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Research on Semiconductor Quantum Structures

Optically active quantum dots (QD) are compound semiconductor structures that confine both electrons and holes in a potential box having a dimension of few tens of nanometers. The electronic and optical properties of these structures are governed by the discrete energy eigenvalues of carriers. The focus of our research has been in strain induced QD's, where a self-assembled stressor island is grown on top of a near surface quantum well (QW). The minimum of the confinement potential is located inside the QW few nanometers below the stressor island. These semiconductor structures have exceptionally high optical quality, which makes them ideal for fundamental research of electronic and optical properties of QD's. In the enclosed project descriptions we describe few topics that have been studied intensively during 2000.

This work includes extensive domestic and international collaboration with the following laboratories: Optoelectronics Laboratory, HUT, VTT Electronics, Instituto Nazionale di Fisica della Materia, University of Lecce, MegaGauss Laboratory, University. of Tokyo, Inst. of Industrial Science, University of Tokyo. Center for Teraherz Science, USCB, Department of theoretical Chemistry, University of Helsinki, and Department of Physics, Åbo Academy University, Department of Physics, Ludwig Maximillians University, Walter Schottky Institut, Technische Universität München, Department of Physics and Materials Research Center, Washington State University.

Carrier capture processes in quantum dot structures

Researchers: Risto Virkkala, and Jukka Tulkki

The special properties of QD's, such as the discrete density of states and low state degeneracy, are of interest for new optoelectronic devices such as QD lasers and charge-storage devices. In such devices, carriers are usually injected via some higher-dimensional barrier states and subsequently captured into the QD's, where they may undergo further energy relaxation between the QD levels. Both capture and relaxation may be slowed down due to the discreteness of the QD level structure, which hinders phonon emission. This slowing down has been predicted to limit the speed of QD devices. An additional problem connected to slow capture and relaxation may arise when parasitic processes, like nonradiative recombination at defects, compete with radiative or switching processes, and reduce the efficiency of devices. In this work we have investigated carrier capture processes in strain-induced quantum dot structures. Using photoluminescence spectroscopy, we show that the rate of carrier capture into the quantum dots increases dramatically when the energetic depth of the confinement potential is reduced by enlarging the quantum well/surface separation D. While carriers in the quantum well region between the quantum dots are found to experience D-dependent nonradiative surface recombination, this process seems to be negligible for carriers in the quantum dots, presumably due to the protecting InP islands. For details see REF[C. Lingk, W. Helfer, G. von Plessen, J. Feldmann, K. Stock, M. W. Feise, D. Citrin, H. Lipsanen, J. Tulkki, J. Ahopelto, Carrier capture processes in strain-induced InGaAs/GaAs quantum dot structures, Phys. Rev. B 62, 13588 (2000)]

Longitudinal Stark effect in parabolic quantum dots

Researchers: Tuukka Drufva, and Jukka Tulkki

The electron and hole confinement in zero-dimensional (0D) heterostructures modifies the density of states bringing about a variety of effects that are interesting from the viewpoint of fundamental physics as well as device application . Due to their nonlinear electro-optical properties, a wide variety of QD systems have been studied to improve the performance of optical devices utilizing the electroabsorption and electrorefraction. Electroabsorption based on the quantum-confined Stark effect (QCSE) is one of the most efficient processes for making optical modulators. The understanding of the electric field induced changes in the optical properties of quantum dots is also very important for the implementation of devices like logic gates for quantum computation.

Theoretical studies suggest that the QCSE could be larger than in semiconductor quantum wells, suggesting the possibility of lower switching energy optical devices. A number of reports have indeed appeared about the QCSE in different quantum dot systems. However in these experients, the electric field is always applied along the growth axis of the dots, with a consequent reduction of the electron and hole wavefunction overlap in the vertical direction. By applying the electric field perpendicularly to the growth axis it is possible to observe the longitudinal QD QCSE induced by the asymmetric modification of the QD lateral confining potential. We have carried out a theoretical and experimental study of the longitudinal QCSE performed on a quantum dot device consisting of two planar metallic contacts with a gap of 800nm (gate) comprising a few strain induced InGaAs quantum dots (of two schottky diodes in series with a gap of 800nm . The changes in the microphotoluminescence (m-PL) spectra induced by the increasing bias.were measured in a submicron gated quantum dot device containing few dots. Due to the device geometry, the electric field profile in the gate is not constant resulting in an asymmetric Stark effect as a function of the applied field polarity. For details see Ref [R.Rinaldi, M.DeGiorgi, M. DeVittorio, P.Visconti, R.Cingolani, H.Lipsanen, M.Sopanen, and J.Tulkki, Longitudinal Stark effect in parabolic quantum dots, submitted to Japanese Journal of Applied Physics. (2000)]

  
Figure 32
Figure 32: Fig. a) shows the effective QD potential of the conduction electrons accounting for strain induced deformation and gate voltage. Fig. b) shows lifting of the first excited QD electron state degeneracy in an electric field.

Effects of few-particle interaction in quantum dots

Researchers: Risto Virkkala, and Jukka Tulkki

The optical excitation of a single quantum dot results in optical spectra reminiscent of atomic transitions with sharp and spectrally narrow lines.The understanding of the complex optical spectra from single dots and their dependence on photoexcitation intensity is, however, still quite poor. The atomiclike lines in the spectra were found to exhibit intriguing red- and blue-shifts, and new features has been seen to appear and disappear depending on the number of injected electron-hole pairs. Such a complicated dynamics originates from the Coulomb correlation among pairs of carriers (excitons) confined in the dot, which is often neglected in the analysis of these experiments. In fact, we expect that whenever an electron-hole pair is added to the dot, the energy states must change because of the resulting additional Coulomb interactions.

In this collaboration with the University of Lecce and University of Modena and Optoelectronics laboratory (HUT) we have investigated the effects of few-particle population of a single strain-induced quantum dot by optical excitation. The low-power photoluminescence spectra os shown to consist of sharp lines with energy separation of a few meV, associated to the formation of excitonic molecules in the single dot. With increasing photoexcitation intensity, the population of higher states is observed; however, we also observe a clear intensity dependence of the transition energies, inconsistent with a simple filling of exciton levels. Based on a theoretical model for interacting electron-hole pairs in the dot, we obtain good agreement with experiment and show that exciton-exciton interactions control the spectral changes as the number of pairs is increased. However more complete correlation calculations including larger number (  20 - 30 ) of e-h pairs are needed to obtain a complete understanding of the screening effects in QD's. See review paper [53].

  
Figure 33
Figure 33: Example of quantum dot two terminal device fabricated by combination of lithographic methods. By wet etching the InP islands were removed everywhere except in the nanogate. In the influence of gate voltage on the luminescence was measured and analyzed theoretically.

Modelling of Transport on Nanoscale Semiconductor Structures

In this project we to study the physical mechanisms that govern the performance of scaled-down microelectronic devices. The project is part of the EMMA program of the Academy of Finland. The consortium partners are VTT Microelectronics Centre, Physics Department, University of Jyväskylä, Optoelectronics Laboratory, Helsinki University of Technology Laboratory of Computational Engineering (LCE), Helsinki University of Technology and Okmetic Ltd. Materials and processes will be developed to fabricate very small prototype devices for future microelectronics, namely quantum point contacts and single electron transistors. Properties of devices fabricated from different materials will be compared, and the effects of scaling down the device size will be studied to gain understanding about the consequences caused by the large surface to active volume ratios in these structures.

The material systems to be studied are Si/SiO2, SiGe/Si and InGaAs/InP. For Si/SiO2 structures, SOI wafers - bonded and SIMOX- will be used as starting material. the project was started in summer 1999. The first topics studied at LCE have focused on modelling the Si/SiO2 interface and its influence on electronic structure in the wave guide and on developing computer programs for calculation of conductance in coherent transport regime.

Modelling of electronic structure in Silicon Quantum Wires and Quantum Point Contacts

Researchers: Fredrik Boxberg, and Jukka Tulkki

The semiconductor technology of today is mainly based on silicon. However, in silicon structures it has shown to be very difficult to obtain a phase coherence length comparable to that of III-V compound semiconductors (for example GaAs). The goal of this project is to understand better the structural and electronic properties of silicon, silicon dioxide and their interface for optimisation of silicon and silicon dioxide based quantum interference devices like quantum wires and point quantum contacts. In this project we study strain and electronic structure silicon and silicon dioxide quantum wires and point contacts fabricated by VTT Microelectronics.

The calculation of strain is based on elastic continuum theory. We have developed a phenomenological model for simulating the strain, caused by thermal oxidation during the fabrication process. This method allows us also to analyse the strain-induced position dependent change of the electronic bands. Figure 34 (a) shows our 3D element model for the structural analysis and (b) is a TEM image of the fabricated device. We have calculated the influence of strain on the electronic states by the envelope wave function method. We have shown that the almost unavoidable oxidation-induced strain leads to localization of charges in the narrow wire and crossing of different electron bands. This causes channelling of the current to the corners and enhances thereby interface and impurity scaattering. It is yet not clear how this will change the operation of the device but we believe that this is the reason to the experimentally observed difficulties with silicon quantum interference devices. These effects are very specific to conducting channels made of silicon and silicon oxide.

  
Figure 34
Figure 34: (a) The element model used in our structural analysis of a quantum wire. (b) A transmission electron micrograph of the cross section of the wire in the active region. The inset shows the whole silicon core which is about 50 nm high.

In the future we will extend our model to account for key features of the oxidation process itself and we will also investigate the charges existing in the isolating oxide and their influence on conductance. This will include theoretical work on the dynamics of the thermal oxidation process using a coupled diffusion reaction model. We have started collaboration with professor T. Hiramoto in the Institute of Industrial Science of the University of Tokyo in order to compare our simulated strains with experiments. For further details, please, see Ref.[F. Boxberg and J. Tulkki, Modelling of oxidation induced strain and its effect on electronic properties of Si Waveguides, submitted to IEEE transactions on electron devices (2000)].

Electron transport in nanostructures

Researchers: Kari Maijala, Pasi Ritaluoto, and Jukka Tulkki

The semiconductor industry will continue downscaling of the CMOS technology. At the point where the nanometer scale has been reached new device concepts will be applied. These can be e.g. single-electron transistors, spintronic devices or resonant tunneling diodes (RTD). RTD's are already commercially exploited.

These new devices, however, present serious challenges for fabrication technology. Even a single atomic defect in the solid state environment can be fatal for the device operation. Extreme purity is thus required in the fabrication process.

  
Figure 35
Figure 35: a) Model of the Silicon QPC b) transmission for conduction bands of different symmetry in Si QPC.

To be able to push the technology forward it is essential to understand the electron transport in these new devices. We use theoretical and computational tools to investigate these properties. The work is done in close collaboration with groups doing experimental research to make sure that our results meet their needs. Quantum point contacts provide a basic example of transport phenomena at the nanoscale.


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