Photon Factory Activity Report 2004 #22 Part B (2005)
Materials Science

18A/2004G002

Photoemission study of the diluted ferromagnetic semiconductor Zn1-xCrxTe
Y. Ishida1, J.I. Hwang2, M. Kobayashi1, A. Fujimori1,2, H. Saito3 and K. Ando3
Department of Physics, University of Tokyo, Kashiwa, Chiba 277-8561, Japan2
Department of Complexity Science and Engineering, University of Tokyo,Kashiwa, Chiba 277-8561, Japan
3
NanoElectronics Research Institute, AIST, Tsukuba, Ibaraki 305-8568, Japan

1

Ferromagnetic (FM) semiconductors have become
one of the key materials for ‘spintronics’ devices, which
aim at the integrated use of spins and carriers insemiconductors. From the application view point, diluted
magnetic semiconductor (DMS) showing high Curie
temperature (TC) and semiconducting transport properties
is desired. Recently, Saito et al. succeeded in thefabrication of DMS Zn1-xCrxTe with TC as high as 300 ±10 K [1]. It is the highest TC ever reported for FM DMSs
showing large magnetic circular dichroism [2].
Furthermore, Zn1-xCrxTe is highly resistive [3], suggesting
the controllability of transport properties in the FM phase.
The mechanism of ferromagnetism in Zn1-xCrxTe is,however, completely open, particularly because its carrier
concentration of ~1×10-15 cm-3 [3] is 3-5 orders smaller
than those of the typical III-V-based DMSs Ga1-xMnxAs
and In1-xMnxAs where carrier-induced ferromagnetism [4]is proposed as the origin of ferromagnetism.
In order to study the electronic structure of Zn1-xCrxTe,we have performed Cr 3p-3d resonant photoemission
spectroscopy (RPES) measurements on Zn0.957Cr0.043Te(TC ~ 70 K) and ZnTe thin films. Samples were preparedby molecular-beam epitaxy method as reported elsewhere[1,3]. Photoemission measurements were performed at
BL-18A of Photon Factory. The energy resolution of thespectrometer including temperature broadening was ~300
meV. Repeated Ar-ion sputtering at 1.0 kV andsubsequent annealing up to 200°C were performed toobtain clean surfaces. Surface cleanliness was checked by
the weakness of the O 1s and C 1s core-level signals.
Measurements were performed at room temperature forZn0.957Cr0.043Te, and at an elevated temperature of ~450 K
for ZnTe in order to minimize charging effects.
Figure 1(a) and (b) show the valence-band spectra of
Zn0.957Cr0.043Te and ZnTe, respectively, in the Cr 3p-3d
absorption region (on resonance; hν ~ 48 eV). The Te 4dcore-level structures due to second order light which weresuperposed on the valence-band spectra taken below hν =44 eV had been subtracted. One can see a large spectral
intensity in the upper region of the valence band in
Zn0.957Cr0.043Te [Fig. 1(a)] compared to that of ZnTe [Fig.
1(b)]. The cross-section of Cr 3d is one order ofmagnitude larger than that of Te 5p in the Cr 3p-3dabsorption region (26 times larger at hν = 50 eV [5]), andtherefore, we conclude that the main structure of Cr 3delectrons resides in the upper region of the host valenceband. Indeed one can see a clear Cr 3p-3d resonanceFig 1. Valence-band photoemission spectra ofZn0.957Cr0.043Te and ZnTe in the Cr 3p-3d absorption
region. (a) Zn0.957Cr0.043Te, and (b) ZnTe.
enhancement around the binding energy (EB) of 1.7 eV.
Careful examination of the constant-initial-state spectra(not shown) indicated another Cr 3p-3d resonanceenhancement at higher EB region of ~ 8 eV, This structure
is assigned to the bonding state of Cr 3d and neighboring
Te 4p orbitals. In the LDA calculations, Zn1-xCrx Te ispredicted to be a half-metallic ferromagnet, but it is clearthat the spectral intensity around the Fermi level isstrongly suppressed, consistent with its semiconducting
behavior.
[1] H. Saito, V. Zayets, S. Yamagata and K. Ando, J. Appl.
Phys. 93 (2003) 6796; H. Saito, V. Zayets, S. Yamagata
and K. Ando, Phys. Rev. Lett. 90 (2003) 207202.
[2] K. Ando, in Magneto-Optics, Springer Series in SolidState Science, edited by S. Sugano and N. Kojima(Springer, Berlin, 2000), Vol. 128, p. 211.
[3] H. Saito, et al., J. Appl. Phys. 91 (2002) 8085.
[4] T. Dietl et al., Science 287(2000) 1019.
[5] J.-J. Yeh and I. Lindau, At. Data Nucl. Data Tables 32(1985) 1.
[6] K. Sato and H. Katayama-Yoshida, Jpn. J. Appl. Phys.
40 (2001) L651; L.M. Sandratskii and P. Bruno, J. Phys.:Condens. Matter 15 (2003) L585.
*ishida@wyvern.phys.s.u-tokyo.ac.jp

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