The performance of nitride-based LEDs was improved by inserting dual stage and step stage InGaN/GaN strain relief layer (SRL) between the active layer and n-GaN template. The influences of step stage InGaN/GaN SRL on the structure, electrical and optical characteristics of GaN-based LEDs were investigated. The analysis of strain effect on recombination rate based k·p method indicated 12.5% reduction of strain in InGaN/GaN MQWs by inserting SRL with step stage InGaN/GaN structures. The surface morphology was improved and a smaller blue shift in the electroluminescence (EL) spectral with increasing injection current was observed for LEDs with step stage SRL compared with conventional LEDs. The output power of LEDs operating at 20 mA was about 15.3 mW, increased by more than 108% by using step stage InGaN/GaN SRL, which shows great potential of such InGaN/GaN SRL in modulating InGaN/GaN MQWs optical properties based on its strain relief function.

This paper presents a visible light positioning system using a single light-emitting diode (LED) and a novel receiver. In the proposed system, the receiver is composed of a horizontal photodetector (PD) and two tilted PDs, and a known calibration point is deployed to improve the accuracy of the positioning. Based upon the multi-PDs' relative positions and the received signal strength, the location of the receiver can be estimated regardless of the LED's and PDs' characteristics. The influences of tilted PDs on the positioning errors are discussed, i.e., the polar angle α and the difference θ between two azimuth angles. The experimental results show that high accuracy can be achieved when α = 20°and θ ranges from 30°to 90°. The average and maximum errors are 2.15 and 4.01 cm, respectively, in the case of α = 20°and θ = 45°. The proposed system avoids the intercell interference caused by different transmitters and can be applied in scenarios with limited LEDs.

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Envelope-function analysis of wurtzite InGaN/GaN quantum well light emitting diodes

The relation between the peak-to-average-power-ratio (PAPR) reduction schemes and the transmission performance of the orthogonal frequency division multiplexing (OFDM) based visible light communication (VLC) system is experimentally investigated. The linear selective mapping (SLM) scheme and the nonlinear logarithmic companding scheme are optimized by considering both PAPR reduction and bit error rate (BER) performance. It is demonstrated that the logarithmic companding scheme, albeit providing larger PAPR reduction when compared to the SLM scheme, may result in worse BER performance due to the additional noise induced in its expanding process. Both numerical and experimental investigations show that, at the expense of increased complexity and reduced efficiency, the VLC system using the linear SLM scheme exhibits better BER performance compared to the system using the nonlinear logarithmic companding scheme. We show that for a 400-Mb/s transmission at a distance of 1 m, the BER can be reduced from 2.02 × 10 to 2.51 × 10 by using logarithmic companding scheme. By using the linear SLM scheme, the BER can be further reduced to 1.77 × 10.

Staggered AlGaN quantum wells (QWs) are designed to enhance the transverse-electric (TE) polarized optical emission in deep ultraviolet (DUV) light- emitting diodes (LED). The optical polarization properties of the conventional and staggered AlGaN QWs are investigated by a theoretical model based on the k·p method as well as polarized photoluminescence (PL) measurements. Based on an analysis of the valence subbands and momentum matrix elements, it is found that AlGaN QWs with step-function-like Al content in QWs offers much stronger TE polarized emission in comparison to that from conventional AlGaN QWs. Experimental results show that the degree of the PL polarization at room temperature can be enhanced from 20.8% of conventional AlGaN QWs to 40.2% of staggered AlGaN QWs grown by MOCVD, which is in good agreement with the theoretical simulation. It suggests that polarization band engineering via staggered AlGaN QWs can be well applied in high efficiency AlGaN-based DUV LEDs.

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