Ultraviolet (UV) pyrometry
Wafer surface temperature for transparent substrates
Real GaN surface temperature is the most critical parameter of the epitaxial process. A temperature variation of less than 1 K across the wafer is necessary to meet the upcoming specs of LED manufacturing for SSL applications. When the wafer bows, the deviation between wafer surface temperature and pocket temperature increases in the case of a 4” sapphire substrate, for example, by up to 20 K or more. This results in a significant indium content variation in the InGaN/GaN multiple quantum wells (MQW) and, as a result, in strong variations in emission wavelength. In other words, surface temperature is fundamental for the homogeneous growth of the active layers and the color uniformity of the final devices.
Transparent substrates: GaN growth on sapphire or SiC
When it comes to bowed transparent substrates like sapphire or SiC, a conventional infrared (IR) pyrometer can measure only the pocket temperature. The GaN surface temperature can only be measured using ultraviolet (UV) pyrometry around 400 nm because the Egap is larger in the UV range. The screenshot below shows surface temperature variations during a laser growth.
The conventional pyrometer (blue) does not "notice" the drop in wafer temperature by 20 K after changing the reactor pressure. But Pyro 400 does! The red curve in the screenshot shows the exact wafer surface temperature as measured by LayTec's Pyro 400.