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

Area of Science:

• Materials Science
• Optics
• Solid-State Physics

Background:

• Gallium nitride (GaN) and indium gallium nitride (InGaN) are crucial materials for optoelectronic devices.
• Understanding piezoelectric fields and defect distributions is vital for optimizing device performance.
• Existing microscopy techniques have limitations in visualizing these properties at the nanoscale.

Purpose of the Study:

• To develop and demonstrate scanning second-harmonic generation (SHG) microscopy for mapping piezoelectric fields in GaN and InGaN.
• To correlate piezoelectric field distribution with defect states and material properties.
• To provide advanced imaging capabilities for GaN and InGaN multiple-quantum-well (MQW) structures.

Main Methods:

• Utilized femtosecond Cr:forsterite and Ti:sapphire lasers for scanning SHG and third-harmonic generation (THG) microscopy.
• Employed electric field-enhanced SHG and resonant effects for sensitive piezoelectric field detection.
• Combined SHG/THG microscopy with two-photon and three-photon fluorescence microscopy for correlative analysis.

Main Results:

• Successfully mapped piezoelectric field distribution in bulk GaN and InGaN MQWs.
• Revealed correlations between defect density, luminescence (bandedge and yellow), bandtail states, and piezoelectric fields.
• Demonstrated that substrate quality directly influences piezoelectric field strength within InGaN quantum wells.

Conclusions:

• Scanning SHG microscopy offers superior imaging of piezoelectric fields and defect states in GaN and InGaN MQWs.
• Correlative microscopy techniques provide new insights into the physical properties of these materials.
• The developed methods enable detailed analysis previously unavailable, aiding material optimization.