Domain matching epitaxy of cubic In 2 O 3 on r -plane sapphire
2015; Wiley; Volume: 212; Issue: 7 Linguagem: Inglês
10.1002/pssa.201431889
ISSN1862-6319
AutoresPatrick Vogt, A. Trampert, M. Ramsteiner, Oliver Bierwagen,
Tópico(s)Ga2O3 and related materials
Resumophysica status solidi (a)Volume 212, Issue 7 p. 1433-1439 Editor's ChoiceFree Access Domain matching epitaxy of cubic In2O3 on r-plane sapphire Patrick Vogt, Corresponding Author Patrick Vogt Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, Germany Corresponding author: e-mail [email protected], Phone: +49-30-20377-348, Fax: +49-30-20377-425Search for more papers by this authorAchim Trampert, Achim Trampert Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, GermanySearch for more papers by this authorManfred Ramsteiner, Manfred Ramsteiner Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, GermanySearch for more papers by this authorOliver Bierwagen, Oliver Bierwagen Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, GermanySearch for more papers by this author Patrick Vogt, Corresponding Author Patrick Vogt Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, Germany Corresponding author: e-mail [email protected], Phone: +49-30-20377-348, Fax: +49-30-20377-425Search for more papers by this authorAchim Trampert, Achim Trampert Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, GermanySearch for more papers by this authorManfred Ramsteiner, Manfred Ramsteiner Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, GermanySearch for more papers by this authorOliver Bierwagen, Oliver Bierwagen Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, GermanySearch for more papers by this author First published: 17 March 2015 https://doi.org/10.1002/pssa.201431889Citations: 13AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Abstract Undoped, Sn-doped, and Mg-doped In2O3 layers were grown on rhombohedral r-plane sapphire (α-Al2O3 (10.2)) by plasma-assisted molecular beam epitaxy. X-ray diffraction and Raman scattering experiments demonstrated the formation of phase-pure, cubic (110)-oriented In2O3 for Sn- and Mg-concentrations up to 2 × 1020 and , respectively. Scanning electron microscopy images showed facetted domains without any surface-parallel (110) facets. High Mg- or Sn-doping influenced surface morphology and the facet formation. X-ray diffraction Φ-scans indicated the formation of two rotational domains separated by an angle Ф = 86.6° due to the substrate mirror-symmetry around the in-plane-projected Al2O3 c-axis. The in-plane epitaxial relationships to the substrate were determined for both domains. For the first domain it is . For the second domain the inplane epitaxial relation is . A low-mismatch coincidence lattice of indium atoms from the film and oxygen atoms from the substrate rationalizes this epitaxial relation by domain-matched epitaxy. Cross-sectional transmission-electron microsopy showed a columnar domain-structure, indicating the vertical growth of the rotational domains after their nucleation. Coincidence structure of In2O3 (110) (In atoms in red) grown on Al2O3 (10.2) (O atoms in blue) showing two rotational domians. 1 Introduction The transparent semiconducting oxide indium sesquioxide (In2O3) possesses a wide range of conventional device applications based on its conductivity and large band-gap of 1-5. Undoped In2O3 is used as active gas-sensor material 6. Indium sesquioxide doped with tin, indium-tin oxide (ITO), In2O3:Sn, is a transparent conducting oxide with unchanged crystal structure, that is used as transparent contact electrode for flat panel displays 7 or solar cells 8. Beyond these conventional applications, In2O3 has the potential for novel or improved conventional applications, when treated with a semi-conductor approach 9. In combination with the related sesquioxides Ga2O3 () and Al2O3 (), band-gap engineering and heterostructure oxide devices are foreseeable 10. By doping In2O3 with the donor Sn or the (deep) acceptor Mg, the n-type conductivity can be controlled from highly conductive 11 to semi-insulating 12. The realization of real semi-conductor devices typically requires highest crystalline quality which is commonly achieved by epitaxial growth methods. During epitaxy, the crystalline film grows aligned to the crystalline substrate. Indium sesqiuoxide solidifies in the stable cubic bixbyite polymorph crystal structure with lattice parameter a = 10.117 Å belonging to space-group 13. It may also crystallize in a metastable rhombohedral phase belonging to space-group with cell parameters and 14. An In2O3 film grown on a different substrate corresponds to the most basic In2O3-based heterostructure. On the cubic substrates, such as InAs, MgO, or ZrO2:Y (YSZ), cubic In2O3 grows in a trivial cube-on-cube fashion 15, i.e., and . A less ordinary epitaxial relation is that of cubic In2O3 on sapphire (α-Al2O3), which has a rhombohedral crystal structure (space-group ) with lattice parameters and 16. Growth of In2O3 on sapphire intuitively suggests the formation of rhombohedral epitaxial In2O3 films, which might, however, be impeded by the large lattice mismatch (above 11%). Cubic In2O3 films could form on the rhombohedral Al2O3 with a non-trivial epitaxial relation and domain structure. Epitaxy in these non-trivial scenarios are often explained in the framework of coincidence lattices by domain matching, i.e., forming a coinciding super-cell for both surface lattices with almost perfect lattice match. The growth of phase-pure cubic In2O3 (111) with rotational domains on rhombohedral α-Al2O3 (00.1) 14, 17 and (11.0) 18 has been reported and rationalized by coincidence lattices. In some cases, a mixture of cubic In2O3 (111) and rhombohedral In2O3 (00.1) on α-Al2O3 (00.1) has been observed 6, 14, and growth conditions to realize phase-pure rhombohedral In2O3 (00.1) on α-Al2O3 (00.1) have been determined 19. Interestingly, doping with Sn concentrations of changed the epitaxial orientation toward (001)-oriented phase-pure cubic ITO (In2O3:Sn with rotational domains 20). On r-plane sapphire, the epitaxial formation of (110)-oriented cubic, Fe-doped In2O3 has been reported 21 without any information on domain structure and in-plane orientation, whereas Co-doped In2O3 has been reported to form additional (111)-oriented fractions 22. Different crystal phases and their epitaxial relationship to the substrate lead to different crystal properties like electronic or heat transport properties, for instance. Therefore, knowing the phase purity of the film and its entire epitaxial relation to the substrate is of great importance. In this letter, we demonstrate the phase-pure growth of undoped, Sn-doped, and Mg-doped (110)-oriented cubic In2O3 on r-plane sapphire (α-Al2O3 (10.2)). The In2O3 domain structure and complete epitaxial relation to the substrate is shown and rationalized by a coincidence lattice approach. The film morphology is shown and discussed in terms of surface free energies. 2 Experimental For this study, undoped, Sn-doped, and Mg-doped indium oxide films were grown heteroepitaxially by plasma-assisted molecular beam epitaxy (MBE) on 2 inch r-plane sapphire wafers as described in literature 23, 12, 11. The oxygen plasma was run at a power of Prf = 200 W. Due to favorable nucleation of the In2O3 film on the Al2O3 substrate, there was no need for a low-temperature nucleation layer. The substrate temperature during growth was Tg = 650 °C (measured by a pyrometer) and oxygen-rich growth conditions at oxygen and indium beam-equivalent pressures of 2 × 10−5 and (In effusion cell temperatures 920 °C), respectively, were chosen. The growth rate was , resulting in film thicknesses of 570 nm for the growth times of 45 min. One undoped sample, 'S0', two Sn-doped samples 'S20' and 'S21' with Sn concentrations of 2 × 1020 and , respectively, and one Mg-doped sample 'M20' with Mg concentration of were grown. The dopants Sn and Mg were provided during growth from effusion cells at (S20), (S21), and (M20). The doping concentrations were determined by secondary ion mass spectroscopy (SIMS). The crystallographic orientation of the In2O3 was determined by X-ray diffraction (XRD). The out-of-plane orientation was analyzed by means of symmetric on-axis 2Θ–ω scans. The crystalline quality of the film was investigated by on-axis rocking curve ω-scans. The phase purity was validated by texture-scans in a symmetric geometry and Raman spectroscopy. The Raman measurements were performed at room temperature using the 473 nm line of a diode-pumped solid-state laser for excitation. The laser line was focused onto the sample surface by a microscope objective. The backscattered signal was collected by the same objective, dispersed spectrally by a 80 cm single monochromator (Horiba/Jobin Yvon), and detected by a liquid nitrogen-cooled CCD. The in-plane epitaxial relationship between the In2O3 film and the substrate were measured by Ф-scans (rotating the sample by angle Ф around its surface normal) in skew symmetric geometry detecting off-axis diffraction peaks at different sample tilt angles . The surface morphology of the film was mapped by scanning electron microscopy (SEM). 3 Results and discussion The XRD 2Θ–ω scans are shown in Fig. 1(a) for undoped and doped In2O3 films. The two strongest diffraction peaks are (20.4) Al2O3 and (440) cubic In2O3 signifying a purely (110)-oriented In2O3 film in all samples but S21. This result suggests the growth of phase-pure cubic In2O3 on r-plane sapphire for Sn- and Mg-doping up to at least 2 × 1020 and , respectively, with an out-of-plane epitaxial relation . Higher Sn-doping leads to formation of additional (100)- and (111)-oriented domains, as indicated by the additional In2O3 diffraction peaks for sample S21. The corresponding full-widths-at-half-maximum (FWHM) of the on-axis (440) ω-rocking curves (not shown) are 0.54°, 0.64°, 0.52°, and 0.34° for S0, S20, S21, and M20, respectively (as growth conditions were not fully optimized for all samples, conclusions on the impact of doping on crystal quality cannot be drawn from these rocking curves). The FWHM of the sapphire substrate was measured being 0.02°. Additional texture scans of the strong In2O3 (400) reflex depicted in Fig. 1(b) and (c) confirm these results for the undoped and Mg-doped sample S0 and M20, respectively. The scan shows four distinct peaks at a tilt angle of , corresponding to the angle between the {110}- and {001}-planes due to the out-of-plane (110)-oriented crystallites. The out-of-plane direction of the substrate and film is (or ). The Raman spectrum of this sample, shown in Fig. 2, exhibits the characteristic phonon lines of cubic In2O3 at 310, 369, 392, 499, and denoted by Ag(3), F2g(5), F2g(6), Ag(4), and F2g(11) 16. The Raman lines of the sapphire substrate are marked with an asterisk in Fig. 2. However, phonon lines from rhombohedral In2O3 expected, e.g., at the frequencies 272, 504, and 593 cm−1 19 have not been resolved with intensities above our detection limit for all samples (not shown). These results strongly suggest the absence of rhombohedral In2O3 in our films. Figure 1Open in figure viewerPowerPoint (a) X-ray diffraction pattern of the In2O3 film grown on the substrate α-Al2O3 (10.2). The peaks refer to the (20.4) of sapphire and the (440) peak of In2O3, respectively. The highly Sn-doped sample S21 shows additional In2O3 phases in directions (100) and (111), respectively. (b) and (c) Texture-scan along the (400) peak of In2O3 of the undoped sample S0- and Mg-doped M20. The axes belong to the azimuth angle Ф and perpendicular to that to the angle . The peaks are located at . The two rotational domains are separated by Ф = 86.6°. The peaks separated by belong to the same domain. Figure 2Open in figure viewerPowerPoint Raman spectrum of In2O3 (110) on Al2O3 (10.2) with the phonon lines of cubic In2O3 denoted by Ag(3), F2g(5), F2g(6), Ag(4) and F2g(11) 16. Expected frequencies for phonon lines of rhombohedral (rh) In2O3 are indicated by arrows. The phonon lines of the sapphire substrate are marked with an asterisk. Figure 3(a) schematically displays the unit cells of the rhombohedral Al2O3 substrate and the cubic In2O3 film. The respective surface planes, r- and (110)-plane, are drawn in blue, and the resulting surface unit cells (SUCs) are drawn in Fig. 3(b). The surface of r-plane sapphire has one-fold rotational symmetry about its normal and consists of rectangular SUCs with side lengths in [01.0] and in direction. The resulting SUC of In2O3 grown in [110] direction has twofold rotational symmetry and rhombic shape with corresponding cell parameters in and direction, respectively. Figure 3Open in figure viewerPowerPoint (a) Unit-cells of Al2O3 and In2O3. The r-plane of sapphire and (110)-plane of In2O3 are indicated in blue. The elements are aluminum (large grey), indium (large red), and oxygen (small blue), respectively. (b) Surface unit-cells (SUCs) of Al2O3 normal to the -plane and of In2O3 normal to the (110)-plane, respectively. The SUCs are indicated in pale blue and red, respectively. The corresponding parameters can be found in the text. Top-view SEM images of the sample surface are shown in Fig. 4. Domains are clearly visible as almost rectangular shacks aligned with a relative angle of almost 90° and terminated by one type of side facets for the undoped sample S0 (Fig. 4(a)) and similarly for S20 (not shown). At the highest Sn- and Mg-concentration, samples S21 (Fig. 4(b) and M20 Fig. 4(c), a different surface morphology evolves, likely reflecting the doping-induced change of facets' free energies, and the domain sizes seem to be larger for M20 than for all other samples. Interestingly, flat {110}-facets (parallel to the surface) are completely absent. In order to capture further details of the microstructure of In2O3 films, cross-sectional transmission electron microscopy (TEM) measurements were conducted and typical images are highlighted in Fig. 4(d). The bright-field image of the undoped In2O3 film (S0) reveals a multi-domain structure with columnar geometry. The average diameter of the domains is around 50–100 nm and rather uniform along the growth direction. The domain surface is almost terminated by inclined facets defining the size of the domain. This is consistent with the domain structure observed by SEM. The domain boundaries run most widely straight as additionally documented by the high-resolution TEM image in Fig. 4(e). Furthermore, as expected for rotational domains, the common (110)-planes are parallel to the interface in both domains. Figure 4Open in figure viewerPowerPoint Scanning electron microscopy images show the surface morphology of In2O3 (110) and the two rotational domains in real space. (a) Undoped S0 (similar to doped with , S20), (b) doped , S21, and (c) doped , M20. The domains in S0 are separated by an angle Ф = 86.6° indicated with a cross drawn in white. The domains are almost rectangular shack-shaped and bounded by facets of mainly one type. Note that (a) and (c) have a magnification of 1 × 105 and (b) 5 × 104. (d) Cross-sectional transmission electron microscopy (TEM) bright-field image of the In2O3 film revealing a multi-domain structure. (e) High-resolution TEM image of the domain boundary between two domains with different in-plane orientations. Figure 5 shows the XRD Ф-scan measurement of the (00.6) peak of Al2O3 drawn in red and the off-axis reflection (400) peaks of the In2O3 film for sample S0, which is representative for all samples. The c- and the r-planes of sapphire are separated by an angle of = 57.36°. The one-fold rotational symmetry of the Al2O3 r-plane is reflected by the single red peak shown in Fig. 5. The film sample was tilted by , being the angle between the cubic {001}- and the {110}-planes. The (110) surface of In2O3 has two-fold rotational symmetry, but we see four peaks, corresponding to those from the texture scan. These four In2O3 (400) peaks arise from the formation of two rotational domains, expected for a two-fold rotational symmetric film on a one-fold symmetric substrate surface with in-plane mirror symmetry 24. The peaks separated by belong to the same domain. In our case, the c-plane of Al2O3 acts as the mirror axis and can be clearly seen in Fig. 5. The two rotational domains are separated by an angle of Ф = 86.6° as shown in Fig. 5. Figure 5Open in figure viewerPowerPoint X-ray diffraction Ф-scan of the Al2O3 (00.6) peak (red) of the one-fold symmetry substrate surface and the (400) peak of the two-fold In2O3 film (black). The mirror axis (c-plane) leads to two rotational domains separated by Ф = 86.6° denoted as D1 and D2. The projection of the c-plane (00.6) peak of Al2O3 onto the r-plane is rotated to the projection of the (400) peak onto the (110)-plane of In2O3 by an angle of . To determine the in-plane epitaxial relation we further evaluated the Ф-scan measurement. The projection of the (00.6) peak onto the r-plane of Al2O3 has the direction in reciprocal- and real-space, respectively. The projection of the (400) peak of In2O3 on the (110)-plane has the direction . Now, the film has to be rotated around the [110] surface normal of the film by an angle of Ф. This angle could also be measured by Ф-scan and is shown in Fig. 5 and describes the rotation between the [01.0] direction of the substrate and the direction of the film. Rotating the film by one obtains the in-plane epitaxial relationship of Al2O3 (10.2) and In2O3 (110): The results for domain 1 (D1) and domain 2 (D2) in the reciprocal- and real-space are as follows: for D1 (1) (2)and for D2 (3) (4) In order to explain the found epitaxial relation, we used a coincidence lattice approach: for multiples of the SUCs of the oxygen-terminated Al2O3 and the indium-terminated In2O3 surface lattices a super-cell (SC) for both lattices could be found with close lattice match, such that the In atoms from the film fit on top of (bond to) the O atoms from the substrate SC lattice points. The two SCs of the coincidence lattice are depicted in Fig. 6(a) and (b) for the oxygen lattice of Al2O3 (10.2) and the indium-surface lattice of In2O3 (110), respectively. Figure 6Open in figure viewerPowerPoint Coincident unit-cells (green frame) for (a) the oxygen lattice of Al2O3 (10.2) drawn with blue oxygen atoms and (b) the indium lattice for In2O3 (110) drawn with red indium atoms. The SUCs are also drawn in pale blue and red, respectively. In parts (c) and (d), the overlapping lattices are shown for both rotational domains D1 and D2 rotated by Ф = 86.6°. The mirror axis is in direction [01.0]. The SC lattice vectors and for the oxygen lattice of the sapphire substrate is the linear combination of the SUC vectors [01.0] and . For both domains the SC lattice vectors are: (5) (6)and (7) (8) The same was done for the In2O3 (110) indium surface, by means of the SUC lattice vectors of the indium lattice. The linear combination in that case is (9)and (10) The lengths of the sides of both the oxygen and indium SCs, respectively, can now be calculated by taking the multiples of the lengths of the SUCs lattice vectors. The result for the SC side lengths for both domains of the Al2O3 oxygen lattice are and . The SC of In2O3 has side length and . The resulting lattice mismatch m for side is (11)and for side : (12) This near lattice match for the SCs explains the preferred out-of-plane growth direction with the resulting in-plane orientation of In2O3 (110) on Al2O3 (10.2). Furthermore, the angle γ of the SCs was also determined being for the oxygen SC, , and for the indium SC, . The mismatch of the angles is (13)and matches almost perfectly, too. D1 is shown in Fig. 6(c). By rotating the indium lattice by an angle of Ф = 86.6° around the surface normal (110) one obtains D2 as shown in Fig. 6(d). 4 Summary and conclusion We have demonstrated that undoped, Sn-doped, and Mg-doped, purely (110)-oriented, compact, cubic bixbyite In2O3 films can be grown phase pure by MBE on r-plane sapphire (10.2) for Sn- and Mg-concentrations up to at least 2 × 1020 and , respectively. For higher Sn concentrations, small cubic fractions with different out-of-plane orientations appear. Due to the mirror symmetry of the substrate surface two rotational domains with a relative in-plane angle of 86.6° are formed. These can be seen as clusters mainly bounded by one facet type on the completely facetted surface. High Sn- and Mg-doping modified the surface morphology, resulting in the appearance of more different facet types. The complete absence of surface-facets parallel to (110) suggests a comparably high surface free energy for the {110} facet. We determined the epitaxial relationship in out-of-plane and in-plane direction for D1 and D2 as and , respectively. This relation can be rationalized by coinciding a super cell of the sapphire oxygen surface lattice and the In2O3 surface indium lattice. This coincidence lattice with almost perfect lattice match facilitates the domain-matched epitaxy of In2O3 on r-plane Al2O3. Future work will consist of plan-view TEM investigations to clarify the configuration and (two-dimensional) geometry of the coincidence site lattice, and in combination with cross-sectional HR-TEM, the local lattice accommodation process within a coincidence unit. Al2O3 (10.2) is a readily available, well-suited substrate to grow phase-pure, (110)-oriented, cubic undoped, or doped In2O3 epitaxial thin films. Improvements of the film's crystal quality would involve the suppression of the rotational domain formation by using substrates with a suitable miscut 24. Acknowledgements We would like to thank Jos Emiel Boschker for critically reading the manuskript and Anne-Kathrin Bluhm for conducting the SEM measurements. Biographies Patrick Vogtdid his first education in applied electronics at the BASF, Ludwigshafen am Rhein. He graduated in physics, specializing in the field of high-temperature superconductors, at the University of Heidelberg in 2013. He is currently working as a PhD student in the group of Oliver Bierwagen at the Paul-Drude-Institut, Berlin. His research interests include semiconducting oxides, especially Ga2O3 and In2O3. He is growing and doping oxides by plasma-assisted molecular beam epitaxy. Oliver Bierwagenreceived his physics diploma (2001) and PhD (Dr. rer. nat., 2007) from Humboldt-Universität zu Berlin, Germany. From 2008–2010 he worked as post-doc at the University of California, Santa Barbara, on molecular beam epitaxy (MBE) of the semiconducting oxides SnO2, In2O3, and Ga2O3, and the transport properties of these oxides and III-nitrides. Since 2011 he has been working as senior scientist at Paul-Drude-Institut, Berlin, where he continued his work on semiconducting oxides. He also started working with rare-earth dielectric oxides, and the in-situ characterization of MBE growth by synchrotron X-ray diffraction and other methods. 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