Bandgap Inhomogeneity Research Articles

Transfer mechanisms and geometry effect on the dynamics of excitons in boron-containing GaAs alloys: Time-resolved photoluminescence investigation

The recombination dynamics in BxGa1-xAs alloys on GaAs (001) grown by metal-organic chemical vapor (MOCVD) are investigated using steady-state photoluminescence and Time-Resolved Photoluminescence (TRPL) measurements in the nanosecond range. The decay time as a function of the emission energy and the geometry of the sample (quantum well, epilayer) and post-growth treatment (annealing) is slower for lower-energy emission. In correlation with a systematic study on the photoluminescence features, we have studied the temperature effect on the decay time of the photogenerated carrier. Depending on the geometry of the sample, the exciton dynamics is assisted by the localization phenomenon in the boron-based ternary alloys. It is shown that, at low temperatures, the transfer is mainly ensured by tunneling through localized states where the recombination process is found to be purely radiative in nature. At intermediate temperatures, a competitive process between radiative and non-radiative recombination takes place. For high temperatures, the transfer is assisted by thermal activation, leading to purely non-radiative recombination. We have shown that the unknown state-of-the-art kinetic parameters of the e stage of carrier relaxation and additional carrier recombination dynamics, divulges the particular role of bandgap inhomogeneity and the below-band gap states for the relaxation dynamics that can contribute to the photogeneration mechanisms in BxGa1-xAs/GaAs alloys. Indeed the research findings of this article will help to gain insight into the preliminary carrier relaxation dynamics in boron-based materials.

Carrier Dynamics in Thin Germanium–Tin Epilayers

The Si-based mid-infrared photonics is an emerging technology in which group-IV germanium–tin (Ge1–xSnx) binary alloys can play a fundamental role in the development of a Si-compatible photonic components including monolithically integrated coherent light sources and detectors, on the same Si or SOI substrate. Although the Ge1–xSnx-on-Si lasers, at low temperatures, have already been demonstrated, the knowledge of the material properties necessary for such device optimization and real-life usage is very limited. In particular, carrier relaxation kinetics, relaxation pathways, and accompanied physical mechanisms, important for the laser’s dynamics, have not been subjected to in-depth research and understanding. In this work, we present detailed spectroscopic studies on photoinjected carrier dynamics in Ge1–xSnx epilayers, as a function of Sn content (6–12%) and temperature (20–300 K), by utilizing time-resolved differential reflectivity and photoluminescence. The latter technique allowed us to track separated electron and hole dynamics with a femtosecond time resolution, while the former experiment exploited a joined electron–hole recombination. This experimental approach allowed us to identify (i) two initial electron relaxation processes after photoexcitation; (ii) radiative electron–hole recombination on below-band gap states; (iii) nonradiative carrier recombination involving the Shockley–Read–Hall mechanism; and (iv) nonradiative recombination through the surface states. The research results significantly expand the knowledge on the initial carrier relaxation dynamics in the Ge1–xSnx epitaxial material. It provides unknown up-to-date kinetic parameters of the initial stage of electron relaxation and further carrier recombination dynamics, unveils the critical role of band gap inhomogeneity for the relaxation dynamics, and highlights the role of below-band gap states that can participate in the light generation process in Ge1–xSnx epilayers.

Perovskite Solar Cells: Imaging Spatial Variations of Optical Bandgaps in Perovskite Solar Cells (Adv. Energy Mater. 7/2019)

In article number 1802790, Daniel Macdonald, Hieu T. Nguyen, and co-workers develop a fast, non-destructive, camera-based method to capture optical bandgap images of perovskite solar cells with micron-scale spatial resolution. This technique allows them to probe relative variations in optical bandgaps across various solar cells, and to resolve optical bandgap inhomogeneity within the same device due to material degradation and impurities. The results are independently confirmed with other optical-based techniques.

Imaging Spatial Variations of Optical Bandgaps in Perovskite Solar Cells

AbstractA fast, nondestructive, camera‐based method to capture optical bandgap images of perovskite solar cells (PSCs) with micrometer‐scale spatial resolution is developed. This imaging technique utilizes well‐defined and relatively symmetrical band‐to‐band luminescence spectra emitted from perovskite materials, whose spectral peak locations coincide with absorption thresholds and thus represent their optical bandgaps. The technique is employed to capture relative variations in optical bandgaps across various PSCs, and to resolve optical bandgap inhomogeneity within the same device due to material degradation and impurities. Degradation and impurities are found to both cause optical bandgap shifts inside the materials. The results are confirmed with micro‐photoluminescence spectroscopy scans. The excellent agreement between the two techniques opens opportunities for this imaging concept to become a quantified, high spatial resolution, large‐area characterization tool of PSCs. This development continues to strengthen the high value of luminescence imaging for the research and development of this photovoltaic technology.

Bandgap Inhomogeneity of a PbSe Quantum Dot Ensemble from Two-Dimensional Spectroscopy and Comparison to Size Inhomogeneity from Electron Microscopy

Femtosecond two-dimensional Fourier transform spectroscopy is used to determine the static bandgap inhomogeneity of a colloidal quantum dot ensemble. The excited states of quantum dots absorb light, so their absorptive two-dimensional (2D) spectra will typically have positive and negative peaks. It is shown that the absorption bandgap inhomogeneity is robustly determined by the slope of the nodal line separating positive and negative peaks in the 2D spectrum around the bandgap transition; this nodal line slope is independent of excited state parameters not known from the absorption and emission spectra. The absorption bandgap inhomogeneity is compared to a size and shape distribution determined by electron microscopy. The electron microscopy images are analyzed using new 2D histograms that correlate major and minor image projections to reveal elongated nanocrystals, a conclusion supported by grazing incidence small-angle X-ray scattering and high-resolution transmission electron microscopy. The absorption bandgap inhomogeneity quantitatively agrees with the bandgap variations calculated from the size and shape distribution, placing upper bounds on any surface contributions.

Effects of In composition on ultraviolet emission efficiency in quaternary InAlGaN light-emitting diodes on freestanding GaN substrates and sapphire substrates

350 - nm -band quaternary InAlGaN quantum-well (QW) structures have been fabricated on low-dislocation-density GaN substrates and sapphire substrates, and the impact of the In composition on ultraviolet emission efficiency has been investigated. Photoluminescence, cathodoluminescence, and atomic force microscopy measurements have revealed that with increasing In concentration, band-gap inhomogeneity in InAlGaN layers is promoted and quantum efficiency becomes less sensitive to dislocation densities. However, this does not necessarily result in the enhancement of QW emission efficiency, because both a decrease in growth temperature and an increase in Al concentration degrade the InAlGaN crystalline quality. It is found that improving the InAlGaN crystalline quality by increasing the growth temperature while maintaining an In fluctuation effect with minimal In composition is essential for highly efficient InAlGaN QWs. The utilization of the optimized InAlGaN growth condition significantly improves the output power of 350-nm-band light-emitting diodes on GaN and sapphire substrates to 7.4mW at 400mA and 2.9mW at 200mA, respectively.

Localized exciton dynamics in InGaN quantum well structures

InGaN multiple quantum well laser diode (LD) wafer that lased at 400 nm was shown to have the InN mole fraction, x, of only 6% in the wells. Nanometer-probe compositional analysis showed that the fluctuation of x was as small as 1% or less, which is the resolution limit. However, the wells exhibited a Stokes-like shift (SS) of 49 meV and an effective localization depth E 0 was estimated by time-resolved photoluminescence (TRPL) measurement to be 35 meV at 300 K. Since the effective electric field due to polarization in the wells is estimated to be as small as 286 kV/cm, SS is considered to originate from an effective bandgap inhomogeneity. Because the well thickness fluctuation was insufficient to produce SS or E 0, the exciton localization is considered to be an intrinsic phenomenon in InGaN material. Indeed, bulk cubic In 0.1Ga 0.9N, which does not suffer any polarization field or thickness fluctuation effect, exhibited an SS of 140 meV at 77 K and similar TRPL results. The origin of the localization is considered to be due to the large bandgap bowing and In clustering in InGaN material. Such shallow and low density localized states are leveled by injecting high density carriers under the lasing conditions for the 400 nm LDs.

Optical and structural studies in InGaN quantum well structure laser diodes

An InGaN multiple-quantum-well laser diode wafer that lased at around 400 nm was shown to have the InN mole fraction, x, of only 6% in the wells. Nanometer-probe compositional analysis showed that the fluctuation of x was as small as 1% or less, which is the resolution limit. However, the wells exhibited a Stokes-like shift (SS) of 49 meV at 300 K, which was approximately 65% of the luminescence linewidth, and effective localization depth, E0, was estimated to be 35 meV at 300 K. Since the effective electric field due to polarization in the wells was estimated to be as small as 300 kV/cm, SS was considered to originate from effective band-gap inhomogeneity. Because the well thickness fluctuation was insufficient to reproduce SS or E0 and bulk cubic In0.02Ga0.98N that does not suffer any polarization field or thickness fluctuation effect exhibited a SS of 140 meV at 77 K, the exciton localization is considered to be an intrinsic phenomenon in InGaN, which is due to the large band-gap bowing and In clustering in InGaN material. The spontaneous emission from the InGaN wells was thus assigned as being due to the recombination of excitons localized at the exponential tail-type potential minima in the density of states. The upper bound of the lateral localization size has been estimated to be 50 nm. Such shallow and low density localized states are leveled by injecting high density carriers under the lasing conditions.

Current-Voltage Characteristics of p-n Structures with Band Gap Inhomogeneity in Base Region

The paper theoretically investigates the peculiarities of the reverse branch of current–voltage characteristics (CVC) of p–n structures whose base region contains a wide band gap layer. The analysis carried out is based on the derived analytical formula for the reverse current connected with the thermal generation of carriers both in the base region and at the Ohmic contact. It is shown that in the case when the wide-gap-layer is remote from the metallurgical edge of p–n junction at a distance comparable with the thickness of the space charge region (SCR) a portion of negative differential resistance (NDR) may appear on the reverse branch of CVC.

Current–Voltage Characteristics of p–n Structures with Band Gap Inhomogeneity in Base Region

Current–Voltage Characteristics of p–n Structures with Band Gap Inhomogeneity in Base Region

Impact of Internal Electric Field and Localization Effect on Quantum Well Excitons in AlGaN/GaN/InGaN Light Emitting Diodes

Strained InxGa1—xN quantum wells (QWs) on thick GaN base layers were investigated to verify the importance of localized QW excitons in their spontaneous emission mechanisms. A strength of the internal piezoelectric field (FPZ) across the QW increases with increasing x up to 1.4 MV/cm for x = 0.25, since the in-plain strain increases. For the QWs with the well thickness L greater than 3 nm, FPZ dominates the emission peak energy due to the quantum-confined Stark effect. Absorption spectra of both hexagonal and cubic InGaN QWs exhibited a broad band-tail regardless of the presence of FPZ normal to the QW plane. The luminescence peak energy of the 3 nm thick QWs was higher than the bandgap energy of the unstrained bulk crystal for x < 0.15, showing that doping of Si in barriers or injection of carriers effectively screens the field. The emission lifetime increased with increasing monitoring wavelength. Also, a temperature-induced change in the luminescence peak energy decreased with increasing x. The real-space variation of the luminescence peak energy was confirmed by the spatially-resolved monochromatic cathodoluminescence mapping method. The localization depth increases with increasing x. The carrier localization is confirmed to originate from the effective bandgap inhomogeneity due to a fluctuation of the local InN mole fraction, which is enhanced by the large and composition-dependent bowing parameter of InGaN material.

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

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.

Role of Localized Quantum Well Excitons in InGaN Quantum Well Structure Correlated with Microstructural Analysis

ABSTRACTInxGa1−xN multiple-quantum-well laser diode structure, which lased at 405 nm, was shown to have atomically-flat interfaces between each layer. Nanometer-probe compositional analysis showed that InN mole fraction, x, in the wells and barriers are approximately 6 % and 2 %, respectively, which agreed with the result obtained from high-resolution x-ray diffraction measurement. The Stokes-like shift (SS) at 300 K was 49 meV, being approximately 65 % of the luminescence linewidth. The localization depth, E0, of qunatum-well (QW) excitons was estimated to be 35 meV at 300 K though the compositional fluctuation in the well was as small as 1 % or less (detection limit) within adjacent 20-30 nm lateral length scale. Since the well thickness fluctuation is insufficient to reproduce SS or E0, effective bandgap inhomogeneity is attributed to be due to large bandgap bowing in InGaN. The spontaneous emission was thus assigned as being due to the recombination of QW excitons weakly localized in exponential tail-type potential minima in the QW. The size of localization is smaller than the quantum-disk [M. Sugawara, Phys. Rev. B 51, 10743 (1995)]-size. Such small bandgap inhomogeneity can be leveled by injecting high density carriers under lasing conditions, which can explain the general experimental finding that the quantum efficiency decreases with increasing carrier density in InGaN QW devices due to free carrier trapping into threading dislocations.

Properties of Quantum Well Excitons in GaN/AlGaN and InGaN/GaN/AlGaN UV, Blue, Green, and Amber Light Emitting Diode Structures

Optical spectra of GaN, In-doped GaN, and InGaN single quantum well (SQW) structures were compared to explore the role of In for the emission mechanisms in the SQW light emitting diodes (LEDs). The internal electric field, F, due to spontaneous and piezoelectric polarization in strained quantum wells (QWs) naturally induces the quantum confined Stark effect (QCSE) that reduces the electron–hole (e–h) wavefunction overlap due to rapid ionization of excitons causing a serious decrease of the emission efficiency especially for the GaN SQW, although a clear QW excitonic absorption peak was observed. The low energy tail of the emission spectrum of InGaN amber SQW LEDs extends below the bulk InN bandgap, indicating that F is not completely screened by injected carriers. However, doping and alloying of In in GaN improve the emission efficiency despite the fact that the effective bandgap inhomogeneity is increased with increasing InN mole fraction. In is considered to support effective localization of QW excitons, and the Coulomb interaction between the e–h pair is maintained provided that the QW thickness is smaller than the free exciton Bohr radius.

Room-temperature continuous-wave operation of GaInNmultiquantum well laser diodes with low indium content

Based on the indium content dependence of optical gain in a GaInN multiquantum well (MQW) structure, room-temperature continuous-wave operation of GaInN MQW laser diodes is demonstrated. Amplified spontaneous emission spectra for Ga/sub 1-x/In/sub y/N (x=0.07 0.11) MQWs suggest that lower indium content leads to smaller bandgap inhomogeneity and an improvement in the laser performance. Ga/sub 0.93/In/sub 0.07/N MQW laser diodes were operated with a threshold current density of 11 kAcm/sup 2/.

Effective band gap inhomogeneity and piezoelectric field in InGaN/GaN multiquantum well structures

The emission mechanisms of strained InxGa1−xN quantum wells (QWs) were shown to vary depending on the well thickness, L, and x. The absorption edge was modulated by the quantum confined Stark effect and quantum confined Franz-Keldysh effect (QCFK) for the wells, in which, for the first approximation, the product of the piezoelectric field, FPZ, and L exceed the valence band discontinuity, ΔEV. In this case, holes are confined in the triangular potential well formed at one side of the well producing the apparent Stokes-like shift. Under the condition that FPZ×L exceeds the conduction band discontinuity ΔEC, the electron-hole pair is confined at opposite sides of the well. The QCFK further modulated the emission energy for the wells with L greater than the three dimensional free exciton Bohr radius aB. On the other hand, effective in-plane localization of carriers in quantum disk size potential minima, which are produced by nonrandom alloy compositional fluctuation enhanced by the large bowing parameter and FPZ, produces a confined electron-hole pair whose wave functions are still overlapped (quantized excitons) provided that L&amp;lt;aB.

Effects of Si-doping in the barriers of InGaN multiquantum well purplish-blue laser diodes

Optical gain spectra of InGaN multiquantum well laser diode wafers having Si-doped or undoped InGaN barriers were compared. Although evidence for effective band-gap inhomogeneity was found in both structures, the wells with the Si-doped barriers exhibited a smaller Stokes-like shift. Si doping suppressed emergence of a secondary amplified spontaneous emission peak at 3.05 eV, which was uncoupled with the primary one at 2.93 eV. Furthermore Si doping reduced the threshold power density required to obtain the stimulated emission.

Exciton localization in InGaN quantum well devices

Emission mechanisms of a device-quality quantum well (QW) structure and bulk three dimensional (3D) InGaN materials grown on sapphire substrates without any epitaxial lateral overgrown GaN base layers were investigated. The InxGa1−xN layers showed various degrees of in-plane spatial potential (band gap) inhomogeneity, which is due to a compositional fluctuation or a few monolayers thickness fluctuation. The degree of fluctuation changed remarkably around a nominal InN molar fraction x=0.2, which changes to nearly 0.08–0.1 for the strained InxGa1−xN. This potential fluctuation induces localized energy states both in the QW and 3D InGaN, showing a large Stokes-like shift. The spontaneous emission from undoped InGaN single QW light-emitting diodes (LEDs), undoped 3D LEDs, and multiple QW (MQW) laser diode (LD) wafers was assigned as being due to the recombination of excitons localized at the potential minima, whose lateral size was determined by cathodoluminescence mapping to vary from less than 60 to 300 nm in QWs. Those structures are referred to as quantum disks (Q disks) or segmented QWs depending on the lateral size. Blueshift of the emission peak by an increase of the driving current was explained to be combined effects of band filling of the localized states by excitons and Coulomb screening of the quantum confined Stark effect induced by the piezoelectric field. The lasing mechanisms of the continuous wave In0.15Ga0.85N MQW LDs having small potential fluctuations can be described by the well-known electron-hole-plasma (EHP) picture. However, the inhomogeneous MQW LDs are considered to lase by EHP in segmented QWs or Q disks. It is desirable to use entire QW planes with small potential inhomogeneity as gain media for higher performance LD operation.

Lasing mechanism of InGaN-GaN-AlGaN MQW laser diode grown on SiC by low-pressure metal-organic vapor phase epitaxy

We studied the lasing mechanism of an InGaN-GaN-AlGaN multiquantum-well (MQW) laser diode by making various optical characterizations on the diode. Excitation power dependence of photoluminescence (PL) intensity was obtained to investigate the carrier recombination process of the laser. Surface emission and edge emission were compared by optical pumping to clarify where the lasing lines were located in relation to the absorption continuum. From the results, we demonstrate that lasing phenomena in our laser are dominated by free carriers. PL mapping was also taken on the same laser chip to examine the in-cavity bandgap inhomogeneity. We found a very large bandgap scattering of 100 meV. We also found that the wavelength distribution has a periodic modulation. We clarified that the various stimulated emission lines observed in our lasers are caused by the in-cavity spatial bandgap inhomogeneity of the InGaN MQW.