Supporting Information Room temperature synthesis of GaN driven by kinetic energy be-yond the limit of thermodynamics Takane Imaoka 1,4,5, Takeru Okada 2,4, Seiji Samukawa 2,3,4*, and Kimihisa Yamamoto 1,4,5* 1 Laboratory for Chemistry and Life Science, Institute of Innovative Research, Tokyo Institute of Technology, Yokohama 225-8503, Japan 2 Institute of Fluid Science, Tohoku University, Sendai 980-8577, Japan 3 WPI-Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan. 4 CREST and 5 ERATO Japan Science and Technology Agency, Chiyoda 102-8666, Japan *Corresponding authors. E-mail: samukawa@ifs.tohoku.ac.jp (S. S.), yamamoto@res.titech.ac.jp (K. Y.) These authors contributed equally to this work. S-1
Table of content Experimental Section 1. General Methods... 3 a) Materials... 3 b) Spin coating of GaCl 3 on a substrate... 3 2. Neutral beam (NB)... 3 a) Apparatus... 3 b) Plasma discharge condition... 3 c) Neutralization of ions and UV elimination... 3 d) Beam energy and flux... 4 e) Sample temperature during the NB Process... 4 3. TEM and EDX analysis... 4 a) Top-view TEM observation and STEM-EDX mapping... 4 b) Cross-section TEM and EDS analysis... 5 4. XPS analysis... 5 a) Wide scan... 5 b) Depth profile... 5 c) N 2 NB irradiation time dependence... 6 5. Surface observation after N 2 NB irradiation... 6 6. Photoluminescence measurement... 6 References... 7 Supporting Figures Figure S1 ~ Figure S15 S-2
Experimental section 1. General Methods a) Materials Anhydrous GaCl 3 (99.999% metal basis) was purchased from AlfaAesar. Other chemical reagents including solvents were purchased from Kanto Kagaku Co., LTD. Quartz substrates, sapphire substrates (c plane), silicon substrates (p-type, (100) surface, 1-10 Ωcm), polyethylene terephthalate (PET) substrates were from Shinetsu, Shinko-sha, Aki Corporation, and Teijin-dupont, respectively. All the substrates were checked beforehand by AFM and photoluminescence measurement. b) Spin coating of GaCl 3 on a substrate GaCl 3 as the precursor of GaN was cast on various substrates (quartz, sapphire, silicon, PET) to form a very thin (~5 nm) film suitable for the neutral beam processing. Typical procedure is as follows. A 10 µl solution of 5 mmol L 1 GaCl 3 in acetonitrile was spin-coated on a quartz substrate (1 cm 2 ) at 2000 rpm for 30 seconds. 2. Neutral beam (NB) a) Apparatus An experimental system of neutral beam (NB) consists of two rooms which are separated by carbon plate with many apertures. The size of aperture and aperture ratio, and thickness of the plate are 1 mm, 50 %, and 10 mm, respectively. The top room is used for plasma generation and the bottom is for sample processing. Sample stage is set 40 mm below from the carbon plate. Temperature of the sample stage is room temperature. 3 turn antenna is set for plasma generation. b) Plasma discharge condition Pure nitrogen gas is introduced into the top chamber through shower-head type gas inlet component. Nitrogen plasma is generated by Inductively Coupled Plasma (ICP) method. The plasma is generated by applying Radio Frequency (RF, 13.56 MHz) with power of 250 W. The pressure of plasma can be controlled by changing nitrogen flow rate as shown in Figure S2. c) Neutralization of ions and UV elimination The ions from plasma can be neutralized by collision with sidewall of the aperture when passing through them to the bottom. The neutralized efficiency is as high as more than 95 %. In addition, more than 90 % of UV photons can be eliminated 1. S-3
d) Beam energy and flux The neutralization ratio of the ions in this NB system is more than 95%. The kinetic energy of the charged particles (residual ions) is believed to be almost equal to that of the neutral particle, and therefore, a beam energy measurement was taken by measuring the residual ions. The beam energy measurement of the NB system has already been established, so a quadrupole mass spectrometer (Q-MS) measurement was carried out in the same way as in previous reports 1. The main charged species from the plasma when using nitrogen is N + 2. Figure S2 shows the beam energy distribution of nitrogen NBs at various pressures. At higher pressures, relatively widely distributed beam energy, resulting from the collision of gas molecules, was observed. At lower pressures, a single sharp peak with FWHM approximately 1 ev was detected. The beam energy is decreased with an increase in pressure. This is due to the collision of particles. We determine the NB energy based on the previous results 2. Note that pressure that described in the Figure S2 indicates the pressure at the plasma generation chamber. For example, the 10 ev beam was generated at the N 2 gas flow rate of 115 sccm. Pressure of the plasma chamber was 1.0 Pa at this flowing condition. Due to the conductance of gas flow by the carbon aperture plate, the pressure at the sample chamber was 0.1 Pa. e) Sample temperature during the NB Process Temperature of the sample surface was measured with thermocouple embedded Si wafer. Plasma discharge for the NB generation was started at the t=0. During typical NB process time, 15 min., the temperature was slightly increased. This is due to the heat radiation from plasma through the aperture and there is no effect on the GaN synthesis process. 3. TEM and EDX analysis a) Top-view TEM observation and STEM-EDX mapping Top-view TEM and electron diffraction images (Figure 2 in the main text) were obtained using a transmission electron microscope (JEOL, JEM-2100F) operated at a 200 kv acceleration. The EDX-STEM mapping image was obtained using the same microscope. The GaCl 3 solution was cast on an elastic carbon film with a Cu mesh (Nisshin EM Co.) followed by the vacuum drying at 40 C overnight. It was then processed with N 2 NB (10 ev) for 10 minutes. The mapping showed that nitrogen atoms were homogeneously introduced onto the GaCl 3 film by the N 2 NB process (Figure S4). The observation and the analysis were carried out using a beryllium double tilt holder. S-4
b) Cross-section TEM and EDS analysis GaCl 3 was deposited onto the Si substrate which is covered with Si native oxide. Then, it was treated by NB using 10 ev condition for GaN formation. To carry out cross-sectional observation with TEM, expoxy resin was coated onto the GaN. Figure S5 shows cross-section TEM image. GaN is formed on the substrate clearly. In this condition, thickness of GaN at observed area is approximately 2 nm. However thickness uniformity is not so high and there still remains improvement of deposition process. For EDX spectrum measurement, electron beam was focused as shown by yellow dotted circle. The EDX spectra were shown in Figure S6. Si and O are originated from Si substrate and its native oxide that is formed before GaCl 3 coating. C is from epoxy resin and contamination due to air exposure. Even after NB irradiation, Ga is still remained on the surface and N is also detected. In addition, Cl is disappeared by N 2 NB irradiation. This implies that our proposed chemical reaction was proceed during the beam irradiation. Peaks at 0.28, 0.40, 0.53, 1.10, 1.84, 2.62, 2.82, and 9.24 kev are assigned C Kα, N Kα, O Kα, Ga L, Si Kα, Cl Kα, Cl Kβ, and Ga Kα, respectively. 4. XPS analysis XPS measurement in this section is used Al Kα line as X-ray source (Theta Probe, Thermos Fischer Scientific). Spot size is φ= 200 mm. For this analysis, GaCl 3 is deposited onto Si substrate. a) Wide scan To survey elemental components in GaN, we carried out XPS wide scan as shown in Figure S7. Major components are Ga, Si, C, N, and O. Small amount of F would be originated from polytetrafluoroethylene (PTFE) sealing cap of glass vessel which is used for GaCl 3 solution or tweezers. All detected elements are originated from GaCl3 and therefore N 2 NB and contribution of other elements on nitridation of Ga compound can be eliminated from discussion. b) Depth profile Depth profile was also measured using Ar ion gun. Although etching rate of our sample cannot be estimated, the rate for thermal-oxided SiO2 on Si wafer is 4.5 nm/min. Note that the concentration N and O contains the effect of air exposure before measurement. Both Ga and N decreased with an etch time and kept constant after 20 s. This means that Ga and N distribute same region in z-direction (perpendicular to sample surface). Thus conversion of GaCl 3 to GaN completely proceeds. The reason why Si and O are observed in the outermost layer is derived from the fact that the surface of the substrate, which is naturally oxidized, is partially exposed S-5
rather than the covering rate by GaN on the substrate surface being 100%. c) N 2 NB irradiation time dependence XPS measurement was carried out. X-ray source was used Mg-Kα and measurement spot size is φ=0.8 mm. Samples are exposed to air after NB treatment. Spectra are shifted because of charge up during measurement, hence it is difficult to analyze quantitatively. For this investigation, GaCl 3 was coated onto Si/SiO 2 substrate. Peak positon of Ga3d is shifted to higher energy. NB-induced conversion from Ga-Cl to Ga-N was not clearly observed due to insufficient peak signal intensity and charge up problem. However, it is found that Ga3d peak was kept after NB treatment. On the contrary, in the N1s spectra, nitrogen component is appeared even after just 2 minutes NB treatment. Peak existence of Ga3d and appearance of N1s indirectly indicate formation of Ga-N from Ga-Cl. By increasing of NB treatment time, sub-peak around 401 ev appears. These peaks are considered that parts of nitrogen species would bind to Si which is used as substrate. These results are consistent with other reports 3,4 and data base. 5. Surface observation after N 2 NB irradiation AFM (atomic force microscope) observations were carried out on an HOPG (highly oriented pyrolytic graphite) substrate. The surface topographies were observed by non-contact atomic force microscopy (AFM) with a scanning probe microscope (SPI3800N, SII) using a standard silicon cantilever with Al coating (SI-DF-40P2, SII). The N 2 NB irradiated surface did not show any significant damage on both substrates and gallium-containing film (Figure S12). The conducting probe measurement was measured using an Au-coated cantilever in contact AFM mode (Figure S13). The I-V curve trace was measured at the same place by an external source meter (Model 2612, Keithley). A constant force (8 16 nn) was kept during the I-V measurement by controlling the deflection value. 6. Photoluminescence measurement Photoluminescence spectra were measured using a fluorescence spectrometer (JASCO, FP-6500). In order to remove the stray light and higher-order diffraction, a long-wavelength cut filter were inserted on the optical path of excitation light source together with a short-wavelength cut filter inserted on the optical path of fluorescence detection. Photoluminescence mapping image was corrected on LabRAM HR-PL (HORIBA JOBIN YVON) using He-Cd laser (325 nm) as the excitation light source at room temperature (25 C). S-6
References 1 Noda, S; Nishimori, H; Ida, T.; Arikado, T.; Ichiki, K.; Ozaki, T.; Samukawa, S. 50 nm gate electrode patterning using a neutral-beam etching system. Journal of Vacuum Science & Technology A 22, 1506-1512, doi:10.1116/1.1723338 (2004). 2 Okada, T. & Samukawa, S. Selective in-plane nitrogen doping of graphene by an energy-controlled neutral beam. Nanotechnology 26, doi:10.1088/0957-4484/26/48/485602 (2015). 3 Yanagisawa, J., Toda, M., Kitamura, T., Matsumoto, H. & Akasaka, Y. Formation of GaN layer on SiN surface using low-energy Ga ion implantation. Journal of Vacuum Science & Technology B 23, 3205-3208, doi:10.1116/1.2134722 (2005). 4 Li, D. S.; Sumiya, M.; Fuke, S. Selective etching of GaN polar surface in potassium hydroxide solution studied by x-ray photoelectron spectroscopy. Journal of Applied Physics 90, 4219-4223, doi:10.1063/1.1402966 (2001). S-7
Supporting Figures Figure S1. Illustration of neutral beam (NB) system. Figure S2. Energy distribution of nitrogen neutral beam (N 2 NB). Signal intensity was normalized. S-8
Figure S3. Sample surface temperature during the N 2 NB (10 ev) irradiation. Figure S4. Comparisons between the experimental electron diffraction image (Figure 2A) and their simulations assuming wurtzite (P6 3 mc) and rocksalt (Fm-3m) structures. S-9
Figure S5. STEM-EDS mapping images of the N 2 NB irradiated GaCl 3 sample. Figure S6. Cross-section TEM image of GaN. Yellow circle indicates the area where electron beam was irradiated for the EDS spectrum measurement (Figure. S6). S-10
Figure S7. An EDS spectrum of GaN on silicon substrate. Figure S8. Wide scan XPS spectrum of GaN. S-11
Figure S9. XPS Ga3d and N1s spectra. Figure S10. Depth profile. GaN forms on Si substrate. S-12
Figure S11. A photograph of N 2 NB irradiated sample through a patterned stencil mask (Figure 3d in the main text) under black light illumination. The inset is the green channel image extracted from the original RGB data to clarify the image. Figure S12. Photoluminescence spectra of as prepared GaN thin layer after irradiation of N 2 NB (10 ev) on a sapphire substrate (C-plane) at room temperature. Excitation wavelength was changed from 230 nm to 300 nm. S-13
(a) (b) (c) Figure S13. Atomic force microscope (AFM) images of (a) blank, (b) GaCl 3 spin-coated and (c) NB irradiated sample on an HOPG substrate. Figure S14. Topography and conduction measurement of N 2 NB-irradiated GaCl 3 on an HOPG substrate. (A) The topographic image. (B) The conduction image observed at the same area. (C) The I-V curve measured at blank area (1: blue) and sample covered area (2: red). The I-V curve indicates the presence of a Schottky barrier. S-14
Figure S15. Photoluminescence spectra of as prepared GaN thin layer after irradiation of N 2 NB (10 ev) on a PET substrates. Figure S16. Photoluminescence spectra of as prepared GaN thin layer after irradiation of N 2 NB (10 ev) in the presence and absence of doped indium (20%). S-15