Transcript Comparison of (1120) and (0001) Surface Orientations in 4H

Modeling, Characterization and Design of
Wide Bandgap MOSFETs for High
Temperature and Power Applications
UMCP: Neil Goldsman
Gary Pennington (Research Associate)
Siddharth Potbhare (MS-Ph.D),
Xiaohu Zhang(Ph.D)
ARL: Skip Scozzie
Aivars Lelis (& UMCP Ph.D)
Bruce Geil (& UMCP MS)
Dan Habersat (& Former Merit)
ARO STAS: Barry Mclean & Jim McGarrity
Personnel Development: Contribution to
ARL
• Gary Pennington: Finished PhD 2003, researching SiC
for ARL
• Steve Powell: Finished PhD 2003
• Gabriel Lopez: Former MERIT, new ARL employee
• Aivars Lelis: ARL employee, PhD under Goldsman
(transferring our software to ARL for use and more
development)
• Bruce Geil: ARL employee, MS under Goldsman
(transferring our software to ARL for use and more
development)
• Steve Risner: Very promising new MERIT student
Outline
•Introduction:
-Benefits of Wide Bandgap Semiconductors
-Difficulties to Overcome
•Atomic Level Analysis of Carrier Transport in 4H & 6H SiC:
-Monte Carlo transport modeling: bulk and surface
• 4H & 6H SiC MOSFETS:
-Developing new simulation methods to extract physic &
propose how to improve performance.
-Effects of High Temperatures & High Voltage
-4H & 6H MOSFET Comparison
Introduction: Benefits of Wide Bandgap
Semiconductors (SiC)
•
•
•
•
•
Extremely High Temperature Operation
Extremely High Voltage
Extremely High Power
Capable of Growing Oxide => MOSFETs
Potential for High Power and High Temperature
Control Logic
• Power IC’s
• High Temperature IC’s
Research Strategy
Device Modeling
Drift-Diffusion
Experiment
SiC Device
Research & Design
Material Modeling
Monte Carlo
Monte Carlo Transport Simulations
for SiC Surfaces
SiC: Monte Carlo Transport
•Utility of Monte Carlo Transport Simulations
•Self-Consistent surface band structure
•Surface Transport for (0001) 4H-SiC
•Comparison of the (0001) and (1120) 4H-SiC
surfaces
•Summary
•Further work
SiC: Utility of Monte Carlo Simulations
• Detailed physics of carrier transport can be compared
directly to measurements
 Change the physical model at will to match experiments
 Interpret experiments in physical terms
 Simulate new experiments using successful physical
models
Surface Band Structure for SiC
•Surface band structure needed at
MOS inversion layer.
E
SiO 2
(V(z) self-consistent)
Inversion Layer
eV(z)
Gate Oxide (Si0 )
2
S
x
Inversion Layer
Subbands
E
D
F
z
z
E(k x)
Substrate (P)
•E(k) continuous
parallel to
interface(x,y)
Subbands
1
2
•Use effective
mass m*
kx
Surface Band Structure for SiC
One ladder for (0001) here
Average penetration
into SiC
Subband energies
4H
6H
Zav
(nm)
6H
4H
Fermi energy is closer to band edge in 4H-SiC
leading to enhanced trap occupation
Surface Scattering for SiC
Scattering model for the Monte Carlo simulations of transport
of inversion layer mobile electrons.
Inversion layer electrons scatter with:
•Near interface trapped charge
(proportional to Nit and screened by Ninv)
•Surface roughness
(scaled roughness parameters of Si)
•Acoustic phonons
(coupling constant from bulk simulations)
•Polar optical phonons
(coupling constant from bulk simulations)
•Ionized impurities
Surface Transport for (0001) 4H-SiC
Problem: mobility degradation
Small mobilities (0-40 cm/Vs) measured at the (0001)
4H-SiC/SiO2 interface.
Likely cause is the large measured density of near
interface traps.
Supported by our simulations of electron transport using
experiment values for the interface charge densities
Surface Transport for (0001) 4H-SiC
Experimental input for Monte Carlo
Saks and Agarwal measured Id and VH to get (
mobile charge (Ninv) with gate potential (VG).
Ninv) the change in
(SA) N. S. Saks and A. K. Agarwal, Appl. Phys. Lett. 77, 3281 (2000)
4H-SiC MOS Hall Bar
Fits to Saks and Agarwal Data
Surface Transport for (0001) 4H-SiC
Threshold voltage increases with decreasing temperature in experiments.
2
Extracted result:
 424
VT  1.5  
V
 T 
Agrees with other experiments on 4H-SiC MOSFETs
S. Harada et al. Mat. Res. Soc. Symp. Proc. 640, H5.37.1 (2001)
Saks and Agarwal Data
Surface Transport for (0001) 4H-SiC
We determine N inv as a function of gate voltage and temperature.
This leads to expressions for the mobile and trapped charge densities:
Ninv (VG , T )  Ninv (VG , T )VG  VT T 
N it (VG , T ) 
Suggests traps near the band edge
Cox
VG  1.5  N inv VG , T 
e
Surface Transport for (0001) 4H-SiC
The scattering rate  is dominated by interface trapped charge scattering
( it). For higher gate voltages or at higher temperatures, Ninv increases
and the ( it) rate is more effectively screened.
Scattering Rate
Surface Transport for (0001) 4H-SiC
Simulation results show a small low-field mobility which increases with increasing
temperature. This agrees with experimental results:
(Saks and Agarwal), (Matsunami, Kimoto, and Yano)
Temperature dependence indicates that the mobility is limited by scattering
from trapped charge. As T decreases, Nit decreases and the screening
increases, both decreasing the mobility.
Saks and Agarwal
Comparison of (0001) and (1120) 4H-SiC
(1120) oriented surfaces show a larger mobility in 4H-SiC. Is
this due to a smaller density of trap states, band structure
differences, or surface quality?
Scaling (0001) results we get (1120)
charge densities:
YHKM
Expt. (0001)
(1120)
MC fits used
N
(1120 )
it
SA
N
VT(1120 )  3.5 
T
440
H. Yano, T. Hirao, T. Kimoto, and H. Matsunami,
Appl. Phys. Lett. 81, 4772 (2002).
(1120 )
inv
(1120 )

 N it
  ( 0001)

 N it
YHKM

 ( 0001)
 N it


VG  VT(1120 )  ( 0001)

N
( 0001)  inv
VG  VT

and the threshold voltage:
VT(1120 )  3.5 
T
440
SA
Comparison of (0001) and (1120) 4H-SiC
2
•Mobility for (1120) 4H-SiC is much larger (90cm /Vs) comparing well
with experiments (81.7-95.9cm 2 /Vs)
H. Matsunami, T. Kimoto, and H. Yano, Mat. Res. Symp. Proc. 640, H3.4.1 (2001).
•Find that the (1120) mobility decreases with temperature, also agreeing with experiments.
•If we again simulate transport for (0001) 4H-SiC but using the density of trap states
measured in (1120) 4H-SiC, a large improvement in the mobility is observed.
Comparison of (0001) and (1120) 4H-SiC
Band structure for (0001) and (1120) 4H-SiC is very similar.
This occurs since the 2 ladder masses in the (1120) orientation are similar to the
mass of the one ladder in the (0001) orientation
If reduce the trap density in (0001) 4H-SiC, expect mobilities similar to those found
in (1120) 4H-SiC.
Results for Surface Monte Carlo for SiC
•Used experimental results for the free and trapped charged densities to
simulate inversion layer electron mobility in (0001) and (1120) 4H-SiC
using a self-consistent surface Monte Carlo simulator.
•For the (0001) orientation the Monte Carlo simulated mobility was found to
2
increase (0-40cm /Vs) with increasing temperature and increasing gate
potential, trends observed in experiments.
(Consistent with strong interface trap scattering subjected to screening
by mobile inversion layer electrons.)
•For the (1120) orientation the Monte Carlo simulated mobility was much larger
(90cm2 /Vs) and decreased with increasing temperature, agreeing with
experiments.
(Phonon scattering plays a larger role for the (1120) orientation.)
•We found Band structure similarities between the (0001) and (1120) orientations
of 4H-SiC. (With commensurate densities of traps, the (0001) and (1120)
surfaces are found to have similar electron transport properties.)
Monte Carlo Simulations of
Transport in SiC: Further
Work
Using the ARL supercomputer:
•Simulation of high-field transport at the MOSFET
interface (Vsat(T)).
•Effect of defects on transport. (stacking faults)
•Effect of tunneling into the oxide.
Characterization &
Simulation of 4H-SiC
MOSFETs
Siddharth Potbhare (MS-Ph.D)
MOSFET Device Simulation
MOSFET Device Structure
Semiconductor Equations
Poisson Equation:

q
     n  p  N D  N A

2

n
   J n  q R  G 
t
Electron current
continuity equation:
q
Hole current
continuity equation:
q
Electron current
equation:
J n  qnn  q(nDn )
Hole current
equation:
J p  qp p   q( pD p )
p
   J p  q R  G 
t
Simulation Methodology
Set up the device
dimensions, material
properties,
temperature, bias
voltages, doping
profile, etc.
Discretization of the
semiconductor
equations
Newton’s Method
for better
accuracy
Y
Converged?
N
Iterative Gummel
Block Method.
Solve for , n, p
Initial
Guess for
, n and p
N
Current
Continuity?
Y
Extract , electron and
hole concentrations,
mobility, current density,
IV characteristics, etc.
Mobility Models
Oxide
Low field mobility:
Matthiessen's rule
1
 LF

1
B

1
 SP

1
 SR

1
Electron Flow
C
LF = Low Field Mobility B = Bulk Mobility
Bulk
SP = Surface Phonon Mobility
Electron
Surface Phonon
SR = Surface Roughness mobility
Trap
Surface Roughness
C = Trapped interface charge mobility
High Field Mobility:
High field mobility:
 HF 
Fixed Charge
 LF
1
   LF E||   
 
1  
  vsat  
Caughey – Thomas Model for bulk mobility:
n 
 300

T


 D
1 
N
 ref
n  
0
T
n
n
  nmin




n
1


 
n
Temperature dependence:
T 
  nmin
Doping dependence:
1
n   
D
n
Surface Phonon Mobility:
 SP
1
 SP

q ac

mc
 n E
 n 
2
1     E 3
T 
ac = Surface acoustic phonon relaxation time
E┴ = Perpendicular E. Field
n, n = calculated from phonon scattering
equation
Surface Roughness Mobility:
1
 SR

SR = Surface roughness parameter.
E2
 SR
Higher the value of SR, smoother the surface and lower
the degradation in total mobility
Interface Trap Charge Mobility:
Corresponds to effect of coulomb scattering of mobile charged carriers
by fixed charge and interface trap charge. The term also accounts for
the screening of these charges by electrons at strong inversion.
1
C

nf  Nit
Temp
 T 


300




ne

 1 
screen_fit


screen_factor
 it
nf = Fixed oxide charge
Nit = Occupied interface trap density
ne = Inversion layer electron concentration
temp = Temperature dependence
screen_fit, screen_factor = fitting parameters
for the screening effect
it = from Coulomb Scattering
model
4H SiC 200m x 200m MOSFET:
Id-Vgs Simulation Fit at T=27oC
4H SiC 200m x 200m MOSFET:
Id-Vds Simulation Fit at T=27oC
Bulk Mobility ….
n 
 300

 T 
 D
1 
N
 ref
n  
0
T
n
  nmin




n
  nmin
Parameter
6H SiC
4H SiC
n0 in cm2/Vs
500.0
1071.0
nmin in cm2/Vs
0.0
5.0
Bulk mobility at
Room Temperature
and D ~ 1015 is
n
2.4
2.5
4H SiC: ~ 800 cm2/Vs
Nref
1.1e18
1.9e17
n
0.45
0.40
6H SiC: ~ 400 cm2/Vs
Values from literature
Surface Roughness Mobility ….
1
 SR

E2
 SR
Parameter
6H
4H
SR (V/s)
1e13
5.82e14
4H SR Value is taken from Linewih (2002) paper
1
 SR

1
Cit
Effect of surface roughness is
negligible as compared to the effect of
interface traps on the total mobility.
Interface Trap Charge Mobility ….
1
C

nf  Nit
Temp
 T 


300




ne

 1 
 screen_fit
screen_factor
 it
6H
4H
nf
5.4 x 1011
2.2 x 1012
Nit at RmT
~ 2 x 1012
~3 x 1012
it
1.5 x 1011
1.5 x 1011
screen_fit
1.5 x 1018
1 x 1018
screen_factor
0.8
0.7
Our Extracted Physical Parameters
Occupied interface trap density (Nit)
Ec
Qit  qNit  q  Dit E   f E dE
Ev
Dit = Density of traps per unit energy
f(E) is the probability density function. It is
directly proportional to the mobile charge
concentration (ne). Hence as MOSFET goes
towards stronger inversion, the occupied
interface trap density increases.
 E  Ec 
Dita E   Ditmid  Ditedge exp

a


f E  
1
1
 E  Ec 
1 Nc

exp
2 ne
k
T
 B 
4H SiC has a higher bandgap than 6H SiC (by 0.2eV). Ditedge value for 4H SiC
is obtained by extrapolating the Dit-E curve for 6H SiC by 0.1eV. This gives a
very high Ditedge value for 4H SiC because of the exponential relation
between Dit and E near the band edge. Hence 4H SiC has much higher
interface traps than 6H SiC.
Extrapolation of Dit-E curve for 6H SiC to get Dit-E
characteristics for 4H SiC
Final Dit-E curve for 4H that is used:
Dit_edge = 2.15 x 1013 cm-2eV-1
Dit_mid = 6.5 x 1011 cm-2eV-1
6H
4H
Ditmid (cm-2eV-1)
1 x 1013
2.19 x 1013
Ditedge (cm-2eV-1)
8 x 1011
8 x 1011
Nit vs. position for different Vgs.
T=27oC
Occupied interface trap
density increases with
increase in Vgs. This is
because the inversion layer
electron concentration
increases with increase in
Vgs causing more traps to
get filled
Device: 4H SiC MOSFET
W/L: 200 m / 200 m
Bias: Vgs = 2 to 4V Vds = 4V
Nit vs. position for different
Temperatures
Occupied interface trap
density decreases with
increase in temperature
because trapped electrons
can escape by gaining
sufficient energy at higher
temperatures.
So as the temperature
increases, effect of interface
trap charge decreases,
increasing overall mobility
Device: 4H SiC MOSFET
W/L: 200 m / 200 m
Bias: Vgs = 6V Vds = 1V
Comparing effects of Surface Roughness and
Interface traps at different Temperatures
The change in Id values for
a tenfold improvement of
the surface roughness
factor, is very small at all
three temperatures. Thus
surface roughness does
not change the current with
change in temperature.
The increase in current
with temperature is caused
by the reduction of filled
interface trap density as
temperature increases.
Device: 4H SiC MOSFET
W/L: 200 m / 200 m
Bias: Vgs = 6V Vds = 0-8V
Key Findings for Recent 4H SiC Technology
• Occupied interface trap density much higher in 4H than 6H SiC
– Increase due mainly to larger bandgap. Interface traps increase
exponentially near band edge
• Average surface mobility approx. half in 4H than 6H (even though
bulk 4H mobility is 2X greater than 6H)
– Mobility degradation due to high interface traps density
• Surface roughness not important mobility degradation mechanism
as compared to interface traps.
• Model predicts current increases with temperature
– Increase due to reduction in occupied interface traps
– Surface roughness becomes very slightly more important at
higher temperatures, but interface states still dominate at 200C.
• Current vs Voltage lower in 4H than 6H.
Future Work…
• Better screening model based on BrooksHerring ionized impurity scattering model
• Surface roughness calculation to get proper
value for SR
• Fitting data at higher temperatures
• High power MOSFET simulation
• Investigating gate leakage in SiC MOSFETs
• Building a Graphical User Interface for the
simulator
Very Recent Publications
1)
G. Pennington, and N. Goldsman, "Empirical Pseudopotential Band Structure of 3C, 4H,
and 6H SiC Using Transferable Semiempirical Si and C Model Potentials,” Phy. Rev. B,
vol 64, pp. 45104-1-10, 2001.
2)
G. Pennington, N. Goldsman, C. Scozzie, J. McGarrit, F.B. Mclean., “Investigation of
Temperature Effects on Electron Transport in SiC using Unique Full Band Monte Carlo
Simulation,” International Semiconductor Device Research Symposium Proceedings, pp.
531-534, 2001.
3)
S. Powell, N. Goldsman, C. Scozzie, A. Lelis, J. McGarrity, “Self-Consistent Surface
Mobility and Interface Charge Modeling in Conjunction with Experiment of 6H-SiC
MOSFETs,” International Semiconductor Device Research Symposium Proceedings, pp.
572-574, 2001.
4)
S. Powell, N. Goldsman, J. McGarrity, J. Bernstein, C. Scozzie, A. Lelis,
“Characterization and Physics-Based Modeling of 6H-SiC MOSFETs”’ Journal of
Applied Physics, V.92, N.7, pp 4053-4061, 2002
5)
S Powell, N. Goldsman, J. McGarrity, A. Lelis, C. Scozzie, F.B McLean., “Interface
Effects on Channel Mobility in SiC MOSFETs,” Semiconductor Interface Specialists
Conference, 2002
6)
G. Pennington, S. Powell, N. Goldsman, J.McGarrity, A. Lelis, C.Scozzie., “Degradation
of Inversion Layer Mobility in 6H-SiC by Interface Charge,” Semiconductor Interface
Specialists Conference, 2002.
Very Recent Publications Continued
7) G. Pennington and N. Goldsman, ``Self-Consistent Calculations for n-Type Hexagonal SiC
Inversion Layers,” Accepted for publication in Journal of Applied Physics, 2003
8) G. Pennington, N. Goldsman, J. McGarrity, A Lelis and C. Scozzie, ``Comparison of 1120
and 0001 Surface Orientation in 4H SiC Inversion Layers,” Semiconductor Interface
Specialists Conference, 2003.
9) S. Potbhare, N. Goldsman, A. Lelis, “Characterization and Simulation of Novel 4H SiC
MOSFETs”, UMD Research Review Day Poster, March 2004.
10) G. Pennington, N. Goldsman, J. McGarrity, A. Lelis, C. Scozzie, ``(001) Oriented 4H-SiC
Quantized Inversion Layers," International Semiconductor Device Research Symposium,
pp. 338-339, 2003.
11) X. Zhang, N. Goldsman, J.B. Bernstein, J.M. McGarrity, S. Powell, ``Numerical and
Experimental Characterization of 4H-SiC Schottky Diodes,” International
Semiconductor Device Research Symposium, pp. 120-121, 2003.