III-nitride-based vertical-cavity surface-emitting lasers have so far used intracavity contacting schemes since electrically conductive distributed Bragg reflectors (DBRs) have been difficult to achieve. A promising material combination for conductive DBRs is ZnO/GaN due to the small conduction band offset and ease of n-type doping. In addition, this combination offers a small lattice mismatch and high refractive index contrast, which could yield a mirror with a broad stopband and a high peak reflectivity using less than 20 DBR-pairs. A crack-free ZnO/GaN DBR was grown by hybrid plasma-assisted molecular beam epitaxy. The ZnO layers were approximately 20 nm thick and had an electron concentration of 1×1019 cm-3, while the GaN layers were 80-110 nm thick with an electron concentration of 1.8×1018 cm-3. In order to measure the resistance, mesa structures were formed by dry etching through the top 3 DBR-pairs and depositing non-annealed Al contacts on the GaN-layers at the top and next to the mesas. The measured specific series resistance was dominated by the lateral and contact contributions and gave an upper limit of ~10-3Ωcm2 for the vertical resistance. Simulations show that the ZnO electron concentration and the cancellation of piezoelectric and spontaneous polarization in strained ZnO have a large impact on the vertical resistance and that it could be orders of magnitudes lower than what was measured. This is the first report on electrically conductive ZnO/GaN DBRs and the upper limit of the resistance reported here is close to the lowest values reported for III-nitride-based DBRs.
ABSTRACT The Vertical-Cavity Surface-Emitting Laser (VCSEL) is an established optical source in short-distance optical communication links, computer mice and tailored infrared power heating systems. Its low power consumption, easy integration into two-dimensional arrays, and low-cost manufacturing also make this type of semiconductor laser suitable for application in areas such as high-resolution printing, medical applications, and general lighting. However, these applications require emission wavelengths in the blue-UV instead of the established infrared regime, which can be achieved by using GaN-based instead of GaAs-based materials. The development of GaN-based VCSELs is challenging, but during recent years several groups have managed to demonstrate electrically pumped GaN-based VCSELs with close to 1 mW of optical output power and threshold current densities between 3-16 kA/cm2. The performance is limited by challenges such as achieving high-reflectivity mirrors, vertical and lateral carrier confinement, efficient lateral current spreading, accurate cavity length control and lateral optical mode confinement. This paper summarizes different strategies to solve these issues in electrically pumped GaN-VCSELs together with state-of-the-art results. We will highlight our work on combined transverse current and optical mode confinement, where we show that many structures used for current confinement result in unintentionally optically anti-guided resonators. Such resonators can have a very high optical loss, which easily doubles the threshold gain for lasing. We will also present an alternative to the use of distributed Bragg reflectors as high-reflectivity mirrors, namely TiO2/air high contrast gratings (HCGs). Fabricated HCGs of this type show a high reflectivity (>95%) over a 25 nm wavelength span.
For GaN-based microcavity light emitters, such as vertical-cavity surface-emitting lasers (VCSELs) and resonant cavity light emitting diodes (RCLEDs) in the blue-green wavelength regime, achieving a high reflectivity wide bandwidth feedback mirror is truly challenging. The material properties of the III-nitride alloys are hardly compatible with the conventional distributed Bragg reflectors (DBRs) and the newly proposed high-contrast gratings (HCGs). Alternatively, at least for the top outcoupling mirror, dielectric materials offer more suitable material combinations not only for the DBRs but also for the HCGs. HCGs may offer advantages such as transverse mode and polarization control, a broader reflectivity spectrum than epitaxially grown DBRs, and the possibility to set the resonance wavelength after epitaxial growth by the grating parameters. In this work we have realized an air-suspended TiO2 grating with the help of a SiO2 sacrificial layer. The deposition processes for the dielectric layers were fine-tuned to minimize the residual stress. To achieve an accurate control of the grating duty cycle, a newly developed lift-off process, using hydrogen silesquioxan (HSQ) and sacrificial polymethyl-methacrylate (PMMA) resists, was applied to deposit the hard mask, providing sub-10 nm resolution. The finally obtained TiO2/air HCGs were characterized in a micro-reflectance measurement setup. A peak power reflectivity in excess of 95% was achieved for TM polarization at the center wavelength of 435 nm, with a reflectivity stopband width of about 80 nm (FWHM). The measured HCG reflectance spectra were compared to corresponding simulations obtained from rigorous coupled-wave analysis and very good agreement was found.
We show numerically that many recently proposed GaN-based VCSEL cavities, with DBR mirrors deposited onto the current aperture, balance dangerously close to the border between the guided and antiguided regime. A guided cavity is often preferred because of its lower optical loss, but a strongly antiguided cavity offers built-in modal discrimination favoring single fundamental mode operation. We show that very small changes in the VCSEL structure are sufficient to strongly change the guiding character of the VCSEL cavity, and that thermal lensing caused by device self-heating under operation can dramatically reduce the optical loss but not the modal discrimination in the antiguided cavities.