Carriers In GaAs Research Articles

Surface-state-related carrier dynamics of GaAs determined by UV-visible pump-probe terahertz spectroscopy

The surface velocity and the bulk lifetime of photo-excited free carriers in GaAs were measured using an optical-pump and THz-probe time-domain technique. By varying the pump laser photon energy from 1.56 to 4.15 eV, we observe that the surface velocity drops abruptly from 0.7×106 cm/s down to 0.2×106 cm/s at 2.5 eV, while the bulk lifetime remains almost constant. We tentatively explain this step-like behavior of the surface velocity vs the photon energy by a trapping of the free carriers at surface states, whose density of states shows a maximum at 2.5 eV.

Terahertz response of ultrafast spin polarization in semi-insulating GaAs

Due to its high sensitivity and time-resolved ability, terahertz time-domain spectroscopy is a powerful tool for investigating ultrafast carrier dynamics in semiconductors. In addition to charges, spins of ultrafast carriers provide an alternate degree of freedom to design modern electronic devices but are rarely studied by terahertz time-domain spectroscopy. Here, ultrafast spin polarization in semi-insulating GaAs is studied by optical-pump terahertz-probe experiments at room temperature. We used circularly and linearly polarized femtosecond laser pulses to inject nonequilibrium carriers in GaAs and observed that both the transmitted and reflected terahertz signals exhibited different dynamical evolutions under the excitations of linearly and circularly polarized laser pulses, which are ascribed to the generation and relaxation of spin-polarized electrons. The lifetime of the ultrafast spin polarization was obtained from our experiments, highlighting the potentialities of terahertz spectroscopy for the investigation of spin relaxation in semiconductors.

Research on the thermal failure mechanism of an opposed-contact gallium arsenide photoconductive semiconductor switch in avalanche mode

The switching mechanism and the thermal process of a gallium arsenide (GaAs) photoconductive semiconductor switch (PCSS) with an opposed-contact is investigated by numerical simulation of the two-dimensional (2D) structure using temperature-dependent physical parameters of the carriers in GaAs. Triggered by a low-energy laser pulse, the PCSS switches on due to the formation and evolution of multiple powerful avalanche domains. The 2D evolving characteristics of the avalanche domains on the two sides of photogenerated plasma during the switching transient are comparatively analysed. It is found that the ionizing centre of each domain moves with the drift of accumulated electrons inside the domain. Meanwhile, the evolution of avalanche domains causes an obvious thermal effect along the drift path of ionizing centres during the switch-on stage of PCSS. Then, the temperature keeps increasing at the edge of the anode and cathode although the switching current starts dropping after the conduction of PCSS, and finally peaks at ∼491 K and ∼541 K, respectively. The simulation results indicate that the 2D filamentary current flows along the drift path of ionizing centres inside the avalanche domains, which finally leads to filamentary erosion after continuous operation of the PCSS. On the basis of numerical simulation, an experiment with opposed-contact GaAs PCSS with 2.5 mm gap at the bias field of ∼90 kV cm−1 is performed. The thermal erosion is found to initially accumulate at the edge of the electrodes and then spreads along the current channel into the GaAs substrate, which is in accordance with the simulation results and analysis.

ОСОБЛИВОСТІ ДОСЛІДЖЕНЬ ПЕРЕХІДНИХ ШАРІВ В НАПІВПРОВІДНИКОВИХ ГЕТЕРОСИСТЕМАХ

Studies of spectral characteristics in Schottky's contact specimens showed that photemf generated by mono­chromatic light, according to the formation mechanism, has a predominantly lateral nature, that is, in a hetero­system there are at least two areas separated by a potential barrier along the interface, with different conductivity levels. The feature of the photoemf spectral characteristics is a variations its appearance when changing the thermal annealing temperature of the studied heterosystems. A significant characteristic and a small amplitude of the characteristic indicates the formation of a transition layer, relatively homogeneous and with insignificant, compared with the volume of GaAs, the doping concentration. If the spectral characteristic has one maximum and amplitude, which several times exceeds the amplitude of a significant characteristic, which means the formation of a transition layer in the Schottky contact depletion area with high conductivity, compared with a quasine-neutral region of a semiconductor. The distribution of lateral photoemf along the sample also has a significant character. In order to obtain the correct results relative to the heterosystem transition layer, it is necessary to measure spectral acute characteristics at a distance from the point change sign of the emf that several times the diffusion length of non-equilibrium charge carriers in GaAs. The problem of the formation of a metal-semiconductor contact and other heterosystems accompanied by the occurrence of heterogeneous transition layers, always paid attention. The use of the proposed photovoltaic method allows to establish the degree of homogeneity of semiconductor layers, components of the structure and predict the redistribution of current density flowing through the physical scope of the device.

Graphene transfer passivates GaAs

Graphene–semiconductor junction interface states influence the carrier recombination processes in emerging optoelectronic devices. The large density of interface states in the graphene–GaAs junction is partly formed by oxidation in air of the GaAs surface. A graphene transfer presented herein reduces the arsenic species in the GaAs oxide and maintains the reduction over a span of at least one year. The photoluminescence and terahertz emission spectra show reduced surface trapping of photogenerated carriers in GaAs with graphene-capped oxide. These findings demonstrate a 2D material transfer that passivates a 3D semiconductor surface. A consequence of the passivation is observed by photoreflectance modulation spectroscopy of graphene covered semi-insulating GaAs. The built-in surface field is sufficiently screened by optically pumped carriers to reveal an enhanced excitonic absorption just below the GaAs bandgap. The absorption critical point anomalously red shifts by 4–6 meV from the bulk exciton characteristic energy, an effect we attribute to the exciton absorption occurring closer to the graphene–GaAs interface and influenced by the near-surface GaAs dielectric polarization.

Research on Time Jitter of GaAs Photoconductive Semiconductor Switches in the Negative Differential Mobility Region

Time jitter of GaAs photoconductive semiconductor switches (PCSS) is investigated at an optical excitation of 1053-nm wavelength and 500-ps pulse duration. The experimental results indicate that the time jitter of the GaAs PCSS exhibits a nonmonotonic variation in the negative differential mobility (NDM) region. All of these time jitters are lower than 4% of the rise time of the switching waveform. The optimum time jitter of ~15 ps is achieved at the onset of the NDM region. The theoretical relationship between the optical excitation parameters, the bias electric field, and the time jitter of the GaAs PCSS is built up. The nonmonotonic behavior of the time jitter with electric field is attributed to the instability of the relative fluctuation of drift velocity caused by inter-valley transition of carriers in GaAs.

Picosecond relaxation of band-gap renormalization induced by the Coulomb interaction of charge carriers in GaAs

The dynamics of the long-wavelength edge of the spectrum of intrinsic stimulated picosecond emission occurring upon the picosecond pumping of GaAs is studied experimentally. The shortage of bandgap renormalization induced by the Coulomb interaction of charge carriers, compared to renormalization in the quasi-steady state, is observed. It is conceived that the shortage is caused by the fact that, under the experimental conditions of the study, the renormalization relaxation time is in the picosecond region.

GaAs光生载流子动力学机制的超快光谱学分析

GaAs光生载流子动力学机制的超快光谱学分析

Research on MeV pulsed radiation detection based on refractive index modulaiton

High time resolution detecting systems for MeV pulsed radiation are essential for inertial confinement fusion diagnostics. Traditional detection of system time resolution is restricted by cable bandwidth. Based on recording excess carrier dynamics in semiconductors, a new detecting mechanism, called RadOptic, was developed by Lawrence Livermore National Laboratory (LLNL). The variation of intensity of pulsed radiation with time was converted into the variation of intensity of infrared laser probe by using this mechanism. The sensing material was InGaAsP quantum wells with severalmicrometer thickness. Picosecond time resolution for several keV pulsed radiation has been demonstrated. The reported system is not suitable for MeV pulses due to its low efficiency to MeV photons. Multiple cascaded structure for MeV photon to electron transformation was proposed by LLNL. Applying bulk material with several-hundredmicrometer thickness is an alternative. Based on transient free carrier absorption, a system recording bulk materials' instantaneous refractive index change is established. The system consists of a probe laser, an interferometer module, a signal transmission module and a signal recording module. The probe is a tunable infrared continuous wave laser whose wavelength is ~1453 nm, guided by single mode fiber to the interferometer. The interferometer consists of a single mode fiber head coupled directly with the polished face of a bulk semiconductor. The interference pattern forms by multiple beams reflected from the front face and the back face of the bulk. Part of interference light is coupled to the single mode fiber and forms the output signal. Pulsed radiation will deposit energy and generate excess carriers in the bulk material. The refractive index of the bulk material changes therewith according to the Drude model. The interference pattern and the light coupled to the single mode fiber also change therewith. The signal is transmitted by a long single mode fiber. The signal recording module consists of photoelectric detectors and a digital oscilloscope. The signal generation process and the time resolution of the system are analyzed. Intrinsic GaAs refractive index change is exploited under electron pulses and X ray pulses. The analysis of signal generation process shows that when the excess carriers recombine much faster/much slower than the pulse width, the output signal/output signal differential can be viewed as a measure of intensity variation with time of the incident pulse. For this prototype system, the time resolution is restricted by the digital oscilloscope to 1 GHz. Bulk intrinsic GaAs demonstrates 30 ns refractive index response time, which is longer than the incident pulse width. The differential signal can be viewed as a measure of incident pulse intensity when GaAs is exposed to 1 ns~0.2 MeV electrons pulses. The differential signal width is shorter than the pulse width when GaAs is exposed to 5 ns~0.2 MeV electrons pulses. Auger recombination process may occur in the pulse duration under this situation. The differential signal width is longer than the pulse width when GaAs is exposed to 1 ns~0.2 MeV X ray pulses. The poor signal to noise ratio affects the signal. The excess carrier generation process may be longer than theoretically estimated one under X ray pulse incident situation. The generation process and recombination process of excess carriers in GaAs show very different characteristics compared with optical excitation. The relationship between the system output signal and the incident pulsed radiation depends on the type of the incident radiation. With carefully considering the effects from incident pulse type and transient carriers density, the system can be used to detect ~MeV pulsed radiation. With an upgraded recording module, the system would demonstrate much higher time resolution.

Ultrafast terahertz Faraday rotation in graphene

Terahertz (THz) Faraday rotation measurements were performed to investigate carrier dynamics in p-type Chemical vapor deposition (CVD) graphene. We used static and time-resolved polarization-sensitive THz transmission measurements in a magnetic field to probe free carriers in GaAs, InP, and Graphene. Static measurements probe the equilibrium carrier density and momentum scattering rate. Time-resolved (optical pump/THz probe) measurements probe the change in these quantities following photoexcitation. In a typical CVD graphene sample, we found that 0.5 ps following photoexcitation with 1 × 1013 photons/cm2 pulses at 800 nm the effective hole scattering time decreased from 37 fs to 34.5 fs, while the carrier concentration increased from 2.0 × 1012 cm−2 to 2.04 × 1012 cm−2, leading to a transient decrease in the conductivity of the film.

Effect of surface treatment with different sulfide solutions on the ultrafast dynamics of photogenerated carriers in GaAs(1 0 0)

The ultrafast carrier dynamics of GaAs(100) surfaces passivated with different sulfide solutions is studied by time-resolved measurements of infrared absorption using the femtosecond visible-pump infrared-probe technique. After passivation of n-GaAs(100) surface with the solution of ammonium sulfide in 2-propanol the three-fold decrease of the surface recombination velocity is observed. The treatment of the n-GaAs(100) surface with the aqueous sulfide solution has a smaller impact on the surface recombination velocity. The different effect of aqueous and alcoholic sulfide solutions on the efficiency of surface passivation is caused by the different mechanisms of charge transfer at the semiconductor/solution interfaces.

Interplay between the hot phonon effect and intervalley scattering on the cooling rate of hot carriers in GaAs and InP

ABSTRACTThe influence of hot phonon effect and intervalley scattering on the hot carrier cooling rate was investigated using femtosecond time‐resolved photoluminescence spectroscopy in bulk GaAs and InP, two electronically similar but vibrationally distinct semiconductors. In both materials, a broad photoluminescence signal that extends from the band gap energy to values larger than the pump pulse energy was observed during the first few picoseconds after photoexcitation, for different excitation energies (1.7, 1.88, and 2.4 eV) at high carrier densities (>1019 cm−3). Different hot carrier relaxation times were observed in GaAs and InP for different excitation energies, demonstrating the influence of intervalley scattering phenomena in GaAs. When electrons were not energetic enough to access satellite valleys, longer decay transients were observed for InP compared with GaAs. This provides experimental evidence of the hot phonon effect in InP. Temperature transients were calculated by analyzing the topography of the two‐dimensional spectra. Copyright © 2011 John Wiley & Sons, Ltd.

Erratum: Quasiequilibrium optical nonlinearities from spin-polarized carriers in GaAs [Phys. Rev. B77, 085202 (2008)

Erratum: Quasiequilibrium optical nonlinearities from spin-polarized carriers in GaAs [Phys. Rev. B<b>77</b>, 085202 (2008)

Cyclic behavior of ultrafast self-modulation of the light-absorption spectrum under conditions of pump and stimulated emission in GaAs

Under picosecond photogeneration of charge carriers in GaAs, accompanied by intense stimulated emission of the semiconductor itself, ultrafast self-modulation of its light-absorption spectrum takes place, which consists in the appearance of regions of local absorption enhancement (bumps) in the spectrum. The ultrafast self-modulation is found to exhibit a cyclic behavior; i.e., the pattern of the self-modulation of the spectrum (the number and spectral position of the bumps) is repeated after a certain time Tc falling in the picosecond range. The cycle period Tc varies over the time span of the pump pulse and depends on the pulse energy, which means that Tc is a function of the pump intensity. Assuming that self-modulation of the absorption reflects self-modulation of the charge-carrier energy distribution in GaAs under pumping, experimental results can be formulated as follows: in the process of the ultrafast self-modulation, deviations of the occupancies of different energy levels from the Fermi distribution evolve with time in a mutually related way; the distribution of the occupancy depletion in the conduction band repeats cyclically in time; and the cycle period decreases as the intensity of the pump increases.

GaAs emission from GaInP/Er,O‐Co doped GaAs/GaInP laser diodes grown by organometallic vapor phase epitaxy

AbstractWe have fabricated GaInP/Er,O‐codoped GaAs (GaAs:Er,O)/GaInP double heterostructure (DH) laser diodes (LDs) by organometallic vapor phase epitaxy (OMVPE) and investigated their lasing characteristics at GaAs band‐edge. Laser emission at the GaAs band‐edge was observed from the DH LDs at room temperature. The threshold current density (Jth) increased by doping of Er. The slope of the Jth against the reciprocal cavity length (1/L) also increased by the Er doping, indicating reduced relaxation time of injected carriers in GaAs:Er,O. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Hot-phonon-assisted absorption at semiconductor heterointerfaces monitored by pump-probe second-harmonic generation

We provide the evidence for the hot-LO-phonon-assisted absorption at semiconductor heterointerfaces. The process is demonstrated with $\mathrm{Ga}\mathrm{As}∕\mathrm{Ga}\mathrm{Sb}$ interface when the photon energy is tuned below the band-gap energy for GaAs, but it is in a great excess for GaSb. The excitation of carriers in GaAs in the vicinity of the heterointerface is shown to be assisted by hot LO phonons generated in GaAs and GaSb within the relaxation of hot carriers in GaSb. The effect has been observed in the ultrafast pump-probe experiment through the interfacial-electric-field-induced second-harmonic generation response.

Publisher's Note: Quasiequilibrium optical nonlinearities from spin-polarized carriers in GaAs [Phys. Rev. B77, 085202 (2008)

Publisher's Note: Quasiequilibrium optical nonlinearities from spin-polarized carriers in GaAs [Phys. Rev. B<b>77</b>, 085202 (2008)

Quasiequilibrium optical nonlinearities from spin-polarized carriers in GaAs

Semiconductor Bloch equations, which microscopically describe the dynamics of a Coulomb interacting, spin-unpolarized electron-hole plasma, can be solved in two limits: the coherent and the quasiequilibrium regimes. These equations have been recently extended to include the spin degree of freedom and used to explain spin dynamics in the coherent regime. In the quasiequilibrium limit, one solves the Bethe-Salpeter equation in a two-band model to describe how optical absorption is affected by Coulomb interactions within a spin unpolarized plasma of arbitrary density. In this work, we modified the solution of the Bethe-Salpeter equation to include spin polarization and light holes in a three-band model, which allowed us to account for spin-polarized versions of many-body effects in absorption. The calculated absorption reproduced the spin-dependent, density-dependent, and spectral trends observed in bulk GaAs at room temperature, in a recent pump-probe experiment with circularly polarized light. Hence, our results may be useful in the microscopic modeling of density-dependent optical nonlinearities due to spin-polarized carriers in semiconductors.

Behaviors of Nonequilibrium Carriers in Er, O-Codoped GaAs for 1.5μm Light-Emitting Devices with Extremely Stable Wavelength

We have fabricated GaInP/Er,O-codoped GaAs (GaAs:Er,O)/GaInP double heterostructure (DH) light-emitting diodes (LEDs) and successfully observed 1.5 µm electroluminescence (EL) due to an Er-2O center under forward bias at room temperature. Er excitation cross section by current injection decreased with increasing GaAs:Er,O active layer thickness, implying reduced diffusion length of injected carriers in the active layer. Carrier dynamics in GaAs:Er,O have also been investigated by means of a pump and probe reflection technique. Time-resolved reflectivity of GaAs:Er,O exhibited a characteristic dip after a steep decrease to negative in less than 10 ps. The analysis of the characteristic dip revealed short lifetime in range of ps for photoexcited carriers. The extremely short lifetime is quite coincident with the reduced diffusion length of injected carriers, and suggests that a trap induced by Er and O codoping would play an important role in dynamics of nonequilibrium carriers in GaAs:Er,O.

Nonequilibrium Carrier Dynamics Studied in Er, O-Codoped GaAs by Pump-Probe Reflection Technique

AbstractCarrier dynamics in Er,O-codoped GaAs (GaAs:Er,O) have been systematically investigated by means of a pump and probe reflection technique with a mode-locked Ti:sapphire laser. In GaAs:Er,O, it has been found that the codoping produces a single atom configuration (Er-20 configuration) as an Er atom located at the Ga sublattice with two adjacent O atoms together with two As atoms, resulting in extremely strong Er luminescence. Time-resolved reflectivity of GaAs:Er,O exhibited an abrupt increase in amplitude, followed by a steep decrease to negative in less than 1 ps and then a gradual increase in approximately 100 ps. The steep decrease is due to bandgap renormalization. The gradual increase in reflectivity depended strongly on Er concentration, indicating that a trap induced by Er and O codoping plays an important role in dynamics of nonequilibrium carriers in GaAs:Er,O.