Journal of Crystal Growth 237–239 (2002) 538–543

Growth and characterization of Ga-doped ZnO layerson a-plane sapphire substrates grown by

molecular beam epitaxy

Hiroyuki Katoa,*, Michihiro Sanoa, Kazuhiro Miyamotoa, Takafumi Yaob

aResearch and Development Center, Stanley Electric Co., Ltd., 1-3-1 Eda-Nishi, Aoba-ku, Yokohama 225-0014, Japanb Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan

Abstract

Gallium-doped ZnO epitaxial layers were grown on a-plane sapphire substrates by molecular beam epitaxy (MBE) at

various Ga cell temperatures from 3501C to 4501C. The ZnO layers grown on a-plane sapphire were c-oriented without

any trace of the 301 rotation domains often observed in ZnO on c-plane sapphire. The Ga concentration in Ga-doped

ZnO increased from 4� 1016 to 7� 1018 cm�3 with increasing Ga cell temperature. The activation ratio of Ga was about

unity when the Ga concentration exceeded 3� 1017 cm�3. The photoluminescence (PL) spectra of Ga-doped ZnO were

dominated by an emission at 3.362 eV which can be assigned to emission of exciton bound to Ga-related neutral donors.

The intensity of this emission was maximum when the Ga concentration was 2� 1018 cm�3. The high crystalline quality

of the Ga-doped ZnO epilayers was confirmed by X-ray diffraction (XRD), Hall effect measurement and Rutherford

backscattering spectrometry. Our results show that high-quality Ga-doped n-type ZnO can be grown on a-plane

sapphire substrates by using MBE. r 2002 Elsevier Science B.V. All rights reserved.

PACS: 81.15.Hi; 81.05.Dz; 61.72.Vv; 78.55.Et; 73.61.Ga

Keywords: A1. Doping; A1. Photoluminescence; A1. Rutherford backscattering spectrometry; A3. Molecular beam epitaxy; B1. Zinc

compounds

1. Introduction

ZnO-based materials are candidates for use inultraviolet (UV) lasers and light-emitting diodesbecause ZnO has a direct energy gap of 3.37 eV atroom temperature (RT). In addition, ZnO has alarge excitonic binding energy of about 60 meVand can exhibit excitonic effects at RT. In fact,optically pumped UV emission from ZnO at RT

has been reported [1,2], and the UV emission wasobserved at temperatures as high as 5501C [3].

Most ZnO epitaxial layers grown by usingmolecular beam epitaxy (MBE) have been studiedusing c-plane sapphire as a substrate [4,5]. How-ever, when c-plane sapphire was used as asubstrate, ZnO epilayers were grown c-orientedwith 301 rotation domains. To overcome thisproblem, other substrates such as a-plane sapphire[6] and c-plane sapphire with GaN [7] or MgObuffer layers [8] have been studied. When thesesubstrates were used, no 301-rotated domains weredetected in ZnO. Therefore, we chose a-plane

*Corresponding author. Tel.: +81-45-911-1111; fax: +81-

45-911-0057.

E-mail address: h.kato@yrd.stanley.co.jp (H. Kato).

0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 1 9 7 2 - 8

sapphire as a substrate which did not require GaNlayer or MgO buffer layer.

Group III elements such as Al, Ga and In, andgroup VII elements such as Cl, Br and I can beused as n-type dopants in ZnO. In comparisonwith the group VII elements, the group IIIelements are easier to control the cell temperature,namely carrier concentration in ZnO, becausevapor pressures of Al, Ga, and In are lower thanthose of Cl, Br and I [9]. Ga is well known to act asan effective n-type donor in ZnO. However,research on Ga-doped ZnO grown on sapphire isscant [10]. The ionic radius and the covalent radiusof Ga are 0.62 and 1.26 (A, respectively, which aresimilar to those of Zn (0.74, 1.31 (A), comparedwith Al (0.50, 1.26 (A) and In (0.81, 1.44 (A) [11].Therefore, we selected Ga as the dopant.

In this study, we report the characteristics ofGa-doped ZnO layers with different Ga concen-trations grown on a-plane sapphire. High-resolu-tion X-ray diffraction (HRXRD), secondary ionmass spectrometry (SIMS), Rutherford backscat-tering spectrometry (RBS), photoluminescence(PL) and Hall effect measurements were used tocharacterize these ZnO layers.

2. Experimental procedure

Undoped and Ga-doped ZnO epitaxial layerswere grown on a-plane substrates by MBE.Elemental zinc (7N grade) and oxygen radiofrequency (RF) plasma (O2 gas with 6N grade)were used as molecular beam sources. The n-typedoping element was 6N grade Ga, and thesubstrate was ð1 1 %2 0Þ a-Al2O3 (a-plane sapphire).A sapphire substrate was etched in a chemicalsolution of H3PO4:H2SO4=1:3 at 1101C for30 min, and then cleaned by heating at 8101C for30 min under an oxygen flux in the growthchamber. Then, undoped ZnO buffer layers (about40 nm thick) were grown on the a-plane sapphiresubstrate at 3501C, and then annealed at 8501C for10 min to improve the crystalline quality andsurface smoothness of the buffer layers. Ga-dopedZnO layers were grown on the ZnO buffer layersat 6001C for five different Ga cell temperatures(350–4501C). The substrate temperatures during

annealing and growth were determined by using anoptical pyrometer. During the epilayer growth, theoxygen plasma flow rate was 3.0 sccm, the RFpower was 150 W, and Zn beam flux measured byquartz thickness monitor was about 3.5 (A/s. Theepilayers during annealing and growth werecharacterized in situ by using reflection high-energy electron diffraction (RHEED). The bufferlayers before annealing showed a spotty RHEEDpattern. In contrast, the buffer layers afterannealing showed a sharp streaky RHEED patternwith (1� 1) reconstruction, similar to the patternseen for Ga-doped ZnO layers.

The growth rate of the epilayers was about0.28 mm/h and the thickness was about 1.4 mm.RHEED observations show that the ZnO layersgrown on a-plane sapphire were c-oriented with-out any trace of the 301-rotation domains oftenobserved in ZnO grown on c-plane sapphire.

Undoped and Ga-doped ZnO epilayers werecharacterized by HRXRD, SIMS, RBS, PL (4.2 K)and Hall effect measurements (RT). The carrierconcentration and mobility of Ga-doped ZnOlayers were measured using the van der Pauwmethod [12]. Gallium concentration in the epi-layers was measured using SIMS. The crystal-lographic quality of the epilayers was evaluated interms of the full width at half maximum (FWHM)of X-ray rocking curves (XRC) measured by using(0 0 0 2) diffraction (X’-pert MRD system, Philips)and RBS measurement. The PL spectra ofundoped and Ga-doped ZnO layers were mea-sured at 4.2 K using an R928 photomultiplier(Hamamatsu Photonics K.K.) and a SPEX 1702monochromator with 1200-grooves/mm gratingblazed at 500 nm. For the PL spectra measure-ments, the specimens were immersed in liquidhelium, and the excitation source was the 325 nmline from a He–Cd laser with an output power of0.1 mW (8 mW/cm2).

3. Results and discussion

The Ga concentration in Ga-doped ZnO epi-layers increased from 4� 1016 to 7� 1018 cm�3

(measured by SIMS) with increasing Ga celltemperature from 3501C to 4501C. Fig. 1 shows

H. Kato et al. / Journal of Crystal Growth 237–239 (2002) 538–543 539

the dependence of carrier concentration andelectron mobility on Ga concentration in ZnO.Undoped ZnO layers grown on a-plane sapphireshowed n-type conductivity with a carrier concen-tration of 3� 1017 cm�3 and an electron mobilityof 26 cm2/Vs. The carrier concentration of Ga-doped ZnO increased with increasing Ga concen-tration. When the Ga concentration exceeded3� 1017 cm�3, the activation ratio of Ga wasabout unity (estimated by dividing the carrierconcentration by the Ga concentration). Thisshows that incorporated Ga atoms in ZnO aresubstituted at the Zn sites. The electron mobility ofGa-doped ZnO was 68 cm2/Vs, which is 2.6 timeshigher than that of undoped ZnO, 26 cm2/Vs. Suchhigher mobility shows that the electrical propertiesof ZnO layers are improved by Ga-doping.

Fig. 2 shows the dependence of FWHM of(0 0 0 2) XRC on Ga concentration. The FWHMof the XRC of undoped ZnO was 272 arcsec, andthat of Ga-doped ZnO increased from 281 to350 arcsec with increasing Ga concentration from4� 1016 to 7� 1018 cm�3. In contrast, the lateralcoherent length of Ga-doped ZnO decreased whenthe Ga concentration exceeded 2� 1018 cm�3.Therefore, the slight increase in FWHM of XRC(from 281 to 350 arcsec) is due to decreasing grainsize. These FWHM values show that ZnO layers

on a-plane sapphire have a good crystallinestructure.

Fig. 3 shows RBS spectra of undoped and Ga-doped ZnO measured with a 1.5 MeV He+ beamincident parallel to the 0 0 0 1h i channel (alignedsignal) and measured with the beam in a randomdirection (random signal). The minimum yield isdefined as the ratio of the aligned signal to therandom signal at the same energy. The minimumZn yield, wmin; of undoped ZnO was 5.6%, whereasthat of Ga-doped ZnO was 2.8%, similar to that ofZnO bulk single crystal [13]. This decrease inminimum yield shows that interstitial Zn atomsare fewer in Ga-doped ZnO than in undoped ZnOand that Ga-doped ZnO is of much bettercrystalline quality than undoped ZnO. As a result,the electron mobility of Ga-doped ZnO was betterthan that of undoped ZnO.

Fig. 4(a) shows PL spectra of undoped and Ga-doped ZnO on a-plane sapphire at 4.2 K, andFig. 4(b) shows details of PL spectra at near bandedge emissions. The PL spectrum of undoped ZnOshowed several emissions at near band edge, suchas 3.378, 3.367, 3.363, 3.353, 3.339 and 3.335 eV,and also showed deep-level emission at about2.25 eV. The emission line at 3.378 eV can beassigned to free exciton, and the emission lines at3.367 and 3.363 eV can be assigned to excitons

1.0E+15

1.0E+16

1.0E+17

1.0E+18

1.0E+19

1.0E+20

1.0.E+15 1.0.E+16 1.0.E+17 1.0.E+18 1.0.E+19 1.0.E+20

Ga concentration (cm-3)

Car

rier

conce

ntr

atio

n (

cm-3

)

0

50

100

150

200

Ele

ctro

n m

obil

ity (

cm2/V

s)

undoped

Fig. 1. Dependence of carrier concentration and electron

mobility on Ga concentration in ZnO epitaxial layers. Solid

line shows that the activation ratio of Ga is unity.

200

250

300

350

400

1.0.E+15 1.0.E+16 1.0.E+17 1.0.E+18 1.0.E+19 1.0.E+20

Ga concentration (cm-3)

FWH

M o

f X

RC

(ar

csec

)

(0002) diffraction

undoped

Fig. 2. Dependence of FWHM of X-ray rocking curves (XRC)

on Ga concentration.

H. Kato et al. / Journal of Crystal Growth 237–239 (2002) 538–543540

bound to neutral donors [14]. Although the originsof the emission lines at 3.353, 3.339 and 3.335 eVare not clarified yet, it is possible to assign them toexcitons bound to neutral acceptors based on theirpositions [15,16]. In contrast, the PL spectrum ofGa-doped ZnO with Ga concentration of2� 1018 cm�3 was dominated by the bound excitonemission to neutral donors at 3.362 eV, and did notshow deep-level emission.

Fig. 5 shows details of the bound excitonemission at neutral donors of Ga-doped ZnO forfive Ga concentrations. The emission lines at 3.367and 3.363 eV observed in undoped ZnO decreasedwith increasing Ga concentration. When the Gaconcentration exceeded 2� 1018 cm�3, these emis-sion lines disappeared and a bound excitonemission emerged at 3.362 eV. This new emission

line dominated the PL spectra of Ga-doped ZnOin the high Ga concentration regime and can beassigned to bound exciton emission at Ga-relatedneutral donors. We note that the line widthremarkably increases with increasing Ga concen-tration.

300 400 500 600 700

Wavelength (nm)PL

Int

ensi

ty (

a.u.

)

undoped ZnO

Ga-doped ZnO

T=4.2KHe-Cd 325nm/0.1mW

[Ga]=2.1×1018 cm-3

360 365 370 375 380 385 390

Wavelength (nm)

PL I

nten

sity

(a.

u.)

undoped ZnO

Ga-doped ZnO

T=4.2KHe-Cd 325nm/0.1mW

[Ga]=2.1×1018 cm-3

3.339 eV

3.363 eV3.367 eV

3.362 eV

3.335 eV

3.353 eV

3.378 eV

(a)

(b)

Fig. 4. (a) PL spectra of undoped and Ga-doped ZnO on

a-plane sapphire at 4.2K. (b) Details of PL spectra at near band

edge emissions of the same specimen.

(a)

(b)

0

4000

8000

12000

16000

0 400 800 1200 1600

Scattering Ion Energy (KeV)

RB

S Y

ield

(a.

u.)

aligned

random

Zn in ZnO

O in ZnO

Al in Sapphire

ZnO

Ga-doped ZnO

0

2000

4000

6000

8000

10000

12000

0 400 800 1200 1600

Scattering Ion Energy (KeV)

RB

S Y

iled

(a.u

.)

aligned

random

undoped ZnO

Fig. 3. Random and aligned RBS spectra of (a) undoped ZnO

and (b) Ga-doped ZnO.

H. Kato et al. / Journal of Crystal Growth 237–239 (2002) 538–543 541

Fig. 6 plots the PL intensities of the boundexciton emission at around 3.36 eV and the deep-level emission at around 2.23 eV against Ga

concentration. When the Ga concentration islower than 3� 1017 cm�3, the bound excitonemission shows only slight increase in intensitywith Ga concentration. The new bound excitonemission at 3.362 eV emerges and eventuallydominates the PL spectrum. This new emissionincreases with Ga concentration, peaks at a Gaconcentration of 2� 1018 cm�3, and then decreaseswith further increase in Ga concentration. Thedeep-level emission intensity monotonically de-creases with Ga concentration. It is most likelythat the new bound exciton emission at 3.362 eV isassociated with doped Ga atoms.

Further research is needed to clarify thecorrelation between structural and electrical prop-erties. The results will be discussed in a futurepaper.

4. Conclusions

Ga-doped ZnO epilayers were grown on a-planesapphire substrates by MBE. The Ga concentra-tion increased from 4� 1016 to 7� 1018 cm�3 withincreasing Ga cell temperature from 3501C to4501C. The activation ratio of Ga was about unitywhen the Ga concentration exceeded3� 1017 cm�3. The high crystalline quality of theepilayers was confirmed by XRD, which showedthat FWHM of XRC ranged from 280 to340 arcsec with increasing Ga concentration. Theelectron mobility of Ga-doped ZnO (determinedby Hall effect measurement) was over two timeshigher than that of undoped ZnO. The minimumyield of Zn signal in aligned RBS spectrum from aGa-doped ZnO, wmin; was 2.8% which indicatesimproved crystallinity in Ga-doped ZnO com-pared to undoped layers. The PL spectra ofGa-doped ZnO were dominated by an emissionat 3.362 eV, which was not detected in the PLspectra of undoped ZnO. This emission can beassigned to emission of excitons bound to Ga-related neutral donors, and its intensity becamemaximum when the Ga concentration was2� 1018 cm�3. Our results show that high-qualityGa-doped n-type ZnO can be grown on a-planesapphire substrates by MBE.

367.5 368 368.5 369 369.5 370 370.5

Wavelength (nm)

PL I

nten

sity

(a.

u.)

T=4.2K,He-Cd 325nm/0.1mW

[Ga]=7.0×1018cm-3

[Ga]=5.0×1018cm-3

[Ga]=2.1×1018cm-3

[Ga]=3.0×1017cm-3

[Ga]=4.0×1016cm-3

undoped

3.362 eV

3.363 eV3.367 eV

Fig. 5. PL spectra at bound exciton to neutral donors of

Ga-doped ZnO with various Ga concentrations.

0

10

20

30

40

1.0E+16 1.0E+17 1.0E+18 1.0E+19

Ga concentration (cm-3)

PL I

nten

sity

at 3

.36

eV (

mV

)

0

1

2

3

4

PL I

nten

sity

at 2

.23

eV (

mV

)

I(edge) at 3.36eV

I(deep) at 2.23eV

Fig. 6. Dependence of PL intensity on Ga concentration.

H. Kato et al. / Journal of Crystal Growth 237–239 (2002) 538–543542

Acknowledgements

We thank Dr. Sato of Hosei University for theRBS measurements.

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