Plasma-assisted Molecular Beam Epitaxy Growth Research Articles

Plasma-assisted molecular beam epitaxy growth diagram of InGaN on (0001)GaN for the optimized synthesis of InGaN compositional grades

A PAMBE (plasma-assisted molecular beam epitaxy) growth diagram of InGaN is established for the metal-face (0001) orientation at 575 °C. This growth diagram allows identifying different growth regimes during the epitaxy of InGaN, namely the N-rich, the Ga-rich, the intermediate In-rich, and the In-accumulation regimes. InGaN compositional grades are then grown by ramping the Ga and In fluxes in parallel to remain in either the intermediate In-rich or the N-rich regime. The two samples exhibit distinct structural characteristics, surface morphologies, and optical properties. The growth diagram proves to be a useful tool in controlling the metal fluxes and thereby the stoichiometry conditions on the surface during the growth of compositionally graded layers.

Reaction kinetics and growth window for plasma-assisted molecular beam epitaxy of Ga2O3: Incorporation of Ga vs. Ga2O desorption

A detailed study of the reaction kinetics of the plasma-assisted molecular beam epitaxy (MBE) growth of the n-type semiconducting oxide Ga2O3 is presented. The growth rate as a function of gallium flux is measured in situ by laser reflectometry at different growth temperatures (TG) and gallium-to-oxygen ratios (rGa). The flux of the suboxide Ga2O desorbed off the growth surface is identified in situ by line-of-sight quadrupole mass spectroscopy. The measurements reveal the influence of TG and rGa on the competing formation of Ga2O3 and desorption of Ga2O resulting in three different growth regimes: (i) Ga transport limited, (ii) Ga2O desorption limited, and (iii) O transport limited. As a result, we present a growth diagram of gallium oxide. This diagram illustrates the regimes of complete, partial, and no Ga incorporation as a function of TG and rGa, and thus provides guidance for the MBE growth of Ga2O3.

Structural and optical characterization of nonpolar (10–10) m-InN/m-GaN epilayers grown by PAMBE

Plasma-assisted molecular beam epitaxy growth of (10–10) m-InN/(10–10) m-GaN was carried out on bare (10–10) m-sapphire substrate. The high resolution X-ray diffraction studies confirmed the orientation of the as-grown films. Nonpolar InN layer was grown at different growth temperatures ranging from 390°C to 440°C and the FWHM of rocking curve revealed good quality film at low temperatures. An in-plane relationship was established for the hetrostructures using phi-scan and a perfect alignment was found for the epilayers. Change of morphology of the films grown at different temperatures was observed using an atomic force microscopy technique showing the smoothest film grown at 400°C. InN optical band gap was found to be vary from 0.79–0.83eV from absorption spectra. The blue-shift of absorption edge was found to be induced by excess background electron concentration.

Impact of varying buffer thickness generated strain and threading dislocations on the formation of plasma assisted MBE grown ultra-thin AlGaN/GaN heterostructure on silicon

Plasma-assisted molecular beam epitaxy (PAMBE) growth of ultra-thin Al0.2Ga0.8N/GaN heterostructures on Si(111) substrate with three buffer thickness (600 nm/400 nm/200 nm) have been reported. An unique growth process has been developed that supports lower temperature epitaxy of GaN buffer which minimizes thermally generated tensile strain through appropriate nitridation and AlN initiated epitaxy for achieving high quality GaN buffer which supports such ultra-thin heterostructures in the range of 10-15Å. It is followed by investigations of role of buffer thickness on formation of ultra-thin Al0.2Ga0.8N/GaN heterostructure, in terms of stress-strain and threading dislocation (TD). Structural characterization were performed by High-Resolution X-Ray Diffraction (HRXRD), room-temperature Photoluminescence (RT-PL), High Resolution Transmission Electron Microscopy (HRTEM) and Atomic Force Microscopy (AFM). Analysis revealed increasing biaxial tensile stress of 0.6918 ± 0.04, 1.1084, 1.1814 GPa in heterostructures with decreasing buffer thickness of 600, 400, 200 nm respectively which are summed up with residual tensile strain causing red-shift in RT-PL peak. Also, increasing buffer thickness drastically reduced TD density from the order 1010 cm−2 to 108 cm−2. Surface morphology through AFM leads to decrease of pits and root mean square value with increasing buffer thickness which are resulted due to reduction of combined effect of strain and TDs.

Plasma assisted molecular beam epitaxy growth and effect of varying buffer thickness on the formation of ultra-thin In0.17Al0.83N/GaN heterostructure on Si(111)

This work reports on the detailed plasma-assisted molecular beam epitaxy (PAMBE) growth of ultra-thin In0.17Al0.83N/GaN heterostructures on Si(111) substrate with three different buffer thickness (600 nm, 400 nm, and 200 nm). Growth through critical optimization of growth conditions is followed by the investigation of impact of varying buffer thickness on the formation of ultra-thin 1.5 nm, In0.17Al0.83N–1.25 nm, GaN–1.5 nm, In0.17Al0.83N heterostructure, in terms of threading dislocation (TD) density. Analysis reveals a drastic reduction of TD density from the order 1010 cm−2 to 108 cm−2 with increasing buffer thickness resulting smooth ultra-thin active region for thick buffer structure. Increasing strain with decreasing buffer thickness is studied through reciprocal space mapping analysis. Surface morphology through atomic force microscopy analysis also supports our study by observing an increase of pits and root mean square value (0.89 nm, 1.2 nm, and 1.45 nm) with decreasing buffer thickness which are resulted due to the internal strain and TDs.

Enhanced catalyst-free nucleation of GaN nanowires on amorphous Al2O3 by plasma-assisted molecular beam epitaxy

We report on plasma-assisted molecular beam epitaxial growth of GaN nanowires (NWs) on Si(111) substrates with a thin amorphous Al2O3 buffer layer deposited by atomic layer deposition. Comparison of nucleation kinetics shows that presence of amorphous Al2O3 buffer significantly enhances spontaneous nucleation of GaN NWs. Slower nucleation was observed on partially amorphous silicon nitride films. No growth of NWs was found on sapphire substrate under the same growth conditions which we explain by a low density of defects on monocrystalline substrate surface where NWs may nucleate. Our finding shows that tuning of substrate microstructure is an efficient tool to control rate of self-induced nucleation of GaN NWs.

Plasma assisted molecular beam epitaxy of GaN with growth rates >2.6 µm/h

Plasma-assisted molecular beam epitaxial (PAMBE) growth of gallium nitride (GaN) was explored with a novel modification of a commercially available nitrogen plasma source. The modified nitrogen plasma source enabled a dramatic increase in the flux of active nitrogen and thus a significantly higher growth rate than has been previously reported. GaN films were grown using N2 gas flow rates between 1 and 8sccm while varying the plasma source's RF forward power from 200 to 600W. The highest growth rate, and therefore the highest active nitrogen flux achieved was ~2.65μm/h. For optimized growth conditions the surfaces displayed a clear step-terrace structure with an average RMS roughness (3µm×3µm) on the order of 1nm. Secondary ion mass spectroscopy (SIMS) impurity analysis demonstrates oxygen and hydrogen incorporation of 1×1016 and 5×1016 respectively, comparable to the metal organic chemical vapor deposition (MOCVD) grown template layer. Initial un-optimized electron mobility measurements of 1µm thick GaN layers have shown a peak mobility of ~705cm2/Vs for an electron concentration of ~3.5×1016cm−3. A revised universal growth diagram is proposed allowing the rapid determination of the metal flux needed to grow in a specific growth regime for any and all active nitrogen fluxes available.

Polarity control and transport properties of Mg-doped (0001) InN by plasma-assisted molecular beam epitaxy

The authors report on the plasma-assisted molecular beam epitaxy growth and carrier transport of Mg-doped In-face (0001) InN. The 1.2 μm thick InN films were grown on GaN:Fe templates under metal rich conditions with Mg concentration from 1 × 1017/cm3 to 3 × 1020/cm3. A morphological transition, associated with the formation of V-shape polarity inversion domains, was observed at Mg concentration over 7 × 1019/cm3 by atomic force microscopy and transmission electron microscopy. Seebeck measurements indicated p-type conductivity for Mg-concentrations from 9 × 1017/cm3 to 7 × 1019/cm3, i.e., as it exceeded the compensating (unintentional) donor concentration.

Molecular beam epitaxy of highly mismatched N-rich GaN1−xSbx and InN1−xAsx alloys

GaN materials alloyed with group V anions form the so-called highly mismatched alloys (HMAs). Recently, the authors succeeded in growing N-rich GaN1−xAsx and GaN1−xBix alloys over a large composition range by plasma-assisted molecular beam epitaxy (PA-MBE). Here, they present first results on PA-MBE growth and properties of N-rich GaN1−xSbx and InN1−xAsx alloys and compare these with GaN1−xAsx and GaN1−xBix alloys. The enhanced incorporation of As and Sb was achieved by growing the layers at extremely low growth temperatures. Although layers become amorphous for high As, Sb, and Bi content, optical absorption measurements show a progressive shift of the optical absorption edge to lower energy. The large band gap range and controllable conduction and valence band positions of these HMAs make them promising materials for efficient solar energy conversion devices.

Photoluminescence and Raman scattering studies of GaN nanowires obtained by top-down and bottom-up approaches

ABSTRACTWe present comparative studies of optical properties of GaN nanowires (NWs) obtained by two different self-formation techniques: Plasma-Assisted Molecular Beam Epitaxy (PAMBE) growth; and plasma etching of GaN layers deposited by Metal-Organic Vapor Phase Epitaxy (MOVPE). The effects of the coalescence process on grown NW and plasma-induced defects in etched NWs have been studied by photoluminescence (PL) and Raman scattering. In MBE grown NWs, the coalescence-associated defects are extended toward the NW top for intermediate Ga flux. Using High Resolution Electron Microscopy of reactive plasma etching (RIE) NWs, it was found that NWs obtained with an optimal combination of inductive (ICP) and capacitive (RF) plasma are free of extended structural defects. The PL efficiency is strongly increased in plasma etched NWs. However, plasma-induced point defects have to be taken into account for explaining the changes of the PL spectra. Less plasma-induced degradation is observed for high ICP/RF power ratios.

Dislocation reduction in GaN film using Ga-lean GaN buffer layer and migration enhanced epitaxy

A GaN buffer layer grown under Ga-lean conditions by plasma-assisted molecular beam epitaxy (PAMBE) was used to reduce the dislocation density in a GaN film grown on a sapphire substrate. The Ga-lean buffer, with inclined trench walls on its surface, provided an effective way to bend the propagation direction of dislocations, and it reduced the dislocation density through recombination and annihilation processes. As a result, the edge dislocation density in the GaN film was reduced by approximately two orders of magnitude to 2 × 10 8 cm − 2 . The rough surface of the Ga-lean buffer was recovered using migration enhanced epitaxy (MEE), a process of alternating deposition cycle of Ga atoms and N 2 radicals, during the PAMBE growth. By combining these two methods, a GaN film with high-crystalline-quality and atomically-flat surface can be achieved by PAMBE on a lattice mismatch substrate.

Control of active nitrogen species used for PA-MBE growth of group III nitrides on Si

A new spiral parallel mesh electrode (PME) is presented to control active nitrogen species in plasma-assisted molecular beam epitaxial (PA-MBE) growth of group III nitrides and their alloys. Direct flux of active nitrogen from radio frequency inductive coupled plasma (rf-ICP) discharge was able to be measured using a mesh electrode for filtering charge particles and electron emission due to the self-ionization of nitrogen atoms on a negatively biased electrode. In situ measurement of direct nitrogen atom fluxes using the spiral PME during PA-MBE growth of GaN and AlN on Si substrates is investigated. A linear rf power dependence of direct flux of active species on atoms such as nitrogen (N+N⁎), where N and N⁎ were ground and excited atoms, respectively, from a rf-ICP was confirmed by the spiral PME. An indirect flux of nitrogen adsorbed (ADS) atoms (N+N⁎) during discharge was also monitored by the spiral PME and received influence of the wall surface of the growth chamber. ADS nitrogen atoms are able to be used for nitridation of Si surface to grow a double buffer layer (DBL) AlN/β-Si3N4/Si.

PAMBE growth and in‐situ characterisation of clean (2 × 2) and (√3 × √3) R30° reconstructed InN(0001) thin films

AbstractThe surface properties of thin InN(0001) films grown by plasma assisted molecular beam epitaxy (PAMBE) were characterised. Two stable surface reconstructions (a (2 × 2) and a (√3 × √3) R30°) are identified which develop depending on the used preparation conditions. The structural, compositional and electronic surface properties were analysed in‐situ using reflection high energy electron diffraction (RHEED) and photoelectron spectroscopy (PES). From the absence of surface contaminants as well as excess indium it can be concluded that these superstructures are formed by single adatom layers. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

MBE GROWTH AND CHARACTERIZATION OF Mg-DOPED III-NITRIDES ON SAPPHIRE

Plasma-assisted molecular beam epitaxial growth of Mg -doped GaN and InGaN on a sapphire substrate is investigated in this study. Electrical characteristics of p -type GaN strongly depend on the flux of Mg acceptors and the growth temperature. Only the intermediate range of Mg fluxes (beam equivalent pressures near 1×10-9T) produce p -type GaN with good electrical properties, and a maximum hole concentration of 3.5 × 1018 cm-3 is obtained with a Hall mobility of 2.1 cm2/V·s. Due to the strong surface accumulation of electrons, Hall measurements do not indicate p -type polarity for In fraction beyond 11%. In contrast, hot probe measurements show that p -polarity can be measured for the entire range of Mg -doped In mole fractions. Electroluminescence also indicates p -polarity for Ga -rich mole fractions. In x Ga1- x N p - n homojunctions are fabricated and tested. All GaN devices show low series resistance (0.03 ohm-cm2) and insignificant parasitic leakage. IV curves of all three InGaN homojunctions show rectifying characteristics under dark conditions and photo-response under outdoor sunlight, indicating the existence of holes in InGaN with up to 40% In content.

PAMBE growth of (1 1 2¯ 2)-oriented GaN/AlN nanostructures on m-sapphire

We report on the plasma-assisted molecular-beam epitaxial growth of (112¯2)-oriented GaN/AlN nanostructures on (11¯00) m-plane sapphire. Moderate N-rich conditions enable to synthesize AlN(112¯2) directly on m-sapphire, with in-plane epitaxial relationships [112¯3¯]AlN∥[0001]sapphire and [11¯00]AlN∥[112¯0]sapphire. In the case of GaN, a Ga-excess of one monolayer is necessary to achieve two-dimensional growth of GaN(112¯2). Applying these growth conditions, we demonstrate the synthesis of (112¯2)-oriented GaN/AlN quantum well structures, showing a strong reduction of the internal electric field. By interrupting the growth under vacuum after the deposition of few monolayers of GaN under slightly Ga-rich conditions, we also demonstrate the feasibility of quantum dot structures with this orientation.

Role of ion damage on unintentional Ca incorporation during the plasma-assisted molecular-beam epitaxy growth of dilute nitrides using N[sub 2]∕Ar source gas mixtures

Unintentional Ca incorporation caused by Ca-contaminated substrate surfaces on as-purchased GaAs wafers are known to limit the efficiency of solar cells based on dilute nitride materials. This article focuses on further understanding the conditions and mechanisms by which these Ca impurities incorporate. Plasma-assisted molecular-beam epitaxy utilizing a 1% N2 in Ar precursor gas mixture was used to grow GaAs at 400 and 580°C, and GaN0.01As0.99 at 400°C. Two plasma operating combinations of rf power and gas flow rate were used to generate different amounts and energies of both ions and other plasma species, while keeping nitrogen incorporation constant. The ions were characterized with a dual-grid, retarding-field ion energy analyzer, and the corresponding ion energy distributions are presented to correlate ions with Ca incorporation. When appropriate, dc-biased deflector plates were used to remove ions during growth. Secondary ion mass spectrometry was used to measure Ca in GaAs and GaN0.01As0.99. Ca incorporation was observed in the dilute nitride samples, but the effects of ions did not exceed other Ca incorporation mechanisms associated with defects due to both low temperature growth and nitrogen incorporation; however, different neutral active nitrogen species (atomic N and metastable N2*) may be a factor. Ca incorporation measured in GaAs grown at 400°C with a pure Ar plasma is predominantly due to defects associated with low temperature growth, as opposed to plasma damage caused by the ions. GaAs growths at 580°C without a plasma did not exhibit Ca incorporation, but growth at 580°C with ions from a pure Ar plasma caused Ca incorporation.

Microstructural characterization of low-temperature grown GaMnN on GaAs(0 0 1) substrates by plasma-assisted MBE

GaMnN layers have been grown on GaAs(0 0 1) substrates by plasma-assisted molecular beam epitaxy (PAMBE), under both N-rich and Ga-rich conditions, at temperatures from 680 °C down to 186 °C. Under N-rich conditions, a reduction in the growth temperature results in a transition from faulted single-crystalline zinc-blende layer growth to mixed-phase zinc-blende and wurtzite polycrystalline layer growth, whilst being also associated with increasing levels of Mn incorporation. Improved layer crystallinities were associated with PAMBE growth under Ga-rich conditions. In particular, a tilted, mixed-phase growth mode allowing for the deposition of comparably uniform GaMnN alloys, at a level of ∼3.5 at% Mn, was identified for slightly Ga-rich conditions at a reduced temperature of ∼265 °C. No correlation could be established between the GaMnN/GaAs (0 0 1) fine-scale defect structures and the layer functional properties, with all the free-standing layers showing p-type conductivities and comparable mobilities.

MBE growth of nitride-based photovoltaic intersubband detectors

In this work, we present the plasma-assisted molecular-beam epitaxial growth of quantum well infrared photodetector (QWIP) structures, including the Si-doped GaN/AlN short-period superlattice of the active region, conductive AlGaN claddings and integration of the final device. The growth of Si-doped GaN/AlN multiple quantum well (QW) structures is optimized by controlling substrate temperature, metal excess and growth interruptions. Structural characterization confirms a reduction of the interface roughness to the monolayer scale. P -polarized intersubband absorption peaks covering the 1.33–1.91 μm wavelength range are measured on samples with QW thickness varying from 1 to 2.5 nm. The absorption exhibits Lorentzian shape with a line width around 100 meV in QWs doped 5×10 19 cm −3 . To prevent partial depletion of the QWs owing to the internal electric field, we have developed highly-conductive Si-doped AlGaN cladding layers using In as a surfactant during growth. Complete ISB photodetectors with 40 periods of 1 nm-thick Si-doped GaN QWs with 2 nm-thick AlN barriers have been grown on conductive AlGaN claddings, the Al mole fraction of the cladding matching the average Al content of the active region. Temperature-dependent photovoltage measurements reveal a narrow (∼90 meV) detection peak at 1.39 μm.

Growth of p-type and n-type m-plane GaN by molecular beam epitaxy

Plasma-assisted molecular beam epitaxial growth of Mg-doped, p-type and Si-doped, n-type m-plane GaN on 6H m-plane SiC is demonstrated. Phase-pure, m-plane GaN films exhibiting a large anisotropy in film mosaic (∼0.2° full width at half maximum, x-ray rocking curve scan taken parallel to [112¯0] versus ∼2° parallel to [0001]) were grown on m-plane SiC substrates. Maximum hole concentrations of ∼7×1018cm−3 were achieved with p-type conductivities as high as ∼5Ω−1cm−1 without the presence of Mg-rich inclusions or inversion domains as viewed by cross-section transmission electron microscopy. Temperature dependent Hall effect measurements indicate that the Mg-related acceptor state in m-plane GaN is the same as that exhibited in c-plane GaN. Free electron concentrations as high as ∼4×1018cm−3 were measured in the Si-doped m-plane GaN with corresponding mobilities of ∼500cm2∕Vs measured parallel to the [112¯0] direction.

Ion damage effects from negative deflector plate voltages during the plasma-assisted molecular-beam epitaxy growth of dilute nitrides

We studied the effects of ion damage on the optical properties of dilute nitrides grown by plasma-assisted molecular-beam epitaxy. A dual-grid retarding field ion energy analyzer was used to measure the ion flux and ion energy distribution at the substrate position from an Applied-EPI UniBuilb™ rf plasma cell. These changes were measured as the negative deflector plate voltage varied from 0 to −800V. The largest ion flux resulted with a −100V setting, while the greatest ion energies occurred with −200V. Deflector plate voltages more negative than −300V resulted in a significant reduction in both the ion flux and ion energy. The damage caused by these ions was determined by measuring the pre- and postanneal photoluminescence properties of Ga0.8In0.2N0.01As0.99 quantum wells. Comparable optical properties were possible with various combinations of ion fluxes and ion energies, which demonstrate how the ion flux and ion energy each impart an individual effect on the sample’s optical properties. An awareness of these effects is crucial because the optical properties of dilute nitrides grown with an improper deflector plate voltage setting can lead to a greater degree of ion damage to the sample.