Predictable quantum efficient detector: I. Photodiodes and predicted responsivity

Abstract

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.

Primary standard of optical power operating at room temperature

“…The type of induced junction photodiodes used in the new detector design are described and characterized in [18] and [19]. In these photodiodes the depletion region is produced by the surface charges in the silicon dioxide layer [10], which improves the collection efficiency of charge carriers.…”

“…The calculated and measured photocurrent ratios at the center of the detector with different combinations of s and p polarized components in the incident laser beam are shown in Figure 5. The nominal value and the uncertainty corridor are calculated using detector and photodiode parameter values and uncertainties obtained from [18]. The most significant contributions are caused by uncertainty of the angle between photodiodes and of the oxide thicknesses of the photodiodes.…”

“…The uncertainty components due to the window transmittance, reflectance and absorption at 1° horizontal tilt angle of the PQED are insignificant in reflectance measurements [18]. Measured specular reflectances of nine PQEDs are shown in Figure 6.…”

“…The effect of reflectance, on the other hand, can be reduced by Brewster-angle operation [13] or by assembling the photodiodes into a lighttrapping configuration [9,[14][15][16][17]. These loss reduction methods are used to construct the Predictable Quantum Efficient Detector (PQED) [18,19]. It consists of two induced junction photodiodes with tailored design parameters suitable for the particular light-trapping configuration.…”

Advantages of white LED lamps and new detector technology in photometry

“…The method relies on an unfiltered detector with a known absolute responsivity and on carrying out the photometric weighting numerically based on the measured relative spectrum of the light source, thus eliminating the need of using photometric filters in the unit realization. When the predictable quantum efficient detector (PQED) 17,18,19 -a new primary standard for optical power that is based on an induced junction photodiode trap and that has a near unity quantum efficiency -is used as the broadband detector, high accuracy can be achieved, because the absolute spectral responsivity of the PQED can be predicted with a relative uncertainty of less than 0.01% 20,21 . We will demonstrate that the PQED method is more accurate than the photometer method when white LED lamps are used as light sources.…”