The nonvolatile memory characteristics of metal-oxide-semiconductor structures containing Pt nanocrystals in SiO 2 gate oxide were studied. The Pt nanocrystals of 2 -3 nm in diameter were self-assembled from reduction of an ultrathin PtO x layer embedded in the SiO 2 by vacuum annealing at 425°C. A large hysteresis loop was found in the capacitance-voltage ͑C -V͒ relation indicating this significant memory effect. However, two different charge storage mechanisms were found for the Pt nanocrystals in devices with different tunnel oxide thickness. A counterclockwise C -V hysteresis was induced from substrate injection for the devices made with a thin tunnel oxide layer 2.5-5.0 nm thick. Contrast, a clockwise behavior attributed to the electron transfer from charged defects in the gate oxide was found for the devices having a tunnel oxide layer 7.5 nm thick. The relatively stable memory characteristics of Pt nanocrystals resulted from substrate injection were also demonstrated.Memory-cell structures employing semiconductor or metal nanocrystals as storage elements in metal-oxidesemiconductor (MOS) field effect transistors have attracted much attention due to their properties as nonvolatile memory devices. [1][2][3][4] Most research has focused on the fabrication of Si or Ge nanocrystals in MOS structure, 1-7 but the metalnanocrystal memories are considered to be more beneficial in many aspects. [8][9][10][11][12] The metal nanocrystals have usually been fabricated by annealing an ultrathin metal film deposited on the thin gate oxide of SiO 2 . 8-12 However, the thermal and mechanical incompatibility between metal films and SiO 2 makes the fabrication of discrete and uniformly sized metal nanocrystals in gate oxide quite difficult. It is known that the oxidized phase of noble metals, such as PtO x , is chemically unstable, and can be easily reduced back to metallic state. 13 Accordingly, a self-assembly process of Pt nanocrystal formation from the reduction of an ultrathin PtO x layer embedded in SiO 2 is reported, and its memory characteristics are studied in this work.A trilayer structure containing a thin tunnel oxide layer of SiO 2 , an ultrathin PtO x layer, and a control oxide layer of SiO 2 was prepared. The tunnel oxide layer 2.5-7.5 nm thick was grown at 800°C in pure oxygen on ͑100͒ p-type ͑1-10 ⍀ cm͒ Si. The PtO x layer ϳ4 nm thick and the control oxide layer 24 or 36 nm thick, were deposited subsequently at room temperature by rf-magnetron sputtering with a low rf power giving a deposition rate of 2 nm/ min. All the MOS devices were then annealed at 425°C for 25 min in vacuum, which resulted in a reduction of the ultrathin PtO x layers into discrete Pt nanocrystals embedded in SiO 2 matrix. Figure 1(a) shows the cross-sectional image of transmission electron microscopy (TEM) of the Pt nanocrystals self-assembled in the SiO 2 matrix. Note that the Pt nanocrystals were mostly spherical with a narrow size distribution in the range of 2 -3 nm, as depicted in Fig. 1(b). The chemical state of Pt in the vacuum-a...