A1257786994VBGXz

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Engineering semiconductors using energetic beams
Oscar D. Dubón Materials Science and
Engineering, UC Berkeley and Lawrence Berkeley
National Laboratory Physics Colloquium Universi
ty of Toronto March 12th, 2009
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Outline

• Semiconductor alloys in the dilute limit
• Ion beams and lasers for materials synthesis
• Highly mismatched alloys
• Ferromagnetic semiconductors
• Summary

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Bandgap engineering

• Control of optical and electrical properties by
  alloying
• Growth of heterostructures by advanced thin-film
  methods (MBE and MOCVD)
• Applications
• high-electron mobility transistor (AlGaAs/GaAs)
• solid-state laser
• multi-junction solar cell

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Multi-junction Solar Cell
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courtesy J. Wu
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Semiconductor thin-film epitaxy
Molecular Beam Epitaxy
LBNL
Bulk equilibrium overcome by surface mediated
growth
Herman, 1986
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Bandgap engineering of highly mismatched systems

• Extraordinary bowing in energy gap
• Tremendously challenging to synthesize due to
  large miscibility gaps

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Bandgap engineering in the dilute alloying limit
Case study GaNxAs1-x

• Reduction of bandgap by 180 meV by replacement of
  1 of As with N
• x above 5 difficult to synthesize
• Bowing modeled by conduction band anticrossing
  (BAC)

W. Shan et al., PRL (1999)
J. Wu et al., Semiconductor Science and
Technology (2002) W. Walukiewicz, Berkeley Lab
(http//emat-solar.lbl.gov/index.html)
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Ion-beam synthesis t,T considerations

• Ion implantation
• Injection of ions to high levels (many atomic )
  into host material
• Availability of a wide range of substrate
  materials (host) and the periodic table
  (implantation species)
• Post implantation annealing required to achieve
  desired phase

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Ion implantation and pulsed-laser melting
(II-PLM)
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GaNxAs1-x formed by N ion implantion and RTA
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Pulsed-laser synthesis of GaNxAs1-x
N ion implanted GaAs

• (a) RTA only (950 ºC, 10 s)
• (b) PLM (0.34J/cm2) followed by RTA (950 ºC,10 s)

Significant enhancement of N incorporation in As
sublattice is achieved by PLM
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IIOxVI1-x a medium for multiband semiconductors
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Multi-Band Solar Cells

• Multi-band
• Single junction
• Add one band ? add many absorptions

• Multi-junction
• Single gap each junction
• Add one junction ? add one absorption

courtesy J. Wu
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II-PLM Multi-band Zn1-yMnyOxTe1-x
An intermediate band is formed in ZnMnTe after
oxygen ion implantation and pulsed-laser melting
K. M. Yu et al., PRL (2003)
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Intermediate-band solar cells

• First single-phase, multi-band semiconductor for
  intermediate-band solar cell
• Other materials discovered GaAsNP, AlGaAsN

K. M. Yu et al., PRL (2003) A. Luque et al., PRL
(1997)
courtesy J. Wu
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Transition-metal doping in the dilute alloy limit
Case study Ga1-xMnxAs

• Ferromagnetism from incorporation dilute amounts
  of Mn into GaAs
• Hole-mediate inter-Mn exchange

H. Ohno et al., APL (1996) JMMM (1999)
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Challenges in synthesis of dilute alloys
Ga1-xMnxAs

• Ga1-xMnxAs is grown exclusively by low-T MBE
• Precipitates (e.g., MnAs) can form by high-T
  growth
• Films are unstable to thermal annealing at
  moderate temperatures (gt300 ºC)
• x is limited to below 10 (equil. solubility
  limitlt1019 cm-3, 0.05)

after H. Ohno, Science (1998).
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Ga1-xMnxAs formed by Mn ion implantation and PLM
TEM

• Mn substitutionality of 50-80
• Non-substitutional Mn at random sites (no
  interstitials)
• No evidence of secondary ferromagnetic phases

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Ga1-xMnxAs ferromagnetism and processing

• Solute trapping is more effective at lower
  fluence due to a higher solidification velocity
• Incorporation of Mn is limited to x5 with
  current II-PLM conditions

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Ga1-xMnxP formed by II-PLM
magnetization
electrical transport

• Non-metallic behavior
• EMn in GaP0.4 eV

TC increases with x
Scarpulla et al., PRL (2005) Farshchi et al.,
SSC (2006).
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TC vs. x

• Maximum TC in Ga1-xMnxP is 65 K at x0.042
• Extrapolated room temperature ferromagnetism is
  reached at x0.12-18
• Hole localization impacts TC

T. Jungwirth et al., PRB (2005) P.R. Stone et
al., PRL (2008)
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Toward planar nanostructures using ion and photon
beams
RTA
Ga dose 3x1013 cm-2
3x1014 cm-2
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Patterned II-PLM
GaNxAs1-x Ga1-xMnxAs
RHall VCD/IAB
T. Kim, JAP (2008)
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Laser patterning of hydrogenated Ga1-xMnxAs

• H passivates Mn ion
• Electrical and ferromagnetic deactivation of Mn
• H occupies bond-centered location

• Effect of H can be reversed by thermal annealing
• H removal leads to reactivation of Mn

T 130C, 3 hrs
R. Bouanani-Rahbi et al., Physica B (2003) M. S.
Brandt et al., APL (2004) L. Thevenard et al.,
APL (2005)
R. Farshchi et. al., Phys. Stat. Sol. (c) (2007)
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Direct writing of ferromagnetism
Mimic effect of furnace locally by focused laser
annealing of Ga1-xMnxAsH
with Grigoropoulos group
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Laser activation of ferromagnetism
Laser conditions Q-switched NdYAG laser (l
532 nm), 4-6 ns, 3000 shots (10 Hz, 5 min)

• Onset of ferromagnetism occurs at fluence gt 55
  mJ/cm2
• TC saturates independent of fluence (and number
  of pulses)

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Femtosecond laser activation C-AFM

• Laser conditions
• mode-locked TiSapphire laser (pulse duration
  100 fs) at a repetition rate of 1 kHz
• The line pattern 50X objective lens, a scan
  speed of 0.5 um/sec, and laser fluence of 40
  mJ/cm2
• dot patterns 2000 pulses, laser fluence of
  20 mJ/cm2 and no scanning

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Femtosecond laser activationmeasurement of
laser-direct-written Hall bar
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Shutter-controlled gap in laser activated
Ga1-xMnxAsH
Require magnetic open (switching) AND conductive
short (spin-injection)
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Summary
Ion implantation and pulsed-laser melting
provides numerous intriguing opportunities for
materials discovery and materials processing
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Acknowledgments

• P.R. Stone
• R. Farshchi
• C. Julaton
• M.A. Scarpulla (Univ. of Utah)
• K. Alberi (NREL)
• S. Tardif (Grenoble)
• K.M. Yu (LBNL)RBS/PIXE
• W. Walukiewicz (LBNL)theory
• C.P. Grigoropoulos group (N. Misra and D.
  Hwang)laser patterning
• P. Ashby (LBNL, Molecular Foundry)c-AFM
• Y. Suzuki and R. Chopdekartransport
• Funding US-DOE and UC Berkeley