Gas Source MBE-Grown Metamorphic InGaAs Photodetectors using InAlAsBuffer and Cap Layers with Cut-off Wavelength up to 2.7

We report on InP-based metamorphic InGaAs photodiodes grown by gas source molecular beam epitaxy （MBE）, in which a relatively thin compositional graded wide band-gap InxAl1-xAs buffer layer is adopted. In the photodiodes, InAiAs is also taken as cap layers, so this structure is suitable for both front and back illuminations. At room temperature the photodiodes show 50% cut-off wavelength of 2.66μm, with measured peak detectivity of 4.91×10^9 cmHz^1/2/W at 2.57μm, and the typical dark current and RoA are 7.68μA/0.94Ωcm^2 and 291 nA/24.29Ωcm^2 at 290 K and 150 K respectively for the devices in diameter 300 μm. Their performances are compared to the 2.5μm cut-off photodiodes with similar structures.     

The leakage current mechanisms in the Schottky diode with a thin Al layer insertion between Al0.245Ga0.755N/GaN heterostructure and Ni/Au Schottky contact

This paper investigates the behaviour of the reverse-bias leakage current of the Schottky diode with a thin Al inserting layer inserted between Al0.245Ga0.755 N/GaN heterostructure and Ni/Au Schottky contact in the temperature range of 25-350°C. It compares with the Schottky diode without Aluminium inserting layer. The experimental results show that in the Schottky diode with Al layer the minimum point of I-V curve drifts to the minus voltage, and with the increase of temperature increasing, the minimum point of I-V curve returns the 0 point. The temperature dependence of gate-leakage currents in the novelty diode and the traditional diode are studied. The results show that the Al inserting layer introduces interface states between metal and Al0.245Ga0.755N. Aluminium reacted with oxygen formed Al2O3 insulator layer which suppresses the trap tunnelling current and the trend of thermionic field emission current. The reliability of the diode at the high temperature is improved by inserting a thin Al layer.     

Low-Threshold and High-Power Oxide-Confined 850 nm AlInGaAs Strained Quantum-Well Vertical-Cavity Surface-Emitting Lasers Based on Intra-Cavity Contacted Structure

The low-threshold and high-power oxide-confined 850 nm AlInGaAs strained quantum-well （QW） vertical-cavity surface-emitting lasers （VCSELs） based on the intra-cavity contacted structure are fabricated. The threshold current of 0.1 mA for a 10-μm oxide-aperture device is obtained with the threshold current density of 0.127kA/cm^2. For a 22-μm oxide-aperture device, the peak optical output power reaches to 14.6mW at the current injection of 25 mA under the room temperature and pulsed operation with a threshold current of 2mA, which corresponds to the threshold current density of 0.526kA/cm^2. The lasing wavelength is 855.4nm. The full wave at half maximum is 2.2 nm. The analysis of the characteristics and the fabrication of VCSELs are also described.     

Temperature dependence on the electrical and physical performance of InAs/AlSb heterojunction and high electron mobility transistors

In this report, the effect of temperature on the In As/Al Sb heterojunction and high-electron-mobility transistors(HEMTs) with a gate length of 2 μm are discussed comprehensively. The results indicate that device performance is greatly improved at cryogenic temperatures. It is also observed that the device performance at 90 K is significantly improved with 27% lower gate leakage current, 12% higher maximum drain current, and 22.5% higher peak transconductance compared to 300 K. The temperature dependence of mobility and the two-dimensional electron gas concentration in the In As/Al Sb heterojunction for the temperature range 90 K-300 K is also investigated. The electron mobility at 90 K(42560 cm2/V·s)is 2.5 times higher than its value at 300 K(16911 cm~2/V·s) because of the weaker lattice vibration and the impurity ionization at cryogenic temperatures, which corresponds to a reduced scattering rate and higher mobility. We also noted that the two-dimensional electron gas concentration decreases slightly from 1.99 × 10~(12) cm~(-2) at 300 K to 1.7 × 10~(12) cm~(-2) at 90 K with a decrease in temperature due to the lower ionization at cryogenic temperature and the nearly constant ?Ec.     

Pressure dissociation of dense hydrogen

The self-consistent fluid variational model （SFVM） has been used to describe the pressure dissociation of dense hydrogen at high temperatures. This paper focuses on a mixture of hydrogen atoms and molecules and is devoted to the study of the phenomenon of pressure dissociation at finite temperatures. The equation of state and dissociation degree have been calculated from the free energy functions in the range of temperature 2000-10,000K and density 0.02-1.0g/cm^3, which can be compared with other approaches and experiments. The pressure dissociation is found to occur in higher density range, while temperature dissociation is a more gradual effect.     

Room-temperature operating extended short wavelength infrared photodetector based on interband transition of InAsSb/GaSb quantum well

Here in this paper,we report a room-temperature operating infrared photodetector based on the interband transition of an In As Sb/Ga Sb quantum well.The interband transition energy of 5-nm thick In As_(0.91)Sb_(0.09) embedded in the Ga Sb barrier is calculated to be 0.53 e V(2.35μm),which makes the absorption range of In As Sb cover an entire range from short-wavelength infrared to long-wavelength infrared spectrum.The fabricated photodetector exhibits a narrow response range from 2.0μm to 2.3μm with a peak around 2.1μm at 300 K.The peak responsivity is 0.4 A/W under-500-m Vapplied bias voltage,corresponding to a peak quantum efficiency of 23.8%in the case without any anti-reflection coating.At 300 K,the photodetector exhibits a dark current density of 6.05×10~(-3)A/cm~2under-400-m V applied bias voltage and3.25×10~(-5)A/cm~2under zero,separately.The peak detectivity is 6.91×10~(10)cm·Hz~(1/2)/W under zero bias voltage at 300 K.     

Optimization of TiO_2/Cu/TiO_2 multilayers as a transparent composite electrode deposited by electron-beam evaporation at room temperature

Highly transparent indium-free composite electrodes of Ti O2/Cu/Ti O2 are deposited by electron-beam evaporation at room temperature. The effects of Cu thickness and annealing temperature on the electrical and optical properties of the multilayer film are investigated. The critical thickness of Cu mid-layer to form a continuous conducting layer is found to be 11 nm. The multilayer with a mid-Cu thickness of 11 nm is optimized to obtain a resistivity of 7.4×10-5Ω·cm and an average optical transmittance of 86% in the visible spectral range. The figure of merit of the Ti O2/Cu(11 nm)/Ti O2 multilayer annealed at 150ΩC reaches a minimum resistivity of 5.9×10-5Ω·cm and an average optical transmittance of 88% in the visible spectral range. The experimental results indicate that Ti O2/Cu/Ti O2 multilayers can be used as a transparent electrode for solar cell and other display applications.     

A low-threshold and high-power oxide-confined 850-nm AlInGaAs strained quantum-well vertical-cavity surface-emitting laser

A low-threshold and high-power oxide-confined 850-nm AlInGaAs strained quantum-well (QW) vertical-cavity surface-emitting laser (VCSEL) based on an intra-cavity contacted structure is fabricated. A threshold current of 1.5 mA for a 22 μm oxide aperture device is achieved, which corresponds to a threshold current density of 0.395 kA/cm². The peak output optical power reaches 17.5 mW at an injection current of 30 mA at room temperature under pulsed operation. While under continuous-wave (CW) operation, the maximum power attains 10.5 mW. Such a device demonstrates a high characteristic temperature of 327 K within a temperature range from -12℃ to 96℃ and good reliability under a lifetime test. There is almost no decrease of the optical power when the device operates at a current of 5 mA at room temperature under the CW injection current.     

Au/Pt/InGaN/GaN Heterostructure Schottky Prototype Solar Cell

A patterned Au/Pt/In0.2Ga0.8N/GaN heterostructure Sehottky prototype solar cell is fabricated. The forward current-voltage characteristics indicate that thermionie emission is a dominant current transport mechanism at the Pt/InGaN interface in our fabricated cell. The Sehottky solar cell has an open-circuit voltage of 0.91 V, short-circuit current density of 7mA/cm^2, and fill factor of 0.45 when illuminated by a Xe lamp with a power density of 300 mW/cm^2. It exhibits a higher short-circuit current density of 30 mA/cm^2 and an external quantum efficiency of over 25% when illuminated by a 20-roW-power He-Cd laser.     

High-Temperature Operation of 8.5 μm Distributed Feedback Quantum Cascade Lasers

We present a distributed feedback quantum cascade laser （DFB-QCL） emitting at a wavelength of 8.5μm and operating up to 420K （147℃） with a low-threshold current density in pulsed mode. The DFB-QCLs studied are based on a four-well active design; the central portion of the waveguide consists of 60 periods of lattice matched InP-based InGaAs/AlInAs. In the design of the device, an active structure with lower doping and a deep-top grating process are utilized to achieve high temperature operation with a lower-threshold current density. At 420K, a low-threshold current density of 3.28 kA/cm^2 and a single mode peak power of 15mW are achieved on an epilayer-up mounting device with ridge width of 26μm and cavity length of 3.0mm. A side mode suppression ratio of 25 dB at 420 K is obtained.     

Carrier Dynamics Determined by Carrier-Phonon Coupling in InGaN/GaN Multiple Quantum Well Blue Light Emitting Diodes

Phonon sidebands in the electrolumiescence(EL) spectra of InGaN/GaN multiple quantum well blue light emitting diodes are investigated. S-shaped injection current dependence of the energy spacing(ES) between the zero-phonon and first-order phonon-assisted luminescence lines is observed in a temperature range of 100–150 K.The S-shape is suppressed with increasing temperature from 100 to 150 K, and vanishes at temperature above200 K. The S-shaped injection dependence of ES at low temperatures could be explained by the three stages of carrier dynamics related to localization states:(i) carrier relaxation from shallow into deep localization states,(ii) band filling of shallow and deep localization states, and(iii) carrier overflow from deep to shallow localization states and to higher energy states. The three stages show strong temperature dependence. It is proposed that the fast change of the carrier lifetime with temperature is responsible for the suppression of S-shaped feature.The proposed mechanisms reveal carrier recombination dynamics in the EL of InGaN/GaN MQWs at various injection current densities and temperatures.     

Electrical characteristics of AlInN/GaN HEMTs under cryogenic operation

Electrical properties of an AlInN/GaN high-electron mobility transistor (HEMT) on a sapphire substrate are investi-gated in a cryogenic temperature range from 295 K down to 50 K. It is shown that drain saturation current and conductance increase as transistor operation temperature decreases. A self-heating effect is observed over the entire range of temperature under high power consumption. The dependence of channel electron mobility on electron density is investigated in detail. It is found that aside from Coulomb scattering, electrons that have been pushed away from the AlInN/GaN interface into the bulk GaN substrate at a large reverse gate voltage are also responsible for the electron mobility drop with the decrease of electron density.     

Effect of Tunneling Current on Schottky Barrier Height in ZnO Varistors at Low Temperature

On the basis of the Schottky barrier and thermionic emission models, the temperature dependence of barrier height in ZnO varistors is investigated by the I - V characteristics in a wide temperature range from 93 K to 373 K. The obtained barrier height decreases with reducing temperature, which is ascribed to the contribution of tunneling current in measured current. From the proposed equivalent circuit, it is suggested that two current components coexist. One is thermionic emission current, which reflects the thermionic emission barrier height. The other is tunneling current, which appears even at low voltage, especially in low temperature ranges, and thus makes the barrier height obtained from measured current vary with temperature.     

Robust Performance of AlGaN-Channel Metal-Insulator-Semiconductor High-Electron-Mobility Transistors at High Temperatures

Superior characteristics of Al Ga N-channel metal-insulator-semiconductor(MIS) high electron mobility transistors(HEMTs) at high temperatures are demonstrated in detail. The temperature coefficient of the maximum saturation drain current for the Al GaN-channel MIS HEMT can be reduced by 50% compared with the Ga N-channel HEMT. Moreover, benefiting from the better suppression of gate current and reduced leakage current in the buffer layer, the Al Ga N-channel MIS HEMT demonstrates an average breakdown electric field of 1.83 MV/cm at25℃ and 1.06 MV/cm at 300℃, which is almost 2 times and 3 times respectively larger than that of the reference Ga N-channel HEMT. Pulsed mode analyses suggest that the proposed device suffers from smaller current collapse when the temperature reaches as high as 300℃.     

Metamorphic InGaAs p-i-n Photodetectors with 1.75 μm Cut-Off Wavelength Grown on GaAs

Top-illuminated metamorphic InGaAs p-i-n photodetectors （PDs） with 50% cut-off wavelength of 1.75 μm at room temperature are fabricated on GaAs substrates. The PDs are grown by a solid-source molecular beam epitaxy system. The large lattice mismatch strain is accommodated by growth of a linearly graded buffer layer to create a high quality virtual InP substrate indium content in the metamorphic buffer layer linearly changes from 2% to 60%. The dark current densities are typically 5 × 10^-6 A/cm^2 at 0 V bias and 2.24 × 10^-4 A/cm^2 at a reverse bias of 5 V. At a wavelength of 1.55 μm, the PDs have an optical responsivity of 0.48 A/W, a linear photoresponse up to 5 mW optical power at -4 V bias. The measured -3 dB bandwidth of a 32 μm diameter device is 7 GHz. This work proves that InGaAs buffer layers grown by solid source MBE are promising candidates for GaAs-based long wavelength devices.     

Dielectric and Pyroelectric Properties of （Pb0.50Sr0.50）TiO3 Ceramics

Lead strontium titanate （Pb0.50Sr0.50）TiO3 （PST） ceramics are prepared by the traditional ceramic processing. The dielectric constants and dielectric loss have been investigated in a temperature range from 25℃ to 300℃. The maximum dielectric constants for unpoled and poled samples are 9924 and 9683, respectively. The temperatures of phase transition for unpoled and poled samples are observed at 153℃ and 157℃, respectively. The phasetransition temperatures for unpoled and poled samples are not equal, which results from the polarization state of the domains. The remnant polarization and the coercive electric field are 18 uC/cm^2 and 6 kV/cm, respectively, from polarization-electric field （P - E） hysteresis loop. The temperature dependence of pyroelectric coefficients of the PST ceramics is measured by a dynamic technique. The dielectric constant and loss Lan δ of the poled PST ceramics are 813 and 0.010, respectively. The pyroelectric coefficients and figure of merit are 294 uC/cm^2 K and 13.6 × 10^-6 pa^-0.5, respectively, at room temperature 25℃and frequency lOOHz.     

Low temperature testing and neutron irradiation of a swept charge device on board the HXMT satellite

We present the low temperature testing of an SCD detector, investigating its performance such as readout noise, energy resolution at 5.9 keV and dark current. The SCD's performance is closely related to temperature, and the temperature range of -80 ℃ to -50 ℃ is the best choice, where the FWHM at 5.9 keV is about 130 eV. The influence of the neutron irradiation from an electrostatic accelerator with fluence up to 1×10⁹ cm^-2 has been examined. We find the SCD is not vulnerable to neutron irradiation. The detailed operations of the SCD and the test results of low temperature are reported, and the results of neutron irradiation are discussed.     

Synthesis and high temperature thermoelectric transport properties of Si-based type-I clathrates

N-type Si-based type-I clathrates with different Ga content were synthesized by combining the solid-state reaction method, melting method and spark plasma sintering (SPS) method. The effects of Ga composition on high temperature thermoelectric transport properties were investigated. The results show that at room temperature, the carrier concentration decreases, while the carrier mobility increases slightly with increasing Ga content. The Seebeck coefficient increases with increasing Ga content. Among all the samples, Ba_7.93Ga_17.13Si_28.72 exhibits higher Seebeck coefficient than the others and reaches -135~μ V.K^-1 at 1000 K. The sample prepared by this method exhibits very high electrical conductivity, and reaches 1.95× 10⁵S.m^-1 for Ba_8.01Ga_16.61Si_28.93 at room temperature. The thermal conductivity of all samples is almost temperature independent in the temperature range of 300--1000~K, indicating the behaviour of a typical metal. The maximum {ZT} value of 0.75 is obtained at 1000~K for the compound Ba_7.93Ga_17.13Si_28.72.     

Large reversible magnetocaloric effect induced by metamagnetic transition in antiferromagnetic HoNiGa compound

The magnetic properties and magnetocaloric effects(MCE) of Ho Ni Ga compound are investigated systematically.The Ho Ni Ga exhibits a weak antiferromagnetic(AFM) ground state below the Neel temperature TNof 10 K, and the AFM ordering could be converted into ferromagnetic(FM) ordering by external magnetic field. Moreover, the field-induced FM phase exhibits a high saturation magnetic moment and a large change of magnetization around the transition temperature,which then result in a large MCE. A large-?S_M of 22.0 J/kg K and a high RC value of 279 J/kg without magnetic hysteresis are obtained for a magnetic field change of 5 T, which are comparable to or even larger than those of some other magnetic refrigerant materials in the same temperature range. Besides, the μ_0H~(2/3)dependence of |?S_M~(pk)| well follows the linear fitting according to the mean-field approximation, suggesting the nature of second-order FM–PM magnetic transition under high magnetic fields. The large reversible MCE induced by metamagnetic transition suggests that Ho Ni Ga compound could be a promising material for magnetic refrigeration in low temperature range.     

Continuous Wave Performance and Tunability of MBE Grown 2.1μm InGaAsSb/AlGaAsSb MQW lasers

InCaAsSb/AlGaAsSb multi quantum well ridge waveguide lasers at 2.1 μm wavelength are fabricated by using molecular beam epitaxy. Continuous wave performance and tunability of the lasers are evaluated in a wide temperature range extend to 80℃. Output power of the laser at 30℃ exceeds 30 m W/facet at driving current of 0.5 A, the characteristic temperature To is 89K in 0-50℃ range. No fast degradation is observed in accelerated aging test at 90℃ for those lasers with lower Al content in cladding layers. Temperature tunability of the lasers is 1.36 nm/K. Single-mode output with side mode suppression ratios greater than 20 dB is achieved in a certain driving current region; current tunability is 8 × 10^-3 nm/mA regardless of mode hopping.