Minority Carrier Diffusion Length Research Articles

Influence of Energetic Particles and Electron Injection on Minority Carrier Transport Properties in Gallium Oxide

The influence of various energetic particles and electron injection on the transport of minority carriers and non-equilibrium carrier recombination in Ga2O3 is summarized in this review. In Ga2O3 semiconductors, if robust p-type material and bipolar structures become available, the diffusion lengths of minority carriers will be of critical significance. The diffusion length of minority carriers dictates the functionality of electronic devices such as diodes, transistors, and detectors. One of the problems in ultrawide-bandgap materials technology is the short carrier diffusion length caused by the scattering on extended defects. Electron injection in n- and p-type gallium oxide results in a significant increase in the diffusion length, even after its deterioration, due to exposure to alpha and proton irradiation. Furthermore, post electron injection, the diffusion length of an irradiated material exceeds that of Ga2O3 prior to irradiation and injection. The root cause of the electron injection-induced effect is attributed to the increase in the minority carrier lifetime in the material due to the trapping of non-equilibrium electrons on native point defects. It is therefore concluded that electron injection is capable of “healing” the adverse impact of radiation in Ga2O3 and can be used for the control of minority carrier transport and, therefore, device performance.

Understanding the light induced hydrophilicity of metal-oxide thin films

Photocatalytic effects resulting in water splitting, reduction of carbon dioxide to fuels using solar energy, decomposition of organic compounds, and light-induced hydrophilicity observed on surfaces of various metal oxides (MOx), all rely on the same basic physical mechanisms, and have attracted considerable interest over the past decades. TiO2 and ZnO, two natively n-type doped wide bandgap semiconductors exhibit the effects mentioned above. In this study we propose a model for the photo-induced hydrophilicity in MOx films, and we test the model for TiO2/Si and ZnO/Si heterojunctions. Experimentally, we employ a wet exposure technique whereby the MOx surface is exposed to UV light while a water droplet is sitting on the surface, which allows for a continuous recording of contact angles during illumination. The proposed model and the experimental techniques allow a determination of minority carrier diffusion lengths by contact angle measurements and suggest design rules for materials exhibiting photocatalytic hydrophilicity. We expect that this methodology can be extended to improve our physical understanding of other photocatalytic surface effects.

Impact of Solid-State Charge Injection on Spectral Photoresponse of NiO/Ga2O3 p–n Heterojunction

Forward bias hole injection from 10-nm-thick p-type nickel oxide layers into 10-μm-thick n-type gallium oxide in a vertical NiO/Ga2O3 p–n heterojunction leads to enhancement of photoresponse of more than a factor of 2 when measured from this junction. While it takes only 600 s to obtain such a pronounced increase in photoresponse, it persists for hours, indicating the feasibility of photovoltaic device performance control. The effect is ascribed to a charge injection-induced increase in minority carrier (hole) diffusion length (resulting in improved collection of photogenerated non-equilibrium carriers) in n-type β-Ga2O3 epitaxial layers due to trapping of injected charge (holes) on deep meta-stable levels in the material and the subsequent blocking of non-equilibrium carrier recombination through these levels. Suppressed recombination leads to increased non-equilibrium carrier lifetime, in turn determining a longer diffusion length and being the root-cause of the effect of charge injection.

Alumina layers deposited by atomic layer deposition with different precursors: Surface photovoltage measurements

The electrical and passivation properties of alumina layers deposited by the atomic layer deposition (ALD) technique with trimethylaluminium (TMA) and two different oxidants (ozone and water) and their mixture under different deposition conditions were investigated by using surface photovoltage. In all alumina layers negatively charged defects were observed after the ALD deposition. The density of interface states (Dit) and the density of negative charge (CFix) in as-deposited alumina layers prepared with water was found to be lower in comparison to wafers prepared with O3 or with a mixture of H2O and O3. The minority carrier diffusion length was also larger in wafers prepared with H2O and TMA. By comparing the deposition conditions of different alumina layers we concluded that interstitial oxygen could be responsible for the appearance of the negatively charged defects. After the broadband illumination we also observed a significant increase of CFix in all alumina layers independent of the precursors used for the deposition and only small changes of Dit were detected. O-related defects could also lead to such changes of CFix after the illumination.

Design of CZTS/CZTSe tandem solar cells with enhanced performance

The replacement of toxic and expensive materials with non-toxic, earth-abundant alternatives is an urgent necessity in the field of solar cells. Due to their environmentally friendly and optical properties, CZTS and CZTSe kesterite materials have been proposed as promising alternative compounds in thin film solar cell applications. In this study, a numerical investigation of a CZTS/CZTSe tandem solar cell structure is conducted using SILVACO software. In this structure, CZTS and CZTSe serve as the main absorbers in the top and bottom sub-cells, respectively. However, the bottom sub-cells require thick absorbers, which leads to a decrease in overall performance due to the limited diffusion length of minority carriers in the applied materials. A separate study of individually top and bottom cells was conducted, and their results closely matched those reported by world champions. Effects of the top absorber thickness were examined for the tandem configuration, which resulted in a conversion efficiency (η) of 15.71%. In order to enhance the performance of the tandem solar cell, a creation of a diffusion electric field in the absorbers of the sub-cells through gradient doping is proposed. The results demonstrated a significant improvement in performance, with gradient doping densities of 2 × 1014-1019 cm−3 and 1.5 × 1015-1019 cm−3 in the CZTS and CZTSe layers, respectively, yielding a remarkable conversion efficiency of 23.38%.

Minority carrier diffusion length in interface-confined PbS colloidal quantum dot (CQD): Effect of CQD surface modification on diffusion length in ultrathin PbS CQD films

Photocurrent microscopy (PCM) is a versatile tool for probing charge carrier transport associated with energy band bending at an interface. For the first time, we report the ligand-dependent electronic properties of interface-confined colloidal quantum dots (CQDs) near a metal electrode and gate dielectric layers using ultrathin lead sulfide (PbS) CQD films embedded in field effect transistors (FETs). We have compared the minority carrier (electron) diffusion length (MCDL) of ultrathin PbS CQD films treated with 1,2-ethanedithiol (EDT) and ammonium sulfide ((NH4)2S) using a bottom-contact FET structure in combination with PCM. We found that the MCDL was greater in the EDT-treated PbS CQDs than in the (NH4)2S-treated PbS CQDs. The MCDL at the 2D carrier accumulation layer was observed to depend on the concentration of the majority carrier (hole). During effective electrical channel formation by electrical gating, the MCDL decreased with hole accumulation (decreasing gate voltage) in the p-channel (NH4)2S-treated PbS CQD FET, whereas it remained similar in the EDT-treated PbS CQD FET. The effect of the majority carrier concentration on the MCDL was more pronounced in the (NH4)2S-treated sample than in the EDT-treated sample. The difference in the MCDL is attributed to a difference in the surface trap density caused by different ligand treatments.

Development of MOVPE grown GaSb-on-GaAs interfacial misfit solar cells

GaSb grown on GaAs through interfacial misfit (IMF) arrays grown via molecular beam epitaxy has been heavily studied; there is limited research, however, on IMF growth through metal-organic vapor phase epitaxy. To demonstrate viability for integration in a multijunction solar cell for terrestrial use, it is imperative to demonstrate high quality GaSb grown on GaAs through metal-organic vapor phase epitaxy. The preferred gallium precursors for n-type and p-type GaSb for longest minority carrier diffusion length were determined to be trimethylgallium and triethylgallium, respectively. A heteroepitaxial GaSb-on-GaAs device attained an open-circuit voltage of 190 mV and an efficiency of 2.2%. Extracted threading dislocation density from the minority carrier lifetime for the heteroepitaxial GaSb-on-GaAs device was determined to be 7.5×106cm−2. In a modeled multijunction solar cell, this device attributes to an overall efficiency of 33.1% under AM1.5g illumination.

Study of porous silicon layer effect in optoelectronics properties of crystalline silicon

In this research, we experimentally and numerically demonstrate the beneficial effect of superficial porous silicon layer in the optoelectronics properties of multi-crystalline silicon. The hydrogen and oxygen-rich porous silicon layer was formed using vapor etching method. From laser beam induced current (LBIC) simulations, we found that the hydrogen and oxygen-rich porous silicon layer used in mc-Si surface acts as a good surface passivation and a potential candidate for electronic quality and optoelectronics properties improvement. As a result, the generated current of treated silicon is 5 times greater as compared to reference substrate, which led to a 50% increase of the minority carrier diffusion length, 25% decrease in the recombination velocity of the minority carrier and the reflectivity reduced from 38 to 3% of the related sample.

Measurement of minority carrier diffusion length in p-GaN using electron emission spectroscopy (EES)

Electron emission spectroscopy was performed on metalorganic chemical vapor deposition grown p-n−-n+ junctions with p-thicknesses ranging from 50 to 300 nm, doped with [Mg] = 3.5 × 1019 cm−3. By measuring the decreasing emitted electron intensity from a cesiated p-GaN surface with increasing p-thickness, we were able to extract the minority carrier diffusion length of electron in p-type GaN, Le = 26 ± 3 nm. The measured value is in good agreement with literature reported values. The extrapolated electron current at the n− region–p-GaN interface is in reasonable agreement with the simulated electron current at the interface.

Hysteresis reduction in planar perovskite solar cell with CH3NH3PbI3 absorber by changing the precursor ratio and electron/hole transport layers

In this paper, we investigate the hysteresis of planar perovskite solar cells by altering the absorber (CH3NH3PbI3) precursor ratio and electron/hole transport layers (ETL/HTL) and their influence on minority carrier diffusion length and electron/hole flux. Field screening due to ion migration and also minority carriers recombination are the significant causes of hysteresis in the perovskite solar cells. Our simulation results indicate that the lowest hysteresis factor has corresponded to the precursor ratio of 0.80. Despite the lowest hysteresis at this precursor ratio, the electron/hole barrier at the interface of the ETL/HTL is high. To reduce these barriers height, we have presented a structure with minimal hysteresis by changing the HTL and the ETL from Spiro-OMeTAD to Cu2O/TCTA and TiO2 to ZnO, respectively. The developed structure increased the carrier flux balance and enhanced the electron/hole diffusion length. By choosing the appropriate ETL and HTL in different ranges of the precursor ratio from 0.30 to 1.70, we also presented structures with clearly reduced hysteresis factor.

Solution-Processed Cu2S Nanostructures for Solar Hydrogen Production.

Cu2S is a promising solar energy conversion material due to its suitable optical properties, high elemental earth abundance, and nontoxicity. In addition to the challenge of multiple stable secondary phases, the short minority carrier diffusion length poses an obstacle to its practical application. This work addresses the issue by synthesizing nanostructured Cu2S thin films, which enables increased charge carrier collection. A simple solution-processing method involving the preparation of CuCl and CuCl2 molecular inks in a thiol-amine solvent mixture followed by spin coating and low-temperature annealing was used to obtain phase-pure nanostructured (nanoplate and nanoparticle) Cu2S thin films. The photocathode based on the nanoplate Cu2S (FTO/Au/Cu2S/CdS/TiO2/RuO x ) reveals enhanced charge carrier collection and improved photoelectrochemical water-splitting performance compared to the photocathode based on the non-nanostructured Cu2S thin film reported previously. A photocurrent density of 3.0 mA cm-2 at -0.2 versus a reversible hydrogen electrode (V RHE) with only 100 nm thickness of a nanoplate Cu2S layer and an onset potential of 0.43 V RHE were obtained. This work provides a simple, cost-effective, and high-throughput method to prepare phase-pure nanostructured Cu2S thin films for scalable solar hydrogen production.

Illuminated from back or front? Insight into factors affecting the efficiency of BiVO4 photoanode

For a long time, it is generally believed that BiVO4 photoanode will obtain higher photocurrent under back-side illumination, the explanation for this is the diffusion length of electrons is shorter in this condition. However, for an n-type semiconductor, the diffusion length of majority carriers, that is, electrons, should not be the main reason affecting the photocurrent, while the diffusion length of minority carriers and the intensity of built-in electric field are the key factors. In this paper, we proved this by comparing the photoelectrochemical performance of BiVO4 films with different thicknesses and densities. More importantly, we found that it produced a potential drop on the polycrystalline boundary, which leads to the interface electric field cannot increased with the increasing of applied potential, this is exactly the reason for limiting the thickness and efficiency of BiVO4 photoelectrode. Based on this, we proposed a carriers transport model more suitable for polycrystalline-electrode.

Fabrication and characterization of high-resolution 4H-SiC epitaxial radiation detectors for challenging reactor dosimetry environments

Reactor dosimetry environments require radiation detectors that are capable of operating at high temperatures in extremely high neutron and gamma-ray dose rates. Silicon carbide (SiC) is one of the most promising wide bandgap semiconductors (3.27 eV) for harsh environment applications due to its radiation hardness, high breakdown voltage, high electron saturation velocity, and high thermal conductivity. In this paper, we summarize the prospect of Schottky barrier radiation detectors, fabricated on highly crystalline low-defect detector-grade n-type 4H-SiC epitaxial layers with thickness ranging from 20 to 250 lm, for harsh environment applications. A comprehensive discussion on the characterization of the parameters that influence the energy resolution has been included. The usage of electrical and radiation spectroscopic measurements for characterizing the junction and rectification properties, minority carrier diffusion lengths, and energy resolution has been elaborated. Characterization of crucial factors that limit the energy resolution of the detectors such as charge trap centers using thermally stimulated transient techniques is summarized. Finally, the effect of neutron fluence on the performance of the 4H-SiC detectors is discussed.

Back-illuminated photoelectrochemical flow cell for efficient CO2 reduction

Photoelectrochemical CO2 reduction reaction flow cells are promising devices to meet the requirements to produce solar fuels at the industrial scale. Photoelectrodes with wide bandgaps do not allow for efficient CO2 reduction at high current densities, while the integration of opaque photoelectrodes with narrow bandgaps in flow cell configurations still remains a challenge. This paper describes the design and fabrication of a back-illuminated Si photoanode promoted PEC flow cell for CO2 reduction reaction. The illumination area and catalytic sites of the Si photoelectrode are decoupled, owing to the effective passivation of defect states that allows for the long minority carrier diffusion length, that surpasses the thickness of the Si substrate. Hence, a solar-to-fuel conversion efficiency of CO of 2.42% and a Faradaic efficiency of 90% using Ag catalysts are achieved. For CO2 to C2+ products, the Faradaic efficiency of 53% and solar-to-fuel of 0.29% are achieved using Cu catalyst in flow cell.

Spectroscopic performance of Ni/4H-SiC and Ti/4H-SiC Schottky barrier diode alpha particle detectors

Advancement in the growth of 4H-SiC with low micropipe densities (∼ 0.11 cm-2) in achieving high pure epitaxial layers, enabled the development of high-resolution 4H-SiC alpha particle Schottky radiation detectors for harsh environments. In particular, the study considers two types of 4H-SiC radiation detectors having Ni and Ti as Schottky contacts. They are fabricated by depositing Ni and Ti on 25 μm thick n-type 4H-SiC by epitaxially growing on 350 μm thick conducting SiC substrates. Electrical characterization and alpha spectral measurements performed on Ni/4H-SiC and Ti/4H-SiC SBDs are reported in this work. The spectral measurements were carried out using 241Am alpha emitting radioactive source. Ni/ 4H-SiC Schottky detector showed a better spectral response with 22.87 keV FWHM (∼ 0.416%) at a reverse bias of 150 V for 5.48 MeV alpha particles while Ti/4H-SiC Schottky detector achieved a resolution of 38.25 keV FWHM (∼ 0.697%) at 170 V reverse bias. This work presented spectral broadening analysis to understand the various factors affecting the energy resolution of the detectors. The extracted charge collection efficiencies (CCEs) are approximately 99% in both the detectors. In addition, polarization effects are not noticed in any of the fabricated detectors. The diffusion length of minority carriers (Lp ) is computed based on the drift-diffusion model by fitting the CCE curve as a function of applied bias, and the values are close to 9 μm and 7 μm for Ni/4H-SiC SBD and Ti/4H-SiC SBD detectors, respectively. Annealing at 400°C for 5 minutes in N2 ambient resulted in resolution of 23.98 keV FWHM (∼ 0.436%) for Ni/4H-SiC SBD detector at -170 V and 36.21 keV FWHM (∼ 0.661%) for Ti/4H-SiC SBD detector at -150 V. Overall Ni/4H-SiC SBD detectors showed superior spectral characteristics and superior resolution when compared to Ti/4H-SiC SBD detectors. However, the Ti/4H-SiC SBD detector fabricated in this work performed better than the previously reported work on a similar device structure. Hence, future work aimed at improving resolution of radiation detectors could also consider Ti/4H-SiC SBDs along with Ni/4H-SiC SBDs.

Room-temperature nonradiative recombination lifetimes in c-plane Al1−xInxN epilayers nearly and modestly lattice-matched to GaN (0.11 ≤ x ≤ 0.21)

Lattice-matched Al1−xInxN / GaN heterostructures with InN mole fraction (x) of 0.18 have attracted considerable interest for use in GaN-based optoelectronic devices. Because the light emission efficiency (ηemission) of Al1−xInxN alloys is far less than that of InxGa1−xN, understanding its causes is essential. For this purpose, room-temperature photoluminescence lifetime (τPLRT), which almost represents the nonradiative recombination lifetime that limits the internal quantum efficiency in low ηemission semiconductors, of c-plane Al1−xInxN epilayers nearly and modestly lattice-matched to GaN (0.11≤x≤0.21) was examined. For the epilayers grown on low threading dislocation density (TDD) GaN substrates (≪107cm−2), τPLRT principally decreased with increasing x, indicating a progressive increase in the concentration of nonradiative recombination centers (NRCs), NNRC. One of the probable causes is the growth temperature (Tg) reduction that is indispensable to incorporate more In, because in insufficient Tg regime higher Tg is preferred for enhancing the surface migration of adatoms to decrease the concentrations of vacancies that compose NRCs. The Al1−xInxN epilayers of the same x but grown on high TDD (>108cm−2) GaN-on-sapphire templates exhibited shorter τPLRT. Because the diffusion length of minority carriers was nearly zero in the Al1−xInxN epilayers, the shorter τPLRT indicates higher bulk NNRC in high TDD epilayers. The Al1−xInxN epilayers of considerably rough surface morphologies exhibited spatially inhomogeneous τPLRT, implying that excited carriers recombined everywhere at InN-rich to InN-poor portions, where NNRC were likely lower to higher, respectively, than the average due to the deviations in the surface stoichiometry at various non-c-plane surfaces at a given Tg.

Electronic properties and photo-gain of UV-C photodetectors based on high-resistivity orthorhombic κ-Ga2O3 epilayers

Electrical and optical properties of nominally-undoped high-resistivity κ -Ga2O3 epitaxial films are presented, along with the ultraviolet detection performance of solar-blind photodetectors fabricated from these epilayers. The photodetectors exhibited a solar-blind rejection ratio higher than 104, reproducible on–off switching times and a remarkable photo-gain (up to 103). The latter can be ascribed to either a strong trapping effect of free holes by deep levels inducing a majority carrier excess Δn with respect to the minority carriers Δp, or to a hole mobility several orders of magnitude lower than the electron mobility, both the hypothesis being consistent with literature data. A saturation of the gain, dependent on detector size, was observed beyond a certain applied voltage, that is consistent with a minority carrier diffusion length of a few 10-5 cm.Non-critical growth conditions, reproducibility and intrinsic spectral selectivity together with good UV-detection performance make κ-Ga2O3 a suitable material for solar-blind ultraviolet photodetectors.

Optimization design of 4H–SiC-based betavoltaic battery using 3H source

This paper describes the theoretical calculation and optimization design of the PN junction betavoltaic batteries with 4H–SiC-based energy converter and titanium tritide source. The self-absorption of radioactive isotope sources and the energy deposition distribution in the semiconductor converter are simulated using the Monte Carlo method. The relationship between doping concentrations and basic factors such as minority carrier diffusion lengths and the width of the depletion region are analyzed via the calculation formulas. Then the maximum output power density and energy conversion efficiency are calculated. The optimal thickness of the titanium tritide film is about 0.7 μm, the doping concentrations are 2.5 × 1016 cm−3, and the junction depth of PN junction is 0.1 μm. The surface recombination velocities of electron and hole are 1 × 106 cm/s, respectively. The maximum output power density and energy conversion efficiency are 0.22 μW/cm2 and 2.37%, respectively.

Over 12% efficient kesterite solar cell via back interface engineering

Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) has attracted considerable attention as a non-toxic and earth-abundant solar cell material. During selenization of CZTSSe film at high temperature, the reaction between CZTSSe and Mo is one of the main reasons that result in unfavorable absorber and interface quality, which leads to large open circuit voltage deficit (VOC-def) and low fill factor (FF). Herein, a WO3 intermediate layer introduced at the back interface can effectually inhibit the unfavorable interface reaction between absorber and back electrode in the preliminary selenization progress; thus high-quality crystals are obtained. Through this back interface engineering, the traditional problems of phase segregation, voids in the absorber and over thick Mo(S,Se)2 at the back interface can be well solved, which greatly lessens the recombination in the bulk and at the interface. The increased minority carrier diffusion length, decreased barrier height at back interface contact and reduced deep acceptor defects give rise to systematic improvement in VOC and FF, finally a 12.66% conversion efficiency for CZTSSe solar cell has been achieved. This work provides a simple way to fabricate highly efficient solar cells and promotes a deeper understanding of the function of intermediate layer at back interface in kesterite-based solar cells.

Over 10% Efficient Cu2CdSnS4 Solar Cells Fabricated from Optimized Sulfurization

AbstractAt present, Kesterite‐based thin‐film solar cells, such as Cu2ZnSnS4 solar cells, involve serious band‐tailed states, which leads to low open‐circuit voltage, thereby hindering the further improvement of device performance. In stannite‐based materials, such as Cu2CdSnS4, the substitution of Zn with Cd can effectively suppress CuCd‐related point defects and defect clusters; thus, the band‐tailing state is few, which has attracted considerable research attention. In this work, on the basis of using optimized sulfurization and optimizing ratios (Cu/Cd+Sn) and temperatures, Cu2CdSnS4 thin films can be obtained with good quality and single‐phase composition, in which the device prepared at a ratio of 0.83 and 590 °C has the highest efficiency. Defect analysis shows that the substitution of Zn with Cd can effectively reduce CuCd‐related defects and defect clusters (such as 2CuCd+SnCd) and also decrease Urbach energy, fluctuations of bandgap, and electrostatic potential compared with kesterite‐based devices. In particular, Cu2CdSnS4 thin‐film solar cell prepared under optimized conditions (the ratio of 0.83 and 590 °C) has the minimum reverse saturation current, red shift, and the maximum minority carrier diffusion length. Therefore, an efficiency over 10% Cu2CdSnS4 thin‐film solar cell is reported, which shows the highest efficiency among stannite‐based solar cells to date.