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• Tandem J-V characteristics reveal tunnel junction failure. • Tunnel junction properties different when grown separately that in tandem device. • Insufficient doping and thermal load as a potential factors responsible for tunnel junction degradation. From tandem solar cell we expect a wide absorption range due to different bandgaps of the subcells, higher open circuit voltage V oc and finally better photo conversion efficiency η than single junction solar cell. The main drawback of it is a current limitation i.e. the short circuit current J sc of whole structure is equal to smaller J sc of both subcells. However, even two subcells with very good photovoltaic performance do not guarantee the correct operation of tandem solar cell. The tunnel junction plays a crucial role on tandem devices as it has to provide low loss electrical and optical connection between top and bottom subcells. Within this paper we present two subcells: top GaAs based, bottom InGaAsN based with good photovoltaic performance and InGaAs tunnel diode used for InGaAsN/GaAs tandem solar cell construction. We compare reference devices with monolithic tandem cell and conclude about tunnel junction failure based on both dark and illuminated J-V characteristics of the InGaAsN/GaAs final device. Based on detailed analysis of tandem performance we propose two most likely effects responsible for tunnel junction induced deterioration of tandem solar cell performance. [ABSTRACT FROM AUTHOR]

The development of low bandgap GaAsNBi solar cells grown using MBE is reported. The devices include a pin heterostructure with GaAsNBi as the i-layer. The substrate rotation is stopped during epitaxy of the GaAsNBi layer (stationary growth), resulting in a continuous variation in the source fluxes over the wafer, enabling the investigation of a wide range of growth conditions in a single growth run. In particular, we investigate the photovoltaic properties as a function of the As/Ga flux ratio around the critical stoichiometric value. For slightly below stoichiometric As/Ga, the lattice mismatch and bandgap energy are minimized to 8 × 10-4 and 0.86 eV, respectively, while the crystal quality remains good. Increasing As/Ga over the stoichiometric ratio leads to a rapid increase in the tensile mismatch and bandgap energy, likely due to reduction in Bi incorporation. Despite a slightly higher open-circuit voltage offset at high As/Ga, this also leads to an clear increase in the external quantum efficiency and short-circuit current, suggesting significant differences in the carrier collection efficiencies. The results demonstrate the importance of the As/Ga flux ratio in controlling the solar cell performance, and the viability of GaAsNBi as a candidate for a low bandgap subjunction in multijunction solar cells. • Low-bandgap pin-GaAsNBi solar cells. • Use of stationary MBE growth to optimize performance. • As/Ga ratio has a drastic effect on the material properties and photovoltaic operation. • Demonstration of photovoltaic operation at 0.86 eV. [ABSTRACT FROM AUTHOR]

Progress in monolithic integration of III-V semiconductor heterostructures on silicon promotes the development of silicon-based integrated optoelectronics. Here, we report on the fabrication of dual-functional light-emitting and photo-detecting heterostructures based on aluminum-free quaternary dilute nitride alloys of GaAsPN, epitaxially grown on silicon wafers, operating in the red part of the visible spectrum. The use of indirect band gap GaP cladding layers minimizes light absorption and simplify fabrication technology. The low-temperature grown GaP/AlGaP nucleation layer has been employed to achieve antiphase domain-free growth of GaP on Si (001). Quaternary GaAsPN alloy layers with bandgap values, ranging from 1.94 to 1.83 eV, were grown by plasma-assisted molecular beam epitaxy at a high temperature of 540 °C, which ensures high crystallinity. The influence of growth conditions on the GaAsPN alloy composition, structure and optoelectronic properties is discussed. The fabricated p-i-n mesa diodes demonstrate low leakage currents and both photocurrent response with an external quantum efficiency in the range of 30–42% and bright red interband electroluminescence at room temperature. [Display omitted] • GaAsPN on Si pin heterostructures show light emission and detection functionality. • High temperature PA-MBE growth provides structural perfection and E g tunability. • Simple two-step procedure provides antiphase domain-free III-V/Si heteroepitaxy. • Bright red electroluminescence at 300 K and low leakage current demonstrated. • Photovoltaic external quantum efficiency of 42% achieved. [ABSTRACT FROM AUTHOR]

Abstract We demonstrate single junction GaInNAsSb solar cells with high nitrogen content, i.e. in the range of 5–8%, and bandgap energies close to 0.7 eV grown by molecular beam epitaxy. A good crystalline quality is demonstrated for the entire range of N concentrations. An average external quantum efficiency of 0.45 is demonstrated for GaInNAsSb solar cell with 6.2% N exhibiting a bandgap of 0.78 eV (no antireflection coatings has been applied). The internal quantum efficiency for the cell is 0.65 at E g + 0.2 eV. The solar cells exhibited bandgap-voltage offsets between 0.55 V (for N = 5.3%) and 0.66 V (for N = 7.9%). When used in a six-junction solar cell architecture under AM1.5D illumination, the estimated short-circuit current density corresponding to the 0.78 eV cell is 8.2 mA/cm2. Furthermore, using the parameters obtained for the GaInNAsSb junction with 6.2% N, we have estimated that such six-junction solar cell architecture could realistically attain an efficiency of over 50% at 1000 suns concentration. Highlights • GaInNAsSb/GaAs bulk heterostructures with high N compositions of 6–8% demonstrated. • Demonstration of highest EQE to date for 0.78 eV GaInNAsSb solar cell. • Simulation of a 6-junction solar cell shows potential for more than 50% efficiency. [ABSTRACT FROM AUTHOR]

Highlights • InGaAsN/AlGaAs QWIPs for e-SWIR. • CBO values reaching up to ∼1 eV with GaAs technology. • Room temperature photocurrent response that spans between 1.1 and 2.2 μm. • Room temperature responsivity of 15 and 1 A/W for 1.65 μm and 2 μm, respectively. • Corresponding detectivity values of ∼1010 and ∼5 × 108 cm·Hz1/2/W at 1.65 μm and 2.0 μm, respectively. Potential detectivity values as high as ∼2 × 1012 and ∼1 × 1011 cm·Hz1/2/W for ∼1.65 μm and 2.0 μm, respectively can be achieved. Abstract We propose and demonstrate a novel concept to address high-performance, extended short wavelength (e-SWIR) photodetectors. Our approach is based on shifting the well-developed GaAs quantum-well infrared photodetector (QWIP) technology to e-SWIR wavelengths. In order to increase the available conduction band offsets (CBOs), we suggest incorporating nitrogen (N) atoms into the quantum well material. The incorporation of N atoms into III-Vs results in dilute-nitride highly mismatched alloys with lower bandgaps and higher CBOs. In our work, we demonstrate CBO values reaching up to ∼1 eV in InGaAsN/AlGaAs QWIPs. This large CBO makes these structures suitable for e-SWIR detection. The large CBO reduces the dark current dramatically and allows efficient detection at room temperature. In our study, we devised two similar InGaAsN/AlGaAs QWIP devices with twofold, 1% and 2% N composition. Based on the measured dark current data, we extracted activation energy barriers of 780 meV and 580 meV for the 1% and 2% N devices, respectively. The dark current and photocurrent spectral response behave in correlation to the change in the barriers' height. The photocurrent response of the 1% N device peaks at ∼2.25 μm and spans at a spectral range between 1.3 and 2.95 μm. The photocurrent response of the 2% N device is blue shifted to ∼1.42 μm and spans at a spectral range between 1.1 and 2.2 μm. The 2% N device exhibits lower dark currents and a strong photoresponse at room temperature, whereas the 1% N device exhibits a clear response only at temperatures below 150 K. The detection mechanism in the InGaAsN/AlGaAs QWIP devices is based on optical excitation of carriers from the quantum wells into highly-localized nitrogen-related E+ defect-like states located energetically above the conduction band edge. For this reason, the photoresponse is insensitive to the radiation polarization and exhibits low absorption efficiency on the order of 0.15% per quantum well. However, at the same time, the responsivity and photocurrent gain are very high due to the long lifetime of the highly localized excited states. The peak responsivity of the 2% N device is ∼70 A/W at room temperature. Significant responsivity is also available at the response tail at longer wavelengths, i.e., ∼15 and 1 A/W for 1.65 μm—the peak wavelength of the night glow emission and 2 μm—at the e-SWIR wavelengths, respectively. Corresponding detectivity values are ∼1010 and ∼5 × 108 cm·Hz1/2/W at 1.65 μm and 2.0 μm, respectively. These values are similar to those of other developing technologies. In-reach potential detectivity is estimated based on the measured data and can arrive easily to values as high as ∼2 × 1012 and ∼1 × 1011 cm·Hz1/2/W for ∼1.65 μm and 2.0 μm, respectively. The presented characteristics and potential performance indicate that InGaAsN/AlGaAs quantum wells are most suitable for efficient e-SWIR photodetection. [ABSTRACT FROM AUTHOR]

This work presents the results of X-ray photoelectron spectroscopy studies on the bonding N configuration in InGaAsN epilayers grown by atmospheric pressure metal organic vapour phase epitaxy. Growth temperature has been tuned in order to obtain both, relaxed and strained layers. The studies were concentrated on analysing the influence of the growth temperature, post growth thermal annealing process and surface quality on the formation of Ga-N and In-N bonds as well as N-related defects. The contamination of InGaAsN films by growth precursor residues and oxides has also been addressed. The growth temperature stands out as a decisive factor boosting In-N bonds formation, while the thermal annealing seems to affect the N-related defects density in the layers. [ABSTRACT FROM AUTHOR]

Using first-principles total energy calculations, we have studied the structural, mechanical and electronic properties of GaAs 1- x N x ternary semiconductor alloys with the zinc-blende crystal structure over the whole nitrogen concentration range (with x from 0 to 1) within density functional theory (DFT) framework. To obtain the ideal band gap, we employ the semi-empirical approach called local density approximation plus the multi-orbital mean-field Hubbard model (LDA+U). The calculated results illustrate the varying lattice constants and band gap in GaAs 1- x N x alloys as functions of the nitrogen concentration x . According to the pressure dependence of the lattice constants and volume, the higher N concentration alloy exhibits the better anti-compressibility. In addition, an increasing band gap is predicted under 20 GPa pressure for GaAs 1- x N x alloys. [ABSTRACT FROM AUTHOR]

We report on a novel lithographic approach for the fabrication of integrated quantum dot (QD)-photonic crystal (PhC) nanocavity systems. We exploit unique hydrogen's ability to tailor the band gap energy of dilute nitride semiconductors to fabricate isolated site-controlled QDs via a spatially selective hydrogenation at the nanometer-scale. A deterministic integration of the realized site-controlled QDs with PhC nanocavities is provided by the inherent realignment precision (~ 20 nm) of the electron beam lithography system used for the fabrication of both QDs and PhC cavities. A detailed description of the fabrication steps leading to the realization of integrated QD-PhC cavity systems is provided, together with the experimental evidence of a weak coupling effect between the single-photon emitter and the PhC cavity. [ABSTRACT FROM AUTHOR]

The effects of 7 MeV electron irradiation at very high doses of 2 × 10 17 and 1.5 × 10 18 electrons / cm 2 and subsequent rapid thermal annealing on photoluminescence from a strain-compensated GaInAsN/GaAsN/GaAs quantum well structure are investigated. A large additional blueshift of photoluminescence has been observed from the lower-dose irradiated sample as compared to the non-irradiated one when annealed after the irradiation. This additional blueshift will become considerably reduced by an ageing effect, which occurs already at room temperature. The mechanism causing the additional blueshift of photoluminescence and its reduction is qualitatively assigned to metastable complex defects promoted by electron irradiation in the nitrogen containing layers. No such additional blueshift of photoluminescence under the thermal treatment has been observed in the higher-dose irradiated sample. [ABSTRACT FROM AUTHOR]

High performance narrow-bandgap GaInNAsSb solar cells are instrumental for the development of lattice-matched GaAs-based solar cells with more than four junctions. To this end a comprehensive optimization process including the effects of growth temperature, As/III beam equivalent pressure ratio, and Sb flux on the performance of 0.8 eV GaInNAsSb solar cells grown by molecular beam epitaxy is reported. For this, sets of GaInNAsSb p-i-n solar cell structures with 5–6% nitrogen compositions were fabricated, while varying the key growth parameters. The quantum efficiency and current generation increased significantly when the narrow gap materials were grown at elevated growth temperatures, close to phase separation. A further improvement in the current generation was observed by employing lower As/III beam equivalent pressure ratios. The best GaInNAsSb cell exhibited about 94% peak external quantum efficiency and generated a short-circuit current of 17.7 mA/cm2 with AM1.5D (1000 W/m2) illumination at wavelengths above 900 nm without employing a back surface reflector. Our analysis indicates that the best cell is already close to being absorption limited. While the N composition should be kept as low as possible (i.e., ≲5%) to achieve high performance, increasing the Sb flux generally results in improved the material quality, i.e., leading to a slight improvement for the open-circuit voltages and fill factors. In addition, it was found that the phase separation observed at the growth temperature of 480 °C could effectively be inhibited by employing higher Sb fluxes. • Optimized MBE growth for 0.8 eV GaInNAsSb solar junctions with 5–6% N content. • High current obtained using high growth temperatures and lower As pressures. • High peak external quantum efficiency of 94% demonstrated. • Background doping level as low as 1.1 × 1015 cm−3 for GaInNAsSb with 6% N. • Phase separation is suppressed, and power output improved by optimizing Sb flux. [ABSTRACT FROM AUTHOR]

In this work, InSbN alloys were hetero-eptiaxially grown on GaSb (100) substrate by metal-organic chemical vapor deposition, expecting large nitrogen addition and long cut-off wavelength of the samples simultaneously. Post annealing treatment was carried out to see the effect on the alloy structural and optical properties. Photoluminescence results indicate that the band gap wavelength of the alloys is reduced to 6.3 μm by the N incorporation. Besides, another peak around 8.3 μm could also be detected, and the peak intensity is comparable to the main band emission, which means much for the application in long wavelength devices. The cause of the defect emission was discussed through the analysis of Photoluminescence and X-ray photoelectron spectroscopy measurement results. [ABSTRACT FROM AUTHOR]

In this work authors present the results of structural and optical characterization of the GaInNAs/GaAs MQW structures grown by AP-MOVPE. The growth conditions of quantum well layers were modified in order to counteract the phase segregation within GaInNAs QWs. Profiles of indium and nitrogen contents across the QWs layers were investigated by using HRXRD. The optical properties were investigated by applying photoluminescence spectroscopy. The inhomogeneous composition of the GaInNAs QW layers is considered as a factor suppressing the PL intensity due to change in the electron and holes wave functions distributions and tuning the transition's selection rules. [ABSTRACT FROM AUTHOR]

Charge transfer excitons are studied in double quantum well structures consisting of a (GaIn)As and a Ga(NAs) layer separated by a GaAs film of variable thickness. With decreasing barrier thickness, the gradual change from a spatially direct exciton within the (GaIn)As well to a charge transfer exciton bound across the GaAs spacing layer is observed. The optical spectra are well reproduced by a fully microscopic theory and band structure calculations based on the k · p method using a weak type-I valence band offset of approximately ( 45 ± 40 ) meV at the Ga(NAs)/GaAs interface. [ABSTRACT FROM AUTHOR]

In the presented work the growth and fabrication process of dilute nitride based solar cells were reported. We fabricated three different solar cells to investigate the influence of top contact on their electrical parameters. Test devices were characterized by the means of current–voltage measurements carried out under the sunlight simulator. The obtained I – V results were scrutinized using a single diode equivalent circuit of a solar cell. We employed the Lambert W approach to find the solvable solution of the modified Shockley equation, in order to determine the basic solar cell electrical parameters such as: ideality factor n , series and shunt resistances ( R s and R sh ), saturation current I 0 and photocurrent I ph generated in the solar cell structure. It was found that electrical parameters obtained from the fitting procedure depend on solar cell design. The type of top electrode influences the values of parasitic resistances, open circuit voltage and short circuit current. [ABSTRACT FROM AUTHOR]

The correlation between the As to group III flux ratio and photovoltaic performance of GaIn 0.1 N 0.03 As solar cells fabricated by molecular beam epitaxy is systematically investigated. The results show that flux ratio has a remarkable influence on the formation of defect traps. Furthermore, the formation of defects at different flux ratios is correlating with the variation of the background doping level and the photovoltaic performance. In particular, this study reveals a linear dependency between current generation, dark saturation current, defect densities, photoluminescence peak intensity and the flux ratio. A significant increase in solar cell performance, exhibiting maximum external quantum efficiency of 90%, is obtained when As/group-III ratio is decreased close to the stoichiometric limit. For optimized growth condition, the 1 eV GaIn 0.1 N 0.03 As solar cell exhibits a short circuit current density as high as 17.9 mA/cm 2 calculated from the external quantum efficiency data (AM0 conditions) with 870 nm high-pass filter. This value reflects the potential of the GaInNAs cell for current matching and power generation in high efficiency solar cells incorporating three- or four- junctions. [ABSTRACT FROM AUTHOR]

We report study on stacked InAs/GaNAs quantum dots heterostructures with dilute nitride GaInNAs strain mediating layers embedded in GaAs p-i-n solar cell structure. The insertion of GaInNAs strain mediating layers in the vicinity of the strain compensated InAs/GaNAs quantum dots heterostructures enhances their surface density, improves and significantly red shifts their light emission. Embedding a stack of the strain-mediated InAs/GaInNAs/GaNAs quantum dots in the i region of a GaAs p-i-n solar cell leads also to a red shift of the absorption edge of the solar cells and improves the solar cell photogenerated currents at longer wavelengths beyond 1200 nm. [ABSTRACT FROM AUTHOR]

III/V semiconductors with cubic zinc-blende crystal structure, for example GaAs, GaP or InP, become metastable if atoms with significantly smaller or larger covalent radius than the matrix atoms are alloyed. Examples are the incorporation of Boron, Nitrogen and Bismuth in the above-mentioned materials. The resulting multinary compound semiconductors, like for example (Ga,In)(N,As), Ga(N,As,P) and Ga(As,Bi), are extremely interesting for several novel applications. The growth conditions, however, have to be adopted to the metastability of the material systems. In addition, structure formation can occur which is different from stable materials. This paper summarizes our current knowledge on growth characteristics of several metastable materials. Mainly examples for Metal Organic Vapor Phase Epitaxy (MOVPE) are given. The MOVPE growth characteristics are compared to selected examples using Molecular Beam Epitaxy growth to highlight that the observed growth characteristics are intrinsic for the studied metastable material systems. Furthermore, structural peculiarities of dilute borides, nitrides and bismides occurring during growth as well as in growth interruptions are summarized and correlated to the growth conditions. [ABSTRACT FROM AUTHOR]

Surface photovoltage (SPV) spectra were measured for GaN 0.014 As 0.986 layers at room temperature and compared with room temperature photoreflectance (PR) and contactless electroreflectance (CER) measurements. Spectral features related to E − and E + transitions were clearly observed in SPV spectra at energies corresponding to PR and CER resonances. In this way it has been shown that SPV spectroscopy is an alternative absorption-like technique to study both the E − and E + transitions in dilute nitrides. The observation of E + transition in SPV spectra means that it is a direct optical transition at the Γ point of GaNAs band structure which can be explained by the band anticrossing interaction between the localized states of N and the extended conduction band states of the GaAs host. [ABSTRACT FROM AUTHOR]

We have investigated the role of the nitrogen content, the growth parameters, and the annealing processes involved in molecular beam epitaxy of GaInNAs solar cells lattice-matched to GaAs. The nitrogen composition was varied between 1% and 5%. The influence of the growth temperature was assessed by performing photoluminescence, atomic force microscopy, X-ray diffraction, reflection high-energy electron diffraction, quantum efficiency and light-biased current–voltage measurements. The growth temperature ensuring the best cell parameters was found to be 440°C. At this temperature we were able to incorporate up to 4% of nitrogen and achieve a good material quality. Further increase of the N composition to 5% led to phase separation. For the lattice matched samples grown within the optimal temperature range, we have identified a clear (1×3) surface reconstruction. Using the optimized growth we have demonstrated a GaInNAs p-i-n solar cell structure containing 4% nitrogen, that exhibited a short-circuit current density as high as 33.8mA/cm2 in respect to effective area illuminated. These measurements have been performed under real sun AM1.5 (~1000W/m2) illumination. [ABSTRACT FROM AUTHOR]

The effects of growth temperature (410 and 470°C) and subsequent rapid thermal annealing (RTA) on luminescence performance of lattice-matched 1-eV GaInNAs-on-GaAs epilayers, grown by molecular beam epitaxy under constant fluxes, has been studied experimentally by 9-K photoluminescence (PL) and 300-K photoreflectance (PR) spectroscopy. The near band edge PL has been found to noticeably decrease and red-shift as the growth temperature (T gr) is reduced from 470 to 410°C. This red-shift is the consequence of a reduction in the alloy band gap with decreasing T gr, as confirmed by PR. An N-related broad PL band, whose intensity remarkably enhances with increasing T gr, has been detected at longer wavelengths in the 9-K PL's. Post-growth RTA (1min at 800°C) significantly enhanced the near band edge PL but it did so at the expense of a PL blue-shift: the more the PL enhancement and blue-shift the lower the T gr. This annealing-induced blue-shift is due to In–N bonds formation, whose magnitude enhanced as the T gr is decreased. The broad PL band also enhanced upon the RTA, but the ratio between its intensity and the intensity of the near band edge PL reduced, especially for the sample grown at 410°C. 7-MeV electron irradiation to the dose of around 1015 cm−2 applied prior to annealing considerably reduced the broad PL band upon annealing. [ABSTRACT FROM AUTHOR]

Abstract: We adapted the atomic layer epitaxy (ALE) technique to grow GaAsN thin films. The effects of the gas flow sequences used on the self-limiting mechanism (SLM), the N incorporation ratios, and the concentrations of residual impurities were investigated. A pulse of the N precursor (monomethylhydrazine) was added after that of the Ga precursor (trimethylgallium) or the As (trisdimethylaminoarsenic) precursor, which were part of the conventional sequence used for the growth of GaAs films via ALE. If it was noticed that the SLM was in effect, the N precursor was supplied on the Ga- or As-terminated surface (the corresponding sequences and their resulting films are labeled as On-Ga and On-As, respectively). The On-As film exhibited rough surfaces, nonuniform N concentration, and disordered lattices owing to defects. On the other hand, in the case of the On-Ga sequence, GaAsN films with high-quality crystals were grown with the SLM in effect and the concentration of N being a few percent. The concentrations of the residual impurities (C and H) in the On-Ga film were low. This demonstrated that the On-Ga sequence was effective for growing GaAsN thin films on precisely controlled surfaces using the ALE technique. [Copyright &y& Elsevier]

Abstract: GaP0.98N0.02 layers having a thickness from 75nm to 600nm are grown on GaP substrates by metalorganic vapor phase epitaxy. Back-reflection X-ray topographs made with a synchrotron radiation of the thick GaP0.98N0.02 layers reveal images of misfit dislocations. The critical thickness of the grown GaP0.98N0.02 layer on GaP substrate is evaluated from the topographs to be about 200nm. The strain relaxation is also observed from high resolution X-ray diffraction curves. X-ray diffraction curves and atomic force microscopy studies indicate that the GaP0.98N0.02 layers with thickness smaller than 200nm possess a very good crystal quality, an atomically smooth surface with a roughness less than 0.3nm and a well-defined interface without misfit dislocations between the layer and the substrate. [Copyright &y& Elsevier]

Abstract: We have investigated the influence of 7-MeV electron irradiation (1.2×1015 and 1.8×1016 electrons/cm−2 doses) and subsequent rapid thermal annealing (RTA) on luminescence efficiency of nearly lattice-matched 1-eV GaInNAs-on-GaAs thin epilayers, grown by molecular beam epitaxy. The study has been done by means of 9K photoluminescence (PL) in conjunction with 300K photoreflectance (PR) spectroscopy and x-ray diffraction (XRD) measurements. An enhancement in PL has been found for the lower dose whereas a drastic deterioration in PL has been seen for the higher dose. When rapid thermally annealed at 800°C for 1min an enhancement in PL has been observed, noticeably for the sample irradiated at a higher dose, as compared to the reference non-irradiated sample. The irradiation-promoted enhancement in PL upon annealing has been accompanied by a small additional blue shift (BS) of the PL. This additional annealing-induced BS has been found, by PR and XRD, to be due to an irradiation-promoted enhancement in In–N bonds formation, whose magnitude appears to not depend on dose within the studied range. [Copyright &y& Elsevier]

Abstract: This paper highlights the electrical characterization of three types of the multiple quantum well (MQW) InGaAsN/GaAs heterostructures labelled NI58n, NI59n and NI66n with the nitrogen concentration of about 0.4% and indium content of 12.1%, 13.5% and 13.0%, respectively, using capacitance methods. These MQW InGaAsN/GaAs heterostructures were grown by atmospheric pressure metal organic vapour phase epitaxy (APMOVPE) at various growth conditions. The capacitance-voltage and DLTS measurements of these structures were performed utilizing the measurement system BIORAD DL8000. The CV characteristics measured at different temperatures on all heterostructures show very fine fluctuations of capacitance in the voltage range around 0V, which is suggested to be a result of free carriers’ emission from quantum wells. The absence of GaAs “cap” layer in one of the samples shifted its CV characteristic to lower values of capacity. According to the DLTS measurement it is highly probable that the similar system of defects is present in all samples. The presence of the deep level ET1 is clear in all samples and its parameters were calculated by multi level evaluation (the activation energy of the defect which is approx. ΔE T =0.62eV, value of effective capture cross-section of the defect σ T is about 10−16 cm2, and defects concentration N T is about 1014 cm−3). [Copyright &y& Elsevier]

Abstract: In this study, we demonstrated the fabrication and characterization of p-GaAs/i-InGaAsN/n-GaAs double hetero-junction solar cells (DHJSCs). The intrinsic InGaAsN absorption layer which is lattice-matched with GaAs substrates was grown by the metal-organic vapor phase epitaxy (MOVPE) system. The metal-organic sources used in the MOVPE growth induced unintentionally-doped carbon adatoms, and this in turn caused some carbon-related defects. This carbon pollution subsequently deteriorated the optical and electrical characteristics of the dilute nitride material. With ex-situ post-annealing being carried out in the furnace, it could not only modify the crystallinity of the InGaAsN material, but the deterioration in the performance of the dilute nitride solar cell due to the nitrogen incorporation could also be mitigated. After conducting the thermal annealing at 550°C for 1h in nitrogen ambience, the conversion efficiency of the DHJSCs was increased from 0.61% to 4.46% under Air Mass 1.5 Global (1000W/m2, AM1.5G) irradiance. The external quantum efficiency (EQE) spectrum further revealed that the efficiency of DHJSCs had been enhanced significantly owing to the crystalline improvement of the intrinsic InGaAsN absorption layer after the ex-situ post-thermal annealing was implemented. This improvement also led to a significant increase in the short-circuit current. The X-ray photoelectron spectroscopy (XPS) measurement result also showed that the ex-situ thermal annealing contributed to the improvement of crystal quality and the reduction of carbon related defects and clusters in the as-grown InGaAsN absorption layer. [Copyright &y& Elsevier]

Abstract: Dilute nitride InAsN epitaxial layers were produced using both liquid phase and molecular beam epitaxial growth techniques. The spectral features in the photoluminescence of samples containing up to 1% N with emission energies in the mid-infrared spectral region are described and compared. The emission intensities of both LPE and MBE grown materials were found to be comparable. Increasing the N content in the InAsN alloys resulted in a significant increase in the activation energy for thermal quenching of the photoluminescence emission due to a combination of lowering of the conduction band edge and Auger de-tuning. [Copyright &y& Elsevier]

Abstract: The photoconductivity of p-i-n GaInNAs/GaAs multiple quantum well (MQW) mesa structures is investigated. When illuminated with photons at energy greater than the GaAs bandgap, a number of oscillations are observed in the current–voltage I–V characteristics. The amplitude and position of the oscillations is shown to depend upon the temperature, as well as upon the exciting wavelength and intensity. Due to the absence of the oscillations in the dark I–V and at temperatures above T =200K, we explain them in terms of photogenerated electrons escaping from quantum wells via tunnelling or thermionic emission. Magnetic fields up to B =11T were applied parallel to the planes of the QWs. A small voltage shift in the position of the oscillations was observed, proportional to the magnetic field intensity and dependent upon the temperature. Calculation of the Landau level energy separation (16meV) agrees with the observed experimental data. Magneto-tunnelling spectroscopy probes in detail the nature of band- or impurity-like states responsible for resonances in first and second subbands, observing the I–V plot in dark condition and under illumination. The field-dependence of the amplitude of the oscillation peaks in I–V has the characteristic form of a quantum mechanical admixing effect. This enhancement is also probably due to the hole recombination with majority electrons tunnelling in the N-related states of the quantum wells. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

Abstract: The electrical and optical characteristics of a set of MBE-grown 1.3μm emitting GaInNAs/GaAs quantum well (QW) resonant cavity light-emitting diodes (RCLEDs) have been analyzed as a function of the growth temperature of the distributed Bragg reflector (DBR) and the contact alloying. The RCLEDs consist of a λ-long microcavity delimited on one side by an 8-pair AlAs/GaAs DBR and on the opposite side by a metallic mirror, with the QW located at one of the antinodes of the standing wave. The structures are designed for extraction of the light through the substrate. Room-temperature electroluminescence at 1.3μm is demonstrated with 38% In and 1.3% N in the quantum well. It is found that the RCLEDs with the DBR grown at the lowest temperature of 670°C show a lower differential slope efficiency and degraded current–voltage characteristics, i.e. higher ideality factor, lower shunt resistance and lower threshold voltage. This degradation can be explained to arise from the higher roughness observed in the DBRs grown at 670°C, which likely enhance the formation of point defects and compositional inhomogeneities in the quantum well. Alloying of the ohmic contacts also results in a widening of the electroluminescence emission, probably due to the degradation of the top metallic mirror sharpness caused by the diffusion of the metal into the GaAs. The best results in terms of differential slope efficiency and electrical characteristics are obtained when the DBRs are grown at 750°C and the contacts are left non-alloyed. [Copyright &y& Elsevier]

Abstract: This paper will describe the first-of-a-kind development and demonstration of dilute nitride strained layer superlattice detectors with detectivity as high as 4×1010 cmHz1/2/W and cut-off wavelength of 11-μm for an LWIR design and a cut-off wavelength of 22-μm for a VLWIR design. The developed dilute nitride SLS detectors are based on ultra-low leakage dilute nitride epitaxial layers and/or strained layer superlattices (SLS) of InAs/InAsSbN and InAs/GaInSbN that could enable high VLWIR detectivities at elevated temperatures and at low cost. [Copyright &y& Elsevier]

Abstract: The effect of atomic hydrogen irradiation during molecular beam epitaxy (MBE) on the electrical properties of GaInNAs thin films and the characteristics of homojunction GaInNAs solar cells are presented. We found that an optimum amount of atomic H flux irradiated during MBE growth of GaInNAs material is effective for suppressing or passivating the defects that act to mainly trap the electrons. Furthermore, an enhanced layer-by-layer growth improves the uniformity of GaInNAs layers thereby reducing the segregation and clustering of N atoms. A p–i–n Ga0.984In0.016N x As1− x solar cell fabricated by atomic H-assisted MBE shows a short-circuit current density of 22.2mA/cm2. [Copyright &y& Elsevier]

Low-bandgap GaInNAsSb single junction solar cells incorporating a planar Au back surface reflector for enhancing the photocurrent generation are reported. In particular, a 700 nm thick GaInNAsSb junction with a bandgap of 0.78 eV (corresponding to 6.2% N) incorporating the back reflector exhibited a short-circuit current density of 15.2 mA/cm2 for AM1.5D (1000 W/m2) illumination, when placed underneath a GaAs-filter mimicking the operation of a multijunction architecture. The corresponding external quantum efficiency represents the highest external quantum efficiency reported so far for dilute nitride solar cells with bandgaps below 0.8 eV. Electrical simulations reveal that the relatively high value of the quantum efficiency is attributed to a low p-type carrier concentration and partially to the varying doping level in the GaInNAsSb region. Moreover, the absorption coefficients for low-bandgap GaInNAsSb materials are estimated and used in a comprehensive optical analysis to understand the photon harvesting performance of the solar cells with reflector. The analysis reveals incomplete absorption in the GaInNAsSb layers for the double-pass configuration enabled by the planar back reflector pointing to the need for deploying more advanced structured reflectors. However, the structures exhibited high collection efficiencies laying the foundation for further improvements of the quantum efficiency values. Moreover, the low-bandgap dilute nitride solar cells show potential to meet the current-matching requirements in next-generation multijunction devices with five or more junctions, thus being a prospective alternative for replacing Ge as the bottom junction. • First demonstration of a low-bandgap (0.8 eV) GaInNAsSb solar junctions incorporating a back surface reflector. • Highest quantum efficiency reported for low-bandgap GaInNAsSb/GaAs-based solar cells. • First report on absorption coefficients for 0.8 eV GaInNAsSb materials. • Proving the potential of low-bandgap GaInNAsSb to achieve current matching in advanced solar cells with 5 or more junctions. [ABSTRACT FROM AUTHOR]

Abstract: Ultra-high-resolution infra-red local vibrational mode (IR LVM) spectroscopy measurements together with density-functional calculations have been used to identify the signatures of close substitutional nitrogen () pairs in alloys with concentrations of . We show that the presence of sub-peaks close to the absorption band can be attributed to nitrogen pairs up to fourth neighbor position. Additionally, we suggest that the nitrogen pairs which give rise to the deepest levels below the conduction band edge are the first to be removed upon annealing. [Copyright &y& Elsevier]

Abstract: Highly mismatched alloys (HMAs) are dilute alloys whose fundamental properties are dramatically modified through the substitution of a relatively small fraction (up to ∼5%) of host atoms with a very different element. Specific examples are dilute III–V nitrides (e.g. GaN x As1−x ) and II–VI oxides (e.g. ZnO x Te1−x ) in which the conduction bands are strongly modified by the anticrossing interaction between localized states of O or N and the extended states of the semiconductor matrix. Using a highly non-equilibrium method: the combination of ion implantation and pulsed laser melting (II-PLM) we have synthesized a wide variety of III-N x -V1−x and II-O x -VI1−x alloys with N or O content up to 2%. The structural and optical properties of these HMAs have been systematically investigated. In particular, we demonstrated that the Zn1−y Mn y O x Te1−y dilute oxide is a multiband alloy that can be used for the realization of the single junction, high efficiency intermediate band solar cells. [Copyright &y& Elsevier]

Abstract: GaIn(N)As/GaAs and GaIn(N)As/GaNAs/GaAs quantum well samples, with and without GaNAs strain-compensating layers (SCL), were grown on GaAs (001) substrates by molecular beam epitaxy. Photoluminescence (PL) was used to study the effects of the GaNAs SCL on the properties of the Ga(In)NAs QWs upon annealing. It is found that an extra blue-shift (ΔE =5.2 meV) in the PL emission from the GaInNAs QW sample at the initial annealing stage (t ann =40 s) was induced by the GaNAs SCLs. However, for the GaInAs QW sample, the blue-shift induced by GaNAs SCL is only 1.1 meV. As the annealing time was increased, the blue-shift of both GaInNAs and GaInAs QWs showed saturations at 16 meV and 8 meV, respectively. The PL blue-shifts were much enhanced by inserting GaNAs SCLs showing a non-saturable behavior. X-ray diffractions from the strain compensated GaIn(N)As QWs before and after annealing show no N atom diffusion but Ga/In atom interdiffusion across the QW interfaces. The Ga/In atom interdiffusion caused by annealing was also confirmed by high-resolution transmission electron microscopy from the GaInNAs/GaAs QW sample. [Copyright &y& Elsevier]

Abstract: GaN x As1−x multiple-quantum-well (MQW) samples (x ≤0.025) grown by molecular beam epitaxy have been studied by photoluminescence (PL) spectroscopy, high-resolution X-ray diffraction (HR-XRD), secondary-ion mass spectrometry (SIMS) and ultra-high-resolution infrared local-vibrational-mode (IR LVM) spectroscopy in order to determine their compositional and structural properties. Compositional data from PL spectroscopy and HR-XRD, derived using simple models of the superlattice structure, show good agreement. SIMS depth profiles show that the wells are triangular, and are wider and shallower than predicted, with nitrogen present in the barrier layers. A comparison between SIMS and HR-XRD suggests the presence of interstitial nitrogen for samples with the highest nominal concentrations. By contrast, IR LVM measurements demonstrate that substitutional nitrogen incorporates linearly with increasing nitrogen fraction. [Copyright &y& Elsevier]

Abstract: We present a theoretical study of the electronic structure and optical properties of zinc-blende GaSb1− x N x semiconducting dilute alloys for small amounts of N (0⩽x⩽0.05). Our calculations are based on the pseudo-potential scheme within the virtual crystal approximation that takes into consideration the effects of compositional disorder. In agreement with previous studies, it is found that the introduction of only a few percent of N into GaSb reduces substantially the energy band gaps. It is shown that GaSb1− x N x can be a direct or an indirect semiconductor depending on the N content. Negative band gaps in the material of interest are predicted to occur when the N concentration exceeds 0.026. The information derived from the present study shows that GaSb1− x N x (0⩽x⩽0.017) should enable access to wavelengths in the 2–4μm range. [Copyright &y& Elsevier]

Abstract: Recently, the growth of patterned surfaces is being used to demonstrate the site control of the three-dimensional nanostructures, and in particular quantum dots. Nevertheless the pre-patterning techniques show some disadvantages. In this work, we report a novel in situ hole-patterning technique which consists of growing by molecular beam epitaxy a dilute nitride GaAsN layer on 1° and 2° towards [2¯11] misoriented GaAs(111)B substrates. Later, we carry out a regrowth of GaAs layers on this patterned surfaces in order to improve the surface quality and the homogeneity of the characteristics of holes (size, depth, etc.). Consecutively, we use these patterned surfaces to grow InAs quantum dots, whose growth on these misorientations results in a greater difficulty. A structural characterization of the resulting samples, both hole-patterns and quantum dots, has been performed. Besides, we have realized studies of the dependence of the surface morphology on some important parameters (including substrate misorientation, thicknesses of the GaAsN and GaAs layers grown and growth conditions). [Copyright &y& Elsevier]

Abstract: Contactless electroreflectance (CER) spectroscopy has been applied to investigate the optical transitions in Ga(In)NAs/GaAs quantum well (QW) structures containing Sb atoms. The identification of the optical transitions has been carried out in accordance with theoretical calculations which have been performed within the framework of the effective mass approximation. Using this method, the bandgap discontinuity for GaN0.027As0.863Sb0.11/GaAs, Ga0.62In0.38As0.954N0.026Sb0.02/GaAs, and Ga0.61In0.39As0.963N0.017Sb0.02/GaN0.027As0.973/GaAs QW structures has been determined. It has been found that the conduction-band offset is ∼50 and ∼80% for GaN0.027As0.863Sb0.11/GaAs and Ga0.62In0.38As0.954N0.026Sb0.02/GaAs QWs, respectively. It corresponds to 264 and 296meV depth QW for electrons and heavy-holes in GaN0.027As0.863Sb0.11/GaAs QW; and 520 and 146meV depth QW for electrons and heavy-holes in Ga0.62In0.38As0.954N0.026Sb0.02/GaAs QW. In the case of the Ga0.61In0.39As0.963N0.017Sb0.02/GaN0.027As0.973/GaAs step-like QW structure it has been shown that the depth of electron and heavy-hole Ga0.61In0.39As0.963N0.017Sb0.02/GaN0.027As0.973 QW is ∼144 and ∼127meV, respectively. [Copyright &y& Elsevier]

Abstract: The pressure dependence of optical transitions in Ga0.64In0.36As/GaAs and Ga0.64In0.36N0.01As0.99/GaAs single quantum well (SQW) structures were studied in photoreflectance (PR) spectroscopy. In order to apply high hydrostatic pressure, up to ∼11kbar, the liquid-filled clamp-pressure cell with a sapphire window for optical access has been adopted in the PR set-up with the so called ‘bright configuration’. It has been found that the linear hydrostatic pressure coefficient for the ground state transition are equal to 8.6 and 7.3meV/kbar for the GaInAs/GaAs and GaInNAs/GaAs SQWs, respectively. This result shows that the incorporation of only 1% of N atoms into GaInAs/GaAs leads to ∼15% decrease in the pressure coefficient. In addition, a non-linearity in the pressure dependence of the ground state transition has been resolved for the GaInNAs/GaAs SQW. [Copyright &y& Elsevier]

Abstract: Raman scattering of coherently strained GaN x As1−x alloys grown on GaAs (001) substrates by molecular beam epitaxy has been investigated in the temperature range of 80–150 K. The intensity of GaAs-like longitudinal optical (LOGaAs) phonon and nitrogen-localized vibration mode (NLVM) decrease with temperature and the line width of both these modes show substantial broadening at higher temperatures. Temperature dependence of phonon line width and peak frequency of LOGaAs modes were analyzed in terms of anharmonic damping effect induced by thermal and compositional disorder. It is found that the anharmonicity is higher for GaNAs than that for GaAs, and increases with N concentration in GaNAs alloys. Both thermal- and disorder-induced anharmonicities can change the line width, line frequency, and lifetime of LOGaAs mode in GaNAs alloys. These results obtained from Raman investigation would lead to a better understanding of the anharmonic effects in dilute nitrides. [Copyright &y& Elsevier]

Abstract: Using the pseudopotential method within the virtual crystal approximation combined with the Harrison bond-orbital model, we have studied the effect of nitrogen content x (0< x ≤0.05) on mechanical properties of GaAs1−x N x dilute alloys in zinc-blende structure. The agreement between our results and the available experimental data is found to be satisfactory. We have found that except for the anisotropy factor and the [100] Poisson ratio, the incorporation of only a few percentage of nitrogen affects the mechanical properties being studied here. In addition, the effect of compositional disorder on the calculated parameters has been examined and found to have an important effect on most studied quantities. [Copyright &y& Elsevier]

Abstract: We present time-resolved photoluminescence (PL) results of a series of samples with x up to 0.01. The temperature dependence, the concentration dependence as well as the temporal behavior indicate that the PL is dominated by excitation transfer processes between the extended band states and localized N-related states (such as the isolated N-impurity, various N-pair states and higher N-clusters) as well as by excitation transfer between the various localized N-related states themselves. The excitation transfer processes in conjunction with the concentration-dependent statistics of the various N-related states alone are sufficient to explain the observed red-shift of the luminescence of with increasing x as well as the spectral dependence of the PL decay times. However, this implies that the PL data alone do not give any conclusive evidence that a hard transformation from an indirect to a direct-gap semiconductor takes place in Ga(N,P) with increasing N up to 2% as often stated. [Copyright &y& Elsevier]

Abstract: (Ga,In)(N,As) lattice matched to GaAs with a band gap of 1eV is employed as active material in high-efficiency III–V solar cells. Te-doped Ga0.934In0.066N0.023As0.977 layers were grown by metal-organic vapor-phase epitaxy on (100) GaAs. The samples were highly doped n-type with carrier concentrations ranging from about 1017–1019 cm−3. Pieces of the samples were hydrogenated with H-doses of 1018 ion/cm2. The optical and electrical properties of the samples before and after hydrogenation were studied by low-temperature photoluminescence and magnetotransport. In undoped samples hydrogen is known to form N–H complexes which strongly reduce the local perturbation of the lattice due to nitrogen and thus reverse the N-induced global changes of the band structure. Combined analysis of photoluminescence and transport measurements on Te-doped samples, however, indicates a competition between N–H formation and passivation of the Te donor favoring the latter. Hardly any band structure changes due to hydrogenation are observed in these Te-doped samples, instead a strong reduction of the free-carrier concentration is observed after hydrogenation. [Copyright &y& Elsevier]

Abstract: We provide a brief review of our recent results from optically detected magnetic resonance studies of grown-in non-radiative defects in two most important dilute nitride systems—Ga(In)NAs grown on GaAs substrates and Ga(Al,In)NP grown on Si and GaP substrates. These results have led to the identification of defect complexes in the alloys, involving intrinsic defects such as AsGa antisites and Gai self-interstitials. They have also shed light on formation mechanisms of the defects and on their role in non-radiative carrier recombination that is harmful to the performance of potential optoelectronic and photonic devices based on these dilute nitrides. [Copyright &y& Elsevier]

Abstract: Three grown-in defects acting as centers of non-radiative recombination (NR) were detected in GaInNP alloys grown on a GaAs substrate using the optically detected magnetic resonance (ODMR) technique. Among them, one was proposed to be either a Gai-related defect or an AsGa-related defect, from the resolved four-line hyperfine structure. The former model was concluded to be more favorable by weighing physical properties of the two defects, e.g. the likelihood for their presence in the studied structures, their spatial location, g-value and effect of rapid thermal annealing (RTA). RTA at 700°C was shown to reduce concentrations of the studied defects but it introduced a new defect that likely directly participates in the monitored radiative recombination process in the RTA-treated samples. [Copyright &y& Elsevier]

Dilute nitrides are promising alloys in view of extending potential micro- and opto-electronics applications of GaAs technology. Orientation effects on nitrogen incorporation in GaAs have been scarcely addressed. Here, GaAsN on (1 0 0) and on As(B)- and Ga(A)-rich (1 1 1) substrates was grown by molecular beam epitaxy at different substrate temperatures. Nitrogen content measured by secondary ion mass spectrometry as a function of the growth temperature highlights the influence of orientation on nitrogen incorporation. Furthermore, thermal annealing is shown to improve the optical quality of GaAsN quantum wells whatever their substrate orientations. [Copyright &y& Elsevier]

We use an sp3s&ast; tight-binding (TB) Hamiltonian to investigate the band anti-crossing model for GaNxAs1−x alloys. This describes the strong band-gap bowing at low N composition x in terms of an interaction between the conduction band edge (E−) and a higher-lying band of localized resonant states (E+). We demonstrate that E− can be described very accurately by the BAC model, in which we treat the nitrogen levels explicitly using a linear combination of isolated nitrogen resonant states (LCINS). We consider several different random supercell structures and show that there is excellent correlation between the results of our BAC and full TB calculations for both the GaNxAs1−x conduction band edge energy and its Γ1c character. We also show that there is excellent replication of the distribution of Γ1c character across the higher conduction band states. Finally, we use the LCINS results to identify E+ in the full TB calculations, showing that at low N composition E+ forms a sharp resonance in the conduction band Γ-related density of states, which broadens rapidly at higher N composition when the E+ energy becomes degenerate with the L-related density of states. [Copyright &y& Elsevier]

The scattering of n-type carriers by three types of nitrogen-associated states (single substitutional N atoms, N–N pairs, and hydrogen passivated N defects) is investigated for the dilute nitride alloy GaNxAs1−x. The connection between the band gap bowing and carrier mobility is derived as a function of temperature and nitrogen concentration. The scattering by substitutional N atoms has been shown to be proportional to the square of the derivative of the band-edge energy with respect to N concentration. In unpassivated samples, the presence of N–N pairs is found to modify the electron mobility by no more than 10%, compared to a model in which scattering by all N atoms is treated independently, in the relevant range of temperatures and N concentration. The temperature dependence of the GaAs conduction band edge gives rise, in conjunction with resonant scattering from weakly temperature-dependent N levels, to an unusually small temperature variation in the defect contribution to carrier mobility. A clear signature of N passivation is predicted to be a simultaneous increase of the band gap and of the carrier mobility. [Copyright &y& Elsevier]

We use a combination of magneto-tunnelling and photoluminescence spectroscopy techniques to explore the admixing of the extended GaAs conduction band states with the localised N-impurity states in dilute GaAs1−yNy quantum wells (QWs) incorporated in resonant tunnelling diodes. When y is increased from 0% to 0.08%, the current resonance due to electrons tunnelling through the lowest quasi-bound state of the QW splits into two main features due to electron tunnelling into the N-induced E− and E+ hybridised subbands of the GaAs1−yNy layer. These subbands have a very well-defined character even for a small N-content (∼0.08%) and have non-parabolic energy–momentum dispersions. A further increase of y smears out the resonances and quenches the current due to electron trapping on strongly localised N-levels. [Copyright &y& Elsevier]