Researchers from Würzburg University, Germany, and Lund University, Sweden, are working together to optimize the HfO2 capping of InP nanowires for use in nanoelectronics. The deposition of a high-k dielectric is a crucial step in the processing of nanowire devices such as wrap-gated field effect transistors (FETs). Initial studies have revealed that dielectric capping has a negative influence on the surface quality and, hence, the transport properties of the nanowires.

In a recent investigation, published in the journal Nanotechnology, the team performed optical studies on InP nanowires capped with HfO2 via atomic layer deposition (ALD). The work shows that optical spectroscopy is a valuable and sensitive tool to identify optimum ALD process parameters for the realization of high quality high-k dielectrics on InP nanowires.

The InP nanowires were grown in a Lund University laboratory using a metal-organic vapour phase epitaxy (MOVPE) system. After growth, the samples were directly transferred to the ALD chamber to realize well defined HfO2 cappings at different deposition temperatures and layer thicknesses. Subsequently, scientists from Würzburg University studied the samples by means of time-resolved micro-photoluminescence (PL) spectroscopy.

Quantitative measure

A careful analysis of the photoluminescence as a function of time and wavelength was performed to evaluate the quality of the HfO2-capped InP nanowires. For this purpose the researchers adopted a model which takes the bending of the energy bands under different levels of excitation into account. This method allowed the researchers to determine the surface recombination velocity (SRV) as a reasonable quantitative measure for the quality of the nanowire surface after the deposition of the high-k dielectric.

Best results in terms of a low SRV (9.5 × 103 cm/s) were obtained for InP nanowires with an additional GaP shell measuring only a few monolayers in thickness and grown on the InP core prior to the HfO2 deposition. An in situ, post-growth annealing step under H2S atmosphere prior to ALD also improved surface quality.

In general, these results show that time-resolved micro-PL spectroscopy is a powerful, non-invasive tool to study the surface quality of nanowires. The presented method of addressing the surface treatment for semiconductors with high-k dielectrics is not only suitable for InP structures, but will also be applicable to nanoelectronic devices based on other III/V materials.

Future studies will focus on optimizing the HfO2 deposition of core-shell nanowires for which a strong enhancement of the InP surface quality is expected.