In-Situ Reflectance Monitoring During III-Nitrides Growth

One of major limitations of III-nitrides technology was absence of homoepitaxial substrate. This limitation was usually overcome by growing a low-temperature (LT)

AlN or GaN nucleation layer on a sapphire substrate before the high-temperature (HT) growth. The LT nucleation layer serves as a template to increase nucleation site and improve the crystal quality of HT growth GaN layer. Although Wu et al. have reported the growth evolution of LT nucleation layer. Amano et al. and Akasaki et al.

have also reported the process of GaN growth with AlN nucleation layer. Their studies were all restricted to ex-situ characterization with interrupted growth. In order to realize the growth information during growing GaN, we need an in-situ monitoring tool to extracting growth information.

The first report of in-situ monitoring during growing GaN was present by Nakamura et al. He investigated the infrared radiation transmission intensity (IR-RTI) oscillation for GaN growth with and without AlN nucleation layer. He concluded that thickness of AlN nucleation layer would strong effected the crystal quality of HT GaN film and verified the model proposed by Amano and Akasaki. This pyrometric interferometry has well developed in MBE for AlGaAs, GaAs, and InAs growths.

However, there are several drawbacks such as infrared detection wavelength limitation, spectrum dispersion due to broad detection band, and the temperature drift during growth.

Recently, Breiland et al. develop a method that optical constants and growth rate can be simultaneously extracted from the in-situ normal incidence reflectance of a

growing thin film. Consider a general III-V compound bulk, we usually grow a thin film on a specific substrate. Suppose the thin film material is different from substrate, the reflectance and interference from the underlying interfaces is much difficult to present. Therefore, Breiland et al. develop a “virtual interface model” to revel the real reflectance in a multiple-layer system.

In this study, we use a spectral reflectivity system to measure the thickness and optical constants of “translucent” thin film layers on the substrates. Because of its wave-like properties, light reflected from the top and bottom surfaces of a thin-film layer can be in-phase so that reflections add, or out-of-phase so that reflections subtract. Whether the reflections in-phase or not depends on the wavelength of the light, as well as the properties of the epitaxial film.

This system is PC-based and integrates measurement and analysis software with a spectrophotometer and fiber optic measurement hardware. Figure 3.3.1 shows the hardware configuration of EpiTune II for the planetary reactor. As mentioned above, this in-situ monitoring technique is a powerful tool for measuring the epitaxial growth rate. Apply this tool we can also do temperature calibrations and growth rate calibration for several different materials in a single run. In the former case, one can perform temperature calibrations with eutectic wafers (Al-coated Si wafers or Silver-coated Si waver) quickly and easily by plotting the reflectance as a function of

temperature, while the temperature of the reactor is ramped slowly. It is well known that the melting poing of aluminum and silver are 660 C and 961 C. As the temperature ramping up to the melting point, the reflectance would be drop down rapidly. Thus one could know the meter temperature is the metal melting point.

In the case of a material system with clearly different indices of refraction between an underlying layer and the layer to be probed, time-resolved Fabry-Perot like reflectance is utilized to determine the growth-rate and the crystalline quality of the growing wafers. There methods, EpiTune I and EpiTune II, can be applied in the case of nitride semiconductor structures where a step in refractive indices is present at the interface between the growing device structure and the sapphire wafer or in selected phosphorus and arsenic containing device structures. In addition, features in the traces at the beginning of the layer growth can be distinguished by the experienced process engineer, thus speeding up time to market for new processer and devices.

Figure 3.3.2 exhibits such a trace for the case of an InGaN MQW structure as described above. The different steps like nucleation, anneal, buffer and MQW growth can be clearly distinguished. In addition to the sole measurement of the reflectivity as a function of time, EpiTune II offers the possibility to measure the emissivity corrected temperature for each wafer. A high repetition rate of the pyrometric temperature measurement and the probing of the reflectivity through the same optical

path facilitate an accurate determination of the emissivity corrected temperature, since pyrometry and reflectometry are practically performed on the same point on the wafer.

In conclusion, as devices become more complex and epitaxial wafer specifications more tighten, the MOCVD equipment for compound semiconductor manufacturing must also advance. The in-situ monitoring technique is simple and powerful, and gives the MOCVD users the really practical tool for looking at epitaxial growth while the growth is in progress.

Fig. 3.2.1 Schematic reactor of production scale EMCORE D180 MOCVD system

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Fig. 3.2.2 Schematic reactor of production scale AIXTRON 2600G3 HT MOCVD system

Fig. 3.2.3 Layout of the AIXTRON 2600G3 HT susceptor in the 242 inch configuration.

Fig. 3.2.4 photo of the reactor chamber of the AIXTRON 2600G3 HT in the 242 inch configuration.

Fig. 3.3.1 Hardware configuration of EpiTune II for planetary reactor.

Fig. 3.3.2 In-situ reflectometry for an InGaN MQW growth run.