What is the Structure of Gallium Oxide?

[Sciencestd]

I have three questions regarding the material Gallium Oxide. I was reading in several articles and they introduced its structure as it has monoclinic structure and it consists tetrahedral and octahedral structures in it. What I can't understand I can connect this structure to the chemical formula: Ga2O3... because in octahedral or tetrahedral the gallium is bonded to 4 or 6 oxygen atoms.

My second question is: how the plane 100 or 001 in the pictures below is crossing the unit cell I can't read this actually?

My last question is regarding the material, that it is a n-type semiconductor... Why it should be n-type if all the oxygen atoms are bonded with gallium atoms? why should be free electrons in the conduction band even there is no doping?!The first image is from the article: https://journals.aps.org/prb/pdf/10.1103/PhysRevB.96.081409
The second image is from the presentation: https://www.spiedigitallibrary.org/...ce-Presentation/10.1117/12.2292778.full?SSO=1

 

[Cthugha]

What I can't understand I can connect this structure to the chemical formula: Ga2O3... because in octahedral or tetrahedral the gallium is bonded to 4 or 6 oxygen atoms.

As you can see in the first figure, the structure is not actually trivial. However, you can relate it to the chemical formula just by counting the atoms in the unit cell. Every atom that is completely within the unit call counts as a full atom. An atom on a surface plane counts as half an atom (since it contributes to two different unit cells). An atom on a surface line counts as a quarter of an atom (since it contributes to four different unit cells) and an atom located at a corner counts as 1/8th of an atom (since it contributes to 8 unit cells).
It might be a bit tedious to actually do the counting in the unit cell in the figure, but when done right, you will find 12 Gallium atoms and 18 Oxygen atoms in total which matches the chemical formula.

My second question is: how the plane 100 or 001 in the pictures below is crossing the unit cell I can't read this actually?

These are the surfaces that form when you cleave the material. The (100)B plane is literally the plane that forms when you cleave the crystal by "removing" the bonds between Ga(II) and O(III). So this is the surface you get when removing all the bonds that cross the line in the figure.

My last question is regarding the material, that it is a n-type semiconductor... Why it should be n-type if all the oxygen atoms are bonded with gallium atoms? why should be free electrons in the conduction band even there is no doping?!

What is your level of prior knowledge on semiconductor physics in general? This question is answered at the very beginning of most courses on semiconductor physics. Maybe it is helpful to have a look at an introductory book.

You have a few free electrons in the conduction band of any semiconductor at finite temperature due to thermal excitation of some electrons to the conduction band. This is why they are semiconductors. Of course the band gap of Gallium Oxide is quite large for a semiconductor, but it is still possible to get thermal excitation to the conduction band. For an intrinsic semiconductor (which means absolutely no doping or defects here), the number of free electrons and holes present will necessarily be the same. However, that does not mean that electron and hole conductivity will be the same. As they originate from different bands their properties may be very different. Within a reasonable approximation these differences are covered by the effective masses of these electrons and holes (which may be derived from the curvature of the bands). Obviously, a small effective mass is good for conductivity and one may get intrinsic semiconductors where conductivity is governed mainly by electrons or mainly by holes.