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

Abstract

Laser diodes (LDs) emitting in the mid-infrared (mid-IR) spectral region (λ= 2 – 3 gm) 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. In the first part of my thesis, I demonstrate InP-based metamorphic type-I QW LDs that take advantage of such multi-functional metamorphic buffers to achieve lasing at λ= 2.63 µm. The metamorphic LDs were grown on n-InP (001) substrates by solid source molecular beam epitaxy (MBE). We first grew Si-doped n-InAsxP1-x metamorphic graded to ensure a low threading dislocation density below 3 x 106/cm 2. For the active region, we utilize a strain-balanced InAs/In 0.54Ga0.46As multi-quantum well (MQW) with low net strain relative to the relaxed n-InAs0.5P0.5 waveguide. After growing the active region sandwiched by InAso.5Po.5 waveguides, a low-index, highly lattice-mismatched p- Al0.5Ga0.5As layer was deposited for the top cladding. Simulation results on fundamental transverse electric field mode show that the asymmetric cladding structure confines light at the center of the MQW with optical confinement factor of 4.14 %. High-resolution x-ray diffraction measurement revealed that the InAs MQW is fully strained, while the n-InAsxP1-x buffers and p-Al0.5Ga0.5As cladding are fully relaxed. The strong satellite peaks from the InAs MQW indicate high crystalline quality, and transmission electron microscopy (TEM) confirms that strain balancing was successful in avoiding misfit dislocation formation in the MQW region. More importantly, the growth of the p-Al0.5Ga0.5As top cladding did not generate TDs penetrating back to the active region, as expected from a highly lattice-mismatched interface where relaxation is dominated by sessile edge dislocations. We fabricated and tested 10 µm ridge-waveguide LDs, observing pulsed mode lasing up to 250 K at λ = 2.63 µm. The threshold current density at 77 K was 160 A/cm2 and increased to 4 kA/cm 2 at 250 K. Relatively low characteristic temperatures of 46-68 K were extracted, indicating possible carrier losses at elevated temperatures. We believe that further optimization in device design will enable lasing at room temperature and above. In the second part of my thesis, I show 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. Self-assembled nanocomposites have been extensively investigated due to the novel properties that can emerge when multiple material phases are combined. Growth of epitaxial nanocomposites using lattice-mismatched constituents also enables strain-engineering, which can be used to further enhance material properties. Ge is a very intriguing material for strain engineered nanocomposites, because strain can dramatically enhance its electrical and optical properties. Spurred by theoretical work showing that tensile strain can convert Ge from an indirect-gap to a direct-gap semiconductor, recent research has focused on applying large biaxial or uniaxial tensions using a range of approaches. For example, epitaxial growth of Ge thin films on template layers with larger lattice constant (e.g. InGaAs or GeSn) has enabled biaxial tensile strain up to 2.33%. Top-down fabrication techniques have also been used to fabricate Ge in biaxial or uniaxial tension using structures such as nanomembranes, bridges, and suspended nanowires. Here, I employ spontaneous phase separation during MBE growth as a fundamentally new approach to forming highly tensile-strained Ge/InAlAs nanocomposite. While the mutual immiscibility of Ge with III-V materials provides the driving force for phase separation, changes in growth kinetics enable significant control over nanostructure morphology, from nanowires to nanosheets. TEM reveals a high density of single-crystalline Ge nanostructures coherently embedded in InAIAs 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. We believe that the group-IV/III-V nanocomposites demonstrated here constitute a new materials platform for investigation of basic aspects of phase-separated growth, as well as offering the ability to create ultra-high strain states and properties that are otherwise inaccessible through conventional growth and processing