Quality Of Epitaxial Layers Research Articles

High-performance GaN ultraviolet polarization-sensitive photodetector based on ferroelectric polarization LiNbO3

High responsivity ultraviolet (UV) photodetectors (PDs) are essential for abundant civilian and military applications. Gallium nitride (GaN) has emerged as an ideal material for UV PD fabrication due to its favorable properties. However, the quality of GaN epitaxial layers significantly impacts device performance and reliability. Sapphire-based GaN epitaxial growth technology enables the realization of high-quality GaN epitaxial layers, making it the preferred choice for GaN substrates. Nonetheless, the thermal expansion coefficient mismatch between sapphire and GaN can lead to crystal mismatch and stress accumulation at high temperatures, affecting device performance and reliability. In contrast, lithium niobate (LiNbO3) exhibits similar coefficients of thermal expansion to GaN, mitigating crystal mismatch and stress accumulation issues. Here, we report the realization of a GaN UV PD by laminating GaN membrane onto ferroelectric LiNbO3 through selective electrochemical etching of the sapphire-based GaN epitaxial film. The LiNbO3-based GaN PD achieves a specific high on/off ratio of 107. At a 5 V bias voltage, the device exhibits a high peak responsivity of 1.712 × 103 A/W under 325 nm laser illumination. Furthermore, the device demonstrates excellent performance for polarization light detection, with a polarization ratio of approximately 54.95. Exploiting the local ferroelectric polarization of x-cut LiNbO3, the photogenerated electron–hole pairs in GaN are efficiently separated by the electrostatic field from the polarization of ferroelectric LiNbO3, resulting in enhanced light-to-electric conversion efficiency. Our work presents a method for fabricating high responsivity GaN-based UV PD, showcasing the potential of integrating ferroelectric LiNbO3 to enhance device performance.

Advanced Mechanical Transfer of Micro-LEDs Enabled by Structurally Modified Wide Sapphire Nanomembranes through Thermal Reflow of Photoresist.

Micro light-emitting diodes (micro-LEDs) are pivotal in next-generation display technologies, driven by the need for high pixel density. This study introduces a novel methodology utilizing wide sapphire nanomembranes (W-SNM) as a dual-purpose template for high-quality epitaxial growth and the mechanical lift-off of individual micro-LEDs. Micro-LEDs grow individually on W-SNM, obviating the chip singulation process. By employing mechanical fracturing of the thin W-SNM, our method facilitates the transfer of micro-LEDs without the conventional laser lift-off (LLO) process. Previously introduced sapphire nanomembranes (SNM) have shown promise in enhancing epitaxial layer quality; however, they encountered challenges in managing micro-LED size variation and achieving efficient mechanical transfer. Here, we apply simple yet effective adjustments to the SNM structure, specifically, its elevation and widening. This strategic modification allows micro-LEDs to endure applied forces without incurring cracks or defects, ensuring that only the targeted W-SNM are selectively fractured. The mechanically transferred vertical 15 × 15 μm2 micro-LED device operates at an optimal turn-on voltage of 3.3 V. Finite element simulations validate the mechanical strain distribution between the W-SNM and GaN when pressure is applied, confirming the efficacy of our design approach. This pioneering methodology offers a streamlined, efficient pathway for the production and mechanical transfer of micro-LEDs, presenting new avenues for their integration into next-generation, high-performance displays.

Displacement damage effect of proton irradiation on vertical β-Ga2O3 and SiC Schottky barrier diodes (SBDs)

In this study, we fabricated vertical Schottky barrier diodes (SBDs) based on wide bandgap semiconductor beta-phase gallium oxide (β-Ga2O3) and silicon carbide (SiC), respectively, and conducted proton irradiation experiments to analyze the radiation hardness of the SBDs comparatively. The effects of proton radiation on the performance of SBDs were assessed through measurements of forward current, capacitance, and breakdown characteristics. Both devices exhibited degradation in current and capacitance characteristics following proton irradiation, attributed to displacement damage (DD). Notably, the β-Ga2O3-based SBD demonstrated more pronounced deterioration compared to the SiC-based device despite similar vacancy distributions as confirmed by SRIM simulation. Moreover, a decrease in contact radius correlated with exacerbated degradation in the current characteristics of the β-Ga2O3-based SBD. Following proton irradiation, breakdown voltages of both devices increased due to elevated resistance induced by displacement damage. While both β-Ga2O3 and SiC-based SBDs experienced displacement damage under high fluence proton irradiation, the extent of performance degradation varied depending on the dimensions and quality of epitaxial and substrate layers.

A Study on the Growth Conditions Role in Defining InGaAs Epitaxial Layer Quality

This study delves into the epitaxial growth and characterization of InxGa1-xAs layers on InP substrate, a critical area in the development of high-performance III-V semiconductor devices. InxGa1-xAs is renowned for its superior electron mobility and broad spectral response, making it indispensable in applications ranging from photodetectors to quantum cascade lasers. Employing a horizontal flow reactor MOVPE (metal-organic vapor phase epitaxy) technique, we meticulously grew n-InxGa1-xAs epilayers under varying conditions to investigate the impact of indium content, growth temperature, and V/III ratio on the material's structural, optical, and electrical properties. HRXRD (High-resolution X-ray diffraction) and Hall-effect measurements provided insights into the correlation between growth parameters and epitaxial layer quality, including dislocation density and carrier mobility. Our findings highlight the delicate balance required in the growth process to optimize the InxGa1-xAs /InP structure's performance for advanced semiconductor applications. The research underscores the potential of tailored InxGa1-xAs layers to push the boundaries of current photonics and optoelectronics technologies, emphasizing the importance of growth condition optimization for enhancing device efficiency and thermal stability.

Mid-infrared Imaging Using Strain-Relaxed Ge1-xSnx Alloys Grown on 20 nm Ge Nanowires.

Germanium-tin (Ge1-xSnx) semiconductors are a front-runner platform for compact mid-infrared devices due to their tunable narrow bandgap and compatibility with silicon processing. However, their large lattice parameter has been a major hurdle, limiting the quality of epitaxial layers grown on silicon or germanium substrates. Herein, we demonstrate that 20 nm Ge nanowires (NWs) act as effective compliant substrates to grow extended defect-free Ge1-xSnx alloys with a composition uniformity over several micrometers along the NW growth axis without significant buildup of the compressive strain. Ge/Ge1-xSnx core/shell NWs with Sn content spanning the 6-18 at. % range are achieved and processed into photoconductors exhibiting a high signal-to-noise ratio at room temperature with a cutoff wavelength in the 2.0-3.9 μm range. The processed NW devices are integrated in an uncooled imaging setup enabling the acquisition of high-quality images under both broadband and laser illuminations at 1550 and 2330 nm without the lock-in amplifier technique.

Wafer level quasi-van der Waals epitaxy of AlGaN/GaN heterojunctions on sp2-bonded BN controlled by AlN nucleation layer

Two dimensional materials, such as sp2-bonded boron nitride (BN) and graphene, have been proven to be excellent templates for the growth and fabrication of independent group III-nitride materials. Herein, wafer level sp2-bonded BN film was successfully grown as a release layer on two-inch sapphire substrate using MOCVD. The different nucleation mechanisms of gallium nitride (GaN) and aluminum nitride (AlN) grown on the BN surface were investigated. We discovered that AlN has a higher nucleation island density compared to GaN on the surface of BN which facilitates better merging of the GaN epitaxial layer. In addition, the quality of the GaN epitaxial layer was optimized by adjusting the growth temperature of AlN. A complete AlGaN/GaN heterojunction structure was subsequently grown on the best quality of GaN, and its average mobility and interlayer carrier concentration was about 1600 cm2 V−1 s−1 and 1 × 1013 cm−2. Finally, the AlGaN/GaN heterojunction was easily transferred from the substrate using tape, which proved the good 2D characteristics of sp2-bonded BN and can undergo non-destructive van der Waals transfer for the upper layer device.

Optical and Electrical Properties of AlxGa1-xN/GaN Epilayers Modulated by Aluminum Content.

AlGaN-based LEDs are promising for many applications in deep ultraviolet fields, especially for water-purification projects, air sterilization, fluorescence sensing, etc. However, in order to realize these potentials, it is critical to understand the factors that influence the optical and electrical properties of the device. In this work, AlxGa1-xN (x = 0.24, 0.34, 0.47) epilayers grown on c-plane patterned sapphire substrate with GaN template by the metal organic chemical vapor deposition (MOCVD). It is demonstrated that the increase of the aluminum content leads to the deterioration of the surface morphology and crystal quality of the AlGaN epitaxial layer. The dislocation densities of AlxGa1-xN epilayers were determined from symmetric and asymmetric planes of the ω-scan rocking curve and the minimum value is 1.01 × 109 cm-2. The (101¯5) plane reciprocal space mapping was employed to measure the in-plane strain of the AlxGa1-xN layers grown on GaN. The surface barrier heights of the AlxGa1-xN samples derived from XPS are 1.57, 1.65, and 1.75 eV, respectively. The results of the bandgap obtained by PL spectroscopy are in good accordance with those of XRD. The Hall mobility and sheet electron concentration of the samples are successfully determined by preparing simple indium sphere electrodes.

Temperature-dependent capacitance-voltage characteristics of β-Ga2O3 Schottky barrier diodes with (001) epitaxial grown layer using MOCVD

We investigate the thin film quality of the epitaxial layer on Sn-doped (001)-oriented β-Ga2O3 single crystal substrates. The homoepitaxial layer is grown by metal organic chemical vapor deposition (MOCVD) at 700 °C, 800 °C, 900 °C, and 1000 °C. To evaluate the grown epitaxial layer at different temperatures, the depletion layer of the Schottky barrier diode (SBD) is studied using the impedance-phase angle (Z-theta) via capacitance-voltage measurements. In addition, the Z-theta value is correlated with X-ray diffraction and transmission electron microscopy. The results show that the epitaxial layer grown at 900 °C has the best epitaxial layer quality. It is presumed that Z-theta measurements are a feasible and reliable approach to evaluate the epitaxial layer in Ga2O3-related devices.

Insights into the effect of susceptor rotational speed in CVD reactor on the quality of 4H-SiC epitaxial layer on homogeneous substrates

In this work, 4H-SiC homoepitaxial layers were grown on 4°off-axis substrates at different susceptor rotation speeds by using a hot-wall horizontal CVD reactor. The effect of different susceptor rotation speed on the quality of 4H-SiC epitaxial layers in terms of thickness, thickness uniformity, crystallinity, surface morphology and morphological defects was investigated via Fourier transform infrared spectroscopy (FTIR), high-resolution X-ray diffraction (XRD), atomic force microscopy (AFM), confocal differential interference contrast microscopy (CDIC), ultra-violet photo-luminescence spectroscopy (UV-PL), scanning electron microscopy (SEM), and micro-Raman spectroscopy, respectively. A flow field simulation was performed to explain the impact of susceptor rotation speed on the film deposition. The FTIR results suggested that the susceptor rotation speed could be an important factor to adjust thickness uniformity and deposition rate. The XRD patterns showed that crystallinity was independent of the susceptor rotation speed. The surface morphology can be improved by changing the susceptor rotation speed. According to CDIC scans, the down-fall related defects were reduced through the increase in the susceptor rotation speed. The origin of down-fall related defects was interpreted by Raman spectroscopy and speculative models. To sum up, the susceptor rotation speed is a crucial factor in increasing growth rate and improving uniformity. Also, the faster susceptor rotation speed helps reduce the number of down-fall related defects in the hot-wall CVD reactor.

Effect of step-flow modulation on the large-size single crystal diamond through mosaic growth

In this paper, large-size single crystal diamond is grown by microwave plasma chemical vapor deposition using mosaic technique. The effects of the step-flow modulation on the surface morphology and crystal quality of epitaxial single crystal diamond layer are systematically investigated. It demonstrates that the two adjacent crystals can be merged into a single crystal layer with good surface morphology and uniform quality when their step-flow orientations are similar. Otherwise, over-lap is observed because of the overgrowth from one seed to another, resulting in stress concentration and a large number of crystalline defects at the mosaic boundaries. Finally, we demonstrate that the step-flow modulation method facilitates the mosaic growth of inch-scale single-crystal diamond with 10 CVD seeds arranged in a T-shape.

Improvement of InGaP solar cells grown with TBP in planetary MOVPE reactor

In this study, the impact of various growth parameters, such as wafer temperature, device structures, and substrate misorientation, on the quality of InGaP epitaxial layers and solar cells grown in a planetary MOVPE reactor using TBP as a P source were investigated and optimized. Results showed that the carrier lifetime in unintentionally n-doped InGaP significantly decreased as the growth temperature increased from 560 to 590 °C. At temperatures above 600 °C, the growth of InGaP was unsuccessful due to extremely rough surface morphology. The study revealed that InGaP rear homojunction and rear heterojunction solar cells using n-type InGaP base layers displayed improved photo-carrier collection and performance compared to traditional front junction solar cells with p-type InGaP base layers. Notably, InGaP grown on GaAs (001) with a 5° miscut towards (111)A was found to have better quality, while InGaP grown on GaAs (001) with a 6° tilt towards (111)B had worse results in terms of carrier lifetime, PL intensity, and solar cell performance.

High‐Brightness GaN‐Based Blue Light‐Emitting Diodes Using AlON Buffer Layer on 4 In.‐Patterned Sapphire Substrate: Low‐Dislocation‐Defect Epitaxy, Growth Mechanisms, and Higher Device Performance

AlN is a common buffer layer in GaN‐based light emitting diode (LED) devices. However, application of AlN on industry‐relevant patterned sapphire substrate (PSS) causes unwanted screw dislocations in the epitaxial layer during GaN nucleation. Industry experience suggests that AlON buffer layer improves the quality of GaN epitaxial layer, but the low‐dislocation‐defect epitaxy mechanisms remain unclear. Herein, high‐brightness GaN‐based blue LEDs with the sputtered AlON buffer layer on industry‐grade four‐inch PSS is demonstrated. The effect of oxygen addition to AlN is revealed by growing GaN epitaxial layers on AlON buffer layers with different oxygen contents and testing these GaN layers in LED devices. Compared with conventional AlN, our AlON buffer layer reduces the density of screw dislocations, thereby improving the quality of the GaN epitaxial layers and the performance of GaN‐based LEDs. These insights allow the explanation of the growth mechanisms of GaN on PSS with AlN and AlON buffer layers and can guide the development of next‐generation optoelectronic technologies.

Dislocation-related leakage-current paths of 4H silicon carbide

Improving the quality of 4H silicon carbide (4H-SiC) epitaxial layers to reduce the leakage current of 4H-SiC based high-power devices is a long-standing issue in the development of 4H-SiC homoepitaxy. In this work, we compare the effect of different type of dislocations, and discriminate the effect of dislocation lines and dislocation-related pits on the leakage current of 4H-SiC by combining molten-KOH etching and the tunneling atomic force microscopy (TUNA) measurements. It is found that both the dislocation lines of threading dislocations (TDs) and the TD-related pits increase the reverse leakage current of 4H-SiC. The dislocation lines of TDs exert more significant effect on the reverse leakage current of 4H-SiC, which gives rise to the nonuniform distribution of reverse leakage current throughout the TD-related pits. Due to the different Burgers vectors of TDs, the effect of TDs on the reverse leakage current of 4H-SiC increases in the order to threading edge dislocation (TED), threading screw dislocation (TSD) and threading mixed dislocation (TMD). Basal plane dislocations (BPDs) are also found to slightly increase the reverse leakage current, with the leakage current mainly concentrated at the core of the BPD. Compared to the effect of TDs, the effect of BPDs on the reverse leakage current of 4H-SiC is negligible. Our work indicates that reducing the density of TDs, especially TMDs and TSDs, is key to improve the quality of 4H-SiC epitaxial layers and reduce the reverse leakage current of 4H-SiC based high -power devices.

GaN/AlInN/AlN/Safir HEMT Yapılar için Mozaik Kusur ve AFM Çalışması

In this study, three samples of GaN/AlInN/AlN/Al2O3 high electron mobility (HEMT) structures are investigated with high resolution X-ray diffraction (HR-XRD) technique. Peak positions and peak broadenings are used in calculations, gained from rocking curves. Structural quality is determined from symmetric and asymmetric peak planes. Mosaic defects such as treadening dilocations (TDs), tilt and twist angles, lateral and vertical crystallite lengths are determined by using Williamson Hall (WH) method. In addition to these, surface morphology is also investigated by atomic force microscopy (AFM). It is noticed that crystal quality of epitaxial layers decrease in the order of samples C, B and A. Al compositions for samples A, B and C are found as %87.4, %86.6 and %86.4, respectively by using Vegard’s law.

Effect of Thickness of InP Nucleation Layer on the Two-Step Growth of InP on Si(001)

InP nucleation layers with different thicknesses were grown on Si(001) substrates by gas-source molecular beam epitaxy (GSMBE), and the two-step growth technique was used to overcome the large lattice mismatch (8%) between the InP nucleation layer and Si substrate. The surface morphology and microstructure were investigated by using an atomic force microscope (AFM) and transmission electron microscopy (TEM). High-resolution X-ray diffraction (HR-XRD) measurements were carried out to characterize the crystal quality. It was found that a too thin nucleation layer will lead to an uneven distribution of atoms on the surface, resulting in a poor crystalline quality of the InP epitaxial layer. The thicker the low-temperature nucleation layer is, the better the crystallization quality of the InP high-temperature layer will be.

The Effect of Si (111) Substrate Surface Cleaning on Growth Rate and Crystal Quality of MOVPE Grown AlN

In this work, the effect of Si (111) substrate surface cleaning by RCA (Radio Corporation of America) method on growth rate and crystalline quality of epitaxially grown AlN thin films by MOVPE (Metal Organic Vapor Phase Epitaxy) technique is investigated. In situ reflectance system and high resolution X-ray diffraction (HRXRD) technique are used for the analysis of growth rate and crystal quality of epitaxial AlN layers, respectively. Also, The Raman measurement is done to show the effect of the RCA cleaning procedure on the position of the peaks that occurred in the Raman spectra. The results have shown that the surface cleaning of Si (111) substrate by the RCA method removes the oxide layer formed on the surface, also helps to decrease the parasitic reactions and increases the adatom efficiency, results in an increased growth rate of the AlN layer. Besides, surface cleaning of Si (111) substrate by the RCA method has reduced the FWHM value ~5% for ω-2θ scan and ~60% for ω scan of AlN epilayer, indicating an improvement in crystal quality.

Review of Highly Mismatched III-V Heteroepitaxy Growth on (001) Silicon.

Si-based group III-V material enables a multitude of applications and functionalities of the novel optoelectronic integration chips (OEICs) owing to their excellent optoelectronic properties and compatibility with the mature Si CMOS process technology. To achieve high performance OEICs, the crystal quality of the group III-V epitaxial layer plays an extremely vital role. However, there are several challenges for high quality group III-V material growth on Si, such as a large lattice mismatch, highly thermal expansion coefficient difference, and huge dissimilarity between group III-V material and Si, which inevitably leads to the formation of high threading dislocation densities (TDDs) and anti-phase boundaries (APBs). In view of the above-mentioned growth problems, this review details the defects formation and defects suppression methods to grow III-V materials on Si substrate (such as GaAs and InP), so as to give readers a full understanding on the group III-V hetero-epitaxial growth on Si substrates. Based on the previous literature investigation, two main concepts (global growth and selective epitaxial growth (SEG)) were proposed. Besides, we highlight the advanced technologies, such as the miscut substrate, multi-type buffer layer, strain superlattice (SLs), and epitaxial lateral overgrowth (ELO), to decrease the TDDs and APBs. To achieve high performance OEICs, the growth strategy and development trend for group III-V material on Si platform were also emphasized.

Enhancement of optical property and crystal structure for GaN films on 2D MoS2 buffer layer by nitridation treatment

The growth of GaN films on layered 2D-MoS2/c-sapphire substrate by plasma-assisted molecular beam epitaxy system was investigated. Pre-nitridation process and deposition time as growth parameters were employed to exploit the heteroepitaxial growth of GaN films at the temperature of 700 °C. The characterizations of surface condition, surface morphology, chemical composition, optical properties, crystal orientation and quality of GaN epitaxial layers were conducted by reflection high energy electron diffraction (RHEED), scanning electron microscope (SEM), atomic force microscope (AFM), X-ray photoelectron Spectroscopy (XPS), Raman spectroscopy, photoluminescence spectroscopy (PL) and high-resolution X-ray diffraction (HR-XRD), respectively. The pre-nitridation process and growth duration can enhance the optical properties and crystal quality of GaN films on 2D-MoS2/c-plane sapphire. The pre-treatment of MoS2 layer by nitrogen plasma before the growth, providing a wetting layer of nitrogen for GaN growth, can eliminate the oxygen contamination and reduce the defects in the films. Band-to-band emission of Wurtzite GaN with the bandgap of 3.5 eV can be improved by pre-nitridation process, and yellow band emission can be suppressed due to the elimination of defect states in the films. Meanwhile, the crystal orientation and quality can be improved by the pre-nitridation process and longer duration of growth time. This study demonstrates a van der Waals epitaxial GaN films on 2D-MoS2 layer with higher crystal quality by pre-nitridation process, and it provides a potential solution for the future applications of GaN-based devices.

Improved epitaxy of ZnO films by regulating the layers of graphene

Growth of ZnO film on graphene buffered sapphire substrate by plasma assisted molecular beam epitaxy (PAMBE) was investigated. Specifically, the surface morphology, crystal orientation and crystal quality of ZnO epitaxial layer and nucleation layer deposited on graphene with various thickness were analyzed by atomic force microscope (AFM), scanning electron microscope (SEM), high resolution X-ray diffraction (HRXRD), and high-resolution transmission electron microscope (HRTEM). The experiments showed that after buffered with double-layer graphene (DLG), six-fold symmetrical pits with the same orientation appeared on the successive ZnO film surface and the full width of half maximum (FWHM) of ZnO (002) plane rocking curve decreased to 769.3 arcsec. By comparing ZnO nucleation layer on graphene with different layers and checking the graphene by Raman spectroscopy, the mechanism for improving crystal quality of ZnO film grown on DLG was revealed. This study demonstrates the potential that high-crystal-quality ZnO film can be grown on graphene by plasma-assisted remote heteroepitaxy without limitation of the crystal mismatch and thermal mismatch between epitaxial film and substrate. It could be meaningful for the development of simplified growth technology of ZnO based films and devices.

Homoepitaxial lateral growth of single-crystal diamond with eliminating PCD rim and enlarging surface area

Avoiding the appearance of polycrystalline diamond (PCD) rim during the epitaxial lateral outward growth of single-crystal diamond (SCD) by microwave plasma chemical vapor deposition (MPCVD) is still full of difficulties and challenges. Herein, the enlarged SCD without PCD rim was synthesized via different growth modes. The core strategy is the self-formation of off-angles on (001) crystal face by regulating the radicals distribution and reducing the temperature at the seed edge. Therefore, the appearance of the secondary nucleation at the edge regions can be inhibited. In a dual-substrate reactor, the SCD surface displayed a typical terrace flow growth pattern. The growth steps on the SCD surface are directed from the center to the edge which coincides with the distribution of carbon-related radicals and temperature. In addition, a spiral growth mode appeared in the homoepitaxial growth of SCD by a single-substrate reactor. Under a spiral growth mode, the top surface of SCD in an off-axis direction was contributed to form a smooth interface. The SCD surface increased by 88.15% from 3 × 3 mm2 to 4.11 × 4.12 mm2 after 82 h. And the crystalline quality of the CVD epitaxial layer is higher than that of the type Ib seed.