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Motion of hydrogen ions in the proton memory
Journal of Non-Crystalline Solids 254 (1999) 57±65
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Motion of hydrogen ions in the proton memory V. Girault *, R.A.B. Devine France Telecom±CNET, BP 98, 38243 Meylan, France
Abstract We have performed a series of measurements on pseudo-metal-oxide-semiconductor ®eld eect transistors which form the basis of a non-volatile memory device involving proton motion in the gate oxide. In particular we have studied the electric ®eld induced proton drift. The protons appear, initially, to accelerate in the electric ®eld then subsequently are subjected to a dispersive form of transport due to their interaction with the oxide network. The dispersive property of proton diusion can be modelled assuming a Gaussian distribution of proton mobilities. We obtain evidence for an initial barrier to proton motion when the protons are close to the Si substrate/gate oxide interface corresponding to an electric ®eld 170 kV cmÿ1 . This ®eld is in reasonable agreement with an estimate of the attractive proton/electron inversion layer Coulomb interaction ®eld. Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Recent studies [1±9] have been carried out on a new type of non-volatile memory which uses mobile hydrogen ions, H , encapsulated in the gate oxide of a classical metal-oxide-semiconductor ®eld eect transistor (MOSFET). In the case of an n-channel (p-channel) MOSFET, the memory is comprised of a two-state system obtained by placing all the ions either at the substrate/oxide interface which is the state ON (state `OFF') or at the polysilicon gate/oxide interface yielding the state OFF (ON). The important industrial applications of this new type of memory are primarily related to its smaller energy consumption required to maintain one state of the device and the technological simplicity involved in the manufacture of this type of memory. In fact, once the hydrogen ions have been placed at one interface, they remain
* Corresponding author. Tel.: +33-4 76 76 45 53; fax: +33-4 76 90 34 43; e-mail: [email protected]
essentially immobile and it is no longer necessary to maintain any electrical polarisation to keep them in this con®guration. Moreover, starting with a classical MOSFET device process ¯ow this memory only requires the addition of a simple forming gas (FG) anneal around 650°C to generate the H ions, this anneal underlines the simplicity of the technology. The proton memory eect was initially demonstrated using silicon on insulator (SOI) substrates made by Separation by IMplantation of OXygen (SIMOX) process. In this paper we present the results of a study of the H motion in a SIMOX oxide under applied electrical stress in order to better understand the physics of the proton motion and to be able to predict performance for technological applications. 2. Experimental The substrates used in our experiments were commercial p-type doped silicon (1 0 0) which had
0022-3093/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 3 7 2 - 5
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V. Girault, R.A.B. Devine / Journal of Non-Crystalline Solids 254 (1999) 57±65
been implanted with oxygen ions at 600°C. The ion energy was (190 keV and the dose 1.8 ´ 1018 ions/cm2 . Subsequent to implantation a sample received a 6 h anneal at 1320°C which resulted in the formation of a 400 nm thick buried SiO2 layer and a 200 nm thick top, single crystal Si layer which is `relatively' defect free. Small islands, 3 mm by 1 mm were de®ned in the top Si layer using elementary photolithography and then the rest of the Si was removed by etching in XeF2 gas. The crucial step which allowed the creation of mobile H in the oxide was a forming gas (N2 :H2 :92:8) anneal [1±9] of the structure in the temperature range 550 to 800°C followed by cooling (usually by pulling samples from a furnace to room temperature), this process is now standard for the integration of protons into the buried oxide. Transistors were then formed using the pseudoMOSFET technique [10]. In this device, two needle probes are placed on the top Si layer, forming Schottky barriers with the Si surface which play the roles of the source (S) and the drain (D) contacts of a classical MOSFET, the Si substrate then acts as the gate electrode (G). This gives rise to an inverted form as compared to the classical MOSFET where the source and drain contacts are usually produced in the Si substrate. For simplicity, in the following when we use the term `substrate' for the pseudo-MOSFET we will mean the top Si layer and when we refer to the gate electrode, this will be physically, the Si starting substrate. The electrical `signature' of the device was obtained through IDS (VG ) measurements, where IDS is the current ¯owing between the source and the drain for an applied source/drain voltage, VDS , and VG is the voltage applied to the gate. Two voltage sweeps were used, one beginning at positive voltage, then swept down, the other beginning at negative voltage, then swept up. The voltage shift between the decreasing and the increasing gate voltage ramp indicates the presence of mobile, positively charged species in the oxide. For all measurements, VDS was equal to 0.2 V. The current, IDS , and the voltage applied on the `gate' were measured using an electrometer (Keithley 617), the voltage source was a dc power supply (HP 6628A system) and a computer (HP 900, series 300) was used to pilot the measuring instruments and for
automatic data acquisition. The approximate measurement time per point was 0.43 s and the full sweep time was 234 s. Pre-polarizations at 30 V and ÿ60 V for 300 s were used initially in inversion and subsequently in accumulation. To study time dependence of the proton motion we concentrated on their movement from the top layer, `substrate'/gate oxide interface to the bottom oxide/`gate' interface induced by application of dierent negative voltages on the gate with respect to the source or drain contacts. The sequence of measurements was ®rst to apply a positive voltage on the gate to make all the protons move to the top layer/oxide interface. Then, a negative voltage pulse was applied to the gate and the current IDS between the source and the drain contacts of the pseudo-MOSFET was measured as a function of time of application. This process was carried out for various voltage pulse amplitudes. 3. Results Fig. 1 shows the shape of a typical hysteresis curve obtained for a sample annealed for 21 min at 650°C in forming gas then quenched. We note
Fig. 1. Electrical hysteresis of the proton memory. Inversion layer current versus gate voltage for a pseudo-MOSFET device made with SIMOX (p-type, with a 400 nm thick oxide) annealed in forming gas at 650°C for 21 min. Polarisation was carried out for 15 min at 30 V before the decreasing ramp and ÿ60 V before the increasing one.
V. Girault, R.A.B. Devine / Journal of Non-Crystalline Solids 254 (1999) 57±65
Fig. 2. Inversion layer current versus time measurements for dierent electric ®elds on a SIMOX sample annealed in forming gas at 700°C for 16 min. Before each measurement a positive polarisation VG 70 V was applied on the gate to move all the protons to the top Si/oxide interface.
clearly that the down sweep curve is displaced by 38 V with respect to the up sweep curve. Note that the voltage sequence was such that after sweeping from positive to negative voltages, the potential was maintained for 15 min at ÿ60 V before beginning the up sweep part of the cycle. Fig. 2 shows the results of a series of time dependent measurements for a SIMOX sample annealed in a FG atmosphere at 700°C for 16 min. Here the gate was polarised positively for 16 min at 70 V before application of the negative voltage resulting in proton drift away from the `substrate' towards the gate electrode.
59
gate voltage. This expression is deduced assuming that VG ÿ VT VDS , VG is typically tens of volts whilst VDS 0:2 V so this assumption is valid. The coecient fG l0 Cox VDS can be extracted from the measurements of IDS at time t 0, when the population of H has not yet had time to evolve, i.e. the threshold voltage is unchanged. By plotting the curve IDS (t 0) with respect to the negative gate voltage pulse height, we can extract the threshold voltages at t 0 and the coecient fG l0 Cox VDS , which is shown in Fig. 3. By extrapolation of the threshold voltage to the condition IDS 0, we are able to estimate the number [11] of H species trapped in the oxide per 2 unit area Q H (expressed as a charge per cm at the top layer/oxide interface): Z dox 1 QH q x; tx dx 2 dox 0 VT f T ; Na ; EG ; Cox ÿ
Q H ; Cox
3
where dox is the oxide thickness, q x; t the H charge density in C/cm3 , T the temperature, Na the doping concentration of the Si `substrate' and EG is the energy band gap of SiO2 . In this example, QH has been found to be equal to 3.7´10ÿ7 C/cm2 , which represents an average density of 2.3 ´ 1012 H /cm2 .
4. Discussion 4.1. Estimation of the number of protons The current IDS in the pseudo-MOSFET can be expressed as [10,11] IDS t fG l0 Cox SVDS VG ÿ VT t;
1
where fG is a geometric factor for the pseudoMOSFET taking into account the size of the device, l0 the electron mobility in the inversion layer, Cox the gate/oxide capacitance per unit area, S the surface of the active device area, VDS the voltage between the source and the drain, and VG is the
Fig. 3. Extraction of the threshold voltage at time t 0 for a SIMOX sample annealed in forming gas at 700°C for 16 min. The coecient fG l0 Cox VDS is equal to 2 ´ 10ÿ7 A/V.
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V. Girault, R.A.B. Devine / Journal of Non-Crystalline Solids 254 (1999) 57±65
4.2. Physical situation of the protons A more accurate determination of the exact number of H in the structure involves the calculation of Z dox q x; t dx: 4 Q 0
If we now consider the state of the memory at time t 0, we assume all the protons are still at the top interface and we can de®ne a small thickness, tox , in which the protons are contained. Assuming also a uniform density: Z dox 1 QH t 0 qx dx qtox ; 5 dox dox ÿtox Z Q t 0
tox
dox ÿtox
q dx qtox :
So that for this speci®c case Q t 0 QH t 0:
6
7
it corIf we assume a distance, tox 2 [10, 100 A], responds to a volume density between 2.3 ´ 1019 and 2.3 ´ 1018 H /cm3 . In both cases the equivalent surface density is equal to 2.3 ´ 1012 H /cm2 . The ®rst question which might be asked is whether this density is consistent with `available space' in the SiO2 network. The samples used were á1 0 0ñ oriented which has a surface density for silicon atoms at the substrate/oxide interface equal to 6.8 ´ 1014 cmÿ2 . We will assume that the protons are stabilised by forming a weak bond [7] with a bridging interfacial oxygen atom, itself linked to an interfacial silicon atom creating the bonding:
For simplicity we assume that 90% of the interface silicon atoms at maximum are linked to an
oxygen atom, which corresponds to a minimum density of 6.10 ´ 1014 oxygen/cm2 which are potential proton `trapping' sites (we assume 90% to allow for interface states Si dangling bonds and others which may be >1013 cmÿ2 ). This number is considerably larger than the H surface density estimated from our experiments equal to 2.3 ´ 1012 / cm2 . We can then reasonably assume that the number of oxygen host atoms at the interface is large enough. We might further consider taking into account the available interstitial volume, the free volume available to accept all the protons must then be estimated. Knowing the oxygen density in SiO2 which is equal to 4.4 ´ 1022 /cm3 and the density of SiO2 which is equal to 2.2 g/cm3 , we are able to calculate the `unused' volume occupied by the vacuum in SiO2 , using the hypothesis of oxygen and silicon hard spheres with an atomic respectively, 66% of the radius of 0.65 and 1.46 A, volume of the SiO2 network is then free. The equivalent surface fraction is equal to 0.76, assuming a homogeneous oxide. The surface occupied by the quantity of protons determined earlier can be estimated from atomic calculations [12]. The O±H bond length is calculated to be (0.1 nm so that we can assume a hydrogen radius approximately equal to (0.1 nm ± the O atom radius 0.065 nm) i.e. 0.035 nm. We then obtain a surface fraction of 8.9 ´ 10ÿ5 . Clearly, both free volume and bridging bonding approaches to H trapping at the Si/oxide interface show that ample space is available to allow the take up of the proton numbers (2.3 ´ 1012 H /cm2 ) observed experimentally. 4.3. Time dependence of the source-drain current, IDS (t) Earlier studies by Hofstein [13] and Snow et al. [14] on the motion of sodium ions in silicon dioxide resulting from the application of a negative potential pulse on MOS capacitors lead them to assume a general property of the source/drain current, IDS as an exponential decrease as a function of time [13] and as a (t)1=2 function [14]. Neither dependence describes the experimental data observed in this work, we argue that this deviation is because of a dispersive motion of the
V. Girault, R.A.B. Devine / Journal of Non-Crystalline Solids 254 (1999) 57±65
H species in the oxide so that no simple law can be demonstrated. We consider now the time dependent motion of the protons in the gate oxide. From Eq. (2) Z dox Q 1 oq x; t x dx: 8 d H dt dox 0 ot The current of a charged species in the presence of both the charge density gradient and an electric ®eld is given [14] oq F ÿD lEq: ox
9
Combining this current expression with the equation of continuity, oq=ot ÿoF =ox, we obtain the equation governing the time-dependent charge (proton) density in the oxide oq o2 q o Elq D 2ÿ : ot ox ox
10
We neglect the term of natural diusion since it appears that the motion is exclusively governed by the electric ®eld in this type of experiment. Indeed, it has been reported [3] that the protons are immobile in the oxide in the absence of electrical stress. This hypothesis modi®es the mathematical description with respect to the solution proposed earlier by Hofstein [13] and Snow et al. [14] where they retained the diusion coecient term. Eq. (8) becomes Z dox dQH 1 ÿ m x; tq x; t dx dt dox 0 ÿ
eN m dox
where lE m x; t;
Taking Eq. (11), we deduce d IDS t 1 1 eN ÿ m; dt IDS 0 Cox VG ÿ VT 0 dox
61
14
with E
VG ÿ VT 0 ; dox
Nm d 2 Cox d ÿ ox e dt E
IDS t : IDS 0
15
Fig. 4 shows N m=E as a function of time using our experimental data (Fig. 2) for three dierent electric ®elds. Since these curves do not superpose, the motion of the protons in the oxide must clearly be dispersive, no general linear relation between the proton speed and the electric ®eld exists. In simple terms this may be evidence for the existence of a range of activation energies for the motion of the protons in the presence of an electric ®eld in the SiO2 network. Considering Fig. 4, we observe two types of dependence of the proton velocity. The velocity increases at the beginning of the application of the electric ®eld, this appears to demonstrate that the major eect of the electric force on the protons is to induce the motion without interaction with the oxide network. This regime might be termed `pseudo-ballistic', during
11
where N is the total number of protons per unit area and m is the average speed for the protons in the presence of the electric ®eld E. From Eq. (1) IDS t VG ÿ VT t ; IDS 0 VG ÿ VT 0 d dt
IDS t IDS 0
1 VG ÿ VT 0
12
dVT t ÿ dt
1 1 dQH : Cox VG ÿ VT 0 dt
13
Fig. 4. N m=E as a function of time. Since the curves do not superpose, we conclude that there is a dispersive proton motion in the oxide.
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V. Girault, R.A.B. Devine / Journal of Non-Crystalline Solids 254 (1999) 57±65
Fig. 5. Maximum proton velocity as a function of applied electric ®eld. This maximum denotes the end of the `pseudoballistic' regime during which the velocity of the protons responds linearly to the electric ®eld. The line is drawn as a guide for the eye.
Fig. 6. The product mmax tmax representing the oxide distance crossed by the protons along which the motion is predominantly governed by the electric ®eld without signi®cant interaction with the oxide network.
which the speed responds linearly to the electric ®eld. In Fig. 5 is plotted mmax as a function of the electric ®eld, which demonstrates a linear regime (for these calculations, N was taken to be equal to 2.3 ´ 1012 H /cm2 ). This observation supports the idea that initial H motion occurs rather unimpeded by interaction with the SiO2 network. However, after a short distance, the proton lattice interaction is no longer negligible and the protons slow. Since we detect the proton motion via the variation of IDS with time, we cannot follow dependence beyond the point IDS ! 0. Fig. 6 shows the product m tmax versus the electric ®eld, where tmax is the time at which the proton velocity is observed to be a maximum (Fig. 4). This product gives us an idea about the maximum distance beyond which the proton motion across the oxide becomes more dicult because of an increased interaction between the protons and the oxide network. The average distance (Fig. 6) is around 45 nm which is about 6 1/7 of the total oxide thickness used in our experiments and is certainly an overestimate of the distance. Beyond this distance and in a constant electric ®eld in the oxide, the velocity decreases monotonically. In the present approximation, this change seems to be the only eect of the amorphous oxide structure. The protons interact with the eective attractive potential associated with bridging oxygen atoms
present in the amorphous network resulting in a `slowing down'. We have further modelled the slowing down process assuming it can be simulated as due to a distribution of proton velocities which result from a distribution of proton mobilities arising from the dierent activation energies for motion encountered by the protons as they cross the oxide. We have simply considered that a proton, i, with a mobility, li and a velocity, mi , crosses the oxide following the law xi t li Et mi t:
16
We also de®ne a time, ti , which represents the time necessary for the i-type proton to cross the whole oxide thickness ti
dox : li E
17
A Gaussian distribution of velocities of the protons, centred around an average velocity, m, is taken (based upon the same distribution of li , Eq. (15)) ! Z 2 1 mi ÿ m pi p exp ÿ ; pi dmi 1: 2r2 2pr 18 In our experiments we measure the inversion channel current IDS variation due to the proton motion and from Eqs. (1)±(3) the sum of interest is
V. Girault, R.A.B. Devine / Journal of Non-Crystalline Solids 254 (1999) 57±65
X i
qi xi :
63
19
At the beginning of the measurement, at time t 0, all the protons are at x dox and their contribution to this sum can be expressed, taking into account an homogeneous charge density q, as X X qi dox qpi dox : 20 i
i
Then, as soon as t 6 0, the protons begin to move with dierent speeds and ®nally, the shape of the inversion layer current curve, IDS (t), is governed by the variations of Eqs. (1)±(3) X X t : 21 qi dox ÿ qi xi / pi 1 ÿ ti i i Fig. 7 shows a series of typical IDS (t) simulations using this model ± the corresponding experimental data is included. We have obtained reasonable ®ts to the measured data corresponding to the applied voltages which enables us to de®ne the average velocities of the protons for each electric ®eld. We have determined a linear relation between the average speed and the electric ®eld leading to the determination of an average mobility for the protons in the SIMOX oxide equal to 2 ´ 10ÿ12 cm2 /s/V, this data is
Fig. 7. Modelization of the inversion layer current IDS versus time induced by a population of protons in the gate oxide drifting in an applied electric ®eld. The ®ts, appearing as full lines, are the result of the evolution of a Gaussian distribution of velocities centred on dierent average velocities.
Fig. 8. The average velocities determined from the Gaussian ®ts (Fig. 7) as a function of the applied electric ®eld.
shown in Fig. 8. We also observe that the average speed is not applicable when the electric ®eld is not zero but equal to 1:7 105 V/cm. This ®eld suggests that there is a threshold electric ®eld necessary for proton motion and this threshold is presumably due to the attractive ®eld between the protons and the inversion layer electrons at the beginning of the measurement when all the protons are at the top interface. A simple electrostatic calculation allows us to estimate the attractive electric ®eld acting on the protons at the beginning of the measurement. If we assume a uniform image surface density of electrons r equal to 2.3 ´ 1012 cmÿ2 in the Si top layer when the protons are at the top interface of a pseudo-MOS, we can calculate the electric ®eld acting on a proton placed in the oxide following Gauss's law: r ; E 2e0 eSiO2 where e0 is the vacuum permittivity and eSiO2 is the silicon dioxide permittivity equal to 3.9. The resulting electrostatic force is equal to 8.5 ´ 10ÿ12 N. On the other hand, the electric ®eld equal to 1.7 ´ 105 V/cm resulting from Fig. 8 leads to an electrostatic force equal to 2.7 ´ 10ÿ12 N. Note that we assume that the appropriate e is that of SiO2 . Since the inversion layer is in the Si, we might assume we should use eSi 12 which leads to a force 2:8 10ÿ12 N, somewhat closer to the ex-
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V. Girault, R.A.B. Devine / Journal of Non-Crystalline Solids 254 (1999) 57±65
perimental force. We suggest that these calculations indicate that the protons have ®rst to overcome an attractive force before leaving the oxide/ gate interface under the application of an exterior electric ®eld.
has been estimated to be 2% of the previously given voltage.
4.4. Early time of IDS (t)
The electrical characterisations carried out in the work presented here on the proton memory device have allowed us to better understand the motion of the protons in the buried oxide and their interaction with the amorphous network. The uptake of a signi®cant amount of H is possible because of a large amount of free volume in the oxide and the large numbers of bridging oxygen bonds at the interface. In the interior of the oxide, motion of the protons under electrical stress is perturbed by collisions resulting in dispersive transport. No general law could describe the shape of the inversion layer current, IDS (t), induced in a pseudo-MOSFET due to proton drift in the presence of an applied electric ®eld. The inversion layer current decay curves can be modelled assuming a Gaussian distribution of proton mobilities in the oxide consistent with a dispersion of site to site activation energies which are involved in the motion of the protons. This distribution is indicative of the strength of the interaction of the protons with the oxide network during their transport. Our experiments further suggest that for a distance up to 45 nm from the oxide/`substrate' interface the protons accelerate through the oxide without interaction with the network. This absence of interaction indicates that for this type of memory it would be useful to develop thinner oxides to obtain the faster switching devices. Finally, we have ascertained that because of the Coulomb interaction between the electron inversion layer in the Si substrate and the protons, a barrier to ®eld induced proton diusion exists which is of the order of 170 kV cmÿ1 . This barrier should be absent for p-channel devices.
In Fig. 9 we replot the data of Fig. 2 for short times (t 6 60 s) and examine the beginning of the curves which show an increase of the current before the expected decrease corresponding to the H motion away from the Si top layer/oxide interface. To explain this dependence, we note that the electron mobility in the Si top layer may be modi®ed and reduced due to the electric ®eld induced by the close proximity of the H species to the interface, this modi®cation causes increased electron scattering. Such a phenomenon is well known [11] for the case of longer electric ®elds. However, as soon as the negative electric ®eld is applied, the H species begin to drift away from the top interface, reducing the electric ®eld at the interface and allowing undisturbed motion of the electrons. In terms of the inversion layer current, IDS , at the beginning of the movement of the protons, it will increase as the electron mobility recovers. The degradation of the electron mobility induced by the proton proximity results in an error in the determination of the threshold voltage, this
5. Conclusion
References Fig. 9. The early time of IDS (t) variation showing an initial current increase. This increase is assumed to result from reduced degradation of the electron mobility in the top silicon layer due to reduction of the electric ®eld induced by the presence of the protons at the Si top layer/oxide interface.
[1] K. Vanheusden, A. Stesmans, Appl. Phys. Lett. 64 (1994) 1575. [2] W.L. Warren, K. Vanheusden, J.R. Schwank, D.M. Fleetwood, P.S. Winokur, Appl. Phys. Lett. 68 (1996) 2993.
V. Girault, R.A.B. Devine / Journal of Non-Crystalline Solids 254 (1999) 57±65 [3] W.L. Warren, D.M. Fleetwood, J.R. Schwank, K. Vanheusden, R.A.B. Devine, L.B. Archer, R.M. Wallace, IEEE Trans. Nucl. Sci. 43 (1996) 1789. [4] K. Vanheusden, W.L. Warren, R.A.B. Devine, D.M. Fleetwood, J.R. Schwank, M.R. Shaneyfelt, P.S. Winokur, Z.J. Lemnios, Nature 386 (1997) 587. [5] K. Vanheusden, J.R. Schwank, W.L. Warren, D.M. Fleetwood, R.A.B. Devine, Microelectronic Eng. 36 (1997) 241. [6] K. Vanheusden, W.L. Warren, R.A.B. Devine, D.M. Fleetwood, J.R. Schwank, P.S. Winokur, Z.J. Lemnios, Mater. Res. Soc. Symp. Proc. 446 (1997) 187. [7] K. Vanheusden, W.L. Warren, R.A.B. Devine, J. NonCryst. Solids 216 (1997) 116. [8] K. Vanheusden, S.P. Karna, R.D. Pugh, W.L. Warren, D.M. Fleetwood, R.A.B. Devine, A.H. Edwards, Appl. Phys. Lett. 72 (1998) 28.
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[9] K. Vanheusden, R.A.B. Devine, L.B. Archer, R.M. Wallace, Appl. Phys. Lett. 73 (1998) 674. [10] S. Cristoloveanu, S.S. Li, Electrical Characterization of Silicon-On-Insulator Materials and Devices, Kluwer, Boston, 1995, pp. 104. [11] S.M. Sze, Physics of Semiconductor Devices, Wiley, New York 1981, pp. 392, 440, 449. [12] A.M. Ferreira, S.P. Karna, C.P. Brothers, R.D. Pugh, B.K. Singaraju, K. Vanheusden, W. Warren, R.A.B. Devine, Mater. Res. Soc. Symp. Proc. 446 (1997) 247. [13] S.R. Hofstein, IEEE Trans. Elec. Dev. ED-13 (1966) 222. [14] E.H. Snow, A.S. Grove, B.E. Deal, C.T. Sah, J. Appl. Phys. 36 (1965) 1664.
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Motion of hydrogen ions in the proton memory V. Girault *, R.A.B. Devine France Telecom±CNET, BP 98, 38243 Meylan, France
Abstract We have performed a series of measurements on pseudo-metal-oxide-semiconductor ®eld eect transistors which form the basis of a non-volatile memory device involving proton motion in the gate oxide. In particular we have studied the electric ®eld induced proton drift. The protons appear, initially, to accelerate in the electric ®eld then subsequently are subjected to a dispersive form of transport due to their interaction with the oxide network. The dispersive property of proton diusion can be modelled assuming a Gaussian distribution of proton mobilities. We obtain evidence for an initial barrier to proton motion when the protons are close to the Si substrate/gate oxide interface corresponding to an electric ®eld 170 kV cmÿ1 . This ®eld is in reasonable agreement with an estimate of the attractive proton/electron inversion layer Coulomb interaction ®eld. Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Recent studies [1±9] have been carried out on a new type of non-volatile memory which uses mobile hydrogen ions, H , encapsulated in the gate oxide of a classical metal-oxide-semiconductor ®eld eect transistor (MOSFET). In the case of an n-channel (p-channel) MOSFET, the memory is comprised of a two-state system obtained by placing all the ions either at the substrate/oxide interface which is the state ON (state `OFF') or at the polysilicon gate/oxide interface yielding the state OFF (ON). The important industrial applications of this new type of memory are primarily related to its smaller energy consumption required to maintain one state of the device and the technological simplicity involved in the manufacture of this type of memory. In fact, once the hydrogen ions have been placed at one interface, they remain
* Corresponding author. Tel.: +33-4 76 76 45 53; fax: +33-4 76 90 34 43; e-mail: [email protected]
essentially immobile and it is no longer necessary to maintain any electrical polarisation to keep them in this con®guration. Moreover, starting with a classical MOSFET device process ¯ow this memory only requires the addition of a simple forming gas (FG) anneal around 650°C to generate the H ions, this anneal underlines the simplicity of the technology. The proton memory eect was initially demonstrated using silicon on insulator (SOI) substrates made by Separation by IMplantation of OXygen (SIMOX) process. In this paper we present the results of a study of the H motion in a SIMOX oxide under applied electrical stress in order to better understand the physics of the proton motion and to be able to predict performance for technological applications. 2. Experimental The substrates used in our experiments were commercial p-type doped silicon (1 0 0) which had
0022-3093/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 3 7 2 - 5
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V. Girault, R.A.B. Devine / Journal of Non-Crystalline Solids 254 (1999) 57±65
been implanted with oxygen ions at 600°C. The ion energy was (190 keV and the dose 1.8 ´ 1018 ions/cm2 . Subsequent to implantation a sample received a 6 h anneal at 1320°C which resulted in the formation of a 400 nm thick buried SiO2 layer and a 200 nm thick top, single crystal Si layer which is `relatively' defect free. Small islands, 3 mm by 1 mm were de®ned in the top Si layer using elementary photolithography and then the rest of the Si was removed by etching in XeF2 gas. The crucial step which allowed the creation of mobile H in the oxide was a forming gas (N2 :H2 :92:8) anneal [1±9] of the structure in the temperature range 550 to 800°C followed by cooling (usually by pulling samples from a furnace to room temperature), this process is now standard for the integration of protons into the buried oxide. Transistors were then formed using the pseudoMOSFET technique [10]. In this device, two needle probes are placed on the top Si layer, forming Schottky barriers with the Si surface which play the roles of the source (S) and the drain (D) contacts of a classical MOSFET, the Si substrate then acts as the gate electrode (G). This gives rise to an inverted form as compared to the classical MOSFET where the source and drain contacts are usually produced in the Si substrate. For simplicity, in the following when we use the term `substrate' for the pseudo-MOSFET we will mean the top Si layer and when we refer to the gate electrode, this will be physically, the Si starting substrate. The electrical `signature' of the device was obtained through IDS (VG ) measurements, where IDS is the current ¯owing between the source and the drain for an applied source/drain voltage, VDS , and VG is the voltage applied to the gate. Two voltage sweeps were used, one beginning at positive voltage, then swept down, the other beginning at negative voltage, then swept up. The voltage shift between the decreasing and the increasing gate voltage ramp indicates the presence of mobile, positively charged species in the oxide. For all measurements, VDS was equal to 0.2 V. The current, IDS , and the voltage applied on the `gate' were measured using an electrometer (Keithley 617), the voltage source was a dc power supply (HP 6628A system) and a computer (HP 900, series 300) was used to pilot the measuring instruments and for
automatic data acquisition. The approximate measurement time per point was 0.43 s and the full sweep time was 234 s. Pre-polarizations at 30 V and ÿ60 V for 300 s were used initially in inversion and subsequently in accumulation. To study time dependence of the proton motion we concentrated on their movement from the top layer, `substrate'/gate oxide interface to the bottom oxide/`gate' interface induced by application of dierent negative voltages on the gate with respect to the source or drain contacts. The sequence of measurements was ®rst to apply a positive voltage on the gate to make all the protons move to the top layer/oxide interface. Then, a negative voltage pulse was applied to the gate and the current IDS between the source and the drain contacts of the pseudo-MOSFET was measured as a function of time of application. This process was carried out for various voltage pulse amplitudes. 3. Results Fig. 1 shows the shape of a typical hysteresis curve obtained for a sample annealed for 21 min at 650°C in forming gas then quenched. We note
Fig. 1. Electrical hysteresis of the proton memory. Inversion layer current versus gate voltage for a pseudo-MOSFET device made with SIMOX (p-type, with a 400 nm thick oxide) annealed in forming gas at 650°C for 21 min. Polarisation was carried out for 15 min at 30 V before the decreasing ramp and ÿ60 V before the increasing one.
V. Girault, R.A.B. Devine / Journal of Non-Crystalline Solids 254 (1999) 57±65
Fig. 2. Inversion layer current versus time measurements for dierent electric ®elds on a SIMOX sample annealed in forming gas at 700°C for 16 min. Before each measurement a positive polarisation VG 70 V was applied on the gate to move all the protons to the top Si/oxide interface.
clearly that the down sweep curve is displaced by 38 V with respect to the up sweep curve. Note that the voltage sequence was such that after sweeping from positive to negative voltages, the potential was maintained for 15 min at ÿ60 V before beginning the up sweep part of the cycle. Fig. 2 shows the results of a series of time dependent measurements for a SIMOX sample annealed in a FG atmosphere at 700°C for 16 min. Here the gate was polarised positively for 16 min at 70 V before application of the negative voltage resulting in proton drift away from the `substrate' towards the gate electrode.
59
gate voltage. This expression is deduced assuming that VG ÿ VT VDS , VG is typically tens of volts whilst VDS 0:2 V so this assumption is valid. The coecient fG l0 Cox VDS can be extracted from the measurements of IDS at time t 0, when the population of H has not yet had time to evolve, i.e. the threshold voltage is unchanged. By plotting the curve IDS (t 0) with respect to the negative gate voltage pulse height, we can extract the threshold voltages at t 0 and the coecient fG l0 Cox VDS , which is shown in Fig. 3. By extrapolation of the threshold voltage to the condition IDS 0, we are able to estimate the number [11] of H species trapped in the oxide per 2 unit area Q H (expressed as a charge per cm at the top layer/oxide interface): Z dox 1 QH q x; tx dx 2 dox 0 VT f T ; Na ; EG ; Cox ÿ
Q H ; Cox
3
where dox is the oxide thickness, q x; t the H charge density in C/cm3 , T the temperature, Na the doping concentration of the Si `substrate' and EG is the energy band gap of SiO2 . In this example, QH has been found to be equal to 3.7´10ÿ7 C/cm2 , which represents an average density of 2.3 ´ 1012 H /cm2 .
4. Discussion 4.1. Estimation of the number of protons The current IDS in the pseudo-MOSFET can be expressed as [10,11] IDS t fG l0 Cox SVDS VG ÿ VT t;
1
where fG is a geometric factor for the pseudoMOSFET taking into account the size of the device, l0 the electron mobility in the inversion layer, Cox the gate/oxide capacitance per unit area, S the surface of the active device area, VDS the voltage between the source and the drain, and VG is the
Fig. 3. Extraction of the threshold voltage at time t 0 for a SIMOX sample annealed in forming gas at 700°C for 16 min. The coecient fG l0 Cox VDS is equal to 2 ´ 10ÿ7 A/V.
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V. Girault, R.A.B. Devine / Journal of Non-Crystalline Solids 254 (1999) 57±65
4.2. Physical situation of the protons A more accurate determination of the exact number of H in the structure involves the calculation of Z dox q x; t dx: 4 Q 0
If we now consider the state of the memory at time t 0, we assume all the protons are still at the top interface and we can de®ne a small thickness, tox , in which the protons are contained. Assuming also a uniform density: Z dox 1 QH t 0 qx dx qtox ; 5 dox dox ÿtox Z Q t 0
tox
dox ÿtox
q dx qtox :
So that for this speci®c case Q t 0 QH t 0:
6
7
it corIf we assume a distance, tox 2 [10, 100 A], responds to a volume density between 2.3 ´ 1019 and 2.3 ´ 1018 H /cm3 . In both cases the equivalent surface density is equal to 2.3 ´ 1012 H /cm2 . The ®rst question which might be asked is whether this density is consistent with `available space' in the SiO2 network. The samples used were á1 0 0ñ oriented which has a surface density for silicon atoms at the substrate/oxide interface equal to 6.8 ´ 1014 cmÿ2 . We will assume that the protons are stabilised by forming a weak bond [7] with a bridging interfacial oxygen atom, itself linked to an interfacial silicon atom creating the bonding:
For simplicity we assume that 90% of the interface silicon atoms at maximum are linked to an
oxygen atom, which corresponds to a minimum density of 6.10 ´ 1014 oxygen/cm2 which are potential proton `trapping' sites (we assume 90% to allow for interface states Si dangling bonds and others which may be >1013 cmÿ2 ). This number is considerably larger than the H surface density estimated from our experiments equal to 2.3 ´ 1012 / cm2 . We can then reasonably assume that the number of oxygen host atoms at the interface is large enough. We might further consider taking into account the available interstitial volume, the free volume available to accept all the protons must then be estimated. Knowing the oxygen density in SiO2 which is equal to 4.4 ´ 1022 /cm3 and the density of SiO2 which is equal to 2.2 g/cm3 , we are able to calculate the `unused' volume occupied by the vacuum in SiO2 , using the hypothesis of oxygen and silicon hard spheres with an atomic respectively, 66% of the radius of 0.65 and 1.46 A, volume of the SiO2 network is then free. The equivalent surface fraction is equal to 0.76, assuming a homogeneous oxide. The surface occupied by the quantity of protons determined earlier can be estimated from atomic calculations [12]. The O±H bond length is calculated to be (0.1 nm so that we can assume a hydrogen radius approximately equal to (0.1 nm ± the O atom radius 0.065 nm) i.e. 0.035 nm. We then obtain a surface fraction of 8.9 ´ 10ÿ5 . Clearly, both free volume and bridging bonding approaches to H trapping at the Si/oxide interface show that ample space is available to allow the take up of the proton numbers (2.3 ´ 1012 H /cm2 ) observed experimentally. 4.3. Time dependence of the source-drain current, IDS (t) Earlier studies by Hofstein [13] and Snow et al. [14] on the motion of sodium ions in silicon dioxide resulting from the application of a negative potential pulse on MOS capacitors lead them to assume a general property of the source/drain current, IDS as an exponential decrease as a function of time [13] and as a (t)1=2 function [14]. Neither dependence describes the experimental data observed in this work, we argue that this deviation is because of a dispersive motion of the
V. Girault, R.A.B. Devine / Journal of Non-Crystalline Solids 254 (1999) 57±65
H species in the oxide so that no simple law can be demonstrated. We consider now the time dependent motion of the protons in the gate oxide. From Eq. (2) Z dox Q 1 oq x; t x dx: 8 d H dt dox 0 ot The current of a charged species in the presence of both the charge density gradient and an electric ®eld is given [14] oq F ÿD lEq: ox
9
Combining this current expression with the equation of continuity, oq=ot ÿoF =ox, we obtain the equation governing the time-dependent charge (proton) density in the oxide oq o2 q o Elq D 2ÿ : ot ox ox
10
We neglect the term of natural diusion since it appears that the motion is exclusively governed by the electric ®eld in this type of experiment. Indeed, it has been reported [3] that the protons are immobile in the oxide in the absence of electrical stress. This hypothesis modi®es the mathematical description with respect to the solution proposed earlier by Hofstein [13] and Snow et al. [14] where they retained the diusion coecient term. Eq. (8) becomes Z dox dQH 1 ÿ m x; tq x; t dx dt dox 0 ÿ
eN m dox
where lE m x; t;
Taking Eq. (11), we deduce d IDS t 1 1 eN ÿ m; dt IDS 0 Cox VG ÿ VT 0 dox
61
14
with E
VG ÿ VT 0 ; dox
Nm d 2 Cox d ÿ ox e dt E
IDS t : IDS 0
15
Fig. 4 shows N m=E as a function of time using our experimental data (Fig. 2) for three dierent electric ®elds. Since these curves do not superpose, the motion of the protons in the oxide must clearly be dispersive, no general linear relation between the proton speed and the electric ®eld exists. In simple terms this may be evidence for the existence of a range of activation energies for the motion of the protons in the presence of an electric ®eld in the SiO2 network. Considering Fig. 4, we observe two types of dependence of the proton velocity. The velocity increases at the beginning of the application of the electric ®eld, this appears to demonstrate that the major eect of the electric force on the protons is to induce the motion without interaction with the oxide network. This regime might be termed `pseudo-ballistic', during
11
where N is the total number of protons per unit area and m is the average speed for the protons in the presence of the electric ®eld E. From Eq. (1) IDS t VG ÿ VT t ; IDS 0 VG ÿ VT 0 d dt
IDS t IDS 0
1 VG ÿ VT 0
12
dVT t ÿ dt
1 1 dQH : Cox VG ÿ VT 0 dt
13
Fig. 4. N m=E as a function of time. Since the curves do not superpose, we conclude that there is a dispersive proton motion in the oxide.
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V. Girault, R.A.B. Devine / Journal of Non-Crystalline Solids 254 (1999) 57±65
Fig. 5. Maximum proton velocity as a function of applied electric ®eld. This maximum denotes the end of the `pseudoballistic' regime during which the velocity of the protons responds linearly to the electric ®eld. The line is drawn as a guide for the eye.
Fig. 6. The product mmax tmax representing the oxide distance crossed by the protons along which the motion is predominantly governed by the electric ®eld without signi®cant interaction with the oxide network.
which the speed responds linearly to the electric ®eld. In Fig. 5 is plotted mmax as a function of the electric ®eld, which demonstrates a linear regime (for these calculations, N was taken to be equal to 2.3 ´ 1012 H /cm2 ). This observation supports the idea that initial H motion occurs rather unimpeded by interaction with the SiO2 network. However, after a short distance, the proton lattice interaction is no longer negligible and the protons slow. Since we detect the proton motion via the variation of IDS with time, we cannot follow dependence beyond the point IDS ! 0. Fig. 6 shows the product m tmax versus the electric ®eld, where tmax is the time at which the proton velocity is observed to be a maximum (Fig. 4). This product gives us an idea about the maximum distance beyond which the proton motion across the oxide becomes more dicult because of an increased interaction between the protons and the oxide network. The average distance (Fig. 6) is around 45 nm which is about 6 1/7 of the total oxide thickness used in our experiments and is certainly an overestimate of the distance. Beyond this distance and in a constant electric ®eld in the oxide, the velocity decreases monotonically. In the present approximation, this change seems to be the only eect of the amorphous oxide structure. The protons interact with the eective attractive potential associated with bridging oxygen atoms
present in the amorphous network resulting in a `slowing down'. We have further modelled the slowing down process assuming it can be simulated as due to a distribution of proton velocities which result from a distribution of proton mobilities arising from the dierent activation energies for motion encountered by the protons as they cross the oxide. We have simply considered that a proton, i, with a mobility, li and a velocity, mi , crosses the oxide following the law xi t li Et mi t:
16
We also de®ne a time, ti , which represents the time necessary for the i-type proton to cross the whole oxide thickness ti
dox : li E
17
A Gaussian distribution of velocities of the protons, centred around an average velocity, m, is taken (based upon the same distribution of li , Eq. (15)) ! Z 2 1 mi ÿ m pi p exp ÿ ; pi dmi 1: 2r2 2pr 18 In our experiments we measure the inversion channel current IDS variation due to the proton motion and from Eqs. (1)±(3) the sum of interest is
V. Girault, R.A.B. Devine / Journal of Non-Crystalline Solids 254 (1999) 57±65
X i
qi xi :
63
19
At the beginning of the measurement, at time t 0, all the protons are at x dox and their contribution to this sum can be expressed, taking into account an homogeneous charge density q, as X X qi dox qpi dox : 20 i
i
Then, as soon as t 6 0, the protons begin to move with dierent speeds and ®nally, the shape of the inversion layer current curve, IDS (t), is governed by the variations of Eqs. (1)±(3) X X t : 21 qi dox ÿ qi xi / pi 1 ÿ ti i i Fig. 7 shows a series of typical IDS (t) simulations using this model ± the corresponding experimental data is included. We have obtained reasonable ®ts to the measured data corresponding to the applied voltages which enables us to de®ne the average velocities of the protons for each electric ®eld. We have determined a linear relation between the average speed and the electric ®eld leading to the determination of an average mobility for the protons in the SIMOX oxide equal to 2 ´ 10ÿ12 cm2 /s/V, this data is
Fig. 7. Modelization of the inversion layer current IDS versus time induced by a population of protons in the gate oxide drifting in an applied electric ®eld. The ®ts, appearing as full lines, are the result of the evolution of a Gaussian distribution of velocities centred on dierent average velocities.
Fig. 8. The average velocities determined from the Gaussian ®ts (Fig. 7) as a function of the applied electric ®eld.
shown in Fig. 8. We also observe that the average speed is not applicable when the electric ®eld is not zero but equal to 1:7 105 V/cm. This ®eld suggests that there is a threshold electric ®eld necessary for proton motion and this threshold is presumably due to the attractive ®eld between the protons and the inversion layer electrons at the beginning of the measurement when all the protons are at the top interface. A simple electrostatic calculation allows us to estimate the attractive electric ®eld acting on the protons at the beginning of the measurement. If we assume a uniform image surface density of electrons r equal to 2.3 ´ 1012 cmÿ2 in the Si top layer when the protons are at the top interface of a pseudo-MOS, we can calculate the electric ®eld acting on a proton placed in the oxide following Gauss's law: r ; E 2e0 eSiO2 where e0 is the vacuum permittivity and eSiO2 is the silicon dioxide permittivity equal to 3.9. The resulting electrostatic force is equal to 8.5 ´ 10ÿ12 N. On the other hand, the electric ®eld equal to 1.7 ´ 105 V/cm resulting from Fig. 8 leads to an electrostatic force equal to 2.7 ´ 10ÿ12 N. Note that we assume that the appropriate e is that of SiO2 . Since the inversion layer is in the Si, we might assume we should use eSi 12 which leads to a force 2:8 10ÿ12 N, somewhat closer to the ex-
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V. Girault, R.A.B. Devine / Journal of Non-Crystalline Solids 254 (1999) 57±65
perimental force. We suggest that these calculations indicate that the protons have ®rst to overcome an attractive force before leaving the oxide/ gate interface under the application of an exterior electric ®eld.
has been estimated to be 2% of the previously given voltage.
4.4. Early time of IDS (t)
The electrical characterisations carried out in the work presented here on the proton memory device have allowed us to better understand the motion of the protons in the buried oxide and their interaction with the amorphous network. The uptake of a signi®cant amount of H is possible because of a large amount of free volume in the oxide and the large numbers of bridging oxygen bonds at the interface. In the interior of the oxide, motion of the protons under electrical stress is perturbed by collisions resulting in dispersive transport. No general law could describe the shape of the inversion layer current, IDS (t), induced in a pseudo-MOSFET due to proton drift in the presence of an applied electric ®eld. The inversion layer current decay curves can be modelled assuming a Gaussian distribution of proton mobilities in the oxide consistent with a dispersion of site to site activation energies which are involved in the motion of the protons. This distribution is indicative of the strength of the interaction of the protons with the oxide network during their transport. Our experiments further suggest that for a distance up to 45 nm from the oxide/`substrate' interface the protons accelerate through the oxide without interaction with the network. This absence of interaction indicates that for this type of memory it would be useful to develop thinner oxides to obtain the faster switching devices. Finally, we have ascertained that because of the Coulomb interaction between the electron inversion layer in the Si substrate and the protons, a barrier to ®eld induced proton diusion exists which is of the order of 170 kV cmÿ1 . This barrier should be absent for p-channel devices.
In Fig. 9 we replot the data of Fig. 2 for short times (t 6 60 s) and examine the beginning of the curves which show an increase of the current before the expected decrease corresponding to the H motion away from the Si top layer/oxide interface. To explain this dependence, we note that the electron mobility in the Si top layer may be modi®ed and reduced due to the electric ®eld induced by the close proximity of the H species to the interface, this modi®cation causes increased electron scattering. Such a phenomenon is well known [11] for the case of longer electric ®elds. However, as soon as the negative electric ®eld is applied, the H species begin to drift away from the top interface, reducing the electric ®eld at the interface and allowing undisturbed motion of the electrons. In terms of the inversion layer current, IDS , at the beginning of the movement of the protons, it will increase as the electron mobility recovers. The degradation of the electron mobility induced by the proton proximity results in an error in the determination of the threshold voltage, this
5. Conclusion
References Fig. 9. The early time of IDS (t) variation showing an initial current increase. This increase is assumed to result from reduced degradation of the electron mobility in the top silicon layer due to reduction of the electric ®eld induced by the presence of the protons at the Si top layer/oxide interface.
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