AlInP Layers Research Articles

Optimisation of four-terminal GaAs//Si tandem solar cells using numerical simulation

III–V//crystalline-silicon (c-Si) multi-junction solar cells exhibit great potential in the development of high-efficiency and cost-effective photovoltaic devices. Numerical simulation tools are fast and convenient approaches to optimise cell operation, compared to optimisation techniques involving costly extending experiments. In this study, optimisation of a four-terminal GaAs//c-Si tandem structure was modelled through automat for simulation of heterostructures (AFORS-HET) simulation. Various window layers, including AlGaAs, InGaP, AlInP, and AlGaInP layers, and absorber thicknesses were examined and compared to obtain the optimal light collection for the GaAs top cell. In addition, the absorber thicknesses and defect densities of the c-Si bottom cells were used to obtain optimal light absorption. The tandem operation was modelled by illuminating the top and bottom cells separately, with different spectral irradiance sources. The results indicated that the tandem structure with a 1.5 μm thick GaAs top and an AlGaAs window layer, in combination with 200-μm thick c-Si bottom cells, showed an optimal efficiency of 32.57%. These simulation results can be used as baselines for strictly experimental processes in the future. The cell operational mechanisms under these variations are detailed herein.

Nonselective etching of As and P based III‒V solar cell heterostructures with aqueous solutions of HIO3 and HCl

Etching characteristics of lattice-matched GaInP/GaAs/GaInNAsSb heterostructures by aqueous solutions of iodic acid (HIO3) and hydrochloric acid (HCl) is reported. The study aims at optimization of mesa fabrication process involved in the development of III‒V multijunction solar cells. The effects of temperature, agitation, etchant composition, and illumination on the etching selectivity are investigated. Varying the etchant composition and agitation rate at room temperature results in various mesa sidewall morphologies, ranging from rough surfaces and significant undercut to smoother sidewall profiles with only minor undercut. Especially, the undercut emerging in the GaInNAsSb junction, as well as in the AlInP layers, was observed for several etching conditions. Increasing the etching temperature also caused an unwanted increase in the selectivity, whereas reducing the temperature below the room temperature enhanced remarkably the formation of smooth morphology. Illumination during the etching resulted in a severe undercut in the GaAs junction, due to an increased selectivity caused by photoetching. This points out the need of controlled illumination condition to avoid unwanted reactions in wet etching of such multijunction structures. The etching process resulting in the best morphology was used for electrical isolation of triple-junction GaInP/GaAs/GaInNAsSb solar cells. The good photovoltaic performance exhibited by these devices proves the suitability of the nonselective etch process in developing multijunction solar cells.

Enhancement in external quantum efficiency of AlGaInP red \u03bc-LED using chemical solution treatment process

To investigate the effects of their surface recovery and optical properties, extremely small sized (12 µm × 12 µm mesa area) red AlGaInP micro light emitting diodes (upmu LED) were fabricated using a diluted hydrofluoric acid (HF) surface etch treatment. After the chemical treatment, the external quantum efficiencies (EQEs) of upmu-LED at low and high injection current regions have been improved by 35.48% and 12.86%, respectively. The different phenomena of EQEs have a complex relationship between the suppression of non-radiative recombination originating from the etching damage of the surface and the improvement of light extraction of the sidewalls. The constant enhancement of EQE at a high injection current it is attributed to the expansion of the active region’s sidewall surface area by the selective etching of AlInP layers. The improved EQE at a low injection current is related to the minimization of the surface recombination caused by plasma damage from the surface. High-resolution transmission electron microscopy (HR-TEM) revealed physical defects on the sidewall surface, such as plasma-induced lattice disorder and impurity contamination damage, were eliminated using chemical treatment. This study suggests that chemical surface treatment using diluted HF acid can be an effective method for enhancing the upmu-LED performance.

Investigation on the materials of heavily Mg-doped AlInP layers for laser diode structure

High concentration p-type AlInP material is an important layer to achieve low resistant and large carrier confinement in short-wavelength red-light laser diodes (LDs). To investigate the Mg doping behavior and other characteristics in the red-light LDs with tensile GaInP well structure, two LDs wafers with different Mg-doped AlInP cladding layers were grown by the metalorganic chemical vapor deposition (MOCVD) method. Incorporation and activation behavior of Mg atoms in heavily-doped AlInP were analyzed by the comparisons between hole concentration and atom concentration. We demonstrate that it is possible to achieve hole concentration above 1018 cm−3 using Mg dopant in AlInP layers just by an in-situ annealing. Furthermore, a hole concentration as high as 2.92 × 1018 cm−3 was achieved after post rapid thermal annealing in the N2 atmosphere, which is the highest hole doping concentrations reported to-date for the p-AlInP material.

GSMBE growth and characterizations of AlInP/InGaAsP strain-compensated multiple-layer heterostructures

We have grown long-period AlInP/InGaAsP strain-compensated multiple-layer heterostructures (SCMLHs) and SCMLHs combined with InAsP/InGaAsP strain-compensated multiple quantum wells (SC-MQWs) by gas source molecular beam epitaxy. Etch pit density (EPD) for both structures are in magnitude of ∼10 5 cm −2. The increment of EPD with increase of period number is small, indicating low sensitivity of the dislocation density to the increase of period number of SCMLHs. High resolution X-ray diffraction and photoluminescence (PL) characterizations of the two structures demonstrate that crystal quality remains high due to strain compensation. Diffusion and segregation of indium were clearly observed in both InGaAsP and AlInP layers by secondary ion mass spectroscopy. As the thickness of epilayers increases In content increases, while Al decreases. PL for the structure of 20-pair AlInP/InGaAsP SCMLHs+InAsP/InGaAsP SC-MQWs shows strong luminescence and narrow line width. The strain-compensated technique can effectively suppress the formation of misfit dislocations in AlInP/InGaAsP SCMLHs although the thicknesses of epilayers are above the critical thickness of consisting materials, which may render its potential application in optoelectronic devices.

Molecular beam epitaxial growth and characterization of strain-compensated Al0.3In0.7P/InP/Al0.3In0.7P metamorphic–pseudomorphic high electron mobility transistors on GaAs substrates

A novel metamorphic high electron mobility transistor (HEMT) structure was grown on GaAs substrates by solid-source molecular-beam epitaxy for potential microwave power applications. The HEMT device layers were strain compensated with pseudomorphic (tensile-strained) Al0.3In0.7P donor–barrier layers and a pseudomorphic (compressive-strained) InP channel layer. Atomic force microscopy measurements of the metamorphic structure yielded a root-mean-square surface roughness of 8 Å. Transmission electron micrographs of the device layers exhibited flat interfaces with the dislocation density estimated to be less than 1×106 cm−2. Room temperature photoluminescence measurements of metamorphic AlInP layers indicated large direct band gaps up to 2.10 eV. Due to the larger conduction band discontinuity at the Al0.3In0.7P/InP heterojunction than the AlGaAs/InGaAs heterojunction in GaAs pseudomorphic HEMTs, significantly higher channel sheet densities were obtained. For Al0.3In0.7P/InP HEMTs, channel sheet densities (cm−2) exceeding 3×1012 for single-pulse-doped, and greater than 4×1012 for double-pulse-doped, structures were readily obtained. Hall measurements on a double-pulse-doped Al0.3In0.7P/InP/Al0.3In0.7P HEMT gave mobilities (cm2/V s) of 4450 at 300 K and 18 500 at 77 K, which are consistent with a high quality InP channel layer. Secondary ion mass spectroscopy depth profiles of a double-pulse-doped structure displayed sharp doping pulses and interfaces indicating that metamorphic growth was not leading to enhanced diffusion or migration. Initial and nonoptimized devices with a gate length of 0.15 μm exhibited a maximum current density of 500 mA/mm and a transconductance of 520 mS/mm, which compare favorably to mature AlGaAs/InGaAs pseudomorphic HEMTs.

Mechanism of Zn and Si diffusion from a highly doped tunnel junction for InGaP/GaAs tandem solar cells

Diffusion of impurities (Zn and Si) from a tunnel junction during epitaxial growth and the effects of impurity diffusion on InGaP/GaAs tandem cell properties have been investigated. Zn diffusion from the tunnel junction has been found to deteriorate the effect of the back-surface field layer on minority carrier reflectance in the InGaP top cell and degrade the quantum efficiency of the top cell. Furthermore, Zn diffusion has been found to be enhanced around the threading dislocations from a GaAs substrate and creates shunt paths only in the top cell region. Si diffusion, which degrades the quantum efficiency of the GaAs bottom cell, has also been observed when a different substrate with high etch pit density was used. Such anomalous diffusion of Zn has been found to be suppressed by using a double-hetero structure InGaP tunnel junction sandwiched by AlInP layers. It has been found that the Zn diffusion occurs as a layer highly doped with Si being formed nearby and Zn diffuses in the opposite direction from the Si-doped layer. The Zn diffusion is thought to be caused by group III self-diffusion which originates in the highly doped n-type layer. The direction of Zn diffusion is thought to be due to Coulombic repulsion between the substitutional Zn on the Ga site and the substitutional Si on the As site. The large energies of the formation and migration of group III vacancies in the AlInP barrier layers and InGaP tunnel junction layers are thought to suppress Zn diffusion from the tunnel junction.

Temperature dependence of photoluminescence intensity from AlGaInP/GaInP-quantum well structures

The photoluminescence (PL) intensity of AlGaInP/GaInP-quantum well structures under a continuous excitation condition was measured as a function of the crystal temperature (10–450 K). The thermal emission of carriers from the well layers to the barrier layers was investigated by generating the carriers only in the wells. The dependence of the PL intensity on the crystal temperature was analyzed by fitting a model function to the experimental results. The rapid decrease in PL intensity at temperatures higher than 200 K was found to be probably due to the emission of carriers into the barrier layers. The effective barrier heights obtained by the fitting are in good agreement with the band-energy alignment schemes of these crystals. When AlInP barrier layers were inserted directly beside the quantum wells, the emission of carriers to the quaternary layers through the AlInP layers was reduced, resulting in stronger PL intensity at higher temperatures and a larger effective barrier height.

Self-assembling InP quantum dots for red lasers

Self-assembling InP quantum dots are prepared by solid source molecular beam epitaxy. The dots have a diameter of 15 to 50 nm and a height of 5 to 15 nm depending on the nominally deposited InP layer thickness between 1.5 and 7 ML. Transmission electron microscopy measurements are presented to provide information about the structural properties. The InP quantum dots are embedded in Ga0.52In0.48P lattice matched to the GaAs (1 0 0) substrate and show a strong and narrow photoluminescence at room temperature in the visible range. Laser structures are prepared with one layer of InP quantum dots within a 160 nm thick Ga0.52In0.48P waveguide region. 0.7 μm thick AlInP layers are used as cladding layers below and above the waveguide. We observe lasing of the InP quantum dots at room temperature in optically pumped cleaved samples with a length of 500 μm.

600-nm-Range GaInP/AlInP Strained Quantum Well Lasers Grown by Gas Source Molecular Beam Epitaxy

GaInP strained single quantum well (SSQW) lasers in the 630–710 nm range grown by gas source molecular beam epitaxy (GSMBE) using a novel shutter control method were systematically investigated. A good controllability in strain amount as well as good in-plane compositional homogeneity over 4-cm-long distance of the laser wafers were confirmed. For +1.1% compressive SSQW lasers (708 nm in wavelength), a very low threshold current density (J th) of 175 A/cm2 was obtained. These facts suggest that the novel shutter control method is very effective for fabricating GaInP SSQW lasers. Furthermore, the post-annealing effect on lasing characteristics of GaInP SSQW lasers was investigated. The increased p carrier density of Be-doped AlInP layers was obtained with the remarkable reduction in J th value of 633 nm tensile SSQW lasers from 850 A/cm2 to 555 A/cm2.

600-nm wavelength range GaInP/AlInP quasi-quaternary compounds and lasers prepared by gas-source molecular-beam epitaxy

GaInP/AlInP quasi-quaternary (QQ) compounds grown by gas-source molecular-beam epitaxy were systematically investigated. Quasi-quaternary properties were obtained with GaInP/AlInP short-period superlattices when the thickness of the AlInP layers was less than 3 ML. Equivalent band-gap values of QQ compounds were defined in terms of the layer thickness ratios of the GaInP and AlInP layers. Using GaInP/AlInP quasi-quaternary compounds, 607–640-nm wavelength QQ lasers with 20-nm QQ active layers were fabricated. Threshold current densities as low as 1.5 kA/cm2 were attained for QQ lasers emitting at 635 nm without the presence of strain or quantum-well effects.

Growth of heavily Be-doped AlInP by gas source molecular beam epitaxy

Heavily Be-doped p-type AlInP layers have successfully grown on (001)GaAs by gas source molecular beam epitaxy (GSMBE) using phosphine (PH3). Net hole concentration (Nh) as high as about 3.5×1018 cm−3 is achieved for the first time. The surface morphology is found to be smooth up to a Be concentration of 3×1019 cm−3. The resistivity for Nh=3.5×1018 cm−3 is as low as 0.3 Ω cm. The improvement of the electrical activity and surface morphology may be ascribed to subhidrides of phosphorus decomposed from PH3 during GSMBE growth.

Phase Separation and Atomic Ordering in Epitaxial Layers of III-V Compound Semiconductors

ABSTRACTMIcrostructural characteristics of phase separated and ordered ternary and quaternary layers of Ill-V compound semiconductors grown by a variety of techniques are highlighted. It is argued that the fine scale contrast modulations seen in a number of layers arise from a two-dimensional, surface spinodal decomposition at the growth temperature. On the other hand, coarse contrast modulations could result from accommodation of asymmetrical strains associated with the twodimensional decomposition.Long range atomic order is observed in a variety of layers grown by different techniques and co-exists with phase separation. In InGaAs and InGaAsP layers, grown on (001) InP substrates, ordering is observed only on the two {111} planes that contain the line of intersection of the {111}in plane with the (001) InP surface. As a result of ordering, periodicity along the 〈111〉 direction is doubled. Also, the influence of growth temperatures and growth rates on ordering characteristics in GalnP and (Ga, Al) InP layers is reported.