Epitaxial Structures of Band-Gap-Engineered $\alpha$-(CrxFe1-x)2O3 ($0 \leq x\leq 1$) Films Grown on $C$-Plane Sapphire

Solid solutions of $\alpha$-Cr 2 O 3 and $\alpha$-Fe 2 O 3 are novel oxide semiconductors exhibiting narrower band gaps than those of the end members. We have investigated the heteroepitaxial growth and crystalline structure of pulsed-laser deposited $\alpha$-(Cr x Fe 1-x ) 2 O 3 films on $c$-plane...

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Published inJpn J Appl Phys Vol. 51; no. 11; pp. 11PG11 - 11PG11-5
Main Authors Mashiko, Hisanori, Oshima, Takayoshi, Ohtomo, Akira
Format Journal Article
LanguageEnglish
Published The Japan Society of Applied Physics 25.11.2012
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Summary:Solid solutions of $\alpha$-Cr 2 O 3 and $\alpha$-Fe 2 O 3 are novel oxide semiconductors exhibiting narrower band gaps than those of the end members. We have investigated the heteroepitaxial growth and crystalline structure of pulsed-laser deposited $\alpha$-(Cr x Fe 1-x ) 2 O 3 films on $c$-plane sapphire substrates. Under the growth condition optimized for $\alpha$-Fe 2 O 3 , reflection high-energy electron diffraction intensity oscillations were observable for both $\alpha$-Fe 2 O 3 and $\alpha$-Cr 2 O 3 , enabling us to control film thickness in a layer-by-layer fashion. The composition dependence of the epitaxial structures, including phase purity, orientation, strain, and in-plane rotation domain, was characterized by X-ray diffraction to reveal a defect-free composition range ($0 \leq x \leq 0.50$). The films with $0.60 \leq x \leq 1$ were found to have 180°-rotation domains. The absorption spectra of $\alpha$-(Cr 0.50 Fe 0.50 ) 2 O 3 solid solution (corundum) and superlattice (nominally composed ilmenite) films are compared with those of end members and an $\alpha$-Fe 2 O 3 /$\alpha$-Cr 2 O 3 bilayer film to elucidate the origin of the band-gap narrowing.
Bibliography:RHEED patterns of (a) $c$-plane sapphire substrate, (b) $\alpha$-Fe 2 O 3 film ($P_{\text{O\scale70%$_{2}$}} = 1$ mTorr, $T_{\text{g}} = 740$ °C), (c) mixture film ($P_{\text{O\scale70%$_{2}$}} = 0.13$ mTorr, $T_{\text{g}} = 730$ °C), and (d) Fe 3 O 4 film ($P_{\text{O\scale70%$_{2}$}} = 1.0 \times 10^{-6}$ Torr, $T_{\text{g}} = 740$ °C). All the patterns for the films were taken along the [$2\bar{1}\bar{1}0$] azimuth for sapphire at approximately 1800 pulses (${\sim}3$ min) after the initial pulse during successive growth in independent runs. $P_{\text{O\scale70%$_{2}$}}$--$T_{\text{g}}$ phase diagram established by in-situ RHEED inspections for FeO x epitaxial films. Solid and broken lines represent the phase boundary between $\alpha$-Fe 2 O 3 ( ) and the mixture of $\alpha$-Fe 2 O 3 and Fe 3 O 4 ($\times$) and the corresponding thermodynamic equilibrium, respectively. Time evolution of normalized RHEED intensity for specular beam during the growth of (a) $\alpha$-Cr 2 O 3 and (b) $\alpha$-Fe 2 O 3 on $c$-plane sapphire under identical conditions ($P_{\text{O\scale70%$_{2}$}} = 1$ mTorr, $T_{\text{g}} = 740$ °C). (a, b) AFM image and RHEED pattern of $\alpha$-Cr 2 O 3 film, respectively. (c, d) AFM image and RHEED pattern of $\alpha$-Fe 2 O 3 film, respectively. Out-of-plane XRD patterns near the (0006) reflection of sapphire ($*$) for as-grown (blue/dark) and annealed (red/light) $\alpha$-(Cr x Fe 1-x ) 2 O 3 films ($0 \leq x \leq 1$). Vertical broken lines indicate positions of the (0006) reflections of bulk single crystals of $\alpha$-Cr 2 O 3 and $\alpha$-Fe 2 O 3 . XRD patterns showing $\{10\bar{1}4\}$ $\phi$ scans of as-grown (blue/dark) and annealed (red/light) $\alpha$-(Cr x Fe 1-x ) 2 O 3 films ($0 \leq x \leq 1$), and sapphire substrate ($*$). Each pattern is obtained by taking the sum of the intensity of three-fold $\{10\bar{1}4\}$ peaks in $\phi$ scans measured in the ranges of 180 to 360°, followed by normalizing the intensity with the peak height of the annealed films. The patterns corresponding to the 180°-rotation domains (plotted in the right panels) have been magnified three-fold along the vertical axis for clarity. Absorption spectra of $\alpha$-Cr 2 O 3 , $\alpha$-Fe 2 O 3 , and $\alpha$-Fe 2 O 3 /$\alpha$-Cr 2 O 3 bilayer films (top); solid-solution and superlattice films of $\alpha$-CrFeO 3 (bottom).
ISSN:0021-4922
1347-4065
DOI:10.1143/JJAP.51.11PG11