Raj Korde

Quantum efficiency stability of silicon photodiodes

The stability of the quantum efficiency of inversion layer, phosphorus-diffused (n on p), and boron-diffused (p on n) photodiodes has been investigated. Unsatisfactory silicon-silicon dioxide interfaces, latent recombination centers in the diffused layers, and moisture absorption by the device were identified as possible causes of instability. Diodes were fabricated using processes in which these sources of instability were carefully controlled. The resulting diodes were subjected to various accelerated aging tests, and the external quantum efficiency of the diodes was monitored during the tests. Diodes made by older procedures, in which some important parameters affecting stability were not controlled, were included in the study for comparison. The major result of this work is the demonstration that n on p photodiodes are inherently more stable than p on n types in the ultraviolet and blue spectral regions, but that stable p on n devices can also be produced with sufficient care.

Stable silicon photodiodes for absolute intensity measurements in the VUV and soft x-ray regions

Abstract Stable silicon photodiodes with 100% internal quantum efficiency have been developed for the vacuum ultraviolet and soft x-ray regions. It is demonstrated that the response of these detectors can be reasonably well represented by a simple model for photon energies above 40 eV. The measured efficiency is consistent with a constant electron-hole pair creation energy for Si above 40 eV. Radiation damage is demonstrated to result in loss of carriers to recombination at the front surface. The uniformity of the diodes is shown to be better than 0.1% RMS at 110 eV.

Measurements of the solar soft X-ray irradiance by the Student Nitric Oxide Explorer: First analysis and underflight calibrations

Beginning on March 11, 1998, the Student Nitric Oxide Explorer (SNOE) satellite has made daily observations of the solar soft X-ray irradiance. These measurements are carried out by a multichannel photometer system. The spectral range between 2 and 20 nm is covered by three channels with bandpasses of 2 – 7 nm, 6–19 nm, and 17 – 20 nm respectively. Absolute sensitivities were measured preflight using the Synchrotron Ultraviolet Radiation Facility of the National Institute of Standards and Technology. The results of the first 1.5 years of SNOE solar measurements are presented. During this time period the F10.7 solar index varied between 80 and 250 × 10−22W m−2 Hz−1 and the 81-day average of the F10.7 solar index varied between 100 and 175 × 10−22W m−2 Hz−1. The solar irradiances in the 2 – 7 nm interval varied between 0.3 and 2.5 mW m−2, while the irradiances in the 6–19 and 17 – 20 nm intervals varied between 0.5 and 3.5 and 1.0 and 3.5 mW m−2, respectively. The measured irradiances are correlated with the F10.7 solar index with a correlation coefficient of ∼0.9 in all three bandpasses. For the levels of activity observed so far the SNOE measurements are typically a factor of 4.0 larger than the irradiances predicted by the Hinteregger et al. [1981] empirical model (hereafter the Hinteregger model). This fact and a long-term trend in the ratio of SNOE measurements to Hinteregger model predictions show that the Hinteregger model underpredicts the long-term variability in the solar soft X-ray irradiance. It is shown that other empirical models provide a reasonable representation of the 27-day variability but also underpredict the magnitude and long term variability. A sounding rocket measurement made on November 2, 1998, by the Thermosphere Ionosphere Mesosphere Energetics and Dynamics Solar EUV Experiment prototype instrument using the same technique measured the solar irradiance in similar wavelength bands and produced results that are in good agreement with the SNOE measurements.

Stability and quantum efficiency performance of silicon photodiode detectors in the far ultraviolet

Recent improvements in silicon photodiode fabrication technology have resulted in the production of photodiodes which are stable after prolonged exposure to short wavelength radiation and which have efficiencies in the far ultraviolet close to those predicted using a value of 3.63 eV for electron-hole pair production in Si. Quantum efficiency and stability data are presented in the 6-124-eV region for several variations on the basic successful design and on devices with extremely thin silicon dioxide antireflecting/passivating layers. The results indicate that the oxide is dominant in determining many of the performance parameters and that a stable efficient far ultraviolet diode can be fabricated by careful control of the Si-SiO(2) interface quality.

Absolute silicon photodiodes for 160 nm to 254 nm photons

Silicon n-on-p photodiodes with 100 % internal quantum efficiency have been studied in the 160 nm to 254 nm spectral range. Preliminary values have been determined for the quantum yield of silicon at these wavelengths. Using these values, a trap detector is presented for absolute flux measurement in this region. The stability under intense 193 nm irradiation, a property of importance in lithography and in photorefractive keratectomy, has been measured, and the diodes tested were found to be several orders of magnitude more stable than p-on-n diodes tested by other investigators at this wavelength. Spatial nonuniformities of the n-on-p diodes were found to be less than 1 % at wavelengths of 254 nm and 161 nm.

Stable, high quantum efficiency, UV-enhanced silicon photodiodes by arsenic diffusion

Abstract Very high quantum efficiency, UV-enhanced silicon photodiodes have been developed by arsenic diffusion into p -type silicon as an alternative to the inversion layer photodiodes commonly used in precise radiometric and spectroscopic measurements. The fabricated diodes had an unbiased internal quantum efficiency that was 100% from 350 to 550 nm, and that exceeded 100% at shorter wavelengths. A typical responsivity at 200 nm was 0.1 A/W. No degradation in responsivity was detected anywhere in the 200–1100 nm range when these devices were exposed to 20 mW/cm 2 of 254 nm radiation for 60 days. Thus the theoretical maximum value of internal quantum efficiency for a diffused photodiode appears to have been achieved in the UV and short wavelength visible, without compromising the diodes long term stability. This is in marked contrast to older types of diffused photodiodes, which either were “dead” in the UV, or exhibited a spectral response vs flux characteristic that changed considerably with UV exposure.

Present status of radiometric quality silicon photodiodes

Evaluation of five types of silicon photodiode was undertaken to verify their suitability for absolute radiometry and also for their use as transfer standards in the spectral region from 1 nm to 1100 nm. Four types of photodiode were fabricated for this study; these were the p-on-n photodiode, n-on-p photodiodes with silicon dioxide front windows and n-on-p photodiodes with a metal-silicide front window. Fabrication of photodiodes with 100% internal quantum efficiency is demonstrated and their necessity for making absolute radiometric measurements with the lowest possible uncertainty is pointed out. The linearity characteristics of these devices, as measured by the ac/dc method, are far superior to those of the p-on-n diodes especially fabricated for this work and also to those exhibited by p-on-n diodes widely used at present by the radiometric community. Results on the stability of the quantum efficiency of the fabricated diodes after exposure to intense radiation of 13 nm, 120 nm, 157 nm, 193 nm and 254 nm radiation will also be presented. Photodiodes with a metal-silicide front window were the only devices stable when exposed to the intense beams of third-generation synchrotrons and UV excimer lasers.

The effect of neutron irradiation on silicon photodiodes

Neutron radiation testing was performed on a total of 125 silicon photodiodes to investigate the changes in the device parameters after neutron exposure. A californium-252 source was used to irradiate the photodiodes with 1-MeV equivalent neutrons having fluences in the range of 5*10/sup 11/ to 10/sup 14/ n/cm/sup 2/. The photodiode forward voltage drop, ideality factor, and series resistance increased after neutron exposure. The increased series resistance caused a degradation in diode photocurrent linearity. An empirical expression for post-neutron-irradiation changes in photodiode linearity is presented. Neutron-induced changes in the photodiode shunt resistance and dark current were modeled using simple expressions that allow device designers to estimate changes in photocurrent linearity, shunt resistance, and dark current after neutron exposure. No postirradiation change in the ultraviolet quantum efficiency of diodes without recombination in the front region was observed. This suggests that neutron irradiation does not affect the Si-SiO/sub 2/ interface recombination velocity of p-n junction diodes. >

Response of a SiC Photodiode to Extreme Ultraviolet through Visible Radiation

The responsivity of a type 6H-SiC photodiode in the 1.5-400 nm wavelength range was measured using synchrotron radiation. The responsivity was 0.20 A/W at 270 nm and was less than 0.10 A/W in the extreme ultraviolet (EUV) region. The responsivity was calculated using a proven optical model that accounted for the reflection and absorption of the incident radiation and the variation of the charge collection efficiency (CCE) with depth into the device. The CCE was determined from the responsivity measured in the 200-400 nm wavelength range. By use of this CCE and the effective pair creation energy (7.2 eV) determined from x-ray absorption measurements, the EUV responsivity was accurately modeled with no free parameters. The measured visible-light sensitivity, although low compared with that of a silicon photodiode, was surprisingly high for this wide bandgap semiconductor.

Silicon Photodiodes With Stable, Near-Theoretical Quantum Efficiency In The Soft X-Ray Region

Silicon photodiodes having practically no carrier recombination at the Si-SiO2 interface or in the front diffused region have been developed by defect-free n-type impurity diffusion into p-type silicon. These photodiodes exhibit very high quantum efficiencies in the 10 eV to 150 eV photon energy region, typically 37 electrons per photon at 150 eV, which is about 300 times the quantum efficiency of the more commonly used photoemissive type soft X-ray detectors. The quantum efficiency of the developed diodes has been found to be stable to a few percent after exposure to photons in the region of 5eV to 200eV, with fluences in excess of 1014/cm2. No significant change in the quantum efficiency was observed after storage in air for several months.