Hot Photocarriers Research Articles

Imaging hot photocarrier transfer across a semiconductor heterojunction with ultrafast electron microscopy

Semiconductor heterojunctions have gained significant attention for efficient optoelectronic devices owing to their unique interfaces and synergistic effects. Interaction between charge carriers with the heterojunction plays a crucial role in determining device performance, while its spatial-temporal mapping remains lacking. In this study, we employ scanning ultrafast electron microscopy (SUEM), an emerging technique that combines high spatial-temporal resolution and surface sensitivity, to investigate photocarrier dynamics across a Si/Ge heterojunction. Charge dynamics are selectively examined across the junction and compared to far bulk areas, through which the impact of the built-in potential, band offsets, and surface effects is directly visualized. In particular, we find that the heterojunction drastically modifies the hot photocarrier diffusivities in both Si and Ge regions due to charge trapping. These findings are further elucidated with insights from the band structure and surface potential measured by complementary techniques. This work demonstrates the tremendous effect of heterointerfaces on hot photocarrier dynamics and showcases the potential of SUEM in characterizing realistic optoelectronic devices.

ZnS/ZnSe heterojunction photocatalyst for augmented hydrogen production: Experimental and theoretical insights

A facile chemical process for the synthesis of ZnS/ZnSe composites and theoretical and experimental insights into their sunlight-driven photocatalytic H2 production via water-splitting reactions are demonstrated. ZnSe systems are varied by synthesizing at various temperatures such as 80, 150, and 180 °C for 12 h via hydrothermal process to tune their crystalline properties while maintaining ZnS as the host material, which is the main driving force to achieve increased photocatalytic H2 production efficiency in this study. The prepared composite photocatalyst is found to have cubic ZnS and wurtzite ZnSe phases with good overall crystalline properties. The morphological investigation revealed that the composite consists of ZnS with a spherical structure coupled with irregular-structured ZnSe particles. The optimized ZnS/ZnSe (ZnSe prepared at 150 °C) photocatalyst showed the highest H2 generation of around 84.8 mmol h−1 g−1cat, with a UTH (i.e., UV–visible-to-H2) conversion efficiency and turnover frequency of 20.4% and 0.097 Atom−1 s−1, respectively. This observed photocatalytic efficiency is presumed to be the formation of type-I heterojunction channelizing the effective transfer of hot photocarriers from ZnS to ZnSe for the rapid production of protons (H+) and their subsequent reduction to H2 molecules. The achieved highest efficiency of the composite is around 56.4 and 4.2 folds higher than the pristine ZnSe and ZnS, respectively. In addition, the recycle experiments of the optimized catalyst showed consistent H2 production for upto 5 cycles. Further, the developed composite systems are investigated via density functional theory and validated through various physio and electrochemical analyses to understand their structure-property relationships and photocatalytic mechanisms toward water splitting for H2 production.

Probing photocarrier dynamics in a Bi2Te3–Te eutectic p–n junction with a laser terahertz emission microscope

Bismuth telluride (Bi2Te3)-based heterostructures have attracted considerable attention owing to their interesting anisotropic properties and expected higher thermoelectric performance. Therefore, exploring the nature of the carrier dynamics in these heterostructures has been an important subject in the design and optimization of advanced materials. In the present study, hot carrier injection and its subsequent spatiotemporal behavior in a multilayered crystalline Bi2Te3–Tellurium (Te) eutectic composite were studied using a laser terahertz (THz) emission microscopy (LTEM). The THz emission electric fields at the Bi2Te3–Te interface were polarized perpendicular to the interface. The polarities of these waveforms reveal the direction of the electric field between the Bi2Te3 and Te regions, indicating the carrier types of these components and the p–n junction formed at the interface. In addition, in the Te region, a strong THz emission with an electric field polarized parallel to the interface was observed. This unique THz emission can be qualitatively explained through hot photocarrier anisotropic transport by considering the effective mass of electrons and holes. LTEM clarified the local carrier dynamics in the microstructures and revealed the potential distribution and anisotropic transport properties. These findings contribute to the exploration of eutectic heterostructures as new functional materials and provide new avenues for cutting-edge thermoelectric and photovoltaic devices.

Impact of photoexcitation on secondary electron emission: A Monte Carlo study

Understanding the transport of photogenerated charge carriers in semiconductors is crucial for applications in photovoltaics, optoelectronics, and photo-detectors. While recent experimental studies using scanning ultrafast electron microscopy (SUEM) have demonstrated that the local change in the secondary electron emission induced by photoexcitation enables direct visualization of the photocarrier dynamics in space and time, the origin of the corresponding image contrast still remains unclear. Here, we investigate the impact of photoexcitation on secondary electron emissions from semiconductors using a Monte Carlo simulation aided by time-dependent density functional theory. Particularly, we examine two photoinduced effects: the generation of photocarriers in the sample bulk and the surface photovoltage (SPV) effect. Using doped silicon as a model system and focusing on primary electron energies below 1 keV, we found that both the hot photocarrier effect immediately after photoexcitation and the SPV effect play dominant roles in changing the secondary electron yield (SEY), while the distribution of photocarriers in the bulk leads to a negligible change in SEY. Our work provides insights into electron–matter interaction under photo-illumination and paves the way toward a quantitative interpretation of the SUEM contrasts.

Ultrafast Momentum-Resolved Hot Electron Dynamics in the Two-Dimensional Topological Insulator Bismuthene.

Two-dimensional quantum spin Hall (QSH) insulators are a promising material class for spintronic applications based on topologically protected spin currents in their edges. Yet, they have not lived up to their technological potential, as experimental realizations are scarce and limited to cryogenic temperatures. These constraints have also severely restricted characterization of their dynamical properties. Here, we report on the electron dynamics of the novel room-temperature QSH candidate bismuthene after photoexcitation using time- and angle-resolved photoemission spectroscopy. We map the transiently occupied conduction band and track the full relaxation pathway of hot photocarriers. Intriguingly, we observe photocarrier lifetimes much shorter than those in conventional semiconductors. This is ascribed to the presence of topological in-gap states already established by local probes. Indeed, we find spectral signatures consistent with these earlier findings. Demonstration of the large band gap and the view into photoelectron dynamics mark a critical step toward optical control of QSH functionalities.

Zero-bias mid-infrared graphene photodetectors with bulk photoresponse and calibration-free polarization detection

Bulk photovoltaic effect (BPVE), featuring polarization-dependent uniform photoresponse at zero external bias, holds potential for exceeding the Shockley-Queisser limit in the efficiency of existing opto-electronic devices. However, the implementation of BPVE has been limited to the naturally existing materials with broken inversion symmetry, such as ferroelectrics, which suffer low efficiencies. Here, we propose metasurface-mediated graphene photodetectors with cascaded polarization-sensitive photoresponse under uniform illumination, mimicking an artificial BPVE. With the assistance of non-centrosymmetric metallic nanoantennas, the hot photocarriers in graphene gain a momentum upon their excitation and form a shift current which is nonlocal and directional. Thereafter, we demonstrate zero-bias uncooled mid-infrared photodetectors with three orders higher responsivity than conventional BPVE and a noise equivalent power of 0.12 nW Hz−1/2. Besides, we observe a vectorial photoresponse which allows us to detect the polarization angle of incident light with a single device. Our strategy opens up alternative possibilities for scalable, low-cost, multifunctional infrared photodetectors.

Inhomogeneous photocarrier dynamics and transport in monolayer MoS2 by ultrafast microscopy

Monolayer MoS2 as a member of two-dimensional transition metal dichalcogenides (TMDs) has attracted considerable attention due to its superior optoelectronic properties. Understanding the photocarrier dynamics and transport in these two dimensional systems is beneficial for applications from photovoltaics to sensing. However, various structural defects strongly impact the dynamics and transport of photocarriers. Especially there lacks a precise measuring and understanding of photocarrier transport in TMDs. Here, femtosecond transient absorption spectroscopy and microscopy were employed to study the photocarrier dynamics and transport in monolayer MoS2. Defect correlated photocarrier dynamics are observed across the monolayer MoS2 where exciton formation and nonradiative recombination are the two dominant decay processes. To the best of our knowledge, we report two distinct photocarrier transport regimes in MoS2 for the first time with diffusion coefficients of cm2 s−1 and cm2 s−1, by taking advantages of ultrafast microscopy with ∼20 nm spatial precision and ∼200 fs temporal resolution. These two regimes are ascribed to fast hot photocarrier diffusion and slow phonon-limited thermal diffusion, respectively. The results indicate that the initial fast photocarrier transport is less dependent on structural defects compared to photocarrier relaxation dynamics which may be useful for hot photocarrier extraction applications.

Spectra of low-temperature photoluminescence in thin polycrystalline CdTe films

The band of intrinsic (e–h) radiation emission by the subsurface potential barriers of crystal grains and the edge doublet band arising as LO-phonon replicas of the e–h band are observed in the spectra of the low-temperature (4.2 K) photoluminescence of fine-grained (with a grain size of dcr ≤ 1 µm) CdTe films. Film doping with the In impurity results in quenching of the doublet band, while heat treatment leads to activation of the intrinsic band, a short-wavelength shift of the red boundary (ΔEr = 16–29 meV) and the halfwidth modulation (ΔA = 6–17 meV) of which correlate with the height of micropotential barriers and the temperature of recombining hot photocarriers.

Time-Resolved Measurements of Photocarrier Dynamics in TiS3 Nanoribbons.

We report synthesis and time-resolved transient absorption measurements of TiS3 nanoribbons. TiS3 nanoribbons were fabricated by direct reaction of titanium and sulfur. Dynamics of the photocarriers in these samples were studied by transient absorption measurements. It was found that following ultrafast injection of nonequilibrium and hot photocarriers, the thermalization, energy relaxation, and exciton formation all occur on a subpicosecond time scale. Several key parameters describing the dynamical properties of photocarriers, including their recombination lifetime, diffusion coefficient, mobility, and diffusion length, were deduced.

Hot photocarrier dynamics in organic solar cells.

Photocurrent in an organic solar cell is generated by a charge transfer reaction between electron donors and acceptors. Charge transfer is expected to proceed from thermalized states, but this picture has been challenged by recent studies that have investigated the role of hot excitons. Here we show a direct link between excess excitation energy and photocarrier mobility. Charge transfer from excited donor molecules generates hot photocarriers with excess energy coming from the offset between the lowest unoccupied molecular orbital of the donor and that of the acceptor. Hot photocarriers manifest themselves through a short-lived spike in terahertz photoconductivity that decays on a picosecond timescale as carriers thermalize. Different dynamics are observed when exciting the acceptor at its absorption edge to a thermalized state. Charge transfer in this case generates thermalized carriers described by terahertz photoconductivity dynamics consisting of an instrument-limited rise to a long-lived signal.

Picosecond real-space imaging of electron spin diffusion in GaAs

We use time-resolved two-color Kerr microscopy to study low-temperature electron spin diffusion in bulk GaAs with high spatial and temporal resolution. This method enables direct real-space imaging of the advancing spread of an optically excited electron spin packet by spin diffusion. We observe a high initial expansion rate of the spin packet, which is strongly reduced with increasing time. Comparison with continuous-wave Kerr spectroscopy demonstrates that this decrease is caused by the influence of the transient cooling of hot photocarriers on the electron spin diffusion coefficient.

Photoconductivity of biased graphene

Graphene is a promising candidate for optoelectronic applications such as photodetectors, terahertz imagers, and plasmonic devices. The origin of photoresponse in graphene junctions has been studied extensively and is attributed to either thermoelectric or photovoltaic effects. In addition, hot carrier transport and carrier multiplication are thought to play an important role. Here we report the intrinsic photoresponse in biased but otherwise homogeneous graphene. In this classic photoconductivity experiment, the thermoelectric effects are insignificant. Instead, the photovoltaic and a photo-induced bolometric effect dominate the photoresponse due to hot photocarrier generation and subsequent lattice heating through electron-phonon cooling channels respectively. The measured photocurrent displays polarity reversal as it alternates between these two mechanisms in a backgate voltage sweep. Our analysis yields elevated electron and phonon temperatures, with the former an order higher than the latter, confirming that hot electrons drive the photovoltaic response of homogeneous graphene near the Dirac point.

Intraband conductivity response in graphene observed using ultrafast infrared-pump visible-probe spectroscopy

Graphene, a truly two-dimensional material with a unique linear energy-momentum dispersion, demonstrates novel photonic properties such as universal absorption and conductivity, with applications including terahertz lasing, broadband midinfrared detectors, and tunable ultrafast lasers. Understanding the ultrafast nonequilibrium dynamics of photocarriers in graphene's unique relativistic band structure is important for the development of such high-speed, graphene-based photonic devices, as well as from a fundamental point of view. Here, our experiments indicate the relativistic nature of a nonequilibrium gas of electrons and holes photogenerated in a graphene monolayer as early as 100 fs after photoexcitation. We observe a nonlinear scaling in the Drude-like optical conductivity of the photocarriers with respect to their density, in striking contrast to the linear scaling expected from conventional materials with parabolic dispersion relations. Our measurements also indicate that hot photocarriers cool on a sub-100-fs time scale via interactions with optical phonons. These results elucidate the unique nature of the ultrafast dynamics of photocarriers in a relativistic material, in contrast to conventional materials, and provide a way to manipulate graphene's optical conductivity for applications in photonics and plasmonics.

Cooling of photoexcited carriers in graphene by internal and substrate phonons

We investigate the energy relaxation of hot carriers produced by photoexcitation of graphene through coupling to both intrinsic and remote (substrate) surface polar phonons using the Boltzmann equation approach. We find that the energy relaxation of hot photocarriers in graphene on commonly used polar substrates, under most conditions, is dominated by remote surface polar phonons. We also calculate key characteristics of the energy relaxation process, such as the transient cooling time and steady state carrier temperatures and photocarriers densities, which determine the thermoelectric and photovoltaic photoresponse, respectively. Substrate engineering can be a promising route to efficient optoelectronic devices driven by hot carrier dynamics.

Excitation processes of trivalent cerium ions in calcium thiogallate crystals by hot photocarriers

Excitation spectra for the 5d → 4f emission of Ce3+ ions in CaGa2S4 crystals have been measured at 300 K by using vacuum ultraviolet (VUV) photons, together with reflectivity and absorption-edge spectra. X-ray photoelectron spectroscopy (XPS) experiment has also been performed in order to clarify the contribution of shallow core states to the excitation and reflectivity spectra. Furthermore, cluster calculation by a relativistic discrete variational Xα (DV-Xα) method has been carried out to elucidate the optical transitions resulting in the creation of multiple e-h pairs in CaGa2S4. The excitation processes of Ce3+ ions in CaGa2S4 by hot photocarriers are discussed on the basis of the electronic structure determined from the experiments and calculation.

Influence of the formation of hot excitons on the quantum efficiency of short-wavelength photoelectric conversion in III–V semiconductors

A new excitonic mechanism for decreasing the quantum efficiency of the photoelectric conversion process in surface-barrier structures is considered. This mechanism involves the formation of hot excitons which have a large ionization energy and are not subject to the influence of the barrier electric field. The spectral dependence of the hot photocarrier losses is derived in an explicit form from experimental data on the basis of a previously proposed model and is analyzed. This dependence has two segments with abrupt increases, which are caused by the formation of excitons in the L and X valleys of the semiconductor.

Effect of the electric field in the space-charge layer on the efficiency of short-wave photoelectric conversion in GaAs Schottky diodes

A study is reported of the quantum efficiency of the short-wave photoelectric effect as function of the reverse bias applied to GaAs Schottky diodes when the light absorption length is much shorter than the width of the space charge region. The quantum efficiency of photoelectric conversion is found to depend strongly on the contact electric field and the photon energy. The field independence of the quantum efficiency is interpreted in terms of a fluctuational trap model. The model can also be used to determine the loss factor for hot photocarriers, which is found to increase in a stepwise manner with increasing photon energy. This effect is explained in terms of the formation of excitons in X-and L-valleys of the semiconductor.

Femtosecond studies of photoinduced bleaching and hot carrier dynamics in a-Si:H and a-Ge:H

The femtosecond pump and probe technique with 2eV and 4eV photons was applied to the study of hot photocarriers with considerable excess energy in extended states of a-Si:H and a-Ge:H. Photoinduced changes in the complex dielectric constant e 1 + ie 2 were calculated from the measured changes in reflectivity ΔR and transmission ΔT. This technique enables us to estimate that the average carrier cooling rates in our a-Si:H and a-Ge:H samples are about 2eV/psec and 1eV/psec, respectively. Photoinduced bleaching associated with the filling of band states dominates over intraband contributions to Δe 1 and Δe 2 for photon energies significantly exceeding the bandgap.

Hot photo-carrier and hot electron effects in p- n junctions

When a p- n junction of semiconductor (with bandgap energy E g ) is illuminated by light beam (with photon energy hv) in the condition of E g > hv, such as in Ge p- n junction illuminated by CO 2 laser beam, an electromotive force (emf) was induced between the terminals of the p- n junction, which indicated the opposite polarity to the ordinary photovoltaic effect like a solar cell. Such an anomalous photovoltaic phenomenon was explained by an optically excited hot carrier effect, through the following experiment with electrical excitation. Using a rod of n- or p-type Ge with a p- n junction at the surface of its center and an ohmic contact at each terminal of the rod, the same kind of phenomena was observed when electric field is applied along the length of the rod. The perpendicularly induced voltage or current had the same polarity instead of the reverse change of the applied electric field, and increases with increasing the applied field strength. The perpendicularly induced emf was caused by warm or hot carriers crossing the potential barrier of the p- n junction, which is very sensitive to the departure from thermally equilibrium velocity distribution of carriers.

Radiative spectra from hot photocarriers

Radiative recombination from non-equilibrium carriers in semiconductors is compared with single-particle scattering in GaAs. The Raman scattering data is obtained by two-photon absorption in bulk material whereas the photoluminescence spectra are obtained by regular one-photon absorption. The results are in close agreement with current theories.