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0022-2313/$ - se

doi:10.1016/j.jlu

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(F. Jiang).

Journal of Luminescence 124 (2007) 162–166

www.elsevier.com/locate/jlumin

Photoluminescence observations of hydrogen incorporationand outdiffusion in ZnO thin films

Fan Li, Li Wang, Jiangnan Dai, Yong Pu, Wenqing Fang, Fengyi Jiang�

Nanchang University, Education Ministry Engineering Research Center for Luminescence Materials and Devices, Nanchang 330047, PR China

Received 13 October 2005; received in revised form 10 January 2006; accepted 22 February 2006

Available online 17 April 2006

Abstract

ZnO thin films were deposited with the addition of H2 to the reaction gas using the atmospheric-pressure metal organic chemical vapor

deposition method. The incorporation and outdiffusion of hydrogen in ZnO films were investigated by comparing the intensity of the

hydrogen-related bound-exciton peak (I4: 3.363 eV) in the photoluminescence spectrum. The intensity of I4 peak was found to be the

strongest in the ZnO film deposited at 680 1C with H2 present. However, for the ZnO films prepared at the same temperature 680 1C but

without H2 present and at the higher temperature of 900 1C with H2 present, respectively, the I4 peak was just a minor shoulder of

another bound-exciton peak (I8: 3.359 eV). The intensity of I4 peak in the ZnO films deposited with H2 present was found to decrease

with the increasing of annealing temperature. These results suggest that it is difficult for hydrogen to incorporate into ZnO thin films

grown at high temperatures even in the hydrogen-present ambient.

r 2006 Elsevier B.V. All rights reserved.

Keywords: ZnO; Hydrogen; Atmospheric-pressure metalorganic chemical vapor deposition (AP-MOCVD); Photoluminescence

1. Introduction

ZnO, a wide-bandgap (3.37 eV at room temperature)semiconductor material, is attracting considerable atten-tion for use in optical devices such as blue/UV light-emitting diodes (LEDs) and next generation laser diodes(LDs) [1,2] because ZnO can provide efficient excitonicemissions at room temperature [3].

Recently, there is a particular interest in the propertiesof hydrogen in ZnO because the density functional theoryand total energy calculations predict that hydrogen inZnO occurs exclusively in the positive-charge state, andinterstitial atomic hydrogen always acts as a shallowdonor [4–7].

Hydrogen is a ubiquitous impurity in most semiconduc-tors and it is present in many of the techniques commonlyused for growth of ZnO, such as vapor-phase transport,hydrothermal growth, or metalorganic chemical vapor

e front matter r 2006 Elsevier B.V. All rights reserved.

min.2006.02.013

ing author.

esses: isfirst@163.com (F. Li), jiangfy@ncu.edu.cn

deposition (MOCVD). Hydrogen-related defects haveattracted significant attention because of their potentialfor tailoring ZnO properties. At present, there have beenmany investigations on the effects of hydrogen on ZnO inseveral ways, including the modification of the electricaland optical properties of ZnO [8–13]. Meanwhile, manymethods have been applied to obtain ZnO materials dopedwith hydrogen, such as direct or remote H2 plasmairradiation [12–15], H2 ion implantation [19,20], mercury-sensitized hydrogen addition [18], etc. A number of authorshave offered strong evidence linking shallow donors withthe presence of hydrogen in ZnO [14–17]. Among theseevidences, the (3.363170.0002) eV peak (called I4 in theliterature) in the low-temperature photoluminescence (PL)spectrum of ZnO is ascribed to the hydrogen-relatedneutral donor-bound exciton. Therefore, the I4 peak canbe used as a sign of the presence of hydrogen in ZnO.There have been some reports about the incorporation

and outdiffusion of hydrogen in single-crystal ZnO dopedwith hydrogen by ion-implantation or hydrogen plasma-exposure [20–22]. These results have shown that hydrogenexhibits a very rapid diffusion in ZnO and subsequent

ARTICLE IN PRESSF. Li et al. / Journal of Luminescence 124 (2007) 162–166 163

annealing at 600–700 1C is sufficient to evolve all of thehydrogen out of the ZnO. However, there are fewinvestigations on the incorporation and outdiffusion ofhydrogen in ZnO thin films, which is originated from thereaction ambient during deposition.

In this work, the atmospheric-pressure metalorganicchemical vapor deposition (AP-MOCVD) method wasused to prepare the hydrogen-doped ZnO thin films(ZnO:H) on c-Al2O3 substrate. A small quantity of H2

gas was added to the reaction gas during the ZnO:H filmdeposition. Meanwhile, the incorporation and outdiffusionof hydrogen in ZnO thin films grown at differentconditions were investigated by comparing the intensityof the hydrogen-related bound-exciton peak (I4) in the PLspectrum.

2. Experimental procedure

Three kinds of uniform ZnO thin film samples labeledS1, S2 and S3 were deposited on c-Al2O3 substrate usingthe self-made AP-MOCVD system [23,24] with rotatingdisk vertical reactor. The precursors were Diethylzinc(DEZn) and H2O, and the carrier gas was 7N-purity N2.Fig. 1. shows the schematic illustration of the growthprocedures of S1, S2 and S3. The growth procedures wereas follows: (1) in-situ cleaning of c-Al2O3 substrates in H2

gas at 900 1C for 5min; (2) deposition of the ZnO bufferlayer at 680 1C for 30 s; (3) annealing of the ZnO buffer inN2 gas at 800 1C for 10min and (4) deposition of the ZnOepitaxial films at different conditions for 30min. Thedetailed growth conditions of S1, S2 and S3 are listed in

0 1000 2000 3000 4000 5000

200

400

600

800

1000S2 sample

S1/S3 sample

43

2

1

Tem

per

atu

re (

°C)

Time (s)

Fig. 1. Schematic illustration of growth procedures of ZnO thin films

deposited on c-Al2O3 substrates.

Table 1

Epitaxial parameters of three samples and related crystal parameters

Sample

no.

H2-flow rate

(sccm)

Substrate temperature

(1C)

S1 10 680

S2 10 900

S3 0 680

Table 1. From Table 1, it can be noted that H2 gas wasadded into the system while growing S1 and S2, but therewas no H2 gas added while growing S3. The depositiontemperature was the same at 680 1C for S1 and S3, while S2had higher deposition temperature at 900 1C.To investigate the PL properties of the three samples,

temperature-dependent PL spectra were measured in thewavelength range of 360–400 nm using a closed-cycle liquidhelium cryogenerator. The 325 nm line of a He–Cd laseroperated at a power of 10mW was used as the excitationsource. The (0 0 2) and (1 0 2) rocking curves of threesamples were measured by a double-crystal X-ray diffract-ometer (QC200, BEDE Instruments, UK). Cu Ka1 line wasused as the source and Ge (0 0 4) was used as themonochromator. The full-widths at half-maximum(FWHM) values of the rocking curves were obtained byPseudo–Voigt fitting. The thickness of S1, S2 and S3 wereapproximately 3.2, 2.3 and 3.8 mm, respectively, which weremeasured by observing the cross-section of the films frominterference microscopy (Olympus BX51).

3. Results and discussion

Fig. 2(a) and (b) show the (0 0 2) and (1 0 2) o-rockingcurves of the three as-grown samples, respectively. TheFWHM values of the (0 0 2) rocking curves of the threesamples are 166, 596 and 183 arcsec, respectively and theFWHM values of the (1 0 2) rocking curves of them are293, 580 and 374 arcsec, respectively. These results indicatethat three samples have good crystalline quality.Fig. 3 shows the temperature-dependent PL spectra of

the three samples from 10 to 100K. These spectra show thesame trends. As the temperature increases, the intensity ofbound-exciton peaks decreases, while the intensity of thefree exciton XA (FXA) emission increases gradually. It canbe observed that the FXA emission becomes the strongestone when the temperature exceeds 100K. The excitonlinewidth is broadened due to scattering of LO phononsand the excitons become thermally ionized on raising themeasured temperature.Although the trends of temperature-dependent PL

spectra are the same, the PL spectra of the three samplesat 10K are different. Fig. 4 shows the PL spectra of threesamples at 10K. From this figure, we can see that the PLspectra of S2 and S3 are almost the same and the PLspectrum of S1 differs from those of S2 and S3. For S1sample, it is dominated by the bound-exciton peak at3.363 eV, which is commonly ascribed to a hydrogen-

(0 0 2) FWHM

(arcsec)

(1 0 2) FWHM

(arcsec)

Thickness

(mm)

166 293 3.2

596 580 2.3

183 374 3.8

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Omega (arcsec)(a) Omega (arcsec)(b)

Inte

nsi

ty (

a.u

.)

FWHM=293

S1 sample (102)

FWHM=580

S2 sample (102)

-1000 -500 0 500 1000

FWHM=374

S3 sample (102)

FWHM=596

S2 sample (002)

FWHM = 166

S1 sample (002)

-1500 -1000 -500 0 500 1000 1500

FWHM=183

S3 sample (002)

Fig. 2. Double-crystal X-ray diffractometer (DCXRD) rocking curves of three as-grown ZnO samples. (a) (0 0 2) rocking curves; (b) (1 0 2) rocking curves.

F. Li et al. / Journal of Luminescence 124 (2007) 162–166164

related donor-bound exciton (D0X1, i.e. I4) according tothe literature [14–17]. At the same time, a strong two-electron satellite (TES1) at 3.331 eV occurs in the spectrum.The slightly weaker line at 3.359 eV is the location ofanother donor-bound exciton (D0X2, i.e. I8) [25]. To thebest of our knowledge, I8 bound-exciton peak is not relatedto the presence of hydrogen in ZnO [17]. Meanwhile, aweak TES2 at 3.321 eV can be observed. However, for S2and S3 samples, the dominant peak is located at 3.359 eV(I8) and the peak at 3.363 eV (I4) is just the shoulder. Aclear TES2 at 3.321 eV appears in the spectra and TES1becomes weak. From these PL spectra, we can observe thatTES1 becomes obvious when I4 is strong; on the otherhand, TES2 becomes visible when I8 is strong.

As mentioned above, the I4 peak is related to thepresence of hydrogen. It is reasonable that the peaks of I4and I8 dominate the spectra of S1 and S3 at 10K,respectively, because H2 gas was added into the growthambient while depositing S1, whereas there was no

hydrogen gas present while depositing S3. It should benoted that H2 is also added into the growth ambient whiledepositing S2. However, I4 peak is not the dominant peakin the PL spectrum of S2 at 10K. The only differencebetween S1 and S2 is the depositing temperature, i.e., thedepositing temperature of S2 (900 1C) which is higher thanthat of S1 (680 1C). From these results, we can deduce thatit is very difficult to introduce hydrogen into ZnO thin filmswhen grown at high temperature (e.g. 900 1C) even inhydrogen-present ambient.Considering that the temperature may be the key factor

as discussed above, annealing experiments were performedon S1 in nitrogen for 30min at 700 and 800 1C,respectively. The low-temperature PL spectra are showedin Fig. 5, in which it is noted that the I4 peak greatlydiminishes with the increasing annealing temperature. Inthe sample annealed at 700 1C, the intensity of I4 peakbecomes weaker than that of I8 peak and the TES1 alsodiminishes slightly. In PL spectra of the sample annealed at

ARTICLE IN PRESS

D0 X

2D

0 X1

FX

A-3

LO

FX

A-2

LO

FX

A-1

LO

FX

A S1 sample

100 K

70 K

50 K

30 K

20 K

10 K

D0 X

2

D0 X

1

FX

A-3

LO

FX

A-2

LO

FX

A-1

LO

FX

A S2 sample

100 K70 K

50 K

30 K

20 K

10 K

3.1 3.2 3.3 3.4 3.5

D0 X

2

D0 X

1

FX

A-3

LO

FX

A-2

LO

FX

A-1

LO

FX

A S3 sample

100 K70 K

50 K30 K20 K10 K

PL

Inte

nsi

ty L

og

, , (a.

u.)

Photon Energy (eV)

Fig. 3. Temperature-dependent PL spectra of three as-grown ZnO thin films measured at temperature from 10 to 100K.

F. Li et al. / Journal of Luminescence 124 (2007) 162–166 165

800 1C, the dominant peak changes to I8, and the TES2 alsobecomes quite clear. These results agree with the results inliteratures [16,17], and indicate that hydrogen will evolveout of ZnO when annealed at high temperature, which maybe the reason that it is difficult for hydrogen to incorporateinto the ZnO thin films when grown at high temperature.

4. Conclusions

In this work, ZnO:H thin films were prepared by addinga small quantity of H2 gas in the reaction gas whendepositing ZnO thin films using AP-MOCVD. The

incorporation and outdiffusion of hydrogen in ZnO thinfilms grown at different conditions were investigated bycomparing the intensity of the hydrogen-related bound-exciton peak (I4) in the PL spectra. The intensity of I4 wasfound to be the strongest in ZnO thin films deposited atlow temperature in the hydrogen-present ambient, whilethe I4 peak was just a shoulder of I8 in ZnO thin films, bothgrown at high growth temperature in the hydrogen-presentambient and grown at low growth temperature in thehydrogen-absent ambient. In addition, hydrogen wasfound to evolve out of ZnO when annealed at hightemperatures. From these results, it can be deduced that it

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3.30 3.31 3.32 3.33 3.34 3.35 3.36 3.37 3.38 3.39 3.40

I8 I4

FX

A-1

LO

T=10K

TE

S2

TE

S1

D0 X

2

D0 X

1

FX

A

S3 sample

S2 sample

S1 samplePL

inte

nsi

ty L

og

(a.

u.)

Photon Energy (eV)

Fig. 4. Low-temperature PL spectra of three as-grown ZnO thin films

measured at 10K.

3.30 3.32 3.34 3.36 3.38 3.40

I8 I4

FX

A-1

LO

TE

X2

TE

X1

D0 X

2

D0 X

1

FX

A

as grown

700°C anneal

800°C anneal

T=10KS1 sample

PL

inte

nsi

ty L

og

(a.

u.)

Photon Energy (eV)

Fig. 5. Low-temperature PL spectra of S1 annealed at different

temperatures measured at 10K.

F. Li et al. / Journal of Luminescence 124 (2007) 162–166166

is very difficult for hydrogen to incorporate into ZnO thinfilms grown at high temperature even in the hydrogen-present ambient.

Acknowledgments

This work was supported by the National High Technol-ogy Research and Development Program of China (863program) with Contract no. 2003AA302160 and the Electro-nic Development Fund of Information Industry in China.

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