Short-period Superlattices Research Articles

Diluted nitride type-II superlattices: Overcoming the difficulties of bulk GaAsSbN in solar cells

We demonstrate type-II GaAsSb/GaAsN superlattices (SL) as a suitable structure to form the lattice-matched 1.0–1.15 eV subcell that would allow the implementation of the optimum monolithic multi-junction solar cell design. The separation of Sb and N atoms during growth leads to an improved composition homogeneity and a lower defect density than in the bulk GaAsSbN counterparts. The type-II band alignment SLs provide long radiative lifetimes that facilitate carrier collection as compared to equivalent type-I SLs. Moreover, the radiative lifetime can be controllably tuned through the period thickness, which is not possible in type-I SLs. A reduced period thickness results in enhanced absorption due to increased wavefunction overlap, as well as in a change in the transport regime from diffusive to quasiballistic, providing improved carrier extraction efficiency. As a result, the short period SL single-junction solar cells show an enhanced power conversion efficiency of 134% over the equivalent bulk devices.

GaN/InAlN chirped short period superlattice as strain free top cladding for blue laser diode

This work presents the theoretical study on strain-free chirped short-period superlattices (C-SPSLs) as the top waveguide core and cladding layers for InGaN laser diodes (LDs) emitting at 450 nm. The total 530 nm thick layers of reference LD containing 100 nm p-GaN waveguide, 30 nm Al0.15Ga0.85N electron blocking layer (EBL) and Al0.065Ga0.935N cladding layers are replaced by 300 nm thick C-SPSL which contains the 33-periods of 10 monolayers (ML) GaN/ 2 ML In0.18Al0.82N, 48-periods of 6 ML GaN/2 ML In0.18Al0.82N, and 97-periods of 2 ML GaN/ 2 ML In0.18Al0.82N. The optical confinement factor of reference LD is 3.28% whereas for C-SPSL LD it is 3.42%. The better optical confinement at reduced thickness using C-SPSL is due to the high refractive index contrast. The strain in the C-SPSL is negligible. The electron leakage has reduced from 2.54 kA cm−2 to 0.166 kA cm−2 at ∼10 kA cm−2 injected current density, whereas the hole transportation has improved by 2.31 kA cm−2. The C-SPSL configuration also favors the electron blocking effect, thus EBL is not required in C-SPSL design. The light output power at a current injection of 500 mA (10 kA cm−2) is 248 mW for the new structure and ∼143 mW for reference LD. The slope efficiency is improved from ∼0.55 to ∼0.89 W A−1. Also, the dynamic resistance from I–V characteristic is ∼1.12 Ω which is lowered to ∼0.94 Ω.

Raman spectra of interface phonons in long-period AlN/GaN superlattices as a tool for determination of the structure period

AlN/GaN superlattices (SL) grown by metalorganic vapor-phase epitaxy with the period of SLs varied from 20 nm to 140 nm, and the thickness of the structures ranged from 0.7 to 1 µm were studied by polarized Raman spectroscopy technique. The peaks within a complex spectral feature of the symmetry A1(TO) observed at about 580 cm−1, were assigned to the interface and quasi-confined phonons. Frequency splitting between the peaks was found monotonously increasing along with the SL period. This dependence was explained using the dielectric continuum model. A method based on this finding was proposed for estimating the period SL using Raman spectroscopy. This method extends the traditional approach based on the studies of folded acoustic phonons in the short-period SLs.

Carrier Transfer of Deep‐Level Localized States in Type‐II InxGa1−xAs/GaNyAs1−y Short‐Period Superlattice

Optical properties of InGaAs/GaNAs superlattice are investigated by the photoluminescence and reflectance measurements. A quantum well (QW) emission PM (centered at ≈1.2 eV) and a broad low‐energy emission (LEE) band, which can be resolved to three peaks PA′, PA, and PB (0.77, 0.83, and 0.92 eV) are observed. The peak positions of the LEE exhibit S‐shaped behavior, whereas the QW emission shows a red shift with increasing temperature. The LEE bands are attributed to N‐related deep‐level localized state. The photoluminescence (PL) peak of QW exhibits a significant blue shift and saturation effect with increasing excitation density, which is interpreted in terms of the band‐bending model in the InGaAs/GaNAs short‐period superlattice (SPSL) with a type II band alignment. Furthermore, carrier transfer among these N‐related localized states is elucidated by examining the temperature‐dependent photoluminescence. Such carrier transfer is also demonstrated by the thermal effect under high excitation density.

Impact of the substrate lattice constant on the emission properties of InGaN/GaN short-period superlattices grown by plasma assisted MBE

Abstract In this work we investigate InGaN/GaN short period superlattices (SPSLs) grown on (0001) GaN and on relaxed In0.2Ga0.8N buffers with varying degree of plastic relaxation. The SPSLs were fabricated by plasma assisted molecular beam epitaxy (PAMBE). Despite the nominal SPSL structure consisted of InN monolayers (MLs) embedded in GaN barriers, we observed single monolayer InGaN layers instead of InN. We study the photoluminescence (PL) of such SPSLs as a function of the substrates lattice constant. Increase of the substrate lattice constant from a = 3.183 A (for GaN grown on sapphire) to a = 3.216 A resulted in a peak emission shift from 379 nm to 419 nm. We attribute the PL red-shift to the increased In incorporation in the ML-thick InGaN on relaxed buffers as a consequence of the reduced lattice mismatch between the substrate and the film. Our experiment confirms that coherent growth of InN is prohibited on GaN and points out the important role of substrate lattice constant engineering for realization of InN/GaN SPSLs.

Evaluation of the optical characteristics of a type-II InAs/GaSb superlattice infrared p–i–n photodetector

We investigated the characteristics of a type-II InAs/GaSb superlattice (SL) with various composition periods based on the k·p band calculation. Considering the application to the infrared photodetectors, the essential material parameters are calculated and evaluated from the electronic band structure as a function of the width of InAs and GaSb layers. Under dark conditions, the smaller effective mass in a short period SL (especially with small GaSb layer width) could result in the reduction of the Shockley–Read–Hall (SRH) leakage current which is proportional to the intrinsic carrier density. However, this merit is also concerned with the increase of the dark current due to the enhanced band-to-band tunneling in high electric field conditions. Moreover, in terms of the photo-absorption property, the smaller effective mass results in the smaller joint density of states, which compensates the contribution of the large optical matrix element to the absorption coefficient in short period SL.

Advanced AlGaAs/GaAs Heterostructures Grown by MOVPE

AlGaAs/GaAs heterostructures are the base of many semiconductor devices. The fabrication of new types of devices demands heterostructures with special features, such as large total thickness (~20 μm), ultrathin layers (~1 nm), high repeatability (up to 1000 periods) and uniformity, for which a conventional approach of growing such heterostructures is insufficient and the development of new growth procedures is needed. This article summarizes our work on the metalorganic vapour-phase epitaxy (MOVPE) growth of AlGaAs/GaAs heterostructures for modern infrared devices. The growth approaches presented allow for the improved output characteristics of different emitting devices such as multi active region lasers, epitaxially integrated via highly doped tunnel junctions (emission wavelength λ ~ 1 μm), quantum cascade lasers (λ ~10 μm) and THz laser (λ ~100 μm), based on short-period superlattice with 500–2000 layers.

Corrections to “Lateral Current Spreading in III-N Ultraviolet Vertical-Cavity Surface-Emitting Lasers Using Modulation-Doped Short Period Superlattices” [Aug 18 Art. no. 2400507

There is an error in Fig. 4 of [1] in the TE 01 optical mode profile. The error is due to the author forgetting to rescale the x-axis when plotting the optical mode profile on the right y-axis. This oversight occurred because the optical mesh is different from the electrical mesh, and the correct figure is shown below. We wish to emphasize that this was merely a plotting error, and hence all of the results and conclusions presented in the manuscript remain unchanged. We sincerely regret the error.

Time-resolved photoluminescence from n-doped GaN/Al0.18Ga0.82N short-period superlattices probes carrier kinetics and long-term structural stability

Heavily n-doped GaN/Al0.18Ga0.82N short-period superlattices with and without SiN protection layers are studied in spectrally and temporally resolved photoluminescence (PL) experiments. The room-temperature PL from a protected sample displays a nonexponential decay with an initial decay time of 150 ps for low excitation levels and an exponential decay with a time constant of ∼300 ps for higher excitation. The PL decays are governed by nonradiative carrier relaxation into deep defect states which are partially saturated at high excitation densities. PL measurements at low sample temperature reveal a marked influence of carrier cooling on the PL kinetics in a time range up to 50 ps and a significant radiative decay component. SiN coatings are shown to provide long-term stabilization of surface morphology while increasing nonradiative carrier relaxation rates.

2.1 μm InGaSb quantum well lasers exhibiting the maximum conversion efficiency of 27.5% with digitally grown AlGaAsSb barriers and gradient layers

Abstract The 2.1-μm-wavelength InGaSb/AlGaAsSb/GaSb double quantum well lasers, exhibiting a high performance with the peak output power of 1.62 W and the maximum conversion efficiency of 27.5%, had been grown by molecular-beam-epitaxy (MBE). Digitally grown AlxGa1-xAsSb barriers and grading layers were employed in the devices to increase the valence-band offset and to improve the simplicity and controllability of MBE growth. The quaternary digital alloys were grown with short-period superlattices (SPS) of AlSb, AlAs and GaSb and varieties of growth conditions such as growth temperature, interface control and growth rate were considered to get the high-quality crystal films. The digitally grown film was parametrically characterized by atomic force microscopy, high resolution X-ray and transmission electron microscope, showing a smooth surface, clear satellite peaks and clear growth cross sections, respectively. Besides the good power performance of the devices, a low threshold current density of 60 A/cm2 was also achieved and the calculated differential resistance was 0.17 Ω.

Influence of Interdiffusion of Heteromaterials on the Diffraction Focusing of Spherical X-Ray Wave in a Superlattice

The influence of interdiffusion of heteromaterials on the diffraction focusing of spherical X-ray wave in a superlattice is investigated. It is shown that in case of dynamical diffraction of spherical X-ray wave in a short period superlattice, the focal lengths of satellites vary depending on the degree of interdiffusion of heteromaterials of the bilayer. The measurement of focal lengths of satellites allows the determination of structural factors of the superlattice and eventually the calculation of the thickness of transition diffuse layer.

Strain-free GaN/InAlN chirped short-period superlattice electron-blocking layer for 450 nm InGaN laser diode

In this study, we optimized the lattice-matched GaN/In0.18Al0.82N chirped short-period superlattice (C-SPSL) electron-blocking layer for a laser diode emitting at 450 nm. The effective bandgap of C-SPSL depends upon the quantum well (QW) and quantum barrier (QB) thickness of C-SPSL. In this study the In0.18Al0.82N QB thickness is constant at 0.5112 nm (1 unit layer (UL) = 1 lattice constant thickness) and the GaN QW thickness is varied as 1 UL, 3 UL, and 5 UL. The estimated effective bandgap for 15 periods of 1 UL GaN/1 UL In0.18Al0.82N SPSL is ~4.2 eV, for four periods of 3 UL GaN/1 UL In0.18Al0.82N SPSL it is ~3.93 eV and for two periods of 5 UL GaN/1 UL In0.18Al0.82N SPSL it is 3.62 eV. Wave-function hybridization and the built-in electric field play an important role in the bandgap behavior of C-SPSL. The electron leakage decreased from 2534.6 A cm−2 to ~14 A cm−2 while hole transportation improved from 7.7 kA cm−2 to 10 kA cm−2 at 10 kA cm−2 injection current density. The light output power per facet improved from 146 mW to 255 mW. Slope efficiency increased from 0.548 W A−1 to 0.924 W A−1 with the C-SPSL design.

Room temperature yellow InGaAlP quantum dot laser

We report simulation of the conduction band alignment in tensile–strained GaP–enriched barrier structures and experimental results on injection lasing in the green–orange spectral range (558–605 nm) in (AlxGa1–x)0.5In0.5P–GaAs diodes containing such barriers. The wafers were grown by metal–organic vapor phase epitaxy side–by–side on (8 1 1)A, (2 1 1)A and (3 2 2)A GaAs substrates, which surface orientations were strongly tilted towards the [1 1 1]A direction with respect to the (1 0 0) plane. Four sheets of GaP–rich quantum barrier insertions were applied to suppress the leakage of non–equilibrium electrons from the gain medium. Two types of the gain medium were applied. In one case 4–fold stacked tensile–strained (In,Ga)P insertions were used. Experimental data shows that self–organized vertically–correlated quantum dots (QDs) are formed on (2 1 1)A– and (3 2 2)A–oriented substrates, while corrugated quantum wires are formed on the (8 1 1)A surface. In the other case a short–period superlattice (SPSL) composed of 16–fold stacked quasi–lattice–matched 1.4 nm–thick In0.5Ga0.5P layers separated by 4 nm–thick (Al0.6Ga0.4)0.5In0.5P layers was applied. Laser diodes with 4–fold stacked QDs having a threshold current densities of ∼7–10 kA/cm2 at room temperature were realized for both (2 1 1)A and (3 2 2)A surface orientations at cavity lengths of ∼1 mm. Emission wavelength at room temperature was ∼599–603 nm. Threshold current density for the stimulated emission was as low as ∼1 kA/cm2. For (8 1 1)A–grown structures no room temperature lasing was observed. SPSL structures demonstrated lasing only at low temperatures <200 K. The shortest wavelength (558 nm, 90 K) in combination with the highest operation temperature (150 K) was realized for (3 2 2)A–oriented substrates in agreement with theoretical predictions.

Picosecond transient thermoreflectance for thermal conductivity characterization

ABSTRACTWe developed a picosecond transient thermoreflectance (ps-TTR) system for thermal property characterization, using a low-repetition-rate picosecond pulsed laser (1064 nm) as the heating source and a 532 nm CW laser as the probe. Low-repetition-rate pump eliminates the complication from thermal accumulation effect. Without the need of a mechanical delay stage, this ps-TTR system can measure the thermal decay curve from 500 ps up to 1 ms. Three groups of samples are tested: bulk crystals (glass, Si, GaAs, and sapphire); MoS2 thin films (157 ~ 900 nm thickness); InGaAs random alloy and GaAs/InAs digital alloy (short period superlattices). Analysis of the thermoreflectance signals shows that this ps-TTR system is able to measure both thermal conductivity and interface conductance in nanostructures. The measured thermal conductivity values in bulk crystals, MoS2 thin films, and InGaAs random alloy are all consistent with literature values. Cross-plane thermal conductivity in MoS2 thin films does not show obvious thickness dependence. Thermal conductivities of GaAs/InAs digital alloys are smaller than InGaAs random alloy, due to the efficient scattering at interfaces. We also discuss the advantages and disadvantages of this newly developed ps-TTR system comparing with the popular time-domain thermoreflectance system.

Double superlattice structure for improving the performance of ultraviolet light-emitting diodes**Project supported by the National Key R&D Program of China (Grant Nos. 2016YFB0400800, 2016YFB0400801, and 2016YFB0400802), the National Natural Science Foundation of China (Grant No. 61634005), and the Fundamental Research Funds for the Central Universities, China (Grant No. JBZ171101).

The novel AlGaN-based ultraviolet light-emitting diodes (UV-LEDs) with double superlattice structure (DSL) are proposed and demonstrated by numerical simulation and experimental verification. The DSL consists of 30-period Mg modulation-doped p-AlGaN/u-GaN superlattice (SL) and 4-period p-AlGaN/p-GaN SL electron blocking layer, which are used to replace the p-type GaN layer and electron blocking layer of conventional UV-LEDs, respectively. Due to the special effects and interfacial stress, the AlGaN/GaN short-period superlattice can reduce the acceptor ionization energy of the p-type regions, thereby increasing the hole concentration. Meanwhile, the multi-barrier electron blocking layers are effective in suppressing electron leakage and improving hole injection. Experimental results show that the enhancements of 22.5% and 37.9% in the output power and external quantum efficiency at 120 mA appear in the device with double superlattice structure.

Parameter transferability, self-doping, and metallicity in LaNiO3/LaMnO3 superlattices

Motivated by recent experiments, we use the $+U$ extension of the generalized gradient approximation to density functional theory to study superlattices composed of alternating layers of LaNiO$_3$ and LaMnO$_3$. For comparison we also study a rocksalt ((111) double perovskite) structure and bulk LaNiO$_3$ and LaMnO$_3$. A Wannier function analysis indicates that band parameters are transferable from bulk to superlattice situations with the exception of the transition metal d-level energy, which has a contribution from the change in d-shell occupancy. The charge transfer from Mn to Ni is found to be moderate in the superlattice, indicating metallic behavior, in contrast to the insulating behavior found in recent experiments, while the rocksalt structure is found to be insulating with a large Mn-Ni charge transfer. We suggest a high density of cation antisite defects may account for the insulating behavior experimentally observed in short-period superlattices.

III-Nitride Short Period Superlattices for Deep UV Light Emitters

III-Nitride short period superlattices (SPSLs), whose period does not exceed ~2 nm (~8 monolayers), have a few unique properties allowing engineering of light-emitting devices emitting in deep UV range of wavelengths with significant reduction of dislocation density in the active layer. Such SPSLs can be grown using both molecular beam epitaxy and metal organic chemical vapor deposition approaches. Of the two growth methods, the former is discussed in more detail in this review. The electrical and optical properties of such SPSLs, as well as the design and fabrication of deep UV light-emitting devices based on these materials, are described and discussed.

Effect of Epitaxial-Structure Design and Growth Parameters on the Characteristics of Metamorphic Lasers of the 1.46-μm Optical Range Based on Quantum Dots Grown on GaAs Substrates

The characteristics of lasers of the 1.44–1.46-μm optical range grown on GaAs substrates using a metamorphic buffer are studied. The active region of the laser contains 10 rows of InAs/In0.4Ga0.6As/In0.2Ga0.8As quantum dots. It is shown that the use of special selective high-temperature annealing along with the application of short-period In0.2Ga0.8As/In0.2Al0.3Ga0.5As short-period superlattices makes it possible to substantially decrease the density of threading dislocations in the active region. A threshold current density of 1300 A cm–2, external differential quantum efficiency of 38%, and maximal pulsed-mode output power of 13 W are attained for a laser with a broad area 3 mm in length.

Phase engineering of rare earth nickelates by digital synthesis

We report on the electronic and magnetic properties of a series of [m EuNiO3/p LaNiO3] superlattices (thickness m and/or p = 1 unit cell) epitaxially grown on single crystalline NdGaO3 substrates. The structural symmetry of these films has been investigated by the combination of in-situ reflection high energy electron diffraction and X-ray diffraction measurements. The metal-insulator transition and the magnetic transition temperatures of the short-period superlattices with m ≥ p are modified from the corresponding bulk Eu1–xLaxNiO3 (x=pm+p) composition. In contrast to the corresponding bulk doped compound with x = 0.67, the [1 EuNiO3/2 LaNiO3] film remains metallic down to at least 2 K without signs of electronic or magnetic transitions. These findings demonstrate the power of the digital synthesis approach to realize electronic and magnetic phases of perovskite nickelates, unattainable in bulk.

Lateral Current Spreading in III-N Ultraviolet Vertical-Cavity Surface-Emitting Lasers Using Modulation-Doped Short Period Superlattices

Lateral hole injection into AlGaN-based ultraviolet (UV) vertical-cavity light-emitting lasers (VCSELs) has been studied via numerical simulation. For blue and violet vertical cavity light emitters, indium tin oxide (ITO) is most commonly used as a transparent current spreading layer to increase the overlap between the optical mode and the radial current profile. However, ITO has very high optical losses in the UV spectrum, so alternative schemes for lateral current spreading have been investigated for use in UV-VCSELs. A modulation doped short-period superlattice (MD-SPSL) has been proposed as a transparent lateral current spreading layer in UV-VCSELs. The narrow bandgap unintentionally doped (uid) material maintains a high mobility due to reduced impurity scattering and has a high free hole concentration due to modulation doping, thus forming highly conductive channels which aid lateral hole transport. This has been shown to partially mitigate current crowding around the current aperture. To account for imperfect modulation doping due to the magnesium memory effects and other factors, the effect of varying the hole mobility in the uid-narrow bandgap layer of the MD-SPSL from 13–300 cm2/( $\text {V}\cdot \text {s}$ ) on the threshold current and slope efficiency has also been studied. Employing an MD-SPSL results in a significant reduction in the threshold current and slope efficiency compared to ITO, and the extent of the improvement depends on the hole mobility in the uid-AlGaN layer.