Detailed evaluation of in-operando potentials in OLED devices: A combined experimental and drift-diffusion study

We report on a technique to determine in-operando transport properties of Organic Light Emitting Diodes (OLEDs). Two types of OLEDs that solely differ in the emission layer but obviously exhibit a different potential distribution are investigated in this study. If the emission layer consists of the isomer TH-A a large shift in onset voltage can be observed in case of layer thickness variation of the emission layer. In case of the isomer TH-B a thickness variation has no impact on the onset voltage. Therefore the voltage developments per layer are determined with the help of IV measurements on a set of devices with varying layer thickness. From an empirical point of view the voltage behaviour in each layer follows a simple power law. A drift-diffusion model is developed that well describes the current density dependent evolution of coefficient and exponent of the power law. From the model we are able to derive the carrier injection mechanism into the respective layer as well as the injection barrier height. Also the carrier mobility is determined. Finally we are able to show that the existence of a large injection barrier can not explain the observed onset voltage shifts in case of TH-A. Instead an electric field at or close to the interface is necessary to describe the TH-A behaviour.     

Atomic mechanism of flat-band voltage shifts at La2O3, Al2O3 and Nb2O5 capping layers

We provide the first fully atomistic model of the origin of the flat band voltage shifts due to La- and Al-based capping layers, which are frequently used to control the flat band voltage in high K metal gate stacks. Supercells with substitutional La, Sr, Al or Nb at an HfO₂–SiO₂ interface create dipole layers which shift the flat band voltage in the observed direction. The shift only occurs for substitutions at the interface, not distributed over the HfO₂ layer. The shift is due to an image charge effect combining the metal electronegativity and oxygen density.     

A novel complementary N+-charge island SOI high voltage device

A new complementary interface charge island structure of SOI high voltage device (CNI SOI) and its model are presented. CNI SOI is characterized by equidistant high concentration n+-regions on the top and bottom interfaces of dielectric buried layers. When a high voltage is applied to the device, complementary hole and electron islands are formed on the two n+-regions on the top and bottom interfaces. The introduced interface charges effectively increase the electric field of the dielectric buried layer (E1) and reduce the electric field of the silicon layer (Es), which result in a high breakdown voltage (BV). The influence of structure parameters and its physical mechanism on breakdown voltage are investigated for CNI SOI. EI = 731 V/μm and BV = 750 V are obtained by 2D simulation on a l-μm-thick dielectric layer and 5-μm-thick top silicon layer. Moreover, enhanced field E1 and reduced field Es by the accumulated interface charges reach 641.3 V/μm and 23.73 V/μm, respectively.     

Hetero-emitter-like characteristics of phosphorus doped polysiliconemitter transistors. Part I: band structure in the polysilicon emitterobtained from electrical measurements

This paper reports on the cause of hetero-emitter-like characteristics recently discovered for a phosphorus doped poly-Si emitter transistor, the poly-Si emitter of which is crystallized from an in-situ phosphorus doped amorphous Si film. The band structure in the poly-Si emitter is investigated using (1) the transistor characteristics and (2) the I-V characteristics of the interface between the poly-Si emitter layer and the Si substrate. As a result, a new kind of potential barriers are observed on the conduction band and the valence band at the interface. The potential barrier on the valence band is proved to be the origin of the hetero-emitter-like characteristics. According to the I-V characteristics of the interface, the formation of the barriers is probably due to band discontinuity at the interface     

InGaP/GaAs0.94Sb0.06/GaAs doubleheterojunction bipolar transistor

A novel InGaP/GaAs_0.94Sb_0.06/GaAs double heterojunction bipolar transistor is presented. It features the use of fully strained pseudomorphic GaAs_0.94Sb_0.06 as the base layer and an InGaP layer as the emitter, which both eliminate misfit dislocations and current blocking, and increase the valence band discontinuity at the InGaP/GaAsSb interface. The device demonstrates a high current gain and a low turn-on voltage     

High current conduction with high mobility by non-radiative charge recombination interfaces in organic semiconductor devices

We report a unique non-radiative p-n-p junction structure to provide high current conduction with high mobility in organic semiconductor devices. The current conduction was improved by increasing p-n junctions made with intrinsic p-type hole transport layer and n-type electron transport layer. The excellent hole mobility of 5.3 × 10^?1 cm²/V s in this p-n-p device configuration is measured by the space charge limited current method with an electric field of 0.3 MV/cm. Enhanced current conduction of 248% at 4.0 V was observed in fluorescent blue organic light-emitting diodes with introduction of non-radiative p-n-p-n-p junction interfaces. Thereupon, the power efficiency at 1000 cd/m² was improved by 22% and the driving voltage also was reduced by 17%, compared to that of no interface device. Such high current conduction with high mobility is attributed to the carrier recombination at p-n-p interfaces through coulombic interaction. This non-radiative p-n-p junction structure suggested in this report can be very useful for many practical organic semiconductor device applications.     

Modulation of work function of ITO by self-assembled monolayer and its effect on device characteristics of inverted perovskite solar cells

In this work, self-assembled monolayers of a series of para-substituted phenylphosphonic acids were formed on the ITO substrate surface for the fabrication of perovskite-based solar cells. With the perovskite layer directly transferred by a stamping method on the SAM-modified ITO surface, followed by spin-coated PCBM layer, an inverted-type, hole-transport layer-free solar cell was fabricated. The device characteristics, such as open circuit voltage V_oc, short circuit current J_sc, and field factor FF, were analyzed with respect to the energy level alignment at the ITO/perovskite interface. The best power conversion efficiency was observed with ITO having work function closest to the valence band maximum of perovskite, giving an efficiency of 13.94 %, considerably higher than the 8.64 % from the bare ITO-based device.     

Record 1.0 V open‐circuit voltage in wide band gap chalcopyrite solar cells

Tandem solar cell structures require a high‐performance wide band gap absorber as top cell. A possible candidate is CuGaSe₂, with a fundamental band gap of 1.7 eV. However, a significant open‐circuit voltage deficit is often reported for wide band gap chalcopyrite solar cells like CuGaSe₂. In this paper, we show that the open‐circuit voltage can be drastically improved in wide band gap p‐Cu(In,Ga)Se₂ and p‐CuGaSe₂ devices by improving the conduction band alignment to the n‐type buffer layer. This is accomplished by using Zn_1−x Sn_x O_y , grown by atomic layer deposition, as a buffer layer. In this case, the conduction band level can be adapted to an almost perfect fit to the wide band gap Cu(In,Ga)Se₂ and CuGaSe₂ materials. With an improved buffer band alignment for CuGaSe₂ absorbers, evaporated in a 3‐stage type process, we show devices exhibiting open‐circuit voltages up to 1017 mV, and efficiencies up to 11.9%. This is to the best of our knowledge the highest reported open‐circuit voltage and efficiency for a CuGaSe₂ device. Temperature‐dependent current‐voltage measurements show that the high open‐circuit voltage is explained by reduced interface recombination, which makes it possible to separate the influence of absorber quality from interface recombination in future studies.     

Photosensitivity of top gate C60 based OFETs: Potential applications for high efficiency organic photodetector

The comparison of light-induced effects in bottom-gate and top-gate organic field effect transistors (OFETs) provide a clear indication, that the nature of interface between the active layer and the gate dielectric plays a major role in the observed light-induced threshold voltage shift. The nature of interface was also analyzed by electron spin resonance (ESR) experiments, which provides a direct evidence for the creation of free radical species when parylene is deposited on the top of the C₆₀ semiconductor layer. The rate of change of light-induced threshold voltage shift strongly depends on the wavelength and intensity of the incident light, and transverse electric field at the interface. The observed effects provide a strong base for the realization of high efficiency organic photodetectors and optical memory devices. The responsivity of organic photodetector was measured up to 1047 A/W.     

Experimental method to thermally deembed pads from R/sub TH/ measurements

Both compound semiconductor and silicon-based bipolar junction transistors or heterojunction bipolar transistors (HBTs) require the efficient removal of heat in order to achieve a maximum level of performance and reliability. In order to satisfy both of these criteria, the electrothermal behavior of each device must be captured in a compact model. The model parameter that determines the junction temperature is R/sub TH/, the thermal resistance. Experimental methods to determine R/sub TH/ often require a relatively small device with a large R/sub TH/ to be attached to a set of relatively large metal pads with a low R/sub TH/. The pads act as a thermal shunt to the substrate and artificially lower the measured R/sub TH/. In order to obtain a suitable R/sub TH/ value for a device located in an IC, the pads must be deembedded from the measured data, much like pad deembedding for an S-parameter measurement. Test structures with various width metal traces between the emitter pad and device's emitter have been fabricated in a 200-GHz InP double HBT process. A method of using the measured R/sub TH/ of these structures and a simple resistive network model to deembed the pads is presented. It is shown that deembedded values can be as much as 30% higher than the measured R/sub TH/.     

Electrical and optical characteristics of phosphorescent organic light-emitting device with thin-codoped layer insertion

In this paper, we demonstrated the changes of electrical and optical characteristics of a phosphorescent organic light-emitting device (OLED) with tris(phenylpyridine)iridium Ir(ppy)₃ thin layer (4 nm) slightly codoped (1%) inside the emitting layer (EML) close to the cathode side. Such a thin layer helped for electron injection and transport from the electron transporting layer into the EML, which reduced the driving voltage (0.40 V at 100 mA/cm²). Electroluminescence (EL) spectral shift at different driving voltage was observed in our blue OLED with [(4,6-di-fluoropheny)-pyridinato-N,C^2′]picolinate (FIrpic) emitter, which came from the recombination zone shift. With the incorporation of thin-codoped Ir(ppy)₃, such EL spectral shift was almost undetectable (color coordinate shift (0.000, 0.001) from 100 to 10,000 cd/m²), due to the compensation of Ir(ppy)₃ emission at low driving voltage. Such a methodology can be applied to a white OLED which stabilized the EL spectrum and the color coordinates ((0.012, 0.002) from 100 to 10,000 cd/m²).     

Hetero-emitter-like characteristics of phosphorus doped polysiliconemitter transistors. Part II: band deformation due to residual stress inthe polysilicon emitter

For part I see ibid., vol.42, no.3, pp.419-26 (1995). The purpose of this paper is to clarify the cause of the hetero-emitter-like characteristics reported for bipolar transistors having a poly-Si emitter which is crystallized from an in-situ phosphorus-doped amorphous Si layer. It has been clarified by X-ray diffraction measurements that there is large lattice deformation at the interface of the poly-Si layers with the Si substrate. This lattice deformation corresponds to tensile stress of about 1.0 GPa. We have calculated the band structure at the interface by the deformation potential theory using the measured lattice deformation. The calculated band structure mostly coincides with that determined electrically in our previous paper, part I of this series. Moreover, the hetero-emitter-like characteristics are found to be stronger in the cases of larger residual stress. We therefore conclude that the origin of the hetero-emitter-like characteristics is the band discontinuity at the interface which is produced by the band deformation due to the residual stress in the poly-Si layer     

Vertical structure permeable-base hybrid transistors based on multilayered metal base for stable electrical characteristics

In this work we present a permeable-base transistor consisting of a 60 nm thick N,N′-diphenyl-N,N′-bis(1-naphthylphenyl)-1,1′-biphenyl-4,4′-diamine layer or a 40 nm thick 2,6-diphenyl-indenofluorene layer as the emitter, a Ca/Al/Ca multilayer as the metal base, and p-Si as collector. In the base, the Ca layers are 5 nm thick and the Al layer was varied between 10 and 40 nm, the best results obtained with a 20 nm thick layer. The devices present common-base current gain with both organic layer and silicon acting as emitter, but there is only observable common-emitter current gain when the organic semiconductor acts as emitter. The obtained common-emitter current gain, ～2, is independent on collector-emitter voltage, base current and organic emitter in a reasonable wide interval. Air exposure or annealing of the base is necessary to achieve these characteristics, indicating that an oxide layer is beneficial to proper device operation.     

Origin of flat band voltage shift in HfO2 gate dielectric with La2O3 insertion

The origin of flat band voltage (V_FB) shift with La₂O₃ insertion for HfO₂ gate dielectrics has been carefully examined. By separating the effect of the fixed charges located at each interface by thickness-dependent V_FB evolution, total voltage shifts (dipole) at metal/high-k and high-k/SiO₂ interfaces have been estimated. Using stacked capacitors of La₂O₃ and HfO₂, it can be concluded that V_FB is mainly determined by the high-k on SiO₂. Therefore, the dipole at La₂O₃ and the interface has an additional dipole of 0.36 eV compared with that of HfO₂/SiO₂. The same trend has also been obtained with a high-k on a Si substrate without a SiO₂ layer. A simple model using electronegativity has been proposed to explain the V_FB shift.     

An investigation of the effect of graded layers and tunneling on the performance of AlGaAs/GaAs heterojunction bipolar transistors

We present the results of theoretical and experimental studies of the heterojunction bipolar transistor. Our calculations are based on a new thermionic field-diffusion model which takes into account the dependence of the emitter efficiency on the height of the interface conduction band spike and tunneling across the spike. Based on this theory we derive analytical expressions for the current-voltage characteristics and relate the short-circuit common emitter current gain to the material parameters, doping levels, grading length, and device temperature. We demonstrate that the thermoemission transport across the interface spike limits the rate of increase in the collector current with the emitter-base voltage and, as a consequence, the maximum common emitter current gain. Tunneling also plays an important role, especially for abrupt heterojunctions. Our calculations reveal an important role played by grading of the composition of the emitter region in the vicinity of the heterointerface. Such grading decreases the barrier height at the interface and greatly enhances the emitter injection efficiency.     

三栅FET的总剂量辐射效应研究

通过对赝MOS进行不同剂量的辐射,得到不同辐射条件下赝MOS器件的I-V特性曲线,并通过中带电压法进行分析,得出在不同辐射下SOI材料的埋氧层中产生的陷阱电荷密度和界面态电荷密度参数。采用这些参数并结合Altal三维器件模拟软件模拟了硅鳍(FIN)宽度不同的三栅FET器件的总剂量辐射效应,分析陷阱电荷在埋氧层的积累和鳍宽对器件电学特性的影响。     

The influence of emitter-base junction design on collectorsaturation current, ideality factor, Early voltage, and device switchingspeed of Si/SiGe HBT's

In advanced Si/SiGe HBT's the base is doped much higher than emitter and collector. Base outdiffusion becomes a problem because of the formation of parasitic barriers that degrade device performance. The simulations and experiments of this paper show that a strong correlation exists between (a) the drop of the collector saturation current, (b) an increase of its ideality factor and (c) a rise of the switching time due to an additional emitter delay which can no longer be neglected. Curves of these three parameters as a function of Si/SiGe heterointerface position and outdiffusion at the base-emitter interface have been calculated and indicate that only a few nm shift may cause severe device degradation. An important result is that the collector current ideality factor or the inverse Early voltage is a very sensitive indicator for the quality of the emitter-base interface. Application of these results have yielded experimental SiGe HBT's with transit frequencies above 60 GHz     

Silicon germanium active layer with graded band gap and µc-Si:H buffer layer for high efficiency thin film solar cells

We investigated solar cells with graded band gap hydrogenated amorphous silicon germanium active layer and hydrogenated microcrystalline silicon buffer layer at the interface of intrinsic and n-type doped layer. A significantly improved, 10.4% device efficiency was observed in this type of single junction solar cell. The intrinsic type microcrystalline silicon buffer layer is thought to play dual roles in the device; as a crystalline seed-layer for growth of n-type hydrogenated microcrystalline silicon layer and helping efficient electron collection across the i/n interface. Based on these, an enhancement in cell parameters such as the open-circuit voltage (V_oc), and fill factor (FF) was observed, where the FF and V_oc reaches up to 69% and 0.85 V respectively. Our investigation shows a simple way to improve device performance with narrow-gap silicon germanium active layer in solar cells in comparison to the conventionally constant band gap device structure.     

A cascade energy band structure enhances the carrier energy in organic vertical-type triodes

Organic vertical-type triodes (OVTs) based on the cascade energy band structure as emitter layer are studied. The electric characteristics were dramatically enhanced while incorporating the cascade energy under current driving and voltage driving modes. The improvement is attributed to that injection carriers can obtain higher energy through a stepwise energy level. When the device has a layered structure of F₁₆CuPC (10 nm)/PTCDI (10 nm)/pentacene (100 nm) in emitter, it exhibits a common-base transport factor of 0.99 and a common-emitter current gain of 225 under current driving mode and exhibits a high current modulation-exceeding ?520 μA for a low collector voltage of ?5 V and a base voltage of ?5 V and the current on/off ratio of 10³ under voltage driving mode. Furthermore, we realized first organic current mirror that exhibited out/in current ratio of 0.75 and output resistance of 10⁵ Ω by using the OVTs.     

Emission from outside of the emission layer in state-of-the-art phosphorescent organic light-emitting diodes

The emission zone profile in an organic light-emitting diode was extracted by fitting the experimentally measured far-field angular electroluminescence spectrum of a purposely designed device. It is based on a thin 10 nm emission layer doped with the red emitting phosphor Ir(MDQ)₂acac. We find strong indications for light emission originating from outside of the emission layer, even though the device has electron and hole blocking layers. These are commonly assumed to completely confine the charge carrier recombination and hence the light emission to the emission layer. Since the calculated internal spectrum of the emission matches the emitter photoluminescence spectrum well, diffusion of the emitter molecules outside of the emission layer is hypothesized.