Wide‐Range‐Tunable p‐Type Conductivity of Transparent CuI_1−xBr_x Alloy

Abstract

Transparent p-type semiconductors with wide-range tunability of the hole density are rare. Developing such materials is a challenge in the field of transparent electronics that utilize invisible electric circuits. In this paper, a CuI-CuBr alloy (CuI 1−x Br x ) is proposed as a hole-density-tunable p-type transparent semiconductor that can be fabricated at room temperature. First-principles calculations predict that the acceptor state originating from copper vacancies in CuBr is deeper than that in CuI, leading to the hypothesis that the hole density in CuI 1−x Br x can be tuned over a wide range by varying x between 0 and 1. The experimental results support this hypothesis. The hole density in CuI 1−x Br x polycrystalline alloy layers can be tuned by over three orders of magnitude (10 17 -10 20 cm −3 ) by varying x. In other words, the p-type conductivity of the CuI 1−x Br x alloy shows metallic and semiconducting properties. Such alloy layers can be prepared at room temperature without sacrificing transparency. Furthermore, CuI 1−x Br x forms transparent p-n diodes with n-type amorphous In-Ga-Zn-O layers, and these diodes have satisfactory rectification performance. Therefore, CuI 1−x Br x alloy is an excellent p-type transparent semiconductor for which the p-type conductivity can be tailored in a wide range.

Characteristics and Electronic Band Alignment of a Transparent p-CuI/n-SiZnSnO Heterojunction Diode with a High Rectification Ratio

“…In addition, it is noteworthy that an exciton absorption peak is found at 407 nm (3.05 eV) (a gray dotted line in Figure 1 c), as frequently observed in the CuI film [ 2 , 4 , 10 , 12 ]. Then, E g of the CuI film is estimated as ≈3.1 eV from the relationship of E g = E Z1,2 + E ex where E Z1,2 is an exciton absorption energy of CuI and E ex is an exciton binding energy of 62 meV [ 20 ].…”

“…It is emphasized that I F / I R = 6.6 × 10 7 is quite a high value among the heterojunction diodes consisting of the p -CuI film. Figure 4 compares I F / I R of the p -CuI/ n -SZTO diodes with those of other CuI-based transparent heterojunction diodes that have a high I F / I R > ≈10 5 ; 2 × 10 7 at ±2 V for p -CuI/ n -ZnO (a polycrystalline CuI film) [ 2 ], 2 × 10 9 at ±2 V for p -CuI/ n -ZnO (an epitaxial CuI film) [ 11 ], 7 × 10 5 at ±2 V for p -CuI/ n -BaSnO 3− δ [ 12 ], 6 × 10 6 at ±2 V for p -CuI/ n -IGZO [ 3 ], and ≈10 9 at ±1.5 V for p -CuI 1− x Br x / n -IGZO ( x = 0.0 − 1.0) [ 10 ]. Albeit the highest value of I F / I R = ≈2 × 10 9 realized in the p -CuI/ n -ZnO diode is higher than that of the present p -CuI/ n -SZTO diode by ≈30 times, to realize such a diode requires a special growth condition, i.e., an epitaxial growth of the CuI film on top of an epitaxial ZnO film.…”

“…This implies that such a high I F / I R is hard to achieve in a flexible device. Although the I F / I R = ≈1 × 10 9 in the p -CuI/ n -IGZO diodes looks also higher than that of the p -CuI/ n -SZTO diode, a proper comparison of I F / I R might be required as the work used the measured I F and the fitted I s [ 10 ].…”

“…I F / I R of the CuI/SZTO diode is estimated by the fitted I F at +2 V and I R at −2 V in this work. I F / I R of the CuI 1− x Br x /IGZO diode is estimated by the measured I F at +1.5 V and the fitted reverse saturation current [ 10 ]. I F / I R of CuI/ZnO [ 2 , 11 ], CuI/a-IGZO [ 3 ], and CuI/BaSnO 3-δ [ 12 ] diodes are obtained from the measured current at ±2 V.…”

“…In order to realize a transparent pn diode based on the CuI, we have been investigating various heterojunction diodes made of p -CuI and n -type semiconductors [ 2 , 8 , 9 , 10 , 11 ]; in our previous work, we have reported a transparent diode made of p -CuI/ n -BaSnO 3− δ (BSO) films and its carrier transport behavior [ 12 ]. The realized p -CuI/ n -BSO diode exhibited a high current rectification ratio ( I F / I R ) of 6.75 × 10 5 at an external voltage bias of ±2 V. On the other hand, a high-quality BSO film often requires an epitaxial growth on a proper substrate such as SrTiO 3 (001), and its ideal growth temperature is rather high (≈800 °C).…”