Pseudomorphic Heterostructures Research Articles

Lattice Strain Relaxation and Compositional Control in As-Rich GaAsP/(100)GaAs Heterostructures Grown by MOVPE.

The fabrication of high-efficiency GaAsP-based solar cells on GaAs wafers requires addressing structural issues arising from the materials lattice mismatch. We report on tensile strain relaxation and composition control of MOVPE-grown As-rich GaAs1-xPx/(100)GaAs heterostructures studied by double-crystal X-ray diffraction and field emission scanning electron microscopy. Thin (80-150 nm) GaAs1-xPx epilayers appear partially relaxed (within 1-12% of the initial misfit) through a network of misfit dislocations along the sample [011] and [011-] in plane directions. Values of the residual lattice strain as a function of epilayer thickness were compared with predictions from the equilibrium (Matthews-Blakeslee) and energy balance models. It is shown that the epilayers relax at a slower rate than expected based on the equilibrium model, an effect ascribed to the existence of an energy barrier to the nucleation of new dislocations. The study of GaAs1-xPx composition as a function of the V-group precursors ratio in the vapor during growth allowed for the determination of the As/P anion segregation coefficient. The latter agrees with values reported in the literature for P-rich alloys grown using the same precursor combination. P-incorporation into nearly pseudomorphic heterostructures turns out to be kinetically activated, with an activation energy EA = 1.41 ± 0.04 eV over the entire alloy compositional range.

High figure of merit extreme bandgap Al0.87Ga0.13N-Al0.64Ga0.36N heterostructures over bulk AlN substrates

We report on high-quality n-Al0.87Ga0.13N-A0.64Ga0.36N heterostructures over single crystal AlN. For these pseudomorphic heterostructures, high-resolution X-ray and X-ray Topographic analysis was used to establish a threading dislocation density of 7 × 103 cm−2. Using reverse composition graded n+-Al x Ga1-x N contact layers, we obtained linear ohmic contacts with 4.3 Ω mm specific resistance. A critical breakdown field >11 MV cm−1 was also measured. In combination with the channel resistance of 2400 Ω sq−1, these translate to a Baliga’s Figure of Merit of 2.27 GW cm−2. This, to the best of our knowledge is the highest reported value for extreme bandgap AlGaN heterostructures.

Всплеск дрейфовой скорости электронов в обращенных транзисторных гетероструктурах с донорно-акцепторным легированием и дополнительными цифровыми потенциальными барьерами

The first results of the electrons drift velocity study in inverted AlGaAs/InGaAs/GaAs pseudomorphic heterostructures with donor-acceptor doping and short-period AlAs/GaAs superlattices are presented. It is theoretically shown that the introduction of superlattices significantly, up to one and a half times, increases the electrons drift velocity overshot when they enter the region of a strong field. Localized states in the superlattice between the quantum well and the substrate have been found. It is shown that this effect leads to an additional increase in the electrons drift velocity overshot up to the theoretical limit for the model used, i.e., a drift velocity overshot in the bulk material of the quantum well.

The electrons drift velocity overshot in inverted transistor heterostructures with donor-acceptor doping and additional digital potential barriers

The first results of the electrons drift velocity study in inverted AlGaAs/InGaAs/GaAs pseudomorphic heterostructures with donor-acceptor doping and short-period AlAs/GaAs superlattices are presented. It is theoretically shown that the introduction of superlattices significantly, up to one and a half times, increases the electrons drift velocity overshot when they enter the region of a strong field. Localized states in the superlattice between the quantum well and the substrate have been found. It is shown that this effect leads to an additional increase in the electrons drift velocity overshot up to the theoretical limit for the model used, i. e., a drift velocity overshot in the bulk material of the quantum well. Keywords: inverted heterostructure, digital barriers, field-effect transistor, gain.

Effect of substrate misorientation on the properties of p-HEMT GaAs-based nanoheterostructures formed during MOCVD epitaxy

As one of the approaches to improve p-HEMT, we studied the effect of misorientation of GaAs substrates on the surface morphology, structure, and electrical properties of pseudomorphic heterostructures, as well as the parameters of transistors based on them. In a single technological cycle, heterostructures were formed on vicinal substrates with (100) orientation and misoriented by 2o to (110) by the method of MOCVD (MOCVD) in a single technological cycle. It has been established that on misoriented substrates, the growth of structurally consistent and stressed epitaxial layers occurs according to a layered-step mechanism with the formation of macrosteps. On vicinal substrates, the formation of monatomic growth steps was observed. The comparative characteristics of p-HEMT obtained using two types of substrates are considered. Keywords: GaAs/InGaAs/GaAlAs heterostructures, MOCVD epitaxy, p-HEMT, substrate misorientation, comparative characteristics of structures and p-HEMT based on them.

Влияние разориентации подложки на свойства p-НЕМТ наногетероструктур на основе GaAs, формируемых в процессе MOCVD эпитаксии

As one of the approaches to improve p-HEMT, we studied the effect of misorientation of GaAs substrates on the surface morphology, structure, and electrical properties of pseudomorphic heterostructures, as well as the parameters of transistors based on them. In a single technological cycle, heterostructures were formed on vicinal substrates with (100) orientation and misoriented by 2° to (110) by the method of MOCVD (MOCVD) in a single technological cycle. It has been established that on misoriented substrates, the growth of structurally consistent and stressed epitaxial layers occurs according to a layered-step mechanism with the formation of macrosteps. On vicinal substrates, the formation of monatomic growth steps was observed. The comparative characteristics of p-HEMT obtained using two types of substrates are considered.

Нелокальная динамика электронов в AlGaN/GaN-транзисторных гетероструктурах

The nonlocal electrons heating in transistor heterostructures based on gallium nitride and arsenide is compared. It is shown that if, in comparison with a pure bulk material, in the case of GaAs double doped pseudomorphic heterostructures, the real space transfer of electrons significantly reduces their drift velocity overshot in the region of a strong field, then for GaN-based heterostructures, the decrease of the drift velocity overshot in the studied cases does not exceed 30%.

Nonlocal electron dynamics in GaN / AlGaN transistor heterostructures

The processes of nonlocal electron heating in transistor heterostructures based on gallium nitride and gallium arsenide were compared. It has been shown that, in the case of GaAs double doped pseudomorphic heterostructures, the real space transfer of electrons significantly reduces their drift velocity overshot in the region of a strong field as compared with pure bulk GaAs, while in GaN-based heterostructures the decrease in the drift velocity overshot does not exceed 30% in the studied cases. Keywords: Real space transfer, field-effect transistor, gain factor

A Millimeter-Wave Field-Effect Transistor Based on a Pseudomorphic Heterostructure with an Additional Potential Barrier

A Millimeter-Wave Field-Effect Transistor Based on a Pseudomorphic Heterostructure with an Additional Potential Barrier

Полевой транзистор миллиметрового диапазона длин волн на основе псевдоморфной гетероструктуры с дополнительными потенциальными барьерами

The results of millimeter – wave field – effect transistors with a 0.14 µm T – gate on pseudomorphic heterostructures Al0.3Ga0.7As – In0.22Ga0.78As – Al0.3Ga0.7As with additional potential barriers based on two-way donor-acceptor channel doping study are presented. At a frequency of 40 GHz in a wide range of gate voltages, a maximum stable gain of more than 15 dB is achieved. The maximum frequency of the device generation is about 250 GHz, the specific current density at the open channel is about 0.7 A / mm, the breakdown voltage of the gate-drain, depending on the version, is 22 -31 V.

Schottky Diodes on Heterostructures with Two-Dimensional Electron Gas

The results of studies of the characteristics of contacts with Schottky barriers on a pseudomorphic AlGaAs/InGaAs heterostructure are presented. The Ti-based Schottky diodes were created by a standard pHEMT technology. It is shown that the parameters of the fabricated contacts with Schottky barriers are close to the characteristics of ideal contacts based on aluminum or molybdenum created in the process of molecular beam epitaxy. It is also shown that as compared to the diodes based on AlGaAs homostructures, the measured Schottky barrier height is smaller and the ideality factor is higher in diodes created on pseudomorphic heterostructures. Along with the well-known methods for determining the Schottky-barrier height, we tested a technique based on the measurement of the threshold voltage in diodes with a two-dimensional electron gas.

Elastic theory of pseudomorphic monoclinic and rhombohedral heterostructures

We calculate analytically the strain tensor for epitaxial, pseudomorphic heterostructures of arbitrary growth plane for monoclinic and rhombohedral crystals. As numerical examples, we treat heterostructures from sesquioxides, namely, monoclinic (Ga-rich) (Al,Ga)2O3 on β-Ga2O3 and rhombohedral (Al-rich) (Al,Ga)2O3 on α-Al2O3 using realistic material parameters. We predict the strains in laboratory and crystal coordinates and derive properties such as lattice constants and tilt of lattice planes for comparison with X-ray data. Our theory also applies to any higher symmetric structure such as orthorhombic, tetragonal, cubic, and hexagonal heterostructures.

Verification of the Hypothesis on the Thermoelastic Nature of Deformation of a (0001)GaN Layer Grown on the Sapphire a-Cut

The deformation of a (0001)GaN epitaxial layer on the (11 $$\bar {2}$$ 0) sapphire a-cut is studied by X-ray diffractometry. Anisotropic-layer deformation is calculated by reference data on the thermal expansion coefficients of gallium nitride and sapphire. A comparison of the calculated and experimental deformation confirms the hypothesis on the thermoelastic character of GaN deformation on the sapphire a-cut. This result makes it possible, in particular, to assess theoretically the elastic deformation and piezoelectric field in pseudomorphic heterostructures with GaN layers on the sapphire a-cut as a virtual substrate or a buffer layer.

Проверка гипотезы о термоупругом характере деформации слоя (0001)GaN, выращенного на a-срезе сапфира

AbstractThe deformation of a (0001)GaN epitaxial layer on the (11 $$\bar {2}$$ 0) sapphire a -cut is studied by X-ray diffractometry. Anisotropic-layer deformation is calculated by reference data on the thermal expansion coefficients of gallium nitride and sapphire. A comparison of the calculated and experimental deformation confirms the hypothesis on the thermoelastic character of GaN deformation on the sapphire a -cut. This result makes it possible, in particular, to assess theoretically the elastic deformation and piezoelectric field in pseudomorphic heterostructures with GaN layers on the sapphire a -cut as a virtual substrate or a buffer layer.

Ab initiosimulations of pseudomorphic silicene and germanene bidimensional heterostructures

Among the novel two-dimensional (2D) materials, silicene and germanene, which are two honeycomb crystal structures composed of a monolayer of Si and Ge, respectively, have attracted the attention of material scientists because they combine the advantages of the new 2D ultimate-scaled electronics with their compatibility with industrial processes presently based on Si and Ge. We envisage pseudomorphic lateral heterostructures based on ribbons of silicene and germanene, which are the 2D analogs of conventional 3D Si/Ge superlattices and quantum wells. In spite of the considerable lattice mismatch ($\ensuremath{\sim}4%$) between free-standing silicene and germanene, our ab initio simulations predict that, considering striped 2D lateral heterostructures made by alternating silicene and germanene ribbons of constant width, the silicene/germanene junction remains pseudomorphic---i.e., it maintains lattice-matched edges---up to critical ribbon widths that can reach some tens of nanometers. Such critical widths are one order of magnitude larger than the critical thickness measured in 3D pseudomorphic Si/Ge heterostructures and the resolution of state-of-the-art lithography, thus enabling the possibility of lithography patterned silicene/germanene junctions. We computed how the strain produced by the pseudomorphic growth modifies the crystal structure and electronic bands of the ribbons, providing a mechanism for band-structure engineering. Our results pave the way for lithography patterned lateral heterostructures that can serve as the building blocks of novel 2D electronics.

Identification of photoluminescence bands in AlGaAs/InGaAs/GaAs PHEMT heterostructures with donor-acceptor-doped barriers

The photoluminescence of AlGaAs/InGaAs/GaAs pseudomorphic high-electron mobility transistor heterostructures with donor-acceptor-doped AlGaAs barriers is studied. It is found that the introduction of additional p +-doped AlGaAs layers into the design brings about the appearance of new bands in the photoluminescence spectra. These bands are identified as resulting from transitions (i) in donor-acceptor pairs in doped AlGaAs layers and (ii) between the conduction subband and acceptor levels in the undoped InGaAs quantum well.

Pseudomorphic GeSn/Ge (001) heterostructures

The synthesis of pseudomorphic GeSn heterostructures on a Ge (001) substrate by molecular-beam epitaxy is described. Investigations by transmission electron microscopy show that the GeSn layers are defect free and possess cubic diamondlike structure. Photoluminescence spectroscopy reveals interband radiative recombination in the GeSn quantum wells, which is identified as indirect transitions between the subbands of heavy electrons and heavy holes. On the basis of experimental data and modeling of the band structure of pseudomorphic GeSn compounds, the lower boundary of the bowing parameter for the indirect band gap is estimated as bL ≥ 1.47 eV.

GaP collector development for SiGe heterojunction bipolar transistor performance increase: A heterostructure growth study

To develop a III/V wide bandgap collector concept for future SiGe heterobipolar transistor performance increase, a heterostructure growth study of GaP on pseudomorphic 4° off-oriented Si0.8Ge0.2/Si(001) substrates was performed. For pseudomorphic GaP/Si0.8Ge0.2/Si(001) heterostructure growth, critical thickness of GaP on Si and maximum thermal budget for GaP deposition were evaluated. A detailed structure and defect characterization study by x-ray diffraction, atomic force microscopy, and transmission electron microscopy is reported on single crystalline 170 nm GaP/20 nm Si0.8Ge0.2/Si(001). Results show that 20 nm Si0.8Ge0.2/Si(001) can be overgrown by 170 nm GaP without affecting the pseudomorphism of the Si0.8Ge0.2/Si(001) layer. The GaP layer grows however partially relaxed, mainly due to defect nucleation at the GaP/Si0.8Ge0.2 interface during initial island coalescence. The achievement of 2D GaP growth conditions on Si0.8Ge0.2/Si(001) systems is thus a crucial step for achieving fully pseudomorphic heterostructures. Anti-phase domain-free GaP growth is observed for film thicknesses beyond 70 nm.

STRAIN INDUCED ACTIVE LAYER DESIGN OF GaN-THz QUANTUM CASCADE LASERS

GaN -based pseudomorphic heterostructures with their demonstrated superior thermal performance suggest an alternative to the standard GaAs -based technology to realize high power lasing at THz frequencies. A larger electron effective mass in GaN based system results in the energy levels lying deeper within the quantum well compared to its GaAs counterpart resulting in longer carrier lifetimes assisting transitions required for THz radiation while reducing tunneling current. However, the presence of spontaneous and piezoelectric polarization and dependence of bandgap and band offsets on structural and bias induced strain reduces carrier lifetime.

Thermal Stability of Pd Gate in Pseudomorphic InGaAs Heterostructures

The characterization of InAs is known to have high spin–orbit interaction as well as high electron velocity, high sheet carrier density. In order to realize Datta–Das type spin field effect transistor (FET), it becomes crucial to develop thermally stable Schottky gate metal process for achieving a non-alloyed ohmic contact with ferromagnetic electrode after gate formation process. We have investigated Pd-gate deposited on In0.81Ga0.19As-based pseudomorphic heterostructures structure in order to check Pd as a good candidate for thermally stable Schottky gate by verifying good Schottky characteristics while maintaining good transport properties during heat treatment. Characteristics of Pd gate on InAlAs barrier layer was found to have Schottky barrier height of around 0.68 and less than 1.10 of ideality factor. Moreover Pd gate was found to endure higher heat treatment than Ti/Pt/Au gate or Ti/Au gate.