Ryoji Katsube

Growth and characterization of indium-doped Zn3P2 bulk crystals

In this paper, we report the crystal growth of indium-doped zinc phosphide bulk crystals to obtain n-type conduction. The crystal growth experiments were carried out by unidirectional solidification from In–Zn–P ternary solution, and n-type Zn3P2 bulk crystals were successfully obtained. It was also revealed that the electrical properties of indium-doped Zn3P2 crystals could be controlled by heat treatment under controlled partial pressure of phosphorus or zinc. The relationship between the electron concentration and the partial pressure of zinc or phosphorus even in indium-doped Zn3P2 can be understood on the basis of defect equilibria, as in the case of undoped Zn3P2. However, the controllable range of the electron concentration was 1010–1013 cm−3, which was too low in terms of the concentration of doped indium. This is due to the low formation energy of the intrinsic acceptor, interstitial phosphorus, and carrier compensation should be discussed. According to recent reports on ab initio calculation, a weakly n-type Zn3P2 is expected to be formed under extremely Zn-rich growth conditions. The results obtained in this study coincide with the prediction based on calculations, and provide useful knowledge for the formation of the p–n junction of Zn3P2.

Thermodynamic considerations on interfacial reactivity concerning carrier transport characteristics in metal/p-Zn3P2 junctions

Carrier transport across a metal/semiconductor interface plays a key role in electronic devices. It is generally affected by some interactions at interfaces, such as atomic interdiffusion and formation of intermediate compounds; however, it is hard to consider all possible chemical reactions at the interface. In this work, we adopted a chemical potential diagram for comprehensive discussions on chemical reactions at the interfaces between the metal (M) and p-type zinc phosphide (p-Zn3P2), which is a promising candidate as an absorber in thin-film solar cells. We revealed that Schottky or Ohmic behavior depended on the reaction at the interfaces through the investigations in Al/p-Zn3P2 and Ag/p-Zn3P2 junctions. The interdiffusion was observed at the interface of Al/p-Zn3P2, while the interfacial structure in Ag/p-Zn3P2 was not changed by annealing. Such behaviors were thermodynamically expected from the chemical potential diagrams and determined the transport characteristics considering the similar work functions of Al and Ag. Furthermore, the discussions on other M–P–Zn (M = Au, In, Mn, Cu) systems based on chemical potential diagrams clarified that phosphide semiconductors as intermediate compounds were formed in the M/p-Zn3P2 junctions, which show Schottky behavior, and that Ohmic M/p-Zn3P2 junctions are stable. Therefore, it is concluded that we should discuss the band structures in heterojunctions under the considerations on reactivity at the interfaces, which is comprehensively understood using chemical potential diagrams.