Auger electrons vs secondary electrons

[superduke1200]

Hello everyone,
the purpose of my question is to find out the way to separate AUGER from secondary electrons. I think? i know the way that each of them is produced but i cannot figure out how can we separate them since both come out of the surface of our material in a SEM let's say experiment. What i think i have understood correctly, is that in a SEM experiment, the secondary electrons come out of the outer in most cases orbitals either by 1) inelastic scattering with the incident beam or 2) in most cases ( 10 times more likely to happen than the 1 case that i just described) by the backscatterd electrons which in their way out of the material ( since the bse electrons tend to go deeper in the material ) excite those outer electrons, which if I am not wrong is the reason why we find most of the secondary electrons at the same place (angle) that we find the bse elctrons. The definition of the AUGER electrons is pretty much everywhere the same and pretty clear i think. And finally why in ELECTRON YIELD vs ELECTRON ENERGY graphs secondary electrons have lower energy than auger electrons? Thanks for your time

 

[ZapperZ]

Look at the energy spectrum of secondary electrons and Auger electrons. Secondary electrons tend to be on a low 10s of eV and continue on with a broad higher energy peak. Auger electrons tend to be in the 100s of eVs and usually have sharp peaks corresponding to the specific core-level transitions.

You should not be seeing both in equal weights in most cases, since one tends to do Auger experiments with primary electrons in the keV range. This is usually outside of the secondary electron energy range that produces greater than 1 secondary electron yield.

Zz.

 

[superduke1200]

Thanks a lot for the immediate response. The way you described it in the first paragraph is something that i have noticed as well, so it was nice to see it being clarified from someone else because it means that i too, have understood it correctly. However i still do not understand why Auger elctrons have in general ten times greater energy than the secondary electrons since both of them originate from the surface of our ( electron or photon )-bombarded material. Does this excess of energy have to do with the fact that, the reason why auger electrons are knocked off the surface is a high energy photon emitted after filling a 1s hole with a 2s electron ( and thus being a high energy one, since the energy gap between the lower orbitals is always big? compared to outer orbital energy gaps ) or is it something else? Or if you wish, let me put it in another, less complicated way: do the photons that cause the auger electrons to come out, have ten times greater energy than the bse electrons or even the incident beam electrons that knock off the secondary ones, like i described in my first post yesterday, or there may be another reason? What i want to say is that, secondary and auger electrons are surface ones. So when one has ten times bigger energy than the other ( =Auger), there must a ten times greater "energy reason" that caused this. Sorry if my english is not so good. Thanks again for your time.

 

[ZapperZ]

This has nothing to do with "surface" or non-surface. It is the ENERGY level that is of significance here. "Core level" means that it is from a deeper energy level. This is what Auger tends to probe. The electrons have to be given a lot of energy to start with so that it can be released and detected clearly (that's why we tend to use x-rays for XPS and Auger).. Otherwise, electrons liberated with low energy are "swallowed" by the noise.

Secondary electrons, on the other hand, are emitted without needed a lot of energy to start with. Look at the energy of the PRIMARY electrons. For the SEY>1, the energy of the primary electrons tends to be less than 1 keV. So how much energy can the secondary have in the first place? This is even worse when the secondary produces more than one electron per incoming primary.

Now, it doesn't mean that when the energy source (photons or electrons) is in the keV range that you don't get any secondary electrons. You probably might get some. But the number is so diminished that, again, they get buried in the noise.

Zz.

 

[DrDu]

I didn't know that also electrons can suffer from BSE.
=> please introduce your abbreviations.

 

[superduke1200]

Well yes, but by saying that it has nothing to do with surface, is like ignoring the work function. By telling that surface has nothing to do with it, is like saying that by doing an XPS analysis for Hydrogen and Carbon you will need the same energy to liberate a K shell electron from the Hydrogen and Carbon. Which i think is wrong since in a Hydrogen atom the work function Φ, is zero if we are talking about single atoms, whereas in a Carbon atom it is certainly bigger than zero. IN an even bigger atom, Fe for example you will have even bigger work function. Why? Because if I am not wrong, one mere electron, considering that we have hydrogen in a sufficient nanoscale sample for an XPS experiment, where we don't have just a single atom, will meet less electrons in its way out of the sample compared to a carbon sample and vastly less than a Fe samle. In other words let's say that we have Fe2O3 and Fe3O4. The symmetry of those materials is exactly the same and that's why their XRD analysis is the same. ONly by using XRD we cannot separate them. In XPS though, you will have a really small difference. Why? Work function again caused by the Fe 2+ and Fe 3+ ions. Just one more electron makes the difference in the case of Fe 2+. S0 a k shell electron in Fe 3+ will be at say, 240 ev while a Fe 2+ will be at 240,1 eV. Because in its way out of a single atom, the core -K shell electron, will meet one more electron in its way out. If core electrons where all about energy and nothing about surface or not, then it would be like saying that at the same time, the same material could have K shell-core electrons with zero work function or greater than zero work function being liberated.
Considering energy: SEM energy is usually about 1000-2000 ev electrons. Xps is in general terms a Xray gun with energy going from 10 up to 1000 ev. And theoretically every single time that you perform a Xps experiment you will have auger electrons which in some cases will bother your image or not depending on the material that you examine. But since auger electrons have hundreds of ev energy they will most likely be electrons originating from the outer orbitals 3s or 2p NOT 1s form example. Statistically though do we expect secondary electrons only in experiments that use INCIDENT ELECTRONS (like tem or sem), or do we expect them in experiments using xray incident beams like AES XPS XRD?

 

[ZapperZ]

What's with the "work function" argument? Compare the values of the work function with the typical binding energy that these are emitted! The work function plays a minor to an insignificant role in affecting the outcome. And you certainly can't use it to argue for why the energies are different between secondary electrons and Auger electrons!

And I think you are confusing the binding energies with "work function" here.

The rest of your post is a jumble of confusion, because I don't know what you are arguing for. Your latest question asked why the energy range/spectrum of secondary electrons are different than Auger electrons. I believe I've explained why. I have no idea why you need to being these other techniques. The fact that XPS and Auger uses higher energy photons exactly matches what I posted before.

Zz.

 

[superduke1200]

First of all excuse for any confusions. It is certainly my wrong and i apologise for that. But allow me to say that i think that i do not confuse at all work function Φ with binding energy. Yes i know that Φ is almost insignificant. Totally insignificant? No way! IF it was you would not be able to separate Fe203 from Fe3O4 in a Xps. It actually is the reason why you can separate them. At least from my knowledge.
Now considering what you suggested yesterday: do we agree that secondary electrons are liberated in most cases by bse electrons? Considering that you did not oppose my original argument i suppose yes. Considering then that bse electrons have usually tsousands of ev energy and secondary up to 100 ev how can you say that secondary electrons do not need enough energy to start with? Exactly opposite to my eyes! Secondary electrons must bee core ones and they certainly need preetty much energy to start with.
Auger? Well since they have ten times bigger than secondary ones energy well, i cannot see how they should not be surface ones or NOT CORE ONES as you might prefare.

 

[ZapperZ]

Thousands of eV?

Have you ever done UV photoemission? Even on a semiconductor/insulator? I have. I can show you, using just 21.2 eV light source, that I can get photoelectrons from a semiconductor and even an insulator. These are NOT core-level electrons! They are electron that either came from a partially-filled conduction band, or from the valence band. These are NOT core-level electrons! They do not have such high binding energy as the ones you probe with XPS/Auger!

Please look at the typical band structure of such material if you do not believe me.

Zz.

 

[superduke1200]

I couldn't agree more with you about what you just said. I totally agree! The thing is that in my exact previous post I didn't ask about anything related with what you just said. I never disagreed or even talked about what you just said. My last statement was completely different and on that exact one, I cannot see what you answered

 

[ZapperZ]

I couldn't agree more with you about what you just said. I totally agree! The thing is that in my exact previous post I didn't ask about anything related with what you just said. I never disagreed or even talked about what you just said. My last statement was completely different and on that exact one, I cannot see what you answered

You were disputing the fact that there can be low-energy electrons in the material that can be liberated. These ARE the electrons involved in secondary electron emissions! I said that they do NOT need such high energy electrons, or photons, to liberate them, and that's why the SEY curve have PRIMARY electrons energies significantly lower than what you would need in XPS or Auger! You seem to think that they ALL need such high energy sources!

Zz.

 

[superduke1200]

I think that there is quite a mess here. So i think that i should post my questions in an a-b-c form so that it is more clear. Before doing that: it is well known that electrons in a single atom tend to "live" in certain energy levels. The core ones which are closer to the nucleus, have the most "negative" energy judging from the way that we measure energy in atomic physics. As we go closer to the surface we "meet" electrons that have less negative energy. What does it mean? That the outer ones need less energy to be liberated. In other words: suppose that we have an incident beam of electrons with energy of 100 ev. When this electron beam meets an atom of any material, many things can happen like: bse electrons, secondary electrons, auger electrons, x rays, temperature raise of the material and other things. In order to liberate an electron at a k shell of aluminum for example which let's suppose has energy of -50 ev, the incident electron which will strike on this specific one, MUST HAVE at least 50 ev of energy in order to liberate that k shell electron. If it will have sufficient energy or not is a whole other story. At the same time this incident 100 ev electron, statistically, can as well directly strike a 3p Aluminum electron with - 8ev energy, which will liberate it with a kinetic energy of 92 ev. Now this 3p electron could be liberated with an incident electron or any electromagnetic photon that has at least 8 ev of energy. Finally i think that after this GENERAL and not super precise example, i have proved that i know that great amounts of energy are not required for liberating any electron. For some of them, which are the core ones, greater energy is required. The outer ones can absorb this great energy and consequently acquire great kinetic energy. But in order to be liberated, no they do not need that great amount of energy. Just their orbital binding energy is enough. I do not think that i could have explained it in words better.

Now let's come back to our main question: AUGER VS SECONDARY. We agree that auger electrons have 100s of ev energy and secondary 10s of ev energy. And because till now not much progress has been made, I am asking a few more things hoping that after that we will finally reach closer to an outcome so that I am not bothering you no more: a) how are secondary electrons emitted b) which is their initial energy IN THE MATERIAL before they get liberated and we finally measure their energy in the 10s of ev range c) are secondary electrons initially in core states or not d) which is the initial energy of auger electrons in the material e) and at last, are auger electrons initially in core states or not.

 

[ZapperZ]

I think that there is quite a mess here. So i think that i should post my questions in an a-b-c form so that it is more clear. Before doing that: it is well known that electrons in a single atom tend to "live" in certain energy levels. The core ones which are closer to the nucleus, have the most "negative" energy judging from the way that we measure energy in atomic physics. As we go closer to the surface we "meet" electrons that have less negative energy. What does it mean? That the outer ones need less energy to be liberated. In other words: suppose that we have an incident beam of electrons with energy of 100 ev. When this electron beam meets an atom of any material, many things can happen like: bse electrons, secondary electrons, auger electrons, x rays, temperature raise of the material and other things. In order to liberate an electron at a k shell of aluminum for example which let's suppose has energy of -50 ev, the incident electron which will strike on this specific one, MUST HAVE at least 50 ev of energy in order to liberate that k shell electron. If it will have sufficient energy or not is a whole other story. At the same time this incident 100 ev electron, statistically, can as well directly strike a 3p Aluminum electron with - 8ev energy, which will liberate it with a kinetic energy of 92 ev. Now this 3p electron could be liberated with an incident electron or any electromagnetic photon that has at least 8 ev of energy. Finally i think that after this GENERAL and not super precise example, i have proved that i know that great amounts of energy are not required for liberating any electron. For some of them, which are the core ones, greater energy is required. The outer ones can absorb this great energy and consequently acquire great kinetic energy. But in order to be liberated, no they do not need that great amount of energy. Just their orbital binding energy is enough. I do not think that i could have explained it in words better.

To be accurate, a solid has "bands of energy" near the Fermi energy, whereas there is such thing as "bands" in single atoms. So already the concept of solids and individual atoms are different. Secondly, these bands are dispersive, i.e. they have a E vs. k dependence, where as core-level states are not, i.e. core-level states are not k-dispersive! So already the bands behave differently from the core level, and the solids are different than the individual atoms.

Now let's come back to our main question: AUGER VS SECONDARY. We agree that auger electrons have 100s of ev energy and secondary 10s of ev energy. And because till now not much progress has been made, I am asking a few more things hoping that after that we will finally reach closer to an outcome so that I am not bothering you no more: a) how are secondary electrons emitted b) which is their initial energy IN THE MATERIAL before they get liberated and we finally measure their energy in the 10s of ev range c) are secondary electrons initially in core states or not d) which is the initial energy of auger electrons in the material e) and at last, are auger electrons initially in core states or not.

(a) Secondary electrons are emitted when the primary electrons interact with either electrons in the conduction band or in the valence band. These are BANDS, not core-level states. Models of this process often invoke random Monte Carlo simulation whereby this interaction transfers part or all of the initial energy to the electrons in the solid. Because of the fact that this involves the conduction/valence band electrons, this is why "low" energy electrons are sufficient.

(b) This is why I asked you to look at the band structure of the material, which will show you the bands of E vs. k. You will not only get the "initial energy" of the electrons in the material before they are liberated, but also their crystal momentum! This map of the low-level dispersion is what we do in ARPES.

(c) see (a).

(d) The initial energy is the whatever the energy state of the core level it is in.

(e) see (d).

Zz.

 

[DrDu]

I think the mechanism behind the generation of secondary electrons, namely from valence electrons (as opposed to core electrons) is Compton scattering.

 

[superduke1200]

Ok now i have to admit that it is much much more clear. The thing is that everything you said about the fact that core electrons are not affected when we create a solid, while all the others are kind of spreading their energies, forming now, valence and conduction bands is something that I am familiar with. And actually i was really surprised when the person that explained me XPS and AUGER techniques ( professor with many years of experience and important achievements in solidstatephysics), insisted on talking about ATOMS and not SOLIDS, a fact that seemed to be wrong to me. That is why i wrote two times about the difference between Fe2O3 and Fe3O4. My argument in that specific issue( which is actually based on what this professor said ), was based clearly on electrons belonging to atoms and not solids. So i think that now you can understand why i insisted on talking all the time about atoms. Thankfull the whole picture is much more clear now, so i think that there are only three things left to ask: a) in which techniques do we expect to have only auger electrons, in which only secondary electrons and in which both of them. When I am saying techniques i mean TEM, SEM, XPS, AUGER etc. b) is it true to say in general, that experiments that involve high energy incident beams are more likely to give auger than secondary and vise versa c) what information do we get about our solid from auger and secondary electrons ( of course i have searched what information can we get, but going from one source to the other, i have encountered some differences so i would like it to be clear if possible ). I know that I am becoming tiresome, but i have to admit that you have really helped me. Thanks again! There is nothing more pleasurable than finally getting to know CORRECTLY AND IN DEPTH something that seemed to be very confusing

 

[ZapperZ]

Ok now i have to admit that it is much much more clear. The thing is that everything you said about the fact that core electrons are not affected when we create a solid, while all the others are kind of spreading their energies, forming now, valence and conduction bands is something that I am familiar with. And actually i was really surprised when the person that explained me XPS and AUGER techniques ( professor with many years of experience and important achievements in solidstatephysics), insisted on talking about ATOMS and not SOLIDS, a fact that seemed to be wrong to me. That is why i wrote two times about the difference between Fe2O3 and Fe3O4. My argument in that specific issue( which is actually based on what this professor said ), was based clearly on electrons belonging to atoms and not solids. So i think that now you can understand why i insisted on talking all the time about atoms. Thankfull the whole picture is much more clear now, so i think that there are only three things left to ask: a) in which techniques do we expect to have only auger electrons, in which only secondary electrons and in which both of them. When I am saying techniques i mean TEM, SEM, XPS, AUGER etc. b) is it true to say in general, that experiments that involve high energy incident beams are more likely to give auger than secondary and vise versa c) what information do we get about our solid from auger and secondary electrons ( of course i have searched what information can we get, but going from one source to the other, i have encountered some differences so i would like it to be clear if possible ). I know that I am becoming tiresome, but i have to admit that you have really helped me. Thanks again! There is nothing more pleasurable than finally getting to know CORRECTLY AND IN DEPTH something that seemed to be very confusing

Y'know, I really wish you learn to break up your paragraph a bit more. Just look at your style of writing. It feels tedious reading such long paragraphs especially when you can easily break it up into various parts.

Secondly, your professor IS CORRECT! Why? Because the techniques that are involved really do often probe the states in the the bands! Those techniques are more sensitive to the core levels! So that's why you can get away with talking about individual atoms. However, it is incorrect to think that these are the ONLY source of electron emission when you employ other techniques! UV-ARPES is one such example where the energy of the photos are meant to study low-lying states below the Fermi energy (often within the first 1 to 2 eV). These low-lying states are the ones involved in secondary electron emission (SEE).

Now, I've asked you, at least twice, to look at the SEY curve. But based on your latest set of questions, I don't now if you have done so, or if you simply do not realize the significance of it. So I have to explain why here.

http://www.stfc.ac.uk/ASTeC/resources/image/jpg/copper-laser-graph.jpg

Use this as example. The SEY curves usually have two "crossover" points, where the yield goes above 1. It is only in between these two points will the secondary electrons be a factor because each incoming primary will kick out more than 1 secondary electron! If you go outside of these two points, i.e. if the primary electron energy is too low or too high, then this phenomenon is suppressed, or at the very least, it is way past the peak of emission yield. It doesn't mean that it is turned off, but the detected signal due to this process will become harder to distinguish from any other phenomena, if present.

Now, guess where the primary electron energy is for a typical Auger process to be detectable? In the high keV! By this time, the SEY for most material (they are different for each material) is already well below 1 or way past the peak yield! The electrons due to SEE can no longer be distinguished from background noise (haven't I said this already?). In that energy regime, the AUGER electrons dominate!

Zz.

 

[superduke1200]

Well

Secondly, your professor IS CORRECT! Why? Because the techniques that are involved really do often probe the states in the the bands! Those techniques are more sensitive to the core levels! So that's why you can get away with talking about individual atoms. However, it is incorrect to think that these are the ONLY source of electron emission when you employ other techniques! UV-ARPES is one such example where the energy of the photos are meant to study low-lying states below the Fermi energy (often within the first 1 to 2 eV). These low-lying states are the ones involved in secondary electron emission (SEE).

I think that this forum is all about learning and accepting to read what someone else claims, regardless his age or experience, regardless if he is right or not. Plus never disrespect or deride nooone. Earlier on, you CORRECTLY say, that atoms and solids are actually different. True. Then I write down for fourth or fifth time about xps and Fe2O3 ( agreeing now after your post about the difference between treating something like an atom or a solid) and you claim that "the techniques that are involved do often probe the states in the brands". Am i supposed to know from all the existing techniques which can probe that? If i was able to do that, well i would be at your very respected level of knowledge, not asking, but answering questions.

Now, I've asked you, at least twice, to look at the SEY curve. But based on your latest set of questions, I don't now if you have done so, or if you simply do not realize the significance of it. So I have to explain why here.

I searched quit a bit about SEY curves and tried to understand it. Inevitably as you as well said, I found out that they are different for each material. What prevents ME then, to suppose, that some material has a graph like the one that you sent, but in the kev range, which consequently would mean that auger and secondary electrons can exist and get detected in the same "technique"?

Now, guess where the primary electron energy is for a typical Auger process to be detectable? In the high keV! By this time, the SEY for most material (they are different for each material) is already well below 1 or way past the peak yield! The electrons due to SEE can no longer be distinguished from background noise (haven't I said this already?). In that energy regime, the AUGER electrons dominate!

Finally I am now sure that unless we do an experiment with primary electrons or photons in the ev range, then we will never see SEY. At least in most cases. May i then ask for example, when I will do a XPS experiment, where i scan from 10-1000ev, what are the electrons that I count? Who tells me if they are secondary electrons or not? And how do I separate in that exact experiment secondary electrons form other elctrons?

Finally. Here is my professor's statement:

"XPS: detects core state electrons, is good for bulk analysis, xps rays penetrate up to 10μm and separates ions like cr3 from cr4, fe2 from fe3..

AUGER spectroscopy: does NOT detect core electrons but outer shell electrons, is good for surface characterisation and Auger electrons ARE OUTER electrons. In a XPS with Fe for example, we will see the normal XPS signal from 1s, 2p etc AND AUGER signal from the d electrons" ( which means that the xps technique is one of those techniques that probes the energy in the bands and allows us to treat our solid like we are looking at individual atoms according to those that you said earlier )!

After that tell me what should I believe? You said that:

The initial energy is the whatever the energy state of the core level it is in.

You claim AUGER are core electrons. My professor says they are surface electrons. And how can someone accept that an electron that originates from a CORE state i.e an AUGER electron according to you, can be great for surface analysis! A big paradox again! How can I not be confused then?

 

[ZapperZ]

Finally I am now sure that unless we do an experiment with primary electrons or photons in the ev range, then we will never see SEY. At least in most cases. May i then ask for example, when I will do a XPS experiment, where i scan from 10-1000ev, what are the electrons that I count? Who tells me if they are secondary electrons or not? And how do I separate in that exact experiment secondary electrons form other elctrons?

You will never get "secondary electrons" with XPS, because you are using photons, not primary electrons.

You also need to look at the energy spectrum to know which regime of the spectrum is dominated by what process. This was something we already discussed way early in this thread, and I thought we had settled it already.

Finally. Here is my professor's statement:

"XPS: detects core state electrons, is good for bulk analysis, xps rays penetrate up to 10μm and separates ions like cr3 from cr4, fe2 from fe3..

AUGER spectroscopy: does NOT detect core electrons but outer shell electrons, is good for surface characterisation and Auger electrons ARE OUTER electrons. In a XPS with Fe for example, we will see the normal XPS signal from 1s, 2p etc AND AUGER signal from the d electrons" ( which means that the xps technique is one of those techniques that probes the energy in the bands and allows us to treat our solid like we are looking at individual atoms according to those that you said earlier )!

After that tell me what should I believe? You said that:

You claim AUGER are core electrons. My professor says they are surface electrons. And how can someone accept that an electron that originates from a CORE state i.e an AUGER electron according to you, can be great for surface analysis! A big paradox again! How can I not be confused then?

I think your professor and I are both talking about the same thing. "core electrons" in my case are electrons not emanating from the conduction/valence band. Your professor refers to the same phrase as something deeper in the atom.

Note that "surface" and "core" electrons are not mutually exclusive! One can obtain core electrons from surfaces! Surface or bulk refers to the real space geometry and location of the material, whereas bands/core/etc. refers to the energy depth! Different dimensions!

Zz.

 

[superduke1200]

You will never get "secondary electrons" with XPS, because you are using photons, not primary electrons.
Could you be a bit more specific about the reason why this would happen? As far as I know we do get secondary electrons for example in a Sem experiment where the incident beam consists of electrons. What changes when the incident beam uses photons?

 

[ZapperZ]

Note the "name" - SECONDARY electrons. It means that this came AFTER the primary, i.e. one or more electrons are ejected after an INITIAL electron came in.

Using photons, you get photoemission!

Zz.

 

[superduke1200]

Why could we not have one or more ejected electrons after an initial photon has come in?
Does a monochromatic radiation of photons with a specific wavelength λ interact in a different way with matter, compared to an incident beam of electrons with the same wavelength?
Let me put it another way :
What would happen if we did a "Xrd" experiment with incident electrons of the same wavelength with the Xrays that are normally used ?
Or "Sem" with photons of the same wavelength instead of using electrons?

 

[superduke1200]

Why could we not have one or more ejected electrons after an initial photon has come in?
Does a monochromatic radiation of photons with a specific wavelength λ interact in a different way with matter, compared to an incident beam of electrons with the same wavelength?
Let me put it another way :
What would happen if we did a "Xrd" experiment with incident electrons of the same wavelength with the Xrays that are normally used ?
Or "Sem" with photons of the same wavelength instead of using electrons?

 

[ZapperZ]

Because they both have different penetration depth AND different interaction mechanisms.

I'm not exactly sure what this thread is all about now. First it was the difference between SEE and Auger electron emission. Now it is SEE and Photoemission? What will it evolve into next? Who has a guess?

Zz.

 

[superduke1200]

Quantum mechanics has one guess for everything! I will post then another thread after your unkind notation

 

[superduke1200]

I didn't know that also electrons can suffer from BSE.
=> please introduce your abbreviations.

DrDu sorry for replying so late at your question. The reason why I wrote that secondary electrons can suffer from bse as well, was based on the things that I came across before posting my question in this thread.
Here is what I had found: When primary electrons collide with the electrons of the atom, some of those which have low binding energy, may escape the atom. These electrons are called secondary electrons and in general it is possible to have more than one SEE per incoming primary high energy electron. Each electron that comes off the atom after a collision with another, high energy one, can theoretically be a secondary electron.
SEE have low energy (50ev) and are emitted from the surface of the specimen since the ones emitted from a bigger depth, are buried in the noise like Zz had noted earlier. That is why SEE are useful for creating IMAGES of our specimen's surface.
SEE are produced with two main mechanisms:
1)When the primary electron beam enters the specimen's surface
2)When the bse come out of the surface.
The second mechanism is ten times more likely to happen than the first one. So we should expect to find many SEE at the places where we find many bse as well.
I hope that was clear enough.