Carrier Dynamics In Semiconductors Research Articles

Transient reflectivity as a probe of ultrafast carrier dynamics in semiconductors: A revised model for low-temperature grown GaAs

We revisit pump-probe transient reflectivity (PPTR) as a probe of ultrafast carrier dynamics in photoconductive materials, using low-temperature grown GaAs (LT-GaAs) as an exemplar. The carrier dynamics in a series of annealed LT-GaAs wafers were measured by PPTR. The wafer growth and anneal conditions were tailored to produce a material system with a sub-picosecond carrier lifetime. The PPTR signals from these wafers are bipolar with time constants on the order of 100 fs and 1 ps, consistent with previous literature reports on LT-GaAs. We examined the utility of numerical simulations of the pump-probe transients described in [V. Ortiz et al., J. Appl. Phys. 102, 043515 (2007)] to model our experimental results. We discovered a discrepancy between the model's predictions and experiment with respect to the scaling of the PPTR response with injected carrier density, and show that this discrepancy is rooted in how the model accounts for the index of refraction change due to band filling (BF) and band gap renormalization (BGR). We demonstrate that any model that includes a non-negligible BGR effect is inconsistent with our experimental observations of LT-GaAs. We present a revised model to simulate PPTR signals that account for BF and incorporate optical absorption from mid-gap states. This model can reproduce the experimental results on LT-GaAs and enables comparative assessments of alternate trapping and recombination hypotheses. For LT-GaAs, we compared point defects and nanoparticles as sites for Shockley-Read-Hall recombination, with the result that nanoparticle trapping and recombination centers most accurately reproduce the PPTR probe of carrier dynamics in LT-GaAs.

Femtosecond time-resolved photoemission electron microscopy for spatiotemporal imaging of photogenerated carrier dynamics in semiconductors.

We constructed an instrument for time-resolved photoemission electron microscopy (TR-PEEM) utilizing femtosecond (fs) laser pulses to visualize the dynamics of photogenerated electrons in semiconductors on ultrasmall and ultrafast scales. The spatial distribution of the excited electrons and their relaxation and/or recombination processes were imaged by the proposed TR-PEEM method with a spatial resolution about 100 nm and an ultrafast temporal resolution defined by the cross-correlation of the fs laser pulses (240 fs). A direct observation of the dynamical behavior of electrons on higher resistivity samples, such as semiconductors, by TR-PEEM has still been facing difficulties because of space and/or sample charging effects originating from the high photon flux of the ultrashort pulsed laser utilized for the photoemission process. Here, a regenerative amplified fs laser with a widely tunable repetition rate has been utilized, and with careful optimization of laser parameters, such as fluence and repetition rate, and consideration for carrier lifetimes, the electron dynamics in semiconductors were visualized. For demonstrating our newly developed TR-PEEM method, the photogenerated carrier lifetimes around a nanoscale defect on a GaAs surface were observed. The obtained lifetimes were on a sub-picosecond time scale, which is much shorter than the lifetimes of carriers observed in the non-defective surrounding regions. Our findings are consistent with the fact that structural defects induce mid-gap states in the forbidden band, and that the electrons captured in these states promptly relax into the ground state.

Application of broadband terahertz spectroscopy in semiconductor nonlinear dynamics

Semiconductor nonlinearity in the range of terahertz (THz) frequency has been attracting considerable attention due to the recent development of high-power semiconductor-based nanodevices. However, the under- lying physics concerning carrier dynamics in the presence of high-field THz transients is still obscure. This paper introduces an ultrafast, time-resolved THz pump/THz probe approach to study semiconductor properties in a nonlinear regime. The carrier dynamics regarding two mechanisms, intervalley scattering and impact ionization, was observed for doped InAs on a sub-picosecond time scale. In addition, polaron modulation driven by intense THz pulses was experimentally and theoretically investi- gated. The observed polaron dynamics verifies the interaction between energetic electrons and a phonon field. In contrast to previous work which reported optical phonon responses, acoustic phonon modulations were addressed in this study. A further understanding of the intense field interacting with solid materials will accelerate the development of semiconductor devices. This paper can be divided into 4 sections. Section 1 starts with the design and performance of a table-top THz spectrometer, which has the advantages of ultra-broad bandwidth (one order higher bandwidth compared to a conventional ZnTe sensor) and high electric field strength ( >100 kV/cm). Unlike the conventional THz time- domain spectroscopy, the spectrometer integrated a novel THz air-biased-coherent-detection (THz-ABCD) techni- que and utilized gases as THz emitters and sensors. In comparison with commonly used electro-optic (EO) crystals or photoconductive (PC) dipole antennas, the gases have the benefits of no phonon absorption as existing in EO crystals and no carrier life time limitation as observed in PC dipole antennas. In Section 2, the newly development THz-ABCD spectrometer with a strong THz field strength capability provides a platform for various research topics especially on the nonlinear carrier dynamics of semiconductors. Two mechanisms, electron intervalley scattering and impact ionization of InAs crystals, were observed under the excitation of intense THz field on a sub-picosecond time scale. These two competing mechanisms were demonstrated by changing the impurity doping type of the semiconductors and varying the strength of the THz field. Another investigation of nonlinear carrier dynamics in Section 3 was the observation of coherent polaron oscillation in n-doped semiconductors excited by intense THz pulses. Through modulations of surface reflection with a THz pump/THz probe technique, this work experimentally verifies the interaction between energetic electrons and a phonon field, which has been theoretically predicted by previous publications, and shows that this interaction applies for the acoustic phonon modes. Usually, two transverse acoustic (2TA) phonon responses are inactive in infrared measurement, while they are detectable in second-order Raman spectroscopy. The study of polaron dynamics, with nonlinear THz spectroscopy (in the far- infrared range), provides a unique method to diagnose the overtones of 2TA phonon responses of semiconductors, and therefore incorporates the abilities of both infrared and Raman spectroscopy. Finally, some conclusions were presented in Section 4. In a word, this work presents a new milestone in wave-matter interaction and seeks to benefit the industrial applications in high power, small scale devices.

Exciton Drift in Semiconductors under Uniform Strain Gradients: Application to Bent ZnO Microwires

Optimizing the electronic structures and carrier dynamics in semiconductors at atomic scale is an essential issue for innovative device applications. Besides the traditional chemical doping and the use of homo/heterostructures, elastic strain has been proposed as a promising possibility. Here, we report on the direct observation of the dynamics of exciton transport in a ZnO microwire under pure elastic bending deformation, by using cathodoluminescence with high temporal, spatial, and energy resolutions. We demonstrate that excitons can be effectively drifted by the strain gradient in inhomogeneous strain fields. Our observations are well reproduced by a drift-diffusion model taking into account the strain gradient and allow us to deduce an exciton mobility of 1400 ± 100 cm(2)/(eV s) in the ZnO wire. These results propose a way to tune the exciton dynamics in semiconductors and imply the possible role of strain gradient in optoelectronic and sensing nano/microdevices.

Competition between the electronic and phonon–mediated scattering channels in the out–of–equilibrium carrier dynamics of semiconductors: an ab-initio approach

The carrier dynamics in bulk Silicon, a paradigmatic indirect gap semiconductor, is studied by using the Baym–Kadanoff equations. Both the electron–electron (e–e) and electron–phonon (e–p) self-energies are calculated fully ab–initio by using a semi–static out–of– equilibrium GW approximation in the e–e case and a Fan self–energy in the e-p case. By using the generalized Baym–Kadanoff ansatz the two–time evolution is replaced by the only dynamics on the macroscopic time axis. The enormous numerical difficulties connected with a real–time simulation of realistic systems is overcome by using a completed collision approximation that further simplifies the memory effects connected to the time evolution. The carrier dynamics is shown to reduce in such a way to have stringent connections to the well–known equilibrium electron–electron and electron–phonon self–energies. This link allows to use general arguments to motivate the relative balance between the e–e and e–p scattering channels on the basis of the carrier energies.

Imaging of Transient Carrier Dynamics in Semiconductors by Nanoscale Pump-Probe Microscopy

Imaging of Transient Carrier Dynamics in Semiconductors by Nanoscale Pump-Probe Microscopy

Carrier dynamics in semiconductors studied with time-resolved terahertz spectroscopy

Time-resolved, pulsed terahertz spectroscopy has developed into a powerful tool to study charge carrier dynamics in semiconductors and semiconductor structures over the past decades. Covering the energy range from a few to about 100 meV, terahertz radiation is sensitive to the response of charge quasiparticles, e.g., free carriers, polarons, and excitons. The distinct spectral signatures of these different quasiparticles in the THz range allow their discrimination and characterization using pulsed THz radiation. This frequency region is also well suited for the study of phonon resonances and intraband transitions in low-dimensional systems. Moreover, using a pump-probe scheme, it is possible to monitor the nonequilibrium time evolution of carriers and low-energy excitations with sub-ps time resolution. Being an all-optical technique, terahertz time-domain spectroscopy is contact-free and noninvasive and hence suited to probe the conductivity of, particularly, nanostructured materials that are difficult or impossible to access with other methods. The latest developments in the application of terahertz time-domain spectroscopy to bulk and nanostructured semiconductors are reviewed.

Ultrafast nanoprobing

Imaging the transient carrier dynamics in semiconductors at both high temporal and spatial resolution has long been a goal for solid-state scientists. Hidemi Shigekawa from the University of Tsukuba in Japan told Nature Photonics how his team accomplished this feat.

Carrier dynamics and terahertz photoconductivity of doped silicon measured by femtosecond pump-terahertz probe spectroscopy

The carrier dynamics and terahertz photoconductivity in the n-type silicon (n-Si) as well as in the p-type Silicon (p-Si) have been investigated by using femtosecond pump-terahertz probe technique. The measurements show that the relative change of terahertz transmission of p-Si at low pump power is slightly smaller than that of n-Si, due to the lower carrier density induced by the recombination of original holes in the p-type material and the photogenerated electrons. At high pump power, the bigger change of terahertz transmission of p-Si originates from the greater mobility of the carriers compared to n-Si. The transient photoconductivities are calculated and fit well with the Drude-Smith model, showing that the mobility of the photogenerated carriers decreases with the increasing pump power. The obtained results indicate that femtosecond pump-terahertz probe technique is a promising method to investigate the carrier dynamics of semiconductors.

Ultrafast Spin Dynamics Including Spin-Orbit Interaction in Semiconductors

This Letter presents a theoretical investigation of ultrafast spin-dependent carrier dynamics in semiconductors due to strong spin-orbit coupling using holes in bulk GaAs as a model system. By computing the microscopic carrier dynamics in the anisotropic hole-band structure including spin-orbit coupling, we obtain spin-relaxation times in quantitative agreement with measured hole-spin relaxation times [Phys. Rev. Lett. 89, 146601 (2002)10.1103/PhysRevLett.89.146601]. We show that different optical techniques for the measurement of hole-spin dynamics yield different results, in contrast to the case of electron-spin dynamics.

Preface: phys. stat. sol. (c) 5/1

AbstractThis issue contains invited and contributed papers of the 15th International Conference on Nonequilibrium Carrier Dynamics in Semiconductors (HCIS15) held 23–27 July 2007 at Hongo Campus, University of Tokyo, Tokyo, Japan.The conference featured 11 invited and 55 con‐tributed talks as well as 80 posters, and an international attendance of more than 160 scientists. Following the tradition of the conference, HCIS15 provided an international forum on the latest progress in the field of physics and applications of nonequilibrium carrier dynamics in bulk semiconductors, nanostructures, and structures of molecular scales. Interesting contributions were made on transport in novel nanostructures and nanodevices, intersubband transitions and terahertz phenomena, ultrafast spectroscopy, and spintronics, which clearly indicates the importance of these topics for the next generation devices.On behalf of the organizers of HCIS15, we thank all the participants who made the conference a successful and pleasant experience. We sincerely appreciate support from the Japan Society of Applied Physics and Electronics Society, IEICE. Generous donations from foundations and companies are gratefully acknowledged. We are also indebted to Michihisa Yamamoto, Kenji Shibata, Sayoko Muta, Mari Ikushima, and Mika Tsukamoto for their invaluable contribution to the conference organization. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Sum-over-states versus quasiparticle pictures of coherent correlation spectroscopy of excitons in semiconductors: Femtosecond analogs of multidimensional NMR

Two-dimensional correlation spectroscopy 2DCS, based on the nonlinear optical response of excitons to sequences of ultrafast pulses may provide some unique insights into carrier dynamics in semiconductors. The most prominent feature of 2DCS, cross peaks, can best be understood using a sum-over-states picture involving the many-body eigenstates. However, the optical response of semiconductors is usually calculated by solving truncated equations of motion for dynamical variables, which result in a quasiparticle picture. In this work we derive Green’s function expressions for the four wave mixing signals generated in various phase-matching directions and use them to establish the connection between the two pictures. The formal connection with Frenkel excitons hard-core bosons and vibrational excitons soft-core bosons is pointed out.

Advantages of the time-resolved four-wave mixing technique for studies of non-equilibrium carrier dynamics in bulk semiconductors and structures

We demonstrate versatility of the four-wave mixing technique in investigating non-equilibrium carrier dynamics in semiconductor crystals and epitaxial structures. Measurements of the effective diffusion coefficient in the presence of a light-induced space charge field have allowed us to determine the photoconductivity type of photorefractive CdTe and InP crystals doped with deep impurities. Hole mobility values of μh=(96±22)cm2/Vs in CdTe and μh=(152±16)cm2/Vs in InP have been found from the ambipolar diffusion coefficient. Studies of a number of differently grown GaN layers with dislocation density ranging from 106 to 1010cm−2 have revealed a correlation of the longer carrier lifetimes with the smaller dislocation density. Competition between radiative and non-radiative recombination channels has been observed in low dislocation density HVPE-grown bulk-like GaN.

Observation of infrared absorption in HgSe and HgTe in transient megagauss fields

Applying infrared-magneto spectroscopy in transient megagauss fields, we were able not only to observe detailed inter- and intraband resonances, but also to study the carrier dynamics due to the pronounced hysteresis effects in the magneto transmission. These time-delay effects are shown only at temperatures above 100K and are strongest at room temperature in the cyclotron-resonance transitions. The effect can be explained by a delay in the thermalization of the carrier population due to a finite spin-lattice relaxation time being in the order of the characteristic time of the field variation.Choosing HgSe and HgTe as examples we demonstrate the transient megagauss fields are a useful tool for the investigation of spin related carrier dynamics in semiconductors.

Carrier Dynamics in Semiconductors under MIR FEL Excitation

Free electron lasers have unique advantages; wide-range wavelength tunability, ultrashort pulse operation and intense peak power. The high-power coherent laser beam in the mid-infrared (MIR) region provides unique opportunities to investigate carrier dynamics in semiconductors. Band-gap luminescence from a variety of compound semiconductors has been observed with an intense MIR radiation beam being directed to the samples. The band-gap luminescence is attributed to impact-ionization following scattering-induced free-electron heating.

Simulation of terahertz generation at semiconductor surfaces

A three-dimensional semiclassical Monte Carlo model is presented to describe fast carrier dynamics in semiconductors after photoexcitation. Far-field terahertz (THz) radiation patterns are calculated for both InAs and GaAs with, and without, application of external magnetic fields. This analysis distinguishes between surface depletion field and photo-Dember mechanisms for generating THz radiation. The theoretical model reproduces experimental data from GaAs and InAs, and demonstrates that a magnetic field enhances THz emission by rotating the emitting dipole with respect to the sample surface, leading to an increased coupling of radiation through the surface.

Picosecond time-resolved cyclotron resonance in semiconductors

A promising method for studying intraband carrier dynamics in semiconductors is monitoring the evolution of far-infrared (FIR) absorption induced by photoexcited carriers. By monitoring the photoinduced FIR absorption as a function of magnetic field, we performed time-resolved cyclotron resonance of photocreated electrons in InSb with picosecond resolution.

Probing ultrafast carrier and phonon dynamics in semiconductors

Over the past 2 decades there has been tremendous advancements in the field of ultrafast carrier dynamics in semiconductors. The driving force behind this movement other than the basic fundamental interest is the direct application of semiconductor devices and the endless need for faster response and faster processing of information. To improve and develop microelectronics devices and address these needs, there must be a basic understanding of the various dynamical processes in the semiconductors which have to be studied in detail. Therefore, the excitation of semiconductors out of their equilibrium and the subsequent relaxation processes with various rates has become a key area of semiconductor research. With the development of lasers that can generate pulses as short as a few femtoseconds the excitation and subsequent probing of semiconductors on an ultrashort timescale have become routine. Processes such as carrier momentum randomization, carrier thermalization, and energy relaxation have been studied in detail using excite-and-probe novel techniques. This article reviews the status of ultrafast carrier and phonon dynamics in semiconductors. Experimental techniques such as excite-and-probe transmission, time-resolved up-conversion luminescence, and pump-probe Raman scattering along with some of the significant experimental findings from probing semiconductors are discussed. Finally, a selfconsistent theoretical model, which correlates the carrier and phonon dynamics in germanium on an ultrashort time scale, is described in detail.

Ultrafast electronic raman spectroscopy in semiconductors

Time-resolved Raman spectroscopy in 100 femtosecond region is developed and applied to investigation of carrier dynamics in semiconductors. Experimental equipments and some results on germanium are presented. Raman scattering due to intervalley scattering from electrons in the L-valley was observed and 1.2 pscc was obtained as a time constant of population build-up in the L valley. From the rise of the inter-valence-band resonant Raman scattering from the photo-excited holes, the cooling rate of the holes in the heavy hole band was obtained.

The Ultrafast Carrier Dynamics in Semiconductors: The Role of Defects

AbstractThe presence of point defects is expected to influence the properties of free carrier in semiconductors. We have used the techniques of ultrafast laser spectroscopy to characterize the dynamics of photoinjected carriers in several III–V semiconductors grown at low temperature. The initial scattering time and the lifetime of the carriers become very short at low growth temperatures. Results obtained with low-temperature grown III–Vs are compared to those obtained with III–Vs grown at normal temperatures and amorphous silicon.