Transcript Network for Computational Nanotechnology (NCN) UC Berkeley, Univ.of Illinois, Norfolk State, Northwestern, Purdue, UTEP Quantum Transport in GaSb/InAs Tunneling FET Yu He, Zhengping Jiang,

Network for Computational Nanotechnology (NCN)
UC Berkeley, Univ.of Illinois, Norfolk State, Northwestern, Purdue, UTEP
Quantum Transport in
GaSb/InAs Tunneling FET
Yu He, Zhengping Jiang, Daniel Mejia, Tillmann Kubis,
Michael Povolotskyi, Jean Michel Sellier, Jim Fonseca,
Gerhard Klimeck
Network for Computational Nanotechnology (NCN)
Electrical and Computer Engineering
Purdue University, West Lafayette IN, USA
Summer School 2012
What is GaSb-InAs TFET
TFET is promising for low-power logic design -> low SS and high Ion/Ioff ratio.
TFET concept (taken from MIND)
L-shape GaSb-InAs tunneling FET
 Broken gap bandstructure – mixture of
electrons/holes
 2D transport (nonlinear geometry)
Conduction band
Conduction band
Valence band
Valence band
GaSb
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InAs
(A/nm)
Set up the simulation task
• We use Meta_nTFET.in
• We will use a sp3s* tight
binding model
cm-3
• GaSb will be p-type doped
with density 4e18 cm-3
InAs will be n-type doped
with density 5e17 cm-3
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• A Lshaped structure is used
• It will produce an I-V curve
and local DOS shown on left
Details of simulation structure
Periodic boundary in plane
Gate
4nm
15nm
GaSb
Source
10nm
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Oxide
InAs
60nm
drain
Define a hetero-structure
Structure
{
Material
{
tag = pGaSb
name = GaSb
crystal_structure = zincblende
Bands:BandEdge:Ec = 1.531
Bands:BandEdge:Eg = Ec - Ev
Bands:BandEdge:Ev = 0.4865
Bands:BandEdge:mstar_v_dos = 1.2523
regions = (1)
doping_type = P
doping_density = 4E18
}
......
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• Define GaSb for regions (1)
• Bands:BandEdge define the
necessary options for
semiclassical density solver
• Doping_type defines the type
of doping: P
• Doping_density defines the
doping density as 4E18
Define a hetero-structure
Structure
{
Material
{
tag = nInAs
name = InAs
crystal_structure = zincblende
Bands:BandEdge:Ec = 0.5337
Bands:BandEdge:Eg = Ec - Ev
Bands:BandEdge:Ev = -0.1929
Bands:BandEdge:mstar_c_dos = 0.1455
regions = (2, 5)
doping_type = N
doping_density = 5E17
}
......
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• Define InAs for regions (2,5)
• Bands:BandEdge define the
necessary options for
semiclassical density solver
• Doping_type defines the type
of doping: N
• Doping_density defines the
doping density as 5E17
Define an Oxide region
Structure
{
Material
{
tag = Oxide
name = SiO2
crystal_structure = zincblende
Lattice:epsilon_dc = 3.9
Lattice:cation = "Si"
Lattice:anion = "O"
regions = (3, 4)
}
......
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• Define SiO2 for regions (3,4)
• Lattice:epsilon_dc define the
dielectric constant
Domains for transport
Domain
{
name = device
……
// names of leads domain
leads = (source_contact, drain_contact)
• Source_contact and
drain_contact domains have to
be defined, and lead_direction
is defined for each lead
• In device domain, we have to
specify the leads as
source_contact, drain_contact
}
Domain
{
name = source_contact
lead_direction = -2
……
}
Domain
{
oxide
InAs
y
name = drain_contact
lead_direction = 1
……
}
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drain
GaSb
source
x
Domains for Poisson
• We have to define a continuum
domain for poisson solver,
whose type is finite_elements
Domain
{
name = continuum
type = finite_elements
mesh_from_domain = device
neglect_periodicity = true
}
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• Finite element mesh is defined
at device domain
• Periodic boundary condition is
not applied to Poisson by
setting neglect_periodicity as
true
Define the Lshaped geometry
Geometry
{
Region // p-GaSb
{
shape
= cuboid
region_number = 1
priority
=1
min
= (-100, -100, -100)
max
= (10.14, 15, 100)
}
……
60nm
30nm
Domains
(device, source ,drain)
15nm
10.14 nm
Region 1
y
x
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Define the Lshaped geometry
Geometry
{
……
Region // n-InAs
{
shape
= cuboid
region_number = 2
priority
=2
min
= (30, 15, -100)
max
= (300,19.1, 100)
}
Region // n-InAs
{
shape
= cuboid
region_number = 5
priority
=2
min
= (-100,15, -100)
max
= (30, 19.1, 100)
}
……
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Region 2 & 5
Domains 4.1 nm
(device, source ,drain)
Region 1
y
x
Define the Lshaped geometry
Geometry
{
……
Region //SiO2
{
Region 3
shape
= cuboid
2 nm
region_number = 3
Region 2 & 5
Domains
priority
=1
(device, source ,drain)
min
= (-100, 19.1, -100)
max
= (20.14,21, 100)
}
Region 1
……
y
x
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Define the Lshaped geometry
Geometry
{
……
Region
{
10nm
Region 4
Region 3
shape
= cuboid
region_number = 4
priority
=1
min
= (20.14, 19, -100)
max
= (30.14, 100, 100)
Region 1
}
oxide
InAs
y
drain
GaSb
source
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Region 2 & 5
Domains
(device, source ,drain)
x
Define the gate for Poisson
Geometry
{
……
Boundary_region // gate
{
shape = cuboid
region_number = 1
priority = 1
min = (-100, 20, -100)
max = (20.5, 100, 100)
}
gate
Region 4
Region 3
Region 2 & 5
Domains
(device, source ,drain)
Region 1
y
x
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Simulation flow
 Ballistic simulation  cannot fill triangular
well  quantum self-consistency not
converge
 Include phonon scattering  numerically
expensive
cm-3
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Due to high doping S/D, depleted channel
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and separation of conduction / valence band
density, semiclassical model provides good
approximation and is much faster.
Semiclassical model: effective mass, quasi-fermi level, quantum corrections
 Simulation flow =>
Step1.
Semiclassical
density + Poisson
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Electrostatic
Potential
Step2. Quantum
transport (NEGF)
Ballistic/Phonon
Impurity
Roughness, etc.
Transport solver options
Solver options:
name = Transport
type = MetaTransportSemiPotential
Transport_type = transfer_matrix
domain = device
active_regions = (1, 2, 5)
output_name = nTFET
contact_domains
= (source_contact, drain_contact,gate)
source_contact_voltages = (0.0, 0.0, …)
drain_contact_voltages = (0.3, 0.3, …)
gate_voltages = (-0.1, 0.0, …)
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Option meaning:
Solver name
Solver type (NEMO5 will look for
“MetaTransportSemiPotential.py”
in. / Meta)
(optional) Default: NEGF
Area the solver will explicitly work on
Defines on which regions the solver works
Prefix for all outputfile names
Names of the lead domains
List of voltages to apply
List of voltages to apply
List of voltages to apply to the gate
(Boundary_region with region_number = 1)
Transport solver options
Solver options:
use_Poisson_potential = true
tb_basis = sp3sstar
charge_self_consistent = false
Option meaning:
if true, Poisson potential is used (otherwise,Φ=0)
Tight binding basis representation
if true, iterative solution (requires
use_potential=true)
use_semiclassical_potential = true if true, use semiclassical density
relative_maximum_energy = -0.9 Emax=max(Ef) - band_margin
relative_minimum_energy = 0.6
Emin=min(Ef) + bandgap_margin
use_adaptive_grid = false
(optional) adaptive mesh for fixed number of
energy points
use_adaptive_grid1 = false
adaptive mesh for variable number
of energy points
number_of_energy_points = 960
(optional) Number of points in energy
add_constant_potential
= 0.0
Add a constant to the potential
momentum_space_degeneracy
=2
momentum_intervals = [(0, 0.2)]
number_of_momentum_points = 31
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degeneracy of k-space (inverse fraction of calculated
Brillouin zone)
List of intervals of resolved k-space
Number of momentum points for each k-interval
Transport solver options
Write multidimensional data to disc:
Poisson potential in 3D, space charge in cm-3 in 3D, transmission energy resolved, Spectral
function energy resolved, electron LDOS in space and energy, hole LDOS in space and energy
output
= (potential, free_charge_cm-3, transmission, spectral_density, ldosn1d, ldosp1d)
Write to disc data along a path:
output_along_path = (cb_band, vb_band, potential, free_charge_cm-3)
path_points = [(5, 0, 0), (9, 15, 0), (11, 17, 0), (70, 17, 0) ]
List of points on the path in nm
number_of_path_points = (80, 16, 120)
List of number of points between
two path points
enable_structure = true
Structure output is added
gate
oxide
drain
source
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Transport solver – output list
Output files:
nTFET.log
nTFET_potential_*
For the first voltage point:
nTFET_ramper_0.vtk
nTFET_ramper_0.xy
nTFET_ramper_0_TRANS_0.dat
nTFET_ramper_0_ldosn1d_0.dat
nTFET_ramper_0_ldosp1d_0.dat
nTFET_ramper_0_nE_0.dat
nTFET_ramper_0_potential.xy
For the second voltage point…
nTFET_ramper_1.vtk
nTFET_ramper_1.xy…
…
nTFET_ramper_current.dat
nTFET_structure.vtk
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File content:
monitored output (defined in global section
preliminary results (overwritten by subsequent
bias points)
all atomistic quantities
transmission
electron LDOS along output path
hole LDOS along output path
energy resolved charge density
potential
IV characteristics
Structure output
Understand the output files
nTFET_ramper_current.dat :
% V_0; I_0; V_1; I_1; ...
0
-4.73015e-10
0.3
0
-1.97807e-23
0.3
0
-8.14723e-27
0.3
0
-1.56303e-18
0.3
0
-1.3812e-15
0.3
……
source
bias
source current
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drain
bias
4.73015e-10
1.97807e-23
8.14723e-27
1.56303e-18
1.3812e-15
-0.1
0
0.1
0.2
0.3
0
0
0
0
0
drain current
gate
bias
gate current
Understand the output files
nTFET_ramper_x.xy:
% NEMO5 1D-interpolated atomistic data:
0
0.985862
-0.0586379
0.194052
0.985862
-0.0586379
0.388104
0.985862
-0.0586379
0.582157
0.985862
-0.0586379
0.776209
0.98568
-0.0588204
……
distance;
CB_band[eV]; VB_band[eV];
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0.545138
0.545138
0.545138
0.545138
0.54532
potential[V];
1.25433e+19
1.25433e+19
1.25433e+19
1.25433e+19
1.24054e+19
free_charge_cm-3;
Understand the output files
nTFET_ramper_x_ldosp1d.dat; nTFET_ramper_x_ldosn1d;
-0.6
3.44E+11
3.44E+11
3.44E+11
-0.599062
3.59E+11
3.59E+11
3.59E+11
-0.598123
3.78E+11
3.78E+11
3.78E+11
-0.597185
3.94E+11
3.94E+11
3.94E+11
-0.596246
4.07E+11
4.07E+11
4.07E+11
……
Energy (eV)
position resolved LDOS at each energy point
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3.44E+11 ……
3.59E+11 ……
3.78E+11 ……
3.94E+11 ……
4.07E+11 ……
Exercise I: Plot I-V curve
• NEMO5 will produce
nTFET_ramper_current.dat
(A/nm)
• Start MATLAB on your workspace
• Load nTFET_ramper_current.dat file
into matlab workspace, enter the
following script:
xlabel('Voltage (V)' )
ylabel(‘Current (A/nm)' )
Semilogy(nTFET_ramper_current(:,1),
nTFET_ramper_current(:,2), ‘rx—’)
• You will have the figure on the left
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Exercise II: Plot I-V curve
• NEMO5 will produce nTFET_ramper_13_ldosp1d.dat
nTFET_ramper_13_ldosn1d.dat nTFET_ramper_13.xy
• Load the three above files into matlab workspace, enter the following script:
pos = nTFET_ramper_13(:,1);
egrid = nTFET_ramper_13_ldosn1d(:,1);
meshgrid(pos,egrid);
[hC hC] = contourf(pos,egrid, log10(nTFET_ramper_13_ldosn1d(:,2:end)+
nTFET_ramper_13_ldosp1d(:,2:end)+1e-3),50);
set(hC,'LineStyle','none');
hold on, plot(nTFET_ramper_13(:,1),nTFET_ramper_13(:,2),'k');
cm-3
hold on, plot(nTFET_ramper_13(:,1),nTFET_ramper_13(:,3),'k');
xlabel('Position (nm)' )
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ylabel('Energy (eV)' )
caxis([13 21]);
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• You will have the figure on the right
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How to interpret your results?
cm-3
Ec
GaSb
Ec
Ev
GaSb
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Ev
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InAs
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(A/nm)
cm-3
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InAs
Conclusion
 Transport calculations
− Calculate quantum transport using NEGF or transfer matrix
method
− Self-consistently iterate with Poisson, or use a semiclassical
density to speed up
− Can handle arbitrary geometries;
− Can be used to study complicated structures like Band-to-Band
tunneling device
 We have more than that …
− Random alloy
− Surface and interface roughness
−…
Yu He
Thank you.