Low-temperature-grown GaAs Layer Research Articles

Efficient THz generation from low-temperature-grown GaAs photoconductive antennas driven by Yb-doped fiber amplifier at 200 kHz repetition rate

We demonstrate the generation of terahertz (THz) pulses with electric field strength reaching 34 kV/cm from low-temperature-grown GaAs (LT-GaAs) interdigitated photoconductive antennas driven by 1030 nm optical pulses delivered by a commercial ytterbium-doped fiber laser operating at a repetition rate of 200 kHz. By probing the Urbach absorption in LT-GaAs layers, we show that the THz generation mechanism predominantly relies on the photoexcitation of electrons from the valence band to shallow defect states arising from the incorporation of excess As during the growth process. Our THz source opens the route toward nonlinear time-resolved study of low-energy excitations in matter with high signal-to-noise ratios.

Effect of heteroepitaxial growth on LT-GaAs: ultrafast optical properties

Epitaxial low temperature grown GaAs (LT-GaAs) on silicon (LT-GaAs/Si) has the potential for terahertz (THz) photoconductive antenna applications. However, crystalline, optical and electrical properties of heteroepitaxial grown LT-GaAs/Si can be very different from those grown on semi-insulating GaAs substrates (‘reference’). In this study, we investigate optical properties of an epitaxial grown LT-GaAs/Si sample, compared to a reference grown under the same substrate temperature, and with the same layer thickness. Anti-phase domains and some crystal misorientation are present in the LT-GaAs/Si. From coherent phonon spectroscopy, the intrinsic carrier densities are estimated to be 1015 cm−3 for either sample. Strong plasmon damping is also observed. Carrier dynamics, measured by time-resolved THz spectroscopy at high excitation fluence, reveals markedly different responses between samples. Below saturation, both samples exhibit the desired fast response. Under optical fluences ⩾54 μJ cm−2, the reference LT-GaAs layer shows saturation of electron trapping states leading to non-exponential behavior, but the LT-GaAs/Si maintains a double exponential decay. The difference is attributed to the formation of As–As and Ga–Ga bonds during the heteroepitaxial growth of LT-GaAs/Si, effectively leading to a much lower density of As-related electron traps.

Enhancement effect on photoelectric conversion efficiency of plasmon-induced terahertz photoconductive antenna

The laser electric field distribution, carrier distribution and photoconductive current densities under different electric field intensities in low temperature grown GaAs (LT-GaAs) layer of photoconductive antenna (PCA) have been simulated by finite element method. When a plasmon-induced PCA is pumped by an 800 nm laser beam, the plasmon on the interface of air and LT-GaAs aids the conversion of more laser photons into photo-generated carriers and thus increases the density of the photo-current leading to a higher-power terahertz pulse. A quadratic function fitting relationship between the laser electric field intensity and the maximum density of photoconductive current has been obtained for the first time. On this basis, a plasmon-induced terahertz PCA has been designed and packaged using electron beam exposure technology. The enhancement effect on photo-electric conversion efficiency of plasmon in PCA is verified by comparing terahertz radiation spectra from conventional and plasmon-induced PCAs. The terahertz radiation properties of two kinds of PCAs fabricated on an identical LT-GaAs layer were evaluated by means of terahertz time-domain spectroscopy.

Atomically-resolved interface imaging and terahertz emission measurements of gallium arsenide epilayers

Semiconductor interfaces are the backbone of modern optoelectronic devices. In terahertz (THz) science, the narrow region of an interface is crucial in the emission process. However, reports on the direct correlation of THz emission with local interface properties remain scarce owing to the inherent difficulty of using the same sample for nanoscale and macroscale studies. In this study, we combined scanning tunneling microscopy/spectroscopy (STM/STS) and THz emission spectroscopy to study the interface between a highly n+-doped and undoped gallium arsenide (GaAs). Using STS, we identify a carrier density of 1×1015 cm−3 in the low-temperature-grown GaAs (LT-GaAs) layer, which we used to visualize the energy band diagram at the interface and the surface of LT-GaAs. THz emission intensity is higher in the LT-GaAs/n+-GaAs structures relative to semi-insulating GaAs owing to the high electric field at the interface regardless of the LT-GaAs layer thickness. Pump fluence dependence of THz showed that the thinner LT-GaAs layers saturate at lower pump fluence compared to thicker LT-GaAs and SI-GaAs. This behavior is explained by the built-in field screening by the photogenerated carriers and the free carriers from the n+-GaAs to the LT-GaAs. Our results demonstrate the utility of STM/STS to the design of semiconductor-based THz emitters.

New Structure for Photoconductive Antennas Based on {LTG-GaAs/GaAs:Si} Superlattice on GaAs(111)A Substrate

The structural characteristics of a new structure for photoconductive antennas have been investigated. This structure is a multilayered epitaxial film grown on a GaAs(111)A substrate; it consists of alternating undoped low-temperature grown GaAs (LTG-GaAs) layers and GaAs layers synthesized in the standard high-temperature regime and doped with silicon (GaAs:Si). The As4/Ga flow ratio γ is chosen such as to produce p-type GaAs:Si layers. LTG-GaAs layers were grown at an enlarged γ value. The samples grown on GaAs(100) substrates were single-crystal, whereas the single-crystal growth on GaAs(111)A substrates changed to polycrystalline when the film thickness reached 320–340 nm. The sizes of As precipitates in annealed samples, their distribution over the film thickness, and specific features of their crystal structure have been analyzed.

Plasmon-enhanced LT-GaAs/AlAs heterostructure photoconductive antennas for sub-bandgap terahertz generation.

Photocurrent generation in low-temperature-grown GaAs (LT-GaAs) has been significantly improved by growing a thin AlAs isolation layer between the LT-GaAs layer and semi-insulating (SI)-GaAs substrate. The AlAs layer allows greater arsenic incorporation into the LT-GaAs layer, prevents current diffusion into the GaAs substrate, and provides optical back-reflection that enhances below bandgap terahertz generation. Our plasmon-enhanced LT-GaAs/AlAs photoconductive antennas provide 4.5 THz bandwidth and 75 dB signal-to-noise ratio (SNR) under 50 mW of 1570 nm excitation, whereas the structure without the AlAs layer gives 3 THz bandwidth, 65 dB SNR for the same conditions.

Simultaneous generation and detection of ultrashort voltage pulses in low-temperature grown GaAs with below-bandgap laser pulses

We present a method that allows for the simultaneous generation and detection of ultrashort voltage pulses, which propagate on a planar transmission line, in the same material with laser pulses of the same wavelength. The generation is accomplished by exciting band-tail states below the fundamental bandgap of a low-temperature grown GaAs layer, while the detection takes advantage of the electro-optic effect in the GaAs material. This simple scheme considerably enhances previous measurement techniques and is capable of generating and measuring frequencies exceeding 1 THz. The optimum wavelength for the combined generation and detection technique is found at ∼900 nm.

High-performance 1.55μm low-temperature-grown GaAs resonant-cavity-enhanced photodetector

A 1.55μm low-temperature-grown GaAs (LT-GaAs) photodetector with a resonant-cavityenhanced structure was designed and fabricated. A LT-GaAs layer grown at 200°C was used as the absorption layer. Twenty- and fifteen-pair GaAs∕AlAs-distributed Bragg reflectors were grown as the bottom and top mirrors. A responsivity of 7.1mA∕W with a full width at half maximum of 4nm was obtained at 1.61μm. The dark current densities are 1.28×10−7A∕cm2 at the bias of 0V and 3.5×10−5A∕cm2 at the reverse bias of 4.0V. The transient response measurement showed that the photocarrier lifetime in LT-GaAs is 220fs.

Interstitial to antisite defect conversion during the molecular beam epitaxial deposition onc(4 3 4) GaAs(001) surfaces

In the model describing the origin of excess arsenic content in low-temperature grown GaAs layers developed by the research group of professor Strunk during the last ten years, formation of an interstitial As atom was identified as precursor to excess arsenic formation. After an As2 molecule interacts with the GaAs surface, a metastable conformation can form, where one of the As atoms of the As2 molecule is located in an interstitial position. Starting from this geometry, during growth stable conformations can easily arise by the assistance of one or two arriving Ga atoms which stabilize the interstitial As atom in its position by forming a half or full cage-like structure. Another model describes how the antisite excess As atoms form in GaAs layers by an incomplete exchange of As atoms in the surface reconstruction layer with arriving Ga atoms. The present article connects these two aspects of the excess As formation by analyzing the stability of the interstitial excess As atom, calculated in four different atomic arrangements according to experimentally observed surface structures. Energies of initial, final and transition states of interstitial to antisite reaction path were calculated by DFT/B3LYP/6-31++G method. Results show that interstitial to antisite conversion happens preferably after a half-cage formation, while after a full cage has been formed, the interstitial As atom remains fixed in its position. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Relaxation process of photoexcited carriers in GaAs structures with low-temperature-grown layers

Spatial relaxation processes of photoexcited carriers in GaAs structures are studied by means of photoluminescence spectroscopy. A single GaAs∕Al0.3Ga0.7As quantum well and a low-temperature-grown GaAs (LT-GaAs) layer containing a high concentration of excess arsenic are placed in a GaAs structure as optical markers; the former serves as the radiative recombination site, while the latter as the trapping site of photoexcited carriers. The photoluminescence intensity from the quantum well is significantly reduced by the presence of a LT-GaAs layer immediately next to a barrier layer. The effect of the LT-GaAs layer is exponentially enhanced as a thickness of the barrier layer decreases. The results suggest that once an excess As point defect is placed within an extent of a wave function of a photoexcited carrier, trapping of the photoexcited carrier occurs at an extremely fast rate. In a structure where a LT-GaAs is placed at a distant location from the quantum well, the photoluminescence intensity from the quantum well is weakly dependent on the location of the LT-GaAs layer as expected from thermal diffusion of photoexcited carriers to trap sites as semiclassical particles.

High-speed 1.55 μm operation of low-temperature-grown GaAs-based resonant-cavity-enhanced p–i–n photodiodes

We report the design, growth, fabrication, and characterization of GaAs-based high-speed p–i–n photodiodes operating at 1.55 μm. A low-temperature-grown GaAs (LT-GaAs) layer was used as the absorption layer and the photoresponse was selectively enhanced at 1.55 μm using a resonant-cavity-detector structure. The bottom mirror of the resonant cavity was formed by a highly reflecting 15-pair GaAs/AlAs Bragg mirror. Molecular-beam epitaxy was used for wafer growth, where the active LT-GaAs layer was grown at a substrate temperature of 200 °C. The fabricated devices exhibited a resonance around 1548 nm. When compared to the efficiency of a conventional single-pass detector, an enhancement factor of 7.5 was achieved. Temporal pulse-response measurements were carried out at 1.55 μm. Fast pulse responses with 30 ps pulse-width and a corresponding 3 dB bandwidth of 11.2 GHz was measured.

Terahertz radiation from n-type GaAs with Be-doped low-temperature-grown GaAs surface layers

We investigate the performance of n-type GaAs plasmon emitters with surface layers grown by low-temperature molecular-beam epitaxy. The emitters have as-grown Be-doped low-temperature-grown GaAs layers on the top surface, which radiate terahertz transients with a power four times greater than reference emitters without low-temperature-grown (LTG) GaAs layers. The power generated by the structures with low-temperature-grown GaAs layers shows much weaker saturation at large excitation densities. The mechanism of the surface field screened by photoexcited holes has been discussed to explain the negative dip in the THz wave form when the emitter is coated by a LTG GaAs layer.

Ultrahigh-power-bandwidth product and nonlinear photoconductance performances of low-temperature-grown GaAs-based metal-semiconductor-metal traveling-wave photodetectors

Maximum-output-power and bandwidth performances are usually two tradeoff parameters in the design of high-speed photodetectors (PDs). In this paper, we report record high-peak output voltage (/spl sim/ 30 V) together with ultrahigh-speed performance (1.8 ps, 190 GHz) observed in low-temperature-grown GaAs (LTG-GaAs)-based metal-semiconductor-metal (MSM) traveling-wave photodetectors (TWPDs) at a wavelength of 800 nm. Ultrahigh-peak output power and ultrahigh-electrical bandwidth performances were achieved due to superior MSM microwave guiding structure, short carrier trapping time, and the capability to take high bias voltage (/spl sim/ 30 V) with a LTG-GaAs layer. Under such a high bias voltage, a significant nonlinear photocurrent increase with the bias voltage was observed. The nonlinear photoconductance and ultrahigh-output power-bandwidth performances opens a new way in the application of high-performance optoelectronic mixers and photomixer devices.

Surface-modified GaAs terahertz plasmon emitter

We studied the THz emission from n-GaAs plasmon emitters modified by low-temperature-grown (LT) GaAs surface layers. The THz emission is increased since the LT GaAs pins the Fermi level at a midgap position, increasing the surface depletion field. For a THz emitter with a 70-nm-thick LT GaAs layer we observe without external fields a THz emission intensity of 140 nW. In addition, the long-term performance of the modified emitters is improved by the LT GaAs surface layer.

Ultrahigh power-bandwidth-product performance of low-temperature-grown-GaAs based metal-semiconductor-metal traveling-wave photodetectors

High-output-power and high-bandwidth performances are usually two tradeoff parameters in the design of high-speed photodetectors. In this letter, we report high peak-output-voltage (∼20 V) and peak-output-current (∼400 mA, 50 Ω load) together with ultrahigh-speed performances (1.5 ps, 220 GHz), observed in low-temperature-grown-GaAs (LTG-GaAs) based metal-semiconductor-metal (MSM) traveling-wave photodetectors (TWPDs) at a wavelength of 800 nm. Ultrahigh-peak-output-power and ultrahigh-electrical-bandwidth performances were achieved due to the superior MSM microwave guiding structure and a short carrier trapping time in the LTG-GaAs layer, which reduced the space-charge screening effect and increased the photoabsorption volume without sacrificing electrical bandwidth significantly. We also observed different bias-dependent nonlinear behaviors in MSM TWPDs under high and low illuminated optical power excitations, which are possibly dominated by the space-charge screening and the lifetime increasing effects, respectively.

X-ray diffraction analysis of LT-GaAs multilayer structures

Multilayer structures of low-temperature-grown GaAs(LT-GaAs) into which ultra-thin layers containing excess As are periodically introduced are grown by molecular beam epitaxy. The concentration of excess As in the ultra-thin layers is determined by the analysis of the intensity of a satellite reflection of the multilayer structure along with a peak shift of the 400 Bragg reflection from that of the GaAs substrate. The analysis shows that the concentration of excess As in the ultra-thin layers is significantly higher than those of thick LT-GaAs layers grown under the identical condition of the fluxes and the substrate temperature.

N-Channel conductance spectroscopy of deep defects in low temperature grown GaAs

We report on a new technique to characterize the deep defects in low temperature grown GaAs (LT-GaAs), based on a thin LT-GaAs layer embedded in the intrinsic zone of a pin diode. At this structure we have performed steady state and time dependent measurements of the n-channel conductance. Thus we deduce information about the activation energy and the capture time constant of the deep defects.

Optical and electrical spectroscopy of defects in low temperature grown GaAs

We report on a technique to characterize the deep defects in low temperature grown GaAs (LT-GaAs), based on a thin LT-GaAs layer embedded in the intrinsic zone of a pin diode. At this structure we have performed steady state and time dependent measurements of the n-channel conductance. Thus we yield information about the density, the activation energy, and the characteristic time constants of the deep defects.

Investigation of deep electronic centers in low-temperature grown GaAs using extremely thin layers

We report on an approach to investigate the deep electronic defect centers in low-temperature grown GaAs (LT-GaAs). Using an extremely thin LT-GaAs layer (comparable with the penetration depth of an electric field in bulk material) incorporated in the i layer of a p-i-n diode, we are able to charge or to deplete the deep centers in the energy gap by applying a reverse bias. The corresponding space charge is monitored by the field changes across the LT-GaAs layer, both optically by Franz–Keldysh experiments and electrically by n-channel conductance changes. From our results, we derive a deep trap density of 1018 cm−3 centered at around 500–700 meV below the conduction band.

Defect-assisted tunneling current: A revised interpretation of scanning tunneling spectroscopy measurements

Scanning tunneling microscopy (STM) is used to study low temperature grown GaAs layers. Excess As gives rise to a high concentration of As antisites (AsGa). On these point defects, tunneling spectroscopy reveals a band of donor states. In fact, the measured tunneling current results from a pure tunneling current between the energy levels of the STM tip and the AsGa energy level Et followed by an exchange of carriers between Et and the bands or between Et and donor states of neighbor point defects. We determine the influence of both contributions on the current. We show that hopping conduction is required to explain the observation of the midgap states in tunneling spectroscopy.