Multilayer Buffer Research Articles

Application of building geometry indexes to assess the correlation between buildings and air temperature

The quantitative exploration of the correlation between buildings and air temperature (Tair) is of widespread research concern. Two building geometry indexes (BGIs), a two-dimensional Area Index (2DAI) and a three-dimensional Volume Index (3DVI) were proposed in this study. Furthermore, we constructed multilayer buffer zones around weather stations and used Spearman's rank correlation to assess the correlation between the 2DAI, 3DVI and Tair to explore the buffer zone with the highest correlation coefficient (Optimal Buffer Zone, OBZ) that exists between BGIs and Tair. The results demonstrated the following. (1) The correlation coefficient between the 3DVI and Tair was higher than that of 2DAI, but this coefficient was affected by the season and month. (2) The OBZ between the BGIs and Tair was the 250 m buffer zone, but the OBZ between the 3DVI and Tair could only be concentrated during the high-temperature period. (3) The change mechanisms of the correlation coefficients and the OBZs between the BGIs and Tair were related to the upper limit of the 2DAI that exists in the buffer zone and the stronger thermal sensitivity of the 3DVI in a high-temperature environment. (4) The upper limits of the 2DAI and 3DVI in 500m×500m grids should be determined according to the actual situation of the BGIs in all grids of the study area. This study provides a new perspective from which to explore the correlation between buildings and Tair and identifies the optimal size of grid units that can be used in urban thermal environment planning units.

On the Origin of the $C_{\text{oss}}$ -Losses in Soft-Switching GaN-on-Si Power HEMTs

The unprecedented performance potential of gallium nitride-on-silicon (GaN-on-Si) high electron mobility transistors (HEMTs) is seen as the key enabler for the design of power converters featuring extreme power density figures, as demanded in next-generation power electronics applications. However, unexpected loss mechanisms, i.e. dynamic $ {R}_{\text {ds,on}}$ phenomena and $ {C}_{\text {oss}}$ -losses, are appearing in currently available GaN transistors and are compromising their operation. In this paper, measurements of $ {C}_{\text {oss}}$ -losses are performed in a dedicated calorimetric measurement setup and, through a systematic approach, the root cause of the loss mechanism is potentially identified. Afterward, with the essential support of a manufacturer of power semiconductors, a novel transistor, featuring an enhanced multilayer III-N buffer, is developed according to the acquired knowledge. A significant reduction in terms of $ {C}_{\text {oss}}$ -losses, i.e. of soft-switching losses, and the absence of dynamic $ {R}_{\text {ds,on}}$ phenomena are verified experimentally on the new device. These achievements enable a significant performance improvement for future soft-switching power converters featuring GaN-on-Si HEMTs.

Plasma assisted-MBE of GaN and AlN on graphene buffer layers

The possibility of using chemical vapor deposition (CVD) graphene as a 2D buffer layer for epitaxial growth of III-nitrides by plasma assisted-MBE on amorphous substrates (SiO2 prepared by thermal oxidation of Si wafer) was investigated. The comparative study of graphene-coated parts of the wafers and the parts without graphene was carried out by scanning electron microscopy and X-ray diffractometry. It was shown that epitaxial GaN and AlN films with close to 2D surface morphology can be obtained by plasma assisted-MBE on amorphous SiO2 substrates with a multilayer graphene buffer using the HT AlN nucleation layer.

GaN-based ultraviolet microdisk laser diode grown on Si

This work reports a demonstration of electrically injected GaN-based near-ultraviolet microdisk laser diodes with a lasing wavelength of 386.3 nm at room temperature. The crack-free laser structure was epitaxially grown on Si substrates using an Al-composed down-graded AlN/AlGaN multilayer buffer to mitigate the mismatches in the lattice constant and coefficient of thermal expansion, and processed into “sandwich-like” microdisk structures with a radius of 12 μm. Air-bridge electrodes were successfully fabricated to enable the device electrical characterization. The electrically pumped lasing of the as-fabricated microdisk laser diodes was evidenced by the rapid narrowing down of electroluminescence spectra and dramatic increase in the light output power, as the current exceeded the threshold of 248 mA.

Enhanced Buffer Management Policy and Packet Prioritization for Wireless Sensor Network

Limited storage of the sensor node is one of the main causes of packet drop in the Wireless sensor network (WSN). Frequent link disconnection is also another cause of packet drop. Due to lack of continues end-to-end connection; two nodes may not be able to communicate with one another. Thus, communication may be established with the help of store and forward approach between the source and destination node. In that case, the sensor node may not be capable of storing a chunk of data since the buffer is available in a small amount. In order to store the data packets in the buffer when the link is down, an effective buffer management scheme is highly needed to keep the data packets for a long time until the link is re-established. This paper proposes a new buffer management scheme called Packet Priority Heterogonous Queue (PPHQ), which based on prioritizing and classifying the packets into different categories to minimize the loss of important packets. Unlike the existing Multi-layer WSN, that treats the data packets differently; we considered the data packets such as; temperature, humidity, and pressure to be same. However, the classification of different packet types is based on the sensor’s information value. We completely divide the whole buffer into different queues, and thus the newly arrived packets are inserted in their corresponding queue. The buffer will then prioritize and schedule on which packet to be stored or transmit first when the buffer is overloaded. Our result exhibits that PPHQ scheme indeed provides minimum packet drop as well as maximum throughput compared to existing Multi-layer WSN buffer management schemes.

Preparation of a double-layer YBa2Cu3O7-x/CeO2 film on a Hastelloy C276 metal substrate with buffer layers of LaMnO3/MgO/Gd2Zr2O7 by laser chemical vapor deposition

A double-layer YBa2Cu3O7-δ/CeO2 film was prepared on a Hastelloy C276 metal substrate with multi-buffer layers of LaMnO3/MgO/Gd2Zr2O7 by laser chemical vapor deposition. The CeO2 film showed a highly (100) preferred orientation with a full width at half maximum of 3.4° of the ω-scan on the (200) reflection and that of 5.0° of the ϕ-scan on the (220) reflection. The YBCO film showed a (110) preferred orientation with coexistence of c-axis-oriented grains. The (110)-oriented YBCO grains showed wedge-shaped morphology while the c-axis-oriented grains had dense surface and cross section.

Some peculiarities at preparation of Bi4Ti3O12 films for bolometric applications

We discuss properties of Bi4Ti3O12 (BTO) films in BTO/CeO2/YSZ multilayered buffer system, on silicon and on silicon-on-insulator (SOI) substrates, employed for the preparation of epitaxial La0.67Sr0.33MnO3 (LSMO) films. We try to find a correlation between the parameters of film preparation and structural properties and morphology of the BTO films with the goal to eliminate defects. We also point out that many morphological defects observed on the final LSMO films prepared for bolometers originate in the bottom BTO layer. We have found out that the actual substrate temperature is a very important parameter during the process of BTO film deposition since deviations of only few degrees from the proper temperature can cause the growth of mixture crystal orientation.

Room-Temperature Electrically Injected AlGaN-Based near-Ultraviolet Laser Grown on Si

This letter reports a successful fabrication of room-temperature electrically injected AlGaN-based near-ultraviolet laser diode grown on Si. An Al-composition step down-graded AlN/AlGaN multilayer buffer was carefully engineered to not only tackle the huge difference in the coefficient of thermal expansion between AlGaN template and Si substrate, but also reduce the threading dislocation density caused by the large lattice mismatch. On top of the crack-free n-AlGaN template, high quality InGaN/AlGaN quantum wells were grown, sandwiched by waveguide and optical cladding layers, for the fabrication of edge-emitting laser diode. A dramatic narrowing of the electroluminescence spectral line-width, an elongated far-field pattern, and a clear discontinuity in the slope of light output power plotted as a function of the injection current provide an unambiguous evidence of lasing.

Effect of hydrogen on the device performance and stability characteristics of amorphous InGaZnO thin-film transistors with a SiO2/SiNx/SiO2 buffer

We investigate the influence of the multi-layered buffer consisting of SiO2/SiNx/SiO2 on amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs). The multi-layered buffer inhibits permeation of water from flexible plastic substrates and prevents degradation of overlying organic layers. The a-IGZO TFTs with a multi-layered buffer suffer less positive bias temperature stress instability compared to the device with a single SiO2 buffer layer after annealing at 250 °C. Hydrogen from the SiNx layer diffuses into the active layer and reduces electron trapping at loosely bound oxygen defects near the SiO2/a-IGZO interface. Quantitative analysis shows that a hydrogen density of 1.85 × 1021 cm−3 is beneficial to reliability. However, the multi-layered buffer device annealed at 350 °C resulted in conductive characteristics due to the excess carrier concentration from the higher hydrogen density of 2.12 × 1021 cm−3.

Selectively Enhanced UV-A Photoresponsivity of a GaN MSM UV Photodetector with a Step-Graded AlxGa1-xN Buffer Layer.

The UV-to-visible rejection ratio is one of the important figure of merits of GaN-based UV photodetectors. For cost-effectiveness and large-scale fabrication of GaN devices, we tried to grow a GaN epitaxial layer on silicon substrate with complicated buffer layers for a stress-release. It is known that the structure of the buffer layers affects the performance of devices fabricated on the GaN epitaxial layers. In this study, we show that the design of a buffer layer structure can make effect on the UV-to-visible rejection ratio of GaN UV photodetectors. The GaN photodetector fabricated on GaN-on-silicon substrate with a step-graded AlxGa−xN buffer layer has a highly-selective photoresponse at 365-nm wavelength. The UV-to-visible rejection ratio of the GaN UV photodetector with the step-graded AlxGa1−xN buffer layer was an order-of-magnitude higher than that of a photodetector with a conventional GaN/AlN multi buffer layer. The maximum photoresponsivity was as high as 5 × 10−2 A/W. This result implies that the design of buffer layer is important for photoresponse characteristics of GaN UV photodetectors as well as the crystal quality of the GaN epitaxial layers.

High-power AlGaN-based near-ultraviolet light-emitting diodes grown on Si(111)

High-power AlGaN-based 385 nm near-ultraviolet light-emitting diodes (UVA-LEDs) grown on Si(111) substrates are reported. The threading dislocation (TD) density of AlGaN was reduced by employing an Al-composition step-graded AlN/AlGaN multilayer buffer. V-shaped pits were intentionally incorporated into the active region to screen the carriers from the nonradiative recombination centers (NRCs) around the TDs and to facilitate hole injection. The light extraction efficiency was enhanced by the surface roughening of a thin-film (TF) vertical chip structure. The as-fabricated TF-UVA-LED exhibited a light output power of 960 mW at 500 mA, corresponding to an external quantum efficiency of 59.7%.

InAs QDs in a metamorphic In0.25Ga0.75As matrix, grown by MOCVD

InAs quantum dots (QDs) in a metamorphic InGaAs matrix are grown by MOVPE (metal-organic vapor-phase epitaxy) on GaAs substrates. The QDs emit light in the range 1380–1400 nm at room temperature. A multilayer metamorphic buffer (MB) consisting of nine sublayers, each having a thickness of ~200 nm, is used to grow the structures. In each of the first seven sublayers, the indium content x successively increases by ~3.5% to reach a final value of 24.5%. Then a compensating sublayer with x = 28% and a final dislocation-free sublayer with x = 24.5% are grown. It is shown that the elastic strain relaxes with dislocation bending at both interfaces in the third, from the surface, sublayer, with the top layer being free of dislocations at both interfaces. The QDs are formed in the metamorphic matrix by the deposition of 2–2.5 InAs single layers at 520°C, with the subsequent overgrowth of a thin InGaAs layer (cap layer) at the same temperature. It is found that, to improve the optical and structural quality of the samples, it is necessary to raise the growth rate and reduce the concentration of indium in the cap layer with respect to the corresponding growth parameters of the last layer of the metamorphic buffer.

Modeling of the wafer bow in GaN-on-Si epiwafers employing GaN/AlN multilayer buffer structures

We built a calculation model for the wafer bow in GaN-on-Si epiwafers employing GaN/AlN multilayer (ML) buffer structures by extending Stoney’s equation. The calculated bow and the derived strain in the epilayers were almost consistent with experimental results. The calculation quantitatively revealed that the ML buffers introduced an in-plane compressive stress in the epitaxial structures. Also, relationships between the epiwafer bow and the stress in the respective layers became clear to a certain extent. For instance, when considering the case where periodic structures with 20 nm thick GaN/5 nm thick AlN pairs were grown as ML buffers on 4-in.-diameter and 525 μm thick Si (111) substrates at a growth temperature (Tg) of 1125 °C, the stress in the MLs was derived to be 2.18 GPa in the in-plane compressive direction in the GaN layers and 5.89 GPa in the in-plane tensile direction in the AlN layers at Tg. Although magnitude of the in-plane stress in the GaN layers is obviously smaller than that in the AlN layers, the restoring force generated in the GaN layers becomes larger than that in the AlN layers, because the force is in proportion to the layer thickness rather than to just the stress. As a consequence, the generated stress in the MLs was considered to produce enough force to suppress the epiwafer bow. The calculation also demonstrated that GaN epilayers on GaN/AlN MLs were strained in the in-plane compressive direction at Tg and almost strain-free at room temperature.

Research on Impact Stress and Fatigue Simulation of a New Down-to-the-Hole Impactor Based on ANSYS

In the present work, a down-to-the-hole electric hammer driven by linear motor is reported for drilling engineering. It differs from the common hydraulic or pneumatic hammers in that it can be applied to some special occasions without circulating medium due to its independence of the drilling fluid. The impact stress caused by the reciprocating motion between stator and rotor and the fatigue damage in key components of linear motor are analyzed by the ANSYS Workbench software and 3D model. Based on simulation results, the hammer’s structure is optimized by using special sliding bearing, increasing the wall thickness of key and multilayer buffer gasket. Fatigue life and coefficient issues of the new structure are dramatically improved. However buffer gasket reduces the impactor’s energy, different bumper structure effect on life improving and energy loss have also been elaborated.

Heterostructures of metamorphic GaInAs photovoltaic converters fabricated by MOCVD on GaAs substrates

Heterostructures of metamorphic GaInAs photovoltaic converters (PVCs) are on GaAs substrates by the metal-organic chemical vapor deposition (MOCVD) method. It is shown that using a multilayer metamorphic buffer with a step of 2.5% in indium content and layer thicknesses of 120 nm provides the high quality of bulk layers subsequently grown on the buffer up to an indium content of 24%. PVCs with a long-wavelength photosensitivity edge up to 1300 nm and a quantum efficiency of ~80% in the spectral range 1050–1100 nm are fabricated. Analysis of the open-circuit voltage of the PVCs and diffusion lengths of minority carriers in the layers demonstrates that the density of misfit dislocations penetrating into the bulk layers increases at an indium content exceeding 10%.

Metamorphic InGaAs photo-converters on GaAs substrates

Metamorphic heterostructure photoconverters (PC) based on InGaAs have been grown on GaAs substrates by the MOCVD technique. It has been shown that using the multilayer metamorphic buffer with layer thickness of 120 nm and In composition step of 2.5% provides a good quality of the bulk layers grown on the buffer with up to 24% of In. A PC with photosensitivity up to 1350 nm and the quantum efficiency of 80% in the range of 1050-1100 nm have been made. The 34.5% efficiency of laser radiation conversion at 1064 nm has been demonstrated.

Effects of multilayer buffer on structural properties of ZnO nanostructures grown using a solvothermal method

This paper reports on a solvothermal method used in the fabrication of vertically aligned two-dimensional (2D) ZnO nanowall networks and one-dimensional (1D) ZnO nanorods with overlapping structures. The introduction of an Al thin film as a doping source was shown to alter the morphology from rods to walls. The proposed multilayer buffer results in the formation of two types of nanostructure (nanowalls and rods) and also increased the length to which the nanorods grow at the same time. Analysis based on transmission electron microscopy (TEM) and selected area electron diffraction (SAED) led us to formulate a possible growth mechanism and elucidate the optical properties of the ZnO nanostructures.

Gradient Al-doped ZnO multi-buffer layers: Effect on the photovoltaic properties of organic solar cells

In this work, gradient Al-doped ZnO multi-buffer layers (MBLs) were used as the electron transport layers (ETLs) of organic solar cells (OSCs) for the first time. Three kinds of MBLs were prepared using a facile sol–gel process and deposited on a homo-buffer layer (ZnO). Compared to a device based on a multi-ZnO buffer layer, devices with the gradient-doped MBL displayed enhanced power conversion efficiency from 2.11% to 3.43%, which were caused by the efficient light transmission and charge carrier transport, derived from the higher film qualities, denser and more uniform surface morphologies, and improved conductivities of the gradient-doped MBLs. Results indicated that a device with ZnO/MBL (0.5:1.0at%(Al/Zn)) simultaneously enhanced the short-circuit current and fill factor. This study presents an innovative and effective method for improving the quality of ETLs for efficient OSCs.

(Invited) High-Power AlGaN/GaN Heterostructure Field-Effect Transistors on 200mm Si Substrates

GaN-based electronic devices have established their niches for both high power switching application and high frequency operation due to their wide-bandgap, excellent transport properties, and high critical electric field, which cannot be achieved by the Si-based electronics. Especially, AlGaN/GaN heterostructure field-effect transistors (HFETs) have shown great promises in a wide range of applications, such as highly efficient power amplifiers for mobile phone base-stations, commercial and military radar/satellite communications, power rectifiers with unprecedentedly low loses, and various circuits in industrial equipment and electric/hybrid vehicles. Among all suitable substrates for III-nitride crystal growth, such as sapphire, SiC, silicon, and GaN bulk, Si substrates have the advantage of the lowest cost, and are available in diameters of beyond 300 mm, although the substrate size available for nitride growth is limited by suitable deposition equipment and the requirement of (111) substrate orientation; they are also easily compatible to be fabricated in well-developed Si–based foundry. To realize the GaN performance in a cost-down platform and to continue using well-developed device fabrication technologies easily scalable to 200 mm and beyond in modern semiconductor industry, the integration of GaN-based electronics with Si platform is not only extremely emergent but also a win-win investment. Despite the tremendous promise and progress in GaN-on-Si technology [1], challenges associated with epitaxial growth of large diameter wafers remain. In this work, we will summarize current status of AlGaN/GaN HFET growth on (111) on-axis silicon substrates varying from 100 to 200 mm in diameter by using metal–organic chemical vapor deposition (MOCVD). All substrates used for this study were per SEMI standard thickness. The total thickness of the HFET epilayers was in the range of 2 – 4.5 µm. The material characterization data will be compared between these III-nitride crystal growths, such as the (002) and (102) x-ray rocking curves, the 2DEG mobility and charge density by Hall effect measurement, etc. The devices analyzed in this study were fabricated using our standard device fabrication technology for AlGaN/GaN HFETs on Si. Ti/Al/Ni/Au multilayer was first deposited and annealed at 850°C in a nitrogen environment to form an Ohmic contact. Multiple doses and energies of nitrogen ion implantation were used for the device isolation and to maintain a planar geometry in the fabricated device to further reduce parasitic leakage current. Ni/Au based Schottky gate metallization was defined by using optical lithography followed by a standard lift-off process of the e-beam deposited metals. The gate dimension was 2 µm × 100 µm. The distances of gate-to-source (Lgs) was fixed at 2 µm while the gate-to-drain distance (Lgd) varied from 2 µm to 10 µm. The isolation characteristics were collected between two 250 µm × 150 µm Ohmic metal pads with 5 µm spacing. Typical DC characteristics will be examined on the similar epitaxial structures grown on 100, 150 and 200 mm Si substrates and also compared between different epitaxial structures on 200 mm Si substrates. For high voltage operation, the two-terminal isolation blocking voltage/leakage, as well as three-terminal off-state breakdown voltage and off-state drain leakage current for AlGaN/GaN HFETs on 200 mm silicon substrates with different thickness of the multi-buffer layers, based on (Al,Ga)N materials, will be discussed. In order to suppress the off-state drain leakage current and improve the off-state breakdown voltage, a highly-resistive blocking buffer layer is needed, and therefore the intentional dopant [2], carbon, is utilized in the multi-buffer layers in this work. As shown in the figure below, the isolation blocking voltage is demonstrated to increase up to 720V at 1µA/mm on 5µm-spacing isolation pads with increasing the thickness of the total epitaxial layers up to 4.5 µm and the isolation leakage current is as low as about 5 nA/mm. Based on the insulating multi-buffer layers, high off-state breakdown voltage over 1000 V and low leakage current of 5×10-8A are achieved with AlGaN/GaN heterostructures on 200 mm Si substrates. 1. H. F. Liu, S. B. Dolmanan, L. Zhang, S. J. Chua, D. Z. Chi, M. Heuken, and S. Tripathy, Journal of Applied Physics 113, pp. 023510 (2013). 2. J. B. Webb, H. Tang, S. Rolfe, and J. A. Bardwell, Applied Physics Letters 75, 953-955 (1999). Figure 1

Application of multi-buffer layer of (Zn,Mg)O/CdS in Cu2ZnSn(S,Se)4 solar cells

(Zn,Mg)O (ZMO) buffer layer has attracted attention for having the potential to control the conduction band offset of buffer layer and large band-gap (Eg) Cu2ZnSn(S,Se)4 (CZTSSe) absorber interface, where the ZMO layer is deposited by the sputtering. However, the solar cell efficiency is decreased with the ZMO layer as compared with the CdS layer. The decrease in conversion efficiency is attributed to the sputtering damage on the absorber and high light reflection from the surfaces of CZTSSe solar cells. To completely suppress the damage, a CdS layer with very thin thickness of 20 nm is inserted between the ZMO layer and the CZTSSe layer. In addition, MgF2 layers are deposited on CZTSSe solar cells as anti-reflection coating. Ultimately, the solar cell with multi-buffer layer of ZMO/thin-CdS is almost same level as that with the CdS layer. Therefore, the multi-buffer layer can be an appropriate buffer layer of the large-Eg CZTSSe layer.