InAlN Layers Research Articles

Avoiding Gallium Pollution in Close‐Coupled Showerhead Reactors, Alternative Process Routes

In contrast to the previous work which solved the problem of gallium pollution using hardware modifications, herein, the changes are examined to process conditions to reduce gallium pollution in InAlN layer. Using a model of GaN decomposition followed by gallium desorption and diffusion to the showerhead, different process conditions are used to limit either the desorption or the diffusion. Reducing the GaN growth temperature gives some improvement by reducing desorption, but affects channel mobility, likely due to increased carbon incorporation. Increasing the wafer–showerhead distance also reduces the gallium pollution, this time by a factor of around 2. Finally, using AlGaN layers instead of GaN as the channel completely removes gallium pollution, even for composition as low as 5%. It is suggested that in addition to reducing desorption due to alloying, the aluminum precursor may be acting as a getter for this gallium, and so stops it getting to the showerhead. A combination of these different approaches suggests that process conditions can significantly reduce gallium pollution in close‐coupled showerhead reactors.

Growth of N-polar In-rich InAlN by metal organic chemical vapor deposition on on- and off-axis sapphire

Metal organic chemical vapor deposition is used to grow N-polar In-rich InAlN layers directly on on- and off-axis (misoriented by 4° towards a plane) c-plane sapphire substrates. During the InAlN growth, trimethylaluminum, ammonia, and the total flow was kept at 4.74 μmol/min, 3 slm and 10 slm, respectively, while trimethylindium (TMIn) flow was selected between 8.42 and 13.48 μmol/min. All samples were grown at about 706 °C however; TMIn flow of 10.95 μmol/min was also tested at the growth temperature of 686 °C. With increasing the TMIn flow, the In molar fraction increased from 0.55 to 0.69, irrespectively of the substrate miscut. For the moderate TMIn flow and In molar fraction of less than 0.63, density of screw and edge dislocations were from 0.3 to 12 ✕ 109 cm-2 and 5 to 7 ✕ 1010 cm-2, respectively, while decisively lower densities appeared on on-axis structures. On the other hand, InAlN surface RMS roughness was from 1 to 8 nm favouring the off-axis substrate and low TMIn flow. Structural and surface deterioration appeared with the highest TMIn flow of 13.48 μmol/min and In molar fraction of 0.69, which hold also for reduced temperature of the growth. Nevertheless, room temperature photoluminescence full-width at half maximum less than 270 meV appeared for all samples, with maxima between 1.9 and 1.5 eV. In-rich N-polar InAlN grown on on-axis sapphire can be used as a buffer layer in new types of heterostructure devices.

Microscopic formation mechanism of Si/Tl5Al1/TiN ohmic contact on non-recessed i-InAlN/GaN heterostructures with ultra-low resistance

In this work, we use Si/Tl5Al1/TiN for a source/drain ohmic contact to demonstrate an ultra-low contact resistance of 0.11 Ω mm (ρc = 2.62 × 10−7 Ω cm2) on non-recessed i-InAlN/GaN heterostructures. The Ti5Al1 alloy was used to suppress the out-diffusion of Al and extract N from the InAlN layer, which aided the formation of ohmic contact by improving the tunneling efficiency of electrons, as we have reported in the past work. A thin Si inter-layer combined with the Ti5Al1 alloy is proposed to further reduce contact resistance. A heavy n-type InAlN layer was obtained through doping with Si atoms to improve the tunneling transport of electrons. Furthermore, the TiN inclusions penetrated into the GaN channel because the in-diffused Si promoted the decomposition of GaN at a high annealing temperature and the in-diffused Ti reacted with GaN. These TiN inclusions provided direct contact with two-dimensional electron gas, offering an additional path for the injection of electrons into the channel. The tunneling and spike mechanism worked alternately to lower the contact resistance at different annealing temperatures (dividing at 900 °C), implying that the joint effect of tunneling and the spike mechanism was initially promoted in the formation of ohmic contact. The mechanism of this Si/Ti5Al1/TiN ohmic contact was fully understood through microscopic and thermodynamic analyses. These results shed light on the mechanism for the formation of ohmic contact in a gold-free metal stack for GaN-based HEMTs.

H2 based etching of GaN for re-grown gallium-free InAlN barriers giving very low 2DEG sheet resistance of 185 Ω/sq

We have studied two approaches to GaN surface preparation before regrowth of InAlN layers, using Cl2 or H2 based etch conditions to prepare the wafer for barrier regrowth. We have shown that the Cl2 based etching induces layer degradation and higher sheet resistance values, while an H2 based etch, or desorption, does not degrade the surface morphology and yields improved sheet resistance. With additional optimization of the InAlN layers using this approach, increasing the growth temperature to 830 °C, we have achieved excellent sheet resistance values down to 185 Ohm/sq while maintaining gallium free growth. This regrowth optimization could equally be applied to AlGaN regrowth structures for normally off GaN based transistors.

Composition Inhomogeneity in Nonpolar (101̅0) and Semipolar (202̅1) InAlN Layers Grown by Plasma-Assisted Molecular Beam Epitaxy

Composition Inhomogeneity in Nonpolar (101̅0) and Semipolar (202̅1) InAlN Layers Grown by Plasma-Assisted Molecular Beam Epitaxy

InN crystal habit, structural, electrical, and optical properties affected by sapphire substrate nitridation in N-polar InN/InAlN heterostructures

Indium nitride (InN) is a very promising direct bandgap semiconductor material for near-infrared optoelectronic devices and high speed transistors, however InN growth is much more challenging compared to other III-N semiconductors. In this study we analysed the impact of pre-growth vicinal c-plane sapphire substrate nitridation on the electrical, optical, and strucutral properties of N-polar InN/InAlN heterostructures and the InN crystal habit. While a short cool-down nitridation phase resulted in a high quality InAlN layer and low quality InN layer, it was the opposite for a long cool-down nitridation phase. A trade-off between the electrical and optical properties was observed: the short nitridation cool-down produced InN layers with a higher electron mobility of 629 cm2 V−1 s−1 and low photoluminescence (PL) emission, and the long one resulted in InN with a ∼21% lower electron mobility of 497 cm2 V−1s−1 but strong PL emission. Also, a very different InN crystal habit was observed for either case. The short nitridation cool-down was conducive to the formation of a continuous layer consisting mostly of coalesced downward-pointing hexagonal pyramids, while the long one led to a layer of upward-pointing hexagonal pyramids. The local decomposition of an AlON layer produced by sapphire nitridation and variations in the InAlN surface morphology resulting in a preferential attraction of growth species and localised change in the surface free energy contributions were most likely responsible for these observations. Our results also suggest N-polar buffer layers are likely to produce coalesced InN layers with a high electron mobility, while metal-polar buffer layers may facilitate the growth of high crystal quality pyramidal InN with a strong optical response.

Growth and Properties of N‐Polar InN/InAlN Heterostructures

N‐polar InN/InAlN heterostructure growth and performance are studied on‐ and off‐axis sapphire substrates misoriented toward the m or a plane by 4°. A high In molar fraction (0.57) in the InAlN layer is chosen to reduce the lattice mismatch. InN growth on the on‐axis InAlN/sapphire system is initiated with random island formation, which coalesce at ≈10 nm thickness smearing grain boundaries. In contrast, on off‐axis sapphires, growth is defined by misorientation‐induced steps and grains remain visible even after the layer coalesces. The best electron mobility of 720 cm2 V−1 s−1 and carrier density of ≈1.5 × 1019 cm−3 are demonstrated on the heterostructure grown on‐axis sapphire, with InN as thin as 20 nm, even though the InN/InAlN interface root mean square roughness is ≈1.3 nm. The concentrations of screw and edge dislocations in the 20 nm thick InN grown on the on‐axis system are extracted to be 4.7 × 109 and 3.5 × 1010 cm−2. In all cases, the InN lattice is still partially strained, performing only ≈85% relaxation. Further lowering of the lattice mismatch, smoothing of the InAlN surface, and achieving semi‐insulating InAlN will provide necessary development steps toward predicted InN‐channel transistors with unprecedented performance.

Effect of band alignment on the n-InAlN/p-Si heterojunction for solar cells: a numerical study

The Inx Al1-xN heterojunction exhibits interesting features that make it a promising candidate for designing solar cells. Nevertheless, there are no studies about how the three types of band alignment (straddled, staggered and broken gap) affect the Inx Al1-x N heterojunction. In this work, Inx Al1-x N on commercial p-type silicon heterojunction was studied using computational simulation in solar cells. The main objective was to study the band alignment to maximize the efficiency of the cell for building experimental devices in the future. The thickness of the silicon layer remained constant at 500 μm. The thickness of Inx Al1-x N was varied between 10 nm and 100 nm, and the concentration of InN between 20% and 90%. It was found that the band alignment impacted on the open circuit voltage, the short circuit current density, the efficiency of the solar cell, and the quantum efficiency. It is theoretically concluded that a n-Inx Al1-x N/p-Si type heterojunction solar cell can be designed, with a maximum efficiency of 26.6%, adjusting the thickness of the InAlN layer to 10 nm and the InN concentration of 70%. This valuable result shows a good prospect for the manufacture of experimental devices.

Role of high nitrogen flux in InAlN growth by plasma-assisted molecular beam epitaxy

We study the impact of increased active nitrogen flux (N-flux) on the indium content and structural properties of InAlN layers grown by plasma-assisted molecular beam epitaxy. It is shown that high N-flux can stabilize In-N bonds, so that In0.18Al0.82N is grown at 605 °C, which is the highest reported temperature so far for the composition lattice-matched (LM) to GaN. Adiagram of InAlN indium content is shown as a function of growth temperature and N-flux. The InAlN layers grown using low and high N-flux had grainy surface morphology typical for N-rich conditions. Inhomogeneity in indium distribution on nanometer scale, i.e. typical honeycomb microstructure, is found for InAlN layers grown using both: low and high N-flux. An increase of average cell size is observed for LM-InAlN when the N-flux and growth temperature are increased.

Features of Radiation Changes in Electrical Properties of InAlN/GaN Hemts

The effect of the proton, electron, gamma - rays, and fast neutron irradiation on the parameters of InAlN/GaN HEMT structures is analyzed. The features of initial electronic properties of the InAlN and AlGaN barrier layers with a change in their composition, as well as the change in these properties when exposed to high-energy radiation are considered with taking into account the compositional dependence of the charge neutrality level energy position in the energy spectrum of these barrier layers.

A systematic study of MOCVD reactor conditions and Ga memory effect on properties of thick InAl(Ga)N layers: a complete depth-resolved investigation

Thick InAlN layers (In-molar fraction >0.37) on GaN buffer layers were prepared using a close-coupled showerhead metalorganic chemical vapor deposition (MOCVD) reactor.

The effect of N-vacancy and In aggregation on the properties of InAlN

Abstract In0.17Al0.83N, lattice matched to GaN, is an attractive solution for applications such as high electron mobility transistors, distributed Bragg reflectors, or light-emitting diodes. It has been reported that InAlN layers lattice matched to GaN on growth along the [0 0 0 1] exhibit crystallographic degradation when the thickness is increased. In our recent work, it was shown, through theoretical modelling as well as transmission electron microscopy, that the N vacancy (VN) plays a critical role in this behavior. In this work, we have carried out a detailed theoretical investigation of the possible modification of the properties of this alloy as brought about by the presence of the nitrogen vacancy and the associated formation of the indium rich regions. After testing the ab initio procedures on the AlN and reproducing the lattice parameters and band gap with high accuracy, it is shown that the indium aggregation in InAlN leads to a reduction of the band gap. In contrast to AlN, where acceptor levels can also form, the combination of the nitrogen vacancy and the favored indium aggregation introduced even more levels at the top of the band gap, moreover examining the whole spin configuration we find a strong asymmetry in the occupation of the spin up and down states with a net moment of around 1 µB per supercell. Most importantly, the presence of a nitrogen vacancy changes significantly the top of the band gap: first, the band edges become very sharp, and second, the introduced states originating from local deformation are occupied. This leads to n type doping of the alloy and nicely agree with the use of Ti for generation of N vacancies in the technology of ohmic contacts on nitride materials.

Growth evolution of N-polar indium-rich InAlN layer on c-sapphire via strain relaxation by ultrathin AlON interlayer

InAlN as a functional inorganic material is a promising alternative to the commonly used InGaN in tunnel diodes and optoelectronic devices, due to its tunable wider range of energy bandgap (0.65–6.2 eV), thus empowering utilization of the whole solar spectrum. Moreover, high electron drift velocity and carrier concentration are considered as the most desirable prerequisite of indium-rich InAlN. N-polar indium-rich InAlN could be more beneficial due to the reverse direction of the polarization compared to Ga-polar. However, unanswered questions persist concerning growth evolution of N-polar indium-rich InAlN grown by organometallic chemical vapor deposition (OMCVD). In this study, energy dispersive X-ray spectroscopy (EDX) and high-angle annular dark-field (HAADF) imaging are used to characterize N-polar In0.60Al0.40N layer at nanometer scale in order to determine the evolution of the layer on (0001) sapphire substrate. Long nitridation of sapphire substrate leading to the formation of ~2 nm AlON ultrathin interlayer, which relaxes strain at the InAlN/sapphire interface with assistance of a low-temperature AlN interlayer is observed. EDX analysis confirms that after strain relaxation of InAlN layer, the indium-incorporation has only a weak dependence on the polarity of the layer. The incorporation of indium at preferential sites is also discussed at length.

The critical role of N-vacancy on chemical composition fluctuations and degradation of InAlN layer

Due to its intrinsic properties and the possible lattice match to GaN, InAlN is expected to allow the fabrication of optimal high electron mobility transistors for high power and high frequency applications. However, the crystal quality of InAlN nearly lattice-matched to GaN degrades when the layer thickness is increased, and this is a strong limitation for the fabrication of devices in which thick barriers need to be used. In this work, we have carried out a detailed theoretical investigation of the behavior of indium atoms in the alloy. It is clearly shown that in the presence of nitrogen vacancies, which are common defects in these materials, indium nitride clusters will present excess formation energy up to diameters around 1.4 nm. In parallel, Z-contrast TEM observations close to the InAlN/GaN interface show that 2–5 nm size indium rich areas form and are systematically connected to the vertical degradation channels. This is at variance with published results, which concluded that the observed degradation was exclusively either due to the underlying threading dislocations or due to a characteristic three-dimensional growth mode.

Solving the problem of gallium contamination problem in InAlN layers in close coupled showerhead reactors

Gallium contamination is a well-known problem for InAlN layers grown in close coupled showerhead metal-organic vapor phase epitaxy reactors, and we have previously suggested a model explaining this gallium incorporation and the associated reduction in indium [Mrad, J. Cryst. Growth 507, 139 (2019)]. Here we propose the hypothesis that increasing the showerhead face temperature during GaN growth should encourage the growth of more stable GaN rather than metallic gallium, and reduce the reactions between tri-methyl indium and gallium on the showerhead. Using a hot deposition shield on the showerhead, we have confirmed this, to grow for the first time almost entirely gallium free InAlN layers in a showerhead reactor.

Ultrathin inserted AlGaN/InAlN heterojunction for performance improvement in AlGaN-based deep ultraviolet light-emitting diodes

We propose an innovative ultrathin AlGaN/InAlN heterojunction (HJ) inserted in AlGaN-based deep ultraviolet light-emitting diode (DUV LED) structure. Theoretical investigations indicate that the AlGaN/InAlN HJ is quite beneficial to suppress electron leakage and improve hole injection. These boosted characteristics strengthen the radiative recombination in the active region leading to remarkably improved optical power. Carrier transfer analysis reveals that the InAlN layer enhances the tunneling process for holes. The distinct AlGaN/InAlN HJ provides a promising alternative to be applied in DUV LED structures, as it not only functions as a sufficient electron blocker, but substantially facilitates the hole transport.

Effects of AlGaN interlayer on scattering mechanisms in InAlN/AlGaN/GaN heterostructures

Recent studies showed that the nominal AlN interlayers in InAlN/AlN/GaN heterostructures had high GaN mole fractions, especially those grown by metalorganic chemical vapor deposition. The Al and Ga mole fraction in the AlGaN interlayer determine the electron wave function and penetration probability, and thus affecting the scattering mechanism related to the InAlN/AlGaN potential layers. In this paper we study the effects of Al mole fraction of the AlGaN interlayer on three scattering mechanisms related to the potential layer, i.e. alloy disorder scattering, subband energy fluctuation scattering and conduction band fluctuation scattering induced by In compositionally inhomogeneous InAlN layer. The wave function and penetration probability in the InAlN/AlGaN/GaN heterostructure are determined by self-consistently calculating the Schrödinger-Poisson equations and then used to calculate the scattering mechanisms. The results show that penetration probabilities in the InAlN and AlGaN both decrease with increasing Al mole fraction. The combination of the contribution of the screening effect and the two-dimensional electron gas (2DEG) density inhomogeneity results in an initial decrease and subsequent increase in the subband energy fluctuation scattering-limited mobility with increasing Al mole fraction, and the heterostructure with a smaller InAlN thickness has a larger mobility increase. The penetration probability and non-periodic arrangement of Al and Ga in the AlGaN predict an Al mole fraction dependence of the alloy disorder scattering-limited mobility similar to the subband energy fluctuation scattering-limited mobility, and the alloy disorder scattering occurs mainly in the AlGaN because the penetration probability in the AlGaN is much higher than in the InAlN. The conduction band fluctuation scattering-limited mobility monotonically increases with increasing Al mole fraction due to the decrease of the penetration probability. The subband energy fluctuation scattering-limited mobility is less sensitive to variation in the Al mole fraction than the other two scattering mechanisms-limited mobilities. In a small Al mole fraction range around 0.1, the alloy disorder scattering is a dominant scattering mechanism, while the subband energy fluctuation scattering dominates the mobility beyond this compositional range. When Al mole fraction is above 0.52, the three scattering mechanisms-limited mobility exceeds that in the InAlN/GaN heterostructure without the AlGaN interlayer, indicating the promotion of the mobility by the AlGaN interlayer. The mobility is raised by more than 50 percent in the InAlN/AlN/GaN heterostructure with an AlN interlayer compared with that in the InAlN/GaN heterostructure without the interlayer.

Uniformity and repeatability of InAlN-barrier HEMTs growth by high-speed-rotation single-wafer MOCVD tool

We fabricated high-electron-mobility transistor structures with InAlN as a barrier layer on 8- or 6-inch Si substrates by using a recently developed high-speed-wafer-rotation single-wafer MOCVD tool. It has been reported that Ga inclusion in the InAlN layer causes serious problems in the control of group III metal composition in some cases, but the samples grown using the tool exhibited an InAlN layer with an abrupt interface and almost no Ga inclusion. Excellent in-wafer uniformity, repeatability, and wafer-to-wafer uniformity of device structure are also reported. The results shown in this paper indicate high performance of the tool in the real production of devices.

Ultrathin Barrier InAlN/GaN Heterostructures for HEMTs

InAlN/AlN/GaN semiconductor heterostructures with a barrier thickness of 5–13 nm have been grown by metalorganic vapor phase epitaxy (MOVPE) on sapphire and SiC substrates. Optimization of GaN buffer and InAlN layers allows fabricating structures with sheet conductivity values below 210 Ohm/sq. High electron mobility transistors (HEMTs) fabricated from such structures show drain current value exceeding 1.25 A/mm with maximum transconductance of 450 mS/mm. Use of thin in situ Si3N4 capping allows to fabricate and compare HEMT and MIS-HEMTs.

Understanding and controlling Ga contamination in InAlN barrier layers

We have shown that in an AIXTRON Close Coupled Showerhead MOVPE system, InAlN layers are strongly contaminated with gallium, giving increasing contamination when thicker GaN layers are grown below the InAlN. We have also shown that as the gallium contamination increases, there is reduced indium in the layers and thicker layers. To explain these phenomena we propose a quantitative model for these observations, suggesting that the TMIn precursor reacts with gallium on the showerhead surface to release TMGa or MMGa, which is then incorporated into the layers.We grew InAlN layers around 14 nm thick with a 1 nm AlN spacer to form High Electron Mobility Transistor (HEMT) structures. For the InAlN barrier layers grown directly on a GaN template, the 2D electron gas (2DEG) was degraded by the regrowth interface, but this 2DEG degradation was reduced by the growth of thin GaN layers. However, for thicker layers of GaN, there was a strong gallium contamination and so a change in alloy composition, which increased the sheet resistance. Thus we need an optimum regrowth thickness to have the best sheet resistance values.