Bandgap Engineering of the Amorphous Wide Band-Gap

Semiconductor (SiC)1-x(AlN)x Doped with Rare Earths and its

Optical Emission Properties

Roland Weingärtner

Departamento de Ciencias – Sección Física – Grupo Ciencias de los Materiales

Pontificia Universidad Católica del Perú (PUCP)

San Miguel, 14th of April 2011

Outline

I Motivation and Introduction

Wide band-gap semiconductors

Band-gap engineering

Rare earth doping and optical emission

II First Results of a-(SiC)x(AlN)1-x

Thin film growth method and structural characterisation

Band-gap engineering of a-(SiC)x(AlN)1-x

III Cathodoluminescense measurements

Spectral emission of rare earth doped a-(SiC)x(AlN)1-x

Thermal activation of rare earth emission

IV Summary and Acknowledgements

1970 1975 1980 1985 1990 1995 2000 2005 20090

20

40

60

80

100

# of ar

ticl

es fro

m A

PS

journ

als

Year

GaN SiC AlN ZnSe

Combine the advantages of an insulator and a semiconductor

Principal idea:

Advantage of a semiconductor:

Advantage of an insulator:

Active electronic devices like diodes, transistors, etc

Due to the wide band-gap the samples are transparent

Why wide band-gap semiconductors ?

Historic development:

GaN based LED

Band-gap engineering

Variation of the band-gap by changing the composition

The band-gap has influence on:

Emission wavelength of an optical device efficiency of the light emission energy level of the dopants etc.

Choose an optimal composition for a specific application

AxB1-x

Small overview of semiconductors

Wid

e b

an

d-g

ap

Why rare earth doping in semiconductors ?

Optical emission properties of rare earths:

emission wavelength does not depend on the host material

Color is typical for a specific rare earth ion

Intensity of rare earth emission depends on the material:

band-gap quenching

temperature quenching

concentration quenching

Colors in rare earth doped GaN

M. Garter et al. Appl. Phys. Lett. 74 (1999) p.182

200 400 600 800 1000

Wavelength [nm]

Yb3+2F5/2 2F7/2

Sm3+4G5/2 6H7/2

4G5/2 6H9/2

CL I

nte

nsitie

s [

a.u

.]

Eu3+5D0 7F1,2

Tb3+5D4 7F5

Tm3+1G4 3H61D2 3H4

Dy3+

4F9/2 6H13/2

Excitation mechanism

1 and 2: excitation pathsa and b: recombination paths

RE3+ Ion

Cathodoluminescense of RE3+ in a-AlN:RE

Intrashell-transitions of f-shells

Temperature quenching of Er3+ doped semiconductors

From Favennec: Electronics Letters 25 (1989) 718

a) In0,16Ga0,38As0,84P0,16

b) Sic) InP d) GaAse) Al0,17Ga0,83Asf) ZnTeg) CdS

Incr

ease

of

ban

d-g

ap

Temperature quenching for Er3+ emission

From Zanatta: Appl. Phys. Lett. 82 1395 (2003)

Temperature quenching in AlN:RE

From Lozykowski and Jadwisienczak: Phys. Stat. Sol. B 244 (2007) 2109

Emission

1 expB

AI

Bk T

Phenomenological description:

Outline

I Motivation and Introduction

Wide band-gap semiconductors

Band-gap engineering

Rare earth doping and optical emission

II First Results of a-(SiC)x(AlN)1-x

Thin film growth method and structural characterisation

Band-gap engineering of a-(SiC)x(AlN)1-x

III Cathodoluminescense measurements

Spectral emission of rare earth doped a-(SiC)x(AlN)1-x

Thermal activation of rare earth emission

IV Summary and Acknowledgements

a-(SiC)x(AlN)1-x:RE

Why a-(SiC)x(AlN)1-x?

Rare earth doping:

Well defined emission color Covering of the whole color range

Wide bandgap semiconductors:

Increase of rare earth emission Lower temperature quenching Transparent Semiconductor devices

Amorphous films:

Inexpensive Simple production Higher incorporation of rare earths

Pseudobinary compound:

Band-gap engineering (3eV to 6eV) one composition parameter Sputtering from SiC and AlN target

Los principios de dc-sputtering

target

ánodo+

+

+

++

+

+

+ ion Ar

Átomo Arelectrón

Plasma frío: ion

electrón

10 3electrón

-4

300K

12000K

10 cm

grado de ionización: 10

T

T

n

10-2 mbarsustrato

-

+

Problemas:

Inestabilidad del plasma Sólo targets metálicos Baja eficiencia

1000 V

Los principios de magnetrón-sputtering

Aumento de densidad de los iones Más rapidez del crecimiento

El magnetrón

magnetrón armado

blindajeportatarget

N

NN

SS

S

Anillo de plasma

target

Schematics of the sputtering system

Turbo-molecular

pump

Mechanical pump

Pressure sensor

Mass spectrometer

control

Ar N2

Mass flow controler

Control of mass

spectrometer

Rf- generator

Rf-generator

shutterH2O

substrate

targets

flexiblemagnetrons

H2O

match

PC control

The rf magnetron sputter system at the PUCP

Vacuum system: residual gas analysis

Gas processing: flow control of N2, H2 and Ar:

0…100 sccm, 5N...6N

working pressure:

Sputter targets: trial magnetron sputtering, 2´´ 3 Rf generators, P<300W felxible target geometry !!

Substrates: Substrate area up to 128 cm2

variable target substrate distance water cooled substrate holder

78 10 mbarp

3 18 10 mbar 1 10 mbarp

A typical film of a-SiC on glas

0 1 2 3 4 5 6 7

Longitud [cm]

Esp

esor

[uu. aa

.]Target material: Silicon Carbide (SiC)

Substrate material: fused glas

Rf power: 100 W

Process gas: Argon, 5N

Gas flow: 80 sccm

Argon pressure: 810-3 mbar

a-SiC

3´

3´

1 2 3 4 5 6 7

Esp

esor [u

u. a

a.]

Longitud [cm]

distancia desde target: 6 cm 5 cm 4 cm 3 cm 2 cm

anillo delplasma

80403

80402

80401

80331

80327

Característica de emisión de un magnetrón I

3argón sputtering120 W, 9 10 mbar, flujo: 50 sccm, 5 hP p t

Característica de emisión de un magnetrón II

N

NN

SS

S

1cm

00.0200.0390.0590.0790.0980.120.140.160.180.200.220.240.260.270.290.310.330.350.370.390.410.430.450.47

emisión en uu. aa. Contorno de emisión

blindaje

plasma

target

imanes

3argón

sputtering

120 W

9 10 mbar

flujo: 50 sccm

5 h

P

p

t

A typical thin film of a-(SiC)x(AlN)1-x

EDX results

highly pure films (i.e. Na content < 8 ppm wt.) no signature of impurities in the film

0 20 40 60 80 100

0

10

20

30

40

50

60

70

80

positio

n (m

m)

content (at. %)

AlNAlN

SiC

Si Al Na Mg Ar Ca

host

substrate

Transmission electron microscopy (TEM):

Structure of a/nc-AlN and a-SiC anealed at 900°C

High resolution transmission electron microscopy (HRTEM):

There are nanocrystals embedded in an amorphous matrix

Substrate (Si)

a-SiC

diffraction

a/nc-AlN

a/nc-AlN

Optical absorption measurements

Determination of the band-gap i.e. a-(SiC)0.25(AlN)0.75 :

0.0

2.0x1010

4.0x1010

6.0x10101 2 3 4 5

1 2 3 4 50

400

800

1200

2-Plot fundamental absorbtion

Energy (eV)2

(cm

-2)

E= (3.7±0.1) eV

Urbach tail

E1

/2(e

V1/2cm

-1/2)

Energy (eV)

ETauc

= (2.4±0.1) eV

Tauc-Plot

Gap 2 11 1E x E x E b x x

0.00 0.25 0.50 0.75 1.000

1

2

3

4

5

6

AlNSiC

a- S

iCc-

SiC

6.2 eV

3C-SiC

6H-SiC4H-SiC

c- AlN

a-/nc-AlN, Ref [2]

a-AlN, Ref [3]

a-(SiC)1-x

(AlN)x, 2-Gap

a-(SiC)1-x

(AlN)x, Tauc-Gap

, c-(SiC)1-x

(AlN)x , Ref. [1]

E (eV

)

composition x

Band-gap engineering of a-(SiC)x(AlN)1-x

[1] Nurmagomedov et al.: Sov. Phys. Semicond. 23 100 (1989)[2] Gurumurugan et al.: Appl. Phys. Lett. 74 3008 (1999)[3] Zanatta et al.: J. Phys. D: Appl. Phys. 42 (2009) 025109

Bowing parameters: ba2=(1.98±0.94) eV , bTauc=(1.96±0.48) eV

Fitting to Vegard´s law:

Outline

I Motivation and Introduction

Wide band-gap semiconductors

Band-gap engineering

Rare earth doping and optical emission

II First Results of a-(SiC)x(AlN)1-x

Thin film growth method and structural characterisation

Band-gap engineering of a-(SiC)x(AlN)1-x

III Cathodoluminescense measurements

Spectral emission of rare earth doped a-(SiC)x(AlN)1-x

Thermal activation of rare earth emission

IV Summary and Acknoledgements

200 400 600 800 1000

Wavelength [nm]

Yb3+2F5/2 2F7/2

Sm3+4G5/2 6H7/2

4G5/2 6H9/2

CL I

nte

nsitie

s [

a.u

.]

Eu3+5D0 7F1,2

Tb3+5D4 7F5

Tm3+1G4 3H61D2 3H4

Dy3+

4F9/2 6H13/2

Emission of rare earth ions in a/nc-AlN and a-SiC

Cathodoluminescense of RE3+ in a-AlN:RE

400 500 600 700 800

a) a-SiC:Tb3+

CL

-In

ten

sity

[a

.u.]

5D4 7F

5

4F9/2

6H13/2

b) a-SiC:Dy3+

5D0 7F

1,2

c) a-SiC:Eu3+

Wavelength [nm]

Cathodoluminescense of RE3+ in a-SiC:RE

Thermal activation of a-/nc-AlN

0 200 400 600 800 1000 1200

C

L P

eak

Inte

nsi

ties

[a.u

.]

Temperature [°C]

Sm3+: 4G5/2

6H7/2

as grown

a/nc-AlN: RE3+

a)

Eu3+: 5D0 7F

1,2

Yb3+: 2F5/2

2F7/2

exponential growth with the anealing temperature there is a saturation of the RE emission at anealing tempertures of 900°C

Thermal activation of a-SiC

0 200 400 600 800 1000

Tb3+

Dy3+

Eu3+

Pe

ak In

ten

sitie

s [a

.u.]

Annealing Temperature [°C]

( )as grown

exponential growth with anealing temperature there is no saturation up to 1000°C there is an optimal anealing temperature for the Tb3+ emission in a-SiC

Thermal activation of a-(SiC)x(AlN)1-x

Thermal activation of a-(SiC)0.83(AlN)0.17:Tb3+

Summary

Wide-bandgap semiconductors

Rare earth doping

bandgap engineering

First results on a-(SiC)x(AlN)1-x thin films

HRTEM investigations

bandgap engineering of a-(SiC)x(AlN)1-x

Cathodoluminescense

optical emission of a-(SiC)x(AlN)1-x

thermal activation of rare earth emission

Conferences/Publications: IMRC 2009 in Cancun, Mexico (invited talk) ICSCRM´2009 in Nuremberg, Germany Five publications in International Journals

Acknowledgements

Materials Department, University of Erlangen, Germany

Prof. Dr. Winnacker

Prof. Dr. H. P. Strunk

Catholic University of Lima, Peru (PUCP)

Prof. F. De Zela

Andrés Guerra, Gonzalo Galvez, Oliver Erlenbach (PhD)

Liz Montañez, Katia Zegarra, (Licenciatura)

This research work is supported by the

• Pontificia Universidad Católica del Peru (PUCP)• Deutsche Forschungsgemeinschaft (DFG) and the• German Service of Academic Interchange (DAAD)

Wide bandgap semiconductors

From Steckl MRS Bull. 24, p. 33 (1999)