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Journal of Crystal Growth 311 (2009) 2167–2171

Contents lists available at ScienceDirect

Journal of Crystal Growth

0022-02

doi:10.1

�Corr

E-m

journal homepage: www.elsevier.com/locate/jcrysgro

Investigation on the ZnO:N films grown on (0 0 0 1) and (0 0 0 1) ZnOtemplates by plasma-assisted molecular beam epitaxy

S.H. Park a,�, J.H. Chang b, T. Minegishi a, J.S. Park a, I.H. Im a, M. Ito c, T. Taishi d, S.K. Hong e,D.C. Oh f, M.W. Cho a, T. Yao a

a Center for Interdisciplinary Research, Tohoku University, Sendai, Miyagi 980-8577, Japanb Major of Nano Semiconductor, Korea Maritime University, Pusan 606-791, Republic of Koreac Institute of Multidisciplinary Research for Advanced Material, Tohoku University, Sendai 980-8577, Japand Institute for Materials Research, Tohoku University, Sendai 980-8577, Japane Department of Nano Information Systems Engineering, Chungnam National University, Daejeon 305-764, Republic of Koreaf Department of Defense Science & Technology, Hoseo University, Asan 336-795, Republic of Korea

a r t i c l e i n f o

Available online 13 November 2008

PACS:

82.70.Uv

Keywords:

A1. Surfaces

A3. Molecular beam epitaxy

B1. Oxides

48/$ - see front matter & 2008 Published by

016/j.jcrysgro.2008.11.018

esponding author. Fax: +81 22 795 7810.

ail address: shpark@imr.tohoku.ac.jp (S.H. Par

a b s t r a c t

Nitrogen-doped ZnO films (ZnO:N) were grown on (0 0 0 1) and (0 0 0 1) ZnO templates by plasma-

assisted molecular beam epitaxy (PA-MBE). High nitrogen concentration (45�1020/cm3) was achieved

from ZnO:N/(0 0 0 1)ZnO films, while relatively low nitrogen concentration (�1019/cm3) was observed

from ZnO:N/(0 0 0 1)ZnO layers at the same growth temperature (Tg). High-resolution X-ray diffraction

(HRXRD) revealed considerable lattice deformation. At Tg=300 1C, the c-lattice expansion in ZnO:

N/(0 0 0 1)ZnO film is almost 36% larger than that in ZnO:N/(0 0 0 1)ZnO due to the high nitrogen

concentration in the film. Electron concentration of the ZnO:N/(0 0 0 1)ZnO films gradually decreases

down to �1016/cm3 as the nitrogen concentration increases. Moreover, donor–acceptor-pair emission

dominates the photoluminescence (PL) spectrum (3.23 eV) of ZnO:N/(0 0 0 1)ZnO films grown at 300 1C

and that emission is 3.1 times stronger than that of ZnO:N/(0 0 0 1)ZnO.

& 2008 Published by Elsevier B.V.

1. Introduction

ZnO is recognized as one of the most promising II–VI oxidesemiconductor for various applications in optoelectronic devicessuch as ultraviolet (UV) light-emitting devices, laser diodes andUV detectors, because it has a wide band-gap of 3.37 eV and alarge exciton binding energy of 60 meV [1].

High-quality ZnO epitaxy has been successfully established byusing various lattice-matched substrates such as ScMgAlO4 [2],GaN templates [3], bulk ZnO [4] and/or two-step growthtechniques such as MgO/c-Al2O3 [5] and Cr2O3/c-Al2O3 [6].Moreover, the band-gap engineering using ternary alloy systemscomposed with Mg [7], Cd [8] atoms has been already achieved.However, although the growth of p-type ZnO is a crucial issue fordevice application based on the p–n junction, it is still verydifficult to produce a reliable p-type ZnO film mainly due totwo reasons.

One is the solubility limit of dopants, which depends ongrowth temperature (Tg). Note that growth temperature stronglyaffects both kinetics of impurity incorporation and crystallinity

Elsevier B.V.

k).

of host crystal. High growth temperature is preferred to growhigh crystalline quality ZnO film [9]. However, a low growthtemperature is preferable to incorporate the acceptor atoms [10].Therefore, suitable growth temperature should be determinedfrom the viewpoint of efficient doping. The other reason is self-compensation by intrinsic defects such as oxygen vacancy (Vo)and zinc interstitial (Zni). Generally, donor-like defects such as Vo

and Zni can be easily formed due to the low formation energy of0.02 and 1.73 eV, respectively. On the other hand, acceptor-likedefects in ZnO such as zinc vacancy (VZn) and oxygen interstitial(Oi) are known to have relatively high formation energy of 5.46and 5.88 eV, respectively [11,12]. Therefore, p-type ZnO has notbeen commercially available so far, even though there werenumerous reports on the achievement of p-type ZnO [13–15]. Thehigh-impurity concentration doping technique in high-qualityfilm is inherently required to obtain reliable p-type conduction ofZnO film.

ZnO material, which is composed of inherent wurtzite crystalstructure, has two distinct faces along the c-axis direction;(0 0 0 1) direction (Zn face) and (0 0 0 1) direction (O face). Thepolarity has strong influence on various physical properties, andespecially acceptor incorporation efficiency strongly depends onthe polarity. Generally, nitrogen doping on Zn face is believed tobe more suitable to obtain p-type conductivity since it provides

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1019

1020

1021

ZnO:N/(0001)ZnO

ZnO:N/(000-1)ZnO

N c

once

ntra

tion

(/cm

3 )

S.H. Park et al. / Journal of Crystal Growth 311 (2009) 2167–21712168

higher incorporation efficiency than O face [16]. However,systematic investigation about the N incorporation dependingon the polarity of ZnO film is still lacking.

Obviously, the difference of nitrogen incorporation efficiencybetween two faces affects various characteristics such as latticestrain, carrier concentration and emission property. Hence,investigation of those parameters is indispensable to understandthe polarity dependence of nitrogen incorporation.

In this paper, nitrogen-doped ZnO (ZnO:N) films were grownon (0 0 0 1) and (0 0 0 1) face ZnO templates (deposited on c-Al2O3

substrates) by plasma-assisted molecular beam epitaxy (PA-MBE).Structural, electrical and optical properties are systematicallydiscussed in terms of different incorporation efficiency oftwo faces.

300 400 500 600 700 800

100

105

O

N Mg

Zn

0 100 200 300 400 500 100

105

NMg

O

Zn

Depth (nm)

SIM

S c

ount

s (a

rb.u

nits

)

Growth temperature (οC)

Fig. 1. (a) Nitrogen concentration of ZnO:N/(0 0 0 1)ZnO and ZnO:N/(0 0 0 1)ZnO

layers grown at various growth temperature. SIMS depth profiles of (b) ZnO:N/

(0 0 0 1)ZnO and (c) ZnO:N/(0 0 0 1)ZnO layers grown at Tg=300 1C, respectively.

2. Experimental procedure

ZnO:N was grown on both (0 0 0 1) and (0 0 0 1)ZnO templatesby PA-MBE, in which oxygen and nitrogen were supplied by usinga RF plasma cell, operated at a power of 350 W. Zinc was suppliedby means of a solid-source dual-filament effusion cell. The Zn fluxwas 1.0 A/s, which is accurately monitored by crystal thicknesscontroller, and O2 and N2 flow rate were 2 and 0.5 sccm,respectively. The background pressure is estimated to be �10�9

Torr before the film growth. It drops down to 4�10�5 Torr duringgrowth by production of oxygen (2 sccm) and nitrogen (0.5 sccm)plasma source. Prior to the growth, c-Al2O3 substrates weredegreased by conventional organic solvents, followed by etchingin a H2SO4:H3PO4=3:1 solution at 160 1C for 10 min.

PA-MBE growth on each face ((0 0 0 1) and (0 0 0 1)) wasrealized by changing the thickness of MgO layer [17]. The (0 0 0 1)low-temperature (LT) ZnO buffer was grown on 10-nm-thick MgOlayer, while (0 0 0 1) ZnO film was grown on 1-nm-thick MgOlayer. After that the LT-ZnO buffer grown on MgO buffer wasannealed at 850 1C for 15 min to obtain a smooth surface. The(0 0 0 1) and (0 0 0 1) face undoped ZnO (u-ZnO) with thethickness of �120 nm are grown continuously. The growth rateof (0 0 0 1) ZnO is 1.7 times faster than that of the (0 0 0 1) ZnO filmdue to the three dangling bonds configuration of Zn face [18].Details of the growth were reported elsewhere [19]. Finally,nitrogen doping was performed for both faces at elevated growthtemperature from 300 to 800 1C. The thickness of ZnO:N films wascontrolled to be 300 nm.

Both c and a axes lattice constants were measured by high-resolution X-ray diffraction (HRXRD). The (0 0 0 2) and (10 11)reflections were investigated to evaluate the c and a axes latticeconstants, respectively. Nitrogen concentration was estimated bysecondary ion mass spectroscopy (SIMS). Carrier concentrationwas estimated by a Hall system in a Van der Pauw configuration,which measured about 5 times per sample in order to obtainaccurate results.

Photoluminescence (PL) spectra of ZnO:N and u-ZnO filmswere measured at 10 K using the 325 nm line from a He–Cdlaser as an excitation source and a monochromator with a focallength of 32 cm.

3. Results and discussion

3.1. Nitrogen incorporation efficiency

We have prepared N-doped (0 0 0 1) and (0 0 0 1) ZnO films atelevated temperature (300–800 1C) in order to investigate in-corporation efficiency. Fig. 1(a) shows N concentration of ZnO:Nfilms grown on the (0 0 0 1) ZnO and (0 0 0 1) ZnO template films

grown at different growth temperature, which are denoted byZnO:N/(0 0 0 1)ZnO and ZnO:N/(0 0 0 1)ZnO, respectively. Amongthose results, Figs. 1(b) and (c) show SIMS depth profiles of ZnO:Nfilms grown on ZnO:N/(0 0 0 1)ZnO and ZnO:N/(0 0 0 1)ZnO filmat 300 1C, respectively. Those two samples were measuredsimultaneously to reduce the measurement errors. Nitrogen con-centration of ZnO:N/(0 0 0 1)ZnO is estimated to be �1021/cm3,and it is remarkably higher than that of ZnO:N/(0 0 0 1)ZnO(2.2�1019/cm3).

However, note that considerable difference was only observedfrom the samples grown at LT range (o500 1C). Such distinctioncan be qualitatively explained in terms of both low-stickingcoefficient of N-atom and dangling bond configuration of eachsurface [16]. For the case of a (0 0 0 1) ZnO growth, the surfacetends to be terminated by Zn-atoms, since it projects only onedangling bond out of the surface. The cation (Zn)-terminatedsurface is apt to adsorb anion atoms (N). Meanwhile, for the caseof a (0 0 0 1) ZnO growth, O-terminated surface is stable. Hence itis not suitable to adsorb N-atoms. Based on this discussion, theobserved high nitrogen incorporation efficiency of a (0 0 0 1)ZnO:N growth front is understood. Another point to be noted isthat Mg seems to diffuse around the interface region. Since theincorporation of Mg makes the resistivity of ZnO higher, theinterface diffusion will not affect the Hall measurements of ZnOlayer except for a slight reduction of the effective thickness of theZnO layer. Although the growth of ZnO on MgZnO buffer layershowed differences in terms of crystallinity [20], it is not yet

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S.H. Park et al. / Journal of Crystal Growth 311 (2009) 2167–2171 2169

certain whether the observed Mg diffusion will affect the growthof the ZnO layer.

3.2. Lattice deformation

The c and a axes lattice strains induced by incorporatednitrogen atoms are evaluated by using X-ray 2y�o scan of(0 0 0 2) and (10 11) reflections. Fig. 2 shows the deviation of (a)a-axis and (b) c-axis lattice constants as a function of growthtemperature. Since we used the lattice constant of undoped ZnOfilm as a reference, observed lattice constant deviation can beascribed to the incorporation of nitrogen atoms.

Here, the lattice expansion in ZnO:N films can be explained interms of ionic bond length. Since the bond length of the Zn–Nbond (2.04 A) [21] is longer than that of the Zn–O bond (1.93 A)[12], compressive lattice strain is expected when nitrogen atomssubstitute into oxygen sites [19].

Furthermore, lattice deformation should be described in detailin terms of two viewpoints: growth temperature and polarity.Lattice deformation of both ZnO:N/(0 0 0 1)ZnO and ZnO:N/(0 0 0 1)ZnO at growth temperature above 600 1C is almostnegligible. However, below 500 1C, a considerable expansion ofc-axis lattice and the contraction of a-axis lattice are observed.Especially, a large lattice deformation (Dc=0.0086 A, Da=�0.0037A) was observed at 300 1C. It clearly shows the effect of growth

-0.005

0.000

0.005

300 400 500 600 700 800-0.005

0.000

0.005

0.010

Δa (a

ZnO

:N-a

u-Zn

O) (

Å)

ZnO:N/(000-1)ZnO

ZnO:N/(0001)ZnO

Δc (c

ZnO

:N-c

u-Zn

O) (

Å)

Growth temperature (οC)

Fig. 2. The variation of (a) a-axis and (b) c-axis lattice constants of ZnO:N/

(0 0 0 1)ZnO and ZnO:N/(0 0 0 1)ZnO layers.

temperature; owing to the increase of sticking coefficient ofnitrogen at low growth temperature region, more nitrogen atomsare incorporated and make a large lattice deformation. Alsonote that ZnO:N/(0 0 0 1)ZnO shows larger lattice deformation,which indicates that the (0 0 0 1)ZnO surface has higherN-incorporation efficiency.

The c-axis lattice constant of ZnO:N/(0 0 01)ZnO film (Tg=300 1C)is almost 36% larger than that of ZnO:N/(0 0 01)ZnO. This factstrongly supports the higher nitrogen incorporation efficiency of(0 0 01)ZnO surface.

3.3. Carrier concentration of ZnO:N films

Fig. 3 shows Hall carrier concentrations of ZnO:N films andundoped ZnO films grown at various temperatures. Although highnitrogen concentration (45�1020/cm3) was observed fromZnO:N/(0 0 0 1)ZnO films, all samples showed n-type conductivity.

Hall concentrations of undoped ZnO films gradually decreaseddown to �1017/cm3 as the growth temperature increases. Sincehigh growth temperature facilitates growth of the high-qualityZnO film by enhancing the migration length of adatoms as well assuppressing point defect [22]. Hence, Fig. 3 indicates that nativedefects such as Vo or Zni are acting as a donor-like defect.

On the other hand, an obvious discrepancy was observed fromZnO:N films. Hall carrier concentrations of ZnO:N/(0 0 0 1)ZnOvaried almost similarly to those of undoped ZnO films. Mean-while, Hall carrier concentration of ZnO:N/(0 0 0 1)ZnO graduallydecreased down to �1016/cm3 as the growth temperaturedecreased (nitrogen concentration increased) as shown in Fig. 3.

Presumably, Hall carrier concentration in ZnO:N/(0 0 0 1)ZnOwill be determined by the competition of two effects; thegeneration of point defects (increase of donor concentration)and nitrogen incorporation (increase of acceptor concentration).Therefore, the Hall carrier concentration of ZnO:N/(0 0 0 1)ZnOfilms in Fig. 3 indicates that when we decrease the growthtemperature, acceptor concentration increases faster than donorconcentration.

All the mentioned data (nitrogen concentration, electricalproperties and lattice discrepancy) are summarized in Table 1.

300 400 500 600 7001015

1016

1017

1018

1019

ZnO:N/(0001) ZnO

ZnO:N/(000-1) ZnO

(0001) ZnO

(000-1) ZnO

Car

rier c

once

ntra

tion

(cm

-3)

Growth temperature (οC)

Fig. 3. Hall carrier concentration of undoped ZnO and ZnO:N layers grown at

various temperatures.

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Table 1Summary of various information on ZnO:N films.

Samples Tg (1C) [N]SIMS (/cm3) nelectron (/cm3) r (O cm) m (cm2/V s) Dc (A) Da (A)

ZnO:N/(0 0 0 1)ZnO 700 4.6E20 4.3E17 0.24 63 0 0

500 6.9E20 1.5E17 0.73 52 0.0031 �0.0009

300 1.3E21 9.2E15 60 9.4 0.0086 �0.0037

ZnO:N/(0 0 0 1)ZnO 500 8.6E18 3.0E17 0.23 80 0.0003 �0.0006

300 2.2E19 6.1E17 0.24 39 0.005 �0.0008

3.15 3.20 3.25 3.30 3.35 3.40

x2

x5

x15

x20

TES

(Tg :300 οC)

(Tg :500 οC)

(Tg : 700 οC)

(Tg :300 οC)

(Tg :500 οC)

(Tg : 700 οC)

10K@PL

He-Cd laser 325 nm

DoX

DAP

PL

inte

nsity

(arb

.uni

ts)

Photon energy (eV)

Fig. 4. 10 K PL spectra of undoped ZnO and ZnO:N layers. (a) ZnO:N/(0 0 0 1)ZnO,

Tg=300 1C, (b) ZnO:N/(0 0 0 1)ZnO, Tg=300 1C, (c) ZnO:N/(0 0 0 1)ZnO, Tg=500 1C, (d)

ZnO:N/(0 0 0 1)ZnO, Tg=500 1C, (e) undoped (0 0 0 1)ZnO, Tg=700 1C and (f) undoped

(0 0 0 1)ZnO, Tg=700 1C.

S.H. Park et al. / Journal of Crystal Growth 311 (2009) 2167–21712170

3.4. Optical properties

Fig. 4 shows the LT (10 K) PL near-band-edge emission (NBE)spectra of ZnO:N films (a–d), which are compared with those ofundoped ZnO (e and f). Both undoped (0 0 0 1) and (0 0 0 1) ZnOfilms show dominant neutral donor-bound exciton emission (D1X,3.361 eV) line [23]. The D1X emission intensities of ZnO:N filmsdecreased rapidly as a consequence of nitrogen incorporation.

Relatively strong emission with narrow linewidth characterizes(0 0 0 1) ZnO film. Also, D1X emission line dominates the PL

spectrum of ZnO:N/(0 0 0 1)ZnO. Note that two peaks at 3.332 and3.367 eV are shown in Fig. 4(d), tentatively assigned to two-electron satellite recombination line of the donor-bound exciton(TES, 3.332 eV) and ionized donor-bound exciton (D+X, 3.367 eV),respectively, which is supported by the previous reports [23,24].On the other hand, relatively weak intensity with broad linewidthand fast appearance of donor–acceptor-pair (DAP) line character-ize undoped (0 0 0 1) ZnO and ZnO:N/(0 0 0 1)ZnO films, respec-tively [25]. Nitrogen-related emission such as DAP (3.23 eV) linewas observed from both samples (Tg=300 1C, Fig. 4(a) and (b)),however, the DAP line intensity of ZnO:N/(0 0 0 1)ZnO is 3.1 timestronger than that of ZnO:N/(0 0 0 1)ZnO.

PL spectra clearly show the fingerprint of high concentration ofnitrogen atoms in ZnO:N/(0 0 0 1)ZnO films, which correspondswell with the results such as nitrogen concentration, latticedeformation, and Hall carrier concentration described above. Alsoit strongly supports high nitrogen incorporation efficiency of(0 0 0 1) ZnO surface.

6. Summary

The effect of polarity on nitrogen incorporation wassystematically investigated. High nitrogen concentration(45�1020/cm3) is achieved in ZnO:N/(0 0 0 1) ZnO layers, whichgenerated additional compressive strain in the ZnO:N film due tosubstitutional incorporation of nitrogen atoms. Nitrogen incor-poration decreases Hall carrier concentration in ZnO:N/(0 0 0 1)ZnO film down to �1016/cm3, meaning that acceptorconcentration increases faster than the donor concentration(pointdefect density). The differences in the emergence of the DAPemission constitute a contrast in the luminescence properties ofthe films grown on different faces. In conclusion, ZnO film withhigh nitrogen concentration is achieved on (0 0 0 1) ZnO surface atlow growth temperature (Tgo500 1C) range. Structural, electricaland optical variations induced by nitrogen atoms are discussed interms of the polarity of template.

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