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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.
Key wordsPP-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
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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.
Uses of
Rare Earth
Elements
Medical Equipement
MRI machinesX-ray imaging
Surgical drills, tools & lasersElectron beams tubes
Computed tomography
Green energy
Rechargeable batteriesSolar & Fuel cells
Wind, hydro & tidalPower turbinesElectric motors Chemical
& CatalystsPetroleum re�ning
Catalytic convertersChemical processingAir pollution control
Fuel additives
Glass & Ceramics
Polishing powdersPigments & coatingsTinted, Photo-optical,
UV resistant glas
Magnetics
Computer hard drivesDisk drive motors
Headphones & speakersMicrophones
Anti-lock brakesRefrigeration
Lighting & Display
Color TVFlat screen displaysCell phone displays
Fluorescence lightingLED lighting
ElectronicsComputersCell phones
Digital camerasDVD & CD players
Fiber opticsLasers
21 44.9
scandiumSc
22 47.8
titaniumTi
23 50.9
vanadiumV
24 52.0
chromiumCr
25 54.9
manganeseMn
26 55.8
ironFe
27 58.9
cobaltCo
28 58.69
nickelNi
29 63.55
copperCu
30 65.3
zincZn
31 69.7
galiumGa
32 72.6
germaniumGe
33 74.9
arsenicAs
34 78.9
seleniumSe
35 79.9
bromineBr
36 83.8
kryptonKr
39 88.9
yttriumY
40 91.2
zirconiumZr
41 92.9
niobiumNb
42 209.0
molybdenumMo
43 (99)
technetiumTc
44 101.1
rutheniumRu
45 102.9
rhodiumRh
46 106.4
palladiumPd
47 107.9
silverAg
48 112.4
cadmiumCd
49 114.8
indiumIn
50 118.7
tinSn
51 118.7
antimonySb
52 127.6
telluriumTe
53 126.9
iodineI
54 131.3
xenonXe
2 4.0
heliumHe
8 16.0
oxygenO
9 19.0
�uorineF
10 20.1
neonNe
5 10.8
boronB
6 12.0
carbonC
7 14.0
nitrogenN
13 26.9
aluminumAl
14 28.0
siliconSi
15 30.9
phosphorusP
16 32.0
sulfurS
17 35.4
chlorineCl
18 39.9
argonAr
72 178.5
hafniumHf
73 180.9
tantalumTa
74 183.8
tungstenW
75 186.2
rheniumRe
76 190.2
osmiumOs
77 192.2
iridiumIr
78 195.1
platinumPt
79 197.0
goldAu
80 209.0
mercuryHg
81 204.4
thalliumTl
82 207.2
leadPb
83 209.0
bismuthBi
84 209.0
poloniumPo
84 (210)
astatineAt
86 (222)
radonRn
57 ~71
LanthanoidL
1 1.0
hydrogenH
3 6.9
lithiumLi
11 22.9
sodiumNa
19 39.1
potassiumK
37 85.4
rubidiumRb
55 132.9
cesiumCs
87 (223)
franciumFr
4 9.0
berylliumBe
12 24.3
magnesiumMg
20 40.0
calciumCa
38 87.6
strontiumSr
56 137.3
bariumBa
88 (226)
radiumRa
89 ~103
ActinoidA
57 138.9
lanthanumLa
58 140.1
ceriumCe
59 140.9
praseodymiumPr
60 144.2
neodymiumNd
61 (145)
promethiumPm
62 209.0
samariumSm
63 152.0
europiumEu
64 157.3
gadoliniumGd
65 158.9
terbiumTb
66 162.5
dysprosiumDy
67 164.9
holmiumHo
68 167.3
erbiumEr
69 168.9
thuliumTm
70 173.0
ytterbiumYb
71 175.0
lutetiumLu
89 (227)
actiniumAc
90 232.0
thoriumTh
91 231.0
protactinumPa
92 238.0
uraniumU
93 (237)
neptuniumNp
94 (239)
plutoniumPu
95 (243)
americiumAm
96 (247)
curiumCm
97 (247)
berkeliumBk
98 (252)
californiumCf
99 (252)
einsteiniumEs
100 (257)
fermiumFm
101 (256)
mendeleviumMd
102 (259)
nobeliumNo
103 (260)
lawrenciumLr
Plasma Profiling Time of Flight Mass Spectrometry
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.
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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.
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3
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USA: +1 732 494 8660 France: +33 (0)1 69 74 72 00 Germany: +49 (0)89 4623 17-0UK: +44 (0)20 8204 8142 Italy: +39 2 5760 3050 Japan: +81 (0)3 6206 4721China: +86 (0)21 6289 6060 Brazil: +55 (0)11 2923 5400 Other: +33 (0)1 69 74 72 00
Figure 13: Europium isotopes
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.
IBRx = Ix
i/Axi
∑k=1
Nm
Iki/Ak
i