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Observation

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									Nanoelectronic Devices
based on Silicon MOS
      structure
               Prof.C.K.Sarkar
               IEEE distinguish lecturer
Dept of Electronics and Telecommunication Engineering
                  Jadavpur University
                    Kolkata- 700032.

                                                        1
         FUNDAMENTALS OF NANOTECHNOLOGY


 Nanotechnology explores and benefit from
   quantum phenomenology in the ultimate
   limit of miniaturization.
 At length-scales comparable to atoms and
   molecules, quantum effects strongly modify
   properties of matter like “color”, reactivity,
   magnetic or dipolar moment, … Besides,
   phenomena characteristic of systems with
   low dimensionality can be use to control
   macroscopic properties.
 Leading Research efforts in
   Nanotechnology
1. Quantum confinement
2. Electronic Transport
3. Quantum confinement

                                                    2
Nanoparticles
  What Is Nanocrystalline Silicon?
  1. It is similar to amorphous silicon (a-Si)
  2. It consists solely of crystalline silicon grains, separated
     by grain boundaries
  3. Nanocrystalline silicon (nc-Si) is an allotropic form of
     silicon
 Advantages of nanosilicon over Silicon
  1. It can have a higher mobility due to the presence of the
     silicon crystallites.
  2. Higher dielectric constant than bulk silicon.
  3. One of the most important advantages of
     nanocrystalline silicon, however, is that it has increased
     stability over a-Si
  4. Mainly used in optoelectronics due to direct band gap.


                                                              3
    nc-Si Embedded MOS structure
   This model consists of
                             Gate Metal
    Si substrate/ pure
                                   nc- Si Layer
    SiO2/ Embedded nc-
    Si layer/ Gate
    electrode
   Voltage applied at the
    gate Terminal
   Electrons tunnel from
    Si-substrate to gate
    through these
    dielectrics.
                                                  4
    Methodology to be adopted and
         Innovative aspects
 Effective dielectric constant
                         
                                         1

          eff   dox  d  dox 
                   ox d  ncSiO d 
                                2   
 Effective barrier height
       b  (Egeff  Egsi ) 2
 Effective mass
              m d mnc  SiO2  d  dox  
      meff   ox ox                     
               d             d           


 Modification of tunneling probability
                                              5
      Maxwell – Garnett Effective medium
           Approximation theory

   Inclusion particles
    randomly dispersed in
    dielectric medium
   Silicon nanocrystallites
    spherical in shape.




                                           6
Maxwell Garnett Theory embedded
            systems
In a binary composite, if the density of silicon
 nanocrystals is small, each particle of the component
 can be treated as being embedded in a large medium
 of SiO2.




                                                   7
       Mathematical formulation
   The effective dielectric function of the composite
    could be expressed as

                a  b    eff   b
           fa           
               a   b  eff   b

=Screening factor depends upon the size and
    orientation of particle. For spherical it is 2 .

    fa = volume fill fraction of the particle
                                                         8
      Tunneling in the model

Low Applied Gate voltage  Direct tunneling


High Applied Gate voltage  Fowler-Nordheim
                             tunneling




                                               9
Direct Tunneling

          At low field when


                 b  E0
            V<
                    q
         The barrier becomes
         Trapezoidal in Shape.



                              10
       Direct tunneling Expression

From Simmon’s model modified at low field

        
        2meff b  E0     
                            1/2
                                   q 2V       
                                                2   2meff b  E0     
                                                                          
 JD                2
                                           exp                          d
                        d                                                
                                                                         
      Where           mox dox mnc  d  dox  
              meff                          
                      d              d        
α = unit less adjustable parameter depends on
effective mass and barrier height.
                                                                              11
Fowler – Nordheim Tunneling

             At high field when

                     b  Eo
                V>
                        q
           The barrier becomes triangular
           in shape




                                     12
Different conditions for Fowler –
       Nordheim equation
                    For this condition

                    qFeffd< Фb-E0

                  Tunneling probability
                                 2 a                      
                 D  En   exp   2m V ( x)  En  dx 
                                                       1/ 2


                                 0                         

                  Where V(x) = -qFs.x x<0


                                                        13
                                              For this condition


                                         Фb-E0< qFeffd< Ф-E0


                                Tunneling probability becomes

                                                                   
                                                                        1
D  En   sin 2 cosh (3 1)  cos 2 cosh 3  1  ln(4)
             2        2              2          2
                                                              
                                         xi                                  1/ 2
                                                                     
                 where          i           2m V
                                                  *
                                                        x    E0  
                                                                    
                                                                                    dx
                                         xi1 



                                    V(x)=Фb-qFeff x                                  0<x<d
                                                                                         14
                                                            Observation
                                                        nc Si total current                   FN tunneling
                                                        SiO2 total current
                    1E-4
                                                        nc Si FN current
                                                        SiO2 FN current
                                                                                             current
                    1E-5
                                                                                             increases
                    1E-6

                                                                                             FN onset
Gate current (A)




                    1E-7


                    1E-8                                                                     voltage
                    1E-9                                                                     decreases
                                                                              FN tunneling
                                                                                             Field emission
                   1E-10       Direct tunneling

                   1E-11
                                                                                             starts at the low
                   1E-12
                           0   5       10          15          20        25           30     applied voltage.
                                            Gate voltage (V)



plot of I g-Vg curve for 30 nm thickness for both pure
SiO2 and proposed dielectric.
                                                                                                              15
                                                                                             1 nm
                                                                                             3 nm                                                                     1 nm
                                                                                             5 nm                    -15                                              3 nm
                           -60
                                                                                                                                                                      5 nm
                                                                                                                     -20

                           -80                                                                                       -25

                                                                                                                     -30
      2
         ln(Jo/F ) A/V




                          -100




                                                                                                     2
                                                                                                     ln (J/F ) A/v
                                                                                                                     -35
      2




                                                                                                     2
                                                                                                                     -40
                          -120
                                                                                                                     -45

                          -140                                                                                       -50

                                                                                                                     -55
                          -160
                                                                                                                     -60

                                                                                                                     -65
                          -180
                                                                                                                           0.1   0.2   0.3      0.4      0.5   0.6   0.7
                                   0.1         0.2          0.3         0.4         0.5    0.6
                                                                                                                                           volume fraction
                                                           Volume fraction




                                                a                                                                                      b

                         -15

                         -20
                                                                                             1 nm
                                                                                             3 nm
                                                                                             5 nm
                                                                                                    The plot of ln(JFN/F2) vs.
                         -25

                         -30
                                                                                                    volume fraction at
                                                                                                    different applied voltages
2
ln (JFN/F ) A/v




                         -35
2




                         -40




                                                                                                    a) 5v b) 10v and c) 15v
                         -45

                         -50

                         -55

                         -60

                         -65
                                 0.1     0.2         0.3          0.4         0.5    0.6   0.7
                                                       volume fraction




                                                     c                                                                                                                       16
FN Tunneling current probability                   Direct Tunneling current density
                                                         2 meff b  E0   q 2V
                                                                            1/ 2
                2 a                         
D  E0   exp      2m V ( x)  E0  dx 
                                         1/ 2
                                                  JD 
                0                            
                                                                       2
                                                                           d
                                                       2     2meff              E0     
                                                  exp                                     d
                                                                               b
    Tunneling current density                                                               
                                                                                            
      J FN  qN I V0 D  E0 
                                                                                       17
Variation of dielectric constant

                      9                                                                                                          1 nm
                                                                                            5.4                                  3 nm
                      8                                                                                                          5 nm
                                                                                            5.2
dielectric constant




                      7                                                                     5.0

                                                                                            4.8
                      6
                                                                                            4.6




                                                                                     eff
                      5
                                                                                            4.4

                      4                                                                     4.2

                                                                                            4.0
                      3
                               28        28            28             28        29
                          2.0x10    4.0x10      6.0x10           8.0x10    1.0x10             0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
                                                             3
                                    nc-Si concentration ( / m )                                        volume fraction ()



                                                                                                                                        18
        Carbon Nanotubes
The Carbon nanotube
   Electronic structure of Carbon nanotube
   The geometry of Carbon nanotube
   Electronic properties of carbon nanotube
   Quantum Modeling & Proposed Design of CNT-
    Embedded Nanoscale MOSFETs
   CNT band structure and electron affinity
   CNT mobility model
   Carrier concentration
   Effective potential due to CNT-Si barrier


                                                 19
Electronic structure of Carbon
           nanotube
              a single atomic layer of
               graphite consists of        2-D
               honeycomb structure
              it has conducting states at, but
               only at specific points along
               certain      directions         in
               momentum space at the
               corners of the first Brillouin
               zone
              Choosing different axes it can
               be used as typical metal or
               semiconductor                20
The geometry of Carbon nanotube
          ** The lattice constant
            a= |a1| = |a2| =3ac-c
          Where ac-c is carbon carbon bond
          length
          ** The vector describe the
          circumference of a nanotube
           Ch = na1 + ma2
          **The chiral angle
            = sin-1{3m / 2(n2+m2+mn)}

                                             21
Different types of carbon
       nanotubes
           The construction of a nanotube
           through the rolling up of a
           graphene sheet leads to three direct
           verities
           These are armchair nanotubes
           which have  = 30o
            These have an indices of the
           form (n,n)[n = m].
           For  = 0o zigzag nanotube
           The indices of the form (n,0)
           For 00 < < 300 chiral nanotube
           Indices of the form (n, m)
                                         22
    From graphene to carbon nanotube
   The only discrete wave-vectors are allowed in radical
    direction and the following condition is
                   Ch . k = 2q
   For an armchair nanotube the circumferential axis lies along x
    direction,
                       |Ch| |kx| = 2q
                      kx = 2q / 3na
   For a zigzag nanotube the azimuthal direction lies along the y
    direction.
                   |Ch| |kx| = 2q
                    kx = 2q / na
                                                                 23
                 Electronic property

the nanotube is metallic or
not can be described by the
m and n indices with the
following rule
 n=m        metallic
 n – m = 3j metallic
 n – m  3j semiconducting


                                       24
Dependence of semiconducting band gap
            with diameter

The energy gap of semiconducting single walled
nanotubes is predicted to be inversely proportional
to the diameter of the nanotube
The best fit equation is of the form is
             Eg = 2oac-c / d
 o = 2.25  0.06 eV is a good arrangement shows a
fundamental energy gap 0.4 – 0.9 eV which lie in
the infrared range
                                                25
CNT-Embedded Nanoscale
     MOSFETs




New design a methodology has been
developed for modeling nanoscale CNT-
MOS-FETs
                                   26
             Fabrication Procedure
Thin HfAlO film was deposited on the Si substrate by the
laser molecular beam epitaxy (MBE)
The ratio of Hf to Al for the ceramic target is 1:2
The commercial CNTs were synthesized by chemical vapor
deposition
The diameter and length are about 2 nm and 1.5µm
respectively.
Finally another layer of HfAlO was deposited to cover
these CNTs and form the structure of HfAlO/CNT/HfAlO/Si.



                                                            27
        Pt/8nmHfAlO/CNT/3nmHfAlO/Si




IV measured at 77K       Actual structure

                                            28
                  Nanotube Parameters
   The dielectric constant of CNT is dependent on its symmetry and tube radius

                                          Where


                                      C~ 1.96       For metallic
                                           2.15      For Semiconducting
According to Maxwell- Garnett Theory the effective dielectric constant can be
written as




                 Where f is the volume fraction and εox is the
                 dielectric constant of HfAlO εox =16
                                                                            29
     C-V measurement of Embedded Carbon Nanotubes


Backward C-V curve
overlaps forward C-V curve
without CNT
A clear hysteresis
between subsequent
forward and backward C-V
curves containing CNTs.
This curve suggests
small number of charge
carriers are stored inside   Typical C-V hysteresis
CNTs.                        characteristics of the CNT based
                             MOS memory devices

                                                                30
                       Observation
                                        Gate leakage current is
                                        direct tunneling current
                                        Two different dielectric,
                                        pure HfAlO and HfAlO
                                        embedded with SWCNTs.
                                        As gate voltages increases
                                        tunneling current density
                                        decreases.
                                        Tunneling current is lower
                                        in embedded CNTs than
                                        pure HfAlO dielectric.
                                        SWCNTs stored charges,
                                        breaks tunneling paths from
Direct tunneling gate leakage current   channel to gate and current
     density at low gate voltage        density decreases.
                                                              31
                    Observations
                                         Field emission or F-N
                                         tunneling current as a
                                         function of applied gate
                                         voltage.
                                         The F-N tunneling onset
                                         voltage is lower in CNT
                                         embedded dielectric than
                                         pure HfAlO oxide
                                         dielectric



F-N Tunneling current as a function of
         high gate voltages
                                                               32
                     Observation
                                 F-N plot is straight line.
                                 Slopes of the two different
                                 dielectrics pure and
                                 embedded are different
                                 For a particular applied
                                 field the F-N tunneling
                                 current density is higher in
                                 CNT embedded dielectric
                                 than pure HfAlO oxide
                                 dielectric.
                                 The dielectric constant is
                                 higher in CNT embedded
                                 dielectric than pure HfAlO
F-N plot of pure HfAlO and CNT   dielectric
  embedded HfAlO dielectric                               33
                      Observation
                                          Gate leakage
                                          current is direct
                                          tunneling current
                                          As applied voltage
                                          increases tunneling
                                          current decreases
                                          As the diameter of
                                          nanotube decreases
                                          direct tunneling
                                          current decreases.

Direct tunneling current with different
         nanotube diameters
                                                              34
                        Observation
                                           F-N tunneling current with
                                           different diameters of
                                           nanotubes
                                           The F-N tunneling onset
                                           voltage decreases with the
                                           increase of the nanotube
                                           diameter.
                                           The diameter in nanometer
                                           regime can cause a highly
                                           localized field across the
                                           nanotube surface. This helps
                                           to increase the Field emission
                                           current.
F-N Tunneling current with the variation
        of nanotube diameters
                                                                     35
                        Observation
                                     High positive gate voltage
                                     nc-Si embedded in SiO2 matrix
                                     SWCNT embedded in high-k
                                     dielectric
                                     High-k dielectric is HfAlO
                                     F-N onset voltage is maximum in
                                     case of pure SiO2 and minimum in
                                     case of embedded CNTs in HfAlO
                                     Embedded CNTs have better
                                     Field emission properties than
                                     embedded nc-Si.
                                     Embedded CNT has highest
F-N tunneling current of different   dielectric constant.
  pure and embedded dielectric                                     36
                             Observation
                                            F-N tunneling current
                                            higher in embedded dielectric
                                            than pure oxide
                                            Tunneling current in
                                            embedded CNTs is higher
                                            than in embedded nc-Si
                                            The value of dielectric
                                            constant is higher in HfAlO
                                            than Pure SiO2
                                            Tunneling current increases
                                            with the increase of dielectric
                                            constant value.

F-N plot with different pure and embedded
                 dielectrics
                                                                     37
Observation
              F-N onset voltage
              is highest in case of
              pure SiO2
              Onset voltage
              decreases with the
              introduction of
              nanoparticles.
              Onset voltage is
              lower in case of CNT
              than in nc_si.

                                   38
Observation




              39
Observation




              40
Observation
         Leakage current is
         lower in high-k dielectric
         HfO2, than pure SiO2
         With embedded
         nanoparticles direct
         tunneling current also
         decreases
         It is lowest in Hf)2
         embedded with CNTs
         All this is due to the
         higher value of
         dielectric constant of
         gate oxide
                                  41
                 Conclusion

   CNT-MOSFET device appears to yield better
    performance than the conventional MOSFET
   The current voltage characteristics predicts that
    the device current of CNT-MOSFET is higher
    than the conventional one.
   The narrow diameter tube shows similar
    performance compared to conventional one.
   CNT-MOSFET may represent the new paradigm
    for devices in the 21st century

                                                  42
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