High electron mobility in (InAs)n(GaAs)n short period superlattices grown by MOVPE for high-electron mobility transistor structure

(InAs)_n(GaAs)_n short period superlattices (SPSs) have been successfully grown by a continuous MOVPE process on InP substrates. Their structural, optical, and electrical properties have been studied. The periodic structures have been confirmed by x-ray measurements and (InAs)₁(GaAs)₁ SPSs have been clearly observed by transmission electron microscopic characterization. The optical quality of the material has been tested by 2K photoluminescence and excitonic recombinations have been observed. Mobilities as high as 10700 cm².V⁻¹.s⁻¹ and 64000 cm². V⁻¹.s⁻¹ for a sheet concentration of 3 × 10¹² cm⁻² have been obtained at 300K and 77K, respectively.     

Ab initio calculations of phonon spectra of (GaP)<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>(AlP)<Subscript><Emphasis Type="Italic">m</Emphasis></Subscript> superlattices

Using the method of linear response, vibrational spectra and densities of states of GaP and AlP crystals and monolayer GaP/AlP superlattices are calculated. Phonon modes of (GaP) _n (AlP) _m superlattices with various numbers of monolayers are calculated for the center of the Brillouin zone. The obtained results are compared with the Raman scattering data and the effect of nonideality of the interface on phonon frequencies is discussed.     

(InSb)m/(HgTe)n短周期超晶格的电子结构

Theelectronicstructureof（InSb）m/（HgTe）nshortperiodsuperlatticesgrownalongthe（001）directionis studiedtheoreticallyusingnorm-conservingpseudo-potentialstogetherwiththelocal-densityapproximationforthe exchange-correlationpotential.Thebandstructuredependsonthevalueofmandn,thenumberofmono-layersand on the ordering of atoms at the InSb/HgTe interface in one unit cell. Our calculation indicates that the superlattice can be a semiconductor having a band gap between the occupied and unoccupied bands, or a metal with no band gap at the Fermi energy. According to the further calculation of total charge density between（InSb）m/（HgTe）nwith different structures, a clearly different behavior happens when the structure changes from a system with a gap and a system without a gap.     

Efficient green luminescence from a type-II neighboring confinement structure realized in an AlP/GaP system

Highly efficient green luminescence was observed from a new class of quantum confined geometry referred to as neighboring confinement structure (NCS). The photoluminescence (PL) intensity of AlP/GaP NCSs was even higher than that of a 300-period AlP/GaP superlattice (SL), and the PL of the NCS exhibited much improved immunity against thermal quenching compared to the SLs. The luminescence origin of the NCS was confirmed from the well width dependence of the PL peak shift, and the main luminescence line was assigned to no-phonon from a phonon-resolved PL study.     

Electrical characterization of thiols self-assembled layers on GaP (1 1 1) structures

The experimental studies on III–V semiconductor compounds surface passivation phenomena are mainly dedicated to solve some technological problems as those regarding the ways to keep the chemical stability of native oxides on surfaces. Self-assembled monolayers (SAMs) provide a simple way to produce relatively ordered structures at a molecular scale, which seems to be capable to protect the clean surface against the evolution of oxidation process. In this respect, thin films of SAMs of aliphatic thiol (dodencanthiol—CH₃(CH₂)₁₁SH) and aromatic thiol (4, 4′ tiobisbenzenthiol-S (C₆H₄SH)₂ have been deposited on the surface of GaP (1 1 1) samples. The electrical properties measurements of some structures based on GaP compound was performed. There were recorded current–voltage (I–V) characteristics for complex structures AuGeNi/R-SH/GaP and AuGeNi/Ar-SH/GaP in darkness and also exposed to a Xe lamp. In dark and in “reverse bias” way, the I–V characteristics present the feature of a Zenner diode for GaP/Ar-SH and a gradual increase of current for GaP/R-SH. In dark and “in forward bias” way, the current increases as for a normal diode for both GaP/Ar-SH and GaP/R-SH structures. The complex structures (e.g.: In/AuGeNi/R-SH/GaP/R-SH/AuGeNi/In) are less sensitive to light. The SEM analysis performed on a GaP/R-SH surface shows a continuous packed up layer while GaP/Ar-SH looks like an inhomogeneous deposition of layers with different thickness regions. The diodes’ ideality factors determined from I–V characteristics are unusually high (n2) as a possible result of inhomogeneous Schottky contacts or due to ageing effects, in the field of degradation.     

Low-frequency low-noise circuits design using an E n-In model

In view of the limitations of a R _n-G_n model in the low frequency range and the defects of an E _n-I_n model in common use now, this paper builds a complete E _n-I_n model according to the theory of random harmonic. The parameters for the low-noise design such as the equivalent input noisy voltage E _ns, the optimum source impedance Z _sopt and the minimum noise figure F _min can be calculated accurately by using this E _n-I_n model because it considers the coherence between the noise sources fully. Moreover, this paper points out that it will cause the maximum 30% miscalculation when neglecting the effects of the correlation coefficient γ. Using the series-series circuits as an example, this paper discusses the methods for the E _n-I_n noise analysis of electronic circuits preliminarily and demonstrates its correctness through the comparison between the simulated and measured results of the minimum noise figure F _min of a single current series negative feedback circuit.     

Semiconductor nanowires surrounded by cylindrical Al2O3 shells

The GaN, GaP, InP, Si₃N₄, SiO₂/Si, SiC, and ZnO semiconductor nanowires were synthesized by a variety of growth methods, and they were wrapped cylindrically with amorphous aluminum oxide (Al₂O₃) shells. The Al₂O₃ was deposited on these seven different semiconductor nanowires by atomic layer deposition (ALD) at a substrate temperature of 200°C using trimethylaluminum (TMA) and distilled water (H₂O). Transmission electron microscopy (TEM) images taken for the nanowires revealed that Al₂O₃ cylindrical shells surround uniformly all these semiconductor nanowires. Our TEM study illustrates that the ALD of Al₂O₃ has an excellent capability to coat any semiconductor nanowires conformally; its coating capability is independent of the chemical component, lattice structure, and growth direction of the nanowires. This study suggests that the ALD of Al₂O₃ on nanowires is one of the promising methods to prepare cylindrical dielectric shells in coaxially gated, nanowire field-effect transistors (FETs).     

Low-pressure pyrolysis studies of a new phosphorus precursor: Tertiarybutylbis(dimethylamino)phosphine

The low pressure decomposition of tertiarybutylbis(dimethylamino) phosphine, (t-Bu)P(NMe₂)₂, (TBBDMAP), has been studied on quartz and deposited GaP and InP surfaces. This new phosphorus precursor has been found to pyrolyze on quartz surfaces at much lower temperatures than the related compounds tertiarybutylphosphine, (t-Bu)PH₂, (TBP) and tris(dimethylamino)phosphorus, P(NMe₂)₃, (TDMAP). In contrast to the results obtained for TDMAP, GaP and InP surfaces decrease the decomposition temperature of TBBDMAP only slightly. The TBBDMAP reaction products were dimethylamine, methylmethyleneimine, and isobutylene, consistent with previous pyrolysis studies of TBP and TDMAP.     

N-methyl-2-pyrrolidone Iodide as Functional Precursor Additive for Record Efficiency 2D Ruddlesden-Popper (PEA)2(Cs)n−1PbnI3n+1 Solar Cells

2D perovskite (PEA)₂(Cs)_n−1Pb_nI_3n+1 (PEA: phenylethylammonium) exhibits more strengthened phase stability than its 3D components under ambient conditions and hence gained great attention in recent years. However, uncontrollable crystallization kinetics in (PEA)₂(Cs)_n−1Pb_nI_3n+1 leads to difficulty in controlling film morphology and phase-orientation regulation, resulting in poor power conversion efficiency (PCE). Herein, by incorporating precursor additive N-methyl-2-pyrrolidone iodide (NMPI), the crystallization rate during the deposition of (PEA)₂(Cs)_n−1Pb_nI_3n+1 film is efficiently regulated. As a result, the 2D or quasi-2D perovskite solar cell (PSC) delivers record PCEs in all reported 2D or quasi-2D CsPbX₃ families, for instance, the quasi-2D (n = 20) CsPbI₃ PSC exhibits a record PCE of 14.59%, showing significantly enhanced stability. Detailed characterization reveals that the NMPI forms hydrogen bonds with dimethylammonium iodide (DMAI) in the precursor to control crystallization rate for a smooth morphology with small fluctuation, which leads to improved carrier lifetime and reduced trap-density. More importantly, femtosecond transient absorption (fs-TA) measurements confirm an improved phase purity and the suppressed nonradiative recombination in quasi-2D perovskite film. It is believed that this simple additive strategy paves a new route for solving phase transition and crystallization kinetic problems in 2D and quasi-2D CsPbX₃.     

Visible light-emitting diodes grown on optimized ∇x[InxGa1−x]P/GaP epitaxial transparent substrates with controlled dislocation density

Epitaxial transparent-substrate light-emitting diodes (ETS-LEDs) have been fabricated on optimized graded buffers of In_xGa_1−xP on GaP (∇_x[In_xGa_1−x]P/GaP) that feature controlled threading dislocation densities of 3×10⁶ cm⁻². The ETSLEDs show increasing efficiency from 575 nm to 655 nm, in marked contrast to previous reports where performance drops above 600 nm, and feature the lowest spectral widths ever reported in ∇_x[In_xGa_1−x]P/GaP. The improvement over earlier reports is attributed to large mean dislocation spacings in optimized ∇_x[In_xGa_1−x]P/GaP, which are an order of magnitude greater than the mean carrier diffusion length. A slight performance decline remains at 655 nm, but the overall performance of this first generation of ETS-LEDs is promising.     

Ag(Ta,Nb)O3 thin-film interdigital capacitors for microwave applications

Epitaxial silver tantalate-niobate Ag(Ta,Nb)O₃ (ATN) films have been grown on LaAlO₃(0 0 1) single crystals by pulsed laser ablation of stoichiometric AgTa_0.38Nb_0.62O₃ ceramic target. Extensive X-ray diffraction analysis reveals epitaxial relationship between the ATN film and LaAlO₃(0 0 1) substrate. Micrometer size interdigital capacitor structures have been defined photolithographically on the top surface of ATN films. ATN/LaAlO₃ thin-film capacitors exhibit superior overall performance: loss tangent as low as 0.0033 at 1 MHz, dielectric permittivity 224 at 1 kHz, weak frequency dispersion of 5.8% in 1 kHz to 1 MHz range. Dielectric permittivity and loss tangent were also measured at the microwave range. Conformal mapping techniques were employed to extract dielectric properties of ATN film on substrate at the microwave frequency range. Theoretical properties of conformal mapping techniques for interdigital capacitors and CPW microstrip lines were discussed.     

Impurity-induced disordering of AlxGa1−xAs-GaAs quantum well heterostructures with (Si2)x(GaAs)1−x barriers

Recent work indicates that the alloy (Si₂)_x(GaAs)_1−x can be formed within the GaAs quantum well of an Al_xGa_1−xAs-GaAs quantum well heterostructure (QWH) and results in a shift of laser operation to higher energy. In this paper we show, by SIMS and EDS measurements, that the Si concentration in the (Si₂)_x(GaAs)_1−x layer far exceeds typical “doping” levels. The stability of these QWHs has been investigated with respect to thermal annealing and Zn impurity-induced layer disordering (Zn-IILD). Data are presented showing that the (Si₂)_x(GaAs)_1−x alloy is stable against thermal annealing unless a rich source of Ga vacancies is provided, and that relatively low temperature Zn diffusion greatly enhances the disordering process of the alloy layer.     

Anomalous anisotropy in the absorption coefficient of vacancy-ordered Ga2Se3

The structural and optical properties of Ga₂Se₃ on (100)GaP and (100)GaAs prepared by molecular beam epitaxy have been investigated. The electron diffraction studies revealed that the superstructure was formed in [011] direction by the spontaneous ordering of native gallium-vacancies in the defect zinc blende structure under a selenium-rich growth condition, and very large absorption anisotropy (Δα>10⁴cm¹) was observed in the vacancy-ordered Ga₂Se₃. Furthermore, polarization dependence of photoconductivity due to the absorption anisotropy was observed in the vacancy-ordered Ga₂Se₃ on (100)GaAs.     

A green emitting iridium(III) complex with narrow emission band and its application to phosphorescence organic light-emitting diodes (OLEDs)

The homoleptic Ir(III) complex, fac-tris{2-(3′-trimethylsilylphenyl)-5-trimethylsilylpyridinato}iridium, has been synthesized and characterized to investigate the effect of the substitution of bulky silyl groups on the photophysical properties and electroluminescence (EL) characteristics of Ir(ppy)₃ (ppy = 2-phenylpyridine). The absorption, emission, cyclic voltammetry and electroluminescent performance of the complex have also been evaluated. A power efficiency of 17.3 lm/W at 10 mA/cm² compared to 11.7 lm/W for Ir(ppy)₃ is achieved with the new complex as a dopant in phosphorescent organic light-emitting diodes (OLEDs). In addition, the complex shows a narrow emission band of a small full width at half-maximum (fwhm, ca. 50 nm) value.     

New plasma chemistries for etching III–V compound semiconductors: Bl3 and BBr3

We investigated inductively coupled plasma (ICP) etching of both In-containing (InP, InAs, and InSb) and Ga-containing compound semiconductors (GaAs, GaP, and GaSb) in two new chemistries: BI₃ and BBr₃ with addition of Ar. Etch rates as high as 2 μm·min⁻¹ were obtained for InP in both types of discharge while for GaAs maximum rates were 1 and 2.5 μm·min⁻¹, respectively, in BI₃ and BBr₃. The rates were strongly dependent on plasma composition, ICP source power and radio frquency chuck power. BI₃ etching produced much smoother surfaces on both GaAs and InP, while maintaining the near-surface stoichiometry. Etch selectivities ≥ 10 were obtained for GaAs and InP over SiO₂ and SiN_x masks. The BI₃ chemistry appears attractive as an universal etchant for In-based and Ga-based compound semiconductors.     

基于再生长欧姆接触工艺的220 GHz InAlN/GaN 场效应晶体管

本文在蓝宝石衬底上研制了具有高电流增益截止频率(f_T)的InAlN/GaN异质结场效应晶体管 (HFETs)。基于MOCVD外延n -GaN欧姆接触工艺实现了器件尺寸的缩小,有效源漏间距(L_sd)缩小至600 nm。此外,采用自对准工艺制备了50 nm直栅。由于器件尺寸的缩小,V_gs= 1 V下器件最大饱和电流(I_ds)达到2.11 A/mm,峰值跨导达到609 mS/mm。小信号测试表明,器件f_T达到220 GHz、最大振荡频率(f_max)达到48 GHz。据我们所知,该f_T值是目前国内InAlN/GaN HFETs器件报道的最高结果。     

A dry-patterned Cu(Mg) alloy film as a gate electrode in a thin-film transistor liquid crystal display

The annealing of a Cu(4.5at.%Mg)/SiO₂/Si structure in ambient O₂ at 10 mtorr and 300–500°C allows for the out-diffusion of the Mg to the Cu surface, forming a thin MgO (15 nm) layer on the surface. The surface MgO layer was patterned and successfully served as a hard mask for the subsequent dry etching of the underlying Mg-depleted Cu films using an O₂ plasma and hexafluoroacetylacetone (H(hfac)) chemistry. The resultant MgO/Cu structure, with a taper slope of about 30°, shows the feasibility of dry etching of Cu(Mg) alloy films using a surface MgO mask scheme. A dry-etched Cu(4.5at.%Mg) gate a-Si:H thin-film transistor (TFT) has a field-effect mobility of 0.86 cm²/Vs, a subthreshold swing of 1.08 V/dec, and a threshold voltage of 5.7 V. A novel process for the dry etching of Cu(Mg) alloy films that eliminates the use of a hard mask, such as Ti, and results in a reduction in the process steps is reported for the first time in this work.     

Epitaxial growth of Dy2O3 thin films on epitaxial Dy-germanide films on Ge(0 0 1) substrates

Ultra-thin films of Dy are grown on Ge(0 0 1) substrates by molecular beam deposition near room temperature and immediately annealed for solid phase epitaxy at higher temperatures, leading to the formation of DyGe_x films. Thin films of Dy₂O₃ are grown on the DyGe_x film on Ge(0 0 1) substrates by molecular beam epitaxy. Streaky reflection high energy electron diffraction (RHEED) patterns reveal that epitaxial DyGe_x films grow on Ge(0 0 1) substrates with flat surfaces. X-ray diffraction (XRD) spectrum suggests the growth of an orthorhombic phase of DyGe_x films with (0 0 1) orientations. After the growth of Dy₂O₃ films, there is a change in RHEED patterns to spotty features, revealing the growth of 3D crystalline islands. XRD spectrum shows the presence of a cubic phase with (1 0 0) and (1 1 1) orientations. Atomic force microscopy image shows that the surface morphology of Dy₂O₃ films is smooth with a root mean square roughness of 10 Å.     

Thick crack-free CaF2 epitaxial layer on GaAs (100) substrate by molecular beam epitaxy

The crystallographic orientation of low temperature (LT) grown CaF₂ on GaAs (100) substrates is investigated. LT epitaxial (100) CaF₂ layers are obtained on a thin (100) oriented CaF₂ template at growth temperatures down to room temperature. This makes it possible to grow crack-free CaF₂ (100) using a multiple-temperature-growth scheme at any desired temperature. The resulting CaF₂ layers, with thickness up to 680 nm, can withstand temperature cycling from RT to 650°C without cracking. Based on these results, a four pair Ga_0.5Al_0.5As/CaF₂ quarter-wavelength Bragg reflector was fabricated with center wavelength at 880 nm. The reflector, with a total CaF₂ thickness of 615 nm, shows broadband high reflectivity with a crack-free surface. This crack-free surface can then be overgrown with further device layers.     

Schottky barrier SOI-MOSFETs with high-k La(2)O(3)/ZrO(2) gate dielectrics

Schottky barrier SOI-MOSFETs incorporating a La₂O₃/ZrO₂ high-k dielectric stack deposited by atomic layer deposition are investigated. As the La precursor tris(N,N′-diisopropylformamidinato) lanthanum is used. As a mid-gap metal gate electrode TiN capped with W is applied. Processing parameters are optimized to issue a minimal overall thermal budget and an improved device performance. As a result, the overall thermal load was kept as low as 350, 400 or 500 °C. Excellent drive current properties, low interface trap densities of 1.9 × 10¹¹ eV⁻¹ cm⁻², a low subthreshold slope of 70-80 mV/decade, and an I_ON/I_OFF current ratio greater than 2 × 10⁶ are obtained.