Visible laser diodes (LDs) based on group III nitride materials have been employed as light sources in many fields such as full-color laser projection, laser lighting, under water communication, and material processing[
Chinese Optics Letters, Volume. 19, Issue 12, 121404(2021)
Reduced threshold current density of GaN-based green laser diode by applying polarization doping p-cladding layer
Absorption induced by activated magnesium (Mg) in a p-type layer contributes considerable optical internal loss in GaN-based laser diodes (LDs). An LD structure with a distributed polarization doping (DPD) p-cladding layer (CL) without intentional Mg doping was designed and fabricated. The influence of the anti-waveguide structure on optical confinement was studied by optical simulation. The threshold current density, slope efficiency of LDs with DPD p-CL, and Mg-doped CL, respectively, were compared. It was found that LDs with DPD p-CL showed lower threshold current density but reduced slope efficiency, which were caused by decreasing internal loss and hole injection, respectively.
1. Introduction
Visible laser diodes (LDs) based on group III nitride materials have been employed as light sources in many fields such as full-color laser projection, laser lighting, under water communication, and material processing[
Distributed polarization doping (DPD) is a newly proposed technique that can provide holes in nitride materials without Mg doping[
In this article, we designed and fabricated GaN-based green LDs with low threshold current density by employing DPD . Optical simulations and calculations were introduced to design and analyze the LDs. Device measurements showed the threshold current density of green LDs was reduced by half (from to ), while the slope efficiency deteriorated. The possible reason for these changes has been explored.
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2. Experimental Details
Two LD structures (named Polar. and Ref. LDs, respectively) were designed, as shown in Fig. 1(a). Low growth temperature and a hybrid layer were used to suppress the thermal budget to QWs during growth, which was demonstrated in our previous work[
Figure 1.(a) Schematic structures of green Polar. and Ref. LDs, respectively. (b) Sample structure for Hall measurements, the dash line shows the current pathway in measurement.
The theoretical hole concentration in the DPD structure can be calculated as follows[
3. Results and Discussion
As the composition-graded layer in Polar. LDs will lead to the anti-waveguide structure, which may influence the optical confinement factor () and substrate mode intensity, the distribution of the optical field was calculated by commercial optical simulation software named MODE SOLUTION developed by Lumerical Inc. More details on the optical simulations and refractive index data for nitride materials at the green wavelength range can be found in Ref. [10]. The simulation results are shown in Fig. 2. The optical confinement factors of QWs are 0.9143% for Polar. LDs and 0.9269% for Ref. LDs, respectively. The difference of is quite small, and thus it will not have obvious impacts on the threshold current density or slope efficiency. We can also find that the intensity of the substrate mode increases from to , which means LDs with DPD may suffer from more severe mode leakage. On the one hand, as the absolute value of the leaked mode and the absorption coefficient of the Si-doped layer are small, the increased substrate mode intensity will not have an obvious impact on LD output power and threshold current density. On the other hand, the enhanced leaked mode will have a negative impact on the far-field pattern of LDs. Both the variation of and substrate mode intensity can be explained by the prominent movement of the optical field to the -side in Polar. LD as the result of higher average Al composition in the DPD layer.
Figure 2.Simulated optical field distribution of (a) LDs with DPD p-CL and (b) LDs with Mg doping p-CL. Insets show the enlarged distribution around p-CL.
Polar. and Ref. structures, respectively, were grown by metal organic chemical vapor deposition. Inductively coupled plasma dry etching was used to form the ridge waveguide LDs. A 200 nm Si dioxide layer was deposited as the insulating layer using inductively coupled plasma chemical vapor deposition on both sides of the ridges. A 200 nm indium tin oxide (ITO) layer was then deposited on top of the ridge using electron beam evaporation. About 100/500 nm of titanium (Ti)/Au was then deposited on top of ITO as a pad and 50/100/50/100 nm of Ti/Al/Ti/Au was deposited on the backside of the wafer to form the electrode. The LD cavity facets were formed by cleaving among the -plane of the GaN crystal and then depositing dielectric films. Then, the ridge LDs were measured under pulse operation at room temperature, and the results are shown in Fig. 3(a). The structure of LDs was the same as the one described above, and the facet reflectivities were 95% and 70%, respectively. The size of the ridge was . As can be seen in Fig. 3(a), the threshold current density has been reduced obviously (from to ). However, the slope efficiency of Polar. LDs decreased greatly compared with that of Ref. LDs (0.07 W/A versus 0.35 W/A). Figure 3(b) shows the lasing spectra of these two LDs. The wavelength difference may be caused by composition variation in InGaN/GaN multi-QWs (MQWs) of these two samples.
Figure 3.(a) Power-current (P-I) curves and (b) laser spectra of green LDs with DPD and conventional Mg doping p-CL, respectively. The solid line in (a) is a guide for the eye.
Then, the reason for improved threshold current density and deteriorative slope efficiency was explored. According to Refs. [24,25], the threshold current density and slope efficiency of the LDs could be expressed as follows:
Applying Eq. (8), the total internal loss in Polar. and Ref. LDs was and , respectively. It should be pointed out that only 150 nm conventional Mg doping near the QW side had been replaced by the DPD layer, and EBL had been removed, but the internal loss of Polar. LDs was almost 50% smaller than that of the Ref. LDs. These results can be well understood by the significant suppression of the overlap of the optical field and Mg doping layer. This indicated the great potential of DPD in decreasing internal loss in nitride LDs. Thus, the threshold current density and slope efficiency can be calculated by Eqs. (5)–(7): and 0.43 W/A for Polar. LDs, and 0.25 W/A for Ref. LDs, respectively. The predicted 50% reduction in threshold current density agrees well with the measurement results.
Since we have confirmed Polar. LDs have smaller internal loss, it is reasonable to speculate that the decreasing injection efficiency is the reason for deteriorative slope efficiency according to Eq. (6). Figure 4 shows the temperature dependent photoluminescence (TDPL) result of Polar. LDs, which indicates that the internal quantum efficiency is 50%. Thus, the injection efficiency is 22%. This value is pretty low compared with the estimated one in the calculations (90%). Actually, GaN-based LDs with DPD show that very low injection efficiency have also been demonstrated in UV-C devices[
Figure 4.TDPL result of Polar. LD. The solid line is the result of Arrhenius fitting.
Figure 5.Current-voltage (I-V) curves of green LDs with DPD and Mg doping p-CL, respectively.
The cause for low injection efficiency has not been figured out yet, and some suppositions have been given in literatures. The consumption induced by point defects in Al-rich AlGaN had been attributed to one of the reasons in UV LDs[
4. Conclusions
GaN-based green LDs with DPD and Mg doping , respectively, were designed and fabricated. LDs with DPD showed reduced threshold current density but lower slope efficiency. Reduction of internal loss by DPD is the main reason for decreasing the threshold current density. The low slope efficiency is attributed to small low injection efficiency, which may be caused by poor hole transportation in the vertical direction. This research proves the possibility to decrease threshold current density by DPD, while further work to improve hole injection is needed.
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Lingrong Jiang, Jianping Liu, Lei Hu, Liqun Zhang, Aiqin Tian, Wei Xiong, Xiaoyu Ren, Siyi Huang, Wei Zhou, Masao Ikeda, Hui Yang, "Reduced threshold current density of GaN-based green laser diode by applying polarization doping p-cladding layer," Chin. Opt. Lett. 19, 121404 (2021)
Category: Lasers, Optical Amplifiers, and Laser Optics
Received: Apr. 20, 2021
Accepted: Jun. 7, 2021
Published Online: Sep. 22, 2021
The Author Email: Jianping Liu (jpliu2010@sinano.ac.cn)