Dopant Passivation Research Articles

Compound doping enables efficient CsPbBr3 nanocrystal light emitting diodes

All-inorganic cesium-based perovskite nanocrystals (PeNCs) attract much attentions in displays due to their facile synthesis protocol and superior luminous performance, while material instability hinders their practical applications. Herein, by inducing a multi-functional dopant, i.e., SrBr2, into the perovskite precursor solution, the as-formed SrBr2-doped CsPbBr3 NCs possess purer phases, suppressed defects and hydrophobic surfaces, which leads to a high photoluminescence quantum yield of ∼90 %, as well as much improved moisture stability. Benefiting from dopant passivation and improved luminescence properties, the doped CsPbBr3 light emitting diodes show five times and two times of current efficiency and external quantum efficiency improvements, respectively, in comparison to their undoped counterparts.

Improve Perovskite Solar Cell Stability and Efficiency via Multifunctional Multiple‐Ring Aromatic Dopant Passivation

Organic–inorganic perovskite solar cells, as a representative of the new generation of solar cells, have made satisfactory advancements in development over the past few years. However, the low carrier transport characteristics and energy loss brought on by defects severely restrict its development. This study develops a new liquid crystal material 1‐naphthalene acetic acid potassium salt (C12H9KO2, K‐NAA) as a dopant into the MA0.6FA0.4PbI3−xClx material. The presence of free K+ ions in K‐NAA modifies the perovskite lattice gap, and the CO structure can efficiently passivate the Pb2+ defect caused by the absence of coordination. Therefore, the grain boundaries, which serve as points of moisture penetration become blurred, hence suppressing charge trapping/recombination centers and enhancing the film quality and stability. The naphthalene in K‐NAA has a conjugated multiple‐ring aromatic structure. The abundant π‐electron delocalization present in the conjugated structure of the benzene ring may provide an electron‐rich environment outside the connected crystal structure, resulting in field‐effect passivation and effectively enhancing carrier transport performance. Hence, the devices deliver a champion power conversion efficiency (PCE) of 19.86%. After 700 h under the dark at 25 °C and 30% humidity, the optimized unencapsulated device PCE maintains at 92% of its initial efficiency.

NMR spectroscopy probes microstructure, dynamics and doping of metal halide perovskites.

Solid-state magic-angle spinning NMR spectroscopy is a powerful technique to probe atomic-level microstructure and structural dynamics in metal halide perovskites. It can be used to measure dopant incorporation, phase segregation, halide mixing, decomposition pathways, passivation mechanisms, short-range and long-range dynamics, and other local properties. This Review describes practical aspects of recording solid-state NMR data on halide perovskites and how these afford unique insights into new compositions, dopants and passivation agents. We discuss the applicability, feasibility and limitations of 1H, 13C, 15N, 14N, 133Cs, 87Rb, 39K, 207Pb, 119Sn, 113Cd, 209Bi, 115In, 19F and 2H NMR in typical experimental scenarios. We highlight the pivotal complementary role of solid-state mechanosynthesis, which enables highly sensitive NMR studies by providing large quantities of high-purity materials of arbitrary complexity and of chemical shifts calculated using density functional theory. We examine the broader impact of solid-state NMR on materials research and how its evolution over seven decades has benefitted structural studies of contemporary materials such as halide perovskites. Finally, we summarize some of the open questions in perovskite optoelectronics that could be addressed using solid-state NMR. We, thereby, hope to stimulate wider use of this technique in materials and optoelectronics research.

Self-aligned local contact opening and n+ diffusion by single-step laser doping from POx/Al2O3 passivation stacks

Laser doping is a promising route to realise industrially compatible processing of local contacts for high-efficiency solar cells, especially when the same film acts as both dopant source and passivation layer. In this work we demonstrate simultaneous local contact opening and n+ laser doping of silicon from positively charged POx/Al2O3 thin-film stacks, which also provide outstanding passivation of n-type silicon surfaces. Local n+ doped regions with sheet resistance ranging from 35 to ~540 Ω/□ are formed using single nanosecond laser pulses with varying fluence. ECV profiling shows net n-type doping in all cases, confirmed by SIMS profiling to be due to phosphorus from the POx layer. J0 of metallised laser-doped regions is consistent with values achieved for state-of-the-art furnace diffusions with similar sheet resistance, confirming that laser-induced recombination-active defects are avoided. A minimum J0 of 540 fA cm−2 is obtained for metallised laser-doped regions formed from POx/Al2O3 passivation stacks having J0 of 2.5 fA cm−2. The combination of outstanding passivation of uncontacted n-type regions offered by POx/Al2O3, with self-aligned formation of locally-diffused contact openings via single-step laser processing, opens up exciting possibilities for simplified fabrication of high-efficiency cell structures.

Dopant passivation by adsorbed water monomers causes high humidity sensitivity in PEDOT: PSS thin films at ppm-level humidity

Thin layers of hole conducting polymer poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT:PSS) are widely utilized in the fabrication of organic light emitting devices. More recently, the material has been considered as a platform for humidity sensor fabrication. This is interesting as the conductive component of this composite, PEDOT, is hydrophobic. Moreover, charge transport mechanism in PEDOT:PSS is not yet fully established and understanding the cause of sensitivity to humidity may shed further light on the obscure charge conduction in this conductive polymer composite. Here, we investigate the humidity sensitivity in PEDOT:PSS thin films in a wide relative humidity (RH) range and report the existence of a distinctly higher humidity sensitivity in the RH < 1% range. The observed sensitivity is electronic in nature, and is described by considering the hydrogen bonding of the airborne water monomers to the surface-exposed sulfonate groups on the hydrophilic PSS chain, which, in turn, passivates the doping process involving hydrogen ion transfer to the PEDOT chain from those sulfonate groups. This dopant passivation mechanism reduces the charge carrier population in the layer and increases its impedance. The results are anticipated to pave the way for the fabrication of a printable organic humidity sensor with an unprecedented wide dynamic range spanning to sub-ppm humidity levels.

Exploration of dopant and surface passivation on optical and morphological properties of AgInS2 nanocrystals

The present work is concerned with synthesis of doped (Cd2+ ions) and surface passivated (ZnS) AgInS2 core/shell nanocrystals by hot-injection method. UV spectral analysis of the nanocrystals thus prepared revealed that compared to dopant, passivation does much better in enhancing the optical property. Among the nanocrystals that were prepared, the growth of shell over core (AgInS2/ZnS) enhanced the optical property of AgInS2 considerably compared to dopant on either core or shell or both core-shell of nanocrystal. On the contrary, photoluminescence study revealed that doped AgInS2 with surface passivation (AgInS2:Cd2+/ZnS) plays a critical role in increasing PL intensity compared to either dopant or passivation alone on AgInS2. This inference is based on the fact that passivation was accompanied by decrease in surface-to-volume ratio that led to change in surface structure and crystal morphology. From XRD measurements we found that all the synthesized nanocrystals exhibited orthorhombic crystal structure with slight change in diffraction peak positions. The diffraction peak was shifted to lower angle with Cd2+ ions dopant and higher angle with surface passivation. The change in diffraction pattern suggests the formation of AgInZnS/ZnS alloy of ZnS on AgInS2 core. TEM images of pure AgInS2/ZnS and doped core/shell nanocrystals revealed that both pure and doped core/shell were irregular and sparingly mono-dispersed nanoparticles. EDX spectrum clearly revealed that pure and doped AgInS2/ZnS nanocrystals were composed of Ag, In, S, Zn and Cd ions in the synthesized systems. The formation of AgInS2/ZnS core/shell nanocrystals were confirmed from the experimental results and these nanocrystals might be a good material in many optoelectronics industrials and bio-imaging application in near future.

Comparative Analyses Between Two Techniques To Understand Metal-Induced Recombination Losses In Industrial N-Type Bifacial PERT Solar Cells

Due to the high cell efficiency, low Light-Induced Degradation (LID) and high bifaciality, n-type bifacial passivated emitter and rear totally diffused (PERT) solar cells are being widely researched and currently being implemented in mass production. With the improvement of dopant profiles and passivation quality, J0,emitter and J0,bsf under passivation area could reduce to lower values. However, the Voc of cell is still not high enough which may be caused by the high recombination loss under metal area. In this paper, two different methods which are suitable for low cell efficiency diagnoses are investigated and compared to evaluate the J0,metal on both boron and phosphorus doping side of solar cells. For the first method, high resistivity n-type mono wafers are prepared with double front side or rear side structure of solar cell. Four different types of metallization ratio areas are designed on the one wafer of single side. By testing the J0,total of these four area, we can fit a line of J0,total with different contact fraction, so the J0,metal can be calculated by the expression of J0,total, J0,passivation and contact fraction. For the second method, Suns-Voc and Suns-PL are used for calculation. Four types of cell structures with metal, without metal, only with front metal and only with rear metal are fabricated to measure the probed Voc and PL derived Voc either use (i) different carrier injection levels or (ii) different light illuminations. A line can be fitted under different Suns, and by contrasting the line of cell structure and other structures, J0,metal of front metal and rear metal can be calculated by the bias of these lines. By comparing this two methods, we get the boron side Ag/Al paste J0,metal and the phosphorus side Ag paste J0,metal are around 2801 fA/cm2 and 964.9 fA/cm2, respectively. By comparing this two methods, the value of second method is lower than the first method and more appropriate for IV simulation using Quokka.

Effect of hot carrier stress on device junctions measured by electron holography and scanning capacitance microscopy

The effect of hot carrier injection on the FET's junction properties has been investigated for CMOS NFET devices. Junction profiles and carrier concentration of stressed and unstressed devices are measured and compared by dual lens electron holography and scanning capacitance microscopy, respectively. The measurements reveal that the width of the junction on the drain side of the device that undergoes hot carrier stressing decreases, indicating a reduction in active carrier concentration. These results are consistent with the hypothesis of phosphorous dopant passivation by the hot carrier induced release of hydrogen into the drain side extension region.

Enhanced Light Emission from N-Doped Ge Microdisks by Thermal Oxidation

We have observed enhanced direct band-gap light emission from highly n-doped germanium microdisks on silicon by thermal oxidation. The microdisks are fabricated by electron beam lithography and dry etching. Thermal oxidation is found to be able to passivate the dry-etched sidewalls, thus efficiently suppress the nonradiative recombination related to the sidewalls and enhance the emission intensity. The enhancement factor increases as the disk size decreases, approaching about 3.42 for microdisks with radii of 0.8 µm. Investigation of light emission dependence on the thermal oxidation temperature from 100 to 550 °C show that there is a trade-off between reduction of doping concentration caused by dopant out-diffusion and surface passivation effect. These results indicate that thermal oxidation is a highly promising yet very simple way to fabricate high quality germanium microresonators, thus to realize low-threshold germanium lasers monolithically integrated on Si substrates.

Application and comparison of band gap narrowing models for passivated phosphorus doped silicon surfaces

In this work, the recently proposed band-gap narrowing model by Yan and Cuevas [J. Appl. Phys. 114, 044508 (2013)] is evaluated by simulations of the recombination pre-factor J0 of highly phosphorus doped, passivated crystalline silicon surfaces, which are particularly relevant for solar cell applications. The results were fitted to experimental J0 data measured on a large range of samples exhibiting different dopant profiles and passivation coatings, both for planar and textured surfaces. For each sample, the surface recombination velocity parameter Sp was extracted by fitting the simulation results to the experimental data. We show that the Yan and Cuevas' model developed for Fermi-Dirac statistics leads to a smooth and monotonically increasing curve for Sp as a function of the surface dopant concentration Nsurf, for both investigated passivation layers. We provide a parameterization for this relation and compare the findings with those obtained with the widely used model by Schenk [J. Appl. Phys. 84, 3684 (1998)]. On the other hand, we show that the apparent band gap narrowing of Yan and Cuevas developed for use with Boltzmann statistics cannot be used to describe the experimental data, requiring unphysical negative Sp values for high Nsurf.

Silicon dopant passivation by nitrogen during molecular beam epitaxy of GaNAs

We have studied the Si doping efficiency in dilute nitride GaNAs by gas-source molecular beam epitaxy across a substrate temperature range from 460 to 570 °C. Particularly, for samples grown at ~480 °C, the doping efficiency changes drastically from 100 to almost 0 % as the N compositions varies from 0 to 3.1 %. By comparing experimental data to Monte Carlo simulation of N adatom surface diffusion during growth, the change in doping efficiency is believed to be due to passivation of Si dopants by N during epitaxy.

Interaction of Boron and Phosphorus Impurities in Silicon Nanowires during Low-Temperature Ozone Oxidation

In doped Si nanowires (SiNWs) boron (B) atoms segregate to the surface oxide layers during thermal oxidation, while phosphorus (P) atoms preferentially pile up in Si crystalline regions close to the Si/SiO2 interface. Here we report on micro-Raman scattering and electron spin resonance (ESR) measurements showing that B atoms can be stabilized at the crystalline Si core region in codped SiNWs with average diameters of 20–30 nm because of the strong interaction between B and P atoms during thermal oxidation below 800 °C. Theoretical calculation clearly demonstrated the effect of B–P pairing, which can stabilize the B atoms in the Si side. In the B–P pairing configuration, dopant passivation—beyond simple compensation—occurs, making the impurities electrically inactive.

Multifunctional PECVD Layers: Dopant Source, Passivation, and Optics

In this study, the feasibility of creating one dielectric layer system acting simultaneously as antireflection coating, phosphorous doping source, masking against metal plating, and surface passivation is presented. Moreover, a similar layer stack is described, which behaves as rear-side surface passivation, boron dopant source, and internal reflection mirror. The optical characteristics of these layers are especially investigated and optimized for a solar cell's front- and rear-side coating. By consequent use of the multifunctional layers, a totally diffused solar cell with rear-side passivation and local rear contacts can be produced using only one wet chemical bath sequence, one multilayer vacuum step, and one high-temperature process. We present the first proof of concept for such a solar cell production using multifunctional plasma-enhanced chemical vapor deposition layers resulting already in a conversion efficiency of 18.3%.

Performance analysis of AlGaAs/GaAs tunnel junctions for ultra-high concentration photovoltaics

An n++-GaAs/p++-AlGaAs tunnel junction with a peak current density of 10 100 A cm−2 is developed. This device is a tunnel junction for multijunction solar cells, grown lattice-matched on standard GaAs or Ge substrates, with the highest peak current density ever reported. The voltage drop for a current density equivalent to the operation of the multijunction solar cell up to 10 000 suns is below 5 mV. Trap-assisted tunnelling is proposed to be behind this performance, which cannot be justified by simple band-to-band tunnelling. The metal-organic vapour-phase epitaxy growth conditions, which are in the limits of the transport-limited regime, and the heavy tellurium doping levels are the proposed origins of the defects enabling trap-assisted tunnelling. The hypothesis of trap-assisted tunnelling is supported by the observed annealing behaviour of the tunnel junctions, which cannot be explained in terms of dopant diffusion or passivation. For the integration of these tunnel junctions into a triple-junction solar cell, AlGaAs barrier layers are introduced to suppress the formation of parasitic junctions, but this is found to significantly degrade the performance of the tunnel junctions. However, the annealed tunnel junctions with barrier layers still exhibit a peak current density higher than 2500 A cm−2 and a voltage drop at 10 000 suns of around 20 mV, which are excellent properties for tunnel junctions and mean they can serve as low-loss interconnections in multijunction solar cells working at ultra-high concentrations.

Dopant passivation and work function tuning through attachment of heterogeneous organic monolayers on silicon in ultrahigh vacuum

Electronic structures of silicon–organic interfaces were studied by the scanning Kelvin probe technique. These surfaces were fabricated by covalent bonding of a range of phenylacetylene-based molecules ( p-X–C 6H 4C CH, where X = CF 3, OCH 3, and H) onto a hydrogen-terminated silicon surface. Organic molecules were bound to the surface under high vacuum conditions by ultraviolet light-induced hydrosilylation. Changes in the electronic structure due to electron-donating ability and dipole moment were analyzed under dark and illuminated conditions. The origin of the silicon band bending was tested to separate the effects of molecular monolayers and unintended dopant passivation. In addition, heterogeneous monolayers were grown by controllably diluting the incoming vapor stream with acetylene during growth. The measured work functions follow a trend linked to dipole moment that can be further tuned by molecular dilution. These results suggest a way to use heterogeneous organic monolayers to tune the electron affinity of silicon and directly alter channel modulation in small semiconductor devices.

Interaction of NiSi with dopants for metallic source/drain applications

This work has a focus on NiSi as a possible metallic contact for aggressively scaled complementary metal oxide semiconductor devices. As the bulk work function of NiSi lies close to the middle of Si bandgap, the Schottky barrier height (SBH) of NiSi is rather large for both electron (∼0.65eV) and hole (∼0.45eV). Different approaches have therefore been intensively investigated in the literature aiming at reducing the effective SBH: dopant segregation (DS), surface passivation (SP), and alloying, in order to improve the carrier injection into the conduction channel of a field-effect transistor. The present work explores DS using B and As for the NiSi∕Si contact system. The effects of C and N implantation into Si substrate prior to the NiSi formation are examined, and it is found that the presence of C yields positive effects in helping reduce the effective SBH to 0.1–0.2eV for both conduction polarities. A combined use of DS or SP with alloying could be considered for more effective control of effective SBH, but an examination of undesired compound formation and its probable consequences is necessary. Furthermore, an analysis of the metal silicides that have a small “intrinsic” SBH reveals that only a very small number of them are of practical interest as most of the silicides require either a high formation temperature or possess a high specific resistivity.

Evidence of Radiation-Induced Dopant Neutralization in Partially-Depleted SOI NMOSFETs

Radiation-induced dopant passivation is evidenced for the first time in partially-depleted SOI n-channel MOSFETs. Isochronal annealing experiments following 10 Mrad(SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) irradiation demonstrate that the neutralization of boron atoms in the NMOSFET body is most pronounced in the 125degC-150degC temperature range. This results in an abrupt decrease of the threshold voltage and the subthreshold swing, due to the transition of the body from partial to full depletion. The SOI four-gate transistor, inherently present in the partially-depleted MOSFET structure, is demonstrated to be a very efficient tool for monitoring dopant neutralization through irradiation and annealing. Radiation-induced dopant passivation has important consequences regarding the reliability of short-channel partially-depleted NMOSFETs.

Investigation of short-term current gain stability of GaInP/GaAs-HBTs grown by MOVPE

Initial current gain variation in GaInP/GaAs-HBTs stressed at high-current density was investigated in dependence on base–dopant concentration and hydrogen passivation. It is shown that current-induced acceptor activation results in short-term current gain reduction, but does not affect emitter–base heterojunction properties. The incorporation of hydrogen during growth and the resulting passivation of carbon acceptors in the base layer depends mainly on carbon doping level and the precursors present during growth and cool-down. Under the growth conditions used, in-diffusion of hydrogen within AsH 3-rich growth steps turned out to be the major source of base–dopant passivation. Utilizing the blocking effect of n-type layers for H + in-diffusion we were able to reduce the base passivation ratio [H]/[C] to 0.1 leading to significant improvements in gain stability. It is shown that current gain instabilities can be completely suppressed by using a high-current density pre-stress treatment.

A hydrogen pathway for electronic processes in amorphous silicon

The characteristic difference in the diamagnetic fractions of hydrogen in doped and undoped amorphous hydrogenated silicon (a-Si:H), as obtained through MuSR modelling, is investigated by first reviewing recent calculations of the electronic energies of interstitial hydrogen. This results firstly in the identification of a charge transfer process between dopant atoms and hydrogen interstitials that provides a new mechanism for dopant passivation. It is concluded, however, that the increase in the diamagnetic fraction with temperature in doped a-Si:H can only be explained by the diffusion of hydrogen interstitials to charged dangling bond sites. These two interactions, of interstitial hydrogen and dopants on the one hand, and hydrogen and dangling bond defects on the other, are then used to propose a hydrogen pathway between dopants and dangling bond defects that explains why the process of doping amorphous silicon is always accompanied by the formation of charged dangling bonds.

Surface Preparation of InAs (110) Using Atomic Hydrogen

Atomic hydrogen cleaning has been used to produce structurally and electronically damage-free InAs(110) surfaces. X-ray photoelectron spectroscopy (XPS) was used to obtain chemical composition and chemical state information about the surface, before and after the removal of the atmospheric contamination. Low energy electron diffraction (LEED) and high-resolution electron-energy-loss spectroscopy (HREELS) were also used, respectively, to determine the surface reconstruction and degree of surface ordering, and to probe the adsorbed contaminant vibrational modes and the collective excitations of the clean surface. Clean, ordered and stoichiometric InAs(110)-(1×1) surfaces were obtained by exposure to thermally generated atomic hydrogen at a substrate temperature as low as 400ºC. Semi-classical dielectric theory analysis of HREEL spectra of the phonon and plasmon excitations of the clean surface indicate that no electronic damage or dopant passivation were induced by the surface preparation method.