Anal. Chem. 1982, 64, 1760-1762
1700
Investigation of Aluminum Gallium Arsenide/Gallium Arsenide Superlattices by Atomic Force Microscopy Gernot Friedbacher'pt and Paul K. Hansma Department of Physics, University of California, Santa Barbara, California 93106
Daniel Schwarzbach and Manfred Grasserbauer Institute for Analytical Chemistry, Technical University Vienna, Getreidemarkt 91151, A-1060 Wien, Austria
Heinrich Nickel Forschungsinstitut der Deutschen Bundespost Telekom, P.O.Box 100003,D-6100 Darmstadt, FRG
We have analyzed AIGaAs/GaAs superlattlces by lmaglng freshly cleaved, untreated sample cross sections wlth the atomlc force microscope In air. We were also able to Image the same superlattice on a sample cross sectlon after etchlng lt for 5 mln In 0.1 M HCI. The reverse contrast of the AlGaAs/ GaAs multllayer system between both samples suggests an explanatlon for the observed corrugation of 0.5 nm on the untreatedsample and 0.2-0.6 nm on the etched sample. The described method might also be useful for analytlcal characterlzatlon of a varlety of other materials and for the study of surface reactlons.
INTRODUCTION Electronic devices based on A1,Gal-,As/GaAs superlattices for high-speed applications are in the forefront of semiconductor development, e.g. for optoelectronic compounds. Their properties] such as electron mobility, depend strongly on the abruptness of the heterointerface and the uniformity of the layers. Accurate characterization methods for thin-layer systems (below20 nm) with good depth resolution are needed to monitor fabrication methods like molecular beam epitaxy (MBE), which is widely used. This presents a challenge for analytical sciences. Many techniques are used to characterize quantum well structures: sputter techniques, like secondary ion mass spectrometry (SIMS) and sputter-assisted Auger electron spectroscopy (AES) allow estimates of the layer thickness and composition, but due to atomic mixing and roughening effects, the depth resolution is limited. Optical methods like photoluminescence (PL) and X-ray diffraction (XRD) give further information] but they impose restrictions on the structure to be investigated. Finally, transmission electron microscopy (TEM) allows the determination of the layer thickness; however, sample preparation is complicated. Muralt et a l . 1 ~ 2and Salemink et a l . 3 have studied the AlGaAs/GaAs heterostructure interface with scanning tunneling microscopy (STM) and scanning tunneling potentiometry (STP). G6mez-Rodr;guez et ala4reported STM images of + Present address: Institute for Analytical Chemistry, Technical University Vienna, Getreidemarkt 9/151,A-1060 Wien, Austria. (1)Muralt, P.; Meier, H.; Pohl, D. W.; Salemink, H. Superlattices Microstruct. 1986,2 (6),519-520. (2) Muralt, P.;Meier, H.; Pohl, D. W.; Salemink, H. W. M. AppLPhys. Lett. 1987,50,1352-1354. (3)Salemink, H. W. M.; Meier, H. P.; Ellialtioglu, R.; Gerritsen, J. W.; Muralt, P. R.M. Appl. Phys. Lett. 1989,54,1112-1114. (4)G6mez-Rodrlguez, J. M.;Bar6, A. M.; Silveira, J. P.; Vizquez, M.; Gonzdlez, Y.;Briones, F. Appl. Phys. Lett. 1990,56, 36-38.
0003-2700/92/0364-1760$03.00/0
' s 1 It"" cantilever
suEte
super lattice
r-7 steeldisc
x y z -pierotranslator
Figure 1. Schematicview of sample and AFM tip. The vertlcai motion of the tip is sensed by laser beam deflectlon off a mlcrofabrlcated cantilever. A feedback loop keeps the force (vertical posltlon of the tip) constant by moving the sample In zdlrection wlth the xyz-plezo translator while scanning laterally. The size of the superlattice perlcd Is exaggerated so It can be seen in this schematic view.
the multiquantum well structure of Alo.3G@,7As/GaAsobtained in vacuum. Tanaka et a l . 5 investigated GaInAs/InP and AlAs/GaAs structures with STM in air. Chalmers et al.6 reported the observation of tilted superlattice structures (GaAs/AlAs) by atomic force microscopy (AFM) operated under HCl solution. Here, we will show how AFM can characterize layer thickness and evenness of an A1,Gal-,As/GaAs superlattice.
EXPERIMENTAL SECTION The images shown in this paper were obtained with a NanoScope I1 (Digital Instruments, Santa Barbara, CA) atomic force microscope, whose operation principle is based on laser beam deflection off a microfabricated cantilever.'-10 Imaging was performed in constant force mode using 100-rmSiBN, cantilevers with integrated tips at a scan rate of 20 Hz,resulting in a constant repulsive force in the order of 10-7-10-9 N. The samples have been prepared by molecular beam epitaxy with a Varian Modular Gen I1 instrument. Thirty alternating layers of A&.35Gao..&s and GaAs have been deposited on a (100) GaAs substrate at a temperature of 620 "Cand a deposition rate (5)Tanaka, I.; Kato, T.; Ohkouchi, S.; Osaka, F. J. Vac. Sci. Technol. 1990,A8, 567-570. (6)Chalmers,S. A.; Gossard, A. C.; Weisenhorn, A. L.;Gould, S. A. C.; Drake, B.; Hansma, P. K. Appl. Phys. Lett. 1989,55,2491-2493. (7)Alexander, S.;Hellemans, L.; Marti, 0.; Schneir, J.; Elings, V.; Hansma, P. K.; Longmire, M.; Gurley, J. J . Appl. Phys. 1989,65,164167. (8) Meyer, G.; Amer, N. M. Appl. Phys. Lett. 1988,53,2400-2402. (9)Amer, N.M.; Meyer, G. Bull. Am. Phys. SOC.1988,33,319. (10)Albrecht, T.R.;Akamine, S.; Carver, T. E.; Quate, C. F. J . Vac. Sci. Technol. 1990,A8, 3386-3396. 0 1992 American Chemical Society
ANALYTICAL CEMISTRY. VOL. 84. NO. 17. SEPTNBER 1. 1992
F l w a 2. (a. Top) AFM image 01 a he8h. unbaated cleavage cross wcllon lhcuoh an Aif2aAsIGaAs superlamce 86ucNe deposned on (100) -As. The suwrlamca consisv) 01 30 Ai,.,Ga..As and 30 &AS layers each 19:9 0.2 nm thick. s he topmost layer is &AS. The &As layers fwm the valleys (dark). The M E s W r e Is clearly r w e d frmthe subsbate ai1the way out to the topmost layer of the water. Its measued total thkknass 01 1194 12 nm is In a g e e m t wHh the manufacturer's ncininai valw (1200 nm). The scan size Is 1500 nm by 1500 nm. the depth scale is 5 nm frm black to whne. (b. Middle) AFM image of the same structure at higher magnlRcatkm. showing the boundary between the GaAs substrate and the supwlattlu, more clearly. The scan size is 613 nm by 613 nm. the depth scab Is 3 nm from black to white. (c. Bonm)AFM image of the same supar'atace sbucture at h i g h magnmcatkm showing a step of 0.5 f 0.03 nm running am058 the bilayer system. The ca'rwatkm hebht Is 0.5 0.03 nm. The scan size Is 500 nm by 500 nm. the depth scab Is 3 nm hm black to whne.
*
of 0.52 nmls for AlGaAs and 0.33 nmls for GaAs. The nominal thickness (from the deposition parameters) of 20 nm for each layer wan confirmed by XRD and PL measurements (20f 2 nm). The topmost layer is GaAs. A piece approximately 4 mm by 3 mm WBB broken from the wafer and glued onto a steel disk, 80 that the fresh cleavage surface,which is the (110) surface, could
1761
Flpur. 3. (a.Top) AFM image 01 the cleavage cross SeCtbn throt@ an AKiaAslGaAs superlanice aner 5 min of etching in 0.1 M HCI. Note the1 the conbast is hreverse of Figure 2a. The scan she Is 1310 nm by 1310 nm. the depth scab Is 7 nm frm black to whiie. (b. Bonm)AFMmageofthesamesampleath~magnlRca~deplcmO
the boundary between substrate and superlanice more clearly. Note thet the flrst layer (AKiaAs) forms a valley in contrast to Figure 2b. The scan she is 31 1 nm by 31 1 nm. the depth scab is 3 nm from black to whne.
be imaged. One sample was imaged without any further treatment. Thecleavagesurfaceoftheeeeondsamplewanetched in 0.1 M HCI for 5 min, rinsed in distilled water, and dried by means of compreased air. The samples were aligned parallel to the y-scan direction of the microscope. and the fresh upper fracture surfaces were then imaged in air (Figure 1). Coarse positioning of the tip to the very edge of the cleavage surface wan done by means of the NanoScope's mechanical xy-stage assisted by observation through a Nikon SMZl binocular microscope. This process allowed positioning of the tip within a few microns of the interesting area. It also turned out that readjustment of the xy-stage was possible even during imagingwithout destroying the tip or 'losing" the images. Tip movementa an small an 5Ce 100 nm could be performed that way allowing imaging without any offset voltage on the piezo suulner.
1762
ANALYTICAL CHEMISTRY, VOL. 64, NO. 17, SEPTEMBER 1, 1992
RESULTS AND DISCUSSION Large scan images on the GaAs substrate (center of the cleavage surface) revealed both atomically flat areas of a few microns on a side and atomically flat terraces (of order 100 nm wide) separated by steps of 0.25-0.5 nm in height. Figure 2a shows a perspective view of an AFM image scanned over the edge of the sample. All 30 AlGaAs and GaAs layers are resolved in the image. Stable images of high quality are obtained as long as most of the scan is performed on top of the substrate. Thus, scanning was always performed with at least 95 5% of the substrate visible in the imaged field. Lower values resulted in increasing image deterioration ("image bending"). The image also shows a number of steps running across the cleavage surface both on the GaAs substrate and the superlattice. Figure 2b shows the step structure more clearly on an image of the boundary at higher magnification. Note that the first layer (which is AlGaAs) adjacent to the Substrate is higher than the substrate and the next GaAs layer. Figure 2c shows an image of the same superlattice at higher magnification. The width of both the valleys (GaAs) and the peaks (AlGaAs) has been determined to be 19.9 f 0.2 nm, being in good agreement with the specifications of manufacturer. The corrugation height is 0.5 f 0.03 nm. This image also clearly shows a step of approximately 0.5 f 0.03 nm, which separates the terrace in the lower left part of the image from the area to its right. Next we imaged a sample etched for 5 min in 0.1 M HC1. Figure 3a shows a topview display of the whole superlattice with the GaAs substrate on the left. Note that the contrast is the reverse of Figure 2a. This can be seen more clearly in Figure 3b. In contrast to the untreated sample the first layer (AlGaAs) forms a valley (compare with Figure 2b). It can also be seen that the surface of the etched sample is less uniform, showing corrugation heights ranging from 0.2 to 0.6 nm. In contrast to G6mez-Rodriguez et al.4 and Tanaka et al.5 who explained their STM images mainly by electronic effects, we are observing the topography with the AFM. From simple model calculations we conclude that a tip with a radius of curvature of 50 nm should be able to follow a maximum corrugation of 1 nm on our structures. The maximum observable corrugation would be smaller than 0.6 nm for tip radii greater than 85 nm. Since we do not know the shape of our particular tip exactly, we have to principally see our corrugation of 0.6 nm as a minimum value; however, the previous comparison makes plausible that the observed corrugations from 0.2 to 0.6 nm correspond to the real topography on the sample. Since we know for sure which phase we are looking at on the superlattice, our comparisons between the untreated and the etched sample suggest an explanation for the AFM contrast. In the first case selective oxidation of the AlGaAs layers forms peaks leading to the observed corrugation in the
AFM images. Selective etching of these oxidized layers then leads to a reverse contrast; see also ref 6. Further AFM experiments-under liquids and also on other systems-for quantitative investigation of the reaction taking place on the surface are underway. Since the atomic resolution imaging capability is a feature of AFM, which is of special interest, we should address that we have also been able to observe silicon &layers in a GaAs substrate 70 nm below the surface using the same procedure as described above. These are preliminary results, but they show that layered systems can be imaged down to monoatomic widths with the described technique.
CONCLUSION Concluding,we have developed a method for imaging multilayer superlattice structures on freshly cleaved samples with AFM in air. Comparison of the results for the untreated and the etched sample confirmed a mechanism for the AFM contrast proposed earlier by Chalmers et al.6 We believe that this technique will also be useful for analyzing surface structures and surface modifications on a variety of other samples-including systems with monoatomic layer thickness-using a comparatively simple sample preparation technique. Especially due to the high resolution imaging capabilities of AFM the shown results demonstrate the value of the described technique for analytical characterization of multilayer systems. Moreover the results are also an important basis for investigations under liquid media. Experiments implementing in situ sample preparation and imaging under liquids, which will allow further study of the chemistry of the observed corrugation on a quantitative basis, are underway. An extension of the technique to other multilayer systems and imaging in different media will not only allow characterization of technologicallyinteresting materiale but will also open up new opportunities for the in situ study of chemical surface reactions.
ACKNOWLEDGMENT We thank P. M. Petroff for valuable discussions. Support of this work by the National Science Foundation, Solid State Physics Grant DMR 89-17164 (P.K.H.), and the Austrian Fonds zur F6rderung der WissenschaftlichenForschung (G.F.) is gratefully acknowledged. Purchases and building of AFM equipment were supported in part by the Office of Naval Research and Digital Instruments. RECEIVED for review December 10, 1991. Revised manuscript received May 6, 1992. Accepted May 14, 1992. Registry No. Alo.~,Ga0.~4s, 106389-87-1; (Al,Ga)As, 3738215-3; GaAs, 1303-00-0.
