Radiation degradation characteristics of component subcells in inverted metamorphic triple‐junction solar cells irradiated with electrons and protons

The radiation response of In_0.5Ga_0.5P, GaAs, In_0.2Ga_0.8As, and In_0.3Ga_0.7As single‐junction solar cells, whose materials are also used as component subcells of inverted metamorphic triple‐junction (IMM3J) solar cells, was investigated. All four types of cells were prepared using a simple device layout and irradiated with high‐energy electrons and protons. The essential solar cell characteristics, namely, light‐illuminated current–voltage (LIV), dark current–voltage (DIV), external quantum efficiency (EQE), and two‐dimensional photoluminescence (2D‐PL) imaging were obtained before and after irradiation, and the corresponding changes due to the irradiations were compared and analyzed. The degradation of the cell output parameters by electrons and protons were plotted as a function of the displacement damage dose. It was found that the radiation resistance of the two InGaAs cells is approximately equivalent to that of the InGaP and GaAs cells from the materials standpoint, which is a result of different initial material qualities. However, the InGaAs cells show relatively low radiation resistance to electrons especially for the short‐circuit current (I sc). By comparing the degradation of I sc and EQE, data, It was confirmed that the greater decrease of minority‐carrier diffusion length in InGaAs compared with InGaP and GaAs causes severe degradation in the photo‐generation current of the InGaAs bottom subcells in IMM3J structures. Additionally, it was found that the InGaP and two InGaAs cells exhibited equivalent radiation resistance of V oc, but radiation response mechanisms of V oc are thought to be different. Further analytical studies are necessary to interpret the observed radiation response of the cells. © 2016 The Authors. Progress in Photovoltaics: Research and Applications published by John Wiley & Sons Ltd.     

Interface trap and interface depletion in lattice-mismatched GaInAs/GaAs heterostructures

The effects of lattice mismatch on the deep traps and interface depletion have been studied for the Ga_0.92In_0.08As(p⁺)/GaAs(N) and Ga_0.92In_0.08As(n)/GaAs(SI) heterostructures grown by molecular beam epitaxy. We have used deep level transient spectroscopy (DLTS) and admittance spectroscopy (AS) and observed two hole traps, one at an energy ranging from 0.1 to 0.4 eV and the other at 0.64 eV, and two electron traps at 0.49 and 0.83 eV in the GalnAs/GaAsp ⁺-N junction sample. The hole trap appeared as a broad peak in the DLTS data and its energy distribution (0.1 ∼ 0.4 eV) was obtained by a simulation fitting of the peak. Concentration of this distributed hole trap increased as the in-plane mismatch increased, suggesting its relation to defects induced by lattice relaxation, whereas the other traps are from the bulk. The misfit dislocations are believed to be responsible for the interface trap. For the Ga_0.92In_0.08As(n)/GaAs(SI) samples, Hall effect measurements showed an increased interface depletion width of about 0.14 Μm for the 0.5 Μm thick layer and about 0.12 /gmm for the 0.25 Μm thick layer, while a corresponding GaAs/GaAs sample had only 0.088 Μm for the interface depletion width.     

Quantitative and Depth-Resolved Investigation of Deep-Level Defects in InGaN/GaN Heterostructures

Deep-level defects in In_0.17Ga_0.83N/In_0.02Ga_0.98N/p-GaN:Mg heterostructures were studied using deep-level optical spectroscopy (DLOS). Depth-resolved DLOS was achieved by exploiting the polarization-induced electric fields to discriminate among defects located in the In_0.17Ga_0.83N and the In_0.02Ga_0.98N regions. Growth conditions for the In _x Ga_1−x N layers were nominally the same as those in InGaN/GaN multi-quantum-well (MQW) structures, so the defect states reported here are expected to be active in MQW regions. Thus, this work provides important insight into defects that are likely to influence MQW radiative efficiency. In_0.17Ga_0.83N-related bandgap states were observed at E _v + 1.60 eV and E _v + 2.59 eV, where E _v is the valence-band maximum, compared with levels at E _v + 1.85 eV, E _v + 2.51 eV, and E _v + 3.30 eV in the In_0.02Ga_0.98N region. A lighted capacitance–voltage technique was used to determine the areal density of deep states. The possible origins of the associated defects are considered along with their potential roles in light-emitting diodes.     

Analysis of temperature coefficients for III–V multi‐junction concentrator cells

The temperature dependence of the I–V parameters of different III–V multi‐junction concentrator cells at several concentration levels was investigated. Moreover, the influence of spectral changes on the temperature coefficients of multi‐junction solar cells was examined. Complete sets of temperature coefficients of a metamorphic Ga_0.35In_0.65P/Ga_0.83In_0.17As dual‐junction cell, a metamorphic Ga_0.35In_0.65P/Ga_0.83In_0.17As/Ge triple‐junction cell and a lattice‐matched Ga_0.50In_0.50P/Ga_0.99In_0.01As/Ge triple‐junction cell determined under well‐controlled laboratory conditions are reported. Copyright © 2012 John Wiley & Sons, Ltd.     

Radiation resistance of MBE-grown GaInP/GaAs-based solar cells

Proton irradiation-based degradation characteristics for molecular beam epitaxy (MBE) grown Ga_0·51In_0·49P/GaAs single-junction tandem solar cells of n/p configuration are reported. The cells were irradiated with 3-MeV protons up to fluences of 10¹³ cm⁻². The cells were characterized with current–voltage (I–V) measurements at AMO conditions, and with spectral measurements. The damage coefficient for the GaAs cells was calculated using numerical modelling by the PC-1D program, and the result was compared with the InP damage coefficient. By using the ‘displacement damage dose’ approach, the degradation characteristics were compared with the published data for InP and GaAs/Ge solar cells. In addition, these MBE results were compared with the radiation behavior of metal-organic chemical vapor deposition (MOCVD)-grown Ga_0·51In_0·49P/GaAs single-, and double-junction solar cells of p/n configuration. © 1998 John Wiley & Sons, Ltd.     

Ohmic contacts ton-type In0.5Ga0.5P

The contact properties of alloyed Ni/Au-Ge/Mo/Au metallization to npoststagger+In_0.5Ga_0.5P epilayers grown by gas-source molecular beam epitaxy on GaAs substrates are reported. A minimum specific contact resistance of 10⁻⁵ Ωcm² was obtained forn = 2 × 10¹⁹ cm⁻³ material after alloying at 360° C for 20 sec. Above this temperature outdiffusion of lattice elements and reactions of the metallization with the In_0.5Ga_0.5P lead to severe morphological changes and degraded contact properties. From the temperature dependence of the contact resistance, thermionic emission was identified as the predominant current transport mechanism in these contacts.     

The effect of selenium doping on the optical and structural properties of Ga0.5ln0.5P

Selenium doping at an electron concentration of 10¹⁸ – 10¹⁹ cm⁻³ is shown to cause an increase in both the band gap and the disorder of Ga_0.5In_0.5P films grown by metalorganic chemical vapor deposition on GaAs substrates. The effect of selenium is shown to be very similar to that of the p-type dopants, zinc and magnesium. Selenium doping is also shown to have a dramatic smoothing effect on the surface morphology of Ga_0.5In_0.5P films.     

Vapor-grown In1−xGaxP electroluminescent junctions on GaAs

In_1?xGa_xP vapor-grown electroluminescent junctions have been deposited directly onto GaAs substrates. For these layers, an alloy composition within a few mole percent of the lattice-matching composition of 51.5 mole percent GaP has been found to be essential for high luminous efficiencies and for the avoidance of microcracks throughout the epitaxial layer. For In_1?xGa_xP alloys near this composition, the electroluminescence characteristics of the diodes have been found to be excellent, with room-temperature external quantum efficiencies as high as 0.2% attained for red emission near 6600 Å. The properties of In_.5Ga_.5P junction structures deposited directly onto GaAs ar? compared with those of In_1?xGa_xP layers previously prepared on GaP substrates.     

Lateral association of vertically coupled quantum dots

The modification produced in the structural and optical properties of vertically coupled In_0.5Ga_0.5As quantum dots in a GaAs matrix by increasing the number of deposited layers of quantum dots has been investigated. It was shown that the deposition of a sequence of In_0.5Ga_0.5As quantum-dot planes separated by narrow (of the order of the height of the quantum dots) GaAs layers gives rise to an interaction between neighboring vertically coupled quantum dots. This interaction shifts the photoluminescence line due to the recombination of nonequilibrium carriers via states of the quantum dots into the region of lower photon energies. Fiz. Tekh. Poluprovodn. 31, 851–854 (July 1997)     

Electrical characterization of self-assembled In0.5Ga0.5As/GaAs quantum dots by deep level transient spectroscopy

We have investigated electron emission from self-assembled In_0.5Ga_0.5As/GaAs quantum dots (QDs) grown by molecular-beam epitaxy (MBE). Through detailed deep level transient spectroscopy comparisons between the QD sample and a reference sample, we determine that trap D, with an activation energy of 100 meV and an apparent capture cross section of 5.4×10⁻¹⁸ cm², is associated with an electron quantum level in the In_0.5Ga_0.5As/GaAs QDs. The other deep levels observed, M1, M3, M4, and M6, are common to GaAs grown by MBE.     

Passivation of carbon acceptors during growth of carbon-doped GaAs,InGaAs, and HBTs by MOCVD

Carbon dopedp-type GaAs and In_0.53Ga_0.47As epitaxial layers have been grown by low-pressure metalorganic chemical vapor deposition using CC1₄ as the carbon source. Low-temperature post-growth annealing resulted in a significant increase in the hole concentration for both GaAs and In_0.53Ga_0.47As, especially at high doping levels. The most heavily doped GaAs sample had a hole concentration of 3.6 × 10²⁰ cm⁻³ after a 5 minute anneal at ≈400° C in N₂, while the hole concentration in In_0.53Ga_0.47As reached 1.6 × 10¹⁹ cm⁻³ after annealing. This annealing behavior is attributed to hydrogen passivation of carbon acceptors. Post-growth cool-down in an AsH₃/H₂ ambient was found to be the most important factor affecting the degree of passivation for single, uncapped GaAs layers. No evidence of passivation is observed in the base region of InGaP/GaAs HBTs grown at ≈625° C. The effect ofn-type cap layers and cool-down sequence on passivation of C-doped InGaAs grown at ≈525° C shows that hydrogen can come from AsH₃, PH₃, or H₂, and can be incorporated during growth and during the post-growth cool-down. In the case of InP/InGaAs HBTs, significant passivation was found to occur in the C-doped base region.     

The effect of temperature on the growth and properties of green light-emitting In0.5Ga0.5N films prepared by reactive sputtering with single cermet target

We have grown In_0.5Ga_0.5N films on SiO₂/Si (100) substrate at 100–400 °C for 90 min by rf reactive sputtering with single cermet target. The target was made by hot pressing the mixture of metallic indium, gallium and ceramic gallium nitride powder. X-ray diffraction (XRD) measurements indicated that In_0.5Ga_0.5N films had wurtzite structure and showed the preferential (1 0 -1 0) diffraction. Both SEM and AFM showed that In_0.5Ga_0.5N films were smooth and had small roughness of 0.6 nm. Optical properties were measured by photoluminescence (PL) spectra from room temperature to low temperature of 20 K. The 2.28 eV green emission was achieved at room temperature for all our InGaN films. The electrical properties of In_0.5Ga_0.5N films on a SiO₂/Si (100) substrate were measured by the Hall measurement at room temperature. InGaN films showed the electron concentration of 1.51×10²⁰–1.90×10²⁰ cm⁻³ and mobility of 5.94–10.5 cm² V⁻¹ s⁻¹. Alloying of InN and GaN was confirmed for the sputtered InGaN.     

Growth and characterization of lattice-matched epitaxial films of GaxIn1−xAs/InP by liquid-phase epitaxy

We determined the conditions for successful lattice-matched growth by liquid-phase epitaxy near T = 620‡ C of Ga_XIn_1−XAs on [111B] InP substrates. We have used the results of the growth of both lattice-matched and intentionally lattice-mismatched epitaxial layers, (0.4 ≤ X ≤ 0.7) to calculate a phase diagram which gives the correct liquidus temperature, (T_L ± 1‡ C), and the correct solid composition, (± 5 % of the nominal composition), for the entire range of growth solutions considered for this important ternary semi-conductor system. The parameters appropriate to this calculation are significantly different from those used to describe the growth of Ga_XIn_1−XAs on GaAs. The results of this calculation play an important part in the better understanding of the quaternary alloy Ga_XIn_1−XAs_yP_1−y. Our measurements show that the ternary alloy lattice-matched to InP is Ga_0.47In_0.53As, semiconductor with a direct band gap about 0.75 eV at room temperature. We have grown p-n junction homostructures and double-heterostructures on InP substrates. These wafers have been used to make detectors in the 1.0 – 1.7/um range of the optical spectrum.     

Dislocation-Induced deep level states in In0.08Ga0.92As/GaAs heterostructures

We have performed luminescence experiments on In_0.08Ga_0.92As/GaAs heterointerfaces to explore the energy distribution of deep level states in the bandgap for two cases: (1) unrelaxed, pseudomorphic In_0.08Ga_0.92As films (200Å thick), which have few if any dislocations at the interface, and (2) partially relaxed In_0.08Ga_0.92As films (1000Å thick) which are expected to have a substantial interfacial dislocation density. A combined photoluminescence and cathodoluminescence technique is used which allows us to profile the sample luminescence through the buried interface region. Our results show the existence of deep level luminescent features characteristic of the GaAs substrate and features common to In_0.08Ga_0.92As and GaAs, as well as the existence of a deep level feature near 1 eV photon energy which undergoes a shift in energy depending upon the degree of strain relaxation in the In_0.08Ga_0.92As film. In addition, a deep level feature near 0.83 eV becomes prominent only in In_0.08Ga_0.92As films which have relaxed, and thus contain misfit dislocations at the interface. These deep level differences may be due to bandgap states associated with the intrinsic dislocation structure, impurities segregated at the dislocation, or bulk point defects, or threading dislocations generated during the strain relaxation. Previous work has determined that a deep level state 0.7 eV above the valence band edge would account for the electrical behavior of relaxed In_0.08Ga_0.92As/GaAs interfaces, which is in good agreement with the range of deep level transitions near 0.8 eV photon energy which we observe. These measurements suggest that photo- and cathodoluminescence measurements of deep level emission in these III-V semiconductors can provide a useful indicator of electrically active defect densities associated with misfit dislocations.     

MOVPE grown Ga1−xInxAs solar cells for GaInP/GaInAs tandem applications

Lattice-mismatched Ga_1−xIn_xAs solar cells with an absorption edge between 900 and 1150 nm have been grown on GaAs substrates. Different graded Ga_1−xIn_xAs buffer layers and solar cell structures were evaluated to achieve a good electrical performance of the device. External quantum efficiencies comparable to our best GaAs solar cells were measured. The best 1 cm² cell with a bandgap energy of 1.18 eV has an efficiency of 22.6% at AM1.5g and a short circuit current density of 36.4 mA/cm². To our knowledge, this is the highest reported efficiency for a Ga_0.83In_0.17As solar cell.     

A first-principles study of the structural and electronic properties of III–V/thermal oxide interfaces

A theoretical study of the structural and electronic properties of the interfaces between a set of III–V compound semiconductors of technological interest and their native oxides is reported. First-principles techniques have been applied to model the reaction of oxidation of the GaAs(0 0 1)–β2(2 × 4) surface and to generate a set of representative models of the atomic structure of a thermally grown GaAs/native oxide interface. The obtained models have been extended to the InAs/ and In_0.5Ga_0.5As/native oxide interfaces case. The impact of indium on both the structural changes occurring during the oxidation of the substrate and the resulting electronic properties has been quantified.     

Degradation study of III–V solar cells for concentrator applications

AlGaAs/GaAs heteroface solar cells with a high aluminium content tend to degrade. The degradation mechanism has been examined and appropriate accelerated ageing procedures have been established. They effectively test the ruggedness of the device against oxidation. Changing the window layer material to (Al_xGa_1−x)_0.51In_0.49P with x = 0, 0.5 or 1 leads to stable devices. In addition, III–V tandem solar cells for concentrator applications were subjected to accelerated ageing tests. They proved to be robust against oxidation. The potential degradation due to the high current density involved in concentrator solar cells was assessed in preliminary experiments. Copyright © 2005 John Wiley & Sons, Ltd.     

Phase formation under the effect of spinodal decomposition in epitaxial alloys of Ga x In1 − x P/GaAs(100) heterostructures

The effect of instability of alloys of Ga _x In_{1 − x} P/GaAs(100) semiconductor epitaxial heterostructures in the composition region x ≈ 0.50 is studied by X-ray diffraction and electron microscopy. The possibility of emergence of modulated relaxation structures on the surface of a Ga _x In_{1 − x} P alloy is shown. This phenomenon is accompanied by the emergence of satellites of main X-ray reflections corresponding to a single-phase structure.     

Fundamental absorption edge of semiconductor alloys with the direct-gap energy-band structure

A procedure convenient for practical application to the calculations of the fundamental absorption spectra of semiconductor alloys with the direct-gap energy-band structure is developed. The procedure is based on the knowledge of the fundamental absorption spectra of binary constituents of the alloy, takes into account the nonparabolic structure of the conduction band, and involves only one adjustable parameter that characterizes the inhomogeneous broadening of the spectra. The procedure is tested by the examples of the best-studied and practically most important Al _x Ga_{1 − x} As alloys, the (Al _x Ga_{1 − x} )_0.5In_0.5P alloys isostructural to GaAs, and the Ga _x In_{1 − x} As alloys isostructural to InP. The procedure can be used in the case of other III–V and II–VI compounds. The results make it possible to calculate the intrinsic luminescence spectra of the alloys.     

Drift velocity of electrons in quantum wells of selectively doped In0.5Ga0.5As/AlxIn1 ? xAs and In0.2Ga0.8As/AlxGa1 ? xAs heterostructures in high electric fields

The field dependence of drift velocity of electrons in quantum wells of selectively doped In_0.5Ga_0.5As/Al _x In_{1 − x} As and In_0.2Ga_0.8As/Al _x Ga_{1 − x} As heterostructures is calculated by the Monte Carlo method. The influence of varying the molar fraction of Al in the composition of the Al _x Ga_{1 − x} As and Al _x In_{1 − x} As barriers of the quantum well on the mobility and drift velocity of electrons in high electric fields is studied. It is shown that the electron mobility rises as the fraction x of Al in the barrier composition is decreased. The maximum mobility in the In_0.5Ga_0.5As/In_0.8Al_0.2As quantum wells exceeds the mobility in a bulk material by a factor of 3. An increase in fraction x of Al in the barrier leads to an increase in the threshold field E _th of intervalley transfer (the Gunn effect). The threshold field is E _th = 16 kV/cm in the In_0.5Ga_0.5As/Al_0.5In_0.5As heterostructures and E _th = 10 kV/cm in the In_0.2Ga_0.8As/Al_0.3Ga_0.7As heterostructures. In the heterostructures with the lowest electron mobility, E _th = 2–3 kV/cm, which is lower than E _th = 4 kV/cm in bulk InGaAs.