MICROSCOPY RESEARCH AND TECHNIQUE 35:363-364 (1996) Brief Communication XTEM sample preparation technique for n-type compound semiconductors using photochemical etching Shigeyasu Tanaka, Hideki Fujii and Michio Hibino* Department of Electronics, Nagoya University, Furo-cho, Chikusa-ku Nagoya 464-01, Japan *Center for Integrated Research in Science and Engineering, Nagoya University, Furo-cho, Chikusa-ku Nagoya 464-01, Japan Abstract A sample preparation technique based on photochemical etching (PCE) is described for cross-sectional transmission electron microscopy (XTEM) of n-type compound semiconductors. XTEM samples of an InGaAsPAnP single-layer structure, prepared by using a moderately focused laser beam and Br2-methanol solution, gave high quality, damage-free XTEM images. The PCE technique is applicable to other n-type compound semiconductors. Introduction Cross-sectional transmission electron microscopy (XTEM) is a commonly used technique for investigating heterostructures of compound semiconductors. For these materials, the final thinning of samples for high resolution XTEM is usually achieved using ion milling. However, conventional Ar+ ion milling has a number of proW 7 s . For example, it is known that a volatile constituent element is preferentially lost e id another element in the compound then accumulates on the sample surface generally in the form of islands, for indium-containing 111-V compound semiconductors [Chew and Cullis 19871. Another problem associated with conventional Ar' ion milling is the formation of an amorphous or damaged crystal layer on the sample surface. Although the use of 1' ions instead of Ar' ions was suggested for an effective solution to the problems mentioned above [Chew and Cullis 19871, conversion of ion species is complex. Recently, photochemical etching (PCE) has been proposed as an another method for preparation of n-type semiconductor plan-view samples for transmission electron microscopy [Lowes and Cassidy 19921. In the PCE technique, a laser beam of energy greater than or equal to the band gap is irradiated onto the semiconductor sample immersed in a suitable electrolyte. For n-type semiconductors, the photogenerated holes at the semiconductor-solution interface induce or accelerate the etching [van de Ven and Nabben 1990, Ruberto et al. 19911, thus leading to a hole in the sample at the laser beam position. The area immediately adjacent to the hole is generally thin enough to be observed by electron microscopy. The PCE technique was shown to be a quick and easy procedure with the added ability to isolate small specific sites [Lowes and Cassidy 19921. We applied the PCE technique to XTEM sample preparation and found that XTEM samples suitable for high resolution imaging can be prepared by proper selection of the laser beam spot size and electrolyte. The preparation method for an InGaAsPAnP single-layer structure is demonstrated. Sample preparation A XTEM sample preparation procedure using PCE is as follows. Two pieces (-8mm x lmm) were cut from the epitaxial wafer and they were glued face-to-face. The sample was lapped and polished on both sides to give a final sample thickness of about 100pm. After polishing, the two pieces were separated in solvent and each piece was placed on a thin wax layer on a glass slide. Then. it was heated to about 15OOC for -30sec and cooled down to room temperature. With this procedure, the side of the piece is partially covered with wax as shown in fig.1, preventing the thin epitaxial layer from being etched away during the PCE process. PCE was carried out using an Ar+ laser (wavelength 514.51~11, 25mW) and 1% Br2-methanol solution. The whole sample was placed in the etching solution and the laser beam was irradiated onto a small part of the edge region including the layer structure as shown in fig.1. When approximately half the sample thickness was etched, etching was stopped. Then the sample was removed from the glass slide, turned over, and remounted as before. PCE was then continued until a hole was formed. Finally, the sample was removed from the glass slide, and mounted on a grid using silver paste. It was found that the spot size of the laser beam affects the angle of the wedge around the hole. For laser spot sizes of 200 - 800pm, the angles of the wedge were -10" - -4" and suitable for XTEM observations. When smaller spot sizes were used, the wedge became too steep, while for larger spot sizes, the whole sample was thinned and handling S. TANAKA ET AL. 364 became very difficult. XTEM observation Samples prepared by PCE were examined using electron microscopes: H-8000 or EM-002B operating at 200kV. Observations were carried out at [I101 incidence. Fig.2(a) shows a XTEM image of an In0.58Ga0.42As0.90P0.1d(001)InP single-layer structure, which was grown by liquid phase epitaxy at a growth m-~. temperature of 625OC. The InP substrate was Sn-doped, n-type with a carrier concentration of about 2 ~ 1 0 ~ ~ c The nominally undoped InGaAsP epitaxial layer was also n-type and the carrier concentration was estimated to be in the high 1016cm-3 to low 1017cm-3 range. This XTEM sample was prepared using a laser beam spot size of about 500pm, and the etching time was about 20min. As can be seen, the sample is clean, fiee from artifacts such as indium islands formed by conventional Ar+ ion milling. Lattice imaging was possible at very thin regions as shown in fig.2(b) where the interface region is imaged. From the change in the background intensity, it is deduced that the interface is abrupt down to the atomic scale. References Chew, N.G. and Cullis, A.G.( 1987) The preparation of transmission electron microscope specimens ffom compound semiconductors by ion milling. Ultramicroscopy 23: 175-198. Lowes, T.D. and Cassidy, D.T.( 1992) InP sample preparation for TEM by photochemical etching, ion milling, and chemical thinning. Microscopy Research and Technique 23 :252-259. Ruberto,M.N., Zhang,X., Scarmozzino,R., Willner,A.E., Podlesnik,D.V., and Osgood, Jr.,R.M.(1991) The lasercontrolled micrometer-scale photochemical etching of 111-V semiconductors. J.Electrochem.Soc. 138:1174-I 185. van de Ven, J. and Nabben, H.J.P.( 1990) Anisotropic Photoetching of 111-V semiconductors. J.Electrochem.Soc. 137: 1603-1610. Acknowledgments The authors are grateful to Dr. T.Tanji of Center for Integrated Research in Science and Engineering, Nagoya University for valuable discussions. We are also grateful to T.Hirayama of Japan Fine Ceramics Center for the use of electron microscope facilities and K.Tomita of Toyota Central R&D Laboratories Inc. for the support and encouragement. \'a'erbe"" Epilayer I Substrate ~~~~~~~~ Glass slide Fig.1 Illustration of XTEM sample preparation using PCE. Fig.2 (a) XTEM image of an InGaAsP /(001)InP single-layer structure. (b)High resolution image of interface region. Interface is marked by triangles.