Every software package contains a full set of examples suitable for that version and are installed with the software. If you see examples here that are not in your installation you should consider updating to a later version of the software. These examples can be used by beginners and advanced users for product evaluation, teaching purposes, and self-training.

Required Modules

Victory Device

• 2D Simulator
• 2D TFT
• 2D Chemistry

In this example, we demonstrate the Positive Bias Temperature Stress simulation due to electron trapping by oxygen related trap. In Victory Device, we can make an electrochemical reaction formula for explaining the positive threshold voltage shift by electron trapping. This electron trapping state is defined as “Oxygen interstitial ” which is acceptor-like state when empty. This can be formulated as “O (neutral) + e <=> O- (single ionized negative charged state) or O2 (neutral) + 2e- <=> O2- (double ionized charged state). ” In this example, we just demonstrate the single ionized charged state. In the other side, we can think of Vo2+ + 2e- <=> Vo reaction, but we omit this electron trapping reaction in this example. So, ionized oxygen vacancy (Vo2+) remains as donor-like trap (NGD).

• Structure formation using Victory Device syntax

• Initial doping concentration of each species for each neutral oxygen and ionized oxygen.

• Material and model settings for bottom gate passivated In-Ga-Zn-O (IGZO TFT).

• Initial DOS (Density Of State) in a-IGZO bulk.

• Reaction formula for electron trapping.

• Initial threshold voltage extraction before applying PBTS.

• Simulation of Equilibrium concentration.

• Positive Bias Temperature Stress Simulation.

• Final threshold voltage shift after applying PBTS.

The bottom gate oxide TFT is initialized by glass substrate thickness of 0.5um. The gate insulator is composed of 0.1um-thick nitride (SiNx) and 0.15um-thick oxide (SiOx). The IGZO channel thickness is 0.05um and then passivation layer (SiOx) is 0.3um. The gate length is 20um and the device width is 20um.

After constructing bottom gate IGZO TFT structure, we define each ion using SPECIES command.

SPECIES formula= “In:Ga:Zn:O ” charge=0 name= “O ”

SPECIES formula= “In:Ga:Zn:O ” charge=-1 name= “O- ”

The formula just defines the element composition of IGZO material. In this simulation, each metal cation (In, Ga, Zn) is not reacted with other species. We will use “name ” flag to distinguish each species. name= “O ” defines the neutral trap state of IGZO configuration. The name= “O- ” defines the ionized charged trap state after reaction.

DOPING region=4 species.name= “O ” uniform concentration=1e17

DOPING region=4 species.name= “O- ” uniform concentration=1e16

which define the initial concentration of each ion species before applying PBTS.

We also added the interface trap concentration using INTERFACE command.

INTERFACE s.i x.min=-$l/2-3 x.max=$l/2+3 y.min=-0.26 y.max=-0.24 species.name= “O ” concentration=2e11

INTERFACE s.i x.min=-$l/2-3 x.max=$l/2+3 y.min=-0.26 y.max=-0.24 species.name= “O- ” concentration=0.2e11

For drift-diffusion simulation of ion during applied bias and temperature, we also need to define the diffusion coefficient as follows.

MATERIAL material=IGZO species.name= “O ” species.ea=1.14 species.af=1e7 species.hop=1e-6

MATERIAL material=IGZO species.name= “O- ” species.ea=1.14 species.af=1e7 species.hop=1e-6

Next, we define the chemical reaction formula using REACTION command.

REACTION formula= “O + e- <=> O- ” forward.ea=0.83 forward.rate=1e-7 reverse.ea=0 reverse.rate=1e-7

PROBE statements measure the concentration of each ion and the occupation probability during stress time.

PROBE x=0.0 y=-0.25 material=IGZO ft.species= “O ” name= “ft_O_front ”

PROBE x=0.0 y=-0.25 material=IGZO species.name= “O ” name= “O_front ”

PROBE x=0.0 y=-0.25 material=IGZO ft.species= “O- ” name= “ft_O-_front ”

PROBE x=0.0 y=-0.25 material=IGZO species.name= “O- ” name= “O-_front ”

To compare the threshold voltage shift, Vth shift, by PBTS, we have simulated the initial transfer curve to obtain initial Vth. After obtaining the initial Vth, we applied actual positive gate bias, and it continue up to 5e3 sec stress time. At each stress time, the solution structure is saved for later use in order to get final Vth shift.
After obtaining forward reaction simulation, we continue to get a backward reaction (reverse recovery) applying zero voltage at gate.
To control the amount of Vth shift and how fast ion react with stress time, user can fit the activation energy (forward.ea/reverse.ea) and reaction coefficient (forward.rate/reverse.rate) in the forward reaction and reverse reaction.

This example demonstrates the positive Vth shift by forward reaction and negative shift from stressed voltage by reverse reaction.

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