Lifetime In GaAs Research Articles

A Hierarchical 3D TiO2 /Ni Nanostructure as an Efficient Hole-Extraction and Protection Layer for GaAs Photoanodes.

Photoelectrochemical (PEC) water splitting is a promising clean route to hydrogen fuel. The best-performing materials (III/V semiconductors) require surface passivation, as they are liable to corrosion, and a surface co-catalyst to facilitate water splitting. At present, optimal design combining photoelectrodes with oxygen evolution catalysts remains a significant materials challenge. Here, we demonstrate that nickel-coated amorphous three-dimensional (3D) TiO2 core-shell nanorods on a TiO2 thin film function as an efficient hole-extraction layer and serve as a protection layer for the GaAs photoanode. Transient-absorption spectroscopy (TAS) demonstrated the role of nickel-coated (3D) TiO2 core-shell nanorods in prolonging photogenerated charge lifetimes in GaAs, resulting in a higher catalytic activity. This strategy may open the potential of utilizing this low-cost (3D) nanostructured catalyst for decorating narrow-band-gap semiconductor photoanodes for PEC water splitting devices.

Characterization of GaAs:Cr sensors using the charge-integrating JUNGFRAU readout chip

Chromium compensated GaAs sensors have been characterized using the charge-integrating readout chip JUNGFRAU. Due to its low noise performance and 75 × 75 μm2 pixel size, JUNGFRAU enables a precise measurement of the charge (of either polarity) with a high spatial resolution. Several sensor parameters like dark current, noise and spectral performance as well as the charge transport properties of the electrons have been determined. The short lifetime of holes in GaAs:Cr gives rise to an effect where pixels adjacent to a pixel with a photon hit show a strong negative signal when being absorbed close to the readout electrode. This so-called `crater effect' has been simulated and allows an estimation of the hole lifetime in GaAs:Cr.

A method for determining the ambipolar diffusion length and carrier lifetime in GaAs p-i-n diodes

It was determined that the forward loss resistance as a function of the radius of the active region of typical microwave p-i-n diodes depends on the ratio of the square of the ambipolar-diffusion length to the square of the radius of the diode active region. Based on this regularity, a method was developed that provides simple determination of the ambipolar-diffusion length and the carrier lifetime.

Effect of annealing on the nonequilibrium carrier lifetime in GaAs grown at low temperatures

GaAs samples grown by molecular-beam epitaxy at low (230°C) temperatures are investigated. One of the samples is subjected to aftergrowth annealing at 600°C. Using an unconventional pump-probe scheme for measuring the dynamic variation in the light refractive index, the nonequilibrium charge-carrier lifetime (275 ± 30 fs before annealing) is determined. Such a short carrier lifetime in the unannealed material is due to the high concentration of point defects, mainly AsGa antisite defects. According to X-ray diffraction and steady-state optical absorption data, the AsGa concentration in the samples is 3 × 1019 cm−3, which corresponds to an arsenic excess of 0.26 at %. Upon annealing at 600°C, the superstoichiometric As defects self-organize and form As nanoinclusions in the GaAs crystal matrix. It is shown that in this case the nonequilibrium charge-carrier lifetime increases to 452 ± 5 fs. This lifetime is apparently ensured by the capture of non-equilibrium charge carriers at metal As nanoinclusions.

Electrically tunable electron spin lifetimes in GaAs(111)B quantum wells

We investigate the electric tunability of the electron spin lifetime in GaAs(111) quantum wells (QWs) inserted in biased p-i-n and n-i-p diode-like structures. Due to the specific symmetry of these QWs, the Rashba contribution to the spin-orbit magnetic field induced by the applied bias is parallel to the intrinsic Dresselhaus contribution for all directions of the electron wavevector. In particular, the voltage applied to the diode, which controls the amplitude of the Rashba contribution, can be adjusted to attenuate the resulting spin-orbit field, thus strongly suppressing spin dephasing due to the Dyakonov-Perel relaxation mechanism for all spin orientations. Spin lifetimes from below 100 ps at an electric field of −20 kV/cm to values exceeding 4 ns at +8 kV/cm have been measured in 25 nm thick multiple QWs at a temperature of 20 K.

Experimental evaluation of the carrier lifetime in GaAs grown at low temperature

The relaxation dynamics of nonequilibrium charge carriers in gallium arsenide epitaxial films grown by molecular-beam epitaxy at low temperatures has been studied. The growth conditions of the epitaxial layer provided an excess arsenic content of 1.2% in the layer. In a material of this kind, the carrier lifetime is <1 ps. To examine carrier relaxation in the femtosecond range, an original scheme for measuring the refractive index dynamics was developed on the basis of the pump-probe technique. The lifetime of nonequilibrium charge carriers was evaluated to be (200 ± 35) fs.

Carrier-mediated optomechanical coupling in GaAs cantilevers

We have investigated optomechanical coupling in $n$-GaAs/$i$-GaAs bilayer cantilevers induced by optical band-gap excitation. The strain-assisted optopiezoelectric effect, which is associated with the separation of electron-hole pairs due to the built-in electric field, causes a time-delayed backaction force and influences the thermal vibration of the cantilevers. Vibration of the [110]-oriented cantilever is amplified by this optopiezoelectric backaction and the self-oscillation is induced for the strong excitation. In contrast, for the [$\overline{1}$10]-oriented cantilever, the optopiezoelectric backaction deamplifies the vibration because the direction of the piezoelectric stress is reversed. We have experimentally extracted the response time of this optopiezoelectric backaction, where the delay is on the order of the nonradiative recombination lifetime in GaAs. This optopiezoelectric backaction is maximized when the laser wavelength matches the optical absorption edge. This is because the strain-induced change in the optical absorption is maximized at the absorption edge and therefore a large force gradient results.

Ultrafast Decay of Surface Photo-Voltage Effect on n-type GaAs(100) Surface

Surface photo-voltage (SPV) effect on n -type GaAs(100) has been investigated with a newly developed time-resolved photoemission spectroscopy, in which the sample surface is excited by one-photon of the pump pulse and the photoemission is measured by two-photon excitation of the probe pulse. Ultrafast decay of the SPV was found in a several picosecond range at 300 and 100 K, corresponding to the hot-electron lifetime in GaAs. The slow SPV decay was also observed in a few hundred picoseconds, due to the thermalized electrons.

Pulsed photoconductive antenna terahertz sources made on ion-implanted GaAssubstrates

In this work we show that improved performances of terahertz emitters can be obtainedusing an ion implantation process. Our photoconductive materials consist of high-resistivityGaAs substrates. Terahertz pulses are generated by exciting our devices with ultrashortnear-infrared laser pulses. The ion implantation introduces non-radiative centres, whichreduce the carrier lifetime in GaAs. The presence of the charged defects also induces aredistribution of the electric field between the antenna electrodes. This effect has a hugeinfluence on the amplitude of the radiated terahertz field. Results obtained asa function of the laser excitation power are discussed and a comparison of theperformance of these devices with a conventional antenna-type emitter is given.

Impact of dislocations on minority carrier electron and hole lifetimes in GaAs grown on metamorphic SiGe substrates

The minority carrier lifetime of electrons (τn) in p-type GaAs double heterostructures grown on GaAs substrates and compositionally graded Ge/Si1−xGex/Si (SiGe) substrates with varying threading dislocation densities (TDDs) were measured at room temperature using time-resolved photoluminescence. The electron lifetimes for homoepitaxial GaAs and GaAs grown on SiGe (TDD∼1×106 cm−2) with a dopant concentration of 2×1017 cm−3 were ∼21 and ∼1.5 ns, respectively. The electron lifetime measured on SiGe was substantially lower than the previously measured minority carrier hole lifetime (τp) of ∼10 ns, for n-type GaAs grown on SiGe substrates with a similar residual TDD and dopant concentration. The reduced lifetime for electrons is a consequence of their higher mobility, which yields an increased sensitivity to the presence of dislocations in GaAs grown on metamorphic buffers. The disparity in dislocation sensitivity for electron and hole recombination has significant implications for metamorphic III-V devices.

The time-resolved spectroscopy of InGaAs/AlGaAs heterostructures with asymmetric funnel-shape quantum wells for near- and mid-IR lasing

The dynamics of interband transitions in InGaAs/AlGaAs heterostructures with funnel-shaped quantum wells, when a narrow and deep well is positioned asymmetrically in wide and shallow ones, has been studied experimentally. These structures have been proposed as a dual-colour laser in mid- and near-IR range simultaneously. The lifetime of electrons in the ground and excited states in the quantum wells has been determined by the time of the photoluminescence intensity decline. The obtained lifetime values allow precise identification of peaks in the photoluminescence spectra and a deep insight into the process of high electron states populating. The experimentally found electron lifetime for the interband transitions corresponds on the order of magnitude with the interband transitions lifetime in GaAs. Analysis of the photoluminescence spectra as well as the time dependence of the photoluminescence intensity decline indicates a possibility of population inversion achievement in these structures under high excitation.

Novel concept for efficient THz-emitters based on quasi-ballistic transport in an asymmetric superlattice

We report on a novel concept for a THz-photomixer, which is not (as usually) limited by the carrier lifetime in GaAs and allows impedance matching to the attached antenna. The concept is based on quasi-ballistic electron transport in a doping superlattice.

Monolithic integration of room-temperature cw GaAs/AlGaAs lasers on Si substrates via relaxed graded GeSi buffer layers

GaAs/Al x Ga (1−x) As quantum well lasers have been demonstrated via organometallic chemical vapor deposition on relaxed graded Ge/GexSi(1−x) virtual substrates on Si. A number of GaAs/Ge/Si integration issues including Ge autodoping behavior in GaAs, reduced critical thickness due to thermal expansion mismatch, and complications with mirror facet cleaving have been overcome. Despite unoptimized laser structures with high series resistance and large threshold current densities, surface threading dislocation densities for GaAs/AlGaAs lasers on Si substrates as low as 2×106 cm−2 permitted continuous room-temperature lasing at a wavelength of 858 nm. The laser structures are uncoated edge-emitting broad-area devices with differential quantum efficiencies of 0.24 and threshold current densities of 577 A/cm2. Identical devices grown on commercial GaAs substrates showed similar behavior. This comparative data agrees with previous measurements of near-bulk minority carrier lifetimes in GaAs grown on Ge/GeSi/Si substrates.

Strategies For Direct Monolithic Integration of AlxGa(1−x)As/InxGa(1−x)As LEDS and Lasers On Ge/GeSi/Si Substrates Via Relaxed Graded GexSi(1−x) Buffer Layers

AbstractAlxGa(1−x)As/GaAs quantum well lasers have been demonstrated via organometallic chemical vapor deposition (OMCVD) on relaxed graded GexSi(1−x) virtual substrates on Si. Despite unoptimized laser structures with high series resistance and large threshold current densities, surface threading dislocation densities as low as 2×106 cm−2 enabled cw room-temperature lasing at a wavelength of 858nm. The laser structures are oxide-stripe gain-guided devices with differential quantum efficiencies of 0.16 and threshold current densities of 1550A/cm2. Identical devices grown on commercial GaAs substrates showed differential quantum efficiencies of 0.14 and threshold current densities of 1700A/cm2. This comparative data agrees with our previous measurements of near-bulk minority carrier lifetimes in GaAs grown on Ge/GeSi/Si substrates. A number of GaAs/Ge/Si integration issues including thermal expansion mismatch and Ge autodoping behavior in GaAs were overcome.

Effective mass dependence of resonant quasi-level lifetime in GaAs–Al xGa 1− xAs double-barrier structures

The present work computes the lifetime of the lowest resonant quasi-level in a GaAs–Al x Ga 1− x As double-barrier structure based on the effective mass theory and takes into account the effective mass mismatch nature of well and barrier layers. The calculations showed a strong dependence of the lowest energy and its associated lifetime on both the effective masses of the well and barrier materials and aluminum concentration in the barrier Al x Ga 1− x As material, which is associated to the barrier height. A comparison between a model based on different masses of well and barrier layers and another which considers same effective mass for both layers is made which showed a discrepancy of about 13% related to the lowest resonant energy and of about 29% associated to the resonant quasi-level lifetime both at barrier to well dimensions ratio of 6 and an aluminum composition of 0.45. The present work raises a question about the correct effective masses of tunneling carriers in resonant tunneling devices.

High minority-carrier lifetimes in GaAs grown on low-defect-density Ge/GeSi/Si substrates

A high bulk minority-carrier lifetime in GaAs grown on Si-based substrates is demonstrated. This was achieved by utilizing a step-graded Ge/GeSi buffer (threading dislocation density 2×106 cm−2) grown on an offcut (001) Si wafer, coupled with monolayer-scale control of the GaAs nucleation to suppress antiphase domains. Bulk minority-carrier lifetimes (τp) were measured using room-temperature time-resolved photoluminescence applied to a series of Al0.3Ga0.7As/GaAs/Al0.3Ga0.7As double-heterojunction structures doped n=1.1×1017 cm−3 with GaAs thicknesses of 0.5, 1.0, and 1.5 μm. A lifetime τp=7.7 ns was determined for GaAs grown on Si. The extracted interface recombination velocity of 3.9×103 cm/s is comparable to recombination velocities found for Al0.3Ga0.7As/GaAs interfaces grown on both GaAs and Ge wafers, indicating that the crosshatch surface morphology characteristic of strain-relaxed Ge/GeSi surfaces does not impede the formation of high-electronic-quality interfaces. These results hold great promise for future integration of III–V minority-carrier devices with Si wafer technologies.

Picosecond carrier lifetime in GaAs implanted with high doses of As ions: An alternative material to low-temperature GaAs for optoelectronic applications

Nonstoichiometric GaAs obtained by implantation with 2 MeV arsenic ions at 1015 cm−2 dose is studied. As-implanted samples show a &amp;lt;200 fs lifetime of photocarriers and low resistivity due to hopping, with mobility less than 1 cm2/V s. Annealing of the samples at 600 °C leads to substantial recovery of postimplant damage, as seen from Rutherford backscattering channeling spectra and mobility increase to about 2000 cm2/V s, but photocarrier lifetime is still about 1 ps. These parameters are similar to those of low-temperature GaAs annealed at 600 °C, and make arsenic implanted GaAs an interesting material for optoelectronic applications.

Subpicosecond carrier lifetimes in GaAs grown by molecular beam epitaxy at low substrate temperature

Time-resolved photoluminescence transients of low-temperature molecular beam epitaxially grown GaAs layers have been measured with femtosecond temporal resolution and compared with numerical Monte Carlo calculations. It has been shown that the shape of these transients measured at different emission energies is determined not only by the carrier lifetime but also by the electron redistribution in the conduction band. Analysis of the experimental results yields the carrier lifetime in the investigated samples of 400 fs.

Carrier densities and electron-hole recombination lifetimes in GaAs-(Ga,Al)As quantum-well photoluminescence

A quantum-mechanical calculation of the carrier densities and electron-hole recombination lifetimes in GaAs-(Ga,Al)As quantum wells is performed, under steady-state optical excitation conditions and in the high-temperature regime. The variables are the continuous-wave (cw) laser intensity, well widths, and acceptor distribution in the well. Radiative recombination of electrons with free holes and holes bound at neutral acceptors are considered. Our calculations for the dependence of the electron density on laser intensity are in quantitative agreement with recent experimental results for multiple asymmetric coupled quantum wells at T=300 K and for intermediate excitation. Also, results for the carrier-density-dependent e-h recombination decay time at T=155 K are in good agreement with recent experimental data in semiconductor quantum wells.

Microsecond lifetimes and low interface recombination velocities in moderately doped n-GaAs thin films

We have observed lifetimes greater than 1 μs in moderately doped, thin film, n-GaAs/Al0.3Ga0.7As double heterostructure membranes formed by etching away the substrate. We attribute these ultralong lifetimes to enhanced photon recycling caused by the removal of the substrate. Nonradiative recombination in the bulk and at the interfaces is very low; the upper limit of the interface recombination velocity is 25 cm/s. Such long lifetimes in GaAs doped at ND=1.3×1017 cm−3 suggest that thin-film solar cells offer a potential option for achieving very high efficiencies.