Jong-Keuk Park

Characterization of efficiency-limiting resistance losses in monolithically integrated Cu(In,Ga)Se2 solar modules

The cell-to-module efficiency gap in Cu(In,Ga)Se2 (CIGS) monolithically integrated solar modules is enhanced by contact resistance between the Al-doped ZnO (AZO) and Mo back contact layers, the P2 contact, which connects adjacent cells. The present work evaluated the P2 contact resistance, in addition to the TCO resistance, using an embedded transmission line structure in a commercial-grade module without using special sample fabrication methods. The AZO layers between cells were not scribed; instead, the CIGS/CdS/i-ZnO/AZO device was patterned in a long stripe to permit measurement of the Mo electrode pair resistance over current paths through two P2 contacts (Mo/AZO) and along the AZO layer. The intercept and slope of the resistance as a function of the electrode interval yielded the P2 contact resistance and the TCO resistance, respectively. Calibration of the parasitic resistances is discussed as a method of improving the measurement accuracy. The contribution of the P2 contact resistance to the series resistance was comparable to that of the TCO resistance, and its origin was attributed to remnant MoSe2 phases in the P2 region, as verified by transmission electron microscopy.

Grain size refinement of the diamond film deposited on the WC-Co cutting inserts using direct current biasing

The effect of negative direct current (DC) bias has been investigated in the deposited diamond on WC–Co insert. SPGN type of WC–Co was used as a substrate where diamond was deposited by hot filament chemical vapor deposition at 60 Torr with H – 2 3%CH gas. DC bias was applied to the insert with the range between 0 and y120 V. The grain size of diamond film deposited 4 on WC–Co insert decreased with the increase in negative bias voltage. The grain size was more refined near the edge than the center of the insert. Compared to other processing parameters such as pressure, temperature and carbon concentration, the biasing is a very effective way to reduce the grain size of diamond film, especially at the edge region where workpiece contacts during machining process. 2003 Elsevier Science B.V. All rights reserved.

Origin of residual stress in the formation of boron nitride film by sputtering with Ar ions

It is demonstrated that the compressive residual stress in turbostratic boron nitride (tBN) film is caused by the incorporation of Ar atoms between the gap of tBN layers. The stress of the film is measured in situ during sputter deposition at various substrate bias voltages. The variation of the stress with increasing film thickness follows that of an Ar concentration profile in the films along the growth direction. This result reveals that the interstitial Ar atoms induce the evolution of the compressive residual stress in the tBN film. Cross-sectional transmission electron microscopic images of the films are examined to explain the penetration behavior of Ar ions during the deposition.

Scalable excitatory synaptic circuit design using floating gate based leaky integrators

We propose a scalable synaptic circuit realizing spike timing dependent plasticity (STDP)—compatible with randomly spiking neurons. The feasible working of the circuit was examined by circuit simulation using the BSIM 4.6.0 model. A distinguishable feature of the circuit is the use of floating-gate integrators that provide the compact implementation of biologically plausible relaxation time scale. This relaxation occurs on the basis of charge tunneling that mainly relies upon area-independent tunnel barrier properties (e.g. barrier width and height) rather than capacitance. The circuit simulations feature (i) weight-dependent STDP that spontaneously limits the synaptic weight growth, (ii) competitive synaptic adaptation within both unsupervised and supervised frameworks with randomly spiking neurons. The estimated power consumption is merely 34 pW, perhaps meeting one of the most crucial principles (power-efficiency) of neuromorphic engineering. Finally, a means of fine-tuning the STDP behavior is provided.

Effect of Oxygen Addition on Residual Stress Formation of Cubic Boron Nitride Thin Films

In this study we investigated the oxygen effect on the nucleation and its residual stress during unbalanced magnetron sputtering. Up to 0.5% in oxygen flow rate, cubic phase (c-BN) was dominated with extremely small fraction of Hexagonal phase (h-BN) of increasing trend with oxygen concentration, whereas hexagonal phase is dominated beyond 0.75% flow rate. Interestingly, the residual stress in cubic-phase-dominated films was substantially reduced with small amount of oxygen () down to a low value comparable to the h-BN case. This may be because oxygen atoms break B-N bonds and make B-O bonds more favorably, increasing bonds preference, as revealed by FTIR and NEXAFS. It was confirmed by experimental facts that the threshold bias voltage for nucleation and growth of cubic phase were increased from -55 V to -70 V and from -50 V to -60 V respectively. The reduction of residual stress in O-added c-BN films is seemingly resulting from the microstructure of the films. The oxygen tends to increase slightly the amount of h-BN phase in the grain boundary of c-BN and the soft h-BN phase of 3D network including surrounding nano grains of cubic phase may relax the residual stress of cubic phase.