Selenium-treatednlicroscopy

GaAs(O01)4 x3 surface studied by scanning tumding

Hidemi Shigekawa,Haruhiro Oigawa, Koji Miyake, Yoshiaki Aiso, and Yasuo Nannichi

Institute of Materials Science, University of Tsukuba, Tsukuba 305, Japan

Tomihiro Hashizume

and Toshio Sakurai

Institute for Materials Research, Tohoku University, Sendai 980, Japan(Received 1 December 1993; acceptedfor publication 18 May 1994)
An Se-passivatedGaAs(001)surfacewas found to be stabilized.by 2X3 reconstruction,previouslyreported as an intermediatestructure, under the condition of a low Se chemical potential. Orderedelliptical protrusions with -0.6-nm periodicity in the [110] direction were observedby scanningtunneling microscopy, the structure of which was in good agreementwith a double-layereddimer
model. A comprehensivemodel to explain the 2X3 structure together with the formation of aGa,Se,-like structure with ordered l/3 ML Ga vacancieswas proposed.

With the progressin the atomic control of fine structures,
modification of the surfaceand interface propertiesby adsorbates has attracted considerableattention in recent years. In
particular, passivation of compoundsemiconductorsurfacesby chalcogenatomshas beenthe focus of many experimental
and theoreticalstudiesbecauseof its beneficial effects on theelectronic propertiesof surfaces/interfaces.l-10
In spite of allthese efforts, the atomic structures of these technologicallyimportant surfacesremain to be clarified.
In most electron diffraction experiments on passivatedsurfaces(2x1) diffraction patternsare observed,indicating adoubling of the surface periodicity in one direction. Of thestructural models proposed for these surfaces, the dimer
modelP which appearsto explain most of the experimental
results, is the most widely supported.In this model, chalcogen atoms are adsorbedat bridge sites in the second layer
forming chalcogen dimers with the 2X1 structure. Firstprinciple study of the passivatedsurfacesalso supportedthe
possibility of such dimerization,5 and, very recently, Sepassivated GaAs(001) surfaces with nearly perfect 2X1
dimer structurewas observedby scanningtunneling microscopy (sTMJ1r However, on the other hand, the Se-treatedsurface has been considered to have a more complicatedstructure; Ga$e,-like structureswith ordered Ga vacancies
of l/4-1/3 ML.a In order to understand the passivation
mechanism,characterizationof the passivatedsurface structure on an atomic scale is urgently needed.Recently, an intermediate 2X3 structure was found to exist on the Setreated GaAs(OO1)surface, and reversible phase transition
was observedbetween the 2X3 and the 2X1 structuresduring heat treatment.6Change in the photoemission spectra
concerning the amount of Ga vacancieswas observedupon
the heat treatment. Therefore, from the standpoint that theGaaSes-likestructure was grown from the original 2X1
dimer structure through phase transition between the 2X1and the 2X3 structures,we studied the surface structuresof
the Se-passivatedGaAs(OOl)-2X3 surface by STM.
Study of the chalcogen-passivatedsurface is also interesting from the standpointof surfacereconstructionbasedon
dimer interaction. Recently, by state-of-the-arttotal energy
calculation,‘” attractive interaction between the As dimers

and relaxation of the As dimer block was shown to be essential to the stability of the As-rich GaAs(OO1)surface.On theother hand, adjacent chalcogen dimers of the passivatedGaAs(OO1)surfaceare expectedto interact repulsively due tothe excess electrons,one electron per dimer, and dimer reconstruction differing from that on the surface of As/
GaAs(OO1)is expected.” Stability of the 2X1 chalcogendimer structuresis a possible result of the repulsive interaction. Therefore,in addition to clarification of the passivation
mechanism,understandingthe surface stabilization mechanism of chalcogen-passivatedcompoundsemiconductorsurfaces is very intriguing from both fundamentaland practicalpoints of view. Among the chalcogen-passivatedGaAs(001)surfaces,the Se/GaAs(OOl)structure is the most interesting
becausethe atomic radius of Se is closest to that of As.13
GaAs(001)surfaces,oriented to within 0.5” of the [OOl]
direction and Si-doped at a dose of -lX10r8 cmm3,werepreparedby thermal cleaning. After observationof 4X2 reflection high-energy electron diffraction @HEED) pattern,selenium, to a thickness of 100-300 nm, was evaporated
onto the GaAs(OO1)surface at room temperature.For theobservationby STM, the sampleswere transferredfrom thepreparation chamber to the STM chamber through air, and .
were heat treatedunder RHEED observation.The basepressure of the chamberwas -4X10-rr Torr.
Generally, heat treatment is performed under Se fiux of-10V6 Torr, and a 2X1 diffraction pattern is reported.However, recently an intermediatephase with a 2X3 diffraction
pattern was observed at higher temperatures(3550 “C),
which suggeststhe importance of the Se-chemicalpotential
in controlling this surface reconstruction similar to thechange reported for As/GaAs(OOl) surfaces.12Therefore,
samples were flash heated and the backpressurewas kept
lower than -lX1O-1o Torr throughout the heat treatmentin
order to suppress the readsorption of desorbedSe atoms.
Thereby,a 2X3 RHEED pattern (Fig. l), previously reported
as an intermediate phase appearing at high temperature(2550 “C) was obtainedby lower temperatureheat treatment
around -400 to -500 “C and was stable even after thesampleswere cooled.13
Figure Z(a) shows an STM image of the Se-passivated0003-6951/94/65(5)/607/3/$6.00
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(4FIG. 1. The 2X3 RHEED patterns obtained for Se/GaAs(OOl): (a) [llO] (b)[iio].

(b)

05.0I0110.0nm(40.11.0I
2.0JnmFIG. 2. (a) STM image of Se/GaAs(OOl) 2X3 structure: V,=-2.0 V, 1,=20pA. (b) Magnified image of (a). (c) Cross sections along a-b and c-d in (a).
(d) Cross sect& along W, in (a).

surface taken after observationof the 2X3 FCHEEDpattern.
The samplebias was -2.0 eV. STM imaging was performedin the constant current mode (setting current: I,=20 PA).
The obtainedsurface is rather rough, but orderedarraysconsisting of elliptical protrusions can be observed in the flatarea.
Figure 2(b) shows a magnified image, including a step,of Fig. 2(a), and the crosssectionsalong a-b and c-d in Fig.
2(b) are shown in Fig. 2(c). The terracelabeledH in Fig. 2(a)is about -0.28 nm higher than the neighboring terrace labeled L. The distancebetweenthe arrays is -0.8 run and theaxes of the rows on both terracesare shifted by about -0.2(-0.6) mn as shown in Fig. 2(b), which agreeswith thedouble-layeredstructure of the dimer model; the elliptical
protrusionsin the arrays are the Se dimers bondedto the Gaunderlayer.
For the most part, the elliptical protrusionsare aligned inthe [llO] direction with -0.6nm periodic@ as shown inFigs. 2(b) and 2(c). In addition, some clusters with 0.4~nm
periodic@ exist in someparts as indicated by arrows in Fig.
2(b). Since reversible phase transition is observedbetween
the 2X3 and the 2X1 structuresduring heat treatment,’weassign the former protrusions with -0.6~nm periodicity to
the 2X3 structure and the latter to the 2X1 structureremaining even after the heat treatment.The 2X 1 dimer cluster WI,
which seemsto have enoughspaceon its side, should change
into the 2X3 structure and the existenceof this type of cluster may indicate an activation processfor dimer reconstruction from the 2X1 to the 2X3 structure. Figure 2(d) shows
the cross section along the 2X1 cluster W, in Fig. 2(b). As isshown in Fig. 2(d), the corrugationof the dimers in the 2X 1cluster is very small (-0.01 nmj comparedto that of the2X3 structure (-0.05 nm).
Since one chalcogenatom has six electrons,Se dimerization for the 2X1 structure will leave one excesselectron
for each Se dirner; the excess electrons occupy Se-Se orGa-Se antibonding states.This configuration rendersthe interaction between the adjacentdimers at the bridge sites repulsive. The 2X3 structure observedis symmetric along thedimer rows in contrast to the clustered dimer structure
formed by As dimers on the GaAs(OO1)surface,r?’which isthought to be causedby the repulsive interactionbetweentheSe dimers; attractive interaction is essential in the case ofAs-dimer reconstruction.
Since the periodicity of the protrusionsis -0.6 nm along
the row in the [llO] direction, two adjacentdimers must beincluded to form the 2X3 structure (1.2 run) in order to explain the observed 2X3 RHEED pattern; two alternative
kinds of dimers or some other structureswhich producethe
1.2-nm period must exist to form the threefold periodicity.
However, the reconstructed2X3 structureis not very different from the 2X1 structure becauseof the finding that the2X3 diffraction pattern transforms easily into the 2X1 pattern again when the substrateis cooled in Se atmosphere.6In
considerationof the result that the difference in the corrugation is not very clear amongthe dimers as shown in Fig. 2(c),we adopt the latter model in this letter; we introduce anotherstructure which producesthe 1.2-mn period.
A possiblestructure exhibiting 2X3 periodicity is shown

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Shigekawa et al.
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1st Se
3rd Asor Se
2nd Ga
(4(b)

W(d)

nm

FIG. 5. Cross section of an Se defect in a dimer row.

in Fig. 3. Here, Ga atoms form dimers in the second layer
following the Se-dimes desorption. Such double-layered
dimer structures with adsorbatesoften appear on semiconductor surfaces.r4The number of excess electrons can thus
be reduced; however, one electron still exists for every twodimers. Therefore, the excess electrons break the Ga-Sebonds on the other sides of Ga dimers, representedby dashedlines in Fig. 3(c), and the electronstransfer into the Ga dangling bond states created by the breaking of Ga-Se bonds.
Actually, Se dimers are buckled as shown in Fig. 4, suggesting the existence of such charge transfer in the dimerlayers.” According to the results obtained by photoelectron
spectroscopy,6phase transition from the 2X1 to the 2X3
structure makes 0.2-0.4 eV shifts of the Se 3d core leveltoward higher binding energies,which agreeswell with thecharge transfer properties in our model. The -0.6~run periodicity in the STM images is formed by Se dimers pulled
toward the Ga dimer as shown in Fig. 3(c). Se dimers in the2X3 regions seem to be slightly higher than those in the 2X1
regions, as shown in Fig. 2(d). Since Ga atoms betweenthe
Ga dimers thus face outward, ordered Ga vacancies of l/3
ML at maximum can be introducedeasily, the value of which
can be comparedto the value obtainedby the analysesof thephotoemissionspectra (l/4 ML) and the results of electron
microscopy (1/3).8
Since the 2X1 clusters still remained after the samples
were flash heated, the phase transition possibly remains atthe initial step, as shown in Fig. 3(c). Actually, corrugations
at the defects are -0.15 mrr, as shown in Fig. 5, which isclose to the height of one GaAs(OO1)layer. When Ga vacancies are created,As atoms in the third layers can be replaced
by Se atoms upon heat treatmentas is shown in Fig. 3(d). In01.0+ [11’]FIG. 3. A model for structural change from 2X 1 (a) via Se-dimer desorptiont.b) to 2X3 (c), (d).

0.52.000.5nm--c [TIO]

FIG. 4. Cross section of a Se dimer in the 2X3 region.

such a case, the structure will remain intact even after thereadsorptionof Se dimers, and the structure becomescloser
to the Ga vacancy zincblende structural model with ordered
Ga vacancies.8’12
In order to characterizethe observedpassivated structures,theoretical calculation is necessary.
In summary, a new 2X3 structure was obtained under
the condition of a low chemical potential, the structure ofwhich observedby means of STM, was in good agreementwith the double-layereddimer model proposedin this letter.
Formation of a Ga,Sea-like structure with ordered l/3-ML
Ga vacancies in the second layer could also be explained
well by the model.
We wish to thank Dr. T. Ohno of NTT LSI Laboratory
for fruitful discussions.This work was supportedin part by aGrant-in-Aid for Scientific Research from the Ministry ofEducation, Science and Culture of Japan.TEPCO Research
Foundation and the ResearchFoundationfor Materials Science are also acknowledged.

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