MODELING OF OPTOELECTRONIC PROCESSES IN SrS:Cu ACTFEL

DISPLAY DEVICES

V.P. Singh

University of Kentucky, Lexington KyA. Garcia

The University of Texas at El Paso, El Paso Tx

A. Aguilera

Hewlett Packard, Ft. Collins Co

D.C. Morton

U.S. Army Research Laboratory

Adelphi, Maryland

-0.004

-0.002

0

0.002

0.004

0.006

0.008

0.01

400 480 560 640 720 800 880 960

Time in Microseconds

a.u

.

Voltage

Current

Luminance

GLASS SUBSTRATE

TRANSPARENT CONDUCTOR

DIELECTRIC STACK

PHOSPHOR

DIELECTRIC STACK

REFLECTIVE ELECTRODE

Va1µm

Typical VIL for ZnS:MnDisplay Device

Device Structure

Insulator

Insulator

Phosphor

Electrode

Electrode Mn

1

24

1) Electron ejection due to high electric field from the cathodic interface2) Electron gains velocity, becoming “hot”3) Electron impact excites activator atom (Mn or Cu)4) Impacting electron reaches anode contributing to the built-in reverse field5) Excited atom relaxes, emitting light

35

e-

SrS:Cu

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0.01

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0 100 200 300 400 500 600 700

Time in Microseconds

a.u

.

Voltage

Current

Luminance

Comparisons

• Secondary Luminance Peaks

• Total Luminance is Small

• Comparable Amount of Tunnel Current

Possible Causes for Low Luminance

• Lack of Tunnel Current (not the case)

• Excessive Impact Ionization - but this can Only Explain low Luminance but does not Explain Secondary Peaks

• “Other” Mechanism Needed to Explain Behavior

Hypothesize

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0 100 200 300 400 500 600 700

Time in Microseconds

a.u

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Voltage

Current

Luminance

We Hypothesize a Mechanism that Causes Luminance with a Field Reduction for a Given Device History

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4.50

350 550 750 950 1150

Time in Microseconds

a.u

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Voltage

Current

Luminance

• Changing the Electric Field without Causing Tunnel Current

Current Characteristics

-2

-1

0

1

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3

4

5

6

7

8

300 500 700 900 1100

Time in Microseconds

Cu

rre

nt

(ma

)

Vapp = 105

Vapp = 120

Vapp = 130

Vapp = 140

Voltage

1

4

1

2

3

4

23

-1

0

1

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5

6

500 700 900 1100 1300

Time in Microseconds

a.u

.

Vapp = 140Vapp = 130

Vapp = 120Vapp = 105

Voltage

1

1

2

2

3

3

4

4

1,2,3

4

Voltage

Luminance Characteristics

• During dent ramp iL(t) 0, however there is an increase in L(t) means that electrons are recaptured by ionized activators

a) Come from Interface State and only Recaptured by nearby Ionized Activators

b) Formed by the Activator and a Defect Produced by the Activators Presence in the Lattice electrically forming a Dipole

Dipoles

• Are Activator-Bulk Trap pair Formed by the Introduction of the Activator in the Phosphor

• Bulk Trap is Localized in Close Vicinity of the Activator

• A Certain Minimum Field is Needed to Create/Maintain the dipole

Positively ChargedActivator

Negatively ChargedTrap

Transient Results

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-1

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0

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1

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2

2.5

0 500 1000 1500 2000 2500 3000 3500 4000 4500

Time in Microseconds

a.u

.

Vapp = 130Vf = 500Hzpw = 200us10/10/99

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

0 500 1000 1500 2000 2500 3000 3500 4000 4500

Time in Microseconds

a.u

.

Vapp = 130Vf = 500Hzpw = 200us10/10/99

Tunnel Current appears in the very first pulse

Luminance becomes noticeable in the 4th voltage pulse

Conclusions

• A more Complete Model is required to study SrS:Cu

• Currently this Dipole Model is able to Explain the Observed Features but

• Further work is required