Vg Measurements Research Articles

Evidence of charged interface states limited scattering in GaN heterostructures

This study provides experimental evidence of charged interface states limited scattering in III-nitride heterostructures. Temperature-dependent Hall measurements and temperature-dependent ID–VG measurements indicate a significant influence of the charged interface states on the electron mobility in different AlGaN/GaN heterostructures where the characteristic of the interface is controlled by modulating the growth conditions. Charged interface states at the AlGaN/GaN heterointerface lead to electron scattering as the distance between the centroid of the two-dimensional electron gas and the interface decreases with increasing electron density. It is observed that a component of experimental Hall mobility, which ranges between 9.2 × 103 and 3.4 × 104 cm2/V s among the three samples, obtained after adding all the scattering events using Matthiessen's rule cannot be explained completely by considering all the conventional scattering mechanisms such as phonon–phonon scattering, interface roughness scattering, and dislocation density scattering. An in-depth analysis reveals a significant scattering of channel electrons by the charged states at the GaN/AlN/AlGaN interface. Furthermore, the estimated interface states from the temperature-dependent subthreshold slopes conducted on the fabricated high electron mobility transistors are in good agreement with the charged interface states extracted from the temperature-dependent Hall measurements. A good understanding on this new scattering mechanism in the GaN heterostructure may help in designing high-performance III-nitride devices in the future.

In vivo assessment of the mechanical properties of crystalline lenses in a rabbit model using ultrasound elastography: Effects of ultrasound frequency and age

PurposeThe occurrence of presbyopia and cataract is closely related to changes in the mechanical properties of the crystalline lens. There are no established methods so far for in vivo assessment. By introducing ultrasound elastography, we proposed group velocity (Vg) of an induced shear wave as a new biomarker to characterize the mechanical properties of the lens in our previous study. Here, we investigated the effect of the ultrasound frequency on measurement accuracy and validated the results with a conventional ex vivo compression testing. We also demonstrated a change trend in Vg and its correlation with age in a rabbit model. MethodsEight New Zealand white rabbits were fed normally from the fourth to seventh month. An ultrasound elastography system was developed to measure Vgin vivo on every eye once per month. The performances when using a high-frequency (L22-11v) and low-frequency (L11-4v) probe were compared. Rabbits were sacrificed after in vivo measurements by the end of the seventh month and this was followed by ex vivo ultrasound measurements and conventional compression tests on the extracted lenses. ResultsThe results demonstrated that there were no significance differences in Vg between measurements with high-frequency (USE-HF) and low-frequency (USE-LF) probes in the same month-age group. The mean Vg and the standard deviation of four rabbits that were 7 months old were 2.37 ± 0.24 m/s, 2.36 ± 0.25 m/s, 2.43 ± 0.26 m/s and 2.44 ± 0.38 m/s, with USE-HF for ex vivo and in vivo measurements and USE-LF for ex vivo and in vivo measurements, respectively. There were no significant differences (p > 0.05) and they were all in agreement with the results of compression tests, which was 16.16 ± 1.84 kPa in Young's modulus. The results also showed that Vg increased with age. In combination with the results of our previous study, Vg showed a relatively sharp increase from 2 to 5 months, while it had a slight increase from 5 to 7 months. ConclusionsThe USE-HF and USE-LF has comparable accuracy in Vg measurements while USE-HF had an advantage regarding better spatial resolution. The change trend of Vg was in accord with the growth phase of New Zealand white rabbits, which usually results in sexual maturity at 5 months old. This implies that Vg can be used as a biomarker parameter for evaluating the mechanical properties of the lens undergoing physiological changes.

Tuning the electronic structure of single-walled carbon nanotube by high-pressure H2 exposure

We report on an electronic structure change of single-walled carbon nanotube (SWNT) on hexagonal boron nitride due to electron doping via high-pressure H2 exposure. The fractional coverage of hydrogenated carbon atom is estimated to be at least θ = 0.163 from the in situ Ids–Vg measurements of the release process. Raman spectroscopy and x-ray photoelectron spectroscopy were carried out to support the in situ electrical measurements. In particular, we used the dissociative Langmuir-type model to yield the desorption coefficient kdes by fitting it to the in situ electrical data. Finally, we applied this hydrogenation method to the SWNT network on the commercial Si/SiO2 substrate to open the possibility of the scalable n-type semiconducting SWNT FETs.

Gate modeling of metal–insulator–semiconductor devices based on ultra-thin atomic-layer deposited TiO2

Metal–insulator–semiconductor devices having different oxide thicknesses (10, 6, 4 and 2 nm) were fabricated using atomic-layer deposited ultra-thin amorphous TiO2 as gate dielectric. From Ig–Vg measurements it was determined that the main conduction mechanism is Schottky emission for all thicknesses and even after passivation of the semiconductor–insulator interface using SiOx. Furthermore, the Schottky barrier height (ΦB) increases when the oxide thickness decreases; this was further corroborated using semi empirical models and SILVACO simulations having excellent agreement. From this analysis, important physical parameters like barrier height (ΦB), effective mass (m*) and optical dielectric constant (er) were extracted, and could be used to effectively understand the performance and reliability of these devices. From the extraction of physical parameters associated to the conduction mechanism, a correlation between materials’ properties with device performance could be obtained (higher barrier height (ΦB) would result in a decrease in leakage current). Also, the high reproducibility enables enhanced performance and therefore, better reliability predictions for electron devices based on these oxides.

Parameter extraction of gate tunneling current in metal–insulator–semiconductor capacitors based on ultra-thin atomic-layer deposited Al2O3

Metal–insulator–semiconductor devices were fabricated using ultra-thin (6 nm) atomic layer deposited Al2O3. From Ig–Vg measurements, it was determined that the main conduction mechanisms for these devices are Ohmic conduction at very low electric fields (E 2 MV cm−1) and finally, just before breakdown, Fowler–Nordheim (E > 5 MV cm−1). From the accurate verification of these conduction mechanisms, physical parameters such as barrier height (ΦB), effective mass (m*) and energy trap level (ΦT) are extracted and could be used to effectively understand the performance and reliability of these devices under different substrate injection conditions.

Dependence of electron mobility on gate voltage sweeping width and deposition temperature in MOSFETs with HfO2/Al2O3/InGaAs gate stacks

In this study, we fabricated MOSFETs with Al2O3/InGaAs or HfO2/Al2O3/InGaAs gate stacks. The surface was subjected to nitrogen plasma and trimethylaluminum cleaning prior to low-temperature atomic layer deposition. Electron mobility was extracted using the capacitance–gate voltage (C–VG) and drain current–gate voltage (ID–VG) characteristics. We determined that the mobility decreased when the gate voltage sweeping width increased during C–VG and ID–VG measurements. In addition, we determined that the lowering of the deposition temperature to 120 °C improved the mobility of MOSFETs with HfO2/Al2O3/InGaAs gate stacks as compared with that corresponding to deposition at 300 °C. Furthermore, HfO2/Al2O3/InGaAs gate stacks with various Al2O3 thicknesses were fabricated. When the number of Al2O3 deposition cycles was more than 4, the mobility of MOSFETs with HfO2/Al2O3/InGaAs gate stacks improved, reaching the value of the Al2O3/InGaAs gate stack.

Modified model of gate leakage currents in AlGaN/GaN HEMTs**Project supported by the National Natural Science Foundation of China (Grant No. 61306113).

It has been reported that the gate leakage currents are described by the Frenkel–Poole emission (FPE) model, at temperatures higher than 250 K. However, the gate leakage currents of our passivated devices do not accord with the FPE model. Therefore, a modified FPE model is developed in which an additional leakage current, besides the gate (III), is added. Based on the samples with different passivations, the III caused by a large number of surface traps is separated from total gate currents, and is found to be linear with respect to (φB−Vg)0.5. Compared with these from the FPE model, the calculated results from the modified model agree well with the Ig–Vg measurements at temperatures ranging from 295 K to 475 K.

Investigation of surface charges and traps in gallium nitride/aluminium gallium nitride/gallium nitride high‐voltage transistors via measurements and technology computer‐aided design simulations of transfer characteristics of metal–insulator–semiconductor field‐effect transistors and high‐electron‐mobility transistors

This study presents a detailed analysis of the Plasma-enhanced chemical vapour deposition (PECVD) silicon-nitride/semiconductor interface of gallium nitride (GaN) transistors through the study of the transfer characteristics of a large-gate-area metal-insulator-semiconductor field-effect transistor (MISFET). Id –Vg measurements were performed on several MISFETs across the wafer and for all of them the authors observed strong hysteresis between forward and reverse sweeps and a double kink. These features indicate the presence of traps beneath the gate electrode. Neither the hysteresis nor the kinks were seen in the measured high-electron-mobility transistor (HEMT) characteristics suggesting that the passivation/semiconductor interface is electrically responsible for them. The transfer characteristics of the MISFET have been reproduced using a technology computer-aided design (TCAD) deck that includes fixed charges and donor traps at the passivation/semiconductor interface. The impact of these charges on the Id –Vg and their influence on the formation of a surface-inversion layer is here explained through extensive TCAD simulations. This study has also been extended to different temperatures between 35 and 75°C to investigate the change in the transfer characteristics at elevated temperatures. It is shown that the hysteresis observed between forward and reverse sweeps and the transconductance decrease significantly with increasing temperature.

Full gate voltage range Lambert-function based methodology for FDSOI MOSFET parameter extraction

A new full gate voltage range methodology using a Lambert W function based inversion charge model, for extracting the electrical parameters in FDSOI nano-MOSFET devices, has been developed. Split capacitance–voltage measurements carried out on 14nm technology FDSOI devices show that the inversion charge variation with gate voltage can be well described by a Lambert W function. Based on the drain current equation in the linear region including the inversion charge described by the Lambert function of gate voltage and the standard mobility equation enables five electrical MOSFET parameters to be extracted from experimental Id–Vg measurements (ideality factor, threshold voltage, low field mobility, first and second order mobility attenuation factors). The extracted parameters were compared with those extracted by the well-known Y-function in strong inversion region. The present methodology for extracting the electrical MOSFET parameters was verified over a wide range of channel lengths on nano-scale FDSOI devices, demonstrating its simplicity, accuracy and robustness.

Accurate surface potential determination in Schottky diodes by the use of a correlated current and capacitance voltage measurements. Application to n-InP

Based on current voltage (I—Vg) and capacitance voltage (C—Vg) measurements, a reliable procedure is proposed to determine the effective surface potential Vd(Vg) in Schottky diodes. In the framework of thermionic emission, our analysis includes both the effect of the series resistance and the ideality factor, even voltage dependent. This technique is applied to n-type indium phosphide (n-InP) Schottky diodes with and without an interfacial layer and allows us to provide an interpretation of the observed peak on the C—Vg measurements. The study clearly shows that the depletion width and the flat band barrier height deduced from C—Vg, which are important parameters directly related to the surface potential in the semiconductor, should be estimated within our approach to obtain more reliable information.

Quantum mechanical modeling of gate capacitance and gate current in tunnel dielectric stack structures for nonvolatile memory application

Multilayered dielectric stack structures, with a layered or crested potential profile, have been proposed for use as the tunnel dielectric of nonvolatile memories for fast low-voltage programming and longer charge retention. In this work, self-consistent quantum mechanical (QM) numerical calculations, using an in-house developed charge quantization simulation program, were conducted to analyze the gate tunneling current and capacitance of metal–insulator–semiconductor (MIS) devices with tunnel dielectric stack structures. The self-consistent QM simulator takes into account polysilicon depletion, quantization effects on the carrier density, and wave penetration effects. The gate current density–gate voltage (Jg–Vg) simulation uses a recursive method for calculating the transmission probability through the dielectric stack structure. The physical model was used to fit with capacitance–voltage and Jg–Vg measurements on MIS devices with different single-layer dielectric and multilayered dielectric stack structures. The simulation of the Jg–Vg characteristics of a layered-barrier structure of HfO2/Al2O3/HfO2, which can be potentially applied as the tunnel dielectric of nonvolatile memory devices, is also presented and compared with results from metal–oxide–semiconductor devices with a single layer of SiO2 or HfO2 as gate dielectric. It was found that the layered-barrier structure has the steepest Jg–Vg characteristics of the three structures with identical equivalent-oxide thickness. This results in a small ratio of program voltage to retention voltage for the layered-barrier structure, which makes it attractive for nonvolatile memory application.

Impairing the Memory of an Electron-Glass by IR Excitation

We study the influence of various excitations on the anomalous field effect observed in insulating indium-oxide films. In conductance G versus gate-voltage Vg measurements one observes a characteristic cusp around the Vg at which the system has equilibrated. In the absence of any disturbance this cusp may persist for a long time after a new gate voltage was imposed on the sample and hence reflects a memory of the previous equilibrium state. This memory is believed to be related to the correlations between electrons. Here we show that exciting the conduction electrons by exposing the sample to IR light degrades this memory. We argue that any excitation that randomizes the system destroys the correlations and therefore impairs the memory.