The Avalanche Drift Diode - A Back-Illuminated Silicon Photomultiplier

Single optical photon detector with an avalanche multiplier integrated into a silicon drift chamber


Development of a new avalanche detector with very high (close to 100%) quantum efficiency and fast time response. Integration of the avalanche structure into the centre of a drift diode one obtains a large area device that focuses the photoelectron produced at the shallow radiation entrance window onto a small “point-like” avalanche region placed on the opposite side. Such a device can be used as a building block for a macroscopic detector.


The motivation for the development of detectors capable of time resolved imaging of optical single photons comes from the experimental particle physics, astrophysics and many other areas. High energy particle showers generated by cosmic radiation are to be reconstructed in the experiments like MAGIC and EUSO by observing fluorescence and Cherenkov light. The detectors used so far, PhotoMultipliers or considered for future experiments, Hybrid Photo Diodes, have limited quantum efficiency especially in the interesting UV range (~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.

By combining our experience in the fabrication of silicon drift detectors with the avalanche diode technology the MPI Halbleiterlabor started to develop a detector which promises raise in the quantum efficiency close to 100%, limited only by the optical reflection of the silicon surface which can be minimized by anti-reflective coating.


The avalanche drift diode consists (like the existing Silicon Photomultipliers - SiPM) 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) electron(s). Composing several of these micro-cells to a macro-cell, by adding their signals, provides a measure for the number of photons detected within the macro-cell. The quantum efficiency of front illuminated SiPMs is limited by the insensitive regions between the micro-cells and by the presence of optically absorbing material needed for connections and circuitry on top of the radiation entrance side. In the new concept 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 (Figure 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 is connected laterally 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. In order to suppress high electric fields in the edge regions of the anode the buried p-layer varies in depth. The negatively biased drift rings R1, R2 will focus the photo-electrons towards the center avalanche region. The optional buried n-layer prevents injection of holes from the deep-p to the back side and improves focusing properties. Due to the longer drift path the charge collection is longer than in front illuminated SiPMs. From device simulations performed up to now for an operation at room temperature (Figure 2) a time resolution of about 1ns is extrapolated if the device is cooled.


In our new photon detector concept the 100% fill factor of the radiation entrance window leads to higher quantum efficiency compared to the front illuminated SiPMs.

Additionally, the detection efficiency of Backside Illuminated Silicon Photomultiplier (BISiPM) for short penetrating radiation in the UV region will be mainly determined by the electrical field properties close to the entrance window and the reflectivity of the detector surface. Special engineering of the ultra shallow homogeneous entrance window and focusing of electrons onto a small avalanche region with the help of a drift diode structure promises high (close to 100%) photon detection efficiency for a wide wavelength region (300nm…1000nm).

Based on the extended device simulations (Figure 3) test structures have been designed. After the evaluation of the test structures the first fully operational prototypes will be produced.


-- JelenaNinkovic - 27 Apr 2006

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Topic revision: r4 - 2006-05-23 - PeterHoll
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