ABSTRACT
As zero-dimensional systems, nanocrystals (NCs) are very attractive candidates for and will play a major role in future electronic devices. This chapter reviews in depth the progress in the growth and physical and electrical characterizations of germanium (Ge) NCs as materials and also their application in floating-gate memory devices. This chapter is mainly divided into three parts. The first part will discuss the synthesis method of Ge NCs and the dependence on concentration, annealing conditions, influence of dielectric material,
etc. In the second part, we will discuss the physical characteristics, such as photoluminescence and electroluminescence, and the stress effect on Ge NCs. The third part will introduce the size-dependent, density-dependent electrical properties of Ge-NC-based floating-gate memory devices. Special care is taken to discuss in detail the fabrication process of memory devices embedding Ge NCs of different sizes, densities, annealing treatments, and dielectric materials and their characterization as transistors. The chapter outlines the challenges faced by Ge-NC-based floating-gate memory devices, with the scope of future development. 5.1 IntroductionThe possibility of integrating group IV elements with silicon (Si)-based technology for optoelectronics has spurred much investigation in the field of low-dimensional systems [1-75]. Germanium (Ge), as a group IV element, is structurally similar to Si [76-125]. The energy difference between the indirect gap (0.66 eV) and the direct gap (0.8 eV) is smaller in Ge (0.14 eV) than in Si (1.1 eV) [126-147]. As Ge also has a larger permittivity and smaller effective masses for electrons and holes than Si, the Bohr radius of the excitons in Ge (24.3 nm) is larger than that of Si (4.9 nm) [32]. The quantum size effect is thus predicted to be stronger in Ge than in Si [8]. These facts had suggested that it is easier to modify the electronic structure around the bandgap of Ge. Hence there has been an intense interest in Ge nanocrystals (NCs) embedded in a silicon oxide (SiO2) matrix as such a structure has potential applications in optoelectronics [126, 127] and electronics [40, 72, 75, 132]. The interest in optoelectronic applications stems from the observation of visible photoluminescence (PL) or electroluminescence (EL) exhibited by Ge NCs in a SiO2 matrix [65, 141]. For electronic applications, Tiwari et al. [132] have proposed a Si NC memory device that can be programmed at hundreds of nanoseconds using low voltages for direct tunneling and storage of electrons in the Si NCs. Several methods have been used in the synthesis of Ge NCs in an oxide matrix, for example, cosputtering [21, 34], chemical vapor deposition (CVD) [39, 118], ion implantation [98], pulsed
laser deposition [35, 36], e-beam irradiation [28], reduction of silicon germanium oxides [69, 112], and the use of anodic alumina membrane (AAM) [15]. We will briefly summarize the results of Ge NCs fabricated by these methods in this section. However, by far, the most common method in the synthesis of Ge NCs is by annealing cosputtered Ge plus silicon oxide. Ion implantation is a versatile technique for forming Ge NCs in the near-surface region of a substrate. For example, Mestanza [96] synthesized Ge NCs in a SiO2 matrix by implanting Ge74+ ions at room temperature using 250 keV energy, at doses of 0.5-4 × 1016cm-2. The samples were subsequently annealed at 1000°C for 1 h in a forming gas ambient. Similar implantation conditions were also adopted by Yang [140] and Barba [4]. However, the disadvantage with the synthesis of Ge NCs by implantation is that the NCs usually have a relatively wide size distribution. Multienergy ion implantation and thermal annealing produce a uniform depth-concentration profile of Ge NCs over the SiO2 film with a narrow size distribution. NCs are usually formed by implanting Ge at doses of 3 × 1016 cm−2 or higher using 100 keV and annealing at temperatures varying from 700°C to 1000°C. Oxidation of Ge due to the diffusion of oxidizing species from the annealing ambient at high temperatures plays an important role in samples with a projected range closer to the surface. E-beam irradiation has been used to synthesize Ge NCs in a SiO2 matrix [28]. The samples consisted of a trilayer structure with a radio frequency (rf)-cosputtered Ge-plus-SiO2 layer sandwiched between two SiO2 layers. It was concluded that the thermal effect of e-beam radiation was only able to initiate Ge clustering as it does not impart enough energy for long-range diffusion for NC formation. The NC formation can only be achieved with a substrate heated to 250°C under e-beam irradiation.
