Metalorganic Chemical Vapor Deposition System
2.5 In-Situ Reflectance Monitoring During III-Nitrides Growth
For many years, high vacuum growth techniques such as MBE have enjoyed the advantage of using in situ diagnostic tools such as RHEED to determine substrate surface conditions and measure growth rates. Now MOCVD has responded with a variety of optical techniques to perform diagnostic on the layers as they grow [22]. Indeed, the in-situ monitoring of optical techniques is very useful for growing optoelectronic devices such as VCSELs due to the stringent requirement of phase matching. The in-situ monitoring is also an important tool to grow III-Nitride materials on foreign substrate, such as sapphire, because the limitation of homoepitaxial substrate absence is usually overcome by growing a low-temperature (LT) AlN [23] or GaN [24] nucleation layer on a sapphire substrate before the high-temperature (HT) growth. A growth mode of GaN film on sapphire substrate covered with a thin AlN buffer layer is shown in Figure 2-9. In order to realize the growth information during growing GaN, the in-situ monitoring tool is usually used to extracting growth information.
In our D-75 system, we use an in-situ normal incidence reflectance method to extract the information of growth rate and crystal quality. The tungsten-lamp reflectometry is applied as the in-situ monitoring in D-75 type MOCVD system. The schematic of the configuration is shown in Figure 2-10. Two optical heads are mounted on the top flow flange of the reactor and monitoring the growth conditions for the upper and lower points
of the wafer surface. An optical head couples two fibers; one is from the light source with the continue band wavelength, and the other is to the detector. Suppose the optic index of thin film materials is different from the substrate, the reflectance and interference from the underlying interface can be described as R= r2, and r can be described as [25]
(2-9) where β2 =4πnd/λ is the phase shift in the film, r∞ and r are the reflectance of an i infinitely thick film of the top layer and the internal layer reflectance, respectively.
Theexp(−i4πβ2) term describe and oscillatory behavior of β2. Thus growth rate can be determined with the refractive index at certain growth temperature at a given wavelength.
The growth rate can be obtained by
(2-10) where λ is the chosen wavelength, Td is the oscillation period of the reflectivity curve, nλ is the refractive index of the epitaxial layer at the growth temperature for λ. This is very useful controlling the thickness of the LT nucleation layer. Figure 2-11 shows the example for the measured reflectivity of a GaN bulk structure.
Furthermore, applying the in-situ monitoring system, we can also do temperature calibrations. In the former case, one can perform temperature calibration with eutectic wafers (Al-coated Si wafers or Silver-coated Si wafers) quickly and easily by plotting the reflectance as a function of temperature, while the temperature of the reactor is ramped
)
slowly. As the temperature ramping up to the melting point of aluminum or silver, 660oC, and 961oC, respectively, the reflectance would be drop down rapidly. Thus one could know the meter temperature is the metal melting point. In the latter case, one can design a
“calibration” structure that contains layers of different compositions or different growth condition in a single growth run without requiring any post growth characterization.
Figure 2-12 shows the typical reflectance plot as a function of time depicting the growth of different layers for calibration of growth rate. The result shows that the growth rates of layers can be clearly judged.
The in-situ monitoring system is PC-based and integrates measurement and analysis software with spectrophotometer and fiber optic measurement hardware. Figure 2-13 shows the schematic diagram of the in-situ monitoring apparatus. Light source is a tungsten-halogen bulb that supplied light form approximately 400nm to 3000nm. This light is delivered to and collected from the sample through a fiber optic cable bundle and a lens. The intensity of the reflected light is measured at 512 different wavelengths with a spectrometer. The spectrometer uses a diffraction grating to disperse the light and a linear photodiode array to measure the light at the different wavelength. The photodiode array operates by integrating the current generated by light falling on each of the 512 pixels.
After a user-selectable integration time, the accumulated charge in each photodiode is read by the computer. Appropriate adjust the integration time to meet the proper signal
level.
References
[1] J. B. Mullin, S. J. C. Irvine, and J. Tunnicliffe, J. Cryst. Growth, 68, 214, 1984 [2] B. Cockayne, and P. J. Wright, J. Cryst. Growth, 68, 223, 1984
[3] H. M. Manasevit, Appl. Phys. Lett. 12, 156, 1968
[4] S. Nakamura, T. Muksi, and M. Senoh, Appl. Phys. Lett. 64, 1687, 1994 [5] H. M. Manasevit, and W. I. Simpson, J. Electrochem. Soc, 116, 1725, 1969 [6] Alan G. Thompson, Materials Letters, 30, 255, 1997
[7] H. P. Maruska, and J. J. Tietjen, Appl. Phys. Lett., 15, 367, 1969.
[8] H. M. Manasevit, F. Erdmann and W. Simpson, J. Electrochem. Soc, 118, 1864, 1971 [9] S.Yu. Karpov, V.G. Prokofjev, E.V. Yakovlev, R.A. Talalaev, Yu.N. Makarov, MRS J.
Nitride Semicond. Res. 4, 4, 1999
[10] S. P. DenBaar, B. Y. Maa, P. D. Dapkus, and H. C. Lee, J. Cryst. Growth, 77, 188, 1986 [11] H. Kawai, I. Hase, K. Kaneko, and N. Watanabe, J. Cryst. Growth, 68, 406, 1984 [12] C. C. Wang and S. H. McFarlane, III, J. Cryst. Growth, 13/14, 262, 1972
[13] E. J. Thrush, J. E. A. Whiteaay, and Wale-Evans, J. Cryst. Growth, 68, 412, 1984 [14] R. D. Dupuis, L. A. Moudy, and P. D. Dapkus, Inst. Phys. Conf, Ser., 45, 1, 1978 [15]J. S. Roberts, and N. J. Mason, J. Cryst. Growth, 68, 422, 1984
[16] M. J. Ludowise, J. Appl. Phys., 58, R31, 1985 [17] S. Yu. Karpov, J. Cryst. Growth, 248, 1, 2003
[18] H. Beneking, A. Escobosa, and H. Kraeutle, J. Electron. Mater., 10, 473, 1981.
[19] C.C. Hsu, R. M. Cohen, and G. B. Stringfellow, J. Cryst. Growth, 63, 8, 1983 [20] J. P. Noda and A. J. SpringThorpe, J. Electron. Mater., 9, 601, 1980.
[21] R. H. Moss and P. C. Spurdens, J. Cryst. Growth, 68, 96, 1984 [22] D. E. Aspnes, IEEE J. Select. Topic Quant Elect., 1, 1054, 1995
[23] H. Amano, N. Sawaki, I. Akasaki, and Y. Toyoda, Appl. Phys. Lett., 48, 353, 1986.
[24] S. Nakamura, Jpn. J. Appl. Phys., 30, L1705, 1991
[25] W. G. Breiland, and K. P. Klleen, J. Appl. Phys., 78, 6726, 1995
Table 2-1 The names and properties of some of a the more commonly used metalorganic (MO) compounds for III-V and II-VI MOCVD ”common” implies a widely used material.
Antimony Trimethylantimony TMSb 50 at 10 L
Arsenic Tertiarybutylarisine Phosphorus Tertiarybutylphosphine TBP 250 at 20 L
Tellurium Diethyltelluride DETe 7 at 20 L
Figure 2-1 Schema of EMCORE D-75 system
Figure 2-2 Major components of a low pressure MOCVD system.
Figure 2-3 Components functions in a low pressure MOCVD system
Figure 2-4 Schema of gas blending system in MOCVD.
Figure 2-5 Two types MOCVD systems (a) Vertical chamber,
(b) Horizontal chamber
200 RPM 400 RPM
800 RPM 1600 RPM
Figure 2-6 Gas flow patterns for high speed rotating disk
3 2 1
Alkyl inject Hydride inject
Double o-ring
Water-cooled flow flange
screen
Wafer Wafer carrier
Reactor wall
Exhaust
Spindle filament
Susceptor
Inner thermal couple Water-cooled
baseplate
Figure 2-7 Schema of reactor design in MOCVD D-75 system.
Figure 2-8 Picture of Filament used in EMCORE D-75
Figure 2-9 Model for the growth of GaN grown by MOCVD using the
LT-buffer layer.
Figure 2-10 Schematic of in-situ monitoring configuration. By collecting the reflection of a Xe-Lamp light perpendicularly impinging on the wafer surface
Xe-Lamp Detector fiber
Wafer
Reactor
GR = λ *T
d/(2*n)
Figure 2-11 The measured reflectivity of GaN bulk layer on sapphire.
0 1000 2000 3000 4000 5000 6000 7000 8000 0.00
0.05 0.10 0.15 0.20
0.25 i ii iii vi v
R e fle c ta n c e
Time (s)
Figure 2-12 The calibration of growth rate for different nitride-based materials by in-situ reflectance measurement.
0 5000 10000
0.00 0.05 0.10 0.15 0.20