Influence of Background Illumination on the Photoresponse of Micropixel Avalanche

Micropixel avalanche photodiodes (MAPD) with deeply buried pixels have no structural components made on its front surface and therefore almost its entire surface is photosensitive. MAPD possesses high photodetection efficiency (PDE), multiplication gain ~105, photosensitivity from a single photon and wide range of dynamic linearity of photoresponse up to 5×103 photons/mm2. The influence of the background light on the gain and linearity of the photoresponse of the MAPD-3N type samples was investigated. External light sources have different effects on the output signal of MAPD whichever it is pulsed or continuous light. It is shown that in contrast to pulsed external light, the light from the permanent source leads to reducing of the output signal. But a constant background light does not deteriorate the linearity of the photoresponse in the whole range of linearity of the device.


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Abstract-Micropixel avalanche photodiodes (MAPD) with deeply buried pixels have no structural components made on its front surface and therefore almost its entire surface is photosensitive.MAPD possesses high photodetection efficiency (PDE), multiplication gain 10 5 , photosensitivity from a single photon and wide range of dynamic linearity of photoresponse up to 5-103 photons/mm 2 .The influence of the background light on the gain and linearity of the photoresponse of the MAPD-3N type samples was investigated.External light sources have different effects on the output signal of MAPD whichever it is pulsed or continuous light.It is shown that in contrast to pulsed external light, the light from the permanent source leads to reducing of the output signal.But a constant background light does not deteriorate the linearity of the photoresponse in the whole range of linearity of the device.

I. INTRODUCTION
With the development of modern electronics the role of high-precision, compact and sensitive photodetectors increases [1]- [3].Micropixel avalanche photodiodes in its qualities are not inferior to the photomultiplier, and in many respects even excels them.
Micropixel avalanche photodiodes (MAPD) are designed prior to detecting of light pulses [4].Generally, for registration of various particles MAPD is used in conjunction with a scintillation crystal.Scintillation crystal generates a directed stream of photons under the influence of the particle.According to the number of photons the energy of particle can be determined or the type of particle can be identified.
Thus, the exact determination of the number of photons is essential in MAPD application.Obviously, ingression of the photons from other light sources is extremely undesirable phenomenon.However, some background light attends in actual applications.
It should be recognized that the influence of the external background illumination depends on whether it has a pulse or permanent type.Apparently, if photons from another source in the pulse mode fall simultaneously with the photon flux from the testing light source, the obtained output signal of the device will exceed the true value.Situation is significant differ if the light from a permanent source continuously falls on the photosensitive surface of device.In this case output signal of MAPD reduces.

II. OUTPUT SIGNAL OF MAPD UNDER A CONSTANT BACKGROUND LIGHT
It was used a sample of MAPD-3N with a deeply buried pixels to study of relation of the photoelectric properties on permanent back ground light [5], [6].Operating voltage of MAPD-3N is 90 V. MAPD-3N has a 3 mm  3 mm photosensitive area, 410 4 pixels/mm 2 , multiplication gain 10 5 and photodetection efficiency (PDE) about 30%.The sample MAPD was made together with the company Zecotek Photonics Inc.
Photosensitive surface of MAPD-3N was illuminated by regular light.The scheme of installation is shown in Fig. 1.Measurements were performed at room temperature.The total number of photons of constant background incident on the photosensitive surface was varied over a wide range from 10 7 to 10 10 photons per seconds.The number of photons in a pulse light Npi was 210 3 , 410 3 , 2010 3 and 4010 3 per seconds.The number of photons in pulse light is selected from the range of dynamic linearity of the studied type of MAPD [7].
Results of studies are presented in Figure 2a.It is seen the amplitude of photoresponse to the pulse light is reduced with increasing of power of constant background light.Above results are interpreted in logarithmic scale in Figure 2b.As seen here the curves plotted for different values of number of photons in pulse light within dynamic range of the linearity of the photoresponse have a similar reducing behavior, ie. a relative decreasing of photoresponse of amplitude is the same.

III. LINEARITY OF MAPD UNDER A CONSTANT BACKLIGHT
The linearity of photoresponse of photodetector assumes a linear proportionality of the amplitude of output signal to the number of photons imbibed by receiving surface of the detector and it is an important characteristic.
The nature of the limitation of dynamic linearity range MAPD lies in the principle of formation of the photoresponse in this kind of photodetector, which is based on the fact that each pixel operates in binary mode and the output signal from one triggered pixel does not depend on the number of primary photoelectrons caused him an avalanche.In this case, even one pixel gets more photons in its region and so formed more than one photoelectron magnitude of structure's output signal will be equal to signal as it was formed only under one photoelectron.Total signal of photodetector is equal to production of signal of one pixel and number of triggered pixels.
The degree of linearity of the photoresponse can be determined by the formula [1]: where, N -number of the triggered pixels; Ntot -the total number of pixels of photodetector; μ = Nph .PDE -the average number of photoelectrons generated; Nph -the number of photons hit on the photosensitive surface of the detector, a PDE -photodetection efficiency.
The range of photoresponse linearity can be increased by reducing of probability of hitting of one pixel by a few photons.For it the density of pixels should be increased.This was managed in MAPD with deeply buried pixels so that on the photosensitive surface there are not any reflective structural components such as resistors for quenching of the avalanche process in the pixel [5].
Various aspects of the dynamic linearity of the photoresponse and the reasons for the high linearity of MAPD with deeply buried pixels discussed in [7].Photoresponse of the sample of MAPD-3N is linear up to 60000 photons in a light pulse.
Figure 3 shows the dependence of the amplitude of photoresponse of MAPD vs. of the number of incident photons in light pulses at different levels of background illumination of the photodiode's surface.It is seen that background light does not reduce the linearity of the photoresponse, but rather increases it.

IV. RESULTS AND DISCUSSION
At a constant background illumination, a continuous flow of photons almost is evenly distributed over time.For example, if the power of constant light source is 10 8 photons per seconds, one photon falls on the surface of the photodetector at the mean every 10 ns.The typical duration of the light pulse, formed upon detecting particle is 5 -30 ns, and the number of photons in this case is hundreds and more.Typical time of opening of electronics' channels for registration (gate) is 100 ns.During this time the number of photons of the background light will be a few units and that number of photons is considerably less than the number of photons generated by detected particle.Therefore, an increase in the signal caused by these photons can be neglected.
However, the background light photons have a significant impact on the amplitude of the output signal and the noise of MAPD because of another reason.Continuously falling on the surface of the detector photons generates photoelectrons, which triggers an avalanche in the appropriate nearest pixels.After avalanche passes it needs some time to pixel for recovering and being able to register of new photoelectron.Thus, under the influence of the continuous light at each instant the part of pixels is in the avalanche passes mode or in the process of its returning to original operating state.These pixels will not be able to respond to the photons generated during registration of the particle, and the output signal of the photodetector will be lower than true.Consequently, for the energy of detected particle it will get less value.Obviously, the more power of background lighting leads to the greater number of "occupied" pixels.
The work of MAPD under the influence of constant background illumination seems like the MAPD with high value of dark current generation.In both cases, some of the pixels are completely or partly inoperative.This state can also be considered as a state with a reduced value of the photodetection efficiency (PDE), since the part of the registered photons decreases.
No deterioration of the linearity of the photoresponse becomes clear when one considers that an increase in power of constant light decreases the coefficient of PDE device.
V. CONCLUSIONS It follows from these experiments; a constant background light leads to a decrease in the magnitude of the useful output signal of the detector.But it does not degrade the dynamic range of the linearity of the photoresponse of the device.These properties are explained by the principles of formation and quenching of the avalanche process in micropixel avalanche photodiodes.

Fig. 1 .
Fig.1.The scheme of installation A LED of blue wavelength was used as a source of constant light.This LED was connected to a DC power supply.The intensity of the emitted light was regulated by changing of electric current through LED by resistance box.The number of photons emitted by the LED was calibrated with a standard photodiode S8664-55 from Hamamatsu Photonics Company.Pulsed light is formed by another LED with a wavelength taken in blue region.This light simulates a photons flow generated by particle in scintillation crystal.Pulse light LED was powered by a generator G5-65.The duration of light