Optimization of Carrier Distributions in Periodic Gain Structures toward Blue VCSELs
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Optimization of Carrier Distributions in Periodic Gain Structures toward Blue VCSELs Kenjo Matsui1, Kosuke Horikawa1, Yugo Kozuka1, Kazuki Ikeyama1, Daisuke Komori1, Tetsuya Takeuchi1, Satoshi Kamiyama1, Motoaki Iwaya1, and Isamu Akasaki1, 2 1 Faculty of Science and Technology, Meijo University, Nagoya, 468-8502, Japan. 2 Graduate School of Engineering and Akasaki Research Center, Nagoya University, Nagoya, 464-8603, Japan. ABSTRACT We have fabricated light emitting diodes (LEDs) in which two active regions separated with a Mg-doped GaN intermediate layer were placed in a single pn junction toward periodic gain structures (PGS) for blue vertical-cavity surface emitting lasers (VCSELs). By current density dependence on a emission intensity ratio from two different active regions, we obtained a very stable emission intensity ratio over 1 kA/cm2. This result is also confirmed with the simulation result. Furthermore, we found that the difference of emission wavelength affect the carrier injection and the emission intensity ratio. On the basis of this result, the optimized wellbalanced Mg concentration in the intermediate layer for the two identical active regions were estimated approximately 5 x 1018 cm-3. INTRODUCTION Recently nitride-based VCSELs have been achieved [1-5]. Some of them reported their continuous-wave operations but with low slope efficiencies. In order to obtain higher gain and higher slope efficiency, a PGS has been proposed [6]. So far the PGS has been applied to GaAsbased and InP-based VCSELs [7]. The PGS consists of two or more active regions which are placed at the antinode positions of the electric field in the VCSEL cavity, resulting in a lager active volume and a lager optical confinement factor. Also, the PGS should be useful to suppress the efficiency droop issue typically observed in GaN-based LEDs since carrier concentrations in the active region will be diluted due to the lager active volume. While the PGS structure has such advantages, a uniform carrier injection into the multiple active regions could be an issue if electrons and holes show asymmetric carrier transport characteristics due to different effective masses and mobility values. Especially GaN-based materials show largely different effective masses and mobility values between electrons and holes, so an imbalanced carrier injection is concerned in nitride-based active regions, for instance, GaInN/GaN multiple quantum wells (MQWs). We have investigated carrier injections into two separated GaInN/GaN MQW active regions in a single pn junction, and then found that a wellbalanced emission intensity ratio was obtained from the two active regions with a Mg-doped intermediate layer. In this experiment we had to use the two active regions which contained different emission wavelengths each other, so that we discriminated the emission intensities from the two active regions. However, the carrier injection could be affected by the emission wavelength, in other word, a potential drop at the active region. That it is not obvious if a uniform
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