Comparison of GaN/AlGaN/AlN/GaN HEMTs Grown on Sapphire with Fe-Modulation-Doped and Unintentionally Doped GaN Buffer:Material Growth and Device Fabrication

AlGaN/GaN high electron mobility transistors （HEMTs） grown on Fe-modulation-doped （MD） and unintentionally doped （UID） GaN buffer layers are investigated and compared. Highly resistive GaN buffers （10^9Ω·cm） are induced by individual mechanisms for the electron traps＇ formation： the Fe MD buffer （sample A） and the UID buffer with high density of edge-type dislocations （7.24×10^9cm^-2, sample B）. The 300K Hall test indicates that the mobility of sample A with Fe doping （2503cm^2V^-1s^-1） is much higher than sample B （1926cm^2V^-1s^-1） due to the decreased scattering effect on the two-dimensional electron gas. HEMT devices are fabricated on the two samples and pulsed I–V measurements are conducted. Device A shows better gate pinch-off characteristics and a higher threshold voltage （-2.63V） compared with device B （-3.71V）. Lower gate leakage current ｜IGS｜ of device A （3.32×10^-7A） is present compared with that of device B （8.29×10^-7A）. When the off-state quiescent points Q_2 （V GQ2=-8V, V DQ2=0V） are on, V th hardly shifts for device A while device B shows ＋0.21V positive threshold voltage shift, resulting from the existence of electron traps associated with the dislocations in the UID-GaN buffer layer under the gate. Under pulsed I–V and transconductance G m–V GS measurement, the device with the Fe MD-doped buffer shows more potential in improving reliability upon off-state stress.

Application of Mulching Materials of Rainfall Harvesting System for Improving Soil Water and Corn Growth in Northwest of China

The ridge and furrow rainfall harvesting（RFRH） system is used for dryland crop production in northwest of China.To determine the effects of RFRH using different mulching materials on corn growth and water use efficiency（WUE）,a field experiment was conducted during 2008-2010 at the Heyang Dryland Experimental Station,China.Four treatments were used in the study.Furrows received uncovered mulching in all RFRH treatments whereas ridges were mulched with plastic film（PF）,biodegradable film（BF） or liquid film（LF）.A conventional flat field without mulching was used as the control（CK）.The results indicated that the average soil water storage at depths of 0-200 cm were 8.2 and 7.3%,respectively higher with PF and BF than with CK.However,LF improved soil water storage during the early growth stage of the crop.Compared with CK,the corn yields with PF and BF were increased by 20.4 and 19.4%,respectively,and WUE with each treatment increased by 23.3 and 21.7%,respectively.There were no significant differences in corn yield or WUE with the PF and BF treatments.The net income was the highest with PF,followed by BF,and the 3-yr average net incomes with these treatments were increased by 2 559 and 2 430 CNY ha-1,respectively,compared with CK.BF and PF had similar effects in enhancing the soil water content,crop yield and net income.Therefore,it can be concluded that biodegradable film may be a sustainable ecological alternative to plastic film for use in the RFRH system in northwest of China.

High Lattice Match Growth of InAsSb Based Materials by Molecular Beam Epitaxy

High lattice match growth of InAsSb based materials on GaSb substrates is demonstrated. The present results indicate that a stable substrate temperature and the optimal flux ratios are of critical importance in achieving a homogeneous InAsSb based material composition throughout the growth period. The quality of these epilayers is assessed using a high-resolution x-ray diffraction and atomic force microscope. The mismatch between the GaSb substrate and InAsSb alloy achieves almost zero, and the rms surface roughness of InAsSb alloy achieves around 1.7A over an area of 28μm × 28μm. At the same time, the mismatches between GaSb and InAs/InAs0.73Sb0.27 superlattices （SLs） achieve approximately 100 arcsec （75 periods） and zero （300 periods）, with the surface rms roughnesses of InAs/InAs0.73Sb0.27 SLs around 1.8 A （75 periods） and 2.1A （300 periods） over an area of 20 μm×20 μm, respectively. After fabrication and characterization of the devices, the dynamic resistance of the n-barrier-n InAsSb photodetector near zero bias is of the order of 10^6Ω·cm^2. At 77K, the positive-intrinsic-negative photodetectors are demonstrated in InAsSb and InAs/InAsSb SL （75 periods） materials, exhibiting fifty-percent cutoff wavelengths of 3.8μm and 5.1μm, respectively.

Growth and Characterization of GaInP Layers on GaAs Substrates by MOCVD

GaInP is an important material in fabrication of optoelectronic and microwave devices. Growth process of GaInP by MOCVD (metal-organic chemical vapor deposition) was investigated, and the effect of growth parameters on the epilayer properties was discussed. It is found that the distribution coefficient of indium is close to unity, while the growth temperature of GaInP is 700°C. The background concentration at 300K is 5.34×1015cm-3 and Hall mobility is 2.27×103cm2&middotV-1&middotS-1.

Growth and Spectral Characteristics of Yb^(3+)-doped LiGd(MoO_4)_2 Crystal

The Yb3＋-doped LiGd（MoO4）2 crystal with the size up to Φ20×30 mm3 has been grown by Czochralski technique.The polarized room temperature absorption and emission spectra have been investigated.This crystal exhibits a broad absorption band centered at 975 nm with an FWHM of 43 and 59 nm for π-and σ-polarization,respectively,and the corresponding maximal absorption cross-sections are 3.36 and 2.42×10-20 cm2.The emission broadband has an FWHM of 47 and 54 nm for π-and σ-polarization,respectively,with the corresponding emission cross sections of 3.92 and 3.34 × 10-20 cm2 at 1020 nm.The measured fluorescence lifetime is 287 μs.

Theoretical studies on alloying of germanene supported on Al (111) substrate

Using density functional theory,we study the alloying of the buckled hexagonal germanene superlattice supported on Al(111)-(3×3),the sheet composed of triangular,rhombic,and pentagonal motifs on Al(111)-(3×3),and the buckled geometry on Al(111)-(√7×√7)(19°),which are denoted,respectively,by BHS,TRP,and SRT7,to facilitate the discussion in this paper.They could be alloyed in the low doping concentration range.The stable configurations BHS,TRP,and SRT7 of the pure and alloyed germanenes supported on both Al(111) and its Al2 Ge surface alloy,except the SRT7 pure germanene on Al2 Ge,could re-produce the experimental scanning tunneling microscopy images.The relatively stable AlGe alloy species are the Al3 Ge5 BHS-2 T,Al3 Ge5 TRP-2 T,and Al3 Ge3 SRT7-1 T on Al(111) while they are the Al4 Ge4 BHS-1 T,Al3 Ge5 TRP-2 T,and A127 Ge27 SRT7-(3×3)-9 T on Al2 Ge(the n in the nT means that there are n Ge atoms per unit which sit at the top sites and protrude upward).In addition,the Al3 Ge5 BHS-2 T and Al4 Ge4 BHS-1 T are the most stable alloy sheets on Al(111) and Al2 Ge,respectively.Comparing with the experimental studies,there exists no structural transition among these alloyed configurations,which suggests that the experimental conditions play a crucial role in selectively growing the pure or the alloyed germanene sheets,which may also help grow the one-atomic thick honeycomb structure on idea Al(111).

Intensities and spectral features of the ~4I_(13/2)-~4I_(15/2) potential laser transition of Er^(3+) centers in CaF_2-CeF_3 disordered crystal

A CaF2-CeF3 disordered crystal containing 1.06% of Er^3＋ ions was grown by the temperature gradient technique.Optical absorption and emission spectra recorded at room temperature and at 10 K, luminescence decay curve recorded at room temperature, and extended x-ray-absorption fine structure spectra were analyzed with an intention to assess the laser potential related to the ^4I13/2→^4I15/2 transition of Er^3＋. In addition, the thermal diffusivity of the crystal was measured at room temperature. The analysis of room-temperature spectra revealed that the ^4I13/2 emission is long-lived with a radiative lifetime value of 5.5 ms, peak emission cross section of 0.73 × 10^-20 cm^2, and large spectral width pointing at the tunability of the emission wavelength in the region stretching from approximately 1480 nm to 1630 nm. The energies of the crystal field components for the ground and excited multiplets determined from low-temperature absorption and emission spectra made it possible to predict successfully the spectral position and shape of the room-temperature ^4I13/2→^4I15/2 emission band. Based on the correlation of the optical spectra and dynamics of the luminescence decay, it was concluded that in contrast to Yb^3＋ ions in heavily doped CaF2 erbium ions in the CaF2-CeF3 crystal reside in numerous sites with dissimilar relaxation rates.

Applying Literature to ELT

Literature is no longer a frightening word to English language learner. Interactive teaching methods and attractive activities can help motivating Chinese university English learners. This essay will first elaborate the reasons to use literature in ELT (English Language Teaching) class and how to apply literature to ELT class.

Recent progress in GeSn growth and GeSn-based photonic devices

The GeSn binary alloy is a new group IV material that exhibits a direct bandgap when the Sn content ex- ceeds 6%. It shows great potential for laser use in optoelectronic integration circuits （OEIC） on account of its low light emission efficiency arising from the indirect bandgap characteristics of Si and Ge. The bandgap of GeSn can be tuned from 0.6 to 0 eV by varying the Sn content, thus making this alloy suitable for use in near-infrared and mid-infrared detectors. In this paper, the growth of the GeSn alloy is first reviewed. Subsequently, GeSn photode- tectors, light emitting diodes, and lasers are discussed. The GeSn alloy presents a promising pathway for the mono- lithic integration of Si photonic circuits by the complementary metal-oxide-semiconductor （CMOS） technology.

Growth and spectral characteristics of a new promising stoichiometric laser crystal: Ca_9Yb(VO_4)_7

A Ca9Yb（VO4）7 crystal with dimensions of Φ23 mm×35 mm was grown successfully by Czochralski method. Its thermal conductivity was 1.06 W/（m？K） at room temperature. The absorption cross-sections at 980 nm were 1.80×10–20 cm2 and 1.28×10–20 cm2 for π- and σ- polarizations, respectively, with a full-width at half-maximum of 34 nm. The crystal had a broad emission at around 1025 nm with a full-width at half-maximum of 67 nm for π- polarization and 70 nm for σ- polarization. The emission cross-sections of the crystal were calculated by using reciprocity method and Füchtbauer-Ladenburg formula. The emission cross-sections at 1025 nm were 3.57×10–20 cm–2 and 1.91×10–20 cm–2 for π- and σ- polarization, respectively. The fluorescence lifetime was 332 μs. The results indicated that the crystal is a promising femtosecond and tunable laser material.

Review on the Progress of AlGaN-based Ultraviolet Light-Emitting Diodes

AlGaN-based materials have exhibited considerable potential for fabricating ultraviolet(UV)light-emitting diodes(LEDs)owing to their direct,wide,and adjustable energy bandgap.AlGaN-based devices have extensive appli-cability owing to their stable physico-chemical properties.With decades of research effort,significant progress has been achieved in enhancing the working efficiency of AlGaN-based LEDs by optimizing the crystalline qual-ity,doping efficiency,and device design.In this review,methods to obtain high-quality AlGaN-based materials,achieve high doping efficiency,and design UV-LED structures are summarized and discussed.Finally,the issues that need to be addressed in AlGaN-based UV-LED devices are highlighted.

Hydrothermal synthesis of monodisperse α-Fe_2O_3 hexagonal platelets

Uniformly sized α-Fe2O3 hexagonal platelets were synthesized by a hydrothermal process using Fe（OH）3 suspension and large amount of NaOH. The reaction products were characterized by X-ray diffraction （XRD）, scanning electron microscopy （SEM）, transmission electron microscopy （TEM）, selected area electron diffraction （SAED）, and a vibrating sample magnetometer （VSM）. The results show that the hexagonaq platelets are fine, monodisperse and consisting of single-crystals. The magnetic hysteresis （M-H） curvel of the samples measured at room temperature indicates that the α-Fe2O3 micro-platelets exhibit ferromagnetic behaviors with relatively low coercivity.

Study of passive Q-switching for flash-lamp-pumped 1.34-μm Nd:GdVO_4 laser

The static laser performance of a-growth Nd：GdVO4 crystal （a-cut, 4 × 4 × 25 （mm）） at 1.34 μm pumped by flash-lamp is investigated with different transmissions of output couplers. With the output coupler transmission of T = 30%, the static output energy of 148 mJ is obtained when the pump energy is 35.2 J, and the corresponding electric-optical conversion efficiency is 0.42%. The Q-switched output of lasers with the output wavelength ranging from 1.3 to 1.6 μm can be realized by using Co^2＋ ：LaMgA11O19 （Co：LMA） as saturable absorber. A flash-lamp-pumped, passively Q-switched Nd：GdVO4 laser with Co：LMA as saturable absorber is demonstrated in piano-concave laser cavity. With the cavity length of 16.3 cm and pump energy of 19.8 J, the single-pulse output energy, pulse width, and peak power are obtained to be 4 m J, 80 ns, and 5 × 10^4 W, respectively.