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Cr impurity-induced electronic states in ZnTe(110) surface

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2015 Jpn. J. Appl. Phys. 54 08LB01(http://iopscience.iop.org/1347-4065/54/8S2/08LB01)
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REGULAR PAPER

Japanese Journal of Applied Physics 54, 08LB01 (2015)
http://dx.doi.org/10.7567/JJAP.54.08LB01

Cr impurity-induced electronic states in ZnTe(110) surface
Ken Kanazawa*, Taku Nishimura, Shoji Yoshida, Hidemi Shigekawa, and Shinji Kuroda
Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan
E-mail: kanazawa@ims.tsukuba.ac.jp
Received November 12, 2014; accepted December 25, 2014; published online June 4, 2015
The impurity states of Cr atoms, which substituted Zn sites in the topmost layer of a p-type ZnTe(110) surface, were investigated by scanningtunneling microscopy/spectroscopy (STM/STS) and we ﬁrstly observed Cr-induced impurity states in the energy gap region of the host ZnTe
including the unoccupied states by STS. Furthermore, we compared the observed energy levels and spatial distributions of the local density ofstates with those in the previous theoretical study [Katayama-Yoshida et al., Phys. Status Solidi A 204, 15 (2007)] and successfully identiﬁed theimpurity states as the respective spin-polarized impurity states predicted by the theoretical study. © 2015 The Japan Society of Applied Physics

1.

Introduction

The investigation of electronic states of the magnetic
impurities in a host semiconductor is essential in understanding the mechanism of ferromagnetism of diluted
magnetic semiconductors (DMSs) and applying and controlling the states in future spintronic devices.1,2) The magnetic
interaction between transition-metal (TM) impurities in amagnetic semiconductor depends on the combination of thehost semiconductor and the magnetic impurity elements. In
order to realize a DMS material with high performance
characteristics such as room-temperature (RT) ferromagnetism, we must understand the impurity states as well as
possible systematically. Furthermore, from the viewpoint oftechnological applications, to advance the new research ﬁeldof nanotechnology combined with magnetism, it is also
important to investigate the magnetic behavior of the material
down-sized to the atomic scale.
Scanning tunneling microscopy=spectroscopy (STM=STS)is one of the most powerful methods of studying such
impurity states at a single-atom scale. Up to now, several
STM studies for DMS systems mainly based on TM-doped
GaAs have been reported.3–15) According to those reports,
distinct impurity states are induced by the hybridization of
the impurity d levels of TM with the host. In particular,
recently, for Fe atoms substituting into the Ga site on theGaAs(110) surface, Mühlenberend et al. observed impurity
states located near the valence band maximum of the host
GaAs, in good agreement with the prediction in an earlier
theoretical study.15)
Cr-doped ZnTe [(Zn,Cr)Te] has been regarded as one ofthe promising materials for spintronic devices because of
its intrinsic RT ferromagnetism when the Cr composition isabout 20%.16) In order to clarify the origin of magnetic
phenomena induced by the doped Cr atoms, it is essential toinvestigate the impurity states such as their energy levels andelectronic spatial distributions. Several theoretical energy
diagrams of the Cr impurity states in the band structure of
the host ZnTe have been proposed.17) However, the numberof experimental reports is very few.16,18,19) In one of ourearlier experimental studies, we performed cross-sectional
STM=STS measurements for (Zn,Cr)Te grown by molecularbeam epitaxy and revealed that a Cr atom formed a localizedstate within the bandgap of the host ZnTe; this state wasbroadened for a pair of Cr atoms within a distance of ∼1 nm
owing to ferromagnetic interaction.19) However, in that study,

the impurity states only in the negative sample bias region
were investigated; it is important to examine all the impurity
states including the empty states in order to understand andapply them. Moreover, considering their application to future
nanotechnology, it is meaningful to study the single TM atomthat is adsorbed and substitutes into the cation site on the
topmost surface of the host semiconductor without any
inﬂuence of neighboring TM impurities. Here, we present theresults obtained in a STM=STS study on Cr atoms adsorbed
on a ZnTe(110) surface. In this study, we successfully
observed Cr atoms substituting for Zn atoms in the topmost
surface, and for the ﬁrst time, report that their impurity states
above the Fermi energy are in good agreement with theresults of an earlier theoretical study. Furthermore, the resultsof our STS measurements suggest the possibility of splittingof the degenerate impurity states owing to the symmetry
breaking at the surface.
2.

Experimental methods

As the substrate, we prepared a ZnTe(110) clean surface bycleaving a single crystal p-type ZnTe(100) wafer (P-doped,∼1 × 1018 cm−3) in a high vacuum (∼10−5 Pa). We deposited
Cr atoms on the surface at room temperature by the electron
beam deposition method. The area covered by Cr atoms wasvery small in order to observe the electronic states of single
Cr atoms. All STM and STS measurements were performed
in an ultrahigh vacuum (∼1 × 10−9 Pa) at a liquid nitrogentemperature (∼80 K) using an electrochemically sharpened W
tip (ϕ = 0.3 mm). STM images were obtained in the constantcurrent mode only at negative sample bias voltages, since theSTM observation of the ZnTe(110) surface with a positive
sample bias voltage is unstable, as reported by Wierts et al.20)
However, we could investigate the electronic states of theunoccupied states by STS using a setpoint at negative
sample bias voltages. Diﬀerential conductance (dI=dV)–V
curves were measured using a lock-in ampliﬁer (3 kHz,ΔVp–p = 20 mV) with open feedback loop conditions.
3.

Results and discussion

Figure 1(a) shows a typical STM image of the ZnTe(110)
surface obtained by the cleaving process. As shown in thisimage, the surface was quite ﬂat and the numbers of defects
and absorbates were very small. Since occupied lone pairstates are formed on the topmost Te atoms owing to chargetransfer from topmost Zn atoms, the Te atoms can be imaged
as bright points at a negative sample bias voltage. In addition

08LB01-1© 2015 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 54, 08LB01 (2015)(a)K. Kanazawa et al.

(a)

[110]
C

[001]
B

C

A

Zn
Te

Fig. 1. (Color online) (a) STM image of ZnTe(110) surface (Vs = −3.0 V,
It = 20 pA) before Cr adsorption. A, B, and C indicate three kinds of point
defects found on the surface. The inset of (a) shows a magniﬁed STM imageof the point defect C. (b) High-resolution image of ZnTe(110) surface(Vs = −2.0 V, It = 20 pA). The inset of (b) shows a schematic structure of theZnTe(110) surface.

to the bright lines of Te atoms along the 〈110〉 direction,
several dark lines along the same direction were also
observed. According to the previous studies,20,21) these darklines are line defects caused by the absence of topmost atoms
and are intrinsic defects of the ZnTe(110) surface. From thediﬀerent brightnesses on the two sides of the dark-line
defects, which are intrinsic defects of the ZnTe(110) surface,
we can determine the indexes of the unit vectors on the directions), as shown in Fig. 1(a).21)
surface ([001] and ½110
We could also determine the correspondence of the STM
image to the atomic arrangement on the surface, as shown in
Fig. 1(b). Even in the clean surface, several point defects
were observed, as indicated by A, B, and C. Since the defectlabeled A is a depression in the single-atom scale, we
identiﬁed it as a single-atom deﬁciency of surface atoms. The
defect labeled B is imaged as a single-atom protrusion, with aheight of about 1 Å. From this result, it is likely to be anadsorbed atom on the surface. These two kinds of defects
should be generated by the cleavage. On the other hand, thedefect labeled C, which is observed as a protrusion withspatial broadening larger than 3 nm diameter in the surface,corresponds to a phosphorus atom doped as an acceptor,
since the appearance is very similar to that of an acceptor
nitrogen atom in ZnTe, as reported in an earlier STM study
by Wierts et al.20)
After the adsorption of Cr atoms, a large number of small
protrusions with bright contrast corresponding to Cr atoms
were observed [Fig. 2(a)]. A typical STM image of adsorbed

100
80
60
40
20
0

Height (pm)

(b)

(b)

0

1
2
3
Position (nm)Fig. 2. (Color online) (a) STM image of Cr-adsorbed ZnTe(110) surface(Vs = −2.5 V, It = 20 pA). (b) STM image of single Cr atoms substituting
into Zn sites in the ﬁrst (110) atomic layer (Vs = −2.5 V, It = 20 pA). The
inset shows the line proﬁle measured along the red line in the STM image.

Cr atoms at Vs = −2.5 V is shown in Fig. 2(b). Most of themwere imaged as bright points on the topmost Te atomic row
with similar size and brightness to those of the Cr atoms
observed in our previous cross-sectional STM study for
MBE-grown (Zn,Cr)Te.19) After comparison with the resultsof the previous study, we concluded that the bright points,
whose STM topographic heights are ∼0.35 Å larger than thatof topmost Te atoms [inset of Fig. 2(b)], correspond to Cr
atoms substituting for Zn atoms in the ﬁrst layer of thesurface. According to the theoretical study performed in ourprevious study,19) when a Cr atom substitutes a Zn atom inthe topmost (110) surface layer, the region around the
topmost two Te atoms neighboring with the Cr is bright in the
STM image at a negative sample bias voltage, reﬂecting theimpurity state induced by the Cr atom. Such substitutions ofadsorbed TM atoms for cation sites in semiconductor
surfaces without any STM manipulation have been observedin several studies.13,15)
Next, in order to investigate the impurity states in moredetail, we carried out STS measurements above a substitutional Cr atom at the points shown in Figs. 3(a) and 3(b).
Since the ZnTe(110) surface is unstable for STM observation
at a positive sample bias voltage, as described above, we
realized STS measurement of the full spectrum by using a set
point at a negative sample bias voltage. Figure 3(c) shows the
dI=dV–V spectra, which correspond to the energy dependenceof the local density of states (LDOS), obtained at thepositions shown in (a) and over the surface without Cr atoms.
The colors of the spectra correspond to those of the closed
circles marking the measurement points in (a). Figure 3(d)08LB01-2© 2015 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 54, 08LB01 (2015)(a)K. Kanazawa et al.

(b)

(d)

Zn

Zn
Te

Cr

(c)

dI/dV(arb. unit)

No Cr

Cr

Te

Topographic-0.97 V

-0.05 V

+1.47 V

+1.88 V0
-2.0 -1.00+2.67 VdI/dV (arb.unit)

large

small

1.0 2.0

Sample Bias (V)Fig. 3. (Color online) (a) STM image of the Cr atom on which STS measurements were carried out (Vs = −2.5 V, It = 20 pA). (b) Schematic of the atomic
structure shown in (a), with the positions where STS measurements were carried out. (c) dI=dV–V spectra obtained at the positions shown in (a). The colors of
the spectra correspond to those of the closed circles marking the measurement points in (a). (d) dI=dV maps over the two substitutional Cr atoms to show the and [001] directions in the topmost
spatial distribution of the energy states shown in (c). White lines correspond to the atomic rows of Te along the ½110
surface. These lines are developed from the topographic STM image of this region (top left). Te atoms in the topmost surface should be located at theintersections of the two lines. The positions of Cr atoms are indicated using a superposed atomic structural model and indicated by colored circles surrounded
by a white line in the images.

shows the dI=dV maps obtained over the two substitutional
Cr atoms to show the spatial distribution of the energy states
shown in (c). Bias voltages slightly diﬀerent from those at thepeak positions shown in (c) were used to enhance the imageof each energy state by reducing the overlap of their spectra.
The positions of atomic rows of topmost Te atoms along the
½110
and [001] directions are shown by white lines inFig. 3(d). As an example of a map at a sample bias voltage
without any speciﬁc electronic states in the dI=dV–V spectra,
dI=dV values in Fig. 3(d#2), which was obtained at Vs =−0.05 V, were almost zero uniformly all over the observedarea.
The spectrum obtained over the area without Cr is almost
zero in the bias voltage range −0.7 < Vs < 2.2 V, reﬂecting
the electronic states of a p-type semiconductor with the eﬀectof tip-induced band bending (TIBB), which becomes larger at
a negative sample bias for a p-type semiconductor. The dI=dV
spectra measured around the Cr atoms clearly exhibited alarger dI=dV than that obtained over the area of ZnTe without
Cr in the small bias region (−1.0 < Vs < +2.5 V). This resultshows the existence of Cr impurity states within the bandgapof the ZnTe.
Here, in order to clarify the origin of the peaks, we
compare the STS result with the results of the previous
theoretical study reported by Katayama-Yoshida et al.17)
They calculated the DOS of the (Zn,Cr)Te crystal, in which
Cr atoms substitute into the tetrahedral Zn site. The
theoretical study has revealed the existence of spin-polarized
impurity states formed by the hybridization between the 3delectrons of substitutional Cr atoms and the 5p electrons of
Te atoms, which mainly contribute to the electronic states ofthe valence band of the host ZnTe, namely, the bonding state(t b), antibonding state (t a), and nonbonding state (e). We

present a simple schematic overview of the energy diagramof the electronic structure of Cr 2+ at the tetrahedral
substitutonal site, as shown in Fig. 4(a), derived from the
DOS calculated in the theoretical study.
Firstly, the dI=dV indicated by the red line in Fig. 3(c) was
measured on the atomic row of the topmost Te atoms and waslarger than that obtained over the area of ZnTe without Cr in
the small negative bias voltage range −0.65 < Vs < −1.5 V,labeled #1 in Fig. 3(c). The LDOS [Fig. 3(d#1)] is large at
the topmost atomic row of Te atoms near the Cr atoms. Fromthe comparison of these results with the energy diagram andthe results of our previous study,19) it is concluded that thelarger dI=dV labeled #1 in Fig. 3(c) reﬂects the t a+ (+ means
up or majority spin) state formed by the hybridization of the
Cr dε and valence electrons of neighboring Te atoms.
On the other hand, the spectra obtained at a positive bias in
the atomic scale are reported for the ﬁrst time in this study.
The peak energies of the dI=dV spectra were diﬀerent fromeach other and depended on the position of the measurement.
From the comparison of our STS result with those of thetheoretical study, we concluded that, in Fig. 3(c), the peaks
at Vs = +1.67 (in brown and purple curves, indicated bya dotted line) and +1.77 V (in a green curve, indicated by abroken line) can be identiﬁed as the energy states derived
from the e− (− means down or minority spin) statesoriginating from Cr dγ states, which very weakly hybridize
with the host valence states. Moreover, the large LDOS areasof these states are not located at the row of topmost Te atoms
but near that of Cr atoms, as shown by #3 and #4 in Fig. 3(d).
This result is also consistent with the conclusion above,
because the e states should be located near the substitutional
Cr atom owing to the weak hybridization with the host. For a
Cr atom in the tetrahedral substitutional site, as in the atomic

08LB01-3© 2015 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 54, 08LB01 (2015)

(a)

Cr 2+
3d electrons

K. Kanazawa et al.

(b) Topmost surface

Bulk
Tetrahedral

Trigonal pyramid

Conduction
Band

ta-

te-

e-

ΔE ~0.1 eV

t a+

t+
e+

e+
t bt b+
Valence
Band

Fig. 4. (Color online) (a) Schematic diagram of energy levels of impurity
states induced by Cr 2+ derived from the results of the previous theoretical
study.17) (b) Schematic explanation for the observed energy splitting of e−states. As a possible mechanism, energy splitting of e− states might be
induced by the symmetry breaking at the surface.

single Cr atoms substituting into Zn sites located in the ﬁrst
layer. Our STS measurement revealed the impurity states
located around the energy gap region of the host ZnTe. We
observed impurity states, which have characteristic energy
levels and LDOS spatial distributions, corresponding to theimpurity states predicted in an earlier theoretical study.17)
Furthermore, our STS measurement revealed that thedegeneracy of the e− states, which are doubly degenerated
in the bulk tetrahedral substitutional site, was clearly resolved
by symmetry breaking at the surface.
We believe that the result of this study provides
fundamental knowledge for realizing the nanomagnetism
based on the DMS of the semiconductor surface in an atomic
scale. In the future, with the reduction in the size of electronic
devices, such impurity states of a single doped atom will
become essential in controlling the performance of thedevice. Furthermore, we expect that the impurity states will
be utilized for the realization and application of single-atomelectronics.22,23)
Acknowledgment−

arrangement of the bulk zinc blende structure, the e states
are doubly degenerated. The results of our STS measurements indicate that the degeneracy was resolved by symmetry
breaking on the surface. For atoms in the topmost (110)
surface, the tetrahedral symmetry is broken into the trigonal
symmetry owing to the absence of one neighboring atom
relative to the bulk conﬁguration. In the trigonal atomic
conﬁguration, the two e− states are not equivalent and split
into two distinct states. Although it is diﬃcult to identify theorigin of the two states at present, these diﬀerent spatial
distributions should give us essential information for solving
the issue. Here, it is also diﬃcult to explain why we could
not observe a split of the t state; this is also expected to beresolved by the breaking of the tetrahedral symmetry on thesurface. In order to clarify these issues, further study must be
carried out from both experimental and theoretical aspects
with the precise description of the surface state.
Finally, we mention the peak labeled #5, which appearedat the highest energy in the gap of p-ZnTe. Its LDOS spatial
distribution [Fig. 3(d#5)] spreads over 5–6 Å from theposition of the doped Cr atoms. From these results, weconsider that the state originates from the conduction band of
the host ZnTe with some energy shift toward a lower energy.
Since the Cr impurity states are formed in the energy gap
region of the host p-ZnTe, the acceptor around the Cr atoms
is compensated and the level of EF is shifted toward a deeper
energy region in the gap. As a result, the relative position of
the energy band of the host p-ZnTe around a Cr atom isshifted toward a lower energy. Since the eﬀect of thecompensation spreads over some spatial extent, we observe
the states within several angstroms around the Cr atom. The
eﬀect of the energy shift appears in the dI=dV–V spectra in
the negative sample bias region; the spectra measured aroundthe Cr atom [green, brown, and purple curves in Fig. 3(c)]rise at larger negative Vs values than that over the area
without Cr.
4.

Conclusions

In this study, we investigated impurity states of Cr atoms in
the p-ZnTe(110) surface by STM. We successfully observed

This work was supported by JSPS KAKENHI Grant
Numbers 22226003, 24310086, and 25870103.

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08LB01-4© 2015 The Japan Society of Applied Physics