Ge (001) system

Surface Science Letters 293 (1993) L821-L825 North-Holland

surface science letters

Surface Science Letters

Surface superstructures of the Pb/Ge(OOl) system Y u n Zhang, R.G. Z h a o and W.S. Yang Department of Physics, Peking University, Belting 100871, China Received 14 April 1993; accepted for publication 27 May 1993

In the present work the Pb/Ge(001) system has been studied with LEED (low-energy electron diffraction), AES (Auger electron spectroscopy), and EELS (electron energy loss spectroscopy). A variety of surface superstructures, i.e. c(4 x 8), 5 x 1, "3 x 6", 4 x 1 and probably c(4 x 2) were observed and their relations as well as the high- and room-temperature phase transformations have been schematically summarized in a figure. Some important clues of the structure of these superstructures have been found. Pb seems to be quite mobile at the P b / G e interfaces.

Metal-semiconductor systems continue to be of great interest for both basic research and industrial applications. Among these, the Pb/Si system has recently attracted particular attention [1-8] as its interfaces are thought to be abrupt [2,3]. However, this conclusion has been challenged by our recent works [9,10]. Through these works it was learned that a variety of surface superstructures may appear for the Pb/Si(001) system and that phase transitions may happen between some of these superstructures even at room temperature [10]. To further enhance our knowledge on the characteristics of Pb in forming metal-semiconductor interfaces, we have recently studied the Pb/Ge(001) system which had never been studied and the P b / G e ( l l l ) system with low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and electron energy loss spectroscopy (EELS) [11]. Here we report the surface superstructures of the Pb/Ge(001) system as well as the phase transitions between some of these superstructures. The experiments were performed in an UHV system consisting of a sample preparation chamber and a main chamber which was equipped with LEED, AES and EELS [10]. Lead deposition was carried out in the sample preparation chamber, and its pressure was maintained at (26) × 10-10 Torr while the Pb source was on. The

sample with a size of 7 x 7 × 0.5 mm 3 was cut from a Sb-doped Ge(001) wafer (18-20 12. cm). After Ar ÷ bombardment (5 x 10 -5 Torr, 600 V, 1.5 h) and annealing (800°C, 15 min) a clean and well ordered Ge(001)2 x 1 surface was always obtained, as verified by the sharp LEED pattern and the very small AES signals of O and N, i.e. the 0(503 eV)/Ge(47 eV) and N(379 eV)/Ge(47 eV) peak-to-peak ratios being below 3 x 10 -4. The construction of the Pb source, the calibration of its deposition rate, as well as the way of annealing the sample and measuring its temperature were the same as those used in the previous work [9-12]. Briefly, the Pb source was a piece of Pb (99.998%) mounted on a Ta ribbon. The deposition rate was controlled by the current passing through the ribbon and calibrated as in a previous Pb/Ni(001) work [12], where the two break-points on the curve of the AES intensity of Pb were assumed as corresponding to one and two (lll)-layers of Pb, respectively. Pb was deposited with a constant rate onto the clean Ge(001)2 x 1 substrate kept below 100°C. Fig. 1 shows that the AES intensity of Pb (MOO, 94 eV, peak-to-peak) increases linearly with Pb deposition till saturation at about 2 monolayers (ML, i.e. 6.24 x 1014 atoms/cm2), which marks the appearance of Pb islands. However, on the EELS spectra [11] the bulk plasmon peak of the

0039-6028/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

L822

LS

}'l~tl Zhang et aL " 5urlace superstructures of the Pb /Ge(O01) system

30.

~ 25. / v 20 %

w

15-

-E Ld
0

~|

Nominal Pb Coveruge (ML)

Fig. 1. Evolutionof the AES intensityof Pb in the courseof deposition.

Pb islands becomes prominent only when the coverage reaches about 6 ML, suggesting that before that the islands are too small to cause such loss. Only at even higher coverages the extra L E E D spots of the Pb islands become visible. In the course of deposition, the surface exhibits a series of superstructures shown in fig. 2. Below 0.5 ML, the surface does not show good ordering and only some dim c(4 x 2) L E E D spots can vaguely be seen. However, further deposition until 1 M L onto this surface gives rise to a nice c(4 x 8) pattern. When the coverage is between 1.3 and 1.7 ML, a very interesting "3 x 6" pattern (more precisely, (3 0)) appears. From 1 to 1.3 ML, this "3 x 6" co-exists with c(4 x 8), while above 1.7 ML it changes into another pattern of 4 x 1 with some dim eighth-order spots seen at some specific incidence geometries and energies. Actually, there is almost no clear-cut boundary of coverage separating the "3 x 6" and 4 x 1 patterns because even at room temperature the former can gradually transform into the latter. The only difference is the higher the Pb coverage the shorter the transition time. In addition to the room-temperature phase transformation from "3 × 6" to 4 x 1, some other transitions may happen at higher temperatures. Upon annealing at about 300°C the c(4 × 8) pattern irreversibly transforms into the 5 x 1 pattern, while no significant desorption of Pb is observed. Annealing in the same temperature range makes the as-deposited "3 × 6" pattern become sharper, but it does not prevent the pattern from transforming into 4 × 1 at room tern-

perature. Annealing the 4 x 1 surface at about 300°C makes the surface become "3 x 6" again, although at room temperature this "3 x 6" will eventually change back to 4 x 1. At temperatures higher than 200°C the "3 x 6" and 4 x 1 patterns become 2 x 1, while the c(4 x 8) and 5 x 1 become 1 x 1. Annealing at 450°C or higher temperatures desorbs the Pb from the sample surface, so that after a period of such annealing, say - 1 0 min or so, the Pb islands may disappear from the 4 x 1 surface and the 4 x 1 may further become "3 x 6", 5 x 1, and eventually the clean 2 x 1. All of the above-mentioned complex processes or transformations are schematically shown in fig. 3. It should b e pointed out that this figure is by no means a precise phase diagram and there are still some interesting facts not reflected in this figure, which might be very important to further investigations of the superstructures. The 4 x 1 surface is highly inert against contamination from the residual gas of the UHV system, although both the clean Ge(001)2 x 1 and the pure Pb sample are much easier to be contaminated. For example, a freshly prepared 4 x 1 L E E D pattern continues to be bright and sharp after being exposed in 10-J0 Torr for one week or in 10-1 Torr for two hours. Recently, we have concluded that strong intermixing does occur at both the Pb/Si(111) and Pb/Si(001) interfaces. The major evidence for this is that on the EELS curves collected from both interfaces there is a new (i.e. neither Si nor Pb) bulk plasmon peak at about 10.8 eV [9]. Very recently, a similar study has shown that at both P b / G e ( l l l ) and P b / G e ( 0 0 1 ) interfaces a new intermixing phase also exists as verified by a new bulk plasmon peak on the EELS curves at 10.8 eV [ll]. However, this peak can only be found in the " 3 x 6 " and 4 x l phases but not in the c(4 x 8) and 5 x 1 phases; therefore, we conclude that the former two and only the former two superstructures belong to the intermixing phase. Or more explicitly, the "3 x 6" and 4 x 1 patterns are surface superstructures of the intermixing phase. From the point of view of LEED, the c(4 x 8) and "3 X 6" superstructures are in some sense

Yun Zhang et aL / Surface superstructuresof the Pb / Ge(O01) system

L823

,--,--,---,--,--,--,---,--, 0

0 0

0 s,S I " , 0

0

0 0 0 s SSO (~ 0 " ~ 0 0 0 0 0 0 ,'SO ',~ 0 " ~ 0 0 s 0 t~ 0 ~ 0 0

0

0 ,'"

0

',

0

(a)

",0

0 o,sS'o 0 0 0 t~ 0 0 0 0",,0 0 s ,S 0 0 .,' 0 0 "'~

(~

0

0

0

0

o 0 0

e)

0

.11. 0 ,,"

o.~

0

-'" 0

0

0

0

0

0

0

(b)

~,.o

oo

o

0

",

oo

(3

~9

0

0 "0.

,"

0

o ",, 0

,0"0

0

0

0

0 ",

o

.--.--.---.--.--.--.---.--. ~

i

~

(c)

0

/'/

~ I

(3"o

o

o

~)

o

o

""-,

(3

o

E>

.-.-.-..-.-.-.-...--... / 0

/

0

/

,"

0 ~ --

"',

(d)

0

11 0

S~

I

® 0 0 0 0 ( 5 0 0 0 0 ® Fig. 2. LEED patterns (left) of the Pb/Ge(001) system and their schematic counterparts (right). The closed, large open, and small open circles in the schematic patterns represent the (00), integral-, and fractional-order beams, respectively, and the dashed lines the mirror planes at the normal incidence cases. (a) c(4 × 8), 31 eV, (b) "3 × 6", 81 eV, (c) 4 × 1, 133 eV, (d) 5 × 1, 132 eV.

different f r o m the 5 × 1 and 4 × 1 patterns. Although all of these superstructures p r o d u c e a quite sharp L E E D pattern, the fractional order

b e a m s o f the c(4 × 8) and "3 × 6" patterns can only be s e e n up to about 100 e V , while for the other two patterns the fractional order b e a m s can

L824

,t[x

Yun Zhang et al. / Surface superstructures of the Pb / Ge(O01) system

500t

400[2 ~ 3001: 2°°h Room

TeMp, { c(Z Pat±ernl ~ O.

C(4XSY. 0.5

1,0

Pb is~.ancls

1,5

20

Nomlnat Pb C o v e r a g e (ML)

Fig. 3. Schematic diagram showing the superstructures of the Pb/Ge(001) system and their relations as well as the phase transformations.

be seen up to 200 eV or higher energies. This difference may suggest that the 5 x 1 and 4 x 1 reconstructions extend down to deeper layers than c(4 x 8) and "3 x 6" do. Now, we discuss the last but very interesting point, i.e. the peculiar symmetry of the "3 x 6" pattern. One may have already noticed from fig. 2 that this pattern has only two mirror planes while the others, i.e. the clean 2 x 1, the c(4 x 8), 5 x 1, and 4 x 1 patterns all have four. Obviously, the symmetry of a pattern is determined by the symmetry of its surface, and as a result of reconstruction the surface symmetry may be lower than that of its bulk, thus the symmetry of a single-domain pattern may also be lower than that of its bulk. However, since the real pattern of a surface is the superposition of all possible single-domain patterns allowed by its bulk symmetry, it may have a synunetry higher than its bulk symmetry [13] or at least not lower. Therefore, the symmetry of the "3 x 6" pattern shown in fig. 2 is peculiar and hard to understand. One possibility we could think of is that the above-mentioned intermixing phase below the "3 x 6" superstructure, somehow, does not allow four of the eight otherwise equally possible domains [13] to appear. At this stage nothing more concrete than this could be imagined and we expect STM might be able to disclose the truth behind the peculiar symmetry.

In summary, in view of the fact that (i) depending on the coverage, the as-deposited Pb/Ge(001) interfaces may have different surface superstructures, i.e. c(4 x 8), "3 x 6", 4 x 1, and probably c(4 x 2), that (ii) the "3 X 6" pattern may gradually transform into 4 x 1 at room temperature, and that (iii) Pb may form an intermixing phase with Ge and the "3 x 6" and 4 x 1 patterns are superstructures of this phase, we suppose Pb is quite mobile at the P b / G e interfaces. Since these facts are very similar to those happening at the Pb/Si interfaces [9-11], we further suppose Pb is quite mobile at interfaces it forms with semiconductors such as Si and Ge. The following facts may be valuable for future investigations of the Pb/Ge(001) system as well as its superstructures: (i) Above room temperature the "3 x 6" and 4 x 1 become 2 x 1 while the c(4X8) and 5 x l become l x l . (ii) The surface reconstructions of the 5 x 1 and 4 x 1 probably extend down more layers than the others do. (iii) Below the "3 x 6" and 4 x 1 there is an intermixing phase of Pb and Ge. (iv) The 4 x 1 is very inert against contamination. (v) The "3 x 6" exhibits a peculiar symmetry. This work was supported by the National Natural Science Foundation of China.

References [1] M. Saitoh, K. Oura, K. Asano, F. Shoji and T. Hanawa, Surf. Sci. 154 (1985) 394. [2] G. Le Lay, J. Peretti, M. Hanbucken and W.S. Yang, Surf. Sci. 204 (1988) 57. [3] G: Le Lay, K. Hricovini and J.E. Bonnet, Appl. Surf. Sci. 41/42 (1989) 25; Phys. Rev. B 39 (1989) 3927; Eisevier's Studies in Surface Science and Catalysis 40 (1988) 210. [4] F. Grey, R. Feidenhans'l, M. Nielsen and R.J. Johnson, J. Phys. (Paris) 50 (1989) 7181. [5] D.R. Heslinga, H.H. Weitering, D.P. van der Weft, T,M. Klapwijk and T. Hibma, Phys. Rev. Len. 64 (1990) 1589. [6] E. Ganz, F. Xiong, I.S. Hwang and J. Golovchenko, Phys. Rev. B 43 (1991) 7316. [7] H.H. Weitering, D.R. Heslinga and T. Hibma, Phys. Rev. B 45 (1992) 5991. [8] C.J. Karisson, E. Landemark, Y.-C. Chao and R.I.G. Uhrberg, Phys. Rev. B 45 (1992) 6321.

Yun Zhang et al. / Surface superstructures of the Pb / Ge(O01)system [9] W.S. Yang, R.G. Zhao and J.F. Jia, in: Proc. 22nd Int. Conf. on the Physics of Semiconductors (1992), in press; R.G. Zhao, J.F. Jia and W.S. Yang, Phys. Rev. B, submitted. [10] R.G. Zhao, J.F. Jia and W.S. Yang, Surf. Sci. Lett. 274 (1992) L519.

L825

[11] R.G. Zhao, Yun Zhang and W.S. Yang, Phys. Rev. B, submitted. [12] R.G. Zhao and W.S. Yang, Acta Phys. Sin. 41 (1992) 1125 [in Chinese]. [13] W.S. Yang, F. Jona and P.M. Marcus, Phys. Rev. B 28 (1983) 2049.