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We present the implementation of an efficient terahertz (THz) photoconductive antenna (PCA) emitter design that utilizes high mobility carriers in the two-dimensional electron gas (2DEG) of a modulation-doped heterostructure (MDH). The PCA design is fabricated with recessed metal electrodes in direct contact with the 2DEG region of the MDH. We compare the performance of the MDH PCA having recessed contacts with a PCA fabricated on bulk semi-insulating GaAs, on low temperature-grown GaAs, and a MDH PCA with the contacts fabricated on the surface. By recessing the contacts, the applied bias can effectively accelerate the high-mobility carriers within the 2DEG, which increases the THz power emission by at least an order of magnitude compared to those with conventional structures. The dynamic range (62 dB) and bandwidth characteristics (3.2 THz) in the power spectrum are shown to be comparable with the reference samples. Drude-Lorentz simulations corroborate the results that the higher-mobility carriers in the MDH, increase the THz emission. The saturation characteristics were also measured via optical fluence dependence, revealing a lower saturation value compared to the reference samples. The high THz conversion efficiency of the MDH-PCA with recessed contacts at low optical power makes it an attractive candidate for THz-time domain spectroscopy systems powered by low power fiber lasers.

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 ~$10^{15}$ 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 $\geq$ 54 $\mu$ J/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., Comment: 8 pages, supplementary info separate

Terahertz (THz) emission increase is observed for GaAs thin films that exhibit structural defects. The GaAs epilayers are grown by molecular beam epitaxy on exactly oriented Si (100) substrates at three different temperatures (Ts = 320ºC, 520ºC and 630ºC). The growth method involves the deposition of two low-temperature-grown (LTG)-GaAs buffers with subsequent in-situ thermal annealing at Ts = 600ºC. Reflection high energy electron diffraction confirms the layer-by-layer growth mode of the GaAs on Si. X-ray diffraction shows the improvement in crystallinity as growth temperature is increased. The THz time-domain spectroscopy is performed in reflection and transmission excitation geometries. At Ts = 320ºC, the low crystallinity of GaAs on Si makes it an inferior THz emitter in reflection geometry, over a GaAs grown at the same temperature on a semi-insulating GaAs substrate. However, in transmission geometry, the GaAs on Si exhibits less absorption losses. At higher Ts, the GaAs on Si thin films emerge as promising THz emitters despite the presence of antiphase boundaries and threading dislocations as identified from scanning electron microscopy and Raman spectroscopy. An intense THz emission in reflection and transmission excitation geometries is observed for the GaAs on Si grown at Ts = 520ºC, suggesting the existence of an optimal growth temperature for GaAs on Si at which the THz emission is most efficient in both excitation geometries. The results are significant in the growth design and fabrication of GaAs on Si material system intended for future THz photoconductive antenna emitter devices.

Carrier dynamics of two LT-GaAs layers grown using similar parameters, but on different substrates were studied. The behavior of the time-resolved terahertz transmission was observed to differ in trapping time constants, as well as in fluence-dependence. The differences are attributed to the difference in layer quality when using different substrates, wherein the use of silicon substrates result in increased structural defect density in the silicon substrate-grown layer, which provide additional scattering and non-recombination pathways, resulting in the reduced carrier lifetime.

GaAs films were grown on Si (100) substrates with a “two-step buffer” growth technique using molecular beam epitaxy, wherein a low temperature GaAs buffer is grown at two substrate temperatures prior to the GaAs layer. Results of GaAs layer growth at two different substrate temperatures are compared using photoluminescence, Raman, and optical-pump terahertz-probe spectroscopy.

A semiconductor terahertz (THz) emitter based on an active i-/n-GaAs layer integrated on a porous silicon (PSi) distributed Bragg reflector (PSi-DBR) is presented. It is specifically designed for the use of a very thin GaAs film of thickness less than the penetration depth of the 800nm laser beam $(\sim 1\mu \mathrm{m})$ . The DBR acts as a reflecting substrate for the excess transmitted photoexcitation. Using a 550nm-thick GaAs, the novel design exhibited a 67% increase in peak-to peak THz signal compared to a similar GaAs on silicon (Si) substrate. The enhancement can be attributed to the increased optical absorption and multiple reflections in the active layer.

Carrier dynamics and photoconductivity in epitaxial-grown low-temperature GaAs on nominal and vicinal Si(1 0 0) substrates ('LT-GaAs/Si') were studied to predict their actual performance as THz photoconductive antenna (PCA) detectors. An optical-pump terahertz-probe technique was used to obtain the transmittance, carrier lifetime and photoconductivity of two LT-GaAs/Si samples, grown using different substrates and different growth protocols. The LT-GaAs grown on Si(1 0 0) substrate with a 4° tilt to 〈1 1 0〉 has better crystallinity, in agreement with other reports; while the LT-GaAs layer grown on nominal Si(1 0 0) substrate, though more structurally defective, has a much faster electron trapping time. Fabricated test PCAs with either dipole or bowtie geometries confirm the characterization results. The photoconductivity and carrier lifetime results manifest in the PCA performance, in responsivity, and in detection bandwidth. The prototypes' sensitivities, bandwidths and dynamic ranges show that with some growth optimization, LT-GaAs/Si can be tailored to create economical, broadband THz detectors.

Two asymmetric double quantum wells of different coupling strengths (barrier widths) were grown via molecular beam epitaxy, both samples allowing tunneling. Photoluminescence was measured at 10 and 300 K to provide evidence of tunneling, barrier dependence, and structural uniformity. Carrier dynamics at room temperature was investigated by optical pump terahertz probe (OPTP) spectroscopy. Carrier population decay rates were obtained and photoconductivity spectra were analyzed using the Drude model. This work demonstrates that carrier, and possibly tunneling dynamics in asymmetric double quantum well structures may be studied at room temperature through OPTP spectroscopy.