Different temperature dependence of carrier transport properties between AlxGa1-xN/InyGa1-yN/GaN and AlxGa1-xN/GaN heterostructures

The temperature dependence of carrier transport properties of AlxGa1-xN/InyGa1-yN/GaN and AlxGa1-xN/GaN heterostructures has been investigated.It is shown that the Hall mobility in Al0.25Ga0.75N/In0.03Ga0.97N/GaN heterostructures is higher than that in Al0.25Ga0.75N/GaN heterostructures at temperatures above 500 K,even the mobility in the former is much lower than that in the latter at 300 K.More importantly,the electron sheet density in Al0.25Ga0.75N/In0.03Ga0.97N/GaN heterostructures decreases slightly,whereas the electron sheet density in Al0.25Ga0.75N/GaN heterostructures gradually increases with increasing temperature above 500 K.It is believed that an electron depletion layer is formed due to the negative polarization charges at the InyGa1-yN/GaN heterointerface induced by the compressive strain in the InyGa1-yN channel,which e-ectively suppresses the parallel conductivity originating from the thermal excitation in the underlying GaN layer at high temperatures.     

Influence of AlN interfacial layer on electrical properties of high-Al-content Al0.45Ga0.55N/GaN HEMT structure

Unintentionally doped high-Al-content Al_0.45Ga_0.55N/GaN high electron mobility transistor (HEMT) structures with and without AlN interfacial layer were grown by metal-organic chemical vapor deposition (MOCVD) on two-inch sapphire substrates. The effects of AlN interfacial layer on the electrical properties were investigated. At 300 K, high two-dimensional electron gas (2DEG) density of 1.66 × 10¹³ cm⁻² and high electron mobility of 1346 cm² V⁻¹ s⁻¹ were obtained for the high Al content HEMT structure with a 1 nm AlN interfacial layer, consistent with the low average sheet resistance of 287 Ω/sq. The comparison of HEMT wafers with and without AlN interfacial layer shows that high Al content AlGaN/AlN/GaN heterostructures are potential in improving the electrical properties of HEMT structures and the device performances.     

Numerical optimization of In-mole fractions and layer thicknesses in AlxGa1−xN/AlN/GaN high electron mobility transistors with InGaN back barriers

The effects of the In-mole fraction (x) of an In_xGa_1−xN back barrier layer and the thicknesses of different layers in pseudomorphic Al_yGa_1−yN/AlN/GaN/In_xGa_1−xN/GaN heterostructures on band structures and carrier densities were investigated with the help of one-dimensional self-consistent solutions of non-linear Schrödinger-Poisson equations. Strain relaxation limits were also calculated for the investigated Al_yGa_1−yN barrier layer and In_xGa_1−xN back barriers. From an experimental point of view, two different optimized structures are suggested, and the possible effects on carrier density and mobility are discussed.     

Magnetotransport, optical, and electronic subband properties in AlxGa1−xN/AlN/GaN heterostructures

The Shubnikov-de Haas (S-dH) results at 1.5 K for Al_xGa_1−xN/AlN/GaN heterostructures and the fast Fourier transformation data for the S-dH data indicated the occupation by a two-dimensional electron gas (2DEG) of one subband in the GaN active layer. Photoluminescence (PL) spectra showed a broad PL emission about 30 meV below the GaN exciton emission peak at 3.474 eV that could be attributed to recombination between the 2DEG occupying in the AlN/GaN heterointerface and photoexcited holes. A possible subband structure was calculated by a self-consistent method taking into account the spontaneous and piezoelectric polarizations, and one subband was occupied by 2DEG below the Fermi level, which was in reasonable agreement with the S-dH results. These results can help improve understanding of magnetotransport, optical, and electronic subband properties in Al_xGa_1−xAs/AlN/GaN heterostructures.     

High temperature electron transport properties of AlGaN/GaN heterostructures with different Al-contents

Electron transport properties in AlGaN/GaN heterostructures with different Al-contents have been investigated from room temperature up to 680 K. The temperature dependencies of electron mobility have been systematically measured for the samples. The electron mobility at 680 K were measured as 154 and 182 cm²/V·s for Al_0.15Ga_0.85N/GaN and Al_0.40Ga_0.60N/GaN heterostructures, respectively. It was found that the electron mobility of low Al-content Al_0.15Ga_0.85N/GaN heterostructure was less than that of high Al-content Al_0.40Ga_0.60N/GaN heterostructure at high temperature of 680 K, which is different from that at room temperature. Detailed analysis showed that electron occupations in the first subband were 75% and 82% at 700 K for Al_0.15Ga_0.85N/GaN and Al_0.40Ga_0.60N/GaN heterostructures, respectively, and the two dimensional gas (2DEG) ratios in the whole electron system were 30% and near 60%, respectively. That indicated the 2DEG was better confined in the well, and was still dominant in the whole electron system for higher Al-content AlGaN/GaN heterostructure at 700 K, while lower one was not. Thus it had a higher electron mobility. So a higher Al-content AlGaN/GaN heterostructure is more suitable for high-temperature applications.     

The effect of InxGa1−xN back-barriers on the dislocation densities in Al0.31Ga0.69N/AlN/GaN/InxGa1−xN/GaN heterostructures (0.05 ≤ x ≤ 0.14)

Al_0.31Ga_0.69N/AlN/GaN/In_xGa_1?xN/GaN heterostructures grown with the metal-organic chemical vapor deposition (MOCVD) technique with different In_xGa_1?xN back-barriers with In mole fractions of 0.05 ≤ x ≤ 0.14 were investigated by using XRD measurements. Screw, edge, and total dislocations, In mole fraction of back-barriers, Al mole fraction, and the thicknesses of front-barriers and lattice parameters were calculated. Mixed state dislocations with both edge and screw type dislocations were observed. The effects of the In mole fraction difference in the back-barrier and the effect of the thickness of front-barrier on crystal quality are discussed. With the increasing In mole fraction, an increasing dislocation trend is observed that may be due to the growth temperature difference between ultrathin In_xGa_1?xN back-barrier and the surrounding layers.     

Numerical optimization of carrier confinement characteristics in (AlxGa1−xN/AlN)SLs/GaN heterostructures

We present numerical optimization of carrier confinement characteristics in (Al_xGa_1−xN/AlN)SLs/GaN heterostructures in the presence of spontaneous and piezoelectrically induced polarization effects. The calculations were made using a self-consistent solution of the Schrödinger, Poisson, potential and charge balance equations. It is found that the sheet carrier density in GaN channel increases nearly linearly with the thickness of AlN although the whole thickness and equivalent Al composition of Al_xGa_1−xN/AlN superlattices (SLs) barrier are kept constant. This result leads to the carrier confinement capability approaches saturation with thicknesses of AlN greater than 0.6 nm. Furthermore, the influence of carrier concentration distribution on carrier mobility was discussed. Theoretical calculations indicate that the achievement of high sheet carrier density is a trade-off with mobility.     

Two-dimensional electron gas densities in AlGaN/AlN/GaN heterostructures

The dependence of two-dimensional electron gas (2DEG) density and distribution in an Al_xGa_1-xN/AlN/GaN heterostructure on the thicknesses of the Al_xGa_1-xN barrier layer and the AlN interfacial layer are investigated theoretically. A competitive contribution of the AlGaN and AlN layers to the 2DEG density is revealed. For an AlN interfacial layer thinner than a critical value d_cAlN, the 2DEG density is dominated by the AlGaN barrier and the 2DEG density increases with the increase of the AlGaN barrier thickness, as in the case of a simple AlGaN/GaN heterostructure. While the AlN interfacial layer will take the dominant contribution to the 2DEG density as its thickness exceeds d_cAlN. In this case, the increase of AlGaN barrier layer thickness leads to the decrease of the 2DEG density. Detailed calculations show that the critical AlN thickness increases with the increase of Al content in the AlGaN barrier. PACS 85.30.De; 73.40.Kp; 02.60.Cb     

Subband transitions in AlxGa1−xN /GaN/ AlxGa1−xN and AlxGa1−xN /InN/ AlxGa1−xN single quantum wells

We have calculated the spectral regime of subband transitions in Al_xGa_1−xN/GaN and Al_xGa_1−xN/InN single quantum wells. We used a simplified model to account for the internal electric fields, which modify the shape of the quantum well. Some of the parameters for these materials have not yet been firmly established. Therefore, we carried out the analysis for the extremes of the reported values of conduction band discontinuities and band gaps (in the case of InN). This analysis shows that the spectral regime of interband transitions for 1–4 nm thick wells has wavelengths above 0.5 μm for AlGaN/InN and above 0.8 μm for AlGaN/GaN and both heterostructures cover several μm wavelengths. The spectral variation with alloy composition is less pronounced in the Al_xGa_1−xN/InN single quantum wells due to the higher electric field present across the InN quantum well as compared to GaN. The results of these calculations are in good agreement with more rigorous theoretical approaches and available experimental values for Al_xGa_1−xN/GaN.     

Structural and local electrical properties of AlInN/AlN/GaN heterostructures

GaN layers and Al_1−xIn_xN/AlN/GaN heterostructures have been studied by scanning probe microscopy methods. Threading dislocations (TDs), originating from the GaN (0 0 0 1) layer grown on sapphire, have been investigated. Using Current-Atomic Force Microscopy (C-AFM) TDs have been found to be highly conductive in both GaN and AlInN, while using semi-contact AFM (phase-imaging mode) indium segregation has been traced at TDs in AlInN/AlN/GaN heterostructures. It has been assessed that In segregation is responsible for high conductivity at dislocations in the examined heterostructures.     

Effect of Al mole fraction on structural and electrical properties of AlxGa1−xN/GaN heterostructures grown by plasma-assisted molecular beam epitaxy

The effect of Al mole fractions on the structural and electrical properties of Al_xGa_1−xN/GaN thin films grown by plasma-assisted molecular beam epitaxy (PA-MBE) on Si (1 1 1) substrates has been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and current-voltage (I-V) measurements. X-ray results revealed that the AlGaN/GaN/AlN was epitaxially grown on Si substrate. By applying Vegard's law, the Al mole fractions of Al_xGa_1−xN samples were found to be 0.11, 0.24, 0.30 and 0.43, respectively. The structural and morphology results indicated that there is a relatively larger tensile strain for the sample with the smallest Al mole fraction; while a smaller compressive strain and larger grain size appear with Al mole fraction equal to 0.30. The strain gets relaxed with the highest Al mole fraction sample. Finally, the linear relationship between the barrier height and Al mole fraction was obtained.     

Alloy effects in Ga1−xInxN/GaN heterostructures

We show that the large band offsets between GaN and InN and the heavy carrier effective masses preclude the use of the virtual crystal approximation to describe the electronic structure of Ga_1−xIn_xN/GaN heterostructures, while this approximation works very well for the Ga_1−xIn_xAs/GaAs heterostructures.     

Semiempirical tight-binding modelling of III-N-based heterostructures

In this work, we present a second nearest neighbour sp³s^* semi-empirical tight-binding theory to calculate the electronic band structure of heterostructures based on group III-N binary semiconductors and their ternaries. The model Hamiltonian includes the second nearest neighbour (2nn) interactions, the spin–orbit splitting and the nonlinear variations of the atomic energy levels and the bond length with ternary mole fraction. Using this sp³s^* tight-binding approach, we investigated the electronic band structure of Al_1−xGa_xN/GaN and In_1−xGa_xN/GaN heterostructures as a function of composition and interface strain for the entire composition range (0≤x≤1). There is an excellent agreement between the model predictions and experiment for the principal bandgaps at Γ, L and X symmetry points of the Brillouin zone for AlN, GaN and InN binaries and Al_1−xGa_xN and In_1−xGa_xN ternaries. The model predicts that the composition effects on the valence band offsets is linear, but on the conduction band offsets is nonlinear and large when the interface strain and deformation potential is large.     

Observation of the transition from diffusive regime to ballistic regime of the 2DEG transport property in AlxGa1−xN/GaN heterostructures

Electron–electron interaction effect of the two-dimensional electron gas (2DEG) in Al_xGa_1−xN/GaN heterostructures has been investigated by means of magnetotransport measurements at low temperatures. From the temperature dependence of the longitudinal conductivity of the heterostructures, a clear transition region has been observed. Based on the theoretical analysis, we conclude that this region corresponds to the transition from the diffusive regime to the ballistic regime of the 2DEG transport property. The interaction constant is determined to be −0.423, which is consistent with the theoretical prediction. However, the critical temperature for the transition, which is 8 K in Al_xGa_1−xN/GaN heterostructures, is much higher than the theoretical prediction.     

The influence of AlN interlayers on the microstructural and electrical properties of p-type AlGaN/GaN superlattices grown on GaN/sapphire templates

AlN with different thicknesses were grown as interlayers (ILs) between GaN and p-type Al_0.15Ga_0.85N/GaN superlattices (SLs) by metal organic vapor phase epitaxy (MOVPE). It was found that the edge-type threading dislocation density (TDD) increased gradually from the minimum of 2.5×10⁹?cm^?2 without AlN IL to the maximum of 1×10¹⁰?cm^?2 at an AlN thickness of 20 nm, while the screw-type TDD remained almost unchanged due to the interface-related TD suppression and regeneration mechanism. We obtained that the edge-type dislocations acted as acceptors in p-type Al _x Ga_1?x N/GaN SLs, through the comparison of the edge-type TDD and hole concentration with different thicknesses of AlN IL. The Mg activation energy was significantly decreased from 153 to 70?meV with a?10-nm AlN IL, which was attributed to the strain modulation between AlGaN barrier and GaN well. The large activation efficiency, together with the TDs, led to the enhanced hole concentration. The variation trend of Hall mobility was also observed, which originated from the scattering at TDs.     

Performance comparison of Pt/Au and Ni/Au Schottky contacts on AlxGa1-x N/GaN heterostructures at high temperatures

In contrast with Au/Ni/Al 0.25 Ga 0.75 N/GaN Schottky contacts,this paper systematically investigates the effect of thermal annealing of Au/Pt/Al 0.25 Ga 0.75 N/GaN structures on electrical properties of the two-dimensional electron gas in Al 0.25 Ga 0.75 N/GaN heterostructures by means of temperature-dependent Hall and temperature-dependent current-voltage measurements.The two-dimensional electron gas density of the samples with Pt cap layer increases after annealing in N 2 ambience at 600℃ while the annealing treatment has little effect on the two-dimensional electron gas mobility in comparison with the samples with Ni cap layer.The experimental results indicate that the Au/Pt/Al 0.25 Ga 0.75 N/GaN Schottky contacts reduce the reverse leakage current density at high annealing temperatures of 400-600℃.As a conclusion,the better thermal stability of the Au/Pt/Al 0.25 Ga 0.75 N/GaN Schottky contacts than the Au/Ni/Al 0.25 Ga 0.75 N/GaN Schottky contacts at high temperatures can be attributed to the inertness of the interface between Pt and AlxGa1-xN.     

Built-in-polarization and thermal conductivity of InxGa1-xN/GaN heterostructures

The effect of built-in-polarization (BIP) field on thermal properties of In_xGa_1−xN/GaN heterostructure has been investigated. The thermal conductivity k of In_xGa_1−xN alloy has been estimated using Callaway's formula including the BIP field for In content x = 0, 0.1, 0.3, 0.5 and 0.9. This study reports that irrespective of In content, the room temperature k of In_xGa_1−xN/GaN heterostructure is enhanced by BIP field. The result predicts the existence of a characteristic temperature T_p at which both thermal conductivities (including and excluding BIP field) show a crossover. This gives signature of pyroelectric nature of In_xGa_1−xN alloy which arises due to variation of polarization with temperature indicating that thermal conductivity measurement can reveal pyroelectric nature. The pyroelectric transition temperature of In_xGa_1−xN alloy has been predicted for various x. The composition dependent nature of room temperature k for x = 0.1 and 0.5 are in line with prior experimental studies. The result can be used to minimize the self heating effect in In_xGa_1−xN/GaN heterostructures.     

First principles study on electronic and optical properties of Al<Subscript>x</Subscript>Ga<Subscript>1−x</Subscript>N and In<Subscript>y</Subscript>Ga<Subscript>1−y</Subscript>N

The band structure, density of states of Al_xGa_1?xN and In_yGa_1?yN was performed by the first-principles method within the local density approximation. The calculated energy gaps of the AlN, Al_0.5Ga_0.5N, GaN, In_0.5Ga_0.5N and InN were 5.48, 4.23, 3.137, 1.274 and 0.504 eV, which were in agreement with the experimental result. The dielectric functions, absorption coefficient and loss function were calculated based on Kramers–Kronig relations. Further more, the relationships between electronic structure and optical properties were investigated theoretically. For Al_xGa_1?xN and In_yGa_1?yN materials, the micromechanism of the optical properties were explained.     

Band offset calculations applied to III–V nitride quantum well device engineering

Band offset calculations for zinc-blende pseudomorphically strained Al_1−xGa_xN/Al_1−yGa_yN and In_xGa_1−xN/In_yGa_1−yN interfaces have been performed on the basis of the model solid theory combined with ab initio calculations. From the results obtained, we have calculated, separately, the valence and conduction band discontinuities of In_xGa_1−xN/GaN and GaN/Al_1−xGa_xN as a function of the indium and gallium contents respectively. Using the latter results, we have extended our study to simulate band discontinuities for strained Ga_1−xIn_xN/relaxed Al_1−yGa_yN heterointerfaces. Information derived from this investigation will be useful for the design of lattice mismatched heterostructures in modeling optoelectronic devices emitting at ultraviolet to near infrared wavelengths.     

Electron spin resonance and Rashba field in GaN-based materials

We discuss problem of Rashba field in bulk GaN and in GaN/Al_xGa_1−xN two-dimensional electron gas, basing on results of X-band microwave resonance experiments. We point at large difference in spin-orbit coupling between bulk material and heterostructures. We observe coupled plasmon-cyclotron resonance from the two-dimensional electron gas, but no spin resonance, being consistent with large zero-field spin splitting due to the Rashba field reported in the literature. In contrast, small anisotropy of g-factor of GaN effective mass donors indicates rather weak Rashba spin-orbit coupling in bulk material, not exceed 400 G, α_BIA<4×10⁻¹³ eVcm. Furthermore, we observe new kind of electron spin resonance in GaN, which we attribute to surface electron accumulation layer. We conclude that the sizable Rashba field in GaN/Al_xGa_1−xN heterostructures originates from properties of the interface.