Liefeng Feng

Size-dependent capacitance study on InGaN-based micro-light-emitting diodes

We report a detailed study on size-dependent capacitance, especially the negative capacitance (NC), in InGaN-based micro-pixelated light-emitting diodes (μLEDs). Similar to conventional broad-area LEDs, μLEDs show NC under large forward bias. In the conventional depletion and diffusion capacitance regimes, a good linear relationship of capacitance with device size is observed. However, the NC under high forward bias shows slight deviation from above-mentioned linear relationship with device size. This behaviour can be understood if the effects of current density and junction temperature on NC are considered. The measured temperature dependence and frequency dispersion of the capacitance underpin this point of view. The NCs of two reference broad-area LEDs were also measured and compared with that of μLED clusters with the same total size. A stronger NC effect is observed in the μLED clusters, which is attributed to the increased number of sidewall defects during fabrication process.

Controlling the microwave characteristic of FeCoB-ZnO granular thin films deposited by obliquely sputtering

A series of FeCoB-ZnO soft magnetic granular films deposited at different oblique angles were prepared by magnetron sputtering system. A variable in-plane uniaxial magnetic anisotropy field from 27.6u2009Oe to 212u2009Oe and an adjustable ferromagnetic resonance frequency from 1.89 GHz to 5.3u2009GHz were obtained in the as-deposited films just by increasing the oblique angle from 15° to 56°. Frequency line-width and effective Gilbert damping factor were both insensitive to the different oblique angles (αeff decreased from 0.036 to 0.03 and Δf decreased from 1.49 to 1.27), which almost satisfied the requirement that fFMR could be tuned independently in a certain frequency range. Besides, the change of dynamic magnetic anisotropy field versus oblique angle was illustrated and analyzed quantitatively compared with the static magnetic anisotropy.

Stimulated emission in GaN-based laser diodes far below the threshold region

We identify that the stimulated emission of GaN laser diodes (LDs) emerges far below the traditionally recognized threshold from both optical and electrical experiments. Below the threshold, the linear-polarized stimulated emission has been the dominating part of overall emission and closely related to resonant cavity. Its intensity increases super linearly with current while that of spontaneous emission increases almost linearly. Moreover, the separation of quasi-Fermi levels of electrons and holes across the active region has already exceeded the photon emission energy, namely, realized the population-inversion.

Electrical Characteristics of GaN-Based Light-Emitting Diodes on Patterned Sapphire Substrates

Patterned sapphire substrate light-emitting diodes display obvious negative capacitance (NC) at large forward biases. This is measured using a method based on a small signal alternating current together with direct I–V plots. The NC in patterned sapphire substrate LEDs grows exponentially with the forward applied voltage. This observation is unexpected and in contrast with Shockley’s p–n junction theory, which only includes an increasing diffusion capacitance and not a NC. However, this result is in good agreement with conventional sapphire substrate LEDs. Furthermore, the negative terminal capacitance confirmed the prediction of Laux and Hess’ theory. The ideal factor of a patterned sapphire substrate LED is about 5, greatly exceeding the traditional theoretical value. The capacitance increased to a maximum and then gradually decreased, which was similar to the results for a p–n junction. Patterned sapphire substrate LEDs can withstand higher voltages than conventional sapphire substrate LEDs. This work could further confirm the existence of NC.

Competitive behavior of photons contributing to junction voltage jump in narrow band-gap semiconductor multi-quantum-well laser diodes at lasing threshold

The junction behavior of different narrow band-gap multi-quantum-well (MQW) laser diodes (LDs) confirmed that the jump in the junction voltage in the threshold region is a general characteristic of narrow band-gap LDs. The relative change in the 1310 nm LD is the most obvious. To analyze this sudden voltage change, the threshold region is divided into three stages by Ithl and Ithu, as shown in Fig. 2; Ithl is the conventional threshold, and as long as the current is higher than this threshold, lasing exists and the IdV/dI-I plot drops suddenly; Ithu is the steady lasing point, at which the separation of the quasi-Fermi levels of electron and holes across the active region (Vj) is suddenly pinned. Based on the evolutionary model of dissipative structure theory, the rate equations of the photons in a single-mode LD were deduced in detail at Ithl and Ithu. The results proved that the observed behavior of stimulated emission suddenly substituting for spontaneous emission, in a manner similar to biological evo...

Well Thickness Dependence of the Internal Quantum Efficiency and Carrier Concentration in GaN-Based Multiple Quantum Well Light-Emitting Diodes

The internal quantum efficiency (IQE) dependence characteristics of seven-well multiple quantum well (MQW) GaN-based light-emitting diodes (LEDs) on well thickness were obtained based on the rate equation without setting specific values for the coefficients. The IQE increased with increasing well thickness until the thickness reached 3.0xa0nm, where the IQE reached a maximum, and then decreased with further increases in well thickness. This IQE well thickness dependence is consistent with that of the measured light emission efficiency. In addition, using various values of the radiative recombination coefficient B, which contained the effects of the carrier density and polarization fields (and was thus dependent on the well thickness), we calculated the rate coefficients. The results indicate that the main factor that is limiting the well thickness dependence of the IQE is Shockley–Read–Hall (SRH) nonradiative recombination. Also, at Bxa0=xa01010 cm3xa0s−1 in a 3.0xa0nm thick well, the optimal values in the rate equation of A, corresponding to the SRH nonradiative recombination, and C, corresponding to the carrier leakage (or Auger recombination), are 2.25xa0×xa0108xa0s−1 and 9.2xa0×xa010−31xa0cm6xa0s−1, respectively. Also, at a given current, the maximum carrier concentration and the minimum radiative recombination lifetime were achieved using a 3.0xa0nm well thickness. Overall, for the seven-well MQW InGaN/GaN LEDs studied, the optimal well thickness was 3.0xa0nm.

First-principles study on the half-metallic properties of the d0 quaternary Heusler compounds: KCaCBr and KCaCI

With ab initio calculations, we studied the structural, electronic, and magnetic properties of quaternary Heusler compounds KCaCX (X = Br and I) adopted stable and metastable phases. We found that the most stable structure is type-3 atomic arrangement configuration where K, Ca, C, and X atoms occupy (0, 0, 0), (0.5, 0.5, 0.5), (0.25, 0.25, 0.25) and (0.75, 0.75, 0.75) positions, respectively. The metastable one is type-1 configuration where K, C, Ca and X occupy (0, 0, 0), (0.5, 0.5, 0.5), (0.25, 0.25, 0.25) and (0.75, 0.75, 0.75) positions, respectively. The results show that they are half-metallic ferromagnets with integer magnetic moments of 2.0 μB at their equilibrium lattice constants. It is also found that the spin-polarization is mainly from the C-2p states. In addition, KCaCBr compound with type-1 configuration shows an electronic transition from ordinary half-metallic state to zero-gap half-metallic state with the changing the lattice parameter.