A team from Tohoku University in Japan have devised a way to improve on photoluminescence spectroscopy techniques using a hollow sphere to measure the electronic and optical properties of large crystals of a semiconductor. [Kojima, K., et al., Appl Phys Express; DOI: 10.7567/1882-0786/ab2165].
Semiconducting crystals are widely used in microelectronic devices. The crystals must be pristine for the fabrication of microprocessors and so the detection of crystal defects is critical as well as the ability to test their energy conversion efficiency. Techniques are available for the measurement of "internal quantum efficiency", their ability to generate photons from electrons excited by an electric current or an exciting laser. Unfortunately, the size of the samples that can be tested with these techniques is rather limited. Tohoku's Kazunobu Kojima hopes to circumvent this problem.
Standard approaches are able to determine the relative amount of light emitted by a semiconductor crystal when it is irradiated with an excitation laser. Energy dissipates through the excitation and emission processes, so the team has been testing whether photoluminescence spectroscopy can be more contained using an "'integrating sphere" to minimize photon losses.
The team explains that their integrating spheres can collect both the excitation light and the light emitted from the sample lying within. The light is diffusively reflected inside the sphere until it becomes uniformly dispersed. This uniform distribution of light improves the accuracy and reproducibility of internal quantum efficiency testing, the team has found. Under normal circumstances, the size of the crystal being tested would be ultimately limited by the size of the sphere. However, Kojima and colleagues have found that they can test the internal quantum efficiency of a crystal when it was placed directly outside the sphere, allowing larger samples to be used.
The team has conducted the tests on gallium nitride, one of the most well-known materials in light-emitting diodes and is anticipated as being a useful component of a wide range of other devices where in some contexts it might even augment the even more well known silicon.
"This 'omnidirectional photoluminescence' spectroscopy can be used to evaluate the quality of large-sized crystals or semiconductor wafers, which are essential for the mass production of power devices," explains Kojima, adding that this could lead to energy savings and hopefully reduce production costs.