AVS 58th Annual International Symposium and Exhibition
    Electronic Materials and Processing Division Wednesday Sessions
       Session EM-WeA

Invited Paper EM-WeA1
Controlling Schottky Barriers and Doping with Native Point Defects

Wednesday, November 2, 2011, 2:00 pm, Room 210

Session: Defects in Electronic Materials
Presenter: Leonard Brillson, The Ohio State Univ.
Authors: L.J. Brillson, The Ohio State Univ.
Y. Dong, The Ohio State Univ.
F. Tuomisto, Helsinki Univ. of Tech., Finland
B. Svensson, Univ. of Oslo, Norway
A.Yu. Kuznetsov, Univ. of Oslo, Norway
D. Doutt, The Ohio State Univ.
H.L. Mosbacker, Traycer Diagnostic
G. Cantwell, ZN Technology
J. Zhang, ZN Technology
J.J. Song, ZN Technology
Z.-Q. Fang, Univ. of Dayton
D.C. Look, Air Force Research Lab
Correspondent: Click to Email
Native point defects in semiconductors have until now not been considered a major factor in Schottky barrier formation or doping due to their relatively low bulk densities. Likewise, efforts to control doping type and density usually treat point defects as passive, compensating donors or acceptors. Recent advances in the rapidly emerging semiconductor ZnO include a deeper understanding into the nature of native point defects at its surfaces, interfaces, and epitaxial films. Key to ZnO Schottky barrier formation is a massive redistribution of native point defects near its surfaces and interfaces. It is now possible to measure the energies, densities and in many cases the type of point defects below the semiconductor free surface and its metal interface with nanoscale precision. Using depth-resolved cathodoluminescence spectroscopy (DRCLS) of deep level emissions calibrated with electrical techniques, we find that native point defects can (i) increase by orders-of-magnitude in densities within tens of nanometers of the semiconductor surface, (ii) alter free carrier concentrations and band profiles within the surface space charge region, (iii) dominate the Schottky barrier formation for metal contacts to ZnO, and (iv) play an active role in semiconductor doping. Among major roadblocks to ZnO optoelectronics have been the difficulty of both n- and p-type doping. Oxygen vacancies (VO), VO complexes, Zn interstitial-related complexes, and residual impurities such as H and Al are all believed to be shallow donors in ZnO, while Zn vacancies (VZn) and their complexes are acceptors. While their impact on free carrier compensation and recombination is recognized, the physical nature of the donors and acceptors dominating carrier densities in ZnO and their effect of carrier injection at contacts is unresolved. How these defects impact ZnO optoelectronics at the nanoscale is only now being explored. We address these issues using a combination of depth-resolved and scanned probe techniques to clearly identify the optical transitions and energies of VZn and VZn clusters, Li on Zn sites, Ga on Zn site donors, the effects of different annealing methods on their spatial distributions in ion-implanted as well as Ga grown-in ZnO, and how VZn, VZn clusters, and VO complexes contribute to near- and sub-surface carrier density. Defects also couple to nanostructures, which form spontaneously on ZnO polar surfaces and create sub-surface VZn locally with Zn diffusion that feeds the growth. These results reveal the interplay between ZnO electronic defects, dopants, polarity, and surface nanostructure, and they highlight new ways to control ZnO Schottky barriers and doping.