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[6] A. E. Berkowitz, J. R. Mitchell, M. J. Carey, A. P. Young, D. Rao, A.Starr, S. Zhang, F. E. Spada, F. T. Parker, A. Hutten, G. Thomas, J.Appl. Phys. 1993, 73, 5320.

[7] M. J. Hall, B. J. Hickey, M. A. Howson, M. J. Walker, J. Xu, D. Greig,N. Wiser, Phys. Rev. B 1993, 47, 12 785.

[8] A. E. Kaloyerose, M. A. Fury, MRS Bull. 1993, 18(6), 22.[9] The Chemistry of Metal CVD (Eds: T. Kodas, M. Hampden-Smith),

VCH, Weinheim 1994.[10] G. J. M. Dormans, G. J. B. M. Meekes, E. G. J. Staring, J. Cryst.

Growth 1991, 114, 364.[11] R. G. Charles, P. G. Haverlack, J. Inorg. Nucl. Chem. 1969, 31, 995.[12] G. A. West, K. W. Beeson, Appl. Phys. Lett. 1988, 537, 40.[13] G. A. West, K. W. Beeson, Mater. Res. Soc. Symp. Proc. 1989, 131, 389.[14] M. E. Gross, D. Schnoes-Kranz, D. Brasen, H. Luftman, J. Vac. Sci.

Technol. B 1988, 6, 1548.[15] W. H. Cheng, J. Pooladdej, S. Y. Huang, K. D. Buehring, A. Appel-

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[16] C. J. Smart, S. K. Reynolds, C. L. Stanis, A. Patel, J. T. Kirleis. Mater.Res. Soc. Symp. Proc.1993, 282, 229.

[17] A. K. Baev, Yu. L. Gubar, K. S. Gasanov, Russ. J. Phys. Chem. (Engl.Transl.) 1982, 56, 1491.

[18] F. Frey, W. Prandl, J. Scheider, C. Zeyen, K. Ziebeck, J. Phys. F 1979,9, 603.

[19] D. M. Donnet, K. M. Krishnan, Y. Yajima J. Phys. D: Appl. Phys.1995, 28, 1942.

[20] T. Fauster, G. Rangelov, J. Stober, B. Eisenhut, Phys. Rev. B 1993, 48,11 361.

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[22] T. Suzuki, D. Weller, C. A. Chang, R. Savoy, T. Huang, B. A. Gurney,V. Speriosu, Appl. Phys. Lett. 1994, 64, 2736.

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[24] K. Brohl, P. Bodeker, N. Metoki, A. Stierle, H. Zabel, J. Cryst. Growth1993, 127, 682.

[25] G. W. C. Kaye, T. H. Laby, Tables of Physical and Chemical Constants,14th ed., Longman, New York 1973.

Suppression of Thermal Convection and ItsEffect on Growth of GaN in Metal OrganicVapor Phase Epitaxy**

By Gang Li,* Soo J. Chua, and Wen Wang

Following the breakthrough in p-type doping and thedemonstration of short-wavelength photonic devices andhigh temperature/high power electronic devices, group IIInitrides have attracted enormous attention in the past fewyears.[1–3] At present, epitaxial growth of group III nitridesis primarily accomplished using either metal organic vaporphase epitaxy (MOVPE) or molecular beam epitaxy(MBE). Further improvement in performance of devicesbased on the group III nitrides will largely rely on advancesin the epitaxial growth technologies for better quality ma-terials.[4]

The growth temperature for GaN in MOVPE is usuallyabove 1000 °C. As a result, hydrodynamics of the gas flowsin the chamber can play an important role in determininggrowth processes and thus the quality of GaN. The detailedmechanism has only recently begun to be elucidated.[4–6] Inthe growth of conventional III–V semiconductors, H2 isusually used as the carrier gas and its flow rate makes upthe major part of the total flow rate in the reactor. Thegroup V flow rate, for example of AsH3, can then be sepa-rately changed to optimize growth conditions with theminimum effect on deposition processes, for example,growth rate. Due to inefficient decomposition of NH3, avery large flow rate of NH3 is required in growth of GaN.Note that, regardless of reactor configuration, the ratio ofNH3 to H2 flow rate is about 1 or above.[4] Thus, the changeof NH3 flow rate not only alters the V/III ratio but alsogreatly influences the hydrodynamics in the reactor.

It is well known that, at a high growth temperature, thethermal convection above the substrate must be sufficientlysuppressed for efficient and uniform growth of GaN. Bylowering the growth temperature, such as in plasmaMOVPE, the effect of the thermal convection can be weak-ened.[7] In conventional MOVPE using NH3 as group Vprecursor, a large gas flow in the direction perpendicular tothe substrate surface is normally used to suppress the ther-mal convection. As in two-flow MOVPE, the use of an ad-ditional inactive gas flow in the vertical direction led togrowth of better quality GaN for photonic device applica-tions.[8]

The thermal convection prevents growing species, i.e.,Ga(CH3)x, from contacting with surface. Thus, the thermalconvection affects growth rate. It is important to reveal thegrowth conditions where the thermal convection is effec-tively suppressed. The optimization of other growth pa-rameters, such as NH3 flow rate, can then be properlycarried out. In this work, the growth rate was investigatedas a function of the gas flow velocity in the verticaldirection at different substrate temperatures. The gas flowvelocity that is required for an effective suppression of thethermal convection is reported and discussed.

Experimental details are given in the Experimental sec-tion. In the vertical reactor configuration, the gas flow veloc-ity in the direction perpendicular to the substrate surfacecan be altered by changing the H2 carrier gas flow rate and/or NH3 flow rate. In this work, the H2 and NH3 flow rateswere actually simultaneously changed to keep the NH3 par-tial pressure constant at 120 torr in the gas phase. Thisminimizes the possible effect of the NH3 partial pressureon the growth rate.[6] The gas flow velocity was approxi-mately estimated using the cross-sectional area of thechamber and the total gas flow rate at a given reactor pres-sure and substrate temperature. The surface of all the sam-ples grown at different gas flow velocities in this work wasspecular. Low temperature photoluminescence measure-ments indicated the dominance of the bandgap emissionover the defects and yellow luminescence. The effect of the

–

[*] Dr. G. Li, Prof. S. J. Chua, W. WangInstitute of Materials Research and Engineering10 Kent Ridge Crescent, Singapore 119260 (Singapore)

Prof. S. J. ChuaThe Center for Opto-electronicsDepartment of Electrical EngineeringNational University of Singapore10 Kent Ridge Crescent, Singapore 119260 (Singapore)

[**] The authors appreciate fruitful discussion with A. Clark (Senior Mate-rial Scientist, EMCORE).

growth conditions on the optical and electrical propertiesof bulk GaN layers will be reported elsewhere.

Figure 1a shows that the growth rate decreases with anincrease of the gas flow velocity at the low growth tempera-ture of 495 °C. At 495 °C, the effect of the thermal convec-tion on epitaxial growth of GaN can be neglected. Due tovery weak decomposition of the reactants (NH3 and tri-methylgallium, TMGa) in the gas phase and/or on the epi-taxial surface, the GaN deposition at low growth tempera-tures is thermodynamically limited.[6,9] Thus, the growthrate becomes very sensitive to the growth temperature. Fig-ure 2 shows that when the temperature rose from 495 °C to550 °C the growth rate was almost doubled. These resultsagree with the previous report.[10]

In this work, the H2 and NH3 flow rates were simulta-neously changed to have different gas flow velocities. As aresult of the fixed TMGa molar flow rate, the TMGa par-tial pressures in the gas phase were actually substantiallydifferent at different gas flow velocities. The larger the gasflow velocity at a higher total flow rate of H2 and NH3, thelower the TMGa partial pressure in the gas phase. There-fore, the reduction of the growth rate with an increase ofthe gas flow velocity shown in Figure 1a could be due tothe decrease of the actual TMGa partial pressure in the gasphase. Figure 1a was therefore re-plotted as a function ofthe TMGa partial pressure in Figure 3a, where it can beseen that the growth rate has an almost linear increase withan increase of the TMGa partial pressure. Also, the growthrate seems to saturate at very large TMGa partial pressure.Note that in the thermodynamically controlled growth sys-tem a significant change of the growth rate with the varia-tion of the TMGa partial pressure such as that seen in Fig-ure 3a would not be expected.

The growth rates, which are shown in Figure 1a, weretherefore divided by the TMGa partial pressure to pre-clude the effect of the TMGa partial pressure. The data arere-plotted as a function of the gas flow velocity in Fig-ure 4a. Interestingly, as can be seen in Figure 4a, the nor-malized growth rate increases linearly with an increase ofthe gas flow velocity. This implies that at 495 °C the decom-position of the growing species, particularly NH3, in the gasphase could be much weaker than that on the growing sur-face. In other words, the growing surface likely plays a cata-lytic role in the decomposition of adsorbed growing species.The surface reactions may play an even more important

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Fig. 1. Growth rates as a function of the gas flow velocity in the directionperpendicular to the substrate surface at a growth temperature of a) 495 °Cand b) 1040 °C.

Fig. 2. Growth temperature dependence of the growth rate. The TMGa par-tial pressure was 0.0152 torr and the gas flow velocity was 0.63 m/s.

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role in the growth of GaN at low temperatures. Hence,pushing the reactants closer to the surface by an increasedvertical gas flow velocity enhances the deposition of GaN.

When the growth temperature was 1040 °C, the growthrate clearly reached a maximum value at a gas flow velocityof ∼ 0.6 m/s (see Fig. 1b). Referring to the discussion above,this effect could be due to the change of the TMGa partialpressure and/or the change of the gas flow velocity. The de-pendence of the growth rate on the TMGa partial pressureis plotted in Figure 3b, which displays a curve similar tothat observed in Figure 1b. The maximum growth rate wasclearly at a TMGa partial pressure of about 0.09 torr. Atvery high growth temperatures, the deposition of GaN ismainly mass-transport limited, which should lead to a lin-ear dependence of the growth rate on the TMGa partial

pressure. This is not the case (see Fig. 3b). Under ourgrowth conditions, the growth rate even declines when theTMGa partial pressure increases above about 0.09 torr.The results in Figures 1b and 3b are then explained by thecompetitive effect of the thermal convection and mass-transport mechanism as follows.

At 1040 °C, the thermal convection above a hot surface isvery strong. Insufficient suppression of the thermal convec-tion can largely prevent the growing species from contact-ing with the surface, which eventually reduces the growthrate at a given TMGa partial pressure. Note that a low gasflow velocity in Figure 1b corresponds to a high TMGa par-tial pressure in Figure 3b, and a low TMGa partial pressurein Figure 3b corresponds to a high gas flow velocity in Fig-

Fig. 3. Dependence of the growth rates on the TMGa partial pressure in thegas phase at a growth temperature of a) 495 °C and b) 1040 °C.

Fig. 4. Growth rate normalized by the TMGa partial pressure as a functionof the gas flow velocity in the direction perpendicular to the substrate sur-face at a growth temperature of a) 495 °C and b) 1040 °C.

ure 1b. In the high TMGa partial pressure region in Fig-ure 3b, the gas flow velocity is not high enough to suppressthe thermal convection. The thermal convection dominatesover the effect of the TMGa partial pressure on the growthrate. As a result, the growth rate increases with an increaseof the gas flow velocity in Figure 1b or decreases with anincrease of the TMGa partial pressure in Figure 3b. In thelow TMGa partial pressure region in Figure 3b, the gasflow velocity appears to be high enough to suppress thethermal convection. The deposition becomes mainly lim-ited by the mass-transport mechanism. The growth rateconsequently increases with an increase of the TMGa par-tial pressure in Figure 3b. The maximum growth rate inFigure 1b or Figure 3b represents the minimum gas flowvelocity for an effective suppression of the thermal convec-tion.

This can be seen even more clearly when the growthrates are divided by the TMGa partial pressure and re-plotted as a function of the gas flow velocity (Fig. 4b). Fig-ure 4b shows that the normalized growth rate increases lin-early with an increase of the gas flow velocity in the lowgas flow velocity region. This indicates the course of sup-pressing the thermal convection by increasing the verticalgas flow velocity. The saturation of the normalized growthrate with the gas flow velocity beyond 0.7 m/s means thatthe thermal convection has been effectively suppressed.Under our growth conditions, the minimum vertical gasflow velocity at 1040 °C for effective suppression of thethermal convection is about 0.7 m/s. Above that gas flowvelocity, a slight increase of the growth rate with the gasflow velocity could also be ascribed to the effect of the re-actants being pushed closer to the surface, as discussedabove for the growth temperature of 495 °C.

GaN grown using the optimized gas flow velocity of0.7 m/s at 1040 °C was characterized using Hall effect mea-surement and photoluminescence. The room temperatureHall mobility was 380 cm2 s–1 V–1 with an electron densityof about 4 × 1017 cm–3. The full width at half maximum ofthe near band-edge emission peak was about 6 meV at 8 Kand 27 meV at room temperature. Compared to the strongnear band-edge emission, the yellow band emission ataround 2.2 eV and the donor–acceptor (DA) emission inthe blue spectral region were both negligible. These resultsindicate the high quality of the GaN. We found that reduc-ing the gas flow velocity to about 0.5 m/s does not signifi-cantly affect the epilayer quality, while the DA emissionbecomes stronger for gas flow velocity below about 0.5 m/s.This is likely due to poor decomposition of NH3 when thethermal convection has not been effectively suppressed.The deterioration of epilayer uniformity was evident fromthe observation of more color fringes on the wafer whenthe gas flow velocity was reduced below about 0.5 m/s.Changing the gas flow velocity in the high region aboveabout 0.5 m/s appears not to affect the epilayer uniformity.Scanning electron microscopy (SEM) measurement indi-cated that the epilayer thickness uniformity was about 5 %

over the 2 inch wafer when the growth was conducted atthe gas flow velocity of 0.7 m/s.

In conclusion, at low growth temperatures (490 °C), thedeposition of GaN on sapphire is thermodynamically lim-ited, thermal convection having a negligible effect. Thegrowth rate is sensitive to the growth temperature. An in-creased gas flow velocity leads to an increased growth rate,probably due to the reactants being pushed closer to theepitaxial surface for enhanced surface reactions. At highgrowth temperatures (1040 °C), the growth of GaN is deter-mined by the competitive effect of the thermal convectionand mass-transport mechanism. At low gas flow velocities,the thermal convection plays a dominant role. The growthrate increases with an increase of the gas flow velocity. Un-der our growth conditions, the thermal convection can beeffectively suppressed at a vertical gas flow velocity ex-ceeding about 0.7 m/s. Using the optimized gas flow veloc-ity, high quality uniform GaN on sapphire was grown.

Experimental

GaN was grown in a multi-wafer vertical reactor at 200 torr. Trimethyl-gallium (TMGa) and NH3 were separately pushed into the reactor from thetop distribution flange. The carrier gas was H2. The wafer carrier was ro-tated at 1000 rpm to generate a sharp temperature gradient normal to thewafer. (0001)-oriented sapphire was used as the substrate. A 250 Å thickGaN buffer layer was grown at 495 °C, prior to the deposition of GaN at1040 °C. The TMGa flow rate was fixed at 1.05 × 10–4 mol/min for the bufferand 3.44 × 10–4 mol/min for the bulk GaN. Growth rate was measured in situusing a photo-reflectance monitor.

Received: February 9, 1998Final version: May 26, 1998

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