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Using Photoluminescence For Characterizing Wide-bandgap Materials And Devices

Using a pulsed short-wavelength laser improves the accuracy and usefulness of photoluminescence for characterizing wide-bandgap semiconductors, according to Colm Mac Mahon and colleagues.
A great deal of research is currently focused on wide-bandgap materials, specifically on the III-nitrides and SiC for applications such as LEDs, laser diodes and photodiodes. The combination of high electric breakdown field and high saturation velocity also makes these materials suitable for high-temperature and high-power RF transistors. The importance of these materials is highlighted by the involvement of DARPA, which has recently launched a program called the Wide Bandgap Semiconductor Technology Initiative (WBSTI) to help accelerate improvements in material quality and device performance. Materials characterization plays a crucial role in such development efforts, and photoluminescence (PL) studies can reveal much about epitaxial structures before expensive device processing steps are carried out (Compound Semiconductor March 2001 p68).

PL studies of wide-bandgap materials are complicated by effects that are not seen, or that are uncommon, in other material systems. The PL from the quantum well (QW) structures required for LEDs and lasers is influenced by the quantum confined Stark effect (QCSE) and elemental clustering. These phenomena are discussed below, and their influences are explained using real data. The type of laser used in PL studies has a great influence on these effects, allowing improved correlation between PL and electroluminescence (EL), which is crucial for manufacturers of LEDs and lasers.
Materials and choice of laser
For PL to occur, the laser excitation needs to have a higher energy than the bandgap of the material under investigation. There are currently a number of lasers available that are suitable for the PL characterization of wide-bandgap semiconductors. The continuous-wave (CW) options include the Ar+ ion (244 nm), Nd:YAG (266 nm), HeCd (325 nm) and Nd:YAG (355 nm). There is also a Q-switched (QS) Nd:YAG option, which emits a pulsed beam at 266 nm. A comparison of the typical parameters of the CW and QS lasers for use in PL characterization is shown in table 1.

CW lasers for use in wide-bandgap characterization tend to be large, and are mounted outside the PL system. Fiber-optic coupling is necessary to get the light to the sample being measured. Over time, fiber-optic coupling can lead to decreased laser power due to fiber degradation, which means frequent coupling changes. CW lasers also have long warm-up and cool-down periods, are expensive and have a short lifetime. The CW 325 nm HeCd commonly used in the characterization of wide-bandgap materials is not suitable for materials with higher bandgaps and for structures requiring high laser power densities. While high power densities can be achieved by focusing the spot to a smaller size (
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