- Email: [email protected]
Nanodot hexagonal ordered arrays on GaSb substrates by LEIS under the presence of chromium diffused impurities
Surface & Coatings Technology 201 (2007) 8456 – 8462 www.elsevier.com/locate/surfcoat
Nanodot hexagonal ordered arrays on GaSb substrates by LEIS under the presence of chromium diffused impurities J.L. Plaza ⁎, Bárbara Capote, E. Diéguez Laboratorio de Crecimiento de Cristales, Departamento de Física de Materiales, Facultad de Ciencias, Universidad Autónoma de Madrid, 28049, Cantoblanco, Madrid, Spain Available online 12 March 2007
Abstract In this work we study the effect of Cr impurities, diffused onto GaSb substrates, on the formation of nanodot hexagonal arrays created by low energy argon ion sputtering. For the same sputtering conditions, the dimensions of the dots in the Cr-diffused sample are smaller than in the case of the pure sample. EDX data revealed that the sputtering induces a more pronounced Ga enrichment in the case of Cr-diffused sample. It is proved that oblique incidence in rotating configuration most probably delays the formation of the nanodots compared to previously reported normal incidence experiments. The diffusion of Cr atoms onto the GaSb surface induced the formation of defects upon sputtering. © 2007 Elsevier B.V. All rights reserved. Keywords: Low energy ion sputtering; Gallium antimonide; Nanodots
1. Introduction Much effort has been devoted in the last decade to the development of periodic structures at the nanometer scale. This is actually one of the main goals in the field of nanotechnology at the present time. As a driving force for this current field of research, we can mention that the formation of controlled nanostructures in semiconducting materials appears to be an important issue in the future development of the next generation of optoelectronic devices. Electron beam lithography (EBL) is at present well known in the field of nanotechnology for being an essential technique to be used in the fabrication of controlled arrays of periodical nanostructures. However, its low throughput makes this technique suitable only for prototype fabrication in the laboratory environment or as support technique in the industry [1]. Difficulties like these are pushing through a great effort in the development of parallel processing techniques to be employed in the fabrication of periodically arranged structures at the nanometer scale. To cite few of them we can mention semiconductor nanocrystal self assembly [2] or the Stransky– Krastanov method for the growth of semiconductor heterostructures and quantum dots [3]. Low energy ion sputtering ⁎ Corresponding author. Tel.: +34 91 4974784; fax: +34 91 4978579. E-mail address: [email protected] (J.L. Plaza). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.02.061
(LEIS) has also been recently presented as a technique appearing to be very promising for the fabrication of nanostructures and quantum dots. Therefore it represents an attractive alternative to the Stranski–Krastanov method. In this way hexagonal arrays of uniform semiconductor quantum dots have been created by LEIS using Ar+ ions [4,5] similar to what Facsko et al. [6] reported for the case of GaSb. In the latter work it is shown how hexagonal dot patterns can be spontaneously formed on GaSb and InSb when the surface of these materials are sputtered with low energy argon ions at normal incidence. The dot density can be easily controlled independently by properly tuning the sputtering time and the energy of the incident ions. Extensive reviews have been devoted to the formation of ordered nanostructures both on metals [7] and semiconductors [8]. The reader is referred to these review works in order to get basic information about the physical processes involved in the formation of periodic nanostructures by LEIS. However, and for what to this work concerns, we must recall that the selforganization of nanodot periodic arrays during sputtering has been attributed to an interplay between surface roughening induced by ion sputtering and smoothing processes of the surface [4]. This work is devoted to the effects of LEIS on pure and doped (impurity diffused) gallium antimonide (GaSb), one of the semiconductors that is becoming extremely relevant in the field of optoelectronics and communications (see for example the review from Dutta et al. [10] and references therein).
J.L. Plaza et al. / Surface & Coatings Technology 201 (2007) 8456–8462
8457
Table 1 Sample notation and sputtering parameters Name of the sample
Description
Sputtering process
Acceleration voltage (V)
Sputteringtime (s)
Ion flux(cm− 2 s− 1)
S0 Sctr1 SCr1 Sctr2 SCr2 Sctr3 SCr3
Pure GaSb Pure GaSb Cr-diffused Pure GaSb Cr-diffused Pure GaSb Cr-diffused
No Sputtered First First Second Second Third Third
– 500 500 600 600 600 600
– 200 200 300 300 1800 1800
– 6 × 1015 6 × 1015 1.5 × 1016 1.5 × 1016 1.5 × 1016 1.5 × 1016
The formation of nanostructures on GaSb by using low energy Argon ion sputtering has been recently reported in the series of papers by Facsko et al. [4–6]. They reported the formation of hexagonal nanostructure arrays and quantum dots on GaSb. An important contribution on this subject has also been published by Panning et al. [9], where the p to n-type conversion on GaSb surfaces is obtained by using the same low energy ion sputtering same technique. However, deeper research must be devoted to the LEIS technique on semiconductor materials. In particular, to the best of the authors’ knowledge, the effect of diffused dopants on the formation of nanostructures created by LEIS has not been investigated yet. In this paper, we study the effect of Cr impurities diffused onto GaSb substrates, on the formation of nanodots fabricated by using low energy Ar+ ion sputtering.
Three different processes were developed with different Ar ion energies and different exposure times. In this way the gradual effect of the LEIS on the surface of the samples could be analysed. The following notation will be used throughout the text: Sctr1, Sctr2, Sctr3, and SCr1, SCr2 and SCr3 to denote the pure GaSb and Cr-diffused samples after the first, second and third sputtering processes respectively. The sputtering parameters of all the samples are given in Table 1 being always 45° the incidence angle of the ion beam on the target (samples). The samples were also continuously rotating during the sputtering at 15 rpm. The success of this technique of rotating the sample under oblique incidence in creating hexagonal arrangements of regular nanodots has already been reported by several researchers [5,11–13]. A Millatron ion miller from Commonwealth Scientific Corp. was used in the sputtering experiments.
2. Experimental
3. Results and discussion
A single wafer from a Bridgman-grown GaSb ingot grown in our laboratory was initially taken and polished with 1 μm alumina powder. The wafer was divided into three pieces of about the same size. Each one of them was used to prepare three different samples. A first as-grown sample, S0, was not submitted to any further process and has been used as a reference sample. The second sample, Sctr, was used as a control sample being exposed to a LEIS process without any other previous treatment. Chromium was thermally evaporated by using an electron beam on the third sample, SCr. An Edwards auto 306 Turbo was employed to this purpose. The evaporation parameters were: evaporation time 5 min; electron beam current 45 mA, base pressure 3.0 × 10− 5 mbar, and the resulting thickness of the evaporated Cr film was 170 nm. An annealing treatment for 9 h was carried out after the evaporation process in order to promote diffusion of the Cr atoms into the surface of the GaSb substrate. This process was performed under 300 cm3 of nitrogen flux. In order to avoid high losses of Sb during the annealing, the temperature was kept at 250 °C. The evaporated Cr film was removed after annealing by using wet chemical etching immersing the sample for 2 h in HCl heated at 60 °C in a hot plate inside a laminar flow fume-cupboard. The acid does not damage the GaSb surfaces. Subsequently, the sample was rinsed in acetone. After the annealing treatment the sputtering process was carried out on the three (as-grown, pure GaSb, and Cr-diffused GaSb) samples by using Ar+ ions.
3.1. Analysis after the first sputtering process Samples Sctr1 and SCr1 were sputtered at the same time in order to be able to establish comparisons between them. Table 1 gives the parameters related to the first sputtering process. Initially, they were chosen to be the same as those used by Facsko et al. for the sputtering of amorphous and crystalline GaSb surfaces [2]. However, two main differences are present in our experimental setup compared to Facsko's. The first one
Fig. 1. Secondary ion mass spectrum of the Cr impurity obtained during the consecutive three sputtering processes from the Cr-diffused sample (SCr1, SCr2, SCr3). The arrows indicate the extent of the first, second and third sputtering process respectively.
8458
J.L. Plaza et al. / Surface & Coatings Technology 201 (2007) 8456–8462
all clear if they are completely equivalent. One of the goals of this work is to clarify this subject. It is also intended to prove weather or not the amount of time needed for the nanodot formation is the same for both configurations and to evaluate the possible differences in their geometry. At the left hand side of Fig. 1 the mass spectrum obtained from the secondary ions corresponding to the first sputtering process is presented. The second and third successive LEIS mass spectra are also shown and will be discussed later on. As Ga and Sb remained constant throughout the process and gave no additional information, only the relevant element, Cr is
Fig. 2. Optical images from (A) S0, (B) Sctr2 and (C) SCr2 after the second sputtering.
corresponds to the ion flux used in our case being 6 × 1015 cm− 2 s− 1. This value is nearly 40% less than that one used by Facsko which was 1 × 1016 cm− 2 s− 1 [2]. The different geometrical arrangement of our sputtering process is the second distinction to be considered, namely while Facsko used normal incidence, we use a combination of oblique incidence (45°) and rotation (15 rpm). Although both configurations have been reported to lead to stable hexagonally ordered dot patterns [2–6], it is not at
Fig. 3. AFM images from (A) S0, (B) Sctr2 and (C) SCr2.
J.L. Plaza et al. / Surface & Coatings Technology 201 (2007) 8456–8462
8459
to Facsko's most probably delay the appearance of this kind of nanostructures. 3.2. Analysis after the second sputtering process A second sputtering process on samples Sctr1 and SCr1, thus becoming samples Sctr2 and SCr2, was carried out onto the two samples with the aim of studying further effects induced by LEIS. Samples Sctr2 and SCr2 were sputtered at the same time as in the previous LEIS process. In order to promote the formation of hexagonal dot patterns, the accelerating voltage, ion flux and the exposure time were further increased compared to the values used in the first sputtering as shown in Table 1. The Cr
Fig. 4. Optical images from (A) Sctr3 and (B) SCr3 after the third sputtering.
shown in Fig. 1. In the portion of the mass spectrum related to the first LEIS process corresponding to sample SCr1, the Cr signal is present, in a relatively remarkable level, up to several nm deep into the GaSb surface, providing a confirmation of the actual formation of a Cr diffusion layer. We can also extract information about the thickness of this impurity layer created during the diffusion process, according to the point where the signal reaches a flat evolution. However the mass spectra corresponding to the second sputtering still indicates a low but still appreciable Cr concentration up to 21 nm where the Cr signal dramatically drops. This point indicates that the diffusion layer has been almost completely removed from the surface by the erosive effect of the sputtering process. The thickness thus obtained for the high concentration Cr layer is about 5 nm, while the total thickness considering the high and low Cr concentration regions is 26 nm. It must be pointed out that the formation of nanodot hexagonal patterns on the surface of the sample has not been observed after the first LEIS process. This fact disagrees with the formation of dot hexagonal patterns reported by Facsko et al. [2] for the same values of the ion energy and sputtering time. However the previously mentioned differences in our setup (lower ion flux and oblique incidence with rotation), compared
Fig. 5. Ultra-high resolution SEM images from (A) Sctr3 and (B) SCr3 after the third sputtering. (C) Detail of a defect found in SCr3.
8460
J.L. Plaza et al. / Surface & Coatings Technology 201 (2007) 8456–8462
Fig. 6. AFM images from (A) Sctr3 and (B) SCr3.
peared due to the erosion of the surface as can be observed on the sputtered surface of sample Sctr2 (Fig. 2B). It must be pointed out that the presence of defects is not observed in this sample. A different behaviour has been observed in the Cr-diffused sample, SCr2. In this case a high density of defects can observed (Fig. 2C). The fact that the defects appear only in SCr2, points to conclusion that their formation is greatly enhanced by the presence of impurities on the GaSb surface. This is probably related to the lattice distortion generated by the impurity diffusion. A threshold sputtering time seems to exist before the formation of this kind of defects as they have not been observed during the shorter first sputtering process. At this point it is worth noting that a preliminary similar work on Cr-diffused GaSb, to be published elsewhere, has revealed the same behaviour. However in that case the defects present a more regular shape and could be more well defined as cracks. In order to determine in more detail the morphology of the sputtered surfaces, an atomic force microscopy (AFM) analysis has been carried out. The aim of using this technique was to provide us with more information about some other features on the surface induced by the sputtering that could still remain unnoticed after the optical microscopy observations. Fig. 3 shows the 3d-images from this AFM analysis corresponding to S0, Sctr2 and SCr2. The polishing marks can be clearly observed in S0 (Fig. 3A). However, in the case of Sctr2 and SCr2 (Fig. 3B,C) the polishing marks are smeared out as a result of the LEIS process. Therefore, the LEIS process effectively reduces the roughness of the sample due to the surface erosion caused by the Ar ion beam. This effect is observed as a partial removal of the polishing marks from surface of the sample. The previous AFM results altogether with further ultrahigh resolution SEM analysis have shown that even after rising the sputtering time and the ion flux, the formation of hexagonal arrays of nanodot structures has not been attained. It is likely that the differences in the sputtering configuration (45° oblique incidence and sample rotation vs. normal incidence used by Facsko et al.) might be the cause for this delay. 3.3. Analysis after the third sputtering process
secondary ion mass spectrum corresponding to this second LEIS process is also presented in Fig. 1 (part of the spectrum covered by the range of the arrow named as “2nd”). As we have previously mentioned in this case the Cr concentration's slightly decreasing plateau indicating that the presence of this element is still detectable even at about 21 nm in depth. A preliminary analysis of the effect of this second low energy Ar ion sputtering process on pure and Cr-diffused GaSb surfaces, several optical images were taken from the samples Sctr2 and SCr2. Optical images taken from the reference sample S0 before sputtering and from the two samples Sctr2 and SCr2 after the second sputtering process are shown in Fig 2. As all the samples were obtained from the same wafer, the image shown in Fig. 2A serves as a reference of the state of the surface of the Sctr2 and SCr2 samples before the sputtering. Polishing scratches were intentionally left on the surface of the samples, serving as an optical reference to reveal the sputtering erosion effects. These polishing marks have practically disap-
Due to the absence of “dot-like” structures after the previous two sputtering processes, a third sputtering process was carried out on the samples Sctr2 and SCr2 (becoming Sctr3 and SCr3). In order to establish some comparisons with the previous data, it was decided to keep the same accelerating voltage and ion flux Table 2 EDX quantitative analysis of the composition of unsputtered pure GaSb and sputtered pure and Cr-diffused GaSb samples Sample
Element
Composite (at.%)
S0
Ga Sb Ga Sb Ga Sb Cr
55.90 44.10 61.04 38.96 74.82 25.18 Not detected
Sctr3 SCr3
J.L. Plaza et al. / Surface & Coatings Technology 201 (2007) 8456–8462
as in the second sputtering process. However, the sputtering time was raised in this third sputtering process up to 30 min (see Table 1). With such a long sputtering time, it was expected to create nanodot hexagonal arrays. Arrow labelled as “3rd” shown in Fig. 1, indicates the mass spectrum corresponding to the third sputtering process. A preliminary optical microscopy analysis offers a rough first idea of the sputtering effects after the third sputtering process. The corresponding optical images are shown in Fig. 4 where the sputtered surface of Sctr3 (Fig. 4A) shows that the polishing scratches have almost disappeared due to the sputtering erosive effect. In the case of sample SCr3 (Fig. 4B), apart from the removal of the polishing marks, a high density of defects can also be observed compared to the previous sputtering processes (Fig. 2C). We have also performed an ultrahigh resolution (UHR) SEM analysis in order to identify possible sputtering-induced nanostructures. As we expected after increasing the sputtering time, it is now observed the formation of dot hexagonal arrays at the nanometer scale (about 50–80 nm in diameter) similar to those reported by Facsko et al. [2–6]. Fig. 5A and B shows SEM images obtained from the surface of the Sctr3 and SCr3 respectively after the third milling process. An statistical analysis of these images gives the following values for the mean area and aspect ratio of the dots: 55 nm2 and 2.2 for Sctr3 and 40 nm2 and 2.2 for SCr3. These values indicate that the size of the dots created on the Cr-diffused sample, SCr3 is smaller than in the case of the pure sample Sctr3 but present similar shape as it is also apparent from Fig 5A and B. The hexagonal arrangement is also similar in both samples. This would mean that the dopant diffusion does seem to have some effect on the dimensions of the nanostructures created by the sputtering process. Some other differences between the pure and Cr-diffused sample have also been observed like the presence of defects as previously observed in the optical analysis (Fig. 4B). The structure of these defects has also been studied in detail by SEM. An example of one of these defects, found on the surface of SCr3 after the third sputtering process is shown in Fig. 5C. It is observed that these defects are present in the form of cracks or voids on the surface with typical lateral dimensions of about 10 μm. However the nanodot formation is observed even at the very boundaries of these defects. In order to complete the SEM observations, an AFM analysis in Tapping Mode has also been developed after this third sputtering. Fig. 6A and B shows two typical AFM images of the surface of Sctr3 and SCr3 respectively. In agreement with the SEM images, the dots created on the surface of the pure GaSb sample Scrt seem to be bigger and higher than those created on the Cr-diffused sample SCr3. The typical lateral dimensions from these AFM pictures are 60 nm and 50 nm in size for Sctr3 and SCr3 respectively. However we must consider somehow smaller values as the convolution of the AFM tip tends to increase the lateral size of these nanodots. However the height of these nanostructures obtained from the AFM measurements (about 15 nm for Sctr3 and 6 nm for SCr3) gives a true confirmation of the smaller size of the dots created on the surface of the Cr-diffused sample.
8461
At this point it is important to make some remarks: It has been previously mentioned that, according to the SIMS analysis, when the nanodot formation starts (after the removal of about 40 nm of surface material), the Cr diffusion layer (only up to 26 nm deep) has almost been removed by the sputtering process. Therefore we should, in principle, not expect any major effects produced by the diffused Cr impurities on the structure of the nanodots. However, as it has been previously shown by SEM and AFM, the Cr impurities induced defects which are still present even when the diffusion layer has been removed. So it can be assumed that these defects could also be responsible for altering the structure of the dots formed during the sputtering, slightly decreasing the dimensions of the dots. However, it is expected that the “penetration depth” of the impurities diffused on the surface of GaSb samples is a key factor in order to induce further changes in the structure of the nanodot structures and probably the particular effect induced very much depends on the nature of the diffused atom. These facts suggests the possibility to diffuse smaller elements or the use of non-equilibrium techniques like ionic implantation in order to increase the thickness of the diffusion layer and to find out whether or not there is any additional effect on the formation of quantum dots induced by the presence of impurities. Such experiments are currently under way. Finally, a compositional study of the different surfaces has been developed by using energy dispersive X-ray analysis (EDX). All the spectra where taken at 20 keV. The results are shown in Table 2 both for pure and diffused samples. We also present, for comparison, the results obtained from the unsputtered sample S0. Four important observations must be pointed out; (i) It can be noticed that the Cr impurity has not been detected at all. This fact is expected as the EDX analysis was performed after the last sputtering process when the nanodots are formed. However, we know from the SIMS results that, after the third sputtering process, the diffusion layer has been almost completely removed and the presence of Cr, if any, must be well below the detection level of our EDX system. (ii) The composition of the standard sample slightly departs from stoichiometry. This is probably due to the formation of SbGa defects. This kind of defect has already been reported in the literature and it is known to be responsible for the high hole concentration (about 1017 cm− 3) which makes the undoped GaSb a p-type semiconductor [10]. (iii) The sputtering process clearly induces a Ga enrichment close to 5% on sample Sctr3. This fact has also been observed by Facsko et al [2] by using Auger spectroscopy in undoped GaSb sputtered samples and it is attributed to Sb losses from the surface during the sputtering. (iv) A more evident Ga enrichment is observed in the case of the Cr-diffused sample, SCr3 where the Ga concentration on the surface is up to 25% more than in S0. 4. Concluding remarks In conclusion we have analysed the effect of Cr diffused impurities on the formation of nanodots on GaSb substrates created by low energy argon ion sputtering. Three separate sputtering processes have been performed on both pure an Cr-
8462
J.L. Plaza et al. / Surface & Coatings Technology 201 (2007) 8456–8462
diffused GaSb samples. This provided us with the threshold of required eroded surface before the nanodots are formed. It is proved that both in pure and diffused samples the nanodots start to be formed after about 40 nm of the surface have been removed by the sputtering, for the experimental conditions used in this work. From the compositional point of view Ga enrichment at the surface has been found more pronounced in the case of Cr-diffused sample. Although the thickness (26 nm) of the Cr diffused layer resulted to be smaller than the formation threshold, the Cr impurities create defects upon sputtering and lowered the size of the dots. AFM and UHR-SEM confirmed the formation of dot hexagonal arrays at the nanometer scale both in pure GaSb and Cr-diffused GaSb samples. Acknowledgements The work was supported by Ministry of Education and Science of Spain under a Ramon y Cajal Contract and by the Projects ESP2004–0041-E, Mat 2003–09873 of the Education and Science Ministry, Spain and MAP-99–035 of the European Space Agency (ESA).
References [1] A.A. Tseng, K. Chen, C.D. Chen, K.J. Ma, IEEE Trans. Electron. Packag. Manuf. 26 (2003) 141. [2] S. Facsko, T. Bobek, H. Kurz, T. Dekorsy, S. Kyrsta, R. Cremer, Appl. Phys. Lett. 80 (2002) 130. [3] V.A. Shchukin, D. Bimberg, Rev. Mod. Phys. 71 (1999) 1125. [4] S. Facsko, H. Kurz, T. Dekorsy, Phys. Rev., B 63 (2001) 165329. [5] S. Facsko, T. Dekorsy, C. Koerdt, C. Trappe, H. Kurz, A. Vogt, H.L. Hartnagel, Science 285 (1999) 1551. [6] S. Facsko, T. Bobek, T. Dekorsy, H. Kurz, Phys. Status Solidi, B Basic Res. 224 (2001) 537. [7] V. Valbusa, C. Boragno, F. Buatier de Mongeot, J. Phys., Condens. Matter. 14 (2002) 8153. [8] G. Carter, J. Phys., D Appl. Phys. 34 (2001) R1. [9] G.N. Panin, P.S. Dutta, J. Piqueras, E. Diéguez, Appl. Phys. Lett. 67 (1995) 3584. [10] P.S. Dutta, H.L. Bhat, V. Kumar, J. Appl. Phys. 81 (1997) 5821. [11] F. Frost, A. Schindler, F. Bigl, Phys. Rev. Lett. 85 (2000) 4116. [12] R.M. Bradley, J.M.E. Harper, J. Vac. Sci. Technol., A, Vac. Surf. Films 6 (1988) 2390. [13] M.A. Makeev, A.L. Barabasi, Appl. Phys. Lett. 71 (1997) 2800.
Nanodot hexagonal ordered arrays on GaSb substrates by LEIS under the presence of chromium diffused impurities J.L. Plaza ⁎, Bárbara Capote, E. Diéguez Laboratorio de Crecimiento de Cristales, Departamento de Física de Materiales, Facultad de Ciencias, Universidad Autónoma de Madrid, 28049, Cantoblanco, Madrid, Spain Available online 12 March 2007
Abstract In this work we study the effect of Cr impurities, diffused onto GaSb substrates, on the formation of nanodot hexagonal arrays created by low energy argon ion sputtering. For the same sputtering conditions, the dimensions of the dots in the Cr-diffused sample are smaller than in the case of the pure sample. EDX data revealed that the sputtering induces a more pronounced Ga enrichment in the case of Cr-diffused sample. It is proved that oblique incidence in rotating configuration most probably delays the formation of the nanodots compared to previously reported normal incidence experiments. The diffusion of Cr atoms onto the GaSb surface induced the formation of defects upon sputtering. © 2007 Elsevier B.V. All rights reserved. Keywords: Low energy ion sputtering; Gallium antimonide; Nanodots
1. Introduction Much effort has been devoted in the last decade to the development of periodic structures at the nanometer scale. This is actually one of the main goals in the field of nanotechnology at the present time. As a driving force for this current field of research, we can mention that the formation of controlled nanostructures in semiconducting materials appears to be an important issue in the future development of the next generation of optoelectronic devices. Electron beam lithography (EBL) is at present well known in the field of nanotechnology for being an essential technique to be used in the fabrication of controlled arrays of periodical nanostructures. However, its low throughput makes this technique suitable only for prototype fabrication in the laboratory environment or as support technique in the industry [1]. Difficulties like these are pushing through a great effort in the development of parallel processing techniques to be employed in the fabrication of periodically arranged structures at the nanometer scale. To cite few of them we can mention semiconductor nanocrystal self assembly [2] or the Stransky– Krastanov method for the growth of semiconductor heterostructures and quantum dots [3]. Low energy ion sputtering ⁎ Corresponding author. Tel.: +34 91 4974784; fax: +34 91 4978579. E-mail address: [email protected] (J.L. Plaza). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.02.061
(LEIS) has also been recently presented as a technique appearing to be very promising for the fabrication of nanostructures and quantum dots. Therefore it represents an attractive alternative to the Stranski–Krastanov method. In this way hexagonal arrays of uniform semiconductor quantum dots have been created by LEIS using Ar+ ions [4,5] similar to what Facsko et al. [6] reported for the case of GaSb. In the latter work it is shown how hexagonal dot patterns can be spontaneously formed on GaSb and InSb when the surface of these materials are sputtered with low energy argon ions at normal incidence. The dot density can be easily controlled independently by properly tuning the sputtering time and the energy of the incident ions. Extensive reviews have been devoted to the formation of ordered nanostructures both on metals [7] and semiconductors [8]. The reader is referred to these review works in order to get basic information about the physical processes involved in the formation of periodic nanostructures by LEIS. However, and for what to this work concerns, we must recall that the selforganization of nanodot periodic arrays during sputtering has been attributed to an interplay between surface roughening induced by ion sputtering and smoothing processes of the surface [4]. This work is devoted to the effects of LEIS on pure and doped (impurity diffused) gallium antimonide (GaSb), one of the semiconductors that is becoming extremely relevant in the field of optoelectronics and communications (see for example the review from Dutta et al. [10] and references therein).
J.L. Plaza et al. / Surface & Coatings Technology 201 (2007) 8456–8462
8457
Table 1 Sample notation and sputtering parameters Name of the sample
Description
Sputtering process
Acceleration voltage (V)
Sputteringtime (s)
Ion flux(cm− 2 s− 1)
S0 Sctr1 SCr1 Sctr2 SCr2 Sctr3 SCr3
Pure GaSb Pure GaSb Cr-diffused Pure GaSb Cr-diffused Pure GaSb Cr-diffused
No Sputtered First First Second Second Third Third
– 500 500 600 600 600 600
– 200 200 300 300 1800 1800
– 6 × 1015 6 × 1015 1.5 × 1016 1.5 × 1016 1.5 × 1016 1.5 × 1016
The formation of nanostructures on GaSb by using low energy Argon ion sputtering has been recently reported in the series of papers by Facsko et al. [4–6]. They reported the formation of hexagonal nanostructure arrays and quantum dots on GaSb. An important contribution on this subject has also been published by Panning et al. [9], where the p to n-type conversion on GaSb surfaces is obtained by using the same low energy ion sputtering same technique. However, deeper research must be devoted to the LEIS technique on semiconductor materials. In particular, to the best of the authors’ knowledge, the effect of diffused dopants on the formation of nanostructures created by LEIS has not been investigated yet. In this paper, we study the effect of Cr impurities diffused onto GaSb substrates, on the formation of nanodots fabricated by using low energy Ar+ ion sputtering.
Three different processes were developed with different Ar ion energies and different exposure times. In this way the gradual effect of the LEIS on the surface of the samples could be analysed. The following notation will be used throughout the text: Sctr1, Sctr2, Sctr3, and SCr1, SCr2 and SCr3 to denote the pure GaSb and Cr-diffused samples after the first, second and third sputtering processes respectively. The sputtering parameters of all the samples are given in Table 1 being always 45° the incidence angle of the ion beam on the target (samples). The samples were also continuously rotating during the sputtering at 15 rpm. The success of this technique of rotating the sample under oblique incidence in creating hexagonal arrangements of regular nanodots has already been reported by several researchers [5,11–13]. A Millatron ion miller from Commonwealth Scientific Corp. was used in the sputtering experiments.
2. Experimental
3. Results and discussion
A single wafer from a Bridgman-grown GaSb ingot grown in our laboratory was initially taken and polished with 1 μm alumina powder. The wafer was divided into three pieces of about the same size. Each one of them was used to prepare three different samples. A first as-grown sample, S0, was not submitted to any further process and has been used as a reference sample. The second sample, Sctr, was used as a control sample being exposed to a LEIS process without any other previous treatment. Chromium was thermally evaporated by using an electron beam on the third sample, SCr. An Edwards auto 306 Turbo was employed to this purpose. The evaporation parameters were: evaporation time 5 min; electron beam current 45 mA, base pressure 3.0 × 10− 5 mbar, and the resulting thickness of the evaporated Cr film was 170 nm. An annealing treatment for 9 h was carried out after the evaporation process in order to promote diffusion of the Cr atoms into the surface of the GaSb substrate. This process was performed under 300 cm3 of nitrogen flux. In order to avoid high losses of Sb during the annealing, the temperature was kept at 250 °C. The evaporated Cr film was removed after annealing by using wet chemical etching immersing the sample for 2 h in HCl heated at 60 °C in a hot plate inside a laminar flow fume-cupboard. The acid does not damage the GaSb surfaces. Subsequently, the sample was rinsed in acetone. After the annealing treatment the sputtering process was carried out on the three (as-grown, pure GaSb, and Cr-diffused GaSb) samples by using Ar+ ions.
3.1. Analysis after the first sputtering process Samples Sctr1 and SCr1 were sputtered at the same time in order to be able to establish comparisons between them. Table 1 gives the parameters related to the first sputtering process. Initially, they were chosen to be the same as those used by Facsko et al. for the sputtering of amorphous and crystalline GaSb surfaces [2]. However, two main differences are present in our experimental setup compared to Facsko's. The first one
Fig. 1. Secondary ion mass spectrum of the Cr impurity obtained during the consecutive three sputtering processes from the Cr-diffused sample (SCr1, SCr2, SCr3). The arrows indicate the extent of the first, second and third sputtering process respectively.
8458
J.L. Plaza et al. / Surface & Coatings Technology 201 (2007) 8456–8462
all clear if they are completely equivalent. One of the goals of this work is to clarify this subject. It is also intended to prove weather or not the amount of time needed for the nanodot formation is the same for both configurations and to evaluate the possible differences in their geometry. At the left hand side of Fig. 1 the mass spectrum obtained from the secondary ions corresponding to the first sputtering process is presented. The second and third successive LEIS mass spectra are also shown and will be discussed later on. As Ga and Sb remained constant throughout the process and gave no additional information, only the relevant element, Cr is
Fig. 2. Optical images from (A) S0, (B) Sctr2 and (C) SCr2 after the second sputtering.
corresponds to the ion flux used in our case being 6 × 1015 cm− 2 s− 1. This value is nearly 40% less than that one used by Facsko which was 1 × 1016 cm− 2 s− 1 [2]. The different geometrical arrangement of our sputtering process is the second distinction to be considered, namely while Facsko used normal incidence, we use a combination of oblique incidence (45°) and rotation (15 rpm). Although both configurations have been reported to lead to stable hexagonally ordered dot patterns [2–6], it is not at
Fig. 3. AFM images from (A) S0, (B) Sctr2 and (C) SCr2.
J.L. Plaza et al. / Surface & Coatings Technology 201 (2007) 8456–8462
8459
to Facsko's most probably delay the appearance of this kind of nanostructures. 3.2. Analysis after the second sputtering process A second sputtering process on samples Sctr1 and SCr1, thus becoming samples Sctr2 and SCr2, was carried out onto the two samples with the aim of studying further effects induced by LEIS. Samples Sctr2 and SCr2 were sputtered at the same time as in the previous LEIS process. In order to promote the formation of hexagonal dot patterns, the accelerating voltage, ion flux and the exposure time were further increased compared to the values used in the first sputtering as shown in Table 1. The Cr
Fig. 4. Optical images from (A) Sctr3 and (B) SCr3 after the third sputtering.
shown in Fig. 1. In the portion of the mass spectrum related to the first LEIS process corresponding to sample SCr1, the Cr signal is present, in a relatively remarkable level, up to several nm deep into the GaSb surface, providing a confirmation of the actual formation of a Cr diffusion layer. We can also extract information about the thickness of this impurity layer created during the diffusion process, according to the point where the signal reaches a flat evolution. However the mass spectra corresponding to the second sputtering still indicates a low but still appreciable Cr concentration up to 21 nm where the Cr signal dramatically drops. This point indicates that the diffusion layer has been almost completely removed from the surface by the erosive effect of the sputtering process. The thickness thus obtained for the high concentration Cr layer is about 5 nm, while the total thickness considering the high and low Cr concentration regions is 26 nm. It must be pointed out that the formation of nanodot hexagonal patterns on the surface of the sample has not been observed after the first LEIS process. This fact disagrees with the formation of dot hexagonal patterns reported by Facsko et al. [2] for the same values of the ion energy and sputtering time. However the previously mentioned differences in our setup (lower ion flux and oblique incidence with rotation), compared
Fig. 5. Ultra-high resolution SEM images from (A) Sctr3 and (B) SCr3 after the third sputtering. (C) Detail of a defect found in SCr3.
8460
J.L. Plaza et al. / Surface & Coatings Technology 201 (2007) 8456–8462
Fig. 6. AFM images from (A) Sctr3 and (B) SCr3.
peared due to the erosion of the surface as can be observed on the sputtered surface of sample Sctr2 (Fig. 2B). It must be pointed out that the presence of defects is not observed in this sample. A different behaviour has been observed in the Cr-diffused sample, SCr2. In this case a high density of defects can observed (Fig. 2C). The fact that the defects appear only in SCr2, points to conclusion that their formation is greatly enhanced by the presence of impurities on the GaSb surface. This is probably related to the lattice distortion generated by the impurity diffusion. A threshold sputtering time seems to exist before the formation of this kind of defects as they have not been observed during the shorter first sputtering process. At this point it is worth noting that a preliminary similar work on Cr-diffused GaSb, to be published elsewhere, has revealed the same behaviour. However in that case the defects present a more regular shape and could be more well defined as cracks. In order to determine in more detail the morphology of the sputtered surfaces, an atomic force microscopy (AFM) analysis has been carried out. The aim of using this technique was to provide us with more information about some other features on the surface induced by the sputtering that could still remain unnoticed after the optical microscopy observations. Fig. 3 shows the 3d-images from this AFM analysis corresponding to S0, Sctr2 and SCr2. The polishing marks can be clearly observed in S0 (Fig. 3A). However, in the case of Sctr2 and SCr2 (Fig. 3B,C) the polishing marks are smeared out as a result of the LEIS process. Therefore, the LEIS process effectively reduces the roughness of the sample due to the surface erosion caused by the Ar ion beam. This effect is observed as a partial removal of the polishing marks from surface of the sample. The previous AFM results altogether with further ultrahigh resolution SEM analysis have shown that even after rising the sputtering time and the ion flux, the formation of hexagonal arrays of nanodot structures has not been attained. It is likely that the differences in the sputtering configuration (45° oblique incidence and sample rotation vs. normal incidence used by Facsko et al.) might be the cause for this delay. 3.3. Analysis after the third sputtering process
secondary ion mass spectrum corresponding to this second LEIS process is also presented in Fig. 1 (part of the spectrum covered by the range of the arrow named as “2nd”). As we have previously mentioned in this case the Cr concentration's slightly decreasing plateau indicating that the presence of this element is still detectable even at about 21 nm in depth. A preliminary analysis of the effect of this second low energy Ar ion sputtering process on pure and Cr-diffused GaSb surfaces, several optical images were taken from the samples Sctr2 and SCr2. Optical images taken from the reference sample S0 before sputtering and from the two samples Sctr2 and SCr2 after the second sputtering process are shown in Fig 2. As all the samples were obtained from the same wafer, the image shown in Fig. 2A serves as a reference of the state of the surface of the Sctr2 and SCr2 samples before the sputtering. Polishing scratches were intentionally left on the surface of the samples, serving as an optical reference to reveal the sputtering erosion effects. These polishing marks have practically disap-
Due to the absence of “dot-like” structures after the previous two sputtering processes, a third sputtering process was carried out on the samples Sctr2 and SCr2 (becoming Sctr3 and SCr3). In order to establish some comparisons with the previous data, it was decided to keep the same accelerating voltage and ion flux Table 2 EDX quantitative analysis of the composition of unsputtered pure GaSb and sputtered pure and Cr-diffused GaSb samples Sample
Element
Composite (at.%)
S0
Ga Sb Ga Sb Ga Sb Cr
55.90 44.10 61.04 38.96 74.82 25.18 Not detected
Sctr3 SCr3
J.L. Plaza et al. / Surface & Coatings Technology 201 (2007) 8456–8462
as in the second sputtering process. However, the sputtering time was raised in this third sputtering process up to 30 min (see Table 1). With such a long sputtering time, it was expected to create nanodot hexagonal arrays. Arrow labelled as “3rd” shown in Fig. 1, indicates the mass spectrum corresponding to the third sputtering process. A preliminary optical microscopy analysis offers a rough first idea of the sputtering effects after the third sputtering process. The corresponding optical images are shown in Fig. 4 where the sputtered surface of Sctr3 (Fig. 4A) shows that the polishing scratches have almost disappeared due to the sputtering erosive effect. In the case of sample SCr3 (Fig. 4B), apart from the removal of the polishing marks, a high density of defects can also be observed compared to the previous sputtering processes (Fig. 2C). We have also performed an ultrahigh resolution (UHR) SEM analysis in order to identify possible sputtering-induced nanostructures. As we expected after increasing the sputtering time, it is now observed the formation of dot hexagonal arrays at the nanometer scale (about 50–80 nm in diameter) similar to those reported by Facsko et al. [2–6]. Fig. 5A and B shows SEM images obtained from the surface of the Sctr3 and SCr3 respectively after the third milling process. An statistical analysis of these images gives the following values for the mean area and aspect ratio of the dots: 55 nm2 and 2.2 for Sctr3 and 40 nm2 and 2.2 for SCr3. These values indicate that the size of the dots created on the Cr-diffused sample, SCr3 is smaller than in the case of the pure sample Sctr3 but present similar shape as it is also apparent from Fig 5A and B. The hexagonal arrangement is also similar in both samples. This would mean that the dopant diffusion does seem to have some effect on the dimensions of the nanostructures created by the sputtering process. Some other differences between the pure and Cr-diffused sample have also been observed like the presence of defects as previously observed in the optical analysis (Fig. 4B). The structure of these defects has also been studied in detail by SEM. An example of one of these defects, found on the surface of SCr3 after the third sputtering process is shown in Fig. 5C. It is observed that these defects are present in the form of cracks or voids on the surface with typical lateral dimensions of about 10 μm. However the nanodot formation is observed even at the very boundaries of these defects. In order to complete the SEM observations, an AFM analysis in Tapping Mode has also been developed after this third sputtering. Fig. 6A and B shows two typical AFM images of the surface of Sctr3 and SCr3 respectively. In agreement with the SEM images, the dots created on the surface of the pure GaSb sample Scrt seem to be bigger and higher than those created on the Cr-diffused sample SCr3. The typical lateral dimensions from these AFM pictures are 60 nm and 50 nm in size for Sctr3 and SCr3 respectively. However we must consider somehow smaller values as the convolution of the AFM tip tends to increase the lateral size of these nanodots. However the height of these nanostructures obtained from the AFM measurements (about 15 nm for Sctr3 and 6 nm for SCr3) gives a true confirmation of the smaller size of the dots created on the surface of the Cr-diffused sample.
8461
At this point it is important to make some remarks: It has been previously mentioned that, according to the SIMS analysis, when the nanodot formation starts (after the removal of about 40 nm of surface material), the Cr diffusion layer (only up to 26 nm deep) has almost been removed by the sputtering process. Therefore we should, in principle, not expect any major effects produced by the diffused Cr impurities on the structure of the nanodots. However, as it has been previously shown by SEM and AFM, the Cr impurities induced defects which are still present even when the diffusion layer has been removed. So it can be assumed that these defects could also be responsible for altering the structure of the dots formed during the sputtering, slightly decreasing the dimensions of the dots. However, it is expected that the “penetration depth” of the impurities diffused on the surface of GaSb samples is a key factor in order to induce further changes in the structure of the nanodot structures and probably the particular effect induced very much depends on the nature of the diffused atom. These facts suggests the possibility to diffuse smaller elements or the use of non-equilibrium techniques like ionic implantation in order to increase the thickness of the diffusion layer and to find out whether or not there is any additional effect on the formation of quantum dots induced by the presence of impurities. Such experiments are currently under way. Finally, a compositional study of the different surfaces has been developed by using energy dispersive X-ray analysis (EDX). All the spectra where taken at 20 keV. The results are shown in Table 2 both for pure and diffused samples. We also present, for comparison, the results obtained from the unsputtered sample S0. Four important observations must be pointed out; (i) It can be noticed that the Cr impurity has not been detected at all. This fact is expected as the EDX analysis was performed after the last sputtering process when the nanodots are formed. However, we know from the SIMS results that, after the third sputtering process, the diffusion layer has been almost completely removed and the presence of Cr, if any, must be well below the detection level of our EDX system. (ii) The composition of the standard sample slightly departs from stoichiometry. This is probably due to the formation of SbGa defects. This kind of defect has already been reported in the literature and it is known to be responsible for the high hole concentration (about 1017 cm− 3) which makes the undoped GaSb a p-type semiconductor [10]. (iii) The sputtering process clearly induces a Ga enrichment close to 5% on sample Sctr3. This fact has also been observed by Facsko et al [2] by using Auger spectroscopy in undoped GaSb sputtered samples and it is attributed to Sb losses from the surface during the sputtering. (iv) A more evident Ga enrichment is observed in the case of the Cr-diffused sample, SCr3 where the Ga concentration on the surface is up to 25% more than in S0. 4. Concluding remarks In conclusion we have analysed the effect of Cr diffused impurities on the formation of nanodots on GaSb substrates created by low energy argon ion sputtering. Three separate sputtering processes have been performed on both pure an Cr-
8462
J.L. Plaza et al. / Surface & Coatings Technology 201 (2007) 8456–8462
diffused GaSb samples. This provided us with the threshold of required eroded surface before the nanodots are formed. It is proved that both in pure and diffused samples the nanodots start to be formed after about 40 nm of the surface have been removed by the sputtering, for the experimental conditions used in this work. From the compositional point of view Ga enrichment at the surface has been found more pronounced in the case of Cr-diffused sample. Although the thickness (26 nm) of the Cr diffused layer resulted to be smaller than the formation threshold, the Cr impurities create defects upon sputtering and lowered the size of the dots. AFM and UHR-SEM confirmed the formation of dot hexagonal arrays at the nanometer scale both in pure GaSb and Cr-diffused GaSb samples. Acknowledgements The work was supported by Ministry of Education and Science of Spain under a Ramon y Cajal Contract and by the Projects ESP2004–0041-E, Mat 2003–09873 of the Education and Science Ministry, Spain and MAP-99–035 of the European Space Agency (ESA).
References [1] A.A. Tseng, K. Chen, C.D. Chen, K.J. Ma, IEEE Trans. Electron. Packag. Manuf. 26 (2003) 141. [2] S. Facsko, T. Bobek, H. Kurz, T. Dekorsy, S. Kyrsta, R. Cremer, Appl. Phys. Lett. 80 (2002) 130. [3] V.A. Shchukin, D. Bimberg, Rev. Mod. Phys. 71 (1999) 1125. [4] S. Facsko, H. Kurz, T. Dekorsy, Phys. Rev., B 63 (2001) 165329. [5] S. Facsko, T. Dekorsy, C. Koerdt, C. Trappe, H. Kurz, A. Vogt, H.L. Hartnagel, Science 285 (1999) 1551. [6] S. Facsko, T. Bobek, T. Dekorsy, H. Kurz, Phys. Status Solidi, B Basic Res. 224 (2001) 537. [7] V. Valbusa, C. Boragno, F. Buatier de Mongeot, J. Phys., Condens. Matter. 14 (2002) 8153. [8] G. Carter, J. Phys., D Appl. Phys. 34 (2001) R1. [9] G.N. Panin, P.S. Dutta, J. Piqueras, E. Diéguez, Appl. Phys. Lett. 67 (1995) 3584. [10] P.S. Dutta, H.L. Bhat, V. Kumar, J. Appl. Phys. 81 (1997) 5821. [11] F. Frost, A. Schindler, F. Bigl, Phys. Rev. Lett. 85 (2000) 4116. [12] R.M. Bradley, J.M.E. Harper, J. Vac. Sci. Technol., A, Vac. Surf. Films 6 (1988) 2390. [13] M.A. Makeev, A.L. Barabasi, Appl. Phys. Lett. 71 (1997) 2800.