Design, fabrication, and characterization of terahertz quantum cascade lasers

• Main Author: Petschke, Adam Christopher
• Format: Dissertation
• Language: English
• Subjects: engineering, electronics and electrical
• ISBN: 1109579527; 9781109579529

Quantum cascade lasers are different from conventional semiconductor lasers. They use only one hand for population inversion, typically the conduction band. The electrons move through many cascaded regions, giving off a photon in each region. In this way, one electron can produce several photons. In addition, by bandstructure engineering, one material system can lase at almost any wavelength in the mid and far infrared spectrum. For terahertz quantum cascade lasers, the GaAs/AlGaAs material system has successfully produced lasing action.
In the terahertz regime, conventional dielectric waveguides cannot be used. Instead, metals and highly doped semiconductors provide confinement using surface plasmons. The two most popular waveguide geometries are the metal-semiconductor-metal (MSM) and the metal-semiconductor-doped semiconductor (MSDS) waveguides. The MSM waveguides provide a larger confinement factor, with the disadvantage of smaller output powers and a more complicated fabrication process.
To model and design the electronic bandstructure of the active region of quantum cascade lasers, we use the finite difference method to solve Schrodinger's equation and Poisson's equation self-consistently. Using the rate equations in steady state, the peak optical gain is calculated. To model the optical modes of the waveguide, we use a propagation matrix approach. Using this method, we can determine the loss and confinement factor, which, with the mirror loss, gives us the threshold material gain. We have found that for the MSDS waveguide, the substrate thickness can have a significant impact on the waveguide characteristics. In addition, under certain circumstances, the first higher-order mode will be the lasing mode.
After the lasers are grown, we fabricate them into ridge waveguides of various lengths and widths. We describe the fabrication and processing of MSM and MSDS quantum cascade lasers. We detail several wafer bonding techniques as well as the details of the etching procedure. We demonstrate our successful fabrication and testing of the lasers near 3 terahertz. We then characterize the electrical and optical properties as a function of temperature from 4.2 K to 55 K. We find that the lasing threshold current density is 187 A/cm 2 and the threshold voltage is 5.4 V at a temperature of 4.2 K for a waveguide with a length of 3 mm. We find that the lasing threshold current density is 200 A/cm2 and the threshold voltage is 2.0 V at a temperature of 4.2 K for a waveguide with a length of 1 mm. The threshold current density increases to 290 A/cm2 and 250 A/cm 2 at 55 K for the 3 mm and 1 mm waveguides, respectively. In addition, we determined that each period had a quantum efficiency of 32% up to 20 K.
We also perform an experimental study of the role of the substrate on the optical and electrical properties. We find that the effect of the substrate thickness on the laser performance is dependent on other waveguide parameters. In particular, if the plasma layer is thick enough, the role of the substrate should be minimal. We use two-dimensional finite element modeling (FEM) to determine the threshold material gain coefficient and compare it to the experimental results of the threshold current density. We find that the threshold current density is roughly constant until the substrate thickness becomes smaller than 150 microm.