On the origin of electrically active defects in AlGaN alloys grown by organometallic vapor phase epitaxy

Abstract

Shallow and deep centers were studied by means of temperature dependent Hall effect and photoluminescence (PL) measurements in two sets of undoped n-AlGaN samples grown by organometallic vapor phase epitaxy. The samples of these two series were grown under different conditions and had, as a result, electron concentrations differing by several orders of magnitude. The composition dependence of ionization energies of dominant donors in these two sets of samples is very different indicating that different types of centers are involved, but in both cases they are most probably related to some native defects. These defects behave as hydrogen-like donors for low Al compositions and become increasingly deeper with increasing Al content. The shallow-deep transition occurs at about x=0.2 in the low conductivity AlxGa1−xN series and at about x=0.5 for the high conductivity series. Several PL bands were detected in AlGaN and it is shown that the band at 3.05 eV is due to a radiative transition between deep donors in the upper part of the band gap and holes in the valence band or on shallow acceptors. For the yellow luminescence band at 2.25 eV it is demonstrated that this band consists of two overlapping bands and that the dominant band is due to a transition between the native donors and a carbon-related deep center.

Carbon doping of GaN with CBr4 in radio-frequency plasma-assisted molecular beam epitaxy

“…8,9 The discrepancy between the two explanations was addressed by Polyakov et al who discussed the likelihood of two overlapping bands responsible for the YL in the context of PL measurements on AlGaN samples of varying composition. 10 Zhang a͒ Author to whom correspondence should be addressed; electronic mail: speck@mrl.ucsb.edu and Kuech then correlated YL to carbon concentration by intentionally doping halide vapor phase epitaxy ͑HVPE͒ grown GaN with propane. 11 Their results also indicate two overlapping YL bands with the carbon doping corresponding to an increase in intensity and shift in peak energy of the YL peak.…”

Photoluminescence of carbon in situ doped GaN grown by halide vapor phase epitaxy

“…The YL was suggested to involve a radiative transition between a shallow donor, with the ionization energy of ϳ25 meV, and a deep acceptor, situated 860 meV above the top of the valence band. 8 Polyakov et al 9 and Niebuhr et al 10 have also attributed the observed YL in GaN grown by MOVPE to carbon impurities. The halide vapor phase epitaxy ͑HVPE͒ technique does not use carbon containing source materials but only uses NH 3 , HCl, and ultrapure Ga to synthesize GaN.…”

“…Carbon impurities are also believed to be related to yellow luminescence ͑YL͒ in GaN. [7][8][9][10] Some of the initial photoluminescence ͑PL͒ studies on C-doped GaN were carried out by Pankove and Hutchby. 7 They reported that C-implanted GaN exhibited a strong yellow luminescence band centered around 2.17 eV at 78 K. This band was attributed to defects arising from implantation damage, since the YL band could also be seen in samples implanted with other elements.…”

Time Resolved Photoluminescence of Si-doped High Al Mole Fraction AlGaN Epilayers Grown by Plasma-Enhanced Molecular Beam Epitaxy

“…[4] A significant number of studies have been dedicated to increasing the n-type doping levels in AlGaN [4][5][6] as well as studying the nature of the compensating acceptors [7,8], DX centers [1,3,9], and Si donors [6,7,10] in these alloys through transport and/or PL studies. Optical studies are especially important for highly doped AlGaN epilayers that exhibit a metallic behavior even at room temperature [6,7], such as the ones studied by Hwang et al [6] who reported temperature independent electron concentrations as high as 1.25*10 20 cm -3 in Al 0.5 Ga 0.5 N and 8.5*10 19 cm -3 in Al 0.8 Ga 0.2 N epilayers grown by rf plasma-enhanced molecular beam epitaxy.…”

“…Optical studies are especially important for highly doped AlGaN epilayers that exhibit a metallic behavior even at room temperature [6,7], such as the ones studied by Hwang et al [6] who reported temperature independent electron concentrations as high as 1.25*10 20 cm -3 in Al 0.5 Ga 0.5 N and 8.5*10 19 cm -3 in Al 0.8 Ga 0.2 N epilayers grown by rf plasma-enhanced molecular beam epitaxy.…”