Growth and properties of Al2O3 and SiO2 nanolayers on III-V semiconductors

Article abstract of DOI:10.1134/S0020168510010097

We describe atomic layer deposition of silica and alumina layers on GaAs, InAs, and InSb substrates. The conditions for layer-by-layer growth of surface nanostructures are established, and some of their dielectric parameters are evaluated.

Full text of DOI:10.1134/S0020168510010097

ISSN 0020ꢀ1685, Inorganic Materials, 2010, Vol. 46, No. 1, pp. 38–42. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © Yu.K. Ezhovskii, V.Yu. Kholkin, 2010, published in Neorganicheskie Materialy, 2010, Vol. 46, No. 1, pp. 44–48.
Growth and Properties of Al2O3 and SiO2 Nanolayers
on III–V Semiconductors
Yu. K. Ezhovskii and V. Yu. Kholkin
St. Petersburg State Technological Institute (Technical University), Moskovskii pr. 26, St. Petersburg, 190013 Russia
eꢀmail: ezhovski@pochta.ru
Received December 23, 2008
Abstract—We describe atomic layer deposition of silica and alumina layers on GaAs, InAs, and InSb subꢀ
strates. The conditions for layerꢀbyꢀlayer growth of surface nanostructures are established, and some of their
dielectric parameters are evaluated.
DOI: 10.1134/S0020168510010097
INTRODUCTION
cal conductivity and dielectric permittivity) of the
nanolayers.
Advances in the fabrication of various nanodevices
depend crucially on the development of processes for
the synthesis of appropriate materials and knowledge
of the fundamental mechanisms underlying the
behavior of nanosystems [1]. In particular, intensive
effort has been concentrated on novel electronic techꢀ
nologies for the atomicꢀscale fabrication of solidꢀstate
nanostructures.
EXPERIMENTAL
The use of surface chemical reactions, basic to the
ALD process, allows one to grow lowꢀdimensional
structures with their composition and thickness conꢀ
trolled on a monolayer scale. In this approach, the
growth of oxide layers involves, as a key step, selfꢀlimꢀ
iting chemisorption of a metal halide (МCln) and
water vapor under limiting surface coverage condiꢀ
tions. For example, the process on hydroxylated surꢀ
The advent of molecular beam epitaxy [2] has conꢀ
siderably extended the possibilities of vacuum techꢀ
nology and has made it possible to overcome a number
of challenges in microprocessor engineering. At the
same time, though offering a number of indisputable
advantages, molecular beam epitaxy requires very
costly equipment, which has stimulated the search for
alternative approaches.
faces (symbol ) can be run according to the schemes
|
|
–OH)m + MCln
|–O–)m MCln
+
m
m
HCl, (1)
–
|–O– )mMCln – m + (n – m)H2O
|–O– )mM(OH)n – m + (n – m)HCl.
The value of depends on the surface density of
(2)
In recent years, atomic layer deposition (ALD) has
been the subject of intense attention as a process for
growing highꢀquality ultrathin layers [3–5]. The
advantages of this technology and its potential for fabꢀ
ricating submicronꢀsized components of integrated
devices have been demonstrated in several studies [6–
8]. ALD takes advantage of surface chemical processes
previously referred to as molecular layering, whose
physicochemical foundations were developed as early
as the 1970s [9]. Chemical processes for the synthesis
of lowꢀdimensional systems are an integral part of the
rapidly growing chemical nanotechnology of novel
materials for a variety of specialty applications, particꢀ
ularly for nanoelectronic systems. Such processes,
which utilize vapor phase delivery of reactants and
take advantage of selfꢀorganization, are readily ameꢀ
nable to largeꢀscale processing and, therefore, comꢀ
mercially viable.
m
hydroxyls; e.g.,
m Ӎ 2 for silicon surfaces [10]. Repeatꢀ
ing reactions (1) and (2) many times and intermitꢀ
tently removing the reaction products and excess reacꢀ
tants, one can grow an oxide layer of predetermined
thickness. In such processes, two important condiꢀ
tions must be met.
First, the growth temperature of AB film,
meet the relation
Ts, must
T•A,T•B ≤ Ts ≤ T•AB
,
(I)
whereT•A,TB• , and TA• B are the critical condensation
temperatures of components A and B and compound
AB, respectively. This condition rules out direct conꢀ
densation of component A or B, so that the process is
limited to the formation of a chemisorbed layer.
Second, the surface reactions (1) and (2) should
In this paper, we report the growth of silica and aluꢀ take place under nonequilibrium conditions in order
mina nanolayers by ALD on GaAs, InAs, and InSb to reach completion. To this end, surface hydroxyls
substrates and the main dielectric properties (electriꢀ must exhibit sufficiently high reactivity. Evaluation of
38

GROWTH AND PROPERTIES OF Al2O3 AND SiO2 NANOLAYERS
39
(a)
d, nm
(b)
1
2
d, nm
3
20
20
16
12
8
1
3
16
12
8
4
4
2
0
10
20
30
40
N
0
20
40
60
80
N
Fig. 1. Layer thickness vs. number of deposition cycles for (a) SiO and (b) Al O layers grown on (100) GaAs at a vapor pressure
2
2 3
of 1.3 Pa and
Т = (1) 403, (2) 453, and (3) 523 K.
their reactivity on gallium arsenide using the Taft tion of the refractive index of the grown structures was
inductive constants gives σI = 5.1 [11]. This value used to assess their composition, which was also deterꢀ
mined by Xꢀray photoelectron spectroscopy (XPS) on
HPꢀ5950A (Al Xꢀray source, ЕK = 1486 eV) and
slightly exceeds that for silica, on which such reactions
occur rather readily [9]. It is, therefore, reasonable to
expect that hydroxyls on such surfaces will exhibit sufꢀ
ficient reactivity with silicon chloride and aluminum
K
α
α
SERꢀ1 (Mg
K Xꢀray source, ЕK = 1253 eV) spectromꢀ
α
α
eters.
The dielectric parameters of the layers were deterꢀ
mined on both the substrates and ~0.1 mꢀthick aluꢀ
minum films grown on silicon by vacuum evaporation.
The electrical properties of the oxide layers were
studied using EMꢀ1 and ITꢀ12 electrometers.
*
chloride vapors (σCl = 2.88). To raise the reaction
ꢀμ
yield in step (2), triethylamine can be used as an
exchange activator, which is introduced together with
water vapor. In addition, this approach makes it possiꢀ
ble to stabilize the hydroxyl layer and to reduce the
layerꢀbyꢀlayer growth temperature [12, 13].
The substrates used were gallium arsenide
((100)ꢀorientated AGChTꢀ23ꢀ17 and (110)ꢀoriented
AGEꢀ4ꢀ16), indium arsenide ((111) and (100)
IMN10/PAIꢀ380ꢀ25.5 epitaxial structures), and
indium antimonide ((111)ꢀoriented ISEꢀ0) wafers,
which were precleaned by etching in a methanolic solution
of bromine (1–5% Br). According to ellipsometric data,
the thickness of the residual oxide layer on all of the subꢀ
strates was within 2 nm.
RESULTS AND DISCUSSION
Our results on the growth kinetics of the oxides
under consideration at various substrate temperatures
(Fig. 1) and reactant vapor pressures indicate that the
thickness of SiO2 and Al2O3 layers,
tion of the number of deposition cycles,
d
, is a linear funcꢀ
N. This indiꢀ
cates that the hydroxides involved retain their reactivꢀ
ity and that the same amount of the synthesized comꢀ
pound is deposited in each cycle. The effect of
substrate orientation was insignificant. In all
instances, the layer thickness was proportional to the
number of surface processing cycles by reactions (1)
and (2):
Oxide nanolayers were grown using reactions (1)
and (2), by sequentially exposing the surface of the
substrates to appropriate metal chloride (p = 1–10 Pa)
and water ( 100 Pa) vapors and intermittently
p
Ӎ
removing the reaction products and excess reactants.
The process was run in a flow reactor evacuated to a
residual pressure no higher than 0.1 Pa.
d
=
d0N
.
(II)
Here, the proportionality factor d0 is the average
increase in layer thickness per cycle of surface processꢀ
ing with one of the reactants and characterizes the
structure of the deposited layer. This parameter is a key
characteristic of the deposition process and allows one
to determine the surface coverage and to gain insight
into the mechanism of film growth.
The thickness of the deposited layers was evaluated
from the ellipsometric parameters
Drude–Tronston oneꢀlayer model [14].
Δ
and
Ψ
in the
were
Δ
and
Ψ
measured using a fixedꢀcompensator ellipsometer in
the PQSA arrangement [15]. As a linearly polarized
light source, we used a 632.8ꢀnm LGꢀ75 laser beam,
which was magnetically modulated to improve the meaꢀ
Analysis of the d(N) data obtained at various subꢀ
surement accuracy. The uncertainty in the ellipsometric strate temperatures allowed us to assess the effect of
parameters was within 0.01'. Ellipsometric determinaꢀ thermal conditions on the layer growth parameter
INORGANIC MATERIALS Vol. 46
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2010

40
EZHOVSKII, KHOLKIN
The most likely reaction is the formation of an actiꢀ
vated complex with the hydroxyl group,
d
, nm
(a)
d , nm
0
0.3
(b)
0
0.3
O
H
Si
Cl
Cl
Cl
1
0.2
0.2
2
1
Cl
0.1
0
0.1
0
3
2
3
which remains stable up to
SiO2 growth was only achieved on GaAs and InAs at
s > 525 K, when d0 approached the silica monolayer
~500 K. Layerꢀbyꢀlayer
350 400 450 500 550
s, K
350 400 450 500 550 600
Ts, K
T
T
thickness. The surface coverage of InSb with silicon–
oxygen groups was within 0.5. The reduction in d0 on
InAs and InSb at Ts > 525 and 550 K, respectively, is
due to both the reduction in the surface density of
hydroxyls and the thermochemical instability of these
materials in silicon halide atmospheres. The latter is
supported by experimental data for GaAs: the
hydroxyl groups on this material are thermally stable
up to 600 K (Fig. 2).
Fig. 2. Effect of substrate temperature on
and (b) Al O layers grown on (
3) InSb at a vapor pressure of 1.3 Pa.
d for (a) SiO
0 2
2) InAs, and
1
) GaAs, (
2
3
(
(Fig. 2), to clarify the growth mechanism of the nanoꢀ
structures, and to identify the layerꢀbyꢀlayer growth
conditions. As seen in Fig. 2, the d0(Ts) curves for SiO2
and Al2O3 differ in shape and are influenced only
slightly by the substrate material. This can be underꢀ
stood primarily in terms of the chemisorption behavꢀ
ior of silicon and aluminum chlorides.
In contrast to SiO2, Al2O3 layer growth exhibited no
activation behavior. The high d0 values for alumina
layers, far above the average size of aluminum–oxygen
tetrahedra, are due to the fact that they grow through
chemisorption of the Al2Cl6 dimer, which prevails in
aluminum chloride vapor. Reactions (1) and (2) may
then follow two major schemes (with the metal preꢀ
The increase in surface reaction yield with increasꢀ
ing temperature in the case of SiO2 layers indicates that
SiCl4 chemisorption is a thermally activated process. dominantly in fourfold coordination):
OH
Cl
Cl
Cl
OH
OH
OH
OH
O
O
O
–HCl
–HCl
–HCl
Al
Al
+ Al2Cl6
OH
Al
Al
(3а)
(3b)
+ H2O
+ H2O
OH
OH
Cl
O
O
O
Cl
Al
Al
Al
Al
–HCl
OH + Al2Cl6
OH
Cl
Cl
OH
OH
O
O
O
O
Which of these surfaceꢀreaction paths prevails Al2O3 and SiO2 correlates with the heats of formation
depends on the topography of the hydroxyl layer. At of GaAs, InAs, and InSb (Fig. 2). Note that the genꢀ
reduced substrate temperatures (Ts < 400 K), where eral trends in the ALD of Al2O3 and SiO2 on GaAs,
conditions (I) are not met, the density of surface InAs, and InSb are similar to those for silicon and
hydroxyls is high, and adsorbed water molecules may other oxide substrates [13], indicating that these proꢀ
be present, Al2O3 layers grow predominantly through cesses follow the same general mechanisms.
reactions in the adlayer. Aluminum chloride dimers
may then have various orientations, and the high d0
values at Ts < 400 K point to polymolecular sorption of
reactants under such conditions. Only at Ts > 400 K
for InAs and InSb or Ts > 475 K for GaAs does the
layer growth parameter approach twice the size of the
aluminum–oxygen tetrahedra. This gives grounds to
assume that, under these conditions, Al2O3 layers grow
in layerꢀbyꢀlayer mode and the process is dominated
Structural characterization of the deposited films
by electron diffraction showed that, over the entire
temperature range studied, the SiO2 layers were amorꢀ
phous, independent of their thickness. The Al2O3 layꢀ
ers less than 100 nm in thickness were amorphous at
T
s < 450 K and contained ꢀAl2O3 at higher temperaꢀ
α
tures. In all cases, increasing the Al2O3 layer thickness
and deposition temperature led to the formation of
crystalline domains as supramolecular structures.
by reaction (3a) with
m
= 2 (Fig. 2b).
From the observed broadening of electron diffracꢀ
The drop in d0 at
Т
> 473 K for InAs and at
Т
> 453 K tion rings, the average crystallite size was evaluated to
for InSb seems to be caused not so much by the reducꢀ be 2 to 20 nm, depending on the layer thickness. The
tion in the density of surface hydroxyls as by the therꢀ crystallites most likely consisted of oxide groups that
mochemical instability of these materials in the halide had formed on periodic arrays of hydroxyl groups on
atmosphere. This is evidenced by the fact that the narꢀ homogeneous surface areas. Heat treatment of aluꢀ
rowing of the layerꢀbyꢀlayer growth ranges of both mina layers on GaAs at 623 K considerably increased
INORGANIC MATERIALS Vol. 46
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GROWTH AND PROPERTIES OF Al2O3 AND SiO2 NANOLAYERS
41
Table 1. Oxygenꢀtoꢀmetal ratio in alumina layers grown at differꢀ
It seems likely that the increased oxygen content at
low and elevated deposition temperatures is due to
hydration of the oxides and to the contribution of the
oxide layer of the substrate, respectively.
ent temperatures
Ts, K
423
4.0
473
2.3
523
1.8
553
1.5
573
1.6
The frustrated total internal reflection IR spectra of
the alumina films grown on GaAs at
Ts = 553 K on the
[O]/[Al]
whole correlated with the XPS results. The spectra
showed wellꢀresolved absorption peaks at
ν
835 cm–1, due to Al–O bonds with the aluminum in
sixꢀ and fourfold coordination [16].
= 744 and
Table 2. Dielectric properties of oxide nanolayers
The dielectric properties of the nanolayers were
studied at thicknesses above 10 nm, because the elecꢀ
trical conductivity was then essentially independent of
the layer thickness and was determined by the deposiꢀ
tion temperature (Fig. 3). In all cases, increasing the
substrate temperature reduced the conductivity to a
level characteristic of layerꢀbyꢀlayer growth of the
nanostructures. The increased conductivity of the layꢀ
Oxide
σ
, S/cm
E
×
10–6, V/cm
ε
tan
δ
SiO2 10–13–10–14
Al2O3 10–13–10–14
4
5
3.9–4.3 0.001–0.01
8–10 0.005
the intensity of the electron diffraction rings from the
layers and sharply increased the crystallite size, leading
to almost complete oxide crystallization. Therefore,
the polycrystalline layers contained an amorphous
phase between the crystallites, which was involved in
the crystallization process. This conclusion is supꢀ
ported by the fact that the deposited films showed no
preferential orientation.
ers at Ts < 400 K is most likely due to the hydration of
the oxides and correlates well with the oxygenꢀtoꢀ
metal ratios extracted from the XPS data for the layers
(Table 1).
Table 2 summarizes the main dielectric properties
of the oxides grown in layerꢀbyꢀlayer mode.
The XPS spectra of the oxide layers did not show
any peaks at binding energies of 190–200 or 260–270 eV,
characteristic of the Cl 2sꢀ and 2p levels. The observed
CONCLUSIONS
broadening of the oxygen levels (530–531 eV) indiꢀ
cated the presence of hydroxyl groups. Note that, at
oxide layer thicknesses above 8 nm (electron escape
depth), the XPS spectra showed no signal from the
substrate, indicating that the layers more than 8 nm in
thickness were continuous.
Silica and alumina nanolayers were grown on
GaAs, InAs, and InSb substrates by ALD.
Comparison of the present experimental data with
previous results for other substrate materials and other
oxides [13] shows that the growth of SiO2 and Al2O3
nanolayers on III–V semiconductor substrates follows
the same general mechanisms, inherent in this proꢀ
cess. In particular, analysis of the effect of deposition
temperature indicates that the growth of oxide nanoꢀ
structures via sequential chemisorption of metal
halide and water vapors may follow three mechanisms:
reaction between components in a polymolecular
adsorbed layer, leading to the formation of hydrous
oxides; sequential growth of monomolecular layers
(layerꢀbyꢀlayer growth mechanism); and formation
and subsequent growth of twoꢀdimensional islands.
Silicon tetrachloride chemisorption under layerꢀbyꢀ
layer growth conditions is a thermally activated proꢀ
cess, and layerꢀbyꢀlayer SiO2 growth occurs only at
From the peak areas for oxygen (
E
b = 530–531 eV)
and aluminum ( b = 74.6 eV for Al2р and
E
Eb = 118.8 eV
for Al2s), we evaluated the oxygenꢀtoꢀmetal ratio in the
oxide layers grown on GaAs at different temperatures.
The results are presented in Table 1.
σ
, S/cm
10–11
10–12
10–13
Ts > 500 K, whereas aluminum chloride chemisorbs in
the form of dimers without thermal activation. This
leads to high d0 values for Al2O3 at any temperature.
2
1
The dielectric properties of SiО2 and Al2O3 films
produced under layerꢀbyꢀlayer growth conditions
meet the requirements for dielectric films. Therefore,
the ALD process can be used to grow dielectric strucꢀ
tures, including multilayers, for submicronꢀsized
components of microꢀ and nanoelectronic systems
(e.g., gate dielectrics in MIS structures), capacitors,
and barrier layers.
10–14
350
400
450
500
550
s, K
T
Fig. 3. Electrical conductivity as a function of deposition
temperature for (1) SiO and (2) Al O layers; d = 25–30 nm,
2 2 3
5
Е
= 10 V/cm.
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EZHOVSKII, KHOLKIN
9. Aleskovskii, V.B., Chemical Assembly of Materials,
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