Quantum Nanostructures Research Articles

Evolution of epitaxial titanium silicide nanocrystals as a function of growth method and annealing treatments

Abstract Titanium silicide grows on silicon in a form of discontinuous layers, which is the most serious obstacle to the formation of high-quality epilayers for VLSI applications. At the same time, nanometric dimensions of the epitaxial silicide islands attract interest as quantum nanostructures. However, for this purpose, nanocrystals in a self-assembled array have to be defect-free, and exhibit high shape and size uniformity. In this work titanium silicide was grown on Si(1 1 1) substrates by reactive deposition epitaxy and by solid-phase epitaxy. Since the reaction and phase-formation kinetics depend on the growth method, accordingly different lattice matching and facet energies may result in different morphological shapes of the nanocrystals. Nanocrystals from reaction in a solid-state could be characterized as highly non-uniform in both shape and size, and their evolution due to post-deposition anneals increased that non-uniformity even further. Relaxation of epitaxial mismatch strain by misfit dislocations could be inferred from a gradual reduction of the nanocrystal vertical aspect ratio and development of flat top facets out of the initially sharp conical crests, in accord with generalized Wulf–Kaishew theorem. On the other hand, the silicide nanocrystals formed by reactive deposition exhibited high uniformity and thermal stability. Significant strain relaxation, as could be judged by the nanocrystal flattening, took place only at temperatures in excess of 650 °C, followed by progressive nanocrystal coalescence. It thus could be inferred, that better titanium silicide nanocrystal arrays (in the sense of uniformity and stability) are more easily obtained by reactive deposition epitaxy than by solid-phase epitaxy. While terminal, stable C54-TiSi2 phase, did eventually form in the epilayers in both methods, different evolution pathways were manifested by different respective morphologies and orientations even in this final state.

Selectively formed GaAs quantum nanostructures and their application to single electron integrated circuits

We propose and fabricate a 1-bit adder circuit having only two single electron transistors (SETs) for AND and XOR logic operations by using SA-MOVPE. Two SETs have two common input gates and different Coulomb oscillation periods. The logic operation can be achieved on the basis of a binary decision diagram (BDD) architecture using Coulomb blockade (CB) in GaAs dots. We also demonstrate Coulomb oscillations caused by input gate voltages for a SET. Oscillation features of two input gates are the same and their phases can be shifted by the control gate. [DOI: 10.1380/ejssnt.2006.180]

Phase-Field Simulation of Shape Evolution and Bimodal Size Distribution of Self-Assembled Quantum Dots

The strain-induced self-assembly of islands in heteroepitaxial systems is a promising approach to the fabrication of quantum nanostructures for optoelectronic devices. In this study, a phase-field model which can simulate the growth process of self-assembled SiGe/Si quantum dots during deposition is developed. The novel feature of this model is that it can simulate the morphological changes of islands, i.e., from single-faceted pyramid to multifaceted dome, by taking a high anisotropy and a sixteen-fold anisotropy of surface energy into account. By using the developed model, two-dimensional simulations are performed on a large computational model. As a result, island nucleation on the surface of a wetting layer, island morphological change and Ostwald ripening due to an interaction between two neighbor islands were well reproduced. The bimodal distribution of island size, which is a very important phenomenon in self-assembled quantum dots, could also be generated. Furthermore, it is clarified that the bimodal distributions are largely affected by island morphological change from pyramid to dome. The variations of island size and energy variations are estimated for single island in detail. As a result, it is concluded that the island morphology transitions occur so as to reduce the elastic strain energy.

ORGANIC SEMICONDUCTOR MICRO-PILLAR PROCESSED BY FOCUSED ION BEAM MILLING

In order to control light, different strategies have been applied by placing an optically active medium into a semiconductor resonator and certain applications such as LEDs and laser diodes have been commercialized for many years. The possibility of nanoscale optical applications has created great interesting for quantum nanostructure research. Recently, single photon emission has been an active area of quantum dot research. A quantum dot is place between distributed Bragg reflectors (DBRs) within a micro-pillar structure. In this study, we shall report on an active layer composed of an organic material instead of a semiconductor. The micro-pillar structure is fabricated by a focused ion beam (FIB) micro-machining technique. The ultimate target is to achieve a single molecule within the micro-pillar and therefore to enable single photon emission. Here, we demonstrate some results of the fabrication procedure of a 5 micron organic micro-pillar via the focused ion beam and some measurement results from this study. The JEOL 6500 dual column system equipped with both electron and ion beams enables us to observe the fabrication procedure during the milling process. Furthermore, the strategy of the FIB micro-machining method is reported as well.

A theoretical model for quantum nanostructures electronic wave functions, magnetic field effects

Analytical solutions of electronic wave functions in symmetric quantum ring (QR), quantum wire (QWR) and quantum dots (QD) structures are given using a parabolic coordinates system. The solutions for low-energy states are combinations of Bessel functions. The density of states of perfect 1D QR and QWR are shown to be equivalent. The continuous evolution from a 0D QD to a perfect 1D QR can be precisely described. The sharp variation of electronic properties, related to the build up of a potential energy barrier at the early stage of the QR formation, is studied analytically. Paramagnetic and diamagnetic couplings to a magnetic field are computed for QR and QD. It is shown theoretically that magnetic field induces an oscillation of the magnetization in QR.

Three-Electron Systems in Quantum Nanostructures: Ground State Transition from a Spin-Doublet State to a Spin-Quartet State

The ground state and the lowest excited state of three electrons confined in quantum structures (quantum dots) are calculated. In general, the ground state is a spin-doublet one in a small dot, level crossing occurs as the dot size increases, and the ground state is a spin-quartet state in a large dot. It is found that the spin-quartet ground state is more likely to be realized in a quantum ring (QR) than in a harmonic confinement. In a narrow QR, two-electron exchanges are unlikely to occur because of the large Coulomb barrier while three-electron exchanges are little suppressed, leading to the fully spin-polarized (spin-quartet) ground state. The present calculation conforms to Herring's theorem that the ground state of three spin-1/2 fermions on a one-dimensional ring is a fully spin-polarized one if the repulsive interaction between fermions is sufficiently strong. The extensions of the present result to more complicated cases are also discussed.

SO phonons in spherical nanostructured quantum dots

AbstractA theoretical model for a nanostructured spherical Quantum Dot (QD) is considered, where to a core sphere of semiconductor “1” it is added a concentric spherical layer of semiconductor “2”, while this structure is assumed as imbedded in an infinite medium made of semiconductor “1”. Using a long‐wave continuum approach, the so‐called dielectric continuum approach, we study the surface optical (SO) phonons associated with the two spherical interfaces of the nanostructure. The possible SO phonon frequencies are analyzed as a function of both the sizes of the structure and the angular momentum quantum number l = 1, 2, …. We also calculate the electric potential provided by the SO phonons and derive an analytical expression for the electron–phonon interaction Hamiltonian. Different possible cases for semiconductors “1” and “2” are discussed, comparisons with previously published works on similar subjects are done and the experimental techniques allowing the possible determination of this kind of phonons are invoked. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Study on ECR dry etching and selective MBE growth of AlGaN/GaN for fabrication of quantum nanostructures on GaN (0 0 0 1) substrates

This paper attempts to form AlGaN/GaN quantum wire (QWR) network structures on patterned GaN (0 0 0 1) substrates by selective molecular beam epitaxy (MBE) growth. Substrate patterns were prepared along 〈 1 1 2 ¯ 0 〉 - and 〈 1 1 ¯ 0 0 〉 -directions by electron cyclotron resonance assisted reactive-ion beam etching (ECR-RIBE) process. Selective growth was possible for both directions in the case of GaN growth, but only in the 〈 1 1 2 ¯ 0 〉 -direction in the case of AlGaN growth. A hexagonal QWR network was successfully grown on a hexagonal mesa pattern by combining the 〈 1 1 2 ¯ 0 〉 -direction and two other equivalent directions. AFM observation confirmed excellent surface morphology of the grown network. A clear cathodoluminescence (CL) peak coming from the embedded AlGaN/GaN QWR structure was clearly identified.

Simulation of Single-Electron Transport in Nanostructured Quantum Dots

Resonant tunneling via discrete energy states in single-electron electronic systems have been observed in many nanostructured configurations. The nodes of such circuits may assume either a continuum of energy states or a set of discrete states with different degeneracy factors. This paper presents a general-purpose simulator intended to provide a unique platform that is able to simulate such circuits. The model is described and then used to study some typical simple circuits. It is shown that even for simple circuits, the interplay between energy spectrum, sensitivity to voltage variations and Coulomb blockade conditions might lead to very complicated current-voltage characteristics.

Nanopowder Management and Plasma Parameters in Nanofabrication of Low-Dimensional Quantum Structures in Reactive Silane-Based Plasmas

Management of nanopowder and reactive plasma parameters in a low-pressure RF glow discharge in silane is studied. It is shown that the discharge control parameters and reactor volume can be adjusted to ensure lower abundance of nanopowders, which is one of the requirements of the plasma-assisted fabrication of low-dimensional quantum nanostructures. The results are relevant to micro- and nanomanufacturing technologies employing low-pressure glow discharge plasmas of silane-based gas mixtures.

Strain, size and composition of InAs quantum sticks, embedded in InP, determined via X-ray anomalous diffraction and diffraction anomalous fine structure in grazing incidence

We report on the study of strain, size and composition of small-size encapsulated semiconductor nanostructures. We show that the partial structure factor of As atoms in InAs stick-like nanostructures (quantum sticks), embedded in InP, can be directly extracted from grazing incidence anomalous X-ray diffraction maps at the As K-edge. We have recovered the average height and strain of the islands and determined their composition. The average height of the quantum sticks (QSs), as deduced from the width of the structure factor profile is 2.54 nm. The InAs out of plane deformation, relative to InP, is equal to 6.1%. Fixed-Q anomalous diffraction spectra, measured at the As K-edge, in grazing incidence provide clear evidence of pure InAs QSs. This is confirmed by the analysis of the diffraction anomalous fine structure (DAFS) that also gives a direct way to recover the strain accomodation inside the quantum sticks. Finite difference method calculations reproduce well the diffraction data. Chemical mixing at interfaces is at most 1 ML. This paper shows that ultimate application of anomalous diffraction and DAFS together with reciprocal space maps is a powerful method to sudy the structural properties of nanostructures.

Carrier distribution in quantum nanostructures by scanning capacitance microscopy

Scanning capacitance microscopy (SCM) was carried out in the angle beveling configuration on B doped, very narrow quantum wells (QWs) of Si0.75Ge0.25 layers strained between Si films. The majority carrier concentration profiles were calculated from the SCM raw data measured on QWs with a minimum width of 5nm, doped with different B concentrations ranging from 2×1016to6×1018cm−3. The equilibrium carrier distribution in the heterostructures has been calculated by different simulation approaches, which will be discussed. Moreover, the effect of the biased tip-sample interaction was studied by accurate simulations of the dC∕dV vs V characteristics for different positions of the tip moving on the beveled sample surface. The agreement between the experimental and simulated SCM profiles is very good. Thus, a spatial SCM resolution of at least 5nm was demonstrated on angle beveled samples, not only in terms of signal sensitivity, but also in terms of quantitative majority carrier profiling.

Formation of two-and one-dimensional solid-phase quantum nanostructures in the CdHgTe-electrolyte system

It is shown experimentally and theoretically that structures that exhibit properties of two-and one-dimensional electron gas at room temperature can be formed on the basis of CdHgTe compounds in a semiconductor-electrolyte system. The methods for fabricating the quantum-confinement structures and in situ monitoring of the physical properties of such structures are described.

Wave packet engineering using a phase-programmable femtosecond optical source

A new optical telecommunication method combining time and frequency domain multiplexing is proposed using phase-controlled femtosecond pulses. Each pulse in a pulse train can be used as a data packet with data bits in the frequency domain. We call the new principle ‘wave packet engineering’, which adjusts the amplitude and phase of the wave function in device materials arbitrarily by controlling the spectral phase of femtosecond pulses. The optical phase-to-amplitude converter is demonstrated with organic dye molecules, in which the phase information in the phase-modulated pulses can be demodulated into the luminescence intensity. The luminescence intensity from cyanine dye molecules is observed to be chirp dependent, and is explained quantum mechanically in terms of coherent population transfer. The design principle of the device using semiconductor coupled quantum nanostructures is also discussed in terms of wave packet engineering.

Formation of one-dimensional surface grooves from pit instabilities in annealed SiGe∕Si(100) epitaxial films

Growth of Si0.7Ge0.3 on Si under kinetically limited conditions results in the formation of shallow strain-relieving pits that only partially penetrate the wetting layer. Upon annealing at the growth temperature of 550°C, these pits elongate in one of the ⟨100⟩ directions and obtain near-{105} facets. The length-to-width aspect ratio of the resulting grooves can be as large as 20. Material ejected from the pits accumulates along the sides of the elongated pit forming shallow islands with a shape that exhibits a monotonic dependence on island size, and eventually evolves to {105} facets. We discuss the origins of this roughening behavior, which may provide a route for self-assembly of highly anisotropic quantum nanostructures.

Control of GaSb/GaAs Quantum Nanostructures by Molecular Beam Epitaxy

GaSb quantum wells (QWs) and quantum dots (QDs) were fabricated on GaAs(001) surfaces by molecular beam epitaxy. By scanning transmission electron microscopy and photoluminescence (PL) measurements, their nanostructures were evaluated. One-monolayer-thick GaSb was precisely formed by the surface exchange reaction between As and Sb atoms, which was monitored by reflection high-energy electron-beam diffraction. In the Stranski-Krastanov growth of the GaSb QDs, low growth rate and low Sb4 pressure were effective conditions for suppressing size fluctuation. As a result, we could obtain the narrowest PL linewidth of 67 meV for GaSb/GaAs QDs.

Growth and photoluminescence of InAlGaN films

Two quaternary InAlGaN films were grown by metal-organic chemical-vapor deposition (MOCVD) on sapphire (0001) substrates with and without high-temperature GaN interlayer, respectively. The structural and optical properties of the quaternary films were investigated by high-resolution X-ray diffraction (HRXRD), high-resolution electron microscopy (HREM), temperature-dependent photoluminescence (PL) spectroscopy and time-resolved photoluminescence (TRPL) spectroscopy. According to the HRXRD and PL results, it is demonstrated that two samples have the same crystal quality. The TRPL signals of both samples were fitted well as a stretched exponential decay from 14 K to 250 K, indicating significant disorder in the materials, which is attributed to recombination of excitons localized in disorder quantum nanostructures such as quantum dots or quantum disks originating from indium (In) clusters or In composition fluctuation. The cross-section HREM measurement further proves that there exist disorder quantum nanostructures in the quaternary. By investigating the temperature dependence of the dispersive exponent beta, it is shown that the stretched exponential decays of the two samples originate from different mechanisms. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Electronic transport in nanosystems

We demonstrate the application of the Landauer–Büttiker formalism (LBF) to MIS (metal–insulator–semiconductor)-type quantum nanostructures and analyze their static and dynamical response properties. For the time-dependent problem, we employ the random phase approximation (RPA) and calculate the irreducible polarization from the one-particle scattering wave functions in the Hartree approximation. For their calculation, we employ the R-matrix approach which we have developed over the past years. We find in the low frequency limit a reasonable agreement with the static Hartree calculations and experimental capacitance data.

Photoluminescence upconversion in colloidal CdTe quantum dots

Photoluminescence ~PL! up-conversion, the observation of luminescence with energies higher than those of the excitation photons, has been widely observed in the bulk 1 and heterostructures 2‐5 of semiconductors, quantum wells, 6 porous silicon, 7 self-assembled quantum dots ~QDs!, 8 and colloidal QDs. 9,10 It was proposed that the existence of intermediate states with energies resonant with or lower than those of the excitation photons is a prerequisite for these upconverted photoluminescence ~UCPL! processes. Carriers must be excited to these intermediate states by the first excitation photon and further to the higher energy states through various underlying mechanisms such as Auger excitation, 7 two-step two-photon absorption ~TS TPA!, 1‐3,5,6 or the thermal effect. 9,10 Most of these intermediate states are ‘‘observable,’’ 2‐7 i.e., in addition to the UCPL emission, one can also see the normal PL emission from these states with energies lower than those of the excitation photons. However, in some cases of self-assembled and colloidal QDs, these intermediate states can hardly be observed since carriers cannot efficiently populate them by the above-band-gap excitation of normal PL. 8 In a way, UCPL can provide a unique method for detecting the existence of these ‘‘defect’’ states in semiconductor quantum nanostructures and investigating their properties, which are usually different from those of the band-gap states. Semiconductor colloidal QDs are characterized by a large surface-to-volume ratio due to their small particle sizes, as well as the surface complexity originating from the passivation ligands used to neutralize the surface dangling bonds. Surface states have long been invoked to be responsible for some complex phenomena that could not be explained by using the concept of band-gap states only. For example, the almost universal occurrence of a biexponential distribution in the radiative lifetime strongly implies the existence of surface states in colloidal QDs. 11,12 In this sense, time-resolved measurements can provide one, but only indirect, way to probe the surface states in colloidal QDs, and consequently the existence of them is still controversial. 13 However, if one can observe a UCPL signal from colloidal QDs, this problem will be easily resolved since the demand of intermediate states can only be fulfilled by the existence of surface states. Recently, UCPL signals were observed in colloidal InP, 9 CdSe, 9,10 and CdTe ~Ref. 10! QDs, which have provided a strong proof of the existence of surface states in colloidal QDs and opened up a new opportunity for the systematic study of these surface states. Here, we report on our optical studies of efficient UCPL in colloidal CdTe QDs with an energy gain of as high as 360 meV. Compared with the normal PL, the UCPL signal shows a redshift of about 80 meV in its peak energy, while the excitation spectroscopy of the UCPL ~UCPLE! measurement at this peak energy showed a broad excitation band with energies below those of the band gap. Time-resolved measurements were utilized to study the time-resolved dynamics of this UCPL, whose radiative lifetime now becomes nearly twice as long as that of the normal PL. Finally this UCPL is attributed to the recombination of carriers from the band-gap states and surface states mainly through a thermal excitation process.

A single-electron circuit as a discrete dynamical system

A single-electron circuit can be operated as a discrete dynamical system because it changes its internal state discontinuously because of electron tunneling. To confirm this idea, we designed a sample circuit for discrete dynamical operation and confirmed by computer simulation that the circuit successfully generated a sequence of discrete-time outputs by following a return map. The concept of discrete dynamical systems will be useful in developing new functional systems that consist of quantum devices and nanostructures.