National Aeronautics and Space Administration

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1 National Aeronautics and Space Administration The work described here was carried out by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. Copyright 2011 California Institute of Technology. Government sponsorship acknowledged. 1

2 1. Detector surfaces and the problem of stability 2. Deltadoped detectors 3. Physics of Deltadoped Silicon 4. Chemistry of the SiSiO2 Interface 5. Physics and Chemistry of Deltadoped Surfaces 1. Compensation 2. Inversion 3. Quantum exclusion 2

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5 George E. Smith, The invention and early history of the CCD, Nuclear Instruments and Methods in Physics Research A, 607: 16,

6 Silicon Imager Architectures: CCD vs. CMOS Chargecoupled Device (CCD) Serial readout device with charge transfer and one (or few) readout amplifiers. CMOS Imaging Array Parallel readout with few charge transfers and one readout amplifier per pixel. CCD Pixel Array Column Processing Circuitry Column Decoder Analog Output CMOS APS Scientific CMOS imagers are catching up with CCDs Jim Janesick,

7 UV Astronomy and the Surface Problem In 1984, quantum efficiency hysteresis was discovered during thermal vacuum testing of Wide Field Camera (WFPC1). Back surface Trapping of photogenerated charge Front surface Backside potential well Buried channel Quantum efficiency hysteresis CCD response depends on prior illumination history Unacceptable Hubble needs stability to 1% over 30 days Passivation of surface defects is necessary to solve the problem. 7

8 Quantum Efficiency Hysteresis on Hubble Light pipe added to WF/PC instrument to expose detectors to UV from sunlight WF/PC1 ( ) Massive UV flood at 250 nm through light pipe WF/PC2 ( ) Flash gate, biased flash gate WF/PC2 ( ) Front illuminated Loral CCDs with lumogen WFC3 (2009present) Back illuminated, ionimplanted CCDs John T. Trauger, Sensors for the Hubble Space Telescope Wide Field and Planetary Cameras (1 and 2), in CCDs in astronomy: Proceedings of the Conference, Tucson, AZ, Sept. 68, 1989 (A ), San Francisco, CA, Astronomical Society of the Pacific, 1990, p

9 The Evolution of CCD Surfaces on Hubble Surface doping early attempts Precision thinning leaves residual p+ (early WF/PC 1) Chemical charging early attempts UV flood (late WF/PC 1) Platinum flash gate (WF/PC 2 never flown) Biased flash gate (WF/PC 2 never flown) Phosphor coatings Frontilluminated with lumogen (WF/PC 2) Chemisorption later evolution Chemisorption (UA/ M. Lesser ACS HRC) Surface doping Ion implantation (WFC3) 11

10 Quantum Efficiency Hysteresis in WFC3 Ionimplanted CCDs on Wide Field Camera 3 Launched in 2009 Instabilities on the order of a few percent Mitigated by onorbit flooding with visible light to fill surface traps Collins et al. SPIE proceedings 7439A10, San Diego, CA, August

11 PingShine Shaw, Rajeev Gupta, Keith R. Lykke, Stability of photodiodes under irradiation with a 157nm pulsed excimer laser, Applied Optics, 44(2): (2005). 13

12 Nanostructured silicon for surface passivation 15

13 Conduction band edge (ev) Atomic layer control over device structure Low temperature process, compatible with VLSI, fully fabricated devices (CCDs, CMOS, PIN arrays) Deltadoped layer (dopant in single atomic layer) original Si deltadoped potential well width ~ 5 Å Native oxide 1 nm CCD frontside circuitry E c MBE growth 3 nm Back Surface Depth from surface (nm) Hoenk et al., Applied Physics Letters, 61: 1084 (1992) Fullyprocessed devices are modified using Molecular Beam Epitaxy (MBE) 16

14 Deltadoping vs. Ion implantation Energy (ev) Deltadoped 2.0 Near Surface Electronic Potential MBE 1.4 Ultrashallow implant Ion implant Hoenk et al., Applied Physics Letters, 61: 1084 (1992) Depth from Surface (nm) Delta doping produces highest surface electric fields of any passivation technology 17

15 Electric field (V/nm) Electric Field 10 Deltadoped 1 MBE Ultrashallow ion implant Ion implant Depth from Surface (nm)

16 Quantum Efficiency (%) Thinned CMOS Array front surface Deltadoped CCD (ARcoated) Back surface Deltadoped CCD (uncoated) Frontilluminated CCD Wavelength (nm) S. Nikzad, Ultrastable and uniform EUV and UV detectors, SPIE Proc., Vol. 4139, pp (2000). J. Trauger (PI WF/PC2) No measurable hysteresis in deltadoped CCDs

17 Standard Deviation (ppm) J. M. Jenkins, W. J. Borucki, E. W. Dunham, J. S. McDonald High Precision Photometry with BackIlluminated CCDs, ASP Conf Ser,1618 Oct STScI the [deltadoped] CCD performed as a nearly shotlimited photometer with only a few ppm of error at an integrated flux of e 20

18 Quantization of states 21

19 Energy (ev) Depth from surface (nm) Deltadoping creates quantum well in silicon Majority carriers confined in quantized subbands Peak electric field ~10 7 V/cm 22

20 Energy (ev) Depth from surface (nm) 20 Quasiisolation Elimination of QEH 23

21 Energy (ev) Surface Bulk Deltalayer Depth from surface (nm) 20 Positively charged cm 2 24

22 Proven performance Lyman alpha flood: No hysteresis! (John Trauger, PI WF/PC 2) Shotnoise limited photometry (Jensen et al., Kepler group) Quasiisolation Extroardinarily high fields: 10 7 V/cm Internal fields decoupled from surface charge Quantum exclusion Signal (QE): trapping of minority carriers suppressed by quantum confinement Noise: Surface dark current suppressed by deltalayer as tunnel barrier 25

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24 Interface is abrupt to 12 monolayers Local chemistry is difficult to resolve Structure is process dependent, including cleaning processes and contaminants Strained SiO 2 near the interface is vulnerable to radiation damage Radiation breaks SiO bonds; mobile defects migrate to surface creating amphoteric traps. Trapping of holes in nearinterfacial region creates fixed positive charge at the interface. F.J. Grunthaner and P.J. Grunthaner, Chemical and Electronic Structure of the SiO 2 /Si Interface, Materials Science Reports, 1 (2, 3): 65160,

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26 XPS data can t differentiate between hydrogen and dangling bonds; Interface densities inferred from XPS measurements are on the order of n 2D ~ 12x10 14 cm 2 29

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30 MBE Surface Passivation Structure Sheet number (x cm 2 ) Delta doping Shallow (15 A cap layer) 0.05 Intermediate (25A cap layer) 0.1 (inversion) Deep deltalayer (150A cap layer)

31 Conduction Band Edge vs. Surface Charge Bulk is independent of Surface Conduction band =1.08nm Depth (nm) Surface Charge (x10 14 cm 2 ) Conduction band =2.025nm Depth (nm) Surface Charge (x10 14 cm 2 ) Delta layer depth = 1nm Delta layer depth = 2nm 34

32 Electron ground states: TS surface state formation vs. Deltalayer depth E lectron ground states shifted D e p th (n m ) Surface Charge (x10 14 cm 2 ) E lectron ground states shifted P o s itio n (n m ) Surface charge (x10 14 cm 2 ) Delta layer depth = 1nm Delta layer depth = 2nm 35

33 Surface charge 2.0x x10 14 MIGS threshold Inversion threshold x x x x x x x10 13 Surface resonance Gap states Surface inversion x Cap layer thickness (ML) 36

34 Distribution of Holes and Electrons vs. Surface charge 2.000E E E E E E E E+20 HL_minus_EL =1.08nm 2.00E E E E E E E E E Depth (nm) Surface Charge (x10 14 cm 2 ) E E E E+21 Mobile charge densities (cm 3 ) 2.00E E E E E E E E E Depth (nm) Surface charge (x1014cm2) E E E E+21 4 Delta layer depth = 1nm Delta layer depth = 2nm 37

35 Integrated sheet densities, holes + electrons Ambipolar conduction Surface Charge (x10 14 cm 2 ) Cap layer (nm)

36 Surface compensation at these densities is surprising, but not unreasonable. 39

37 1. Quantum confinement of electrons and holes dominates the behavior of deltadoped surfaces 2. Stability of deltadoped detectors: Deltalayer creates a ~1 ev tunnel barrier between bulk and surface 3. At high surface charge densities, TammShockley states form at the surface 4. Surface passivation by quantum exclusion: Nearsurface deltalayer suppresses TS trapping of minority carriers 5. The SiSiO2 interface compensates the surface 6. For deltalayers at intermediate depth, surface inversion layer forms 7. Density of SiSiO2 interface charge can be extremely high (>10 14 cm 2 ) 40