# Refraction of IR "light" in CCD sensors SiO2 layer

• Balint Kovats
In summary, the article explains that the photoelectric effect is dependent on the wavelength of the photons that are impacting the photodiode. If the photons have a higher energy than the bandgap energy, then an electron is excited and the CCD is sensitive to IR light. The article also explains that the depth at which 90% of photons are absorbed is dependent on the wavelength of the photons.
Balint Kovats
Dear friends I am new at this forum thank you for accepting my application first of all.

My question is that I don't understand the optics/physics behind the reason why Si-based CCDs are not sensitive for IR-light (above 1000-1100 nm) if on the top of the p-type Si there is a SiO2 layer which has a refractive index of 1.449 at 1100 nm wavelength. Taking into consideration that EM waves entering into a new medium because of their frequency is constant their wavelenght and speed will change can we calculate in this way: wavelength = speed of light in vacuum/frequency >> frequency = speed of light in vacuum/wavelength in our case f = 3*10^8[m/s]/1100*10^-9[m] = 2,73*10^14 [1/s] and as written in this post (https://www.physicsforums.com/threads/changing-wavelength-of-light-ps-hello.1833/) new wavelength in the new medium = speed of light in vacuum/frequency*refraction index >> new wavelength in the new medium = 3*10^8[m/s]/2,73*10^14[1/s]*1,449 = 7,58*10^-7 [m] = 758 [nm] There we go: if the Si is perfectly coated with SiO2 layer why isn't the incoming IR-light impacting the surface of the Si at 758 nm wavelength and so it should generate photoelectric effects and the CCD should be sensitive for that range too. Can anyone explain this? Is there something wrong with my knowledge about CCD manufacturing or the basic optical principles?

Thank you very much

rbelli1 said:
This has to do with the critical wavelength of the photoelectric effect.

http://hamamatsu.magnet.fsu.edu/articles/quantumefficiency.html

BoB

Whoa! 1100 nm light penetrates 7600 μm into the silicon! That's most of the way through the chip!

Balint Kovats said:
Can anyone explain this? Is there something wrong with my knowledge about CCD manufacturing or the basic optical principles?

From the article linked by rbelli1:

In cases where the photon energy is greater than the band gap energy, an electron has a high probability of being excited into the conduction band, thus becoming mobile. This interaction is also known as the photoelectric effect, and is dependent upon a critical wavelength above which photons have insufficient energy to excite or promote an electron positioned in the valence band and produce an electron-hole pair. When photons exceed the critical wavelength (usually beyond 1100 nanometers), band gap energy is greater than the intrinsic photon energy, and photons pass completely through the silicon substrate. Table 1 lists the depth (in microns) at which 90 percent of incident photons are absorbed by a typical CCD.

Note that the energy per photon is not wavelength dependent, but frequency dependent. This is a requirement of energy conservation. Otherwise light would gain or lose energy as it passed through mediums of different refractive indices.

Drakkith said:
That's most of the way through the chip!

That's all the way through a handful of chips. It seems that for silicon something on the order of a microchip's thickness is essentially transparent.

BoB

Drakkith
Thank you for the great article and explanation rbelli1 and Drakkith. What do you think is there any way to increase somehow the energy of a single photon or a beam of photons higher than the band gap energy to make them energized enough to cause photoelectric effect in Si?

Balint Kovats said:
What do you think is there any way to increase somehow the energy of a single photon or a beam of photons higher than the band gap energy to make them energized enough to cause photoelectric effect in Si?

Not that I know of. We usually have to choose a different material with a different bandgap energy if we want to detect IR beyond 1100 nm.

You could use a phosphor coating and increase your illuminance to induce two photon fluorescence. Are you in control of the light source? Can your subject withstand the additional infrared irradiance?

BoB

Yes I am in control of the source and the subject can withstand the IR radiance, though inducing two photon fluoresence does change the energy of a single photon?

And to get back to the original question: on the surface of the Si photodiode the SiO2 coating should change the visible light's wavelength too based on the above mentioned optical equations. Is there an other medium between the photodiode and the SiO2 layers in which the light's wavelength changes back for example air or vacuum?

The changes are all cumulative. The wavelength in the silicon is the same no matter how many different materials it passes through. The intensity may be altered due to absorption.

BoB

## 1. What is the SiO2 layer in a CCD sensor?

The SiO2 layer, also known as silicon dioxide or silica, is a thin layer of silicon dioxide that is present on top of the CCD sensor. This layer is essential for the proper functioning of the sensor as it helps to prevent light from leaking into neighboring pixels.

## 2. How does the SiO2 layer affect the refraction of IR light in CCD sensors?

The SiO2 layer has a high refractive index, meaning it can bend or refract light at a greater angle compared to air. This affects the refraction of IR light in CCD sensors by allowing more light to reach the sensor and improving the sensor's sensitivity to IR light.

## 3. Why is IR light important in CCD sensors?

IR light is important in CCD sensors because it can capture images in low light conditions and can reveal details that are not visible to the naked eye. It is also used in various applications such as night vision, security cameras, and astronomy.

## 4. How does the thickness of the SiO2 layer affect the refraction of IR light in CCD sensors?

The thickness of the SiO2 layer is crucial in determining the refraction of IR light in CCD sensors. A thicker layer will cause more refraction, resulting in better sensitivity to IR light. However, if the layer is too thick, it can also cause unwanted reflections and distortions in the image.

## 5. Can the SiO2 layer be modified to improve the refraction of IR light in CCD sensors?

Yes, the SiO2 layer can be modified through a process called doping, where impurities are added to the layer to change its properties. Doping can be used to improve the refraction of IR light in CCD sensors by altering the refractive index of the layer. It can also help reduce unwanted reflections and improve the overall performance of the sensor.

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