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SIMULATION OF RESONANT TUNNELING DIODES FOR NANOELECTRONIC APPLICATIONS Palla Pavankumar, Perumalla V. N. Pradeep Kumar, and Reddy Bhanu Prakash

Department of Electronics and Communication Engineering,SRM University, Kattankulathur , Chennai, Tamil Nadu-603203, India .

Guided by: Dr . P. Aruna Priya.

ABSTRACT

Molecular electronics is the utilisation of a molecule or a group of molecules as an electronic device in a circuit. Experiments have shown how such devices could be fabricated with useful properties such as rectification, hysteresis and negative differential resistance (NDR). In this paper we investigate the transport behaviour of the molecular resonant tunneling diode (RTD) addressed by gold electrodes from both sides. A sulphur atom at each joins the molecular resonant tunneling diode to the bulk Au (111) electrode. The molecule with the discrete energy levels is connected by two semi-infinite electrodes with the continuous band. To describe the interaction correctly at the interface between the sulphur atoms and the gold electrodes, we treat an extended molecule exactly by including one gold atom on each side. RTD exhibits a negative differential resistance (NDR) region in its current-voltage characteristics which can be exploited for high speed and low power circuits. Here we design a new intramolecular complex with donor and acceptor molecular subunits to explore the possibility of its working as an RTD. We calculate the I-V behaviours using a self consistent method and calculate the current and the differential conductance. 

The molecule which consists of appropriate donor and acceptor, if placed between suitable electrode contacts, forms a diode. That is, electron flow through the structure should be strongly preferred only in one direction.

One  fundamental  step  in  molecular  electronics  is  to  study  the electronic  properties  of  functional molecules  and  explore  potential molecules for device implementation.

The negative differential resistance characteristic of these devices, achieved  due  to  resonant  tunneling,  is  also  ideally  suited  for  the design of highly compact, self-latching logic circuits.

we are going  to design a new  intramolecular  complex with donor and  acceptor  molecular  subunits  and  to  show  how  it  works  as  a RTD. 

We have substituted –NO2 group which acts as acceptor and –NH2 as donor in the benzene ring. This introduces a redox centre in the middle benzene ring. 

The  electron  withdrawing  nitro  group  is  responsible  for  NDR behaviour, whereas the electron-donating amine group gives rise to a bound state in the molecule.

Fig.1: Structure of molecular RTD 

INTRODUCTION

FORMALISM AND COMPUTATIONAL SCHEME

The    gold  electrodes    are  attached  at  both  sides  and  a  sulphur atom  at  each  end  joins  the  molecular  diode  to  the  bulk Au  (111) electrode. 

Fig 2. Molecule attached with gold electrodes.

An electron incident from the source with energy E has a probability T  (E)  of  being  transmitted  through  the  molecule  to  the  drain.  By calculating  this  transmission  probability  for  a  range  of  energies around the Fermi function Ef of the lead, current is calculated using the Landauer formula .

T (E) = trace (Γ1G Γ2 G+)

Electrochemical potential µ1, 2 = E ± 

Broadening Γ1, 2 = i (∑1, 2 -∑1, 2+).

The self energy  functions ∑1, 2    are used to describe the effect of contacts on the device. The molecular Green’s function G [9] [10] is given by

G (E) = (ES – H + USCF - ∑1 - ∑2)-1

The  device  is  described  by  a  Hamiltonian  matrix  H  and  overlap matrix S. The self  consistent potential is given by

USCF =U (N-Neq)

U  is  the  charging  energy;  N-Neq  is  the  change  in  the  number  of electrons from the equilibrium value Neq.

Optimising the geometry of the molecule is the most time consuming process. The time taken depends upon the number of atoms in the molecule, may be 4 to 10 days.

Calculating the Equilibrium properties of the molecule is the time consuming process of the simulation. For PDT molecule it takes 10-15 minutes to build the Dos_TE.mat file. It may take up to 1-2 hours for bigger molecules.

The contacts are assumed to have a constant density of states in the energy range of   interest.

Structural changes in the molecule under bias are not considered.

RESULTS AND DISCUSSION

Our  calculations  are  based  on  the  Huckel-IV  codes  and  it  is assumed  that  the  molecule  under  investigation  forms  symmetric contact with two semi-infinite gold<111> electrodes.

  A  resonant-tunneling  diode  is  made  by  placing  two  insulating barriers in a molecule, creating between them an island or potential well where electrons can reside. 

Whenever  electrons  are  confined  between  two  such  closely spaced barriers, quantum mechanics restricts their energies to one of a finite number of discrete "quantized" levels.

Fig 3. Current-Voltage characteristics of molecular Resonant tunneling diode.

Fig 4. Conductance-Voltage characteristics of molecular Resonant tunneling diode.

When the energy of the incoming electrons aligns with that of one of the internal energy levels, the energy of the electrons outside the well is said to be "in resonance" with the allowed energy inside the well.  Then,  maximum  current  flows  through  the  device  at  this resonant voltage or peak voltage (Vp) called the peak current (Ip).

The measured peak-to-valley ratio is approximately 1.33: 1.

[1] M. A. Reed, “Molecular-scale electronics,” Proc. IEEE, vol. 87, no. 4, pp. 652–658, Apr. 1999. [2] G. L. Fisher, A. E. Hooper, R. L. Opila, D. L. Allara, and N. Winograd, “The interaction of vapor-deposited Al atoms with COOH groups at the surface of a self-assembled alkanethiolate monolayer on gold”, J. Phys. Chem. B, vol. 104, pp. 3267–3273, 2000.

[3]S.Datta, Quantum Transport :Atom to Transistor, Cambridge University Press, Cambridge, UK, 2005.

REFERENCES

REALISTIC CONSTRAINTS