D. Y. Lin

Optical characteristics of a-plane ZnO/Zn0.8Mg0.2O multiple quantum wells grown by pulsed laser deposition

We reported optical properties of a-plane ZnO/ZnMgO multiple quantum wells (MQWs) structure grown by the pulse laser deposition system. The emission peak energy of a-plane ZnO/ZnMgO MQWs kept invariant in the power-dependent photoluminescence (PL) measurement, indicating the nonpolar characteristics due to the lack of built-in electric fields. Large exciton binding energy of 68 meV was deduced and no apparent S-curve appeared in temperature-dependent PL results, demonstrating less carrier localization effect in a-plane ZnO/ZnMgO MQWs. Large difference in electronic transition levels of 45 meV due to the valence band splitting was observed in the polarization dependent absorption spectrum. Furthermore, the high degree of polarization of 92% and 56% at 20 and 300 K in PL emission of a-plane ZnO/ZnMgO MQWs were obtained.

Temperature dependence of quantized states in an In0.86Ga0.14As0.3P0.7/InP quantum well heterostructure

Piezoreflectance (PzR) and contactless electroreflectance (CER) measurements of an In0.86Ga0.14As0.3P0.7/InP quantum well heterostructure as a function of temperature in the range of 20–300 K have been carried out. A careful analysis of the PzR and CER spectra has led to the identification of various excitonic transitions, mnH(L), between the mth conduction band state and the nth heavy (light)-hole band state. The parameters that describe the temperature dependence of EmnH(L) are evaluated. A detailed study of the temperature variation of excitonic transition energies indicates that the main influence of temperature on quantized transitions is through the temperature dependence of the band gap of the constituent material in the well. The temperature dependence of the linewidth of 11H exciton is evaluated and compared with that of the bulk material.

Temperature dependent photoreflectance and photoluminescence characterization of GaInNAs∕GaAs single quantum well structures

Ga0.69In0.31NxAs1−x∕GaAs single quantum well (SQW) structures with three different nitrogen compositions ( x=0%, 0.6%, and 0.9%) have been characterized, as functions of temperature in the range 10–300K, by the techniques of photoreflectance (PR) and photoluminescence (PL). In PR spectra, clear Franz-Keldysh oscillations (FKOs) above the GaAs band edge and the various excitonic transitions originating from the QW region have been observed. The built-in electric field in the SQW has been determined from FKOs and found to increase with N concentration. The PR signal has been found to decrease for nitrogen incorporated samples when the temperature was lowered due to a weakening of the modulation efficiency induced by carrier localization. A careful analysis of PR and PL spectra has led to the identification of various excitonic transitions, mnH(L), between the mth conduction band state and the nth heavy (light)-hole band state. The anomalous temperature dependent 11H transition energy and linewidth observed ...

Optical and electrical properties of MoS2 and Fe-doped MoS2

We prepared undoped and Fe-doped MoS2 layered crystals using a chemical vapor transport method to compare their optical and electrical properties. Optical behaviors of carrier transitions were observed successfully in both undoped and Fe-doped MoS2 samples using reflectance and piezoreflectance. Frequency-dependent photoconductivity (PC) measurements reveal an additional deep Fe doping level for the Fe-doped MoS2 sample. In addition, a longer carrier lifetime was calculated for the Fe-doped MoS2 sample than for the undoped MoS2 sample through PC analysis. Hall measurements were also performed for both samples and indicated that the Fe-doped MoS2 sample exhibited a higher carrier concentration and a lower mobility owing to the effect of Fe dopants. Furthermore, both samples were confirmed to have n-type carriers.

Surface photovoltage spectroscopy characterization of the GaAlAs/InGaAs/GaAs pseudomorphic high electron mobility transistor structures with varied quantum well compositional profiles

Abstract Using room-temperature surface photovoltage spectroscopy (SPS) we have characterized four GaAlAs/InGaAs/GaAs pseudomorphic high electron mobility transistor (pHEMT) structures with varied quantum well compositional profiles fabricated by molecular beam epitaxy. Signals have been observed from every region of the samples. The normalized first derivative of the surface photovoltage (SPV) signal with respect to photon energy from the InGaAs quantum well channel can be accounted for by a line shape function which is the first derivative of a step-like two-dimensional density of states and a Fermi level filling factor. A detailed line shape fit makes it possible to evaluate the Fermi energy, and hence the concentration of two-dimensional electron gas in addition to the energies of the intersubband transitions. The differences of intersubband transition energies are attributed to the surface segregation effects of indium atoms. In addition, other important parameters of the system such as the Al composition and the properties of the GaAs/GaAlAs superlattice (SL) buffer layer also are obtained from the SPV spectra. The results demonstrate the considerable potential of SPS for the contactless and nondestructive characterization of pHEMT structures at room temperature.

Anisotropy of Photoluminescence in Layered Semiconductors ReS2 and ReS2:Au

The optical anisotropy properties of rhenium disulphide (ReS2) and Au-doped rhenium disulphide (ReS2:Au) have been investigated by polarization-dependent photoluminescence (PL). Because the excitonic transitions show strong polarization dependences in the near-infrared region, we used polarization-dependent PL measurements in the range between 0° and 180° to characterize the unique polarization property of ReS2 and ReS2:Au, and identify the origin of the excitonic transitions. It was observed that the variation in the amplitude of PL excitonic transitions with different polarization characteristics follows the Malus rule. In comparison with the undoped ReS2, the PL spectra of Au-doped sample show not only the main excitonic transitions near the direct band edge but also some extra transitions owing to the doping effects.

A comprehensive study of temperature-dependent reflectance and photoluminescence of Zn1−xMnxO thin films grown on c-Al2O3

We present a systematic study of temperature-dependent reflectance (R) and photoluminescence (PL) measurements on ZnMnO films grown by plasma-assisted molecular beam epitaxy. For the first time, the three free-exciton transitions FXA (Γ7c-Γ7vu), FXB (Γ7c-Γ9v), and FXC (Γ7c-Γ7vl) and the longitudinal-optical phonon replicas of FXB and FXC of ZnMnO films have been clearly observed in the R spectra. The parameters describing the activation energy and the temperature dependence of the transition energy and broadening have been extracted by fitting the experimental R and PL spectra. The spectral data of ZnMnO films not only show the deterioration of crystalline quality with increasing Mn composition fraction but also indicate the Mn clustering caused by Mn atom segregation.

Contactless Electroreflectance and Photoluminescence Study of the Sb Surfactant Effects on InGaAsN Multiple Quantum Wells

The InGaAsN/GaAs multiple quantum wells structure grown on GaAs substrate with Sb surfactant treatment by metalorganic vapor-phase epitaxy has been characterized as a function of temperature in the temperature range between 15 and 300 K by the techniques of contactless electroreflectance (CER) and photoluminescence (PL). The interband transition energies are determined via a lineshape fit to the CER spectra and exhibit a blue-shift with the incorporation of Sb. A careful analysis of the CER and PL spectra has led to the identification of various excitonic transitions. The parameters that describe the temperature dependence of the interband transition energies are evaluated. The optical behavior results from the Sb surfactant treatment are presented and discussed.

Photoreflectance Study of Barrier-Width Dependence of Above-Barrier States in GaAs-AlxGa 1-xAs Multiple Quantum Wells

The optical transitions of the quasibound states at the above-barrier region in GaAs/Ga0.77Al0.23As multiple quantum wells have been observed at room temperature by photoreflectance measurement. It is found that the barrier-width dependence of the above-barrier transition energies can be described quite well by the modified Messiahs calculation. However, the simple calculation using the constructive interference condition can only explain the transitions at lower energies, and fails with increasing transition energy.