Cl2 Plasma Research Articles

Self‐Powered Graphene/Black‐Ge Photodetectors Enhanced by Simultaneous Nanotexturing and Self‐Passivation

AbstractBlack germanium (Ge) exhibits exceptional light absorption, holding significant promise for optoelectronic applications. However, achieving self‐powered photodetection performance in black Ge is challenging due to its high surface recombination rate. Herein, this challenge is addressed by demonstrating self‐powered Graphene (Gr)/black‐Ge Schottky photodiodes, achieved through simultaneous nanotexturing and high‐quality self‐passivation. This approach involves utilizing reactive ion etching with Cl2 and BCl3 to achieve Cl‐passivated black Ge. Optical analysis reveals excellent optical characteristics in both Cl2‐treated and BCl3‐treated samples, including a high aspect ratio of 1.9 and a low reflectance of 1.5%. Notably, the Cl2‐treated black Ge exhibits a higher carrier lifetime of 20.4 µs compared to the 11.7 µs lifetime of the BCl3‐treated black Ge, attributed to the self‐passivation induced by Cl2 plasma, effectively mitigating defects. Surface composition analysis further confirms the substantial role of Cl in passivation. Significantly, these improved properties translate into notable advancements in device performance, including an enhancement in responsivity from 21 to 276 mA W−1 when compared to planar Gr/Ge devices. These findings underscore the potential of Cl2 RIE for developing high‐performance Ge‐based optoelectronic devices.

Fabrication of high aspect ratio AlN nanopillars by top-down approach combining plasma etching and wet etching

This study proposes a top-down approach for fabricating high aspect ratio AlN pillars with m-oriented nonpolar sidewalls, which will serve as the first building block for the fabrication of core-shell UV-LEDs. Such structures are achieved through a two-step process, combining a chlorine plasma etching, followed by a wet chemical etching with KOH. In this work, the mechanisms driving the AlN etching in chlorine plasma are discussed. In particular, we highlight the impact of the ratio between ion flux and radical flux on AlN etch rates, pillar profiles, and crystal orientation-dependent etching. We also identify two mechanisms of passivation layer formation on the pillar sidewalls that contribute to the pattern slope: redeposition of the carrier wafer etch by-products and Aluminium line of sight redeposition, both phenomena are also driven by the ionic bombardment. Low ionic bombardment (either low ion over radical flux ratio or low ion energy) has been identified as plasma conditions allowing the patterning of anisotropic AlN pillars but with the formation of a-nonpolar facets. Due to the inability to obtain m-oriented sidewalls using Cl2 plasma etching alone, we show that the use of a KOH wet etching treatment allows verticalizing the pillars and revealing smooth m-facets on the pillars’ sidewalls if suitable m-oriented hard mask is used. In this wet etching step, we highlight the key role played by the hard mask and its initial shape during the wet etching if straight and m-oriented pillars are desired.

BCl3/Cl2 plasma etching process to fabricate a ferroelectric gate structure for device integration

Despite the significant potential of ferroelectric devices in overcoming the challenges faced by conventional high-k-based CMOS devices owing to the scaling of CMOS processes, most ferroelectric devices are not implemented in practical circuits yet. For practical application, integrating them into a circuit is essential, and the development of a reliable etching process is crucial for the integration of individual devices into circuits. Therefore, this study proposes a process for etching hafnium zirconium oxide (HZO)-based gate stacks to fabricate a gate structure and integrate HZO-based devices into circuits. First, poly-Si/TiN/HZO/TiN/SiO2 was deposited on a Si substrate and etched via Cl2 and BCl3/Cl2 plasma etchings. Cl2 plasma etching was found to be less effective, whereas BCl3/Cl2 plasma etching exhibited a higher etching rate. The optimal etching time for the BCl3/Cl2 plasma at which the entire stack was successfully removed was 50 s. Furthermore, the optimal ratio of Ar:Cl2:BCl3 that resulted in minimal damage to the Si surface was determined to be 1:1:3. These results led to the successful formation of an HZO-based gate structure and provided the potential to integrate ferroelectric devices into the circuit, thereby enabling their practical utilization.

Fine-line formation of cobalt thin films via cyclic etching using two-step process

Co thin films were etched via cyclic etching using a two-step process of Cl2 plasma and Ar sputtering. In the first step, the Co surface was chlorinated by exposing the Co film to Cl2 plasma to form a CoCl2 layer, and the chlorinated Co film was removed via Ar sputtering in the second step. The saturation point of chlorination in the first step provided a 2 nm-thick CoCl2 layer. In the second step, the dc bias voltage and Ar sputtering time were varied to optimize the removal of the CoCl2 layer. A close relationship between the thickness of the CoCl2 layer and the sputtering conditions was revealed, which was responsible for the etch profiles of the Co films. Finally, 300 nm patterns of the Co films were successfully etched via cyclic etching without redeposition on the sidewall.

Low damage atomic layer etching technology for gate recessed fabrication

In this work, the damage induced by etching to AlGaN surface employing atomic layer etching (ALE), which use Cl2 plasma and Ar plasma for etching and removal, compared with conventional etching method was investigated. The Hall-effect measurements indicate that a higher two-dimensional electron gas (2DEG) concentration of 1.54 × 1013 cm−2 is achieved and an electron mobility of 1756 cm2/V is obtained on the ALE sample by reducing the bombardment of BCl2+ and BCl + on the surface. Simultaneously, the damage caused by ALE to the etching surface is lower than the conventional etching method according to Raman spectroscopy analyses and atomic force microscopy (AFM) scans. The X-ray photoelectron spectroscopy (XPS) spectrum and Transmission Electron Microscope (TEM) measurements were further carried out to prove that ALE process results in a higher 2DEG concentration due to maintaining stable Al composition and inducing lower damage on the surface.

Etching characteristics and surface modification of InGaSnO thin films under Cl2/Ar plasma

Indium gallium tin oxide (IGTO) thin films have the potential for high mobility and low-temperature processing, which makes them suitable for applications such as display backplanes and high-voltage switching devices. However, very few studies have investigated the plasma-etching characteristics of IGTO and changes in its properties after etching. In this study, the etching characteristics of IGTO were investigated using Cl2/Ar plasma, and changes in surface properties were analyzed. Results showed that the etch rate increased with an increase in the proportion of Cl2, with the highest etch rate observed at 69 nm min−1 in pure Cl2 plasma with a gas flow rate of 100 sccm. Furthermore, increased radio-frequency power caused a rise in the etch rate, while a process pressure of 15 mTorr was optimal. The primary etching mechanism for IGTO thin films under Cl2 plasma was a chemical reaction, and an increased work function indicated the occurrence of defects on the surface. In addition, the etching process reduced the surface roughness of Cl2-containing plasma, whereas the etching process in pure Ar plasma increased surface roughness. This study contributes to a better understanding of the plasma-etching characteristics of IGTO and changes in its properties after etching, providing valuable insights for IGTO-based applications.

Hydrogen iodide (HI) neutral beam etching characteristics of InGaN and GaN for micro-LED fabrication

We investigated the etching characteristics of hydrogen iodide (HI) neutral beam etching (NBE) of GaN and InGaN and compared with Cl2 NBE. We showed the advantages of HI NBE versus Cl2 NBE, namely: higher InGaN etch rate, better surface smoothness, and significantly reduced etching residues. Moreover, HI NBE was suppressed of yellow luminescence compared with Cl2 plasma. InCl x is a product of Cl2 NBE. It does not evaporate and remains on the surface as a residue, resulting in a low InGaN etching rate. We found that HI NBE has a higher reactivity with In resulting in InGaN etch rates up to 6.3 nm min−1, and low activation energy for InGaN of approximately 0.015 eV, and a thinner reaction layer than Cl2 NBE due to high volatility of In-I compounds. HI NBE resulted in smoother etching surface with a root mean square average (rms) of 2.9 nm of HI NBE than Cl2 NBE (rms: 4.3 nm) with controlled etching residue. Moreover, the defect generation was suppressed in HI NBE compared to Cl2 plasma, as indicated by lower yellow luminescence intensity increase after etching. Therefore, HI NBE is potentially useful for high throughput fabrication of μLEDs.

Investigation of the dual-frequency bias effect on inductively coupled Cl2 plasmas by hybrid simulation

In the etching process, a bias source is usually applied to the bottom electrode in inductively coupled plasmas (ICPs) to achieve independent control of the ion flux and ion energy. In this work, a hybrid model, which consists of a global model combined bi-directionally with a fluid sheath model, is employed to investigate the dual-frequency (DF) bias effect on the inductively coupled Cl2 plasmas under different pressures. The results indicate that the DC self-bias voltage developed on the biased electrode is approximately a linear function of the phase shift between the fundamental frequency and its second harmonic, and the value only varies slightly with pressure. Therefore, the ion energy on the bottom electrode can be modulated efficiently by the bias voltage waveform, i.e. the fluctuation of the ion energy with phase shift is about 40% for all pressures investigated. Besides, the ion energy and angular distribution functions (IEADFs) in DF biased inductive discharges is complicated, i.e. the IEADFs exhibits a four-peak structure under certain phase shift values. Although the species densities and ion fluxes also evolve with phase shift, the fluctuations are less obvious, especially for Cl2 + ions at low pressure. In conclusion, the independent control of the ion energy and ion flux are realized in DF biased ICPs, and the results obtained in this work are of significant importance for improving the etching process.

Preferential crystal orientation etching of GaN nanopillars in Cl2 plasma

The ability to fabricate organized, dense arrays of GaN nanostructures with high aspect ratios is of great interest for improving light extraction and absorption in optoelectronic devices, such as light-emitting diodes (LEDs) and Lasers.However, achieving this requires a patterning method that allows for the fabrication of GaN nanostructures with a controlled final shape and specific crystallographic facets. Our study shows that a top-down approach combining lithography and Cl2-based plasma etching processes offers the possibility of anisotropically transferring a pattern while revealing smooth nonpolar facets.Although GaN etching in Cl2 is driven by ion-enhanced chemical etching mechanisms, the chemical component is very strong, and Cl2 plasma leads to preferential crystallographic orientation etching of GaN. The mechanisms that drive facet formation are very similar to those involved in wet KOH etching. The etching ability of a specific crystallographic plane by chlorine atoms depends on the planar density and the N dangling bonds present on the plane. Predictably, GaN crystallographic planes will be etched in Cl2 plasmas as follows: a-type semipolar > m-type semipolar planes > a nonpolar plane > m polar planes > c polar planes. Crystallographic etching usually reveals the slowest etching planes, meaning the m-planes. However, consistently with the wet mechanisms, our study shows that the final revealed plane in Cl2 plasma will strongly depend on the surface curvature: in concave surfaces, fast-etch facets persist, while in convex surfaces, slow-etch facets persist. Consequently, Cl2 plasma etching of pillars (convex surfaces) will result in a-type facets, while etching of holes (concave surfaces) result in m-type facets.

GaN damage-free cyclic etching by sequential exposure to Cl2 plasma and Ar plasma with low Ar+-ion energy at substrate temperature of 400 °C

Damage-free atomic layer etching (ALE) of GaN was demonstrated using a cyclic process in which the chlorinated layer formed by Cl2 plasma exposure was removed by exposure to Ar plasma with energy-controlled ions when the substrate temperature was maintained at 400 °C. The layer chlorinated at 400 °C by Cl2 plasma exposure was found to be thinner than that chlorinated at 25 °C. Therefore, in the case of an Ar+-ion energy of 70 eV, the “ALE synergy” parameter, which quantifies the degree to which a process approaches the ideal ALE regime, decreased from 86% at a substrate temperature of 25 °C to 24% at a substrate temperature of 400 °C. A substrate temperature of 400 °C promoted etching even at the lower ion energy of 40 eV, thereby resulting in a higher ALE synergy of 62%. The vertical etching profile with no degradation of photoluminescence near the band edge on the etched surface was then observed. The proposed high-temperature ALE method is promising for realizing a vertical pattern profile via damage-free etching of GaN.

Monolithic Cl-Modified Nanoporous Ag Nanowires for Electrochemical CO2 Reduction to CO

Nanoporous catalysts with large surface areas are promising electrocatalysts to reduce carbon dioxide (CO2). However, previous methods of producing nanoporous catalysts require a high temperature (>500 °C) or a long processing time (∼12 h). Herein, we develop a facile method to produce monolithic nanoporous Ag nanowires (NWs) within a few minutes. The Cl2 plasma treatment on a Ag-coated plastic film induces the rapid growth of vertically aligned AgCl NWs due to the highly reactive Cl radicals and strain-relaxation-driven growth of NWs. Then, AgCl NWs are electrochemically reduced to produce nanoporous Ag NWs with a high electrochemical surface area. Moreover, the residual Cl– anions on the surface of the nanoporous Ag NWs increase CO2 reduction activity. Consequently, the Ag NWs have a 90 times higher electrochemical surface area but less than 0.06 times as much charge-transfer resistance as the Ag film. As a result, the Ag NWs achieve a high selectivity of CO (FECO = 98.7%) and high CO partial current density of 3.16 mA/cm2 at −0.8 VRHE, which is 20 times higher than that of Ag foil (0.16 mA/cm2). Moreover, the Ag NWs had a good long-term stability even after 60 h of CO2 reduction.

Influence of the carrier wafer during GaN etching in Cl2 plasma

In this study, we have performed a thorough characterization of the GaN surface after etching up to 100 nm in Cl2 plasma under various bias voltages and according to the carrier wafer used (Si, SiO2, Si3N4, and photoresist). The objective of this article is to evaluate the etch damage and contamination of the GaN surface when materials with other chemical nature are present during etching. The effects of etching conditions on surface morphology and chemical compositions of etched GaN films are studied in detail using XPS and AFM measurements. To this aim, a universal methodology is proposed to estimate accurately by XPS the stoichiometry of the GaN surface exposed to reactive plasmas when only an Al Kα x-ray source is available. The results indicate that the GaN etching mechanisms are very sensitive to the chlorine radical density present in the plasma, the latter being strongly influenced by the carrier wafer. Substrates that are more chemically reactive with Cl2 plasma such as silicon or photoresist compared to SiO2 or Si3N4 will lead to a greater loading of atomic chlorine, which in turn will lead to lower GaN etch rates. Moreover, the GaN surface contamination will depend on the etch by-products ejected by the carrier wafer. The GaN surface exposed to Cl2 plasma shows a Ga-depleted surface because of the more important reactivity of Cl with Ga rather than N, except in the SiO2 carrier wafer case. In this latter case, the formation of Ga–O bond limits the Ga removal. Regarding the surface roughness, it seems that the contaminants play a little role in the roughness formation except for the oxygen released by the SiO2 carrier wafer. On the other hand, the surface roughness evolution is clearly driven by the chlorine radical flux reaching the GaN surface. At low bias voltage, a preferential crystalline orientation etching driven by the Cl radicals leads to the formation of hexagonal shaped defects that are associated to screw-type threading dislocations already present in the pristine GaN material. At higher bias, the enlargement of the defects is limited, leading to a very low surface roughness value but to amorphized surfaces.

Realization of electrically driven AlGaN micropillar array deep-ultraviolet light emitting diodes at 286 nm

We report on the realization of top-down fabricated, electrically driven, deep-ultraviolet (DUV) AlGaN micropillar array light emitting diodes (LEDs) with high output power density. Ordered arrays of micropillars with the inverse-taper profile were formed from an AlGaN epitaxial stack (epistack) using a Ni-masked Cl2 plasma dry etch and KOH-based wet etching. Following deposition of the n-contact, polydimethylsiloxane was spin-coated and etched-back to reveal the tips of the pillars to allow for formation of the p-contact. The DUV LEDs were tested at the wafer-level using a manual probe station to characterize their electrical and optical properties, revealing stable electroluminescence at 286 nm with a narrow 9-nm linewidth. Optical output power was found to be linearly related to current density, with output power densities up to 35 mW/cm2, comparable to the results reported for epitaxially grown DUV nanowire LEDs. Simulations revealed that the inverse-taper profile of the micropillars could lead to large enhancements in light extraction efficiency (ηEXT) of up to 250% when compared to micropillars with vertical sidewalls. The realization of ordered, electrically driven, top-down fabricated micropillar DUV LEDs with competitive output power represents an important step forward in the development of high-efficiency, scalable DUV emitters for a wide range of applications.

Cyclic Plasma Halogenation of Amorphous Carbon for Defect-Free Area-Selective Atomic Layer Deposition of Titanium Oxide.

As critical dimensions in integrated circuits continue to shrink, the lithography-based alignment of adjacent patterned layers becomes more challenging. Area-selective atomic layer deposition (ALD) allows circumventing the alignment issue by exploiting the chemical contrast of the exposed surfaces. In this work, we investigate the selective deposition of TiO2 by plasma halogenation of amorphous carbon (a-C:H) acting as a growth-inhibiting layer. On a-C:H, a CF4 or Cl2 plasma forms a thin halogenated layer that suppresses the growth of TiO2, while nucleation remains unaffected on plasma-treated SiO2. The same halogenating plasmas preferentially etch TiO2 nuclei over films and thus enable the restoration of the halogenated surface of amorphous carbon. By embedding the intermediate plasma treatments in the ALD TiO2 sequence, an 8 nm TiO2 layer could be deposited with a selectivity of 0.998. The application of the cyclic process on a 60 nm half-pitch line pattern resulted in the defect-free deposition of TiO2 at the bottom of the trenches. Cyclic fluorination demonstrated better growth inhibition compared to chlorination due to more efficient defect removal and retention of the favorable surface composition during plasma exposure. While exploring the TiO2 nucleation defects at the limit of detection for conventional elemental analysis techniques (<1 × 1014 at/cm2), we additionally highlight the value of imaging techniques such as atomic force microscopy for understanding defect formation mechanisms and accurately assessing growth selectivity.

Model analysis of the feature profile evolution during Si etching in HBr-containing plasmas

Feature profiles of Si etched in HBr-containing plasmas have been analyzed through a comparison between experiments and simulations. The emphasis was placed on a mechanistic understanding of the difference in the evolution of profile anomalies (such as tapering, footing, and microtrenching) during Si etching between HBr- and Cl2-based plasmas. Experiments were made with Cl2/O2/HBr chemistry by varying the HBr mixing ratio, using a commercial ultrahigh-frequency electron cyclotron resonance plasma etching reactor, where HCl/O2 chemistry was also employed to compare with that of Cl2/O2 and HBr/O2. Numerical simulations of feature profile evolution were made using a semiempirical atomic-scale cellular model based on the Monte Carlo method that we developed for Si etching in Br2, HBr, and Cl2 plasmas, where surface chemistry and kinetics include the effects of ion reflection from and/or penetration into feature surfaces on incidence. The experiments showed more vertical sidewalls with less footing and microtrenching with HBr; concretely, with increasing HBr mixing ratio in Cl2/O2/HBr plasmas, the tapering is reduced and minimized at 80% HBr where slight lateral or side etching tends to occur, the footing is reduced gradually, and the microtrenching fades away at more than 20% HBr. A comparison with simulations, with the help of separate analyses of ion reflection from surfaces on incidence, indicated that the smaller reflection probability and reflected energy fraction of Br+ on tapered sidewalls (compared to Cl+) are responsible for reduced tapering, footing, and microtrenching in HBr-containing plasmas; moreover, chemical etching effects of neutral H atoms at the feature bottom and sidewalls, arising from the larger reaction probability of H (compared to Cl), are also responsible for reduced microtrenching and for reduced tapering (and the lateral or side etching induced) therein.

Sheet Resistance Reduction of MoS₂ Film Using Sputtering and Chlorine Plasma Treatment Followed by Sulfur Vapor Annealing

Sheet resistance (R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sheet</sub> ) reduction of a-few-layered molybdenum disulfide (MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) film using sputtering is investigated in this study. To enhance the carrier density, chlorine (Cl2) gas excited by inductively coupled plasma is introduced as a substitute for sulfur. To electrically activate the Cl dopants and simultaneously prevent out-diffusion of sulfur, a furnace annealing was performed in sulfur-vapor ambient. Consequently, the R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sheet</sub> in the MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> film with the Cl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> plasma treatment remarkably reduced by one order lower than that without one, because of the activation of Cl dopants in the MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> film.

Electron beam evaporated Au islands as a nanoscale etch mask on few-layer MoS2 and fabrication of top-edge hybrid contacts for field-effect transistors

Metal contacts to two-dimensional layered semiconductors are crucial to the performance of field-effect transistors (FETs) and other applications of layered materials in nanoelectronics and beyond. In this work, the wetting behavior of very thin Au films on exfoliated MoS2 flakes was studied and evaluated as a nanoscale, self-assembled dry etch mask. Etching nanoscale pits into MoS2 flakes prior to metallization from the top of the flake forms edge sites that contribute some fraction of edge contacts in addition to top contacts for additional carrier injection and lower contact resistance. The morphology and thickness of Au islands and MoS2 were studied with scanning electron microscopy and atomic force microscopy before and after etching with low-power plasmas. A Cl2 plasma etch of 10 s with a Au island mask of 6 nm (nominal) showed the best resulting morphology among the plasma conditions studied. Back-gated MoS2-based FETs on SiO2/p +-Si with Ti/Au contacts were fabricated using a Cl2 etch of only the contact regions, and they yielded devices with ON currents of 100s µA/µm, ON/OFF ratios ⩾106, and contact resistance <10 kΩ µm. The best set of devices had a very low contact resistance of ∼1 kΩ µm with almost no dependence of contact resistance on gating. Using nanoscale etch masks made from metal islands could be highly customizable and shows promise for engineering FETs with low contact resistance.

Silver film etching using halogen gas plasma

Silver (Ag) film etching was studied with a focus on suppressing the surface roughness induced by Cl2 and CF4 plasmas. After Cl2 plasma etching, roughening of the Ag surface was observed. From in situ x-ray photoelectron spectroscopy and atomic force microscopy analyses using a plasma beam system, the Ag surface was roughened with AgCl formation after Cl2 plasma treatment before exposure to air. In capacitively coupled Cl2 plasma, it seemed that many voids were formed on the Ag surface at a high bias power and cathode temperature. This was considered to be characteristic of agglomeration. In contrast, severe surface roughness was not observed after CF4 plasma etching, even at a high bias power and cathode temperature. Secondary ion mass spectrometry analysis showed high chlorine accumulation near the Ag film surface after Cl2 plasma etching. Possible agglomeration of the Ag film during Cl2 plasma etching was volume expansion caused by chlorine accumulation assumed to enhance the compressive stress of the Ag film, and this resulted in increased boundary grooving and, finally, agglomeration. In contrast, fluorine accumulation was unlikely during CF4 plasma etching, resulting in less Ag film stress, which suppressed grain boundary grooving and agglomeration.

Characteristics of etching residues on the upper sidewall after anisotropic plasma etching of silicon

Anisotropic Si nano-trench structures were fabricated using inductively coupled HBr + Cl2 plasmas for sidewall etching residue analysis. The sidewall etching residues formed inside the Si nano-trench patterns were analyzed via tilted X-ray photoelectron spectroscopy. The changes in the chemical composition and binding state of the etching residues formed on the Si nano-trench sidewalls were investigated with various gas mixing ratios in HBr + Cl2 plasma etching processes. The sidewall chemistry of the plasma-etched Si nano-trench patterns was examined at various take-off angles. SiO2 and suboxide groups (SixOy, x ≤ 2, y ≤ 3) were formed on the Si nano-trench sidewalls after the plasma etching. An increase in the chlorine content of the gas plasma resulted in the increased formation of SiO2 and suboxide residual groups on the Si nano-trench sidewalls. Additionally, the changes in the chemical states of the Si nano-trench sidewalls after a wet-cleaning process were examined using our designed experimental technique.

Plasma Induced Addition of Active Functional Groups to Biochar for Elemental Mercury Removal

The mercury removal capacity of biochar can be improved by plasma modification and the functional group is an important factor affecting the mercury removal. This paper examines factors such as discharge voltages, gas flow rates, chlorine concentrations and discharge times that may affect functional groups on the surface of biochar. The mercury removal performance of the tobacco stem biochar prepared under different Cl2 plasma modification conditions was investigated using a fixed bed reactor. The number of C–Cl bonds and carboxyl increased after modification and decreased in mercury removal. Longer discharge times can destroy the biochar surface and possibly cause a decrease in the number of active sites. Increasing the discharge voltage promotes the formation of C–Cl bonds and carboxyl groups. Excessive gas flow rates cause active chlorine to be carried out of the reactor quickly which reduces the formation of C–Cl. The amount of C–Cl bonds increases with increased chlorine concentration. In the modified biochar, C–Cl and carboxyl take part in the adsorption of mercury to form HgCl2 and HgO, while the unmodified biochar is mainly physically adsorbed Hg0. C–Cl is the main functional group participated in mercury removal and the carboxyl is a secondary functional group. The proportion of HgCl2 is at least 68.2% in the used modified biochar. Under optimum conditions, the initial mercury removal efficiency of modified tobacco biochar was 99.1%, and the efficiency was still 79.1% after 100 min.