Pulsed Laser Deposition of In_0.1Ga_0.9N Nanoshapes by Nd:YAG Technique

Abstract

In_0.1Ga_0.9N thin film was grown on a cheap glass substrate by the Nd:YAG pulsed laser deposition technique. The In_0.1Ga_0.9N thin films show the semi-crystalline structure as observed with X-ray diffraction (XRD). The surface morphology has a non-dense layer with both scattered nanospheres and agglomerated particles. These nanospheres tended to grow randomly on the glass substrate, as observed with field emission scanning electron microscopy (FESEM). The direct bandgap energy for In_0.1Ga_0.9N thin film was 2.08 eV, which is calculated using photoluminescence (PL) measurements. The Raman measurements illustrated two sets of phonon modes as A1(LO) and E2 high vibrational modes that are observed. The resonance behavior of the A1(LO) mode is experimentally verified and studied under laser light energy of 532 nm.

1. Introduction

The III-nitrides, such as InN, GaN and AlN, have recently received great attention because of their major contribution to various optoelectronic applications such as light-emitting and laser diode applications [1]. The structures of these materials depend on the stoichiometry of the elements and preparation conditions [2]. The In_xGa_1−xN was driven by tailoring their bandgap according to the composition x [3]. The binary nitrides crystallize in the wurtzite (wz) structure and show the lowest direct optical transition across their fundamental band gaps of 0.7 eV (InN), 3.5 eV (GaN) and 6 eV (AlN) [4]. Thus, these alloys cover the electromagnetic spectrum from the infrared to the ultraviolet [5]. The In_xGa_1−xN thin films have been prepared by several techniques such as molecular beam epitaxy (MBE), hybrid vapor phase epitaxy (HVPE), and pulsed laser deposition (PLD) [6]. PLD is a highly non-equilibrium evaporation process, which can be operated at low temperature to increase indium incorporation [7]. Most of the In_xGa_1−xN targets used in PLD are fabricated by pressing the alloy powders at high pressure, followed by sintering at high temperatures [8]. From our previous work [9], In_xGa_1−xN bulk alloys have been prepared as a target by a simple novel method. III-nitride group has been deposited on sapphire (Al₂O₃), silicon, Sic and GaN which play an important role in the stoichiometry of the thin film substrates [10]. In this work, In_xGa_1−xN will be deposited for the first time by an Nd:YAG laser technique on glass substrate, giving a yellow emission that could be used in LED applications.

2. Material and Method

In_0.1Ga_0.9N film is grown on a glass substrate under a heating temperature of 250 °C in nitrogen plasma by an Nd:YAG laser technique. The target consists of In_0.1Ga_0.9N alloy mounted on a holder and it is prepared as illustrated in our previous work [11]. An Nd:YAG laser (Nd:YAG, Continuum Company pl9000, Boston, MA, USA) at λ = 532 nm was employed as the ablation source, and operated with a repetition rate of 10 Hz and pulse energy of 60 mJ. The working pressure was 1.13 × 10⁻⁴ Torr with the injection of N₂ plasma, and the deposition time was 10 min. During the deposition process, the target was rotated at 25 rpm. The ablation laser was directed onto the target to generate In_0.1Ga_0.9N vapor. The structure and morphology of the In_0.1Ga_0.9N thin film were systematically analyzed by X-ray diffraction (XRD-Shimadzu XRD 7000 maxima powder diffractometer, Kyoto, Japan), field emission scanning electron microscopy (FESEM-Quanta 250, Hillsboro, OR, USA), photoluminescence (PL-Perkin Elmer Luminescence Spectrometer Model LSS, Waltham, MA, USA) and Raman spectroscopy (Raman Senterra II, Karlsruhe, Germany).

3. Results and Discussion

Semi-crystalline nature is observed for the studied material as shown in Figure 1A. The thin film exhibits diffraction peaks located at different angles for (GaN, InN, and In_0.1Ga_0.9N). GaN is located at 2Ɵ = 35.14°, 35.94°, 36.98°, 38.56° [PDF CARD 01-080-0011, Cubic]; [PDF CARD 01-079-2499, Hexagonal], [PDF CARD 01-076-0703,Hexagonal]; [01-079-2499, Hexagonal]. InN is located at 2Ɵ = 29.33° [PDF CARD 01-088-2362, Hexagonal] and In_0.1Ga_0.9N is located at 2Ɵ = 32.39° (Hexagonal), 38.18° (Cubic), 38.81° (Cubic), which is matched with the results of other works [12,13,14]. In PLD, a pulse from the ablation laser is impacted with the In_0.1Ga_0.9N target which is dissociated, so In and Ga vapors were reacted with the nitrogen plasma, resulting in the formation of InN, GaN and In_0.1Ga_0.9N as a result of the reaction between indium and Gallium vapors [15]. The average crystallite size of the particles is 1.17 nm, which is calculated for 2Ɵ = 32° by Scherer’s equation [16]:

D = 0.9 λ/β cosθ

(1)

Figure 1B shows the Raman spectra peak at 437.41 cm⁻¹ for InN, which is slightly shifted from 431.41 to 437 cm⁻¹ [17]. A broad peak at 538.9 cm⁻¹ is observed, which is corresponding to GaN formation that is shifted from 533.9 cm⁻¹.Two other peaks are observed; one peak at 621.98 cm⁻¹ belongs to In_0.1Ga_0.9N (cubic), which is shifted from 617.98 cm⁻¹ [18] and the second peak at 694.94 cm⁻¹ belongs to In_0.1Ga_0.9N (cubic), which is shifted from 690.94 cm⁻¹ [19]. The broad peak of 746.2 cm⁻¹ belongs to In_0.1Ga_0.9N (hexagonal), which is shifted from 742.2 cm⁻¹ [19]. The shifting process of Raman peaks may be referring to the strain in GaN, InN and In_0.1Ga_0.9N samples [20].

Figure 2 shows the PL spectra, which present one strong emission peak of In_0.1Ga_0.9N centered at 596.22 nm with a band gap of 2.08 eV. The PL peak is observed in the yellow region. The deposited In_0.1Ga_0.9N (with energy bandgap of 2.08 eV) shows a red shift of the visible PL form the bulk target alloy (E_g = 2.65 eV) [9]. This shift may be related to application of the Nd:YAG technique. Through the deposition process, the bulk material is heated and transferred into vapor phase with the same composition, and the nucleation process is started, and crystal growth [21] occurs on the glass substrate but with disordered distribution of elements on sites of the parent lattice. This disorder distribution has a deep effect on the band gap, which caused this reduction in the band gap value from its bulk state [22]. Thus, the conduction band edge is pushed down while the valence band edge is pushed upward, which causes a reduction of the band gap from 2.65 to 2.08 eV. Additionally, the strain and relaxation may cause a mismatch between the deposited layer and substrate and have an effect on the energy gap of the deposited thin film [22].

Figure 3A shows the surface morphology of the In_0.1Ga_0.9N Bulk alloy (order distribution) [9] and deposited layer on glass substrate, with bright spots standing out against a darker background (disorder distribution). From our previous work [9], the bulk material had a nanowire structure with diameter of 69 nm and length of 341 nm. Moreover, the morphology of the film is not very dense, is free of cracks and has a relatively inhomogeneous distribution. The average grain sizes are in the range of 79.43 nm, which is determined by using image analyzer software. The film thickness is 94.7 nm. The difference between the XRD crystallite size calculated and FESEM may be due to the nature of the particles themselves that are agglomerated, having dense particles in some areas of the thin film.

The chemical composition of the film is examined using EDX analysis as shown in

Table 1. EDX results of In_0.1Ga_0.9N film.

EDX	In%	Ga%	N%
	20.34	70.85	8.80

. The weight percent of the composition of In_0.1Ga_0.9N is doubled to the calculated one. There is an excess of indium and a decrease of gallium, because the evaporation temperature of In is less than the evaporation temperature of gallium, which appears clearly in the difference between the large lattice mismatch between InN and GaN. This is confirmed by the XRD results according to the semi-crystalline nature of the thin film [23,24].

4. Conclusions

We reported the fabrication of In_0.1Ga_0.9N thin film using a Nd:YAG laser technique, which preserved a semi-crystalline structure with both hexagonal and cubic phases. FESEM images show agglomerated nanoparticles with an average grain size of 79.43 nm. PL concludes that there is a large emission peak with a distinctive yellow band. Raman spectra prove the strain of the In_0.1Ga_0.9N through the peak shift from 746.2 to 742.2 cm⁻¹. Future work will involve the use indium composition (x ≥ 0.2) in order to tolerate the In_xGa_1−xN band gaps, which are of importance for optoelectronic, sensor and photovoltaic applications.