How to precisely control the cavity length of gallium nitride based vertical cavity surface emitting lasers?

Gallium nitride (GaN) vertical cavity surface emitting laser (VCSEL) is a semiconductor laser diode with broad application prospects in various fields such as adaptive headlights, retinal scanning displays, nursing point testing systems, and high-speed visible light communication systems. Their high efficiency and low manufacturing costs make them particularly attractive in these applications.

Gallium nitride purple surface emitting laser with a power conversion efficiency exceeding 20%. Source: Tetsuya Takeuchi/Minato University

GaN-VCSEL consists of two special semiconductor mirrors called Distributed Bragg Reflectors (DBRs), separated by an active GaN semiconductor layer in the middle, forming an optical resonant cavity where laser is generated. The length of the resonant cavity is crucial for controlling the target laser wavelength (i.e. resonant wavelength).

So far, two VCSEL structures based on gallium nitride have been developed: one is the bottom dielectric DBR, and the other is the bottom aluminum indium nitride (AlInN)/gallium nitride DBR. Both structures can generate VSCEL with optical output power exceeding 20 milliwatts and wall plug efficiency (WPE) exceeding 10%. However, the stopping wavelength bandwidth of AlInN/GaN DBR is narrow, so VCSEL can only emit light within a narrow wavelength range.

In addition, traditional cavity length control methods require pre experiments on the test cavity layer to determine its growth rate, which can lead to errors between the estimated and final thickness of the VCSEL cavity. This error can cause the resonance wavelength to exceed the narrow stopping bandwidth of AlInN/GaN DBR, seriously affecting performance.

Innovation in cavity length control
To address this issue, in a recent study, researchers led by Professor Tetsuya Takeuchi from the Department of Materials Science and Engineering at Nagagi University in Japan developed a new in-situ cavity length control method for gallium nitride based VCSEL optical cavities. By using in-situ reflectance spectroscopy to accurately control the growth of gallium nitride layers, researchers achieved precise cavity length control with a deviation of only 0.5% from the target resonant wavelength. Now, they have further expanded this innovative technology and demonstrated the full cavity length control of VSCEL.

Professor Takeuchi explained, "The cavity of VCSEL not only contains a gallium nitride layer, but also an indium tin oxide (ITO) electrode and a niobium pentoxide (Nb2O5) spacer layer, which cannot be controlled by the same in situ reflectance spectroscopy measurement system. In this study, we developed a technique for accurately calibrating the thickness of these additional layers to achieve efficient VCSEL." Their research findings were published in the Journal of Applied Physics Letters, Volume 124, Issue 13.

Calibration techniques for additional layers
In order to calibrate the thickness of the additional layer, researchers first deposited ITO electrodes of different thicknesses and Nb2O5 spacer layers on GaN test structures grown using in-situ cavity control. Considering that in-situ reflectance measurements cannot be used for these additional layers, they directly used in-situ reflectance spectroscopy measurements to evaluate the resonance wavelength of these test cavity structures. The obtained resonance wavelength undergoes a redshift, meaning that as the thickness of the ITO and Nb2O5 layers increases, the wavelength also increases.

Next, the researchers plotted the functional relationship between resonance wavelength shift and the thickness of ITO and Nb2O5 layers, thereby obtaining accurate information about their optical thickness. They used this information to accurately calibrate the ITO layer and Nb2O5 layer thickness of the target VCSEL resonance wavelength. The resonance wavelength control deviation generated by this method is very small, within 3%, and can be comparable to on-site control methods in terms of optical thickness.

Finally, researchers fabricated GaN VCSEL with pore sizes ranging from 5 to 20 µ m by adding tuned ITO electrodes and Nb2O5 spacer layers to VCSEL cavities grown using in-situ cavity control technology. The deviation between the peak emission wavelength of these VCSELs and the design resonance wavelength is only 0.1%. It is worth noting that thanks to precise cavity length control, VCSEL with a 5-micron aperture achieved 21.1% WPE, which is a significant achievement.

Professor Takeuchi summarized, "Just like high-precision rulers can manufacture fine frames, precise in-situ thickness control of gallium nitride layers, combined with thickness calibration of ITO electrodes and Nb2O5 interlayer, can achieve highly controllable manufacturing of VCSEL. It is a powerful tool for obtaining high-performance and highly repeatable gallium nitride based VCSEL, which can be used in efficient optoelectronic devices."

Source: cnBeta