Engineering semiconductors using energetic beams Oscar D. Dubón Materials Science and Engineering,...

Engineering semiconductors using energetic beams

Oscar D. Dubón

Materials Science and Engineering, UC Berkeleyand

Lawrence Berkeley National Laboratory

Physics ColloquiumUniversity 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

• 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

Ga0.35In0.65P/Ga0.83In0.17As/Ge ( 5.09 mm²)

www.ise.fraunhofer.de1 μm

www.nobelprize.org

4

Tunnel Junction

InGaAs Middle Cell

AR CoatingFront Contact

Back Contact

InGaP Top Cell

Buffer Layer

n+ (In)GaAsn+ AlInP [Si]n+ InGaP [Si]p InGaP [Zn]

p AlInP [Zn]p++ AlGaAs [C]n++ InGaP [Si]n+ AlInP [Si]n+ (In)GaAs [Si]

p (In)GaAs [Zn]

p+ InGaP [Zn]

p Ge Substrate

p++ AlGaAs [C]n++ InGaP [Si]

n+ GaAs : 0.1µmn+ (In)GaAs [Si]

n

Tunnel Junction

Ge Bottom Cell

Structure of Triple-Junction (3J) Cell

Multi-junction Solar Cell

power concentrationcourtesy J. Wu

Semiconductor thin-film epitaxy

Herman, 1986

LBNL

Molecular Beam Epitaxy

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

J. Wu et al., Semiconductor Science and Technology (2002)

W. Walukiewicz, Berkeley Lab (http://emat-solar.lbl.gov/index.html)

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)

0.9

1

1.1

1.2

1.3

1.4

1.5

0 0.01 0.02 0.03 0.04 0.05

Uesugi, et. al.Keyes, et. al.Malikova, et. al.Bhat, et. al.BAC theory

Nitrogen fraction, x

GaNxAs

1-x @ 295KVCA

-1 -0.5 0.5 1

-1

-0.5

0.5

1

1.5

2

2.5

VB

E(k)

EN

E

k

-1 -0.5 0.5 1

-1

-0.5

0.5

1

1.5

2

2.5

VB

E+

E-

E

k

conduction band restructuring

bandgap

W. Shan et al., PRL (1999)

xCEkEEkEkE NCNC 22 4

2

1

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

Non-equilibrium growth

Kinetically limited growth

Furnace annealing (FA)

Rapid thermal annealing (RTA)

Pulsed laser melting (PLM)

Regrowth Time

>103 s

102-101 s

<10-6 s

≈

Post-implantation processing

Ion implantation and pulsed-laser melting (II-PLM)

Liquid-phase epitaxy at submicrosecond time scales

Outcome

•Growth of epitaxial, single crystal

•Solute trapping of implanted species

•Suppression of second phases

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

• III-Mn-V ferromagnetic semiconductors

N ion implanted GaAs

Homogenized excimer laser pulse (=248 nm, 25 ns FWHM, ~0.2-0.8 J/cm2)

N ions

GaAs

Liquid MeltFront

GaAs

GaNxAs1-x

GaAs

Ga1-xMnxAsion induced damage

Time resolved reflectivity (TRR)

GaNxAs1-x formed byN ion implantion and RTA

J. Wu, 2002

N ion implanted GaAs

N ions

GaAs

GaNxAs1-x

GaAs

Ga1-xMnxAsion induced damage

Rapid thermal annealing (RTA)

Pulsed-laser synthesis of GaNxAs1-x

(a) (b)

(c) melted/recrystallized

unmelted

100 nm100 nm

50 nm

5 nm

GaN0.02As0.98

J. Jasinski et al., APL (2001)

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

IIOxVI1-x: a medium for multiband semiconductors

courtesy J. Wu

Multi-Band Solar Cells

junction1

junction2

junction3

I valence band

“intermediate” band

“conduction” band

I

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

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

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)

Zn0.88Mn0.12OxTe1-x

Intermediate-band solar cells

K. M. Yu et al., PRL (2003)A. Luque et al., PRL (1997)

•First single-phase, multi-band semiconductor for intermediate-band solar cell

•Other materials discovered: GaAsNP, AlGaAsN

courtesy J. Wu

Transition-metal doping in the dilute alloy limit

H. Ohno et al., APL (1996); JMMM (1999)

Case study: Ga1-xMnxAs

• Ferromagnetism from incorporation dilute amounts of Mn into GaAs

• Hole-mediate inter-Mn exchange

Challenges in synthesis of dilute alloys

Ga1-xMnxAs

after H. Ohno, Science (1998).

• 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 (>300 ºC)

• x is limited to below 10% (equil. solubility limit<1019 cm-3, ~0.05%)

300

200

100

subs

trat

e te

mpe

ratu

re (

ºC)

0 0.02 0.04 0.05

x

polycrystalline

roughening

metallic (Ga,Mn)As

insulating(Ga,Mn)As

insulating(Ga,Mn)As

secondary phase formation

roughening

Molecular beam epitaxy (MBE)

• Mn substitutionality of 50-80%

• Non-substitutional Mn at random sites (no interstitials)

• No evidence of secondary ferromagnetic phases

1000 Å GaAs

Ga1-xMnxAs

D. Zakharov and Z. Liliental-Weber

TEM

Ga1-xMnxAs formed by Mn ion implantation and PLM

-100

-50

0

50

100

-400 -200 0 200 400

5 K 100 K

H (Oe)

M (

em

u/g

Mn

)

Magnetism

Transport

• 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-xMnxAs: ferromagnetism and processing

Ga1-xMnxP formed by II-PLM

Scarpulla et al., PRL (2005); Farshchi et al., SSC (2006).

electrical transport

•Non-metallic behavior•EMn in GaP=0.4 eV

magnetization

TC increases with x

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)

• Focused ion beam (FIB) patterning

• Ga+ implantation into GaNxAs1-x GaNxAs1-x quantum dots & wires

Ga+ implanted lines

GaNxAs1-x

GaAs

GaNxAs1-x wires

FIB patterning RTACB

localized amorphization

nitrogen release

RTA

Ga+ dose: 3x1013 cm-2 3x1014 cm-2

Toward planar nanostructures using ion and photon beams

Size of previously amorphized region

Protective Pt layer

film thickness

50 nm

Patterned II-PLM

TC

R=VDE/IAB

RHall =VCD/IAB

A B

D

C

E

T. Kim, JAP (2008)

GaNxAs1-x Ga1-xMnxAs

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

R. Bouanani-Rahbi et al., Physica B (2003)M. S. Brandt et al., APL (2004) L. Thevenard et al., APL (2005)

T = 130°C, 3 hrs

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

GaAs:Mn-H

Ga1-xMnxAs

with Grigoropoulos group

Laser activation of ferromagnetism

• Onset of ferromagnetism occurs at fluence > 55 mJ/cm2

• TC saturates independent of fluence (and number of pulses)

Laser conditions:Q-switched Nd:YAG laser ( = 532 nm),4-6 ns, 3000 shots (10 Hz, 5 min)

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

Require: magnetic open (switching) AND conductive short (spin-injection)

40 x40 μm2

40

30

20

10

0

µm

403020100

µm

2000

1500

1000

500

0

nA

8 sec40

30

20

10

0

µm

403020100

µm

2000

1500

1000

500

0

nA

40

30

20

10

0

µm

403020100

µm

2000

1500

1000

500

0

nA

40

30

20

10

0

µm

403020100

µm

2000

1500

1000

500

0

nA

10 sec

13 sec20 sec

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