Density Of States In Band Gap Research Articles

Impact of the Gate Dielectric on Contact Resistance in High‐Mobility Organic Transistors

AbstractThe impact of the gate dielectric on contact resistance in organic thin‐film transistors (OTFTs) is investigated using electrical characterization, bias‐stress stability measurements, and bandgap density of states (DOS) analysis. Two similar dielectric materials, namely Cytop and poly[4,5‐difluoro‐2,2‐bis(trifluoromethyl)‐1,3‐dioxole‐co‐tetrafluoroethylene] (Teflon AF2400), are tested in top‐gate bottom‐contact OTFTs. The contact resistance of Cytop‐based OTFTs is found to be greater than that of the AF2400‐based devices, even though the metal/OSC interface remains identical in both systems. The Cytop devices are also found to perform worse in bias‐stress stability tests which, along with the DOS calculations, suggests that charge trapping at the OSC/dielectric interface is more prevalent with Cytop than AF2400. This increased charge trapping at the Cytop OSC/dielectric interface appears to be associated with the higher contact resistance in Cytop OTFTs. Differences in the molecular structure between Cytop and AF2400 and the large difference in the glass transition temperature of the two polymers may be responsible for the observed difference in the transistor performance. Overall, this study highlights the importance of the gate dielectric material in the quest for better performing OTFTs and integrated circuits.

(Invited) Passivation of High K/GaN Interfaces for GaN Tunnel FETs

A combined wet and dry cleaning process for GaN(0001) has been investigated with XPS, capacitance voltage measurement. Capacitance–voltage characterization showed that combined ex-situ wet sulfide passivation and in-situ cyclic trimethylaluminum (TMA)/hydrogen plasma exposure led to reductions in the densities of both interface traps and border traps. In situ XPS studies show that after the wet sulfur treatment on GaN(0001), sulfur desorbs in vacuum at 25C prior to gate oxide deposition. Ex-situ depth profiling ARXPS post-ALD deposition shows that the a-Al2O3 gate oxide bonds directly to the GaN substrate leaving both the gallium surface atoms and the oxide interfacial atoms with bulk-like charge. DFT calculations predict that the oxide/GaN(0001) interface will have bulk-like charges and a low density of band gap states. This passivation is consistent with the oxide restoring the surface gallium atoms to tetrahedral bonding by eliminating the gallium empty dangling bonds on bulk terminated GaN(0001). A key application of gate oxide on GaN is GaN tunnel FETs. Tunneling FETs (TFETs) are one promising option for reducing power consumption per logic function due to their potential for subthreshold slopes steeper than the thermionic limit of 60 mV/decade. While TFETs based on narrow-gap semiconductors have been demonstrated, the narrow band gaps of these materials also lead to limited on-off current ratios, compromising their power efficiency. GaN-based TFETs offer a potential solution to this problem, since the large band gap effectively reduces off-state current, and the large polarization fields present in III-N heterostructures (which arise from the lack of inversion symmetry in the wurtzite crystal structure of the III-N system)—and GaN/InGaN/GaN heterojunctions in particular—can be used to dramatically increase tunneling currents (and thus TFET on-state currents). However, implementation of a GaN-based TFET required a low defect gate oxide/GaN interface. This has been achieved using the two-step wet-dry surface passivation.

Preparation of gallium nitride surfaces for atomic layer deposition of aluminum oxide.

A combined wet and dry cleaning process for GaN(0001) has been investigated with XPS and DFT-MD modeling to determine the molecular-level mechanisms for cleaning and the subsequent nucleation of gate oxide atomic layer deposition (ALD). In situ XPS studies show that for the wet sulfur treatment on GaN(0001), sulfur desorbs at room temperature in vacuum prior to gate oxide deposition. Angle resolved depth profiling XPS post-ALD deposition shows that the a-Al2O3 gate oxide bonds directly to the GaN substrate leaving both the gallium surface atoms and the oxide interfacial atoms with XPS chemical shifts consistent with bulk-like charge. These results are in agreement with DFT calculations that predict the oxide/GaN(0001) interface will have bulk-like charges and a low density of band gap states. This passivation is consistent with the oxide restoring the surface gallium atoms to tetrahedral bonding by eliminating the gallium empty dangling bonds on bulk terminated GaN(0001).

Scanning tunneling spectra for the etched surface of p-type HgCdTe

PDF HTML阅读 XML下载 导出引用 引用提醒 液相外延p型碲镉汞表面区与腐蚀凹坑的不同扫描隧道谱特征 DOI: 作者: 作者单位: 上海大学理学院物理系和微结构实验室,上海市上大路号,上海大学理学院物理系和微结构实验室,上海市上大路号,上海大学理学院物理系和微结构实验室,上海市上大路号,上海大学理学院物理系和微结构实验室,上海市上大路号,上海大学理学院物理系和微结构实验室,上海市上大路号,上海大学理学院物理系和微结构实验室,上海市上大路号 作者简介: 通讯作者: 中图分类号: 基金项目: 上海市重点学科建设无线电物理专项基金（S30105） Scanning tunneling spectra for the etched surface of ptype HgCdTe Author: Affiliation: Dept of Physics and Laboratory for Microstructures,Shanghai University,Dept of Physics and Laboratory for Microstructures,Shanghai University,Dept of Physics and Laboratory for Microstructures,Shanghai University;China,Dept of Physics and Laboratory for Microstructures,Shanghai University,China,Dept of Physics and Laboratory for Microstructures,Shanghai University,Dept of Physics and Laboratory for Microstructures,Shanghai University Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:利用超高真空扫描隧道显微镜对经过溴甲醇溶液腐蚀处理的液相外延碲镉汞材料进行了表征.发现经过腐蚀处理（3%浓度，2.5 min）的样品表面出现高密度的凹坑结构，凹坑深度约几十纳米，横向尺度在几十到几百纳米之间.扫描隧道谱测量表明，腐蚀样品表面平坦区呈现较大表观带隙，需考虑针尖诱导的能带弯曲效应，而凹坑区在零偏压区的扫描隧道谱线则近似为线性变化，说明该区域包含较高的带隙态并直接参与隧穿，从而掩盖了带隙信息. Abstract:Ultrahigh vacuum scanning tunneling microscopy（STM） and spectroscopy （STS）were used to characterize Hg1-xCdxTe grown by liquidphase epitaxy (LPE) method. The sample was etched with 3% Brominemethanol in 2.5 minutes. The STM images display submicrometersized pit structures with depths ranging from a few tens to a few hundreds nanometers. The scanning tunneling spectra show a larger apparent gap than the energy band gap of the bulk material due to the tipinduced band bending effect. In contrast, the scanning tunneling spectra of the pits show a finite slope through zero volt, implying the contribution of high density of band gap states which blur out the band gap information. 参考文献 相似文献 引证文献

ROOM TEMPERATURE AND ELEVATED TEMPERATURE CHARACTERIZATION OF NANOCRYSTALLINE SnO2 SURFACES BY SCANNING TUNNELING MICROSCOPY AND SPECTROSCOPY

Semiconductor gas sensors based on nanocrystalline SnO 2 offer many advantages over current technologies for detecting reducing gases, such as low cost, long lifetime, and high selectivity and sensitivity. However, the local surface properties on the nanoscale of SnO 2 nanocrystals are not fully understood, which impedes the exploitation of the full potential of SnO 2 for gas sensing applications. In this paper, we present a scanning tunneling microscopy and spectroscopy (STM/STS) study of nanocrystalline SnO 2 at room temperature, and under standard sensing conditions at 120°C. STS data indicate that the electronic surface properties change with nanoparticle size, temperature and exposure to gas. The surface density of states in the band gap is shown to increase with temperature while CO exposures induce a large drop in the density of band gap states as the CO molecules react with chemisorbed oxygen species.

Structural and electrical defects in amorphous silicon probed by positrons and electrons

The structure of pure amorphous Si, prepared by ion implantation, has been investigated by variable-energy positron annihilation spectroscopy (PAS) and lifetime measurements of optically generated free carriers. In general, PAS measurements are thought to be sensitive to vacancy-type defects while the carrier lifetime depends on the density of band-gap states (e.g., dangling bonds). The PAS measurements indicate that the density of positron-trapping defects can be reduced by thermal annealing at 500 °C. Concurrent with the removal of structural defects the density of band gap states is reduced as indicated by an increased photocarrier lifetime by a factor of 10. Some material has been implanted with H+ and annealed at a low temperature (150 °C). The hydrogen is expected to passivate electrical defects associated with strained and dangling bonds and indeed the photocarrier lifetime is increased in this material. Moreover, the PAS measurements cannot distinguish this material from 500 °C annealed amorphous Si, indicating that (some of) the electrical defects are associated with positron-trapping, and therefore possibly vacancy-type, structural defects. Finally, both methods have been used to detect small amounts of ion irradiation damage in annealed amorphous Si.

Numerical simulation and experimental verification of the parameters determining the base current in an a-Si : H/c-Si emitter-base heterojunction bipolar transistor

We have modelled the electrical conduction at room temperature of an hydrogenated amorphous silicon/crystalline silicon pn junction ( n type a-Si : H/ p type c-Si) by the numerical integration of Poisson's equation and of the two conduction current continuity equations. In the a-Si : H material, the band gap density of states is the main input in the modelling. This density takes into account (i) the two exponential band tails near the conduction and the valence band edges, (ii) gaussian state distribution for the dangling bonds, (iii) some single trap levels. The dopant level is given by an acceptor or donor trap, with a gaussian shape, near one of the two bands. The solution of the full set of equations with integration of the ionized trap densities over the entire band gap gives the local electron and hole currents inside the simulated structure. The experimental base current curves are well fitted by this modelling which gives the relative contribution of the technological parameters to the electrical behaviour of the structures.

Vacancy-type and electrical defects in amorphous silicon probed by positrons and electrons

Amorphous Si has been investigated by variable-energy positron annihilation spectroscopy (PAS) and lifetime measurements of optically generated free carriers. The density of positron-trapping defects can be reduced by thermal annealing at 500°C. Simultaneously, the density of bandgap states is reduced as indicated by an increased photocarrier lifetime. Hydrogen, implanted and annealed at 150°C, leads to an increased photocarrier lifetime and reduced positron trapping. It appears that (some of) the electrical defects are associated with positron-trapping, and therefore possibly vacancy-type, structural defects. Finally, both methods have been used to detect small amounts of ion irradiation damage in amorphous Si.