Ultrascaled GaN HEMTs with thin AlN barriershxing/research/pdfs/201108... · Huili (Grace) Xing...
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Ultrascaled GaN HEMTs with thin AlN barriers
Huili (Grace) Xing
Electrical Engineering Department, University of Notre Dame
Huili (Grace) Xing ([email protected]) TWHM-Gifu-2011 1
Ft – transport physics in GaN
Outline • AlN/GaN HEMTs
– Highest mobility with highest 2DEG (D. Jena) – Ohmic contacts
• InAlN/AlN/GaN HEMTs – Very high mobility with very high 2DEG – Ohmic contacts – Passivation – Emode – Phonon-limited injection velocity (D. Jena)
• Summary
Huili (Grace) Xing ([email protected]) TWHM-Gifu-2011 2
Important for high power and high speed – High ns, mobility and saturation velocity – Thin barrier, short gate length, large barrier height
Barrier
Channel
Source Drain
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Gate
Ultra-scaled WBG semiconductor-based devices (e.g. Lg < 50 nm, Cg > 1.6 µF/cm2 – corresponding to ~ 5 nm thick AlN, ~ 1.6 S/mm assuming 1e7 cm/s)
0 100 200 300 400 500 600-5-4-3-2-101234
Ener
gy (e
V)
Thickness (A)
(3 nm) AlN/GaN
Ultra-thin AlN barrier GaN HEMTs
EOT = 1.4 nm
Huili (Grace) Xing ([email protected])
AlN/GaN by MBE
• An AlN/GaN superlattice
Streak pattern due to SL
5nm
20nm
TEM By Kejia (Albert) Wang and Tom Kosel (U. of Notre Dame)
AlN (13 monolayers)
GaN
GaN
Huili (Grace) Xing ([email protected]) TWHM-Gifu-2011 4
4 nm AlN/GaN (MBE)
Highest 2DEG and mobility in III-V nitrides
I. Smorchkova et al, 2000, APL Y. Cao et al, 2007, APL Y. Cao et al, 2008, APL
Huili (Grace) Xing ([email protected]) TWHM-Gifu-2011 5
High conductivity window: 2-5 nm AlN Best metrics among all III-V heterostructures: ns = 2x1013 cm-2 with u = 1900 cm2/Vs Y. Cao et al, 2011, JCG
1.5 nm 6 nm AlN thickness
• RT Hall measurement: – ns ~ 2.75e13 cm-2
- µ ~ 1367cm2/Vs - Rsh ~ 166 ohm/sq
3.5 nm AlN
Sapphire substrate
GaN layers
S D
G S G & 3 nm Al2O3 gate dielectric
I. Ultrathin barrier AlN/GaN HEMT: structure Early studies
Zimmermann et al. IEEE EDL. 2008
Huili (Grace) Xing ([email protected]) TWHM-Gifu-2011 6
S S
D
G
• Gate oxide used to prevent leakage in 3.5 nm AlN/GaN HEMTs. • Current densities & transconductance are high. • Breakdown ~ 25 Volts. • As measured ft/fmax ~ 52/60 GHz
Zimmermann et al. IEEE EDL. 2008 gm,int ~ 1 S/mm
I. Ultrathin barrier AlN/GaN HEMTs: performance Early Studies
Huili (Grace) Xing ([email protected]) TWHM-Gifu-2011 7
Without de-embedding
I. Ultrathin barrier AlN/GaN HEMTs: performance Early studies
Zimmermann et al. IEEE EDL. 2008
The first reported AlN/GaN HEMT with mobility > 1000 cm2/Vs
Performance limited by poor ohmics, gate and buffer leakage
Also see Higashiwaki’s work in 2007 and HRL’s work in 2010/2011 Huili (Grace) Xing ([email protected]) TWHM-Gifu-2011
Ohmic contact challenge
Huili (Grace) Xing ([email protected]) TWHM-Gifu-2011
9
Solved by MBE regrown contacts
on metal-face GaN HEMTs, see HRL DRC 2010, UND ICNS 2011
On N-face GaN HEMTs, see UCSB APL 2009
T. Zimmermann et al,
PSS, 2008
D. Deen, PhD
dissertation, UND 2010
Outline
• AlN/GaN HEMTs
– Highest mobility with highest 2DEG – Ohmic contacts
• InAlN/AlN/GaN HEMTs – Very high mobility & ns
– Ohmic contacts – Passivation – Emode
• Summary
Huili (Grace) Xing ([email protected]) TWHM-Gifu-2011 10
D. Jena et al, PSS 2011
InAl(Ga)N/AlN/GaN HEMTs • Attractive to cap the thin AlN barrier
– GaN: substantially reduce ns (< 1.2 x 1013 cm-2)
– InAlN: maintain high ns (> 1.5x1013 cm-2)
Huili (Grace) Xing ([email protected]) TWHM-Gifu-2011 11
0 50 100 150 2000.00E+000
1.00E+013
2.00E+013
3.00E+013
4.00E+013
5.00E+013
6.00E+013
ns=2.5x1013 cm-2
AlGaN 30% AlInN 17% AlN
n s (cm
-2)
Barrier Thickness (A)
ns=1.4x1013 cm-2
d ~ 40 A
d = 180 A
IQE, IWN 2010
AlN/GaN
(Y. Cao
JCG’11)
InAlN
(IQE,
EPFL)
SiN passiva)on SiN passiva)on
In0.17Al0.83N (4.7 nm)
Alloyed ohmic contacts with Si and recess etch
In0.13Al0.83Ga0.04N 10 nm
GaN
SiC
AlN 1.0 nm
DS
2DEG GaN
SiC
AlN 1.0 nm DS
2DEG
ü Mesa isola)on; ü Alloyed Ohmics:
(ohmic recess) + (Si)/ Ti/Al/Ni/Au, RTP
• Reliable Rc extrac)on necessitates passiva)on • Smooth morphology
(Si)/Ti/Al/Ni/Au
0 5 10 15 20 25 300
20
40
60
80
R c = 0.23 Ω-‐mm,
R s h = 190 Ω/s q
As -‐proc es s ed S iN pas s ivated
R (Ω)
d (um)
R c = 0.17 Ω-‐mm,
R s h = 220 Ω/s q
TL M: InA lG aN, 860 C , 18 s
800 820 840 860 8800.0
0.3
0.6
0.9
1.2
1.5 InA lN (#B -‐1), w/o reces s
InA lN (#B -‐2), 3 nm reces s
InA lN (#B -‐3), 7 nm reces s
InA lN (#B -‐4), 15 nm reces s
InA lG aN (#C _311), w/o reces s
InA lG aN (#D _310), ~ 6 nm reces
~11 nm barrier, 18 s
Rc (Ω
-‐mm)
A nnealing Temperature (C )
5.7 nm barrier, 15 s ;
w/ 2 nm S iR. Wang et al., APEX 2011.
Huili (Grace) Xing ([email protected]) TWHM-Gifu-2011 12
Alloyed Ohmics with Si and ohmic recess
InAlGaN
ü No obvious Rc improvement for thin barriers; contact via sidewall is not efficient aIer etching away the en)re barrier;
ü Rc improvement for thick barriers (Rc as low as 0.23 Ω•mm); no metal penetra)on into GaN;
ü Ul)mate limit for tunneling: 1-‐nm AlN spacer.
InAlGaN, 860 C, 18 s
R. Wang et al., APEX 2011.
Huili (Grace) Xing ([email protected]) TWHM-Gifu-2011 13
HEMTs with SiN passivation (I)
With increasing SiN thickness:
Ø Mobility is nearly constant Ø Carrier concentra)on increases and saturates (> 30 nm) Ø Sheet resistance decreases and saturates (> 30 nm) Ø gm increases. Why does gm increase with > 30 nm SiN? Ø I increases, saturates (70 – 100 nm) and then decreases.
InAlN 4.7 nm
GaN
SiC
AlN 1.0 nm
DSG
2DEG
# A
Ni/Au
0 20 40 60 80 100500
550
600
650
700
f T (GHz)
Pea
k gm (mS/m
m)
S iN Thic knes s (nm)
30
40
50
60
70
80
90
100
(a)
0 20 40 60 800.0
0.5
1.0
1.5
2.0
2.5
3.0
µ (x 103
cm
2 /V.s)/ns (x10
-‐13 cm
-‐2)
Rsh (ohm/sq)
S iN Th ic knes s (nm)
As -‐g rwon
-‐400
-‐200
0
200
400
600 (c )
Lg = 160 nm
Huili (Grace) Xing ([email protected]) TWHM-Gifu-2011
R. Wang et al., ISCS 2011. (presenta:on only)
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HEMTs with SiN Passivation (II)
0 2 4 6 8 10 120.0
0.5
1.0
1.5
2.0
2.5
+3 V+2 V
0 V
A s -‐proc es s ed 30 nm S iN 70 nm S iN
I d (A/m
m)
V ds (V)
S tep = -‐1 V
-‐2 V
(a)
-‐4 -‐3 -‐2 -‐1 0 1 2 30.0
0.5
1.0
1.5
2.0 As -‐proc es s ed 30 nm S iN 70 nm S iN
I d (A/m
m)
gm (mS/m
m)
V g s (V)
Vds = 6 V
(b)
0
200
400
600
800
1000
-‐4 -‐3 -‐2 -‐1 0 1 2 310-‐8
10-‐6
10-‐4
10-‐2
100
10-‐8
10-‐6
10-‐4
10-‐2
100
A s -‐proc es s ed 30 nm S iN 70 nm S iN
Vds = 6 V I g (A/m
m)
V g s (V)
I d (A/m
m)
(c )
SiN 30 nm è 70 nm, both intrinsic delay and drain delay decreased – consistent with higher gm. Possible reason for higher gm: 1. Higher carrier velocity? 2. Shorter effective gate length Lg, eff thus higher carrier velocity?
✗
✔
Huili (Grace) Xing ([email protected]) TWHM-Gifu-2011
R. Wang et al., ISCS 2011. (presenta:on only)
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Pulsed I-Vs
With 100 nm SiN passivation ~ 4% gate delay and ~ 7% drain delay
As-fabricated (without any passivation) ~ 30% gate delay and ~ 10% drain delay
Lg ~ 60 nm
Huili (Grace) Xing ([email protected]) TWHM-Gifu-2011
R. Wang et al., ISCS 2011. (presenta:on only)
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HEMTs with Dielectric-Free Passivation (DFP)
InAl(Ga)N ~11 nm
GaN
SiC
AlN 1.0 nm
DS
G
2DEG
Plasma (DFP) O2
# B
Before DFP After DFP
Rsh (Ω/sq)
ns (x 1013 cm-2)
µ (cm2/V.s)
Rsh (Ω/sq)
ns (x 1013 cm-2)
µ (cm2/V.s)
InAlN_313 290 1.62 1330 257 1.86 1300
InAlGaN_310 227 1.45 1900 190 1.83 1790
With DFP in the access region only:
Ø Mobility decreases slightly Ø Carrier concentra)on increases Ø Sheet resistance decreases Ø gm increases slightly Ø ft increases from 125 GHz to 210-‐220 GHz -‐8 -‐6 -‐4 -‐2 0 2
0.0
0.5
1.0
1.5
2.0
2.5 w/o DF P w/ DF P
I d (A/m
m)
gm (mS/m
m)
V g s (V)
Vds = 6 V
0
100
200
300
400
500
600 (a)
Lg ~ 60 nm
Huili (Grace) Xing ([email protected]) TWHM-Gifu-2011
R. Wang et al., EDL 2011. 17
Dielectric-Free Passivation (DFP): I. InAlN
-‐50 -‐40 -‐30 -‐20 -‐10 010-‐13
10-‐11
10-‐9
10-‐7
10-‐5
10-‐3
10-‐1
Vbr = 50 V
w/o DF P w/ DF P
Vds = 0 V
Vg s (V)
I g (A/m
m)
V br = 12 V
108 109 1010 10110
10
20
30
40
50
60
/ |h21|2
/ U
w/o DF PVg s / Vds = -‐4.1 / 4.0 V
fT / fmax = 125 / 46 G Hz
w/ DF PVg s / Vds = -‐4.0 / 4.8 V
fT / fmax = 210 / 55 G Hz
Gain (dB)
F requenc y (Hz )
L g ~ 60 nm
-‐20 dB /dec
All delay components droped. Lg,eff > 120 nm prior to DFP
R. Wang et al. IEEE EDL, 2011
Huili (Grace) Xing ([email protected]) TWHM-Gifu-2011
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Dielectric-Free Passivation (DFP): II. InAlGaN
109 1010 10110
10
20
30
40
|h21|2
U
fT = 220 G Hz
fmax= 60 G Hz
Gain (dB)
F requenc y (Hz )
Vds = 4.7 V ,
Vg s = -‐3.7 V
L g ~ 66 nm
0 10 20 30 40 50 60 700
2
4
6
8
10
12
14
16
18
20
C alc ulations AlG aN, 2 nm S iN, T -‐g ate AlG aN, no pas s ivation, T -‐g ate AlN, 50 nm S iN, T -‐g ate InAlN, 50 nm S iN, T -‐g ate InAlN, 10 nm Al2O 3, I-‐g ate
InAlN, DF P , I-‐g ate InAlG aN, DF P , I-‐g ate
f T. L
g (GHz.µm
)
L g /tbar (un itles s )
Th is work 0 2 4 6 8 100.0
0.5
1.0
1.5
2.0
2.5
B ias po int: DC (Vg s , Vds ) = (0 V , 0 V )
(Vg s , Vds ) = (-‐8 V , 0 V )
(Vg s , Vds ) = (-‐8 V , 10 V )
I d (A/m
m)
V ds (V)
Vg s = 0 V
300 ns pu ls e width
R. Wang et al. EDL, 2011
Little dispersion observed in HEMTs with DFP Other attributes of DFP: 1. Air stable 2. Little parasitic capacitance
3. Large signal performance yet to be tested. ?
✔ ✔
W DFP Lg = 66 nm
Huili (Grace) Xing ([email protected]) TWHM-Gifu-2011
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R. Wang et al., IEEE EDL 2010
Huili (Grace) Xing ([email protected]) TWHM-Gifu-2011
Characterization of TQT HEMTs at UND (2): Device Fabrication
Post Annealing Parameters: 400 C, 10 min, in forming gas (5% H2, balanced with Ar) * Annealing in N2 resulted in similar behaviors too.
RT Hall transport data: ns ~ 2.0 x 1013 cm-‐2
µ ~ 1160 cm2/Vs Rsh ~ 270 Ω/sq
Gate geometry Lg ~ 150 nm, Wg ~ 100 -‐ 600 µm Lsd ~ 2 µm, 3 µm Gate metal: Pt/Au
Ti based ohmic contact Rc ~ 0.6 Ω-‐mm
SiN InAlN 4.8 nm
GaN
SiC
AlN 1 nm S D
G
21 Huili (Grace) Xing ([email protected]) TWHM-Gifu-2011
Characterization of TQT HEMTs at UND (3): E-mode
0 2 4 6 8 100.0
0.5
1.0
1.5
2.0Vg s = 4 V , S tep = -‐0.5 V
I d (A
/mm)
V ds (V )
L g = 150 nm, Wg = 3x50 um, L s d = 2 um
R. Wang, P. Saunier, et al., DRC, 2010; IEEE EDL, vol. 31 (12) 2010.
AlN InAlN
Pt Lg = 150 nm
SiN
GaN
0 1 2 3 40.0
0.5
1.0
1.5
2.0
g m (m
S/mm)
I d (A
/mm)
V g s (V)
Vds = 5 V
L g = 150 nm, Wg = 2x50 um
L s d = 2 um
0
200
400
600
800
ft/fmax ~ 70/105 GHz, without deembedding
Cross-‐sec)onal STEM
22 Huili (Grace) Xing ([email protected]) TWHM-Gifu-2011
Characterization of TQT HEMTs at UND (4): Effect of Annealing
0 1 2 3 4 5 6 7 8 9 10
0.0
0.2
0.4
0.6
0.8
Wg = 8x50 um w/o annea ling w/ annea ling
I d (A/m
m)
V ds (V)
Vg s = 2 V , S tep = -‐0.5 V
-‐1 0 1 2 30.0
0.2
0.4
0.6
0.8
1.0
1.2
Vth = 0.6 V Vth = 1.2 V
Wg = 8x50 um
L s d = 3 um
w/o annealing w/ annealing
I d (A/m
m)
gm (mS/m
m)
V g s (V)
Vds = 6 V
0
100
200
300
400
500
600
InAlAs
K. Chen, et al., IEEE EDL, 1996. Pt/InAlAs HEMT
A. Fricke et al., APL, 1994.
Gate sink
aIer annealing In0.52Al0.48As
Pt
In0.52Al0.48As
Pt PtIn/PtAs2 AlAs
23 Huili (Grace) Xing ([email protected]) TWHM-Gifu-2011
Characterization of TQT HEMTs at UND (5): pre-annealing Dit
• High Dit (> 1x1013cm-2) observed in the as-fabricated E-mode devices
SS = (1+Cq + Cit
Cb
)kBTq
ln10
R. Wang et al., IEEE EDL 2010
24 Huili (Grace) Xing ([email protected]) TWHM-Gifu-2011
Gate diode breakdown
Ø Forward effec)ve barrier height increases from 0.67 to 0.93 eV, akributed to an insula)ng layer formed aIer post annealing.
Characterization of TQT HEMTs at UND (6): Leakage Reduction
-‐1 0 1 2 310-‐15
10-‐13
10-‐11
10-‐9
10-‐7
10-‐5
10-‐3
10-‐1
101
10-‐15
10-‐13
10-‐11
10-‐9
10-‐7
10-‐5
10-‐3
10-‐1
101
S S ~ 62 mV/dec
Ion/Ioff = 1012
S S ~ 84 mV/dec
Ion/Ioff = 107
Vds = 6 V
w/o annealing w/ annealing
I g (A/m
m)
V g s (V)
I d (A/m
m)
(a)
Log-‐scale transfer curve
Ø Dit decreases from 1.5 x 1013 to 1.1 x 1012 cm-‐2eV-‐1, extracted from subthreshold slopes, meaning post-‐annealing repairs the interface;
5
110
×
R. Wang et al., IEEE EDL 2011
25 Huili (Grace) Xing ([email protected]) TWHM-Gifu-2011
Characterization of TQT HEMTs at UND (7): Leakage Mechanism
Trap density decrease and barrier height increase are responsible for the gate leakage reduc)on. More work are being taken, such as STEM, EDX, modeling on leakage…
AlN GaN Pt
DT
Ef, s Ef, g
Interlayer (PtxIny, Al-‐O, …) Post-‐annealing
AlN GaN Pt
Direct tunneling (DT)/FN tunneling (FN)
Trap-‐assisted tunneling (TAT)
Ef, s Ef, g
φb
dieEv
20
exp( )exp( )dieTTAT die
qEqJ AEkT kT rε
πε= −
3/22 exp( )b
FN diedie
CJ BEEφ
= −
A depends on trap density.
B, C are constants. Pre-‐annealing
R. Wang et al., IEEE EDL 2011
26 Huili (Grace) Xing ([email protected]) TWHM-Gifu-2011
GaN
300 nm
300 nm
Thickness measured by a-step
metal
AFM
MBE
MBE regrown contact process flow
RMS=0.6 nm
MBE Regrown n+ GaN surface
Control regrowth (45 nm n+GaN): Sheet charge ~2e15/cm2
Mobility ~ 52 cm2/Vs Rsh~58 Ohm/sq
MB
E
crys
talli
ne
MBE amorphous
Barrier
GaN
Barrier
GaN
Barrier
Mask
MB
E
crys
talli
ne
MB
E
crys
talli
ne
GaN
Barrier
MB
E
crys
talli
ne
MB
E
crys
talli
ne
GaN
Barrier
MB
E
crys
talli
ne
• Planar regrowth of high quality • Lateral regrowth interface needs to be further characterized SEM
Regrown n+GaN on mask
27 Huili (Grace) Xing ([email protected]) TWHM-Gifu-2011
G. Jia et al., ICNS 2011 (manuscript under preparation)
Regrown contacts with n+GaN/2DEG Rc ~ 0.05 ohm-mm
Huili (Grace) Xing ([email protected]) TWHM-Gifu-2011 28
1. Regrowth interface resistance ~ 0.05 ohm-mm
2. Total Rc ~ 0.27 ohm-mm, dominated by metal/n+GaN resistance (~ 0.15 ohm-mm).
3. We have also demonstrated metal/n+InGaN Rc < 0.02 ohm-mm.
G. Jia et al., ICNS 2011
Injection point phonon model and Peak injection velocity
29
• Saturation currents from the phonon model fit experimental state-of-the-art • The peak injection velocity is vp~1.3x107cm/s at room temperature • The peak injection velocity occurs when ns~4x1012/cm2 @ source injection pt.
Op:cal phonon emission
Op:cal phonon Emission blocked
Peak injec:on velocity
Investigate here
T. Fang et al., DRC 2011
Huili (Grace) Xing ([email protected]) TWHM-Gifu-2011
Peak fT with barrier and gate length scaling
30
• Intrinsic peak fT is ‘fundamental’, limited by the onset of phonon emission • Gate length scaling improves peak fT faster than vertical scaling • Peak fT improvement is severely stalled by source/drain resistances
Lg=50 nm
fT peaks at phonon emission
T. Fang et al., DRC 2011
?
Huili (Grace) Xing ([email protected]) TWHM-Gifu-2011
Summary
• Ohmic contacts have been solved for GaN HEMTs – Rc comparable to that of InAs and Si FETs, < 50 ohm-um
• Low injection velocity in GaN limits the device speed – Low mobility (phonon scattering, interface scattering) – High effective mass (wide bandgap) – Locked by optical phonons
• Plasma treatments need to be better understood for reliable device operation
• Novel ideas are necessary for GaN THz transistors
Huili (Grace) Xing ([email protected]) TWHM-Gifu-2011 31
Acknowledgement
Students and postdocs Yu Cao David Deen Tian Fang Zongyang Hu Fazia Faria Jia Guo Guowang Li Chuanxin Lian Berardi Sensale-Rodriguez John Simon Ronghua Wang Tom Zimmermann Yuanzheng Yue Vladimir Protasenko
Huili (Grace) Xing ([email protected]) TWHM-Gifu-2011 32
Collaborators (University of Notre Dame) Debdeep Jena Tom Kosel Gregory Snider Patrick Fay (Triquint Semiconductor) Paul Saunier (IQE RF LLC) Shiping Guo (Kopin) Wayne Johnson
GaN research Sponsored DARPA ONR AFOSR
Special thanks to
Umesh Mishra & Debdeep Jena