Preparation and structure of ultra-thin GaN (0001) layers on In0.11Ga0.89N-single quantum wells

Multiple surface reconstructions have been observed on ultra-thin GaN (0001) layers of 1–10 nm thickness, covering a 3 nm thick In_0.11Ga_0.89N single quantum well in a GaN matrix. Low energy electron diffraction patterns show (2×2) and (√3×√3)-R30° symmetries for samples annealed in nitrogen plasma, and (2×2), (3×3), (4×4), and (6×6) symmetries for samples overgrown with an additional monolayer-thin GaN film by molecular beam epitaxy under Ga-rich growth conditions. Photoelectron spectroscopy shows that the InGaN quantum wells and capping layers are stable for growth temperatures up to 760 °C, and do not show formation of indium or gallium droplets on the surface. The photoluminescence emission from the buried InGaN SQWs remains unchanged by the preparation process, demonstrating that the SQWs do not undergo any significant modification.     

Growth and Characterization of In x Ga1−x N Multiple Quantum Wells Without Phase Separation

Efficient conversion of photon energy into electricity is a crucial step toward a sustainable solar-energy economy. Likewise, solid-state lighting devices are gaining prominence because of benefits such as reduced energy consumption and reduced toxicity. Among the various semiconductors investigated, In _x Ga_1–x N alloys or superlattices are fervently pursued because of their large range of bandgaps between 0.65 eV and 3.4 eV. This paper reports on the fabrication of multiple quantum wells on LiGaO₂ (001) substrates by plasma-assisted molecular beam epitaxy. Metal modulated epitaxy was utilized to prevent formation of metal droplets during the growth. Streaky patterns, seen in reflection high-energy electron diffraction, indicate two-dimensional growth throughout the device. Postdeposition characterization using scanning electron microscopy also showed smooth surfaces, while high-resolution x-ray diffraction and high-resolution transmission electron microscopy confirm the epitaxial nature of the overall quantum well structure.     

Evolution of the deformation state and composition as a result of changes in the number of quantum wells in multilayered InGaN/GaN structures

The methods of high-resolution X-ray diffraction have been used to study the multilayered structures in an In _x Ga_{1 − x} N/GaN system grown by the method of metal-organic chemical-vapor deposition. A correlation between the strain state (relaxation) of the system, the indium content within quantum wells, the ratio of the barrier/well thicknesses, and the number of quantum wells in the active superlattice is established. It is shown that partial relaxation is observed even in a structure with one quantum well. The results we obtained indicate that the relaxation processes are bound to appreciably affect the optical characteristics of devices.     

Critical layer thickness of strained-layer InGaAs/GaAs multiple quantum wells determined by double-crystal x-ray diffraction

Double-crystal x-ray diffraction (DCXD) is shown to reveal the onset of relaxation in strained-layer InGaAs/GaAs multiple quantum well (MQW) structures. The MQW structures contain 10 nm thick In_0.16Ga_0.84As quantum wells and 55 nm thick GaAs barrier layers. As the number of periods in the structure was increased from to 3 to 15, the x-ray rocking curves were characterized by increasing distortion of superlattice interference fringes, broadening of superlattice peaks, and reduction in peak intensity. The x-ray diffraction data are correlated with an asymmetric crosshatched surface pattern as observed under Nomarski contrast microscopy. By using DCXD and Nomarski microscopy, the onset of strain relaxation in InGaAs/GaAs MQW structures was established for samples with various GaAs barrier layer thicknesses. For MQW structures in which the thickness of the barrier layers is the same or greater than that of the strained quantum wells, the critical layer thickness can be calculated according to the Matthews and Blakeslee force-balance model with dislocation formation by the single-kink mechanism.     

Optical and Structural Properties of In<Subscript>0.08</Subscript>GaN/In<Subscript>0.02</Subscript>GaN Multiple Quantum Wells Grown at Different Temperatures and with Different Indium Supplies

The optical and structural properties of In_0.08Ga_0.92N/In_0.02Ga_0.98N multiple quantum wells (MQWs) grown at different temperatures and with different supplies of indium were analyzed by atomic force microscopy and spectrally resolved cathodoluminescence (CL). By comparing the contrasts of monochromatic CL images with high-resolution secondary-electron images of the sample surface, it is shown that almost all contrasts of the CL images can be explained by lateral inhomogeneities of both the thickness and the InN mole fraction of the InGaN layers. Dark contrasts in the CL images solely related to dislocations were not observed, indicating very weak nonradiative recombination correlated with threading dislocations in the InGaN quantum wells. The lateral inhomogeneities of layer thickness and indium incorporation depend strongly on the growth conditions.     

Near ultraviolet optically pumped vertical cavity laser

Optically pumped near ultraviolet vertical cavity laser operation (VCSEL) has been obtained under quasi-continuous wave conditions at room temperature near 383 nm from shallow InGaN/GaN multiple quantum wells (MQWs). Low loss optical resonators were fabricated by using in-situ grown (Al,Ga)N distributed Bragg reflectors that featured strain engineering design for high optical morphology, in combination with low-loss dielectric multilayer mirrors     

Structural and optical investigation of InGaN/GaN multiple quantum well structures with various indium compositions

We have studied the influence of indium (In) composition on the structural and optical properties of In_x Ga_1−xN/GaN multiple quantum wells (MQWs) with In compositions of more than 25% by means of high-resolution x-ray diffraction (HRXRD), photoluminescence (PL), and transmission electron microscopy (TEM). With increasing the In composition, structural quality deterioration is observed from the broadening of the full width athalf maximum of the HRXRD superlattice peak, the broad multiple emission peaks oflow temperature PL, and the increase of defect density in GaN capping layers and InGaN/GaN MQWs. V-defects, dislocations, and two types of tetragonal shape defects are observed within the MQW with 33% In composition by high resolution TEM. In addition, we found that V-defects result in different growth rates of the GaN barriers according to the degree of the bending of InGaN well layers, which changes the period thickness of the superlattice and might be the source of the multiple emission peaks observed in the In_xGa_1−xN/GaN MQWs with high in compositions.     

Enhanced Emission of (In,Ga) Nitride Nanowires Embedded with Self‐Assembled Quantum Dots

We report the structure and emission properties of ternary (In,Ga)N nanowires (NWs) embedded with self‐assembled quantum dots (SAQDs). InGaN NWs are fabricated by the reaction of In, Ga and NH₃ via a vapor–liquid–solid (VLS) mechanism, using Au as the catalyst. By simply varying the growth temperature, In‐rich or Ga‐rich ternary NWs have been produced. X‐ray diffraction, Raman studies and transmission electron microscopy reveal a phase‐separated microstructure wherein the isovalent heteroatoms are self‐aggregated, forming SAQDs embedded in NWs. The SAQDs are observed to dominate the emission behavior of both In‐rich and Ga‐rich NWs. Temperature‐dependent photoluminescence (PL) measurements indicate relaxation of excited electrons from the matrix of the Ga‐rich NWs to their embedded SAQDs. A multi‐level band schema is proposed for the case of In‐rich NWs, which showed an anomalous enhancement in the PL peak intensity with increasing temperature accompanies with red shift in its peak position.     

Active region based on graded-gap InGaN/GaN superlattices for high-power 440- to 470-nm light-emitting diodes

The structural and optical properties of light-emitting diode structures with an active region based on ultrathin InGaN quantum wells limited by short-period InGaN/GaN superlattices from both sides have been investigated. The dependences of the external quantum efficiency on the active region design are analyzed. It is shown that the use of InGaN/GaN structures as limiting graded-gap short-period superlattices may significantly increase the quantum efficiency.     

Growth and properties of HgSe:Fe quantum wells and superlattices

Single quantum wells (SQW) and superlattices of mercury-iron selenide (HgSe:Fe) have been grown by molecular beam epitaxy (MBE) and characterized by in-situ high-energy electron diffraction (RHEED) and magneto spectroscopy investigations. The influence of the structural parameters of the ZnTe buffer layer on the properties of the HgSe:Fe microstructures has been investigated by RHEED-mosaic structure analysis. The onset of strain relaxation at the critical thickness has been determined by intensity-profile analysis of different reflexes in the RHEED pattern. These results are compared with high resolution X-ray measurements (HRXRD) of the lattice relaxation of the HgSe layer. Different types of HgSe:Fe/HgSe quantum well structures and superlattices have been characterized by magneto transport investigations. The clear resolved plateau structure of the Hall voltage (Quantum Hall Effect) indicates the existence of a Q2D electron gas in the layers which is also corroborated by the cosine dependence of the peak shift of the Shubnikov de Haas (SdH)-oscillations in tilted magnetic fields.     

Strain induced lateral ordering in Ga0.22In0.78As/Ga0.22In0.78P short period superlattices on (001) InP

The application of the strain induced lateral ordering process to the strain-compensated (Ga_0.22In_0.78As)_m (Ga_0.22In_0.78P)_m short period superlattices is investigated. The superlattices have been grown at low temperatures by solid source molecular beam epitaxy (MBE) on (001) InP. These superlattices have been used in multiquantum well heterostructures using InP as barriers. The anisotropic polarization of photoluminescence shows the existence of lateral modulation. Dark-field images using the 220 reflection gives modulated contrast in the superlattice layers. High-resolution transmission electron microscopy shows local variations of the interplanar spacing of the (200) planes as well as the angles they form with the (002) planes.     

100‐period, 1.23‐eV bandgap InGaAs/GaAsP quantum wells for high‐efficiency GaAs solar cells: toward current‐matched Ge‐based tandem cells

Major challenges for InGaAs/GaAsP multiple quantum well (MQW) solar cells include both the difficulty in designing suitable structures and, because of the strain‐balancing requirement, growing high‐quality crystals. The present paper proposes a comprehensive design principle for MQWs that overcomes the trade‐off between light absorption and carrier transport that is based, in particular, on a systematical investigation of GaAsP barrier effects on carrier dynamics that occur for various barrier widths and heights. The fundamental strategies related to structure optimization are as follows: (i) acknowledging that InGaAs wells should be thinner and deeper for a given bandgap to achieve both a higher absorption coefficient for 1e‐1hh transitions and a lower compressive strain accumulation; (ii) understanding that GaAs interlayers with thicknesses of just a few nanometers effectively extend the absorption edge without additional compressive strain and suppress lattice relaxation during growth; and (iii) understanding that GaAsP barriers should be thinner than 3 nm to facilitate tunneling transport and that their phosphorus content should be minimized while avoiding detrimental lattice relaxation. After structural optimization of 1.23‐eV bandgap quantum wells, a cell with 100‐period In_0.30GaAs(3.5 nm)/GaAs(2.7 nm)/GaAsP_0.40(3.0 nm) MQWs exhibited significantly improved performance, showing 16.2% AM 1.5 efficiency without an anti‐reflection coating, and a 70% internal quantum efficiency beyond the GaAs band edge. When compared with the GaAs control cell, the optimized cell showed an absolute enhancement in AM 1.5 efficiency, and 1.22 times higher efficiency with 38% current enhancement with an AM 1.5 cut‐off using a 665‐nm long‐pass filter, thus indicating the strong potential of MQW cells in Ge‐based 3‐J tandem devices. Copyright © 2013 John Wiley & Sons, Ltd.     

Microstructural Aspects of Nucleation and Growth of (In,Ga)As-GaAs(001) Islands with Low Indium Content

Molecular beam epitaxy growth of multilayer In _x Ga_1-x As/GaAs(001) structures with low indium content (x = 0.20–0.35) was studied by X-ray diffraction and photoluminescence in order to understand the initial stage of strain-driven island formation. The structural properties of these superlattices were investigated using reciprocal space maps, which were obtained around the symmetric 004 and asymmetric 113 and 224 Bragg diffraction, and ω/2θ scans with a high-resolution diffractometer in the triple axis configuration. Using the information obtained from the reciprocal space maps, the 004 ω/2θ scans were simulated by dynamical diffraction theory and the in-plane strain in the dot lattice was determined. We determined the degree of vertical correlation for the dot position (“stacking”) and lateral composition modulation period (LCM) (lateral ordering of the dots). It is shown that initial stage formation of nanoislands is accompanied by LCM only for [110] direction in the plane with␣a period of about 50 to 60 nm, which is responsible for the formation of a quantum wire like structure. The role of In _x Ga_1-x As thickness and lateral composition modulation in the formation of quantum dots in strained In _x Ga_1-x As/GaAs structures is discussed.     

The effect of temperature on the growth and properties of green light-emitting In0.5Ga0.5N films prepared by reactive sputtering with single cermet target

We have grown In_0.5Ga_0.5N films on SiO₂/Si (100) substrate at 100–400 °C for 90 min by rf reactive sputtering with single cermet target. The target was made by hot pressing the mixture of metallic indium, gallium and ceramic gallium nitride powder. X-ray diffraction (XRD) measurements indicated that In_0.5Ga_0.5N films had wurtzite structure and showed the preferential (1 0 -1 0) diffraction. Both SEM and AFM showed that In_0.5Ga_0.5N films were smooth and had small roughness of 0.6 nm. Optical properties were measured by photoluminescence (PL) spectra from room temperature to low temperature of 20 K. The 2.28 eV green emission was achieved at room temperature for all our InGaN films. The electrical properties of In_0.5Ga_0.5N films on a SiO₂/Si (100) substrate were measured by the Hall measurement at room temperature. InGaN films showed the electron concentration of 1.51×10²⁰–1.90×10²⁰ cm⁻³ and mobility of 5.94–10.5 cm² V⁻¹ s⁻¹. Alloying of InN and GaN was confirmed for the sputtered InGaN.     

Study of dual-blue light-emitting diodes with asymmetric AlGaN graded barriers

A dual-blue light-emitting diode （LED） with asymmetric A1GaN composition-graded barriers but without an AlGaN electron blocking layer （EBL） is analyzed numerically. Its spectral stability and efficiency droop are improved compared with those of the conventional InGaN/GaN quantum well （QW） dual-blue LEDs based on stacking structure of two In0.18Ga0.szN/GaN QWs and two In0.12Ga0.88N/GaN QWs on the same sapphire substrate. The improvement can be attributed to the markedly enhanced injection of holes into the dual-blue active regions and effective reduction of leakage current.     

（英）组分过冲对InP基InAlAs递变缓冲层的影响

通过在InP基InxAl1-xAs 递变缓冲层上生长In0.78Ga0.22As/In0.78Al0.22As量子阱和In0.84Ga0.16As探测器结构,研究了缓冲层中组分过冲对材料特性的影响。原子力显微镜结果表明,在InAlAs缓冲层中采用组分过冲可以使量子阱及探测器样品表面粗糙度都得到降低。对于相对较薄的量子阱结构,X射线衍射倒易空间扫描图和光致发光谱的测量表明,使用组分过冲可以增加弛豫度、减小剩余应力并改善光学性质。而对于较厚的探测器结构,X射线衍射和光致发光谱测试发现使用组分过冲后的材料性质没有明显的变化。量子阱和探测器结构的这些不同特性需要在器件设计应用中加以考虑。     

Optical and structural properties of MOVPE grown GaxIn1−xAs/InP strained multiple quantum well atructures

This paper presents a study of the structural and optical properties of strained GaInAs/ InP multiple quantum well (MQW) structures fabricated by LP-MOVPE. The composition of the Ga _x In_1−x As films ranged fromx = 0.17 tox = 1.0 and was determined by sputtered neutral mass spectrometry (SNMS) on thick layers. The structures of the MQW samples with well widths from 1.5 to 5 nm were investigated by high resolution x-ray diffraction (HR-XRD). Simulations of the diffraction patterns showed that transition layers of approximately 2 monolayer (ML) thickness with high lattice mismatch exist at the interfaces. Photoluminescence (PL) measurements indicate well widths of a multiple of a monolayer with local variations of one monolayer. The PL peak energies vary smoothly with the Ga concentration. These results were confirmed by optical absorption measurements.     

The form of the profile of heterointerfaces in (311)Ga GaAs/AlAs structures

The steady-state photoluminescence and kinetics of photoluminescence of the (100)-oriented and (311)Ga-oriented type II GaAs/AlAs superlattices are studied under the effect of the electric field of the surface acoustic wave. It is found that, in the (100)-oriented structures, the drop of intensity of steady-state photoluminescence and acceleration of photoluminescence kinetics are independent of the direction of the electric field of the surface acoustic wave with respect to crystallographic directions, while in the (311)Ga-oriented structures these effects are anisotropic. It is shown that all variations in the steady-state photoluminescence and in kinetics of photoluminescence of (100)-oriented and (311)Ga-oriented structures under the effect of the electric field of the acoustic wave are associated with transfer and capture by the nonradiative recombination centers of nonequilibrium charge carriers, which are initially localized in wide quantum wells formed by fluctuations of the thickness of the layers of the structures. From the obtained experimental data, the parameters of the profile of heterointerfaces of the (311)Ga GaAs/AlAs superlattices are determined. It is established that the lateral sizes of microgrooves in the [011] direction on the direct and inverse heterointerfaces of the (311)Ga superlattices exceed 3.2 nm, while the modulation of the thickness of the AlAs layers is from 0.8 to 1.2 nm.     

Parametric Analysis of Spin-Polarized VCSELs

Calculations of spin-injected vertical-cavity surface-emitting lasers (VCSELs) are presented using an efficient algorithm for solution of the steady-state rate equations in the spin-flip model. The effects of spin relaxation, birefringence, electron and photon lifetimes, linewidth factor, and the magnitude and ellipticity of the pumping are investigated. After a review of published values for spin relaxation rates in semiconductors of interest for VCSELs, the dependence of spin relaxation times in GaInNAs quantum wells on N content and well width is calculated for the Elliot–Yafet process. The results, which show good agreement with experiment, are used in simulations of spin-injected GaInNAs VCSELs to determine the dependence of polarization control on well width.      

Band structure,magneto-transport,and magneto-optical properties of lnAs-Ga1-xlnxSb superlattices

We have theoretically and experimentally investigated the electronic properties of InAs-Ga_1-xIn_xSb superlattices. It is found that a strong repulsion between the El and HI bands in superlattices with thin Ga_1-xIn_xSb layers leads to dispersion relations that closely resemble those in HgTe-CdTe superlattices. Temperature-dependent magneto-transport and magneto-optical measurements on samples with a range of InAs layer thicknesses confirm several of the theoretically predicted consequences, e.g., the coexistence of two electron species in semimetallic superlattices and a very light electron cyctron mass in narrow-gap semiconducting samples. The electron mobility is found to be dominated by interface roughness scattering under nearly all conditions of interest. Implications for this system as an infrared detector material are discussed.