Mobility Of Two-dimensional Electron Gas Research Articles

Over 3000 cm2 V−1 s−1 room temperature two-dimensional electron gas mobility by annealing Ni/Al deposited on AlGaN/GaN heterostructures

We investigated sheet electron density (ns) and electron mobility (μ) in AlGaN/GaN heterostructures deposited with Ni/Al by annealing in vacuum. We observed that both ns and μ at room temperature increased by annealing at 1020 K and the amount of increase depended on the Ni/Al stack thickness. We achieved a room temperature Hall electron mobility of 3050 cm2 V−1 s−1, which is the highest value ever reported for AlGaN/GaN heterostructures. The origin of ns increase was attributed to the additionally induced tensile strain in AlGaN layer, which was confirmed by Raman spectroscopy.

Growth of ultra-high mobility In0.52Al0.48As/InxGa1−xAs (x ≥ 53%) quantum wells on Si substrates using InP/GaAs buffers by metalorganic chemical vapor deposition

InGaAs quantum wells (QWs) cladded by InAlAs barriers were grown on Si by metalorganic chemical vapor deposition. InP/GaAs/Si buffer templates were first prepared using a two-step growth method. We were able to significantly reduce the dislocation density in the upper InP buffer and obtain smooth surface morphology by fine-tuning the growth parameters and inserting an InGaAs interlayer in the InP buffer. On these InP/GaAs/Si compliant substrates, we investigated InGaAs QWs with various well/barrier parameters and Si-delta doping. We obtained two-dimensional electron gas mobilities over 10,000 cm2 V−1 s−1 at 300 K and above 39,000 cm2 V−1 s−1 at 77 K on Si substrates.

Interface roughness scattering considering the electrical field fluctuation in undoped AlxGa1−xN/GaN heterostructures

The roughness existing in the interface between the AlxGa1−xN barrier and the GaN well gives rise to two events: the thickness fluctuation of the GaN well and the AlxGa1−xN barrier. The former results in displacement of the electron and the latter leads to fluctuation of the electrical field in the well. The mobility of two-dimensional electron gas limited by the interface roughness scattering was calculated considering the two phenomena in undoped AlxGa1−xN/GaN heterostructures. We found that the fluctuation of the electrical field caused the reduction of mobility at small barrier thickness or low sheet carrier concentrations.

Elimination of columnar microstructure in N-face InAlN, lattice-matched to GaN, grown by plasma-assisted molecular beam epitaxy in the N-rich regime

The microstructure of N-face InAlN layers, lattice-matched to GaN, was investigated by scanning transmission electron microscopy and atom probe tomography. These layers were grown by plasma-assisted molecular beam epitaxy (PAMBE) in the N-rich regime. Microstructural analysis shows an absence of the lateral composition modulation that was previously observed in InAlN films grown by PAMBE. A room temperature two-dimensional electron gas (2DEG) mobility of 1100 cm2/V s and 2DEG sheet charge density of 1.9 × 1013 cm−2 was measured for N-face GaN/AlN/GaN/InAlN high-electron-mobility transistors with lattice-matched InAlN back barriers.

Electrical and structural characteristics of metamorphic In0.38Al0.62As/In0.37Ga0.63As/In0.38Al0.62As HEMT nanoheterostructures

The influence of the metamorphic buffer design and epitaxial growth conditions on the electrical and structural characteristics of metamorphic In0.38Al0.62As/In0.37Ga0.63As/In0.38Al0.62As high electron mobility transistor (MHEMT) nanoheterostructures has been investigated. The samples were grown on GaAs(100) substrates by molecular beam epitaxy. The active regions of the nanoheterostructures are identical, while the metamorphic buffer InxAl1 − xAs is formed with a linear or stepwise (by Δx = 0.05) increase in the indium content over depth. It is found that MHEMT nanoheterostructures with a step metamorphic buffer have fewer defects and possess higher values of two-dimensional electron gas mobility at T = 77 K. The structures of the active region and metamorphic buffer have been thoroughly studied by transmission electron microscopy. It is shown that the relaxation of metamorphic buffer in the heterostructures under consideration is accompanied by the formation of structural defects of the following types: dislocations, microtwins, stacking faults, and wurtzite phase inclusions several nanometers in size.

Effects of plasma-induced defects on electrical characteristics of AlGaN/GaN heterostructure before and after low-temperature annealing

We investigated the electrical characteristics of an AlGaN/GaN heterostructure exposed to Ar plasma. In the near-surface region of the AlGaN/GaN heterostructure, we found that plasma-induced defects reduced the two-dimensional electron gas (2DEG) density and mobility at the AlGaN/GaN interface with increasing exposure time. The decrease in 2DEG density suggests that plasma-induced disordering partly extinguishes the piezo-polarization of the AlGaN layer, that the effective Schottky barrier height is increased by the introduction of negatively changed defects, or that the negatively charged defects induced during plasma exposure deactivate or compensate Si donors. In addition, we investigated the postannealing behavior of plasma-induced defects in the AlGaN/GaN heterostructure as well as in the n-GaN layer under an applied bias voltage.

Comparative Analysis of Nano-Scale Structural and Electrical Properties in AlGaN/GaN High Electron Mobility Transistors on SiC and Sapphire Substrates

A comparison of AIGaN/GaN HEMTs fabricated on both 4-in SiC and sapphire substrates was performed. Due to the high crystalline quality with one order lower dislocation density of GaN on a SiC substrate, a better two-dimensional electron gas (2DEG) mobility with high values of drain current density (780 mA/mm) and a better extrinsic transconductance (240 mS/mm) were observed. We demonstrate GaN-on-SiC HEMTs with a periphery gate width of 200 microm, exhibiting a unity-gain cut-off frequency (f(T)) = 29.6 GHz, a maximum frequency of oscillation (f(MAX)) = 63.2 GHz, and an output power density of 6.4 W/mm with a 55% power added efficiency (PAE) at 10 GHz. A surface roughness of 0.828 nm and 1.025 nm and an X-ray diffraction (XRD) GaN (0002) full-width at half-maximum (FWHM) of 120 s and 919 s were measured for the SiC and sapphire-based AIGaN/GaN HEMTs, respectively. The SiC substrate has been shown to be an optimal solution for fabricating HEMTs for X-band high-power applications, which require excellent performances.

Growth of AlGaN/GaN heterostructures with a two-dimensional electron gas on AlN/Al2O3 substrates

The possibility of using AlN/Al2O3 substrates to grow AlGaN/GaN hetero-epitaxial structures with a two-dimensional electron gas is studied. A method of calibrating the temperature of the substrates by measuring the thermal radiation spectrum is proposed. Differences between AlN/Al2O3 substrates that lead to differences in the electrophysical parameters of the grown structures are determined. AlN/Al2O3 substrates were used to grow AlGaN/GaN samples with a two-dimensional electron gas mobility in excess of 1300 cm2/(V · s) at an electron concentration in the channel higher than 1013 cm−2.

Magnetoresistance in a High Mobility Two-Dimensional Electron System as a Function of Sample Geometry

In a high mobility two-dimensional electron gas (2DEG) realized in a GaAs / Al0.3Ga0.7As quantum well we observe changes in the Shubnikov-de Haas oscillations (SdHO) and in the Hall resistance for different sample geometries. We observe for each sample geometry a strong negative magnetoresistance around zero magnetic field which consists of a peak around zero magnetic field and of a huge magnetoresistance at larger fields. The peak around zero magnetic field is left unchanged for different geometries.

Tuning of the Landé g-factor in AlxGa1−xAs/AlAs single and double quantum wells

We report on the spin dynamics of a high mobility two-dimensional electron gas in a AlxGa1−xAs/AlAs double quantum well structure. For high electron density samples, the g-factor was measured using time-resolved Kerr rotation technique. The g-factor tuning capability was observed by changing the aluminum content x independently in each well. Experiments demonstrated an unusual spin dephasing time robustness for high excitation power. The effect of the interaction between wells was analyzed in samples with different tunneling barriers. Results were compared with experiments on single well systems demonstrating higher spin polarization generation, longer spin dephasing time, and coupling for the double structures.

Nitrogen-related effects on low-temperature electronic properties of two-dimensional electron gas in very dilute nitride GaNxAs1−x/AlGaAs (x = 0 and 0.08%) modulation-doped heterostructures

The low-temperature (4 K) two-dimensional (2D) electron gas mobility data versus carrier concentration in the modulation-doped dilute nitride GaAs1−xNx/Al0.3Ga0.7As (x = 0 and 0.08%) heterostructures are analyzed. Theoretical analysis is based on Fermi–Dirac statistics for the occupation of the quantum confined electronic states in the triangular quantum wells and the width of the quantum well versus 2D concentration. In addition, the mobility analysis is based on Matthiessen's rule for various scattering mechanisms. We found that the N-related neutral cluster alloy scattering together with crystal dislocations created at the interface strongly affects the electrons' mobility in the N-contained channel sample. We also found that as the electron concentration in the well increases from ∼1 × 1011 to 3.5 × 1011 cm−2 the carriers mainly occupy the first subband, tending to remain closer and closer to the hetero-interface.

Coherent spin dynamics of different density high mobility two-dimensional electron gas in a GaAs quantum well

We have addressed the dependence of quasi-two-dimensional electron spin dephasing time on the electron gas density in a 17-nm GaAs quantum well using the time-resolved magneto-optical Kerr effect. A superlinear increase in the electron dephasing time with decreasing electron density has been found. The degree of electron spin relaxation anisotropy has been measured and the dependence of spin-orbit splitting on electron gas density has been determined.

Dielectric and barrier thickness fluctuation scattering in Al2O3/AlGaN/GaN double heterojunction high-electron mobility transistors

The two-dimensional electron gas (2DEG) mobility limited by dielectric and barrier thickness fluctuations (TF) scattering in Al2O3/AlGaN/GaN double heterojunction high-electron mobility transistors (HEMTs) is calculated. Calculation shows that thickness fluctuation scattering is the main limitation in Al2O3/AlGaN/GaN double heterojunction HEMTs with thin Al2O3 layer thicknesses. In addition, a study of 2DEG mobility as a function of 2DEG density, ns, shows that TF scattering acts as the main limitation when ns exceeds 2×1012cm−2. The results may be used to design HEMTs to obtain higher 2DEG mobilities by modulating the dielectric layer and barrier thicknesses or 2DEG density.

Atom probe analysis of AlN interlayers in AlGaN/AlN/GaN heterostructures

Atom probe tomography was used to characterize AlN interlayers in AlGaN/AlN/GaN heterostructures grown by plasma-assisted molecular beam epitaxy (PAMBE), NH3-based molecular beam epitaxy (NH3-MBE), and metal-organic chemical vapor deposition (MOCVD). The PAMBE-grown AlN interlayer had the highest purity, with nearly 100% of group-III sites occupied by Al. The group-III site concentrations of Al for interlayers grown by NH3-MBE and MOCVD were ∼85% and ∼47%, respectively. Hall measurements were performed to determine the two-dimensional electron gas mobility and sheet concentration. Sheet concentrations were ∼25%–45% higher with molecular beam epitaxy than with MOCVD, and these results matched well with atom probe data.

Effect of electron irradiation on AlGaN/GaN and InAlN/GaN heterojunctions

AlGaN/AlN/GaN/sapphire, AlGaN/GaN/sapphire, AlGaN/GaN/Si, and InAlN/GaN/sapphire heterojunctions (HJs) were irradiated with 10 MeV electrons to fluences of 2 × 1015 to 3.3 × 1016 cm−2. The main effects on the electrical properties were a decrease in two-dimensional electron gas (2DEG) mobility and the shift of capacitance–voltage (C-V) characteristics to more positive values. The 50% 2DEG mobility decrease occurred at a similar fluence of 3.3 × 1016 cm−2 for all AlGaN/GaN and AlGaN/AlN/GaN HJs, but at a much lower fluence of 1.3 × 1016 cm−2 for InAlN/GaN, which is in line with previous observations for neutron irradiated HJs. The shift of C-V characteristics is due to increased concentration of deep acceptor traps in the barrier/interface region. In AlGaN/GaN/Si transistors, the increase of concentration of deep barrier/interface traps with activation energy of 0.3, 0.55, and 0.8 eV was observed. This increase correlates with the observed degradation of gate lag characteristics of transistors after irradiation with 1.3 × 1016 cm−2 electrons.

Acoustic phonon scattering on the two-dimensional electron gas in wurtzite AlxGa1−xN/GaN/AlyGa1−yN double heterostructures: The ternary mixed crystal effect and size effect

Based on the elastic continuum model and Fermi golden rule, the ternary mixed crystal effect and size effect on the mobility of two-dimensional electron gas influenced by the scattering from acoustic phonons via deformation potential in a wurtzite AlxGa1−xN/GaN/AlyGa1−yN double heterostructure at 77K are discussed theoretically. The results show that the increase of the Al component in AlxGa1−xN barrier changes the symmetry and vibration modes of acoustic phonons, leading an abrupt increase of the total mobility in a certain Al component region for an asymmetrical heterostructure. Besides, the total mobility reaches a maximum around a certain thickness of channel layer GaN. These effects give a guidance to improve the properties of high electron mobility transistors.

Surface Morphology and Electric Property of the AlGaN/AlN/GaN Multilayers with Varying the AlN Thickness

ABSTRACTThe surface morphologies of AlGaN/AlN/GaN HEMT structures were examined by using the atomic force microscopy (AFM).These HEMT structures have been grown by metalorganic chemical vapor deposition (MOCVD) onto the sapphire (0001) substrates where the thicknesses of AlN interlayers were varied from 0 to 5 nm. After the growth of GaN buffer layer, the AlN intermediate and AlGaN top layers were subsequently deposited at 1130°C in an trimethylaluminum (TMAl) and ammonia (NH3) atmosphere. The surface of AlGaN layers shows the thick- and fine-thread patterns.It was found that the root-mean-square (RMS) roughness of the samples with 0, 0.5, 2.5 and 5nm AlN interlayer thickness are 0.503, 0.534, 0.534 and 0.601nm, respectively. Although the cracks and rougher surfaces show that the qualities of AlGaN barrier layers have slightly degraded in the samples with thicker AlN layer. These phenomena could be attributed to the lattice mismatch and the growth temperature. In addition, the room-temperature two-dimensional electron gas (2DEG) mobility and density analysis were performed on the AlGaN surfaces and the measured results were discussed in detail.

Al组分对MOCVD制备的AlxGa1-xN/AlN/GaN HEMT电学和结构性质的影响

采用金属有机化合物化学气相沉积(MOCVD)方法制备了不同Al组分(x=0.19,0.22,0.25,0.32)的Al x Ga1-x N/AlN/GaN结构的高电子迁移率晶体管(HEMT)材料。研究了Al x Ga1-x N势垒层中Al组分对HEMT材料电学性质和结构性质的影响。研究结果表明,在一定的Al组分范围内,二维电子气(2DEG)浓度和迁移率随着Al组分的升高而增大。然而,过高的Al组分导致HEMT材料表面粗糙度增大,2DEG迁移率降低,该实验现象在另一方面得到了原子力显微镜测试结果的验证。在最佳Al组分(25%)范围内,获得的HEMT材料的2DEG浓度和室温迁移率分别达到1.2×1013cm-2和1 680 cm2/(V·s),方块电阻低至310Ω/□。

Decoherence in a Landau Quantized Two Dimensional Electron Gas

We have studied the dynamics of a high mobility two-dimensional electron gas as a function of temperature. The presence of satellite reflections in the sample and magnet can be modeled in the time-domain.

Cyclotron resonance spectroscopy in a high mobility two dimensional electron gas using characteristic matrix methods

We develop a new characteristic matrix-based method to analyze cyclotron resonance experiments in high mobility two-dimensional electron gas samples where direct interference between primary and satellite reflections has previously limited the frequency resolution. This model is used to simulate experimental data taken using terahertz time-domain spectroscopy that show multiple pulses from the substrate with a separation of 15 ps that directly interfere in the time-domain. We determine a cyclotron dephasing lifetime of 15.1 ± 0.5 ps at 1.5 K and 5.0 ± 0.5 ps at 75 K.