High-temperature AlN Interlayer Research Articles

Influence of high Mg doping on the microstructural and opto-electrical properties of AlGaN alloys

Mg-doped AlxGa1-xN (x = 0.23 and 0.35) alloys have been grown on GaN templates with high temperature AlN (HT-AlN) interlayer by metalorganic chemical vapor deposition (MOCVD). A combination of secondary ion mass spectrometry (SIMS) and transmission electron microscopy (TEM) indicates the formation of more inversion domains in the high Al mole fraction Mg-doped AlGaN alloys at Mg concentration ∼1020 cm−3. For Mg-doped Al0.23Ga0.77N epilayer, the analysis of cathodoluminescence (CL) spectra supports the existence of self-compensation effects due to the presence of intrinsic defects and Mg-related centers. The energy level of Mg is estimated to be around 193 meV from the temperature dependence of the resistivity measured by Hall effect experiments. And hole concentration and mobility are measured to be 1.2 × 1018 cm−3 and 0.56 cm2/V at room temperature, respectively. The reduction of acceptor activation energy and low hole mobility are attributed to inversion domains and self-compensation. Moreover, impurity band conduction is dominant in carrier transport up to a relatively higher temperature in high Al content Mg-doped AlGaN alloys.

Phase separation-suppressed and strain-modulated improvement of crystalline quality of AlGaN epitaxial layer grown by MOCVD

High crystalline quality AlGaN films were grown on GaN templates by metalorganic chemical vapor deposition (MOCVD). Inhomogeneous distributions of Al compositions in both the vertical and lateral growth direction caused by the strong gas-phase pre-reaction occurring between Al precursors and NH3 were suppressed by optimizing the growth conditions. The residual strain in AlGaN/GaN induced by lattice mismatch, which results in the degradation of crystallinity, was modulated by varying thickness of a high temperature (HT) AlN interlayer (IL) inserted between AlGaN and GaN layers. Both the crystalline quality and Al-incorporation into the AlGaN are influenced by the residual strain related to the AlN IL thickness. Lastly, the understanding of the AlGaN growth and of the strain modification was employed to grow an AlGaN/GaN HEMT structure showing good electrical characteristics and uniformity.

Structural characterization of Al0.55Ga0.45N epitaxial layer determined by high resolution x-ray diffraction and transmission electron microscopy**Project supported by the National Key Research and Development Project of China (Grant No. 2016YFB0400100), the Hi-tech Research Project of China (Grant Nos. 2014AA032605 and 2015AA033305), the National Natural Science Foundation of China (Grant Nos. 61274003, 61422401, 51461135002, and 61334009), the Natural Science Foundation of

Structural characteristics of AlN epilayer were investigated by high resolution x-ray diffraction (HRXRD) and transmission electron microscopy (TEM); the epilayer was grown on GaN/sapphire substrates using a high-temperature AlN interlayer by metal organic chemical vapor deposition technique. The mosaic characteristics including tilt, twist, heterogeneous strain, and correlation lengths were extracted by symmetric and asymmetric XRD rocking curves as well as reciprocal space map (RSM). According to Williamson–Hall plots, the vertical coherence length of AlGaN epilayer was calculated, which is consistent with the thickness of AlGaN layer measured by cross section TEM. Besides, the lateral coherence length was determined from RSM as well. Deducing from the tilt and twist results, the screw-type and edge-type dislocation densities are 1.0 and 1.8, which agree with the results observed from TEM.

Mechanism of stress control for GaN growth on Si using AlN interlayers

For the purpose of controlling the wafer bow of GaN-on-Si structure, in situ curvature transient during the growth of a GaN layer on an AlN interlayer was investigated systematically by estimating the compressive strain applied to the GaN layer with the progress of the layer growth. The compressive strain was dependent on the morphology of the GaN surface prior to the growth of the AlN interlayer. It was found that the transition sequence from GaN growth to AlN growth induces roughening of the GaN surface and both high NH3 partial pressure and the short transition time were effective for reducing the roughness of the GaN surface beneath the AlN interlayer. The improved transition sequence increased the compressive strain in GaN by a factor of 2.5. The AlN grown at the same temperature as that of GaN was beneficial in both better surface morphology and the reduction of the transition time between GaN growth and AlN growth. With this high-temperature AlN interlayer, its thickness is another important factor governing the compressive strain in GaN. To get AlN relaxed for applying the compressive strain to GaN, the AlN layer should be thicker but too thick layer after relaxation results in surface roughening, which in turn introduces defects to the overlying GaN layer and reduces the compressive strain by partial lattice relaxation of GaN.

Structural and optical properties of AlxGa1−xN (0.33 ≤ x ≤ 0.79) layers on high-temperature AlN interlayer grown by metal organic chemical vapor deposition

High-Al-content AlxGa1−xN films with x varying from 0.33 to 0.79 were grown on GaN templates with the high temperature AlN (HT-AlN) interlayer by metal organic chemical vapor deposition (MOCVD). The best crystalline quality, among these AlxGa1−xN alloys, can be obtained for an AlN mole fraction x = 0.55, where the full-width at half-maximum of the Al0.55Ga0.45N (0002) diffraction peak was measured to be 259 arcsec by high resolution X-ray diffraction (HRXRD). The screw threading dislocation (TDs) density was 2 × 108 cm−2 evaluated by transmission electron microscope (TEM), which agreed with the calculations from Williamson-Hall plots. Moreover, cross-sectional TEM indicated that the HT-AlN interlayer could sufficiently reduce the threading dislocations (TDs) through generation of V trenches in the HT-AlN interlayer, since the TDs propagated along the V trenches, then bent into basal planes and annihilated with other dislocations. The study of optical properties indicated that obvious S-shape of temperature dependence on emission energy was observed for Al0.55Ga0.45N layers, which was attributed to exciton localization with energy (Eloc) ∼14.95 meV at 10 K resulting from potential fluctuation and band tail states. The time-resolved photoluminescence (TRPL) curves showed a bi-exponential decay at low temperature. The fast decay time implied the presence of the localized excitons enhancing radiative recombination, while the quite slow one was due to the dominance of trapping mechanisms originating from cation vacancy complexes and the VIII-related complexes.

Crack-free Si-doped n-AlGaN film grown on sapphire substrate with high-temperature AlN interlayer

The crack-free Si-doped n-Al0.28Ga0.72N epitaxial films with high-temperature (HT) AlN interlayer were successfully grown on c-plane sapphire substrate by metal-organic chemical vapor deposition (MOCVD). The high resolution X-ray diffraction (XRD) measurement results demonstrate that the density of dislocations was reduced significantly with the insertion of the HT-grown AlN interlayer between the low-temperature (LT)-grown AlN buffer layer and the n-AlGaN epitaxial film. Room temperature (RT) Raman scattering spectra show a blue-shift for the E2 (high) phonon mode, implying that the Si-doped n-Al0.28Ga0.72N epitaxial film is under compressive strain. Moreover, the photoluminescence (PL) peak position slightly red-shifted from 311 to 315nm, while the PL intensity decreased remarkably as the temperature was increased from 10 to 300K. The Hall effect measurement results reveal that a carrier density in the Si-doped n-Al0.28Ga0.72N epitaxial film as high as 6.84×1018cm−3 has been achieved.

Performance improvements of AlGaN/GaN HEMTs by strain modification and unintentional carbon incorporation

An advanced AlGaN/GaN HEMT structure, grown on a sapphire substrate by MOCVD utilizing a high temperature (HT) AlN interlayer (IL) and a multilayer high-low-high temperature (HLH) AlN buffer layer, demonstrates a superior performance both in breakdown voltage (>200 V) and maximum drain current (IDSS = 667 mA/mm). The HT AlN IL produces an additional compressive strain into the above GaN layer. Accordingly, an AlGaN barrier, grown on the more compressive GaN, introduces less tensile strain leading to an improvement in surface morphology (RMS = 0.19 nm in 2 × 2 μm2), a remarkable increase in 2DEG mobility by 46% (μ s = 1900 cm2/Vs) and a decrease in densities of defects acting as paths for the leakage current through the AlGaN barrier. A high semi-insulating buffer is achieved by eliminating leakage paths both through the buffer layer and the buffer-substrate interfacial layer. These result from an increase in unintentional carbon introduced by AlN layers, especially by a low temperature AlN layer; which are grown under low pressure (50 Torr). Lastly, the decrease in AlGaN barrier tensile strain and low leakage current in the advanced HEMTs structure using an HT AlN IL and an HLH AlN buffer are promising for an improvement in AlGaN/GaN HEMTs’ reliability.

Barrier Strain and Carbon Incorporation‐Engineered Performance Improvements for AlGaN/GaN High Electron Mobility Transistors**

AbstractThe improvements in electrical characteristics of AlGaN/GaN high electron mobility transistors (HEMTs) grown using metal‐organic (MO)CVD by engineering structure, barrier strain, and unintentional carbon incorporation, are demonstrated in this work. Both normal HEMT structure (with a high temperature (HT) AlN buffer) and advanced HEMT structure (with a high‐low‐high temperature (HLHT) AlN buffer, and a HT AlN interlayer (IL)) present a breakdown voltage higher than 200 V, while a much smaller breakdown voltage of 17 V is measured on the conventional structure using a low‐temperature GaN buffer. The HT AlN IL inserted in the middle of the conventional HEMT structure introduces a reduction in the tension of the AlGaN barrier, which results in an improvement of the surface morphology (0.46 nm). As a consequence, the two‐dimensional electron gas (2DEG) mobility increases by remarkable 46% (1900 cm2 V−1 s−1). The HLHT AlN buffer, substituting for the HT AlN buffer, leads to the enhancement of GaN crystalline quality, which contributes to the performance improvement for HEMTs. The advanced HEMT, using both an AlN IL and an HLHT AlN buffer, produces increases in the DC maximum drain current by 35.5% (∼680 A mm−1), and in the transconductance by 15% (114 mS mm−1) in comparison with the normal HEMT with an AlN buffer. The very low leakage current in the advanced HEMTs is caused by optimizing the design of the buffer and modifying growth parameters. Lastly, the reduction of AlGaN barrier tensile strain by inserting the HT AlN IL is promising for an improvement in AlGaN/GaN HEMT reliability.

Effect of growth temperature of AlN interlayers on the properties of GaN epilayers grown on c‐plane sapphire by metal organic chemical vapor deposition

AbstractThe effect of growth temperature of AlN interlayers on the properties of GaN epilayers grown on c‐plane sapphire by metal organic chemical vapor deposition has been investigated by high resolution X‐ray diffraction (HRXRD) and Raman spectroscopy. It is concluded that the crystalline quality of GaN epilayers is improved significantly by using the high temperature AlN (HT‐AlN) interlayer in GaN buffers. The density of threading dislocation is reduced especially for edge type dislocations. Higher compressive stress exists in GaN epilayers with HT‐AlN interlayer than with low temperature AlN (LT‐AlN) interlayer, which is related to the reduction of strain relaxation caused by the formation of misfit dislocation. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Improved electrical properties of the two-dimensional electron gas in AlGaN/GaN heterostructures using high temperature AlN interlayers

The electrical properties of two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructures using high temperature (HT) AlN interlayers (ITs) grown on c-plane sapphire substrate by metal organic chemical vapor deposition (MOCVD) have been investigated. It is found that the electrical properties (electron mobility and sheet carrier density) are improved compared with those in the conventional AlGaN/GaN heterostructures without HT AlN ITs, and the improved 2DEG properties result in the reduction of the sheet resistance. The results from high resolution X-ray diffraction (HRXRD) and Raman spectroscopy measurements show that HT AlN ITs increase the in-plane compressive strain in the upper GaN layer, which enhances the piezoelectric polarization in it and consequently causes increasing of 2DEG density at the AlGaN/GaN interface. Meanwhile, the compressive strain induced by HT AlN ITs leads to a less tensile strain in AlGaN barrier layer and causes positive and negative effects on the sheet carrier density of 2DEG, which counteract each other. The HT AlN ITs reduce the lattice mismatch between the GaN and AlGaN layers and smooth the interface between them, thus increasing the electric mobility of 2DEG by weakening the alloy-related interface roughness and scattering. In addition, the surface morphology of AlGaN/GaN heterostructures is improved by the insertion of HT AlN ITs. The reason for the improved properties is discussed in this paper.

Comparative study of different properties of GaN films grown on (0001) sapphire using high and low temperature AlN interlayers

Comparative study of high and low temperature AlN interlayers and their roles in the properties of GaN epilayers prepared by means of metal organic chemical vapour deposition on (0001) plane sapphire substrates is carried out by high resolution x-ray diffraction, photoluminescence and Raman spectroscopy. It is found that the crystalline quality of GaN epilayers is improved significantly by using the high temperature AlN interlayers, which prevent the threading dislocations from extending, especially for the edge type dislocation. The analysis results based on photoluminescence and Raman measurements demonstrate that there exists more compressive stress in GaN epilayers with high temperature AlN interlayers. The band edge emission energy increases from 3.423 eV to 3.438 eV and the frequency of the Raman shift of E2(TO) moves from 571.3 cm−1 to 572.9 cm−1 when the temperature of AlN interlayers increases from 700 °C to 1050 °C. It is believed that the temperature of AlN interlayers effectively determines the size, the density and the coalescence rate of the islands, and the high temperature AlN interlayers provide large size and low density islands for GaN epilayer growth and the threading dislocations are bent and interactive easily. Due to the threading dislocation reduction in GaN epilayers with high temperature AlN interlayers, the approaches of strain relaxation reduce drastically, and thus the compressive stress in GaN epilayers with high temperature AlN interlayers is high compared with that in GaN epilayers with low temperature AlN interlayers.

Study of the leakage current mechanism in Schottky contacts to Al0.25Ga0.75N/GaN heterostructures with AlN interlayers

The leakage current mechanism in Schottky contacts (SCs) to Al0.25Ga0.75N/GaN heterostructures incorporated by a thin high-temperature (HT) AlN interlayer has been investigated using current–voltage measurements, atomic force microscopy and deep level transient spectroscopy. It is found that the HT AlN interlayer thickness has a significant effect on the leakage current in SCs. The leakage current density decreases to 1.1 × 10−4 A cm−2 when the growth time of the AlN interlayer increases from 0 to 10 s, and then changes to increase with increasing growth time. Correspondingly, the heterostructure with the AlN growth time of 10 s has the least number of surface pinholes. The thickness of the HT AlN also influences the density of electron traps with the activation energy of 0.762 eV in an Al0.25Ga0.75N barrier. It is suggested that the HT AlN interlayer adjusts the microstructure and the defect state density in the Al1−xGaxN barrier, and the leakage via these defect states makes the main contribution to the leakage current in SCs to Al1−xGaxN/GaN heterostructures.

Influence of high-temperature AlN interlayer on the electrical properties of AlGaN/GaN heterostructure and HEMTs

GaN buffer with insertion of a 40-nm-thick high-temperature （HT） AlN interlayer is studied. The HT-AlN interlayer enhances the in-plane compressive strain of GaN film and thereby improves the electrical properties of AlGaN/GaN high electron mobility transistors （HEMTs）. Base on the precise measurement of Bragg angle, the strain states of GaN are calculated. It is found that the increased compressive stress enhances the piezoelectric polarization field in GaN, which consequently causes accumulation of more electrons at the AlGaN/GaN interface. On the other hand, the influence of AlGaN layer induced by the enhanced compressive stress on the two-dimensional electron gas （2DEG） sheet carrier density, both positive and negative, are proved to counteract each other. Meanwhile, the employment of the HT-AlN interlayer reduces the lattice mismatch between the GaN and AlGaN and smoothes the AlGaN/GaN interface, thus increases the 2DEG mobility by weakening the interface roughness scattering. The 1-μm gate-length HEMTs by using the GaN buffer layer with the HT-AlN interlayer are fabricated. The measurements show that the maximum drain current and transconductance are increased by 42% and 20% respectively compared with the conventional HEMTs without HT-AlN interlayer.

High-temperature AlN interlayer for crack-free AlGaN growth on GaN

This paper presents a study of the transformation of high-temperature AlN (HT-AlN) interlayer (IL) and its effect on the strain relaxation of Al0.25Ga0.75N/HT-AlN/GaN. The HT-AlN IL capped with Al0.25Ga0.75N transforms into AlGaN IL in which the Al composition increases with the HT-AlN IL thickness while the total Ga content keeps nearly constant. During the HT-AlN IL growth on GaN, the tensile stress is relieved through the formation of V trenches. The filling up of the V trenches by the subsequent Al0.25Ga0.75N growth is identified as the Ga source for the IL transformation, whose effect is very different from a direct growth of HT-AlGaN IL. The a-type dislocations generated during the advancement of V trenches and their filling up propagate into the Al0.25Ga0.75N overlayer. The a-type dislocation density increases dramatically with the IL thickness, which greatly enhances the strain relaxation of Al0.25Ga0.75N.

Al incorporation, structural and optical properties of Al xGa 1−xN (0.13⩽ x⩽0.8) alloys grown by MOCVD

The alloy Al x Ga 1− x N was grown by metal-organic chemical vapor deposition (MOCVD) using a high-temperature AlN interlayer on a thick GaN template. The Al composition ( x) of the Al x Ga 1− x N was varied in the range 0.13⩽ x⩽0.8. The in-situ reflectance spectra indicate that the growth process of AlGaN alloys is dominated by trimethylgallium (TMGa) molar flux when the molar flux of trimethylaluminium (TMAl) is kept constant. The Al compositions and growth rates of AlGaN alloys were determined by Rutherford backscattering, which indicates that the incorporation efficiency of TMAl is improved remarkably by decreasing the TMGa molar flux. The crystalline quality of these AlGaN alloys is evaluated by measuring the symmetric (0 0 2) and asymmetric (1 0 2) ω-scan X-ray diffraction peak widths. The best crystalline quality, among these Al x Ga 1− x N alloys, is for an Al composition of x=0.54 where the full-width at half-maximums of the AlGaN (0 0 2) and (1 0 2) diffraction peaks are 265 and 797 arcsec, respectively. This conclusion is consistent with the surface morphology of the AlGaN alloys probed by atomic force microscopy. Room temperature cathodoluminescence spectra show pronounced near band edge emission from these AlGaN alloys. The optical band gaps ( E g) are found to deviate from linear interpolation between E g GaN and E g AlN with a bowing parameter b=0.89.

Influence of AlN thickness on strain evolution of GaN layer grown on high-temperature AlN interlayer

The strain evolution of a GaN layer grown on a high-temperature AlN interlayer with varying AlN thickness by metalorganic chemical vapour deposition is investigated. In the growth process, the growth strain changes from compression to tension in the top GaN layer, and the thickness at which the compressive-to-tensile strain transition takes place is strongly influenced by the thickness of the AlN interlayer. It is confirmed from the x-ray diffraction results that the AlN interlayer has a remarkable effect on introducing relative compressive strain to the top GaN layer. The strain transition process during the growth of the top GaN layer can be explained by the threading dislocation inclination in the top GaN layer. Adjusting the AlN interlayer thickness could change the density of the threading dislocations in the top GaN layer and then change the stress evolution during the top GaN layer's growth.

Structural, morphological, and optical properties of AlGaN/GaN heterostructures with AlN buffer and interlayer

Al x Ga 1 − x N ∕ GaN ( x ∼ 0.3 ) heterostructures with and without a high-temperature (HT) AlN interlayer (IL) have been grown on sapphire (Al2O3) substrates and AlN buffer/Al2O3 templates by metal organic chemical vapor deposition. The effects of an AlN buffer layer (BL) grown on an Al2O3 substrate and an AlN IL grown under the AlGaN ternary layer (TL) on structural, morphological, and optical properties of the heterostructures have been investigated by high-resolution x-ray diffraction, spectroscopic ellipsometry, atomic force microscopy, and photoluminescence measurements. The AlN BL improves the crystal quality of the AlGaN TL. Further improvement is achieved by inserting an AlN IL between GaN BL and AlGaN TL. However, experimental results also show that a HT AlN IL leads to relatively rough surfaces on AlGaN TLs, and an AlN IL changes the strain in the AlGaN TL from tensile to compressive type. In addition, an AlN BL improves the top surface quality of heterostructures.

Spatial distribution of deep level defects in crack-free AlGaN grown on GaN with a high-temperature AlN interlayer

The deep level luminescence of crack-free Al0.25Ga0.75N layers grown on a GaN template with a high-temperature grown AlN interlayer has been studied using spatially resolved cathodoluminescence (CL) spectroscopy. The CL spectra of Al0.25Ga0.75N grown on a thin AlN interlayer present a deep level aquamarine luminescence (DLAL) band at about 2.6eV and a deep level violet luminescence (DLVL) band at about 3.17eV. Cross-section line scan CL measurements on a cleaved sample edge clearly reveal different distributions of DLAL-related and DLVL-related defects in AlGaN along the growth direction. The DLAL band of AlGaN is attributed to evolve from the yellow luminescence band of GaN, and therefore has an analogous origin of a radiative transition between a shallow donor and a deep acceptor. The DLVL band is correlated with defects distributed near the GaN∕AlN∕AlGaN interfaces. Additionally, the lateral distribution of the intensity of the DLAL band shows a domainlike feature which is accompanied by a lateral phase separation of Al composition. Such a distribution of deep level defects is probably caused by the strain field within the domains.

Evolution of threading dislocations in MOCVD-grown GaN films on (1 1 1) Si substrates

We have quantitatively compared the evolution of threading dislocations (TDs) in GaN films grown on (1 1 1) Si substrates using different buffer/interlayer structures: a compositionally graded Al x Ga 1− x N (0⩽ x⩽1) buffer layer, a thin high-temperature (HT) AlN interlayer (IL), and an AlN/GaN/Al x Ga 1− x N multilayer. Plan-view transmission electron microscopy (TEM) shows a reduction in TD density in GaN films grown on graded Al x Ga 1− x N buffer layers, and an increase in TD density in GaN films grown on HT AlN ILs, in comparison with those grown directly on an AlN buffer layer. Cross-sectional TEM reveals bending and annihilation of TDs within the graded Al x Ga 1− x N buffer layer, which lead to a decrease of TD density in the overgrown GaN films. On the other hand, a high density of TDs forms at the GaN/AlN IL interface, resulting in an increase in TD density in the GaN film. In addition, growing a thin AlN+GaN bilayer before growing the compositionally graded Al x Ga 1− x N buffer layer significantly reduces the TD density in the Al x Ga 1− x N buffer layer, which subsequently further reduces the TD density in the overgrown GaN film.

Strain evolution in GaN layers grown on high-temperature AlN interlayers

The strain evolution of the GaN layer grown on a high-temperature AlN interlayer with GaN template by metal organic chemical vapor deposition is investigated. It is found that the layer is initially under compressive strain and then gradually relaxes and transforms to under tensile strain with increasing film thickness. The result of the in situ stress analysis is confirmed by x-ray diffraction measurements. Transmission electron microscopy analysis shows that the inclination of edge and mixed threading dislocations rather than the reduction of dislocation density mainly accounts for such a strain evolution.