Comparison of Optical Properties of GaN/AlGaN and InGaN/AlGaN Single Quantum Wells

Abstract

Static, field-modulated and time-resolved spectroscopies were carried out to compare the electronic states between GaN/AlGaN binary and InGaN/AlGaN ternary single quantum wells (SQWs). The internal field that exists across the quantum well (QW) naturally induces a quantum-confined Stark effect (QCSE), namely the redshift of the QW resonance energy and decrease of the electron-hole wavefunction overlap. The GaN/AlGaN SQW exhibited a weak emission due to QCSE. However, optical absorption and degenerate pump-probe measurements revealed that excitonic character was maintained for thin QWs with the well width nearly the same as the bulk free exciton Bohr radius even under an electric field as high as 0.73 MV/cm. A slightly In-alloyed InGaN SQW exhibited a bright luminescence peak in spite of an effective bandgap inhomogeneity in the QW, which was confirmed by the point excitation and monochromatic cathodoluminescence (CL) mapping method. The lateral interval of each light-emitting area was estimated from the spatial resolution of the CL mapping to be smaller than 60 nm. Such local potential minima is considered to be formed due to the presence of a structure similar to quantum-disks [M. Sugawara: Phys. Rev. B 51 (1995) 10743]. Carriers generated in the InGaN QWs are effectively localized in these regions to form localized QW excitons exhibiting highly efficient spontaneous emissions.

Recent developments in the III‐nitride materials

“…The structural problems present in these QWs can be simply tracked from the linewidth of the PL spectrum. As an example, with large spot excitation at low temperature for a low In composition of x = 0.1, in some work the FWHM value is about 35-50 meV [45,46], while other authors have material with the same composition with a half-width about 100 meV or larger [47][48][49]. These structural inhomogeneities have different origin: interface roughness and composition fluctuations.…”

Violet to deep-ultraviolet InGaN∕GaN and GaN∕AlGaN quantum structures for UV electroabsorption modulators

“…With the use of InGaN / GaN quantum structures, optoelectronic devices operating in visible to near UV are feasible, whereas deep-UV optoelectronic devices require the use of GaN / AlGaN quantum structures. [10][11][12] AlGaN based quantum structures, however, exhibit technical difficulties such as relatively slow growth rates, high dislocation densities of Al, and insufficient conductivity of doped layers. 13 In this work, to this end, we demonstrate four different quantum electroabsorption modulators with their operating wavelengths spanning from 400 to 270 nm by using InGaN and AlGaN based quantum structures in their active region as required for operating in visible to near-UV and deep-UV spectral ranges, respectively.…”

Photoluminescence in n-doped In0.1Ga0.9N/In0.01Ga0.99N multiple quantum wells

“…There are many investigations of such multiple quantum well (MQW) systems, with different conclusions about the physical origin of the optical emissions from such structures [2] [3] [4] [5] [6] [7]. A very common suggestion is that the main PL emission is due to zero-dimensional nm size features in the QWs, of a higher In content than the main matrix [2] [3] [4] [5] [6] [7] [8]. Evidence from structural investigations for the presence of such quantum dot (QD) like structures in many samples has been presented [9] [10].…”

“…It is generally agreed that the low temperature PL spectra of such MQWs are influenced by both localization and the internal polarization induced electric fields [2] [12], but the detailed physics is still under debate. The experimentally derived internal electric fields vary within wide limits, with some authors finding large fields [6], while in most cases the fields are generally much smaller than the theoretical predictions in the case of InGaN/GaN QWs [2] [13] [14].…”

“…It is generally agreed that the low temperature PL spectra of such MQWs are influenced by both localization and the internal polarization induced electric fields [2] [12], but the detailed physics is still under debate. The experimentally derived internal electric fields vary within wide limits, with some authors finding large fields [6], while in most cases the fields are generally much smaller than the theoretical predictions in the case of InGaN/GaN QWs [2] [13] [14]. The reason for the large spread in extracted values for the polarization fields, and the resulting disagreement with the predicted theoretical values [15] [16] [17] [18], is not understood, since for the AlGaN/GaN MQW system there seems to be a rather good agreement between experimental data and theoretical predictions concerning polarization induced electric fields [14].…”