Rf-plasma Molecular Beam Epitaxy Research Articles

RF-plasma MBE growth of epitaxial metallic TaNx transition metal nitride films on SiC

RF-plasma molecular-beam epitaxy was used to epitaxially grow 3–150-nm-thick metallic TaNx thin films on hexagonal SiC substrates. Single-phase hexagonal γ-Ta2N films were obtained when the starting substrate temperature was ∼900 °C and the active N to Ta ratio was ∼2.5–3. The films were characterized using in situ reflection high-energy electron diffraction and ex situ atomic force microscopy, contactless sheet resistance, x-ray diffraction, and cross-sectional transmission electron microscopy. Smooth, single-crystal, low-resistivity films of epitaxial, hexagonal γ-Ta2N on SiC are demonstrated for films at least ∼50-nm-thick.

Structural and magnetic phase transitions in chromium nitride thin films grown by rf nitrogen plasma molecular beam epitaxy

A magneto-structural phase transition is investigated in single crystal CrN thin films grown by rf plasma molecular beam epitaxy on MgO(001) substrates. While still within the vacuum environment following MBE growth, $\it in-situ$ low-temperature scanning tunneling microscopy, and $\it in-situ$ variable low-temperature reflection high energy electron diffraction are applied, revealing an atomically smooth and metallic CrN(001) surface, and an $\it in-plane$ structural transition from 1$\times$1 (primitive CrN unit cell) to $\mathrm{\sqrt{2}\times\sqrt{2}-R45^\circ}$ with a transition temperature of $\sim$ 278 K, respectively. $\it Ex-situ$ temperature dependent measurements are also performed, including x-ray diffraction and neutron diffraction, looking at the structural peaks and likewise revealing a first-order structural transition along both [001] and [111] $\it out-of-plane$ directions, with transition temperatures of 256 K and 268 K, respectively. Turning to the magnetic peaks, neutron diffraction confirms a clear magnetic transition from paramagnetic at room temperature to antiferromagnetic at low temperatures with a sharp, first-order phase transition and a N$\'{e}$el temperature of 270 K or 280 K for two different films. In addition to the experimental measurements of structural and magnetic ordering, we also discuss results from first-principles theoretical calculations which explore various possible magneto-structural models.

(Invited) ScAlN: A Novel Barrier Material for High Power GaN-Based RF Transistors

GaN-based high-electron-mobility transistors (HEMTs) have established a ten-fold increase in power density over incumbent GaAs technology over a wide range of RF operating frequencies. The improvement in power density comes as a result of both increased breakdown voltage in the wide-bandgap nitrides and increased channel current available via polarization engineering. Next generation high-power RF HEMTs will require further increases in channel sheet charge density (n s ) to improve output power while requiring thinner barrier layers for higher frequency operation in the millimeter-wave regime. Sc x Al1-x N is a novel materials system consisting of an alloy between group-IIIa (transition metal) nitrides and group-IIIb nitrides, with a stable wurtzite crystal structure for x < 0.55 and a lattice-match to GaN with x ∼ 0.2. Thin films with x up to 0.43 have been reported having a factor of five enhancement of the piezoelectric response relative to AlN [1], and similar predicted enhancement in spontaneous polarization [2]. The calculated polarization discontinuity between a Sc0.2Al0.8N barrier layer and GaN channel gives a predicted n s as high as 5 × 1013 cm-2, a factor of five higher than a conventional Al0.25Ga0.75N channel device. In this talk we will show initial development of ScAlN for high-power HEMTs, focusing on molecular beam epitaxy (MBE) growth and materials characterization of ScAlN, plasma etching of ScAlN and its effectiveness as an etch stop layer (ESL), and dc characterization of ScAlN-barrier HEMTs. Sc x Al1-x N thin films were grown on freestanding GaN and SiC substrates using an Omicron PRO-75 RF-plasma MBE system, equipped with an Al effusion cell and an e-beam evaporator to supply Sc. Several series of samples were grown to investigate the impact of growth temperature (360–890 °C) and III-V ratio (0.6–1.1) on Sc x Al1-x N crystal quality and composition. The measured ScN fraction was constant between 360–810 °C and increased at 890 °C due to Al re-evaporation from the growth surface. X-ray diffraction (XRD) rocking curve full width at half maximum values were below 300 arcsec for 80-nm-thick Sc0.18Al0.82N thin films for growth temperatures between 520–730 °C, indicating a wide growth window and high crystal quality. Surface rms roughness measured by atomic force microscopy was generally under 1 nm and as low as 0.7 nm at a growth temperature of 730 °C. XRD measurements indicated single crystalline phase epitaxial Sc x Al1-x N for N-rich samples (III/V ratio < 1) and the emergence of additional phases for samples grown metal-rich. In addition to having high spontaneous and piezoelectric polarization, Sc x Al1-x N also has a relatively low etch rate in Cl2-based dry etching commonly used for GaN and AlN. The etch selectivity is as high as 11.2 relative to AlN and 18.6 relative to GaN. The etched surface remains smooth with no increase in the rms roughness or evidence of pitting or micromasking. There are several etch methodologies that allow selective etching of Al-containing layers relative to GaN, but this is the first demonstration of a conventional dry etch chemistry with a working ESL relative to AlN, leading to a variety of applications in AlN-based electronic devices and deep-UV optoelectronics. Using a 25-nm-thick Sc0.14Al0.86N barrier layer in a GaN-based HEMT structure, we demonstrate the first ScAlN-barrier HEMTs with n s as high as 3.4 × 13 cm-2 and mobility of 910 cm2\\V∙s, resulting in a sheet resistance of 213 Ω/□. Reducing the ScAlN barrier thickness to only 3 nm results in an n s of 2.0 × 1013 cm-2 with a mobility of 1060 cm2/V∙s. Both devices included both AlN and GaN interlayers to improve the mobility. These results demonstrate the potential for ScAlN as a barrier material in a highly-scaled, high charge density HEMT for high power millimeter-wave amplifiers. [1] M. Akiyama, K. Kano, and A. Teshigahara, "Influence of growth temperature and scandium concentration on piezoelectric response of scandium aluminum nitride alloy thin films," Appl. Phys. Lett., vol. 95, p. 162107, 2009. [2] M. A. Caro, S. Zhang, T. Riekkinen, M. Ylilammi, M. A. Moram, O. Lopez-Acevedo, et al., "Piezoelectric coefficients and spontaneous polarization of ScAlN," J. Phys.: Condens. Matter, vol. 27, p. 245901, 2015.

Surface preparation of freestanding GaN substrates for homoepitaxial GaN growth by rf-plasma MBE

The authors have investigated different methods for preparing the surfaces of freestanding, Ga-polar, hydride vapor-phase epitaxy grown GaN substrates to be used for homoepitaxial GaN growth by plasma-assisted molecular beam epitaxy (MBE). Cross-sectional transmission electron microscopy and secondary ion mass spectroscopy, respectively, were used to characterize the microstructure and to measure the concentrations of impurities unintentionally incorporated in the MBE-grown homoepitaxial GaN layers. Heating Ga-polar substrates to ∼1100 °C is as effective as a wet chemical clean for reducing impurity concentrations of oxygen, silicon, and carbon. The combination of an aggressive ex situ wet chemical clean with in situ Ga deposition and thermal desorption results in homoepitaxial GaN layer growth with very low residual impurity concentrations and without generating additional threading dislocations.

Fe-doped semi-insulating GaN with solid Fe source grown on (110) Si substrates by NH3 molecular beam epitaxy

Iron doped GaN layers were grown on (110) Si substrates by ammonia molecular beam epitaxy (MBE) using solid elemental iron as a source. Specular films with concentrations up to 1×1020cm−3, as determined by secondary ion mass spectroscopy, were grown, unlike a limited incorporation of Fe into GaN by metal-rich rf plasma MBE. The Fe concentration in the film showed an exponential dependence on the inverse of source temperature with an activation energy of 3.4eV, which agrees well to the reported value for the sublimation of Fe. A 1.5µm thick GaN film with a sheet resistance of 1GΩ/sq. was obtained by compensating unintentional residual donors with a small Fe concentration of 1×1017cm−3. X-ray diffraction rocking curves indicated high crystalline quality, very similar to an undoped film, showing that the Fe incorporation required to obtain the semi-insulating film properties did not affect the structural properties of the film. The low-temperature PL spectra of highly resistive and semi-insulating Fe:GaN in the range of 1017~1018cm−3 show dominant exciton emissions and enhanced donor-acceptor-pair (DAP) emissions, implying that Fe ions contribute to the DAP transition between donor levels and Fe-related acceptor levels, possibly compensating the residual donors to achieve the semi-insulating electrical properties.

Probing the correlation between structure, carrier dynamics and defect states of epitaxial GaN film on (112̄0) sapphire grown by rf-molecular beam epitaxy

A systematic study has been performed to correlate structural, optical and electrical properties with defect states in the GaN films grown on a-plane (112̄0) sapphire substrate via rf-plasma molecular beam epitaxy.

Growth of crystallized AlOx on AlN/GaN heterostructures by in-situ RF-MBE

We report successful growth of a crystallized AlOx layer on top of AlN/GaN heterostructures by using RF-plasma molecular-beam epitaxy for exploring a new-type oxide/nitride heterostructure system. The insertion of an AlOx buffer layer, which was formed by following three steps of (i) an Al metal deposition at 150°C, (ii) an oxidation of the Al metal by oxygen plasma irradiation, and (iii) an annealing of the oxidized layer at 800°C, facilitated the formation of a crystalline AlOx layer on top of the AlN/GaN structures. Surface morphologies observed by atomic force microscope showed that the AlOx buffer layer was directly formed on the nitride structure and fully covered the AlN layer. The AlOx top layer grown on the buffer layer had a flat and smooth surface. A cross-sectional transmission electron microscopy micrograph revealed that the AlOx thin film grown at 800°C on the nitride structure was fully crystallized.

Homoepitaxial N-polar GaN layers and HEMT structures grown by rf-plasma assisted molecular beam epitaxy

The authors have investigated the growth and structural and electrical properties of homoepitaxial GaN layers and GaN/AlGaN heterostructures grown on free-standing, hydride vapor phase epitaxy grown, N-polar GaN:Fe substrates by rf-plasma molecular beam epitaxy. Secondary-ion mass spectroscopic analysis of unintentionally doped and Be-doped N-polar GaN layers indicate that oxygen is the dominant impurity in all layers and is largely insensitive to growth temperature in the range investigated (675 °C < TS < 760 °C). Transmission electron microscopy (TEM) indicates that threading dislocations are generated at the regrowth interface in these samples; in contrast to homoepitaxial growth on Ga-polar GaN, and that the density of threading dislocations diminishes as the growth temperature increases. However, examination by TEM indicates that threading dislocations are not generated at the regrowth interface of samples subjected to pregrowth substrate surface cleaning by gallium deposition and desorption and subsequent growth of ultrathin (15 Å) initial AlN layers. N-polar GaN/AlGaN heterostructures grown on Be-doped homoepitaxial N-polar GaN buffers exhibit low buffer leakage and Hall mobilities up to 1680 cm2/Vs at sheet densities of 1.3 × 1013 cm−2. High electron mobility transistors have been fabricated on these structures; drain current densities over 700 mA/mm and breakdown voltages as high as 70 V have been measured.

Highly Efficient and Bright LEDs Overgrown on GaN Nanopillar Substrates

We presented a study of high-performance GaN-based light emitting diodes (LEDs) using a GaN nanopillars (NPs) structure grown on sapphire substrate by integrating RF-plasma molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD). Nanoscale air voids were clearly observed at the interface between GaN NPs and the overgrown GaN layer by cross-sectional scanning electron microscopy. It can increase the light-extraction efficiency due to additional light scattering. The transmission electron microscopy images suggest the air voids between GaN NPs introduced during nanoscale epitaxial lateral overgrowth of GaN can suppress the threading dislocation density. Moreover, Raman spectrum demonstrated that the strain of the GaN layer grown on GaN NPs was effectively eliminated, resulting in the reduction of quantum-confined Stark effect in InGaN/GaN quantum wells. Consequently, the LEDs fabricated on the GaN NPs template exhibit smaller electroluminescent peak wavelength blue shift and great enhancement of the light output (70% at 20 mA) compared with the conventional LEDs.

Carbon doping for p‐type GaP and GaPN in molecular beam epitaxy using carbon tetrabromide source

AbstractCarbon doping for p ‐type GaP and GaPN using carbon tetrabromide (CBr4) source in rf plasma molecular beam epitaxy was investigated. The doping concentration could be easily controlled by adjusting CBr4 pressure, and a wide region of hole concentration form ∼1017 to ∼1019 cm‐3 was obtained in both GaP and GaPN. It was found that the activation ratio of carbon in GaPN is lower than that in GaP. The hole concentration and mobility in GaPN do not change with nitrogen content at a wide region from 0.5 to 3%. The Hall mobility values in GaP and GaPN were ∼120 and ∼50 cm2/Vs at a hole concentration of ∼1 × 1017 cm‐3. Photoluminescence measurement show that the carbon incorporation reduces the intensity of luminescence, and the relative peak intensity decreases with the increase of hole concentration. This implies the existence of carbon‐related defects that quench the luminescence (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Lasing Actions in GaN Tiny Hexagonal Nanoring Resonators

We achieved optically pumped lasing actions of extremely small optical resonators, which have 1.4-2-μm-diameter GaN hexagonal nanorings with wall widths of 60-140 nm. The hexagonal nanorings were grown by rf-plasma molecular beam epitaxy (rf-MBE) with Ti-mask selective area growth (SAG). Due to the nanometer scale of the wall width, the propagation of threading dislocations through the wall was suppressed, and high crystalline quality of the nanoring resonators was achieved. The hexagonal nanorings possessed a fine cavity configuration of crystallographically flat outer walls and consisted of hexagonally connected sixfold side sections. The periodic boundary condition for each section with a short length limits the number of longitudinal resonance wavelengths, and narrowing the wall suppresses the occurrence of lasing in higher transverse modes. Therefore, both contribute to the lasing action in a single longitudinal mode of the hexagonal nanoring.

Proximity effects of beryllium-doped GaN buffer layers on the electronic properties of epitaxial AlGaN/GaN heterostructures

AlGaN/GaN/Be:GaN heterostructures have been grown by rf-plasma molecular beam epitaxy on free-standing semi-insulating GaN substrates, employing unintentionally-doped (UID) GaN buffer layers with thicknesses, d UID, varying between 50 nm and 500 nm. We have found that the heterostructures with UID buffers thicker than 200 nm exhibit much improved Hall properties and inter-device isolation current compared to heterostructures with d UID < 200 nm. The output conductance of devices fabricated on these heterostructures increases as d UID decreases below 200 nm, and devices with gate lengths of 240 nm and 1 μm exhibited no significant difference in output conductance. Evidence of buffer trapping is observed in devices for which d UID ⩽ 100 nm. The observed effects are tentatively explained by the presence of parallel conduction paths in samples with non-optimized UID buffer thickness.

Growth of free-standing GaN layer on Si(1 1 1) substrate

This investigation demonstrates the epitaxial growth of a free-standing GaN layer on Si(1 1 1) using a funnel-like GaN nano-rod buffer structure. The funnel-like GaN nano-rods were directly grown on Si substrates by RF-plasma molecular beam epitaxy. Free-standing GaN layers were achieved through the coalescence of funnel-like GaN nano-rods by the metalorganic chemical vapor deposition. This study examines the structure, optical characteristics and stress of GaN nano-rods and free-standing GaN layers. The c-axis lattice constant of the strain-free Ga-face GaN layer on Si is 5.1844 Å, as determined by high-resolution X-ray diffraction. The fully relaxed band edge at 3.468 eV without deep-level emission around 2.3 eV, was revealed in a free-standing GaN layer on Si, using photoluminescence.

Effect of substrate surface on GaN dot structure grown on Si(111) by droplet epitaxy

AbstractThe effect of substrate surface on GaN dot structure grown on Si(111) by droplet epitaxy using an rf plasma molecular beam epitaxy (MBE) system is reported. Compared with nitridated substrates, both enhancement of Ga migration and poorer wettability were observed on oxidized substrates, inducing enlargement of Ga droplets during nitridation. We demonstrated that the size and density of GaN dots grown by droplet epitaxy are strongly affected by pre‐treatment of the substrate. In the fabrication of GaN dots by droplet epitaxy, the importance of optimizing nitridation conditions according to the substrate surface is demonstrated. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Thermodynamic analysis of cation incorporation during molecular beam epitaxy of nitride films using metal-rich growth conditions

The conventional approach to growth of the nitride films GaN, AlN, InN, and their alloys by rf plasma molecular beam epitaxy uses metal-rich surface conditions due to improved material quality compared to nitrogen-rich conditions. The surface metal may incorporate into the growing film, act as a surfactant, and/or react with the underlying film or substrate. Using a simple chemical exchange reaction model and tabulated thermodynamic data at molecular beam epitaxy growth temperatures the predicted preferential incorporation series on the column III site under metal-rich conditions is found to be Al&amp;gt;B,Be,Si, Mg&amp;gt;Ga&amp;gt;In,Fe. This series is consistent with the observed ternary growth behavior and surfactant order. The series is also consistent with silicon migration in AlN but not GaN, sharper beryllium transitions in GaN than AlN, the significant migration of iron in GaN, and the reactivity of AlN nucleation layers with SiC surfaces. The model is used to predict boron incorporation under metal-rich conditions in BGaN and BAlN and should prove useful as a tool in predicting the incorporation behavior of other cations during metal-rich epitaxial growth of nitride films and possibly other materials.

Properties of CdZnO/ZnO Heterostructures for UV Light Emitters

In the paper we present band-gap alignment diagrams for type-II/type-I heterostructures incorporating the strong piezoelectric and spontaneous polarization fields in ZnCdMgO and AlGaN-based materials and interfaces. The paper describes some of the recent progress made in the development of high quality ZnCdO layers and CdZnO/ZnO heterostructures grown epitaxially by RF-plasma molecular beam epitaxy (MBE). We summarize optical and electrical properties of high quality CdxZn1-xO alloys with Cd mole fraction from 0.02 to 0.78 and discuss phase separation phenomenon, which may be present in ternary alloys. A single-crystal wurtzite structure of CdZnO alloys has been confirmed. We achieved a strong optical emission at RT in the 380 nm to 574 nm spectral range from CdxZn1-xO with various compositions. Dependence of the fundamental optical band gap on the composition of CdxZn1- xO alloys, band gap bowing, and the possible effect of composition micro-fluctuations in ternary CdxZn1-xO alloys on the optical bandgap is also discussed. Time resolved photoluminescence shows multi-exponential decay with 21 psec. and 49{plus minus}3 psec. lifetimes, suggesting that composition micro- fluctuations may be present in Cd0.16Zn0.84O film. We also report on crystallographic and optical properties of CdZnO/ZnO multiple quantum wells.

Ga adsorbate on (0001) GaN: In situ characterization with quadrupole mass spectrometry and reflection high-energy electron diffraction

We have investigated the adsorption and subsequent desorption of Ga on (0001) GaN using simultaneous line-of-sight quadrupole mass spectrometry (QMS) and reflection high-energy electron diffraction (RHEED). The in situ QMS and RHEED desorption transient measurements demonstrate the Ga flux dependent accumulation of the theoretically predicted laterally contracted Ga bilayer [J. E. Northrup et al., Phys. Rev. B 61, 9932 (2000)] under conditions similar to those used during GaN growth by rf-plasma molecular beam epitaxy. We correlated bioscillatory RHEED desorption transients [C. Adelmann et al., J. Appl. Phys. 91, 9638 (2002)] to QMS-measured Ga-adsorbate coverage and found both to be consistent with layer-by-layer desorption of the Ga-adsorbate bilayer. The QMS-measured steady-state Ga-adlayer coverage exhibited a continuous increase from 0 to 2.4 ML (monolayer) with respect to impinging Ga flux at substrate temperatures of 640–700°C. We observed an exponential dependence of the Ga flux corresponding to 1.0 ML Ga-adsorbate coverage on substrate temperature and we measured an apparent activation energy of 2.43±0.11eV and an attempt prefactor of 6.77×1012nm∕min (4.36×1011Hz) for this transition.

Epitaxy of nonpolar AlN on 4H-SiC (1-100) substrates

AlN has been grown on 4H-SiC (1-100) substrates by rf-plasma molecular beam epitaxy. The epilayers assume a metastable 4H structure to match the in-plane stacking arrangement of the substrate. Initial two-dimensional nucleation of 4H-AlN is revealed by reflection high-energy electron diffraction. The epitaxial quality is evidenced by narrow x-ray diffraction ω-scan linewidths less than 70 arcsec for both symmetric and asymmetric reflections. The AlN growth surface exhibits a smooth and anisotropic morphology similar to that of GaN (1-100). Large residual stress is present in the epilayers, consistent with incomplete relaxation of misfit strain during growth.

Growth of Nonpolar AlN and AlGaN on 4H-SiC (1-100) by Molecular Beam Epitaxy

AlN and AlGaN have been grown on 4H-SiC (1-100) substrates by rf-plasma molecular beam epitaxy. AlN assumes a metastable 4H structure to match the in-plane stacking arrangement of the substrate. Initial 2D nucleation of 4H-AlN is revealed by reflection high-energy electron diffraction. The epitaxial quality is evidenced by narrow x-ray diffraction ω-scan line widths less than 100 arcsec for symmetric and asymmetric reflections. Structural characterization results for AlGaN/AlN multiple quantum wells are also presented.

Molecular beam epitaxy of InAlN∕GaN heterostructures for high electron mobility transistors

We describe the growth of InAlN∕GaN heterostructures by rf-plasma molecular beam epitaxy. Due to the weak In–N bond, the InAlN growth temperature must be below about 460°C for In to incorporate reliably into the film. Thus far, a thin AlN spacer layer has been required to form a low resistance two dimensional electron gas (2DEG) at the InAlN∕GaN interface. The thin AlN barrier is believed to reduce alloy scattering of carriers in the 2DEG. The best HEMT material with an InAlN barrier and a thin AlN spacer layer has a sheet resistance of 980Ω∕◻ with a sheet electron density of 1.96×1013cm−2.