Controlled Doping in Zinc Oxide Thin Films via Oxygen Plasma-Assisted Sol-Gel (OPASG) Processing

1. Controlled Doping in Zinc Oxide Thin Films via Oxygen Plasma-Assisted Sol-Gel (OPASG) Processing Elizabeth Michael Candidacy Fall 2012 October 25, 2012 Topic: Stoichiometry control in oxide thin films, reduction of defect concentration as well as activation of dopants are considered major roadblocks towards their utilization as semiconductors. Inhomogeneous distribution of dopants or electronically active defects can complicate the extraction of material parameters such as carrier mobility and concentration. Develop an experimental program to address these challenges in a model oxide material of your interest. Be specific how you want to achieve controlled p and n doping in your material of choice.

2. Outline • Motivation • Background • Hypothesis • Experimental Plan • Summary

3. Oxide Semiconductors • Potential as low-cost, nontoxic semiconductors • Transparent to visible light • Integration into industry is challenged by poor control over stoichiometry, defects, and dopants • Difficulties with carrier concentration and mobility

4. Challenges of ZnO • Potential for ZnO in industry • Challenge in defect control • The cause of inherent n-type conductivity in ZnO is unknown, but may be due to: • Oxygen vacancies • Zinc interstitial atoms • Antisite defects • Residual hydrogen atoms

5. Shockley-Read-Hall Recombination • Deep-level defects are the most effective centers for recombination • Conclusion: minimize deep-level defects

6. Self-Compensation in ZnO • p-Type doping is suppressed by self-compensation: • If defects are removed before doping is attempted, p-type doping may achieve more success

7. Dopant Control in ZnO • Controlled n-type doping has been achieved • Reproducible p-type doping has not been achieved due a self-compensation mechanism • If defects can be minimized, it may be possible to achieve p-type doping • Key issue: Control oxygen vacancies

8. Motivation: Oxygen Vacancy Control • Minimization of defects may allow controlled doping • Reduces self-compensation • Liu and Kim showed that treatment of sputtered ZnO thin films with oxygen plasma reduces the concentration of oxygen vacancies • Next step: Introduce p-type dopants M. L. a. H. K. Kim, "Ultraviolet detection with ultrathin ZnO epitaxial films treated with oxygen plasma," Applied Physics Letters, vol. 84, no. 173, pp. 173-175, 2004.

9. Plasma-Film Interactions • Plasma couples kinetic energy into a film deposition process • When plasma strikes the surface of a film, the species may: • Be reflected due to structural or size constraints • Cause chemical activation, removing any residual organic species • Cause atomic displacement both within and on the surface of the film • Penetrate the film • Key Issue: Controlling the power of the plasma

10. Hypothesis Oxygen plasma-assisted sol-gel (OPASG) techniques will be used to decrease the concentration of oxygen vacancies formed during the growth of ZnO thin films.  If the minimization of these deep level defects via OPASG processing suppresses the self-compensation mechanism in ZnO, then excellent control over dopant homogeneity will be achieved. 

11. Objectives • Growth of ZnO thin films using both traditional and plasma-assisted spin-casting techniques • X-Ray Diffraction (XRD) to determine phase purity • Dynamic Secondary Ion Mass Spectrometry (SIMS) to verify stoichiometry • Deep-Level Transient Spectroscopy (DLTS) to detect electrically active defects • Growth of extrinsically n- and p-doped ZnO thin films • Measure dopant distribution using Dynamic SIMS • Carrier concentration and mobility measured via van der Pauw-Hall measurements

12. Growth and Deposition • Concentration of zinc: 0.25 M • May be decreased to ensure that oxygen radicals are able to penetrate the entire layer • Plasma treatment time to be varied depending on Zn:O • Plasma with these attributes can penetrate 20 nm of ZnO • Dopant concentration: 2 wt.% • Concentration will be varied between 0 and 5 wt.% • Final film thickness: 200 nm T. H. D. C. O. T. M. H. G. G. F. J. S. P. I. H. I. J. H. C. M. W. C. a. T. Y. S. H. Park, "Lattice relaxation mechanism of ZnO thin films grown on c-Al2O3 substrates by plasma-assisted molecular-beam epitaxy," Applied Physics Letters, vol. 91, p. 231904, 2007.

13. Plasma Treatment • Oxygen plasma treatment of sputtered ZnO improved the stoichiometry of the film through the reduction of oxygen vacancies • It was suggested that oxygen radicals occupied the oxygen vacancies • Oxygen radicals exhibit high diffusivity in oxides • In ZnO, oxygen will traverse 20 nm • Thus, we should plasma treat after the deposition of each layer

14. Choice of Dopant Atoms • Large dopant atoms in ZnO have exhibited segregation to the interface of the film • Gallium and nitrogen were chosen as n- and p-dopantsdue to their close match in radius • These elements also have appropriately high solubility limits in ZnO

15. Dynamic Secondary Ion Mass Spectrometry Probes composition as a function of depth Profile several areas to verify lateral uniformity Time of flight analyzer will be used due to its high sensitivity (N. O. I. S. S. H. H. H. Manabu Komatsu, "Ga, N solubility limit in co-implanted ZnO measured by secondary ion mass spectrometry," Applied Surface Science, vol. 189, pp. 349-352, 2002.) Dr. Adair, MatSE 514 Notes

16. Deep-Level Transient Spectroscopy • Determines density of states (DOS) and energy level of a defect population • Measures capacitance transients produced after a short forward bias pulse τe = carrier emission time constant σn = capture cross-section for electrons vth = thermal velocity Nc= conduction band effective DOS Ec-Et = difference between the conduction band minimum and trap level k= Boltzmann constant T= temperature F.D. Auret, J.M. Nel, M. Hayes, L. Wu, W. Wesch, E. Wendler,”Electrical characterization of growth-induced defects in bulk-grown ZnO,” Superlatticesand Microstructures, 39:1–4, January–April 2006, 17–23.

17. van der Pauw-Hall Measurements • Extracts carrier concentration and mobility • To calculate resistivity (ρ), measure R12,34and R14,23 • To calculate the Hall coefficient (RH), must measure ∆R13,24 • Dopant concentration can be optimized d= film thickness R= resistance B= magnetic field n= carrier concentration q= charge of an electron

18. Summary • ZnO is an oxygen-deficient, n-type semiconductor • Oxygen vacancies impede stoichiometry • Stoichiometric, intrinsically undoped ZnO can be controllably doped • Extraction and optimization of carrier concentration and mobility

19. Supplementary Slides

21. Important Constants

22. Electromagnetic Spectrum Goody and Walker, Planetary Atmospheres, 1972.

23. Schottky Barrier • Forms when a metal with a high work function is brought into contact with an n-type semiconductor • Electrons diffuse into the metal to minimize their energy, leaving behind holes in the semiconductor • The excess electrons at the interface prevent more electrons from crossing over DacidLeadley, Department of Physics, University of Warsaw

24. Sol-Gel Chemistry Modified from: L. Znaidi, "Sol–gel-deposited ZnO thin films: A review," Materials Science and Engineering B, vol. 174, pp. 18-30, 2010.

25. Substrate-Film Geometry • ZnO undergoes a 30° rotation with respect to the sapphire substrate to reduce lattice mismatch • Mismatch is 18% • Interaction is dominated by Zn-O bond between Zn-plane of ZnO and O-plane of sapphire

26. X-Ray Diffraction • Used for phase identification and crystallite orientation • nλ=2dh,k,l *sinθ Institute of Physics, Teaching Advanced Physics

27. X-Ray Photoelectron Spectroscopy(XPS) • Eb = hν -Ek – φ • Eb = binding energy of the electron, hv= energy of a photon, Ek= kinetic energy of the ejected electron, φ= work function of the spectrometer • Peaks are sensitive to the local bonding environment • The ejected photoelectrons can only pass through a few nanometers of a solid • Ability to depth probe if an ion beam is used to etch away layers

28. Ellipsometry • Measures the change in polarization as light interacts with a material • Input linearly polarized light, while output is typically elliptically polarized • Film thickness is determined from the interference between light reflected from the surface of the film and light travelling through the film • Difference in the phase of the light waves

29. Tauc Plots Direct Transition: Indirect Transition: Absorption Coefficient:

30. DLTS Continued • By varying the sample temperature, we vary the decay time constant while repetitively pulsing the device between zero and reverse bias • A peak is produced in the DLTS spectrum • τe = (t2-t1)/ln(t2/t1) • Signals are obtained only at the temperatures for traps with the emission rate corresponding to the time constant • Must vary the rate window to obtain a full spectrum

31. Kröger-Vink Diagram • ∆G = -RTln(k) • G= Gibbs free energy, R= ideal gas constant, T= temperature, k= rate constant • Relates defect equations to ∆G via Brouwer approximations • Slopes relate changes in pO2 to defect concentration Lukas Schmidt-Mende, Judith L. MacManus-Driscoll. “ZnO – nanostructures, defects, and devices,” Materials Today, Volume 10, Issue 5, May 2007, Pages 40–48

32. Shockley-Read-Hall Recombination • In an n-type semiconductor, the trap first captures a hole • 1/τp = NTvthσp • This is the rate limiting step • The filled trap then captures an electron • This is a nonradiative recombination process • Energy is released in the form of phonons (heat) S. J. Fonash, Solar Cell Device Physics, Burlington: Elsevier, 2010.

33. Other Recombination Mechanisms Band-to-Band Recombination Auger Recombination S. J. Fonash, Solar Cell Device Physics, Burlington: Elsevier, 2010.

34. Nucleation and Growth Layer-by-layer (Frank-van der Merwe) Layer and island (Stranski-Kastranov) Island (Volmer-Weber)

35. ZnO Wurtzite Crystal Structure • a= 3.25 Å, c= 5.20 Å

36. ZnO Phase Diagram

37. Choice of Substrate • ZnO films will be grown on c-plane sapphire (α-Al2O3) • Corundum structure CrystalMaker