Metal-oxide catalyzed epitaxy (MOCATAXY): the example of the O plasma-assisted molecular beam epitaxy of β-(Al _x Ga_{1− x} )₂O₃/β-Ga₂O₃ heterostructures

Abstract

We demonstrate a marked increase in the possible growth domain and growth rate of the O plasma-assisted molecular beam epitaxy of β-(AlxGa1−x)2O3, by adding the element In during growth. We explain these enhancement results from a metal-exchange catalytic effect. This mechanism allows us to synthesize β-(AlxGa1−x)2O3/β-Ga2O3 heterostructures at growth conditions that are not accessible in the absence of In, stabilizing the monoclinic β-phase. We demonstrate the growth of β-(AlxGa1−x)2O3 at growth temperatures up to 900 °C. Moreover, we illustrate how additional In on the β-(AlxGa1−x)2O3 surface acts as a surface active agent, improving the crystal quality of the synthesized β-(AlxGa1−x)2O3/β-Ga2O3 heterostructures. These structures are shown to be of the highest crystal quality up to an Al concentration of x = 0.2. We predict the novel growth mode introduced for ternary III–O thin film synthesis — shown by the example of β-(AlxGa1−x)2O3 — to be applicable for a wide range of thin film materials, whose individual constituents possess material properties similar to those discussed for the constituents contributing to β-(AlxGa1−x)2O3.

Extending the kinetic and thermodynamic limits of molecular-beam epitaxy utilizing suboxide sources or metal-oxide catalyzed epitaxy

“…We begin by deriving a growth-rate model for the growth of films of In 2 O 3 can be grown at high Γ is larger than that for Ga 2 O 3 . This result is also similar to the growth of Ga 2 O 3 and In 2 O 3 by conventional MBE [13,15,18].…”

“…For decades, the singlestep reaction mechanism occurring during the MBE growth of III-V (e.g., GaAs, GaN, AlN) [6][7][8] and II-VI (e.g., ZnSe, ZnO) [9,10] compound semiconductors has defined MBE as a rather simple and straightforward thin film technique, especially when compared with chemical vapor deposition methods [11]. In contrast to the growth of III-V and II-VI materials, the surface kinetics of III-O (e.g., Ga 2 O 3 and In 2 O 3 ) [12][13][14][15][16][17][18] and IV-O (e.g., SnO 2 ) [19] compounds is governed by a complex reaction pathway, resulting in a two-step reaction mechanism to form the intended compound. The formation and subsequent desorption of ad-molecules, called suboxides (e.g., Ga 2 O, In 2 O, SnO), define the growth-limiting step for these classes of materials.…”

“…The formation and subsequent desorption of ad-molecules, called suboxides (e.g., Ga 2 O, In 2 O, SnO), define the growth-limiting step for these classes of materials. The result is a rather narrow growth window within the adsorption-controlled regime [14,15,[18][19][20].…”

“…Using the -MBE approach, the growth-limiting step occurring during the growth of III-O and IV-O materials by conventional MBE is bypassed, enabling the growth of films with excellent structural quality and surface smoothness at growth rates exceeding 1 μm hr −1 and comparatively low growth temperature ( G ) [21]. The second variant is metaloxide catalyzed epitaxy (MOCATAXY), a method involving the introduction of a catalyst into oxide growth systems [5,17,18,22]. Using this technique, the growth-limiting step of III-O compounds is bypassed by the catalyst [5].…”

“…Using this technique, the growth-limiting step of III-O compounds is bypassed by the catalyst [5]. It has been proposed that MOCATAXY results from metalexchange catalysis (MEXCAT) [5] and has been observed on various growth surfaces for the formation of Ga 2 O 3 and (Al Ga 1− ) 2 O 3 [5,18,[22][23][24][25] as well as during other physical vapor deposition methods [26]. Nevertheless, the underlying physics leading to the observed catalysis has been explained differently-experimentally [5,22] and theoretically [27]-and thus remains disputable.…”