Plasma-assisted Molecular Beam Epitaxy Growth Research Articles

Influence of dislocation and ionized impurity scattering on the electron mobility in GaN/AlGaN heterostructures

The mobility of electrons forming the two-dimensional gas in GaN/AlGaN heterostructures is reviewed in connection with recent technological achievements in plasma-assisted molecular beam epitaxy (MBE) growth. We discuss the dependence of the room temperature and liquid helium temperature mobilities on the dislocation density, impurity concentration and compensation. This allows proposing directions of technological developments leading to an improvement of the electron mobility in GaN/AlGaN heterostructures. We also present results on the record mobility observed in GaN/AlGaN heterostructures grown by plasma-assisted MBE on bulk GaN crystals.

Critical RF damage conditions for the plasma-assisted molecular beam epitaxy growth of GaInNAs with dilute N 2/Ar gas mix

In this work, we investigate the operating conditions for RF plasma sources that employ nitrogen–argon gas mixtures for the molecular beam epitaxy growth of dilute nitride, narrow-bandgap semiconductors. The effects of RF power and gas flow rate variations are discussed, wherein a ‘critical’ RF power level is identified at 300 W. Growths performed at RF powers below this critical setting resulted in noticeably better quality material, which was determined from a comparison of pre- and post-anneal room temperature photoluminescence measurements. In addition to the RF power, the gas flow rate directly influences the growth chamber pressure, which in turn also affects the material quality. A modified pumping configuration was used to decouple the chamber pressure from the gas flow rate. In all growths, lower chamber pressures resulted in better material quality, irregardless of the power setting used. These results suggest that operating the nitrogen RF plasma source at its lowest possible power setting and lowest gas flow rate, while still maintaining a stable plasma, will result in the best quality of dilute nitride materials.

Role Of Active Oxygen Species On Growth Of ZnO Using RF-PAMBE

ABSTRACTThe effect of different oxygen species on the RF plasma-assisted molecular beam epitaxy growth of ZnO films has been investigated. By varying the geometry of the aperture plate and also the RF power, the relative atomic content in the discharge was altered, and this is found to be corre-lated to the film quality. Further, growth rate studies performed in tandem with in-situ laser interferometry suggest that stoichiometric conditions may not result in saturation of growth rates.

Plasma-assisted MBE growth and characterization of InN on sapphire

We report on a close correlation between the growth conditions of InN/Al 2O 3 (0 0 0 1) epilayers grown by plasma-assisted molecular beam epitaxy (MBE); their optical properties in the IR range, and the In clustering in the layers. High-spatial resolution techniques, namely micro-cathodoluminescsence, back-scattered electron imaging and energy dispersive X-ray analysis, were used to establish this correlation. The In-rich growth conditions, achieved by increasing either the growth temperature or the effective In/N flux ratio, causes the In clustering in InN, responsible in our samples for the 0.7–0.8 eV luminescence and IR optical absorption. Growth under In/N=1:1 conditions slightly shifted to the N-rich side generally produces InN layers without visible In clusters, having an optical absorption edge around 1.4 eV. A possible mechanism of In cluster formation is suggested on the basis of thermodynamic considerations for InN MBE growth.

Formation of single-phase oxide nanoclusters: Cu2O on SrTiO3(100)

Selective formation of the single-phase nanoclusters of Cu2O on SrTiO3(100) substrates in the size range of 10–50 nm is found to occur only in a very narrow oxygen plasma-assisted molecular-beam epitaxy growth parameter window, in comparison with the bulk phase diagram (for oxygen pressure versus temperature). There are distinctive parameter regions, where multiple phaselike forms coexist (CuO/Cu2O and Cu2O/Cu), in agreement with theoretical prediction for small systems, and as opposite to the sharp phase boundaries for the bulk. Observed changes in the nanocluster composition are found to correlate with differences in cluster morphologies.

New product developments

The recent development of group III nitrides allows researchers world-wide to consider AlGaN based light emitting diodes as a possible new alternative deep ultra–violet light source for surface decontamination and water purification. In this paper we will describe our recent results on plasma-assisted molecular beam epitaxy (PA-MBE) growth of free-standing wurtzite AlxGa1−xN bulk crystals using the latest model of Riber's highly efficient nitrogen RF plasma source. We have achieved AlGaN growth rates up to 3 µm/h. Wurtzite AlxGa1−xN layers with thicknesses up to 100 μm were successfully grown by PA-MBE on 2-inch and 3-inch GaAs (111)B substrates. After growth the GaAs was subsequently removed using a chemical etch to achieve free-standing AlxGa1−xN wafers. Free-standing bulk AlxGa1−xN wafers with thicknesses in the range 30–100 μm may be used as substrates for further growth of AlxGa1−xN-based structures and devices. High Resolution Scanning Transmission Electron Microscopy (HR-STEM) and Convergent Beam Electron Diffraction (CBED) were employed for detailed structural analysis of AlGaN/GaAs (111)B interface and allowed us to determine the N-polarity of AlGaN layers grown on GaAs (111)B substrates. The novel, high efficiency RF plasma source allowed us to achieve free-standing AlxGa1−xN layers in a single day's growth, making this a commercially viable process.

Nitrogen-plasma study for plasma-assisted MBE growth of 1.3 μm laser diodes

We have studied the influence of radio-frequency plasma cell operating conditions on the plasma-assisted molecular-beam epitaxy growth of bulk GaAsN and GaInAsN quantum wells. For low N 2 flow rates, the species that mostly incorporate are atoms and the dissociation fraction of N 2 for the lowest flow rate and the highest RF power has been found to be up to 0.7. For high N 2 flow rates, the species that mostly incorporate are vibrationally excited molecules in the N 2( X, v) ground state. The vibrational temperature of the N 2( X, v) ground state has been found to be higher than 10 4 K. The ion damage has been decreased by increasing the N 2 pressure which induces a decrease of the electron temperature and consequently a decrease of the plasma ionization. Oxygen has been evidenced to incorporate proportionally to nitrogen and its emission line has been identified in the plasma emission spectrum. Finally, quantum-well structures emitting at around 1.3 μm at 300 K have been fabricated and characterized by photoluminescence and photocurrent spectroscopy.

GaN thin film growth on GaAs (0 0 1) by CBE and plasma-assisted MBE

GaN thin films were deposited on GaAs (0 0 1) substrates using chemical beam epitaxy (CBE) and plasma-assisted molecular beam epitaxy (MBE) methods. The effect of in situ substrate cleaning and pre-treatment on the interfacial structure was investigated. Time-of-flight mass spectroscopy of recoiled ions was utilized to measure in situ the surface composition. The microstructure of GaN films was studied with selected area electron diffraction, and both conventional and high-resolution transmission electron microscopy. It was found that simultaneous electron cyclotron resonance (ECR) plasma treatment using an Ar/N 2 mixture increases the yield of a cubic phase in GaN films during CBE growth. Resulting films are less defective than mixed phase GaN films grown after pure N 2 plasma nitridation. In the case of plasma-assisted MBE growth, a low-temperature annealing followed by N 2 ECR nitridation process yields cubic GaN thin films while N 2 ECR nitridation without a sample annealing to 600°C yields hexagonal phase GaN films.

X-ray diffraction characterization of MBE grown Pr 1− xSr xMnO 3 thin films on NGO(1 1 0)

Oxygen plasma-assisted molecular beam epitaxial (MBE) growth of Pr 1− x Sr x MnO 3 (PSMO) thin films has been carried out on NdGaO 3(1 1 0) (NGO) substrates. The growth parameters have been optimized to realize 2D layer-by-layer growth. XRD results of the epilayers show that the PSMO/NGO(1 1 0) thin films are of high crystal quality, as clear diffraction peaks can be observed belonging to the film and the substrate, respectively. Based on analysis of the peaks, it was concluded that epitaxial relation is PSMO(1 1 0)//NGO(1 1 0), i.e., the c-axis being parallel to the surface. Both single scans ( ω scan, 2 θ/ ω scan) and 2-axis reciprocal space mapping (RSM) were performed in an effort to assess the crystal structure, crystalline quality, surface and interface properties of the epitaxial layers. High temperature annealing effects on lattice structure and crystal quality have been studied and discussed. Transport property measurement of the PSMO thin film samples has been carried out and main features discussed.

Review of Structure of Bare and Adsorbate-Covered GaN(0001) Surfaces

A review of surface structures of bare and adsorbate-covered GaN (0001) and (000) surfaces is presented, including results for In, Mg, Si, and H adsorbates. Emphasis is given to direct determination of surface structure employing experimental techniques such as scanning tunneling microscopy, electron diffraction, and Auger electron spectroscopy, and utilizing first principles computations of the total energy of various structural models. Different surface stoichiometries are studied experimentally by varying the surface preparation conditions (e.g. Ga-rich compared to N-rich), and the stoichiometry is included in the theory by performing calculations for various chemical potentials of the constituent atoms. Based on the work reviewed here, surface reconstructions for plasma-assisted molecular beam epitaxy growth of GaN (0001) and (000) surfaces are fairly well understood, but reconstructions for reactive molecular beam epitaxy or for metal-organic vapor phase epitaxy (both involving H, at moderate and high temperatures, respectively) are less well understood at present.

Growth and characterizations of InGaN on N- and Ga-polarity GaN grown by plasma-assisted molecular-beam epitaxy

Plasma-assisted molecular-beam epitaxial (RF-MBE) growth of InGaN films on both N- and Ga-polarity GaN underlayers was carried out. Clear evidence was obtained for the difference in the film quality between InGaN grown on the N- and Ga-polarity GaN underlayers, to which the Ga-polarity GaN is preferred. Based on this point, high-quality InGaN films were obtained on the Ga-polarity GaN underlayers with an In composition up to 0.36. Intense photoluminescence (PL) emissions from the InGaN films were observed. Comparing the PL peak energy and the In composition determined by X-ray diffraction, it was found that the band-gap bowing parameter of InGaN films depends on the In composition.

Strain relaxation in (0001) AlN/GaN heterostructures

The strain-relaxation phenomena during the early stages of plasma-assisted molecular-beam epitaxy growth of lattice-mismatched wurtzite (0001) AlN/GaN heterostructures have been studied by real-time recording of the in situ reflection high-energy electron diffraction (RHEED), ex situ transmission electron microscopy (TEM), and atomic-force microscopy. A pseudo-two-dimensional layer-by-layer growth is observed at substrate temperatures of 640--660 \ifmmode^\circ\else\textdegree\fi{}C, as evidenced by RHEED and TEM. However, the variation of the in-plane lattice parameter during growth and after growth has been found to be complex. Three steps have been seen during the deposition of lattice-mismatched AlN and GaN layers: they were interpreted as the succession of the formation of flat platelets, 3--6 monolayers high (0.8--1.5 nm) and 10--20 nm in diameter, their partial coalescence, and gradual dislocation introduction. Platelet formation leads to elastic relaxation as high as 1.8%, i.e., a considerable part of the AlN/GaN lattice mismatch of 2.4%, and can be reversible. Platelets are always observed during the initial stages of growth and are almost insensitive to the metal/N ratio. In contrast, platelet coalescence and dislocation introduction are very dependent on the metal/N ratio: no coalescence occurs and the dislocation introduction rate is higher under N-rich conditions. In all cases, the misfit dislocation density, as measured by the irreversible relaxation, is initially of the order of $7\ifmmode\times\else\texttimes\fi{}{10}^{11}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ and decreases exponentially with the layer thickness. These results are interpreted in the framework of a model that emphasizes the important role of the flat platelets for dislocation nucleation.

ZnO and related materials: Plasma-Assisted molecular beam epitaxial growth, characterization and application

This paper will address features of plasma-assisted molecular beam epitaxial growth of ZnO and related materials and their characteristics. Two-dimensional, layer-by-layer growth is achieved both on c-plane sampphire by employing MgO buffer layer growth and on (0001) GaN/Al2O3 template by predepositing a low-temperature buffer layer followed by high-temperature annealing. Such two-dimensional growth results in the growth of high-quality heteroepitaxial ZnO epilayers. Biexciton emission is obtained from such high quality epilayers The polarity of heteroepitaxial ZnO epilayers is controlled by engineering the heterointerfaces. We achieved selective growth of Zn-polar and O-polar ZnO heteroepitaxial layers. The origin of different polarities can be successfully explained by an interface bonding sequence model. N-type conductivity in Gadoped ZnO epilayers is successfully controlled. High conductivity, enough to be applicable to devices, is achieved. MgxZn1-xO/ZnO heterostructures are grown and emission from a ZnO quantum well is observed. Mg incorporation in a MgZnO alloy is determined by in-situ reflection high-energy electron diffraction intensity oscillations, which enables precise control of the composition. Homoepitaxy on commericial ZnO substrates has been examined. Reflection high-energy electron diffraction intensity oscillations during homoepitaxy growth are observed.

Atomic layer-by-layer growth of superconducting Bi–Sr–Ca–Cu–O thin films by molecular beam epitaxy

In situ reflection high-energy electron diffraction (RHEED) is employed to investigate the growth kinetics, and monitor the crystal surface evolution, during plasma-assisted molecular beam epitaxy growth of Bi 2Sr 2Ca n−1 Cu n O (BSCCO) compounds. By varying the growth parameters such as operating pressure, substrate temperature, cation flux and shutter opening pulse duration, it is found that the crystal growth front exhibits surface reconstructions with (1×1), (2×2), c(2×2) and (3×1) symmetries for the Sr, Ca and Cu species, and a RHEED pattern characteristic of twinning for Bi. Through manipulation of these surface reconstructions, and use of an adapted growth mode, it was possible to achieve a monolayer coverage for each species supplied. For the n=1, 2 and 3 compounds the resulting films exhibit a crystal quality characterised by an X-ray diffraction rocking curve width of 0.03° and an atomic force microscope mean surface roughness of 0.9 nm [over 10×10 μm] for 40 nm thick films.

Investigation into the influence of buffer and nitrided layers on the initial stages of GaN growth on InSb (100)

Radio frequency plasma-assisted molecular beam epitaxy (MBE) growth of GaN on InSb (100) has been investigated. This combination is interesting because a 45° rotation of a cubic epitaxial GaN layer could result in a nearly “lattice-matched” system. The growth of low-temperature buffer layers and initial substrate nitridation at 275°C on the morphology of the subsequent growth at 450°C were considered. Nitridation produced a smooth, mixed InN and Sb–N layer, whilst annealing to 450°C resulted in the loss of the Sb nitride component and disruption of the InN, causing exposure of the underlying substrate and surface roughening. Similarly thin buffer layers (∼8 Å) were found to crystallise and island at 450°C but allowed substrate damage. By contrast, thicker buffer layers (∼80 Å) remained smooth and continuous and protected the substrate but did not crystallise. Subsequent growth morphologies reflected the surface quality of the underlying layers, however all layers were polycrystalline wurtzite GaN and no evidence was found for crystalline cubic GaN formation.

Optical Properties of an AlInN Interface Layer Spontaneously Formed in Hexagonal InN/Sapphire Heterostructures

We report on the spontaneous formation of an AlInN interlayer in plasma-assisted molecular beam epitaxial growth of InN on sapphire substrates. Under our growth conditions, and with an initial growth stage corresponding to the best attainable InN film quality, the AlInN layer has a thickness of ≈︂100 nm and an aluminum content of around 0.3. The layer possesses a good crystalline quality as manifested by transmission electron microscopy and X-ray diffraction (XRD) studies. No evidence of binary phase separation within the layer is found by XRD and optical absorption measurements. The energy gap bowing of the AlInN is well approximated by the expression for a disordered alloy. The possibility of AlInN to emit light is demonstrated by photo- and cathodoluminescence measurements.

Effect of substrate temperature and V/III flux ratio on In incorporation for InGaN/GaN heterostructures grown by plasma-assisted molecular-beam epitaxy

Reflection high-energy electron diffraction (RHEED) and laterally spatially resolved high resolution x-ray diffraction (HRXRD) have been used to identify and characterize rf plasma-assisted molecular-beam epitaxial growth factors which strongly affect the efficiency of In incorporation into InxGa1−xN epitaxial materials. HRXRD results for InxGa1−xN/GaN superlattices reveal a particularly strong dependence of average alloy composition x̄ upon both substrate growth temperature and incident V/III flux ratio. For fixed flux ratio, results reveal a strong thermally activated behavior, with over an order-of-magnitude decrease in x̄ with increasing growth temperature within the narrow range 590–670 °C. Within this same range, a further strong dependence upon V/III flux ratio is observed. The decreased In incorporation at elevated substrate temperatures is tentatively attributed to In surface-segregation and desorption processes. RHEED observations support this segregation/desorption interpretation to account for In loss.

Microscopic processes during electron cyclotron resonance microwave nitrogen plasma-assisted molecular beam epitaxial growth of GaN/GaAs heterostructures: Experiments and kinetic modeling

A set of δ-GaNyAs1−y/GaAs strained-layer superlattices grown on GaAs (001) substrates by electron cyclotron resonance (ECR) microwave plasma-assisted molecular beam epitaxy (MBE) was characterized by ex situ high resolution X-ray diffraction (HRXRD) to determine nitrogen content y in the nitrided GaAs monolayers as a function of growth temperature T. A first order kinetic model is introduced to quantitatively explain this y(T) dependence in terms of an energetically favorable N for As anion exchange and thermally activated N-surface desorption and surface segregation processes. The nitrogen surface segregation process, with an estimated activation energy Es∼0.9 eV appears to be significant during the GaAs overgrowth of GaNyAs1−y layers, and is shown to be responsible for strong y(T) dependence.

Kinetic modeling of microscopic processes during electron cyclotron resonance microwave plasma-assisted molecular beam epitaxial growth of GaN/GaAs-based heterostructures

Microscopic growth processes associated with GaN/GaAs molecular beam epitaxy (MBE) are examined through the introduction of a first-order kinetic model. The model is applied to the electron cyclotron resonance microwave plasma-assisted MBE (ECR-MBE) growth of a set of δ-GaNyAs1−y/GaAs strained-layer superlattices that consist of nitrided GaAs monolayers separated by GaAs spacers, and that exhibit a strong decrease of y with increasing T over the range 540–580 °C. This y(T) dependence is quantitatively explained in terms of microscopic anion exchange, and thermally activated N surface-desorption and surface-segregation processes. N surface segregation is found to be significant during GaAs overgrowth of GaNyAs1−y layers at typical GaN ECR-MBE growth temperatures, with an estimated activation energy Es∼0.9 eV. The observed y(T) dependence is shown to result from a combination of N surface segregation/desorption processes.