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

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

Alternative Calibration Methods of Radiometric Detectors

The usual way of calibrating detectors or devices is to treat them as black boxes. We measure the response when the device is excited with a well-known signal. With this approach we throw away a lot of information that can be used to develop new calibration techniques. In the common European project chipS∙CALe (running from June 2019 – May 2022) we aim to develop self-induced silicon photodiodes capable of calibrating themselves in possibly remote operation. The trick is to exploit the intrinsic quantum properties of photodiodes that each photon generates exactly one electron hole pair. This is a valid assumption to about 99.9% of common calibration standard photodiodes used in laboratories today when correcting for reflectance. Two fundamentally different approaches are explored in the chipS∙CALe project. We are developing new simple structure photodiodes with improved quantum efficiency beyond the usual 99.9 % photon to electron conversion efficiency. Because the photodiodes have a simplified structure, their losses can be simulated with 3D simulation models. With simple I-V measurements at one wavelength only, a 3D model fit can be applied, and the responsivity from 400 nm to 850 nm can be predicted. The second method is based on exploiting the photodiode as an electrical substitution radiometer in addition to its usual quantum mode. The incoming radiation is converted either to a photocurrent, as in a traditional photodiode, or to heat, using electrical substitution to determine the power of the absorbed radiation. The true internal quantum deficiency is measured by this method as it is the same absorber used in both application modes and heat is generated by forward bias of the photodiode using the same ammeter. Special type of packaging is required to operate the photodiode in dual mode and the technique is not in general limited to the use of self-induced photodiodes. However, if using a self-induced photodiode both approaches can be applied independently on the same device and the radiometric measurement of fundamental constants e/hc can be measured as a validation of the equivalence between the two independent methods. As these fundamental constants are three of the defining constants of the new SI, the dual-mode detector will provide a direct link between two practical primary radiometric measurement techniques and the SI. Acknowledgement The chipS∙CALe project 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

Predictable quantum efficient detector: I. Photodiodes and predicted responsivity

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