Cross-sectional transmission electron microscopy (XTEM) has been used to analyze phase separation in the III-V semiconductor alloy InGaAs grown by molecular beam epitaxy (MBE) on (001) InP substrates with a 2 ° offcut, as well as InGaP grown upon on-axis (001) GaAs. Both films were deposited using a time of 20 seconds for each substrate rotation, with a deposition rate of 0.278 nm/s. Phase separation resulting in lateral composition modulation (LCM) along the [110] direction was analyzed via XTEM for InGaAs grown at the temperatures 400, 450, and 500 °C. An Arrhenius analysis of InGaAs XTEM LCM measurements, based on the phase separation model of Malyshkin and Shchukin, yields the activation energy of 0.54+/-0.05 eV associated with cation adatom surface diffusion, which compares favorably with the activation energy of 0.55+/-0.02 eV determined by in-situ MBE In droplet epitaxy surface diffusion measurements. XTEM analysis of vertical composition modulation (VCM) for InGaP grown upon an on-axis (001) GaAs substrate reveals a wavelength of 5.3 nm, that comes close to matching 5.5 nm, which is product of the rotation time and the deposition rate. However, for InGaAs grown on an off-axis InP (001) substrate, the VCM wavelengths observed by XTEM deviates significantly from the expected 5.5 nm and instead ranges from 7-9 nm, due to the step-bunching promoted by deposition on the off-axis InP substrate. Both the effects of LCM and VCM phase separation in InGaAs can be understood in terms of the greater surface diffusivity of In adatoms relative to Ga adatoms as revealed by droplet epitaxy measurements. Trends in surface diffusion in ternary alloys are also applied to quaternary alloys, leading to a more accurate theoretical estimate of the critical temperature for phase separation.

Inclusion of Si atoms to the growth surface during the molecular beam epitaxy of Ge and Sn to form a SiGeSn alloy was identified as a reactive surface species and as a means to compensate strain, which allowed for the subsequent growth of GeSn alloys with high Sn content. The development of a SiGeSn virtual substrate having a 15% Sn concentration and lattice parameter larger than 5.72 Å is demonstrated, using atomic force microscopy, x-ray reciprocal space mapping, and transmission electron microscopy, as a method for the direct growth of thick (>500 nm) fully relaxed GeSn alloys with greater than 10% Sn. This buffer layer enables the monolithic integration of GeSn with silicon for optoelectronic applications, as the SiGeSn virtual substrate allows for selective chemical etching of GeSn, which is important for device fabrication.

Long-wavelength infrared (IR) III-V based devices have long been of interest for potential applications such as chemical sensing and large format IR imaging. Within the III-V family, only the InAs 1-x Sb x bulk alloy in composition range 0.45 ≤ x ≤ 0.8, offers the required bandgap energy ( E g ) from 100 to 125 meV at an operating temperature of 80K and below [1]. Over this composition range however, the InAs 1-x Sb x lattice constant varies from 6.24 to 6.39 Å, where a lack of conventional substrates has restricted progress on the growth and study of this material system. To bridge this lattice-constant gap while maintaining relatively low defect densities, we employed a metamorphic step-graded InAs 1-x Sb x buffer on GaSb, enabling the study of low- E g InAs 1-x Sb x as a function of growth conditions. Using this method, we investigated the effect of substrate temperature ( T sub ) and group V to group III flux ratio (beam equivalent pressure, V/III) on Sb incorporation of the lowest- E g cap layer [2]. We also used x-ray reciprocal space mapping (RSM) to examine the effect of growth conditions on strain and dislocation dynamics. Following these growth studies, we employed the metamorphic InAs 1-x Sb x in an InAs/InAsSb superlattice designed with a cutoff wavelength of 9 µm which leads to improved absorption compared with the lattice-matched counterpart. We first grew, via molecular beam epitaxy, several InAs 1-x Sb x step-graded structures in which the Sb/(As+Sb) flux ratio was varied from 0.05 to 0.50 in 0.05 increments (see figure), under various T sub and V/III. Nomarski imaging revealed smoother surfaces under a V/III=10, the highest ratio we attempted. At this higher V/III, we observed the cross-hatch morphology expected for metamorphic materials and found that the cross-hatch spacing changes, implying a change in dislocation dynamics, with T sub . We then used photoluminescence (PL) to measure the Sb-content in the cap layer as well as compare intensities between samples. We found the highest Sb-incorporation to occur when T sub =415 C and V/III=10, while the most intense samples used T sub =415-430 C and V/III=10 [2]. Using RSM along [110] with (004) and (115) reflections, we identified the Sb composition in each layer. This allowed comparison of Sb-content as a function of Sb/(As+Sb) for various T sub and V/III. The results suggest that V/III has little effect on Sb incorporation, in direct conflict with our previous PL results [2]. To understand the discrepancy between PL and RSM, we measured (004) RSM of the same three samples with the x-ray beam incident along [1-10], revealing extremely different strain relaxation compared to the [110] case (see figure). Asymmetric strain relaxation has been observed in other III-V graded buffer systems and has been explained by different dislocation formation energies and glide velocities along each direction resulting from the core structure of the dislocation being terminated with either a group-III or a group-V element [3]. Transmission electron microscopy is ongoing to further understand the dislocation dynamics in these samples. Taking this all together allowed us to investigate the effect of substrate lattice-constant on strain-balanced InAs/InAsSb superlattices designed for 9 µm cutoff wavelength [4]. Theoretically, by using a larger substrate lattice-constant the superlattice design results in larger electron-hole wavefunction overlap, ultimately increasing photon absorption. Our experimental results confirm this theory, even in the presence of increased threading dislocations inherent to the required lattice-mismatch. [1] I. Vurgaftman et al. JAP 89 , 5815-5875 (2001). [2] Tomasulo et al. J. Vac. Sci. and Technol. B 36 , 02D108 (2018). [3] France et al. J. Appl. Phys. 107 , 103530 (2010); Gelczuk et al., J. Cryst. Growth 310 , 3014 (2008). [4] Affouda, Tomasulo et al., Appl. Phys. Lett. 110 , 181107 (2017). Figure 1

Amorphous films of Nb2O5 composition were deposited by thermal atomic layer deposition on (001) Si substrates and subsequently crystallized by annealing in forming gas at temperatures ranging from 550 °C to 1000 °C. After subjecting these films to an 850 °C anneal, cross-sectional transmission electron microscopy revealed the presence of B-Nb2O5 and T-Nb2O5 phases in the matrix, as well as reduced R-NbO2 in floret-shaped regions. Annealing at 1000 °C completed the reduction process, resulting in the insulator-to-metal transition (IMT) capable T-NbO2 phase throughout the film. ALD films of composition Nb2O5 were also deposited on electron-transparent SiN membranes and then subjected to 550 °C and 1000 °C anneals. Here, the 550 °C anneal induced the B-Nb2O5 and T-Nb2O5 phases without inducing the R-NbO2 phase. The 1000 °C anneal of the films deposited on SiN membranes retained B-Nb2O5 while inducing the R-NbO2 phase, but without bringing the process to completion and inducing the T-NbO2 phase. The effectiveness of the 1000 °C reducing annealing treatment to induce the T-NbO2 phase for Nb2O5 films deposited on (001) Si substrates, while stopping short of this transition for films deposited on SiN membranes, suggests the importance of the SiO2 layer on the Si substrate in contributing to the reduction reaction that results in the technologically important insulator-to-metal transition (IMT)-capable T-NbO2 phase.

One strategy for decreasing the density of threading dislocations penetrating the surface of a heteroepitaxial semiconductor film is that in which the greater mechanical stiffness of a dislocation blocking layer acts to reduce the Peach–Koehler image forces acting on the leading segment of the half loop generated by dislocation multiplication sources at the heteroepitaxial interface situated below the blocking layer. Reducing the Peach–Koehler force, drawing the half loop to the film surface, helps prevent the two threading arms of the half loop from becoming threading dislocations once the half loop penetrates the film surface. The calculation of the Peach–Koehler force employs an analytical continuation formalism using anisotropic elasticity theory for treating dislocation image forces generated by three heteroepitaxial interfaces corresponding to the top and bottom interfaces of the blocking layer and the film surface. The system used in this calculation is that of a Ge film grown on a (001) Si substrate, using a SiGe blocking layer just below the critical thickness for dislocation generation. It is found that the dislocation blocking is favored by thinner blocking layers of greater mechanical stiffness, rather than thicker blocking layers of moderate mechanical stiffness. Specifically, for the blocking layers of composition Si0.2Ge0.8, Si0.3Ge0.7, and Si0.4Ge0.6, of thickness 50, 18, and 10 nm, respectively, it is the thinnest (and mechanically stiffest) layer (Si0.4Ge0.6, 10 nm) that brings about the greatest reduction in the Peach–Koehler force, drawing the leading segment of the half loop to the surface of the film.

The Sb concentration profile in an nBn photodetector containing an InAs/InAsSb type-II superlattice is collected and analyzed using atom probe tomography. A 3D reconstruction comprises the full composition of 31 periods. The Sb concentration profile is evaluated for the entire 31 period stack, as well as each individual period using segregation models from Muraki and Wood. Trends in the asymmetric Sb profile show a consistent non-negligible Sb concentration in the InAs layers and a lower Sb concentration in the InAsSb with respect to the target concentration.

Niobium oxide (NbOx) materials of various compositions are of interest for neuromorphic systems that rely on memristive device behavior. In this study, we vary the composition of NbOx thin films deposited via atomic layer deposition (ALD) by incorporating one or more in situ hydrogen plasma exposure steps during the ALD supercycle. Films with compositions ranging from Nb2O5 to NbO2 were deposited, with film composition dependent on the duration of the plasma exposure step, the number of plasma exposure steps per ALD supercycle, and the hydrogen content of the plasma. The chemical and optical properties of the ALD NbOx films were probed using spectral ellipsometry, X-ray photoelectron spectroscopy, and optical transmission spectroscopy. Two-terminal electrical devices fabricated from ALD Nb2O5 and NbO2 thin films exhibited memristive switching behavior, with switching in the NbO2 devices achieved without a high-field electroforming step. The ability to controllably tune the composition of ALD-grown NbOx films opens new opportunities for realizing a variety of device structures relevant for neuromorphic computing and other emerging electronic and optoelectronic applications.

Heterostructures of InAs/GaSb have served as the archetypal system for the study of the type II semiconductor band alignment. The precarious balance that must be maintained in molecular beam epitaxy growth may also be undermined by the complexities of heteroepitaxy. The nature of the growth surface itself may contribute to the difficulties in establishing a smooth and abrupt heteroepitaxial interface. The chapter presents X-ray diffraction and Raman spectroscopy data for InAs/GaSb superlattice, and reviews the cross-sectional scanning tunneling microscopy results of Feenstra et al. Shanabrook and coworkers identified the bonding at the interface through the use of Raman spectroscopy. One of the best-known consequences of the translational symmetry of a crystal lattice is Bloch's Theorem, which states, in part, that the electron density in such a solid must have the same periodicity as the lattice. High-resolution transmission electron microscopy imaging experiments found that there was a degree of disorder in InAs/GaSb interfaces grown on GaSb buffer layers.

We present molecular beam epitaxy growth of novel InAlAsSb lattice-matched to InP for use in an all lattice-matched triple-junction design. Given the unexplored nature of this material, we studied the effect of substrate temperature (T sub ) and group-V to group-III flux ratio (V/III) on surface morphology, Sb-incorporation, and peak photoluminescence (PL) energy. Decreased T sub and increased V/III ratio, i.e. reduced adatom mobility at the growth front, increased the peak PL energy, likely due to a reduction in phase separation.

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InAs1-xSbx is a unique semiconductor as it possesses the lowest bandgap (Eg) of the conventional III–V materials, yielding emission out to nearly 15 μm at room temperature. As such, it is well-suited as the absorber material in long-wavelength infrared (IR) applications such as chemical sensing and large format IR imaging. However, the compositions at the longest wavelengths are significantly lattice-mismatched to conventional substrates. Overcoming this lattice-mismatch requires using thick graded buffers, and to date, little work has focused on the effect of growth conditions on the Sb-incorporation or optical quality of the longest-wavelength InAs1-xSbx materials. Here, the authors investigate the molecular beam epitaxy (MBE) growth of this potential absorber material by using solid source MBE to grow InAs1-xSbx on step-graded buffers on GaSb substrates. The authors have achieved cap-layer compositions as high as x ≈ 0.55 under various substrate temperatures (Tsub) and V/III beam equivalent pressure ratios. By using InAs1-xSbx as the grading material as well as the cap material, the authors can measure Sb-content as a function of growth conditions over many compositions. The author's results show good agreement between x and Eg compared with previous reports, obtaining T = 8 K photoluminescence (PL) emission approaching 11 μm. However, the authors also found that Sb-incorporation falls off for x > 0.2 and worsens with increasing Tsub, suggesting that Sb-incorporation rates increase with reduced adatom mobility. Additionally, the PL of the samples show improved intensity when either Tsub or V/III were increased, while cross-sectional transmission electron microscopy revealed a significant decrease in threading dislocation density with increased V/III. Ultimately, the authors found that under the proper growth conditions, optically active InAs1-xSbx with x over 0.55 can be attained, and with an optimized grading structure, could be a low-cost alternative to HgCdTe for long-wavelength infrared optoelectronics.

The narrow band gap and staggered band alignment of InAsSb alloys make it possible to engineer type-II superlattices (T2SLs) for mid-wave and long-wave (LW) infrared sensors operating in the 3–12 μm range. However, InAs/InAsSb T2SLs that are strain balanced to the underlying GaSb substrate have much lower absorption coefficients for LWIR operation because of the larger superlattice (SL) period, leading to reduced electron-hole overlap. The absorption coefficient of T2SLs can be greatly improved by growing on metamorphic buffers (MBs) with reduced lattice mismatch to the InAsSb layers, which allows the SL period to be reduced. For this study, MBs were capped with InAs/InAsSb T2SLs to assess the suitability of the materials for detector applications by X-ray diffraction and photoluminescence lifetime measurements. We show that the absorption of T2SLs can be significantly increased with no apparent degradation in the minority-carrier lifetime.

Defect sensitive etching (DSE) was developed to estimate the density of non-basal plane dislocations in hexagonal boron nitride (hBN) single crystals. The crystals employed in this study were precipitated by slowly cooling (2–4 °C/h) a nickel-chromium flux saturated with hBN from 1500 °C under 1 bar of flowing nitrogen. On the (0001) planes, hexagonal-shaped etch pits were formed by etching the crystals in a eutectic mixture of NaOH and KOH between 450 °C and 525 °C for 1–2 min. There were three types of pits: pointed bottom, flat bottom, and mixed shape pits. Cross-sectional transmission electron microscopy revealed that the pointed bottom etch pits examined were associated with threading dislocations. All of these dislocations had an a-type burgers vector (i.e., they were edge dislocations, since the line direction is perpendicular to the [211¯0]-type direction). The pit widths were much wider than the pit depths as measured by atomic force microscopy, indicating the lateral etch rate was much faster than the vertical etch rate. From an Arrhenius plot of the log of the etch rate versus the inverse temperature, the activation energy was approximately 60 kJ/mol. This work demonstrates that DSE is an effective method for locating threading dislocations in hBN and estimating their densities.

Journal Article Assessing Hexagonal Boron Nitride Crystal Quality by Defect Sensitive Etching Get access T Hoffman, T Hoffman Dept of Chemical Engineering, Kansas State University, Manhattan, Kansas, USA Search for other works by this author on: Oxford Academic Google Scholar Y Zhang, Y Zhang Dept of Chemical Engineering, Kansas State University, Manhattan, Kansas, USA Search for other works by this author on: Oxford Academic Google Scholar S Liu, S Liu Dept of Chemical Engineering, Kansas State University, Manhattan, Kansas, USA Search for other works by this author on: Oxford Academic Google Scholar N Khan, N Khan School of Science and Technology, Georgia Gwinnett College, Lawrenceville, Georgia, USA Search for other works by this author on: Oxford Academic Google Scholar ME Twigg, ME Twigg United States Naval Research Laboratory, Washington, DC, USA Search for other works by this author on: Oxford Academic Google Scholar ND Bassim, ND Bassim Dept of Materials Science and Engineering, JHE 357, McMaster University, Hamilton, Ontario, CANADA Search for other works by this author on: Oxford Academic Google Scholar JH Edgar JH Edgar Dept of Chemical Engineering, Kansas State University, Manhattan, Kansas, USA Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 23, Issue S1, 1 July 2017, Pages 1518–1519, https://doi.org/10.1017/S143192761700825X Published: 04 August 2017

Although the crystalline α and γ phases are the most stable forms of alumina, small-diameter (<6 nm) nanoparticles are known to be completely amorphous, due to the surface energy being correspondingly lower for the less stable non-crystalline phase. Al2O3 films with a thickness of 5 nm grown by low temperature (200 °C) atomic layer deposition (ALD) on small-diameter (<20 nm) Ga2O3 nanowires (NWs), however, are identified by transmission electron microscopy as belonging to the α, γ, and possibly θ crystalline phases of Al2O3, while films deposited on larger diameter (>20 nm) NWs are found to be amorphous. Indeed, until recently, all Al2O3, films deposited by low-temperature ALD using trimethylaluminum and water have been reported to be amorphous, regardless of film thickness or substrate. The formation of a crystalline ALD film can be understood in terms of the energetics of misfit dislocations that maintain the registry between the ALD film and the NW substrate, as well as the influence of strain and surface energy. The decreasing energy of co-axial misfit dislocations with NW diameter results in a corresponding decrease in the contribution of the Al2O3/Ga2O3 interface to the free energy, while the interfacial energy for an amorphous film is independent of the NW diameter. Therefore, for NW cores of sufficiently small diameter, the free energy contribution of the Al2O3/Ga2O3 interface is smaller for crystalline films than for amorphous films, thereby favoring the formation of crystalline films for small-diameter NWs. For ALD Al2O3 films of 10 nm thickness deposited on small-diameter Ga2O3 NWs, however, only the first 5 nm of the ALD film is found to be crystalline, possibly due to well-established kinetic limitations to low temperature epitaxial growth.

In order to mitigate the formation of threading dislocations in an In0.1Ga0.9Sb heteroepitaxial buffer layer grown on a (001) GaSb substrate, an AlSb blocking layer was grown within the buffer layer. Transmission electron microscopy measurements revealed that the film with the AlSb blocking layer had a significantly lower threading dislocation density than a buffer layer of equivalent thickness in the absence of the blocking layer. Because the AlSb blocking layer is of greater mechanical stiffness than the In0.1Ga0.9Sb buffer layer, attractive image forces that act to draw dislocations to the film surface are countered by repulsive image forces traceable to the In0.1Ga0.9Sb/AlSb interface. These experimental results are consistent with calculations based on anisotropic elasticity theory for repulsive image forces acting on a dislocation beneath the blocking layer.

In this work, we examine the thermal diffusivity of Si/SiGe thin-film superlattice (SL) structures and use these results with pervious thermal conductivity results to calculate the heat capacity from 374 K to 674 K. The thermal properties of semiconductor layered structures can be altered through the use of SL structures. This alteration occurs through two possible mechanisms: increased phonon scattering due to rough interfaces and phonon zone folding due to boundary conditions of the propagating waves. Examining the heat capacity allows for the observation of phonon zone folding effects while limiting effects due to scattering. Structures studied here consist of SiGe SLs grown at different temperatures and with varying SL spacing allowing the examination of period and crystallinity effects on thermal properties. Previously reported results show that for SL structures both crystalline and polycrystalline have a thermal conductivity of approximately 1 W/mK measured over temperatures ranging from 374 K to 674 K. In this work, thermal diffusivity was measured through laser flash analysis, with crystalline SL structures showing values <1 mm2/s, while the thermal diffusivity of the polycrystalline structure was found to be twice that of the crystalline structure over the temperature range. In all instances, the heat capacities for the SL structures are found to be lower than that for a uniform thin film alloy, indicating a significant contribution of phonon dispersion modification to the heat capacity.

An understanding of the growth of (110) quantum wells (QWs) is of great importance to spin systems due to the observed long spin relaxation times. In this article, we report on the metamorphic growth and characterization of high mobility undoped InAs (110) QWs on GaAs (110) substrates. A low-temperature nucleation layer reduces dislocation density, results in tilting of the subsequent buffer layer and increases the electron mobility of the QW structure. The mobility varies widely and systematically (4000–16 000 cm2/Vs at room temperature) with deposition temperature and layer thicknesses. Low-temperature transport measurements exhibit Shubnikov de-Haas oscillations and quantized plateaus in the quantum Hall regime.

InAs quantum wells grown in the (110) direction are an important materials system in the field of spin systems. The major hurdle in realizing such devices is structural defects arising from the large mismatch, 7.2%, between the InAs film and its GaAs substrate. Thus, we embark on a study utilizing transmission electron microscopy (TEM) and electron channeling contrast imaging (ECCI) to understand the density and nature of these defects.