Thor A. Garcia

High detectivity short-wavelength II-VI quantum cascade detector

We demonstrate the first II-VI based short-wave (λ ≤ 4 μm) Quantum Cascade Detector. Peak responsivity and background limited detectivity of 0.1 mA/W and 2.5×10¹⁰ cm√Hz/W, respectively, were measured at 80 K.

Hybrid GaN LED with capillary-bonded II–VI MQW color-converting membrane for visible light communications

The rapid emergence of gallium-nitride (GaN) light-emitting diodes (LEDs) for solid-state lighting has created a timely opportunity for optical communications using visible light. One important challenge to address this opportunity is to extend the wavelength coverage of GaN LEDs without compromising their modulation properties. Here, a hybrid source for emission at 540 nm consisting of a 450 nm GaN micro-sized LED (micro-LED) with a micron-thick ZnCdSe/ZnCdMgSe multi-quantum-well color-converting membrane is reported. The membrane is liquid-capillary-bonded directly onto the sapphire window of the micro-LED for full hybridization. At an injection current of 100 mA, the color-converted power was found to be 37 μW. At this same current, the −3 dB optical modulation bandwidth of the bare GaN and hybrid micro-LEDs were 79 and 51 MHz, respectively. The intrinsic bandwidth of the color-converting membrane was found to be power-density independent over the range of the micro-LED operation at 145 MHz, which corresponds to a mean carrier lifetime of 1.9 ns.

Molecular Beam Epitaxial Growth and Properties of Bi2Se3 Topological Insulator Layers on Different Substrate Surfaces

Growth of high-quality Bi2Se3 films is crucial not only for study of topological insulators but also for manufacture of technologically important materials. We report a study of the heteroepitaxy of single-crystal Bi2Se3 thin films grown on GaAs and InP substrates by use of molecular beam epitaxy. Surface topography, crystal structure, and electrical transport properties of these Bi2Se3 epitaxial films are indicative of highly c-axis oriented films with atomically sharp interfaces.

Molecular beam epitaxial growth and characterization of Bi2Se3/II-VI semiconductor heterostructures

Surfaces of three-dimensional topological insulators (TIs) have been proposed to host quantum phases at the interfaces with other types of materials, provided that the topological properties of interfacial regions remain unperturbed. Here, we report on the molecular beam epitaxy growth of II-VI semiconductor–TI heterostructures using c-plane sapphire substrates. Our studies demonstrate that Zn0.49Cd0.51Se and Zn0.23Cd0.25Mg0.52Se layers have improved quality relative to ZnSe. The structures exhibit a large relative upward shift of the TI bulk quantum levels when the TI layers are very thin (∼6nm), consistent with quantum confinement imposed by the wide bandgap II-VI layers. Our transport measurements show that the characteristic topological signatures of the Bi2Se3 layers are preserved.

MBE growth of ZnCdSe/ZnCdMgSe quantum-well infrared photodetectors

The authors report the growth of quantum well infrared photodetectors (QWIPs) made from wide band gap II-VI semiconductors. ZnCdSe/ZnCdMgSe QWIPs in both medium-wave infrared and long-wave infrared regions were grown by molecular beam epitaxy on InP substrates. High-resolution x-ray diffraction and photoluminescence measurements showed that the as-grown samples have high structural and optical quality. Spectral responses with peaks at 8.7 μm and 4.0 μm have been obtained.

Improved electrical properties and crystalline quality of II–VI heterostructures for quantum cascade lasers

The authors report on investigations of the doping, lattice mismatch, and interface quality for the molecular beam epitaxial growth of ZnCdSe/ZnCdMgSe/InP quantum cascade structures with improved electrical, structural, and spectral properties. An improved doping strategy, the control of the lattice mismatch to less than 0.25%, and the incorporation of growth interruptions have led to quantum cascade structures with good I–V characteristics and electroluminescence emission up to room temperature, with an emission energy of 230 meV (5.4 μm) and a full-width at half maximum of 41 meV at 80 K, the best device properties reported so far for this material system. It is expected that the addition of waveguide layers in the structure will lead to the observation of lasing.

Characterization of the three-well active region of a quantum cascade laser using contactless electroreflectance

Quantum cascade (QC) lasers with emission at wavelengths below 4 μm are difficult to achieve from conventional III-V materials systems lattice matched to GaAs and InP due to the limited conduction band offset (CBO) of those materials that results from the presence of intervalley scattering. The II-VI materials ZnCdSe/ZnCdMgSe, with a CBO as high as 1.12 eV and no intervalley scattering, are promising candidates to achieve this goal. Using molecular beam epitaxy (MBE), the authors grew a QC laser structure with a three-well active region design made of ZnCdSe and ZnCdMgSe multilayers closely lattice matched to InP. A test structure, which contains only the active region of the QC laser separated by quaternary barrier layers, was also grown. The test structure was characterized by contactless electroreflectance (CER). Photoluminescence measurements and a model based on the transfer matrix method were used to identify the CER transitions. The energy levels obtained for the test structure were then used to pr...

Reduced twinning and surface roughness of Bi2Se3 and Bi2Te3 layers grown by molecular beam epitaxy on sapphire substrates

The authors investigate the structural properties of Bi2Se3 and Bi2Te3 topological insulator layers grown on sapphire (0001) substrates by molecular beam epitaxy, using various pregrowth optimization methods. Samples of Bi2Se3 grown on sapphire with a particular combination of pregrowth surface treatment steps showed evidence of greatly reduced twinning and significantly reduced surface roughness. Evidence of twinning, and its suppression by the appropriate choice of pregrowth steps, is obtained from x-ray diffraction Φ-scan measurements as well as atomic force microscopy (AFM) images. Improved surface roughness is also evident from the AFM images. Growth of Bi2Te3 on sapphire exhibited similar reduced twinning by the application of the pregrowth surface preparation steps, although the surface roughness was significantly greater than that of Bi2Se3. Additional improvements in surface roughness of the Bi2Te3 were achieved by growing the Bi2Te3 on a Bi2Se3 buffer layer.

Bi2Se3 van der Waals Virtual Substrates for II–VI Heterostructures

We report on the growth and characterization of optical quality multiple quantum well structures of Zn x Cd1-x Se/Zn x Cd y Mg1-x-y Se on an ultra-thin Bi2Se3/CdTe virtual substrate on c-plane Al2O3 (sapphire). Excellent quality highly oriented films grown along the (111) direction were achieved as evidenced by reflection high energy electron diffraction and X-ray diffraction studies. We also observed room temperature and 77 K photoluminescence emission with peak energies at 77 K of 2.407 eV and linewidths of 56 meV comparable to those achieved on structures grown on InP. Exfoliation of the structures is also possible due to the van der Waals bonding of Bi2Se3. Exfoliated (substrate free) films exhibit photoluminescence emission nearly identical to that of the supported film. Additionally, contactless electroreflectance measurements show good agreement with simulations of the multiple quantum well structure as well as evidence of excited state levels. These results open new avenues of research for substrate independent epitaxy and the possibility of ultra-thin electronics.

Growth and properties of wide bandgap (MgSe)n(ZnxCd1−xSe)m short-period superlattices

We report the molecular beam epitaxy (MBE) growth and properties of (MgSe)n(ZnxCd1-x Se)m short-period superlattices(SPSLs) for potential application in II-VI devices grown on InP substrates. SPSL structures up to 1 μm thick with effective bandgaps ranging from 2.6 eV to above 3.42 eV are grown and characterized, extending the typical range possible for the ZnxCdyMg1-x-ySe random alloy beyond 3.2 eV. Additionally, ZnxCd1-xSe single and multiple quantum well structures using the SPSL barriers are also grown and investigated. The structures are characterized utilizing reflection high-energy electron diffraction, X-ray reflectance, X-ray diffraction and photoluminescence. We observed layer-by-layer growth and smoother interfaces in the QWs grown with SPSL when compared to the ZnxCdyMg1-x-ySe random alloy. The results indicate that this materials platform is a good candidate to replace the random alloy in wide bandgap device applications.