Indium distribution in epitaxially grown InGaN layers analyzed by transmission electron microscopy

Abstract

An overview is given about microstructure and composition analyses of InGaN quantum wells embedded in Ga(Al)N barriers to study the mechanisms which determine the In distribution in epitaxially grown InGaN layers. The applied technique is transmission electron microscopy (TEM). The main prerequisite for this work was the development of a technique based on high-resolution lattice fringe images that allows quantitative chemical analyses of InGaN on an atomic scale. A large variety of samples was investigated that were produced by molecular beam epitaxy (MBE) and metal-organic vapor phase epitaxy (MOVPE). The effect of the deposition temperature, growth rate, strain and high-temperature annealing treatments on the average In concentration and In distribution was studied to assess the influence of phase separation, In surface segregation and In desorption. Composition fluctuations in InGaN are always observed on two different lateral scales independent of the growth technique and particular set of growth parameters but the strength of the composition fluctuations can be influenced by the details of the growth.

Does In form In-rich clusters in InGaN quantum wells?

“…In the light of the Smeeton et al [16,17] papers, which suggested that the gross indium-rich clusters in InGaN quantum wells reported by many researchers might be due to electron beam damage, the Gerthsen group revised their earlier conclusions [11,12]. They observed that the indium concentration in the clusters increased with increasing irradiation time in the electron microscope.…”

“…By using Vegard's law, the lattice parameter map can be converted to a composition map. For InGaN quantum wells grown with 10-20% indium, the indium-rich clusters are typically found to contain at least 80% indium [10][11][12][13], although the projection problem in TEM makes it difficult to quantify the indium content. We will call such clusters "gross indium-rich clusters".…”

Comparison of the In distribution in InGaN/GaN quantum well structures grown by molecular beam epitaxy and metalorganic vapor phase epitaxy

“…The details of the procedure are outlined in Ref. [10]. Optimized imaging conditions for the formation of lattice-fringe images were obtained by tilting the sample by about 5 from the /1 0 % 1 0S or /1 1 % 2 0S zone-axis orientation to excite only {0 0 0 2n} beams.…”

“…The characterization of the QWs was carried out by (HR)TEM. Local and average In concentrations were obtained on an atomic scale based on HRTEM images using the digital analysis of lattice images (DALI) evaluation method [8][9][10].…”

Influence of growth kinetics on the indium distribution during MOVPE growth of GaInN quantum wells

“…The interference of only two beams leads to fringe images which are obtained in our case by a two-beam excitation condition with strongly excited (0000) and (0002) beams. It can be shown by image simulations that artifacts due to variations of the sample thickness and objective lens defocus ∆f are negligible if the (0002) beam is centered on the optic axis and if only the (0000) and (0002) beams are transmitted through the objective aperture [10]. The apparent loss of resolution along the (0002) fringes compared to a conventional zone-axis image does not present a problem because only the distances between the (0002) fringes need to be measured.…”