arXiv subjects

John S. Jamison

Publications and source records attributed to John S. Jamison.

4 recordsLinked to original sources

arXiv2020

Deep-recessed $β$-Ga$_2$O$_3$ delta-doped field effect transistors with in situ epitaxial passivation

We introduce a deep-recessed gate architecture in $β$-Ga$_2$O$_3$ delta-doped field effect transistors for improvement in DC-RF dispersion and breakdown properties. The device design incorporates an unintentionally doped $β$-Ga$_2$O$_3$ layer as the passivation dielectric. To fabricate the device, the deep-recess geometry was developed using BCl$_3$ plasma based etching at ~5 W RIE to ensure minimal plasma damage. Etch damage incurred with plasma etching was mitigated by annealing in vacuum at temperatures above 600 $°$C. A gate-connected field-plate edge termination was implemented for efficient field management. Negligible surface dispersion with lower knee-walkout at high V$_\mathrm{DS}$, and better breakdown characteristics compared to their unpassivated counterparts were achieved. A three terminal off-state breakdown voltage of 315 V, corresponding to an average breakdown field of 2.3 MV/cm was measured. The device breakdown was limited by the field-plate/passivation edge and presents scope for further improvement. This demonstration of epitaxially passivated field effect transistors is a significant step for $β$-Ga$_2$O$_3$ technology since the structure simultaneously provides control of surface-related dispersion and excellent field management.

cond-mat.mtrl-sci↗

arXiv2019

Ferromagnetic Epitaxial μ-Fe$_{2}$O$_{3}$ on β-Ga$_{2}$O$_{3}$: A New Monoclinic form of Fe$_{2}$O$_{3}$

Here we demonstrate a new monoclinic iron oxide phase (μ-Fe$_{2}$O$_{3}$), epitaxially stabilized by growth on (010) β-Ga$_{2}$O$_{3}$. Density functional theory (DFT) calculations find that the lattice parameters of freestanding μ-Fe$_{2}$O$_{3}$ are within ~1% of those of β-Ga$_{2}$O$_{3}$ and that its energy of formation is comparable to that of naturally abundant Fe$_{2}$O$_{3}$ polytypes. A superlattice of μ-Fe$_{2}$O$_{3}$/β-Ga$_{2}$O$_{3}$ is grown by plasma assisted molecular beam epitaxy, with resulting high-resolution x-ray diffraction (XRD) measurements indicating that the μ-Fe$_{2}$O$_{3}$ layers are lattice-matched to the substrate. The measured out-of-plane (b) lattice parameter of 3.12 $\pm$ 0.4 Å is in agreement with the predicted lattice constants and atomic-resolution scanning transmission electron microscopy (STEM) images confirm complete registry of the μ-Fe$_{2}$O$_{3}$ layers with β-Ga$_{2}$O$_{3}$. Finally, DFT modeling predicts that bulk μ-Fe$_{2}$O$_{3}$ is antiferromagnetic, while the interface region between μ-Fe$_{2}$O$_{3}$ and β-Ga$_{2}$O$_{3}$ leads to ferromagnetic coupling between interface Fe$^{3+}$ cations selectively occupying tetrahedral positions. Magnetic hysteresis persisting to room temperature is observed via SQUID measurements, consistent with the computationally predicted interface magnetism.

cond-mat.mtrl-sci↗

arXiv2019

Long lifetime of thermally-excited magnons in bulk yttrium iron garnet

Spin currents are generated within the bulk of magnetic materials due to heat flow, an effect called intrinsic spin-Seebeck. This bulk bosonic spin current consists of a diffusing thermal magnon cloud, parametrized by the magnon chemical potential ($μ_{m}$), with a diffusion length of several microns in yttrium iron garnet (YIG). Transient opto-thermal measurements of the spin-Seebeck effect (SSE) as a function of temperature reveal the time evolution of $μ_{m}$ due to intrinsic SSE in YIG. The interface SSE develops at times < 2 ns while the intrinsic SSE signal continues to evolve at times > 500 $μ$s, dominating the temperature dependence of SSE in bulk YIG. Time-dependent SSE data are fit to a multi-temperature model of coupled spin/heat transport using finite element method (FEM), where the magnon spin lifetime ($τ$) and magnon-phonon thermalization time ($τ_{mp}$) are used as fit parameters. From 300 K to 4 K, $τ_{mp}$ varies from 1 to 10 ns, whereas $τ$ varies from 2 to 60 $μ$s with the spin lifetime peaking at 90 K. At low temperature, a reduction in $τ$ is observed consistent with impurity relaxation reported in ferromagnetic resonance measurements. These results demonstrate that the thermal magnon cloud in YIG contains extremely low frequency magnons (~10 GHz) providing spectral insight to the microscopic scattering processes involved in magnon spin/heat diffusion.

cond-mat.mtrl-sci↗

arXiv2017

Thermally Driven Long Range Magnon Spin Currents in Yttrium Iron Garnet due to Intrinsic Spin Seebeck Effect

The longitudinal spin Seebeck effect refers to the generation of a spin current when heat flows across a normal metal/magnetic insulator interface. Until recently, most explanations of the spin Seebeck effect use the interfacial temperature difference as the conversion mechanism between heat and spin fluxes. However, recent theoretical and experimental works claim that a magnon spin current is generated in the bulk of a magnetic insulator even in the absence of an interface. This is the so-called intrinsic spin Seebeck effect. Here, by utilizing a non-local spin Seebeck geometry, we provide additional evidence that the total magnon spin current in the ferrimagnetic insulator yttrium iron garnet (YIG) actually contains two distinct terms: one proportional to the gradient in the magnon chemical potential (pure magnon spin diffusion), and a second proportional to the gradient in magnon temperature ($\nabla T_m$). We observe two characteristic decay lengths for magnon spin currents in YIG with distinct temperature dependences: a temperature independent decay length of ~ 10 $μ$m consistent with earlier measurements of pure ($\nabla T_m = 0$) magnon spin diffusion, and a longer decay length ranging from about 20 $μ$m around 250 K and exceeding 80 $μ$m at 10 K. The coupled spin-heat transport processes are modeled using a finite element method revealing that the longer range magnon spin current is attributable to the intrinsic spin Seebeck effect ($\nabla T_m \neq 0$), whose length scale increases at lower temperatures in agreement with our experimental data.

cond-mat.mtrl-sci↗