ALD deposited bipolar HfOx films for silicon surface passivation

Hafnium oxide thin films consist of trap states which can degrade or enhance the device performance depending upon the application. These trap states play a crucial role in silicon surface passivation when using HfOx films. This paper investigates the dependence of crucial surface passivation parameters on different film growth components of thermal ALD process. Our study is focused on understanding different oxide charges present at the HfOx/Si interface using capacitance-voltage and constant voltage stress studies; particularly trap states introduced during thermal atomic layer deposition process. This study is performed for different film growth conditions to understand the origin of traps states. We show that the interfacial layer (IL) grows during the film deposition and therefore two interfaces are formed, i.e., HfOx/SiOx and SiOx/Si. Acting as the tunnelling oxide, this interlayer consists of trap states which are intrinsic in origin. The investigation on MOS structures fabricated using n and p type silicon substrates gives an understanding towards amphoteric nature of the trap states. Their nature (acceptor/donor) depends on the substrate doping type (n-Si/p-Si) giving it bipolar characteristic. However, the interaction of such trap states with the majority carrier band edge of the substrate strongly depends on the IL thickness. Additionally, the trap charge density increases with metal pulsing duration, suggesting that the metal precursor may be the source of these trap charges rather than the oxidant. By shedding light on the nature and behaviour of trap states in HfOx films, our study contributes to a deeper understanding of their impact on device performance. This knowledge can aid in optimizing surface passivation strategies and improving the overall performance of HfOx-based devices in various applications.