Programmable 3-Dimensional Memories Based on Current Induced Conductivity in Hydrogenated Amorphous Silicon Nitride
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ABSTRACT Changes of several orders of magnitude in the low field conductivity of hydrogenated amorphous silicon alloy metal-semiconductor-metal MSM devices can be obtained by current stressing. This feature is suitable for memory applications, since the device can be programmed from an unstressed state to a stressed state with a ratio greater than 10 . The change in conductivity is attributed to the creation of a large concentration of silicon dangling bond states during current stressing which form a defect band with a low activation energy for current transport. In this paper, we consider the use of current induced conductivity in hydrogenated amorphous silicon nitride (a-SiNx:H) multi-layer structures and show that, in principle, highly complex three dimensional circuits could be made. In particular, the potential of this approach is illustrated by using a simple MSMSM device with two silicon rich silicon nitride layers. An array with lxi0 elements has been fabricated and programmed to store 28 bits of binary information with three outputs per input. INTRODUCTION It has been shown that the conductivity of simple metal-semiconductor-metal (MSM) structures made using silicon rich silicon nitride or carbide can be changed over many orders of magnitude by current stressing [1].
The speed of the current stressing depends on the current
density used, since the change of conductivity depends on the formation of a defect band containing charged silicon dangling bond states which in turn are generated by the energy released during hole-electron recombination in the MSM structure [2]. In general the writing speed is of the order of 10 ms for silicon nitride. The defect bands can be annealed out by annealing above the equilibration temperature and the MSM device can be reprogrammed [1]. In this paper we examine the use of current induced conductivity (CIC) for three dimensional memories. In particular we are concerned about whether the on-state (1) and off-state (0) can be programmed and selected with a useful relation between them. Results on a simple I x 10 array containing two layers of a-SiNx:H are described to illustrate how this may be achieved. SOME RELEVANT FEATURES OF CIC The main features of CIC in the context of programmable MSM devices have been described elsewhere [1]. Current through the devices is at first controlled by the metalsemiconductor barriers but after severe current stressing there is a large change in the conductivity of the device (Fig. 1) and the current becomes dominated by current transport through a defect band via the Poole-Frenkel effect. Because the current increases exponentially with voltage and the defect generation rate increases with the square of the current, reading the current at a low voltage does not significantly change the conductivity of a device. To introduce a defect band the current density needs to be high (> 5 Acm-2) and a large field has to be applied 339 Mat. Res. Soc. Symp. Proc. Vol. 507 ©1998 Materials Research Society
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