Ramanathaswamy Pandian

Influence of capping layers on the crystallization of doped SbxTe fast-growth phase-change films

Isothermal crystallization of doped SbxTe fast-growth phase-change films, with and without capping layers, was investigated using transmission electron microscopy, which provided direct and quantitative information on nucleation and growth processes separately. Two types of amorphous dielectric layers, ZnS–SiO2 and GeCrN, were used for sandwiching the SbxTe films to form typical trilayer stacks, which are the active part in applications. The nucleation and growth parameters of SbxTe films were found to be influenced by the dielectric capping layers. The crystal growth rate is temperature dependent and it reduces when the film is sandwiched between the dielectric layers. The reduction in growth rate differs with the capping layer type. The capping layer influence on the growth rate is pronounced at lower temperatures ∼160°C, but tends to vanish at higher temperatures ∼200°C. The activation energy for crystal growth is 2.4±0.3eV for an uncapped film and it increases ∼40% when the capping layers, GeCrN or Zn...

Polarity-dependent reversible resistance switching in Ge–Sb–Te phase-change thin films

In this paper, we demonstrate reversible resistance switching in a capacitorlike cell using a Ge–Sb–Te film that does not rely on amorphous-crystalline phase change. The polarity of the applied electric field switches the cell resistance between lower- and higher-resistance states, as was observed in current-voltage characteristics. Moreover, voltage pulses less than 1.25V showed this switching within time scales of microseconds with more than 40% contrast between the resistance states. The latter are found to be nonvolatile for months. The switching could also be achieved at nanoscales with atomic force microscopy with a better resistance contrast of three orders of magnitude.

Influence of electron beam exposure on crystallization of phase-change materials

Isothermal crystallization of amorphous SbxTe films capped with ZnS-SiO2 or GeCrN layers was performed using in situ heating within a transmission electron microscope. The effect of the electron beam of the microscope on the crystallization process was investigated. It was found that electron irradiation during the crystallization process leads to a continuous increase in the crystal growth velocity. For SbxTe sandwiched between ZnS-SiO2 the effect of the electron beam was equivalent to a temperature rise of about 10 K, without affecting the activation energy for growth. However, for SbxTe sandwiched between GeCrN the activation energy for growth was also decreased due to electron beam exposure. The observed variations in the crystal growth rates are attributed to relaxations within the initial amorphous phase initiated by thermal energy and/or electron irradiation.

Polarity-dependent resistance switching in GeSbTe phase-change thin films: The importance of excess Sb in filament formation

We show that polarity-dependent resistance switching in GeSbTe thin films depends strongly on Sb composition by comparing current-voltage characteristics in Sb-excess Ge2Sb2+xTe5 and stoichiometric Ge2Sb2Te5 samples. This type of switching in Ge2Sb2+xTe5 films is reversible with both continuous and pulsed dc voltages less than 1.5 V. Low and high resistance states of this switching can be attributed to formation and rupture, respectively, of electrically conductive Sb-bridges between the Ge2Sb2Te5 crystals and electrodes through the resistive amorphous phase. The coexistence of polarity-dependent resistance switching with amorphous-crystalline phase-changes renders great opportunities to expand the applicability of GeSbTe films for data storage applications.

In situ transmission electron microscopy study of the crystallization of fast-growth doped SbxTe alloy films

Crystallization of amorphous thin films composed of doped SbxTe with x = 3.0, 3.6, and 4.2 and constant dopant level was studied by in situ heating in a transmission electron microscopy. Magnetron sputtering was used to deposit 20-nm-thick films sandwiched between two types of 3-nm-thick dielectric layers on 25-nm-thick silicon-nitride membranes. One type of dielectric layer consists of ZnS-SiO2 (ZSO), the other of GeCrN (GCN). Crystallization was studied for temperatures in-between 150 and 190 degrees C. The type of dielectric layer turned out to strongly influence the crystallization process. Not only did the nucleation rate appear to depend sensitively on the dielectric layer type, but also the growth rate. The velocity of the crystalline/amorphous interface is about 5 times higher for the x = 4.2 film than for the x = 3.0 film if ZSO is used. In case of GCN, the interface velocity is about 2 times higher for the x = 4.2 film than for the x = 3.0 film. The activation energy for crystal growth is not significantly dependent on the Sb/Te ratio but is clearly different for ZSO and GCN-2.9 eV and 2.0 eV, respectively. The incubation time for the crystal nuclei formation is longer for ZSO than for GCN. Although the effects of the Sb/Te ratio and the dielectric layer type on the growth rates are strong, their effects on the nucleation rate are even more pronounced. A higher Sb/Te ratio results in a lower nucleation rate and the use of GCN instead of ZSO leads to higher nucleation rates.

Optical properties of AlGaN nanowires synthesized via ion beam techniques

AlGaN plays a vital role in hetero-structure high electron mobility transistors by employing a two-dimensional electron gas as an electron blocking layer in multi-quantum well light emitting diodes. Nevertheless, the incorporation of Al into GaN for the formation of the AlGaN alloy is limited by the diffusion barrier formed by instant nitridation of Al adatoms by reactive atomic N. The incorporation of Al above the miscibility limit, however, can be achieved by the ion beam technique. The well known ion beam mixing (IBM) technique was carried out with the help of Ar+ irradiation for different fluences. A novel approach was also adopted for the synthesis of AlGaN by the process of post-irradiation diffusion (PID) as a comparative study with the IBM technique. The optical investigations of AlGaN nanowires, synthesized via two different methods of ion beam processing, are reported. The effect of irradiation fluence and post-irradiation annealing temperature on the random alloy formation was studied by the vi...