AlGaN-based material characterizations and recent development of related solar-blind ultraviolet detectors
Author(s):
Xiangyang Li;
Jintong Xu;
Yan Zhang;
Yonggang Yuan;
Kaihui Chu;
Chao Li;
Xichang Bao;
Ling Wang
Show Abstract
Recently, AlGaN-based material and related devices have been investigated intensively because of their applications in
ultraviolet solar-blind detectors, blue light-emitting diodes, UV laser diodes, and high-power-temperature devices. Due
to intrinsic difficulty to grow high aluminum fraction material, achieving reasonable good quality of wafers is the key
aspect to fabricate high performance solar-blind AlGaN-based detectors. Transmission spectra, XRD, and a two
dimension transmission scanning system were employed to evaluate the properties of AlGaN-based material. Wet
chemical etching process of n-type AlGaN in 20% aqueous KOH solutions was performed to reduce dry etching
damages and the average leakage current. I-V characterization indicated that the average leakage current of the wet
etching treated detectors was lower than that of the detectors without treatment by about one order of magnitude.
Ti/Al/Ti/Au contact on n-type Al0.63Ga0.37N was optimized to get a low series resistance. 128×128 solar-blind AlGaN UV
Focal Plane Arrays (FPAs) were fabricated and the performance were characterized. A CTIA (capacitive-transimpedance)
readout circuit architecture has been proven to be well suited for AlGaN detectors arrays. The results show that 128×128
back-illuminated AlGaN PIN detector SNR is as high as 74 db at a speed of above 30 frames per second.
Bright and color-saturated quantum dot light-emitting diodes: new star for next generation displays and solid state lighting
Author(s):
Zhanao Tan;
Jian Xu;
Ting Zhu;
Andrew Y. Wang;
Qingjiang Sun;
Yongfang Li;
Yongping Yang
Show Abstract
The emergence of solution-processed semiconductor quantum dot (QD)-light emitting diodes (LEDs) has
recently offered a great prospect for developing low-cost, efficient, bright, and large-area colorful displays compatible
with flexible substrates. The band-edge electroluminescence (EL) of colloidal QDs of cadmium compounds, exhibits
size-tunable spectral emission (450-760nm) and narrow bandwidth (FWHM~15-30nm), allowing for the design and
fabrication of color-saturated red, green and blue (RGB) QD-LEDs with simple device configurations and high spectral
purities that outperform those of liquid crystal displays and organic light-emitting diodes. Additionally, high fluorescence
quantum yield and photochemical stability can be achieved by engineering the nanocrystal surfaces with wide-bandgap
shells, which favors the efficiency and stability of QD-LEDs. We report in this conference of our recently developed red,
orange, yellow, green, blue and white quantum dot light-emitting diodes with high brightness, high efficiency, saturated
color on both indium tin oxides (ITO) coated glass and on flexible PET substrate.
Cadmium sulfide quantum dots grown by chemical bath deposition for sensitized solar cell applications
Author(s):
D. W. Zhang;
S. Chen;
X. D. Li;
Z. A. Wang;
J. H. Shi;
Z. Sun;
X. J. Yin;
S. M. Huang
Show Abstract
Cadmium sulfide (CdS)-sensitized solar cells have been investigated. Chemical bath deposition (CBD) based on an
aqueous medium involving cadmium acetate, thiourea, ammonium acetate and ammonia was utilized to assemble CdS
quantum dots (QDs) onto mesoporous TiO2 films for dye-sensitized solar cell (DSC) applications. Physical and chemical
properties of the formed TiO2/CdS bilayers were analyzed using field emission scanning electron microscope (FE-SEM),
Raman spectroscope, ultraviolet-visible light (UV-VIS) spectrometer and X-ray diffraction (XRD). An efficiency as high
as 1.50% for the CdS Q dots-sensitized DSC was achieved using the present method. The results have demonstrated the
potential applications of CBD CdS Q dots for sensitized solar cells.
Centrotherm's high end CIGS thin film turnkey solution
Author(s):
Dieter Schmid;
Immo Koetschau;
Andreas Kampmann;
Thomas Hahn;
Jan Hinze;
Wei Zhang
Show Abstract
Centrotherm photovoltaics AG has chosen Cu(In,Ga)Se2 (CIGS) as material system for turnkey thin-film photovoltaic
production lines because of its great potential concerning conversion efficiencies and economical production. Within
CIGS thin-film technology, centrotherm overcomes several disadvantages of former technologies with a unique
technology for the CIGS absorber layer formation. The new technology is based on sputter technology on one hand side
and a rapid thermal process on the other side. Precursor metal layers are sputtered on large area and transformed to semiconducting
CIGS in an ultra rapid thermal process step, which is performed under atmospheric pressure conditions. This
leads to unreached short process time on the order of 1 minute and hence to a very high productivity, and prerequisites
for real mass production on the GW scale.
Density functional study of neutral and charged titanium dioxide clusters
Author(s):
Dongmei Li;
Zhihua Xiong;
Qixin Wan;
Yan Lv;
Jianfei Peng
Show Abstract
With density functional theory (DFT), the structural and electronic properties of both neutral and different
negatively charged (TiO2)n clusters with n=1-5 have been investigated. The HOMO-LUMO (highest occupied molecular
orbital- lowest unoccupied molecular orbital) energy gaps as a function of the charge have been presented. The
calculated results suggest that the least change of energy gap between (TiO2)4 and (TiO2)4
2- happens, and the following is
that of (TiO2)3 clusters. Distribution of extra electrons in (TiO2)n
- and (TiO2)n
2- has also been performed. The calculated
results indicate that in both these two kinds of negative clusters, the excess charge is localized around the titanium with
the lowest coordinated number. These theoretical results may be helpful for understanding the influence of electron
transport on the small (TiO2)n clusters better.
Design and analysis of solar energy photovoltaic system
Author(s):
Tianze Li;
Chuan Jiang;
Xia Zhang;
Luan Hou
Show Abstract
First of all, the paper represents three main components, structure and working principle of solar energy photovoltaic
system. It analyzes the principles and design elements of photovoltaic power generation system design, represents Steps
and ideas of the design of the photovoltaic system, illustrates the acquiring informationand the method of software,
hardware design at the same time. Finally, it discusss the applications of the photovoltaic technology.
Effects of substrate temperature on properties of the aluminum-doped zinc oxide thin films deposited by RF magnetron sputtering
Author(s):
Yanyan Yang;
Xiangbin Zeng
Show Abstract
The 3wt% aluminum-doped zinc oxide film (AZO) was sputtered on glass by radio frequency (RF)
magnetron sputtering using ceramic target. What's more, sputtering was carried out at different
substrate temperatures from 200°C to 350°C with normal deposition. The effect of deposition
temperature on structural, optical and electrical properties of the AZO films was studied. It was found
that the films become more uniform and compactness as the deposition temperature increased from
200°C to 300°C. Highly oriented AZO films in the (002) direction were observed in specimens
deposited at 250°C and 300°C. In addition, It was showed that the average optical transmittance of
specimens was about 85%. And there was an increase in electrical conductivity when deposition
temperature increased from 200°C to 300°C. The lowest resistivity of 1.05×10-3Ω•cm was obtained in
specimen deposited at 300°C, which makes them available as transparent conductive electrodes in
photonics devices including LCD flat panel displays and solar cells.
Effects of the thickness of low-temperature AlN interlayers on GaN layers grown on Si(111) substrates by MOCVD
Author(s):
R. F. Xiang;
J. N. Dai;
L. Zhang;
Y. Gao;
Z. H. Wu;
C. Q. Chen;
Q. Feng;
Y. Hao
Show Abstract
Silicon is a promising substrate for GaN growth due to its low cost, large size and potential applications in the
integration of optoelectronic and microelectronic devices. However, a high lattice (~17%) and thermal (~56%) mismatch
between Si substrate and GaN epilayer causes large tensile stress and leads to the formation of cracks when the GaN
layer thickness is above 1.0 μm during the cooling down to room temperature. Therefore, the growth of high quality and
crack-free GaN layers on silicon substrates is still a challenging issue. It has been widely reported that cracking problem
can be mitigated by using techniques like low-temperature (LT) AlN interlayers, AlxGa1-xN transition layer, thin SixNy
interlayers, AlN/GaN superlattices, etc. In this work, we used the LT-AlN interlayers for strain engineering during the
growth of GaN layers by low-pressure metal-organic chemical vapor deposition (LP-MOCVD) on Si(111) substrates.
And we investigated the fluence of the thickness of LT-AlN interlayers, which is considered as one of the most important
factors in reducing tensile stress and controlling density of micro-cracks in GaN layer. Optical microscopy, atomic force
microscopy, X-ray diffraction and Raman spectrum were employed to characterize these samples of GaN epilayers. The
results demonstrate that the crystal quality of GaN depends strongly on the thickness of the epilayers, and crack-free
GaN layers with thickness exceeding 1.5 μm can be achieved by using LT-AlN interlayers with an optimized thickness.
Efficiency enhancement of GaN-based LED using nanotechnology
Author(s):
C. H. Chiu;
M. A. Tsai;
Peichen Yu;
H. C. Kuo
Show Abstract
We had demonstrated several novel methods to improve the luminescence efficiency of the GaN-based light emitting
diodes (LEDs). The high-aspect-ratio GaN nanorods, formed by spun nano-spheres and inductively coupled plasma (ICP)
etching, contributed to an enhancement in light output power and better light directionality. Nevertheless, the etching
process would affect the electrical properties. We then attempt to rough the surface by synthesizing ZnO nanorods in
liquid solution at room temperature. The LEDs with ZnO nanorods enjoyed high extraction efficiency and comparable
electric performance than that without nanorods. At third part, we fabricate a high efficiency GaN-based LED by
regrowth on SiO2 nanorod patterned sapphire substrate. It could improve the light extraction and internal efficiencies
simultaneously.
Electrochemical study on the TiO2 porous electrodes for metal-free dye-sensitized solar cells
Author(s):
D. W. Zhang;
S. Chen;
X. D. Li;
Z. A. Wang;
J. H. Shi;
Z. Sun;
X. J. Yin;
S. M. Huang
Show Abstract
Nanocrystalline TiO2 porous electrodes were prepared by screen-printing method in order to efficiently control the
fabrication process. TiO2 viscous pastes were prepared from commercial TiO2 nano powder using ethyl cellulose as a
porosity controlling agent. A metal-free organic dye (indoline dye D102) was used as a sensitizer. TiO2 porous
electrodes with different thicknesses were investigated. The optical and physical properties of the TiO2 films, dye
adsorption behavior and performance of dye-sensitized solar cells (DSCs) were investigated systemically. The electronic
and ionic processes in DSCs were analysized and discussed by electrochemical impedance spectroscopy (EIS). High
conversion efficiencies over 8.00 % under illumination of simulated AM1.5 sunlight (60mW/cm2) were achieved.
Field-emission SEM characterization of novel ZnO thin films grown by magnetron sputtering on the different substrates
Author(s):
Tangchao Peng;
Xiangheng Xiao;
Changzhong Jiang
Show Abstract
Zinc oxide is a direct, wide bandgap semiconductor material with many promising properties for blue/UV
optoelectronics, transparent electronics, spintronic devices and sensor applications. In this work, zinc oxide films were
deposited by RF magnetron sputtering on different substrates. The images of field-emission SEM are used to analyse the
structures of the samples, and some novel structures of the ZnO thin films grown on quartz glass and normal glass
substrate are found. The x-ray diffraction is used to analyse the grain structure of these samples, it is shown that films
grow mainly with the hexagonal c-axis perpendicular to the substrate surface.
First-principles calculations of crystal structure of AgxZn1-xO alloys
Author(s):
Qixin Wan;
Zhihua Xiong;
Dongmei Li;
Guodong Liu
Show Abstract
In this paper we have studied AgxZn1-xO alloys by the method of the density functional theory with the generalized
gradient approximation and the projector augmented wave pseudopotentials. In order to calculate the crystal structure of
the AgxZn1-xO alloys with wurtzite structure, we adopt a 32-atom AgnZn16-nO16 supercell which allows the simulation of
the silver composition x=0.0, 0.0625, 0.125, 0.25, 0.375, 0.50, 0.625, 0.75, 0.875, and 1.0. The calculated results show
that the formation energies and the calculated lattice constants of AgxZn1-xO increases by the x increasing. As a result, the
doping of Ag becomes more difficult with the increment of Ag concentration. Furthermore,the solid solubility of Ag in
wurtzite ZnO is small.
First-principles study on distribution of Ag in ZnO
Author(s):
Qixin Wan;
Zhihua Xiong;
Dongmei Li;
Guodong Liu;
Jianfei Peng
Show Abstract
Except for the group-V dopants, Ag, as a group IB element, could also act as an acceptor in ZnO, if incorporated on
substitutional Zn sites. In this paper, first-principles density-functional calculations have been performed to investigate
various distributions of Ag in ZnO. The first-principles calculations were carried out using the density functional theory
with the generalized gradient approximation (GGA) and the projector augmented wave (PAW) pseudopotentials. The
supercell employed contained 32 atoms that corresponded to a 2×2×2 supercell of ZnO. The various distributions of Ag
in ZnO have been calculated corresponding to each possible location. In conclusion, the calculation results show that the
formation energies of Ag on the substitutional Zn site (AgZn) and incorporation in the interstitial site (Agi) are smaller
than that of Ag on the O site (AgO). When AgZn and Agi coexist and are partitioned by an oxygen atom layer, the
formation energy and the total energy is the smallest. As a result, Ag prefers to distribute discretely in Ag doped ZnO. It
is also found that our results are in agreement with other experimental results.
GaP single crystal layers grown on GaN by MOCVD
Author(s):
Shuti Li;
Jianxing Cao;
Guanghan Fan;
Yong Zhang;
Shuwen Zheng;
Huiqing Sun;
Jun Su
Show Abstract
The GaP layers grown on GaN layer by Metalo-organic chemical vapour deposition (MOCVD)
were studied. The results show that the GaP layers are single crystals when the growth temperature
varied from 350°C to 500°C. However, the crystal quality of GaP grown at low temperature is not
good. Using a 40 nm GaP buffer layer grown at 400°C, the crystal quality of GaP grown at 680°C
improves evidently. The p-type GaP layers with hole carrier concentration of 4.2×1018 cm-3 were
obtained by CP2Mg doping.
High-resolution x-ray diffraction studies of highly curved GaN layers prepared by hydride vapor phase epitaxy
Author(s):
J. Q. Liu;
Y. X. Qiu;
J. F. Wang;
X. Guo;
K. Huang;
K. Xu;
H. Yang
Show Abstract
In addition to dislocations, the wafer curvature can also affect the broadening of x-ray rocking curve (XRC) peaks and it
may deteriorate the accuracy of the tilt and twist angle measurements. In this paper, the radial-distribution of curvature
and the effects of wafer curvature on XRC of highly curved (the radius curvature of r less than 2.5 m) GaN layers were
studied. Curvature-related effects both on symmetric geometry and on the (102) skew symmetric geometry were studied
by the use of adjustable beam slits and 'antis.slit' placed before the sample and before the detector, respectively. The
acceptable approaches to minimize the curvature-related effects in determination of a reliable tilt or twist angle were
proposed. It is found that the curvature-related effects can be eliminated by the use of the methods we proposed. The
dislocation densities obtained by high resolution x-ray diffraction (HRXRD) are fairly consistent with that obtained by
cathodoluminescence (CL) and atomic force microscope (AFM).
ICP etching of high Al mole fraction AlGaN
Author(s):
Zhao Meng;
Libo Yu;
Xiao Li;
Qibin Liu;
Huiqiang Duan;
Chenhui Yu;
Changqing Chen
Show Abstract
The etching process has great influence on the performance of solar blind detector based on AlxGa1-xN epitaxial
layers on sapphire substrate with high Al mole fraction grown by metal organic chemical vapor deposition
(MOCVD) method. Traditional etching methods, including wet or reactive ion etching (RIE) are hard to achieve
good result due to the high chemical-stability of AlGaN films with high Al mole fraction. In this paper, we studied
on the inductively coupled plasma reactive ion etching (ICP-RIE) of high Al mole fraction AlxGa1-xN films (x>0.4)
for fabricating high performance solar blind detectors. SiN was used as mask, and Cl2 and BCl3 were used as
etching gas. Etching systems was selected from Oxford Inc. DC bias was controlled automatically. A 2.5:1 of
selectivity on AlGaN and SiN was obtained with suitable flux and component of etching gas, RF power and ICP
power. Etching velocity was adjusted mainly by RF power. The role of Ar, Cl2, and BCl3 in the etching process was
also discussed.
Improved performance of organic light emitting devices using triazole/ Cs2CO3/Al cathode
Author(s):
Jiarong Lian;
Haitao Xu;
Fangfang Niu;
Yawei Liu;
Pengju Zeng
Show Abstract
In this study, a new cathode system of triazole (TAZ)/Cs2CO3/Al has been proposed. Obvious
efficiency improvement and sharp enhanced electron injection were maintained in the devices with this
new cathode. TAZ owns deep the highest occupied molecular orbital (HOMO) and wide band gap, so
as to block the hole leakage effectively and confine the exciton in Alq3 emissive layer. An abrupt
lowering of the vacuum level was found in the mixture of TAZ and Cs2CO3, which indicated that the
electron injection barrier from metal cathode into Alq3 was greatly reduced. Our experimental proved
that this new cathode may have a wide application in OLEDs emitting light with short wavelength.
Influence of alumina coating on transport and recombination in DSSCs with 1-methylbenzidazole as electrolyte additives
Author(s):
Xueqing Xu;
Eva Maria Barea;
Francisco Fabregat-Santiago;
Juan Bisquert;
Gang Xu
Show Abstract
Nanocrystalline TiO2 electrodes have been coated with alumina by being immersed into the solutions of
aluminum isopropoxide and aluminum acetylacetonate respectively. The current-voltage characteristics of the TiO2 and
TiO2/Al2O3 electrodes with 1-methylbenzimidazole (MBI) as additives in the electrolytes have been detected. It is found
that the TiO2/Al2O3 electrodes immersed in the solutions of aluminum isopropoxide (acetylacetone as coordinate ligands)
for 1 hour have obtained great increase of Jsc, and owned the improved cells efficiency by 17%. To discover the origins
of this improved photovoltaic performance and investigate the electron properties of the cells, the electrochemical
impedance spectroscopy (EIS) has been used both in the dark and under illumination at different applied potentials. It is
proposed that with the coating of Al2O3, the adsorption of MBI cations at the surface of TiO2 electrodes decreased with
the decrease of the applied potentials, which resulted in the downwards shift of the conduction band position of the
TiO2/Al2O3 electrodes at the lower potentials. As a result, the charge transfer resistance Rct decreased, and chemical
capacitance Cfilms increased. Under illumination, the decrease of Rct became smaller and the conductivities became larger,
which was attributed to the recombination inhibition effect of Al2O3 coating to the photoinjected electrons. As a
consequence, the electron lifetime and the effective diffusion length for the TiO2/Al2O3 electrodes increased, leading to
the great increase of Jsc. It is indicated that with the coating of Al2O3, the decrease of Jsc could be overcome when MBI is
used as the additives of the electrolyte.
Influence of different parameter profiles on the formation of aluminum back surface field (Al-BSF) using for HIT solar cell
Author(s):
Xiangbin Zeng;
Yu Zeng;
Qusay Assim Hanna AL-Naser;
Chunlan Zhou;
Xiao Zhang;
Qiankun Chen;
Yanyan Yang;
Luo Liu
Show Abstract
The aluminum back surface field used in p-type substrate hetero-junction with intrinsic thin film (HIT) solar cell is
studied in this paper. The enhancement of material quality and the decrease of wafer thickness will make it necessary to
passivate the back surface. It simply states the principle and formation process of aluminum back surface field, and
studies the evenness of back surface field because it is necessary for high efficiency solar cell. Screen-printing and rapid
thermal annealing were used to make aluminum back surface field to gain low recombination on the backside of solar
cells. In this experiment, we analyze Al-BSF formed by taking different times and temperatures in which the temperature
varied in the range between 620°C and 940 °C step 80°C and the time varied from 60 seconds to 180 seconds with a step
of 30 seconds. Minority carrier lifetime with the back surface field was measured by microwave photoconductive decay
(μ-PCD) device, analyze and get the optimal parameters of forming Al-BSF and further improve conversion efficiency of
silicon solar cell.
Influence of the base layer thickness and the graded buffer layer thickness on the conversion efficiency of a metamorphic triple-junction solar cell
Author(s):
Minghui Song;
Hu Wang;
Yankai Xiong;
Yuqin Sun;
Chenhui Yu;
Zhihao Wu;
Changqing Chen
Show Abstract
Metamorphic multiple-junction soalr cells based on III-V compound semiconductor have advantages of more degree
of freedom in selection of bandgaps of subcells. Recently, Spectrolab, Inc. reported a high conversion efficiency of
40.7% in a triple-junction Ga0.44In0.56P/Ga0.92In0.08As/Ge solar cell where the top and middle cells are lattice-mismatched
to Ge substrate. Optimization of device structure of such metamorphic Ga0.44In0.56P/Ga0.92In0.08As/Ge solar cell is
important to increase its efficiency. In this work, two-dimensional simulation has been performed on the metamorphic
Ga0.44In0.56P/Ga0.92In0.08As/Ge solar cell. Efficiency dependence of the Ga0.44In0.56P/Ga0.92In0.08As/Ge triple-junction solar
cell on the base layer thickness and the InGaAs graded buffer layer thickness has been investigated. It has been found
that the efficiency depends significantly on the thickness of the GaInP base layer. The metamorphic triple junction solar
cell has hightest efficiency when the thicknesses of the GaInP base layer, the GaInAs base layer and the GaInAs graded buffer layer connecting the Ge substrate and GaInAs middle-cell are 0.35 μm, 2.5 μm and 0.15 μm, respectively.
Investigation of photoelectrochemical etching of textured silicon in solar cells
Author(s):
Jinchuan You;
Jing Shi;
Bobo Peng;
Lianwei Wang;
Paul K. Chu
Show Abstract
Light trapping is an important method to increase the efficiency of solar cells. In this work, we study the
combined effects of photoelectrochemical etching and texturing to enhance light trapping. Porous silicon (PSi) as an
antireflection coating is fabricated on the surface of a textured silicon layer by photoelectrochemical etching. The
effects of different electrochemical conditions such as acidic solutions, illumination, current densities, and etching time
on the solar cell parameters and surface reflectance from the silicon are evaluated. Our results indicate that the average
reflectance of the surface is lower when electrochemical etching is combined with texturing and the PSi thus formed can
serve as an antireflection layer in single-crystal silicon solar cells. Our SEM results acquired from samples prepared
under different conditions show that the morphology of the textured surface can be improved by electrochemical etching.
Junction temperature measurement on light-emitting diodes and its application
Author(s):
S. M. He;
B. Zhang;
N. Li;
S. S. Liu;
T. Zhang;
W. Lu
Show Abstract
Recently, the GaN based LED white light illumination technology has been developed rapidly. It is required to measure
the p-n junction temperature of LED which has been encapsulated in the lamp with the temperature measurement error
less than 1K. We present a spectroscopy approach to measure this p-n junction temperature of LED encapsulated in the
lamp. The electroluminescence peak shift is used to determine the temperature difference between the junction and the
ambient. At ambient temperature of 300K, the junction temperature can be determined by the formula of
T=300+27.8▵λp-1.22▵λp
2 with the unit of temperature T in K and electroluminescence peak shift ▵λp in nm. This
method has been used on the high power lamp made by LED. The LED lamp has been fabricated for the illuminated
application on the satellite and the spacecraft in China.
Minority carrier mobility measurement in HgCdTe with light-modulated Hall effect
Author(s):
Fei Yin;
Bo Zhang
Show Abstract
The light modulated Hall measurement system was set up in order to measure the minority carrier mobility of the p-type
HgCdTe material. Minority carrier mobility is one of the main parameters for HgCdTe infrared photodetectors, because
it determines the diffusion length of minority carriers, which plays a big role in the performance of optoelectronic
devices. Therefore, it is important to get the minority carrier mobility in HgCdTe, so as to evaluate the material property
before fabrication of photodetectors. By adding a modulated laser to the Hall system, the modulated Hall voltage was
measured on p-type Hg1-xCdxTe(x=0.233) sample over a range of 0-1.8T in the magnetic field at temperature of 75K. The
modulated signal is generated by the movement of excess photocarriers in the magnetic field, so it has a relationship with
the magnetic field, the photogenerated carrier concentration and the electron and hole mobilities. Since the majority
concentration and mobility can be derived from the Hall effect, the minority mobility and the photogenerated carrier
concentration were obtained from fitting the light-modulated Hall voltage ▵VH as a function of the magnetic field B. As
compared to the references, the minority mobility we have obtained is reasonable, so the light-modulated Hall effect is
an effective way to obtain the minority carrier mobility.
Novel materials for high-efficiency solar cells
Author(s):
Nobuaki Kojima;
Masato Natori;
Hidetoshi Suzuki;
Makoto Inagaki;
Yoshio Ohshita;
Masafumi Yamaguchi
Show Abstract
Our Toyota Technological Institute group has investigated various novel materials for solar cells from organic
to III-V compound materials. In this paper, we report our recent results in conductivity control of C60 thin films by
metal-doping for organic solar cells, and mobility improvement of (In)GaAsN compounds for III-V tandem solar cells.
The epitaxial growth of Mg-doped C60 films was attempted. It was found that the epitaxial growth of
Mg-doped C60 film was enabled by using mica (001) substrate in the low Mg concentration region (Mg/C60 molar ratio <
1). The crystal quality of the epitaxial Mg-doped C60 film was improved drastically in compared with micro-crystalline
film on glass substrate. Such drastic improvement of crystal quality in the epitaxial films resulted significant increase in
conductivity. This result may indicate the significant increase of carrier mobility.
Crystal quality improvement of CBE-grown GaAsN materials was investigated. We achieved the reduction of
residual impurity concentration by chemical reaction control on the growing surface by modifying flow sequence of
precursors and by increasing step density on the surface by using a vicinal GaAs substrate. Furthermore, the
improvement in carrier mobility was observed, and it was suggested that the reduction of both residual impurities and
N-related defects leads this improvement.
Numerical simulation study of a distributed fiber-optic lighting system
Author(s):
Shuhua Li;
Huaping Gong;
Yan Hong;
Jixuan Chen
Show Abstract
A distributed fiber lighting system with double-ended light sources is simulated numerically by using the software
Matlab 7.0. The key factors affecting the lighting power and uniformity of the distributed fiber lighting system, such as
light leakage rate, the distance of adjacent fiber loops and the number of the fiber loops are analyzed. The simulation
results show that, the high lighting power and good uniformity are conflictive objects. The compromising effect is
obtained on the condition that the number of fiber loops is 20, light leakage rate is 4.0% and the adjacent distance is 5
meters.
Optical properties of BiFeO3 films grown by Sol-gel method
Author(s):
Gang Sheng;
Long Zeng;
Zhipeng Liu;
Zhenlun Zhang;
Pingxiong Yang
Show Abstract
BiFeO3 thin films on different substrates were prepared via Sol-gel method. X-ray diffraction patterns
indicated all the films were well-crystallized and gave X-ray peaks corresponding to BiFeO3. The influence of different
annealing temperatures and substrates to the Raman spectra was investigated. The Raman spectra of the 750 °C-annealed
films gave almost the completed E modes. Compared with the film deposited on Si substrate, the positions of E modes
peaks in Raman spectra of the film deposited on LNO-coated Si substrate shifted to higher frequency regions and the
existence of the stress between the BiFeO3 films and the substrates was considered to cause the shift.
Preparation and photoelectric properties of Fe-doped mesoporous TiO2 thick films used in DSSC
Author(s):
Yian Xie;
Yue Shen;
Feng Gu;
Huina Lu;
Mingming Wu;
Linjun Wang
Show Abstract
Fe-doped mesoporous TiO2 (M-TiO2-Fe) thick films were prepared by sol-gel and screen printing process. Raman
characteristics results show that the M-TiO2-Fe thick film possesses a certain degree of the anatase phase, which may
have advantages on photocatalysis and photovoltaic ability. Derived from small angel X-Ray diffraction (SAXRD), the
films exhibit mesoporous structure with pore size around 7-8 nm. Eg of the films was obviously narrowed from 3.4 eV to
3.0 eV, which allows the thick films using more light to initiate photovoltaic process. Dye-sensitized solar cell (DSSC)
based on M-TiO2-Fe was structured and chlorophyl was used as sensitizers. The solar cells have an open circuit voltage
above 260mV.
Preparation and photoelectric properties of mesoporous ZnO/TiO2 composite films for DSSC
Author(s):
Mingming Wu;
Yue Shen;
Feng Gu;
Yun Zhang;
Yian Xie;
Jiancheng Zhang
Show Abstract
Mesoporous TiO2/ZnO (M-TiO2-xZn (x=10, 30, 50, 70, 90)) composite films have been prepared in sol-gel method
by spin coating process. The films have sponge-like microcrystalline phase structures, as evidenced from TEM. The
SAXRD patterns of M-TiO2-xZn thin films suggest that the films exhibit mesoporous structure. The diameter/d value of
thin films is around 11.76 nm, 7.88 nm, 6.89 nm, 10.02 nm and 13.78 nm, respectively. UV/vis transmittance spectra
indicate that the fundamental transmittance edge of M-TiO2-xZn films has a blue shift from 350 nm to 310 nm firstly,
and then has a red shift. Photoluminescence (PL) spectra of M-TiO2-xZn have three emission peaks, which are caused by
near-band edge (NBE) emission and structural defects or/and impurities M-TiO2-xZn thin films display a positive
photovoltaic effect with the Zn content increased above 70 mol %, which may improve the energy conversion efficiency
of M-TiO2-xZn in Dye-sensitized solar cells (DSSC).
Preparation of BST ferroelectric thin films by pulsed laser deposition for infrared sensor
Author(s):
Long Zeng;
Gang Sheng;
Zhenlun Zhang;
Pingxiong Yang
Show Abstract
Barium strontium titanate (Ba1-xSrxTiO3) shows great potential in optical device applications, especially in
infrared uncooled focal plane arrays (UFPAs) fabrication. In this paper, Ba0.5Sr0.5TiO3 ferroelectric thin films were
successfully deposited on different substrates by pulsed laser deposition (PLD) with an aim to fabricate dielectric
bolometer type infrared (IR) sensor. The microstructure, crystalline characterization and surface morphology of the film
were studied by XRD pattern and Atomic Force Microscope (AFM). The fabricated thin films show a similar perovskite
structure, have evenly distributed grains and dense, crack-free surface. The dielectric properties and ferroelectric
properties of the films are also studied after deposition of the top Pt electrodes. Results show that the film has fine
dielectric and ferroelectric properties and could be used for infrared sensor fabrication. Buffer layer effect on film
properties is also discussed. The use of different buffer layers could strongly influence the film performance. As in
optical device applications, the buffer layer effect should be considered in film fabrication.
Removal and passivation of surface defects in perforated GaN-based light-emitting diodes
Author(s):
Y. Yang;
X. A. Cao
Show Abstract
Perforated GaN-based light-emitting diodes (LEDs) with an array of plasma-etched microholes penetrating through the
active region were fabricated using lithography and plasma etching. Plasma damage on the microhole sidewalls led to an
increase in junction leakage by up to seven orders of magnitude and a reduced light emission in the low injection regime.
It was found that KOH can etch off the plasma-damaged materials, leading to a complete suppression of surface leakage
currents. It however attacked metal contacts and increased the forward turn-on voltage. Thermal annealing removed
damage in the near-surface bulk region, whereas (NH4)2S treatment only passivated the defect states at the immediate
surfaces. Both methods produced a partial restoration of the forward-bias characteristics. It has been demonstrated that
annealing at 700 °C used in conjunction with prolonged sulfide passivation can remove or passivate all plasma-induced
defects and result in a complete suppression of surface leakage in the perforated LEDs. This work is an important step
toward developing high-efficiency photonic crystal-integrated LEDs, in which light can only be coupled to radiation
modes but the undesirable guided light emission is inhibited.
Research on light capture of solar cell and its application
Author(s):
Tianze Li;
Luan Hou;
Chuan Jiang;
Lixiu Ma
Show Abstract
First of all, the article represents the character of photovoltaic effect and solar energy of semiconductor devices and the
current situation of solar panel. It analyzes two difficuitys of the appplication of solar energy, the ways of enhancing
solar cell performance and the barriers of producing high-efficiency PV modules. Solar photovoltaic technology, the
technology use of solar energy and industry development trend are preticted.
Research on transparent conductive AZO film fabricated by PECVD method
Author(s):
Zhaoquan Chen;
Minghai Liu;
Yuping Liu;
Peng Hu;
Liang Tang;
Xiwei Hu
Show Abstract
In this paper, AZO (ZnO:Al) polycrystalline thin films is produced with
strong adhesion to the substrate, which deposit on the glass substrate by PECVD
(plasma enhanced chemical vapor deposition) method. The film with the sheet
resistance as low as 89Ω/(see manuscript), and with transmittance as high as 85% in visible light
spans has been obtain. The measurements of the film structure and the experiments to
examine the influence of the film electro-optical property have been shown. This
AZO film fabricated by PECVD method is a useful attempt, and the results are very
important to the choice of actual process of the solar cell.
Structural and optical properties of Bi2VO5.5 thin films deposited on silicon substrates
Author(s):
Zhenlun Zhang;
Zhipeng Liu;
Ming Guo;
Pingxiong Yang
Show Abstract
Ferroelectric Bi2VO5.5 (BVO) thin films have been successfully fabricated on p-type Si (100) substrates by sol-gel
method. The microstructures and surface morphologies of BVO thin films were studied by X-ray diffraction and atomic
force microscopy, respectively. Bi2VO5.5 thin films show c-preferred orientation, which indicates that the films match
very well with the p-type Si substrate. Raman spectra measurements were carried out to study the lattice vibration modes
of BVO thin films. Optical properties of the BVO were investigated due to their potential optical applications. And the
optical properties of the BVO thin films have been studied by spectroscopic ellipsometric measurements in the
wavelength from 400 nm to 1700 nm. The optical constant refractive index and extinction coefficient have been obtained.
The resulting refractive index and extinction coefficient of BVO films show difference due to the different annealing
process of the films, which implies the importance of the post annealing process. The increase of the refractive index
could be understood by the higher density of the BVO films caused by the crystallization after annealing process. The
Optical properties indicate that BVO films have potential applications in optical devices.
Structure and electrical properties of [0001] GaN nanowires
Author(s):
Zhihua Xiong;
Lanli Chen;
Dongmei Li;
Qixin Wan
Show Abstract
We perform density functional theory within the generalized gradient approximation to investigate infinitely wurtzite
bare GaN nanowires in the [0001] direction. We report atomic and electronic structure of GaN nanowires with diameters
of 10 and 16 Å. We find that relaxations on the facets are very similar to the ones in nonpolar (10-10) surfaces and play
an important role in stabilizing the wires, and the average Ga-N bond length of the GaN nanowires decreases compared
with bulk GaN. Both wires are found to be semiconducting and have a direct gap, with band gaps slightly smaller than
that in bulk GaN. The shape of the band edge remains unaltered as the size of the nanowire increases. It is also found, for
Ga and N atoms at the edge of the nanowires, the Ga 3p predominantly contribute to the edge states near the conduction
band minimum, while the N 2p contribute mainly to the edge states near the valence band maximum. The present
calculated results are helpful to gain a systematic understanding of structure, electrical properties of wurtzite GaN
nanowires.
Study on performance of p-Si thin film fabricated by aluminum induced lateral crystallization at low temperature
Author(s):
Qiankun Chen;
Xiangbin Zeng;
Yu Zeng;
Luo Liu;
Yanyan Yang
Show Abstract
Nowadays, low temperature polycrystalline silicon thin film transistor (poly-Si TFT) has been the most popular
subject in active matrix liquid display (AMLCD) and active matrix organic light emitting display (AMOLED). Relative
to the other crystallization technique, aluminum-induced crystallization have certain advantages in lower temperature,
and of course, lower cost than any other methods. This paper gives a research on the performance of poly-Si thin film
fabricated by aluminum-induced electric field enhancing lateral crystallization at low temperature (<600°C). Raman
spectra, X-ray diffraction and scan electron microscope were used to analyze the crystallization state, crystal structure
and surface morphology of the poly-Si thin film. Results show that the poly-Si thin film has good crystallinity, and the
electric field has the effect of enhancing the crystallization when direct current electric voltage is added to the film
during annealing. In addition to this, the metal aluminum induced crystallization was monitored in the whole progress
when Al was diffusing into the a-Si thin film by measuring resistance of crystallizing thin film. The poly-Si thin film
fabricated by this low temperature electric enhancing aluminum-induced crystallization technique was obtained. The
results show that it is suitable for P-Si TFTs which is widely used in AMLCD and AMOLED.
The design of back surface field layer for a single GaAs solar cell
Author(s):
Yankai Xiong;
Kemin Tang;
Minghui Song;
Chao Feng;
Yuqin Sun;
Chenhui Yu;
Zhihao Wu;
Hui Xiong;
Changqing Chen
Show Abstract
The inclusion of a back surface field (BSF) can significantly improve the efficiency of GaAs solar cells due to a better
collection of photogenerated minority carriers. In this work, the efficiency of a single GaAs solar cell has been optimized
by varying the material properties of AlGaAs back surface field layer using an advanced commercial software,
Crosslight APSYS simulator. By optimizing the Al composition and width of the back surface field layer, a maximum
efficiency 25.660% is obtained when Al composition is 0.25 and the width is 0.3 μm. The band diagrams, 2-dimensional
relative energy density profile and I-V curve of the device structure with the optimal back surface field layer are also
obtained by APSYS.
The electrical and optical properties of doped BTO thin films
Author(s):
Zhipeng Liu;
Zhenlun Zhang;
Gang Sheng;
Pingxiong Yang;
Junhao Chu
Show Abstract
BaTiO3 thin films with different dopants of Fe and Co were prepared by sol-gel technique on Si and LaNiO3 coating Si
substrates respectively. The x-ray diffraction patterns showed that all the films were perovskite structure and the change
of lattice constant caused by dopants was undetectable. Atomic Force Microscope (AFM) was used to characterize the
surface morphology of the thin films. The surfaces of films were uniform and dense except Co-doped BaTiO3 film on Si
substrate. The Raman spectra showed the Fe dopant could improve the crystal quality of BTO films on these two
different substrates. The results of UV reflectance for BTO thin films prepared on Si substrates showed that the
reflectance peaks of Fe-doped BTO film moved to the low frequency. The Pt top electrodes were fabricated on
BaTiO3/LaNiO3/Si to form a metal-ferroelectrics-metal structure and the dielectric properties of the thin films were
carried out by impedance analyzer. It was found that Co dopant could increase the dielectric constant of BTO film to
some extent and the Fe dopant could decrease the dielectric constant of film below 30 kHz. Both of the dopants could
increase the dielectric loss of the films with the frequency increasing.
The evaluation and measurement of AlGaN epitaxial layer with high Al mole fraction
Author(s):
Zhao Meng;
Libo Yu;
Xiao Li;
Qibin Liu;
Huiqiang Duan;
Chenhui Yu;
Changqing Chen
Show Abstract
The epitaxial layer quality of AlxGa1-xN (x>0.6) on sapphire substrate grown by metal organic chemical vapor
deposition (MOCVD) needs to be further improved. In this paper, we evaluated the properties of defects, lattice
mismatch between epitaxial layer and substrate, crystal quality and conductivity for these high Al mole fraction
materials from the viewpoint of fabricating high performance solar blind detectors by comprehensive utilizing
various undamaged measurements. The measurement of transmission spectrum was used to evaluate the absorption
edge, band gap, mole fraction of Al content, hetero-epitaxial interface, and transmissivity in the ultraviolet spectral
range. X-ray diffraction (XRD) was used to measure the component of the AlGaN material, uniformity of the
material and crystal quality. The conductivity of the surface layer of the AlGaN film material was obtained by using
high precision current-voltage curve measurement. In short, the material quality, optical and electrical properties,
and uniformity for high Al mole fraction AlGaN epitaxial layers were qualitatively or quantitatively measured and
analyzed. These works lay the foundation for manufacturing high performance solar blind ultraviolet detectors
based on high Al mole fraction AlGaN epitaxial materials on sapphire substrate.
The improvement of ohmic contact of Ti/Al/Ni/Au to AlGaN/GaN HEMT by multi-step annealing method
Author(s):
Qian Feng;
Li-mei Li;
Yue Hao;
Zhi-wei Bi
Show Abstract
A multi-step rapid thermal annealing process of Ti/Al/Ni/Au was investigated for
ohmic contact of AlGaN/GaN high electron mobility transistor(HEMT).The samples
were studied by Transmission Line Model(TLM),Scanning Electron
Microscopy(SEM) and Auger Electron Spectroscopy(AES) measurements. By the
multi-step annealing process, the specific contact resistance was decreased and the surface
morphology was improved. The AES measurements showed that the limitation
indiffusion of Au and outdiffusion of Al were account for the surface morphology
improvement.
The optical properties of rare-earth multi-ferroelectric thin-film LuFeO3 and LuFe1-xMnxO3 fabricated with sol-gel method
Author(s):
Tingjing Xu;
Lipin Zhu;
Pingxiong Yang;
Junhao Chu
Show Abstract
RMO3 (R=rare earth, M=Fe, Mn, Ni) is a kind of multi-ferroelectric material1 while Lu is
a element which has been widely used in electroluminescence area. Material constituted with
Fe and Lu is thought to have special properties on electromagnetic coupling due to the
reactivity of Lu and the magnetism of Fe. For example, LuFe2O4 is a new type of electric
ferroelectric material which has become more and more concerned after Ikeda reported it in
the Nature magazine in 2005.2 We successfully fabricated another LuFeO3 thin film with
sol-gel method on Si substrates with different constituents, and analyzed the structural, optical
properties of the samples. We investigated the optical properties with UV-Vis, PL, etc. After
Mn was doped, the properties were compared with the previous ones.
Theoretical investigation of three dimensional p-n junctions for improvement of silicon solar cell efficiency
Author(s):
Jing Shi;
Jinchuan You;
Lianwei Wang;
Paul K. Chu
Show Abstract
Silicon is one the most extensively used photovoltaic materials because of its relatively low cost and well established
fabrication processes. Therefore, development of technologies that can improve the energy efficiency of silicon solar
cells is important. In this report, a novel solar cell structure based on 3D (three-dimensional) macroporous silicon p-n
junctions and the corresponding mathematical model are proposed. The structure and electrical performance of the
silicon solar cell with the 3D p-n junction are analyzed. Significantly increased efficiency can be achieved due to the
better carrier collection and light absorption. Our results reveal improved photocurrent and other characteristics,
thereby demonstrating the validity of the improved solar cell structure.
Visible luminescence from Dy3+ doped tellurite glasses
Author(s):
Feng Zhang;
Zhisong Xiao;
Lu Yan;
Fang Zhu;
Anping Huang
Show Abstract
This paper reports on luminescence property of the Dy3+-doped tellurite glasses. The samples are studied by absorption
and visible emission spectra. The measured emission spectrum of Dy3+ glass has revealed two main bands at 484 nm
(4F9/2→6H15/2) and 574 nm (4F9/2→6H13/2) when pumping with the wavelength of 325 nm. The concentration quenching
occurred as Dy3+ concentration increased beyond 2 mol%. The chromaticity coordinates of these samples are close to
white light, which implies that the glasses might be a potential candidate for white lighting through an appropriate
combination.
Wet chemical etching of Al0.65Ga0.35N in aqueous KOH solutions
Author(s):
Jintong Xu;
Jie Chen;
Zhiguo Su;
Yan Zhang;
Xiangyang Li
Show Abstract
We have investigated wet chemical etching of Al0.65Ga0.35N in 20% (by weight) aqueous KOH solutions at 85, 95
and 102 °C. The etch rates were 0.48, 0.64 and 0.96 μm/min, respectively. Scanning electron microscope, atomic force
microscopy, Auger electron spectroscopy and x-ray photoelectron spectroscopy were employed to characterize the
surface morphology and stoichiometry before and after wet chemical etching. Experimental results show that aqueous
KOH solutions can effectively etch Al0.65Ga0.35N. SEM results show that the root-mean-square (rms) roughness increased
from 16.189 to 50.496 nm. Degradation in surface smoothness is attributed to selective anisotropic wet chemical etching.
The AES measurements show that after the untreated sample was wet chemical treated in a 20% KOH solution at 85 °C
for 15 seconds, the oxide formed in the surface was removed and a lower content of oxide layer near the surface was
formed and the N content is higher. XPS measurements show that the banding energies of Ga 3d shifted to low energy
direction after the untreated sample was successively wet chemical treated. It indicated that the atom Ga tended to form
Ga-N rather than Ga-O bonding on the surface after KOH solution treatment. The damage and oxide layer induced by
dry etching processes was effectively removed by wet chemical treatment and the damage surface was refreshed.
Synthesis and characterization of two kind of deep blue emission organic small molecular materials
Author(s):
Hongyu Zhao;
Fujun Zhang;
Lifei Cai
Show Abstract
Two kinds of deep blue emission materials (** and *** )were synthesized by reduction
coupling Method. The characteristic of the two materials have been studied by UV-vis
absorption spectra, thermogravimetric analysis(TGA), differential scanning calorimetry(DSC)
and fluorescence spectroscopy. The two materials emit strong blue fluorescence under the
excitation of UV light. The induction of Br ion may induce the red-shifted 15 nm of absorption
in the UV-visible light range. UV absorption spectra show the absorption energy of complexes
mostly from ligands.
AIN based diluted magnetic semiconductors from first-principles study
Author(s):
Kun Zeng;
Zhi-You Guo;
Xiao-Qi Gao
Show Abstract
Using a variety of experimental, alumina nitride (AlN) based diluted magnetic semiconductors (DMSs) are found to
exhibit ferromagnetic properties at room temperature by many groups. The origin of ferromagnetism in DMSs is
ambiguous and difficult to be clearly identified. The geometrical structure of V doped 32-atom supercell of AlN was
optimized by using the ultra-soft pseudopotential method of total-energy plane wave based on the density functional
theory (DFT) . Density of states and band structure were calculated and discussed in detail. The results revealed that the
V dopants were found spin-polarized. Band structures show a half metallic behaviour. The ferromagnetic ground state in
V-doped AlN can be explained in terms of p-d hybridization mechanism. These results suggest that V-doped AlN may
present a promising dilute magnetic semiconductor.
Electronic structure and the optical properties of GaN (0001) surface from first-principles study
Author(s):
Hua-xiong Zhao;
Zhi-you Guo;
Kun Zeng;
Xiao-qi Gao
Show Abstract
The geometrical structure of GaN (0001) surfaces supercell of GaN was optimized by using the ultra-soft
pseudopotential method of total-energy plane wave based on the density functional theory (DFT) .Electronic structure,
Density of states , band structure and optical properties were calculated and discussed in detail.Geometry optimization
indicates that largest atomic relaxation occurs to metal atoms(Ga) in surface layers where all atoms are displaced
inward,and the interlayer distances vary large, total polarization strength is strong. The band gap of GaN (0001) surface
is obviously narrower than that of GaN, the luminescence spectra shift red.The two aspects go against making high
efficiency white-light LED.This paper provides theoretical foundation of GaN (0001) surface and the direction fo making
high efficiency white-light LED form first-principles calculation.
Numerical modeling of thermionic electrons in abrupt isotype heterojunction for the light emitting transistor
Author(s):
Huaxiong Zhao;
Zhiyou Guo;
Kun Zeng;
Xiaoqi Gao
Show Abstract
Light can emit from light emitting transistor of III-V materials. The pn-n+ structure of light emitting
transistor, which has an additional n-n+ heterojunction between the base and electron emitter, is suggested.
The base voltage can control the light output effectively. The functional principle of light emitting transistor is
illuminated. A model about the abrupt isotype heterojunction in the base has been utilized to describe
thermionic electron transport, current density and potential barrier in such a device. The model is used to
explain the underlying mechanisms of the present devices.
Simulation and analysis of GaN-based light-emitting diodes with diamond shaped
Author(s):
Bin Cao;
Pei Wang;
Zhiyin Gan;
Sheng Liu
Show Abstract
Improving light extraction efficiency (LEE) is important to enhance efficiency of light-emitting diodes (LEDs) and
the issue has been studied comprehensively. A GaN-based LED chip with diamond shaped is presented in this letter
and LEE was analyzed. The LEEs of the conventional chip and flip chip with different angle of slant from 0° to
20° were obtained through Monte-Carlo ray tracing method, which is a useful and efficient way in studying the
LEE of LED. The results show that the LEE of the chip with obliquity enhanced by 20.2% compared with
conventional rectangle chip with mirror bottom surface, however, there is no improve of the LEEs of flip chip and
conventional chip with diffuse surface.
Theoretical study on optimization of high efficiency GaInP/GaInAs/Ge tandem solar cells
Author(s):
Gui Jiang Lin;
Sheng Rong Huang;
Jyh Chiarng Wu;
Mei Chun Huang
Show Abstract
This paper investigates which dopping concentration or layer thickness should be
used to design practical GaInP/GaInAs/Ge triple-junction cells in order to optimize their
performance. A rigorous model includes optical and electrical modules is developed to
simulate the external quantumn efficiency, photocurrent and photovoltage of the
GaInP/GaInAs/Ge tandem solar cells. It is found that cell efficiency strongly dependend on
the top cell thickness and doping concentration at base and emitter layers. Proper structures of
the tandem cell operating under AM0 ("air mass zero") illumination are suggested to obtain
high efficiency.