Question

7.7. (a) To increase the electron (n-type) conductivity of ZnO, which would you add, Al2O3 or Li,O? Explain. Answer: Al2O3 (not a typo)

Answer 1

Answer: Al2O3

ZnO with a wurtzite structure is naturally an n-type semiconductor because of a deviation from stoichiometry due to the presence of intrinsic defects such as O vacancies O(v) and Zn interstitials Zn(i).

Undoped ZnO shows intrinsic n-type conductivity with very high electron densities of about 1021 cm−3. Although it is experimentally known that unintentionally doped ZnO is n type, whether the donors are Zn(i )and O(v) is still controversial. The first-principles study suggested that none of the native defects show high concentration shallow donor characteristics. However, Look Zhang et al. suggested that Zn(i) rather than O(v) is the dominant native shallow donor in ZnO with an ionization energy of about 30– 50 meV. It has also been suggested that the n-type conductivity of unintentionally doped ZnO films is only due to hydrogen H, which, acts as a shallow donor with an ionization energy about 30 meV. This assumption is valid since hydrogen is always present in all growth methods and can easily diffuse into ZnO in large amounts due to its large mobility. First-principles calculations also suggested that unintentionally incorporated hydrogen acts as a source of conductivity and behaves as a shallow donor in ZnO. n-type doping of ZnO is relatively easy compared to p-type doping(That is why not Li2O).

Group-III elements Al, Ga, and In as substitutional elements for Zn and group-VII elements Cl and I as substitutional elements for O can be used as n-type dopants.

The group-III impurities B, Al, Ga and In when substituted on the Zn site act as shallow donors in ZnO. The extra valence electron of these impurities is loosely bound and occupies effective-mass states near the conduction-band minimum (CBM) at low temperatures. As the temperature rises this extra electron is excited to the conduction band and is free to move. For Al, Zhang et al calculated an ionization energy of 120 meV and a low formation energy, referenced to the elemental Al phase. This formation energy would be raised if equilibration with Al2O3 were taken into account. Hu and Gordon obtained carrier concentrations up to 8×1020 cm−3 with Al doping in chemical vapor deposition of ZnO.