Nanoscale Electrostructural Characterization of Compositionally Graded Al_xGa_1–xN Heterostructures on GaN/Sapphire (0001) Substrate

Abstract

We report on AlxGa1-xN heterostructures resulting from the coherent growth of a positive then a negative gradient of the Al concentration on a [0001]-oriented GaN substrate. These polarization-doped p-n junction structures were characterized at the nanoscale by a combination of averaging as well as depth-resolved experimental techniques including: cross-sectional transmission electron microscopy, high-resolution X-ray diffraction, Rutherford backscattering spectrometry, and scanning probe microscopy. We observed that a small miscut in the substrate orientation along with the accumulated strain during growth led to a change in the mosaic structure of the AlxGa1-xN film, resulting in the formation of macrosteps on the surface. Moreover, we found a lateral modulation of charge carriers on the surface which were directly correlated with these steps. Finally, using nanoscale probes of the charge density in cross sections of the samples, we have directly measured, semiquantitatively, both n- and p-type polarization doping resulting from the gradient concentration of the AlxGa1-xN layers.

Investigation of the Structural and Optical Properties of Compositionally V‐Graded Strained InxGa1–xN Layers

“…Comparison of the two samples shows that for increased maximum indium composition the tail of the fringes broadens and shifts to lower ω values. These techniques and results have been validated using several chemically sensitive techniques in AlGaN V‐graded samples …”

The effect of ion implantation on structural damage of сompositionally graded AlGaN layers

“…2) were previously determined in Ref. [10] and are used in this work to study the effect of ion implantation on the strain distribution in the Al x Ga 1-x N/GaN heterostructures. It is well known that ion implantation is accompanied with formation of a high density of point defects, which generally leads to a vertical hydrostatic expansion of the unit cell (Fig.…”

“…This also explains the weaker change of the microdefect state of the sample S2 in comparison with the sample S1. The defect concentration in the as-grown sample S2 is higher due to the higher Al concentration in the Al x Ga 1-x N layer, and higher lattice mismatch [10].…”

“…The full width of half maximum (FWHM) of ωscans for symmetrical (0002) reflection is commonly measured to estimate the density of screw-type dislocations [10]. FWHMs for the Al x Ga 1-x N layers and GaN substrates of the samples S1 and S2 are shown in Fig.…”

“…First, the 400-nm thick GaN buffer layer was grown under gallium-rich conditions at a substrate temperature of 690 °C. Next, the substrate temperature was raised up to 710 °C, and compositionally graded Al x Ga 1-x N layers were grown by linearly changing the temperature of the Al effusion cell [10]. The AlN molar fraction in the Al x Ga 1-x N layers was graded from ~7 up to ~22% (sample S1) and from ~7 up to ~32% (sample S2).…”