Inx Ga1 Research Articles

High efficient step graded Inx Ga1−x N/GaN superlattice solar cell

High efficient step graded Inx Ga1−x N/GaN superlattice solar cell

Simultaneous Effects of Hydrostatic Pressure and Temperature on Donor Impurity States in Wurtzite Inx Ga1 – x N/GaN Concentric Double Quantum Rings

Simultaneous Effects of Hydrostatic Pressure and Temperature on Donor Impurity States in Wurtzite In<i>_x</i> Ga₁ –<i> x</i> N/GaN Concentric Double Quantum Rings

Radial composition of single InGaN nanowires: a combined study by EDX, Raman spectroscopy, and X‐ray diffraction

Abstractmagnified imageThe radial alloy distribution of Inx Ga1–xN nanowires grown by plasma‐assisted molecular beam epitaxy has been investigated by three different techniques with nanometric spatial resolution and capability to study single nanowires. Energy‐dispersive X‐ray spectroscopy radial line‐scans revealed a gradient in the alloy composition of individual nanowires. Resonant Raman scattering and spatially resolved X‐ray diffraction showed the existence of three distinctive regions with different alloy composition. The combination of the three techniques provides robust evidence of the spontaneous formation of a core–shell structure with a thin Ga‐richer shell wrapping an In‐rich core at the bottom part of the nanowires. This composition‐modulated nanostructure offers an attractive way to explore new device concepts in fully epitaxial nanowire‐based solar cells. (© 2013 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Observation of A1(LO), E2(high) and B1(high) phonon modes in Inx Ga1–xN alloys with x = 0.11–0.54

AbstractPhonon spectra of A1(LO), E2(high) and B1(high) modes were observed in a series of hexagonal Inx Ga1–xN alloy films (x = 0.11–0.54) by Raman scattering using a deep‐UV laser at 266 nm for excitation. The A1(LO) and E2(high) modes both showed one‐mode type variation with x. The B1(high) mode, which is Raman‐forbidden in ideal binary wurtzite compounds, has been first observed in Inx Ga1–xN alloy in a clear manner. Its frequency variation is in line with a theoretical prediction of one‐mode type behavior. (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Carrier dynamics of InxGa1−xN quantum disks embedded in GaN nanocolumns

Time-integrated and time-resolved microphotoluminescence studies have been performed on Inx Ga1−xN quantum disks at the tips of GaN nanocolumns. The results are analyzed in the context of current theories regarding an inhomogeneous strain distribution in the disk which is theorized to generate lateral charge separation in the disks by strain induced band bending, an inhomogeneous polarization field distribution, and Fermi surface pinning. It is concluded that no lateral separation of carriers occurs in the quantum disks under investigation. Internal field screening by an increased carrier density in the QDisks at higher excitation densities is observed via a blue-shift of the emission and a dynamically changing decay time. Other possible explanations for these effects are discussed and discounted. Cathodoluminescence studies have also been carried out on the nanocolumns to provide insight into the physical origin of the luminescence.

Direct observation of elemental segregation in InGaN nanowires by X‐ray nanoprobe

AbstractUsing synchrotron radiation nanoprobe, this work reports on the elemental distribution in single Inx Ga1–xN nanowires (NWs) grown by molecular beam epitaxy directly on Si(111) substrates. Single NWs dispersed on Al covered sapphire were characterized by nano‐X‐ray fluorescence, Raman scattering and photoluminescence spectroscopy. Both Ga and In maps reveal an inhomogeneous axial distribution inside sin‐ gle NWs. The analysis of NWs from the same sample but with different dimensions suggests a decrease of In segregation with the reduction of NW diameter, while Ga distribution seems to remain unaltered. Photoluminescence and Raman scattering measurements carried out on ensembles of NWs exhibit relevant signatures of the compositional disorder. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

InGaN/GaN multiple quantum well concentrator solar cells

We present the growth, fabrication, and photovoltaic characteristics of Inx Ga1−xN/GaN(x∼0.35) multiple quantum well solar cells for concentrator applications. The open circuit voltage, short circuit current density, and solar-energy-to-electricity conversion efficiency were found to increase under concentrated sunlight. The overall efficiency increases from 2.95% to 3.03% when solar concentration increases from 1 to 30 suns and could be enhanced by further improving the material quality.

“Step‐graded interlayers” for the improvement of MOVPE InxGa1‐xN (x ∼ 0.4) epi‐layer quality

AbstractThis paper reports the effect of step‐graded interlayers on MOVPE InGaN epi‐layer quality. Multilayer epitaxial structures consisting of Inx Ga1–xN layers with various compositions have been successfully grown. HRXRD measurements reveal that the insertion of step‐graded interlayers significantly reduces the twist while little effects are observed on tilt. Decreasing of twist with increasing the number of interlayers is found to be remarkable up to a certain value and then maintain nearly the same even with increasing the In composition. Different types of dislocation density have also been studied using the X‐ray diffraction analysis. Dislocation densities, particularly the edge dislocation density, decrease considerably with the insertion of interlayers for higher In composition. An edge dislocation of 3 × 1010 cm–2 is obtained for a smple with 0.25 In composition, that seems to come from the GaN layer beneath the step‐graded interlayers (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

In$_x$Ga$_{1-x}$N–GaN-Based Solar Cells With a Multiple-Quantum-Well Structure on SiCN–Si(111) Substrates

In this letter, the author describes an effective method of obtaining a high photovoltaic efficiency of In <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-</sub> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> N-GaN-based solar cells with a multiple-quantum-well (MQW) structure using a SiCN-Si (111) substrate. The MQW region had a higher solar cell absorption than the non-MQW In <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-</sub> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> N-GaN sample. Under an air mass 1.5 global solar spectrum, a maximum photovoltaic efficiency of 5.43% in the MQW sample over the control sample was observed. It was found that the device and fabrication technology developed in this study are applicable to the realization of solar cells with high open-circuit voltages ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">oc</sub> ) of 2.86-2.96 V, high short-circuit current densities ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sc</sub> ) of 2.40-2.45 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , and high fill factors of 74.8%-76.5%.

Built‐in fields in non‐polar InxGa1–xN quantum dots

AbstractWe present a detailed analysis of the internal built‐in potential in polar and non‐polar Inx Ga1–x N quantum dots (QDs). The applied model is based on a surface integral method to calculate the three‐dimensional electrostatic built‐in potential in semiconductor QDs of arbitrary shape. This technique allows for a detailed and systematic analysis of the different contributions to the total built‐in field. Here, we also investigate the impact of the QD geometry on the built‐in potential in structures grown along non‐polar directions of the crystal.Since there is uncertainty in the literature as to the sign of the piezoelectric constant e15 associated with the shear strain, we study the influence of this constant in detail. While recent results on non‐polar GaN/AlN QDs indicate that only a negative e15 leads to significant reduction in the built‐in field, we show that for InN/GaN QDs there should still be a significant field present, independent of the sign of the piezoelectric constant e15. However, this field is strongly reduced with increasing Ga concentration in the nanostructure. (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Structural, electrical and optical characterization of InGaN layers grown by MOVPE

We present a study on n-type ternary InGaN layers grown by atmospheric pressure metalorganic vapour phase epitaxy (MOVPE) on GaN template/(0001) sapphire substrate. An investigation of the different growth conditions on n-type Inx Ga1 − x N (x = 0.06 − 0.135) alloys was done for a series of five samples. The structural, electrical and optical properties were characterized by high resolution x-ray diffraction (HRXRD), Hall effect and photoluminescence (PL). Experimental results showed that different growth conditions, namely substrate rotation (SR) and change of total H2 flow (THF), strongly affect the properties of InGaN layers. This case can be clearly observed from the analytical results. When the SR speed decreased, the HRXRD scan peak of the samples shifted along a higher angle. Therefore, increasing the SR speed changed important structural properties of InGaN alloys such as peak broadening, values of strain, lattice parameters and defects including tilt, twist and dislocation density. From PL results it is observed that the growth conditions can be changed to control the emission wavelength and it is possible to shift the emission wavelength towards the green. Hall effect measurement has shown that the resistivity of the samples changes dramatically when THF changes.

N-type doping of InGaN by high energy particle irradiation

This article reviews our extensive studies of the effects of native defects introduced by high energy particles on the electrical and optical properties of InGaN alloys. We show that the electronic properties of irradiated InGaN can be well described by the amphoteric defect model. Because of the extremely low position of the conduction band edge of InN the formation energy of native donor defects is very low in In-rich InGaN alloys. High energy particle irradiation of InN and In-rich InGaN, will therefore produce donor defects and result in more n-type materials. As the irradiation dose increases, the electron concentration increases until the Fermi energy EF approaches the Fermi stabilization energy EFS. At this point both donor and acceptor-type defects are formed at similar rates, and compensate each other, leading to stabilization of EF and a saturation of the electron concentration. Hence a large increase and then saturation in the Burstein–Moss shift of the optical absorption edge is also observed. Furthermore we also found that mobilities in the irradiated films can be well described by scattering from triply charged defects, providing strong evidence that native defects in InN are triple donors. The excellent agreement between the experimental results and predictions based on the ADM suggests that particle irradiation can be an effective and simple method to control the doping (electron concentration) in In-rich Inx Ga1–x N via native point defects. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Direct determination of intrinsic Inx Ga1–xP (x = 0.49) band‐gap deformation potentials by cathodoluminescence piezo‐spectroscopy

AbstractWe determined to a degree of precision the complete set of three deformation potentials of zinc‐blende‐like (intrinsic) Inx Ga1–x P (x = 0.49) and found that their variation with chemical composition does not follow a linear rule‐of‐mixture between values of InP and GaP, as usually assumed in the published literature. Measurements were made according to a microscopic cathodoluminescence piezo‐spectroscopic (CL/ PS) procedure, previously calibrated for paradigm materials, which deformation potentials are known according to macroscopic calibration calibration procedures. The procedure followed in this study allowed us to obtain the desired deformation potential values directly on a thin film (i.e., without the need of a bulk sample), by exploiting the high spatial resolution of the electron probe. Microscopic strain fields were used, which were preliminary introduced by a pyramidal indentation print on the surface of the epitaxially grown film. The methodology enabled accurate deformation potential assessments in small material volumes for both tensile and compressive stress fields. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Origin of the broad lifetime distribution of localized excitons in InGaN/GaN quantum dots

AbstractWe derive an energy‐dependent decay‐time distribution function from the multi‐exponential decay of the ensemble photoluminescence (PL) of Inx Ga1–x N/GaN quantum dots (QDs), which agrees well with recently published single‐QD time‐resolved PL measurements. Using eight‐band k · p modelling, we show that the built‐in piezo‐ and pyroelectric fields within the QDs cause a sensitive dependence of the radiative lifetimes on the exact QD geometry and composition. Moreover, the radiative lifetimes also depend heavily on the composition of the direct surrounding of the QDs. A broad lifetime distribution occurs even for moderate variations of the QD structure. Thus, for unscreened fields a multi‐exponential decay of the ensemble PL is generally expected in this material system. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Electron and exciton energy spectra in self‐assembled InGaAs/GaAs ring‐like nanostructures

AbstractTheoretical analysis of the electron energy spectrum and the magnetization of an electron in a strained Inx Ga1–x As/GaAs self‐assembled quantum ring (SAQR) is performed with realistic parameters, determined from the cross‐sectional scanning‐tunneling microscopy characterization of that nanostructure. The effects of the Coulomb interaction on the energy spectrum of an exciton in a SAQR as well as on the optical‐transition probabilities are studied within our model of a SAQR. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Properties of Ga2O3‐based (Inx Ga1–x)2O3 alloy thin films grown by molecular beam epitaxy

AbstractA series of Ga2O3‐based (Inx Ga1–x )2O3 alloy thin films have been grown on c‐plane sapphire substrates with a thin Ga2O3 buffer layer by plasma‐assisted molecular beam epitaxy. At growth temperatures of 700 °C and higher, even with a slight inclusion of In2O3 to Ga2O3, for example, the film of (In0.08Ga0.92)2O3, exhibited a rough surface and degraded transmission spectrum resulting from phase separation of In2O3. Due to low temperature growth at 600 °C, however, the phase separation was suppressed for the In composition up to 35%, which was confirmed by X‐ray diffraction measurement, and the films exhibited high transmittance over 85% with sharp absorption edges. The bandgap could be tuned form 5.0 to 4.0 eV. The results encourage the application of (Inx Ga1–x )2O3 thin films in short‐wavelength optical devices. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Growth, fabrication, and characterization of InGaN solar cells

AbstractThe InGaN alloy system offers a unique opportunity to develop high efficiency multi‐junction solar cells. In this study, single junction solar cells made of Inx Ga1–x N are successfully developed, with x = 0, 0.2, and 0.3. The materials are grown on sapphire substrates by MBE, consisting of a Si‐doped InGaN layer, an intrinsic layer and an Mg‐doped InGaN layer on the top. The I –V curves indicate that the cell made of all‐GaN has low series resistance (0.12 Ω cm2) and insignificant parasitic leakage. Contact resistances of p and n contacts are 2.9 × 10–2 Ω cm2 and 2.0 × 10–3 Ω cm2, respectively. Upon illumination by a 200 mW/cm2, 325 nm laser, Voc is measured at 2.5 V with a fill factor of 61%. Clear photo‐responses are also observed in both InGaN cells with 0.2 and 0.3 Indium content when illuminated by outdoor sunlight. But it is difficult to determine the solar performance due to the large leakage current, which may be caused by the material defects. A thicker buffer layer or GaN template can be applied to the future growth process to reduce the defect density of InGaN films. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Atom probe reveals the structure of InxGa1–xN based quantum wells in three dimensions

AbstractThe three‐dimensional atom probe has been used to characterize InxGa1–xN based multiple quantum well structures emitting from the green to the ultra‐violet with sub‐nanometre resolution over a 100 nm field of view. The results show gross discontinuities and compositional variations within the UV‐emitting quantum well layers on a 20–100 nm length scale. We propose that these may contribute to the high efficiency of this structure. In addition, the analysis shows the presence of indium in the barrier layers of all the samples, whether or not there was an indium precursor present during barrier growth. The distribution of indium within the blue‐ and green‐emitting InxGa1–xN quantum wells is also analyzed, and we find no evidence that InxGa1–xN with a range of compositions is not a random alloy, and so rule out indium clustering as the cause of the reported carrier localization in these structures. The upper interface of each quantum well layer is shown to be rougher and more diffuse than the lower, and the existence of monolayer steps in the interfaces that could effectively localize carriers at room temperature is revealed. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Mg‐doped InN and InGaN – Photoluminescence, capacitance–voltage and thermopower measurements

AbstractThe bandgap range of InGaN extends from the near‐IR (InN, 0.65 eV) to the ultraviolet. To exploit this wide tuning range in light generation and conversion applications, pn junctions are required. The large electron affinity of InN (5.8 eV) leads to preferential formation of native donor defects, resulting in excess electron concentration in the bulk and at surfaces and interfaces. This creates difficulties for p‐type doping and/or measuring of the bulk p‐type activity. Capacitance–voltage measurements, which deplete the n‐type surface inversion layer, have been used to show that Mg is an active acceptor in InN and Inx Ga1–x N for 0.2 &lt; x &lt; 1.0, i.e. over the entire composition range. Mg acceptors can be compensated by irradiation‐induced native donors. Thermopower measurements were used to provide definitive evidence that Mg‐doped InN has mobile holes between 200 K and 300 K. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Surface electronic properties of n‐ and p‐type InGaN alloys

AbstractX‐ray photoemission spectroscopy is utilised to determine the variation in surface Fermi level pinning across the composition range of n‐ and p‐type c ‐plane Inx Ga1–x N alloys. The pinning relative to the charge neutrality level is used to explain a change in band bending direction causing a transition from surface electron accumulation (In‐rich) to depletion (Ga‐rich) at x ≈ 0.43 for n‐type alloys and a transition from surface inversion to hole depletion at x ≈ 0.59 for p‐type alloys where downward band bending occurs across the composition range. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)