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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.

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 .

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 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.

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.

Temperature and excitation power dependent time-integrated photoluminescence of Si doped InN thin films are investigated. Photoluminescence (PL) spectra at low temperatures are described by single emission peak ensued due to “free-to-bound” recombination; whereas PL spectra at higher temperatures above 150 K are characterized by both “band-to-band” and “free-to-bound” transition. Carrier dynamics of Si doped InN thin films is studied using pump-probe reflection spectroscopy at room temperature. The hot electron cooling process is well described by electron-electron scattering. The dependence of the hot electron cooling rate on total electron density shows sublinear to linear behavior with increase of background electron density. The variation of the carrier recombination lifetime with total electron density implicates the dominance of the defect-related nonradiative recombination channel over other recombination processes.

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.