The influence of the strain-induced electric field on the charge distribution in GaN-AlN-GaN structure

Abstract

We show that strongly pronounced piezoelectric properties play a key role in GaN-AlN-GaN semiconductor-insulator-semiconductor (SIS) and related structures. In sufficiently thin AlN layers, the lattice constant mismatch is accommodated by internal strains rather than by the formation of misfit dislocations. These lattice-mismatch-induced strains generate polarization fields. We demonstrate that, in a GaN-AlN-GaN SIS structure with the growth axis along a (0001) crystallographic direction, the strain-induced electric fields can shift the flat band voltage and produce an accumulation region on one side and a depletion region on the other side of the AlN insulator. On which side of the insulator the accumulation region is produced depends on the type of atomic plane at the heterointerface (Ga or N). The surface charge density caused by the piezoeffect is on the order of 1012 cm−2. As a consequence of the asymmetry in the space charge distribution, the capacitance-voltage (C-V) characteristics of the SIS structure become asymmetrical. The asymmetrical shift of the C-V characteristics with respect to the origin is on the order of 1.5 V for a 30 Å AlN film. This asymmetry should vanish in a relaxed film. Hence, capacitance-voltage measurements of GaN-AlN-GaN SIS structures can be used for quantitative characterization as to the degree of AlN film relaxation, depending on the film thickness. This and related techniques should become important tools for the characterization of piezoelectric layered semiconductor films.

Mechanism of mobility increase of the two-dimensional electron gas in AlGaN∕GaN heterostructures under small dose gamma irradiation

“…It is known that the most peculiar feature of nitride heterostructures is the presence of huge spontaneous and piezoelectric polarization fields. [32][33][34] As a consequence, a high-density 2DEG appears at the AlGaN / GaN interfaces even without doping. Any changes in polarization fields will cause changes in 2DEG concentrations.…”

Morphology and surface reconstructions of GaN(11̄00) surfaces

“…Films with these surface orientations have well known pyroelectric and piezoelectric properties [2,3]. Although such properties can have useful device applications (such as confining carriers in a two-dimensional electron gas [2]), for other applications these properties are undesirable.…”

“…Films with these surface orientations have well known pyroelectric and piezoelectric properties [2,3]. Although such properties can have useful device applications (such as confining carriers in a two-dimensional electron gas [2]), for other applications these properties are undesirable. For this reason a few growth studies of (10 1 0) oriented (m-plane) GaN have been performed since for this orientation the crystal symmetry precludes pyroelectric and piezoelectric effects (at least in the absence of shear stresses in the growth plane) [4].…”

“…Several reconstructions on the GaN(1 1 00) surfaces are found, with the most common having approximate symmetry of "4×5". We propose a model in which this structure consists of 2 ≥ monolayers of Ga terminating the GaN surface.…”

Induced strain mechanism of current collapse in AlGaN/GaN heterostructure field-effect transistors

“…If the GaN layer is not strained then the AlGaN barrier layer simply adjusts its lattice constant to that of the underlying GaN layer. 11 Thus, the change in the electric field with gate biasing should not affect the strain. However, we believe that the surface region of the GaN layer is strained to a certain degree as well.…”