Current-transport mechanisms in the AlInN/AlN/GaN single-channel and AlInN/AlN/GaN/AlN/GaN double-channel heterostructures

Current-transport mechanisms were investigated in Schottky contacts on AlInN/AlN/GaN single channel (SC) and AlInN/AlN/GaN/AlN/GaN double channel (DC) heterostructures. A simple model was adapted to the current-transport mechanisms in DC heterostructure. In this model, two Schottky diodes are in series: one is a metal-semiconductor barrier layer (AIInN) Schottky diode and the other is an equivalent Schottky diode, which is due to the heterojunction between the AlN and GaN layer. Capacitance-voltage studies show the formation of a two-dimensional electron gas at the AlN/GaN interface in the SC and the first AlN/GaN interface from the substrate direction in the DC. In order to determine the current mechanisms for SC and DC heterostructures, we fit the analytical expressions given for the tunneling current to the experimental current-voltage data over a wide range of applied biases as well as at different temperatures. We observed a weak temperature dependence of the saturation current and a fairly small dependence on the temperature of the tunneling parameters in this temperature range. At both a low and medium forward-bias voltage values for Schottky contacts on AlInN/AlN/GaN/AlN/GaN DC and AlInN/AlN/GaN SC heterostructures, the data are consistent with electron tunneling to deep levels in the vicinity of mixed/screw dislocations in the temperature range of 80-420 K. © 2013 Elsevier B.V. All rights reserved

Transport properties of epitaxial graphene grown on SiC substrate

In this study, the Hall effect measurement of graphene on SiC substrate was carried out as a function of temperature (12-300 K). Hall data were first analyzed to extract the temperature dependent mobilities and carrier densities of the bulk (3D) and two dimensional (2D) channels using a Simple Parallel Conduction Extraction Method (SPCEM) successfully. High carrier mobility 2.296 cm2/V.s from the graphene layer and low carrier mobility 813 cm2/V.s from the SiC were obtained at room temperature. By using SPCEM extracted data, 3D and 2D scattering mechanisms were analyzed and the dominant scattering mechanisms in low and high temperature regimes were determined. It was found that the transport was mainly determined by scattering processes in 2D graphene. © 2017, National Institute of Optoelectronics. All rights reserved

The effect of AIN interlayer thicknesses on scattering processes in lattice-matched AllnN/GaN two-dimensional electron gas heterostructures

The scattering mechanisms governing the transport properties of high mobility AllnN/AlN/GaN two-dimensional electron gas (2DEG) heterostructures with various AIN spacer layer thicknesses from zero to 2 nm were presented. The major scattering processes including acoustic and optical phonons, ionized impurity, interface roughness, dislocation and alloy disorder were applied to the temperature-dependent mobility data. It was found that scattering due mainly to alloy disorder limits the electron mobility for samples having spacer layer thicknesses up to 0.3 nm. On the other hand, alloy scattering is greatly reduced as the AlN spacer layer thickness increases further, and hence the combination of acoustic, optical and interface roughness become operative with different degrees of effectiveness over different temperature ranges. The room-temperature electron mobility was observed to increase gradually as the AlN spacer layer increases. A peak electron mobility of 1630 cm2 V-1s -1 was realized for the sample consisting of a 1 nm AlN spacer layer. Then, the electron mobility decreased for the sample with 2 nm AlN. Moreover, the measured 2DEG densities were also compared with the theoretical predictions, which include both piezoelectric and spontaneous polarization components existing at AlN/GaN interfaces. The experimental sheet carrier densities for all AllnN/AlN/GaN HEMT structures were found to be in excellent agreement with the theoretical predictions when the parasitic (unintentional) GaN layer deposited between AlN and AllnN was taken into account. From these analyses, 1 nm AlN spacer layer thickness is found to be the optimum thickness required for high electron mobility and hence low sheet resistance once the sheet carrier density is increased to the theoretically expected value for the sample without unintentional GaN layer. © IOP Publishing Ltd and Deutsche

Mobility limiting scattering mechanisms in nitride-based two-dimensional heterostructures with the InGaN channel

The scattering mechanisms limiting the carrier mobility in AlInN/AlN/InGaN/GaN two-dimensional electron gas (2DEG) heterostructures were investigated and compared with devices without InGaN channel. Although it is expected that InGaN will lead to relatively higher electron mobilities than GaN, Hall mobilities were measured to be much lower for samples with InGaN channels as compared to GaN. To investigate these observations the major scattering processes including acoustic and optical phonons, ionized impurity, interface roughness, dislocation and alloy disorder were applied to the temperature-dependent mobility data. It was found that scattering due mainly to interface roughness limits the electron mobility at low and intermediate temperatures for samples having InGaN channels. The room temperature electron mobilities which were determined by a combination of both optical phonon and interface roughness scattering were measured between 630 and 910 cm2 (V s)-1 with corresponding sheet carrier densities of 2.3-1.3 × 1013 cm-2. On the other hand, electron mobilities were mainly limited by intrinsic scattering processes such as acoustic and optical phonons over the whole temperature range for Al0.82In 0.18N/AlN/GaN and Al0.3Ga0.7N/AlN/GaN heterostructures where the room temperature electron mobilities were found to be 1630 and 1573 cm2 (V s)-1 with corresponding sheet carrier densities of 1.3 and 1.1 × 1013 cm-2, respectively. By these analyses, it could be concluded that the interfaces of HEMT structures with the InGaN channel layer are not as good as that of a conventional GaN channel where either AlGaN or AlInN barriers are used. It could also be pointed out that as the In content in the AlInN barrier layer increases the interface becomes smoother resulted in higher electron mobility. © 2010 IOP Publishing Ltd

The analysis of scattering mechanisms in GaN by relaxation time approximation and the comparison by the transport to quantum scattering time ratios

The effects of conventional scattering mechanisms on the electron Hall mobility in GaN are calculated and analysed. The ratios of the transport to quantum scattering time are also calculated and the ratio is evaluated in closed form without any fitting parameters. The common interpretation of the transport and quantum lifetime ratios by using the analytical equation solutions gives us dominant scattering mechanisms. It has been observed that the ratio is larger for dislocation scattering than for impurity scattering. These both results are compared and summarized that the Coulombic scattering from a charged dislocation core is more dominant than impurity scattering

Energy relaxations of hot electrons in AlGaN/AlN/GaN heterostructures grown by MOCVD on sapphire and 6H-SiC substrates

In this work, we investigated the hot-electron dynamics of AlGaN/GaN HEMT structures grown by MOCVD on sapphire and SiC substrates at 80 K. High-speed current-voltage measurements and Hall measurements over the temperature range 27–300 K were used to study hot-electron dynamics. At low fields, drift velocity increases linearly, but deviates from the linearity toward high electric fields. Drift velocities are deduced as approximately 6.55 × 106 and 6.60 × 106 cm/s at an electric field of around E ~ 25 kV/cm for samples grown on sapphire and SiC, respectively. To obtain the electron temperature as a function of the applied electric field and power loss as a function of the electron temperature, we used the so-called mobility comparison method with power balance equations. Although their low field carrier transport properties are similar as observed from Hall measurements, hot carrier energy dissipation differs for samples grown on sapphire and SiC substrates. We found that LO-phonon lifetimes are 0.50 ps and 0.32 ps for sapphire and SiC substrates, respectively. A long hot-phonon lifetime results in large non- equilibrium hot phonons. Non-equilibrium hot phonons slow energy relaxation and increase the momentum relaxation. The effective energy relaxation times at high fields are 24 and 65 ps for samples grown on sapphire and SiC substrates, respectively. They increase as the electron temperature decreases

Celiac disease and COVID-19 pandemic : should we worry?

Background and study aims : The Coronavirus Disease 2019 (COVID-19) epidemic especially worries people with chronic diseases the entire world. In this study, the frequency, and clinical course of COVID-19 infection in patients with Celiac disease (CD) were investigated. CD patients' adherence to purchasing gluten free products (GFPs), the strict diet, and how patients' anxiety affects CD symptoms during the COVID-19 outbreak were also examined.Patients and methods : A detailed-questionnaire was administered by telephone, and e-mail to the CD patients to determine the status of these patients in obtaining GFPs, and dietary compliance during the COVID-19 pandemic. State and trait anxiety levels of patients were evaluated using the State-trait Anxiety Inventory- (STAI) scale. Additionally, whether patients with CD were diagnosed with COVID-19, and if diagnosed, their clinical course of the disease were investigated.Results : One hundred and one patients were included in the study. The total number of patients who could obtain GFPs decreased significantly in the pandemic than before the pandemic. The patients' state anxiety index was 40.7 +/- 7.9, and the trait anxiety index was 44.5 +/- 8.5, and all patients were evaluated as mildly anxious. During the pandemic, two female patients were diagnosed with COVID-19.Conclusion : CD patients did not have any additional risk compared to other individuals in terms of becoming infected with COVID-19 for patients under gluten free diet, and these patients will have a similar clinical course as individuals without CD

Comparison of the transport properties of high quality AlGaN/AlN/GaN and AlInN/AlN/GaN two-dimensional electron gas heterostructures

The transport properties of high mobility AlGaN/AlN/GaN and high sheet electron density AlInN/AlN/GaN two-dimensional electron gas (2DEG) heterostructures were studied. The samples were grown by metal-organic chemical vapor deposition on c -plane sapphire substrates. The room temperature electron mobility was measured as 1700 cm2/V s along with 8.44× 10 12 cm-2 electron density, which resulted in a two-dimensional sheet resistance of 435 / for the Al0.2Ga 0.8 N/AlN/GaN heterostructure. The sample designed with an Al 0.88In0.12 N barrier exhibited very high sheet electron density of 4.23× 1013 cm-2 with a corresponding electron mobility of 812 cm2 /V s at room temperature. A record two-dimensional sheet resistance of 182 / was obtained in the respective sample. In order to understand the observed transport properties, various scattering mechanisms such as acoustic and optical phonons, interface roughness, and alloy disordering were included in the theoretical model that was applied to the temperature dependent mobility data. It was found that the interface roughness scattering in turn reduces the room temperature mobility of the Al0.88In 0.12 N/AlN/GaN heterostructure. The observed high 2DEG density was attributed to the larger polarization fields that exist in the sample with an Al0.88In0.12 N barrier layer. From these analyses, it can be argued that the AlInN/AlN/GaN high electron mobility transistors (HEMTs), after further optimization of the growth and design parameters, could show better transistor performance compared to AlGaN/AlN/GaN based HEMTs. © 2009 American Institute of Physics

The Effect of Gan Thickness Inserted Between Two Aln Layers on the Transport Properties of a Lattice Matched Alinn/Aln/Gan/Aln/Gan Double Channel Heterostructure

One AlInN/AlN/GaN single channel heterostructure sample and four AlInN/AlN/GaN/AlN/GaN double channel heterostructure samples with different values of the second GaN layer were studied. The interface profiles, crystalline qualities, surface morphologies, and dislocation densities of the samples were investigated using high resolution transmission electron microscopy, atomic force microscopy, and high-resolution X-ray diffraction. Some of the data provided by these measurements were used as input parameters in the calculation of the scattering mechanisms that govern the transport properties of the studied samples. Experimental transport data were obtained using temperature dependent Hall effect measurements (10-300 K) at low (0.5 T) and high (8 T) magnetic fields to exclude the bulk transport from the two-dimensional one. The effect of the thickness of the second GaN layer inserted between two AlN barrier layers on mobility and carrier concentrations was analyzed and the dominant scattering mechanisms in the low and high temperature regimes were determined. It was found that Hall mobility increases as the thickness of GaN increases until 5 nm at a low temperature where interface roughness scattering is observed as one of the dominant scattering mechanisms. When GaN thicknesses exceed 5 nm, Hall mobility tends to decrease again due to the population of the second channel in which the interface becomes worse compared to the other one. From these analyses, 5 nm GaN layer thicknesses were found to be the optimum thicknesses required for high electron mobility. (C) 2013 Published by Elsevier B.V.Wo