Mobility of two-dimensional electrons in AlGaN/GaN modulation-doped field-effect transistors

R. Oberhuber, G. Zandler, P. Vogl
1998 Applied Physics Letters  
We present quantitative calculations of the electron drift mobility in wurtzite ͑WZ͒ and zincblende ͑ZB͒ structure n-type AlGaN/GaN modulation-doped field-effect transistors. The two-dimensional character of the quantum confined carriers as well as spontaneous and piezoelectric electric field effects are fully taken into account. For given doping concentration, we find that the internal electric fields lead to a much stronger carrier confinement and higher channel densities than in standard
more » ... han in standard III-V materials. For high quality n-type heterostructures, we predict a room temperature mobility at high densities close to 2000 cm 2 /V s. The rapid advance in fabricating high quality sub-m III-nitride modulation-doped field-effect transistors 1 calls for reliable and predictive device simulations. While published transport studies of nitride compounds so far focused on bulk properties, 2-9 the prediction of efficient AlGaN/GaN heterostructure devices requires accurate modeling of quantum confinement effects of the carriers in the twodimensional ͑2D͒ channels. This is a prerequisite for understanding, predicting, and optimizing the effects of remote doping, interface roughness, or the temperature dependence 1,9,10 of the electron mobility. In this letter, we present detailed predictions of the electron drift mobility in wurtzite ͑WZ͒ and zincblende ͑ZB͒ structure AlGaN/GaN single heterostructure devices, taking fully into account the 2D character of the quantum confined carriers. We find that the carrier density and mobility depend crucially on the AlGaN/GaN interface charge that is caused by the spontaneous and piezoelectric polarization in the wurtzite materials. In addition, wurtzite material is found to have superior transport properties as compared to zincblende material, in spite of the smaller effective mass in the latter case. For high quality n-type heterostructures, we predict a room temperature mobility close to 2000 cm 2 /V s. The carrier transport has been calculated in two steps. First, the confining potential, the electronic subband energies and wave functions near the AlGaN/GaN interface are computed self-consistently from a coupled 1D Schrödinger and Poisson equation. The bulk conduction band is modeled by a nonparabolic isotropic band centered at wave vector kϭ0 in wurtzite and zincblende structure GaN. 11-13 The Poisson equation includes the total electric dipole field in the wurtzite phase. All scattering rates 14,15 among the subband states are determined consistently in terms of the confined electronic states. In the second step, the mobility is calculated from the full Boltzmann equation for the transport along the channel, employing a k-space Monte Carlo method. We have included 40 subbands to compute the electron-phonon, impurity and surface roughness scattering rates ͑interand intrasubband processes͒. The general procedure is analogous to the one developed previously. 14-17 Interface roughness scattering has been modeled analogously to Refs. 14, 16, and 17. Piezoelectric scattering has been treated elastically with an angle averaged coupling coefficient, 18 and the appropriate piezoelectric and elastic constants for the wurtzite 19,20 or zincblende structure 4,9,12 have been taken into account. The remaining material parameters have been taken from the compilation given in Ref. 7. As a concrete application, we have modeled two different WZ-Al 0.15 Ga 0.85 N/GaN modulation-doped field effect transistors ͑MODFET͒ structures. 1,21 These ͓0001͔ oriented structures have been grown on sapphire substrate, followed by a GaN nucleation layer and a 0.3 m thick, nominally undoped, strain relaxed, GaN layer with an estimated electron density of 5ϫ10 16 cm Ϫ3 . The channel is formed by subsequent growth of a 300 Å Al 0.15 Ga 0.85 N barrier layer that we assume to be pseudomorphic and therefore biaxially strained. 21 In one case, 1 the Al 0.15 Ga 0.85 N contains a 150 Å thick ␦-doped supply layer that is separated from the channel by a 30 Å spacer and has a doping density of 3ϫ10 18 cm Ϫ3
doi:10.1063/1.122011 fatcat:qmybf77gsjbj7jseiga2cxwkhi