Polarization-Induced 3-Dimensional Electron Slabs in Graded AlGaN Layers
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0892-FF17-04.1
Polarization-Induced 3-Dimensional Electron Slabs in Graded AlGaN Layers John Simon1 , Kejia (Albert) Wang, Huili Xing, Debdeep Jena Department of Electrical Engineering University of Notre Dame IN, 46556 Siddharth Rajan Department of Electrical and Computer Engineering University of California, Santa Barbara CA, 93106 Abstract By compositionally grading AlGaN layers over different thicknesses, high mobility electron gases are produced by polarization-induced doping. Temperature dependent Hall and capacitance-voltage measurements were performed on these AlGaN layers, and two degrees of freedom are found for choosing the carrier concentration of these slabs. Carrier mobilities determined from Hall measurements are observed to be much higher than impurity doped structures of similar carrier densities. Alloy and phonon scattering are determined to be the major contributors limiting the mobility of the electron in the graded layers. This form of polarization-induced doping offers an attractive alternative to the traditional doping techniques, and may be used for highly conductive AlGaN layers with high Al composition, both for lateral and vertical transport.
1
Introduction
In recent years there has been a large interest in III-V nitride semiconductors for applications in high power electronic devices and short-wavelength optical devices. Optical devices have gained much popularity due to the large span in bandgaps ranging from 0.7 (InN) to 6.2 eV(AlN)[1]. Doping in most devices has been traditionally obtained by conventional methods, such as substitutional or modulation doping. Activation energies for traditional dopants in GaN are in the order of 10 meV (Si for n-type) and 160 meV[2] (Mg for p-type). P-type doping is more challenging than n-type in GaN. When the same dopants are introduced into the wider bandgap AlGaN, both the activation energies of n-type and p-type increase[3]. The need for high quality dopants in high aluminum composition AlGaN for UV-detectors/emmitters and heterojunction bipolar transistors (HBT’s) has been a matter of extensive research. Large polarization fields[4] have been observed in wurtzite III-V nitrides that are not present in other semiconductor materials. Both spontaneous and piezoelectric components of the polarization contribute to the total polarization charge in the crystal. By engineering these polarization fields it is possible to produce regions of fixed charge ρπ given by: 1
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0892-FF17-04.2 n(z) 19
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Figure 1:
Sample structure of measured samples. Composition was linearly graded AlGaN over different thicknesses d. Positive polarization charges induce a 3-dimensional electron concentration in the graded layer. Room tempe
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