Oxygen Doped ZnTe Phosphors for Synchrotron X-Ray Imaging Detectors - - PowerPoint PPT Presentation

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Oxygen Doped ZnTe Phosphors for Synchrotron X-Ray Imaging Detectors Z. T. Kang, H. Menkara, B. K. Wagner and C. J. Summers Georgia Institute of Technology, Atlanta, GA 30332 R. Durst, Y. Diawara, G. Mednikova and T. Thorson Bruker AXS 5465 East

SLIDE 1

Oxygen Doped ZnTe Phosphors for Synchrotron X-Ray Imaging Detectors

Z. T. Kang, H. Menkara, B. K. Wagner and C. J. Summers

Georgia Institute of Technology, Atlanta, GA 30332

R. Durst, Y. Diawara, G. Mednikova and T. Thorson

Bruker AXS 5465 East Cheryl Parkway, Madison WI 53711 September 20, 2005

SLIDE 2

Sep.20, 2005 II-VI Workshop 2

Outline

Introduction

X-ray image phosphors

Objective

Synthesis of ZnTe:O for biological imaging

Experimental

Dry doping by ball-milling in O2 Dry etching in H2 atmosphere

Results and Discussion

Optical and structural analysis

Comparison with standard X-ray phosphors

Conclusions and Future Work

SLIDE 3

Sep.20, 2005 II-VI Workshop 3

X-ray luminescence and phosphors

High x-ray absorption (α α α αx-ray) and large density; Low cost per e-h pair (small <Eeh> and Eg) Efficient electron-hole transport (η η η ηeh) High luminescent efficiency (QEl) Low optical self absorption (α α α αph)

Absorbing of an X-ray photon Producing an energetic photoelectron Generating secondary e-h pairs E-h recombining at luminescent centers Emitting visible photons

Principle of x-ray luminescence Principle of x-ray luminescence Characteristics of efficient x-ray phosphors Characteristics of efficient x-ray phosphors

SLIDE 4

Sep.20, 2005 II-VI Workshop 4

Quantum gain of X-ray phosphors

370 32.3 7.0 4.6 CaWO4 700 17.2 3.9 4.4 Gd2O2S 750 16.0 2.5 6.4 CsI 755 15.9 2.7 5.9 NaI 1040 11.0 2.9 3.8 ZnS 2040 5.9 2.2 2.7 ZnSe 2400 5.0 2.2 2.3 ZnTe geh <Eeh> (eV) β β β β Eg (eV) Host material <Eeh>: mean creation energy to form an e-h pair; η η η ηeh: e-h transport efficiency; QEl: luminescent center conversion efficiency.

Expression of quantum gain geh (light photons/X-ray photon) of X-ray phosphors:

Inoue et al., J. Appl. Phys., 55, 1558 (1984)

geh = ExrayηehQEl Eeh = ExrayηehQEl βEg geh

SLIDE 5

Sep.20, 2005 II-VI Workshop 5

X-ray phosphors for biological imaging

ZnSe:Cu,Ce,Cl has the highest known x-ray conversion efficiency

1.7 times higher than Gd2O2S:Tb
However, not suitable for

imaging of biologic cells because

f Se edge

ZnTe under development for macromolecular applications

Efficiency potentially superior to

ZnSe

No Se edge, suitable for MAD

experiments

Applications: Efficient and fast X-ray phosphors needed for CCD detectors used for synchrotron-based structural biology Macromolecular imaging such as biologic cells, protein, ribosome

fiberoptic taper X-ray w indow phosphor screen CCD

CCD Fiber Optic taper X-ray window Phosphor screen

SLIDE 6

Sep.20, 2005 II-VI Workshop 6

Synthesis issues for ZnTe:O X-ray phosphors Issues: ZnTe is very sensitive to moisture during synthesis. Tellurium oxides are formed on the particle surface. Dry synthesis process:

Ball-milling in O2 or Ar/ O2 Drying Crystallization in N2-5%H2 Annealing in Zn vapor Coating

Ball-milling of ZnTe in O2 can lead to mechanically stimulated ion implantation of oxygen into the crystal lattice; Doping ZnTe with a gas media through ball milling is much more effective than doping by solid or liquid medias.

Conventional wet doping process used for ZnS and ZnSe phosphors synthesis is very difficult for ZnTe; A dry doping process is needed. Conventional wet doping process used for ZnS and ZnSe phosphors synthesis is very difficult for ZnTe; A dry doping process is needed.

SLIDE 7

Sep.20, 2005 II-VI Workshop 7

PL properties of ZnTe:O phosphors

600 650 700 750 800 850 200 400 600 800 1000 1200 1400 1: Ball milled in O2 2: 0.2% ZnO doped 3: 1% ZnO doped 4: 5% ZnO doped 4 3 2 1

Intensity (a.u.) Wavelength (nm)

450 500 550 600 650 700 750 800 850 1000 2000 3000 4000 5000 6000

PL PLE Intensity (a.u.) Wavelength (nm)

570 580 840 880

PL PLE Valence Band Conduction Band 2.25 eV Excitation Emission O 1.8 eV Excitation 680nm emission from oxygen centers

SLIDE 8

Sep.20, 2005 II-VI Workshop 8

4 6 8 10 12 14 16 18 20 22 0.0 0.2 0.4 0.6 0.8 1.0 1.2 0.2% ZnO doped

Relative CL Efficiency (a.u.) Voltage (kV)

Ball milled in O2 1 2 3 4 5 0.01 0.1 1 Integrated intensity 8kV, 4.3µ

µ µ µs pulses

1.1µ

µ µ µs

2.6µ

µ µ µs

10% 1/e

Relative Intensity Decay Time (µ

µ µ µs)

CL properties of ZnTe:O phosphors

CL Decay CL O2 doping significantly improved the CL efficiency compare to ZnO doping. Fast CL exponential decay time of 1.1µs was observed.

SLIDE 9

Sep.20, 2005 II-VI Workshop 9

590 585 580 575 570 3: Vacuum 2: N2 1: 95% N2 / 5% H2 3 2 1 Surface Oxide Te 3d

Bingding Energy (eV)

1 µ µ µ µm 25 µ µ µ µm

(b) (a)

50 µ µ µ µm 2 µ µ µ µm

Particle morphology by SEM Surface chemistry by XPS

(a) ZnTe:O annealed in Vacuum; (b) ZnTe:O annealed in N2/5%H2.

Smoothed surface morphology Effect of N2/5%H2 annealing on surface property Removal of surface tellurium oxides

SLIDE 10

Sep.20, 2005 II-VI Workshop 10

4 6 8 10 12 14 16 18 20 22 0.0 0.5 1.0 1.5 2.0

Relative CL Efficiency (a.u.) Voltage (kV)

95% N2 / 5% H2 Vacuum N2

Improvement of optical property by H2 annealing

550 600 650 700 750 800 850 900 400 800 1200 1600 2000 2400 2800 3 2 1

PL intensity (a.u.) Wavelength (nm)

1: 95% N2 / 5% H2 2: Vacuum 3: N2

76.4 56.1 95%N2/5%H2 ZT120 12.4 9.2 N2 ZT11 21.9 11.6 Vacuum ZT05 % gain (Gd2O2S:Tb) % gain (ZnSe: Cu,Cl) Annealing atmosphere Sample No.

X-ray luminescent efficiency

CL Luminescent efficiency of ZnTe:O improved ~5 times after H2 annealing PL

SLIDE 11

Sep.20, 2005 II-VI Workshop 11

Preliminary X-ray imaging results

Resolution: 2.5 lines/mm Mo (17 KeV) radiation is used ZnTe:O screen x-ray imaging

SLIDE 12

Sep.20, 2005 II-VI Workshop 12

The emission spectrum of ZnTe:O is an very good match

to the spectral sensitivity of front-illuminated CCD

Comparison with standard phosphors (1)

400 500 600 700 800 900 1000 10 20 30 40 50 ZnTe:O Gd2O2S:Tb ZnSe:Cu,Ce,Cl QECCD

Quantum Efficiency (%) Wavelength (nm) Instensity (a.u.)

SLIDE 13

Sep.20, 2005 II-VI Workshop 13

163.6 100 111.3 Gain% (Cu, 8KeV) 7× × × ×10-4 1× × × ×10-4 1× × × ×10-4 Afterglow (10ms later) 470 8.9 1.1 1/e Decay time (µ µ µ µs) 2.5 2.5 2.5 Resolution (lines/mm) 12 45 46 Screen density (mg/cm2) 9 20 51 Particle size (um) 73.4 100 56.1 Gain% (Mo, 17 KeV) 545 650 680 Peak wavelength (nm) Gd2O2S:Tb ZnSe:Cu ZnTe:O Phosphor Material

High efficiency, high resolution, fast decay, low afterglow and

improved spectral match to the CCD detector, indicate that ZnTe:O is a promising phosphor candidate for X-ray imaging applications.

Comparison with standard phosphors (2)

SLIDE 14

Sep.20, 2005 II-VI Workshop 14

Conclusions and Future work

Conclusions

ZnTe:O powder phosphors successfully prepared by dry

synthesis using gaseous doping and etching

– Red emission centered at 680nm; decay time 1.1µ

µ µ µs.

5 times improvement of X-ray luminescent efficiency was

bserved after annealing in a forming gas atmosphere,

attributed to the removal of surface tellurium oxides.

The X-ray luminescent properties were evaluated and

compared to standard commercial phosphors.

– Efficiency equivalent to 76% of Gd2O2S:Tb – An equal resolution of 2.5 lines/mm Future Work

Optimize doping & annealing to further improve QE Develop dry coating technique

SLIDE 15

Sep.20, 2005 II-VI Workshop 15

Acknowledgement

Financial support from Molecular Beam Consortium Support