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

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2000

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nA

8 sec40

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µm

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nA

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nA

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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