Improved multi-element silicon photodiodes for detection at 1.06 µm

The advent of the Neodymium-doped laser has created a demand for multi-element detection devices having large sensitive-areas, high uniform quantum-efficiencies, fast response, low capacitance, and minimum dead-space between adjacent elements. Silicon p-i-n photodiodes can be made to satisfy these requirements, provided the starting material is either sufficiently pure, or the degree of compensation sufficiently good, to enable the desired depletion-depths to be obtained. Multi-element devices have been described previously in which the lithium-ion-drift technique was used to achieve the described degree of compensation. These devices performed adequately at room-temperature but were found to deteriorate with age when used at elevated temperatures. In this paper improved photodiodes are described which are fabricated from hyperpure silicon. Techniques have been developed for maintaining the net concentration of donor and acceptor ions in the intrinsic layer to less than 5 × 10¹¹/cm³, so that fully depleted p-i-n structures with intrinsic layers up to 1 mm in thickness are now possible. Since the diffused p- and n- regions are very thin, practically all carriers photo-generated in the device are collected quickly. The quantum-efficiency is further enhanced through the use of an antireflection coating on the front surface and a reflective coating on the back. For a typical device 0.7 mm thick with a bias of 200V, a quantum-efficiency greater than 0.85 (electrons/ incident photon), a capacitance of 15 pf per cm²of sensitive-area, and a response-time of less than 30 nanoseconds have been measured. For a device 0.25 mm thick, a quantum-efficiency of 0.50, a capacitance of 45 pf/cm², and a response-time less than 5 nanoseconds have been achieved. Multi-element devices with sensitive areas up to 6 cm²have been fabricated. Typical multi-element devices have less than 1% optical cross-talk, separation between adja- ent elements of less than 0.005" impedance between adjacent elements of greater than 10⁸ohms, and variation of quantum-efficiency of less than a few percent (depending on the size of the test light-spot). These devices have been subjected to environmental tests without deterioration. Other properties of these devices, including variation of dark-current and quantum-efficiency with temperature, will be discussed.