Mapping piezoelectric-field distribution in gallium nitride with scanning second-harmonic generation microscopy

Taking advantage of the electric field-enhanced second-harmonic generation effect in bulk gallium nitride (GaN) and indium gallium nitride (InGaN) qua ntum wells, we demonstrated the piezoelectric field distribution mapping in bulk GaN and InGaN multiple-quantum-well (MQW) samples using scanning seco nd-harmonic generation (SHG) microscopy. Scanning SHG microscopy and the ac companying third-harmonic generation (THG) microscopy of the bulk GaN sampl e were demonstrated using a femtosecond Cr:forsterite laser at a wavelength of 1230 nm. Taking advantage of the off-resonant electric field-enhanced S HG effect and the bandtail state-resonance THG effect, the second- and thir d-harmonic generation microscopic images obtained revealed the piezoelectri c field and bandtail state distributions in a GaN sample. Combined with 720 nm wavelength excited two-photon fluorescence microscopy in the same sampl e, the increased defect density around the defect area was found to suppres s bandedge photoluminescence, to increase yellow luminescence, to increase bandtail state density, and to decrease residue piezoelectric field intensi ty. Scanning SHG microscopy of the InGaN MQW sample was resonant excited wi th 800 nm femtosecond pulses from a Ti:sapphire laser in order to suppress SHG contribution from the bulk GaN substrate. Taking advantage of the stron g piezoelectric field inside the InGaN quantum well, the wavelength resonan t effect, and the electric field-enhanced SHG effect of InGaN quantum wells , resonant scanning SHG microscopy revealed the piezoelectric field distrib ution inside the wells. Combined with accompanying three-photon fluorescenc e microscopy from the bulk GaN substrate underneath the quantum wells, the direct correspondence between the piezoelectric field strength inside the q uantum well and the substrate quality can be obtained. According to our stu dy, the GaN substrate area with bright bandedge luminescence corresponds to the area with strong SHG signals indicating a higher stained-induced piezo electric field. These scanning harmonic generation microscopies exhibit sup erior images of the piezoelectric field and defect state distributions in G aN and InGaN MQWs not available before. Combining with scanning multiphoton fluorescence microscopy, these techniques open new ways for the physical p roperty study of this important material system and can provide interesting details that are not readily available by other microscopic techniques.