Electronic Materials Letters, Vol. 7, No. 2 (2011), pp. 115-119

DOI: 10.1007/s13391-011-0605-x Published 29 June, 2011

Enhanced p-type Conductivity of Nitrogen Doped ZnO by Nano/Micro Struc-tured Rods and Zn-Rich Co-Doping Process

Wei Mu1, Lei L. Kerr

1,*, and David C. Look2

1Dept. of Chemical and Paper Engineering, Miami University, Oxford, OH, 450562Semiconductor Research Center, Wright State University, Dayton, OH, 45435

(received date: 29 October, 2010 / accepted date: 25 March, 2011)

We report the formation of p-type nitrogen doped ZnO (ZnO:N) grown by thermal-evaporation deposition.We found that the ZnO nano/micro structured rods can improve nitrogen doping. In addition, co-doping withmetal (excess Zn in this study) can enhance nitrogen doping through a Zn-rich process. The effects of nitrogenprecursors on the electrical and optical properties of ZnO:N were investigated. This study shows that growthprocess plays a critical role in the electrical properties of ZnO:N.

Keywords: nitrogen doped ZnO, p-type, nano/micro rods, co-doping

1. INTRODUCTION

For many applications, ZnO has certain advantages over

GaN, such as a high exciton binding energy (60 meV), low

cost, and the availability of large-area substrates.[1,2]

How-

ever, the development of ZnO for optoelectronic devices is

hindered by the difficulty of synthesizing p-type ZnO to

make a p-n homojunction. Group-V elements are predicted

to be the most promising dopant candidates, especially those

involving nitrogen.[3-13]

However, despite great efforts, there

is no recipe for making good p-type ZnO in a reproducible

manner. This includes the research groups who use expen-

sive molecular beam expitaxy (MBE) to produce high qual-

ity ZnO.[4,9-11]

Not only N but also other dopants, such as Sb

and Li, have not been successful even with high quality

films. The p-type ZnO community has put a great effort into

making defect-free ZnO with the expectation of producing

good p-type doping. However, this has not been demon-

strated to be a viable approach. Thus, it is time to develop

alternative approaches to improve the dopant effectiveness

instead of aiming at making defect free ZnO.

In this work, we have made the following important find-

ings which might serve as tools for future p-type ZnO devel-

opment:

i. ZnO nano/micro structured rods can improve p-type

conductivity

ii. Co-doping with metal (excess Zn in this study) can

enhance p-type conductivity

2. EXPERIMENTS

A low-cost thermal evaporation technique was developed

to grow ZnO:N for its flexibility in making semiconductors

from continuous film to nanostructures. Two separate pro-

cesses of thermal evaporation deposition were used to grow

ZnO:N. All the samples were grown on glass substrates.

Process I – The Zn-rich process contains two steps. In the

first step, Zn powder is evaporated in a low oxygen ambient

at 500°

C for 2 h. The sample from this step is called the “as-

grown ZnO.” In the second step, the as-grown ZnO film was

annealed in dopant gas for an hour to form ZnO:N.

Process II – The O-rich process has an additional step

between the 1st and 2

nd steps of process I. In the additional

step, a saturated-O2 is used to anneal the as-grown ZnO sam-

ple for an hour. The motivation here was to eliminate O

vacancies, which are donors and will compensate for accep-

tors such as NO. Hall-effect measurements were performed

at room temperature in the van der Pauw configuration, with

an Accent 5500 apparatus. Photoluminescence measure-

ments were performed at 4.2 K, and excitation, dispersion,

and detection were accomplished, respectively, with a 45-

mW HeCd laser, a Spex 1269 1.26-m spectrometer, and a

photomultiplier detector. Resolution was better than 0.01

meV in the spectral range important for this study. X-ray dif-

fraction (XRD) was used to characterize the crystal structure

of the ZnO films. Scanning electron microscopy (SEM) and

energy dispersive X-ray (EDX) were used to obtain the sur-

face morphology and elemental composition of the film,

respectively. The acceleration voltage was 5 keV for SEM

and 20 kev for EDX.*Corresponding author: kerrll@muohio.edu.©KIM and Springer

116 W. Mu et al.: Enhanced p-type Conductivity of Nitrogen Doped ZnO by Nano/Micro Structured Rods and...

Electron. Mater. Lett. Vol. 7, No. 2 (2011)

3. RESULTS AND DISCUSSION

The Hall-effect measurements in Table 1 indicate that

annealing ZnO film in 5%NO/N2 is a feasible method of

synthesizing p-type ZnO:N. Each sample was run at least

twice, and at different currents, to validate the electrical

properties. When p-type conductivity was measured in both

runs, it was recorded as p-type. When both runs indicated n-

type, it was recorded as n-type. When one run indicated p-

type and the other indicated n-type, it was recorded as

ambiguous-type. The conductivity of the film depended not

only on the dopant gas but also on the growth process. For

example, under a 350oC anneal, the ZnO:N film annealed in

a 5%NO/N2 mixture exhibited p-type behavior by process I

but n-type behavior by process II. When increasing the

annealing temperature to 400oC, both Process I and II pro-

duced n-type films for 5%NO/N2 dopant gas. On the other

hand, for the films annealed in pure NO, n-type films were

generated at 350 °C and ambiguous-type films at 400oC, for

both processes. This indicates that N2 plays a critical role

during the annealing process. The common belief that pure

NO is better than 5%NO/N2 for producing p-type ZnO is

evidently not correct in every situation.

Figure 1 shows the cross section SEM of the as-grown

ZnO. The ZnO film thickness is about 2 µm. The ZnO rods

are connected at the substrate. Please note that the ZnO mor-

phology in this study is a hybrid structure of nano/micro rods

integrated into a continuous film at the substrate, which

allows the Hall measurement. The EDX was taken along the

cross section and indicated that the as-grown ZnO contained

excess Zn existing at the film surface, perhaps mostly at the

tips of the nanorods shown in Fig. 2(a). The Zn/O atomic

ratio of the as-grown ZnO film was 3:1 at the surface, which

proves the existence of excess Zn at the surface of the as-

grown film. The excess Zn might be deposited onto the ZnO

rods by the Zn vapor during the cooling. These sharp tips

shown in Fig. 2(a) disappeared or became dull after anneal-

ing in O2 or in any dopant gas (Fig. 2(b)) and the EDX

showed that after annealing in pure O2, the Zn/O atomic ratio

was reduced to 3:4 at the film surface.

The following two observations can then be drawn from

the Hall, SEM and EDX analysis:

Observation 1: The ZnO rods or nano/micro structure can

improve and stabilize the nitrogen doping. ZnO nanostruc-

tures exhibit high potential for fabricating novel nanodevices

associated with energy harvesting, conversion and stor-

age.[14-16]

Furthermore, there are indications that it is easier to

produce p-type materials from ZnO nanorods.[16]

Observation 2: Co-doping with metal (excess Zn in this

study) can enhance nitrogen doping. It has been shown that

dual acceptor doping will create shallower acceptors and

improve the p-type conductivity.[17]

Here, we provide an

explanation for the Zn co-doping with N (the Zn-rich pro-

Table 1. Hall-effect measurements of ZnO:N

Annealing Gas P AT typeResis Mob

Carrier concentra-

tion

Ω · cm cm2/V-s per cm

3

As-grown ZnO - - n 1.08 × 102

0.519 1.11 × 1017

Pure O2 II 500 n 4.35 × 103 0.914 1.57 × 10

15

5% NO/N2 I 350 p 1.58 × 101

2.47 1.60 × 1017

5% NO/N2 II 350 n 4.96 × 101 2.72 4.62 × 10

16

5% NO/N2 I 400 n 3.83 × 101 1.57 1.04 × 10

17

5% NO/N2 II 400 n 2.81 × 102 0.797 2.79 × 10

16

Pure NO I 350 n 6.57 × 101 1.07 8.85 × 10

16

Pure NO II 350 n 7.19 × 102 0.827 1.05 × 10

16

Pure NO I 400 Ambiguous 2.42 × 103 - -

Pure NO II 400 Ambiguous 1.15 × 103 - -

P: Process. AT: Annealing temperature (oC).

Fig. 1. SEM cross section of the as-grown ZnO film which showsthat ZnO nano/micro rods have integrated to a continuous film.

W. Mu et al.: Enhanced p-type Conductivity of Nitrogen Doped ZnO by Nano/Micro Structured Rods and... 117

Electron. Mater. Lett. Vol. 7, No. 2 (2011)

cess) to enhance p-type conductivity from the chemical reac-

tion point of view. The possible chemical reactions involv-

ed in the growth and annealing processes are presented in

Table 2. The p-type conductivity of the films annealed in

5%NO/N2 in Process I is attributed to the formation of Zn3N2

due to the presence of N2 in the 5%NO/N2 mixture by reac-

tion 3 (R3). The formation of Zn3N2 helps to capture the N2

in the 5%NO/N2 and converts N2 to form the Zn-N bond in

Zn3N2. Zn3N2 further reacts with NO to form ZnO:N by

reaction R4. In other words, reactions R3 and R4 enhance

the nitrogen incorporation into the ZnO film. This is in

agreement with Kaminska et al.[18]

Reaction R4 is affected in

two different ways with an increasing temperature. At a

higher temperature the sticking coefficient of the NO in the

5%NO/N2 gas is lower, thus inhibiting R4. On the other

hand, all of the chemical reactions, including R4, are temper-

ature dependent and usually are enhanced at higher tempera-

tures. Apparently, 350oC is a critical temperature for the

efficacy of 5%NO/N2 in process I. At temperatures lower

than 350oC, all of the films annealed in 5%NO/N2 are n-type,

evidently because the chemical reactivities of R3 and R4 are

lower. At 350oC, the increased chemical reactivities of R3

and R4 produce p-type material. When further increasing the

temperature to 400oC, n-type material is obtained due to the

poor sticking coefficient of NO, which inhibits R4. When

pure NO is used, the sticking coefficient does not play a sig-

nificant role due to the oversaturation of NO. The film con-

ductivity is thus determined by the temperature dependence

of reaction R5. At 350oC, the films are n-type for both pro-

cesses I and II, but then become “ambiguous” type (indeter-

minate) at 400oC. In process II, films grown with 5%NO/N2

are n-type at any temperature. This is because the excess Zn

from the 1st step reacts with the pure O2 in the 2

nd step, by

reaction R2, before the introduction of the nitrogen dopant

gas. Thus, the N2 in the 5%NO/N2 mixture does not have

any excess Zn with which to react and form Zn3N2 and thus

creates only n-type behavior at both 350°

C and 400°

C. Our

previous EXFAS study[19]

of process II indicated that the N2

molecules will be trapped in the surface when there is no free

Zn to react with and will contribute to the (N-N)O donor

defects.

Table 3 presents some of the photoluminescence proper-

ties. The interesting lines can be roughly grouped into seven

regions: 1.9 eV (red band); 2.5 eV (green band); 3.31 eV

(acceptor band); 3.33 – 3.35 eV (defect lines); 3.360 (Al/Ga

Fig. 2. (a): The SEM plane view of the as-grown ZnO and 2 (b): TheSEM plane view of the ZnO after annealing in O2 (O-rich process)

Table 2. Possible chemical reactions in Processes I and II

Process I Process II

5% NO/N2 Pure NO 5% NO/N2 Pure NO

Step 1 Zn (s) Zn (g) (R1)

Zn + 1/2O2 ZnO (R2)

Step 2 Zn + 1/2O2 ZnO (R2)

(Residual Zn reaction with oxygen)

Step 3

Zn + 1/3N2 1/3Zn3N2 (R3)

Zn3N2+NO ZnO:N(R4)

ZnO + NO ZnO:N (R5)

ZnO + NO ZnO:N (R5) ZnO + NO ZnO:N (R5)

R: reaction

→

→

→

→→

→ → →

118 W. Mu et al.: Enhanced p-type Conductivity of Nitrogen Doped ZnO by Nano/Micro Structured Rods and...

Electron. Mater. Lett. Vol. 7, No. 2 (2011)

lines); 3.362 eV (X lines); and 3.366 eV (surface band). The

3.31-eV band was first seen in p-type material[10]

and has

been observed many times since.[1]

Although there is wide

disagreement as to the detailed nature of the transition,

almost all workers believe that an acceptor is involved. The

3.33 eV to 3.35 eV lines can have one to five or even more

components, depending on the growth process, and since

some of these are produced by electron irradiation, they are

designated “defect lines” in this work[20]

The sharp lines near

3.360 eV are mainly donor-bound excitons due to Al and Ga,

although a defect-related line (at 3.3607 eV) is also known to

exist in this region.[20]

The 3.362 eV line is also a donor-

bound-exciton transition, but its origin is unknown. Finally, a

3.366 eV band, similar to ours, has been shown to be sur-

face-related.[21]

From the results listed in Table 3, the relative

strengths of the acceptor band (3.31 eV) seem to be corre-

lated with the presence of N, except for the sample annealed

only in O2 at 500oC. We can explain this result as follows:

the as-grown ZnO has excess Zn at the film surface as

described earlier. Upon annealing in pure O2, most of the

excess Zn will react with O2 but some of the Zn atoms will

diffuse into the bulk, attaching themselves to various types

of acceptors. One such ZnI-acceptor complex which has

been proposed is ZnI-NO,[20]

and even though the sample in

question has not been annealed in N, still it likely contains N,

a very common background impurity in ZnO.

4. CONCLUSIONS

In conclusion, film conductivity depends not only on the

dopant gas but also on the film morphology and growth pro-

cess. ZnO nano/micro rods can enhance the nitrogen doping.

The formation of Zn2N3 and its further reaction with NO are

thought to be important elements in this process. The use of

Zn2N3 as a starting material might further enhance nitrogen

incorporation into the films.

ACKNOWLEDGMENTS

The authors are grateful for funding under DOE BES

Grant DE-FG02-07ER46389 administrated by Dr. Refik

Kortan, and would like to thank T.A. Cooper and W. Rice for

the Hall-effect and PL measurements, respectively. The

author also would like to thank another graduate student,

Ms. Lei Guo for her help in SEM measurement. The author

also feel grateful for the free access to electron microscopy

facility (directed by Dr. Richard E. Edelmann) at Miami

University.

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Pure NO II 350 N 3 2 1 Strong Strong

Pure NO I 400 Amb-iguous 3 1 2 Weak Strong

Pure NO II 400 Amb-iguous 3 1 2 medium Strong

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