TFT_ex12.in : NBIS (Negative Bias Illumination Stress)

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TFT_ex12.in : NBIS (Negative Bias Illumination Stress)

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Required Modules

Victory Device

2D Simulator
2D TFT
2D Chemistry
2D Optical

In this example, we demonstrate the NBIS (Negative Bias Illumination Stress) simulation of In-Ga-Zn-O (IGZO) TFT with an ionization of oxygen vacancy. In Victory Device, we simulate an electrochemical reaction formula for explaining the negative threshold voltage (Vth) shift by the ionization process under the illumination.

The oxygen vacancy ionization is described in a chemical ion reaction formula as “Vo + 2h+ <-> Vo2+, ” where Vo is the neutral oxygen vacancy and Vo2+ is the double positive-charged oxygen vacancy.

Victory Device / Ion Reaction and Transport Simulation

First step is to define each ion’s charge state.

SPECIES formula= “In:Ga:Zn:O ” charge=+2 name= “Vo2+ ”

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

Next step is to define the initial species concentration before reaction in IGZO region.

DOPING region=4 species.name= “Vo ” uniform concentration=1.3e17

DOPING region=4 species.name= “Vo2+ ” uniform concentration=2e16

In addition, a complex refractivity of the IGZO is loaded from a data file of “igzo_nk.dat ” via “index.file ” option of MATERIAL command.

MATERIAL material=IGZO index.file=igzo_nk.dat

For drift-diffusion simulation, we need to define the diffusion coefficient. The ion’s diffusion coefficient is defined in MATERIAL statement. Because the ion species is assumed to be immobile, the small hopping length (species.hop) is employed.

MATERIAL material=IGZO species.name= “Vo ” species.ea=2.36 species.af=1e7 species.hop=1e-6

MATERIAL material=IGZO species.name= “Vo2+ ” species.ea=1.7 species.af=1e7 species.hop=1e-6

Finally, we define the chemical reaction of “Vo + 2h+ <=> Vo2+ ” formula of using REACTION command.

REACTION formula= “Vo + 2h+ <=> Vo2+ ” forward.ea=0.2 forward.rate=1e-20 reverse.ea=1 reverse.rate=1e-25

The simulation starts to get an initial Vth. Then, NBIS simulation proceeds with stress voltage of “Vgs=-10 V + initial Vth ” to maintain the constant stress voltage. The light power [W/cm2] and wavelength [um] of the NBIS are specified by “SET ipw= ” and “SET wL=, ” respectively. Structure files whose names end with “_VD_IdVg*.str ” (* between 1 and 7) include 2D distribution of Vo and Vo2+ concentration at each NBIS stress time. Log files whose names end with “_VD_IdVg*.log ” show the transfer curve at each NBIS stress time. The calculated Vth shift due to the NBIS is saved in a data file whose name ends with “_dvth_vv.dat. ” When the low light power (ex. “SET ipw=1e-5 “) is applied, the Vth shift is small because of the low hole concentration via photo-induced e-h pair generation.