A key method in understanding fundamental semiconductor material properties and device performance is optical characterization. At NaMLab, various techniques such as photoluminescence (PL, Fig. 1), Raman and Fourier transform infrared spectroscopy are applied to determine defect types and energies, phonon energies and transmission and reflection characteristics in semiconductor materials. Furthermore, photoactive electrical devices can be characterized.
Special focus of NaMLab’s optical activities over the past two years was placed on low-temperature PL measurements of gallium nitride (GaN) material with its band gap of 3.4 eV. The HeCd excitation laser emitting at 325 nm had to be refurbished after reaching the end of its lifetime and now features an output power in excess of 50 mW again. A challenge was and still is to resolve very narrow excitonic features originating from defects in epitaxial GaN. As an example, in low-temperature PL spectra the oxygen-bound donor exciton has been resolved with a record-breaking line width of ~ 150 µeV (Fig. 2) demonstrating both, the outstanding sample properties and the capability of the PL set-up.
With the Raman set-up, degradation mechanisms of silicon nanowire anodes integrated in lithium ion batteries could be extensively investigated in situ during Si lithiation/delithiation. With these measurements a relationship between structural and electrochemical properties over electrode cycling could be established. As a result, amorphous as well as liquid and transient species in a battery cell were observed and the difference between silicon nanowires and its carbon-coated counterpart as anode materials became immediately visible (Fig. 3).
In summary, established optical methods at NaMLab include:
- Low-temperature photoluminescence (15 – 300 K) in the UV – visible spectral range (340 nm – 800 nm) with UV (325 nm) laser excitation
- Spectral response with and without bias illumination for photosensors and solar cells
- Micro-Raman mapping with 457, 514 and 785 nm excitation wavelength (spatial resolution: 1 µm) for e.g. local strain analysis
- IR and VIS-NIR ellipsometry and VIS-NIR reflectometry
- Microwave detected photoconductivity for 2D mapping of minority carrier lifetimes in silicon
