Mechanism for Heteroepitaxial Growth of Transparent P-Type Semiconductor: LaCuOS by Reactive Solid-Phase Epitaxy Hidenori Hiramatsu,*,† Hiromichi Ohta,† Toshiyuki Suzuki,‡ Chizuru Honjo,‡ Yuichi Ikuhara,§ Kazushige Ueda,|,⊥ Toshio Kamiya,| Masahiro Hirano,† and Hideo Hosono|
CRYSTAL GROWTH & DESIGN 2004 VOL. 4, NO. 2 301-307
Transparent Electro-Active Materials Project, ERATO, Japan Science and Technology Agency, KSP C-1232, 3-2-1 Sakado, Takatsu-ku, Kawasaki 213-0012, Japan, Japan Fine Ceramics Center, 2-4-1 Mutsuno, Atsuta-ku, Nagoya 456-0023, Japan, Engineering Research Institute, University of Tokyo, 2-11-16 Yayoi, Bunkyo-ku, Tokyo 113-8656, Japan, and Materials and Structures Laboratory, Tokyo Institute of Technology, 4259, Nagatsuta, Midori-ku, Yokohama 226-8503, Japan Received August 27, 2003;
Revised Manuscript Received September 16, 2003
ABSTRACT: A unique epitaxial growth method, reactive solid-phase epitaxy, was used to fabricate heteroepitaxial thin films of LaCuOS, a transparent p-type semiconductor with layered structure. The epitaxial growth mechanism is examined through microscopic observations. A thin metallic Cu layer deposited between the amorphous LaCuOS (a-LaCuOS) and the yttria-stabilized zirconia (YSZ) single-crystal substrate before thermally annealing is a key to realizing the epitaxial LaCuOS films in this process. To grow the epitaxial films, it is critical for the Cu layer to have a discontinuous structure with triple junctions among the Cu, a-LaCuOS layer, and the YSZ substrate. The Cu layer is needed to create seed grains for the epitaxial LaCuOS at the junctions. The resulting seed grains work as an epitaxy template for subsequent epitaxial growth from the substrate to the film’s top surface, and high temperatures such as 1000 °C are necessary to completely convert the a-LaCuOS layer to an epitaxial layer. Introduction Transparent semiconductors with band gap energies g3.1 eV have a large opportunity for optoelectronic application since they have optical transparency in the visible region, wide controllability of electrical conductivity,1,2 and emission properties at short wavelengths.3-6 Recently, the electrical and optical properties of transparent p-type semiconductors such as nitrides and complex oxides have been intensively studied7-22 because they are indispensable for fabricating optoelectronic devices based on a pn junction.23-29 Although transparent n-type conduction is often obtained in various materials, transparent p-type conduction, especially high performance, is rather difficult. This is true even for p-type GaN because the acceptor level is deep (∼160 meV), and the carrier activation ratio remains ∼1%.4 An oxysulfide, LaCuOS, is a transparent p-type semiconductor,30 which was found following our material design concept for transparent p-type semiconductors.13,16,19 The design concept proposes that the delocalization of positive holes in the valence band maximum (VBM) enhances p-type conductivity. The VBM in most transparent oxides is primarily composed of O 2p orbitals, which are localized on oxygen ions. Thus, selecting an appropriate cation with orbitals, whose * To whom correspondence should be addressed. E-mail:
[email protected]. † Japan Science and Technology Agency. ‡ Japan Fine Ceramics Center. § University of Tokyo. | Tokyo Institute of Technology. ⊥ Present address: Department of Materials Science, Kyushu Institute of Technology, 1-1 Sensuicho, Tobata, Kitakyushu, 804-8550, Japan.
Figure 1. Crystal structure of a layered oxysulfide, LaCuOS. Crystal symmetry: tetragonal, space group: P4/nmm. Each layer is constituted by edge-shared La4O and CuS4 tetrahedra. The solid rectangle describes a unit cell.
energy levels are comparable to that of O 2p orbitals, that has a closed shell configuration, which is essential for optical transparency, will realize p-type transparent oxides. Cu1+ ions with 3d10 electronic configuration meet these criteria, which is why transparent p-type oxide materials are based on Cu1+.13 When Cu1+ is chosen, the S 3p level is energetically closer to Cu 3d levels than O 2p.31 Consequently, a higher hole mobility than in oxides may be expected for LaCuOS. Figure 1 shows the crystal structure of LaCuOS.32,33 This material has alternate layers of (La2O2)2+ and (Cu2S2)2- stacked along the c-axis. This type of layered structure may be loosely regarded as a natural superlattice composed of wide band gap oxide layers, La2O3 (∼5.5 eV),34 and narrow band gap sulfide layers, Cu2S (500 °C and decomposes into La2O3, La2O2S, and Cu2S. Therefore, it was concluded that conventional vapor phase epitaxy techniques are inappropriate for LaCuOS. Figures 4 and 5 show the XRD patterns and AFM images of (a) a-LaCuOS/YSZ (001) (without the thin Cu layers, simple annealed) and (b) a-LaCuOS/Cu/YSZ (001) (R-SPE) films annealed at 1000 °C, respectively. Simple annealing results in a single-phase film, but the polycrystals are randomly oriented (Figure 4a). AFM
Figure 5. AFM images of (a) a-LaCuOS/YSZ (001) and (b) a-LaCuOS/Cu/YSZ (001) films annealed at 1000 °C. Images a and b indicate an annealed film without Cu and an R-SPE processed film, respectively.
also measured the surface morphology of the polycrystalline features (Figure 5a). The film surface is rather rough with the root-mean-square roughness (Rrms) of ∼30 nm. For the R-SPE sample, the film was grown heteroepitaxially on a YSZ (001) substrate as shown in Figure 4b. The epitaxial growth was examined in more detail by four-axes high-resolution XRD to further substantiate the heteroepitaxial growth.45,54 Figure 5b shows an AFM image of the R-SPE film, and highly oriented tetragonal grains are clearly observed. The grain sizes are ∼500 nm, which indicates that the film is epitaxially grown, but contains multiple domains with grain or domain boundaries. The Rrms of the films is ∼8 nm, revealing that the epitaxial film is much smoother than the randomly oriented polycrystalline film prepared by simple annealing. These results demonstrate that the R-SPE is very effective for the epitaxial growth of LaCuOS and that a thin metallic Cu layer is necessary to grow epitaxial LaCuOS films. Microstructure Evolution of a-LaCuOS/Cu/YSZ (001) Films during Thermal Annealing. The variations in the microstructure, including those of the Cu and a-LaCuOS layers, were observed by changing the annealing temperature. Figure 6 shows the XRD patterns of a-LaCuOS/Cu/YSZ (001) films as-deposited and
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Figure 6. XRD patterns of annealed a-LaCuOS/Cu/YSZ (001) films at various temperatures. The annealing temperatures are shown on the right side of the diffraction patterns. Arrows indicate newly emerged diffraction peaks.
annealed at various temperatures. The as-deposited (nonannealed) film exhibits a 111 diffraction from Cu (2θ ) 43.3°) and two broad halo peaks around 2θ ) ∼25 and ∼55° arising from the a-LaCuOS layer. The Cu layer does not have an in-plane orientation. The diffraction peak from the Cu layer is distinctly observed when the annealing temperature is e400 °C. When annealed at 500 °C, the Cu diffraction peak disappears, and new broad diffraction peaks are observed with nearly identical angle spacing as indicated by the arrows in Figure 6. Although the peaks are slightly shifted to a lower angle, the peak positions of these periodical
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diffractions are close to those of 00l diffractions from LaCuOS. Diffraction peaks of LaCuOS appear upon increasing the annealing temperature, their intensities increase, and the widths become narrower, indicating that the crystal quality is improved and/or crystalline size is increased. The film annealed at 800 °C strongly shows preferential orientation along the c-axis. Finally, a heteroepitaxial film is obtained at 1000 °C. These results suggest that the solid-state reaction between the thin Cu layer and the a-LaCuOS layer initially occurs at temperature between 400 and 500 °C, and subsequent solid-phase epitaxy proceeds at the higher temperature. Figure 7 shows cross-sectional TEM images of aLaCuOS/Cu/YSZ (001) samples as-deposited (a) and annealed at 500 °C (b), 800 °C (c), and 1000 °C (d). Island structures are formed at the a-LaCuOS-YSZ substrate interface in the as-deposited sample (Figure 7a). When annealed at 500 °C (Figure 7b), thin (∼10 nm) crystallites, exhibiting a layered structure, are observed at the interface. The number density of this crystallite is similar to that of the Cu island observed in Figure 7a, suggesting that the Cu island is converted to the crystallite without coagulating. Figure 7c shows that the crystallite serves as a seed for growing largegrained epitaxial layers at temperatures g500 °C. However, nonoriented polycrystalline LaCuOS grains are still seen in the film surface region. Finally at 1000 °C, a LaCuOS epitaxial crystal grows in the entire area, as seen in Figure 7d. Figure 8 shows a magnified TEM (left) and its Fourier transformed images (right) of a-LaCuOS/Cu/YSZ (001) samples annealed at 500 °C (a) and 1000 °C (b) (corresponding to panels b and d in Figure 7, respectively). The TEM image in Figure 8a is from the crystallite observed in Figure 7b. Both the figures show similar structures, strongly suggesting that the crystallites
Figure 7. Cross-sectional TEM images of annealed a-LaCuOS/Cu/YSZ (001) at various temperatures. (a) As-deposited (nonannealed), (b) 500 °C, (c) 800 °C, and (d) 1000 °C. The incident direction of the electron beam is [010] of YSZ substrate.
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Figure 8. Magnified TEM images (left) and Fourier transformed images (right) of a-LaCuOS/Cu/YSZ (001) films annealed (a) at 500 °C and (b) at 1000 °C. The diffraction images for a and b are identical, indicating that the layered grains grown at 500 °C are epitaxial LaCuOS.
formed by annealing at 500 °C are heteroepitaxially grown LaCuOS. This idea is consistent with the XRD results in Figure 6. We also performed a TEM observation on a-LaCuOS/YSZ (001) films (i.e., without the Cu layer) annealed at 500 and 800 °C for comparison, confirming that the epitaxial LaCuOS crystallites were not formed at the interface. Figure 9 depicts a mechanism for heteroepitaxial growth of LaCuOS from these observations. The thin metallic Cu layer deposited between a-LaCuOS and YSZ (Figure 9a) works as an initiator to form epitaxial LaCuOS seeds at temperatures g500 °C (Figure 9b). Single-crystalline grain growth proceeds from the epitaxial LaCuOS seeds toward the top of film surface at 800 °C (Figure 9c) to 1000 °C (Figure 9d). Influence of the Microstructure in Cu Layer. The effect of the microstructure of the Cu layer was examined to investigate the mechanism for forming the seed layer. Figure 10 shows AFM images (left) and surface roughness profiles (right) of the metallic Cu layers prepared on YSZ (001) under various growth conditions. The surface roughness profiles of the film are described based on the height from the substrate surface. Table 1 summarizes the microstructure variations with deposition conditions and thickness. The sample a, deposited at 25 °C, is amorphous. Samples b-d, deposited at 400 °C, are polycrystalline. The orientation of Cu is (111) for the 5 nm thick samples (b and c), but the grain size and the surface roughness are much larger for c. It should be noted that sample c was used to grow heteroepitaxial LaCuOS in previous sections. The Cu layer in c has a discontinuous structure as seen in Figure 7a, which is also seen in AFM image c. The sample d is a thick film (∼30 nm) and contains (100)and (111)-oriented grains. This kind of orientation
Figure 9. Schematically drawn mechanism for heteroepitaxial growth of LaCuOS as the annealing temperature increases. (a) As-deposited (nonannealed), (b) 500 °C, (c) 800 °C, and (d) 1000 °C.
change with thickness is often observed in growing polycrystalline films.55 Then, a-LaCuOS layers were deposited at 25 °C on these different Cu layers and annealed at 1000 °C. However, an epitaxial LaCuOS film was obtained only when the 5 nm thick, (111)-oriented, discontinuous polycrystalline Cu layer (c) was used. Randomly oriented polycrystalline LaCuOS was obtained in other case. These observations indicate that the choice of the microstructure of the metallic Cu layer is crucial for obtaining epitaxial LaCuOS in R-SPE, and only Cu layers with an appropriate microstructure can assist epitaxial growth. Comparing the Cu layers (b and c), there is a striking difference in the surface morphologies such as grain size and surface roughness. That is, it is evident that the orientation of Cu crystalline does not control the subsequent heteroepitaxial growth. Some parts of the substrate surface are exposed (indicated by arrows in Figure 10c) and directly contact the upper a-LaCuOS layer, forming a triple junction of YSZ (001), Cu, and a-LaCuOS. The previous results lead to a model that explains the role of the thin Cu layer. Figure 7a shows that the Cu layer has triple junctions when a 5 nm thick, largegrained film was deposited. Epitaxial LaCuOS seeds initially form at the triple junction and convert the Cu grains to single-domain LaCuOS, maintaining the het-
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ible with those of LaCuOS crystallographic planes. That means that heteroepitaxial growth from a Cu surface would be unlikely or very difficult. These results indicate that the final crystallographic orientation of the LaCuOS seeds should be determined by the surface of the substrate, not by Cu layer, for epitaxial growth. Therefore, it is concluded that the triple junctions are necessary for determining the heteroepitaxial orientation of the LaCuOS seed layers. The LaCuOS seed layer subsequently determines the orientation of the upper layer through solid-phase epitaxy. It is noteworthy that a rather small grain size, ∼10 nm, of the Cu layer also enhances the solid-state reaction forming the LaCuOS seeds at a low temperature such as 500 °C. We have reported the mechanism in R-SPE for other materials, InGaO3(ZnO)m (m ) integer)42,56 and ZnO/ ZnRh2O4,27 which demonstrates the concept of R-SPE. These also used a thin template layer and distinctly separated the thin film deposition and annealing, which is common for many material systems. However, detailed investigations of LaCuOS and InGaO3(ZnO)m have revealed that the microscopic mechanisms and the appropriate choice of the template layers are very different. For example, an epitaxial ZnO template layer must be formed, but its thickness does not affect the epitaxial growth for InGaO3(ZnO)m. Further, the composition parameter, m, can be controlled by altering the thickness of the ZnO template layer. Thus, for InGaO3(ZnO)m, the thickness of the template layer is not crucial for epitaxial growth, which is vastly different from the present observations (i.e., only a 5 nm thick discontinuous Cu layer works as a template for LaCuOS, and the Cu layer is a preferentially oriented polycrystalline). Conclusion
Figure 10. AFM images (left, 1 × 1 µm) and surface roughness profiles (right) of Cu layers grown on YSZ (001) under various growth conditions. The height in surface roughness profiles is measured from the substrate-film interface. The microstructural characteristics and growth conditions are summarized in Table 1. Table 1. Microstructural Characteristics and Deposition Conditions of Metallic Cu Layers Shown in Figure 10 AFM images in Figure 10 average thickness (nm) crystallographic orientation Rrms (nm) deposition temperature (°C) KrF power (J/cm2/pulse)
b
c
d
∼5
a
∼5
∼5
∼30
amorphous
(111)
(111)
0.3 25
1 400
5 400
(111) + (001) 3 400
∼1
∼1
∼2
∼2
eroepitaxial orientation to the substrate. If the Cu layer is thicker or fine-grained, the resulting Cu film is continuous, and fewer triple junctions are formed. In this case, the solid-state reaction between the Cu layer and the a-LaCuOS layer starts from the surface of the Cu layer. Similar R-SPE growth on single-crystal Cu (001) substrates was also examined. However, the resultant film was polycrystalline. This result is reasonable since Cu does not have a crystal structure compat-
Reactive solid-phase epitaxy (R-SPE) was used to fabricate epitaxial films of LaCuOS, a transparent p-type semiconductor, on (001)-oriented yttria-stabilized zirconia (YSZ) substrates. Although a conventional pulsed laser deposition technique yielded only amorphous or polycrystalline films, epitaxial films were successfully obtained by R-SPE. Deposition of a 5 nm thick Cu layer before depositing a thick amorphous LaCuOS (a-LaCuOS) layer on YSZ (001) is indispensable. The epitaxial LaCuOS films were formed by annealing the bilayer films in a controlled atmosphere. The epitaxial growth mechanism was examined, and the crucial role of the thin Cu layer in the epitaxial growth was clarified through microscopic observations, which focused on interface between the YSZ substrate and the film. A discontinuous Cu layer was required to form the epitaxial LaCuOS seed, and the data strongly suggest that nucleation of the LaCuOS seed occurs at the triple junctions of the YSZ (001), Cu, and a-LaCuOS. The LaCuOS seed layer formed at the interface controls the orientation of the crystal growth in a-LaCuOS layer, leading to epitaxial growth of the film. Although LaCuOS and InGaO3(ZnO)m both have a layered crystal structure, the role of thin sacrificial layer for the epitaxial growth is vastly different. In the latter, a triple junction is not needed and the ZnO thin epitaxial layer itself works as a seed. Choosing the appropriate thin layers between the substrate and the precursor of the material to be grown is the heart of
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