Materials Transactions, Vol. 46, No. 8 (2005) pp. 1908 to 1910 #2005 The Japan Institute of Metals Epitaxial Growth of Vanadium-Doped ZnSe by MOVPE Masahiro Tahashi, Shinsuke Ito, Toshiyuki Ido and Hideo Goto Department of Electrical Engineering, Chubu University, Kasugai 487-8501, Japan Vanadium-doped ZnSe, which is theoretically predicted to induce ferromagnetism above room temperature without carrier doping, was epitaxially grown on (100)GaAs substrate by metal-organic vapor phase epitaxial method in an atmospheric pressure. Vanadium concentration in the film obtained under the condition where the substrate and the vanadocene temperatures are 500 and 140C, respectively, was 6.0 at% at maximum. The full width at half maximum (FWHM) of the peak diffracted from ZnSe(400) face increased with the increase of a vanadium concentration. (Received April 18, 2005; Accepted June 3, 2005; Published August 15, 2005) Keywords: metal-organic vapor phase epitaxy, vanadium doping, zinc selenide 1. Introduction Infrared heater As a new material, diluted magnetic semiconductors Gas flow Gas flow Substrate (DMS) have attracted much interest and have been studied V(C5H5)2 actively for the purpose of the use of both charge and spin of electrons in semiconductors with doping of magnetic Band heater elements. In some diluted magnetic materials, Curie temper- ature has been reported to be higher than room temperature Fig. 1 Schematic view of experimental apparatus. and is given in several groups of semiconductors as follows; 1) 2) 285 K for Ge1ÀxMnx in IV group, 300 K for Zn1ÀxCrxTe 15min. 3,4) 640 Substrate temp.: Ts and Zn1ÀxVxO in II–VI group, and 300 K for Ga1Àx- 30min. 5) 6) 7) C ° MnxN, Ga1ÀxCrxN, Al1ÀxCrxN in III–V group, respec- / 550 T 120min. tively. However, Curie temperature for most of DMS is lower Ts Ts=450~550°C than room temperature so that it has been desired to get DMS 15min. with Curie temperature higher than room temperature for Tv=130~180°C practical applications. Vanadium-doped ZnSe is expected as Temperature, Tv DMS inducing ferromagnetism above room temperature Vanadocene temp.: Tv 8) without carrier doping. ZnSe also has been investigated for R.T. a candidate to a blue light emitting diode and a blue laser one Time, t/min. for a long time. Therefore, vanadium-doped ZnSe would Fig. 2 Temperature sequences for the substrate and vanadocene. have not only properties of both semiconductor and magnetic materials but also optical one so that vanadium-doped ZnSe has been expected as a device material having new functions. shows temperature sequences for the substrate and the The purpose of this study is to fabricate vanadium-doped vanadocene. First of all, GaAs substrate was heated at ZnSe films by using metal-organic vapor phase epitaxial 640C for 15 min in hydrogen atmosphere as a surface method in an atmospheric pressure. treatment to remove oxide layers, which is essential to enhance epitaxial growth of ZnSe. Next, undoped ZnSe film 2. Experimental Procedure was grown on GaAs substrate at 550C for 30 min for homogeneous epitaxial growth between ZnSe and vanadium- Vanadium-doped ZnSe films were grown by metal-organic doped ZnSe, and then vanadium-doped ZnSe was grown for vapor phase epitaxial (MOVPE) method in an atmospheric 120 min. In vanadium doping, the temperature of the pressure. Figure 1 illustrates the schematic view of the vanadocene was varied from 130 to 180C and that of the experimental apparatus. A reactor tube of the apparatus was substrate from 450 to 550C. The vanadium concentration in horizontal and a substrate was heated by an infrared lamp. ZnSe film and the crystallinity evaluated from the full width (100)GaAs single crystals were used as substrates. Dimethyl at half maximum (FWHM) of ZnSe(400) peak intensity were Se (DMSe, Se(CH3)2) and dimethyl Zn (DMZn, Zn(CH3)2) investigated by using energy dispersive X-ray spectrometer were used as source materials. Both materials were kept at (EDX) and four-crystals X-ray diffractometer (XRD), re- 0C in thermo-baths. The flow rates of DMSe and DMZn spectively. were 30 mmol/min and 12 mmol/min, respectively. The carrier gas was hydrogen at the flow rate of 200 ml/min. 3. Results and Discussion Vanadocene (V(C5H5)2) was used as a dopant source for vanadium. The boat filled with vanadocene that is in solid 3.1 The dependence of vanadium concentration in ZnSe state at room temperature was set into the reactor and was on vanadocene and substrate temperatures heated by a band-heater wrapped around the reactor. Figure 2 Figure 3 shows the dependence of EDX spectra on the Epitaxial Growth of Vanadium-Doped ZnSe by MOVPE 1909 Substrate temperature, Ts/°C zinc 450 500 550 selenium vanadium ) 6 Tv=140°C Tv=130°C 4 Intensity (arb. unit) undoped 2 vanadocene 01020 Vanadium concentration (at substrate Energy, E/keV 0 130 140 Fig. 3 The vanadocene temperature dependence of EDX spectra obtained at the substrate temperature of 500C. Vanadocene temperature, Tv/°C Fig. 5 The dependence of vanadium concentrations on vanadocene and substrate temperatures. The substrate temperature was maintained at vanadocene temperature obtained at the constant substrate 500C for the study on the vanadocene temperature dependence. The temperature of 500 C. There are only peaks due to zinc and vanadocene temperature was maintained at 140C for the study on the selenium in undoped ZnSe. On the other hand, no peaks substrate temperature dependence. resulted from vanadium are seen in the vanadium doped sample grown at the vanadocene temperature of 130C. At that of 140C, the peaks resulted from vanadium were seen at depending on the vanadocene and the substrate temperatures. 0.511 keV due to L -line, at 4.949 keV due to K 1;2-line, and In the case where the substrate temperature was maintained at at 5.426 keV due to K 1-line, respectively. At 150 C and 500 C, the vanadium concentration increases with an higher temperatures, a surface of the film was covered with increase of the vanadocene temperature. On the other hand, black powders and the film thickness was extremely thin so vanadium concentration increases up to 6.0 at% at maximum that results obtained at these conditions are excluded from and then decreases by increasing of the substrate temperature here. at the constant vanadocene temperature of 140C. Next, the dependence of EDX spectra on the substrate As a result, vanadium concentration in films takes temperature at the constant vanadocene temperature of maximum value of 6.0 at% at the condition where substrate 140C is shown in Fig. 4. There are no peaks from vanadium and vanadocene temperatures are 500 and 140C, respec- for the sample grown at 450C. This is the same for undoped tively. It is theoretically predicted that ferromagnetism would ZnSe. Though the peaks from vanadium are seen in both be induced above room temperature if a vanadium concen- 500C and 550C, the intensity of vanadium peaks in 550C tration was higher than 5 at%.8) In this study we have is relatively small in comparison with that in 500C. obtained vanadium concentration higher than 5 at%. Figure 5 shows the vanadium concentration in ZnSe films 3.2 The effect of vanadium doping on crystallinity of ZnSe The crystal quality of the ZnSe was investigated by the zinc XRD measurement. ZnSe(400) peak was appeared regardless selenium of vanadium doping, this experimental result indicated that vanadium ZnSe film was epitaxially grown on GaAs substrate. The relation between vanadium concentration and the FWHM of Ts=550°C ZnSe(400) diffraction is shown in Fig. 6. The FWHM of undoped ZnSe was about 250 arc seconds. In the case vanadium doping, the FWHM increases rapidly up to 500 arc Ts=500°C seconds and then increases gradually up to 600 arc seconds with an increase of vanadium concentration. The crystallinity Intensity (arb. unit) of ZnSe became worse by vanadium doping, but it was found Ts=450°C that vanadium doped ZnSe films were epitaxially grown on GaAs substrate regardless of vanadium doping. Furthermore, the state of vanadium element in ZnSe 02010 crystal was investigated by the wide range X-ray diffraction Energy, E/keV measurement and the area scan with EDX showing distribu- Fig. 4 The substrate temperature dependence of EDX spectra obtained at tion of vanadium element. No peaks except for ZnSe the vanadocene temperature of 140C. epitaxial layer and GaAs substrate were detected in all 1910 M. Tahashi, S. Ito, T. Ido and H. Goto cene as the vanadium dopant source, where the substrate and 600 the vanadocene temperatures were 500 and 140C, respec- tively. ZnSe films were epitxially grown on GaAs substrate regardless of vanadium doping. However, FWHM of X-ray diffraction peak of ZnSe(400) increased with an increase of vanadium concentration. 400 Acknowledgements This research was partly supported by a grant of the vanadium-doped ZnSe Academic Frontier Promotion Project from Ministry of undoped ZnSe Education, Culture, Sports, Science and Technology. FWHM of X-ray diffraction (arc sec) 200 0246 Vanadium concentration (at ) REFERENCES Fig. 6 FWHM of X-ray diffraction of ZnSe(400) as a function of the 1) S. Cho, S. Choi, S. C. Hong, Y. Kim, Y. C. Ketterson, B. J. Kim, Y. C. vanadium concentration. Kim and J. H. Jung: Phys. Rev. B 66 (2002) 033303. 2) H. Saito, V. Zayets, S. Yamagata and K. Ando: Phys. Rev. Lett. 90 (2003) 207202. 3) H. Saeki, H. Tanabe and T. Kawai: Solid State Commun. 120 (2001) samples investigated in this study. Also the area scan in the 439. area of 5 mm  5 mm suggests that vanadium is distributed 4) H.