High-mobility two-dimensional electron gases at oxide interfaces:Origin and opportunities

Our recent experimental work on metallic and insulating interfaces controlled by interfacial redox reactions in SrTiO3-based heterostructures is reviewed along with a more general background of two-dimensional electron gas （2DEG） at oxide interfaces. Due to the presence of oxygen vacancies at the SrTiO3 surface, metallic conduction can be created at room temperature in perovskite-type interfaces when the overlayer oxide ABO3 has Al, Ti, Zr, or Hf elements at the B sites. Furthermore, relying on interface-stabilized oxygen vacancies, we have created a new type of 2DEG at the heterointerface between SrTiO3 and a spinel γ-Al2O3 epitaxial film with compatible oxygen ion sublattices. This 2DEG exhibits an electron mobility exceeding 100000 cm2·V-1·s-1, more than one order of magnitude higher than those of hitherto investigated perovskite-type interfaces. Our findings pave the way for the design of high-mobility all-oxide electronic devices and open a route toward the studies of mesoscopic physics with complex oxides.

A comparison of the transport properties of bilayer graphene,monolayer graphene,and two-dimensional electron gas

We studied and compared the transport properties of charge carriers in bilayer graphene, monolayer graphene, and the conventional semiconductors （the two-dimensional electron gas （2DEG））. It is elucidated that the normal incidence transmission in the bilayer graphene is identical to that in the 2DEG but totally different from that in the monolayer graphene. However, resonant peaks appear in the non-normal incidence transmission profile for a high barrier in the bilayer graphene, which do not occur in the 2DEG. Furthermore, there are tunneling and forbidden regions in the transmission spectrum for each material, and the division of the two regions has been given in the work. The tunneling region covers a wide range of the incident energy for the two graphene systems, but only exists under specific conditions for the 2DEG. The counterparts of the transmission in the conductance profile are also given for the three materials, which may be used as high-performance devices based on the bilayer graphene.

Effects of donor density and temperature on electron systems in AlGaN/AlN/GaN and AlGaN/GaN structures

It was reported by Shen et al that the two-dimensional electron gas （2DEG） in an AlGaN/AlN/GaN structure showed high density and improved mobility compared with an AlGaN/GaN structure, but the potential of the AlGaN/AlN/GaN structure needs further exploration. By the self-consistent solving of one-dimensional Schroedinger- Poisson equations, theoretical investigation is carried out about the effects of donor density （0-1×10^19 cm^-3） and temperature （50-500 K） on the electron systems in the AlGaN/AlN/GaN and AlGaN/GaN structures. It is found that in the former structure, since the effective △Ec is larger, the efficiency with which the 2DEG absorbs the electrons originating from donor ionization is higher, the resistance to parallel conduction is stronger, and the deterioration of 2DEG mobility is slower as the donor density rises. When temperature rises, the three-dimensional properties of the whole electron system become prominent for both of the structures, but the stability of 2DEG is higher in the former structure, which is also ascribed to the larger effective △Ec. The Capacitance-Voltage （C - V） carrier density profiles at different temperatures are measured for two Schottky diodes on the considered heterostructure samples separately, showing obviously different 2DEG densities. And the temperature-dependent tendency of the experimental curves agrees well with our calculations.

MBE生长高2-DEG面密度InP基PHEMT外延材料

利用MBE技术生长了InP基InAlAs/InGaAs PHEMT结构,使用原子力显微镜(AFM)、霍耳测试系统研究了影响二维电子气(2-DEG)面密度和电子迁移率的因素,着重分析了隔离层厚度、沟道层In组分的影响。在保持较高迁移率的基础上,生长出了高μn×ns的InP PHEMT外延材料。

Breakdown voltage analysis of Al_(0.25)Ga_(0.75)N/GaN high electron mobility transistors with partial silicon doping in the AlGaN layer

In this paper,two-dimensional electron gas（2DEG） regions in AlGaN/GaN high electron mobility transistors（HEMTs） are realized by doping partial silicon into the AlGaN layer for the first time.A new electric field peak is introduced along the interface between the AlGaN and GaN buffer by the electric field modulation effect due to partial silicon positive charge.The high electric field near the gate for the complete silicon doping structure is effectively decreased,which makes the surface electric field uniform.The high electric field peak near the drain results from the potential difference between the surface and the depletion regions.Simulated breakdown curves that are the same as the test results are obtained for the first time by introducing an acceptor-like trap into the N-type GaN buffer.The proposed structure with partial silicon doping is better than the structure with complete silicon doping and conventional structures with the electric field plate near the drain.The breakdown voltage is improved from 296 V for the conventional structure to 400 V for the proposed one resulting from the uniform surface electric field.

Two-dimensional metallic behavior at polar MgO/BaTiO_3(110) interfaces

The first-principles calculations are employed to investigate the electrical properties of polar MgO/BaTiO3（110）interfaces. Both n-type and p-type polar interfaces show a two-dimensional metallic behavior. For the n-type polar interface,the interface Ti3d electrons are the origin of the metallic and magnetic properties. Varying the thickness of Ba TiO3 may induce an insulator–metal transition, and the critical thickness is 4 unit cells. For the p-type polar interface, holes preferentially occupy the interface O 2p y state, resulting in a conducting interface. The unbalance of the spin splitting of the O 2p states in the interface Mg O layer leads to a magnetic moment of about 0.25μB per O atom at the interface.These results further demonstrate that other polar interfaces, besides LaAlO3/SrTiO3, can show a two-dimensional metallic behavior. It is helpful to fully understand the role of polar discontinuity on the properties of the interface, which widens the field of polar-nonpolar interfaces.

2-D Theoretical Model for Current–Voltage Characteristics in AlGaN/GaN HEMT’s

A threshold-voltage-based 2-D theoretical model for the Current–Voltage characteristics of the AlGaN/GaN high electron mobility transistors (HEMT’s) is developed. The present work proposes an improved charge-control model by employing the Robin boundary condition when introduced the solution of the 2-D Poisson’s equation in the density of charge depleted in the AlGaN layer. The dependence of 2-DEG sheet carrier concentration on the aluminum composition and AlGaN layer thickness has been investigated in detail. Current–voltage characteristics developed from the 2-DEG model in order to take into account the impact of gate lengths. The relation between the kink effect and existing deep centers has also been confirmed by using an electrical approach, which can allow to adjust some of electron transport parameters in order to optimize the output current.

Abnormal phenomenon of source-drain current of AlGaN/GaN heterostructure device under UV/visible light irradiation

We report an abnormal phenomenon that the source-drain current(I_(D))of AlGaN/GaN heterostructure devices decreases under visible light irradiation.When the incident light wavelength is 390 nm,the photon energy is less than the band gaps of GaN and AlGaN whereas it can causes an increase of ID.Based on the UV light irradiation,a decrease of I_(D) can still be observed when turning on the visible light.We speculate that this abnormal phenomenon is related to the surface barrier height,the unionized donor-like surface states below the surface Fermi level and the ionized donor-like surface states above the surface Fermi level.For visible light,its photon energy is less than the surface barrier height of the AlGaN layer.The electrons bound in the donor-like surface states below the Fermi level are excited and trapped by the ionized donor-like surface states between the Fermi level and the conduction band of AlGaN.The electrons trapped in ionized donor-like surface states show a long relaxation time,and the newly ionized donor-like surface states below the surface Fermi level are filled with electrons from the two-dimensional electron gas(2DEG)channel at AlGaN/GaN interface,which causes the decrease of ID.For the UV light,when its photon energy is larger than the surface barrier height of the AlGaN layer,electrons in the donor-like surface states below the Fermi level are excited to the conduction band and then drift into the 2DEG channel quickly,which cause the increase of ID.

Electrical property effect of oxygen vacancies in the heterojunction of LaGaO3/SrTiO3

Density functional theory within the local density approximation is used to investigate the effect of the oxygen va- cancy on the LaGaO3/SrTiO3 （001） heterojunction. It is found that the energy favorable configuration is the oxygen vacancy located at the 3rd layer of the STO substrate, and the antiferrodistortive distortion is induced by the oxygen vacancy introduced on the SrTiO3 side. Compared with the heterojunction without introducing oxygen vacancy, the heterojunction with introducing the oxygen vacancy does not change the origin of the two-dimensional electron gas （2DEG）, that is, the 2DEG still originates from the dxy electrons, which are split from the t2g states of Ti atom at interface; however the oxygen vacancy is not beneficial to the confinement of the 2DEG. The extra electrons caused by the oxygen vacancy dominantly occupy the 3dx2-y2 orbitals of the Ti atom nearest to the oxygen vacancy, thus the density of carrier is enhanced by one order of magnitude due to the introduction of oxygen vacancy compared with the density of the ideal structure heterojunction.

Electrical characteristics of AlInN/GaN HEMTs under cryogenic operation

Electrical properties of an AIlnN/GaN high-electron mobility transistor （HEMT） on a sapphire substrate are investigated in a cryogenic temperature range from 295 K down to 50 K. It is shown that drain saturation current and conductance increase as transistor operation temperature decreases. A self-heating effect is observed over the entire range of temperature under high power consumption. The dependence of channel electron mobility on electron density is investigated in detail. It is found that aside from Coulomb scattering, electrons that have been pushed away from the AIInN/GaN interface into the bulk GaN substrate at a large reverse gate voltage are also responsible for the electron mobility drop with the decrease of electron density.

Topological to trivial insulating phase transition in stanene

Electronic properties of stanene, the Sn counterpart of graphene are theoretically studied using first-principles simulations. The topological to trivial insulating phase transition induced by an out-of-plane electric field or by quantum confinement effects is predicted. The results highlight the potential to use stanene nanoribbons in gate-voltage controlled dissipationless spin-based devices and are used to set the minimal nanoribbon width for such devices, which is typically approximately 5 nm.

Real-time decay of fluorinated fullerene molecules on Cu（001） surface controlled by initial coverage

In this study, the evolution of C60F18 molecules on a Cu（001） surface was studied by means of scanning tunneling microscopy and density functional theory calculations. The results showed that fluorinated fuUerenes （tortoise-shaped polar C60F18） decay on Cu（001） surfaces by a step-by-step detachment of F atoms from the C60 cage. The most favorable adsorption configuration was realized when the F atoms of C60F18 pointed towards the Cu surface and six F atoms were detached from it. The results also showed that a further decay of C60F12 molecules strongly depended on the initial C60F18 coverage. The detached F atoms initially formed a two-dimensional （2D） gas phase which then slowly transformed into F-induced surface structures. The degree of contact between the C60F12 molecules and the Cu（001） surface depended on the density of the 2D gas phase. Hence, the life-time of fluorinated fullerenes was determined by the density of the 2D gas phase, which was affected by the formation of new F-induced structures and the decay of C60F12 molecules.