Effects of Zn Doping on the Surface Structure and Initial Growth Processes of InP Thin Film Layers on InP(111)B Substrate

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The effects of Zn atoms as a dopant on the surface structure and initial growth processes on InP(111)B substrate are theoretically investigated on the basis of surface phase diagrams which are obtained by comparing the adsorption energy given by ab initio calculations with the gas-phase chemical potentials. The calculated surface phase diagrams demonstrate that the Zn-incorporated surface is stabilized under the growth conditions with high Zn supply ratio. Furthermore, In atoms tend to easily adsorb on the Zn-incorporated surface as an In–P dimer due to the formation of Zn–P and P–P bonds, implying that the adsorption of In atoms and resultant growth processes are promoted on the Zn-incorporated InP(111)B surface which is stabilized under Zn-rich conditions. These results suggest that effects of Zn doping on the surface structure and initial growth processes are crucial for understanding the bidirectional growth mechanisms of InP nanowires on InP(111)B substrate. [DOI: 10.1380/ejssnt.2015.147]

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Effects of Zn Doping on the Surface Structure and Initial Growth Processes of InP

Thin Film Layers on InP(111)B Substrate _∗_

Masataka Kato, _†_ Toru Akiyama, Kohji Nakamura, and Tomonori Ito

_Depertment_ _of_ _Physics_ _Engineering,_ _Mie_ _University,_ _Tsu_ _514-8507,_ _Japan_
(Received 10 November 2014; Accepted 3 March 2015; Published 11 April 2015)

The effects of Zn atoms as a dopant on the surface structure and initial growth processes on InP(111)B substrate
are theoretically investigated on the basis of surface phase diagrams which are obtained by comparing the adsorption
energy given by _ab initio_ calculations with the gas-phase chemical potentials. The calculated surface phase diagrams
demonstrate that the Zn-incorporated surface is stabilized under the growth conditions with high Zn supply ratio.
Furthermore, In atoms tend to easily adsorb on the Zn-incorporated surface as an In–P dimer due to the formation
of Zn–P and P–P bonds, implying that the adsorption of In atoms and resultant growth processes are promoted
on the Zn-incorporated InP(111)B surface which is stabilized under Zn-rich conditions. These results suggest
that effects of Zn doping on the surface structure and initial growth processes are crucial for understanding the
bidirectional growth mechanisms of InP nanowires on InP(111)B substrate. [DOI: 10.1380/ejssnt.2015.147]

Keywords: Computer simulations; Indium phosphide; Zinc; Surface structure

I. INTRODUCTION

Semiconductor nanowires (NWs) have attracted much
attention because of their unique electrical and optical
properties applicable for various nanoscale devices [1-3].
In particular, InP-based NWs are promising for highspeed electrons, optoelectronic, and photovoltaic devices
because of their superior material property and possibility to develop various kinds of heterostructures. One of
important issues for fabricating such applications is to
control nanowire shapes and crystal structure. Indeed,
many types of NWs such as hexagonal pillar, tapered pillar, and irregular wire structures have been synthesized
depending on the growth conditions. There have been intensive experimental studies to control nanowire shapes
and structures in InP NWs, where growth temperature
and V/III supply ratio are crucial factors [4-6].

Recently, various types of morphology of InP NWs have
been observed in the selective-area metalorganic vapor
phase epitaxy (SA-MOVPE) on the InP(111)B substrate

[7, 8]. InP NWs tend to form a tripod structure that consists of three NWs pointing toward three-fold symmetric

[111]A direction. However, the structure of InP NWs is
dramatically changed by introducing Zn atoms which are
used as _p_ -type dopant. Under high Zn supply (high II/III
ratio) conditions, hexagonal pillar structures with vertical
sidewall grown along the [111]B direction have been fabricated while tripod structures are obtained under low Zn
supply (low II/III ratio) conditions. These results indicate
that the growth directions of InP NWs during the SAMOVPE on InP(111)B substrate switch from the [111]A
direction to the [111]B direction by changing Zn supply
ratio, exhibiting bidirectional behavior. Despite these experimental findings, little is known about the mechanisms
of this bidirectional behavior.

In order to obtain qualitative insights for the bidirectional growth of InP NWs on InP(111)B substrate, we
here investigate the growth of InP thin film layers on

_∗_ This paper was presented at the 7th International Symposium

on Surface Science, Shimane Prefectural Convention Center (Kunibiki Messe), Matsue, Japan, November 2-6, 2014.

_†_ Corresponding author: 413m608@m.mie-u.ac.jp

InP(111)B substrate. We assume that the topmost layer
of nanowire growth is similar to that of InP(111)B surface. Effects of Zn atoms on the initial surface structure
and growth processes are discussed on the basis of _ab_ _ini-_
_tio_ calculations.

II. COMPUTATIONAL METHODS

The total-energy calculations are performed within the
generalized gradient approximation [9] in the densityfunctional theory. We use norm-conserving pseudopotentials [10] with partial core correction [11]. The
conjugate-gradient minimization technique is used for
both electronic-structure calculation and geometry optimization [12,13]. In the optimized geometries the remaining forces acting on the atoms are less than 5 _×_ 10 _−_ 3 Ry _/_ ˚ A.
The valence wave functions are expanded by the planewave basis set with a cut-off energy of 20.25 Ry, which
gives enough convergence of total energy to discuss the
relative stability. We take (2 _×_ 2) slab models consisting
of seven atomic layers with psudohydrogen atoms [14] and
a 10 ˚ A vacuum region [15]. The atoms belonging to bottom bilayer are fixed at their ideal positions. 10- _k_ points
sampling in the surface Brillouin zone is used throughout
this study.

Surface phase diagrams are obtained by considering the
adsorption-desorption behavior, which can be described
by comparing the free energy of ideal gas per particle
(chemical potential) _µ_ gas to the adsorption energy per
atom _E_ ad. The chemical potential of a molecule is expressed as [16, 17]

where _k_ B is Boltzmann’s constant, _T_ is the gas temperature, _g_ is the degree of degeneracy of the electron energy level, and _p_ is the pressure. _ξ_ trans, _ξ_ rot and _ξ_ vib are
the partition functions for translational, rotational and vibrational motions, respectively. Details of these partition
functions are expressed elsewhere [16, 17]. In the chemical potential calculations, we assume the form of In and
Zn (P) in gas phase as In and Zn atoms (P2 molecule), respectively. The relative stability between two structures

)

_µ_ gas = _−k_ B _T_ ln

( _k_ B _T_

_× g × ξ_ trans _× ξ_ rot _× ξ_ vib
_p_

_,_ (1)

ISSN 1348 0391 _⃝_ c 2015 The Surface Science Society of Japan (http://www sssj org/ejssnt) 147

FIG. 1. Calculated surface phase diagram of InP(111)B surface as functions of temperature and P2 pressure. Solid lines
show boundaries between stable surface structures. The experimental growth condition in Ref. 8 is also shown by cross
mark. In and P atoms are represented by gray and yellow
circles, respectively.

is determined by comparing _µ_ gas to _E_ ad, _i.e._, the structure corresponding to the adsorbed surface is favorable
when _E_ ad is less than _µ_ gas, whereas the desorbed surface
is stabilized when _µ_ gas is less than _E_ ad.

As a typical example of surface phase diagram on
InP(111)B surface, Figure 1 shows the calculated phase
diagram of the InP(111)B surface as functions of temperature and P2 pressure. This figure reveals that there are
several reconstructions depending on temperature and P2
pressure. The surface with P-trimer (P-trimer, hereafter)
is stabilized over the wide range of temperatures and P2
pressure. This trend is consistent with previous calculations in which the P-trimer is favorable over the wide
range of phosphorus chemical potential [18]. The stability of this surface structure can be explained in terms of
the electron counting rule [19, 20]. Considering the experimental condition for the growth of InP NWs in the
SA-MOVPE (cross mark in Fig. 1), the P-trimer emerges
under the growth conditions of the SA-MOVPE [7,8].

III. RESULTS AND DISCUSSION

Figure 2 shows the calculated surface phase diagrams
of Zn-incorporated surfaces as functions of temperature
and P2 pressure under low and high Zn pressure conditions (Figs. 2(a) and 2(b), respectively). As shown in
Fig. 2(a), under low Zn pressure conditions the surface
with Zn–P pair (Zn–P pair, hereafter) is stabilized below
850 _∼_ 910 K, whereas the P-trimer is favorable beyond
850 _∼_ 910 K. It is thus likely that the P-trimer is stable in
the SA-MOVPE with low II/III ratio [8]. In contrast, the
stable temperature and pressure area for the stabilization
of Zn–P pair under high Zn pressures is larger than that

FIG. 2. Calculated surface phase diagrams of Zn-incorporated
InP(111)B surfaces as functions of temperature and P2 pressure under (a) low (6 _._ 0 _×_ 10 _−_ 4 Torr) and (b) high (4 _._ 6 _×_
10 _−_ 3 Torr) Zn pressures. Note that low and high Zn pressures are obtained by the experimental partial pressure of Zn
in Ref. 8. The experimental growth condition in Ref. 8 is
also shown by cross mark. In, P, and Zn atoms are represented by gray, yellow, and brown circles, respectively. The
Zn-incorporated surfaces are stabilized in the regions emphasized by shaded area.

under low Zn pressures. As shown in Fig. 2(b), the Zn–P
pair is stabilized beyond 880 _∼_ 970 K. Therefore, the
Zn–P pair is expected to be favorable in the SA-MOVPE
with high II/IIII ratio [8]. The stabilization of Zn–P pair
can be interpreted in terms of the number of P dangling
bonds _N_ DB on the surface. The number of P dangling
bonds in Zn–P ( _N_ DB = 2) is smaller than that of P trimer
( _N_ DB = 4), so that the number surface states originating
from P dangling bonds, which are located above the valence band maximum [18], becomes small in the Zn–P
pair. Therefore, the electronic contribution is decisive for
the relative stability between Zn–P pair and P-trimer.

148 http://www sssj org/ejssnt (J Stage: http://www jstage jst go jp/browse/ejssnt/)

FIG. 3. Calculated surface phase diagrams for the adsorption
of In–P dimer on InP(111)B surface as functions of temperature and P2 pressure under (a) low (6 _._ 0 _×_ 10 _−_ 4 Torr) and (b)
high (4 _._ 6 _×_ 10 _−_ 3 Torr) Zn pressures. Shaded area denotes stable temperature and pressure range for the surface with In–P
dimer. The experimental growth condition in Ref. 8 is also
shown by cross mark. In, P, and Zn atoms are represented by
gray, yellow, and brown circles, respectively. Blue and orange
circles denote In and P atoms of In–P dimer, respectively.

On the basis of the calculated surface structures, we furthermore investigate the initial growth processes on the
InP(111)B surfaces. This is accomplished by considering the adsorption behavior of In and P atoms. Table
I shows the calculated adsorption energies of In and P
atoms, along with In–P dimer on the P-timer and Zn–P
pair, which are favorable under low and high Zn pressures, respectively. It is found that the adsorption energies of In and P atoms on the Zn–P pair ( _−_ 2 _._ 48 and _−_ 1 _._ 30
eV, respectively) are lower than those on the P-trimer by
0.42 and 1.74 eV, respectively. However, these values are
too large to adsorb on the surface under the growth condition around 930 K. In the case of adsorption as In–P
dimer, the adsorption energy on the Zn–P pair is much

TABLE I. Calculated adsorption energies _E_ ad (in eV) of In
and P atoms, and of In–P dimer on the InP(111)B surfaces
with P trimer and Zn–P pair.

Surface In P In–P
P-trimer _−_ 2 _._ 06 0.44 _−_ 2 _._ 29
Zn–P pair _−_ 2 _._ 48 _−_ 1 _._ 30 _−_ 4 _._ 37

lower than that on the P-trimer, and this In–P dimer can
adsorb on the surface even under the growth condition.
Figure 3 shows the calculated surface phase diagrams for
the adsorption of In–P dimer on the InP(111)B surface as
functions of temperature and P2 pressure under low and
high Zn pressure conditions (Figs. 3(a) and 3(b), respectively). As shown in Fig. 3(a), the stable region for In–P
dimer is ranging below 500 _−_ 580K. This indicates that
In–P dimer on the P-trimer desorbs from the surface in
the SA-MOVPE with low II/III ratio [8]. On the other
hand, as shown in Fig. 3(b), In–P on the Zn–P pair is
stabilized below 910 _∼_ 960 K, implying that In–P dimers
can adsorb on the surface in the SA-MOVPE with high
II/III ratio [8]. The low adsorption energy of In–P dimer
on the Zn–P pair originates from the formation of covalent bonds. On the Zn–P pair the formation of Zn–P and
P–P bonds occurs in addition to the formation of three
In–P bonds, which are also generated in the adsorption of
In–P dimer on the P-trimer.

The calculated results on the surface structures and
adsorption-desorption behavior on the InP(111)B surfaces
mentioned above offer a possible explanation for the bidirectional growth of InP NWs [8]. Under low II/III ratio
conditions in the SA-MOVPE, the P-trimer is the most
stable surface structure and In atoms hardly adsorb on
the surface. In this case, the growth along the [111]A
direction, in which the adsorption of In and P atoms easily occurs on the InP(111)A surface [21], preferentially
proceeds by forming tripods tilted to the InP(111)B substrate. In contrast, under high II/III ratio, the Zn–P pair
is the most stable and In atoms adsorb on the surface
as In–P dimers. The formation of Zn–P pair enables the
vertical growth of InP layers on the InP(111)B surface.
Although details of growth processes on these surfaces
should be carefully examined, our scenario obtained by
the calculated results is consistent with the experimental
results.

IV. CONCLUSIONS

We have theoretically investigated the effects of Zn
atoms on the surface structure and initial growth processes on InP(111)B substrate by performing firstprinciples total-energy calculations. We have revealed
that the surface reconstruction strongly depends on the
growth conditions such as Zn pressure. The surface with
Zn–P pair is found to be stabilized under high Zn pressure.
The formation of Zn-incorporated surface leads to the adsorption of In–P dimer due to the formation of Zn–P and
P–P bonds, suggesting that InP layers on the InP(111)B
substrate under high Zn pressures can be formed more

http://www sssj org/ejssnt (J Stage: http://www jstage jst go jp/browse/ejssnt/) 149

easily than the surface under low Zn pressures. We have
proposed that role of Zn doping on the surface structure
and initial growth processes are crucial for understanding
the bidirectional growth mechanisms of InP nanowires on
InP(111)B substrate.

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ACKNOWLEDGMENTS

This work was supported in part by a Grant-in-Aid for
Scientific Research (No. 24560025) from the Japan Society for the Promotion of Science. Computations were
performed at RCCS (National Institutes of Natural Sciences).

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[15] In this study, we use the (2 _×_ 2) surface to determine lo

cal atomic structures of the surfaces with Zn atom. The
effe _~~√~~_ ct of u _~~√~~_ nit cell size have been checked by using the
(2 3 _×_ 2 3) supercell, and found that the adsorption

ct of u _~~√~~_

3 _×_ 2

_~~√~~_

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