerate pump-probe technique. We have observed a rising process in the transient reflectivity, following the initial fast relaxation in a GaAs matrix, and assigned this rising process to the carrier capture from the GaAs barriers to the InAs layers. The assignment was modeled using the Kramers-Kronig relations. The capture time was found to depend strongly on the InAs layer thickness as well as on the excitation density and photon energy

Step‐like I‐V Characteristics and Period‐adding Bifurcations in External Ac‐driven GaAs/AlAs Superlattices

Step‐like current‐voltage characteristics are observed in ac‐driven dynamic voltage bands under certain ac conditions. Fourier analysis reveals that the current steps correspond to frequency‐locked states, which exhibit a period‐adding bifurcation with the dc voltage as a control parameter. This bifurcation is also found to strongly depend on the external ac conditions. It indicates that the vertical transport of ac‐driven weakly‐coupled superlattices provides a rich testing ground for nonlinear physics.

Resonant Raman scattering with the E+ band in a dilute GaAs1−xNx alloy (x=0.1%)

Resonant Raman scattering has been applied to a dilute GaAs1−xNx alloy with 0.1% N. The Raman lines of GaAs and GaN related modes, their combinations, and multiple order replicas of GaAs-like longitudinal-optical modes have been observed with a lower N composition than those studied previously. All these Raman features are found to be strongly enhanced with excitations in resonance with a broad photoluminescence band that is associated with the so-called E+ transition. This study provides additional insights into how the GaAs host conduction band states are perturbed and thus the electron-phonon interaction is affected by the N doping.

Static and dynamic electric field domain formation in a doped GaAs/AlAs superlattice

We have observed the transition from static to dynamic electric field domain formation induced by a transverse magnetic field and the sample temperature in a doped GaAs/AlAs superlattice. The observations can be very well explained by a general analysis of instabilities and oscillations of the sequential tunnelling current in superlattices based solely on the magnitude of the negative differential resistance region in the tunnelling characteristic of a single barrier. Both increasing magnetic field and sample temperature change the negative differential resistance and cause the transition between static and dynamic electric field domain formation