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Abstract
Synthesis and characterization of gallium nitride (GaN) thin films via microwave plasma-assisted atomic layer deposition (MPALD) is reported in this research. The GaN thin films grown by this technique were amorphous or nanocrystalline as it was demonstrated by electron microscopy. The optical response of these GaN thin films showed a broad peak between 400 nm and 750 nm wavelengths due mostly to carbon and oxygen impurities as has been demonstrated by XPS. That emission from these GaN thin films is located at the border of white and yellow-green emission according to CIE 1931 chromaticity diagram with coordinates, x = 0.3491 and y = 0.4312.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
GaN has been relevant for optoelectronic applications such as LEDs, lasers, solar cells, and other [1,2]. Moreover, GaN-InN alloys has a few special properties that make it applicable for photovoltaic applications, namely; a tunable bandgap that ranges throughout the visible spectrum and internal piezoelectric fields that may help to separate carriers and lead to devices of higher efficiency. InGaN films has been synthetized by many techniques such as MOCVD, MBE and ALD [3]. Particularly, plasma-enhanced ALD had demonstrated several advantages over thermal ALD and other vapor phase techniques. In a MPALD process a plasma generates higher reactive species that can interact successfully with the surface of the substrate, allowing to expand into new experimental conditions for producing materials with a wide range properties [4]. Over the last few years, the synthesis of GaN at relative low temperatures (100–350°C) using Plasma-enhanced ALD have gained interest [4–8]. GaN is one of the most widely commercialized materials applied for color-tunable and solid state white-light lamps [9]. This work presents the research of GaN thin films deposited by means of a homemade MPALD. These GaN thin films show an emission located in the border of white and yellow-green region, making these kinds of GaN thin films feasible candidates for daylight white emission applications.
2. Experimental procedure
The MPALD system consists of a quartz growth chamber operating as a hot-wall reactor with a reaction temperature of 230 °C used to synthetize GaN. Figure 1 shows the schematic of homemade MPALD system. Precursor and purge gases are fed into the chamber at 100 and 30 sccm flow rates, respectively. Trimethylgallium (TMG) doses are controlled by opened lapse-time of the ALD-valves and temperature (−16 °C) of the container. The ALD cycles are performed using doses of 50 ms for TMG and 30 s for N2, each one alternated by an Ar purge of 15 s. The final GaN thin films were obtained after 1100 cycles achieved at a working pressure of 450 mTorr and 300 watts of microwave generator power. In order to improve the growth of GaN on silicon, the substrates were submitted to a pre-treatment of nitrogen radicals for 30 minutes before each deposition.
3. Characterization
The thicknesses of GaN samples used to perfom the saturation curve study of TMG were measured by a multi-wavelength rotating analyzer instrument model M-44 by J.A. Woollam Co., covering the 1.625 to 4.405 eV photon-energy range. The optical properties of the GaN thin films synthesized in this work were studied by cathodoluminescence (CL) carried out at room temperature in a JEOL JIB 4500 SEM with a MonoCL4 detector. The CL spectra were obtained using an acceleration voltage of 1 kV and a spot size of 50 µm. Thickness and crystalline structure were estimated on a cross-section of a GaN thin film sample by a JEOL JEM-2010 TEM at 200 kV. Chemical composition of the GaN thin films were determined using a PHOIBOS WAL electron energy analyzer XPS system equipped with a non-monochromatic Al Kα (1486.7 eV) X-ray sour ce provided by SPECS. The X-ray source was operated at 100 W and the analyzer at a constant pass energy of 20 eV. The spectra were calibrated with respect to the main C 1s peak located at 284.8 eV. An accurate XPS peak-fitting was performed to explore chemical species in the GaN compound [10,11].
4. Results and discussion
In order to demonstrate the self-limiting growth which is characteristic of the ALD technique, a saturation curve study for TMG dosing was performed by a series of short deposits (100 cycles). Figure 2 shows the optimum dosing time used on the deposition of GaN thin films which was obtained at 50 ms. The growth rate for GaN samples was found in the range from 0.5 Å/cycle using this homemade system.
The GaN thin films synthesized in this work showed only the luminescence attributable to the impurities within the thin films. Unlike the typical emissions of bulk GaN samples, the emission of these thin films did not present the near band-to-band emission, due to that the radiation by defects is much higher that the bandgap or the donor-acceptor pairs (DAP) emissions. Figure 3(A) shows the CL spectrum of the GaN thin films acquired in the range between 300 nm to 800 nm wavelength at room temperature. A broad emission, which covers the entire visible region from 375 nm to 750 nm has been observed. This wide peak has been attributed to several combination of emissions due to impurities in the GaN thin films. In order to analyze the internal signals that contribute to the optical response, an accurate peak-fitting was used employing Gaussian profiles, where the peak area, position and FWHM are treated as free variables. The best fitting shows that the CL spectrum is composed of four peaks where the contribution of each individual signal was quantified using the following equation:
Where $\%{C_{ie}}$ is the contribution percentage of the individual emission, ${A_t}$ is the total area under the broad curve, and ${A_{ie}}$ is the area under the individual curve. According to Reshchicov and Morkoc [12], the luminescence of GaN films, except that related to its band edge, can be associated to the presence of defects in the GaN lattice. As a result, the blue luminescence (BL), yellow luminescence (YL) and red luminescence (RL) emissions have been assigned to GaN unintentional doped with carbon and oxygen in different positions inside the crystal lattice. The percentage of the contributions were calculated as following: 42.3% for YL band (545–585 nm), 35.6% for GL band (485–520 nm), 19.8% for RL band (615–660 nm) and 2.2% for BL band (400–425 nm) from the CL spectrum. The color purity of emission is around 43% at 564 nm wavelength, which is far from pure yellow emission, and combined with other emission contributions it is more closely to white emission.Figure 3(B) shows the Comission Internationale de ĺÉclairage (CIE) 1931 chromatographic coordinates system of broad GaN luminescence and its individual emission. The emission of GaN is located in the border of white and yellow-green section of CIE 1931 color space [13]. The CCT of GaN emission was determined by the McCamy’s approximation [14]:
Where $n = ({x - {x_e}} )/({y - {y_e}} )$ is the reciprocal slope and $({{x_e} = 0.332,\; \; {y_e} = 0.186} )$ are the coordinates of epicenter of convergence. As a result, the CCT of GaN was obtained at 5061 K. The Color Purity (CP) of the broad emission was calculated using the standard source C (0.3101, 0.3162), the color coordinates of GaN and, the dominant wavelength. The later was obtained drawing a straight line from standard source C through the color coordinates of GaN emission until the spectral edge of CIE 1931 color space. CP was calculated by the expression:The XPS spectra of the GaN thin films are shown in Fig. 4. Each XPS peak is resolved into their principal contributions, employing the active background approach [16]. The Ga 2p XPS window reveals the spin-orbit splitting the Ga 2p1/2 and Ga 2p3/2 signals, which can be resolved in two contributions with a binding energy of 1117.65 eV (with a spin-orbit splitting of 26.91 eV) and 1118.69 eV (with a spin-orbit splitting of 27.24 eV), attributed to Ga-N and Ga-O bonds respectively [17–19]. Additionally, the fitting procedure features several loss peaks that are related to inelastic scattering of photoelectrons as they travel through the film, which may be a sign of a poor crystalline quality of the GaN film. The photoemission spectrum of the N 1s region reveals contribution from the Ga-N (397.38 eV) and C-N (399.28 eV) bonds, overlapped with Auger electron signals originated from Ga LMM emission [18,19]. A small contribution at 402.27 eV can be attributed to the presence of absorbed NOx.
The spectra reveal the existence of carbon impurities in the film which are associated to C-C and C-N (285.83 eV) contributions and some adventitious carbon complexes. The Ga 3d XPS peak, in the same way to that for Ga 2p, reveals a principal signal of Ga-N with binding energy now at 21.20 eV, accompanied with traces of oxygen as impurities such as Ga-O at 21.20 eV and O 2s peaks at 22.94, 24.30 and 26.02 eV binding energy.
The chemical composition was determined by correction of the photoemission signal using the appropriate physical parameters (for better detail please refer to Ref. [10,11,20]) and an approach that allows the determination of chemical composition of stacked conformal layers as explained in detail in Ref. [10,21]. The results show that the GaN film, here considered as a bulk layer, is composed of GaN0.96 equating to 66.5% of the film, 13.0% corresponding to the C-N compound and 15.5% to oxygen-compounds related to gallium oxide and impurities. The atomic percentages employed to determine the latters is found in Table 1.
Table 1. Atomic percent of the GaN film as determined by an approach that considers that scattering attenuates photoemission signal of each core level [10]. It is noteworthy, that only these signals are considered to pertain the GaN film, which is evaluated as a bulk film possessing a carbon overlayer of approximately 18 Å.
It is important to note that the carbon impurities C-N can be related to the film per se and not to superficial carbon in the film; resulting in bulk carbon impurities yielding up to 5.3% of the film. Peaks related to C-C, C-O-C, and O-C = O are excluded from the chemical composition of the bulk GaN film, because they belong to the surface and not the bulk resulting in an adventitious carbon layer of 18 Å on top of the GaN film.
Figure 5(A) shows a HRTEM image of a cross-section of the GaN thin film grown in this work. This film exhibits a thickness of 52 nm. Figure 5(B) shows two crystallographic planes (002) and (101) which are characteristic of the wurtzite structure for GaN. These arrangements correspond to (002) planes with an interplanar distance of 2.5 Å and (101) planes with an interplanar distance of 2.4 Å. The GaN crystallites were found embedded into an amorphous Ga-O-N matrix in agreement to the results reported by Ozgit et al [3].
Fig. 5. HRTEM images (A) interfaces GaN and substrate, (B) two crystallographic planes (002) and (101) characteristic of the wurtzite structure for GaN.
5. Conclusions
GaN thin films were synthesized by means of a homemade MPALD. The luminescence response of these GaN thin films, which comprises the entire visible spectrum, was obtained by cathodoluminescence. The carbon and oxygen impurities unintentional doped GaN films and produced interbands emissions such as BL, GL, YL and RL, which results in a near-white emission. CIE1931 coordinates of this emission $({x = 0.3491,\; \; y = 0.4312} )$ are located at the border of white and yellow-green sections. The CCT of 5061 K is close to daylight white emission. The main contribution of CL peak was determined at yellow wavelength by CP and YL band with 43% and 42.3%, both at 564 nm, respectively. Using the XPS peak-fitting results, the contaminations of 13% for carbon and 15.5% for oxygen was calculated for these GaN thin films. The presence of C and O impurities in the GaN can be attributed to the interaction of organometallic precursors with the plasma and the residual oxygen within the growth chamber.
Funding
Consejo Nacional de Ciencia y Tecnología (Basic Science 242508, FORDECYT 272894, National Laboratories 294452, Scholarship 407524).
Acknowledgements
The author acknowledges the CONACyT projects: Scholarship 407524, FORDECYT 272894, Basic Science 242508 and National Laboratories 294452 for the support, and valuable technical support of David Dominguez.
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