Electrical properties of GaN (Fe) buffers for AlGaN∕GaN high electron mobility transistor structures

Abstract

The electrical properties of AlGaN∕GaN high electron mobility transistor structures grown on composite GaN(Fe)∕GaN buffers by molecular beam epitaxy were reported. The concentration of Fe in the GaN(Fe) layer ranged from 8×1016to3×1017cm−3 as established by secondary ion mass spectrometry. The thickness of the undoped GaN layer of the buffer was varied from 2.2to4.1μm. For thinner buffers and higher Fe concentration, the buffer was semi-insulating, with the Fermi level pinned near Ec-0.57eV. For thicker buffers and lower Fe concentration, the top part of the buffer was conducting. Admittance spectra measured in conducting buffers also showed a prominent contribution from Ec-(055–0.6)eV electron traps. Despite the universal prominence of these traps in all our films, the behavior of their concentration with Fe doping and with increased distance from the GaN (Fe)∕GaN boundary is not compatible with the assumption that they are due to substitutional Fe acceptors. Possible compensation mechanisms in the studied structures were discussed.

Deep traps in GaN-based structures as affecting the performance of GaN devices

“…[181,182] showed that the resistivity of the GaN films consisting of the heavily Fe doped layer near sapphire and an undoped portion of the film higher up increased with increased Fe doping in the Fe-doped subbuffer and with decreased thickness of the undoped portion of the buffer [181]. The semi-insulating samples with combined buffer showed the Fermi level pinned near E c À0.6 eV.…”

“…These data suggest the participation of major technologically important contaminants (most likely, O, C) forming complexes with native defects created by irradiation. At the same time, in AlGaN/GaN, AlN/GaN with semi-insulating GaN(Fe) buffers grown by MBE [181,182,264] the centers dominant in reverse DLTS were the 0.25 and 0.6 eV prevalent in the GaN buffer.…”

“…37-40 can in many cases be determined by the reverse DLTS method in which the partially depleted heterostructure is pulsed to high reverse voltage to fill the traps in question and then returned to the quiescent bias [181,182,261,264] (the procedure is similar to filling the traps in HEMTs by reverse bias pulsing to determine parameters of the traps in HEMTs responsible for the gate lag or drain lag, see next section). The capacitance relaxations are then processed in the usual DLTS manner.…”

Thermal stability of semi-insulating property of Fe-doped GaN bulk films studied by photoluminescence and monoenergetic positron annihilation techniques

“…The Fermi level pinning approximately 0.5-0.6 eV below the conduction band minimum ͑CBM͒ has also been confirmed using a capacitancevoltage method. 15 Since the charge state of Fe Ga 3+/2+ level is transferred from Fe Ga 3+ to Fe Ga 2+ by capturing an electron, an Fe atom acts as a compensating deep acceptor in GaN. As the Fe concentration, ͓Fe͔, is increased sufficiently in moderately n-type GaN, the Fermi level shifts toward the midgap and Fe Ga 3+/2+ level is inversely transferred from Fe Ga 2+ to Fe Ga 3+ by releasing an electron.…”

Electron irradiation of AlGaN∕GaN and AlN∕GaN heterojunctions

“…The 50% degradation of the 2DEG conductivity happens at several times higher doses ͑close to 3 ϫ 10 16 cm −2 versus 6.5ϫ 10 15 High electron mobility transistor ͑HEMT͒ structures based on AlGaN / GaN and AlN / GaN are of interest for high-power microwave applications. The irradiation increases the resistivity of the GaN buffer due to compensation by radiation defects with levels near E c − 1 eV and decreases the mobility of the two-dimensional electron gas ͑2DEG͒ near the AlGaN / GaN ͑or AlN / GaN͒ interface.…”