Formation Of Two-dimensional Electron Gas Research Articles

Reconstruction of electrostatic field at the interface leads to formation of two-dimensional electron gas at multivalent(110)LaAlO3/SrTiO3interfaces

Reconstruction of electrostatic field at the interface leads to formation of two-dimensional electron gas at multivalent<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mrow><mml:mo>(</mml:mo><mml:mn>110</mml:mn><mml:mo>)</mml:mo></mml:mrow><mml:mspace width="0.28em" /><mml:mi>LaAl</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>3</mml:mn></mml:msub><mml:mtext>/</mml:mtext><mml:mi>SrTi</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math>interfaces

Model of two-dimensional electron gas formation at ferroelectric interfaces

The formation of a two-dimensional electron gas at oxide interfaces as a consequence of polar discontinuities has generated an enormous amount of activity due to the variety of interesting effects it gives rise to. Here we study under what circumstances similar processes can also take place underneath ferroelectric thin films. We use a simple Landau model to demonstrate that in the absence of extrinsic screening mechanisms a monodomain phase can be stabilized in ferroelectric films by means of an electronic reconstruction. Unlike in the LaAlO$_3$/SrTiO$_3$ heterostructure, the emergence with thickness of the free charge at the interface is discontinuous. This prediction is confirmed by performing first principles simulations of free standing slabs of PbTiO$_3$. The model is also used to predict the response of the system to an applied electric field, demonstrating that the two-dimensional electron gas can be switched on and off discontinuously and in a non-volatile fashion. Furthermore, the reversal of the polarization can be used to switch between a two-dimensional electron gas and a two-dimensional hole gas, which should, in principle, have very different transport properties. We discuss the possible formation of polarization domains and how such configuration competes with the spontaneous accumulation of free charge at the interfaces.

Two-dimensional electron gas at the Ti-diffused BiFeO3/SrTiO3 interface

Oxide heterostructures with the broken translational symmetry often trigger a two-dimensional quantum confinement and associated unique electronic properties that cannot be observed in bulk constituents. Particular interest is devoted to the formation of two-dimensional electron gas (2DEG) at heterointerfaces between two insulators, which offers a fertile ground for fabricating advanced electronic devices. Here, we combine atomic force microscopy, transmission electron microscopy, and atomistic first-principles calculations to demonstrate that the (100) BiFeO3/SrTiO3 interface takes on a metallic nature and a 2DEG is generated at this interface. Our findings also reveal that the electronic reconstruction due to the polar discontinuity and the variation in valence state of Ti arising from diffusion of Ti cations in SrTiO3 to Fe sites of BiFeO3 are critical to the formation of 2DEG at the heterointerface.

Formation of two-dimensional electron gas at AlGaN/GaN heterostructure and the derivation of its sheet density expression**Project supported by the National Natural Science Foundation of China (Grant Nos. 61377020, 61376089, 61223005, and 61176126) and the National Science Fund for Distinguished Young Scholars, China (Grant No. 60925017).

Models for calculating the sheet densities of two-dimensional electron gas (2DEG) induced by spontaneous and piezoelectric polarization in AlGaN/GaN, AlGaN/AlN/GaN, and GaN/AlGaN/GaN heterostructures are provided. The detailed derivation process of the expression of 2DEG sheet density is given. A longstanding confusion in a very widely cited formula is pointed out and its correct expression is analyzed in detail.

Comparative investigation of GaAsSb/InGaAs type-II and InP/InGaAs type-I doped-channel field-effect transistors

DC performance of GaAsSb/InGaAs type-II and InP/InGaAs type-I doped-channel field-effect transistors (DCFETs) is demonstrated and compared by two-dimensional simulated analysis. As compared with the traditional InP/InGaAs DCFET, the GaAsSb/InGaAs DCFET exhibits a higher drain current of 8.05 mA, a higher transconductance of 216.24 mS/mm, and a lower gate turn-on voltage of 0.25 V for the presence of a relatively large conduction band discontinuity (ΔEc ≈ 0.4 eV) at GaAsSb/InGaAs heterostructure and the formation of two-dimensional electron gas in the n+-InGaAs doping channel. However, due to the tunneling effect under large gate-to-source bias, it results in considerably large gate leakage current in the GaAsSb/InGaAs DCFET.

RF and microwave characteristics of a 10 nm thick InGaN-channel gate recessed HEMT

A new depletion-mode gate recessed AlGaN/InGaN/GaN-high electron mobility transistor (HEMT) with 10 nm thickness of InGaN-channel is proposed. A growth of AlGaN over GaN leads to the formation of two-dimensional electron gas (2DEG) at the heterointerface. High 2DEG density (ns) is achieved at the heterointerface due to a strain induced piezoelectric effect between AlGaN and GaN layers. The electrons are confined in the InGaN-channel without spilling over into the buffer layer, which also reduces the buffer leakage current. From the input transfer characteristics the threshold voltage is obtained as −4.5 V and the device conducts a current of 2 A/mm at a drain voltage of 10 V. The device also shows a maximum output current density of 1.8 A/mm at Vds of 3 V. The microwave characteristics like transconductance, cut-off frequency, max frequency of oscillation and Mason's Unilateral Gain of the device are studied by AC small-signal analysis using a two-port network. The stability and power performance of the device are analyzed by the Smith chart and polar plots respectively. To our knowledge this proposed InGaN-channel HEMT structure is the first of its kind.

Atmospheric pressure plasma jet annealed ZnO films for MgZnO/ZnO heterojunctions

Rf-sputtered ZnO films, annealed by atmospheric pressure plasma jet (APPJ), are characterized and used for MgZnO/ZnO heterostructures. The highly reactive N2 plasma generated by APPJ allows much shorter treatment time compared with conventional thermal anneals. The APPJ treatment can increase the crystallinity of ZnO films and release the compressive residue stresses, verified by XRD and UV–Vis transmission measurements. In our previous studies, we demonstrate that thermal anneal is the critical step for the formation of two-dimensional electron gases in defective rf-sputtered MgZnO/ZnO heterostructures. This paper reports the experimental results that APPJ treatments can be used for the same purpose with a much shorter processing time. A thirty-second APPJ anneal on ZnO can be used to replace 400 °C × 30 min furnace-anneal to promote the formation of 2DEGs in MgZnO/ZnO heterostructure. The ultra-short processing time is attributed to the synergy of plasma reactivity and temperature of APPJ.

Enhancement in electron transport and light emission efficiency of a Si nanocrystal light-emitting diode by a SiCN/SiC superlattice structure

We report an enhancement in light emission efficiency of Si nanocrystal (NC) light-emitting diodes (LEDs) by employing 5.5 periods of SiCN/SiC superlattices (SLs). SiCN and SiC layers in SiCN/SiC SLs were designed by considering the optical bandgap to induce the uniform electron sheet parallel to the SL planes. The electrical property of Si NC LED with SiCN/SiC SLs was improved. In addition, light output power and wall-plug efficiency of the Si NC LED with SiCN/SiC SLs were also enhanced by 50% and 40%, respectively. This was attributed to both the formation of two-dimensional electron gas, i.e., uniform electron sheet parallel to the SiCN/SiC SL planes due to the conduction band offset between the SiCN layer and SiC layer, and an enhanced electron transport into the Si NCs due to a lower tunneling barrier height. We show here that the use of the SiCN/SiC SL structure can be very useful in realizing a highly efficient Si NC LED.

Tuning the Surface Charge Properties of Epitaxial InN Nanowires

We have investigated the correlated surface electronic and optical properties of [0001]-oriented epitaxial InN nanowires grown directly on silicon. By dramatically improving the epitaxial growth process, we have achieved, for the first time, intrinsic InN both within the bulk and at nonpolar InN surfaces. The near-surface Fermi-level was measured to be ∼0.55 eV above the valence band maximum for undoped InN nanowires, suggesting the absence of surface electron accumulation and Fermi-level pinning. This result is in direct contrast to the problematic degenerate two-dimensional electron gas universally observed on grown surfaces of n-type degenerate InN. We have further demonstrated that the surface charge properties of InN nanowires, including the formation of two-dimensional electron gas and the optical emission characteristics can be precisely tuned through controlled n-type doping. At relatively high doping levels in this study, the near-surface Fermi-level was found to be pinned at ∼0.95-1.3 eV above the valence band maximum. Through these trends, well captured by the effective mass and ab initio materials modeling, we have unambiguously identified the definitive role of surface doping in tuning the surface charge properties of InN.

Two-dimensional electron gas in Zn-polar ZnMgO/ZnO heterostructure grown by metal-organic vapor phase epitaxy

We report the formation of two-dimensional electron gas (2DEG) at the Zn1−xMgxO/ZnO interface grown by metal-organic vapor phase epitaxy on sapphire substrates. The existence of the 2DEG is confirmed by the observation of Shubnikov–de Haas oscillations and the integer quantum Hall effect. In particular, the Zn0.8Mg0.2O/ZnO heterostructure shows a high Hall mobility of 2138 cm2/V s with a carrier sheet density of 3.51×1012 cm−2 at 1.4 K. We attribute the origin of 2DEG to be the donor states on ZnMgO surface. The dependence of carrier sheet density of 2DEG on ZnMgO layer thickness and Mg composition (x) are also investigated.

Two dimensional electron gases in polycrystalline MgZnO/ZnO heterostructures grown by rf-sputtering process

This paper reports the formation of two-dimensional electron gas (2DEG) in rf-sputtered defective polycrystalline MgZnO/ZnO heterostructure via the screening of grain boundary potential by polarization-induced charges. As the MgZnO thickness increases, the sheet resistance reduces rapidly and then saturates. The enhancement of the interfacial polarization effect becomes stronger, corresponding to a larger amount of resistance reduction, when the Mg content in the cap layer increases. Monte Carlo method by including grain boundary scattering effect as well as 2D finite-element-method Poisson and drift-diffusion solver is applied to analyze the polycrystalline heterostructure. The experimental and Monte Carlo simulation results show good agreement. From low temperature Hall measurement, the carrier density and mobility are both independent of temperature, indicating the formation of 2DEG with roughness scattering at the MgZnO/ZnO interface.

The effects of cap layers on electrical properties of indium nitride films

The unintentional n-type doping in the indium nitride thin films was investigated. The electron density decreases from 3.5×1019 to 9×1018 cm−3 and the mobility increases from 4 to 457 cm2 V−1 s−1 when the thickness increases from 50 to 350 nm. This can be explained by assuming the film consists of a surface accumulation layer and a bulk layer. It was found that the accumulation layer can be eliminated by capping the surface with silicon nitride, GaN or zinc nitride of 2 nm each, respectively; while an AlN cap layer will cause the formation of two-dimensional electron gas at the AlN/InN interface.

Utilizing Polarization Induced Band Bending for InGaN Solar Cell Design

AbstractStrong polarization effects observed in III-nitride materials can invert the surface carrier type. The corresponding band bending can be used to design InGaN solar cells. Similar surface inversion was observed in the past with silicon-based Schottky-barrier solar cells, but was limited by Fermi level pinning. The formation of two-dimensional electron gas by polarization fields in III-nitrides has been reported. Using a similar idea, the growth of a thin AlN capping layer on p-InGaN has resulted in band bending, hence depletion region, under the surface that can be used separate any generated photo-carriers. Hall measurements at different depths on these structures confirm the inversion of surface carrier type. Solar cells based on this concept have resulted in an open circuit voltage of 2.15 V and short circuit current of 21.8 μA.

Temperature Stability of Heteropolytypic 6H/3C FETs

The formation of two-dimensional electron gases (2DEG) at the polytypic 6H/3C heterojunction is investigated. The main of study was to obtain the properties of the 3C/6H structure using Technology Computer Aided Design (TCAD) software. The electron-density distribution and conduction band profile in 6H/3C SiC heterojunction are calculated as a function of temperature. Simulation of these hetero-junctions has concentrated on the I-V behaviour over a range of temperatures between 350 and 650 K. We show that the device characteristics are substantially degraded at high temperatures and this will limit the use of these devices to moderate temperature applications.

Photoluminescence Investigation of Two-Dimensional ElectronGas in an Undoped AlxGa1−xN/GaN Heterostructure

Low-temperature photoluminescence measurement is performed on anundoped AlxGa1−xN/GaN heterostructure.Temperature-dependent Hall mobility confirms the formation oftwo-dimensional electron gas (2DEG) near the heterointerface. A weakphotoluminescence (PL) peak with the energy of ~79 meV lower thanthe free exciton (FE) emission of bulk GaN is related to the radiativerecombination between electrons confined in the triangular well and theholes near the flat-band region of GaN. Its identification is supportedby the solution of coupled one-dimensional Poisson and Schrödingerequations. When the temperature increases, the red shift of the 2DEGrelated emission peak is slower than that of the FE peak. The enhancedscreening effect coming from the increasing 2DEG concentration and thevarying electron distribution at two lowest subbands as a function oftemperature account for such behaviour.

Formation of two-dimensional electron gases in polytypic SiC heterostructures

The formation of two-dimensional electron gases (2DEGs) at polytypic (hexagonal/cubic) SiC heterojunctions (4H/3C SiC and 6H/3C SiC) is investigated by numerical self-consistent solutions of the Schrödinger and Poisson equations. The free-electron-density distributions and conduction-band profiles in the SiC heterostructures are calculated and compared to those occurring at AlGaN/GaN interfaces. Spontaneous and piezoelectric polarization effects in both SiC/SiC and AlGaN/GaN structures are taken into account. The combined effect of the polarization-induced bound charge and conduction-band offset between the hexagonal and cubic SiC polytypes results in the formation of 2DEGs with very high electron sheet concentration. 2DEG sheet densities about 20% larger than that in Al0.3Ga0.7N/GaN structures are calculated for 4H/3C SiC heterostructures. We also find that the 2DEG densities in the 4H/3C are much less sensitive to variations of the barrier layer thickness. The influence of the barrier doping layer on the 2DEG densities is also investigated.

A study of the interface and the related electronic properties in n-Al 0.35Ga 0.65N/GaN heterostructure

The interface and the related electronic properties in n-Al 0.35Ga 0.65N/GaN heterostructure were studied non-destructively by the spectral and transient photovoltage (PV) technique. A defect level as recombination center was observed in the buffer layer of GaN at the side close to the substrate. It was found that the GaN layer of 3 μm was completely depleted owing to the formation of two-dimensional electron gas. When the sample was illuminated at the front or the back side, with a laser radiation of 355 nm, the electron-hole pairs were generated in GaN layer and then separated there. By the action of the electric field in GaN layer, the holes moved to the substrate, while the electrons moved to the AlGaN/GaN interface. In the PV transients under the front illumination, a fast and a slow component, related to the initiative separation of the electron-hole pairs and the drift of the resultant holes, respectively, were observed. At higher illumination intensities, the bimolecular recombination happened to the fast component, while in that period of time it did not to the slow one owing to the long separation distance between the electrons and the holes.

Electron localization and resonant tunneling in uniform nanocrystalline silicon quantum dot systems

In this paper, we review our recent investigation of the electronic transport in hydrogenated nanocrystalline silicon thin film grown on crystalline silicon substrate. The variable magnetic-field Hall measurements have been applied to extract the separated carrier information about the nc-Si:H, c-Si and their interface, where the formation of two-dimensional electron gas is clearly observed. With a decrease in the temperature, the nc-Si:H thin film shows a transition from the extended state to the band-tail state, together with a positive magneto-conductivity at extremely low temperatures. These results reveal the good uniformity of the quantum dots in the nc-Si:H thin film and the electron localization. Furthermore, we have successfully observed clear step-like and spike-like structures for the electrons resonant tunneling through the quasi-2D and 0D states in the I– V curves of the nc-Si:H/ c-Si diode under strong electric field at low temperatures.

Effects of polarization field on formation of two-dimensional electron gas in (0 0 0 1) and [formula omitted] plane AlGaN/GaN heterostructures

Photoluminescence (PL) and temperature-dependent Hall effect measurements were carried out in (0 0 0 1) and (1 1 2 ̄ 0) AlGaN/GaN heterostructures grown on sapphire substrates by metalorganic chemical vapor deposition. There are strong spontaneous and piezoelectric electric fields (SPF) along the growth orientation of the (0 0 0 1) AlGaN/GaN heterostructures. At the same time there are no corresponding SPF along that of the (1 1 2 0) AlGaN/GaN. A strong PL peak related to the recombination between two-dimensional electron gas (2DEG) and photoexcited holes was observed at 3.258 eV at room temperature in (0 0 0 1) AlGaN/GaN heterointerfaces while no corresponding PL peak was observed in (1 1 2 ̄ 0) . The existence of a 2DEG was observed in (0 0 0 1) AlGaN/GaN multi-layers with a mobility saturated at 6000 cm 2/V s below 80 K, whereas a much lower mobility was measured in (1 1 2 ̄ 0) . These results indicated that the SPF was the main element to cause the high mobility and high sheet-electron-density 2DEG in AlGaN/GaN heterostructures.

Simulation of hot electron and quantum effects in AlGaN/GaN heterostructure field effect transistors

We report on simulations of electrical characteristics of AlGaN/(InGaN)/GaN heterostructure field effect transistors with quantum and hot electron effects taken into account. Polarization charges lead to quantum confinement of electrons in the channel and to the formation of two-dimensional electron gas. The electron quantization leads to the spread of the electron wave function into the barrier and bulk but does not have significant impact on dc electrical characteristics. Hot electrons play an important part in the charge transport by spilling over into the bulk GaN where they are captured by traps. This leads to negative differential conductivity, which is also observed experimentally. The simulation results are in good agreement with measured dc characteristics.