Ga Adatoms Research Articles

Tracking Charge Carrier Paths in Freestanding GaN/AlN Nanowires on Si(111).

Functional and abundant substrate materials are relevant for applying all sophisticated semiconductor-based device components such as nanowire arrays. In the case of GaN nanowires grown by metalorganic vapor phase epitaxy, Si(111) substrates are widely used, together with an AlN interlayer to suppress the well-known Ga-based melt-back-etching. However, the AlN interlayer can degrade the interfacial conductivity of the Si(111) substrate. To reveal the possible impact of this interlayer on the overall electrical performance, an advanced analysis of the electrical behavior with suitable spatial resolution is essential. For the electrical investigation of the nanowire-to-substrate junction, we used a four-point probe measurement setup with sufficiently high spatial resolution. The charge separation behavior of the junction is also demonstrated by an electron beam-induced current mode, while the n-GaN nanowire (NW) core exhibits good electrical conductivity. The charge carrier-selective transport at the NW-to-substrate junction can be attributed to different, local material compositions by two main effects: the reduction of Ga adatoms by shadowing of the lower part of the NW structure by the top part during growth, i.e. the protection of the pedestal footprint from Ga adsorption. Our combination of investigation methods provides direct insight into the nanowire-to-substrate junction and leads to a model of the conductivity channels at the nanowire base. This knowledge is crucial for all future GaN bottom-up grown nanowire structure devices on conductive Si(111) substrates.

Comparison of AlN/GaN heterojunctions grown by molecular beam epitaxy with Al and Ga assistance

Although conventional III-nitride molecular-beam-epitaxy growth theories indicate that high-quality AlN should be prepared in Al-assisted regimes, AlN/GaN heterojunctions grown in Ga-assisted regimes actually demonstrate superior performances. In this letter, we compared Al-assisted and Ga-assisted grown AlN/GaN heterojunctions and explored the underlying mechanisms. Aberration-corrected transmission electron microscope, energy dispersive spectrometer, and atomic force microscopy measurements indicate that the Al-assisted growth results in partly-relaxed AlN of high-density defects, deteriorating AlN/GaN interfaces, and surface morphology consisting of irregular and truncated atomic steps. The Ga-assisted growth produces high-quality pseudomorphic AlN with excellent AlN/GaN interfaces, surface morphology, and electrical properties. It is suggested that the better crystalline quality and higher performances of the Ga-assisted grown samples are attributed to a lower N subsurface diffusion barrier resulting from a higher energy of the fcc adsorption positions in the adlayer-enhanced-lateral-diffusion model and intense thermal motions of Ga adatoms. It helps to understand the kinetics of metal-adlayer-enhanced growth of AlN and AlN/GaN heterojunctions at atomic scale and optimize the growth processes. Meanwhile, it is demonstrated that amorphization and Al diffusion near the AlN surfaces occur due to Pt bombardment on highly tensile-strained AlN during focused-ion-beam fabrications. It implies that large strain in AlN/GaN heterojunctions might be an important issue to be considered for device reliability.

Nucleation-Limited Kinetics of GaAs Nanostructures Grown by Selective Area Epitaxy: Implications for Shape Engineering in Optoelectronics Devices.

The growth kinetics of vertical III-V nanowires (NWs) were clarified long ago. The increasing aspect ratio of NWs results in an increase in the surface area, which, in turn, enhances the material collection. The group III adatom diffusion from the NW sidewalls to the top sustains a superlinear growth regime. In this work, we report on the growth of different GaAs nanostructures by selective area MOVPE on GaAs (111)B substrates. We show that the opening dimensions and geometry qualitatively alter the morphology and height evolution of the structures. We compare the time evolution of vertical GaAs NWs stemming from circular holes and horizontal GaAs nanomembranes (NMs) growing from one-dimensional (1D) rectangular slits on the same substrate. While NW heights grow exponentially with time, NMs surprisingly exhibit sublinear kinetics. The absence of visible atomic steps on the top facets of both NWs and NMs suggests layer-by-layer growth in the mononuclear mode. We interpret these observations within a self-consistent growth model, which links the diffusion flux of Ga adatoms to the position- and shape-dependent nucleation rate on top of NWs and NMs. Specifically, the island nucleation rate is lower on top of the NMs than that on the NWs, resulting in the total diffusion flux being directed from the top facet to the sidewalls. This gives a sublinear height evolution for the NMs. These results open innovative perspectives for shape engineering of III-V nanostructures and new avenues for the design of optoelectronics and photonic devices.

(Invited) Ultrathin Gap Nanoantenna: Crystal Phase Transitions and Luminescent Properties

We report Te-doped GaP nanowires (NWs) with positive tapering and radii measuring as low as 5 nm grown by the self-assisted vapor-liquid-solid mechanism using selective-area molecular beam epitaxy. The occurrence of the ultrathin nanoantenna showed a dependence on pattern pitch (separation between NWs) with a predominance at 600 nm pitch, and exhibited radius oscillations that correlate with polytypic zincblende/wurtzite segments. A growth model explains the positive tapering of the NW leading to an ultrathin tip from the suppression of surface diffusion of Ga adatoms on the NW sidewalls by Te dopant flux. The model also provides a relationship between the radius modulations and the oscillations of the droplet contact angle, predicting the quasi-periodic radius oscillations and corresponding crystal phase transitions. The GaP NWs showed strong low temperature micro-photoluminescence exciton-related emission, indicating a bandgap difference between the zincblende/wurtzite segments, with phonon replicas due to the transverse optical phonon mode. These results establish a link between dopants and the ability to control NW morphology, crystal phase and luminescent properties with possible applications in thermoelectrics, quantum emitters and photodetectors.

Demonstration of MOCVD based in situ etching of β-Ga2O3 using TEGa

In this work, we demonstrate an in situ etch technique for β-Ga2O3 inside a metalorganic chemical vapor deposition (MOCVD) reactor using triethylgallium (TEGa) as the etching agent. At sufficiently high substrate temperatures (Tsub), TEGa is introduced into the MOCVD reactor which undergoes pyrolysis, resulting in the deposition of Ga on the β-Ga2O3 surface. These Ga adatoms react with Ga2O3 to form gallium suboxide (Ga2O), which desorbs from the β-Ga2O3 surface resulting in the etching of the epilayer. MOCVD chamber parameters such as TEGa molar flow rate, substrate temperature, and chamber pressure were shown to be key in controlling the etch rate and surface morphology. A wide range of etch rates from ∼0.3 to 8.5 μm/h is demonstrated by varying the etch parameters. In addition, smooth surface morphology on (010) and (001) β-Ga2O3 substrates is also demonstrated. This new etch technique could enable damage free fabrication of 3D structures like fins and trenches, which are key components in many β-Ga2O3 device structures.

Sn-doped n-type GaN layer with high electron density of 1020 cm−3 grown by halide vapor phase epitaxy

A Sn-doped n-type GaN layer with a high electron density of 2 × 1020 cm−3 and a low resistivity of 8.7 × 10−4 Ω∙cm was grown by halide vapor phase epitaxy (HVPE). Sn doping was performed through the reaction between Sn metal and HCl gas. The Sn concentration markedly increased with decreasing growth temperature and the activation energy of Sn desorption from the GaN surface was found to be 4.1 eV. Smooth surfaces were obtained by introducing the Sn precursor even though the samples were grown at a low temperature of 905 °C, suggesting that Sn atoms act as surfactants and promote the migration of Ga adatoms. Almost all the Sn atoms act as donors in GaN below the Sn concentration of 2 × 1020 cm−3. These results indicate that using the Sn donor is promising for fabricating low-resistivity n-type GaN substrates by HVPE.

Polar GaN Surfaces under Gallium Rich Conditions: Revised Thermodynamic Insights from Ab Initio Calculations.

This paper presents an improved theoretical view of ab initio thermodynamics for polar GaN surfaces under gallium-rich conditions. The study uses density functional theory (DFT) calculations to systematically investigate the adsorption of gallium atoms on GaN polar surfaces, starting from the clean surface and progressing to the metallic multilayer. First principles phonon calculations are performed to determine vibrational free energies. Changes in the chemical potential of gallium adatoms are determined as a function of temperature and surface coverage. Three distinct ranges of Ga coverage with very low, medium, and high chemical potential are observed on the GaN(000-1) surface, while only two ranges with medium and high chemical potential are observed on the GaN(000-1) surface. The analysis confirms that a monolayer of Ga adatoms on the GaN(000-1) surface is highly stable over a wide range of temperatures. For a second adlayer at higher temperatures, it is energetically more favorable to form liquid droplets than a uniform crystalline adlayer. The second Ga layer on the GaN(0001) surface shows pseudo-crystalline properties even at a relatively high temperature. These results provide a better thermodynamic description of the surface state under conditions typical for molecular beam epitaxy and offer an interpretation of the observed growth window.

From Layer-by-Layer Growth to Nanoridge Formation: Selective Area Epitaxy of GaAs by MOVPE.

Selective area epitaxy at the nanoscale enables fabrication of high-quality nanostructures in regular arrays with predefined geometry. Here, we investigate the growth mechanisms of GaAs nanoridges on GaAs (100) substrates in selective area trenches by metal-organic vapor-phase epitaxy (MOVPE). It is found that pre-growth annealing results in the formation of valley-like structures of GaAs with atomic terraces inside the trenches. MOVPE growth of GaAs nanoridges consists of three distinct stages. Filling the trench in the first stage exhibits a step-flow growth behavior. Once the structure grows above the mask surface, it enters the second stage of growth by forming {101} side facets as the (100) flat top facet progressively shrinks. In the third stage, the fully formed nanoridge begins to overgrow onto the mask with a significantly reduced growth rate. We develop a kinetic model that accurately describes the width-dependent evolution of the nanoridge morphology through all three stages. MOVPE growth of fully formed nanoridges takes only about 1 min, which is 60 times faster than in our set of molecular beam epitaxy (MBE) experiments reported recently, and with a more regular, triangular cross-sectional geometry defined solely by the {101} facets. In contrast to MBE, no material loss due to Ga adatom diffusion onto the mask surface is observed in MOVPE until the third stage of growth. These results are useful for the fabrication of GaAs nanoridges of different dimensions on the same substrate for various applications and can be extended to other material systems.

Homoepitaxial growth of (100) Si-doped β-Ga2O3 films via MOCVD

Homoepitaxial growth of Si-doped β-Ga2O3 films on semi-insulating (100) β-Ga2O3 substrates by metalorganic chemical vapor deposition (MOCVD) is studied in this work. By appropriately optimizing the growth conditions, an increasing diffusion length of Ga adatoms is realized, suppressing 3D island growth patterns prevalent in (100) β-Ga2O3 films and optimizing the surface morphology with [010] oriented stripe features. The slightly Si-doped β-Ga2O3 film shows smooth and flat surface morphology with a root-mean-square roughness of 1.3 nm. Rocking curves of the (400) diffraction peak also demonstrate the high crystal quality of the Si-doped films. According to the capacitance–voltage characteristics, the effective net doping concentrations of the films are 5.41 × 1015 – 1.74 × 1020 cm−3. Hall measurements demonstrate a high electron mobility value of 51 cm2/(V·s), corresponding to a carrier concentration of 7.19 × 1018 cm−3 and a high activation efficiency of up to 61.5%. Transmission line model (TLM) measurement shows excellent Ohmic contacts and a low specific contact resistance of 1.29 × 10-4 Ω·cm2 for the Si-doped film, which is comparable to the Si-implanted film with a concentration of 5.0 × 1019 cm−3, confirming the effective Si doing in the MOCVD epitaxy.

Importance of As and Ga Balance in Achieving Long GaAs Nanowires by Selective Area Epitaxy

We report on the selective area growth (SAG) of GaAs nanowires (NWs) by the catalyst-free vapor-solid mechanism. Well-ordered GaAs NWs were grown on GaAs(111)B substrates patterned with a dielectric mask using hydride vapor phase epitaxy (HVPE). GaAs NWs were grown along the ⟨111⟩B direction with perfect hexagonal shape when the hole’s opening diameter in SiNx or SiOx mask was varied from 80 to 340 nm. The impact of growth conditions and the hole size on the NW lengths and growth rates was investigated. A saturation of the NW lengths was observed at high partial pressures of As4, explained by the presence of As trimers on the (111)B surface at the NW top surface. By decreasing As4 partial pressure and decreasing the hole size, high aspect ratio NWs were obtained. The longest and thinnest NWs grew faster than a two-dimensional layer under the same conditions, which strongly suggests that surface diffusion of Ga adatoms from the NW sidewalls to their top contributes to the resulting axial growth rate. These findings were supported by a dedicated model. The study highlights the capability of the HVPE process to grow high aspect ratio GaAs NW arrays with high selectivity.

Controlling metal adatoms on InGaN growing front for defect suppression and high-stability visible-light photodetection

Despite the successful commercialization of InGaN-based light emitting diodes, the further application of this material system in photosensitive devices remains hindered by complex crystal defects and lacking understanding of them. Herein, the InGaN epilayers are grown through the synchronous control of In and Ga adatoms on the growing front. Reducing excess In adatoms lowers the surface strain and suppresses the formation of misfit dislocation (MD) networks, while enhancing Ga adatom migration reduces the trench defects as well as the related In-rich hillock regions. Therefore, the nonradiative centers (NRCs) and the localized states played by these surface-visible defects, are both effectively reduced, leading to superior optical performance of the InGaN alloy. Furthermore, optoelectronic characterization using coplanar Schottky-contact photodetectors reveals that the localized states and the deep-level traps associated with these surface defects cause the persistent photoconductive (PPC) effect, resulting in unstable and slow photo-response. Therefore, the suppression of these complex defects contributes to the realization of high-performance InGaN visible-light photodetectors (VLPDs) with high speed and high-temperature stability, exhibiting great potential in wide visible-light applications.

First-principles study for self-limiting growth of GaN layers on AlN(0001) surface

The GaN thickness dependence of surface structural stability and adsorption behavior of Ga adatom in GaN layers on a AlN(0001) surface are investigated on the basis of first-principles calculations to clarify the self-limiting growth on AlN(0001) surface during metal-organic vapor phase epitaxy. The calculations demonstrate that the stability of reconstructed GaN layers on a AlN(0001) surface is similar to that of a GaN(0001) surface irrespective of the GaN film thickness. Furthermore, we find that the adsorption of a Ga adatom on the AlN(0001) surface easily occurs compared with that on AlN(0001) surface with GaN layers. The difference in the adsorption behavior implies that the growth of GaN layers on a AlN(0001) surface is suppressed. The calculated results provide theoretical guidance for understanding the self-limiting growth of GaN layers, resulting in the formation mechanism of GaN quantum wells.

Insight into the step flow growth of gallium nitride based on density functional theory

We report the first-principles calculations based on the density-functional theory that elucidate the mechanism of the epitaxial growth on the mixed GaN (0001) surface which consists of the Ga-adatom and the Ga-H structural motifs. We find that the source-gas molecule NH3 is decomposed on the Ga-adatom site to an NH unit and the resulting NH unit diffuses in a newly found concerted way in which the Ga adatom assists in the NH diffusion by annihilating dangling bonds nearby. We also identify the structures of the H-covered step edges and obtain their formation energies. We find plausible reaction pathways near the step edges in which the NH units and Ga adatoms are incorporated in the epitaxial films, forming new units of GaN. The obtained final structures are certainly the manifestation of the step-flow mechanism of the epitaxial growth of GaN.

Tuning the p-type doping of GaN over three orders of magnitude via efficient Mg doping during halide vapor phase epitaxy

The precise control of Mg concentration ([Mg]) in p-type GaN layers from 2.3 × 1016 to 2.0 × 1019 cm−3 was demonstrated by halide vapor phase epitaxy (HVPE) on n-type GaN (0001) freestanding substrates. [Mg] in GaN layers could be controlled well by varying the input partial pressure of MgCl2 formed by a chemical reaction between MgO solid and HCl gas under the thermodynamic equilibrium condition. In the sample with [Mg] of 2.0 × 1019 cm−3, a step-bunched surface was observed because the surface migration of Ga adatoms was enhanced by the surfactant effect of Mg atoms. The samples show high structural qualities determined from x-ray rocking curve measurements. The acceptor concentration was in good agreement with [Mg], indicating that almost all Mg atoms act as acceptors. The compensating donor concentrations in the samples were higher than the concentrations of Si, O, and C impurities. We also obtained the Mg acceptor level at a sufficiently low net acceptor concentration of 245 ± 2 meV. These results show that the HVPE method is promising for fabricating GaN vertical power devices, such as n-channel metal–oxide–semiconductor field-effect transistors.

Compositional degradation of the electron blocking layers through solid-solution in GaN-based laser diodes

Electron leakage currents seriously hinder GaN-based blue laser diodes (LDs) from high wall-plug efficiencies. Inserting an ultra-thin AlGaN electron blocking layer (EBL) in the epitaxy structure is a major technique to suppress the leakage currents for which a high Al composition in the EBL is necessary. Despite many studies on the optimization of the compositions of EBLs, it is questionable whether they reach the designed value in real growths by metal-organic vapor phase epitaxy. We investigate the influence of the growth conditions of upper cladding layers (CLs) on the underlying EBLs. A strong composition degradation of the EBL is observed when the growth rate of the CL is low, which drastically reduces the output performance of both LEDs and LDs. A 30-nm fast-growing protecting layer can efficiently prevent the EBL from such degradation. The phenomenon cannot be explained by a composition pulling effect nor an etch effect by hydrogen, but by a mutual solid solution between the EBL and the adjacent CL. The solution process is found thermally favored by calculating the Gibbs energy where strain and entropies are considered. It is inferred that the chemically active Ga adatoms at the surface play an important role in accelerating the solution process. Based on these considerations, we introduce a random walk model to clarify the kinetic influence of CL growth rates on EBL degradation semi-quantitatively. The results help to understand the subtle process in the growth of heterostructures and the transport process of GaN-based LDs.

An atomistic insight into reactions and free-energy profiles of NH3 and Ga on GaN surfaces during the epitaxial growth

Precursor molecules (NH3 and Ga compounds) along with carrier gas (H2 or N2) used to grow GaN structures bring a large amount of hydrogen atoms which affect the growing mechanism of GaN. This has a non-negligible effect of the chemistry and diffusivity of precursors and dissociation products. To encompass the experimentally difficulty in of unraveling such a complicated reaction mechanism, we resort to first principles molecular dynamics modeling, providing an atomistic insight into two major issues. The first one is the evolution of H atoms after the adsorption and dissociation of NH3 on the growing GaN surface. The second issue is to shed light on the role of passivating hydrogen at growth conditions for a typical GaN Ga-rich (0001) surface. In the first case, reaction pathways alternative to the product of molecular hydrogen (H2) can be realized, depending on the initial conditions and morphology of the surface, resulting in an adsorption of H atoms, thus contributing to its hydrogenation. In the second one, instead, we show how the presence of passivating H atoms at the surface, corresponding to a relatively high degree of hydrogenation, contribute to limit the diffusivity of Ga adatoms at the typical growth temperatures.

Monte Carlo Simulation of Alternate Pulsed Epitaxial Growth of GaAs Nanowires

The Monte Carlo simulation of a self-catalyzed GaAs nanowire (NW) growth regime is proposed and realized, where gallium and arsenic are deposited in pulsed intervals. Such a pulsed epitaxial growth mode allows low-temperature epitaxial growth of long GaAs NWs. The influence of gallium and arsenic pulse durations and sequences on the NW morphology is investigated. The catalyst droplet size and contact angle, as well as the diffusion length of Ga adatoms along the NW sidewalls, determine the wire morphology. For a dense NW array, the arsenic readsorption starts to affect the NW growth. The influence of arsenic readsorption on the formation of Ga liquid droplets on the NW sidewalls is investigated with the help of annealing simulations. The significant role of the mass transfer of evaporated materials is demonstrated for GaAs NW morphology.

Effect of initial condition on the quality of GaN film and AlGaN/GaN heterojunction grown on flat sapphire substrate with ex-situ sputtered AlN by MOCVD

The growth condition of initial medium (870 °C–920 °C) temperature (MT) layer is crucial to the quality of high temperature (HT) GaN layer and AlGaN/GaN heterojunction grown on flat sapphire substrate (FSS) with ex-situ sputtered AlN. The superior wetting of GaN and AlN makes the MT layer tend to directly two-dimensional (2D) growth mode at the initial stage. In this study, it was found that high pressure was effective in lowering Ga adatom diffusion and promoting early three-dimensional (3D) growth, which resulted in reducing (102) X-ray full-width-at-half-maximum as well as edge dislocation density significantly. The lowest X-ray rocking curve full-width-at-half-maximum of (002) and (102) were 35 arcsec and 220 arcsec respectively, which were comparable with GaN layer grown on patterned sapphire substrate (PSS). High quality AlGaN/GaN heterojunction was achieved with sheet electron density of 1.18×1013cm−2, Hall mobility of 1909 cm2/V⋅s and sheet resistance of 336 Ω/□. Controlled growth interruption was carried out to study the evolution mechanism of MT layer growth.

Planar and three-dimensional damage-free etching of β-Ga2O3 using atomic gallium flux

In situ etching using Ga flux in an ultra-high vacuum environment like molecular beam epitaxy is introduced as a method to make high aspect ratio three-dimensional (3D) structures in β-Ga2O3. Etching of β-Ga2O3 due to excess Ga adatoms on the epilayer surface had been viewed as non-ideal for epitaxial growth especially since it results in plateauing and lowering of the growth rate. In this study, we use this well-known reaction from epitaxial growth to intentionally etch β-Ga2O3. We demonstrate etch rate ranging from 2.9 to 30 nm/min with the highest reported etch rate being only limited by the highest Ga flux used. Patterned in situ etching is also demonstrated and used to study the effect of fin orientation on the sidewall profiles and dopant (Si) segregation on the etched surface. Using in situ Ga etching, we also demonstrate 150 nm wide fins and 200 nm wide nanopillars with high aspect ratio. This etching method could enable future development of highly scaled vertical and lateral 3D devices in β-Ga2O3.

Formation of wurtzite sections in self-catalyzed GaP nanowires by droplet consumption

Wurtzite GaP nanowires are interesting for the direct bandgap engineering and can be used as templates for further growth of hexagonal Si shells. Most wurtzite GaP nanowires have previously been obtained with Au catalysts. Here, we show that long (∼500 nm) wurtzite sections are formed in the top parts of self-catalyzed GaP nanowires grown by molecular beam epitaxy on Si(111) substrates in the droplet consumption stage, which is achieved by abruptly increasing the atomic V/III flux ratio from 2 to 3. We investigate the temperature dependence of the length of wurtzite sections and show that the longest sections are obtained at 610 °C. A supporting model explains the observed trends using a phase diagram of GaP nanowires, where the wurtzite phase is formed within a certain range of the droplet contact angles. The optimal growth temperature for growing wurtzite nanowires corresponds to the largest diffusion length of Ga adatoms, which helps to maintain the required contact angle for the longest time.