Dopant Compensation Research Articles

Fabrication of low loss and high speed silicon optical modulator using doping compensation method

Compared with an optical modulator based on lithium niobate, the total loss of the current high speed silicon modulator is still too high for commercial use. Reduction of the total loss always comes along with the degradation of the other two characteristics including modulation efficiency or switching speed. In this paper, we reduce the phase shifter loss through optimizing the doping level out of the depletion region while keeping the modulation efficiency and switching speed at a high level. Compensated doping method is utilized to optimize the doping level on the cross section of the phase shift. With doping compensation, the Loss·Efficiency figure-of-merit (FOM) of 4 mm phase shifter is reduced from 25.8 dB·V to 19.4 dB·V while still keeping the small signal 3 dB-bandwidth at about 10 GHz. After doping profile optimizing, the measured bandwidth of the phase shifter with doping compensation can even reaches 17 GHz with a Loss·Efficiency FOM of about 25.4 dB·V.

Charge compensation in trivalent cation doped bulk rutileTiO2

Doping of TiO2 is a very active field, with a particularly large effort expended using density functional theory (DFT) to modeldoped TiO2; this interest has arisen from the potential for doping to be used in tuning the band gap ofTiO2 for photocatalytic applications. Doping is also of importance for modifying the reactivity ofan oxide. Finally, dopants can also be unintentionally incorporated into an oxide duringprocessing, giving unexpected electronic properties. To unravel properly how dopingimpacts on the properties of a metal oxide requires a modelling approach that candescribe such systems consistently. Unfortunately, DFT, as used in the majority ofstudies, is not suitable for application here and in many cases cannot even yield aqualitatively consistent description. In this paper we investigate the doping of bulk rutileTiO2 with trivalent cations, Al, Ga and In, using DFT, DFT corrected for on-site Coulomb interactions (DFT + U, withU on oxygen 2p states) and hybrid DFT (the screened exchange HSE06 exchange correlationfunctional) in an effort to better understand the performance of DFT in describing suchfundamental doping scenarios and to analyse the process of charge compensation with thesedopants. With all dopants, DFT delocalizes the oxygen hole polaron that results fromsubstitution of Ti with the lower valence cation. DFT also finds an undistortedgeometry and does not produce the characteristic polaron state in the band gap.DFT + U and hybrid DFT both localize the polaron, and this is accompaniedby a distortion to the structure around the oxygen hole site.DFT + U and HSE06 both give a polaron state in the band gap. The band gap underestimation present inDFT + U means that the offset of the gap state from both the valence and the conductionband cannot be properly described, while the hybrid DFT offsets should becorrect. We have investigated dopant charge compensation by formation ofoxygen vacancies. Due to the large number of calculations required, we useDFT + U for these studies. We find that the most stable oxygen vacancy site has either a very smallpositive formation energy or is negative, so under typical experimental conditions, anionvacancy formation will compensate for the dopant.

Drain Current Collapse in Nanoscaled Bulk MOSFETs Due to Random Dopant Compensation in the Source/Drain Extensions

We reveal a new statistical variability phenomenon in bulk n-channel metal-oxide-semiconductor field-effect transistors scaled down to 18-nm physical gate length. Rare but dramatic on-current degradation is observed in 3-D simulations of large ensembles of transistors that are subject to random dopant fluctuations. Physically, it originates from the random compensation of donors (from the source or drain extension) and acceptors (from halo implants) around the access regions to the channel, leading to mobile charge starvation, dramatic increase in the access resistance, and corresponding current collapse. The estimated frequency of occurrence of the phenomenon is higher than one in a hundred for a square device and higher than 10-4 for two-times-wider devices, as demonstrated by simulations of 10 000-device ensembles.

Effect of compensation doping on the electrical and optical properties of mid-infrared type-II InAs/GaSb superlattice photodetectors

This paper presents a theoretical study on the electrical and optical properties of mid-infrared type-II InAs/GaSb superlattices with different beryllium concentrations in the InAs layer of the active region. Dark current, resistance-area product, absorption coefficient and quantum efficiency characteristics are thoroughly examined. The superlattice is residually n-type and it becomes slightly p-type by varying beryllium-doping concentrations, which improves its electrical performances. The optical performances remain almost unaffected with relatively low p-doping levels and begin to deteriorate with increasing p-doping density. To make a compromise between the electrical and optical performances, the photodetector with a doping concentration of 3 × 1015 cm−3 in the active region is believed to have the best overall performances.

Dual temperature process for reduction in regrowth interfacial charge in AlGaN/GaN HEMTs grown on GaN substrates

AbstractThe effects of growth temperature and Mg compensation doping on the structural and electrical properties of AlGaN/AlN/GaN high electron mobility transistor (HEMTs) structures grown on low threading dislocation density bulk GaN substrates by metalorganic chemical vapor deposition were investigated. The background electron concentration in the regrown GaN was found to decrease from 1.5x1018 cm‐3 to 2x1016 cm‐3 as the growth temperature was reduced from 1100 °C to 950 °C. A dual temperature process was then employed for growth of the GaN base layer and AlGaN/AlN/GaN top heterostructure in the HEMT along with Mg doping of the GaN base to compensate residual Si donors at the regrown interface. Using this approach, AlGaN/AlN/GaN HEMTs were produced that had a sheet carrier density as high as 1.1x1013 cm‐3 with a room temperature mobility of 1600 cm2/Vs. The incorporation of a thin AlN layer at the regrown interface was found to reduce the sheet carrier density of the overall structure (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Low temperature‐coefficient for solar cells processed from solar‐grade silicon purified by metallurgical route

ABSTRACTThis study is dedicated to the temperature (T)‐variation of the photovoltaic performances of solar cells made from solar‐grade silicon directly purified by metallurgical route (SoGM‐Si). Experimental results were systematically compared with those for standard electronic‐grade silicon (EG‐Si) solar cells. We showed that the conversion efficiency (η) of SoGM‐Si cells decreases much less when T increases than the η of EG‐Si cells. This major difference is due to a strong increase with T of the short‐circuit current density (Jsc) of the SoGM‐Si solar cells. We showed that this a priori unexpected result could be described and explained by numerical simulations, by taking into account the main particularities of SoGM‐Si: dopant compensation, moderate minority carrier diffusion length and larger amount of boron–oxygen complexes. These results are significant since T of a solar module under illumination being generally higher than 25°C, modules made from low‐cost SoGM‐Si cells should have performances closer to those of standard EG‐Si solar panels. Copyright © 2011 John Wiley & Sons, Ltd.

Charge Compensation and Ce3+Formation in Trivalent Doping of the CeO2(110) Surface: The Key Role of Dopant Ionic Radius

In this paper, we use density functional theory corrected for on-site Coulomb interactions (DFT + U) and hybrid DFT (HSE06 functional) to study the defects formed when the ceria (110) surface is doped with a series of trivalent dopants, namely, Al3+, Sc3+, Y3+, and In3+. Using the hybrid DFT HSE06 exchange-correlation functional as a benchmark, we show that doping the (110) surface with a single trivalent ion leads to formation of a localized MCe/ + OO• (M = the 3+ dopant), O− hole state, confirming the description found with DFT + U. We use DFT + U to investigate the energetics of dopant compensation through formation of the 2MCe′ +VO·· defect, that is, compensation of two dopants with an oxygen vacancy. In conjunction with earlier work on La-doped CeO2, we find that the stability of the compensating anion vacancy depends on the dopant ionic radius. For Al3+, which has the smallest ionic radius, and Sc3+ and In3+, with intermediate ionic radii, formation of a compensating oxygen vacancy is stable. On the...

Demonstration of a nipi‐diode photovoltaic

AbstractThe simulation, growth, processing, and characterization of a three‐period GaAs n‐type/intrinsic/p‐type/intrinsic … (nipi) doping solar cell is demonstrated. A V‐groove etching process is characterized and used to expose the multiple n‐type and p‐type layers for electrical connection made by interdigitated grid‐finger electrodes. A five‐layer photolithographic process flow is developed and used to make 1 × 1 cm2 devices with varying grid‐finger separation. Device simulations of the structure indicate that strong rectification can be achieved in the parallel‐connected three period nipi GaAs solar cell structure provided the necessary semiconductor doping compensation is achieved in the region near the metal‐semiconductor interfaces. Experimentally, the improvements observed in the open circuit voltage, short circuit current, and ideality of the devices following thermal annealing suggests the formation of barriers near the contacts, which support the simulation results. A comparison of the short circuit current and series resistance under illumination indicate a tradeoff between shadowing and series resistance, which may be overcome with modification to the device structure. Ultimately, these results show promise towards the development of high efficiency solar cells or radioisotope batteries, and offer a novel device structure for the incorporation of nano‐structures such as quantum wells or quantum dots. Copyright © 2010 John Wiley & Sons, Ltd.

The impact of dopant compensation on the boron–oxygen defect in p‐ and n‐type crystalline silicon

AbstractWe review recent results relating to the boron–oxygen defect in compensated crystalline silicon for solar cells. The experimental observations are not easily explained by the standard model for the boron–oxygen defect, which involves substitutional boron. In addition, the proposed presence of boron–phosphorus pairs as a possible explanation for these findings is inconsistent with numerous other results. A recently proposed new model for the defect, based on interstitial boron, appears to resolve these problems. In this paper we attempt to extend this model to the case of boron‐containing n‐type silicon. The model predicts that the defect will occur in such material, as has been observed experimentally. However, the tentatively predicted impact of the defect on carrier lifetimes in such material does not appear to be consistent with recent experimental results.

CO-free hydrogen production over Au/CeO2–Fe2O3 catalysts: Part 2. Impact of the support composition on the performance in the water-gas shift reaction

Gold catalysts were prepared by deposition-precipitation method on mixed Ce–Fe composite oxides, synthesized by the urea gelation coprecipitation pathway. The impact of the support composition on the catalytic performance for the water-gas shift reaction was studied by varying the Ce/(Ce+Fe) ratio. The following activity order was observed: Au/CeO2>Au/Ce50Fe50>Au/Ce75Fe25>Au/Ce25Fe75>Au/Fe2O3. The differences in the gold particle size (1–25nm) and in the concentration of surface Ce3+ defect sites could explain the WGS activity order. The analysis of the HRTEM data combined with a FTIR study suggested the presence of highly dispersed gold clusters on the surface of Au/CeO2. Additionally, a higher amount of Ce3+ sites was registered by FTIR spectroscopy on the surface of this catalyst. Mössbauer spectra collected at room temperature and 90K evidenced that different amounts of cubic CeO2-like solid solution (CeO2FeSS) and hematite coexisted in CeO2–Fe2O3 supports. The analysis of the characterization data suggested that Fe3+ were distributed at Ce4+ and interstitial sites by a dopant compensation mechanism. This mechanism of solid solution formation accounted for the lower oxygen vacancy concentration and Ce3+ amount on all gold catalysts supported on mixed CeO2–Fe2O3 materials, which affected the performance in the WGSR.

Transport properties of p‐type compensated silicon at room temperature

AbstractDeliberate dopant compensation has proven to be an effective way to control ingot resistivity, although the impact of compensation on carrier recombination and mobilities remains under investigation. This paper summarizes recent findings regarding the carrier transport properties of compensated silicon. The capacity of common mobility models to describe compensated silicon is reviewed and compared to experimental data. The observed reduction of both majority and minority carrier mobility due to dopant compensation is described in terms of the underlying scattering mechanisms. The related problem of conversion between resistivity and dopant density in compensated silicon is discussed, and published values of the Hall Factor in compensated silicon are reviewed. Copyright © 2010 John Wiley & Sons, Ltd.

CO-free hydrogen production over Au/CeO2–Fe2O3 catalysts: Part 1. Impact of the support composition on the performance for the preferential CO oxidation reaction

Ce–Fe mixed oxides were prepared by urea gelation coprecipitation method and used as supports of gold catalysts. The impact of the support composition on the catalytic performance for the preferential CO oxidation (PROX) was studied by varying the Ce/(Ce+Fe) ratio. A deep characterization study by different tools such as XRD, HRTEM, TPR and FTIR spectroscopy was undertaken in order to correlate the structural characteristics of the catalysts and the gold oxidation state and dispersion with the catalytic properties. The results revealed that the variation of the support composition led to significant differences in the gold particles size (in the range 1–25nm), which affected strongly the CO oxidation activity of Au/CeO2–Fe2O3 catalysts under PROX conditions. The following activity order was observed: Au/CeO2≈Au/Ce50Fe50>Au/Ce75Fe25>Au/Ce25Fe75>Au/Fe2O3. The support with composition 50wt.% CeO2–50wt.% Fe2O3 appeared beneficial not only for nucleation and growth of highly dispersed gold particles (1–1.8nm), but also for activation of oxygen and its mobility. Moreover, the presence of Fe2O3 in the supports composition improved the resistance towards deactivation by CO2. The CeO2–Fe2O3 supports comprised different amount of two phases: cubic CeO2-like solid solution and hematite. The analysis of the characterization data suggested that the solid solution formation probably proceeded via a dopant interstitial compensation mechanism.

Hall mobility reduction in single-crystalline silicon gradually compensated by thermal donors activation

This letter focuses on the variation of the Hall majority carrier mobility with the dopant compensation level in purely Boron-doped Czochralski grown silicon single crystals. Compensation was varied continuously at the sample scale via a step by step activation of the oxygen-based thermal donors. At room temperature, we show a strong drop in mobility for high compensation levels in both p- and n -type Si. Mobility models taking into account carrier scattering on ionized impurities and phonons could not reproduce this drop. We conclude that a specific effect of compensation must be taken into account to explain the observed behaviour. We qualitatively discuss physical mechanisms susceptible to reduce mobility in highly compensated Si.

Preparation and properties of heavily doped and strongly compensated Ge films on GaAs

We present and generalize the preparation conditions and properties of heavily doped and strongly compensated (HDSC) Ge films obtained by deposition in the vacuum onto the semi-insulating GaAs (100) substrates. A possibility of formation of Ge films with various doping levels and compensation degrees (in particular, fully compensated) is demonstrated. Heavily doped and fully compensated Ge single-crystalline thin (∼0.1 μm) films obtained have high resistivity (up to 140 Ω cm), conductance activation energy as high as half the bandgap of Ge, low free charge carrier mobility (∼50 cm2/V s), and concentration (∼1014–1015 cm−3). The electrical and optical properties of the films are explained with allowance made for the presence of large-scale fluctuations of electrostatic potential in Ge. Under certain conditions, a two-dimensional potential relief may exist in thin HDSC Ge films, as well as two-dimensional percolation may occur.

Impact of dopant compensation on the deactivation of boron-oxygen recombination centers in crystalline silicon

The boron-oxygen recombination center responsible for the light-induced degradation of Czochralski silicon solar cells can be permanently deactivated by illumination at elevated temperature. In this study, we examine the impact of dopant compensation on the deactivation process. The experimental results show that the deactivation rate depends inversely on the total boron concentration instead of the net doping concentration, suggesting that boron is directly involved in the deactivation process. A linear dependence of the activation energy on the total boron concentration further supports this conclusion.

Drift-Diffusion Modeling for Impurity Photovoltaic Devices

A 1-D drift-diffusion modeling for impurity photovoltaics is presented. The model is based on the self-consistent solution of Poisson's equation and carrier continuity equations incorporating generation and recombination mechanisms including the intermediate states. The model is applied to a prototypical solar cell device, where strong space charge effects and reduced conversion efficiency are identified for the case of lightly doped absorption regions. A doping compensation scheme is proposed to mitigate the space charge effects, with optimal doping corresponding to one-half the concentration of the intermediate states. The compensated doping device design provides calculated conversion efficiencies of approximately 40%, which is similar to the maximum expected values from prior 0-D models. The carrier transport between intermediate levels is shown to be noncritical for achieving the efficiency limit predicted by 0-D models. The qualitative behavior of the model is compared to existing experimental data on quantum dot solar cells.

Influence of sputtering a ZnMgO window layer on the interface and bulk properties of Cu(In,Ga)Se2 solar cells

The authors studied the influence of sputtering a ZnMgO window layer for Cu(In,Ga)Se2 solar cells on bulk and interface electrical properties. Admittance spectroscopy reveals deep levels at the ZnMgO∕CdS interface whose activation energy (∼0.4eV) increases with reverse bias, indicating an unpinned quasi-Fermi level at the interface. The Cu(In,Ga)Se2 carrier concentration determined by capacitance-voltage measurements decreases to 3×1014cm−3, compared to 1×1016cm−3 in a device with a ZnO window. Scanning Kelvin probe force microscopy verifies the increased depletion region width and indicates that the junction location is unaltered by ZnMgO. Secondary-ion mass spectroscopy shows the presence of Mg near the top and bottom surfaces of the Cu(In,Ga)Se2 film. They hypothesize that the decrease in carrier concentration is due to compensation doping of the Cu-poor Cu(In,Ga)Se2 by Mg. Optimizing sputtering conditions to reduce surface damage and Mg migration eliminates the interface states and restores the carrier concentration, resulting in device performance comparable to those with a ZnO window.

Compensation Doping in Conjugated Polymers: Engineering Dopable Heterojunctions for Modulating Conductivity in the Solid State

Compensation doping of conjugated polymers is used to create a composite that contains a sufficient level of ions to support the doped form of the polymer upon removal of the compensating ion. Interfacing this form of conjugated polymer with another semiconductor capable of becoming doped upon uptake of the compensating ion allows a field-driven change in conductivity to be achieved in the solid state. As a result, this system is capable of rectification as well as the storage of charge and can be highly tunable. The electrodeposition of the heterojunctions is scalable to the nanometer range and provides a means for creating devices on existing crossbar structures outside strictly controlled fab lines.

Long carbon nanotubes intercrossed Cu/Zn/Al/Zr catalyst for CO/CO 2 hydrogenation to methanol/dimethyl ether

Vertically aligned carbon nanotube (CNT) arrays were dispersed in Na 2CO 3 solution and co-precipitated with metal nitrite to form long CNTs (over 500 μm) intercrossed Cu/Zn/Al/Zr catalyst (CD703). The catalyst was used for methanol synthesis from CO/CO 2 hydrogenation. The space time yield (STY) of methanol on CD703 increased to 0.94 and 0.28 g/(gcat h), with an increment of 7% and 8% compared with that on the Cu/Zn/Al/Zr catalyst without CNTs for CO and CO 2 hydrogenation, respectively. Furthermore, when combined with γ-Al 2O 3 catalyst and HZSM-5, dimethyl ether (DME) was obtained with a STY of 0.90 and 0.077 g/(gcat h) at 270 °C from one step CO and CO 2 hydrogenation, respectively. Due to the phase separation, ion doping and valence compensation, hydrogen reversibly adsorption and storage on CNTs promoting hydrogen spillover, CD703 catalyst show good activity for methanol synthesis. Owing to the high thermal conductivity of CNTs, the stability of CD703 was improved. This indicated that long CNTs were a good promoter on the Cu/Zn/Al/Zr catalyst for methanol/DME production from CO/CO 2 hydrogenation.

Features of hopping conductivity in weakly doped and weakly compensated semiconductor quantum wells

Main features of hopping transport in quantum wells in the conditions of low doping and low compensation are considered. Based on the features of the classical law of interaction between the impurities and on the percolation problem in quantized wells, it is shown that the specific resistance and activation energy of donor centers of the well are determined by the doping parameters and sharply depend on the width of the well.