Junkang Wang

Bio: Junkang Wang is an academic researcher from Université Paris-Saclay. The author has contributed to research in topics: Materials science & Wafer. The author has an hindex of 6, co-authored 15 publications receiving 100 citations. Previous affiliations of Junkang Wang include Centre national de la recherche scientifique & École Polytechnique.

Anticrossing of axial and planar surface-related phonon modes in Raman spectra of self-assembled GaN nanowires

Abstract: GaN columnar nanostructures usually called nanowires have been investigated by micro-Raman spectroscopy. In addition to conventional Raman scattering by confined optical phonons of a wurtzite structure (i.e., ${E}_{2h}$ and $QLO$ modes), an unusual two peaks band centered near 700 cm${}^{\ensuremath{-}1}$ is observed and analyzed as a function of several experimental parameters (polarization, filling factor, incidence angle). The surface character of these two modes is experimentally confirmed by their high sensitivity to the dielectric constant of the as-grown nanowires surrounding medium. Calculations describing the nanowires' environment by means of an effective dielectric function are in good agreement with this two peaks structure and suggest that these two peaks result from the anticrossing of axial and planar surface-related phonons, which should be observed in all anisotropic polar semiconductors.

Experimental demonstration of multifrequency impedance matching for tailored voltage waveform plasmas

Abstract: Driving radiofrequency capacitively coupled plasmas by multiharmonic tailored voltage waveforms (TVWs) has been shown to allow considerable control over various plasma properties for surface processing applications. However, industrial adoption of this technology would benefit from more efficient solutions to the challenge of impedance matching the radiofrequency power source to the load simultaneously at multiple harmonic frequencies. The authors report on the design and demonstration of a simple, practical multifrequency matchbox (MFMB) based on a network of LC resonant circuits. The performance of the matchbox was quantified in terms of a range of matchable impedances (when matching a single frequency at a time), as well as for the independence of each match to changes at adjacent harmonics. The effectiveness of the MFMB was demonstrated experimentally on an Ar plasma excited by a three-frequency TVW with a fundamental frequency of 13.56 MHz. Under the plasma conditions studied, the power coupling efficiency (at the generator output) was increased from less than 40% (without impedance matching) to between 80% and 99% for the different exciting frequencies.

Effect of Ion Energy on Microcrystalline Silicon Material and Devices: A Study Using Tailored Voltage Waveforms

Abstract: The use of tailored voltage waveforms to excite a plasma has been shown to be an effective technique to decouple maximum ion energy from the ion flux on the electrode. We use it here as a way to scan through the maximum ion energy in order to study this quantity's role in the growth of μc-Si:H. We find that at critical energies (30 and 70 eV), a stepwise increase in the a-Si:H/μc-Si:H transition thickness is observed, together with change in the surface morphology. These thresholds correspond to SiH x + - and H 3 + -induced displacement energies, respectively. A model is proposed to account for the impact of these ions on the morphology of μc-Si:H growth and is confirmed by comparison with epitaxial growth on a crystalline wafer.

Stress distribution of 12 μm thick crack free continuous GaN on patterned Si(110) substrate

Abstract: The stress distribution on crack free thick continuous GaN film (12 µm) grown by Metal organic chemical vapour deposition (MOCVD) on the arrays of different sizes of the patterned silicon substrate is investigated by micro-Raman (µRaman) spectroscopy. On the largest crack free mesa (400 μm) both µ-photoluminescence (µPL) at low temperature and µRaman measurements are performed. The µRaman shift of the GaN E2 mode shows the U-shape in-plane stress distribution across the mesa. The center of the mesa is under tensile stress and it relaxes near the corner and edges. A similar trend is observed also from the µPL spectra. The size of the mesa, the trench height and the trench width of the patterned silicon are varied to study the stress of the thick epitaxial crack free GaN layer. The size and trench height of the mesa have a large influence on the GaN film stress but the trench width does not show any significant effect. The maximum stress is saturated for the large sizes of mesas. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoluminescence spectrum from heterojunction with intrinsic thin layer solar cells: An efficient tool for estimating wafer surface defects

Abstract: In heterojunction with intrinsic thin layer “HIT” solar cells, care has to be taken to passivate the defects on the wafer surface before the deposition of the doped amorphous emitter and back surface field layers. Otherwise, these defects would lead to a significant loss in cell performance. It is therefore important to estimate the magnitude of these defects. In the passivation studies undertaken experimentally, both wafer surfaces were cleaned either by a dip in a 5% HF solution for 30 s or by various dry plasma processes, before depositing intrinsic a-Si:H layers. Detailed electrical–optical modeling of the measured PL signal from these wafers (in a solar cell configuration) not only allows us to quantify the surface defects in each case, relative to e.g., the sample showing the highest PL signal, but also to calculate the solar cell output parameters and link them to the intensity of the PL signal and the defect density on the wafer surface. Dry plasma cleaning has the advantage that it can be done in situ. Our preliminary plasma cleaning studies in conjunction with modeling indicate that it is possible in this way to achieve the levels of surface passivation comparable to standard HF dip cleaning.

Tailored-waveform excitation of capacitively coupled plasmas and the electrical asymmetry effect

Abstract: Unequal areas of the powered and grounded electrodes in single-frequency capacitively coupled plasmas (CCPs) are well-known to generate a DC self-bias voltage and an asymmetric plasma response. By instead applying non-sinusoidal waveforms composed of multiple harmonics—referred to in the literature as arbitrary waveforms, multi-harmonic waveforms or tailored waveforms—an asymmetric plasma response and a DC self-bias can also be produced; even for perfectly geometrically symmetric systems. This electrical asymmetry effect (EAE) has opened the doors to a wide range of novel ideas and interesting new physics that could allow limitations between the control of the ion flux and ion energy in traditional CCPs to be broken; thus helping to develop next-generation industrial plasma processing reactors. This review is dedicated to the current status of the EAE, and highlights important theoretical, numerical and experimental work in the field that has contributed to our understanding.

Strong Ionization Asymmetry in a Geometrically Symmetric Radio Frequency Capacitively Coupled Plasma Induced by Sawtooth Voltage Waveforms

Abstract: The ionization dynamics in geometrically symmetric parallel plate capacitively coupled plasmas driven by radio frequency tailored voltage waveforms is investigated using phase resolved optical emission spectroscopy (PROES) and particle-in-cell (PIC) simulations. Temporally asymmetric waveforms induce spatial asymmetries and offer control of the spatiotemporal dynamics of electron heating and associated ionization structures. Sawtooth waveforms with different rise and fall rates are employed using truncated Fourier series approximations of an ideal sawtooth. Experimental PROES results obtained in argon plasmas are compared with PIC simulations, showing excellent agreement. With waveforms comprising a fast voltage drop followed by a slower rise, the faster sheath expansion in front of the powered electrode causes strongly enhanced ionization in this region. The complementary waveform causes an analogous effect in front of the grounded electrode.

p-Type Doping of GaN Nanowires Characterized by Photoelectrochemical Measurements

Abstract: GaN nanowires (NWs) doped with Mg as a p-type impurity were grown on Si(111) substrates by plasma-assisted molecular beam epitaxy. In a systematic series of experiments, the amount of Mg supplied during NW growth was varied. The incorporation of Mg into the NWs was confirmed by the observation of donor–acceptor pairs and acceptor-bound excitons in low-temperature photoluminescence spectroscopy. Quantitative information about the Mg concentrations was deduced from Raman scattering by local vibrational modes related to Mg. In order to study the type and density of charge carriers present in the NWs, we employed two photoelectrochemical techniques, open-circuit potential and Mott–Schottky measurements. Both methods showed the expected transition from n-type to p-type conductivity with increasing Mg doping level, and the latter characterization technique allowed us to quantify the charge carrier concentration. Beyond the quantitative information obtained for Mg doping of GaN NWs, our systematic and comprehens...

Cracking effects in squashable and stretchable thin metal films on PDMS for flexible microsystems and electronics

Abstract: Here, we study cracking of nanometre and sub-nanometre-thick metal lines (titanium, nickel, chromium, and gold) evaporated onto commercial polydimethylsiloxane (PDMS) substrates. Mechanical and electromechanical testing reveals potentially technologically useful effects by harnessing cracking. When the thin film metal lines are subjected to uniaxial longitudinal stretching, strain-induced cracks develop in the film. The regularity of the cracking is seen to depend on the applied longitudinal strain and film thickness—the findings suggest ordering and the possibility of creating metal mesas on flexible substrates without the necessity of lithography and etching. When the metal lines are aligned transversally to the direction of the applied strain, a Poisson effect-induced electrical ‘self-healing’ can be observed in the films. The Poisson effect causes process-induced cracks to short circuit, resulting in the lines being electrically conducting up to very high strains (~40%). Finally, cracking results in the observation of an enhanced transversal gauge factor which is ~50 times larger than the geometric gauge factor for continuous metal films—suggesting the possibility of high-sensitivity thin-film metal strain gauge flexible technology working up to high strains.

Helium metastable species generation in atmospheric pressure RF plasma jets driven by tailored voltage waveforms in mixtures of He and N2

Abstract: Spatially resolved tunable diode-laser absorption measurements of the absolute densities of He-I (23S1) metastables in a micro atmospheric pressure plasma jet operated in He/N2 and driven by "peaks"- and "valleys"-type tailored voltage waveforms are presented. The measurements are performed at different nitrogen admixture concentrations and peak-to-peak voltages with waveforms that consist of up to four consecutive harmonics of the fundamental frequency of 13.56 MHz. Comparisons of the measured metastable densities with those obtained from Particle-in-Cell/Monte Carlo collisions simulations show a good quantitative agreement. The density of helium metastables is found to be significantly enhanced by increasing the number of consecutive driving harmonics. Their generation can be further optimized by tuning the peak-to-peak voltage amplitude and the concentration of the reactive gas admixture. These findings are understood based on detailed fundamental insights into the spatio-temporal electron dynamics gained from the simulations, which show that Voltage Waveform Tailoring allows to control the electron energy distribution function to optimize the metastable generation. A high degree of correlation between the metastable creation rate and the electron impact excitation rate from the helium ground state into the He-I ((3s)3S1) level is observed for some conditions which may facilitate an estimation of the metastable densities based on Phase Resolved Optical Emission Spectroscopy measurements of the 706.5 nm He-I line originating from the above level and metastable density values at proper reference conditions.