Low Indium Content Research Articles

Effects of Growth Temperature on Indium Incorporation in InAlN Alloys Grown by GSMBE on Si(111)

InxAl1−xN alloys with low indium content (0.025 < x < 0.080) were grown on Si(111) substrates, with an AlN buffer layer, using gas source molecular beam epitaxy with ammonia under nitrogen-rich conditions. Composition was varied by changing the growth temperature from 580°C to 660°C. Growth temperature in excess of 580°C was found to be necessary to obtain compositional uniformity. As temperature was varied from 590°C to 660°C, both the growth rate and indium incorporation decreased substantially. Rising In content observed near the surface of each sample was attributed to native indium oxide formation.

Suppressed lattice relaxation during InGaAs/GaAsP MQW growth with InGaAs and GaAs ultra-thin interlayers

InGaAs/GaAsP strain-compensated multiple quantum wells (MQW) with an effective bandgap of 1.2eV are promising candidate materials for solar cells applications. Since high indium content in wells is needed to achieve such a low bandgap, lattice relaxation can easily occur during crystal growth. Here, we propose graded interlayers, consisting of ultra-thin GaAs and InGaAs with lower indium content, inserted between wells and barriers to prevent relaxation during growth and to achieve better crystal quality. Lattice relaxation was detected by in-situ monitoring of surface reflectance including an anisotropic component. Our method allowed the number of MQW periods before detectable lattice relaxation to reach approximately three times that without interlayers. Over 100 periods of In0.26Ga0.74As (7.0nm)/GaAs0.60P0.40 (9.2nm) MQW, with an absorption edge at 1.2eV, were successfully piled up as a result. Lattice relaxation without interlayers was caused dominantly by dislocations due to the large lattice mismatch between a well and a barrier, and graded interlayers made the hetero-interfaces smoother so that the subsequent layers could be more easily grown. Transmission electron microscopy confirmed the existence of the interlayer at both ends of In0.26Ga0.74As layer, with graded indium content and the thickness of 1.2–1.8nm.

Towards anode with low indium content as effective anode in organic solar cells

In2O3 thin films (100nm thick) have been deposited by reactive evaporation of indium, in an oxygen partial atmosphere. Conductive (σ=3.5×103S/cm) and transparent films are obtained using the following experimental conditions: oxygen partial pressure=1×10−1Pa, substrate temperature=300°C and deposition rate=0.02nm/s. Layers of this In2O3 thick of 5nm have been introduced in AZO/In2O3 and FTO/In2O3 multilayer anode structures. The performances of organic photovoltaic cells, based on the couple CuPc/C60, are studied using the anode as parameter. In addition to these bilayers, other structures have been used as anode: AZO, FTO, AZO/In2O3/MoO3, FTO/In2O3/MoO3 and FTO/MoO3. It is shown that the use of the In2O3 film in the bilayer structures improves significantly the cell performances. However the open circuit voltage is quite small while better efficiencies are achieved when MoO3 is present. These results are discussed in the light of surface roughness and surface work function of the different anodes.

Synthesis, Crystal Structure, and Thermoelectric Properties of Clathrates in the Sn‐In‐As‐I System

AbstractClathrates of the general formula Sn24–x–δInxAs22–yI8 have been synthesized with xmax = 10.7. They crystallize in the clathrate‐I type of crystal structure, space group Pm$\bar{3}$n, with the cubic unit cell parameter ranging from a = 11.088 (x = 0) to a = 11.343 (x = 10.7) Å. The crystal structure refined for five compositions with different indium contents revealed that despite the linear dependence of the unit cell parameter upon x the details of the crystal structure change twice upon varying the indium contents. These changes occur without altering the space group and show up in positional disorder of atoms within the framework and the concomitant change of the coordination of part of tin atoms from 3+2 to 3+3, which is additionally confirmed by the Mössbauer spectroscopy data. In general, these changes reflect the transition from the Sn24P19.3I8 structure type for high indium contents to the Sn20.5As22I8 structure type for low indium contents. It is shown that the Sn24–x–δInxAs22–yI8 clathrates have n‐type conductivity with high values of the Seebeck coefficient ranging from –322 to –594 μV·K–1 at room temperature. They display an abnormally low thermal conductivity at 293 K, from 0.36 to 0.57 W·m–1·K–1. The calculated thermoelectric figure‐of‐merit for Sn24–x–δInxAs22–yI8 has its room‐temperature maximum at 0.041 for x = 6.5.

Two types of quantum-confinement characters for the bound states in the InGaN/GaN quantum wells

The ( Ga 1 − x In x N ) Nw ( GaN ) Nb single and multiple quantum wells (SQWs and MQWs) are investigated theoretically using the sp 3 s ⁎ tight-binding (TB) method with inclusion of spin–orbit interaction. This study explores the effects of barrier thickness L b , well width L w , indium content x and valence-band offset (VBO) on the quantum confinement (QC) characteristics of the bound states in the well and on the electronic transitions. The calculations are based on the validity of two assumptions: the virtual crystal approximation (VCA) for the structure of the alloyed Ga 1 − x In x N wells, and the macroscopic theory of elasticity (MTE) for the structure of the computational supercell as a whole. The results demonstrate the following main trends: (1) the existence of two types of QC characteristics for the bound states in the GaInN alloyed wells. The nitrogen p-level ( E p N = 2.71 eV , which is associated with InN TB parametrization), displays a threshold/edge that divides the bound states into two types: (i) block-like localized states (in the energy range E p N < E < E g GaN , where E g GaN = 3.3 eV is the energy gap of zinc-blende GaN) and (ii) singlet-like localized states (in the energy range E g InN < E < E p N , where E g InN = 0.71 eV is the energy gap of zinc-blende InN). The confinement energy versus well width L w is found to follow an exponential rule in the former energy region and a power-law rule in the latter one. A stronger localization should be expected as the level becomes deeper in the quantum well. (2) The TB results of E g were compared with the available photoluminescence (PL) data of 1-ML and 2-ML thick SQWs. Taking into account the error bars due to the lattice relaxation and interface specific effects, the TB results provide evidence that the high-energy emissions ( E 0 1 W = 3.125 eV and E 0 2 W = 2.845 eV , for 1-ML- and 2-ML-thick wells, respectively) must have originated from wells with fractional filling (i.e., low indium contents). (3) The TB results predict that the indium content would rise as the well width increases. Unfortunately, overcoming this limitation of fractional monolayers is likely to remain beyond the capability of the currently existing growth techniques. The indium content being kept low is a natural authenticity which is the compromise to make in growing ultrathin GaInN/GaN quantum wells free of misfit dislocations.

Strained Active Regions in GaAs-based Quantum Cascade Lasers

This paper explores the effects of low indium concentration in deep-well AlGaAs/GaAs quantum cascade laser structures. Photon emission from these strained active regions involves nonlinear optical processes, which reduce the emission wavelength. Designs that incorporate strain in one and two wells of the active region, respectively, on (111) GaAs are compared. Results demonstrate wavelength extendibility in compressively strained deep-well GaAs-based devices.

Synthesis and thermoelectric properties of oxygen deficient fluorite derivative Ga3−xIn5+xSn2O16

Exploring the possibility to obtain n-type oxide thermoelectrics, the transparent conducting oxide Ga3−xIn5+xSn2O16 (0.3&amp;lt;x&amp;lt;1.6) has been investigated taking into consideration its structural relationships with In2O3 and its much lower indium content, more promising for applications. The absolute value of the Seebeck coefficient decreases almost linearly with x, whereas improvement in the power factor is obtained by increasing the indium content. The figure of merit ZT value reaches about 0.28 at 1000 K for x=0.6. The observed results are explained taking into account the peculiarities of the oxygen-deficient fluorite structure.

Effect of indium doping on zinc oxide films prepared by chemical spray pyrolysis technique

We report the conducting and transparent In doped ZnO films fabricated by a homemade chemical spray pyrolysis system (CSPT). The effect of In concentration on the structural, morphological, electrical and optical properties have been studied. These films are found to show (0 0 2) preferential growth at low indium concentrations. An increase in In concentration causes a decrease in crystalline quality of films as confirmed by X-ray diffraction technique which leads to the introduction of defects in ZnO. Indium doping also significantly increased the electron concentrations, making the films heavily n type. However, the crystallinity and surface roughness of the films decreases with increase in indium doping content likely as a result of the formation of smaller grain size, which is clearly displayed in AFM images. Typical optical transmittance values in the order of (80%) were obtained for all films. The lowest resistivity value of 0.045 Ω-m was obtained for film with 5% indium doping.

Pros and cons of green InGaN laser on c‐plane GaN

AbstractThe challenges of green InGaN lasers are discussed concerning material quality as a function of InGaN composition, quantum well design and piezoelectrical fields. Investigations of polar quantum well designs and comparison with simulated non‐polar structures demonstrate that the quality of the indium rich layers is more important than the influence of interface charges. A high risk of dark spots at high In concentrations of 26–33% is observed. Small changes of about 2% of In significant reduce or increase the quantity and size of dark luminescence areas. Polar designs are a trade‐off between low indium concentrations of 4 nm wide quantum wells and high overlap of electrons and holes in 2 nm narrow designs. Furthermore, our single quantum wells have less non‐radiative defects than indium rich multi‐quantum well structures. Optimized active layer designs and the material qualities enable us to get green InGaN lasers on c‐plane substrates for cw operation at 515–524 nm and wall plug efficiencies of 3.9–2.3%. Slope efficiency of 0.3–0.4 W/A allows up to now highest optical output power of 50 mW.

Effects of InGaN barriers with low indium content on internal quantum efficiency of blue InGaN multiple quantum wells

Blue In0.2Ga0.8N multiple quantum wells (MQWs) with InxGa1 – xN (x = 0.01–0.04) barriers are grown by metal organic vapour phase epitaxy. The internal quantum efficiencies (IQEs) of these MQWs are studied in a way of temperature-dependent photoluminescence spectra. Furthermore, a 2-channel Arrhenius model is used to analyse the nonradiative recombination centres (NRCs). It is found that by adopting the InGaN barrier beneath the lowest well, it is possible to reduce the strain hence the NRCs in InGaN MQWs. By optimizing the thickness and the indium content of the InGaN barriers, the IQEs of InGaN/InGaN MQWs can be increased by about 2.5 times compared with conventional InGaN/GaN MQWs. On the other hand, the incorporation of indium atoms into the intermediate barriers between adjacent wells does not improve IQE obviously. In addition, the indium content of the intermediate barriers should match with that of the lowest barrier to avoid relaxation.

Double subband occupation of the two-dimensional electron gas in In xAl 1 − xN/AlN/GaN/AlN heterostructures with a low indium content (0.064 ≤ x ≤ 0.140) barrier

We present a carrier transport study on low indium content (0.064 ≤ x ≤ 0.140) In x Al 1 − x N/AlN/GaN/AlN heterostructures. Experimental Hall data were carried out as a function of temperature (33–300 K) and a magnetic field (0–1.4 T). A two-dimensional electron gas (2DEG) with single or double subbands and a two-dimensional hole gas were extracted after implementing quantitative mobility spectrum analysis on the magnetic field dependent Hall data. The mobility of the lowest subband of 2DEG was found to be lower than the mobility of the second subband. This behavior is explained by way of interface related scattering mechanisms, and the results are supported with a one-dimensional self-consistent solution of non-linear Schrödinger–Poisson equations.

Thermal stability of thin InGaN films on GaN

The thermal stability of ∼200-nm-thick InGaN thin films on GaN was investigated using isothermal and isochronal post-growth anneals. The In x Ga 1− x N films ( x=0.08–0.18) were annealed in N 2 at 600–1000 °C for 15–60 min, and the resulting film degradation was monitored using X-ray diffraction (XRD) and photoluminescence (PL) measurements. As expected, films with higher indium concentration showed more evidence for decomposition than the samples with lower indium concentration. Also for each alloy composition, decreases in the PL intensity were observed starting at much lower temperatures compared to decreases in the XRD intensity. This difference in sensitivity of the PL and XRD techniques to the InGaN decomposition suggest that defects that quench luminescence are generated prior to the onset of structural decomposition. For the higher indium concentration films, the bulk decomposition proceeds by forming metallic indium and gallium regions as observed by XRD. For the 18% indium concentration film, measurement of the temperature-dependent InGaN decomposition yields an activation energy, E A , of 0.87±0.07 eV, which is similar to the E A for bulk InN decomposition. The InGaN integrated XRD signal of the 18% film displays an exponential decrease vs. time, implying InGaN decomposition proceeds via a first-order reaction mechanism.

Efficiency Dependence on Degree of Localization States in GaN-Based Asymmetric Two-Step Light-Emitting Diode With a Low Indium Content InGaN Shallow Step

GaN-based asymmetric two-step light-emitting diodes (LEDs) with a low indium content (LIn) InGaN shallow step was proposed and fabricated. It was found the LIn-InGaN shallow step can significantly enhance phase separation and/or inhomogeneous indium distribution in the active In0.27Ga0.73N layer. By inserting an In0.08Ga0.92N shallow step, it was found that we can enhance LED output power by a factor of 2.27 with an injection current of 20 mA.

In-Dopants Effect on the Photoluminescence Properties of YVO4 Nanophosphors

The powders of Yttrium vanadate (YVO4) with In-dopants were synthesized by solid-state reactions, and X-ray diffraction (XRD), transmission electron microscopy (TEM), ultraviolet-visible (UV-vis) absorption spectra, photoluminescence (PL) spectra, and the luminescence intensity change were used to characterize the samples. The results of XRD indicated that the YVO4:In3+ samples remained in pure cubic phase. TEM illustrated that the powders mainly consisted of grains with an average size of 100 nm. Under the excitation of 320 nm, the YVO4:In3+ single-crystalline samples exhibited emission ranging from 350 to 700 nm. The emission intensity of YVO4:In3+ increased with increasing indium concentration in the lower indium concentration region until the saturated PL intensity was reached, and the strongest white fluorescence was observed when the In3+ doping concentration was 2% at 900 degrees C. The luminescent intensity of YVO4:In3+ (2%) was 9.6 times as strong as that of non-doped YVO4. The nanophosphors emit white luminescence owing to broad charge transfer in crystal lattice is due to the addition of In3+ to capture the UV radiation.

Effect of bonding and static atomic displacements on composition quantification inInxGa1−xNyAs1−y

This study addresses the influence of static atomic displacements (SAD) and chemical bonding in (InGa)(NAs) alloys on structure factors for electron scattering and stresses their importance for compositional analysis. First, SAD are derived using valence force field (VFF) methods and their reliability is demonstrated by calculating residual atomic forces using density-functional theory (DFT). A systematic study of structure factors for low indium and nitrogen contents is given by means of full DFT calculations on the one hand and atomistic models on the other. We show that the consideration of SAD via VFF together with the inclusion of bonding via modified atomic scattering amplitudes is in best agreement with a full DFT calculation, providing the possibility to include bonding effects also if large cells containing ${10}^{6}$ atoms are considered. Second, a technique is presented, which allows the extraction of atomically resolved indium and nitrogen composition maps by evaluating strain and contrast in transmission electron microscopic two-beam images simultaneously. As the fringe contrast is compared with Bloch wave simulations, which in turn require accurate structure factors, we exemplify the influence of SAD and bonding on composition profiles regarding an ${\text{In}}_{0.08}{\text{Ga}}_{0.92}{\text{N}}_{0.03}{\text{As}}_{0.97}$ quantum well structure. In this respect, imaging conditions may be chosen for which SAD are of minor influence for conventional transmission electron microscopy, whereas relative errors larger than 25% can arise for the compositions if bonding is neglected.

Effect of nitrogen on the InAs/GaAs quantum dot formation

AbstractWe studied the influence of nitrogen incorporation on the InAsN/GaAs quantum dot formation using cross‐sectional scanning tunneling microscopy. Upon nitrogen exposure during InAs growth, rather strong dissolution effects occur, leading to the formation of extended almost spherical InGaAs quantum dots with a low indium content and almost free of nitrogen. In this paper we will discuss in particular the detailed stoichiometry determination of the quantum dots. Furthermore, we will demonstrate that nitrogen atoms are located in the surrounding GaAs matrix.Indium atoms are found underneath the nominal base plane, where quantum dot growth started, demonstrating strong strain‐induced intermixing already during quantum dot formation. The observed strong separation of indium and nitrogen is related to a rather low solubility of nitrogen within InAs as compared with GaAs. Finally, a schematic model for the growth of InAs and InAsN quantum dots in GaAs will be discussed in detail (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Contactless electroreflectance of InGaN layers with indium content ≤36%: The surface band bending, band gap bowing, and Stokes shift issues

Contactless electroreflectance (CER) supported by photoluminescence (PL) has been applied to study (i) the surface band bending, (ii) the band gap bowing, and (iii) the Stokes shift for InGaN layers grown by molecular beam epitaxy with 0.14≤In≤0.36. The type of surface band bending has been investigated on the basis of the shape of CER resonance. It has been found that the surface band bending changes from n-type for layers with low indium content (In&amp;lt;27%) to flatband (or weak p-type band) for layers with In∼35%. The band gap bowing has been determined to be 1.4±0.2 and 2.1±0.3 eV for CER data with and without strain corrections, respectively. From this analysis it has been concluded that the reliable value of the bowing parameter for unstrained InGaN should be between 1.4 and 2.1 eV. Comparing CER with PL data it has been found that the Stokes shift rises from 20 to 120 meV when the indium concentration increased from 14% to 36%. In addition, it has been observed that the intensity of PL from InGaN layers decreased exponentially with the increase in the indium content. The last two findings are attributed to an easier formation of native point defects and stronger indium segregation in InGaN alloys with higher indium concentrations.

Low-indium content bilayer transparent conducting oxide thin films as effective anodes in organic photovoltaic cells

Bilayer In-doped CdO/Sn-doped In 2O 3 (CIO/ITO) transparent conducting oxide (TCO) thin films were prepared by depositing thin ITO films by ion-assisted deposition on CIO films grown by metal-organic chemical vapor deposition. The optical, electrical, and microstructural properties of these bilayer TCO films were investigated in detail. A low sheet resistance of ~ 4.9 Ω/□ is achieved for the CIO/ITO (170/40 nm) bilayers without annealing. With a significantly lower In content (20 vs. ~ 93 at.%) and a much higher conductivity (> 12,000 vs. 3000–5000 S/cm) than commercial ITO, these bilayer films were investigated as anodes in bulk-heterojunction organic photovoltaic (OPV) devices having a poly(2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene) + [6,6]-phenyl C 61 butyric acid methyl ester active layer. Device performance metrics in every way comparable to those of devices fabricated on commercial ITO are achieved, demonstrating that CIO/ITO bilayers are promising low-In content, highly conductive and transparent electrode candidates for OPV cells.

GaN‐based nanocolumn LEDs: Impact of strain engineering on the electro‐optical performance

AbstractA computation of the electronic properties of GaN nanocolumn LEDs with optically active InGaN quantum disks is presented. Using the simulation framework AQUA/tdkp, the relaxation of intrinsic strains in the selected 3D structure and its effects on the built‐in piezoelectric potential and strain dependent bandedge shifts are computed.The results show a strong relaxation of the compressive strains in the InGaN quantum disk compared to usual broad area grown structures suggesting that lower wavelength emission can be achieved at considerable lower indium contents (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Nitride-Based Asymmetric Two-Step Light-Emitting Diode With In$_{0.08}$Ga$_{0.92}$N Shallow Step

A nitride-based asymmetric two-step light-emitting diode (LED) with In0.08 Ga0.92N shallow step was proposed and fabricated. It was found that the low indium content In0.08 Ga0.92N layer can significantly enhance phase separation and/or inhomogeneous indium distribution in the active In0.27Ga0.73N layer. It was also found that we can enhance LED output power by a factor of 2.27 by simply inserting an In0.08 Ga0.92N shallow step.