Thick AlN Layers Research Articles

Fabrication of vertical Schottky barrier diodes on n-type freestanding AlN substrates grown by hydride vapor phase epitaxy

Thick Si-doped AlN layers were homoepitaxially grown by hydride vapor phase epitaxy on AlN(0001) seed substrates. Following the removal of the seed substrate, an n-type AlN substrate with a carrier concentration of 2.4 × 1014 cm−3 was obtained. Vertical Schottky barrier diodes were fabricated by depositing Ni/Au Schottky contacts on the N-polar surface of the substrate. High rectification with a turn-on voltage of approximately 2.2 V was observed. The ideality factor of the diode at room temperature was estimated to be ∼8. The reverse breakdown voltage, defined as the leakage current level of 10−3 A/cm2, ranged from 550 to 770 V.

Demonstration of isotype GaN/AlN/GaN heterobarrier diodes by NH3-molecular beam epitaxy

The results of vertical transport through nitride heterobarrier structures grown by ammonia molecular beam epitaxy are presented. Structures are designed with binary layers to avoid the effects of random alloy fluctuations in ternary nitride barriers. The unintentional incorporation of Ga in the AlN growth is investigated by atom probe tomography and is shown to be strongly dependent on both the NH3 flowrate and substrate temperature growth parameters. Once nominally pure AlN layer growth conditions are achieved, structures consisting of unintentionally doped (UID) GaN spacer layers adjacent to a nominally pure AlN are grown between two layers of n+ GaN, from which isotype diodes are fabricated. Varying the design parameters of AlN layer thickness, UID spacer layer thickness, and threading dislocation density show marked effects on the vertical transport characteristics of these structures. The lack of significant temperature dependence, coupled with Fowler-Nordheim and/or Milliken-Lauritsen analysis, point to a prevalently tunneling field emission mechanism through the AlN barrier. Once flatband conditions in the UID layer are achieved, electrons leave the barrier with significant energy. This transport mechanism is of great interest for applications in hot electron structures.

Oxidation behavior of magnetron sputtered Mo2N/AlN multilayer coatings during heat treatment

The Mo2N/AlN multilayer coatings with AlN layer thickness in the range of 0.5nm to 3.7nm were deposited by dc magnetron sputtering. The Mo2N/AlN multilayer coatings exhibited columnar growth structure. As the AlN layers’ thickness exceeds 1.2nm, AlN layers changed from fcc structure to amorphous structure, which breaks off the crystallization integrity of multilayer coatings. The coatings oxidized at 400°C, 550°C, 700°C in air for 1h were investigated by X-ray diffractions (XRD), X-ray photoelectron spectroscopy (XPS) and Scanning electron microscopy (SEM). It was found that with the increase of AlN layer thickness, the oxidation resistance of Mo2N/AlN multilayer coatings was improved by reducing the density of columnar grain boundaries and forming dense Al2O3 oxide layer on the surface. The addition of AlN to Mo2N coatings also leads to the lower coefficient of friction (0.36–0.42) of Mo2N/AlN multilayer coatings than that (0.62) of Mo2N coatings.

Compatibility of the selective area growth of GaN nanowires on AlN-buffered Si substrates with the operation of light emitting diodes

AlN layers with thicknesses between 2 and 14 nm were grown on Si(111) substrates by molecular beam epitaxy. The effect of the AlN layer thickness on the morphology and nucleation time of spontaneously formed GaN nanowires (NWs) was investigated by scanning electron microscopy and line-of-sight quadrupole mass spectrometry, respectively. We observed that the alignment of the NWs grown on these layers improves with increasing layer thickness while their nucleation time decreases. Our results show that 4 nm is the smallest thickness of the AlN layer that allows the growth of well-aligned NWs with short nucleation time. Such an AlN buffer layer was successfully employed, together with a patterned SiOx mask, for the selective-area growth (SAG) of vertical GaN NWs. In addition, we fabricated light-emitting diodes (LEDs) from NW ensembles that were grown by means of self-organization phenomena on bare and on AlN-buffered Si substrates. A careful characterization of the optoelectronic properties of the two devices showed that the performance of NW-LEDs on bare and AlN-buffered Si is similar. Electrical conduction across the AlN buffer is facilitated by a high number of grain boundaries that were revealed by transmission electron microscopy. These results demonstrate that grainy AlN buffer layers on Si are compatible both with the SAG of GaN NWs and LED operation. Therefore, this study is a first step towards the fabrication of LEDs on Si substrates based on homogeneous NW ensembles.

Investigation, characterization and effect of substrate position on thick AlN layers grown by high temperature chemical vapor deposition

Thick AlN layers are grown on c-sapphire at 1,300 °C in turbulent, laminar and 45° oblique laminar injection respectively by a homebuilt hot-wall high temperature chemical vapor deposition system (HTCVD) and characterized by SEM, XRD, Raman and optical absorption spectra. The results demonstrate that high quality and 20 μm thick (002) AlN epilayer with full width at half maximum of 1,010 arcsec is grown in 45° oblique laminar injection at growth rate of 24 μm/h, while 28 μm thick (002) AlN epilayer and 46 μm thick (002)-oriented columnar AlN layer with rough surface and larger misorientation are grown in laminar and turbulent injection at growth rate of 34 and 55 μm/h respectively. Meanwhile, the epilayer grown in 45° oblique laminar injection exhibits sharper absorption edge and larger optical band gap of 5.93 eV than that of other two layers. Furthermore, the proposed model suggests that column growth mode governs the growth in turbulent injection with high source concentrations, resulting in (002)-oriented columnar layer with large misorientation. In contrast, Stranski–Krastanov mode governs the growth in laminar injection and 45° oblique laminar injection with decreased source concentrations, leading to nonuniform and uniform (002) epilayer, respectively. Therefore, the HTCVD with 45° oblique laminar injection can be used to grow high quality thick (002) AlN epilayer at high growth rate and low cost.

Single phase (112¯2) AlN grown on (101¯0) sapphire by metalorganic vapour phase epitaxy

Heteroepitaxial growth of AlN buffer layers directly on (101¯0) sapphire substrates by metalorganic vapour phase epitaxy has been investigated. A single-step growth procedure without a sapphire nitridation was employed resulting in mirror-like crack free ≈1.1–1.6μm thick AlN layers of single phase (112¯2) orientation. Trimethylaluminum pre-dose time and reactor pressure were optimized for surface roughness and crystal quality. The crystal quality was found to degrade with increasing pre-dose time and also reactor pressure. The smallest full width at half maximum value for on-axis X-ray rocking curve of the (112¯2) AlN layers was about 610arcsec and 1480arcsec along [1¯1¯23]AlN and [11¯00]AlN, respectively. The surface roughness, measured by atomic force microscopy using a 10×10μm2 area, was in the range 2.6–3.5nm. A basal stacking fault density of (7±1)×105cm−1 was estimated by transmission electron microscopy.

Defect analysis in AlGaN layers on AlN templates obtained by epitaxial lateral overgrowth

The defect distribution in thick AlN layers obtained by epitaxial lateral overgrowth (ELO-AlN) has been analyzed as a function of the miscut direction of the patterned sapphire substrate. A 0.25° miscut toward the sapphire a-plane leads to formation of smooth ELO-AlN layers containing vertical coalescence grain boundaries and exhibiting an almost homogeneous threading dislocation (TD) distribution with a TD density ranging from 5×108cm−2 to 8×108cm−2. In contrast, a 0.25° miscut toward the sapphire m-plane results in formation of periodically arranged macrosteps on the surface of the coalesced ELO-AlN layers as well as formation of inclined coalescence grain boundaries leading to an inhomogeneous TD distribution. A subsequent AlxGa1−xN deposition onto ELO-AlN template with surface macrosteps leads to Ga enrichment on the step sidewalls and, for lower Al-contents (e.g. x=0.5), even to additional defect formation. For higher Al contents (e.g. x=0.8) no additional threading dislocations are formed in the AlGaN layers and the observed TD density corresponds to that of the ELO-AlN template: less than 108cm−2 in the wing regions and from 6×108cm−2 to 9×108cm−2 above the ridges. Compressive strain during growth of Al0.8Ga0.2N on ELO-AlN tends to be compensated by threading dislocation inclination. However, due to the low TD densities the inclination angles are more than 3 times larger than those observed in Al0.8Ga0.2N layers on planar AlN/sapphire templates.

Simulation Study of AlN Spacer Layer Thickness on AlGaN/GaN HEMT

High electron mobility transistor (HEMT)Two-dimensional electron gas (2DEG) formed at AlGaN/GaN interface is a critical part to tune the characteristic of AlGaN/GaN HEMT devices. Introduction of AlN spacer layer in between AlGaN and GaN layer is one of the way to improve 2DEG density, mobility, and drain current. Carrier concentration, mobility and conduction band offset for different spacer layer thickness was simulated by using Silvaco simulation tool. Our device simulations showed that carrier concentration, mobility are enhance on introduction of AlN spacer layer in HEMT. In addition, carrier properties of HEMT also depend on thickness of spacer layer. Our simulation showed that the mobility of 2DEG attains its maximum value at the 0.5 nm thick AlN layer but carrier concentration increases with spacer thickness. Finally, drain current increases with increasing spacer layer thickness and reach maximum value at 1.2nm thick spacer layer.The Himalayan Physics Vol. 4, No. 4, 2013 Page: 14-17 Uploaded date: 12/22/2013

Cathodoluminescence and TEM investigations of structural and optical properties of AlGaN on epitaxial laterally overgrown AlN/sapphire templates

Surface steps as high as 15 nm on up to 10 μm thick AlN layers grown on patterned AlN/sapphire templates play a major role for the structural and optical properties of AlxGa1−xN layers with x ≥ 0.5 grown subsequently by metalorganic vapour phase epitaxy. The higher the Ga content in these layers is, the stronger is the influence of the surface morphology on their properties. For x = 0.5 not only periodic inhomogeneities in the Al content due to growth of Ga-rich facets are observed by cathodoluminescence, but these facets give rise to additional dislocation formation as discovered by annular dark-field scanning transmission electron microscopy. For AlxGa1−xN layers with x = 0.8 the difference in Al content between facets and surrounding material is much smaller. Therefore, the threading dislocation density (TDD) is only defined by the TDD in the underlying epitaxially laterally overgrown (ELO) AlN layer. This way high quality Al0.8Ga0.2N with a thickness up to 1.5 μm and a TDD ≤ 5x108 cm−2 was obtained.

Epitaxial growth of AlN on c-plane sapphire by High Temperature Hydride Vapor Phase Epitaxy: Influence of the gas phase N/Al ratio and low temperature protective layer

AlN is epitaxially grown on c-plane sapphire by High Temperature Hydride Vapor Phase Epitaxy (HT-HVPE) at constant growth rate and thickness, while varying the N/Al ratio in the gas phase at 1500°C. The influence of an additional low temperature (1200°C) protective layer on AlN crystal quality is also assessed. The experiments and thermodynamic calculations show that the sapphire substrate is unstable at high temperature under hydrogen and ammonia while it is stable at low temperature or under a few hundred nanometers of AlN protective layer even at high temperature. In terms of AlN crystal quality, the optimal process developed here consists in depositing a 170nm low temperature protective AlN layer with N/Al=3 followed by a high temperature thick AlN layer grown with N/Al=1.5. In this case, the interface between AlN and sapphire remains continuous (no etching) and the stress in the grown layer at room temperature is minimized by a balance of the growing tensile stress with the cooling compressive stress.

The AlN layer thickness dependent coherent epitaxial growth, stress and hardness in NbN/AlN nanostructured multilayer films

NbN/AlN nano-multilayer films with AlN layer thickness (lAlN) ranging from 2.2 to 12.2nm have been deposited on Si(100) substrate by reactive magnetron sputtering in Ar/N2 mixtures. The lAlN dependent structural and mechanical properties for resulting NbN/AlN multilayers have been evaluated by means of low-angle X-ray reflectivity, X-ray diffraction, transmission electron microscope, pole figure measurements and nanoindentation tests. The finding is that at small lAlN both hexagonal wurtzite-AlN(0002) and face-centered cubic (fcc) AlN(111) are coherent with the fcc NbN(111), and the increase of lAlN can promote coherent growth of fcc-NbN(111)/w-AlN(0002) due to minimization of total energy and formation of a strong NbN(111)/AlN(0002) fiber texture. The hardness of all NbN/AlN multilayers lies in between 32.7 and 37.5GPa with increasing lAlN from 2.2 to 12.2nm, which is obviously higher than that calculated by using a simple rule of mixture, showing that the remarkable hardness enhancement implements at a wide range of lAlN from 2.2 to 12.2nm for NbN/AlN multilayer system. The high-hardness value in a wide range of lAlN can be mainly attributed to Koehler mechanism and structural barriers (fcc/hexagonal) to dislocation motion between NbN and AlN layers.

Influence of CrN and AlN layer thicknesses on structure and mechanical properties of CrN/AlN superlattices

Based on their outstanding of chemical, physical and mechanical properties, CrN/AlN multilayer coatings are the topic of many research activities. Our results for CrN/AlN superlattice coatings with AlN layer thicknesses of 1, 2 and 3nm combined with CrN layer thicknesses ranging from 1 to 10nm suggest that the CrN layer thicknesses need to be at least as thick as the targeted AlN layer thicknesses to provide sufficient strength for a fully stabilization of the AlN layers in their metastable cubic structure by epitaxial strain. Otherwise, the AlN layers tend to crystallize in their stable hexagonal (wurtzite type) structure, leading to the loss of coherency between alternating CrN and AlN layers. Hardness maxima of 31.0GPa are obtained at a bilayer period Λ of 3 and 5.5nm, for the fully cubic superlattice coatings composed of 1- and 2-nm-thin AlN layers, respectively. Due to the reduced number of interfaces per coating thickness and hence less obstacles for dislocations, only a hardness maximum of ~28.5GPa at Λ=6.3nm can be obtained for the coatings composed of 3-nm-thin AlN layers.Our study clearly demonstrates that the structural and mechanical properties of superlattice CrN/AlN coatings not just depend on the bilayer period but on the individual layer thicknesses.

Improved injection efficiency in 290 nm light emitting diodes with Al(Ga)N electron blocking heterostructure

The effect of different Al(Ga)N electron blocking heterostructures (EBH) on the emission spectra and light output power of 290 nm light emitting diodes (LEDs) has been investigated. The carrier injection and internal quantum efficiency of the LEDs is simulated and compared to electroluminescence measurements. The highest light output power has been found for LEDs with an Mg-doped AlN/Al0.7Ga0.3N EBH with an AlN layer thickness >3 nm. The output power of these LEDs was 8.5-times higher compared to LEDs without EBH. This effect is attributed to an improved carrier injection and confinement which prevents electron leakage into the p-doped region of the LED with a simultaneously enhanced hole injection into the active region.

Impact of AlN Spacer on Electron Mobility of AlGaN/AlN/GaN Structures on Silicon

The impact of the thickness of an AlN spacer in AlGaN/AlN/GaN high electron mobility transistor (HEMT) structures on the Hall mobility was investigated in a range of 30 K - 340 K. The AlN spacer has a strong impact on the mobility at temperatures below 150 K. This effect is linked to a reduction of alloy scattering. Optical and scanning electron microscopy revealed hexagonal shaped defects which also have an effect on the mobility. These defects can be avoided by an appropriate adjustment of the AlN layer thickness.

EXAFS study of GaN/AlN multiple quantum wells grown by ammonia MBE

AbstractExtended X‐ray absorption fine structure above the Ga–K edge has been used to study the local structure of heterointerfaces in GaN/AlN multiple quantum wells (MQWs) grown by ammonia molecular beam epitaxy. The thickness of AlN and GaN layers in MQWs were evaluated using transmission electron microscopy, X‐ray diffraction and Raman spectroscopy. Two sets of MQWs structures with the number of period about of 20 and more than 100 were studied. The Ga‐Ga interatomic distance in MQWs is lower than that in bulk GaN due to elastic compression of GaN layers in MQWs. This effect is less pronounced in thick structures because of almost full relaxation of GaN layers. The Ga‐Al interatomic distance in thick MQWs is too long, about 0.1 Å, longer than typical values for AlGaN alloys. Then number of Ga cations in the second coordination shell of Ga atoms is lower than that predicted for abrupt heterointerfaces that evidences intermixing of heterointerfaces. The intermixing degree depends on the total thickness of MQWs (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High precision pressure sensors based on SAW devices in the GHz range

In this paper, an AlN/free-standing nanocrystalline diamond (NCD) system is proposed in order to process high frequency surface acoustic wave (SAW) resonators for sensing applications. The main problem of synthetic diamond is its high surface roughness that worsens the sputtered AlN quality and hence the device response. In order to study the feasibility of this structure, AlN films from 150nm up to 1200nm thick have been deposited on free-standing NCD. We have then analysed the influence of the AlN layer thickness on its crystal quality and device response. Optimized thin films of 300nm have been used to fabricate of one-port SAW resonators operating in the 10–14GHz frequency range. A SAW based sensor pressure with a sensibility of 0.33MHz/bar has been fabricated.

Electrically active defects at AlN/Si interface studied by DLTS and ESR

AbstractA combined deep‐level transient spectroscopy (DLTS) and electron spin resonance (ESR) study is performed to identify the electrically active defects at the AlN/Si (111) interface. It is shown that the density of deep‐level states not only depends on the thermal budget of the epitaxial deposition but also on the strain built up during growth and upon cooling to room temperature (RT). At the same time, diffusion of Si into the 200 nm thick AlN layer produces a thin crystalline Si3N4 interfacial layer, identified by transmission electron microscopy (TEM). This gives rise to so‐called dangling bond Pb centres at the Si3N4/Si (111) interface. In addition, a strong evolution of the electrically active defect clusters in the silicon substrate close to the interface has been observed both in DLTS and ESR.

Layer-by-layer thermal conductivities of the Group III nitride films in blue/green light emitting diodes

Thermal conductivities (k) of the individual layers of a GaN-based light emitting diode (LED) were measured along [0001] using the 3-omega method from 100-400 K. Base layers of AlN, GaN, and InGaN, grown by organometallic vapor phase epitaxy on SiC, have effective k much lower than bulk values. The 100 nm thick AlN layer has k = 0.93 ± 0.16 W/mK at 300 K, which is suppressed >100 times relative to bulk AlN. Transmission electron microscope images revealed high dislocation densities (4 × 1010 cm−2) within AlN and a severely defective AlN-SiC interface that cause additional phonon scattering. Resultant thermal resistances degrade LED performance and lifetime making layer-by-layer k, a critical design metric for LEDs.

Thin-film heteroepitaxy by the formation of the dilatation dipole ensemble

It is shown that in substantially anisotropic media, such as a crystal with cubic lattice symmetry, the analogous dilatation centers can strongly attract each other considerably decreasing the total elastic energy. Such attractive centers form stable objects of a new type, the elastic dilatation dipoles. By the example of heteroepitaxy of the film of silicon carbide SiC on a silicon substrate, the calculations of elastic energy of the system are performed. It is shown that the elastic energy can relax completely only due to the ensemble of dilatation dipoles. A new growth method of SiC on Si is suggested and implemented experimentally. In this method, the elastic energy relaxes due to the ensemble of elastic dilatation dipoles. It is shown experimentally that the heteroepitaxial SiC films grown do not contain cracks and lattice-misfit dislocations despite the tremendous difference in lattice parameters. Thick low-defect GaN and AlN layers are grown on the SiC/Si templates obtained. The laboratory model of the operating light-emitting diode is originally obtained based on these structures.

Role of surface trap states on two-dimensional electron gas density in InAlN/AlN/GaN heterostructures

In order to clarify the effect of charged dislocations and surface donor states on the transport mechanisms in polar AlInN/AlN/GaN heterostructures, we have studied the current-voltage characteristics of Schottky junctions fabricated on AlInN/AlN/GaN heterostructures. The reverse-bias leakage current behaviour has been interpreted with a Poole-Frenkel emission of electrons from trap states near the metal-semiconductor junction to dislocation induced states. The variation of the Schottky barrier height as a function of the AlN layer thickness has been measured and discussed, considering the role of the surface states in the formation of the two dimensional electron gas at AlN/GaN interface.