Rf Plasma-assisted Molecular Beam Epitaxy Research Articles

(Invited) InGaN/AlGaN Red-Emitting Nanowire LEDs for Monolithic Micro-LED Displays

High efficiency, high color rendition and low-cost light-emitting diodes (LEDs) with low power consumption, long lifetime and high reliability are highly expected for general lighting illumination, smartphones, smartwatches, virtual reality (VR), augmented reality (AR) headsets, and micro-display applications. Nonetheless, the achievement of deep green to red-emitting LEDs using conventional III-nitride quantum well heterostructures has been difficult, due to the presence of large densities of dislocations, strong polarization fields, poor hole transport, and carrier delocalization [1]. The external quantum efficiency (EQE) of the blue and green InGaN LEDs surpassed 80% and 53%, respectively. However, due to the aforementioned issues, the current InGaN red LEDs with high Indium composition exhibit extremely low EQE which is less than 3% [2]. In this regard, LEDs using nanowire structures offer dramatically reduced strain-induced polarization fields and dislocation densities, providing ideal material structure for high efficiency full-color and even white light emission without using phosphor-converters. In this study, we have successfully designed, and fabricated high efficiency red-emitting nanowire heterostructures and demonstrated micro-LEDs with stable and strong emission at ~645 nm. Moreover, the micro-LEDs have high internal quantum efficiency of >40%.The InGaN/AlGaN nanowire micro-LED structures are grown by RF plasma-assisted molecular beam epitaxy (MBE) under nitrogen-rich condition. The LED structure consists of a 250 nm n-GaN nanowire template, 10 couples of 3 nm AlGaN quantum barrier (QB)/ 3 nm InGaN quantum well (QW) served as the active region, and a 200 nm p-GaN. During the epitaxial growth of AlGaN barriers, an AlGaN shell spontaneously forms, enabling a unique InGaN/AlGaN core-shell structure [3]. The emission color of the micro-LEDs can be defined by controlling the ratio of Ga/In flux and the substrate temperature during the MBE growth process. Detailed growth conditions and the device fabrication can be found elsewhere [3-5]. The nanowires are uniformly arranged on Si substrates, as illustrated in Figure 1(a). Figure 1(b) shows the schematic structure of the fabricated micro-LEDs. Strong red emissions were measured from the InGaN/AlGaN core-shell LEDs, as shown in Figure 1(c). At injection current of 400mA, the peak emission wavelength is at ~645nm. The red-emitting micro-LEDs exhibit stable emissions with a blue-shift of only ~ 4nm under injection current from 50 mA to 400 mA, attributed to the significantly reduced quantum-confined Stark effect (QCSE) in the nanowire structures. Moreover, full color micro-LEDs with device size from 10x10 µm2 to 100x100 µm2 have been fabricated using similar approach. Such high efficiency, high color rendering properties, and low power consumption micro-LEDs are promising candidates for emerging AR/VR devices and micro-LED display applications.

Self-Organized Eu-Doped GaN Nanocolumn Light-Emitting Diode Grown by RF-Molecular-Beam Epitaxy

High-concentration doping of Eu ions working as luminescent centers in GaN is required to improve light output of optical devices. GaN nanocolumns can be used to achieve high crystallinity in high-concentration Eu-doped GaN (GaN:Eu). In this study, optical properties of a GaN:Eu nanocolumn grown by RF-plasma-assisted molecular-beam epitaxy (RF-MBE) are investigated and GaN:Eu nanocolumn light-emitting diodes (LEDs) are demonstrated. Suppression of yellow luminescence and concentration quenching of PL intensity are observed at RT for GaN:Eu nanocolumns. When the effect of Eu concentration on optical properties is analyzed using rate equation, an increase in the Eu concentration is observed to affect 5D0 lifetime corresponding to back transfer, and not the effective cross-section corresponding to transfer efficiency. Based on these results, GaN:Eu nanocolumn LEDs are fabricated on an n-type (111) Si substrate. Clear rectification characteristics and sharp red luminescence are observed. The light output increases with increasing Eu concentration up to a concentration of 6 × 1020 cm−3. These results suggested that GaN:Eu nanocolumns provide feasible crystal to realize novel optical devices.

AlGaN/GaN High Electron Mobility Transistor Grown and Fabricated on Lattice Matched Metallic Layers

AlGaN/GaN high electron mobility transistors (HEMTs) showed its promising features in high power and high frequency applications such as inverter units in hybrid electric vehicles, advanced radar systems, and satellite-based communication networks with its high density of sheet carrier concentration, high electron mobility, and radiation hardness. In order to lower the cost of HEMTs, larger diameters substrates are needed. Currently, the HEMT structures are grown on non-native sapphire, SiC and Si substrates or on native substrate of vapor phase epitaxy (VPE) GaN wafers. HEMTs grown on sapphire substrate suffer from poor heat dissipation and higher defect density due to the relative larger lattice mismatch to GaN. SiC has a very good thermal conductivity and is less lattice mismatch to GaN, but high quality semi-insulating SiC substrates are quite expensive. Although, Si substrates are fair low cost and available for larger diameter substrates, due to larger lattice mismatch to GaN, it requires to grow thick complex AlGaN buffers prior to the growth of GaN layer to reduce the threading dislocation density. These thick AlGaN layers are very defective and thermal resistive, which will hinder device heat dissipation. VPE grown GaN wafers are limited to 2-3” diameter and very expensive. Recently, it was reported that high quality GaN layers were grown by RF plasma-assisted molecular beam epitaxy (PA-MBE) or metal organic chemical vapor deposition (MOCVD) on high-quality, single crystal ZrTi refractory metal alloys deposited by plasma sputtering on c-plane sapphire and Si substrates. These ZrTi refractory metal alloys are not only lattice match to GaN, but have a similar coefficient of thermal expansion (CTE) as the CTE of GaN [1]. By employing these ZrTi alloys, there is no need to grow thick AlGaN buffer, both photonic and electronic GaN based device structure can be grown on large area Si substrates to achieve low cost GaN based device fabrication. In this work, we have demonstrated AlGaN/GaN HEMTs grown on ZrTi refractory metal alloys, which were deposited on both Si and sapphire substrates. HEMTs with a gate dimension of 1 µm × 100 um were fabricated and characterized. Both dc and rf performance of AlGaN/GaN HEMTs fabricated on the ZrTi metal alloys will be presented. 1. “Epitaxial Growth of High Quality GaN Films on Lattice Matched Metallic Layers”, A.M. Dabiran, F. Machuca, I. De, and R. Weiss, ECS Trans. 66, 113-117 (2015).

Microstructures of InN film on 4H-SiC (0001) substrate grown by RF-MBE**Project supported by the Thailand Center of Excellence in Physics (ThEP) and the King Mongkut's University of Technology Thonburi under The National Research University Project. One of the authors (S. Sanorpim) was supported by the National Research Council of Thailand (NRCT) and the Thai Government Stimulus Package 2 (TKK2555), under

InN film was grown on 4H-SiC (0001) substrate by RF plasma-assisted molecular beam epitaxy (RF-MBE). Prior to the growth of InN film, an InN buffer layer with a thickness of ∼5.5 nm was grown on the substrate. Surface morphology, microstructure and structural quality of InN film were investigated. Micro-structural defects, such as stacking faults and anti-phase domain in InN film were carefully investigated using transmission electron microscopy (TEM). The results show that a high density of line contrasts, parallel to the growth direction (c-axis), was clearly observed in the grown InN film. Dark field TEM images recorded with diffraction vectors and g = 0002 revealed that such line contrasts evolved from a coalescence of the adjacent misoriented islands during the initial stage of the InN nucleation on the substrate surface. This InN nucleation also led to a generation of anti-phase domains.

As4 overpressure effects on the phase purity of cubic GaN layers grown on GaAs substrates by RF-MBE

Cubic GaN samples on GaAs(001) substrates were grown by RF-plasma-assisted molecular beam epitaxy (RF-MBE), an As4 overpressure was employed during the nucleating layer. We found that the relaxation process of c-GaN during the nucleating layer, affects the purity of cubic phase. Higher cubic phase of GaN was obtained for samples that initiated the relaxation process after some monolayers (8ML) of growth, as a result of better stabilization of the cubic layer. The relaxation process depends on the As4 overpressure over the growth surface and on the growth temperature. The structural properties of c-GaN characterized by transmission electron microscopy (TEM) and high resolution X-ray diffraction (HR-XRD) have evidenced the purity of cubic phase in the first nanometer of growth, and also the formation of the hexagonal inclusions after several nanometers of growth from the stacking faults on the {111} planes. From an estimation of the hexagonal content by X-ray reciprocal space mapping (RSM), we found a lower hexagonal component on the (11¯1)N and (1¯11)N planes. The highest purity of cubic GaN was 98.7% along the {111}N planes for sample grown at 720°C. Photoluminescence measurements also demonstrated the high purity of the cubic phase of this sample, no emission of hexagonal inclusion was detected.

Ultraviolet random lasing from Mg0.12Zn0.88O:N/ZnO:Ga single-heterostructure diode

A heterostructure device consisting of nitrogen-doped Mg0.12Zn0.88O and gallium-doped ZnO thin films was grown on c-plane sapphire substrate using RF plasma-assisted molecular beam epitaxy. Current–voltage and photocurrent characteristics indicate the formation of a p–n junction. Random lasing behavior with lasing modes centered at 356 nm was observed. A low-threshold current of 6 mA was determined, and an output power of 34 nW was measured at an injection current of 8 mA. The film contains columnar structures with much air gaps, which assist in light scattering to achieve necessary gain for random lasing.

Photoluminescence study of nitrogen-doped p-type Mg x Zn1−x O nanocrystalline thin film grown by plasma-assisted molecular beam epitaxy

Temperature-dependent photoluminescence of nitrogen-doped p-type Mg x Zn1−x O nanocrystalline thin film grown on c-plane sapphire substrate by rf plasma-assisted molecular beam epitaxy is investigated. P-type behavior is confirmed by both Hall effect and Seebeck measurements. Structural defect-related bound excitonic emission peak is distinguished at around ~50 meV lower than peak energy of the near band edge neutral acceptor bound excitonic emission. Typical ‘S shape’ behavior of energy position versus temperature is observed due to polarization-induced internal field. Nitrogen-related acceptor ionization energy is calculated to be ~180–200 meV.

Nanoscratch characterization of indium nitride films

Abstract In this study we used RF plasma-assisted molecular beam epitaxy for the epitaxial growth of single-crystalline indium nitride (InN) thin films on aluminum nitride buffer layers/Si (111) substrates. We then used scratch techniques to study the influence of the c-axis orientation of the InN films and the beam interactions on the tribological performance of these samples. When grown at 440, 470, and 500 °C, the coefficients of friction were 0.18, 0.22, and 0.26, respectively, under a normal force (F n) of 2000 μN; 0.19, 0.23, and 0.27, respectively, under a value of F n of 4000 μN; and 0.21, 0.24, and 0.28, respectively, under a value of F n of 6000 μN. These measured values increased slightly upon increasing the growth temperature because of the resulting smaller sizes of the apertures and/or pores in the inner films. The sliding resistance of the ploughed area was observed. The contact sliding line became increasingly noticeable upon increasing the value of F n; the plot of the friction with respect to the penetration depth revealed a significant relation in its adhesion properties presentation.

Study of structural properties of cubic InN films on GaAs(001) substrates by molecular beam epitaxy and migration enhanced epitaxy

InN epitaxial films with cubic phase were grown by rf-plasma-assisted molecular beam epitaxy (RF-MBE) on GaAs(001) substrates employing two methods: migration-enhanced epitaxy (MEE) and conventional MBE technique. The films were synthesized at different growth temperatures ranging from 490 to 550 °C, and different In beam fluxes (BEPIn) ranging from 5.9 × 10−7 to 9.7 × 10−7 Torr. We found the optimum conditions for the nucleation of the cubic phase of the InN using a buffer composed of several thin layers, according to reflection high-energy electron diffraction (RHEED) patterns. Crystallographic analysis by high resolution X-ray diffraction (HR-XRD) and RHEED confirmed the growth of c-InN by the two methods. We achieved with the MEE method a higher crystal quality and higher cubic phase purity. The ratio of cubic to hexagonal components in InN films was estimated from the ratio of the integrated X-ray diffraction intensities of the cubic (002) and hexagonal (101¯1) planes measured by X-ray reciprocal space mapping (RSM). For MEE samples, the cubic phase of InN increases employing higher In beam fluxes and higher growth temperatures. We have obtained a cubic purity phase of 96.4% for a film grown at 510 °C by MEE.

Ultrathin-barrier AlN/GaN heterostructures grown by rf plasma-assisted molecular beam epitaxy on freestanding GaN substrates

We report the structural and electrical properties of ultrathin-barrier AlN/GaN heterostructures grown on freestanding GaN substrates by rf plasma-assisted molecular beam epitaxy. Structures with barrier thicknesses between 1.5nm and 7.5nm were grown and characterized. We observe that AlN/GaN structures with barriers of 3.0nm exhibit the highest Hall mobility, approximately 1700cm2/Vs. Furthermore, the Hall mobility is much diminished in heterostructures with AlN barriers thicker than 4.5nm, coincident with the onset of strain relaxation.

Growth of self-assembled InGaN quantum dots on Si (111) at reduced temperature by molecular beam epitaxy

Self-assembled InGaN quantum dots (QDs) were grown by RF plasma-assisted molecular beam epitaxy on Si (111). The growth of InGaN QDs has been investigated in situ by reflection high energy electron diffraction. It is found that a low-temperature growth of InGaN instantaneously leads to the formation of three-dimensional islands. Field emission scanning electron microscopy and atomic force microscopy revealed that diameters were small enough to expect a zero-dimensional quantum effect which is around 20nm and the height of the dots was around 6.0nm. X-ray diffraction (XRD) measurement indicated that the InGaN was epitaxially grown on silicon substrate. Photoluminescence measurement exhibits a sharp and intense band edge emission of InGaN at 1.7 eV. There was an excellent agreement with the result of mole fraction measured from XRD by employing Vegard's law which is In molar fraction is 0.5.

Influence of off-cut angle of (0001) 4H-SiC on the crystal quality of InN grown by RF-MBE

The effect of the off-cut angle of a 4H-SiC (0001) substrate on the growth of InN thick layer by RF-plasma assisted molecular beam epitaxy (RF-MBE) has been investigated. The off-cut angle used in this study was inclined from 0° (just surface) toward the [11–20] direction of 4° and 8°. Crystalline quality and surface morphology were remarkably sensitive to the value of off-angle. Higher off-cut angles result in a reduction of the full-widths at half-maximum of HRXRD (0002) ω-scans, compared to that of the layer on the (0001)-just surface. In addition, the full-widths at halfmaximum of μ-Raman scattering spectra at 490cm-1, which is attributed to E2 (high) phonon mode, was decreased with increase in off-cut angle. Furthermore, In-droplets, which are commonly observed on the (0001) InN grown surface under In rich-growth condition, were found to be suppressed owing to an improvement of a nucleation on the off-cut angle surface. In our case, the use of 8°-off substrate increased film density and growth rate, while a surface roughness was reduced. These results clearly demonstrate that the larger off-cut angles improve the crystalline quality of InN film with reducing the In-droplets due to a higher step surface density on the off-cut angle surface.

Nanoindention study of indium nitride thin films grown using RF plasma-assisted molecular beam epitaxy

In this study, we used an RF plasma-assisted molecular beam epitaxy (RF-MBE) system to grow single-crystalline indium nitride (InN) films onto aluminum nitride (AlN) buffer layers on Si (111) substrates. We then used nanoindentation techniques and reflection high-energy electron diffraction (RHEED) to study the influence of the c-axis-oriented InN films on the mechanical performance. From morphological observations, we compared the stiffness and resistance against contact-induced damage of the InN films in the presented shrinkage of the area. InN films prepared at growth temperatures of 440, 470, and 500 °C had nanohardnesses (H) of 3.6 ± 0.2, 4.5 ± 0.25, and 9.1 ± 0.8 GPa, respectively, and Young’s moduli (E) of 97.4 ± 1.2, 147.7 ± 1.8, and 176.0 ± 2.3 GPa, respectively.

Effect of Mg Doping on the Photoluminescence of GaN:Mg Films by Radio-Frequency Plasma-Assisted Molecular Beam Epitaxy

We investigate undoped GaN and Mg-doped GaN grown by rf plasma-assisted molecular beam epitaxy (MBE) with different Mg concentrations by photoluminescence (PL) at low temperature, Hall-effect and XRD measurements. In the PL spectra of lightly Mg-doped GaN films, a low intensity near band edge (NBE) emission and strong donor-acceptor pair (DAP) emission with its phonon replicas are observed. As the Mg concentration is increased, the DAP and NBE bands become weaker and a red shift of these bands is observed in the PL spectra. Yellow luminescence (YL) is observed in heavily Mg-doped GaN. The x-ray diffraction is employed to study the structure of the films. Hall measurement shows that there is a maximum value (3.9 × 1018 cm−3) of hole concentration with increasing Mg source temperature for compensation effect. PL spectra of undoped GaN are also studied under N-rich and Ga-rich growth conditions. Yellow luminescences of undoped Ga-rich GaN and heavily Mg-doped GaN are compared, indicating the different origins of the YL bands.

GaGdN/AlGaN multiple quantum disks grown by RF‐plasma‐assisted molecular‐beam epitaxy

AbstractIII–V‐based diluted magnetic semiconductor (DMS) GaGdN/AlGaN multiple quantum disks (MQDisks) were fabricated on Si (001) substrates with native silicon oxides by RF‐plasma‐assisted molecular‐beam epitaxy (RF‐MBE). It was found that the degree of the c‐axis orientation of GaGdN/AlGaN MQDisks and the Ga atomic configuration around Gd atom were improved due to the growth of AlGaN disks, comparing with the GaGdN nanorods grown without AlGaN disks. Additionally, GaGdN/AlGaN MQDisks showed larger saturation magnetisation than that of the GaGdN nanorods. With increasing the height of AlGaN disk, saturation magnetisation was increased.

Effect of Growth Mode on Eu-Incorporation and Luminescence of Eu-Doped GaN Epitaxial Film Grown by Plasma-Assisted Molecular Beam Epitaxy

The growth mode of europium (Eu)-doped GaN epitaxial films grown on a GaN template by rf plasma-assisted molecular beam epitaxy (PAMBE) was investigated with different III/V ratios under a constant Eu beam equivalent pressure ratio [PEu/(PEu+PGa)]. The reflection high-energy electron diffraction (RHEED) patterns and atomic force microscopy (AFM) images revealed the transition of the growth mode from three-dimensional (3D) to step-flow/two-dimensional (2D) by increasing the III/V ratio. When the films were grown in the 3D growth mode, Eu concentrations estimated by Rutherford backscattering spectrometry/channeling (RBS/channeling) were almost constant, although the III/V ratios varied. However, when the growth mode was transferred from 3D to step-flow/2D, precipitates on the surface abruptly increased while the Eu concentration abruptly decreased, indicating the abrupt degradation of Eu-incorporation in the film. Luminescence sites of Eu3+ were sensitive to the III/V ratio, and Eu atoms have different luminescence sites in both growth modes. Furthermore, luminescence efficiency abruptly increased when the growth mode was transferred from 3D to step-flow/2D.

Droplet Epitaxy of InN Quantum Dots on Si(111) by RF Plasma-Assisted Molecular Beam Epitaxy

InN quantum dots (QDs) were fabricated on Si(111) substrate by droplet epitaxy using an RF plasma-assisted MBE system. Variation of the growth parameters, such as growth temperature and deposition time, allowed us to control the characteristic size and density of the QDs. As the growth temperature was increased from 100 C to 300 degrees C, an enlargement of QD size and a drop in dot density were observed, which was led by the limitation of surface diffusion of adatoms with the limited thermal energy. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to assess the QDs size and density. The chemical bonding configurations of InN QDs were examined by X-ray photo-electron spectroscopy (XPS). Fourier transform infrared (FTIR) spectrum of the deposited InN QDs shows the presence of In-N bond. Temperature-dependent photoluminescence (PL) measurements showed that the emission peak energies of the InN QDs are sensitive to temperature and show a strong peak emission at 0.79 eV.

Epitaxial Overgrowth of Gallium Nitride Nano-Rods on Silicon (111) Substrates by RF-Plasma-Assisted Molecular Beam Epitaxy

Strain-free gallium nitride (GaN) overgrowth on GaN nano-rods is realized by RF-plasma assisted molecular beam epitaxy (RF-MBE) on silicon (Si) substrate. The strain-free condition was identified by the strong free A exciton (FXA) photoluminescence (PL) peak at 3.478 eV and the E2 high phonon Raman shift of 567 cm-1. It is clearly demonstrated that the critical diameter of GaN nano-rods is around 80 nm for the overgrowth of strain-free GaN. The blue-shift of PL peak energy and phonon Raman energy with decreasing the diameter of nano-rod result from the strain relaxation of overgrowth GaN.

Effect of growth conditions on Eu 3+ luminescence in GaN

Eu-doped GaN thin films were in situ grown on sapphire substrates by RF plasma-assisted solid-source molecular beam epitaxy technique. Strong red emission at ∼622 nm from 5D 0– 7F 2 radiative transitions in Eu 3+ ions was observed for all samples. The effects of important growth parameters, such as III/V ratio (Ga flux), Eu cell temperature (Eu flux) and growth temperature, on Eu 3+ photoluminescence were studied. X-ray diffraction and secondary ion mass spectroscopy measurements were performed to investigate thin film quality and Eu doping profiles. The strongest Eu 3+ luminescence was obtained from GaN:Eu thin films grown under slightly N-rich condition (III/V<1), while the highest Eu 3+ emission efficiency was obtained in thin films grown under Ga-rich condition (III/V≥1). The optimum Eu doping concentration for Eu 3+ luminescence is ∼(0.1–1.0) at% for III/V≤1 ratio condition. Higher growth temperature (>750 °C) was also found to enhance Eu 3+ luminescence intensity and efficiency.

Depth Dependence of Tetragonal Distortion of a ZnO/Mg0.1Zn0.9O/ZnO Heterostructure Studied by Rutherford Backscattering/Channeling

Rutherford backscattering and channeling are used to characterize the structure of a ZnO/Mg0.1Zn0.9O/ZnO heterostructure grown on a sapphire (0001) substrate by rf plasma-assisted molecular beam epitaxy. The results show that the Mg0.1Zn0.9O layer has the same hexagonal wurtzite structure as the underlying ZnO layer, and the heterostructure has a good crystalline quality with χmin = 5%, which is the ratio of the backscattering yields of aligned and random spectra in the near-surface region. Using the channeling angular scan around an off-normal (12̄13) axis in the {101̄0} plane of both ZnO and MgZnO layer, the tetragonal distortion eT, which is caused by the elastic strain in the epilayer, is determined. The depth dependence of eT is obtained by using this technique. It can clearly be seen that the elastic strain rapidly decreases with the increase in thickness of the ZnO film in the early growth stage and becomes slightly larger in the region of the Mg0.1Zn0.9O layer.