Engineering semiconductors using
energetic beams
Oscar D. Dubón
Materials Science and Engineering, UC Berkeley
and
Lawrence Berkeley National Laboratory
Physics Colloquium
University of Toronto
March 12th, 2009
Outline
• Semiconductor alloys in the dilute limit
• Ion beams and lasers for materials synthesis
• Highly mismatched alloys
• Ferromagnetic semiconductors
• Summary
Bandgap engineering
Ga0.35In0.65P/Ga0.83In0.17As/Ge ( 5.09 mm²)
• Control of optical and electrical
properties by alloying
• Growth of heterostructures by
advanced thin-film methods
(MBE and MOCVD)
1 μm
www.ise.fraunhofer.de
www.nobelprize.org
• Applications
–high-electron mobility
transistor (AlGaAs/GaAs)
–solid-state laser
–multi-junction solar cell
Multi-junction Solar Cell
power concentration
4
courtesy J. Wu
Semiconductor thin-film epitaxy
Molecular Beam Epitaxy
LBNL
Herman, 1986
Bulk equilibrium overcome
by surface mediated growth
Bandgap engineering of highly
mismatched systems
• Extraordinary bowing in energy gap
• Tremendously challenging to synthesize due to large miscibility gaps
Bandgap engineering in the dilute
alloying limit
Case study: GaNxAs1-x
E(k)
2.5
E
2
1.5
EN
VCA
1
0.5
-1
-0.5
0.5
k
1
-0.5
VB
-1
conduction band
restructuring
E+
2.5
E
2
1.5
E-
1
0.5
bandgap
k
-1
• 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)
-0.5
VB
E  k  
1
2
 E
0.5
1
-0.5
-1
C
k   E N   EC k   E N  2  4C 2  x 
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)
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
Post-implantation processing
Regrowth
Non-equilibrium
growth
Kinetically limited
growth
Time
Furnace
annealing (FA)
Rapid thermal
annealing (RTA)
Pulsed laser
melting (PLM)
>103 s
102-101 s
≈
<10-6 s
Ion implantation and pulsed-laser
melting (II-PLM)
N ion implanted GaAs
N ions
ion induced
Ga1-xMndamage
xAs
GaAs
Homogenized excimer laser pulse
Liquid-phase epitaxy at
submicrosecond
time scales
Time resolved reflectivity
(TRR)
Outcome
•Growth of epitaxial, single crystal
•Solute trapping of implanted species
•Suppression of second phases
(=248 nm, 25 ns FWHM, ~0.2-0.8 J/cm2)
Liquid
GaAs
Melt
Front
Route for the synthesis of new materials
•III-N-V & II-O-VI highly mismatched alloys
(w/ K.M. Yu & W. Walukiewicz, LBNL)—ZnTeO for
intermediate band solar cells
GaNxAs1-x
GaAs
•III-Mn-V ferromagnetic semiconductors
GaNxAs1-x formed by
N ion implantion and RTA
N ion implanted GaAs
N ions
ion induced
Ga1-xMndamage
xAs
GaAs
Rapid thermal
annealing (RTA)
GaNxAs1-x
GaAs
J. Wu, 2002
Pulsed-laser synthesis of
GaNxAs1-x
N ion implanted GaAs
GaN0.02As0.98
(a)
(b)
100 nm
(c)
100 nm
melted/recrystallized
J. Jasinski et al., APL (2001)
(a) RTA only (950 ºC, 10 s)
unmelted
(b) PLM (0.34J/cm2) followed
by RTA (950 ºC,10 s)
Significant enhancement of N incorporation in As
sublattice is achieved by PLM
5 nm
50 nm
IIOxVI1-x: a medium for multiband
semiconductors
Multi-Band Solar Cells
“conduction” band
junction3
I
junction2
“intermediate” band
I
valence band
junction1
Multi-junction
• Single gap each junction
• Add one junction  add one absorption
Multi-band
• Single junction
• Add one band  add many absorptions
courtesy J. Wu
II-PLM Multi-band Zn1-yMnyOxTe1-x
Zn0.88Mn0.12OxTe1-x
An intermediate band is formed in ZnMnTe after oxygen ion
implantation and pulsed-laser melting
K. M. Yu et al., PRL (2003)
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
Transition-metal doping in the
dilute alloy limit
Case study: Ga1-xMnxAs
• Ferromagnetism from incorporation
dilute amounts of Mn into GaAs
H. Ohno et al., APL (1996); JMMM (1999)
• Hole-mediate inter-Mn exchange
Challenges in synthesis of
dilute alloys
substrate temperature (ºC)
Ga1-xMnxAs
Molecular beam epitaxy (MBE)
secondary phase formation
300
• Precipitates (e.g., MnAs) can form
by high-T growth
metallic (Ga,Mn)As
insulating
(Ga,Mn)As
200
insulating
(Ga,Mn)As
roughening
polycrystalline
100
0
0.02
0.04
• Ga1-xMnxAs is grown exclusively
by low-T MBE
0.05
x
after H. Ohno, Science (1998).
• Films are unstable to thermal
annealing at moderate
temperatures (>300 ºC)
• x is limited to below 10% (equil.
solubility limit<1019 cm-3, ~0.05%)
Ga1-xMnxAs formed by Mn ion
implantation and PLM
Magnetism
100
50
5K
TEM
100 K
0
-400
-200
-50
-100
0
200
400
Ga1-xMnxAs
H (Oe)
GaAs
1000 Å
D. Zakharov and Z. Liliental-Weber
Transport
• Mn substitutionality of 50-80%
• Non-substitutional Mn at
random sites (no interstitials)
• No evidence of secondary
ferromagnetic phases
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 x~5% with current II-PLM conditions
Ga1-xMnxP formed by II-PLM
magnetization
electrical transport
TC increases with x
•Non-metallic behavior
•EMn in GaP=0.4 eV
Scarpulla et al., PRL (2005); Farshchi et al., SSC (2006).
TC vs. x
• Maximum TC in Ga1-xMnxP is
~65 K at x~0.042
• Extrapolated room temperature
ferromagnetism is reached at
x~0.12-18
• Hole localization impacts TC
T. Jungwirth et al., PRB (2005)
P.R. Stone et al., PRL (2008)
Toward planar nanostructures
using ion and photon beams
• Focused ion beam (FIB) patterning
• Ga+ implantation into GaNxAs1-x
GaNxAs1-x quantum dots & wires
Ga+ implanted lines
GaNxAs1-x
GaAs
FIB patterning
GaNxAs1-x wires
RTA
CB
nitrogen
release
localized
amorphization
Size of previously
amorphized region
RTA
Ga+ dose: 3x1013 cm-2
3x1014 cm-2
film
thickness
Protective Pt
layer
50 nm
Patterned II-PLM
GaNxAs1-x
Ga1-xMnxAs
C
A
B
D
E
R=VDE/IAB
TC
RHall =VCD/IAB
T. Kim, JAP (2008)
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 = 130°C, 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)
Direct writing of ferromagnetism
Mimic effect of furnace locally by focused laser annealing of Ga1-xMnxAs:H
Ga1-xMnxAs
GaAs:Mn-H
with Grigoropoulos group
Laser activation of ferromagnetism
Laser conditions:
Q-switched Nd:YAG laser ( = 532 nm),
4-6 ns, 3000 shots (10 Hz, 5 min)
• Onset of ferromagnetism occurs at
fluence > 55 mJ/cm2
• TC saturates independent of fluence
(and number of pulses)
Femtosecond laser activation: C-AFM
Laser conditions
• mode-locked Ti:Sapphire 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
Femtosecond laser activation:
measurement of laser-direct-written Hall bar
H
Shutter-controlled gap in laser
activated Ga1-xMnxAs:H
40
40
8 sec
10 sec
2000
2000
30
30
1000
1000
500
10
20
500
10
0
0
0
0
0
10
20
µm
30
40
0
40
10
20
µm
30
40
40
13 sec
20 sec
2000
2000
30
30
1500
500
10
20
1000
500
10
0
0
0
0
0
10
20
µm
30
40
0
10
20
µm
30
40 x40
40
μm2
Require: magnetic open (switching) AND conductive short (spin-injection)
nA
1000
nA
20
µm
1500
µm
nA
20
µm
1500
nA
µm
1500
Summary
Ion implantation and pulsed-laser melting provides
numerous intriguing opportunities for materials
discovery and materials processing
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. Chopdekar—transport
• Funding: US-DOE and UC Berkeley