I come across a question when I try to simulate the optical absorption process in extrinsic 6H-SiC semiconductor.
The intrinsic semiconductor can be only excited with above-bandgap light, but the extrinsic one can be excited by sub-bandgap light, as optical absorption occurs at impurity and defect levels within the bandgap. Extrinsic light absorption has higher depth, as the impurity and defect concentration is much lower than intrinsic matrix. Thus more uniform photocurrent can be obtained.
The sample is a semi-insulating 6H-SiC, which is at first a n-type semiconductor, as shallow impurity donors such as nitrogen (N) are unavoidable during growth, then deep level accepter (∼0.7 eV) vanadium (V) is introduced to compensate the N impurity. Since concentration of V is higher, the substrate is an insulator under room temperature. When shined by a 532 nm laser (sub-bandgap light), the electrons trapped at V accepter level will be released and the substrate turns into a photoconductor [1].
I want to simulate the process of light absorption in this extrinsic semiconductor, and I consulted the manual of Silvaco Atlas (Section 3.6.3, April 10, 2018), but I did not find one: the carrier generation/recombination models do not include sub-bandgap absorption. The only optical absorption model assumes that an electron moves from the valence band to the conduction band. It is indeed above-bandgap absorption. Other models such SRH models deal with phonons; Auger recombination models deal with three particles.
Can anyone provide a hint or point out what I have missed? Thanks in advance.
[1] Wu, Qilin, et al. "Initial test of optoelectronic high power microwave generation from 6H-SiC photoconductive switch." IEEE Electron Device Letters 40(7): 1167 – 1170, 2019.
Dear Hanwu,
welcome,
You are right the major photon electron interaction occurs between valence band and conduction band. This because the valence band is normally filled and the conduction band is normally empty. So, the traffic of electrons between the two bands is dominant.
In case of wide gap intrinsic materials the intrinsic concentration is very small such that any excess electrons or holes will be detectable.
This makes the underband gap electron generation process from the traps is sensible.
I think you can yourself build a model for the under band gap optical activity of the material. You need to assume the trap distributions in the energy gap and determine the rates of their occupation in thermal equilibrium and under the excitation.
In principle the rate of photo emission will be determined by the position of the traps and their density.
The generation recombination theory will help you build a model for the optical processes.
When i have more time i will come again to this as it is very interesting.
Best wishes
Dear Hanwu,
welcome,
You are right the major photon electron interaction occurs between valence band and conduction band. This because the valence band is normally filled and the conduction band is normally empty. So, the traffic of electrons between the two bands is dominant.
In case of wide gap intrinsic materials the intrinsic concentration is very small such that any excess electrons or holes will be detectable.
This makes the underband gap electron generation process from the traps is sensible.
I think you can yourself build a model for the under band gap optical activity of the material. You need to assume the trap distributions in the energy gap and determine the rates of their occupation in thermal equilibrium and under the excitation.
In principle the rate of photo emission will be determined by the position of the traps and their density.
The generation recombination theory will help you build a model for the optical processes.
When i have more time i will come again to this as it is very interesting.
Best wishes
Hi Abdelhalim ,
Thank you for the suggestions. I will try to build a model, but as I am more of an engineer who tries to understand how to improve the efficiency of the SiC PCSS than a researcher who focuses on semiconductors, it will be hard. I will try anyway.
I am still reading the technical manual of Silvaco Atlas, and in section 3.9.6 it says there are band structure dependent optoelectronic models. I wll also try in this direction.
Looking forward to your work.
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