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

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

Outline � Introduction � X-ray image phosphors � Objective � Synthesis of ZnTe:O for biological imaging � Experimental � Dry doping by ball-milling in O 2 � Dry etching in H 2 atmosphere � Results and Discussion � Optical and structural analysis � Comparison with standard X-ray phosphors � Conclusions and Future Work Sep.20, 2005 II-VI Workshop 2

X-ray luminescence and phosphors Principle of x-ray luminescence Principle of x-ray luminescence Absorbing of an X-ray photon Producing an energetic photoelectron Generating secondary e-h pairs E-h recombining at luminescent centers Emitting visible photons High x-ray absorption ( α α α α x-ray ) and large density; Characteristics of Low cost per e-h pair (small <E eh > and E g ) Characteristics of Efficient electron-hole transport ( η η eh ) η η efficient x-ray efficient x-ray High luminescent efficiency ( QE l ) phosphors phosphors Low optical self absorption ( α α ph ) α α Sep.20, 2005 II-VI Workshop 3

Quantum gain of X-ray phosphors Expression of quantum gain g eh (light photons/X-ray photon) of X-ray phosphors: g eh = E xray η eh QE l = E xray η eh QE l β E g E eh β β β β Host material E g (eV) <E eh > (eV) g eh ZnTe 2.3 2.2 5.0 2400 ZnSe 2.7 2.2 5.9 2040 ZnS 3.8 2.9 11.0 1040 g eh NaI 5.9 2.7 15.9 755 CsI 6.4 2.5 16.0 750 Gd 2 O 2 S 4.4 3.9 17.2 700 CaWO 4 4.6 7.0 32.3 370 < E eh >: mean creation energy to form an e-h pair; η η η eh : e-h transport efficiency; η QE l : luminescent center conversion efficiency. Inoue et al., J. Appl. Phys., 55, 1558 (1984) Sep.20, 2005 II-VI Workshop 4

X-ray phosphors for biological imaging 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 X-ray � ZnSe:Cu,Ce,Cl has the highest Fiber Optic X-ray w indow window known x-ray conversion efficiency fiberoptic taper taper 1.7 times higher than Gd 2 O 2 S:Tb � However, not suitable for � imaging of biologic cells because of Se edge � ZnTe under development for macromolecular applications Efficiency potentially superior to � ZnSe CCD CCD No Se edge, suitable for MAD � Phosphor experiments phosphor screen screen Sep.20, 2005 II-VI Workshop 5

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. Conventional wet doping process used for ZnS and ZnSe Conventional wet doping process used for ZnS and ZnSe phosphors synthesis is very difficult for ZnTe; phosphors synthesis is very difficult for ZnTe; A dry doping process is needed. A dry doping process is needed. Dry synthesis process: Ball-milling in Crystallization Annealing Drying Coating O 2 or Ar/ O 2 in N 2 -5%H 2 in Zn vapor Ball-milling of ZnTe in O 2 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. Sep.20, 2005 II-VI Workshop 6

PL properties of ZnTe:O phosphors Conduction Band 1400 1 1: Ball milled in O 2 PL 2: 0.2% ZnO doped 1200 3: 1% ZnO doped Intensity (a.u.) 4: 5% ZnO doped 1000 800 O 600 400 2 Excitation 3 2.25 eV 200 4 0 Emission 600 650 700 750 800 850 Excitation 1.8 eV Wavelength (nm) 6000 PLE 880 5000 840 Intensity (a.u.) 4000 570 580 PLE PL 3000 2000 Valence Band 1000 0 680nm emission from oxygen centers 450 500 550 600 650 700 750 800 850 Wavelength (nm) Sep.20, 2005 II-VI Workshop 7

CL properties of ZnTe:O phosphors 1 Relative CL Efficiency (a.u.) CL Decay 1.2 CL 1/e Relative Intensity Ball milled in O 2 1.0 1.1 µ µ s µ µ 0.8 10% 0.1 0.6 2.6 µ µ µ s µ 0.2% ZnO doped 8kV, 4.3 µ µ s pulses µ µ 0.4 Integrated intensity 0.2 0.0 0.01 4 6 8 10 12 14 16 18 20 22 0 1 2 3 4 5 Decay Time ( µ µ µ µ s) Voltage (kV) O 2 doping significantly improved the CL efficiency compare to ZnO doping. Fast CL exponential decay time of 1.1 µ s was observed. Sep.20, 2005 II-VI Workshop 8

Effect of N 2 /5%H 2 annealing on surface property Particle morphology by SEM Surface chemistry by XPS (a) (b) 1: 95% N 2 / 5% H 2 Te 3d 2: N 2 1 µ µ µ m µ 3: Vacuum 1 50 µ µ µ µ m 25 µ µ µ µ m 2 µ µ m µ µ 2 (a) ZnTe:O annealed in Vacuum; (b) ZnTe:O annealed in N 2 /5%H 2 . 3 Surface Oxide Smoothed surface morphology 590 585 580 575 570 Bingding Energy (eV) Removal of surface tellurium oxides Sep.20, 2005 II-VI Workshop 9

Improvement of optical property by H 2 annealing CL PL 2800 Relative CL Efficiency (a.u.) 2.0 1 2400 95% N 2 / 5% H 2 PL intensity (a.u.) 1: 95% N 2 / 5% H 2 Vacuum 2000 2: Vacuum 1.5 N 2 3: N 2 1600 2 1.0 1200 800 3 0.5 400 0 0.0 4 6 8 10 12 14 16 18 20 22 550 600 650 700 750 800 850 900 Wavelength (nm) Voltage (kV) X-ray luminescent efficiency Annealing % gain % gain Sample No. atmosphere (ZnSe: Cu,Cl) (Gd 2 O 2 S:Tb) ZT05 Vacuum 11.6 21.9 ZT11 N 2 9.2 12.4 ZT120 95%N 2 /5%H 2 56.1 76.4 Luminescent efficiency of ZnTe:O improved ~5 times after H 2 annealing Sep.20, 2005 II-VI Workshop 10

Preliminary X-ray imaging results ZnTe:O screen x-ray imaging Resolution: 2.5 lines/mm Mo (17 KeV) radiation is used Sep.20, 2005 II-VI Workshop 11

Comparison with standard phosphors (1) 50 QE CCD Quantum Efficiency (%) 40 Instensity (a.u.) ZnSe:Cu,Ce,Cl 30 Gd 2 O 2 S:Tb 20 ZnTe:O 10 0 400 500 600 700 800 900 1000 Wavelength (nm) � The emission spectrum of ZnTe:O is an very good match to the spectral sensitivity of front-illuminated CCD Sep.20, 2005 II-VI Workshop 12

Comparison with standard phosphors (2) Phosphor Material ZnTe:O ZnSe:Cu Gd 2 O 2 S:Tb Peak wavelength (nm) 680 650 545 Gain% (Mo, 17 KeV) 56.1 100 73.4 Gain% (Cu, 8KeV) 111.3 100 163.6 1 × × × × 10 -4 1 × × 10 -4 × × 7 × × 10 -4 × × Afterglow (10ms later) 1/e Decay time ( µ µ s) µ µ 1.1 8.9 470 Resolution (lines/mm) 2.5 2.5 2.5 Screen density (mg/cm 2 ) 46 45 12 Particle size (um) 51 20 9 � 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. Sep.20, 2005 II-VI Workshop 13

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 1.1 µ µ s. µ µ � 5 times improvement of X-ray luminescent efficiency was observed 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 Gd 2 O 2 S:Tb – An equal resolution of 2.5 lines/mm � Future Work � Optimize doping & annealing to further improve QE � Develop dry coating technique Sep.20, 2005 II-VI Workshop 14

Acknowledgement � Financial support from Molecular Beam Consortium � Support by Ga Tech Research Institute Shackelford GRA Fellowship Sep.20, 2005 II-VI Workshop 15