A bottom-up method is described to grow patterned polycrystalline diamond on silicon substrates using polymer assisted seeding and photolithography. Diamond seeding densities were > 1011 cm−2. The pattern consists of well-defined features ranging from microns to millimeters in size. A systematic study was carried out to investigate the effects of CH4 concentration (1.5, 2.0, 2.5, and 3.0

Direct bandgap group IV materials could provide intimate integration of lasers, amplifiers, and compact modulators within complementary metal–oxide–semiconductor for smaller, active silicon photonics. Dilute germanium carbides (GeC) with ∼1 at. % C offer a direct bandgap and strong optical emission, but energetic carbon sources such as plasmas and e-beam evaporation produce defective materials. In this work, we used CBr4 as a low-damage source of carbon in molecular beam epitaxy of tin-free GeC, with smooth surfaces and narrow x-ray diffraction peaks. Raman spectroscopy showed substitutional incorporation of C and no detectable sp2 bonding from amorphous or graphitic carbon, even without surfactants. Photoluminescence shows strong emission compared with Ge.

High quality diamond, similar to 10 mu m thick, is deposited on AlN/Si by hot filament chemical vapor deposition with low (10(8) cm(-2)) and high seeding density (>10(12) cm(-2)). The higher seeding density suppresses three-dimensional growth resulting in films with reduced roughness, improved uniformity, and a more abrupt transition without voids between the diamond and AlN compared to films grown at lower seeding density. Furthermore, micro - Raman results of diamond films grown using higher seeding density show improved quality extending from the substrate to the surface.

Industry demands for power switching, amplification and photonic applications are continually pushing the limits of semiconductor device technologies leading to increasing applied voltages and, thus, higher electric fields while simultaneously requiring reduction in the size, weight and power consumption. In addition, recent heightened interest in higher temperature (and other extreme environment) applications, while simultaneously maintaining high-voltage and high-power performance, has inexorably led to ultrawide bandgap (UWBG) semiconductor materials. This interest in UWBG materials stems from superior properties that include high material hardness, breakdown strength to withstand large electric fields and mitigation of power dissipation during device switching, deliver high current density while operating at higher junction and ambient temperature. Among these materials are boron nitride in its hexagonal and cubic phases, gallium oxide, diamond, and aluminum gallium nitride. The versatile alloys of wurtzite AlxGa1-xN have long been demonstrated across the full composition range. The variation in Eg with AlGaN composition, along with select WBG and UWBG materials, is shown in Figure 1 versus (in-plane for non-cubic) lattice constant. Premier WBG materials to date for electronics and photonics are SiC (Eg = 3.3 eV) and GaN (Eg = 3.4 eV). The latter has been successfully applied to high-power transistors and near ultraviolet photonics. In both electronics and photonics applications, the polarization fields of the wurtzite crystal structure, generally grown along (0001) direction, produce design challenges. In photonics, for example, the fields and band alignment couple to spatially separate electrically injected electrons and holes to reduce wavefunction overlap in quantum wells. In electronics, the internal fields have contributed to difficulties in the design of logic and bipolar transistor devices based on these materials. Despite this, the inherent polarization field and band offsets were successfully employed in the design of high electron-mobility transistors (HEMTs). The inherent AlGaN/GaN HEMT band offset produces a high electron concentration at the interface that spontaneously forms a two-dimensional electron gas (2DEG). Because charge flow in these devices is restricted to the 2DEG, current crowding results in intense self-heating. The local heating decreases electrical conductivity to produce a runaway scenario for elevating the Joule heating. Dissipating this heat, therefore, is the principal limiting factor in operating at power higher than 40 W/mm. The various temperature dependences of semiconductor device parameters are the principal drivers in the operation of semiconductor devices. The physics has inextricably led to UWBGs for future high temperature applications, > 300 °C, due to their lower intrinsic carrier density and thus the effects of leakage are orders of magnitudes less than Si-based devices. Furthermore, AlGaN and diamond offer superior high temperature mechanical stability due to their structural hardness and chemical inertness. Thermal management is essential for achieving higher power densities. Diamond is highly attractive for many applications due to its high hardness, transparency, chemical inertness, promising semiconductor properties, and its high thermal conductivity (κ) to 2500 W/m·K in natural material. Factors related to the consequences of self-heating in devices have motivated extensive research to improve the properties of diamond and produce better approaches to determine the thermal properties. Laboratory-grown diamond films exhibit significantly poorer κ than natural diamond and, when integrated with other electronic materials, a low thermal boundary conductance (TBC in W/m2·K) that inhibits heat transfer between materials. The former originates from the ubiquitous polycrystalline nature of chemical vapor deposition (CVD) diamond and the presence of non-diamond carbon (NDC) in the layer. The low TBC stems from fundamental barriers to phonon propagation between two materials and from practical issues of poor diamond quality in the initial growth regime. Continued research is needed to improve κ in the polycrystalline diamond and mitigate the negative impact of the initial diamond layer on TBC. The current status of hetero-integration of UWBG AlGaN and diamond will be reviewed. Particular emphasis is given to high Al content AlGaN for near-bandgap matching with diamond. Patterned diamond grown using hot-filament CVD to realize unique three-dimensional lateral device structures in a self-aligned, as-grown fabrication process is employed. Engineering the AlGaN-diamond bandgap alignment is noteworthy at the nonpolar AlGaN – diamond interface including diamond surface terminations that produce either negative or positive electron affinity. The review will focus on improvements in material properties, especially the interfaces, that are critical to both charge and heat transport for UWBG device structures. Figure 1

Ultrathin Al2O3 interlayers have been grown on silicon (111) substrates using atomic-layer deposition (ALD) and investigated as interlayers prior to AlN growth by metalorganic chemical vapor deposition. The ALD process is carried out at low temperatures with the interlayer thickness systematically varied from 0.9 to 5.1 nm. A reference sample with the standard SiNx interlayer is also investigated. Thin Al2O3 layers (<2 nm) are found to significantly improve the crystal quality of AlN. X-ray diffraction measurements show the total dislocation density is decreased by nearly one order of magnitude for an Al2O3 thickness of 1.7 nm compared with the standard SiNx interlayer. The impact of the interlayer on the AlN strain is studied by x-ray diffraction and Raman spectroscopy measurements. Some reduction in stress is observed when incorporating the 1.7 nm interlayer. A Raman stress factor of −2.6 ± 0.1 cm−1/GPa is obtained for AlN. Surface and interface analysis studied by atomic force microscopy, high-resolution transmission electron microscopy, and x-ray photoelectron spectroscopy indicates sharp atomic alignment between AlN and silicon with a 1.7 nm Al2O3 interlayer.

GeSnC alloys offer a route to direct bandgap semiconductors for CMOS-compatible lasers, but the use of CBr4 as a carbon source was shown to reduce Sn incorporation by 83%–92%. We report on the role of thermally cracked H in increasing Sn incorporation by 6x–9.5x, restoring up to 71% of the lost Sn, and attribute this increase to removal of Br from the growth surface as HBr prior to formation of volatile groups such as SnBr4. Furthermore, as the H flux is increased, Rutherford backscattering spectroscopy reveals a monotonic increase in both Sn and carbon incorporation. X-ray diffraction reveals tensile-strained films that are pseudomorphic with the substrate. Raman spectroscopy suggests substitutional C incorporation; both x-ray photoelectron spectroscopy and Raman suggest a lack of graphitic carbon or its other phases. For the lowest growth temperatures, scanning transmission electron microscopy reveals nanovoids that may account for the low Sn substitutional fraction in those layers. Conversely, the sample grown at high temperatures displayed abrupt interfaces, notably devoid of any voids, tin, or carbon-rich clusters. Finally, the surface roughness decreases with increasing growth temperature. These results show that atomic hydrogen provides a highly promising route to increase both Sn and C to achieve a strongly direct bandgap for optical gain and active silicon photonics.

Tensile-strained pseudomorphic Ge1–x–ySnxCy was grown on GaAs substrates by molecular beam epitaxy using carbon tetrabromide (CBr4) at low temperatures (171–258 °C). High resolution x-ray diffraction reveals good crystallinity in all samples. Atomic force microscopy showed atomically smooth surfaces with a maximum roughness of 1.9 nm. The presence of the 530.5 cm−1 local vibrational mode of carbon in the Raman spectrum verifies substitutional C incorporation in Ge1–x–ySnxCy samples. X-ray photoelectron spectroscopy confirms carbon bonding with Sn and Ge without evidence of sp2 or sp3 carbon formation. The commonly observed Raman features corresponding to alternative carbon phases were not detected. Furthermore, no Sn droplets were visible in scanning electron microscopy, illustrating the synergy in C and Sn incorporation and the potential of Ge1–x–ySnxCy active regions for silicon-based lasers.

A multistep deposition technique is developed to produce highly oriented diamond films by hot filament chemical vapor deposition (HFCVD) on Si (111) substrates. The orientation is produced by use of a thin, 5–20 nm, Ni interlayer. Annealing studies demonstrate diffusion of Ni into Si to form nickel silicides with crystal structure depending on temperature. The HFCVD diamond film with Ni interlayer results in reduced non-diamond carbon, low surface roughness, high diamond crystal quality, and increased texturing relative to growth on bare silicon wafers. X-ray diffraction results show that the diamond film grown with 10 nm Ni interlayer yielded 92.5% of the diamond grains oriented along the (110) crystal planes with ~ 2.5 µm thickness and large average grain size ~ 1.45 µm based on scanning electron microscopy. Texture is also observed to develop for ~ 300 nm thick diamond films with ~ 89.0% of the grains oriented along the (110) crystal plane direction. These results are significantly better than diamond grown on Si (111) without Ni layer with the same HFCVD conditions. The oriented growth of diamond film on Ni interlayers is explained by a proposed model wherein the nano-diamond seeds becoming oriented relative to the β 1 -Ni 3 Si that forms during the diamond nucleation period. The model also explains the silicidation and diamond growth processes. Article Highlights High quality diamond film with minimum surface roughness and ~93% oriented grains along (110) crystallographic direction is grown on Si substrate using a thin 5 to 20 nm nickel layer. A detailed report on the formation of different phases of nickel silicide, its stability with different temperature, and its role for diamond film texturing at HFCVD growth condition is presented. A diamond growth model on Si substrate with Ni interlayer to grow high quality-oriented diamond film is established.

In situ metal-organic chemical vapor deposition growth of SiNx passivation layers is reported on AlGaN/GaN high-electron-mobility transistors (HEMTs) without surface damage. A higher SiNx growth rate, when produced by higher SiH4 reactant gas flow, enables faster lateral coverage and coalescence of the initial SiNx islands, thereby suppressing SiH4-induced III-nitride etching. The effect of in situ SiNx passivation on the structural properties of AlGaN/GaN HEMTs has been evaluated using high-resolution X-ray diffraction. Electrical properties of the passivated HEMTs were evaluated by clover-leaf van der Pauw Hall measurements. The key findings include (a) a correlation of constituent gas chemistry with SiNx stoichiometry, (b) the degree of suppression of strain relaxation in the barrier layer that can be optimized through the SiNx stoichiometry, and (c) optimum strain relaxation by tailoring the SiNx passivation layer stoichiometry that can result in near-ideal AlGaN/AlN/GaN interfaces. The latter is expected to reduce the carrier scatterings and improve electron mobility. Under optimized conditions, low sheet resistance and high electron mobility are obtained. At 10 K, a sheet resistance of 33 Ω/sq and a mobility of 16,500 cm2/V-s are achieved. At 300 K, the sheet resistance is 336 Ω/sq and mobility is 2020 cm2/V-s with a sheet charge density of 0.78 × 1013 cm-2.

A systematic study is reported on the effects of nano-diamond seeding density on the growth, quality, and morphology of diamond films. A process is described to examine nano-diamond seeding densities 4 × 108, 8 × 1010, and 2 × 1012 cm−2 on silicon wafers. The diamond film is grown using hot-filament chemical vapor deposition with CH4/H2/O2 feed gases and varying growth time to determine properties at coalescence and as thickness increases. Polycrystalline morphology is examined by scanning electron and atomic force microscopy. Both vertical and lateral growth rates are found to be higher for sparse seeding prior to coalescence. Following coalescence, the growth rate is similar for all densities. The development of polycrystals is found to be influenced by the initial growth with smaller mean lateral size at higher seeding density and reduced surface roughness that also improves with thickness to reach ≲90 nm at a thickness of 6.4 μm. The crystal quality is examined by micro-Raman spectroscopy from the sample surfaces and line images from cross sections. Narrowing of the diamond phonon peak shows material quality to improve with the thickness, at a given seed density, and as density increases. Concomitant improvements are seen from the relative intensity of the diamond phonon and Raman bands from non-diamond carbon. Cross-section micro-Raman results suggest improved diamond film quality and crystallinity near the substrate interface as well as at the growth surface for the film grown with 2 × 1012 cm−2 seed density compared to 4 × 108 and 8 × 1010 cm−2. X-ray photoelectron spectroscopy confirms these trends at the diamond surface.

We report the results of two studies of the growth and physical properties of AlGaN-based short-period superlattices (SPSLs), each aimed at improving light emission. In the first experiment, we grow structures on bulk AlN substrates. We observe ~ 3 times higher luminescence efficiency than identically grown structures on sapphire. In the second experiment, we grow structures on sapphire while controlling the growth mode. We observe a significant improvement in the room temperature cathodoluminescence efficiency (at least by factor of 10) of AlGaN quantum wells when the 3D growth mode is induced by reduced flux of ammonia over identically prepared structures grown in the 2D mode.

Direct measurements are reported of the thermal conductivity (κ) for gold nanowires and nanoribbons fabricated on a support substrate. Two thickness values of 50 and 100 nm are investigated with lateral dimensions ranging from 74 to 720 nm, thereby spanning the nanowire to nanoribbon scales. Average polycrystalline grain sizes were 27 ± 5 nm for thickness 50 nm and 35 ± 4 nm for 100 nm thickness. Systematic decrease in κ is observed as film thickness is reduced and, for given thickness, as width decreases. At large lateral width studied here, corresponding to the microscale, κ values at room temperature are 280 and 200 W/m K for thicknesses of 100 and 50 nm, respectively. These are to be compared with the accepted value of κ = 317 W/m K for bulk gold. For the smallest case investigated, 50 nm in thickness and 74 nm in width, a value of κ = 56 W/m K is obtained. A small decrease is observed for κ when measured with ambient temperature 100 °C. The systematic dependence on width, for each thickness and both ambient temperatures studied, followed a simple exponential dependence with a characteristic length parameter of ∼150 nm. This length parameter exceeds the accepted electron mean-free path (∼40 nm), the grain sizes, and the film thicknesses. The decrease in κ with diminishing size was examined based on the Boltzmann transport equation (BTE) methods. Agreement and limitations of the BTE are discussed.

The effect of precursor stoichiometry is reported on morphology, phase purity, and texture formation of polycrystalline diamond films. The diamond films were deposited on 100-mm Si (100) substrates using hot filament chemical vapor deposition at substrate temperature 720–750 °C using a mixture of methane and hydrogen. The gas mixture was varied with methane concentrations 1.5% to 4.5%. Diamond film thickness and average grain size both increase with increasing methane concentration. Diamond quality was checked using surface and cross-section by ultraviolet micro-Raman spectroscopy. The data show consistent diamond properties across the surface of the film and along the cross-section. XRD pole figure analyses of the films show that 3.0% methane results in preferential orientation of diamond in the 〈111〉 direction, whereas films deposited with 4.5% methane showed texture along the 〈220〉 direction in addition to 〈111〉 which was tilted ~ 23° with respect to the surface normal.

High crystal quality crack-free AlN on sapphire was grown by low pressure metal organic vapor phase epitaxy (MOVPE). Growth experiments combine two recent approaches: the ammonia pulse-flow method and ammonia continuous-flow growth mode by varying the V/III ratio. The detailed aspects of MOVPE, employing the periodic multilayer approach at low, intermediate, and high temperatures are described. This method yields significant reduction of screw dislocation density and provides very smooth surface for thin AlN layers.

Joint analysis studies of open-circuit voltage and photoluminescence intensity (PL-I) are reported for CuIn1−xGaxSe2 (CIGSe) solar cells. A range of compositions are investigated, including constant x = 0.35 and x = 0.55 as well as a graded composition profile having a minimum of x = 0.25. Both the open-circuit voltage and PL-I are measured as functions of temperature and illumination intensity. With these two measurements, a full model-based fitting of the temperature and illumination dependence allows extraction of the effects of window layer band offset and shunt resistance, in addition to bulk and interface recombination parameters. To quantitatively analyze the two distinct measurements jointly, the absolute PL-I is measured to obtain quasi-Fermi-level splitting.

The effects of post-deposition CdCl2 annealing temperature on the electronic properties of CdTe solar cells were investigated. CdTe devices incorporate Mg doped ZnO as a buffer layer and selenization using a CdSe interlayer for reducing the buffer/absorber interface recombination and increasing solar energy absorption respectively. The post-deposition CdCl2 annealing treatments were done under separate, inert atmospheres of nitrogen and helium across the temperature range 380-430 degrees C. Electrical characterization of devices is carried out including temperature dependent current-voltage characteristics, admittance spectroscopy, and Shockley-Read-Hall recombination analysis. The best improvements in device efficiency are obtained upon annealing at temperature 410 degrees C. This anneal correlated with reduced back contact barrier in CdTe and reduced grain-boundary barrier height which is beneficial for enhanced charge transport.

We describe the growth of high quality AlN and GaN on Si(111) by gas source molecular beam epitaxy (GSMBE) with ammonia (NH3). The initial nucleation (at 1130−1190K) of an AlN monolayer with full substrate coverage resulted in a very rapid transition to two-dimensional (2D) growth mode of AlN. The rapid transition to the 2D growth mode of AlN is essential for the subsequent growth of high quality GaN, and complete elimination of cracking in thick ( > 2 μm) GaN layers. We show, using Raman scattering (RS) and photoluminescence (PL) measurements, that the tensile stress in the GaN is due to thermal expansion mismatch, is below the ultimate strength of breaking of GaN, and produces a sizable shift in the bandgap. We show that the GSMBE AlN and GaN layers grown on Si can be used as a substrate for subsequent deposition of thick AlN and GaN layers by hydride vapor phase epitaxy (HVPE).

Growth of single-crystalline GaN on polycrystalline diamond is reported for the first time. The structure was achieved using a combined process including selective diamond growth on GaN/Si wafers using hot filament chemical vapor deposition (CVD) and epitaxial lateral overgrowth of GaN on the window region between then above the diamond stripes via metal organic CVD. Optimization of the growth was performed by varying the ammonia to trimethylgallium mole ratio (V/III), chamber pressure, and temperature in the range of 8000-1330, 40-200 Torr, and 975-1030 °C, respectively. A lower pressure, higher V/III ratio, higher temperature, and GaN window mask openings along [11̅00] resulted in enhanced lateral growth of GaN. Complete lateral coverage and coalescence of GaN were achieved over a [11̅00]-oriented 5 μm-wide GaN window between 5 μm diamond stripes when using V/III = 7880, P = 100 Torr, and T = 1030 °C. The crystalline quality of overgrown GaN was confirmed using cross-sectional scanning electron microscopy, high-resolution X-ray diffraction, micro-Raman spectroscopy, transmission electron microscopy, and selective-area electron diffraction.

A new technique is reported for selective growth of polycrystalline diamond by hot filament chemical vapor deposition (HFCVD) on AIGaN/GaN-on-Si (111) wafers without degradation of the underlying layers. Selective diamond seeding is accomplished by dispersing nanodiamond seeds in photoresist and patterned lithographically prior to HFCVD growth. A thin layer of plasma enhanced CVD SiNx, deposited prior to seeding and diamond deposition, was found to be essential to protect the AIGaN/GaN wafer. A methane concentration of 3.0% was used to achieve an increased diamond growth rate and faster surface coverage. Excellent selectivity and minimal AIGaN surface damage were achieved with increased methane concentration. Damage mitigation was confirmed by comparison of atomic force microscopy, X-ray diffraction, and Raman spectroscopy, each conducted before and after diamond deposition, and by SEM images of the final structures.