Amorphous/Nanocrystalline Silicon Thin Films for Particle Detectors James Kakalios School of Physics and Astronomy The University of Minnesota Minneapolis, MN

p-i-n semiconductor devices - “solar cells” for high energy particles Figure courtesy of Ph.D. thesis of Matthieu Despeisse (Lyon, 2006)

Silicon Solar Cells metal back contact p-type Si ~250  m diffused n-type dopant metal grid glass substrate ITO p a-Si(C):H i a-Si(C):H ~400 nm n a-Si(C):H metal back contact monocrystalline Si solar cell amorphous Si solar cell sun

Amorphous Silicon Solar Cells Advantages: – Uniform deposition over large areas on wide variety of substrates – Completely fabricated by thin film technology – Strong absorption in visible light – Radiation Hard – Inexpensive! PECVD Amorphous Silicon

Commercial Solar Cell Production

a-Si:H p-i-n detector integrated with ASIC device Figure courtesy of Ph.D. thesis of Matthieu Despeisse (Lyon, 2006)

Drift Length (Schubweg) L D L D =  x  x   = mobility  = recombination lifetime  = built-in electric field

Challenges Ideal semiconductor material for p-i-n structures for particle detectors: Large charge carrier mobility Long recombination lifetime Radiation Hard Low Cost Easy to deposit over large areas on wide variety of substrates

Hydrogenated amorphous silicon containing silicon nanocrystallite inclusions (a/nc-Si:H) Goal is to combine large area deposition advantages of a-Si:H with superior opto- electronic properties of crystalline silicon Preliminary results indicate enhancement of mobility and recombination lifetime in a/nc-Si:H Mixed-Phase Thin Films

Experimental System R elec = 8 cm, d gap = 5 cm

Dual Chamber Co-Deposition System Nanocrystals are produced in one capacitively coupled PECVD chamber In the second chamber a-Si:H film is produced The nanoparticles are entrained by a carrier gas and injected in the second chamber where mixed phase film is deposited Growth parameters can be separately optimized for both the particles and the amorphous film

High Resolution TEM confirms production of nanocrystals Pr ~ 1.5 Torr RF Power ~ 50 W

High Resolution TEM Si particle ~ 1.5 nm x 3 nm Images taken by Prof. C. B. Carter with a Philips CM 200 FEG with a spherical aberration corrector with Dr. Markus Lentzen and Prof. Knut Urban (Research Center Jülich, Germany). HRTEM image of the 1450 mTorr a/nc-Si:H sample

Energy (eV) Log Density of States Defect States Mobility Edges Band Tail States Delocalized States Localized States Amorphous eV -1 cm cm -3 eV -1

Energy (eV) Log Density of States Delocalized States Localized States Amorphous EFEF E A =activation energy EcEc

450 K 320 K E C -E F ~0.73 eV

Large Enhancements in Conductivity with nc Inclusions n-type doped a/nc-Si:H n-type doped a-Si:H

Conductivity  = n e 

Electrode Transport Picture

Photoluminescence Sensitive to Recombination Lifetime of Photo-Excited electron-hole pairs Energy (eV) Log Density of States Strongly absorbed light in EvEv EcEc photo-excited e-h recombine - light out

Large Enhancements in Recombination Lifetime with nc Inclusions

Dual Chamber Co-Deposition System

Summary Mixed-phase thin films of amorphous silicon containing nanocrystalline inclusions have been synthesized using dual chamber co-deposition system Preliminary results indicate enhancements in both mobility and recombination lifetime with nc inclusions Further studies needed to optimize materials properties for particle detection applications

Acknowledgements All of this research performed in collaboration with Prof. U. R. Kortshagen, and our students C. Blackwell, Y. Adjallah, C. Anderson, L. R. Wienkes, K. Bodurtha, J. Trask Research Supported by NSF (NER-DMI , DMR , IGERT DGE , MRSEC DMR ), Xcel (RD3-25), NREL (XEA ), NINN Characterization Facility and The University of Minnesota

Ge nc in a-Si:H Matrix

GeH 4 + Ar SiH MHz Substrates Germanium nanocrystals Amorphous silicon film To pump

Si-Ge a-Si:H nc-Ge

Figure courtesy of Ph.D. thesis of Matthieu Despeisse (Lyon, 2006)

Mixed-Phase Thin Films courtesy of Reinhard Carius decreasing R = [H 2 ]/[SiH 4 ] P. R. Cabarrocas, Thin Solid Films (2002)  Recent reports suggest a-Si:H containing silicon nanocrystalline inclusions display enhanced resistance to light-induced defect creation

X-ray detection for medical applications Figure courtesy of Ph.D. thesis of Matthieu Despeisse (Lyon, 2006)

Disadvantages of a/nc-Si:H films synthesized in single chamber PECVD (1)Poor control of nc concentration (2) nc deposition conditions far from optimal conditions for a-Si:H (3) Only capable of growing nc-Si and a-Si:H

Dual Chamber Co-Deposition System Particle Synthesis Chamber can inject nanocrystallites from top Concentration in a-Si:H film controlled through gas convection in second chamber

Energy scale of hop exceeds phonon energy  need multiple phonons Observed in a-Ge, a-C and Metal-Oxide glasses Multi-phonon Hopping 100K ~ Hz (acoustic phonon)

High Xc (> 15%) – Mod. Low Temp (300 < T < 100K) - Power Law Temp Dependence

Raman Spectroscopy XRD – 8.5 nm x 20 nm

High Resolution TEM of Silicon Nanocrystals Embedded Within an a-Si:H Matrix Anderson, Blackwell, Deneen, Carter, JK, Kortshagen, MRS (2006) Particle Chamber ONParticle Chamber OFF

Amorphous Silicon’s History Strained Silicon Bonds Dangling Bonds (a-Si:H) T > 1975 (a-Si) T < 1975

How Does Conductivity Depend on nc Concentration?

Conductivity as Xc Increases E A ~0.9 eV X C =18% A B C E A ~0.9 eV X C <1% E A ~0.6 eV X C =1.4%

Charge Donation Model E CB E VB EFEF Nanocrystallites 1.8 eV 1.4 eV 0.1 eV 0.3 eV a-Si:H Thermal Excitation

Charge Donation Model E CB E VB EFEF Nanocrystallites 1.8 eV 1.4 eV 0.1 eV 0.3 eV a-Si:H Electron Injection E’ F

Charge Donation Model Donated electrons in a-Si:H matrix will occupy defect states in gap – shifting E F for low nc films At high nc concentration – dangling bond density increases – soaking up excess charge and shifting E F back to mid-gap Dangling bond density can be obtained from optical absorption spectrum measured using Constant Photocurrent Method (CPM)

Optical Absorption Measurement Spectral dependence of optical absorption coefficient  sensitive to DOS, N(ε) Energy (eV) Log Density of States A B C EvEv EcEc

Optical Absorption Coefficient against Photon Energy Energy (eV) Absorption Coefficient (1/cm) A B C Measure Photocurrent as function of wavelength to obtain optical absorption coefficient Constant Photocurrent Method

A Film E A ~0.9 eV X C <1% Urbach ~ 78 meV CPM ~ 7x10 16 cm -3 ESR ~ cm -3 NC ≤ cm -3 A

A Film – Density of States Log (g(ε)) (au) CBT VBT

B Film – Density of States Log (g(ε)) (au) CBT VBT

A Film E A ~0.9 eV X C <1% Urbach ~ 78 meV CPM ~ 7x10 16 cm -3 ESR ~ cm -3 NC ≤ cm -3 A

B Film E A ~0.6 eV X C =1.4% CPM ~ 6x10 16 cm -3 ESR ~ cm -3 Urbach ~ 77 meV NC ~ 2x10 17 cm -3 A B

B Film – Density of States Log (g(ε)) (au) CBT VBT

C Film – Density of States Log (g(ε)) (au) CBT VBT

B Film E A ~0.6 eV X C =1.4% CPM ~ 6x10 16 cm -3 ESR ~ cm -3 Urbach ~ 77 meV NC ~ 2x10 17 cm -3 A B

C Film Urbach C ~ 100 meV E A ~0.9 eV X C =18% A B C CPM ~ 4x10 17 cm -3 ESR ~ cm -3 NC ~ 3x10 18 cm -3

Influence of Light Soaking of Dark Conductivity

Photosensitivy  ph /  d Decreases with X c