UV Light-Induced Response Degradation Characteristics of Silicon-Based Detectors ^†

1. Introduction

High energy radiation is known to potentially impact silicon-based optical sensors adversely, either permanently or reversibly. We have studied the impact of UV light (190–300 nm) concerning the spectral response of silicon photodiodes.

We found that the first and obvious prerequisite for degradation is a sufficiently high quantum efficiency in the energy range that the light may damage. If the light is already absorbed before it reaches the silicon, or if the detector has no sensitivity, due to its junction design, both lead to degradation-free devices. Most of the silicon detectors in use today are of such kind and are “immune” to UV-caused degradation, due to their poor UV response capabilities.

Nevertheless, there are special use cases where the detection of short wavelength light (below 300 nm) is required and for these, the degradation is to be avoided or at least known [1].

2. Materials and Methods

To generate UV light, mercury lamps are mostly used, but they provide a poorly defined energy spectrum (see Figure 1). In publications concerning the subject of degradation, often an intensity value per area is given without a complete spectrum. The wavelength at which the stress is provided and at what intensity remains unknown. A mercury lamp does not stress at all wavelengths of the spectrum.

To investigate the potential damage, we use a laser driven light source from Energetiq, a monochromator, and a solarization robust fiber for illumination. By that, we characterize the photon-energy-related degradation (see Figure 2, left).

3. Discussion

3.1. Spectral Dependency of UV Caused Degradation

It was expected that shorter wavelength light has higher degradation potential. This behavior is affirmed. Interestingly, we see no significant damage for wavelengths longer than 300 nm, and around 280 nm, the degradation is not a steady function of photon energy. This very likely originates from the refractive index properties and the high reflection around these wavelengths.

3.2. Exceptional High UVC Response

It is a challenge to engineer sensors with high UV response without degradation. Based on our long experience [3], we can show silicon sensors with UV response down to the vacuum UV (200 nm), which show very little degradation and can be used for sensing applications in that special range. Our newest research shows a significant response improvement (see Figure 2, right) for the spectral range around 260 nm, which is important for the application of disinfection.

4. Patents

Three patents are pending to protect the IP used to realize the new silicon photodiode with exceptional high UV response.