Unraveling Electron Dynamics in p-type Indium Phosphide (100): A Time-Resolved Two-Photon Photoemission Study.

Renewable ("green") hydrogen production through direct photoelectrochemical (PEC) water splitting is a potential key contributor to the sustainable energy mix of the future. We investigate the potential of indium phosphide (InP) as a reference material among III-V semiconductors for PEC and photovoltaic (PV) applications. The p(2 × 2)/c(4 × 2)-reconstructed phosphorus-terminated p-doped InP(100) (P-rich p-InP) surface is the focus of our investigation. We employ time-resolved two-photon photoemission (tr-2PPE) spectroscopy to study electronic states near the band gap with an emphasis on normally unoccupied conduction band states that are inaccessible through conventional single-photon emission methods. The study shows the complexity of the p-InP electronic band structure and reveals the presence of at least nine distinct states between the valence band edge and vacuum energy, including a valence band state, a surface defect state pinning the Fermi level, six unoccupied surface resonances within the conduction band, as well as a cluster of states about 1.6 eV above the CBM, identified as a bulk-to-surface transition. Furthermore, we determined the decay constants of five of the conduction band states, enabling us to track electron relaxation through the bulk and surface conduction bands. This comprehensive understanding of the electron dynamics in p-InP(100) lays the foundation for further exploration and surface engineering to enhance the properties and applications of p-InP-based III-V-compounds for, e.g., efficient and cost-effective PEC hydrogen production and highly efficient PV cells.

Polyimide@Ketjenblack Composite: A Porous Organic Cathode for Fast Rechargeable Potassium-Ion Batteries.

Potassium-ion batteries (PIBs) configurated by organic electrodes have been identified as a promising alternative to lithium-ion batteries. Here, a porous organic Polyimide@Ketjenblack is demonstrated in PIBs as a cathode, which exhibits excellent performance with a large reversible capacity (143 mAh g-1 at 100 mA g-1 ), high rate capability (125 and 105 mAh g-1 at 1000 and 5000 mA g-1 ), and long cycling stability (76% capacity retention at 2000 mA g-1 over 1000 cycles). The domination of fast capacitive-like reaction kinetics is verified, which benefits from the porous structure synthesized using in situ polymerization. Moreover, a renewable and low-cost full cell is demonstrated with superior rate behavior (106 mAh g-1 at 3200 mA g-1 ). This work proposes a strategy to design polymer electrodes for high-performance organic PIBs.

Femtosecond cooling of hot electrons in CdSe quantum-well platelets.

Semiconductor quantum wells are ubiquitous in high-performance optoelectronic devices such as solar cells and lasers. Understanding and controlling of the (hot) carrier dynamics is essential to optimize their performance. Here, we study hot electron cooling in colloidal CdSe quantum-well nanoplatelets using ultrafast two-photon photoemission spectroscopy at low excitation intensities, resulting typically in 1-5 hot electrons per platelet. We observe initial electron cooling in the femtosecond time domain that slows down with decreasing electron energy and is finished within 2 ps. The cooling is considerably faster at cryogenic temperatures than at room temperature, and at least for the systems that we studied, independent of the thickness of the platelets (here 3-5 CdSe units) and the presence of a CdS shell. The cooling rates that we observe are orders of magnitude faster than reported for similar CdSe platelets under strong excitation. Our results are understood by a classic cooling mechanism with emission of longitudinal optical phonons without a significant influence of the surface.

Two-photon photoemission study of competing Auger and surface-mediated relaxation of hot electrons in CdSe quantum dot solids.

Solids composed of colloidal quantum dots hold promise for third generation highly efficient thin-film photovoltaic cells. The presence of well-separated conduction electron states opens the possibility for an energy-selective collection of hot and equilibrated carriers, pushing the efficiency above the one-band gap limit. However, in order to reach this goal the decay of hot carriers within a band must be better understood and prevented, eventually. Here, we present a two-photon photoemission study of the 1Pe→1Se intraband relaxation dynamics in a CdSe quantum dot solid that mimics the active layer in a photovoltaic cell. We observe fast hot electron relaxation from the 1Pe to the 1Se state on a femtosecond-scale by Auger-type energy donation to the hole. However, if the oleic acid capping is exchanged for hexanedithiol capping, fast deep hole trapping competes efficiently with this relaxation pathway, blocking the Auger-type electron-hole energy exchange. A slower decay becomes then visible; we provide evidence that this is a multistep process involving the surface.

Impact of the Tip-to-Semiconductor Contact in the Electrical Characterization of Nanowires.

Well-defined semiconductor heterostructures are a basic requirement for the development of high-performance optoelectronic devices. In order to achieve the desired properties, a thorough study of the electrical behavior with a suitable spatial resolution is essential. For this, various sophisticated tip-based methods can be employed, such as conductive atomic force microscopy or multitip scanning tunneling microscopy (MT-STM). We demonstrate that in any tip-based measurement method, the tip-to-semiconductor contact is decisive for reliable and precise measurements and in interpreting the properties of the sample. For that, we used our ultrahigh-vacuum-based MT-STM coupled in vacuo to a reactor for the preparation of nanowires (NWs) with metal organic vapor phase epitaxy, and operated our MT-STM as a four-point nanoprober on III-V semiconductor NW heterostructures. We investigated a variety of upright, free-standing NWs with axial as well as coaxial heterostructures on the growth substrates. Our investigation reveals charging currents at the interface between the measuring tip and the semiconductor via native insulating oxide layers, which act as a metal-insulator-semiconductor capacitor with charging and discharging conditions in the operating voltage range. We analyze in detail the observed I-V characteristics and propose a strategy to achieve an optimized tip-to-semiconductor junction, which includes the influence of the native oxide layer on the overall electrical measurements. Our advanced experimental procedure enables a direct relation between the tip-to-NW junction and the electronic properties of as-grown (co)axial NWs providing precise guidance for all future tip-based investigations.

Epitaxial III-V films and surfaces for photoelectrocatalysis.

Efficient photoelectrochemical devices for water splitting benefit from the highest material quality and dedicated surface preparation achieved by epitaxial growth. InP(100)-based half-cells show significant solar-to-hydrogen efficiencies, but require a bias due to insufficient voltage. Tandem absorber structures may provide both adequate potential and efficient utilization of the solar spectrum. We propose epitaxial dilute nitride GaPNAs photocathodes on Si(100) substrates to combine close-to-optimum limiting efficiency, lattice-matched growth, and established surface preparation. Prior to a discussion of the challenging III-V/Si(100) heterojunction, we describe the closely related epitaxial preparation of InP(100) surfaces and its beneficial impact on photoelectrochemical water-splitting performance. Analogies and specific differences to GaP(100) surfaces are discussed based on in situ reflectance anisotropy and on two-photon photoemission results. Preliminary experiments regarding GaP/Si(100) photoelectrochemistry and dilute nitride GaPN heteroepitaxy on Si(100) confirm the potential of the GaPNAs/Si tandem absorber structure for future water-splitting devices.

P-Terminated InP (001) Surfaces: Surface Band Bending and Reactivity to Water.

Stable InP (001) surfaces are characterized by fully occupied and empty surface states close to the bulk valence and conduction band edges, respectively. The present photoemission data show, however, a surface Fermi level pinning only slightly below the midgap energy which gives rise to an appreciable surface band bending. By means of density functional theory calculations, it is shown that this apparent discrepancy is due to surface defects that form at finite temperature. In particular, the desorption of hydrogen from metalorganic vapor phase epitaxy grown P-rich InP (001) surfaces exposes partially filled P dangling bonds that give rise to band gap states. These defects are investigated with respect to surface reactivity in contact with molecular water by low-temperature water adsorption experiments using photoemission spectroscopy and are compared to our computational results. Interestingly, these hydrogen-related gap states are robust with respect to water adsorption, provided that water does not dissociate. Because significant water dissociation is expected to occur at steps rather than terraces, surface band bending of a flat InP (001) surface is not affected by water exposure.

Charge transport in GaAs nanowires: interplay between conductivity through the interior and surface conductivity.

The charge transport through GaAs nanowires, partially p-doped and partially intrinsic, is analyzed by four-point resistance profiling along freestanding nanowires using a multip-STM. The charge transport channel in the undoped segment is assigned to the surface conductivity, while the interior of the nanowire is the conductance channel in the p-doped segment. The convoluted interplay between conduction through the interior of the nanowire and surface state conduction is studied in detail. Measurements of the I-V curves along the nanowires provide the experimental basis for the proposed charge transport model for the transition of the conduction from the interior to the surface of the nanowire. A voltage drop along the surface state conduction channel leads to an upward shift of the band edges at the surface. This results, for higher applied voltages, in the removal of the depletion layer and an opening of a conductance channel between the interior of the nanowire and the surface states.

Impact of N Incorporation on VLS Growth of GaP(N) Nanowires Utilizing UDMH.

III-V nanowires (NWs) possess great potential for use in future semiconductor technology. Alloying with dilute amounts of nitrogen provides further flexibility in tuning their material properties. In this study, we report on successful in situ nitrogen incorporation into GaP(N) NWs during growth via the Au-catalyzed vapor-liquid-solid (VLS) mechanism. The impact of the nitrogen precursur unsymmetrical dimethyl hydrazine (UDMH) on morphology was found to be overall beneficial as it strongly reduces tapering. Analysis of the crystal structure of NWs with and without N reveals zinc blende structure with an intermediate amount of stacking faults (SF). Interestingly, N incorporation leads to segments completely free of SFs, which are related to dislocations transverse to the growth direction.

Impact of Rotational Twin Boundaries and Lattice Mismatch on III-V Nanowire Growth.

Pseudomorphic planar III-V transition layers greatly facilitate the epitaxial integration of vapor-liquid-solid grown III-V nanowires (NW) on Si(111) substrates. Heteroepitaxial (111) layer growth, however, is commonly accompanied by the formation of rotational twins. We find that rotational twin boundaries (RTBs), which intersect the surface of GaP/Si(111) heterosubstrates, generally cause horizontal NW growth and may even suppress NW growth entirely. Away from RTBs, the NW growth direction switches from horizontal to vertical in the case of homoepitaxial GaP NWs, whereas heteroepitaxial GaAs NWs continue growing horizontally. To understand this rich phenomenology, we develop a model based on classical nucleation theory. Independent of the occurrence of RTBs and specific transition layers, our model can generally explain the prevalent observation of horizontal III-V NW growth in lattice mismatched systems and the high crystal quality of horizontal nanowires.

Comparative analysis on resistance profiling along tapered semiconductor nanowires: multi-tip technique versus transmission line method.

The detection of doping dependent values like contact- and path resistances along nanowires (NWs) still proves to be rather challenging compared to planar structures. Unfortunately, the usually used and well established TLM (transmission line measurement) setup exhibits some drawbacks. Complex preliminary preparation steps and the necessity of ohmic contacts limit the investigation to certain semiconductor materials. The simultaneous determination of contact- and path resistances with an unknown distribution makes an analysis on complex structures like tapered nanowires very challenging. Our approach is the utilization of a multi-tip scanning tunneling microscope (MT-STM) as a four point prober, which allows the investigation of freestanding nanowires with an increased spatial resolution. Here, the used measurement setup allows a local separation of current injection and potential measurement and thus a highly precise determination of path resistances. Tapered p-doped GaAs-NWs were used to compare both techniques. Whereas the evaluation of the axial doping profile by MT-STM was rather simple, correction factors had to be introduced for the TLM measurement to calculate the specific resistances and transfer length. By comparing the results of both methods for the very same NW-sample, the precision and accuracy of MT-STM measurements was demonstrated. We found an agreement, which allows the conclusion that both methods exhibit advantages; however the MT-STM was determined as the more precise setup, which enables additional characterization capabilities, such as surface, temperature or light dependent measurements.

Efficient direct solar-to-hydrogen conversion by in situ interface transformation of a tandem structure.

Photosynthesis is nature's route to convert intermittent solar irradiation into storable energy, while its use for an industrial energy supply is impaired by low efficiency. Artificial photosynthesis provides a promising alternative for efficient robust carbon-neutral renewable energy generation. The approach of direct hydrogen generation by photoelectrochemical water splitting utilizes customized tandem absorber structures to mimic the Z-scheme of natural photosynthesis. Here a combined chemical surface transformation of a tandem structure and catalyst deposition at ambient temperature yields photocurrents approaching the theoretical limit of the absorber and results in a solar-to-hydrogen efficiency of 14%. The potentiostatically assisted photoelectrode efficiency is 17%. Present benchmarks for integrated systems are clearly exceeded. Details of the in situ interface transformation, the electronic improvement and chemical passivation are presented. The surface functionalization procedure is widely applicable and can be precisely controlled, allowing further developments of high-efficiency robust hydrogen generators.

Time-Resolved In Situ Spectroscopy During Formation of the GaP/Si(100) Heterointerface.

Though III-V/Si(100) heterointerfaces are essential for future epitaxial high-performance devices, their atomic structure is an open historical question. Benchmarking of transient optical in situ spectroscopy during chemical vapor deposition to chemical analysis by X-ray photoelectron spectroscopy enables us to distinguish between formation of surfaces and of the heterointerface. A terrace-related optical anisotropy signal evolves during pulsed GaP nucleation on single-domain Si(100) surfaces. This dielectric anisotropy agrees well with the one calculated for buried GaP/Si(100) interfaces from differently thick GaP epilayers. X-ray photoelectron spectroscopy reveals a chemically shifted contribution of the P and Si emission lines, which quantitatively corresponds to one monolayer and establishes simultaneously with the nucleation-related optical in situ signal. We attribute that contribution to the existence of Si-P bonds at the buried heterointerface. During further pulsing and annealing in phosphorus ambient, dielectric anisotropies known from atomically well-ordered GaP(100) surfaces superimpose the nucleation-related optical in situ spectra.

Formation of GaP/Si(100) Heterointerfaces in the Presence of Inherent Reactor Residuals.

Adequate silicon preparation is a prerequisite for defect-free III-V growth on Si. We transfer the silicon processing from clean to GaP containing metalorganic vapor phase epitaxy reactors, where we monitor the entire process in situ with reflection anisotropy spectroscopy and analyze the chemical composition of the surface with X-ray photoelectron spectroscopy. Beyond a certain submonolayer threshold value of (Ga,P) residuals found on the Si(100) surface, GaP grows with an inverted majority sublattice. Analogously to III-V growth on two-domain substrates, the coexistence of Si-Ga and Si-P interfacial bonds at terraces of the same type causes antiphase disorder in GaP epilayers.