Synthesis and Characterization of Gallium nitride (GaN) thin films

Manipal Research Colloquium 2015 (TS-31)

Synthesis and Characterization of Gallium nitride (GaN) thin films deposited by Low pressure chemical vapor deposition (LPCVD) technique. Umesh Rizal1, Bhabani S. Swain2, Bibhu P. Swain1* 1.Nano-Processing Laboratory, Centre for Material Science and Nanotechnology, Sikkim Manipal Institute of Technology, Majitar, Rangpo, East Sikkim, India-737136 2. School of Advanced Material Engineering, Koomin University, Republic of Korea Email of corresponding author: [email protected] Abstract Gallium nitride (GaN) thin films have attracted increasing attention due to their great prospects in the fundamental Physical sciences and Nanotechnology applications. Thin films of GaN were grown by using Low pressure chemical vapor deposition (LPCVD) method on p-type Si substrate. A variety of techniques, including Scanning Electron Microscope (SEM), Raman spectroscopy, Fourier Transform Infrared (FTIR) Spectroscopy, X-ray photoelectron spectroscopy and Photoluminescence were used to characterize the grown materials. The vibrational signatures from FTIR are found at 458 cm-1, 515 cm-1, 514 cm-1, 567 cm-1, 611 cm-1, 671 cm-1, 740 cm-1 for zone boundary phonon, A1 (TO), E2 (high), GaN-C bond, SO Phonon, and A1 (LO) modes. Apart from these, Raman active phonon modes appeared at 365 cm-1, 519 cm-1, 623 cm-1 corresponding to acoustic overtone, A1 (TO), A1 (overtone) modes. The synthesized GaN Thin film shows a pronounced and broad exciton peak at 2.85 eV in photoluminescence spectrum. The role of H2 flow rate on GaN thin films revealed decreased deposition rate; however, surface roughness has been increased resulted into formation of nanostructure thin films. Keywords: GaN thin film, SEM, FTIR, Raman, PL, and XPS. 1. Introduction. Gallium nitride (GaN) is a direct wide band-gap (3.4 eV) semiconductor material, which has been intensely studied over the last decade due to their unique structures, properties and great potential application in the areas of microelectronic and optoelectronic devices such as light emitting diodes and field-effect-transistors [1]. Many research efforts have been allocated to analyze, Microstructural, Vibrational, Optical and Electromechanical property of GaN thin film. However, such property of H2 assisted CVD synthesized GaN thin films have rarely studied. To date, several synthesis methods were employed for the synthesis of NH3 or N2 assisted GaN thin film by MBE [2], Laser ablation [3], APCVD [4], LPCVD [5], PECVD [6], MOCVD [7], HWCVD [8]. Here, we have made an attempt to optimally synthesize GaN thin film by LPCVD method. The advantage of employing LPCVD comes from two aspects: a) It facilitates the deposition of materials via chemical process during which a chemical precursor is delivered at a controlled temperature and pressure. b) Absence of deleterious electrons, ions and surface charges. Wang et al. investigated the structure and surface effect of field emission from GaN thin film deposited under N2/H2 atmosphere by HWCVD [9]. Wang et al. successfully synthesized wurtzite GaN NWs on silicon substrate using GaN powder, Nitrogen and Hydrogen as a reactive gas by PE-HFCVD [10]. In this work, we focus on detailed

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investigation of Microstructural and Vibrational properties of H2 diluted GaN thin film deposited on p- type Si substrate. 2. Experimental detail. GaN thin film were deposited by Low pressure chemical vapour deposition (LPCVD) system using GaN powder as a Ga source and N2, H2 as reactive gases. The process parameter is given in Table 1. The substrates were cleaned using an ultrasonic treatment in acetone, etched by a 2% hydrogen fluoride solution for 2 minute, rinsed in deionised water and dried on N2 gas prior to loading into the CVD system. Thereafter, the cleaned Si substrate was placed inside a Low pressure chemical vapor deposition (CVD) system. After deposition, the samples were handled carefully to avoid moisture and oxidation from the environment. Briefly, there is a heating system constructed with one long ceramic tube covered with heating blanket in the CVD chamber, which is heated about 1050oC for the heating of the substrate and the decomposition of the reaction gases and the GaN powder. Films were deposited on ptype Si substrate to facilitate characterisation by Field Emission Scanning Electron Microscopy (FESEM), Raman spectroscopy, and Fourier Transforms Infrared Spectroscopy (FTIR). The microstructure of GaN thin film was observed by a Field Emission scanning electron microscopy (FE-SEM, JEOL-JEM-3000F) in low as well high resolution modes. The Raman spectrum was recorded at room temperature with an optical microscope using the 488 nm line of an Ar+ laser as excitation source. A long-working-distance 50x laser was employed in the confocal microscope to focus and collect the laser light with a spatial resolution of about 1–2 μm. The laser power on the spot of GaN thin film sample was 1 mW to avoid laser crystallization during analysis. FTIR spectra were collected with a Fourier Transform Infrared Perkin Elmer 5000 spectrometer (model: spectrum two) operating in transmission mode between 450- 1500 cm-1 with a resolution of 4 cm-1. The deconvolution of Raman and FTIR spectra was carried out by an origin 6.0 computer program. Table 1: Deposition parameter for preparation of GaN thin film samples. Precursor gases N2 = 20 sccm H2 = 10-40 sccm o Processing temperature (Tp) ( C) 1050 H2 dilution (sccm) 10-40 Process pressure (mtorr) 2 X 10 -2 Deposition time (min) 180 Filament to substrate distance (cm) 8 3. Result and discussion. Deposition rate of LPCVD deposited thin films is lower than the APCVD deposited thin films. However, high quality GaN thin films are deposited by proper optimization of processing temperature, gas flow rates and process pressure. 3.1. Microstructure of GaN Thin Film. Figure 1 shows the FESEM images of GaN thin film synthesized at different H2 flow rate while keeping N2 flow rate remaining constant. H2 dilution is important parameter to modify chemical network in GaN Thin film while N2 gas enrich the thin film with much needed to maintain nitrogen species.

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As can be seen in Figure 1 (a), first, surface morphology of thin film is rough. The low flow rate of hydrogen gas results in a slow rate of decomposition of GaN. Thus due to low growth rate, the Ga and N atoms in the thin film cannot be well arranged. Secondly, there are small grains in the film which grows with 20 sccm H2 flow rate. This indicates that the mobility of Ga atoms is not large enough to make the grains grow large, so the crystallite size is limited by the diffusion length of Ga atoms. Figure 1 (b-c) shows husky and granular microstructure that was grown at higher H2 flow rates which indicates surface structure are fractal in nature. Here, the growth of GaN thin film was accelerated due to the fast motion of Ga and N ions under the high flow rates of H2. It can be seen that the crystalline size is larger than that of films shown in Figure 1 (a) this is because the mobility of Ga atoms becomes larger with the increasing H2 flow, thus it is possible to form larger grains. In the same way, the grains shown in Figure 1(b-c) are much larger than those in Figure 1(a) due to the higher hydrogen flux. The grain size of the films is found to be about 200 nm. H2 makes bond with every particle in film, thus forming numerous grains [11]. Therefore, well-defined grains attribute the faster hydrogenation. The rough surfaces offer active sites to attach hydrogen to each flake and enhance the hydrogenation capacity. The H2 dilution plays vital role in following aspects of thin films: a) Passivates the dangling bonds at the growing surface. b) Increase the surface mobility of ad-atoms. c) Reduced the film deposition rate but it increase surface roughness. d) In hydrogen environment, GaN has a low decomposition temperature and oxidation of Ga can be restrained, thus hydrogen was used in the growth process of thin film [12].

Figure 1.FESEM of GaN Thin Film at H2 flow rates of (a) 20 sccm, (b) 30 sccm (c) 40sccm. 3


200

A1 (overtone)

A1 (TO)

Acoustic overtone

ZB Phonon

Intensity (real unit)

3.2 Raman spectroscopy. Fig.2 (a) shows Raman spectra of GaN thin film network with different Hydrogen flow rates. Due to wide bandgap, the Raman scattering study of GaN thin film is easily performed using visible lines of Ar+ laser. The strong bond in GaN and light nitrogen atom results in high phonon energies limiting the range of possibly observable local vibrational modes of impurities to even lighter elements at higher frequencies. As seen from the Raman spectra, the various peaks centered at 292 cm-1, 400 cm-1, 528 cm-1, 624 cm-1, 673 cm-1 respectively [13]. All observable Raman modes are affected by strain resulting in shifts of the phonon modes. The band at 292 cm-1 corresponds to zone boundary phonon [14]. The vibrational band at 400 cm-1 corresponds to acoustic overtones [15]. The phonon vibration at 528 cm-1 corresponding to A1 (TO) [16]. The vibrational band spectra at 624 cm-1 and 673 cm-1 corresponds to A1 overtone and surface optical phonon of GaN thin film [17]. The major observations in Raman characterization of GaN thin film are follows: (a) The Raman modes at 292 and 400 cm-1 are assigned to the zone boundary phonon activated by crystal imperfection and acoustic overtone of wurtzite GaN. (b) The presence of intense A1 (TO) mode confirms the phonon scattering from all the facets of the thin film. A slight red shift with respect to the literature values in most cases indicates that there is residual strain in the thin film. (c) The peaks at 624, 673 cm-1 corresponds to surface optical phonons modes indicate appreciable surface effects spread out from thin film.

40 sccm 20 sccm 10 sccm 30 sccm

S.O. Phonon

400 600 -1 Wavenumber (cm )

800

Figure 2.Raman spectra of GaN Thin film deposited on Si substrate with different H2 flow rates. For the case of simplicity, we further deconvoluted A1 (TO) phonon mode into two peaks based on the assumption that each peak consists of the Guassian / Lorentzian sum function. The deconvolution of GaN phonon modes are also described by Wei et al. [18]. The A1 (TO) peak intensity increases monotonously with increasing hydrogen dilution indicating modification of vibration network in GaN network with the presence of hydrogen atom. It is worth to point out that, with increase in H2 4


dilution, A1 (TO) peak consistently shifted to lower wavenumber region. This could be due to (a) H2 dilution suppresses the amorphous fraction and increases the crystalline fraction (b) the change of bonding configuration of hydrogen environment around Gallium Nitride network. However, the relatively decrease of nitrogen content and increase of more electropositive hydrogen atom might shifts peak towards lower energy. We found that the Full width half maxima (FWHM) decrease from 21.341 cm-1 to 20.63 cm-1. This is the combined effect of increase in hydrogen content and structural changes takes place among H – N- Ga network. Moreover, the decreasing of full width half maxima (FWHM) of A1 (TO) phonon band with increasing of H2 flow rate indicated enhancement of Microstructural network in the film.

A1 (LO)

E2 (high)

A1 (T0)

ZB

% Transmittance (real unit)

3.3 FTIR Spectroscopy. To reveal the Ga – N chemical bonding configuration in the film, FTIR spectroscopy was performed. Fig. 3 (a) shows the different vibration spectra of a GaN thin film after base line correction. The major vibration bands are observed in the range of 400 – 800 cm-1. The vibrational signature is at 465 cm-1 corresponds to zone boundary phonons [19]. The vibration signature at 515 cm-1 is corresponding to A1 (TO) match with previous research groups [20]. The vibration from 566 cm-1 generally appeared at the Raman signatures. However in our case we have found out as in FTIR signature. The vibrational signature at 566 cm-1 is corresponding to E2 (high) match with various groups [21]. The vibration signature at 610 cm-1 is corresponding to attachment of carbon atom to Ga-N bond in the GaN thin film [17]. The vibration signatures at 739 cm-1 corresponds to A1 (LO) of the GaN network [22].

GaN-C

450

550 650 -1 Wavenumber (cm )

10 sccm 20 sccm 40 sccm 30 sccm

750

Figure 3.FTIR spectra of GaN Thin film deposited on Si substrate with different H2 flow rates. The major observations in FTIR characterisation of GaN thin film are follows: (a) The intensity of A1 (LO) is higher than that of the E2 (high) mode. This anomaly arises may be due to surface roughness of thin film associated with their crystallinity.

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(b) The intensity of vibrational band centered at 515 cm-1 does not show appreciable gain in its position and intensity with increase in H2 dilution. This inert behavior of phonon may be due to passivation effect from incorporated hydrogen environment. (c) The presence of zone boundary phonon at 465 cm-1 activated by surface disorders and internal stress in the film. (d) The intense peak at 610 cm-1 increases consistently with increasing H2 dilution. This may be due to high electron affinity of carbon atom with electropositive gallium atom. The inherent limitation of thermal CVD might be the source of carbon contamination at high temperature.

% Transmittance

Raw data Fitted Area N-Ga-H stretching Ga-N stretching(A1 (LO)) H-Ga-Ga stretching Ga-N- H stretching

700

720

740

760

780

800

-1

Wavenumber (cm ) Figure 4. Deconvuluted FTIR spectrum of the GaN thin film between 700 – 800 cm-1 associated with N- Ga-H,Ga – N, H- Ga-Ga, Ga- N- H), phonon stretching mode of GaN network. Fig. 4 shows deconvolution of A1(LO) peak of GaN thin film for 30 sccm H2 flow rates. For the case of simplicity, the spectra could be decomposed into four or five peaks based on the assumption that each peak consists of the Guassian / Lorentzian sum function. The peak positions for different chemical bonding are appeared at 720.28 cm-1, 736 cm-1, 755.46 cm-1, 775.06 cm-1corresponding to N–Ga-H stretching, Ga-N stretching [23, 13], H-Ga-Ga stretching, Ga-N-H stretching respectively. Being lightest atom, H2 will occupy most of the silicon substrate prohibiting nitrogen atom to stay close with gallium atom. Therefore bond length of Ga – N bond increases. Hence the A1 (LO) peak intensity increases monotonously with increasing hydrogen dilution. With low H2 flow rate, the FWHM decreases while in high flow rate it increases from 18.433 to 18.445. This gives us clear cut indication of increase of bonding attachment of Ga-N with other atoms. Therefore, increase of hydrogen content in the thin film increases the microstructural network in GaN thin film.

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3.4 Photoluminescence analysis. The GaN films were grown by LPCVD on Silicon substrate shows broad emission peak from 2.4 to 3.1 eV. The observed peaks may have originated due to defects and impurities introduced in the middle of band gap [24]. The PL spectrum shows an emission peak centered at 2.85 eV with a FWHM of 0.46 nm for room temperature measurement. The resulting film exhibit a red shift in the optical band gap relative to bulk GaN (3.4 eV). It may be explained by quantum confinement model [25].

Intensity (real unit)

Excitation at 320 nm H2 flow rates 10 sccm 20 sccm 30 sccm 40 sccm

2.0

2.2

2.4

2.6 2.8 3.0 Photon energy (eV)

3.2

Figure 5. Room Temperature PL spectra of GaN Thin film deposited on Si substrate with different H2 flow rates. Both the optical excitation and recombination take place in the nanometer grain, and the energy gap of the grain is enlarged due to quantum confinement effect. Existence of near band gap emission (NBE) can be assigned to the presence of Ga and N vacancies, deep level impurities and structural defects. 3.5 X-ray Photo electron spectroscopy Fig 5: shows the general scan in the binding energy ranging from 0 eV to 1000 eV and the core orbital spectra of Ga, C, N, and O with XPS peaks at the location of Ga (3d), N (1s), O (1s) and C (1s) are found at 22 eV, 400 eV, 532 eV, 285 eV respectively.

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4000 3500 N(1s)

Counts/sec

3000

C(1s)

O(1s)

2500

OKLL

Ga(3d)

2000 1500 1000 500 0

200

400

600

800

1000

Binding Energy (eV) Figure 5.Broad XPS spectra of GaN Thin film deposited on Si substrate. The width and slight asymmetry of the N (1s) peak are attributed to N-H2 formation due to the interaction between GaN and H2 at the GaN film surface. The elements of C and O arise from the surface pollution of the sample. The O1s peak centered at 532 eV. Generally, the O1s peak had been observed in the binding energy region of 529535 eV, and the peak around 529-530 eV is ascribed to lattice oxygen [26]. The percentage of composition of the GaN are found out by using ⎛ %=⎜

∑

⎞ ⎟ 100

⎝ ⎠ Where Ai is the area under core orbital spectra of Ga (3d) and N (1s), Si is the atomic sensitivity factor of Ga and N are 0.31 and 0.42 respectively. The above calculation indicates that the composition of Ga and N are 57.5 % and 42.5% respectively. 4. Conclusions GaN thin films with large grain were synthesized with different H2 flow rate. An attempt has been made to study the microstructure and vibrational properties of GaN thin film in H2 environment. The FTIR vibrational signatures of GaN bonds are found at 465 cm-1, 515 cm-1, 566 cm-1, 739 cm-1 for zone boundary phonon, A1 (TO), E1 (TO), and A1 (LO) modes. Raman spectra confirmed the surface optical phonons are at 648 cm-1of GaN thin films. Apart from these, other phonon modes also appeared at 292 cm-1, 400cm-1, 528 cm-1 and 624 cm-1 corresponding to zone boundary phonon, acoustic overtone, A1 (TO), surface optic (SO) Phonon. 5. Acknowledgement This work has been carried out under a (DBT and DST) funded project, Govt of India.

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