Structural and photoluminescence properties of Ga2-2xIn2xO3 films prepared on α-Al2O3 (0001) by

MOCVD

Fan Yang1,2, Jin Ma1, Caina Luan1, Lingyi Kong1, Zhen Zhu1

1. School of Physics Shandong University,

Jinan, , P R China

2. School of Information Engineering Hebei University of Technology

Tianjin, P R ChinaE-mail address: yang_fan@hebut.edu.cn E-mail address: yang_fan@hebut.edu.cn

Abstract—Ga2-2xIn2xO3 films with different indium content x

[In/(Ga+In) atomic ratio] have been prepared on α-Al2O3 (0001) substrates by the metalorganic chemical vapor deposition (MOCVD) method. The preparation, structural and photoluminescence (PL) properties of the Ga2-2xIn2xO3 films were investigated. The XRD analysis revealed that the film with high Ga content exhibited monoclinic structure of Ga2O3 and transformed to body-centered cubic (bcc) structure of In2O3 with increasing In content. UV PL peaks located at 333 nm (x<0.5) and 338nm (x≥0.5) for the Ga2-

2xIn2xO3 films were observed at room temperature (RT) and the corresponding mechanisms were discussed.

Keywords- Ga2-2xIn2xO3 films; MOCVD; Photoluminescence

I. INTRODUCTION

Intense interest has been paid to wide band gap semiconductors in recent years owing to their potential applications in short-wavelength luminescent devices, such as blue and UV light-emitting diodes and laser diodes. In the past, several binary oxides such as In2O3, SnO2, ZnO, MgO, and Ga2O3 and multicomponent oxides consisting of a combination of them have been investigated extensively [1-7]. It should be emphasized that band gap engineering is one of the key issues for constructing various electronic and optical devices using compound semiconductors. By alloying ZnO with MgO, the band gap of the ternary MgZnO alloys can be tuned from 3.3 eV to 4.65 eV and higher [8,9], which facilitates the fabrication of heteroepitaxial ultraviolet light emitting devices based on ZnO. Both In2O3 and Ga2O3 are good transparent oxide semiconductors (TOSs) with direct band gap energy of 3.7 and 4.9 eV, respectively. Ga2-2xIn2xO3 is considered to be an alloy of In2O3 and Ga2O3. The band gap of Ga2-2xIn2xO3 could be tuned from 3.7eV to 4.9eV by controlling the composition of the films [10]. Hence, modulation of the band gap is one of the keys to lift this material to higher potential in heteroepitaxial ultraviolet light emitting devices.

In the former literature, transparent conducting thin films of doped GaInO3 deposited by pulsed laser deposition (PLD) and the phase diagram in β-Ga2O3-In2O3 system as well as the optical properties of Ga2O3-based (InxGa1-x)2O3 thin films grown by molecular beam epitaxy (MBE) method have been

reported [11-14]. However, to our knowledge, little work has been published on the structural and PL properties of Ga2-

2xIn2xO3 films depending on the composition, especially by means of MOCVD. In this paper, Ga2-2xIn2xO3 films with different In content prepared on α-Al2O3 (0001) by MOCVD technique are reported. The structural and photoluminescence (PL) properties for the obtained films were investigated.

II. EXPERIMENTAL The Ga2-2xIn2xO3 films with a series of compositions

were prepared on α-Al2O3 (0001) substrates by an MOCVD system. Trimethylindium [In(CH3)3] and trimethylgallium [Ga(CH3)3] were used as organometallic (OM) source. And the OM vapors were delivered to the reactor by ultra high purity nitrogen (99.9999999% in purity). High purity O2 was injected using a separate delivery line into the reactor. Table 1 presents detailed deposition parameters of the procedure.

The structural properties were determined by a

RIGAKU D/MAX-γB X-ray diffractometer (XRD) with Cu Kα radiation. Compositions and thickness of the films were determined by Rutherford backscattering spectrometry (RBS) using 2MeV He2+ ion beam. High-resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED) with a Technai F30 transmission electron

Project supported by the National Natural Science Foundation of China (No. 50672054).

TABLE 1. DEPOSITION PARAMETERS OF GA2-2XIN2XO3 FILMS BY MOCVD

Substrate temperature 700 oC

Working pressure 50 Torr

OM precursor

In(CH3)3 Temperature 28˚C

Source supply 12~50 sccm

Ga(CH3)3 Temperature -14˚C

Source supply 5.6~20.2 sccm

O2 flow rate 50 sccm

Growth time 120min

978-1-4244-6554-5/11/$26.00 ©2011 IEEE

microscope operated at 300 kV were used to study the microstructure of the films. The optical transmittance measurements were performed using a Shimadzu TV-1900 double-beam UV-vis-NIR spectrophotometer in the wavelength range of 200-800nm. The PL spectra of the films were measured with an SP-2500i fluorescence spectrometer. Pulsed radiation with 266 nm wavelength is generated by a Ti-sapphire laser (Mira 900-F, Coherent Lasers) pumped with a solid-state laser (Verdi 6, Coherent Lasers).

III. RESULTS AND DISCUSSION The X-ray diffraction patterns of the films with different

In content are depicted in Fig. 1. For the film deposited with x=0.2, as exhibited in Fig. 1(a), the α-Al2O3 (0006) substrate diffraction peak located at 2θ =41.7° along with a single diffraction peak corresponding to Ga2O3 (401) reflection of monoclinic structure are observed. No extra peaks corresponding to In2O3 structure have been detected from the XRD pattern. This indicates that the obtained Ga1.6In0.4O3 film exhibited the structure of β-Ga2O3. For the film deposited with x=0.4 [Fig. 1(b)], three diffraction peaks corresponding to In2O3 (222) and (400) reflections of body-centered cubic (bcc) structure and β-Ga2O3 (401) reflection are observed, which implies that two crystal structures coexist. It should be noted that the location of the Ga2O3 (401) diffraction peak shifts to low θ direction compared with the Ga1.6In0.4O3 film. This result indicates that the interplane spacing of Ga2O3 (401) increases when elevating the In content. Because the ionic radius of Ga3+ (0.62 Å) is smaller than that of In3+ (0.81 Å), when the In content of the film increases, more In3+ ions come into the Ga2O3 lattice resulting in the increase of the (401) interplane spacing. Similar results have been observed by Minami et al. [6]. For the higher In content films (x=0.6 and 0.8), as exhibited in Fig. 1(c) and (d), only a single diffraction peak located at about 2θ=30.8° corresponding to bcc-In2O3 (222) plane is observed. The peak locations of the two samples shift to a higher angle compared with the standard diffraction spectra of indium oxide JCPDS no. 06-0416. The offset of the diffraction peak suggests that some In3+ ions in the lattice are replaced by Ga3+ ions. To sum up, as the In content increases, the structure of the obtained films changes from β-Ga2O3 structure to mixed-phase form, and finally to bcc-In2O3 structure.

Fig. 2 shows the cross-sectional HRTEM image of the interface area between the α-Al2O3 substrate and the Ga1.6In0.4O3 film. From Fig. 2, it can be seen that the obtained film is polycrystalline with β-Ga2O3 structure.

3 0 4 0 5 0 6 0 7 0 8 0

α − A l2 O 3 ( 0 0 0 6 )

Inte

nsity

(a.u

.)

2 θ (d e g r e e )

x = 0 .2

a

Ga 2O

3 (401

)

α − A l2 O 3 ( 0 0 0 6 )

x = 0 .4

b

Ga 2O

3 (401

)

In2O

3 (400

)

In2O

3 (222

)

In2O

3 (222

)

x = 0 .6

c

x = 0 .8

In2O

3 (222

)d

Fig. 1. XRD spectra of Ga2-2xIn2xO3 films with different In content x: (a) 0.2,

(b) 0.4, (c) 0.6 and (d) 0.8.

Fig. 3 gives the typical RBS of Ga2-2xIn2xO3 films with enactment values x = 0.2, 0.4, 0.6 and 0.8, respectively. The In and Ga content in Ga2-2xIn2xO3 films can be estimated by the RBS [15]. The calculated values of In content in Ga2-2xIn2xO3 samples a, b, c and d are about 0.37, 0.65, 0.81 and 0.92, respectively, which are larger than the experiment enactment values. This is because that Ga is less reactive and more resistive to oxidation compared to In during the growth process.

Fig. 2. HRTEM image of the interface area between the film with x=0.2 and the sapphire substrate with corresponding SAED pattern in inset.

200 400 600 800 1000

RB

S Yi

eld

(arb

. uni

ts)

C hannels

In

G ad

x=0 .8

In

G a

c

x=0 .6

In

G ab

x= 0 .4

a

H InH G a

InG a

x=0 .2

Fig. 3. RBS of Ga2-2xIn2xO3 films with different indium content x: (a) 0.2, (b)

0.4, (c) 0.6, (d) 0.8.

The optical transmittance spectra of Ga2-2xIn2xO3 films with different In content are shown in Fig. 4. The thickness of the films is in the range of 70 to 95nm measured by RBS. It is exhibited that the average transmittance of all the samples in the visible range exceed 90%. The optical gap (Eg) of the films (0.1≤x≤0.9) determined from the spectra are summarized in Table 2. It can be seen that the Eg of the Ga2-

2xIn2xO3 films varies with increasing Ga content from 3.72eV to 4.58eV.

200 300 400 500 600 700 8000

20

40

60

80

100

x

Wavelength (nm)

Tran

smis

sion

( %

)

Ga2(1-x)In2xO3

0.2 0.4 0.6 0.8

Fig. 4 Transmittance spectra of Ga2-2xIn2xO3 films with different In content.

TABLE 2. THE IN CONTENT X AND THE BAND GAPS OF INVESTIGATED SAMPLES CALCULATED FROM OPTICAL TRANSMISSION SPECTRA.

In content x 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Eg (eV) 4.58 4.33 4.25 4.09 4.02 3.93 3.87 3.78 3.72

Fig. 5 shows the PL spectra of the Ga2-2xIn2xO3 films with different In content measured under shorter UV light irradiation (266nm, λex) at room temperature. From Fig. 5a, an UV PL peak around 338nm (3.67eV) is observed and the intensity of the peaks decreases with decreasing the In content x from 0.9 to 0.5. The obtained films are n type semiconductor and the value of 3.67eV is smaller than the bandgap energy of the samples, so the origin of this UV emission is attributed to the electron transition from the donor levels to valance band. The intensity of the UV emission decreasing with In content decrease is ascribed to the degradation of the crystal structure of the films.

For the lower In content (x<0.5) samples, as exhibited in Fig. 5b, a sharp UV PL peak located at 333nm (3.72eV) is observed, and the relative intensity of the peak rises with decreasing the In content. The UV PL properties for β-Ga2O3 nanowires and nanobelts have been previously reported by Zhang et al. [16] and Gundiah et al. [17]. Binet and Gourier [18] have put forward a UV PL model of the β-Ga2O3 single crystals. They thought that the electrons on the donors could be detrapped to conduction band; the holes on the acceptor can be detrapped to form hole acceptors via the valence band at the same time. The electron and the hole recombine via a self-trapped exciton to emit the UV photon, and this process depends on the temperature consumingly. The donor would be formed by O vacancy and the acceptor would be formed by Ga vacancies or gallium–oxygen vacancy pairs [19]. In our case, the structure defects such as O vacancies (VO), Ga vacancies (VGa) and Ga-O vacancy pairs (VO, VGa) could be produced during the films growth. We attribute the PL peak of the samples (x<0.5) to the donor-acceptor pair transition in the Ga2O3 grains. The enhancement of the peak intensity with decreasing the In content is due to the increase of Ga2O3 grains and the improved polycrystalline structure of the resulting films when elevating the Ga content.

The PL spectra of the Ga1.4In0.6O3 film measured at different temperatures are shown in Fig. 6. As the measurement temperature decreases from RT to 120K and 13K, the UV peak has a blue shift of 4nm (45meV) and 5nm (57meV), respectively. The band gap of the film gets wider as the temperature decreases, which leads to the blue shift of the UV peak.

200 300 400 500 600 700 800

2.0x10-5

4.0x10-5

6.0x10-5

8.0x10-5

1.0x10-4

1.2x10-4

1.4x10-4

λex= 266nm

x

Ga2-2xIn2xO3

Inte

nsity

(a.u

.)

Wavelength(nm)

0.9 0.8 0.5

a338nm

200 300 400 500 600 700 800

0.0

2.0x10-4

4.0x10-4

6.0x10-4

8.0x10-4

1.0x10-3

Ga2(1-x)In2xO3

In

tens

ity (a

.u.)

Wavelength(nm)

0.4 0.3 0.2

λex= 266nm

x

b333nm

Fig. 5 Room temperature PL spectra of Ga2-2xIn2xO3 films with different

In content.

300 400 500 600 700 800

c3 2 8n m

W avelength (nm )

Inte

nsity

(a.u

.) 1 3K

b3 29 n m

12 0K

R T

3 3 3 n ma

Fig. 6. PL spectra of the Ga1.4In0.6O3 film measured at (a)RT, (b)120K and

(c)13K, respectively.

IV. CONCLUSIONS Ga2-2xIn2xO3 films were deposited on α-Al2O3 (0001)

substrates by MOCVD. The structure of the deposited films changed from β-Ga2O3 structure to mixed-phase form, and finally to In2O3 structure with increasing the In content. The

average transmittance of the films in the visible range exceeded 90%. The band gap was tuned from 3.72 eV to 4.58eV as the Ga content (1-x) increased from 0.1 to 0.9. UV emissions located at 338nm (x≥0.5) and 333nm (x<0.5) for the Ga2-2xIn2xO3 films were observed at RT. The UV PL peak near 338nm was originated from the donor-valance band transition for the Ga2-2xIn2xO3 films with In2O3 structure and the 333nm PL peak for the higher Ga content samples can be ascribed to the donor-acceptor pair transition in the Ga2O3 grains. These results indicate that Ga2-2xIn2xO3 films may have potential for the ultraviolet light emitting materials.

ACKNOWLEDGMENT This work is financially supported by the National Natural

Science Foundation of China (Grant No. 50672054).

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