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Register a new userThermionic emission or tunneling? The universal transition electric field for ideal Schottky reverse leakage current in $beta$-Ga$_{2}$O$_{3}$
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The reverse leakage current through a Schottky barrier transitions from a thermionic-emission dominated regime to a barrier-tunneling dominated regime as the surface electric field increases. In this study, we evaluate such transition electric field ($E_{rm T}$) in $beta$-Ga$_{2}$O$_{3}$ using a numerical reverse leakage model. $E_{rm T}$ is found to have very weak dependence on the doping concentration and barrier height, thus a near-universal temperature dependence suffices and is given by a simple empirical expression in Ga$_{2}$O$_{3}$. With the help of a field-plate design, we observed experimentally in Ga$_{2}$O$_{3}$ Schottky barrier diodes a near-ideal bulk reverse leakage characteristics, which matches well with our numerical model and confirms the presence of the transition region. Near the transition electric field, both thermionic emission and barrier tunneling should be considered. The study provides important guidance toward accurate design and modeling of ideal reverse leakage characteristics in $beta$-Ga$_{2}$O$_{3}$ Schottky barrier diodes.
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Injection from metallic electrodes serves as a main channel of charge generation in organic semiconducting devices and the quantum effect is normally regarded to be essential. We develop a dynamic approach based upon the surface hopping (SH) algorithm and classical device modeling, by which both quantum tunneling and thermionic emission of charge carrier injection at metal/organic interfaces are concurrently investigated. The injected charges from metallic electrode are observed to quickly spread onto the organic molecules following by an accumulation close to the interface induced by the built-in electric field, exhibiting a transition from delocalization to localization. We compare the Ehrenfest dynamics on mean-field level and the SH algorithm by simulating the temperature dependence of charge injection dynamics, and it is found that the former one leads to an improper result that the injection efficiency decreases with increasing temperature at room-temperature regime while SH results are credible. The relationship between injected charges and the applied bias voltage suggests it is the quantum tunneling that dominates the low-threshold injection characteristics in molecular crystals, which is further supported by the calculation results of small entropy change during the injection processes. An optimum interfacial width for charge injection efficiency at the interface is also quantified and can be utilized to understand the role of interfacial buffer layer in practical devices.
Holding promising applications in power electronics, the wide band gap material gallium oxide has emerged as a vital alternative to materials like GaN and SiC. The detailed study of phonon modes in $beta$-Ga$_{2}$O$_{3}$ provides insights into fundamental material properties such as crystal structure and orientation and can contribute to the identification of dopants and point defects. We investigate the Raman active phonon modes of $beta$-Ga$_{2}$O$_{3}$ in two different oxygen isotope compositions ($^{16}$O,$^{18}$O) by experiment and theory: By carrying out polarized micro-Raman spectroscopy measurements on the (010) and ($bar{2}$01) planes, we determine the frequencies of all 15 Raman active phonons for both isotopologues. The measured frequencies are compared with the results of density functional perturbation theory (DFPT) calculations. In both cases, we observe a shift of Raman frequencies towards lower energies upon substitution of $^{16}$O with $^{18}$O. By quantifying the relative frequency shifts of the individual Raman modes, we identify the atomistic origin of all modes (Ga-Ga, Ga-O or O-O) and present the first experimental confirmation of the theoretically calculated energy contributions of O lattice sites to Raman modes. We find that oxygen substitution on the O$_{mathrm{II}}$ site leads to an elevated relative frequency shift compared to O$_{mathrm{I}}$ and O$_{mathrm{III}}$ sites. This study presents a blueprint for the future identification of different point defects in Ga$_{2}$O$_{3}$ by Raman spectroscopy.
Experimental observations have long-established that there exists a smooth roll-off or knee transition between the temperature-limited (TL) and full-space-charge-limited (FSCL) emission regions of the emission current density-temperature J-T (Miram) curve, or the emission current density-voltage J-V curve for a thermionic emission cathode. In this paper, we demonstrate that this experimentally observed smooth transition does not require frequently used a priori assumptions of a continuous distribution of work functions on the cathode surface. Instead, we find the smooth transition arises as a natural consequence of the physics of nonuniform thermionic emission from a spatially heterogeneous cathode surface. We obtain this smooth transition for both J-T and J-V curves using a predictive nonuniform thermionic emission model that includes 3-D space charge, patch fields (electrostatic potential nonuniformity on the cathode surface based on local work function values), and Schottky barrier lowering physics and illustrate that a smooth knee can arise from a thermionic cathode surface with as few as two discrete work function values. Importantly, we find that the inclusion of patch field effects is crucial for obtaining accurate J-T and J-V curves, and the further inclusion of Schottky barrier lowering is needed for accurate J-V curves. This finding, and the emission model provided in this paper have important implications for modeling electron emission from realistic, heterogeneous surfaces. Such modeling is important for improved understanding of the interplay of emission physics, cathode materials engineering, and design of numerous devices employing electron emission cathodes.
We investigated the origin of the high reverse leakage current in light emitting diodes (LEDs) based on (In,Ga)N/GaN nanowire (NW) ensembles grown by molecular beam epitaxy on Si substrates. To this end, capacitance deep level transient spectroscopy (DLTS) and temperature-dependent current-voltage (I-V) measurements were performed on a fully processed NW-LED. The DLTS measurements reveal the presence of two distinct electron traps with high concentrations in the depletion region of the p-i-n junction. These band gap states are located at energies of $570pm20$ and $840pm30$ meV below the conduction band minimum. The physical origin of these deep level states is discussed. The temperature-dependent I-V characteristics, acquired between 83 and 403 K, show that different conduction mechanisms cause the observed leakage current. On the basis of all these results, we developed a quantitative physical model for charge transport in the reverse bias regime. By taking into account the mutual interaction of variable range hopping and electron emission from Coulombic trap states, with the latter being described by phonon-assisted tunnelling and the Poole-Frenkel effect, we can model the experimental I-V curves in the entire range of temperatures with a consistent set of parameters. Our model should be applicable to planar GaN-based LEDs as well. Furthermore, possible approaches to decrease the leakage current in NW-LEDs are proposed.
$beta$-Ga$_2$O$_3$ is a next-generation ultra wide bandgap semiconductor (E$_g$ = 4.8 eV to 4.9 eV) that can be homoepitaxially grown on commercial substrates, enabling next-generation power electronic devices among other important applications. Analyzing the quality of deposited homoepitaxial layers used in such devices is challenging, in part due to the large probing depth in traditional x-ray diffraction (XRD) and also due to the surface-sensitive nature of atomic force microscopy (AFM). Here, a combination of evanescent grazing-incidence skew asymmetric XRD and AFM are investigated as an approach to effectively characterize the quality of homoepitaxial $beta$-Ga$_2$O$_3$ layers grown by molecular beam epitaxy at a variety of Ga/O flux ratios. Accounting for both structure and morphology, optimal films are achieved at a Ga/O ratio of $sim$1.15, a conclusion that would not be possible to achieve by either XRD or AFM methods alone. Finally, fabricated Schottky barrier diodes with thicker homoepitaxial layers are characterized by $J-V$ and $C-V$ measurements, revealing an unintentional doping density of 4.3 $times$ 10$^{16}$ cm$^{-3}$ - 2 $times$ 10$^{17}$ cm$^{-3}$ in the epilayer. These results demonstrate the importance of complementary measurement methods for improving the quality of the $beta$-Ga$_2$O$_3$ homoepitaxial layers used in power electronic and other devices.
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