Cardona Symposium 20101 A Tale of Two Vacancies Peter Y. Yu Department of Physics, University of...

Cardona Symposium 2010 1

A Tale of Two VacanciesPeter Y. Yu

Department of Physics, University of California&

Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720

Acknowledgments: Collaborators are Lei Liu, Wei Cheng, Zixun Ma and Samuel S. Mao. This work was supported by the Us Department Of Energy NNSA/NA-22, under Contract No. De-Ac02-05ch11231


OUTLINE• INTRODUCTION• MOTIVATIONS• VACANCIES IN GROUP III-NITRIDES

– FERROMAGNETISM DUE TO Ga VACANCIES– DOPING BY Gd

• VACANCIES IN Cd-CHALCOGENIDES– CODOPING WITH OXYGEN– EVIDENCE OF O2 MOLECULES

• CONCLUSIONS• ACKNOWLEDGMENTS


INTRODUCTION• Vacancies are often introduced during

crystal growth at high temperature• Vacancies are important in determining

the quality and electrical properties of semiconductors:– Vacancies allow impurities to diffuse more

easily throughout a crystal– Vacancies involve dangling bonds and are

electrically active, vacancies can cause self-compensation


MOTIVATIONS• Vacancies are detected mainly by two methods:

High Resolution TEM and Positron Annihilation. Otherwise they are difficult to detect

• Recent advances in First-Principle Density Functional Theory (DFT) make it possible to calculate the properties of vacancies

• Present work attempts to study their role in– (1) room temperature Ferromagnetism in GaN:Gd– (2) incorporation of Oxygen into CdTe in the

formation of O2


STORY 1

How Vacancies Produce Room Temperature Ferromagnetism In

GaN


Ferromagnetism in GaN:GdRoom temperature Ferromagnetism was reported by at least 2 groups in GaN doped with Gd :“Magnetic, optical and electrical properties of GaN and AlN doped with rare-earth element Gd” by S. W. Choi, Y. K. Zhou, S. Emura, X. J. Lee, N. Teraguchi, A. Suzuki, and H. Asahi (2002-2006):

[Gd]:2-6%Tc~400Ksample n-type with [e]>5x1019 cm-3.

“Gd-doped GaN: A very dilute ferromagnetic semiconductor with a Curie temperature above 300 K” by S. Dhar, L. Pérez, O. Brandt, A. Trampert, and K. H. Ploog (2005-2007)[Gd]:1016-1019 cm-3

Tc>300Kmagnetic moment /Gd~4000B

magnetic moment/Gd increases with defect concentration since ion-implanted sample has large moment than sample after annealing


Results of Ploog’s Group suggested that intrinsic defects played an important role in the Ferromagnetism.The nature and role of the intrinsic defect are, however, unclear.

Our First-Principle Calculation suggests that:GaN:Gd is paramagnetic

GaN containing GdGa+VN is also paramagnetic

GaN containing GdGa+VGa is ferromagnetic

Surprisingly GaN containing only VGA is also ferromagnetic!

Ferromagnetism in GaN:Gd


Computation Method•Use spin-polarized DFT within the Generalized Gradient Approximation (GGA). •Electron correlation important for the d and f electrons of Gd ions are included (approximation known as GGA+U )•Use Full-potential Linearized Augmented Plane Wave (FLAPW) as basis functions for calculating the electron eigenvalues and functions.Lei Liu, Peter Y. Yu, Zhixun Ma, and Samuel S. Mao. Ferromagnetism in GaN:Gd: A Density Functional Theory Study. Phys. Rev. Lett. 100, 127203 (2008)


Supercell model:GdGa7N8

Gd atoms are separated by 3 atom layers to simulate long range interaction between Gd atoms.


f electrons

GaN:Gd is Paramagnetic

Band Structure of GaN:Gd: f-electrons in Gd ions are magnetized but the coupling between the moments is paramagnetic


GdGa6N8 Supercell containing Gd and VGa


GaN containing GdGa+VGa is ferromagnetic!


Top valence bands are 100% Polarized!


What is the source of the strong coupling between the Gd ions?

Answer: coupling with the spin of holes introduced by Ga vacancies


Pratibha Dev, Yu Xue, and Peihong Zhang. Defect-induced intrinsic magnetism in wide-gap III-nitrides. Phys. Rev. Lett. 100, 117204 (2008).

Spin-resolved DOS of Ga Vacancy in GaN


How come the Gd magnetic moment is so large?


(1)Local strain of Gd ion induces vacancies in the vicinity

(2)The magnetic moment of Gd is enhanced by the 3 spins of each Ga vacancy nearby

GdGa6N8 10 0.7


Summary:Gd introduces Ga vacancies by producing tensile local

strain Ga vacancies produce holesWhen the hole wave functions are localized enough (as in

case of nitrides) they become spin polarized according to Hund’s Rule

Coupling between the Gd and hole spins produce a strong ferromagnetic state.

When the hole or Ga vacancy concentration is much higher than the Gd concentration the magnetic moment of Gd appears to be enhanced



STORY 2

How Cd Vacancies allow

O2 Molecules to be incorporated into CdTe


• CdTe is an important semiconductor for thin film solar cells

• Oxygen is a common impurity in the manufacture of solar cells.

• O replacing Te (OTe) is an isovalent impurity. Since electronegativity of O>>electronegativity of Te, O will attract an electron to form O- which is a shallow acceptor.

• Mass of O<<Mass of Te so vibration of OTe is highly localized around O. These are called local vibration mode (LVM).

• LVM are very sharp and, therefore, sensitive probes of light impurities

BACKGROUND

Cardona Symposium 2010

(G. Chen, I. Miotkowski, S. Rodriguez, and A. K. Ramdas, Phys. Rev. B 75, 125204 (2007).)

Infrared Absorption Spectra of CdTe:O

340 344 348 352 356 360

W ave number (cm -1)

0

10

20

30

Ab

so

rpti

on

Co

eff

icie

nt

(cm

-1)

O

C d

C d

C d

C d

T = 5 KR es. = 0 .0 2 cm -1

F W H M = 0 .2 4 cm -1

Sample Growth strategies:CdO to provide oxygen and ExcessExcess Cd to suppress VCd

A single sharp line is observed:

0 = 349.8 cm-1

FWHM = 0.24 cm-1

Selection rule:1 5


1092 1096 1100 1104 1108 1112

W ave num ber (cm -1)

0

20

40

60

Ab

sorp

tio

n C

oef

fici

ent

(cm

-1)

T = 5 KR es. = 0 .0 1 cm -1

F W H M 1 = 0 .1 6 5 cm -1

F W H M 2 = 0 .1 3 7 cm -1

I2 / I1 = 1 .7

O T e-V C d

Chen et al. observe 2 high frequency modes at :

1 = 1096.78 cm-1; 2 = 1108.35 cm-1.

At high T 1 and 2 merged into one mode with frequency:1104cm-1.

They attributed these 2 modes to vibration of a complex: OTe-VCd

High Frequency Mode when VCd Is Present

In CdTe:O without excess Cd


Model Proposed by Chen et al.

22

TeTe

Te

Cd

Cd

Cd

VCd

O

c1 2

N = 0

N = 1

1

1

3

E || c E c

1

2


Dynamic Switching of VCd-O complex

23

0 100 200 300

Temperature (K)

1100

1104

1108

Wa

ve

nu

mb

er

(cm

-1)

( + 2 * ) / 30

*O

VCd

c

Cd

Cd

Cd1

2

3

4

As T increases Oxygen switches between sites: 1, 2 , 3 and 4


Activation Energy for Switching

24

0 0.1 0.2 0.3 0.4 0.51 / T (1 / K)

-6

-4

-2

0

In [

1 -

(

-

)T/(

-

)0 ]

0.0036 0.004 0.0044 0.0048-0.8

-0.6

-0.4

-0.2

0

1/T

]1[)()( /01212

kTWT e W=42 meV


L. Zhang, J. T-Thienprasert, M.-H. Du, D. J. Singh, and S. Limpijumnong, Phys. Rev. Lett. 102, 209601 (2009) calculated, from first principle, the frequency of the OTe-VCd complex and obtained <500 cm-1.

Similar high frequency modes (>1000 cm-1)have also been observed by Chen et al. in CdSe (G. Chen, J. S. Bhosale, I. Miotkowski, and A. K. Ramdas, Phys. Rev. Lett. 101, 195502 (2008)) so this complex is rather common in Cd chalcogenides.

What is the identity of this complex of O in CdTe and CdSe? How to explain the dynamic switching?

MOTIVATION FOR PRESENT WORK


Computational Method

26

First-Principle density-functional theory based on the GGA-PBE potential (J.P. Perdew, K. Burke, and M.

Ernzerhof, Phys. Rev. Lett. 77, 3865 (1996) )Two commercial softwares:

VASP (Vienna ab initio simulation package ) and MedeA (by Material Design )


Tests Of Softwares

27

Our result Experiment

CdTe Lattice Constant (nm)

0.641 0.646

O-O bond length (nm) & stretching mode frequency (cm-1)

0.1236 ; 1548.20

0.1208 ;

1580

Local mode frequency of OTe

(cm-1)

331.86 349.8


New Model of Oxygen-VCd Complex in CdTe

28

Ball-and-Stick model of the cell: Cd31Te32O2 containing a VCd (blue ball) and a O2 molecule (red balls). The golden and green balls represent Te and Cd atoms, respectively.

O-O is oriented along the [111] axis and is displaced from the Cd site.Symmetry of complex is C3v


Defect Formation Energies in CdTe

29

Defect Formation Energy (our

result) eV

Formation

Energy (Wei, Zhang

and Zunger*) eV

VoCd 2.1 2.30

V-Cd 2.42

V-2Cd 2.69

VCd-O2 1.2*S. H. Wei, S. B. Zhang, and A. Zunger J. Appl. Phys. 87, 1304 (2000).


Normal Modes of O2 Molecule in CdTe Vacancy

30

A1 Modes

E Modes

O-O stretch:1112.5 cm-1 Rocking of the O2 molecule:192.1 cm-1

Libration Mode at 315.7 cm-1 Rocking of the O2 molecule:176.6 cm-1


IR activity of O-O Stretching Mode in CdTe

31

• O-O Stretching Mode of O2 in gas form is not infrared-active since it has even parity

• O2 in VCd of CdTe has no inversion symmetry and therefore can be infrared-active.

• The calculated charge difference between the two O atoms in CdTe is ~0.05e and the bond length is ~0.13 nm (0.1208 nm in gaseous O2) giving an electric dipole moment of ~3 Debye.


Existence of Two IR modes in VCd-O2

1 peak (singlet) 2 peak (doublet)>1

The energy to rotate the O2 molecule by 90o~ energy of the libration mode=39 meV


Summary:Oxygen replacing Te has a LVM at ~350 cm-1.In the presence of VCd Oxygen prefers to form molecule inside the VCd

The O2 molecule is oriented along the [111] direction but displaced from the center of the vacancy. The two O atoms occupy in-equivalent sites so the O-O stretching mode is IR-active

Charge transfer to the neighboring Te atoms weakens the O-O bond and lowers the O-O stretching mode frequency. The calculated O-O stretching mode frequency is in good agreement with experiment

The existence of two modes at low T and their convergence at high T are also explained by theory

LVM of Oxygen in CdTe


Using first-principle density-functional theory we have studied the electronic and vibrational properties of vacancies in CdTe and GaN.

In GaN vacancies can be induced by replacing the cations with large rare-earth ions like Gd. The Ga vacancies produce holes which are spin polarized.They strongly coupled to each other and to the Gd spins. These results explains recently reported observation of

ferromagnetism in GaN:Gd above room temperature and the enhancement of the magnetic moment per Gd by intrinsic defects.

In CdTe the cation vacancies are large thus allowing small molecules like oxygen to be located inside them and forming a new kind of molecular complex. The vibrational modes of these molecular-vacancy complexes in

CdTe explain the sharp high frequency local vibration modes reported in CdTe.

CONCLUSIONS


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