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0022-0248/$ - se

doi:10.1016/j.jc

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Journal of Crystal Growth 287 (2006) 7–11

www.elsevier.com/locate/jcrysgro

MBE growth of ZnO layers on sapphire employing hydrogen peroxideas an oxidant

A. Bakina,�, A. El-Shaera, A. Che Mofora, M. Kreyea, A. Waaga, F. Bertramb,J. Christenb, M. Heukenc, J. Stoimenosd

aInstitute of Semiconductor Technology, Technical University Braunschweig, Hans-Sommer-Street 66, 38106 Braunschweig, GermanybInstitute of Experimental Physics, Otto-von-Guericke-University Magdeburg, Germany

cAixtron AG, Aachen, GermanydPhysics Department, Aristotele University Thessaloniki, Greece

Abstract

The growth of ZnO epitaxial layers by using molecular beam epitaxy (MBE) is presented in this paper. We employed a modified

Varian Gen II MBE system using H2O2 as an oxidant. ZnO layers with thickness from 100 to 600 nm were grown on (0 0 0 1)-sapphire

using an MgO buffer. The surface morphology of the samples was studied by atomic force microscopy (AFM), optical microscopy with

Nomarski contrast and scanning electron microscopy (SEM). The crystalline quality of the layers was investigated using X-ray

diffractometry (XRD) and transmission electron microscopy (TEM). Optical properties of the ZnO layers were studied by

photoluminescence (PL). The surface roughness (rms) measured by AFM for the best layers is about 0.2 nm. The formation of

hexagonal ZnO pyramids was also observed in some cases. XRD measurements of the obtained ZnO layers show the excellent quality of

the single crystalline ZnO heteroepitaxially grown on sapphire. The FWHM of the XRD (0 0 0 2) rocking curve measured for our best

layers is as low as 27 arcsec.The influence of growth parameters on structural properties as well as on surface morphology of the zinc

oxide layers on sapphire is investigated and discussed. High-quality ZnO-on-sapphire epiwafers with a very good lateral uniformity

revealed both by XRD and PL measurements could provide an alternative to bulk ZnO wafers.

r 2005 Elsevier B.V. All rights reserved.

PACS: 68.55.�a; 68.55.Ac; 68.55.Jk; 68.65.Ac

Keywords: A3. Molecular beam epitaxy; B1. Oxides; B2. Semiconducting materials

1. Introduction

ZnO is a promising material for gas sensors, opticalapplications (ultraviolet and blue light emitting devices),transparent electrodes, piezoelectric resonators, etc. Mucheffort has been made to produce high-quality ZnOepilayers. In the past years, different methods have beenemployed to grow single-crystalline ZnO layers on differentsubstrates: metal-organic vapour-phase epitaxy [1], pulsed-laser deposition [2] and plasma-assisted molecular beamepitaxy (MBE) [3–5]. The latter method is typically used formanufacturing of high-quality epitaxial layers, providing

e front matter r 2005 Elsevier B.V. All rights reserved.

rysgro.2005.10.033

ing author. Tel.: +49 531 391 3779; fax: +49 531 391 5844.

ess: A.Bakin@tu-bs.de (A. Bakin).

excellent control of layer thickness, composition, interfaceand surface uniformity and sharp dopant profiles. MBE isthe method of choice for the manufacturing of high-qualitysuperlattices and heterostructures comprising differentmaterials. Much effort has been made to improve thequality and doping control of MBE-grown ZnO layers[6–9]. One of the most successful approaches which led tothe improvement of the ZnO layer quality is theimplementation of a thin MgO buffer layer grown directlyon sapphire with a consequent low-temperature ZnObuffer layer [10].The most simple approach to provide oxygen species for

ZnO growth is to employ O2, but the molecular bondingenergy of O2 is as high as 5.16 eV. This fact makes thermaldissociation of oxygen directly at the substrate surface

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Fig. 1. RHEED patterns of: sapphire substrate (a), MgO buffer layer (b),

ZnO buffer layer (c, d), and ZnO layer (e) with the electron beam incident

along the 1 1 2 0� �

and 1 1 0 0� �

directions.

A. Bakin et al. / Journal of Crystal Growth 287 (2006) 7–118

practically impossible [11]. So, it is necessary to providespecial sources of reactive oxygen species for effective ZnOMBE growth. One of the possibilities to obtain reactiveoxygen species is by employing plasma source, but theimplementation of plasma sources can cause surfacedamage by high-energy particles, contamination of grow-ing layers by source materials [12] and possible degradationof plasma sources due to the high reactivity of oxygenradicals. So, further attempts to find alternative sources ofreactive oxygen species have been made to solve theseproblems. Such alternatives to oxygen plasma cell can bethe implementation of reactive oxygen-containing com-pounds, like ozone [13], electron cyclotron resonancesource [14] or the recently presented implementation ofH2O2 as an oxidant for MBE growth of ZnO [15,16]. Theimprovements of the quality of ZnO layers grown by MBEon c-sapphire by employing hydrogen peroxide as anoxidant are described in the present paper.

2. Experimental procedure

We use a modified Varian Gen II MBE system for thegrowth of ZnO layers on sapphire. The chamber wasinitially evacuated to a base pressure of 10�8 Torr. 6Npurity elemental Zn was evaporated using a double zoneeffusion cell in order to reduce the oxidation of the Znsource material. H2O2 placed in a stainless-steel vessel anda stabilized temperature water bath was used as oxidant. Aquartz pipe connected to a leak valve was employed todirect the vapour flow from the stainless-steel vessel withH2O2 to the sapphire substrate mounted on the rotatingholder of the substrate manipulator. The pressure in thechamber during the ZnO epitaxial growth was maintainedat the level of 1� 10�5–5� 10�5 Torr by setting differentH2O2 flows and this pressure. Thus, in our case, thepressure in the chamber is mostly defined by H2O2

pressure, and this pressure is a very convenient choice tomeasure and control the extremely low H2O2 flow. We usedthe Zn deposition rate (determined prior to growth usingquartz crystal monitor) as a parameter that defines the Znflow during growth.

The initial surface state, deposition of both MgO andZnO were monitored in situ by reflection high-energyelectron diffraction (RHEED) (Fig. 1).

We used c-sapphire (0 0 0 1) nominally ‘‘on-axis’’ sub-strates. Ti was deposited on the backside of the substratesin order to provide a more effective heating during thegrowth. After the cleaning procedure the substrates weremounted on molybdenum holders for the substratemanipulator, and loaded into the buffer chamber andthermally cleaned at a temperature about 800 1C for 1 hand then transferred into the growth chamber, where it wasfurther treated in oxygen at 750 1C for 20min to producean oxygen terminated c-sapphire surface showing a sharpstreaky RHEED pattern indicating a clean and flat surface(cf. Fig. 1a).

After the above described surface treatment, a 2.5 nmMgO buffer layer was grown at 700 1C with a growth rateof about 0.01 nm s�1. A diffused streaky RHEED patterncorresponding to the MgO lattice appears after a fewmonolayers growth (cf. Fig. 1b). Thereafter, a low-temperature (LT) ZnO buffer layer was grown at 300 1Cat a growth rate of about 0.02 nm s�1. After the growth ofabout 6 nm LT ZnO buffer, the RHEED patternsgradually changed from streaky to spotty. This indicatesa roughening of the growth surface (cf. Fig. 1c). Thegrowth was stopped and annealing started at 700 1C forabout 5min. Then a sharp streaky RHEED pattern of ZnOappears again (cf. Fig. 1d). This procedure was repeatedseveral times in order to produce a thicker LT ZnO bufferlayer. The main ZnO layer was grown at differenttemperatures ranging from 350 to 700 1C and employingvarious Zn/O ratios to optimize the growth conditions.ZnO layers with thickness from 50 to 600 nm wereobtained.Investigations on the structural quality were carried out

employing X-ray diffraction (XRD), transmission electronmicroscopy (TEM) and photoluminescence (PL) measure-ments. XRD measurements were performed by employinga high-resolution diffractometer with Cu Ka radiation,operated at a voltage of 40 kV and a current of 40mA. Weused a detector without a slit for the rocking curvemeasurements of the obtained ZnO epitaxial layers.Atomic force microscopy (AFM) was employed for thesurface morphology investigations of the ZnO epilayers.We used the narrowing of the layer XRD rocking curveand roughness of the ZnO surface as the optimizationparameters in our investigations of the influence of growthparameters on the ZnO layer quality. We reported on theinfluence of Zn deposition rate and H2O2 pressure on theFWHM of the (0 0 0 2) rocking curve for the obtained ZnOlayers in our previous work [15].

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Fig. 3. AFM images of surfaces of ZnO layers grown at different

temperatures: (a) 450 (b) 550 1C, and (c) 700 1C for the constant Zn�1 �5

A. Bakin et al. / Journal of Crystal Growth 287 (2006) 7–11 9

3. Results and discussions

According to RHEED, ZnO layers grown at tempera-tures from 350 to 650 1C are single-crystalline. Typicalstreaky RHEED patterns of the ZnO layer with theelectron beam incident along the 1 1 2 0

� �and 1 1 0 0

� �

directions were observed (cf. Fig. 1e). Rotation of thesample around its axis showed that each pattern wasrepeated every 601 proving the hexagonal wurtzite struc-ture of the growing film.

Fig. 2 presents the FWHM of the XRD (0 0 0 2) rockingcurves corresponding to the ZnO layers grown at differentgrowth temperatures (substrate temperature Ts) but at aconstant Zn rate of 0.1 nm s�1, and at a constant H2O2

pressure 3.5� 10�5 Torr which were found to be optimalvalues for our growth approach [16]. As clearly seen fromthe figure, there is an optimal temperature Ts of 550 1Cproviding the growth of high-quality ZnO layers. If thegrowth temperature is increased or decreased from thisvalue, the FWHM increases revealing a worsening of thelayer quality.

In situ RHEED investigations showed that the layersgrown at temperatures below 450 1C reveal a three-dimensional nucleation mode, although at the initial stageof the growth a 2D mode was observed. The 2D RHEEDpatterns were observed in the case of ZnO growth attemperatures above 500 1C and, in this case the RHEEDpatterns, remained 2D during further growth. These resultsare in a good agreement with the AFM measurementresults. AFM micrographs of the surface show islands withirregular and rough steps in the case of growth at 450 1Cwith a surface roughness of 5.0 nm (Fig. 3a). Increase of thegrowth temperature to 550 1C led to a decrease in rough-ness from 5.0 to 0.2 nm, and a coalescence of the hexagonalislands leading to the regular step edge was observed (Fig.3b). Further increase of the growth temperature up to700 1C led to the formation of hexagonal pits on anatomically flat surface with a roughness of 0.45 nm (cf. Fig.3c). These results show that the broadening of the FWHMof the XRD rocking curve at low temperatures is caused bylow surface diffusion and 3D growth mode. The increase of

200 300 400 500 600 700 8000

200

400

600

800

1000

1200

1400

FW

HM

of (

0002

)roc

king

cur

ve(a

rcse

c)

Growth temprature (°C)

Fig. 2. FWHM of (0 0 0 2) XRD rocking curve for ZnO layers grown at

different temperatures for the constant Zn deposition rate of 0.1 nm s�1,

and H2O2 pressure of 3.5� 10�5 Torr.

deposition rate of 0.1 nm s and H2O2 pressure of 3.5� 10 Torr.

the surface roughness and the respective broadening of theFWHM of XRD rocking curves (cf. Fig. 3) with increasinggrowth temperature above 550 1C can be explained asfollows: increasing the growth temperature causes Zndesorption from the surface and leads to oxygen-richgrowth conditions. According to the described investiga-tions as well as our investigations on the ZnO growth rateas a function of the Zn deposition rate and H2O2 pressure,the optimal growth conditions were obtained in the casewhen II/VI ratio is about 1 with a slight oxygen excess.The results of cross-sectional TEM investigations of the

obtained ZnO epitaxial layers are shown in Fig. 4,revealing the presence of threading dislocations. It is worth

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Fig. 4. Cross-sectional TEM image of the ZnO layer grown on sapphire:

(a) high quality ZnO on sapphire layer; (b) surface pyramidal defect.

17.02 17.04 17.06 17.08 17.10 17.12

10

100

1000

FW HM=27 arcsec

Inte

nsity

(a.

u.)

ω (degree)

Fig. 5. The (0 0 0 2) XRD rocking curve of ZnO layer grown in optimized

conditions on a 2 in sapphire substrate (FWHM is about 30 arcsec).

3.20 3.25 3.30 3.35 3.40

Energy (eV)

PL

Inte

nsity

(ar

b.u.

)

T = 11K

(X)

I3

I6I9

LO

(a)

3.34 3.35 3.36 3.37 3.38

T = 11K

Energy (eV)

PL

nsity

(ar

b.u.

)

(b)

(X)

I3

I6I9

Fig. 6. PL spectra of 200 ZnO-on-sapphire epiwafer measured at 11K: (a)

wide range spectrum; (b) detailed spectrum with I9, I6, I3 bound excitons

and X free exciton.

A. Bakin et al. / Journal of Crystal Growth 287 (2006) 7–1110

noting the perfection of the ZnO/MgO/Sapphire interface.In spite of the large misfit in the heteroepitaxial system,most of the misfit dislocations are well-confined on theinterface (cf. Fig. 4a). In some cases where the II/VI ratiowas around 1 but with a slight Zn excess, the formation ofhexagonal ZnO pyramids was also observed. A cross-sectional TEM image of such a pyramid is shown in Fig.4b. Surprisingly, dislocation semi-loops starting from thesurface extended downwards to the interface, as shown byarrows in Fig. 4b. A pair of threading dislocationsemanating from the interface typical for the layers withoutpyramids is also shown in the Fig. 4b, and is denoted by theletter P at the right side of Fig. 4b. The rest of the layerwhere no pyramids were observed was of high quality.These hexagonal pyramids were, supposingly, formed atthe final stage of the non-optimized growth when itoccurred in slightly Zn-rich conditions. Growth in opti-mized condition with a proper control of the II/VI ratioduring the whole growth process resulted in ZnO layerswithout pyramidal defects.

High quality ZnO layers on 2 in sapphire substrates weregrown in optimized conditions employing the results of theinvestigations described above. Fig. 5 shows an XRD(0 0 0 2) rocking curve obtained from one of the manufac-tured 2 in ZnO epiwafers. The thickness of the ZnOepilayer in this case is about 200 nm. The FWHM of therocking curve shown is 27 arcsec and XRD measurementsin different areas of the ZnO epiwafer revealed a very highlateral homogeneity across the whole 2 in epiwafer. TheFWHM of the XRD (0 0 0 2) rocking curve measured forour best layers is as low as 27 arcsec.

PL investigations of the 200 ZnO on sapphire wafersare presented in Figs. 6 and 7. The low-temperature(T ¼ 11K) PL spectrum is dominated by the impuritybound excitons I6 (E ¼ 3:3578 eV, FWHMo4meV) and I9(E ¼ 3:3537 eV, FWHMo2.5meV) (cf. Fig. 6b). The freeexciton X (3.3728 eV) and the bound exciton I3 (3.3644 eV),as well as the first two LO phonon replica from X and I6are clearly visible (cf. Figs. 6a, b). All lines are 3meV red-shifted with respect to their fully relaxed position knownfrom bulk crystals, indicating minor substrate-inducedtensile strain. The PL spectra taken from different samplepositions show a slight change in I6 intensity, whereas allother lines remain constant. Results of PL mapping of 2 inZnO epiwafer are presented in Fig. 7. High lateralhomogeneity of the epiwafer is clearly seen from both PL

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Fig. 7. PL mapping of 200 ZnO-on-sapphire epiwafer: (a) PL intensity mapping; (b) PL FWHM mapping.

A. Bakin et al. / Journal of Crystal Growth 287 (2006) 7–11 11

intensity distribution (cf. Fig. 7a) and PL FWHMdistribution (cf. Fig. 7b).

4. Conclusion

In conclusion, we described recent advancement in theMBE growth of ZnO epilayers on (0 0 0 1) sapphiresubstrates by employing hydrogen peroxide as an oxidantand a thin MgO buffer layer. The quality of the layers wasimproved by employing optimized growth conditions. High-quality ZnO-on-sapphire epiwafers with a very good lateraluniformity revealed both by XRD and PL measurementscould provide an alternative to bulk ZnO wafers.

Acknowledgements

The authors would like to thank M. Karsten, K-H.Lachmund and D. Rummler for their technical assistance.This work was supported by Deutsche Forschungsge-meinschaft project DFG-WA870/71.

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