High Indium Composition Research Articles

InxGa1−xN/GaN Multiple Quantum Well Solar Cells with Conversion Efficiency of 3.77%**Supported by the National Basic Research Program of China (No 2012CB619303), and the National High-Technology Research and Development Program of China (No 2011AA050514).

We report on fabrication and photovoltaic characteristics of InxGa1−xN/GaN multiple quantum well solar cells with different indium compositions and barrier thicknesses. The as-grown samples are characterized by high-resolution x-ray diffraction and reciprocal space mapping. The results show that the sample with a thick barrier thickness (10.0nm) and high indium composition (0.23) has better crystalline quality. In addition, the dark current density-voltage (J–V) measurement of this device shows a significant decrease of leakage current, which leads to high open-circuit voltage Voc. Through the J–V characteristics under an Air Mass 1.5 Global (AM 1.5 G) illumination, this device exhibits a Voc of 1.89 V, a short-circuit current density Jsc of 3.92 mA/cm2 and a fill factor of 50.96%. As a result, the conversion efficiency (η) is enhanced to be 3.77% in comparison with other devices.

Double superstructures in InGaN/GaN nano-pyramid arrays

We have fabricated and investigated InGaN/GaN nanopyramid arrays grown on c-plane sapphire by selective area growth technique. The double InGaN/GaN superstructures were formed in one GaN pyramid observed in transmittance emission microscopy. The two growth faces on GaN nanopyramind showed different growth rates, resulted in double superheterostructures of semi-polar facets for InGaN/GaN multiple quantum wells (MQWs) and c-plane InGaN/GaN nano-hetero-disk (NHD) under the nanoscale growth condition. The c-plane InGaN/GaN NHD had high Indium composition due to the pulling effect, exhibiting double wavelength emissions from GaN nanopyramid. And InGaN/GaN MQWs and c-plane InGaN/GaN NHD showed extremely low quantum confinement Stark effect, yielding great light emission efficiency. This result is beneficial for the development of blue, green and white light-emitting diodes.

Influence of piezoelectric fields on InGaN based intermediate band solar cells

As it is practically infeasible to fabricate multiple-junction InGaN based tandem solar cells due to an intrinsic limit, intermediate-band solar cells (IBSCs) provide an alternative option for the fabrication of single-junction solar cells with their performance potentially equivalent to that of multiple-junction solar cells. InGaN quantum dots (QD) could be used for designing an IBSC structure. More importantly, it is well-known that there exist very strong piezoelectric fields in an InGaN/GaN system with a high indium composition, which becomes more pronounced for InGaN based QDs. The built-in piezoelectric fields can lead to a significant increase in the open circuit voltage and thus improved performance of solar cells, which has not yet been considered in designing III-nitride based solar cells so far. An optimized InGaN based QD-IBSC structure has been designed, combining the major advantages from the IBSC structure and the benefits due to the strong piezoelectric fields. A conversion efficiency, open-circuit voltage and short-circuit current have been calculated, and a highest conversion efficiency of 55.4% is obtained. The combination of the single-junction IBSC structure and the piezoelectric fields paves the way for the fabrication of InGaN based single-junction solar cells with ultra-high energy efficiency.

Abnormal selective area growth of irregularly-shaped GaN structures on the apex of GaN pyramids and its application for wide spectral emission

We report on the growth and the characterization of three-dimensional randomly-shaped InGaN/GaN structures selectively grown on the apex of GaN pyramids for the purpose of enlarging the emission spectral range. We found that the variations in the shape and the size of the three-dimensional GaN structures depend on the growth temperature and the surface area for selective growth under intentional turbulence in the gas stream. The selectively grown GaN structures grown at 1020 °C have irregular shape, while the samples grown at 1100 °C have rather uniform hexagonal pyramidal shapes. Irregularly shaped GaN structures were also obtained on the apex of GaN pyramids when the SiO2 mask was removed to 1/10 of the total height of the underlying GaN pyramid. When only 1/5 of the SiO2 mask was removed, however, the selectively grown GaN structures had similar hexagonal pyramidal shapes resembling those of the underlying GaN pyramids. The CL (Cathodoluminescence) spectra of the InGaN layers grown on the randomly shaped GaN structures showed a wide emission spectral range from 388 to 433 nm due to the non-uniform thickness and spatially inhomogeneous indium composition of the InGaN layers. This new selective growth method might have great potential for applications of non-phosphor white light emitting diodes (LEDs) with optimized growth conditions for InGaN active layers of high indium composition and with optimum process for fabrication of electrodes for electrical injection.

Guidelines and limitations for the design of high-efficiency InGaN single-junction solar cells

Astract Indium gallium nitride (InGaN) alloys offer great potential for high-efficiency photovoltaics, yet theoretical promise has not been experimentally demonstrated. Several major challenges remain including polarization effects, suitable p-type doping, improved surface passivation, and growth of thick, high-quality InGaN layers. In this paper, we present numerical simulations of InGaN p-i-n single-junction solar cells to provide guidelines for performance improvement through optimization of device structures given achievable material characteristics. The performance of both InGaN/GaN heterojunction devices that are presently achievable and InGaN homojunction solar cells that should be feasible in the future are investigated through the calculation of characteristic parameters: short-circuit current density, open-circuit voltage, and conversion efficiency. These simulations study the effect of indium content, thickness, and background doping of the unintentionally-doped InGaN absorbing layer on the performance of both InGaN solar-cell designs. While the maximum efficiency of a p-i-n InGaN/GaN heterojunction solar cell with low indium composition is 11.3%, the conversion efficiency of heterojunction devices with high indium composition needed for longer wavelength absorption drastically reduces because of polarization effects. Above an indium composition of 45%, the modeled heterojunction devices do not operate as solar cells. Using presently achievable values of minority carrier lifetime and surface recombination velocity, the maximum efficiency of InGaN single-homojunction solar cells with optimized parameters is ~17%, which is significantly smaller than the theoretical maximum energy-conversion efficiency for single-junction cells.

Investigation of properties of InGaN-based vertical-type solar cells with emission wavelengths in ultraviolet-blue-green regions

We investigate the properties of InGaN-based vertical-type solar cells having wavelengths ranging from the ultraviolet to green regions. It is well known that InGaN-based solar cells require a high indium composition to obtain high conversion efficiency. However, although InGaN-based solar cells with a high indium composition have been fabricated, their conversion efficiency has not sufficiently increased. Therefore, to further understand carrier transport, we measured the bias-dependent external quantum efficiency. For vertical-type green solar cells with a high indium composition, we confirmed that they have a higher short circuit current than other samples tested due to their broader overlapping region with the solar spectrum, though their fill factor remained low due to their high barrier height and strong piezoelectric field, which caused a reduction in the carrier tunneling rate.

Photo-Electroless-Etching of Wide Band Gap Material for Flexible Solid-State Devices

We report on photo electroless chemical (PEC) etching of III-Nitride material for thin layer lift-off for flexible solid-state light emitting devices. We investigate three different structures: 30 µm unintentionally doped GaN on sapphire, 30 µm n-type GaN on sapphire, and 200 nm InGaN on 4 µm unintentionally doped GaN on sapphire with different indium composition. Initially the three structures were etched as-grown for the same duration and the same conditions in a HF/CH3OH/H2O2 solution under UV illumination. Scanning electron microscopy (SEM) measurements performed on the etched samples reveals porous structure and high density nanowire on the surface, as well as the formation of V-shaped pits. The etch rate in each structure were compared. Optical study were performed using photoluminescence (PL) and Raman spectroscopies to analyze PL peak shift and phonon frequency shift compared to the as-grown, which we attributed to the compressive strain relaxation and localized impurity formed in the crystalline structure after PEC etching. We also fabricated microcolumns of diameters ranging from 5 to 100-µm via inductively coupled plasma (ICP) etching process in the InGaN on GaN samples. Dry etching of micropillars exposes unintentionally doped GaN to the wet etching. We study the selectivity of GaN over InGaN in the PEC etching process. SEM images reveal lateral etching and porous GaN layer formation under the InGaN after wet etching. We compare etching process in the InGaN structure with and without microcolumns. PEC etching is sensitive to band bending in the surface of the semiconductor and electrolyte, when InGaN surface is in contact with HF solution, oxidation and reduction reaction happens at slow rate and leads to the porous structure in the surface. However, when the GaN under the InGaN is exposed, the etching in this layer is preferred due to the low resistivity and the confinement of the photogenerated minority in the upward band bending depleted region near the contact surface between GaN and electrolyte.Undercut InGaN microdisks were lifted off for optical study via flexible polydimethylsiloxane (PDMS) material. Microdisks were then transferred on to a Si wafer for PL measurements. We also performed Raman study on the microdisks and observed a shift in the phonon frequency attributed to the relaxation of the compressive strain with respect to the as-grown. InGaN microdisks with higher indium composition were transferred on a blue LED for optical pumping. Optical measurements on transferred microdisks revealed typical prominent visible peaks. These observations confirm the promising application of this technique to selectively etch III-V semiconductor. The lift-off process is a compelling technique for next generation transferred substrate devices and III-Nitride micro phosphor fabrication.

Superatmospheric MOCVD Growth of Bulk InGaN for Template and Optoelectronic Applications

InGaN, a III-Nitride semiconductor, has bandgaps between 0.7 eV (InN) and 3.39 eV (GaN), covering the entire visible spectrum, which makes InGaN an important material for photonic applications. To utilize the full range of InGaN’s bandgap, methods for material growth with increased indium content must be established. MOCVD synthesis of InGaN with indium content higher than approximately 20% has been difficult to achieve while maintaining high crystal quality. There is evidence that superatmospheric MOCVD growth can establish such a method. Work by Cheng Li et. al. indicates that the photoluminescent intensity increases with pressure, while thermodynamics indicate favorable conditions for superatmospheric growth. Growths of GaN and InGaN were done at 1.5 bar-a, with promising results. GaN was successfully grown with low yellow band luminescence and high indium composition material was grown, confirmed by X-Ray diffraction. The possibility of superatmospheric growth of high indium content InGaN was confirmed.

Mobility limited by cluster scattering in ternary alloy quantum wires

The mobility limited by cluster scattering in ternary alloy semiconductor quantum wire (QWR) is theoretically investigated under Born approximation. We calculate the screened mobility due to clusters (high indium composition InGaN) scattering in the InxGa1−xN QWR structure. The characteristics of the cluster scattering mechanism are discussed in terms of the indium composition of clusters, the one-dimensional electron gas (1DEG) concentration, and the radius of QWR. We find that the density, breadth of cluster, and the correlation length have a strong effect on the electron mobility due to cluster scattering. Finally, a comparison of the cluster scattering is made with the alloy-disorder scattering. It is found that the cluster scattering acts as a significant scattering event to impact the resultant electron mobility in ternary alloy QWR.

Projected Efficiency of Polarization-Matched p-In$_{\bm x}$Ga$_{\bm {1-x}}$N/i-In $_{\bm y}$Ga$_{\bm{1-y}}$N/n-GaN Double Heterojunction Solar Cells

Traditional p-GaN/i-InGaN/n-GaN double heterojunction solar cells have limited power conversion efficiency due to large polarization charges that accumulate at the heterojunction interfaces, leading to severe band bending that, in turn, hinders the carrier transport. In this study, we proposed the use of a p-type InGaN layer to reduce the polarization field and projected the power conversion efficiencies of p-In xGa1-xN/i-In yGa1-y N/n-GaN double heterojunction solar cells that are grown on a c-facet sapphire substrate with various indium components. Numerical simulations predict that a maximal power conversion efficiency that is close to 7% with a short-circuit current density of 4.05 mA/cm2 and an open-circuit voltage of 1.94 V can be achieved with a p-In0.2 Ga0.8N/i-In0.2Ga0.8N/n-GaN structure due to a polarization-matched p-i interface. Further efficiency enhancement with a higher indium composition over 20% is also possible via the redistribution of the built-in potential with n-GaN doping.

Temperature-Controlled Symmetry of Linear Polarization of Photoluminescence from InGaAs-Buried InAs/GaAs Quantum Dots

We have succeeded in adjusting the symmetry of the linear polarization of exciton emission from self-assembled InAs/GaAs quantum dots by controlling the combination of temperature and composition of the InGaAs burying layer. The anisotropic shape of the Stranski–Krastanow-type quantum dot is a drawback to the generation of a polarization-entangled photon pair. We found that the polarization symmetry of the intensity and wavelength of photon emission depends on the sample temperature and the composition of the burying layer. The ground-state emission peaks in two linear polarization directions were tuned to overlap by lowering the temperature and using a high indium composition of the burying layer. Our results will aid in the development of an entangled-photon generator using the emission of exciton molecule in self-assembled quantum dots.

Study of high indium InXGa1-XN alloys with synchrotron radiation

InGaN thin films with near entire indium composition range have been successfully grown on GaN/sapphire (0001) by molecular beam epitaxy (MBE). X-ray absorption fine structure have been used to study the local structure of some typical In x Ga 1-x N alloys with high indium (In) composition of x=0.78 and 0.86. A detailed analysis of the extended x-ray absorption fine structure of In K-edge by using the IFEFFIT program, and the chemical bonds of In-N are obtained. The x-ray absorption near-edge structure of In K- and L-edge and N K-edge are investigated, and the electronic structure of In x Ga 1-x N are determined with these high In content In x Ga 1-x N ternary compounds. The calculated XANES spectra of N K-edge, based on first principle method, are consistent with the observed spectra. DOI: http://dx.doi.org/10.11591/telkomnika.v11i2.2050

Simulation of High-Efficiency GaN/InGaN p-i-n Solar Cell With Suppressed Polarization and Barrier Effects

The photovoltaic characteristics of Ga-face GaN/InGaN p-i-n solar cells are investigated numerically. The severe polarization and barrier effects induced by the GaN/InGaN hetero-interfaces are demonstrated to be detrimental for the carrier collection. The conversion efficiency could be degraded to be out of application when the degree of polarization and/or indium composition are high. To efficiently eliminate both critical issues, the solar cell structure with appropriate band engineering is introduced. In the proposed structure, the photovoltaic characteristics not only show high-grade performance but also become insensitive to the degree of polarization, even in the situation of high indium composition.

Energy states and carrier transport processes in metamorphic InAs quantum dots

Photoluminescence excitation spectroscopy is used to probe energy states and carrier transport in InAs quantum dot structures grown on InGaAs metamorphic layers, designed for room temperature emission at 1.3, 1.4, or 1.5 μm. The dominant spectral feature is shown to arise from the partially relaxed InGaAs confining layer. In structures with a low indium composition or thin InGaAs layer, a clear wetting layer feature is observed which acts as the dominant reservoir for carriers thermally excited from the quantum dots. Structures with high indium composition and/or thick InGaAs lack a wetting layer and carriers escape directly to the InGaAs layers.

High indium content InGaN films grown by pulsed laser deposition using a dual-compositing target

High indium compositions InGaN films were grown on sapphires using low temperature pulse laser deposition (PLD) with a dual-compositing target. This target was used to overcome the obstacle in the InGaN growth by PLD due to the difficulty of target preparation, and provided a co-deposition reaction, where InGaN grains generated from the indium and GaN vapors deposit on sapphire surface and then act as nucleation seeds to promote further InGaN growth. The effects of co-deposition on growth mechanisms, surface morphology, and electrical properties of films were thoroughly investigated and the results clearly show promise for the development of high indium InGaN films using PLD technique with dual-compositing targets.

Investigation of InGaN green light-emitting diodes with chirped multiple quantum well structures

The effect of using chirped multiple quantum-well (MQW) structures in InGaN green light-emitting diodes (LEDs) is numerically investigated. An active structure, which is with both thick QWs with low indium composition on the p-side and thin QWs with high indium composition next to the n-region, is presented in this study. The thickness and indium composition in each single QW is specifically tuned to emit the same green emission spectrum. Comparing with conventional active structure design of green LEDs, which is using uniform MQWs, the output power is increased by 27% at 20 mA, and by 15% at 100 mA current injections. This improvement is mainly attributed to the enhanced efficiency of carrier injection into QWs and the improved capability of carrier transport.

Investigation of the optical properties of InGaN/GaN nanorods with different indium composition

AbstractIn this paper, the optical properties of our nanorod array structures fabricated from InGaN/GaN multiple quantum wells (MQWs) epiwafers with two different kinds of indium compositions have been investigated. The optical performance of our nanorod samples are significantly enhanced compared to the as‐grown epiwafers, in particular, such improvement is much more pronounced in the nanorod array structure with a higher indium composition. This is mainly attributed to effective strain relaxation as a result of fabrication into nanorod structure, leading to significant reduction in quantum confined Stark effect (QCSE). X‐ray reciprocal space mapping (RSM) measurements have been performed in order to quantitatively evaluate the stain relaxation in the nanorod samples, confirming that the majority of the strain in InGaN/GaN MQWs can be relaxed as a result of fabrication into nanorods. Finally, excitation‐power dependant photoluminescence measurements performed at a low temperature also support the conclusion (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optimization towards reduction of efficiency droop in blue GaN/InGaN based light emitting diodes

Light emitting diodes (LEDs) based on GaN/InGaN material suffer from efficiency droop at high current injection levels. We propose multiple quantum well (MQW) GaN/InGaN LEDs by optimizing the barrier thickness and high–low–high indium composition to reduce the efficiency droop. The simulation results reflect a significant improvement in the efficiency droop by using barrier width of 10nm and high–low–high indium composition in MQW LED.

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.

Influence of strain relaxation on the optical properties of InGaN/GaN multiple quantum well nanorods

Optical investigation has been carried out on InGaN/GaN nanorod structures with different indium compositions, fabricated from InGaN/GaN multiple quantum well (MQW) epitaxial wafers using a self-organized nickel nano-mask and subsequent dry etching techniques. In comparison with the as-grown InGaN/GaN MQWs, the internal quantum efficiencies of the nanorods are significantly improved, in particular, for the green InGaN/GaN nanorods with a high indium composition, the internal quantum efficiency is enhanced by a factor of 8, much larger than the enhancement factor of 3.4 for the blue InGaN/GaN nanorods. X-ray reciprocal space mapping (RSM) measurements have been performed in order to quantitatively evaluate the stain relaxation in the nanorods, demonstrating that the majority of strain in InGaN/GaN MQWs can be relaxed as a result of fabrication into nanorods. The excitation-power-dependent photoluminescence measurements have also clearly shown that a significant reduction in the strain-induced quantum confined stark effect has occurred to the nanorod structures.