Infrared optical properties of aged porous GaAs
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T.E. Tiwald J.A. Woollam Co., Inc., 645 M Street, Suite 102, Lincoln, Nebraska 68508
J.A. Woollama) Center for Microelectronic and Optical Materials Research, and Department of Electrical Engineering, University of Nebraska, Lincoln, Nebraska 68588 (Received 20 September 2000; accepted 23 February 2001)
Aging properties of porous GaAs were investigated nondestructively using variable angle of incidence infrared spectroscopic ellipsometry. In addition to the thickness and volume porosity, properties of the solid part of the porous material are investigated in terms of the long-wavelength dielectric function and chemical composition. The high sensitivity is employed to detect and identify infrared resonant absorptions related to different vibration modes of cubic and amorphous As2O3. Resonances centered at 333.3, 480, 785.8, 838, and 1045.5 cm−1 are from cubic As2O3, whereas resonances centered at 350, 490, and 808.5 cm−1 are from amorphous As2O3. Recent widespread interest in luminescence properties of porous silicon,1,2 has led to efforts to investigate properties of other semiconductor-based porous materials such as porous SiC and GaAs.3–7 The prospects of adoption of electronic, thermal, and optical properties of the bulk semiconductors by the corresponding high surface area porous structures, provides application possibilities in optics, sensor technology, and electronics. The advantageous electronic properties of GaAs compared to silicon have led to its use in high-speed diodes and field effect transistors and circuits. Furthermore, anodization of GaAs results in formation of porous layers that luminesce in the visible, at higher photon energies than porous silicon.8,9 Hence, development of nondestructive tools for optical and microstructural characterization of porous GaAs and its aging is of importance. Variable angle of incidence infrared spectroscopic ellipsometry (IRSE), being intrinsically sensitive to the presence of thin films, has advantages compared with the more traditional reflectance spectroscopy. For example and imaginary parts of the dielectric function of the material can be determined simultaneously without need for additional measurements on a reference sample or employment of the Kramers–Kronig relation requiring extrapolation of experimental data beyond the measurement range. IRSE has been shown to be effective for characterization of porous silicon and porous SiC.3,10 In this work, infrared optical properties of freshly prepared and aged porous GaAs formed in highly degenerate
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Address all correspondence to this author. J. Mater. Res., Vol. 16, No. 5, May 2001
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p-type GaAs are investigated. Optical properties of bulk material are compared with the solid phase of the porous material. Porous GaAs samples were prepared using p-type, Zndoped, wafers with (100) crystal orientation. Highly degenerate wafers were used to obtain uniform porous layers.5 Wafers were obtained from Materials Technology International (MTI) Corporation. Electr
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