- Email: [email protected]
In situ monitoring of the c( 4 × 4) to the 2 × 4 surface phase transformation on GaAs(001) by grazing incidence X-ray diffraction
:,;;.:.,: .... ;
..
‘.‘.‘....,:
‘b-face science : .,,. (.,:: .f ?.::.:::.: ..,,.. ,. ,:... ELSEVIER
Surface Science 320 (1994) 252-258
In situ monitoring of the c( 4 X 4) to the 2 X 4 surface phase transformation on GaAs( 001) by grazing incidence X-ray diffraction V.H. Etgens a,b, M. Sauvage-Simkin aYb,*, R. Pinchaux aYc,J. Massies agd, N. Jedrecy a,b,A. Waldhauer aYe,N. Greiser a aLURE, CNRS-MEN-CEA, Bat. 209d, F-91405 Orsay, France b Laboratoire de Mintralogie-Cristallographie, 4 Place Jussieu, F-75252 Paris Cedex 05, France ’ Uniuersitt! P. et M. Curie, 4 Place Jussiey F-75252 Paris Cedex 05, France ’ LPSES-CNRS, rue B. Grkgory, F-06560 Valbonne, France e DRECAM-SRSIM, CEA, F-91191 Gif-sur-Yvette, France Received 30 April 1994; accepted for publication 30 July 1994
Abstract The transition between the arsenic saturated c(4 X 4) and the As stabilised
2 X 4 reconstructed
GaAs(001)
surfaces has
been followed in situ on a UHV grazing incidence X-ray diffractometer stage. X-ray diffraction lines specific of either structure have been recorded as a function of temperature. The intensity and lineshape evolution has enabled to propose a model for the transformation involving a homogeneous disordering of the c(4 X 4) surface through random As desorption followed by nucleation and growth of 2 X 4 domains. Under UHV conditions, the irreversible transition is observed over a temperature interval ranging from 330°C to 380°C.
1. Introduction Compound semiconductors have the specific feature of presenting a variety of surface structures controlled by the stoichiometry. In the particular case of GaAs(OOl), the sequence of phases has been thoroughly investigated by the surface characterisation methods coupled with molecular beam epitaxy (MBE) [l]. The symmetries of the various phases have been identified through their electron diffrac-
* Corresponding author. [email protected].
0039-6028/94/$07.00
Fax:
+33
1 6446
4148;
E-mail:
0 1994 Elsevier Science B.V. All rights reserved
SSDI 0039-6028(94)00524-9
tion patterns whereas the trends in the variation of the arsenic coverage were derived from Auger and photoelectron spectroscopies [2]. However, it is only recently that the atomic arrangement at the surface could be established for some of these phases by grazing incidence X-ray diffraction (GIXD) [3] and (or) scanning tunnelling microscopy (STM) [4,_5]. In addition, an estimate of the surface composition changes in terms of the As/Ga ratio in the topmost layers has been obtained from calibrated Ga incorporation monitored by reflection high energy electron diffraction (RHEED) intensity [6]. We present here the first in situ structural assessment of the transition between the c(4 X 4) and the
2 X 4 phases. Since structures with different 2D sieves diffract in separate reciprocal space locations, X-ray di~a~on is a unique tool to monitor simultaneously the decay of the ~(4 X 4) order and the onset of the 2 X 4 arrangement. Moreover, by following the thermal behaviour of fractional and integer diffraction peaks, a layer-by-layer propagation of the phase transformation could be established: two steps were identified, an order-disorder transition from the c(4 X 4) to a 1 X 1 state completed at 36o”C, followed by nucleation and growth of a new 2 X 4 phase. In the UHV conditions of the experiment the ~ansfo~ation is not reversible, in order to observe a true phase tr~sition from the the~od~~i~ point of view an arsenic pressure should be maintained over the sample.
2. Experimental Semi-insulating GaAs wafers &nnitomo, qudlity “ready for epitaxy”) were in~oduced in the molecular beam epitaxy CMBE) chamber coupled to the UHV compatible diffractometer of the D25 surface diffraction beam line at the LURE-DC1 sync~o~on radiation facility l&say, France) 171.A few thousand A thick C&As Layer was then grown in standard As stabilised conditions. When brought down to room temperature, the sample showed the expected cf4 X 4) reconstruction and was transferred on to the diffractometer stage. All the diffraction peaks presented in this work are transverse d, scans collected at the critical incidence angle for total reflexion:
@ Chemisorbed
Aa
6 A8 bulk termination layer 0 Ga in the Player
cyc= 0.28” for GaAs at A = 0.1488 rim.
Fig. 1. Structural modeis for the Gaas(OO1~44 X 4) reconstructed surface: (a) three-dimer &sters; ib) two-dimer clusters (afterRef. f31).
A set of ~ac~on lines typical of the ~$4 X 4) structure was first collected with the sample kept at 250°C. These data scale ~t~facto~ly with those recorded in a previous work [3], confirming the description of the surface in terms of a mixing of two ordered structures (chemisorbed clusters of three or two As dinners) (Figs. la and lb). STM experiments [4] have confirmed the presence of broad areas with the three-dirtier structure but did not confirm the two-dimer model. The temperature was then increased by steps of about 5 K. At each step and after stabilisation of the the~~uple read-out, several reflection profiles
pertaining to either the 44 X 4) or the 2 X 4 reciprocal lattice were recorded at regular intervals in order to detect a possible influence of the annealing time: no time dependence was observed for the decay of the c(4 X 4) order whereas a clear effect was detected on the onset of the 2 X 4 ordering. After completion of the phase transformation, at 38O”C, the sample was brought down to 200°C and a set of 2 X 4 reflections was collected which equally scales with previous data f8]. Direct STM observations of the 2 X 4 (or c2 X 8) phase [4,5] support a
254
V.H. Elgens et af./Surface Science320 11994)2.52-258
model with one missing As dimer every fourth row, already proposed on the basis of RHEED data and energy minimisation calculations [9] (Fig. 2). Another model involving two missing dimers in the As top layer, associated to vacancies and dimers in deeper layers was also shown to be energetically stable [lo], however, as mentioned earlier [8], there is still an inconsistency between these models and the surface diffraction data which could not be lifted by introducing subsurface layer displacements or phase mixing.
3. Results Ideally, the phase transformation should be followed by recording the variation of reflection lines belonging to either structure as a function of temperature. If one uses the usual 1 X I surface cell with unit vectors: a, = q2z[iio], a2= a/2[1107,
a3 = ~[ooif,
referred to the fee lattice, with the [l?O] direction parallel to the dangling bonds of the arsenic bulk top layer, fourth-order reflections (K/4, k//4, 0 with h’ and k’ both odd are specific of the c(4 X 4) phase whereas reflections with h’ multiple of 4 and k’ odd arise from 2 X 4 reconstructed areas. Integer orders
@ Dimerized As C
Ga Bulk
l
As
termination tayer
in the
third layer
Fig. 2. Possible model for the GaAs(OO1) 2X4 reconstructed surface (after Ref. [9]).
140120
-
100
-
60
-
604020
-
Fig. 3. Variation of the (0, 3/2) reflection peak with temperature. (a) 250°C; (b) 32OT; (c) 330°C; (d) 340°C; (e) 350°C; (f) 360°C. The axes labels are referred to curve f, the other curves are arbitrarily shifted for the sake of clarity.
and half-order lines are common to both structures, however, reflection (0, 3/2, I> which is the strongest in the ($4 X 4) phase is v~ishin~y small in the 2 X 4 structure. Consequently, the measurements have been focused on the two strongest lines of either phase, namely the (0, 3/2, Z) for the ~$4 X 4) and (2, 5/4, I) for the 2 X 4. Due to the grazing incidence geometry, the index 1 was kept constant and extremely low (I = 0.03), it will be omitted in the following where a two-index surface reciprocal cell notation will be used for both fractional and integer orders. The evolution of integer orders, resulting from the interference between the surface and bulk diffracted ampli~des, has been followed in the same temperature range and is discussed in a separate section. Figs. 3 and 4 display raw diffraction data in a pseudo-3D representation. It appears clearly that the c(4 X 4) reconstruction vanishes before a 2 X 4 order starts to develop. A more quantitative analysis is performed through lineshape fitting. Between 250°C and 35O”C, the integrated intensity of line (0, 3/2) is constantly decreasing whereas the linewidth remains constant, which can be interpreted as a progressive disordering induced by random desorption of As dimers in the chemisorbed layer (as As, molecule$, within domains of constant size S (about 140 A) given by the AQ transverse width of the reflexion peaks through the formula S = 2?r/AQ (Fig. 5). An
255
V.H.Etgensetal./SurfaceScience320(1994) 252-258
400
~160
i; t
(2,5/4)
r
2x4
z
5 n120 1) E : ’ 80 E t aI 5 40 -
300
200
100
fff$
TI L 3
l
2x4
.
c(4x4)
-
f ?
-
i
0 50
-30 .* \
50
-10 *;;I,
70
90
(Xl&
Fig. 4. Variation of the (2, S/4) reflection: (a) 360°C; (b) 365°C; (c) 370°C; (d) 375’C; (e) 38O’C; (f) 380°C. Labels refers to curve a.
appealing alternate interpretation would be a transition from the initially mixed surface with three and two-dimer cluster structural basis (Figs. la and lb) towards a single phase with a lower As coverage (two-dimer clusters only, Fig. lb). However, although we did not collect the (3/2, 0) reflection, expected to be very weak in the two-dimer structure, which would have been the optimal flag for such a transition, the perfect scaling of the several c(4 X 4) fractional peaks measured at different temperatures did not support the assumption of a structural change. In addition, in this temperature range, the intensity of the reflection lines were not time-dependent, which would more likely be the case for a rearrangement towards a different order unless a fully co-operative dimer desorption was taking place. Below 350°C the (2, 5/4) reflection was not detectable. When the temperature reached 360°C a very broad peak appeared (Fig. 4). The integrated intensities normalised to the maximum values and the coherent domain size deduced from the peak width are plotted in Fig. 5 as a function of temperature for the reflexions of interest. The data can be interpreted in terms of nucleation and growth of 2 X 4 ordered domains covering progressively the whole surface which explains the increase of the integrated intensity, although the latter is probably underestimated at the beginning due to a too small scanning range. The reduction of linewidth corresponds to a coherent domain size growing from 30 to
;1 0 " 0.8 P iO.6 0
.Z
.!0.4 zN jo.2 G =
--c-(0
312)
---0;.-(1,2)
0 ! 240
260
280
300 320 T PC)
340
360
380
Fig. 5. Lower panel: integrated intensity normalised to the maximum values for the reflexions (0,3/2) 0itll circles), (1,2) (empty triangles) and (2,5/4) (diamonds) as a function of temperature. The lines are only guides for the eye. Upper panel: coherent domain size deduced from the linewidth of reflexion (0,3/2) in the c(4 X 4) phase and reflexion (2, S/4) in the 2 X 4 phase. The two values at 380°C correspond to the annealing treatment.
160 A. Contrary to the c(4 X 4) case, a significant time dependence was observed for the integrated intensity and linewidth (see Table 1). This behaviour is compatible with an ordering process controlled by
Table 1 Effect of the annealing time at 380°C on the reflection (2,5/4) belonging to the 2 X 4 surface phase Annealing
time
peak
Integrated intensity
FWHM
Domain size
(h)
(au)
(deg)
(A)
0.0 1.3 11.0
115 114 123
1.05 0.9 0.6
93 108 160
V.H. E&ens et al. /Surface
256
Science 320 (1994) 252-258
600
a
500 400 300 200 100 0
-1
-2
-3
0
1
angle
2
3
surface diffusion which can be activated even in this low-temperature range. It should be remarked that the peaks presented in Fig. 4, except peak f, were collected at the same stage of each temperature step. The identical size of the quasi-isotropic coherent domains, before and after the transformation can be interpreted as controlled by the initial width of the terra$es on the substrate surface. This average size of 160 A in the high-temperature phase corresponds to an integer number of (m X 4) unit cells in agreement with the STM data on 2 X 4 islands nucleation during epitaxial growth [ll].
(“)
160
b
3.1. Surface versus subsurface transformation kinetics
(1,2) 2x4
140 120
Separating the m X n reconstructed surface layer behaviour from that of the underlying bulk is made possible by the analysis of bulk forbidden integer orders (h and k of different parity in the present case) [12]. The total structure factor reads
100 80 60 40
k‘ -4
I -2
I
0
2
angle
450
_
I
4
(“)
,
I R
350
(2,1) 2x6
where the phase 4 accounts for a possible different choice of the origin for the evaluation of the surface and bulk contributions. For the GaAs(001) surface the presence of a strong bulk contribution in either Fhk or Fkh is a signature of the As or Ga nature of the bulk top layer according to the following rule [8]:
. Y
300
As-terminated
Y ;: rr)
250
F:” = i( fAs + ei*k fG,,).
In that case a strong bulk component for k even
m
200
bulk
is present
in
Fhk
v
Ga-terminated
100
and then a strong bulk component appears in Fhk for h even, as long as the surface axes are chosen as stated previously. In the present study, the integer orders (1, 2) and (2, 1) have been recorded during the phase transformation between the c(4 X 4) and the 2 X 4 structures and also after a flash annealing up to 600°C leading to a 2 X 6 phase. For the c(4 X 4) structure, with three-dimer clusters, calculations show that the surface contribution is about one third of the total structure factor in the
501 -2
-1
1 angli
2
(“)
Fig. 6. Integer orders in successive surface phases: (a) 32OOC (1, 2) reflection in the c(4 X 4) structure, experimental data (dots) and fitted curve with two components; inset: variation with temperature illustrated with the fitted curves; full line 320°C dotted line 355”C, full line 360°C. (b) (1, 2) line at 380°C in the 2X4 phase, experimental data (dots) and fitted curve showing a splitting not understood yet. (c) (2, 1) line in the (2 X 6) surface with a sharp bulk component.
bulk
F:l’ = i( fGa + eimh fAs)
150
V.H. Etgens et al. /Sur,Cace Science 320 (1994) 252-258
(1, 2) line which is very strong, an evidence for an As bulk top layer below the dimers. The experimental rocking curves show clearly two components of unequal widths (Fig. 6a). The broader curve, fitted with a Lorentzian shape leadsot a surface correlation length (2/AQ> equal to 50 A related to the terrace distribution [13] and fully compatible with the 150 w average coherent domain size derived from the fractional peak width. During the thermal treatment, the integrated intensity decreases up to the transformation temperature of 360°C while the lineshape remains constant thus confirming the assumption of a structure factor reduction within an unchanged domain size (inset in Fig. 6a). However, the thermal behaviour is markedly different from what is observed with the fractional peak (Fig. 5) and supports the interpretation presented in the previous section. Between 300°C and 350°C changes occur only in the chemisorbed layer where surface dimers are randomly desorbed leading to a more pronounced decay for the fractional than for the integer orders, where the volume contribution is still strong; above 350°C the first As bulk layer equally starts to be disrupted and the whole c(4 X 4) diffraction pattern vanishes. In the same temperature range, the (2, 1) reflection is extremely weak. Above 360°C and up to 400°C in the 2 X 4 phase temperature domain, both (1, 2) and (2, 1) lines show an increase of integrated intensity with complex lineshapes for which there is no clear interpretation (Fig. 6b). The in t e rf erence effect between surface and bulk which can be calculated with the different models does not allow a straightforward assignment of the bulk top layer. After the flash annealing at 600°C the strong sharp component is again visible, this time in the (2, 1) peak (Fig. 5~). For the 2 X 6 structure, X-ray diffraction data are then in favour of a Ga-terminated bulk below the reconstructed layer which is not contradictory with the STM data [4]. It should be noted that X-ray diffraction, performed here far from an absorption edge cannot tell As from Ga atoms. The present work is thus sensitive to the back-bond sequence from the top surface towards the inner bulk but would not reveal substitutional gallium in the As terminal plane, as is found in a recent MEIS (medium energy ion scattering) experiment [ 141.
257
4. Conclusion Although we are not dealing here with a true phase transition since the system is not in equilibrium with an As pressure, our observations compare satisfactorily with previous results obtained by RHEED under a finite As flux except for an overall downward shift of the temperature scale: the reversible transition between the two phases takes place at 400°C under an As flux of about 10” As, molecules/cm*/s (instead of 360°C in UHV). The analysis of both X-ray intensity and lineshape for fractional and integer orders has enabled us to propose a mechanism for the phase transformation: the c(4 i< 4) structure is destroyed by random disordering of the As dimer layer after which nucleation and growth of the 2 X 4 order takes place. A similar interpretation in terms of random dimer breaking has been proposed in the case of the 2 X 1 to 1 X 1 transition on the Ge(OO1) surface at 955 K [15], however instead of desorbing, the Ge atoms migrate from a dimer configuration to an adatom-vacancy pair thus increasing the surface roughness. A desorption-induced continuous phase transformation has indeed been observed on the Ge(ll1) 1 X 1 surface at 1070 K, evidenced by mass spectrometer measurements and resulting in a smooth decrease of the integer orders intensities with a constant FWHM
Ml. Acknowledgement One of us (V.H.E.) gratefully acknowledges the financial support from the Brazilian CAPES during the completion of this work.
References [I] J. Massies, P. Etienne, F. Dezaly and N.T. Linh, Surf. Sci. 99 (1980) 121. [2] P.K. Larsen, J.H. Neave, J.F. van der ken, P.J. Dobson B.A. Joyce, Phys. Rev. B 27 (1983) 4770. [3] M. Sauvage-Sin&in, R. Pinchaux, J. Massies, P. Claverie, Jedrecy, J. Bonnet and I.K. Robinson, Phys. Rev. Lett. (1989) 563. [4] M.D. Pashley, K.W. Haberern W. Friday, J.M. Woodhall P. Kirshner, Phys. Rev. Lett. 60 (1988) 2176.
and N. 62 and
2.58
KH. Etgens et al. /Surface Science 320 ~199412.52-258
[5] D.K. Biegelsen, R.D. Bringans, J.E. Northrup and L.E. Swartz, Phys. Rev. B 41 (1990) 5701. [6] C. Deparis and J. Massies, J. Cryst. Growth 108 (1991) 157. [7] P. Claverie, J. Ma&es, R. Pinchaux, M. Sauvage-Simkin, J. Frouin, J. Bonnet and N. Jedrecy, Rev. Sci. Instrum. 60 (1989) 2369. [8] M. Sauvage-Simkin, R. Pinchaux, J. Massies, P. Claverie, J. Bonnet, N. Jedrecy and LK. Robinson, Surf. Sci. 211 212 (1989139. [9] P.K. Larsen and D.J. Chadi, Phys. Rev. B 37 (1988) 8282. IO] D.J. Chadi, J. Vat. Sci. Technol. A 5 (1987) 834. :ll] M.D. PashIey, K.W. Haberem and J.M. Gaines, AppI. Phys. Lett. 58 (1991) 406.
[12] LK. Robinson, in: Handbook on Synchrotron Radiation, Vol. 3, Eds. D.E. Moncton and G.S. Brown (North-Holland, Amsterdam, 1991). [13] E. Vlieg, J.F. van der Veen, S.J. Gurman, C. Norris and J.E. Macdonald, Surf. Sci. 210 (1989) 301. [141 J. Falta, R.M. Tromp, M. Cope], G.D. Pettit and P.D. Kirchner, Phys. Rev. Lett. 69 (1992) 3068. [15] A.D. Johnson, C. Norris, J.W.M. Frenken, H.S. Derbyshire, J.E. Macdonald, R.G. van Silfhout and J.F. van der Veen, Daresbury Laboratory preprint DL/SCI/P692E. I161 A. Mak, K.W. Evans-Lutterodt, K. Blum, D.Y. Noh, J.D. Brock, G.A. Held and R.J. Birgeneau, Phys. Rev. Lett. 66 11991) 2002.
..
‘.‘.‘....,:
‘b-face science : .,,. (.,:: .f ?.::.:::.: ..,,.. ,. ,:... ELSEVIER
Surface Science 320 (1994) 252-258
In situ monitoring of the c( 4 X 4) to the 2 X 4 surface phase transformation on GaAs( 001) by grazing incidence X-ray diffraction V.H. Etgens a,b, M. Sauvage-Simkin aYb,*, R. Pinchaux aYc,J. Massies agd, N. Jedrecy a,b,A. Waldhauer aYe,N. Greiser a aLURE, CNRS-MEN-CEA, Bat. 209d, F-91405 Orsay, France b Laboratoire de Mintralogie-Cristallographie, 4 Place Jussieu, F-75252 Paris Cedex 05, France ’ Uniuersitt! P. et M. Curie, 4 Place Jussiey F-75252 Paris Cedex 05, France ’ LPSES-CNRS, rue B. Grkgory, F-06560 Valbonne, France e DRECAM-SRSIM, CEA, F-91191 Gif-sur-Yvette, France Received 30 April 1994; accepted for publication 30 July 1994
Abstract The transition between the arsenic saturated c(4 X 4) and the As stabilised
2 X 4 reconstructed
GaAs(001)
surfaces has
been followed in situ on a UHV grazing incidence X-ray diffractometer stage. X-ray diffraction lines specific of either structure have been recorded as a function of temperature. The intensity and lineshape evolution has enabled to propose a model for the transformation involving a homogeneous disordering of the c(4 X 4) surface through random As desorption followed by nucleation and growth of 2 X 4 domains. Under UHV conditions, the irreversible transition is observed over a temperature interval ranging from 330°C to 380°C.
1. Introduction Compound semiconductors have the specific feature of presenting a variety of surface structures controlled by the stoichiometry. In the particular case of GaAs(OOl), the sequence of phases has been thoroughly investigated by the surface characterisation methods coupled with molecular beam epitaxy (MBE) [l]. The symmetries of the various phases have been identified through their electron diffrac-
* Corresponding author. [email protected].
0039-6028/94/$07.00
Fax:
+33
1 6446
4148;
E-mail:
0 1994 Elsevier Science B.V. All rights reserved
SSDI 0039-6028(94)00524-9
tion patterns whereas the trends in the variation of the arsenic coverage were derived from Auger and photoelectron spectroscopies [2]. However, it is only recently that the atomic arrangement at the surface could be established for some of these phases by grazing incidence X-ray diffraction (GIXD) [3] and (or) scanning tunnelling microscopy (STM) [4,_5]. In addition, an estimate of the surface composition changes in terms of the As/Ga ratio in the topmost layers has been obtained from calibrated Ga incorporation monitored by reflection high energy electron diffraction (RHEED) intensity [6]. We present here the first in situ structural assessment of the transition between the c(4 X 4) and the
2 X 4 phases. Since structures with different 2D sieves diffract in separate reciprocal space locations, X-ray di~a~on is a unique tool to monitor simultaneously the decay of the ~(4 X 4) order and the onset of the 2 X 4 arrangement. Moreover, by following the thermal behaviour of fractional and integer diffraction peaks, a layer-by-layer propagation of the phase transformation could be established: two steps were identified, an order-disorder transition from the c(4 X 4) to a 1 X 1 state completed at 36o”C, followed by nucleation and growth of a new 2 X 4 phase. In the UHV conditions of the experiment the ~ansfo~ation is not reversible, in order to observe a true phase tr~sition from the the~od~~i~ point of view an arsenic pressure should be maintained over the sample.
2. Experimental Semi-insulating GaAs wafers &nnitomo, qudlity “ready for epitaxy”) were in~oduced in the molecular beam epitaxy CMBE) chamber coupled to the UHV compatible diffractometer of the D25 surface diffraction beam line at the LURE-DC1 sync~o~on radiation facility l&say, France) 171.A few thousand A thick C&As Layer was then grown in standard As stabilised conditions. When brought down to room temperature, the sample showed the expected cf4 X 4) reconstruction and was transferred on to the diffractometer stage. All the diffraction peaks presented in this work are transverse d, scans collected at the critical incidence angle for total reflexion:
@ Chemisorbed
Aa
6 A8 bulk termination layer 0 Ga in the Player
cyc= 0.28” for GaAs at A = 0.1488 rim.
Fig. 1. Structural modeis for the Gaas(OO1~44 X 4) reconstructed surface: (a) three-dimer &sters; ib) two-dimer clusters (afterRef. f31).
A set of ~ac~on lines typical of the ~$4 X 4) structure was first collected with the sample kept at 250°C. These data scale ~t~facto~ly with those recorded in a previous work [3], confirming the description of the surface in terms of a mixing of two ordered structures (chemisorbed clusters of three or two As dinners) (Figs. la and lb). STM experiments [4] have confirmed the presence of broad areas with the three-dirtier structure but did not confirm the two-dimer model. The temperature was then increased by steps of about 5 K. At each step and after stabilisation of the the~~uple read-out, several reflection profiles
pertaining to either the 44 X 4) or the 2 X 4 reciprocal lattice were recorded at regular intervals in order to detect a possible influence of the annealing time: no time dependence was observed for the decay of the c(4 X 4) order whereas a clear effect was detected on the onset of the 2 X 4 ordering. After completion of the phase transformation, at 38O”C, the sample was brought down to 200°C and a set of 2 X 4 reflections was collected which equally scales with previous data f8]. Direct STM observations of the 2 X 4 (or c2 X 8) phase [4,5] support a
254
V.H. Elgens et af./Surface Science320 11994)2.52-258
model with one missing As dimer every fourth row, already proposed on the basis of RHEED data and energy minimisation calculations [9] (Fig. 2). Another model involving two missing dimers in the As top layer, associated to vacancies and dimers in deeper layers was also shown to be energetically stable [lo], however, as mentioned earlier [8], there is still an inconsistency between these models and the surface diffraction data which could not be lifted by introducing subsurface layer displacements or phase mixing.
3. Results Ideally, the phase transformation should be followed by recording the variation of reflection lines belonging to either structure as a function of temperature. If one uses the usual 1 X I surface cell with unit vectors: a, = q2z[iio], a2= a/2[1107,
a3 = ~[ooif,
referred to the fee lattice, with the [l?O] direction parallel to the dangling bonds of the arsenic bulk top layer, fourth-order reflections (K/4, k//4, 0 with h’ and k’ both odd are specific of the c(4 X 4) phase whereas reflections with h’ multiple of 4 and k’ odd arise from 2 X 4 reconstructed areas. Integer orders
@ Dimerized As C
Ga Bulk
l
As
termination tayer
in the
third layer
Fig. 2. Possible model for the GaAs(OO1) 2X4 reconstructed surface (after Ref. [9]).
140120
-
100
-
60
-
604020
-
Fig. 3. Variation of the (0, 3/2) reflection peak with temperature. (a) 250°C; (b) 32OT; (c) 330°C; (d) 340°C; (e) 350°C; (f) 360°C. The axes labels are referred to curve f, the other curves are arbitrarily shifted for the sake of clarity.
and half-order lines are common to both structures, however, reflection (0, 3/2, I> which is the strongest in the ($4 X 4) phase is v~ishin~y small in the 2 X 4 structure. Consequently, the measurements have been focused on the two strongest lines of either phase, namely the (0, 3/2, Z) for the ~$4 X 4) and (2, 5/4, I) for the 2 X 4. Due to the grazing incidence geometry, the index 1 was kept constant and extremely low (I = 0.03), it will be omitted in the following where a two-index surface reciprocal cell notation will be used for both fractional and integer orders. The evolution of integer orders, resulting from the interference between the surface and bulk diffracted ampli~des, has been followed in the same temperature range and is discussed in a separate section. Figs. 3 and 4 display raw diffraction data in a pseudo-3D representation. It appears clearly that the c(4 X 4) reconstruction vanishes before a 2 X 4 order starts to develop. A more quantitative analysis is performed through lineshape fitting. Between 250°C and 35O”C, the integrated intensity of line (0, 3/2) is constantly decreasing whereas the linewidth remains constant, which can be interpreted as a progressive disordering induced by random desorption of As dimers in the chemisorbed layer (as As, molecule$, within domains of constant size S (about 140 A) given by the AQ transverse width of the reflexion peaks through the formula S = 2?r/AQ (Fig. 5). An
255
V.H.Etgensetal./SurfaceScience320(1994) 252-258
400
~160
i; t
(2,5/4)
r
2x4
z
5 n120 1) E : ’ 80 E t aI 5 40 -
300
200
100
fff$
TI L 3
l
2x4
.
c(4x4)
-
f ?
-
i
0 50
-30 .* \
50
-10 *;;I,
70
90
(Xl&
Fig. 4. Variation of the (2, S/4) reflection: (a) 360°C; (b) 365°C; (c) 370°C; (d) 375’C; (e) 38O’C; (f) 380°C. Labels refers to curve a.
appealing alternate interpretation would be a transition from the initially mixed surface with three and two-dimer cluster structural basis (Figs. la and lb) towards a single phase with a lower As coverage (two-dimer clusters only, Fig. lb). However, although we did not collect the (3/2, 0) reflection, expected to be very weak in the two-dimer structure, which would have been the optimal flag for such a transition, the perfect scaling of the several c(4 X 4) fractional peaks measured at different temperatures did not support the assumption of a structural change. In addition, in this temperature range, the intensity of the reflection lines were not time-dependent, which would more likely be the case for a rearrangement towards a different order unless a fully co-operative dimer desorption was taking place. Below 350°C the (2, 5/4) reflection was not detectable. When the temperature reached 360°C a very broad peak appeared (Fig. 4). The integrated intensities normalised to the maximum values and the coherent domain size deduced from the peak width are plotted in Fig. 5 as a function of temperature for the reflexions of interest. The data can be interpreted in terms of nucleation and growth of 2 X 4 ordered domains covering progressively the whole surface which explains the increase of the integrated intensity, although the latter is probably underestimated at the beginning due to a too small scanning range. The reduction of linewidth corresponds to a coherent domain size growing from 30 to
;1 0 " 0.8 P iO.6 0
.Z
.!0.4 zN jo.2 G =
--c-(0
312)
---0;.-(1,2)
0 ! 240
260
280
300 320 T PC)
340
360
380
Fig. 5. Lower panel: integrated intensity normalised to the maximum values for the reflexions (0,3/2) 0itll circles), (1,2) (empty triangles) and (2,5/4) (diamonds) as a function of temperature. The lines are only guides for the eye. Upper panel: coherent domain size deduced from the linewidth of reflexion (0,3/2) in the c(4 X 4) phase and reflexion (2, S/4) in the 2 X 4 phase. The two values at 380°C correspond to the annealing treatment.
160 A. Contrary to the c(4 X 4) case, a significant time dependence was observed for the integrated intensity and linewidth (see Table 1). This behaviour is compatible with an ordering process controlled by
Table 1 Effect of the annealing time at 380°C on the reflection (2,5/4) belonging to the 2 X 4 surface phase Annealing
time
peak
Integrated intensity
FWHM
Domain size
(h)
(au)
(deg)
(A)
0.0 1.3 11.0
115 114 123
1.05 0.9 0.6
93 108 160
V.H. E&ens et al. /Surface
256
Science 320 (1994) 252-258
600
a
500 400 300 200 100 0
-1
-2
-3
0
1
angle
2
3
surface diffusion which can be activated even in this low-temperature range. It should be remarked that the peaks presented in Fig. 4, except peak f, were collected at the same stage of each temperature step. The identical size of the quasi-isotropic coherent domains, before and after the transformation can be interpreted as controlled by the initial width of the terra$es on the substrate surface. This average size of 160 A in the high-temperature phase corresponds to an integer number of (m X 4) unit cells in agreement with the STM data on 2 X 4 islands nucleation during epitaxial growth [ll].
(“)
160
b
3.1. Surface versus subsurface transformation kinetics
(1,2) 2x4
140 120
Separating the m X n reconstructed surface layer behaviour from that of the underlying bulk is made possible by the analysis of bulk forbidden integer orders (h and k of different parity in the present case) [12]. The total structure factor reads
100 80 60 40
k‘ -4
I -2
I
0
2
angle
450
_
I
4
(“)
,
I R
350
(2,1) 2x6
where the phase 4 accounts for a possible different choice of the origin for the evaluation of the surface and bulk contributions. For the GaAs(001) surface the presence of a strong bulk contribution in either Fhk or Fkh is a signature of the As or Ga nature of the bulk top layer according to the following rule [8]:
. Y
300
As-terminated
Y ;: rr)
250
F:” = i( fAs + ei*k fG,,).
In that case a strong bulk component for k even
m
200
bulk
is present
in
Fhk
v
Ga-terminated
100
and then a strong bulk component appears in Fhk for h even, as long as the surface axes are chosen as stated previously. In the present study, the integer orders (1, 2) and (2, 1) have been recorded during the phase transformation between the c(4 X 4) and the 2 X 4 structures and also after a flash annealing up to 600°C leading to a 2 X 6 phase. For the c(4 X 4) structure, with three-dimer clusters, calculations show that the surface contribution is about one third of the total structure factor in the
501 -2
-1
1 angli
2
(“)
Fig. 6. Integer orders in successive surface phases: (a) 32OOC (1, 2) reflection in the c(4 X 4) structure, experimental data (dots) and fitted curve with two components; inset: variation with temperature illustrated with the fitted curves; full line 320°C dotted line 355”C, full line 360°C. (b) (1, 2) line at 380°C in the 2X4 phase, experimental data (dots) and fitted curve showing a splitting not understood yet. (c) (2, 1) line in the (2 X 6) surface with a sharp bulk component.
bulk
F:l’ = i( fGa + eimh fAs)
150
V.H. Etgens et al. /Sur,Cace Science 320 (1994) 252-258
(1, 2) line which is very strong, an evidence for an As bulk top layer below the dimers. The experimental rocking curves show clearly two components of unequal widths (Fig. 6a). The broader curve, fitted with a Lorentzian shape leadsot a surface correlation length (2/AQ> equal to 50 A related to the terrace distribution [13] and fully compatible with the 150 w average coherent domain size derived from the fractional peak width. During the thermal treatment, the integrated intensity decreases up to the transformation temperature of 360°C while the lineshape remains constant thus confirming the assumption of a structure factor reduction within an unchanged domain size (inset in Fig. 6a). However, the thermal behaviour is markedly different from what is observed with the fractional peak (Fig. 5) and supports the interpretation presented in the previous section. Between 300°C and 350°C changes occur only in the chemisorbed layer where surface dimers are randomly desorbed leading to a more pronounced decay for the fractional than for the integer orders, where the volume contribution is still strong; above 350°C the first As bulk layer equally starts to be disrupted and the whole c(4 X 4) diffraction pattern vanishes. In the same temperature range, the (2, 1) reflection is extremely weak. Above 360°C and up to 400°C in the 2 X 4 phase temperature domain, both (1, 2) and (2, 1) lines show an increase of integrated intensity with complex lineshapes for which there is no clear interpretation (Fig. 6b). The in t e rf erence effect between surface and bulk which can be calculated with the different models does not allow a straightforward assignment of the bulk top layer. After the flash annealing at 600°C the strong sharp component is again visible, this time in the (2, 1) peak (Fig. 5~). For the 2 X 6 structure, X-ray diffraction data are then in favour of a Ga-terminated bulk below the reconstructed layer which is not contradictory with the STM data [4]. It should be noted that X-ray diffraction, performed here far from an absorption edge cannot tell As from Ga atoms. The present work is thus sensitive to the back-bond sequence from the top surface towards the inner bulk but would not reveal substitutional gallium in the As terminal plane, as is found in a recent MEIS (medium energy ion scattering) experiment [ 141.
257
4. Conclusion Although we are not dealing here with a true phase transition since the system is not in equilibrium with an As pressure, our observations compare satisfactorily with previous results obtained by RHEED under a finite As flux except for an overall downward shift of the temperature scale: the reversible transition between the two phases takes place at 400°C under an As flux of about 10” As, molecules/cm*/s (instead of 360°C in UHV). The analysis of both X-ray intensity and lineshape for fractional and integer orders has enabled us to propose a mechanism for the phase transformation: the c(4 i< 4) structure is destroyed by random disordering of the As dimer layer after which nucleation and growth of the 2 X 4 order takes place. A similar interpretation in terms of random dimer breaking has been proposed in the case of the 2 X 1 to 1 X 1 transition on the Ge(OO1) surface at 955 K [15], however instead of desorbing, the Ge atoms migrate from a dimer configuration to an adatom-vacancy pair thus increasing the surface roughness. A desorption-induced continuous phase transformation has indeed been observed on the Ge(ll1) 1 X 1 surface at 1070 K, evidenced by mass spectrometer measurements and resulting in a smooth decrease of the integer orders intensities with a constant FWHM
Ml. Acknowledgement One of us (V.H.E.) gratefully acknowledges the financial support from the Brazilian CAPES during the completion of this work.
References [I] J. Massies, P. Etienne, F. Dezaly and N.T. Linh, Surf. Sci. 99 (1980) 121. [2] P.K. Larsen, J.H. Neave, J.F. van der ken, P.J. Dobson B.A. Joyce, Phys. Rev. B 27 (1983) 4770. [3] M. Sauvage-Sin&in, R. Pinchaux, J. Massies, P. Claverie, Jedrecy, J. Bonnet and I.K. Robinson, Phys. Rev. Lett. (1989) 563. [4] M.D. Pashley, K.W. Haberern W. Friday, J.M. Woodhall P. Kirshner, Phys. Rev. Lett. 60 (1988) 2176.
and N. 62 and
2.58
KH. Etgens et al. /Surface Science 320 ~199412.52-258
[5] D.K. Biegelsen, R.D. Bringans, J.E. Northrup and L.E. Swartz, Phys. Rev. B 41 (1990) 5701. [6] C. Deparis and J. Massies, J. Cryst. Growth 108 (1991) 157. [7] P. Claverie, J. Ma&es, R. Pinchaux, M. Sauvage-Simkin, J. Frouin, J. Bonnet and N. Jedrecy, Rev. Sci. Instrum. 60 (1989) 2369. [8] M. Sauvage-Simkin, R. Pinchaux, J. Massies, P. Claverie, J. Bonnet, N. Jedrecy and LK. Robinson, Surf. Sci. 211 212 (1989139. [9] P.K. Larsen and D.J. Chadi, Phys. Rev. B 37 (1988) 8282. IO] D.J. Chadi, J. Vat. Sci. Technol. A 5 (1987) 834. :ll] M.D. PashIey, K.W. Haberem and J.M. Gaines, AppI. Phys. Lett. 58 (1991) 406.
[12] LK. Robinson, in: Handbook on Synchrotron Radiation, Vol. 3, Eds. D.E. Moncton and G.S. Brown (North-Holland, Amsterdam, 1991). [13] E. Vlieg, J.F. van der Veen, S.J. Gurman, C. Norris and J.E. Macdonald, Surf. Sci. 210 (1989) 301. [141 J. Falta, R.M. Tromp, M. Cope], G.D. Pettit and P.D. Kirchner, Phys. Rev. Lett. 69 (1992) 3068. [15] A.D. Johnson, C. Norris, J.W.M. Frenken, H.S. Derbyshire, J.E. Macdonald, R.G. van Silfhout and J.F. van der Veen, Daresbury Laboratory preprint DL/SCI/P692E. I161 A. Mak, K.W. Evans-Lutterodt, K. Blum, D.Y. Noh, J.D. Brock, G.A. Held and R.J. Birgeneau, Phys. Rev. Lett. 66 11991) 2002.