Hole Barrier Height Research Articles

Indium gallium zinc oxide (IGZO)-based Ohmic contact formation on n-type gallium antimony (GaSb)

In this paper, Ohmic-like contact on n-type GaSb with on/off-current ratio of 1.64 is presented, which is formed at 500 °C by inserting IGZO between metal (Ni) and GaSb. The resulting Ohmic contact is systematically investigated by TOF-SIMS, HSC chemistry simulation, XPS, TEM, AFM, and J–V measurements. Two main factors contributing to the Ohmic contact formation are (1) InSb (or InGaSb) with narrow energy bandgap (providing low electron and hole barrier heights) formed by In diffusion from IGZO and Sb released by Ga oxidation, and (2) free Sb working as traps that induces tunneling current.

Performance Enhancement of Blue InGaN Light-Emitting Diodes With InGaN Barriers and Dip-Shaped Last Barrier

The characteristics of blue InGaN light-emitting diodes (LEDs) with InGaN barriers and dip-shaped last barrier are investigated numerically. The simulation results show that the newly designed LEDs have a better performance over the conventional InGaN/GaN and InGaN/InGaN counterparts attributed to the enhancement of carriers confinement induced by the improved potential barrier height for electrons and holes, and the amelioration of electron-hole spatial overlap caused by the reduced polarization effect between the well and barrier. In addition, the efficiency droop and the radiative recombination rate are markedly improved by employing the LEDs with InGaN barriers and dip-shaped last barrier.

Antimonide-Based Heterostructure p-Channel MOSFETs With Ni-Alloy Source/Drain

In this letter, we study the formation and electrical properties of Ni-GaSb alloys by direct reaction of Ni with GaSb. It is found that several properties of Ni-antimonide alloys, including low thermal budget processing (300°C), low Schottky barrier height for holes (~0.1 eV), low sheet resistance of Ni-InGaSb (53 Ω/\square), and low specific contact resistivity (7.6×10) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-7</sup> Ω cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ), show good progress toward antimonide-based metal source/drain (S/D) p-channel metal-oxide-semiconductor field-effect transistors. Devices with a self-aligned metal S/D were demonstrated, in which heterostructure design is adopted to further improve the performance, e.g., ON/OFF ratio , subthreshold swing (140 mV/decade), and high effective-field hole mobility of ~510 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Vs at sheet charge density of 2×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> .

(Invited) Development of Metal Source/Drain Ge-CMOS Using TiN/Ge and HfGe/Ge Contacts

Ge is of great interest as a candidate channel material for future CMOS devices due to its high intrinsic carrier mobility. To translate this potential into CMOS devices, it is desirable that the source/drain (S/D) junctions are composed by metal/Ge contacts. However, the Fermi-level pinning impedes the embodiments, by which the contact formation showing a low electron barrier height (ΦBN) is very difficult. We have found TiN on Ge is superior as a low ΦBN contact. In this paper, we present the detailed fabrication and the electrical performance of a TiN/Ge contact. Furthermore, we present the excellence of a HfGe/Ge contact showing a low hole barrier height (ΦBP). Finally, we present the fabrication and device performance of metal S/D n- and p-MOSFETs using TiN/Ge and HfGe/Ge contacts, respectively.

Photoelectrochemical Studies on CdS/Poly Ethyl-Aniline Interfaces

An inorganic/organic/interface (IOI) consisting of CdS/poly ethyl aniline (PEA) assemblies was subjected to photoelectrochemical studies in nanoparticle suspensions and in thin solid film forms . The effects that PEA modifiers have on the photoelectrochemical behavior of the IOI were investigated using [Fe(CN)6]4-as a photoactive hydrated electron donor agent. Results show that the adsorption process of [Fe(CN)6]3- (the photolysis product) controls the photoactivity outcome of IOI assemblies. CdS/PEA shows lower heterogeneous photochemical response than native CdS. Native CdS amorphous nanoparticles adsorb more [Fe(CN)6]3- with a very steady adsorption/desorption process than the CdS/PEA. The interface activities were explained by analyzing the IOI junction’s properties such as electron affinity, work function and hole/electrons barrier heights. The lower hole barrier height versus electron barrier height suggests that charge injection is mediated at the IOI interface through hole transfer. The aqueous nano systems retained moderate stability as indicated by the reproducibility of their photocatalytic activities. Both [Fe(CN)6]4- and PEA contributed to the stability of native CdS surfaces.

Improvement of UV electroluminescence of n-ZnO/p-GaN heterojunction LED by ZnS interlayer

n-ZnO/p-GaN heterojunction light emitting diodes with different interfacial layers were fabricated by pulsed laser deposition. The electroluminescence (EL) spectra of the n-ZnO/p-GaN diodes display a broad blue-violet emission centered at 430 nm, whereas the n-ZnO/ZnS/p-GaN and n-ZnO/AlN/p-GaN devices exhibit ultraviolet (UV) emission. Compared with the AlN interlayer, which is blocking both electron and hole at hetero-interface, the utilization of ZnS as intermediate layer can lower the barrier height for holes and keep an effective blocking for electron. Thus, an improved UV EL intensity and a low turn-on voltage (~5V) were obtained. The results were studied by peak-deconvolution with Gaussian functions and were discussed using the band diagram of heterojunctions.

Hetero-Tunnel Field-Effect-Transistors With Epitaxially Grown Germanium on Silicon

The epitaxially grown Ge (epi-Ge)/Si hetero-tunnel field-effect transistors (HTFETs) is demonstrated to enhance band-to-band tunneling (BTBT) current because of effective bandgap reduction. The epi-Ge HTFET has a drain current as high as 11 μA/μm at V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GS</sub> =V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DS</sub> =-2 V, which enhances 2.3× as compared with that of Si, and the drain-induced barrier thinning (DIBT) shows 28 mV/V. The smaller subthreshold swing of epi-Ge HTFET is obtained as compared with Si. The current mechanism of BTBT is confirmed at the interface of epi-Ge/Si hetero-tunneling occurring in the n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -region by hole barrier height extraction. The gate/source overlap lengths for parallel-plate tunneling region are discussed for current and DIBT.

Comparative studies on the inorganic and organic p-type dopants in organic light-emitting diodes with enhanced hole injection

Comparative studies on inorganic and organic electron acceptors used as p-dopants in N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB) in organic light-emitting diodes (OLEDs) are carried out. It demonstrates that 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) doped hole-injecting layer is superior to molybdenum trioxide (MoO3) doped one in device efficiency and stability. Combining with absorption spectral measurement, the effectiveness of a p-doped NPB in OLEDs does not solely rely on the generation of charge-transfer complexes in the doped NPB. The detailed difference between MoO3 and HAT-CN as p-dopants in NPB is further investigated by evaluating the hole injection efficiency, hole barrier height, and surface morphology of the doped films.

Contact Resistance Reduction for Strained N-MOSFETs With Silicon-Carbon Source/Drain Utilizing Aluminum Ion Implant and Aluminum Profile Engineering

We demonstrate a novel technique to reduce the nickel silicide (NiSi) contact resistance <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">con</sub> in strained n-channel MOSFETs (n-FETs) with silicon carbon (Si:C) stressors, where a presilicide aluminum (Al) implant is performed and the Al profile is found to be affected by carbon (C). Al diffusion during silicidation is retarded by the presence of C and a high Al concentration is retained within the NiSi:C film, which is considered to be the main reason for electron barrier height Φ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Bn</i> reduction in NiSi:C contacts. Ge preamorphization implant prior to Al implant further reduces the Φ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Bn</i> to 0.44 eV. Integration of this technique in n-FETs with Si:C stressors achieves a 50% reduction in source/drain series resistance and 12% enhancement in saturation drive current. Negligible impact on the device short-channel effects is observed. When Al segregates at the NiSi/Si interface, the hole barrier height Φ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Bp</i> is lowered, and such an Al profile can be used for the p-FETs. Al profile engineering shows a promise as a single-metal-silicide solution for selective <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">con</sub> optimization in CMOS.

Graphene as transparent electrode for direct observation of hole photoemission from silicon to oxide

We demonstrate the application of graphene as collector material in internal photoemission (IPE) spectroscopy, which enables direct observation of both electron and hole injections at a Si/Al2O3 interface and overcomes the long-standing difficulty of detecting holes in IPE measurements. The observed electron and hole barrier heights are 3.5 ± 0.1 eV and 4.1 ± 0.1 eV, respectively. Thus, the bandgap of Al2O3 can be deduced to be 6.5 ± 0.2 eV, in good agreement with the value obtained by ellipsometry analysis. Our modeling effort reveals that, by using graphene, the carrier injection from the emitter is significantly enhanced and the contribution from the collector electrode is minimal.

Origin of Rectification in Boron Nitride Heterojunctions to Silicon

Cubic and hexagonal boron nitride (cBN and hBN) heterojunctions to n-type Si are fabricated under low-energy ion bombardment by inductively coupled plasma-enhanced chemical vapor deposition using the chemistry of fluorine. The sp2-bonded BN/Si heterojunction shows no rectification, while the cBN/sp2BN/Si heterojunction has rectification properties analogue to typical p-n junction diodes despite a large thickness (∼130 nm) of the sp2BN interlayer. The current-voltage characteristics at temperatures up to 573 K are governed by thermal excitation of carriers, and mostly described with the ideal diode equation and the Frenkel-Poole emission model at low and high bias voltages, respectively. The rectification in the cBN/sp2BN/Si heterojunction is caused by a bias-dependent change in the barrier height for holes arising from stronger p-type conduction in the cBN layer and enhanced with the thick sp2BN interlayer for impeding the reverse current flow at defect levels mainly associated with grain boundaries.

Charge re-distribution and ZT in PEDOT-related thermoelectrics

Recently it has been reported that poly(3,4-ethylenedioxythiphene (PEDOT))-related polymers can attain values of ZT comparable to their inorganic counterpart. In particular, it was shown that the thermal conductivity κ could be lowered to a value found in nano-structured semiconductors. In this work we studied the charge transport properties of PEDOT-related polymers and examined how the materials parameters were linked to κ and ZT based on data reported in the open literature. Using a two-region model and taking into account charge migration from the more conducting PEDOT islands to the resistive PSS sheaths in poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate), we were able to demonstrate that ZT in the PSS sheaths could increase by orders of magnitude. Furthermore, ZT could approach a value of 20 or more when σ the bandwidth of the density of states of the transport sites was very small (<3 meV). Outward charge migration from the islands can in theory be enhanced by adding negatively-charged oxygen ions to the sheaths as observed in oxidized Poly(3,4-ethylenedioxythiophene-tosylate). This lowers the barrier height for holes and enhances ZT in the sheaths.

Band alignment between Ta2O5 and metals for resistive random access memory electrodes engineering

Band alignment of resistive random access memory (RRAM) switching material Ta2O5 and different metal electrode materials was examined using high-resolution X-ray photoelectron spectroscopy. Schottky and hole barrier heights at the interface between electrode and Ta2O5 were obtained, where the electrodes consist of materials with low to high work function (Φm,vac from 4.06 to 5.93 eV). Effective metal work functions were extracted to study the Fermi level pinning effect and to discuss the dominant conduction mechanism. An accurate band alignment between electrodes and Ta2O5 is obtained and can be used for RRAM electrode engineering and conduction mechanism study.

ITO/p+nc-Si:H contact barrier effects on n-i-p′-p silicon solar cell performances

Abstract The computer program AMPS-1D (analysis of microelectronic and photonic structures) has been used to explore the effect of front contact barrier heights for electrons ( φ b 0 ) or holes ( φ h ) on the performances of n-i-p′-p amorphous/nanocrystalline silicon based solar cell, with a p type hydrogenated nanocrystalline silicon double layer and with no back reflector. φ b 0 is the result of band bending at the indium tin oxide (ITO)/p + doped hydrogenated nanocrystalline silicon (p nc-Si:H) interface. This paper presents results for a n-i-p′-p device, when the p nc-Si:H layer is used as a window and the p′-nc-Si:H layer as a buffer. Band diagram at thermodynamic equilibrium and current–voltage characteristics ( J – V ), under dark and illumination conditions, for the considered solar cell structure, are calculated. The modeling showed that the reverse bias currents do not depend on the front contact barrier heights. However, in the forward direction, this contact barrier influences strongly the J – V characteristic in the dark and under illumination. As a result, when φ b 0 increases, output cell parameters, like open circuit voltage ( V OC ), fill factor (FF) and efficiency ( E ff ) increase. The best values obtained are 0.893 V, 0.757 and 8.04%, respectively. These values correspond to a front contact barrier height ( φ b 0 ) equal to 1.65 eV. Such a value of φ b 0 can be realized experimentally by using an indium tin oxide (ITO) front contact electrode, with a work function value about 5.35 eV.

Nanoscale quantification of charge injection and transportation process in Si-nanocrystal based sandwiched structure

Si nanocrystals are formed by using KrF pulsed laser crystallization of an amorphous SiC/ultrathin amorphous Si/amorphous SiC sandwiched structure. Electrons and holes are injected into Si nanocrystals via a biased conductive AFM tip and the carrier decay and transportation behaviours at the nanoscale are studied by joint characterization techniques of Kelvin probe force microscopy (KPFM) and conductive atomic force microscopy (CAFM). Quantification of the surface charge density is realized by solving the Poisson equation based on KPFM measurements. Besides, the asymmetric barrier height for electrons and holes is considered to play a dominant role in controlling the charge retention and transportation characteristics. The methodology developed in this work is promising for studying the charge injection and transportation process in other materials and structures at the nanoscale.

A photovoltaic effect in the metal–high-resistive GaAs:Cr contact

The results of studies of photovoltaic effect in the contacts of a number of metals with high-resistive GaAs:Cr are reported. High-resistive (HR) GaAs was obtained by diffusion of chromium in n-GaAs. V, Cr, and Al were used as metals. In was employed in order to produce ohmic contacts. Photovoltage was excited by red light (hν = 1.85 eV), and the excitation intensity amounted to 1.5⋅1021 сm–2⋅s–1. Photovoltage was measured in the presence of asymmetric pairs of contacts to HR-GaAs: V–In, Cr–In, and Al–In. It is shown that V, Cr, and Al form barriers for electrons in the contact with high-resistive GaAs:Cr. The photovoltage of the contacts is determined by the inversion of conductivity type of the near-surface GaAs layer under the metal contact. The hole concentration in the inversion layer can be as high as ≈1015 сm–3. An In contact to high-resistive GaAs:Cr is an ohmic injecting contact for electrons with the barrier height for holes ≈0.9 eV.

Low Electron Schottky Barrier Height of NiGe/Ge Achieved by Ion Implantation After Germanidation Technique

In this letter, an extremely low electron Schottky barrier height (SBH) of NiGe/Ge contact has been experimentally demonstrated with P+ ion implantation after germanidation. The results show that the current characteristics of NiGe/p-Ge diode changes from ohmic to well rectifying with Ion/Ioff ratio over 105 and the corresponding hole barrier height increases to 0.56 eV, which indicates that a record-low electron barrier height of 0.10 eV is achieved. The remarkable Schottky barrier modulation is attributed to phosphorus segregation near the NiGe/Ge interface. In addition, the slight dependence of the SBH on drive-in annealing temperature is also observed ranging from 350 °C to 500 °C, and the wide temperature window is beneficial for the process integration of Ge MOS device. The low electron SBH achieved in this letter is very critical to reduce the source/drain resistance and may provide new ideas for the performance improvement of Ge devices, particularly for nMOSFETs.

Enhanced nanoscale resistive switching memory characteristics and switching mechanism using high-Ge-content Ge0.5Se0.5 solid electrolyte

We demonstrate enhanced repeatable nanoscale bipolar resistive switching memory characteristics in Al/Cu/Ge0.5Se0.5/W, as compared with Al/Cu/Ge0.2Se0.8/W structures, including stable AC endurance (>105 cycles), larger average SET voltage (approximately 0.6 V), excellent data retention (>105 s) at 85°C, and a high resistance ratio (>104) with a current compliance of 8 μA and a small operation voltage of ±1.5 V. A small device size of 150 × 150 nm2 and a Cu nanofilament with a small diameter of 30 nm are both observed by high-resolution transmission electron microscope in the SET state. The GexSe1 − x solid electrolyte compositions are confirmed by both energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. The switching mechanism relies on the smaller barrier heights for holes rather than for electrons; the positively charged Cuz+ ions (i.e., holes) migrate through the defects in the GexSe1 − x solid electrolytes during SET/RESET operations. Hence, the Cu nanofilament starts to grow at the Ge0.5Se0.5/W interface, and starts to dissolve at the Cu/Ge0.5Se0.5 interface, as illustrated in the energy band diagrams. Owing to both the higher barrier for hole injection at the Cu/Ge0.5Se0.5 interface than at the Cu/Ge0.2Se0.8 interface and greater thermal stability, the resistive switching memory characteristics of the Al/Cu/Ge0.5Se0.5/W are improved relative to the Al/Cu/Ge0.2Se0.8/W devices. The Al/Cu/Ge0.5Se0.5/W memory device can also be operated with a low current compliance of 1 nA, and hence, a low SET/RESET power of 0.61 nW/6.4 pW is achieved. In addition, a large memory size of 1,300 Pbit/in2 is achieved with a small nanofilament diameter of 0.25 Å for a small current compliance of 1 nA.

Enhanced Optical Power of InGaN/GaN Light-Emitting Diode by AlGaN Interlayer and Electron Blocking Layer

We report on the effect of an AlGaN interlayer with a low Al composition, which is inserted between multiple quantum wells (MQWs) and an AlGaN electron blocking layer (EBL) in light-emitting diodes (LEDs). The band diagram shows that this interlayer reduces the hole barrier height between the last GaN barrier in MQWs and the AlGaN EBL to enhance the hole injection. The optical output power at 20 mA of LEDs with interlayer is increased by 20% more than that of LEDs without an interlayer. This improvement is attributed to the enhanced radiative recombination rate, which is due to a more uniform hole distribution and higher hole concentration in MQWs.

Photoemission Study of the Morphology and Barrier Heights at the Interface between Benz[a]anthracene and Noble Metal (111) Surfaces

Various coverages of benz[a]anthracene (BA) thin films prepared by depositing the molecules in several steps on Au(111) and Ag(111) were investigated using ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS). XPS and UPS were taken after each deposition. A Schottky junction formed between BA/Au(111) and BA/Ag(111). A hole barrier height of 1.89 eV in BA/Ag(111) was determined directly from the observed highly occupied molecular orbital (HOMO) peak cutoff however a HOMO peak cutoff in BA/Au(111) 1.63 eV was indirectly determined. When combined with previous measurements of the BA/Cu(111) interface, the slope S of the plot of hole barrier height as a function of metal workfunction (WF) was calculated to be 0.39, which is close to the S value of pentacene. Discussion on film growth mode, band bending, band diagram, and comparisons of interface parameter S of BA with S of other molecules condensed on metal surfaces are presented.