Advanced electronic device structures using semiconductor structures and superlattices
Semiconductor structures and methods for forming those semiconductor structures are disclosed. For example, a p-type or n-type semiconductor structure is disclosed. The semiconductor structure has a polar crystal structure with a growth axis that is substantially parallel to a spontaneous polarization axis of the polar crystal structure. The semiconductor structure changes in composition from a wider band gap (WBG) material to a narrower band gap (NBG) material or from a NBG material to a WBG material along the growth axis to induce p-type or n-type conductivity.
1. A method of forming a p-type or n-type semiconductor superlattice comprising a plurality of unit cells each comprising at least two distinct layers formed of a substantially single crystal semiconductor, the method comprising:
growing along a growth axis the superlattice having a polar crystal structure, the growth axis being substantially parallel to a spontaneous polarization axis of the crystal structure; and
changing an average composition of the unit cells of the superlattice monotonically from an average composition corresponding to a wider band gap (WBG) material to an average composition corresponding to a narrower band gap (NBG) material or from an average composition corresponding to a NBG material to an average composition corresponding to a WBG material along the growth axis to induce p-type or n-type conductivity.
2. The method of claim 1 , wherein the p-type conductivity is induced by:
growing the superlattice with a cation-polar crystal structure and changing the average composition of the unit cells monotonically from an average composition corresponding to a WBG material to an average composition corresponding to a NBG material along the growth axis; or
growing the superlattice with an anion-polar crystal structure and changing the average composition of the unit cells monotonically from an average composition corresponding to a NBG material to an average composition corresponding to a WBG material along the growth axis.
3. The method of claim 1 , wherein the n-type conductivity is induced by:
growing the superlattice with a cation-polar crystal structure and changing the average composition of the unit cells monotonically from an average composition corresponding to a NBG material to an average composition corresponding to a WBG material along the growth axis; or
growing the superlattice with an anion-polar crystal structure and changing the average composition of the unit cells monotonically from an average composition corresponding to a WBG material to an average composition corresponding to a NBG material along the growth axis.
4. The method of claim 2 , wherein:
the anion-polar crystal structure is a nitrogen-polar crystal structure or an oxygen-polar crystal structure; and
the cation-polar crystal structure is a metal-polar crystal structure.
5. The method of claim 3 , wherein:
the cation-polar crystal structure is a metal-polar crystal structure; and
the anion-polar crystal structure is a nitrogen-polar crystal structure or an oxygen-polar crystal structure.
6. The method of claim 1 , wherein the average composition of the unit cells is changed in a stepwise manner along the growth axis.
7. The method of claim 1 , wherein the average composition of the unit cells is changed by changing a thickness of one or more of the at least two distinct layers of the unit cells.
8. The method of claim 1 , wherein a thickness of the unit cells is constant along the growth axis.
9. The method of claim 1 , wherein a composition of one or more of the at least two distinct layers of the unit cells is selected from the following:
gallium nitride (GaN);
aluminium nitride (AlN);
aluminium gallium nitride (Al x Ga 1-x N) where 0≦x≦1;
boron aluminium nitride B x Al 1-x N where 0≦x≦1; and
aluminium gallium indium nitride (Al x Ga y In 1-x-y N) where 0≦x≦1, 0≦y≦1 and 0≦(x+y)≦1.
10. The method of claim 1 , wherein a composition of one or more of the at least two distinct layers of the unit cells is selected from the following:
magnesium oxide (MgO);
zinc oxide (ZnO); and
magnesium zinc oxide (Mg x Zn 1-x O) where 0≦x≦1.
11. The method of claim 1 , wherein the at least two distinct layers of each unit cell each have a thickness that is less than the de Broglie wavelength of a charge carrier in the respective layer.
12. The method of claim 1 , wherein the at least two distinct layers of each unit cell each have a thickness that is less than or equal to a critical layer thickness required to maintain elastic strain.
13. The method of claim 1 , further comprising:
including impurity dopants in one or more of the least two distinct layers of each unit cell to enhance the induced p-type or n-type conductivity.