E. A. DeCuir

Intersublevel Infrared Photodetector with Strain-Free GaAs Quantum Dot Pairs Grown by High-Temperature Droplet Epitaxy

Normal incident photodetection at mid infrared spectral region is achieved using the intersublevel transitions from strain-free GaAs quantum dot pairs in Al(0.3)Ga(0.7)As matrix. The GaAs quantum dot pairs are fabricated by high temperature droplet epitaxy, through which zero strain quantum dot pairs are obtained from lattice matched materials. Photoluminescence, photoluminescence excitation optical spectroscopy, and visible-near-infrared photoconductivity measurement are carried out to study the electronic structure of the photodetector. Due to the intersublevel transitions from GaAs quantum dot pairs, a broadband photoresponse spectrum is observed from 3 to 8 microm with a full width at half-maximum of approximately 2.0 microm.

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.

Near-infrared intersubband absorption in nonpolar cubic GaN∕AlN superlattices

Optical absorption spectra related to intersubband transitions in molecular beam epitaxially grown nonpolar cubic-GaN∕AlN superlattices were observed in the spectral range of 1.5–2.00μm. The background doping was measured using an electrochemical capacitance-voltage technique and found to be on the order of 1018cm−3. This doping level yields a Fermi energy level slightly above the ground state energy level enabling intersubband transitions to occur. The existence of the intersubband transition is verified in several samples with different well widths. The observed peak position energy of the intersubband transition is compared to those calculated using a transfer matrix method.

Cubic GaN/AlN multiple quantum well photodetector

Photodetectors based on intersubband transitions in molecular beam epitaxially grown cubic GaN∕AlN multiple quantum wells were fabricated and tested. The presence of the intersubband transition was confirmed by using the optical absorption technique for structures with different well widths. Samples were polished into waveguide configuration on which the devices were fabricated. The photoresponse spectra were collected in the temperature range of 77–215K under the influence of small bias voltages. All devices exhibit photovoltaic effect where the photoresponse is observed at zero bias voltage. Theoretical calculations of the intersubband transition were performed and found to be in agreement with the observed results.

Tuning In0.3Ga0.7As∕GaAs multiple quantum dots for long-wavelength infrared detectors

Optical absorption spectra of intersubband transitions in In0.3Ga0.7As∕GaAs multiple quantum dots were investigated using the optical absorption as a function of the number of In0.3Ga0.7As monolayers deposited using the molecular-beam epitaxy Stranski–Krastanow technique. The peak position energy reached 13.7μm for a sample containing 50 monolayers of In0.3Ga0.7As. The lack of the observation of intersubband transitions in small quantum dots, where the number of the deposited monolayer is less than 15 monolayers, is an indication of the absence of quantum confinement. On the other hand, the presence of high dislocations density in larger quantum dots, where the deposited number of monolayers exceeds 50, could be the reason of why the intersubband transitions are degraded.

Mechanism of strain-influenced quantum well thickness reduction in GaN/AlN short-period superlattices

We report on the mechanism of strain-influenced quantum well (QW) thickness reduction in GaN/AlN short-period superlattices grown by plasma-assisted molecular beam epitaxy. Density functional theory was used to support the idea of a thermally activated exchange mechanism between Al adatoms and Ga surface atoms that is influenced by the strain state of the GaN QWs. These ab initio calculations support our experimentally observed reduction in QW thickness for different intrinsic strains.

Influence of template type and buffer strain on structural properties of GaN multilayer quantum wells grown by PAMBE, an x-ray study

The influence of template type and residual strain of the buffer layer on the structural properties of GaN/AlN superlattices (SLs) was studied using high resolution x-ray diffraction. Using sapphire substrates, an effective thinning of the GaN quantum wells and the corresponding thickening of the AlN barriers were observed in SL structures grown on thin, strained AlN templates as compared with SL structures grown on thick, relaxed GaN templates. Moreover, a bimodal strain relaxation of SL structures in dependence of template type was observed. The SLs grown on AlN templates relax predominantly by the formation of misfit dislocations, while the SLs grown on GaN templates relax predominantly by cracking of the layers. We explain these effects by the influence of residual strain in the buffer/template systems used for the growth processes of SL layers. A correlation is made between the strain state of the system and the cracking processes, the dislocation density, the radius of curvature and the layer thickness.

Enhanced photoluminescence from InAs/GaAs surface quantum dots by using a Si-doped interlayer

Photoluminescence (PL) of InAs/GaAs surface quantum dots (QDs) is enhanced by implanting a silicon-doped GaAs interlayer beneath surface QDs. It is observed that setting the doping concentration to 2.3 ? 1017?cm?3 in the doped GaAs interlayer spaced 10?nm from the surface QDs results in optimal QD PL, i.e., highest intensity and narrowest linewidth. This improvement is attributed to the effective enhancement of the photo-excited carrier capture into the surface QDs and a filling of surface states due to free-carrier transfer from the doped GaAs layer to the surface QDs.

Confocal Raman depth-scanning spectroscopic study of phonon−plasmon modes in GaN epilayers

Coupled longitudinal-optical (LO)-phonon-plasmon excitations were studied using confocal micro-Raman spectroscopy. The high-quality Si-doped GaN epilayers were grown in a Gunn diode structure on (0001) sapphire substrates by plasma assisted molecular beam epitaxy. Depth-profiled Raman spectra exhibit a spatial variation of both low, ω-, and high, ω+, frequency coupled phonon−plasmon modes (CPPMs) in the n-GaN layers. To describe the features of the CPPMs in the Raman spectra a self-consistent model that includes both the electro-optic and deformation-potential as well as charge-density fluctuation mechanisms as important processes for light scattering in n-GaN has been proposed. An agreement between the theoretical and experimental line shapes of the Raman spectra is obtained. From the best line-shape fitting of the CPPMs the depth profiles of the plasmon and phonon damping, plasmon frequency, free carrier concentrations, and electron mobility as well as the contributions of the electron−phonon interactio...

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.