Search

We report a novel Sc/Al/Ni/Au metal scheme for Ohmic contacts to InAlN/GaN HEMT structures on silicon. A contact resistance of $0.39~\Omega $ -mm with a low surface roughness of 20± 3 nm of the annealed contact has been achieved using this metal scheme. The microstructure of the region under the contacts revealed the formation of ~60 nm deep ScGaN inclusions in the GaN channel layer. A thin (~3-5 nm) non-uniform layer of ScInAlN is observed on top of InAlN barrier. Field-emission is found to be the dominant conduction mechanism. Polarization mismatch arising due to the structural modifications is used to explain the possible mechanism related to Ohmic contact formation.

The integration of BaTiO3 with Si(100) is essential to exploit its ferroelectric capabilities in the well-established Si-complementary metal–oxide–semiconductor (CMOS) technological platform. To en...

This experimental study reports first observations of (i) SOA boundary shift in AlGaN/GaN HEMTs and (ii) early time-to-fail of vertical AlGaN/GaN epi-stack under fast changing (sub-10ns rise time) cyclic pulse transient stress, which otherwise qualified for 600 V DC stress. It is shown that a epi stack qualified for 10 years lifetime under DC stress, fails faster under cyclic transient stress. The drain-to-substrate leakage exhibits different trends under DC and pulse stress. Integrated electrical and mechanical stress characterization routine involving Raman/ PL mapping, PFM and CL spectroscopy along with atomistic simulations reveals material limited unique failure physics under transient stress condition. Failure analysis using cross-sectional SEM and TEM investigations reveal signature of different degradation and failure mechanism under transient and DC stress conditions. A failure model is proposed for rapid breakdown of AlGaN/GaN epi-stack under cyclic transient stress and it is experimentally validated.

This article reports on the experimental and analytical determination of the optimum carbon concentration in GaN to achieve enhanced breakdown in AlGaN/GaN high-electron mobility transistors (HEMTs). The lateral breakdown voltage increases when carbon doping is increased from $3\times 10^{{18}}$ to 1019 cm−3 beyond which it decreases, whereas there is no substantial enhancement in the vertical breakdown voltage with carbon doping. We invoke carrier statistics in a compensated semiconductor vis-a-vis the formation energy of carbon-occupying Ga (or N) vacancies to explain the observed buffer leakage. Temperature-dependent data indicate that the buffer leakage current is due to hopping transport, the activation energy of which yields the positions of the defect states within the bandgap. The increase in buffer leakage beyond optimum C concentration is attributed to the formation of shallow donor traps by carbon atoms occupying Ga vacancies (CGa). The observations correlated with the relative intensities of the defect-mediated peaks in the cathodoluminescence (CL) data of the samples. Based on our findings, a C doping beyond 1019 cm−3 is not recommended for GaN buffers in order to achieve high breakdown voltages.

This letter demonstrates heterogeneous stacking through postfabrication transfer of a thin layer of AlGaN/GaN high-electron-mobility transistors (HEMTs) and Si metal–oxide–semiconductor field-effect transistors (MOSFETs) onto a copper heat spreader. Both devices were fabricated on silicon substrate using separate process flows. Transfer of ~3- $\mu \text{m}$ GaN layer grown on silicon, however, leads to cracks due to the high-stress gradients arising from large lattice and thermal mismatch. Electroplated copper was used to improve mechanical strength, allowing transfer of the GaN layer using Cu–In bonding. Use of electroplated copper ensures high thermal conductivity to the bonded heat spreader. Next, an ultrathin silicon layer ( $\sim 1.5~\mu \text{m}$ ) with functional NMOS transistors was stacked above the GaN devices using a cost-effective epoxy bonding approach. The Cu–In bonding not only improved thermal dissipation but also led to enhanced performance of the GaN device due to the compressive stress induced by the bonding process. This compressive stress will also provide reliability against coefficient of thermal expansion (CTE) mismatch at higher operating temperatures.

For semiconductor industry to replace silicon CMOS integrated circuits by 2-D semiconductors or transition metal dichalcogenides (TMDs), TMD-based n-FETs as well as p-FETs having performance better than Si FETs are a must. While a lot of literature demonstrates n-channel characteristics, the major roadblocks in the realization of TMD-based CMOS integrated circuit are the lack of approach to realize p-channel transistors having performance comparable to n-channel transistors, all realized over the same TMD substrate. To address this, we propose a new technique by engineering WSe2/metal interface to realize WSe2-based high-performance p- and n-channel transistors and therefore unveil its potential toward CMOS-integrated technology. The technique involves a dry process, based on the chemistry between the sulfur atom and WSe2 surface, that induces unique metal-induced gap states in the source/drain (S/D) contact area, which causes improved hole (electron) injection when Cr (Ni) as S/D metal was used. This has enabled the controlled realization of high-performance WSe2 FETs with desired polarity (N, P, or ambipolar), which solely depends on the contact metal used and contact engineering (CE)/surface engineering. Fundamental investigations on the effect of the proposed CE on metal–WSe2 interface revealed interesting and counter-intuitive facts, which very well corroborate with experimental observations.

This experimental study reports new aspects of electrostatic discharge (ESD) behavior in AlGaN/GaN HEMTs. The role of Schottky gate and MESA is investigated using special test structures. Influence of piezoelectric field, carrier trapping, and self-heating on ESD behavior is studied. A unique power-law-like behavior is found. Linear scaling of failure current with source–drain spacing is reported. Spot measured drain-to-source DC current is realized as an important parameter to monitor degradation. Unique degradation trends are observed for the first time and a correlation between snapback depth and % degradation is established. Cumulative nature of device degradation is discovered. Change from soft to hard failure with an increase in pulsewidth (PW) is reported. Finally, the cause of snapback instability observed in device, at low PW, is discussed.

This paper reports the very first systematic study on the physics of avalanche instability and safe operating area (SOA) reliability in AlGaN/GaN high-electron-mobility transistor (HEMT) using submicroseconds pulse characterization, poststress degradation analysis, well-calibrated TCAD simulations, and failure analysis by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Impacts of electrical and thermal effects on SOA boundary and avalanche instability are investigated. Trap-induced cumulative nature of degradation is studied in detail. The root cause for avalanche instability in AlGaN/GaN HEMTs is investigated. Postfailure SEM, energy dispersive X-ray (EDX), and TEM analysis reveal distinct failure modes in the presence and absence of carrier trapping.

In this paper, for the first time, we have experimentally demonstrated enhancement mode (e-mode) AlGaN/GaN high-electron-mobility transistor (HEMT) operation by integrating p-type high- $\kappa {\mathrm {Al}}_{x}{\mathrm {Ti}}_{1-x}$ O based gate stack. Concentration of Al in Al-Ti-O system was found to be a tuning parameter for the threshold voltage of GaN HEMTs. The high- $\kappa $ properties of ${\mathrm {Al}}_{x}{\mathrm {Ti}}_{1-x}$ O as a function of Al % are studied. Superiority of AlTiO over other p-oxides such as CuO and NiO x is proven statistically. Using the high- $\kappa $ and p-type AlTiO, in conjunction with a thinner AlGaN barrier under gate, 600-V e-mode GaN HEMTs are demonstrated with superior ON-state performance ( $\text{I}_{ \mathrm{\scriptscriptstyle ON}}~\sim ~400$ mA/mm and $\text{R}_{ \mathrm{\scriptscriptstyle ON}} ={8.9}\,\,\Omega $ -mm) and gate control over channel ( $\text{I}_{ \mathrm{\scriptscriptstyle ON}}/\text{I}_{ \mathrm{\scriptscriptstyle OFF}} = {10}^{{7}}$ , SS = 73 mV/dec, and gate leakage

This experimental study reports behavior of Ni/Au-based AlGaN/GaN Schottky diodes under ESD conditions. A comparative study of diodes with/without recessed Schottky contact unfolds different degradation physics in each case. The impact of different current conduction mechanisms, device degradation and trap generation on its robustness are analyzed. The role of interface traps under ESD failure of the GaN Schottky diode is investigated. The transition from soft-to-hard failure, which is found to depend on the presence of traps, and diode design is discussed. New insights into degradation trends, cumulative nature of degradation, and trap-assisted failure modes are discovered.

We investigate the vertical leakage mechanism in metal–organic chemical vapor deposition-grown carbon (C)-doped AlGaN/GaN High Electron Mobility Transistors (HEMTs) on 6-in silicon wafer. Substrate bias polarity-dependent ${I}$ – ${V}_{s}$ , temperature-dependent fitting, and band diagram analysis pointed to the Poole–Frenkel (P–F) type of conduction mechanism for vertical transport in the devices with breakdown as high as 580 V for a buffer of $\textsf {4}~\mu \text{m}$ . Trap activation energy of 0.61 eV was estimated from the P–F fitting which matches well with values reported in the literature. We propose that higher dislocation density leads to shallower traps in the buffer and build an analytical model of dislocation-mediated vertical leakage around this. The variation in leakage as a function of dislocation density at a given field is predicted and is found to be the most abrupt in the range from $\sim 10^{\textsf {7}}$ to $\sim 10^{\textsf {9}}$ cm $^{-\textsf {2}}$ of dislocation density. This can be attributed to a sharp decrease in trap activation energy in the above range of dislocation density, possibly due to complex formation between point defects and dislocations.

The talk will describe the defect-to-device performance correlations for optoelectronic and power devices on hetero-epitaxially grown III-nitride epi-layers on silicon and sapphire. While UV detectors and power electronic devices might seem far apart, we see similarities between the defect origins of dark current in the former and leakage currents in the latter. Both have their origins in crystal growth. The first part of the work focuses on the correlation between performance parameters of vertical deep-ultraviolet photodetectors on c-plane sapphire and the density of extended defects vis-a-vis screw and edge dislocations. Through a careful optimization of nucleation density on the growth surface, state-of-the-art crystalline quality AlN epi-layers were grown. This led to the realization of record performance 289 nm p-i-n photodiodes with zero-bias quantum efficiency of 92 %, leakage current below 1 nA at 100 V and breakdown field in excess of 6 MV/cm. Performance of III-nitride power devices on silicon, on the other hand, was found to be more sensitive to structural defects such as surface pits as compared to dislocations. By utilizing a two-temperature growth technique which involved an AlN layer grown at high temperatures, a significant reduction in the pit density could be achieved. This resulted in three-orders of magnitude reduction in lateral as well as vertical leakage in AlGaN/GaN high electron mobility transistors (HEMTs) on silicon. To achieve a further reduction in the reverse leakage current and an enhancement in the device breakdown voltage, point defect control also becomes important. Carbon doping to reduce the background n-carrier concentration in the GaN epilayers is discussed as one of the possible approaches. Reduction of pit density and C-incorporation cumulatively led to the realization of 2 MV/cm vertical breakdown field in AlGaN/GaN HEMTs on silicon.

We report on the band offset and interfacial electronic properties of e-beam evaporated HfO2 gate dielectrics on III-nitride device stacks on Si. A conduction band offset of 1.9 eV is extracted for HfO2/GaN along with a very low density of fixed bulk and interfacial charges for optimally annealed samples. Normally-ON HfO2/AlGaN/GaN metal–insulator–semiconductor high-electron-mobility transistors exhibit negligible shifts in threshold voltage, transconductances of 120 mS/mm for 3- $\mu \text{m}$ gate length devices, and three-terminal off-state gate leakage currents of 55 nA/mm at a ${V}_{D}$ of 100 V. Dynamic capacitance dispersion measurements in the temperature range of 25 °C–200 °C show two peaks at the AlGaN/GaN interface corresponding to slow and fast interface traps with a peak ${D}_{\text {it}}$ of ${5.5}\times {10}^{13}$ and ${1.5} \times {10}^{13}$ eV−1cm−2 respectively as a function of Fermi level position above ${E}_{C}$ . The HfO2/AlGaN interface exhibits a reasonably constant peak ${D}_{\text {it}}$ of ${2} \times {10}^{13}$ – ${4.4} \times {10}^{13}$ eV−1cm−2 at trap levels of 0.42–0.72 eV below ${E}_{\text {C}}$ . Hysteretic pulsed $I_{D}$ – $V_{G}$ measurements revealed a negative shift in threshold voltages indicative of unoccupied donor-like trap states at the HfO2/AlGaN interface and comparable ${D}_{\text {it}}$ to that inferred from conductance measurements.

In this work, we have investigated the material properties of PECVD (Plasma Enhanced Chemical Vapor Deposition) amorphous Silicon nitride (SiN) films and the influence of deposition conditions on gate-leakage increase of AlGaN/GaN HEMT’s (High Electron Mobility Transistor) after passivation. We have studied the effect of gas flow ratio (SiH4/NH3) on the structural and compositional properties of SiNx deposited on crystalline Silicon (Si). Based on the inference, electrical properties of the SiNx films were examined by depositing on AlGaN/GaN HEMT as a passivation layer. The optimized SiNx which is N-rich with a refractive index of 1.83, tensile stress of 681 MPa and NH3/SiH4 ratio of 18 shows only a 1.2x variation in gate leakage, 1.4x increase in gm, minimum left shift of 0.03 V in Vth and 1.26x increase Ion/Ioff ratio after passivation. We believe that the 3x reduction in Si-H bonds resulting in reduced interface traps at SiN/AlGaN interface is the reason for the minimal increase in gate leakage after passivation.

BaBiO3 is presented as a potential absorber for all-oxide solar cells. Thin-films of BaBiO3 were deposited by pulsed laser deposition. As-deposited thin-films were amorphous, but a rapid thermal anneal at 600 degrees C for 10 min yields polycrystalline thin-films. X-ray photoelectron spectroscopy measurements show that deposited films have Barium in Ba-12 state and bismuth in both Bi-13 and Bi-15 state. UV-Vis spectrophotometer (UV-Vis) shows that BaBiO3 films have direct and indirect bandgap (E-g) of 2.25 eV and 2.02 eV respectively, while ultraviolet photoelectron spectroscopy (UPS) shows that valence band maxima (E-V), conduction band minima (E-C) and work-function (phi) of BaBiO3 is at 5.82 eV, 3.8 eV and 4.22 eV respectively. BaBiO3 thin-film devices with a FTO/c-TiO2/BaBiO3/Au structure show rectifying current-voltage characteristics in dark. Under illumination, a photo response with a J(light)/J(dark) ratio of 1.75 is observed. (C) 2017 Elsevier B.V. All rights reserved.

We report on the estimation of trap capture cross section in AlGaN/GaN HEMTs as a function of bias and temperature. Conductance dispersion technique was employed to study the AlGaN/GaN interface of the devices with a carbon-doped GaN buffer grown on 6-in silicon. While a negligible shift in the threshold voltage ( $V_{\mathrm {{\textsf {TH}}}}$ ) was observed in temperature-dependent $I_{\mathrm {{\textsf {DS}}}}$ – $V_{\mathrm {{\textsf {GS}}}}$ sweeps, we observed a spread in the capacitance-voltage ( $C$ – $V$ ) measurements, indicating a contribution of interface traps. When biased near depletion, G/ $\omega $ versus frequency plot for AlGaN/GaN interface exhibits two peaks which correspond to a pair of trap density ( $D_{\mathrm {{\textsf {it}}}}$ ) and trap time constant ( $T_{\mathrm {{\textsf {it}}}}$ ) values. This was explained using a circuit model in conjunction with energy band diagram. The $D_{\mathrm {{\textsf {it}}}}$ and $T_{\mathrm {{\textsf {it}}}}$ values for one peak were in the range from ~ 0.3– $7\times 10^{12}$ /eV $\cdot $ cm2 and 0.6– $10~\mu \text{s}$ while for the other peak, $D_{\mathrm {{\textsf {it}}}}$ – $T_{\mathrm {{\textsf {it}}}}$ were in the range of ~0.1– $35\times 10^{12}$ /eV $\cdot $ cm2 and ~0.06– $0.3~\mu \text{s}$ at 25 °C. From the $T_{\mathrm {{\textsf {it}}}}$ values, electron capture cross section ( $\sigma )$ for both the traps was extracted and was found to be decreasing with increasing temperature in the range of $1.1\times 10^{-20}$ – $1\times 10^{-19}$ cm $^{\mathrm {{\textsf {2}}}}$ and $4.5\times 10^{-20}$ – $1\times 10^{-17}$ cm2 for slow traps and fast traps, respectively. A multiphonon emission effect was invoked to explain the temperature dependence of capture cross section.

We report on the characterization of the interfaces of Al2O3/InAlN/GaN HEMT structure grown on 200 mm diameter silicon using conductance dispersion technique. Irreversible threshold voltage (VTH) shift of up to +similar to 2.5 V was observed due to the gate stress induced activation of acceptor states. Further, frequency dependent VTH shift during capacitance voltage measurements were also recorded due to the presence of slow traps at InAlN/GaN interface. The conductance dispersion indicated the presence of acceptor traps of the order of similar to 4 x 10(12) to 7 x 10(13) cm(-2) eV(-1) with a time constant of similar to 10 to 350 ls at the InAlN/GaN interface. Trap density at the Al2O3/InAlN was found to be in similar range but with a time constant of similar to 2 ms. The presence of high density of traps at InAlN/GaN interface is attributed to the unavoidable growth interruption before the start of InAlN growth. (C) 2017 Elsevier Ltd. All rights reserved.

Spin coated perovslcite thin films are known to have an issue of pinholes & poor morphology control which lead to poor device-to-device repeatability, that is an impediment to scale-up. In this work, Methylamine vapor annealing process is demonstrated which consistently leads to high-quality perovskite thin-films with an average grain-size of 10-15 mu m. The improvement in film morphology enables improvement in effective carrier recombination lifetime, from 21 its in as-deposited films to 54 mu s in vapor-annealed films. The annealed films with large-grains are also more stable in ambient conditions. Devices made on annealed perovskite films are very consistent, with a standard deviation of only 0.7%. Methylamine vapor annealing process is a promising method of depositing large-grain CH3NH3PbI3 films with high recombination lifetime and the devices with improved performance. (C) 2017 Elsevier B.V. All rights reserved.

Germanium is considered an important electronic material which can serve both as a photo-voltaic material as well as a substrate for low cost III-V solar cells on silicon. However, growth of high quality germanium on silicon is challenging due to 4% lattice mismatch. In this work, we present growth of epitaxial, large grain polycrystalline germanium on silicon using liquid phase crystallization. The method involves re-crystallization of PECVD deposited amorphous Ge on Si using a two-step annealing process by melting (937oC) amorphous thin film and cooling it right below the melting point leading to high quality and large grains. SEM XRD and hall measurements were done to evaluate the material and electronic quality of the films. The films cooled near melting point were found to have largest grains of size ~14 μm and XRD rocking curve FWHM of as low as ~300 arc-sec. Films also demonstrated record high electron mobility of 500-1000 cm2Vs-1.The process is scalable and orientation agnostic with comparable film quality for all orientations.

Semiconducting oxides are often stable, non-toxic, and can potentially be deposited at a low cost. The existence of oxides with appropriate bandgap (2V2O7, MVO). Films deposited using pulsed laser deposition show no photoluminescence (PL). After annealing in a nitrogen ambience, sharp PL (FWHM ∼ 100 µeV) peaks are observed at low temperatures, which indicates improved optoelectronic quality. Our analysis suggests these emissions are from carriers trapped in very shallow native acceptor and donor type defects. This improvement is correlated with the reduction in Urbach energy, which also suggests reduced electronic disorder. Coupled with low bandgap and easy processing, MVO seems to be an exciting material for optoelectronics and photovoltaics.

Pulsed Nd:YAG (1064 nm) laser was used to crystallize amorphous germanium thin-films on silicon (0 0 1) wafer to get epitaxial Ge. Film thickness and laser fluence played a crucial role. Thicker films (≥300 nm) are polycrystalline while thinner films are epitaxial. Rocking curve measurements show improvement in crystallinity with increase in laser fluence. The full 2-in. laser crystallized wafer has omega scan of 0.20° and implied threading dislocation density of 6 × 108 cm−2. Hence, a fast, cost-effective, and low thermal budget process is used to achieve epitaxial Ge on Si for large area applications.

Yttria stabilized zirconia (YSZ) films are being used both as functional oxide and buffer layers for integration of various other functional oxide films on Si substrates. As functional properties of these oxides are highly anisotropic in nature, highly oriented films are essential to realizing their fascinating properties. (111) and (100) textured YSZ films have been deposited on Si substrates by reactive-direct current (R-DC) sputtering. Annealing of these films leads to grain growth and improvement in texture. However, it strongly depends on the growth stresses developed during deposition of these films. Depending on stress harnessing in films/stacks, the texture was improved from rocking curve FWHM of 16 degrees to 7 degrees and 25 degrees to 15 degrees for (111) and (100) YSZ films respectively. A detailed analysis of the relation between stress and grain growth is carried out using an energy balance model. We have found that grain growth is limited by kinetics, though it should be possible from a thermodynamic viewpoint. It is observed that higher initial compressive stress aids significant grain growth (similar to 150%) and texture-improvement (similar to 57%) on annealing.

We report on the demonstration of (InxGa1-x)2O3 (InGaO)-based UV photodetectors realized using a low temperature (∼200 °C) microwave irradiation-assisted deposition technique. By irradiating a solution of the substituted acetylacetonate (acac) complex, namely In0.6Ga0.4(acac)3, employed as the “single-source precursor”, InGaO film was deposited on sapphire substrate, and found to be poly(nano)crystalline, with root mean square (r.m.s). roughness of 8.9 nm. However, the indium content of the film (0.26 mol fraction) was considerably less than in the metal complex (0.6 mol fraction). The optical band gap of the film was found to be 4.5 eV from Tauc’s plot, indicative of a low indium mole fraction. This was confirmed using X-ray photoelectron spectroscopy measurements, from which the indium mole fraction was found to be 0.26. Further, the nature of band gap was determined and defect analysis was carried out using, respectively, Tauc’s plot and cathodoluminescence (CL) measurements. A planar, interdigitated metal-semiconductor-metal (MSM) photodetector fabricated with the InGaO film exhibited a high responsivity of 16.9 A/W at a bias of 20 V, corresponding to a band edge at ∼ 276 nm, with a high photo-to-dark current ratio of ∼105.

Limiting buffer layer current leakage is essential for the realization of high breakdown fields in GaN-on-Si high electron mobility transistors (HEMTs). In this report, we demonstrate the importance of controlling the surface morphology of the AlN nucleation layer (NL) in limiting this leakage. Testing on a self-consistent series of samples grown under two different AlN NL conditions revealed the presence of leakage paths within the epilayers grown using a single temperature AlN NL owing to the presence of surface pits. The introduction of a higher temperature AlN in the NL drastically reduced the pit density and led to a large reduction (>103) in the lateral and vertical buffer leakage in HEMT structures. Using conductive atomic force microscopy, secondary ion mass spectroscopy, and temperature-dependent carrier transport measurements, we confirm that these pits—which originate in the AlN NL, thread vertically, and propagate into the device structures—are associated with leakage paths, thus reducing the field that can be dropped across the epilayers. This is explained by invoking preferential oxygen segregation at their side-facets. It is shown that when a pit-free surface is maintained, a vertical field of 1.6 MV/cm can be achieved for HEMTs. This study is expected to benefit the development of high-performance GaN HEMTs in moving toward the theoretical breakdown field of III-nitrides.

We report the fabrication and characterization of a new design of 3D nano Capacitors using Alumina nanopores as the dielectric material. Nickel nanowires grown inside the nanopores act as high surface area electrodes. These wires are combined together in the form interdigitated capacitor structure, to achieve very high capacitance density.

Flexible and transparent moisture barrier films have been climacteric importance for the further development of the polymer solar cells (PSCs). Herein, a monolayer graphene (2D atomic crystal) embedded in a flexible polymer (GEPs) was used to encapsulate the solar cells. The improvement of long-term stability of PSCs still needs to be overcome for their commercialization to be feasible.Degradation mechanisms studied in ITO/ZnO/PTB7:PC 70 BM/MoO 3 /Ag inverted PSCs by Capacitance-voltage, Capacitance-frequency and light intensity dependent measurements is reported here. Measurements was performed on encapsulated and nonencapsulated (controlled and ambient environment) cells under AM1.5 illumination. However, the degradation studies performed with the ISOS D1 protocol suggested the increased lifetimes of the PSCs encapsulated with GEPs barrier films. A promising lifetime of more than 20000 hours was demonstrated with less than 22 % degradation for encapsulated devices.

Microstructure evolution of GaN grown on c-sapphire by MOCVD has been systematically studied using transmission electron microscopy based techniques. Individual samples have been derived by interrupting the GaN growth at various steps, starting from nitridation at 530° C, followed by the standard two step growth comprising of first deposition of low temperature GaN nucleation layer (LT-GaN NL) and then ramping up the temperature followed by high temperature GaN epilayer growth. Effect of nitridation, and the microstructure of the nitride layer for various nitridation temperatures has been recently reported by our group [1], where we have shown that the nitride layer formed at this nitridation temperature is cubic spinel Al x O y N z . In this paper it will be shown, that the LT-GaN grown on this Al x O y N z (after nitridation at 530° C) is primarily cubic zinc blende (zb) in structure, with multiple twin variants existing about various {111} planes. Its crystallographic orientation relationships with the underlying nitride layer and the sapphire substrate will be shown. The transformation of LT-GaN and the regions of transformation from cubic zb phase to the wurtzite (w) phase during the annealing step will be presented. Subsequent effects on the GaN epilayer growth due to the microstructural evolution of these underlying layers along with the evolution of defects will also be discussed.

Nitridation of sapphire substrates is used as a precursor to the growth of GaN films to provide a wetting layer which is closer in terms of structure and chemistry to the overlayer. Nitridation has been carried out by metal-organic chemical vapor deposition at 530, 800, and 1100 degrees C in an environment of NH3 and H-2. The structure and chemistry of the nitrided layer grown at these different temperatures have been studied by X-ray photoelectron spectroscopy, electron diffraction, high resolution electron microscopy, and electron energy loss spectroscopy. The low temperature nitridation process results in a nitrided layer in which oxygen has been partially replaced by nitrogen to form a cubic spinel-AL(x)O(y)N(z) structure. Nitridation at 800 degrees and 1100 degrees C results in complete substitution of oxygen atoms by nitrogen to form a cubic rock salt AIN structure. These structures are stable on thermal annealing at 1000 degrees C prior to epitaxial GaN growth.

Germanium integration on silicon is required for fabricating high efficiency III-V low-cost solar cells on silicon. In this work, we present a single step thermal annealing of amorphous germanium films as a method of monolithic integration of epitaxial germanium on crystalline silicon (110) wafers. The re-crystallization progresses via liquid phase epitaxy by heating the germanium layer in inert ambient to right above its melting temperature of $937 ^{0}C$. On slow cooling; the Ge layer gets oriented by the underlying Si (100) wafer, yielding an epitaxial Ge film on Si. SEM show that the films are continuous and crack free whereas XRD measurements indicated highly oriented films. 1 micron thick film had rocking curve FWHM of 0.15 degree corresponding to dislocation densities of low 109 cm-2. Also, AFM measurements indicate that 1 micron films have RMS roughness of ˜ 80nm for scan area of 25 μm2.

A simple microstructural rationale for successful anodization of metallic films into ordered oxide nanostructures has been identified. It applies to three of the most commonly studied systems, Zr, Ti and Al films and can be extended to other such oxides. A dense Zone T or II microstructure, in sputtered films, is the most critical ingredient. While T-substrate > 0.3T(melting) Ching is the simplest route, pressure and plasma heating can also be exploited. Such microstructures are also associated with a unique growth stress signature. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

An efficient buffer layer scheme has been designed to address the issue of curvature management during metalorganic chemical vapour deposition growth of GaN on Si (111) substrate. This is necessary to prevent cracking of the grown layer during post-growth cooling down from growth temperature to room temperature and to achieve an allowable bow (

The authors report a detailed investigation of the flicker noise (1/f noise) in graphene films obtained from chemical vapour deposition (CVD) and chemical reduction of graphene oxide. The authors find that in the case of polycrystalline graphene films grown by CVD, the grain boundaries and other structural defects are the dominant source of noise by acting as charged trap centres resulting in huge increase in noise as compared with that of exfoliated graphene. A study of the kinetics of defects in hydrazine-reduced graphene oxide (RGO) films as a function of the extent of reduction showed that for longer hydrazine treatment time strong localised crystal defects are introduced in RGO, whereas the RGO with shorter hydrazine treatment showed the presence of large number of mobile defects leading to higher noise amplitude.

Low cost high-power electronics era has begun with the fabrication of Gallium Nitride (GaN) based High Electron Mobility Transistors (HEMTs) on Silicon substrates. Silicon is however not an optimum material from the viewpoint of the final substrate. Having GaN based devices on other substrates would enable better performing RF and high-power devices. Here we report the process development of the transfer technology, of a fabricated GaN HEMT transistors to a glass wafer using temporary bonding material. Epoxy has been used to finally bond the GaN stack and the glass wafer. Transfer and output characteristics of the HEMT and the capacitance measurements of the capacitors were performed on the identical devices before and after the transfer process. Also, to understand the stress before and after the transfer process Raman measurements were carried out.

The performance of GaN-on-Silicon electronic devices is severely degraded by the presence of a parasitic conduction pathway at the nitride-substrate interface which contributes to switching losses and lower breakdown voltages. The physical nature of such a parasitic channel and its properties are however, not well understood. We report on a pronounced thickness dependence of the parasitic channel formation at AlN/Si interfaces due to increased surface acceptor densities at the interface in silicon. The origin of these surface acceptors is analyzed using secondary ion mass spectroscopy measurements and traced to thermal acceptor formation due to Si-O-N complexes. Low-temperature (5K) magneto-resistance (MR) data reveals a transition from positive to negative MR with increasing AlN film thickness indicating the presence of an inversion layer of electrons which also contributes to parasitic channel formation but whose contribution is secondary at room temperatures., Comment: 23 pages, 7 figures

High-efficiency III-V solar cells on low-cost substrates such as steel, are limited because a) the overlying III-V semiconductor stack gets contaminated with iron, b) it is not easy to deposit crystalline Ge thin-films on amorphous steel substrates. In this work, we report a TiN-based barrier layer that inhibits Fe diffusion into the overlying semiconductor. We also report a method to deposit crystalline germanium thin-film on steel on which high quality III-V layers can be deposited using laser annealing. PECVD amorphous germanium films were irradiated with a 532 nm laser with a fluence of 4 - 8 x 108 Jcm-2 which melted and re-crystallized the irradiated amorphous film into nano-crystalline germanium film. The recrystallized films were characterized using SEM and Raman spectroscopy. Films irradiated at higher fluence and films with higher thicknesses, had more crystalline order. The work enables future development of lost-cost GaAs solar cells.

All-oxide solar cells are attractive due to their stability, low cost and ease of fabrication. Very few oxide-absorbers are known and the efficiencies are limited by low mobility and lifetimes. In this work, for the first time, photovoltaic properties of a new oxide semiconductor, Mn2V207 (MVO) are described. Optical measurements show that MVO has an indirect bandgap of 1.6 eV and a direct bandgap of 1.75 eV, which suggests that it absorbs efficiently in the visible region of the solar spectrum. The valence and conduction band positions of the intrinsically n-doped MVO film are determined to be at 5.5 eV and 3.9 eV. Schottky solar cells fabricated using Pt/MVO heterojunction show low short circuit current (Jsc) and open circuit voltage (Voc). Annealing in nitrogen ambience results in a Jsc of 0.46 mA/cm2 a 50x increase and of Voc 0.21 V, almost 10x increase compared to un-annealed device. A maximum power of 24.1 μcm2is obtained, which is two orders of magnitude higher than un-annealed devices.

Integration of high quality germanium on silicon is prerequisite for fabricating high efficiency low-cost III-V solar cells on silicon. In this work, we present a method of monolithic integration of epitaxial germanium on crystalline silicon wafers where a single-step anneal in hydrogen ambient is used to recrystallize PECVD-deposited amorphous germanium into crystalline germanium. The re-crystallization progresses via liquid phase epitaxy by heating the germanium layer in inert ambient to just above its melting temperature of 937 °C. On cooling, the Ge layer gets oriented by the underlying Si (100) wafer, yielding an epitaxial Ge film on Si. SEM show that the films are continuous and void free. XRD pattern indicates highly oriented films. Symmetric Rocking curve measurements showed a thickness dependence with 1 µm thick film showing the lowest threading dislocation density (TDD) of the order of 108 cm−2. AFM measurements indicate surface measurements ~10 nm. Quality of epitaxy depends on thickness of film.

Mg is the most common p-type dopant in III-nitride devices and is becoming increasingly important in the design and development of transistors for power applications. The diffusion of Mg atoms to adjacent layers during growth has been a persistent problem. We report on a simple method involving in situ etching by hydrogen, the most commonly used carrier gas in nitride growth, for suppressing Mg diffusion. This method can be implemented during growth itself without removing the wafer from the chamber and can be controlled by in situ monitoring. A Mg concentration decay rate of 24 nm/dec, is reported for the etched sample compared to 160 nm/dec for the unetched one. An increase in 2DEG mobility in AlGaN/GaN HEMT structures on silicon substrate from 591 cm(2) V-?1 s(?1) for the unetched sample to 1214 cm(2) V(?1 )s(?1 )for the etched sample was observed. Capacitance?voltage studies to understand the effect of diffused Mg atoms on conduction channel are also reported.

Alloy composition of InxGa1-xN layer is investigated using photoluminescence (PL), and photoluminescence excitation (PLE) techniques. InGaN layers were grown on Si(111) substrates using metal organic vapour phase epitaxy (MOVPE) technique under variable flow of either ammonia (NH3) or trimethyl gallium (TMGa) with corresponding V/III ratios in the range of 3.6 × 103 to 1.2 × 104. We observed that though room temperature PL measurements give an indication about the growth of InGaN, it doesn't help in the estimation of band gap or alloy composition of layer. It is restricted mainly by the presence of multiple peaks in the PL spectra which is rather severe at low temperature. A comparison of PL and reflectivity spectra confirmed that such peaks are attributed to interference oscillations and are not related with multiple compositions of InGaN. Further, in such a scenario PLE is found to be quite effective where the measured PLE spectrum is consisted of a clear onset related to InGaN layer in the sub band gap region of GaN. It is quite helpful in a precise determination of band gap and hence the alloy composition of InGaN layer. The indium composition is estimated to be in the range of 3.5% ≤ x ≤ 20.5%, however its trends with respect to V/III ratio as a function of NH3 or TMGa flow are found to be rather opposite.

We report on the demonstration of UV/visible and UV/near-IR photodetectors of high spectral responsivity (SR) in a non-conventional heterojunction, realized by combining multi-layered materials with wide band gap Gallium Nitride (GaN). Multi-layer MoS2 and β-In2Se3 flakes were exfoliated separately on epitaxial GaN-on-sapphire, followed by fabrication of photodetectors in a lateral inter-digitated metal semiconductor metal geometry with Ti/Au contacts. Devices exhibited distinct steps in SR graph at 365 nm with responsivity value of 127 A W−1 and at ~685 nm with responsivity value of 33 A W−1 for MoS2/GaN heterostructure. Whereas, similar steps exhibited at 365 nm with responsivity value of 1.6 A W−1 and at ~850 nm with responsivity value of 0.03 A W−1 in case of β-In2Se3/GaN heterostructure. The wavelength dependent I–V characteristics showed photo-to-dark current ratio of ~30 at 685 nm in case of MoS2/GaN heterostructure and ratio of ~2 at 850 nm for β-In2Se3/GaN heterostructure. The reasons which limit the performance of the devices were also investigated using transient analysis and power dependent responsivity analysis. In summary, current work paves the way for futuristic layered-materials/3D heterostructure devices.

This experimental study reports ESD failure analysis of AlGaN/GaN HEMTs. Effect of MESA isolation, gate and parasitic MESA Schottky diode on ESD robustness is studied. Cause of snapback instability, multiple NDCs and transition from soft-to-hard failure are discussed. Unique leakage trends and cumulative nature of degradation are discovered. Post failure analysis reveals role of inverse piezoelectric effect, fringing electric field, contact resistivity, temperature and field induced contact metal migration in degradation of AlGaN/GaN HEMTs under ESD conditions.

A two-step growth method for high-quality epitaxial germanium films on silicon (100) is presented. During the first step a seed layer of crystalline Ge is deposited on silicon. The seed layer is a 50–100 nm germanium thin-film crystallized by melting amorphous germanium deposited using PECVD. During the second step germanium is epitaxially deposited on top of the re-crystallized seed layer using LPCVD at 600 °C. Scanning electron microscopy (SEM) reveal that the resulting germanium films are continuous, conformal and crack-free. X ray diffraction (XRD) pattern in the θ-2θ configuration shows a strong Bragg's peak corresponding to Ge (400). XRD rocking curve pattern in the vicinity of the Bragg peak for Ge(400) shows a sharp peak with a FWHM of only 512 arc sec, indicating highly-oriented Ge film. XRD phi-scan for (221), (111) and (110) planes of germanium show sharp peaks at 45° interval, proving the in-plane epitaxy of germanium which have a four-fold cubic symmetry. The epitaxial germanium films on silicon are useful for heterogeneous CMOS devices, photonics, IR photodetectors, and low-cost III-V photovoltaics.

Most methods for optical visualization beyond the diffraction limit rely on fluorescence emission by molecular tags. Here, we report a method for visualization of nanostructures down to a few nanometers using a conventional bright-field microscope without requiring additional molecular tags such as fluorophores. The technique, Bright-field Nanoscopy, is based on the strong thickness dependent color of ultra-thin germanium on an optically thick gold film. We demonstrate the visualization of grain boundaries in chemical vapour deposited single layer graphene and the detection of single 40 nm Ag nanoparticles. We estimate a size detection limit of about 2 nm using this technique. In addition to visualizing nano-structures, this technique can be used to probe fluid phenomena at the nanoscale, such as transport through 2D membranes. We estimated the water transport rate through a 1 nm thick polymer film using this technique, as an illustration. Further, the technique can also be extended to study the transport of specific ions in the solution. It is anticipated that this technique will find use in applications ranging from single-nanoparticles resolved sensing to studying nanoscale fluid-solid interface phenomena.

A wafer-scale method to obtain epitaxial germanium (Ge) on crystalline silicon (Si) using liquid-phase-crystallization (LPC) is presented. The technique provides a simple yet versatile method to grow epitaxial germanium on silicon with any crystallographic orientation: (100), (110) or (111). The process starts with amorphous Ge, which is melted and cooled in a controlled manner to form epitaxial germanium. LPC Ge films are continuous with an average grain-size of 2-5 μm. Rocking scan confirms that the LPC Ge is oriented with a threading dislocation density of ∼109 cm-2. The phi-scan confirms that LPC germanium is epitaxial with Ge (100), Ge (110) and Ge (111) showing four-fold, two-fold, and three-fold symmetry, respectively. The epitaxial quality of the Ge is influenced by the cleanliness of the Ge/Si interface; rate of cooling and ambient gas during LPC; and Ge layer thickness. Best films are obtained for 1 μm thick LPC Ge(100), cooled at ∼3-4 C/min in hydrogen ambient. Electron Hall mobility in these LPC Ge films is 736cm2/Vs, a high value that confirms the electronic quality of LPC Ge film.

Flexible, transparent, and moisture-impermeable materials are critical for packaging applications in electronic, food, and pharmaceutical industries. Here, we report that a single graphene layer embedded in a flexible polymer reduces its water vapor transmission rate (WVTR) by up to a million-fold. Large-area, transparent, graphene-embedded polymers (GEPs) with a WVTR as low as 10(-6) g m(-2) day(-1) are demonstrated. Monolayered graphene, synthesized by chemical vapor deposition, has been transferred onto the polymer substrate directly by a very simple and scalable melt casting process to fabricate the GEPs. The performances of the encapsulated organic photovoltaic (OPV) devices do not vary even after subjecting the GEPs to cyclic bending for 1000 cycles. Accelerated aging studies of working OPV devices encapsulated in the GEPs show a 50% lifetime of equivalent to 1 000 000 min, which satisfies the requirements of organic electronics.

We have quantitatively compared the evolution of threading dislocations (TDs) in GaN films grown on (111) Si substrates using different buffer/interlayer structures: a compositionally graded Al x Ga 1-x N (0≤x≤1) buffer layer, a thin high-temperature (HT) A1N interlayer (IL), and an A1N/GaN/Al x Ga 1-x N multilayer. Plan-view transmission electron microscopy (TEM) shows a reduction in TD density in GaN films grown on graded Al x Ga 1-x N buffer layers, and an increase in TD density in GaN films grown on HT A1N ILs, in comparison with those grown directly on an A1N buffer layer. Cross-sectional TEM reveals bending and annihilation of TDs within the graded Al x Ga 1-x N buffer layer, which lead to a decrease of TD density in the overgrown GaN films. On the other hand, a high density of TDs forms at the GaN/AIN IL interface, resulting in an increase in TD density in the GaN film. In addition, growing a thin A1N + GaN bilayer before growing the compositionally graded Al x Ga 1-x N buffer layer significantly reduces the TD density in the Al x Ga 1-x N buffer layer, which subsequently further reduces the TD density in the overgrown GaN film.

The availability of large-area substrates imposes an important constraint on the technological and commercial realization of devices made of layered materials. Aluminum nitride films on silicon are shown to be promising candidate materials as large-area substrates for such devices. Herein, the optical contrast of exemplar 2D layers - MoS2and graphene - on AlN films has been investigated as a necessary first step to realize devices on these substrates. Significant contrast enhancements are predicted and observed on AlN films compared to conventional SiO2films. Quantitative estimates of experimental contrast using reflectance spectroscopy show very good agreement with predicted values., 17 pages, 8 figures, supplementary information

We are reporting the 2DEG mobility and sheet carrier concentration behavior of AlGaN/GaN HEMT stacks that are grown on different kinds of transitions from AlN buffer to epi-GaN. The role of each transition in terms of interfacial roughness and epi-GaN crystallinity are discussed. Further, the observed material parameters are correlated with the HEMT 2DEG property. Enhancement in 2DEG mobility was observed from the stacks those exhibit low surface roughness irrespective of the scattering seen in HRXRD FWHMs. This results show that other than the dislocation reduction and crack free film, the low surface roughness is essential for a better confinement of 2DEG and an optimum carrier concentration.

Zirconia doped with 6-8 wt% (3.2-4.2 mol%) yttria (6-8YSZ), the most common thermal barrier coating material, relies mostly on oxygen vacancies to provide the phonon scattering necessary for low thermal conductivity. The present study examines whether specific substitutional defects—in addition to, or instead of, oxygen vacancies—can provide similar or greater reductions in conductivity. To this end a series of zirconia samples co-doped with varying levels of yttrium (trivalent) and tantalum/niobium (pentavalent) oxides were synthesized, thereby allowing oxygen vacancy and substitutional atom concentration to be varied independently. The results show that Nb-Y and Ta-Y co-doped zirconia samples containing only substi- tutional defects produce stable single-phase tetragonal materials with thermal conductivities very close to that of the conventional 6-8YSZ. In these samples, Nb 51 and Td 51 are similarly effective in lowering thermal conductivity, in contradiction to phonon scattering theories that consider primarily mass effects and thereby predict significantly greater conductivity reduction due to Ta 51 doping than Nb 51 doping. Finally, Nb 51 /Ta 51 - Y 31 doped samples, which contain both oxygen vacancies and substitutional defects, are found not to be stable in single-phase form; however, the thermal conductivities of the two-phase tetragonal 1 cubic mixtures are again as low as that of the conventional 6-8YSZ. © 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

AlGaN/GaN high electron mobility transistor stacks deposited on a single growth platform are used to compare the most common transition, AlN to GaN, schemes used for integrating GaN with Si. The efficiency of these transitions based on linearly graded, step graded, interlayer, and superlattice schemes on dislocation density reduction, stress management, surface roughness, and eventually mobility of the 2D-gas are evaluated. In a 500 nm GaN probe layer deposited, all of these transitions result in total transmission electron microscopy measured dislocations densities of 1 to 3 x 10(9)/cm(2) and