Atte Haapalinna

Spectral reflectance of silicon photodiodes

A precision spectrometer was used to measure the spectral reflectance of a silicon photodiode over the wavelength range from 250 to 850 nm. The results were compared with the corresponding values predicted by a model based on thin-film Fresnel formulas and the known refractive indices of silicon and silicon dioxide. The good agreement at the level of 2 x 10(-3) in the visible wavelength range verifies that the reflection model can be used for accurate extrapolation of the spectral reflectance and responsivity of silicon photodiode devices. In addition, characterization of the photodiode reflectance in the ultraviolet region improves the accuracy of the spectral irradiance measurements when filter radiometers based on trap detectors are used.

Nonlinearity measurements of silicon photodetectors

Nonlinearities of the responsivity of various types of siliconphotodetectors have been studied. These detectors are based onphotodiodes with two sizes of the active area (10 x 10 mm(2) and 18 x 18 mm(2)). The detectorconfigurations investigated include single photodiodes, two reflectiontrap detectors, and a transmission trap detector. For all devices, the measured nonlinearity was less than 2 x 10(-4) forphotocurrents up to 200 muA. The diameter of themeasurement beam was found to have an effect on thenonlinearity. The measured nonlinearity of the trap detectorsdepends on the polarization state of the incident beam. Theresponsivity of the photodetectors consisting of the large-areaphotodiodes reached saturation at higher photocurrent values comparedwith the devices based on the photodiodes with smaller activearea.

Measurement of the absolute linearity of photodetectors with a diode laser

An automated instrument based on the beam-addition technique has been developed for measurement of the linearity of photodetectors. The system is designed for absolute characterization of a transfer standard photodetector, against which the linearity of other detectors can be measured. The measurement set-up has been made as simple as possible. A diode laser at 633 nm is used as the light source due to the good stability and high power output available with the device. Two measurement beams of 0.9 mm diameter are aligned to intercept on the photodetector. As an example, a silicon photodiode of the type S1227 has been studied and found to be linear within 3 × 10-5 up to 7 mW of optical power.

Characterisation of optical detectors using high-accuracy instruments

The facilities of the Metrology Research Institute at the Helsinki University of Technology, and methods for characterisation of optical detectors for spectral radiant intensity and irradiance responsivity, are described. The instrumentation for such characterisations includes a reference spectrometer with a number of auxiliary set-ups, and equipment for the spectral irradiance measurements with a filter radiometer based on a trap detector. The methods of realising the spectral responsivity scales based on an absolute cryogenic radiometer in house are addressed. The procedures and results of characterisation of a multipoint measuring system of photosynthetically active radiation, by employing the available facilities, are briefly described. The absolute irradiance responsivity of the device is determined by using a photometric lamp, whose spectral irradiance has been measured with the filter radiometer. The combined standard uncertainty of this set of calibrations is 3.6% at the 1σ level. The uncertainty is caused almost completely by the multipoint measuring system.

Filter radiometry based on direct utilization of trap detectors

A new design for a filter radiometer based on a trap detector, a set of temperature-controlled filters, and a precision aperture is described. This filter radiometer can be used to realize high-accuracy scales for illuminance, luminous intensity, and spectral irradiance. The new filter radiometer is an improved version of our earlier designs. It has been improved in such a way that the filter can be easily and reliably changed. This makes it more suitable for spectral irradiance measurements, where lamps usually have to be measured at several wavelengths. The results of the characterization of the filter radiometer equipped with a lambda filter using two different methods are presented. The results are in agreement at the level of 0.3%.

High-accuracy measurement of specular spectral reflectance and transmittance

The realization of spectrophotometric quantities at the Helsinki University of Technology is based on our reference spectrometer. The reference spectrometer is a high-accuracy instrument developed for measuring spectral specular transmittance and reflectance in a wavelength range extending from ultraviolet to near-infrared. The relative uncertainty estimates for transmittance measurements of neutral-density filters are ca. 0.05%. For spectral reflectance the estimated uncertainties are between 0.14% and 0.34% depending on the sample reflectance and the measurement geometry. We have derived and verified equations that enable both the reflectance and transmittance of various samples to be predicted. Utilizing these equations, the reflectance and transmittance can be accurately calculated for samples with known refractive index. For precise calculations, the characteristics of the measurement beam must be taken into account.

Precision spectrometer for measurement of specular reflectance

The HUT reference spectrometer was modified for measuring specular reflectance in the wavelength range of 300 to 850 nm. The instrument is based on a diffraction-grating monochromator, reflecting optics, sample control mechanics and detection systems with linear responsitivities. Relative standard uncertainties between 0.14% and 0.32% were estimated for the reflectance measurements. For spectral reflectance between 1.5% and 15%, the results of test measurements using samples with known reflectances confirm that for all geometries the relative deviations are less than 0.36%. A set of ultraviolet (UV)-interference filters was measured in the UVB wavelength range, and the results are used as a part of filter radiometer characterization.

Chapter One – Properties of Silicon

Publisher Summary In this chapter, properties of silicon are explained in detail. Silicon is an abundant element found in the Earths crust in various compounds. Semiconductor and MEMS applications use more than 20000 tons of high-purity silicon. Quartz or silicon dioxide, is the most common starting raw material for purified silicon for semiconductor and sensor applications, and the Siemens process is the most commonly used in semiconductor-grade silicon production. Silicon crystallizes into a diamond cubic crystal structure in which the atoms are covalently bonded. Silicon is a hard, brittle material, and at room temperature under stress silicon single crystal elongates elastically until fracture stress appears without significant plastic deformation. Silicon is a group IV element in the periodic table and is a semiconductor with a bandgap of 1.12 eV, which means that pure silicon at room temperature is almost an insulator. By doping with group III or group V elements, the resistivity of silicon can be varied over a wide range. In this chapter, mechanical and electrical properties of silicon are explained in detailed. Schematic diagrams help to better understand the reaction of silicon and its various properties.