arXiv subjects

J. M. Zavada

Publications and source records attributed to J. M. Zavada.

17 recordsLinked to original sources

arXiv2018

Room-temperature lasing action in GaN quantum wells in the infrared 1.5 micron region

Large-scale optoelectronics integration is strongly limited by the lack of efficient light sources, which could be integrated with the silicon complementary metal-oxide-semiconductor (CMOS) technology. Persistent efforts continue to achieve efficient light emission from silicon in the extending the silicon technology into fully integrated optoelectronic circuits. Here, we report the realization of room-temperature stimulated emission in the technologically crucial 1.5 micron wavelength range from Er-doped GaN multiple-quantum wells on silicon and sapphire. Employing the well-acknowledged variable stripe technique, we have demonstrated an optical gain up to 170 cm-1 in the multiple-quantum well structures. The observation of the stimulated emission is accompanied by the characteristic threshold behavior of emission intensity as a function of pump fluence, spectral linewidth narrowing and excitation length. The demonstration of room-temperature lasing at the minimum loss window of optical fibers and in the eye-safe wavelength region of 1.5 micron are highly sought-after for use in many applications including defense, industrial processing, communication, medicine, spectroscopy and imaging. As the synthesis of Er-doped GaN epitaxial layers on silicon and sapphire has been successfully demonstrated, the results laid the foundation for achieving hybrid GaN-Si lasers providing a new pathway towards full photonic integration for silicon optoelectronics.

physics.app-ph↗

arXiv2016

Photoluminescence quantum efficiency of Er optical centers in GaN epilayers

We report the quantum efficiency of photoluminescence processes of Er optical centers as well as the thermal quenching mechanism in GaN epilayers prepared by metal-organic chemical vapor deposition. High resolution infrared spectroscopy and temperature dependence measurements of photoluminescence intensity from Er ions in GaN under resonant excitation excitations were performed. Data provide a picture of the thermal quenching processes and activation energy levels. By comparing the photoluminescence from Er ions in the epilayer with a reference sample of Er-doped SiO2, we find that the fraction of Er ions that emits photon at 1.54 micron upon a resonant optical excitation is approximately 68%. This result presents a significant step in the realization of GaN:Er epilayers as an optical gain medium at 1.54 micron.

cond-mat.mtrl-sci↗

arXiv2015

Excitation Mechanisms of Er Optical Centers in GaN Epilayers

We report direct evidence of two mechanisms responsible for the excitation of optically active Er3+ ions in GaN epilayers grown by metal-organic chemical vapor deposition. These mechanisms, resonant excitation via the higher-lying inner 4f shell transitions and band-to-band excitation of the semiconductor host, lead to narrow emission lines from isolated and the defect-related Er centers. However, these centers have different photoluminescence spectra, decay dynamics, and excitation cross sections. The isolated Er optical center, which can be excited by either mechanism, has the same decay dynamics, but possesses a much higher cross-section under band-to-band excitation. In contrast, the defect-related Er center can only be excited through band-to-band excitation but has the largest cross-section. These results explain the difficulty in achieving gain in Er doped GaN and indicate new approaches for realization of optical amplification, and possibly lasing, at room temperature.

cond-mat.mtrl-sci↗

arXiv2011

Strong Substrate Dependence of Joule Heating in Graphene

The Joule heating effect on graphene electronic properties is investigated by using full-band Monte Carlo electron dynamics and three-dimensional heat transfer simulations self-consistently. A number of technologically important substrate materials are examined: SiO2, SiC, hexagonal BN, and diamond. The results illustrate that the choice of substrate has a major impact via the heat conduction and surface polar phonon scattering. Particularly, it is found that the poor thermal conductivity of SiO2 leads to significant Joule heating and saturation velocity degradation in graphene (characterized by the so-called 1/\surd n decay). Considering the overall characteristics, BN appears to compare favorably against other substrate choices for graphene in electronic applications.

cond-mat.mes-hall↗

arXiv2011

Weak ferromagnetism of antiferromagnetic domains in graphene with defects

Magnetic properties of graphene with randomly distributed magnetic defects/vacancies are studied in terms of the Kondo Hamiltonian in the mean field approximation. It has been shown that graphene with defects undergoes a magnetic phase transition from a paramagnetic to a antiferromagnetic (AFM) phase once the temperature reaches the critical point $T_{N}$. The defect straggling is taken into account as an assignable cause of multiple nucleation into AFM domains. Since each domain is characterized by partial compensating magnetization of the defects associated with different sublattices, together they reveal a super-paramagnetic behavior in a magnetic field. Theory qualitatively describe the experimental data provided the temperature dependence of the AFM domain structure.

cond-mat.mtrl-sci↗

arXiv2010

Electron-Phonon Interactions in Bilayer Graphene: A First Principles Approach

Density functional perturbation theory is used to analyze electron-phonon interaction in bilayer graphene. The results show that phonon scattering in bilayer graphene bears more resemblance with bulk graphite than monolayer graphene. In particular, electron-phonon scattering in the lowest conduction band is dominated by six lowest (acoustic and acoustic-like) phonon branches with only minor contributions from optical modes. The total scattering rate at low/moderate electron energies can be described by a simple two-phonon model in the deformation potential approximation with effective constants Dac $\approx$ 15 eV and Dop $\approx 2.8 \times 108$ eV/cm for acoustic and optical phonons, respectively. With much enhanced acoustic phonon scattering, the low field mobility of bilayer graphene is expected to be significantly smaller than that of monolayer graphene.

cond-mat.mes-hall↗

arXiv2010

Surface Polar Phonon Dominated Electron Transport in Graphene

The effects of surface polar phonons on electronic transport properties of monolayer graphene are studied by using a Monte Carlo simulation. Specifically, the low-field electron mobility and saturation velocity are examined for different substrates (SiC, SiO2, and HfO2) in comparison to the intrinsic case. While the results show that the low-field mobility can be substantially reduced by the introduction of surface polar phonon scattering, corresponding degradation of the saturation velocity is not observed for all three substrates at room temperature. It is also found that surface polar phonons can influence graphene electrical resistivity even at low temperature, leading potentially to inaccurate estimation of the acoustic phonon deformation potential constant.

cond-mat.mes-hall↗

arXiv2010

Influence of Carrier-Carrier Scattering on Electron Transport in Monolayer Graphene

The influence of electron-electron scattering on the distribution function and transport characteristics of intrinsic monolayer graphene is investigated via an ensemble Monte Carlo simulation. Due to the linear dispersion relation in the vicinity of the Dirac points, it is found that pair-wise collisions in graphene do not conserve the ensemble average velocity in contrast to conventional semiconductors with parabolic energy bands. Numerical results indicate that electron-electron scattering can lead to a decrease in the low field mobility by more than 80% for moderate electron densities. At high densities, the impact gradually diminishes due to increased degeneracy.

cond-mat.mes-hall↗

arXiv2010

Graphene spin capacitor for magnetic field sensing

An analysis of a novel magnetic field sensor based on a graphene spin capacitor is presented. The proposed device consists of graphene nanoribbons on top of an insulator material connected to a ferromagnetic source/drain. The time evolution of spin polarized electrons injected into the capacitor can be used for an accurate determination at room temperature of external magnetic fields. Assuming a spin relaxation time of 100 ns, magnetic fields on the order of $\sim 10$ mOe may be detected at room temperature. The observational accuracy of this device depends on the density of magnetic defects and spin relaxation time that can be achieved.

cond-mat.mes-hall↗

arXiv2010

Electron spin relaxation in carbon nanotubes

The long standing problem of inexplicably short spin relaxation in carbon nanotubes (CNTs) is examined. The curvature-mediated spin-orbital interaction is shown to induce fluctuating electron spin precession causing efficient relaxation in a manner analogous to the Dyakonov-Perel mechanism. Our calculation estimates longitudinal (spin-flip) and transversal (decoherence) relaxation times as short as 150 ps and 110 ps at room temperature, respectively, along with a pronounced anisotropic dependence. Interference of electrons originating from different valleys can lead to even faster dephasing. The results can help clarify the measured data, resolving discrepancies in the literature.

cond-mat.mtrl-sci↗

arXiv2009

First Principles Analysis of Electron-Phonon Interaction in Graphene

The electron-phonon interaction in monolayer graphene is investigated by using density functional perturbation theory. The results indicate that the electron-phonon interaction strength is of comparable magnitude for all four in-plane phonon branches and must be considered simultaneously. Moreover, the calculated scattering rates suggest an acoustic phonon contribution that is much weaker than previously thought, revealing the role of optical phonons even at low energies. Accordingly it is predicted, in good agreement with a recent measurement, that the intrinsic mobility of graphene may be more than an order of magnitude larger than the high values reported in suspended samples.

cond-mat.mes-hall↗

arXiv2009

Electrically Controlled Magnetic Memory and Programmable Logic based on Graphene/Ferromagnet Hybrid Structures

It has been shown that the combining of the electrical effect on the exchange bias field with giant magneto-resistance effect of the graphene/ferromagnet hybrid structures reveals a new non-volatile magnetic random access memory device conception. In such device an electric bias realizes the writing bits instead a magnetic field of remote word line with high energy consumption. Interplay of two graphene mediated exchange bias fields applied to different sides of free ferromagnet results in programable logic operations that depends on specific realization of the structure.

cond-mat.mtrl-sci↗

arXiv2008

Electrical Control of Exchange Bias Mediated by Graphene

The role of graphene in mediating the exchange interaction is theoretically investigated when it is placed between two ferromagnetic dielectric materials. The calculation based on a tight-binding model illustrates that the magnetic interactions at the interfaces affect not only the graphene band structure but also the thermodynamic potential of the system. This induces an indirect exchange interaction between the magnetic layers that can be considered in term of an effective exchange bias field. The analysis clearly indicates a strong dependence of the effective exchange bias on the properties of the mediating layer, revealing an effective mechanism of electrical control even at room temperature. This dependence also results in qualitatively different characteristics for the cases involving mono- and bilayer graphene.

cond-mat.mtrl-sci↗

arXiv2008

Ferromagnet proximity effects and magnetoresistance of bilayer graphene

A drastic modification of electronic band structure is predicted in bilayer graphene when it is placed between two ferromagnetic insulators. Due to the exchange interaction with the proximate ferromagnet, the electronic energy dispersion in the graphene channel strongly depends on the magnetization orientation of two ferromagnetic layers, $\mathbf{M_{1}}$ and $\mathbf{M_{2}} $. While the parallel configuration $\mathbf{M_{1}}= \mathbf{M_{2}}$ leads to simple spin splitting of both conduction and valence bands, an energy gap is induced as soon as the angle $θ$ between $\mathbf{M_{1}}$ and $% \mathbf{M_{2}}$ becomes non-zero with the maximum achieved at $θ=π$ (i.e., antiparallel alignment). Consequently, bilayer graphene may exhibit a sizable magnetoresistive effect in the current-in-plane configuration. A rough estimate suggests the resistance changes on the order of tens of percent at room temperature. This effect is expected to become more pronounced as the temperatures decreases.

cond-mat.mtrl-sci↗

arXiv2007

Graphene based spin field effect transistor

A spin field effect transistor (FET) is proposed by utilizing a graphene nanoribbon as the channel. Similar to the conventional spin FETs, the device involves ferromagnetic metals as a source and drain; they, in turn, are connected to the graphene channel. Due to the negligible spin-orbital coupling in the carbon based materials, the bias can accomplishes spin manipulation by means of electrical control of electron exchange interaction with a ferromagnetic dielectric attached to the nanoribbon between source and drain. The numerical estimations show the feasibility of graphene-based spin FET if a bias varies exchange interaction on the amount around 5 meV. It was shown that the device stability to the thermal dispersion can provide the armchair nanoribbons of specific width that keeps the Dirac point in electron dispersion law.

cond-mat.mtrl-sci↗

arXiv2002

Enhancement of hole injection for nitride-based light-emitting devices

A novel device design is proposed for a strong enhancement of hole injection current in nitride-based light-emitting heterostructures. Preliminary calculations show orders of magnitude increase in injected hole current when using the proposed superlattice hole injector device based on the real-space transfer concept.

cond-mat↗

arXiv2002

Laterally-doped heterostructures for III-N lasing devices

To achieve a high-density electron-hole plasma in group-III nitrides for efficient light emission, we propose a planar two-dimensional (2D) p-i-n structure that can be created in selectively-doped superlattices and quantum wells. The 2D p-i-n structure is formed in the quantum well layers due to efficient activation of donors and acceptors in the laterally doped barriers. We show that strongly non-equilibrium 2D electron-hole plasma with density above $10^{12} cm^{-2}$ can be realized in the i-region of the laterally biased p-i-n structure, enabling the formation of interband population inversion and stimulated emission from such a LAteral Current pumped Emitter (LACE). We suggest that implementation of the lateral p-i-n structures provides an efficient way of utilizing potential-profile-enhanced doping of superlattices and quantum wells for electric pumping of nitride-based lasers.

cond-mat↗