Dependence of Hooge parameter of InAs heterostructure on temperature

The InAs quantum well heterostructure was successfully grown on a semi-insulating GaAs substrate by MBE. On the GaAs substrate, the semi-insulating AlGaAsSb was grown to a thickness of 600 nm as the buffer layer, followed by a 15 nm InAs channel layer and a 35 nm AlGaAsSb doped layer together with 10 nm GaAsSb cap layer successively. The electron mobility and the sheet carrier concentration of the 15 nm InAs heterostructure was about 1 m² V⁻¹ s⁻¹ and 4.5×10¹² cm⁻², respectively, at room temperature. This heterostructure is equivalent to the heavily doped InAs substrate with little change of the electron mobility on temperature. This device has typical 1/f noise characteristics without any large bulge throughout the frequency and the temperature ranges observed. The Hooge parameter was α_H=1×10⁻³ at room temperature, decreasing monotonically with the decreasing temperature down to 5×10⁻⁴ at 50 K, indicating characteristics of the virtually constant mobility and constant carrier concentration device.     

Molecular beam epitaxy growth of InAs-AlSb-GaSb interband tunneling diodes

This paper presents our studies of the growth of InAs/GaAs and GaSb/GaAs heterojunctions by molecular beam epitaxy and their applications in fabricating the InAs-AlSb-GaSb interband tunneling devices. The Hall effect and x-ray diffraction were used to determine the optimum growth conditions for the layers. In addition, the qualities of the InAs and GaSb epilayers, grown under their optimum conditions, were compared. The full width at half maximum (FWHM) from the x-ray diffraction for a GaSb epilayer is about 50 arcsec narrower than an InAs epilayer of the same thickness. The narrower FWHM and excellent surface morphology of the GaSb layer have led us to grow the polytype heterostructure on a p⁺-GaAs substrate using a p⁺-GaSb as the buffer layer. The polytype tunneling structures grown on GaAs substrates under these conditions show good negative differential resistance properties. Five different interband tunneling structures are compared and discussed in terms of their peak-current densities and peak-to-valley current ratios.     

AlGaSb Buffer Layers for Sb-Based Transistors

InAs quantum wells can serve as the channel for high-electron-mobility transistors. Structures are typically grown on semi-insulating GaAs substrates with 1.5 μm to 3.0 μm buffer layers of AlSb and AlGaSb accommodating the lattice mismatch. We demonstrate that high electron mobility in the InAs (>20,000 cm²/V s at 300 K) and smooth surfaces can be achieved with Al_0.8Ga_0.2Sb buffer layers as thin as 600 nm, grown at rates of 1.5 monolayers/s to 2.0 monolayers/s. The use of thinner buffer layers reduces molecular beam epitaxial growth time and source consumption. The buffer layers also exhibit higher resistivity, which should reduce excess gate leakage current and improve device isolation.     

Intense Terahertz Radiation from InAs Thin Films

Terahertz (THz) radiation from InAs thin films grown by molecular-beam epitaxy on closely lattice-matched p-type GaSb (100) substrates and lattice-mismatched semi-insulating GaAs (100) substrates was investigated. The THz radiation intensity was measured from InAs films with thicknesses between 100 nm and 1.5 μm excited by a femtosecond laser pulse with a wavelength of approximately 780 nm. The radiation intensity increased as the InAs film thickness increased and it exceeded that from a bulk n-type InAs substrate with an electron concentration of 2.3 × 10¹⁶ cm⁻³ when the InAs film thickness was greater than about 500 nm. In addition, the THz intensity from a 1-μm-thick InAs film was greater than that from a bulk p-type InAs substrate. We ascribe this enhanced THz intensity to the wave reflected from the lower interface between the InAs film and the layer grown beneath it. We confirmed this by observing an increased pulse width due to constructive overlap of the reflected wave. The results demonstrate that InAs thin films are promising materials for THz emitting devices.     

Molecular beam epitaxial growth of gaas on gadolinium-gallium garnet

Single-crystal GaAs has been grown by molecular beam epitaxy on Gd₃Ga₅0₁₂ (GGG) using an InAs buffer layer and an InAs/GaAs multilayer structure between the GGG and the GaAs. The x-ray diffraction spectrum shows that both the InAs and GaAs epitaxial layers are oriented in the (111) direction when grown on a (100) GGG substrate. The unintentionally doped InAs layers aren-type and have donor concentrations in the range of 7 × 10¹⁶ to 2 × 10¹⁸ cm^-3 which vary inversely with growth temperature. The corresponding carrier mobilities vary from 3.5 × 10³ to 1 × 10³ cm²/V s. The GaAs was also found to be conducting. The 77-K photoluminescence (PL) spectrum of the GaAs grown on the GGG differs from that of homoepitaxial GaAs in that the heteroepitaxial GaAs PL intensity is approximately 50 times lower, its linewidth is five times broader, and its peak energy is blue shifted by 10 meV.     

Laser and electron beam scanning of GaAs FETS

The results of work to study the effects of laser and electron beam stimulation of GaAs FETs are reported. Studies were made using biassed and unbiassed devices and also using devices with the gate removed by etching. Several mechanisms can be effective but the major cause of the very large induced drain current sensitivities is the photovoltaic effect of the beam on the interface between the semi-insulating substrate and the channel and consequent channel depth changes. A reduction in drain current on illumination observed under negative bias conditions is attributed to the presence of a surface channel on the semi-insulating substrate. The two methods of stimulation produce essentially the same results despite the large differences in charge and input energy. The implications for reliability analysis are discussed.     

Molecular beam epitaxy of AlGaAs/Zn(Mn)Se hybrid nanostructures with InAs/AlGaAs quantum dots near the heterovalent interface

Features of the growth of InAs quantum dots in an Al_0.35Ga_0.65As matrix by molecular beam epitaxy at different substrate temperatures, deposition rates, and amounts of deposited InAs are studied. The optimum conditions for growing an array of low-density (≤2 × 10¹⁰ cm^?2) small (height of no more than 4 nm) self-organized quantum dots are determined. The possibility of the formation of optically active InAs quantum dots emitting in the energy range 1.3–1.4 eV at a distance of no more than 10 nm from the coherent heterovalent GaAs/ZnSe interface is demonstrated. It is established that inserting an optically inactive 5-nm GaAs quantum well resonantly coupled with InAs quantum dots into the upper AlGaAs barrier layer enhances the photoluminescence efficiency of the quantum-dot array in hybrid heterostructures.     

Low frequency noise physical analysis for the improvement of the spectral purity of GaAs FETs oscillators

Low frequency (L.F.) noise in GaAs FETs was investigated both theoretically and experimentally. The main contribution to the overall noise at frequencies over 10³ Hz was found to be flicker noise generated in the gradual region of the channel.A new simple relationship is proposed to derive the noise voltage intensity referred back to the input at normal operating conditions: it is reported that this noise spectral intensity does not depend on bias voltages for micrometer or submicrometer devices. This relationship provides a fast and easy way for assessing devices for their L.F. noise: an improvement in the spectral purity of GaAs FETs oscillators designed with low L.F. noise FETs is reported.     

Comparison of 80-200 nm gate lengthAl0.25GaAs/GaAs/(GaAs:AlAs), Al0.3GaAs/In0.15GaAs/GaAs, and In0.52AlAs/In0.65GaAs/InPHEMTs

In this paper we present a comparative study of the high frequency performance of 80-200 mm gate length Al_0.25GaAs/GaAs/(GaAs:AlAs) superlattice buffer quantum well (QW) HEMTs, Al_0.3GaAs/In_0.15GaAs/GaAs pseudomorphic HEMTs and In_0.52AlAs/In_0.65GaAs/InP pseudomorphic HEMTs. From an experimental determination of the delays associated with transiting both the intrinsic and parasitic regions of the devices, effective electron velocities in the intrinsic channel region under the gate of the HEMT's were extracted. This analysis showed no evidence of any systematic increase in the effective channel velocity with reducing gate length in any of the devices. The effective electron velocity in the channel of the pseudomorphic In_0.65GaAs/InP HEMTs, determined to be at least 2.5×10⁵, was was around twice that of either the Al_0.25GaAs/GaAs quantum well or pseudomorphic In_0.15GaAs/GaAs HEMTs, resulting in 80 nm gate length devices with f_T's of up to 275 GHz. We also show that device output conductance is strongly material dependent. A comparison of the different buffer layers showed that the (GaAs:AlAs) superlattice buffer was most effective in confining electrons to the channel of the Al_0.25GaAs/GaAs HEMTs, even for 80 nm gate length devices. We propose this may be partly due to the presence of minigaps in the superlattice which provide a barrier to electrons with energies of up to 0.6 eV. The output conductance of pseudomorphic In_0.65GaAs/InP HEMTs was found to be inferior to the GaAs based devices as carriers in the channel have greater energy due to their higher effective velocity and so are more difficult to confine to the 2DEG     

A simple model for determining effects of negative differential mobility and magnitude of saturated velocity on the performance of Schottky-barrier field effect transistors

The effects of nonuniform fields in the channel of GaAs MESFETs, and the resulting range of velocities with which carriers drift, are here considered using a simple model in order to determine the effects of negative differential mobility and saturated velocity on the performance of these devices, and of devices made of other materials. On the basis of the equilibrated velocity-field ν(E) characteristic for GaAs, carriers in short-channel FETs drift over much of the source to drain length with the saturated high field velocity, i.e. a velocity about half that used in most previous analytical solutions for GaAs FETs, and one about equal to the maximum carrier velocity in Si. Neglecting transient effects in electron transport, fields at which a negative differential mobility occurs exist over only a very small portion of the channel. The value of low field mobility of the MESFET material is therefore important primarily in determining source resistance.     

High-breakdown-voltage AlSbAs/InAs n-channel field-effecttransistors

InAs channel field-effect transistors of 1-μm gate length were grown by molecular beam epitaxy and observed to operate at channel electric fields (20 kV/cm) higher than previously demonstrated and several times greater than the threshold for impact ionization in bulk InAs. Voltage gains on the order of 10 were observed with transconductances as high as 414 mS/mm and output conductances as low as 33 mS/mm. These voltage gains are comparable to those of GaAs-based devices and are the highest observed for InAs channel devices. The results demonstrate the potential for practical room-temperature operation of InAs FETs     

Printed and Electrochemically Gated,High‐Mobility,Inorganic Oxide Nanoparticle FETs and Their Suitability for High‐Frequency Applications

Solution‐processed or printed n‐channel field‐effect transistors (FETs) with high performance are not reported very often in the literature due to the scarcity of high‐mobility n‐type organic semiconductors. On the other hand, low‐temperature processed n‐channel metal oxide semiconductor (NMOS) transistors from electron conducting inorganic‐oxide nanoparticles show reduced‐performance and low mobility because of large channel roughness at the channel‐dielectric interface. Here, a method to produce ink‐jet printed high performance NMOS transistor devices using inorganic‐oxide nanoparticles as the transistor channel in combination with a 3D electrochemical gating (EG) via printed composite solid polymer electrolytes is presented. The printed FETs produced show a device mobility value in excess of 5 cm² V^?1 s^?1, even though the root mean square (RMS) roughness of the nanoparticulate channel exceeds 15 nm. Extensive studies on the frequency dependent polarizability of composite polymer electrolyte capacitors show that the maximum attainable speed in such printed, long channel transistors is not limited by the ionic conductivity of the electrolytes. Therefore, the approach of combining printable, high‐quality oxide nanoparticles and the composite solid polymer electrolytes, offers the possibility to fully utilize the large mobility of oxide semiconductors to build all‐printed and high‐speed devices. The high polarizability of printable polymer electrolytes brings down the drive voltages to ≤1 V, making such FETs well‐suited for low‐power, battery compatible circuitry.     

Growth of InAs-AlSb quantum wells having both high mobilities and high concentrations

Low-temperature mobilities in InAs-AlSb quantum wells depend sensitively on the buffer layer structures. Reflection high energy electron diffraction and x-ray diffraction show that the highest crystalline quality and best InAs transport properties are obtained by a buffer layer sequence GaAs → AlAs → AlSb → GaSb, with a final GaSb layer thickness of at least 1 μm. Using the improved buffer scheme, mobilities exceeding 600,000 cm²/Vs at 10 K are routinely obtained. Modulation δ-doping with tellurium has yielded electron sheet concentrations up to 8 × 10¹² cm⁻² while maintaining mobilities approaching 100,000 cm²/Vs at low temperatures.     

Coupling of electron states in the InAs/GaAs quantum dot molecule

Deep level transient spectroscopy (DLTS) is used to study electron emission from the states in the system of vertically correlated InAs quantum dots in the p-n InAs/GaAs heterostructures, in relation to the thickness of the GaAs spacer between the two layers of InAs quantum dots and to the reverse-bias voltage. It is established that, with the 100 Å GaAs spacer, the InAs/GaAs heterostructure manifests itself as a system of uncoupled quantum dots. The DLTS spectra of such structures exhibit two peaks that are defined by the ground state and the excited state of an individual quantum dot, with energy levels slightly shifted (by 1–2 eV), due to the Stark effect. For the InAs/GaAs heterostructure with two layers of InAs quantum dots separated by the 40 Å GaAs spacer, it is found that the quantum dots are in the molecule-type phase. Hybridization of the electron states of two closely located quantum dots results in the splitting of the levels into bonding and antibonding levels corresponding to the electron ground states and excited states of the 1s ⁺, 1s ^?, 2p ⁺, 2p ^?, and 3d ⁺ types. These states manifest themselves as five peaks in the DLTS spectra. For these quantum states, a large Stark shift of energy levels (10–40 meV) and crossing of the dependences of the energy on the electric field are observed. The structures with vertically correlated quantum dots are grown by molecular beam epitaxy, with self-assembling effects.     

Nanoheterostructures with improved parameters for high-speed and efficient plasmon-polariton light emitting Schottky diodes

As a result of theoretical and experimental analyses, the parameters of heterostructures with InAs quantum dots in a GaAs matrix are determined, which provide the development of high-speed and efficient plasmon-polariton near-infrared light-emitting Schottky diodes based on such structures. The quantum dots should be arranged on a heavily doped (to a dopant concentration of 10¹⁹ cm^–3) GaAs buffer layer and be separated from the metal by a thin (10–30 nm thick) undoped GaAs cap layer. The interface between the metal (e.g., gold) and GaAs provides the efficient scattering of surface plasmon-polaritons to ordinary photons if it contains inhomogeneities shaped as metal-filled cavities with a characteristic size of ~30 nm and a surface concentration above 10¹⁰ cm^–2.     

Molecular beam epitaxial growth of site-controlled InAs quantum dots on pre-patterned GaAs substrates

Ex situ electron-beam lithography followed by conventional wet etching has been used to pattern small holes 60–150 nm wide, 13 nm deep in GaAs substrates. These holes act as preferential nucleation sites for InAs dot growth during subsequent overgrowth. By varying either the InAs deposition amount or the thickness of a GaAs buffer layer, the occupancy over the patterned sites can be controlled. Comparison with growth on a planar substrate shows that preferential nucleation occurs due to a reduction in the apparent critical thickness above the pattern site; the magnitude of this reduction is dependent on the dimensions of the initial pattern.     

Photoreflectance study of electric field distributions in semiconductors heterostructures grown on semi-insulating substrates

We have studied the photoref lectance (PR) spectra from a MBE grown heterostructure consisting of 200 nm of Ga_0.83Al_0.17As, a 800 nm GaAs buffer layer on a semi-insulating (100) LEC GaAs substrate. By varying both the pump beam wavelength and modulation frequency (up to 100 kHz) we are able to identify the component layers, their quality and the properties of the various interfaces. In this study we find evidence for a low density of interface states between the GaAs buffer layer and GaAlAs layer and a relatively large density of interface states between the substrate and buffer regions. These states, previously observed by Deep Level Transient Spectroscopy of doped structures, are presumably associated with the interface produced by MBE growth on etched and air exposed substrates. However, in our material, since the substrate is semi-insulating and the buffer layer is undoped, it is difficult to resolve these states spatially by C-V techniques. Our results show that the PR technique can be used to characterize low conductivity or semi-insulating structures such as enhancement mode MESFET and HEMT type devices and it may be useful for the in-situ characterization of epigrown surfaces and interfaces     

Arsenic cluster superlattice in gallium arsenide grown by low-temperature molecular-beam epitaxy

Molecular-beam epitaxy at 200 °C is used to grow an InAs/GaAs superlattice containing 30 InAs delta-layers with a nominal thickness of 1 monolayer, separated by GaAs layers of thickness 30 nm. It is found that the excess arsenic concentration in such a superlattice is 0.9×10²⁰ cm⁻³. Annealing the samples at 500 and 600 °C for 15 min leads to precipitation of the excess arsenic mainly into the InAs delta-layers. As a result, a superlattice of two-dimensional sheets of nanoscale arsenic clusters, which coincides with the superlattice of the InAs delta-layers in the GaAs matrix, is obtained. Fiz. Tekh. Poluprovodn. 32, 1161–1164 (October 1998)     

Reduction of backgating effect in MBE-Grown GaAs/AlGaAs HEMT's

We have investigated the backgating effect in high electron mobility transistors (HEMT's) fabricated on MBE-grown GaAs/AlGaAs layers, which is undesirable for LSI fabrication. Comparing five different types of devices, we related the backgating effect to the interface between the GaAs substrate and the undoped GaAs buffer layer. By using a thermally etched GaAs substrate, we successfully reduced the backgating to the same order as that of ion-implanted GaAs MESFET's.     

22 nm In0.75Ga0.25As channel-based HEMTs on InP/GaAs substrates for future THz applications

In this work, the performance of L_g=22 nm In_0.75Ga_0.25As channel-based high electron mobility transistor (HEMT) on InP substrate is compared with metamorphic high electron mobility transistor (MHEMT) on GaAs substrate. The devices features heavily doped In_0.6Ga_0.4As source/drain (S/D) regions, Si double δ-doping planar sheets on either side of the In_0.75Ga_0.25As channel layer to enhance the transconductance, and buried Pt metal gate technology for reducing short channel effects. The TCAD simulation results show that the InP HEMT performance is superior to GaAs MHEMT in terms of f_T, f_max and transconductance (g_{m_max}). The 22 nm InP HEMT shows an f_T of 733 GHz and an f_max of 1340 GHz where as in GaAs MHEMT it is 644 GHz and 924 GHz, respectively. InGaAs channel-based HEMTs on InP/GaAs substrates are suitable for future sub-millimeter and millimeter wave applications.