The wide-bandgap gallium nitride (GaN)’s one-dimensional semiconductor-nanowires are leading contenders for nanoscale devices such as power/high-temperature electronics and short wavelength emitter optoelectronic devices. Hence, it is necessary to measure the composition and homogeneity of such strongly anisotropic materials and to correlate these characteristics with their dimensions and optical properties.
This article discusses the application of a confocal microscope equipped with a high-resolution piezoelectric stage to perform high- resolution Raman measurements on a single GaN nanowire. The high-resolution piezoelectric stage facilitates an accurate and reproducible positioning.
The experiment started with cutting and positioning of a single  nanowire with a Wurtzite-like hexagonal cross section on a microscope glass slide. The AFM measurement revealed that the diameter and length of the nanowire were about 170nm and 41µm, respectively (Figure 1). The Raman instrument used was a combination of a LabRAM spectrometer (grating 600 grooves/mm, resolution 4 cm-1) from HORIBA Scientific and an inverted microscope (Olympus IX 71).
Figure 1. Cross section (a) and atomic force microscopy topographical image (b) of the  GaN nanowire deposited on a glass substrate.
The piezoelectric stage was used to scan the sample with roughly 1nm intrinsic accuracy in the lateral directions. The input polarization was chosen with a half- waveplate and the light beam input excitation wavelength was set to 514.5nm (λ) Ar+ ion laser. A 100X objective (Olympus MPL 100X-NA=0.90) was used to perform focusing on the sample and the Y(ZZ)Y, Y(XX)Y and Y(XZ)Y polarized spectra were measured by an analyzer just in front of the entrance slit of the spectrometer.
A liquid-nitrogen cooled CCD camera was used the detector. Although the acquisition time set was 1s per spectrum in the imaging mode, an acquisition time of 20s per spectrum was applied to capture most spectra.
For GaN single nanowire, four main signals were detected at 142, 530, 557 and 568cm and were assigned to E2 (low), A1(TO), E1(TO) and E2(high) symmetry type modes, respectively (Figure 2). Step-spectra were recorded at every 200nm and at 1s integration time to perform the mapping of the nanowire. The differences in the Raman signal across the entire nanowire were probed by intensity integration of the [509-552 cm-1] spectral domain around the A1(TO) mode (530 cm-1). The images exhibited a lateral resolution superior than 200nm, with an acquisition time of roughly 1h for a polarized image.
Figure 2. Video image (a) and polarized Raman spectra of the GaN nanowire in the Y(ZZ) Y, Y(XX)Y and Y(XZ) Y polarization configurations (b-d).
From Figure 3a, a maximum signal was observed on the straight portions of the nanowire fo the Y(ZZ)1 polarization configuration, while the signal almost disappeared on the bent portion of the nanowire. The complementary image was acquired by integration of the E2 (high) mode at 568 cm-1 (Figure 3b).
Figure 3. Polarized Raman images generated by integration of the [509-552 cm- 1] (a) and [558-575cm-1] (b) spectral ranges for Y(ZZ)Y polarization configuration. Polarized Raman images generated by integration [509- 552 cm-1] (c) and [558-575cm-1] (d) spectral ranges for Y(XX)Y polarization configuration.
For Y(ZZ)Y polarization configuration, a maximum signal was observed for the A1 mode in the horizontal part of the nanowire and a maximum signal of the 568 cm-1 mode for the opposite spectroscopic contrast with in the straight portions of the nanowire (Figure 3c).
The experiment clearly demonstrate the capability of a confocal microscope equipped with a high-resolution stage to perform a complete Raman polarized study. The optical properties of nano-objects were clearly imaged with a resolution superior than 200nm, thanks to the novel combination of the high spatial resolution Raman confocal instrument and the piezoelectric stage. This instrument setup maintains the full benefits of the polarization control under a confocal microscope.
This information has been sourced, reviewed and adapted from materials provided by HORIBA Scientific.
For more information on this source, please visit HORIBA Scientific.