Density Of Inversion Domains Research Articles

Определение толщины зародышевого слоя AlN, сформированного на поверхности Al-=SUB=-2-=/SUB=-O-=SUB=-3-=/SUB=-(0001) в процессе нитридизации, методами РФЭС и ИК-спектроскопии

The effect of different degrees of the sapphire surface nitridation process completion on the AlN buffer layer morphology has been studied. It was found that ~85% completion of the AlN crystalline phase formation promotes the growth of a two dimensional AlN buffer layer with a smooth surface morphology, regardless of the substrate temperature and ammonia flux. In contrast, during the AlN nucleation layer formation as a result of weak or excessive sapphire nitridation, a polycrystalline or three-dimensional AlN structures with a high density of inversion domains, respectively, were formed. Using independent methods of X-ray photoelectron spectroscopy and infrared spectroscopy of surface polaritons, the thickness of the AlN nucleation layer was determined at ~85% degree of the nitridation process completion, which amounted to ~1 monolayer.

Determination of the AlN nucleation layer thickness formed on the Al-=SUB=-2-=/SUB=-O-=SUB=-3-=/SUB=-(0001) surface during nitridation process by XPS and IR spectroscopy

The effect of different degrees of the sapphire surface nitridation process completion on the AlN buffer layer morphology has been studied. It was found that ~85% completion of the AlN crystalline phase formation promotes the growth of a two dimensional AlN buffer layer with a smooth surface morphology, regardless of the substrate temperature and ammonia flux. In contrast, during the AlN nucleation layer formation as a result of weak or excessive sapphire nitridation, a polycrystalline or three-dimensional AlN structures with a high density of inversion domains, respectively, were formed. Using independent methods of X-ray photoelectron spectroscopy and infrared spectroscopy of surface polaritons, the thickness of the AlN nucleation layer was determined at ~85% degree of the nitridation process completion, which amounted to ~1 monolayer. Keywords: ammonia molecular beam epitaxy, AlN, sapphire, reflection high-energy electron diffraction, nitridation, inversion domains, X-ray photoelectron spectroscopy, surface polaritons.

(Invited) Unidirectional Epitaxy of TMD Monolayers on Sapphire

Wafer-scale synthesis of semiconducting transition metal dichalcogenide (TMDs) monolayers is of significant interest for device applications to circumvent size limitations associated with the use of exfoliated flakes. Promising results have been demonstrated for epitaxial films deposited by vapor phase techniques such as CVD and MOCVD. However, the three-fold symmetry of TMDs such as MoS2 and WSe2, results in two energetically equivalent domain alignments, often referred to as 0o and 60o domains, when grown on substrates such as c-plane sapphire and graphene. The oppositely oriented domains give rise to inversion domain boundaries (IDBs) upon coalescence which exhibit a metallic character and are generally undesirable. In this study, we demonstrate the epitaxial growth of unidirectional TMD monolayers on 2” diameter c-plane sapphire substrates with a significantly reduced density of inversion domains. Steps on the sapphire surface are shown to break the surface symmetry giving rise to a preferred domain orientation.Metalorganic chemical vapor deposition (MOCVD) was used for the epitaxial growth of WSe2 and WS2 monolayers on c-plane sapphire in a cold-wall horizontal quartz-tube reactor. The as-received sapphire substrates, which are miscut ~0.2o toward <112 ̅0>, consist of steps with sub-1 nm step height separated by 50-70 nm wide terraces. A three-step nucleation-ripening-lateral growth process, carried out at temperatures ranging from 850oC to 1000oC, was used to achieve epitaxial films using W(CO)6, H2Se and H2S as precursors in a H2 carrier gas. Nucleation was observed to occur at the terrace edge and the growing domains align epitaxially with the underlying (0001) sapphire lattice. As a result of the nucleation process, the domains grow with a zig-zag edge facing the terrace edge which imparts a preferential direction to the domains. The percentage of domains with a preferred direction ranges from 75%-86% depending on MOCVD growth conditions. Continued lateral growth for times ranging from 10-30 minutes results in fully coalesced TMD monolayers that are epitaxially oriented on the sapphire, as assessed by in-plane x-ray diffraction, with a reduced density of inversion domain boundaries. The results demonstrate the important role of surface structure in nucleation and epitaxial growth of TMD monolayers.

High quality AlN for deep UV photodetectors

We have prepared large-area, 0.50×0.55mm2, metal-semiconductor-metal photodetectors based on AlN layers with different density of inversion domains (IDs). AlN layers were grown on (0001) sapphire substrates using gas source molecular beam epitaxy. The introduction of AlN∕GaN short period superlattices after growth of AlN nucleation layer yields significant reduction in the ID density. Photodetectors with ID density of 106cm−2 exhibit a very low dark current of 0.5fA at zero bias, which remains below 50fA up to a bias of ±30V. The peak responsivity of 0.08A∕W was obtained at a wavelength of ∼202nm.

Electron microscopy investigation of extended defects in a‐plane gallium nitride layers grown on r‐plane sapphire by molecular beam epitaxy

AbstractThe interfacial structure and extended defects in a‐plane (1120)‐oriented GaN layers are investigated using conventional and high resolution transmission electron microscopy (HRTEM). The a‐plane GaN epilayers have been grown by nitrogen rf plasma source molecular beam epitaxy (RFMBE) on r‐plane sapphire substrates, which have been nitrided either at 200 °C or at 800 °C for one hour. A thin interfacial layer of strained AlN, with an average thickness of about 1 nm, was formed during nitridation of the sapphire surface at both temperatures. The extended defects observed in the GaN layers are predominantly threading dislocations and basal stacking faults (BSFs), which originate from the GaN/sapphire interface. The threading dislocation density is slightly reduced in the film nitrided at 800 °C, as is the density of inversion domains, although stacking fault density does not appear to change with nitridation conditions. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Epitaxial condition and polarity in GaN grown on a HfN-buffered Si(111) wafer

Single-crystal GaN thin films have been deposited epitaxially on a HfN-buffered Si(111) substrates by molecular-beam epitaxy. The microstructural and compositional characteristics of the films were studied in detail by transmission electron microscopy (TEMs). Cross-sectional TEM investigations have revealed the crystallographic orientation relationship in different GaN∕HfN∕Si layers. GaN film polarity is studied by conventional TEM and convergent beam electron diffraction simulations, and the results show that the GaN film has a Ga polarity with relatively high density of inversion domains. Based on our observations, growth mechanisms related to the structural properties are discussed.

Comparative study of Ga- and N-polar GaN films grown on sapphire substrates by molecular beam epitaxy

We report the surface, structural, and optical properties of typical Ga- and N-polar GaN films grown on sapphire substrates by molecular beam epitaxy. The Ga-polar films were grown on AlN buffer while the N-polar films were grown on GaN buffer layers. Atomic force microscopy imaging shows that the as-grown and chemically etched Ga-polar films have a flat and pitted surface while the N-polar surface is rougher with isolated columns or islands. Transmission electron microscopy demonstrates a low density of inversion domains in the Ga-polar films, while a much higher density of inversion domains was observed in the N-polar films. X-ray diffraction curves show a narrower (002) peak for Ga-polar films than that for N-polar films. On the other hand, both Ga- and N-polar films show a similar width of (104) peak. Despite their rough surfaces, high density of inversion domains, and broader (002) x-ray diffraction peaks, N-polar films with low dislocation density were demonstrated. In addition, higher PL efficiency for the N-polar films than that for the Ga-polar films was observed.

Structure Analysis of GaN Thin Film with Inversion Domains by High Voltage Atomic Resolution Microscopy

The atomic structures and surface morphologies of three types of GaN films were investigated by high voltage atomic resolution microscopy (HVARM). By HVARM, each atomic column of Ga and N could clearly be resolved and the polarity of the film and inversion domains could be directly determined. The GaN film was grown on a sapphire substrate by molecular beam epitaxy (MBE) after nitridation of the sapphire surface. Inversion domains (IDs) crossed the whole film to the surface and made small pyramids on the surface. The small pyramids had Ga-polarity and the rest had N-polarity. A GaN film with In exposure during film growth had an almost Ga-polarity flat surface. In exposure process reduced the density of inversion domains that have a N-polarity. While a GaN film grown on an AlN buffer layer was unipolar, with a Ga-polarity. HVARM observation revealed that the density of the IDs determine the qualities and the polarity of the film.

Polarity of GaN Grown on Sapphire by Molecular Beam Epitaxy with Different Buffer Layers

We report on polarity of GaN films grown on sapphire substrates by molecular beam epitaxy using different buffer layers and growth conditions. On high temperature AlN or GaN buffer layers, the GaN films typically show Ga or N-polarity, respectively. When low temperature (either AlN or GaN) buffer layers are employed, GaN films of both polarities can be grown, but these films have high density of inversion domains. Insertion of additional GaN/AlN quantum dot layers between the buffer layers and the GaN films provides strain relief and a significant improvement in the quality of the GaN epilayers.

Polarity of GaN Grown on Sapphire by Molecular Beam Epitaxy with Different Buffer Layers

We report on polarity of GaN films grown on sapphire substrates by molecular beam epitaxy using different buffer layers and growth conditions. On high temperature AlN or GaN buffer layers, the GaN films typically show Ga or N-polarity, respectively. When low temperature (either AlN or GaN) buffer layers are employed, GaN films of both polarities can be grown, but these films have high density of inversion domains. Insertion of additional GaN/AlN quantum dot layers between the buffer layers and the GaN films provides strain relief and a significant improvement in the quality of the GaN epilayers.

Optical properties of GaN with Ga and N polarity

AbstractWe compared photoluminescence (PL) and cross-sectional transmission electron microscopy (TEM) characteristics of GaN samples with Ga and N polarities grown by molecular beam epitaxy (MBE) on sapphire substrates. Ga-polar films grown at low temperature typically have very smooth surfaces, which are extremely difficult to etch with acids or bases. In contrast, the N-polar films have rougher surfaces and can be easily etched in hot H3PO4 or KOH. The quality of the X-ray diffraction spectra is also much better in case of Ga-polar films. Surprisingly, PL efficiency is always much higher in the N-polar GaN, yet the features and shape of the PL spectra are comparable for both polarities. We concluded that, despite the excellent quality of the surface, MBE-grown Ga-polar GaN layers contain higher concentration of nonradiative defects. From the analyses of cross-sectional TEM investigations, we have found that Ga-polar films have high density of threading dislocations (5x109 cm-2) and low density of inversion domains (1x107 cm-2). For N-polar GaN the situation is the reverse: the density of dislocations and inversion domains are 5x108 and ~1x1011 cm-2, respectively. One of the important conclusions derived from the combined PL and TEM study is that inversion domains do not seem to affect the radiative efficiency very adversly, whereas dislocations reduce it significantly.

Comparative Study of Typical Defects in III-Nitride Thin Films and Their Alloys

AbstractThe microstructure and typical defects in GaN and GaN/GaAlN thin film heterostructures grown on (0001)α-A12O3 were investigated using different transmission electron microscopy (TEM) techniques including diffraction contrast analysis, multiple dark field imaging, and highresolution TEM. The films were grown by metal-organic chemical vapor deposition (MOCVD) technique. All of the films exhibited good electrical/optical properties. Yet, films were found to be of two distinctive types in terms of the microstructure. Films of the first type (A) were found to contain high, up to the 109 cm2, density of inversion domains (IDs) as well as pure edge (b=l/3[11 20]), screw and mixed type dislocations with the average density of 109 - 1010 cm−2. Smoother surface of the film, absence of IDs, and low (down to 107 cm−2 in the device quality layers) density of screw and mixed type dislocations were found to be characteristic for the second type (B) microstructure. The majority of defects present in these B-type GaN and GaN/GaA1N thin films were found to be threading pure edge dislocations associated with low angle tilt sub-grain boundaries. Despite the 1010 cm−2 density of the edge dislocations, the films displayed the devicequality electrical characteristics. Type A and type B microstructure can be obtained by the variation of growth conditions. The correlation between the optical and structural properties are discussed.

Defect structure of metal-organic chemical vapor deposition-grown epitaxial (0001) GaN/Al2O3

Defect structures were investigated by transmission electron microscopy for GaN/Al2O3 (0001) epilayers grown by metal-organic chemical vapor deposition using a two-step process. The defect structures, including threading dislocations, partial dislocation bounding stacking faults, and inversion domains, were analyzed by diffraction contrast, high-resolution imaging, and convergent beam diffraction. GaN film growth was initiated at 600 °C with a nominal 20 nm nucleation layer. This was followed by high-temperature growth at 1080 °C. The near-interfacial region of the films consists of a mixture of cubic and hexagonal GaN, which is characterized by a high density of stacking faults bounded by Shockley and Frank partial dislocations. The near-interfacial region shows a high density of inversion domains. Above ∼0.5 μm thickness, the film consists of isolated threading dislocations of either pure edge, mixed, or pure screw character with a total density of ∼7×108 cm−2. The threading dislocation reduction in these films is associated with cubic to hexagonal transformation of the nucleation layer region during high temperature growth.