Influence of Doping on the Lattice Dynamics of Gallium Nitride
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Cite this article as: MRS Internet J. Nitride Semicond. Res. 4S1, G3.57 (1999) Abstract We present results of Raman-scattering experiments on GaN doped with Si, C, and Mg, respectively, grown by molecular beam epitaxy (MBE). The influence of the different dopants on strain and freecarrier concentration was investigated. Furthermore, we report on several local vibrational modes (LVM) around 2200 cm-I in Raman spectra of highly Mg-doped GaN. A possible explanation of these high-energy modes in terms of hydrogen-related vibrations is given. We also found a variety of new structures in the range of the GaN host lattice phonons. Secondary ion mass spectroscopy (SIMS) was applied to determine the concentration of magnesium and unintentionally incorporated hydrogen.
Introduction Much attention has been paid to the wide-bandgap material GaN due to its high potential for optoelectronic and high-power electronic applications [1]. Controlled p- and n-doping is a major issue for the fabrication of electronic devices based on group III-Nitrides. The growth of doped GaN epilayers and heterostructures by molecular beam epitaxy offers the advantage of lower unintentional dopant concentration in the material. Among the optical properties of doped GaN the influence of doping on the lattice dynamics is of special interest. Raman-spectroscopy is a powerful tool to investigate the correlation between doping, strain [2], and free-carrier concentration [3]. Moreover, dopant atoms can give rise to local vibrational modes (LVM) due to their different masses compared with those of the substituted elements [4]. High-energy modes are reported for Mg-doped GaN [5, 61 which were assumed to be related to hydrogen complexes. This is of special interest, because hydrogen is known as a compensating center for magnesium acceptors. To our knowledge, no LVM for Si-, C-, or Mg-doped GaN in the range of the host lattice phonons have been reported in literature so far.
Experiment The Raman-scattering experiments were carried out in backscattering geometry with a triple-grating spectrometer equipped with a cooled charge-coupled device detector. The 488 nm line of an Ar+/Kr+ mixed-gas laser was used for excitation. The Raman shifts were determined with an accuracy better than 1 cm . Microscope optics allowed spatially-resolved measurements with a resolution of about 0.7 pm. G 3.57 Mat. Res. Soc. Symp. Proc. Vol. 537 ©1999 Materials Research Society
The samples under study were Si-, C-, and Mg-doped GaN films of about 1 pm thickness grown on sapphire (0001) substrates by MBE; the details are given elsewhere [7]. The Mg-doped samples A and B were p-conductive with a hole concentration at room temperature of 3.7.1017 cm-3 and 1.4. 1017 cm-3, respectively, whereas the other two samples were compensated but n-conductive. Secondary ion mass spectroscopy was applied to determine the concentration of magnesium and hydrogen. The measured concentrations are accurate to within 20% deviation. The dopant concentration of the Si- and C-doped GaN films were estimated
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