Transcript Slide 1
PL as indicator of material quality 50000 PL intensity (a.u.) • Better quality of epilayer means higher intensity and narrower FWHM • Also true for quantum wells where the interface fluctuations controls the FWHM of PL peaks • AlGaN epilayers grown on superlattice (SL) buffered GaN layers produces the best quality. This is because SL helps to reduce strain in the epilayers grown on non-lattice matched substrates. B1098 B1025 B1001 40000 SL buffered Al.2Ga.8N: 4.5 m 30000 20000 SL buffered Al.2Ga.8N: 2.1 m 10000 conventional Al.2Ga.8N: 1.2 m 0 260 280 300 320 340 360 380 400 420 Wavelength (nm) Slide # 1 Summary of Photoluminescence • Information on bandgap and hence material composition (peak position). Direct or indirect bandgap (from intensity) • Information on dopant density and their energy levels (FWHM and peak position) • Information on the quality of material, both substrate and epitaxial layers (poor quality material has more states giving rise to non-radiative recombination, or radiative recombination at a different wavelength) • Information on material properties such as phonon energies, effective mass, and dielectric constant (from spectra of the hydrogenic model of the impurities) • Information on the energy levels of quantum wells, their interface roughness, alloy disorder, and built-in electric field (from the peak position, FWHM, and the variation of lineshape with width of the QWs) Slide # 2 Common scanning probe modes • Basic mode: surface morphology – Contact mode – Non-contact – Tapping mode • Secondary modes: – – – – – – Surface potential Capacitance derivative and actual Capacitance Surface conductivity Scanning gate Magnetic force microscopy Thermal conductivity and temperature Trivia: AFM was discovered after the inventor of STM (Gerd Binnig and Heinrich Rohrer were awarded Nobel Prize in1986) wanted to develop a technique for mapping non-conductive samples. Slide # 3 Scanning probe microscopy • Atomic force microscopy (AFM) and associated modes: Applicable for conductive and nonconductive samples. Not as high resolution as STM. Best resolution possible in conductive AFM. • Scanning tunneling microscopy (STM): Only applicable for conductive samples. True atomic resolution possible. Slide # 4