Presentation Transcript:
CoulombBlockade Oscillations in Semiconductor Nanostructures
(Part I & II)
PHYS 503: Physics Seminar
Fall 2008
Deepak Rajput
Graduate Research Assistant
Center for Laser Applications
University of Tennessee Space Institute
Tullahoma, Tennessee 373889700
Email: drajput@utsi.edu Web: http://drajput.com
Part I
> Introduction to Coulombblockade oscillations
> Basic properties of semiconductor nanostructures
Introduction
* Coulombblockade Oscillations: A manifestation of singleelectron tunneling through a system of two tunnel junctions in series.
* They occur when the voltage on a nearby gate electrode is varied.
* Tunneling is blocked at low temperatures where the charge imbalance jumps from + e/2 to ? e/2 (except near the degeneracy points).
Introduction
* Semiconductor nanostructures are fabricated by lateral confinement of the 2DEG in Siinversion layers, or in GaAsAlGaAs heterostructures.
* First type of semiconductor nanostructure found to exhibit Coulombblockade oscillations: A narrow disordered wire, defined by a splitgate technique.
* Second type of semiconductor nanostructure: A small artificially confined region in a 2DEG (a quantum dot), connected by tunnel barriers.
Basic Properties of Semiconductor Nanostructures
* Electrons in a 2DEG move in a plane due to a strong electrostatic confinement at the interface between two semiconductor layers or a semiconductor and an insulator.
* The areal density can be continuously varied by changing the voltage on a gate electrode deposited on the top semiconductor layer or on the insulator.
* The gate voltage is defined with respect to an ohmic contact to the 2DEG.
Basic Properties of Semiconductor Nanostructures
* The density under a gate electrode of large area changes linearly with the electrostatic potential of the gate ?gate, according to the plate capacitor formula:
* A unique feature of a 2DEG is that it can be given any desired shape using lithographic techniques.
* The energy of noninteracting conduction electrons in an unbounded 2DEG is given by:
Basic Properties of Semiconductor Nanostructures
* The density of states per unit area is independent of the energy:
where gs and gv account for the spin and valleydegeneracy.
* In equilibrium, the states are occupied according to the FermiDirac distribution function:
Basic Properties of Semiconductor Nanostructures
* At low temperatures kBT ? EF , the Fermi energy EF of a 2DEG is directly proportional to its sheet density ns , according to:
* The Fermi wave number is related to the density by:
Basic Properties of Semiconductor Nanostructures
* When 2DEG is confined laterally to a narrow channel, its conduction band splits itself into a series of onedimensional subbands. The total energy of En(k) of an electron in the nth 1D subband in zero magnetic field is given by:
* Two frequently used potentials to model analytically the lateral confinement are square well potential and the parabolic potential well. The confinement levels are given by:
Basic Properties of Semiconductor Nanostructures
* Transport through a very short quantum wire (~ 100 nm, much shorter than the mean free path) is perfectly ballistic: quantum point contact.
* The conductance G of a quantum point contact is quantized in units of 2e2/h. This effect requires a unit transmission probability for all of the occupied 1D subbands in the point contact, each of which then contributes 2e2/h to the conductance (gsgv = 2).
* Quantum wires are extremely sensitive to disorder, since the effective scattering crosssection, being of the order of Fermi wavelength, is comparable to the width of the wire.
Basic Properties of Semiconductor Nanostructures
* A quantum dot is formed in a 2DEG if the electrons are confined in all three directions and its energy spectrum is fully discrete.
* Transport through the discrete states in a quantum dot can be studied if tunnel barriers are defined at its perimeter.
* The quantum point contacts are operated close to pinchoff (G < 2e2/h), where they behave as tunnel barriers of adjustable height and width.
* The shape of such barriers differs from that encountered in metallic tunnel junctions.
Part II
Theory of Coulombblockade oscillations
> Periodicity of the oscillations
> Amplitude and lineshape
Periodicity of the oscillations
* In a weakly coupled quantum dot, transport proceeds by tunneling through its discrete electronic states.
* In the absence of charging effects, a conductance peak due to resonant tunneling occurs when the Fermi energy EF in the reservoirs lines up with one of the energy levels in the dot.
* The probability to find N electrons in the quantum dot in equilibrium with the reservoirs is given by:
Periodicity of the oscillations
* F(N) is the free energy of the dot, T is the temperature, EF is the reservoir Fermi energy.
* At T=0, P(N) is nonzero for only a single value of N (for the value which minimizes the thermodynamic potential ?(N) = F(N) ? NEF).
* A nonzero G (conductance) is possible only if P(N) and P(N+1) are both nonzero for some N. A small applied voltage is then sufficient to induce a current through the dot.
* To have P(N) and P(N+1) both nonzero at T=0 requires that both N and N+1 minimize the thermodynamic potential in way that F(N+1) ? F(N) = EF.
Periodicity of the oscillations
* At T=0, the free energy F(N) equals the ground state energy of the dot, for which we take the simplified form:
Here U(N) is the charging energy, and Ep (p=1,2,?) are singleelectron energy levels in ascending order. The term U(N) accounts for the charge imbalance between dot and reservoirs.
* The sum over energy levels accounts for the internal degrees of freedom of the quantum dot, evaluated in a meanfield approximation.
Periodicity of the oscillations
* Each level contains either one or zero electrons. The energy levels Ep depend on gate voltage and magnetic field, but are assumed to be independent of N. A peak in the low temperature conductance occurs whenever:
Here, U(N) is written as:
Periodicity of the oscillations
* The capacitance C is assumed to be independent of N and the charging energy then takes the form:
* The periodicity is given by the equation:
In the absence of charging effects, ?EF is determined by the irregular spacing ?E of the single electron levels in the quantum dot.
Periodicity of the oscillations
* To determine the periodicity in case of Coulombblockade oscillations, we need to know how EF and the set of energy levels EP depend on ?ext.
* In a 2DEG, the external charges are supplied by ionized donors and by a gate electrode (with an electrostatic potential difference ?gate between gate and 2DEG reservoir) and can be expressed as:
* The period of the oscillations can expressed as:
Periodicity of the oscillations
Periodicity of the oscillations
* The gate voltage Vgate is the electrochemical potential difference between gate and leads. The oscillation period ?Vgate is given by:
Amplitude and lineshape
* The equilibrium distribution function of electrons among the energy levels is given by the Gibbs distribution in the grand canonical ensemble:
* Where {ni}={n1, n2, ?} denotes a specific set of occupation numbers of the energy levels in quantum dot. The number of electrons in the dot is N=?ni and Z is the partition function:
Amplitude and lineshape
* The joint probability Peq (N, np=1) that the quantum dot contains N electrons and that level p is occupied is:
* In terms of this probability, the conductance is given by:
Amplitude and lineshape
* The conductance of the quantum dot in the high temperature limit is simply that of the two tunnel barriers in series:
* The conductances Gl and Gr of the left and the right tunnel barriers are given by the thermally averaged Landauer formula:
Amplitude and lineshape
* The transmission probability of a barrier T(E) equals the tunnel rate ?(E) divided by the attempt frequency ?(E)=1/h?(E):
* If the height of the tunnel barriers is large, the energy dependence of the tunnel rates and of the density of states in the dot can be ignored. The conductance of each barrier the becomes:
Amplitude and lineshape
* The conductance of the quantum dot becomes:
If ?E, e2/C << kBT << EF
* For energyindependent tunnel rates and density of states, one obtains a line shape of individual conductance peaks given by:
Amplitude and lineshape
* The width of the peaks increases with T in the classical regime, whereas the peak height is temperature independent.
Thank You
