The self-assemble GaN:Mg inverted hexagonal pyramids formatted by photoelectrochemical wet-etching process

Small sized self-assembled inverted hexagonal pyramids consisting of GaN:Mg and InGaN/GaN multi-quantum-well (MQW) structures were formed using photoelectrochemical wet etching. Lateral etching, bottom-up etching, and anisotropic etching are the formation mechanism of the pyramids during the etching process sequentially. The dimension of these inverted hexagonal pyramids was measured as 245 nm in width and 184 nm in height, and the angle between the top GaN:Mg surface and the pyramid sidewall was calculated at about 56.3°. Due to the strain relief in the nano-disk MQW structure we induced an emission peak of photoluminescence at the tip of the inverted hexagonal pyramid which had a strong blue shift of 244 meV at 100 K.     

Luminescence and lasing in GaN epitaxial layers and InGaN/GaN quantum well heterostructures under optical and electron-beam excitation

Interrelation between stimulated and excitonic emission intensity of GaN epitaxial layers and yellow luminescence intensity as well as correlation between photoluminescence and laser properties of InGaN based multiple quantum well heterostructures was investigated. It was found among all studied undoped GaN epitaxial layers that the higher intensity of the yellow luminescence and so the higher concentration of the yellow luminescence related centres the higher is the excitonic, electron–hole plasma and stimulated emission intensity. It was shown that a small Stokes shift and a high ratio of the luminescence intensity from InGaN quantum well layers to the photoluminescence intensity from GaN barrier layers indicate high laser quality of the multiple quantum well heterostructures. The lowest full width at half maximum of the laser line was 0.04 nm, the highest operating temperature was 585 K, the lowest threshold was 100 kW cm⁻², the highest characteristic temperature was 164 K and the highest wavelength was 442.5 nm. The far-field patterns of the laser emission from the MQW lasers consist of two approximately symmetrical high brightness spots localized at angles =±30–35°.     

The status and future development of innovative optoelectronic devices based on III-nitrides on SiC and on III-antimonides

High-brightness nitride-based LEDs have been grown on SiC substrates, which offers many advantages in the production both from epitaxial, chip and device processing point of view. Optimized InGaN/GaN/GaAlN MQW structures and improved chip and package designs were developed, resulting in optical outputs that exceed 7 mW at 20 mA in a 5 mm axial lamp. InGaN oxide stripe lasers (450 μm×3.5 μm) with an emission wavelength around 420 nm were fabricated showing threshold currents of 330 mA and turn-on voltages of about 21V operated under pulse current injections at room temperature. Strained layer GaInAsSb/AlGaAsSb quantum well lasers operating near room temperature with emission wavelengths up to 2.26 μm and a cw output of 240 mW were demonstrated. Short-period InAs/GaInSb superlattices with different InAs layer widths have been used for the fabrication of photodiodes, showing responsivity spectra with cut-off wavelengths of 4.5 and 10 μm, respectively.     

Microstructural defect properties of InGaN/GaN blue light emitting diode structures

In this paper, we study structural and morphological properties of metal-organic chemical vapour deposition-grown InGaN/GaN light emitting diode (LED) structures with different indium (In) content by means of high-resolution X-ray diffraction, atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), photoluminescence (PL) and current–voltage characteristic (I–V). We have found out that the tilt and twist angles, lateral and vertical coherence lengths of mosaic blocks, grain size, screw and edge dislocation densities of GaN and InGaN layers, and surface roughness monotonically vary with In content. Mosaic defects obtained due to temperature using reciprocal lattice space map has revealed optimized growth temperature for active InGaN layer of MQW LED. It has been observed in this growth temperature that according to AFM result, LED structure has high crystal dimension, and is rough whereas according to PL and FTIR results, bandgap energy shifted to blue, and energy peak half-width decreased at high values. According to I–V measurements, it was observed that LED reacted against light at optimized temperature. In conclusion, we have seen that InGaN MQW structure’s structural, optical and electrical results supported one another.     

Investigation of V-defects formation in InGaN/GaN multiple quantum well grown on sapphire

In the growth of InGaN multiple quantum well structure, V-pits has been observed to be initiated at the threading dislocations which propagate to the quantum well layers with high indium composition and substantially thick InGaN well. A set of samples with varying indium well thickness (3-7.6 nm) and composition (10-30%) are grown and characterized by photoluminescence (PL), X-ray diffraction, transmission electron microscopy and atomic force microscopy. The indium content and the layer thicknesses in InGaN/GaN quantum well are determined by high-resolution X-ray diffraction (XRD) and TEM imaging. With indium composition exceeding 10%, strain at the InGaN/GaN interface leads to the generation of V-pits at the interlayers of the MQW. Higher indium composition and increase in thickness of a period (InGaN well plus the GaN barrier) appear to enhance pits generation. With thicker InGaN well and reduction in thickness of GaN to InGaN (or the R ratio), pit density is substantially reduced, but it results in greater inhomogeneity in the distribution of indium in the InGaN well. This leads to a broadened PL emission and affect the PL emission intensity.     

Multi-quantum-well nanowire heterostructures for wavelength-controlled lasers

Rational design and synthesis of nanowires with increasingly complex structures can yield enhanced and/or novel electronic and photonic functions. For example, Ge/Si core/shell nanowires have exhibited substantially higher performance as field-effect transistors and low-temperature quantum devices compared with homogeneous materials, and nano-roughened Si nanowires were recently shown to have an unusually high thermoelectric figure of merit. Here, we report the first multi-quantum-well (MQW) core/shell nanowire heterostructures based on well-defined III-nitride materials that enable lasing over a broad range of wavelengths at room temperature. Transmission electron microscopy studies show that the triangular GaN nanowire cores enable epitaxial and dislocation-free growth of highly uniform (InGaN/GaN)n quantum wells with n=3, 13 and 26 and InGaN well thicknesses of 1-3 nm. Optical excitation of individual MQW nanowire structures yielded lasing with InGaN quantum-well composition-dependent emission from 365 to 494 nm, and threshold dependent on quantum well number, n. Our work demonstrates a new level of complexity in nanowire structures, which potentially can yield free-standing injection nanolasers.     

Defect Engineering in Few‐Layer Phosphorene

Defect engineering in 2D phosphorene samples is becoming an important and powerful technique to alter their properties, enabling new optoelectronic applications, particularly at the infrared wavelength region. Defect engineering in a few‐layer phosphorene sample via introduction of substrate trapping centers is realized. In a three‐layer (3L) phosphorene sample, a strong photoluminescence (PL) emission peak from localized excitons at ≈1430 nm is observed, a much lower energy level than free excitonic emissions. An activation energy of ≈77 meV for the localized excitons is determined in temperature‐dependent PL measurements. The relatively high activation energy supports the strong stability of the localized excitons even at elevated temperature. The quantum efficiency of localized exciton emission in 3L phosphorene is measured to be approximately three times higher than that of free excitons. These results could enable exciting applications in infrared optoelectronics.     

Linearly polarized light emission from InGaN/GaN quantum well structure with high indium composition

We fabricated yellow (575 nm) emitting a-plane InGaN/GaN light emitting diode (LED). Microstructure and stress relaxation of the InGaN well layer were observed from the images of dark field transmission electron microscopy. The LED chip was operated at 3.7 V, 20 mA, and the polarization-free characteristic in nonpolar InGaN layer was confirmed from a small blue-shift of approximaely 1.7 nm with increase of current density. The high photoluminescence (PL) efficiency of 30.4% showed that this non-polar InGaN layer has a potential of application to green-red long wavelength light emitters. The PL polarization ratio at 290 K was 0.25 and the energy difference between two subbands was estimated to be 40.2 meV. The low values of polarization and energy difference were due to the stress relaxation of InGaN well layer.     

Luminescence and ultrafast phenomena in InGaN multiple quantum wells

High quality In_0.13Ga_0.87N/GaN multiple quantum wells (MQWs) on (0001) sapphire substrate were fabricated by MOCVD method. The quantum well thickness is as thin as 10 Å, and the barrier thickness is 50 Å. We have investigated these ultrathin MQWs by continuous wave (cw) and time-resolved spectroscopy in the picosecond time scales in a wide temperature range from 10 to 290 K. In the luminescence spectrum at 10 K, we observed a broad peak at 3.134 eV which was attributed to the quantum wells emission of InGaN. The full width at half maximum of this peak was 129 meV at 10 K and the broadening at low temperatures which was mostly inhomogeneous was thought to be due to compositional fluctuations and interfacial disorder in the alloy. We also observed an intense and narrow peak at 3.471 eV due to the GaN barrier. The temperature dependence of the luminescence was studied and the peak positions and the intensities of the different peaks were obtained. The activation energy of the InGaN quantum well emission peak was estimated as 69 meV. From the measurements of luminescence intensities and lifetimes at various temperatures, radiative and non-radiative recombination lifetimes were deduced. The results were explained by considering only the localization of the excitons due to potential fluctuations.     

Growth of InGaN quantum dots with AlGaN barrier layers via modified droplet epitaxy

The growth of c-plane InGaN quantum dots via modified droplet epitaxy with AlGaN barrier layers is reported. The growth of the AlGaN layer underlying the InGaN quantum dot layer was carried out under H₂ at 1050 °C, while the capping AlGaN layer was grown at the same temperature (710 °C) and using the same carrier gas (N₂) as that used to grow the InGaN quantum dot layer to prevent decomposition of the InGaN. Atomic force microscopy of InGaN epilayers grown and annealed on high temperature AlGaN using identical growth conditions used for the quantum dot samples highlighted a narrower distribution of nanostructure heights than that obtained for similar growth on GaN. Scanning transmission electron microscopy (STEM) imaging combined with energy dispersive X-ray (EDX) analysis revealed the presence of a thin high aluminium content layer at the surface of both AlGaN layers, which is believed to be related to loss of Ga during temperature ramping processes. Micro-photoluminescence studies carried out at low temperature revealed near resolution-limited peaks while time-resolved measurements on these peaks demonstrated mono-exponential decay times between 1 and 4 ns, showing that quantum dots had successfully been formed between the AlGaN barriers. Temperature-dependant measurement of the emission lines revealed that quenching of the peak often occurred at ∼60–70 K, with some of the peaks exhibiting significant line broadening whilst others remained narrow.     

Deep‐Ultraviolet Hyperbolic Metacavity Laser

Given the high demand for miniaturized optoelectronic circuits, plasmonic devices with the capability of generating coherent radiation at deep subwavelength scales have attracted great interest for diverse applications such as nanoantennas, single photon sources, and nanosensors. However, the design of such lasing devices remains a challenging issue because of the long structure requirements for producing strong radiation feedback. Here, a plasmonic laser made by using a nanoscale hyperbolic metamaterial cube, called hyperbolic metacavity, on a multiple quantum‐well (MQW), deep‐ultraviolet emitter is presented. The specifically designed metacavity merges plasmon resonant modes within the cube and provides a unique resonant radiation feedback to the MQW. This unique plasmon field allows the dipoles of the MQW with various orientations into radiative emission, achieving enhancement of spontaneous emission rate by a factor of 33 and of quantum efficiency by a factor of 2.5, which is beneficial for coherent laser action. The hyperbolic metacavity laser shows a clear clamping of spontaneous emission above the threshold, which demonstrates a near complete radiation coupling of the MQW with the metacavity. This approach shown here can greatly simplify the requirements of plasmonic nanolaser with a long plasmonic structure, and the metacavity effect can be extended to many other material systems.     

Thermal effect of multi-quantum barriers within InGaN/GaN multi-quantum well light-emitting diodes

We introduce the InGaN/GaN multi-quantum barriers (MQBs) into InGaN/GaN multi-quantum well (MQW) heterostructures to improve the performance of light-emitting diodes. The temperature and injection current dependent electroluminescence were carried out to study the thermal effect of InGaN/GaN MQWs. We observe the enhancement of carrier confinement in the active layer and the inhibited carrier leakage over the barrier for the sample with MQBs. In addition, the external quantum efficiency of the samples is obtained. It is found that the radiative efficiency of the sample possessing MQBs exhibits less sensitive temperature dependence and leads to an improved efficiency in the high temperature and high injection current range.     

Largely extended light-emission shift of ZnSe nanostructures with temperature

ZnSe nanowires and nanobelts with zinc blende structure have been synthesized. The morphology and the growth mechanisms of the ZnSe nanostructures will be discussed. From the photoluminescence (PL) of the ZnSe nanostructures, it is interesting to note that red color emission with only a single peak at the photon energy of 2 eV at room temperature is obtained while the typical bandgap transition energy of ZnSe is 2.7 eV. When the temperature is reduced to 150 K, the peak wavelength shifts to 2.3 eV with yellowish emission and then blue emission with the peak at 2.7 eV at temperature less than 50 K. The overall wavelength shift of 700 meV is obtained as compared to the conventional ZnSe of about 100 meV (i.e., sevenfold extension). The ZnSe nanostructures with enhanced wavelength shift can potentially function as visible light temperature-indicator. The color change from red to yellowish and then to blue is large enough for the nanostructures to be used for temperature-sensing applications. The details of PL spectra of ZnSe at various temperatures are studied from (i) the spectral profile, (ii) the half-width half-maximum, and (iii) the peak photon energy of each of the emission centers. The results show that the simplified configuration coordinate model can be used to describe the emission spectra, and the frequency of the local vibrational mode of the emission centers is determined.     

Optical and Magnetic Properties of Ten-Period InGaMnAs/GaAs Quantum Wells

Ten layers of InGaMnAs/GaAs multiquantum wells (MQWs) structure were grown on high resistivity (100) p-type GaAs substrates by molecular beam epitaxy (MBE). A presence of the ferromagnetic structure was confirmed in the InGaMnAs/GaAs MQWs structure, and have ferromagnetic ordering with a Curie temperature, T _C=50 K. It is likely that the ferromagnetic exchange coupling of the sample with T _C=50 K is hole-mediated resulting in Mn substituting In or Ga sites. PL emission spectra of the InGaMnAs MQWs sample grown at a temperature of 170 °C show that an activation energy of the Mn ion on the first quantum confinement level in InGaAs QW is 32 meV and impurity Mn is partly ionized. The fact that the activation energy of 32 meV of Mn ion in the QW is lower than an activation energy of 110 meV for a substitutional Mn impurity in GaAs, indicating an impurity band existing in the bandgap due to substitutional Mn ions.     

High-efficiency InGaN/GaN/AlGaN light-emitting diodes with short-period InGaN/GaN superlattice for 530–560 nm range

A new method of forming the active region in high-efficiency InGaN/GaN/AlGaN light-emitting diode (LED) structure for long-wave green range is described. The introduction of a short-period InGaN/GaN superlattice situated immediately under the emitting quantum well and overgrown with GaN layer at reduced temperature leads to a more than tenfold increase in the efficiency of emission. For the proposed LEDs, the maximum quantum efficiency was 12% at 552 nm and 8% at 560 nm.     

Efficient Red Perovskite Light‐Emitting Diodes Based on Solution‐Processed Multiple Quantum Wells

This paper reports a facile and scalable process to achieve high performance red perovskite light‐emitting diodes (LEDs) by introducing inorganic Cs into multiple quantum well (MQW) perovskites. The MQW structure facilitates the formation of cubic CsPbI₃ perovskites at low temperature, enabling the Cs‐based QWs to provide pure and stable red electroluminescence. The versatile synthesis of MQW perovskites provides freedom to control the crystallinity and morphology of the emission layer. It is demonstrated that the inclusion of chloride can further improve the crystallization and consequently the optical properties of the Cs‐based MQW perovskites, inducing a low turn‐on voltage of 2.0 V, a maximum external quantum efficiency of 3.7%, a luminance of ≈440 cd m^?2 at 4.0 V. These results suggest that the Cs‐based MQW LED is among the best performing red perovskite LEDs. Moreover, the LED device demonstrates a record lifetime of over 5 h under a constant current density of 10 mA cm^?2. This work suggests that the MQW perovskites is a promising platform for achieving high performance visible‐range electroluminescence emission through high‐throughput processing methods, which is attractive for low‐cost lighting and display applications.     

An approach to suppress the blue-shift of photoluminescence peaks in coupled multilayer InAs/GaAs quantum dots by high temperature post-growth annealing

Effect of post-growth annealing on 10 layer stacked InAs/GaAs quantum dots (QDs) with InAlGaAs/GaAs combination capping layer grown by molecular beam epitaxy has been investigated. The QD heterostructure shows a low temperature (8 K) photoluminescence (PL) emission peak at 1267 nm. No frequency shift in the peak emission wavelength is seen even for annealing up to 700 °C which is desirable for laser devices requiring strict tolerances on operating wavelength. This is attributed to the simultaneous effect of the strain field, propagating from the seed layer to the active layer of the multilayer QD (MQD) and the indium atom gradient in the capping layer due to the presence of a quaternary InAlGaAs layer. Higher activation energy (of the order of ∼250 meV) even at 650 °C annealing temperature also signifies the stronger carrier confinement potential of the QDs. All these results demonstrate higher thermal stability of the emission peak of the devices using this QD structure.     

Impact of substrate nitridation on the photoluminescence and photovoltaic characteristics of GaN grown on p-Si (100) by molecular beam epitaxy

This report focuses on the structural and optical properties of the GaN films grown on p-Si (100) substrates along with photovoltaic characteristics of GaN/p-Si heterojunctions fabricated with substrate nitridation and in absence of substrate nitridation. The high resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), Raman and photoluminescence (PL) spectroscopic studies reveal that the significant enhancement in the structural as well as in the optical properties of GaN epifilms grown with silicon nitride buffer layer when compared with the sample grown without silicon nitride buffer layer. The low temperature PL shows a free excitonic (FX) emission peak at 3.51 eV at the temperature of 5 K with a very narrow line width of 35 meV. Temperature dependent PL spectra follow the Varshni equation well and peak energy blue shifts by ~63 meV from 300 to 5 K. Raman data confirms the strain free nature and reasonably good crystallinity of the films. The GaN/p-Si heterojunctions fabricated without substrate nitridation show a superior photovoltaic performance compared to the devices fabricated in presence of substrate nitridation. The discussions have been carried out on the junction properties. Such single junction devices exhibit a promising fill factor and conversion efficiency of 23.36 and 0.12 %, respectively, under concentrated AM1.5 illumination.     

Generation-recombination noise in pseudomorphic Modulation-doped Al/sub 0.2/Ga/sub 0.8/As/In/sub 0.1/Ga/sub 0.9/As/GaAs micro-Hall devices

The noise spectrum in micro-Hall devices based on pseudomorphic Al/sub 0.2/Ga/sub 0.8/As/In/sub 0.1/Ga/sub 0.9/As/GaAs modulation-doped heterostructures was measured between 4 Hz and 65 kHz, allowing components due to thermal, 1/f, and generation-recombination to be characterized. Applying deep level noise spectroscopy (DLNS) in the temperature range of 77-300 K to analyze the generation-recombination part of the spectrum, two electron traps contributing to noise density were identified. An emission activation energy of 474 meV was measured for the dominant trap, corresponding to the well-known DX center originating from the AlGaAs barrier. The other deep level, with an emission activation energy of 242 meV, is probably related to defects in the InGaAs layer. The structures under investigation resulted in high-performance micro-Hall devices: a supply-current-related sensitivity up to 725 V/spl middot/A/sup -1//spl middot/T/sup -1/ at 77 K independent of bias current, a signal-to-noise sensitivity of 155 dB/spl middot/T/sup -1/ and a detection limit of 340 pT/spl middot/mm/spl middot/Hz/sup -1/2/ at 77 K were measured.     

Photoluminescence study of type-II InGaPN/GaAs quantum wells

Nearly lattice-matched In(0.528)Ga(0.472)P(1-y)Ny bulk layer and In(0.528)Ga(0.472)P(1-y)Ny/GaAs and GaAs/ In(0.528)Ga(0.472)P(1-y)Ny quantum wells with higher N content, y = 0.027, were grown on GaAs(001) substrates by metalorganic vapor phase epitaxy. High-resolution X-ray diffraction results demonstrated the high quality of both the layer and quantum wells with fairly flat interfaces. Temperature dependent photoluminescence results showed that a near-band-edge emission is dominant in the bulk In(0.528)Ga(0.472)P(0.973)N(0.027) layer, which at low temperature (T < 100 K) is associated with localized emissions centered at approximately 1.73 eV. Bandgap of In(0.528)Ga(0.472)P(0.973)N(0.027) was examined to be 1.81 and 1.78 eV at 10 K and room-temperature, respectively. Low temperature (10 K)-photoluminescence spectrum obtained from the GaAs/InxGa(1-x)P(1-y)Ny quantum well also exhibited red emission at 1.73 eV attributed to the emission from the InGaPN barrier. In addition, there are the extra weak peaks appear in a near-infrared energy range at 1.357 and 1.351 eV for InxGa(1-x)P(1-y)Ny/GaAs and GaAs/InxGa(1-x)P(1-y)Ny quantum wells, respectively. Such optical transitions are considered as an indirect transition between electrons located in the InGaPN and holes located in the GaAs regions. This situation suggested that both the In(0.528)Ga(0.472)P(0.973)N(0.027)/GaAs and GaAs/In(0.528)Ga(0.472)P(0.973)N(0.027) quantum wells exhibits a type-II quantum structure. This interpretation is justified when the valence and conduction band offsets of the type-II band alignment, which are relatively approximated to be 450 and 160 meV, are properly taken into account.