Hf O2 and Al2 O3 gate dielectrics on GaAs grown by atomic layer deposition

Article abstract of DOI:10.1063/1.1899745

High-performance metal-oxide-semiconductor field effect transistors (MOSFETs) on III-V semiconductors have long proven elusive. High-permittivity (high- κ) gate dielectrics may enable their fabrication. We have studied hafnium oxide and aluminum oxide grown on gallium arsenide by atomic layer deposition. As-deposited films are continuous and predominantly amorphous. A native oxide remains intact underneath Hf O2 during growth, while thinning occurs during Al2 O3 deposition. Hydrofluoric acid etching prior to growth minimizes the final interlayer thickness. Thermal treatments at ~600 °C decompose arsenic oxides and remove interfacial oxygen. These observations explain the improved electrical quality and increased gate stack capacitance after thermal treatments.

Full text of DOI:10.1063/1.1899745

Hf O 2 and Al 2 O 3 gate dielectrics on GaAs grown by atomic layer deposition
Martin M. Frank, Glen D. Wilk, Dmitri Starodub, Torgny Gustafsson, Eric Garfunkel, Yves J. Chabal, John Grazul
,
and David A. Muller
Citation: Applied Physics Letters 86, 152904 (2005); doi: 10.1063/1.1899745
View online: http://dx.doi.org/10.1063/1.1899745
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APPLIED PHYSICS LETTERS 86, 152904 ͑2005͒
HfO and Al O gate dielectrics on GaAs grown by atomic layer deposition
2
2
3
a͒
Martin M. Frank
IBM Semiconductor Research and Development Center (SRDC), Thomas J. Watson Research Center,
Yorktown Heights, New York 10598
Glen D. Wilk
ASM America, Phoenix, Arizona 85034
Dmitri Starodub, Torgny Gustafsson, Eric Garfunkel, and Yves J. Chabal
Rutgers University, Departments of Physics and Chemistry, and Laboratory for Surface Modification,
Piscataway, New Jersey 08854
John Grazul and David A. Muller
Cornell University, Center for Materials Research, Ithaca, New York 14853
͑
Received 13 January 2005; accepted 26 February 2005; published online 4 April 2005͒
High-performance metal-oxide-semiconductor field effect transistors ͑MOSFETs͒ on III–V
semiconductors have long proven elusive. High-permittivity ͑high-␬͒ gate dielectrics may enable
their fabrication. We have studied hafnium oxide and aluminum oxide grown on gallium arsenide by
atomic layer deposition. As-deposited films are continuous and predominantly amorphous. A native
oxide remains intact underneath HfO2 during growth, while thinning occurs during Al O3
2
deposition. Hydrofluoric acid etching prior to growth minimizes the final interlayer thickness.
Thermal treatments at ϳ600 °C decompose arsenic oxides and remove interfacial oxygen. These
observations explain the improved electrical quality and increased gate stack capacitance after
thermal treatments. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.1899745͔
7
Metal-oxide-semiconductor
MOSFETs͒ on III–V semiconductor would offer a number
field-effect
transistors
substrates. Gate stacks for depletion mode MOSFETs with
1
2
−2
−1
͑
D Ͻ10 cm eV were recently fabricated by ALD of
Al O onto native oxide covered GaAs.
anneal in O minimized current–voltage hysteresis and fre-
quency dispersion, and maximized gate stack capacitance.
Transmission electron microscopy ͑TEM͒ pointed to a re-
markably sharp Al O /GaAs interface, prompting the specu-
lation that the native oxide is removed during the ALD pro-
cess. ALD-grown Al O also results in good gate stack
properties on InGaAs and AlGaN/GaN and may be used
to coat compound semiconductor nanowires conformally.
By contrast, very few studies have been published regarding
the most important high-␬ gate dielectric, HfO , on III–V
it
8
–10
of advantages over Si-based devices. The higher carrier mo-
bility would lead to faster complementary MOS ͑CMOS͒
logic operation, higher breakdown fields would support high-
power/temperature applications, band structure engineering
would offer design flexibility, and monolithic optoelectronic
circuits would likely become manufacturable.
However, MOSFETs on high-carrier-mobility substrates
compound semiconductors as well as germanium͒ have
been elusive due to the lack of high-quality gate insulators,
mostly because of the absence of stable passivating native
A 600–650 °C
2
3
2
2
3
2
3
1
1
12
͑
1
3
oxides and the high density of slow interface traps ͑D ͒ with
it
2
1
14,15
deposited oxides. For example, Fermi level pinning upon
semiconductors.
GaAs oxidation has been attributed to oxygen-induced dis-
placement of surface As atoms, where doubly O-coordinated
second-layer Ga atoms give rise to gap states. Excess inter-
In this letter, we characterize the structure and composi-
tion of Al O /GaAs and HfO /GaAs, to help develop an
2
3
2
2
understanding of the impact of material and processing con-
ditions on the quality of ALD-grown high-␬/III–V stacks. We
address successive stages of stack formation: starting sur-
face, temperature ramp-up, ALD process, and anneal. Both
Al O and HfO were grown on oxide-covered ͑epiready͒
facial As occupying As antisite defects causes gap states as
Ga
2
,3
well. Such As may be formed via decomposition of As O3
2
in the vicinity of GaAs, resulting from the reaction: As O
2
3
4
+
2GaAs→Ga O +4As. Until recently, oxide/III–V inter-
2
3
2
3
2
faces with low D had to be fabricated by molecular beam
it
and HF-etched GaAs͑100͒. For high-␬ dielectric growth, we
epitaxy ͑MBE͒ of Ga O ͑where Gd was added to reduce
2
3
followed a procedure that recently yielded high-quality
1
,5
6
8
–10
leakage͒
or Al O . Fermi level unpinning in the
2 3
Al O /GaAs gate stacks.
Depositions were performed
2
3
Ga O /GaAs system has been attributed to
a
2
3
using alternating exposures of the common ALD precursors
Ga O/GaAs-like interface in which the Ga and As surface
2
Al͑CH ͒ +H O or HfCl +H O in a N carrier gas at
3
3
2
4
2
2
2
atoms are restored to near-bulk charge.
3
00 °C, using a commercial ASM Pulsar3000™ ALD reac-
Atomic layer deposition ͑ALD͒ or chemical vapor depo-
sition ͑CVD͒ is technologically more attractive than MBE.
The recent development of high-quality ALD- and CVD-
grown high-permittivity ͑high-␬͒ gate dielectrics on Si justi-
fies some optimism regarding integration on III–V
tor. C or Al served as caps for microscopy. Films were char-
acterized ex situ by scanning TEM ͑STEM͒, electron energy
loss spectroscopy ͑EELS͒, transmission infrared spectros-
copy ͑70° off-normal͒, medium energy ion scattering
͑
MEIS͒, and x-ray photoelectron spectroscopy ͑XPS͒.
The epiready oxide on GaAs was found by STEM to
have a thickness of 20–25 Å ͓Fig. 1͑a͔͒. The layer is Ga-rich
a͒Electronic mail: mmfrank@us.ibm.com
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003-6951/2005/86͑15͒/152904/3/$22.50 86, 152904-1 © 2005 American Institute of Physics
28.59.222.12 On: Sat, 29 Nov 2014 08:55:31
0
1

152904-2
Frank et al.
Appl. Phys. Lett. 86, 152904 ͑2005͒
FIG. 1. ͑Color online͒ ͑a͒ STEM image and ͑b͒ STEM-EELS spectra in the
Ga and As L2,3 regions from C-capped epiready oxide on GaAs͑100͒. Spec-
tra were recorded within the native oxide and in the GaAs substrate.
FIG. 3. Transmission infrared spectra from ͑a͒ 40 Å Al O and ͑b͒ 40 Å
2
3
HfO deposited onto Ga͑As͒O/GaAs͑100͒. “a” and “m” denote amorphous
2
and monoclinic oxides, respectively.
͑
As:Ga=0.17͒, as determined by STEM-EELS ͓Fig. 1͑b͔͒,
with an increased As concentration near the GaAs substrate.
2
0
be attributed to monoclinic HfO2, indicating crystalline in-
clusions in an amorphous matrix.
Areal densities of O and ͑Ga+As͒ in the oxide are 3.2 and
1
5
−2
3
.3ϫ10 cm , respectively, as measured by MEIS, consis-
The structure and composition of the high-␬/GaAs stack
depends on the nature of the starting surface. At a target
high-␬ thickness of 40 Å, the areal densities of metal ions
are slightly higher on HF-etched substrates than on oxide-
covered substrates ͑by 14% for Al2O3 and by 6% for HfO2;
MEIS, not shown͒. This may indicate high-␬ agglomeration
during initial growth on HF-etched GaAs, similar to what
tent with a porous ͑50% of bulk density͒ mixture of known
oxides containing a high fraction of Ga O. In the following,
2
we will denote this substrate Ga͑As͒O/GaAs. HF-etched
1
6
substrates, on the other hand, are mostly oxide-free.
During a vacuum preanneal of Ga͑As͒O/GaAs to the
ALD temperature of 300 °C, there is only a 7% reduction in
O content, while most surface hydrocarbons desorb ͑as mea-
sured by MEIS, not shown͒. The lack of any substantial thin-
ning of Ga͑As͒O during inert anneal to 300 °C is consistent
with the higher onset temperatures of reactions in, e.g., ther-
2
1
has been observed for HF-etched Ge.
Interfacial layer thicknesses after Al O and HfO depo-
2
3
2
sition onto Ga͑As͒O/GaAs are ϳ10 and 20–25 Å, respec-
tively ͓Figs. 2͑a͒ and 2͑b͔͒. When native oxide removal by
an initial HF wet etch is performed instead, both Al O and
1
7
4
mal ͑300–390 °C͒, anodic ͑430 °C͒, and wet-chemical/
1
8
native ͑320 °C͒ oxides on GaAs.
2
3
The as-deposited high-␬ films are continuous, i.e., not
islanded, and appear amorphous in TEM images ͑Fig. 2͒.
Infrared spectroscopy confirms that Al O films on
HfO deposition result in low interfacial layer thicknesses of
2
only 3–8 Å ͓Figs. 2͑c͒ and 2͑d͒,͔. MEIS data support this
finding, indicating near-stoichiometric O:Al and O:Hf ratios
of 1.51 and 2.04, respectively, for etched substrates, as op-
posed to oxygen-rich ratios of 1.70 and 2.33 for oxide-
covered substrates, with excess oxygen attributed to interfa-
cial Ga͑As͒O.
2
3
Ga͑As͒O/GaAs are amorphous, as for ALD-grown Al O on
2
3
1
9
Si. This is evidenced by absorption features at 720 and
9
55 cm− ͓Fig. 3͑a͔͒ which may be assigned to transverse-
1
optical ͑TO͒ and longitudinal-optical ͑LO͒ phonon modes of
amorphous Al O , respectively. In contrast, for HfO a signal
2
3
2
The TEM observation of only ϳ10 Å interfacial oxide
at 700 cm− and its high-frequency shoulder ͓Fig. 3͑b͔͒ may
1
for Al O /Ga͑As͒O/GaAs indicates that the initial oxide is
2
3
thinned during the Al O ALD growth process. This points
2
3
to volatilization of Ga͑As͒O or its conversion into Al O . By
2
3
contrast, the HfO growth process does not cause interface
2
thinning, even though the standard Gibbs energies of forma-
tion per O atom are nearly identical ͑Al O : −527 kJ/mol;
2
3
HfO : −544 kJ/mol͒ and both higher than for Ga and As
2
4
,22
oxides. The different degree of interface thinning likely is
due to the much higher reactivity of Al͑CH ͒ compared to
3
3
HfCl , reflected in standard enthalpies of formation of
4
2
2,23
−
74 kJ/mol and −990 kJ/mol, respectively.
Also, ALD
reaction with hydroxylated oxides has been shown to be
2
4–26
much more favorable for Al͑CH ͒ than for HfCl .
3
3
4
The residual interfacial oxide remains Ga-rich during
ALD growth, as shown by STEM-EELS, with As oxides still
present in the case of Al O on Ga͑As͒O/GaAs ͑XPS data in
2
3
2
7
Fig. 4; As oxide signal at 44.8 eV͒. For all interfaces, TEM
images indicate disorder or random strain in the topmost 1–2
GaAs unit cells. The presence of amorphous interfacial GaAs
FIG. 2. STEM images from 40 Å Al O ͑top͒ and HfO ͑bottom͒ deposited
2
3
2
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onto Ga͑As͒O-covered ͓͑a͒ and ͑b͔͒ and HF-etched ͓͑c͒ and ͑d͔͒ GaAs͑100͒.
is confirmed by excess visible GaAs in MEIS.
1
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152904-3
Frank et al.
Appl. Phys. Lett. 86, 152904 ͑2005͒
oxygen. This may explain the improved electrical quality and
increased capacitance of such structures.
Film growth as well as STEM and infrared analysis were
performed at and supported by Bell Laboratories MEIS was
performed at Rutgers University and supported by NSF and
SRC. M.M.F. was supported by a fellowship from DAAD.
D.R.M. and J.G. were supported by the NSF through Cor-
nell’s MRSEC Program
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21
8
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3
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0
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22
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3
0
G. D. Wilk and P. D. Ye ͑unpublished͒.
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