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Contract No. DE-AC36-99-GO10337 

X. Li, S.E. Asher, B.M. Keyes, H.R. Moutinho, 

J. Luther, and T.J. Coutts 

Prepared for the 31st IEEE Photovoltaics Specialists 

Conference and Exhibition 

Lake Buena Vista, Florida 

January 3–7, 2005 

February 2005 • NREL/CP-520-37378 

p-Type ZnO Thin Films Grown 

by MOCVD 
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p-TYPE ZnO THIN FILMS GROWN BY MOCVD 

X. Li, S.E. Asher, B.M. Keyes, H.R. Moutinho, J. Luther, 

and T. J. Coutts 

National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, CO 80401 

ABSTRACT 

ZnO has demonstrated a possibility to

be doped as a p-type by using nitrogen and 

other group-V elements. A high nitrogen 

doping concentration by metalorganic 

chemical vapor deposition (MOCVD) with 

nitric oxide (NO) gas has been achieved. 

However, the processing window for 

obtaining the p-type ZnO:N film is very 

narrow, and the hole concentration is typically

low. Possible compensation and passivation 

effects have been studied. Hydrogen and 

carbon elements are detected by secondary-ion 

mass spectroscopy (SIMS). Considering the 

other experimental and modeling results, we 

believe that the impurities inadvertently

incorporated with the zinc precursor could be 

compensating or passivating the nitrogen 

acceptor and result in the low hole 

concentration. 

INTRODUCTION 

ZnO, a wide-bandgap semiconductor 

material, is widely used in various 

applications. It can be easily doped to n-type, 

but is difficult to dope to p-type [1]. Low 

impurity solubility, excessive accepter 

ionization energy, and possible compensating 

mechanisms are the three main factors that 

could make p-type doping of ZnO difficult. In 

addition to being of interest for ultraviolet 

light emitters, p-type ZnO has a potential 

advantage in the fabrication of novel solar cell 

structures: inverted CdTe devices, improved 

contacts to both p-type absorbers, and organic 

semiconductors. The recent reports on p-type 

ZnO brings this potential close to reality [2-

9]. 

In this report, we discuss the 

achievement and the issues that have been 

observed on the nitrogen-doped p-type ZnO 

films formed by MOCVD. Special focus is 

put on the low hole concentration and 

possible compensation mechanisms. 

EXPERIMENTAL 

P-type ZnO films have been 

fabricated by MOCVD, using diethylzinc 

(DEZn) and nitric oxide (NO) precursors. 

NO gas serves as both the oxidizer and

nitrogen dopant in this reaction. The 

substrate is Corning 1737 glass. More 

information on ZnO:N sample fabrication can 

be found in previous papers [10]. 

The composition of the ZnO films was 

analyzed by a CAMECA IMS 5f SIMS. Film 

topography was taken by atomic force

microscopy (AFM, Auto probe LS from

Park Scientific Instruments with Si 

Cantilevers). Electrical and optical

properties were measured using Hall analysis 

(BioRad Model HL5500), capacitancevoltage (double-Schottky surface contact 

with mercury probe), and a Cary 5G 

spectrophotometer. 

RESULTS AND DISCUSSION 

The MOCVD-formed ZnO:N films

on glass substrate are randomly oriented 

polycrystalline with a light yellow color. The 

1 
transparent spectra of the ZnO:N compare 

with the ZnO film shown in Fig. 1. The

increased absorption around the band edge of 

the ZnO:N made the film look yellow. The

topography of the ZnO:N on glass substrate is 

illustrated in Fig. 2. 

100

80

60

40

20

0

 Transmittance (%)

400 800 1200 1600 2000

 Wavelength (nm)

 ZnO 

 ZnO:N

Figure 1. Optical transmittance spectra of

ZnO and ZnO:N films. 

Figure. 2. The topography of a 0.74-μmthick ZnO:N film grown on a glass substrate. 

An O-poor growth ambient is required 

for nitrogen doping. Therefore, all the ZnO:N

films studied here were formed with NO gas

as the oxidizer. When NO gas is the only 

oxidizer, nitrogen concentration levels in the 

range of 1.75x1021 ~2.60x1021

 cm-3 have 

been achieved. The high nitrogen 

concentration level can be achieved in a wide

processing range, but the growth window for

fabricating the p-type ZnO:N is narrow. The 

p-type films are observed only in a 

temperature range of 400°-440°C [10]. 

Although some of the ZnO:N films deposited 

in this temperature range are remain 

insulating. 

Results of Hall probe analysis of 

several ZnO samples are shown in Table 1. 

Undoped ZnO is n-type with a carrier 

concentration in the range of 1016~1018 cm-3. 

The nitrogen-doped ZnO samples are no long 

n-type but are insulator or weak p-type. The 

hole concentration is around 1013 to 1018 cm-3

and the resistivity is in the range of 2 to few 

hundred Ω−cm. The capacitance-voltage (CV) technique was used to confirm the Hall 

results. Without post-deposition heat 

treatment, the C-V analysis indicated that the

as-deposited ZnO:N sample is weak p-type. 

Table 1. Electrical properties of ZnO films 

deposited at a temperature of 400°C. 

Doping C. C. 

(cm-3) 

μ 

(cm2

/V) 

ρ 

(Ω-cm)

ZnO -8.42 x 1018 3.2 0.235

ZnO -8.38 x 1016 6.3 11.8

ZnO:N 9.24 x 1013 236 286

ZnO:N 8.36 x 1017 4.55 1.64

The measured carrier concentration is

several orders of magnitude lower than the 

nitrogen concentration. With the achieved

nitrogen doping concentrations, it is clear that

the limitation is not due to the dopant 

solubility. Several other reasons could be: (1) 

a small fraction of the nitrogen acts as an

acceptor. There is a possibility that the 

nitrogen forming the [(N2)O] (two nitrogen 

atoms substitute on an oxygen site) double 

shallow donor instead of (NO) single deep 

acceptor [11]. (2) The nitrogen acceptors’ 

energy levels lie well above the valence band 

2 
edge. And (3), a strong compensation or 

passivation mechanism exists. The second 

possibility has been discussed previously [10]. 

The calculations indicated that the ionization

energy needs to be as much as ~350 meV to 

explain the observed difference between the 

nitrogen and the carrier concentration.

Considering the recent experimental result that 

the ionization energy NO is about 160 meV, a 

much larger carrier concentration should result 

[12]. With the knowledge of the possible 

impurities introduced by MOCVD procedures,

in this study, we shall focus our attention on 

the third possibility: compensation or 

passivation effects by possible impurities

introduced during MOCVD deposition. 

The depth profiles of hydrogen and 

carbon in ZnO:N films were obtained using 

SIMS. In both ZnO and ZnO:N films, both 

hydrogen and carbon have been found. Figure 

3 indicates hydrogen and carbon 

concentrations are strongly affected by 

deposition temperature. This observation 

indicates that the hydrogen and carbon 

elements are possibly introduced as the 

products of the zinc precursor ((C2H5)2Zn)

decomposition process. 

It has been found that the other zinc 

metal-organic precursor ((CH3)2Zn) 

introduced some carbon into the ZnO film

[13]. There are not many studies on the 

function of the carbon in ZnO film. In 

nitrogen-doped ZnO, there is evidence that 

carbon has possibly bonded with nitrogen 

[14]. The recent theoretical study conducted

at NREL indicated that the carbon-related

defect in ZnO:N film most possible are donor 

type. Thus, the carbon is an undesirable 

impurity for p-type ZnO film [15]. 

The role of hydrogen in ZnO has been 

well studied [16-18]. It has been indicated 

that hydrogen acts only as a donor in ZnO. 

There are not many studies on the function of 

hydrogen in a nitrogen-doped ZnO film. 

Considering the difficulty associated with

nitrogen doping by MOCVD and the high 

level of hydrogen concentration in the ZnO 

film, we believe that hydrogen could be one 

of the important factors in determining the

efficiency of nitrogen doping. 

Figure 3: The hydrogen and carbon 

impurity levels varied with deposition 

temperature in MOCVD-formed ZnO films. 

Our study by Fourier transform 

infrared spectroscopy (FTIR) and firstprinciples calculations indicated that there is

a strong tendency for NO

-

 and H+ defects to

form a neutral combined defect complex 

[19]. In undoped ZnO samples, hydrogen 

bonded with oxygen. With nitrogen doping, 

the intensity of the absorption peak due to OH decreased, and the absorption line at 3007 

cm-1 due to the NO-H (anti-bonding) 

emerged. This observation implies that one 

of the possible reasons for low hole 

concentration in heavily doped ZnO:N is due 

to the hydrogen passivation effect. 

In summary, we have fabricated ptype, nitrogen-doped ZnO by MOCVD. The

nitrogen concentration achieved using this 

method is as high as 2.60x1021

 cm-3. 

However, low hole concentration is observed. 

Hydrogen and carbon elements have been 

detected by SIMS analysis. This, together 

3 

0.0E+00

1.0E+05

2.0E+05

3.0E+05

4.0E+05

5.0E+05

100 200 300 400 500 600

Deposition Temperature (°C)

Secondary Ion Counts (cts/s)

C
7. T. Aoki, Y. Shimizu, A. Miyake, A. 

Nakamura, Y. Nakanishi, and Y. Hatanaka, 

Phys. Stat. Sol. (b) 229, 911, (2002) 

with FTIR, XPS, and modeling results, 

provides strong evidence that compensation or 

passivation of nitrogen acceptors by 

inadvertently incorporated impurities exist in

ZnO:N films, thereby resulting in the low hole

concentration. 

8. A.B.M. Almamum Ashrafi, Ikuo Suemune, 

Hidekazu Kumano, and Satoru Tanaka, Japan 

Journal of Applied Physics, 41, L1281. 

(2002) 

9. B.S. Li, Y.C. Liu, Z.Z. Zhi, D.Z. Shen, Y.M. 

Lu, J.Y. Zhang, X.W. Fan, R.X. Mu, and Don 

O. Henderson, J. Mater. Res. 18, 9 (2003). 

ACKNOWLEDGEMENTS 

The authors would like to 

acknowledge James Keane for sample

preparation and Matthew Young for SIMS

characterization. This work was supported by 

the U.S. Department of Energy under Contract 

No. DE-AC36-99GO10337. 

10. X. Li, Y . Yan, T.A. Gessert, C.L. Perkins, D. 

Young, C. Dehart, and T.J. Coutts, J. Vac. 

Sci. Technol. A21, 1342, (2003).

11. Yanfa Yan and S.B. Zhang, Physical Review 

Letters, 86, 5723, (2001). 

12. B.K. Meyer, H. Alves, D.M. Hofmann, W. 

Kriegseis, D. Forster, F. Bertram, J. Chresten, 

A. Hoffmann, M. Strabburg, M. Dworzak, U. 

Haboeck, and A.V. Rodina, Phys. Stat. Sol. 

(b) 241, 231, (2004) 

REFERENCES: 

1. S.B. Zhang, S.-H. Wei, and A. Zunger, J. 

Appl. Phys., 83, 3192 (1998). 

13. Teresa M. Barnes, Steve Hand, Jackie Leaf, 

and Colin A. Wolden, J. Vac. Sci. Technol. 

A22, 2118. 2. Kazunori Minegishi, Yasushi Koiwai, 

Kikuchi, Koji Yano, Masanobu Kasuga, Japan 

Journal of Applied Physics, 36, L1453, (1997) 

14. C.L. Perkins, S.-H. Lee, X. Li, S.E. Asher, 

and T.J. Coutts, Journal of Applied Physics, 

(submitted). 3. Mathew Joseph, Hitoshi Tabata, and Tomoji 

Kawai, J. JAP 38 L1205, (1999) 15. S. Limpijumnong, X. Li, S-H. Wei, S.B. 

Zhang, (submitted). 4. Y.R. Ryu, S. Zhu, D.C. Look, J.M. Wrobel, 

H.M. Jeong, H.W. White, Journal of Crystal 

Growth 216 330 (2000) 

16. Chris G. Van De Walle, Phys. Stat. Sol (b) 

229, 221 (2002). 5. X. Li, Y . Yan, T.A. Gessert, C. Dehart, C.L. 

Perkins, D. Young, and T.J. Coutts, 

Electrochemical and Solid Letters, 6(4) C56-

C58 (2003). 

17. E.V. Lavrov, J. Weber, and F. Borrnert, 

Physical Review B 66, 165205, (2002). 

18. C. H. Seager and S.M. Myers, Journal of 

Applied Physics, 94, 2888, (2003). 6. Xin-Li Guo, Hitoshi Tabata, Tomoji Kawai, 

Journal of Crystal Growth, 223 , 135 (2001). 

19. X. Li, S. Limpijumnong B.M. Keyes, S.E. 

Asher, S.B. Zhang, S-H. Wei,C.G. Van de 

Walle, and T.J. Coutts (submitted). 

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p-Type ZnO Thin Films Grown by MOCVD 

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14. ABSTRACT (Maximum 200 Words)

ZnO has demonstrated a possibility to be doped as a p-type by using nitrogen and other group-V elements. A high 

nitrogen doping concentration by metalorganic chemical vapor deposition (MOCVD) with nitric oxide (NO) gas has 

been achieved. However, the processing window for obtaining the p-type ZnO:N film is very narrow, and the hole 

concentration is typically low. Possible compensation and passivation effects have been studied. Hydrogen and 

carbon elements are detected by secondary-ion mass spectroscopy (SIMS). Considering the other experimental and 

modeling results, we believe that the impurities inadvertently incorporated with the zinc precursor could be 

compensating or passivating the nitrogen acceptor and result in the low hole concentration. 

15. SUBJECT TERMS

PV; thin films; p-type; nitrogen doping; metalorganic chemical vapor deposition (MOCVD); nitric oxide (NO); 

secondary-ion mass spectroscopy (SIMS); 

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