InGaN Insertions Research Articles

Analysis and improvement of self-heating effect based on GaN HEMT devices

Gallium nitride high electron mobility transistor (GaN HEMT) applications in high-power and high-frequency environments can lead to high device temperatures due to the self-heating effect, thus limiting device performance and reliability. In order to address this problem, this paper changes the material and structure of the device. It successfully reduces the maximum temperature of the device to 335 K by using a new structure of the diamond substrate, diamond heat sink layer, and InGaN insertion layer. Simulation results show that the new structure has a 35% reduction in maximum temperature, a 61% increase in current, a 37% improvement in maximum transconductance, and a 35% improvement in current collapse. At the same time, the new structure also improves the electron mobility of the channel.

Enhancing performance of GaN-based LDs by using GaN/InGaN asymmetric lower waveguide layers

The effects of GaN/InGaN asymmetric lower waveguide (LWG) layers on photoelectrical properties of InGaN multiple quantum well laser diodes (LDs) with an emission wavelength of around 416 nm are theoretically investigated by tuning the thickness and the indium content of InGaN insertion layer (InGaN-IL) between the GaN lower waveguide layer and the quantum wells, which is achieved with the Crosslight Device Simulation Software (PIC3D, Crosslight Software Inc.). The optimal thickness and the indium content of the InGaN-IL in lower waveguide layers are found to be 300 nm and 4%, respectively. The thickness of InGaN-IL predominantly affects the output power and the optical field distribution in comparison with the indium content, and the highest output power is achieved to be 1.25 times that of the reference structure (symmetric GaN waveguide), which is attributed to the reduced optical absorption loss as well as the concentrated optical field nearby quantum wells. Furthermore, when the thickness and indium content of InGaN-IL both reach a higher level, the performance of asymmetric quantum wells LDs will be weakened rapidly due to the obvious decrease of optical confinement factor (OCF) related to the concentrated optical field in the lower waveguide.

High Piezoelectric Conversion Properties of Axial InGaN/GaN Nanowires.

We demonstrate for the first time the efficient mechanical-electrical conversion properties of InGaN/GaN nanowires (NWs). Using an atomic force microscope equipped with a modified Resiscope module, we analyse the piezoelectric energy generation of GaN NWs and demonstrate an important enhancement when integrating in their volume a thick In-rich InGaN insertion. The piezoelectric response of InGaN/GaN NWs can be tuned as a function of the InGaN insertion thickness and position in the NW volume. The energy harvesting is favoured by the presence of a PtSi/GaN Schottky diode which allows to efficiently collect the piezo-charges generated by InGaN/GaN NWs. Average output voltages up to 330 ± 70 mV and a maximum value of 470 mV per NW has been measured for nanostructures integrating 70 nm-thick InGaN insertion capped with a thin GaN top layer. This latter value establishes an increase of about 35% of the piezo-conversion capacity in comparison with binary p-doped GaN NWs. Based on the measured output signals, we estimate that one layer of dense InGaN/GaN-based NW can generate a maximum output power density of about 3.3 W/cm2. These results settle the new state-of-the-art for piezo-generation from GaN-based NWs and offer a promising perspective for extending the performances of the piezoelectric sources.

Site-controlled GaN nanocolumns with InGaN insertions grown by MBE

The site-controlled plasma-assisted molecular beam epitaxy (PA MBE) has been developed to fabricate the regular array of GaN nanocolumns (NCs) with InGaN insertions on micro-cone patterned sapphire substrates (μ-CPSSs). Two-stage growth of GaN NCs, including a nucleation layer grown at metal-rich conditions and high temperature GaN growth in strong N-rich condition, has been developed to achieve the selective growth of the NCs. Microcathodoluminescence measurements have demonstrated pronounced emission from the InGaN insertions in 450-600 nm spectral range. The optically isolated NCs can be used as effective nano-emitters operating in the visible range.

Influences of stress on the properties of GaN/InGaN multiple quantum well LEDs grown on Si (111) substrates**Project supported by the National Natural Science Foundation of China (Grant Nos. 61274039 and 51177175), the National Basic Research Program of China (Grant Nos. 2010CB923201 and 2011CB301903), the Ph. D. Program Foundation of the Ministry of Education of China (Grant No. 20110171110021), the International

The influences of stress on the properties of InGaN/GaN multiple quantum wells (MQWs) grown on silicon substrate were investigated. The different stresses were induced by growing InGaN and AlGaN insertion layers (IL) respectively before the growth of MQWs in metal–organic chemical vapor deposition (MOCVD) system. High resolution x-ray diffraction (HRXRD) and photoluminescence (PL) measurements demonstrated that the InGaN IL introduced an additional tensile stress in n-GaN, which released the strain in MQWs. It is helpful to increase the indium incorporation in MQWs. In comparison with MQWs without the IL, the wavelength shows a red-shift. AlGaN IL introduced a compressive stress to compensate the tensile stress, which reduces the indium composition in MQWs. PL measurement shows a blue-shift of wavelength. The two kinds of ILs were adopted to InGaN/GaN MQWs LED structures. The same wavelength shifts were also observed in the electroluminescence (EL) measurements of the LEDs. Improved indium homogeneity with InGaN IL, and phase separation with AlGaN IL were observed in the light images of the LEDs.

Advances in MBE Selective Area Growth of III-Nitride Nanostructures: From NanoLEDs to Pseudo Substrates

The aim of this work is to provide an overview on the recent advances in the selective area growth (SAG) of ( In ) GaN nanostructures by plasma assisted molecular beam epitaxy, focusing on their potential as building blocks for next generation LEDs. The first three sections deal with the basic growth mechanisms of GaN SAG and the emission control in the entire ultraviolet to infrared range, including approaches for white light emission, using InGaN disks and thick segments on axial nanocolumns. SAG of axial nanostructures is developed on both GaN /sapphire templates and GaN -buffered Si (111). As an alternative to axial nanocolumns, section 4 reports on the growth and characterization of InGaN / GaN core-shell structures on an ordered array of top-down patterned GaN microrods. Finally, section 5 reports on the SAG of GaN , with and without InGaN insertion, on semi-polar (11-22) and non-polar (11-20) templates. Upon SAG the high defect density present in the templates is strongly reduced as indicated by a dramatic improvement of the optical properties. In the case of SAG on non-polar (11-22) templates, the formation of nanostructures with a low aspect ratio took place allowing for the fabrication of high-quality, non-polar GaN pseudo-templates by coalescence of these nanostructures.

Selective area growth and characterization of GaN nanocolumns, with and without an InGaN insertion, on semi-polar (11–22) GaN templates

The aim of this work is the selective area growth (SAG) of GaN nanocolumns, with and without an InGaN insertion, by molecular beam epitaxyon semi-polar (11–22) GaN templates. The high density of stacking faults present in the template is strongly reduced after SAG. A dominant sharp photoluminescence emission at 3.473 eV points to high quality strain-free material. When embedding an InGaN insertion into the ordered GaN nanostructures, very homogeneous optical properties are observed, with two emissions originating from different regions of each nanostructure, most likely related to different In contents on different crystallographic planes.

Recombination dynamics in InGaN/GaN nanowire heterostructures on Si(111)

We have performed room-temperature time-resolved photoluminescence measurements on samples that comprise InGaN insertions embedded in GaN nanowires. The decay curves reveal non-exponential recombination dynamics that evolve into a power law at long times. We find that the characteristic power-law exponent increases with emission photon energy. The data are analyzed in terms of a model that involves an interplay between a radiative state and a metastable charge-separated state. The agreement between our results and the model points towards an emission dominated by carriers localized on In-rich nanoclusters that form spontaneously inside the InGaN insertions.

P-InGaN/AlGaN electron blocking layer for InGaN/GaN blue light-emitting diodes

In this work, the advantages of the p-InGaN/AlGaN electron blocking layer (EBL) for InGaN/GaN light-emitting diodes (LEDs) were studied numerically and experimentally. The LEDs with p-InGaN/AlGaN EBL exhibited better optical performance over a wide range of carrier concentration due to the enhancement of holes' injection and electrons' confinement. The values of A, B, C, and D coefficients had been iteratively obtained by fitting quantum efficiency in the modified rate equation model. The analysis indicated that the improvement in the device properties could be attributed to the relatively small band gap and p-type doping of InGaN insertion layer.

Role of InGaN Insertion Layer on Nitride-Based Light-Emitting Diodes

In this study, we systematically investigate the effect of InGaN insertion layer (IL) on nitride-based light-emitting diodes. First, a series of samples with different InGaN ILs (Si doping level, thickness) were fabricated and investigated. An optimized condition of the IL was obtained based on current–voltage and electroluminescence measurements. Furthermore, in order to investigate the dominant mechanism for the improved performance of the samples with IL, the optimized sample and a control sample without InGaN IL were compared by means of X-ray diffraction, atomic force microscopy, photoluminescence, injection-current-dependent electroluminescence measurements and infrared camera images. Based on the discussion of these measurement results, we conclude that the performance improvements of samples with InGaN IL are due to both the effects of strain relaxation and better current spreading.

Role of InGaN Insertion Layer on Nitride-Based Light-Emitting Diodes

In this study, we systematically investigate the effect of InGaN insertion layer (IL) on nitride-based light-emitting diodes. First, a series of samples with different InGaN ILs (Si doping level, thickness) were fabricated and investigated. An optimized condition of the IL was obtained based on current–voltage and electroluminescence measurements. Furthermore, in order to investigate the dominant mechanism for the improved performance of the samples with IL, the optimized sample and a control sample without InGaN IL were compared by means of X-ray diffraction, atomic force microscopy, photoluminescence, injection-current-dependent electroluminescence measurements and infrared camera images. Based on the discussion of these measurement results, we conclude that the performance improvements of samples with InGaN IL are due to both the effects of strain relaxation and better current spreading.

Growth mechanism and properties of InGaN insertions in GaN nanowires

We demonstrate the strong influence of strain on the morphology and In content of InGaN insertions in GaN nanowires, in agreement with theoretical predictions which establish that InGaN island nucleation on GaN nanowires may be energetically favorable, depending on In content and nanowire diameter. EDX analyses reveal In inhomogeneities between the successive dots but also along the growth direction within each dot, which is attributed to compositional pulling. Nanometer-resolved cathodoluminescence on single nanowires allowed us to probe the luminescence of single dots, revealing enhanced luminescence from the high In content top part with respect to the lower In content dot base.

The Influence of a Piezoelectric Field on the Dynamic Performance of GaN-Based Green Light-Emitting Diodes With an InGaN Insertion Layer

In this letter, the mechanism for improvement of the dynamic performance of GaN-based light-emitting diodes with an InGaN insertion layer is investigated using the very fast electrical-optical pump-probe technique. Our measurements indicate that, when the bias current is relatively low (100 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ), the device with the InGaN insertion layer (device A) exhibits a shorter response time than does the control (device B) without such a layer. However, when the bias current density reaches 0.5 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , devices A and B exhibit exactly the same response time during operation from room temperature to 200 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C. These results indicate that, under low current density (100 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ), the piezoelectric (PZ) field inside device A will be stronger, which should result in a lower effective barrier height with a shorter carrier escape time than is the case for device B. On the other hand, under high bias current density, both devices have the same internal response time, which indicates the screening of the PZ field inside due to injected carriers. These dynamic measurement results suggest that the origin of the efficiency droop in our device under low and high bias current densities is carrier leakage and the Auger effect, respectively.

Very-High Temperature (200 $^{\circ}$C) and High-Speed Operation of Cascade GaN-Based Green Light- Emitting Diodes With an InGaN Insertion Layer

We demonstrate a novel type of linear cascade green light-emitting diode (LED) arrays as a light source for in-car or harsh environment plastic optical fiber (POF) communications. To further enhance its dynamic and static performance, an InGaN layer is inserted between an n-type GaN cladding layer and InGaN-GaN multiple quantum wells as an efficient current spreading layer. Compared with the control device without that layer, our three-LED cascade array demonstrates a smaller turn-on voltage (9.3 versus 11 V at 20 mA) and a larger output power (25.5 versus 22.5 mW at 180 mA), corresponding to an enhancement of around 31% in wall-plug efficiency. Furthermore, under the constant voltage bias of an in-car battery (12 V), our three-LED array exhibits an electrical-to-optical 3-dB bandwidth (100 versus 40 MHz) performance superior to that of the control device. Even under high-temperature dynamic operation, we observe that the InGaN insertion layer gives strong enhancement of modulation speed with negligible degradation of the output power, unlike the red resonant-cavity LEDs conventionally used for POF. We achieve 200-Mb/s error-free transmission at 200°C which is the highest operation temperature among all the reported high-speed LEDs.

Improved Performance of GaN-Based Blue LEDs With the InGaN Insertion Layer Between the MQW Active Layer and the n-GaN Cladding Layer

In this study, we demonstrate the effect of GaN-based blue light-emitting diodes (LEDs), using an InGaN layer inserted between the n-type GaN cladding layer and the active layer (InGaN/GaN multiple quantum well), on improving device performances. With a 20-mA current injection, the results indicate that the typical output power (or forward voltage) of light-emitting diodes grown with, and without, the InGaN insertion layer are approximately 18.1 (3.1) and 15.3(3.5) mW (V), respectively. This corresponds to an enhancement in output power (wall-plug efficiency) of around 18% (33%), with the use of the InGaN insertion layer. In addition, the electrostatic discharge (ESD) endurance voltages increased from 1000 V to 6000 V when the InGaN insertion layer was applied to the GaN/sapphire-based LEDs. The improvement of output power and ESD endurance voltage could be mainly due to the fact that the Si-doped InGaN insertion layer played the role of a current-spreading layer, which led to a lower possibility of junctions suffering a large current density in specific local sites.

Analysis of the local indium composition in ultrathin InGaN layers

Experimental photoluminescence (PL) spectra and structural properties of the ultrathin InGaN insertions in an AlGaN matrix grown on a sapphire substrate were investigated. The PL emission mechanism was shown to be governed by the quantum dot (QD)-like indium-rich areas with size about 3 nm, which was determined from high resolution transmission electron microscopy image analysis. The local indium composition in QDs of the samples was estimated as about 26% using the suggested model of PL transition energy, which accounts for quantum confinement energy, spontaneous and piezoelectric fields.

A Study of Carrier Statistics in InGaN∕GaN LED Structures

The carrier statistics in LED structures with ultrathin multilayer InGaN insertions in a GaN matrix was studied. The optical data obtained indicate that an array of quantum dots (QDs) is formed in these structures. The QDs are scattered in size, which leads to an inhomogeneous broadening of the energy spectrum of carriers localized in the QDs. It is shown that, despite the suppressed transport of carriers between QDs, carriers are distributed among the levels of the QD array quasi-statistically at temperatures of about room temperature and higher. This makes it possible to describe the carrier injection and recombination in the device structures studied in terms of quasi-Fermi levels for electrons and holes.

Influence of metalorganic chemical vapor deposition growth conditions on In-rich nanoislands formation in InGaN/GaN structures

The influence of different growth conditions on the In distribution in ultrathin InGaN insertions in a GaN matrix is investigated by high-resolution transmission electron microscopy and an appropriate image evaluation technique. It is demonstrated that the indium distribution represents dense arrays of In-rich nanodomains inserted in a layer with a lower indium concentration. The sizes of the In-rich regions are about 4–5 nm at a growth temperature of 720 °C. Increasing the growth temperature leads to a strong decrease in the of nanoisland density and, also, a moderate decrease in their lateral size. Increasing the trimethylindium/trimethylgallium ratio strongly increases the density of the islands, but the lateral size remains weakly effected. The observations are in agreement with a thermodynamic model of island formation including entropy effects.

Quantum dots formed by ultrathin insertions in wide-gap matrices

We report on experimental and theoretical studies on a new type of quantum-dot (QD) structures obtained using ultrathin, i.e. below the critical thickness for 2D–3D transition, strained narrow gap insertions in wide bandgap matrices. We concentrate on submonolayer (SML) or slightly above 1 ML CdSe insertions in a wide-gap II–VI matrices and give the first results on ultrathin InGaN insertions in a GaN matrix. A discussion on detailed comparison of our original results with the results of other authors is presented. The formation of dense arrays (up to 1012 cm−2) of nanoscale two-dimensional (2D) islands is revealed in processed high-resolution transmission electron microscopy images. In the case of stacked sheets of SML insertions, the islands in the neighboring sheets are formed predominantly in correlated or anticorrelated way for thinner and thicker spacer layers, respectively. Different polarization of photoluminescence (PL) emission recorded in edge geometry for vertically-uncoupled and coupled QDs confirms the QD nature of excitons. By monitoring of sharp lines due to single QDs using cathodoluminescence the 3D confinement is manifested. We demonstrate significant squeezing of the QD exciton wavefunction in the lateral direction using magneto-optical experiments. We point to complete suppression of lateral motion of excitons bound to islands in case of wide-gap (ZnMgSSe) matrices, as follows from PL excitation studies. A resonant (0-phonon) lasing is observed in ultrathin CdSe insertions and proves the lifting of the k-selection rule for QD excitons. We show that lack of exciton screening in QDs up to high excitation densities enables strong resonant modulation of the refractive index in stacked ultrathin insertions and allows realization of resonant (excitonic) waveguiding and lasing. This enables the realization of a new type of heterostructure laser operating without external optical confinement by layers having lower average refractive indices. Ultrahigh QD excitonic gain in dense arrays of stacked QDs allows a new type of surface-emitting laser.

Optical Properties of Structures with Single and Multiple InGaN Insertions in a GaN Matrix

Optical Properties of Structures with Single and Multiple InGaN Insertions in a GaN Matrix