arXiv subjects

Zlatko Sitar

Publications and source records attributed to Zlatko Sitar.

4 recordsLinked to original sources

arXiv2026

High sub-bandgap response and fast switching enabled by thermal quenching in carbon-doped semi-insulating GaN

Carbon-doped GaN is a promising material for sub-bandgap triggered optical switches. When incorporated in GaN, carbon introduces deep compensating centers that enable defect-mediated extrinsic photoconductivity. Here, we investigate the optical responsivity and switching kinetics of semi-insulating carbon-doped GaN actuated by sub-bandgap blue illumination. A high ON/OFF ratio exceeding $\mathrm{10^7}$ is achieved under low-irradiance 405-nm excitation. Temperature-dependent transient measurements reveal that the photocurrent decay is thermally quenched above a crossover temperature of ~300 K. This behavior is attributed to hole-emission-assisted recombination. The extracted activation energies vary across samples; a commonly observed value of ~0.83 eV is attributed to the $\mathrm{C_N}$ defect. Notably, when heating above the crossover temperature, thermal quenching accelerates the photocurrent decay by up to a factor of five, enabling significantly faster switching.

cond-mat.mtrl-sci↗

arXiv2025

On the Origin of Carrier Loss in Mg-Doped N-Polar GaN

The neutral $(V_N-3Mg_{Ga})^0$ complex was found to be the primary compensator in Mg-doped, N-polar GaN. The experimental data showed a sharp drop in hole concentration once [Mg] exceeded ~$10^{19} cm^{-3}$. Temperature-dependent Hall measurements, in conjunction with a charge balance model, revealed that the carrier loss was due to a drastic reduction in acceptor concentration ($N_A$), suggesting that a significant fraction of Mg atoms incorporated in an electrically neutral configuration. A quantitative semi-empirical model based on the grand canonical formalism pointed to the formation of $(V_N-3Mg_{Ga})^0$ complexes as the primary cause for the observed carrier loss.

cond-mat.mtrl-sci↗

arXiv2017

Structure of Native Two-dimensional Oxides on III--Nitride Surfaces

When pristine material surfaces are exposed to air, highly reactive broken bonds can promote the formation of surface oxides with structures and properties differing greatly from bulk. Determination of the oxide structure, however, is often elusive through the use of indirect diffraction methods or techniques that probe only the outer most layer. As a result, surface oxides forming on widely used materials, such as group III-nitrides, have not been unambiguously resolved, even though critical properties can depend sensitively on their presence. In this work, aberration corrected scanning transmission electron microscopy reveals directly, and with depth dependence, the structure of native two--dimensional oxides that form on AlN and GaN surfaces. Through atomic resolution imaging and spectroscopy, we show that the oxide layers are comprised of tetrahedra--octahedra cation--oxygen units, similar to bulk $θ$--Al$_2$O$_3$ and $β$--Ga$_2$O$_3$. By applying density functional theory, we show that the observed structures are more stable than previously proposed surface oxide models. We place the impact of these observations in the context of key III-nitride growth, device issues, and the recent discovery of two-dimnesional nitrides.

cond-mat.mtrl-sci↗

arXiv2017

Probing collective oscillation of $d$-orbital electrons at the nanoscale

Here we demonstrate that high energy electrons can be used to explore the collective oscillation of $s$, $p$, and $d$ orbital electrons at the nanometer length scale. Using epitaxial AlGaN/AlN quantum wells as a test system, we observe the emergence of additional features in the loss spectrum with increasing Ga content. A comparison of the observed spectra with ab--initio theory reveals the origin of these spectral features is attributed to 3$d$--electrons contributed by Ga. We find that these modes differ in energy from the valence electron plasmons in Al$_{1-x}$Ga$_x$N due to the different polarizability of the $d$ electrons. Finally, we study the dependence of observed plasmon modes on Ga content, lending insight into plasmon coupling with electron--hole excitations.

cond-mat.mtrl-sci↗