Gallium Nitride Films Research Articles

Effect of Different Etching Time on Fabrication of an Optoelectronic Device Based on GaN/Psi

Gallium nitride (GaN)/porous silicon (PSi) film was prepared using a pulsed laser deposition method and 1064 nm Nd: YAG laser for optoelectronic applications and a series of Psi substrates were fabricated using a photoelectrochemical etching method assisted by laser at different etching times for 2.5–15 min at 2.5 min intervals. X-ray diffraction, room-temperature photoluminescence, atomic force microscopy and field emission scanning electron microscopy images, and electrical characteristics in the prepared GaN on the Psi film were investigated. The optimum Psi substrate was obtained under the following conditions: 10 min, 10 mA/cm2, and 24% hydrofluoric acid. The substrate exhibited two highly cubic crystalline structures at (200) and (400) orientations and yellow visible band photoluminescence, and homogeneous pores formed over the entire surface. The pores had steep oval shapes and were accompanied by small dark pores that appeared topographically and morphologically. The GaN/Psi film fabricated through PLD exhibited a high and hexagonal crystallographic texture in the (002) plane. Spectroscopic properties results revealed that the photoluminescence emission of the deposited nano-GaN films was in the ultraviolet band (374 nm) related to GaN material and in the near-infrared band (730 nm) related to the Psi substrate. The topographical and morphological results of the GaN films confirmed that the deposited film contained spherical grains with an average diameter of 51.8 nm and surface roughness of 4.8 nm. The GaN/Psi surface showed a cauliflower-like morphology, and the built-in voltage decreased from 3.4 to 2.7 eV after deposition. The fabricated GaN/Psi film exhibited good electrical characteristics.

Growth of GaN Thin Films Using Plasma Enhanced Atomic Layer Deposition: Effect of Ammonia-Containing Plasma Power on Residual Oxygen Capture.

In recent years, the application of (In, Al, Ga)N materials in photovoltaic devices has attracted much attention. Like InGaN, it is a direct band gap material with high absorption at the band edge, suitable for high efficiency photovoltaic devices. Nonetheless, it is important to deposit high-quality GaN material as a foundation. Plasma-enhanced atomic layer deposition (PEALD) combines the advantages of the ALD process with the use of plasma and is often used to deposit thin films with different needs. However, residual oxygen during growth has always been an unavoidable issue affecting the quality of the resulting film, especially in growing gallium nitride (GaN) films. In this study, the NH3-containing plasma was used to capture the oxygen absorbed on the growing surface to improve the quality of GaN films. By diagnosing the plasma, NH2, NH, and H radicals controlled by the plasma power has a strong influence not only on the oxygen content in growing GaN films but also on the growth rate, crystallinity, and surface roughness. The NH and NH2 radicals contribute to the growth of GaN films while the H radicals selectively dissociate Ga-OH bonds on the film surface and etch the grown films. At high plasma power, the GaN film with the lowest Ga-O bond ratio has a saturated growth rate, a better crystallinity, a rougher surface, and a lower bandgap. In addition, the deposition mechanism of GaN thin films prepared with a trimethylgallium metal source and NH3/Ar plasma PEALD involving oxygen participation or not is also discussed in the study.

Deposition Mechanism and Properties of Plasma-Enhanced Atomic Layer Deposited Gallium Nitride Films with Different Substrate Temperatures

Gallium nitride (GaN) is a wide bandgap semiconductor with remarkable chemical and thermal stability, making it a competitive candidate for a variety of optoelectronic applications. In this study, GaN films are grown using a plasma-enhanced atomic layer deposition (PEALD) with trimethylgallium (TMG) and NH3 plasma. The effect of substrate temperature on growth mechanism and properties of the PEALD GaN films is systematically studied. The experimental results show that the self-limiting surface chemical reactions occur in the substrate temperature range of 250-350 °C. The substrate temperature strongly affects the crystalline structure, which is nearly amorphous at below 250 °C, with (100) as the major phase at below 400 °C, and (002) dominated at higher temperatures. The X-ray photoelectron spectroscopy spectra reveals the unintentional oxygen incorporation into the films in the forms of Ga2O3 and Ga-OH. The amount of Ga-O component decreases, whereas the Ga-Ga component rapidly increases at 400 and 450 °C, due to the decomposition of TMG. The substrate temperature of 350 °C with the highest amount of Ga-N bonds is, therefore, considered the optimum substrate temperature. This study is helpful for improving the quality of PEALD GaN films.

Direct growth of flexible GaN film via van der Waals epitaxy on mica

A flexible gallium nitride (GaN) thin film was obtained on fluorphlogopite-mica through van der Waals epitaxy using an optimized two-step growing process in a hydride vapor phase epitaxy system. The interfacial stress caused by the mismatches of thermal expansion coefficient was significantly reduced via van der Waals epitaxy. The proposed two-step growth allows such low-stressed GaN thin film forms on mica directly where a critical (minimum) growth pressure was defined and the temperatures chosen were optimized. The transmittance was controlled by surface roughness and growth temperature, as high density of defects near their interface caused by GaN decomposition leading the GaN film non-transparent. In the measurement region, the average transmittance decreased linearly as the GaN surface became rougher. The bending of the flexible GaN thin film caused only slight PL peak shift. Thus, such direct grown GaN film on fluorphlogopite-mica can be evaluated as a reliable optical device.

Morphological and structural characteristics of Gallium Nitride (GaN) porosity using image processing

Attaining a generalized porosity measurement based on the conventional measuring methods is unfeasible due to thin and complex layers on Gallium Nitride (GaN) film. Therefore, there is an impetus for developing a method of estimating general fabric porosity via image processing techniques. This study is aimed to investigate and validate a new technique for extracting the GaN porosity in terms of morphological and structural characteristics using an image processing technique. The anodization on porous GaN films is prepared by using direct current photo-assisted electrochemical etching (DC-PECE) etching technique. The quantitative structural characteristics based on mathematical morphology is analyzed using Field Emission Scanning Electron Microscopy (FESEM) and Atomic Force Microscopy (AFM). To validate the method, the evaluation of porous GaN quality is performed through a non-destructive investigation of its nanostructures using adapting image analysis techniques. Porosity of the structures obtained by calculating the areas occupied by the pores. The quantitative results were obtained and showing good agreement between two modes of measurement and calculation (percentage porosity and pore depth) based on the image-processing data with 91 % and 78 % correlation coefficient.

Downstream Electric Field Effects during Film Deposition with a Radio Frequency Plasma and Observations of Carbon Reduction

Strong electric fields are generated by radio frequency (RF) plasma sources, and though the RF portion is too high a frequency for ions to react, the direct current (DC) portion of these fields has been shown to cause the atomic migration of metals, which can influence film morphology even downstream of the plasma where ionized plasma species are absent. In particular, we have observed the growth of nanopillars due to metal atoms migrating toward the positive field of the remote plasma. A biased grid placed between the plasma and the substrate can shield the substrate from these fields so that, when grounded, smooth films can be grown to a root mean square roughness of less than 1 nm. Positively biasing the grid returns the growth of nanocolumns. Interestingly, negatively biasing the grid significantly reduced the carbon and hydrocarbon content of gallium nitride films grown at a low temperature (~660 °C) using a nitrogen plasma, as observed using secondary ion mass spectroscopy (SIMS) and optical absorption measurements. The films also showed a notable improvement in conductivity and visible appearance.

Fabrication of hexagonal microstructure on gallium nitride films by wet etching assisted femtosecond laser ablation

Nitride-based microstructures are highly desirable for next-generation photonic and micro-electronic technologies. However, the efficient realization of regular microstructures for gallium nitride (GaN) is still technically challenging. This paper demonstrates the formation of hexagonal microstructures on GaN films via femtosecond laser ablation and wet chemical (hot KOH) etching. The fabricated hexagonal microstructures are shown to undergo anisotropic ablation. The structures with tunable profiles and dimensions are obtained by varying the laser energy and etching time. Additionally, we demonstrate that the microstructures were formed by selectively etching away the laser-irradiated region. This work could lead to a new approach to fabricating nitride-based microstructures using wet etching-assisted ultrafast laser processing technologies.

Hierarchically porous GaN thin films fabricated using high fluence Ar ion implantation of epitaxial GaN on sapphire

• We show the fabrication of hierarchical pores in GaN by means of Ar ion implantation. • This approach is free of the limitations of anodic etching of porous GaN. • There is no need for uniform current spreading to obtain uniform porosity. • The submission focuses on the material properties not on the implantation process. • We demonstrate different damage behavior for single and double ionized Ar. We show a method to fabricate porous gallium nitride (GaN) films without the need for wet chemical etching nor high donor concentrations in the starting material. The pores in epitaxial GaN films were formed using 200 keV Ar + ion implantation at fluences ranging from 2.5∙10 16 Ar + ∙cm −2 to 10 17 Ar + ∙cm −2 . The properties of the created nanostructures are discussed by means of complementary structural, optical and electrical studies. Electrochemical capacitance of the samples in an aqueous electrolyte is studied to deconvolute information about the surface area available after the implantation. Single and double ionized implantation is compared in the discussion and different damage in the two cases is shown. The results show information complementary to the current models of damage buildup in gallium nitride films upon high fluence irradiation as well as present a means of fabricating on-wafer porous areas for future GaN-based devices. The method overcomes the drawbacks of the commonly applied anodic etching: it does not require a high GaN donor concentrations nor difficult setups for achieving uniform etching. Furthermore, it can be applied locally using appropriate masking of the selected areas with a thick photoresist or other strippable inorganic ion stopping film.

Precessed electron diffraction study of defects and strain in GaN nanowires fabricated by top-down etching

Gallium nitride (GaN) nanowires (NWs) have been fabricated by top-down etching from GaN heteroepitaxial films, which provides an accurate control of their position and dimensions. However, these NWs contain, similar to the initial GaN films, high density of structural defects such as threading dislocations (TDs). In this work, different strategies to reduce the density of defects along the NWs have been compared based on two different wet etching approaches followed by a rapid thermal annealing (RTA) at 750 °C. The addition of a 30 nm SiNx coating is also explored. The defects and strain/stress along the NWs have been studied by high resolution transmission electron microscopy, diffraction contrast imaging in two-beam conditions and 4D STEM, as well as strain maps calculated from scanning precession electron diffraction measurements. RTA reduced the density of TDs at the middle of GaN NWs with bare surfaces by approximately 25%. The reduction increased to approximately 70% by RTA of GaN NWs with surfaces coated by amorphous SiNx, which is attributed to enhancement of dislocation movements by stresses induced from differential thermal expansion of GaN and SiNx. These results suggest a process route that, if optimized and combined with reduction of NW diameter, could establish etching as an efficient fabrication method for high crystal quality GaN NWs.

Ultrafast processes simulation under femtosecond laser irradiation of Gallium Nitride thin films

Lateral epitaxial growth is an effective method to reduce dislocations in Gallium Nitride (GaN) films. Compared to heteroepitaxy, the homoepitaxial growth films have lower stress. However, its application is limited due to the complex processes and high cost. In recent years, femtosecond laser technology has been applied in wide bandgap semiconductor processing increasingly due to its high instantaneous power density and excellent photon absorption rate. In the homoepitaxial growth of GaN, the femtosecond laser technology used for processing in-situ patterns is an effective method to reduce dislocations. Moreover, the laser equipment is relatively simple and compact, easy to be integrated into an in-situ growth cavity. However, there is still a lack of research on the mechanism of ultrafast laser processing of pattern windows on GaN substrates. In our study, the model of the transient nonlinear evolutions of femtosecond laser acting on GaN is established to solve partial differential equations with coefficients of the GaN film and the laser. The optical and electrical properties, such as the laser intensity and reflectivity, can be obtained. In addition, the ablation morphology of GaN can be predicted. The results show that under the action of the femtosecond laser with a power intensity of 0.6 J/cm2 and a wavelength of 1030 nm, numerous free electrons are generated on the GaN surface, and the reflectivity increases to 0.94. The electron temperature rises to 9750 K and then decreases. While the lattice temperature rises slowly, reaching 2060 K around 4 ps, which is in equilibrium with the electron temperature. The ablation pit on the thin film surface has a radius of 1 μm and a depth of 0.2 μm. The simulation results are consistent with the current theoretical and experimental results, which are helpful in further revealing the transient nonlinear mechanism of ultrafast laser acting on GaN. Under our simulation conditions, the femtosecond laser technology can be applied to process the sub-micron-sized patterns, which provides a more theoretical basis for preparing the GaN in-situ growth patterns.

Thin film transistors and metal–semiconductor–metal photodetectors based on GaN thin films grown by inductively coupled plasma metal-organic chemical vapor deposition

Gallium nitride (GaN) films with an almost single-crystalline quality, smooth surface, and high optical transmission were prepared at 580 °C using inductively coupled plasma metal-organic chemical vapor deposition (ICP-MOCVD) based on a showerhead structure. The GaN thin films with unintentional doping were used as the active layer in the production of thin-film transistors (TFTs) and metal–semiconductor–metal (MSM) photodetectors. TFTs had on-to-off current ratios of 6.3 × 103 and their output characteristics demonstrated the proper saturation behavior of transistors. The field-effect mobility was 3.79 cm2 (V s)−1. MSMs were visible blind, with a contrast of almost three orders of magnitude between ultraviolet and green light. The maximum responsivity was 34 mA W−1 with a 10 V bias voltage and a 325 nm illumination. When the bias voltage was >2 V, the quantum efficiency and noise equivalent power under 266 nm illumination reached >10% and <1, respectively. These findings suggest that the ICP-MOCVD technique has the potential to be used to fabricate (opto)electronic devices at low temperatures.

Optical properties of LMBE grown c-axis oriented GaN thin films using Surface Plasmon Resonance technique

Present work reports the growth and optimisation of thin films of Gallium nitride (GaN) with preferred c-axis (0002) orientation on Si (111) substrate using the Laser Molecular Beam Epitaxy technique. For the development of optically efficient GaN thin film, the nitrogen gas flow during film deposition has been optimised and film properties are measured using the highly sensitive Surface Plasmon Resonance (SPR) technique in OTTO configuration. Structural, morphological and optical properties of the GaN thin films were studied using XRD, AFM, PL and FTIR spectroscopy. Varying the deposition gas flow from 1.3 sccm to 4 sccm led to a decrease in refractive index values from 2.77 to 2.21. By using the SPR technique, the refractive index (n) dispersion of GaN thin films was also analysed with the incident radiated wavelength.

GaN growth on (0 0 1) and (1 1 0) MgO under different Ga/N ratios by MBE

The growth of gallium nitride (GaN) films by molecular beam epitaxy (MBE) at 700 °C on (0 0 1) and (1 1 0) magnesium oxide (MgO) under different Ga/N ratios was investigated by high-resolution X-ray diffraction (HRXRD). While the lowest Ga/N ratio of 1.0 % led to a complete hexagonal GaN film (h-GaN), the other ratios show cubic films with small hexagonal inclusions. On (1 1 0) MgO, only cubic GaN (c-GaN) can be grown with a Ga/N ratio of 3.7 %. A Ga/N ratio of 1.9 % results in a mixture layer of cubic and hexagonal GaN. Independent of the substrate orientation, a cube-on-cube epitaxial relationship can be detected by ϕ-scans for c-GaN to MgO. The hexagonal GaN shows a 12-fold symmetry.

Two-Step Deposition of an Ultrathin GaN Film on a Monolayer MoS2 Template

Ultrathin gallium nitride (GaN) application can be profoundly influenced by its quality, especially the issue of amorphous interfacial layers formed on conventional substrates. Herein, we report a two-step deposition of an ultrathin GaN film via the plasma-enhanced atomic layer deposition (PEALD) technique on a mono-MoS2 template over a SiO2/Si substrate for quality improvement, by starting the deposition temperature at 260 °C and then ramping it to 320 °C. It was found that a lower initiating deposition temperature could be conducive to maintaining the mono-MoS2 template to support the subsequent growth of GaN. Compared to the control group of one-step high-temperature deposition at 320 °C, ideal layer-by-layer film growth is achieved at the low temperature of the two-step method instead of island formation, leading to the direct crystallization of GaN on the substrate with a rather sharp interface. Structural and chemical characterizations show that this two-step method produces a preferred [0001] orientation of the film originating from the interface region. Additionally, the improved two-step ultrathin GaN displays a smooth surface roughness as low as 0.58 nm, a low oxygen impurity concentration of 3.6%, and a nearly balanced Ga/N stoichiometry of 0.95:1. Our work paves a possible way to the feasible fabrication of ultrathin high-quality PEALD-GaN, and it is promising for better performance of relevant devices.

Surface Structures from NH3 Chemisorption in CVD and ALD of AlN, GaN, and InN Films

Aluminum nitride (AlN), gallium nitride (GaN), and indium nitride (InN) form a family of technologically important semiconductors of high importance to light-emitting diodes and high-frequency electronics. Although thin films of these materials are routinely manufactured by chemical vapor deposition (CVD) and atomic layer deposition (ALD), these methods are far from optimal and knowledge of the underlying chemical processes is lacking. In this work, we performed ab initio investigations of the surface coverage of these materials under an ammonia-rich atmosphere. Periodic density functional theory calculations were used to test the probable surface structures, and their electronic and thermal energies were used to calculate their contribution to the surface composition under the temperature and pressure conditions relevant for CVD and ALD processes of these materials. The results show similarities between the group of materials with a similar NHx surface structure present for all three. Comparison of the coverage showed that at low growth temperatures, the surface is expected to be covered by NH2, while at high temperatures, most surface sites would be vacant. The surface structures were all found to be the most stable on AlN and least stable on InN. These results are important for further investigations of the material growth mechanisms.

Polarization‐Driven‐Orientation Selective Growth of Single‐Crystalline III‐Nitride Semiconductors on Arbitrary Substrates

AbstractIII‐nitride semiconductor films are usually achieved by epitaxial growth on single‐crystalline substrates (sapphire, silicon (Si), and silicon carbide). It is important to grow these films on non‐epitaxial substrates of interest such as polycrystalline substrates for exploring novel applications in electronics and optoelectronics. However, single‐crystalline III‐nitride films with uniform orientation on non‐epitaxial substrates have not yet been realized, due to the lack of crystallographic orientation of the substrates. Here, this work proposes a strategy of polarization‐driven‐orientation selective growth (OSG) and demonstrate that single‐crystalline gallium nitride (GaN) can in principle be achieved on any substrates. Taking polycrystalline diamond and amorphous‐silicon dioxide/Si substrates as typical examples, the OSG is demonstrated by utilizing a composed buffer layer consisting of graphene and polycrystalline physical vapor deposited (PVD) aluminium nitride (AlN). The polarization of the PVD AlN can effectively tune the strength of interfacial orbital coupling between AlN nuclei and graphene at different rotation angles, as confirmed by atom‐scale first‐principles calculations, and align the AlN nuclei to form a uniform orientation. This consequently leads to continuous single‐crystalline GaN films. The ability to grow single‐crystalline III‐nitrides onto any desired substrates would create unprecedented opportunities for developing novel electronic and optoelectronic devices.

Morphological study of thin films: Simulation and experimental insights using horizontal visibility graph

We studied the morphological nature of various thin films such as silicon carbide (SiC), diamond (C), germanium (Ge), and gallium nitride (GaN) on silicon substrate Si(100) using the pulsed laser deposition (PLD) method and Monte Carlo simulation. We, for the first time, systematically employed the visibility algorithm graph to meticulously study the morphological features of various PLD grown thin films. These thin-film morphologies are investigated using random distribution, Gaussian distribution, patterned heights, etc. The nature of the interfacial height of individual surfaces is examined by a horizontal visibility graph (HVG). It demonstrates that the continuous interfacial height of the silicon carbide, diamond, germanium, and gallium nitride films are attributed to random distribution and Gaussian distribution in thin films. However, discrete peaks are obtained in the brush and step-like morphology of germanium thin films. Further, we have experimentally verified the morphological nature of simulated silicon carbide, diamond, germanium, and gallium nitride thin films were grown on Si(100) substrate by pulsed laser deposition (PLD) at elevated temperature. Various characterization techniques have been used to study the morphological, and electrical properties which confirmed the different nature of the deposited films on the Silicon substrate. Decent hysteresis behavior has been confirmed by current-voltage (IV) measurement in all the four deposited films. The highest current has been measured for GaN at ∼60 nA and the lowest current in SiC at ∼30 nA level which is quite low comparing with the expected signal level (μA). The HVG technique is suitable to understand surface features of thin films which are substantially advantageous for the energy devices, detectors, optoelectronic devices operating at high temperatures.

Low‐Energy UV Ultrafast Laser Controlled Lift‐Off for High‐Quality Flexible GaN‐Based Device (Adv. Funct. Mater. 8/2022)

Ultrafast Laser Lift-Off In article number 2111920, Lingfei Ji and co-workers present a novel one-step laser lift-off (LLO) for an ultra-smooth, low-stress patterned gallium nitride (GaN) film and GaN-based light-emitting diode device without affecting the electroluminescence performance of the low-energy ultrafast laser, showing great promise in industrial fabrication of flexible GaN-based electronics.

Cascade mechanism and mechanical property of the dislocation loop formation in GaN twin crystal-induced crystallization

Single-crystal gallium nitride (GaN) is considered as the key to future high-performance light emitters and power devices. Nevertheless, a mixed phase with micro twins often exists in GaN films. The formation of twin strongly affects the electronic properties of GaN-based devices. Based on the molecular dynamics (MD) simulation, we built a perfect nano-twinned cubic GaN (NT-cGaN) and created a solid-liquid (S/L) model in order to study the growth characteristics of the NT-cGaN at different isothermal temperatures. We observed an interesting dislocation loop (DL) in some systems. Our work not only reveals the formation process of DLs and its effect on mechanical properties, but also proposes some parameters to describe the length and shape of DLs. The results indicated that a perfect DL is coalesced by other dislocation lines after some dislocation reactions. A system with a DL is more stable than a system with a dislocation network (DN), and the length and shape of the DLs are closely relative to the isothermal time and temperatures. Moreover, increasing the isothermal time and temperatures can relieve the DL. In general, understanding nucleation and coalescence of dislocations is crucial in decreasing the dislocation density in a GaN crystal.

Deterministic modeling of hybrid nonlinear effects in epsilon-near-zero thin films

In nonlinear optics, significant effort is concentrated on improving the strength and efficiency of interactions; however, experimentally investigating nonlinear materials is a complex, time-consuming, and costly investment. Moreover, it is often challenging to isolate, study, and optimize material parameters in an experiment due to complexities in the growth process. Recently, epsilon-near-zero materials have received a great deal of attention as promising nonlinear optical materials, but like many up-and-coming materials, the ability to explore and optimize their properties has been challenging. Here, we establish a framework to rapidly evaluate the performance of nonlinear epsilon-near-zero materials for both inter- and intraband effects in silico, requiring only an energy–momentum (E–k) diagram, linear optical properties, and experimental conditions. Measured nonlinear reflection and transmission in gallium-doped zinc oxide films are compared to the numerical framework for both intra- and interband excitation to verify accuracy across wavelength and irradiance while two figures of merit (FoMs) are introduced to quickly evaluate the performance of films without a full numerical framework. This capability is used to predict the performance of highly doped gallium nitride, cadmium oxide, zinc oxide, and indium tin oxide films, and efficient intra- and interband operation conditions are identified. Through this numerical framework and the FoMs, the exploration of unstudied epsilon-near-zero materials is enabled without the need for a nonlinear experiment, thereby accelerating the search for more efficient nonlinear materials and excitation conditions.