Since the beginning, the only semiconductor material the VCSEL industry has used is GaAs, whether for datacom or 3D sensing applications. GaN-based VCSELs that emit in the blue and green regions are much more difficult to produce, but are starting to emerge. Yole Développement is interviewing one of the leading players that we identified in our recent report “VCSELs – Market and technology trends 2020” – Sony Corporation.
Yole Développement VCSEL analyst Pierrick Boulay talked with Tatsushi Hamaguchi, senior scientist, and Rintaro Koda, senior manager at the company’s R&D center. Discover the details of the discussion below.
Yole Développement (YD): Can you please introduce yourself, your activities and the activities of Sony?
Sony Corporation (Sony): We are members of the Sony’s Tokyo laboratory, working on GaN-based VCSELs. These devices are suitable for blue, green and UV- emitting lasers due to GaN’s wide band-gap covering those wavelengths, though lasing has been difficult to be achieve so far. A goal of ours is to find a way to fabricate blue and green VCSELs whose process is stable enough to be used in mass production. Adding those to existing red VCSELs, we can attain full color light sources using VCSELs. The merits of VCSELs include low power consumption, circular beam profile, high switching rate, and arraying capability, all of which classic edge emitting lasers (EELs) lack.
YD: GaN-Based VCSELs are relatively difficult to produce. Can you explain to our readers how they differ from GaAs-based VCSELs?
Sony: In short, VCSELs essentially should have a cavity that consists of upper and bottom mirrors and some structure for lateral optical confinement. GaAs-based materials have properties that make them possible.
Laminating numerous AlGaAs layers with two different compositions corresponding to high and low indices of refraction accumulates light through reflection between the layers. This can be enhanced by using more pairs. This structure is called a distributed Bragg reflector (DBR). Varying the composition of AlGaAs leads to negligible shift in its lattice constant compared to that of the GaAs substrate. Therefore AlGaAs DBRs can be grown with large numbers of pairs, which let mirrors achieve reflectance reaching almost 100%. Ending the top and bottom of the device layers on the wafer with those DBRs gives the cavity can have an efficient vertical resonance of light. Also, an AlAs layer placed inside the cavity can be laterally oxidized to leave a circular non-oxidized area in the core of the cavity. This structure allows lateral confinement of the light.
Moreover, these materials provide suitable current paths as well. AlGaAs DBRs are electrically conductive for both p- and n-type layers. Therefore, a metallic electrode deposited on the outside of each DBR can offer a good path for current injection vertically into the cavity. In there, the laterally oxidized AlAs layer offers lateral current confinement, allowing devices to concentrate carriers in the core of the cavity. Placing the active layers just beneath the AlAs layer, population inversion of carrier, an essential phenomenon for lasers, occurs.
The other advantage of GaAs-based VCSELs is that all layers, including DBRs, p-type and n-type layers, and quantum wells, are grown directly on the GaAs substrate by an epitaxial growth technique, such as metal organic chemical vapor deposition (MOCVD). Thus, the fabrication process be very simple.
Those merits do not apply to GaN-based materials.
YD: One of the many challenges for GaN-based VCSELs has been the formation of high-reflectivity DBR mirrors. Why is it so difficult for VCSELs, when GaN-based edge-emitting EELs can easily be found?
Sony: Yes, one difficulty about GaN-based VCSELs is formation of DBR mirrors. In GaN-based material systems, it is difficult to find good pairs of materials to form them. AlInN and GaN are almost the only choice for making a DBR, because those materials have the same lattice constant, like GaAs and AlAs do. However, AlInN/GaN layer pairs have much a smaller refractive index difference than GaAs/AlAs. This weakens the reflectivity of the DBRs, and is sometimes insufficient for laser oscillation.
So, some researchers take another approach, namely dielectric DBR mirrors. The fabrication process they adopt is as follows. First, they form an LED-like structure, containing p and n-type GaN layers sandwiching active regions, also called multi quantum wells or MQW. Second, they thin the back side of the LED substrate to obtain a flat surface just beneath the MQW. Thirdly, they deposit a DBR made of dielectric materials on the top and the bottom of the specimen, forming the cavity only with dielectric mirrors. Because pairing dielectric materials can have larger reflective index difference so that those dielectric DBRs can offer high enough reflectivity for the laser action. However, the thinning process sometimes causes local tilt of mirrors, which causes destruction of resonance. This often prevents laser operation.
In EELs, the normal direction of the mirror is in the plane of the wafer. Generally, semiconductor single crystals have a lot of crystal planes with right angles to their wafer surface. Those are exposed by applying mechanical stress. This process is widely known as cleaving. By depositing dielectric mirrors onto those exposed crystal planes, EELs can obtain two mirrors perfectly parallel to each other. Thus, GaN-based EELs can be free from mirror tilting that affects GaN-based VCSELs.
YD: In 2016, Sony Corporation demonstrated the first GaN-based VCSEL with a peak output power over 1 mW. The lasing wavelength was 453.9 nm. Have you made progress since?
Sony: Yes, in 2016, we reported 1 mW optical output power from GaN-based VCSELs having two flat dielectric DBR mirrors. This was a milestone. However, this structure only attained efficiency less than 1% and showed arbitrary values for threshold current, because of mirror tilting. We considered that this would hinder mass production.
Thus, in 2017, we adopted a drastically different approach to form GaN-based VCSELs. We placed a curved mirror on the bottom side of the device while using a flat mirror for the top side. The reflecting characteristics of curved mirror is almost indifferent to its tilt. This relieved GaN-based VCSELs from the concern of mirror tilting. Moreover, the cavity with a curved and a flat mirror has a function of lateral optical confinement. This eliminates undesirable lateral dissipation of light to help the device have better optical characteristics. With this structure, we attained output power more than 10mW with wall plug efficiency around 10% in 2018.
YD: We have counted less than 10 companies and universities involved in GaN VCSELs and most of them are focused on blue VCSELs. Are green VCSELs even more difficult to manufacture? Why is that?
Sony: Green is the most difficult color to obtain with semiconductor-based light emitting devices. To attain green emission from GaN-based materials, devices need InGaN-based quantum wells, or QW, with high In content. The addition of In to InGaN changes its crystal structure, resulting in bigger lattice mismatch with GaN, the common substrate material used for GaN-based laser fabrication. Thus, InGaN QWs tend to contain serious amounts of crystal defects, causing low emission efficiency. Moreover, addition of In to InGaN causes mechanical stress in and around it. This leads to piezoelectric fields. Piezoelectric fields drag holes and electrons in opposite directions, inactivating the recombination of carriers, and suppressing light emission. The intensity of the piezo field in InGaN layers depends on the crystal orientation of the substrate used for device fabrication. This field is most prominent when the layers are formed over c-plane GaN-substrates and weaker on GaN-substrates orientated in other crystal directions. Thus, the other substrates are more convenient for obtaining optical devices with green emission. On the other hand, c-plane GaN substrates are very common and commercially available worldwide. Thus, a lot of research institutes use this kind of substrate and avoid researching green VCSELs. A few use GaN substrates orientated in other crystal directions. For example, we adopted (20-21) orientated GaN-substrates for VCSEL fabrication and achieved green emission from them in 2020.
YD: GaN-based VCSELs are still at the R&D level. What would be the trigger to launch GaN-based VCSELs into commercial production? When will it be possible?
Sony: Efficiency could be a prime issue for GaN-based VCSEL commercialization. At this moment, blue VCSEL efficiency is about 10%. This is smaller than EELs and LEDs. I hope research soon improves this situation. Conversely, one might say that GaN-based VCSELs are almost ready for applications that are not sensitive to efficiency of the light source.
YD: What kind of applications could use such VCSELs? Is the low output power an obstacle for commercial applications?
Sony: We recently attained green emission from GaN-based VCSELs. Adding a green VCSEL to existing blue and red ones, we could have full-color light sources only with VCSELs. This should facilitate development of future displays with VCSELs. Because VCSEL power consumption is low, this kind of light source should be suitable for mobile displays. As is well known, those displays are inevitable with the augmented/virtual reality eco-system. Because VCSELs have color purity and brightness better than LEDs, we could expect a lot of display applications with VCSELs, such as projectors and back lights. Arraying those VCSELs will enable super-watt class optical output that enables huge, bright displays.
YD: Do you have a final word for our readers?
We are open for discussion about potential applications of our GaN-based VCSELs. We especially look for chances to talk with customers. Please feel free to contact us. Email Tatsushi.firstname.lastname@example.org for more information.
Tatsushi Hamaguchi received the B.S. and M.S. degrees in material science and engineering from Kyoto University in 2004 and 2006. He received the Ph.D. degree in electronic engineering from Sophia University in 2019. From 2006 to 2008, he worked at the Nitride Semiconductor Research Laboratory of Nichia Corporation. After 2008, he worked for Sony corporation, where he did development of nitride-based edge-emitting lasers and vertical-cavity surface-emitting lasers (VCSELs). He has acted as the leader of the Sony’s project team for nitride-based VCSELs during the recent years.
Rintaro Koda is a senior manager at R&D center of Sony Corporation. He received B. S. degree in Physics from UC Berkeley in 1999 and Ph. D in electrical engineering from UC Santa Barbara under supervision of Professor Coldren in 2005. He has been working for Sony more than 15 years working on different projects such as GaAs-based VCSEL for optical communication, GaN-based mode-locked laser and optical amplifier. Now he is managing Sony’s research activities of VCSELs. He has recently awarded incentive award toward his research on GaN-based master oscillator power amplifier development from the laser society of Japan.
As part of the Photonics, Sensing & Display division at Yole Développement (Yole), Pierrick Boulay works as Market and Technology Analyst in the fields of Solid State Lighting and Lighting Systems to carry out technical, economic and marketing analysis. Pierrick has authored several reports and custom analysis dedicated to topics such as general lighting, automotive lighting, LiDAR, IR LEDs, UV LEDs and VCSELs.
Prior to Yole, Pierrick has worked in several companies where he developed his knowledge on general lighting and on automotive lighting. In the past, he has mostly worked in R&D department for LED lighting applications. Pierrick holds a master degree in Electronics (ESEO – Angers, France).
VCSELs – Market and Technology Trends 2020
VCSEL market growth is triggered now but still under evolution. Changes are happening at design, manufacturing, supply chain and application levels.
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