multiple-QW Structure Research Articles

The effects of varying threading dislocation density on the optical properties of InGaN/GaN quantum wells

AbstractThe effects of the threading dislocation (TD) density on the optical properties of a series of comparable InxGa1–xN/ GaN multiple QW structures were studied. The TD density ranged from 2 × 107 cm–2, for a structure grown on a free‐standing GaN substrate, to 5 × 109 cm–2 grown on a sapphire substrate. Room temperature internal quantum efficiencies (IQEs) were determined by temperature dependent photoluminescence (PL); no systematic dependence of the IQE on the TD density was found. The excitation power density dependence of the efficiency was investigated, which also showed no systematic dependence on TD density. PL excitation spectroscopy was used to verify that equivalent carrier densities were generated within the QWs of each structure. The lack of systematic dependence of the optical properties on TD density is attributed to the strong carrier localisation in InGaN/GaN QWs. At the highest density of TDs studied, it is estimated that the average defect separation greatly exceeds the in‐plane diffusion lengths of electrons and holes; consequently the majority of carriers in the system are isolated from the TDs. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Thickness evaluation of InGaAs/InAlAs quantum wells

This work proposes a new optoelectronic measurement of quantum well (QW) thickness and applies it to doped and undoped In0.53Ga0.47As/In0.52Al0.48As multiple-QW structures. Near-infrared spectroscopic identification of the interband optical transition at 100–300 K gave the eigenenergies of the conduction band in the QW. Evaluation of the QW thickness involved analysis of the effective mass at the corresponding eigenenergy. QW thicknesses in the range of 5.45–20.8 nm were determined in six different wafers. These thicknesses agreed well with the QW thicknesses estimated by double-crystal x-ray diffraction within almost two monolayers. This measurement was used to determine the distance of potential boundaries confining the electron wave functions.

Modeling of injection characteristics of polar and nonpolar III-nitride multiple quantum well structures

Carrier confinement and injection characteristics of polar and nonpolar III-nitride quantum well (QW) light-emitting diode or laser diode structures are compared. We demonstrate that strongly inhomogeneous QW injection in multiple-QW (MQW) active region is one of the possible reasons holding back the advance of nonpolar laser structures. In polar structures, strong interface polarization charges induce the nonuniform carrier distribution among the active QWs so that the extreme p-side QW always dominates the optical emission. On the contrary, in nonpolar MQW structures, the inhomogeneity of QW populations is supported mainly by QW residual charges and the prevailing QW is the one closest to the n-side of the diode. For both polar and nonpolar structures, the QW injection inhomogeneity is strongly affected by the QW carrier confinement and becomes more pronounced in longer wavelength emitters with deeper active QWs. We show that in nonpolar structures indium incorporation into optical waveguide layers improves the uniformity of QW injection. On the contrary, QW injection in polar structures remains inhomogeneous even at high-indium waveguide layer compositions. We show, however, that polarization-matched design of the electron-blocking layer can noticeably improve the injection uniformity in polar MQW structure and enhance the structure internal quantum efficiency.

Time-resolved cathodoluminescence study of carrier relaxation, transfer, collection, and filling in coupledInxGa1−xN∕GaNmultiple and single quantum wells

We have examined in detail the optical properties and carrier capture dynamics of coupled ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}∕\mathrm{Ga}\mathrm{N}$ multiple and single quantum well (MQW and SQW) structures that possess various numbers of QWs in the confinement region adjacent to a SQW. The aim is to study the influence of the structure of an InGaN MQW confinement region on carrier transfer and collection into a coupled SQW. By applying in a complementary way temperature- and excitation-dependent cathodoluminescence (CL) spectroscopy and time-resolved CL measurements, we have analyzed the carrier dynamics and state filling in the SQW and the adjacent MQW. We solved self-consistently the nonlinear Poisson-Schr\"odinger equation for wurtzite materials including strain, deformation potentials, and piezoelectric field of our ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}∕\mathrm{Ga}\mathrm{N}$ single and multiple QW structures to obtain the excitation-dependent eigenstates that are used to calculate band filling, excitonic lifetimes, and exciton binding energies. We show that it is possible to treat a coupled ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ single and multiple QW system in a way that allows for a determination of the quasi-Fermi levels, carrier densities in separated QWs, luminescence efficiencies, thermal activation energies for carrier transfer, and carrier capture and recombination rates. We demonstrate in this unique method an improved determination of the piezoelectric field and In composition $x$ by a non-contact optical means alone. The results demonstrate an enhanced luminescence efficiency and yet decreased carrier capture rate by the SQW as the number of QWs increases in the adjacent MQW confinement region.

AlGaN-GaN UV light-emitting diodes grown on SiC by metal-organic chemical vapor deposition

We report the study of the electrical and optical characteristics of AlGaN-GaN quantum-well (QW) ultraviolet light-emitting diodes grown on SiC by metal-organic chemical vapor deposition. These devices exhibit room-temperature electroluminescence emission peaked at /spl lambda/ = 363 nm with a narrow linewidth of /spl Delta//spl lambda/ = 9 nm under high-current-density dc injection. We have also applied a Mg-doped AlGaN-GaN superlattice structure as a p-cladding layer and vertical-geometry hole conduction improvement has been verified. A comparative study of the performance of light-emitting devices with single-QW and multiple-QW structures indicates that the single-QW structure is preferred.

Built-in electric-field effects in wurtzite AlGaN/GaN quantum wells

AlGaN/GaN quantum well (QW) structures are grown on c-plane sapphire substrates by molecular beam epitaxy. Control at the monolayer scale of the well thickness is achieved and sharp QW interfaces are demonstrated by the low photoluminescence linewidth. The QW transition energy as a function of the well width evidences a quantum-confined Stark effect due to the presence of a strong built-in electric field. Its origin is discussed in terms of piezoelectricity and spontaneous polarization. Its magnitude versus the Al mole fraction is determined. The role of the sample structure geometry on the electric field is exemplified by changing the thickness of the AlGaN barriers in multiple-QW structures. Straightforward electrostatic arguments well account for the overall trends of the electric-field variations.

Photoluminescence characteristics and pit formation of InGaN/GaN quantum-well structures grown on sapphire substrates by low-pressure metalorganic vapor phase epitaxy

We have examined how a growth interruption, caused by closing group-III sources, affects the crystalline quality of InGaN/GaN quantum-well (QW) structures grown by metalorganic vapor phase epitaxy. The QW samples were characterized by their photoluminescence (PL), and by atomic force microscopy (AFM), transmission electron microscopy (TEM), and energy dispersive x-ray (EDX) microanalysis. The PL peak wavelength was strongly dependent on the duration of the growth interruption and on the number of QW layers. AFM measurements revealed that the size of the open hexagonally shaped pits in the QW structures increased dramatically as the interruption duration was lengthened. Through TEM and EDX microanalysis, we found that the formation of these hexahedronal pits, formed due to the growth interruption, causes a large fluctuation in the In composition, especially around the pits, and the presence of such pits in an underlying QW layer strongly affects the In incorporation into the upper QW layers, leading to significant growth-rate variation in an InGaN QW layer and red-shifting of the PL spectra when a multiple-QW structure is grown.