Development of Avalanche-Drift and Avalanche-Pixel Detectors for Single Photon Detection and Imaging in the Optical Regime

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1 Development of Avalanche-Drift and Avalanche-Pixel Detectors for Single Photon Detection and Imaging in the Optical Regime G. Lutz * a), P. Holl c), M. Laatiaoui a), C. Merck a), H.G. Moser a), N. Otte a), R.H. Richter a), L. Strüder b) a) Max-Planck-Institut für Physik, München b) Max-Planck Institut für extraterrestrische Physik, München *) c) PNSensor GmbH, München All at MPI Semiconductor Laboratory, Otto Hahn Ring 6, D München, Germany Abstract A new avalanche detector concept is presented that promises very high (close to 100%) quantum efficiency. Integrating the structure into the centre of a drift diode one obtains a large area device that focuses the photoelectron onto a small point-like avalanche region. Such a device can be used as building block for a silicon photomultiplier. Implementing the structure into the output node of a pnccd a pixel detector sensitive to single optical photons can be constructed. II. THE AVALANCHE DRIFT DIODE: CONCEPT The concept of the avalanche drift diode was motivated by the shortcomings of the silicon photo multiplier, a device consisting of many small area avalanche diodes working in the limited Geiger mode. Each of these micro-cells provides a standard pulse when an avalanche is initiated by one (or several) Keywords: Single photon detector, avalanche detector, drift diode I. INTRODUCTION The motivation for the development of detectors capable of time resolved imaging of optical photons comes from experiments in particle physics and astrophysics. High energy particle showers generated by cosmic radiation are to be reconstructed in MAGIC [1] and EUSO [2] by observing fluorescence and Cherenkov light. The devices used (photomultipliers) or considered (SiPMs) for this purpose, have so far unsatisfactory quantum efficiency of the order of 25%. In time resolved astronomy rapidly varying astronomical objects are to be observed by looking at the time dependence of their optical emission. Also here single photon efficiency, position and time resolution is of utmost importance. The devices under development make use of a new concept which promises to raise the quantum efficiency close to 100%, limited only by the optical reflection of the silicon surface which can be minimized by anti-reflective coating. *) Corresponding author: Gerhard Lutz; Gerhard.lutz@hll.mpg.de; tel: ; fax: Fig.1: Concept of the avalanche drift diode electron(s). Combining several of these micro-cells to a macro-cell by adding their signals provides a measure for the number of electrons generated within the macro-cell. The low quantum efficiency of SiPMs is due to the insensitive regions between the microcells and to the optically absorbing material needed for connections and circuitry on top of the radiation entrance side. In the new concept the radiation enters from the backside of a fully depleted wafer and the photoelectrons are focused onto a small point-like avalanche region located on the front side (Fig. 1). A large thin homogeneous diode on the fully depleted n-type wafer forms the radiation entrance window. The avalanche structure is formed by the n+-doped anode and the buried p-type layer (deep p) which can

2 be connected through the innermost p+-doped ring (R0). The doping of the buried layer has to be chosen in such a way that avalanche conditions are reached when it is fully depleted by applying a sufficiently high reverse bias voltage between anode (A) and R0. Notice that the buried p-layer varies in depth. Only the center part will deplete completely and there will be the avalanche region. The negatively biased drift rings R1, R2 will focus the photo-electrons towards the centre avalanche region. The optional buried n layer prevents injection of holes from the front to the back side and improves field and focusing properties. close-up look of the avalanche region. One notices in Fig. 3 the presence of holes in the buried p-region outside the immediate centre III. THE AVALANCHE DRIFT DIODE: DEVICE SIMULATIONS Device simulations have been performed for a structure as shown in Fig. 1 with 48µm radius and 50µm wafer thickness using the TeSCA program Fig.3: Simulated hole distribution in and around the avalanche region. where the p-doping layer has been moved closer to the surface. Fig. 4 demonstrates that a very uniform electric high field region up to several micrometer radius can be obtained with such an Fig.2: Simulated potential distribution in the cylindrically symmetric avalanche drift diode shown in Fig. 1. One half of the device (r=0 to 48µm) is presented, with the radiation entrance window at the top and the anode located at the left lower corner [6,7]. This program simultaneously solves Poisson and continuity equations for electrons and holes. Charge generation processes including avalanche generation are also modeled Fig. 2 shows an isoline presentation of the potential distribution under typical operation conditions. Electrons generated anywhere in the fully depleted bulk will move into and then along the electron potential valley towards the avalanche region. Figs. 3 and 4 give a Fig.4: Isoline presentation of the simulated electric field strength in the small avalanche region situated at the lower left corner of Fig.1. The nearly uniform high field region extends to more than 3 µm radius arrangement. Focusing the photo-electrons onto a point-like avalanche region leads to differences in path length and drift time, depending on the entrance position of the photon. Diffusion additionally contributes to the

3 time jitter. These effects were presented in another paper at this conference [8]. would be distributed over a large volume once it arrives at the avalanche region. Therefore charge generation was put to the immediate vicinity of the avalanche region. The signal response to an electron on the right side of Fig.5 is compared with that to 10 6 electrons with the avalanche mechanism turned off in the simulation. The simulation program allows to observe the onset of the avalanche in the limited Geiger mode, not however its ending due to fluctuation to zero of the number of charge carriers in the avalanche region Fig.5: Simulation of response to photoelectron (top). For comparison the lower plot shows current (continuous) and voltage (dashed line) response to 10 6 electrons with the avalanche mechanism artificially turned off. IV. AVALANCHE PIXEL DETECTORS This development was initiated by the demands from high time resolution astronomy. The light of faint rapidly periodically changing objects was to be observed. The desired repetition rate of 1000 picture frames per second makes it mandatory to detect single photons in a pixel. This is possible when combining our new avalanche structures with the pn-ccd developed and produced already in our laboratory. The expected response of the device connected with a 1MΩ resistor and 50fF capacitance in parallel to the virtual ground of a read-out amplifier has also been simulated. Here it should be noted that the simulation program does not know quantization of electric charge. An electron produced at the entrance window Fig.7: Quantum efficiency for two different antireflective coatings. Fig.6: Photo of a recent production of pnccds in the MPI Semiconductor laboratory. The colors are due to different antireflective coatings of the radiation entrance windows. Fig. 6 shows a photograph of a pnccd from a resent production that was optimized for applications in adaptive optics [9]. The CCDs have parallel columnwise readout, each column with its own amplifier. The CCD is divided in an image area and a frame store area. After integration the image can be transported very fast to this frame store area and the next image can be taken while the previous image is read out. The anti reflective coating on the radiation entrance windows leads to a very high quantum efficiency (Fig. 7).

4 V. SINGLE PHOTON COUNTING PNCCD For the intended application a high frame rate at low light intensity is requested. This means that only a small fraction of pixels will contain photoelectrons and that the probability of having two photoelectrons in the same pixel cell is minute. Storage and transport of electrons within a column of the pnccd is not a problem. Therefore the task is the detection of single electrons with the readout electronics of our pnccd. Fig.8 shows a cross section of the proposed device along the last pixels of a column and the readout structure of the channel. Here one recognizes the avalanche structure already introduced with the avalanche drift diode. The deep buried n-doping introduced for preventing the injection of holes from the top side to the back, simultaneously serves to form the transfer channel of the pnccd. The signal photoelectron collected in a pixel cell is moved in the transfer channel at a depth of several micrometers towards the avalanche structure where it turns upwards into the high field region. The anode signal is further amplified with the help of an (optional) integrated field effect transistor. The avalanche structure can be operated either in proportional or in Fig.8: Cut along the transfer channel of the proposed avalanche pnccd. limited Geiger mode, depending on the reverse bias voltage of the avalanche structure. Avalanche amplification can also be turned off completely by further reduction of reverse biasing. In that case the device works like a normal pnccd. Since in the foreseen application single photoelectrons should be detected and high readout speed is required, proportional operation mode is the preferred option. Only yes/no decisions are required and the value of charge multiplication is not critical. Proportional mode operation not only avoids dead time for recharging but also cross talk from photons generated in the avalanche process. VI. SUMMARY Our new concept of avalanche structure promises high (close to 100%) photon detection efficiency over energy range of optical photons due to the thin homogeneous entrance window on backside of fully depleted silicon substrate and focusing of electrons onto small (point-like) avalanche region with the help of an drift diode structure. Low detector capacitance will allow operation in moderate gain limited Geiger mode thereby limiting optical cross talk. The structure will be used as building block for a Silicon Photomultiplier Tube (PMT) and an amplification stage for a high time resolution single optical photon sensitive pn-ccd. Extended simulations have shown the validity of the concept and the layout of the new structures is on its way. The devices will be produced in the MPI Semiconductor Laboratory. A test production for finding optimized technology parameters will precede first prototype production. VII. ACKNOWLEDGEMENTS We thank our colleagues from the Semiconductor Laboratory for the support and encouragement given to our project. References [1] J. A. Barrio, et al., Design report on MAGIC, MPI-PhE/98-5 (March 1998) [2] M. Teshima, et al., EUSO (The Extreme Universe Space Observatory)- Scientific Objectives, Proceedings ICRC 2003, pp [3] V. Golovin and V. Sveliev. Novel type of avalanche photodetector with Geiger mode operation, NIM A 518 (2004), p [4] B. Dolgoshein, et al., Silicon photomultiplier and its possible applications, NIM A 504 (2003), p [5] D. Bisello, et al., NIM A 367 (1995) pp. 212 H. [6] Gajewski et al., "TeSCA Two Dimensional Semiconductor Analysis Package", Handbuch, Weierstraß -Institute for Applied Analysis and Stochastics WIAS, Berlin, 1997 [7] H. Gajewski, H.-Chr. Kaiser, H. Langmach, R. Nürnberg and R. H. Richter: "Mathematical Modeling and Numerical Simulation of Semiconductor Detectors", Springer-Verlag, Berlin (2001) [8] C.Merck etal.: Timing properties of an avalanche diode for single photon counting, presented at this conference [9] R. Hartmann et al.: A high speed pnccd detector system for optical applications, presented at the 10 th European Symposium of Semiconductor Detectors, Wildbad Kreuth, June 12-16, 2005

5 Figure captions Fig.1: Concept of the avalanche drift diode Fig.2: Simulated potential distribution in the cylindrically symmetric avalanche drift diode shown in Fig. 1. One half of the device (r=0 to 48µm) is presented, with the radiation entrance window at the top and the anode located at the left lower corner Fig.3: Simulated hole distribution in and around the avalanche region. Fig.4: Isoline presentation of the simulated electric field strength in the small avalanche region situated at the lower left corner of Fig.1. The nearly uniform high field region extends to more than 3 µm radius Fig.5: Simulation of response to photoelectron (top). For comparison the lower plot shows current (continuous) and voltage (dashed line) response to 10 6 electrons with the avalanche mechanism artificially turned off. Fig.6: Photo of a recent production of pnccds in the MPI Semiconductor laboratory. The colors are due to different antireflective coatings of the radiation entrance windows. Fig.7: Quantum efficiency for two different antireflective coatings. Fig.8: Cut along the transfer channel of the proposed avalanche pnccd