InAlGaN Quaternary Alloys Research Articles

Highly Si‐Doped GaN Regrown by Metal–Organic Vapor‐Phase Epitaxy for Ohmic Contact Applied to Quaternary Barrier‐Based High‐Electron‐Mobility Transistors

The quaternary barrier InAlGaN is suitable for GaN high‐electron‐mobility transistors (HEMT) power microwave applications. High doping of the semiconductor under the drain and source is a known suitable solution to achieve low ohmic contact resistance. However, InAlGaN quaternary alloys require a low thermal budget to avoid indium desorption from the active layer during regrowth and thus deteriorating the barrier. Herein, a selective‐area growth technique at 850 °C by metal–organic vapor‐phase epitaxy (MOVPE) to achieve low contact resistance with respect to temperature constraint is presented. Regrowth temperature and mask geometry are investigated to achieve selectivity and control of the regrowth rate. The use of H2 as carrier gas decomposes the GaN buffer layer and damages the surface, creating material cluster during regrowth. Growth with N2 carrier gas shows nonselective epitaxy, as there are deposits on the entire surface of the dielectric mask. Switching from one carrier gas to another depending on the step in the MOVPE reactor helps to control both the morphology and selectivity. The resulting high doping levels of 8 × 1019 cm−3 lead to a low contact resistance of 0.26 Ω mm.

A Composition-Dependent Unified Analytical Model for Quaternary InAlGaN/GaN HEMTs for pH Sensing

Group III–V material alloys such as gallium nitride (GaN) with piezoelectric and pyroelectric properties are used for designing advanced devices suitable for harsh surroundings such as high temperature and acidic ambience. In the present paper, we introduce a unified model using InAlGaN quaternary alloy in the barrier layer to analyze the performance of pH sensors based on high electron-mobility transistors (HEMTs) Our model is used to accurately calculate the threshold voltage, the sheet charge concentration and thus the drain current, in response to changes in the pH values of the electrolyte put in the gate area of the InAlGaN/GaN heterostructure based sensor. We have taken three devices for our studies with varying In mole fraction from 0% to 16% and Al mole fraction from 23% to 74%. The sensitivity of a HEMT based pH sensor depends on its transconductance. The maximum transconductance values of the InAlGaN/GaN devices were found to be much higher than those of the AlGaN/GaN HEMT devices. A theoretical sensitivity of 1.3 mA/pH was achieved for the quaternary structures. Our model shows good agreement with the experimental data available in literature, presenting less than 1.2% root mean square error in almost all the devices. We observe that HEMTs based on the InAlGaN/GaN structures have better sensitivity than the AlGaN/GaN structures in pH sensing applications. Our findings may be used in designing quaternary HEMT based novel pH sensors.

Growth of InAlGaN Quaternary Alloys by Pulsed Metalorganic Chemical Vapor Deposition**Supported by the National Science and Technology Major Project of China under Grant No 2013ZX02308-002, and the National Natural Science Foundation of China under Grant Nos 11435010, 61474086 and 61334002.

Epitaxial growth of InAlGaN/GaN structures are performed on the c-plane sapphire by pulsed metal organic chemical vapor deposition with different triethylgallium (TEGa) flows in the growth process of InAlGaN quaternary alloys. X-ray photoelectron spectroscopy results show that the Al/In ratio of the samples increases as the TEGa flows increase in the InAlGaN quaternary growth process. High-resolution x-ray diffraction results show that the crystal quality is improved with increasing TEGa flows. Morphology of the InAlGaN/GaN heterostructures is characterized by an atomic force microscopy, and the growth mode of the InAlGaN quaternary shows a 2D island growth mode. The minimum surface roughness is 0.20 nm with the TEGa flows equaling to 3.6 μmol/min in rms. Hall effect measurement results show that the highest electron mobility μ is 1005.49 cm2 /Vs and the maximal two-dimensional electron gas is 1.63 × 1013 cm−2.

Characterization of temperature-dependent photoluminescence properties of InAlGaN quaternary alloys

Abstract Samples containing thin InAlGaN layers grown on c -Al 2 O 3 substrates by metalorganic chemical vapor deposition have been studied experimentally and theoretically with the help of photoluminescence measurements between 10 and 300 K. From the analysis of the temperature dependence photoluminescence spectra with the modified Bose–Einstein equation, the localization energy of excitons was estimated successfully and the value is found out to be well correlated to the strength of the electron/exciton-phonon interaction as well as the stress observed in the X-ray diffraction spectra. These results are also confirmed that the c -axis length decreases as the increasing Al/In ratio corresponding to the decreased compressive stress which would be attributed to the increased tensile stress formed by Al atoms incorporated.

Raman spectra investigation of InAlGaN quaternary alloys grown by metalorganic chemical vapor deposition

Raman analysis of the A1 (LO) mode of AlGaN-like phonons of the InAlGaN/GaN heterostructures in the composition range 1.38% ≦ In ≦ 2.73% and 8.01% ≦ Al ≦ 13.97% is presented. The line shape of A1 (LO) mode of AlGaN-like phonons was observed to exhibit a significant asymmetry and Raman linewidth toward the lower energy side. The spatial correlation model is discussed and is shown to account the line shape. The spatial correlation model calculations also indicate the lack of a long-range order in the higher Al to In ratio of InAlGaN/GaN alloys. These results were confirmed by x-ray diffraction and the correlation length L decreases as the increasing of Al to In ratio corresponding to the absence of the long-range order in the alloy. The Raman linewidth of the AlGaN-like A1 (LO) mode was found to exhibit a maximum at the higher Al to In ratio indicative of a random disordered alloy system.

Effects of elastic energy on the spinodal decomposition in InAlGaN materials

AbstractThe purpose of our study is to determine the influence of elastic energy on the spinodal decomposition ranges of InAlGaN alloys lattice‐matched to GaN. The distinction in lattice constants of the binary compounds constituting these alloys gives rise to their deformation energies. Such energies provide the tendency to disintegration that can lead to an appearance of the thermodynamically unstable states with respect to the decomposition. The initial stage of spinodal decomposition of the InAlGaN quaternary alloys is accompanied by transfers of atoms over distances of a lattice parameter. Accordingly, these transfers produce a thin two‐layer region with negligibly small differences in their compositions. Formation of these layers causes the elastic energy. The alloys are represented as quaternary regular solutions. The internal deformation energy is calculated with the interaction parameters estimated by the valence force field model. The spinodal decomposition ranges of these materials are demonstrated up to 580 °C (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Polarization Engineering in GaN Power Devices

This paper reviews novel design of epitaxial structure for GaN-based power devices taking advantages of the material's unique polarization. This can be called as polarization engineering which enables low on-state resistances and high breakdown voltages. AlGaN/GaN superlattice and polarization-matched InAlGaN quaternary alloy capping layers effectively reduce the series resistance of AlGaN/GaN heterojunction field effect transistors (HFETs) by reducing the potential barriers above the heterojunction. Natural super junction (NSJ) model is proposed, which well explains the limitless increase of the breakdown voltages of GaN transistor by the extension of the gate-drain spacing. This model is applied to diodes with multi channels of AlGaN/GaN resulting in low on-state resistances and high breakdown voltages. The presented polarization engineering is very promising for future GaN power devices with the improved performances.

Polarization engineering in GaN power transistors

This paper reviews novel design of epitaxial structure for GaN power transistors to reduce the on-state resistance and to increase the breakdown voltage. It takes advantages of the fixed charges at the interfaces caused by the material's inherent polarization, and thus it can be called as polarization engineering. Both AlGaN/GaN superlattices and polarization-matched InAlGaN quaternary alloy effectively reduce the on-state resistance by using them as capping layers over AlGaN/GaN combined with the recessed-gate structure. These capping layers successfully eliminate the electron depletion which is caused by the balancing of the fixed charges. The series resistance across the interface is fully reduced so that these capping layers enable low on-state resistance. Proposed natural super junction (NSJ) model well explains the fact that the breakdown voltages of AlGaN/GaN hetero-junction field effect transistors (HFETs) are increased by the extension of the gate and drain spacing. The model assumes that the hetero-junction can act an insulator since the polarization induced charges are always balanced at the top and the bottom surfaces. Taking advantages of the model, the fabricated multi-channel AlGaN/GaN diode with dual-recessed structure exhibits high breakdown voltage of 9400 V with low on-state resistance of 52 mΩ cm2, which reaches the predicted performance limit of GaN. The presented polarization engineering is very promising for future improvement of the device performances.

Structural and Optical Properties of In-Rich InAlGaN/InGaN Heterostructures for White Light Emission

The effect of trimethylaluminium (TMAl) flow rate and the reactor pressure on the structural and optical properties of In-rich InAlGaN quaternary alloy grown on InGaN/GaN by metal organic chemical vapor deposition (MOCVD) were investigated. The addition of Al atoms to InGaN layers at an optimized growth reactor pressure, 200 mbar in our case, resulted in a broad emission from green to red because of the spatial fluctuation of local In concentration and also the formation of three-dimensional (3D) island structures due to low surface mobilities of Al atoms. When the reactor pressure is decreased below 200 mbar, the In-rich phase separation and related green to red emission have not been observed. Combining the blue emission from InGaN layer with the green to red emission from In-rich InAlGaN alloy layer, white light emission has been obtained. The addition of small amount of Al atoms at an optimum reactor pressure was found to enhance the In-rich phase formation in InAlGaN quaternary alloy.

RBS/PIXE channeling for InAlGaN epitaxial layers

Characteristics of InAlGaN quaternary alloy epitaxial layers have for the first time been studied by Rutherford backscattering spectrometry (RBS) and particle induced X-ray emission (PIXE). We have evaluated the crystalline quality from the normalized yield as a function of aluminium content of the InAlGaN quaternary alloy epitaxial layers with the same indium content. The compositions of the samples were estimated to be In0.02Al0.12Ga0.86N and In0.02Al0.24Ga0.74N from RBS measurements. The normalized yield for an In0.02Al0.12Ga0.86N epitaxial layer and an In0.02Al0.24Ga0.74 epitaxial layer was calculated to be 2.44% and 4.20%, respectively. This indicates that they have excellent crystalline quality and that the crystalline quality of the epitaxial layer becomes worse with increase of the aluminum content. PIXE channeling has also been performed for both samples. Individual signals of indium and aluminum atoms in InAlGaN epitaxial layers were obtained by PIXE measurements. The normalized yield of the indium was smaller than that of the aluminum in both InAlGaN epitaxial layers which suggest the differences of their lattice vibration or atomic displacements.

The growth of high-Al-content InAlGaN quaternary alloys by RF-MBE

High-Al-content InxAlyGa1−x−yN (x = 1–10%, y = 34–45%) quaternary alloys were grown on sapphire by radio-frequency plasma-excited molecular beam epitaxy. Rutherford back-scattering spectrometry, high resolution x-ray diffraction and cathodoluminescence were used to characterize the InAlGaN alloys. The experimental results show that InAlGaN with an appropriate Al/In ratio (near 4.7, which is a lattice-match to the GaN under-layer) has better crystal and optical quality than the InAlGaN alloys whose Al/In ratios are far from 4.7. Some cracks and V-defects occur in high-Al/In-ratio InAlGaN alloys. In the CL image, the cracks and V-defect regions are the emission-enhanced regions.

Energy gaps and bowing parameters of InAlGaN ternary and quaternary alloys

AbstractWe report a study of band gap energies and bowing parameters of all possible families of ternary and quaternary nitride alloys. The experimental band gap energies (E g) measured by photoluminescence, optical transmission and reflection methods, were strain‐corrected in order to obtain for every sample the unstrained value of its E g. For Alx Ga1–x N the bowing parameter is equal to 1 eV, but the alloys Inx Ga1–x N and Inx Al1–x N exhibited strong dependence of the bowing parameter on alloy composition. Finally, we investigated many quaternaries alloys Inx Aly Ga1–x –y N and we observed that the bowing parameter appears also to be very sensitive on In content. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Temperature dependence of the formation of nano-scale indium clusters in InAlGaN alloyson Si(111) substrates

The temperature dependence of the formation of nano-scale indium clusters inInAlGaN quaternary alloys, which are grown by metalorganic chemical vapourdeposition on GaN/Si(111) epilayers, is investigated. Firm evidence is provided tosupport the existence of phase separation, or nano-scale In-rich clusters, by thecombined results of high-resolution transmission electron microscopy (HRTEM),high-resolution x-ray diffraction (HRXRD) and micro-Raman spectra. The resultsof HRXRD and Raman spectra indicate that the degree of phase separation isstrong and the number of In clusters in the InAlGaN layers on silicon substrate ishigher at lower growth temperatures than that at higher growth temperatures,which limits the In and Al incorporated into the InAlGaN quaternary alloys. Thedetailed mechanism of luminescence in this system is studied by low temperaturephotoluminescence (LT-PL). We conclude that the ultraviolet (UV) emission observed inthe quaternary InAlGaN alloys arises from the matrix of a random alloy, and thesecond emission peak in the blue–green region results from the nano-scale indiumclusters.

Recessed-Gate AlGaN/GaN HFETs With Lattice-Matched InAlGaN Quaternary Alloy Capping Layers

In this paper, we present recessed AlGaN/GaN heterojunction field-effect transistors (HFETs) with lattice-matched InAlGaN capping layers, which reduce both ohmic contact resistance and series resistance between the AlGaN and the capping layer. The lattice-matched alloy epitaxial layer with both In and Al high compositions are successfully grown by metal-organic chemical vapor deposition. The grown lattice-matched In/sub 0.09/Al/sub 0.32/Ga/sub 0.59/N capping layer has close total polarization and bandgap to those of the underlying Al/sub 0.26/Ga/sub 0.74/N layer. The balanced polarization eliminates the depletion of electrons at the In/sub 0.09/Al/sub 0.32/Ga/sub 0.59/N/Al/sub 0.26/Ga/sub 0.74/N interface, which can reduce the series resistance across it. It is also noted that the fabricated HFET exhibits very low ohmic contact resistance of 1.0/spl times/10/sup -6/ /spl Omega//spl middot/cm/sup 2/ or less. Detailed analysis of the source resistance reveals that the series resistance at the In/sub 0.09/Al/sub 0.32/Ga/sub 0.59/N/Al/sub 0.26/Ga/sub 0.74/N interface is one fifth as low as the resistance at the conventional GaN/Al/sub 0.26/Ga/sub 0.74/N interface.

Crack-free InAlGaN quaternary alloy films grown on Si(1 1 1) substrate by metalorganic chemical vapor deposition

Crack-free In 0.08Al 0.25Ga 0.67N quaternary films, with and without thick (>1.5 μm) high-temperature-GaN (HT-GaN) interlayer, have been grown on Si(1 1 1) substrates by a low-pressure metalorganic chemical vapor deposition (MOCVD) system. Mole fractions of In and Al in quaternary alloy layers are determined by Energy dispersive spectroscopy (EDS) and Rutherford backscattering spectrometry (RBS), which are recorded as ∼8% and ∼25–27%, respectively. High-resolution X-ray diffraction (HRXRD) and room temperature photoluminescence (RT-PL) results evidence the film's single crystal structure and the existence of local In- and/or Al-rich regions. Compared with GaN film grwon on Si(1 1 1) substrate, no crack is observed in the quaternary ones. Two explanations are proposed. First, mismatch-induced strain is relaxed significantly due to gradual changes of In concentration. Second, the weak In–N bond is likely to break when the sample is cooled down to the room temperature, which is expected to favor the releasing of thermal stress.

Characterizations of In xAl yGa 1− x− yN alloy systems grown on GaN substrates by molecular-beam epitaxy

In 0.05Al 0.10Ga 0.85N epilayers and Al 0.10Ga 0.90N epilayers have been grown on bulk GaN single crystals and GaN templates by radio-frequency (RF) molecular-beam epitaxy (MBE). Photoluminescence (PL) spectra at different temperatures ranged from 8 to 300 K were measured for these epilayers. The decreasing rates of PL peak intensity of the In 0.05Al 0.10Ga 0.85N epilayers were smaller than those of the Al 0.10Ga 0.90N epilayers. The fluctuations of emission intensities were not observed in the In 0.05Al 0.10Ga 0.85N epilayers by cathodoluminescence observations at 77 K. Our results indicate that In-related effects exist in InAlGaN quaternary alloys on substrates with low-dislocation densities, however, expect that the localization effect related to In-segregation is weak.

III-nitride ultraviolet light-emitting diodes with delta doping

We present the results on the fabrication and characterization of 340 nm UV light-emitting diodes (LEDs) based on InAlGaN quaternary alloys grown by metalorganic chemical vapor deposition. By employing δ doping in the n- and p-type layers, we have demonstrated enhanced LED structural quality and emission efficiency. Combining with our interconnected microdisk LED architecture, the output power of a 300×300 μm2 bare LED chip measured from the sapphire side reached 50 μW under a standard dc operation condition (20 mA) at 4.6 V and 1.6 mW under a pulsed driving current.