Strain States and Relaxation for α-(Al_xGa_1-x)₂O₃ Thin Films on Prismatic Planes of α-Al₂O₃ in the Full Composition Range

Ga₂O₃ received tremendously increasing interest within the last decade. This is due to its large bandgap of about 5 eV and the correspondingly large expected electric breakdown field of about 8 MV/cm rendering it promising for power device applications [1]. Ga₂O₃ can be stabilized in at least four different polymorphs, the thermodynamically stable monoclinic β-phase as well as the metastable rhombohedral α-, orthorhombic κ-, and the defect spinel γ-modification. Among those, the α-phase exhibits the largest bandgap of about 5.3 eV as well as the largest expected electric breakdown field. Additionally, it features the same crystal structure as α-Al₂O₃ and can therefore be fabricated with high crystalline quality on all common epitaxial planes of cost-effective sapphire substrates. Especially the prismatic m-plane shows promising properties of α-Ga₂O₃ thin films such as increased stability as well as larger electron mobilities [2,3]. Sophisticated power devices such as HEMTs require high-quality heterostructures comprising α-Ga₂O₃ as well as α-(Al<i>_x</i>Ga_1-<i>x</i>)₂O_3, whose bandgap increases with <i>x</i>. For the controlled growth of such ideally pseudomorphically strained heterostructures, knowledge of the elastic tensor and the relaxation processes of the material is required, which is lacking up to now in literature especially for the growth on the prismatic a- and m-planes of sapphire.<br/>Here, we present a detailed X-ray diffraction study on α-(Al<i>_x</i>Ga_1-<i>x</i>)₂O₃ thin films deposited by combinatorial pulsed laser deposition in the full composition range 0≤<i>x</i>≤1 on a- as well as m-plane sapphire substrates. We investigate both pseudomorphically strained and relaxed thin film layers with different thickness. We achieved pseudomorphic growth on m-plane sapphire for Al-contents as low as <i>x</i>~0.45 for layers of about 10 nm thickness. By reciprocal space map measurements, we will demonstrate a fundamental difference for the growth on a- and m-epitaxial planes. Pseudomorphic α-(Al<i>_x</i>Ga_1-<i>x</i>)₂O₃ layers on m-plane sapphire show a distinct shear strain <i>e</i>’₅ along the in-plane c-axis direction, which is in contrast to samples on a-plane sapphire. Similarly, relaxed m-plane α-(Al<i>_x</i>Ga_1-<i>x</i>)₂O₃ exhibits a global lattice tilt in c-axis direction that is missing on the a-plane. We modeled the lattice constants as well as the shear strain of the pseudomorphic layers as function of <i>x</i> for both epitaxial planes based on our elastic theory for rhombohedral heterostructures [4]. We will show that the reason for this behavior is the non-vanishing <i>C</i>₁₄ component of the stress-strain tensor both for α-Ga₂O₃ and α-Al₂O₃ that contributes strongly only for the m-epitaxial plane. Additionally, we are able to confirm the up to this date only theoretically calculated value of <i>C</i>₁₄ for α-Ga₂O₃ of about 17.3 GPa [5]. By geometric considerations of the possible slip planes and Burger’s vectors for a- and m-epitaxial planes as well as the modeling of the angle of the global lattice tilt as function of <i>x</i>, we will explain the occurrence of the lattice tilt only for the m-plane as well as identify possible microscopic relaxation mechanisms. The results are summarized in [6].<br/>[1] Zhang <i>et al</i>., APL Mater. <b>8</b>, 020906 (2020)<br/>[2] Jinno <i>et al</i>., Sci. Adv. <b>7</b>, eabd5891 (2021)<br/>[3] Akaiwa <i>et al</i>., Phys. Status Solidi A <b>217</b>, 1900632 (2020)<br/>[4] Grundmann, J. Appl. Phys. <b>124</b>, 185302 (2018)<br/>[5] Furthmüller, Phys. Rev. B <b>93</b>, 115204 (2016)<br/>[6] Kneiß <i>et al</i>., J. Mater. Res., online (2021), DOI: 10.1557/s43578-021-00375-3