High-density Two-dimensional Electron Gas Research Articles

Polarization engineering of two-dimensional electron gas at ε-(AlxGa1–x)2O3/ε-Ga2O3 heterostructure

In this study, we present an investigation of the spontaneous and piezoelectric polarization in the ε-(AlxGa1–x)2O3/ε-Ga2O3 heterostructures using density functional theory calculations. The spontaneous polarization (Psp) was found to increase from 23.93 to 26.34 μC/cm2 when Al-content increase from 0% to 50%. With Al-content increasing, the strain-induced piezoelectric polarization (Ppe) increases, which negates the Psp, causing the total polarization (Ptot) of the epitaxy layer in the AlGaO/GaO heterostructure to remain almost constant across all Al compositions. Additionally, due to the ferroelectric nature of ε-Ga2O3, a high-density polarization-induced two-dimensional electron gas (2DEG) can be formed at the interface of polarization reversed ε-(AlxGa1–x)2O3/ε-Ga2O3 when an electric field is applied. Using the 1D Schrödinger–Poisson model, the 2DEG of polarization reversed ε-(Al0.125Ga0.875)2O3/ε-Ga2O3 was found to be 2.05 × 1014 cm−2, which is nearly ten times larger than that of GaN-based structures. Our work indicates that ε-(AlxGa1–x)2O3/ε-Ga2O3 heterostructure could play a key role attributed to the large polarization and capability in modulating the polarization for high-power electronic and radio frequency device applications.

Propagation of surface magnetoplasmon polaritons in a symmetric waveguide with two-dimensional electron gas

The properties of surface magnetoplasmon polaritons (SMPPs) in a symmetric structure, composed of two semi-infinite regions of high-density two-dimensional electron gas (2DEG) separated by a thin film in Voigt configuration, are investigated. The normal and absorption dispersion relations for the transverse magnetic polarization are derived by correlating Maxwell’s equation and the boundary conditions. It is demonstrated that the features of SMPPs are greatly influenced by the external magnetic field, collision frequency of 2DEG, the dielectric constant, and the thickness of the thin film, suggesting that the locations and propagation lengths of SMPPs can be governed accordingly. It is shown that the symmetry of the physical geometry preserves the symmetry of the dispersion relations of SMPPs. Furthermore, it is discovered that as the external magnetic field increases, the penetration depth of SMPPs decreases, while their energy loss reduces, implying that plasmons can propagate for longer distances. Additionally, it is observed that SMPPs in the symmetric configuration have a longer lifetime than those in the asymmetric configuration.

Chiral Berry plasmon dispersion of the two-dimensional electron gas based on a quantum hydrodynamic model

We employ the quantum hydrodynamic model to investigate the properties of chiral Berry plasmons (CBPs) generated by a high-density two-dimensional electron gas (2DEG) film covering a semi-infinite dielectric substrate with a finite Berry flux. By coupling Maxwell's equations, Poisson's equation, and the boundary conditions, we derive the dispersion relations of plasmon with a nonzero Berry flux in detail. We present the difference in the dispersion of CBPs with quantum effects and the classical model. We show how the quantum effect, Berry curvature, the dielectric constant of the substrate, and electron density in 2DEG affect the behavior of the CBPs. Parameter dependence of the effects is calculated and discussed.

Investigation of AlGaN/GaN Schottky barrier diodes on free-standing GaN substrate with low leakage current

Benefiting from the excellent properties of GaN with a wide bandgap of 3.4 eV as well as high critical field of 3.3 MV/cm, GaN-based devices prove to be a promising candidate in extreme conditions. Especially, high-density high-mobility two-dimensional electron gas (2DEG) induced by spontaneous piezoelectric polarization in AlGaN/GaN heterostructure enables AlGaN/GaN device to lower on-resistance (<i>R</i><sub>ON</sub>). However, owing to the lack of free-standing GaN substrate with large size and high quality, the epitaxis of GaN is always based on hetero-substrate such as Al<sub>2</sub>O<sub>3</sub>, Si and SiC, which shows large lattice mismatch and thermal mismatch. The large mismatch between GaN and substrate leads to high dislocation as well as high leakage current (<i>I</i><sub>R</sub>) of GaN devices. In this work, high-performance AlGaN/GaN Schottky barrier diode with low <i>I</i><sub>R</sub> and low turn-on voltage (<i>V</i><sub>ON</sub>) is fabricated on a 3-inch free-standing GaN substrate with C-doping GaN buffer layer to suppress <i>I</i><sub>R</sub>. Owing to the suppressed dislocation density of the AlGaN/GaN epitaxial wafer on free-standing substrate, low Ohmic contact resistance (<i>R</i><sub>C</sub>) is difficult to achieve the suppressed penetration of Ohmic metal into 2DEG channel, which is adverse to the high current density. In this work, a low <i>R</i><sub>C</sub> of 0.37 Ω·mm is obtained by one-step self-aligned Ohmic process, including the etching of partial AlGaN barrier layer and lift-off of Ohmic metal. The 2DEG is formed under the effect of residual AlGaN barrier layer, and the short distance between 2DEG and Ohmic metal contributes to lowering the value of <i>R</i><sub>C</sub>. The groove anode region is defined by the low damaged inductively coupled plasma process with a low etching rate of 1 nm/min, and the total depth is 35 nm, confirmed by atomic force microscope. Fully removing the AlGaN barrier layer from the anode region makes the anode metal directly contact the 2DEG channel, thereby improving the performance of the fabricated AlGaN/GaN Schottky barrier diode (SBD) with a low <i>V</i><sub>ON</sub> of 0.67 V, low <i>I</i><sub>R</sub> of 3.6 × 10<sup>–8</sup> A/mm, and an <i>I</i><sub>ON</sub>/<i>I</i><sub>OFF</sub> ratio of up to 3 × 10<sup>7</sup>. The values of differential <i>R</i><sub>ON,sp</sub> are calculated to be 0.44, 0.86, 1.59, 2.55 mΩ·cm<sup>2</sup> for GaN SBDs with various values of <i>L</i><sub>AC</sub> of 6, 10, 15, 20 μm, and the values of <i>R</i><sub>ON,sp</sub> determined at an anode current density of 100 mA/mm are 1.27, 2.08, 3.29, 4.63 mΩ·cm<sup>2</sup>, respectively. As the measured temperature increases from 300 to 425 K, the <i>I</i><sub>R</sub> is increased only by 3 times to 1.6 × 10<sup>–7</sup> A/mm, which shows the great potential for next-generation power electronics.

Self-screening of the polarized electric field in wurtzite gallium nitride along [0001] direction

The strong polarization effect of GaN-based materials is widely used in high-performance devices such as white-light-emitting diodes (white LEDs), high electron mobility transistors (HEMTs), and GaN polarization superjunctions. However, the current researches on the polarization mechanism of GaN-based materials are not sufficient. In this paper, we studied the influence of polarization on electric field and energy band characteristics of Ga-face GaN bulk materials by using a combination of theoretical analysis and semiconductor technology computer-aided design (TCAD) simulation. The self-screening effect in Ga-face bulk GaN under ideal and non-ideal conditions is studied respectively. We believe that the formation of high-density two-dimensional electron gas (2DEG) in GaN is the accumulation of screening charges. We also clarify the source and accumulation of the screening charges caused by the GaN self-screening effect in this paper and aim to guide the design and optimization of high-performance GaN-based devices.

Surprising stability of polar (001) surfaces of the Mott insulator GdTiO3

Using first-principles techniques based on hybrid density functional calculations, we study the stability, energetics, and electronic structure of the (001) surface of the Mott insulator GdTiO3 (GTO), which has an orthorhombic perovskite structure. Interestingly, we find the bare unreconstructed (but relaxed) polar surface terminated by a TiO2 plane to be very stable with a low surface energy (71 meV/Å2). As a test for stability of the TiO2 termination against reconstructions, we studied the influence of an H adatom. Hydrogen is known to form strong bonds with surface O atoms and passivate surface states, but contrary to expectations, hydrogen does not lead to a lowering of the GTO surface energy. We explain the energetics based on the surface electronic structure. We also address the interaction between the TiO2-terminated GTO surface and the high-density two-dimensional electron gas (2DEG) that can be formed at an SrTiO3 (STO)/GTO heterointerface. Unlike the situation in STO/LaAlO3 (LAO) heterostructures, where the LAO surface acts as a sink for electrons, the GTO surface does not drain electrons away from the 2DEG.

High-resistivity nitrogen-polar GaN for GaN/AlGaN high electron mobility transistors by metalorganic chemical vapor deposition

Nitrogen-polar (N-polar) III-nitride has natural advantages in the fabrication of electronic devices. In this work, unintentionally doped N-polar GaN films were grown on sapphire substrates by metalorganic chemical vapor deposition. By optimizing the growth parameters including growth temperature, pressure, and V/III ratio, N-polar GaN with a relatively high sheet resistivity of 3.6 × 105 Ω/sq was achieved. The secondary ion mass spectroscopy and x-ray diffraction measurement results demonstrate that the increased carbon impurity concentration is primarily responsible for the high resistivity. Moreover, N-polar GaN/Al0.25Ga0.75N heterojunctions were deposited on the high-resistivity N-polar GaN template. An extremely high-density two-dimensional electron gas of up to 1.12 × 1013 cm−2 was realized at the interface between the Al0.25Ga0.75N and the GaN. It is reasonably believed that the experimental results obtained here are beneficial for the development of high-performance N-polar GaN-based electronic devices.

Band offsets at metalorganic chemical vapor deposited β-(AlxGa1−x)2O3/β-Ga2O3 interfaces—Crystalline orientation dependence

The β-(AlxGa1−x)2O3 alloy represents an emerging ultrawide bandgap semiconductor material for applications in high-power electronics and deep ultraviolet optoelectronics. The recent demonstrations of orientation-dependent epitaxial growth of high quality β-(AlxGa1−x)2O3 films have unlocked prospects for the development of ultrahigh frequency β-(AlxGa1−x)2O3/Ga2O3 based transistors with high-power tolerance. To control the electronic and optical properties of β-(AlxGa1−x)2O3/Ga2O3 heterostructure-based devices, an understanding of the band offsets between β-(AlxGa1−x)2O3 and β-Ga2O3 is crucial. However, there have been no systematic experimental studies on the evolution of the band offsets between differently oriented β-(AlxGa1−x)2O3/Ga2O3 heterointerfaces as a function of Al compositions. This work presents the valence and conduction band offsets at (010) and (2¯01) oriented β-(AlxGa1−x)2O3/Ga2O3 interfaces using x-ray photoelectron spectroscopy. β-(AlxGa1−x)2O3 films with x ≤ 0.35 and x ≤ 0.48 were grown by metalorganic chemical vapor deposition on (010) and (2¯01) oriented β-Ga2O3 substrates, respectively. The determined band offsets reveal the formation of a type-II (staggered) band alignment at (010) oriented β-(AlxGa1−x)2O3/Ga2O3 and a type-I (straddling) heterojunction between (2¯01) β-(AlxGa1−x)2O3 and β-Ga2O3. For both crystalline orientations, the valence and conduction band offsets are found to increase with increasing Al content with a much weaker variation in the valence band offsets as compared to the conduction band offsets. Among different orientations investigated, such as (010), (2¯01), and (100), the largest conduction band offset occurs at a β-(AlxGa1−x)2O3/Ga2O3 interface with (100) orientation, providing opportunities for excellent electron confinement with a high-density two-dimensional electron gas. Results from this study on the tunable and orientation-dependent band offsets with the variation of the Al alloy fraction will provide guidance for the design of heterostructures in future β-(AlxGa1−x)2O3/Ga2O3 based devices.

Modeling of Bias-Dependent Effective Velocity and Its Impact on Saturation Transconductance in AlGaN/GaN HEMTs

In this article, we present a surface-potential-based approach to model the bias-dependent effective velocity observed in AlGaN/GaN high-electron-mobility transistors (HEMTs) due to optical phonon scattering. Our model precisely reproduces the progressive decrease in the saturation velocity in GaN HEMTs with increasing gate voltages, as reported in the literature, which is predominantly due to the scattering of electrons, forming the high-density two-dimensional electron gas (2DEG), by optical phonons at high overdrive voltages. We show that this dependence differs from the traditional mobility degradation models in terms of its impact on the device current-voltage (I- V) characteristics and illustrate how the inclusion of a velocity saturation model also provides the model users with an additional handle for parameter extraction. The model is explicit and, by virtue of its Simulation Program with Integrated Circuit Emphasis (SPICE) compatibility, is readily implemented in the industry-standard Advanced SPICE Model for HEMTs (ASM-HEMTs) model and has been validated against experimental dc I-V and RF S-parameter measurements of an in-house GaN device.

The Leakage Mechanism of the Package of the AlGaN/GaN Liquid Sensor

Wide bandgap gallium nitride (GaN)-based devices have attracted a lot of attention in optoelectronics, power electronics, and sensing applications. AlGaN/GaN based sensors, featuring high-density and high-mobility two-dimensional electron gas (2DEG), have been demonstrated to be effective chemical sensors and biosensors in the liquid environment. One of the key factors limiting the wide adoption of the AlGaN/GaN liquid sensor is the package reliability issue. In this paper, the reliability of three types of sensor packaging materials (SiO2/Si3N4, PI, and SiO2/Si3N4/PI) on top of 5-μm metal are tested in Phosphate buffer saline (PBS) solution. By analyzing the I-V characteristics, it is found that the leakage currents within different regimes follow distinct leakage models, whereby the key factors limiting the leakage current are identified. Moreover, the physical mechanisms of the package failure are illustrated. The failure of the SiO2/Si3N4 package is due to its porous structure such that ions in the solution can penetrate into the packaging material and reduce its resistivity. The failure of the PI package at a relatively low voltage (<3 V) is mainly due to the poor adhesion of PI to the AlGaN surface such that the solution can reach the electrode by the “lateral drilling” effect. The SiO2/Si3N4/PI package achieves less than 10 μA leakage current at 5 V voltage stress because it combines the advantages of the SiO2/Si3N4 and the PI packages. The analysis in this work can provide guidelines for the design and failure mechanism analysis of packaging materials.

Broadband optical modulation in a zinc-oxide-based heterojunction via optical lifting

Broadband electro-absorptive optical modulation in a zinc-oxide (ZnO)-based heterojunction is demonstrated. The presence of high-density two-dimensional electron gas (2DEG) at the bottom of conduction band offset in a MgZnO-CdZnO heterojunction is shown to exhibit electrically tunable optical absorption in the regions well below the material bandgap. Electrons confined near the heterojunction are lifted across the potential barrier on optical excitation. Optical modulation with an extinction ratio of 8 dB is reported at an operating wavelength of 527 nm. The extinction ratio remains around 7–8 dB over a wide bandwidth of 115 nm, providing an excellent opportunity to explore ZnO-based heterojunctions to realize a broadband optical modulator for applications in optical communication and interconnects.

Carrier-Density-Induced Ferromagnetism in EuTiO_{3} Bulk and Heterostructures.

EuTiO_{3} is an antiferromagnetic (AFM) material showing strong spin-lattice interactions, large magnetoelectric response, and quantum paraelectric behavior at low temperatures. Using electronic-structure calculations, we show that adding electrons to the conduction band leads to ferromagnetism. The transition from antiferromagnetism to ferromagnetism is predicted to occur at ∼0.08 electrons/Eu (∼1.4×10^{21} cm^{-3}). This effect is also predicted to occur in heterostructures such as LaAlO_{3}/EuTiO_{3}, where ferromagnetism is triggered by the formation of a high-density two-dimensional electron gas in the EuTiO_{3}. Our analysis indicates that the coupling between Ti 3d and Eu 5d plays a crucial role in lowering the Ti 3d conduction band in the ferromagnetic (FM) phase, leading to an almost linear dependence of the energy difference between the FM and AFM ordering on the carrier concentration. These findings open up possibilities in designing field-effect transistors using EuTiO_{3}-based heterointerfaces to probe fundamental interactions between highly localized spins and itinerant, polarized charge carriers.

Structural and electrical properties of AlN thin films on GaN substrates grown by plasma enhanced-Atomic Layer Deposition

Aluminum nitride (AlN) thin films have been deposited by Plasma Enhanced Atomic Layer Deposition (PE-ALD) onto GaN-Sapphire substrates. The morphological, structural and electrical properties of AlN films with different thickness (from 5 to 15 nm) have been investigated. They uniformly cover the underlying GaN substrate without pinholes and cracks. All the AlN thin films show c-axis orientation and their in-plane crystalline arrangement perfectly matches the hexagonal structure of GaN substrate. In particular, the cross-sectional TEM analysis demonstrated that the first AlN layers are well aligned with respect to the GaN (0001) substrate, while stacking faults formation is observed in the upper part of the films. Finally, electrical measurements by Hg-probe on as-deposited AlN showed very low current leakage across these layers and the presence of a high density two-dimensional electron gas (>2 × 1013 cm−2) at the AlN/GaN interface.

Two-dimensional electron gases in MgZnO/ZnO and ZnO/MgZnO/ZnO heterostructures grown by dual ion beam sputtering

This work reports on the formation of high-density (~1013–1014 cm−2) two-dimensional electron gas (2DEG) in ZnO-based heterostructures, grown by a dual ion beam sputtering system. We probe 2DEG in bilayer MgZnO/ZnO and capped ZnO/MgZnO/ZnO heterostructures utilizing MgZnO barrier layers with varying thickness and Mg content. The effect of the ZnO cap layer thickness on the ZnO/MgZnO/ZnO heterostructure is also studied. Hall measurements demonstrate that the addition of a 5 nm ZnO cap layer results in an enhancement of the 2DEG density by about 1.5 times compared to 1.11 1014 cm−2 for the uncapped bilayer heterostructure with the same 30 nm barrier thickness and 30 at.% Mg composition in the barrier layer. From the low-temperature Hall measurement, the sheet carrier concentration and mobility are both found to be independent of the temperature. The capacitance–voltage measurement suggests a carrier density of ~1020 cm−3, confined in 2DEG at the MgZnO/ZnO heterointerface. The results presented are significant for the optimization of 2DEG for the eventual realization of cost-effective and large-area MgZnO/ZnO-based high-electron-mobility transistors.

Investigation of high density two-dimensional electron gas in Zn-polar BeMgZnO/ZnO heterostructures

Zn-polar BeMgZnO/ZnO heterostructures grown by molecular beam epitaxy on high resistivity GaN templates producing high-density two-dimensional electron gas (2DEG) are investigated. This is motivated by the need to reach plasmon-longitudinal optical (LO) phonon resonance for attaining minimum LO phonon lifetime. Achievement of high 2DEG concentration in MgZnO/ZnO heterostructures requires growth of the MgZnO barrier at relatively low temperatures, which compromises the ternary quality that in turn hinders potential field effect transistor performance. When this ternary is alloyed further with BeO, the sign of strain in the BeMgZnO barrier on ZnO switches from compressive to tensile, making the piezoelectric and spontaneous polarizations to be additive in the BeMgZnO/ZnO heterostructures much like the Ga-polar AlGaN/GaN heterostructures. As a result, a 2DEG concentration of 1.2 × 1013 cm−2 is achieved in the Be0.03Mg0.41Zn0.56O/ZnO heterostructure. For comparison, a 2DEG concentration of 7.7 × 1012 cm−2 requires 2% Be and 26% Mg in the barrier, whereas the same in the MgZnO/ZnO system would require incorporation of more than 40% Mg into the barrier, which necessitates very low growth temperatures. Our results are consistent with the demands on achieving short LO phonon lifetimes through plasmon-LO phonon resonance for high carrier velocity.

Suppression of surface-originated gate lag by a dual-channel AlN/GaN high electron mobility transistor architecture

A dual-channel AlN/GaN high electron mobility transistor (HEMT) architecture is demonstrated that leverages ultra-thin epitaxial layers to suppress surface-related gate lag. Two high-density two-dimensional electron gas (2DEG) channels are utilized in an AlN/GaN/AlN/GaN heterostructure wherein the top 2DEG serves as a quasi-equipotential that screens potential fluctuations resulting from distributed surface and interface states. The bottom channel serves as the transistor's modulated channel. Dual-channel AlN/GaN heterostructures were grown by molecular beam epitaxy on free-standing hydride vapor phase epitaxy GaN substrates. HEMTs fabricated with 300 nm long recessed gates demonstrated a gate lag ratio (GLR) of 0.88 with no degradation in drain current after bias stressed in subthreshold. These structures additionally achieved small signal metrics ft/fmax of 27/46 GHz. These performance results are contrasted with the non-recessed gate dual-channel HEMT with a GLR of 0.74 and 82 mA/mm current collapse with ft/fmax of 48/60 GHz.

Kinetic magnetism at the interface between Mott and band insulators

We show that the interplay of a high density two-dimensional electron gas and localized electrons in a neighboring Mott insulator leads to kinetic magnetism unique to the Mott/band insulator interface. Our study is based upon a bilayer Hubbard model at $U=\infty$ with a potential difference between the two layers. We combine analytic results with DMRG simulations to show that magnetism, and especially ferromagnetism, is greatly enhanced relative by the proximity of the two subsystems. The results are potentially relevant to recent experiments suggesting magnetism in $R$TiO$_3$/SrTiO$_3$ heterostructures.

BaSnO3 as a channel material in perovskite oxide heterostructures

BaSnO3 (BSO) is a transparent perovskite oxide with high room-temperature mobility, a property that is highly desirable for a channel material in transistors. However, its low density of states (DOS) makes it challenging to confine a high-density two-dimensional electron gas (2DEG). Using hybrid density functional theory, we calculate the band structure of BSO, its DOS, and its band offsets with candidate barrier materials, such as SrTiO3 (STO), LaInO3, and KTaO3. With the calculated material parameters as input, Schrödinger-Poisson simulations are then performed on BSO heterostructures to quantitatively address the issue of 2DEG confinement. The BSO/STO interface with a conduction-band offset of 1.14 eV limits the 2DEG density confined within BSO to 8×1013 cm−2. Strategies to improve the confinement via band-offset engineering are discussed.

Impact of electric-field dependent dielectric constants on two-dimensional electron gases in complex oxides

High-density two-dimensional electron gas (2DEG) can be formed at complex oxide interfaces such as SrTiO3/GdTiO3 and SrTiO3/LaAlO3. The electric field in the vicinity of the interface depends on the dielectric properties of the material as well as on the electron distribution. However, it is known that electric fields can strongly modify the dielectric constant of SrTiO3 as well as other complex oxides. Solving the electrostatic problem thus requires a self-consistent approach in which the dielectric constant varies according to the local magnitude of the field. We have implemented the field dependence of the dielectric constant in a Schrödinger-Poisson solver in order to study its effect on the electron distribution in a 2DEG. Using the SrTiO3/GdTiO3 interface as an example, we demonstrate that including the field dependence results in the 2DEG being confined closer to the interface compared to assuming a single field-independent value for the dielectric constant. Our conclusions also apply to SrTiO3/LaAlO3 as well as other similar interfaces.

Droop Improvement in InGaN/GaN Light-Emitting Diodes by Polarization Doping of Quantum Wells and Electron Blocking Layer

-Polarization doping in quantum wells and electron blocking layer is proposed to improve the emission intensity and efficiency droop of InGaN light-emitting diodes (LEDs). Band diagrams and the internal quantum efficiencies of LEDs are theoretically studied by the ATLAS simulation program. Numerical results show that the internal quantum efficiency of polarization doped LED structures are improved by 25% as compared to un-doped structures which can be due to accumulation of high density two-dimensional electron gas in quantum wells.