PP-TOFMS Depth Profiling of ZnO Thin layers co-doped with Rare Earths for Photonic Materials Abstract This note reports on an example of depth analysis by plasma profiling time of flight mass spectrometry of rare earth doped materials: codoped Eu and Tb ZnO thin layers developed for making white LEDs. Key words PP-TOFMS, fast depth profile, rare earths, thin films, photoluminescence, magnetron sputtering, photonics Introduction The rare earth elements (REEs) form a chemically uniform group and include yttrium (Y), lanthanum (La) and the lanthanides cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu) (they are highlighted in the periodic table shown in Figure 1). REE unique electronic, optical, luminescent, and magnetic properties have made them attractive for a variety of applications[1] (Figure 2). Recently, considerable research activity is being carried out on rare earth (RE) doped materials for photonics[2]. Among these materials, the optical properties of ZnO studied extensively for various applications in the photoelectrochemical cells, diluted magnetic semiconductors (DMS), field effect transistors, and photoluminescence devices may be tailored by RE doping for wide range electroluminescent devices (white LEDs). In RE-doped ZnO, the intra-ionic 4f transitions of RE ions form luminescent centers which generate narrow and intense emission lines at infrared and visible wavelengths. Figure 1: Periodic table Explore the future Automotive Test Systems | Process & Environmental | Medical | Semiconductor | Scientific Dr Agnès Tempez, HORIBA Scientific, rue de la Vauve, 91120 Palaiseau, France, [email protected]PPTOF 01 Figure 2: Use of REE For rapid optimisation of thin film deposition processes, direct analysis techniques (with no sample preparation) are highly in demand. Plasma Profiling Time of Flight Mass Spectrometry (PP-TOFMS TM ) provides direct measurement of the chemical composition of materials as a function of depth, with nanometre resolution and the capability to measure both thin and thick layers. It consists in a pulsed radio frequency glow discharge plasma source fed with pure Ar and created under a pulsed RF potential coupled to a time of flight mass spectrometer (TOFMS). The instrument is shown in Figure 3 and schematically in Figure 4. Uses of Rare Earth Elements Medical Equipement MRI machines X-ray imaging Surgical drills, tools & lasers Electron beams tubes Computed tomography Green energy Rechargeable batteries Solar & Fuel cells Wind, hydro & tidal Power turbines Electric motors Chemical & Catalysts Petroleum refining Catalytic converters Chemical processing Air pollution control Fuel additives Glass & Ceramics Polishing powders Pigments & coatings Tinted, Photo-optical, UV resistant glas Magnetics Computer hard drives Disk drive motors Headphones & speakers Microphones Anti-lock brakes Refrigeration Lighting & Display Color TV Flat screen displays Cell phone displays Fluorescence lighting LED lighting Electronics Computers Cell phones Digital cameras DVD & CD players Fiber optics Lasers 21 44.9 scandium Sc 22 47.8 titanium Ti 23 50.9 vanadium V 24 52.0 chromium Cr 25 54.9 manganese Mn 26 55.8 iron Fe 27 58.9 cobalt Co 28 58.69 nickel Ni 29 63.55 copper Cu 30 65.3 zinc Zn 31 69.7 galium Ga 32 72.6 germanium Ge 33 74.9 arsenic As 34 78.9 selenium Se 35 79.9 bromine Br 36 83.8 krypton Kr 39 88.9 yttrium Y 40 91.2 zirconium Zr 41 92.9 niobium Nb 42 209.0 molybdenum Mo 43 (99) technetium Tc 44 101.1 ruthenium Ru 45 102.9 rhodium Rh 46 106.4 palladium Pd 47 107.9 silver Ag 48 112.4 cadmium Cd 49 114.8 indium In 50 118.7 tin Sn 51 118.7 antimony Sb 52 127.6 tellurium Te 53 126.9 iodine I 54 131.3 xenon Xe 2 4.0 helium He 8 16.0 oxygen O 9 19.0 fluorine F 10 20.1 neon Ne 5 10.8 boron B 6 12.0 carbon C 7 14.0 nitrogen N 13 26.9 aluminum Al 14 28.0 silicon Si 15 30.9 phosphorus P 16 32.0 sulfur S 17 35.4 chlorine Cl 18 39.9 argon Ar 72 178.5 hafnium Hf 73 180.9 tantalum Ta 74 183.8 tungsten W 75 186.2 rhenium Re 76 190.2 osmium Os 77 192.2 iridium Ir 78 195.1 platinum Pt 79 197.0 gold Au 80 209.0 mercury Hg 81 204.4 thallium Tl 82 207.2 lead Pb 83 209.0 bismuth Bi 84 209.0 polonium Po 84 (210) astatine At 86 (222) radon Rn 57 ~71 Lanthanoid L 1 1.0 hydrogen H 3 6.9 lithium Li 11 22.9 sodium Na 19 39.1 potassium K 37 85.4 rubidium Rb 55 132.9 cesium Cs 87 (223) francium Fr 4 9.0 beryllium Be 12 24.3 magnesium Mg 20 40.0 calcium Ca 38 87.6 strontium Sr 56 137.3 barium Ba 88 (226) radium Ra 89 ~103 Actinoid A 57 138.9 lanthanum La 58 140.1 cerium Ce 59 140.9 praseodymium Pr 60 144.2 neodymium Nd 61 (145) promethium Pm 62 209.0 samarium Sm 63 152.0 europium Eu 64 157.3 gadolinium Gd 65 158.9 terbium Tb 66 162.5 dysprosium Dy 67 164.9 holmium Ho 68 167.3 erbium Er 69 168.9 thulium Tm 70 173.0 ytterbium Yb 71 175.0 lutetium Lu 89 (227) actinium Ac 90 232.0 thorium Th 91 231.0 protactinum Pa 92 238.0 uranium U 93 (237) neptunium Np 94 (239) plutonium Pu 95 (243) americium Am 96 (247) curium Cm 97 (247) berkelium Bk 98 (252) californium Cf 99 (252) einsteinium Es 100 (257) fermium Fm 101 (256) mendelevium Md 102 (259) nobelium No 103 (260) lawrencium Lr Plasma Profiling Time of Flight Mass Spectrometry
4
Embed
PP-TOFMS Depth Profiling of ZnO thin layers co-doped with ... · PP-TOFMS Depth Profiling of ZnO Thin layers co-doped with Rare Earths for Photonic Materials Abstract This note reports
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
PP-TOFMS Depth Profiling of ZnO Thin layers co-doped with Rare Earths for Photonic Materials
AbstractThis note reports on an example of depth analysis by plasma profiling time of flight mass spectrometry of rare earth doped materials: codoped Eu and Tb ZnO thin layers developed for making white LEDs.
The rare earth elements (REEs) form a chemically uniform group and include yttrium (Y), lanthanum (La) and the lanthanides cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu) (they are highlighted in the periodic table shown in Figure 1). REE unique electronic, optical, luminescent, and magnetic properties have made them attractive for a variety of applications[1] (Figure 2). Recently, considerable research activity is being carried out on rare earth (RE) doped materials for photonics[2]. Among these materials, the optical properties of ZnO studied extensively for various applications in the photoelectrochemical cells, diluted magnetic semiconductors (DMS), field effect transistors, and photoluminescence devices may be tailored by RE doping for wide range electroluminescent devices (white LEDs). In RE-doped ZnO, the intra-ionic 4f transitions of RE ions form luminescent centers which generate narrow and intense emission lines at infrared and visible wavelengths.
Figure 1: Periodic table
Explore the future Automotive Test Systems | Process & Environmental | Medical | Semiconductor | Scientific
Dr Agnès Tempez, HORIBA Scientific, rue de la Vauve, 91120 Palaiseau, France, [email protected]
PPTOF 01
Figure 2: Use of REE
For rapid optimisation of thin film deposition processes, direct analysis techniques (with no sample preparation) are highly in demand. Plasma Profiling Time of Flight Mass Spectrometry (PP-TOFMSTM) provides direct measurement of the chemical composition of materials as a function of depth, with nanometre resolution and the capability to measure both thin and thick layers. It consists in a pulsed radio frequency glow discharge plasma source fed with pure Ar and created under a pulsed RF potential coupled to a time of flight mass spectrometer (TOFMS). The instrument is shown in Figure 3 and schematically in Figure 4.
The key strength of PP-TOFMS is to record a full and continuous spectrum over a flexible mass range at any depth point (sampling as low as sub-nanometer/point). In addition the high mass range benefits from low background, which makes PP-TOFMS high mass sensitive. As a result, PP-TOFMS is well suited for measuring REE composition distribution in thin films. In this note, PP-TOFMS data on Eu-Tb co-doped ZnO layers are presented. PP-TOFMS profiles complemented with structural characterisation (XRD) allow for interpreting PL data for a better understanding of the emission mechanisms of the as grown and post-growth treated ZnO layers.
Figure 3: PP-TOFMS instrument
Figure 4: PP-TOFMS Principle
ExperimentalUndoped ZnO, RE co-doped ZnO thin films and multilayer structures were grown on (100) silicon substrates by RF magnetron sputtering using a pure ZnO target (figure 5). Magnetrons make use of the fact that a magnetic field parallel to the target surface can constrain secondary electron motion to the vicinity of the target. As a result, trapped electrons enhance collision and thereby ionisation creating a more dense plasma in the target region for higher deposition rates. Eu and Tb codoping was achieved by arranging europium oxide (Eu
2O
3) and terbium oxide (Tb
4O
7)
calibrated pellets on the target surface. Samples were post-growth an-nealed at 1200°C for 1 min under N
2 to optimise dopant distributions
and activate dopants.
Figure 5: RF magnetron sputtering deposition system
Depth profile analysis was carried out with PP-TOFMSTM. The erosion plasma was created between a cylindrical copper anode and the sample (used as cathode) fed with RF from its back surface. The initial sample dimension was 10 mm x 10 mm. The anode was 4 mm diameter creating a 4 mm diameter crater (probed region). The Argon pressure was maintained constant at 150 Pa; RF excitation (50 W) was pulsed with a pulse width and period of 1 ms and 4 ms, respectively. The transient ion signals of the pulsed plasma were recorded over 1.8 ms by 65 successive TOF mass spectra. The resulting “source profiles” were summed over 50 RF periods giving a point in depth profile every 200 ms. For each sample, thickness and refractive index n were determined using a UVISEL ellipsometer (1.5 - 4.5 eV range) (Figure 6). Photoluminescence emissions (PL and PLE) were collected by using a HORIBA Scientific Fluorolog spectrometer with a 450 W lamp as source excitation (Figure 7).
Figure 6: UVISEL 2 Ellipsometer
Figure 7: Fluorolog principle & Fluorolog PL
DiscussionFigure 8 shows the depth profile of an as-deposited ZnO thin film in which inputs of Eu and Tb co-dopants are varied within the layer (every 50 nm). Eu and Tb are shown as atomic % and major elements, namely, O, Zn, and Si are shown as raw signals in counts per extraction.
Explore the future Automotive Test Systems | Process & Environmental | Medical | Semiconductor | Scientific
2
10-7 mbar
ZnO sputtering
Ar
Substrate
Tb4O7 and/or Eu2O3 pellets
ZnO target
Heating stage
Anode
Cathode
Magnetron system
Cooled system
We have used the thickness determined by ellipsometry to convert the raw PP-TOFMS measurement time (X axis) to depth (in nm). Tb and Eu concentrations are obtained by simple calculation of ion beam ratio* without any calibration. IBR is the ratio between signal of a peak corrected for isotopic abundance of corresponding isotope and the sum of ion matrix signals corrected for isotopic abundance. Here, 30Si, 67Zn, and 16O are used as matrix ions . It is important to note that this profile was obtained in less than 1 min.
Figure 8: ZnO thin film depth profile
In this study, photoluminescence signal was detected on annealed samples whereas as-deposited samples were non-active (Figure 9). As shown in Figure 10 PP-TOFMS depth profiles of both as-deposited and annealed samples evidence the high Si diffusion in Zn upon annealing and formation of a new matrix. This study extends to several doped and undoped ZnO thin layers. More details may be found in reference [3].
Figure 9: Photoluminescence of ZnO layer
Figure 10: Depth profiles of as-deposited & annealed samples
The high PP-TOFMS sensitivity in rare earth elements is explained by the unique TOFMS capability of recording a complete mass spectrum every 30 µs and the ultra-low background for high mass elements. This is illustrated by the full mass spectrum taken from a single point of the depth profile (i.e. integrated over a 0.3 nm depth) shown in both linear and semi logarithmic scales in Figures 11 and 12. Below mass 50, ions such as C, O, OH
x, CO
X … are present.
Figure 11: Mass elements spectrum (linear scale)
Figure 12: Mass elements spectrum (logarithmic scale)
The acquisition of a continuous full spectrum gives access to signals of all isotopes. The spectrum in Figure 13 zoomed on the two Europium isotopes shows perfect fit between 151Eu and 153Eu signals (violet line) and the natural isotopic distribution (green line) of Europium. Such isotopic abundance matching is readily checked with PP-TOFMS software and allows for picking non-interfered isotope.
Explore the future Automotive Test Systems | Process & Environmental | Medical | Semiconductor | Scientific
3
Explore the future Automotive Test Systems | Process & Environmental | Medical | Semiconductor | Scientific
ConclusionsThis example shows that PP-TOFMS is a fast and reliable technique for depth profiling of rare earth doped ZnO thin films. Tb and Eu profiles are obtained with high sensitivity and high depth resolution. This type of information is typically provided by SIMS, RBS or depth profiling XPS but not as rapidly and readily and at a higher cost. Such profiles turn out to be powerful complementary information to un-derstand photoluminescence data. This example extends to similar materials for photonics (lighting, display, solar energy industries) applications such as other wide bandgap semi-conductors (SiC,GaN…), nitrides and oxynitrides layers, silicon nano-ob-jects, glasses… doped with Yb, Y, Sm, Er, Nd, Pr, and Tm...
Definition of Ion Beam Ratio or IBR
N: Number of all elements (ion peaks being investigated)N
m: Number of main elements included in a matrix
Iij: Signal of an isotope i of an element j
Aij: Abundance of an isotope i of an element j
x: An element of interest
References[1] The Rare Earth Elements: Fundamentals and Applications, David A. Atwood, John Wiley & Sons, 2013.[2] Andries Maijerink, Lanthanide Ions as Photon Managers for Solar Cells, Material Matters, 6 (4) 113 (2011).[3] A. Ziani, A. Tempez, C. Frilay, C. Davesnne, C. Labbé, Ph. Marie, S. Legendre and X. Portier, Concentration determination and activation of rare earth dopants in zinc oxide thin films, EMRS Fall 2013 Proceedings, physica status solidi
Close ApplicationsAll materials containing rare earth elements: nanometer thick thin films at level down to 1017 atom/cm3 and atomic ppb level in thicker films or bulk materials.Examples: GaN, SiC, glass, hard coating, steels…
Check on other HORIBA techniques in this fieldStructural properties: Raman SpectroscopyThickness and optical properties: Spectroscopic EllipsometryPurity determination: ICP-OESDefect studies: Cathodoluminescence
AcknowledgmentsC. Frilay, A. Ziani, and F. Gourbilleau from the Research Center for Ions, Materials and Photonics (CIMAP, University of Caen, France) are kindly thanked for allowing the use of results on ZnO samples.