Status of III-V and Nano-Scale Photo- Cathodes at ANL The ANL: Thomas Prolier Matthew Wetstein Igor Veryovkin Zikri Yusof Alexander Zinovev Bernhard Adams Klaus Attenkofer Matthieu Chollet Zeke Insepov Anil Mane Quing Peng

Overview  Physics & Technological Challenges –Different Applications have Different Needs –The Specific Challenge of a Large Photocathode –Technological Challenges: Price, Simplicity, Materials- Process-Compatibility  The Description of the Scientific and Engineering Program –GaN-Family –GaAsP-Family –Nano-Structures  The Path: –Sample Preparation –Characterization  The Goals and “Measure of Success” Large Area Detector Project: 1. Collaboration Meeting 2 10/15/09

Physics & Technological Challenges D i f f e r e n t A p p l i c a t i o n s h a v e D i f f e r e n t N e e d s  Required spectral response still not clear (main application)  Future applications (combination with scintillators) will require response optimization  III-V are complex to grow, but: –Full developed industry available for production and production tools –Large efforts worldwide in refining growth-technology –This effort can be a milestone for future device development Large Area Detector Project: 1. Collaboration Meeting 3 SuffixPhotocathodeInput Window -71GaAsBorosilicate Glass -73 Enhanced Red GaAsP Borosilicate Glass -74GaAsPBorosilicate Glass -76InGaAsBorosilicate Glass NonMultialkaliSynthetic Silica Enhanced Red Multialkali Synthetic Silica -02BialkaliSynthetic Silica -03Cs-TeSynthetic Silica 10/15/09 Hamamatsu:

Physics & Technological Challenges The Specific Challenge of a Large Photocathode  Good conductivity layer to avoid charging effects  Good homogeneity of the cathode over the full size  No “insitu” activation possible -> in vacuum fabrication and sealing necessary  Cathode has to be process compatible to sealing process and final assembly  Cathode has to work under “relaxed” vacuum conditions  The price of the detector will be largely by cathode processing determined  Well established doping methods available  Foundries with large throughput and wafer-sizes available (process parameters can be developed on lab-sizes systems)  High temperature resistivity (about 550C)  Emerging nano-technologies are available  Industrial standards available (yield, homogeneity) Large Area Detector Project: 1. Collaboration Meeting 4 10/15/09 III-V are an appropriate approach:

Physics & Technological Challenges T e c h n o l o g i c a l C h a l l e n g e s : P r i c e, S i m p l i c i t y, M a t e r i a l s - P r o c e s s - C o m p a t i b i l i t y Scalable production tools available Large Area Detector Project: 1. Collaboration Meeting 5 10/15/09 Process parameters can be developed on lab-system and transferred to production systems Complex fabrication (layer-system) will be performed in foundry with quality control Ready-to-mount cathode (on window) will be transported in air, chemically cleaned and finally brought in the vacuum assembly chamber. Activation requires high temperature (~ C) and small amounts of Cs (sub-monolayer)

Description of the Scientific & Engineering Program Physics of Semiconductor Cathodes  Interface layer between window/substrate and active area: –Defines how much light gets into the active light (reflection) –Important for compatibility (growth on glass, bonding, transfer printing….) –Conductivity-layer  Active area: –Light absorption (multilayer options) –Electron transport (scattering/trapping) –Noise-suppression layers  Surface: –Electron escape –Responsible for dark-current –Surface states extreme sensitive to chemical changes 10/15/09 Large Area Detector Project: 1. Collaboration Meeting 6 The Three Functions of a Cathode:

Description of the Scientific & Engineering Program The Negative Electron Affinity 10/15/09 Large Area Detector Project: 1. Collaboration Meeting 7 What are surface states:

Description of the Scientific & Engineering Program Tunability of III-V 10/15/09 Large Area Detector Project: 1. Collaboration Meeting 8 Two “families”: N-based and As-based Wide tunability of band-gap Only for specific materials -combinations NEA available No cross combination of families possible “Good materials” are direct band gap

Description of the Scientific & Engineering Program G a N - F a m i l y  Largest variation in band-gap  Growth on  -Al 2 O 3 (sapphire)  GaN NEA-layer exist  GaN is UV active  Perfect combination would be Ga x In (x-1) N, but: large strain -> high defect density -> large losses  Direct growth on ALD coated  -Al 2 O 3 (sapphire) glass  InN/GaN multilayer system to adjust band-gap and minimize strain  Cascade structures?  Optimizing surface reconstruction (growth direction, temperature, coating) Large Area Detector Project: 1. Collaboration Meeting 9 10/15/09 The Challenge The Research Program Jim Buckley & Daniel Leopold (Wash University)

Description of the Scientific & Engineering Program GaAsP-Family  Largest family  Growth on GaAs substrate  GaAs too much red!  GaAsP large strain (Similar to GaInN)  Alternative: AlGaAs/GaAs multilayer  No NEA system known for AlGaAs  Finding best bonding or transfer printing technique  Optimizing AlGaAs/GaAs film structure and doping profile  Surface doping & NEA layer  Delta-doping? Large Area Detector Project: 1. Collaboration Meeting 10 10/15/09 The Challenge The Research Program Xiuling Li and colleagues (UIUC)

Description of the Scientific & Engineering Program Nano-Structures  Largest variety of growth combinations  Radial and longitudinal growth possible  Ion-edging is no issue  Not demonstrated (but various groups have projects)  Growth on glass is possible  Dark current and field enhancement  Developing of a delta-doped radial structure  Most likely GaInN, first test structures GaAs Large Area Detector Project: 1. Collaboration Meeting 11 10/15/09 The Challenge The Research Program Jonas Johansson (university of Lund)

The Path: Who is involved? 10/15/09 Large Area Detector Project: 1. Collaboration Meeting 12 People involved (so far): Klaus Attenkofer Zeke Insepov Matth Wetstein Zikri Yusof (Thomas Prolier) “Bernhard Characterization”: Bernhard Adams Matthieu Chollet Matth Wetstein Berkeley Activity (Ossi) Common Meetings By Matth Wetstein Characterization Group Regular Meeting Technical coordination By Dean Walters Potential sample fabrication: Xiuling Li (UIUC) Jim Buckley & Daniel Leopold (Wash University) Jonas Johansson (first samples are waiting for characterization) “Novosibirsk connection” (Zeke Insepov) Thomas Prolier (ALD and?) Igor Veryovkin Alex Zinovev

The Path: S a m p l e P r e p a r a t i o n  Production of “raw-cathode” at collaboration partner (later perhaps also own fabrication capabilities)  Cathode Activation in Argonne (currently work on chamber design)  Compatible to characterization group  Large Area Detector Project: 1. Collaboration Meeting 13 10/15/09 Standard according Dean Walter Surface cleaning Chamber: HCL at 1mbar Heating Ni-chamber or glass? Simple thermal coating facility Cs-source Insitu in-plane resistivity Insitu QE-measurement Characterization of: Quantitative QE(E) Noise/QE Field enhancement Time response

The Path: C h a r a c t e r i z a t i o n Large Area Detector Project: 1. Collaboration Meeting 14 10/15/09 Characterization: QE(E) quantitative Noise/QE I(E Ph,U external,T) (Photo current) I/µd (Photo current versus absorption) Calibration of simple light sources Timing characterization (up to 8/25/50/70GHz?) Properties: Transportable Fully computer controlled “Bernhard compatible” “small” optical table Progress & Status: Optics components ordered Electronics components ordered Calibration diodes available Data-acquisition system in progress Current design of vacuum system, chamber, evaporators

The Goals and “Measure of Success”  Establishing of collaboration and growth of “small samples (1x1cm 2 )”  Assembly of high throughput activation/characterization chamber  Automatic data-acquisition and analysis system  Modeling of timing behavior  Demonstration of successful activation of the three cathode systems  Demonstration of QE = 15% for the three cathode systems  GaN –Evaluation of growth on ALD grown Al 2 O 3 -films –Demonstration and characterization (dark current/QE) of multilayer approach –Standard NEA-approach  GaAsP –Demonstration and characterization of transfer-printing –AlGaAs/GaAs verus GaAsP evaluation –Investigating NEA-effect and surface reconstruction/coating effects  Nano-structure –Feasibility test (dark current) Large Area Detector Project: 1. Collaboration Meeting 15 10/15/09 First Year: