Large Modulation of Charge Carrier Mobility in Doped Nanoporous Organic Transistors

Molecular doping of organic electronics has shown promise to sensitively modulate important device metrics. One critical challenge is the disruption of structure order upon doping of highly crystalline organic semiconductors, which significantly reduces the charge carrier mobility. This paper demonstrates a new method to achieve large modulation of charge carrier mobility via channel doping without disrupting the molecular ordering. Central to the method is the introduction of nanopores into the organic semiconductor thin films via a simple and robust templated meniscus‐guided coating method. Using this method, the charge carrier mobility of C₈‐benzothieno[3,2‐b]benzothiophene transistors is boosted by almost sevenfold. This paper further demonstrates enhanced electron transport by close to an order of magnitude in a diketopyrrolopyrrole‐based donor–acceptor polymer. Combining spectroscopic measurements, density functional theory calculations, and electrical characterizations, the doping mechanism is identified as partial‐charge‐transfer induced trap filling. The nanopores serve to enhance the dopant/organic semiconductor charge transfer reaction by exposing the π‐electrons to the pore wall.     

Electron Mobility in Silicon Nanowires

The low-field electron mobility in rectangular silicon nanowire (SiNW) transistors was computed using a self-consistent Poisson-Schroumldinger-Monte Carlo solver. The behavior of the phonon-limited and surface-roughness-limited components of the mobility was investigated by decreasing the wire width from 30 to 8 nm, the width range capturing a crossover between two-dimensional and one-dimensional electron transport. The phonon-limited mobility, which characterizes transport at low and moderate transverse fields, is found to decrease with decreasing wire width due to an increase in the electron-phonon wavefunction overlap. In contrast, the mobility at very high transverse fields, which is limited by surface roughness scattering, increases with decreasing wire width due to volume inversion. The importance of acoustic phonon confinement is also discussed briefly     

Conjugated Polymer Photovoltaic Materials with Broad Absorption Band and High Charge Carrier Mobility

Polymer solar cells (PSCs) have attracted great attention in recent years because of their advantages of easy fabrication, low cost, light weight, and potential for flexible devices. However, the power conversion efficiency (PCE) of the PSCs needs to be improved for future commercial applications. Factors limiting the PCE of the PSCs include the low exploitation of sunlight due to the narrow absorption band of conjugated polymers, and the low charge‐transport efficiency in the devices due to the lower charge‐carrier mobility of the polymer photovoltaic materials. In this Research News article, recent progress in new conjugated polymer photovoltaic materials fabricated by our group and others is reviewed, including polythiophene (PT) and poly(thienylene vinylene) derivatives with conjugated side chains for a broad absorption band, crosslinked PT derivatives with conjugated bridges for higher hole mobility, and low‐bandgap donor–acceptor copolymers for broad, red‐shifted absorption to match the solar spectrum.     

Carrier dynamics in coalescence overgrowth of GaN nanocolumns

Coalescence overgrowth of pattern-grown GaN nanocolumns on c-plane sapphire substrate with metal organic chemical vapor deposition is demonstrated. The subsequent coalescence overgrowth opens a possibility for dislocation reduction due to the lateral strain relaxation in columnar geometry. We present further growth optimization and innovative characterization of metal organic chemical vapor deposition layers, overgrown on the columnar structure with varying diameters of columns. Nano-imprint lithography was applied to open circular holes of 250, 300, 450, and 600 nm diameter on the SiO₂ layer, deposited on the GaN layer on the c-plane sapphire template. After the growth of ~ 1 μm high GaN nanocolumns, the further coalescence conditions led to an overgrown layer ~2 μm thickness. Photoelectrical and optical properties of the overgrown layers and a reference sample were investigated by time-resolved picosecond transient grating and time-integrated photoluminescence. We note a 3-4 fold increase in carrier lifetime in the overgrown epilayers when the diameter of columns increased from 250 to 450 nm. This feature is a clear indication of an ~4-fold reduced defect density.     

半导体Si载流子迁移率的统计模型计算模拟

半导体材料栽流子迁移率是表征其电导特性的重要参数,在半导体材料载流子散射机制理论基础上,基于玻耳兹曼方程和Mathiessen规则,通过对具有不同热运动速度的载流子漂移速度求统计平均值,建立了Si材料栽流子迁移率的玻耳兹曼统计模型.计算了模拟载流子迁移率的影响因素和变化规律,并得出电场作用下的饱和漂移速度为-Vdn=1.1×107cm/s, -Vdn=8.7×106cm/s,与基于实验数据的经验公式导出的结果基本一致,表明玻耳兹曼统计模型具有良好的适用性.     

Charge Transfer at the Mn Acceptor Level in GaN

MBE-grown GaN : Mn layers with Mn doping concentrations around 10²⁰ cm^–3 were investigated by photoconductivity measurements. From electron spin resonance (ESR), Mn is known to be mostly present in the neutral Mn³⁺ or Mn²⁺ + h⁺ state, which leads to a reassignment of the known optical absorption features to charge transfer from Mn³⁺, either by direct photoionization at about 1.8 eV or by a photothermal ionization process via an excited state (Mn³⁺)^* at 1.42 V higher internal energy than the Mn³⁺ ground state. It is proposed that the Mn³⁺/Mn²⁺ acceptor level is located about 1.8 eV above the valence band edge of GaN so that the nature of the acceptor wavefunction is very different from an effective-mass-like state such as the Mn²⁺ + h⁺ complex in GaAs : Mn. According to these experimental results, the realization of carrier-mediated ferromagnetism becomes rather unlikely in not co-doped GaN : Mn.