Design and fabrication of 940 nm vertical-cavity surface-emitting lasers

Abstract

A high slope efficiency vertical-cavity surface-emitting laser (VCSEL) is described. The InGaAs/GaAsP strain compensated multiple quantum wells (MQWs) are designed by PICS3D. The wavelength redshift occurs due to the thermal effect, the lasing wavelength of MQWs is designed to be around 928 nm. The active region consists of five compressively strained 4.4 nm thick In<sub>0.16</sub>Ga<sub>0.84</sub>As quantum wells separated and surrounded by 6.2 nm thick GaAs<sub>0.88</sub>P<sub>0.12</sub> tensile strained compensation layers to obtain the high quantum efficiency and ensure the stress release. Subsequently, the MQWs are grown by metal-organic chemical vapor deposition (MOCVD) and the photoluminescence (PL) spectrum is measured using an Nd:YAG laser (532 nm excitation), of which the peak wavelength is approximately 928 nm and the full width at half maximum is nearly 17.1 nm. The resonant cavity is surrounded by p- and n-DBRs. The n-DBRs are designed to be a 28-period AlAs/Al<sub>0.12</sub>Ga<sub>0.88</sub>As and 3.5-period Al<sub>0.90</sub>Ga<sub>0.10</sub>As/Al<sub>0.12</sub>Ga<sub>0.88</sub>As, and the p-DBR is designed to be a 23-period Al<sub>0.90</sub>Ga<sub>0.10</sub>As/Al<sub>0.12</sub>Ga<sub>0.88</sub>As. The thickness of each a material is <inline-formula><tex-math id="M2">\begin{document}$\lambda/4n$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20181822_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20181822_M2.png"/></alternatives></inline-formula> (<inline-formula><tex-math id="M3">\begin{document}$\lambda$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20181822_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20181822_M3.png"/></alternatives></inline-formula> = 940 nm, <i>n</i> represents refractive index), and 20 nm graded layer is inserted in the interface between two types of materials. The p-/n-DBRs’ experiment PL reflection spectra (using a white illuminant) are carried out, the central wavelength is around 938.7 nm, and the reflectivity values of p-/n-DBRs are nearly 99.0% and 99.7%, respectively. The VCSELs are grown by MOCVD technique, and treated by dry etching, wet oxidation, metal electrode technology and other processes. In the process of dry etching, the top mesa is treated by inductively coupled plasma with BCl<sub>3</sub> and Cl<sub>2</sub> chemistry. In order to expose the oxide layer the wet oxidized process is carried out, and the etching depth is nearly 3500 nm. An oxidation furnace is heated for 15 min prior to oxidation. Then the oxide aperture is shaped by the wet nitrogen oxidation furnace at 425 °C with an N<sub>2</sub> flow of 200 sccm, and the oxide rate is approximately 0.40 <inline-formula><tex-math id="M4">\begin{document}${\text{μm}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20181822_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20181822_M4.png"/></alternatives></inline-formula>/min for A<sub>0.98</sub>Ga<sub>0.02</sub>As. The diameter of oxide aperture is made into an 8 <inline-formula><tex-math id="M5">\begin{document}${\text{μm}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20181822_M5.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20181822_M5.png"/></alternatives></inline-formula> diameter. In the process of metal electrode technology, AuGeNi alloy is sputtered on the top surface to form p-type ohmic contact, and Ti/Pt/Au is evaporated on the back surface of substrate to form an n-type ohmic contact. Rapid thermal annealing at 350 °C in a nitrogen atmosphere is carried out subsequently to obtain a good-quality ohmic contact. Finally, we test the VCSELs’ <i>L-I-V</i> characteristics and spectra in different areas. In area 1, room-temperature lasing at around 940 nm is achieved with a threshold current of 0.95 mA, a slope efficiency of 0.96 W/A, and an output power of 4.75 mW. In area 2, threshold current is 1 mA, a slope efficiency is 0.81 W/A at 25 °C and threshold current is 1.9 mA, slope efficiency is 0.57 W/A at 25 °C. The output power values reach up to 3.850 mW and 2.323 mW at 25 °C and 80 °C, respectively.