Thermal oxidation of amorphous GaSe thin films

Accepted Manuscript Thermal oxidation of amorphous GaSe thin films T. Siciliano, M. Tepore, A. Genga, G. Micocci, M. Siciliano, A. Tepore PII:

S0042-207X(12)00505-2

DOI:

10.1016/j.vacuum.2012.12.001

Reference:

VAC 5911

To appear in:

Vacuum

Received Date: 17 September 2012 Revised Date:

30 November 2012

Accepted Date: 1 December 2012

Please cite this article as: Siciliano T, Tepore M, Genga A, Micocci G, Siciliano M, Tepore A, Thermal oxidation of amorphous GaSe thin films, Vaccum (2013), doi: 10.1016/j.vacuum.2012.12.001. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Thermal oxidation of amorphous GaSe thin films T. Sicilianoa*, M. Teporea, A. Gengab, G. Micoccia, M. Sicilianob, A. Teporea Dipartimento di Beni Culturali, Università del Salento, 73100 Lecce, Italy

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DiSTeBA: Dipartimento di Scienze e Tecnologie Biologiche e Ambientali, Università del Salento,

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73100 Lecce, Italy

ABSTRACT

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In this work the results of the thermal oxidation of GaSe thin films in air at different temperatures are presented. The structural and morphological characteristics of the thermally annealed products were studied by X-ray diffraction (XRD), Raman spectroscopy and scanning electron microscopy (SEM). The as–deposited GaSe films were amorphous and they transformed into polycrystalline GaSe films with a hexagonal crystal structure at a temperature around 400 °C.

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Thermal oxidation at 650 °C resulted in the formation of mixed Ga2Se3 and Ga2O3 compounds both in the monoclinic phase. At higher temperatures, Ga2Se3 disappeared and complete oxidation of the initial compound occurred. The optical energy gaps of products were determined at room

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temperature by transmittance measurements using UV–vis–NIR spectroscopy.

Keywords: Characterization; Thin film; Gallium oxide.

*corresponding author: Dott.ssa Tiziana Siciliano e-mail: [email protected] tel. +39 0832 297073 fax +39 0832 297062

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1. Introduction III-VI layered semiconductors such as GaSe, GaS and InSe have highly anisotropic electrical, optical and mechanical properties. Their high anisotropy arises from the fact that the layer

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interaction is considerably weaker than the bonding force within the layer. It has been suggested that these compounds have the potential characteristics for the fabrication of optoelectronic devices [1-4]. The recent increasing interest in these compounds is also due to their prospects for the

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fabrication of various nanostructures including nanotubes, nanowires, nanobelts and nanorods [5-9]. Owing to the low density of dangling bonds on the surface of layered materials, they can be used as

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substrates for the formation of heterostructures based on such compounds. It is well known that oxidation is a common method used to obtain the formation of heterostructures. The oxidation processes of different III-VI semiconductors in single crystal form grown by the Bridgman technique have been described in earlier papers [10-14]. It was established that the oxidation of III– VI compounds is a technique that is not very reliable due to the complexity of the sequence of

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intermediate phases. For example, the experimental data of InSe oxidation show that In2(SeO4)3, In2Se3 and In2O3 intermediate phases sequentially appeared in the annealed samples along with the progressive temperature increase. Moreover, the oxidation of GaSe single crystal involves the

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formation of phases with a higher selenium content (Ga2Se3), while no other intermediate phases such as Ga2S3 were detected in the oxidation of GaS crystals.

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In this work, we study the evolution of the structural, morphological and optical features of the thermally evaporated amorphous GaSe thin films during their oxidation at different annealing temperatures in air atmosphere.

2. Experimental details GaSe films were deposited on unheated clean quartz substrates by vacuum thermal evaporation of polycrystalline GaSe in a Varian vacuum coating unit by using a molybdenum boat.

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The films thickness was about 250 nm as measured by the quartz crystal monitor and confirmed by a Tencor (model Alpha-Step 200) computerized surface profilometer. The deposition rate of the film was kept constant at about 0.3 nm/s. After deposition, thermal oxidation of films was carried

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out in air at different annealing temperatures (400, 650 and 900 °C) for six hours. The morphology of as–deposited films was analyzed with scanning electron microscopy (SEM) using a Tescan VEGA-LMU. The crystal structure was determined by X-ray diffraction

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(XRD) using a Rigaku MiniFlex diffractometer with Cu-Kα radiation (λ ~ 1.54056 Å). The 2θ range used in this measurement ranged from 10° to 80° in steps of 0.02° with a count time of 1 s. Raman

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spectra were recorded by a Renishaw InVia Raman microscope in open air using standard backscattering geometry. Raman scattering experiments were carried out at room temperature by illuminating the annealed sample with a 514.5 nm Ar+ laser. The laser output power was fixed at 15 mW. The optical transmittance spectra were measured in the UV–vis–NIR regions using a Varian Cary 5 double-beam spectrophotometer with unpolarized light at normal incidence and at room

3. Results and discussion

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temperature.

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Fig. 1 shows the XRD patterns of the films annealed in air at temperatures of 400, 650 and 900 °C along with the one of the as–deposited sample. As it is evident in Fig. 1(a), the as–deposited

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film is amorphous because no peaks are observed in the XRD pattern. The XRD pattern of the film treated at 400 °C (Fig. 1(b)) shows five well defined diffraction peaks at 11.08°, 22.26°, 27.56°, 48.50° and 57.85° indicating the onset of phase transformation into the polycrystalline state of the sample. According to JCPDS card No. 37-0931, the observed diffraction peaks correspond, respectively, to the (002), (004), (100), (110) and (202) reflection planes of the hexagonal crystal structure of GaSe. The most intense peak corresponding to the (004) plane clearly indicates that a preferential c–axis orientation of crystallites. The sample thermally annealed in air at 650 °C exhibits a completely different structure, indicating that after the thermal treatment, the GaSe film is 3

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transformed into a new crystal structure. At this annealing temperature, the XRD pattern (Fig. 1(c)) shows two main peaks at 28.22° and 47.28° corresponding to the (13-1) and (13-3) reflection planes of monoclinic crystal structure of Ga2Se3 (JCPDS card No. 44-1012), indicating that GaSe

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transforms into Ga2Se3 phase. Other diffraction peaks corresponding to the planes (-202), (111), (311) and (-221) of monoclinic Ga2O3 (β-Ga2O3) are observed in XRD pattern (JCPDS card No. 431012). As these peaks are very weak, we may conclude that the β-Ga2Se3 is the dominating phase in

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the sample. Fig. 1(d) shows that the XRD pattern for the sample annealed at 900°C only consists of monoclinic Ga2O3 phase with diffraction peaks corresponding to (400), (-202), (-111), (111), (-

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311), (-112), (112), (510), (-313), (020) and (-712) reflection planes, indicating that at this temperature the Ga2Se3 is completely converted into Ga2O3.

Raman scattering measurements were carried out to further investigate the structure of the synthesized material. Figures 2(a-d) show the Raman spectra at room temperature of the as– deposited film and the same film annealed at different temperatures in air atmosphere. Fig. 2(a)

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shows the Raman spectrum of the untreated amorphous film, where two broad peaks at 136 and 254 cm-1 are evident. These two peaks trace their origins back to the strong modes between 130 and 260 cm-1 in the Raman spectrum of crystalline GaSe. The Raman spectrum of the sample annealed at

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400°C (Fig. 2(b)) contains four sharp lines at 132, 208, 251 and at 305 cm-1 and they are characteristic of the spectrum of the hexagonal ε-GaSe [15] which has the following Raman peaks:

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E’’ mode at 208 cm-1, E’(LO) mode at 250 cm-1 and A’1 modes at 132 and 304 cm-1. Raman peaks at 119, 155, 180, 247 and 286 cm-1 are observed for the sample annealed at 650 °C (Fig. 2(c)). The peak positions are in good agreement with those reported in the literature for the monoclinic crystalline phase of Ga2Se3 [16,17]. The modes at low (<200 cm-1) and at high frequency of the Raman spectrum can be assigned to folded acoustic and optical phonon modes in the zinc-blende structure of β-Ga2Se3, respectively. XRD spectra of the thin film annealed at 650 °C showed the presence of β-Ga2Se3 dominating phase, along with minor Ga2O3 phase. The typical micro-Raman spectrum of such a sample confirmed the presence of β-Ga2Se3 phase. This apparent discrepancy 4

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arises from the fact that, while micro-Raman spectroscopy can provide local information because of the shorter coherence length and time scale of the phonons [18], XRD yields structural information that averages over all the sample hit by the beam. Fig. 2(d) shows the Raman spectrum of the film

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annealed at 900°C where eleven peaks can be observed. The obtained data are in excellent agreement with the vibrational modes of β-Ga2O3 structures. β-Ga2O3 belongs to the C32h space group, which predicts 27 sets of optical modes at k = 0 given by Γ= 10Ag + 5Bg + 8Bu + 4 Au.

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Fifteen of them (Ag and Bg) are Raman active and 12 of them (Bu and Au) are IR active [19]. In particular, the Raman peaks recorded from Fig. 2(d) and reported in Table I well agreed with the

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experimentally observed frequencies reported in literature by Dohy et al. [20].

The absorption coefficient α of films was determined in the strong absorption region from transmittance measurements (T) using the expression T = exp(− αt ) where t is the film thickness. These absorption coefficient values were used to determine the optical energy gap. Fig. 3 shows the transmittance spectra recorded in the wavelength ranging 200–1500 nm as a function of annealing

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temperature. The top solid curve is the transmittance of the quartz substrate which indicates that the energy of the optical absorption edge of quartz is larger than that of the examined films. The fringes are due to the interference between the air–film and the film–substrate interfaces.

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According to the theory of interband absorption [21], the photon energy dependence of the absorption coefficient in the strong optical absorption region can be described by the following

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relation:

αhν = K (hν − E g )n

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where Eg is the optical energy gap of the investigated film, hν is the photon energy, K is a

parameter that depends on the transition probability and n is a constant that can take different values depending on the type of electronic transition involved. In accordance to Eq. (1), the usual method for determining the value of Eg involves a plot of (αhν )1 n versus photon energy hν. If an appropriate value of n is used to obtain a linear plot, the value of Eg will be given by the intercept 5

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on the hν axis. As it can be observed from Fig. 4, the photon energy dependence of α in the region of the absorption edge for the as–deposited film and for the film annealed at 400 °C is well fitted by the relation (1) with n = 2, while for the films annealed at 650 and 900 °C the optical absorption

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coefficient is well fitted with n = 1/2. The values of the optical energy gap Eg are reported in Table 2 together with the annealing temperatures and detected products. The optical gap of as–deposited film is in agreement with that reported by Ohyama [22] (1.8 eV) for amorphous GaSe film

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fabricated by radio-frequency (RF) magnetron sputtering in argon on fused quartz substrates at 80 °C. The energy gap of the sample annealed at 400 °C agrees with that obtained by Kepinska [23]

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(2.09 eV) for indirect allowed transitions of GaSe single crystals grown by the Bridgman– Stockbarger’s method. It is close to the value obtained by Thamilselvan et al. [24] (2.03 eV) for polycrystalline GaSe thin films prepared on glass substrates at 573 K by thermal heating method. According to the Mott and Davis [25] model, the unsaturated bonds in amorphous GaSe film are responsible for the formation of some defects in the material. Such defects produce localized

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states in the band gap near the mobility edge. As these defects are increased, the density of localized states increases and this fact is responsible for the low values of the optical gap Eg in case of the as– deposited GaSe films. During the process of thermal annealing, the unsaturated defects are

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gradually annealed out producing larger number of saturated bonds. The reduction in the number of unsaturated defects decreases the density of localized states in the band structure and consequently

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increases the optical gap.

The optical energy gap of the sample annealed at 650 °C agrees with that reported by Afifi et

al. [26] (2.65 eV) for direct allowed transitions of Ga2Se3 films obtained on glass substrates by thermal evaporation under vacuum of bulk material. This value also agrees with the one obtained by Rusu et al. [27] (2.56 eV) for Ga2Se3 films grown on glass substrates by means of the chemical close-spaced vapour transport (CCSVT) deposition technique using gaseous HCl/H2 as a volatilization and transport agent. Moreover, it is close to the value theoretically determined by Peressi et al. [28] for direct transitions (2,4 eV). The energy gap of the films thermally annealed at 6

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900 °C is in agreement with the value reported by Hao et al. [29] (4.75 eV) for no annealed and undoped Ga2O3 thin films deposited by spray pyrolysis on different substrates. It is close to the value reported by Zhang et al. [30] (4.83 eV) for direct allowed transitions of polycrystalline Ga2O3

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thin films deposited on quartz substrates by RF magnetron sputtering. Fig. 5 presents the surface morphology of the as–deposited film and of the films annealed at various temperatures in air atmosphere. It can be observed the as–deposited film shows a relatively

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smooth and two–dimensional surface without any cracks or pits on the surface (Fig. 5(a)), which is consistent with the XRD results. After annealing at 400°C (Fig. 5(b)), the surface morphology changes with the appearance of a grain-like structure. When the annealing temperature is 650 ºC,

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the films show a porous structure (Fig. 5(c)) with a maximum pore size of 0.015 µm2. On the other hand, the film treated at 900 °C shows a dense grain distribution characterized by an average size of about 50 nm (Fig. 5(d)).

According to the experimental results obtained in this work we retain the oxidation of GaSe

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films deposited through thermal evaporation is an evolutionary process consisting of the following stages. The as–deposited GaSe thin films are amorphous in nature. The amorphous structure of the as–deposited films on the unheated substrate is expected because the evaporating molecules

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precipitate randomly on the surface of the substrate and the following condensed molecules also adhere randomly, leading to disordered films of increased thickness. The loss of adequate kinetic

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energy for the precipitated molecule keeps them unable to orient themselves to produce the chain structure required for a crystalline structure. After thermal annealing in air at 400 °C for 6 h, the GaSe films are polycrystalline. No other Ga-Se phases, gallium or selenium oxides are observed in the corresponding X-ray diffraction and Raman spectra. When the annealing temperature is 650 °C, both Ga2Se3 and Ga2O3 polycrystalline products are present in the film due to the oxidation process of the GaSe according to possible reactions [10]: GaSe + 1 4 O2 = 1 3Ga 2 Se3 + 1 6 Ga 2 O3 GaSe + 3 4 O2 = 1 2 Ga 2 O3 + Se

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with subsequent evaporation of the volatile component. At higher temperatures the Ga2Se3 disappears because of the complete oxidation of the initial compound according to the possible reaction:

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Ga 2 Se3 + 3 2 O2 = Ga 2 O3 + 3Se

Our results are consistent with the thermodynamic analysis and experimental investigations reported in literature for GaSe single crystals thermally oxidized in air and checked by X-ray diffraction and luminescence methods [31, 32]. In particular, these works reported the formation

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Ga2Se3 and β-Ga2O3 phases on GaSe single crystal surfaces, at temperatures higher than 450 °C

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and 650 °C, respectively. In our work, we described the crystallization and oxidation processes of amorphous GaSe thin films thermally evaporated on quartz substrates. In particular, we described the effect of thermal annealing in air not only on GaSe thin film structure (by XRD and Raman spectroscopy) but also on its surface morphology (by SEM investigations) and optical properties

4. Conclusion

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(by UV-vis-NIR spectroscopy).

In summary, we have described the oxidation processes of GaSe thin films thermally

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evaporated on quartz substrates. XRD, Raman spectroscopy, SEM and optical analysis indicated

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that as-deposited samples had an amorphous structure. The heat treatment at the temperature of 400 °C in air atmosphere led to the formation of polycrystalline hexagonal GaSe films. No detectable traces of oxides were observed at this temperature. The additional annealing at 650 °C led to the formation of monoclinic Ga2Se3 and Ga2O3, where the Ga2Se3 phase was the dominating phase in the samples. At higher temperatures, the Ga2Se3 disappeared and complete oxidation of the initial compound occurred. No selenium oxides were observed under any of the oxidation conditions. The evolution of the optical band gap of the films was also estimated from optical absorption coefficient

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spectra. The band gap increased from 1.8 eV to 4.6 eV when the oxidation temperature increased from room temperature to 900 °C.

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Acknowledgements

The authors thank A.R. De Bartolomeo and G. D’Elia for their technical assistance during the

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measurements.

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Highlights Thermal oxidation of amorphous GaSe thin films deposited on quartz was studied.

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The oxidation at 400 °C led to the formation of hexagonal GaSe film.

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The oxidation at 650 °C led to the formation of monoclinic Ga2Se3 and Ga2O3 film.

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The oxidation at 900 °C led to the formation of monoclinic Ga2O3 film.

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The band gap increased with increasing temperature of oxidation.

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Figure captions

Fig. 1.

XRD patterns of GaSe thin films annealed in air for 6 h at different

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temperatures. The peaks A and B correspond to Ga2Se3 and Ga2O3 phases, respectively. Fig. 2.

Raman spectra of GaSe thin films annealed in air for 6 h at different

Fig. 3.

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temperatures.

Optical transmittance spectra of amorphous GaSe thin films annealed

Fig. 4.

Bad gap energy of amorphous GaSe thin films annealed at different temperatures.

Fig. 5.

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in air for 6 h at different temperatures.

SEM images of (a) as–deposited GaSe thin film and the films annealed in air atmosphere for 6 h at temperature of (b) 400 °C, (c)

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650 °C and (d) 900 °C.

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Table captions

Experimental frequencies (cm-1) and attribution of the peaks in Raman spectra of β-Ga2O3 thin film

Table 2

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Table 1

Optical energy gap and main detected phase of the GaSe thin films at

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different temperatures.

Expt. Data (cm-1)

Expt. Data (cm-1)

111 114 147 169 199 318 346 353 415 475 … 628 651 657 763

… 115 145 171 201 321 348 … 417 476 … 631 654 … 768

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This work

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Ag Bg Bg Ag Ag Ag Ag Bg Ag Ag Bg Ag Bg Ag Ag

Dohy et al.

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Mode simmetry

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Eg (eV)

Main detected phase

As-deposited

1.86

Amorphous

400

2.08

GaSe

650

2.66

Ga2Se3

900

4.64

Ga2O3

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Annealing Temperature (°C)