Anal. Chem. 1995, 67, 1530-1535
In Situ Dnvestigation of Aluminum Gallium Arsenide/Gallium Arsenide Multilayer Structures under Dnert and Reactive Media by Atomic Force Microscopy Thomas Pmhaska, Gemot Friedbacher,* and Manfred Grasserbauer Institute of Analytjcal Chemistry, Vienna University of Technology, Getreidemadd 9/151, A- 1060 Wien, Austria
Heinrich Nickel, Rainer LOsch, and Winfried Schlapp DBP-Telekom-R, P.O. Box 100003, 0-64276 Darmstadt, Germany
In this paper we describe the analytical benefits of in situ preparation and imaging under inert and reactive media using an AlGaAs/GaAs superlattice structure as a welldefined model sample. Imaging under inert organic liquids like toluene has been made possible by means of a home-builtsample holder which has been designed for mechanical mounting of small-scale, cleaved specimens and which works without polymer seals in the liquid cell. The results show that sensitive surfaces can be protected from atmospheric influences without the need for ultrahigh-vacuum conditions. Contrast development on the AlGaAs/GaAs system by preferential oxidation on the AlGaAs layers could be completely suppressed by covering the cross-sectional cleavage surface with toluene. In situ studies of corrosion processes on oxidized AIGaAs/ GaAs cleavage surfaces have shown that 0.01 and 0.001 M HCl lead to pitting, while 0.1 M HCl results in a clear contrast inversion of the original corrugation of 0.3-0.6 nm. The described in situ imaging capability significantly enhances the information content of atomic force microscopy (AFM) images, since it allows assignmentof chemical information to topographical features primarily seen in AFM images. Because they allow study of materials under ambient conditions, both scanning tunneling microscopy (STM) and atomic force microscopy (AFM)-6 bear the potential for in situ investigation of surface processes down to the atomic scale. Since AFM also allows imaging of insulators and irnages can usually be interpreted straightforwardly as topographical information, this method exhibits the more general applicability to analytical problems. A drawback of the technique is that the images do not contain direct chemical information. One goal of this work was to show that this shortcoming can be overcome by assigning chemical information to topographical domains via in situ observa(1) Binnig, G.; Rohrer, H.; Gerber, Ch.; Weibel, E. Phys. Reu. Left. 1982,49, 57-61. (2) Frommer, J. Angew. Chem. 1992,104, 1325-1357. (3) Magonov, S. N. Appl. Spectrosc. Rev. 1993,28, 1-121. (4) Binnig, G.; Quate, C. F.; Gerber, Ch. Phys. Reu. Lett. 1986,56, 930-933. (5) Rugar, D.; Hansma, P. Phys. Today 1990,43, 23-30. (6) Friedbacher, G.; Prohaska, T.; Grasserbauer, M Mikrochim. Acta 1994, 113, 179-202.
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tion of changes introduced by selective chemical reactions. Moreover, it should be tested how efficiently sensitive surfaces can be protected from atmospheric influences by preparing and imaging them under inert liquids without the need for ultra-highvacuum 0conditions. Cross-sectional cleavage surfaces through semiconductor multilayer systems were chosen for methodological development and evaluation of the analytical scope and limitations of this concept. The first cross-sectional analysis on AlGaAs/GaAs layer structures by STM and scanning tunneling potentiometry (STP) in UHV was reported by Muralt et aL7 Further investigations have been performed and discussed for AlGaAs/GaAs heteroand other system^.'^-'^ It was shown that STM is a valuable tool for determination of the evenness of the superlattice structure and for investigation of alloy fluctuations and interface roughness on the atomic s~a1e.l~ Since the tunneling current is sensitive to both tip sample separation and local density of states near the Fermi level, it is dif6cult to extract the surface topography on the atomic scale from STM measurements alone. This problem can be overcome by using AFM, since the images obtained contain mainly topographical information. Howells et al.'* have compared AFM with STM results obtained on an InGaAs/InP superlattice. AFM has also been described as a tool for characterizing layer thickness and evenness of an AlGaAs/GaAs superlattice via imaging of both etched and untreated cross-sectiocal cleavage (7) Muralt, P.; Meier, H.; Pohl, D. W.; Salemink. H. Superlattices Microstruct. 1986,2 (6), 519-520. (8) Gomez-Rodriguez, J. M.; Bard, A. M.; Silveira, J. P.; Vazquez, M.; GonzAlez, Y.; Briones, F. Appl. Phys. Lett. 1990,56, 36-38. (9) Albrektsen, 0.;Arent, D. J.; Meier, H. P.; Salemink, H. W. M. Appl. Phys. Lett. 1990,57 (l),31-33. (10) Dagata, J. A; Tseng, W.: Bennett, J.; Schneir, J.; Harary, H. H. Ultramicroscopy 1992,42-44, 1288-1294. (11) Gwo, S.; Chao, K J.; Shih, C. K Appl. Phys. Lett. 1994,64 (41, 493-495. (12) Pinnington, T.: Patitsas, S. N.: Lavoie, C.; Sanderson, A; Tiedje, T./. Vat. Sci. Technol. 1993,Bll (3), 908-911. (13) Johnson, M. B.; Maier. E.: Meier, H. P.; Salemink, H. W. M. Appl. Phys. Left. 1993,€3 (9), 1273-1275. (14) Gwo, S.; Chao, K J.; Smith, A R; Shih, C. IC; Sadra, K; Streetman, B. G. /. Vac. Sci. Technol. 1993,B11 (4), 1509-1513. (15) Kato, T.: Osaka, F.; Tanaka, 1.Jpn.J. Appl. Phys. 1989,28 (6).1050-1053. (16) Kato, T.; Osaka, F./pn. /. Appl. Phys. 1991,30 (SA), L1586-Ll587. (17) Yu,E. T.; Halbout, J. M.; Powell, A. R.; Iyer. S. S.Appl. Phys. Lett. 1992.61 (26), 3166-3168. (18) Howells, S.; Gallagher, M.J.; Chen, T.; Pax, P.; Sarid, D. Apgl. Phys. Lett. 1992,61 (7), 801-803.
0003-2700/95/0367-1530$9.00/0 0 1995 American Chemical Society
Table 1. Composition of the Investigated AIGaAs/GaAs Sample Along with Nominal and Measured Thicknesses of the Layers
layer L1
LZ L3
LA a
composition and nominal thickness
thickness measd by AFM (nm)
40 x (15nm 15 f 1: 13 f lb Alo.4Gao.&/l3 nm GaAs) 100nmGaAs 100f2 100 nm Alo.4Gao.sAs 100 f 2 2pmGaAs
cantilever holder
substrate
\ 0 1 I 0,
GaAs
cantilever
I
AlGaAs. GaAs.
surfaces in air.Ig Chalmers et al. have used AFM to image tilted superlattice structures under HC1 solution.20 Here, we have used cross-sectional cleavage surfaces through such multilayer systems as well-defined model samples to test and develop the analytical potential of AFM with respect to studying oxidation and corrosion processes under ambient conditions starting from freshly prepared surfaces protected by inert organic liquids. EXPERIMENTAL SECTION
Layer sequence and specifications of the AlGaAs/GaAs multilayer system grown by molecular beam epitaxy (MBE) are listed in Table 1. Investigations were performed with a NanoScope I11 atomic force microscope (Digital Instruments, Santa Barbara, CA), whose detection scheme is based on laser beam deflection off a microfabricated Si3N4 cantilever with an integrated pyramidal tip.21t22 Measurements were performed in constant force mode with an imaging force in the range of 10-7-10-9 N. The scan rate for the fast scan direction of the tip was kept at 9 Hz. For the cross-sectional studies, the wafers were cleaved into 5 mm x 5 mm squares by scratching the rear of the wafer with a diamond scriber. This procedure led to a sharp cleavage edge without destroying the topmost region of the multilayer structure. The sample was then mounted mechanically in the slot of the aluminum cylinder of our home-built sample holder, as shown in Figure 1. In the case of aqueous media and ethanol, the cell was sealed with a silicone O-ring in the conventional way (not shown in Figure 1). The AFM probe was positioned over the area of interest by means of a mechanical xy-stage and observation through a stereozoom microscope. For the investigations under HCI, instrument alignment was carried out under ethanol. In this way it was guaranteed that images could be recorded immediately after flooding the cell with HCl in order to monitor the first steps of interaction. During scanning, ethanol was exchanged by HCl. Thus, the same sample area could be observed from the very first contact with the acid up to several hours afterward. During measurements, fresh HCl was pressed through the liquid cell to guarantee constant etching conditions. For the in situ preparation under toluene, the wafer was cleaved in a beaker under toluene by the same procedure as in air. The (19) Friedbacher, G.; Hansma, P. IC;Schwarzbach, D.; Grasserbauer, M.; Nickel, H. Anal. Chem. 1992.64,1760-1762. (20) Chalmers, S. A; Gossard, A C.; Weisenhom, A. L.; Gould, S. A C.; Drake, B.;Hansma, P. K Af$l. Phys. Lett. 1989.55(24), 2491-2493. (21) Meyer, G.; h e r , N. M. Appl. Phys. Lett. 1988,53,2400-2402. (22) Albrecht, T. R; Akamine, S.; Carver, T. E.; Quate, C. F.J. Vac. Sci. Technol. 1990,A8, 3386-3396.
spring
Figure 1. Schematic view of the NanoScope Ill liquid cell together with the home-built sample holder designed for imaging small-scale samples under organic and inorganic liquids.
home-built sample holder (Figure 1)was immersed into toluene, too. After fkation of the wafer, the sample holder was mounted on the AFM scanner, while the cleavage surface was kept covered with toluene. During the adjustment of the tip near the edge of the wafer, fresh toluene was injected into the liquid cell, keeping the surface covered with toluene throughout the adjusting procedure. Lateral positioning of the tip on top of the layer structure was performed by means of the mechanical xy-stage while the tip was scanning across the sample surface. To carry out measurements under toluene, a new sample holder was needed (Figure 1). This sample holder was designed for imaging under a variety of organic liquids which cannot be used in the commercial cell. It consists of two concentric cylinders made of aluminum. The sample can be fixed in the slot of the inner cylinder by means of a screw. This cylinder is glued onto a thin steel disk which allows mounting on the magnetic top of the scanner. The outer ring is moveable in height and is gently pressed toward the cantilever holder by means of a spring. This allows motion of the sample in the x-, y, and z-directions. The important feature of this sample holder is that it works without polymer seals, which swell under the influence of a number of organic liquids (e.g., toluene), making stable measurements impossible. Moreover, due to the mechanical fkation capability, no glues are necessary to mount small samples, thus introduction of impurities into the liquid medium and finally on the sample surface can be avoided. RESULTS AND DISCUSSION
In air, the freshly cleaved AlGaAs/GaAs multilayer structure (Table 1) shows reproducible corrugation between 0.3 and 0.6 nm, where the AlGaAs layers form the peaks (Figure 2). Throughout this paper, the term corrugation is used for the height differences between the different layers of the multiquantum well structure. Even after several days of exposure to air, the structure was still visible with an unchanged corrugation. This can be explained as a result of formation of a passivating surface aluminum oxide layer through out-diffusion of aluminum.23 The width of the single layers has been determined via the full width at half-maximum (fwhm) of the corrugated structure at different (23) Mesanvi, A; Ignatiev, A J. Appl. Phys. 1992,71 (4), 1943-1948.
Analytical Chemistry, Vol. 67, No. 9, May 1, 1995
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$ 1 D
O O
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40
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distance [nm]
E
0.5
Y
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0
0
20
40.
60
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c) Figure 2. AFM image of the oxidized AIGaAs/GaAs cleavage surface in air. The corrugation is 0.3-0.6 nm. The measured width of the AlGaAs and GaAs layers is 15 f 1 and 13 f 1 nm, respectively. Image size, 1 pm x 1 pm; depth scale, 1 nm from black to white.
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,
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sample positions. These values are in good agreement with the nominal values from the manufacturer derived from deposition rates (I‘able 1). The corrugation of 0.3-0.6 nm observed in the topographical AFM images can be interpreted as preferential oxidation of A G ~ A s . ~Similar ~ results were obtained on an InGaAs/InAlAs sample. In this case, the corrugation across the multilayer structure was also between 0.3 and 0.6 nm. In further systems, it turned out that chemical differences of p and ndoped layers (1017-1018 cm-3) were not pronounced sufficiently to distinguish these layers by this contrast mechanism. To make sure that the observed corrugation is not affected by convolution of the sample topography with the tip geometry, different convolutions of a rectangular profile (spacing, 20 nm) with spherical tips (radius of curvature, 20 and 50 nm) were compared with the experimental profile (Figure 3). From that comparison and from the fact that the experimental profiles were reproducible with different tips (even with commercially available silicon which are generally sharper than the standard SiN4 tips2?, it can be concluded that the AFM images represent the real sample topography. Even tips with a hypothetical radius of curvature of 50 nm should be able to follow a maximum corrugation of 1 nm on the rectangular profile of F‘igure 3a. Moreover, AFM profiles across monoatomic steps on different single crystals show much steeper slopes than that shown in Figure 3d, which further supports that this profile represents the real topography of the sample. The width of the interface between the alternating layers, as determined through the standard deviation of the fwhm values on the oxidized structure, was about 2-3 nm. This scatter could be explained by preferential oxidation of the A-rich regions of the interface and subsequent outdiffusion of aluminum into the 0xide.13” At the interface between the wafer substrate and the multilayer structure steps were plainly visible in the AFM images. The heights of these steps varied from 1nm up to several nanometers on different cleavage surfaces. This could be explained as a result of an increased number of defects at that interface due to residual impurities like carbon and oxygen on the initial wafer substrate prior to MBE growth. (24) Nanoprobe, Wetzlar-Blankenfeld,Germany, 1992.
1532 Analytical Chemistry, Vol. 67, No. 9, May 1, 1995
distance [nm]
Figure 3. Convolution of a rectangular profile with different spherical tips compared with the experimental profile obtained with AFM. (a) Rectangular profile, spacing 20 nm; (b) convolution of (a) with a spherical tip of r = 50 nm; (c) convolution of (a) with a spherical tip of r = 20 nm; (d) experimental profile.
As already mentioned, a major goal of our work was to show that sensitive surfaces can be protected by in situ preparation and investigation under inert organic media. Figure 4a shows the surface of the AGaAs/GaAs system prepared by in situ cleaving and mounting of the sample in the homebuilt sample holder under toluene. The image has been obtained under toluene without any prior exposure to the atmosphere. The roughness (root-meansquare roughness, 0.05 nm) of the uncorrugated surface is comparable to that of a freshly cleaved Si wafer, indicating that we are looking at an atomically flat surface. This shows that preparation and imaging under inert toluene is an appropriate procedure to protect the sample from oxidative attack by atmospheric oxygen, which opens up the potential of simulating UHV under ambient conditions. After that image was taken, toluene was removed from the cell, and the cleavage surface was exposed to air for 5 min. The sample was then imaged again under toluene at the same position, and contrast formation was observed Figure 4b). Corrugation and layer spacings are comparable to the results obtained on wafers cleaved in air; however, the surface is less uniform, which can be explained as a result of hindered oxidation through unidentified residues from the organic solvent suppressing contrast development in some areas. Next we tried to examine the potential of in situ investigations under reactive media to extract additional information from the topographical images obtained primarily by AFM. The first step was to visualize directly the contrast inversion on our AGaAs/ GaAs system by etching with hydrochloric acid, which has been described earlier.lg Figure 5 shows a timeresolved sequence of the contrast inversion under 0.1 M HCl. After injection of HCl,
Figure 4. AFM images of the AIGaAs/GaAs cleavage cross section taken under toluene. (a, top) Sample prepared by in situ cleavage under toluene without exposure to air shows an atomically flat surface. (b, bottom) Same sample position after removal of toluene, exposure to air for 5 min, and refilling with toluene shows a contrast development due to preferential oxidation on the AlGaAs layers. Image size, 510 nm x 510 nm; depth scale, 20 nm from black to white.
the development of ongoing etching was monitored for 2 h. Dissolution of oxide started on the GaAs layers. After removal of HCl, etching of the oxide on the AlGaAs layers started from the interface, leading to ridges of decreasing thickness. After 5 min, the fwhm of these oxide ridges was 8 nm; after 14 min the ridges were removed, and the contrast of the layered system was not visible any more on L1 (see Table 1). Preferential etching on the AlGaAs layers then led to an inverse contrast with a corrugation of about 1 nm in the L1 region. The whole inversion process took approximately 15-20 min. Figure 6 shows the development of the corrugation versus time. The curve also confirms that oxide removal starts on the GaAs layers, leading to an initial enhancement of the original corrugation. After that, etching on the AlGaAs leads to a drop in the contrast, and finally an inverse corrugation develops steadily up to a maximum in the range of 2 nm, which, however, could be already affected by the depth resolution due to the bluntness of the AFM tip. As can be seen in Figure 5, the development of the contrast inversion on layers L2 and L3 is somewhat slower. On L2, the oxide was removed after 2 min, and also part of the GaAs beneath was etched off, leading to a 100 nm ridge on the AlGaAs layer L3 with an elevation of 5 nm. Etching then proceeded on L3,and complete contrast inversion in that region could be observed after
2 h of exposure to HCl. The contrast seen in Figure 5d was stable for a prolonged period of time. Under HCl of lower concentration, a completely different behavior was observed. In 0.01 M HCl, etching of oxide started on the GaAs in the interface region with the AlGaAs,followed by dissolution of oxide on both AlGaAs and GaAs. This process led to pitting, mainly in the part of L1 adjacent to L2. After 10 min (Figure 7),craters with diameters up to 200 nm and depths up to 30 nm were observed. Only small areas inside the craters near their edges showed a reversed contrast of approximately 2 nm, as described for 0.1 M HCI. The part of L1 adjacent to the edge of the wafer remained unaffected by the acid treatment, showing the original corrugation even after prolonged exposure to HCl. The phenomenon was reproducible and was also visible under 0.001 M HCI. This indicates that chemical and crystallographic (e.g., dislocations) inhomogeneities exist mainly in the epilayer region closer to the wafer substrate, which can be explained as a result of incorporation of residual impurities on the substrate in the first cycles of the MBE process. On the 100 nm GaAs layer (L2), the oxide was etched off 1 min after exposure to HCl, and etching proceeded into AlGaAs (L3), leading to a ridge on that layer, which was stable up to 1 h in a few areas. Under 0.001 M HCI, a similar pitting process was observed. Figure 8 shows a cross section of a fresh pit after 5 min of exposure to HCl. The unaffected structure near the pit shows a corrugation of 0.6 nm in that cross-sectional profile. The hole in the GaAs layer is 1.9 nm deep, indicating that 1.3 nm has been etched off. The fwhm of the AlGaAs peak left of the hole is only 8 nm, compared to 14 nm in the original structure. This clearly shows that etching of oxide starts from these holes and proceeds into the AlGaAs layers. After 20 min, the holes grew together to form craters 50-200 nm wide and 5 nm deep, as was observed under 0.01 M HCl. Again, only the area closer to the wafer substrate was affected by this pitting process. On the 100 nm GaAs layer (L2), the oxide was removed up to 10 nm from the interface to the neighboring layers after 10 min, leading to a step of approximately 0.9 nm in height, which can be interpreted as the thickness of the oxide layer on GaAs. This value is in good agreement with the expected thickness (1-2 nm) of the oxide layer on a freshly cleaved GaAs ~urface.2~ CONCLUSION We have shown that the potential of AFM for both topographical and chemical surface analysis can be enhanced significantly by means of in situ investigations under different liquids. An AlGaAs/GaAs multilayer system served as a well-defined and wellsuited model sample for evaluating this concept. As a first important result, it was shown that in situ preparation and imaging of cross-sectional cleavage surfaces under an inert organic medium like toluene is an appropriate way to investigate surfaces unaffected by atmospheric influences, but still under ambient conditions. In this way it could be proven unequivocally that the observed corrugation on our specimen of about 0.3-0.6 nm in air is caused by preferential oxidation of the chemically different layers, since this contrast development could be completely suppressed by cleaving and imaging under toluene. Such in situ studies have been made possible by means of a home built sample holder designed for imaging small samples under a (25) Lakes, F.Sut$ Sci. 1972,30,91-100.
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Figure 5. Series of AFM images showing the time-resolved contrast inversion on our AIGaAs/GaAs system under 0.1 M HCI (a, top left) 2, (b, bottom left) 14, (c, top right) 37,and (d, bottom right) 210 min after injection of HCI. Image size, 750 nm x 750 nm; depth scale, 20 nm from black to white.
100
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Figure 6. Corrugation on the AIGaAdGaAs system versus time of exposure to 0.1 M HCI. HCI has been injected at point 1. From point 1 to point 2, the oxide on GaAs is being removed, enhancing the original contrast. At point 2, etching of oxide on the AlGaAs begins, finally leading to an inverse contrast.
variety of organic liquids, which lead to unstable imaging conditions in the conventional setup. Secondly, we have shown also that time-resolved in situ observation of corrosion processes is possible on fresh cleavage edges using that sample holder. This allows imaging of the first steps of interaction immediately after the first contact with the corrosive medium. Here, we have observed that HCl of low concentration (0.01 and 0.001 M) leads to pitting in certain areas of the AlGaAs/GaAs cleavage surface, indicating a higher number of defects and thus a lower quality of the multiquantum well structure in those regions. At a higher concentration of HCI (0.1 M), a clear inversion of the initial contrast without pitting was observed and evaluated on a quantitative basis. 1534 Analytical Chemistry, Vol. 67, No. 9, May 1, 1995
Figure 7. AFM image of the AIGaAs/GaAs system after 10 min of exposure to 0.01 M HCI. In contrast to Figure 5, pronounced pitting and crater formation can be observed. Image size, 1.6 pm x 1.6 pm; depth scale, 50 nm from black to white.
The technique described greatly enhances the potential of
AFM due to combination of topographical and chemical information. Thus, material properties of nanodomains, which cannot be obtained from topographical images alone, are accessible analytically. Furthermore, UHV conditions can be simulated under inert liquids, allowing imaging of sensitive surfaces under ambient conditions and study of surface modifications starting from such protected samples by exchanging the medium in the cell.
14nm 0
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L1 Figure 8. Cross-sectional analysis of a freshly etched pit under 0.001 M HCI. Pitting on the GaAs led to a 1.9 nm deep hole. The fwhm of the AlGaAs ridge left of the pit has decreased from 14 to 8 nm, indicating that some oxide on AlGaAs has been etched off already, too.
Investigations of corrosion processes on other technologically relevant materials and further surface studies using the described potential are currently underway.
wissenschaftlichen Forschung (p906SPHY), and the osterreichische Nationalbank is gratefully acknowledged.
ACKNOWLEDGMENT
Received for review July 8, 1994. Accepted February 20,
The authors thank Dr. W. Vandeworst at the Interuniversitair Micro Elektronica Centrum vzw/Belgium (IMEC)for providing part of the samples. Support of this work by the Austrian Ministry for Science and Research, the Austrian Fonds zur Forderung der
AC9406874
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Abstract published in Advance ACS Abstracts, April 1,1995.
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