Near-field imaging and spectroscopy of locally strained GaN using an IR broadband laser

Abstract

Scattering-type scanning near-field optical microscopy (SNOM) offers the possibility to analyze material properties like strain in crystals at the nanoscale. In this paper we introduce a SNOM setup employing a newly developed tunable broadband laser source with a covered spectral range from 9 µm to 16 µm. This setup allows for the first time optical analyses of the crystal structure of gallium nitride (GaN) at the nanometer scale by excitation of a near-field phonon resonance around 14.5 µm. On the example of an artificially induced stress field within a GaN wafer, we present a method for a 2D visualization of small deviations in the crystal structure, which allows for fast qualitative characterizations. Subsequently, the stress levels at chosen points were quantified by recording complex near-field spectra and correlating them with theoretical model calculations. Applied to the cross-section of a heteroepitaxially grown GaN wafer, we finally demonstrate the capability of our setup to analyze the relaxation of the crystal structure along the growth axis with a nanometer spatial resolution.

Exploiting Phonon‐Resonant Near‐Field Interaction for the Nanoscale Investigation of Extended Defects

“…As previously reported for grain boundaries in SrTiO 3 , an accumulation of charge carriers at extended defects can lead to a similar damping of the phonon near‐field resonance . Furthermore, local stress and strain can significantly influence the phonon spectra as well 27b,38. Strain is likely to play a significant role around lattice defects, and locations with increased strain have been observed around the extended carrot defect.…”

“…In our case no such reference region was available close to the defects investigated. To derive quantitative values for the local stress/strain, charge carrier accumulations, or crystal quality around the defects in future studies, nonresonant reference materials such as lithographically deposited Au patches with additional direct comparison to TEM or KOH etch‐pit analysis are necessary.…”

“…Both s‐SNOM and nano‐FTIR would be beneficial additions to current investigations of semiconductor materials for high temperature and high power applications, such as SiC and emerging materials 2a,3a,b. Specifically, an investigation of additional wide bandgap semiconductors, such as GaN or different Ga 2 O 3 polytypes would be possible with current techniques, as at least a portion of the Reststrahlen bands for each is above 560 cm −1 , which is the low‐frequency cutoff for current compact, broadband IR lasers employed in nano‐FTIR (Figure S2, Supporting Information) 35a‐c,38,55,62. Such studies would be valuable as the scientific community seeks to advance these materials for future power applications as well as for emerging single‐photon emission in quantum optics applications and mid‐ and far‐IR optical and nanophotonic concepts.…”

“…The s‐SNOM technique has already demonstrated that near‐field studies of SiC can distinguish between different polytypes,27a strain,27b and crystalline quality within the Reststrahlen band, suggesting that these near‐field techniques will also allow for insightful characterization of extended defects. Additionally, while the s‐SNOM technique has recently been employed to study fundamental material properties of semiconductors and/or phase change materials other than SiC, such as GaN, hexagonal boron nitride (hBN),24c,d,28,39 vanadium dioxide (VO 2 ), Ge 3 Sb 2 Te 6 (GST),32c and Ag 4 In 3 Sb 67 Te 26 (AIST), a detailed, correlative study of extended defects in wide‐bandgap materials has not been performed up until now.…”

Nearly diffraction limited FTIR mapping using an ultrastable broadband femtosecond laser tunable from 133 to 8 µm

“…State-of-the-art devices also use interferometric spectroscopy (nano FTIR), wherefore it would be interesting to transfer our system from the far-field to the near-field. Furthermore our laser system might be suited for such applications, due to its bandwidth, low noise, tuning range and long-term stability, as previous publications on mid-IR sources for s-SNOM have indicated [27][28][29]. The laser system might ultimately even enable a low-noise mid-infrared femtosecond pump-probe configurations.…”