Scanning Tunneling Microscopy/Spectroscopy and Low-Energy Electron Diffraction Investigations of GaTe Layered Crystal Cleavage Surface
Scanning tunnelling microscopy and spectroscopy (STM/STS) and low-energy electron diffraction (LEED) techniques are used in combination to study the surface structure of GaTe cleavages. Two different structures, hexagonal one on macroscale and monoclinic one randomly distributed on nanoscale, are id...
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Інститут металофізики ім. Г.В. Курдюмова НАН України
2015
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irk-123456789-1122542017-01-19T03:03:19Z Scanning Tunneling Microscopy/Spectroscopy and Low-Energy Electron Diffraction Investigations of GaTe Layered Crystal Cleavage Surface Galiy, P. Nenchuk, T. Ciszewski, A. Mazur, P. Zuber, S. Yarovets, I. Дефекты кристаллической решётки Scanning tunnelling microscopy and spectroscopy (STM/STS) and low-energy electron diffraction (LEED) techniques are used in combination to study the surface structure of GaTe cleavages. Two different structures, hexagonal one on macroscale and monoclinic one randomly distributed on nanoscale, are identified on the crystal cleavage surface. The hexagonal unit cell parameters, a=b≡4.08Å, c≡16Å, determined by STM are in a good agreement with the bulk ones and, besides, with planar parameters a, b obtained using LEED. The monoclinic unit cell parameters, a≡24Å, b≡4Å, c≡10Å, are consistent with ones of the known monoclinic modifications. LEED and STS data indicate that the GaTe surface is not flat, but is characterized by a well-developed staircase structure formed by cleavage. As concluded, the possibility of partial on nanoscale reconstruction of base hexagonal structure to the monoclinic one is directly related to the number of surface defects such as loosely arranged steps of one single Te—Ga—Ga—Te packet height. За допомогою комплексу метод сканівної тунельної мікроскопії та спектроскопії (СТМ/СТС) і дифракції повільних електронів (ДПЕ) досліджено структуру поверхонь сколювання кристалів GaTe. Встановлено існування двох різних структур на поверхні сколювання кристалу: гексагональної в макромасштабі і моноклінної, яка є випадковим чином розподіленою на поверхні в наномасштабі. Параметри гексагональної ґратниці a=b≡4,08Å, c≡16Å, яких одержано за допомогою СТМ, добре узгоджуються з даними для об’єму кристалу, а також із параметрами a, b для поверхні кристалу, одержаними з використанням ДПЕ. Параметри моноклінної ґратниці a≡24Å, b≡4Å, c≡10Å є такими ж, як і для однієї з відомих моноклінних структурних модифікацій. Відповідно до результатів, одержаних за допомогою ДПЕ і СТС, встановлено, що поверхня GaTe не є пласкою і характеризується наявністю добре розвиненої східчастої структури, яка утворюється внаслідок сколювання кристалу. Зроблено висновок про те, що можливість локальної в наномасштабі реконструкції базової гексагональної структури в моноклінну пов’язана з кількістю поверхневих дефектів, таких, як довільно розміщені сходинки з висотою, що дорівнює товщині одного пакета Te—Ga—Ga—Te. С помощью комплекса методов сканирующей туннельной микроскопии и спектроскопии (СТМ/СТС), а также дифракции медленных электронов (ДМЭ) исследована структура поверхностей скалывания кристаллов GaTe. Установлено существование двух различных структур на поверхности скалывания кристалла: гексагональной в макромасштабе и моноклинной, которая случайным образом распределена по поверхности в наномасштабе. Параметры гексагональной решётки a=b≡4,08Å, c≡16Å, полученные с помощью СТМ, хорошо согласуются с данными для объёма кристалла, а также с параметрами a, b для поверхности кристалла, полученными с использованием ДМЭ. Параметры моноклинной решётки a≡24Å, b≡4Å, c≡10Å – такие же, как и в одной из известных моноклинных структурных модификаций. Согласно результатам, полученным с помощью ДМЭ и СТС, установлено, что поверхность GaTe не является плоской и характеризуется наличием хорошо развитой ступенчатой структуры, которая образуется в результате скалывания кристалла. Сделано заключение о том, что возможность локальной в наномасштабе реконструкции базовой гексагональной структуры в моноклинную связана с количеством поверхностных дефектов, таких, как случайно расположенные ступеньки с высотой, которая равняется толщине одного пакета Te—Ga—Ga—Te. 2015 Article Scanning Tunneling Microscopy/Spectroscopy and Low-Energy Electron Diffraction Investigations of GaTe Layered Crystal Cleavage Surface / P. Galiy, T. Nenchuk, A. Ciszewski, P. Mazur, S. Zuber, I. Yarovets // Металлофизика и новейшие технологии. — 2015. — Т. 37, № 6. — С. 789-801. — Бібліогр.: 9 назв. — англ. 1024-1809 PACS: 07.79.Cz, 61.05.jh, 68.35.bg, 68.37.Ef, 68.47.Fg, 71.20.Nr, 73.20.At http://dspace.nbuv.gov.ua/handle/123456789/112254 en Металлофизика и новейшие технологии Інститут металофізики ім. Г.В. Курдюмова НАН України |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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| language |
English |
| topic |
Дефекты кристаллической решётки Дефекты кристаллической решётки |
| spellingShingle |
Дефекты кристаллической решётки Дефекты кристаллической решётки Galiy, P. Nenchuk, T. Ciszewski, A. Mazur, P. Zuber, S. Yarovets, I. Scanning Tunneling Microscopy/Spectroscopy and Low-Energy Electron Diffraction Investigations of GaTe Layered Crystal Cleavage Surface Металлофизика и новейшие технологии |
| description |
Scanning tunnelling microscopy and spectroscopy (STM/STS) and low-energy electron diffraction (LEED) techniques are used in combination to study the surface structure of GaTe cleavages. Two different structures, hexagonal one on macroscale and monoclinic one randomly distributed on nanoscale, are identified on the crystal cleavage surface. The hexagonal unit cell parameters, a=b≡4.08Å, c≡16Å, determined by STM are in a good agreement with the bulk ones and, besides, with planar parameters a, b obtained using LEED. The monoclinic unit cell parameters, a≡24Å, b≡4Å, c≡10Å, are consistent with ones of the known monoclinic modifications. LEED and STS data indicate that the GaTe surface is not flat, but is characterized by a well-developed staircase structure formed by cleavage. As concluded, the possibility of partial on nanoscale reconstruction of base hexagonal structure to the monoclinic one is directly related to the number of surface defects such as loosely arranged steps of one single Te—Ga—Ga—Te packet height. |
| format |
Article |
| author |
Galiy, P. Nenchuk, T. Ciszewski, A. Mazur, P. Zuber, S. Yarovets, I. |
| author_facet |
Galiy, P. Nenchuk, T. Ciszewski, A. Mazur, P. Zuber, S. Yarovets, I. |
| author_sort |
Galiy, P. |
| title |
Scanning Tunneling Microscopy/Spectroscopy and Low-Energy Electron Diffraction Investigations of GaTe Layered Crystal Cleavage Surface |
| title_short |
Scanning Tunneling Microscopy/Spectroscopy and Low-Energy Electron Diffraction Investigations of GaTe Layered Crystal Cleavage Surface |
| title_full |
Scanning Tunneling Microscopy/Spectroscopy and Low-Energy Electron Diffraction Investigations of GaTe Layered Crystal Cleavage Surface |
| title_fullStr |
Scanning Tunneling Microscopy/Spectroscopy and Low-Energy Electron Diffraction Investigations of GaTe Layered Crystal Cleavage Surface |
| title_full_unstemmed |
Scanning Tunneling Microscopy/Spectroscopy and Low-Energy Electron Diffraction Investigations of GaTe Layered Crystal Cleavage Surface |
| title_sort |
scanning tunneling microscopy/spectroscopy and low-energy electron diffraction investigations of gate layered crystal cleavage surface |
| publisher |
Інститут металофізики ім. Г.В. Курдюмова НАН України |
| publishDate |
2015 |
| topic_facet |
Дефекты кристаллической решётки |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/112254 |
| citation_txt |
Scanning Tunneling Microscopy/Spectroscopy and Low-Energy Electron Diffraction Investigations of GaTe Layered Crystal Cleavage Surface / P. Galiy, T. Nenchuk, A. Ciszewski, P. Mazur, S. Zuber, I. Yarovets // Металлофизика и новейшие технологии. — 2015. — Т. 37, № 6. — С. 789-801. — Бібліогр.: 9 назв. — англ. |
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Металлофизика и новейшие технологии |
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789
ДЕФЕКТЫ КРИСТАЛЛИЧЕСКОЙ РЕШЁТКИ
PACS numbers:07.79.Cz, 61.05.jh,68.35.bg,68.37.Ef,68.47.Fg,71.20.Nr, 73.20.At
Scanning Tunnelling Microscopy/Spectroscopy and Low-Energy
Electron Diffraction Investigations of GaTe Layered Crystal
Cleavage Surface
P. Galiy, T. Nenchuk, A. Ciszewski*, P. Mazur*, S. Zuber*, and I. Yarovets’
Ivan Franko Lviv National University, Electronics Department,
50 Dragomanov Street,
79005 Lviv, Ukraine
*University of Wrocław, Institute of Experimental Physics,
Maxa Borna Plac, 9,
50—204 Wrocław, Poland
Scanning tunnelling microscopy and spectroscopy (STM/STS) and low-energy
electron diffraction (LEED) techniques are used in combination to study the
surface structure of GaTe cleavages. Two different structures, hexagonal one
on macroscale and monoclinic one randomly distributed on nanoscale, are
identified on the crystal cleavage surface. The hexagonal unit cell parameters,
a b 4.08 Å, c 16 Å, determined by STM are in a good agreement with the
bulk ones and, besides, with planar parameters a, b obtained using LEED. The
monoclinic unit cell parameters, a 24 Å, b 4 Å, c 10 Å, are consistent with
ones of the known monoclinic modifications. LEED and STS data indicate that
the GaTe surface is not flat, but is characterized by a well-developed staircase
structure formed by cleavage. As concluded, the possibility of partial on-
nanoscale reconstruction of base hexagonal structure to the monoclinic one is
directly related to the number of surface defects such as loosely arranged
steps of one single Te—Ga—Ga—Te packet height.
За допомогою комплексу метод сканівної тунельної мікроскопії та спект-
роскопії (СТМ/СТС) і дифракції повільних електронів (ДПЕ) досліджено
структуру поверхонь сколювання кристалів GaTe. Встановлено існування
двох різних структур на поверхні сколювання кристалу: гексагональної в
макромасштабі і моноклінної, яка є випадковим чином розподіленою на
поверхні в наномасштабі. Параметри гексагональної ґратниці a b
4,08 Å, c 16 Å, яких одержано за допомогою СТМ, добре узгоджуються з
даними для об’єму кристалу, а також із параметрами a, b для поверхні
кристалу, одержаними з використанням ДПЕ. Параметри моноклінної
ґратниці a 24 Å, b 4 Å, c 10 Å є такими ж, як і для однієї з відомих мо-
ноклінних структурних модифікацій. Відповідно до результатів, одержа-
Металлофиз. новейшие технол. / Metallofiz. Noveishie Tekhnol.
2015, т. 37, № 6, сс. 789—801
Оттиски доступны непосредственно от издателя
Фотокопирование разрешено только
в соответствии с лицензией
2015 ИМФ (Институт металлофизики
им. Г. В. Курдюмова НАН Украины)
Напечатано в Украине.
790 P. GALIY, T. NENCHUK, A. CISZEWSKI et al.
них за допомогою ДПЕ і СТС, встановлено, що поверхня GaTe не є пласкою
і характеризується наявністю добре розвиненої східчастої структури, яка
утворюється внаслідок сколювання кристалу. Зроблено висновок про те,
що можливість локальної в наномасштабі реконструкції базової гексаго-
нальної структури в моноклінну пов’язана з кількістю поверхневих дефе-
ктів, таких, як довільно розміщені сходинки з висотою, що дорівнює тов-
щині одного пакета Te—Ga—Ga—Te.
С помощью комплекса методов сканирующей туннельной микроскопии и
спектроскопии (СТМ/СТС), а также дифракции медленных электронов
(ДМЭ) исследована структура поверхностей скалывания кристаллов GaTe.
Установлено существование двух различных структур на поверхности
скалывания кристалла: гексагональной в макромасштабе и моноклинной,
которая случайным образом распределена по поверхности в наномасшта-
бе. Параметры гексагональной решётки a b 4,08 Å, c 16 Å, полученные
с помощью СТМ, хорошо согласуются с данными для объёма кристалла, а
также с параметрами a, b для поверхности кристалла, полученными с ис-
пользованием ДМЭ. Параметры моноклинной решётки a 24 Å, b 4 Å,
c 10 Å – такие же, как и в одной из известных моноклинных структур-
ных модификаций. Согласно результатам, полученным с помощью ДМЭ и
СТС, установлено, что поверхность GaTe не является плоской и характери-
зуется наличием хорошо развитой ступенчатой структуры, которая обра-
зуется в результате скалывания кристалла. Сделано заключение о том, что
возможность локальной в наномасштабе реконструкции базовой гексаго-
нальной структуры в моноклинную связана с количеством поверхностных
дефектов, таких, как случайно расположенные ступеньки с высотой, ко-
торая равняется толщине одного пакета Te—Ga—Ga—Te.
Key words: gallium telluride, layered crystal, scanning tunnelling microsco-
py, scanning tunnelling spectroscopy, low-energy electron diffraction.
(Received March 17, 2015)
1. INTRODUCTION
Gallium telluride semiconductor crystal belongs to the layered III—VI
group chalcogenides and has been characterized for a long time [1—3].
It is known that these crystals exhibit anisotropic properties [4].
Moreover, the results of structural studies of bulk GaTe crystal indi-
cate the presence of hexagonal and monoclinic structural modifica-
tions. The latter one is characterized by a rather wide variety of values
of lattice constant. At least, one hexagonal phase of GaTe with unit cell
parameters of a b 4.06 Å, c 16.96 Å [1, 3] and several monoclinic
phases of a 23.64 Å, b 4.077 Å, c 10.46 Å, 134.47; a 17.45 Å,
b 10.47 Å, c 4.09 Å, 104.5; a 17.34 Å, b 4.06 Å, c 10.61 Å,
104.44 [3, 5] have previously been reported. The results of GaTe
thin-film studies are interesting for their model representations of the
STM/S AND LEED INVESTIGATIONS OF GaTe LAYERED CRYSTAL CLEAVAGE 791
structural hexagonal—monoclinic transformations [2].
It is well known that GaTe differs from other III—VI group chalco-
genides of similar chemical compositions, such as GaS, GaSe, InSe, by a
stronger planar anisotropy that characterizes a single layer of GaTe
[4]. In fact, apparently, it might result in the possibility of total or
partial reconstructions of the cleavage surfaces, which are known not
to be typical of layered crystals and, as it was reported, lead to a broad
number of structural phases with different lattice parameters.
The basic prerequisites for this GaTe surface study are the follow-
ing. Firstly, the conclusion contained in the article [3] about the possi-
bility of phase transformation in the bulk crystal from hexagonal to
monoclinic one upon grinding a single crystal to powder for XRD pow-
der diffraction analysis. Obviously, this might be due to an increase in
the surface-to-bulk ratio, and thus, a surface structure study is essen-
tial. However, it should be noted that paper [6] reports on a completely
reverse partial structural transformation on the cleavage surface of
GaTe crystal. Secondly, the known result of [2] that hexagonal-to-
monoclinic phase transformation occurs in the polycrystalline thin
films at elevated temperatures.
A variety of existing structural unit cell parameters is reported.
Nevertheless, there is lack of ‘direct’ observation of evident surface
phase transformations at the nanoscale by scanning probe methods
such as scanning tunnelling microscopy/spectroscopy (STM/STS). The
phase state of the GaTe surface is obviously important to characterize
the crystal, which is the aim of our study employing the STM/STS and
LEED methods.
2. EXPERIMENTAL
The STM/STS data were acquired using the Omicron NanoTechnology
STM/AFM System, at room temperature under ultrahigh vacuum
(UHV) conditions (410
9
Pa). The STM images were taken in the con-
stant-current mode. Considering the importance of quantitative data
obtained in the step height analyses the STM scanner vertical move-
ment was calibrated using commercial (NT-MDT) 6H-SiC test sample
with half of lattice constant (0.75 nm) in [0001] direction steps and in
situ prepared Si (100) substrate with single atomic steps (height of
0.14 nm). The STM/STS data processing was carried out using the
WSM v.4.0 of Nanotec Electronica [7].
The LEED data were analysed using the SPECS SAFIRE Diffraction
Image Acquisition and Processing System for LEED and RHEED.
LEED analysis was carried out using incident beam energies 10—
150 eV, and spot size less than 1 mm. Heating the sample in situ was
possible up to 500—600C.
GaTe single crystals were grown by the Bridgman method. The sam-
792 P. GALIY, T. NENCHUK, A. CISZEWSKI et al.
ples for the STM/STS and LEED studies had dimensions of 346 mm3
and a special shape suitable for multiple cleavage directly before intro-
ducing into the vacuum chamber. Previous bulk-structure investiga-
tions indicated the hexagonal structure of the studied GaTe crystals
with unit cell parameters a b 4.06 Å, c 16.96 Å.
3. RESULTS AND DISCUSSION
Recent GaTe crystal cleavage STM observations revealed only a rela-
tively poor-resolution set of sixfold chains composed of clusters with
an average size of 6—8 nm [6]. This size of the experimentally observed
structures is clearly not associated with the LEED data dimensions
given in the same paper. However, a combination of STM and LEED
methods is productive in terms of cleavage surface characteristics and
structural phase transformations inherent in GaTe crystals. General-
ly, integral LEED results confirm the crystallographic data obtained
by STM on the nanoscale. STM measurements are crucial, particularly,
to identify and assign the satellite spots and streaks to coexistent dif-
ferent superstructures, which might be superposed incoherently in the
diffraction pattern.
3.1. STM/STS Results
Figure 1, a shows a typical 2D STM image with a very weak periodic
pattern that could potentially be associated with the GaTe crystal lat-
tice order. We were able to obtain such lattice resolution usually for
large-scale images, 2121 nm2, as shown in Fig. 1, a, and a better one
for a negative bias. A lot of areas on the different cleavage surfaces
were investigated with a view to dropout a ‘tunnel version’. It is also
important to see that self-correlation filtering pattern (see inset in
Fig. 1, a) exhibits a good periodicity with the period a 11.85 Å. Fig-
ure 1, b presents known GaTe structural models described elsewhere
[2]. Comparison of these models suggests that the surface of the mono-
clinic phase in general is less smooth than the hexagonal one. It would
manifest itself by the characteristic furrows, and therefore can be eas-
ier ‘captured’ in STM studies.
In accordance with the model presented in [2] on conversion of h-
GaTe to m-GaTe, the sheets are still parallel to the surface except now
the lattice parameters have changed and some geometric features of
the layers are different. The localized twist of every third Ga—Ga moie-
ty in the h-GaTe structure occurs on going to m-GaTe. Thus, furrows
observed in top and bottom boundary regions in Fig. 1, a and con-
firmed in the self-correlation filtering pattern are the result of such
lattice reorganization when the (201) plane becomes as the surface
STM/S AND LEED INVESTIGATIONS OF GaTe LAYERED CRYSTAL CLEAVAGE 793
one instead of (001).
Figures 1, c, e show 2.82.6 nm2
and 5.34.5 nm2
fragments, taken
within the ‘smooth’ central and ‘furrowed’ upper near boundary areas
in Fig. 1, a, respectively, obtained by 2dFFT filtering. Figure 1, c dis-
plays honeycomb structure, which is typical for h-GaTe crystal struc-
ture [2]; cf. Fig. 1, g. Since we are dealing with the surface of the crys-
Fig. 1. STM study of GaTe cleavage surface: 2121 nm2
2D image obtained at
2.3 V bias, 0.122 nA. Inset: self-correlated filtered image (a); structural
models for GaTe monoclinic m-GaTe and hexagonal h-GaSe or h-GaTe phases
[2] (b); 2.82.6 nm2
and 5.34.5 nm2
2D FFT filtered fragments taken within
the ‘smooth’ central and ‘furrowed’ upper near boundary areas of (a) image
(respectively, c, e), and their corresponding 3D visualization (d, f). The sche-
matic models of the hexagonal and monoclinic GaTe surface unit cells are
drawn as guide to the eye. The model hexagons have 4.08 Å length sides. The
size of model rectangular is 1.30.408 nm2. Schematic diagram showing the
relative directions of unit cells’ vectors, a and b, in the monoclinic and hexag-
onal GaTe phases (g).
794 P. GALIY, T. NENCHUK, A. CISZEWSKI et al.
tal, which consists of two kinds of atoms, and it is known that the sur-
face is not flat, for better identification of the surface structure, Fig.
1, d shows 3D image of Fig. 1, c represented by the shaded texture,
which allows better visualization of hexagonal structure. The presence
of hexagonal structure on the surface of GaTe crystal cleavage basical-
ly confirms data previously obtained for the bulk of this crystal and as
shown below is also confirmed in the LEED study. Analysis of the peri-
odicities observed in Fig. 1, e, which actually represents visually ‘fur-
rowed’ structure in Fig. 1, a, suggests the presence of another pattern,
that is characteristic in accordance with GaTe structure model repre-
sentations in [2] for monoclinic GaTe structure. 3D visualization of
this pattern (see Fig. 1, g) clearly illustrates the typical structure of
the monoclinic GaTe phase surface, as can be seen in Fig. 1, b.
Figures 1 c, e are supplemented by schematic models of hexagonal
and monoclinic unit cells, respectively, that are superimposed on the
corresponding structures observed on the surface. The model hexagons
have 4.08 Å side lengths. The size of model rectangular is 1.30.408
nm2. However, it should be taken into account that, in fact, the cleav-
age surface in any of the structural phases is not flat, as can be seen
from the Fig. 1, b, but, even in this case, there is a good correlation of
STM image and relevant unit cells models. Thus, as seen from Figs. 1, c,
e and more strikingly from Figs. 1, d, f, which are their appropriate 3D
images, the structure of GaTe surface is not homogeneous even on na-
noscale.
The evaluated parameters of 4.08 Å in STM studied hexagonal struc-
ture in the surface plane correlate well with lattice constants a and b of
the hexagonal GaTe phase. As for the parameters of the monoclinic
structure derived from STM study, the value 0.408 nm is consistent
with b lattice constant in one of the known monoclinic phases. As for
the 1.3 nm periodicity, which apparently does not correlate with any of
known bulk structures, it should be noted that the monoclinic unit cell
of GaTe, as well as the hexagonal one, consists of two Te—Ga—Ga—Te
packets with a different stacking sequence relative to each other (see
Fig. 1, b). Considering the above, it is possible to estimate the parame-
ter of the monoclinic structure toward the a axis, giving a value of
2.6 nm for the GaTe monoclinic lattice parameter. When evaluating a
unit cell parameter for the monoclinic structure by STM, it should be
also considered a certain angle between [201] and [100] axes that de-
pends on one or another monoclinic phase. Therefore, we can assume
that we were able to identify the monoclinic phase with a equal to
23.64 Å.
The schematic diagram showing the relative directions of the sur-
face unit cells a and b vectors, however, in ‘projection’ on the certain
surface plane, for the monoclinic and hexagonal GaTe phases is pre-
sented in Fig. 1, g.
STM/S AND LEED INVESTIGATIONS OF GaTe LAYERED CRYSTAL CLEAVAGE 795
The corresponding unit cells models are taken from Figs. 1, c and e
and reduced to a common scale. According to the model representa-
tions of h-GaTe towards m-GaTe phase transformation described in [2],
two hexagonal cells on the surface are involved in the process of rear-
rangement into surface monoclinic unit cell. Thus, as mentioned
above, the direction and size of the lattice vectors a and b are changing
as it is shown in Fig. 1, g.
It should be noted that patterns shown in Figs. 1, c and e actually
obtained for individually selected areas on ‘raw’ STM image, like Fig.
1, a, give reason to believe that we are dealing with two locally distinct
‘topographic’ structures on the surface of GaTe cleavage, rather than
‘electronic’ features like local density of states (DOS) oscillation shown
by Moiré pattern.
Thus, the detailed STM study of the surface and comparing the ob-
tained distances with known data for bulk structures suggests the co-
existence of both hexagonal and monoclinic phase on the surface of the
GaTe cleavage, at least on nanoscale.
Another major issue is the possibility of direct observation of not
only planar a and b lattice vectors of GaTe crystal, but also c vector
which, for the hexagonal structure is normal to the surface plane, and
for the monoclinic one does not form normal angles with the plane of
the surface (there are several monoclinic structures as mentioned
above). For such an STM study, it is important that, besides the differ-
ence in the c lattice-vector angle relative to the surface plane, the geo-
metric heights of the h-GaTe and m-GaTe layers are different. Thus,
according to the obtained height differences, which can be quite well
identified in the vicinity of steps, we succeeded in determining the
value of the c lattice parameter and, consequently, the structural
phase of the surface layer associated with it.
Figures 2, a, d show 2D and 3D STM images of rather an elongated
step on the GaTe cleavage surface. It should be noted that stepped
structure is characteristic of the GaTe crystal cleavage surfaces on
macroscale as it will be shown in the LEED study discussed below. Fig-
ure 2, b shows a typical height profile of the step, which indicates that
the average height difference in the vicinity of the step tends to 8 Å.
The statistical evaluation of the height differences in the array of im-
age pixels, which could be obtained via standard feature of WSM ap-
plication such as that for roughness analysis, is even more reliable.
Figure 2, c shows a corresponding histogram for height intervals of im-
age pixels in the surface region marked with rectangle in Fig. 2, a. It is
clear that there is some height distribution of pixels along both sides of
the step, which is depicted in Fig. 2, c by the ‘Bactrian structure’, but
the actual height difference between the maxima of the distributions is
about 8 Å. The above suggests that the monoclinic or hexagonal unit
cell of GaTe contains a double Te—Ga—Ga—Te packet. Taking into ac-
796 P. GALIY, T. NENCHUK, A. CISZEWSKI et al.
count the known data on the c parameter, we can conclude that have ob-
served a hexagonal structure of the surface layer. Moreover, only indi-
vidual Te—Ga—Ga—Te packet could form the observed step, because the
actual thickness of the GaTe hexagonal packet structure is a multiple of
8 Å, rather than 5 Å as would be in the case of a monoclinic one.
The question arises about the nature of the possible partial recon-
struction of the hexagonal bulk structure at the GaTe cleavage surface
toward the monoclinic one. Thus far, we analysed the height distribu-
tion of image pixels along the narrow area that is selected in the cen-
tral region of Fig. 1, a, also by using a roughness analysis tool of
WSM. Such choice of the analysed area is reasonable, since the ob-
served pattern of the central region of Fig. 1, a exhibits lack of perio-
dicity, which is inherent in monoclinic structure, while the hexagonal
pattern is present. From the obtained height distribution histograms
(see Fig. 2, e), we were able to identify two characteristic heights sepa-
Fig. 2. STM study of the c axis unit cell parameter of GaTe crystal: 400400
nm2
2D (a) and 3D images (d), obtained at 2.3 V bias, 0.303 nA; the subse-
quent typical height profile indicating the single Te—Ga—Ga—Te packet thick-
ness in hexagonal structure (b); height distribution of image pixels obtained
from roughness analysis, including both sides in the vicinity of the step as
marked with rectangle in Fig. 2, a (c); height distribution of image pixels ob-
tained from roughness analysis made along the narrow area of Fig. 1, a and
indicated by inset (only for better visualization the inset shows appropriate
3D image of 2D FFT filter of Fig. 1, a) (e).
STM/S AND LEED INVESTIGATIONS OF GaTe LAYERED CRYSTAL CLEAVAGE 797
rated by size of about 5 Å. This value is approximately equal to the typ-
ical thickness of a single Te—Ga—Ga—Te packet in the majority of
known monoclinic structures, particularly, such as that in Fig. 1.
Consequently, the STM results show that the surface of the crystal
cleavage contains local areas of both hexagonal and monoclinic struc-
ture, which is indicated by the presence of steps with different heights.
Regarding STS studies of GaTe surfaces, it is known that one of
their direct outcomes is the ability to evaluate the energy band gap of
semiconductor crystal. However, the obtained value may be character-
istic of local points or rectangular regions of the surface, after averag-
ing over some array of points on the nanoscale. It should be noted that
local structural inhomogeneity, particularly the steps, could signifi-
cantly affect the local DOS and, consequently, result in the band gap.
Figure 3, a shows a typical set of the normalized (dI/dV)/(I/V) f(V)
curves spatially averaged each over an area of 5050 nm2. The corre-
sponding STS derived band gap ranges from 1.65 to 1.75 eV.
There are a number of known, but ‘integrated’ data on GaTe band
gap magnitudes obtained by different methods. The values range be-
tween 1.65 and 1.7 eV [3, 8, 9] for bulk crystals and between 1.7 and
1.9 eV for GaTe films [2]; and there is evidence of only slight differ-
ence in band gap values between monoclinic and hexagonal structures.
Our STS data on band gap values (see Fig. 3, a and Fig. 3, b, curve 2),
obtained for a local nanoscale surface area, correlate well with these
data. Thus, the wide apparent band gap, shown as a magnified frag-
ment in Fig. 3, c, curve 2, suggests the absence or a negligible quantity
of intrinsic surface states for the fundamental bulk band gap.
Curve 1 in Figure 3, b represents the spatially averaged normalized
(dI/dV)/(I/V) f(V) STS spectrum from the averaged electronic struc-
ture of the stepped 11 m2
surface. It is not possible to distinguish
spectra of the step edges and the centre of relatively flat areas from
each other due to a poor image resolution. However, even the averaged
STS spectra from the whole 11 m2
area give evidence of a significant
inhomogeneity of the surface on macroscale. This assertion arises from
comparing the zoomed up band gap regions with the normalized dI/dV
STS curves averaged over the 11 m2
and 5050 nm2
areas (see Fig. 3,
c). One could find that the band gap is significantly less for a larger ar-
ea than the one, which is obtained for a local 5050 nm2
surface area
and ‘narrows’ to about 1 eV. Certainly, it must be considered that this
effect could arise from the decrease of the transmission coefficient for
electron tunnelling with tip/sample separation, leading to a broader
range around the zero bias voltage without a detectable tunnel current.
However, the actual tip/sample separation for all curves changed only
slightly. Thus, so significant differences in the average values of band
gap over the studied areas could be explained by availability of signifi-
cant tails of DOS near the bottom of conduction band and the top of va-
798 P. GALIY, T. NENCHUK, A. CISZEWSKI et al.
lence band. Thus, the peculiarities of the local DOS spatial distribu-
tion, particularly, in the vicinity of zero bias, are determined by the
inhomogeneous distribution of structural defects on the stepped sur-
face of cleavage. The fact that a highly stepped surface provides the
major impact on the acquired STS data is also confirmed by our LEED
studies of GaTe crystal cleavage.
3.2. LEED Results
In order to check the structure of GaTe cleavage surface on the mac-
roscale and get more information about the structures observed by
STM on nanoscale, the LEED experiment was performed. Some of the
Fig. 3. STS data obtained from the GaTe cleavage given by the normalized
(dI/dV)/(I/V) f(V) curves: STS curves, each one shows the averaged data of
6400 local curves from three different 5050 nm2
areas within the relatively
large 11 m2
area, of which an averaged spectrum is given by curve 1 in Fig.
3, b (a); comparison of averaged STS 1 and 2 curves typical of the large
11 m2
and small 5050 nm2
areas of the surface, respectively (b); zoomed up
fragment of figure (b) showing in detail the region of the GaTe energy gap (c).
STM/S AND LEED INVESTIGATIONS OF GaTe LAYERED CRYSTAL CLEAVAGE 799
structural periodicities measured by STM are confirmed by diffraction
patterns. First of all, the spots of the hexagonal pattern are intense
and clearly visible anytime.
Figure 4, a shows a typical hexagonal pattern obtained from cleav-
ages, which presents several orders of spots. Diffraction spots indicate
a formation of long-range order as concluded from their sharpness.
Dimensions of the hexagonal structure lattice vectors, as derived from
the reciprocal ones observed in the LEED pattern, are equal to about
4 Å. Thus, they are in accordance with the reported STM local data ob-
served on the nanoscale.
However, we were not able to observe the spots characteristic of the
Fig. 4. Typical LEED patterns from different areas of the GaTe cleavage sur-
face taken at the primary electron beam energy of: 121.5 eV (a), 28 eV (b),
52.5 eV (c).
800 P. GALIY, T. NENCHUK, A. CISZEWSKI et al.
monoclinic structure, which, in accordance with our STM data, could
be present on the surface of cleavage. Obviously, this one could be re-
lated to the random distribution of local areas, where the monoclinic
structure could be seen. The additional reason for the lack of monoclin-
ic structure spots under observation might be a relatively weak inten-
sity of this structure spots caused by the fact that, at least, one of the
monoclinic structure lattice parameters is sufficiently larger than that
of hexagonal one to contribute to a smaller proportion in the surface
and, consequently, its spots are much less intense than those of the
hexagonal structure and below the camera sensitivity.
However, the detailed STM/STS data on surface structure defects,
including the steps on the GaTe surface, are confirmed by LEED data.
Generally, the LEED technique provides statistical information about
the distribution of defects, because structural defects strongly influ-
ence the spot profiles. The randomly distributed height of the steps or
terraces has an impact on all diffraction spots in such a way that their
width periodically varies between sharp and diffuse appearance, par-
ticularly with observation of streaks, with increasing electron energy.
Otherwise, the LEED pattern, which shows some integral order spots
split into doublets, indicates terraces on the surface that have a single
orientation, regular spacing and height. Besides, all spots might ap-
pear alternately as doublets and single spots with changes in electron
energy. It is obvious that spacing of the doublets depends on the ter-
race width and the doublets direction is normal to the step direction.
Typical LEED patterns recorded from the GaTe cleavage surfaces,
which concern the above common considerations, are shown in Figs. 4,
b, c. Figure 4, b shows very prominent threefold streaks along the
[110] , [010] and [100] directions, and Fig. 4, c exposes clear doublets
associated with the spots of hexagonal structure. As already men-
tioned, the streaks in Fig. 4, b are caused by random distributed ter-
races, while the spot splitting demonstrated in Fig. 4, c is caused by
regular arranged step arrays. The observed characteristic spot split-
ting is about 8.55% of BZ (Brillouin zone) that corresponds to the
width of about 48 Å (6 single packet distances in hexagonal structure).
The possibility of structural transformations on GaTe cleavage sur-
faces as a result of thermal heating up to 500—600C has also been in-
vestigated by LEED, considering the known results on the hexago-
nal/monoclinic transformations in GaTe polycrystalline thin films [2]
at elevated temperatures. The characteristic hexagonal reflections,
which were observed directly after the cleavage at room temperature,
never disappeared also after consecutive heating at 200, 300, 400C up
to 500—600C. However, diffraction spots of monoclinic structure
were not observed in the LEED pattern even after the heating, thus,
one could not state about the reconstruction of the cleavage surface, at
least on macroscale.
STM/S AND LEED INVESTIGATIONS OF GaTe LAYERED CRYSTAL CLEAVAGE 801
4. CONCLUSIONS
The results of the combined scanning probe and electron diffraction
study of the GaTe cleavage surface indicate the presence of different
structures: hexagonal one on macroscale and local monoclinic one ran-
domly distributed at the nanoscale. Furthermore, it is found that the
surface structure is not perfectly flat, but is characterized by a well-
developed staircase structure that is formed by cleavage. All this con-
firms the possibility of partial hexagonal/monoclinic surface recon-
struction after the cleavage, when the local stepped structures on the
surface appear as thick as one Te—Ga—Ga—Te packet.
We believe that such reconstruction, generally atypical of layered
crystals, is determined by the presence of double Te—Ga—Ga—Te pack-
ets in the structure of the unit cell and from the corresponding anisot-
ropy, which is higher for GaTe than for similar structures of GaS,
GaSe, and InSe layered crystals. The possibility of partial on-nanoscale
reconstruction of the basic hexagonal structure towards a monoclinic
one is directly related to the number of surface defects, such as the
loosely distributed steps, which are as high as a single Te—Ga—Ga—Te
packet. This helps to understand why the structural monoclinic phase
cannot be detected in LEED patterns, namely, the cause is the random-
ness and incoherence of the above-mentioned stepped areas that are
formed on the surface due to cleavage.
Data of this study might be directly applied to explain experimental-
ly obtained results such as those of paper [3], for the GaTe hexagonal-
to-monoclinic structural transformations upon grinding due to the in-
crease of the surface-to-bulk ratio in the powder.
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/NLD (Gebruik deze instellingen om Adobe PDF-documenten te maken die zijn geoptimaliseerd voor prepress-afdrukken van hoge kwaliteit. De gemaakte PDF-documenten kunnen worden geopend met Acrobat en Adobe Reader 5.0 en hoger.)
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/UKR <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>
/ENU (Use these settings to create Adobe PDF documents best suited for high-quality prepress printing. Created PDF documents can be opened with Acrobat and Adobe Reader 5.0 and later.)
>>
/Namespace [
(Adobe)
(Common)
(1.0)
]
/OtherNamespaces [
<<
/AsReaderSpreads false
/CropImagesToFrames true
/ErrorControl /WarnAndContinue
/FlattenerIgnoreSpreadOverrides false
/IncludeGuidesGrids false
/IncludeNonPrinting false
/IncludeSlug false
/Namespace [
(Adobe)
(InDesign)
(4.0)
]
/OmitPlacedBitmaps false
/OmitPlacedEPS false
/OmitPlacedPDF false
/SimulateOverprint /Legacy
>>
<<
/AddBleedMarks false
/AddColorBars false
/AddCropMarks false
/AddPageInfo false
/AddRegMarks false
/ConvertColors /ConvertToCMYK
/DestinationProfileName ()
/DestinationProfileSelector /DocumentCMYK
/Downsample16BitImages true
/FlattenerPreset <<
/PresetSelector /MediumResolution
>>
/FormElements false
/GenerateStructure false
/IncludeBookmarks false
/IncludeHyperlinks false
/IncludeInteractive false
/IncludeLayers false
/IncludeProfiles false
/MultimediaHandling /UseObjectSettings
/Namespace [
(Adobe)
(CreativeSuite)
(2.0)
]
/PDFXOutputIntentProfileSelector /DocumentCMYK
/PreserveEditing true
/UntaggedCMYKHandling /LeaveUntagged
/UntaggedRGBHandling /UseDocumentProfile
/UseDocumentBleed false
>>
]
>> setdistillerparams
<<
/HWResolution [2400 2400]
/PageSize [612.000 792.000]
>> setpagedevice
|