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Carrier transport in semiconductors with different dimensionalities, i.e., 3D (bulk), 2D (QW), and 0D (QD), were investigated via temperature-dependent terahertz time-domain spectroscopy (THz-TDS). The optical properties and recombination dynamics in the samples were probed via photoluminescence spectroscopy. The temperature-dependence of the THz emission from the samples was explained in the context of the drift-diffusion model using the dominant THz radiation mechanism. The THz emission from diffusion-type THz emitters such as p- and n-InAs decreases as temperature increases due to mobility decrease. Conversely, the THz emission from drift-type THz emitters such as SI-GaAs, GaAs QW, and InAs QD was found to increase with temperature due to the increase in the driving electric field. In summary, THz-TDS can be utilized to gain qualitative insights on the temperature-dependent transport characteristics and establish dominant THz radiation mechanisms.

The power and temperature dependent photoluminescence (PL) of epitaxially grown In-rich CuInSe2 (CIS) and Cu-rich CIS deposited on N-polar GaN and InN were investigated in this paper. The In-rich CIS/GaN has two PL emissions characterized by a donor-acceptor pair (DAP) peak at 0.92 eV and an excitonic peak at 1.08/1.1 eV. On the other hand, the Cu-rich CIS/GaN has four PL emissions characterized by two DAP peaks at 0.86 eV and 0.94 eV, free-to-bound recombination peak at 0.97 eV and an excitonic peak at 1.03 eV. Identification of these defects in the CIS absorber layer is crucial for the improvement of the device efficiency. The PL emission of the CIS/InN closely resembles that of the Cu-rich CIS/GaN. For all the samples, the PL intensity increased with excitation power while the PL intensity decreased with temperature. The obtained power coefficients and activation energies support the proposed mechanism causing the luminescence. The defects present in CIS/GaN and CIS/InN were also identified. The results from this study are consistent with those of CIS single crystals and show that the CIS absorber layer can be incorporated with III-nitride materials, and its absorption can be extended beyond the spectrum covered by plain CIS.

GaSe1-xSx mixed crystals have recently drawn worldwide attention owing to their wide bandgap, which can be tuned by varying the ratio between Se and S. We have addressed the scarcity of comprehensive studies on its dynamic properties by investigating the photoluminescence (PL) and time-resolved photoluminescence (TRPL) of GaSe1-xSx (x = 0, 0.1, 0.2, 0.5) mixed crystals at different temperatures (14 – 300 K) using 404 nm excitation from a Ti:Sapphire laser. This paper aims to show that the PL spectra and the lifetime of GaSe1-xSx could be used to determine the stacking type and composition of the sample. Three Gaussian peaks were deconvoluted from the obtained PL spectrum and were attributed to excitons from the direct and indirect bandgaps. As the sulfur composition increases, the color of the emission changes and the PL spectra exhibits a blueshift. Selenium-rich samples, which have a e-stacking, have an orange emission while sulfur-rich samples, which have a e-β mixed stacking, have a yellow to green emission. TRPL results reveal that the lifetime becomes longer as the sulfur composition increases. The stacking type influences the separation and transfer of the carriers. Hence, sulfur-rich samples have a significantly longer lifetime than selenium-rich samples due to their β- stacking. As the temperature increases, the lifetime also becomes longer due to the increase of the non-radiative recombination rate, which plays a dominant role in the PL emission. These results are useful for the development of GaSe1-xSx-based optoelectronic devices in the red to the blue visible region.

Optical properties of multi-layer InAs/GaAs quantum dots (QDs) with different GaAs spacer layer thicknesses (dGaAs) of 20, 15 and 10 nm are investigated by time-integrated and time-resolved photoluminescence (PL) spectroscopy. The energy spacing between ground and excited state is observed to be decreased with decrease in dGaAs and increased with increase in excitation intensity. The rate of carrier capture and energy relaxation in QDs is found to be slower at excitation energy of 3.06 eV than that at 1.53 eV which is attributed to intervalley scattering in GaAs. The effect of intervalley scattering process in GaAs on decay time is negligible for dGaAs of 20 nm and is prominent for dGaAs ≤ 15 nm.

Effect of GaAs spacer layer thickness ( d GaAs ) on multi-stacked InAs/GaAs quantum dots is investigated by photoluminescence (PL) and excitation wavelength ( λ exc ) dependent pump–probe reflection spectroscopy. Dominance of light hole exciton transition in the PL spectra is observed at smaller d GaAs ( R / R ) max1 and (Δ R / R ) max2 appear in the differential reflection spectra (DRS) at intermediate λ exc beyond which positive to negative reversal of the DRS is observed due to dominating effect of inter band absorption in InAs wetting layer. The λ exc at which double maxima occur, and the positive to negative reversal starts is found to be dependent on d GaAs .

Optical excitations of monolayer bismuthene present the rich and unique absorption spectra. The threshold frequency is not equal to an indirect energy gap, and it becomes zero under the critical electric field. The frequency, number, intensity and form of the absorption structures are dramatically changed by the external field. The prominent peaks and the observable shoulders, respectively, arise from the constant-energy loop and the band-edge states of parabolic dispersions. These directly reflect the unusual electronic properties, being very different from those in monolayer graphene., 15 pages, 4 figures, 34 conferences

We study the photoluminescence (PL) and terahertz (THz) emission characteristics of MBE-grown InAs/GaAs single-layered quantum dots. Results from temperature-dependent and excitation power-dependent PL spectroscopy have revealed the presence of energy peaks corresponding to two possible quantum dot size distributions. The THz emission was found to increase as temperature increases, which we attribute to the increase in the number of carriers undergoing transport.

The photocarrier dynamics in molecular beam epitaxy (MBE)-grown single- (SLQD) and multi-layered (MLQD) InAs/GaAs quantum dots were studied. Photoluminescence (PL) spectroscopy has shown that the MLQD has more uniform QD size distribution as compared to the bimodal SLQD. Correlation between PL and THz-TDS has shown that photocarrier transport is more favored in the MLQD owing to this uniform QD size distribution, resulting to higher THz emission. The THz emission from the QD samples were found to be proportional to temperature. A drift-related photocarrier transport mechanism is proposed, wherein photocarriers generated in the QDs are accelerated by an interface electric field.

In2O3 epitaxial thin films with heavy Nb doping have been investigated in expectation of achieving higher doping efficiency per doping atom than what has been achieved by Sn-doping because Nb has one more available valence electron. The films were deposited by co-sputtering of Nb and In2O3 on (001) YSZ substrates, and are found to follow the epitaxial relations of [001]In2O3||[001]YSZ and [110]In2O3||[110]YSZ aligned within Δω∼0.31–0.41° of full rocking width at half maximum (FWHM). The doped thin films present optical transparencies of 97–99 % with electrical resistivities down to 10−4 Ω cm, 100 times lower than the as-deposited pristine thin films of no intentional doping. Optimal doping efficiency of ηmax∼1 charge carrier per Nb-atom added, not the hoped 2, suggests an effective ionization state of Nb·In, much like Nb·Sn, rather than the anticipated Nb··In. This singly-charged state is associated with the formation of Nb2O4 molecules by drawing extra interstitial Oi atoms to gather around the substitutional NbIn sites, as confirmed by XPS based on the chemical-shift of the X-ray photoelectron energy that measures the binding energy of core–shell electrons. Plasma oscillation analysis by FTIR optical spectroscopy shows an anomalously-high effective mass, about 10 times larger than the reported m⁎=0.35me. A dual-band model is proposed to reconcile with the findings.

Nematic liquid crystals (NLCs) can be easily reoriented by the laser beam at the temperature closed to the nematic–isotropic phase transition (TNI). At the temperature closed to TNI, the propagating mode of laser beam and the optically induced phase transition were explored by using a microscopic conoscope technique. This investigation demonstrates the formation of soliton at particular beam propagation modes. The interaction between nematic liquid crystals and the laser beam with different polarizations showed the nonlinearity in optical alignment.

The energy relaxations of Zn0.91Cd0.09Se multiquantum wells and epilayer structures were studied with an ultrafast time-resolved photoluminescence apparatus. The increase in the rise time of photoluminescence with decreasing photon energy and the redshift of the peak energy of time-resolved photoluminescence spectra with the delay time are attributed to the band-filling effect and energy relaxation of hot carriers. The derived carrier temperature decreases rapidly within the first 10 ps after photoexcitation and at a much slower rate thereafter. The fast carrier cooling can be explained by the longitudinal optical (LO) phonon emissions by carriers through the Fr?hlich interaction. The obtained effective scattering times of carrier and LO phonons are comparable to the theoretical prediction of 20 fs for multi-quantum well (MQW) of 20 nm well thickness and the epilayer. The slow carrier capture process may account for the long effective scattering time of 35 fs for the MQWs of 5 nm well thickness.

The dependence of photoluminescence (PL) decay time on the thickness of spacer in InAs/GaAs multi-stacked QDs (MSQDs) has been studied using time-resolved PL upconversion. The decay time of PL increases with decreasing spacer thickness and indicates that vertical coupling in MSQD is significant. The separation of the two peaks of PL for 30 nm QDs results from excited and ground states indicates that the QDs sizes may be different from those of QDs with 10, 15, and 20 nm spacer thicknesses. The band filling effect is evident in MSQD from the study of time-resolved photoluminescence. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper, we report the investigation of the electrochemical properties of nano-structured diamond thin-film electrodes on porous silicon (PSi) synthesized by microwave plasma chemical vapor deposition (MPCVD). For the application, boron-doped and undoped diamond thin film has been performed and fabricated into an electrode device, and its microstructure, electrical and chemical properties have been studied. In order to enlarge the surface area of diamond electrodes, a negative bias was applied to the MPCVD process to deposit diamond thin film in a nano-structured form, so that its surface remained rough and nano-fine structured. Diamond thin films were analyzed by Raman spectroscopy and SEM. The morphology of boron-doped diamond thin films on PSi reveals nano-rods in the shape of diamond crystallites. Their electrochemical properties were evaluated by performing cyclic voltammetry (CV) measurement in inorganic K 4 [Fe(CN) 6 ] in a K 2 HPO 4 buffer solution. Boron-doped diamond thin film on PSi has demonstrated good electrochemical properties, with a larger redoxidation current of CV, due to its rough surface, which provides a more active electrochemical interface.

The application of a low-frequency oscillating magnetic field perpendicular to a magnetic fluid thin film leads to the separation of a phase that is concentrated in particles from a dilute phase. The concentrated phase forms cylindrical columns that construct two-dimensional lattices. The ordered structure of magnetic fluid thin films is the basis for potential optical applications. We investigate the dynamical ordering formation of columnar lattices in magnetic fluid thin films subjected to oscillating perpendicular magnetic fields.

The origin of two distinct emissions, P1 and P2 observed in the PL spectra of m-plane GaN thin films grown on γ -LiAlO2 substrate were investigated at different temperatures by photoluminescence (PL) and time-resolved photoluminescence (TRPL) spectroscopy. The temperature dependence of the radiative lifetime for P2 emission is discussed.

The carrier cooling and the carrier relaxation of an InN thin film illuminated with two excitation energies of 1.53 and 3.06 eV were studied by an ultrafast time-resolved photoluminescence upconversion apparatus. The hot phonon effect could be accounted for longer effective phonon emission times as compared to the theoretical prediction. The rise time and the LO phonon emission time for 3.06 eV excitation were much smaller than those for 1.53 eV excitation. These differences were attributed to the intervalley scattering between the Γ 1 and Γ 3 valleys in InN when carriers were excited with the energy of 3.06 eV. The intervalley scattering times of 250 fs and 2 ps were estimated for the intervalley scattering from the Γ 1 to Γ 3 valley and the reversed scattering process, respectively.

Effect of GaAs spacer layer thickness (dGaAs) on carrier capture, and the relaxation process is studied in multi-stacked InAs/GaAs quantum dots by photoluminescence and time-resolved photoluminescence. Auger scattering is the dominating process for carrier relaxation above dGaAs of 15 nm. At dGaAs of 10 nm, the carrier relaxation process is faster due to the combined effect of both single longitudinal optical phonon and Auger scattering resulting in higher photoluminescence intensity. The photoluminescence rise time corresponding to carrier capture and relaxation in quantum dots is longer at 3.06 eV excitation than that at 1.53 eV due to the effect of intervalley scattering in GaAs.

Green emitting InGaN/GaN multi-quantum well samples were investigated using transmission electron microscopy, photoluminescence (PL), and time-resolved photoluminescence (TRPL) spectroscopy. Weak carrier localization with characteristic energy of ∼12 meV due to an inhomogeneous distribution of In in the InGaN quantum (QW) layer is observed. The temperature dependence of the PL peak energy exhibits S-shape phenomenon and is comparatively discussed within the framework of the Varshni's empirical formula. The full width at half maximum of the PL emission band shows an increasing-decreasing-increasing behavior with increasing temperature arising from the localized states caused by potential fluctuations. The radiative life time, τr, extracted from the TRPL profile shows ∼T3/2 dependence on temperature above 200 K, which confirms the absence of the effect of carrier localization at room temperature.

M-plane GaN thin films grown on γ-LiAlO2 substrate were investigated at different temperatures by photoluminescence (PL) and time-resolved photoluminescence (TRPL) spectroscopy. The origin of two distinct emissions, P1 and P2 observed in the PL spectra were established by analyzing their PL and TRPL properties at different temperatures. The P1 emission is attributed to the excitons bound to the stacking faults (SFs). The P2 shows an anomalous “S-shaped” emission shift with increasing temperature (T), and the associated mechanism is discussed. The radiative life time ‘τr’ for P2 emission exhibits the T3/2 dependence at higher temperatures and deviates at lower temperatures whereas the radiative life time ‘τr’ for P1 emission does not show the T3/2 dependence with temperature. The polarization-dependent PL study reveals that P2 emission involves free holes in the transition at room temperature.

Time-integrated and time-resolved photoluminescence (PL) of InN thin films of different background carrier concentrations are investigated. The PL formation mechanism is attributed to the “free-to-bound” transition by analyzing the time-integrated PL spectra at different pump fluences. The dependence of the PL decay time with emission energy is investigated using a theoretical model which speculates upon the carrier localization in InN thin films. The radiative lifetime, mobility edge, and carrier localization energy are obtained from the dependence of the PL decay time on emission energy and are studied at different background carrier concentrations. The effect of intervalley scattering between the Γ1 and Γ3 valley on the radiative lifetime, mobility edge, and carrier localization energy is discussed. The longer radiative lifetime and smaller values of the mobility edge and localization energy for 3.06 eV excitation are observed than that for the 1.53 eV excitation due to the intervalley scattering process.

Spin-splitting energies of wurtzite AlN and InN are calculated using the linear combination of atomic orbital method, and the data are analyzed utilizing the two-band k⋅p model. It is found that in the k⋅p scheme, a spin-degenerate surface exists in the wurtzite Brillouin zone. Consequently, the D’yakonov-Perel’ spin relaxation mechanism can be effectively suppressed for all spin components in the [001]-grown wurtzite quantum wells (QWs) at a resonant condition through application of appropriate strain or a suitable gate bias. Therefore, wurtzite QWs (e.g., InGaN/AlGaN and GaN/AlGaN) are potential structures for spintronic devices such as the resonant spin lifetime transistor.

Auger recombination is studied in InN thin films using an ultrafast time-resolved photoluminescence apparatus. The decay rates are analyzed with nonlinear dependence of the photoluminescence intensity on the carrier concentration. The fitted radiative recombination coefficients at a temperature of 35K are consistent with the theoretical prediction. The Auger rates are small at low carrier concentrations but increase quadratically with the carrier concentration. The Auger activation energies of 4.3 and 9.0meV obtained from the temperature-dependent Auger coefficient indicate that Auger recombination is weakly dependent on temperature and is a phonon-assisted process.

The energy relaxation of InN thin films has been studied by ultrafast time-resolved photoluminescence technique. The obtained carrier cooling curves can be explained by carriers releasing excessive energy through the carrier–LO-phonon interaction. The extracted effective phonon emission times decrease as the photoexcited carrier concentration reduces and come close to the theoretical prediction of 23fs at small carrier concentration. The reduction of energy loss rate at high photoexcited carrier density is attributed to the hot phonon effect.

The spin-splitting energies of the conduction band for ideal wurtzite materials are calculated within the nearest-neighbor tight-binding method. It is found that ideal wurtzite bulk inversion asymmetry yields not only a spin-degenerate line (along the kz axis) but also a minimum-spin-splitting surface, which can be regarded as a spin-degenerate surface in the form of bkz2−k‖2=0 (b≈4) near the Γ point. This phenomenon is referred to as the Dresselhaus effect (defined as the cubic-in-k term) in bulk wurtzite materials because it generates a term γwz(bkz2−k‖2)(σxky−σykx) in the two-band k∙p Hamiltonian.

holography technique involving various polarizations of excitation and probe beams has been employed to identify the strong asymmetry of excitation and diffraction in the guest-host system. In this paper, we report the temporal profiles of diffraction efficiency for transient gratings and their temperature and polarization dependences in azo-dye-doped liquid crystals. The dynamics of molecular reorientation phenomena in transient gratings is investigated by analyzing the buildup time of the diffraction maximum and diffractive decay. The study of the polarization dependence of the first-order diffraction efficiency and its corresponding temperature effect allows us to understand the underlying mechanism of laser-induced transient gratings. 2. Experimental

The morphology and the size-dependent photoluminescence (PL) spectra of the type-II ZnTe quantum dots (QDs) grown in a ZnSe matrix were obtained. The coverage of ZnTe varied from 2.5 to 3.5 monolayers (MLs). The PL peak energy decreased as the dot size increased. Excitation power and temperature-dependent PL spectra are used to characterize the optical properties of the ZnTe quantum dots. For 2.5- and 3.0-ML samples, the PL peak energy decreased monotonically as the temperature increased. However, for the 3.5-ML sample, the PL peak energy was initially blueshifted and then redshifted as the temperature increased above 40K. Carrier thermalization and carrier transfer between QDs are used to explain the experimental data. A model of temperature-dependent linewidth broadening is employed to fit the high-temperature data. The activation energy, which was found by the simple PL intensity quenching model, of the 2.5, 3.0, and 3.5 MLs were determined to be 6.35, 9.40, and 18.87meV, respectively.