InAlN Layers Research Articles

Effect of Growth Temperature on Structural Quality of InAlN Layer Lattice Matched to GaN Grown by Metal Organic Chemical Vapor Deposition

Effect of Growth Temperature on Structural Quality of InAlN Layer Lattice Matched to GaN Grown by Metal Organic Chemical Vapor Deposition

Effect of Growth Temperature on Structural Quality of InAlN Layer Lattice Matched to GaN Grown by Metal Organic Chemical Vapor Deposition

The authors report the effect of growth temperature on the structural quality of an InAlN layer lattice matched to GaN. InAlN/GaN structures were grown on a sapphire substrate using a low-temperature-grown GaN buffer layer. The X-ray diffraction and Atomic force microscopy results indicate that the quality of the InAlN layer is strongly affected by the three-dimensionally grown surface morphology, depending on the growth temperature. The lattice matching of InAlN to GaN with a flat surface and a smooth interface are obtained at temperatures of 790–800 °C. The Schottky diode fabricated on InAlN grown at 800 °C shows high-quality characteristics with the leakage current as low as 2.3 ×10-9 A at -5 V, corresponding to a current density of 1.2 ×10-6 A/cm2 and 6.2 ×10-8 A at -10 V.

Optimization of AlGaN‐based spacer layer for InAlN/GaN interfaces

AbstractAlGaN‐based spacer layers for lattice‐matched and nearly lattice matched InAlN/GaN interfaces were examined in Al2O3/InAlN/AlGaN/AlN/GaN structures. An Al2O3 overlayer was deposited to investigate the characteristics under positive bias by capacitance‐voltage (C‐V) measurement. The C‐V characteristic for a sample with an Al0.38Ga0.62N/AlN double spacer layer indicated unfavorable electron accumulation at the InAlN/AlGaN interface inside the barrier under positive bias. To suppress the unfavorable accumulation, attempts were made to increase the Al molar fraction of the AlGaN layer to reduce the conduction band discontinuity and interface charge at InAlN/AlGaN interface. An Al0.44Ga0.56N/AlN double spacer layer and an Al0.44Ga0.56N single spacer layer of almost the same total thickness were investigated. Although both spacer layers result in normal C‐V characteristics without the indication of unfavorable electron accumulation, the InAlN layer on a 1.5‐nm‐thick Al0.44Ga0.56N single spacer layer exhibited superior surface morphology without deteriorating the mobility of the two‐dimensional electron gas despite the absence of the AlN layer. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

(V)EELS Characterization of InAlN/GaN Distributed Bragg Reflectors

Ten-period InAlN/GaN distributed Bragg reflectors are examined by aberration corrected scanning transmission electron microscopy and by valence electron energy-loss spectroscopy (VEELS) with sub-nanometric spatial resolution and sub-eV energy dispersion. Deconvolution and peak subtraction methods, implemented in Matlab routines, are applied to the low loss region of the obtained VEEL spectra to retrieve information about the band gap energy and chemical composition, whereas a Kramers-Kronig transformation is used to retrieve the complex dielectric function of the examined material. The VEEL measurements reveal significant compositional variations in InAlN layers and show a ∼2nm thick InAlN layer with high indium content at each GaN/InAlN interface.

High quality InAlN single layers lattice-matched to GaN grown by molecular beam epitaxy

We report on properties of high quality ∼60 nm thick InAlN layers nearly in-plane lattice-matched to GaN, grown on c-plane GaN-on-sapphire templates by plasma-assisted molecular beam epitaxy. Excellent crystalline quality and low surface roughness are confirmed by X-ray diffraction, transmission electron microscopy, and atomic force microscopy. High annular dark field observations reveal a periodic in-plane indium content variation (8 nm period), whereas optical measurements evidence certain residual absorption below the band-gap. The indium fluctuation is estimated to be ± 1.2% around the nominal 17% indium content via plasmon energy oscillations assessed by electron energy loss spectroscopy with sub-nanometric spatial resolution.

Growth and characterization of InAlN layers nearly lattice‐matched to GaN

AbstractA set of InxAl1‐xN films lattice‐matched (LM) to GaN/sapphire substrates were grown by molecular beam epitaxy (MBE) and studied using X‐ray diffraction and transmission electron microscopy with the aim of implementing barrier and channels in high electron mobility transistors (HEMTs). Although all InAlN epilayers grow pseudomorphic to GaN, two sublayers with different compositions formed when a direct deposition onto the bare GaN buffer was carried out. On the other hand, heterostructures having single‐layered In∼0.18Al∼0.82N are achieved when a spacer consisting of an AlN interlayer or an AlN/GaN/AlN stack is placed between the InAlN and the buffer. These spacers not only yield a better compositional and structural homogeneity of the InAlN, but also improve electrical properties with respect to HEMT applications. Compared to one single AlN interlayer, the use of a triple AlN/GaN/AlN multilayer further improves the structural quality of the InAlN film (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Investigation of polarization‐induced electric field in ultrathin InAlN layers on GaN by X‐ray photoelectron spectroscopy

AbstractThe polarization‐induced electric field in ultrathin InxAl1‐xN (0.17 ≤ x ≤ 0.30) layers on GaN was investigated by using X‐ray photoelectron spectroscopy (XPS). The core‐level energy position, ECL, and the full width at half maximum (FWHM) of the Al2p, In4d, and In3d spectra from 2.5‐nm‐thick InAlN layers increased with the increase in the photoelectron exit angle (elevation angle). These increases were well reproduced with numerical calculations assuming polarization‐induced internal fields combined with surface Fermi level pinning. The magnitudes of the internal field decreased as the In molar fraction increased. The Ga3d spectra from the host GaN layers markedly shifted by 530 meV depending on the molar fraction of InAlN layers, which was independent of the exit angle. This indicated that the Fermi level was unpinned at the interfaces, or GaN surfaces, and shifted due to the potential drop in the InAlN layers. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Measurement of valence-band offsets of InAlN/GaN heterostructures grown by metal-organic vapor phase epitaxy

The valence band offsets, ΔEV, of In0.17Al0.83N/GaN, In0.25Al0.75N/GaN, and In0.30Al0.70N/GaN heterostructures grown by metal-organic vapor phase epitaxy were evaluated by using x-ray photoelectron spectroscopy (XPS). The dependence of the energy position and the full width at half maximum of the Al 2p spectrum on the exit angle indicated that there was sharp band bending caused by the polarization-induced electric field combined with surface Fermi-level pinning in each ultrathin InAlN layer. The ΔEV values evaluated without taking into account band bending indicated large discrepancies from the theoretical estimates for all samples. Erroneous results due to band bending were corrected by applying numerical calculations, which led to acceptable results. The evaluated ΔEV values were 0.2±0.2 eV for In0.17Al0.83N/GaN, 0.1±0.2 eV for In0.25Al0.75N/GaN, and 0.0±0.2 eV for In0.30Al0.70N/GaN. Despite the large decrease of around 1.0 eV in the band gap of InAlN layers according to the increase in the In molar fraction, the decrease in ΔEV was as small as 0.2 eV. Therefore, the change in band-gap discontinuity was mainly distributed to that in conduction band offset.

High electron mobility lattice-matched InAlN/GaN materials

InAlN can be in-plane lattice matched (LM) to GaN, and the formed InAlN/GaN heterostructure is one kind of materials with high conductivity to be used in GaN-based high electron mobility transistors (HEMTs). It is reported that the high-mobility InAlN/GaN material is grown by using pulsed metal organic chemical vapor deposition (PMOCVD) on sapphire, and the Hall electron mobility reaches 949 and 2032 cm2/Vs at room temperature and 77 K, respectively. The two-dimensional electron gas (2DEG) is formed in the sample. When 1.2 nm thick AlN space layer is inserted to form InAlN/AlN/GaN structure, the Hall electron mobility increases to 1437 and 5308 cm2/Vs at room temperature and 77 K, respectively. It is shown by analyzing the results of X-ray diffraction and atomic force microscopy and the features of PMOCVD that the crystal quality of InAlN/GaN material is quite high, and the InAlN layer LM to GaN has smooth surface and interface. The high mobility characteristics of InAlN/GaN and InAlN/AlN/GaN materials are ascribed to the fact that the 2DEG has a comparatively low sheet density (1.61013-1.81013 cm-2), the alloy disorder scattering is weakened in the high-quality InAlN crystal since its compositions are evenly distributed, and the interface roughness scattering is alleviated at the smooth interface where the 2DEG is located.

High quality InAlN/GaN heterostructures grown on sapphire by pulsed metal organic chemical vapor deposition

High quality InAlN/GaN heterostructures are successfully grown on the (0 0 0 1) sapphire substrate by pulsed metal organic chemical vapor deposition. The InAlN barrier layer with an indium composition of 17% is observed to be nearly lattice matched to GaN layer, and a smooth surface morphology can be obtained with root mean square roughness of 0.3 nm and without indium droplets and phase separation. The 50 mm InAlN/GaN heterostructure wafer exhibits a mobility of 1402 cm 2/V s with a sheet carrier density of 2.01×10 13 cm −2, and a low average sheet resistance of 234 Ω/cm 2 with a sheet resistance nonuniformity of 1.22%. Compared with the conventional continual growth method, PMOCVD reduces the growth temperature of the InAlN layer and the Al related prereaction in the gas phase, and consequently enhances the surface migration, and improves the crystallization quality. Furthermore, indium concentration of InAlN layer can be controlled by adjusting the pulse time ratio of TMIn to TMAl in a unit cycle, the growth temperature and pressure, as well as the InAlN layer thickness by the number of unit cycle repeats.

Indium incorporation into InGaN and InAlN layers grown by metalorganic vapor phase epitaxy

Experimental data on indium incorporation in InGaN and InAlN layers grown by metalorganic chemical vapor epitaxy (MOVPE) on bulk GaN substrates are presented and discussed. For the step-flow growth mode, realized for InGaN layers grown at relatively high temperatures (around 800°C), incorporation of indium increases with growth rate, and similarly, with a decrease in GaN substrate misorientation. Both dependences are explained by a higher velocity of flowing steps incorporating the indium atoms. For InAlN layers, three-dimensional nucleation takes place, and thus no significant changes of indium incorporation versus either growth rate or GaN substrate misorientation were observed.

Formation of composite InGaN/GaN/InAlN quantum dots

Composite InGaN/GaN/InAlN quantum dots (QDs) have been formed and studied. The structural properties of thin InAlN layers overgrown with GaN have been analyzed, and it is shown that 3D islands with lateral sizes of ∼(20–30) nm are formed in structures of this kind. It is demonstrated that deposition of a thin InGaN layer onto the surface of InAlN islands overgrown with a thin GaN layer leads to transformation of the continuous InGaN layer to an array of isolated QDs with lateral sizes of 20–30 nm and heights of 2–3 nm. The position of these QDs in the growth direction correlates with that of InAlN islands.

Structural and optical properties of InAlN/GaN distributed Bragg reflectors

Structural and optical properties of InAlN/GaN distributed Bragg reflectors grown by MOCVD on sapphire substrates are studied. The influence of growth conditions and thicknesses of the InAlN layers on structural properties of distributed Bragg reflectors is studied. It is shown that optimization of the conditions of epitaxial growth makes it possible to design InAlN/GaN distributed Bragg reflectors with a reflectance of more than 99% and a reflection maximum in the wavelength range of 460-610 nm.

Metal-related gate sinking due to interfacial oxygen layer in Ir/InAlN high electron mobility transistors

We report on an annealing-induced “gate sinking” effect in a 2-nm-thin In0.17Al0.83N/AlN barrier high electron mobility transistor with Ir gate. Investigations by transmission electron microscopy linked the effect to an oxygen containing interlayer between the gate metal and the InAlN layer and revealed diffusion of oxygen into iridium during annealing. Below 700 °C the diffusion is inhomogeneous and seems to occur along grain boundaries, which is consistent with the capacitance-voltage analysis. Annealing at 700 °C increased the gate capacitance over a factor 2, shifted the threshold voltage from +0.3 to +1 V and increased the transconductance from 400 to 640 mS/mm.

Threshold voltage control of InAlN/GaN heterostructure field-effect transistors for depletion- and enhancement-mode operation

We describe a method to change the threshold voltage of heterostructure field-effect transistors (HFETs) using InxAl1−xN/GaN heterostructures by using polarization and strain modification in the InAlN barrier layer to realize enhancement-mode operation. The threshold voltage and electronic band structure of the heterostructures were calculated for different indium compositions in the InAlN layer. Band structure calculations predict the enhancement-mode operation of compressively strained InAlN/GaN HFETs with an indium composition higher than 0.25. Studies of InAlN/GaN HFETs with different In alloy compositions show that the sheet resistance increases and the carrier concentration decreases for the heterostructures with increasing indium composition due to changes in the compressive strain and polarization in the InAlN barrier layer. Fabricated HFETs show threshold voltages of −2.5, −0.75, and +0.2 V for In∼0.18Al∼0.82N/GaN, In∼0.22Al∼0.78N/GaN, and In∼0.25Al∼0.75N/GaN HFETs, respectively, corresponding to a shift from depletion-mode to enhancement-mode operation.

Photoluminescence studies of high-quality InAlN layer lattice-matched to GaN grown by metal organic chemical vapor deposition

Temperature-dependent photoluminescence (PL) measurements have been performed on high-quality InAlN layers lattice-matched (LM) to GaN with different thicknesses. It is found that the PL is consisted of two components denoted as IH (high-energy side) and IL (low-energy side), respectively. IH is attributed to exciton luminescence of bulk InAlN with linewidth comparable to those calculated under the assumption of perfect random alloy. While IL is attributed to the emission from the quantum-dotlike structure on the surface of InAlN, revealing the importance of surface effect to the investigations related to InAlN LM to GaN.

Improvement of quantum efficiency by employing active-layer-friendly lattice-matched InAlN electron blocking layer in green light-emitting diodes

Improvement of the internal quantum efficiency in green-light emitting diodes has been achieved using lattice-matched InAlN electron-blocking layers. Higher electroluminescence intensities have been obtained due to better electron confinement in the device active region. The device efficiency has also been found to significantly depend on the InAlN growth temperature. Optimized InAlN growth at ∼840 °C results in a lower growth rate and longer growth times than at ∼780 °C. The observed reduction in emission efficiency for InAlN layers grown at higher temperatures is possibly attributed to thermal damage in the green active region.

Growth and characterization of In-polar and N-polar InAlN by metal organic chemical vapor deposition

In x Al 1 − x N layers, with 0.09≤x≤0.23, were grown on GaN on both the In-polar and N-polar orientations by metal organic chemical vapor deposition. The impact of growth conditions, including temperature and the group-III flow rates, on the surface morphology and indium mole fraction was investigated. In-polar layers had a smooth surface morphology characterized by mounds which decreased in size with increasing supersaturation during growth. Smooth N-polar InAlN was achieved through the use of vicinal sapphire substrates with misorientation angles of 3°–5° toward ⟨101¯0⟩GaN, and a trend of an increase in step bunching with decreasing supersaturation was observed for N-polar InAlN layers. The indium incorporation increased with decreasing growth temperature and increasing growth rate for both In-polar and N-polar layers. The indium incorporation was similar for both orientations on samples which were coloaded in our reactor.

Optical and structural properties of Eu-implanted InxAl1−xN

Off-axis implantation of 80 keV Eu ions into epitaxial c-plane InAlN/GaN bilayers confines rare-earth (RE) doping largely to the InAlN layer. Rutherford backscattering spectrometry and x-ray diffraction show good correlations between the Eu3+ emission linewidth and key structural parameters of InxAl1−xN films on GaN in the composition range near lattice matching (x∼0.17). In contrast to GaN:Eu, selectively excited photoluminescence (PL) and PL excitation spectra reveal the presence of a single dominant optical center in InAlN. Eu3+ emission from In0.13Al0.87N:Eu also shows significantly less thermal quenching than GaN:Eu. InAlN films are therefore superior to GaN for RE optical doping.

Optimisation of GaN overgrowth of InAlN for DBRs

AbstractThe growth of InAlN lattice matched to GaN is a recent development with a major application in nitride distributed Bragg reflectors (DBRs). In this work we investigate three ways of capping lattice matched InAlN layers to make the GaN layers in a DBR, while ensuring that InAlN is not damaged in the process and that the top GaN surface is suitable for further growth. Growing the GaN caps in a nitrogen atmosphere at the same temperature as the InAlN does not observably affect the InAlN epilayer but the cap has a top surface with large undulations unsuitable for DBRs. Growing the GaN at 1000 °C in a hydrogen atmosphere results in good GaN surfaces without altering the bulk of the InAlN epilayer but does alter the InAlN surface, possibly by causing the desorption of In. The advantages of both methods can be combined by growing the first 10 nm of the cap at the same temperature as the InAlN and the rest at 1000 °C. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)