Nondestructive Characterization of GaN Films Grown at Low and High Temperatures
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j.
HANc), J.M.
ABSTRACT GaN films grown on GaAs and sapphire substrates by molecular beam epitaxy (MBE) and metalorganic vapor phase epitaxy (MOVPE) at both low and high temperatures (LT and HT) were characterized by Raman scattering and variable angle spectroscopic ellipsometry (VASE). Optical phonon spectra of GaN films are obtained through back-scattering geometry. Crystal quality of these films was qualitatively examined using phonon line-width. Phonon spectra showed that the HT GaN has wurtzite crystal structure, while LT GaN and GaN/GaAs have cubic-like structures. Thickness nonuniformity and defect-related absorption can be characterized by pseudo dielectric functions directly. Surface roughness also can be determined by using an effective-medium approximation (EMA) over-layer in a VASE analysis. Anisotropic optical constants of GaN, both ordinary and extraordinary, were obtained in the spectral range of 0.75 to 6.5 eV with the consideration of surface roughness, through the small and large angles of incidence, respectively. The film thickness of the GaN was accurately determined via the analysis as well. INTRODUCTION GaN and related ITl-nitrides wide band gap semiconductors have attracted much attention recently due to the successes of blue light emitting diode (LED) and laser diode (LD) fabrications [1]. However, there are many unsolved problems in the nitride film growth field, such as high dislocation density, cracks, and lack of surface flatness [2]. In order to improve the film quality, more advanced and sensitive film characterizations are highly required, especially in nondestructive ways. By measuring the phonon spectrum of a film, Raman scattering can reveal its crystalline quality, crystal structure, alloy composition, optical anisotropy, and many others. Spectroscopic ellipsometry, on the other hand, not only can measure the film optical constants and its thickness, but also determine the surface roughness, thickness uniformity, and defect related absorption. In this paper, we report phonon spectra of LT and HT GaN films. The anisotropic optical constants along with film thickness, surface roughness, and thickness nonuniformity of the HT GaN are studied by VASE as well. THEORY It is well known that a-GaN is optically anisotropic (uniaxial) material due to its wurtzite crystal structures. Group theoretical analysis shows that the irreducible representation for the optical modes of Wurtize GaN [3] F = A,(z) + 2B1 + El(x, y) + 2E 2 ,
(1)
for phonon propagating along or perpendicular to the optical axis . Where x, y, z in parentheses repent the directions of phonon polarization. The A, and El modes are both Raman and infrared active, two E2 modes are only Raman active, and B, modes are both Raman and IR silent. Spectroscopic ellipsometry is another powerful nondestructive method that can be used to study the optical and structural properties of crystals. Two standard ellipsometric parameters V and A are
313 Mat. Res. Soc. Symp. Proc. Vol. 5910 2000 Materials Research Society
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