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Wurtzite-derived ternary I – III – O 2 semiconductors

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ABSTRACT

Ternary zincblende-derived I–III–VI2 chalcogenide and II–IV–V2 pnictide semiconductors have been widely studied and some have been put to practical use. In contrast to the extensive research on these semiconductors, previous studies into ternary I–III–O2 oxide semiconductors with a wurtzite-derived β-NaFeO2 structure are limited. Wurtzite-derived β-LiGaO2 and β-AgGaO2 form alloys with ZnO and the band gap of ZnO can be controlled to include the visible and ultraviolet regions. β-CuGaO2, which has a direct band gap of 1.47 eV, has been proposed for use as a light absorber in thin film solar cells. These ternary oxides may thus allow new applications for oxide semiconductors. However, information about wurtzite-derived ternary I–III–O2 semiconductors is still limited. In this paper we review previous studies on β-LiGaO2, β-AgGaO2 and β-CuGaO2 to determine guiding principles for the development of wurtzite-derived I–III–O2 semiconductors.

No MeSH data available.


Variation of optical band gap of the (1 − x)ZnO–x(AgGaO2)1/2 alloys (red dots and line) as a function of the alloying level, x, together with that reported for the (1 − x)–xCdO alloys (green dots and line) for comparison.
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Figure 4: Variation of optical band gap of the (1 − x)ZnO–x(AgGaO2)1/2 alloys (red dots and line) as a function of the alloying level, x, together with that reported for the (1 − x)–xCdO alloys (green dots and line) for comparison.

Mentions: Unlike the x(LiGaO2)1/2–(1 − x)ZnO alloys, the x(AgGaO2)1/2–(1 − x)ZnO alloys cannot be fabricated by a conventional solid state reaction between ZnO and β-AgGaO2, because Ag+ ions are easily reduced to metallic silver at high temperature. The alloy films are fabricated by rf-magnetron sputtering using mixed ZnO and β-AgGaO2 powders as target materials [54]. Wurtzite-type alloy films form when x ≤ 0.33. This alloying range is wider than that of the xCdO–(1 − x)ZnO system with x < 0.17. This is expected from the structural similarity between ZnO and β-AgGaO2. Nevertheless, the alloying range is slightly smaller than that of the β-LiGaO2–ZnO system because of lattice mismatch between ZnO and β-AgGaO2 (4.6% in the ab-plane and 5.2% along the c-axis of the wurtzite structure) [54], which is larger than that between ZnO and β-LiGaO2 (3.0% in the ab-plane and 3.8% along the c-axis) [37]. Figure 4 shows the change in optical band gap of the x(AgGaO2)1/2–(1 − x)ZnO alloys as a function of the alloying level, x, together with that reported for the xCdO–(1 − x)ZnO alloys for comparison [55]. The band gap decreases with an increase in alloying level and was 2.55 eV for x = 0.33. Compared with the CdO–ZnO system, the narrowest band gap was approximately the same. In figure 4, comparatively large reduction in the band gap is evident for the composition with a small AgGaO2 concentration at x ≤ 0.1. However, a near linear decrease occurs with an increase in alloying level. Therefore, band gap bowing in the β-AgGaO2–ZnO alloys is much smaller than that in the CdO–ZnO system. The large reduction in the band gap during the early alloying stage can be explained by the introduction of the Ag 4d contribution around the valence band maximum (VBM), which mainly consists of the O 2p states in pure ZnO. The introduction of the Ag 4d contribution highly modulates the electronic configuration of the VB of ZnO because of the higher energy of the Ag 4d atomic orbitals compared with that of the O 2p atomic orbitals.


Wurtzite-derived ternary I – III – O 2 semiconductors
Variation of optical band gap of the (1 − x)ZnO–x(AgGaO2)1/2 alloys (red dots and line) as a function of the alloying level, x, together with that reported for the (1 − x)–xCdO alloys (green dots and line) for comparison.
© Copyright Policy - open-access
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC5036475&req=5

Figure 4: Variation of optical band gap of the (1 − x)ZnO–x(AgGaO2)1/2 alloys (red dots and line) as a function of the alloying level, x, together with that reported for the (1 − x)–xCdO alloys (green dots and line) for comparison.
Mentions: Unlike the x(LiGaO2)1/2–(1 − x)ZnO alloys, the x(AgGaO2)1/2–(1 − x)ZnO alloys cannot be fabricated by a conventional solid state reaction between ZnO and β-AgGaO2, because Ag+ ions are easily reduced to metallic silver at high temperature. The alloy films are fabricated by rf-magnetron sputtering using mixed ZnO and β-AgGaO2 powders as target materials [54]. Wurtzite-type alloy films form when x ≤ 0.33. This alloying range is wider than that of the xCdO–(1 − x)ZnO system with x < 0.17. This is expected from the structural similarity between ZnO and β-AgGaO2. Nevertheless, the alloying range is slightly smaller than that of the β-LiGaO2–ZnO system because of lattice mismatch between ZnO and β-AgGaO2 (4.6% in the ab-plane and 5.2% along the c-axis of the wurtzite structure) [54], which is larger than that between ZnO and β-LiGaO2 (3.0% in the ab-plane and 3.8% along the c-axis) [37]. Figure 4 shows the change in optical band gap of the x(AgGaO2)1/2–(1 − x)ZnO alloys as a function of the alloying level, x, together with that reported for the xCdO–(1 − x)ZnO alloys for comparison [55]. The band gap decreases with an increase in alloying level and was 2.55 eV for x = 0.33. Compared with the CdO–ZnO system, the narrowest band gap was approximately the same. In figure 4, comparatively large reduction in the band gap is evident for the composition with a small AgGaO2 concentration at x ≤ 0.1. However, a near linear decrease occurs with an increase in alloying level. Therefore, band gap bowing in the β-AgGaO2–ZnO alloys is much smaller than that in the CdO–ZnO system. The large reduction in the band gap during the early alloying stage can be explained by the introduction of the Ag 4d contribution around the valence band maximum (VBM), which mainly consists of the O 2p states in pure ZnO. The introduction of the Ag 4d contribution highly modulates the electronic configuration of the VB of ZnO because of the higher energy of the Ag 4d atomic orbitals compared with that of the O 2p atomic orbitals.

View Article: PubMed Central - PubMed

ABSTRACT

Ternary zincblende-derived I&ndash;III&ndash;VI2 chalcogenide and II&ndash;IV&ndash;V2 pnictide semiconductors have been widely studied and some have been put to practical use. In contrast to the extensive research on these semiconductors, previous studies into ternary I&ndash;III&ndash;O2 oxide semiconductors with a wurtzite-derived &beta;-NaFeO2 structure are limited. Wurtzite-derived &beta;-LiGaO2 and &beta;-AgGaO2 form alloys with ZnO and the band gap of ZnO can be controlled to include the visible and ultraviolet regions. &beta;-CuGaO2, which has a direct band gap of 1.47 eV, has been proposed for use as a light absorber in thin film solar cells. These ternary oxides may thus allow new applications for oxide semiconductors. However, information about wurtzite-derived ternary I&ndash;III&ndash;O2 semiconductors is still limited. In this paper we review previous studies on &beta;-LiGaO2, &beta;-AgGaO2 and &beta;-CuGaO2 to determine guiding principles for the development of wurtzite-derived I&ndash;III&ndash;O2 semiconductors.

No MeSH data available.