Increasing Indium Composition Research Articles

InGaN based Schottky barrier solar cell: Study of the temperature dependence of electrical characteristics

The indium gallium nitride (InGaN) semiconductor alloy shows great promise for high-efficiency thin-film solar cells due to its intrinsic characteristics. However, there are major challenges in the development of high-quality p-doped and indium-rich InGaN layers to fabricate a pn or pin solar cell. This study focuses on the development of an alternative structure, not requiring p-doping, with a Schottky contact on n-type InGaN. The InGaN absorber was grown by metalorganic chemical vapor deposition and its structural, morphological and optical properties studied. Structural analysis by X-ray diffraction showed the growth of single crystal layers without phase separation. Indium compositions which vary up to around 8.3 % were confirmed by transmission measurements. The formation of V-shaped pits was observed with increased indium composition at lower growth temperatures. Schottky diodes based on InGaN with platinum metallic contacts were fabricated and their current–voltage characteristics studied in detail as a function of temperature. This analysis of the conduction mechanisms in Schottky/InGaN devices permitted in particular the extraction of the diode parameters as a function of the temperature showing a high rectification with an ideality factor of 1.15 with reverse saturation current of 6.7×10−9A at room temperature for an indium composition of 5.52 %. In addition, the analysis revealed the presence of a dual Gaussian distribution of barrier heights in a specific temperature range. Finally, the incident photon-to-electron conversion efficiency (IPCE) of the fabricated InGaN Schottky solar cells, measured using intensity-modulated photocurrent spectroscopy, reached a peak value of 51 %, which is among the highest published values for InGaN based solar cells.

Theoretical analysis of intersubband optical absorption in asymmetric AlInN/GaN/AlInN quantum well heterostructures

The intersubband optical absorption coefficients were theoretically investigated in asymmetric AlInN/GaN/AlInN quantum wells. The electron band parameters are calculated by solving, self-consistently, the one-dimensional Poisson and Schrödinger equations under Hartree and effective mass approximations. The intersubband optical absorption between the two lowest levels was calculated based on the density matrix formalism. The effects of polarization, indium composition and delta-doping on the optical absorption coefficients in AlInN/GaN double heterostructures were studied. Results have revealed: i) an enhancement of the total (linear plus nonlinear) absorption coefficient peak and a reduction of wavelength due to the polarization effects ii) that the peak of the total optical absorption coefficient increased and showed a red shift upon increasing indium composition iii) that the peak of the total absorption coefficient increased and red-shifted or blue-shifted through optimizing the concentration of the planar doping in the AlInN barriers. Furthermore, the use of a back doping with a delta-shaped profile was identified as a consistent approach to carry out the transition 1.55 µm. The results in this study could be useful for the design and applications of optoelectronic devices operating at 1.55 µm.

Investigation on the effect of indium composition on ultrafast carrier dynamics in InGaN alloys

In this letter, we investigated the effect of indium composition on carrier relaxation mechanisms in InGaN alloys. Four high quality alloys with indium composition from 25% to 75% were fabricated using energetic neutral atomic-beam lithography/epitaxy molecular beam epitaxy. Using sub-picosecond resolved photoluminescence at high carrier density, it was found the effective carrier lifetime extends with increasing indium composition. Moreover, the calculated initial carrier temperature also rises with higher indium composition. These results are consistent with the theoretical prediction that a greater phonon bandgap could reduce the carrier cooling rate to a certain extent via hot carrier bottleneck effect.

Theoretical study on the electronic and optical properties of bulk and surface (001) InxGa1-xAs

The optical properties of surface and bulk InxGa1-xAs materials are compared systematically first time in this paper. The band structures, density of states and optical properties including dielectric function, reflectivity, absorption coefficient, loss function and refractive index of bulk and surface InxGa1-xAs materials are investigated by first-principles based on plane-wave pseudo-potentials method within the LDA approximation. The results agree well with the available theoretical and experimental studies and indicate that the electronic and optical properties of bulk and surface InxGa1-xAs materials are much different, and the results show that the considered optical properties of the both materials vary with increasing indium composition in an opposite way. The calculations show that the optical properties of surface In0.75Ga0.25As material are unexpected to be far from the other two indium compositions of surface InxGa1-xAs materials while the optical properties of bulk InxGa1-xAs materials vary with increasing indium composition in an expected regular way.

The role of indium composition on thermo-electric properties of InGaN/GaN heterostructures grown by MOCVD

Thermo Electric (TE) properties of InGaN/GaN heterostructure with different Indium compositions grown by Metal Organic Chemical Vapour Deposition (MOCVD) are investigated. Room temperature thermoelectric studies reveals that increasing indium composition (from 6% to 19%) in the InGaN/GaN heterostructure lead to a decrease in Seebeck coefficient (S) due to sharpening of bandgap which in turn increases TE figure of merit. Seebeck coefficient, Power factor and Figure of Merit of the InGaN/GaN heterostructured thin films shows significant enhancement at higher temperature up to 420 K as compared to room temperature measurement. Promising results on the TE properties of as grown InGaN/GaN heterostructures were observed with Figure of Merit (ZT) value of 0.15 at 420 K for In0.19Ga0.81N sample. The results indicate that InGaN material system could be potentially imparted for high temperature TE devices.

Anisotropically biaxial strain in non-polar (112\u20130) plane InxGa1\u2212xN/GaN layers investigated by X-ray reciprocal space mapping

In this study, the indium composition x as well as the anisotropically biaxial strain in non-polar a-plane InxGa1−xN on GaN is studied by X-ray diffraction (XRD) analysis. In accordance with XRD reciprocal lattice space mapping, with increasing indium composition, the maximum of the InxGa1−xN reciprocal lattice points progressively shifts from a fully compressive strained to a fully relaxed position, then to reversed tensile strained. To fully understand the strain in the ternary alloy layers, it is helpful to grow high-quality device structures using a-plane nitrides. As the layer thickness increases, the strain of InxGa1−xN layer releases through surface roughening and the 3D growth-mode.

Influence of TMIn flow rate on structural and optical quality of AlInGaN/GaN epilayers grown by MOCVD

In this paper, nearly lattice-matched AlInGaN/GaN epilayers grown on sapphire substrate by metal organic chemical vapor deposition have been studied. The structural and optical properties of these epilayers have been investigated and compared. The TMIn flow is believed to have a direct influence on the quality of the AlInGaN epilayer. By increasing indium composition in AlInGaN epilayer, dislocations have been found to decrease, indicating lower threading dislocation density confirmed by HRXRD. Reciprocal space mapping analysis has confirmed that the growth of AlInGaN on GaN buffer is a fully strained growth. Room temperature photoluminescence from AlInGaN exhibits two emissions originating from In(Ga)N clusters and the AlInGaN random matrix, respectively. Upon increasing the TMIn flow, red shift has been observed in the photoluminescence peak confirming the decrease in bandgap of AlInGaN epilayers. In the Raman spectra, the mode at 751 cm−1 is attributed to the A1 (LO) mode of AlInGaN. The mode at 632 cm−1 is attributed to In(Ga)N clustering and assigned as A1 (LO) mode. V-shaped defect pits with different density and size have been observed on the surface of AlInGaN epilayers.

Efficiency droop improvement in InGaN light-emitting diodes with graded InGaN barriers of increasing indium composition

In this work, graded barrier InGaN light-emitting diodes with increasing indium composition barriers are proposed and investigated numerically. When the conventional GaN barriers are replaced by this unique graded InGaN barrier design, the forward voltage at 100 mA is reduced from 3.32 V to 3.27 V, and the efficiency droop is improved from 46.6% to 7.5%. The simulation results observed in this work indicate that these improvements can be attributed to increased electron confinement and enhanced hole injection efficiency caused by the modified energy band diagram.

Comparative study of polar and semipolar (112¯2) InGaN layers grown by metalorganic vapour phase epitaxy

InGaN layers were grown simultaneously on (112¯2) GaN and (0001) GaN templates by metalorganic vapour phase epitaxy. At higher growth temperature (≥750 °C), the indium content (<15%) of the (112¯2) and (0001) InGaN layers was similar. However, for temperatures less than 750 °C, the indium content of the (112¯2) InGaN layers (15%–26%) were generally lower than those with (0001) orientation (15%–32%). The compositional deviation was attributed to the different strain relaxations between the (112¯2) and (0001) InGaN layers. Room temperature photoluminescence measurements of the (112¯2) InGaN layers showed an emission wavelength that shifts gradually from 380 nm to 580 nm with decreasing growth temperature (or increasing indium composition). The peak emission wavelength of the (112¯2) InGaN layers with an indium content of more than 10% blue-shifted a constant value of ≈(50–60) nm when using higher excitation power densities. This blue-shift was attributed to band filling effects in the layers.

Influence of InxGa1-xAs Underlying Layer on the Structural of the In0.5Ga0.5As Quantum Dots Grown by MOCVD

The single layer In0.5Ga0.5As quantum dots (QDs) were grown on a thin InxGa1-xAs underlying layer by metal-organic chemical vapor deposition(MOCVD) via Stranski-Krastanow growth mode. The effect of different indium composition in the InxGa1-xAs underlying layer was investigated usingatomic force microscopy (AFM). AFM images show that the QDs structures were formed on the surface. The dots formation on the surface changes withdifferent composition of InxGa1-xAs underlying layer. Increasing indium composition in the underlying layer resulted to formation of higher density andsmaller size dots. Several large dots were also formed on the surface. Growing of underlying layer reduces the lattice mismatch between In0.5Ga0.5As andGaAs, and decreases the critical thickness of the dots. This strongly influences the dots nucleation on the surface. Growth of quantum dots usingunderlying layer is one way to modify dot formation in order to achieve uniform QDs of right size and high density, which are essential for QDs deviceapplications.

Improvement of light power and efficiency droop in GaN-based LEDs using graded InGaN hole reservoir layer

InGaN-based light-emitting diodes with graded indium composition p-type InGaN hole reservoir layer (HRL) are numerically investigated using the APSYS simulation software. It is found that by gradient increasing indium composition in growth direction of the p-InGaN HRL can improve light output power, lower current leakage and efficiency droop. Based on numerical simulation and analysis, these improvements on the electrical and optical characteristics are attributed mainly to tailoring energy band in p–n junction vicinal region, and finally enhanced the hole injection efficiency and electron blocking efficiency.

Effects of exciton localization on internal quantum efficiency of InGaN nanowires

The optical properties of InGaN nanowires with different emission wavelengths of 485, 515, 555, and 580 nm have been studied by means of photoluminescence (PL) and time-resolved PL (TRPL) spectroscopy. The PL peak energy of the nanowires exhibited an anomalous shift to higher energy and then to lower energy with increasing temperature. Analysis of the temperature-dependent variations in the PL peak energy let us evaluate the localization energies of excitons, which increased with increasing indium composition. TRPL measurements also revealed that the PL decay time of the nanowires increased and then became constant with decreasing emission energy, which was typical of localized excitons and enabled us to evaluate the characteristic energies of localized states. The characteristic energy increased with increasing indium composition, indicating that the density of localized states broadened with increasing indium composition. In addition, a correlation was clearly observed between the internal quantum efficiency (IQE) and localization energy of the nanowire: the IQE increased with increasing localization energy. The increase in the IQE was attributed to the increase in the degree of exciton localization as the indium composition of the nanowire increased. Moreover, it was found that with increasing excitation power density, a reduction in the IQE occurred simultaneously with a PL blue shift. This indicated that the reduction in the IQE was associated with saturation of localized states.

Great emission enhancement and excitonic recombination dynamics of InGaN/GaN nanorod structures

Excitonic recombination dynamics has been investigated on a series of InxGa1−xN/GaN (0.10 ≤ x ≤ 0.30) nanorod (NR) structures with a diameter of ∼220 nm by time-revolved photoluminescence (PL). The NR structures are fabricated by means of a post-growth etching technique. Compared with their corresponding as-grown samples, the time-integrated PL intensities of the NR samples show a remarkable enhancement with a factor of up to 52 at room temperature. The ratios of the radiative to non-radiative recombination lifetime of the NR structures are much less sensitive to temperature than those of their corresponding as-grown samples. This becomes more prominent with increasing indium composition. A distinct delay in transition temperature, where the dominating emission mechanism changes from radiative to non-radiative recombination, has been observed on the NR structures. The great enhancement in optical properties is attributed to both strain relaxation and extra in-plane excitonic confinement due to the nanostructures.

InGaN light-emitting diodes: Efficiency-limiting processes at high injection

The authors discuss a relatively comprehensive theoretical and experimental study aimed on unveiling the dominant efficiency loss mechanism at high injection levels in InGaN light-emitting diodes(LEDs), which still limits their application for general lighting despite the breathtaking performance demonstration. A large body of theoretical and experimental data ascribes the observed efficiency loss to overflow of hot electrons aggravated by nonuniform distribution of carriers in the active region as the primary origin of the efficiency droop-phenomenon, but Auger recombination has also been invoked as the genesis of the efficiency loss. The electron overflowand the associated efficiency loss can be reduced substantially by inserting, in the n-side of the InGaN active region, an InGaN stair-case electron injector (SEI) with a step-like increased indium composition to operate as an “electron cooler.” In contrast to electron-blocking layer usually employed to prevent the electron leakage from the active region, the SEI does not impede hole injection due to the absence of valence band offset with p-GaN. Moreover, SEI does not generate piezoelectric polarizationfield in addition to differential spontaneous polarization field that pulls down the conduction band at the AlGaN/GaN interface aggravating the electron rollover. In terms of the active region design, owing to their high three-dimensional density of states, it is argued that double heterostructures (DHs) are more attractive for general-lighting LEDs than necessarily quantum wells. The authors demonstrate that DH-based LED active regions,particularly wide ones and those composed of multiple DHs separated by thin (3 nm)In0.06Ga0.94N barriers of reduced barrier height, meant to allow efficient hole transport across the active regions, naturally act as an electron cooler, thus considerably reducing the electron overflow at high injection. However, a wide separation of electron and hole distribution functions in DHs wider than 6 nm substantially reduces the radiative recombination efficiency at injection current densities below∼200 A/cm2. Consequently, the LEDs with dual 6 nm and quad (4×) 3 nm DHs separated by 3-nm In0.06Ga0.94N barriers exhibit the highest external quantum efficiency with substantially reduced efficiency degradation at injection current densities of special interest for low-voltage general-lighting applications. The authors conclude that, for achieving the highest possible LED efficiency, it is imperative that optimum the SEI and the active region should be designed to operate in unison.

Emission Wavelength Dependence of Internal Quantum Efficiency in InGaN Nanowires

The internal quantum efficiency (IQE) of InGaN nanowires with different emission wavelength of 485, 515, 555, and 580 nm has been studied by means of photoluminescence (PL) spectroscopy. It was found from the analysis of IQE as a function of excitation power density that the IQE was unchanged at about 100% under weak excitation conditions at low temperature. This indicated that the effects of nonradiative recombination processes were negligibly small at low temperature. Moreover, the IQE increased from 5 to 12% with increasing emission wavelength from 485 to 580 nm. Since the clear correlation between the IQE and the PL blue shift due to band filling effects of localized states was observed, the increase in the IQE reflected the increase in the effect of exciton localization with increasing indium composition.

The impact of AsH3 overflow time and indium composition on the formation of self-assembled InxGa1 − xAs quantum dots studied by atomic force microscopy

We have performed atomic force microscopy to investigate the effect of various indium compositions and various AsH3 flow times during cooling on the formation of self-assembled In x Ga1 − xAs quantum dots (QDs). The In x Ga1 − xAs QDs were grown by metal-organic chemical vapour deposition using the Stranski-Krastanow (S-K) growth mode. The migration of group III species in the growth of In x Ga1 − xAs QDs is influenced by the AsH3 flow during the cooling period due to the increasing surface population of the active arsenic species. It influences the size and density of the dots on the surface. For various indium compositions, an increase in In x Ga1 − xAs QD density with increasing indium composition is observed. It indicates that the dot density depends on lattice parameters. The dot density is inversely proportional to surface diffusion (ρ ∝ R/D), with D = (2kT/h)/a2 exp(−ED/kT). In the growth of In x Ga1 − xAs QDs using the S-K growth mode, the dots were formed on the surface as the effect of elastic strain relaxation due to the lattice mismatch. Increasing indium composition affects the lattice mismatch of the In x Ga1 − xAs/GaAs QD system, which influences the dot formation on the surface. However, due to the stochastic nature of the nucleation of self-assembled growth, control of the spatial ordering of the QDs has proved to be extremely challenging.

High temperature thermoelectric properties of optimized InGaN

The effects of carrier concentration, composition, and temperature on the thermoelectric properties of high quality n-type InGaN grown by metal organic chemical vapor deposition (MOCVD) were systematically investigated. The Seebeck coefficient was found to decrease and electrical conductivity increase with increasing carrier concentration, while both were found to decrease with increasing indium composition. Additionally, thermal conductivity was found to decrease by over an order of magnitude as indium composition was increased from 0 to 19%. These trends resulted in optimum carrier concentration and indium composition of 1.1×1019 cm−3 and 17%, respectively, with a room temperature ZT of 0.04. Increasing temperature resulted in a rapidly increasing ZT, reaching a maximum value of 0.34 at 875 K. This significantly improved ZT demonstrates the potential of InGaN and other III-Nitride materials for high temperature thermoelectric applications.

Droop improvement in blue InGaN/GaN multiple quantum well light-emitting diodes with indium graded last barrier

Blue InGaN/GaN multiple quantum well light-emitting diodes with the conventional and graded last barriers (GLB) are numerically investigated. When the last GaN barrier is replaced by a linearly graded InxGa1 − xN barrier with increasing indium composition in the growth direction, the forward voltage is reduced from 3.60 V to 3.25 V, and the efficiency droop is improved from 36% to 13%. Simulation results indicate that these improvements can be attributed to the formation of a deep potential well in the GLB which enhances the electron confinement and improves the hole injection efficiency.

Effects of an InGaAs Cap Layer on the Optical Properties of InAs Quantum Dot Molecules

Self-assembled InAs quantum dot molecules are grown on GaAs substrates without following any special protocols by using metal-organic chemical vapor deposition. The effects of indium composition and the thickness of the InGaAs cap layer on the optical properties of InAs quantum dot molecules are investigated by photoluminescence. With increasing indium composition and thickness of the InGaAs cap layer, the ground-state wavelength of the emission spectrum redshifts and the peak intensity decreases. In addition, the structural and optical properties of quantum dots and quantum dot molecules are comparatively studied, and the results show that when quantum dots turn into quantum dot molecules, the emission wavelength red shifts.

Photoluminescence and surface photovoltage spectroscopy characterization of highly strained InGaAs/GaAs quantum well structures grown by metal organic vapor phase epitaxy

Photoluminescence (PL) and surface photovoltage spectroscopy (SPS) are used to characterize a series of highly strained In x Ga 1− x As/GaAs quantum well (QW) structures grown by metal organic vapor phase epitaxy with different indium compositions (0.395 ≤ x ≤ 0.44) in the temperature range of 20 K ≤ T ≤ 300 K. The PL features show redshift in peak positions and broadened lineshape with increasing indium composition. The S-shaped temperature dependent PL spectra have been attributed to carrier localization effect resulting from the presence of indium clusters at QW interfaces. A lineshape fit of features in the differential surface photovoltage (SPV) spectra has been used to determine the transition energies accurately. At temperature below 100 K, the light-hole (LH) related feature shows a significant phase difference as compared to that of heavy-hole (HH) related features. The phase change of the LH feature can be explained by the existence of type-II configuration for the LH valence band and the process of separation of carriers within the QWs together with possible capture by the interface defect traps. A detailed analysis of the observed phenomena enables the identification of spectral features and to evaluate the band lineup of the QWs. The results demonstrate the usefulness of PL and SPS for the contactless and nondestructive characterization of highly strained InGaAs/GaAs QW structures.