International Radiation Detectors, Inc.

Responsivity Stability

It has been known for a number of years that silicon photodiodes show degradation in responsivity or linearity when exposed to intense UV flux [1,2].

The major UV induced instability arises because of inferior quality of the silicon-silicon dioxide (Si-SiO₂) interface [1]. Passivating layers other than silicon dioxide have been investigated recently to improve the interface quality of silicon devices. Among the many alternative passivating coatings reported, silicon oxynitrides (nitrided oxides) look promising. Oxynitrides have been shown to have higher resistance to ionizing radiation and impurity diffusion compared to pure oxides [3]. It has been postulated that the energy required to break a Si-N bond is much greater than that required to break Si-H or Si-OH bonds. Thus, less interface states are created in a nitrided device after exposure to UV radiation.

The above point was verified by us recently when diodes with 1 G-rad (SiO₂) hardness [4] were fabricated by incorporating nitrogen in the passivating oxide. It may be noticed here that this hardness is about 10,000 times the hardness of commonly used p-on-n photodiodes, and is the highest hardness ever reported or is known to exist in any silicon device.

Two other causes may be pointed out for quantum efficiency instability of silicon photodiodes. The first cause is formation of latent recombination centers by metallic impurities like silver [5]. These recombination centers become active over a period of several years causing a long term loss in quantum efficiency.

The other cause for quantum efficiency degradation is moisture penetration into the device over a long period of time. Moisture is suspected of causing recombination centers near the oxide-silicon interface leading to the quantum efficiency loss [6].

To minimize the effects of the above quantum efficiency instability mechanisms, UVG photodiodes were fabricated in an extremely clean environment to have negligible latent recombination centers and a trap-free, moisture insensitive Si-SiO₂ interface. Nitrogen incorporation in the Si-SiO₂ interface is known to make it insensitive to impurity penetration. Fig. 1 shows a quantum efficiency plot of our UV-enhanced diodes before and after exposure to 100% relative humidity. These diodes were fabricated by nitrogen incorporation at the interface and hence have exhibited no change in the 50 to 250 nm quantum efficiency even after 4 weeks of 100% relative humidity exposure at room temperature [4].

Fig. 1: Quantum efficiency of UV-enhanced photodiodes with 60 Å oxynitride passivating front window. O Before exposure, X after exposure to 100% relative humidity for 4 weeks.

Figures 2 and 3 show the responsivity stability of the UVG series diodes after exposure to intense radiation at 254 nm and 193 nm respectively. The 254 nm exposure was performed by a 20 mW/cm² low pressure mercury lamp. A Lambda Physik ArF excimer laser with 100 Hz pulse repetition rate and an energy density of 200 mJ/cm² (3.9 W at 100 Hz) was used to carry out the 193 nm stability test.

Fig. 2: Stability of a UVG series diode
compared to other types of diodes after
exposure to 254 nm radiation

Fig. 3: Stability of a UVG series diode compared to p-on-n diode
when exposed to 193 nm radiation.

Fig. 4 shows the response of three types of silicon photodiodes used to test the stability of Nichia 378 nm LED. Current through the photodiodes was about 61 nA. It is interesting to note that the p-on-n and the inversion layer diodes indicate that the long term stability of Nichia UV LED is questionable!

Fig. 4: Nichia 378 nm LED stability measured with p-on-n, inversion layer,
and the UVG-100 photodiodes [7].

References:

1] R. Korde and J. Geist, "Quantum Efficiency Stability of Silicon Photodiodes", Applied Optics, Vol. 26, 5284-5290 (1987).

2] K. D. Stock, "Regeneration of the Internal Quantum Efficiency of Silicon Photodiodes," Inst. Phys. Conf. Ser. No. 92, 167-171 (1988).

3] For example, see : "Ultra-thin Dielectrics for Semiconductor Applications - Growth and Characteristics" H. R. Harrison and S. Dimitrijev, Microelectronics Journal, Vol. 22, 3-38 (1991).

4] R. Korde, J. Cable and R. Canfield, "100% Internal Quantum Efficiency Silicon Photodiodes with One G-rad Passivating Silicon Dioxide" IEEE Trans. on Nuclear Sciences, Vol. 40, no. 6, 1655-1659 (1993).

5] V.G. Weizer et al., "Photon Degradation Effects in Terrestrial Silicon Solar Cells," J. Appl. Phys. Vol. 50, 4443 (1979).

6] L. Manchandra, "Hot Electron Trapping Generic Reliability of p+ Polysilicon/SiO2 /Silicon Structures for Fine Line CMOS Technology," in 24th Annual Proceedings, Twenty-Fourth Annual Conference on Reliability Physics IEEE, 183-186 (1986).

7] Courtesy of Donald F. Heath, Research Support Instruments, Boulder Co.