Coulomb Impurity in Semiconductor Nanowires

1. Coulomb Impurity in Semiconductor Nanowires T.Tchelidze, T. Kereselidze, T. Nadareishvili Ivane Javakhishvili Tbilisi State University, Faculty of Exact and Natural Sciences Ten Years of German-Georgian Science Bridge - 2014

2. Motivation: • how space confinement affects conductivity of materials • Is it possible to obtain desired conductivity type in 1-D structures, which impossible to obtain in bulk materials? Object of investigation is ZnO, which is naturally n-type and p-type doping is still difficult. Ten Years of German-Georgian Science Bridge - 2014

3. Why ZnO? • ZnO is a direct wide band-gap (3.37 eV) compound semiconductor that is suitable for short wavelength optoelectronic applications. • ZnO is transparent to visible light and can be made highly conductive by doping . • Exciton binding is very high energy – 60 meV, which is especially important for optoelectronic application. Because of the lack of p-type samples the application is limited with areas where unipolar conductivity is required, such as gas sensors and transparent conductive oxide Ten Years of German-Georgian Science Bridge - 2014

4. What makes difficult achieving hole conductivity in ZnO – What is the reason of dopping assymetry ? Difficulties of p-type doping can arise from a variety of causes. One of the main reasons that makes difficult obtaining low ohmic hole conductivity is compensation of dopants by low energy native defects, such as VO or Zni, or background impurities (H). Ten Years of German-Georgian Science Bridge - 2014

5. Difficulties of obtaining hole conductivity (and generally doping asymmetry in wide and-gap materials) is also explained by valence and conductive band alignment - ZnO posses very low laying valence band The valence band can be raised by replacing oxygen with other elements of VI group. The valence band of ZnO0.81S0.19 410meV higher than that of ZnO. Acceptor activation energy decreases from 220 meV to 5 meV. Maksimov, Rev. Adv. Mater. Sci. v. 24, 26,2010 Ten Years of German-Georgian Science Bridge - 2014

6. Compensation – crystal responses to acceptor Inclusion by creation of donor defect- hole killer. Ten Years of German-Georgian Science Bridge - 2014

7. Problem of p-conductivity is related to: • Low formation enthalpy of donors • Low activation energy of donors • High activation energy of acceptors To suppress formation of compensations donors (interstitial zinc Zni and oxygen vacancy VO treatment in high oxygen pressure is used. Ten Years of German-Georgian Science Bridge - 2014

8. Ag doped ZnO • Group Ib elements - Ag, Cu, Au do not prefer to occupy interstitial sites in ZnO, and impurity self compensation is very small • From the same reason defect chemistry quite simple Kroger method of quasi-chemical equations – Bulk ZnO:Ag pressure needed for suppressing compensating (with intrinsic donor) processes is expressed as This value of oxygen partial pressure can be taken as a measure of the strength of compensating processes. For bulk ZnO at t=750K P(02)=109 atm Ten Years of German-Georgian Science Bridge - 2014

9. How space confinement can alter conductivity and compensation • It can change (increase) acceptor ionization energy - bad • It can change (increase) donor ionization energy - good This changes depend on nanowire radius and there should be optimal radius range where the “bad” change is much less than the “good” one Ten Years of German-Georgian Science Bridge - 2014

10. Calculation of activation energies of donors and acceptors In quasi-one-dimensional structures the ionization energies of defects and impurities are strongly enhanced because of space and dielectric confinement. Ten Years of German-Georgian Science Bridge - 2014

11. Calculation of activation energies of donors and acceptors If one assumes that the magnitude of Coulomb interaction is less than total nanowire confinement energy of electron (hole) the three-dimensional Coulomb potential can be reduced to a one-dimensional potential, by averaging it over the electron (hole) wave function describing electron (hole) motion in defect-free nanowires: Ten Years of German-Georgian Science Bridge - 2014

12. Calculation of activation energies of donors and acceptors Binding energies of electron (hole) to donors (acceptor) centre are obtained by solving Schrödinger equation with averaged 1D attractive potential With this effective potential SE isreduced to Whittaker’s equation: Ten Years of German-Georgian Science Bridge - 2014

13. Results of calculation of activation energies Ionization energy of compensating donor remains higher that corresponding bulk value until a<8nm , while ionization energy of acceptor impurity returns to its bulk value for a=3.5nm. As shown below, this fact should condition weakening of compensation by oxygen vacancy in nanowires with radius less than 8nm. Ten Years of German-Georgian Science Bridge - 2014

14. Results of calculation of the oxygen partial pressure that is needed for suppressing the compensation by native donors For nanowires 1- 5 nm radius oxygen pressure much less than that for bulk crystal at whole range of processing temperature. Ten Years of German-Georgian Science Bridge - 2014

15. Conclusion We studied perspectives of achieving impurity controlled hole conductivity in free standing Zno nanowires. Results show that for nanowirs of 1-5 nm radius oxygen pressure needed for suppressing creation of acceptor killer defects much less than that for bulk crystal at those range of treatment temperature. Therefore we can expect that there should exist optimal nanowire radius ranging between 4-7 nm, where uncompensated hole conductivity can be obtained. Ten Years of German-Georgian Science Bridge - 2014

16. Thank you for attention! Acknowledgements This work has been supported by joint research grant #MTCU/37/6-265/11 from STCU (Science and Technology Center in Ukraine) and SRNDF (Shota Rustaveli National Science Foundation, Georgia) Ten Years of German-Georgian Science Bridge - 2014