Conductivity Measurements on GaN Grown by OMVPE and HVPE
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D. Volm*,
H. Amano**,
*Technical University Munich, Department of Physics, 85747 Garching, Germany "**Department of Electrical and Electronic Engineering, Meijo University, 1-501 Shiogamaguchi, Tempaku-ku, Nagoya 468, Japan ***Department of Electronics, School of Engineering, Nagoya University, Furocho, Chikusa-ku, Nagoya 468-01, Japan ABSTRACT We report on conductance and cyclotron resonance (CR) experiments on GaN epitaxial films grown by the OMVPE and HVPE techniques. From a precise determination of the electron effective mass the donor binding energy in the effective mass approximation (EMT) is calculated. We obtain 31.7 meV. The transport experiments on the HVPE films show that the conductance is thermally activated with an activation energy of 15 meV in contrast to the OMVPE films which showed temperature independent conductivity for temperatures between 4 and 100 K.
INTRODUCTION Undoped GaN epitaxial films commonly show n-type conduction[l] with free carrier concentrations at room temperature ranging between 1017 and 1019 cm-3 . There is no clear correlation between the free carrier concentrations and the number of residual impurities as found, e.g. by SIMS. Therefore, there is an ongoing discussion whether intrinsic defects, such as nitrogen vacancies or gallium interstitials, are responsible for the autodoping behavior. Recent total energy calculations demonstrated [2,3] that both centers create shallow effective-mass type levels in GaN. Oxygen and silicon might be good candidates for extrinsic shallow donors in GaN when substituting on nitrogen or gallium sites, respectively. However the binding energy determined for the defects causing n-type conduction scatters between 20 and 40 meV [4,5]. A lower limit for extrinsic origin can be given by an effective mass theory calculation, if the electron effective mass is known with high precision. Here we will report on a precise determination of the electron effective mass in GaN with cyclotron resonance.
EXPERIMENTAL Several GaN films grown by different techniques were investigated. A 400 jtm thick film was grown by hydride vapor phase expitaxial (HVPE) without a buffer layer. The films grown by organometallic vapor phase epitaxy (OMVPE) had a 35 nm AIN buffer layer, whereas the GaN film thickness was 3 gm. All films were deposited on c-plane sapphire substrates. 467 Mat. Res. Soc. Symp. Proc. Vol. 395 01996 Materials Research Society
For the conductivity measurements four contacts were soldered onto the sample using Ga/In eutecticum. Current-voltage measurements exhibited good ohmic behavior. For the electric measurements the sample temperature was varied between 30 K and 300 K. The luminescence experiments used the 325 nm line of the HeCd laser as excitation. The CR experiments were performed in transmission and with additional optical detection using a CO 2 pumped far-infrared laser. The wavelength was 432 gm. For details see ref.[6]
EXPERIMENTAL RESULTS AND DISCUSSION The temperature dependence of the conductivity of the HVPE layer is shown in Fig. 1.
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