A study on Si/Al2O3 paramagnetic point defects

Abstract

In this contribution, negative charges and electronic traps related to the Si=Al 2 O 3 interface were measured and related to paramagnetic point defects and molecular vibrations. To this end, contactless capacitance voltage measurements, X-band electron paramagnetic resonance (EPR), and infrared spectroscopy were carried out, and their results were compared. A change in the negative charge density and electron trap density at the Si=Al 2 O 3 interface was achieved by adding a thermally grown SiO 2 layer with varying thicknesses and conducting an additional temperature treatment. Using EPR, five paramagnetic moments were detected in Si=ðSiO 2 Þ=Al 2 O 3 samples with g values of g 1 ¼ 2:008160:0002; g 2 ¼ 2:005460:0002; g 3 ¼ 2:000360:0002; g 4 ¼ 2:002660:0002, and g 5 ¼ 2:002960:0002. Variation of the Al 2 O 3 layer thickness shows that paramagnetic species associated with g 1 , g 2 , and g 3 are located at the Si=Al 2 O 3 interface, and those with g 4 and g 5 are located within the bulk Al 2 O 3 . Furthermore, g 1 , g 2 , and g 3 were shown to originate from oxygen plasma exposure during Al 2 O 3 deposition. Comparing the g values and their location within the Si=Al 2 O 3 system, g 1 and g 3 can be attributed to P b0 centers, g 3 to Si dangling bonds (Si-dbs), and g 4 and g 5 to rotating methyl radicals. All paramagnetic moments observed in this contribution disappear after a 5-min temperature treatment at 450 C. The deposition of an additional thermal SiO 2 layer between the Si and the Al 2 O 3 decreases the negative fixed charge density and defect density by about one order of magnitude. In this contribution, these changes can be correlated with a decrease in amplitude of the Si-db signal. P b0 and the methyl radical signals were less affected by this additional SiO 2 layer. Based on these observations, microscopic models for the negative fixed charge density (Q tot ) and the interface trap density (D it ) and the connection between these values are proposed. Published by AIP Publishing. [http://dx

Intrinsic charge trapping in amorphous oxide films: status and challenges

“…Introducing significant band bending at the silicon side of the Si/a-Al 2 O 3 interface leads to reduction of the surface recombination, thus improving the solar cell efficiency. Formation of negative charges in alumina is seen as a result of electron transfer from silicon to the oxide traps thought to be either oxygen interstitials [170] or aluminum vacancies [18]. However, these trapping site models can hardly explain the thermally-activated increase of the negative charge observed when annealing a-Al 2 O 3 layers on Si at temperatures below 500 °C [171] which are insufficient for alumina crystallization.…”

The origin of negative charging in amorphous Al2O3 films: the role of native defects

“…The major challenge for atomic identification of electron trapping sites in a-Al 2 O 3 concerns the absence of electron spin resonance (ESR) signals associated with these electron states. Despite several attempts to observe electrons trapped in a-Al 2 O 3 using ESR, only signals stemming from its interface with the silicon substrate (silicon dangling bonds at the surface of the Si crystal or in the near-interface Si oxide layer) or with contaminants, mostly carbon-related, have been detected so far 7,10,[15][16][17][18][19] . We note that crystallization of alumina as a result of high temperature annealing does not eliminate electron trapping sites, suggesting that they are not caused by disorder as in a-HfO 2 The EPDS measurements were confirmed by gate side trap spectroscopy when injecting electrons from silicon and sensing' (GS-TSCIS) measurements which show a peak defect density of 1.6 × 10 19 /cm 3 at approximately 3.4 eV below the Al 2 O 3 CBM, with a significant distribution of traps from 3.0 eV below the CBM 4 .…”

“…It has been suggested that the formation of negative charges in alumina is result of electron transfer from silicon to energetically deep electron traps inside the oxide. A broad variety of models for the electron traps have been introduced, ranging from oxygen interstitials [10] to oxygen vacancies [11] and aluminum vacancies [7]. Testing the validity of these models requires understanding of how these trapping site models can explain the thermally-activated increase of the negative charge observed when annealing a-Al 2 O 3 layers on Si at temperatures below 500 °C [12].…”