InN Films Research Articles

Enhancing indium bilayer nitridation through novel hydrogen insertion process in InN epitaxy: A kinetic mechanism

InN holds great promise for a wide range of applications, including broadband optical devices, high-frequency electronic devices, and serving as a substrate for highly sensitive gas detectors and efficient catalysts. Nevertheless, production of high-quality InN thin films through epitaxy has remained a significant challenge. This is primarily due to complexities arising from the low dissociation temperature and the high N2 equilibrium pressure of InN, which lead to formation of numerous nitrogen vacancies and a propensity to generate indium metal nanoparticles. Efficacy of the indium bilayer pre-deposition method for InN film growth has been established in prior studies. In this work, we introduce a specialized hydrogen insertion method to further enhance nitridation of the indium bilayer. The corresponding kinetic mechanism has been demonstrated through theoretical simulations and practical epitaxy experiments, leading to the development of an optimized hydrogen insertion process. This research represents a substantial improvement over existing InN epitaxial methods that involve control of the indium bilayer and introduces a novel mechanism for enhancing InN heteroepitaxy.

Temperature dependent correlation of Hall effect and optical measurements of electron concentration in degenerate InN thin film

In this work, we study the thermal evolution of the optical and electrical features of an InN thin film. By correlating photoluminescence (PL) and Hall effect results, we determine the appropriate values of the correlation parameter to be used in the empirical power law that associates the electron concentration with the linewidth of the PL spectrum, in the scope of the Burstein–Moss effect across a wide range of temperatures. Additionally, by associating Raman and PL results, we observe the thermally induced compressive strain widening of the bandgap of the InN film. Our findings demonstrate the reliability of optical methods in providing contactless measurements of electrical and structural features of semiconductors.

Microwave plasma-assisted reactive HiPIMS of InN films: Plasma environment and material characterisation

This work focuses on the low temperature fabrication process of InN thin films via microwave plasma-assisted reactive high power impulse magnetron sputtering (MAR-HiPIMS). The influence of microwave plasma on the HiPIMS discharge process at various nitrogen flows and microwave powers was monitored and characterised through in situ diagnostics, including following HiPIMS I(V,t) curves, optical emission spectroscopy (OES), as well as performing time-resolved Langmuir probe and time-of-flight mass spectroscopy (ToF-MS) measurements. This was followed by the deposition of InN films via standard reactive HiPIMS (reference sample) and MAR-HiPIMS and their characterisation via X-ray diffraction (XRD), reflectometry (XRR), as well as scanning and transmission electron microscopy (SEM, TEM). It was found that the microwave plasma facilitates the dissociation/activation of nitrogen species and supplies seed electrons to the magnetron discharge plasma. Furthermore, the energy of the incoming ions was determined via ToF-MS, and it was possible to identify their plasma origin and temporal behaviour. The produced R-HiPIMS sample was highly metallic, with no nitride phase detected. The MAR-HiPMS film, however, was stoichiometric and exhibited (0002) direction texturing, with an optical bandgap of approx. 1.5 eV, electron concentration of 2.72 × 1020 cm−3 and electron mobility of 7.16 cm2V−1 s−1 (in the range for polycrystalline InN).

Revealing the simultaneous increase in transient transmission and reflectivity in InN

The ultrafast transient behavior of InN under intensive laser irradiation remains unclear. The simultaneous measurements of pump–probe transient transmission and reflectivity are reported in this study. The irradiation-induced change in the dielectric constant of InN films due to the generation of thermalized carriers gives rise to complex transient behavior, and simultaneous increase in both transient transmission and reflectivity are observed at certain wavelengths. Herein, transient transmission is interpreted as the occupation probability of thermalized electrons at the probing level originating from a hot Fermi–Dirac distribution, and our calculation results are in good agreement with the experiments. Likewise, the Drude-like response due to the collective motion of thermalized carriers causes the increase in transient reflectivity, which depends on the change of dielectric constant caused by the collective motion of thermalized carriers. The ultrafast carrier dynamics is modeled by calculating the temporal evolution of the occupation probability of thermalized electrons in the conduction band. On the basis of the two-temperature model, the electron–phonon scattering time is extrapolated to be ∼300 fs in InN, which dominates the cooling of excited electrons.

Nitrogen Radical Beam Irradiation on InN Film for Surface Modification

Nitrogen Radical Beam Irradiation on InN Film for Surface Modification

Molecular beam epitaxy growth of high mobility InN film for high-performance broadband heterointerface photodetectors

III-Nitrides (III-N) heterojunction broadband photodetectors have attracted substantial attention in optoelectronic applications because of their large band tuneability from ultraviolet (UV) to near-infrared (NIR), high temperature, and high-power sustainability as compared to traditional semiconductor materials. InN is extensively studied as optoelectronic material in recent years due to its unique characteristics such as short bandgap, high electron mobility, and high sensitivity in the NIR region. The InN/GaN junction may exhibit exceptional optoelectronic properties due to spontaneous and piezoelectric polarization at the interface. In this work, we fabricated high quality InN/GaN heterointerface with high electron mobility and incorporate it for high-performance broadband photodetector with a simple InN/GaN heterostructure. As a result, a high responsivity of 37.07 A/W, and a specific detectivity of 1.82 × 1013 Jones at 50 µWcm−2 are calculated. The results are verified numerically with Technology Computer-Aided Design (TCAD) software which is comparable with experimental measurements. The proposed InN/GaN can be a potential candidate for the future broadband photodetector market.

Study on the Performance Impact of Introducing an InN Buffer Layer at Various Deposition Temperatures on InN Film Grown by ECR-PEMOCVD on Free-Standing Diamond Substrate

In this study, InN films are grown at a relatively low temperature by electron cyclotron resonance plasma-enhanced metal organic chemical vapor deposition (ECR-PEMOCVD) on free-standing diamond substrates. Due to the high lattice mismatch rate between InN film and the free-standing diamond substrate, the function of a buffer layer is to build a bridge between the substrate and film to reduce the lattice mismatch between them. Therefore, here, we study the performance impact of introducing an InN buffer layer at various deposition temperatures and explore the optimal buffer layer deposition temperature used to grow relatively high-quality InN films. The experimental results show that when an InN buffer layer is introduced at a deposition temperature of 100 °C, the growth direction of the InN film is perpendicular to the substrate with a high c-axis preferred orientation, the roughness of the surface is minimal, and the particle sizes are consistent with growth in the same direction. Additionally, the carrier mobility is highest, and the carrier concentration is lowest compared with other conditions.

Low temperature growth of semi-polar InN [formula omitted] on non-crystalline substrate by plasma-assisted laser ablation technique

We report low-temperature growth of semipolar (101¯1) InN films on a non-crystalline substrate (quartz) using a novel route i.e. plasma-assisted laser ablation technique. The structural, morphological, optical, electrical and chemical/elemental environment of these films was investigated using respective techniques. Dramatic changes associated with the surface morphology of these films/ nanostructures are observed to depend on the growth temperature which is changed only by the difference of 100 °C. These changes are mainly the transformation from a continuous planar 2D nanostructured film (deposited at RT) to 3D faceted nanostructured film (deposited at 300 °C). Observed changes/transformations are attributed to the surface diffusion processes, which are temperature-dependent. These structural transformations are observed to affect the optical as well as electrical properties of the films.

Infrared stimulated emission with an ultralow threshold from low-dislocation-density InN films grown on a vicinal GaN substrate

Near-infrared stimulated emission from a high-quality InN layer under optical pumping was observed with a threshold excitation power density of 0.3 and 4 kW cm−2 at T = 8 and 77 K, respectively. To achieve such a low threshold power density, vicinal GaN substrates were used to reduce the edge-component threading dislocation (ETD) density of the InN film. Cross-sectional transmission electron microscopy images reveal that the annihilation of ETDs can be divided into two steps, and the ETD density can be reduced to approximately 5 × 108 cm−2 near the surface of the 5-µm-thick film. The well-resolved phonon replica of the band-to-band emission in the photoluminescence spectra at 9 K confirm the high quality of the InN film. As a result, the feasibility of InN-based photonic structures and the underlying physics of their growth and emission properties are demonstrated.

Repeatability and Mechanisms of Threading Dislocation Reduction in InN Film Grown with In Situ Surface Modification by Radical Beam Irradiation

The objective of this study was to investigate the repeatability of in situ surface modification by radical beam irradiation to reduce threading dislocation density in InN film. The growth of InN template and N radical irradiation processes were repeated twice in situ in the radio-frequency plasma-excited molecular beam epitaxy chamber before the regrowth of InN film on the N radical irradiated template. Transmission electron microscopy was applied to study dislocation behaviors of the InN film grown. In this letter, we show cross-sectional-view transmission electron microscopy evidence of the threading dislocation reduction from ~2.8×1010 cm-2 in the first irradiated InN layer to ~2.0×1010 cm-2 in the second irradiated InN layer, and to ~1.3×1010 cm-2 in the top regrown InN layer. The mechanisms of threading dislocation reduction were also studied.

Preparation of InN films at different substrate temperatures and the effect of operating temperatures on the carrier transmission characteristics of p-NiO/n-InN heterojunction

The good properties of ITO substrate endow the as-grown InN thin films especially attractive for various optoelectronic applications. Herein, the InN thin films were prepared on the ITO substrates by magnetron sputtering. The effects of substrate temperature (Ts) on the structural, morphological, and optical properties of InN films were assessed by X-ray diffractometer, scanning electron microscope and ultraviolet–visible spectrophotometer. The combination of structural and morphological investigation suggested that the better InN thin film was obtained at 200 °C. Then, the variation of the optical band gap and transmittance with Ts was discussed. What's more, the p-NiO/n-InN heterojunction was trial-produced, and the carrier transport characteristics were studied in the range of 30–120 °C. The heterojunction exhibited excellent I–V characteristics at all operating temperatures. This work enriches the description of InN materials and devices, and provides a useful reference for the practical application of InN.

Correlation of Morphology Evolution with Carrier Dynamics in InN Films Heteroepitaxially Grown by MOMBE

In this study, we report the catalyst-free growth of n-type wurtzite InN, along with its optical properties and carrier dynamics of different surface dimensionalities. The self-catalyzed epitaxial growth of InN nanorods grown by metal–organic molecular-beam epitaxy on GaN/Al2O3(0001) substrates has been demonstrated. The substrate temperature is dominant in controlling the growth of nanorods. A dramatic morphological change from 2D-like to 1D nanorods occurs with decreasing growth temperature. The InN nanorods have a low dislocation density and good crystalline quality, compared with InN films. In terms of optical properties, the nanorod structure exhibits strong recombination of Mahan excitons in luminescence, and an obvious spatial correlation effect in phonon dispersion. The downward band structure at the nanorod surface leads to the photon energy-dependent lifetime being upshifted to the high-energy side.

InN Quantum Dots by Metalorganic Chemical Vapor Deposition for Optoelectronic Applications

This review will cover recent work on InN quantum dots (QDs), specifically focusing on advances in metalorganic chemical vapor deposition (MOCVD) of metal-polar InN QDs for applications in optoelectronic devices. The ability to use InN in optoelectronic devices would expand the nitrides system from current visible and ultraviolet devices into the near infrared. Although there was a significant surge in InN research after the discovery that its bandgap provided potential infrared communication band emission, those studies failed to produce an electroluminescent InN device in part due to difficulties in achieving p-type InN films. Devices utilizing InN QDs, on the other hand, were hampered by the inability to cap the InN without causing intermixing with the capping material. The recent work on InN QDs has proven that it is possible to use capping methods to bury the QDs without significantly affecting their composition or photoluminescence. Herein, we will discuss the current state of metal-polar InN QD growth by MOCVD, focusing on density and size control, composition, relaxation, capping, and photoluminescence. The outstanding challenges which remain to be solved in order to achieve InN infrared devices will be discussed.

Enhancement of interface transportation for quantum dot solar cells using ultrathin InN by atomic layer deposition

Quantum dot-sensitized solar cells have gained rapid development which could produce potential applications. Although they have a theoretical photoelectric conversion efficiency of 44%, there is still a considerable gap in comparison with corresponding practical solar cells, which is mainly due to the fact that the interface transfer, stability and efficiency improvement are still facing some problems. In particular, the carrier recombination loss at the cell interface seriously hinders the quantum dot-sensitized solar cells from developing. In this work, an ultra-thin layer of InN prepared by plasma-enhanced atomic layer deposition is inserted into the FTO/TiO<sub>2</sub> interface of the photoanode of CdSeTe based quantum dot-sensitized solar cells to improve the performance of the photoanode structure, and physical mechanism behind the device is discussed. We first investigate the effects of different deposition temperatures (170, 200 and 230 ℃) on the cell performance of InN films. While the InN ultra-thin layer is deposited at 200 ℃, an enhancement of 16.9% in conversion efficiency is achieved as compared with the reference group. Then, the effects of different thickness (5, 10, and 15 cycles) on the cell are investigated at a fixed deposition temperature of 200 ℃. Additionally, an improvement of fill factor for the device after an introduction of InN layer is observed. This enhancement is further convinced by an apparent reduction of series resistance extracted by the Nyquist curve. The significant increase in fill factor indicates that the introduction of InN accelerates the extraction, transfer and separation of electrons, and reduces the possibility of photon-generated carriers recombination. However, the insertion of InN deposition temperature and thickness have a certain range of enhancement in the cell performance, and further investigation of the mechanism will be carried out.

Impact of the Deposition Temperature on the Structural and Electrical Properties of InN Films Grown on Self-Standing Diamond Substrates by Low-Temperature ECR-MOCVD

The progress of InN semiconductors is still in its infancy compared to GaN-based devices and materials. Herein, InN thin films were grown on self-standing diamond substrates using low-temperature electron cyclotron resonance plasma-enhanced metal organic chemical vapor deposition (ECR-PEMOCVD) with inert N2 used as a nitrogen source. The thermal conductivity of diamond substrates makes the as-grown InN films especially attractive for various optoelectronic applications. Structural and electrical properties which depend on deposition temperature were systematically investigated by reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and Hall effect measurement. The results indicated that the quality and properties of InN films were significantly influenced by the deposition temperature, and InN films with highly c-axis preferential orientation and surface morphology were obtained at optimized temperatures of 400 °C. Moreover, their electrical properties with deposition temperature were studied, and their tendency was correlated with the dependence on micro- structure and morphology.

Optical studies of nitrogen plasma for molecular beam epitaxy of InN

We report on the optical characterization of a nitrogen plasma source based on radiofrequency (RF) used to grow III-nitride materials by molecular beam epitaxy (MBE). Optical emission spectroscopy (OES) was used to study the nitrogen plasma response as a function of the RF power applied and the flow rate of molecular nitrogen. Analysis of the intensities of spectral signals assigned to atomic and molecular species and the ratio of these intensities is performed in detail. The OES results show that the plasma source studied is sensitive to the RF power applied to produce an atomic nitrogen signal, while varying the incoming flow impacts the signal of metastable nitrogen molecules; this outcome allows for the determination of conditions under which certain types of nitrogen species are favored over others. InN films were grown on AlN-buffered Si(111) substrates by MBE under different plasma operational parameters, where, according to the OES studies, atomic nitrogen or excited molecular nitrogen is favored in the plasma. In situ reflection high-energy electron diffraction, scanning electron microscopy, and x-ray diffraction techniques were employed to characterize the InN samples. It is found that the surface morphology of the InN films is highly sensitive to the plasma conditions. A transition in the growth mode from smooth compact films to coalesced islands and columnar structures is observed when the dominant reactive species is atomic nitrogen or excited molecular nitrogen. The results of the characterization are discussed and correlated with the reactive nitrogen species present in the plasma.

Atomic Layer Epitaxy of III-Nitrides: A Microscopic Model of Homoepitaxial Growth.

We develop a microscopic theoretical model of AlN, GaN, and InN film growth by atomic layer epitaxy. To make the model realistic, we take into account the atomic hydrogen that is created by the hydrogen plasma commonly used in plasma-assisted atomic layer epitaxy. This growth technique relies on separate deposition steps for nitrogen and the group-III cation. Our model addresses the processes that occur after a complete monolayer of nitrogen has formed, that is, the deposition, adsorption, surface diffusion, island nucleation, and island growth of group-III cations. According to our model, the three nitrides grow in a standard and qualitatively similar manner: during a brief nucleation phase, a modest attractive interaction leads to stable island nuclei consisting of just a few atoms. These then grow in place at a nearly constant island density and with an island size distribution which obeys an expected universal scaling relationship that depends only on the critical island size.

A Simple Approach to Growth Mode of InN and InGaN Thin Films on GaN(0001) Substrate

Group-III nitride semiconductors have attracted much attention for application in optoelectronic devices such as light emitting diodes and laser diodes. In particular, highly efficient light emitting devices using self-assembled quantum dots have been pained much attention. For InN and InGaN grown on GaN(0001) substrate, it has been reported that three-dimensional (3D) islands are formed after two-dimensional (2D) wetting layers, resulting in Stranski-Krastanow (SK) growth mode [1,2]. Moreover it has been reported that the growth modes of InN on GaN(0001) substrate during molecular beam epitaxy depend on growth conditions such as V/III ratio and substrate temperature[3]: N-rich condition favors 3D growth, whereas In-rich condition leads to 2D growth. However, it is still unclear what is the crucial factor for determining the growth modes of InN and InGaN depending on the growth condition. In this study, the growth mode of InN and InGaN thin films on GaN(0001) substrate are investigated using the macroscopic theory using free energy formula [4] with the aid of empirical potential calculations [5] as well as ab initio calculations [6]. Effects of pyramids island with {1-103} facets and misfit dislocation (MD) formation are taken into account for the evaluation of free energies of InN and InGaN thin films on GaN(0001) substrate.The calculations of cohesive energy difference ΔE as a function of InN film thickness for 2D coherent growth (2D-coherent) and 2D growth with MD (2D-MD) using empirical interatomic potentials reveals that 2D-MD is more stable than 2D-coherent for InN films beyond 0.7 monolayers (ML). On the basis of these results, the MD formation energy Ed and the effective elastic constant M are estimated to be 1.31 eV/Å and 5.60×1011 N/m2. The calculated result of Ed reasonably agrees with previous calculations for dislocation core energy in bulk InN (1.51 eV/Å) [7]. Moreover, the analysis of free energies using the value of Ed and M and surface energies obtained by ab initio calculations (~ 0.074 eV/ Å2) demonstrate that the critical thickness between growth 2D-coherent and 2D-MD under In-rich conditions is 0.45 ML. The calculated results are qualitatively consistent with the experimental results [3]. The calculated phase diagram for growth mode as functions of surface energy of (0001) and surface energy difference between (0001) and (1-103) planes suggest that growth mode boundary between 2D-MD and 3D coherent growth with hexagonal pyramid island formation (3D-coherent) strongly depends on the surface energy difference between (0001) and (1-103) planes. 2D-MD is favorable when the surface energy of InN(1-103) is larger than that of InN(0001), whereas 3D-coherent is favorable when the surface energies of InN(1-103) is smaller than InN(0001). These results imply that the relative stability between (0001) and (1-103) planes depending on the growth condition is decisive for the growth modes of InN on GaN(0001) substrate.[1] S. Ploch et al., Phys. Status. Solidi C 6, 574 (2009). [2] C. Adelmann et al., Appl. Phys. Lett. 76, 1570 (2000). [3] Y. F. Ng et al., Appl. Phys. Lett. 81, 3960 (2002). [4] K. Shiraishi et al., J. Cryst. Growth 237, 206 (2002). [5] K. E. Khor, and S. Das Sarma, Phys. Rev. B 38, 3318 (1988). [6] T. Akiyama et al., Phys. Status Solidi B 255, 1700329 (2018). [7] T. Ito et al., J. Cryst. Growth 298, 186 (2007).Figure: Phase diagrams of growth modes boundary between 2D-MD and 3D-coherent of InN on GaN(0001) substrate as functions of surface energy of (0001) and surface energy difference between (0001) and (1-103) planes. Figure 1

A Simple Approach to Growth Mode of InN and InGaN Thin Films on GaN(0001) Substrate

The growth of InN and InGaN thin films on GaN(0001) substrates are simply interpreted by using the macroscopic theory using free energy formula with the aid of empirical interatomic potential calculations as well as ab initio calculations. Our empirical interatomic potential calculations reveal that the system with 2D growth with misfit dislocation (2D-MD) is more stable than that with 2D coherent growth (2D-coherent) for InN films beyond 0.7 monolayers (ML), and the thickness for the stabilization of 2D-MD decreases with In composition. Moreover, the analysis of free energies demonstrate that 2D-MD is favored under In-rich conditions while three-dimensional growth with both islands and misfit dislocations is favored under N-rich conditions. The difference in growth mode is due to the surface energy anisotropy depending on the growth condition. The results suggest the surface energy anisotropy is a crucial factor for determining the growth modes of InN and InGaN on GaN(0001) substrate.

Effect of multiple-step annealing on the structural, optical and electrical properties of ZnO:In-N films

The intrinsic defects in ZnO:In-N films can be regulated efficiently by multiple-step annealing together with the ambient atmosphere. Correspondingly, the conductivity of films changes from n to p-type, then to n-type (n → p → n). The experimental results indicate high-quality films annealed at 600 °C for 8 min in N2 ambience can be obtained by the analysis of X-ray diffraction (XRD) patterns and the concentrations of donor defects (Zni, VO) are dramatically decreased from Raman and photoluminescence (PL) spectra. When the films are further annealed in air for 3 min, the concentrations of acceptors such as oxygen interstitials (Oi) are increased slightly. Meanwhile, N-related acceptor complexes in the films are dominantly activated in terms of relative X-ray photoelectron spectroscopy (XPS) peak area ratio of P1 (395.5 eV) and P2 (397.3 eV) to P3 (403.5 eV), which plays an important role in realizing the conversion of the ZnO p-type conductivity. Noted that the p-type ZnO:In-N films with the hole concentration of 1017 cm−3 can be obtained in this way.