The Effect of Si and Mg Doping in the Microstructure of Epitaxially Grown GaN
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T. D. MOUSTAKAS(c) (a)Dept. of Physics, Aristotle Univ. of Thessaloniki, 54006 Thessaloniki, Greece (b)Hahn-Meitner Institute (A.S.), Glienicker 100, 14109 Berlin, Germany. (c)Photonics Center & Dept. of Electrical and Computer Engineering, Boston University, Boston MA 02215, U.S.A.
ABSTRACT The effect of p- and n-type doping (using Mg and Si, respectively) in the microstructure of GaN, grown epitaxially on (0001)A120 3 and (11l)Si, is studied with X-ray absorption measurements at the N-K-edge. A distortion in the local microstructure around the N atom is detected in the undoped and the Mg doped samples. The N atom is 4-fold coordinated with n Ga atoms in the expected distance and 4-n atoms at a distance longer by 0.28A, where 2.9 < n < 3.3. Such a distortion, which is attributed to the inward relaxation and the strong interaction between the Ga atoms surrounding the nitrogen vacancies (VN), does not exist in the Si doped sample (carrier concentration= .57x10 18cm-3 ) where the formation of VN is suppressed due to the n-type doping. However, in GaN:Si the N atom is undercoordinated with 3.3 nearest neighbors instead of 4. This undercoordination indicates the presence of VGa and/or NGa antisite defects. Finally, from the nearest neigbohr distances the lattice parameters were calculated and it is found that although the a and c vary by about 1.5%, the ratio of the lattice constants, c/a, remains constant and equal to 1.63. I.
INTRODUCTION
GaN is a wide-band gap semiconductor which finds applications in UV- and visible- light emitters and detectors, lasers as well as in data storage devices[1,2]. GaN is also used in microelectronics because of its good thermal and chemical stability and physical hardness[3], while alloying of GaN with AIN and InN permits band-gap engineering. The fabrication of high brightness blue LEDs, based on GaN/InGaN/AJGaN heterostructures, was recently reported[4]. Epitaxially grown GaN films are characterized by high densities of point and extended defects due to: i) the lattice mismatch with the widely available substrates; ii) the large differences between the thermal expansion coefficients of the film and the substrates; iii) the low reactivity of N with Ga and iv) the large size mismatch between the N and Ga atoms. Furthermore, GaN is characterized by high background n-type conductivity which has been attributed to the presence of native point defects, such as nitrogen vacancies (VN) or impurities. Native defects such as VN[5], N antisite (NGa)[6,7,8], Ga vacancies (Vca)[9,10], or Ga interstitials[1 1] are held responsible for the yellow luminescence (-2.2eV) of the GaN films, which is associated to strong phonon coupling. Among the native defects in GaN, the VN has the lowest formation energy[12,13], which could be further 381 Mat. Res. Soc. Symp. Proc. Vol. 482 © 1998 Materials Research Society
decreased during far-from-equilibrium growth conditions (N- or Ga-rich)[13]. Extended X-ray absorption fine structure (EXAFS) spectroscopy measures the variation of the absorption coefficient
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