Design of Shallow p-type 

in ZnO

Su-Huai Wei, J. Li, and Y

National Renewable Energy Labora

NREL/PR-520-43248Presented at the 33rd IEEE Photovoltaic Specialist Conference held May 11-16, 2008 in San 
The work at NREL is sup

by the U.S. DOE under cNo. DE-AC36-99GO103
3⁄4 ZnO is a promising material for short wave-length opto-

such as UV lasers and LEDs due to its large exciton bin

material cost

3⁄4 ZnO can be doped easily n-type, but the realization of st

rather difficult

Using first-principles band structure methods

we will address:

3⁄4 What causes the p-type doping difficulty in ZnO3⁄4 How to overcome the p-type doping difficulty in ZnO

Introduction
Atomic potentia

negative when thincreases from lePeriodic Table

Chemical Trends of Atomic Ene
Method of Calculati
• Band structure and total energy are calculated u

principles band structure method (FLAPW, PP) approximation (LDA) 

• Defects are described using the supercell appro

background charge is added for charged defect 

• All the internal structural parameters are optimi

the quantum mechanical forces

• Band in different supercell calculations are alig

core levels or average potentials

Method of calculatio

S.-H. Wei, Computation
Origin of the p-type do

difficulty in ZnO
Main reasons of failure-to-

• The impurity atom has limited solubilit

material, so not enough dopants are int

• The defect transition energy levels are 

enough charge carrier are generated at temperature

• Spontaneous formation of opposite-cha

defects” (cation vacancy, anion vacanc

pins the Fermi energy

S.-H. Wei, Computation
Origin of p-type doping difficult

high acceptor formation en

Calculate miformation enHf(N

O

)min = 1

Due to the strong bonding and large formation energy of ZnO, intrinsic defect formation energies are large

μ

O-poor

O

O-rich

ΔHf (eV)

02468

10

02468

10

V

O

N

O

VZn

Oi

Zni

Zn

O
Acceptor energy levels in ZnO

(0/-) 

O

OO

N 

O 0.35

0.701.100.90

Sb

As

P

CBM VBM 

Zn

Ag Zn 0.40Cu 0.74

V Zn 0.34 0.18 (0/2-) Zn (0/-) Li Zn 0.09 Na 0.17

N Zn

O

VBM 

Formation energy 

Origin of p-type doping difficult

high acceptor ionization en

• For group V on oxygen site acceptor, the lowest 

level N

O is 0.35 eV above the VBM

• For group IB on Zn site acceptor, the lowest tran

is also deep, at 0.40 eV for AgZn

• Group IA on Zn site has relatively shallow defect 

compensation limits their use as effective acceptor
3⁄4 An acceptor 

has a wavefunctto the VBM, i.eand cation 

d orb

3⁄4 Oxygen 

p or

low, there are nthat are more el3⁄4 p-d coupling 

elements and do

Zn or CuZn with 

Origin of p-type doping difficult

high acceptor ionization en

Na

Li

Zn

Mg

Be

Sb

As

P

N

O

F

Zn

-0.8

-1.1-1.3-1.5-2.2

-5.1

-5.4-5.7-7.3-9.3

-10.5

-11.4

p

-orbital

d

-orbital

Atomic Orbital Energy ( eV)

0

-

2

-

4

-

6

-

8

-10-12

-7.7-5.4

Ag

Cu
Strategies to overcome the do

3⁄4 Increase defect solubility by “defeatin

thermodynamics

3⁄4 Reduce defect ionization level throug

codoping techniques

3⁄4 Reduce defect compensation and ion

by modifying the band edge states
Improve the Dopant Solu

Adjusting Dopant Chemical 
Increase defect solubility using non

thermodynamics

3⁄4 What controls the dopant solubility is the dopan

potential, 

μ

A. Therefore, the key to enhance the so

dopant is to raise the chemical potential and avoid 

the precipitates of the dopants

3⁄4 Choose the optimal host element che3⁄4 Enhance solubility by metastable mo3⁄4 Enhance solubility by epi-growth (e.g

Δ

H(α,q)(E

F,μ) = 

Δ

E(α,q)(E

F

=

0,μi=0) + Σ 

ni

μi + qE

S.-H. Wei, Computation
Choose the optimal host element chem

 The formation energy of N

O is the lowest under 

whereas VZn is the lowest under the O-rich cond

Δ

H

f = E(ZnO:N) - E(ZnO) + 

μ

O

Δ

H

f = E(ZnO:VZn) - E(ZnO) + 

μ

μ

O-poor

O

O-rich

ΔHf (eV)

02468

10

02468

10

V

O

N

O

VZn

Oi

Zni

Zn

O
ΔHf = E(ZnO:N) - E(ZnO) + μO ‒ 

ΔHf= E(ZnO:N) - E(ZnO) + 2μO ‒ 

μN(N2) < μN(N2O) < μN(NO) < μN(

N chemical potentidoping sources (N2

N solubility in ZnO NO or NO2 is used 

NO or NO2 doping formation of (N2)O

compensation dono

Enhanced solubility by molecular dop

Yan et al., Phys

O-poor O-rich

-4-2

0246

NO2

NON2O

N2

Δ

H

f

(eV

)

μO
Single N atom in Z

N

Zn

O Transition e

E(0/-) = VBM 

Calculate mformation e

Hf(N

O

)min = 

Hf(N

O

)min = 0
Design shallow defect leve

using band structure calculat
Effects of conventional co-

• Can co-doping lower the defect transitio

CBM VBM 

VZn + GaZn

2N

O, t

2

CBM VBM 

VZn, t

2

GaZn, a1

2

A 

3⁄4 The level repulsion effect is rather small becstate and the acceptor states have different symm3⁄4 The defect transition energy level may be lowdefect complex consists a single donor and a sin
Design shallow p-type dopants 

3⁄4 Defe

large neigh

3⁄4 Repl

electexpeenerg

3⁄4 Rem

betw

by reor Be reducenerg

N

O,p

Zn,d

Zn,dA
Design shallow p-type dopants 

3⁄4 The calculated defect ionization energy leveF and Be could be good p-type co-dopant for Z

[J. Li, S.-H. Wei, S.-S. Li, J. B. Xia, Phys. 
0 123

-2-0

246

EF (eV)

AuZn

CuZn

AgZn

8 Aui

Cui

Agi

ΔH

f (eV)

Reduce self-compensation by int

Group-IB acceptors in Zn

3⁄4 CuZn has very deep acceptor level because of the large phas relatively shallower levels3⁄4 IBi is highly unstable, so self-compensation for IB dopa

Y. Yan, M. M. Al-Jassim, and 
Large size mismatched p-type doping 

3⁄4 p-type conductivity in As and P-doped ZnO have been obserdoping model attributed the dopants to As

O and P

O

3⁄4 We have show that As

O and P

O are unlikely to be the measur

- The formation energy is high (As and P is much lar

- The ionization energy of As

O and PO are very high 

Background:

Acceptor energy levels in ZnO

(0/-) 

O

OO

N 

O 0.40

0.701.100.90

Sb

As

P
- Atomic size of As and Zn are similar

- AsZn has relatively lower formation energy but 

it is a (triple) donor- VZn is a native (double) acceptor with low formation energy- One AsZn and two VZn bind strongly and form 

a new acceptor complex (AsZn-2VZn)

- The complex has low formation energy and 

low ionization energy (~ 150 meV)

The new model:

[As

V

Large size mismatched p-type doping 

Limpijumnong, Zhang, Wei, a
Calculated binding energy and defect formation en

As-related defect complexes in ZnO

Large size mismatched p-type doping 

μ

O= 0

μAs=0

3⁄4 Strong Couloand strain compthe formation enAsZn-2VZn defec

3⁄4 Coupling be

donor states and acceptor states lionization energcomplex relativ
Modify the host band stru

reduce ionization energ

compensation
Universal approach to overcome 

asymmetry in wide-band-gap sem

3⁄4 First, throug

of mutually pairs, we intcompensated the VBM or 

3⁄4 Second, afte

compensated is formed, uto dope the pby ionizing t

Excess dopant states

Defect band

Host band Y. F. Yan, et al., P
Modify the valence band edge 

passivate doping of Ga w

3⁄4 N combined with Ga creates a passivateabove the host ZnO VBM
Create shallow acceptor level by 

passivated ZnO:(Ga+N) syste

excess N

3⁄4 The calculated defect level of N is about 0.1 above the defect band

[Y. Yan, J. Li, S.-H. Wei, M. M. Al-Jassim, Phys. Re
Possible dopants or dopant com

p-type doping in ZnO

3⁄4 Based on defect wavefunction analysis, varioumodels have been proposed to reduce the ionizatiacceptor level in ZnO

AgZn ; VZn + F

O

MgZn + N

O ; BeZn + N

O

AsZn + 2VZn ; PZn + 2VZn

3⁄4 Doping of defect band is an effective and univto doped wide band gap materials such as ZnO

N

O + (N

O + GaZn

)defect band
Summary

We have analyzed the origin of p-type doping difSeveral strategies have been proposed to overcomdifficulty

• Increase defect solubility by “defeating” bulk defect the

• optimized host elements chemical potential• surface enhanced defect solubility• molecular doping• large size-mismatched antisite doping

• Reduce defect ionization level by 

• combining donor with acceptor to modify defect wa

• reducing p-d coupling between defect level and hos

• Design new dopable materials by adjusting the band edgpassivated doping and subsequent doping using the same 
Thank

You 

for

Your 

Atte