MBE growth of GaAs photcathodes for the Cornell ERL photoinjector and effect of roughness on emittance Ivan Bazarov Bruce Dunham Siddharth Karkare Xianghong Liu Tobey Moore William Schaff CLASSE - Cornell Laboratory for Accelerator-based ScienceS and Education

Schaff et. Al., Photocathode Physics for Photoinjectors 2012 Contents Motivation Relate GaAs Surface Roughness to thermal emittance (TE) MBE growth of GaAs Photocathodes Arsenic protective caps protect surface from oxidation roughening GaAs epitaxial structures are implemented to test models of TE A speculative question - How much does surface roughness impact optical absorption? Summary 10/08/2012 Schaff et. Al., Photocathode Physics for Photoinjectors 2012

Surface roughening of GaAs Primary origins of Surface Roughness Ex-situ processing of cathodes in atmosphere Native oxidation promotes a roughening mechanism* In-situ heating to remove oxides and other contaminants High-temperature * Recommended reference : “Monitoring epiready semiconductor wafers” Allwood et. al., Thin Solid Films 412 (2002) 76–83 After: 350°C hydrogen clean 550°C 1 hour anneal Before: Siddharth Karkare et al., APPLIED PHYSICS LETTERS 98, 094104 2011 10/08/2012 Schaff et. Al., Photocathode Physics for Photoinjectors 2012

Reflection High Energy Electron Diffraction (RHEED) Side view of Phosphor screen Looking down from the top of the MBE machine Ga As GaAs Front view of Phosphor screen 10/08/2012 Schaff et. Al., Photocathode Physics for Photoinjectors 2012

Reflection High Energy Electron Diffraction (RHEED) of GaAs One goal for GaAs photocathodes is to create atomically flat surfaces Uniform intensity of diffraction streaks along their length only occurs for flat surfaces

RHEED of as-loaded GaAs wafer The bands of rings are diffraction from an amorphous (but fairly flat) surface This is an oxide on GaAs There is a faint diffraction line from underlying GaAs The oxide must be very few monolayers (ML) thick Substrate package opened just prior to loading No surface preparation was performed 10/08/2012 Schaff et. Al., Photocathode Physics for Photoinjectors 2012

RHEED image changes with substrate temperature 10/08/2012 Schaff et. Al., Photocathode Physics for Photoinjectors 2012

Flattening of GaAs surface 15 sec 2.1nm 2.1 nm of GaAs growth quickly fills in and around monolayer-scale pits and bumps to create an atomically flat surface. 10/08/2012 Schaff et. Al., Photocathode Physics for Photoinjectors 2012

Schaff et. Al., Photocathode Physics for Photoinjectors 2012 MBE wafer mounting 3inch wafer Arsenic deposited through mask Arsenic mask As4 through mask: 4 hours at 3x10-6T Tsub 580°C to -30°C 10/08/2012 Schaff et. Al., Photocathode Physics for Photoinjectors 2012

Photocathode wafer images 1 3/8 inch cathode in 3 inch wafer holder prior to arsenic deposition Wafer G20222 after arsenic deposition 10/08/2012 Schaff et. Al., Photocathode Physics for Photoinjectors 2012

Photocathode wafer images Wafer G20222 after arsenic deposition 10/08/2012 Schaff et. Al., Photocathode Physics for Photoinjectors 2012

Compare uniformly doped and undoped near-surface layers 12nm p-GaAs 5x1018cm-3 P-GaAs substrate surface P-GaAs substrate p-GaAs 5x1018cm-3 GaAs 100nm undoped 101nm Create more carriers in a longer high-field region: Faster turn-off time Hotter or colder emission? 10/08/2012 Schaff et. Al., Photocathode Physics for Photoinjectors 2012

Thermal Emittance for GaAs Photocathodes Bazarov, et Al., J. Appl.. Phys. 103, 054901 2008 This work: wafer G20222 MBE grown photocathode with 100nm undoped region at the top surface 10/08/2012 Schaff et. Al., Photocathode Physics for Photoinjectors 2012

Thermal emittance and 520-532nm quantum efficiency Measurements performed at different stages of activation cycles 10/08/2012 Schaff et. Al., Photocathode Physics for Photoinjectors 2012

Quantum efficiency speculation Why is QE low? Is band bending different than expected? The surface was protected by an arsenic cap – is it flatband? Is optical absorption different for a smooth surface? Does less optical scattering result in lower near-surface light intensity? Do rough surfaces contain plasmonic elements? 10/08/2012 Schaff et. Al., Photocathode Physics for Photoinjectors 2012

High temperature surface roughening Wafers heated in MBE Specular opitcal scattering measured at 514nm Increased roughness measured by AFM Increased roughness also observed by RHEED 10/08/2012 Schaff et. Al., Photocathode Physics for Photoinjectors 2012

Schaff et. Al., Photocathode Physics for Photoinjectors 2012 Thermal cleaning example Hypothesis: increased surface roughness increases quantum efficiency due to increased light absorption Roughness? Figure 2.16 The quantum efficiency dependent on heat cleaning temperature Submitted to the Graduate School in partial fulfillment of the requirements for the degree of Doctor of Philosophy in School of Physics, Peking University January, 2012 10/08/2012 Schaff et. Al., Photocathode Physics for Photoinjectors 2012

Optical coupling increase due to roughness Photoluminescence (PL) from Ga0.2In0.8N surface roughened by scanning laser exposure CAD pattern IR Image Inside patterning PL Intensity (AU) Outside of patterning Line scan location 1560nm PL Intensity (AU) Roughening increases PL by a factor of 8 X. Chen, W. J. Schaff, and L. F. Eastman, JVST B 25 3, 974-977 2007 Line scan location 10/08/2012 Schaff et. Al., Photocathode Physics for Photoinjectors 2012

Optical coupling enhancment - Roughness? Plasmonics? Both? In solar cells and light emitting diodes surfaces are intentionally roughened on a similar size scale to improve light coupling efficiency Plasmonic coupling can increase near-surface optical intensity by 10x (as routinely seen in Raman spectroscopy) Could Cs clusters form plasmonic structures? Metal balls of a few nm in size give rise to large plasmonic coupling 10/08/2012 Schaff et. Al., Photocathode Physics for Photoinjectors 2012

Schaff et. Al., Photocathode Physics for Photoinjectors 2012 Summary 100nm undoped As-capped GaAs photocathodes have low emittance Questions: Is surface roughening during heating partially responsible for increased QE? Is surface smoothing during operation partially responsible for decreased QE? How do we figure this out? 10/08/2012 Schaff et. Al., Photocathode Physics for Photoinjectors 2012

Schaff et. Al., Photocathode Physics for Photoinjectors 2012 Summary The end 10/08/2012 Schaff et. Al., Photocathode Physics for Photoinjectors 2012

MBE in-situ activation Observe quantum efficiency during Cs/NF3activation. Simultaneous RHEED observations may reveal the surface structure required for high QE Observe optical scattering with changing roughness 10/08/2012 Schaff et. Al., Photocathode Physics for Photoinjectors 2012

Schaff et. Al., Photocathode Physics for Photoinjectors 2012 10/08/2012 Schaff et. Al., Photocathode Physics for Photoinjectors 2012

Conduction band energies and electric fields Doping - 5x1018cm-3 0nm vs 100nm undoped cap 10/08/2012 Schaff et. Al., Photocathode Physics for Photoinjectors 2012

Schaff et. Al., Photocathode Physics for Photoinjectors 2012 Is this cap protective enough? Is it thick enough to survive air exposure? Is it free of holes or cracks that might expose the surface to air? The answer is YES ! Arsenic begins to desorb at 350°C and peaks out at about 370°C during wafer heating (as seen on the RGA facing the wafer). The desorbing flux falls by about a factor of 10 when the surface is completely clear The wafer is then raised to a temperature below oxide desorption temp (580°C) and GaAs is grown. If any residual contamination was present (even though not visible in RHEED), the surface would become rough within 1-2 monolayers. Instead, perfect growth was observed. G20192 after warm arsenic cap removal 10/08/2012 Schaff et. Al., Photocathode Physics for Photoinjectors 2012

Schaff et. Al., Photocathode Physics for Photoinjectors 2012 RHEED oscillations for GaAs and AlAs (previous work) 10/08/2012 Schaff et. Al., Photocathode Physics for Photoinjectors 2012

Cross section, CAD and SEM images 10nm 78nm 18nm 5W 20kHz 1424mm/Sec 771 passes 675sec 2.5nm In0.8Ga0.2N 540nm 5W 20kHz 1424mm/Sec 59 passes 25sec GaN buffer AlN buffer Sapphire substrate backscattered electron image Lines are very sharp despite being written 771 times over 11 minutes Individual spots can be seen - spacing is approximately 70µm corresponding to 1424mm/Sec z-contrast shows bright regions (more In) surrounding dark regions (more Ga)

SEM images Lines are very sharp despite being written 771 times over 11 minutes Individual spots can be seen - spacing is approximately 70µm corresponding to 1424mm/Sec z-contrast shows bright regions (more In) surrounding dark regions (more Ga)