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, agnes.tempez@horiba.com

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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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