Mobility Of Two-dimensional Electron Gas Research Articles

The Effect of the Barrier Layer on the Uniformity of the Transport Characteristics of AlGaN/GaN Heterostructures on HR-Si(111).

The high transport characteristics of AlGaN/GaN heterostructures are critical components for high-performance electronic and radio-frequency (RF) devices. We report the transport characteristics of AlGaN/GaN heterostructures grown on a high-resistivity (HR) Si(111) substrate, which are unevenly distributed in the central and edge regions of the wafer. The relationship between the composition, stress, and polarization effects was discussed, and the main factors affecting the concentration and mobility of two-dimensional electron gas (2DEG) were clarified. We further demonstrated that the mechanism of changes in polarization intensity and scattering originates from the uneven distribution of Al composition and stress in the AlGaN barrier layer during the growth process. Furthermore, our results provide an important guide on the significance of accomplishing 6 inch AlGaN/GaN HEMT with excellent properties for RF applications.

Effect of annealing on the electrical performance of N-polarity GaN Schottky barrier diodes

A nitrogen-polarity (N-polarity) GaN-based high electron mobility transistor (HEMT) shows great potential for high-frequency solid-state power amplifier applications because its two-dimensional electron gas (2DEG) density and mobility are minimally affected by device scaling. However, the Schottky barrier height (SBH) of N-polarity GaN is low. This leads to a large gate leakage in N-polarity GaN-based HEMTs. In this work, we investigate the effect of annealing on the electrical characteristics of N-polarity GaN-based Schottky barrier diodes (SBDs) with Ni/Au electrodes. Our results show that the annealing time and temperature have a large influence on the electrical properties of N-polarity GaN SBDs. Compared to the N-polarity SBD without annealing, the SBH and rectification ratio at ±5 V of the SBD are increased from 0.51 eV and 30 to 0.77 eV and 7700, respectively, and the ideal factor of the SBD is decreased from 1.66 to 1.54 after an optimized annealing process. Our analysis results suggest that the improvement of the electrical properties of SBDs after annealing is mainly due to the reduction of the interface state density between Schottky contact metals and N-polarity GaN and the increase of barrier height for the electron emission from the trap state at low reverse bias.

Review of the AlGaN/GaN High-Electron-Mobility Transistor-Based Biosensors: Structure, Mechanisms, and Applications.

Due to its excellent material performance, the AlGaN/GaN high-electron-mobility transistor (HEMT) provides a wide platform for biosensing. The high density and mobility of two-dimensional electron gas (2DEG) at the AlGaN/GaN interface induced by the polarization effect and the short distance between the 2DEG channel and the surface can improve the sensitivity of the biosensors. The high thermal and chemical stability can also benefit HEMT-based biosensors' operation under, for example, high temperatures and chemically harsh environments. This makes creating biosensors with excellent sensitivity, selectivity, reliability, and repeatability achievable using commercialized semiconductor materials. To synthesize the recent developments and advantages in this research field, we review the various AlGaN/GaN HEMT-based biosensors' structures, operations mechanisms, and applications. This review will help new researchers to learn the basic information about the topic and aid in the development of next-generation of AlGaN/GaN HEMT-based biosensors.

Vacancy-type defects in AlInN/AlN/GaN structures probed by monoenergetic positron beam

Vacancy-type defects in AlInN(10 nm)/AlN(1–2 nm)/GaN were probed by using a positron annihilation technique. The crystal quality of the AlInN layer and atomic diffusion near heterointerfaces were also studied by x-ray diffraction reciprocal space mapping, transmission electron microscopy, and energy-dispersive x-ray spectroscopy. For an as-deposited sample without an AlN spacer layer (AlInN/GaN), Ga atoms diffused into the AlInN layer, and as a result, the concentration of Ga-vacancy-type defects in the GaN layer increased. The vacancy concentration was decreased by inserting the AlN layer, which was attributed to the suppression of out-diffusion of Ga from the GaN layer. The effect of the thickness of the AlN layer on the mobility of two-dimensional electron gas is discussed in terms of the introduction of vacancies into the channel region. The annealing behaviors of vacancies in the GaN layer and atomic exchange near heterointerfaces are also discussed.

Impact of Notch Structures on Transfer Characteristics of AlGaN/GaN HEMTs: A Simulation Study

Lateral GaN devices, with a substantial critical breakdown field and increased mobility of two-dimensional electron gas (2DEG), are particularly promising for future power applications. Despite low power consumption by design, further improvements are required in numerous areas, including reliability concerns and switching loss, usually contributing to significant power loss. The research objective concerns the impact of different notch structures on the transfer characteristics of GaN-based high-electron-mobility transistor (HEMT) devices. Thirteen simulated models were produced using COMSOL Multiphysics, incorporating the electrical, structural, and piezoelectric effects of the device. From the results, notch-structure devices demonstrated better electrical characteristics than devices using conventional architecture, particularly a model with a double-notch design. Higher transconductance and maximum drain current were among the improvements, benefiting from the notch structure at the barrier layer.

Nonlinear transport phenomena and current-induced hydrodynamics in ultrahigh mobility two-dimensional electron gas

We report on nonlinear transport phenomena at high filling factor and DC current-induced electronic hydrodynamics in an ultrahigh mobility $\text{(}\ensuremath{\mu}=20\ifmmode\times\else\texttimes\fi{}{10}^{6}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}/\mathrm{Vs}\text{)}$ two-dimensional electron gas in a narrow ($15\ensuremath{-}\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{m}$ wide) GaAs/AlGaAs Hall bar for DC current densities reaching 0.67 A/m. The various phenomena and the boundaries between the phenomena are captured together in a two-dimensional differential resistivity map as a function of magnetic field (up to 250 mT) and DC current. This map, which resembles a phase diagram, demarcate distinct regions dominated by Shubnikov-de Haas (SdH) oscillations (and phase inversion of these oscillations) around zero DC current; negative magnetoresistance and a double-peak feature (both ballistic in origin) around zero field; and Hall field-induced resistance oscillations (HIROs) radiating out from the origin. From a detailed analysis of the data near zero field, we show that increasing the DC current suppresses the electron-electron scattering length that drives a growing hydrodynamic contribution to both the differential longitudinal and transverse (Hall) resistivities. Our approach to induce hydrodynamics with DC current differs from the more usual approach of changing the temperature. We also find a significant (factor of two to four) difference between the quantum lifetime extracted from SdH oscillations, and the quantum lifetime extracted from HIROs. In addition to observing HIRO peaks up to the seventh order, we observe an unexpected HIRO-like feature close to midway between the first-order and the second-order HIRO maxima at high DC current.

Effect of strain on the effective mass of GaN and the mobility of AlGaN/GaN two-dimensional electron gas

By performing first-principles calculations based on density functional theory, we have investigated the effect of strain on the effective mass of GaN and the two-dimensional (2D) electron gas mobility of AlGaN/GaN. Our calculated results predict that the effective mass of GaN under strain decreases with increasing tensile strain while increases with increasing compressive strain. The 2D electron gas mobility increases with increasing tensile strain and decreases with increasing compressive strain at constant surface density, reaching a value of 3.40×103 cm2/V·s under the tensile strain of 5 %. However, the 2D electron gas mobility decreases with increasing surface density, and reaching a value of 3.05×103 cm2/V·s at a surface density of 1.0×1012 cm−2. It is expected that our results will serve as a guide to improve the electrical properties of AlGaN/GaN HEMTs.

Research on nano-scale AlN nucleation layer growth and GaN HEMT characteristics based on MOCVD technology

The AlN nucleation layers of different thicknesses were grown on a 100 mm 4H-SiC substrate in a low-pressure MOCVD. By using in-situ high-temperature etching of the substrate steps, the AlN nucleation layer could achieve layered growth as soon as possible, and finally achieve 11.6 nm. The complete AlN film was prepared, and the surface morphology and interface quality of AlN were analyzed with atomic force microscope and transmission electron microscope. A 1.8 μm thick GaN HEMT epitaxial material was prepared based on the ultra-thin AlN nucleation layer. The (002) and (102) plane rocking curves of GaN had a FWHM of 138arcsec and 231arcsec, and the electrons mobility of the heterojunction two-dimensional electron gas reached 2200 cm2/V·s. The AlN interface thermal resistance measured by the high-precision 3ω method was reduced to 5.53 × 10−9 m2K/W.

Interface-state suppression of AlGaN/GaN Schottky barrier diodes with post-anode-annealing treatment

Owing to the high density and high mobility of two-dimensional electron gas (2DEG) induced by strong spontaneous polarization and piezoelectric polarization effect, AlGaN/GaN Schottky barrier diodes (SBDs) with high output current density and low on-resistance have proved to be a promising candidate. Anode of GaN SBD is the core structure, which affects the device performance such as turn-on voltage, reverse current, on-resistance, and breakdown voltage. Therefore, idealized Schottky junction with low interface state density is very important in achieving high-performance GaN SBD. In this work, AlGaN/GaN SBD with low work-function metal W as anode is fabricated, and the post-anode-annealing (PAA) treatment is found to be effective in promoting the bonding reaction between anode metal and GaN in the anode region. Comparing with GaN SBDs without PAA treatment, the interface state density decreases from 9.48×10<sup>15</sup> eV<sup>–1</sup>·cm<sup>–2</sup> to 1.77×10<sup>13</sup> eV<sup>–1</sup>·cm<sup>–2</sup> after PAA treatment. The reverse leakage current is reduced by two orders, which ascribes to the idealized anode interface with low interface state density. Meanwhile, the influence of interface state on carriers in the forward conduction process is also suppressed, and the differential on-resistance of the fabricated GaN SBDs decreases from 17.05 Ω·mm to 12.57 Ω·mm. It is obvious that the PAA process proves to be an effective method to suppress the interface states density at M/S interface, thus significantly improving the performance of GaN SBD, which is the key technology in fabricating the high-performance GaN device.

Determination of 2DEG parameters in LED heterostructures with three quantum wells In-=SUB=-x-=/SUB=-Ga-=SUB=-1-x-=/SUB=-N/GaN by terahertz time-domain spectroscopy (THz-TDs)

Terahertz time-domain spectroscopy (THz-TDs) has been used to record the resonant frequencies of plasmon oscillations excited in samples of heterostructures with three InxGa1-xN/GaN quantum wells (QWs) by laser pulses with a duration of 130 fs in the temperature range from 90 to 170 K. Fast Fourier transform (FFT) of the time dependence of the electric field of THz-pulses made it possible to obtain the frequency spectra of the power and phase shift of THz-radiation, the interpretation of which made it possible to estimate the pulse relaxation time, mobility and effective mass of two-dimensional electron gas (2DEG) in the heterostructures. Using a series of frequency spectra of the power and phase shift of THz-radiation, the temperature dependences of the effective mass and relaxation time of the 2DEG pulse were obtained. Mobility value 2DEG obtained by the THz-TDs is in good agreement with the data of Hall measurements. Keywords: heterostructures, pulse relaxation time, 2DEG, terahertz radiation, terahertz spectroscopy.

Low leakage current and high breakdown voltage of AlGaN/GaN Schottky barrier diodes with wet-etching groove anode

AlGaN/GaN heterojunction epitaxies with wide bandgap, high critical electric field as well as high density and high mobility two-dimensional electron gas have shown great potential applications in the next-generation high-power and high-frequency devices. Especially, with the development of Si-based GaN epitaxial technique with big size, GaN devices with low cost also show great advantage in consumer electronics. In order to improve the rectification efficiency of AlGaN/GaN Schottky barrier diode (SBD), low leakage current and low turn-on voltage are important. The GaN Schottky barrier diode with low work-function metal as anode is found to be very effective to reduce turn-on voltage. However, the low Schottky barrier height makes the Schottky interface sensitive to damage to groove surface, which leads to a high leakage current. In this work, a novel wet-etching technique with thermal oxygen oxidation and KOH corrosion is used to prepare the anode groove, and the surface roughness of groove decreases from 0.57 to 0.23 nm, compared with that of the dry-etching surface of groove. Meanwhile, the leakage current is suppressed from 1.5 × 10<sup>–6</sup> to 2.6 × 10<sup>–7</sup> A/mm. Benefiting from the great corrosion selectively of hot KOH solution to AlGaN barrier layer and GaN channel layer after thermal oxygen oxidation, the spikes of the edge of groove region caused by the nonuniform distribution of plasma in the cavity is improved, and the breakdown voltage of the fabricated AlGaN/GaN SBDs is raised from –1.28 to –1.73 kV.

Molecular beam epitaxy of InAs quantum wells on InP(001) for high mobility two-dimensional electron gases

For InAs quantum-well structures grown on InP, the dislocations generated in the strain relaxation is confined in the compositionally graded buffer layer, leaving the two-dimensional electron gases nearly unscattered by the defects.

Predominant Scattering Mechanisms in Quaternary AlInGaN/GaN Heterostructures

Scattering mechanisms that limit the mobility of two-dimensional electron gas (2DEG) in AlInGaN/GaN heterojunctions with three different barrier layer thicknesses of 37.2 (sample A), 10.6 (sample B) and 4.30 (sample C) nm were studied. Hall measurements were performed between 12 and 350 K. Mobilities limited by scattering due to acoustic and optic phonons, dislocation, interface roughness, and alloy disorder were used in the calculation. It was found that scattering, predominantly due to interface roughness, determine the Hall mobility for all samples at different strengths. The highest electron mobility of 492 cm2V-1s-1 at room temperature is obtained for sample B with a high sheet density of about 4.43x1013 cm-2 and a corresponding sheet resistance of 287 Ω/.

Effects of Si-doped GaN insert layer in AlGaN/GaN/GaN:Si/AlN DH-HEMT structure

In this study, we investigate the effects of Si-doped GaN insert layer on the structural and electrical characteristics of AlGaN/GaN/GaN:Si/AlN double-heterostructure high-electron-mobility transistor (DH-HEMT). Compared with an AlGaN/GaN/AlN HEMT without a Si-doped GaN layer, the AlGaN/GaN/GaN:Si/AlN DH-HEMT showed an improved root-mean-square roughness of 0.57 nm and a decreased GaN channel compressive stress of –1.44 GPa. Also, an increased two-dimensional electron gas mobility of 1845 cm2/V∙s and a reduced specific contact resistance of 1.49 × 10−6 Ω∙cm2 were obtained. The direct-current output results showed an increased maximum drain current of 600 mA/mm and transconductance of 130 mS/mm, which was attributed to the enhanced 2DEG mobility and reduced contact resistance by the insertion of Si-doped GaN layer in the DH-HEMT.

High Mobility Two-Dimensional Electron Gas at the BaSnO3/SrNbO3 Interface.

Oxide two-dimensional electron gases (2DEGs) promise high charge carrier concentrations and low-loss electronic transport in semiconductors such as BaSnO3 (BSO). ACBN0 computations for BSO/SrNbO3 (SNO) interfaces show Nb-4d electron injection into extended Sn-5s electronic states. The conduction band minimum consists of Sn-5s states ∼1.2 eV below the Fermi level for intermediate thickness 6-unit cell BSO/6-unit cell SNO superlattices, corresponding to an electron density in BSO of ∼1021 cm-3. Experimental studies of analogous BSO/SNO interfaces grown by molecular beam epitaxy confirm significant charge transfer from SNO to BSO. In situ angle-resolved X-ray photoelectron spectroscopy studies show an electron density of ∼4 × 1021 cm-3. The consistency of theory and experiments show that BSO/SNO interfaces provide a novel materials platform for low loss electron transport in 2DEGs.

Electrical characterization and extraction of activation energies of the defect states in the LaAlO3/SrTiO3 heterostructure

In this work, we study the electronic properties of defects in the LaAlO3/SrTiO3 heterostructure, which is known to host a high mobility two-dimensional electron gas (2DEG) at the interface. This 2DEG also shows photoconductance, which could be related to defects that act as deep center trapping and releasing carriers by interaction with light. This phenomenon has raised an interest for the identification of deep energy levels in the LaAlO3/SrTiO3 heterostructure. We have studied the defect state properties using electrical characterization such as capacitance–voltage (C–V), current–voltage (I–V) measurements, and deep-level transient Fourier spectroscopy (DLTFS). From C–V and I–V analyses, a hysteresis was observed indicating an effect of mobile charges in the LaAlO3. Using DLTFS, we identify three defect states located at around 0.17 eV below conduction band and at 0.23 and 0.26 eV above the valence band. These defect states were attributed to defects in SrTiO3 such as strontium vacancies or titanium vacancies. We identify a fourth defect state having an energy of about 0.69 eV below the conduction band that could be related to oxygen vacancies in LaAlO3 or in SrTiO3. In addition, the observation of an effect of the electric field with DLTFS indicated that oxygen vacancies might be involved in Fowler–Nordheim or trap-assisted tunneling through the LaAlO3 layer.

Investigation on Temperature Behavior for a GaAs E-pHEMT MMIC LNA.

In order to investigate the temperature behavior for monolithic microwave integrated circuits (MMICs) under alpine conditions, the performance parameters of a 0.4–3.8 GHz gallium arsenide (GaAs) enhancement pseudomorphic high-electron-mobility transistor (E-pHEMT) low-noise amplifier (LNA) are tested at different temperatures. The typical temperatures of −39.2 °C, −32.9 °C, −25.3 °C, −11.3 °C, −4.9 °C, 0 °C and 23 °C are chosen as the alpine condition. The major performance indexes including the direct current (DC) characteristics, S-parameters, stability, radio frequency (RF) output characteristics, output third-order intersection point (OIP3) and noise figure (NF), which were inspected and analyzed in detail. The results show that the DC characteristics, small-signal gain (S21), RF output characteristics and NF all deteriorate with the rising temperature due to the decrease in two-dimensional electron gas mobility (). Contrary to this trend, the special design makes stability and OIP3 increase. For further application of this MMIC LNA under alpine conditions, several measures can be utilized to remedy performance degradation. This paper can provide some significant engineering value for the reliable design of MMICs.

Enhancement of two-dimensional electron gas mobility using strain reduced AlGaN barriers grown by metalorganic vapor phase epitaxy under nitrogen atmosphere

We demonstrated high-electron-mobility transistors (HEMTs) with enhanced two-dimensional electron gas (2DEG) mobility using a low-strain AlGaN barrier grown by metalorganic vapor phase epitaxy under a nitrogen atmosphere. We investigated the effects of the growth temperature under a nitrogen atmosphere on the electrical properties of AlGaN-HEMT structures, focusing on 2DEG mobility. At growth temperatures below 855 °C, the 2DEG mobility decreased with decreasing growth temperature owing to an increase in the threading dislocation density. However, at growth temperatures above 855 °C, the 2DEG mobility decreased with increasing growth temperature. This finding was attributed to the compressive strain in the GaN channel, which increased with increasing growth temperature owing to the increased tensile strain in the AlGaN barriers. We concluded that temperatures around 855 °C are suitable for AlGaN barrier growth under nitrogen atmosphere. Finally, we achieved the highest 2DEG mobility of 2182 cm2 V−1 s−1 with a low sheet resistance of 406 Ω sq−1. using an Al0.41Ga0.59N barrier.

Research on the Synergistic Effect of Total Ionization and Displacement Dose in GaN HEMT Using Neutron and Gamma-Ray Irradiation.

This paper studies the synergistic effect of total ionizing dose (TID) and displacement damage dose (DDD) in enhancement-mode GaN high electron mobility transistor (HEMT) based on the p-GaN gate and cascode structure using neutron and 60Co gamma-ray irradiation. The results show that when the accumulated gamma-ray doses are up to 800k rad(Si), the leakage-current degradations of the two types of GaN HEMTs with 14 MeV neutron irradiation of 1.3 × 1012 n/cm2 and 3 × 1012 n/cm2 exhibit a lower degradation than the sum of the two separated effects. However, the threshold voltage shifts of the cascode structure GaN HEMT show a higher degradation when exposed to both TID and DDD effects. Moreover, the failure mechanisms of the synergistic effect in GaN HEMT are investigated using the scanning electron microscopy technique. It is shown that for the p-GaNHEMT, the increase in channel resistance and the degradation of two-dimensional electron gas mobility caused by neutron irradiation suppresses the increase in the TID leakage current. For the cascode structure HEMT, the neutron radiation-generated defects in the oxide layer of the metal–oxide–semiconductor field-effect transistor might capture holes induced by gamma-ray irradiation, resulting in a further increase in the number of trapped charges in the oxide layer.

Fermi level tuning and band alignment in Mn-doped InAs/GaSb

InAs/GaSb hosts a broken gap band alignment that has been shown to generate helical topological edge states. Upon the introduction of Mn into the structure, it has been predicted to host a quantized anomalous Hall effect. Here, we show that dilute Mn doping on InAs in InAs/GaSb, allows a tuning of the Fermi level, the introduction of paramagnetism, but also has a non-trivial impact on the band alignment of the system. The measurement of Shubnikov-de-Haas oscillations, cyclotron resonance, and a non-linear Hall effect in Mn-doped samples indicate the coexistence of a high mobility two-dimensional electron gas and a hole gas. Conversely, in undoped InAs/GaSb, pure-n-type transport is observed. We hypothesize that Mn acceptor levels can pin the Fermi energy near the valence band edge of InAs, far from the interface, which introduces a strong band bending to preserve the band offset at the InAs/GaSb interface. The realization of the QAHE in this structure will thus require a careful control of the band alignment to preserve topological insulating character.