Hall and photoluminescence studies of effects of the thickness of an additional In0.3Ga0.7As layer in the center of In0.15Ga0.85As/Al0.25Ga0.75As/GaAs high electron mobility transistors

In order to reduce the noise and carrier–donor scattering and thereby increase the carrier mobility of the pseudomorphic AlGaAs/InGaAs high electron mobility transistors (pHEMTs), we have grown Al_0.25Ga_0.75As/In_0.15Ga_0.85As/In_0.3Ga_0.7As/GaAs pHEMTs with varied In_0.3Ga_0.7As thickness, and studied the effects of the In_0.3Ga_0.7As thickness on the electron mobility and sheet density by Hall measurements and photoluminescence measurements. We calculated the electron and hole subbands and obtained good agreement between calculated and measured PL energies. It was found that the additional In_0.3Ga_0.7As layer could be used to reduce the carrier–donor scattering, but due to the increased interface roughness as the In_0.3Ga_0.7As layer becomes thicker, the interface scattering reduced the electron mobility. An optimal thickness of the In_0.3Ga_0.7As was found to be 2 nm.     

Super-flat interfaces in In0.53Ga0.47As/In0.52Al0.48As quantum wells grown on (411)A InP substrates by molecular beam epitaxy

Effectively atomically flat interfaces over a macroscopic area (“(411)A super-flat interfaces”) were successfully achieved in In_0.53Ga_0.47As/In_0.52Al_0.48As quantum wells (QWs) grown on (411)A InP substrates by molecular beam epitaxy (MBE) at a substrate temperature of 570°C and V/III=6. Surface morphology of the In_0.53Ga_0.47As/In_0.52Al_0.48As QWs was smooth and featureless, while a rough surface of those simultaneously grown on a (100) InP substrate was observed. Photoluminescence (PL) linewidths at 4.2 K from the (411)A QWs with well width of 0.6–12 nm were 20–30 % narrower than those grown on a (100) InP substrate and also they are almost as narrow as each of split PL peaks for those of growth-interrupted QWs on a (100) InP substrate. In the case of the (411)A QWs, only one PL peak with very narrow linewidth was observed from each QW over a large distance (7 mm) on a wafer.     

Photoluminescence studies of In0.7Al0.3As/In0.75Ga0.25As/In0.7Al0.3As metamorphic heterostructures on GaAs substrates

The influence of the design of the metamorphic buffer of In_0.7Al_0.3As/In_0.75Ga_0.25As metamorphic nanoheterostructures for high-electron-mobility transistors (HEMTs) on their electrical parameters and photoluminescence properties is studied experimentally. The heterostructures are grown by molecular-beam epitaxy on GaAs (100) substrates with linear or step-graded In _x Al_{1 ? x} As metamorphic buffers. For the samples with a linear metamorphic buffer, strain-compensated superlattices or inverse steps are incorporated into the buffer. At photon energies ?ω in the range 0.6–0.8 eV, the photoluminescence spectra of all of the samples are identical and correspond to transitions from the first and second electron subbands to the heavy-hole band in the In_0.75Ga_0.25As/In_0.7Al_0.3As quantum well. It is found that the full width at half-maximum of the corresponding peak is proportional to the two-dimensional electron concentration and the luminescence intensity increases with increasing Hall mobility in the heterostructures. At photon energies ?ω in the range 0.8–1.3 eV corresponding to the recombination of charge carriers in the InAlAs barrier region, some features are observed in the photoluminescence spectra. These features are due to the difference between the indium profiles in the smoothing and lower barrier layers of the samples. In turn, the difference arises from the different designs of the metamorphic buffer.     

High electron mobility 18300 cm2/V·s in the InAIAs/lnGaAs pseudomorphic structure obtained by channel indium composition modulation

The highest electron mobility yet reported for an InP-based pseudomorphic structure at room temperature, 18300 cm²/V·s, has been obtained by using a structure with an indium composition modulated channel, namely, In_0.53Ga_0.47As/ In_0.8Ga_0.2As/InAs/In_0.8Ga_0.2As/In_0.53Ga_0.47As. Although the total thickness of the high In-content layers (In_0.8Ga_0.2As/InAs/In_0.8Ga_0.2As) exceeds the critical thick-ness predicted by Matthews theory, In_0.8Ga_0.2As insertion makes it possible to form smooth In_0.53Ga_0.47As/In_0.8Ga_0.2As and In_0.8Ga_0.2As/InAs heterointerfaces. This structure can successfully enhance carrier confinement in the high In-content layers. This superior carrier confinement can be expected to lead to the highest yet reported electron mobility.     

Highly strained ln0.35Ga0.65As/GaAs layers grown by molecular beam epitaxy for high hole mobility transistors

We have grown highly strained In_0.35Ga_0.65As layers on GaAs substrates by molecular beam epitaxy to improve the performance of high hole mobility transistors (HHMTs). The mobility and sheet hole concentration of double side doped pseudomorphic HHMT structures at room temperature reached 314 cm2/V-s and 1.19 × 10¹² cm⁻², respectively. Photoluminescence measurements at room temperature show good crystalline quality of the In^0.35Ga_0.65As layers. This study suggests that the performance of HHMTs can be improved by using high-quality In_0.35Ga_0.65As layers for the channel of double side doped heterostructures pseudomorphically grown on GaAs substrates.     

In0.48Ga0.52P/In0.20Ga0.80As/GaAs pseudomorphic high electron mobility transistors grown by solid-source molecular-beam epitaxy using a valved phosphorus cracker cell

In_0.48Ga_0.52P/In_0.20Ga_0.80As/GaAs pseudomorphic high electron mobility transistor (p-HEMT) structures were grown by solid-source molecular beam epitaxy (SSMBE) using a valved phosphorus cracker cell. The sheet carrier density at room temperature was 3.3 × 10¹²cm^?2. A peak transconductance (G _m) of 267 mS mm^?1 and peak drain current density (I _ds) of 360 mA mm^?1 were measured for a p-HEMT device with 1.25 µm gate length. A high gate-drain breakdown voltage (BV _gd) of 33V was measured. This value is more than doubled compared with that of a conventional Al_0.30Ga_0.70As/In_0.20Ga_0.80As/GaAs device. The drain-source breakdown voltage (BV _ds) was 12.5V. Devices with a mushroom gate of 0.25 µm gate length and 80 µm gate width achieved a peak transconductance (G _m) of 420 mS mm^?1 and drain current density of nearly 500mA mm^?1. A high cutoff frequency (f _T) of 58GHz and maximum oscillation frequency (f _max) of 120 GHz were obtained. The results showed that the In_0.48Ga_0.52P/In_0.20Ga_0.80As/GaAs material system grown by SSMBE using the valved phosphorus cracker cell for the In^0.48Ga^0.52P Schottky and spacer layers is a viable technology for high frequency p-HEMT device applications.     

Halogen lamp rapid thermal annealing of Si- and Be-implanted In0.53Ga0.47As

Halogen lamp rapid thermal annealing was used to activate 100 keV Si and 50 keV Be implants in In_0.53Ga_0.47As for doses ranging between 5 × 10¹²−4 × 10¹⁴ cm⁻². Anneals were performed at different temperatures and time durations. Close to one hundred percent activation was obtained for the 4.1 × 10¹³ cm⁻² Si-implant, using an 850° C/5 s anneal. Si in-diffusion was not observed for the rapid thermal annealing temperatures and times used in this study. For the 5 × 10¹³ cm⁻² Be-implant, a maximum activation of 56% was measured. Be-implant depth profiles matched closely with gaussian profiles predicted by LSS theory for the 800° C/5 s anneals. Peak carrier concentrations of 1.7 × 10¹⁹ and 4 × 10¹⁸ cm⁻³ were achieved for the 4 × 10¹⁴ cm⁻² Si and Be implants, respectively. For comparison, furnace anneals were also performed for all doses.     

Effect of GaAs (100) substrate misorientation on the electrical parameters and surface morphology of metamorphic In0.7Al0.3As/In0.75Ga0.25As/In0.7Al0.3As HEMT nanoheterostructures

The results of studies of the effect of GaAs (100) substrate misorientation on the electrical parameters and surface morphology of high electron mobility In_0.7Al_0.3As/In_0.75Ga_0.25As/In_0.7Al_0.3As/GaAs nanoheterostructures are reported. Using molecular-beam epitaxy, two identical structures with a stepped compositional profile of the metamorphic In _x Al_{1 ? x} As (Δ _x = 0.05) buffer are grown on substrates of two types: a singular GaAs substrate with the orientation (100) ± 0.5° and a GaAs (100) substrate misoriented by (2 ± 0.5)° in the $\left[ {0\bar 1\bar 1} \right]$ direction. It is found that, in the case of the misoriented substrate, the concentration of the two-dimensional electron gas is ～40% higher. Broadening of the photoluminescence spectra and a shift of the peaks to lower photon energies, as experimentally observed in the case of the misoriented substrate, are attributed to the increased roughness of the heterointerfaces and strengthened fluctuations of the quantum-well width.     

Persistent photoconductivity and electron mobility in In0.52Al0.48As/In0.53Ga0.47As/In0.52Al0.48As/InP quantum-well structures

The influence of the width of the quantum well L and doping on the band structure, scattering, and electron mobility in nanoheterostructures with an isomorphic In_0.52Al_0.48As/In_0.53Ga_0.47As/In_0.52Al_0.48As quantum well grown on an InP substrate are investigated. The quantum and transport mobilities of electrons in the dimensionally quantized subbands are determined using Shubnikov-de Haas effect measurements. These mobilities are also calculated for the case of ionized-impurity scattering taking into account intersub-band electron transitions. It is shown that ionized-impurity scattering is the dominant mechanism of electron scattering. At temperatures T < 170 K, persistent photoconductivity is observed, which is explained by the spatial separation of photoexcited charge carriers.     

InAlSb/InAs/AlGaSb Quantum Well Heterostructures for High-Electron-Mobility Transistors

Heterostructures for InAs-channel high-electron-mobility transistors (HEMTs) were investigated. Reactive AlSb buffer and barrier layers were replaced by more stable Al_0.7Ga_0.3Sb and In_0.2Al_0.8Sb alloys. The distance between the gate and the channel was reduced to 7–13 nm to allow good aspect ratios for very short gate lengths. In addition, n⁺-InAs caps were successfully deposited on the In_0.2Al_0.8Sb upper barrier allowing for low sheet resistance with relatively low sheet carrier density in the channel. These advances are expected to result in InAs-channel HEMTs with enhanced microwave performance and better reliability.     

OMVPE growth of In0.53Ga0.47As on InP using tertiarybutylarsine

We have successfully grown bulk In_0.53Ga_0.47As on InP using tertiarybutylarsine (TBA), trimethylindium and trimethylgallium. The growth temperature was 602° and the V/III ratio ranged from 19 to 38. Net carrier concentrations were 2 – 4 × 10¹⁵ cm^-3, n-type, with a peak 77 K mobility of 68,000 cm²/V. sec. Increasing compensation was observed in In_0.53Ga_0.47As grown at higher V/III ratios. PL spectra taken at 5 K revealed strong near bandgap emission at 0.81 eV—with the best sample having a FWHM of 2.5 meV. At lower energies, donor-acceptor pair transitions were evident. Strong and sharp 5 K PL emission was observed from InP/In_0.53Ga_0.47As/InP quantum wells grown with TBA.     

The behavior of unintentional impurities in Ga0.47 In0.53As grown by MBE

A number of factors contribute to the high n-type background carrier concentration (high 10¹⁵ to low 10¹⁶ cm⁻³) measured in MBE Ga_0.47In_0.53As lattice-matched to InP. The results of this study indicate that the outdiffusion of impurities from InP substrates into GalnAs epitaxial layers can account for as much as two-thirds of the background carrier concentration and can reduce mobilities by as much as 40%. These impurities and/or defects can be gettered at the surfaces of the InP by heat treatment and then removed by polishing. The GalnAs epitaxial layers grown on the heat-treated substrates have significantly improved electrical properties. Hall and SIMS measurements indicate that both donors and acceptors outdiffuse into the epitaxial layers during growth resulting in heavily compensated layers with reduced mobilities. The dominant donor species was identified by SIMS as Si, and the dominant acceptors as Fe, Cr and Mn.     

Thermal characteristics of InP, InAlAs, and AlGaAsSb metamorphic buffer layers used in In0.52Al0.48As/In0.53Ga0.47As heterojunction bipolar transistors grown on GaAs substrates

In_0.52Al_0.48As/In_0.53Ga_0.47As heterojunction bipolar transistors (HBTs) were grown metamorphically on GaAs substrates by molecular beam epitaxy. In these growths, InAlAs, AlGaAsSb, and InP metamorphic buffer layers were investigated. The InAlAs and AlGaAsSb buffer layers had linear compositional grading while the InP buffer layer used direct binary deposition. The transistors grown on these three layers showed similar characteristics. Bulk thermal conductivities of 10.5, 8.4, and 16.1 W/m K were measured for the InAlAs, AlGaAsSb, and InP buffer layers, as compared to the 69 W/m K bulk thermal conductivity of bulk InP. Calculations of the resulting HBT junction temperature strongly suggest that InP metamorphic buffer layers should be employed for metamorphic HBTs operating at high power densities.     

Electrical characterization of self-assembled In0.5Ga0.5As/GaAs quantum dots by deep level transient spectroscopy

We have investigated electron emission from self-assembled In_0.5Ga_0.5As/GaAs quantum dots (QDs) grown by molecular-beam epitaxy (MBE). Through detailed deep level transient spectroscopy comparisons between the QD sample and a reference sample, we determine that trap D, with an activation energy of 100 meV and an apparent capture cross section of 5.4×10⁻¹⁸ cm², is associated with an electron quantum level in the In_0.5Ga_0.5As/GaAs QDs. The other deep levels observed, M1, M3, M4, and M6, are common to GaAs grown by MBE.     

MHEMT with a power-gain cut-off frequency of f max = 0.63 THz on the basis of a In0.42Al0.58As/In0.42Ga0.58As/In0.42Al0.58As/GaAs nanoheterostructure

The method of molecular-beam epitaxy is used to grow a In_0.42Al_0.58As/In_0.42Ga_0.58As/In_0.42Al_0.58As nanoheterostructure with a step-graded metamorphic buffer on a GaAs substrate. The root-mean-square value of the surface roughness is 3.1 nm. A MHEMT (metamorphic high-electron-mobility transistor) with a zigzag-like gate of a length of 46 nm is fabricated on the basis of this nanoheterostructure; for this MHEMT, the cutoff frequencies for the current and power gain are f _T = 0.13 THz and f _max = 0.63 THz, respectively.     

Dislocation-Induced deep level states in In0.08Ga0.92As/GaAs heterostructures

We have performed luminescence experiments on In_0.08Ga_0.92As/GaAs heterointerfaces to explore the energy distribution of deep level states in the bandgap for two cases: (1) unrelaxed, pseudomorphic In_0.08Ga_0.92As films (200Å thick), which have few if any dislocations at the interface, and (2) partially relaxed In_0.08Ga_0.92As films (1000Å thick) which are expected to have a substantial interfacial dislocation density. A combined photoluminescence and cathodoluminescence technique is used which allows us to profile the sample luminescence through the buried interface region. Our results show the existence of deep level luminescent features characteristic of the GaAs substrate and features common to In_0.08Ga_0.92As and GaAs, as well as the existence of a deep level feature near 1 eV photon energy which undergoes a shift in energy depending upon the degree of strain relaxation in the In_0.08Ga_0.92As film. In addition, a deep level feature near 0.83 eV becomes prominent only in In_0.08Ga_0.92As films which have relaxed, and thus contain misfit dislocations at the interface. These deep level differences may be due to bandgap states associated with the intrinsic dislocation structure, impurities segregated at the dislocation, or bulk point defects, or threading dislocations generated during the strain relaxation. Previous work has determined that a deep level state 0.7 eV above the valence band edge would account for the electrical behavior of relaxed In_0.08Ga_0.92As/GaAs interfaces, which is in good agreement with the range of deep level transitions near 0.8 eV photon energy which we observe. These measurements suggest that photo- and cathodoluminescence measurements of deep level emission in these III-V semiconductors can provide a useful indicator of electrically active defect densities associated with misfit dislocations.     

Highly confined two-dimensional electron gas in an In0.52AI0.48As/In0.52Ga0.47As modulation-doped structure with a strained InAs quantum well

This paper examines a detailed analysis by Shubnikov-de Haas measurements of the effective mass of two-dimensinal electron gas (2DEG) in an In_0.52Al_0.48As/ In_0.53Ga_0.47As modulation-doped (MD) structure with an InAs quantum well inserted into the InGaAs channel (InAs-inserted channel). The measured effec-tive mass of 2DEG in the InAs-inserted-channel MD structure is in good agreement with the calculated one of the strained InAs layer on In_0.53Ga_0.47As. This indicates that almost all of the 2DEG forms in the strained InAs quantum well. These results show that the InAs-inserted-channel MD structure improves the electron confinement, since the 2DEG is confined in the InAs quantum well with the thickness of 4 nm.     

High Performance (fT~500GHZ) In0.52Al0.48As/In0.53Ga0.47As/InP Quantum Wire MODFETs Employing Asymmetric Coupled-Well Channels

A coupled-well InAlAs/InGaAs quantum wire MODFET structure is proposed, for which simulations predict improved frequency performance (>500 GHz), over a wider range of V_g, as compared to well/wire devices with a standard MODFET heterointerface. A comparison of several transverse potential well profiles, obtained by varying the placement of a thin barrier within a 100 Å finite well, is presented. In all cases, the quantum wires consist of a 0.1 μm long channel and a 150 Å finite-square-well lateral profile. It has been found that the peak of the electron distribution for the first confined state, as measured from the heterointerface, changes dramatically depending on the location of the thin barrier. For quantum wire structures, realized in the lattice matched system of In_0.52Al_0.48As/In_0.53Ga_0.47As/InP, a change in the barrier location of 25 Å is accompanied by a shift in the carrier peak of more than 40 Å (~20 Å closer to or farther from the spacer-well interface than in the standard MODFET profile). Implications of this are reflected in the current-voltage characteristics (I_d-V_d) and frequency responses (f_T-V_g) of the proposed structures.     

Interfacial Chemical Bond and Oxygen Vacancy-Enhanced In2O3/CdSe-DETA S-scheme Heterojunction for Photocatalytic CO2 Conversion

The S-scheme heterojunctions have great potential for photocatalytic carbon dioxide reduction due to their unique carrier migration pathways, superior carrier separation efficiencies, and high redox capacities. However, the precise process of the oriented powerful electron transport remains a great challenge. Herein, an In O Cd bond-modulated S-scheme heterojunction of In₂O₃/CdSe-DETA is synthesized by a simple microwave-assisted hydrothermal method for the accelerated photogenerated electron transfer. Meanwhile, the oxygen vacancies (Vo) of In₂O₃ have an electron capture effect. Consequently, thanks to the synergistic effect of this In-Vo-In-O-Cd structural units at the interface, electrons are extracted and rapidly transferred to the surface-active sites, which improves the electronic coupling of CO₂. This finding precisely adjusts the electron transfer pathway and shortens the electron transfer distance. The synergistic effect of this chemical bond established in the S-scheme heterostructure with oxygen vacancies in In₂O₃ (Vo-In₂O₃) provides new insights into photocatalytic CO₂ reduction.     

Application of photoluminescence spectroscopy to studies of In0.38Al0.62As/In0.38Ga0.62As/GaAs metamorphic nanoheterostructures

The results of studies of the surface morphology, electrical parameters, and photoluminescence properties of In_0.38Al_0.62As/In_0.38Ga_0.62As/In_0.38Al_0.62As metamorphic nanoheterostructures on GaAs substrates are reported. Some micron-sized defects oriented along the [011] and \([0\bar 11]\) directions and corresponding to regions of outcropping of stacking faults are detected on the surface of some heterostructures. The Hall mobility and optical properties of the samples correlate with the surface defect density. In the photoluminescence spectra, four emission bands corresponding to the recombination of charge carriers in the InGaAs quantum well (1–1.2 eV), the InAlAs metamorphic buffer (1.8–1.9 eV), the GaAs/AlGaAs superlattice at the buffer-substrate interface, and the GaAs substrate are detected. On the basis of experimentally recorded spectra and self-consistent calculations of the band diagram of the structures, the compositions of the alloy constituents of the heterostructures are established and the technological variations in the compositions in the series of samples are determined.