Memory characteristics of Pt nanocrystals self-assembledfrom reduction of an embedded PtOx ultrathin film in metal-oxide-semiconductor structures

Abstract

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...

Charge storage and data retention characteristics of forming gas-annealed Gd2O3-nanocrystal nonvolatile memory cell

“…The ability to realize smaller and more portable next-generation electronic devices is very important; however, there are significant obstacles to scaling down continuously conductive polysilicon FG memories [1]. For NVM devices, a simple way to replace the conventional FG memories is to use discrete dielectric or metallic charge storage islands, termed nanocrystals (NCs) [2][3][4][5][6][7][8]. Among all investigations which focused on the nanocrystal memories, the silicon nanocrystals (Si-NCs) have been attracted for the application of memory device [2].…”

Metal nanocrystals as charge storage nodes for nonvolatile memory devices

“…The memory effect will be seen when the effective potential well depth (d eff ) is higher. As a result, the memory effects of the Pt and Au are better than the Ag [26][27][28][29]. The work functions of various metallic materials are listed in Table 1.…”

“…The memory effects of the Ag, Au, Pt, W, Co, Ni, NiSi 2 , Ni 1 − x Fe x , TiN and Al metal nanocrystals were investigated [26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43]. Devices with Au, Ag and Pt nanocrystals working in the F-N tunneling regime have been investigated and compared with Si nanocrystals memory devices.…”

Charge storage characteristics of nonvolatile memories with chemically-synthesized and vacuum-deposited gold nanoparticles

“…It is known that the silicon nanoparticles (Si-NPs) have been used and implemented into mass production level for some embedded memory devices [7]. Moreover, metal nanoparticles with high work-function, like platinum (Pt) and gold (Au) NPs, are promising for the use of nanoparticletype memory due to the deep potential well, high density of states (DOS) around Fermi level, and small perturbation against the quantum confinement effect [8,9]. The first merit of metal NPs theoretically leads to a long-term retention time because the electrons are confined in a three-dimensional quantum well and hardly escape.…”