Jianwu Sun

Nitrogen-related recombination mechanisms in p-type ZnO films grown by plasma-assisted molecular beam epitaxy

The recombination mechanisms of nitrogen-related emissions in p-type ZnO films were investigated by photoluminescence (PL) measurements. The enhanced peak at 3.260 eV was confirmed as a donor-acceptor pair (DAP) transition and the emission around 3.310 eV was assigned to the free electron to acceptor (FA) recombination by studying the temperature evolution of DAP and FA luminescence, calculating the energy level of the corresponding nitrogen acceptor, and measuring the decay time of DAP transition. By comparing the PL spectra of the samples with various hole concentrations, it can be found that as the hole concentration increases, the DAP peak significantly dominates the spectra. In addition, the acceptor binding energy is estimated to be about 120 meV from the FA transition, which is in good agreement with the value calculated by a hydrogenic acceptor model.

Excitonic electroluminescence from ZnO-based heterojunction light emitting diodes

This work reports on the fabrication and characteristics of n-ZnO/p-GaN and n-ZnO/n-MgZnO/n-ZnO/p-GaN heterojunction light emitting diodes (LEDs). Both devices exhibited diode-like rectifying current-voltage characteristics. Room temperature electroluminescence (EL) spectra for both LEDs consisted of dominant emission at 375 nm and two weaker bands centred at 415 nm and 525 nm, which were attributed to ZnO excitonic transition and defect-related emissions from GaN and ZnO, respectively. Moreover, it was demonstrated that the single heterojunction required a higher injection current to obtain an excitonic EL than that for the n-ZnO/n-MgZnO/n-ZnO/p-GaN LEDs. This indicated that the insertion of the MgZnO layer confined the injection carriers and thus increased the intensity of excitonic emission in the ZnO active region.

Strained germanium quantum well p-FinFETs fabricated on 45nm Fin pitch using replacement channel, replacement metal gate and germanide-free local interconnect

Strained Ge p-channel FinFETs on Strain Relaxed SiGe are integrated for the first time on high density 45nm Fin pitch using a replacement channel approach on Si substrate. In comparison to our previous work on isolated sGe FinFETs [1], 14/16nm technology node compatible modules such as replacement metal gate and germanide-free local interconnect were implemented. The ION/IOFF benchmark shows the high density strained Ge p-FinFETs in this work outperform the best published isolated strained Ge on SiGe devices.

15nm-W FIN high-performance low-defectivity strained-germanium pFinFETs with low temperature STI-last process

An STI-last integration scheme was successfully developed to fabricate low-defectivity and dopant-controlled SiGe SRB / sGe Fins. For the first time, 15 nm fin-width SiGe SRB/highly-strained Ge pFinFETs are demonstrated down to 35 nm gate length. With a CET_INV-normalized G_M,SAT,INT of 6.7 nm.mS/μm, the Si_0.3Ge_0.7 / sGe pFinFETs presented in this work improve the performance by ~90% as compared to the state-of-the-art relaxed-Ge FinFETs.

Hole transport in p-type ZnO films grown by plasma-assisted molecular beam epitaxy

The hole transport properties of nitrogen doped p-type ZnO grown on c-plane sapphire (c-Al2O3) were investigated by temperature-dependent Hall-effect measurements. The experimental Hall mobility was found to be considerably lower than the calculated mobility including ionized impurity scattering, acoustic-mode deformation potential scattering, piezoelectric potential scattering, and polar optical phonon scattering. Atomic force microscopy and x-ray diffraction measurements demonstrated that p-type ZnO on c-Al2O3 consisted of two kinds of 30°-rotated domains surrounded by grain boundaries. Thus, taking the effect of inhomogeneous microstructure on the mobility into account, the calculated mobility agreed favorably with the experimental data. This agreement indicates that besides ionized impurity and acoustic deformation potential scattering at low temperatures and the polar optical phonon scattering at high temperatures, the effects of the inhomogeneous microstructure in p-type ZnO films play a more import...

Fluorescent SiC as a new material for white LEDs

Current III–V-based white light-emitting diodes (LEDs) are available. However, their yellow phosphor converter is not efficient at high currents and includes rare-earth metals, which are becoming scarce. In this paper, we present the growth of a fluorescent silicon carbide material that is obtained by nitrogen and boron doping and that acts as a converter using a semiconductor. The luminescence is obtained at room temperature, and shows a broad luminescence band characteristic of donor-to-acceptor pair recombination. Photoluminescence intensities and carrier lifetimes reflect a sensitivity to nitrogen and boron concentrations. For an LED device, the growth needs to apply low-off-axis substrates. We show by ultra-high-resolution analytical transmission electron microscopy using aberration-corrected electrons that the growth mechanism can be stable and that there is a perfect epitaxial relation from the low-off-axis substrate and the doped layer even when there is step-bunching.

Considerably long carrier lifetimes in high-quality 3C-SiC(111)

As a challenge and consequence due to its metastable nature, cubic silicon carbide (3C-SiC) has only shown inferior material quality compared with the established hexagonal polytypes. We report on ...

Room temperature excitonic spontaneous and stimulated emission properties in ZnO/MgZnO multiple quantum wells grown on sapphire substrate

ZnO/Mg0.2Zn0.8O multiple quantum wells (MQWs) were grown on a c-plane sapphire (Al2O3) substrate by plasma-assisted molecular beam epitaxy. Both the spontaneous and the stimulated emission properties at room temperature (RT) in MQWs were studied under lower and higher excitation densities. The strong emission at 3.330 eV was observed in the MQWs at RT, which was attributed to the free exciton transition by the temperature-dependent photoluminescence measurement. Significantly, RT stimulated emission caused by inelastic exciton–exciton scattering was observed in the ZnO/Mg0.2Zn0.8O MQWs grown on Al2O3. Thus, the exciton binding energy was determined to be 122 meV in our MQWs.

Combined effects of Ga, N, and Al codoping in solution grown 3C–SiC

We report on Ga-doped 3C–SiC epitaxial layers grown on on-axis (0001) 6H–SiC substrates using the vapor-liquid-solid technique and different Si1−xGax melts. The resulting samples have been investigated using secondary ion mass spectroscopy (SIMS), micro-Raman spectroscopy (μ-R) and, finally, low temperature photoluminescence (LTPL) spectroscopy. From SIMS measurements we find Ga concentrations in the range of 1018 cm−3, systematically accompanied by high nitrogen content. In good agreement with these findings, the μ-R spectra show that the Ga-doped samples are n-type, with electron concentrations close to 2×1018 cm−3. As expected, the LTPL spectra are dominated by strong N–Ga donor-acceptor pair (DAP) transitions. In one sample, a weak additional N–Al DAP recombination spectrum is also observed, showing the possibility to have accidental codoping with Ga and Al simultaneously. This was confirmed on a non-intentionally doped 3C–SiC (witness) sample on which, apart of the usual N and Al bound exciton lines,...

Fluorescent silicon carbide as an ultraviolet-to-visible light converter by control of donor to acceptor recombinations

As an alternative to the conventional phosphors in white LEDs, a donor and acceptor co-doped fluorescent 6H-SiC can be used as an ultraviolet-to-visible light converter without any need of rare-earth metals. From experimental data we provide an explanation to how light can be obtained at room temperature by a balance of the donors and acceptors. A steady-state recombination rate model is used to demonstrate that the luminescence in fluorescent SiC can be enhanced by controlling the donor and acceptor doping levels. A doping criterion for optimization of this luminescence is thus proposed.