jap1998 nonvolatilememory ac nitrogendoped

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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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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


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