Dual-mode room temperature self-calibrating photodiodes approaching cryogenic radiometer uncertainty

The room temperature dual-mode self-calibrating detector combines low-loss photodiodes with electrical substitution radiometry for determination of optical power. By using thermal detection as a built-in reference in the detector, the internal losses of the photodiode can be determined directly, without the need of an external reference. Computer simulations were used to develop a thermal design that minimises the electro-optical non-equivalence in electrical substitution. Based on this thermal design, we produced detector modules that we mounted in a trap structure for minimised reflection loss. The thermal simulations predicted a change in response of around 280 parts per million per millimeter when changing the position of the beam along the centre line of the photodiode, and we were able to reproduce this change experimentally. We report on dual-mode internal loss estimation measurements with radiation of 488 nm at power levels of 500 μW, 875 μW and 1250 μW, using two different methods of electrical substitution. In addition, we present three different calculation algorithms for determining the optical power in thermal mode, all three showing consistent results. We present room temperature optical power measurements at an uncertainty level approaching that of the cryogenic radiometer with 400 ppm (k = 2), where the type A standard uncertainty in the thermal measurement only contributed with 26 ppm at 1250 μW in a 6 hour long measurement sequenc

Predictable quantum efficient detector based on n-type silicon photodiodes

The 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 SiO2 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 Al2O3 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 spatially 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.Peer reviewe

Strengthening the Radiometric Link to the SI: Achievements from the chipSCALe Project

The EURAMET European Metrology Programme for Innovation and Research (EMPIR) project chipS·CALe aims to improve and simplify radiometric traceability with a focus on strengthening the link between radiometric measurements and the international system of units (SI). In this project, several national metrology institutes (NMIs) and research institutions in Europe have collaborated to develop improved low-loss Predictable Quantum Efficient Detector (PQED) photodiodes with an external quantum deficiency in the 10 ppm range or below from 400 nm to 850 nm. These induced-junction photodiodes are simple in their structure, making them suitable for 3D computer simulations. In a 2 minute animation video developed in the chipS·CALe project, we will show the photodiode structure, the working principle, and how to use simple I-V measurements combined with a 3D model fit to extract photodiode defining loss parameters. Once the parameters are known, the fitted model is used to predict the responsivity of the photodiode in the spectral range from 400 nm to 850 nm. The chipS·CALe photodiodes have also been combined with thermal detection, in a dual-mode self-calibrating detector. By using thermal detection as a built-in reference in the detector, the internal losses of the photodiode can be determined directly, without the need of an external reference. We will present results for room temperature, with an uncertainty of 0.04 %, and our latest results of the ongoing measurements at cryogenic temperatures. By combining the 3D model fit and the dual-mode methods, we can extract the fundamental constants ratio e/hc from our measurements. This makes the dual-mode detector self-assured, serves as a validation of the two primary methods through a cryogenic high-accuracy comparison on one device, and provides a direct link between radiometric measurements and the new SI. This project 18SIB10 chipS·CALe has received funding from the EMPIR programme co-financed by the Participating States and from the European Union’s Horizon 2020 research and innovation programme

Self-Calibration of Photodiodes using the Dual-Mode Method

In 2014, White et al suggested to combine the measurement principle of the two primary standards for optical power measurements [1]; the cryogenic radiometer and the predictable quantum efficient detector (PQED). The result was the dual mode operated induced junction photodiode, for which the internal losses of the diode are estimated by comparing the two modes of operation, i.e. electrical substitution and photocurrent measurements. During the electrical substitution measurements, the optical power is found through the temperature at the diode, which is measured using a thermistor located close to the diode. The diode is cycled between electrical heating (forward bias) where the power is precisely defined, and optical heating (using the diode as a passive absorbing element). Since the initial demonstration, significant improvements in the room temperature dual mode measurements have been achieved, bringing the uncertainties close to that given by cryogenic radiometer. [2] The measurement system presented in this work consists of an induced junction photodiode which is connected to a heat sink through a weak heat link. The module is placed in a vacuum chamber, and a laser beam (HeNe laser with a wavelength of 633 nm) is directed at the diode through an Ar-coated window. Building on the system from ref [2], we present an improved measurement setup where the electrical coupling and grounding considerations have been improved. This has resulted in more accurate estimates of internal losses over a broader dynamic range. The internal losses are position dependent, with an average of 0.00% ± 0.04% for optical power as low as 366 μW, compared to 0.08% ± 0.04% at 500 μW, as reported in [2]. The uncertainties are limited by the thermal non-equivalence between electrical and optical heating. In this work we also propose an improved temperature drift compensation, resulting in a reduction of uncertainties by a factor of four compared to previously reported results [2]. In addition, refinements made in the data analysis provide a clearer and more transparent interpretation, facilitating a better understanding of system variability. These refinements have resulted in a robust methodology for estimating uncertainties which corresponds well with observed variation in the measurements. We also demonstrate the importance of propagating absolute uncertainties, as the observed standard deviation in the internal quantum deficiency depends on the relative position of optical power related to the electrical power even if the noise level is constant. This work demonstrates increased absolute radiometric measurement capability of room temperature dual mode detectors with uncertainties as low as 0.04 % limited by the heat equivalence of the dual mode assembly design. Work is ongoing to design and produce dual mode modules with better heat equivalence. The dual mode method represent a robust absolute radiometric measurement of any type of photodiodes, not just limited to PQEDs, with the potential to bring self-calibration directly into instruments and remote locations over a wide spectral range. References: [1] White, M. et al. (2014). Metrologia, 51(6), S245. [2] Ulset, M. et al. (2022). Metrologia, 59(3), 035008. Tuesday, June 11, 202

Room Temperature The Self-Calibrating Dual-Mode Detector - Say Goodby to the Long Traceability Chain

Low cost, high-accuracy calibration standards are requested by the radiometry community [1]. In the EMPIR-funded project chipS·CALe, we are developing self-calibrating dual-mode detectors for high-accuracy optical power measurements to meet this need. The dual-mode detector combines two primary standard techniques (PQEDs and electrical substitution radiometer [2]) into one device. This means the detector can be calibrated against its own internal primary reference. This eliminates the usually long and cumbersome traceability chain, and makes the detector suitable for calibrations in remote locations. The absorbing element for both measurement modes is an induced-junction photodiode [3]. A photodiode has internal losses that make it deviate from ideal responsivity. These losses vary with temperature and wavelength, and change over time as material properties of the photodiode change. As they affect the responsivity of the photodiode, the internal losses must be determined before high-accuracy measurements can be done. In the dual-mode detector these internal losses are determined by using thermal mode as a reference. In addition, an independent method for determining the internal losses is available, by fitting a charge carrier simulation model to IV curves [4]. To have the dual-mode detector work in thermal mode, special packaging of the photodiode is required [5]. The silicon photodiode is mounted on a carrier with a weak thermal heat link. The thermal design of the detector is optimised to minimise non-equivalence between optical and electrical heating, by the use of COMSOL heat transfer simulations. Vacuum conditions are necessary, as heat convection through air introduces complicated and unpredictable effects on the thermal equivalence. The largest contributor to non-equivalence is radiation losses, due to different thermal gradients in electrical and optical heating mode. Halfway through the project, we have already reduced the type A uncertainty below the project aim of 0.05 % in room temperature, getting close to uncertainty levels comparable to primary standard cryogenic radiometers. We are continuously making improvements in thermal packaging, thermal readout, electrical readout and calculation algorithms, and the latest results will be presented at the conference. This project 18SIB10 chipS·CALe has received funding from the EMPIR programme co-financed by the Participating States and from the European Union’s Horizon 2020 research and innovation programme. Figure 1: Two of our dual-mode detector modules. During operation they are mounted in a trap configuration in the construction visible in the background, to minimise reflection losses. References: [1] CCPR Strategy Document, Ch. 5.2.1 https://www.bipm.org/utils/en/pdf/CCPR-strategy-document.pdf [2] BIPM. Mise en pratique for the definition of the candela in the SI, Ch 5.1 http://www.bipm.org/en/publications/mises-en-pratique, 2019. [3] T. E. Hansen. Silicon UV-Photodiodes Using Natural Inversion Layers. Physica Scripta, 18:471-475, 1978. [4] J. Gran, T. Tran and T. Donsberg. Three dimensional modelling of photodiode responsivity. 14th International Conference on New Developments and Applications in Optical Radiometry (NEWRAD 2021) [5] E. Bardalen, M. U. Nordsveen, P. Ohlckers, and J. Gran. Packaging of silicon photodiodes for use as cryogenic electrical substitution radiometer. 14th International Conference on New Developments and Applications in Optical Radiometry (NEWRAD 2021

Enhanced surface passivation of predictable quantum efficient detectors by silicon nitride and silicon oxynitride/silicon nitride stack

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Determination of the responsivity of a predictable quantum efficient detector over a wide spectral range based on a 3D model of charge carrier recombination losses

Funding Information: This project (18SIB10 chipS.CALe) has received funding from the EMPIR programme co-financed by the Participating States and from the European Union’s Horizon 2020 research and innovation programme. Authors of Aalto University acknowledge the support by the Academy of Finland Flagship Programme, Photonics Research and Innovation (PREIN), decision number: 320167. Publisher Copyright: © 2022 BIPM & IOP Publishing Ltd.We present a method to determine the internal quantum deficiency (IQD) of a predictable quantum efficient detector (PQED) based on measured photocurrent dependence on bias voltage and a 3D simulation model of charge carrier recombination losses. The simulation model of silicon photodiodes includes wafer doping concentration, fixed charge of SiO2 layer, bulk lifetime of charge carriers and surface recombination velocity as the fitted parameters. With only one set of physical photodiode defining parameters, the simulation shows excellent agreement with experimental data at power levels from 100 μW to 1000 μW with variation in illumination beam size. We could also predict the dependence of IQD on bias voltage at the wavelength of 476 nm using photodiode parameters determined independently at 647 nm wavelength. The fitted values of doping concentration and fixed charge extracted from the simulation model are in close agreement with the expected parameter values determined earlier. At bias voltages larger than 5 Vat the wavelength of 476 nm, the internal quantum efficiency of one of the tested PQEDs is measured to be 0.999 970 ± 0.000 027, where the relative expanded uncertainty of 0.000 027 is one of the lowest values ever achieved in spectral responsivity measurement of optical detectors.Peer reviewe