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GaN epitaxial layers have been grown by molecular beam epitaxy (MBE). Low (200 degreesC) and high (700 degreesC) temperature nitrided sapphire substrates, and GaN and AlN buffer layers have been used as the platforms for investigating polarity selection processes of GaN grown by RF-MBE, aimed at presenting a comprehensive understanding of the issues of GaN polarity and growth conditions. The results show that Ga-polar GaN results from the use of AlN buffer layers, and when the substrate/bulk interface has a Ga- or Al-rich interface. In contrast, a large density of N-polar domains is found when GaN epitaxial films are grown on GaN buffer layers on sapphire when nitrided at 200 degreesC. The impact of the polarity of GaN on the interaction of GaN surfaces with atomic hydrogen is also studied. A different reaction rate and reaction extent is found for N- and Ga-polar GaN with atomic hydrogen, with N-polar GaN exhibiting greater reactivity. It is also demonstrated that the reactivity of GaN with atomic hydrogen coupled with surface potential measurements exploiting Kelvin probe microscopy is an alternative approach to determining the polarity of GaN films, and identifying the presence of inversion domains. Finally, the impact of GaN polarity on electrical properties of AlGaN/GaN heterojunction structures is also shown

The impact of the nitridation temperature on sapphire/GaN interface modifications and the structural, chemical, and optical properties of GaN epitaxial thin films with N plasma radicals is investigated. Based on ex situ spectroscopic ellipsometry and x-ray photoelectron spectroscopy analysis, it is found that the sapphire nitridation chemistry, specifically AlN versus oxynitride (NO) production, depends on the surface temperature. Nitridation at 200 °C produces a very thin AlN layer with 90% coverage, while high temperature nitridation leads to a 70% coverage of AlN layer containing NO. These initial stages of growth significantly impact the characteristics of the layers following the nitridation step, specifically the low temperature buffer, annealed buffer, and the GaN epitaxial layer. The annealed buffer on a 200 °C nitridation provides a homogeneous GaN thin layer covering most of the sapphire surface. This homogeneous GaN layer after annealing produces a superior template for subsequent growth, resulting in improved structural and optical properties of GaN epitaxial films. On the other hand, the annealed buffer grown on sapphire nitrided at temperatures lower or higher than 200 °C, has islands of GaN nuclei revealing the sapphire substrate, and ultimately, resulting in degraded GaN epitaxial film quality as demonstrated by photoluminescence and x-ray diffraction measurements. The results can be traced back to the chemistry of the nitridation process.

The effect of sapphire nitridation temperature on the chemistry and microstructure of the sapphire substrate/GaN interface, nucleation layer, and of the GaN epilayers grown by rf plasma assisted molecular beam epitaxy is investigated. It is found that a sapphire nitridation temperature as low as 200 °C improves the structural and optical quality of GaN epilayers. This result can be explained by the chemistry of the sapphire nitridation process, which is discussed in the framework of a model considering the competitive formation of AlN and oxynitride (NO). In particular, at 200 °C, NO desorbs from the sapphire surface, yielding an homogeneous 6 A AlN layer upon N2 plasma nitridation. This low temperature AlN template favors the nucleation of hexagonal GaN nuclei which coalesce completely resulting in a hexagonal GaN buffer layer that homogeneously covers the sapphire substrate. This condition promotes the growth of a high quality GaN epilayer. In contrast, high nitridation temperatures result in a mixed AlN/NO nitrided sapphire surface which induce a perturbed and more defected interface with the occurrence of cubic crystallites in the GaN buffer. A sapphire surface with random GaN islands is found upon annealing of the GaN buffer and this condition results in a low-quality GaN epilayer.

We report on the optical and structural properties of strained InAlAs/InAsxP1−x heterostructures grown by solid source molecular beam epitaxy. The incorporation of As4 into strained InAsP has been studied using x-ray diffraction and photoluminscence measurements. We find that the incorporation of As decreases as x in the InAsxP1−x increases due, presumably, to increased strain. In addition, we have examined the effect of interface growth conditions and barrier material on the quantum well properties. The 4.2 K photoluminescence spectra of InAlAs/InAsP multi-quantum wells grown with different As and P flux switching conditions at the interfaces were compared with that of higher quality InP/InAsP multi-quantum wells. This comparison shows that the interface quality of InAlAs/InAsP multi-quantum wells is highly dependent on the switching conditions at the interfaces, especially the exposure time of InAsP surface to an As4 overpressure. This sensitivity is due to the As–P exchange reaction at the interfaces of InAlAs/InAsP.

The interaction of atomic nitrogen with 4H- and 6H-SiC(0001) Si-face surfaces is investigated. Clean and atomically smooth terraced surfaces obtained by in situ cleaning using atomic hydrogen have been exposed at 200 degreesC and 750 degreesC to atomic nitrogen produced by a rf remote N-2 plasma. Spectroscopic ellipsometry is used for real-time monitoring of the kinetics of SiC surface modifications, and determining the thickness and properties of the nitrided layer. Surface potential measurements reveal the band bending of the nitrided SiC surface. An improvement in the heteroepitaxy of GaN on the low-temperature nitrided SiC surface is found

The density and energy distribution of InxAl1−xN/GaN surface donor states are studied for InxAl1−xN structures with varying indium compositions. The results support a surface states model with a constant energy distribution of 2.17–2.63 eV below the conduction band minimum and a concentration of 4.64–8.27 × 1013 cm−2 eV−1. It is shown that the properties of the surface states are affected by the surface indium composition xs, as opposed to the bulk composition, xb (InxAl1−xN). Higher surface indium composition xs increases the density of surface states and narrows their energy distribution.

The controlled incorporation of Bi into GaAs is a key challenge to synthesizing dilute Bi materials. This work reveals the importance of the surface step density and direction on Bi incorporation. Steps in the [110] direction are demonstrated to enhance Bi incorporation, but at the cost of reduced photoluminescence intensity at a red-shifted peak position, while steps in the [1¯10] direction yield the opposite behavior. A qualitative model based on the competitive incorporation of As and Bi at different step edges is used to rationalize the observed differences in Bi incorporation.

Angle-resolved X-ray photoelectron spectroscopy (XPS) is used in this work to experimentally determine the valence band offsets of molecular beam epitaxy (MBE)-grown InAlN/GaN heterostructures with varying indium composition. We find that the internal electric field resulting from polarization must be taken into account when analyzing the XPS data. Valence band offsets of 0.12 eV for In0.18Al0.82N, 0.15 eV for In0.17Al0.83N, and 0.23 eV for In0.098Al0.902N with GaN are obtained. The results show that a compositional-depended bowing parameter is needed in order to estimate the valence band energies of InAlN as a function of composition in relation to those of the binary endpoints, AlN and InN.

The growth, fabrication, and characterization of ultraviolet metal–semiconductor–metal (MSM) GaN photodetectors grown on LiGaO2 by molecular-beam epitaxy are reported herein. GaN/LiGaO2 material with dislocation densities of approximately 108 cm−2 and x-ray diffraction (00.4) full width at half maximum of 75 arcsec results in MSMs showing high responsivity, 0.105 A/W, at a reverse bias voltage of 20 V at 308 nm, and low dark currents of 7.88 pA at a 60 V reverse bias. Given the etch selectivity of the GaN/LiGaO2 system and the excellent performance of these devices, GaN device integration onto alternative substrates appears promising.

The effects of high temperature rapid thermal annealing processes on carrier concentration and mobility of bulk AlInAs and AlInAs/GaInAs high electron mobility transistor structures with planar Si doping are studied. At annealing temperatures of 700 °C and 800 °C, slight reduction in mobilities and carrier concentration are observed in samples annealed with a Si3N4 cap or GaAs pieces in close proximity. The reduction in mobility is thought to be due to enhanced diffusion of the donor Si atoms towards the two‐dimensional electron gas channel. Preferential vacancy enhanced diffusion of Si atoms towards the surface is projected to be responsible for the loss in carrier concentration. At these annealing temperatures, the reduction in mobility in the samples annealed with SiO2 capping is more pronounced, and is as high as 80% at the measurement temperature of 15 K. This behavior is attributed to the outdiffusion of Ga and In atoms into the oxide thereby creating vacancies and resulting in interface mixing. Red...

We describe dramatically decreased Mg incorporation in GaN above a critical Mg flux. Secondary ion mass spectroscopy analysis showed a linear increase in Mg concentration up to a flux equivalent to 8.0×10−10 Torr beam equivalent pressure (BEP) and 1.6×10−9 Torr BEP at 550 and 615 °C respectively, beyond which the Mg incorporation was reduced by factors of 10 for 550 °C, and 2 for 615 °C. In a transition region between this critical flux and higher flux, a time dependent incorporation phenomenon was observed. An increase in the GaN growth rate was also observed in the presence of Mg above the critical flux.

A configuration of misfit dislocation dipoles is observed in a Ga0.5In0.5P heterostructure grown by solid-source molecular-beam epitaxy on GaAs. The dipole dislocations are mostly of 60° type, separated by ∼3.5 nm. The dislocations are not produced by conventional lattice mismatch, rather, they could be the result of lateral compositional modulation in the Ga0.5In0.5P epilayer.

In this letter, we report on the properties of a AlxGa1−xN/GaN heterostructure grown on LiGaO2. A two-dimensional electron gas (2DEG) is observed with mobility of 731 cm2/V s at room temperature and 2166 cm2/V s at 77 K. A comparison of the structural quality of the heterostructure as determined by x-ray diffraction shows significant improvement in comparison to a similar structure grown on a sapphire substrate. Secondary ion mass spectroscopy analysis indicates that lithium diffuses into the GaN during growth. The concentration decreases by two orders of magnitude from the substrate to the surface in a 0.8 μm thick GaN film. The enhancement of the mobility of the 2DEG compared to that of electrons in a uniformly doped film is due, in part, to the proximity of the 2DEG to the film surface, where the Li concentration is lower. In addition, we believe that the surface roughness plays a role in the mobility of the 2DEG. Despite these extrinsic factors, the good conductivity of the 2DEG shows the promise of LiGaO2 as a substrate for device-quality GaN.

Thin films of the wide bandgap quaternary semiconductor InxAlyGa(1−x−y)N with low In (x = 0.01–0.05) and high Al composition (y = 0.40–0.49) were synthesized on GaN templates by plasma-assisted molecular beam epitaxy. High-resolution X-ray diffraction was used to correlate the strain accommodation of the films to composition. Room temperature ultraviolet B (280 nm–320 nm) photoluminescence intensity increased with increasing In composition, while the Stokes shift remained relatively constant. The data suggest a competition between radiative and non-radiative recombination occurs for carriers, respectively, localized at centers produced by In incorporation and at dislocations produced by strain relaxation.

The strain dynamic of thin film AlN is investigated before and after the deposition of a GaN epitaxial layer using plasma assisted molecular beam epitaxy. X-ray diffraction ω/2θ-scan and asymmetric reciprocal space mapping analysis show that the deposition of GaN alters the strain state of the underlying AlN template. The in-plane lattice constant of the AlN is found to increase upon growth of GaN, giving rise to a more relaxed GaN epitaxial layer. Hence, the subsequent GaN epitaxial thin film possesses better structural quality especially with lower screw dislocation density and flat surface morphology which is evidenced by the X-ray diffraction ω-scan, room temperature photoluminescence, and atomic force microscopy analysis. Such relaxation of AlN upon GaN deposition is only observed for relatively thin AlN templates with thicknesses of 20 nm–30 nm; this effect is negligible for AlN with thickness of 50 nm and above. As the thicker AlN templates already themselves relax before the GaN deposition, the lo...

Bottom-patterned compliant substrates can be used to laterally modulate the properties of a mismatched epilayer. At temperatures where strain-dependent growth kinetics are significant, the GaAs bottom pattern compliant substrates affected the growth of strained InGaAs epilayers by causing lateral modulation in material height. Spatially aligned mounds and quantum dots were observed on the samples grown at high temperatures. A new mounding mechanism was observed on InGaAs layers grown on the compliant substrates. Strain-dependent growth kinetics and the bottom patterned compliant substrate are used to explain these effects.

A metastability model for GaAs compliant substrates is developed using the compliant substrate partitioning formula and experimental strain relief data. The developed model agrees with compliant substrate strain relief data deduced from double crystal x-ray diffraction and indicates that, for a set of growth conditions and compliant substrate thicknesses, layers of InGaAs of any thickness can be grown free of dislocations. The model developed in this letter is also compared to other compliant substrate critical thickness models, and the authors discuss the mechanisms of partitioning in mismatched layers grown on compliant substrates.

Double crystal x-ray diffraction data is presented from the most extensive compliant substrate experiment to date. Five consecutive InGaAs–GaAs growths were performed simultaneously on GaAs-based thin film compliant substrates and thick reference substrates. The In0.07Ga0.93As layers were grown to thicknesses below and above the conventional critical thickness. It was found that InGaAs films grown on the compliant substrates have a larger critical thickness and slower strain relief than InGaAs grown on conventional GaAs substrates.

Critical interdependent effects have been observed in the growth of AlGaAs/InGaAs quantum-well structures by molecular beam epitaxy. It is shown that statistical experimental design is an effective method for quickly optimizing complex device structures. This technique is very useful for the optimization of processes with a large number of interdependent parameters, and allows for the clear visualization and separation of complex interwoven effects. In the present work, we show the importance of the oxide desorption process for the optimal growth of AlGaAs-containing structures.

We have observed a degradation in the epitaxial layer quality of AlInAs when it is grown by molecular beam epitaxy at a substrate temperature of about 400 °C compared to that for alloys grown at 300 and 500 °C. The barrier height and ideality factor of Ti– and Au–AlInAs Schottky diodes also exhibit large spatial variations and dependence on growth temperatures. The observed phenomena can be explained by invoking a kinetic growth model or thermodynamic phase equilibria in the growing surface layer.

The evolution of defect structures and surface reconstruction of Ga0.47In0.53As epitaxial layers grown by molecular beam epitaxy on InP substrate have been investigated by TEM and RHEED over a wide growth temperature range (150 °C≤Tg≤450 °C) before and after annealing. In the growth temperature range 400 °C≤Tg≤450 °C, extensive segregation of In near the layer surface takes place. The maximum of In concentration was found to lie under the layer surface. The kinetic of surface pit formation after annealing was explained in terms of surface reconstruction and As clustering. Extensive As clustering was observed after annealing for the growth temperature range 175 °C≤Tg≤300 °C. Analysis of diffuse scattering and dark‐field images made it possible to propose an atomic model of cluster structure. Precipitate formation close to the interface was observed only for the samples grown at 150 °C and annealed at 500 °C. Differences in electrical properties between LT‐grown GaAs and Ga0.47In0.53As are explained in term...

An increase of the Al‐ content of AlInAs layers above that of the composition which is lattice matched to InP (Al0.48In0.52As) has been shown to lead to increased Schottky barrier height [Lin et al., Appl. Phys. Lett. 49, 1593 (1986)]. This technique has been used to realize improved gate‐to‐drain breakdown voltage in AlInAs‐GaInAs modulation‐doped transistors designed for power applications. This letter reports the observation of an optimum growth temperature regime for the Al‐rich Schottky layers in the modulation‐doped structure. Growth in this regime results in the highest conductivity for modulation‐doped structures as well as the highest quality interface formation as determined from low temperature photoluminescence measurements.

Recent work has shown that Bi incorporation increases during molecular beam epitaxy (MBE) when surface processes are kinetically limited through increased growth rate. Herein we explore how the structural and optical properties of GaAs1−xBix films are modified when grown under conditions with varying degrees of kinetic limitations realized through growth temperature and growth rate changes. Within the typical window of MBE growth conditions for GaAs1−xBix, we compare films with similar (∼3%) compositions grown under conditions of reduced kinetic limitations, i.e., relatively low gallium supersaturation achieved at higher temperatures (∼350 °C) and lower growth rates (∼0.5 μm/h), to those grown farther from equilibrium, specifically, higher supersaturation achieved at lower growth temperatures (∼290 °C) and higher growth rates (∼1.4 μm/h). Both the x-ray diffraction full width at half maximum of the omega-2theta scan and the 300 K photoluminescence intensity increase when samples are grown under less kinet...

Phase shift epitaxy (PSE) is a periodic growth scheme, which desynchronizes host material growth process from dopant incorporation, allowing independent optimization. p-type doping of GaN with Mg by PSE is accomplished with molecular beam epitaxy by periodic shutter action (in order to iterate between Ga- and N-rich surface conditions) and by adjusting time delays between dopant and Ga shutters. Optimum PSE growth was obtained by turning on the Mg flux in the N-rich condition. This suppresses Mg self-compensation at high Mg concentration and produces fairly high hole concentrations (2.4 × 1018 cm−3).

A study of the relationship between strain and the incorporation of group III elements in ternary InGaN and InAlN grown by molecular beam epitaxy is reported. Using X-ray Photoelectron Spectroscopy compositional depth profiles with x-ray diffraction, we are able to find a clear relationship between strain and In incorporation including tensile-strained InAlN which has, to date, not been studied. The results show that fully strained films contain homogeneous indium composition while partially relaxed films have a non-homogeneous indium composition with depth. These results can be interpreted by considering the impurity formation energies of indium in host lattices.

The interaction of hydrogen with InN epitaxial films has been investigated by exposing InN to a remote hydrogen plasma and monitoring changes in the InN optical properties in real time via spectroscopic ellipsometry. Atomic hydrogen reacts swith InN causing nitrogen depletion with formation of indium clusters at the sample surface. After terminating the hydrogenation process, the hydrogen remains stable in the InN host lattice at T < 200 degrees C, while at higher temperatures it partially desorbs as NHx and InHx (x=1-3).

p-type InGaAs∕Si heterojunctions were fabricated through a wafer fusion bonding process. The relative band alignment between the two materials at the heterointerface was determined using current-voltage (I-V) measurements and applying thermionic emission-diffusion theory. The valence and conduction band discontinuities for the InGaAs∕Si interface were determined to be 0.48 and −0.1eV, respectively, indicating a type-II band alignment.

Liquid Ga nanoparticles have been deposited on sapphire substrates at room temperature. The optical evolution of Ga nanoparticle surface plasmon resonance during deposition has been characterized by in situ real-time spectroscopic ellipsometry to control and tune the plasmon resonance photon energy. The existence of both longitudinal and transverse modes for spheroidal Ga nanoparticles supported on a sapphire substrate is demonstrated and the dependence of the longitudinal and transverse plasmon energies on particle size is discussed. Stability of the Ga surface plasmon resonance to air exposure and high temperature is also demonstrated.

InN epitaxial films were grown by N-2 plasma-assisted molecular beam epitaxy on 4H- and 6H-SiC substrates using low-temperature InN nucleation layers. InN films grown at various In fluxes under the In-rich regime show improved crystal quality, surface morphology, and optical properties, without sizable metallic In incorporation. Photoluminescence measurements show emission up to room temperature, band gap values as low as 0.64 eV at T=10 K, and carrier concentrations of the order of 8x10(17) cm(-3).

A study of strained InAs–AlxGa1−xAsySb1−y quantum well structures produced by molecular beam epitaxy is presented. The ability to manipulate quantum well strain by way of the AlxGa1−xAsySb1−y buffer is examined using statistical experimental design. Results show that anion composition in the buffer (with a target lattice constant, a=6.12A) varies by as much as 3% in the 450–500°C growth temperature range. The data reveal interrelationships between strain, structural characteristics, and conductivity. Results demonstrate that these relationships exist and can be modeled empirically and exploited for the design of near-infrared optoelectronic devices.

The complex dielectric function of hexagonal InN has been determined in the 0.72–6.50eV photon energy range using spectroscopic ellipsometry. The InN films have been synthesized using molecular beam epitaxy on Si-face 6H-SiC(0001) substrates. The fundamental band gap E0 and higher energy interband critical points have been identified at room temperature. The impact of indium clusters and of the InN native oxide on the dielectric function is discussed.

Thin layers of AlN and GaN have been grown by plasma-assisted molecular beam epitaxy on Si-face 4H- and 6H-SiC(0001)(Si) substrates. The impact of the SiC surface preparation and oxide removal via a Ga deposition and desorption process on the chemistry and structure of the GaN/SiC and AlN/SiC interfaces, and on the GaN/SiC subsurface reactivity is characterized. We also investigate the impact of this process on growth mode evolution.

The quality of GaInAs‐AlInAs epitaxial layers is found to be critically dependent on the degree of (100)‐InP substrate misorientation. The alloy quality of both materials is improved when the substrate is misoriented 4° off the (100). The heterojunction interface quality as determined by the full width at half‐maximum of quantum‐well photoluminescence is also improved when a substrate misoriented by 4° is used. A degradation of both alloy and interface quality as compared to material on (100) InP is observed when the misorientation is 2°. These effects are also observed for strained quantum‐well structures.

The effects of substrate misorientation on the interface quality of Ga0.47In0.53As/Al0.48In0.52As quantum well structures grown by molecular beam epitaxy lattice matched on InP substrates have been investigated. Transmission electron microscopy and low‐temperature (15 K) cathodoluminescence spectra have been used to characterize the GaInAs/AlInAs layers and analyze the effects of nucleation and growth kinetics on the heterojunction interface quality. The quantum well luminescence line shape correlates with the presence of compositional fluctuations in the AlInAs layers and thickness variations of the quantum wells for structures deposited on misoriented substrates. Rapid thermal annealing of these samples improves the quantum well luminescence characteristics.

GaInAs‐AlInAs quantum well structures have been analyzed by low‐temperature photoluminescence. The photoluminescence linewidth (full width at half‐maximum) of thicker quantum wells (>10 nm) grown directly on AlInAs buffer layers shows that an inverse relationship exists between interface quality and AlInAs alloy quality in agreement with the theoretical analysis of J. Singh, S. Dudley, B. Davies, and K. K. Bajaj [J. Appl. Phys. 60, 3167 (1986)]. Thinner wells show much improved luminescence properties due to a growth of previous wells.