Investigation of phase separation in InGaN alloys by plasmon loss spectroscopy in a TEM

Abstract

Phase separation of InxGa1-xN alloys into Ga-rich and In-rich regions was observed by a number of research groups for samples grown with high indium content, x. Due to the radiation sensitivity of InGaN to beam damage by fast electrons, high-resolution imaging in transmission electron microscopy (TEM) or core-loss electron energy-loss spectroscopy (EELS) may lead to erroneous results. Low-loss EELS can yield spectra of the plasmon loss regions at much lower electron fluxes. Unfortunately, due to their delayed edge onset, the low energetic core losses of Ga and In partially overlap with the plasmon peaks, all of which shift with indium content.Here we demonstrate a method to quantify phase separation in InGaN thin films from the low-loss region in EELS by simultaneously fitting both plasmon and core losses over the energy range of 13-30eV. Phase separation is shown to lead to a broadening of the plasmon peak and the overlapping core losses, resulting in an unreliable determination of the indium concentration from analyzing the plasmon peak position alone if phase separation is present. For x=0.3 and x=0.59, the relative contributions of the binary compounds are negligibly small and indicate random alloys. For xnom.=0.62 we observed strong broadening, indicating phase separation.

Study of phase separation in an InGaN alloy by electron energy loss spectroscopy in an aberration corrected monochromated scanning transmission electron microscope

“…NION UltraSTEM, 100kV, 30mrad convergence semi-angle, 90 mrad inner collection angle (HA-ADF), 45 mrad collection angle (EELS), 50ms dwell time per pixel, 128 x 128 pixel, 0.5eV/channel, 7nm field of view. [11][12][13][14][15][16][17][18][19][20] =0.16nm) along the vertical are visible. The apparent waviness of the lattice planes is due to slight thermal drift using the acquisition.…”

“…This directly meant that the database of InGaN reference spectra previously acquired for 197kV in the JEOL microscope (see figure 4a) had to be compiled afresh, now for monochromated measurements at 60kV. Our approach was to extract high quality spectra from the GaN buffer region and the InGaN layer, and to subtract the Ga core losses from figure 3(c) from the latter to obtain references for pure InN, then interpolating InGaN data in a way analogous to the one described in [17]. This resulted in the new set of compiled InGaN reference spectra shown in figure 4(b).…”

“…Comparison of GaN reference spectra for x=0 to x=1 at (a) 197kV [17] and (b) 60kV. Note the different scale chosen to correlate with the experimental dispersions of (a) 0.05eV/channel (1024 channels) and (b) 0.015eV/channel (2048 channels).…”

“…We have recently found first evidence for phase separation in several In x Ga 1-x N thin films with x>0.5 by fitting both plasmon and low-energy core losses over the energy range of 13-30eV in single electron energy-loss spectra acquired at 197keV at low electron beam density [17] and now extend this method to multiple linear-least squares (MLLS) regression to complete InGaN EELS spectrum images, including monochromated valence electron energy loss spectra (VEELS). We also assess the bandgap of the alloys using different fitting routines for the region of valence electron excitation, demonstrating both the possibility to measure bandgaps down to <2eV with a monochromator and the significance of scattering delocalization at such low energies, which means a spatial resolution of ~10nm is achieved despite a much smaller electron beam size.…”

“…a) displays a high-angle annular dark-field (HA-ADF) image of a section ~7nm wide near the bottom of the InGaN thin film, close to a <1-100> zone axis orientation. The growth direction points upwards, and clear (0002) lattice fringes (d 0002 =0.26nm) running along the horizontal and in some areas weak(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) lattice fringes (d…”