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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.

Carrier transport in semiconductors with different dimensionalities, i.e., 3D (bulk), 2D (QW), and 0D (QD), were investigated via temperature-dependent terahertz time-domain spectroscopy (THz-TDS). The optical properties and recombination dynamics in the samples were probed via photoluminescence spectroscopy. The temperature-dependence of the THz emission from the samples was explained in the context of the drift-diffusion model using the dominant THz radiation mechanism. The THz emission from diffusion-type THz emitters such as p- and n-InAs decreases as temperature increases due to mobility decrease. Conversely, the THz emission from drift-type THz emitters such as SI-GaAs, GaAs QW, and InAs QD was found to increase with temperature due to the increase in the driving electric field. In summary, THz-TDS can be utilized to gain qualitative insights on the temperature-dependent transport characteristics and establish dominant THz radiation mechanisms.

Terahertz (THz) photoconductive antennas (PCA’s) from SI-GaAs substrates having one-dimensional (1D) and two-dimensional (2D) micron-size metal line arrays (MLA's) were fabricated. Photolithography and electron beam deposition of Ni/Au were used to fabricate spiral PCA’s and 1D/2D MLA's on the transmission side of the PCA. Compared to a reference bare PCA, the THz time-domain signal enhanced ~6x for 1DMLA and ~11x for 2DMLA, with their corresponding bandwidths broadened. The origin of the enhancement is being investigated but is currently attributed to spoof surface plasmon phenomena. Integrating MLA’s with PCA’s demonstrates a more cost-effective alternative to nanostructure fabrication within the PCA gap.

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

An externally applied magnetic field was used to induce increased photocarrier transport along the high mobility channel in GaAs/AlGaAs modulation-doped heterostructures (MDH). The terahertz (THz) emission from GaAs/AlGaAs MDH increases with increasing magnetic field, applied parallel to the heterojunction. The THz emission enhancement factors due to the magnetic field in MDH are higher than in undoped GaAs/AlGaAs heterojunction and in bulk SI-GaAs. This demonstrates that properly utilizing the high-mobility channel for carrier transport promises to be a viable design consideration for efficient THz photoconductive antenna (PCA) devices. Moreover, it was observed that for MDH, as well as for an undoped GaAs/AlGaAs heterojunction, the enhancement for one magnetic field direction is greater than the enhancement for the opposite direction. This is in contrast to the symmetric enhancement with magnetic field direction observed in a bulk SI-GaAs. An analysis of photocarrier trajectories under an external magnetic field supports the explanation that the enhancement asymmetry with magnetic field direction in MDH is due to the cycloid motion of electrons as affected by the GaAs/AlGaAs interface.

We investigate the hydrothermal growth of zinc oxide (ZnO) nanorods to realize their potential scintillator applications. Vertically aligned ZnO nanorods were successfully fabricated on seeded silicon (Si) substrates at a relatively low temperature of 85 °C using different hexamethylenetetramine [(CH2)6N4, HMTA] and zinc acetate dihydrate [Zn(CH3COO)2·2H2O, ZnAc] concentration ratios. Varying the precursor concentration ratio affects the nanorod dimensions which, in turn, influence the apparent densities and observed emissions. But regardless of the HMTA and ZnAc concentration ratio used, the fabricated nanorods exhibit well-defined morphologies, hexagonal crystal structure, preferential c -axis orientation, and 24–120-ps ultraviolet (UV) emissions. With these excellent structural and optical properties, the hydrothermal-grown ZnO nanorods are suggested to be used as scintillator materials which offer not only fast scintillation response but also high spatial resolution for future radiation detectors.

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.

Terahertz (THz) photoconductive antenna (PCA) fabricated on low-temperature-grown GaAs (LT-GaAs) with layer structure consisting of a doped buffer exhibited enhanced THz emission for LT-GaAs grown at lower temperature. As THz emitter, the LT-GaAs grown at 270°C with doped buffer generated THz radiation with amplitude twice as that of its undoped counterpart. Similar effect is not observed when the LT-GaAs is grown at 320°C with doped buffer, which is expected to have higher THz emission. As THz detector, both LT-GaAs with doped buffer exhibited identical and improved detection sensitivity regardless of the LT-GaAs growth temperature.

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.

We report on the influence of chamber design on the gas sensing performance of a graphene field-effect-transistor (GFET). A conventional chamber (V = 400 ml) and a cap chamber (V = 1 ml), were used to perform dynamic measurements on a GFET. To gain a-priori knowledge on the gas flow in the chambers, Naiver–Stokes and convection-diffusion equations were numerically-solved using COMSOL Multiphysics. We numerically and experimentally observed two main factors that can affect the GFET performance: (1) the gas flow direction through the chamber and (2) the chamber volume. At 5-min exposure time, at least 200% higher GFET sensitivity was calculated from the cap chamber, which is expected since the conventional chamber is 400 times larger. Interestingly, even when the conventional chamber is fully saturated (at 90-min exposure time), the GFET sensitivity in the cap chamber is still better by 28.57%. We attributed this behavior to the swirling vapor flow in the cap chamber brought about by the U-shaped path. This effect causes multiple interaction of H2O molecules with the GFET, resulting to higher computed sensitivity. However, at higher relative humidity, the GFET becomes populated, reducing the number of H2O molecules that can re-interact with the sensor. In terms of GFET transient characteristics, a 154% and 86.9% faster response and recovery, respectively, were observed in the cap-design. This was due to its smaller volume that minimized poorly purged region in the chamber. But if the chambers have the same volumes, we may infer a faster GFET response and recovery from the conventional chamber where the gas flow is unperturbed. These results could contribute in designing a time efficient and cost-effective gas sensing system.

The poor gas selectivity problem has been a long-standing issue for miniaturized chemical-resistor gas sensors. The electronic nose (e-nose) was proposed in the 1980s to tackle the selectivity issue, but it required top-down chemical functionalization processes to deposit multiple functional materials. Here, we report a novel gas-sensing scheme using a single graphene field-effect transistor (GFET) and machine learning to realize gas selectivity under particular conditions by combining the unique properties of the GFET and e-nose concept. Instead of using multiple functional materials, the gas-sensing conductivity profiles of a GFET are recorded and decoupled into four distinctive physical properties and projected onto a feature space as 4D output vectors and classified to differentiated target gases by using machine-learning analyses. Our single-GFET approach coupled with trained pattern recognition algorithms was able to classify water, methanol, and ethanol vapors with high accuracy quantitatively when they were tested individually. Furthermore, the gas-sensing patterns of methanol were qualitatively distinguished from those of water vapor in a binary mixture condition, suggesting that the proposed scheme is capable of differentiating a gas from the realistic scenario of an ambient environment with background humidity. As such, this work offers a new class of gas-sensing schemes using a single GFET without multiple functional materials toward miniaturized e-noses.

We present threefold enhancement of terahertz emission from silicon nanowire (SiNW)-coated gallium-arsenide photoconductive antenna over its uncoated counterpart. The enhancement is attributed to the increased photoabsorption, and possibly additional photoconductive pathways induced by the SiNWs. © 2020 The Author(s)

The deposition of n-GaAs cap on low-temperature GaAs (LT-GaAs) improved the THz emission of LT-GaAs grown at a much lower temperature (< $$300\,^{\circ }$$ C), where the defect density is high, without compromising the spectral bandwidth and carrier lifetimes necessary for ultrafast THz detection. The LT-GaAs grown at $$220$$ and $$270\,^{\circ }$$ C showed a 192 and 10% enhancement THz emission peak-to-peak intensity, respectively, while the sample grown at $$310\,^{\circ }$$ C showed a 49% reduction. The n-GaAs cap reduced the As-related defects density in the LT-GaAs resulting to improved THz emission. The THz emission from the sample grown at $$310\,^{\circ }$$ C with already low defect density suffered possibly due to the free carrier absorption by the n-GaAs cap. The results are relevant in future material design of LT-GaAs based photoconductive antenna.

The influence of crystal thickness of metal-coated -cut GaAs (M-G-M) on Cherenkov-phase-matched terahertz (THz) pulse detection was studied. The M-G-M detectors were utilized in conjunction with a metallic tapered parallel-plate waveguide (TPPWG). Polarization-sensitive measurements were carried out to exemplify the efficacy of GaAs in detecting transverse magnetic (TM)- and transverse electric (TE)-polarized THz waves. The reduction of GaAs’ thickness increased the THz amplitude spectra of the detected TM-polarized THz electro-optic (EO) signal due to enhanced electric field associated with a more tightly-focused and well-concentrated THz radiation on the thinner M-G-M. The higher-fluence THz beam coupled to the thinner M-G-M improved the integrated intensity of the detected THz amplitude spectrum. This trend was not observed for TE-polarized THz waves, wherein the integrated intensities were almost comparable. Nevertheless, good agreement of spectral line shapes of the superposed TM- and TE-polarized THz-EO signals with that of elliptically polarized THz-EO signal demonstrates excellent polarization-resolved detection capabilities of M-G-M via Cherenkov-phase-matched EO sampling technique.

Photocarrier dynamics in GaAs/AlGaAs modulation-doped heterostructures (MDH) having i-AlGaAs spacer layers of different thicknesses were investigated using temperature-dependent terahertz time-domain spectroscopy (THz-TDS) and photoreflectance spectroscopy. In particular, results are discussed in the framework of the temperature dependence of the heterojunction electric field and photocarrier velocity for two i-AlGaAs spacer layer thickness values. The junction electric field, THz emission intensity and bandwidth of the MDH samples all decrease as temperature decreases. In contrast, the THz emission intensity and bandwidth of a reference bulk undoped GaAs does not significantly vary with temperature. These results imply that THz emission of MDH’s originates primarily from carrier drift due to the GaAs/AlGaAs junction electric field. A general decrease in the THz emission bandwidth of the MDH’s is attributed to a decrease in carrier velocity at lower temperatures, presumably due to the weaker electric field. Moreover, the MDH sample with thinner spacer layer exhibited a higher junction electric field. This work demonstrates the study of temperature-dependent photocarrier transport and junction electric field measurements. The results may provide useful insights in the design of MDH-based devices.

We report the molecular beam epitaxy growth of high-quality p-InAs thin films evaluated in the context of 1.55 μm femtosecond laser-excited THz emission efficiency. The presence of p-InAs is confirmed via scanning electron microscopy and X-ray diffraction. Using a GaAs buffer layer, the epitaxial growth of p-InAs layers was successfully achieved. Initiating GaAs deposition by growth interruption, we find that GaAs adheres to the GaSb substrate and provides a quasi-planar surface for the subsequent layers. We also find a significant enhancement in the THz radiation intensity of p-InAs films that is approximately twice compared to that of bulk p-InAs for 1.55 μm wavelength.

We report the interplay of zinc acetate [Zn(CH3COO)2, Zn(OAc)2] concentration, morphology, and emission in hydrothermal-grown ZnO nanostructures. Highly oriented nanorods with intense, 54 to 96 ps near-band-edge ultraviolet (UV) emissions are successfully fabricated using 25 to 75 mM concentrations. On the other hand, larger and randomly oriented nanocrystals are fabricated using 100 to 150 mM concentrations. Although all nanostructures exhibit hexagonal wurtzite crystal structures, increasing the Zn(OAc)2 concentration affects their preferential orientation, morphology, and UV emission. The peculiar nanostructure emissions observed are also attributed to a reabsorption phenomenon among adjacent nanostructures. For potential scintillator applications, particularly for x-ray diagnostics, our results suggest that less than 100 mM Zn(OAc)2 concentrations should be used to reduce the UV emission reabsorption in hydrothermal-grown ZnO nanostructures.

Spintronic THz emission from Ni/Pt bilayer grown on MgO is reported based on the novel THz emitter using metallic structures. The Ni metal was deposited first on a MgO substrate and capped with a thin Pt metal via electron beam deposition. The THz emission data was obtained using a standard terahertz time-domain spectroscopy setup using a Ti: sapphire laser excitation source. Initial measurements were done using 800nm excitation with 7 mW and 185 mW pump powers under upward and downward magnetic field orientations. Polarity reversal of the terahertz signal was observed upon changing the orientation of the magnetic field. Maximum amplitude was found at 0.5 THz with bandwidth up to ~6 THz. A saturation fluence of 85.04 mJ/cm2 was calculated from the pump fluence-dependence plot of the THz peak-to-peak signal. The results are consistent with the spintronic THz emission due to the inverse spin-Hall effect and provide insights for future development and optimizations.

We have demonstrated experimental evidence of non-resonant tunneling at room temperature in GaAs/AlGaAs asymmetric coupled double quantum wells (ACDQW’s) using time-resolved photoluminescence (TRPL) spectroscopy at 300 K. Two ACDQW samples (A and B) with a barrier thickness of 25 A were grown via molecular beam epitaxy. The energy separation (ΔE) between the ground state of the conduction band of the wide well and that of the narrow well are 42.7 meV and 19.5 meV, for samples A and B respectively. The TRPL measurement revealed a double decay rate in sample A whose ΔE is greater than one GaAs longitudinal optical phonon energy (36 meV), suggesting a phonon assisted tunneling mechanism. The evidence of tunneling was supported by measuring the relative intensity of the PL contributions from the narrow and wide well at 10 K.

We report increased radiation in the visible and terahertz (THz) regimes in silicon(Si)-based nanostructures. The nanostructures, Si nanowires (SiNWs) and porous Si (PSi), were synthesized via electroless and electrochemical surface modification, respectively. In particular, picosecond (ps) radiative lifetimes in the order of 250 ps were obtained from time-resolved photoluminescence (PL) measurements. The fast radiative lifetimes are associated with increased surface defect density in PSi. Reflectance measurements confirmed that optical absorption of the nanostructured Si samples increased relative to bulk Si. Both nanostructured Si exhibit THz emission, albeit weaker in PSi due to higher density of defects. An inverse relationship between PL and THz emission strength was therefore observed. Lastly, the wider bandwidth of the THz emission in SiNWs is attributed to the directionality of the transient photocurrent compared to the more disordered carrier transport in PSi.

Silicon nanowires (SiNWs) were etched on Si (100), (110), and (111) substrates. Slanted nanowires with respect to the surface normal were produced on the Si (110) and (111) substrates, and vertical nanowires were produced on the Si (100) substrate. Photoluminescence spectroscopy exhibited luminescence lines attributed to oxide defects from the nanowires. A comparison of the transient photocurrent as measured using terahertz time-domain spectroscopy revealed an increase in the THz emission from the SiNWs on Si (110) substrate compared to those grown on (100) substrate. Reorienting the dipole moment by applying an external 650 mT magnetic field suggested that the carrier transport was confined along the axis of the nanowires. Understanding the photocarrier and transport recombination properties in SiNWs may prove useful in the design considerations for future SiNW photovoltaic cell applications.

We investigate the effects of an externally applied magnetic field on the terahertz (THz) emission of Gallium Manganese Arsenide (GaMnAs) films grown via molecular beam epitaxy (MBE). Results show that low Mn-doping in GaMnAs resulted to increased THz emission as compared with a SI-GaAs substrate. Further increase in Mn-doping content resulted to a comparably less THz emission, which is attributed to reduced crystallinity and higher free-carrier absorption. Under an external magnetic field, the contributions of the B u p and B d o w n -related THz emission were observed to be asymmetric: possibly due to intrinsic magnetic properties of GaMnAs.

ZnO microrods are fabricated by a simple hydrothermal growth route using zinc acetate dihydrate [Zn(CH3COO)2·2H2O] and hexamethylenetetramine [(CH2)6N4] aqueous solutions. The as-prepared microrods exhibit uniform dimensions, well-faceted surfaces, and hexagonal crystal structure. The microrods also have an intense ultraviolet (UV) emission at 392 nm with an average lifetime of 80 ps. No peaks are observed at the visible wavelengths that can be attributed to defect-related emissions. With excellent structural and optical properties and with loose adhesion to their substrates, the ZnO microrods can be isolated, harvested, and manipulated and can be integrated as building blocks of a microstructured scintillator screen. The proposed scintillator screen possibly offers efficient and precise detection with high resolution. Hydrothermal-grown ZnO microrods then hold a promise towards radiation detector innovation and integrated optoelectronic microsystems.

Porosity dependent terahertz emission of porous silicon (PSi) was studied. The PSi samples were fabricated via electrochemical etching of boron-doped (100) silicon in a solution containing 48% hydrofluoric acid, deionized water and absolute ethanol in a 1:3:4 volumetric ratio. The porosity was controlled by varying the supplied anodic current for each sample. The samples were then optically characterized via normal incidence reflectance spectroscopy to obtain values for their respective refractive indices and porosities. Absorbance of each sample was also computed using the data from its respective reflectance spectrum. Terahertz emission of each sample was acquired through terahertz - time domain spectroscopy. A decreasing trend in the THz signal power was observed as the porosity of each PSi was increased. This was caused by the decrease in the absorption strength as the silicon crystallite size in the PSi was minimized.

This work reports distinctive gas sensing patterns for water vapor and methanol using graphene FET (Field Effect Transistors) functionalized with atomic layer deposition (ALD) RuO 2 . Compared with the state-of-art, three distinctive advancements have been achieved: (1) enhanced sensitivity using the scheme of electron mobility characterizations by a hybrid structure of graphene and ALD-RuO 2 base layer; (2) first demonstration of gas sensing by means of the 4-dimentional (4D) physical properties vectors of graphene FETs; (3) using the 16-dimensional (16D) characteristic gas sensing pattern to distinguish water vapor and methanol. As such, the device structure and the multi-dimensional physical properties vectors could offer robust gas classification schemes for gas sensing applications.

Density functional theory based calculations with Hubbard correction (DFT + U) were performed to investigate the effects of varying coverage and different adsorption sites on hydrogen (H) adsorption on zinc oxide (ZnO) ( 10 1 ¯ 0 ) surface. Results show that H adsorption on top of oxygen (O) at low coverage (0.25 monolayer, ML) shifts the conduction band below the Fermi level and narrows the band gap. These phenomena are attributed to the charge transfer between H and the surface zinc (Zn) and O atoms. On the other hand, the H adsorption on top of Zn at low coverage (0.25 ML) shows an overlapping of H, Zn, and O states while maintaining the semiconductor nature of the system. At high coverage (1.0 ML), a charge accumulation layer on the surface forms, and the mechanisms that govern the interactions of H atoms when adsorbed exclusively on top of Zn or top of O are found to be similar with the low coverage cases. Lastly, at full coverage (2.0 ML), the effect of H on top of Zn is more evident as the system retained its semiconducting property. The adsorption energy is enhanced due to the reinforced overlapping of the H, Zn, and O states and due to the possible attraction between the adsorbed H atoms. The properties and stability of full-coverage adsorption were explained based on the findings on high- and low- coverages adsorption. The findings of the study will aid in understanding the interaction of H with the ZnO surface toward the further development of ZnO's optoelectronic applications.

With the use of a thermoelectric material, terrestrial heat can be harvested then converted to electrical power. The advent of these devices has led to the idea of self-powering wherein devices are driven by heat from their working environment. The focus of this study is to fabricate low cost thermoelectric materials, such as aluminum-doped ZnO (ZnO:Al) and nitrogen-doped Cu x O (Cu x O:N) that can effectively harvest heat for power generation. ZnO:Al (n-type) and Cu x O:N (p-type) thin films with nanocrystallites were deposited in (1.27×0.64) cm 2 glass substrates via spray pyrolysis technique. These materials exhibit significantly high thermoelectric properties, which is comparable to previous works on thermoelectric materials. ZnO:Al showed to have a maximum Seebeck coefficient (S) of 448 μV/K ranging from 300 to 330 K. Cu x O:N exhibited a significantly much larger |S| of 1002 μV/K at the same temperature range. A prototype of a thermoelectric device was constructed based from these grown thin films and showed to generate a maximum of 32.8 mV at 28 K temperature difference.

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.

We reveal the As-antisite (AsGa) defect close to the surface of GaAs(1 1 0) with bulk characteristics using first-principles methods with experimental verifications. We found that the AsGa in the third-layer mimics the geometry, partial charge density and more importantly, the density of states of AsGa in bulk GaAs. Notably, the mid-gap state induced by AsGa in bulk GaAs is well-reproduced by the AsGa in the third layer of GaAs(1 1 0). Simulated and experimental STM images show an “asymmetric two-lobe” feature in the region around the defect. Using local density of states (LDOS) and STS spectra, we propose three prominent peaks with characteristic energy levels corresponding to the third layer AsGa. The above results present the first report of surface electronic signatures of true bulk defect near the surface of GaAs(1 1 0).

We study the photoluminescence (PL) and terahertz (THz) emission characteristics of MBE-grown InAs/GaAs single-layered quantum dots. Results from temperature-dependent and excitation power-dependent PL spectroscopy have revealed the presence of energy peaks corresponding to two possible quantum dot size distributions. The THz emission was found to increase as temperature increases, which we attribute to the increase in the number of carriers undergoing transport.

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.

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 × 10 15 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.

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.

The photocarrier dynamics in molecular beam epitaxy (MBE)-grown single- (SLQD) and multi-layered (MLQD) InAs/GaAs quantum dots were studied. Photoluminescence (PL) spectroscopy has shown that the MLQD has more uniform QD size distribution as compared to the bimodal SLQD. Correlation between PL and THz-TDS has shown that photocarrier transport is more favored in the MLQD owing to this uniform QD size distribution, resulting to higher THz emission. The THz emission from the QD samples were found to be proportional to temperature. A drift-related photocarrier transport mechanism is proposed, wherein photocarriers generated in the QDs are accelerated by an interface electric field.

Room-temperature amplified spontaneous emission (ASE) has been observed from aqueous chemical grown zinc oxide (ZnO) microrods. The well-faceted microrods have only a single narrow ultraviolet (UV) emission at 390 nm (3.2 eV) with average lifetimes as fast as 85–100 ps. The characteristic ASE also exhibits blue-shifted peaks and shortened lifetimes. At present, the peak shifting and the lifetime shortening are attributed to the band filling and photo-induced screening effects and to the nonradiative relaxation process, respectively. Results indicate that the ZnO microrods have good structural and optical quality which leads to their suitable use for optoelectronic applications.

The Cardona equation describing the temperature behavior of a bulk semiconductor is utilized to fit experimental photoluminescence data for both strained and unstrained quantum wells (QWs). To account for confinement energy and strain energy, the Cardona equation was modified to include the respective energy offsets. Results of modeling experimentally obtained PL of GaAs/AlGaAs QWs for the unstrained case, and PL of InGaAs/GaAs QWs for the strained case, shows that the modified Cardona equation gives excellent fits to the temperature behavior even for QW bandgaps. Thus, the equation is utilized to extract QW eigenenergies, which are verifiable using the usual effective mass approximation method in solving Schrodinger’s equation for a finite potential well.

Graphene is one of the most promising allotropes of carbon with wide applications in nanotechnology. Modification of graphene by chemical means can further expand its uses. Here, liquid-exfoliated graphene is functionalized with p-aminophenyl moiety using p-nitrophenyl diazonium salts which were diazotized in situ then reduced by tin(II) chloride. The aminophenyl-graphene thus produced is further modified to form polyaniline-graphene (PANI-GNH) by in situ oxidative graft polymerization of aniline using ammonium peroxydisulfate as oxidizing agent. The properties of the PANI-GNH were compared with polyaniline/graphene blends by Raman, infrared and UV-Visible spectroscopy, and cyclic voltammetry. Indeed, PANI-GNH registered different spectrochemical and electrochemical properties compared with the physically blended PANI and GNH, a manifestation of the effect of chemical grafting on the overall property of the modified graphene.

Photoluminescence properties of ZnO microstructures are investigated for potential scintillator applications. Samples are successfully prepared by the carbothermal reduction method. A single free-standing ZnO microribbon exhibits UV and visible emissions which behave differently along the microstructure. Fast UV emission lifetimes of 40 and 150 ps have been observed for the microstructures compared to 440 ps and 2 ns for a bulk crystal. With waveguided emissions and fast lifetimes, the ZnO microstructures have promising applications as future XFEL scintillators.

We report on the shell-to-core carrier-transfer in GaAs/Al 0.1 Ga 0.9 As core-shell nanowires grown on Si(1 0 0) substrates via molecular beam epitaxy. The nanowires are dominantly zincblende and are tilted with respect to the substrate surface. Photoluminescence (PL) excitation spectrosocopy at 77 K revealed an abrupt increase in the GaAs PL intensity at excitation above the Al 0.1 Ga 0.9 As shell bandgap which is attributed to shell to core carrier-transfer. More carriers from the Al 0.1 Ga 0.9 As transfer to the GaAs at T >90 K, as observed in the time-resolved PL and temperature dependence of the relative PL intensities of GaAs and Al 0.1 Ga 0.9 As due to the ionization of the traps within the Al 0.1 Ga 0.9 As. Using a coupled rate equation model that takes into account shell to core carrier-transfer, the average recombination time constants of Al 0.1 Ga 0.9 As shell τ rec , s =400 ps (580 ps) and GaAs core τ rec , c =600 ps (970 ps) were obtained from the time-resolved PL at 300 K (77 K). Carrier-transfer time constants τ CT =50 ps (55 ps) at 300 K (77 K) were also obtained.

Porous silicon was fabricated through electrochemical etching and is used as an optical template for liquid sensing application. Using reflectance spectroscopy, change in optical properties such as refractive index and reflectivity upon liquid introduction were obtained. Chromatic dispersion of porous silicon upon detection of transparent liquids such as absolute ethanol, methanol, 2-propanol and distilled water were determined for spectral range 450nm to 1100nm. Mixture of the organic transparent liquid and water was also tested. In this study, porosity and liquid’s concentration were varied to establish the sensitivity of detection. Expected ideal values were also simulated for comparison and correction factor computation. Results provide calibration basis for water quality assessment, environmental monitoring, and diagnostic application.

We report on the enhanced terahertz (THz) emission from SI-GaAs and n-GaAs wafer surfaces grown with aluminum nitride (AlN) thin films, by more than a factor of 2 and 4, respectively. This is attributed to the increased GaAs surface electric field as confirmed by calculations and photoreflectance measurements. The deposition of AlN on an n-InAs substrate, however, produces no change in the THz emission despite the presence of a strong field as obtained from calculations. These results show that for semiconductors in which the THz radiation is primarily due to drift current, an increase in interface electric field results to an increase in the THz emission. For semiconductors whose primary source of THz radiation is carrier diffusion however, an increase in the interface electric field has a no effect in THz emission intensity.