Structural and optical properties of Ga2(1�x)In2xO3 films prepared on a-Al2O3

(0 0 0 1) by MOCVD

Fan Yang, Jin Ma *, Caina Luan, Lingyi Kong

School of Physics, Shandong University, Shanda South Road No. 27, Jinan, Shandong 250100, People’s Republic of China

Applied Surface Science 255 (2009) 4401–4404

A R T I C L E I N F O

Article history:

Received 1 July 2008

Received in revised form 22 October 2008

Accepted 22 October 2008

Available online 27 November 2008

PACS:

61.10.Nz

68.37.Lp

81.15.Gh

Keywords:

Ga2(1�x)In2xO3 films

Microstructure

Optical properties

MOCVD

A B S T R A C T

Ga2(1�x)In2xO3 thin films with different indium content x [In/(Ga + In) atomic ratio] were prepared on a-

Al2O3 (0 0 0 1) substrates by the metal organic chemical vapor deposition (MOCVD). The structural and

optical properties of the Ga2(1�x)In2xO3 films were investigated in detail. Microstructure analysis

revealed that the film deposited with composition x = 0.2 was polycrystalline structure and the sample

prepared with x up to 0.8 exhibited single crystalline structure of In2O3. The optical band gap of the films

varied with increasing Ga content from 3.72 to 4.58 eV. The average transmittance for the films in the

visible range was over 90%.

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1. Introduction

Recently, intense interest has been paid to wide band gapsemiconductors due to their potential applications in lightemitting devices in the blue to ultraviolet (UV) region. It shouldbe emphasized that band gap engineering is one of the key issuesfor constructing various electronic and optical devices usingcompound semiconductors. In the past, ternary compounds andmulticomponent oxides consisting of a combination of binarycompounds such as In2O3, SnO2, ZnO, MgO, and Ga2O3 have beeninvestigated extensively [1–6]. By alloying ZnO with MgO, theband gap of the ternary MgZnO alloys can be tuned from 3.3 to4.65 eV and higher [7,8], which facilitates the fabrication ofheteroepitaxial ultraviolet light emitting devices based on ZnO.

Both In2O3 and Ga2O3 are good transparent oxide semiconduc-tors (TOSs) with direct band-gap energy of 3.7 and 4.9 eV,respectively. In2O3 with the bixbyite structure is a very importantn-type transparent semiconductor that shows excellent optoelec-trical properties [9,10] and has been widely used in many fieldssuch as solar cells and flat-panel displays. Ga2O3 with themonoclinic structure (b-Ga2O3) is a promising candidate for

* Corresponding author. Tel.: +86 531 88361057; fax: +86 531 88564886.

E-mail addresses: yangfanemily@mail.sdu.edu.cn (F. Yang), jinma@sdu.edu.cn

(J. Ma).

0169-4332/$ – see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2008.10.129

deep-UV transparent conductive oxides [11,12]. But Ga2O3 as adeep-UV transparent material presents some difficulties. Since theposition of conduction band bottom is relatively high, the donorlevels become deep levels. Introducing shallow donor levels intothe wide band-gap compounds for release of electrons into theconduction band becomes more difficult than in narrow gapcompounds [13]. Ga2(1�x)In2xO3 is considered to be an alloy ofIn2O3 and Ga2O3. According to the theory of Hill [14], the band gapof Ga2(1�x)In2xO3 could be tuned from 3.7 to 4.9 eV by controllingthe composition of the alloy suitably. Hence, modulation of theband gap is one of the keys to lift this material to higher potential inthe field of UV optoelectronics. To the best of our knowledge, littlework has been done to investigate the structural and opticalproperties of Ga2(1�x)In2xO3 alloy film depending on the composi-tion, especially by using the MOCVD method. In this study,Ga2(1�x)In2xO3(0.1 � x � 0.9) films were prepared on a-Al2O3

(0 0 0 1) by MOCVD technique. The structural and optical proper-ties as well as compositions of these films were investigated indetail.

2. Experimental details

The Ga2(1�x)In2xO3 films with a series of compositions wereprepared on a-Al2O3 (0 0 0 1) substrates by an MOCVD system.Commercially available trimethylindium (TMIn) and trimethylgal-lium (TMGa) were used as organometallic (OM) source. High-purity

Table 1Investigated samples and values of In content determined from the RBS spectra.

Sample number [In/(Ga + In)]

atomic ratio

Calculated

composition x0

1 [Ga2(1�0.1)In2(0.1)O3] 10 at.% 0.22

2 [Ga2(1�0.2)In2(0.2)O3] 20 at.% 0.37

3 [Ga2(1�0.3)In2(0.3)O3] 30 at.% 0.51

4 [Ga2(1�0.4)In2(0.4)O3] 40 at.% 0.65

5 [Ga2(1�0.5)In2(0.5)O3] 50 at.% 0.74

6 [Ga2(1�0.6)In2(0.6)O3] 60 at.% 0.81

7 [Ga2(1�0.7)In2(0.7)O3] 70 at.% 0.87

8 [Ga2(1�0.8)In2(0.8)O3] 80 at.% 0.92

9 [Ga2(1�0.9)In2(0.9)O3] 90 at.% 0.95

F. Yang et al. / Applied Surface Science 255 (2009) 4401–44044402

N2 (purity, 99.9999999%) passed through the OM bubblers anddelivered OM vapor to the reactor. The TMIn and TMGa bubblerswere maintained at the temperature of 28 8C and �14 8C,respectively. High-purity O2 (purity, 99.999%) with the flow rateof 50 sccm was injected using a separatedelivery line into the reactoras the oxidant. The Ga2(1�x)In2xO3 films were synthesized in thereactor with the growth pressure of 50 Torr at 700 8C. Compositionsand the thickness of the films were determined by Rutherfordbackscattering spectrometry (RBS) using 2 MeV He+ ion beam. Thestructural properties were determined by a RIGAKU D/MAX-gB X-ray diffractometer (XRD) with Cu Ka radiation. High-resolutiontransmission electron microscopy (HRTEM) and selected-areaelectron diffraction (SAED) with a Technai F30 transmission electronmicroscope operated at 300 kV were used to study the micro-structure of the films. The optical transmittance measurements wereperformed using a Shimadzu TV-1900 double-beam UV–vis–NIRspectrophotometer in the wavelength range of 200–800 nm.

3. Results and discussion

Fig. 1 shows the typical RBS of Ga2(1�x)In2xO3 films withenactment values x = 0.1, 0.3, 0.5, 0.7 and 0.9, respectively. Thearrows indicate the element signals of our samples. The In and Gacontent in Ga2(1�x)In2xO3 films can be estimated by using theformula [15]:

x

1� x¼ NIn

NGa¼ HInsGa½eo�GaInO

In

HGasIn½eo�GaInOGa

; (1)

where the NIn and NGa are the number of In and Ga atoms per unitarea, sIn and sGa are the scattering cross-section for indium andgallium, ½eo�GaInO

In and ½eo�GaInOGa are the stopping cross-section factor

for the element in the compounds, HIn and HGa are the signal heightin the spectrum for In and Ga, respectively. The experimentenactment atomic ratios x [In/(Ga + In)] and the actual value of In

Fig. 1. RBS of Ga2(1�x)In2xO3 films prepared at 700 8C with different indium content

x: (a) 0.1, (b) 0.3, (c) 0.5, (d) 0.7 and (e) 0.9.

content x0 in Ga2(1�x)In2xO3 samples calculated from the formulaare summarized in Table 1. It can be clearly seen that the In contentin our samples is larger than the experiment enactment value. Thisis because that Ga is less reactive and more resistive to oxidationcompared to In during the deposition process.

The X-ray diffraction patterns of the alloy films deposited at700 8C with different In content are depicted in Fig. 2. It can be seenthat, for the film deposited with x = 0.1, the a-Al2O3 (0 0 0 6)substrate diffraction peak located at 2u = 41.78 along with threediffraction peaks corresponding to Ga2O3 ð2̄ 0 2Þ, ð3̄ 1 1Þ and ð6̄ 0 3Þreflections of monoclinic structure are observed. This indicatesthat the obtained Ga2(1�x)In2xO3 films were polycrystalline withthe b-Ga2O3 structure. From Fig. 2(a), it can be seen that theintensities of the three diffraction peaks are weak and the fullwidth at half maximum (FWHM) of these peaks are relativelybroad. These results imply that the grain size of Ga2(1�x)In2xO3

(x = 0.1) film is smaller than other samples. As increasing the Incontent, for x = 0.3 and x = 0.5 samples, two diffraction peakscorresponding to (2 2 2) and (4 0 0) of body centered cubic (bcc)structure of In2O3 are observed. No extra peaks corresponding tothe Ga2O3 structure are detected from the XRD patterns. For the

Fig. 2. XRD spectra of Ga2(1�x)In2xO3 films prepared at 700 8C with different In

content x: (a) 0.1, (b) 0.3, (c) 0.5, (d) 0.7 and (e) 0.9.

Fig. 4. Transmittance spectra of Ga2(1�x)In2xO3 films with different In content.

F. Yang et al. / Applied Surface Science 255 (2009) 4401–4404 4403

higher In content (x > 0.5), as exhibited in Fig. 1(d) and (e), only asingle diffraction peak corresponding to In2O3 (2 2 2) plane of bcc

structure is observed.Fig. 3a and b shows the cross-sectional HRTEM image of the

interface area between the a-Al2O3 substrate and the Ga2(1�x)In2xO3

films with In content x0 = 0.37 and x0 = 0.92, respectively. FromFig. 3a, it can be seen that the obtained film is polycrystalline with b-Ga2O3 structure. Fig. 3b reveals that the prepared film is structurallyuniform single crystalline of In2O3, and the growth direction is[2 2 2]. For the substrate, the interface spacings marked by thearrows are 0.217 and 0.348 nm which are consistent with a-Al2O3

(0 0 0 6) and (0 1 2), respectively. For the film, the interplane spacingis 0.293 nm corresponding to bcc-In2O3 (2 2 2). The selected-areaelectron diffraction (SAED) pattern of the film with x0 = 0.92 is shownin the inset of Fig. 3b, which includes both the Ga2(1�x)In2xO3 film andthe sapphire substrate. These results indicate that, as increasing theGa content, the film keeps single crystalline structure initially andthen degrades to polycrystalline with b-Ga2O3 structure.

Fig. 3. Cross-sectional HRTEM image of Ga2(1�x)In2xO3 film with different In content.

(a) x0 = 0.37, (b) x0 = 0.92 (inset: the SAED pattern of the film).

Fig. 5. Band-gap variation of Ga2(1�x)In2xO3 alloys as a function of Ga content (1 � x).

Fig. 4 shows the optical transmittance of Ga2(1�x)In2xO3 filmswith different In content. The thickness of the films is in the rangeof 70–95 nm measured by RBS. It is exhibited that the sampleshave an absorption edge in the UV region and the averagetransmittance in the visible range exceed 90%. From the spectra, itis observed that the absorption edge of the films shifts to shorterwavelength with increasing the Ga content (1 � x) from 10 to80 at.%. The optical gap (Eg) of the films can be obtained by plottinga2 vs. hn (a is the absorption coefficient and hn is the photonenergy) and extrapolating the straight-line portion of this plot tothe energy axis. The dependence of band gap on the Ga content inall of our samples is shown in Fig. 5. The band gap of pure In2O3 andGa2O3 are also lined out in the pattern. Eg of Ga2(1�x)In2xO3 filmsincreases from 3.72 to 4.58 eV for 0.1 � (1 � x) � 0.9. When the Gacontent (1 � x) exceeds 0.8, the Eg deviates from the linear fit. Thismay be due to that the obtained Ga2(1�x)In2xO3 film (1 � x = 0.9) ispolycrystalline structure with smaller grain size, which results inthe band-gap energy of the film increasing further.

4. Conclusions

Ga2(1�x)In2xO3 (0.1 � x � 0.9) films were deposited on a-Al2O3

(0 0 0 1) substrates by MOCVD. In the case of Ga0.16In1.84O3, theHRTEM measurement showed that the sample was single crystal-line thin film with bcc structure of In2O3 and for Ga1.26In0.74O3,polycrystalline film with b-Ga2O3 structure was obtained. The

F. Yang et al. / Applied Surface Science 255 (2009) 4401–44044404

band gap was tuned from 3.72 to 4.58 eV as the Ga content (1 � x)increased from 0.1 to 0.9. The average transmittance of the filmsin the visible range exceeds 90%. These results indicate thatGa2(1�x)In2xO3 films may have higher potential for the constructionof superlattice quantum well and heteroepitaxial ultraviolet lightemitting devices.

Acknowledgement

This work is financially supported by the National NaturalScience Foundation of China (Grant No. 50672054).

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