Vertical-cavity surface-emitting lasers (VCSELS) operating at visible wavelengths have a number of applications, including optical data storage, laser printing, and communications over plastic optical fiber. Output power, of course, is always an issue. Now, researchers at the Ferdinand-Braun Institute (Berlin, Germany) have produced red VCSELs capable of generating up to 10 mW in the red spectral region at room temperature.1 "Precise control of the growth process and device structure is the key," says team member Markus Weyers. The group worked with LayTec (Berlin, Germany) to implement and test a new in-situ monitoring system. The project demonstrates the importance of collaborative development in producing both optimized visible VCSELS and improved process control.
The red VCSELS are very sensitive to the alignment of the Bragg mirrors with respect to the active region. In addition, optimum performance of a VCSEL requires that every layer in the stack be grown to the desired thickness. The Ferdinand-Braun group used a real-time optical sensor during fabrication to obtain both spectroscopic reflectance and reflectance anisotropy measurements, quantities that provide important data on properties like thickness, composition, and doping for all buried layers.
The device structure consists of an active-layer cavity sandwiched between an n-doped bottom distributed Bragg reflector (DBR) with 55 pairs of aluminum gallium arsenide/aluminum arsenide (AlGaAs/AlAs) and a p-doped top DBR with 35 pairs of AlGaAs/AlAs layers. The cavity has three compressively strained gallium indium phosphide (GaInP) quantum wells embedded within barriers and spacer layers. This is surrounded by a quarter-wave DBR with linearly graded interfaces.
The electroluminescence wavelength is 650 to 660 nm and describes the maximum emission at the wafer center. The team obtained 10 mW from a 670-nm VCSEL operating at 20°C, for a 18% wall-plug efficiency. A separate set of devices operating at 650 nm produced 4.3 mW at 20°C. The group also demonstrated stable performance at elevated temperatures for both sets of devices.
Maintaining tight control over layer thickness throughout the lengthy low-pressure metalorganic vapor phase epitaxy (MOVPE) growth process is one of the biggest challenges. The key criteria are the peak reflectance wavelength of both DBR mirrors and the resonance wavelength of the cavity, both of which must be adjusted precisely to the same desired value. The group achieved these results with an in-situ sensor that performed normalized reflectance and reflectance anisotropy spectroscopy (RAS) measurements.
Reflectance fingerprint of the complete VCSEL growth process: (a) as measured and (b) as calculated. The dotted lines indicate the alignment between the top and bottom DBR mirrors and the cavity. (Courtesy Ferdinand-Braun Institute, Berlin)
Normalized reflectance provided information on the reflectance wavelength and composition of the DBRs, the resonant wavelength of the cavity, and the cap-layer thickness. The RAS sensor assessed the doping level of the respective layers and the surface stoichiometry and surface morphology during growth.
The group performed the measurements with the LayTec spectrometer. "Compensating for rotational wobble, phase, and orientation is the key feature that allows the precision in-situ monitoring required for these devices," says Kolja Haberland, manager of technical services at LayTec.
RAS measures the difference in reflectance for linearly polarized light along the two principle axes on the surface. The data is captured directly from the layer in real time using a reference GaAs reflector in the chamber for normalization, yielding measurements accurate to within ± 0.2%.
The group compared and correlated the data obtained in situ during the VCSEL growth process with the ex-situ measured data and the simulation (see figure). Measurements on different structures and processes showed that alignment tuning of the lower and upper DBRs can be correlated with different electro-optic performances of the VCSELS. These optical properties cannot be inferred from the ex-situ data. With in-situ data, DBR reflectance, cavity resonance wavelength, doping level, and layer thickness can be measured and controlled, making these optimized results possible.
1. A. Knigge, M. Zorn, et al., Electronics Letters, 38, pp.882-883.