InAlN Films Research Articles

Effects of growth temperature on structural and electrical properties of in-rich InAlN–GaN heterostructures by radio-frequency metal–organic molecular beam epitaxy

In-rich InAlN is a promising nitride semiconductor alloy for high-efficiency solar cells and wide-range light-emitting diodes due to its tunable bandgap from 0.7 to 6.2 eV. However, incomplete characterization has led to inconsistent fundamental properties in some studies. The aim of this study was to comprehensively investigate the structural, optical, and electrical properties of In-rich InAlN films grown on GaN/Al2O3 templates by RF-MOMBE at various temperatures. The methodology involved state-of-the-art metrology techniques, such as high-resolution x-ray diffraction (HRXRD), scanning electron microscopy (FE-SEM), Hall effect measurements, and transmission electron microscopy (TEM). The results showed that all InxAl1-xN films were epitaxially grown on the GaN/Al2O3 template, with the indium composition (x) decreasing with increasing growth temperature. Furthermore, phase separation of the In-rich InAlN films occurred at high growth temperatures(>550 °C), resulting in a relatively smooth surface. The optical absorption method measured the band-gap of the InxAl1-xN films, which ranged from 1.7 to 1.9 eV for x values between 0.77 and 0.91. The mobility and carrier concentrations of all In-rich InAlN films were measured at ∼60−277 cm2 V−1-s−1 and 2–7 × 1019 cm3 in the growth temperature of range 450 °C–610 °C, respectively. In conclusion, our comprehensive characterization using advanced metrology methods provides valuable insights into the properties of In-rich InAlN films, which can inform future optimization of these materials for various applications.

Oxidation Behavior of InAlN during Rapid Thermal Annealing

Leakage currents in InAlN/GaN‐based high‐electron‐mobility transistors considered for normally‐off devices critically depend on the oxidation behavior of InAlN thin films. Herein, lattice‐matched InAlN thin films deposited on GaN (0001) are rapid thermally annealed at 800 °C in an oxygen‐rich environment. Aberration‐corrected scanning transmission electron microscopy combined with electron energy‐loss spectroscopy is used to systematically characterize the oxidation behavior of InAlN films as a function of annealing time. Initial growth of oxide layers is found to be reaction limited, which is replaced by a diffusion limited growth process once a critical thickness of the oxide layer is obtained. Growing oxide layers are amorphous and become porous with increasing annealing time.

Band gap bowing for high In content InAlN films

The InAlN alloy system has bandgap energies that span the entire visible range and is a promising candidate for bandgap engineering. To facilitate bandgap engineering, the bandgap bowing for the entire composition range must be well characterized. In rich InxAl1−xN (x > 0.60), films were grown via plasma assisted molecular beam epitaxy on freestanding GaN substrates. The InxAl1−xN film compositions were determined using high resolution X-ray diffraction. On-axis 0002 ω-2θ scans were used to determine the c spacing of the films, reciprocal space maps of the 1¯015 peaks were used to determine that the films were completely relaxed, and the film compositions were calculated from that information assuming Vegard's law. The bandgaps were independently measured using absorbance spectroscopy and fitting to the linear region of a Tauc plot (dependence of α2 hν on hν) as determined by interpolating the second derivative of the data and selecting a region of low curvature. Compared with the literature, the data were fit to a composition-independent bandgap bowing model, and a bowing parameter of b = 4.0 ± 0.2 eV was calculated, which is consistent with previous results.

AlN/InAlN thin-film transistors fabricated on glass substrates at room temperature

In this study, InAlN was grown on glass substrates using pulsed sputtering deposition (PSD) at room temperature (RT) and was applied to thin-film transistors (TFTs). The surface flatness of the InAIN films was improved by reducing the growth temperature from 350 °C to RT. Further, the electron mobility and concentration of the InAlN film that was grown at RT were observed to be strongly dependent on the In composition. It was also observed that the electron concentration could be reduced during the introduction of Al atoms into InN, which could most likely be attributed to the reduction in the position of the Fermi level stabilization energy with respect to the conduction band edge. Further, InAlN-TFT was fabricated, and successful operation with a field-effect mobility of 8 cm2 V−1 s−1 was confirmed. This was the first demonstration of the operation of TFTs based on the growth of InAlN on an amorphous substrate at RT.

Microstructure variation in thick AlInN films grown on c-plane GaN on sapphire by metalorganic chemical vapor deposition

300-nm-thick AlInN films with InN molar fractions ranging from 0.114 to 0.197 were grown by metalorganic chemical vapor deposition on c-plane GaN on sapphire. It was confirmed that no lattice relaxation occurred for samples with InN molar fractions from 0.144 to 0.197, and the InAlN films with low InN molar fractions showed a relative smooth surface. However, it turned into a granular surface morphology resulting from a columnar polycrystalline structure when the InN molar fraction exceeded a compositional boundary of in-plane lattice matching. As for the smooth-surface AlInN single-layers, the optical constants as well as energy bandgaps were determined.

Evolution of indium segregation in metal-polar In0.17Al0.83N lattice-matched to GaN grown by plasma assisted molecular beam epitaxy

The impact of growth conditions on indium segregation in metal-polar In0.17Al0.83N lattice-matched to GaN grown by plasma assisted molecular beam epitaxy is reported. A series of lattice-matched InAlN films was systematically grown over a wide range of temperatures and active nitrogen fluxes. The growth conditions include those reported in the literature as conducive to homogeneous In0.17Al0.83N (Kaun et al., 2014). However, honeycomb indium composition modulation was found in all InAlN films. For samples grown at the highest temperature and nitrogen flux, InAlN layers exhibit larger and more irregular cells with emergence of open cell boundaries. We conclude that the studied growth conditions alone are not sufficient to eliminate the ‘honeycomb’ structure in metal-polar InAlN lattice-matched to GaN, and that the physical process necessary for reproducible growth of homogeneous In0.17Al0.83N requires further clarification.

Structural, morphological, electrical and optical properties of amorphous InxAl1-xN thin films for photovoltaic applications

The indium and aluminum nitride (InxAl1-xN) semiconductor material was postulated in 2008 as an excellent candidate to produce solar cells. In this research, amorphous layers of InxAl1-xN (0.55 ≤ x ≤ 0.60) were synthesized using the RF sputtering magnetron technique, and then studied for their possible use in photovoltaic applications. Physical, structural, morphological, optical, and electrical properties were studied. The amorphousness was checked by XRD difractograms. Samples have very smooth surfaces, with very low roughness values, according to AFM and SEM techniques.Electrical properties of the InAlN films were studied using Van Der Pauw technique and Hall Effect parameters. The volumetric carrier concentration decreases when InN fraction mole increases. The mobility values range between 6 × 10−2 and 5 × 10−1 cm2 V−1 s−1, which are lower than those obtained for crystalline and polycrystalline samples. The obtained band gap values could be used in both the absorbing layer and the window layer in solar cells (1.9 eV–2.3 eV). Interestingly, this figures of Eg are very close to the previously obtained values for polycrystalline samples. The optical absorption coefficients were high compared to the materials currently used in solar cells (~105 cm−1). This implies the possibility of using thinner layers in photovoltaic devices based on InxAl1-xN.

Epitaxial growth of semipolar InAlN films on yttria-stabilized zirconia

We report on the epitaxial growth of semipolar InAlN (11¯03) on yttria-stabilized zirconia (YSZ) by pulsed sputtering deposition (PSD). Unlike the direct growth of InAlN on YSZ that resulted in c-plane InAlN, the insertion of an InN buffer layer favored the growth of InAlN in the semipolar direction. Phase separation of semipolar InAlN taking place in the films with intermediate indium contents can be suppressed by low-temperature (less than 400 °C) PSD growth. These semipolar InAlN films grown at low temperatures possessed sufficient optical quality to demonstrate photoluminescence at room temperature.

DEPOSITION AND CHARACTERIZATION OF MAGNETRON CO-SPUTTERED InAlN FILM AT DIFFERENT Ar:N2 GAS FLOW RATIOS

This work presents the influence of changing Ar:N2 gas ratio on the growth and properties of InAlN films. InAlN films were deposited on [Formula: see text]-type Si(111) substrates by using magnetron co-sputtering method in 6:12, 10:10, 12:8 and 12:6 Ar:N2 mixtures at 300[Formula: see text]C. The surface, structural, electrical and optical properties of the deposited films were evaluated at different Ar:N2 ratios. The grain size and film thickness were increased by increasing the Ar flow with respect to N2. Structural characterization by X-ray diffraction (XRD) revealed an improvement in the crystalline quality of the [Formula: see text]-axis-oriented InAlN film by adjusting the Ar:N2 ratio to 12:8, however no diffraction peak corresponding to InAlN was detected at 6:12 Ar:N2 mixture. The surface roughness of InAlN film exhibited an increasing trend whereas the electrical resistivity of the film was decreased by increasing the Ar:N2 ratio. The bandgap of InAlN film was calculated from the optical reflectance spectra and it was found to change by changing the Ar:N2 gas ratio. The analysis of results from this work shows that the InAlN film with improved physical properties can be obtained through reactive magnetron co-sputtering method by adjusting the Ar:N2mixture to 12:8.

Fabrication and characterization of In0.83Al0.17N based MSM visible photodiode

In this work, we report on the growth of In 0.83 Al 0.17 N film on p -type Si (1 1 1) substrate for metal-semiconductor-metal (MSM) photodiode application. The film was synthesized by reactive magnetron co-sputtering technique on Si (1 1 1) substrate in Ar and N 2 mixture at 300 °C. The X-ray diffraction analysis revealed (0 0 2) oriented diffraction peak corresponding to nanocrystalline InAlN. The band gap of In 0.83 Al 0.17 N film was estimated from UV-vis reflectance measurement and it was found to be 2.38 eV. To fabricate In 0.83 Al 0.17 N based MSM photodiode, Pt contacts were deposited on the film through RF magnetron sputtering. Upon exposure to 520 nm light, the Pt/In 0.83 Al 0.17 N/Pt photodiode displayed a sharp rise in the value of current. The photodiode exhibited a high sensitivity (4.8×10 3 ) and current gain (48.1) at a bias voltage of 5 V. The response and recovery time were calculated to be 0.62 and 0.63 s respectively. The results of present work demonstrate that the sputtered grown InAlN film is a promising material for the MSM photodiode application.

High indium content homogenous InAlN layers grown by plasma-assisted molecular beam epitaxy

InAlN grown by plasma-assisted molecular beam epitaxy often contains a honeycomb microstructure. The honeycomb microstructure consists of 5–10nm diameter aluminum-rich regions which are surrounded by indium-rich regions. Layers without this microstructure were previously developed for nominally lattice-matched InAlN and have been developed here for higher indium content InAlN. In this study, InAlN was grown in a nitrogen-rich environment with high indium to aluminum flux ratios at low growth temperatures. Samples were characterized by high-resolution x-ray diffraction, atomic force microscopy, high-angle annular dark-field scanning transmission electron microscopy, and atom probe tomography. Atomic force microscopy showed InAlN layers grown at temperatures below 450°C under nitrogen-rich conditions were free of droplets. InAlN films with indium contents up to 81% were grown at temperatures between 410 and 440°C. High-angle annular dark-field scanning transmission electron microscopy and atom probe tomography showed no evidence of honeycomb microstructure for samples with indium contents of 34% and 62%. These layers are homogeneous and follow a random alloy distribution. A growth diagram for InAlN of all indium contents is reported.

The immiscibility of InAlN ternary alloy.

We have used two models based on the valence force field and the regular solution model to study the immiscibility of InAlN ternary alloy, and have got the spinodal and binodal curves of InAlN. Analyzing the spinodal decomposition curves, we obtain the appropriate concentration region for the epitaxial growth of the InN-AlN pseudobinary alloy. At a temperature most common for the epitaxial growth of InAlN (1000 K), the solubility of InN is about 10%. Then we introduce the mismatch strain item into the Gibbs free energy, and the effect of different substrates is taken into consideration. Considering Si, Al2O3, InN, GaN, AlN as a substrate respectively, it is found that all the five systems are stabilized with the upper critical solution temperature largely reduced. Finally, InN and GaN are potential substrates for In-rich InAlN, while AlN and GaN substrates are recommended in the Al-rich region. Si and Al2O3 may be ideal substrates for thin InAlN film.

A comparative study on the growth of InAlN films on different substrates

This work investigates the growth of InAlN films on Si (111), sapphire (001), GaAs (100) and glass substrates and compares the structural, morphological, electrical and optical properties of these films. One micron thick InAlN films were synthesized on these substrates at 300 °C by using reactive magnetron co-sputtering system. The structural analysis showed the formation of polycrystalline InAlN films on all the substrates having preferred orientation along (101) plane. The films grown on sapphire and silicon displayed better structural quality than the films grown on GaAs and glass. The morphological results revealed identical granular features on all the substrates with small variation in the grain size. The electrical resistivity of InAlN film on sapphire was the lowest one (8×10−3Ω-cm) whereas the highest carrier concentration (8×1020cm−3) was obtained for the film deposited on glass. The energy band gap of InAlN films was determined through UV–vis absorption and reflectance spectroscopy. The band gap value obtained on the glass was slightly higher as compared to its value on the other substrates. The changes in InAlN properties on different substrates were explained on the basis of lattice mismatch, crystallite size, residual strain and orientation of the substrates.

Morphological and microstructural stability of N-polar InAlN thin films grown on free-standing GaN substrates by molecular beam epitaxy

The sensitivity of the surface morphology and microstructure of N-polar-oriented InAlN to variations in composition, temperature, and layer thickness for thin films grown by plasma-assisted molecular beam epitaxy (PAMBE) has been investigated. Lateral compositional inhomogeneity is present in N-rich InAlN films grown at low temperature, and phase segregation is exacerbated with increasing InN fraction. A smooth, step-flow surface morphology and elimination of compositional inhomogeneity can be achieved at a growth temperature 50 °C above the onset of In evaporation (650 °C). A GaN/AlN/GaN/200-nm InAlN heterostructure had a sheet charge density of 1.7 × 1013 cm−2 and no degradation in mobility (1760 cm2/V s) relative to 15-nm-thick InAlN layers. Demonstration of thick-barrier high-electron-mobility transistors with good direct-current characteristics shows that device quality, thick InAlN layers can be successfully grown by PAMBE.

Influence of substrate temperature on the growth and properties of reactively sputtered In-rich InAlN films

Indium-rich InAlN films were prepared on Si (111) substrates by using reactive co-sputtering in a mixed Ar-N2 atmosphere. The substrate temperature was varied from room temperature to 300 °C to investigate the film’s growth and properties at different temperatures. Structural and optical properties of the films were evaluated through high resolution XRD and Raman spectroscopy respectively, surface morphology and roughness analysis was performed by using FE-SEM and AFM respectively, whereas the electrical characterizations were made through resistivity and current–voltage (I–V) measurements respectively. Highly c-axis oriented nanocrystalline InAlN films with wurtzite structure were obtained at a substrate temperature of 100 °C and above. Structural quality of the films was improved with increase of the substrate temperature. The Raman spectroscopy revealed A1 (LO) modes which became more intense by the increasing the substrate temperature. The electrical studies indicated n-type nature of InAlN film having electron concentration in the range 3 × 1019–20 × 1019 cm−3. The electrical resistivity exhibited a decreasing trend with increase of the deposition temperature. The I–V measurements showed a noticeable increase in the value of current by increasing the substrate temperature to 300 °C.

Effect of film thickness on the surface, structural and electrical properties of InAlN films prepared by reactive co-sputtering

InAlN films of different thicknesses (150nm, 250nm, 380nm, 750nm and 1050nm) were grown on Si (111) by means of reactive co-sputtering at 300°C. Surface morphology results indicated an increase in the grains size and their spacing with increase of the film thickness. The surface of InAlN remained smooth with a slight variation in its RMS roughness from 1.29nm to 6.62nm by varying the film thickness. X-ray diffraction patterns exhibited InAlN diffraction peaks with preferred orientation along (002) plane in the thickness range 250nm to 750nm, however, the preferred orientation of the film was changed towards (101) plane at 1050nm. An improvement in the crystallinity of InAlN was observed with increase of the film thickness. Electrical characterization revealed a decrease in the film's resistivity by increasing its thickness to 750nm, however, the resistivity was found to increase at 1050nm. The electron concentration indicated an increasing trend whereas changes in the electron mobility were found to be inconsistent with increase of the film thickness.

Effects of indium mole fraction on the physical characteristics of magnetron sputtered InxAl1−xN films

InxAl1 − xN films with composition 0.25 ≤ x ≤ 0.86 were grown on p-type Si (111) substrates by using reactive magnetron co-sputtering technique in an ambient of argon and nitrogen at 200 °C. X-ray diffraction results at x = 0.25 and x = 0.40 indicated the formation of nanostructured InAlN films along (101) plane with low diffraction intensities. However, by increasing the indium mole fraction ‘x’ from 0.40 to 0.86, crystallinity of the InAlN film was considerably improved along with a change in its preferred orientation towards the c-plane. Surface morphology analysis revealed an under-dense grains structure at x = 0.25, which was changed into a compact structure with increase of the ‘x’ value. Surface roughness of the films exhibited a decreasing trend with increase of the indium contents whereas the Raman spectroscopy analysis specified the existence of A1 (LO) and E2 (high) phonon modes in the films at x = 0.40 and x = 0.86. The Hall measurements indicated n-type nature of InxAl1 − xN film along with a decrease in its electrical resistivity with increase of the indium composition ‘x’. The current–voltage (I–V) characteristics at x = 0.25 displayed Schottky behavior at the metal/film interface which was changed into the ohmic one with increase of indium mole fraction in the film.

INFLUENCE OF ANNEALING TREATMENT ON THE PHYSICAL PROPERTIES OF InAlN FILMS

In this work, pure indium and aluminum targets were co-sputtered in a reactive argon–nitrogen environment at 200°C to deposit InAlN film on the GaAs substrate in the presence of a ZnO buffer layer. The as-grown film was annealed at 750°C for 1 h in a high temperature furnace under nitrogen ambient. XRD pattern of the as-grown film did not display any diffraction peak relating to the InAlN due to its poor structural crystallinity, however, the annealed film exhibited InAlN diffraction peaks corresponding to (002), (101) and (102) planes. A significant increase in the grain size and the surface roughness was observed after the films' annealing. Raman spectroscopy revealed A1 (LO) and E2 (high) phonon modes whereas the PL analysis showed a luminescence peak at 2 eV in the annealed film. The Hall measurements indicated an increase in the carrier concentration and electron mobility after the annealing which was accompanied by a decrease in electrical resistivity of the film. The dark current–voltage (I–V) characteristics of the as-grown and the annealed films were also recorded to investigate the barrier height and the ideality factor.

Post-deposition annealing of magnetron sputtered InAlN film at different temperatures

Effects of post-deposition annealing on the structural, surface and optical properties of InAlN films have been investigated in this paper. InAlN films were grown on ZnO buffer layers on GaAs (100) substrates by magnetron co-sputtering of In and Al targets in Ar–N2 plasma at 100°C. The as-grown films were annealed at 250°C, 500°C and 700°C for six hours in a quartz tube furnace. X-ray diffraction (XRD) analysis confirmed the formation of InAlN films with preferred orientation along (101¯1) plane. Crystallinity of the films was improved with increase of the annealing temperature. A substantial increase in the crystallinity of InAlN film along (101¯1) plane was noticed at 700°C. Compositional analyses from the XRD and energy dispersive X-ray spectroscopy (EDS) specified higher In contents in the films as compared to the Al. Surface morphology of the films indicated grains growth whereas changes in the surface roughness, calculated using WSxM 5.0 develop 7.0 Nanotech Software, were found to be inconsistent with increase of the annealing temperature. Raman spectroscopy revealed A1 (LO) and E2 (high) phonon modes in the film annealed at 700°C. Photoluminescence spectra of InAlN films were also recorded to determine the band gap.

Room temperature photoluminescence from InxAl(1−x)N films deposited by plasma-assisted molecular beam epitaxy

InAlN films deposited by plasma-assisted molecular beam epitaxy exhibited a lateral composition modulation characterized by 10–12 nm diameter, honeycomb-shaped, columnar domains with Al-rich cores and In-rich boundaries. To ascertain the effect of this microstructure on its optical properties, room temperature absorption and photoluminescence characteristics of InxAl(1−x)N were comparatively investigated for indium compositions ranging from x = 0.092 to 0.235, including x = 0.166 lattice matched to GaN. The Stokes shift of the emission was significantly greater than reported for films grown by metalorganic chemical vapor deposition, possibly due to the phase separation in these nanocolumnar domains. The room temperature photoluminescence also provided evidence of carrier transfer from the InAlN film to the GaN template.