Thermal Transport in Materials and Structures Pertinent to Quantum Cascade Lasers
Open Access
- Author:
- Perez, Carlos
- Graduate Program:
- Mechanical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 23, 2023
- Committee Members:
- Robert Kunz, Professor in Charge/Director of Graduate Studies
Venkatraman Gopalan, Outside Unit & Field Member
Sukwon Choi, Major Field Member
Luke Mawst, Special Member
Linxiao Zhu, Major Field Member
Bladimir Ramos Alvarado, Chair & Dissertation Advisor - Keywords:
- Quantum cascade laser
Thermal conductivity
Superlattice
Phonons - Abstract:
- Since their initial demonstration in 1994, quantum cascade lasers (QCLs) have undergone enormous advancements. However, inefficient dissipation of heat can hinder the performance of QCLs. The complex structure of QCL devices creates a difficult thermal problem. The multilayer and multi-material nature of QCL designs results in a highly resistive thermal pathway to the heatsink due to various phonon-scattering mechanisms. Given the large proportion of electrical pump power that dissipates as heat within these arrays, the thermal design needs to be optimized to minimize the temperature increase within the array and avoid thermal issues, such as reduced maximum power output, decreased wall-plug efficiency, thermal lensing, and increased lasing threshold current. To maintain the advancement of QCLs, the thermal dissipation issues of these devices must be addressed. It is necessary to characterize the different scattering mechanisms that affect the thermal conductivity of materials and structures, such as superlattices (SLs), relevant to these devices. In this work, time-domain thermoreflectance was used to measure the thermal properties of SLs, silicon-doped indium phosphide (Si-InP), iron-doped indium phosphide (Fe-InP), InGaAs, and InAlAs at various temperatures, concentrations, interface densities (# interfaces/total thickness), and thicknesses. In this work, the thermal conductivity of thin films of Si-InP and Fe-InP from 80 to 450 K was measured. The phonon gas model and sensitivity analysis were used to characterize the role of various scattering mechanisms in Si-InP and Fe-InP. The effect of film thickness on the thermal conductivity of these materials was quantified. In addition, the thermal conductivity of In0.63Ga0.37As/In0.37Al0.63As SLs with interface densities ranging from 0.0374 to 2.19 nm-1 in the temperature range of 80–450 K was measured. Time-domain thermoreflectance measurements of the thermal conductivity of III-V alloy SLs as a function of interface density demonstrate the presence of a minimum, which is an indication of a crossover from incoherent to coherent phonon transport as the interface density increases. This minimum continues with increasing temperature, showing the continued dominance of the temperature-independent interface and alloy disorder scattering over the temperature-dependent three-phonon scattering in thermal transport through III–V alloy SLs. The In0.5275Ga0.4725As measurements indicate that thermal conductivity decreases as thickness decreases, primarily due to boundary scattering, until a critical thickness of ~100 nm is reached, below which the thermal conductivity appears to flatten out at ~2.75 W m-1 K-1. The In0.521Al0.479As measurements seem to be insensitive to sample thickness, similar in some ways to what has been shown in amorphous materials such as SiO2, where the effects of thin film size are nonexistent. This study fills the knowledge gap regarding the thermal properties of materials and structures important to QCLs.