Atomic layer epitaxy of rare earth oxide films on GaAs(111)A and their device properties
Yiqun Liu1)*, Min Xu2), Jaeyeong Heo1), Peide D. Ye2), and Roy G. Gordon1)** 1) Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts, U.S.A. 2) School of Electrical and Computer Engineering, Purdue University, West Lafayette, Indiana, U.S.A. Email:* yiqunliu@fas.harvard.edu, ** gordon@chemistry.harvard.edu

The aggressive scaling of MOSFETs has created interest in using high-mobility III-V channel materials to replace traditional strained Si. However, it has been challenging to form highdielectrics that can passivate III-V surfaces with a low interface state density (Dit). We deposited LaLuO3 high- dielectric layer by ALD on sulfur-passivated GaAs substrates. The precursors lanthanum tris(N,N'-diisopropylformamidinate), and lutetium tris(N,N'-diethylformamidinate) reacted with water vapor at 350 oC. The compositional ratio of La:Lu was about 1:1 by using one cycle of La2O3 followed by one cycle of Lu2O3 in one complete cycle of LaLuO3. Both highresolution XRD analysis and TEM showed that ALD LaLuO3 formed epitaxially on GaAs(111)A substrates, as shown in Figures 1 and 2, respectively. The epitaxial layer exhibited a cubic structure with a lattice constant smaller than GaAs by 3.8%. The LaLuO3 film had a high degree of crystalline perfection and was relaxed and not strained. Electrical characterizations showed the measured dielectric constant of around 30, which is close to its bulk crystalline value. The interface had a low interface state density (Dit) of ~7×1011 cm-2eV-1. The amount of lattice mismatch can be engineered by choosing various rare-earth oxides. ALD La2O3 formed cube-oncube epitaxy on GaAs(111)A with a lattice constant just +0.9% larger than that of the substrate. The mismatch can be reduced to zero by adding some Y2O3 to the La2O3, using yttrium tris(N,N'diisopropylactamidinate)/H2O cycles. Perfect zero-mismatched epitaxy was achieved on GaAs(111)A by depositing La1.7Y0.3O3, as shown in Figure 3. The effects of mismatch on the electrical properties of epi-LaYO3 on GaAs(111)A were studied. These results suggest that atomic layer epitaxy of rare-earth oxides/GaAs(111)A is a promising structure for future generations of high-power/high-frequency analog devices or high-speed logic devices.

Figure 1 (a) High-resolution X-ray omega-two theta scan for LaLuO3 film on GaAs(111)A. (b) and (c) The rocking curves of the LaLuO3 (222) peak and the GaAs(111) peak, respectively. Figure 2 (a) Crosssectional TEM of LaLuO3/GaAs(111)A heterostructure. (b) A magnified image of the interface. Figure 3. Cross-sectional TEM of La1.7Y0.3O3/GaAs(111)A. The mismatch is zero for this composition with 15% Y2O3

Atomic Layer Epitaxy of Rare Earth Oxide Films on GaAs and Device Properties
2011 ALD Meeting

Yiqun Liu1), Min Xu2), Jaeyeong Heo1), Peide D. Ye2), and Roy G. Gordon1)
1) Department of Chemistry and Chemical Biology, Harvard University 2) School of Electrical and Computer Engineering, Purdue University

1

Motivation

• High

and high mobility for future scaling

• GaAs: high electron mobility

• I Issue: small IDS i G A NMOSFET Í F ll in GaAs Fermi l l pinning i level i i

• Fermi level unpinning: GaAs(111)A with ALD Al2O3* (Prof. Ye’s group at Purdue)

* “Metal oxide semiconductor field effect transistors on GaAs (111)A surface with atomic layer deposited Al2O3 as gate dielectrics.” M. Xu, Y. Q. Wu, O. Koybasi, T. Shen, and P. D. Ye, Appl. Phys. Lett. 94, 212104 (2009).
2

High

on GaAs for EOT scaling

on G A GaAs

Band B d gap (eV)

Ec E
(eV) 1.8 1.8 1.6 2 2 2.1

Ev E
(eV) 3.4 3 2.8 2.3 2.3 2.2

Al2O3 12 15 22 23 32 (bulk) 5.6 5.6 5.7 5.8 6.2

8

6.5

HfO2

LaAlO3

GdScO3

LaScO3

LaLuO3

“Band offsets of high K gate oxides on III V semiconductors” J. Robertson and B. Falabretti, J. Appl. Phys. 100, 014111(2006)
3

ALD process for LaLuO3 films

• Metal amidinate precursor for ALD La2O3:

• Lanthanum tris(N,N’ diisopropylformamidinate) or La(iPr2 fAmd)3 tris(N,N i H Pr • white solid i Pr N • most volatile La compound known, N iPr N 60 mTorr at 100 oC o 00 La H N N iPr • high reactivity to H2O, O2 and NH3
i

Pr

N i Pr

H

• Metal amidinate precursor for ALD Lu2O3:
Et Et N H N Et N Et N Lu

• Lutetium tris(N,N’ diethylformamidinate) or Lu(Et2 fAmd)3 • more volatile than La(ipr2 fAmd)3

H N Et N Et H
4

ALD process for LaLuO3 films
H2O Oven (118 oC) O N2 purge

Furnace Heater (350 (

oC) )

Lu(Et2 fAmd)3 La(iPr2 f d)3 ( fAmd)

N2 carrier

ALD process condition for LaLuO3
Oxidant DI H2O 118 oC Bubbler temp. Deposition temp. 300 350 oC La2O3 : Lu2O3 1:1 Exposure 0.003 Torr s for metal precursor p 0.06 Torr s for H2O

MO precursors

( ) La(iPr2 fAmd)3 Lu(Et2 fAmd)3

5

Properties of ALD LaLuO3
top

• Composition (RBS) • La:Lu=1:0.9 • 80% of bulk density • C and N not d t t bl d t detectable • Amorphous on Si with abrupt interface ( ~26)

• Highly conformal (90% step coverage)
middle
1.85 1.80

Energy (MeV)

30

1.70

1.75

25

LaLuO3 (14 nm)

RBS LaLuO LaLuO3 3 Si
Lu

20

Glass carbon

15

Normalized Yield

10

Si(111) 5 nm 5 nm
200 nm 10 nm

bottom

5

La

0 1240
1320 1340 1360 1380

1260

1280

1300

Channel

*H. Wang, J. J. Wang, R. Gordon, J. S. M. Lehn, H. Li, D. Hong, and D. V. Shenaic, Electrochem. Solid State Lett. 12, G13 (2009).

TEM, aspect ratio: 65:1*

6

Properties of ALD LaLuO3 on GaAs(111)A
Plan view

Pretreatment:

Step 1: HCl:H2O=1:1 for 30 sec (native oxide removal) Step 2: (NH4)2S 20% for 10 min (S passivation)

Epitaxy!

6 nm ALD LaLuO3 (350 oC)

GaAs(111)A

5 nm
Zone axis [111]

• Lattice mismatch ~ 4%* • Relaxed • Epitaxy: cubic, not orthorhombic! p y ,

* defined as (aoxide 2a)/2a

“Hetero epitaxy of single crystal LaLuO3 on GaAs(111)A by atomic layer deposition” Yiqun Liu, Min Xu, Jaeyeong Heo, 7 Peide D. Ye, and Roy G. Gordon, App. Phys. Lett. 97, 162910 (2010).

Quality of ALD epi LaLuO3 on GaAs(111)A
300

• High resolution XRD measurements
LaLuO3(222)
200

10
100

5

16.5 arcsec

10

3

Inte ensity

GaAs(111)

LaLuO3(222)
14.28 14 28

0 14.29 14 29 14.30 14 30

log(intensity y)

10 14.0 14.5 15.0

1

o Z( )

13.5

150 GaAs(111) 100 50 0 13.66

ZT ( )

o

16.2 arcsec

• Crystalline uniformity

Inte ensity (X10 00)

• Mismatch ~ 3.8%

• FWHM of the film peak

o Z( )

13.67

The quality of epitaxy is comparable to GaAs
8

Substrate dependent crystal structures
Glue LaLuO3 (222)

LaLuO3 (400) GaAs(111) (200)

GaAs(100)

5 nm
c

Zone axis [110] Z i

GaAs (100) LaLuO3 (440)

GaAs (200)

• Cubic polycrystalline with (110) preferential orientation on GaAs(100)
9

Yiqun Liu, Min Xu, Jaeyeong Heo, Peide D. Ye, and Roy G. Gordon, App. Phys. Lett. 97, 162910 (2010).

Substrate dependent crystal structures

LaLuO3 Si(111) 5 nm 5 nm

LaLuO3

Si(100)

• Amorphous on Si (0.18% mismatch) (0 18% • Silicate IL formation Æ Disturb ordering of substrate.

Epi LaLuO3/GaAs(111)A: no amorphous IL desirable for EOT scaling
10

1 J.

Kwon, M. Dai, M. D. Halls, E. Langereis, Y. J. Chabal, and R. G. Gordon, J. Phys. Chem. C 113, 654 (2009).

2

Yiqun Liu, Shaoping Shen, Leonard J. Brillson, and Roy G. Gordon, , App. Phys. Lett. 98, 122907 (2011).

Electrical properties
8 nm Al2O3

• Epi LaLuO3 v.s. a Al2O3 on GaAs(111)A
GaAs(111)A
8
2

6 nm Al2O3 10 nm LaLuO3
1 kHz 10 kHz 100 kH kHz 1 MHz
1 kHz 10 kHz 100 kH kHz 1 MHz

GaAs(111)A
6 4 2 0
300 K 2

8

6

1 kHz 10 kHz 100 kH kHz 1 MHz

1 kHz 10 kHz 100 kH kHz 1 MHz

C C (fF/Pm2)

2
-2 0
423 K 2

C C (fF/Pm )
-2

4

-2

0

300 K 2

-2

0

423 K 2

Gate Bi (V) G t Bias(V)
2

Gate Bias (V)
LaLuO3/GaAs(111)A Al2O3/GaAs(111)A

9Dielectric constant is ~30

2 Dit (x10 cm-2eV-1)
13

9Smaller VFB & frequency dispersions

1

9Interface state density (Dit) ~ 7×1011 cm 2 eV 1

LaLuO3/GaAs(111)A : better interface

0

1.05

1.10

1.15

1.20

E-EV (eV)

11

Lattice mismatch engineering

• ALD La2O3 on GaAs(111)A • nearly zero lattice mismatch (+0.8%) • cubic phase (not tetragonal), ~23 23
La2O3 (440) ( )

La2O3

GaAs (220)

GaAs(111)A

Zone axis [111]

12

Lattice mismatch engineering
Sc

Adjust lattice parameters to get perfect lattice match
Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb
C N N

aY2O3 =1.061 nm Y aLa2O3 =1.141 nm La 2*a 2* GaAs = 1 130 nm Ce 1.130

Lu

• Metal amidinate precursor for ALD Y2O3
Y(iPr2 amd)3
N C N

• Yttrium tris(N,N’ diisopropylacetamidinate) or

• White solid, less volatile

Y Yb
N

N C

• ALD LaYO3 @300 oC
Lattice mismatch +0.8% 0 2.5% 6%

Subcycle ratio Composition La2O3 : Y2O3 La2O3 La1.7Y0.3O3 La1Y1 O3 Y2O3

1:0

3:1

1:2

0:1

13

Summary

• Substrate dependent structures of ALD LaLuO3

• Epitaxy on GaAs(111)A • Poly. on GaAs(100) • Amorph on Si Amorph.

• Promising electrical properties of epi LaLuO3/GaAs(111)A

• High and low Dit • Fermi level is unpinned

• Lattice mismatch engineering

• La1.7Y0.3O3 on GaAs(111)A: perfect lattice match

14

Acknowledgements

• Prof. Roy Gordon and all group members

• Prof Peide D Ye and Min Xu Prof. D.

• Dr. Jun Jieh Wang prepared Lu precursor

• Dow Chemical provides La and Y precursor

• Center for Nanoscale Systems (CNS) at Harvard
15

Electrical properties
6 nm Al2O3 10 nm LaLuO3

• Effect of sulfur passivation Eff f lf i i

GaAs(111)A Without passivation

Sulfur passivation

8

1 kHz 10 kHz 100 kHz

(a)

6

C (fF/Pm2) m

4

2 -1 0 1 2 3

-3

-2

Voltage (V)

All devices: RTA in N2 at 700 oC before Ni electrodes were patterned on the devices.

16

Electrical properties
6 nm Al2O3

• GaAs(111)A v.s. GaAs(100) G A (111)A G A (100)
10 nm LaLuO3

6 nm Al2O3

10 nm LaLuO3

GaAs(111)A
1 kHz 10 kHz 100 kHz

GaAs(100)
8 6 4 2
1 kHz 10 kHz 100 kHz

8

6

C (fF/Pm2) m

2 -3 -2 -1 0 1 2 3

C (fF/Pm2) m
-3

4

-2

-1

0

1

2

3

Voltage (V)

Voltage (V)

‰ Fermi level is partially pinned and Dit is higher on GaAs(100).

‰ Fermi level is unpinned on GaAs(111)A.
17