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Novel approach to optimize quantum dot (QD) materials for specific optoelectronic applications is based on engineering of nanoscale potential profile, which is created by charged QDs. The nanoscale barriers prevent capture of photocarriers and drastically increase the photoelectron lifetime, which in turn strongly improves the photoconductive gain, responsivity, and sensitivity of photodetectors and decreases the nonradiative recombination losses of photovoltaic devices. QD charging may be created by various types of selective doping. To investigate effects of selective doping, we model, fabricated, and characterized AlGaAs/InAs QD structures with n-doping of QD layers, doping of interdot layers, and bipolar doping, which combines p-doping of QD layers with strong n-doping of the interdot space. We have measured spectral characteristics of photoresponse, photocurrent and dark current. The experimental data show that providing the same electron population of QDs, the bipolar doping creates the most contrasting nanoscale profile with the highest barriers around dots.
Group III-Sb compound semiconductors are promising materials for future CMOS circuits. Especially, In1-xGaxSb is considered as a complimentary p-type channel material to n-type In1-xGaxAs MOSFET due to the superior hole transport properties and similar chemical properties in III-Sb’s to those of InGaAs. The heteroepitaxial growth of In1-xGaxSb on Si substrate has significant advantage for volume fabrication of III-V ICs. However large lattice mismatch between InGaSb and Si results in many growth-related defects (micro twins, threading dislocations and antiphase domain boundaries); these defects also act as deep acceptor levels. Accordingly, unintentional doping in InGaSb films causes additional scattering, increase junction leakages and affects the interface properties. In this paper, we studied the correlations between of defects and hole carrier densities in GaSb and strained In1-xGaxSb quantum well layers by using various designs of metamorphic superlattice buffers.
The paper reports on the growth of group III-Sb’s on silicon, substrate preparation, optimization of AlGaSb metamorphic buffer, formation of defects (threading dislocations, microtwins and anti-phase boundaries) and their effect on the surface morphology and electrical properties of these high hole mobility materials for future III-V CMOS technology. Defect density was found to be 2-3x higher than in similar structures grown on GaAs, resulting in 2x higher roughness. Defects also result in background p-type doping well above 1017 cm-3 causing inversion of polarity from n-type to p-type in thin n-type doped GaSb. MOS Capacitors fabricated on these buffers demonstrate similar characteristics to higher quality GaSb-on-GaAs. The highest hole mobility obtained in a strained InGaSb QW MOS channel grown on silicon is ∼630 cm2/V-s which is ∼30% lower than similar channels grown on GaAs substrates.
Group III-V semiconductor materials are being studied as potential replacements for conventional CMOS technology due to their better electron transport properties. However, the excess scattering of carriers in MOSFET channel due to high-k gate oxide interface significantly depreciates the benefits of III-V high-mobility channel materials. We present results on Hall electron mobility of buried QW structures influenced by remote scattering due to InGaAs/HfO2 interface. Mobility in In0.77Ga0.23As QWs degraded from 12000 to 1200 cm2/V-s and the mobility vs. temperature slope changed from T-1.2 to almost T+1.0 in 77-300 K range when the barrier thickness is reduced from 50 to 0 nm. This mobility change is attributed to remote Coulomb scattering due to charges and dipoles at semiconductor/oxide interface. Elimination of the InGaAs/HfO2 interface via introduction of SiOx interface layer formed by oxidation of thin a-Si passivation layer was found to improve the channel mobility. The mobility vs. sheet carrier density shows the maximum close to 2×1012 cm-2.
We present experimental results on the effect of strain on hole transport in InGaAs quantum well (QW) structures. Indium content was varied from lattice matched to high compressive stress in InGaAs/InP QW and the transport properties were analyzed at various temperatures (T = 77-300 K) using Hall measurements. The effect of QW thickness (4-20 nm) on hole transport is also presented. The current best results include room temperature mobility and sheet resistance of 390 cm2/V-s and 8500 Ω/sq., respectively. It was observed that the mobility had a T-1.8 dependence indicating similar scattering mechanism in almost all of the samples with prominent mechanism being due to interface and barrier scattering. Further optimization of p-channel for InGaAs CMOS needs to be performed using the above results as guidelines.
Structures of tunnel-coupled pairs consisting of InGaAs (In composition was varied from 29 to 36%) quantum wells (QW) grown on top of shape-engineered self-assembled InAs quantum dots (QW-on-QDs) were employed to increase the maximum saturated gain of QD-based laser active medium. Room temperature optical properties of tunnel-coupled well-on-dots structures at low excitation were found to be sensitive to energy separation between GS energies of QDs and QW. The spectra also show that QD-related photoluminescence (PL) tends to peak at discrete energy separations from the QW peak, in this case multiples of ∼ 35meV (LO phonon energy). The optimized GS energy separation between QW and QDs was found to be close to the energy of the LO phonon. This structure demonstrated narrowing of the QD PL line down to 21.6 meV at T=77K, indicating efficient resonant tunneling of carriers from QW into QD ensemble states. All-epitaxial vertical cavity surface emitting lasers (VCSELs) with triple-pair tunnel QW-on-QDs as active medium demonstrated continuous wave mode lasing. Tunnel QDs-QW VCSELs exhibited 1.8 mA (Jth ∼ 800 A/cm2) minimum threshold current at QD GS emission wavelength, 1135 nm, with 0.7mW optical power and 12% light-current efficiency.
In order to develop nanoengineering methods to control electronic spectrum of self-assembled InAs quantum dots (QDs) grown by molecular beam epitaxy, we have utilized atomic force microscopy (AFM), photoluminescence (PL) and TEM methods to investigate the effects of capping layer growth on the physical/chemical properties as well as the optical/electronic performance of QD device structures. Capping layer material choice (or its absence all together) has been found to directly influence QD dimensions (size, height), and subsequently, to affect QD emission wavelength. We report results of QD lateral size and height as well as densities of InAs QDs capped with 2ML (monolayers) of AlAs or GaAs grown at various rates. Our AFM results are complemented by PL measurements, where the optical properties of capped versus non-capped QDs have been explored and direct correspondence between structural differences induced by capping and the electronic/optical properties of QDs is demonstrated. Analysis of the data shows that the results can be explained by two competing surface processes. The first of which is the redistribution of indium between QDs on top of the 2D wetting layer, resulting in the increase of QD size with time. The second effect is the diffusion of indium out of the QDs and onto the top of the capping layer. TEM with multislice image simulation has supported our AFM and PL observations with the demonstration of “indium driven” alloy intermixing in the overlayer as well as significant alloying in the InAs wetting layer.
A method for hybrid integration of III-V optoelectronic components on Si substrate using BCB was demonstrated. The method included bonding, selective wet etching of the GaAs substrate, components separation by wet etching, two-level metallization and lateral oxidation to form optical apertures. Simulations of thermal behavior and mechanical stresses of this integration scheme were performed using finite element analysis, which revealed adequate heat dissipation. Simulations show that this bonding protocol allows reduction of overheating and mechanical stress that enhances the optoelectronic device performance and increases reliability. Electro-luminescence spectrum, I-V and P-T characteristics were measured and compared with a reference homoepitaxial structure and the results of the simulations. Measured thermal impedance was found to be less then two times higher than that for the devices on a host GaAs wafer. Novel method of substrate removal named oxidation lift-off was proposed and demonstrated. This process allows to release a VCSEL structure with epitaxial DBRs and separate individual components on Si, reduces the number of process steps and eventually reduces cost of the fabricated devices. Au/Ge alloy was used for the metal bonding of the test oxidation lift-off structure. Substrate removal, device separation, bonding and formation of the oxide apertures were done within a single processing step.
A quantum dot (QD) medium is expected to demonstrate superior performance in various devices when compared with quantum wells (QWs). One area of interest has been the improved defect tolerance of QD media, though it was demonstrated at low temperatures so far. In this study, the defect tolerance of shape-engineered QD structures is compared with that of a QW structure at temperatures up to 300 K. To create high defect densities both QD and QW structures were irradiated with high energy (1.5 MeV) protons (with doses up to 3×1014 cm-2). Then, the relative luminescence efficiency was measured by variable temperature photoluminescence. The shape-engineered QD structure withstood two orders of magnitude higher defect density than the QWs at room temperature. This improvement is correlated with the activation energy for thermal evaporation of 390 meV, acquired through a kinetic model.
We have studied the first phases of post-growth evolution of InAs quantum dots (QDs) using in-situ Auger electron spectroscopy in conjunction with Reflection High Energy Electron Diffraction (RHEED). Direct evidence for InAs intermixing with about 6ML (monolayers) of the matrix material is found from Auger signal behavior during MBE overgrowth of InAs nanostructures. Re-establishment of 2D growth mode by overgrowth with GaAs or AlAs was monitored in single-layer and multi-layer QD structures using RHEED. Decay process of InAs QDs on the surface is found to have activation energy of about 1.1 eV that corresponds to In intermixing with the matrix rather than evaporation from the surface.
Integration of dense arrays of high frequency III-V photoemitters and photodetectors with Si platform is one of the challenging tasks for realization of novel chip-level optical interconnects. These interconnects require the resolution of numerous problems of compatibility of materials. Comparison of monolithic and hybrid integration technologies highlights the advantages of hybrid approaches for emitters highly sensitive to growth defects. A novel protocol for fabrication of III-V optoelectronic components on a Si platform is proposed. Reversed vertical cavity surface emitting laser (VCSEL) structures were grown homoepitaxialy by MBE on a GaAs substrate, and then bonded to a Si wafer using a benzocyclobutene (BCB) polymer. The GaAs substrate was subsequently removed by selective etching down to an AlAs etch stop layer. This reduces thermal stresses in order to enhance the optoelectronic device performance and increase lifetime. A 10 μm-thick high frequency VCSEL with coplanar metallization is processed on Si with PMGI reflow planarization. Electro-luminescence spectrum, I-V and P-T characteristics were measured and compared with a reference structure. It was found that measured thermal resistance is about five times higher than for devices on a host GaAs wafer.
The kinetics of the wet oxidation process of MBE-grown high-Al-content AlAs/Al0.6Ga0.4As short-period superlattices (SPSLs) was investigated and compared to AlGaAs alloys and pure AlAs. We found that alloys and superlattices (SLs) have different oxidation characteristics. These differences were attributed to traces of the superlattice structure in the oxidized material. The microstructure and chemistry of SPSLs with an equivalent composition of Al0.98Ga0.02As was studied, using transmission electron microscopy, energy-dispersive x-ray spectroscopy, Rutherford backscattering, and nuclear reaction analysis for hydrogen-profiling. We also report on the mechanical stability of oxidized SPSL layers in optoelectronic device structures.
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