Ekaterina Dikarov (Suhovoy ) Development of high sensitivity, high resolution ESR and its applications for studying solar cells

What is ESR? ESR- Electron Spin Resonance Spin & Magnetic Field interaction. MW induces transition between two energy levels. MW induces transition between two energy levels. m s =+1/2 m s =-1/2 B field Energy Paramagnetic centers

ESR Imaging Optical image 2D image with 3µm resolution 100 µm B0B0 Frequency B0B0 E1E1 E2E2

The sensitivity and resolution challenge Resonator volume Magnetization Quality factor Sample volume Temperature Detection bandwidth MW frequency Signal-to-noise-ratio (SNR) (Pulsed induction detection ESR) : Conventional ESR: Spin sensitivity : ~ spins. Imaging resolution : Limited to ~ μm Conventional ESR: Spin sensitivity : ~ spins. Imaging resolution : Limited to ~ μm

What is needed: Increased sensitivity : electron spins. Improved image resolution: nm. What is needed: Increased sensitivity : electron spins. Improved image resolution: nm. ESR and solar cells: requirements and challenges ESR and solar cells: requirements and challenges Dangling bonds defects Impurities Dopants Less than spins/cm 3 Thin layers ~1μm Heterogeneous samples Short relaxation times Background signals

Pulsed ESR imaging system Microwave reference source Pulsed microwave bridge Gradient coils drivers Electromagnet Temperature controller Control PC Cryogenics Cryogenic probe Gradient coils’ structure Resonator and sample holder

Dielectric resonator - minimization GHz GHz Microwave dielectric resonator (TiO 2 ) High permittivity dielectric resonators Conventional ESR resonator 17 GHz – 10 7 spins in 1 h at RT 35 GHz – 10 6 spins in 1 h at RT Relatively high Q ~ 1000 Dielectric resonator (first generation)

Going down further in size – New surface resonators Twig, Y., Suhovoy, E., and Blank, A., Review of Scientific Instruments 81 (2010) Twig, Y., Suhovoy, E., Hutchison, W. D., and Blank, A., Review of Scientific Instruments, 82 (2011) Twig Y., Dikarov E., and Blank A., Journal of Magnetic Resonance, 218 (2012) ~14 GHz – 10 4 spins in 1 h at 5K

Poly-crystalline silicon thin layers (PV) 1.2 μm thin layer. Grain size ~ 0.1μm. Defect density = 9x10 17 cm -3 ~ 10 7 defects in measured volume. (in the surface resonator). 1.2 μm thin layer. Grain size ~ 0.1μm. Defect density = 9x10 17 cm -3 ~ 10 7 defects in measured volume. (in the surface resonator). Conventional ESR resonator (10K): Surface resonator (10K): Sample Surface resonator

Imaging results 2D ESR image for 10 µm thick layer of phosphorous doped 28 Si, taken at 10K.

Future plans  Smaller resonators.  Superconducting surface resonators.  Higher frequencies.

Magnetic Resonance Group members: Prof. Aharon Blank Dr. Ygal Twig Dr. Lazar Shtirberg Alon Plattner Michael Levit Alex katchlis Ksenia Sirota Omri Arbiv Gubrail Shakkour Rami Maymon Mada Hashem Itai Katz Helen Wolfson