GaN Layers Research Articles

Impacts of specific element-treated three-dimensional GaN layer on characteristics of nonpolar a-plane GaN film

Nonpolar a-plane GaN films with three-dimensional (3D) GaN layers have successfully grown on r-plane sapphire substrates by metal organic chemical vapor deposition. The inserted 3D GaN layers were grown accompanied with the treatment by various kinds of elements such as Si, Mg, and In to further improve the characteristics of the subsequently lateral overgrown nonpolar a-plane GaN films. Scanning electron microscopy, x-ray rocking curve, and room temperature photoluminescence spectroscopy were used to examine the effects of the introduction of the specific element-treated 3D GaN layer on the crystalline quality, the anisotropy, and the optical property of the nonpolar a-plane GaN film. It was found that significant improvements in the crystalline quality and optical property as well as a remarkable reduction in anisotropy have been achieved for the nonpolar a-plane GaN film grown on the r-plane sapphire substrate by inserting a 3D GaN layer treated with Si. In fact, evident reduction in full width at half maximum of x-ray rocking curves from 972 to 651 arcsec along the c-axis (φ = 0°) and from 1234 to 752 arcsec along the m-axis (φ = 90°), and a notable decrease in anisotropy from 27.0% to 15.6% were obtained with the employment of Si treatment to the 3D GaN layer.

Ultra-high pressure annealing of Mn-implanted HVPE-GaN

Manganese diffusion in ion implanted gallium nitride crystals was investigated. The ions were implanted into a sample composed of a GaN layer grown using halide vapor phase epitaxy on an ammonothermal gallium nitride substrate. The sample was polished into an atomically flat state prior to the implantation. Manganese ions were implanted into the surface of a (0001) plane (also known as the c-plane) using the beam energy of 230 keV and ion fluence of 5E15 cm−2. The beam was tilted relative to the sample’s normal to prevent channeling. After the implantation, ultra-high pressure annealing method was used to both, remove the post-implantation damage and induce the diffusion of implanted Mn ions. Manganese profiles were analyzed along the c-direction using secondary ion mass spectrometry after 5 and 30 h of annealing in temperatures ranging from 1473 K to 1753 K. Two diffusion mechanisms were observed. The diffusion parameters, activation energy and temperature-independent diffusion coefficients, are presented for each mechanism. Additionally, the structural quality of the samples is assessed after 5 and 30-hour annealing in the aforementioned temperature range through the means of X-ray diffractometry.

Effects of Buffer Layer on Structural Properties of Nonpolar (112¯0)-Plane GaN Film

Nonpolar (112¯0) a-plane GaN films were grown on semipolar (11¯02) r-plane sapphire substrates using various buffer layers within a low-pressure metal organic chemical vapor deposition system. The structural properties of nonpolar a-plane GaN films were intensively investigated by X-ray diffraction and Raman spectra measurements. A set of buffer layers were adopted from a GaN layer to a composite layer containing a multiple AlN layers and a gradually varied-Al-content AlGaN layer, the full width at half maximum of the X-ray rocking curves measured along the [0001] and [101¯0] directions of a-plane GaN were reduced by 35% and 37%, respectively. It was also found that the basal-plane stacking faults (BSFs) density can be effectively reduced by the heterogeneous interface introduced together with the composite buffer layer. An order of magnitude reduction in BSFs density, as low as 2.95 × 104 cm−1, and a pit-free surface morphology were achieved for the a-plane GaN film grown with the composite buffer layer, which is promising for the development of nonpolar GaN-based devices in the future.

Acceptor activation of Mg-doped GaN—Effects of N2/O2 vs N2 as ambient gas during annealing

Here, we investigate the effects of O2:N2 (1:1) as ambient gas as compared with pure N2 during activation annealing of Mg as p-type doping in GaN layers grown by MOCVD. The purpose is to understand the impact of O2 on the resulting free hole concentration and hole mobility using SIMS, XRD, STEM, AFM, and Hall effect measurements. Even though the presence of O2 in the ambient gas during annealing is very effective in reducing the H level of the Mg-doped GaN layers, the maximum achievable hole concentration and mobility is still higher with pure N2. The differences are explained by an in-diffusion of O to the GaN layer acting as n-dopant and, thus, giving rise to a compensation effect. The Mg–H complexes at substitutional (MgGa), i.e., the electrically active acceptor sites that provide free holes, are preferentially activated by annealing with N2 only as ambient gas, while annealing with O2:N2 (1:1) also dissociates electrically inactive Mg–H complexes resulting in much less residual H. At the lower growth pressure of 150 mbar compared to higher growth pressure of 300 mbar, an increasing carbon incorporation leads to a compensation effect drastically reducing the free hole concentration while the mobility is unaffected.

Spalling induced van der Waals lift-off and transfer of 4-in. GaN epitaxial films

Epitaxial lift-off (ELO) of high-quality GaN layers allows for integration with a variety of materials enabling improved performance, reduced costs, and development of new electronics. Of the ELO technologies, two-dimensional (2D) material-based lift-off offers great promise but is still in the early stages of development and has yet to demonstrate the scale and yield of other ELO technologies. Here, we demonstrate the potential of this process's scalability, speed, and yield through epitaxial growth and lift-off of 4-in. GaN films using a 2D boron nitride (BN) van der Waals (vdW) buffer layer. Since the BN layer acts as the growth template and the mechanical release layer, both the quality and adhesion of the GaN layer are correlated with the BN morphology and uniformity. Detailed spectroscopic mapping demonstrates excellent BN uniformity, which translates into growth of high-quality GaN as shown in mapping of the x-ray rock curves (XRCs), atomic force microscopy, and photoluminescence. Scanning transmission electron microscopy and electron energy loss spectroscopy reveal abrupt chemically distinct interfaces between the sapphire, BN, and AlN/GaN layers essential for efficient lift-off. Combined with the BN/GaN vdW heterostructure, Ni spalling is used to efficiently lift-off and transfer a full 4-in. GaN layer. Post transfer characterization of a 1.9 μm thick GaN layer transferred to a SiO2/Si wafer shows a very minimal change in the XRC and photoluminescence. Strain measurements before and after transfer show that the process fully relaxes residual strain formed in the GaN during high-temperature growth. This work highlights the potential for industry scalability of an exciting 2D material-based lift-off technology, which can facilitate higher power and more efficient radio frequency devices.

Understanding the influence of physical properties on the mechanical characteristics of Mg-doped GaN thin films

This study investigates the impact of Mg doping concentration in GaN thin films on their physical and mechanical properties, specifically on their conductivity transition, luminescence, hardness, Young's modulus, and creep behavior. Mg-doped GaN layers were grown on sapphire substrates using the MOCVD technique, with trimethylgallium (TMG) and bis(cyclopentadienyl) magnesium (Cp2Mg) as precursors for Ga and Mg, respectively. The TMG flow rate was set at 20 μmol/min, while the Mg flow rate was adjusted by tuning the temperature of the thermostatic bath of Cp2Mg. Three different Cp2Mg flow rates: 2, 7 and 9 μmol/min were utilized to explore their effects on the physical and mechanical properties of the grown epilayers. The results demonstrate that an increase in the Mg concentration leads to a conductivity transition from n-type to p-type, with the highest hole concentration (4.5 ± 0.2) × 1017 cm−3 and blue luminescence attained with Mg concentration equal to 2.1 × 1019 atoms/cm3. Moreover, the hardness, Young's modulus and the intensity of compressive stress increase with the Cp2Mg flow rate due to the evolution of pre-existing dislocation density and point defects incorporation. Additionally, this study evaluates the creep behavior of the Mg-doped GaN thin films. It shows that both the creep stress exponent and the maximal creep depth decrease with the increase of the Cp2Mg flow rate. This is attributed to dislocation glides and climbs governing the creep mechanism. Accordingly, this investigation highlights the importance of understanding the relationship between physical properties and mechanical characteristics of Mg-doped GaN thin films, which have potential implications for various technological applications.

Abnormal on Current Tendency in Saturation Region Between High and Light Carbon Doped Buffer Layer in p-GaN HEMT

To reduce drain leakage current, carbon doping is introduced in the GaN layer of the p-GaN HEMT device. The focus of this study is on the discussion of the abnormal current behavior that occurs in the saturation region of carbon-doped p-GaN HEMT. This abnormal phenomenon disappears when the device is heated up to 150°C. The abnormal current behavior corresponds to the hot electron stress (HES) result, which indicates that electron trapping causes this abnormal current behavior. An energy band is proposed to describe the lesser trapping effect that occurs in the saturation region.

Analysis of Dislocation Density for GaN Based HEMTs in Screw Mod

Quick response is an important feature in design of optoelectronic cards. So in this study, structural properties of GaN/AlN/AlGaN HEMTs structures grown on sapphire by the chemical vapor adjustment method are analyzed by the X-ray diffraction method. The main property of these kind of materials is that they are resistant to high voltage, temperature, and pressure. Although their performance is worse compared silicon, for forcing limit standards, they present wide research field. In this study, the focus of investigation is dislocation density stemming from lattice mismatch between layers and wafer causing cracks on the surface. In HEMT structure calculation of dislocation density for GaN and AlN represents all structure. High dislocation density for AlN layer is determined because of aggressive behavior of Al element in the structure. Also, quantized GaN layers stop moving of dislocations and prevents surface cracks.

SiC/Si Hybrid Substrate Synthesized by the Method of Coordinated Substitution of Atoms: A New Type of Substrate for LEDs

This paper proposes a new type of substrate for manufacturing LEDs based on AlInGaN heterostructures. Instead of depositing SiC layers on the surface of Si using the conventional method, a new method involving the coordinated substitution of atoms (MCSA) to form the SiC layer is proposed. This new approach enables the growth of epitaxial GaN layers with low defect content and facilitates transfer to any surface. The paper details the technology of manufacturing LEDs on SiC/Si substrates obtained by the MCSA and elaborates on the benefits of using these substrates in LED production. Additionally, the advantages of the growth interface between SiC and Si materials are discussed. Moreover, it is found that thinner SiC layers (<200 nm) contribute to the scattering of the LED’s own radiation in the heterostructure waveguide, which decreases its absorption by silicon. For flip-chip LEDs with the substrate removed, substrates with thicker SiC layers (~400 nm) and a growth porous layer of several microns at the SiC-Si interface is utilized to simplify Si substrate removal and enhance the manufacturing process’s cost-effectiveness.

Detailed band alignment of high-B-composition BGaN with GaN and AlN

The electronic structure of B0.097Ga0.903N was determined by examining its bandgap and valence band offset (VBO) in detail. The BGaN sample was grown using a horizontal reactor metalorganic chemical vapor deposition. For bandgap determination, three different techniques were utilized yielding similar results, which are: UV–Vis spectroscopy, Schottky photodiodes, and electron energy-loss spectroscopy. The bandgap was determined to be ∼3.55 eV. For measuring the VBO, the valence edges and the core levels of Al 2s and Ga 2p were measured using x-ray photoelectron spectroscopy (XPS). The valence edges were then fitted and processed along with the core levels using the standard Kraut method for VBO determination with AlN. The BGaN/AlN alignment was found to be −1.1 ± 0.1 eV. Due to core level interference between GaN and BGaN, the Kraut method fails to provide precise VBO for this heterojunction. Therefore, a different technique is devised to analyze the measured XPS data which utilizes the alignment of the Fermi levels of the BGaN and GaN layers when in contact. Statistical analysis was used to determine the BGaN/GaN alignment with decent precision. The value was found to be −0.3 ± 0.1 eV.

Quantitative analysis of carbon impurity concentrations in GaN epilayers by cathodoluminescence

In this work, a technique for quantifying carbon doping concentrations in GaN:C/AlGaN buffer structures using cathodoluminescence (CL) is presented. The method stems from the knowledge that the blue and yellow luminescence intensity in CL spectra of GaN varies with the carbon doping concentration. By calculating the blue and yellow luminescence peak intensities normalised to the peak GaN near-band-edge intensity for GaN layers of known carbon concentrations, calibration curves that show the change in normalised blue and yellow luminescence intensity with carbon concentration in the 1016 − 1019 cm−3 range were derived at both room temperature and 10 K. The utility of such calibration curves was then examined by testing against an unknown sample containing multiple carbon-doped GaN layers. The results obtained from CL using the normalised blue luminescence calibration curves are in close agreement with those from secondary-ion mass spectroscopy (SIMS). However,the method fails when applying calibration curves obtained from the normalised yellow luminescence likely due to the influence of native VGa defects acting in this luminescence region. Although this work shows that indeed CL can be used as a quantitative tool to measure carbon doping concentrations in GaN:C, it is noted that the intrinsic broadening effects innate to CL can make it difficult to differentiate between the intensity variations in thin ( < 500 nm) multilayered GaN:C structures such as the ones studied in this work.

Influence of Pulse Atomic-Layer Epitaxy (PALE) AlN Buffer Layer on Quality of MOCVD Grown GaN on Si(111) Substrate.

A crack-free GaN-on-Si with a high capability to grow a thick GaN layer is important for the performance of optoelectronic devices. Regrettably, a large lattice and thermal mismatch degraded the quality of GaN-on-Si. Therefore, we introduce the pulse atomic-layer epitaxy (PALE) AlN buffer layer to overcome the surface crack issues and improve the quality of GaN epilayers. The effect of PALE AlN buffer layer with 0, 35, 70, and 140 cycle numbers towards the quality of grown GaN on Si(111) substrate is studied. All samples have been observed using AFM, FESEM, XRD and Raman spectroscopy. AFM imaging revealed a fully-coalesced and clear step flow GaN layer with the lowest RMS roughness of 0.606 nm. FESEM surface morphology shows a crack occurs on a sample without AlN PALE (0 pulse cycle) while crack-free GaN is obtained for samples with AlN PALE layer (35, 70 and 140 cycles). It was observed by XRD that the AlN PALE pulse cycle number greatly affects the structural properties of the top GaN layer where the lowest x-ray rocking curve for (002) and (102) achieved at 70 cycles, indicating the reduction of threading dislocations density in the growth structure. Further, the transition of the GaN E2 (high) from the Raman spectroscopy revealed a tensile strain in the GaN epilayers reduces with the increasing number of pulse cycles.

Mechanical Properties and Creep Behavior of Undoped and Mg-Doped GaN Thin Films Grown by Metal–Organic Chemical Vapor Deposition

This paper investigates the mechanical properties and creep behavior of undoped and Mg-doped GaN thin films grown on sapphire substrates using metal–organic chemical vapor deposition (MOCVD) with trimethylgallium (TMG) and bis(cyclopentadienyl)magnesium (Cp2Mg) as the precursors for Ga and Mg, respectively. The Mg-doped GaN layer, with a [Mg]/[TMG] ratio of 0.33, was systematically analyzed to compare its mechanical properties and creep behavior to those of the undoped GaN thin film, marking the first investigation into the creep behavior of both GaN and Mg-doped GaN thin films. The results show that the incorporated [Mg]/[TMG] ratio was sufficient for the transition from n-type to p-type conductivity with higher hole concentration around 4.6×1017 cm−3. Additionally, it was observed that Mg doping impacted the hardness and Young’s modulus, leading to an approximately 20% increase in these mechanical properties. The creep exponent is also affected due to the introduction of Mg atoms. This, in turn, contributes to an increase in pre-existing dislocation density from 2 × 108 cm−2 for undoped GaN to 5 × 109 cm−2 for the Mg-doped GaN layer. The assessment of the creep behavior of GaN and Mg-doped GaN thin films reveals an inherent creep mechanism governed by dislocation glides and climbs, highlighting the significance of Mg doping concentration in GaN thin films and its potential impact on various technological applications.

Microstructural evolution of extended defects in 25 μ m thick GaN homo-epitaxial layers

Defect origins and their propagation behavior were investigated in 25 μm thick homo-epitaxial GaN layers grown on ammono-thermal and void-assisted separation (VAS) substrates using multi-vector x-ray topography in both transmission and reflection geometries. Complex inclusions were identified and their microstructure was analyzed. Additionally, generation of threading dislocation clusters during epitaxial growth is analyzed. Various defects are delineated from the substrate vs epitaxial layers. Growth on the ammono-thermal substrate led to less defective and flatter epitaxial layers compared to the growth on the VAS substrate. Determining the origins and microstructure of defects is crucial toward developing defect mitigation strategies for reliable GaN devices.

Vacancy-type defects in AlInN/AlN/GaN structures probed by monoenergetic positron beam

Vacancy-type defects in AlInN(10 nm)/AlN(1–2 nm)/GaN were probed by using a positron annihilation technique. The crystal quality of the AlInN layer and atomic diffusion near heterointerfaces were also studied by x-ray diffraction reciprocal space mapping, transmission electron microscopy, and energy-dispersive x-ray spectroscopy. For an as-deposited sample without an AlN spacer layer (AlInN/GaN), Ga atoms diffused into the AlInN layer, and as a result, the concentration of Ga-vacancy-type defects in the GaN layer increased. The vacancy concentration was decreased by inserting the AlN layer, which was attributed to the suppression of out-diffusion of Ga from the GaN layer. The effect of the thickness of the AlN layer on the mobility of two-dimensional electron gas is discussed in terms of the introduction of vacancies into the channel region. The annealing behaviors of vacancies in the GaN layer and atomic exchange near heterointerfaces are also discussed.

Investigation of DC and RF characteristics of spacer layer thickness engineered recessed gate and field‐plated III‐nitride nano‐HEMT on β‐Ga2O3 substrate

AbstractIn this work, a field plated recessed gate III‐nitride HEMT on β‐Ga2O3 substrate is proposed using AlN spacer between AlGaN and GaN layers. The two‐dimensional electron gas (2DEG) transport characteristics, input/output, and RF characteristics of the proposed HEMT with AlN spacer layer of different thickness (1, 2, 3, 4, and 5 nm) are numerically simulated and compared with HEMT without spacer layer. The 2DEG, which is created at the junction of AlGaN/GaN, plays a crucial role in the operation of GaN HEMTs. With the help of AlN as spacer layer between AlGaN and GaN, an augmented 2DEG concentration is accomplished owing to its large conduction band offset, high polarization effect, and higher barrier. Additionally, the inclusion of AlN spacer layer shifts the 2DEG density away from interface, resulting minimized interface scattering which results into greater mobility. The HEMT exhibited a mobility of 1261 cm2/Vs, threshold voltage of −0.45 V, drain current of 1.12 A/mm, breakdown voltage of 149 V, and excellent RF characteristics, that is, cut‐off frequency (maximum oscillation frequency) of 384 GHz (546 GHz) using 2‐nm AlN spacer layer. The proposed III‐nitride HEMT on β‐Ga2O3 substrate with 2‐nm AlN spacer can be used for RF/microwave and high‐power nanoelectronics applications.

An Experimental Study of Dislocation Dynamics in GaN.

The dynamics of dislocations introduced through indentation or scratching at room temperature into a few GaN layers that were grown using the HVPE, MOCVD and ELOG methods and had different dislocation densities were studied via the electron-beam-induced current and cathodoluminescence methods. The effects of thermal annealing and electron beam irradiation on dislocation generation and multiplication were investigated. It is shown that the Peierls barrier for dislocation glide in GaN is essentially lower than 1 eV; thus, it is mobile even at room temperature. It is shown that the mobility of a dislocation in the state-of-the-art GaN is not entirely determined by its intrinsic properties. Rather, two mechanisms may work simultaneously: overcoming the Peierls barrier and overcoming localized obstacles. The role of threading dislocations as effective obstacles for basal plane dislocation glide is demonstrated. It is shown that under low-energy electron beam irradiation, the activation energy for the dislocation glide decreases to a few tens of meV. Therefore, under e-beam irradiation, the dislocation movement is mainly controlled by overcoming localized obstacles.

Crystal-field analysis and models of Eu-emission centers with C3v symmetry in insitu Eu- and Mg-codoping GaN layers

Gallium nitride active layers co-doped in situ with magnesium and europium, GaN(Mg):Eu, show strong red photoluminescence (PL), promising for red light emitting diode (LED) application (Eu@621 nm). Polarized PL lines excited by either above or below band-gap energy for the GaN(Mg):Eu layers are assigned to the 5D0→7FJ (J = 0, 1, 2) transitions lines from several Eu centers with C3v symmetry, which consist of single line, double lines and triple lines related to splitting of 7FJ (J = 0, 1, 2) by the C3v crystal-field, respectively. Cascade luminescence line-pattern of the 5D1→7FJ (J = 0, 1, 2) transitions is a complete replica of the PL line-pattern and gives evidence of the correct assignment of the polarized PL lines. Crystal-field splitting and Zeeman splitting of the PL lines have been analysed using the C3v crystal-field equivalent operator Hamiltonian. Structural Eu models with the C3v symmetry have been proposed on the basis of the analysis: the dominant Eu center is an Eu complex with defect-free perfect surrounding: the minor Eu centers are Eu complexes accompanied by nitrogen vacancies with different charge states on the c-axis in the closest vicinity of Eu. An important role of Mg atoms in the GaN(Mg):Eu active layers has been proposed to suppress creation of gallium vacancies, giving rise to enhancement of the PL emitted from the defect-free Eu centers.

Development of a novel immunoFET technology-based POC assay for detection of Leishmania donovani and Leishmania major.

Leishmaniasis is considered as one of the 20 neglected tropical diseases. Current methods of leishmanial diagnosis depend on conventional laboratory-based techniques, which are time-consuming, costly and require special equipment and trained personnel. In this context, we aimed to provide an immuno field effect transistors (ImmunoFET) biosensor that matches the conventional standards for point-of-care (POC) monitoring and detection of Leishmania (L.) donovani/Leishmania major. Crude antigens prepared by repeated freeze thawing of L. donovani/L. major stationary phase promastigotes were used for ELISA and ImmunoFETs. Lesishmania-specific antigens were serially diluted in 1× PBS from a concentration of 106 -102 parasites/mL. A specific polyclonal antibody-based sandwich ELISA was established for the detection of Leishmania antigens. An immunoFET technology-based POC novel assay was constructed for the detection of Leishmania antigens. Interactions between antigen-antibody at the gate surface generate an electrical signal that can be measured by semiconductor field-effect principles. Sensitivity was considered and measured as the change in current divided by the initial current. The final L. donovani/L. major crude antigen protein concentrations were measured as 1.50 mg/mL. Sandwich ELISA against the Leishmania 40S ribosomal protein detected Leishmania antigens could detect as few as 100 L. donovani/L. major parasites. An immunoFET biosensor was constructed based on the optimization of aluminium gallium nitride/gallium nitride (AlGaN/GaN) surface oxidation methods. The device surface was composed by an AlGaN/GaN wafer with a 23 nm AlGaN barrier layer, a 2 μm GaN layer on the silicon carbide (SiC) substrate for Leishmania binding, and coated with a specific antibody against the Leishmania 40S ribosomal protein, which was successfully detected at concentrations from 106 to 102 parasites/mL in 1× PBS. At the concentration of 104 parasites, the immunoFETs device sensitivities were 13% and 0.052% in the sub-threshold regime and the saturation regime, respectively. Leishmania parasites were successfully detected by the ImmunoFET biosensor at a diluted concentration as low as 150 ng/mL. In this study, the developed ImmunoFET biosensor performed well. ImmunoFET biosensors can be used as an alternative diagnostic method to ELISA. Increasing the sensitivity and optimization of immuno-FET biosensors might allow earlier and faster detection of leishmaniasis.

Impact of high-temperature Mg-implantation on defects and dopants distribution in GaN

We have investigated the impact of high-temperature Mg-implantation in GaN layers on distribution of Mg-enriched defects using scanning transmission electron microscopy and atom probe tomography. For this, 1 × 1019 cm−3 Mg ions have been implanted in GaN layers at room temperature (RT) and 1000 °C, followed by annealing at 1300 °C. A smaller number of Mg-enriched defects were observed in the sample implanted at 1000 °C in comparison to the sample implanted at RT. The implantation of Mg ions at 1000 °C resulted in a higher amount of randomly distributed Mg in the GaN matrix, which, in turn, leads to more uniform and enhanced donor–acceptor pair emission, leading to higher Mg activation.