**Photodiodes as Optical Radiation Measurement Standards**

Ana Luz Muñoz Zurita, Joaquín Campos Acosta, Alejandro Ferrero Turrión and Alicia Pons Aglio

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/51462

## **1. Introduction**

Aseev, A. L. (2001). MWIR and LWIR detectors based on HgCdTe/CdZnTe/GaAs

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[26] Andreeva, E. V., Varavin, V. S., Vasiliev, V. V., Gumenjuk-Sichevska, J. V., Dvoret‐ sky, S. A., Mihajlov, N. N., Tsybtii, Z. F., & Sizov, F. F. (2009). Comparison of current characteristics of CdHdTe photodiodes grown by MBE and LPE methods. *Journal of*

[27] Vasiliev, V. V., Varavin, V. S., Dvoretsky, S. A., Mikhailov, N. N., Ovsyuk, V. N., Si‐ dorov, Yu. G., Suslyakov, A. O., Yakushev, M. V., & Aseev, A. L. (2003). HgCdTe epi‐

layers on GaAs: growth and devices. *Opto-Electronics Review*, 11, 99-111.

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172 Photodiodes - From Fundamentals to Applications

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Photodiodes for optical radiation measurements are used without reverse bias in most ap‐ plications since this operation yields the lowest dark current. To obtain photodiodes that op‐ erate at a low bias and have a low dark current, it is necessary to produce epitaxial layers that are pure and have few defects (such as dislocations, point defects, and impurity precipi‐ tates). Furthermore, a planar device structure requires that a guard ring be used to keep the electric field around the photoreceptive area from increasing too much. Fabrication and processing technologies such as impurity diffusion, ion implantation, and passivation play important roles in the production of reliable photodetectors.

From a radiometric point of view, the photodetectors important characteristics are: Speed of response (characterized by the bandwidth of the frequency response or the Full Width Half Maximum (FWHM) of the pulse response), responsivity (determined as the ratio of current out the detector to the incident optical power on the device), sensitivity (defined as the mini‐ mal input power that can still be detected which, as a first approximation, is defined as the optical power which generates an electrical signal equal to that due to noise of the diode) and response linearity. These quantities defined the basic radiometrical behavior of any de‐ tector. For those detectors having large area, as it may be the case for some photodiodes, knowing the response uniformity of the sensitive area is important too, especially when the incident beam diameter is much smaller than the detector sensitive surface. A high nonuni‐ formity would produce measurement errors when the detector is used at different positions, errors that have to be taken into account for the final accuracy of the measurement.

© 2012 Luz Muñoz Zurita et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Luz Muñoz Zurita et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

To determine those radiometric features in photodiodes and learn how they change with wavelength, for instance, it is a good approach to start by analyzing. The physical phenom‐ ena involved in the detection. When light impinges on a detector, various physical processes occur; part of the incident light is reflected at the sensitive surface, while the rest passes in‐ side the detector, where can be partially, because of losses due to absorption, converted into an electronic signal. Then the photodetector response is conditioned by the amount of absor‐ bed light, but for evaluating the incident power one has to know the ratios of the reflected, absorbed, and converted power as well. Taking into account these phenomena, the short cir‐ cuit response of a photodiode can be written as

$$I(\lambda) = I\_0 + (1 - \rho(\lambda))\varepsilon(\lambda)\frac{\lambda}{k} \mathcal{Q}\left(\lambda\right) \tag{1}$$

Therefore the attainable scope at present is just to obtain a model to be able to calculate spec‐ tral responsivity values at any wavelength. To get this, a model has been developed to cal‐ culate reflectance values from experimental ones at some wavelengths and another model has been developed to interpolate spectral internal quantum efficiency values from some values got from reflectance and responsivity measurements at some wavelengths. Both

**2. Spectral responsivity scale in the visible range based on single silicon**

A spectral responsivity scale means that the responsivity is known at every wavelength within the response range of interest and it would be desirable to know it for all the other

Aspectral responsivity scale in the visible range can be created by calibrating a silicon trap detector at several laser wavelengths against ahigh accuracy primary standard such as an electrically calibrated cryogenic radiometer. This method provides a very certain value for the responsivity at specific wavelengths as those of lasers (for instance 406.7 nm, 441.3 nm, 488.0 nm, 514.5 nm, 568.2 nm, 647.1 nm and 676.4 nm). From there single elements detectors, most suitable for some applications, can be calibrated against that trap detector at those

parameters associated with a beam: angle of incidence, divergence or polarization.

The spectral responsivity of silicon photodiodes is given by the well-known equation

*R*(*λ*)=(1 - *ρ*(*λ*))*ε*(*λ*)

**3. Reflectance evaluation of silicon photodiodes**

This chapter describes the results obtained for the responsivity of the photodiodes by using a model to calculate the diode's reflectance from experimental measurements and a model for the internal quantum efficiency, which is also fitted to experimental values. Based on the mod‐ els, the fitting errors and the uncertainty of reflectance and responsivity measurements, the uncertainty of the responsivity scale is calculated according to the ISO recommendations.

From the reflectance point of view, a silicon photodiode can be considered as a system formed by a flat transparent film over an absorbing medium. The flat film is the silicon ox‐ ide and the absorbing medium is the silicon substrate. The reflectance of such a system is

*λ*

*<sup>k</sup>* (2)

Photodiodes as Optical Radiation Measurement Standards

http://dx.doi.org/10.5772/51462

175

models will be presented in this chapter.

wavelengths to define the working scale.

**photodiodes.**

given by [5]

Where *I <sup>0</sup>* is the dark response, ρ(λ) is the photodiode's reflectance, λ is the radiation wave length, ε (λ) is the photodiode's internal quantum efficiency,*k* is a constant that takes into ac‐ count other fundamental physical constants and ϕ (λ) is the spectral radiant flux incident on the photodiode. According to this equation, the incident radiant flux can be determined from measuring the photodiode's response as far as its spectral reflectance and internal quantum ef‐ ficiency are known. Then photodiodes are good devices for radiant flux standards.

Silicon and InPphotodiodes from different manufacturers have got rather low noise lev‐ el, good response uniformity over the sensitive surface and a wide dynamic range. There‐ fore they are good devices to build radiometers in the visible and NIR spectral region in many different applications, particularly for building up spectroradiometric scales for ra‐ diant flux measurements.

Back to equation (1), if photodiode's reflectance and internal quantum efficiency were known, the photodiode's responsivity would be known without being compared to another standard radiometer; i. e. the photodiode would be an absolute standard for optical radia‐ tion measurements [1, 2, 3].

This idea was firstly developed for silicon photodiodes in the eighties, once the technology was able to produce low defects photodiodes [4]. Following this reference, the reflectance could be approached from a superimposed thin layers model. By knowing the thicknesses of the layers and the optical constants of the materials, it is possible to determine the device reflectance. However, this information is not completely available for InP photodiodes: the actual thickness of the layers is not known and optical constants of materials are only ap‐ proximately known for bulk. Nevertheless it's possible to measure reflectance at some wave‐ lengths and to fit the thicknesses of a layer model that would reproduce those experimental values.

The internal quantum efficiency cannot be determined as for Si. Since InP photodiodes are hetero-junctions rather than homo-junctions as silicon photodiodes are. In the other hand, since the internal structure is not accurately known, it is not possible to model the internal quantum efficiency without having experimental values for it.

Therefore the attainable scope at present is just to obtain a model to be able to calculate spec‐ tral responsivity values at any wavelength. To get this, a model has been developed to cal‐ culate reflectance values from experimental ones at some wavelengths and another model has been developed to interpolate spectral internal quantum efficiency values from some values got from reflectance and responsivity measurements at some wavelengths. Both models will be presented in this chapter.

To determine those radiometric features in photodiodes and learn how they change with wavelength, for instance, it is a good approach to start by analyzing. The physical phenom‐ ena involved in the detection. When light impinges on a detector, various physical processes occur; part of the incident light is reflected at the sensitive surface, while the rest passes in‐ side the detector, where can be partially, because of losses due to absorption, converted into an electronic signal. Then the photodetector response is conditioned by the amount of absor‐ bed light, but for evaluating the incident power one has to know the ratios of the reflected, absorbed, and converted power as well. Taking into account these phenomena, the short cir‐

*λ*

Where *I <sup>0</sup>* is the dark response, ρ(λ) is the photodiode's reflectance, λ is the radiation wave length, ε (λ) is the photodiode's internal quantum efficiency,*k* is a constant that takes into ac‐ count other fundamental physical constants and ϕ (λ) is the spectral radiant flux incident on the photodiode. According to this equation, the incident radiant flux can be determined from measuring the photodiode's response as far as its spectral reflectance and internal quantum ef‐

Silicon and InPphotodiodes from different manufacturers have got rather low noise lev‐ el, good response uniformity over the sensitive surface and a wide dynamic range. There‐ fore they are good devices to build radiometers in the visible and NIR spectral region in many different applications, particularly for building up spectroradiometric scales for ra‐

Back to equation (1), if photodiode's reflectance and internal quantum efficiency were known, the photodiode's responsivity would be known without being compared to another standard radiometer; i. e. the photodiode would be an absolute standard for optical radia‐

This idea was firstly developed for silicon photodiodes in the eighties, once the technology was able to produce low defects photodiodes [4]. Following this reference, the reflectance could be approached from a superimposed thin layers model. By knowing the thicknesses of the layers and the optical constants of the materials, it is possible to determine the device reflectance. However, this information is not completely available for InP photodiodes: the actual thickness of the layers is not known and optical constants of materials are only ap‐ proximately known for bulk. Nevertheless it's possible to measure reflectance at some wave‐ lengths and to fit the thicknesses of a layer model that would reproduce those experimental

The internal quantum efficiency cannot be determined as for Si. Since InP photodiodes are hetero-junctions rather than homo-junctions as silicon photodiodes are. In the other hand, since the internal structure is not accurately known, it is not possible to model the internal

quantum efficiency without having experimental values for it.

*<sup>k</sup>* ∅(*λ*) (1)

*I*(*λ*)= *I*<sup>0</sup> + (1−*ρ*(*λ*))*ε*(*λ*)

ficiency are known. Then photodiodes are good devices for radiant flux standards.

cuit response of a photodiode can be written as

174 Photodiodes - From Fundamentals to Applications

diant flux measurements.

tion measurements [1, 2, 3].

values.
