Jeffrey G. Cederberg

Efficient Terahertz Emission from InAs Nanowires

Abstract : We observe intense pulses of far-infrared electromagnetic radiation emitted from arrays of InAs nanowires. The terahertz radiation power efficiency of these structures is 15 times higher than a planar InAs substrate. This is explained by the preferential orientation of coherent plasma motion to the wire surface, which overcomes radiation trapping by total-internal reflection.We present evidence that this radiation originates from a low-energy acoustic surface plasmon mode of the nanowire. This is supported by independent measurements of electronic transport on individual nanowires, ultrafast terahertz spectroscopy, and theoretical analysis. Our combined experiments and analysis further indicate that these plasmon modes are specific to high aspect ratio geometries.

Transport characterization in nanowires using an electrical nanoprobe

Electrical transport in semiconductor nanowires is commonly measured in a field effect transistor configuration, with lithographically defined source, drain and in some cases, top gate electrodes. This approach is labor intensive, requires high-end fabrication equipment, exposes the nanowires to extensive processing chemistry and places practical limitations on minimum nanowire length. Here we describe an alternative, simple method for characterizing electrical transport in nanowires directly on the growth substrate, without any need for post growth processing. Our technique is based on contacting nanowires using a nano-manipulator probe retrofitted inside of a scanning electron microscope. Using this approach, we characterize electrical transport in GaN nanowires grown by catalyst-free selective epitaxy, as well as InAs and Ge nanowires grown by a Au-catalyzed vapor solid liquid technique. We find that in situations where contacts are not limiting carrier injection (GaN and InAs nanowires), electrical transport transitions from Ohmic conduction at low bias to space-charge-limited conduction at higher bias. Using this transition and a theory of space-charge-limited transport which accounts for the high aspect ratio nanowires, we extract the mobility and the free carrier concentration. For Ge nanowires, we find that the Au catalyst forms a Schottky contact resulting in rectifying current‐voltage characteristics, which are strongly dependent on the nanowire diameter. This dependence arises due to an increase in depletion width at decreased nanowire diameter and carrier recombination at the nanowire surface. (Some figures in this article are in colour only in the electronic version)

Midinfrared doping-tunable extraordinary transmission from sub-wavelength Gratings

The authors demonstrate doping-tunable extraordinary transmission from subwavelength apertures in a periodic two-dimensional metallic grating deposited upon n-doped GaAs. By varying the doping of the underlying GaAs epilayer, they demonstrate wavelength tunability of ∼23cm−1 or approximately 0.15μm. The authors have achieved transmission peaks as narrow as 18cm−1 with such gratings, which suggests that devices based on midinfrared extraordinary transmission gratings could be used in external cavity structures for quantum cascade lasers, or as tunable filters or modulators for midinfrared applications.

Electrically tunable extraordinary optical transmission gratings

We report a semiconductor based mechanism for electrically controlling the frequency of light transmitted through extraordinary optical transmission gratings. In doing so, we demonstrate active control over the surface plasmon (SP) resonance at the metal/dielectric interface. The gratings, designed to operate in the midinfrared spectral range, are fabricated upon a doped GaAs epilayer. Tuning of over 25cm−1 is achieved, and the devices are modeled to investigate the physical origin of the tuning mechanism. Though our structures are designed for the midinfrared, the tuning mechanism demonstrated could be applied to other wavelength ranges, especially the visible and near infrared.

Development of high quantum efficiency GaAs/GaInP double heterostructures for laser cooling

We report on the growth and characterization of high external quantum efficiency (EQE) GaAs/GaInP double heterostructures. By properly treating the GaAs/GaInP interface, we are able to produce structures measuring a record EQE of 99.5% ± 0.1% in GaAs. This efficiency exceeds the requirement for achieving laser cooling in GaAs. However, net cooling has not yet been realized due to residual below gap background absorption.

The preparation of InGa(As)Sb and Al(Ga)AsSb films and diodes on GaSb for thermophotovoltaic applications using metal-organic chemical vapor deposition

InGaAsSb thermophotovoltaic cells and Al(Ga)AsSb cell isolation diodes have been successfully grown by MOCVD. Epitaxial growth of antimonide films is quite sensitive to small changes in growth conditions, but modern MOCVD technology has sufficient control to reproducibly generate antimonide-based devices. In this work In 0.1 Ga 0.9 As 0.09 Sb 0.91 thermophotovoltaic cells exhibit open circuit voltages of 250 mV with 60% external quantum efficiency. The electrical properties of Al(Ga)AsSb are dominated by background oxygen. Oxygenated contaminates from TESb are the major contributors to this background oxygen concentration. Both Al 0.4 Ga 0.6 As 0.04 Sb 0.96 and AlAs 0.08 Sb 0.92 cell isolation diodes have been tested. AlAs 0.08 Sb 0.92 cell isolation diodes show a reverse breakdown of 7.9 V at 0.1 A/cm 2 . An antimonide-based thermophotovoltaic monolithic interconnected module has been fabricated.

Heterogeneous metasurface for high temperature selective emission

We demonstrate selective emission from a heterogeneous metasurface that can survive repeated temperature cycling at 1300 K. Simulations, fabrication, and characterization were performed for a cross-over-a-backplane metasurface consisting of platinum and alumina layers on a sapphire substrate. The structure was stabilized for high temperature operation by an encapsulating alumina layer. The geometry was optimized for integration into a thermophotovoltaic (TPV) system, and was designed to have its emissivity matched to the external quantum efficiency spectrum of 0.6 eV InGaAs TPV material. We present spectral measurements of the metasurface that result in a predicted 22% optical-to-electrical power conversion efficiency in a simplified model at 1300 K. Furthermore, this broadly adaptable selective emitter design can be easily integrated into full-scale TPV systems.

Enhancement-mode buried strained silicon channel quantum dot with tunable lateral geometry

We propose and demonstrate a relaxed-SiGe/strained-Si enhancement-mode gate stack for quantum dots. A mobility of 1.6 × 105 cm2/Vs at 5.8 × 1011/cm2 is measured in Hall bars that witness the same device process flow as the quantum dot. Periodic Coulomb blockade measured in a double-top-gated lateral quantum dot nanostructure terminates with open diamonds up to ±10 mV of dc voltage across the device. The devices were fabricated within a 150 mm Si foundry setting that uses implanted ohmics and chemical-vapor-deposited dielectrics. A modified implant, polycrystalline silicon formation and annealing conditions were utilized to minimize the thermal budget that potentially leads to Ge/Si interdiffusion.

Optically pumped DBR-free semiconductor disk lasers

We report high power distributed Bragg reflector (DBR)-free semiconductor disk lasers. With active regions lifted off and bonded to various transparent heatspreaders, the high thermal impedance and narrow bandwidth of DBRs are mitigated. For a strained InGaAs multi-quantum-well sample bonded to a single-crystalline chemical-vapor deposited diamond, a maximum CW output power of 2.5 W and a record 78 nm tuning range centered at λ≈1160 nm was achieved. Laser operation using a total internal reflection geometry is also demonstrated. Furthermore, analysis for power scaling, based on thermal management, is presented.

Empirical correlation for minority carrier lifetime to defect density profile in germanium on silicon grown by nanoscale interfacial engineering

High-quality Ge-on-Si heterostructures have been explored for many applications, including near infrared photodetectors and integration with III–V films for multijunction photovoltaics. However, the lattice mismatch between Ge and Si often leads to a high density of defects. Introducing annealing steps prior to and after full Ge island coalescence is found to reduce the defect density. The defect density in Ge is also found to decrease with increasing dopant density in Si substrates, likely due to the defect pinning near the Ge-Si interface by dopants. The authors establish an empirical correlation between the minority carrier lifetime (τG) and the defect density in the Ge film (ρD) as a function of distance from the Ge-Si interface: τGe = C/ρD, where C is a proportionality constant and a fitting parameter which is determined to be 0.17 and 0.22 s/cm2 for Ge films grown on low-doped, high-resistivity Si substrates and high-doped, low-resistivity Si substrates, respectively. The effective minority carrier ...