Introduction
Cross-point memory with 4F2 cell is ideal for high density memory. To ensure adequate sense margin, a diode must be integrated at each cross-point to eliminate the leakage current through the half-selected cells (Fig. 1). In this paper, we demonstrate, for the first time, a pn-junction diode integrated with a phase change memory cell. The large programming currents needed in phase change memory require large access device widths. This is a major obstacle that must be overcome to fully utilize intrinsic superb scalability of phase change material [1]. Many ideas have been proposed to reduce the bottom electrode contact area and therefore reduce the programming current. These include edge contact [2], lateral cell [3], and trench cell [4]. Compared to these methods, the nanowire bottom electrode contact approach [5] can easily provide a small contact area due to the small diameter of the nanowire. In this paper, we use phosphorus doped germanium nanowires (GeNW) as the bottom electrode contact (BEC). At the same time, the n-doped nanowire forms a pn junction with the p-type substrate. This nanowire pn-junction diode plus phase change memory cell combination results in a low programming current and eliminates the leakage current of half-selected cells. The size of nanowire is determined by the catalyst volume and growth condition, which can be reduced below the lithographic limit. Schematic of PCM memory arrays with nanowire diodes as memory cell selelction devices. The upper right array is without the diodes, which has leakage paths due to half-selected memory cells. The lower right array with diodes eliminates the leakage paths.