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Cu(In,Ga)Se2 (CIGS) thin films, a promising photovoltaic architecture, have mainly relied on Molybdenum for the bottom contact. However, the opaque nature of Molybdenum (Mo) poses limitations in module level fabrication by laser scribing as a preferred method for interconnect. We examined the P1, P2, and P3 laser scribing processes on CIGS photovoltaic architecture on the indium tin oxide (ITO) bottom contact with a cost-effective nanosecond pulsed laser of 532 nm wavelength. Laser illuminated from the substrate side, enabled by the transparent bottom contact, facilitated selective laser energy deposition onto relevant interfaces towards high-quality scribing. Parametric tuning procedures are described in conjunction with experimental and numerical investigation of relevant mechanisms, and preliminary mini-module fabrication results are also presented.

Current efforts of guided growth of carbon nanotubes alone cannot make large-scale and directed assembly of them. The nanopelleting concept overcomes this limitation by embedding carbon nanotubes in microscale pellets that can be transplanted readily. This technique includes vertical growth of carbon nanotubes, pellet casting, planarization, pellet separation, and transplantation. A specific manufacturing process is developed and tested with favorable results. This technology will enable directed assembly of carbon nanotubes in a long-range order.

We investigated the impact of incorporating 2% oxygen during intrinsic ZnO sputtering on the efficiency of Cu(In,Ga)Se2 solar cells. The added oxygen not only reduced the optical absorption loss of the Al-doped ZnO overlaying layer but also improved the electronic properties of the underlying CdS/Cu(In,Ga)Se2 by increasing carrier density, lowering defect level, and increasing diffusion length, eventually enhancing JSC, VOC, and fill factor. It was found that the Na doping concentration was significantly increased around the CdS/Cu(In,Ga)Se2 junction due to the plasma-activated oxygen. The improved electronic properties are better explained by the increased Na concentration than simply the oxygen-related defect passivation.

An investigation was conducted to examine the high RESET-current (IRESET) problem of phase-change memory (PCM) using a fast growth-dominated Ge-doped SbTe (GeST). By examining material and device characteristics of GeST with varying Sb-to-Te ratio from 1.80 to 3.82, the growth rate of crystallization was found to play an important role in determining IRESET and SET speed of the device. Lower IRESET obtained with decreasing Sb-to-Te ratio was ascribed to lower growth rate leading to smaller degree of recrystallization during melt-quenching. With shrinkage of device dimensions, GeST-PCM of a lower Sb-to-Te ratio may become increasingly promising due to its lower IRESET and scaled SET speed.

Ge-doped SbTe (Ge–ST) was compared with Ge2Sb2Te5 (GST) for its potential use in the phase-change memory with improved stability at varying ambient temperature (TA). Device characteristics such as RESET current, RESET resistance, and SET resistance of Ge–ST devices were found to vary significantly less with TA than those of GST devices. From measured carrier density, mobility, and optical band gaps, these findings are interpreted to derive from a metallic nature of the crystalline Ge–ST in contrast with a semiconducting nature of the crystalline GST as well as a relatively weaker covalent bonding in amorphous Ge–ST.

We investigated the temperature dependence of the threshold switching characteristics of a memory-type chalcogenide material, Ge2Sb2Te5. We found that the threshold voltage (Vth) decreased linearly with temperature, implying the existence of a critical conductivity of Ge2Sb2Te5 for its threshold switching. In addition, we investigated the effect of bias voltage and temperature on the delay time (tdel) of the threshold switching of Ge2Sb2Te5 and described the measured relationship by an analytic expression which we derived based on a physical model where thermally activated hopping is a dominant transport mechanism in the material.

We investigated the temperature dependence of characteristics of phase change memory devices composed of Ge2Sb2Te5 (GST). We found that the RESET resistance (RR), SET resistance (RS), and SET time (tSET) decreased with increasing ambient temperature (Tamb.) while RESET current (IR) increased with Tamb. These results were explained in terms of the enhanced thermal activation of electrical conduction in GST and the increased thermal conductivity of the bottom electrode with Tamb.. Besides, threshold voltage (Vth) was found to decrease linearly with Tamb., seemingly in support of the relaxation semiconductor model and hopping-dominated transport model both of which predicted the existence of critical conductivity for threshold switching in chalcogenide glasses.

A semiquantitative characterization of the interfacial adhesion of phase change materials is developed, which consists of determining critical adhesion temperature (TCA) via measuring the probability of adhesion failure with temperature using patterned films. By comparison of TCA values, Ge-doped SbTe (Ge-ST) is shown to have weaker adhesion than Ge2Sb2Te5 (GST), which results from its limited ability in relaxation of crystallization-induced stress. Nitrogen or oxygen doping in Ge-ST produces significant increase in TCA, close to that of GST. This improvement is due to smaller grain size of N-/O-doped Ge-ST, which facilitates the relaxation of the stress via grain boundary diffusion or sliding.

We investigated the bias polarity dependence of the characteristics of a phase change memory device and found that the device showed higher resistance both at programed and erased states, extended erasing time, and higher threshold voltage (Vth) under negative bias than those under the conventional positive bias. Taking advantage of this dependence, we were able to obtain four highly distinguishable resistance states in such a reproducible manner that may be utilized for a reliable multilevel storage. We explain this polarity dependence in terms of the difference in the density of trap states at the interfaces between the phase change material and electrodes.

An experimental investigation was carried out on the kinetic nature of the set process in a phase change memory device by combined analyses of set voltage wave forms and time-resolved low-field resistances. As it turned out, the progress of a set process may be measured in terms of three characteristic times in sequence i.e., threshold switching time tth, incubation time for crystallization tinc, and complete set time tset. These characteristic times are supposed to demarcate, in some measure, different stages of crystallization in the memory material during a set process. Each of these times has a strong dependence on input pulse voltage and particularly threshold switching time tth was found to have an exponentially decaying dependence. The latter may be related to the decreasing capacitance of an amorphous phase-change material with approaching threshold switching.