Ⅲ족 Nitride 증착을 위한 OMVPE 공정에 관한 연구

박진호, Timothy J. Anderson*

영남대학교 응용화학공학부, 플로리다대학교 화학공학과*

Investigation of the OMVPE Process for Group Ⅲ Nitrides Deposition

Chinho Park and Timothy J. Anderson*

School of Chemical Engineering & Technology, Yeungnam University

Department of Chemical Engineering, University of Florida*

Introdution

Group Ⅲ Nitrides such as GaN, InN, AlN, and their alloys are promising materials for the applications in the new-generation optoelectronic devices. These materials have direct bandgaps covering the whole visible spectrum. Thus in principle, natural color display devices can be all made from these materials. Especially, GaN has recently pulled a world-wide attention due to its ability to make blue-emitting devices such as LED's and LD's1)

In spite of these recent popularities, many fundamental aspects related to this material system are not fully investigated yet. The growth techniques, properties, and issues involving homoepitaxy have still to be addressed.

This study reports an effort to elucidate the OMVPE (organometallic vapor phase epitaxy) process used for the deposition of group Ⅲ nitrides from the fundamental view point. The focus of this work was mainly put on the understanding of the gas dynamics of the OMVPE reactor and pyrolysis reactions of film precursors occurring inside the reactor. Based on these results, actual film growth was performed and the  properties of grown films were analyzed.

Experimental

The experimental apparatus consisted of an OMVPE reactor system interfaced with a Raman spectrometer. Details of the reactor schematics are already reported elsewhere2-3), and the main features of the system are summarized as follows: A resistance heated furnace body was inserted into the reactor through a top flange, and two separate gas flow streams were provided to the reactor. The center flow supplied reactants diluted in the carrier gas (N2 or H2), while a sweeping flow containing only the carrier gas surrounded the center flow. An upflow configuration was adopted to reduce the buoyancy driven flow recirculations. The reactor was mounted on a micrometer controlled x-y-z stage located inside the macro-chamber of Raman system  to permit spatially resolved measurement of the gas temperature and reactant concentration. The gas manifold had a run/vent capability for fast reactant switching, and it also contained the make-up carrier gas supply to minimize flow disturbances during the gas switching operation4-5).

The 488 nm light from an Ar-ion laser was used as the excitation source at a power of 1 W. A photomultiplier tube (PMT) or CCD was used as a detector, and photon counting electronics were employed. The spatial resolutions of about 30 □ in the directions perpendicular to the laser beam path and about 3000 □ in the direction of the laser beam path were easily obtained. With this setup, spatially well resolved measurements could be performed for both temperatures and concentrations. The growth studies of GaN were performed on substrate materials such as Al2O3 and LiGaO2, and the properties of grown films were analyzed by various analytical tools.

Results and Discussion

(1) Gas Dynamics Studies

The gas temperature was measured by pure rotational Raman scattering from the carrier gas molecules (N2 or H2) as a function of the inlet gas velocity and the aspect ratio (AR: ratio of the distance between the susceptor and the center inlet tube to the center tube diameter). The technique is well established for randomly oriented (e.g.,  gas) diatomic molecules6).

Fig. 1 shows the effect of changing the inlet gas velocity on the measured axial centerline temperature profile in the case of a susceptor set point temperature (Tset) of 650 ℃. As expected, increasing the inlet gas velocity produced a steeper temperature gradient primarily because of the reduced residence time. At low inlet gas velocities, the inlet gas temperature is observed to be greater than the ambient temperature due to radiative heat transfer to the inlet manifold exceeding convective or conductive losses. The extrapolated surface temperature of the susceptor is seen to be almost 200 ℃ less than the set point temperature measured by the control thermocouple, apparently because of thermal contact resistance between the heating block and the susceptor.

The switching response of the gas manifold was characterized by monitoring the vibrational Raman spectrum of the tracer molecules (CH4), and an example is shown in Fig. 2. The figure represents response of the reactor to a 20 second pulse of carrier gas containing the tracer species for two different flow conditions. In the case of matched flow velocities (Fig. 2a), the response curve shows the expected rectangular shape. But in the case of unmatched flow velocities (Fig. 2b), a second shoulder spike was clearly observed when the sweeping flow velocity was reduced to 0.75 cm/s from the matched velocity of 3.0 cm/s, which indicated that a recirculating flow was developed in the reactor. This result was consistent with an observation in a steady flow conditions.

Fig. 3 shows the transient temperature rise of the N2 carrier gas was turned on. As shown in the figure, gas phase temperature responds very quickly to the susceptor temperature, but oscillatory temperature response was also observed.

(2) Decomposition Studies

Fig. 4 shows the concentration profiles of In-bearing species along the axial centerline of reactor when TMIn was introduced into the center flow. As shown in the figure, the decomposition of TMIn begins at approximately 5 mm (220 ℃) below the susceptor surface and its concentration decreases monotonically to a level below the detection limit at 0.5 mm (530 ℃) from the surface. It is interesting to note that the temperature range over which TMIn decomposition occurs, namely 220 to 530 ℃, is wider than the range 300 to 400 ℃ previously reported for TMIn decomposition in a He carrier using a flow tube apparatus7). This can be understood by noting that the residence time of TMIn in the thermal boundary layer is considerably shorter than that in the flow tube. Consequently, a higher temperature is required to completely decompose TMIn.

On the other hand, the lower threshold for TMIn decomposition determined in the present study may be a result of the dispersion of reactive chemical species(e.g., CH3) formed upstream, which could serve to accelerate the decomposition reactions. The participation of reactive intermediates in the decomposition of TMIn is also proposed for the formation of atomic indium.

An examination of the MMIn pyrolysis behavior in the thermal boundary layer also gives insight into the TMIn decomposition mechanism. From the results shown in Fig. 4, it is seen that the formation of MMIn coincides with the onset of TMIn disappearance. This observation is consistent with the homolytic fission mechanism8), which propose to yield MMIn as a stable intermediate at a high rate. It is also observed that the MMIn concentration reaches a maximum value at approximately 1.5 mm below the susceptor. This observation can be explained by the following proposed pyrolysis mechanism: It is noted that MMIn is produced by the homolytic fission reactions of TMIn, while being depleted simultaneously by further decomposition reactions. Close to the susceptor, the combined effects of a depleted TMIn concentration and increased rates of decomposition of MMIn causes the disappearance of MMIn and produces a maximum in the detected MMIn concentration.

Atomic indium, on the other hand, exhibits a monotonic increase in concentration as the probe position is moved towards the heated susceptor surface. The formation of atomic indium also occurs at the same onset as TMIn decomposition and MMIn formation (at a local temperature of approximately 220 ℃). The production of atomic indium by homolytic fission8) is believed to be too slow to be significant at this low temperature. A possible explanation for the low temperature appearance of atomic In  is the methyl radical attack of MMIn.

It is believed that the gas phase species detected in this study are the result of homogeneous reactions. If the reaction intermediate (e.g., MMIn) was produced by heterogeneous decomposition of TMIn, the maximum concentration of MMIn would be expected to occur at the susceptor surface rather than some distance away from the surface. Similarly, if heterogeneous reactions were responsible for MMIn depletion, atomic indium transitions would not have been detected, given the low vapor pressure of indium at the surface temperature (approximately 10-8 Torr at 575 ℃). Thus, it is concluded that the species monitored in this study were predominantly produced or consumed by homogeneous reactions.

Studies of other precursors such as TMGa and NH3 were also performed and the data analyses are under progress.

(3) Growth Studies

Growth of GaN on (0001) Al2O3 substrates were performed using the information obtained from the above-mentioned studies. A TEM picture near the interface between the substrate and the grown GaN film is shown in Fig. 5. As shown in the figure excellent epi-layer could be formed on Al2O3 substrates. Measurements of the film properties are under progress.

Acknowledgements

This work is supported in part by KOSEF Grant #96-0300-14-01-3.

References

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8.

Fig. 1. The effect of varying the inlet gas velocity on the axial centerline temperature profile in the reactor for N2 carrier gas: AR = 1, Tset = 650 ℃.		Fig. 2. Time response of the reactor for matched and unmatched H2 carrier velocities: pulse width = 20 s, Tset = 25 ℃, AR = 1, Probe position = 12 mm below susceptor; (a) vc = 3.0 cm/s, vs = 3.0 cm/s; (b) vc = 3.0 cm/s, vs = 0.75 cm/s.
Fig. 3. Transient temperature rise after heater on for N2 carrier gas: AR = 1, vc = vs = 3.0 cm/s, Tset = 1125 K.		Fig. 4. Concentration profiles of In-bearing species in N2 ambient along the axial center line of the reactor.
Fig. 5. Cross-sectional HRTEM at the GaN/(0001)Al2O3 interface.

M.G. Jacko and S.J.W. Price, Can. J. Chem. 42, 1198 (1964).