A molecular dynamics study of the causes of defects at dielectric/substrate interface of Ge MOSFETs

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The superior electrical properties of Germanium (Ge), such as higher electron and hole mobility, compared to that of other semiconductor materials, have been one of the factors to cause germanium to return to be of focal interest in R&D. However, the inferior quality of dielectric/substrate interface of Ge-based MOSFETs compared to its Si counterparts has been reported to be a challenge in manufacturing Ge-based MOSFETs, causing reliability issues for the devices (including Negative-Bias Temperature Instability – NBTI). Ge dangling bonds at the dielectric/substrate interface will induce defects known as interface states. Hence, reducing interface states density is one of the major concerns regarding Ge MOSFETs. These states lead to higher leakage currents and increased threshold voltage over time. Specifically, the number of Ge dangling bonds at the interface can play a key role in deteriorating electrical properties over time. Many factors in the process of MOSFETs fabrication can affect the number of interface states. These include oxidation temperature, oxide thickness, dopant nature and concentration, fabrication ambient, etc. Our work reported here used classical molecular dynamics simulations based on reactive forcefield (ReaxFF) to gain insight into the interfacial properties of Germanium dioxide in Ge channel MOSFETs. For example, our simulations established that the oxide thickness has an inverse relationship with the number of Ge dangling bonds at the interface of dielectric/substrate, i.e. thinner oxides tend to generate more interfacial defects. This indicates that there is an optimum oxide thickness affecting the number of dangling bonds. Beyond a certain thickness however, this effect gradually fades out. Our findings reveal that there are four different types of Ge dangling bonds at GeO2/Ge interface. Firstly, there is what we denote as Ge0+ which is threefold coordinated and only back bonded to Ge atoms. Secondly, there are Ge1+ and Ge2+ which are threefold coordinated with one- and two-oxygen back bonded respectively. Finally, there is Ge3+, which is bonded to three oxygen atoms. Although Ge3+ is not present at the interface, it is more prone to appear near the interface. We also show that the closer to the interface, the more GeO is found. GeO generation has been recognized to degrade C-V characteristics due to leaving a large amount of defects at the interface. Additionally, our study shows that the sputtering temperature of the oxide on the substrate is inversely proportional to the number of Ge dangling bonds at the interface. Therefore, elevating the sputtering temperature decreases the density of Ge dangling bonds. Incidentally, it has been reported in other works that the oxygen vacancy in the dielectric (resulted from Ge3+) plays a key role in having a positive fixed charge near the Ge/GeO2 interface. We also qualitatively show a dipole effect at the interface (which has been shown to reduce electron mobility), characterized by a positive charge at the channel and an equal negative charge within the oxide. Similar to interface defects, these fixed charges have been identified to cause NBTI in Ge MOSFETs.

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Journal: TechConnect Briefs
Volume: 4, Informatics, Electronics and Microsystems: TechConnect Briefs 2018
Published: May 13, 2018
Pages: 55 - 58
Industry sector: Sensors, MEMS, Electronics
Topicss: Advanced Manufacturing, Nanoelectronics
ISBN: 978-0-9988782-1-8