Low-temperature Liquid-phase Epitaxy Research Articles

Surface Photovoltage Method for Photovoltaic Quality Control of GaAs-Based Solar Cells

In this paper, we demonstrate the potential of the contactless surface photovoltage (SPV) method for fast and reliable control of GaAs-based solar cells directly on epitaxial heterostructures before metallization and photolithography processes. The magnitude of the SPV corresponds to the generated photovoltage in the photoactive region, which is related to the open circuit voltage of the cell. The focus of this investigation is the potential of dilute nitride compounds grown by low-temperature liquid-phase epitaxy (LPE) for application as intermediate cells in multijunction solar cells. First, SPV spectroscopy is used to determine the photosensitivity spectral range and bandgap of the grown dilute nitride compound layers. Further, the photovoltaic quality of the grown solar cell heterostructures is evaluated by comparing the magnitude of their SPV signals with that of a reference GaAs solar cell. A drastic reduction in the measured SPV is observed for nitrogen-containing solar cell structures, which correlates with the lowering of solar cell open-circuit voltage values measured under standard test conditions. Finally, solar cell structures based on nitrogen-free GaAsSb compounds with the same long-wavelength photosensitivity limit as GaAsSbN are grown by LPE. They show one order of magnitude higher SPV signal and, therefore, have a great potential for solar cell application.

Doping of dilute nitride compounds grown by liquid phase epitaxy

The doping of dilute nitrides is an important point of the growth and processing technology for different optoelectronic devices based on these compounds. In this paper, both intentional and nonintentional doping of InGaAsN and GaAsSbN have been investigated by temperature-dependent Hall effect measurements. Dilute nitrides layers have been grown by low-temperature (Tcryst < 600 °C) liquid-phase epitaxy (LPE). The chemical elements Sn and Mg have been chosen as n-type and p-type dopants, respectively. All nominally undoped InGaAsN layers are n-type with free carrier concentration about one order of magnitude higher than layers not containing nitrogen and grown by LPE. This makes it difficult to obtain epitaxial layers with p-type conductivity with these compounds. However, high-quality n-type, p-type and nearly compensated GaAsSbN layers have been successfully grown covering a large range of carrier concentrations from 1015 to 6×1018 cm−3. The quality of the GaAsSbN layers has been evidenced by a good photoresponse with a low energy threshold extended down to 1.2 eV as obtained by surface photovoltage spectroscopy.

Low-temperature LPE growth and characterization of GaAsSb layers for photovoltaic applications

In this paper, we present an original study on metamorphic GaAsSb layers grown by liquid phase epitaxy (LPE) on GaAs substrates for photovoltaic applications. To ensure a controlled growth of the layers with reproducible composition and properties a low-temperature (below 600 °C) variant of LPE was used combined with a precise choice of the technological conditions. This allowed obtaining layers with ∼8% Sb in the As sublattice. The crystal structure, composition, surface morphology, local arrangement and chemical bonding of Sb in the obtained layers were investigated by a variety of measurement methods including X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, atomic force microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy. Hall-effect measurements were performed in the temperature range of 80–300 K to clarify the influence of defects on the transport properties. Temperature-dependent photoluminescence spectra at low excitation power were measured to evaluate the optical quality and identify localized states in the grown layers. The optical absorption properties and the transport of the photogenerated carriers in the grown samples were investigated using surface photovoltage (SPV) spectroscopy at room temperature. The SPV measurements have shown that the long-wavelength photosensitivity of the GaAsSb layers is extended down to 1.2 eV.

Calculation of the Bandgap of Dilute Nitride GaAsSbN Alloys

Dilute nitrides are semiconductor alloys, obtained from the conventional III-V compounds by incorporating a small amount of nitrogen. In this work, we focus on GaAsSbN considered as a perspective material for incorporation in multijunction solar cells. Nitrogen creates a localized level inside the conduction band continuum. The interaction of this level with the conduction band is usually described by the single band anti-crossing (BAC) model. The double BAC model of GaAsSbN considers both the N and the Sb localized levels in the conduction and the valence band, respectively. We calculate the bandgap energy of GaAsSbN employing the double BAC model for different concentrations of Sb and N. Parameters of the BAC model taken from different literature sources are used in the calculations and their influence on the final result is explored. Finally, the calculated bandgap energies are compared to experimental data of GaAsSbN layers grown on n-GaAs substrates by low-temperature liquid phase epitaxy. These data include the optical absorption edge of the material determined by surface photovoltage spectroscopy and the energy position of the photoluminescence peak at room temperature.

Single-junction solar cells based on p-i-n GaAsSbN heterostructures grown by liquid phase epitaxy

In this paper, we present single heterojunction p-i-n GaAsSbN/GaAs solar cells grown by low-temperature liquid-phase epitaxy (LPE) – this is of interest as a component of multi-junction solar cell devices. The quaternary absorber layer was characterized by low excitation power photoluminescence to give the temperature dependence of the bandgap. This conformed to the Varshni function at low temperatures to within 10 meV, indicating relatively small alloy potential fluctuations. The absorption properties and the transport of the photogenerated carriers in the heterostructures were investigated using surface photovoltage method. A power conversion efficiency of 4.15% (AM1.5, 1000 W·m−2) was measured for p-i-n GaAsSbN/GaAs solar cells, which is comparable to the efficiency of MOCVD grown devices of this type. This is promising for the first report of LPE grown GaAsSbN/GaAs solar cells since the current record efficiency for the cells based on these compounds grown by MBE stands just at 6%. The long-wavelength photosensitivity of the cells determined from external quantum efficiency and surface photovoltage measurements was shown to be extended to 1040 nm.

Liquid-Phase Quasi-Epitaxial Growth of Highly Stable, Monolithic UiO-66-NH2 MOF thin Films on Solid Substrates.

Invited for this month's cover is the group of Dr. Tawheed Hashem from the Karlsruhe Institute of Technology. The cover picture shows a successful synthesis of high quality, monolithic UiO-66-NH2 MOF thin films on diverse solid substrates via a low-temperature liquid phase epitaxy method. The achievement of continuous MOF-coatings with low defect densities and pronounced stability against high temperatures and hot water was proven. The new type of coatings clearly outperforms other reported types of MOF thin films. Read the full text of their Communication at https://doi.org/10.1002/open.201900324.

Dilute nitride InGaAsN and GaAsSbN layers grown by liquid-phase epitaxy for photovoltaic applications

Dilute nitride InGaAsN and GaAsSbN layers are grown by low-temperature liquid-phase epitaxy (LPE) on GaAs substrates and characterized in view of application in solar cells. The composition of the layers is determined by energy-dispersive X-ray spectroscopy. X-ray photoelectron spectroscopy measurements confirm the Sb content and provide information about the chemical bonding of the N atoms. The band gap values at room temperature assessed from surface photo-voltage and photoluminescence measurements are in good agreement. The experimental results show that the layers exhibit reproducible properties, including a good optical quality and a photosensitivity red limit extended in comparison to GaAs down to about 1.33 – 1.37 eV for InGaAsN and 1.19 – 1.23 eV for GaAsSbN layers. The results obtained highlight the capacity of the LPE for growing dilute nitride layers with good optical quality for photovoltaic applications.

Liquid-Phase Quasi-Epitaxial Growth of Highly Stable, Monolithic UiO-66-NH2 MOF thin Films on Solid Substrates.

High quality, monolithic UiO‐66‐NH2 thin films on diverse solid substrates have been prepared via a low temperature liquid phase epitaxy method. The achievement of continuous films with low defect densities and great stability against high temperatures and hot water is proven, clearly outperforming other reported types of MOF thin films.

GaAsSbN-based p-i-n heterostructures for solar cell applications grown by liquid-phase epitaxy

We present an original study on quaternary GaAsSbN layers and multilayer heterostructures grown by low-temperature liquid-phase epitaxy (LPE) on GaAs substrates aiming to explore their potential for photovoltaic applications. We choose Sn and Mg as suitable dopants for practical use. Our experiments on doping reproducibly show the introduction of controlled concentrations of doping impurities in the range applicable to device fabrication. High-quality n-, p- and nearly compensated GaAsSbN layers covering a large range of carrier concentrations from 1015 to 6 × 1018 cm−3 have been grown at temperatures lower than 600 °C. The successful LPE growth of p-i-n GaAs/GaAsSbN/GaAs heterostructures based on closely compensated i-GaAsSbN is demonstrated for the first time. Temperature-dependent photoluminescence and room temperature surface photovoltage contactless characterization techniques have been used for investigation of the grown structures. These measurements have revealed high optical quality with a low concentration of localized states and red photoresponse limit extended down to 1.2 eV.

Study of GaAsSb:N bulk layers grown by liquid phase epitaxy for solar cells applications

We present an original study of bulk epitaxial GaAsSb:N layers in view of photovoltaic applications of this material. The layers are grown on n-GaAs substrates by low-temperature liquid phase epitaxy (LPE). The grown GaAsSb:N layers exhibit reproducible properties and good optical quality. A number of experimental methods including x-ray diffraction, energy dispersive x-ray spectroscopy, atomic force microscopy, x-ray photoelectron spectroscopy and Raman spectroscopy are applied for investigation of the structural properties, surface morphology, local arrangement and chemical bonding of Sb and N in the obtained compounds. The band gap values at room temperature assessed from surface photovoltage and photoluminescence (PL) measurements are in good agreement and are ∼20 meV lower than those of reference GaAsSb layers. PL spectra measured at different temperatures (10–300 K) show a very weak S-shape-like behaviour of the PL peak energy position indicating minimal carrier localization. The obtained results reveal the capacity of the LPE for growing bulk GaAsSb:N layers with good optical quality.

AlGaAs/GaAs Photovoltaic Converters of Tritium Radioluminescent-Lamp Radiation

Electrical supply sources based on photovoltaic converters and tritium radioluminescent lamps with blue and green glow are developed and fabricated. AlxGa1 –xAs/n-GaAs p–n heterostructures with various band-gap widths (Eg = 1.4–1.9 eV) and compositions of the active region (x = 0.1–0.35) are prepared by low-temperature liquid-phase epitaxy. The photovoltaic converter with an area of S = 0.12 cm2 upon irradiation by a green tritium lamp (wavelength λ = 550 nm) has the short-circuit current density 180 nA/cm2 and output electrical power >100 nW/cm2.

Effect of Sb in thick InGaAsSbN layers grown by liquid phase epitaxy

Dilute nitride InGaAsSbN layers grown by low-temperature liquid phase epitaxy are studied in comparison with quaternary InGaAsN layers grown at the same growth conditions to understand the effect of Sb in the alloy. The lattice mismatch to the GaAs substrate is found to be slightly larger for the InGaAsSbN layers, which is explained by the large atomic radius of Sb. A reduction of the band gap energy with respect to InGaAsN is demonstrated by means of photoluminescence (PL), surface photovoltage (SPV) spectroscopy and tight-binding calculations. The band-gap energies determined from PL and ellipsometry measurements are in good agreement, while the SPV spectroscopy and the tight-binding calculations provide lower values. Possible reasons for these discrepancies are discussed. The PL spectra reveal localized electronic states in the band gap near the conduction band edge, which is confirmed by SPV spectroscopy. The analysis of the power dependence of the integrated PL has allowed determining the dominant radiative recombination mechanisms in the layers. The values of the refraction index in a wide spectral region are found to be higher for the Sb containing layers.

Growth and characterization of broad spectrum infrared emitting GaInAsP/InP heterostructures

Broad spectrum InGaAsP/InP light emitting heterostructures were grown by low temperature liquid phase epitaxy (LPE). The structure of the LED wafers was investigated by optical transmission measurements, and the layer thicknesses were also measured by electron microscopy. Two quaternary light emitting layers of different composition were built in one device structure in order to broaden the usable wavelength range of the emission spectrum. One of the layers is electrically, whereas the other is optically excited as a result of internal absorption and re-emission of the LED light. As a result of this absorption and re-emission process the modified LED chips have substantially broader emission spectra and higher radiance than the conventional surface emitting multi-wavelength NIR LED structures. The two emission peaks of the spectrum were designed for matching the first and second harmonic wavelength of the fundamental absorption band of C-H bonds. The internal quantum efficiency of the wavelength conversion in this type of LEDs is nearly 100%.

Low-temperature liquid-phase epitaxy of YBa2Cu3Oy films by the molten KOH method

We fabricated high-performance YBa2Cu3Oy (Y123) films by the liquid-phase epitaxy method using molten KOH flux at low temperatures. The Y123 films fabricated in N2 atmosphere showed biaxial orientation above 500 °C; moreover, the surface morphology of the Y123 films fabricated above 650 °C suggested spiral growth. The Y123 films fabricated above 650 °C showed a sharp transition with a of 90 K.

Structural and waveguiding characteristics of Er3+:Yb3Al5−yGayO12 films grown by the liquid phase epitaxy

Erbium (Er3+) doped ytterbium garnet (Er:Yb3Al5−yGayO12; y=0, 0.55 and 1.1) single crystalline thick films have been grown by the low-temperature liquid phase epitaxy method (LPE). The composition of the films was determined using the high resolution XRD, the particle-induced X-ray emission spectroscopy (PIXE) and the particle-induced gamma-ray emission spectroscopy (PIGE). The lattice mismatch between films and substrates was investigated by the high-resolution X-ray diffraction. The surface analysis was carried out by the atomic force microscopy (AFM). Pure infrared emission of Er3+ ions was observed in all films containing gallium. The characteristics such as refractive index, thickness and light propagation were studied by the m-line spectroscopy (MLS) using several wavelengths (633, 964, 1311 and 1552nm). All samples, where y=1.1, were multimode waveguides. For these reasons, the Er:Yb3Al3.9Ga1.1O12 seems to be a promising material for light amplifiers in the IR region.

High-efficiency (η = 39.6%, AM 1.5D) cascade of photoconverters in solar splitting systems

A concentrator photovoltaic module with spectral splitting of solar radiation is developed. The module is based on a Fresnel lens and two dichroic filters. Solar cells based on AlGaAs and GaAs are grown by low-temperature liquid-phase epitaxy. GaSb photoconverters are fabricated by zinc gas-phase diffusion into a base epitaxial layer or an n-type GaSb substrate. The total efficiency of three solar cells developed for the spectral splitting module reached 39.6% (AM 1.5D spectrum).

Low-temperature LPE growth and characterization of InGaAsN thick layers

Abstract This work demonstrates the possibility for low-temperature Liquid-Phase Epitaxy (LPE) growth of lattice matched to GaAs substrate dilute nitride InGaAsN layers with good crystalline quality and high Hall electron mobility. X-ray microanalysis and X-ray diffraction methods have been used to determine the composition and crystalline quality of the grown InGaAsN layers. Surface roughness is examined by atomic force microscopy. N-related local vibration modes are observed by Raman scattering. The Hall electron mobility and free carrier concentration have been measured in the temperature range 80-300K by conventional Van der Pauw method.

Solar cell technology on the base of III–V heterostructures

Low-cost single-junction GaAs solar cells on the base of p-AlGaAs/p-GaAs/ n-GaAs heterostructures grown by low-temperature liquid-phase epitaxy have been fabricated. For analyzing the factors contributing to solar cell efficiency the program products PC1D have been used. The optimal doping concentrations and thicknesses of the layers which correspond to the maximum values of the solar cell output parameters have been determined. The multilayer solar cell structure has been optimized on the base of this simulation. Low cost technological processes and masks, necessary for fabrication of the cells on the base of the grown heterostructures, have been developed, too. The front and back metallization of the cell is Ni/Al system deposited by magnetron sputtering. A double layer Al2O3 /ZrO2 antireflection coating is deposited on the front side of a solar cell structure by the spin coating technique. The conversion efficiencies of 19 % at one sun AM1.5 conditions have been measured.

The kinetics of low-temperature liquid phase epitaxy in the Zn-Cd-Te system

The kinetic corrections for the supersaturation of the initial liquid phase providing the preparation of ZnxCd1 − xTe solid solution layers at low deposition temperatures (500–600°C) were found. The results of kinetic modeling were in satisfactory agreement with the experimental data. It was shown that the inclusion of corrections for supersaturation of the initial liquid phase to the data on phase equilibria in the Zn-Cd-Te system was necessary for finding the temperature-time conditions of the crystallization of layers with the required composition.

Temporal dynamics of upconversion luminescence in Er 3+, Yb 3+ co-doped crystalline KY(WO 4) 2 thin films

Crystalline Er 3+ and Yb 3+ singly and doubly doped KY(WO 4) 2 thin films were grown by low-temperature liquid-phase epitaxy. Absorption, luminescence, excitation and temporal evolution measurements were carried out for both Er 3+ and Yb 3+ transitions from 10 K to room temperature. Green Er 3+ upconversion luminescence was observed after Yb 3+ and Er 3+ excitation. The mechanisms responsible for the upconversion phenomena detected in each case were identified.