GaN Thin Films Research Articles

Spatial Distribution Control of Room‐Temperature Single Photon Emitters in the Telecom Range from GaN Thin Films Grown on Patterned Sapphire Substrates

Spatial Distribution Control of Room‐Temperature Single Photon Emitters in the Telecom Range from GaN Thin Films Grown on Patterned Sapphire Substrates

Formation of decorated LIPSS on GaN thin film by UV picosecond laser beam in air and under vacuum

Formation of decorated LIPSS on GaN thin film by UV picosecond laser beam in air and under vacuum

Fabrication of Al/n-GaN/p-Si/Al diodes by thermal evaporation and evaluation of effect of gamma irradiation on device properties

This study investigates the impact of γ-irradiation on the material and device properties of Al/n-GaN/p-Si/Al heterojunction diodes. GaN thin films were deposited on glass and p-Si substrates using thermal evaporation, followed by annealing at 450 °C. The diodes were subjected to γ-irradiation doses of 0, 3, and 6 kGy. X-ray diffraction (XRD) measurements revealed significant structural changes, including phase transitions influenced by radiation. Electrical characteristics were assessed through current–voltage (I-V) measurements within the ± 2 V range. Notably, the ideality factor for the annealed diodes improved from 6.60 to 4.62 and 3.85 with increased γ-irradiation. The barrier height was determined to be 0.85 eV, and it did not exhibit a significant change upon γ-irradiation dose. The results provided valuable insights into the response of heterojunction diodes to radiation exposure, aiding in the understanding and potential improvement of the radiation resistance of GaN-based electronic devices.

Control polarity of gallium nitride on Si (1 1 1) using atomic layer annealing and thermal atomic layer deposition

Ga-polar GaN thin films and N-polar GaN thin films were fabricated by using thermal ALD and plasma assisted ALA processes, respectively. The polarity of GaN wurtzite thin films was determined by cross-sectional STEM analysis of as-deposited films and top-view SEM images after KOH wet-etching. The crystallinity and grain size of thermal ALD GaN were drastically increased with the GaN ALA buffer layer. The ALA Ar plasma pulse in each ALD cycle decreased the oxygen and carbon impurity content and improved the crystallinity of GaN thin films by removing organic ligands. High purity GaN thin films were successfully deposited on oxide-free Si (111) substrate by using the ALA process with 100 ms of TDMAGa with push gas, 3 × 800 ms of N2H4, and 15 s of 25 W ALA plasma pulse with −25 V stage DC bias at 275 °C. The lowest FWHM value (0.59°), smooth surface morphology (RMS=1.0 nm), and 10–20 nm grain size of columnar GaN thin films were obtained by tuning process parameters.

Sulfur-doped g-C3N4/GaN n-n heterojunction for high performance low-power blue-ultraviolet photodetector with ultra-high on/off ratio and detectivity

Polymeric graphitic carbon nitride (g-C3N4) shares a structural similarity to graphene, yet it possesses a distinctive electronic band structure, rendering it promising for photoelectric applications. However, g-C3N4 faces inherent limitations such as inadequate crystalline quality, limited conductivity, and a short lifespan of photogenerated charge carriers, all of which impede the photoelectric conversion efficiency. In this work, we introduce a two-step thermal vapor condensation method for depositing compact, crack-free sulfur-doped g-C3N4 thin films. The doped thin films exhibit reduced interlayer spacing, which leads to improved conductivity and charge transfer capabilities. These processes culminate in the creation of sulfur-doped g-C3N4/GaN thin film heterojunction blue-ultraviolet photodetectors with remarkable photodetection performance, including an ultrahigh on/off ratio of 7.3 × 107 and specific detectivity of 2.06 × 1014 Jones under a low bias of −0.4 V. Moreover, it is found that the n-n heterojunction undergoes a transition from photodiode to photoconduction behavior, attributed to the photoinduced electron trapping effects by sulfur-doped sites, which outcomes a notable gain with the responsivity as high as 581 mA/W. These findings underscore the vast potential of high-quality sulfur-doped g-C3N4 thin films for photoelectric applications.

Ultrastable Zn3N2 Thin Films via Integration of Amorphous GaN Protection Layers

AbstractZinc nitride (Zn3N2) is a promising semiconductor for a range of optoelectronic and energy conversion applications, offering a direct bandgap of 1.0 eV, large carrier mobilities, and abundant constituent elements. However, the material is prone to bulk oxidation in ambient environments, which has thus far impeded its practical deployment. While previous approaches have focused on stabilizing the material via integration of ZnO surface layers, these strategies introduce additional challenges regarding elevated processing temperatures and limited control of interface properties. In this study, it is shown that amorphous GaN thin films can serve as highly stable protection layers on Zn3N2 surfaces and can be deposited at the same growth temperature and in the same deposition system as the underlying semiconductor. The GaN‐capped Zn3N2 structures exhibit long‐term stability, surviving over 3 years of exposure to ambient conditions with no discernible alterations in composition, structure, or electrical properties. Notably, the amorphous GaN coatings can even impede Zn3N2 oxidation under prolonged aqueous exposure. Thus, this study offers a solution to stabilize Zn3N2 in ambient conditions, providing a viable pathway to its utilization in robust and high‐performance electronic devices, such as thin film transistors and solar energy conversion systems.

Optical, Structural, and Synchrotron X-ray Absorption Studies for GaN Thin Films Grown on Si by Molecular Beam Epitaxy.

GaN on Si plays an important role in the integration and promotion of GaN-based wide-gap materials with Si-based integrated circuits (IC) technology. A series of GaN film materials were grown on Si (111) substrate using a unique plasma assistant molecular beam epitaxy (PA-MBE) technology and investigated using multiple characterization techniques of Nomarski microscopy (NM), high-resolution X-ray diffraction (HR-XRD), variable angular spectroscopic ellipsometry (VASE), Raman scattering, photoluminescence (PL), and synchrotron radiation (SR) near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. NM confirmed crack-free wurtzite (w-) GaN thin films in a large range of 180-1500 nm. XRD identified the w- single crystalline structure for these GaN films with the orientation along the c-axis in the normal growth direction. An optimized 700 °C growth temperature, plus other corresponding parameters, was obtained for the PA-MBE growth of GaN on Si, exhibiting strong PL emission, narrow/strong Raman phonon modes, XRD w-GaN peaks, and high crystalline perfection. VASE studies identified this set of MBE-grown GaN/Si as having very low Urbach energy of about 18 meV. UV (325 nm)-excited Raman spectra of GaN/Si samples exhibited the GaN E2(low) and E2(high) phonon modes clearly without Raman features from the Si substrate, overcoming the difficulties from visible (532 nm) Raman measurements with strong Si Raman features overwhelming the GaN signals. The combined UV excitation Raman-PL spectra revealed multiple LO phonons spread over the GaN fundamental band edge emission PL band due to the outgoing resonance effect. Calculation of the UV Raman spectra determined the carrier concentrations with excellent values. Angular-dependent NEXAFS on Ga K-edge revealed the significant anisotropy of the conduction band of w-GaN and identified the NEXAFS resonances corresponding to different final states in the hexagonal GaN films on Si. Comparative GaN material properties are investigated in depth.

CVD-Grown Monolayer MoS2 and GaN Thin Film Heterostructure for a Self-Powered and Bidirectional Photodetector with an Extended Active Spectrum.

Photodetector technology has evolved significantly over the years with the emergence of new active materials. However, there remain trade-offs between spectral sensitivity, operating energy, and, more recently, an ability to harbor additional features such as persistent photoconductivity and bidirectional photocurrents for new emerging application areas such as switchable light imaging and filter-less color discrimination. Here, we demonstrate a self-powered bidirectional photodetector based on molybdenum disulfide/gallium nitride (MoS2/GaN) epitaxial heterostructure. This fabricated detector exhibits self-powered functionality and achieves detection in two discrete wavelength bands: ultraviolet and visible. Notably, it attains a peak responsivity of 631 mAW-1 at a bias of 0V. The device's response to illumination at these two wavelengths is governed by distinct mechanisms, activated under applied bias conditions, thereby inducing a reversal in the polarity of the photocurrent. This work underscores the feasibility of self-powered and bidirectional photocurrent detection but also opens new vistas for technological advancements for future optoelectronic, neuromorphic, and sensing applications.

Optimization of high-quality gallium nitride thin films deposited on silicon substrates with an aluminum nitride buffer layer through radio-frequency magnetron sputtering

In this study, high-quality GaN thin films were deposited on Si (111) substrates with an AlN buffer layer through radio-frequency (RF) magnetron sputtering. The sputtering parameters for the AlN buffer layer were optimized to enable subsequent high-quality GaN deposition. GaN thin films with a thickness of 300 nm were deposited in situ by using Ga metal as the sputtering target with N2 gas for performing reactive sputtering at a temperature of 600 °C and an RF power of 350 W. X-ray diffraction analysis confirmed the presence of a crystalline phase in the (002) plane at 2θ = 34.5° for the sputtered GaN thin films. Photoluminescence spectroscopy conducted at 100 K revealed that the GaN thin film had an emission peak at 3.37 eV (367 nm). The results of transmission electron microscopy and X-ray photoelectron spectroscopy verified that high-quality GaN thin films were formed over AlN buffer layers. The findings of this study suggest that RF-sputtered GaN films on Si substrates can be applied in electronic and optoelectronic devices.

Synthesis and Structure of Thin GaN Films by Radio-Frequency Sputtering

Synthesis and Structure of Thin GaN Films by Radio-Frequency Sputtering

Identification of Elastoplastic Constitutive Model of GaN Thin Films Using Instrumented Nanoindentation and Machine Learning Technique

This study presents a novel inverse identification approach to determine the elastoplastic parameters of a 2 µm thick GaN semiconductor thin film deposited on a sapphire substrate. This approach combines instrumented nanoindentation with finite element (FE) simulations and an artificial neural network (ANN) model. Experimental load–depth curves were obtained using a Berkovich indenter. To generate a comprehensive database for the inverse analysis, FE models were constructed to simulate load–depth responses across a wide range of GaN thin film properties. The accuracy of both 2D and 3D simulations was compared to select the optimal model for database generation. The Box–Behnken design-based data sampling method was used to define the number of simulations and input variables for the FE models. The ANN technique was then employed to establish the complex mapping between the simulated load–depth curves (input) and the corresponding stress–strain curve (output). The generated database was used to train and test the ANN model. Then, the learned ANN model was used to achieve high accuracy in identifying the stress–strain curve of the GaN thin film from the experimental load–depth data. This work demonstrates the successful application of an inverse analysis framework, combining experimental nanoindentation tests, FE modeling, and an ANN model, for the characterization of the elastoplastic behavior of GaN thin films.

Narrow-band n-GaN/n-Si isotype heterojunction photodiode: A simplified approach for photodiode development

An innovative approach has been explored to fabricate a high-performance photodiode in semiconductor device production, utilizing the thermal evaporation technique to grow GaN thin films. Structural analyzes were conducted on both as-deposited and annealed samples, and their properties were determined using various characterization techniques, including FESEM, XRD, XPS, Hall effect, photoluminescence, and UV-Vis spectrophotometer measurements. Valuable information has been gained about the potential of this alternative approach for fabricating GaN-based electronic devices. In this study, Al/n+-GaN/n-Si/Ag (as-deposited GaN) and Al/n-GaN/n-Si/Ag (annealed GaN) structures have been fabricated. It was found that the device produced from the as-deposited GaN thin film exhibited better performance than the annealed one. The isotype heterojunction n+-GaN/n-Si photodiode exhibited exceptional performance in detecting NIR light at a wavelength of 1050 nm under a 0 V bias. It has a full-width at half-maximum (FWHM) value of 120 nm and achieved a photoresponsivity value of 355 mA/W at a light output of 10 mW/cm². It is further supported by its external quantum efficiency of 42% and detectivity value of 6.93×1011 Jones at the peak wavelength and very fast response speed with ∼33 µs rise/fall times. These findings highlight the potential of photodiode as an efficient narrow-band in the NIR region.

Direct growth of highly oriented GaN thin films on silicon by remote plasma CVD

We report on low-temperature (500 °C) and low-pressure (0.3 mbar) direct growth of GaN thin films on silicon (100) substrates using remote plasma chemical vapour deposition (RP-CVD). In the custom-designed reactor, an RF inductively coupled plasma is generated remotely from the substrate’s area to facilitate the decomposition of group-V precursor, N2 with added H2, while group-III precursor trimethylgallium (TMGa), is directly injected into the growth chamber mixed with H2 carrier gas. Growth parameters such as RF power, process pressure and gas flow rates have been optimized to achieve a film growth rate of about 0.6 µm h−1. Several characterization techniques were used to investigate the plasma and the properties of the grown thin films in terms of their crystallinity, morphology, topography, and composition. The films are highly textured with a preferential orientation along the c-axis of the wurtzite structure. They present a small roughness in the nanometer range and a columnar microstructure with a grain size of one hundred nanometer, and a gallium polarity (+c plane oriented). Rutherford backscattering spectrometry and nuclear reaction analysis show that the chemical composition is homogeneous through the depth of the layer, with a III/V ratio close to 1, a very low content of oxygen (below the detection limit ∼1%) and a carbon content up to 11%. It was shown that the increase of plasma power helps to reduce this carbon contamination down to 8%. This research paves the way for a growth method compatible with cost reduction of III–V thin film production achieved through reduced gas consumption facilitated by RP-CVD operation at low pressure.

Elucidating the formation mechanisms of the parasitic channel with buffer-free GaN/Si hetero-bonding structures

Driven by the increasing demand for 5G communication, GaN radio frequency (RF) device on Si technology has been flourishing attributable to the large size, low cost, and compatibility with complementary metal–oxide–semiconductor technology. However, a significant challenge is that a high-conductance parasitic channel forms at the interface between the III-N epitaxial layers and the Si substrate, leading to severe RF loss, which has been considerably impairing both the performance and advancement of RF GaN-on-Si technologies. Despite continuing controversies concerning the physical mechanisms engendering the parasitic channel, clarification is critically needed. Standing apart from traditional studies on RF loss in III-N epilayers grown on Si, this article comprehensively investigates the bonding interface of GaN thin film and Si(100) substrate realized via direct surface activated bonding and ion-cutting technologies. It was clearly determined that substantial diffusion of gallium (Ga) atoms into the Si substrate at the bonding interface occurred even at an annealing temperature as low as 350 °C. Subsequent high-temperature post-annealing at 800 °C intensified this diffusion, activating Ga atoms to form a p-type highly conductive parasitic channel. Simultaneously, it triggered Ga atoms aggregation and incited melt-back etching within the Si substrate at the interface. Contrasting with the conventional hetero-epitaxy, this study presents a compelling view based on the bonding technique. It conclusively elucidates the physical mechanisms of the formation of the primary source of RF loss—the p-type highly conductive parasitic channel.

Disorder induced in GaN thin films by 200 MeV silver ions

GaN is a material of strategic importance due to its versatility in high temperature, radiation prone and chemical environments. The present study explores evolution of defects in GaN thin films in swift heavy ion radiation regime. GaN thin films grown by metal organic chemical vapour deposition techniques were irradiated with 200 MeV Ag ions at various fluences starting from 5 × 1010 to 5 × 1012 ions/cm2. Structural analysis revealed accumulation of lattice damage with increasing fluence. XRD analysis reveals that swift heavy ions of silver (Ag) deposit more energy into the target lattice, inducing localized damage at higher fluences, leading to broader peaks in irradiated samples. Recrystallization becomes evident at fluences of 1 × 1011 ions/cm2 of Ag 200 MeV ion irradiation, as indicated by a decrease in Full Width at Half Maximum (FWHM). The compressive stress identified in Raman analysis is associated with the c-axis and elevates the wavenumber of the E2(H) mode following Ag ion irradiation. Meanwhile, the pristine sample exhibits narrow A1 (LO) phonon mode peaks, indicating good crystallinity. Photoluminescence investigations demonstrate that Ag ions induce point defects in GaN, evident through analysis of YL, GL, and BL defect-related emission peaks following irradiation at various fluences.

Investigation of Nondestructive and Noncontact Electrical Characterization of GaN Thin Film on ScAlMgO4 Substrate Using Terahertz Time‐Domain Spectroscopic Ellipsometry with Characteristic Impedance Analytical Model

Semiconductor evaluation methods frequently require sample processing and carry the risk of defects and property changes. Therefore, nondestructive and noncontact electrical property measurement techniques are necessary. Terahertz time‐domain spectroscopic ellipsometry (THz‐TDSE) can simultaneously estimate the electrical properties and film thickness of a sample. However, whether this method can simultaneously determine electrical properties and film thickness for semiconductor thin films of 1 μm or less remains unclear. This study uses THz‐TDSE to evaluate the electrical properties and film thickness of a GaN thin film (≈0.8 μm thick) on a ScAlMgO4 substrate. Using the conventional analysis based on Fresnel's formula (FR model), which is used in the field of optics, uniquely determining the electrical properties and film thickness is not possible owing to multiple optimal solutions. Therefore, a model combining the characteristic impedance model used in the radio‐wave region with the FR model is employed in the analysis. The obtained values of electrical properties of the GaN thin film evaluated using THz‐TDSE are consistent with those using Hall effect measurement. Moreover, the film thickness is consistent with that measured from the cross‐sectional transmission electron microscopy observation.

Crystal Structure and Surface Morphology of GaN Thin Films Grown on Different Patterned AlN Substrate Surfaces

Crystal Structure and Surface Morphology of GaN Thin Films Grown on Different Patterned AlN Substrate Surfaces

Low-field transport properties and scattering mechanisms of degenerate n-GaN by sputtering from a liquid Ga target

In this work, degenerate n-type GaN thin films prepared by co-sputtering from a liquid Ga-target were demonstrated and their low-field scattering mechanisms are described. Extremely high donor concentrations above 3 × 1020 cm−3 at low process temperatures (<800 °C) with specific resistivities below 0.5 mΩcm were achieved. The degenerate nature of the sputtered films was verified via temperature-dependent Hall measurements (300–550 K) revealing negligible change in electron mobility and donor concentration. Scattering at ionized impurities was determined to be the major limiting factor with a minor contribution of polar optical-phonon scattering at high temperatures.

Observation of structural defects in GaN/InGaN multi-quantum wells grown on semipolar (112¯2) substrate using cathodoluminescence in transmission electron microscopy

Defect structures of semipolar GaN thin film, grown on the m-plane of sapphire, were investigated using the home-built cathodoluminescence (CL) stage in TEM. The CL maps with high spatial resolution identify the type of basal plane stacking faults (BSFs) and visualize their distribution. I1-BSF type defects had the highest population and the I2-BSF type defect was revealed in the form of thin strips with less population. Partial dislocations and basal stacking fault-related defects existed in the form of segmented lines but with strong luminescence characteristics. TEM-CL was able to identify the distribution and the characteristic luminescence from the defects, which were difficult to discern in the typical defect analysis.

Investigation into the growth mode of GaN thin films on 4H–SiC substrates via plasma-enhanced atomic layer deposition

GaN ultra-thin films are deposited on 4H–SiC substrates using plasma-enhanced atomic layer deposition (PEALD). Our investigation reveals a remarkably narrow temperature window (225–275 °C) for GaN PEALD growth, characterized by substrate hindered nucleation retardation. Within this temperature window, the growth process exhibits three distinct stages, which are demarcated by the 35th and 200th cycles, each displaying unique characteristic, especially in aspects growth rate, film strain, surface topography, and crystallization. Notably, the instantaneous growth rate, which is often overlooked, provided a better representation of the growth state compared with growth per cycle (GPC) and shows better consistency with film strain and surface roughness. Additionally, we observed a step filling effect during the early stages (10 cycles) of ALD growth, which reduces surface roughness to 0.12 nm form 0.20 nm of substrate SiC. X-ray photoelectron spectroscopy (XPS) analysis reveals the formation of N–Si and N–C bonds on the sample surface following 20 cycles of plasma bombardment. Further analysis using high-resolution X-ray diffraction (HRXRD) and high-resolution transmission electron microscopy (HRTEM) consistently confirms the presence of a hexagonal wurtzite crystalline structure in GaN films deposited under varying cycle counts.