Experimental study of trimethyl aluminum decomposition

Journal of Crystal Growth 473 (2017) 6–10

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Experimental study of trimethyl aluminum decomposition Zhi Zhang a, Yang Pan b, Jiuzhong Yang b, Zhiming Jiang b, Haisheng Fang a,⇑ a b

State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science & Technology, Wuhan, Hubei 430074, China National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, China

a r t i c l e

i n f o

Article history: Received 22 April 2017 Received in revised form 14 May 2017 Accepted 20 May 2017 Available online 23 May 2017 Communicated by T.F. Kuech Keywords: A1. Photolysis A1. Pyrolysis A1. Synchrotron radiation B1. Trimethyl aluminum B1. Superlattice

a b s t r a c t Trimethyl aluminum (TMA) is an important precursor used for metal-organic chemical vapor deposition (MOCVD) of most Al-containing structures, in particular of nitride structures. The reaction mechanism of TMA with ammonia is neither clear nor certain due to its complexity. Pyrolysis of trimethyl metal is the start of series of reactions, thus significantly affecting the growth. Experimental study of TMA pyrolysis, however, has not yet been conducted in detail. In this paper, a reflectron time-of-flight mass spectrometer is adopted to measure the TMA decomposition from room temperature to 800 °C in a special pyrolysis furnace, activated by soft X-ray from the synchrotron radiation. The results show that generation of methyl, ethane and monomethyl aluminum (MMA) indicates the start of the pyrolysis process. In the low temperature range from 25 °C to 700 °C, the main product is dimethyl aluminum (DMA) from decomposition of TMA. For temperatures larger than 700 °C, the main products are MMA, DMA, methyl and ethane. Ó 2017 Elsevier B.V. All rights reserved.

1. Introduction Metal-organic chemical vapor deposition (MOCVD) is a popular technique for synthesis of thin films [1]. During deposition, complex chemical reactions between the precursors make the study of the mechanism very challenging [2,3]. Trimethyl aluminum (TMA) (abbreviations of some species are given in Table 1 in Part 2) is an important source, in particular, for growth of nitride based LEDs. Ammonia and TMA are carried into the reactor by carrier gas (nitrogen or hydrogen), and are pyrolysized above the hightemperature substrate (homogeneous gas phase reactions), into reactant precursors. The reactants reach the substrate, attach onto the surface by adsorption; heterogeneous reactions on the surface then form the films layer by layer. The by-products of the surface reactions desorbed from the surface into the main stream by diffusion are carried out of the reactor by the carrier gas [4]. Some advanced thin-film growth methods on different substrates were invented and characteristics of growth process were discovered [5–7]. Therefore, from the above description of the thin-film deposition, pyrolysis of the source gases, TMA among them, is a key step of the whole chain of reactions. To optimize the deposition process, researchers have put much effort on the mechanisms of the reactions between trimethyl metals and ammonia. In the early studies, reactions of ammonia had ⇑ Corresponding author. E-mail address: [email protected] (H. Fang). http://dx.doi.org/10.1016/j.jcrysgro.2017.05.020 0022-0248/Ó 2017 Elsevier B.V. All rights reserved.

not been involved. Mashita et al. investigated pyrolysis of triethyl gallium (TEG) in the TEG/TMA/H2 system using a quadrupole mass spectrometer [8]. They found that the pyrolysis temperature of TEG was raised by 25 °C in the presence of TMA. Nobumasa et al. studied a vapor phase decomposition mechanism for the TMA by infrared spectroscopy, and obtained the activation energy of the decomposition of about 0.4 eV [9]. Kiyoshi et al. showed that the pyrolysis of TMA produced Al4C3 at 950 °C [10]. Ivanov et al. investigated the process of TMA laser-induced pyrolysis using a mass spectrometer [11]. They summarized that the production ratio of ethane/ ethylene, methane and hydrogen is 1:5:1.75. However, the parasitic chemistry reaction with the ammonia introduced had a bad influence on film deposition. Creighton et al. analyzed kinetics of metal organic–ammonia adduct decomposition, and found that adducts and metal organic can form into nanoparticles, which consumes a significant fraction of the group-III growth precursor [12,13]. Coltrin et al. developed a series of gas-phase reaction mechanisms to simulate the gas-phase nucleation and the particle growth, and achieved results in good agreement with the experiments [14]. Due to the complexity added by the presence of ammonia, only specific theoretical studies have been conducted on the reaction mechanisms. Mihopoulos et al. investigated the formation of Lewis acid-base adducts of the organometallic precursors [TMG:NH3 and TMA:NH3] using hybrid density functional theory and transition state theory [15]. Formation of dimers and trimers containing Ga, Al, and N was identified as the major pathway affecting the growth efficiency. Hirako et al. investigated

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Z. Zhang et al. / Journal of Crystal Growth 473 (2017) 6–10 Table 1 Abbreviation of species. Abbreviation

Chemical formula

TMG/TMA DMG/DMA MMG/MMA TMG:NH3/TMA:NH3 DMG:NH2/DMA:NH2 [DMG:NH2]n/[DMA:NH2]n

Ga(CH3)3/Al(CH3)3 Ga(CH3)2/Al(CH3)2 GaCH3/AlCH3 Ga(CH3)3:NH3/Al(CH3)3:NH3 Ga(CH3)2:NH2/Al(CH3)2:NH2 [Ga(CH3)2:NH2]n/[Al(CH3)2:NH2]n

gallium nitride (GaN) growth by MOCVD considering formation of polymers, such as [Ga-N]n and [MMGaNH]n (n = 2–6), in the reaction model for a TMG/NH3/H2 system [16]. They concluded that the type of reactive molecule changes with temperature, with the formation of [MMGaNH]n at 600–750 K and [Ga-N]n at higher temperatures. Sengupta et al. presented a methodology combining the density functional theory with rate theories to determine the reaction pathways and rates for GaN growth from TMG and ammonia [17]. Parikh et al. reviewed GaN growth chemistry, and suggested to select the competing reaction pathways according to the reactor geometry [18].

From the above analysis, one can find that pyrolysis of the trimethyl metal is the prerequisite step of the growth chemistry of thin films. However, few studies regarding pyrolysis of trimethyl metal have been conducted. The available results have neither presented the decomposition temperature of TMA exactly, nor determined the key species characterizing the pyrolysis process. The purpose of the paper is to obtain the start temperature of TMA pyrolysis, as well as to determine the species issued from pyrolysis. The current studies provide important information for evaluating the prerequisite species originated from TMA for the whole epitaxial growth. 2. Experimental facilities and background measurements To describe the species conveniently, abbreviations of the species are given in Table 1. The pyrolysis of TMA is investigated using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) at National Synchrotron Radiation Laboratory (NSRL) in Hefei, China [19–21]. The experimental setup is shown in Fig. 1. The pyrolysis system consists of a pyrolysis chamber with the reactor, a pumped chamber carrying the molecular beam to the

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Fig. 1. The experimental setup.

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Fig. 2. The background measurements: (a) at room temperature (25 °C) without nitrogen, (b) at room temperature (25 °C) with nitrogen, (c) at 100 °C with nitrogen for a long time, and (d) at 500 °C with nitrogen.

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Z. Zhang et al. / Journal of Crystal Growth 473 (2017) 6–10

ing the chamber. The data collection lasts 100 s after the mass spectrometer gets the measuring signals. The abscissa m/z indicates mass-to-charge ratio. At room temperature, the major peak appears at the m/z of 134 without nitrogen (Fig. 2a). The additional nitrogen peak of 28 rises with nitrogen (Fig. 2b). The measurements were repeated several times, and similar profiles were obtained. Therefore, the 134 peak is a type of impurity in the chamber. For further investigation of the impurity, the chamber temperature is increased to 100 °C, and the nitrogen gas is kept flowing for a long time until the 134 peak disappears (Fig. 2c). This is immediately followed by the background measurement at 500 °C with nitrogen as illustrated in Fig. 2d. Including the 28 peak, the 134 peak appears again, and a 279 peak strengthens. Conclusively, a gas impurity evaporated from the chamber wall (Peak 134) and a possible impurity (Peak 279) exists in the background of the system, with an enhanced signal at high temperature.

Relative Intensity

photoionization region, and a photoionization chamber where the pyrolysis products are ionized by the synchrotron’s VUV light. The VUV light is a part of the ultraviolet light and its wavelength range is from 10 to 200 nm. This light of the wavelength only can transport a few millimeters due to strongly absorption in air ambient [22]. The activated ions are detected by a reflectron time-of-flight mass spectrometer (RTOF-MS). Since the SVUV-PIMS was initially designed for pyrolysis and combustion of hydrocarbon gases, the gas channel was redesigned as shown in the left of Fig. 1. The liquefied TMA stored in a highpressure cylinder is isolated by a SWAGELOK diaphragm valve (Valve 1). To facilitate installation of Valve 2, a ‘‘pigtail” shaped tube is designed for connection. Valve 2 is a needle valve to limit the flow rate of TMA into the chamber. The high-purity carrier gas (N2) controlled by a ball valve (Valve 3) dilutes the gas source (TMA), and brings it into the heater chamber for pyrolysis. After checking the tightness of the sealing, the chamber is repeatedly evacuated and purged by high-purity nitrogen to remove the residual impurities. Since the saturated vapor pressure of TMA at 15 °C is 6.57 Torr, pressure of the pyrolysis chamber is kept to 1 Torr to ensure TMA vapor flowing in continuously. The photoionization chamber pressure is controlled by a vacuum pump to 1 mTorr, which permits only tiny amounts of the pyrolysis products enter-

1.0 0.8 0.6 0.4 0.2 0.0

3. Results and discussion At room temperature and 1.04 Torr pressure, two tests as shown in Fig. 3 are conducted. With a nitrogen flow rate of 100 sccm, the peaks of 15, 28, 57 (58), 72 (73), 129 and 134 are

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Fig. 3. TMA decomposition measurements at 25 C with (a) a nitrogen flow rate of 100 sccm, and (b) without nitrogen.

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(b) Fig. 4. TMA decomposition measurements at 500 °C. The flow rate of TMA in (a) is less than that in (b).

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Z. Zhang et al. / Journal of Crystal Growth 473 (2017) 6–10

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Fig. 5. TMA decomposition measurements at 600 °C (a) and 700 °C (b).

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(b) Fig. 6. TMA decomposition measurements at 800 °C. The flow rate of TMA in (a) is less than that in (b).

detected. The small peak one m/z unit after a big one is regarded as its isotope. After turning off the nitrogen flow, the observed peaks are similar except for peak 28 denoting nitrogen. When the temperature is increased to 500 °C, the measured results are basically the same as shown in Fig. 4 with different TMA injections. Comparing to the case at room temperature, the extra peaks of 94, 96 and 98 become obvious, while the relative intensity of 129 decreases. Additional tests show the extra peaks strengthen as the major peak at 57 rises that is controlled by injecting more TMA gas. Conclusively, the peaks at 94, 96 and 98 are the reaction products. According to the characteristics of TMA, at room temperature it is in the form of dimeric polymer and, in the absence of heating, decomposition of TMA may only be caused by the light from the sodium lamp in the chamber. The photolysis products are materials with m/z of 57, 72, 94, 96, 98 and 129. The m/z of 57 and 72 are possibly DMA and TMA, respectively. The peaks at 94, 96 and 98 are C-Al-CAl-C chain molecules after methyl or hydrogen atom removal from the dimeric TMA. The measured peaks at temperatures of 600 °C and 700 °C are respectively given in Fig. 5a and b. At 600 °C, the main peaks are 57 (58), 72 (73), 94, 96, 98 and 129. Comparing to 500 °C, the proportion of 129 is further reduced, and the m/z at 94, 96, and 98 also decrease. The main products at 700 °C are 15, 28, 40, 42, 54, 56, 57 (58), 72 (73). It can be deduced that TMA pyrolysis obviously

Table 2 The detected products at different temperatures. Temperature

m/z of the main detected products

25 (°C) 500 (°C) 600 (°C) 700 (°C) 800 (°C)

15, 15, 15, 15, 15,

28, 28, 28, 28, 28,

57, 57, 57, 40, 40,

72, 72, 72, 42, 42,

129 94, 96, 98, 129 129 57, 72 57, 72, 78

happens, as the emergence of new products 40 and 42. The 42 is from 57 (DMA) losing a methyl during pyrolysis, known as MMA. Further loss of two hydrogen atoms from MMA forms the m/z of 40. The products with m/z of 94, 96, 98 and 129 found at low temperature are not detected any more. The methyl (ACH3) portion of the 15 peak goes up significantly due to TMA pyrolysis. For the temperature up to 800 °C, the peaks of 15, 28, 40, 42, (52), (54), (56), 57, (58), (66), (68), (70), 72, (73), 78, (79), (80), (84), (92) and (106) are monitored in Fig. 6. The variety of the products dramatically increases. The ACH3 (15) portion significantly increases due to ramp-up of temperature that stimulates pyrolysis of TMA. Furthermore, the amount of 28 and 40 becomes sharply larger than that at low temperature. Hence, at 800 °C thermal decomposition of TMA becomes dominant. At low temperature,

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Z. Zhang et al. / Journal of Crystal Growth 473 (2017) 6–10

Fig. 7. The decomposition path at low temperature (a) and at high temperature (b).

peak 28 is considered as the carrier gas (nitrogen), while at high temperature, the intensity of the peak 28 becomes several orders higher. It is considered as ethylene (C2H4). More MMA and DMA are determined from amount of the m/z of 40. A new product 78 is captured. As a summary, the detected products are listed in Table 2. The determined species are CH3 (15), N2 (28), C2H4 (28), MMA (42), DMA (57), TMA (72), Al2(CH3)5 (129) and C-Al-C-Al-C (94, 96, 98). The product with the weight of 40 is possibly MMA2 (losing two hydrogen atoms of MMA). The m/z of 78 is undetermined, but should be some chain molecules formed during pyrolysis. The decomposition paths are summarized in Fig. 7. At low temperature (less than 600 °C), the photolysis dominates, characterized by the chain molecules, C-Al-C-Al-C, with weights of 94, 96 and 98. At high temperature (larger than 700 °C), the presence of the peaks of 40, 42 and 57 indicates pyrolysis as the major mechanism. 4. Conclusions In this paper, the pyrolysis of TMA is carefully investigated using SVUV-PIMS device. The pyrolysis temperature and products are determined. From the analysis, the following conclusions can be drawn. Firstly, at low temperature range from 25 to 500 °C, the reactions of dimeric TMA decomposition and methyl removal of TMA can easily occur by photolysis. The product of C-Al-C-AlC gets high as the temperature increases. Secondly, the main thermal decomposition products of TMA are DMA, MMA, CH3 and C2H4 at high temperature (800 °C). The pyrolysis pathway of TMA follows as (TMA)2 ? TMA ? DMA ? MMA. The intensity of the methyl group determines whether the pyrolysis occurs. Finally, at transition temperature range (600–700 °C), Al2(CH3)5 decreases, and CH3, C2H4 and MMA increase by temperature. The reaction process of TMA is gradually switched from photolysis to pyrolysis as the system temperature goes up. Acknowledgement This work is mainly supported by the grants from National Natural Science Foundation of China (No. 51476068). References [1] G. Dhanaraj et al., Springer Handbook of Crystal Growth, Springer Science & Business Media, 2010.

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