Surface Atomic Arrangement of Aluminum Ultra-Thin Layers Grown on Si(111).

Surface atomic arrangement and physical properties of aluminum ultrathin layers on c-Si(111)-7 × 7 and hydrogen-terminated c-Si(111)-1 × 1 surfaces deposited using molecular beam epitaxy were investigated. X-ray photoelectron spectroscopy spectra were collected in two configurations (take-off angle of 0° and 45°) to precisely determine the surface species. Moreover, 3D atomic force microscopy (AFM) images of the air-exposed samples were acquired to investigate the clustering formations in film structure. The deposition of the Al layers was monitored in situ using a reflection high-energy electron diffraction (RHEED) experiments to confirm the surface crystalline structure of the c-Si(111). The analysis of the RHEED patterns during the growth process suggests the settlement of aluminum atoms in Al(111)-1 × 1 clustered formations on both types of surfaces. The surface electrical conductivity in both configurations was tested against atmospheric oxidation. The results indicate differences in conductivity based on the formation of various alloys on the surface.

Band Offsets of the MOCVD-Grown ß-(Al0.21Ga0.79)2O3/ß-Ga2O3 (010) Heterojunction.

The band offsets for the ß-(Al0.21Ga0.79)2O3/ß-Ga2O3 (010) heterojunction have been experimentally measured by X-ray photoelectron spectroscopy. High-quality ß-(Al0.21Ga0.79)2O3 films were grown by metal-organic chemical vapor deposition for characterization. The indirect band gap of ß-(Al0.21Ga0.79)2O3 was determined by optical transmission to be 4.69 ± 0.03 eV with a direct transition of 5.37 ± 0.03 eV, while ß-Ga2O3 was confirmed to have an indirect band gap of 4.52 ± 0.03 eV with a direct transition of 4.94 ± 0.03 eV. The resulting band alignment at the heterojunction was determined to be of type II with the valence and conduction band edges of ß-(Al0.21Ga0.79)2O3 being -0.26 ± 0.08 and 0.43 ± 0.08 eV, respectively, above those of ß-Ga2O3 (010). These values can now be used to help better design and predict the performance of ß-(AlxGa1-x)2O3 heterojunction-based devices.

Coherent-interface-induced strain in large lattice-mismatched materials: A new approach for modeling Raman shift.

Strain engineering as one of the most powerful techniques for tuning optical and electronic properties of Ill-nitrides requires reliable methods for strain investigation. In this work, we reveal, that the linear model based on the experimental data limited to within a small range of biaxial strains (< 0.2%), which is widely used for the non-destructive Raman study of strain with nanometer-scale spatial resolution is not valid for the binary wurtzite-structure group-III nitrides GaN and AlN. Importantly, we found that the discrepancy between the experimental values of strain and those calculated via Raman spectroscopy increases as the strain in both GaN and AlN increases. Herein, a new model has been developed to describe the strain-induced Raman frequency shift in GaN and AlN for a wide range of biaxial strains (up to 2.5%). Finally, we proposed a new approach to correlate the Raman frequency shift and strain, which is based on the lattice coherency in the epitaxial layers of superlattice structures and can be used for a wide range of materials. Electronic Supplementary Material: Supplementary material (Table S1: Values of bulk phonon deformation potentials and elastic constants for GaN and AlN from each reference used in Table 1, Fig. S1: Lattice parameters of SL layers using Eq. (8), and Fig. S2: Raman mapping using Eq. (7)) is available in the online version of this article at 10.1007/s12274-021-3855-4.

Photoluminescence of InAs/GaAs quantum dots under direct two-photon excitation.

Self-assembled quantum dots grown by molecular beam epitaxy have been a hotbed for various fundamental research and device applications over the past decades. Among them, InAs/GaAs quantum dots have shown great potential for applications in quantum information, quantum computing, infrared photodetection, etc. Though intensively studied, some of the optical nonlinear properties of InAs/GaAs quantum dots, specifically the associated two-photon absorption of the wetting and barrier layers, have not been investigated yet. Here we report a study of the photoluminescence of these dots by using direct two-photon excitation. The quadratic power law dependence of the photoluminescence intensity, together with the ground-state resonant peak of quantum dots appearing in the photoluminescence excitation spectrum, unambiguously confirms the occurrence of the direct two-photon absorption in the dots. A three-level rate equation model is proposed to describe the photogenerated carrier dynamics in the quantum dot-wetting layer-GaAs system. Moreover, higher-order power law dependence of photoluminescence intensity is observed on both the GaAs substrate and the wetting layer by two-photon excitation, which is accounted for by a model involving the third-harmonic generation at the sample interface. Our results open a door for understanding the optical nonlinear effects associated with this fundamentally and technologically important platform.

Type-II GaSb quantum dots grown on InAlAs/InP (001) by droplet epitaxy.

GaSb quantum dots (QDs) have been grown by droplet epitaxy within InAlAs barrier layers on an InP (001) substrate. The droplet growth mode facilitates a larger size (average height ∼4.5 nm) and a lower density (∼6.3 × 109 cm-2) for the QDs than would be expected for the 4% lattice mismatch between GaSb and InAlAs. A type-II band alignment between the GaSb QDs and the InAlAs barriers is revealed by photoluminescence (PL) through a prominent blue-shift of ∼0.11 eV resulting from a six orders of magnitude increase in excitation power. Further confirmation of the type-II nature of these QDs is found through time-resolved PL studies showing a biexponential decay with a long carrier lifetime of ∼10.9 ns. These observations reveal new information for understanding the formation and properties of GaSb/InAlAs/InP QDs, which may be an optimum system for the development of both efficient memory cells and photovoltaic devices.

Interplay Effect of Temperature and Excitation Intensity on the Photoluminescence Characteristics of InGaAs/GaAs Surface Quantum Dots.

We investigate the optical properties of InGaAs surface quantum dots (SQDs) in a composite nanostructure with a layer of similarly grown buried quantum dots (BQDs) separated by a thick GaAs spacer, but with varied areal densities of SQDs controlled by using different growth temperatures. Such SQDs behave differently from the BQDs, depending on the surface morphology. Dedicated photoluminescence (PL) measurements for the SQDs grown at 505 °C reveal that the SQD emission follows different relaxation channels while exhibiting abnormal thermal quenching. The PL intensity ratio between the SQDs and BQDs demonstrates interplay between excitation intensity and temperature. These observations suggest a strong dependence on the surface for carrier dynamics of the SQDs, depending on the temperature and excitation intensity.

Correlation between photoluminescence and morphology for single layer self-assembled InGaAs/GaAs quantum dots.

Single layer self-assembled InGaAs quantum dots (QDs) are manipulated by using different arsenic species on GaAs (100) surface. The As4 molecules are experimentally observed to be more promising than As2 to promote the formation of one-dimensionally-aligned QD-chain arrays. The lateral alignment of QDs and the corresponding formation of dot chains are explained by the anisotropic surface kinetics in combination with the different reactivities of the two molecules with bonding sites on the GaAs (100) surface. Photoluminescence (PL) measurements demonstrate that the spectra of the QD-chains broaden to higher energy and increases in intensity with increasing excitation laser power. The PL band of the QD-chains also exhibits a 9 meV reduction in linewidth as temperature increases starting from 8 K. These observations confirm an efficient lateral coupling between neighboring QDs and thereafter polarized QD emission, whereas the randomly distributed QDs grown with As2 show no preferential polarization. Such QD-chains exhibiting anisotropic properties have the potential for nanophotonics applications like electro-optic modulators with very low drive voltage and ultra-wide bandwidth operation.

Polarization Effects in Graded AlGaN Nanolayers Revealed by Current-Sensing and Kelvin Probe Microscopy.

We experimentally demonstrate that the conductivity of graded AlxGa1-xN increases as a function of the magnitude of the Al concentration gradient (%Al/nm) due to polarization doping effects, without the use of impurity dopants. Using three up/down-graded AlxGa1-xN nanolayers with Al gradients ranging from ∼0.16 to ∼0.28%Al/nm combined in one structure, the effects of polarization engineering for localized electric fields and current transport were investigated. Cross-sectional Kelvin probe force microscopy and conductive atomic force microscopy were used to directly probe the electrical properties of the films with spatial resolution along the thickness of the growth. The experimental profiles of the built-in electric fields and the spreading current found in the graded layers are shown to be consistent with simulations of the field distribution as well as of the electron and hole densities. Finally, it was directly observed that for gradients less than 0.28%Al/nm the native n-type donors still limit polarization-induced hole doping, making p-type conductivity still a challenge due to background impurities and defects.

Infrared Reflectance Analysis of Epitaxial n-Type Doped GaN Layers Grown on Sapphire.

Infrared (IR) reflectance spectroscopy is applied to study Si-doped multilayer n+/n0/n+-GaN structure grown on GaN buffer with GaN-template/sapphire substrate. Analysis of the investigated structure by photo-etching, SEM, and SIMS methods showed the existence of the additional layer with the drastic difference in Si and O doping levels and located between the epitaxial GaN buffer and template. Simulation of the experimental reflectivity spectra was performed in a wide frequency range. It is shown that the modeling of IR reflectance spectrum using 2 × 2 transfer matrix method and including into analysis the additional layer make it possible to obtain the best fitting of the experimental spectrum, which follows in the evaluation of GaN layer thicknesses which are in good agreement with the SEM and SIMS data. Spectral dependence of plasmon-LO-phonon coupled modes for each GaN layer is obtained from the spectral dependence of dielectric of Si doping impurity, which is attributed to compensation effects by the acceptor states.

Photoluminescence Study of the Interface Fluctuation Effect for InGaAs/InAlAs/InP Single Quantum Well with Different Thickness.

Photoluminescence (PL) is investigated as a function of the excitation intensity and temperature for lattice-matched InGaAs/InAlAs quantum well (QW) structures with well thicknesses of 7 and 15 nm, respectively. At low temperature, interface fluctuations result in the 7-nm QW PL exhibiting a blueshift of 15 meV, a narrowing of the linewidth (full width at half maximum, FWHM) from 20.3 to 10 meV, and a clear transition of the spectral profile with the laser excitation intensity increasing four orders in magnitude. The 7-nm QW PL also has a larger blueshift and FWHM variation than the 15-nm QW as the temperature increases from 10 to ~50 K. Finally, simulations of this system which correlate with the experimental observations indicate that a thin QW must be more affected by interface fluctuations and their resulting potential fluctuations than a thick QW. This work provides useful information on guiding the growth to achieve optimized InGaAs/InAlAs QWs for applications with different QW thicknesses.

Investigation of electrically active defects in InGaAs quantum wire intermediate-band solar cells using deep-level transient spectroscopy technique.

InGaAs quantum wire (QWr) intermediate-band solar cell-based nanostructures grown by molecular beam epitaxy are studied. The electrical and interface properties of these solar cell devices, as determined by current-voltage (I-V) and capacitance-voltage (C-V) techniques, were found to change with temperature over a wide range of 20-340 K. The electron and hole traps present in these devices have been investigated using deep-level transient spectroscopy (DLTS). The DLTS results showed that the traps detected in the QWr-doped devices are directly or indirectly related to the insertion of the Si δ-layer used to dope the wires. In addition, in the QWr-doped devices, the decrease of the solar conversion efficiencies at low temperatures and the associated decrease of the integrated external quantum efficiency through InGaAs could be attributed to detected traps E1QWR_D, E2QWR_D, and E3QWR_D with activation energies of 0.0037, 0.0053, and 0.041 eV, respectively.

Comparative study of photoluminescence from In0.3Ga0.7As/GaAs surface and buried quantum dots.

The optical properties of In0.3Ga0.7As/GaAs surface quantum dots (SQDs) and buried QDs (BQDs) are investigated by photoluminescence (PL) measurements. The integrated PL intensity, linewidth, and lifetime of SQDs are significantly different from the BQDs both at room temperature and at low temperature. The differences in PL response, measured at both steady state and in transient, are attributed to carrier transfer between the surface states and the SQDs.

The Peculiarities of Strain Relaxation in GaN/AlN Superlattices Grown on Vicinal GaN (0001) Substrate: Comparative XRD and AFM Study.

Superlattices (SLs) consisting of symmetric layers of GaN and AlN have been investigated. Detailed X-ray diffraction and reflectivity measurements demonstrate that the relaxation of built-up strain in the films generally increases with an increasing number of repetitions; however, an apparent relaxation for subcritical thickness SLs is explained through the accumulation of Nagai tilt at each interface of the SL. Additional atomic force microscopy measurements reveal surface pit densities which appear to correlate with the amount of residual strain in the films along with the appearance of cracks for SLs which have exceeded the critical thickness for plastic relaxation. These results indicate a total SL thickness beyond which growth may be limited for the formation of high-quality coherent crystal structures; however, they may indicate a growth window for the reduction of threading dislocations by controlled relaxation of the epilayers.

X-ray Reciprocal Space Mapping of Graded Al x Ga1 - x N Films and Nanowires.

The depth distribution of strain and composition in graded Al x Ga1 - x N films and nanowires (NWs) are studied theoretically using the kinematical theory of X-ray diffraction. By calculating [Formula: see text] reciprocal space maps (RSMs), we demonstrate significant differences in the intensity distributions from graded Al x Ga1 - x N films and NWs. We attribute these differences to relaxation of the substrate-induced strain on the NWs free side walls. Finally, we demonstrate that the developed X-ray reciprocal space map model allows for reliable depth profiles of strain and Al composition determination in both Al x Ga1 - x N films and NWs.

Defect-Free Self-Catalyzed GaAs/GaAsP Nanowire Quantum Dots Grown on Silicon Substrate.

The III-V nanowire quantum dots (NWQDs) monolithically grown on silicon substrates, combining the advantages of both one- and zero-dimensional materials, represent one of the most promising technologies for integrating advanced III-V photonic technologies on a silicon microelectronics platform. However, there are great challenges in the fabrication of high-quality III-V NWQDs by a bottom-up approach, that is, growth by the vapor-liquid-solid method, because of the potential contamination caused by external metal catalysts and the various types of interfacial defects introduced by self-catalyzed growth. Here, we report the defect-free self-catalyzed III-V NWQDs, GaAs quantum dots in GaAsP nanowires, on a silicon substrate with pure zinc blende structure for the first time. Well-resolved excitonic emission is observed with a narrow line width. These results pave the way toward on-chip III-V quantum information and photonic devices on silicon platform.

Nanoscale Electrostructural Characterization of Compositionally Graded Al(x)Ga(1-x)N Heterostructures on GaN/Sapphire (0001) Substrate.

We report on AlxGa1-xN heterostructures resulting from the coherent growth of a positive then a negative gradient of the Al concentration on a [0001]-oriented GaN substrate. These polarization-doped p-n junction structures were characterized at the nanoscale by a combination of averaging as well as depth-resolved experimental techniques including: cross-sectional transmission electron microscopy, high-resolution X-ray diffraction, Rutherford backscattering spectrometry, and scanning probe microscopy. We observed that a small miscut in the substrate orientation along with the accumulated strain during growth led to a change in the mosaic structure of the AlxGa1-xN film, resulting in the formation of macrosteps on the surface. Moreover, we found a lateral modulation of charge carriers on the surface which were directly correlated with these steps. Finally, using nanoscale probes of the charge density in cross sections of the samples, we have directly measured, semiquantitatively, both n- and p-type polarization doping resulting from the gradient concentration of the AlxGa1-xN layers.

State filling dependent luminescence in hybrid tunnel coupled dot-well structures.

A strong dependence of quantum dot (QD)-quantum well (QW) tunnel coupling on the energy band alignment is established in hybrid InAs/GaAs-In(x)Ga(1-x)As/GaAs dot-well structures by changing the QW composition to shift the QW energy through the QD wetting layer (WL) energy. Due to this coupling a rapid carrier transfer from the QW to the QD excited states takes place. As a result, the QW photoluminescence (PL) completely quenches at low excitation intensities. The threshold intensities for the appearance of the QW PL strongly depend on the relative position of the QW excitonic energy with respect to the WL ground state and the QD ground state energies. These intensities decrease by orders of magnitude as the energy of the QW increases to approach that of the WL due to the increased efficiency for carrier tunneling into the WL states as compared to the less dense QD states below the QW energy.

Substrate effects on the strain relaxation in GaN/AlN short-period superlattices.

We present a comparative study of the strain relaxation of GaN/AlN short-period superlattices (SLs) grown on two different III-nitride substrates introducing different amounts of compensating strain into the films. We grow by plasma-assisted molecular beam epitaxy (0001)-oriented SLs on a GaN buffer deposited on GaN(thick)-on-sapphire template and on AlN(thin)-on-sapphire template. The ex-situ analysis of strain, crack formation, dislocation density, and microstructure of the SL layers has established that the mechanism of strain relaxation in these structures depends on the residual strain in substrate and is determined mainly by the lattice mismatch between layers. For growth on the AlN film, the compensating strain introduced by this film on the layer prevented cracking; however, the densities of surface pits and dislocations were increased as compared with growth on the GaN template. Three-dimensional growth of the GaN cap layer in samples with pseudomorphly grown SLs on the AlN template is observed. At the same time, two-dimensional step-flow growth of the cap layer was observed for structures with non-pseudomorphly grown SLs on the GaN template with a significant density of large cracks appearing on the surface. The growth mode of the GaN cap layer is predefined by relaxation degree of top SL layers.