Polarization-induced charge and electron mobility in AlGaN/GaN heterostructures grown by plasma-assisted molecular-beam epitaxy

Abstract

The formation of the two-dimensional electron gas (2DEG) in unintentionally doped AlxGa1−xN/GaN (x⩽0.31) heterostructures grown by rf plasma-assisted molecular-beam epitaxy is investigated. Low-temperature electrical-transport measurements revealed that the two-dimensional electron gas density strongly depends on both the thickness of the AlGaN layer and alloy composition. The experimental results agree very well with the theoretical estimates of the polarization-induced 2DEG concentrations. Low-temperature electron mobility was found to be much higher in the structures with lower electron sheet densities. Interface roughness scattering or alloy disorder scattering are proposed to be responsible for this trend. A maximum mobility of 51 700 cm2/V s (T=13 K) was obtained in the Al0.09Ga0.91N/GaN structure with a two-dimensional electron gas density of 2.23×1012 cm−2.

High electron mobility in AlGaN/GaN heterostructures grown on bulk GaN substrates

“…For the heterostructure grown on the GaN single-crystal substrate, the Hall measurements yield a 77 K mobility of 30 000 cm 2 /V s and a 1.5 K mobility of 60 100 cm 2 /V s. For comparison, we also show in Fig. 1 the best results obtained for the heterostructures grown on sapphire by Smorchkova et al 4 and on 6H-SiC by Gaska et al 3 As it is seen in Fig. 1, heterostructures grown on GaN and sapphire substrates show a strong decrease of the electron concentration and an increase of carrier mobility with decreasing temperature.…”

“…The comparison of the homoepitaxial sample of this work and the sample of Ref. 4, grown on a sapphire substrate, allows us to comment on the role of dislocations in the low-temperature mobility of 2DEG. Although the structure investigated in this work is dislocation-free, its lowtemperature mobility is only slightly higher than that of the structure grown on sapphire, which has a dislocation density of 10 8 cm Ϫ2 .…”

“…For example, Gaska et al 3 reached a room-temperature mobility slightly over 2000 cm 2 /V s (n s ϭ1.3ϫ10 12 cm Ϫ2 ) in an Al 0.2 Ga 0.8 N/GaN structure deposited on 6H-SiC substrate by low-pressure metalorganic vapor-phase epitaxy. Smorchkova et al 4 succeeded in obtaining a mobility of 51 700 cm 2 /V s at 13 K in Al 0.09 Ga 0.91 N/GaN structure grown on sapphire by rf plasma-assisted molecular-beam epitaxy ͑MBE͒, with a 2DEG density of 2.2ϫ10 12 cm Ϫ2 . In these devices, a very large lattice mismatch between the substrate and the active layer leads to a high density of threading dislocations.…”

“…These measurements yield a low-temperature two-dimensional ͑2D͒ electron mobility that is one of highest ever reported for GaNbased heterojunctions. 4 GaN single crystals were grown by a self-seeding process under high-N 2 pressures of 10-20 kbar and at temperatures ranging from 1400 to 1700°C from an atomic solution in Ga. Usually, such crystals are highly n doped and can be made semi-insulating by adding compensating magnesium atoms to the Ga solution. The typical dimensions of the bulk crystals are 8ϫ8ϫ0.1 mm with the c axis perpendicular to the surface.…”

Dipole scattering in polarization induced III–V nitride two-dimensional electron gases

“…It is well worth noticing that this is much lower than the record low temperature mobilities (≈ 10 7 cm 2 /V s) of AlGaAs/GaAs modulation doped heterostructures, and an order of magnitude higher than the record high mobilities in AlGaN/GaN HEMTs observed till date (51, 700cm 2 /V s) [Ref. 19,21]. It hints at some more severe scattering mechanism(s) that determine the low temperature mobility in the III-V nitrides.…”