"Novel mid-infrared materials and devices on InP: from metamorphic lasers to self-assembled nanocomposites"

Daehwan Jung, Electrical engineering, Yale University

May 5th (Thursday), 11:00am
Engineering Science Building (ESB), Room 2001

Laser diodes (LDs) emitting in the mid-infrared (mid-IR) spectral region (λ= 2 – 3 μm) are important for applications including molecular spectroscopy and gas detection. Quantum cascade lasers on InP have reached λ=3.0 μm continuous wave (CW) lasing at room temperature (RT), while type-I InAs quantum well (QW) LDs have reached λ= 2.4 μm. However, due to extremely high strain in the active regions for both technologies, demonstration of CW RT lasing at 2.4 – 3.0 μm remains difficult for InP-based lasers. A metamorphic InAsxP1-x graded buffer on InP can perform multiple functions in addressing this challenge, as it not only increases the critical thickness of InAs QWs to enable longer wavelength emission, but also functions as graded-index bottom cladding for optical confinement. We demonstrate room-temperature metamorphic type-I QW LDs on InP that take advantage of such multi-functional metamorphic buffers to achieve lasing at λ= 2.76 μm. The metamorphic LDs were grown on n-InP (001) substrates by solid source molecular beam epitaxy (MBE). We fabricated and tested 10 µm ridge-waveguide LDs, observing pulsed mode lasing up to 300 K at λ = 2.76 μm. The threshold current density at 77 K was 200 A/cm2 and increased to 14 kA/cm2 at 300 K.

In the second part of my talk, we present self-assembled growth of highly tensile-strained Ge nanostructures, coherently embedded in an InAlAs matrix (i.e. Ge/InAlAs nanocomposite) by using spontaneous phase separation. Ge is a very intriguing material for strain engineered nanocomposites, because strain can dramatically enhance its electrical and optical properties. Here, I employ spontaneous
phase separation during MBE growth as a fundamentally
new approach to forming highly tensile-strained Ge/InAlAs nanocomposite. Transmission electron microscopy reveals a high density of single-crystalline Ge nanostructures coherently embedded in InAlAs without extended defects, and Raman spectroscopy reveals a 3.8% biaxial tensile strain in the Ge nanostructures. I demonstrate that the strain in the Ge nanostructures can be tuned to 5.3% by altering the lattice constant of the matrix material, illustrating the versatility of epitaxial nanocomposites for strain engineering and the largest biaxial tension realized in Ge to date; the cross-over from indirect to direct is predicted at ~2% biaxial tension. Photoluminescence and electroluminescence results are then discussed to illustrate the potential for realizing devices based on this novel nanocomposite material.

About Daehwan Jung:

Daehwan Jung received his B.S. degree in Electrical Engineering from the University of Texas at Dallas in 2010 and expects to receive his Ph.D. from Yale University in May of 2016. His Ph.D. research focuses on growth and characterizations of novel group III-V and IV materials using molecular beam epitaxy for mid-infrared lasers and solar cells.

Hosted by: Professor John Bowers