GaP/GaNP Heterojunctions for Efficient Solar‐Driven Water Oxidation

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/137529/1/smll201603574_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137529/2/smll201603574.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137529/3/smll201603574-sup-0001-S1.pd

Dilute Nitride GaNP Wide Bandgap Solar Cells Grown by Gas-Source Molecular Beam Epitaxy

Integration of III-V semiconductors and Si is a very attractive means to achieve low-cost high-effi ciency solar cells. A promising configuration is to utilize a dual-junction solar cell, in which Si is employed as the bottom junction and a wide-bandgap III-V semiconductor as the top junction. The use of a III-V semiconductor as a top junction offers the potential to achieve higher efficiencies than today's best Si solar cell. Dilute nitride GaNP is a promising candidate for the top cell in dual-junction solar cells because it possesses several extremely important attributes: a direct-bandgap that is also tunable as well as easily-attained lattice-match with Si. As a first step towards integration of GaNP solar cells onto Si, the goal of this dissertation is to optimize and demonstrate GaNP solar cells grown by gas-source molecular beam epitaxy (GSMBE) on GaP (001) substrate.The dissertation is divided into three major parts. In the first part, we demonstrate ~ 2.05 eV ([N]~ 1.8%) dilute nitride GaNP thin film solar cells, in which the GaNP is closely lattice-matched to Si, on GaP substrates. From transmission electron microscopy (TEM), the device exhibits defects only at the GaNP/GaP interface, and no threading dislocations in an active layer are observed. Our best GaNP solar cell achieved an efficiency of 7.9% with anti-reflection (AR) coating and no window layer. This GaNP solar cell's efficiency is higher than the most efficient GaP solar cell to date and higher than other solar cells with similar direct bandgap (InGaP, GaAsP). Through a systematic study of the structural, electrical, and optical properties of the device, effi cient broadband optical absorption and enhanced solar cell performance using GaNP are demonstrated.In the second part, we demonstrate the successful fabrication of GaP/GaNP core/shell microwires utilizing a novel technique: top-down reactive-ion etching (RIE) to create the cores and MBE to create the shells. Systematic studies have been performed over a series of microwire lengths, array periods, and microwire sidewall morphology. For a fixed length, short circuit current (Jsc) increases as physical fill factor (PFF) of microwires increases, while, for a fixed array period, Jsc increases as microwire length increases. Our studies show that the open circuit voltage (Voc) is degraded primarily due to defects at the GaP/GaNP interface and in the shells, not surface recombination. The best efficiency we achieved using our microwire solar cell is 3.2% using an AR coating. Compared to thin film solar cells in the same growth run, the microwire solar cells exhibit greater Jsc but poorer Voc. This results from greater light absorption and a greater number of defects in the microwire structure, respectively.In the final part, vertical self-catalyzed GaP, GaNP, and GaNP/GaNP core/shell nanowires are demonstrated. The growth window of GaP nanowires is comparable to the growth window of GaNP nanowires. The diameter of nanowires (cores) can be controlled by adjusting substrate temperature (Tsub). The shells can be grown by decreasing Tsub and increasing the V/III incorporation ratio to reduce adatom mobility. The crystal structures of GaP and GaNP nanowires are mixtures of cubic zincblend phase and hexagonal wurtzite phase along the [111] growth direction. According to photoluminescence measurements, the broad spectrum of nanowire arrays do not result from the variation of N composition among nanowires, but from the mechanism of light emission of GaNP

Dilute Nitride GaNP Wide Bandgap Solar Cells Grown by Gas-Source Molecular Beam Epitaxy

Integration of III-V semiconductors and Si is a very attractive means to achieve low-cost high-effi ciency solar cells. A promising configuration is to utilize a dual-junction solar cell, in which Si is employed as the bottom junction and a wide-bandgap III-V semiconductor as the top junction. The use of a III-V semiconductor as a top junction offers the potential to achieve higher efficiencies than today's best Si solar cell. Dilute nitride GaNP is a promising candidate for the top cell in dual-junction solar cells because it possesses several extremely important attributes: a direct-bandgap that is also tunable as well as easily-attained lattice-match with Si. As a first step towards integration of GaNP solar cells onto Si, the goal of this dissertation is to optimize and demonstrate GaNP solar cells grown by gas-source molecular beam epitaxy (GSMBE) on GaP (001) substrate.The dissertation is divided into three major parts. In the first part, we demonstrate ~ 2.05 eV ([N]~ 1.8%) dilute nitride GaNP thin film solar cells, in which the GaNP is closely lattice-matched to Si, on GaP substrates. From transmission electron microscopy (TEM), the device exhibits defects only at the GaNP/GaP interface, and no threading dislocations in an active layer are observed. Our best GaNP solar cell achieved an efficiency of 7.9% with anti-reflection (AR) coating and no window layer. This GaNP solar cell's efficiency is higher than the most efficient GaP solar cell to date and higher than other solar cells with similar direct bandgap (InGaP, GaAsP). Through a systematic study of the structural, electrical, and optical properties of the device, effi cient broadband optical absorption and enhanced solar cell performance using GaNP are demonstrated.In the second part, we demonstrate the successful fabrication of GaP/GaNP core/shell microwires utilizing a novel technique: top-down reactive-ion etching (RIE) to create the cores and MBE to create the shells. Systematic studies have been performed over a series of microwire lengths, array periods, and microwire sidewall morphology. For a fixed length, short circuit current (Jsc) increases as physical fill factor (PFF) of microwires increases, while, for a fixed array period, Jsc increases as microwire length increases. Our studies show that the open circuit voltage (Voc) is degraded primarily due to defects at the GaP/GaNP interface and in the shells, not surface recombination. The best efficiency we achieved using our microwire solar cell is 3.2% using an AR coating. Compared to thin film solar cells in the same growth run, the microwire solar cells exhibit greater Jsc but poorer Voc. This results from greater light absorption and a greater number of defects in the microwire structure, respectively.In the final part, vertical self-catalyzed GaP, GaNP, and GaNP/GaNP core/shell nanowires are demonstrated. The growth window of GaP nanowires is comparable to the growth window of GaNP nanowires. The diameter of nanowires (cores) can be controlled by adjusting substrate temperature (Tsub). The shells can be grown by decreasing Tsub and increasing the V/III incorporation ratio to reduce adatom mobility. The crystal structures of GaP and GaNP nanowires are mixtures of cubic zincblend phase and hexagonal wurtzite phase along the [111] growth direction. According to photoluminescence measurements, the broad spectrum of nanowire arrays do not result from the variation of N composition among nanowires, but from the mechanism of light emission of GaNP

Enhancement of the performance of GaP solar cells by embedded In(N)P quantum dots

Improving the utilization of solar spectra of wide bandgap semiconductors that can potentially provide enough free energy is one of the promising strategies for realizing efficient and spontaneous integrated conversion of solar energy to chemical fuels. We demonstrate herein that nitrogen doped InP quantum dots (QDs) embedded in wide bandgap GaP could improve the solar energy conversion performance. Photoelectrochemical experiments in contact with a nonaqueous, reversible redox couple indicated that the QD-embedded devices exhibited improved performance relative to devices without QDs, with short-circuit current densities increasing from 0.16 mA cm^(−2) for GaP-only devices to 0.23 and 0.29 mA cm^(−2) for InP and InNP QD-embedded devices, respectively. Additionally, the open-circuit voltages increased from 0.95 V for GaP-only devices to 1.11 and 1.14 V for InP and InNP QD-embedded devices, respectively, and the external quantum yield of the devices was also enhanced by the embedded QDs. The improvement is attributed to the absorption of sub-bandgap photons by the In(N)P QDs

Origin of strong photoluminescence polarization in GaNP nanowires

The III-V semiconductor nanowires (NWs) have a great potential for applications in a variety of future electronic and photonic devices with enhanced functionality. In this work, we employ polarization resolved micro-photoluminescence (µ-PL) spectroscopy to study polarization properties of light emissions from individual GaNP and GaP/GaNP core/shell nanowires (NWs) with average diameters ranging between 100 and 350 nm. We show that the near-band-edge emission, which originates from the GaNP regions of the NWs, is strongly polarized (up to 60 % at 150 K) in the direction perpendicular to the NW axis. The polarization anisotropy can be retained up to room temperature. This polarization behavior, which is unusual for zinc blende NWs, is attributed to local strain in the vicinity of the N-related centers participating in the radiative recombination and to preferential alignment of their principal axis along the growth direction. Our findings therefore show that defect engineering via alloying with nitrogen provides an additional degree of freedom to tailor the polarization anisotropy of III-V nanowires, advantageous for their applications as nanoscale emitters of polarized light

Optical properties of GaP/GaNP core/shell nanowires: a temperature-dependent study Optical properties of GaP/GaNP core/shell nanowires: a temperature-dependent study

Abstract Recombination processes in GaP/GaNP core/shell nanowires (NWs) grown on Si are studied by employing temperature-dependent continuous wave and time-resolved photoluminescence (PL) spectroscopies. The NWs exhibit bright PL emissions due to radiative carrier recombination in the GaNP shell. Though the radiative efficiency of the NWs is found to decrease with increasing temperature, the PL emission remains intense even at room temperature. Two thermal quenching processes of the PL emission are found to be responsible for the degradation of the PL intensity at elevated temperatures: (a) thermal activation of the localized excitons from the N-related localized states and (b) activation of a competing non-radiative recombination (NRR) process. The activation energy of the latter process is determined as being around 180 meV. NRR is also found to cause a significant decrease of carrier lifetime

Effects of Polytypism on Optical Properties and Band Structure of Individual Ga(N)P Nanowires from Correlative Spatially Resolved Structural and Optical Studies

III–V semiconductor nanowires (NWs) have gained significant interest as building blocks in novel nanoscale devices. The one-dimensional (1D) nanostructure architecture allows one to extend band structure engineering beyond quantum confinement effects by utilizing formation of different crystal phases that are thermodynamically unfavorable in bulk materials. It is therefore of crucial importance to understand the influence of variations in the NWs crystal structure on their fundamental physical properties. In this work we investigate effects of structural polytypism on the optical properties of gallium phosphide and GaP/GaNP core/shell NW structures by a correlative investigation on the structural and optical properties of individual NWs. The former is monitored by transmission electron microscopy, whereas the latter is studied via cathodoluminescence (CL) mapping. It is found that structural defects, such as rotational twins in zinc blende (ZB) GaNP, have detrimental effects on light emission intensity at low temperatures by promoting nonradiative recombination processes. On the other hand, formation of the wurtzite (WZ) phase does not notably affect the CL intensity neither in GaP nor in the GaNP alloy. This suggests that zone folding in WZ GaP does not enhance its radiative efficiency, consistent with theoretical predictions. We also show that the change in the lattice structure have negligible effects on the bandgap energies of the GaNP alloys, at least within the range of the investigated nitrogen compositions of <2%. Both WZ and ZB GaNP are found to have a significantly higher efficiency of radiative recombination as compared with that in parental GaP, promising for potential applications of GaNP NWs as efficient nanoscale light emitters within the desirable amber-red spectral range

Optimizing GaNP Coaxial Nanowires for Efficient Light Emission by Controlling Formation of Surface and Interfacial Defects

We report on identification and control of important nonradiative recombination centers in GaNP coaxial nanowires (NWs) grown on Si substrates in an effort to significantly increase light emitting efficiency of these novel nanostructures promising for a wide variety of optoelectronic and photonic applications. A point defect complex, labeled as DD1 and consisting of a P atom with a neighboring partner aligned along a crystallographic ⟨111⟩ axis, is identified by optically detected magnetic resonance as a dominant nonradiative recombination center that resides mainly on the surface of the NWs and partly at the heterointerfaces. The formation of DD1 is found to be promoted by the presence of nitrogen and can be suppressed by reducing the strain between the core and shell layers, as well as by protecting the optically active shell by an outer passivating shell. Growth modes employed during the NW growth are shown to play a role. On the basis of these results, we identify the GaP/GaNyP1–y/GaNxP1–x (x &lt; y) core/shell/shell NW structure, where the GaNyP1–y inner shell with the highest nitrogen content serves as an active light-emitting layer, as the optimized and promising design for efficient light emitters based on GaNP NWs.Funding Agencies|Swedish Research Council [621-2010-3815]; U.S. National Science Foundation [DMR-0907652, DMR-1106369]; Royal Government of Thailand</p