Thermal management of ultraviolet LEDs and VCSELs: computer-aided multiphysics optimization

AlGaN alloys have emerged as the most promising semiconductor compounds for deep-ultraviolet (DUV) applications. Indeed, electronic bandgap engineering can be performed by changing the alloy composition, enabling the production of semiconductor light sources covering most of UVA, UVB and UVC spectra. The main interest in pushing the emission toward such short wavelengths relies on the wide variety of applications ranging from the biomedical field, for dermatological therapy (i.e. psoriasis, vitiligo), to UV curing (mainly UVA and UVB), to plant illumination systems for green-houses (UVB), and water/surface purification and disinfection (UVC and below). The main limitation of a realistic implementation of deep-UV emitting devices relies in the thermal management. Several studies have reported an output power conversion efficiency for blue-VCSELs and UVB-LEDs lower than the 10%, meaning that all the input power that is not converted into optical power is dissipated into heat. Even more challenging is the implementation of UV-emitting vertical-cavity surface-emitting lasers (VCSELs). In fact, the poor refractive index difference and hole transport properties prevents the use of AlGaN distributed Bragg reflectors (DBRs). On the other hand, VCSELs based on dielectric DBRs exhibit mediocre heat conduction properties, leading to dramatic increases of the cavity temperature. This explains why no emission below 340 nm is reported for a VCSEL. Thus, it is straightforward to notice how thermal management is a crucial issue for the development of UV-emitting devices. In this master thesis, the thermal characteristics of UV-emitting thin-film flip-chip LEDs and VCSELs are studied by analysing different device geometries, in order to enhance the thermal dissipation through the structure and study the thermal effects in terms of cavity resonance shifts for the VCSELs, aiming to provide concrete inputs for the optimization of the thermal management of the next generation of deep-UV emitting devices. In the first part of this work the temperature profile and the heat flow in UVB/UVC VCSELs and UV-emitting thin-film flip-chip LEDs is simulated in COMSOL Multiphysics by implementing 2D and 3D steady-state simulations. Different geometries are explored for each device in order to minimize the temperature rise in the active region and improve the heat flow from the active region toward the heat sink. The heat capability of the different devices is then studied by calculating the thermal resistance, used as a figure of merit to compare all the different geometries. The second part of the thesis is aimed at studying the effects of temperature rise in the cavity of a VCSEL in terms of temperature-induced resonance wavelength shift. A one-dimensional transfer matrix method is implemented to solve the Helmholtz's equation along the VCSEL optical axis and extract the standing wave pattern. The temperature profile along the optical axis is extracted from the thermal simulation implemented in COMSOL and used to update the refractive index of each layer of the resonator according to the thermo-optic coefficient of the corresponding material. Different VCSEL structures are investigated: UVB and UVC emitting VCSELs with a double dielectric DBR scheme and a blue VCSEL with hybrid DBR configuration. In the latter case, the calculated shift rate is compared to literature values, obtaining great accordance between simulations and measurements.