Polarization induced ultra-high electron concentration up to 1020 cm-3 in graded AlGaN

Abstract

Carrier concentration and mobility of materials are key factors affecting device performance. Hall tests at different temperatures demonstrate that the carrier concentration and mobility in impurity-doped AlGaN decrease with temperature decreasing. However, carrier concentration and mobility obtained by polarization-induced doping are independent of temperature. Using quasi-insulating GaN as substrate, the electron concentration obtained in the linearly graded AlGaN film through impurity-doping is only 10-17 cm-3 or less. In this study, using unintentional impurity doped (n-type, 10-16 cm-3) GaN template, graded AlGaN film is grown by molecular beam epitaxial, in which polarization induced ultra-high electron concentration is up to 1020 cm-3 in graded AlGaN film without using any dopant. Using quasi-insulating GaN as substrate, only the surface of the free electrons serves as polarization dopant, while unintentionally doped GaN template is used as a substrate, in addition to free electrons on surface/interface, it is also reasonable to surmise more negative charges attracted by polarization electric field to be the source of polarization doping, in the unintentional doped GaN template, thereby achieving an ultra-high carrier concentration via polarization induced n-type doping.

Low-resistance Ohmic contact on polarization-doped AlGaN/GaN heterojunction

“…Previous studies have indicated that the unintentionally doped GaN template functions as a reservoir of free electrons, thus more free electrons are available for the polarization charge field. [24,25] However, this factor is not included in the calculated models, Therefore, the polarizationinduced sheet density in the present work is much higher than the value (10 12 cm −2 and/or low 10 13 cm −2 range) obtained by calculating and using semi-insulating GaN as substrate. [4,26] This clearly shows that high Al composition can enhance the carrier concentration.…”