Glass-ceramics for photonic applications: an example

(Reference based optical characterization of glass-ceramic converter for high power white LEDs)

Authors: A. Engela, M. Letza, T. Zachaua, E. Pawlowskia, K. Seneschal-Merza, T. Korba,

D. Enselinga, B. Hoppea, U. Peucherta, J.S. Haydenb

Presented by: U. Fotheringhama

aSCHOTT AG, Hattenbergstr. 10, 55014 Mainz, GermanybSCHOTT North America, RDD, 400 York Avenue, Duryea, PA 18642 USA

� motivation

� glass ceramic at Schott (definition of glass ceramics)

� glass ceramic converter materials

� description of fluorescence measurements together with the corresponding scattering regimes

� used metrology for lifetime determination

� lifetime measurements on our glass ceramics

� conclusion

Outline

Goal: development of converter materials for HB-WLEDs

blue LED + yellow converter = white LED

Standard White LED light generation: conversion using YAG:Ce

wavelength in nm

Usually this phosphor is applied as a powder which is embedded in a polymer or

silicone matrix.

This solution has besides some processing steps two major drawbacks.

1)

large refractive index of n=1.836 (YAG crystals) are embedded in a material

with a much lower refractive index in the order of 1.45 – 1.55.

���� Large difference in refractive index leads to high amount of scattering light

���� decreasing by strongly reducing the size of the crystallites recommended

���� leading to the development of nano-YAG.

2)

Organics materials involved suffer from thermal and radiation stabilty

- strong radiation

- high temperature and

- non negleglible part of UV radiation

� Therefore glass ceramics with small ∆∆∆∆n as solely solid state solutions for converter materials can be promising candidates for color conversion.

Motivation

Li2O

Na2O

Al2O3

SiO2

TiO2

ZrO2

•

•

•

compositiontexture

Properties (mechanical,

physical, chemical)

„Greenglass“fabrication

Ceramization

processing

GC‘s property combination due to interaction of synthesis, process & texture

ArcticFire Hightrans

Products made from glass ceramics exhibit...

� high optical transparency

� high temperature resistance

� high temperature shock resistance

� adjustable CTE

� high chemical durability

�...

Glass ceramics offers promising product opportunities

... yielding low-expansion SCHOTT products for different application fields

Home appliances

Aerospace

ArcticFire HightransArcticFire Hightrans

CERAN

ROBAX

Projection

ZERODUR

Lithography

ZERODUR

500 600 700 800 900 1000 1100 1200 1300 1400 1500500 600 700 800 900 1000 1100 1200 1300 1400 1500

130

140

150

160

170

180

T / °C

DT

A [@

5 K

min

-1; a

rb. u

n.]

E / G

Pa

CTE30-300 ~5 ppm/K

CTE30-300 ~6,5 ppm/K

CTE30-300 ~6 ppm/K

The Core: Crystallization and Crystal Phases ( MgO-Al2O3-SiO2-GK)

spinel → sapphirine → cordierite

type and amount determines:

� mechanical properties

� thermal expansion

� thermal stability

� chemical stability

� toughness

� transparency

Glass ceramics from the Y2O3-Al2O3-SiO2-system

Yttriumpyrosilicate(Keiviite) Mullite

Yttrium Aluminium Garnet

� �

We realize a pure YAG phase in our YAS-glass ceramic

electron microscopy

shows crystallites

and residual glass

example:

X-ray diffraction proves

pure YAG as crystalline

phase!!

samples turn yellow

Light conversion on 0.1-0.5mm thick samples possible:

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

250 300 350 400 450 500 550 600 650 700 750 800 850

Wavelength (nm)

Re

mis

sio

n

3% YAC:Ce in Silicone phosphor powder Schott 5 0.328 mm

Remission (diffuse reflection): YAG:Ce phosphor and YAG:Ce glass ceramics

� glass ceramic shows a remission at 460 nm which is comparable to Ce:YAG powder

� On the other hand the remission spectra that the spectral profile of the glass

ceramic shows less structuring in comparison to the phosphor.

� This may be explained by the different scattering regime of the glass ceramic

together with the vicinity of the band-gap of the host glass.

Schott glass ceramic

YAG:Ce in Silicone

YAG:Ce powder

0

500000

1000000

1500000

2000000

2500000

3000000

3500000

4000000

4500000

290 390 490 590 690 790Wavelength in nm

Inte

nsit

y

in c

ps

1% YAG:Ce in Silicone emissio em 460 nm

1% YAG:Ce in Silicone emissio ex 545 nm

Schott 5 em 460 nm

Schott 5 ex 545 nm

Excitation & emission spectra of YAG:Ce phosphor & glass ceramics

� We observe the expected spectral absorption and emission profiles.

� For the phosphor we find absorption and excitation for maximum at 350

and 450 nm, whereas for the glass ceramics we see mainly the 450 nm

absorption.

� This reduction of the 350 nm excitation is determined by the band-

gap absorption of the host glass system.

The lifetime ττττl of the Ce3+ fluorescence level is completely independent on the scattering regime !

ττττl : measured lifetime,

ττττid : lifetime of YAG:Ce in a perfect single crystal, approximately 67ns

ττττNR : originates from non radiative processes, which are e.g. coupling to phonon modes or coupling to impurities in the material.

Lifetime measures internal quantum efficiency !

NRidlτττ

111+=

FLUOLOG 3 with IBH lifetime module with Time correlated single photon counting TCSPC

FLUOLOG 3 with lifetime module

� detection performed using identical double monochromators of the FLUOLOG 3.

� Synchronization of pulsed ignition and detection performed via a controller, which

determines the timing of excitation and that of emitted photons and

calculates the respective time of delay.

� decay time of a fluorescent state is defined as the time interval after which the

initial intensity has decreased to 27 % (= 1/e part).

fluorescence signal

scattering

fitting accuracy

fluorescence signal

scattering

fitting accuracy

Cts

Std

. D

ev.

Au

to-c

orr

.

Channels (1 Ch = 0.28 ns)

Decay curve and evaluation of YAG:Ce glass ceramics.

� periodic excitation by the LEDs can reconstruct the single decay profile from

single photon events collected over many cycles.

� reference for the timing: corresponding excitation pulse.

� The method is based on the repetitive precise timing registration of single photons.

0 10 20 30 40 50 60 70 80

0

500

1000

1500

2000

2500

3000

3500

Cts

Delay in ns

CaF2:Pb; em: 540 nm

ExpDec fit

Decay curve of CaF2:Pb for the emission of 540 nm

� Testing procedure for the fluorescence decay measurement using a doped CaF2:Pb

crystal. The sample itself is transparent in the visible range.

� Comparative measurements using different diagnostics and excitation sources

at Piqoquant, PTI and Schott� mono-exponential decay with a decay time between 24.3 ± 0.3 ns and 21.60 ± 0.49 ns

respectively.

� differences due to non-identical set of evaluated data for mono-exponential fitting

Decay curve of CaF2:Pb for the emission of 540 nm

19

20

21

22

23

24

25

1 2 3 4 5

Lab

Decay t

ime f

or

540 n

m

min

max

[ns

]

Lifetime evaluation using mono exponential fitting procedures

� A mono-exponential fit is very sufficient to determine the lifetime of the

observed emission

� The shape of the decay curves is very similar, but the intensity is different

between the phosphor and the glass ceramic material.

� This may be explained by the different absorption mechanism, but nevertheless

the observed samples show the same decay characteristics.

45

50

55

60

65

70

-50 0 20 60 100 130 150

Temperature in °C

De

ca

y t

ime

in

ns

Schott 5 Schott 1 Schott 1 repro 3 % in Silicone

Lifetime dependency versus temperature

� The temperature dependency shows no significant changes within the accuracy

� In order to check how temperature dependent effects occur, as already reported for

different types of phosphors, we have to improve the accuracy of our

measurements for these types of material.

Single crystal

lifetime at RT

67 +/- 6 nsE.Zych et al.J. Lum. 75, 193 (1997)

60 - 65 nsE. Mihokova et al.J. Lum. (2006)

� The lifetime measurement of the Ce3+ fluorescence in our glass ceramic has

been performed for the first time.

� Accurate lifetime measurements in our glass ceramic show a lifetime of

62 +/- 6 ns and suggest that our material is at least comparable to existing

YAG:Ce phosphors (67 +/- 5 ns) with respect to internal conversion efficiency.

� We believe to reach an accuracy below ± 10 %.

� In the nearest future round robin tests have to be performed in order to check

the consistency of the fitting procedures.

� Next steps will be further temperature dependent measurements in order to

prove the observed shifts of the life time

Conclusion Lifetime measurements

Conclusion

� A glass ceramic material shows large potential as a converter material for

blue-white light conversion in a white LED.

� Our glass ceramic is a moderate scattering, translucent material.

� it allows freedom of design to adjust the scattering level

� it offers a challenging regime for measuring and quantifying fluorescence.

� As a first step we showed fluorescence as well as excitation spectra and

measurements of the life time for the Ce3+ fluorescence.

� We conclude from lifetime measurements that our material has practically no

additional channels for non radiative decay.

� There is no principle reason that this glass ceramic material will not reach

best conversion efficencies know today from phosphor powders.