Gap Layer Research Articles

Thin film GaP for solar cell application

A new approach to the silicon based heterostructures technology consisting of the growth of III-V compounds (GaP) on a silicon substrate by low-temperature plasma enhanced atomic layer deposition (PE-ALD) is proposed. The basic idea of the method is to use a time modulation of the growth process, i.e. time separated stages of atoms or precursors transport to the growing surface, migration over the surface, and crystal lattice relaxation for each monolayer. The GaP layers were grown on Si substrates by PE-ALD at 350°C with phosphine (PH3) and trimethylgallium (TMG) as sources of III and V atoms. Scanning and transmission electron microscopy demonstrate that the grown GaP films have homogeneous amorphous structure, smooth surface and a sharp GaP/Si interface. The GaP/Si heterostructures obtained by PE-ALD compare favourably to that conventionally grown by molecular beam epitaxy (MBE). Indeed, spectroscopic ellipsometry measurements indicate similar interband optical absorption while photoluminescence measurements indicate higher charge carrier effective lifetime. The better passivation properties of GaP layers grown by PE-ALD demonstrate a potential of this technology for new silicon based photovoltaic heterostructure

GaP/Si anisotype heterojunction solar cells

Multijunction solar cells grown on Si substrates using III-V compounds are very promising due to potentially high efficiency (over 40%). In that case homojunction in silicon can be replaced with GaP heterojunctions which can be more effective in terms of lowering surface recombination. Silicon substrates of both n- and p-type doping were used to form GaP/Si anisotype heterostructures. It was demonstrated that for p-GaP/n-Si structures charge carrier transport is mainly blocked due to large value of valence band offset at the heterointerface. For n-GaP/p-Si solar cells optimal thickness and doping level were calculated using AFORS-HET software. The influence of interface states density and GaP layer parameters was investigated.

Multi Band Gap Cu(In,Ga)(S,Se)<sub>2</sub> Thin Films Deposited by Spray Pyrolysis for High Performance Solar Cell Devices

The performance of copper indium gallium disulfoselenide (CIGSSe) solar cells strongly depends on the band bap of absorbing layer of CIGSSe. The device performance can be improved by fabricating multi band gap layer of CIGSSe. However, the fabrication of multi band gap CIGSSe using non-vacuum techniques is challenging. In this study, we fabricated solar cell devices which consisted of multi band gap Cu (In,Ga)(S,Se)2 thin films. The CIGS thin films were prepared by the spray-pyrolysis of aqueous precursor solutions of gallium (gallium chloride; GaCl3), copper (indium chloride; CuCl2), indium (indium chloride; InCl3), and Sulphur (thiourea; (SC(NH2)2) sources on Mo-coated glass substrate. The as-sprayed thin films were then selenized at 500 °C for 10 minutes.After selenization, CIGS films were transformed to Cu (In,Ga)(S,Se)2 (CIGSSe). The CIGS films with different composition were deposited again on top of selenized CIGSSe films and selenization process was repeated, hence multi band gap CIGSSe films were fabricated. The Chemical bath deposition (CBD) process was used to deposit cadmium sulphide (CdS) buffer layer. The solar cell fabricated with the device configuration of glass/Mo/CIGSSe/CdS/i-ZnO/AZO showed a power conversion efficiency of 6.51%.

Gap-plasmon based broadband absorbers for enhanced hot-electron and photocurrent generation.

Plasmonic hot-electron generation has recently come into focus as a new scheme for solar energy conversion. So far, however, due to the relatively narrow bandwidth of the surface plasmon resonances and the insufficient resonant light absorption, most of plasmonic photocatalysts show narrow-band spectral responsivities and small solar energy conversion efficiencies. Here we experimentally demonstrate that a three-layered nanostructure, consisting of a monolayer gold-nanoparticles and a gold film separated by a TiO2 gap layer (Au-NPs/TiO2/Au-film), is capable of near-completely absorbing light within the whole visible region. We show that the Au-NPs/TiO2/Au-film device can take advantage of such strong and broadband light absorption to enhance the generation of hot electrons and thus the photocurrent under visible irradiation. As compared to conventional plasmonic photocatalysts such as Au-NPs/TiO2 nanostructures, a 5-fold-enhanced incident photon-to-current conversion efficiency is achieved within the entire wavelength range 450–850 nm in the Au-NPs/TiO2/Au-film device. Simulations show good agreements with the experimental results, demonstrating that only the plasmon-induced losses contribute to the enhanced photocurrent generation of the Au-NPs/TiO2/Au-film device.

Thick orientation-patterned growth of GaP on wafer-fused GaAs templates by hydride vapor phase epitaxy for frequency conversion

Quasi-phase-matched (QPM) GaP layers up to 300 μm thick have been produced by low-pressure hydride vapor phase epitaxy (LP-HVPE) overgrowth on orientation-patterned GaAs (OPGaAs) templates fabricated using a wafer-fusion bonding technique. The growth on the OPGaAs templates resulted in up to 200 μm thick vertically propagating domains, with a total GaP thickness of 300 μm. The successful thick growth on OPGaAs templates is the first step towards solving the material problems associated with unreliable material quality of commercially available GaP wafers and making the whole process of designing QPM frequency conversion devices molecular beam epitaxy free and more cost-effective.

Influence of surface passivation on electric properties of individual GaAs nanowires studied by current–voltage AFM measurements

Current–voltage (I–V) characteristics of vertical p-GaAs nanowires (NWs) covered by different surface passivation materials were experimentally measured by conductive atomic force microscopy (C-AFM). The obtained I–V curves for individual NWs with a diameter of 100 nm covered with AlGaAs, GaN, GaP or InP shell layers were compared to analyse the influence of surface passivation on the density of surface states and choose the most beneficial passivating material for technological applications. We have found the absence of a Schottky barrier between the golden catalytic cap on the top of a NW and the nanowire situated below and covered with an ultrathin GaP passivating layer. It was suggested that passivating material can arrange the heterostructure configuration with the GaAs NW near the Au cap. The latter mechanism was proposed to explain a strong energy barrier found in nanowires covered with InP passivation. AlGaAs passivation affected the forward threshold voltage of nanowires for NWs, which was measured simultaneously with the resistivity of each individual vertical structure from an array by means of AFM in the regime of measuring the I–V curves and onefold calculations. We made an attempt to develop the methodology of measurement and characterization of electric properties of passivated NWs.

Complete photonic bandgaps in self-assembled square bilayer structures

The photonic bandgap properties of colloidal photonic crystals assembled under confinement are simulated. Confinement self-assembly provides a rich diversity of phases for anisotropic particles as a function of confinement height due to incommensurate gap height and layer number. In particular, we investigate the square bilayer phase of cut spheres, which displays high Brillouin zone isotropy and a complex basis. These features are known to open complete photonic bandgaps. Here, a wide range of structures with complete bandgaps is obtained as a function of the dielectric contrast, dielectric filling fraction, and slab height. A large, complete bandgap (gap-to-midgap ratio, 10.2%) is found between the tenth and eleventh bands in the direct structure at a dielectric contrast of 12. In the inverted structure, two smaller bandgaps between the fourth and fifth and the eighth and ninth bands are found. The electric displacement field density shows that the bandgaps arise due to shifts in the mode field concentration to complementary high dielectric regions. Such bandgaps are often associated with Mie scattering; however, the particle basis is tangent, promoting a strong Bragg scattering component. An equifrequency contour analysis is also performed to demonstrate the potential of the square bilayer slab for negative refraction. Right-handed negative refraction is predicted in the second band for the direct structure.

Expansion of the spectral sensitivity range of the silicon photocells by growing a solid (Si2)1–x (GaP) x (0 ≤ x ≤ 1) solution

The article concerns the growth of a hetero-epitaxial GaP layer on a silicon substrate via a buffer layer containing a continuous solid substitutional solution (Si2)1–x (GaP) x (0 ≤ x ≤ 1) from the liquid phase. Epitaxial films grown under 950–830°С have n-type conductivity and specific resistance of ~0.01 Ohm•cm. The thickness of epitaxial films is 10–15 μm. The spectral sensitivity of the pSi-n(Si2)1–x (GaP) x (0 ≤ x ≤ 1) heterostructure, which allows expanding the region of the spectral sensitivity of silicon photoreceivers and photocells, is studied.

Antiphase domain tailoring for combination of modal and 4¯ -quasi-phase matching in gallium phosphide microdisks.

We propose a novel phase-matching scheme in GaP whispering-gallery-mode microdisks grown on Si substrate combining modal and 4¯ -quasi-phase-matching for second-harmonic-generation. The technique consists in unlocking parity-forbidden processes by tailoring the antiphase domain distribution in the GaP layer. Our proposal can be used to overcome the limitations of form birefringence phase-matching and 4¯ -quasi-phase-matching using high order whispering-gallery-modes. The high frequency conversion efficiency of this new scheme demonstrates the competitiveness of nonlinear photonic devices monolithically integrated on silicon.

Defect characterization of proton irradiated GaAs pn-junction diodes with layers of InAs quantum dots

In order to expand the technology of III-V semiconductor devices with quantum structures to both terrestrial and space use, radiation induced defects as well as native defects generated in the quantum structures should be clarified. Electrically active defects in GaAs p+n diodes with embedded ten layers of InAs quantum dots (QDs) are investigated using Deep Level Transient Fourier Spectroscopy. Both majority carrier (electron) and minority carrier (hole) traps are characterized. In the devices of this study, GaP layers are embedded in between the QD layers to offset the compressive stress introduced during growth of InAs QDs. Devices are irradiated with high energy protons for three different fluences at room temperature in order to characterize radiation induced defects. Seven majority electron traps and one minority hole trap are found after proton irradiation. It is shown that four electron traps induced by proton irradiation increase in proportion to the fluence, whereas the EL2 trap, which appears before irradiation, is not affected by irradiation. These defects correspond to electron traps previously identified in GaAs. In addition, a 0.53 eV electron trap and a 0.14 eV hole trap are found in the QD layers before proton irradiation. It is shown that these native traps are also unaffected by irradiation. The nature of the 0.14 eV hole trap is thought to be Ga-vacancies in the GaP strain balancing layers.

Development of a new thin film composition for SnS solar cell

The work is devoted to the development of a new thin film composition for SnS solar cell based on semiconductor heterojunction n-Zn(O,S)/p-SnS by the combination of two low-cost liquid methods, namely electrodeposition and Successive Ionic Layer Adsorption and Reaction (SILAR). Structure and optical properties of the obtained layers have been investigated. The p-SnS absorber is fabricated by combining the two methods. The creation of the main SnS film was carried out by sulfurization of the electrodeposited Sn thin film. The elimination of the shunt leakage was made by SILAR deposition of nanocrystalline SnS into pores of the main SnS. All SnS films are single-phase and consisted of tin monosulfide with orthorhombic structure of Herzenbergite. Wide band gap and transparent buffer layer n-Zn(O,S) is created by air annealing of ZnS thin film prepared by SILAR. Series and shunt resistances as well as a height of the rectifying barrier of this n-Zn(O,S)/p-SnS heterostructure were investigated due dark current–voltage characteristics. The diode and electronic parameters of this new thin film composition is superior ones for the obtained by using tin sulfide absorber produced only by SILAR.

Cross-sectional scanning tunneling microscopy of antiphase boundaries in epitaxially grown GaP layers on Si(001)

In a fundamental cross-sectional scanning tunneling microscopy investigation on epitaxially grown GaP layers on a Si(001) substrate, differently oriented antiphase boundaries are studied. They can be identified by a specific contrast and by surface step edges starting/ending at the position of an antiphase boundary. Moreover, a change in the atomic position of P and Ga atoms along the direction of growth is observed in agreement with the structure model of antiphase boundaries in the GaP lattice. This investigation opens the perspective to reveal the orientation and position of the antiphase boundaries at the atomic scale due to the excellent surface sensitivity of this method.

Plasmonic modes in a conductor-corrugated gap-dielectric system at telecommunication wavelength

We propose a conductor-corrugated gap-dielectric structure to realize broadband plasmonic modes at telecommunication wavelength. The structure consists of a corrugated low-index dielectric gap layer sandwiched by high-index dielectric and conductor layers. The electric field of the plasmonic modes is primarily concentrated in the corrugated gap, which not only reduces the metal absorption resulting in long propagation length but also causes good field confinement. It is proved that periodic modulation introduced into the gap can significantly increase the cutoff gap thickness of the structure. The effective medium theory and numerical simulation demonstrate that the cutoff gap thickness can be further increased by decreasing the filling factor of the low-index dielectric in the corrugated gap. Meanwhile, the scattering effects induced by the periodic modulation are also investigated. The results not only provide a new understanding of the surface plasmonic modes but also benefit the designing of compact devices.

Narrow gap laser welding by multilayer hot wire addition

Welding of thick section butt joints experiences limitations for different techniques. One option is to fill a narrow gap layer by layer with laser melted wire, a laser metal deposition technique where the complexity of a keyhole is avoided. The presented results show that wire addition can enable relatively thick layers. In particular, when electrically preheating the wire the process becomes more energy-efficient and favorable wetting conditions might be achieved. Since the wire was preheated by an electric current conducted through the wire to the workpiece, high speed imaging has shown that the wire tip can occasionally ignite small electric arcs. The wire deposited in the narrow gap also shows a fluctuating but self-stabilizing movement of the tip. Imperfections that have to be avoided are hot cracks, cavities, lack of fusion, and an irregular final weld surface topology. The technique shows high potential.

Микрорезонатор с РБО зеркалами для эффективного терагерцевого излучения из оптически накачиваемого GaP слоя: численное исследование методом единого выражения

Микрорезонатор с РБО зеркалами для эффективного терагерцевого излучения из оптически накачиваемого GaP слоя: численное исследование методом единого выражения

Local field enhancement on demand based on hybrid plasmonic-dielectric directional coupler.

The concept of local field enhancement using conductor-gap-dielectric-substrate (CGDS) waveguide structure is proposed. The dispersion equation is derived analytically and solved numerically. The solution of the dispersion equation reveals the anti-crossing behavior of coupled modes. the optimal gap layer thickness and the coupling length of the guided modes are obtained. The mechanism of the CGDS works as follows: Light waves are guided by conventional low-loss dielectric waveguides and, upon demand, they are transformed into highly confined plasmonic modes with strong local field enhancement, and get transformed back into low-loss dielectric modes. As an example, in a representative CGDS structure, the optimal plasmonic gap size is 17 nm, the local light intensity is found to be more than one order of magnitude stronger than the intensity of the dielectric mode at the film surface. The coupling length is only 2.1 μm at a wavelength of 632.8 nm. Such a local field confinement on demand is expected to facilitate efficient light-matter interaction in integrated photonic devices while minimizing losses typical for plasmonic structures.

Theoretical analysis and experiment of subwavelength structure-integrated red AlGaInP light-emitting diodes for uniform field distribution and enhanced light extraction efficiency

We report theoretical and experimental analysis of antireflective subwavelength structures (SWSs) on GaP substrates to enhance the light output with a uniform light distribution of AlGaInP-based red light-emitting diodes (LEDs). Scaling issues on the optical performance are analyzed by the calculation results from finite-difference time-domain (FDTD) and rigorous coupled-wave analysis (RCWA) methods. The optical simulation reveals that SWSs with a period of 100–150 nm achieve highest optical output power, while maintaining a uniform light field distribution, in the subwavelength regime. To validate our theoretical results, disordered SWSs with a tapered shape were fabricated on a GaP layer of AlGaInP red LEDs by lithography-free dry etching of Ag nanoparticles. The SWS-integrated LED shows a uniform light output distribution with an improved light output power compared with the conventional LED.

InAs/GaAs quantum dot lasers with GaP strain-compensation layers grown by molecular beam epitaxy

The strain-compensation (SC) technique to reduce the accumulation of strain is a promising approach to increase the design flexibility as well as the performance of quantum dot (QD) lasers. Here we have studied the application of tensile-strained ultra-thin GaP layers into multiple stacked InAs/GaAs QD grown by MBE. XRD analysis shows the controllability of the average strain in multiple-stacked QD active layer, revealing a reduction in accumulated strain. Fabricated QD laser diodes including thinner QD active layers realized by SC technology show a narrower vertical far-field angle and an increased small signal modulation bandwidth without loss of gain. Cross-sectional SEM image of SC-QD LD active region

Dopant in Near-Surface Semiconductor Layers of Metal–Insulator–Semiconductor Structures Based on Graded-Gap p-Hg0.78Cd0.22Te Grown by Molecular-Beam Epitaxy

Peculiarities in determining the dopant concentration and dopant distribution profile in the near-surface layer of a semiconductor are investigated by measuring the admittance of metal–insulator–semiconductor structures (MIS structures) based on p-Hg0.78Cd0.22Te grown by molecular beam epitaxy. The dopant concentrations in the near-surface layer of the semiconductor are determined by measuring the admittance of MIS structures in the frequency range of 50 kHz to 1 MHz. It is shown that in this frequency range, the capacitance–voltage characteristics of MIS structures based on p-Hg0.78Cd0.22Te with a near-surface graded gap layer demonstrate a high-frequency behavior with respect to the recharge time of surface states located near the Fermi level for an intrinsic semiconductor. The formation time of the inversion layer is decreased by less than two times, if a near-surface graded-gap layer is created. The dopant distribution profile in the near-surface layer of the semiconductor is found, and it is shown that for structures based on p-Hg0.78Cd0.22Te with a near-surface graded-gap layer, the dopant concentration has a minimum near the interface with the insulator. For MIS structure based on n-Hg0.78Cd0.22Te, the dopant concentration is more uniformly distributed in the near-surface layer of the semiconductor.

Coexistence of Scattering Enhancement and Suppression by Plasmonic Cavity Modes in Loaded Dimer Gap-Antennas.

Plasmonic nanoantenna is of promising applications in optical sensing and detection, enhancement of optical nonlinear effect, surface optical spectroscopy, photoemission, etc. Here we show that in a carefully-designed dimer gap-antenna made by two metallic nanorods, the longitudinal plasmon antenna mode (AM) of bonding dipoles can compete with the transverse plasmonic cavity modes (CMs), yielding dramatically enhanced or suppressed scattering efficiency, depending on the CMs symmetry characteristics. More specifically, it is demonstrated that an appropriately loaded gap layer enables substantial excitation of toroidal moment and its strong interaction with the AM dipole moment, resulting in Fano- or electromagnetically induced transparency (EIT)-like profile in the scattering spectrum. However, for CMs with nonzero azimuthal number, the spectrum features a cumulative signature of the respective AM and CM resonances. We supply both detailed near-field and far-field analysis, showing that the modal overlap and phase relationship between the fundamental moments of different order play a crucial role. Finally, we show that the resonance bands of the AM and CMs can be tuned by adjusting the geometry parameters and the permittivity of the load. Our results may be useful in plasmonic cloaking, spin-polarized directional light emission, ultra-sensitive optical sensing, and plasmon-mediated photoluminescence.