Exciton-assisted tunneling transport in heterojunction microstructures

Longitudinal tunneling transport in the low-dimensional heterojunction structures induced by the excitonic Coulomb interaction has been formulated and discussed in the framework of Fermi's golden rule. We have investigated the tunneling transition of free carriers to quantum-well Wannier–Mott excitons incorporated in the sequential tunneling Hamiltonian. The modeling is evaluated by a set of coupled rate equations involving subband states of electron, hole and exciton. The exciton-assisted tunneling (EAT) phenomenon has its characteristic fingerprint causing tunneling current prior to the resonance electric fields, and a significant modulation of the I–V characteristics. It is also found that the bias offset and the FWHM of the EAT current spectrum can be comparable to that of resonant tunneling (RT) current, depending both on the 2D hole density of the confined subband and the excitonic properties in the active region. The EAT effect has a different I–V spectral line shape, compared to that of the RT or its replica, tailing off in the resonance regime induced by the exciton binding energy.     

Resonant Raman scattering of discrete hole states in self-assembled Si/Ge quantum dots

We report the first resonant electronic Raman spectroscopy study of discrete electronic transitions within small p-doped self-assembled Si/Ge quantum dots (QDs). A heavy hole (hh) to light hole (lh) Raman transition with a dispersionless energy of 105 meV and a resonance energy of the hh states to virtually localised electrons at the direct band gap of 2.5 eV are observed. The hh–lh transition energy shifts to lower values with increasing annealing temperature due to significant intermixing of Si and Ge in the QDs. Structural parameters of the small Si/Ge dots have been determined and introduced into 6-band k·p valence band structure calculations. Both the value of the electronic Raman transition of localised holes as well as the resonance energy at the E₀ gap are in excellent agreement with the calculations.     

Quantum box resonant tunneling spectroscopy: Experiments and modelisation

We have calculated the potential profile and the electronic levels in resonant tunneling double barrier structures with nanometric lateral dimensions (≤ 500 nm) for various contact doping. At biases for which the box states (laterally confined quantum well) are resonant with the emitter Fermi level, fine structures are expected in the resonant tunneling current. Comparison with I(V) characteristics measured on nanometric GaAs/GaAlAs and GaAs/GaAlAs/InGaAs resonant tunneling diodes shows that our model accounts for the resonance bias voltage and explains the shape of the current peak. The fine structure observed in the current peak provides a spectroscopy of the confined states in the quantum box.     

Exchange-mediated anisotropy of (ga,mn)as valence-band probed by resonant tunneling spectroscopy

We report on experiments and theory of resonant tunneling anisotropic magnetoresistance (TAMR) in AlAs/GaAs/AlAs quantum wells (QW) contacted by a (Ga,Mn)As ferromagnetic electrode. Such resonance effects manifest themselves by bias-dependent oscillations of the TAMR signal correlated to the successive positions of heavy (HH) and light (LH) quantized hole energy levels in GaAs QW. We have modeled the experimental data by calculating the spin-dependent resonant tunneling transmission in the frame of the 6 x 6 valence-band k.p theory. The calculations emphasize the opposite contributions of the (Ga,Mn)As HH and LH subbands near the Gamma point, unraveling the anatomy of the diluted magnetic semiconductor valence band.     

Effect of strain on hole tunneling dynamics in double barrier heterostructures

A study of the effects of strain due to external stress and lattice mismatch on hole tunneling times in double barrier heterostructures is presented. The band structure of holes is calculated based on a k·p method within the envelope function approximation, including band mixing effects. The phase delay time is obtained from the energy derivative of the total phase shift of the wave function upon tunneling and is used to estimate the hole tunneling time. The results demonstrate that strain can be utilized to tailor hole tunneling times by changing the energy separation and, consequently, the mixing between heavy hole and light hole states in the quantum well.     

Infrared absorption in SiGe/Si multiple quantum wells

Infrared absorption has been used to investigate the subband structures in SiGe/Si quantum wells. The quantum wells are prepared using RRH/VLP-CVD and consist of 20 periods of and 60 periods of . The good periodical and interface sharpness of the SiGe/Si quantum wells have been shown by Auger Electron Spectroscopy (AES). The absorption peaks due to transitions between the hole subbands and the conduction band have been observed in infrared absorption spectra. The transverse photocurrent spectrum parallel to the growth plane have also shown absorption peaks due to transitions between the heavy and light hole band states and the conduction band states in quantum wells.     

Macroscopic resonant tunneling in an rf-SQUID flux qubit

We have observed the macroscopic resonant tunneling of magnetic flux between macroscopically distinct quantum states in a superconducting flux qubit.The dependences of the macroscopic resonant tunneling on the barrier height of the potential well,the flux bias and the initial state are investigated.Detailed measurements of the tunneling rate as a function of the flux bias reveal the feature of the quantum noise in the superconducting flux qubit.     

Photocurrent response in a double barrier structure with quantum dots–quantum well inserted in central well

We report the photocurrent response in a double barrier structure with quantum dots–quantum well inserted in central well. When this quantum dots–quantum well hybrid heterostructure is biased beyond +1 or −1 V, the photocurrent response manifests itself as a step-like enhancement, increasing linearly with the light intensity. Most probably, at proper bias condition, the pulling down of the X minimum of GaAs at the outgoing interface of the emitter barrier by the photovoltaic effect in GaAs QW will initiate the Γ–X–X tunneling at much lower bias as compared with that in the dark. That gives rise to the observed photocurrent response.     

Multicomponent Structure of an Electron-Hole Liquid in Shallow SiGe/Si Quantum Wells

The fine structure of the emission spectrum lines is studied for a quasi-two-dimensional electronhole liquid (EHL) embedded in SiGe/Si quantum wells. It is shown that the patterns observed in the spectra of the two-particle (IR band) and four-particle (visible band) recombination can be explained by the presence of both heavy and light holes in the condensed phase. Comparison of the experimental data and calculated photoluminescence spectra of EHL allow determination of its main parameters, e.g., the equilibrium density, work function for pairs of particles, and light hole–heavy hole splitting at the G-point of the Brillouin zone for quantum wells that are 3.7–5.1% Ge.     

Lateral transport in boron doped SiGe quantum wells

The temperature dependence of lateral conductivity and hole mobility in boron doped Si/SiGe/Si quantum well structures were studied. The conductivity at the temperatures below 20 K is shown to be due to hopping over B centers while at higher temperatures it is due to two-stage excitation consisting of thermal activation of holes from the ground to strain-split B states and the next hole tunneling into the valence band.     

表面偶极子对扫描电子显微镜共振电子注入的影响

用共振电子注入法和第一性原理计算研究了硒(Se)单原子在Si(111)-7×7表面的吸附. 理论结果表明由于不同的电负性,表面Si原子会向吸附的Se原子发生电子转移,从而导致一个0.61 eV的表面偶极子形成. 该表面偶极子改变了Si表面的有效隧道能垒同时导致在样品和扫描电子显微镜针尖之间真空间隙中共振态能级的移动. 并且0.61 eV的表面偶极子会引起共振电子注入偏压向高电位移动0.45 V.     

Fine structure of the emission spectrum of a two-dimensional electron–hole liquid in SiGe/Si quantum wells

The fine structure of the emission spectrum of a quasi-two-dimensional electron–hole liquid in shallow SiGe/Si quantum wells is observed experimentally. This fine structure is explained by the occurrence of steps in the density of states resulting from the coexistence of light and heavy holes in the liquid phase and by the interaction of charge carriers with charge-density oscillations in the liquid.     

Acoustically driven charge separation in semiconductor heterostructures sensed by optical spectroscopy techniques

We demonstrate a method of using a two-layer sandwich structure, which includes a LiNbO₃ plate and a semiconductor heterostructure to create an inhomogeneous stress and piezoelectric harmonic potential in the semiconductor. Both the GaAs/AlGaAs quantum well (QW) structures and SiGe/Si heterostructures are attempted, working with and without using a piezoelectric field in the semiconductor layer. The standing-wave fields generated in the semiconductor and the electron and hole distributions driven by the piezoelectric field are computed by finite element method (FEM) techniques. It is experimentally shown that, in a GaAs/Al_xGa_1-x As asymmetric double quantum well structure, the resonance enhancement of the narrower QW photoluminescence band is observed, which may be explained by the resonant charge transfer between the wider and narrower QWs. It is also shown that the piezoelectric fields quench the pure LO-phonon lines in the Raman spectra, whereas the coupled LO-phonon-plasmon mode strengthens. Experimental results indicate that the charge separation occurs in the plane of the QWs due to the piezoelectric fields. The recombination of carriers in the SiGe/Si heterostructures can be effectively enhanced by the presence of ultrasonic stress, displaying features consistent with varying electrical activity at dislocations.     

耦合锗量子点中空穴态对称特性研究

分别采用单带重空穴近似和六带Kronig-Penney模型, 对垂直耦合锗量子点在不同耦合距离下的空穴态特性进行了计算, 并探讨了自旋-轨道的相互作用对空穴态对称性的影响. 计算结果表明: 多带耦合的框架下, 随着量子点垂直间距的增大, 空穴基态从成键态转变为反键态, 而且价带基态能级和第一激发态能级发生反交叉现象, 这与单带模型下得到的相应结果存在较大差异. 通过分析六带模型计算得到的成、反键态波函数, 轻、重空穴态和自旋-轨道分裂态对特征空穴态波函数的贡献比例随着量子点垂直间距的增大发生了转变, 并最终导致量子点空穴基态波函数由成键态转变为反键态. 关键词： 耦合量子点 空穴态 成健态-反健态 自旋-轨道     

Surface-field induced interband tunneling in InAs

Metal/insulator/semiconductor junctions are prepared on degeneratep-type InAs substrates with hole concentrations ranging from 2.3×10¹⁷ cm^–3 to 2.7×10¹⁸ cm^–3. The low work function of the top metal Yb, Al, or Au and charged interface states influence a two-dimensional (2D) electron inversion layer at the InAs surface. The insulator barrier that is formed by thermal oxidation is designed sufficiently thin, so that the bias voltage applied at the metal electrode mainly drops across the depletion layer separating the electron channel from the bulk. The current-voltage (I–V) characteristics exhibit strong negative differential conductance due to interband, tunneling from the 2D subband into the 3D valence band with peak-to-valley current ratios up to 3.1, 18, and 32 at 300 K, 77 K, and 4.2 K, respectively. In agreement with a theoretical model based on coherentelastic tunneling, the form of the I–V curves resembles those of double-barrier resonant tunnel devices rather than those of 3D Esaki diodes. The series resistance is obtained from the saturation of the differential conductance dI/dV at high forward bias and from the shift of structures in d² I/dV ² arising from phonon assisted tunneling.Dedicated to G. Lautz on the occasion of his 65th birthday     

Magneto-tunneling spectroscopy of GaAs---AlGaAs double barrier tunneling devices in pulsed high magnetic fields up to 40T

The magneto-tunneling effect was investigated in GaAs---AlGaAs double barrier resonant tunneling devices in pulsed high magnetic fiels up to 40T applied parallel(B) and perpendicular (B) to the barrier layers. In a sample with , oscillatory structures due to the 2D electrons in the emitter and the LO phonon assisted resonant tunneling were observed when the magnetic field (B) was swept at constant bias voltages. A large drop of the current was found in the quantum limit at applied voltages below the negative differential conductivity region. A striking hysteresis was observed in the voltage-current (V - I) curves. In a wide well sample with , rich structures were observed in the V - I curve for B, corresponding to the tunneling to different cyclotron orbits from the emitter.     

半导体量子器件物理讲座第四讲 共振隧穿器件及其电路应用

当一个电子的能量低于势垒高度时,它仍可以隧穿通过势垒,在一定条件下,双势垒结构中电子的隧穿几率甚至可以接近1,利用这种共振隧穿现象可以做成共振隧穿二极管,它的电流－电压特性曲线中会出现负微分电阻,利用这种负阻效应可以做成高频振荡器和倍频器等电子器件,双势垒结构与通常的双极晶体管结合可以做成共振隧穿双极晶体管,它们可以用来做成多态记忆器和模数转换器等器件。     

Room-temperature observation of current bistability and fine structures in germanium quantum dots/SiO2 resonant tunneling diodes

The steady-state and time-dependent current–voltage (I–V) characteristics are experimentally investigated in Ge quantum dot (QD)/SiO₂ resonant tunneling diodes (RTDs). Ge QDs embedded in a SiO₂ matrix are naturally formed by thermal oxidation of Si_0.9Ge_0.1 nanowires (30 nm×50 nm) on silicon-on-insulator substrates. The average dot size and spacing between dots are 9±1 and 25 nm, respectively, from TEM observations, which indicate that one or two QDs are embedded between SiO₂ tunneling barriers within the nanowires. Room-temperature resonant oscillation, negative differential conductance, bistability, and fine structures are observed in the steady-state tunneling current of Ge-QD/SiO₂ RTDs under light illumination. Time-dependent tunneling current characteristics display periodic seesaw features as the Ge-QDs RTD is biased within the voltage regime of the first resonance peak while they exhibit harmonic swing behaviors as the RTD is biased at the current valleys or higher-order current peaks. This possibly originates from the interplay of the random telegraph signals from traps at the QD/SiO₂ interface as well as the electron wave interference within a small QD due to substantial quantum mechanics effects.     

Influence of off-set energy on the electrical characteristics of Si resonant tunneling MOST (SRTMOST)

The effects of the off-set energy between the dielectric films which are formed at the both edges of the channel and the Si substrate on the electrical characteristics are examined for the Si resonant tunneling MOST (SRTMOST). The barrier height of the dielectric films has a great influence on the operation of the SRTMOST. The relationship between the transmission coefficient and the off-set energy of the double barrier/Si is calculated as well as the relationships between the gate-off currents, the transition time from the source to the drain and the off-set energy. It was found that the critical off-set energy of dielectric film/Si is approximately 1.0–1.5 eV.     

A Green's function solution to the image and multiple image interactions for hyperboloidal geometry: Application to metallic pointcontact infrared detectors

Using the Mehler-Fock transformation to solve Poisson's equation in prolate spheroidal coordinates, we have obtained an exact Green's function solution for all multiple image corrections to the vacuum tunneling barrier for a hyperboloidal tip-planar-anode model of a point-contact junction consisting of identical or dissimilar metals.These calculations show that the image corrections significantly modify both the form and area of the barrier, producing an enhancement in the rectification and tunneling currents at low bias.I–V characteristics have also been obtained for the hyperboloidal tip model using estimates of the emission and collection regions based on field emission experiments for whisker tips of comparable dimensions. These results are compared with earlier calculations whichThis research was supported in part by the NATO Research Grants Program, Grant No. 1902, Scientific Affairs Division, Brussels, BELGIUM.