High-quality InN Research Articles

Simulation and analysis of solar cells based on InN/p-Si: influence on thickness, doping concentration, and temperature dependence

ABSTRACT The current research project intends to enhance solar cells' power and conversion efficiency based on InN/p-Si utilizing the PC1D simulator. A broad direct bandgap of Indium nitride (0.65 eV) makes it suitable for various applications. The InN-based solar cells show an excellent candidate for generating a higher efficiency device, incorporating well-established silicon substrate technology. The open-source PC1D is well-known software for simulating future solar devices without the need to fabricate real devices. The simulated area was adjusted to 10 cm2. The Si substrate and InN layer thicknesses were designed to be 350 μm and 1×10-3 μm, respectively. The n- and p-regions have doping concentrations of 1×1021 cm-3 and 1×1017 cm-3, respectively. This work analyses the influence of geometrical and technological aspects such as both n-p regions thickness, doping concentrations, and temperature dependency to enhance the conversion efficiency of these structures under the AM1.5G solar spectrum with intensity 0.1 W/cm2. It has been demonstrated that the growth of high-quality InN layers and p-type doping persists to be problematic. It appears challenging to find the most suitable material substrate for InN solar. To produce compatible solar cells with simple structures and cost-effective, however, extremely thin layers of n-layer material are required due to the high absorption coefficient of type III-nitrides. The results illustrate that by adjusting the optimized parameter at room temperature to the lowest temperature (200 K), the solar efficiency may increase up from 19.18% to 27.67%.

MOLECULAR BEAM EPITAXY GROWTH OF HIGH-QUALITY InN NANORODS ON Si (111) SUBSTRATE

The growth of high-quality InN nanorods (NRs) on Si (111) by plasma-assisted molecular beam epitaxy (PAMBE) is reported. X-ray diffraction and Raman spectroscopy investigations indicates that the NRs are wurtzite, c-axis oriented and single crystalline. Low temperature photoluminescence emissions with peak energy of ~ 0.75eV was observed indicating the high quality of the nanostructures. This study unravels a novel strategy for the successful growth of high-quality InN NRs on Silicon which is highly promising for applications in next generation nanodevices.

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.

Inhomogeneity of the Current Flow in High-Quality InN Nanowires

Inhomogeneity of the Current Flow in High-Quality InN Nanowires

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.

High electron mobility in nearly-dislocation-free hexagonal InN

We demonstrate a recorded directed-probed electron mobility of ∼4850 cm2 V−1s−1 in nearly-dislocation-free hexagonal InN at room temperature by Hall-effect measurement. Those extremely high-quality InN are achieved through droplet-assisted epitaxy on a GaN/sapphire template by molecular beam epitaxy. They behave as crystals with a diameter of several micrometers, being confirmed to be nearly free of threading dislocation by transmission electron microscopy. The achievement of such high-mobility InN provides promising opportunities for fabricating high-speed electronic devices.

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.

Explore the correlation between intervalley scattering and phonon bottleneck effect on the hot carrier relaxation in bulk GaSb and InN for hot carrier solar cells

Slowed carrier intervalley scattering (IVS) may trap some photo-excited carriers within side valleys and reduce the overall thermalization rate. On the other hand, the thermalization rate could also be significantly retarded within the gamma valley by slowing the Klemens decay due to a large phononic bandgap via the phonon bottleneck effect (PBE). However, the correlation between IVS and PBE on slowing the carrier relaxation rate is still unclear. Herein, high-quality bulk InN and GaSb with similar electronic bandgap but remarkably different phononic structures are studied to clarify the above correlation through picosecond time-resolved photoluminescence. The occurrence of IVS is controlled by using different pump energies at high carrier density (∼1019 cm−3). The relaxations of carriers excited at the band edge just above and far above the side valley threshold are, therefore, studied in GaSb. It shows that the effective hot carrier lifetime (τ1) of GaSb gets significantly extended with higher pump energy mainly due to more carriers being trapped in side valleys through IVS. Nevertheless, the longest τ1 of GaSb is still shorter than the shortest τ1 of InN with a more polar nature and larger phononic bandgap. This implies that PBE plays a more decisive role in retarding Γ valley carrier relaxation than the slowed IVS. It indicates that the large phononic bandgap and Fröhlich coupling constant are keys to the slow thermalization rate if the Fröhlich interaction dominates. If the deformation potential interaction dominates this process, then low side valley threshold energy and the Fröhlich constant determine the reduction of the overall thermalization rate.

Fabrication of InN on epitaxial graphene using RF-MBE

This paper reports the fabrication of InN layers on the epitaxial graphene (EG) using radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE). Prior to the fabrication of InN, single crystalline EG with step and terrace structure was formed on 6H-SiC (0001) substrate in an Ar ambient by the Si sublimation method. Single crystalline epitaxial layers of InN with smooth surfaces are successfully fabricated on the EG using RF-MBE. InN layers with terrace and step structure are grown on the graphene surface up to 2MLs, and InN are grown in a layer by layer 2D growth mode. If the number of layers is increased above 3 MLs, the terrace and steps disappear, and the growth mode changes to 3D mode. The Raman spectroscopy analysis shows that the interfacial stress is reduced for the InN layer grown on the EG surface. The quality of the grown InN layer on the EG surface achieved at present is comparable to the InN film grown on sapphire. This work opens the possibility of growing high-quality InN layers on the EG surface in the near future.

Significant Carrier Extraction Enhancement at the Interface of an InN/p-GaN Heterojunction under Reverse Bias Voltage.

In this paper, a superior-quality InN/p-GaN interface grown using pulsed metalorganic vapor-phase epitaxy (MOVPE) is demonstrated. The InN/p-GaN heterojunction interface based on high-quality InN (electron concentration 5.19 × 1018 cm−3 and mobility 980 cm2/(V s)) showed good rectifying behavior. The heterojunction depletion region width was estimated to be 22.8 nm and showed the ability for charge carrier extraction without external electrical field (unbiased). Under reverse bias, the external quantum efficiency (EQE) in the blue spectral region (300–550 nm) can be enhanced significantly and exceeds unity. Avalanche and carrier multiplication phenomena were used to interpret the exclusive photoelectric features of the InN/p-GaN heterojunction behavior.

Threading dislocation reduction in InN grown with in situ surface modification by radical beam irradiation

In this study, the applicability of in situ surface modification by radical beam irradiation method to reduce threading dislocation density in InN was investigated. The dislocation behavior, crystallographic quality, electrical properties, and photoluminescence properties of InN grown with this method were studied. Transmission electron microscopy observation revealed that dislocation density in the InN layer reduced by a factor of 3 in certain regions and showed good agreement with the results from photoluminescence spectroscopy. If this technology is established, high-quality InN with low threading dislocation density can be achieved with a very simple and repeatable growth process.

High-quality InN films on GaN using graded InGaN buffers by MBE

The growth of high-quality thick InN films is challenging because of the lack of native substrates. In this work, we demonstrate the use of a linearly graded InGaN buffer layer for the growth of InN films on GaN substrates. A 500 nm InN film with <0.1 nm RMS roughness is obtained with a peak mobility of 1410 cm2/(V·s) at 300 K. A strong room temperature photoluminescence showing a bandgap of 0.65 eV with 79 meV linewidth is observed. A graded InGaN buffer is found to lead to extremely smooth and high-quality InN films.

Effect of strain relaxation and the Burstein–Moss energy shift on the optical properties of InN films grown in the self-seeded catalytic process

For the first time, high optical quality InN films were grown on sapphire substrate using atmospheric chemical vapour deposition technique in the temperature range of 560-650 oC. Self-catalytic approach was adopted to overcome the nucleation barrier for depositing InN films. In this process, seeding of the nucleation sites and subsequent growth was performed in the presence of reactive NH3. We investigated the simultaneous effect of strain and Burstein-Moss (BM) energy shift on optical properties of InN films using Raman and photoluminescence spectroscopy. Existence of compressive strain in all films is revealed by Raman spectroscopic analysis and is found to relax with increasing growth temperature. The asymmetric broadening of the A1(LO) phonon mode is observed with the onset of plasmon-phonon interaction for films grown at 620 oC. Large blue shift of the band gap of InN (1.2 eV) is observed as a collective result of compressive strain in films as well as BM shift. Carrier density is calculated using the BM shift in the photoluminescence spectra. Finally, blue shift in band edge emission is observed further because of the presence of compressive strain in the films along with the BM effect.

Excitation dependent Raman studies of self-seeded grown InN nanoparticles with different carrier concentration.

High quality InN nanoparticles are grown using an atmospheric chemical vapour deposition technique via a self-seeded catalytic approach in the temperature range of 580-650 °C. In this temperature region, the nucleation barrier of InN is overcome by seeding low density In nanoparticles prior to introduction of reactive NH3. Samples with increasing carrier densities are grown, with the help of increasing growth temperature, to understand the role of carrier density in the optical phonon structure. Near-resonance Raman spectra show completely different phonon pictures compared to those for the off-resonance spectra. A Raman forbidden mode of B1(high), because of the possible breakdown of selection rules in the near-resonance conditions, is invoked for the first time. The intensity and frequency of this mode strongly depend on the carrier concentration in the sample. In off-resonance conditions, the A1(LO) mode for the sample with higher carrier concentration is dominated by Fano interference rather than plasmon-phonon coupling. Variation of the intensity of the B1(high) mode is correlated with a band filling effect, which is substantiated by the luminescence studies of the InN samples with different carrier concentrations.

Effects of the group V/III ratio and a gan inter-layer on the crystal quality of InN grown by using the hydride vapor-phase epitaxy method

The hydride vapor-phase epitaxy (HVPE) method was used to deposit high-quality InN layers on GaN inter-layer/sapphire (0001) structures that had been fabricated by using either the HVPE method or the metal-organic chemical-phase deposition (MOCVD) method. The effects of the group V/III ratio and different GaN inter-layers on the crystal quality of the InN layers were systemically investigated. The InN layer grown at a low Group V/III ratio revealed a high crystal quality with a two-dimensional (2D) growth mode. Also, the 110.7-nm-thick InN layer grown by using HVPE on a GaN inter-layer/sapphire (0001) substrate structure that had been fabricated by using MOCVD had a high crystal quality, with the full width at half maximum (FWHM) of the InN X-ray diffraction (XRD) peak being about 844 arcsec, and a smooth surface with an atomic force microscopy (AFM) roughness of about 0.07 nm. On the other hand, the 145.7-nm-thick InN layer grown by using HVPE on a GaN inter-layer/sapphire (0001) substrate structure that had been fabricated by using the HVPE method had a lower crystal quality, a FWHM value for the InN (0002) peak of about 2772 arcsec, and a surface roughness of about 3.73 nm. In addition, the peak of the E2 (high) phonon mode for the 110.7-nm-thick InN layer grown by using HVPE on a GaN inter-layer/sapphire (0001) structure that had been fabricated by using MOCVD was detected at 491 cm -1 and had a FWHM of 9.9 cm-1. As a result, InN layers grown by using HVPE on GaN inter-layer/sapphire (0001) substrate structures fabricated by using MOCVD have a high crystal quality and a reduced Raman value, which agrees well with the results of the XRD analysis.

Bulk properties of InN films determined by experiments and theory

Bulk properties of InN are determined by combining experimental and theoretical studies. In this work, we produced high quality InN film deposited on GaN templates by a modified ion beam assisted deposition technique confirmed by low temperature photoluminescence and absorption. The density of states, real and imaginary parts of the complex dielectric function and the absorption coefficient are calculated by means of first-principles beyond density-functional theory. The quasi-particle aspect is described in the framework of a quasi-particle method (the GW approximation). The calculated band-gap energy is ~0.8eV whereas significance in the optical absorption occurs at ~1.2eV, which are consistent with both luminescence and absorption results. The Bethe–Salpeter equation is utilized to model the two-particle exciton interactions, revealing a strong excitonic peak just below the absorption edge of InN.

High-quality InN films on MgO (100) substrates: The key role of 30° in-plane rotation

High crystalline layers of InN were grown on MgO(100) substrates by gas source molecular beam epitaxy. Good quality films were obtained by means of an in-plane rotation process induced by the annealing of an InN buffer layer to minimize the misfit between InN and MgO. In situ reflection high-energy electron diffraction showed linear streaky patterns along the [011¯0] azimuth and a superimposed diffraction along the [112¯0] azimuth, which correspond to a 30° α-InN film rotation. This rotation reduces the mismatch at the MgO/InN interface from 19.5% to less than 3.5%, increasing the structural quality, which was analyzed by high-resolution X-ray diffraction and Raman spectroscopy. Only the (0002) c plane diffraction of α-InN was observed and was centered at 2θ = 31.4°. Raman spectroscopy showed two modes corresponding to the hexagonal phase: E1(LO) at 591 cm−1 and E2(high) at 488 cm−1. Hall effect measurements showed a carrier density of 9 × 1018 cm−3 and an electron Hall mobility of 340 cm2/(V s) for a film thickness of 140 nm.

(Invited) Structural and Optical Properties of InN Quantum Dots Grown by an Alternating Supply of Source Precursors

The InN quantum dots (QDs) grown on a GaN buffer layer have been widely studied for their potential applications, such as solar cells, infrared light-emitting diodes and biosensors etc.. However, the InN growth temperature was restricted due to the issue of low dissociation temperature. Leading In (indium) adatoms are less mobile in finding the energetically favored site, resulting in high density structure defects that generally exist in the conventional metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy growth of InN QDs. The flow rate modulation epitaxy (FME) technique was proposed to overcome the In adatoms migration issue and was proven to be a suitable method for preparing high optical quality InN QDs. Nevertheless, the low density of InN QDs grown by the FME technique is a drawback for most optoelectronic devices based on the QDs’ active layer which prefer high density QDs. In other words, the growth technique for high density QDs plays an important role. It was well known that the adatom kinetic processes, such as adsorption, desorption, and diffusion at surfaces, are considered to be key parameters controlling the dot density, material quality and surface morphology. Recently, the pulsed-mode (PM) MOCVD method was reported as a useful approach for preparing high density InN QDs on GaN buffer layers. However, most of these reports could only achieve one benefit, either density or optical quality. Therefore, in this study we proposed a new method to grow high density InN QDs while maintaining the optical quality.The two-step growth method consists of one growth cycle for step-1 and two growth cycles in step-2. The duration of one growth cycle is 50 sec, including a 10 s TMIn flow process, a 20 s NH3 process, and two 10 s purge processes intervened in between. The two-step growth included step-1 for high density InN nucleation at 575 oC, and then, the reactor temperature was raised to 650 oC in 5 min increments to decompose the poor material quality InN islands. The aim of the step-1 growth was only to create high density nucleation sites on the GaN buffer layer. The reactor temperature was then reduced to 600oC to enable the growth of InN QDs with high density and high optical property in step-2 of the process. The experimental result show that the high density 1.5×1010 cm− 2 truncated hexagonal pyramidal InN QDs can be grown by the two-step growth method. The InN nucleation, InN decomposition and In adatom diffusion in the step-1, annealing and the step-2 process, respectively, determined the density, shape and size distribution of the InN QDs. The bimodal size distribution of the two-step growth InN QDs can be attributed to the high mobility of the incoming adatoms or/and existing islands, which favors coalescence process. The double-peak feature in the photoluminescence (PL) is related to the ground-state emission from the InN QDs with bimodal size-distribution. The low residual carrier concentration (1.8×1018 cm− 3), large confinement energy (342 meV) and intense PL emission for 6.1 nm high InN QDs indicated that a two-step growth is suitable for QD device applications.

GaN-based substrates and optoelectronic materials and devices

In order to solve the problems of GaN heteroepitaxy on sapphire substrate, some techniques were explored. Freestanding GaN substrates have been made by hydride vapor phase epitaxy (HVPE), laser lift-off (LLO), and chemical mechanical polishing techniques. Wafer bending and cracking in the HVPE growth were partly settled by pulsed flow modulation method. High-crystal quality was established for 1.2 mm thick GaN substrate by X-ray diffraction measurement, in which the full width of half maximum values were 72, 110 arcsec for (102), (002) peaks. A novel micro-size patterned sapphire substrate (PSS) and a nano PSS were also fabricated. High-power vertical structure light emitting diodes (VSLEDs) have been developed by Au–Sn eutectic wafer bonding, homemade micro-area LLO, and light extraction structure preparation. The high-injection-level active region with low temperature GaN sandwiched layers was used for low-efficiency droop. The light output power of VSLED was achieved as 400 mW driven at 350 mA, and the dominant wavelength is about 460 nm. The structures and properties of strain modulated superlattices (SLs) and quantum wells as well as advanced simulation of carriers transport across the electron blocking layer were investigated in laser diodes. The hole concentration was achieved as high as 1.6 × 1018 cm−3 in AlGaN/GaN SLs:Mg by inserting an AlN layer. High-quality AlGaN epilayers and structures were grown by MOCVD. Some device structures of UV LEDs and detectors were demonstrated. The emission wavelength of 262 nm UV LED has been successfully fabricated. At last, high-quality InN and InGaN materials for solar cell were grown by boundary-temperature-controlled epitaxy and growth-temperature-controlled epitaxy. Hall-effect measurement showed a recorded electron mobility of 3,280 cm2/(V s) and a residual electron concentration of 1.47 × 1017 cm−3 at 300 K.

Growth of high crystal quality InN by ENABLE‐MBE

AbstractIndium nitride is of interest as a small band gap material for hot carrier solar cells and for alloying of III‐Nitrides in conventional solar cells. Growth of photovoltaic device quality InN on cost effective sapphire wafers is challenging. Lattice mismatch between sapphire and indium nitride (∼25%) makes growth of epitaxial crystals with low crystalline imperfection problematic. InN films with promising XRD results were grown by MBE using a well‐aligned AlN buffer layer to grow InN on sapphire substrates. Crystal quality, analyzed by HRXRD, gives evidence that films of both AlN on sapphire and InN on AlN are almost fully relaxed by edge dislocations created on the interfaces with a rather low density of closed dislocation loops, created subsequently in the film volume of both epitaxial layers. These films exhibit threading dislocation densities of 3 x 107 cm‐2 and 2 × 108 cm‐2 for AlN and InN. Additional metrics quantifying film quality of AlN and InN characterized by XRD and TEM will be presented. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)