Joshua M. O. Zide

Ultrafast optical switching of terahertz metamaterials fabricated on ErAs/GaAs nanoisland superlattices

We demonstrate optical switching of electrically resonant terahertz planar metamaterials fabricated on ErAs/GaAs nanoisland superlattice substrates. Photoexcited charge carriers in the superlattice shunt the capacitive regions of the constituent elements, thereby modulating the resonant response of the metamaterials. A switching recovery time of 20 ps results from fast carrier recombination in the ErAs/GaAs superlattice substrates.

Comparison of the 3ω method and time-domain thermoreflectance for measurements of the cross-plane thermal conductivity of epitaxial semiconductors

The 3ω technique and time-domain thermoreflectance (TDTR) are two experimental methods capable of measuring the cross-plane thermal conductivity of thin films. We compare the cross-plane thermal conductivity measured by the 3ω method and TDTR on epitaxial (In0.52Al0.48)x(In0.53Ga0.47)1−xAs alloy layers with embedded ErAs nanoparticles. Thermal conductivities measured by TDTR at low modulation frequencies (∼1 MHz) are typically in good agreement with thermal conductivities measured by the 3ω method. We discuss the accuracy and limitations of both methods and provide guidelines for estimating uncertainties for each approach.

Hybrid metamaterials enable fast electrical modulation of freely propagating terahertz waves

We demonstrate fast electrical modulation of freely propagating terahertz waves at room temperature using hybrid metamaterial devices. The devices are planar metamaterials fabricated on doped semiconductor epitaxial layers, which form hybrid metamaterial—Schottky diode structures. With an applied ac voltage bias, we show modulation of terahertz radiation at inferred frequencies over 2MHz. The modulation speed is limited by the device depletion capacitance which may be reduced for even faster operation.

Effect of nanoparticle scattering on thermoelectric power factor

The effect of nanoparticles on the thermoelectric power factor is investigated using the relaxation time approximation. The partial-wave technique is used for calculating the nanoparticle scattering cross section exactly. We validate our model by comparing its results to the experimental data obtained for ErAs:InGaAlAs samples. We use the theory to maximize the power factor with respect to nanoparticle and electron concentrations as well as the barrier height. We found that at the optimum of the power factor, the electron concentration is usually higher in the sample with nanoparticles, implying that Seebeck is usually unchanged and conductivity is increased.

Enhanced terahertz detection via ErAs:GaAs nanoisland superlattices

We demonstrate enhanced terahertz detection using photoconductive antennas fabricated on superlattices of self-assembled ErAs nanoislands and GaAs. Low-temperature GaAs and radiation-damaged silicon-on-sapphire based detectors are compared in terms of bandwidth, signal-to-noise ratio, and optical efficiency.

Cross-plane lattice and electronic thermal conductivities of ErAs:InGaAs/InGaAlAs superlattices

We studied the cross-plane lattice and electronic thermal conductivities of superlattices made of InGaAlAs and InGaAs films, with the latter containing embedded ErAs nanoparticles (denoted as ErAs:InGaAs). Measurements of total thermal conductivity at four doping levels and a theoretical analysis were used to estimate the cross-plane electronic thermal conductivity of the superlattices. The results show that the lattice and electronic thermal conductivities have marginal dependence on doping levels. This suggests that there is lateral conservation of electronic momentum during thermionic emission in the superlattices, which limits the fraction of available electrons for thermionic emission, thereby affecting the performance of thermoelectric devices.

High efficiency semimetal/semiconductor nanocomposite thermoelectric materials

Rare-earth impurities in III–V semiconductors are known to self-assemble into semimetallic nanoparticles which have been shown to reduce lattice thermal conductivity without harming electronic properties. Here, we show that adjusting the band alignment between ErAs and In0.53Ga0.47−XAlXAs allows energy-dependent scattering of carriers that can be used to increase thermoelectric power factor. Films of various Al concentrations were grown by molecular beam epitaxy, and thermoelectric properties were characterized. We observe concurrent increases in electrical conductivity and Seebeck coefficient with increasing temperatures, demonstrating energy-dependent scattering. We report the first simultaneous power factor enhancement and thermal conductivity reduction in a nanoparticle-based system, resulting in a high figure of merit, ZT=1.33 at 800 K.

Optical and electrical characterization of InGaBiAs for use as a mid-infrared optoelectronic material

In0.53Ga0.47BixAs1−x films were grown on InP:Fe substrates by molecular beam epitaxy, with Bi concentrations up to x = 3.60%. Bi content in the epilayers was determined by Rutherford backscattering spectroscopy, and channeling measurements show Bi incorporating substitutionally. Unlike previous work, electrical and optical data are obtained for all samples. A redshift in peak wavelength of about 56 meV/%Bi was observed using spectrophotometry. The valence band anti-crossing model is applied, showing InyGa1−yBixAs1−x lattice-matched to InP is possible by varying the composition, with a theoretical cutoff wavelength of about 6 μm.

Cross-plane Seebeck coefficient of ErAs:InGaAs/InGaAlAs superlattices

We characterize cross-plane and in-plane Seebeck coefficients for ErAs:InGaAs∕InGaAlAs superlattices with different carrier concentrations using test patterns integrated with microheaters. The microheater creates a local temperature difference, and the cross-plane Seebeck coefficients of the superlattices are determined by a combination of experimental measurements and finite element simulations. The cross-plane Seebeck coefficients are compared to the in-plane Seebeck coefficients and a significant increase in the cross-plane Seebeck coefficient over the in-plane Seebeck coefficient is observed. Differences between cross-plane and in-plane Seebeck coefficients decrease as the carrier concentration increases, which is indicative of heterostructure thermionic emission in the cross-plane direction.

Temperature and Bi-concentration dependence of the bandgap and spin-orbit splitting in InGaBiAs/InP semiconductors for mid-infrared applications

Replacing small amounts of As with Bi in InGaBiAs/InP induces large decreases and increases in the bandgap, Eg, and spin-orbit splitting, ΔSO, respectively. The possibility of achieving ΔSO > Eg and a reduced temperature (T) dependence for Eg are significant for suppressing recombination losses and improving performance in mid-infrared photonic devices. We measure Eg(x, T) and ΔSO (x, T) in In0.53Ga0.47BixAs1−x/InP samples for 0 ≤ x ≤ 0.039 by various complementary optical spectroscopic techniques. While we find no clear evidence of a decreased dEg/dT (≈0.34 ± 0.06 meV/K in all samples) we find ΔSO > Eg for x > 3.3–4.3%. The predictions of a valence band anti-crossing model agree well with the measurements.