III-V Alloys Research Articles

Organic and perovskite solar cells for space applications

For almost sixty years, solar energy for space applications has relied on inorganic photovoltaics, evolving from solar cells made of single crystalline silicon to triple junctions based on germanium and III-V alloys. The class of organic-based photovoltaics, which ranges from all-organic to hybrid perovskites, has the potential of becoming a disruptive technology in space applications, thanks to the unique combination of appealing intrinsic properties (e.g. record high specific power, tunable absorption window) and processing possibilities. Here, we report on the launch of the stratospheric mission OSCAR, which demonstrated for the first time organic-based solar cell operation in extra-terrestrial conditions. This successful maiden flight for organic-based photovoltaics opens a new paradigm for solar electricity in space, from satellites to orbital and planetary space stations.

Alloying strategy for two-dimensional GaN optical emitters

The recent progress in formation of two-dimensional (2D) GaN by a migration-enhanced encapsulated technique opens up new possibilities for group III-V 2D semiconductors with a band gap within the visible energy spectrum. Using first-principles calculations we explored alloying of 2D-GaN to achieve an optically active material with a tunable band gap. The effect of isoelectronic III-V substitutional elements on the band gaps, band offsets, and spatial electron localization is studied. In addition to optoelectronic properties, the formability of alloys is evaluated using impurity formation energies. A dilute highly mismatched solid solution 2D-${\mathrm{GaN}}_{1\ensuremath{-}x}{\mathrm{P}}_{x}$ features an efficient band gap reduction in combination with a moderate energy penalty associated with incorporation of phosphorous in 2D-GaN, which is substantially lower than in the case of the bulk GaN. The group-V alloying elements also introduce significant disorder and localization at the valence band edge that facilitates direct band gap optical transitions, thus implying the feasibility of using III-V alloys of 2D-GaN in light-emitting devices.

Droplet induced compositional inhomogeneities in GaAsBi

Compositional inhomogeneities in III-V alloys heavily influence the device performance. This work presents evidence for Ga droplets inducing inhomogeneities in the Bi composition, which we propose is due to a variation in the Ga flux across the surface. These inhomogeneities may be manipulated through the use of growth interrupts, which eliminate the buildup of Ga at the growth front.

Novel Band-Gap Engineered III-V Alloys for Unassisted Water Photoelectrolysis

Photoelectrochemical water splitting cells comprising the alloys of III-V semiconductors hold record solar-to-hydrogen efficiencies. However, the performance of state-of-the art III-V alloy-based single absorber devices, particularly of InGaP2, InGaAs, GaPN, and GaAsN, comes short of unassisted water due to a) inadequate alignment of band edges with respect to the HER and OER redox potentials, b) insufficient photovoltage to favor charge separation at the solid electrolyte liquid junction, and/or iii) recombination. In order to circumvent these limitations considerable research efforts have been directed at creating more complex architectures involving e.g. tandem cells, nanostructures, and buried junctions to correctly position the bands and provide enough driving force to surmount overpotentials and the 1.23 eV energetic barrier of the water splitting reactions. Therefore, developing novel III-V photoabsorbers with adequate band energetics is key to conceiving simpler, yet efficient single semiconductor PEC cells for production of solar fuels at a competitive cost. First-principles DFT+U calculations incorporating the local density approximation and generalized gradient approximation have shown that incorporation of Sb narrows the band gap in Ga(Sbx)N1-x and changes the electronic band gap from indirect to direct in GaSbP. Theoretical computations predict that, with band gaps in the order of 2 eV, these materials straddle the potential window for water oxidation and proton reduction in acidic solution. Single crystalline films of these two materials have been deposited by metal organic chemical vapor deposition and halide vapor phase epitaxy, in a wide range of Sb incorporation without phase segregation. Experimental results corroborate the significant band gap reduction in GaSbN from 3.4 to 1.5 eV(Fig 1a) and suggest the reduction of the indirect band gap of GaP when Sb is incorporated(Fig 1b) as determined by Tauc plot analysis of diffuse reflectance data, low temperature photoluminescence spectroscopy, and photocurrent spectroscopy. Electrodes comprising Ga(Sbx)N1-x and Ga(Sbx)P1-x have been benchmarked employing 2- and 3-electrode standard methods for assessment of their characteristic attributes, i.e. flat-band and onset potentials, photovoltage, zero-bias photocurrent density, fill factors, carrier concentrations, and most importantly, their ability for gas evolution by in situ fluorescence probing. New insights into these materials regarding the donor concentration in GaP alloys, and the effect of incorporation help in fundamentally understanding the thermodynamics at a solid liquid electrolyte interface and work function of GaSbP to realize the full potential of these novel III-V alloys. This poster will highlight recent advances in the understanding of the inter-relationship of processing/synthesis, material structure and photoelectrochemical properties of this new class of materials. Acknowledgements: Financial support from US Department of Energy (DE-FG02-07ER46375) and NSF (DMS1125909).

Enhanced Coupling Strength Gratings for Couplers in III–V Compound Dielectric Waveguides

The use of a thin, low-index liner layer over a surface grating covered by a high-index cover layer in a III-V alloy waveguide significantly improves the coupling efficiency of grating couplers, and therefore, reduces the length of such couplers by an order of magnitude. The same low-index liner and high-index cover layer can also reduce losses between transition sections of photonic integrated circuits or between a laser region and a distributed Bragg reflector or grating coupler region.

Stable hybrid organic/inorganic photocathodes for hydrogen evolution with amorphous WO3 hole selective contacts.

Photoelectrochemical H2 production through hybrid organic/inorganic interfaces exploits the capability of polymeric absorbers to drive photo-induced electron transfer to an electrocatalyst in a water environment. Photoelectrode architectures based on solution-processed organic semiconductors are now emerging as low-cost alternatives to crystalline inorganic semiconductors based on Si, oxides and III-V alloys. In this work, we demonstrate that the stability of a hybrid organic/inorganic photocathode, employing a P3HT:PCBM blend as photoactive material, can be considerably improved by introducing an electrochemically stable WO3 hole selective layer, paired with a TiO2 electron selective layer. This hybrid photoelectrode exhibits a photocurrent of 2.48 mA cm-2 at 0 VRHE, +0.56 VRHE onset potential and a state-of the art operational activity of more than 10 hours. This work gives the perspective that photoelectrodes based on organic semiconductors, coupled with proper inorganic selective contacts, represent a sound new option for the efficient and durable photoelectrochemical conversion of solar energy into fuels.

Optical characterisation of III-V alloys grown on Si by spectroscopic ellipsometry

Increased interest in novel substrates for III-V material growth has led to the use of alloy fractions other than those lattice-matched to traditional GaAs and Ge substrates. The optical constants of these materials are generally not available in the literature. We characterise the unconventional alloy fractions used in a GaAsP/SiGe tandem on Si by spectroscopic ellipsometry on specially prepared samples, with consideration of transparent and opaque spectral regions during analysis. Complex indices of refraction for GaAs.84P.16,Ga.59In.41P, and Al.65In.35P are determined. These values allow improvement of the solar cell and will enable future optical modelling and design of this and other devices and materials.

(Electrodeposition Division Research Award) Using Liquid Metals for Advanced Electrodepositions

This presentation will summarize the recent development of the hybrid electrodeposition process known as electrochemical liquid-liquid-solid (ec-LLS) crystal growth by our group. Three aspects will be covered in this presentation. First, the general premise behind ec-LLS and how ec-LLS melds conventional electrodeposition and melt growth for the purposes of controlled and low-temperature growth of crystalline inorganic solids. Second, the status of ec-LLS as a method to produce Group IV (Ge, Si, and alloys thereof) and Group III-V (GaAs, GaSb, InAs, InSb, and alloys thereof) semiconductor crystals will be presented. Special emphasis will be given to the deposition of nanowires, microwires, and epitaxial films. Third, a general outlook on the possibilities ec-LLS offers as a unique electrodeposition method will be given. Specifically, opportunities for quantitative melt crystal growth studies, the synthesis of other inorganic materials, and the prospects for ec-LLS as a means to produce directly functional devices will be discussed.

(Invited) Novel Band-Gap Engineered III-V Alloys for Unassisted Water Photoelectrolysis

Photoelectrochemical water splitting cells comprising the alloys of III-V semiconductors hold record solar-to-hydrogen efficiencies. However, the performance of state-of-the-art III-V alloy-based single absorber devices, particularly of InGaP2, InGaAs, GaPN, and GaAsN, comes short of unassisted water splitting due to a) inadequate alignment of band edges with respect to the HER and OER redox potentials, b) insufficient photovoltage to favor charge separation at the solid electrolyte liquid junction, and/or iii) recombination. In order to circumvent these limitations considerable research efforts have been directed at creating more complex architectures involving e.g. tandem cells, nanostructures, and buried junctions to correctly position the bands and provide enough driving force to surmount overpotentials and the 1.23 eV energetic barrier of the water splitting reactions. Therefore, developing novel III-V photoabsorbers with adequate band energetics is key to conceiving simpler, yet efficient single semiconductor PEC cells for production of solar fuels at a competitive cost. First-principles DFT+U calculations incorporating the local density approximation and generalized gradient approximation have shown that incorporation of Sb narrows the band gap in Ga(Sbx)N1-xand changes the electronic band gap from indirect to direct in GaSbP. Theoretical computations predict that, with band gaps in the order of 2 eV, these materials straddle the potential window for water oxidation and proton reduction in acidic solution. Single crystalline films of these two materials have been deposited by metal organic chemical vapor deposition and halide vapor phase epitaxy, in a wide range of Sb incorporation without phase segregation. Experimental results corroborate the significant band gap reduction in GaSbN from 3.4 to 1.5 eV (Fig 1a) and suggest the conversion of excitonic transitions in the electronic band structure of GaSbP (Fig 1b) as determined by Tauc plot analysis of diffuse reflectance data and low temperature photoluminescence spectroscopy. Electrodes comprising Ga(Sbx)N1-x and Ga(Sbx)P1-x have been benchmarked employing 2- and 3-electrode standard methods for assessment of their characteristic attributes, i.e. flat-band and onset potentials, photovoltage, zero-bias photocurrent density, fill factors, carrier concentrations, and most importantly, their ability for gas evolution by in situfluorescence probing. New insights into these materials raise questions regarding the native conductivity type of GaP alloys and the effect of incorporation or defects in the extent of band bending at a solid liquid electrolyte interface and work function of GaSbN. This presentation will highlight recent advances in the understanding of the inter-relationship of processing/synthesis, material structure and photoelectrochemical properties of this new class of materials. Acknowledgements: Financial support from US Department of Energy (DE-FG02-07ER46375) and NSF (DMS1125909).

Ultraviolet optoelectronic devices based on AIGaN alloys grown by molecular beam epitaxy

AbstractThis paper reviews progress in ultraviolet (UV) optoelectronic devices based on AIGaN films and their quantum wells (QWs), grown by plasma-assisted molecular beam epitaxy. A growth mode, leading to band-structure potential fluctuations and resulting in AIGaN multiple QWs with internal quantum efficiency as high as 68%, is discussed. Atomic ordering in these alloys, which is different from that observed in traditional III-V alloys, and its effect on device performance is also addressed. Finally, progress in UV-light-emitting diodes, UV lasers, UV detectors, electroabsorption modulators, and distributed Bragg reflectors is presented.

Temperature dependence of the photoluminescence polarization of ordered III-V semiconductor alloys

We studied the linear polarization of the photoluminescence (PL) emission of atomically ordered GaInAsP and GaInP alloys with different ordering parameters in the temperature range from 10 to 300 K. The epitaxial layers of these alloys were grown on GaAs and Ge (001) substrates by metal organic vapor phase epitaxy. The polarization of the PL emission propagating along different crystallographic axes depends on the value of biaxial strain in the layer and changes with temperature. We calculated the PL polarization patterns for different propagation directions as a function of biaxial strain using an existing model developed for ternary atomically ordered III-V alloys. Comparing the calculated PL polarization patterns with those obtained experimentally, we separated the variation of the PL polarization due to change of biaxial strain with temperature.

Critical thickness of atomically ordered III-V alloys

The critical thickness model is modified with a general boundary energy that describes the change in bulk energy as a dislocation regularly alters the atomic structure of an ordered material. The model is evaluated for dislocations gliding through CuPt-ordered GaInP and GaInAs, where the boundary energy is negative and the boundary is stable. With ordering present, the critical thickness is significantly lowered and remains finite as the mismatch strain approaches zero. The reduction in critical thickness is most significant when the order parameter is greatest and the amount of misfit energy is low. The modified model is experimentally validated for low-misfit GaInP epilayers with varying order parameters using in situ wafer curvature and ex situ cathodoluminescence. With strong ordering, relaxation begins at a lower thickness and occurs at a greater rate, which is consistent with a lower critical thickness and increased glide force. Thus, atomic ordering is an important consideration for the stability of lattice-mismatched devices.

Compound semiconductor alloys: From atomic-scale structure to bandgap bowing

Compound semiconductor alloys such as InxGa1−xAs, GaAsxP1−x, or CuInxGa1−xSe2 are increasingly employed in numerous electronic, optoelectronic, and photonic devices due to the possibility of tuning their properties over a wide parameter range simply by adjusting the alloy composition. Interestingly, the material properties are also determined by the atomic-scale structure of the alloys on the subnanometer scale. These local atomic arrangements exhibit a striking deviation from the average crystallographic structure featuring different element-specific bond lengths, pronounced bond angle relaxation and severe atomic displacements. The latter, in particular, have a strong influence on the bandgap energy and give rise to a significant contribution to the experimentally observed bandgap bowing. This article therefore reviews experimental and theoretical studies of the atomic-scale structure of III-V and II-VI zincblende alloys and I-III-VI2 chalcopyrite alloys and explains the characteristic findings in terms of bond length and bond angle relaxation. Different approaches to describe and predict the bandgap bowing are presented and the correlation with local structural parameters is discussed in detail. The article further highlights both similarities and differences between the cubic zincblende alloys and the more complex chalcopyrite alloys and demonstrates that similar effects can also be expected for other tetrahedrally coordinated semiconductors of the adamantine structural family.

New III-V Semiconductor Alloys for Solar Hydrogen Production

Dilute III-V alloys have garnered immense interest as suitable materials for solar hydrogen generation due to their compositional tunable band gaps, high carrier mobilities and high absorption coefficients. Till date there are no III-V materials that can satisfy all the stringent criteria for photoelectrochemical water splitting. In this presentation, we report two new III-V materials GaSbN and GaSbP that addresses current materials bottleneck for photoelectrochemical water splitting is developed for photoelectrochemical water splitting. Applicability of the Ga(Sb x )N1−x alloys for practical realization of photoelectrochemical water splitting is investigated using first-principles density functional theory and experiments. Theoretical results predict that Ga(Sb x )N1−x , GaSbP alloys with 2 eV band gaps straddle the potential window at moderate to low pH values, thus indicating that dilute Ga(Sb x )N1−x alloys could be potential candidates for splitting water under visible light irradiation.1,2 Theoretical computations with Sb composition beyond 7% change the electronic band gap from direct to indirect. In the case of GaSbP a small amount of Sb incorporation will change the indirect GaP band gap to direct band gap.3 Experimental synthesis is carried out using metal organic chemical vapor deposition using trimethyl gallium (TMGa) and Trimethyl Antimony (TMSb) and ammonia. Crystalline GaSbxN1-x films were obtained at x values ranging from 0-5%. The synthesis was carried out on different planar substrates and GaN nanowires. Optical measurements confirm that severe band gap reduction occurs with incorporation of antimony in to GaN as predicted by the theoretical calculations. X-Ray Diffraction (XRD) results confirm the lattice expansion at small concentrations of antimony. This work is the first successful attempt on the experimental synthesis of crystalline GaN and GaP based alloy in the low antimonide regime and photoelectrochemical data on activity, band edge energetics and stability show high suitability for direct solar water splitting. The fundamental insights gained from this work on the growth mechanisms and formation of stacking faults can be extended to other ternary alloy nanowires of interest. From a scientific point of view, the large band gap bowing and corresponding optical properties with very small alloying compositions is interesting and can be exploited for band gap engineering. This presentation will highlight our results with both synthesis and photoelectrochemical characterization of GaSbN and GaSbP alloys. Acknowledgements: Financial support from US Department of Energy (DE-FG02-07ER46375) and NSF (DMS1125909). R.M. Sheetz, E. Richter, A.N. Andriotis, C. Pendyala, M.K. Sunkara and M. Menon, “Visible light absorption and large band gap bowing in dilute alloys of gallium nitride with antimony”, Phys. Rev. B, 84, 075304 (2011)S. Sunkara, V.K. Vendra, J.B. Jasinski, T. Deutsch, A.N. Andriotis, K. Rajan and M.K. Sunkara, “New Visible Light Absorbing Materials for Solar Fuels, Ga(Sbx)N1-x”, Adv. Mater., 26 (18), 2878-2882 (2014). H.B. Russell, A.N. Andriotis, M.Menon, J. Jasinski and M.K. Sunkara, “Direct Band Gap Gallium Antimony Phosphide (GaSbxP1-x) Alloys”, Submitted (2015).

The Blue LED Nobel Prize: Historical context, current scientific understanding, human benefit

The Blue LED Nobel Prize: Historical context, current scientific understanding, human benefit

Mobility and Capacitance Comparison in Scaled InGaAs Versus Si Trigate MOSFETs

In this letter, we study the electronic properties of InGaAs and Si trigates using the nonparabolic effective mass approximation and up-to-date mobility models. Our comprehensive simulations estimate a strong reduction of the InGaAs electron mobility to values even lower than those achieved with Si. Considering the reduction of the gate capacitance due to the low density-of-states of the III-V alloy, no apparent benefit would be obtained from using InGaAs trigates instead of Si ones unless high quality interfaces can be achieved. We conclude that the potential application of InGaAs trigates as a reference device for future technological nodes is seriously jeopardized by the quality of the semiconductor-insulator interface.

Observation of atomic ordering of triple-period-A and -B type in GaAsBi

We report the observation of atomic ordering of triple-period (TP)-A and -B type in low temperature (LT) grown GaAsBi alloy using transmission electron microscopy (TEM). In addition to previous reports, where only TP-A ordering was identified in III-V alloys, here, we confirm by electron diffraction, high-resolution (HR) TEM, and HR Z-contrast scanning TEM that two ordering variants coexists for LT-GaAsBi. We find that the TP-A ordering variant dominates over the TP-B variant. TP-A domains extend over 50–100 nm (projected lateral width) and are of higher perfection compared to TP-B domains. HR Z-contrast scanning TEM on different domains reveals a variation in the Bi occupancy in the {111} planes with triple period sequence. Since the formation of ordered phases has been directly linked to the occurrence of specific surface reconstructions, our results suggest a correlation between the TP-A and B type domains and the multiple stability of n × 3 and 3 × n reconstructions on the (001) surface of GaAsBi under low temperature growth.

In situ measurement of CuPt alloy ordering using strain anisotropy

The optical and electrical properties of many III-V alloys change with the degree of CuPt atomic ordering, which is very sensitive to growth conditions. The bulk ordered alloy is elongated along the normal to the ordered planes, and is asymmetrically strained when coherent to a cubic substrate. Here, we demonstrate in situ measurement of the anisotropic strain due to ordering using two-dimensional wafer curvature. The measurement is sensitive to bulk anisotropies, and so is complementary to other in situ measurements that are sensitive to surface anisotropies. Using ab initio calculations, we determine a maximum strain anisotropy of 0.27% between [110] and [1¯10] when perfectly ordered single-variant GaInP2 is coherent to a (001) cubic substrate. We relate the in situ measurement of strain anisotropy on various GaInP2 samples to ex situ measurements of the order parameter to validate the measurement and confirm the capability to predict material properties. The measurement monitors change in ordering during growth, useful for quickly determining the growth condition dependence of ordering or monitoring order-disorder transitions. More generally, this measurement technique could, in principle, be used to monitor phase changes in any epitaxial system for which the strain anisotropy of the two phases differs.

Ordering-enhanced dislocation glide in III-V alloys

Ordering-induced effects on dislocations in metallic alloys have been extensively studied due to their importance in technology applications. We demonstrate that dislocation behavior in ordered III-V semiconductor alloys can be drastically different. This is because ordering in bulk metallic alloys is generally stable, whereas the surface-stabilized group-III sublattice ordering of a III-V alloy is only metastable in the bulk. Here, we show that dislocation glide can release some of the energy stored by ordering of III-V alloys, enhancing the glide of any dislocation which cuts through the ordered layers to create an antiphase boundary in the ordering pattern. This leads to an experimentally observed glide-plane switch which is unique to ordered III-V alloys. Implications for other unique strain-relaxation processes in III-V ordered alloys are also discussed.

The III–V Alloy p–n Diode Laser and LED Ultimate Lamp

In this paper, an account is presented of the semiconductor, because of the energy gap bipolar with electron (e) and hole (h) conductivity, becoming ingeniously with little or no technology (Bardeen and Brattain, Dec. 1947) the transistor, a triode (at first just a germanium “base” crystal and a point contact “emitter” and “collector”), a low impedance “emitter” minority carrier input (IE) into a (the) vital central “base” active region, the base supporting along with essential recombination current I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> > 0) carrier transport to a higher impedance “collector” output I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> ( I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</sub> +I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> +I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> =0, I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> /I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</sub> =α ~ 0.9), hence gain, thus revealing at last that a current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> ) can increase the electron-hole (e-h) population, and as a further consequence, yield band-to-band (k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sub> =k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">h</sub> ) excess carrier recombination radiation (light). The transistor established (1947) a basis for the study of the light-emitting diode (LED). The path to an “ultimate lamp,” the p-n diode laser and LED, is described from semiconductor, to energy gap, e-h bipolarity, transistor, the p-n transition, excess carriers, e-h recombination, direct gap (k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sub> =k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">h</sub> ) crystal, autocatalytic e-h/photon interaction, spectral narrowing, stimulated recombination (laser), need for resonator (cavity Q, how?), direct-gap (k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sub> =k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">h</sub> ) III-V alloy, visible spectrum III-V alloy, III-V alloy gap change (ΔE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> ), quantum-well (QW) e-h recombination, stimulated emission solving the problem of photon extraction ( → 100% quantum efficiency), the p-n diode “ultimate lamp,” return to the 1947 triode, to the transistor, and the base importance informing the realization of a QW-base transistor laser.