The design and construction of a predictable quantum efficient detector (PQED), suggested to be capable of measuring optical power with a relative uncertainty of 1 ppm (ppm = parts per million), is presented. The structure and working principle of induced junction silicon photodiodes are described combined with the design of the PQED. The detector uses two custom-made large area photodiodes assembled into a light-trapping configuration, reducing the reflectance down to a few tens of ppm. A liquid nitrogen cryostat is used to cool the induced junction photodiodes to 78 K to improve the mobility of charge carriers and to reduce the dark current. To determine the predicted spectral responsivity, reflectance losses of the PQED were measured at room temperature and at 78 K and also modelled throughout the visible wavelength range from 400 nm to 800 nm. The measured values of reflectance at room temperature were 29.8 ppm, 22.8 ppm and 6.6 ppm at the wavelengths of 476 nm, 488 nm and 532 nm, respectively, whereas the calculated reflectances were about 4 ppm higher. The reflectance at 78 K was measured at the wavelengths of 488 nm and 532 nm over a period of 60 h during which the reflectance changed by about 20 ppm. The main uncertainty components in the predicted internal quantum deficiency (IQD) of the induced junction photodiodes are due to the reliability of the charge-carrier recombination model and the extinction coefficient of silicon at wavelengths longer than 700 nm. The expanded uncertainty of the predicted IQD is 2 ppm at 78 K over a limited spectral range and below 140 ppm at room temperature over the visible wavelength range. All the above factors are combined as the external quantum deficiency (EQD), which is needed for the calculation of the predicted spectral responsivity of the PQED. The values of the predicted EQD are below 70 ppm between the wavelengths of 476 nm and 760 nm, and their expanded uncertainties mostly vary between 10 ppm and 140 ppm, where the lowest uncertainties are obtained at low temperatures.

The predictable quantum efficient detector (PQED) is intended to become a new primary standard for radiant power measurements in the wavelength range from 400 nm to 800 nm. Characterization results of custom-made single induced junction photodiodes as they are used in the PQED and of assembled PQEDs are presented. The single photodiodes were tested in terms of linearity and spatial uniformity of the spectral responsivity. The highly uniform photodiodes were proved to be linear over seven orders of magnitude, i.e. in the radiant power range from 100 pW to 400 µW. The assembled PQED has been compared with a cryogenic electrical substitution radiometer with a very low uncertainty of the order of 30 ppm. Experimental results show good agreement with the modelled response of the PQED to optical radiation and prove a near unity external quantum efficiency.

The spectral responsivity of a predictable quantum efficient detector (PQED) is calculated based on the responsivity of an ideal quantum detector and taking into account reflection losses from the surface of the photodiode and internal charge-carrier gains/losses inside the diode. The internal quantum deficiency (IQD) is obtained from simulations with the PC1D software using the material data of the produced PQED photodiodes. The results indicate that at room temperature the predicted IQD of the PQED is close to zero with an uncertainty of about 100 ppm over the visible range. It is further concluded that a primary standard of visible optical power with an uncertainty of approximately 1 ppm is achievable using the PQED at low temperatures.

Ikonen, E. (2017). Predictable quantum efficient detector based on n-type silicon photodiodes. Metrologia, 54(6) AbstractThe predictable quantum efficient detector (PQED) consists of two custom-made induced junction photodiodes that are mounted in a wedged trap configuration for the reduction of reflectance losses. Until now, all manufactured PQED photodiodes have been based on a structure where a SiO 2 layer is thermally grown on top of p-type silicon substrate. In this paper, we present the design, manufacturing, modelling and characterization of a new type of PQED, where the photodiodes have an Al 2 O 3 layer on top of n-type silicon substrate. Atomic layer deposition is used to deposit the layer to the desired thickness. Two sets of photodiodes with varying oxide thicknesses and substrate doping concentrations were fabricated. In order to predict recombination losses of charge carriers, a 3D model of the photodiode was built into Cogenda Genius semiconductor simulation software. It is important to note that a novel experimental method was developed to obtain values for the 3D model parameters. This makes the prediction of the PQED responsivity a completely autonomous process. Detectors were characterized for temperature dependence of dark current, spatial uniformity of responsivity, reflectance, linearity and absolute responsivity at the wavelengths of 488 nm and 532 nm. For both sets of photodiodes, the modelled and measured responsivities were generally in agreement within the measurement and modelling uncertainties of around 100 parts per million (ppm). There is, however, an indication that the modelled internal quantum deficiency may be underestimated by a similar amount. Moreover, the responsivities of the detectors were spatiallyOriginal content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. uniform within 30 ppm peak-to-peak variation. The results obtained in this research indicate that the n-type induced junction photodiode is a very promising alternative to the existing p-type detectors, and thus give additional credibility to the concept of modelled quantum detector serving as a primary standard. Furthermore, the manufacturing of PQEDs is no longer dependent on the availability of a certain type of very lightly doped p-type silicon wafers.

We have investigated the optical properties of self-induced inversion-layer silicon photodiodes using spectroscopic ellipsometric measurement techniques. We report a self-consistent data set and dispersion relation for the optical constants of the thermally grown oxide layer. The oxide layer thickness and spatial uniformity of a series of custom manufactured 22 mm x 11 mm rectangular diodes are evaluated. These photodiodes are used in a light trapping arrangement and exhibit predictable quantum efficiency and thus, predictable spectral responsivity. For comparison, we report measurements of the absolute spatial uniformity of the oxide layer on commercial "S6337" and "S1337" radiometric quality photodiodes