Can anyone explain this to me?

[klaviernista]

Was hoping someone out there could explain the difference between quantum confined (QC) doping and normal doping of semiconductors.

My rather basic understanding is that with normal doping, a carrier (an electron or a hole) is bumped up from the valence band into the conduction band via varius methods, i.e. photoexcitation, electrical current, or the insertion of elements into the semiconductor that either donate or withdraw electrons.

However, QC doping is apparently different. My understanding is that in QC doping a carrier is inserted in the lowest unoccupied quantum confined orbital (LUQCO) as opposed to a surface or trapped state.

OK, you should note a couple things before you answer. I am a chemist, not a semiconductor engineer. If you could explain what the difference is between a surface state and a trapped state in you answer as well as the difference the LUQCO and LUMO (lowest unocupied molecular orbital) I would appreciate it. Also, if you use acronyms, please identify what they stand for, and please exaplin them if possible.

Thanks.

 

[MobiusPizza]

The first thing I need to state is that I am only 18 as a student, but I have a book I just read about quantum dots which I found has the relavent information. I will try to type some info out. Since I don't master in this field I will try my best to get something relavent. The book is huge and covered with many topics in this field. Therefore I cannot garantee what I typed out is what you want.

Anyway this book is a nice read. I shall also spell it out as the author gets the credit not me.
ISBN 0-7167-4517-8
The quantom dot
Richard Turton

It is something to do with superlattices.

Some important definitions:
A superlattice is an artificial structure formed by constructing a large number of quantum wells side by side, seperated by thin barrier layers. Interaction of electrons in neighbouring wells produces a series of allowed minibands.

Quantum Well: An artificial structure in which the carriers are confined in 1 dimention. I.E. the electrons exhibit wavelike properties in one dimension but behave as a free electrons in the other 2 dimensions.

P-type semiconductor: A material doped with acceptor impurities. The majority carriers are therefore holes.

N-type semiconductor: A material doped with donor impurities. The majority carriers are therefore conduction electrons.

Nipi superlattice:
"A superlattice of quite different property previously mentioned, can be formed by a single material, such as gallium arsenide , and doping thin layers alternatively with donars and acceptors. It is usual in such system to separate the doped layers with reagion which are undoped, and the alternate layer of n-type, intrinsic, p-type, intrisic gives rise to the name 'nipi superlattice'"

Main quote:
"Concentrate on a small section of nipi superlattice. Suppose we focus on a thin layer of n-type material somewhere on the middle of this system. On the either side there is a layer of intrinsic material, followed by a thin layer containing p-type doping. Consider solely on the conduction electrons. We can imagine that these are initially confined to the narrow n-type layer. However, the principle of diffusion ensures that they do not stay there for long. The electrons moves outwards, and as they do, some will reach the p-layers and recombine with the holes. This makes the p-type reagion negatively charged, and at the same time the donor ions which are left behind in the n-type layers render this reagions positively charged. As further electrons move across to the p-type layer, it becomes increasingly difficult for the electrons to leave the n-type region. As the electrons are transferred to the p-type reagions, those still contained in the n-type layers need increasingly more energy to escape. They are trapped in this reagion just as effectively as the electrons in gallium arsenide layer sandwiched between two algas layers. If the region is small enough - in another words, if it is comparable with the wavelength of an electron - then quantum effects will be important. This means that we can create a quantum well simply by precise doping of a single type of material.

A similar argument applies if we consider the holes in the valence band. Some of the holes diffuse outwards from the p-type layers and recombine with electrons in the n-type layer. The difference in charge also produces a quantum well to trap the holes. This again appears to be similar to the gallium arsenide quantum well. However, there's a distinct difference. In this case the electrons are confined in one region (the layer doped with donors), while the holes are confined in different regions (those doped with acceptors)"

 

[MobiusPizza]

Oh yeah, if you explain more detailly your knowledge of QC doping I might be able to find more relavent information. From what I've read.

 

[klaviernista]

Thanks for the reply. As I'm sure you realize from your book this topic is extremely complicated.

You wished clarification as to my general knowledge of semiconductor doping, so here goes. I am quite fluent in doping technologies for bulk semiconductors, and I am familiar with most of the terminology associated with that area (i.e. I know what p tyope and n type semiconductors and dopants are, what the HOMO an LUMO are, etc...). I guess I should have been more direct and just asked for an explanation of the distinction between quantum confined states and surface/trapped states. So feel free to throw lingo out at me, but if you use somethign related to quantum confined states thats not intuitively obvious, please explain.

reason I am asking about this is because I've been reading a bunch of articles lately about people who have been doping electrons into II-IV semiconductor nanocrystals by exposing them to a reducing agent, and they report that the electron is in a quantum confined state as opposed to a trapped or surface state. I can post a link to the article(s) if you want, but I'm not sure you'd be able to read them beause that may require a subscription which you probably don't have (the articles are available through the online versions of Nature and Science Magazine).

Its interesting because these articles indicate that these doped nanocrystals exhibit fluoresence which is quenched on the picosecond time frame. The article nots that this has tremendous potential for next generation displays, both because they would allow a reduction in pixel size and because they would allow an exponential increase in refresh rate.

Thanks for your help.

 

[klaviernista]

Hmmm... the book stuff you cited seems to help a bit, but it seems to be desribing a quantum well, which is as far I remember a description of an electron in a trapped state as opposed to a quantum confined state. See where my confusion comes from?