Growth of p-type ZnOS films by pulsed laser deposition
Introduction
White light-emitting diodes (LED) have been developed by using Ga1−xInxN-based blue LEDs and yellow phosphors. From viewpoints of resources, the substitutes for Ga and In atoms are required. ZnO is one of promising substitutes because of its abundant raw materials and non-toxicity. An n-type ZnO film was easily prepared by doping of Al and Ga atoms [1], whereas the growth of p-type ZnO films is very difficult [2], [3], [4], [5], [6]. Such a difficulty is related to the deep energy level of the top of the valence band of ZnO. In recent, it was found that the energy level of the valence band can be moved upward by doping of S into ZnO [7]. A mixed crystal of ZnO1−xSx has the strong bowing effect of band gap energy: As S-content, x, increases from 0 to 0.5, the top of the valence band moves upward whereas the bottom of the conduction band is unchanged [7]. ZnO1−xSx films have been grown by several techniques, e.g., chemical vapor deposition [7], sputtering [8] and PLD [9]. However, no one succeeded in the growth of p-type ZnO1−xSx films. The suppression of oxygen vacancies and sulfur vacancies are necessary to realize the growth of p-type ZnO1−xSx films. In the present work, we have prepared ZnO1−xSx films by PLD of ZnO1−xSx targets synthesized by sulfuring ZnO targets at several temperatures. The crystal structure and the lattice constants of ZnO1−xSx films are determined by X-ray diffraction (XRD) measurements, and the S-contents of the films are calculated from the lattice constants using the Vegard’ rule. The electrical and optical properties of the ZnO1−xSx films are investigated as a function of the S-content. Furthermore, in order to incorporate excess S atoms into a film, evaporation of Sulfur was simultaneously performed during PLD process. The resistivity, the optical band gap, and the carrier types of ZnO1−xSx films grown by PLD accompanying the S-evaporation are examined as a temperature of the S-evaporation. The S-contents of ZnO1−xSx films are determined by X-ray photoelectron spectroscopy (XPS) and are compared with those estimated from XRD. The chemical situation of S atoms in ZnO1−xSx films is clarified from XPS of S 2p.
Section snippets
Experimental
ZnO1−xSx films were grown on quartz substrates heated at 200 °C by PLD with a KrF excimer laser (248 nm). A laser repetition rate was 10 Hz and the energy per pulse was 100 mJ. The film growth was carried out at atmospheres of N2 + O2, in which the N2-partial pressure was kept to be 1.6×10−2 Torr and an O2-partial pressure was changed from 4×10−3 to 3×10−2 Torr. ZnO1−xSx targets were prepared by sulfuring ZnO targets at several temperatures; a ZnO target was kept in a small crucible and subsequently
Results
Films were deposition on the quartz substrates by PLD of the ZnO1−xSx targets sulfured at 200 and 500 °C. XRD patterns of the films grown in an atmosphere containing an O2-partial pressure of 4×10−3 Torr are shown in Fig. 1. The film prepared from the target sulfured at 200 °C shows a peak at 34.0°, which are lower than 34.4° of ZnO (002). The shift of the diffraction peak is due to the insertion of large S atoms into the ZnO lattice. It was reported that the linear relationship between wurtzite
Discussion
As seen in Table 2, the formation of the p-type ZnO1−xSx films is achieved by the supply of excess S atoms. In PLD accompanying S-evaporation, a ZnO1−xSx film was deposited in a short period by the ablation of the ZnO1−xSx target with a low S-content, and subsequently the film surface was sulfured for 0.1 s, which is an interval between laser pulses. The sulfuring process results in the replacement of O atoms at anion sites by S atoms. The replacement reaction is an endothermic process, so that
Conclusions
ZnO1−xSx films were deposited on quartz substrates by PLD of the ZnO1−xSx targets sulfured at 200 and 500 °C in atmospheres containing O2-partial pressures of 4.0×10−3 to 3.4×10−2 Torr. The S-content of the ZnO1−xSx film increases with the sulfuring temperature of the target, but oppositely decreases with an O2-partial pressure. Although the resistivity of the films increases with both the sulfuring temperature and an O2-partial pressure in an atmosphere, all of the films are n-type. In order to
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