Epitaxial growth of A1N film by low-pressure MOCVD in gas-beam-flow reactor

Journal of Crystal Growth 115 (1991) 643—647 North-Holland J

CRYSTAL GROWTH

Epitaxial growth of A1N film by low-pressure MOCVD in gas-beam-flow reactor S. Kaneko, M. Tanaka, K. Masu, K. Tsubouchi and N. Mikoshiba Research Institute of Electrical Communication, Tohoku Uniicrsitv, Katahira 2-I-i, Aaba-ku, Sendai 980, Japan

We suggested “gas-beam-flow’ condition and ‘gas-beam-flow” type reactors for epitaxial AIN growth using trimethylaluminum and ammonia. By flow visualization experiments, two gas flows of the sheath and the source were found to be laminar at atmospheric and at low pressures in the “gas-beam-flow” condition. The source gas flowed without touching the reactor wall. Using the “gas-beam-flow” type reactor, we succeeded in growing epitaxial AIN films at low temperature of 6900 C with a superior surface morphology.

1. Introduction Aluminum nitride (A1N) thin film is an attractive Ill—V compound because of its wide band gap of 6.2 eV, high surface acoustic wave (SAW) velocity, high thermal conductivity and its cornpatibility with the thermal expansion coefficient of Si and GaAs. AIN thin film has a potential use in UV light emitting devices, SAW devices, passivation and packaging materials, etc. We developed zero-temperature coefficient SAW delay lines over 1 GHz on epitaxial AIN/aA1203 combination, which was grown by metalorganic chemical vapor deposition (MOCVD) at 12000 C using trimethylaluminum (TMA; (CH3)3 Al) and ammonia (NH1) [1]. However, the hillock-like surface morphology of the AIN film was not smooth enough to fabricate a device with less than a 1~m rule by photolithography. In order to grow an epitaxial film of AIN with a smooth sruface morphology and good crystallinity, we developed an initial-nitriding method in which the a-A1203 surface was nitrided in NH3 + H2 ambient before AIN growth [2]. The initial-nitriding AIN layer used, as a buffer layer, is able to relax the large lattice mismatch bePresent

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—

Laboratories Japan,

tween the epitaxial A1N film and the a-A1703 substrate. However, if the gas flow in a reactor is convective, unexpected vapor phase reaction occurs and then the surface morphology of AIN can not be improved. In this paper, we report on the flow visualization and “gas-beam-flow” type reactors in section 2. and on low temperature growth of high-quality epitaxial AIN films using the “gas-beam-flow” type reactor in section 3.

2. Flow visualization in MOCVD system Flow visualization in the reactor was performed using the MOCVD system. When growing AIN films, TMA-loaded H2 and NH3 were separately introduced into the vertical quartz reactor in order to avoid vapor phase nucleation in the gas supply line. For flow visualization, TiCI4loaded and water vapor-loaded He were introduced into the reactor instead of TMA-loaded H2 and NH3, respectively. The flow pattern in the reactor was observed with TiO, smoke which was formed by reacting TiCI4 and H20 [3,4]. It is noted that the mixed region of TiC14 and H20 is visualized as Ti07 smoke. A graphite susceptor was heated by RF inductive heating. Fig. 1 shows the flow patterns for three types of water-cooled vertical reactors. The reactor

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44

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center nozzle flows without touching the reactor wall. We named this condition “gas-beam-flow’ The center gas flow was called “source-flow and the surrounding gas flow was “sheath—flow’’. From atmospheric pressure to 5(1 Torr, we observed the “gas-beam-flow’’ condition. However, below 50 Torr, the center gas flow touched the reactor wall because the surrounding flow was insufficient to suppress spreading of the center gas flow.

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the surrounding flow velocity i’~ was lower than the center flow velocity l’~\ and the total gas flow was surface notand sufficiently thermal convection exhausted occurred at the wide because gap between the susceptor and the reactor wall. In the “slender” reactor of fig. lb. the surrounding flow velocity i~ was larger than that for the ‘‘hold” reactor and the gas flow was sufficiently exhausted at the gap. The center flow was not reflected at the susceptor surface and thermal was flow convection realized.did Note notthat occur, the gas so flow that from laminar the

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he faster than that in the ‘‘slender” reactor by using a conical part above the susceptor. I he laminar flow was also realized at 150 1 orr and 1000 C as shown in fig. I c. Both at atmospheric and at low pressures, we observed that the gas flow was laminar at 1000 C in the ‘‘gas—beam—

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pressure was 1St) Torr and the temperature of the susceptor was 10000 C. In the “hold” reactor of fig. Ia, the gas flow was convective both at atmospheric and at low

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3. Epitaxial growth of AIN films We deposited AIN films using the leak-tight MOCVD appaiatus with load-lock chamhei. The He leak rate at the SUS Joint of gas supply lines and at the sealing between the quartz reactor 3/s. and the SUS chamber was below 2 >< 10 0 atm cm The dew-point of H gas was below 100 We used high purity TMA and NH~.The residual oxygen in TMA (Uhe Corp.. Japan) was helow 2 ppm. The residual moisture and oxygen in NH 3 was below 0.5 ppm. The TMA-Ioaded H~ and NH3 were separately introduced to the cen—

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Fig. 2. SEM micrographs of AIN epitaxial films on 0)001) w-Al,O~.Thickness I urn. (a) Under “gas-beam-flow’ conditions P 0,, = 5)) Torr, T~01,= 12)))) 0 C). (h ) Under convential conditions ( P1,,1 = 5 Torr. T01, = 12)))) 0 C).

ter nozzle and the surrounding inlet of the quartz reactor, respectively. A substrate placed on a SiC-coated graphite susceptor was heated by RF inductive heating. The AIN films were grown on 2 inch diameter (0001) and (0112) a-A1203 substrates. A polished a-A1203 substrate was cleaned with organic solvents, the substrate was then dipped in HCI to remove ionized metals Ofl the surface and finally rinsed in ultrapure water, The growth procedure was as follows: the substrate was annealed in atmospheric H2 ambient for 30 mm at 1200°C, then the substrate was cooled down to the room temperature. The substrate was heated up to the growth temperature in a mixed flow of NH3 and H, at growth pressure for 5 mm. During this 5-minute heating, the a-AI,03 surface was nitrided to form a thin AIN single crystal layer. After the initial-nitriding penod, TMA-loaded hydrogen was introduced into the reactor, and the growth of AIN was started. The flow rates of TMA, H, and NH3 were 5.9 x 10~’, 4.9 X i0~ and 1.3 X 10’ mol/min, respectively. The molar ratio of NH3/TMA was 2>< io~. Fig. 2 shows SEM micrographs of AIN epitaxial films grown on (0001) a-AI,03 at 1200°C under the “gas-beam-flow” condition (50 Torr) and the conventional condition (5 Torr) using the “slender” reactor. In the conventional condition, the total pressure was 5 Torr so that the source flow touched the reactor wall. The AIN film grown under the “gas-beam-flow” condition cxhibited a surface morphology superior to one grown under the conventional condition. At the

initial growth stage on (0112) a-Al 203, island-like nuclei of 20() A in diameter were generated under the conventional condition. On the other hand, under the “gas-beam-flow” condition, the island-like nuclei were not observed. For (1120) AIN epitaxial films on (0112) aA1203 using the “gas-beam-flow” type reactor I of fig. lc, the half width of the X-ray rocking curve (.~O)became narrower; e.g., iO 0.300 for the “gas-beam-flow” type reactor 1 (1200 C, 50 Torr) and iO 0.42 for the conventional condition (1200 C, 5 Torr) using the “slender” reactor at 1.6 ,am thick AIN. An AIN film with good crystallinity was successfully grown using the “gas-beam-flow” type reactor I. In order to further improve the “gas-beamflow” condition in a wide pressure range, we proposed the “gas-beam-flow” type reactor 11 shown in fig.3. In order that the gas flow is exhausted smoothly at the gap between the susceptor and the reactor wall, the reactor has a sloping shoulder at the gap and the top of the susceptor is tapered. Furthermore, in order that —

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lOOwr.th (‘(3 MN growth rate as a function of substrate temPera~ tUN.

the source flow spreads uniformly on the substrate surface, the length of the conical part is larger than that in the “gas-beam-flow” type actor I. Fig. 4 shows the AIN growth rate as a function ~‘~>

of substrate temperature. The growth rate was scattered for the slender reactor under the conventional flow condition. On the other hand, using the “gas-beam-flow” type reactor II, the growth rate (OR) was constant above 600°Cand had the form OR = OR,, exp( —E/kT) where E = 0.14 eV below 600°C.Hence, AIN was wown under the mass-transfer-limited condition above 600°C and under the surface-reaction-limited condition below 600°C. Fig. 5 shows the growth rate as a function of thetotalpressureat 1200°C.Thegrowthratefor the “gas-beam-flow” type reactor I and the “slender” reactor depended strongly on the total pressure. In the restricted pressure range, the growth rate was constant. On the other hand, the growth rate for the “gas-beam-flow” type reactor II did not depend on pressure in the wide range of 5-60 Ton, i.e., OR — constant. Here, we discuss the pressure dependence of the growth rate. In the stagnant layer model, the growth rate isgivenas[51 —

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the ispartial pressure ofpartial reactant in the of gasreactant phase. p,~ the equilibrium pressure on the surface, R is the gas constant and T is the growth temperature. 1) and given as I)where =0,, mand 8 = 8k are (tjx/ps)”-. (P,,/I’,~1XT/T,,) 0,, is the diffusion coefficient at a pressure P~, and at temperature T,,. ~ is the growth pressure, in is the constant value (usually 1.5—2). q is the viscosity. p is the density of gas, r is the gas velocity. x is the arbitrary distance on the substrate. and k is a constant. In our experimental conditions, the gas flow rates of TMA, NH~and H, were not changed while the total pressure was changed by controlling the evacuation speed of the pump. so that z’a P~2 1.p a ~ and ~h a p Since P,, is larger than P~,OR = constant. The stagnant layer model assumes laminar flow. The pressure dependence of the growth rate for the “gas-beam-flow” type reactor II at 5—60 Torr agrees with the result derived from the stagnant layer model. Accordingly, the laminar flow is thought to be realized for the “gas-beam-

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Fig. 5. MN growth rate as a function of total pressure.

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Fig. 6. RHEED patterns and SEM micrographs of(l 120) AIN/(0112) a-Al 03. (a) “Gas-beam-flow” type reactor II (T.~5= 690°C.

(h) “Slender” reactor (T,~5= 1200°C).

flow” type reactor II under the wide pressure range. Fig. 6 shows the reflection high-energy dcctron diffraction (RHEED) patterns and SEM micrographs of epitaxial AIN films grown on (0112) a-A1203. The AIN film grown using the “gasbeam-flow” type reactor II at 690°Cexhibited a surface morphology superior to films grown at 1200°Cusing the “slender” reactor. In our previous investigations, the epitaxial A1N film was grown above 1005 C by atmospheric MOCVD [1] while the epitaxial AIN film was grown above 850 C by low-pressure or plasma-enhanced MOCVD [6]. By using the “gas-beam-flow” type reactor II, we have successfully grown epitaxial AIN films with a superior surface morphology at as low temperature as 690 C. °

°

4. Conclusion

We suggested “gas-beam-flow” condition and “gas-beam-flow” type reactors. Under the “gasbeam-flow” conditions, two gas flows, the first of

the sheath and the second of the source, were found to be laminar. The source gas was found to flow without touching the reactor wall at atmospheric and at low pressures from the flow visualization experiment. Using the “gas-beam-flow” type reactor, we succeeded in growing epitaxial A1N films with a smooth surface morphology at as low temperature as 690 C. °

References It]

K. Tsubouchi and N. Mikoshiba, IEEE Trans. Sonics Ultrasonics SU-32 (1985) 634. [2] 11. Kawakami, K. Sakurai, K. Tsubouchi and N. Mikoshiba, J. AppI. Phys. (1988) C.H.J. Ll61. van den Brekel and [3] Japan. F.C. Eversteyn, P.J.W.27Severin, HE. Peek, J. Electrochem. Soc. 117 (1970) 925. [4]R. Takahashi, Y. Koga and K. Sugawara, J. Electrochem. Soc. 119 (1972) 1406. [5] J. Bloem and L.J. Giling, in: Current Topics in Materials Science, Vol. 1, Ed. E. Kaldis (North-Holland, Amster[6] ~ Umino, K. Tsubouchi and N. Mikoshiba 1985 Ultrasonics Symp. Proc. (San Francisco) IEEE 85CH220955 (1985) 192.