Optically induced gate voltage spectroscopy in a GaAs/AlGaAs heterostructure

We report on measurements of optically induced gate voltage spectroscopy in a GaAs/AlGaAs heterostructure with a high mobility 2-dimensional electron gas (2DEG) in a thin (55 nm) GaAs layer. The optically induced gate voltage between the front gate and the 2DEG is sensitive to excess electron concentrations below 10⁷ cm⁻². In the gate voltage spectrum we observe a peak below the bandgap energy of GaAs, which is not observed in the photocurrent, luminescence or excitation spectra. Due to the extremely high sensitivity of this technique we attribute this below bandgap signal to very weak absorption lines below the GaAs bandgap energy by impurity bands or defect absorption. The fall-off of the below bandgap signal varies as exp (hω/E₀), where E₀ is an indicative for the quality of the heterostructure.     

Capacitance fluctuations in quantum dots

We discuss fluctuations of the charging energy E_C and gate voltage spacings between Coulomb oscillation conductance peaks, as computed within spin density functional theory for a realistic GaAs–AlGaAs dot. We explicitly exhibit the fluctuations in the portion of the total free energy which incorporate the interaction between the dot and its surroundings. These variations in the dot capacitance show a dispersion which is typically greater than the dispersion of the total dot charging energy.     

Temperature dependence of the inelastic scattering in a GaAs/n-AlGaAs selectively doped heterojunction with InGaAs quantum dots

Inelastic scattering processes of two-dimensional electron gas (2DEG) have been investigated in a inverted GaAs/n-AlGaAs heterojunction with self-organized InGaAs quantum dots (QDs) embedded near the 2DEG channel where the electron population in the QDs is controllable by the gate voltage V_g. By analyzing magnetoresistance, the inelastic scattering time τ_ε have been evaluated as functions of V_g at 0.6, 0.8, 1.2, and 1.7 K. It is found that τ_ε increases with V_g below 0.8 K and decreases above 1.2 K, which suggests that the dominant scattering mechanisms below 0.8 K and above 1.2 K are different. To interpret this behavior, we have calculated the inelastic scattering time theoretically. It is found that the experimental data are well explained by a theoretical model where a 2D electron is considered to be inelastically scattered both by the other 2D electrons and by the trapped electrons in QDs. It is also found that the 2DEG–2DEG scattering is dominant at low temperature, while the 2DEG-QDs scattering becomes important as the temperature increases.     

Coulomb effects in a ballistic one-channel S-S-S device

We develop a theory of Coulomb oscillations in superconducting devices in the limit of small charging energy E _C ≪Δ. We consider a small superconducting grain with finite capacitance connected to two superconducting leads by nearly ballistic single-channel quantum point contacts. The temperature is assumed to be very low, so there are no single-particle excitations on the grain. Then the behavior of the system can be described in terms of the quantum mechanics of the superconducting phase on the island. The Josephson energy as a function of this phase has two minima that become degenerate when the phase difference on the leads equals to π, the tunneling amplitude between them being controlled by the gate voltage on the grain. We find the Josephson current and its low-frequency fluctuations, and predict their periodic dependence with period 2e on the induced charge Q _x =CV _g . Zh. éksp. Teor. Fiz. 114, 640–653 (August 1998) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor     

The influences of gas and electron temperatures on electron attachment in gas electrical discharges

Various electron attachment processes are reviewed, emphasising the way in which the rates and products of some selected reactions vary with the attaching gas temperatureT _g, the temperature,T _e, and the energy of the attaching electrons. The examples illustrating the variety of reactions are the efficient dissociative attachment reaction to CCl₄, attachment to SF₆ which involves both dissociative and non-dissociative attachment, attachment to CHCl₃ which requires activation energy, and attachment to CCl₃Br which results in both Cl- and Br- product ions. A model has been presented which is able to quantitatively explain the difference influences ofT _g andT _e on the rates of some of these reactions. Also described are the unusually efficient attachment properties of the fullerene molecules C₆₀ and C₇₀ as revealed by our FALP experiments, noting that these molecules have potential importance as efficient suppressers of electrical breakdown through gases such as those used to insulate high voltage devices. We emphasise throughout this paper the importance of an understanding of the separate influences of gas and electron temperature on attachment reactions for the modelling of practical gas discharge media such as etchant plasmas. We dedicate this paper to Professor Jan Janča on the occasion of his sixtieth birthday in recognition of his major contributions to gas discharge physics.     

Electron g-factor in a gated InAs-inserted-channel In0.53Ga0.47As/In0.52Al0.48As heterostructure

We report on electron g-factor in an InAs-inserted In_0.53Ga_0.47As/In_0.52Al_0.48As heterostructure. The gate voltage dependence of g-factor is obtained from the coincidence method. The obtained g-factor values are surprisingly smaller than the g-factor value of bulk InAs, and it is close to the bare g-factor value of In_0.53Ga_0.47As. The large change in g-factor is observed by applying the gate voltage. The obtained gate voltage dependence is not simply explained by the energy dependence of g-factor.     

Charging effects of ErAs islands embedded in AlGaAs heterostructures

Self-assembled ErAs islands were grown on GaAs between a two-dimensional electron gas (2DEG) and a backgate electrode by molecular-beam epitaxy. The islands have overlapping Schottky barriers that form an insulating potential barrier. A TiAu topgate was added by shadow mask evaporation. Thermal activation and charging experiments were employed to gain insight into the electronic properties of the ErAs island systems. In addition the 2DEG was characterized as a function of topgate and backgate voltage.     

Influence of caged noble-gas atom on the superatomic and valence states of C60−

Using high-level ab initio coupled cluster methods, we have conducted a detailed study of two endofullerene anions, He@C^?₆₀ and Ne@C^?₆₀. The main focus was on elucidating the effect of the noble-gas atom on the energy and electronic structure of the stable anion states of the parent C^?₆₀. Our study revealed that the noble-gas atom has no influence on the valence-like states of C^?₆₀, but strongly affects the superatomic-like ²A_g state. The latter was found to undergo pronounced destabilisation in both the He@C^?₆₀ and Ne@C^?₆₀ species, losing more than 70% of its original electron binding energy. The observed destabilisation is due to perturbation of the excess electron density of the ²A_g state inside the C₆₀ cage by the caged noble-gas atom. We show how the results of our analysis can be used to predict the behaviour of the ²A_g state in other closed-shell endohedral complexes. Some possible implications of the observed features of the ²A_g state are discussed.     

Coherent mesoscopic transport through a quantum dot-carbon nanotube system under two-photon irradiation

Mesoscopic transport through an ultrasmall quantum dot (QD) coupled to two single-wall carbon nanotube (SWCN) leads under microwave fields (MWFs) is investigated by employing the nonequilibrium Greens function (NGF) technique. The charging energy and junction capacitances influence the output characteristics sensitively. The MWFs applied on the leads and gate induce novel photon-assisted tunnelling, strongly associated with the density of states (DOS) of the SWCN leads. The SWCN leads act as quantum wires, and the compound effect induces nonlinear current behavior and resonant tunnelling in a larger region of energy scale. Negative differential conductance (NDC) is clearly observed, as the source-drain junction capacitances C _L , and C _R are large enough. The multi-resonant NDC oscillation appears due to the charging and photon-electron pumping effects associated with the contribution of multi-channel quantum wires.Received: 5 July 2004, Published online: 14 December 2004PACS: 73.40.-c Electronic transport in interface structures - 73.63.Fg Nanotubes - 73.61.Wp Fullerenes and related materials - 73.22.-f Electronic structure of nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals     

Single electron charging effects in semiconductor quantum dots

We have studied charging effects in a lateral split-gate quantum dot defined by metal gates in the two dimensional electron gas (2 DEG) of a GaAs/AlGaAs heterostructure. The gate structure allows an independent control of the conductances of the two tunnel barriers separating the quantum dot from the two 2 DEG leads, and enables us to vary the number of electrons that are localized in the dot. We have measured Coulomb oscillations in the conductance and the Coulomb staircase in current-voltage characteristics and studied their dependence on the conductances of the tunnel barriers. We show experimentally that at zero magnetic field charging effects start to affect the transport properties when both barrier conductances are smaller than the first quantized conductance value of a point contact at 2e ²/h. The experiments are described by a simple model in terms of electrochemical potentials, which includes both the discreteness of the electron charge and the quantum energy states due to confinement.     

AlGaN/GaN异质结载流子面密度测量的比较与分析

对MOCVD技术在蓝宝石衬底上生长的不同Al组分AlGaN/GaN异质结进行了范德堡法Hall测量和电容-电压(C-V)测量，发现Hall测量载流子面密度值大于C-V测量值，并且随着AlGaN层Al组分的增加，两种测量值都在增加，同时它们的差值也在增加.认为产生这一结果的原因有两方面.一方面，Ni/Au肖特基金属淀积在AlGaN/GaN异质结上，改变了AlGaN势垒层的表面状态，使得一部分二维电子气(2DEG)电子被抽取到空的施主表面态中，从而减小了AlGaN/GaN异质结界面势阱中的2DEG浓度.随着势垒层Al组分的增加，AlGaN层产生了更多的表面态，从而使得更多的电子被抽取到了空的表面态中.另一方面，由于C-V测量本身精确度受到串联电阻的影响，使得测量电容小于实际电容，从而低估了载流子浓度.     

Dynamics and correlated performance of a photo-triggered discharge-pumped HF laser using SF6 with hydrogen or ethane

3 , has been performed in Ne/SF₆/H₂ and Ne/SF₆/C₂H₆ mixtures. Parameters involved have been the storage line capacitance and the circuit inductance, the capacitors charging voltage, the RH-molecule type and partial pressure, and the X-ray dose for the preionization. High laser performance has been achieved with C₂H₆: an output energy up to 3 J corresponding to a specific energy of 9.6 J/l at an efficiency of 4.7%, which strengthens the advantage of the photo-triggering technique to energize high-power HF lasers. However the optimum performance achieved with H₂, 5.75 J/l and 3.5%, are lower. It is shown, through a time-resolved study of the electrical discharge and spatial dynamics correlated to laser power and energy measurements, that discharge instabilities are responsible for the poor laser performance of the mixture with H₂. These instabilities, which lead to arc development, are characteristics of the discharge in Ne/SF₆. It is demonstrated for the first time that addition of a heavy hydrocarbon, such as C₂H₆, to that mixture induces the discharge stabilization so that the laser emission arises in a homogeneous active medium. This effect allows us to achieve better laser performance than with H₂. Received: 17 March 1998/Revised version: 13 July 1998     

Sr/Si(100)表面TiSi2纳米岛的扫描隧道显微镜研究

为了解半导体衬底与氧化物之间存在的相互作用,以及量子尺寸效应对不同再构体的影响,制备了1—2个原子层厚的TiSi₂/Si(100)纳米岛,并使用扫描隧道显微镜(STM)表征手段详细地研究了TiSi₂ /Si(100)纳米岛的电子和几何特性. 结果发现:这些纳米岛表面显示出明显的金属性;其空态STM图像具有典型的偏压依赖性:在高偏压下STM 图像由三聚物形成的单胞构成,并在低偏压下STM 图像显示为密堆积的图案,这些不同的图案反映出不同能量位的态密度有明显差异. 关键词： 2纳米岛')" href="#">TiSi₂纳米岛 Sr/Si(100)表面 扫描隧道显微镜     

Wave function mapping of self-assembled quantum dots by capacitance spectroscopy

We use frequency-dependent capacitance–voltage spectroscopy to study the dynamic charging of self-assembled InAs quantum dots. With increasing frequency, the AC charging becomes suppressed, beginning with the low-energy states. By applying an in-plane magnetic field, we generate an additional magnetic confinement that alters the tunneling barrier and hence the charging dynamics. In traveling through the potential barrier, the electrons acquire an additional momentum k₀, proportional to the magnetic field B. As the tunneling is enhanced, when k₀ matches the maximum of the electronic wave function Ψ (in momentum representation), we are able to map out the shape of Ψ by varying B.     

UV photoprotection of Osthol

In the present paper, Osthol (molecular formula: C₁₅H₁₆O₃; molecular mass: 244) was obtained from the plant Prangos pabularia by column chromatography technique. This compound was simultaneously exposed to short- (254?nm) and long-wavelength (365?nm) ultraviolet (UV) radiations for different time periods. These irradiated samples were characterized by UV–Visible spectroscopy. No significant variation in optical absorbance/transmittance in broad (200–365?nm) spectrum is observed with increasing dose. The effect of temperature on optical absorption is also insignificant. Effect of UV radiation and temperature on optical band gap (E_g), Urbac energy, or disorder energy (E_u) was also analyzed. A small impact on these optical parameters (E_g and E_u) was observed. This molecule seems very stable (at molecular level) under the influence of UV radiations and temperature. These observed properties shown by this compound projects it as a potential UV protection material.     

DX-center energy calculation with quantitative mobility spectrum analysis in n-AlGaAs/GaAs structures with low Al content

Experimental Hall data that were carried out as a function of temperature (60–350 K) and magnetic field (0–1.4 T) were presented for Si-doped low Al content (x=0.14) n–Al_xGa_1−xAs/GaAs heterostructures that were grown by molecular beam epitaxy (MBE). A 2-dimensional electron gas (2DEG) conduction channel and a bulk conduction channel were founded after implementing quantitative mobility spectrum analysis (QMSA) on the magnetic field dependent Hall data. An important decrease in 2DEG carrier density was observed with increasing temperature. The relationship between the bulk carriers and 2DEG carriers was investigated with 1D self consistent Schrödinger–Poisson simulations. The decrement in the 2DEG carrier density was related to the DX-center carrier trapping. With the simulation data that are not included in the effects of DX-centers, 17 meV of effective barrier height between AlGaAs/GaAs layers was found for high temperatures (T>300 K). With the QMSA extracted values that are influenced by DX-centers, 166 meV of the DX-center activation energy value were founded at the same temperatures.     

Topological quantization of current in quantum tunnel contacts

It is shown that an account of the Berry phase (a topological ϑ-term), together with a dissipative term in the effective action S[ϕ] of the tunnel contacts, induces a strong quantization of the tunnel current at low temperatures. This phenomenon, as the Coulomb blockade, reflects a discrete charge structure of the quantum shot noise and can ensure a quantization of the tunnel current without a capacitive charging energy E _C when the latter is strongly suppressed by quantum fluctuations. Since the value of the ϑ parameter is determined by the gate voltage, this effect allows us to control the current through the contact. A possible physical application of this effect is proposed. The text was submitted by the author in English.     

Coupled ab initio potential energy surfaces for the two lowest 2A′ electronic states of the C2H molecule

Realistic two-valued potential energy surfaces for the reaction C(³P) + CH(X²Π) → C₂ + H have been constructed from a set of high level ab initio data describing the first two ²A′ electronic states of the C₂H system. These states have linear equilibrium configurations, known as the X ²Σ⁺ and A²Π states, and are coupled by a conical intersection. They lead to the formation of C₂(X¹Σ⁺ _g) and C₂(a³Π_u) considering an adiabatic dissociation process. The ab initio calculations are of the multireference configuration interaction variety and were carried out using a polarized triple-zeta basis set. Using the ab initio adiabatic energies and the matrix elements of the dipole moment, a 2 × 2 diabatic representation of the electronic Hamiltonian was built. Each element of this Hamiltonian matrix was expressed within the double many-body expansion (DMBE) scheme which is based, in this case, on the extended Hartree-Fock approximate correlation energy model (EHFACE). The analytical adiabatic potential energy surfaces are then obtained as the eigenvalues of this matrix, and display correctly the Σ/Π conical intersection. Moreover, the non-adiabatic couplings given by our analytical model are compared with the ab initio ones, and good qualitative agreement is observed.     

多间隙耦合腔中注-波同步与耦合

本文基于空间电荷波理论,导出了N间隙耦合腔中注-波耦合系数和电子注电导计算公式.通过计算分析多间隙耦合腔中工作模式(2π模)耦合系数和归一化电子注电导随间隙数目N、直流电压和导流系数的变化,研究多间隙耦合腔中注-波耦合和同步关系,对分布互作用速调管的理论研究有指导意义. 关键词： 多间隙耦合腔 耦合系数 电子注电导 同步与耦合     

Impacts of fluorine on GaN high electron mobility transistors: Theoretical study

We investigate the role of fluorine (F) in GaN‐based high electron mobility transistors (HEMTs) with first principle calculations. Formation energy calculations of F in GaN and AlN reveal that energetically favored interstitial F (F_i) and substitutional F at N sites (F_N) could play important roles in the performance of HEMTs. F_i is responsible for positive threshold voltage (V_th) shift by forming F^– anion and depleting 2DEG carriers. The degradation of device performance at high temperature is ascribed to the defect energy state near conduction band edge of F_N. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)