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Nonvolatile memory effects in nitrogen doped tetrahedral amorphouscarbon thin films
E. G. Gerstnera) and D. R. McKenzieSchool of Physics, University of Sydney, New South Wales 2006, Australia
Received 21 April 1998; accepted for publication 20 August 1998
Electrical measurements of nitrogen doped tetrahedral amorphous carbon ( t a-C:N thin films have
revealed a reversible nonvolatile memory effect, related to the excitation and de-excitation ofelectrons between deep acceptor states and shallow donor states within the mobility gap. This effect
is characterized by changes in the small signal film conductivity of up to 10 times, and has been used
to fabricate 1-bit memory cells with effective memory retention times in the order of several
months. 1998 American Institute of Physics. S0021-89799805522-4
I. INTRODUCTION
Electron states located within the band gap of a semicon-
ductor are normally detrimental to its performance as they
act as traps for mobile carriers. In this article we show
that these states can act as a means of storing binary infor-
mation in a nonvolatile manner and may therefore form thebasis for potential memory devices. Furthermore, we show
that these memory effects, observed in nitrogen doped tetra-
hedral amorphous carbon thin films, are a new phenomenon
brought about by a switching between two electrical states in
which different conduction mechanisms dominate: a high re-
sistance state in which hopping conduction dominates; and a
low resistance state in which PooleFrenkel conduction1
dominates.
Amorphous carbon thin films with a high tetrahedral
content are produced by condensation onto a substrate from
energetic carbon ion fluxes.2 The properties and formation of
this material, referred to as tetrahedral amorphous carbon
( ta-C, have recently been reviewed.3 In a material of high
s p 3 a band gap of approximately 2.5 eV opens up between
the residual and * states. Ta-C is weakly p type,4 and
can be doped n type by the incorporation of phosphorus or
nitrogen.5
With the emergence of improved thin-film deposition
technologies during the late 1960s and early 1970s, there
appeared in the literature several reports on the observation
of memory effects in various amorphous insulators and semi-
conductors. The bulk of these can be classed into two differ-
ent categories ofcurrent- and voltage-controlledeffects, de-
pending on the mechanism by which switching between
states occurs. Ovonic switching, reported by Ovshinskyet al.6 in chalcogenide glasses is probably the best known of
current-controlled effects. This type of switching is believed
to occur by a current controlled transition between a high
conductivity polycrystalline and a low conductivity amor-
phous phase of the material see Mott and Davis.7 The sec-
ond category of voltage-controlled switching has been re-
ported in a range of insulating oxide, sulfide, and fluoride
thin films810 and has been reviewed by Simmons
and Verderber.11 This type of switching is typified by a re-
duction in resistance after the application and rapid removal
within 0.1 ms of voltage bias beyond some threshold, with
the low resistance state persisting until the bias again ex-
ceeds the threshold and removed more gradually 0.1 ms.
The operation of both current- and voltage-controlled effects
are symmetric with respect to the direction of bias. The ef-
fect on which we report here lies in the voltage-controlled
category, however, the direction of bias plays a critical role
in the switching process. We therefore consider it to be a
new type of effect, qualitatively different to those previously
observed in other amorphous materials.
Currently, the majority of commercial nonvolatile
memories operate by the storage of electrons in the charge
traps of a floating silicon nitride gate, by either hot electron
injection or tunnelling across a thin silicon dioxide film.12
The gate is usually embedded between the channel and the
gate of a metal oxide semiconductor field effect transistor
MOSFET, with the charge state of a cell measured by test-ing the turn-on voltage of the FET. The disadvantages of
such metal-nitride-oxide-siliconMNOStechnology are low
information density, fabrication complexity, and expense.
Conversely, the simplicity of the structure of t a-C memory
cells suggests the potential for overcoming these disadvan-
tages.
II. EXPERIMENT
Nitrogen doped t a-C denoted ta-C:N thin films were
deposited by means of a filtered cathodic vacuum arc onto
aluminum films which had been thermally evaporated onto
both glass and silicon substrates. Nitrogen was incorporatedinto these films by the introduction of nitrogen gas into the
vacuum chamber during deposition. The films studied here
were deposited with nitrogen base pressures of between 104
and 103 Torr, with optimal results occurring at 6104
Torr. The nitrogen flow rates required to maintain these pres-
sures were between 2 and 12 standard cubic centimeters per
minute sccm. The atomic concentration of nitrogen in these
films was found to be approximately 5%, measured using
electron energy loss spectroscopy. The substrates were water
cooled to keep them at room temperature during deposition,aElectronic mail: [email protected]
JOURNAL OF APPLIED PHYSICS VOLUME 84, NUMBER 10 15 NOVEMBER 1998
56470021-8979/98/84(10)/5647/5/$15.00 1998 American Institute of Physics
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and no substrate bias was applied. The thickness of the films
lay was between 50 and 100 nm. The films were transparent
and displayed strong interference colors indicating a rela-
tively small optical absorption. The conductivity of the films
was approximately 6104 cm.
All electrical measurements were made through film of
sandwich structures of ta-C:N films between a sprung gold
point contact on the surface, and an underlying aluminum
film. A Hewlett Packard HP4145b semiconductor parameter
analyzer was used to collect the currentvoltage characteris-
tics. The area of contact between the top contact and the
surface of the ta-C:N film was approximately 103 mm2.
The biases quoted are those of the top contact with respect tothe underlying contact.
III. RESULTS AND DISCUSSION
A. Switching in ta-C:N films
We first observed memory effects in t a-C in the
currentvoltage I V) characteristics oft a-C:N films. When
collecting the through film I-V characteristics between a
sprung gold point contact and an underlying thin film alumi-
num contact, a kink in the forward direction positive bias
of the point contactwas observedsee Fig. 1. In the reverse
directionpassing from positive to negative biases this kinkwas then found to disappear. Further investigation revealed
that the kink in the forward part of the I Vcharacteristic
only appeared after the application of a certain negative
threshold bias, and disappeared after the application of a cer-
tain positive threshold bias. In other words, the device had a
memory of whether a negative or a positive bias had been
applied to it, by the presence or absence respectively of a
kink in the forward I V curve. Furthermore it was found
that upon the application of a negative voltage pulse beyond
some threshold amplitude herein referred to as the write
bias, the small signal resistance measured at voltages 0.5
Vdecreased by a factor of between 5 and 10 times see Fig.
2, which could be reversed simply by the application of a
positive pulse of similar amplitude
the erase bias.It is the transition from the high conductivity state to the
low conductivity state with the application of bias above a
given positive voltage which causes the kink in the forward
characteristic. By extending the I V characteristic of the
high conductivity state at negative bias to be symmetrical at
positive bias the dotted line in Fig. 1 this transition is
clearly illustrated. It should be noted that the transition from
the high to the low resistance states is not due to electrical
avalanche breakdown of the material. Such breakdown of the
material can occur at voltages around twice those used here,
however the resulting resistance afterwards is of the order of
100 times lower than that after the application of a negative
write bias.When voltages well below either the write or erase
thresholds were applied to a device, no noticeable change
occurred in the state of the device. As a result, small voltages
can be used to test the resistance of a device without affect-
ing the state, providing a method ofreadinga device nonde-
structively. We can therefore write a 1 or ON with a
negative threshold bias, erase this to 0 or OFF with
positive threshold bias, and read the state by measuring the
resistance with a small bias between these thresholds. Fur-
thermore, the ON low resistivity state of the device was
found to persist for extended periods of time see Sec. III C.
B. Conduction mechanisms
In order to investigate the switching mechanism and its
temperature dependence, a device was cycled between its
ON and OFF state at various temperatures between 40 and
360 K. Switching between states was achieved with write
and erase voltages of3 and 3 V, respectively. In order to
avoid changing the state during measurement, ON currents
were collected at negative biases after writing, and OFF
currents at positive biases after erasing. Arrhenius plots of
the currents versus inverse temperature were then plotted at
various biases see Fig. 3. At temperatures much below 250
K no measurable change in the memory state was observed.
FIG. 1. I V characteristic of a t a-C:N memory cellarrows indicate the
direction of voltage sweep sweep rate 0.83 V/s. Dotted line represents a
symmetric projection of the forward swept reverse characteristic to positive
voltages to highlight the transition from the low to the high resistance state
with forward bias.
FIG. 2. Small signal resistance of a t a-C:N memory cell as a function of
pulse amplitude for a 100 s negative voltage pulse. The device was erased
between write pulses with a positive erase bias.
5648 J. Appl. Phys., Vol. 84, No. 10, 15 November 1998 E. G. Gerstner and D. R. McKenzie
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At such temperatures, conduction mechanisms present in
both the OFF and ON states dominate, allowing no discrimi-
nation between the two conduction states. At temperatures
above 250 K, however, significant differences between the
ON and OFF state, in both the temperature and voltage de-
pendence of the conductivity, were found, reflecting the
dominance of different conduction mechanism in each of the
two states. In the OFF state, the conductivity exhibited a
weak temperature dependence, with an activation energy of
0.070.01 eV which was independent of bias. Conduction
in this state is expected to proceed via hopping between
states near the Fermi energy, as occurs in many amorphous
semiconductors.13 Conversely, in the ON state, the conduc-
tivity was found to have a stronger dependence on tempera-
ture, with a voltage dependent activation energy decreasinglinearly with the square root of the applied bias see Fig. 4,
consistent with single center PooleFrenkel conduction.1
Such conduction occurs by the field assisted thermionic
emission of electrons from donor traps positively charged
when unoccupied which lie just below the conduction band
edge. The activation energy for PooleFrenkel conduction at
high fields is given by,1
EaEiV/t1/2, 1
where Ei is the activation energy at zero field, is the
PooleFrenkel constant for the material, t is the film thick-
ness, and V is the applied voltage. Therefore, using Eq. 1
and extrapolating the voltage dependence in the ON state to
zero field an activation energy ofEi0.25 eV is found, and
represents the position of the donor states below the conduc-
tion band edge.
With these two conduction mechanisms in mind it there-
fore appears that switching to the ON state proceeds by the
promotion of electrons held in deep acceptor traps to shal-
lower unoccupied donor traps 0.25 eV below the conduc-
tion band, with the increase in conductivity due simply to the
greater efficiency of PooleFrenkel conduction which emits
electrons to the extended states of the conduction band, as
opposed to hopping conduction comprising transitions be-
tween localized states at the Fermi level. The very slow de-
cay of the ON state is explained by the fact that the rate of
direct transitions from donor states to acceptor states deeper
in the gap will be very low as they are between extremely
localized states as opposed to other trap related processes
such as carrier recombination in which at least one of the
states is extended. Furthermore, it is expected that the rate
of capture of electrons from the conduction band is greater
for unoccupied donor traps than for unoccupied acceptor
traps, owing to the positive coulombic attraction associated
with only the donor traps. With respect to nitrogen doping of
ta-C, the filling of deeper acceptor states by the decay of
electrons from nitrogen donor states just below the conduc-
tion band, explains both the movement of the Fermi energywith increased nitrogen doping and the poor doping effi-
ciency observed by Amaratunga et al.4,14
The write process in the above model is analogous to the
creation of a population inversion in a laser. In the high
resistance ground state the majority of donor electrons
will be held in acceptor traps deep within the mobility gap,
with the majority of donor states being ionized. In the lower
resistance state some number of the donor states are re-
filled, representing a nonequilibrium excited state of the
material. Herein, we present a possible explanation for the
bias dependent switching between the two states. We first
assume a situation in which the top contact is more rectifying
than the bottom contact, as in the case with many other n-type semiconductors with Au and Al contacts, and then con-
sider the effect of a negative write bias on the region di-
rectly underneath the top contact. Band bending will cause
this region to be forced into depletion, and any electrons
which undergo thermionic emission to the conduction band
will be immediately swept from the region. This will cause a
net ionization of electrons from acceptor traps in the region
see Fig. 5a. When the voltage is removed the excess elec-
trons will be recaptured preferentially by positively charged,
unoccupied donor traps 0.25 eV below the conduction band
edge as determined from the voltage dependence of the ac-
tivation energy in the ON state, resulting in a metastable
FIG. 3. The current in the ON points and OFF linesstates of a memory
device as a function of 1/T, at different applied voltages.
FIG. 4. The activation energy for conduction in a device in the ON state as
a function of V1/2, calculated from the temperature dependence shown in
Fig. 3. The linear dependence of Ea with respect to V1/2 is indicative of
single center PooleFrenkel conduction Ref. 1.
5649J. Appl. Phys., Vol. 84, No. 10, 15 November 1998 E. G. Gerstner and D. R. McKenzie
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population inversion of electrons higher in the mobility gap,
and more importantly, above the Fermi energy. It should be
noted that given the dimensions of the thin film 100 nm,
and its insulating nature, the region of depletion can extend
well into the bulk of the film under conditions of significant
bias.
Turning now to the erase process, a positive bias applied
to the top contact will accumulate electrons in the region
enhancing the filling of all traps throughout the gapsee Fig.
5b. When most donor traps are filled acceptor traps will
proceed to fill. At the same time a narrow region of positive
space charge will form in the metal. Thus, when the applied
field is removed, a field will be generated so as to neutralize
this charge in the metal. Since the electrons held in the shal-low donor traps are more mobile than those in deeper accep-
tor traps, such traps will be emptied before deeper traps, and
the distribution will be returned to the OFF state.
C. Memory retention
In order to determine the duration of effective memory
retention of the devices a cell was written with a 2.8 V
write pulse of width 5 ms, and the small signal resistance
monitored at regular intervals over a period of several weeks.
The resulting decay profile see Fig. 6 was found to have a
power law dependence on time, and is well described by the
expression,15
R tR ONROFFR ON
1 R
1R t/t0
m
, 2
where R ON is the resistance immediately after writing, ROFFis the resistance when completely erased, R is the tolerance
level for distinguishing the ON states resistance of the device
from the OFF state, t0 is the time at which the device decays
to this tolerance level, and m is the exponent of decay related
to the ON state charge distribution and the temperature. This
expression was fitted to the experimental data by a least-
squares algorithm with only two free parameters t0 and m ,
yielding excellent agreement indicated by the solid line in
Fig. 6. If we extend this fit to longer times and use a toler-
ance level of 20% (R0.2, the fraction of the total differ-ence between the ON and OFF states, below the OFF state
we find effective retention times in the order of several
months.
An important question relating to the possible applica-
tion of this memory effect in the fabrication of commercial
devices is the effect of temperature on the rate of memory
decay. In a previous article,15 the write and erase currents
measured as a function of time at constant bias were found to
have a power law dependence. Furthermore, the erase current
has a similar form to that of the memory decay, suggesting
that the erase and decay processes proceed by similar mecha-
nismsalbeit at different rates, with similar temperature de-
pendencies. Therefore by measuring the temperature depen-dence of the erase current, we can infer the temperature
dependence of the memory decay.
The expression for the erase current is given by,
Idischarge tII0Atm 3
where I is the final current after the device is completely
discharged,I0 is the initial difference in the current before
discharging, A is a constant related to the donor electron
emission rate, and m is the time dependence exponent. By
collecting erase currents as a function of time at constant
erase voltages, at different temperatures, and then fitting Eq.
3 to the data, the temperature dependencies of the coeffi-
FIG. 5. Write and erase processes of a ta -C:N memory cella a net
ionization of electrons from acceptor traps (D) is achieved by the deple-
tion of electrons from the region directly underneath the top contact, these
electrons are then recaptured by positively charged donor traps D), and
ba net recapture of electrons is achieved by the accumulation of electronsin the region underneath the top contact.
FIG. 6. Resistance of a ta -C:N memory cell as a function of time after a
2.8 V write pulse of 5 ms duration. The solid line represents a least-
squares fit of Eq. 2 to the experimental data.
5650 J. Appl. Phys., Vol. 84, No. 10, 15 November 1998 E. G. Gerstner and D. R. McKenzie
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7/27/2019 JAP1998 Nonvolatilememory aC Nitrogendoped
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cientsm and A can be found. The results see Fig. 7indicate
a linear relationship with temperature for both coefficients,
withm increasing and A decreasing with increasing tempera-
ture. From these, the time to erase the device can be calcu-latedhere defined as the time to reduce the conductivity of
the device to within 20% of its OFF state value. The results
see Fig. 8 give a relatively weak exponential dependence
on temperature, with around a ten times decrease in the erase
time between 293 and 360 K. The implication for the
memory retention is that at 310 K the decay proceeds at
approximately twice the rate of that at room temperature.
IV. CONCLUSION
A memory effect which involves switching between
high and low resistance states in nitrogen doped tetrahedral
amorphous carbon thin films has been observed. The two
states are characterized by two different dominant conduc-tion mechanisms: in the high resistance state by hopping
conduction via states near the Fermi level within the mobility
gap; and, in the low resistance state by PooleFrenkel con-
duction from donor states 0.25 eV below the conduction
band edge. Switching is believed to involve the promotion of
electrons from acceptor states deep in the gap to higher do-
nor states, and a mechanism by which this may occur has
been proposed. Furthermore, switching is found to be criti-
cally dependent on the direction of applied bias, distinguish-
ing it apart from other voltage-controlled memory effects
reported previously in other amorphous semiconductors in
the literature.
The 1-bit memory cells were fabricated with write timesdown to 100 s and effective memory retention in the order
of several months, opening the possibility for the use of this
effect as a means of nonvolatile digital information storage.
1 R. M. Hill, Philos. Mag. 23, 591971.2 D. R. McKenzie, D. A. Muller, and B. A. Pailthorpe, Phys. Rev. Lett. 67,
7731991.3 D. R. McKenzie, Rep. Prog. Phys. 59, 1611 1996.4 G. A. J. Amaratunga, D. E. Segal, and D. R. McKenzie, Appl. Phys. Lett.
59, 691991.5 D. R. McKenzie, D. A. Muller, B. A. Pailthorpe, Z. H. Wang, E. Kravtch-
inskaia, D. Segal, P. B. Lukins, P. D. Swift, P. J. Martin, G. A. J. Ama-
ratunga, P. H. Gaskell, and A. Saeed, Diamond Relat. Mater. 1, 51 1991.6 S. R. Ovshinsky, Phys. Rev. Lett. 21, 4501968.7
N. F. Mott and E. A. Davis, in Electronic Processes in Non-CrystallineMaterials, 2nd ed. Clarendon, Oxford, 1979, Chap. 9, pp. 507512.8 G. S. Kreynina, Radio Eng. Electron. Phys. 7, 19491962.9 T. W. Hickmott, J. Appl. Phys. 36, 1885 1965.
10 J. G. Simmons and R. R. Verderber, Radio Electron. Eng. 34, 811967.11 J. G. Simmons and R. R. Verderber, Proc. R. Soc. London, Ser. A 301, 77
1967.12 H. E. Maes, G. Groeseneken, H. Lebon, and J. Witters, Microelectron. J.
20, 91989.13 N. F. Mott and E. A. Davis, Electronic Processes in Non-Crystalline
Materials, 2nd ed. Clarendon, Oxford, 1979.14 G. A. J. Amaratunga, V. Veerasamy, W. I. Milne, and D. R. McKenzie,
2nd International Conference on Applications of Diamond Films and Re-
lated Materials MYU, Tokyo, 1993.15 E. G. Gerstner and D. R. McKenzie, Diamond Relat. Mater. to be pub-
lished.
FIG. 7. Erase coefficientsA and m obtained from least squares fits of Eq. 3
to erase current data collected at various temperatures.
FIG. 8. Erase time to reduce the conductivity of a device to within 20% of
its OFF state, as a function of temperature, as calculated from value for A
and m obtained from least squares fits of Eq. 3 to erase current data
collected at various temperatures.
5651J. Appl. Phys., Vol. 84, No. 10, 15 November 1998 E. G. Gerstner and D. R. McKenzie
Downloaded 01 Oct 2009 to 132.234.251.211. Redistribution subject to AIP license or copyright; see http://jap.aip.org/jap/copyright.jsp