Stopping Resistance Drift in Phase Change Memory Cells and Analysis of Charge Transport in Stable Amorphous Ge 2Sb2Te5 Md Tashfiq Bin Kashem Raihan Sayeed Khan ABM Hasan Talukder Faruk Dirisaglik and Ali Gokirmak

2025-05-02 0 0 2.46MB 7 页 10玖币

Stopping Resistance Drift in Phase Change Memory Cells and

Analysis of Charge Transport in Stable Amorphous Ge2Sb2Te5

Md Tashfiq Bin Kashem*, Raihan Sayeed Khan*, ABM Hasan Talukder, Faruk Dirisaglik and Ali Gokirmak

Department of Electrical and Computer Engineering, University of Connecticut, Storrs, CT 06269, USA. *Equal contribution.

Email: ali.gokirmak@uconn.edu

Abstract—We stabilize resistance of melt-quenched amorphous

Ge2Sb2Te5 (a-GST) phase change memory (PCM) line cells by

substantially accelerating resistance drift and bringing it to a

stop within a few minutes with application of high electric field

stresses. The acceleration of drift is clearly observable at electric

fields > 26 MV/m at all temperatures (85 K – 300 K) and is

independent of the current forced through the device, which is a

strong function of temperature. The low-field (< 21 MV/m) I-V

characteristics of the stabilized cells measured in 85 K – 300 K

range fit well to a 2D thermally-activated hopping transport

model, yielding hopping distances in the direction of the field

and activation energies ranging from 2 nm and 0.2 eV at 85 K

to 6 nm and 0.4 eV at 300 K. Hopping transport appears to be

better aligned with the field direction at higher temperatures.

The high-field current response to voltage is significantly

stronger and displays a distinctly different characteristic: the

differential resistances at different temperatures extrapolate to a

single point (8.9×10-8 ohm.cm), comparable to the resistivity of

copper at 60 K, at 65.6 ± 0.4 MV/m. The physical mechanisms

that give rise to the substantial increase in current in the high-

field regime also accelerate resistance drift. We constructed

field and temperature dependent conduction models based on

the experimental results and integrated it with our electro-

thermal finite element device simulation framework to analyze

reset, set and read operations of PCM devices.

I. INTRODUCTION

Spontaneous increase of resistance with time in amorphous

phase of phase change materials, known as resistance drift, can

cause erroneous inter-mixing of intermediate states in multi-

level cells and thus act as a bottleneck for denser storage in PCM

[1], [2]. A significant effort has been devoted so far to identify

the cause of drift and minimize it [3]–[8], however a

comprehensive solution has not been produced yet. There is also

a number of different transport mechanisms proposed for

amorphous phase change materials [9]–[13]. In this work, (i) we

experimentally demonstrate substantial acceleration of

resistance drift and stabilization of device resistance in melt-

quenched a-GST line-cells with application of high-field stresses

(> 26 MV/m) in 85 K to 200 K range, (ii) characterize field and

temperature dependent current conduction in stabilized devices,

(iii) construct a 2D temperature dependent hopping transport

model for low-field regime and extract the hopping distances,

hopping angles and activation energies associated with

percolation transport and (iv) construct an empirical model for

electronic conductivity in the high-field regime. We integrate the

resulting electric field and temperature dependent electrical

conductivity for stable a-GST with our finite element simulation

framework to model reset, set and read operations of PCM

devices.

II. DEVICE FABRICATION AND CHARACTERIZATION

GST line cells used for this study are fabricated by co-

sputtering from elemental targets on thermally grown SiO2 atop

Si substrates with tungsten back contacts, patterned using

photo-lithography and reactive ion etching, and capped by

Si3N4, as described in [14] (Fig. 1a). The dimensions of the cells

are: length (l) × width (w) × thickness (th) = ~600-700 nm ×

~70-150 nm × ~50±5 nm. The cells are first crystallized to the

hexagonal close packed (hcp) phase by annealing at 675 K and

then amorphized using a single 100 ns pulse with 50 ns rise and

fall times (Fig. 1b,c) in 85 K to 300 K temperature range in a

Janis ST-500-UHT cryogenic probe station under vacuum

(~0.01 mTorr). Pulse width and rise/fall times are chosen to

minimize reflections and parasitic contributions in the

measurement setup while ensuring amorphization of the cells

without substantial distortion of the waveforms. After the

amorphizing reset pulse, five DC I-V sweeps are performed at

each temperature using an Agilent 4156C parameter analyzer

(0 V to 25 V and back to 0 V in 0.1 V steps) with current

compliance set to 50 nA. The width of the devices in the

analysis and the construction of the models is ~152 nm.

Fig. 1 (a) SEM image of a GST line cell with TiN bottom contacts.

(b) Electrical measurement setup. (c) Voltage across the GST wire

(—) and measured current through the wire (—) during reset pulse.

Function

Generator

Oscilloscope

50 Ω

Parameter Analyzer

GST

1 kΩ

B50 Ω

50 Ω

(a) (c)

050 100 150 200 250

0.0

0.5

1.0

1.5

2.0

2.5

3.0

VGST (V), IGST (mA)

t (ns)

(b)

100 ns

trise = 50 ns

tfall = 50 ns

III. STOPPAGE OF RESISTANCE DRIFT

The first I-V sweeps after reset show a clear hysteresis

behavior with larger hysteresis windows at lower temperatures

(Fig. 2). The subsequent sweeps for T < 200K display

significantly smaller hysteresis windows and devices stabilize

within 3 sweeps (Fig. 3). We observe a stronger response to

electric field above ~21 MV/m, transitioning from a low-field

response to a high-field response, and substantial acceleration

in resistance drift at ~26 MV/m. Devices reach their final

resistance value within minutes with high electric-field stress,

which normally takes place in months without stress (Fig. 4).

As the current compliance is small (50 nA), self-heating and

thermally induced structural relaxation are not expected in the

devices as a whole. However, filamentary conduction is

expected and higher temperatures may be reached along these

filaments. The stress induces changes on the percolation paths,

device resistance increases and fluctuations in current decrease

(Fig. 5). These changes may be due to relaxation of the charges

left in the traps within a-GST as devices quench [4] or

annihilation of unstable trap sites that assist percolation

transport. The first being only due to charge relaxation and the

second being due to movement of unstable atoms in the

structure. Both of these processes would lead to substantial

distortion of the potential profiles and change the trapped-

charge emission rates.

IV. MODELING CHARGE TRANSPORT IN STABLE

AMORPHOUS GST

A. Low field transport

We observe two distinct exponential responses in the I-V

characteristics of the stable cells in the low-field and high-field

regimes (Fig. 7). Low-field response can be modeled as

thermionic emission over a barrier with an activation energy

(Ea), where the symmetry between the forward transmission

Fig. 2 I-V sweeps immediately after amorphization for four different

devices at indicated temperatures. Device dimensions are indicated

in nm as w × l. Current compliance was set to 1 μA for 305K and 50

nA for the rest.

Fig. 3 All five I-V sweeps after amorphization at 85K. Inset shows a

zoomed version of the plot in 11V to 17.5V voltage range, the up and

down arrow indicate the upswing and downswing currents of the first

I-V sweep.

Fig. 4 Bi-logarithmic plot of voltage required for current to reach 0.3

nA versus time at 200K. Required voltage increases following a

power law behavior () before the application of 25V sweep (),

after which the voltage increases substantially (acceleration of drift)

and becomes stable () afterwards (stoppage of drift).

Fig. 5 Current versus time plot showing the effect of electrical stress

and light at 200K. The yellow and grey circles indicate the LED being

on and off during that time period. Inset shows the histograms before

and after the application of high voltage stress from 0 s to 200 ns time

period under dark condition and in 600 ns to 800 ns time interval

under light. The device dimensions are: l ~ 690 nm and w ~ 146 nm.

0 5 10 15 20 25

10-14

10-12

10-10

10-8

10-6

130´690

140´620

130´700

152´710

I (A)

V (V)

305K

200K

150K

85K

w(nm) ´ l(nm)

0 5 10 15 20 25

10-14

10-13

10-12

10-11

10-10

10-9

10-8

12 14 16

I (A)

V (V)

T = 85K

1st

2nd

3rd

4th

5th

I (pA)

V (V)

102103104105106107

6V sweep (before)

25V sweep 1

25V sweep 2

6V sweep (after)

V (V) @ 0.3nA

t (s)

35 days

T = 200K

0 200 400 600 800 1000

10-16

10-15

10-14

10-13

10-12

10-11

Before stress

After stress

I (A)

t (ns)

0 3 6 9 12 15

105

140

Count

I (pA)

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StoppingResistanceDriftinPhaseChangeMemoryCellsandAnalysisofChargeTransportinStableAmorphousGe2Sb2Te5MdTashfiqBinKashem*,RaihanSayeedKhan*,ABMHasanTalukder,FarukDirisaglikandAliGokirmakDepartmentofElectricalandComputerEngineering,UniversityofConnecticut,Storrs,CT06269,USA.*Equalcontribution.Email:al...

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Stopping Resistance Drift in Phase Change Memory Cells and Analysis of Charge Transport in Stable Amorphous Ge 2Sb2Te5 Md Tashfiq Bin Kashem Raihan Sayeed Khan ABM Hasan Talukder Faruk Dirisaglik and Ali Gokirmak

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