Interface barriers at the interfaces of polar GaAs(111) faces with Al 2O3

Article abstract of DOI:10.1063/1.3698461

Internal photoemission measurements of barriers for electrons at interfaces between GaAs(111) and atomic-layer deposited Al₂O₃ indicate that changing the GaAs polar crystal face orientation from the Ga-terminated (111)A to the As-terminated (111)B has no effect on the barrier height and remains the same as at the non-polar GaAs(100)/Al₂O ₃ interface. Moreover, the presence of native oxide on GaAs(111) or passivation of this surface with sulphur also have no measurable influence on the GaAs(111)/Al₂O₃ barrier. These results suggest that the orientation and composition-sensitive surface dipoles conventionally observed at GaAs surfaces are effectively compensated at GaAs/oxide interfaces.

Full text of DOI:10.1063/1.3698461

Interface barriers at the interfaces of polar GaAs(111) faces with Al2O3
H. Y. Chou, E. O’Connor, P. K. Hurley, V. V. Afanas’ev, M. Houssa et al.
Citation: Appl. Phys. Lett. 100, 141602 (2012); doi: 10.1063/1.3698461
View online: http://dx.doi.org/10.1063/1.3698461
View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v100/i14
Published by the American Institute of Physics.
Additional information on Appl. Phys. Lett.
Journal Homepage: http://apl.aip.org/
Journal Information: http://apl.aip.org/about/about_the_journal
Top downloads: http://apl.aip.org/features/most_downloaded
Information for Authors: http://apl.aip.org/authors
Downloaded 29 Apr 2013 to 216.47.136.20. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissions

APPLIED PHYSICS LETTERS 100, 141602 (2012)
Interface barriers at the interfaces of polar GaAs(111) faces with Al O
2
3
1
,a)
2
E. O’Connor, P. K. Hurley, V. V. Afanas’ev, M. Houssa, A. Stesmans,
2
1
1
1
H. Y. Chou,
P. D. Ye , and S. B. Newcomb
3
4
1
Department of Physics and Astronomy, University of Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium
Tyndall National Institute, University College Cork, Lee Maltings, Prospect Row, Cork, Ireland
School of Electrical and Computer Engineering and Brick Nanotechnology Center, Purdue University,
2
3
West Lafayette, Indiana 47907, USA
Glebe Scientific Ltd., Newport, Tipperary, Ireland
4
(Received 15 December 2011; accepted 12 March 2012; published online 2 April 2012)
Internal photoemission measurements of barriers for electrons at interfaces between GaAs(111) and
atomic-layer deposited Al O indicate that changing the GaAs polar crystal face orientation from the
2
3
Ga-terminated (111)A to the As-terminated (111)B has no effect on the barrier height and remains
the same as at the non-polar GaAs(100)/Al O interface. Moreover, the presence of native oxide on
2
3
GaAs(111) or passivation of this surface with sulphur also have no measurable influence on the
GaAs(111)/Al O barrier. These results suggest that the orientation and composition-sensitive
surface dipoles conventionally observed at GaAs surfaces are effectively compensated at GaAs/oxide
2
3
interfaces. VC 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3698461]
Degraded electron transport properties of AIIIBV
semiconductor materials at the interfaces with insulators remain
a major obstacle hindering development of high-mobility
channel structures for future generations of metal-oxide-
semiconductor (MOS) devices. Recently, a significantly
improved MOS transistor performance was achieved by replac-
ing the traditional (100)GaAs or In0.53Ga0.47As surface orienta-
tion by the polar (111)A (Ga or In-rich) crystal face in
combination with thermal atomic-layer deposition (ALD) of
prepared on both n- or p-type GaAs single crystals with dop-
17
ꢂ3
ant concentration of ꢀ 6 ꢁ 10 cm and, for each dopant
type, two polar surface plane orientations, GaAs(111)A (Ga-
terminated) and GaAs(111)B (As-terminated) were explored.
This set of samples is contrasted with the previously studied
8
case of the non-polar GaAs(100) face. Three different GaAs
surface conditions were compared: A surface covered with
native oxide, a surface with native oxide on passivated by
(NH ) S treatment (20 min in a 10% polysulfide water solu-
4
2
1,2
2,9
insulating Al O . Moreover, the pre-ALD surface treatment
2
tion at room temperature), and a surface from which native
oxides were removed by HCl cleaning (3.7% water solution)
followed by the above indicated (NH ) S treatment. Insulat-
3
in (NH ) S leads to further enhancement of the electron mobil-
2
4
ity suggesting a lower scattering rate. These results were
explained by elimination of hypothetically present interface
4
2
ing Al O films of 20 nm thickness were deposited on these
2
3
2
ꢃ
surfaces at 300 C by ALD using Al(CH ) and H O precur-
3 3 2
dipoles, operating as the major factor in electron scattering.
Indeed, GaAs surface dipoles are long known for their strong
sors with a Al(CH3)3 pulse being injected first. Cross-
sectional transmission electron microscopy (TEM) analysis
indicates that the samples obtained by ALD of Al O onto
3–6
sensitivity to the composition and processing of the surface.
However, if applied to an interface, the surface dipole
concept must be extended beyond the first layer of inter-
atomic bonds: A charge transfer may also occur between
atomic layers located further away from the semiconductor
surface plane which might give rise to an additional contri-
bution(s) to the electrostatic potential. Aiming at evaluation
of these interface dipoles, we addressed the effect of GaAs
crystal face orientation, (100) versus (111)A (Ga-terminated)
and (111)B (As-terminated), as well as of surface chemical
treatment on the interface barrier height for electrons
between GaAs and ALD-grown Al O . Within an accuracy
2
3
native GaAs oxide for both the A and B faces of GaAs(111)
[see examples shown in panels (a) and (b) in Fig. 1] exhibit a
ꢄ1-nm thick interfacial layer (IL). Though the HCl etching
of the native oxide enhances the GaAs surface roughness,
TEM images obtained under different defocusing conditions
suggest that the IL becomes thinner [panel (c)] or even unde-
tectable [panel (d)] in the sulphur passivated samples, in this
way indicating structural differences of the interfaces as
affected by the pre-ALD GaAs surface treatment. More
10
TEM images can be found in the supplemental material.
The energy barrier height for electrons at the GaAs/
Al O interfaces was determined using the spectroscopy of
2
3
of 0.1 eV, we found no measurable contribution of
orientation-dependent dipoles, suggesting that the crystal
face sensitive charge transfers in the first layer of GaAs-
2
3
internal photoemission (IPE) of electrons from the valence
band (VB) of GaAs into the conduction band (CB) of the
7
oxide bonds are compensated by the dipole moments stem-
11
ming from the next-to-the-first atomic layers at the GaAs/
Al O interfaces.
In the present work we address the possible impact of
Al O insulator. These measurements were performed at
2 3
room temperature on MOS capacitors fabricated by thermor-
esistive evaporation of semitransparent (13-15-nm thick) Au
2
3
2
electrodes of 0.5 mm area onto the Al O layer. The photo-
both the GaAs crystal face orientation and the surface chemi-
cal treatment on the dipole component of the interface bar-
rier with ALD Al O insulator films. The samples were
2
3
current across the oxide was measured as a function of pho-
ton energy (hꢀ) in the spectral range from 2.0 to 6.8 eV and
then recalculated to the quantum yield (Y) by normalizing to
the incident photon flux. The interface barrier height (U) was
2
3
a)
Electronic mail: HsingYi.Chou@fys.kuleuven.be.
0003-6951/2012/100(14)/141602/4/$30.00
100, 141602-1
VC 2012 American Institute of Physics
Downloaded 29 Apr 2013 to 216.47.136.20. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissions

141602-2
Chou et al.
Appl. Phys. Lett. 100, 141602 (2012)
metry points in the Brillouin zone of the GaAs crystal
v
c
7
0
at 300 K, [C -C at E ¼ 4.4 eV, and X -X /R -R at
8
0
7
6
v
c
E ¼ 4.9 eV (Ref. 14)]. These features point to optical excita-
2
tion in GaAs as the dominant source of photocurrent, thus
allowing us to associate this photocurrent with electron IPE
from the GaAs VB into the CB of Al O . In the case of
2
3
p-type GaAs [panel (a)], the sulphur passivation is seen to
shift the spectral curves towards lower photon energy as
indicated by the horizontal arrow. By contrast, in n-type
GaAs samples [panel (b)] no such clear shift is observed,
suggesting that the (NH ) S treatment results in electric field
4
2
penetration into the p-type MOS samples that leads to a sub-
stantial (ꢀ 0.4 V) variation of the electrostatic potential
across the GaAs surface layer over a depth comparable to the
15
mean photoelectron escape depth. This variation in band
bending in p-type GaAs corresponds to the shift of the Fermi
level from the position close to the VB top in the samples
with native oxide towards the CB bottom in the sulphur pas-
sivated samples, suggesting that the (NH ) S treatment has
4
2
eliminated the high density of interface traps in the lower
portion of the GaAs gap. This results in un-pinning of the
Fermi level and allows one to shift it across the GaAs
16
bandgap. Then the positive bias applied to the top metal
electrode during IPE measurements would give rise to a
large band bending in p-GaAs as illustrated in the inset in
Fig. 2(a). Most important, however, is the observation that
no measurable change in the IPE spectra occurs when the
GaAs(111)A face is changed to GaAs(111)B, irrespective of
the initial GaAs surface treatment and the kind of semicon-
ductor conductivity type—even so, as known, with the den-
sity of interface defects being significantly different between
the samples with native GaAs oxide and those with GaAs
surfaces subjected to S-passivation.
FIG. 1. Cross-sectional TEM images of the interfaces prepared by ALD of
Al onto p-type GaAs(111)A and GaAs(111)B surfaces covered with
native oxide [panels (a) and (b), respectively] and of the p-GaAs(111)A/
Al interface prepared by etching the native oxide in HCl followed by sul-
2 3
O
2 3
O
phur passivation (c). For comparison is shown the image of an IL-free inter-
face obtained on a sulphur passivated n-GaAs(111)A surface (d).
inferred from the dependence of Y on hꢀ as the spectral
1
2,13
threshold of electron IPE.
Semi-logarithmic plots of the IPE yield spectra meas-
ured under þ2 V bias on the metal in GaAs(111)/Al O /Au
2
3
samples with different pre-ALD GaAs surface treatment are
compared in Fig. 2 for both the (111)A and (111)B GaAs
faces. The spectra are seen to be modulated by the features
universally observed at hꢀ ꢀ 4.4 and hꢀ ꢀ 4.9 eV which cor-
respond to the direct optical transitions between high sym-
The spectral threshold of electron IPE from the GaAs
1
/3
VB into the CB of Al
O
(U
by extrapolating the yield to the constant level of
the sub-threshold signal, where the latter is related to sample
) was determined from Y -hꢀ
2
3
e
1
2,13
plots
1
7
heating by incident light. This is illustrated in Fig. 3 for p-
FIG. 2. Semi-logarithmic spectral plots of the IPE yield as a function of
photon energy measured, under þ2.0 V bias applied to the Au electrode, on
FIG. 3. Cube root of the IPE yield, measured with the average strength of
electric field in the oxide of 1 MV/cm, at the interfaces of p-doped (a) and n-
2 3
p-type (a) and n-type (b) GaAs(111)/Al O /Au samples for different surface
orientations and pre-ALD treatments. The vertical lines indicate energies of
optical singularities in the GaAs crystal. Insets show a schematic of the
observed electron transitions.
doped (b) GaAs(111) samples with an Al
entations and pre-ALD treatments. The vertical lines mark the inferred
thresholds, U , of electron IPE from GaAs into Al
2 3
O layer for different surface ori-
e
2 3
O .
Downloaded 29 Apr 2013 to 216.47.136.20. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissions

141602-3
Chou et al.
Appl. Phys. Lett. 100, 141602 (2012)
[
panel (a)] and n-type [panel (b)] samples. In p-type samples
effect might be related to the fact that, unlike the case of an
uncovered GaAs surface, at an interface charge transfer may
occur not only between the atoms at the very surface of the
semiconductor but also between atomic layers located in the
oxide more remote from the geometrical plane of the inter-
face. For instance, in the model proposed in Fig. 11(a) of
Ref. 2, the additional partial charges may be positioned on
Al atoms making up the layer next to the group VI (oxygen
or sulfur) atomic plane. Importantly, this second dipole layer
will be of opposite orientation than that formed at the GaAs
surface and, hence, will compensate it. As the results of the
present work suggest, this compensation appears to be com-
plete within the accuracy of the IPE measurements.
fabricated by deposition of Al O on native oxide (*, h),
2
3
the yield rises up at U ꢀ 3.25 eV for both GaAs surface
e
orientations, indicating the absence of crystallographically
sensitive dipoles. The same threshold U is observed on all
e
n-type samples as shown in panel (b). Treatment of p-GaAs
in (NH ) S results in lowering of the threshold by ꢄ0.4 eV
4
2
which, as already discussed above, is likely to be caused by
penetration of electric field into the GaAs photoemitter.
Next, the inferred U values were plotted as a function of
e
the square root of the electric field (F) across the Al O layer
(the Schottky plot), calculated by simultaneously taking into
2
3
account the built-in voltage. The latter was determined as the
bias voltage corresponding to zero photocurrent, i.e., to the flat
bands in Al O . The results are summarized in Fig. 4 which
Here, it should be added that this conclusion does not
19
contradict the earlier reported lowering of the interface
electron barrier by ꢀ 0.3 eV upon high-temperature anneal-
ing of In Ga As(100)/Al O entities: This barrier varia-
2
3
also shows the previously reported results for the non-polar n-
GaAs(100) interfaces with an Al O layer grown either by ther-
2
3
0.53
0.47
2 3
8
mal (") (as applied here) or plasma-assisted ( ) ALD. Except
for the S-passivated p-GaAs samples (!, ~, ^, 3), which,
as mentioned, are affected by the electric field penetration in
the GaAs, the thresholds of electron IPE at the GaAs(111)A/
Al O and GaAs(111)B/Al O interfaces fall (n, h, and the
tion is probably caused by the oxide CB bottom shift due to
in-diffusion of In or Ga into the Al O film since a compara-
ble red shift is also found at the opposite Al/Al O interface
2
3
2
3
in the same MOS structures. Also, the sensitivity of the IPE
20
spectral curves to the GaAs(100) surface treatments is
unlikely to be due to interface dipoles because, as already
2
3
2 3
encircled symbols in Fig. 4) on the same trendline as those
observed at the GaAs(100)Al O interfaces (", ), indicating
8
discussed in detail, development of a low energy IPE band
2
3
that the energy barrier between the top of the GaAs VB and the
bottom of the Al O CB remains the same. Extrapolation to
correlates with the growth of a narrow gap Ga O -like inter-
2
3
layer between GaAs and Al O .
2 3
2
3
zero field yields the barrier U (F ¼ 0) ¼ 3.46 0.1 eV—one
From the practical point of view, the revealed absence
of significant (>0.1 eV) orientation-sensitive dipoles at
GaAs/Al O3 interfaces represents good news for A B
e
coinciding value, irrespective of the GaAs surface orientation
and the pre-ALD surface treatment. Moreover, given that
defect generation during GaAs oxidation is seen to be a result
2
III V
MOS channel design: The MOS devices can be fabricated on
the GaAs face delivering the highest carrier mobility without
worrying about a dipole-induced threshold voltage shift.
Moreover, the non-planar A B MOS transistor design
18
of strain relief occurring through ejection of surface atoms,
oxidation-induced variation of the surface atomic composition
may also be excluded as the possible source of interface dipole
III
V
21–23
formation since no U variation is found at interfaces with dif-
becomes more feasible
as no additional compensation is
e
ferent trap density.
required for potentially different threshold voltage at the dif-
ferently oriented faces of a 3D channel.
From a more general perspective, the results of the pres-
ent work suggest that the orientation and processing-
sensitive surface dipole formation well established before for
To conclude, the IPE experiments reveal that the elec-
tron barrier height between the top of the GaAs VB and the
bottom of the Al O CB shows no measurable variation
3–6
clean GaAs surfaces cease to work at the interface with an
insulating Al O layer. The possible explanation of the latter
2
3
when changing the surface orientation of the GaAs substrate
crystal and its chemical treatment prior to Al O deposition.
2
3
2
3
This result suggests that the surface dipoles known from pre-
vious studies at the free GaAs surfaces are largely compen-
sated by charge transfer between atoms in the oxide layer.
The authors acknowledge Ian Povey and Aileen O’Mah-
ony from Tyndall for work on the InGaAs surface prepara-
tion and ALD oxide growth and the authors PKH and EO’C
gratefully acknowledge the financial support of the Science
Foundation Ireland strategic research cluster FORME under
Project No. 07/SRC/I1172.
1
M. Xu, Y. Q. Wu, O. Koybasi, T. Shen, and P. D. Ye, Appl. Phys. Lett.
94, 212104 (2009).
2
3
4
FIG. 4. The Schottky plot of the electron IPE thresholds measured on the
Y. Urabe, N. Miyata, H. Ishii, T. Itatani, T. Maedam T. Yasuda, H. Yamada,
N. Fukuhara, M. Hata, M. Yokoyama, N. Taoka, N. Takenaka, and S.
Takagi, Tech. Dig. -Int. Electron Devices Meet. 6-8 Dec (2010), p. 142.
I. M. Vitomirov, A. Raisanen, A. C. Finnefrock, R. E. Viturro, L. J. Bril-
lson, P. D. Kirchner, G. D. Prtti, and J. M. Woodall, Phys. Rev. B 46,
13293 (1992); J. Vac. Sci. Technol. B 10, 1898 (1992).
differently prepared GaAs(111)/Al interfaces, on comparison with the
2
O
3
values observed at GaAs(100)/Al O interfaces with the oxide grown by
2 3
thermal (") or the plasma-assisted ( ) ALD. Filled and open symbols corre-
spond to n-type GaAs and p-type GaAs samples, respectively. Encircled
datapoints correspond to the overlapping spectral threshold results as meas-
ured on different samples under þ2 V bias applied to the top Au electrode.
Line illustrates determination of the zero-field barrier.
W. Chen, M. Dumas, D. Mao, and A. Kahn, J. Vac. Sci. Technol. B 10,
1886 (1992).
Downloaded 29 Apr 2013 to 216.47.136.20. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissions

141602-4
Chou et al.
Appl. Phys. Lett. 100, 141602 (2012)
5
16
R. Duszak, C. J. Palmstrom, and L. T. Florez, Y. N. Yang, and J. H.
Weaver, J. Vac. Sci. Technol. B 10, 1891 (1992).
I. Jimenez, F. J. Palomares, and J. L. Sacedon, Phys. Rev. B 49, 11117
E. O’Connor, B. Rennan, V. Djara, K. Cherkaoui, S. Monaghan, S. B.
Newcomb, R. Contreras, M. Milojevic, G. Hughes, M. E. Pemble, R. M.
Wallace, and P. K. Hurley, J. Appl. Phys. 109, 024101 (2011).
R. Williams, Semiconductors and Semimetals. Volume 6: Injection Phe-
nomena, edited by R. K. Willardson and A. C. Beer (Academic, New
York, 1970), pp. 103–105.
6
1
7
(
1994).
7
8
G. Hegde, G. Klimeck, and A. Strachan, Appl. Phys. Lett. 99, 093508
2011).
(
18
V. V. Afanas’ev, M. Badylevich, A. Stesmans, G. Brammertz, A. Delabie,
S. Sionke, A. O’Mahony, I. M. Povey, M. E. Pemble, E. O’Connor, P. K.
Hurley, and S. B. Newcomb, Appl. Phys. Lett. 93, 212104 (2008).
Y. Nanichi, J. Fan, H. Oigawwa, and A. Koma, Jpn. J. Appl. Phys. 27,
M. Scarrozza, G. Pourtois, M. Houssa, M. Caymax, A. Stesmans, M. Meu-
ris, and M. M. Heyns, Appl. Phys. Lett. 95, 253504 (2009).
N. V. Nguyen, M. Xu, O. A. Kirillov, P. D. Ye, C. Wang, K. Cheung, and
J. S. Suehle, Appl. Phys. Lett. 96, 052107 (2010).
N. V. Nguyen, O. A. Kirillov, W. Jiang, W. Wang, J. S. Suehle, P. D. Ye,
Y. Xuan, N. Goel, K.-W. Choi, W. Tsai, and S. Sayan, Appl. Phys. Lett.
93, 082105 (2008).
M. Radosavljevic, G. Dewey, J. M. Fastenau, J. Kavalieros, R. Kotlyar, B.
Chu-Kung, W. K. Liu, D. Lubyshev, M. Metz, K. Millard, N. Mukherjee,
L. Pan, R. Pillarisetty, W. Rachmady, U. Shah, and Robert Chau, Tech.
Dig. -Int. Electron Devices Meet. 6-8 Dec (2010), p. 126.
J. J. Gu, O. Koybasi, Y. Q. Wu, and P. D. Ye, Appl. Phys. Lett. 99,
112113 (2011).
1
2
9
0
9
2
0
367 (1988).
1
See supplementary material at http://dx.doi.org/10.1063/1.3698461 for
additional TEM images.
1
1
1
1
1
2
3
4
21
V. V. Afanas’ev and A. Stesmans, J. Appl. Phys. 102, 081301 (2007).
R. J. Powell, J. Appl. Phys. 41, 2424 (1970).
J. S. Helman and F. Sanchez-Sinencio, Phys. Rev. B 7, 3702 (1973).
P. Y. Yu and M. Cardona, Fundamentals of Semiconductors: Physics and
Materials Properties, 2nd ed (Springer, New York, 1999).
V. V. Afanas’ev, H. Y. Chou, A. Stesmans, C. Merckling, and X. Sun,
Appl. Phys. Lett. 98, 072102 (2011).
2
2
2
3
1
5
J. J. Gu, A. T. Neal, and P. D. Ye, Appl. Phys. Lett. 99, 152113 (2011).
Downloaded 29 Apr 2013 to 216.47.136.20. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissions