Surface Of Epitaxial Layer Research Articles

Characteristics of graphene/4H-SiC/graphene photodetector based on hydrogenated multilayer-graphene electrode

By utilizing a Joule heating decomposition method, hydrogenated multilayer graphene (MLG) has been grown on the surface of a semi-insulation 4H-SiC epitaxial layer. The Raman spectra have indicated that the sublimation speed of Si atoms, which corresponds positively to the Joule heat, had great influence on the hydrogenation degree and the layer number of the MLG. Then, a graphene/4H-SiC/graphene photodetector was fabricated and studied, showing hydrogenation-dependent dark current and photocurrent, depicting the influence of hydrogenation degree and the layer number on the Schottky barrier high (varying from 0.59 to 0.99 eV). Moreover, the sheet resistance of MLG and the specific contact resistance of graphene/Cr/Au came out to be ∼3.2 kΩ / sq and 7.5 × 10 − 3 Ω · cm2.

Analysis of the Photoluminescence Spectra of Heterostructures with AlGaAs/GaAs Quantum Wells

A technique for the examination of the photoluminescence spectra of multilayer heteroepitaxial structures with AlGaAs/GaAs quantum wells grown from molecular-beam epitaxy was developed. The size-quantization levels in quantum wells were calculated. Heat maps of distributions of peak wavelengths and intensities of photoluminescence spectra over the surface of epitaxial layers of different compositions were plotted. This mapping provided an opportunity to estimate the uniformity of distributions of the composition and thickness of epitaxial layers over the surface of samples. The results suggest possible ways to improve the techniques for acceptance and interoperational inspection of multilayer heteroepitaxial structures used in infrared photodetector arrays.

Construction of Uniform Cobalt-Based Nanoshells and Its Potential for Improving Li-Ion Battery Performance.

Surface cobalt doping is an effective and economic way to improve the electrochemical performance of cathode materials. Herein, by tuning the precipitation kinetics of Co2+, we demonstrate an aqueous-based protocol to grow uniform basic cobaltous carbonate coating layer onto different substrates, and the thickness of the coating layer can be adjusted precisely in nanometer accuracy. Accordingly, by sintering the cobalt-coated LiNi0.5Mn1.5O4 cathode materials, an epitaxial cobalt-doped surface layer will be formed, which will act as a protective layer without hindering charge transfer. Consequently, improved battery performance is obtained because of the suppression of interfacial degradation.

Stabilizing Cathode Materials of Lithium-Ion Batteries by Controlling Interstitial Sites on the Surface

Summary Lithium-ion batteries with high energy density are being intensively pursued to meet the ever-growing demand for energy storage. However, the increase in energy density often comes with an elevated instability of electrode materials, causing major concerns about the reliability and safety of lithium-ion batteries. Here, we report a strategy for stabilizing cathode materials by modulating the vacant lattice sites on the particle surface. Using the high-voltage Li[Ni0.5Mn1.5]O4 as an example, we demonstrate that introduction of a 10-nm epitaxial surface layer with Al3+ in the empty 16c octahedral sites of the spinel Li[Ni0.5Mn1.5]O4 suppresses structural degradation during cycling by increasing the surface stability without interfering with the Li+ diffusion around the Al3+ sites. Control of the Al3+ concentration in the surface region was shown to be a facile process. The process was shown to stabilize long-term cycling of Li[Ni0.5Mn1.5]O4 to 5 V versus Li+/Li0.

Electrical and Material Properties Analysis of 5kV 4H-SiC Schottky Barrier Diodes

Silicon carbide is an attractive material for next-generation power devices, it has wide band gap and high thermal conductivity. SiC power devices can operate at higher temperatures and voltages compared to silicon devices. But, electrical characteristics of the SiC power devices can be deteriorated by SiC defects. The killer defects such as micropipes and stacking faults have been reduced through the development of SiC growth technique. But still the threading dislocations (TDs) and basal plane dislocations (BPDs) lowered the breakdown voltage and increased the leakage current during device operation [1]. In this study, we fabricated the 5 kV 4H-SiC Schottky barrier diode (SBD) to study the effect of SiC defect on electrical properties of SiC SBDs. The thickness of epitaxial layer was 50 um and the doping concentration is 1E15 cm-3. The device was fabricated with various length of junction termination extension (JTE). Ion implantation was performed to form p surge (Al, 500 °C, 5E19 cm-3) and JTE (B, 500 °C, 2E18 cm-3) regions. Activation annealing was performed at 1700 °C for 60 m. 100 and 30-nm-thick Ni were deposited as the cathode and front-side ohmic metal respectively, and then annealed at 950 °C for 90 s in RTA system. Finally, 100-nm-thick Ni was sputtered on front-side as a Schottky contact metal. We investigated the forward, reverse characteristics, and reverse breakdown voltage of the fabricated 4H-SiC SBDs. The diode characteristics, ideality factor (IF) and Schottky barrier height (SBH) were obtained from the forward electrical characteristics, about the 1.23 and 1.3 eV, respectively. The relative highly ideality factor may be due to rough surface of thick epitaxial layer. The breakdown voltage was obtained above 5 kV with JTE length longer than 60 um. During reverse characteristics the electrically weak points were observed using electroluminescence (EL). The EL spot was mainly aroused from electrically leaky points, it mostly observed at the edge of JTE. Some EL spots were observed inside the JTE or active region. The defects of the fabricated 4H-SiC 5 kV SBDs were analyzed using non-destructive analysis method, X-ray topography (XRT) and electron beam induced current (EBIC). We tried to determine the correlation between the electrical characteristics of the device and the crystal defects in the device. [1] H. Fujiwara, H. Naruoka, M. Konishi, K. Hamada, T. Katsuno, T. Ishikawa, Y. Watanabe, and T. Endo, Appl. Phys. Lett. 100, 242102 (2012). Figure 1

Impact of substrate off-angle on the m-plane GaN Schottky diodes

We investigated the effects of the substrate off-angle on the m-plane GaN Schottky diodes. GaN epitaxial layers were grown by metal–organic chemical vapor deposition on m-plane GaN substrates having an off-angle of 0.1, 1.1, 1.7, or 5.1° toward . The surface of the GaN epitaxial layers on the 0.1°-off substrate consisted of pyramidal hillocks and contained oxygen (>1017 cm−3) and carbon (>1016 cm−3) impurities. The residual carbon and oxygen impurities decreased to <1016 cm−3 when the off-angle of the m-plane GaN substrate was increased. The leakage current of the 0.1°-off m-plane GaN Schottky diodes originated from the +c facet of the pyramidal hillocks. The leakage current was efficiently suppressed through the use of an off-angle that was observed to be greater than 1.1°. The off-angle of the m-plane GaN substrate is critical in obtaining high-performance Schottky diodes.

Study of Nanoscale Profiling Modes of GaAs Epitaxial Structures by Focused Ion Beams

The nanoscale profiling modes of epitaxial GaAs layers are experimentally studied through focused ion beams (FIB). The regularities of the influence of ion current and single FIB exposure time on the geometric characteristics of the forming nanosized profile and the etching rate of the surface of GaAs epitaxial layers are determined. It is established that, within the range of FIB modes used, the rate of normal etching of GaAs(001) is on average an order of magnitude lower than the rate of lateral etching. It is shown that the FIB formation of structures with sizes of up to 100 nm atop the GaAs epitaxial layers necessitates the use of ion currents of up to 102 pA and single exposure times at a point from 10 to 100 μs. The results of the present work can be used in the development of technological processes for manufacturing promising elements of nanoelectronics and nanophotonics based on a combination of the FIB method and various types of growth methods.

Wide (0001) terrace formation due to step bunching on a vicinal 4H-SiC (0001) epitaxial layer surface

The wide (0001) terrace formation due to step bunching on a vicinal 4H-SiC (0001) epitaxial layer surface was investigated using low-energy electron channeling contrast (LE-ECC) imaging and atomic force microscopy. LE-ECC imaging revealed that step bunching resulted in the formation of wide atomically-flat (∼200 nm) (0001) terraces on the surface, and the terraces tended to form in pairs. Terraces in a pair had almost the same width and often showed the same LE-ECC; moreover, the contrast of the two terraces, either bright or dark, appeared to be determined by the orthogonal misorientation of substrates. On the basis of these results, the formation mechanism of the paired terraces with the same LE-ECC on a vicinal 4H-SiC (0001) surface is discussed herein.

SEM and ECC Imaging Study of Step-Bunched Structure on 4H-SiC Epitaxial Layers

Step bunching on a vicinal 4H-SiC (0001) epitaxial layer surface was investigated using low-voltage electron scanning microscopy (LVSEM) and electron channeling contrast (ECC) imaging. LVSEM observations revealed that the step bunching resulted in the formation of atomically flat wide (~250 nm) terraces on the surface, and the terraces tended to form in pairs. The two terraces in paired terraces often showed the same electron channeling contrast as each other, and the contrast of the two terraces, either bright or dark, appeared to be determined by the orthogonal misorientation of substrates. On the basis of these results, the formation mechanism of the step-bunched structure on a vicinal 4H-SiC (0001) surface is discussed.

Electrical properties of Ni/n-GaN Schottky diodes on freestanding m-plane GaN substrates

The electrical properties of m-plane Ni/n-GaN Schottky diodes grown via metalorganic chemical vapor deposition were investigated. Under growth at 1,120 °C with a V/III ratio of 1,000 (growth rate of 100 nm/min), the residual Si, O, and C impurity concentrations in the m-plane GaN layer were below the secondary-ion mass spectroscopy detection limit. The surface of the Si-doped n-GaN epitaxial layer on the 5°-off m-plane GaN substrate consisted of steps and terraces. A linear correlation between the carrier concentration and the Si atomic concentration was clearly observed from 1 × 1017 to 5 × 1015 cm−3. The reverse current–voltage curves were fitted using the thermionic field-emission model at the measured carrier concentration and qϕB. The leakage current of the diodes under a reverse bias was effectively suppressed at a low carrier concentration of 4.6 × 1015 cm−3.

Electron microscopy characterization of AlGaN/GaN heterostructures grown on Si (111) substrates

AlGaN/GaN buffer heterostructures were grown on “on axis” and 4 deg off Si (111) substrates by MOVPE. The electron microscopy study reveals the very good epitaxial growth of the layers. Almost c-plane orientated nucleation grains are achieved after full AlN layer growth. Step-graded AlGaN layers were introduced, in order to prevent the stress relaxation and to work as a dislocation filter. Thus, a crack-free smooth surface of the final GaN epitaxial layer is achieved in both cases, making the buffer structure ideal for the forthcoming growth of the heterostructure (used for HEMT device applications). Finally, the growth of the AlGaN/GaN heterostructure on top presents characteristic and periodic undulations (V-pits) on the surface, due to strain relaxation reasons. The AlN interlayer grown in between the heterostructure demonstrates an almost homogeneous thickness, probably reinforcing the 2DEG electrical characteristics.

The Influence of Surface Pit Shape on 4H-SiC MOSFETs Reliability under High Temperature Bias Tests

The influence of surface pit shape on 4H-SiC double implanted MOSFETs (DMOSFETs) reliability under a high temperature drain bias test has been investigated. Threading dislocations formed two types of pit shapes (deep pit and shallow pit) on an epitaxial layer surface. The cause of the failure was revealed to be an oxide breakdown above the pit generated at the threading mixed dislocation in both pit shapes. Weibull distributions between two types of pits were different. Although the DMOSFETs on the epitaxial layer with the deep pit show longer lifetime than those with the shallow pit, the epitaxial layer with the shallow pit is suitable to estimate the lifetime of the DMOSFETs because of a linearity of the Weibull plot. The lifetime of the DMOSFETs with the shallow pit was dominated by an oxide electric field. The maximum oxide electric field required to obtain the lifetime of more than 10 years was estimated to be 2.7 MV/cm.

Formation and reconstruction of Se nanoislands at the surface of thin epitaxial ZnSe layers grown on GaAs substrates

Strained epitaxial ZnSe layers are grown on GaAs substrates by the method of vapor-phase epitaxy from metal-organic compounds. It is found that Se nanoislands with a density of 108 to 109 cm–2 are formed at the surface of such layers. It is established that an increase in the size of Se islands and a decrease in their density take place after completion of growth. Annealing in a H2 atmosphere at a temperature higher than 260°C leads to the disappearance of Se islands and to a decrease in the surface roughness. It is shown that annealing does not lead to deterioration of the structural perfection of the epitaxial ZnSe films; rather, annealing gives rise to a decrease in the intensity of impurity–defect luminescence and to an increase in the intensity of intrinsic radiation near the bottom of the exciton band.

A nitride-based epitaxial surface layer formed by ammonia treatment of silicene-terminated ZrB2.

We present a method for the formation of an epitaxial surface layer involving B, N, and Si atoms on a ZrB2(0001) thin film on Si(111). It has the potential to be an insulating growth template for 2D semiconductors. The chemical reaction of NH3 molecules with the silicene-terminated ZrB2 surface was characterized by synchrotron-based, high-resolution core-level photoelectron spectroscopy and low-energy electron diffraction. In particular, the dissociative chemisorption of NH3 at 400 °C leads to surface nitridation, and subsequent annealing up to 830 °C results in a solid phase reaction with the ZrB2 subsurface layers. In this way, a new nitride-based epitaxial surface layer is formed with hexagonal symmetry and a single in-plane crystal orientation.

GaN-based multi-two-dimensional-electron-gas-channel diodes on sapphire substrates with breakdown voltage of over 3 kV

We investigated the achievability of low specific on-resistance and high breakdown voltage by GaN diodes consisting of three, five, and eight two-dimensional-electron-gas (2DEG) channels. The anode Schottky electrode and cathode Ohmic electrode were formed on each side wall of the multi-2DEG-channel and the n-type region was formed by Si-ion implantation in the cathode electrode-formation area of each multi-2DEG-channel. With increasing number of 2DEG channels of the diodes, specific on-resistance (RonA) showed a tendency to decrease; RonA of eight-2DEG-channel diodes was as low as 12.1 mΩ cm2. The breakdown voltage of all the fabricated diodes exceeded 3 kV. Although the electrical characteristics of the multi-2DEG-channel diodes fabricated on sapphire substrates were demonstrated, the number of cracks appearing on the epitaxial layer surface was found to increase with increasing number of 2DEG channels. Such crack formation was concluded to govern the practical limit for the number of 2DEG channels.

Competitive Interaction between Segregation Gettering and Surface Precipitation of Nickel in p/p+ Silicon Epitaxial Wafers

The segregation gettering mechanism for nickel in p/p+ silicon epitaxial wafers is investigated using heavily boron-doped substrates with different resistivities in the range of 7 to 10 mΩcm. It is found that the segregation gettering becomes active when the nickel contamination level is lower than 1 × 1013 cm−2, resulting in the decrease of the nickel precipitation at the surface of epitaxial layers when the resistivity of p+ substrates decreases. The gettering model based on the enhanced solubility of nickel in p+ substrates could explain the competitive interaction between the segregation effect and the nickel precipitation at the surface below 500°C during the cooling process after heat treatments. It is suggested by these results that the gettering effect is useful for the low temperature process in the fabrication of a future device.

Epitaxial growth and optical characterization of AlInGaN quaternary alloys with high Al/In mole ratio

AlInGaN quaternary epitaxial layers with high Al/In mole ratio were grown on GaN/sapphire templates by metalorganic chemical vapor deposition (MOCVD). The optical and structural properties were characterized by various kinds of technical tools. The full width at half-maximum of the (0002) X-ray diffraction rocking curves was found to decrease as the Al/In mole ratio was increased. The density and size of V-shaped defect pits observed on the surface of AlInGaN epitaxial layers with energy-dispersive X-ray spectroscopy could be reduced by lowering the MOCVD growth pressure. It was also discovered with the variable temperature photoluminescence spectra that the emission peak energy for the AlInGaN epitaxial layers demonstrated an S-shaped shift (red shift–blue shift–red shift) behavior with increasing the measurement temperature. This feature was more significant for those AlInGaN epitaxial layers with lower Al/In mole ratio.

Liquid Phase Epitaxy (LPE) Formation of Localized High Quality and Mobility Ge & SiGe by High Dose Ge-Implantation with Laser Melt Annealing for 10nm and 7nm Node CMOS Technology

Localized Ge and SiGe high quality/mobility device surface material region on bulk-Si and SOI wafers are needed for 10nm and 7nm node CMOS technology. Traditionally SiGe and Ge device surface material on bulk-Si or SOI wafers are realized by CVD epitaxial growth or Vapor Phase Epi (VPE) at elevated temperatures. Selective deposition can be achieved with hard mask. To reduce defects strain relaxed buffer (SRB) or aspect ratio trapping (ART) have been employed with bulk-Si or layer transfer wafer bonding for SiGeOI or GeOI. An alternative to VPE is to use either SPE (solid phase epitaxy) or LPE (liquid phase epitaxy) to form SiGe or Ge epitaxial surface layers. Using high dose Ge implantation >E16/cm2 with photoresist soft mask as proposed in 2004 by Borland et al [1,2]. Localized amorphous Ge surface regions can be formed after Ge-infusion doping by Gas Cluster Ion Beam (GCIB) technique followed by low temperature SPE to form single crystal thin surface Ge epitaxial layers but residual end-of-range (EOR) damage remained. Today, laser melt annealing of implanted junctions are currently being used in production for high quality back-side illuminated CMOS image sensors used in smart phone cameras by several IC and foundry semiconductor manufacturers to completely eliminate any residual implant damage/defects with 100% dopant activation provided the melt depth exceeds the implant damage depth [3]. Last year at IWJT-2013 Borland et al [4] reported using Ge-plasma ion implantation at 1E16/cm2 and 1E17/cm2 doses with laser melt annealing to realize up to 55% SiGe by LPE with >4x higher mobility at 160cm2/Vs. One limitation they noted with plasma implantation was poor retained dose due to surface sputtering at low energies which limited the Ge content to <55% for 1E17/cm2 dose. Therefore to overcome this retained dose problem we investigated Ge beamline ion implantation in this paper which should give 100% retained dose. The Ge surface amorphous layer thickness measured by elipsometry was 15nm while the Ge-SIMS depth profile measures 7nm Ge at 100%. Therma-wave analysis was used to monitor the Ge-implant damage recovery and Ge or SiGe recrystallized epitaxial surface layer after various laser melt annealing conditions of varying power level and pulse duration. The Ge-SIMS depth profiles shows we could vary the melt depth for LPE Ge layer from 10nm to >400nm and the Ge layer content from 97% down to 2% respectively. We will also show X-TEM of the Ge-LPE regions. Sb implantation was used to investigate n-type dopant activation in Ge at low and high doses of 3E13/cm2 and 3E15/cm2. Rs measurements were done using a special Hx 4 point probe and SRP (spreading resistance profile) depth profiles. In summary, using high dose beam-line Ge-implantation we realized >7nm amorphous Ge deposition. Epitaxial growth of Ge was achieved by liquid phase epitaxy using shallow laser melt annealing between 10nm and >400nm melt depths.

Chemical Mechanical Polishing of III-V Materials for Wafer Bonding Applications

ABSTRACT Wafer bonding of III-V cells offers opportunities to integrate materials with different bandgaps and different lattice parameters without forming high defect densities. However, growth conditions that produce the best cells do not necessarily produce epitaxial layer surfaces that are suitable for wafer bonding. The chemical mechanical polishing of III-V materials including GaAs, InP, InAs, and alloy layers used in high efficiency III-V solar cell structures is investigated with sodium hypochlorite and citric acid solutions. It is found that the surfaces can be polished to below 0.5 nm RMS surface roughness without the induction of crystalline damage using similar abrasive-free polishing solutions for all the materials with controlled polishing rates of 10 nm / min. The low removal rate is key for applications that include epitaxial layers that must be subsequently polished without l=excessive loss of material. The actual composition of the slurry is adjusted for the particular alloy composition. A balance between reduced sub-surface mechanical damage and smooth surface morphology is obtained by adjusting the amount of citric acid similar to Br-methanol polishing. However, the damage-free planarization in these cases may be aided by an oxide formation as predicted by Pourbaix diagrams. Subsequent bonding suing sulfur passivation on the CMP surface shows excellent electrical characteristics with low resistivity as well as high strength bonding after annealing at 400 °C. Additionally, triple axis x-ray diffraction is found to be an extremely sensitive, non-destructive method for evaluating damage induced by the CMP process.

Mossbauer Study of Yttrium Iron Carnet Epitaxial Films Magnetic Microstructure

The magnetic microstructure of the yttrium iron garnet epitaxial films surface layer were studied by conversion electronic Mossbauer spectroscopy method. The presence of two magnetically non-equvivalent positions of Fe3+ ions in tetrahedral sites of garnet structure and a paramagnetic phase formed by Fe2+ ions were fixed. Using the model of mixed magnetic and quadrupole interaction by the diagonalization of the nuclear Hamiltonian matrix the information about the spatial orientation of the cation sublattices magnetic moments was obtained and the components of electric field gradient tensor at 57Fe nuclei in different crystal non-equvivalent positions were calculated.