mit Kumar Singh Chauhana,b, Nirosh M. Eldosea, Monu Mishraa, Asad Niazib,ekha Nairb, Govind Guptaa,∗
Physics of Energy Harvesting, (CSIR-NPL), Dr. K.S. Krishnan Road, New Delhi 110012, IndiaDepartment of Physics, JMI, New Delhi 110025, India
r t i c l e i n f o
rticle history:eceived 9 May 2014ccepted 26 June 2014vailable online 8 July 2014
eywords:i(5 5 7)ndium
a b s t r a c t
This paper introduces issue of kinetically controlled and temperature driven superstructural phase tran-sition of Indium (In) on atomically clean high index Si(5 5 7)-7 × 1 surface. Auger electron spectroscopyanalysis reveals that at room-temperature (RT) with a controlled incident flux of 0.002 ML/s; In over-layers evolve through the Frank-van der Merwe growth mode and yield a (1 × 1) diffraction pattern forcoverage ≥1 ML. For substrate temperature <500 ◦C, growth of In follows Stranski–Krastanov growthmode while for temperature >500 ◦C island growth is observed. On annealing the In/Si(5 5 7) interfacein the temperature range 250–340 ◦C, clusters to two dimensional (2D) layer transformation on top of
dsorptionhermal desorptionESEED
a stable monolayer is predominated. In-situ RT and HT adsorption and thermal desorption phenomenarevealed the formation of coverage and temperature dependent thermally stable In induced superstruc-tural phases such as (4 × 1) at 0.5 ML (520 ◦C), (
√3 × √
3-R30◦) at 0.3 ML (560 ◦C) and (7 × 7) at 0.1 ML(580 ◦C). These indium induced superstructures could be utilized as potential substrate for the growth ofvarious exotic 1D/2D structures.
. Introduction
The epitaxial growth of metals on Silicon (Si) substrates hasxtensively been studied due to their involvement in various exotichysical phenomena at metal/semiconductor interfaces and poten-ial application in nanoscale devices [1–4]. Recently, it has alsoeen reported that the bonding between Lead (Pb) or Indium (In)etal atomic layer film on Si and adsorbate induced superstructural
hases can enhance the 2-dimensional (D) superconductivity at thenterface [5]. Moreover, the controlled kinetics of metal adsorptionn various Si surfaces has shown interesting result regarding dif-erent growth modes and formation of a number of superstructuralhases [4,6–8]. Under the kinetic and thermodynamically con-rolled conditions, the epitaxial growth of metal/Si interface followne of the three main growth modes i.e. Frank Van der Merwe,transki Krastanov or Volmer Weber [9] and form several thermallytable adsorbate induced interfacial phases, which are developed
ue to interfacial interactions or self assembly of adsorbate atoms6]. These modified adsorbate induced structural phases can be uti-ized as template to form interfaces with desired properties [10].
The condition of growth causes formidable issue of repeatability;therefore, the systematic understanding of the adsorbate–substrateinteraction is crucial to understand the growth kinetics. Besides theplanner Si surfaces, vicinal or stepped Si(1 1 1) surfaces also exhibitthe formation of atomic structures at submonolayer coverage forvarious metal adsorbates [11–15]. This is a more flexible tech-nique to fabricate self organized atomic structures via utilizing thestepped surface as “template”: an ordered step array of high index Sisubstrate easily stabilizes anisotropic 1D structure [12,13]. The wellknown systems are Au induced atomic chain structure on stepped Sisurfaces such as Si(5 5 7), Si(5 5 3) and Si(5 5 12) [16–20]. Recently,the formation of adsorbate induced 1D/2D structures of 3rd groupmetals on high index stepped Si surfaces have been reported[13,21,22]. Indium (In) growth on Si surfaces is a very interest-ing research area as In/Si system provides a suitable arena for thestudy of coverage and temperature controlled surface reconstruct-ions [23]. The In/Si interface is nonreactive, but adatom–substratecoupling strength is sufficient to support a various ordered surfacestructures from low to high In coverages [23]. Recently, variousIn induced facet formation and surface reconstructions (such as
2 × (337), 2 × (225) and 1 × 1, 3 × 2, 1 × 3, 1 × 2 respectively) onhigh index vicinal Si(1 1 1) surfaces such as: Si(5 5 12), Si(1 1 3) andSi(5 5 7) surfaces and Si(5 5 3)–Au interface [8,21,22,24] has beenreported. In the present study, we focus on the controlled kinetics of
Fig. 1. (a) Cross-sectional view of the bulk Si showing the Si(5 5 7) surface which lies9 ◦
sw
gsamwrF5tiecagscIiepdS
2
ibstsbwa1cocfract
.45 from the (1 1 1) plane to (0 0 1) plane. (b) Schematic drawing of the Si(5 5 7)urface, which self-possessed an alternating arrangement of (1 1 1) and (1 1 2) facetsith angle 19.5◦ between them. (1 1 2) facet is composed of three atomic steps.
rowth of In metal atoms on high index Si(5 5 7) surface. A Si(5 5 7)urface is unique among the vicinal Si(1 1 1) surfaces because it has
well ordered reconstructed surface with an alternative arrange-ent of (1 1 1) and (1 1 2) terraces (composed of triple atomic step)ith finite widths [25]. The Si(5 5 7) surface is inclined at 9.45◦
elative to the (1 1 1) plane toward[1 1̄ 2̄
]direction as shown in
ig. 1(a). The period of the staircase of triple steps on this surface is.73 nm [26] (comprising 111 and 112 terrace) and it could poten-ially be utilized as template for the growth of various adsorbatenduced superstructure/nanostructures. The formation of 1D het-rogeneous In–Si adatom chain structure on Si(5 5 7)-1 × 3 phase atoverage ∼0.12 ML has been reported by Song et al. [22]. However,
systematic study is essential to understand the kinetics of therowth and adsorbate–substrate interaction for adsorbate inducedurface reconstruction. In the present paper, we have reported ouronsolidated results of In growth on Si(5 5 7), thermal stability ofn/Si(5 5 7) interface and adsorbate induced surface reconstruct-ons, which is probed by Auger electron spectroscopy (AES) and lownergy electron diffraction (LEED) techniques. The various LEEDatterns, resultant of different In coverage, are combined in a phaseiagram which demonstrate In adsorption/desorption pathways oni(5 5 7) surface.
. Experimental
The experiments for In/Si(5 5 7) system have been carried outn a custom designed ultra high vacuum (UHV) system at aase pressure <1 × 10−10 torr and equipped with various surfaceensitive techniques such as; AES, LEED, electron energy loss spec-roscopy and residual mass spectrometer. The Si(5 5 7) sample ofize 20 × 8 × 0.35 mm3 was cut and cleaned in ambient conditionsy employing modified Shiraki process [27]. The cleaned sampleas then transferred to the UHV system and prepared in-situ by
nnealing at 600 ◦C for approximately 6 h and repeated flashing at250 ◦C (vacuum ∼2 × 10−9 torr during the flash) [4]. The atomicleanliness of the sample surface was ascertained via the absencef carbon and other contaminations by AES (having a single passylindrical mirror analyzer with 0.18% resolution), and the sur-ace order observation of the characteristic 7 × 1 LEED pattern of
econstructed Si(5 5 7) surface. A W-Re (5–25%) thermocouple isttached to the sample for precise temperature measurement andalibrated with the help of an optical pyrometer. A homemade tan-alum Knudsen cell (K-cell) is used to evaporate In metal (purity
Fig. 2. AES uptake curve for RT growth of In on Si(5 5 7) surface where AES intensityratio (IIn/Si) of In MNN and Si LVV peak plot as a function of deposition time. Insetshowing the schematic modeling for Frank vander Merwe (FM) growth mode.
level 99.999%) and degassed thoroughly before evaporation of thematerial. In flux is controlled by regulating the current through celland is calibrated in terms of the adsorbed monolayers by measuringthe In (MNN)/Si (LVV) Auger peak intensity ratio. The AES spectraand the LEED pattern are acquired and analyzed at every stage ofadsorption and thermal desorption, which leads to the extractionof a complete phase diagram for the In/Si(5 5 7) interface for thefirst time.
3. Results and discussions
RT-adsorption of In atoms is carried out on clean reconstructedSi(5 5 7)-7 × 1 surface, where the coverage is limited to few mono-layers. The Auger uptake curve for RT adsorption is shown in Fig. 2,which plots the AES peak intensity ratio (IIn/Si) of In MNN (404 eV)peak to Si LVV (92 eV) peak as a function of deposition time. It isevident from Auger uptake curve that the value of IIn/Si increases ina linear manner for In deposition time of 9 min and on further depo-sition of In, a change in slope of Auger uptake curve is observed. Thechange in the slope (at IIn/Si ∼0.2) is confirmed by plotting the sumof square of errors (SSQ) [28] in the least-square fit of a set of twostraight lines near the change in slope, minima of which identifiesthe inflexion point that suggest the completion of 1 monolayer (ML){1 ML ∼7.8 × 1014 atoms/cm2, bulk truncated Si(1 1 1) density [7]}.The change in slope arises due to the attenuation of Si (LVV) signalby the presence of In adatoms on 1 ML covered Si(5 5 7) surface. Theformation of 1 ML gives a calibration of In flux rate as ∼0.11 ML/min(∼0.002 ML/s). On further adsorption of In atoms on top of 1 ML cov-ered Si(5 5 7) surface, another break point is observed at 18 min ofdeposition time (IIn/Si ∼0.4) which implies the completion of sub-sequent second ML (bilayer). On further deposition of In on bilayercovered Si(5 5 7) surface a similar trend is observed. This impliesthat the RT growth of In atoms on Si(5 5 7) surface, with consider-able low In flux rate, evolves in the layer-by-layer i.e. Frank vanderMerwe growth mode. To understand the RT growth kinetics of Inatoms on Si(5 5 7) surface, a schematic model is presented as aninset of Fig. 2. Similar layer-by-layer growth pattern has also beenreported for RT growth of In metal on low index planner recon-structed Si(1 1 1)-7 × 7 [7] and high index trenched reconstructed
Si(5 5 12)-2 × 1 surface [8].
The thermal stability of RT grown In/Si(5 5 7) system is investi-gated by thermal desorption (TD) experiments, where In/Si systemis subjected to anneal at elevated temperatures for a fixed period
Fig. 3. Thermal desorption curve of In adsorbed Si(5 5 7) surface plots the IIn/Si asa function of temperature. The coverage after annealing the In/Si(5 5 7) system isr
oIpRdtI(wbartiatrpiI[tmsiiriai(oa
lso
At
>1/2 ML could not be attained. These observations indicate that the
epresented by schematic models along with the desorption curve.
f 1 min. Fig. 3 shows the corresponding TD curve for RT grownn/Si(5 5 7) system which plots IIn/Si against the annealing tem-erature. On increasing the substrate temperature initially fromT to 250 ◦C, steady decrease in the IIn/Si value is observed. Theecrease in Auger intensity ratio is either because of desorp-ion of In atom from the surface or initial rearrangement ofn atoms into small 3D islands. On further rise in temperature∼340 ◦C), the value of IIn/Si increases and attains its initial valuehich indicate that the possibility of In desorption is negligi-
le below 340 ◦C. This indicates that low temperature (<250 ◦C)nnealing provides thermal energy (mobility) to In adatoms toe-arrange themselves into closely packed islands. Annealing inhe temperature range of 250–340 ◦C, In atoms in closely packedslands got higher mobility which cause segregation of clustersnd convert them into pseudomorphic flat layer. Therefore, inhe temperature range from RT to 340 ◦C temperature inducedearrangement (clusters to layer transformation) of In atoms tooklace without In desorption. The unusual phenomenon of cluster-
ng to layering occurs due to the gradual reduction in strain ofn/Si(5 5 7) system which facilitates the layering of the In islands8]. Annealing in the temperature range of 250–340 ◦C, due toheir respective thermal expansion coefficient, the lattice mis-
atch between In and Si is reduced which lead to lowering oftrain and assist in the layering of In clusters. Such temperaturenduced cluster to layer transformation has also been observed dur-ng In desorption from planner Si(1 1 1)-7 × 7 in the temperatureange 400–500 ◦C [7] and from trenched Si(5 5 12)-2 × 1 surfacen the temperature range 400–550 ◦C [8]. Further increase in thennealing temperature to 360 ◦C, a sharp decrease in IIn/Si values observed which attributes to the desorption of In multilayeradsorbate–adsorbate atom bonding) followed by stabilized valuef IIn/Si ∼ 0.3 which corresponds to 1.5 ML coverage in the temper-ture range 360–500 ◦C.
On increasing the desorption temperature beyond 500 ◦C, multi-ayer and monolayer (adsorbate–substrate atom bonding) desorbsharply and complete desorption of In atoms from Si(5 5 7) isbserved at 600 ◦C.
The thermal desorption is usually described in terms of an
rrhenius expression, often called Polanyi–Wigner equation [6,31],
herefore, desorption of multilayer and monolayer of In/Si(5 5 7)
Fig. 4. AES uptake curve for HT In overlayer growth on Si(5 5 7) surface at substratetemperatures (i) 300 ◦C (ii) 400 ◦C (iii) 500 ◦C.
system is calculated by means of Arrhenius equation which isexpressed as:
ln I(T) ∝ −ED
kBT(1)
where I(T) is the change in density of adatoms (related to adatomcoverage), ED is the desorption energy, kB is Boltzmann constantand T is temperature. By plotting ln I(T) vs. 1/T, the resultantslope (–ED/kB) of the curve determines the desorption energy. Thedesorption energy for RT grown In/Si(5 5 7) for multilayer (In–In)desorption is calculated to be 1.01 eV, while for monolayer (In–Si)desorption energy is found to be 2.41 eV.
To understand the growth kinetics of In atoms on Si(5 5 7)surface at higher substrate temperature, HT adsorption experi-ments are performed at temperatures 300 ◦C, 400 ◦C and 500 ◦C.AES uptake curve for these HT growth of In atoms are shown inFig. 4. For substrate temperature 300 ◦C (Fig. 4(i)), it is observedthat the value of IIn/Si increases linearly upto ∼0.3 (corresponds to1.5 ML In coverage) and on further adsorption no appreciable incre-ment in the coverage is observed. This suggest that the interactionbetween In–In atoms at 300 ◦C temperature reduces, and conse-quently In overlayers evolve in the Stranski–Krastanov (SK) growthmode, where 2D/3D islands are formed on top of the 1 ML coveredIn/Si(5 5 7) surface.
For substrate temperatures 400 ◦C, IIn/Si increases linearly andattains the value 0.2 (1 ML coverage) and get saturated on furtherIn deposition (shown in Fig. 4(ii)) which indicates the formationof In islands on top of the single monolayer. This revealed that thegrowth of In overlayer at substrate temperature 400 ◦C also followsSK growth mode. For further higher substrate temperature (500 ◦C),the growth kinetics seems to be very interesting as the value ofIIn/Si remains well within the monolayer coverage even though Inatoms were continuously deposited for 18 min which implies thatIn did not adsorb appreciably on Si(5 5 7) surface for temperatures≥500 ◦C. From Fig. 4(iii), it is observed that the IIn/Si increases ini-tially upto value ∼0.09 and on further In deposition it saturates inthe sub-monolayer coverage (<1/2 ML). The low adsorption of Intend to suggests the formation of incomplete layer or, more pre-cisely, 2D-islands on the Si(5 5 7) surface at 500 ◦C, which may bedue to reduced sticking probability of In atoms on Si(5 5 7) surface.From the AES data, it can also be concluded that for substrate tem-peratures ≥500 ◦C, the desorption rate of In atom dominates andadatoms form small islands or partial layer and even the coverage
In adsorption on Si(5 5 7) at HT (≥500 ◦C) follows Volmer Webergrowth mode.
A.K.S. Chauhan et al. / Applied Surfac
FI
FgHuft5(ictwpstlastits
aTbm<tfbHtcTisalpIrtidi
ig. 5. Desorption curve which plotted IIn/Si vs. annealing temperature for HT grownn/Si(5 5 7) system (i) 300 ◦C (ii) 400 ◦C (iii) 500 ◦C.
The TD curves of HT grown In/Si(5 5 7) system, are shown inig. 5, which plots IIn/Si with annealing temperature for samplesrown at substrate temperatures 300 ◦C, 400 ◦C and 500 ◦C. ForT-300 ◦C system, it is observed that In adatoms desorbed grad-ally in the temperature range 300–380 ◦C, while thermal stabilityor monolayer coverage is observed in the annealing tempera-ure range 380–520 ◦C (Fig. 5(i)). On subsequent annealing beyond20 ◦C, In monolayer desorbs from Si(5 5 7) surface. By means of Eq.1), bilayer and monolayer desorption energy for HT-300 ◦C systems calculated and found to be 0.35 eV and 2.38 eV, respectively. Thealculated bilayer desorption energy for HT-300 ◦C In/Si(5 5 7) sys-em is found to be significantly lower than that of RT grown system,hile the value of monolayer desorption energy is found to be com-arable in both HT-300 ◦C and RT cases. The TD curve for HT-400 ◦Cystem is shown in Fig. 5(ii), where bilayer desorption is absent ashe In coverage is limited to 1 ML only, but similar trend for mono-ayer stability is perceived in the temperature range 400–500 ◦Cs observed for HT-300 ◦C. The monolayer desorption energy islightly lower and calculated to be 2.1 eV. A similar desorptionrend, stability at 1/2 ML coverage prior to its complete desorption,s observed for HT-500 ◦C as shown in Fig. 5(iii). In addition to that,he complete desorption of In atoms from Si(5 5 7) for all HT grownystem is observe at 600 ◦C, which is similar to RT grown system.
A competition between kinetics and thermodynamics of Intoms on Si(5 5 7) surface is exhibited during RT and HT adsorption.his shows that the kinetics of In on Si(5 5 7) surface is governedy the substrate temperature i.e. RT growth pursued FM growthode while SK and VB growth modes are followed in case of HT
500 ◦C and HT ≥500 ◦C, respectively. For RT growth, the interac-ion between In–Si dominates and In atoms grow in layer-by-layerashion on to Si(5 5 7) surface, while during desorption, stability inilayer and monolayer coverage is observed. On the other side, forT growth (<500 ◦C), formation of 1st ML took relatively similar
ime as RT, but after that 2nd ML could not be completed and theoverages are saturated near to 1.5 ML (300 ◦C) and 1 ML (400 ◦C).his indicates that the In–In interaction at HT is altered in compar-son to RT grown In on Si(5 5 7) system. For RT grown In/Si(5 5 7)ystem, the occurrence of clustering and layering phenomenonttributed to the temperature induced mobility to In atoms whicheads to the segregation of clusters and formation of pseudomor-hic flat layer. As the respective thermal expansion coefficient for
n and Si are different, the lattice mismatch between In and Si iseduced in annealing temperature range of 250–340 ◦C, which lead
o lowering of strain and assist in layering of In clusters. Interest-ngly, the calculated desorption energy for bilayers is found to beifferent in case of RT and HT grown system while it is comparable
n case of monolayer desorption. This suggests that the substrate
e Science 314 (2014) 586–591 589
temperature play significant role in controlling the dynamics ofIn atoms. In addition to that, in the present study the completedesorption temperature (TD) for In/Si(5 5 7) system is found to behigher than the reported values of TD in case of In on planar Si(1 1 1)(570 ◦C) surface while it is lower in comparison to In on trenchedSi(5 5 12) (820 ◦C) surface [7,8]; i.e. TD Si(5 5 12)> TD Si(5 5 7)> TD
Si(1 1 1). This may be ascribed to the terrace and stepped surfacemorphology of Si(5 5 7) surface, furnished with intermediate dan-gling bond density and hence provide stability to In atoms higherthan Si(1 1 1)-7 × 7 surface but lower than Si(5 5 12)-1 × 2 surfacewhich consists of trenched surface morphology.
In order to establish a structural correlation between variousIn induced superstructural phases on Si(5 5 7) surface, LEED pat-terns with primary beam energy of 70 eV, are obtained, as shown inFig. 6(a)–(f). These images illustrate the In induced surface variousreconstructions on high index Si(5 5 7) surface during adsorptionand subsequent desorption process. The surface reconstructionoccurs due to the change in equilibrium position of each individualatom near to the surface (result from the dangling bonds of ter-minated crystal surface) which creates a different structure on thesurface compared to the bulk structure (due to change in atomicforce). Moreover, these surface reconstructions can be induced oraltered by the adsorption of foreign atoms onto the surface as theinter-atomic forces are changed. Fig. 6(a) corresponds to the charac-teristic 7 × 1 surface reconstruction on the atomically clean Si(5 5 7)surface. The streaks or vertical chains of spots between integralorder spots are due to the step array, i.e. the alternative arrange-ment of hill and valley structure. The 7 × 7 spots are elongatedalong [1 1 2̄] direction, due to the step structure and superstructuraldomains formed on (1 1 1) and (1 1 2) facets [29,30].
For the coverage 0.11 ML (RT), the fractional order spots whichare elongated in
[1 1 2̄
]direction becomes more clearer and In-
induced 7 × 7 reconstruction is observed which remain stable till0.33 ML coverage (Fig. 6(b)). Similar 7 × 7 pattern has also beenobserved during HT growth for coverage ∼0.33 ML. On furtherdeposition of In (1 ML coverage), fractional order spots becameweak and a sharp 1 × 1 LEED pattern (Fig. 6(c)) is appeared forboth RT and HT growth (except for HT-500 ◦C). On increasing thecoverage above 1 ML, diffused 1 × 1 phase is observed. Similar In-induced 1 × 1 reconstruction has been reported by Song et al. [22]for In/Si(5 5 7) system for coverage ≥1.0 ML. Beside this, no recon-struction on (1 1 2) facet of Si(5 5 7) is observed which impliesthat (1 1 2) facet act as step bunch instead of three atomic-stepand, thus, the In-induced reconstructions on Si(5 5 7) surface arelimited to (1 1 1) facet only. During the thermal desorption pro-cess, no significant change in LEED pattern is observed for RT andHT grown system <300 ◦C, i.e. diffuse 1 × 1 phase remain intact.However, on increasing the substrate temperature beyond 300 ◦Can intense 1 × 1 LEED pattern is emerged. On increasing the sub-strate temperature to 450 ◦C for HT-300 ◦C and HT-400 ◦C grownsystem, the observed 1 × 1 pattern converted into a weak 4 × 1reconstruction (1 ML) where the intensity of the 4 × 1 (Fig. 6(d))fractional spot increases on increasing the annealing temperaturebeyond 500 ◦C. A sharp 4 × 1 diffraction pattern has been observedfor HT-500 ◦C system at 520 ◦C (0.5 ML) which remains stable upto 550 ◦C while no such reconstruction was observed in case of RT-grown In/Si(5 5 7) system. However, Song et al. [22] has recentlyreported the formation of 1D atomic chain structure during desorp-tion process of In/Si(5 5 7) system, where In induced Si(5 5 7)-1 × 3phase was observed after annealing at 530 ◦C. The 4 × 1 LEED pat-tern depicts the formation of In induced quasi-1D chain on (1 1 1)facet of Si(5 5 7) surface. Lander and Morrison [32], has reported
the formation of In induced Si(1 1 1)-(4 × 1) reconstruction by LEEDwhich was later confirmed by Scanning tunneling microscopyinvestigations where zig-zag chains in the filled-state images andlinear chains in the empty-state images [33] were observed. As
F lean Si(5 5 7)-7 × 1 reconstruction, (b) 7 × 7 pattern at coverage 0.11 ML and (c) 1 × 1 atc desorption process are: (d) 1 × 4 pattern after annealing at 520 ◦C (∼0.5 ML coverage), (e)√
× 7 pattern after annealing at 580 ◦C (<0.1 ML coverage).
tdtLsis[fSR(Isaat[s1psaois
sdtwimIbtpSIvsc
ig. 6. LEED patterns obtained at primary beam energy of 70 eV. (a) Atomically coverage 1 ML during adsorption. The In/Si(5 5 7) interfacial phases during thermal
3 × √3-R30◦ pattern after annealing at 560 ◦C (∼0.3 ML coverage) and (f) intense 7
he annealing temperature increases beyond 500 ◦C, In overlayeresorbs steadily for all RT and HT grown systems. For annealingemperature ≥550 ◦C, the coverage reduces to ∼0.3 ML and the 4 × 1EED pattern disappeared and after annealing the surface at 560 ◦C,harp LEED pattern corresponding to
√3 × √
3-R30◦ reconstructions appeared as shown in Fig. 6(e). The well known
√3 × √
3-R30◦
tructure has been reported earlier on planner Si(1 1 1) surface34,35] with a general agreement that In adatoms occupy the three-old hollow positions (at 0.3 ML coverage) above the second layeri atom i.e. T4 sites, to obtain substantial In/Si(1 1 1)-
√3 × √
3-30◦ interface relaxation. Since, the Si(5 5 7) surface contains a1 1 1) facet, thus, for relaxed In/Si(5 5 7)-
√3 × √
3-R30◦ interface,n adatoms (coverage 0.3 ML) occupy the energetically favorable T4ites on Si(5 5 7) surface. In-induced
√3 × √
3-R30◦ phase emergesfter annealing at 560 ◦C (with coverage ≤1/3 ML) which is in goodgreement with planner Si(1 1 1) surface where this reconstruc-ion appears after annealing at 550 ◦C with same residual coverage34]. Recently, Pb and Ag induced Si(5 5 7)-
√3 × √
3-R30◦ super-tructural phases has been reported with ∼0.3 ML coverage, whereD-structures show the metallic properties [30,36]. As the tem-erature increases above 570 ◦C (coverage ∼0.1 ML) a sharp 7 × 7uperstructural phase, as shown in Fig. 6(f), is observed whichlso suggest that the In induced reconstruction at low coverage isccurred on (1 1 1) facet of the Si(5 5 7) surface. On further anneal-ng the surface ∼600 ◦C, In atoms desorbs completely from theurface and the characteristic Si(5 5 7) 7 × 1 pattern is reappeared.
To establish the sequence of superstructures corresponding tourface coverage and temperature, a complete phase diagram iseduced and shown in Fig. 7 which gives a schematic picture ofhe sequence of order of In induced surface structures on Si(5 5 7)ith increasing In coverage (Fig. 7(A)) for adsorption and with
ncreasing substrate annealing temperature (Fig. 7(B) during ther-al desorption process. The solid lines in the phase diagram for
n adsorption (as shown in Fig. 7(A)) indicates the clear boundaryetween two phases, while the shaded regions (in Fig. 7(B)) showhe coexistence of more than one phase during the desorptionrocess. The adsorption of In changes the 7 × 1 reconstructedi(5 5 7) surface to 7 × 7 structure at initial coverage of ∼0.1 ML.
n adsorption on atomically clean Si(5 5 7)-7 × 1 surface faded theertical chains of spot between integral order spots and a 7 × 7uperstructural phase begin to appear which maintained in theoverage range 0.1–1.0 ML. On further adsorption, the fractional
Fig. 7. Phase diagram for adsorption (A) and desorption (B) route of In/Si(5 5 7)system.
order spots in 7 × 7 phase became weak and 7 × 7 phase convertedinto an ordered 1 × 1 phase for coverage 1.0 ML. On increasing thecoverage >1.0 ML, weak 1 × 1 phase is observed. Similar, In inducedphases has also been observed for the HT-300 ◦C and HT-400 ◦Cgrown In/Si(5 5 7) interfaces. The thermal desorption region startwith weak 1 × 1 phase at RT, which transformed into the ordered1 × 1 phase after annealing the In/Si(5 5 7) system at 300 ◦C. Thisphase will remain stable in the temperature range of 300–420 ◦C.After annealing the interface beyond 450 ◦C, In coverage reducedto below 1.0 ML and 4 × 1 phase is observed in the temperaturerange 450–550 ◦C. A narrow region of coexistence of 4 × 1 and1 × 1 phase is also observed for 420–450 ◦C temperature range.As the annealing temperature increases >550 ◦C, the In coveragereduced to less than 0.5 ML and In-induced
√3 × √
3-R30◦ phase isobserved. A narrow region of phase coexistence is also observed inbetween these two phases (4 × 1 and
√3 × √
3-R30◦) in the tem-perature range of 530–550 ◦C. An ordered 7 × 7 phase is observedon annealing the
√3 × √
3-R30◦ surface beyond 570 ◦C till 590 ◦Cwhich converted into characteristic 7 × 1 surface reconstruction ofSi(5 5 7) surface at temperature 600 ◦C.
4. Conclusions
The present study illustrates the evolution of In/Si(5 5 7) inter-face, with precise control over the flux and temperature, during In
adsorption at various substrate temperatures (RT, 300 ◦C, 400 ◦C,500 ◦C) where RT adsorption follows the layer-by-layer growthmode while Stranski–Krastanov and Volmer Weber growth modesare being followed for HT In adsorption. In subsequent thermal
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esorption study, the anomalous behavior of temperature inducedayering-to-clustering and vice versa rearrangements of In atomsn the Si(5 5 7) surface are observed for RT while during desorp-ion of HT adsorbed systems, no such anomalous rearrangementas been observed. In additional to this, all desorption processesre found to be similar, with subtle differences in bilayer desorp-ion temperature range and energy. The bilayer desorption energyor RT and HT-300 ◦C grown In/Si(5 5 7) system are found to be dif-erent, while monolayer desorption energy is comparable in bothhe cases. The commencement of desorption and the temperatureegime in which In completely desorbed from the Si(5 5 7) sur-ace is attributed to the strain relaxation at the interface. At highernnealing temperature, the lattice mismatch between In and Si iseduced (as thermal expansion coefficient are different) which leado the lowering of strain and assist in the layering of In clusters.he surface symmetry of interface during adsorption and thermalesorption is also monitored and several In induced superstruc-ural phases on Si(5 5 7) has been observed which are consolidatedn a complete phase diagram where
√3 × √
3-R30◦ and 4 × 1 onn/Si(5 5 7) interface has been reported for the first time, whereuasi 1D metallic chains are formed on In/Si(5 5 7)-4 × 1 and
n/Si(5 5 7)-√
3 × √3-R30◦ interfaces. These experiments using con-
entional techniques revealed interesting surface phenomena byontrolling kinetics and thermodynamics of the growth and coulde helpful for the understanding of growth kinetics of In atoms onarious high-index Si surfaces.
cknowledgements
The authors gratefully acknowledge Director, CSIR-NPL, Newelhi for their constant encouragement and support. One of theuthors (AKSC) is thankful to the CSIR-India for -JRF/SRF fellowship.his work is supported by Inorganic Solid State Lighting activity atPL under CSIR-TAPSUN project NWP-55.
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