Ph.D Thesis

Ph.D StudentKaminski Yelena
SubjectThe Effect of Interfaces on the Collection Efficiency
of Carriers in Silicon Photodiodes
DepartmentDepartment of Chemical Engineering
Supervisors PROF. Yaron Paz
DR. Eitan Shauly
Full Thesis textFull thesis text - English Version


Recombination on silicon interfaces, associated with dark current, is a topic being addressed for various applications such as photosensors and solar cells. In 0.18mm-technology CMOS Image sensors, it is observed at silicided areas as well as at Shallow Trench Isolation (STI) interfaces. Recombination on the contact surfaces is also one of the main limiting factors for silicon solar cells. The aim of this research was to study the effect of interfaces on carrier collection efficiency in silicon photodiodes. Several different interfaces were explored and significant enhancement in photodiode performances was obtained.

Suppressing recombination on n-type salicided silicon contact interfaces by applying polysilicon buffer layers was examined. The contact incorporated the advantage of low contact resistance due to silicidation, with the advantage of reduced recombination by virtue of formation of carrier selective junction at the polysilicon interface. The introduction of a polysilicon interlayer was found to increase the short circuit current and the fill factor and to decrease the saturated leakage current. The advantage of the polysilicon contact was much more pronounced under high intensity illumination conditions.

The superior low leakage characteristics of the polysilicon salicided contacts enabled to increase the contact area without degrading the leakage current. As a result trenched polysilicon salicided contacts were developed, enabling vertical P/N junction design. These contacts can be prepared up to 10mm in depth, without showing any leakage current associated with the increase in the contact area. Consequently, the trenched photodiodes revealed better performance than no-trench photodiodes. A simple two dimensional model was developed, allowing to estimate the conditions under-which a vertical design has the potential to have better performance than that of a planar design.

This work showed that the introduction of a polysilicon interlayer may have a benevolent effect on photodiodes' characteristics, and, in particular in photodiodes that, due to their design, are more sensitive to the properties of their contacts. This includes, for example, salicided polysilicon trenched contacts, where the use of polysilicon interlayer open new opportunities for designing of solar cells and image sensors.

Another aspect of the study had to do with p-type silicon - metal contacts. Here, the same approach of creating carrier selective junction was applied by grafting hole conducting molecules on a silicon surface. The results showed significant improvement in photodiode performance.

The STI interface is known to be one of the major reasons for dark current phenomena, and its quality may be affected by chemical pretreatments prior to STI formation. Assessing the effect of pre-treatments can be quite cumbersome and may require to complete the production of the CIS prior to measuring. In this context, we developed a method for analyzing the quality of cleaning and for optimizing the sequence of chemical treatments, based on analyzing the performance of a simple STI interface- sensitive high resistance structure (SSHR). The method was demonstrated by comparing various cleaning treatments and following the resistivity and breakdown voltage of the SSHR device. Results were correlated with STI-sensitive final products such as CIS and Lateral Diffused Metal Oxide Semiconductor transistor.