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Hybrid organic–inorganic halide perovskites have sub-band-gap absorption and photocurrent spectra which are difficult to explain using existing models, particularly in light of the small Urbach ene...

The previously developed bistable amphoteric native defect (BAND) model is used for a comprehensive explanation of the unique photophysical properties and for understanding the remarkable performance of perovskites as photovoltaic materials. It is shown that the amphoteric defects in donor (acceptor) configuration capture a fraction of photoexcited electrons (holes) dividing them into two groups: higher energy bright and lower energy dark electrons (holes). The spatial separation of the dark electrons and the dark holes and the k-space separation of the bright and the dark charge carriers reduce electron hole recombination rates, emulating the properties of an ideal photovoltaic material with a balanced, spatially separated transport of electrons and holes. The BAND model also offers a straightforward explanation for the exceptional insensitivity of the photovoltaic performance of polycrystalline perovskite films to structural and optical inhomogeneities. The blue-shifted radiative recombination of bright electrons and holes results in a large anti-Stokes effect that provides a quantitative explanation for the spectral dependence of the laser cooling effect measured in perovskite platelets. National Research Foundation (NRF) Accepted version This work (JWA, WW) was supported by the Singapore National Research Foundation through the Intra-CREATE Collaborative Grant NRF2018-ITC001-001. B. W. acknowledges the support from the National Natural Science Foundation of China (NFSC) (grant No. 51802331), Science and Technology Program of Guangzhou (No. 2019050001). T. C. S. acknowledges the support of Nanyang Technological University under its internal grant (M4082480); and the National Research Foundation (NRF) Singapore under its Competitive Research Program (NRF-CRP14-2014-03) and its NRF Investigatorship (NRF-NRFI-2018-04).

Properties of semiconductors are largely defined by crystal imperfections including native defects. Van der Waals (vdW) semiconductors, a newly emerged class of materials, are no exception: defects exist even in the purest materials and strongly affect their electrical, optical, magnetic, catalytic and sensing properties. However, unlike conventional semiconductors where energy levels of defects are well documented, they are experimentally unknown in even the best studied vdW semiconductors, impeding the understanding and utilization of these materials. Here, we directly evaluate deep levels and their chemical trends in the bandgap of MoS2, WS2 and their alloys by transient spectroscopic study. One of the deep levels is found to follow the conduction band minimum of each host, attributed to the native sulfur vacancy. A switchable, DX center - like deep level has also been identified, whose energy lines up instead on a fixed level across different hosts, explaining a persistent photoconductivity above 400 K., The optical and electronic properties of semiconducting transition metal dichalcogenides are strongly dependent on native defects within their crystal lattice. Here, the authors use transient spectroscopy to identify defect-related deep levels in the bandgap of MoS2, WS2 and their alloys.

We have grown Zn1−xCdxO (ZnCdO) thin films by molecular beam epitaxy on α-Al2O3 (0001) substrates across the full composition range of 0 ≤ x ≤ 1. The wurtzite phase is observed in the composition x ≤ 0.44 whereas the rocksalt phase is observed for x ≥ 0.58. Within the composition region of 0.44 3.0 eV. A large optical gap of 3.0 eV (an intrinsic gap of ∼2.7 eV) is achieved for RS-ZnCdO alloys at x ∼ 0.6. The stability of the RS phase with a low Cd content can be attributed to the low temperature MBE process. The upward shift of the band gap energy is observed in the mixed phase region due to the phase transition, and the largest band gap energy is obtained as 3.27 eV in the WZ (x = 0) and 3.0 eV in RS (x = 0.6) phase. RS-ZnCdO thin films exhibit a low resistivity of 5 × 10−4 Ω cm with a maximum mobility of ∼90 cm2 V−1 s−1 and a high carrier concentration of 4 × 1020 cm−3.

As-grown Ar-deposited Cd1−xVxO and Ar/O2-deposited Cd1−yVyO feature lower and higher electron concentrations than 4 × 1020 cm−3, respectively. After isothermal and isochronal annealing under N2 ambient, we find that the two series exhibit a decrease or increase in electron concentrations until ~4 × 1020 cm−3 which is close to Fermi stabilization energy (EFS) level of CdO, with the assistance of native defects. An amphoteric defects model is used to explain the changing trends in electron concentrations. The tendencies in mobility further confirm our results. This work may provide some strategies to predict the electrical properties in CdO.

Photoreflectance spectroscopy is used to study optical properties of GaNPAs for the intermediate band (IB) solar cell absorbers. The IB is created in GaNPAs by the incorporation of ~2% of N atoms and the required partial occupation of the intermediate band with electrons is realized by doping with Si donors. Undoped and p-type doped (Be doped) GaNPAs films are studied as a references. Observation of all three absorption bands: VB → IB (a transition between the valence band and the IB), VB → CB (a transition between the valence band and the conduction band), and IB → CB (a transition between the IB and the conduction band), demonstrate that n-type doped GaNPAs layers act as intermediate band solar cell absorbers.

Highly mismatched alloys are a class of materials whose fundamental properties are dramatically modified through the substitution of a relatively small fraction of host atoms with an element of very much different electronegativity. In ZnTe, the incorporation of a small amount of isoelectronic O leads to the formation of a narrow, O-derived intermediate band (IB, E-) located well below the conduction band (CB, E+) edge of the ZnTe through an anticrossing interaction between localized states of O and the CB of the ZnTe matrix. Therefore, ZnTe1-xOx (ZnTeO) alloy is one of the potential candidates for an absorber material in a bulk intermediate band solar cell (IBSC). We have previously demonstrated the generation of photocurrent induced by two-step photon absorption (TSPA) in ZnTeO IBSCs using n-ZnO window layer. Here, we review our recent progress on the development of ZnTeO based IBSCs using n-ZnS window layer and Cldoped ZnTeO. With n-ZnS window having a small conduction band offset with ZnTe, the open circuit voltage of ZnTeO IBSC was improved. Cl-doping was performed to introduce electrons into the IB of ZnTeO that is required to be halffilled with electrons for the efficient operation of an IBSC. Low temperature photoluminescence spectra indicated that the doped Cl atoms act as donors in ZnTeO. The improved photovoltaic properties were demonstrated in the IBSC using Cl-doped ZnTeO.

We report the growth of layered GaSexTe1–x mesostructures across the whole composition range. For compositions up to x = 0.32 (the Te-rich region), mesocrystals form predominantly in the monoclinic structure, similar to naturally occurring GaTe. However, the hexagonal crystal structure, similar to naturally occurring GaSe, begins growing at the x = 0.28 composition and grows almost exclusively in the range of x = 0.32 to pure GaSe, establishing a region of composition where both monoclinic and hexagonal crystals exist. While the optical bandgap of the monoclinic phase increases linearly from 1.65 to 1.77 eV with increasing Se content, the incorporation of Te in the hexagonal phase reduces the optical gap from 2.01 (pure GaSe) to 1.38 eV (x = 0.28). Specifically, a bandgap difference of ∼0.35 eV between monoclinic and hexagonal crystals is observed in the composition range where both crystal structures can be grown. These observations are in good agreement with direct-gap trends calculated by density funct...

In this work, we have synthesized Cd1–xGaxO1+δ alloy thin films at room temperature over the entire composition range by radio frequency magnetron sputtering. We found that alloy films with high Ga contents of x > 0.3 are amorphous. Amorphous Cd1–xGaxO1+δ alloys in the composition range of 0.3 < x < 0.5 exhibit a high electron mobility of 10–20 cm2 V–1 s–1 with a resistivity in the range of 10–2 to high 10–4 Ω cm range. The resistivity of the amorphous alloys can also be controlled over 5 orders of magnitude from 7 × 10–4 to 77 Ω cm by controlling the oxygen stoichiometry. Over the entire composition range, these crystalline and amorphous alloys have a large tunable intrinsic band gap range of 2.2–4.8 eV as well as a conduction band minimum range of 5.8–4.5 eV below the vacuum level. Our results suggest that amorphous Cd1–xGaxO1+δ alloy films with 0.3 < x < 0.4 have favorable optoelectronic properties as transparent conductors on flexible and/or organic substrates, whereas the band edges and electrical co...

Doping of traditional semiconductors has enabled technological applications in modern electronics by tailoring their chemical, optical and electronic properties. However, substitutional doping in two-dimensional semiconductors is at a comparatively early stage, and the resultant effects are less explored. In this work, we report unusual effects of degenerate doping with Nb on structural, electronic and optical characteristics of MoS2 crystals. The doping readily induces a structural transformation from naturally occurring 2H stacking to 3R stacking. Electronically, a strong interaction of the Nb impurity states with the host valence bands drastically and nonlinearly modifies the electronic band structure with the valence band maximum of multilayer MoS2 at the Γ point pushed upward by hybridization with the Nb states. When thinned down to monolayers, in stark contrast, such significant nonlinear effect vanishes, instead resulting in strong and broadband photoluminescence via the formation of exciton complexes tightly bound to neutral acceptors.

We have grown the full composition range (0 ≤ x ≤ 1) of Zn1-xCdxO (ZnCdO) thin films on MgO (100) substrates by molecular beam epitaxy. We obtained the wurtzite (WZ) phase in the Cd composition x ≤ 0.51, whereas in the Cd composition x ≥ 0.59, the rocksalt (RS) phase is obtained. A mixed phase of both the WZ and RS phases are observed inside the Cd composition range of 0.59> x >0.51. At this mixed phase region, we observed the WZ phase is transited to RS phase with an increasing band gap energy. We obtained the largest optical band gap energy in WZ-ZnCdO (x = 0) as 3.25 eV and RS-ZnCdO (x = 0.59) as 3.08 eV. In spite of high carrier concentration above 1.3 × 1020 cm−3, low resistivity around 2.5 × 10−4 Ωcm and high mobility of 51.7 cm2/Vs of maximum are observed on RS-ZnCdO thin films.

Quaternary Zn 1- x Cd x Te 1- y O y (ZnCdTeO) alloys with Cd and O contents of x y 1- x Cd x Te matrix. Measurements of the photovoltaic performance of two intermediate band solar cell structures indicate an importance of the conduction band alignment and the location of the intermediate band relative to the upper conduction band.

Al-doped ZnS thin films have been grown by molecular beam epitaxy with confirmed n-type conductivity under a wide range of Al cell temperatures between 810 °C and 910 °C at a substrate temperature of 250 °C. An electron concentration up to 4.2 × 1019 cm−3 was achieved with the Al cell temperature of 890 °C. An n-ZnS/i-ZnTe/p-ZnTe solar cell structure was fabricated and under 1 sun illumination this structure showed an improved V OC of 0.77 V, a large short-circuit current density of 6.7 mA/cm2 with a fill factor of 0.60, yielding the power conversion efficiency of 3.1%. The n-type ZnS layer was also applied to the ZnTeO-based intermediate band solar cell structure and results were compared to a similar structure with an n-ZnO layer.

Below we present semiempirical modeling of conceptually new three-sublayer photoanode, composed of Absorber, Grading and Barrier sublayers, for highly efficient photoelectrochemical water dissociation. The modeling resulted into Absorber (Sub-A) made of Cd 0.55 Zn 0.45 O due to its favorable positions of the band extrema to the water splitting potentials and a band gap ~2.0 eV. The Grading layer (Sub-G) was composed of Cd x Zn 1−x O with a gradual decrease of x across the profile, changing from 0.2 to 0.55, aiming to photon absorption from 2.0 to 3.0 eV. At the same time, Sub-G furnishes the profile with an implanted electrical field that improves the hole-transport. The electron Barrier layer (Sub-B) deposited above the Sub-A, was engineered to provide 1 eV high barrier in the conduction band. It comprised of a 50 nm thick Ni 0.4 Cd 0.6 O film with E g ~3.0 eV with a valence band aligned to the one of the Sub-A, providing a barrier-free hole-flow. In this paper, we provide evidence that the proposed three-sublayer concept clearly represents a new paradigm for an improved efficiency for photocatalytic water dissociation. The highest photocatalytic activity of the optimized profile was achieved with an optimized electrolyte: 87% 1 M K 2 HPO 4 and 13% 1 M Na 2 SO 3 (known to act as a hole scavenger or sacrificial agent) at pH=10. A noteworthy feature of this study is that under optimized profile parameters and customized electrolyte conditions the photocurrent yields increased from ~0.05 mA/cm 2 to ~20 mA/cm 2 at +1.2 V for visible light. The observed Incident Photon-to-Current Efficiency (IPCE) was about 50% measured at a photon energy of 3 eV.

We report the effect of O concentration in Cldoped ZnTeO on the photovoltaic (PV) properties of intermediate band solar cells (IBSCs) with a n-ZnS/ZnTe/Cl-doped ZnTeO/ ZnTe/p-ZnTe structure. With increasing O concentration in ZnTeO, an open circuit voltage does not change largely but a short circuit current density decreases significantly. On the other hands, the photocurrent induced by two-step photon absorption (TSPA) through the IB increases with the increase of O concentration, probably due to the increased density of states in IB.

Three-dimensional band structure of rock-salt (rs) CdxZn1−xO (x = 1.0, 0.83, and 0.60) have been determined by angle-resolved photoemission spectroscopy (ARPES) using synchrotron radiation. Valence-band features shift to higher binding energy with Zn content, while the conduction band position does not depend strongly on Zn content. An increase of the indirect band gap with Zn-doping is larger than that of the direct band gap, reflecting a weaker hybridization between Zn 3d and O 2p than that between Cd 4d and O 2p. Two-dimensional electronic states due to the quantization along surface normal direction are formed in the surface accumulation layer and show non-parabolic dispersions. Binding energy of the quantized two-dimensional state is well reproduced using an accumulation potential with the observed surface band bending and the characteristic width of about 30 Å.

NiO-CdO alloy films over the entire composition range were synthesized by RT magnetron sputtering in both pure Ar (Ni x Cd 1-x O) and $\text{Ar} +\mathrm{O}2(\text{Ni}_{\mathrm{x}}\text{Cd}_{1-\mathrm{x}}\mathrm{O}_{1+\delta})$ environments. These alloy films are nanocrystalline with rocksalt structure. We demonstrated that in the alloy composition range of $0.38\leq \mathrm{x}\leq 0.52, \text{Ni}_{\mathrm{x}}\text{Cd}_{1-\mathrm{x}}\mathrm{O}_{1+\delta}$ films with bipolar (i.e., ${n-}$ or $p$ -type) conductivity can be achieved by controlling the oxygen stoichiometry. Within this composition range the band gap of the alloy is ∼3-3.3 eV with transmittance >50% in the visible range. The unusual electrical and optical properties of $\text{Ni}_{\mathrm{x}}\text{Cd}_{1-\mathrm{x}}\mathrm{O}_{1+\delta}$ alloy thin films can be explained by the modifications of the electronic band structure due to anticrossing interactions between localized Ni d levels and extended valence and conduction band states of the alloy. In addition, we studied the transport mechanism of $p$ -type $\text{Ni}_{\mathrm{x}}\text{Cd}_{1-\mathrm{x}}\mathrm{O}_{1+\delta}$ alloy films and found that the hole mobility increases with temperature in the range of 400-450 K. This is consistent with hole transport via a small polaron hopping process with an activation energy $Ea$ of 0.174 eV ( $\mathrm{x}=1$ ) to 0.331 eV ( $\mathrm{x}=0.47$ ). These results strongly suggest that the nanocrystalline $\text{Ni}_{\mathrm{x}}\text{Cd}_{1-\mathrm{x}}\mathrm{O}_{1+\delta}$ alloy system has great technological potential for applications in transparent optoelectronic device.

A systematic investigation on the influence of oxygen flow rates and substrate positions on the structural, optical and electrical properties of copper oxide (Cu x O) thin films deposited by reactive radio frequency sputtering to establish the deposition condition for Cu 4 O 3 . Detailed analyses on x-ray diffraction (XRD) and Raman spectroscopy were performed, revealing that different single or bi-phase of Cu 2 O, Cu 4 O 3 and CuO were grown at the different O 2 flow rate and substrate position. From the optical absorption analyses, Cu 2 O films showed a direct forbidden transition at $\mathrm{E}_{\mathrm{g}}=2.28\sim 2.37\ \text{eV}$ in addition to a direct allowed transition at $\mathrm{E}_{\mathrm{g}}=2.50\sim 2.53\ \text{eV}$ . In Cu 4 O 3 and CuO films, a direct forbidden transition at $\mathrm{E}_{\mathrm{g}}=1.58\ \text{eV}$ and 1.51 ∼ 1.54 eV was observed respectively with a high absorption coefficient, demonstrating the possibility to use these compounds as an absorber for single junction solar cells. Moreover, single phase Cu 4 O 3 showed the resistivity from 38 to 460 ohm cm with p-type conductivity, which was the lowest resistivity for the single phase of Cu 4 O 3 films reported so far.

Three-dimensional band structure of rock-salt (rs) CdxZn1−xO (x = 1.0, 0.83, and 0.60) have been determined by angle-resolved photoemission spectroscopy (ARPES) using synchrotron radiation. Valence-band features shift to higher binding energy with Zn content, while the conduction band position does not depend strongly on Zn content. An increase of the indirect band gap with Zn-doping is larger than that of the direct band gap, reflecting a weaker hybridization between Zn 3d and O 2p than that between Cd 4d and O 2p. Two-dimensional electronic states due to the quantization along surface normal direction are formed in the surface accumulation layer and show non-parabolic dispersions. Binding energy of the quantized two-dimensional state is well reproduced using an accumulation potential with the observed surface band bending and the characteristic width of about 30 Å.

The availability of wide-band-gap oxides with bipolar conductivity is critical to developing transparent optoelectronic technologies. Unfortunately, most metal oxides show a strong propensity for $n$-type conductivity, while $p$-type doping remains extremely challenging. Focusing on CdO and NiO, which have type-III band offset, this study reveals that their alloy Ni${}_{x}$Cd${}_{1\ensuremath{-}x}$O${}_{1+\ensuremath{\delta}}$ can exhibit bipolar conductivity. Electronic band structure and native defects are manipulated by varying the oxygen stoichiometry. The results strongly suggest that these alloys have great technological potential for applications requiring transparent conducting oxides.

Composites are a class of materials that are formed by mixing two or more components. These materials often have new functional properties compared to their constituent materials. Traditionally composites are formed by self-assembly due to structural dissimilarities or by engineering different layers or structures in the material. Here we report the synthesis of a uniform and stoichiometric composite of CdO and SnTe with a novel nanocomposite structure stabilized by the dissimilarity of the electronic band structure of the constituent materials. The composite has interesting electronic properties which range from highly n-type in CdO to semi-insulating in the intermediate composition range to highly p-type in SnTe. This can be explained by the overlap of the conduction and valence band of the constituent compounds. Ultimately, our work identifies a new class of composite semiconductors in which nanoscale self-organization is driven and stabilized by charge transfer between constituent materials.

We have synthesized Indium doped Cd1-xZnxO alloys across the full composition range using magnetron sputtering method. The crystallographic structure of these alloys changes from rocksalt (RS) to wurtzite (WZ) when the Zn content is higher than 30%. The rocksalt phase alloys in the composition range 0

Cu4O3 thin films have been synthesized in an ambient of Ar and O2 plasma using a pure Cu target by radio frequency magnetron sputtering. The structural, electrical, and optical properties of the films were studied systematically as a function of O2 gas flow. The study reveals that O2 flow rate ( R O 2) during sputtering has major impacts on both the composition and functional properties of the resultant Cu4O3 thin films. X-ray diffraction and Raman spectroscopy measurements suggest that the parameter window for the growth of single-phase Cu4O3 thin films was very narrow. Oxygen partial pressure of 7.9%–9.1% was required to grow the pure phase of Cu4O3. From optical absorption analyses, pure phase Cu4O3 films exhibited a direct transition at Eg = 1.52–1.62 eV. All the Cu4O3 thin films showed p-type conductivity with resistivities in the order of 102–103 Ω cm. An increase of R O 2 resulted in the increase of the Hall mobility from 0.01 to 0.25 cm2/V s, which is the highest mobility reported so far for this material. These results indicate clearly that Cu4O3 is a semiconductor with a high potential as absorber materials in low-cost thin film photovoltaics.

The past few years have witnessed unprecedented rapid improvement of the performance of a new class of photovoltaics based on halide perovskites. This progress has been achieved even though there is no generally accepted mechanism of the operation of these solar cells. Here we present a model based on bistable amphoteric native defects that accounts for all key characteristics of these photovoltaics and explains many idiosyncratic properties of halide perovskites. We show that a transformation between donor-like and acceptor-like configurations leads to a resonant interaction between amphoteric defects and free charge carriers. This interaction, combined with the charge transfer from the perovskite to the electron and hole transporting layers results in the formation of a dynamic n-i-p junction whose photovoltaic parameters are determined by the perovskite absorber. The model provides a unified explanation for the outstanding properties of the perovskite photovoltaics, including hysteresis of J-V characteristics and ultraviolet light-induced degradation., 21 pages, 7 figures

Highly mismatched ZnTe1-xOx (ZnTeO) alloy is one of the potential candidates for an absorber material in a bulk intermediate band solar cell (IBSC) because a narrow, O-derived intermediate band IB (E-) is formed well below the conduction band CB (E+) edge of the ZnTe. We have previously demonstrated the generation of photocurrent induced by two-step photon absorption (TSPA) in ZnTeO IBSCs using n-ZnO window layer. However, because of the large conduction band offset (CBO) between ZnTe and ZnO, only a small open circuit voltage (Voc) was observed in this structure. Here, we report our recent progress on the development of ZnTeO IBSCs with n-ZnS window layer. ZnS has a large direct band gap of 3.7 eV with an electron affinity of 3.9 eV that can realize a smaller CBO with ZnTe. We have grown n-type ZnS thin films on ZnTe substrates by molecular beam epitaxy (MBE), and demonstrated ZnTe solar cells and ZnTeO IBSCs using n-ZnS window layer with an improved VOC. Especially, a n-ZnS/i-ZnTe/p-ZnTe solar cell showed an improved Voc of 0.77 V, a large short-circuit current density of 6.7 mA/cm2 with a fill factor of 0.60, yielding the power conversion efficiency of 3.1 %, under 1 sun illumination.

In this work, we have synthesized Cd

PN junctions are basic building blocks of many electronic devices and their performance depends on the structural properties of the component layers and on the type and the amount of the doping impurities incorporated. Magnesium is the common p-type dopant for nitride semiconductors while silicon and more recently germanium are the n-dopants of choice. In this paper, therefore we analyze the quantitative limits for Mg and Ge incorporation on GaN and InGaN with high In content. We also discuss the challenges posed by the growth and characterization of InGaN pn-junctions and we discuss the properties of large area, long wavelength nanocolumn LEDs grown on silicon (1 1 1) by PA-MBE.

Simultaneous increases in electrical conductivity (up to 200%) and thermopower (up to 70%) are demonstrated by introducing native defects in Bi2 Te3 films, leading to a high power factor of 3.4 × 10(-3) W m(-1) K(-2). The maximum enhancement of the power factor occurs when the native defects act beneficially both as electron donors and energy filters to mobile electrons. They also act as effective phonon scatterers.

Improved open-circuit voltages have been achieved in dilute nitride thin-film intermediate-band solar cells by optimizing intermediate-band barrier layers. The blocking properties of the AlGaAs electron barrier are found to critically depend on the barrier-doping level. Open-circuit voltages $V_{{\rm OC}}$ improved when electron-doping levels in the AlGaAs barriers were reduced. This is ascribed to the increased association of $V_{{\rm OC}}$ with the larger gap defined by the valence and conduction bands due to the improved electrical isolation of the intermediate band.

The electronic band structure of phosphorus-rich GaNxPyAs1−x−y alloys (x ~ 0.025 and y ≥ 0.6) is studied experimentally using optical absorption, photomodulated transmission, contactless electroreflectance, and photoluminescence. It is shown that incorporation of a few percent of N atoms has a drastic effect on the electronic structure of the alloys. The change of the electronic band structure is very well described by the band anticrossing (BAC) model in which localized nitrogen states interact with the extended states of the conduction band of GaAsP host. The BAC interaction results in the formation of a narrow intermediate band (E− band in BAC model) with the minimum at the Γ point of the Brillouin zone resulting in a change of the nature of the fundamental band gap from indirect to direct. The splitting of the conduction band by the BAC interaction is further confirmed by a direct observation of the optical transitions to the E+ band using contactless electroreflectance spectroscopy.

The subband features E ‒ and E + for the conduction band of III-V dilute nitride alloys make them promising for intermediate band solar cell application. However, presence of bandgap states can limit the two-step photon absorption activity, a necessary requirement for IBSC functionality. A model analysis is performed to characterize the density of states. The sub-band tails states are characterized using a temperature-dependent map of photo-modulated reflectance spectroscopy for GaNAs thin films grown on GaAs substrates using molecular beam epitaxy. The effect of indium and antimony incorporation on the subband features were investigated. Marked improvements in the thin films were observed both for the lower ( E ‒ ) and the upper ( E+ ) conduction bands (CB) when In was introduced with marginal enhancement by Sb. These improvements are associated with suppression of tail states below both the E ‒ and E + bands. Sb rather mainly plays a surfactant role improving the abruptness of the GaNAs/GaAs hetero-interface.

Photoluminescence and photomodulated reflectivity measurements of ZnOSe alloys are used to demonstrate a splitting of the valence band due to the band anticrossing interaction between localized Se states and the extended valence band states of the host ZnO matrix. A strong multiband emission associated with optical transitions from the conduction band to lower E− and upper E+ valence subbands has been observed at room temperature. The composition dependence of the optical transition energies is well explained by the electronic band structure calculated using the kp method combined with the band anticrossing model. The observation of the multiband emission is possible because of relatively long recombination lifetimes. Longer than 1 ns lifetimes for holes photoexcited to the lower valence subband offer a potential of using the alloy as an intermediate band semiconductor for solar power conversion applications.

GaNsub1-x/subSbsubx/subwith x∼5%-7% is a highly mismatched alloy predicted to have favorable properties for application as an electrode in a photoelectrochemical cell for solar water splitting. In this study, we grew GaNsub1-x/subSbsubx/subunder conditions intended to induce phase segregation. Prior experiments with the similar alloy GaNsub1-x/subAssubx/sub, the tendency of Sb to surfact, and the low growth temperatures needed to incorporate Sb all suggested that GaNsub1-x/subSbsubx/suballoys would likely exhibit phase segregation. We found that, except for very high Sb compositions, this was not the case and that instead interdiffusion dominated. Characteristics measured by optical absorption were similar to intentionally grown bulk alloys for the same composition. Furthermore, the alloys produced by this method maintained crystallinity for very high Sb compositions and allowed higher overall Sb compositions. This method may allow higher temperature growth while still achieving needed Sb compositions for solar water splitting applications.

Intermediate-band semiconductors have attracted much interest in the photovoltaic community, as they are expected to allow higher solar-cell efficiencies than traditional absorbers. The authors directly measure minority-carrier lifetimes in Ga(P,As,N), only to find that these times are too short for efficient solar cells. The observed lifetimes seem not to be fundamental material properties, though, but rather the result of recombination at defects. This identifies material quality as possibly the most important issue preventing the development of efficient solar cells based on these semiconductors.

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.

Magnesium is the only known effective p-type dopant for nitride semiconductors. Although the p-doping is challenging for AlN and GaN, requiring the activation of the Mg acceptors, in the case of the MOCVD growth, methods for obtaining reliable and high level doping have been developed. Similarly for n-type doping silicon has been used successfully for more than a decade. More recently germanium has been found to be as effective for n-type doping, with the additional advantage of inducing less strain in the host lattice due its size similarity to gallium. Doping of InGaN is more challenging due the difficulty in controlling the donor background associated with material extended defects and the incorporation of impurities. Although successful p-type doping of InGaN with Mg has been demonstrated, quantitative limits for the magnesium incorporation and its behavior have not been analyzed. In this paper we investigate both, Mg and Ge doping of InGaN. We also discuss the challenges posed by the growth and measurement of the InGaN pn-junctions characteristics as well as we demonstrate the fabrication of large area long wavelength LEDs on silicon (111) by PA-MBE. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present the results of systematic experimental studies on ZnTeO intermediate band solar cells (IBSCs) with a n-ZnO window layer. In order to understand photovoltaic (PV) activities of ZnTeO IBSCs, we first describe PV properties of ZnO/ZnTe solar cells without the intermediate band (IB). The improved efficiency of 1.38% is demonstrated by using a n+-ZnO/ i-ZnO/ i-ZnTe/ p-ZnTe structure. Then, the PV properties of ZnTeO IBSCs fabricated using n-ZnO window layer with and without a blocking barrier for IB are compared. The device with a blocked IB shows higher open-circuit voltage than that without the blocking barrier. High external quantum efficiency (EQE) is observed in the photon energy range in which electron transitions from the valence band to the IB take place in ZnTeO IBSC without the blocking layer, whereas the device with the blocked IB shows a small EQE at the same energy range, implying the electron accumulation in IB. Finally, the production of photogenerated current by two-step photon excitation via IB is demonstrated.

The electrical and optical properties of cadmium oxide indicate that thin films of the material could be good transparent electrodes for high-efficiency multijunction solar cells, though its complex dielectric function depends drastically on carrier concentration. This comprehensive study correlates CdO's optical gap, free-carrier absorption, refractive index, high-frequency dielectric constant, and electron effective mass with dopant concentration and species. These results are crucial for engineering optoelectronic devices based on this transparent conducting oxide.

The electronic band structure of highly mismatched alloys (HMAs) was very successfully explored using electromodulation (EM) spectroscopy, i.e., photoreflectance (PR), electroreflectance, and contactless electroreflectance (CER). With these techniques, the optical transitions between the valence band and the E− and E+ bands, which are formed in the conduction band of dilute nitrides and dilute oxides, were observed and used to formulate the band anticrossing model, which well describes the electronic band structure of HMAs. In this tutorial, principles of EM spectroscopy are presented and shortly discussed. Special attention is focused on PR and CER techniques, which are nondestructive and have recently been widely applied to study the electronic band structure of HMAs and low dimensional heterostructures containing HMAs. For these methods, experimental setups are described, and theoretical approaches to analyze the experimental data are introduced. Finally, to show the utility of EM spectroscopy, selected examples of the application of this method to study various issues in HMAs are presented and briefly discussed.The electronic band structure of highly mismatched alloys (HMAs) was very successfully explored using electromodulation (EM) spectroscopy, i.e., photoreflectance (PR), electroreflectance, and contactless electroreflectance (CER). With these techniques, the optical transitions between the valence band and the E− and E+ bands, which are formed in the conduction band of dilute nitrides and dilute oxides, were observed and used to formulate the band anticrossing model, which well describes the electronic band structure of HMAs. In this tutorial, principles of EM spectroscopy are presented and shortly discussed. Special attention is focused on PR and CER techniques, which are nondestructive and have recently been widely applied to study the electronic band structure of HMAs and low dimensional heterostructures containing HMAs. For these methods, experimental setups are described, and theoretical approaches to analyze the experimental data are introduced. Finally, to show the utility of EM spectroscopy, selecte...

The amphoteric native defect model phenomenologically explains and categorizes observed background doping concentrations and accounts for the doping limitations in compound semiconductors. Here, we test and verify the model's capabilities as a quantitative, predictive tool. We demonstrate that the nature, (donor or acceptor), of non-stoichiometry related vacancy-like defects in the (Al,Ga,In)(As,Sb) system is determined by the position of the intrinsic Fermi energy, E Fi at the growth temperature relative to the Fermi level stabilization energy, E FS , a universal energy reference located at 4.9 eV below the vacuum level. We show that a dramatic and predictable conduction type-flip from n- to p-type occurs when E Fi is deliberately shifted from below to above E FS .

Stoichiometric gallium oxide sulfide Ga2(O1 − xSx)3 thin-film alloys were synthesized by pulsed-laser deposition with x ≤ 0.35. All deposited Ga2(O1 − xSx)3 films were found to be amorphous. Despite the amorphous structure, the films have a well-defined, room-temperature optical bandgap tunable from 5.0 eV down to 3.0 eV. The optical absorption data are interpreted using a modified valence-band anticrossing model that is applicable for highly mismatched alloys. The model provides a quantitative method to more accurately determine the bandgap as well as an insight into how the band edges are changing with composition. The observed large reduction in energy bandgap with a small sulfur ratio arises from the anticrossing interaction between the valence band of Ga2O3 and the localized sulfur level at 1.0 eV above the Ga2O3 valence-band maximum.

Interband optical transitions in highly mismatched ZnTe1−xOx and Zn1−yCdyTe1−xOx alloys with Cd content y = 0.1 and 0.32 and oxygen content x

Photoreflectance (PR) and microwave-reflectance photoconductivity-decay (μ-PCD) have been applied to study the surface band bending in Mg-doped InN layers with various Mg concentrations (∼1 × 1017 to ∼7 × 1019 cm−3) and thicknesses (∼400 to ∼3600 nm). A PR resonance related to the band-to-band transition at the Γ point below the surface electron accumulation is observed for moderately doped samples ([Mg]: 1.3 × 1017 cm−3 to 8.7 × 1017 cm−3). Samples with the Mg doping concentration in the range of ∼5 × 1018 ≤ [Mg] ≤ 7 × 1019 cm−3 are p-type and show very weak or no PR resonance. It is shown that the results correlate with the surface layer depletion thickness. The strongest PR signal is observed for thick depletion layers. The results are confirmed by μ-PCD measurements that show the longest carrier lifetimes for moderately doped samples with the thickest surface depletion layer.

Cl doping in highly mismatched ZnTe1-xOx (ZnTeO) alloys was achieved by molecular beam epitaxy using ZnCl2 as a dopant source. The temperature dependence of the bandgap energies for both upper and lower subbands, E+ and E−, of ZnTeO was determined by photoreflectance, and it was found to vary in accordance with those expected by the band anticrossing model. Secondary ion mass spectroscopy indicates that the Cl concentration increases with increasing ZnCl2 cell temperature. Low temperature photoluminescence (PL) spectra show the broad PL peak near the E− band. Intermediate band solar cells (IBSCs) fabricated using ZnTeO:Cl exhibit improved short circuit current and open circuit voltage. The results suggest that the Cl doping can effectively improve the PV properties of ZnTeO-based IBSCs and, therefore, is potentially useful for the realization of high efficiency IBSCs.