Session N12: Mechanical and Dynamical Properties

Cavity optimization for AlGaN heterostructure deep-ultraviolet lasers

The challenges for AlGaN based UV emitters revolve around the wide-bandgap semiconductor materials limitations such as low carrier mobilities, high dopant activation energies and asymmetries between electron and hole transport [1]. For the purpose of a laser diode, these properties translate into the challenge of achieving population inversion, or optical gain, by electrical injection. Since lasing occurs when the optical gain of the laser diode equals the optical losses, the minimization of the optical losses is of paramount importance. The optical losses consist of intrinsic material losses, as well as cavity losses. Here, we quantify the effects of mirror imperfections including slant and roughness on the cavity loss and show that it is a superlinear function of the slant angle and RMS roughness, and scales as the inverse wavelength squared of the principal lasing mode [2]. This highlights the importance of device processing optimization as Fabry-Pérot cavities couple to shorter wavelengths. With the development of a complementary dry and wet etch recipe for both etched and cleaved facets [3], we demonstrate an optically pumped AlGaN double heterostructure laser grown by molecular beam epitaxy (MBE) on bulk AlN, exhibiting peak gain at 284 nm [2]. However, the reflectivity for a smooth and vertical facet is still limited by a single semiconductor-air interface. To reduce the cavity loss further, we develop sidewall deposited oxide Distributed Bragg Reflectors (DBRs) which are compatible with deep-UV coupled Fabry-Pérot cavities.   

Quantum Plasmonics of Few Electrons in Strongly Confined Doped Semiconducting Oxide Nanocrystal: A DFT+U Study of ZnGaO

Photodoping experiments by Faucheaux and Jain [1] show that electrons as few as 4 to 5 may sustain localized surface plasmonic oscillations in a nanocrystal. Gamelin et al [2] also claim that about as low as 3 electrons in nanocrystal are in involved in their photodoping experiment. Theoretical explanation by Jain [3] is based on free conduction electrons confined to a potential well to get qualitative results. On the other hand, Gamelin et al [2] used a semiclassical approach that involves quantum corrections to explain their results. In this talk, we present our DFT+U results of ZnO doped by Ga to find out if plasmonic oscillation are indeed sustained with 3, 4, 5, or 6 electrons in a spherical nanocrystal of 1.4 nm in diameter. We optimized the bulk unit cell to get a band gap of 3.37 eV and built the supercell from the optimized unit cell. Following geometric optimization of the supercell, optical calculations were performed in the independent particle approximation (IPA) as implemented in VASP code. The doping by Ga instead of the often-used Al to dope ZnO is to minimize local effects to justify IPA. Results of extinction and absorptions calculations will be presented.    

 

[1] J. A. Faucheaux and P. K. Jain, J. Phys. Chem. Lett. 4, 3024 (2013)

[2] A. M. Schimpf, N. Thakkar, C. E. Gunthardt, D. Massiello, and D. R. Gamelin, ACS Nano 8, 1065 (2014)

[3] P. K. Jain, J. Phys. Chem. Lett. 5, 3112 (2014)   

Carrier-phonon scattering in the optoelectronic response of laser-generated electron-hole plasmas in a GaAs nanowire

We examine the role of carrier-phonon scattering in the ultrafast optoelectronic response of a subwavelength a GaAs nanowire.  The wire is exposed to a low bias voltage when it is excited by an ultrashort resonant laser pulse.  For low bias voltages resulting in DC fields less than 0.6 kV/cm within the nanowire, the laser-excited carrier distributions reach a quasi-equilibrium within 100 ps, from which we calculate a stable conductivity that is nonlinear with the applied DC field.  At higher DC field strengths, phonons do not contain the carrier distributions near the gamma-point.  Instead, they break free and traverse the band structure. Further calculations including Coulomb scattering and dephasing show that these effects are not negligible for these high DC field cases once the carrier distributions reach higher energies.  We also confirm that Coulomb scattering and dephasing are safely negligible for the low DC field cases.   

Investigation of shock-compressed III-V semiconductors

We report postmortem studies on shock-compressed direct band-gap semiconductors. Shock waves are immediate jumps of pressure that are generated by high-speed impact or explosion. They usually lead to permanent changes in the material structures and properties. Here, commercial III-V wafers were characterized before and after shock wave compression. Shock compressions were performed on a laser-driven flyer plate system at impact velocities of 2 – 3.5 km/s, corresponding to pressures of 14-24 GPa. The semiconductors were recovered after shock for postmortem analysis with x-ray diffraction, photoluminescence, and Raman measurements at room temperature. The results have shown evidence of permanent changes in the crystal structures of the compressed materials. Considering the wide usage of semiconductor bulk substrates in the hi-tech industry, we emphasize the significance of practical, shock-induced band structure engineering pathways.   

Polarized Infrared Spectroscopy of FeTe2 Single Crystals

There is renewed interest in thermoelectric performances of transition metal dichalcogenides (FeTe₂, FeS₂, FeSe₂), and in their potential for optoelectronics applications, as they are narrow gap semiconductors with large electron mobility and strong absorption in visible/ultraviolet spectral regions. With the goal of addressing some of the open questions regarding their electronic properties, we performed measurements of polarized infrared reflectance on single crystals of FeTe₂, focusing in particular on samples with vacancy defects, which were found to suppress long range magnetic order. Here, we discuss the in-plane anisotropy of optical conductivity and optical band gap, as well as additional optical transitions observed at low energies, below the gap. We show that our data is in good agreement with recent band structure calculations, indicating spin-splitting of the e_g and t_g orbitals of the Fe-3d shell.   

The Effect of Ge-doping on Electronic and Lattice Vibrational Properties of FeGa3

Studied initially for its large thermopower effect, the intermetallic semiconductor FeGa₃ has recently been shown to provide a suitable platform for studying electron correlations, such as metal-insulator transition, non Fermi liquid behavior and ferromagnetism. In particular, with Ge-doping at Ga site the ground state is tuned from a Kondo insulator to a paramagnetic and further to a ferromagnetic metal. Here we present optical reflectance measurements on single crystals of FeGa_3-xGe_x, for different values of x. The temperature was varied between 300 K and 5 K, and the frequency ranged from 40 cm^-1 to 50 000 cm^-1. Involving the Kramers-Kronig transformation, we obtained various optical functions and discuss the electronic and lattice vibrational properties of different ground states resulting from Ge substitution.   

Charge transfer and dynamics in van der Waals trilayer hybrid heterostructures

Ultrafast interlayer charge transfer has been generally observed in a wide range of van der Waals heterostructures formed by two monolayer semiconductors. Previous studies have revealed that the transferred carriers form interlayer excitons with rather large binding energies. While such interlayer excitons are interesting systems on their own, generating free charge carriers is desired for various optoelectronic applications. For this goal, charge transfer and dynamics in heterostructures formed by F8ZnPc, few-layer transition metal dichalcogenide, and graphene are investigated by femtosecond pump-probe measurements. In such F8ZnPc/WS2/graphene samples, holes excited in F8ZnPc are found to transfer to graphene to minimize their energy. Electrons, however, reside in F8ZnPc due to the energy barrier presented by the WS2 layer. By increasing the thickness of WS2 from monolayer to 4 layers, the Coulomb interaction between the layer-separated electrons and holes is reduced, resulting in unipolar-like diffusion of holes in graphene, as evident by spatially resolved transient absorption microscopy. The finding illustrates the possibility of using band-alignment engineering with thickness control to generate free charge carriers in van der Waals heterostructures.   

Effects of Device Geometry and Illumination on Gunn Threshold Voltage and Gunn Oscillation Properties in GaAs Gunn Devices

We fabricate submillimeter-sized GaAs Gunn devices to study the effects of device geometry and illumination on the Gunn threshold voltage and the Gunn oscillation properties. Threshold voltage reduction is observed in wedge-shape-tapered devices and is attributed to the electric field focusing effect towards the narrower end of the devices at the cathode. In addition, illumination could alter the local electron concentration and the global electric field profile of the devices. This leads to a modulation of the Gunn threshold voltage, as well as the magnitude and the coherency of the Gunn oscillations, with the modulation being dependent on the illumination position. The experimental findings of the effects of device geometry and illumination are further corroborated with finite element method simulation of the electric field profile along the devices.   

First principles investigation of ultrahigh thermal conductivity materials

Materials with high thermal conductivity(k) are critical for efficient heat dissipation in power electronics, electronics cooling, data center, energy storage and energy conversion technologies in order to improve the system reliability. Especially, k at nanoscale is crucial for increasing the number of transistors embedded in a given chip to meet the ever-increasing demand of computational power without sacrificing its durability. In this work, we are reporting some advanced high thermal conductivity materials such as h-BC6N, h-GeC, h-BC2P and BC5. Interestingly, these materials also have a good thermal conductivity in nanostructured systems (L=50 -100 nm) due to its contributions from optical phonons with phonon meanfreepaths lower than its system size. We also report thermal conductivity enhancement through biaxial strain. We report 710% and 35% enhancement in k for 2D-GeC (6% biaxial tensile strain) and zinc-blende boron phosphide(4% biaxial compressive strain) respectively through biaxial strain. This work provides an avenue to explore and tune k of high thermal conductivity materials in both bulk and nanoscale.

Keywords: Thermal management systems, Phonons, Semiconductors, First principles calculations, Nanostructures   

Ultra-strong coupling of electron tunneling and mechanical motion in carbon nanotube electromechanical resonators

A single quantum dot electrostatically defined in a carbon nanotube-based mechanical resonator is a promising testbed for exploring electron-phonon coupling. The tunnelling of electrons generates back action on the nanomechanical resonator, an effect known as softening of the mechanical frequency. The dynamics of these electromechanical resonators are determined by two main energy scales: the tunnel coupling of the quantum dot with the leads and the resonance frequency of the mechanical oscillator. We study the magnitude of electromechanical coupling in various regimes. We find an ultra-strong coupling regime for fast electron tunnelling under the adiabatic approximation. I will discuss how these findings establish carbon nanotube-based electromechanical devices as a fertile platform for exploring non-equilibrium thermodynamics.   

Strong anharmonicity in a carbon nanotube electromechanical oscillator populated with a few tens of quanta

It has been a long-sought goal to prepare nonlinear quantum states in a mechanical oscillator. However, intrinsic anharmonicity due to geometric nonlinearity in conventional micro and nano-electromechanical oscillators is extremely small at the level of zero-point motion. Here we demonstrate the ultra-strong coupling of the vibrations of a carbon nanotube resonator to single electron tunnelling.  The results in the suppression of the mechanical resonance frequency by up to 30%, which is one order higher than previous reported results.  Our device is in the so-called ultrastrong coupling regime, where the electromechanical coupling per phonon is one order of magnitude larger than the resonance frequency. This coupling results in a strong anharmonicity in the mechanical oscillator. More than 20% of the thermal energy is stored in the quartic potential when the number of quanta is about 80.  The strong electromechanical coupling in nanotube resonators opens up the possibility to realize a mechanical qubit.   

Role of Defects in the Mechanical Properties of Graphene-Copper Heterostructures

Through molecular dynamics simulations of tensile tests, the effect of vacancies and Stone-Wales defects on the mechanical properties of sandwich-like heterostructures, composed of graphene and two symmetrical copper layers at the nanoscale, is studied. Chirality dependence of the graphene layer is also investigated. During elastic deformation, defects adversely affect the mechanical response. However, defective systems can show an improvement of the plastic properties. Vacancies have a stronger impact compared to Stone-Wales defects. Elasticity, toughness, and ductility are enhanced along the zigzag direction, while stiffness is improved along the armchair edge. The Poisson's ratio was calculated for all graphene-copper heterostructures. At a critical strain it becomes negative along the thickness direction, preserving the auxetic property at higher strain values. In general, the heterostructure behavior is driven by the graphene response. Our findings may be useful to understand the strengthening mechanism induced by this two-dimensional material in metals such copper and for the design of similar systems.   

Investigating Nonlinear Dynamic Response of Zinc Oxide Piezoelectric Micromachined Ultrasonic Transducers

Ultrasonic transducers have shown tremendous potential with wide-ranging applications in medical diagnostics, sensing and energy harvesting. Piezoelectric micromachined ultrasonic transducers (PMUTs) are ideal due to their miniature size, low power consumption, ease of fabricating transducer arrays, and integration with CMOS circuitry. We utilize highly oriented ZnO thin films grown by the microwave-assisted hydrothermal method as the piezo material to fabricate ZnO-based PMUTs. The PMUTs achieve high vibrational amplitude and depict nonlinearity.

In this work, we investigate the effect of fabrication-induced curvature in the circular PMUTs on the dynamics and nonlinearity of the diaphragm. We observe a clear transition from hardening to softening nonlinearity for devices of identical radial diameter (500μm) and increasing pre-curvature. Increasing the driving strength leads to a mixed nonlinear response depicting softening followed by hardening nonlinearity. The ability to change the curvature of a device through annealing and its associated transition in nonlinear response further confirms our observation of tuning of nonlinearity through curvature. We verify our observed experimental results through numerical simulations of the device response using a reduced-order model.   

Analyzing the electronic structure of the Sm3Fe5O12 garnet via experimental and ab-initio methods

Rare earth iron garnets (R₃Fe₅O₁₂) are fascinating materials with various exciting properties, almost as diverse as their compositions. They are remarkably stable ferrimagnetic insulators with the highest Faraday rotation known to date. Furthermore, the presence of the rare earth ion induces spin-orbit effects. These properties have made them relevant for the spintronics community. However, computational studies for these compositions are scarce. This talk presents a computational and experimental study of Sm₃Fe₅O₁₂, analyzing its crystal and electronic structures. Density functional theory (DFT) under the generalized gradient approximation (GGA) agrees with the Rietveld refined lattice parameter of 12.5231(3) Å from X-ray diffraction. Meanwhile, introducing the Hubbard-U correction (DFT+U) results in a band gap of 2.27 eV, which matches the 2.26-2.27 eV from UV-Vis spectra, and is identified as a direct transition between minority spin states. Effective Hubbard-U values were calculated analytically at 4.3092 eV for tetrahedral iron and 6.0926 eV for octahedral iron. We thus propose a model to identify the band-gap in Sm₃Fe₅O₁₂, taking into account the structure's ferrimagnetism and energy level distribution, enabling a better understanding of its electronic structure.