Investigations of CdTe and (Cd,Zn)Te crystals grown by the Bridgman method M. Fiederle, A. Fauler, V. Babentsov, J. Franc, J. Ludwig, K.W. Benz Freiburger Materialforschungszentrum FMF Albert-Ludwigs-Universität Stefan-Meier-Straße 21, D-79104 Freiburg www.2-6.uni-freiburg.de; Email: michael.fiederle@fmf.uni-freiburg.de • growth of 25 and 75 mm CdTe • defects and compensation • material properties and detector performance Freiburger Materialforschungszentrum FMF M. Fiederle Why is doping of CdTe/(Cd,Zn)Te necessary? • there are three posibilities to obtain high resistivity: – intrinsic material (problem of shallow impurities) – shallow compensation (problem of stability) • Cl, In, ... – three level compensation • deep intrinsic defect (TeCd, VCd, ??) • deep donor – V, Ge, Fe, ... (problem of deep traps) • doping is mandatory to obtain high resistivity! Freiburger Materialforschungszentrum FMF M. Fiederle configuration of growth • ampoule – inner diameter 24 and 75 mm – semiconductor quality HSQ300 / HSQ800 Heraeus – graphitized ampoule • furnace – ceramic tube (safety) – 4 PtRh-Pt thermocouples – online monitoring (4 values / 10 sec.) • material – 7N Cd, Te, Zn (6N) – dopants: Ge, Sn, Al, In, Cl – C[dopants]: 1018 - 1019 Freiburger Materialforschungszentrum FMF M. Fiederle temperature timeline 75 mm CdTe 1200 melting point CdTe 800 o temperature / C 1000 600 exothermic reaction (400 K in 10 sec) 400 200 melting Cd and Te 0 0 20000 40000 60000 time 80000 Freiburger Materialforschungszentrum FMF 100000 120000 M. Fiederle (Cd,Zn)Te undoped crystal 24 mm Freiburger Materialforschungszentrum FMF M. Fiederle grains and twins – problems of melt growth CdTe:Ge wafer grown with modifications Freiburger Materialforschungszentrum FMF M. Fiederle resistivity of 75 mm CdTe:Ge wafer (TDCM) resistivity in cm 70 6,6E9 6,6E9 6,3E9 6E9 5,7E9 5,4E9 5,1E9 4,9E9 4,6E9 4,3E9 4E9 3,7E9 3,4E9 3,2E9 2,9E9 2,6E9 2,3E9 2E9 1,7E9 1,4E9 1,2E9 8,8E8 60 length / mm 50 40 30 20 10 0 0 10 20 30 40 50 60 70 length / mm Freiburger Materialforschungszentrum FMF axial distribution: 2.0 – 8.0 x109 cm M. Fiederle chemical analysis GDSM and LIMS cooperation with Li B Na Mg Al Si P S Cl K Ca Fe Co Ni Cu Zn As Se Ag In S1 < 0.001 0.0004 0.01 0.014 0.015 0.013 0.0003 0.003 < 0.001 0.002 0.006 0.034 < 0.0001 0.004 0.002 0.003 0.003 < 0.01 < 0.05 0.01 R. James, Brookhaven National Lab. and E. Cross, Sandia National Labs. S2 < 0.023 0.0004 0.02 0.004 0.021 0.016 0.002 0.002 0.002 0.006 0.004 0.047 0.0001 0.0005 0.39 0.010 0.010 < 0.01 < 0.05 0.01 S3 0.013 0.0007 0.027 0.004 0.007 0.001 0.003 0.005 0.006 0.001 0.005 0.13 < 0.0001 0.009 2.9 0.005 0.003 < 0.01 < 0.05 0.01 • • • • • concentration of impurties in ppm GDMS data of three different locations of CdTe:Ge crystal concentration of Ge (AAS): – 2 – 3 x 1016 cm-3 total concentration of shallow impurities less than 3 ppm (~ 4 x 1016 cm-3) concentration of possible deep level: – Fe 47 ppb (~ 7 x 1014 cm-3) Freiburger Materialforschungszentrum FMF M. Fiederle deep levels in CdTe:Ge Level Energy (eV) Capture cross section (cm2) Beginning End P1 P2 P4 0.11 0.21 1.10-18 1.10-15 P5 P7 P8 0.35 0.75 0.95-1.05 5.10-13 1.10-11 6.10-8 0.16 0.34 0.42eV 0.74 0.95-1.05 1.10-7 8.10-13 3.10-16 1.10-11 6.5.10-8 transition transition transition transition Fe or Cu related VCd-2 - V band Ge related defect- Fe related Tentative transition identification A center- V band unknown defect-V band defect-C band C band Freiburger Materialforschungszentrum FMF M. Fiederle CdTe:Ge PL maps at 80K A-centre FE 0.8 eV 1.0 eV 70 50 12 40 9 30 5 20 PL Intensity in colors Crystal length (mm) 60 2 10 0 0 10 20 Crystal diameter (mm) 0 10 20 0 Crystal diameter (mm) 10 20 Crystal diameter (mm) 0 10 20 Crystal diameter (mm) Freiburger Materialforschungszentrum FMF M. Fiederle Resistivity and PL signal of undoped (Cd,Zn)Te PL map at 80K Resistivity map at RT ( h nmax = 1.636 eV ) 1E10 3,000 7E9 25 5,5E9 20 4E9 15 2,5E9 10 25 Resistivity ( cm) 30 Crystal length (mm) 30 8,5E9 2,449 20 1,760 15 10 1,209 5 1E9 5 0,6578 0 5 10 15 20 25 Crystal diameter (mm) 30 0 0 5 10 15 20 Crystal diameter (mm) Freiburger Materialforschungszentrum FMF M. Fiederle PL Intensity in pseudo-colors 35 Detektor properties • Setup: – Amptek A250 Preamplifier – module with bonded detector – detector with guarding properties (alpha particles) detector mete t. b. measured (10-4 cm2/V) mhth 6 x 10-5 cm2/V resolution bias Freiburger Materialforschungszentrum FMF 10 % 30 – 200 V M. Fiederle technolgy • detector processing – lithography – metalization – wire bonding – passivation (BCB) • fabrication of strip detectors – 127 µm pitch – 24 x 14 mm detector array 500 µm 14 x 24 mm detection area crystal board Freiburger Materialforschungszentrum FMF M. Fiederle X-ray analysis of a copper-cable ( 500 µm diameter) 1 Dunkelwert- und Verstä rkungsnormiert Photocurrent a.u. 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 2.2 2.4 2.6 2.8 3 3.2 3.4 Weg in mm X [mm] 3.6 3.8 4 ©Sikora Industrieelektronik GmbH Freiburger Materialforschungszentrum FMF M. Fiederle Technology: Flip Chip Bonding CdTe • cooperation with Prof. G. Anton, H. Braml Friedrich-Alexander-Universität Erlangen-Nürnberg • in-house processing of CdTe and (Cd,Zn)Te Quelle: Pac Tech Freiburger Materialforschungszentrum FMF M. Fiederle Technology: Flip Chip Bonding CdTe • technology: Solder bumps • material: PbSn and In • processing: Electroplating (Cu+PbSn) Deposition (Indium) • backend: manual Flip-Chip-Bonding Freiburger Materialforschungszentrum FMF M. Fiederle Summary • successful growth of 75 mm and 24 mm CdTe and (Cd,Zn)Te – large single crystalline areas • high resistivity material (>5 x 109 cm) – successful compensation by • deep donor Ge, Sn and undoped (Cd,Zn)Te – homogeneous distribution of material properties – deep levels correlated with Ge, Fe and Cu were detected • detector performance suffient – reduction of impurities / dopants necessary • successful detector technology – pixel detectors under developing Freiburger Materialforschungszentrum FMF Acknowledgement: • European Space Agency – MAP programm • Humboldt Stiftung M. Fiederle compensation in CdTe:Ge • 3 level compensation – shallow acceptor NA(Cu, ...) – shallow donor ND(Cl, In, ..) – deep donor NDD • germanium N DD N DD E EDD 1 exp F kT p N D N DD n N A Freiburger Materialforschungszentrum FMF M. Fiederle compensation in CdTe:Ge • 3 level compensation – shallow acceptor NA(Cu, ...) – shallow donor ND(Cl, In, ..) – deep donor NDD • germanium N DD N DD E EDD 1 exp F kT p N D N DD n N A Freiburger Materialforschungszentrum FMF M. Fiederle photoluminescence of CdTe:Ge • mapping of 45 mm wafer • selection of different energy levels corresponding to: – A-center – exciton – deep level at 1.0 eV • comparison with resitivity mapping Freiburger Materialforschungszentrum FMF M. Fiederle Comparison resistivity and photosensitivity Freiburger Materialforschungszentrum FMF M. Fiederle photoluminescence spectra (77K) of CdTe:Ge • found emissions: – near band gap emmision (FE) – A-center – 1.1 eV emission – 0.82 eV • slightly reduction of crystal quality from beginning of growth to the end Freiburger Materialforschungszentrum FMF M. Fiederle growth of 25 mm (Cd,Zn)Te • • • ampoule – inner diameter 24 mm – semiconductor quality HSQ800 Heraeus – graphitized ampoule furnace – ceramic tube (safety) – 4 PtRh-Pt thermocouples – online monitoring (4 values / 10 sec.) material – 7 N Cd, Te – 6 N Zn – 5 percent Zn Freiburger Materialforschungszentrum FMF M. Fiederle P L I n t e n s i ty ( a r b . u n .) P L s p e c tru m o f C Z T c ry s ta l DX 6 1. 22 eV 5 4 .2 K 1. 115 eV 4 3 x20 1 P L In te n s i t y ( a r b . u n . ) FE AX 2 0 0 .8 0 .9 1 .0 1 .1 1 .2 1 .3 1 .4 1 .5 1 .6 0 1 .6 5 1 .7 0 1 .5 80K D -h 1. 09 eV G a u s s ia n f it 1 .0 FE 0 .5 x40 0 .0 0 .8 0 .9 1 .0 1 .1 1 .2 1 .3 1 .4 1 .5 1 .6 0 1 .6 5 1 .7 0 hn, eV Freiburger Materialforschungszentrum FMF M. Fiederle Resistivity and Zn concentration • strong correlation between zinc concentration and resistivity 5,2 Zn (at.%) 4,8 • resistivity and zinc show the same behaviour 4,4 a 4,0 3,6 3,2 10 2,0x10 ( cm) • experimental data close to the theoretical ones 1 10 1,6x10 theoretical 10 1,2x10 9 8,0x10 2 9 4,0x10 0,0 0 5 10 15 20 X (mm) Freiburger Materialforschungszentrum FMF M. Fiederle 25 35 zinc concentration in the lower part 30 25 1E10 Crystal length (mm) 20 7E9 15 5,5E9 10 4E9 2,5E9 Resistivity ( cm) 8,5E9 5 1E9 10 15 20 25 0 30 Crystal diameter (mm) 10 4,800 zn1 y (mm) 4,444 8 4,089 6 4 3,733 Zn (at. %) 5 2 2 4 6 8 10 12 14 16 18 20 22 24 3,378 x (mm) Freiburger Materialforschungszentrum FMF M. Fiederle deep levels in CdTe:Ge Conduction band Shallow donors 0.9eV 0.6eV Thermal transitions 1 eV Eg = 1.59eV 1.21eV Optical transitions Ge2+/Ge3+ (R-center) 1.47eV 0.69eV Fe2+/Fe3+ (R-center) Phonons 0.12eV 0.38eV A center Freiburger Materialforschungszentrum FMF Valence band M. Fiederle