I-III-VI semiconductors

I-III-VI₂ semiconductors are solid semiconducting materials that contain three or more chemical elements belonging to groups I, III and VI of the periodic table. They usually involve two metals and one chalcogen. Some of these materials have a direct bandgap, E_g, of ~ 1.5 eV, which makes them efficient absorbers of sunlight and thus potential solar cell materials. A fourth element is often added to a I-III-VI₂ material to tune the bandgap for maximum solar cell efficiency. A representative example is copper indium gallium selenide (CuIn_xGa_(1–x)Se₂, E_g = 1.7–1.0 eV for x = 0–1^[1]), which is used in copper indium gallium selenide solar cells.

Optical absorption spectrum of β-CuGaO₂ powder (top left inset) obtained from diffuse reflection measurements. The right inset shows the Shockley-Queisser limit for the efficiency of a single-junction solar cell under unconcentrated sunlight.^[2]

CuGaO₂

CuGaO₂ exists in two main polymorphs, α and β. The α form has the delafossite crystal structure and can be prepared by reacting Cu₂O with Ga₂O₃ at high temperatures. The β form has a wurtzite-like crystal structure (space group Pna2₁); it is metastable, but exhibits a long-term stability at temperatures below 300 °C.^[3] It can be obtained by an ion exchange of Na⁺ ions in a β-NaGaO₂ precursor with Cu⁺ ions in CuCl under vacuum, to avoid the oxidation of Cu⁺ to Cu²⁺.^[2]

Unlike most I-III-VI₂ oxides, which are transparent, electrically insulating solids with a bandgap above 2 eV, β-CuGaO₂ has a direct bandgap of 1.47 eV, which is favorable for solar cell applications. In contrast, β-AgGaO₂ and β-AgAlO₂ have an indirect bandgap. Undoped β-CuGaO₂ is a p-type semiconductor.^[2]

AgGaO₂ and AgAlO₂

Bandgap in AgGaO₂-ZnO and CdO-ZnO alloys.^[2]

Similarly to CuGaO₂, α-AgGaO₂ and α-AgAlO₂ have the delafossite crystal structure while the structure of the corresponding β phases is similar to wurtzite (space group Pna2a). β-AgGaO₂ is metastable and can be synthesized by ion exchange with a β-NaGaO₂ precursor. The bandgaps of β-AgGaO₂ and β-AgAlO₂ (2.2 and 2.8 eV respectively) are indirect; they fall into the visible range and can be tuned by alloying with ZnO. For this reason, both materials are hardly suitable for solar cells, but have potential applications in photocatalysis.^[2]

Contrary to LiGaO₂, AgGaO₂ can not be alloyed with ZnO by heating their mixture because of the Ag⁺ reduction to metallic silver; therefore, magnetron sputtering of AgGaO₂ and ZnO targets is used instead.^[2]

LiGaO₂

Bandgap in LiGaO₂-ZnO alloys.^[2]

Pure single crystals of β-LiGaO₂ with a length of several inches can be grown by the Czochralski method. Their cleaved surfaces have lattice constants that match those of ZnO and GaN and are therefore suitable for epitaxial growth of thin films of those materials. β-LiGaO₂ is a potential nonlinear optics material, but its direct bandgap of 5.6 eV is too wide for visible light applications. It can be reduced down to 3.2 eV by alloying β-LiGaO₂ with ZnO. The bandgap tuning is discontinuous because ZnO and β-LiGaO₂ do not mix but form a Zn₂LiGaO₄ phase when their ratio is between ca. 0.2 and 1.^[2]

Room-temperature properties of I-III-VI₂ semiconductors^[4]

Formula	a (Å)	b (Å)	c (Å)	Space group	Density (g/cm^3)	Melting point (K)	Bandgap (eV)
α-LiGaO2[5]	2.92	2.92	14.45	R3m	5.07	m	5.6d
β-LiGaO2[6]	5.406	6.379	5.013	Pna21	4.18	m	5.6d
CuAlS2	5.323	5.323	10.44	I42d	3.47	2500	2.5
CuAlSe2	5.617	5.617	10.92	I42d	4.70	2260	2.67
CuAlTe2	5.976	5.976	11.80	I42d	5.50	2550	0.88
β-CuGaO2[3]	5.46004(1)	6.61013(2)	5. 27417(1)	Pna21		m	1.47d
CuGaS2	5.360	5.360	10.49	I42d	4.35	2300	2.38
CuGaSe2	5.618	5.618	11.01	I42d	5.56	1970	0.96; 1.63
CuGaTe2	6.013	6.013	11.93	I42d	5.99	2400	0.82; 1.0
CuInS2	5.528	5.528	11.08	I42d	4.75	1400	1.2
CuInSe2	5.785	5.785	11.56	I42d	5.77	1600	0.86; 0.92
CuInTe2	6.179	6.179	12.365	I42d	6.10	1660	0.95
CuTlS2	5.58	5.58	11.17	I42d	6.32
CuTlSe2	5.844	5.844	11.65	I42d	7.11	900	1.07
CuFeO2	3.035	3.035	17.166	R3m	5.52
CuFeS2	5.29	5.29	10.32	I42d	4.088	1135	0.53
CuFeSe2				I42d		850	0.16
CuLaS2	5.65	5.65	10.86	I42d
β-AgAlO2						m	2.8i
AgAlS2	5.707	5.707	10.28	I42d	3.94
AgAlSe2	5.986	5.986	10.77	I42d	5.07	1220	0.7
AgAlTe2	6.309	6.309	11.85	I42d	6.18	1000	0.56
α-AgGaO2				P63mc			4.12d[7]
β-AgGaO2				Pna2a		m	2.2i
AgGaS2	5.755	5.755	10.28	I42d	4.72		1.66
AgGaSe2	5.985	5.985	10.90	I42d	5.84	1120	1.1
AgGaTe2	6.301	6.301	11.96	I42d	6.05	990	1.9
AgInS2	5.828	5.828	11.19	I42d	5.00		1.18
AgInSe2	6.102	6.102	11.69	I42d	5.81	1053	0.96; 0.52
AgInTe2	6.42	6.42	12.59	I42d	6.12	965
AgFeS2	5.66	5.66	10.30	I42d	4.53		0.88 [8]

• m stands for metastable, d for direct and i for indirect bandgap

See also

• List of semiconductor materials

References

1. Tinoco, T.; Rincón, C.; Quintero, M.; Pérez, G. S. N. (1991). "Phase Diagram and Optical Energy Gaps for CuIn_yGa_1−ySe₂ Alloys". Physica Status Solidi (a). 124 (2): 427–434. Bibcode:1991PSSAR.124..427T. doi:10.1002/pssa.2211240206.
2. Omata, T.; Nagatani, H.; Suzuki, I.; Kita, M. (2015). "Wurtzite-derived ternary I–III–O₂ semiconductors". Science and Technology of Advanced Materials. 16 (2): 024902. Bibcode:2015STAdM..16b4902O. doi:10.1088/1468-6996/16/2/024902. PMC 5036475. PMID 27877769.
3. Nagatani, H.; Suzuki, I.; Kita, M.; Tanaka, M.; Katsuya, Y.; Sakata, O.; Miyoshi, S.; Yamaguchi, S.; Omata, T. (2015). "Structural and Thermal Properties of Ternary Narrow-Gap Oxide Semiconductor; Wurtzite-Derived β-CuGaO₂". Inorganic Chemistry. 54 (4): 1698–704. doi:10.1021/ic502659e. PMID 25651414.
4. Haynes, William M., ed. (2011). CRC Handbook of Chemistry and Physics (92nd ed.). Boca Raton, FL: CRC Press. pp. 12.82 and 12.87. ISBN 1439855110.
5. Hoppe, R. (1965) Bull. Soc. Chim. Fr., 1115–1121
6. Konovalova E.A., Tomilov N.P. (1987) Russ. J. Inorg. Chem. 32, 1785–1787
7. Vanaja, K. A.; Ajimsha, R. S.; Asha, A. S.; Rajeevkumar, K.; Jayaraj, M. K. (2008). "Pulsed laser deposition of p-type α-AgGaO₂ thin films". Thin Solid Films. 516 (7): 1426–1430. Bibcode:2008TSF...516.1426V. doi:10.1016/j.tsf.2007.07.207.
8. Sciacca, B.; Yalcin, A. O.; Garnett, E. C. (2015). "Transformation of Ag Nanowires into Semiconducting AgFeS₂ Nanowires". Journal of the American Chemical Society. 137 (13): 4340–4343. doi:10.1021/jacs.5b02051.