Infrared photodetectors in heterostructure nanowires

We report on spectrally resolved photocurrent measurements on single self-assembled nanowire heterostructures. The wires, typically 3 microm long with an average diameter of 85 nm, consist of InAs with a 1 microm central part of InAsP. Two different sets of wires were prepared with phosphorus contents of 15+/-3% and 35+/-3%, respectively, as determined by energy-dispersive spectroscopy measurements made in transmission electron microscopy. Ohmic contacts are fabricated to the InAs ends of the wire using e-beam lithography. The conduction band offset between the InAs and InAsP regions virtually removes the dark current through the wires at low temperature. In the optical experiments, interband excitation in the phosphorus-rich part of the wires results in a photocurrent with threshold energies of about 0.65 and 0.82 eV, respectively, in qualitative agreement with the expected band gap of the two compositions. Furthermore, a strong polarization dependence is observed with an order of magnitude larger photocurrent for light polarized parallel to the wire than for light polarized perpendicular to the wire. We believe that these wires form promising candidates as nanoscale infrared polarization-sensitive photodetectors.     

Improved subthreshold slope in an InAs nanowire heterostructure field-effect transistor

An n-type InAs/InAsP heterostructure nanowire field-effect transistor has been fabricated and compared with a homogeneous InAs field-effect transistor. For the same device geometry, by introduction of the heterostructure, the threshold voltage is shifted 4 V, the maximum current on-off ratio is enhanced by a factor of 10,000, and the subthreshold swing is lowered by a factor 4 compared to the homogeneous transistor. At the same time, the drive current remains constant for a fixed gate overdrive. A single nanowire heterostructure transistor has a transconductance of 5 muA/V at a low source-drain voltage of 0.3 V. For the homogeneous InAs transistor, we deduced a high electron mobility of 1500 cm2/Vs.     

InAs1-xPx nanowires for device engineering

We present the growth of homogeneous InAs(1-x)P(x) nanowires as well as InAs(1-x)P(x) heterostructure segments in InAs nanowires with P concentrations varying from 22% to 100%. The incorporation of P has been studied as a function of TBP/TBAs ratio, temperature, and diameter of the wires. The crystal structure of the InAs as well as the InAs(1-x)P(x) segments were found to be wurtzite as determined from high-resolution transmission electron microscopy. Furthermore, temperature-dependent electrical transport measurements were performed on individual heterostructured wires to extract the conduction band offset of InAs(1-x)P(x) relative to InAs as a function of composition. From these measurements we extract a value of the linear coefficient of the conduction band versus x of 0.6 eV and a nonlinear coefficient, or bowing parameter, of 0.2 eV. Finally, homogeneous InAs(0.8)P(0.2) nanowires were shown to have a nondegenerate n-type doping and function as field-effect transistors at room temperature.     

A radio frequency single-electron transistor based on an InAs/InP heterostructure nanowire

We demonstrate radio frequency single-electron transistors fabricated from epitaxially grown InAs/InP heterostructure nanowires. Two sets of double-barrier wires with different barrier thicknesses were grown. The wires were suspended 15 nm above a metal gate electrode. Electrical measurements on a high-resistance nanowire showed regularly spaced Coulomb oscillations at a gate voltage from -0.5 to at least 1.8 V. The charge sensitivity was measured to 32 microe rms Hz(-1/2) at 1.5 K. A low-resistance single-electron transistor showed regularly spaced oscillations only in a small gate-voltage region just before carrier depletion. This device had a charge sensitivity of 2.5 microe rms Hz(-1/2). At low frequencies this device showed a typical 1/f noise behavior, with a level extrapolated to 300 microe rms Hz(-1/2) at 10 Hz.     

生长温度对InAs/GaAs横向异质结构纳米线形貌及晶体结构的影响

低维半导体材料因其超常的物理性能而受到了广泛关注和研究。本文采用金属有机物化学气相沉积（MOCVD）技术,利用金作催化剂制备了InAs/GaAs横向异质结构纳米线,并讨论了不同生长温度情况下InAs横向异质材料对纳米线形貌及晶体结构的影响。提高InAs材料的生长温度,可以有效地抑制纳米线的纵向生长,使其实现横向异质结构的生长。在异质结构纳米线横向生长时发生了侧面晶面旋转的现象,这是纳米线表面重构后侧面趋向能量更低的晶面的结果。本文的研究工作为推动微纳技术的发展提供了相应的理论基础和科学依据。      

Remote p-doping of InAs nanowires

We report on remote p-type doping of InAs nanowires by a p-doped InP shell grown epitaxially on the core nanowire. This approach addresses the challenge of obtaining quantitative control of doping levels in nanowires grown by the vapor-liquid-solid (VLS) mechanism. Remote doping of III-V nanowires is demonstrated here with the InAs/InP system. It is especially challenging to make p-type InAs wires because of Fermi level pinning around 0.1 eV above the conduction band. We demonstrate that shielding with a p-doped InP shell compensates for the built-in potential and donates free holes to the InAs core. Moreover, the off-current in field-effect devices can be reduced up to 6 orders of magnitude. The effect of shielding critically depends on the thickness of the InP capping layer and the dopant concentration in the shell.     

Solution-liquid-solid (SLS) growth of ZnSe-ZnTe quantum wires having axial heterojunctions

Heterostructured ZnSe-ZnTe quantum wires are grown by the solution-liquid-solid (SLS) mechanism. The nature of the axial or radial heterostructure obtained is strongly influenced by the growth sequence and related synthetic conditions. Compositionally graded ZnSe(x)Te(1-x) wires, ZnSe-ZnTe axial heterostructures containing a ZnSe(x)Te(1-x) transitional segment, ZnSe-ZnTe wires with sharp axial heterojunctions, and radial core-shell ZnSe-ZnTe quantum wires have been selectively prepared. The axial and radial quantum-wire heterostructures are characterized microscopically and spectroscopically.     

Growth of InAs/InP core-shell nanowires with various pure crystal structures

We have studied the epitaxial growth of an InP shell on various pure InAs core nanowire crystal structures by metal-organic vapor phase epitaxy. The InP shell is grown on wurtzite (WZ), zinc-blende (ZB), and {111}- and {110}-type faceted ZB twin-plane superlattice (TSL) structures by tuning the InP shell growth parameters and controlling the shell thickness. The growth results, particularly on the WZ nanowires, show that homogeneous InP shell growth is promoted at relatively high temperatures (～500?°C), but that the InAs nanowires decompose under the applied conditions. In order to protect the InAs core nanowires from decomposition, a short protective InP segment is first grown axially at lower temperatures (420-460?°C), before commencing the radial growth at a higher temperature. Further studies revealed that the InP radial growth rate is significantly higher on the ZB and TSL nanowires compared to WZ counterparts, and shows a strong anisotropy in polar directions. As a result, thin shells were obtained during low temperature InP growth on ZB structures, while a higher temperature was used to obtain uniform thick shells. In addition, a schematic growth model is suggested to explain the basic processes occurring during the shell growth on the TSL crystal structures.     

Faceting, composition and crystal phase evolution in III-V antimonide nanowire heterostructures revealed by combining microscopy techniques

III-V antimonide nanowires are among the most interesting semiconductors for transport physics, nanoelectronics and long-wavelength optoelectronic devices due to their optimal material properties. In order to investigate their complex crystal structure evolution, faceting and composition, we report a combined scanning electron microscopy (SEM), transmission electron microscopy (TEM), and scanning tunneling microscopy (STM) study of gold-nucleated ternary InAs/InAs(1-x)Sb(x) nanowire heterostructures grown by molecular beam epitaxy. SEM showed the general morphology and faceting, TEM revealed the internal crystal structure and ternary compositions, while STM was successfully applied to characterize the oxide-free nanowire sidewalls, in terms of nanofaceting morphology, atomic structure and surface composition. The complementary use of these techniques allows for correlation of the morphological and structural properties of the nanowires with the amount of Sb incorporated during growth. The addition of even a minute amount of Sb to InAs changes the crystal structure from perfect wurtzite to perfect zinc blende, via intermediate stacking fault and pseudo-periodic twinning regimes. Moreover, the addition of Sb during the axial growth of InAs/InAs(1-x)Sb(x) heterostructure nanowires causes a significant conformal lateral overgrowth on both segments, leading to the spontaneous formation of a core-shell structure, with an Sb-rich shell.     

Focused ion beam creation and templating of InAs and InAs/InP nanospikes

Ion beam irradiation has been examined as a method for creating nanoscale semiconductor pillar and cone structures, but has the drawback of inaccurate nanostructure placement. We report on a method for creating and templating nanoscale InAs spikes by focused ion beam (FIB) irradiation of both homoepitaxial InAs films and heteroepitaxial InAs on InP substrates. These 'nanospikes' are created as In droplets, formed due to FIB irradiation, act as etch masks for the underlying InAs. By pre-patterning the InAs to influence In droplet movement, nanospike locations on homoepitaxial InAs may be controlled with limited accuracy. Creating nanospikes using an InAs/InP heterostructure provides an additional measure of control over where the spikes form because nanospikes will not form on exposed regions of InP. This effect may be exploited to accurately control nanospike placement by pre-patterning an InAs/InP heterostructure to control the location of the InAs/InP interface. Using this heterostructure templating method it is possible to accurately place nanospikes into regular arrays that may be useful for a variety of applications.     

Growth of straight InAs-on-GaAs nanowire heterostructures

One of the main motivations for the great interest in semiconductor nanowires is the possibility of easily growing advanced heterostructures that might be difficult or even impossible to achieve in thin films. For III-V semiconductor nanowires, axial heterostructures with an interchange of the group III element typically grow straight in only one interface direction. In the case of InAs-GaAs heterostructures, straight nanowire growth has been demonstrated for growth of GaAs on top of InAs, but so far never in the other direction. In this article, we demonstrate the growth of straight axial heterostructures of InAs on top of GaAs. The heterostructure interface is sharp and we observe a dependence on growth parameters closely related to crystal structure as well as a diameter dependence on straight nanowire growth. The results are discussed by means of accurate first principles calculations of the interfacial energies. In addition, the role of the gold seed particle, the effect of its composition at different stages during growth, and its size are discussed in relation to the results observed.     

The growth of infrared antimonide-based semiconductor lasers by metal-organic chemical vapor deposition

We describe the metal-organic chemical vapor deposition (MOCVD) growth of InAsSb/InAs and GaAsSb/GaAs(P) multiple quantum well (MQW) and InAsSb/InAsP and InAsSb/InPSb strained-layer superlattice (SLS) active regions for use in mid-infrared emitters. We also describe the growth and initial characterization of GaAsSbN/GaAs MQW structures. By changing the layer thickness and composition of the InAsSb SLSs and MQWs, we have prepared structures with low temperature (<20 K) photoluminescence wavelengths ranging from 3.2 to 6.0 m. We have made gain-guided, injection lasers using undoped, p-type AlAs_0.16Sb_0.84 for optical confinement and both strained InAsSb/InAs MQW and InAsSb/InAsP and InPSb SLS active regions. The lasers and LEDs utilize the semi-metal properties of a p-GaAsSb/n-InAs heterojunction as a source for electrons injected into the active regions. Cascaded, semi-metal, mid-infrared, injection lasers with pseudomorphic InAsSb multiple quantum well active region lasers and LEDs are reported. We also report on GaAsSb/GaAs(P) lasers and LEDs emitting at 1.1 to 1.2 m grown on GaAs substrates and using AlGaAs layers for confinement.     

Coaxial metal-oxide-semiconductor (MOS) Au/Ga2O3/GaN nanowires

Coaxial metal-oxide-semiconductor (MOS) Au-Ga2O3-GaN heterostructure nanowires were successfully fabricated by an in situ two-step process. The Au-Ga2O3 core-shell nanowires were first synthesized by the reaction of Ga powder, a mediated Au thin layer, and a SiO2 substrate at 800 degrees C. Subsequently, these core-shell nanowires were nitridized in ambient ammonia to form a GaN coating layer at 600 degrees C. The GaN shell is a single crystal, an atomic flat interface between the oxide and semiconductor that ensures that the high quality of the MOS device is achieved. These novel 1D nitride-based MOS nanowires may have promise as building blocks to the future nitride-based vertical nanodevices.     

InAs/InP radial nanowire heterostructures as high electron mobility devices

Radial core/shell nanowires (NWs) represent an important class of one-dimensional (1D) systems with substantial potential for exploring fundamental materials electronic and photonic properties. Here, we report the rational design and synthesis of InAs/InP core/shell NW heterostructures with quantum-confined, high-mobility electron carriers. Transmission electron microscopy studies revealed single-crystal InAs cores with epitaxial InP shells 2-3 nm in thickness, and energy-dispersive X-ray spectroscopy analysis further confirmed the composition of the designed heterostructure. Room-temperature electrical measurements on InAs/InP NW field-effect transistors (NWFETs) showed significant improvement in the on-current and transconductance compared to InAs NWFETs fabricated in parallel, with a room-temperature electron mobility, 11,500 cm(2)/Vs, substantially higher than other synthesized 1D nanostructures. In addition, NWFET devices configured with integral high dielectric constant gate oxide and top-gate structure yielded scaled on-currents up to 3.2 mA/microm, which are larger than values reported for other n-channel FETs. The design and realization of high electron mobility InAs/InP NWs extends our toolbox of nanoscale building blocks and opens up opportunities for fundamental and applied studies of quantum coherent transport and high-speed, low-power nanoelectronic circuits.     

Elastic and Piezoelectric Properties of Zincblende and Wurtzite Crystalline Nanowire Heterostructures

The elastic and piezoelectric properties of zincblende and wurtzite crystalline InAs/InP nanowire heterostructures have been studied using electro‐elastically coupled continuum elasticity theory. A comprehensive comparison of strains, piezoelectric potentials and piezoelectric fields in the two crystal types of nanowire heterostructures is presented. For each crystal type, three different forms of heterostructures—core‐shell, axial superlattice, and quantum dot nanowire heterostructures—are considered. In the studied nanowire heterostructures, the principal strains are found to be insensitive to the change in the crystal structure. However, the shear strains in the zincblende and wurtzite nanowire heterostructures can be very different. All the studied nanowire heterostructures are found to exhibit a piezoelectric field along the nanowire axis. The piezoelectric field is in general much stronger in a wurtzite nanowire heterostructure than in its corresponding zincblende heterostructure. Our results are expected to be particularly important for analyzing and understanding the properties of epitaxially grown nanowire heterostructures and for applications in nanowire electronics, optoelectronics, and biochemical sensing.     

Self-assembling processes involved in the molecular beam epitaxy growth of stacked InAs/InP quantum wires

The growth mechanism of stacked InAs/InP(001) quantum wires (QWRs) is studied by combining an atomic-scale cross-sectional scanning tunnelling microscopy analysis with in situ and in real-time stress measurements along the [110] direction (sensitive to stress relaxation during QWR formation). QWRs in stacked layers grow by a non-Stranski-Krastanov (SK) process which involves the production of extra InAs by strain-enhanced As/P exchange and a strong strain driven mass transport. Despite the different growth mechanism of the QWR between the first and following layers of the stack, the QWRs maintain on average the same shape and composition in all the layers of the stack, revealing the high stability of this QWR configuration.     

Influence of Barrier Material on Spin Splitting Due to Inversion Asymmetry in Heterostructures

Influence of barrier material on the spin splitting of conduction subbands in heterostructures because of structure inversion asymmetry (Bychkov–Rashba splitting) is studied. The spin splitting at a vanishing magnetic field is calculated for two heterostructures: InAs/SiO₂ and InAs/In_0.8Al_0.2As, having the same well material InAs but very different barrier materials. It is demonstrated that the barrier material strongly influences the spin splitting of the ground conduction subband in InAs. The spin splittings for both heterostructures are computed as functions of electron density, we obtain the splitting in InAs/SiO₂ almost twice larger than that in InAs/In_0.8Al_0.2As. The influence of spin-dependent part of the boundary conditions on the spin spin splitting is studied and it is shown that for considered heterostructures it changes the splitting up to 25% of its value. It is emphasized that the Bychkov–Rashba spin splitting is not proportional to the average electric field in heterostructure.     

Mott–Schottky Effect in Core–Shell W@WxC Heterostructure: Boosting Both Electronic/Ionic Kinetics for Lithium Storage

The dynamics rate of traditional metal carbides (TMCs) is relatively slow, severely limiting its fast-charging capacity for lithium-ion batteries (LIBs). Herein, the core–shell W@W_xC heterostructure is developed to form Mott–Schottky heterostructure, thereby simultaneously accelerating the electronic and ionic transport kinetics during the charging/discharging process. The W nanoparticles are partially reduced into W_xC to form a particular core–shell structure with abundant heterogeneous interfaces. Benefiting from the Mott–Schottky effect, the electrons at the metal/semiconductor heterointerface can migrate spontaneously to realize an equal work function on both sides. In addition, the independent nanoparticle as well as the unique core–shell structure facilitate the ionic diffusion kinetics. As expected, the W@W_xC electrode exhibits excellent electrochemical stability for LIBs, whose capacity can be maintained at 173.8 mA h g⁻¹ after 1600 cycles at a high current density of 5 A g⁻¹. When assembled into a full cell, it can achieve an energy density of 360.2 Wh kg⁻¹. This work presents a new avenue to promote the electronic and ionic kinetics for LIBs anodes by constructing the unique Mott–Schottky heterostructure.     

In(Ga)As quantum dots on Ge substrate

The effects of growth temperature and deposition thickness on the formation, size, density, and uniformity of InAs and In_0.5Ga_0.5As islands grown by metalorganic vapor phase epitaxy on a germanium substrate are investigated. Atomic force microscopy images show InAs islands when 0.8–2.0 monolayers are deposited. At the nominal deposition thickness of 2.0 monolayers an island density of 2.5×10¹⁰ cm^–2 is achieved. InAs islands covered with a GaAs layer show low-temperature luminescence at around 1.15 eV. The In_0.5Ga_0.5As islands grown at 550 °C show a maximum density of 3.5×10¹⁰ cm^–2 at a nominal three monolayers deposition thickness.     

Thermal stability of the peak emission wavelength in multilayer InAs/GaAs QDs capped with a combination capping of InAlGaAs and GaAs

The effect of rapid thermal annealing (RTA) on the optical properties of a 10 layer stacked InAs/GaAs quantum dot (QD) heterostructure where the QDs are overgrown with a combination of quaternary InAlGaAs and GaAs capping have been investigated. TEM micrographs showed that the shape of the QDs is preserved for annealing temperatures up to 800 degrees C. The peak emission wavelength of the investigated heterostructures remains stable on annealing at temperatures upto 750 degrees C, which is unusual in QD samples. This phenomenon is attributed due to the suppression of the strain-enhanced intermixing in such structures. One of the reasons behind such suppression is the strain driven phase separation of Indium from the overgrown quaternary alloy, which maintains an In rich region across the QD periphery thereby checking the out-diffusion of Indium from the dots. The overlapping vertical strain from the under lying dot layers in the QD stack also maintains a strain relaxed state at the QD base, thereby preventing the material mixing at the base of the pyramidal QDs. This stability of wavelength is of paramount importance in optoelectronic devices where the design is based on the emission wavelength of the active region.