"From your data, is the bandgap of ZnSe direct or indirect?"

In summary: S.P. Sen (2007), p. 456.In summary, the bandgap energy of semiconductor ZnSe was found to be 2.63 eV. The absorption peak at this wavelength was found to be strong.
  • #1
chrononaut 114
10
0
media%2Ff25%2Ff253eb59-4b7a-44ae-a1aa-c46e8fad9684%2FphpeFVIND.png


(urgent)

Hi,

This question was apart of an assignment sheet that I was given in 'Experimental Physics III' after having completed and obtained data for the practical called 'The Bandgap Energy of Semiconductor ZnSe'.
Cheers

Below is some screenshots of the (Matlab-processed) data we obtained, for some context.
1_zpsfuegkzuv.png
2_zpselc7m5ay.png


We observed a cutt-off wavelength at approx λ = 475 nm, and then after calibration using known spectrum of Mercury, we calculated the bandgap of ZnSe with the equation E = hƒ = h*c/λ. Giving E = 2.63 eV. (Where the accepted value is 2.7 eV).

We obtained the data by focusing (with two lenses) the light from a given lamp onto the entrance slit of a monochromator, which came out of the exit slit onto a photodiode, with or without the ZnSe glass 'window'/filter/sample slotted infront of the photodiode. The photodiode was connected to a lock-in amplifier, and an oscilloscope to read the voltage off from.

I added in the orange dashed arrows on the plots to emphasize what the significance of Fig 2 is and what it means with respect to Fig 1. And am I correct in saying that Fig 2 is basically the 'transmission' or the transmittance, or something different?

Thanks
 

Attachments

  • media%2Ff25%2Ff253eb59-4b7a-44ae-a1aa-c46e8fad9684%2FphpeFVIND.png
    media%2Ff25%2Ff253eb59-4b7a-44ae-a1aa-c46e8fad9684%2FphpeFVIND.png
    8 KB · Views: 427
  • 1_zpsfuegkzuv.png
    1_zpsfuegkzuv.png
    25 KB · Views: 551
  • 2_zpselc7m5ay.png
    2_zpselc7m5ay.png
    24.4 KB · Views: 602
  • media%2Ff25%2Ff253eb59-4b7a-44ae-a1aa-c46e8fad9684%2FphpeFVIND.png
    media%2Ff25%2Ff253eb59-4b7a-44ae-a1aa-c46e8fad9684%2FphpeFVIND.png
    8 KB · Views: 1,236
  • 2_zpselc7m5ay.png
    2_zpselc7m5ay.png
    24.4 KB · Views: 1,232
  • 1_zpsfuegkzuv.png
    1_zpsfuegkzuv.png
    25 KB · Views: 1,593
Last edited:
Physics news on Phys.org
  • #2
A direct band gap material will absorb the light of wavelength equal to its band-gap whereas an in-direct band gap material will not. You found the band gap energy to be 2.63 eV so with ## hc=1.239842 \frac{eV}{\mu m}## ##\lambda = \frac{1.2139842}{2.63}=.4714\mu m##. Do you see a strong absorption at that wavelength?
 
  • #3
Plot the absorption coefficient as function of light frequency. The functional dependence on frequency should differ for semiconductors with a direct or indirect band gap. I think the relevant equations can be found in textbooks on semiconductor physics or by searching the internet.
 
  • #4
Fred Wright said:
A direct band gap material will absorb the light of wavelength equal to its band-gap whereas an in-direct band gap material will not.

Thanks for the response, really appreciate it. Ok that makes sense, I think; so if it were an indirect bandgap it wouldn't 'line up' with the absorption peak since it has a different momentum value or something like that?

Fred Wright said:
You found the band gap energy to be 2.63 eV so with ## hc=1.239842 \frac{eV}{\mu m}## ##\lambda = \frac{1.2139842}{2.63}=.4714\mu m##. Do you see a strong absorption at that wavelength?

So, I think I see a strong absorption at that wavelength in the form of the minimum peak shown in Figure 2 of my original post? Is it correct to say that said peak in Fig 2 is an absorption peak?

Thanks
 
  • #5
Lord Jestocost said:
Plot the absorption coefficient as function of light frequency. The functional dependence on frequency should differ for semiconductors with a direct or indirect band gap. I think the relevant equations can be found in textbooks on semiconductor physics or by searching the internet.

Cheers. I found this expression for the absorption coefficient:

image050.png


(from: http://www.pveducation.org/pvcdrom/absorption-coefficient)

It can easily be converted to a function of frequency by using λ = c/f .

But, here, 'k' is referred to as the 'extinction coefficient', which I couldn't find an expression for. Are you familiar with the term 'extinction coefficient'?
Thanks
 
  • #6
I just found this version of an expression for the absorption coefficient:

cbc8418d-5c8c-46f4-9b95-c6be725dcd71.jpg


(from: http://file.scirp.org/Html/2-7700668_17248.htm)

-But it doesn't specify what K is, rather it just says that it is a constant, so not really sure what to do with that
-Here E_g is the bandgap energy, as per usual
-Apparently n depends on the nature of the 'optical transition' (n = 1/2 for direct, n = 2 for indirect bandgap)

A larger screenshot from the website for a bit more context:

23405850_1720440754657081_1593829704380403323_o.jpg
 

Attachments

  • cbc8418d-5c8c-46f4-9b95-c6be725dcd71.jpg
    cbc8418d-5c8c-46f4-9b95-c6be725dcd71.jpg
    5.3 KB · Views: 962
  • 23405850_1720440754657081_1593829704380403323_o.jpg
    23405850_1720440754657081_1593829704380403323_o.jpg
    60 KB · Views: 2,179

1. What is the bandgap of ZnSe?

The bandgap of ZnSe refers to the energy difference between the top of the valence band and the bottom of the conduction band in the material. It is an important parameter in determining the electrical and optical properties of ZnSe.

2. What does it mean for a bandgap to be direct or indirect?

A direct bandgap means that the minimum energy required for an electron to transition from the valence band to the conduction band is the same at all points in the material's momentum space. In an indirect bandgap, the minimum energy required for this transition varies depending on the electron's momentum.

3. How is the bandgap of ZnSe determined from data?

The bandgap of ZnSe can be determined through various experimental techniques such as optical absorption spectroscopy, photoluminescence, and electrical measurements. These techniques provide information on the energy levels and transitions in the material, which can be used to calculate the bandgap.

4. What factors can affect the directness or indirectness of the bandgap in ZnSe?

The directness or indirectness of the bandgap in ZnSe can be influenced by factors such as strain, temperature, and doping. These factors can alter the band structure of the material and affect the minimum energy required for electron transitions.

5. Is the bandgap of ZnSe generally direct or indirect?

ZnSe is known to have an indirect bandgap, but this can vary depending on the specific conditions and properties of the material. It is important to carefully analyze data and conduct further experiments to accurately determine the directness or indirectness of the bandgap in ZnSe.

Back
Top