Group one impurities in single crystalline Zinc Oxide
Appears in the following Collection
- Fysisk institutt 
AbstractZinc Oxide (ZnO) has been used as a material in many different technologies from pharmaceuticals to electronics. This exciting material can also be utilized as a wide band gap semiconductor for application in optoelectronic devices. The availability of Zn, the possibility to grow single crystal bulk material and the exitonic binding energy of 60 meV makes this material especially interesting. Even though the material has been studied already since the late 1920s there are still some fundamental properties of ZnO as a semiconductor which are not yet fully understood e.g. the inherent n-type activity and the challenge of achieving stable p-type doping.
The present work is a study of dopants and impurities in ZnO, with a focus on H and Li and their interplay in hydrothermally grown (HT) ZnO. Doping with H typically increases the electron concentration in the material, either by direct donor activity or by passivation of acceptors. Doping by Li on the other hand typically results in material with high resistivity, which is explained by Li contributing both as acceptor and donor on the substitutional Zn-site and interstitial site, respectively. Both elements may therefore significantly contribute to the electrical properties of ZnO.
Both H and Li are light elements with small ionic radius, and thus the diffusivity along the interstitial route is expected to be high. In this work H diffusion has been studied by secondary ion mass spectrometry (SIMS). From this the H diffusion in hydrothermally grown material was found to be trap limited with an activation energy of 0.8-0.9 eV and with a dissociation energy from the diffusion trap of approximately 1.5 eV. Since H and Li are known to form a complex together and the concentration of Li is close to the observed trap concentration Li was suggested to be the diffusion trap for H, however contribution from other defects or impurities could not be ruled out. To approach this question from a different angle, Fourier transformed infrared absorption spectroscopy (FTIR) was employed to study the 3577 cm−1 local vibrational mode, previously identified as a OH-Li-complex. The 3577 cm−1 line had been found to survive heat treatments of several hours at 1200◦C, which is inconsistent with the estimated dissociation energy of the H diffusion trap. However, in this thesis it is shown that the reason for the high thermal stability is efficient re-trapping of H (and possibly Li) when the sample is let to cool down slowly to room temperature. In this way the complex responsible for the 3577 cm−1 vibrational mode is reformed and the peak can therefore be observed even after heat treatment at temperatures far above the thermal stability of the complex. Furthermore, by comparing results from both SIMS and FTIR, no correlation between the as-grown 3577 cm−1 peak intensity and the Li-concentration was found, indicating that most of the Li atoms in the as-grown state are not passivated by H. SIMS-measurements also revealed an in-homogeneous distribution of Li repeated in the lateral directions in some of the studied wafers. It is speculated that these in-homogeneities are related to the presence of c-axis inversion domains as observed by transmission electron microscopy (TEM) and it is speculated that they might contribute to the high apparent thermal stability of the 3577 cm−1 vibrational mode.
The third angle of approach employed in this thesis is the combination of SIMS and positron annihilation spectroscopy (PAS) together with theoretical estimates. The PAS-technique is very well suited to study open volume defects. One of the main results in this work is the identification of the positron annihilation signature of Li occupying the Zn-site, which showed that most of the Li atoms in as-grown n-type HT-ZnO is in the acceptor-state (substitutional Zn-site). It also showed that in melt grown ZnO doped with 1.5×1019 Li/cm3 more than 2-3×1018 cm−3 of the Li atoms resides on the Zn-site, showing the possibility of obtaining large concentrations of Li on the Zn-site in the acceptor state.
The identification of the annihilation signature of LiZn also opened the possibility to study the effect of hydrogenation of HT-ZnO by PAS, where H is shown to efficiently passivate the Liacceptors leaving Zn-vacancies as the dominant positron trap. However, in material where Li had been removed prior to hydrogenation H is found to be trapped by Zn-vacancies.
This work shows that there is a strong interplay between H and Li in ZnO. There are strong arguments that Li is the main trapping site for H-diffusion based on the combination of results from techniques like SIMS, FTIR and PAS, however other sites may not be completely excluded and there is also evidence of relatively complicated dynamics related to the behavior of H and Li in ZnO, involving several different sites for H. This work has also confirmed that several properties of hydrothermally grown ZnO may vary from wafer to wafer and using good reference samples are necessary to be able to draw correct conclusions.
List of papers.
Paper I Hydrogen Migration in Single Crystalline ZnO K. M. Johansen, J. S. Christensen, E. V. Monakhov, A. Yu. Kuznetsov and B. G. Svensson Materials Research Society Symposium Proceedings 1035, L03-10 (2008) doi:10.1557/PROC-1035-L03-10
Paper II Deuterium diffusion and trapping in hydrothermally grown single crystalline ZnO K. M. Johansen, J. S. Christensen, E. V. Monakhov, A. Yu. Kuznetsov and B. G. Svensson Applied Physics Letters 93, 152109 (2008) Copyright 2008 American Institute of Physics doi:10.1063/1.3001605
Paper III Li and OH-Li complexes in hydrothermally grown single crystalline ZnO K. M. Johansen, H. Haug, Ø. Prytz, P. T. Neuvonen, K. E. Knutsen, L. Vines, E. V. Monakhov, A. Yu. Kuznetsov, and B. G. Svensson Journal of Electronic Materials Volume 40, Number 4, 429-432 doi:10.1007/s11664-010-1404-0
Paper IV Thermal stability of the OH-Li complex in hydrothermally grown single crystal ZnO K. M. Johansen, H. Haug, E. Lund, E. V. Monakhov and B. G. Svensson Applied Physics Letters 97, 211907 (2010) Copyright 2010 American Institute of Physics doi:10.1063/1.3522886
Paper V Identification of substitutional Li in n-type ZnO and its role as an acceptor K. M. Johansen, A. Zubiaga, I. Makkonen, F. Tuomisto, P. T. Neuvonen, K. E. Knutsen, E. V. Monakhov, A. Yu. Kuznetsov and B. G. Svensson Physical Review B 83, 245208 (2011) Copyright 2011 The American Physical Society doi:10.1103/PhysRevB.83.245208
Paper VI H passivation of Li on Zn-site in ZnO K. M. Johansen, A. Zubiaga, I. Makkonen, F. Tuomisto, E. V. Monakhov, A. Yu. Kuznetsov and B. G. Svensson. Manuscript, published in: Physical Review B 84, 115203 (2011) Copyright 2011 The American Physical Society doi:10.1103/PhysRevB.84.115203