In this study, a report is prepared on significantly low specific contact resistivity of alloyed and non-alloyed ohmic contacts fabricated on an as-grown n+-GaN layer and measured with the transfer length method. A low rho c = 8 x 10-8 Omega cm2 is extracted for the alloyed Ti/Al/Ni/Au, and rho c = 4 x 10-7 Omega cm2 for the unannealed Ti/Pd/Au. To achieve these, a highly doped n+-GaN layer with ND = 1.5 x 1019 cm-3 is used. The results are derived from a study of three different metal contact stacks, namely Ti/Al/Ni/Au (20 nm/300 nm/20 nm/400 nm), Ti/Pd/Au (2 nm/5 nm/200 nm), and Mo/Au (30 nm/200 nm). The Ti/Al/Ni/Au metal contact is studied in both annealed and non-annealed conditions, whereas for the Ti/Pd/Au and Mo/Au ohmic contacts, a study is conducted without annealing. Their performance and thermal stability are evaluated with a four-probe TLM, with temperatures ranging from 25 to 150 degrees C. Finally, a theoretical model based on thermionic emission theory is employed to gain a deeper understanding of the physical mechanisms governing the behavior of the ohmic contacts.

Herein, it is shown that a fully vertical GaN FinFET can reach close to ideal Baliga's figure of merit (BFOM) for thicker drift layers. Devices with drift layers between 1 and 8 mu m are simulated using TCAD, and the device geometry is varied to find optimal device performance. For 8 mu m drift layers, device operation at the BFOM limit is found with VB = 1550 V and Ron,sp = 0.53 m Omega cm2. An optimal switching FOM at 2.36 nOC is found.

Assessing short channel effects (SCEs) is crucial in the high-frequency optimization of downscaled field-effect transistors (FETs) such as GaN high electron mobility transistors (HEMTs). Drain-induced barrier lowering (DIBL) is commonly used for quantifying the ability of the gate to modulate the drain-source current at high drain voltages. DIBL is traditionally extracted from the relative shift of the threshold voltage at different drain-source voltages. In this article, we propose a new method based on a drain current injection technique (DCIT) to assess DIBL. This method facilitates a direct measure of the threshold voltage over a wide range of drain-source voltages in a single measurement. The method is demonstrated and compared to the conventional method using AlGaN/GaN and InAlGaN HEMTs with a Fe-doped buffer and a C-doped AlGaN back-barrier, respectively. Furthermore, the impact of different gate lengths and GaN channel layer thicknesses is presented. The measurements are analyzed and discussed with supporting technology computer-aided design (TCAD) simulations. The proposed method facilitates a more general and detailed measurement of the DIBL for HEMTs.

The impact of different carbon concentrations in the Al 0.06 Ga 0.94 N graded back-barrier and GaN buffer of high electron mobility transistors (HEMTs) is investigated. Four epi-wafers with different carbon concentrations, ranging from 1 x 10(17) to 5 x 10(17) cm( -3) , were grown by metal organic chemical vapor deposition (MOCVD). HEMTs with 100 and 200 nm gate lengths were fabricated and characterized with dc, Pulsed-IV, drain current transient spectroscopy (DCTS), and large-signal measurements at 30 GHz. It is shown that the back-barrier effectively prevents buffer-related electron trapping. The highest C-doping provides the best 2DEG confinement, while lower carbon doping levels are beneficial for a high output power and efficiency. A C-doping of 1 x 10(17)cm( -3) offers the highest output power at maximum power added efficiency (PAE) (1.8 W/mm), whereas 3 x 10(17) cm( -3) doping provides the highest PAE ( > 40%). The C-profiles acquired by using secondary ion mass spectroscopy (SIMS), in combination with DCTS, is used to explain the electron trapping effects. Traps associated with the C-doping in the back-barrier are identified and the bias ranges for the trap activation are discussed. The study shows the importance of considering the C-doping level in the back-barrier of microwave GaN HEMTs for power amplification and generation.

Modest channel carrier mobility in SiC-MOSFETs with NO annealed gate oxides has been the main factor hampering development of low power devices (300 – 650 V). A very fast interface trap, noted as NI, has been suggested to be the main culprit for poor inversion channel carrier mobility. The origin of the NI trap is unknown, but it is likely a property of the SiO2 and it is enhanced during post nitridation. In this study we show that the NI trap is also detected in 4H-SiC/AlN and 4H-SiC/Al2O3 MIS-capacitors. Observations are done using conductance spectroscopy and capacitance voltage measurements at cryogenic temperatures. This strongly suggests that the NI trap is a property of the SiC surface and not the dielectric used to form the SiC/dielectric interface. Furthermore, a scanning transmission electron microscopy (STEM) was performed to confirm that there are no SiO2 layers or islands present at the 4H-SiC/AlN and 4H-SiC/Al2O3 interfaces.

Electron effective mass is a fundamental material parameter defining the free charge carrier transport properties, but it is very challenging to be experimentally determined at high temperatures relevant to device operation. In this work, we obtain the electron effective mass parameters in a Si-doped GaN bulk substrate and epitaxial layers from terahertz (THz) and mid-infrared (MIR) optical Hall effect (OHE) measurements in the temperature range of 38-340 K. The OHE data are analyzed using the well-accepted Drude model to account for the free charge carrier contributions. A strong temperature dependence of the electron effective mass parameter in both bulk and epitaxial GaN with values ranging from (0.18 +/- 0.02) m(0) to (0.34 +/- 0.01) m(0) at a low temperature (38 K) and room temperature, respectively, is obtained from the THz OHE analysis. The observed effective mass enhancement with temperature is evaluated and discussed in view of conduction band nonparabolicity, polaron effect, strain, and deviations from the classical Drude behavior. On the other hand, the electron effective mass parameter determined by MIR OHE is found to be temperature independent with a value of (0.200 +/- 0.002) m(0). A possible explanation for the different findings from THz OHE and MIR OHE is proposed. (c) 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/)

Polar dielectrics are key materials of interest for infrared (IR) nanophotonic applications due to their ability to host phonon-polaritons that allow for low-loss, subdiffractional control of light. The properties of phonon-polaritons are limited by the characteristics of optical phonons, which are nominally fixed for most "bulk" materials. Superlattices composed of alternating atomically thin materials offer control over crystal anisotropy through changes in composition, optical phonon confinement, and the emergence of new modes. In particular, the modified optical phonons in superlattices offer the potential for so-called crystalline hybrids whose IR properties cannot be described as a simple mixture of the bulk constituents. To date, however, studies have primarily focused on identifying the presence of new or modified optical phonon modes rather than assessing their impact on the IR response. This study focuses on assessing the impact of confined optical phonon modes on the hybrid IR dielectric function in superlattices of GaSb and AlSb. Using a combination of first principles theory, Raman, FTIR, and spectroscopic ellipsometry, the hybrid dielectric function is found to track the confinement of optical phonons, leading to optical phonon spectral shifts of up to 20 cm-1. These results provide an alternative pathway toward designer IR optical materials. Optical phonons are known to become confined when the size of the host crystal approaches atomic limits. This confinement offers a unique yet underexplored pathway toward the modification and design of optical phonons in tailored atomic-scale devices. This study sheds light on the criteria for, and ramifications of phonon confinement, paving the way for designer phonon applications.image

In this work, we present the fabrication and investigation of the properties of quasi-vertical gallium nitride (GaN) fin field effect transistors (FinFETs) on silicon carbide (SiC) substrates and the influence of a postgate metallization annealing (PMA). The devices reveal low subthreshold swings (SSs) down to around 70 mV/dec. For a 1- μm -thick drift layer, a low ON-resistance below 0.05 mΩ⋅ cm2 (normalized on the fin area) and a breakdown voltage of 60 V were obtained. Devices with included PMA show a decreased threshold voltage and ON-resistance and by several orders of magnitude reduced gate leakage current compared to non-annealed devices. The devices show ohmic contact behavior and slightly negative threshold voltages, which indicates normally- ON behavior. The effective and field-effect mobility of the fin channel was obtained with a modeled carrier concentration and reveal to around 70 and 13 cm2/(Vs) at high gate voltages, which is in a good comparison to so far reported similar devices.

Nanostructures of conventional metals offer manipulation of light at the nanoscale but are largely limited to static behavior due to fixed material properties. To develop the next frontier of dynamic nano-optics and metasurfaces, this study utilizes the redox-tunable optical properties of conducting polymers, as recently shown to be capable of sustaining plasmons in their most conducting oxidized state. Electrically tunable conducting polymer nano-optical antennas are presented, using nanodisks of poly(3,4-ethylenedioxythiophene:sulfate) (PEDOT:Sulf) as a model system. In addition to repeated on/off switching of the polymeric nanoantennas, the concept enables gradual electrical tuning of the nano-optical response, which was found to be related to the modulation of both density and mobility of the mobile polaronic charge carriers in the polymer. The resonance position of the PEDOT:Sulf nanoantennas can be conveniently controlled by disk size, here reported down to a wavelength of around 1270 nm. The presented concept may be used for electrically tunable metasurfaces, with tunable farfield as well as nearfield. The work thereby opens for applications ranging from tunable flat meta-optics to adaptable smart windows.

The hot-wall metalorganic chemical vapor deposition (MOCVD) concept, previously shown to enable superior material quality and high performance devices based on wide bandgap semiconductors, such as Ga(Al)N and SiC, has been applied to the epitaxial growth of beta-Ga2O3. Epitaxial beta-Ga2O3 layers at high growth rates (above 1 mu m/h), at low reagent flows, and at reduced growth temperatures (740 degrees C) are demonstrated. A high crystalline quality epitaxial material on a c-plane sapphire substrate is attained as corroborated by a combination of x-ray diffraction, high-resolution scanning transmission electron microscopy, and spectroscopic ellipsometry measurements. The hot-wall MOCVD process is transferred to homoepitaxy, and single-crystalline homoepitaxial beta-Ga2O3 layers are demonstrated with a 201 rocking curve width of 118 arc sec, which is comparable to those of the edge-defined film-fed grown (201) beta-Ga2O3 substrates, indicative of similar dislocation densities for epilayers and substrates. Hence, hot-wall MOCVD is proposed as a prospective growth method to be further explored for the fabrication of beta-Ga2O3.