Radio-frequency Plasma-assisted Molecular Beam Research Articles

Controlling the size and density of InN quantum dots formed on sapphire substrate by droplet epitaxy

The growth of InN quantum dots (QDs) on c-plane sapphire by droplet epitaxy (DE) using radio frequency plasma-assisted molecular beam epitaxy is reported here. The QD growth process from liquid In droplets to the InN QDs is described with a focus on the effect ambient nitrogen from an active RF-plasma source has on the formation of In droplets as a function of substrate temperatures. The variation in the shape and size of InN QDs is explained in terms of the In atom surface diffusion and the migration of droplets. Additionally, two nitridation procedures were used to investigate the crystallization of In droplets. The droplet formation was determined to follow well known principles of nucleation theory with ripening. The resulting activation energy for In surface diffusion on sapphire was found to be 0.62 ± 0.07 eV in ultra-high vacuum, ∼10−10 Torr, and 0.57 ± 0.08 eV in ambient N2, ∼10−5 Torr. The growth of InN QDs using the DE method has many advantages over the classical Stranski–Krastanov technique, including the ability to control a wide range of QD shapes, sizes, and densities.

Direct Observation of Al Migration Enhancement by Changing the Al and N Source Relative Position in the Molecular Beam Epitaxy of AlGaN Nanowires

Aluminum (Al)-containing nitrides, including AlN, AlScN, and AlGaN, are emerging material platforms for future-generation photonic, electronic, ferroelectric, and multifunctional semiconductor devices. Nonetheless, the low Al migration mobility in a nitrogen environment is a notorious hindrance to obtaining high-quality Al-containing nitride epilayers. This is particularly the case for the epitaxy of Al-containing nitrides with radio frequency (RF) plasma-assisted molecular beam epitaxy (MBE), due to the use of relatively low substrate temperatures. As such, various approaches have been employed in promoting Al migration such as metal-modulated epitaxy (MME). Despite the progress, the effect of the relative position of the Al and N sources on the Al migration mobility has remained unclear. In this study, we show that MBE-grown AlGaN nanowires can be an effective probe to examine the effect of the Al and N source relative position on the Al migration mobility and demonstrate clearly that Al adatoms appear to be more mobile from the Al source that is far away from the N source, compared to those from the Al source that is near the N source. This study therefore can potentially impact the development of emerging Al-containing semiconductor devices by MBE. Moreover, the demonstrated correlated Al source dependent morphological and optical properties of AlGaN nanowires can naturally help the development of AlGaN-nanowire-based UV light-emitting diodes (LEDs) and lasers.

Ohmic-Schottky conversion of ZnO/metal contact modulated by a plasma surface treatment method

Herein, we present a systematical investigation with regard to the Ar/N2/O2 plasma surface treatments on the ZnO thin films prepared by a radio frequency plasma-assisted molecular beam epitaxy (RF MBE) technique. The as-deposited ZnO thin films are exposed to the Ar, N2 and O2 plasma atmospheres under different RF powers. The conductivity and the surface morphology of the post-treated ZnO thin films are strongly depending on the treatment species and the RF powers. Higher RF power results in lower resistance and reduces the grain boundaries of the surface. Indium and gold metals are designed as the Ohmic and Schottky contact electrodes, respectively. The ZnO/Au contacts based on both pre-treated films and Ar/N2 plasma-treated films show obvious Ohmic contact behaviors. However, In/ZnO/Au the devices fabricated on the ZnO thin film which exposes to the O2 plasma show distinct rectified behavior, indicating the conversion of ZnO/Au contact from Ohmic type to Schottky type. The O2 plasma treatment may play two key roles within the contact conversion: 1, removing the surface contaminants such as carbon, absorbed oxygen and hydroxide; 2, repairing the defects (Vo), thus reducing the near surface donor density and defect assisted hopping transportation.

III-Nitrides Resonant Cavity Photodetector Devices.

III-nitride resonant cavity-enhanced Schottky barrier photodetectors were fabricated on 2 µm thick GaN templates by radio frequency plasma-assisted molecular beam epitaxy. The optical cavity was formed by a bottom distributed Bragg reflector based on 10 periods of Al0.3Ga0.7N/GaN, an Au-based Schottky contact as top mirror, and an active zone of 40 nm-thick GaN layer. The devices were fabricated with planar geometry. To evaluate the main benefits allowed by the optical cavity, conventional Schottky photodetectors were also processed. The results revealed a planar spectral response for the conventional photodetector, unlike the resonant devices that showed two raised peaks at 330 and 358 nm with responsivities of 0.34 and 0.39 mA/W, respectively. Both values were 80 times higher than the planar response of the conventional device. These results demonstrate the strong effect of the optical cavity to achieve the desired wavelength selectivity and to enhance the optical field thanks to the light resonance into the optical cavity. The research of such a combination of nitride-based Bragg mirror and thin active layer is the kernel of the present paper.

Selective area growth of GaN nanowires on Si(1 1 1) substrate with Ti masks by molecular beam epitaxy

We present results on the selective area growth (SAG) of Gallium Nitride (GaN) nanowires on Si substrate without any buffer layer by radio frequency plasma-assisted molecular beam epitaxy. Full selectivity was achieved with a thin Ti metal film used as a mask. The mask was fabricated with a lift-off method to avoid damage to the mask or Si substrate. The growth on the Ti film was totally suppressed at a low substrate temperature of 840 °C which is by far the lowest critical temperature for SAG. An InGaN thin layer was embedded in the GaN nanowires to realize a site controllable single photon source. Transmission electron microscopy (TEM) indicates single crystalline quality of defect-free GaN nanowires and the embedded InGaN quantum dot structure.

Epitaxial growth of semipolar InN() on LaAlO3 substrate: Epitaxial relationship analysis

Epitaxial semipolar InN() crystals were grown on a LaAlO3(112) substrate using radio frequency plasma-assisted molecular beam epitaxy without any interlayer. The lattice mismatch between InN and LaAlO3 was estimated to be −7.75% along the []InN direction and 0.2% along the []InN direction. The InN film is epitaxic with the LaAlO3 substrate, with orientation relationships between InN() ∥ LaAlO3(112) and []InN ∥ []LAO. InN grown on LaAlO3(112) appears () plane orientated, with two types of domains. Semipolar InN() layers can be grown at 510 °C and show X-ray rocking curve FWHMs of 1830 and 1408 arcsec for InN(0002) and InN(), respectively. However, InN grown at 510 °C has the highest peak intensity and the narrowest FWHM of the prepared samples, indicating high-quality crystal growth.

(Invited) High Efficiency Solar-to-Hydrogen Conversion on InGaN Nanowire Arrays

Recently, III-nitride nanowire structures have been intensively investigated for applications in solar water splitting and hydrogen generation. Compared to the conventional oxide-based photoelectrodes, III-nitride nanowires offer distinct advantages including tunable energy bandgap across nearly the entire solar spectrum, conduction and valence band edges that can straddle water redox potentials under deep visible and near-infrared light irradiation, and high efficiency charge carrier separation and extraction. Spontaneous overall water splitting under UV, blue, and green light irradiation has been demonstrated on InGaN/GaN nanowire structures. Moreover, multi-band InGaN nanowire photocatalyst and photoanode have been developed to effectively harvest a large part of the solar spectrum. Such nanowire arrays also exhibit a high level of stability in aqueous solution. To further improve the efficiency of solar-to-hydrogen conversion, we have investigated the design, epitaxial growth, and testing of paired InGaN/GaN photoanode and photocathode. The devices exhibit relatively high incident photon-to-current conversion efficiency (up to 65%). The energy conversion efficiency in the wavelength range of 300-600 nm is also significantly improved, compared to a single electrode. Here, we report on the design principle and operation of such novel photoelectrodes. In this design, the photocathode and the photoanode are connected side by side, wherein the solar light can be split spectrally and spatially. Different split light spectra are spread out to strike the corresponding photoelectrodes. n-InGaN/GaN and p-InGaN/GaN decorated with Pt nanoparticles are used as the photoanode and photocathode, respectively. The design of paired InGaN nanowire photoelectrodes can provide high photovoltage without trading off on the harvesting of longer wavelength of sunlight. Catalyst-free, vertically aligned InGaN/GaN nanowire arrays were grown on n-type Si(111) substrate by radio frequency plasma-assisted molecular beam epitaxy under nitrogen rich conditions. After an in situ oxide desorption at ~ 770 °C, A GaN nanowire template with a height of ~ 200 nm was first grown on Si substrate. To enhance the carrier transport between the n-type Si substrate and p-InGaN nanowire, a tunnel junction consisting of n +-GaN/p +-InGaN/p +-GaN was implemented beneath the p-InGaN nanowire. The nanowires are doped n-type or p-type using silicon or magnesium, respectively. The photoelectrochemical (PEC) properties of the photocathode and the photoanodes were investigated separately using a cell with a three-electrode configuration in 1 mol/L HBr. A linear sweep voltammogram for n-InGaN/GaN photoanode was performed under both dark and illuminated conditions. The maximum incident photon-to-current conversion efficiency (IPCE) measured at 1 V vs. Ag/AgCl for the n-InGaN/GaN photoanode is ~ 50% at 350 nm, while the maximum IPCE for p-InGaN/GaN photocathode (at -0.6 V vs. Ag/AgCl) is ~ 65% at 350 nm. The IPCE shows a decreasing trend with increasing wavelength, limited by the effective light absorption of the nanowire structures. In the paired configuration, the incident light was split by using long- and short-pass optical filters. The parallel-connected photoanode was illuminated with light with wavelengths in the range of 300-480nm. The photocathode was illuminated with light with wavelengths of 485-600 nm. Using a two-electrode configuration, the photoanode was considered to be the working electrode, and the photocathode p-InGaN/GaN was connected as the counter electrode in 1 mol/L HCl. With this novel design, the power conversion efficiency in the range of 300-600 nm has been increased by nearly one order of magnitude, compared to efficiency of a single electrode. The maximum power conversion efficiency of this device can reach >5%. Work is currently in progress on the integration of InGaN nanowires with solar cells to achieve further increased efficiency for solar-to-hydrogen conversion.

(Invited) High Power Phosphor-Free InGaN/GaN/AlGaN Core-Shell Nanowire White Light Emitting Diodes on Si Substrates

Recently, III-nitride nanowire structures have been intensively investigated for applications in solid state lighting and full-color displays. Compared to the conventional GaN based planar LEDs, III-nitride nanowires offer distinct advantages including greatly reduced dislocation densities, polarization fields, and quantum-confined Stark effect, due to the effective lateral stress relaxation, thereby promising superior performance LEDs for lighting and display applications. However, the performance of these devices suffers severely from nonradiative recombination due to the large densities of surface states and defects. As a consequence, currently reported nanowire LEDs still show relatively low output power, which is often in the range of μW, or smaller. It is expected that, by reducing nonradiative surface recombination, the device performance including the light output power and the quantum efficiency can be significantly enhanced. Here, we report that by passivating the InGaN/GaN dot-in-a-wire structures with an in-situgrown AlGaN shell, a record high output power of ~ 1.5mW can be measured, which is more than 100 times stronger than that of InGaN/GaN nanowire white LEDs without using an AlGaN shell.Vertically aligned InGaN/GaN dot-in-a-wire LED heterostructures were grown on Si(111) substrates by radio frequency plasma-assisted molecular beam epitaxy (MBE) under nitrogen-rich conditions. Shown in figure 1(a), the dot-in-a-wire LED structure consists of ~ 0.4 µm GaN:Si, ten vertically coupled InGaN/GaN dots, and ~ 0.2 µm GaN:Mg segments. The emission wavelengths can be varied across the entire visible spectral range by controlling the indium compositions and sizes of the dots. To reduce/prevent electron overflow, a ~10nm p-doped AlGaN electron blocking layer was incorporated between the InGaN/GaN quantum dot active region and the GaN:Mg segment. To effectively reduce nonradiative surface recombination, an AlGaN layer of ~ 80nm was grown for the formation a shell surrounding the InGaN/GaN core region, due to the diffusion-controlled growth process. The thickness and Al content of the AlGaN shell can be controlled by the growth duration and Al/Ga flux ratio, respectively. Figure 1(b) is a 45 degree-titled scanning electron microscopy (SEM) image for typical InGaN/GaN/AlGaN dot-in-a-wire core-shell LED heterostructures grown on Si (111) substrate, showing the high density and high degree of size uniformity. Illustrated in figure 2, the energy dispersive x-ray (EDX) spectrometry elemental mapping image of InGaN/GaN/AlGaN active region shows clear evidence of AlGaN layer passivating the InGaN/GaN core region, forming a unique core-shell nanowire heterostructure. Moreover, dislocations or stacking faults were not observed in the InGaN/GaN quantum dot active region and in the AlGaN shell.Illustrated in figure 3(a), the InGaN/GaN/AlGaN core-shell LED structure shows significantly improved photoluminescence (PL) intensity compared to that of non-core-shell LED sample. The significantly improved light intensity is attributed to the greatly reduced nonradiative surface recombination and the effective lateral confinement offered by the large bandgap AlGaN shell. Shown in figure 3(b), under the same current injection conditions, an output power of ~1.5mW was measured for the core-shell LEDs, which is more than two orders of magnitude larger than that of nanowire LEDs without using AlGaN shell, which is attributed to the increased carrier injection efficiency offered by the core-shell structure. Moreover, truly white emission with stable performance, shown in figure 3(c), was achieved, with the derived x and y values in the ranges of ~0.35-0.36 and 0.39-0.40 in the CIE diagram, respectively. Shown in the inset is an optical image of the phosphor-free core-shell nanowire white LED.In conclusion, the unique core-shell LED structures can lead to significantly reduced surface recombination and enhanced carrier injection efficiency. A record high power of 1.5mW was further measured for InGaN/GaN/AlGaN core-shell phosphor-free white LEDs, showing the tremendous potential for future solid-state lighting applications.

Micro-Raman characterization of InGaN/GaN single quantum well nanocolumns on Si(111) substrate

The phonon mode behavior of InGaN/GaN single quantum well (SQW) nanocolumns grown on silicon (111) substrate was investigated by micro-Raman scattering technique. The samples were synthesized using radio frequency (RF) plasma-assisted molecular beam epitaxy (MBE). High temperature (1020°C) grown AlN (about 5 nm) was used as a buffer. Raman spectra were recorded at room temperature with a confocal micro-Raman spectrometer in the backscattering geometry. The Raman spectra showed that the $ E_2^{\mathrm{high}} $ mode of GaN and the AO phonon mode of silicon were clearly visible. On the basis of these studies, the tensile stress and size of the GaN nanocolumns were calculated with respect to the $ E_2^{\mathrm{high}} $ mode of GaN. The nanosize dependence of the peak shift of the $ E_2^{\mathrm{high}} $ mode was discussed. These results were in good agreement with the calculated spatial correlation model.

Solar-blind wurtzite MgZnO alloy films stabilized by Be doping

MgxZn1−xO alloy films were deposited on c-plane sapphire substrates by radio frequency plasma-assisted molecular beam epitaxy (rf-PMBE). The phase segregation occurred when x was larger than 33%. Be doping was found experimentally able to stabilize the high-Mg-content MgZnO alloy. By alloying 1–2% Be into MgZnO, the band gap of as-prepared quaternary alloys can be raised to the solar-blind range (4.5 eV). Calculated formation energy of the alloys based on first principle reveals that a small amount of Be incorporation can reduce the formation energy of high-Mg-content MgZnO alloys and results in a more stable system, which justifies our experimental observations.

Molecular beam epitaxial growth of ZnCdTeO epilayers for intermediate band solar cells

We report the growth and characterization of the lattice-matched Zn1-xCdxTe1-yOy (ZnCdTeO) layers on ZnTe substrates by radio frequency plasma-assisted molecular beam epitaxy technique. The Cd composition increases linearly with increasing Cd/(Zn+Cd) flux ratio, indicating a controllability of Cd composition by Cd flux. Introduction of O radical during the growth of ZnCdTe resulted in the formation of ZnCdTeO layer. At particular O and Cd compositions lattice-matched ZnCdTeO epilayers on ZnTe substrate were obtained. Photoreflectance (PR) spectroscopy on the lattice-matched ZnCdTeO layer revealed two distinct PR features in the energy regions at 2.2–2.5eV and 1.5–1.8eV, which can be attributed to transitions from the valence band to the two conduction subbands, E+ and E−, respectively.

Strong atomic ordering in Gd-doped GaN

Gd-doped GaN (Ga1−xGdxN) thin films were grown on a GaN(001) template by radio frequency plasma-assisted molecular beam epitaxy and characterized by means of x-ray diffraction (XRD) and transmission electron microscopy (TEM). Three samples with a different Gd composition were prepared in this study: x = 0.02, 0.05, and 0.08. XRD and TEM results revealed that the low Gd concentration GaN possesses the wurtzite structure. On the other hand, it was found that an ordered phase with a quadruple-periodicity along the [001] direction in the wurtzite structure is formed throughout the film with x = 0.08. We proposed the atomistic model for the superlattice structure observed here.

High-Resolution TEM Observation of AlN/GaN Grown on Si Substrates

A high-quality crack-free AlN cap layer on GaN layer has been achieved using an AlN buffer layer directly grown on a silicon substrate at high temperature by radio frequency (RF) plasma-assisted molecular beam epitaxy. A two dimensional (2D) growth process guide to AlN cap layer of high grade crystal quality. The nucleation and the growth dynamics have been studied by in situ reflection high energy electron diffraction (RHEED) and ex situ by high resolution transmission electron microscopy (HR-TEM). The microstructure was investigated by energy-dispersive X-ray spectroscopy (EDX). It was disclosed that AlN is single crystalline with low defect. High densities of V-shaped pits were not detected at the interface between AlN and GaN layers. Contradictory the earlier reported V-shaped defects in nitride-based alloys; these V-shaped pits were condensed on top of the AlN layer because of H2 etching of the surface when a high temperature growth discontinuity between AlN and GaN layers.

Self-assembled flower-like nanostructures of InN and GaN grown by plasma-assisted molecular beam epitaxy

Nanosized hexagonal InN flower-like structures were fabricated by droplet epitaxy on GaN/Si(111) and GaN flower-like nanostructure fabricated directly on Si(111) substrate using radio frequency plasma-assisted molecular beam epitaxy. Powder X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to study the crystallinity and morphology of the nanostructures. Moreover, X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) were used to investigate the chemical compositions and optical properties of nano-flowers, respectively. Activation energy of free exciton transitions in GaN nano-flowers was derived to be ∼28.5 meV from the temperature dependent PL studies. The formation process of nano-flowers is investigated and a qualitative mechanism is proposed.

Optimization of the structural and optical quality of InN nanowires on Si(111) by molecular beam epitaxy

The authors report on the growth and characterization of high quality InN nanowires on Si(111) by radio frequency plasma-assisted molecular beam epitaxy. InN nanowires with nontapered morphology were achieved with the use of an in situ deposited In seeding layer. They further examined the effects of the growth temperature, In/N flux ratio, as well as the thickness of the In seeding layer on the morphological, structural, and optical properties of InN nanowires grown directly on Si(111). It was observed that nontapered InN nanowires exhibit significantly improved optical quality compared to the conventionally grown InN nanowires.

Controlled growth of Zn-polar ZnO film on MgAl2O4 (1 1 1) substrate using MgO buffer layer

A pure rocksalt MgO buffer layer was used to modify the surface structure of MgAl2O4 (1 1 1) substrates to achieve growth of a Zn-polar ZnO film by radio frequency plasma-assisted molecular beam epitaxy. It is found that this pure rocksalt MgO buffer layer plays a crucial role in 30° rotation domain elimination, surface morphology improvement and Zn-polarity control of the ZnO film, as demonstrated by in situ reflection high-energy electron diffraction and ex situ transmission electron microscopy. Atomic force microscopy observation also illustrates a smooth surface for the ZnO film.

Epitaxial growth, electrical and optical properties of a-plane InN on r-plane sapphire

The heteroepitaxy of a-plane (112¯0) InN films on r-plane (11¯02) sapphire substrates, by nitrogen radio frequency plasma-assisted molecular beam epitaxy, has been investigated and compared to that of c-plane (0001) InN. The epitaxial growth of a-plane InN proceeded through the nucleation, growth, and coalescence of three-dimensional islands, resulting in surface roughness that increased monotonically with epilayer thickness. The full width at half maximum of (112¯0) x-ray diffraction rocking curves decreased significantly with increasing InN thickness, characteristic of structural improvement, and it reached the value of 24 arcmin for a 1 μm thick film. Hall-effect measurements exhibited a similar dependence of electron concentration and mobility on thickness for both the a- and c-plane InN films. The analysis of the Hall-effect measurements, by considering the contribution of two conducting layers, indicates a similar accumulation of low mobility electrons with Ns>1014 cm−2 at the films’ surface/interfacial region for both the a- and c-plane InN films. From optical transmittance measurements, the absorption edge of 0.768 eV was determined for the 1 μm a-plane film, consistent with the expected Burstein–Moss effect. Photoluminescence spectra exhibited a lower energy peak at 0.631 eV, suggesting defect-related transitions.

High responsivity GaNAsSb p-i-n photodetectors at 13µm grown by radio-frequency nitrogen plasma-assisted molecular beam epitaxy

GaNAsSb/GaAs p-i-n photo notdetectors with an intrinsic GaNAsSb photoabsorption layer grown at 350 degrees C, 400 degrees C, 440 degrees C and 480 degrees C, have been prepared using radio-frequency nitrogen plasma-assisted molecular beam epitaxy in conjunction with a valved antimony cracker source. The i-GaNAsSb photoabsorption layer contains 3.3% of nitrogen and 8% of antimony, resulting in DC photo-response up to wavelengths of 1350 nm. The device with i-GaNAsSb layer grown at 350 degrees C exhibits extremely high photoresponsivity of 12A/W at 1.3 microm. These photodetectors show characteristics which strongly suggest the presence of carrier avalanche process at reverse bias less than 5V.

Molecular beam epitaxy of InN dots on nitrided sapphire

A series of self-assembled InN dots are grown by radio frequency (RF) plasma-assisted molecular beam epitaxy (MBE) directly on nitrided sapphire. Initial nitridation of the sapphire substrates at 900 °C results in the formation of a rough AlN surface layer, which acts as a very thin buffer layer and facilitates the nucleation of the InN dots according to the Stranski–Krastanow growth mode, with a wetting layer of ∼0.9 nm. Atomic force microscopy (AFM) reveals that well-confined InN nanoislands with the greatest height/width at half-height ratio of 0.64 can be grown at 460 °C. Lower substrate temperatures result in a reduced aspect ratio due to a lower diffusion rate of the In adatoms, whereas the thermal decomposition of InN truncates the growth at T>500 °C. The densities of separated dots vary between 1.0×10 10 and 2.5×10 10 cm −2 depending on the growth time. Optical response of the InN dots under laser excitation is studied with apertureless near-field scanning optical microscopy and photoluminescence spectroscopy, although no photoluminescence is observed from these samples. In view of the desirable implementation of InN nanostructures into photonic devices, the results indicate that nitrided sapphire is a suitable substrate for growing self-assembled InN nanodots.

Elastic strain in Mg0.28Zn0.72O layer: Combined Rutherford backscattering/channeling and X-ray diffraction

A 290nm Mg0.28Zn0.72O layer was grown by radio frequency plasma-assisted molecular beam epitaxy on a sapphire (0001) substrate using a 910nm thick ZnO intermediate layer. The Mg composition, which cannot be unambiguously determined by X-ray diffraction (XRD), was determined by Rutherford backscattering (RBS). The perpendicular and parallel elastic strain of the Mg0.28Zn0.72O layer, e⊥=−0.10% and e∥=0.15%, respectively, were derived using a combination of XRD and RBS/channeling. The ratio ∣e⊥/e∥∣=0.67 indicates that the Mg0.28Zn0.72O layer is a little stiffer along the c axis than along the a axis.