Amorphous SiGe:H TFTStructure and Operation

By,

S ARCHANA

Introduction

Objectives

Working of IR touch screen

Basic structure and operation of TFT

Amorphous Si and DOS model

Cross section of a-SiGe:H TFT

Optical characteristics

Photon incidence on tft

Results using TCAD Silvaco

Photo-stress effects

Limitation and future work

Conclusion

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Investigate application of hydrogenated amorphous SiGe thin film transistors (TFTs) as photo sensor for touch sensing displays.

Study the structure, operation and characteristics of a-SiGe:H TFT.

Study the optical characteristics, photo response and stress on device.

Study the results of photon generation of active layer using TCAD Silvaco.

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Touch-Screen displays-user-friendly interface for portable electronic devices and large-sized public displays.

Photo-type touch sensing (a-Si:H)-limitations

Difficult to distinguish between the shadow of, touching object and of a proximate one in bright ambient light.

Touch sensing impossible if on-screen image is black in backlight reflection mode.

Key to solving such problem-infrared (IR) as a touch sensing light source

a-SiGe:H photosensor-good photo sensitivity to the wavelength of IR range.

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Inverted-type OLED integrated TFT

Hydrogen passivated dangling silicon bonds, reduced the defect density

Fairly high carrier mobility

Strong photoconductivity

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PARAMETERS a-Si

Field Effect Mobility

(CM2 /VS)

0.5~1

Process Temperature (0 C) ~3500 C

TRANSPARENCY

(%)

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Density of electronic

states

Three main mechanisms of electron

transport

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Transfer characteristics

under IR and visible light

illumination.

(a)(c): Images from an a-Si visible sensor. (d)(f) images from an a-SiGe IR sensor under various

ambient conditions, the bright [470 lx, (a) and (d)], normal [170

lx, (b) and (e)], and dark [1 lx, (c) and (f)].

(g)The original images of stripe and QR code, and (h) their

scanned touch images captured from the image

grabber.

Double-gated TFT with an etch-back process

SiNx deposited on the glass substrate by

PECVD

a-SiGe:H deposited as an active layer

Mo deposited by sputtering and patterned,

forming source/drain electrodes

SiNx as a gate insulator and top gate

electrode with Mo

a-SiGe:H layer not only activates to the IR but

also to the visible light, causing the significant

photo current

To avoid the noise by visible light bottom gate

made of amorphous Si:H based material was

added

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Photo response curves in a-SiGe:H TFT stacked with

(a) normally white and

(b) normally black LCD panel in order to observe the effect of display content

dependence during the operation of LCD.

Twisted Nematic LCD

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Schematic 4

blocks operation

drawings

(a) Sensor image

when IR

LED was on

(b) sensor image

with IR LED off

(c) Ambient light

cancelled image

by subtracting the

previous two

images.

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Schematic band diagrams of a-SiGe:H double gate TFT for

(a) vertical and

(b) horizontal cross section in the active area with the bias condition of VG < VFB and Vds > 0.

(c) Corresponding region in TFT.

(d) Principle of touch sensing by photosensor embedded in display panel.

2D Simulation using SILVACO

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The photon

is incident from

(a) top-down

direction

(b) bottom-up

directions.

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Transfer characteristics

from the TFT photosensor

with source/drain

electrodes made of

(a) metal

(b) ITO for the purpose of

identifying the

dependence of the photon

incidence directions.

The external light was

given by both the bottom-

up and the top-down

directions.

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Transfer curves under IR

illumination (850 nm) and

dark state with respect to

the thickness of a-SiGe:H

active layer

Dependence of a-SiGe:H

thickness on the change of

photo sensitivity with

varying gate biases at

Vds = 5 V.

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(a) Change in the drain current as a function of stress time under a bias of Vgs=-5 V, Vds=5V with IR light soaking.

(b) The net change in the drain current in a-SiGe:H TFTs as a function of IR light soaking time. The isothermal

annealing was conducted at 50, 75, and 100 C, respectively.

(c) Logarithmic lifetime plots as a function of the reciprocal of the annealing temperature.

(d) The predicted lifetime as a function of the net changes in the drain current at 40 C

with electrical biases and IR light soaking. The captured image from the image grabber is included in the inset.

Low mobility

High dark current

Current a-SiGe:H TFT structure should be

further improved and optimized

Further study of electrical properties

Methods to reduce dark current

Modelling of defect states

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A a-SiGe:H TFT structure and various

properties was studied.

Thus a-SiGe is a strong candidate material

for an IR photosensor for future embedded

touch screen panel.

Device exhibits potential for broad use in

applications of large-sized touch screen

displays such as indoor e-boards as well as

outdoor information displays.

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