the 3R polymorph, and a change in color from light gray for 3R to dark gray for 2H

AN ABSTRACT OF THE THESIS OF Robert Kykyneshi for the degree of Master of Science in Physics presented on May 27. 2004. Title: Transport Properties o...
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AN ABSTRACT OF THE THESIS OF Robert Kykyneshi for the degree of Master of Science in Physics presented on May 27. 2004.

Title: Transport Properties of CuSc1Mg

and BaCu2$2 Transparent Semiconductors.

Redacted for Privacy

Abstract approved:

Bulk properties of CuSc1..MgO2, CuSci..MgO2+, BaCu2S2, Bai..KCu2S2, BaCu2Se2

and Bai.KCu2Se2 are investigated supporting the search for highly conductive p-type thin films. Mg is an efficient dopant in CuScO2 with conductivity up to l.5.102 S/cm. Oxidation of CuScO2:Mg leads to further increase in conductivity up to 0.5 S/cm. The amount of oxygen entering the material is dependent on the Mg content with an observed

maximum of y=0.23 in the 5 at. % Mg doped sample. Mg doping above 6 at. % leads to saturated or slightly decreased conductivities in CuScO2. In the case of BaCu2S2:K and BaCu2Se2:K, conductivities of 420 S/cm and 740 S/cm are achieved at room temperature

with 10 at. % potassium doping with no observable saturation, 2 orders of magnitude higher than in the corresponding undoped forms. A change from activated to metallic type conduction mechanism is observed in Bai.KCu2S2 at x>0.005. The Seebeck effect

measurements demonstrate p-type conductivity for all samples. The temperature dependence of the Seebeck coefficient exhibits hopping type transport for CuSc1.. MgO2+ and metallic type for BaCu2S2:K. The optical properties of the 2H polymorph of CuScO2 and CuScO2:Mg show introduction of defects into the material compared to

the 3R polymorph, and a change in color from light gray for 3R to dark gray for 2H CuScO2:Mg was observed. Potassium doping and preparation temperature converts cxBaCu2S2 to 1.8 eV.

-BaCu2S2 which results in a change of measured band gap from 2.3 eV to

© Copyright by Robert Kykyneshi

May 27, 2004

All Rights Reserved

Transport Properties of CuSci..MgO2 and BaCu2S2 Transparent Semiconductors

by

Robert Kykyneshi

A THESIS

submitted to

Oregon State University

in partial fulfillment of the requirements for the

degree of Master of Science

Presented May 27, 2004 Commencement June 2005

Master of Science thesis of Robert Kykyneshi presented on May 27, 2004

APPROVED: Redacted for Privacy

Maj or Professor, representing Physics Redacted for Privacy

Chair of the Department of physics Redacted for Privacy

Dean of the Graduate School

I understand that my thesis will become part of the permanent collection of Oregon State University libraries. My signature below authorizes release of my thesis to any reader upon request. Redacted for Privacy

Robert Kykyneshi, Author

ACKNOWLEDGEMENTS

I would like to express my sincere appreciation to my major professor, Dr. Janet Tate, for believing in me, for her support, and for her expert guidance over the years. Dr.

Douglas Keszler, Dr. Arthur Sleight, and Dr. William Warren, Jr. all generously contributed their ideas to my project, and for that I am grateful. In particular, I am thankful for the way in which they shared their knowledge of solid state physics and inorganic chemistry, especially in the field of semiconducting materials.

I always appreciated the interest Dr. Henry Jansen expressed in my progress as a

student and for kindly providing me with his signature on many forms and papers. And thanks to Debbie Snow, Paula Rhodaback, and Verna Paullin-Babcock for their kind help in school business problem solving.

Thanks to the members of my research group, Benjamin Nielsen, Dara Easley, Paul Newhouse, and James Osborn for creating a friendly work environment. Special thanks to Jun Li and Choel-Hee Park for their friendship and for providing samples and

data for the project. Also, I enjoyed sharing an office with Jim Ketter and Scott Berry

we spent many late night study sessions together and I appreciated their humorous attitudes.

My life here was substantially richer and better thanks to all my Eugene and

Corvallis friends, the camping trips, get-togethers, terrific conversations, and great anecdotes.

And last, but not least, to my parents, who gave me the guidance, which made it possible for me to be here.

TABLE OF CONTENTS

Chapter1. Introduction .........................................................................................

1

l.1.Purposeofthe Work ..........................................................................

1

1.2. Background on Transparent Conductors ...........................................

1

1.3. Organization of the Thesis .................................................................

3

Chapter 2. Characterization Techniques ...............................................................

5

2.1. Conductivity Measurement ................................................................

5

2.2. Seebeck Coefficient Measurement.....................................................

12

2.3. Diffuse Reflectance Measurement .....................................................

20

Chapter 3. Properties of Bulk CuScO2 .................................................................

23

3.1. Paper Preprint Submitted to Phys. Rev. B .........................................

23

3.2. Further Results ...................................................................................

48

3.2.1. Structural Properties ............................................................

48

3.2.2. Electrical Properties ............................................................

50

TABLE OF CONTENT (Continued) Page

3.2.3. Optical Properties................................................................

55

3.3. Conclusions ........................................................................................

57

Chapter 4. Properties of Bulk BaCu2S2 And BaCu2S2:K......................................

59

4.1. Introduction ......................................................................................... 59 4.2. Experimental ......................................................................................

61

4.3. Results ................................................................................................

62

4.3.1. Optical properties ................................................................

63

4.3.2. Effect of Preparation Temperature ......................................

64

4.3.3. Transport properties ............................................................

66

4.4. Conclusions ........................................................................................

72

Chapter 5. Conclusions and Suggestions for Further Study .................................

74

References .............................................................................................................

77

LIST OF FIGURES Figure

Page

2.1. Conductivity measurement experimental setup of rectangular (a)

andround (b) pellets ...................................................................................... 2.2. Van der Pauw symmetric contact geometries ................................................

7

10

2.3. The inline technique systematically underestimates the conductivity

of the samples. Shown is a set of four Sri..KCuSF samples with different

dopinglevels ..................................................................................................

11

2.4. Sketch of the Seebeck effect in an n-type material ........................................

13

2.5. Seebeck coefficient measurement: experimental setup .................................

17

2.6. The Seebeck coefficient of chromel (red circles) and alumel (blue squares) with a linear fit superimposed ................................................

18

2.7. Diffuse reflectance measurement ...................................................................

21

3.1 .Part of the AMO2 delafossite structure showing the site of the 0 dopant

38

3.2.(Color online) X-ray diffraction patterns of unoxidized (light/orange line) CuSco.95Mgo.0502 and oxidized (dark/blue line) CuSco.95Mgo.o5O2+ bulk

Powders ...........................................................................................................

39

3.3 .(Color online) X-ray diffraction patterns of unoxidized CuSco.99Mgooi 02

(light/orange line) and oxidized

CuSco.99Mgo.0102.5 (dark/blue

line) films .....

40

LIST OF FIGURES (Continued) Figure

Page

3.4.(Color online) Variation of the aaxis lattice parameter in CuSclMgO2+ sputtered films (closed symbols) as a function of intercalation pressure

41

3.5. Room temperature conductivity (closed symbols, left scale) and

Seebeck coefficient (open symbols, right scale) of unoxidized

CuSci..MgO2 pellets ....................................................................................

42

3.6. Room temperature conductivity (closed symbols, left scale) and Seebeck coefficient (open symbols, right scale) of oxidized

CuSciMgO2+ pellets .................................................................................

43

3.7. Room temperature conductivity (closed symbols, left scale) and oxygen content as determined by TGA (open symbols, right scale) of oxidized

CuSciMgO2+ pellets as a function of Mg dopant concentration ..............

44

3.8. Room temperature conductivity of oxidized CuSc 1XMgO2 pellets as a function of average excess oxygen content ................................................

45

3.9. Room temperature conductivity of CuSci..MgO2+ films ............................

46

3.10. Diffuse reflectance spectra of CuSci..MgO2 powders ................................

47

3.11. Doping with Mg on the Sc site changes the a lattice parameter (a) and c lattice parameter (b) (Jun Li) ..............................................................

48

3.12. 2H and 3R polymorths of CuScO2 ...............................................................

49

LIST OF FIGURES (Continued) Figure

3.13. Oxygen intercalation levels of CuSci..MgO2+ measured by TGA. (Jun Li) .........................................................................................................

50

3.14. Temperature dependent conductivity measurements of not oxidized CuScO2:Mg samples ....................................................................................

51

3.15. Temperature dependent conductivity measurements of oxidized CuScO2+:Mg samples. The activation energies are about 70 meV ............

52

3.16. The conductivity of the samples is not noticeably correlated with their densities. Shown are a) not oxidized and b) oxidized samples ...........

52

3.17. Temperature dependent Seebeck coefficient measurement of CuScO2+:Mg thin films between 10-300 K ................................................

54

3.18. Powder colors of the intercalated 3R CuScO2 (left), unoxidized

3R and 2H polytypes, and 4 at. % Mg doped ..............................................

56

3.19. Oxygen intercalation makes the material black, hence no band gap structure can be observed via diffuse reflectance ........................................

57

4.1. Structure of(a) a- and (b) 3-BaCu2S2 ...........................................................

60

4.2. Diffuse-reflectance optical measurements for band gap determination of a- BaCu2S2 (green) and Ba09K01Cu2S2 (red) (Cheol-Hee Park) ............... 63

LIST OF FIGURES (Continued) Figure

Pag

4.3. Higher preparation temperature results in more conductive materials of BaCu2S2 and K-doped derivative ..............................................

65

4.4. Temperature dependent conductivity measurements of Ba1.KCu2S2..........

67

4.5. Estimated carrier density (blue) and defect creation efficiency (green) with K doping in BaCu2S2 .............................................................................

68

4.6. Room temperature conductivity and Seebeck coefficient measurement results of Ba1 .KCu2S2, O( a) a)

x

0.10

0.05

0

1 02

0

0.02

0.04

0.06

0.08

0.1

Mg doping, x

FlU. 3.7. Room temperature conductivity (closed symbols, left scale) and oxygen

content as determined by TGA (open symbols, right scale) of oxidized CuSci.. MgO2+ pellets as a function of Mg dopant concentration.

45

0.6

0.5

0.4

C)

0.2

0.1

0.0

0

0.05

0.1

0.15

0.2

0.25

Oxygen excess, y

FIG. 3.8.

Room temperature conductivity of oxidized CuSciMgO2+ pellets as a function of average excess oxygen content. The conductivity of each corresponding unoxidized sample has been subtracted. The straight line is a fit.

46

1 .OE+2

1.OE+1 1.OE+O

1.OE-1

1.OE-2 1.OE-3 1.OE-4 1.OE-5

0

5

10

Mg doping in target (at.

FIG. 3.9.

15

%)

Room temperature conductivity of CuSciMgO2+ films. The Mg content is

indicated as the percentage of Mg in the sputter target.

Three different

intercalation procedures are indicated - high pressure (circles), intermediate

pressure (diamonds), and no intercalation (squares), and two different 350°C (closed symbols) and 150°C (closed deposition temperatures symbols). The lines are guides.

47

1.0

0.6

0.6 cl

0.4

0.2

0.0

1.5

2

2.5

3

3.5

E, eV

FIG. 3.10. Diffuse reflectance spectra of CuSc1MgO2 powders.

4

4.5

48

3.2. Further results

3.2.1. Structural properties Structure refinement reveals that the a lattice parameter decreases by about 0.04%

(fig.3.11, a) upon the incorporation of the smaller Mg atom on the Sc site compared to

the 2H polymorph. The c lattice parameter changes even less (0.006 %) shown on fig.3.1l (b). Therefore, the incorporation of Mg on the Sc sites does not have much influence on the crystalline lattice parameters. This can be explained by the similar values

of the ionic radii (-'3% different) of Sc (0.745 coordination

and Mg (0.720

A)

A)

in six-fold

56

S

11.4040

3.2200 3.2195

.SSSS

11.4035

a)

.

11.4030 11.4025

3.2190

S

S

11.4020

3.2185

S

b)

11.4015

3.2180

11.40 10

11.4005

3.2175

0 1 23456 78910111213141516

0 1 23 45 6 78910111213141516

Mg doping, at.%

Mg doping, at.%

Fig.3.1 1. Doping with Mg on the Sc site changes the a lattice parameter (a) and c lattice parameter (b) (Jun Li).

The 3R polymorph was obtained only when no Mg was incorporated and a slight

Sc excess was present, while the 2H polymorph results upon Mg doping or even in the presence of excess Cu (Fig.3.12). The structural similarities between the Mg-doped and the "Cu-doped" material indicates that the excess Cu necessary to stabilize the 2H phase substitutes as Cu2 on the

Sc3

site.

49

P63/mmc

1

2H

aR=aH CII

2CR=3CH

Fig.3.12. 2H and 3R polymorths of CuScO2. Sc is in the center of the octahedra.

The oxygen intercalation level of the CuSclMgO2 samples was measured by TGA (Fig.3.13). As discussed above, regardless of the simultaneous oxidation process of

all Mg doped samples, a maximum intercalation level of y around 0.22 is observable for CuSc1MgO2+, 0.04 0.005 show degenerate semiconductor behavior.

A carrier concentration calculation was carried out based on the assumptions that

the mobility of the intrinsic carriers in the undoped material is 3.5 cm2V

and

s1 71

remains unchanged with doping (Fig.4.5). Along with the measured conductivity, such a calculation yields a carrier concentration of 3.5.1019 cm3 in the undoped BaCu2S2 and increases to 7.7.1020 cm3 for 10 at. % K doping, using:

p=cT/(1U.e)

where c.r is the conductivity of the sample, p is the mobility of the carriers, and e

(4.1)

is

the

charge carried by the carrier. The doping efficiency, i.e. the number of potassium atoms

68

100

1021 I

I

A

-

-

CD

H

.4 Vi)

/

CD

CD

U C)

CD

CD

CD

.4

E

102°

C) CD

.4 CD

Q. CD

ACI C)

n,lbr3

.4

'
0.005. The values of the Seebeck coefficient and their temperature dependence for BaCu2S2:K are typical for degenerate semiconductors.

BaCu2Se2 forms only one phase, similar to that of 13-BaCu2S2. The K doping is

found to enhance the transport properties even more than for the sulfide samples. The highest conductivity (740 S/cm) in p-type pressed powder pellets of Bao,9K0.iCu2Se2 is

reported. BaCu2Se2 has an unusually high Seebeck coefficient in the undoped form and

falls of rather sharply when doped by potassium. Although temperature dependent transport measurements are not available, the high conductivity and low Seebeck coefficient of the BaCu2Se2:K are likely to confirm the degenerate semiconducting nature of the samples.

74

Chapter 5. Conclusions and suggestions for further work.

In this work the structural, optical and transport properties of two material systems are presented and discussed. This includes systematic studies of the effect of substitutional and interstitial dopants on p-type conductors. The relevance of the study lies in the possibility of quite precise control of the doping level and determination of its influence on the properties of the materials.

Substitutional doping of CuScO2 with Mg on the Sc sites to form CuSci..MgO2 with 0