1.
Introduction
GaN-based laser diodes (LDs), owing to their broad stimulated wavelength range and small device size, have attracted a great deal of attention to the applications in ultraviolet (UV) Raman spectroscopy[1], optical coherence tomography[2], biomedical science[3], underwater wireless optical communication[4, 5], and laser-based televisions (TVs) and small portable projectors[6–9] since Nakamura and his coworkers demonstrated the first GaN-based blue-violet LDs in the 1990s[10, 11]. Especially, GaN-based blue-violet LDs can be used in high-density optical data storage systems[12–15] and the next generation optical atomic clocks[16–18]. Therefore, GaN-based LDs with a steady output and a long working lifetime are expected. In fact, in 2013 and 2017 our group had successively demonstrated the blue-violet LD array with a maximum optical power of 7.5 W[19] and the blue-violet LD with a peak optical power of 20 W[20], respectively, under a pulsed injection current. Their threshold current density was around 3.5 kA/cm2. In this paper, we demonstrate the room temperature (RT, 25 °C) continuous-wave (CW) operation of GaN-based blue-violet LD with a lifetime longer than 1000 h.
2.
Experiments
In this work, GaN-based blue-violet LDs are grown on c-plane GaN substrate by metal organic chemical vapor deposition (MOCVD). The trimethylgallium (TMGa) or triethylgallium (TEGa), trimethylaluminum (TMAl), trimethylindium (TMIn), NH3, dicyclopentadienyl magnesium (Cp2Mg), and SiH4 are used as the Ga, Al, In, N, Mg, and Si sources, respectively. The LDs are grown in proper sequence, and the epitaxial structure is shown in Fig. 1. Firstly, a thick n-GaN layer is grown on GaN substrate, and then an n-AlGaN cladding layer (CL), an n-GaN lower waveguide (LWG) layer, InGaN/GaN multi-quantum wells (MQWs), a p-AlGaN electron-blocking layer (EBL), a p-GaN upper waveguide (UWG) layer, a p-AlGaN cladding layer, a p-GaN layer, and a very thin heavily Mg-doped GaN (p++-GaN) layer are grown subsequently. In addition, a 10-μm-wide ridge stripe is formed by dry etching along the <1-100> direction, and a 600- μm-long cavity is fabricated by cleaving along the {1-100} plane after grounding and polishing the GaN substrate to reduce its thickness. The front and rear cleaved cavity facets are coated, and the reflectivity is 10% and 90%, respectively. Moreover, Ti/Pt/Au and Pd/Pt/Au are used to form Ohmic contacts with n-GaN substrate and p-GaN contact layer, respectively.
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Figure1.
Schematic diagram of the epitaxial structure for the GaN-based blue-violet LDs.
3.
Results and discussions
The optical spectrum of stimulated emission from the GaN-based LD under CW operation is shown in Fig. 2. It can be seen that the peak wavelength is around 413 nm, and the full width at half maximum (FWHM) of the spectrum is small, i.e. about 0.4 nm. In addition, the inset of Fig. 2 shows the far field pattern of the laser beam when the blue-violet LD illuminates a page of white paper. These results indicate the success in the fabrication of GaN-based blue-violet LD with a lasing wavelength of around 413 nm and CW operated at room temperature.
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Figure2.
(Color online) The optical spectrum of stimulated emission for a GaN-based blue-violet LD. The inset shows the far field pattern of laser beam.
Fig. 3 shows the voltage and optical power of GaN-based blue-violet LD as a function of the forward direct-current (P–I–V) measured at room temperature. It is observed that the optical power increases sharply and lasing starts when the injection current is higher than 87.6 mA. Thus, the threshold current is taken as 87.6 mA, and the corresponding threshold current density is 1.46 kA/cm2. In addition, the peak optical power is larger than 600 mW under an injection current of 800 mA, and the corresponding current density is around 13.33 kA/cm2.
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Figure3.
(Color online) Power–current–voltage (P–I–V) curves of a GaN-based blue-violet LD at room temperature.
It is found that the operating voltage under the threshold current, i.e. threshold voltage, is as low as 4.1 V. Such a low threshold voltage may be mainly attributed to our previous works on the improvement of the p-GaN Ohmic contact. In our previous study, we demonstrated the influence of residual carbon impurities in the heavily Mg-doped GaN (p++-GaN) layer on the performance of p-GaN Ohmic contact. A low specific contact resistivity (ρc) of 6.8 × 10?5 Ω·cm2 is obtained by properly controlling the residual carbon impurity incorporation in the p++-GaN layer[21, 22]. Recently, the p-GaN Ohmic contact has been significantly improved further. The specific contact resistivity is characterized by current–voltage (I–V) measurements based on the circular transmission line model (CTLM) with a contact inner radius (r) of 200 μm and eight different out radius (R) ranged from 215 to 260 μm. The details about the CTLM experimental method can be found in Refs. [21, 22]. As shown in Fig. 4, the I–V curves are straight and the specific contact resistance is as low as 1.1 × 10?6 Ω·cm2. It implies that an excellent Ohmic contact is obtained, and it is beneficial to achieving a small threshold voltage for GaN-based LD.
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Figure4.
(Color online) (a) Current–voltage characteristics obtained for different values of R between 215 and 260 μm, and (b) the measured data of total resistance (Rt, black squares) as a function of ln(R/r) and the fitting line (red), by using the circular transmission line model for p-GaN Ohmic contact in the GaN-based blue-violet laser diode. The specific contact resistance is as low as 1.1 × 10?6 Ω·cm2.
We have taken the aging experiment to check the lifetime of blue-violet LDs, which is performed at room temperature under a fixed current injection. Fig. 5 shows the optical output power as the function of aging time. It can be seen that the optical power decreases slightly along with the increasing aging time, and the optical power keeps larger than 100 mW as the aging time reaches up to around 1067 h. It demonstrates that the lifetime of GaN-based blue-violet LD is longer than 1000 h under a room-temperature continuous-wave operation. It is noted that output power has declined after the LD operates for the right time, which is supposed to be caused by two reasons. First, the increasing temperature due to the accumulation of the heat process can decrease the slope efficiency and reduce the output power. Second, it may be that there are a few dislocations in GaN-based materials, which can reduce the output power along with increasing aging time. Actually, some necessary technological procedures have been taken in the device fabrication process in order to avoid any failure happening in the early stage of the device operation. We think the long lifetime obtained for the blue-violet LDs is due to our previous work for the improvement of Ohmic contact and material growth, and due to the new LD structure design. In details, first, as mentioned above, an excellent Ohmic contact with a very low specific contact resistance is obtained, which is good for reducing operation voltage and contact resistance. Second, the good p-AlGaN materials are grown by reducing the carbon and hydrogen concentration through controlling the growth conditions[23–25]. Third, the emission efficiency of InGaN/GaN MQWs is enhanced by reducing the carbon concentration in MQWs[26]. Fourth, a new LD structure to reduce optical loss and leakage current is proposed and which is beneficial to decreasing the threshold current[27]. Therefore, low threshold current density and voltage can be obtained for the GaN-based blue-violet LD in this study.
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Figure5.
Optical power of blue-violet LD as a function of the aging time with a fixed injection current at room temperature.
4.
Conclusion
In summary, the continuous-wave GaN-based blue-violet LDs with a lifetime longer than 1000 h operated at room temperature are demonstrated. The blue-violet LDs are grown on c-plane GaN substrate by MOCVD, and a 10 × 600 μm2 ridge waveguide structure device is fabricated. The stimulated emission wavelength of LD is about 413 nm, and the peak optical power is larger than 600 mW. Moreover, the threshold current density and voltage are as low as 1.46 kA/cm2 and 4.1 V, respectively. The fabricated GaN-based blue-violet LDs successfully show a long cw working lifetime.
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Acknowledgements
This work was supported by the National Key R&D Program of China (Nos. 2016YFB0401801, 2016YFB0400803), the Science Challenge Project (No. TZ2016003), the National Natural Science Foundation of China (Nos. 61674138, 61674139, 61604145, 61574135, 61574134, 61474142, 61474110), and the Beijing Municipal Science and Technology Project (No. Z161100002116037).
