Optical Pump Terahertz Research Articles

Towards higher electron mobility in modulation doped GaAs/AlGaAs core shell nanowires.

Precise control over the electrical conductivity of semiconductor nanowires is a crucial prerequisite for implementation of these nanostructures into novel electronic and optoelectronic devices. Advances in our understanding of doping mechanisms in nanowires and their influence on electron mobility and radiative efficiency are urgently required. Here, we investigate the electronic properties of n-type modulation doped GaAs/AlGaAs nanowires via optical pump terahertz (THz) probe spectroscopy and photoluminescence spectroscopy over the temperature range 5 K-300 K. We directly determine an ionization energy of 6.7 ± 0.5 meV (T = 52 K) for the Si donors within the AlGaAs shell that create the modulation doping structure. We further elucidate the temperature dependence of the electron mobility, photoconductivity lifetime and radiative efficiency, and determine the charge-carrier scattering mechanisms that limit electron mobility. We show that below the donor ionization temperature, charge scattering is limited by interactions with interfaces, leading to an excellent electron mobility of 4360 ± 380 cm2 V-1 s-1 at 5 K. Above the ionization temperature, polar scattering via longitudinal optical (LO) phonons dominates, leading to a room temperature mobility of 2220 ± 130 cm2 V-1 s-1. In addition, we show that the Si donors effectively passivate interfacial trap states in the nanowires, leading to prolonged photoconductivity lifetimes with increasing temperature, accompanied by an enhanced radiative efficiency that exceeds 10% at room temperature.

Low temperature-grown GaAs carrier lifetime evaluation by double optical pump terahertz time-domain emission spectroscopy.

We present the use of a "double optical pump" technique in terahertz time-domain emission spectroscopy as an alternative method to investigate the lifetime of photo-excited carriers in semiconductors. Compared to the commonly employed optical pump-probe transient photo-reflectance, this non-contact and room temperature characterization technique allows relative ease in achieving optical alignment. The technique was implemented to evaluate the carrier lifetime in low temperature-grown gallium arsenide (LT-GaAs). The carrier lifetime values deduced from "double optical pump" THz emission decay curves show good agreement with data obtained from standard transient photo-reflectance measurements on the same LT-GaAs samples grown at 250 °C and 310 °C.

Optical pump-terahertz probe analysis of long-lived d-electrons and relaxation to self-trapped exciton states in MnO

The dynamics of photoexcited electrons in various excited d-states was investigated in a transition metal oxide MnO by tunable optical pump-terahertz probe measurements. Photoexcited electrons in the lowest excited d-state showed the longest relaxation time among the three excited d-states that are accessible in MnO at room temperature. The relaxation rate in the lowest excited d-state showed a drastic increase below the Neel temperature TN = 120 K in MnO. We conclude that this increase is caused by the appearance of a decay channel related to magnetic-excitation-assisted photoluminescence from self-trapped exciton (STE) states. The opening of relaxation channels to the STE states in an antiferromagnetic phase suggests that it may be possible to control photocarrier lifetime by magnetic order in transition metal oxides.

Ultrafast terahertz probe of photoexcited free charge carriers in organometal CH3NH3PbI3 perovskite thin film

By using optical pump–terahertz probe (OPTP) experiments, we study the free charge carrier dynamics in photoexcited drop-cast CH3NH3PbI3-based perovskite thin film at room temperature. Compared with the pump photon energy at 1.55 eV, the measured OPTP signal following excitation of 3.1 eV shows an additional fast decay channel of the photoconductivity. Our experimental results demonstrate that effective carrier lifetime can be strongly modulated by surface recombination. In addition, the Drude-Smith-like transient terahertz photoconductivity spectra suggest that photogenerated free carriers experience backscattering at grain boundaries in our solution-processed perovskite films studied here.

Investigation of insulator-metal transition in Ti4O7 using terahertz probe pulse

The ultrafast photo-induced insulator-metal transition in a Ti4O7 single crystal is investigated with different excitation densities using an optical-pump terahertz (THz)-probe technique. The oxide shows a two-step phase transition at 130 K from a low temperature (LT) phase to an intermediate phase and to a high temperature metallic phase at 150 K. The photo-irradiation in the LT phase induces an ultrafast decrease in the transmittance of the THz probe within 3.6 ps, corresponding to the generation of a metallic domain. This is followed by an additional slow decrease when the excitation is above the threshold level. The nonlinear dependence of the amplitude and lifetime on the excitation density indicates that the metallic domains grow spontaneously and are stabilized depending on their resultant amount.

Microscopic insight into the pump–probe relaxation dynamics of superconductors: Model study of MgB2 relaxation within nonlinear response theory

Here we present a quantum‐statistical formulation of relaxation dynamics of superconductors related to pump–probe (PP) spectroscopy. The method is based on the perturbation expansion of the non‐equilibrium density matrix for calculation of multi‐time correlation functions, and the corresponding response function. As a model for our study, the high‐temperature superconductor MgB2 has been selected. Knowledge of the electronic structure and of the corresponding Eliashberg function of electron–phonon (EP) interactions enabled us to distinguish non‐equilibrium processes in a sequence of interactions with laser pulses on a microscopic level. Temperature‐dependent relaxation dynamics as a function of delay time between the pump and probe pulses have been derived. We have shown that an abrupt increase of the relaxation time at Tc is the direct consequence of sudden changes in the character of EP coupling in transition from an adiabatic to a stabilized superconducting anti‐adiabatic state, as it predicts the anti‐adiabatic theory of electron–vibration interactions. The BCS model, which is based on adiabatic character of EP coupling, is basically unable to reflect the enormous sudden increase of the relaxation time at Tc. Differences in the optical pump–optical probe and the optical pump–terahertz probe settings of PP relaxation dynamics are discussed within diagrammatic perturbation theory.

Visualization of Photoexcited Carrier Responses in a Solar Cell Using Optical Pump—Terahertz Emission Probe Technique

We observed photoexcited carrier responses in solar cells excited by femtosecond laser pulses with spatial and temporal resolution using an optical pump-terahertz emission probe technique. We visualized the ultrafast local variation of the intensity of terahertz emission from a polycrystalline silicon solar cell using this technique and clearly observed the change in signals between a grain boundary and the inside of a grain in the solar cell. Further, the time evolution of the pump–probe signals of the polycrystalline and monocrystalline silicon solar cells was observed, and the relaxation times of photoexcited carriers in the emitter layers of crystalline silicon solar cells were estimated using this technique. The estimated relaxation time was consistent with the lifetime of the Auger recombination process that was dominant in heavily doped silicon used as an emitter layer for the silicon solar cells, which is difficult to obtain with photoluminescence method commonly used for the evaluation of solar cells.

Thin film polycrystalline Si solar cells studied in transient regime by optical pump–terahertz probe spectroscopy

We used time-resolved terahertz spectroscopy to study ultrafast photoconductivity of polycrystalline thin-film silicon solar cells. We selected a series of samples, which exhibited variable conversion efficiencies due to hydrogen plasma passivation under various technological conditions. The decay of the transient terahertz conductivity shows two components: the fast one is related to the charge recombination at interfaces, while the slow nanosecond one is attributed to the trapping of photocarriers by defects localized at grain boundaries or at dislocations in the polycrystalline p− layer of the structure. We observed a clear correlation between the open-circuit voltage and the nanosecond-scale decay time of the transient terahertz conductivity of the solar cells. Thus, the terahertz spectroscopy appears to be a useful contactless tool for inspecting the local photoconductivity of solar cells including, in particular, various nanostructured schemes.

Probing Photoexcited Carriers in a Few-Layer MoS2 Laminate by Time-Resolved Optical Pump-Terahertz Probe Spectroscopy.

We report the dynamics of photoinduced carriers in a free-standing MoS2 laminate consisting of a few layers (1-6 layers) using time-resolved optical pump-terahertz probe spectroscopy. Upon photoexcitation with the 800 nm pump pulse, the terahertz conductivity increases due to absorption by the photoinduced charge carriers. The relaxation of the non-equilibrium carriers shows fast as well as slow decay channels, analyzed using a rate equation model incorporating defect-assisted Auger scattering of photoexcited electrons, holes, and excitons. The fast relaxation time occurs due to the capture of electrons and holes by defects via Auger processes, resulting in nonradiative recombination. The slower relaxation arises since the excitons are bound to the defects, preventing the defect-assisted Auger recombination of the electrons and the holes. Our results provide a comprehensive understanding of the non-equilibrium carrier kinetics in a system of unscreened Coulomb interactions, where defect-assisted Auger processes dominate and should be applicable to other 2D systems.

Effect of Na presence during CuInSe2 growth on stacking fault annihilation and electronic properties

While presence of Na is essential for the performance of high-efficiency Cu(In,Ga)Se2 thin film solar cells, the reasons why addition of Na by post-deposition treatment is superior to pre-deposition Na supply—particularly at low growth temperatures—are not yet fully understood. Here, we show by X-ray diffraction and electron microscopy that Na impedes annihilation of stacking faults during the Cu-poor/Cu-rich transition of low temperature 3-stage co-evaporation and prevents Cu homogeneity on a microscopic level. Lower charge carrier mobilities are found by optical pump terahertz probe spectroscopy for samples with remaining high stacking fault density, indicating a detrimental effect on electronic properties if Na is present during growth.

Excitation correlation photoluminescence in the presence of Shockley-Read-Hall recombination

Excitation correlation photoluminescence (ECPL) measurements are often analyzed in the approximation of a cross correlation of charge carrier populations generated by the two delayed pulses. In semiconductors, this approach is valid for a linear non-radiative recombination path, but not for a non-linear recombination rate as in the general Shockley-Read-Hall recombination scenario. Here, the evolution of the ECPL signal was studied for deep trap recombination following Shockley-Read-Hall statistics. Analytic solutions can be obtained for a fast minority trapping regime and steady state recombination. For the steady state case, our results show that the quadratic radiative term plays only a minor role, and that the shape of the measured signal is mostly determined by the non-linearity of the recombination itself. We find that measurements with unbalanced intense pump and probe pulses can directly provide information about the dominant non-radiative recombination mechanism. The signal traces follow the charge carrier concentrations, despite the complex origins of the signal, thus showing that ECPL can be applied to study charge carrier dynamics in semiconductors without requiring elaborate calculations. The model is compared with measurements on a reference sample with alternating layers of InGaAs/InAlAs that were additionally cross-checked with time resolved optical pump terahertz probe measurements and found to be in excellent agreement.

Evolution of hot carrier dynamics in graphene with the Fermi level tuned across the Dirac point

The ultrafast dynamics of photoexcited carriers closely depends on the excitation processes pertaining to the energy band of the materials and the relevant relaxation pathway relies on the interactions between hot carriers and lattice phonons. By using ultrafast optical-pump terahertz (THz)-probe spectroscopy with an ion-gel gate to tune the Fermi energy level ${E}_{F}$ in graphene, we are able to reveal the relaxation dynamics of hot carriers at different ${E}_{F}$. It is found that the relaxation time increases while the pump-induced differential transmission decreases as ${E}_{F}$ approaches the Dirac point. Through self-consistent model calculations, we quantitatively interpret that a temperature-dependent scattering rate is responsible for a negative photoinduced conductivity, and the relaxation transient directly manifests the Dirac spectrum dependence of the optical phonon emission and the carrier scattering rate. More interestingly, the scattering rate of hot carriers also exhibits a strong ${E}_{F}$ dependence, which is the most likely to originate from charged impurities inevitably present in graphene. The diminution of photoresponse efficiency across the Dirac point implies that the graphene-based optoelectronic devices may be operable only in the highly doped regime.

Modulation doping of GaAs/AlGaAs core-shell nanowires with effective defect passivation and high electron mobility.

Reliable doping is required to realize many devices based on semiconductor nanowires. Group III-V nanowires show great promise as elements of high-speed optoelectronic devices, but for such applications it is important that the electron mobility is not compromised by the inclusion of dopants. Here we show that GaAs nanowires can be n-type doped with negligible loss of electron mobility. Molecular beam epitaxy was used to fabricate modulation-doped GaAs nanowires with Al0.33Ga0.67As shells that contained a layer of Si dopants. We identify the presence of the doped layer from a high-angle annular dark field scanning electron microscopy cross-section image. The doping density, carrier mobility, and charge carrier lifetimes of these n-type nanowires and nominally undoped reference samples were determined using the noncontact method of optical pump terahertz probe spectroscopy. An n-type extrinsic carrier concentration of 1.10 ± 0.06 × 10(16) cm(-3) was extracted, demonstrating the effectiveness of modulation doping in GaAs nanowires. The room-temperature electron mobility was also found to be high at 2200 ± 300 cm(2) V(-1) s(-1) and importantly minimal degradation was observed compared with undoped reference nanowires at similar electron densities. In addition, modulation doping significantly enhanced the room-temperature photoconductivity and photoluminescence lifetimes to 3.9 ± 0.3 and 2.4 ± 0.1 ns respectively, revealing that modulation doping can passivate interfacial trap states.

Influence of structural symmetry on resonant property and optical sensitivity in terahertz metamaterials

By utilizing optical pump terahertz probe spectroscopy we have investigated four samples with terahertz subwavelength structures, which derived from split ring resonators. The experimental results show that adding the same length bar in the middle of the symmetric samples has less influence on resonant properties. However, asymmetric samples could present coupling and splitting effect in transmission spectra. Additionally, compared with the symmetric structures the asymmetry structures are more sensitive to pump excitation. Under pump excitation for different resonant mechanisms, we find the LC resonance has higher sensitivity than dipole resonance to the pump excitation. Our obtained results indicate that the photogenerated carriers in the substrate play a very important role in optical modulation properties.

Ultrafast carrier dynamics in GaAs nanowires

The results of investigation of electronic transport in GaAs nanowires (NWs), which were grown by MBE and MOCVD on n-type GaAs(111) substrates are presented. The time-resolved dynamics of photocarriers were studied by opticalpump terahertz generation-probe time-domain spectroscopy. The dynamics of THz generation for different levels of excitation for the GaAs NWs grown by MBE can be explained by separation of photoinduced carriers in the contact field with subsequent screening of the contact field and with non-saturable carrier recombination. The dynamics of THz generation for the GaAs NWs grown by MOCVD is due to the growth of the mobility of carriers as they are being grabbed to the non-radiative centres and as a consequence of carrier retrapping. The shift of THz pulse is connected with the changes of a phase in transmission and reflection THz pulse through the border between the absorbing and non-absorbing medium.

Ultrafast carrier dynamics in GaAs nanowires

The results of investigation of electronic transport in GaAs nanowires (NWs), which were grown by MBE and MOCVD on n-type GaAs(111) substrates are presented. The time-resolved dynamics of photocarriers were studied by opticalpump terahertz generation-probe time-domain spectroscopy. The dynamics of THz generation for different levels of excitation for the GaAs NWs grown by MBE can be explained by separation of photoinduced carriers in the contact field with subsequent screening of the contact field and with non-saturable carrier recombination. The dynamics of THz generation for the GaAs NWs grown by MOCVD is due to the growth of the mobility of carriers as they are being grabbed to the non-radiative centres and as a consequence of carrier retrapping. The shift of THz pulse is connected with the changes of a phase in transmission and reflection THz pulse through the border between the absorbing and non-absorbing medium.

Density-dependent electron scattering in photoexcited GaAs in strongly diffusive regime

In a series of systematic optical pump–terahertz probe experiments, we study the density-dependent electron scattering rate in photoexcited GaAs in the regime of strong carrier diffusion. The terahertz frequency-resolved transient sheet conductivity spectra are perfectly described by the Drude model, directly yielding the electron scattering rates. A diffusion model is applied to determine the spatial extent of the photoexcited electron-hole gas at each moment after photoexcitation, yielding the time-dependent electron density, and hence the density-dependent electron scattering time. We find that the electron scattering time decreases from 320 to 60 fs, as the electron density changes from 1015 to 1019 cm−3.

Electronic properties of GaAs, InAs and InP nanowires studied by terahertz spectroscopy

We have performed a comparative study of ultrafast charge carrier dynamics in a range of III–V nanowires using optical pump–terahertz probe spectroscopy. This versatile technique allows measurement of important parameters for device applications, including carrier lifetimes, surface recombination velocities, carrier mobilities and donor doping levels. GaAs, InAs and InP nanowires of varying diameters were measured. For all samples, the electronic response was dominated by a pronounced surface plasmon mode. Of the three nanowire materials, InAs nanowires exhibited the highest electron mobilities of 6000 cm2 V−1 s−1, which highlights their potential for high mobility applications, such as field effect transistors. InP nanowires exhibited the longest carrier lifetimes and the lowest surface recombination velocity of 170 cm s−1. This very low surface recombination velocity makes InP nanowires suitable for applications where carrier lifetime is crucial, such as in photovoltaics. In contrast, the carrier lifetimes in GaAs nanowires were extremely short, of the order of picoseconds, due to the high surface recombination velocity, which was measured as 5.4 × 105 cm s−1. These findings will assist in the choice of nanowires for different applications, and identify the challenges in producing nanowires suitable for future electronic and optoelectronic devices.

Ultrafast dynamics of hot electrons and phonons in chemical vapor deposited graphene

The relaxation dynamics of photoexcited carriers in a chemical vapor deposited graphene transferred on quartz substrate has been investigated by using ultrafast optical-pump terahertz (THz)-probe spectroscopy. Terahertz transmission through graphene sample is reduced by optical pumping. The change of transmission decays exponentially after the optical pulse. We find the relaxation time is insensitive to the substrate temperature from 10 K to 300 K but increases sublinearly with pump fluence. We model the relaxation process involving electron-phonon coupling together with a set of rate equations to describe the transient responses of quasi-particles and optical phonons. The increases of the extracted carrier temperature and the measured relaxation time with pump fluence are associated with the fact that high pump fluence significantly increases the carrier temperature and broadens the carrier distribution. As a result, it leads to the reduction of optical phonon emission efficiency and the decrease of cooling rate as well.

Defect Engineering in CdSxSe1–x Nanobelts: An Insight into Carrier Relaxation Dynamics via Optical Pump–Terahertz Probe Spectroscopy

Defects in nanomaterials often induce dramatic changes in the photoelectrical properties of semiconducting II–VI compound nanomaterials. The relationship between defects and carrier dynamics is pivotal in material engineering for potential applications. A thorough understanding of the dynamics of defect-related free carrier depletion is particularly important for the fabrication and optimization of nano-optoelectronic devices. In this work, optical pump–terahertz probe spectroscopy was employed to investigate the carrier dynamics in CdS and Se-alloyed CdS nanobelts. The dynamics are dominated by the surface defect trapping in the case of CdS and structural-defect-related recombination for the Se-alloyed CdS. The conclusion is also supported by temperature-dependent photoluminescence spectroscopic studies. Our results indicate that congeneric element replacement is an effective approach for defect-distribution restructuring, which modifies the physical properties of nanomaterials through defect engineering.