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The unique electronic band structure of indium nitride InN, part of the industrially significant III-N class of semiconductors, offers charge transport properties with great application potential due to its robust n-type conductivity. Here, we explore the water sensing mechanism of InN thin films. Using angle-resolved photoemission spectroscopy, core level spectroscopy, and theory, we derive the charge carrier density and electrical potential of a two-dimensional electron gas, 2DEG, at the InN surface and monitor its electronic properties upon in situ modulation of adsorbed water. An electric dipole layer formed by water molecules raises the surface potential and accumulates charge in the 2DEG, enhancing surface conductivity. Our intuitive model provides a novel route toward understanding the water sensing mechanism in InN and, more generally, for understanding sensing material systems beyond InN.

Deep-ultraviolet emitting structures based on Al0.65Ga0.35N/Al0.8Ga0.2 N multiple quantum wells (MQWs), embedded in compositionally graded AlxGa1-xN films in the form of graded-index-separate-confinement heterostructure, were grown by molecular beam epitaxy. The graded AlGaN layer blocked threading defects (TDs) in the vicinity of the AlN/AlGaN heterointerface, resulting in a reduction of the TDs in the active layer. The intensity of the transverse-magnetic polarized amplified spontaneous emission spectra with peak emission at 270 nm was found to increase linearly with the number of QWs. Furthermore, while the peak intensity for devices with a single QW varies linearly with the pump fluence, it varies superlinearly for larger number of QWs, suggesting light amplification by stimulated emission. These results are consistent with numerical simulations, which indicate that the confinement of the optical mode in this device structure increases with the number of QWs and simultaneously the threshold under optical pumping decreases with increasing of the number of QWs. Finally, the band structure simulations indicate that the device is in the form of a p-n junction due to polarization-induced p- and n-doping in the compositionally graded AlxGa1-xN films on either side of the MQWs, and, thus, could be used for the development of an electrically pumped deep-ultraviolet laser by further optimizing the structures.

AlGaN based multiple quantum wells (MQWs) were grown on 8° vicinal 4H p-SiC substrates by plasma-assisted molecular beam epitaxy. The MQWs were designed to emit near 300 nm using the wurtzite k.p model. The MQW periodicity and strain state were measured with X-ray diffraction. The optical properties were characterized with temperature dependent photoluminescence (PL). The internal quantum efficiency was estimated from the ratio of room temperature to 18K integrated PL intensity. Internal quantum efficiency up to 48% was achieved. These data are encouraging for future vertical and inverted ultraviolet light emitting diodes grown on p-SiC substrates.

This paper reviews progress in ultraviolet (UV) optoelectronic devices based on AIGaN films and their quantum wells (QWs), grown by plasma-assisted molecular beam epitaxy. A growth mode, leading to band-structure potential fluctuations and resulting in AIGaN multiple QWs with internal quantum efficiency as high as 68%, is discussed. Atomic ordering in these alloys, which is different from that observed in traditional III-V alloys, and its effect on device performance is also addressed. Finally, progress in UV-light-emitting diodes, UV lasers, UV detectors, electroabsorption modulators, and distributed Bragg reflectors is presented.

In this paper we report on the growth mode by plasmaassisted MBE of nitride films on nano-patterned sapphire substrates. Such substrates were fabricated using nano-sphere and nano-imprint lithographies, which led to cone-and cylinder-shaped patterns respectively. The data indicate that the pillars of the patterned substrate act a seeds for the subsequent growth and that the growth proceeds through the formation of a fully faceted pyramidal top surface. As the growth proceeds to more than 3 µm, the growth is dominated by the lateral growth of the top (0001) facets since the fluxes of the precursors on the inclined facets are lower. Eventually, these hexagonal (0001) facets coalesce leading to a continuous film with a smooth surface morphology. This multifaceted growth mode at the initial stages has the potential of lateral expulsion of threading dislocation by the inclined facets leading to nitride pillars with minimum number of threading dislocations. Cathodoluminescence measurements indicate significant increase in the emission intensity from the films grown on the nanopatterned sapphire substrates, compared to the emission from the films grown on the un-patterned sapphire substrates. This increase is partly due to improvements in light extraction and partly to the improved crystalline quality of the films. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The effects of 2-D hexagonal-lattice TiO 2 photonic crystal layers on the directional light extraction efficiency of low-scattering (Y 1-xCe x) 3Al 5O 12 (YAG:Ce 3+) ceramic converters have been investigated. The hexagonal-lattice TiO 2 patterns were fabricated by an electron beam lithography process. The optimized yellow directional light extraction enhancement by a factor of 4.4 is achieved with a 2-D photonic crystal layer having a diameter of 430 nm, a lattice constant of 590 nm, and a height of 350 nm. A 3-D finite-difference time-domain model has also been developed to study the geometric parameters of the photonic crystal layer, yielding results that are consistent with our measured data.

Extensive high resolution transmission and scanning transmission electron microscopy observations were performed in In(Ga)N/GaN multi-quantum well short period superlattices comprising twodimensional quantum wells (QWs) of nominal thicknesses 1, 2, and 4 monolayers (MLs) in order to obtain a correlation between their average composition, geometry, and strain. The high angle annular dark field Z-contrast observations were quantified for such layers, regarding the indium content of the QWs, and were correlated to their strain state using peak finding and geometrical phase analysis. Image simulations taking into thorough account the experimental imaging conditions were employed in order to associate the observed Z-contrast to the indium content. Energetically relaxed supercells calculated with a Tersoff empirical interatomic potential were used as the input for such simulations. We found a deviation from the tetragonal distortion prescribed by continuum elasticity for thin films, i.e., the strain in the relaxed cells was lower than expected for the case of 1 ML QWs. In all samples, the QW thickness and strain were confined in up to 2 ML with possible indium enrichment of the immediately abutting MLs. The average composition of the QWs was quantified in the form of alloy content.

AlGaN-based graded-index-separate-confinement-heterostructures (GRINSCHs) with Al0.65Ga0.35N/Al0.8Ga0.2N MQWs in their active region were grown by plasma-assisted MBE on (0001) 6H-SiC substrates. The MQWs were grown under excess Ga. Indium flux was used in addition in some of the samples in order to study the role of indium on the growth mode of such MQWs. Transmission electron microscopy (TEM) provides evidence that the bottom compositionally graded AlGaN layer in the GRINSCH structure may also be serving as a strain transition buffer layer, by blocking threading defects in the vicinity of the AlN/AlGaN heterointerface. X-ray diffraction (XRD) indicates that the interfaces in the MQWs grown in the presence of indium are sharper. However, photoluminescence (PL) studies indicate evidence of stimulated emission at relatively low threshold only in the sample grown without indium, which is attributed to deep band structure potential fluctuations in this sample. The presence of indium during growth led to more homogeneous AlGaN MQWs, which require higher excitation flux to produce population inversion.

Article history:Received 13 June 2014Accepted 5 October 2014Available online 12 October 2014Keywords:Angle resolved photoemissionAlkali-metalCharge accumulationMolecular beam epitaxyInN surface “We present angle resolved photoemission study of quantized electron accumulation subbands obtained fromboth clean and potassium deposited InN(0001) surfaces. Shifting of the quantized accumulation states towardhigherbindingenergiesuponlowenergyN 2+ ionbombardmentorasmallamountofpotassiumadsorptionisex-plainedbythemodificationoftheIn-adlayerinducedsurfacestates.N 2+ ionbombardmentleadstoahigherden-sity of donor-type surface states by creating nitrogen vacancies near the surface. On the other hand, a smallamount of K adsorbates initially donate their free electron to InN and result in more pronounced downwardbandbending.Eventually,furtherKadsorptionleadstopassivation ofsurfacestatesandreduction ofthesurfaceelectron accumulation.With theincrease of the electron density, enhancedmany-body interactionsof electronswithin the electron accumulation layer are observed.”© 2014 Elsevier B.V. All rights reserved.

We report the development of terahertz intersubband photodetectors based on GaN/AlGaN quantum wells, covering the frequency range that is fundamentally inaccessible to existing III-V semiconductor devices due to Reststrahlen absorption. Two different approaches have been employed to mitigate the deleterious effects of the intrinsic polarization fields of nitride heterostructures: the use of suitably designed double-step quantum wells, and epitaxial growth on semipolar GaN substrates. Promising results are obtained with both approaches, which could be extended to other device applications as a way to utilize the intrinsic advantages of nitride semiconductors for THz intersubband optoelectronics.

Conference on Lasers and Electro-Optics (CLEO) -- MAY 14-19, 2017 -- San Jose, CA WOS: 000427296202271 Terahertz intersubband photodetectors are developed based on GaN/AlGaN quantum wells grown on a semi-polar GaN substrate, covering the frequency range that is fundamentally inaccessible to existing III-V semiconductor devices due to Reststrahlen absorption. IEEE NSFNational Science Foundation (NSF) [ECS-0824116] This work was partly supported by NSF under Grant ECS-0824116.

Thick GaN templates (>50 µm) were grown by hydride-vapor-phase epitaxy (HVPE) on the r-plane of sapphire. While X-ray diffraction indicates that the GaN grows with its a-plane parallel to the r-plane of sapphire, the surface has a triangular morphology of 120o, which is the result of the faster growth rate of the a-plane compared to the m-plane. Electron microscopy studies indicate that these GaN templates have a high concentration of threading defects (>1010 cm–2) and basal plane stacking faults (7x105 cm–1). Several identical deep-UV emitting Al0.7Ga0.3N/AlN multiple quantum wells (MQWs) were grown on such GaN templates by molecular-beam epitaxy (MBE). The wells were grown under more gallium than required by stoichiometry, which changes the growth mode from physical vapor deposition to liquid phase epitaxy. This growth mode leads to deep band structure potential fluctuations. The structure and microstructure of these MQWs were evaluated by transmission electron microscopy (TEM) and found to have even higher density of threading defects than the GaN template. In spite of this, the internal quantum efficiency (IQE) of these MQWs was estimated from the temperature dependence of the cathodoluminescence spectra to be 87%. This remarkable result was attributed to the deep band structure potential fluctuations introduced by the liquid-phase epitaxy growth mode as well as the high concentration of basal plane stacking faults. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The electronic structures of nonpolar short-period InN/GaN superlattices grown in the wurtzite a -direction, have been calculated and compared to earlier calculations for polar superlattices (grown in the c-direction). For the nonpolar superlattices it is found that the calculated band gaps and their pressure coefficients are quite similar to those of bulk InGaN alloys with an equivalent In/Ga composition ratio. Also, they are much closer than the values calculated for polar superlattices to the photolu-minescence emission energies and their pressure coefficients measured on polar structures. Possible explanations of the observed phenomena are suggested and discussed. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper I am addressing the fundamental question as to why the performance of optoelectronic devices based on nitride semiconductors is insensitive to high concentration of extended defects, which is not the case for traditional III–V compounds. There are three important differences between these two families of semiconductors, which contribute to this finding: (a) the chemical bonds in nitrides are strongly ionic while are mostly covalent in III–V compounds. This leads to the bunching of the surface states as well as the states associated with dangling bonds in edge dislocations near the band edges, which prevents them from being non-radiative recombination centers. Furthermore, the surface states have less effect on the surface Fermi level position; (b) the nitrides can exist in the wurtzite structure (equilibrium) and the cubic structure (metastable) and the enthalpy of formation of the two allotropic forms differs only by a few meV. Thus, conversions between the two phases occur easily by creation of stacking faults along the closed packed (0001) and (111) planes. As a result basal plane stacking faults are abundant in both compounds and alloys. This leads to strong band structure potential fluctuations; (c) additional band structure potential fluctuations exist in InGaN and AlGaN alloys, due to phase separation and alloy ordering. Both types of potential fluctuations contribute to exciton localization and thus efficient radiative recombination even at room temperature.

We demonstrate electronic sequential tunneling transport through the ground-state subbands of GaN/Al0.15Ga0.85N quantum cascade structures grown on free-standing GaN substrates. A pronounced transition from a high- to a low-resistance state is observed as the applied voltage is increased. The measured current-voltage characteristics are found to vary with temperature, structure design, and polarity of the applied bias, in a way that is fully consistent with theoretical expectations. The dominant transport mechanism is determined to be scattering-assisted tunneling. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper we present studies and compare the performance of UV electroabsorption modulators based on bulk GaN films and GaN/AlGaN multiple quantum wells. In both types of devices, the absorption edge at room temperature is dominated by excitonic effects and can be strongly modified through the application of an external electric field. We find that when similar active layer thicknesses and device geometries are employed, the electroabsorption modulators based on bulk nitride films can provide similar performance levels as those of GaN/AlGaN multiple quantum well devices. This should be compared to the case of arsenide, phosphide, or antimonide semiconductors, where the electroabsorption effect in quantum wells is an order of magnitude stronger than in the bulk. This is primarily due to the strong ionic character of the nitride semiconductors, which lead to very large exciton binding energies at room temperature (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report the development of efficient AlGaN-based deep ultraviolet (UV) LEDs emitting in the wavelength range from 320 nm to 265 nm by plasma-assisted molecular beam epitaxy (MBE). By growing the AlGaN well layers under Ga-rich conditions to produce strong band structure potential fluctuations, the internal quantum efficiency (IQE) of the quantum wells, which are used as the active region of the UV LEDs, was found to be as high as 32%. Unpackaged deep UV-LEDs emitting at 273 nm were found to have an optical output power of 0.35 mW and 1.3 mW under DC and AC injection current of 20 mA and 100 mA respectively. The maximum external quantum efficiency under DC injection was measured to be 0.4%. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We propose that the growth of III-Nitride semiconductors by plasma-assisted MBE under Ga-rich conditions takes place though the saturation of the liquid Ga covering the surface of the growing film with nitrogen, alloy constituents (aluminum and indium) and impurities. Thus, the proposed growth mode is a liquid phase epitaxy (LPE) rather than physical vapor phase epitaxy. While in traditional LPE growth the driving force is the gradient between the liquid and seed, in the proposed mode of MBE growth of nitrides the driving force is the concentration gradient due to the constant supply of active nitrogen, alloy components and impurities. The proposed LPE growth mechanism explains the relative temperature stability of GaN growth, the efficient doping with silicon and magnesium, and the selective desorption of oxygen and other impurities. Finally, the proposed LPE growth model can also account for the introduction of band structure potential fluctuations in AlGaN alloys, which improves their emission properties and leads to efficient deep UV-LEDs. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The plasmonic excitations of square-periodic arrays of silver nanocylinders are used to increase the radiative efficiency of nearby InGaN/GaN quantum wells emitting at near-green wavelengths. These nanostructures allow for an accurate control of the plasmonic resonance wavelength, scattering-absorption ratio, and electromagnetic near-field enhancement via geometrical tuning. In turn, these properties are found to play a critical role in the measured light-emission efficiency enhancements.

In this paper we propose that under Ga-rich conditions the growth of III-Nitride semiconductors by plasma-assisted MBE takes place though saturation of the metallic Ga covering the surface of the growing film with nitrogen, alloy constituents and impurities. According to this model the growth process is a liquid phase epitaxy (LPE) rather than vapor phase epitaxy. Experimental evidence is presented in support of this model. Such include, for example, that the growth rate of GaN does not decrease at temperatures in excess to 800 ºC, a result attributed to higher solubility of active nitrogen in Ga at higher temperatures. Also the metallic Ga in the surface of the film controls the incorporation of impurities and is responsible for the introduction of compositional inhomogeneities and thus band structure potential fluctuations in AlGaN due to lateral statistical fluctuations of the thickness of Ga in the surface.

Due to their large conduction-band offsets of up to 1.75 eV, GaN/Al(Ga)N quantum wells can support intersubband transitions at record short wavelengths, well into the low-loss transmission window of optical fibers. As a result, these quantum structures are promising to enable new intersubband device applications, including all-optical switching for ultra-broadband fiber-optic networks and emission of short-wave infrared radiation. Here we report our work on the development of high-quality GaN/Al(Ga)N quantum well structures designed for such applications. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper we discuss the growth of InN, GaN and InGaN QDs by MBE on either GaN or AIN templates. InN QDs on GaN templates were found to occur without an InN wetting layer, a result consistent with the large lattice mismatch of 11% between InN and GaN. Self-assembled GaN QDs were grown on AIN templates, using the modified Stranski-Krastanov mode of growth. The microstructure and the size distribution of such QDs in a single layer or a superlattice structure were investigated by electron microscopy and atomic force microscopy. Finally, the self assembly of InGaN QDs on GaN templates using the Stranski-Krastanov mode and the applications of such QDs to blue-green LEDs are addressed. The results indicate that InGaN / GaN multiple quantum dots (MQDs) are highly strained and their emission at low injection is red shifted with respect to that of a single layer of QDs due to quantum confined Stark effect.

Thick GaN films, with (1 1 2 0) or (1 1 2 6) planes parallel to the r -plane of sapphire, were grown by molecular beam epitaxy using AlN or GaN buffer layers. Characterization by transmission electron microscopy revealed a high density of basal-plane stacking faults (BSFs) in the (1 1 2 0) non-polar GaN ( a -GaN) films. {1 1 2 0} and {1 0 1 0} prismatic-plane and {1 1 0 2} pyramidal-plane stacking faults (SFs) were observed to terminate some BSFs. The SFs on {1 0 1 0} and {1 1 0 2} planes generally formed closed domains. For (1 1 2 6) semi-polar GaN ( s -GaN) films, most threading dislocations were located at small-angle grain boundaries. Many BSFs were observed close to the AlN/GaN interface but, in comparison with a -GaN, the s -GaN films had much lower BSF density at the top surface. Inversion domain boundaries (IDBs) on {1 0 1 0} planes were observed to form closed domains. GaN/AlGaN multiple quantum wells (MQWs) grown on s -GaN or a -GaN followed the morphology of the GaN surface. Some IDBs in the s -GaN propagated through the GaN/AlGaN MQWs to the top surface.

Near-ultraviolet light emitting diodes (LEDs), based on one monolayer thick InN/GaN multiple quantum wells, were grown by radio-frequency plasma-assisted molecular beam epitaxy. The active region was grown at 685 °C, a temperature where the growth of thick InN layers is not possible. High resolution transmission electron microscopy revealed that the InN well layers were grown pseudomorphically in between the GaN barriers and were characterized by well defined interfaces and uniform thickness. The LED structures exhibited electroluminescence emission at 384 nm, with a FWHM of about 14 nm. The peak emission wavelength was independent of the injection current, indicating the absence of quantum confined Stark effect in these structures. Output optical power of 0.84 mW was measured at 200 mA in a bare die configuration. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper, we present detailed studies of growth of AlN and GaN films on R-plane sapphire substrates. Contrary to previously reported work that GaN grows with its A-plane (110) parallel to the R-plane (102) of sapphire, our studies indicate that the crystallographic orientation of the III-nitride films grown on such substrates depends strongly on the kinetic conditions of growth of the AlN buffer. Specifically, the group III-rich conditions of growth of AlN buffer lead to nitride films having the (110) planes parallel to the sapphire surface, while growth of the buffer under N rich conditions leads to nitride films with the (116) planes parallel to the sapphire surface. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This paper addresses the formation of freestanding GaN substrates by a natural separation mechanism, effectively eliminating the need for post-growth processes such as laser liftoff, chemical etching or mechanical lapping to form freestanding GaN substrates. A number of GaN thick films were grown onto sapphire substrates by the hydride vapor-phase epitaxy (HVPE) method with thickness varying from 200 μm to 3.8 mm using either a low-temperature GaN or an AlN buffer as the nucleation step. We have found that samples grown on a low temperature GaN buffer naturally delaminate from the sapphire substrate post-growth over the entire thickness range studied. Furthermore, we have observed that the thinner films have high crack densities leading to the delamination of several smaller freestanding pieces. As the GaN thickness increases, the area of the delaminated pieces also increases, ultimately leading to a 1-to-1 correlation between initial sapphire substrate area and freestanding GaN area. However, the GaN films grown on AlN buffers did not delaminate. These results were accounted for by calculating the thermal stresses in the GaN film and substrate as a function of film thickness using Stoney's equation and assuming that the GaN buffer undergoes decomposition at the growth temperature.

Resonance effects at the Ga 3p photoabsorption threshold have been observed in photoemission spectra recorded from thin film InxGa1-xN alloys. The spectra display satellites of the main Ga 3d emission line, and the intensity of these satellites resonate at this threshold. The satellites are associated with a 3d(8) state, and have previously been observed for the semiconductors GaN, GaAs, and GaP. The resonance behavior has been studied for a variety of InxGa1-x thin films with differing In concentration and band gap. The photon energy where the maximum resonance is observed varies with band gap within the alloy system, but does not follow the trend observed for binary Ga semiconducting compounds. We also observe that the threshold resonant energy increases slightly as the In content increases. (c) 2006 Elsevier B.V. All rights reserved. (Less)

This paper describes the growth, structure, transport and optical properties of InN films grown by MBE using either nitrogen radicals (N* 2 ) produced by a RF plasma source or clusters containing on the average 2000 nitrogen molecules (N 2 ) 2000 . The InN films were grown at temperatures between 300 and 600 °C on (000 1) sapphire substrates using either an InN buffer or a GaN template. It was found that the conversion of the surface of the sapphire from Al 2 O 3 to AlN, by exposing it to active nitrogen, is essential for the growth of single-crystalline InN films. Thick films (2-3 μm), produced by the plasma source, tend to delaminate from the substrate presumably due to extreme compressive stresses. On the other hand, films adhere better if nitridation of the sapphire substrate and the low-temperature InN buffer are grown by the cluster source. All films are auto-doped n-type with carrier concentration higher than 3 x 10 18 cm -3 and best room temperature mobility 1130 cm 2 /V s. The energy gap of InN was determined to be 0.77eV. The plasma frequency was measured by infrared reflectivity and the data were used to determine the electron effective mass (0.09m 0 ). We found that the measured optical gap and the electron effective mass are in qualitative agreement with the predictions of the k p method for direct semiconductors.

The surface electronic structure of thin film n-type GaN( 0 0 0 1 ¯ ) has been studied using high resolution angle resolved photoemission spectroscopy. Clean surfaces, sulfur-exposed surfaces, and oxygen-exposed surfaces were investigated. Spectra were recorded for the Ga 3d shallow core states and for the valence band states. The sulfur covered surface was found to be inert with respect to subsequent oxygen exposure. For the clean Ga-adlayer terminated ( 0 0 0 1 ¯ ) surface, the Fermi level is pinned by surface states at least 1.4 eV below the bottom of the conduction band. Sulfur and oxygen covered surfaces exhibit smaller values for surface band bending, with the Fermi edge lying 0.7–1.0 eV below the conduction band minimum. Finally, we discuss a possible new interpretation for the asymmetry in the shape of Ga 3d photoemission feature.

AlN/GaN multiple quantum wells were grown by RF plasma-assisted molecular beam epitaxy on (0 0 0 1) sapphire substrates. Intersubband (ISB) absorption in the range of 1.5–1.9mm was investigated in these structures as a function of the main quantum well design parameters. It is found that the ISB transition line width does not depend strongly on the degree of strain relaxation in the investigated structures, but rather is sensitive to both the well and barrier widths. This is explained by considering well/barrier thickness fluctuations. A reduction in the ISB absorption line width was

In this paper we report the growth and Silicon doping of AlGaN films in the entire alloy composition by RF-plasma-assisted MBE. The films (0.7 to 0.8 µm thick) were grown with the (0001) polarity on C-plane sapphire substrates using a thin AlN buffer and were doped n-type with the silicon cell at a constant temperature (1200 °C). The composition and the structure of the films were determined by XRD, the silicon and oxygen impurities were determined by SIMS measurements, and the transport coefficients were obtained from Hall effect measurements. The FWHM of the on- and off-axis XRD rocking curves indicate that the microstructure of the films is better at low AlN mole fraction ( 70%). The resistivity of the films varies from 10–3 to 25 Ω.cm as the AlN mole fraction increases from 0 to 96%. Reliable Hall effect measurements were carried out in films with up to 77% Al and were found to be degenerately n-type doped with room temperature carrier concentration varying from 1 × 1020 to 7.7 × 1018 cm–3 as the AlN mole fraction increased from 0 to 77%. Correspondingly, the electron mobility is reduced by almost an order of magnitude, a result which is consistent with alloy scattering. The effective mass of some of the samples was determined by combining infrared reflectivity and Hall Effect measurements and these data were used to determine the Si ionization energies. We concluded that the Si is the main donor in the entire alloy composition and that the gradual reduction of conductivity with Al content is due to the increase of the Si ionization energy. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoemission spectra recorded near the Ga 3p photothreshold from GaN have been found to contain satellites of the main Ga 3d emission line. The intensity of these satellites resonate at this threshold, and are associated with a 3d(8) state. The correlation energies and binding energies for the satellite multiplet have been measured for the satellite and related Auger transitions. The satellite multiplet contains additional constant binding energy features not observed in previous studies of other Ga compounds. The present results are compared with those for Gal? and GaAs.

We report on the investigation of excitons in Al0.2Ga0.8N/GaN multiple quantum wells (MQWs) by lateral photocurrent, and photoluminescence (PL) spectroscopies over the temperature range from 9 to 300 K. The MQWs were deposited homoepitaxially by plasma-assisted molecular-beam epitaxy on a (0001) GaN template grown by hydride vapor phase epitaxy. Excitonic peaks in the photocurrent spectra due to the bulk GaN template and the MQW structure were observed up to room temperature. The PL excitonic peak in the MQWs was strongly Stokes shifted over the whole temperature range, a result attributed to recombination via disorder-induced excitonic band-tail states in the MQWs, due primarily to well/barrier interface roughness fluctuations. A theoretical calculation estimates these fluctuations to be 0.7±0.2 monolayers. The temperature dependence of the Stokes shift indicates that, at higher temperatures, the excitons in the PL experiments are in thermal equilibrium with the lattice before recombining. At lower tempe...

The effects of nitrogen ion sputtering and thermal anneal processing on the surface electronic structure of the ternary III–V semiconductor In0.12Ga0.88N have been studied using scanning photoemission microscopy. No phase separation of the material is observed for annealing temperatures up to 650 °C. However, samples annealed at 700 °C for 5 h show clear evidence of phase separation. Furthermore, annealing at these temperatures with the sample surface directly exposed to ultrahigh vacuum produces a surface greatly deficient in In and with considerable surface roughness. This can be circumvented by using a sacrificial sample in physical contact with the film to artificially increase the local vapor pressure of Ga, In, and N during annealing.

Atomic ordering in AlGaN films produced by plasma assisted molecular beam epitaxy (PA-MBE) was investigated with X-ray diffraction (XRD) and transmission electron microscopy selected area diffraction (TEM-SAD). Under nitrogen-rich growth conditions, films undergo 1×1 monolayer cation ordering along the [0001] growth direction. This type of ordering is stronger in N-polar films than in Ga-polar films and is sensitive to the growth ratio of group-III to group-V flux. These observations are in qualitative agreement with the first principal calculations by Northrup and co-workers. Under group-III rich growth conditions of Ga-polar films, the XRD data is consistent with the quasi-random stacking of spontaneously formed 14-ML and 12-ML superlattice structures. Reciprocal space mapping shows a small tilt (0.25°) between the crystallographic c-axis of the wurtzite structure and the superlattice axis. This result can be explained by the presence of c-axis oriented superlattice domains with a parallel step translation between neighboring domains. This result is in qualitative agreement with a recently proposed model by Venezuela and co-workers of self-organized superlattice formation during step flow growth of strained alloys on vicinal surfaces.

Structural and optical properties of GaN/AlGaN multiple quantum wells (MQWs) grown by plasma-assisted molecular-beam epitaxy on ( 1 1 0 0 ) plane free-standing GaN substrates and (0 0 0 1) GaN quasi-substrates have been compared. Atomic force microscopy studies indicate that the films and MQW structures grown on both substrates replicate the surface morphology of the substrates. MQWs with AlGaN barriers grown in the presence of In flux have stronger photoluminescence (PL) intensity than those with AlGaN barriers without In. X-ray diffraction spectra of MQWs grown on the (0 0 0 1) GaN substrates show larger number of superlattices peaks than those grown on ( 1 1 0 0 ) substrates suggesting that the former have smoother interfaces. The PL spectra of MQWs deposited on (0 0 0 1) GaN substrates, where the growth is in a polar direction, exhibit a red-shift as well as a decrease in peak intensity with increase in well widths. Similar MQW structures on the ( 1 1 0 0 ) GaN, on which the growth is in a non-polar direction, do not exhibit this phenomenon, which we attribute to the absence of internal electric fields in these structures. PL intensity of MQWs with a well width of 75 A is 20 times stronger for those grown on the ( 1 1 0 0 ) plane than on the (0 0 0 1) plane GaN substrate.

A structural assessment of various interfaces formed between successive GaN/AlN layers epitaxially grown on (0001) sapphire, as well as between n-type and p-type GaN is presented, using transmission and high-resolution electron microscopy. The structure of the interfaces between the thick GaN and the thin AlN interlayers (ILs) is investigated in terms of misfit difference and elastic fit of the two crystal lattices. Dense threading dislocations are observed to originate from the substrate/buffer layer interface and their interaction with the successive AlN ILs is studied. Furthermore, basal inversion domain boundaries localized in the n-GaN/p-GaN interface are evaluated.

This paper reviews the device physics and technology of optoelectronic devices based on semiconductors of the GaN family, operating in the spectral regions from deep UV to Terahertz. Such devices include LEDs, lasers, detectors, electroabsorption modulators and devices based on intersubband transitions in AlGaN quantum wells (QWs). After a brief history of the development of the field, we describe how the unique crystal structure, chemical bonding, and resulting spontaneous and piezoelectric polarizations in heterostructures affect the design, fabrication and performance of devices based on these materials. The heteroepitaxial growth and the formation and role of extended defects are addressed. The role of the chemical bonding in the formation of metallic contacts to this class of materials is also addressed. A detailed discussion is then presented on potential origins of the high performance of blue LEDs and poorer performance of green LEDs (green gap), as well as of the efficiency reduction of both blue and green LEDs at high injection current (efficiency droop). The relatively poor performance of deep-UV LEDs based on AlGaN alloys and methods to address the materials issues responsible are similarly addressed. Other devices whose state-of-the-art performance and materials-related issues are reviewed include violet-blue lasers, 'visible blind' and 'solar blind' detectors based on photoconductive and photovoltaic designs, and electroabsorption modulators based on bulk GaN or GaN/AlGaN QWs. Finally, we describe the basic physics of intersubband transitions in AlGaN QWs, and their applications to near-infrared and terahertz devices.

We present a detailed structural investigation of self-assembled indium gallium nitride nanodots grown on c-plane aluminum nitride templates by the droplet heteroepitaxy technique in a plasma-assisted molecular beam epitaxy reactor. Various growth parameters, including the total coverage of the metal species, relative and total metal effusion fluxes, and nitridation temperature were investigated. Analyses of in situ reflection high-energy electron diffraction patterns and comparison with simulations showed that the resulting crystal structure was a mixture of wurtzite and twinned zinc blende phases, with the zinc blende phase increasingly dominant for lower metal coverages and lower nitridation temperatures, and the wurtzite phase increasingly dominant for higher nitridation temperature. Studies by field emission scanning electron microscopy and atomic force microscopy revealed that the nanodots exhibit trimodal size distributions, with the dot morphologies of the intermediate size mode often resembling a...

Jacques was born in Chernihiv, Ukraine, on 23 November 1922. A year later he and his family immigrated to Constantinople, and the following year they moved to Marseilles, France. After the Nazis oc ...

We report in this letter experimental results that confirm the two-dimensional nature of the electron systems in a superlattice structure of 40 InN quantum wells consisting of one monolayer of InN embedded between 10 nm GaN barriers. The electron density and mobility of the two-dimensional electron system (2DES) in these InN quantum wells are 51015cm2 (or 1.251014cm2 per InN quantum well, assuming all the quantum wells are connected by diffused indium contacts) and 420 cm2/Vs, respectively. Moreover, the diagonal resistance of the 2DES shows virtually no temperature dependence in a wide temperature range, indicating the topological nature of the 2DES.

Aluminum gallium nitride alloys were grown by molecular beam epitaxy and their film composition, structure and microstructure were investigated by Rutherford backscattering spectroscopy, Atomic Force Microscopy, X-ray diffraction, Transmission Electron Microscopy and High Resolution Electron Microscopy. The kinetics of growth was investigated and the result show that at the growth temperature of 750 °C the sticking coefficient of Ga varies monotonically from practically 0 to 1 as the growth varies from group-III to -V rich regime. Correspondingly, the surface morphology changes from atomically smooth in the group-III regime to relatively rough in the nitrogen-rich regime. The X-ray diffraction and TEM studies revealed the existence of three types of spontaneously formed superlattice structures, along the [0001] direction, with periodicities of 2-, 7- and 12-monolayers. While the two-monolayer ordering is preferred under group-V rich conditions of growth, the 7- and 12-monolayer ordering were observed under group-III rich conditions of growth. Off-axis X-ray diffraction shows the absence of in-plane ordering and that periodic stacking faults are not present in the superlattice structures.

It is demonstrated that the NEXAFS spectra are a fingerprint of the symmetry and the composition of the binary nitrides GaN, AlN and InN, as well as of their ternary alloys In 0.16 Ga 0.84 N and Al y Ga 1-y N. From the angular dependence of the N-K-edge NEXAFS spectra, the hexagonal symmetry of the under study compounds is deduced and the (p x , p y ) or p z character of the final state is identified. The energy position of the absorption edge (E abs ) of the binary compounds GaN, AlN and InN is found to red-shift linearly with the atomic number of the cation. The E abs of the Al y Ga 1-y N alloys takes values in between those corresponding to the parent compounds AlN and GaN. Contrary to that, the E abs of In 0.16 Ga 0.84 N is red-shifted relative to that of GaN and InN, probably due to ordering and/or phase separation phenomena. The EXAFS analysis results reveal that the first nearest-neighbour shell around the N atom, which consists of Ga atoms, is distorted in both GaN and Al x Ga 1-x N for x < 0.5.

We report the development of AlGaN-based deep ultraviolet LEDs by plasma-assisted molecular beam epitaxy. The devices were evaluated under bare-die configuration, placed outside an integrated sphere, and were found to have optical power of 1.3 mW and 1.2 mW at 300 nm and 277 nm, under pulsed driving current of 200 mA and 500 mA, respectively. The external quantum efficiency (EQE) of the 300 nm LED is 0.16% at drive current of 90 mA. These data suggest that AlGaN alloys grown by MBE under excess Ga are capable of producing deep UV-LEDs with high IQE. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The surface electronic structure of p-type wurtzite GaN(0001) 1×1 grown by molecular beam epitaxy has been investigated using synchrotron radiation excited angle resolved photoemission spectroscopy. Four discrete surface states were clearly identified and characterized. All the states were removed by exposure of the surface to atomic hydrogen. Three of the states were of pz orbital character, while the fourth is derived from s orbitals. Three of the surface states lie below the bulk valence band maximum throughout the surface Brillouin zone, and one surface state was observed to disperse into the bulk optical band gap. Comparison with theory suggests that this surface state is Ga-derived, consistent with a model of Ga-terminated, but N polar, GaN. The other three surface states compare well with states observed earlier from n-type GaN.

In this paper we report on the fabrication and characterization of GaN diodes (Schottky and p-n junctions) grown by plasma assisted MBE. We observed that Schottky diodes improve both in reverse as well as forward bias when deposited on 5 μm thick HVPE n+-GaN/sapphire instead of bare sapphire substrates. These improvements are attributed to the reduction of disloctions in the MBE homoepitaxially grown GaN. Similar benefits are observed in the reverse bias of the p-n junctions which according to EBIC measurements are attributed to the reduction of etch pits in the MBE grown p-GaN.

Lateral and vertical electron transport parameters were investigated in lightly doped n-GaN films, grown by MBE. Diodes were fabricated by forming Schottky barriers on n−-GaN films using a mesa-etched vertical geometry. Doping concentrations and barrier heights were determined, from C-V measurements, to be 8-9×1016 cm−3 and 0.95-1.0 eV respectively. Reverse saturation current densities were measured to be in the 1-10×10−9 A/cm2 range. Using the diffusion theory of Schottky barriers, vertical mobility values were determined to be 950 cm2/V-s. Lateral mobility in films grown under similar conditions was determined by Hall effect measurements to be 150-200 cm2/V-s. The significant increase in mobility for vertical transport is attributed to reduction in electron scattering by charged dislocations.