Supersonic Jet Epitaxy: An Improved Method for Nitride Deposition
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G= ((r+ 1)/2)r/(r- 1)
Here r is the ratio of the heat capacity at constant pressure to the heat capacity at constant volume of the gas, and G is less than 2.1 for all gases. At this point the pressure at the exit of the nozzle becomes pi/G, independent of Po. After leaving the nozzle the gas continues
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Mat. Res. Soc. Symp. Proc. Vol. 395 01996 Materials Research Society
to expand, gas velocity continues to increase, and M becomes greater than 1. When this occurs, the gas flow becomes a supersonic jet. The translational energy of this monochromatized supersonic molecular beam can be monitored by controlling the nozzle temperature and gas seeding according to the formula. 1 2 Vt = (2RT(YXi(ri/(ri- 1))/yXiWi))( / )
(3)
Here Vt is terminal velocity of the gas, R is the gas constant, T is the nozzle temperature, Xi and Wi is the mole fraction and molecular weight of the component gas in the mixture. It is clear that the terminal energy is proportional to the stagnation temperature inside the nozzle and also inversely proportional to the molar average molecular weight. This means that heating the nozzle could increase the translational energy of the molecules, and gas seeding, e. g., mixing heavy molecules in larger quantity lighter molecules, results in a larger terminal velocity for the heavier molecules[61, resulting in 0.5-5.0 eV kinetic energy. After expansion, the molecules in the supersonic jet will collide with other background gas molecules in the growth chamber. The collision probability depends on the chamber pressure and gas constituents. The mean free path in nitrogen of 10 mTorr is around 5 mm. To avoid excessive cooling of the beam, the growth chamber pressure should be low so as the mean free path of the molecules is comparable to the distant to the substrate. The unique characteristics of the supersonic jet arise from the physical processes taking place at the nozzle as the molecules squeeze through the orifice from the high pressure inside the nozzle to the chamber vacuum. When the mean free path inside the nozzle is smaller than the orifice diameter, the expanding molecular beam possesses monochromatic nature and directionality. The monochromatic high energy of the reactant molecules impinging to the growing surface enhances the sticking efficiency, helps to break chemical bonds and promotes surface migration, and makes it possible to reduce the growth temperature and to control the kinetic energy to less than damage threshold of 5xEg of the growing crystal films[lI]. The supersonic jet beam also effectively delivers the source molecules to the growing surface so that growth efficiency and rate can be increased. But the ultimate growth rate is limited by the pumping speed required to keep the molecular mean free path large enough for the reactant molecules to reach the substrate without excessive cooling. T0-600"C 5 H511 oatS1(100)
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