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

Elizabeth MichaelCandidacy Fall 2012October 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.

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

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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

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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:1. Oxygen vacancies2. Zinc interstitial atoms3. Antisite defects4. Residual hydrogen atoms

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Shockley-Read-Hall Recombination

• Deep-level defects are the most effective centers for recombination

• Conclusion: minimize deep-level defects

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• p-Type doping is suppressed by self-compensation:

• If defects are removed before doping is attempted, p-type doping may achieve more success

Self-Compensation in ZnO

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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

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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.

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Plasma-Film Interactions• Plasma couples kinetic energy

into a film deposition process • When plasma strikes the

surface of a film, the species may: 1. Be reflected due to

structural or size constraints2. Cause chemical activation,

removing any residual organic species

3. Cause atomic displacement both within and on the surface of the film

4. Penetrate the film • Key Issue: Controlling the

power of the plasma

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HypothesisOxygen 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. 

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Objectives1. 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 defects2. Growth of extrinsically n- and p-doped ZnO thin

films• Measure dopant distribution using Dynamic SIMS

3. Carrier concentration and mobility measured via van der Pauw-Hall measurements

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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.

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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

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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-dopants due to their close match in radius▫These elements also have appropriately high

solubility limits in ZnO

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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

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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=1

𝜎𝑛𝑣 h𝑡 𝑁 𝑐𝑒

(𝐸𝐶−𝐸 𝑡)𝑘𝑇

τe = carrier emission time constantσn = capture cross-section for electronsvth = thermal velocityNc = conduction band effective DOSEc-Et = difference between the conduction band minimum and trap levelk= Boltzmann constantT= temperatureF.D. Auret, J.M. Nel, M. Hayes, L. Wu, W. Wesch, E. Wendler,”Electrical characterization of

growth-induced defects in bulk-grown ZnO,” Superlattices and Microstructures, 39: 1–4, January–April 2006, 17–23.

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van der Pauw-Hall Measurements• Extracts carrier

concentration and mobility▫ To calculate resistivity (ρ),

measure R12,34 and R14,23

▫ To calculate the Hall coefficient (RH), must measure ∆R13,24

• Dopant concentration can be optimized

exp (− 𝜋 𝑑𝜌 𝑅12,34)+exp (− 𝜋 𝑑

𝜌 𝑅14,23)=1

𝑅𝐻=𝑑𝐵 ∆𝑅13,24=

1𝑛𝑞

d= film thickness R= resistanceB= magnetic fieldn= carrier concentrationq= charge of an electron

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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

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Supplementary Slides

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Important ConstantsProperty ValueZnO Thermal Expansion Coefficient

4*10-6 /K

ZnO Lattice Parameters a= 3.25 Å, c= 5.20 Å Sapphire Thermal Expansion Coeff.

5*10-6 /K

Sapphire Lattice Parameters a= 4.79 Å, c= 12.99 Å N Solubility in ZnO 3.8*1017 ions/cm3 Ga Solubility in ZnO 5.8*1019 ions/cm3

Zn Radius 74 pmO Radius 124 pmGa Radius 62 pmN Radius 132 pmZnO Band Gap, Electron Affinity, WF

3.34 eV, 4.29 eV, 4.45 eV

Au Work Function 5.1 eVNi/Au on ZnO Resistivity 2.06*10-4 Ω*cm2

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Electromagnetic Spectrum

Goody and Walker,  Planetary Atmospheres, 1972.

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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

Dacid Leadley, Department of Physics, University of Warsaw

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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.

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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

M. Liu, "Study of Ultra-Thin Zinc Oxide Epilayer Growth and UV Detection Properties," University of Pittsburgh Ph.D. Thesis, 2003.

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X-Ray Diffraction

• Used for phase identification and crystallite orientation

• nλ=2dh,k,l *sinθ

Institute of Physics, Teaching Advanced Physics

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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

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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

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Tauc Plots

Direct Transition:

Indirect Transition:

Absorption Coefficient:

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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

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Kröger-Vink Diagram

Lukas Schmidt-Mende, Judith L. MacManus-Driscoll. “ZnO – nanostructures, defects, and devices,” Materials Today, Volume 10, Issue 5, May 2007, Pages 40–48

• ∆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

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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.

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Other Recombination Mechanisms

Band-to-Band Recombination Auger Recombination

S. J. Fonash, Solar Cell Device Physics, Burlington: Elsevier, 2010.

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Nucleation and GrowthLayer-by-layer (Frank-van der Merwe)

Layer and island (Stranski-Kastranov)

Island(Volmer-Weber)

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ZnO Wurtzite Crystal Structure

• a= 3.25 Å, c= 5.20 Å

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ZnO Phase Diagram

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Choice of Substrate

• ZnO films will be grown on c-plane sapphire (α-Al2O3)▫ Corundum structure

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