Jung-Hwan In

Measurements of electron energy distribution functions and electron transport in the downstream region of an unbalanced dc magnetron discharge

In this study, electron energy distribution functions (EEDFs) are measured using a Langmuir probe in conjunction with the ac superposition method in the downstream region of a planar and unbalanced magnetron argon discharge and the effects of an anode sheath boundary on the discharge characteristics are investigated. The potential of the anode sheath can be controlled by applying a dc voltage to the substrate, and the nonlinear behaviour of the plasma potential with respect to the dc substrate voltage causes the distinctive evolution of the potential of the anode sheath. It is found that when the potential of the anode sheath reaches a specific value, which is related to the threshold energies of argon for the inelastic collisions, an outstanding EEDF transition from a bi-Maxwellian distribution to a single Maxwellian distribution occurs. We introduce the concept of the total electron bounce frequency as an indicator of how the electron collisions such as the electron–electron collision and the inelastic collisions affect the EEDF features as the potential of the anode sheath changes. This result provides the decisive clue to explaining the appearance of the bi-Maxwellian distribution in magnetron discharges. The results of the spatially resolved measurements of EEDF and plasma characteristics are also presented. From these results, we will discuss the electron transport in the downstream region in detail.

Selective removal of CuIn1?xGaxSe2 absorber layer with no edge melting using a nanosecond Nd?:?YAG laser

This paper reports that selective removal of a CuIn1−xGaxSe2 (CIGS) thin film on a Mo-coated glass substrate can be achieved with no edge melting or damage of the Mo layer using a nanosecond Nd:YAG laser with a wavelength of 1064nm. It is shown that the two crucial parameters that determine the possibility of clean removal of only the CIGS layer are Ga concentration of the CIGS film and laser fluence. For CIGS films with Ga/(Ga+In) ratio greater than about 0.2 for which the band gap energy is close to or over the photon energy (1.17eV), laser-induced thermal expansion proved to be the mechanism of film removal that drives an initial bulging of the film and then fracture into tens of micrometre sized fragments as observed in in situ shadowgraph images. The fracture-type removal of CIGS films was further verified by scanning electron micrographs of the craters showing that the original shapes of the CIGS polycrystals remain intact along the crater rim. A numerical simulation of film temperature under the irradiation conditions of selective removal was carried out to show that the magnitude of induced thermal stress within the film closely agreed to the yield strength of the CIGS thin film. The results confirmed that a nanosecond laser could be a better choice for P2 and P3 scribing of CIGS thin films if process conditions are properly determined. (Some figures may appear in colour only in the online journal)

Effect of duty cycle on plasma parameters in the pulsed dc magnetron argon discharge

The time-resolved probe measurements of the plasma parameters and the electron energy distribution function are carried out in a unipolar pulsed dc magnetron argon discharge. The cathode target is driven by the 20kHz midfrequency unipolar dc pulses at three operating modes, such as constant voltage, constant power, and constant current with the duty cycles ranging from 10% to 90%. It is observed that as the duty cycle is reduced, the electron temperature averaged during the pulse-on period rapidly increases irrespective of the operating mode although the average electron density strongly depends on the operating mode. The comparison of the measured electron energy distribution functions shows that the electron heating during the pulse-on period becomes efficient in the pulse operation with short duty cycle, which is closely related to the deep penetration of the high-voltage sheath into the bulk during the pulse-on period.

Influence of Ar buffer gas on the LIBS signal of thin CIGS films

Laser induced breakdown spectroscopy (LIBS) measurement of a thin Cu(In,Ga)Se2 (CIGS) absorber layer was performed under the conditions of open air and argon (Ar) gas jet flow (25 L min−1). A second harmonic Q-switched Nd:YAG laser (λ = 532 nm, τ = 5 ns, top-hat profile) was employed for the ablation of co-evaporated CIGS samples. The use of an Ar gas jet for the LIBS measurement of CIGS thin films leads to two significant differences from the open air measurement results that are important from the precision point of view: (i) the increase of signal intensity and (ii) decrease of relative standard deviation (RSD) of measured intensity or intensity ratios. From calculation of the degree of intensity enhancement, it was shown that the enhancement of signal intensity is primarily due to the increase of plasma temperature and electron density under Ar buffer gas environment. It was also found that in addition to the influence of Ar gas itself the gasdynamic effects of the Ar gas jet contribute significantly to the reduction of RSD.

Quantitative analysis of CuIn1−xGaxSe2 thin films with fluctuation of operational parameters using laser-induced breakdown spectroscopy

To enable a quantitative analysis of CuIn1−xGaxSe2 (CIGS) thin solar cell films using laser-induced breakdown spectroscopy (LIBS), a new method based on the internal standard principle is proposed to identify the spectral line pairs unaffected by the fluctuations of measurement conditions. Due to nonlinear calibration curves resulting from the high concentration of constituent elements of CIGS, the standard deviation of measured line intensity ratios divided by the slope of calibration curves was introduced as a new figure of merit to determine the precision of LIBS analysis along with the linearity of data points in the correlation plots. From the analyses of 39 Cu, 10 In, and 5 Ga emission lines, Cu 324.754 nm–In 325.608 nm, Cu 219.975 nm–Ga 287.424 nm, and In 303.935 nm–Ga 287.424 nm line pairs were identified as the spectral lines of which intensity ratios were least influenced by the fluctuations of spot size, the position of collection optics, and/or laser energy. The selected line pairs, except the Cu 219.975 nm line, had similar upper level energy and consisted of strong resonance lines. It was found that the use of these strong lines has advantages of better reproducibility and precision for the analysis of elements with high concentration for which the calibration curve may become nonlinear due to self-absorption.

Experimental investigation of plasma dynamics in dc and short-pulse magnetron discharges

The spatiotemporal evolution of the electron energy distribution function (EEDF) and of plasma parameters such as the electron density, the electron temperature and the plasma and floating potentials has been investigated using spatially and temporally resolved single Langmuir probe measurements in dc and mid-frequency, short-pulse magnetron discharges with a repetition frequency of 10 kHz and a duty cycle of 10%. In the pulsed discharge of the short duty cycle, a peak electron temperature higher than 10 eV was observed near the cathode fall region during the early phase of the pulse-on, which is about three times higher than the steady-state value of the electron temperature in the dc discharge. The temporal evolution of the measured EEDFs showed the initial efficient electron heating during the early phase of the pulse-on and the subsequent relaxation of electron energy by the inelastic collisions and the diffusive loss. The high-energy electrons generated during the pulse-on phase diffused the downstream region toward the grounded substrate, resulting in a bi-Maxwellian EEDF consisting of the background low-energy electrons and the high-energy electrons. The results of the spatially and temporally resolved probe measurements will be presented and the enhanced efficiency of the electron heating in the short-pulse discharge will be explained on the basis of the global model of a pulsed discharge.

Reproducibility of CIGS thin film analysis by laser-induced breakdown spectroscopy

The reproducibility of an elemental analysis of thin CIGS solar cell films by laser-induced breakdown spectroscopy (LIBS) is assessed. A Q-switched Nd:YAG laser (λ = 532 nm, τ = 5 ns, top-hat profile) and an ICCD detector were used for the LIBS measurements. It was shown that the concentration ratios and depth profile of a CIGS sample with unknown composition could be predicted with high precision by using the calibration curves generated with reference CIGS samples. The spot-to-spot relative standard deviation of 1.4% could be achieved for (Ga + In)/Cu at the gate delay of 0.1 μs condition, demonstrating that LIBS can be applied for real time monitoring of the elemental composition of CIGS thin films.

Independence of elemental intensity ratio on plasma property during laser-induced breakdown spectroscopy

The authors report that the elemental composition ratio of Ga to In in a CuIn(1-x)Ga(x)Se(2) compound semiconductor, a thin-film solar cell material, can be measured with little influence of plasma property changes by laser-induced breakdown spectroscopy (λ = 1064 nm, τ = 5 ns). It is shown that the similarity in excitation energy levels of the selected Ga and In emission lines and the fact that these elements belong to the same group of the periodic table are the critical factors ensuring the independence of intensity ratio on plasma conditions.

Depth profiling analysis of CuIn 1-x Ga x Se 2 absorber layer by laser induced breakdown spectroscopy in atmospheric conditions

This work reports the capability of depth profile analysis of thin CuIn1-xGa(x)Se2 (CIGS) absorber layer (1.89 μm) with a sub-hundred nanometer resolution by laser induced breakdown spectroscopy (LIBS). The LIBS analysis was carried out with a commercial CIGS solar cell on flexible substrate by using a pulsed Nd:YAG laser (λ = 532 nm, τ = 5 ns, top-hat profile) and an intensified charge-coupled device spectrometer in atmospheric conditions. The measured LIBS elemental profiles across the CIGS layer agreed closely to those measured by secondary ion mass spectrometry. The resolution of depth profile analysis was about 88 nm. Owing to the short measurement time of LIBS and the capability of in-air measurement, it is expected that LIBS can be applied for in situ analysis of elemental composition and their distribution across the film thickness during development and manufacturing of CIGS solar cells.

Improvement of selenium analysis during laser-induced breakdown spectroscopy measurement of CuIn1−xGaxSe2 solar cell films by self-absorption corrected normalization

The analysis of Se emission lines (196.089 nm and 203.984 nm) of CuIn1−xGaxSe2 (CIGS) thin solar cell films by laser-induced breakdown spectroscopy (LIBS) is reported. With CIGS thin films fabricated by co-evaporation or sputtering methods, LIBS measurement was carried out using a nanosecond laser (λ = 532 nm, τ = 5 ns, top-hat profile) and a CCD spectrometer (gate width = 1.05 ms, gate delay = 0.2 μs) in a flowing Ar gas environment (20 L min−1). From the comparison of calibration curves, it was found that the correlation between the Se/Cu LIBS intensity ratio and the Se/Cu concentration ratio could be improved significantly by applying the self-absorption correction procedure introduced in this study, demonstrating the significance of self-absorption in those Se emission lines. It was shown that normalization alone could reduce the relative standard deviation but was insufficient for accurate LIBS analysis of Se in CIGS thin films.