⇦Chinese Physics Letters, 2017, Vol. 34, No. 1, Article code 017101 Performance Improvement of GaN-Based Violet Laser Diodes * De-Gang Zhao(赵德刚)1,2**, De-Sheng Jiang(江德生)1, Ling-Cong Le(乐伶聪)1, Jing Yang(杨静)1, Ping Chen(陈平)1, Zong-Shun Liu(刘宗顺)1, Jian-Jun Zhu(朱建军)1, Li-Qun Zhang(张立群)3 Affiliations 1State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083 2University of Chinese Academy of Sciences, Beijing 100049 3Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou 215123 Received 13 October 2016 *Supported by the National Key Research and Development Program of China under Grant No 2016YFB0401801, the National Natural Science Foundation of China under Grant Nos 61574135, 61574134, 61474142, 61474110, 61377020, 61376089, and 61223005, and the One Hundred Person Project of the Chinese Academy of Sciences.
**Corresponding author. Email: dgzhao@red.semi.ac.cn Citation Text: Zhao D G, Jiang D S, Le L C, Yang J and Chen P et al 2017 Chin. Phys. Lett. 34 017101 Abstract The influences of InGaN/GaN multiple quantum wells (MQWs) and AlGaN electron-blocking layers (EBL) on the performance of GaN-based violet laser diodes are investigated. Compared with the InGaN/GaN MQWs grown at two different temperatures, the same-temperature growth of InGaN well and GaN barrier layers has a positive effect on the threshold current and slope efficiency of laser diodes, indicating that the quality of MQWs is improved. In addition, the performance of GaN laser diodes could be further improved by increasing Al content in the AlGaN EBL due to the fact that the electron leakage current could be reduced by properly increasing the barrier height of AlGaN EBL. The violet laser diode with a peak output power of 20 W is obtained. DOI:10.1088/0256-307X/34/1/017101 PACS:71.20.Nr, 71.55.Eq, 78.55.Cr © 2017 Chinese Physics Society Article Text Due to their extensive applications in optoelectronics and microelectronics, the study on gallium nitride (GaN) and related nitrides has been focused for many years, and great successes have been obtained so far.[1-4] For example, GaN-based ultraviolet photodetectors focal plane arrays have been fabricated, GaN-based high brightness white light-emitting-diodes have been commercialized, and even high performance GaN laser diodes (LD) have been realized.[5,6] In particular, GaN laser diodes could be used in the fields of high-density storage, full-color displays, medical and industrial applications. However, high performance of GaN laser diodes can only be fabricated by a few groups, such as Nichia chemical industries. This is because the technique of fabricating GaN laser diodes is still kept as a challenge. It is well-known that the high performance GaN laser diodes are usually grown on GaN substrates instead of sapphire ones because of the low-density defects and simple device processing. InGaN/GaN multiple quantum wells (MQWs) are the active region layer in the GaN-based laser diodes.[7-9] Numerous research works have been reported about the growth of InGaN MQWs on more popular and cheap sapphire substrate. However, there are some property differences between sapphire and GaN substrates. In particular, the stress is very different in these two kinds of substrates.[10,11] As a result, to obtain high quality GaN LD MQW structures, even though we have known how to grow them on the sapphire substrate very well, the research on their growth on GaN substrate is still very important. In addition, the injection current density of GaN-based laser diodes is very high, thus it is very important to reduce the electron leakage through adjusting the structure parameters of laser diodes.[12-15] High quality InGaN MQWs and optimized structure parameters are helpful to improve the performance of GaN laser diodes. In this Letter, the GaN-based violet laser diodes grown on GaN substrate are investigated. Compared with the InGaN MQWs grown with different growth temperatures for the InGaN wells and GaN barriers, it is found that the InGaN MQWs sample grown using the same growth temperature for both wells and barriers has a much higher electro-luminescence intensity. It is thought that such an improvement may result from overcoming the negative substrate bowing effect. It is also found that increasing the Al content in AlGaN electron-blocking layer (EBL) leads to a lower leakage current and a better device performance. The simulation results confirm the positive effect of EBL. The GaN-based violet laser diodes in this work are grown on the $c$-plane GaN substrate by metal-organic chemical vapor deposition (MOCVD). Firstly, thick GaN layers are grown on GaN substrate, then the LD structure is grown in proper sequence, which is shown in Fig. 1. The LD is composed of n-AlGaN cladding layer, n-GaN waveguide layer, InGaN MQWs, p-AlGaN EBL, p-GaN waveguide layer, p-AlGaN cladding layer, and p-GaN Ohmic contact layer. The TMGa, TMAl, TMIn and NH$_{3}$ are used as Ga, Al, In and N sources, respectively. Five broad-area stripe emitters are integrated monolithically into one laser chip. Broad-area stripes are used for laser diodes with high power output due to their higher level of the catastrophic optical damage on the front laser facet. In our laser diode's chip, the width of each stripe emitter is 30 μm and the interval between every two stripes is 10 μm. After the GaN substrate is polished and its thickness is reduced to less than 100 μm, the cavity of the laser diode with a length of 800 μm is fabricated by cleaving the epitaxial film and substrate together along the {1100} plane. The front and rear cleaved facets are coated to reflectivities of 10% and 90%, respectively. As a part of the vertical structure, the n-type electrode is made on the bottom of the GaN substrate. The Pd/Pt/Au and Ti/Pt/Au metals are used for the p-type and n-type electrodes, respectively. The laser diodes are operated under a pulsed injection current with a frequency of 10 kHz, and the width of the pulse is set to be 100 ns. Three typical GaN violet laser diodes are prepared in this experiment.
cpl-34-1-017101-fig1.png
Fig. 1. Schematic diagram of the GaN-based violet laser diode epitaxial layer structure.
Figure 2 shows the output light peak power as a function of the pulsed injection current of the three GaN-based violet LDs: LD1, LD2 and LD3. The InGaN MQW in LD1 is grown at different temperatures, which are 760$^{\circ}\!$C and 840$^{\circ}\!$C for the InGaN wells and GaN barriers, respectively. However, the InGaN MQWs in LD2 and LD3 are grown at the same temperature, both well and barrier layers are at 760$^{\circ}\!$C. The structure parameters and other growth conditions for LD1 and LD2 are the same, while LD2 and LD3 have a structural difference in the Al composition $x$ of Al$_{x}$Ga$_{1-x}$N EBL. It is 0.10 for the former and 0.18 for the latter. The inset shows the optical spectrum of LD3, the peak wavelength is around 411 nm. We will discuss the optical characteristics of LD1 and LD2 firstly. As shown in Fig. 2, the three output power curves are very different, and we will firstly investigate the characteristic difference between LD1 and LD2. The threshold currents of LD1 and LD2 are 7.4 kA/cm$^{2}$ and 4.2 kA/cm$^{2}$, respectively. The slope efficiencies of LD1 and LD2 are 0.32 A/W and 0.59 A/W, respectively. The growth temperature and characterized results are listed in Table 1. Obviously, the performance of LD2 is much better than LD1. As the active region layer, it seems that the same-temperature-grown InGaN MQW is much better than that grown with different temperatures for the InGaN wells and GaN barriers.
cpl-34-1-017101-fig2.png
Fig. 2. The output light peak power as a function of the pulsed injection current of the three GaN-based violet LDs: LD1, LD2 and LD3. The inset shows the optical spectrum of LD3, and the peak wavelength is around 411 nm.
Table 1. Growth condition and characteristic results of three GaN violet laser diodes.
Samples Growth temperature ($^\circ\!$C) Al content in AlGaN EBL layers Threshold current (kA/cm$^{2}$) Slope efficiency (W/A)
LD1 760 840 10% 7.4 0.32
LD2 760 760 10% 4.2 0.59
LD3 760 760 18% 3.3 0.92
We have investigated the electro-luminescence (EL) spectra of LD1 and LD2 before stimulated emission. As shown in Fig. 3, the peak wavelengths of two LDs are all about 413 nm, while the integrated intensity of LD2 is about 3 times stronger than that of LD1, which are about 116.1 and 381.5 for LD1 and LD2, respectively. In fact, we have also studied LEDs with the InGaN MQWs grown on sapphire substrates by two different methods. One method is the growth at the same-temperature, just like the active region of LD2, the other is the two-temperature-growth, just like the active region of LD1. However, there is nearly no difference in the EL intensity output for the two samples (not shown here). This implies that the GaN substrate has certain different properties from the sapphire substrate to induce influences on the quality of grown MQW structures. One possible reason is that the same-temperature growth is beneficial to reducing the incorporation of unwanted impurities during the growth, but it cannot explain why the growth on GaN substrates is different from that on sapphire substrates. Another more plausible reason to explain such a phenomenon is as follows. As was reported, the GaN substrate has different curvatures under different growth temperatures due to the thermal expansion effect.[10,11] It is deduced that the GaN substrates used in this work may have different curvatures when the growth temperature changes during the well and barrier layer growth. In fact, the GaN substrate has more serious bowing level at 840$^{\circ}\!$C than that at 760$^{\circ}\!$C. Then the real temperature distribution or the gas flow over the GaN substrate will also be changed under two different growth temperatures. In particular, the incorporation condition of indium into the InGaN well layers and the related defects will be different. As a result, the In content distribution in the InGaN layer will be more inhomogeneous and it will be very difficult for the interface of InGaN MQWs to become smooth. The EL intensity is thus reduced. Instead, if the same-temperature growth method is used, although the bowing of GaN substrate still exists, the gas flow distribution over the substrate and the substrate temperature is almost the same whether the growth layer is well or barrier. It is noted that the surface curvature of the quartz substrate holder in our MOCVD equipment is modified to be compatible to the substrate's curvature at the growth temperature. In this case, the interface of InGaN MQWs will become much smoother, and the EL intensity will be improved. Thus the EL intensity of LD2 is much stronger than that of LD1. By using the same-temperature growth of InGaN MQW, the lasing performance of LD2 is also improved in comparison with LD1. It seems that even the growth temperature of GaN barrier layer is much lower to grow high quality GaN, the negative bowing effect of the GaN substrate could be overcome by employing the same-temperature-grown InGaN MQWs, and the quality of the MQW active region layer in the structure of laser diodes can be essentially improved.
cpl-34-1-017101-fig3.png
Fig. 3. The EL spectra of LD1 and LD2 before stimulated emission. The inset shows that GaN substrates have different curvatures for the high-temperature growth.
From the experimental result shown in Fig. 2, it could also be observed that the performance of LD3 is better than that of LD2. The threshold current densities are 4.2 kA/cm$^{2}$ and 3.3 kA/cm$^{2}$ for LD2 and LD3, respectively. The slope efficiencies are 0.59 and 0.92 W/A for LD2 and LD3, respectively. The peak output power of LD3 is about 20 W. The detailed characterization results and the difference of LD2 and LD3 are listed in Table 1. The growth conditions of InGaN MQWs for LD2 and LD3 are the same, while the Al contents in their AlGaN EBLs are different, which are 10% and 18% for LD2 and LD3, respectively. This indicates that the AlGaN EBL also plays an important role in determining the performance of GaN violet laser diodes. To obtain a deep insight into the effect of AlGaN EBL on the performance of GaN violet laser diodes, a theoretical calculation is carried out. Figure 4 shows the output optical power-current curves of GaN violet laser diodes with different AlGaN EBLs simulated by commercial software LASTIP, a powerful tool in analysis of the performance of semiconductor laser diodes.[16,17] As shown in Fig. 4, the calculated threshold current and slope efficiency of GaN laser diodes are strongly dependent on the Al content in AlGaN EBL. The threshold currents are 71.8, 54.1, and 48.5 mA for the Al contents of 10%, 20% and 25%, respectively. The slope efficiencies are 0.44, 0.92 and 1.1 A/W for the Al contents of 10%, 20% and 25%, respectively. Clearly, with the increase of Al content, the threshold current decreases and the slope efficiency increases. Therefore, the simulation results demonstrate that the performance of GaN violet laser diodes could be improved by suitably increasing the Al content in AlGaN EBL.
cpl-34-1-017101-fig4.png
Fig. 4. The output optical power-current curves of GaN violet laser diodes with different AlGaN EBLs simulated by commercial software LASTIP, where the Al contents in AlGaN are 10%, 20% and 25%, respectively.
Due to the high injection current density in the GaN laser diodes, it is necessary to block the electrons from escaping from the InGaN MQWs by employing AlGaN EBL. It is reasonable to assume that more electrons will be restricted in the InGaN MQWs and less leakage current will occur after a higher barrier is formed by using the AlGaN EBL with a higher Al content. We have simulated the dependence of the leakage current on the Al content of AlGaN EBL. As shown in Fig. 5, it is found that the LDs have different leakage currents with different AlGaN EBLs. The leakage currents are 1029.41, 439.11 and 112.64 A/cm$^{2}$ for Al contents of 10%, 20% and 25%, respectively. Obviously, the leakage current decreases with the increase of Al content in AlGaN EBL. The inset of Fig. 5 shows the electron distribution in the InGaN MQW. It is observed that the first and second wells almost have the same electron accumulation. However, the electron distribution in the third well is quite different. With the increase of Al content in AlGaN EBL, the electron concentration in the third well increases. The electron accumulation before the AlGaN EBL also increases. It is obtained that the performance of GaN violet laser diodes could be improved by using AlGaN EBL with a relatively higher Al content up to 0.25, even as yet during the experimental test of MQW diodes we still did not grow an EBL with Al content as high as 0.25.
cpl-34-1-017101-fig5.png
Fig. 5. The dependence of the leakage current on the Al content of AlGaN EBL in GaN-based violet LDs simulated by LASTIP, where the Al contents in AlGaN are 10%, 20% and 25%, respectively.
In summary, we have investigated the influences of the growth condition of InGaN MQWs and AlGaN EBLs on the performance of GaN based violet laser diodes grown on GaN substrates. It is found that the same-temperature-grown InGaN MQWs for the InGaN well and GaN barrier layers have a positive influence on the threshold current and slope efficiency of laser diodes, mainly due to the fact that the negative bowing effect of GaN substrates is suppressed. It is also found that the performance of GaN violet laser diodes could be further increased by properly increasing the Al content in AlGaN EBL. The simulation results prove that the electron leakage current could be reduced by increasing the barrier height of AlGaN EBL. Overcoming the influence of bowing of GaN substrates during the growth and reducing the electron leakage current are very important for the fabrication of high quality GaN-based laser diodes. Finally, the violet laser diode with a peak output power of 20 W is obtained. References Metalorganic vapor phase epitaxial growth of a high quality GaN film using an AlN buffer layerGaN Growth Using GaN Buffer LayerGaN-on-Si blue/white LEDs: epitaxy, chip, and packageVariation of efficiency droop with quantum well thickness in InGaN/GaN green light-emitting diodeTable of contentsThe Roles of Structural Imperfections in InGaN-Based Blue Light-Emitting Diodes and Laser DiodesHigh-power blue-violet AlGaN-cladding-free m-plane InGaN/GaN laser diodes8 W single-emitter InGaN laser in pulsed operationRealization of InGaN laser diodes above 500 nm by growth optimization of the InGaN/GaN active regionUniformity of the wafer surface temperature during MOVPE growth of GaN-based laser diode structures on GaN and sapphire substrateStudies about wafer bow of freestanding GaN substrates grown by hydride vapor phase epitaxyHigh-power GaN-based blue-violet laser diodes with AlGaN∕GaN multiquantum barriersOrigin of InGaN light-emitting diode efficiency improvements using chirped AlGaN multi-quantum barriersAn improved multi-layer stopper in a GaN-based laser diodeSuppression of electron leakage by inserting a thin undoped InGaN layer prior to electron blocking layer in InGaN-based blue-violet laser diodesSuppression of electron leakage in 808 nm laser diodes with asymmetric waveguide layer
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