A study of the surface structure and composition of annealed Ga0.96Mn0.04As(1 0 0)

Applied Surface Science 222 (2004) 23–32

A study of the surface structure and composition of annealed Ga0.96Mn0.04As(1 0 0) A. Mikkelsena, J. Gustafsona, J. Sadowskic, J.N. Andersena, J. Kanskib, E. Lundgrena b

a Department of Synchrotron Radiation Research, Institute of Physics, Lund University, Box 118, S-221 00 Lund, Sweden Department of Experimental Physics, Chalmers University of Technology and Go¨teborg University, S-41296 Go¨teborg, Sweden c Niels Bohr Institute, Copenhagen University, DK-2100 Copenhagen, Sweden

Received 27 May 2003; received in revised form 29 July 2003; accepted 29 July 2003

Abstract The surface structure and chemical composition of annealed Ga0.96Mn0.04As(1 0 0) have been studied by scanning tunneling microscopy (STM), auger electron spectroscopy (AES) and low energy electron diffraction (LEED). The samples were As capped and subsequently transferred in-air from the MBE system to the STM chamber. After annealing to 600 K it is found that the Mn segregates to the surface and forms a compound, which is stable up to annealing temperatures of 790 K. For annealing temperatures above 825 K a well-ordered phase exists signified by a LEED pattern consisting of a superposition of a ð1  6Þ and a ð4  2Þ pattern. LEED and STM measurements demonstrate that the surface is dominated by ð1  6Þ domains coexisting with small patches of ð4  2Þ domains. By comparing the STM images of the high temperature phase found on Ga0.96Mn0.04As(1 0 0) with the high temperature phases found on ordinary GaAs(1 0 0), we demonstrate differences between annealed Ga0.96Mn0.04As(1 0 0) and GaAs(1 0 0) in both surface morphology and atomic structure. We argue that the Ga0.96Mn0.04As surface is more As rich than the GaAs surface prepared in a similar fashion. Reasons for these differences are discussed. # 2003 Elsevier B.V. All rights reserved. Keywords: STM; LEED; AES; GaAs; GaMnAs; Manganese; Surface; Structure

1. Introduction The combination of ferromagnetic materials and semiconductor technology, is expected to lead to new device applications introducing magnetic degrees of freedom into semiconductor electronics. One approach to creating such spintronics technology, which have attracted considerable attention, is the attempt to combine GaAs and Mn. This includes growing structures of ferromagnetic materials such as for example MnAs and GaMn on GaAs, and the development of new ferromagnetic materials such as the low temperature grown GaMnAs compounds [1],

which belongs to a new group of dilute magnetic semiconductors with a Mn contents of 0–10%. Hetero-structures of MnGa/GaAs and MnAs/GaAs have been grown successfully by MBE, indicating one approach for combining magnetic structures and GaAs [2–4]. The GaMnAs compounds has been found to become ferromagnetic at around 1 0 0 K depending on the Mn concentration (around 5%), and the abundance of various defects [1]. Apart from the obvious advantage that GaMnAs and similar compounds easily can be integrated with ordinary GaAs technology, it has also been found that it is possible to control the magnetic properties of magnetic semiconductors by

0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0169-4332(03)00964-4

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applying an electric field across the material [5]. As the properties of these new materials and superstructures depend strongly on the structure and morphology of defects and interfaces, it is important to obtain a detailed knowledge and understanding of their surface structure and morphology. In the present study, we have focused on the (1 0 0) surface, which is the technologically most important GaAs surface, as it is used as template for the growth of most GaAs based technologies. In order to understand our experimental observations on Ga0.96Mn0.04As(1 0 0) we compare them with the structure and composition found on ordinary GaAs(1 0 0) grown in the same MBE system using similar procedures. The GaAs(1 0 0) surface has already been the subject of a large number of scanning tunneling microscopy (STM) studies, as reviewed recently by Xue et. al. [6]. Focusing on the results obtained from the As capped 1 0 0 surface it is found that the desorption of the As capping at around 600 K leads to a ð2  4Þ low energy electron diffraction (LEED) pattern, which actually cover three different structures the a, b and g ð2  4Þ phases, as first established by RHEED [7]. The three surface phases are all terminated by rows of As dimers, the differences lie in the ordering of the rows and the As dimers in the rows [6]. The structure of the especially wellordered b phase has been determined by both surface X-ray diffraction (SXRD) [8] and ab initio calculations which included simulated STM images [9]. At higher annealing temperatures a number of ðn  6Þ structures can be found which also consist of more or less disordered As dimer rows separated by six lattice spacings. While the 6 periodicity is well established, the disorder along the rows have lead to the suggestion of models with periodicities ranging from ð2  6Þ to ð6  6Þ. In LEED however only clear diffraction spots representing a ð1  6Þ structure has been observed, while for example ð6  6Þ structure is only evidenced by some weak streaks. Generally we will use the term ð1  6Þ to denote the experimentally found structure. Finally a ð4  2Þ structure terminated by rows of Ga dimers can be found after annealing to around 900 K. It should be noted that the ð1  6Þ and ð4  2Þ structures often exists together, in a phase termed the ð4  6Þ phase, with the relative amounts of the two depending on annealing temperature and cooling rates. While the ð2  4Þ phase found at lower temperatures

has been determined with some confidence using a number of techniques, there is still some discussion concerning the structure of the ð1  6Þ phase. This can to some extent be attributed to the large disorder found in the As rows of this structure, however, the most reasonable ideal model of the structure in the As rows, which has been suggested by Biegelsen et al. [10], consists of a zig-zag pattern of As dimers. The Biegelsen model and some modifications of it, will be discussed further below. Little is known about the surface structure and morphology of GaMnAs(1 0 0), which is usually characterized by RHEED measurements indicating that well-defined surfaces and interfaces can be formed [11]. Very little STM work exists for this surface to our knowledge. One AFM study showing only large scale images indicate that the morphology of the GaMnAs surface is similar to low temperature grown GaAs at low temperatures, while growth at higher temperatures results in a roughening of the surface perhaps due to the formation of MnAs clusters [12]. A combined RHEED and STM study of GaMnAs and adsorption of Mn [13], indicated that Mn can improve the epitaxy of this material, as compared to the growth of low temperature GaAs. Finally it should be mentioned that no work investigating the surface structure and composition of GaMnAs as a function of annealing temperature has been carried out previously. Thus, the almost non-existent amount of information on how the Mn influences the structure and morphology of the GaMnAs(1 0 0) surface as a function of temperature, clearly motivates a detailed structural investigation. In the present study, we specifically investigate the surface properties of Ga0.96Mn0.04As(1 0 0), with an As capping, at annealing temperatures in the range between 300 and 900 K. Two reasons can be given for this study. Firstly, as some Mn diffusion is already found at 500 K, the formation of MnAs precipitates can be expected after annealing to higher temperatures [14]. Therefore, this study explores the area between GaMnAs compounds and GaAs combined with MnAs. Secondly, As capping is a very important technique for preservation of the GaAs(1 0 0) surface during ex-vacuum transfer, thus it is important to explore how As capped GaMnAs samples behaves. The As capped samples were transferred in air from the MBE system to the STM vacuum system, the

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samples were never exposed to air for more than 24 h. The As capping of the GaAs(1 0 0) samples could be removed by annealing to temperatures around 600 K, and the well-ordered crystalline surface structure could be re-established. As discussed below the structure found in our experiments for the GaAs(1 0 0) surface agree well with previous studies of As capped GaAs(1 0 0). In the case of Ga0.96Mn0.04As compounds, the present investigation indicates that this procedure results in significant diffusion of the Mn at 600 K and higher temperatures. In principle, such a diffusion could lead to a redistribution of Mn at the surface or as precipitates in the bulk. Our investigation indicates that Mn instead migrates to the oxidized As capping layer and disappears in conjunction with the capping layer at approximately 825 K, a temperature considerably higher than in the case of the pure GaAs compound. Further, we demonstrate that differences in the surface morphology and atomic structure between the Ga0.96Mn0.04As(1 0 0) and GaAs(1 0 0) surfaces after annealing.

2. Experimental details The Ga0.96Mn0.04As samples, containing 4% of Mn, were grown by low temperature MBE as described previously [11], and capped with an As film as signified by the disappearance of the RHEED diffraction pattern. For comparison, pure GaAs films were grown by similar procedures in the same MBE system and also capped in a similar fashion. Samples were transferred between the MBE system and the STM within 24 h. STM, auger electron spectroscopy (AES) and LEED measurements were carried out in a separate vacuum chamber, with a base pressure below 1  1010 Torr. The STM experiments were performed with a commercial Omicron UHV-STM, using etched Tungsten tips. AES measurements were performed using a PerkinElmer cylindrical electron analyzer, with an estimated sensitivity of about 0.02 ML. The error on the normalized peak-to-peak AES signal is estimated to be 14%, from repeated measurements. The LEED measurements were carried out on a Omicron LEED optics, images could be recorded by a 8-bit CCD camera. The annealing procedure used for both GaAs(1 0 0) and Ga0.96Mn0.04As(1 0 0) samples studied in this work, consisted of the following steps: Initially the

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sample was annealed to 600 K for 30 min, to remove the As capping. Then the samples were flashed to higher and higher temperatures in intervals of about 100 K, until a temperature of 870 K was reached as the final flashing temperature. The samples were cooled below 400 K after each annealing. We estimate that the cooling rate was between 1 and 2 K/s.

3. Results In order to study the surface composition of the GaAs and Ga0.96Mn0.04As after each step in the annealing sequence, we carried out AES measurements as well as LEED observations. In Fig. 1a and b we show the AES measurements displaying the peak-to-peak height between the O (506 eV), As (1225 eV), and Mn (583 eV) peaks, normalized by the peak-to-peak height of Ga (1068 eV), additionally the observed LEED pattern is given for each temperature. The results from the As capped pure GaAs samples are shown in Fig. 1a, and it is seen that prior to annealing, the measurements indicate the presence of O, As and Ga. Below a temperature of 600 K, no significant changes of the O signal is observed, and no ordered diffraction can be observed in LEED, indicating the presence of an amorphous partially oxidized As capping layer. The oxygen signal disappears after annealing at 600 K, in conjunction with the appearance of a well-ordered ð2  4Þ LEED pattern. This compares well to previous LEED observations from the GaAs(1 0 0) surface annealed to this temperature [6]. As mentioned in Section 1, the ð2  4Þ structure consists of more or less well-ordered rows of As dimers separated by a distance of four lattice spacings. These findings demonstrates the removal of the As capping layer at around 600 K. It should be noted that the small change in the ratio of the As signal to the Ga signal after removal of the capping indicates that the ˚ , however, the fact that we capping is quite thin 10 A are able to obtain similar structures as found in previous LEED studies indicates that the capping is still sufficiently thick to protect the surface. It has also been found previously that even after a month of exposure in air, a conventional As capping has only ˚ [15]. Our samples been oxidized in the first 10 A were never exposed to air for more than about 24 h. Annealing the GaAs(1 0 0) samples to 870 K results in

no LEED pattern

2x4

+ 1x6

2.5

AES peak to peak / Ga peak to peak

AES peak to peak / Ga peak to peak (a)

4x2

2.0

1.5

1.0

0.5

0.0 300

LEED pattern

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LEED pattern

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1x6 no LEED pattern

+ 4x2

4

3

2

1

0 500

700

900

Temperature/K

(b) 300

500

700

900

Temperature/K

Fig. 1. The peak-to-peak height of the O (506 eV), As (1225 eV), and Mn (583 eV) AES peaks (denoted by squares, circles, and triangles, respectively), as a function of annealing temperature for (a) GaAs(1 0 0) and (b) Ga0.96Mn0.04As(1 0 0). All AES peaks are normalized by the peak-to-peak height of the Ga (1068 eV) AES peak. Error bars are derived from estimated error on the normalized signal of 14% (see text). It is noted that prior to annealing a small Mn component is found in the AES spectra. The corresponding LEED patterns as a function of annealing temperature are also given in the figure.

the appearance of a LEED pattern consisting of a ð1  6Þ and a ð4  2Þ super-positioned on top of each other. The spots belonging to the ð4  2Þ pattern are the strongest as observed by LEED. This phase has been argued to consist of a Ga terminated ð4  2Þ structure and a As terminated ð1  6Þ structure, as discussed in the introduction. Turning to the composition of the Ga0.96Mn0.04As, the AES measurements shown in Fig. 1b indicates a similar composition as the GaAs compound prior to annealing apart from the presence of a very small Mn component. Annealing of the Ga0.96Mn0.04As sample to 600 K lead to segregation of Mn to the surface. The Mn signal is at this temperature found to be about seven times larger than prior to annealing, while the O and As signals remains unchanged. No LEED pattern is observed from the surface at this temperature. As the sample is annealed to 825 K both the O peak and the Mn peak disappears. Further, a sharp LEED pattern is also observed, consisting of a weak ð4  2Þ and a strong ð1  6Þ pattern super-imposed on top of each other. Further flashing to 870 K leads to no observable changes in neither the AES spectra nor the LEED pattern. These observations demonstrates that in the

case of the GaMnAs sample, amorphous oxidized layers are still present at 790 K but removed after annealing at 825 K. One explanation could be that an amorphous manganese oxide or manganese–arsenic oxide is formed on top of the Ga0.96Mn0.04As surface. It is not possible to deduce the structure of the disordered oxide on the surface, however, it seems quite clear that Mn plays an important role in stabilizing the oxide compound as the oxygen would otherwise evaporate from the surface at 600 K.

4. The high temperature Ga0.96Mn0.04As(1 0 0) surface structure ˚ large STM In Fig. 2a we show a 2500 2500 A image from the Ga0.96Mn0.04As(1 0 0) surface after annealing to 870 K. The image confirm the LEED observations presented above, and reveals that the surface indeed has of two kinds of domains. The most abundant type of domain consists of rows along the ˚ (6 [1 1 0] direction with a separation of about 24 A the surface unit-cell in the (1 1 0) direction). The less abundant domain consists of rows along the [110]

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˚ filled state STM image of the ð1  6Þ Fig. 2. (a) 2500 2500 A and ð4  2Þ domains obtained after annealing the Ga0.96Mn0.04As(1 0 0) surface to 870 K as described in the text. (b) ˚ filled state STM image of the ð1  6Þ and ð4  2Þ 1500 1500 A domains obtained after annealing the GaAs(1 0 0) surface to 870 K.

˚ . The rows direction with a separation of about 16 A in the two types of domains are perpendicular to each other. These observations are consistent with the observation of a ð1  6Þ þ ð4  2Þ LEED pattern.

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The morphology of the high temperature annealed As capped Ga0.96Mn0.04As can be compared to the As capped GaAs(1 0 0), annealed in a similar fashion. It ˚ can be seen from Fig. 2b, showing an 1500 1500 A STM image of the GaAs(1 0 0) annealed to 870 K, that this surface also displays ð4  2Þ and ð1  6Þ perpendicular domains, however, the ð4  2Þ domains are in this case the most abundant type. As mentioned above these observations are in good agreement with a number of previous STM measurements of As capped GaAs(1 0 0) annealed to around 870 K [6], and also consistent with the LEED observation presented above. In previous work it was concluded that the high temperature ð4  2Þ reconstruction is observed at Ga rich surfaces while the ð1  6Þ reconstruction appears on more As rich surfaces [6]. Since the annealed As capped Ga0.96Mn0.04As surface does not contain any significant amount of Mn (see above), we will have to conclude that this surface is more As rich as compared to the pure GaAs(1 0 0) surface. We will now turn to the differences in the detailed atomic arrangements observed between the Ga0.96Mn0.04As and the pure GaAs samples. We will focus on the ð1  6Þ reconstruction since no difference between the two samples is observed in the case of the ð4  2Þ reconstruction. Since the surface structure of the Ga0.96Mn0.04As does not contain any significant amount of Mn, as evidenced by the AES measurements mentioned above, the rows observed in the ð1  6Þ reconstruction have to consist of either As or Ga dimers. As in the case of the GaAs-ð1  6Þ structure, the rows in the Ga0.96Mn0.04As-ð1  6Þ structure are likely to consist of As dimers. This is indicated by the difference between filled state and empty state images of the Ga0.96Mn0.04As-ð1  6Þ structure, as seen on Fig. 3. The dimer rows behaves in a similar fashion as observed in previous STM work on GaAs(1 0 0) [16], which also show that the brighter rows of the ð1  6Þ reconstruction appear broad in the empty state images, while they appear much sharper in the filled state images. A comparison between the two ð1  6Þ structures of GaAs and Ga0.96Mn0.04As in more detail is shown in Fig. 4a and b, resolving the individual dimers of the ð1  6Þ structures. Firstly it should be noted, that in both cases the As dimers are somewhat disordered along the rows, however, especially for the Ga0.96Mn0.04As(1 0 0) As rows there is clearly some local

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˚ STM image of the ð1  6Þ phase obtained after annealing Ga0.96Mn0.04As(1 0 0) to 870 K. (a) Filled state image Fig. 3. (a) 400 400 A recorded with a negative bias of about 2 V, which results in an improved resolution of the top layer dimers [16]. (b) Empty state image recorded with a positive bias of about 2 V and a current of 0.1 nA, note that the As dimers light up as compared to (a). The arrow indicates the most common defect found in this structure, where part of a As row in the top layer is missing, revealing the second layer Ga structure.

ordering spanning up to about 10 As dimers. From Fig. 4a and b the difference between the density of dimers in the As rows may be seen, assuming that each white dot is a single As dimer, as indicated for a number of dimers on Fig. 4a and b. The rows of the GaAs-ð1  6Þ structure are narrow consisting of very few and single dimers, generally separated by ˚ , forming both zig-zag patterns and straight about 8 A sections, as seen in Fig. 4a. The rows of the Ga0.96Mn0.04As-ð1  6Þ structure are much broader and consist of many more As dimers, usually separated

˚ , generally forming a zig-zag pattern as by about 4 A seen in Fig. 4b. The images resolving the individual dimers of the ð1  6Þ structures seen on Fig. 4a and b can be explained as imperfect versions of the ideal Biegelsen model [10] shown in Fig. 5a. By utilizing the observed differences in the STM images between the GaAs and Ga0.96Mn0.04As surfaces, we schematically indicate the reason for the observed difference between the two ð1  6Þ structures in Fig. 5b and c. In the case of the GaAs-ð1  6Þ about half of the As dimers are missing

Fig. 4. Filled state STM images of the phases obtained after annealing: (a) GaAs(1 0 0) to 870 K. (b) Ga0.96Mn0.04As(1 0 0) to 870 K. The images were recorded with a bias of 2 V and a current of 0.1 nA. Dimers are indicated on both (a) and (b) as black circles with a white center. The rows of the ð1  6Þ structure in (b) contain noticeably more dimers than the rows of the ð1  6Þ structure in (a).

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(a)

4Å

[110]

[110]

Top view

Side view

As (b)

Ga (c)

Top view

Top view

As

Ga

Fig. 5. (a) The basic model of the ð1  6Þ phase found on GaAs(1 0 0) suggested by Biegelsen et al [10]. (b) On ordinary GaAs(1 0 0) the As rows of the ð1  6Þ structure are usually missing some of the As dimers as suggested in the drawing. (c) On the Ga0.96Mn0.04As(1 0 0) surface the As rows of the ð1  6Þ structure seems to contain more As dimers than the basic model of Biegelsen et al. as suggested on the drawing.

compared to the Biegelsen model, while the Ga0.96Mn0.04As-ð1  6Þ has all the As dimers of the Biegelsen model as well as sites with additional As dimers as compared to the Biegelsen model.

Information about the second layer Ga atoms can be deduced from the empty state images such as in Fig. 3b, because the Ga dimers in the second layer can be seen in the troughs between the As rows, as have

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4Å

also been noted recently by Xu et al. [17]. The structure of the Ga dimers is especially clear in the defect areas of the Ga0.96Mn0.04As-ð1  6Þ structure, as noted by the arrow on Fig. 3b. The defect areas in Fig. 3b can be interpreted as simply the top layer As atoms missing, thus, the dotted structure now seen in the troughs represents the second layer Ga dimers. The distance ˚ , which is in good agreement between the dots is 12 A with the recent observation by Xu et al. [17] of the ð1  6Þ structure on the GaAs(1 0 0) surface. This indicates that the position of the second layer Ga atoms in the model suggested by Biegelsen et al. cannot be correct for the ð1  6Þ structure on Ga0.96˚ periodicity is found in Mn0.04As(1 0 0), as no 12 A their second Ga layer. However, by rearranging the second layer Ga atoms as also indicated by Xu et al. [17] for GaAs(1 0 0), it is possible to construct an As rich modified Biegelsen model with a new second layer structure, as shown in Fig. 6. While the positioning of the Ga atoms is based on ˚ periodicity the finer details can be the observed 12 A obtained by assuming that the electron counting model

Top view

Side view

As

(ECM) holds for this system. The ECM states that the number of electrons available due to the broken bulk bonds at the surface should exactly match the number of electrons used in new dangling bonds and dimer bonds formed by the surface atoms. The model has previously been used successfully to explain a number of the reconstructions on the clean GaAs(1 0 0) surface [6]. In the present case of a ð6  6Þ unit-cell a framework for checking the ECM has been derived by McLean et al. and Xu et al. [17,19]. A similar strategy will be applied here to the model shown in Fig. 6. Counting first the broken bulk bonds we have 12 As second layer broken bonds, 32 broken Ga bonds in the second layer and 28 broken As bonds of the first layer. This yields 74 electrons. The number of electrons needed to fill dimer and dangling bonds are 24 electrons for the second As layer, 8 electrons for the first Ga layer and 42 for the first As layer, also resulting in 74 electrons. This shows that the ECM is fulfilled. Comparing the present model to those of Xu et al. and McLean et al. it can be seen that the main difference is in the number of top layer As dimers. While our model has seven As dimers in a ð6  6Þ unit-cell, the model of Xu has five, while only three As dimers are found in the McLean unit-cell. Thus, the models of Mclean and Xu can be seen as As poor versions of a model with the highly symmetric 6 As dimers, similar to the reduced Biegelsen model of Fig. 5b. Our model can be seen as an As rich model with more than 6 As dimers like the As enriched Biegelsen model of Fig. 5c. Interestingly it seems to be difficult to construct high symmetry models with six As dimers as in the Biegelsen model, ˚ . Thus, when assuming a Ga dimer separation of 12 A the finding of ð1  6Þ surfaces with varying number of top layer As dimers is not surprising within the framework of ECM, as models with varying number of As dimers can be constructed, especially when models of lower symmetry than for example the Biegelsen model is considered—a lower symmetry which is in accordance with the disorder observed in the STM images.

Ga

Fig. 6. The refined model of the As rich version of the Biegelsen model (shown in Fig. 5c). Instead of the zig-zag pattern of Ga dimers in the Biegelsen model, pairs of Ga dimers are now introduced as suggested by the periodicity in the troughs between the As rows, and previously found for GaAs(1 0 0) by several authors [17,19]

5. Discussion The phase diagram of the GaAs(1 0 0) surface contains two structures at higher temperatures: the ð1  6Þ at around 700 K and the ð4  2Þ at around 800 K.

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The transition between the two phases is reversible, thus usually after annealing to 800 K and cooling down slowly the two structures are found to co-exist [6]. The ð1  6Þ structure then dominates at slow cooling rates while the ð4  2Þ structure dominates at higher cooling rates. This shows that the structure found on GaAs(1 0 0) after annealing to high temperatures is influenced by kinetics, i.e. annealing times and cooling rates are quite important as just indicated. There is some disagreement in the literature about the temperature and cooling rates needed in order to produce the different phases. Nevertheless, Verheij et al. [18] states that cooling rates at 0.3 K/s from 850 to 700 K followed by 2000 s of annealing at 700 K is needed to obtain a surface consisting mostly of the ð1  6Þ structure, while McLean et al. [19] anneal to similar temperatures but they only need a constant cooling rate of about 0.8 K/s to obtain a ð1  6Þ dominated surface. In the present system we find that by annealing the GaAs(1 0 0) surface at 870 K and then turning of the heating generally leads to the formation of a ð4  2Þ dominated surface, with small patches of ð1  6Þ at the step edges. This observation is similar to those by Resch-Esser et al. [20] We estimate that we have a cooling rate around 1–2 K/s in the temperature range of 870 K to 400 K. The finding that we may obtain a ð1  6Þ dominated surface after annealing Ga0.96Mn0.04As at 870 K followed by the rapid cooling rate as described above could be attributed to the existence of a disordered manganese oxide or manganese–arsenide oxide layer on top of the surface acting as a protective layer on the surface. The formation of the Ga rich structures at high temperatures is usually attributed to the depletion of the As in the top surface layer due to desorption. We therefore suggest that the manganese-oxygen–arsenic compound hinders As depletion from the surface resulting in a more As rich surface. Thus, when the surface oxide finally desorbs a As rich surface remains. However, as the surface oxide desorbs at a temperature of around 825 K, the exposed As should also desorb at this temperature, which would mean that the structure found in this study is perhaps also due to kinetics, i.e. a different annealing procedure could lead to the formation of the As poor surface found on ordinary GaAs(1 0 0) after annealing to such temperature. As mentioned previously we have tried to anneal both the Ga0.96Mn0.04As and

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GaAs(1 0 0) surface first to 825 K and then to 870 K without observing any changes in the structure of the surfaces. As to the question whether the structure found for Ga0.96Mn0.04As can be obtained on the pure GaAs(1 0 0) surface, we note that previous STM investigations [6,17] as well as our own, has found structures with a lower density of top layer As dimers than the structure found on Ga0.96Mn0.04As. However, as the high temperature surface structure of Ga0.96Mn0.04As contains only Ga and As it cannot be ruled out that it is possible to form this on pure GaAs(1 0 0) using particular annealing conditions. One should mention, that it has been found previously for other semiconductors, as for example Si, that even very small amounts of metal impurities on the surface may also dramatically change the surface structure and morphology [21]. Finally we note that our investigation do not find any evidence for the formation of MnAs precipitates at the surface region of the Ga0.96Mn0.04As sample, as was proposed for bulk GaMnAs previously [22]. A situation as found by Boeck et al. [14], were the MnAs ˚ below the surprecipitates are buried several 1000 A face after annealing of Ga0.96Mn0.04As, would be in agreement with our findings. It should further be noted that the study by Boeck et al. indicate that although evaporation of the Mn could occur, Mn generally diffuses into the bulk after annealing and forms MnAs precipitates. Our investigation depends on the formation of ordered and relatively flat surface structures. In order to obtain this, it is necessary to anneal the samples. In the process of annealing, it is clear that if any MnAs precipitates exists near the surface region, they will vanish when the Mn is depleted from the surface region, as evidenced from AES. If MnAs precipitates exists in our samples, they will have to be cleaved rather than annealed. Such investigations are currently being performed.

6. Summary In summary we have studied the surface of Ga0.96Mn0.04As(1 0 0) as a function of annealing temperature. It was found that the Mn segregates to the surface at 600 K, forming a disordered oxide on top of the surface. After annealing to 825 K the oxygen desorbs while the Mn presumably goes into the bulk. An ordered

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structure is found on the Ga0.96Mn0.04As(1 0 0) surface after annealing to 825 K which consists of a ð1  6Þ and a ð4  2Þ structure. In contrast to ordinary GaAs(1 0 0) annealed in a similar fashion the ð1  6Þ structure dominates the surface, while the ð4  2Þ structure dominates on the GaAs(1 0 0) surface. In detail the ð1  6Þ structure on Ga0.96Mn0.04As(1 0 0) is also found to be more As rich than the ð1  6Þ structure on GaAs(1 0 0). Acknowledgements Support of this work by the Swedish Natural Science Research Council is gratefully acknowledged. References [1] H. Ohno, Science 281 (1998) 951. [2] M. Tanaka, J.P. Harbison, J. DeBoeck, T. Sands, B. Philips, T.L. Cheeks, V.G. Keramidas, Appl. Phys. Lett. 62 (1993) 1565. [3] M. Tanaka, J.P. Harbison, M.C. Park, Y.S. Park, T. Shin, G.M. Rothberg, Appl. Phys. Lett. 65 (1994) 1964. [4] T. Plake, M. Ramsteiner, V. Kaganer, B. Jenichen, M. Kaestner, L. Daeweritz, K. Ploog, Appl. Phys. Lett. 80 (2002) 2523. [5] H. Ohno, D. Chiba, F. Matsukura, T. Omiya, E. Abe, T. Dietl, YoHno, K. Ohtani, Nature 408 (2000) 944. [6] Q. Xue, T. Hashizume, T. Sakurai, Appl. Surf. Sci. 141 (1999) 244.

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