Characterization on DBR structure of VCSEL laser cavity by u-PLM-EX

The DBR structure is the most important process for VCSELs. The design and production of the DBR deeply affect the performance of the VCSEL. Enli Tech u-PLM-EX adopts non-destructive optical detection methods to accurately detect and analyze DBR epitaxial structures. With user-friendly analyzing software users can efficiently conduct researches on VCSEL.

Enli Tech u-PLM-EX

What is VCSEL laser?

Vertical-cavity surface-emitting lasers (VCSELs) made their debut over 10 years ago. Since then, they have become a staple component for a wide range of applications, especially in networks. In general, VCSEL is a type of semiconductor laser device that emits a highly efficient optical beam vertically from its top surface. It has an active medium and two high reflectance mirrors in semiconductor process. Compare with traditional laser diode, VCSEL has the following advantages:

1. Surface-Normal output
2. Circular output beam
3. Low beam divergence
4. Extremely small active volumes
5. Low threshold currents
6. Single frequency
7. Thermally stable operation
8. High-speed 
9. 1-D and 2-D dimensional array
10. On-wafer testing
11. Ease of integration

For now, VCSEL applications include fiber optic communications, 3D sensing, precision sensing, computer mice, laser printers and augmented reality.

Structure diagram of VCSEL
▲Fig 1. Structure diagram of VSCEL

What is DBR structure and why it is important?

To archive a SLM cavity in VCSEL, the laser resonator consists of two distributed Bragg reflector (DBR) mirrors parallel to the wafer surface with an active region consisting of one or more quantum wells for the laser light generation. The planar DBR-mirrors consist of layers with alternating high and low refractive indices. Each layer has a thickness of a quarter of the laser wavelength in the material to make excellent constructive interference, yielding intensity reflectivity above 99 %. High reflectivity mirrors are required in VCSELs to balance the short axial length of the gain region.

structure of DBR

Fig. 2 structure of DBR

In general, the upper and lower mirrors of VCSELs are doped as p-type and n-type materials, forming a diode junction. In more complex structures, the p-type and n-type regions may be embedded between the mirrors, requiring a more complex semiconductor process to make electrical contact to the active region, but eliminating electrical power loss in the DBR structure. Therefore, it is very important that to check the optical properties of DBR in VCSEL devices.

How to probe DBR ?

To check the optical properties of DBR, Enlitech uses an Interferometer structure to analyze reflectance of DBR. In general, using a broadband source to illuminate sample and then measure the reflectance spectrum. Basic on resonator structure, the reflectance of each-layer of DBR will be seen as interference patterns which is shown on Fig. 3.

Principle of DBR maeasurement

Fig. 3 Principle of DBR measurement

According to measured results, researcher/engineers can check finger number, height of center-band, width of center-band and F-P dip to fit the original design. On the other hand, it could be integrated with mapper to further perform quality control in manufacturing process.

What’s our feature?

1. Fast mapping:
 Enli Tech had developed an algorithm to increase acquisition time for spectral mapping and DBR mapping can be done in a few minutes with high resolution step. In general, for 4-inch wafer with 1 mm resolution, it takes less than 5 minutes.
2. Broad wavelength range: 350 nm-2500 nm
 To measure reflectance mapping in DBR analysis, it must be equipped a highly stable and broadband light. Meanwhile, the collecting optics for spectral measurement also need be considered in an excellent mapper.The system equipped a long life (>10000 hrs) halogen lamp which spectral range is from 350 nm to 2500 nm. For detector part, it can installed visible or NIR array sensor for customer’s need.
3. Friendly analyzing Software: 
 To quickly analyzing the results of sample, software not only have basic tools for spectroscopy such as FWHM, Peak lambda, Peak intensity, but also have SB height, SB center, SB width and F-P dip analysis function. It could help researcher or engineer to quickly find out the defects and verify the processing parameters in their design.

1. Application Note AN-2137 FINISAR
2. Emanuel P. Haglund et al., “Hybrid vertical-cavity laser integration on silicon”, SPIE (2017)