Theoretical study of resonant tunneling in rectangular double-, triple-, quadruple-, and quintuple-barrier structures

The transmission coefficient and the resonance condition in the one-dimensional rectangular double-, triple-, quadruple-, and quintuple-barrier structures are derived theoretically under the assumption of the constant tunneling effective mass. It is found that the resonance energies are different from the eigenvalues in the quantum well due to coupling between wells in the multiple-barrier (much more than triple-barrier) structures. It is confirmed that the transmission spectrum is a Lorentzian near to energies of resonance.     

Dynamics of resonant and non-resonant tunneling in asymmetric coupled quantum wells

Within the framework of the effective mass approximation, coherent oscillations of a photoexcited electron wave packet in an asymmetric coupled quantum well structure have been studied using a time-dependent Schrödinger equation. In the method of calculation, the continuity of the current across a semiconductor heterojunction is considered. The amplitude and period of the electronic is obtained and in the case of high bias, it is found the existence of electric field-induced tunelling to semiconductor bulk.     

Time domain capacitance spectroscopy of tunneling electrons in the two-dimensional electron gas

We have developed a technique capable of measuring the tunneling current into both localized and conducting states in a 2D electron system (2DES). The method yields I-V characteristics for tunneling with no distortions arising from low 2D in-plane conductivity. We have used the technique to determine the pseudogap energy spectrum for electron tunneling into and out of a 2D system and, further, we have demonstrated that such tunneling measurements reveal spin relaxation times within the 2DEG. Pseudogap: In a 2DEG in perpendicular magnetic field, a pseudogap develops in the tunneling density of states at the Fermi energy. We resolve a linear energy dependence of this pseudogap at low excitations. The slopes of this linear gap are strongly field dependent. No existing theory predicts the observed behavior. Spin relaxation: We explore the characteristics of equilibrium tunneling of electrons from a 3D electrode into a high mobility 2DES. For most 2D Landau level filling factors, we find that electrons tunnel with a single, well-defined tunneling rate. However, for spin-polarized quantum Hall states (ν=1, 3 and 1/3) tunneling occurs at two distinct rates that differ by up to two orders of magnitude. The dependence of the two rates on temperature and tunnel barrier thickness suggests that slow in-plane spin relaxation creates a bottleneck for tunneling of electrons.     

In-gap density of states in hole- and electron-doped CuO2 planes probed by scanning tunneling spectroscopy

The low-energy electronic structure of a c-axis Sr_xA_yCuO₂ (A is alkaline earth cation, x+y≦1, hole- and electron-doped infinite layer) thin film, grown by laser-molecular-beam epitaxy on a SrTiO₃ (001) substrate, has been studied using ultrahigh-vacuum scanning tunneling microscopy/spectroscopy. Images have been obtained for co-deposited Sr_xA_yCuO₂ thin films, which show the surface consisting of flat terraces separated by steps that are unit cell high. Tunneling spectra of undoped Sr_0.3Ca_0.7CuO₂ indicate a wide band gap of 1.8 eV which is consistent with the charge transfer gap. Hole-doped Sr_0.85CuO₂ shows in-gap states appearing at both the valence and conduction band edges. In contrast, for the electron-doped Sr_0.9La_0.1CuO₂, in-gap states appear predominantly above the Fermi level, and the spectral shape becomes asymmetric around the Fermi level. When these two systems are compared, barrier-height measurements reveal that there is no apparent shift of the Fermi level measured from the vacuum level. This suggests that the framework of the rigid-band picture might break down implying a strongly correlated electron system. Received: 20 August 1998 / Accepted: 15 February 1999 / Published online: 5 May 1999     

THz-field induced nonlinear transport and dc voltage generation in a semiconductor superlattice due to Bloch oscillations

We report on a theoretical analysis of terahertz (THz-) field induced nonlinear dynamics of electrons in a semiconductor superlattice that are capable to perform Bloch oscillations. Our results suggest that for a strong THz-field a dc voltage should be generated. We have analyzed the real-time dynamics using a balance equation approach to describe the electron transport in a superlattice miniband. Taking account of both Bloch oscillations of electrons in a superlattice miniband and dissipation, we studied the influence of a strong THz-field on currently available superlattices at room temperature. We found that a THz-field can lead to a negative conductance resulting in turn in a THz-field induced dc voltage, and that the voltage per superlattice period should show, for varying amplitue of the THz-field, a form of wisted plateaus with the middle points being with high precision equal to the photon energy divided by the electron charge. We show voltage to the finite voltage state, and that in the finite voltage state dynamic localization of the electrons in a miniband occurs.     

Resonant tunneling in electrically biased multibarrier systems

The effect of a uniform electric field on the resonant tunneling across multibarrier systems (GaAs/Al_xGa_1−xAs and GaN/Al_xGa_1−xN) is exhaustively explored by a computational model using exact Airy function formalism and the transfer-matrix technique. The numerical computation takes care of the common problems of numerical inefficiency and overflow associated with the Airy functions for low-applied voltages. The model presents the study of both the field-free and field-dependent tunneling across multibarrier systems using a single formalism. The current-voltage characteristics, studied for the multibarrier systems with different number of barriers, exhibit all the experimentally observed features like resonant peaks, negative differential conductivity regimes, etc. Our results have both qualitative and quantitative agreement with the reported experimental findings.     

Capacitance analysis for a metal–insulator– semiconductor structure with an ultra-thin oxide layer

Applied Physics A - We have studied theoretically the capacitance characteristics of a metal–insulator–semiconductor structure with an ultra-thin oxide layer by self-consistently...     

Theoretical study of resonant-tunneling and confining phenomena with mass variation in unsymmetrical rectangular double-barrier structures

Analytical expressions for the transmission coefficient and the resonance condition in unsymmetrical rectangular double-barrier structures are derived theoretically by taking into account the mass difference between well and barrier layers. It is found that resonant tunneling with a transmission peak equal to 1 (unity resonance) and resonant tunneling with a transmission peak less than 1 (below unity resonance) may occur in the unsymmetrical double-barrier structures. Two independent conditions are required for unity-resonant transmission: One is the Phase-Difference Condition for Resonance (PDCR) and the other is the Maximum Condition for the Peak Value (MCPV). The below-unity resonant transmission occurs when only condition PDCR holds. It is believed that the two conditions are useful for calculating values of the transmission coefficient and the resonance energy for the unsymmetrical double-barrier structures. They may be useful for resonant tunneling-device fabrication. Furthermore, wave functions of an electron at resonance level are calculated and the confining phenomenon is confirmed.     

Low contact resistance of n+-Si/Ti/WNx/Al submicron contact structures

+ -Si/Ti/WN_x/Al multi-layer metallization scheme. The contact resistance has been strongly related to the plasma nitridation of the Ti surface because the contact resistance of n⁺-Si/Ti/WN_x/Al with contact size of 0.49 μm² about 100–130 Ω, whereas without the nitridation of the Ti surface the contact resistance rises up to 200–390 Ω. ¹⁹F (p,αγ) nuclear resonance analysis and Auger electron spectroscopy reveal that F adatoms on the Ti surface are successfully removed by the 30 s nitridation and as a result, the low contact resistance can be achieved. Received: 16 July 1996/Accepted: 5 November 1996     

The incorporation of Nickel and Phosphorus dopants into Boron-Carbon alloy thin films

5 C) alloy thin films grown by plasma-enhanced chemical vapor deposition have been examined. The Ni-doped boron–carbon alloys were grown using closo-1,2-dicarbadodecaborane (C₂B₁₀H₁₂) as the boron–carbon source compound and nickelocene(Ni(C₅H₅)₂) as the nickel source. The phosphorus-doped alloys were grown using the single-source compound: dimeric chloro-phospha(III)-carborane ([C₂B₁₀H₁₀PCl]₂). Nickel doping increased the conductivity, relative to undoped B₅C, by six orders of magnitude from 10^-9 to 10^-3 (Ω cm)^-1 and transformed the material from a p-type semiconductor to an n-type. Phosphorus doping decreased the conductivity, relative to undoped B₅C, by two orders of magnitude and increased the band gap from 0.9 eV for the undoped material to 2.6 eV. Infrared absorption spectra of the nickel- and phosphorus-doped B₅C alloys were relatively unchanged from those of undoped B₅C. X-ray diffraction suggests that the phosphorus-doped material may be a different polytype from the Ni-doped and undoped B₅C alloys. Received: 23 April 1997/Accepted: 3 November 1997     

Mechanism and control of current transport in GaN and AlGaN Schottky barriers for chemical sensor applications

Strong interests are recently emerging for development of integrated high-performance chemical sensor chips. In this paper, the present status of understanding and controlling the current transport in the GaN and AlGaN Schottky diodes is discussed from the viewpoint of chemical sensor applications. For this purpose, a series of works recently carried out by our group are reviewed in addition to a general discussion. First, current transport in GaN and AlGaN Schottky barriers is discussed, introducing the thin surface barrier (TSB) model to explain the anomalously large leakage currents. Following this, attempts to reduce the leakage currents are presented and discussed. Then, as an example of gas-phase sensors using Schottky barriers, a Pd/AlGaN/GaN Schottky diode hydrogen sensor developed recently by our group is presented with a discussion on the sensing mechanism and related current transport. On the other hand, in liquid-phase sensors, contact is made between liquid and semiconductor which is regarded as a kind of Schottky barrier by electrochemists. As one of such liquid-phase sensors, open-gate AlGaN/GaN heterostructure field effect transistor (HFET) pH sensor developed recently by our group is presented. Finally, a brief summary is given together with some remarks for future research.     

Nonlinear transport properties of quantum dots

The influence of excited levels on nonlinear transport properties of a quantum dot weakly coupled to leads is studied using a master-equation approach. A charging model for the dot is compared with a quantum mechanical model for interacting electrons. The currentvoltage curve shows Coulomb lockade and additional finestructure that is related to the excited states of the correlated electrons. Unequal coupling to the leads causes asymmetric conductance peaks. Negative differential conductances are predicted due to the existence of excited states with different spins.     

Rapid communicationPhotoelectron emission in femtosecond laser assisted scanning tunneling microscopy

Direct illumination of the tunneling gap in an ultrahigh vacuum scanning tunneling microscope with ultrashort pump-probe laser pulses may offer ultimate spatial and temporal resolution in surface experiments. The electronic bandwidth of the tunneling gap ( 1 THz) does not limit the time resolution. Our experiments show that multiphoton photoelectron emission from the sample limits the application of this detection scheme at high laser fluence. However, a substrate specific pump-probe effect in the photoelectron yield with femtosecond transients is observed on Tantalum and on GaAs(110) surfaces. Received: 5 November 1996     

Formation and charge control of a quantum dot by etched trenches and multiple gates

We have fabricated a GaAs/InGaAs/AlGaAs-based single-electron transistor (SET) formed by etched trenches and multiple gates. Clear Coulomb-blockade oscillations have been observed when the gate biases are scanned. By self-consistently solving three-dimensional Schr?dinger and Poisson equations, we have studied the energy-band structure and the carrier distribution of our SET. General agreement between numerical simulation results and measurement data has been obtained, thus indicating the effectiveness of our SET-device design as well as the necessity of a complete three-dimensional quantum-mechanical simulation. Received: 18 October 2001 / Accepted: 6 January 2002 / Published online: 20 March 2002     

Stark shifts of charged particles in semiconductor quantum wells

 We calculate the effect of a homogeneous electric field on electrons, holes and excitons confined in a quantum well structure consisting of alternate thin layers of well and barrier material. The electric field which acts perpendicular to the quantum well is taken as a perturbation on the quantum well structure confining the charges. The electron and hole energies in the conduction and valence subbands are calculated by solving a one-dimensional Schr?dinger equation. The exciton binding energy is calculated using an improved excitonic model. Results obtained indicate the importance of higher-order excitons in optical transitions at high electric fields. Received: 29 February 1996/Accepted: 19 August 1996     

M-waveguide structures for direct phase matching in AlGaAs

In this paper the potential of M-waveguide structures for direct phase matching in AlGaAs is investigated by numerical simulations. Principal waveguiding characteristics are discussed. The impact of the optical layer thicknesses is analyzed with respect to phase matching and conversion efficiency for second harmonic generation (SHG). An optimization of the M-waveguide parameters yields a normalized conversion efficiency of 153 or 214 %/W respectively. Received: 16 May 2001 / Revised version: 2 August 2001 / Published online: 23 October 2001     

Electronic structure of single-wall carbon nanotubes studied by resonant inelastic X-ray scattering

F in the nanotubes, indicating that metallic nanotubes are present in the sample. Received: 2 April 1998     

Influences of thermal annealing, the electrolyte pH, and current density on the interface state density distribution of anodic MOS structures

2 /p-Si MOS structures were prepared in 0.1 M K₂SO₄ electrolyte with a pH of 7 (the 0.1 M KOH solution was buffered with H₂SO₄) at current densities of 3, 5, and 7 mA/cm² and with four different pH values of the electrolyte at 3 mA/cm². It is found that thermal annealing at a relatively low temperature can be used to improve the anodic MOS characteristics. Moreover, of the pH and current density it followed that the pH has a dominant role in the interface electrical properties. The lowest interface state densities at the maximum and the midgap positions are 7.1×10¹¹ and 2.7×10¹⁰ eV^-1cm^-2 for a sample made with pH=7, J=3 mA/cm². The characteristics of this sample seem satisfactory for device applications of anodized p-Si. Received: 8 July 1996/Accepted: 22 January 1997     

Electrical characteristics of hole resonant tunneling diodes with high Ge fraction (x > 0.4) Si/strained Si1−xGex/Si(1 0 0) heterostructure

Effectiveness of a Ge fraction modulated spacer in hole resonant tunneling diodes (RTDs) with Si/strained Si_1−xGe_x heterostructures epitaxially grown on Si(1 0 0) was investigated to improve the electrical characteristics at higher temperatures. Electrical characteristics measured for 30 RTDs, with the modulated spacer at higher Ge fraction (x = 0.48) on a single wafer, show that the deviation of the peak current and voltage at the resonant peak falls in ranges of ±25% and ±10%, respectively. For the RTDs, negative differential conductance (NDC) characteristics are obtained even at higher temperatures around 230 K than that for the RTDs with x = 0.42. The result indicates that the introduction of higher Ge fraction is effective for NDC in RTD at higher temperature.     

Single-electron quantum dots in silicon MOS structures

We present experimental results for two types of quantum dots, which are embedded within a silicon metal-oxide-semiconductor structure. Evidence is found for single-electron charging at low temperature, and for an asymmetric shape of the dot. First results of simulations of these dots are presented. Received: 14 April 2000 / Accepted: 17 April 2000 / Published online: 6 September 2000