The effects of oxidation temperature on the capacitance-voltage characteristics of oxidized AIN films on Si

Article abstract of DOI:10.1063/1.120510

The thermal oxidation of AlN thin films produces a high quality insulator which exhibits the gate voltage-controlled charge regimes of accumulation, depletion, and inversion on Si surfaces. The temperature dependence of oxidation is important for device processing. We report on the composition, structure, and electrical properties of the AlN versus the oxidization temperature. AlN layers 500 nm thick were deposited by rf sputtering on p-type Si (100) substrates, followed by oxidation in a furnace at temperatures from 800 to 1100°C with O₂ flow. An oxidation time of 1 h produced layers of Al₂O₃ with small amounts of N having a thickness of 33 nm at 800°C, and 524 nm at 1000°C. Electrical measurements of metal-oxide-semiconductor capacitors indicated that the dielectric constant of the oxidized AlN was near 12. The best layer had a flatband voltage near zero with a net oxide trapped charge density less than 10¹¹ cm^-2. These results show that oxidized AlN has device-grade characteristics for the gate regions of field effect transistors, and for optoelectronic applications.

Full text of DOI:10.1063/1.120510

The effects of oxidation temperature on the capacitance–voltage characteristics of
oxidized AlN films on Si
J. Kolodzey, E. A. Chowdhury, G. Qui, J. Olowolafe, C. P. Swann, K. M. Unruh, J. Suehle, R. G. Wilson, and J.
M. Zavada
Citation: Applied Physics Letters 71, 3802 (1997); doi: 10.1063/1.120510
View online: http://dx.doi.org/10.1063/1.120510
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The effects of oxidation temperature on the capacitance–voltage
characteristics of oxidized AlN films on Si
J. Kolodzey,a) E. A. Chowdhury, G. Qui, and J. Olowolafe
Department of Electrical and Computer Engineering, University of Delaware, Newark, Delaware 19716
C. P. Swann
Bartol Research Institute, University of Delaware, Newark, Delaware 19716
K. M. Unruh
Department of Physics and Astronomy, University of Delaware, Newark, Delaware 19716
J. Suehle
National Institute of Standards and Technology, Gaithersburg, Maryland 20899
R. G. Wilson
Hughes Research Laboratories, Malibu, California 90265
J. M. Zavada
ARO, Research Triangle Park, North Carolina 27709
͑Received 10 October 1997; accepted for publication 29 October 1997͒
The thermal oxidation of AlN thin films produces a high quality insulator which exhibits the gate
voltage-controlled charge regimes of accumulation, depletion, and inversion on Si surfaces. The
temperature dependence of oxidation is important for device processing. We report on the
composition, structure, and electrical properties of the AlN versus the oxidization temperature. AlN
layers 500 nm thick were deposited by rf sputtering on p-type Si ͑100͒ substrates, followed by
oxidation in a furnace at temperatures from 800 to 1100 °C with O2 flow. An oxidation time of 1 h
produced layers of Al2O3 with small amounts of N having a thickness of 33 nm at 800 °C, and 524
nm at 1000 °C. Electrical measurements of metal-oxide-semiconductor capacitors indicated that the
dielectric constant of the oxidized AlN was near 12. The best layer had a flatband voltage near zero
with a net oxide trapped charge density less than 1011 cmϪ2. These results show that oxidized AlN
has device-grade characteristics for the gate regions of field effect transistors, and for optoelectronic
applications. © 1997 American Institute of Physics. ͓S0003-6951͑97͒04652-4͔
High quality insulators such as thermally grown SiO2 are
crucial for optoelectronic device and circuit applications.
The thickness of gate insulators in field effect transistors are
being scaled to below 4 nm,1 and tunneling leakage currents
affect circuit operation and degrade reliability by time de-
pendent dielectric breakdown.2 Insulators with high dielec-
tric constants can be made thicker for the same stored
charge, producing a lower electric field and potentially fewer
problems due to leakage and breakdown. Al2O3 has a high
dielectric constant and may be an ideal alternative to SiO2
depending on its properties.3–5 We have found that thermally
oxidized AlN produces Al2O3 with excellent electrical and
optical properties. In contrast, the oxidization of AlAs has
also given interesting results,6,7 but this oxide may contain
residual As, unlike oxidized AlN.
The oxidized AlN reported here was prepared using
techniques described previously.5 Layers of AlN 500 nm
thick were reactivity sputtered onto p-type Si ͑100͒ sub-
strates, which were prepared by degreasing, an RCA etch,
and an HF dip prior to sputtering. Oxidation was performed
under dry O2 flow in a quartz furnace tube at temperatures
ranging from 800 to 1100 °C for durations of 1 and 2 h. The
oxide structure was determined using ␪/2␪ powder x-ray dif-
FIG. 1. XRD intensity vs scattering angle showing the trend of less AlN and
fractometry ͑XRD͒. Figure 1 shows scans of XRD intensities
over the angles associated with AlN and Al2O3 for a series of
more Al2O3 vs increasing oxidation temperature and time. The peak labels
correspond reasonably to the angles expected for the following crystal
phases and planes: ͑I͒ AlN ͑002͒; ͑II͒ AlN ͑101͒; ͑III͒ AlN ͑102͒; ͑A͒
␪-Al2O3 ͑several planes of the ␪ phase occur near this angle͒; ͑B͒ ␣-Al2O3
͑104͒; ͑C͒ ␣-Al2O3 ͑113͒.
a͒
Electronic mail: Kolodzey@ee.edel.edu
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FIG. 3. Plot of layer thickness vs reciprocal temperature, based on RBS
simulations using three layers. With increasing oxidation temperature, Al2O3
is grown at the expense of AlN ͑500 nm as-grown thickness͒. The Al2O3
growth is thermally activated as discussed.
FIG. 2. RBS data of normalized ion counts vs ion energy using a
2.0 MeV Heϩ ion beam, for sample oxidized at 1000 °C. The spectrum
shows Al at 1.1 MeV, Si from substrate at 0.9 MeV, and O from the Al2O3
at 0.7 MeV. If present, N would appear at 0.6 MeV. The smooth curve is the
RUMP simulation which yielded the composition profiles vs temperature
shown in Fig. 3.
son, the dry oxidation thickness of Si has activation energies
of 1.24 eV in the diffusion-limited regime, and 2.0 eV in the
linear regime.9 For temperatures less than 1100 °C, the pres-
ence of the SiO2 layer is interpreted as an artifact of the
simulation which attempted to minimize the error using only
three layers in the presence of composition mixing, and is
not considered to be physically real. RBS indicated that the
oxidation of the AlN began at 800 °C. No unoxidized AlN
remained above 1000 °C for a 500 nm layer, and our results
indicate that a 120 nm layer of AlN would fully oxidize at
900 °C for 1 h. At 1100 °C, some SiO2 was produced at the
Si surface, showing that the Al2O3 layer was not a barrier to
Si oxidation at this temperature. The RBS compositions ver-
sus temperature were reasonably consistent with the x-ray
results of Fig. 1.
To measure the dielectric properties of the oxidized AlN
insulator, metal-insulator-silicon ͑MIS͒ capacitors were fab-
ricated and measured versus bias and frequency.10 Electrical
contacts of Al metal 100 nm thick were sputtered onto the
top of the oxide and the bottom of the Si substrate. The top
contacts were patterned by conventional photolithographic
liftoff into arrays of circular dots of area 8ϫ10Ϫ4 cm2. As
shown in Fig. 4, the capacitance–voltage C–V characteris-
tics indicated that the gate bias brought the Si surface charge
into the regimes of accumulation at negative gate voltages,
and depletion and inversion at progressively more positive
voltages. This is similar to the well known behavior of SiO2
capacitors.9 The surface inversion at measurement frequen-
cies below 1 MHz is pivotal for transistor operation because
it indicates that there are no defects which pin the Fermi
level near mid-gap and prevent the appearance of mobile
electrons at the Si surface. At frequencies higher than the
carrier thermal generation rate ͑near 1 MHz͒, the semicon-
ductor remains depleted with correspondingly lower capaci-
tance.
oxidation temperatures and times. The presence of relatively
weak multiple diffraction lines indicates that the as-deposited
AlN and the oxidized layers consisted of amorphous and
crystalline grains with several orientations. Several phases of
Al2O3 were observed including sapphire (␣-Al2O3). With
increasing oxidation temperature and time, the XRD intensi-
ties of the AlN lines decreased and the intensities of the
Al2O3 lines increased. The Al2O3 is evident at 1000 °C, and
at 1100 °C there is little evidence for AlN. The XRD scans
were made to reveal structure, however, and were not in-
tended for revealing volume fractions of the compositions.
The layer compositions were measured using Rutherford
backscattering spectrometry ͑RBS͒, and secondary ion mass
spectrometry ͑SIMS͒. To estimate the composition versus
depth, the RBS data were simulated using RUMP software.8
Figure 2 shows the RBS ion counts versus energy for a layer
oxidized at 1000 °C, along with a RUMP simulation for a
structure containing Al2O3 at the surface, a sublayer of un-
oxidized AlN, and a layer of SiO2 next to the Si ͑to account
for the possibility of oxidizing the Si substrate͒. For a simple
estimate of the oxidation rates, we used RUMP simulations
with only three abrupt layers having uniform, fixed compo-
sitions, with the layer thickness adjusted to minimize the
simulation error. The simulations provided a reasonable
match to the RBS data, and are in qualitative agreement with
the SIMS results. The amount of N in the Al2O3 was esti-
mated to be less than 10% because it was not detectable by
RBS. In principle, more complicated simulations could be
obtained by adding sublayers of intermediate compositions
accounting for mixing at interfaces by interdiffusion and in-
complete oxidation.
The thickness of the layers is plotted versus reciprocal
temperature in Fig. 3, with an estimated accuracy of Ϯ3 nm.
Straight lines on a log scale indicate thermally activated,
diffusive behavior. The Al2O3 thickness slope yields 1.9 eV
in the range between 900 °C and 1000 °C, which we inter-
pret as the activation energy of AlN oxidation. By compari-
The C–V data are summarized in Table I. The flatband
voltage is:
V
FBϭ⌽MSϪQOX /COX ,
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are significantly higher than the value of 3.9 for SiO2. The
reason for the increase in ␬ as temperature decreased is
OX
not yet clear, but may be partly due to thickness errors. The
sample oxidized at 1000 °C had an oxide leakage current
density of 1.2ϫ10Ϫ7 A cmϪ2 at a field of 1 MV/cm, corre-
sponding to a resistance of 5.8ϫ1011 ⍀.
These results show that AlN thin films on Si can be
oxidized at standard process temperatures, producing a
device-grade insulator with sufficient quality for the gates of
field effect transistors. The oxidation produced Al2O3 with
no N detected by RBS. C–V measurements of MIS capaci-
tors on Si exhibited surface charge inversion, showing that
oxidized AlN has low defect densities and is comparable to
device-grade SiO2. The dielectric constant is higher than that
of SiO2, and oxidized AlN can be used as a thicker gate
insulator for Si-based field effect transistors with less tunnel-
ing leakage and greater reliability for the same stored charge.
Other possible applications include native oxides for AlGaN-
based MIS field effect transistors, and dielectrics for optical
devices.
Special thanks go to Dave Smith for valuable advice and
help with metallization, to Y. Chen for help with the C–V
measurements, and to Dr. J. Comas for valuable advice. This
work was supported by the ARO under Grant No. DAAH04-
95-1-0625, and DARPA under Contract No. F49620-96-C-
0006.
FIG. 4. Capacitance–voltage characteristics vs frequency for AlN oxidized
at 1000 °C for 1 h. Surface charge inversion of the Si occurs for positive
bias at frequencies below 1 MHz.
where ⌽MS is the metal–semiconductor work function dif-
ference ͑near Ϫ0.9 V for Al metal and p-type Si͒, QOX is the
net oxide trapped charge ͑the first moment of the oxide
charge distribution divided by the oxide thickness͒, and COX
is the capacitance of the oxide layer. Samples oxidized at
1000 °C and above have QOX/q near 1011 cmϪ2, which is
comparable to that of device-grade SiO2.9 Lower tempera-
tures yielded higher values of VFB and QOX which were at-
tributed to the presence of unoxidized AlN, because we ob-
served that thinner samples in which the AlN was fully
oxidized at 900 °C had low defect densities. We emphasize
that QOX includes contributions from the entire oxidized
layer, and would not be reduced merely by the presence of
defect-free SiO2 at the Si interface.
1 C. Hu, Tech. Dig. Int. Electron Devices Meet. , 319 ͑1996͒.
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The effective dielectric constant of the oxide ␬OX , given
in Table I, was obtained from the capacitance measurements.
These values are comparable to the values for ␣-Al2O3
(␬sapphireϭ12),11 and unoxidized AlN (␬AlNϭ9.14)12 which
5 E. A. Chowdhury, J. Kolodzey, J. Olowolafe, G. Qui, G. Katulka, D. Hits,
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TABLE I. The dielectric properties of oxidized AlN thin films from C–V
measurements of MIS capacitors. The sublayer thickness was determined
from RUMP software simulations to the RBS data, assuming a three layer
structure. The as-grown AlN was near 500 nm thick for all samples. The
sample number, oxidation temperature and time, dielectric constant, net ox-
ide trapped charge, and flatband voltage are indicated.
7 E. F. Schubert, M. Passlack, M. Hong, J. Mannerts, R. L. Opila, L. N.
Pfeiffer, K. W. West, C. G. Bethea, and G. J. Zydzik, Appl. Phys. Lett. 64,
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Proceedings of the 1994 Reliability Physics Symposium, April 12–14,
1994, San Jose, pp. 120–125.
QOX/q
TOX /dOX
͑ °C/h͒
VFB
͑V͒
Sample
␬
(cmϪ2
)
OX
100601a
100601b
100601c
100601d
100601e
1100/2
1100/1
1000/1
900/1
10
8.6
12.6
15.6
14.2
5.56ϫ1010
1.36ϫ1011
2.5ϫ1011
Ϫ1.67
Ϫ2.96
ϩ1.13
Ϫ18.1
Ϫ19.4
11 The dielectric constant of ␣-Al2O3 has components 11.5 and 9.65 parallel
and perpendicular to the c axis, respectively, at the frequency of 106 Hz at
298 K; Hand Book of Thin Film Technology, edited by L. I. Maissel and
R. Glang ͑McGraw-Hill, reissue, New York, 1983͒, pp. 6–12.
12 Semiconductors—Basic Data, edited by O. Madelung ͑Springer, Berlin,
1991͒.
2.84ϫ1012
2.85ϫ1012
800/1
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