Abstract
In recent years, visible light communication (VLC) technology has attracted intensive attention due to its huge potential in superior processing ability and fast data transmission. The transmission rate relies on the modulation bandwidth, which is predominantly determined by the minority-carrier lifetime in III-group nitride semiconductors. In this paper, the carrier dynamic process under a stress field was studied for the first time, and the carrier recombination lifetime was calculated within the framework of quantum perturbation theory. Owing to the intrinsic strain due to the lattice mismatch between InGaN and GaN, the wave functions for the holes and electrons are misaligned in an InGaN/GaN device. By applying an external strain that “cancels” the internal strain, the overlap between the wave functions can be maximized so that the lifetime of the carrier is greatly reduced. As a result, the maximum speed of a single chip was increased from 54 MHz up to 117 MHz in a blue LED chip under 0.14% compressive strain. Finally, a bandwidth contour plot depending on the stress and operating wavelength was calculated to guide VLC chip design and stress optimization.
Highlights
The compound semiconductor devices mentioned above operate based on minority-carrier transport and recombination, which are characterized by the minority-carrier lifetime[30]
It is reported that a built-in electric field as high as 2.45 MV/ cm is generated in an In0.2Ga0.8N/GaN quantum well due to the internal strain along the c-axis caused by the large lattice mismatch of GaN and InGaN34
These samples were characterized with continuous-wave photoluminescence (PL) and atomic force microscopy (AFM) measurements
Summary
InGaN/GaN LEDs commonly operate in the wide wavelength band from blue to green. Two blue and green InGaN/GaN LEDs were chosen as typical examples to evaluate the device performance in the available wavelength band. The potential well in the InGaN layer tends to the flat-band condition; the overlap of the wave functions is maximized, and the transition probability is greatly increased (Fig. 4c) This is our proposed mechanism for enhancing the rate of electron-hole recombination for highly efficient LEDs. Internal strain-induced piezoelectric fields have been already demonstrated to modify the band structure in CdSe/CdS Strak superlattices[46] and III-nitride quantum dots[47]. The strain dependence of the 3 dB modulation bandwidth of single-chip blue and green InGaN/GaN LEDs is shown in Fig. 6a computationally and experimentally. The LED modulation bandwidth gradually increases with the decreasing operating wavelength This means that the weakened piezoelectric field rather than the stronger quantum confinement in high indium content QW dominates the optical transition under the strain field. With this simple strain-enhanced technology, the modulation bandwidth can theoretically increase to GHz
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