INDUSTRY MOCVD
Figure 3. Uniformity across the wafer, evaluated in terms of the standard deviation, can be less than 0.5 nm. This wafer is numbered 14 in Figure 2
the volume of gas within the reaction chamber compared with a conventional, barrel-type chamber with a full-platter injector. Consequently, residual gas is swept out of the system very quickly.
with the pocket; and good work practice to ensure particle free placement of the wafer in the pocket. Characterisation of LED epiwafers has not been limited to photoluminescence studies. We have also investigated the thickness uniformity, crystalline quality and electrical properties of other key layers, such as the n-type GaN, p-type AlGaN and p-type GaN layers, wherein material quality is very high, validating the high level of capability associated with the reactor design.
Influencing material quality with reactor design
Figure 5. The left and right plots show photoluminescence intensity before and after improved temperature ramping and stability during quantum well growth. These improvements can lead to better interface quality
Although excellent wavelength uniformity is an essential attribute of any production-worthy MOCVD reactor, the factors that contribute to good uniformity don’t necessarily guarantee a brighter LED. There are many factors that influence the quantum efficiency of this device. Basic quality of the epitaxial material, dislocation density and background contamination levels are obvious examples. The quality of the MQW interfaces is also a major contributor. In a typical MQW LED the InGaN active layers must be grown at a precise temperature while the GaN barrier layers are grown at a temperature that may be over 100 °C higher. To obtain atomically sharp interfaces with an absolute minimum of inter- diffusion or contamination between the layers demands gas switching that is rapid and abrupt; fast temperature ramping and stabilization and, when the growth temperature is reached, rock- solid stability. Our reactor, thanks to its novel design, excels in all these areas. The small injector in combination with the curved wall profile reduces
The thermal response is also very rapid, thanks to an over-powered heater and a very light wafer carrier. Thermal reflectors, built into the flow guide surrounding the wafer carrier, maximise the efficiency of the heater while also shielding exhaust gases from being directly exposed to the heating elements. The end result is a system that is capable of very rapid gas switching, very rapid thermal ramping and very rapid temperature stabilisation – leading to sharp and clean interfaces and uniform indium concentration within the QWs. Both these effects can be observed in the detail of high-quality triple-axis X-ray diffraction data (see Figure 4). The intensity and widths of the higher-order satellite peaks are influenced by the ‘sharpness’ of the QW structure. Fast thermal cycling and stabilization also has a throughput benefit. For an eight-period MQW, total growth time can be reduced by as much as 40 minutes.
Figure 4. Triple axis X-ray diffraction data and simulation from an eight period MQW structure with 20-period strain- relieving superlattice. Note the narrow and symmetrically shaped superlattice peaks and how well they match the simulated data
But the biggest benefit is the effect on photoluminescence or electroluminescence intensity. After optimisation of the MQW growth temperature control and cycle time, we have seen up to 20 percent higher intensities from identical structures grown under otherwise identical conditions (see Figure 5).
This gain in emitted intensity, along with a faster growth time that contributes to a higher throughput and up to a 40 percent reduction in gas and alkyl consumption, demonstrates that our reactor may be ideal for LED manufacturing. In short, we believe that chipmakers armed with this tool can build LEDs cheaper and brighter.
© 2013 Angel Business Communications. Permission required.
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www.compoundsemiconductor.net August / September 2013
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