Emission cross sections and spectroscopy of Ho3+ laserchannels in KGd(WO4)2 single crystal

The spectroscopic properties of Ho³⁺ laser channels in KGd(WO₄)₂ crystals have been investigated using optical absorption, photoluminescence, and lifetime measurements. The radiative lifetimes of Ho³⁺ have been calculated through a Judd-Ofelt (JO) formalism using 300-K optical absorption results. The JO parameters obtained were Ω₂=15.35×10^-20 cm², Ω ₄=3.79×10^-20 cm², Ω₆ =1.69×10^-20 cm². The 7-300-K lifetimes obtained in diluted (8·10¹⁸ cm^-3) KGW:0.1% Ho samples are: τ(⁵F₃)≈0.9 μs, τ( ⁵S₂)=19-3.6 μs, and τ(⁵F₅ )≈1.1 μs. For Ho concentrations below 1.5×10²⁰ cm^-3, multiphonon emission is the main source of non radiative losses, and the temperature independent multiphonon probability in KGW is found to follow the energy gap law τ_ph ^-1(0)=βexp(-αΔE), where β=1.4×10^-7 s^-1, and α=1.4×10³ cm. Above this holmium concentration, energy transfer between Ho impurities also contributes to the losses. The spectral distributions of the Ho³⁺ emission cross section σ_EM for several laser channels are calculated in σ- and π-polarized configurations. The peak a σ_EM values achieved for transitions to the ⁵I₈ level are ≈2×10^-20 cm² in the σ-polarized configuration, and three main lasing peaks at 2.02, 2.05, and 2.07 μm are envisaged inside the ⁵I₇→⁵I₈ channel     

原子层沉积方法制备低温多层Al2O3/TiO2复合封装薄膜的研究

原子层沉积(ALD)方法可以制备出高质量薄膜,被认为是可应用于柔性有机电致发光器件(OLED)最有发展前景的薄膜封装技术之一。本文采用原子层沉积(ALD)技术,在低温(80℃)下,研究了Al₂O₃及TiO₂薄膜的生长规律,通过钙膜水汽透过率(WVTR)、薄膜接触角测试等手段,研究了不同堆叠结构的多层Al₂O₃/TiO₂复合封装薄膜的水汽阻隔特性,其中5 nm/5 nm×8 dyads(重复堆叠次数)的Al₂O₃/TiO₂叠层结构薄膜的WVTR达到2.1×10^-5 g/m²/day。采用优化后的Al₂O₃/TiO₂叠层结构薄膜对OLED器件进行封装,实验发现封装后的OLED器件在高温高湿条件下展现了较好的寿命特性。     

Effect of hot-electron injection of high-frequency characteristicsof abrupt In0.52(Ga1-xAlx)0.48As/InGaAs HBT's

The effect of hot-electron injection energy (E_i) into the base on the high-frequency characteristics of In_0/52(Ga_1-xAl_x)_0.48 As/InGaAs abrupt heterojunction bipolar transistors (HBTs) is investigated by changing the composition of the emitter. There exists an optimum E_i at which a maximum current gain cutoff frequency (ft) is obtained. Analysis of hot-electron transport in the base and collector by Monte Carlo simulation is carried out to understand the above phenomenon. The collector transit time (τ_c ) increases with E_i, because electrons with higher energy transfer from the Γ valley into the upper L and X valleys. At first, the base transit time (τ_b ) decreases with E_i at the low E_i region. However, τ_b does not decrease monotically with E_i, because of the nonparabolicity in the energy-band structure of InGaAs. Consequently, there exists a minimum in the sum of τ_b and τ_c , in other words a maximum f_t, at an intermediate value of E_i     

Carrier transport related analysis of high-power AlGaN/GaN HEMTstructures

Shubnikov-de Haas (SdH) oscillation and Hall measurement results were compared with HEMT DC and RF characteristics for two different MOCVD grown AlGaN-GaN HEMT structures on semiinsulating 4H-SiC substrates. A HEMT with a 40-nm, highly doped AlGaN cap layer exhibited an electron mobility of 1500 cm²/V/s and a sheet concentration of 9×10¹² cm at 300 K (7900 cm²/V/s and 8×10¹² cm^-2 at 80 K), but showed a high threshold voltage and high DC output conductance. A 27-nm AlGaN cap with a thinner, lightly doped donor layer yielded similar Hall values, but lower threshold voltage and output conductance and demonstrated a high CW power density of 6.9 W/mm at 10 GHz. The 2DEG of this improved structure had a sheet concentration of n_SdH=7.8×10¹² cm^-2 and a high quantum scattering lifetime of τ_q=1.5×10^-13 s at 4.2 K compared to n_SdH=8.24×10¹² cm^-2 and τ_q=1.72×10^-13 s for the thick AlGaN cap layer structure, Despite the excellent characteristics of the films, the SdH oscillations still indicate a slight parallel conduction and a weak localization of electrons. These results indicate that good channel quality and high sheet carrier density are not the only HEMT attributes required for good transistor performance     

Metal base transistor ofIn/Bi(Ba,Rb)O3/SrTiO3 (Nb)

A metal-base transistor of the MOMOM type with large current gain is reported. It uses Bi(Ba,Rb)O₃ and oxide semiconductors. I-V curves for a Bi(Ba,Rb)O₃ base transistor in the common-base configuration were studied from room temperature to 30 K. Current gain α~1 was obtained at 50 K. Transport behavior is determined by analysis of threshold voltages and derivatives dI_c/dV_cb     

Low Dit, thermodynamically stable Ga2O3-GaAs interfaces: fabrication, characterization, and modeling

Thermodynamically stable, low D_it amorphous Ga₂ O₃-(100) GaAs interfaces have been fabricated by extending molecular beam epitaxy (MBE) related techniques. We have investigated both in situ and ex situ Ga₂O₃ deposition schemes utilizing molecular beams of gallium oxide. The in situ technique employs Ga₂O₃ deposition on freshly grown, atomically ordered (100) GaAs epitaxial films in ultrahigh vacuum (UHV); the ex situ approach is based on thermal desorption of native GaAs oxides in UHV prior to Ga₂O₃ deposition. Unique electronic interface properties have been demonstrated for in situ fabricated Ga₂O₃-GaAs interfaces including a midgap interface state density D_it in the low 10¹⁰ cm^-2 eV^-1 range and an interface recombination velocity S of 4000 cm/s. The existence of strong inversion in both n- and p-type GaAs has been clearly established. We will also discuss the excellent thermodynamic and photochemical interface stability. Ex situ fabricated Ga₂O₃-GaAs interfaces are inferior but still of a high quality with S=9000 cm/s and a corresponding D_it in the upper 10¹⁰ cm^-2 eV^-1 range. We also developed a new numerical heterostructure model for the evaluation of capacitance-voltage (C-V), conductance-voltage (G-V), and photoluminescence (PL) data. The model involves selfconsistent interface analysis of electrical and optoelectronic measurement data and is tailored to the specifics of GaAs such as band-to-band luminescence and long minority carrier response time τ_R. We will further discuss equivalent circuits in strong inversion considering minority carrier generation using low-intensity light illumination     

Low-temperature fabrication of amorphous BaTiO3thin-film bypass capacitors

Amorphous BaTiO₃ thin-film capacitors suitable for integration into a multichip module packaging process were fabricated. The multilayer capacitor structure consisted of an adhesion layer (TiO _xN_y or Ti), a bottom electrode (Cu), a dielectric (amorphous BaTiO₃), and a top electrode (Cu). A 3000-Å amorphous BaTiO₃ film was deposited onto the electrode by the reactive partially ionized beam (RPIB) technique at near room temperature. After a 300°C postdeposition anneal, the capacitors had the following properties: ε_r=17-18 and tanδ<0.01 up to 600 MHz, J_leak=0.06-0.5 μA/cm² at 0.5 MV/cm, and breakdown field E_max=3.3 MV/cm     

Electrical characteristics of high quality La2O3 gate dielectric with equivalent oxide thickness of 5 Å

Electrical and reliability properties of ultrathin La₂O ₃ gate dielectric have been investigated. The measured capacitance of 33 Å La₂O₃ gate dielectric is 7.2 μF/cm² that gives an effective K value of 27 and an equivalent oxide thickness of 4.8 Å. Good dielectric integrity is evidenced from the low leakage current density of 0.06 A/cm² at -1 V, high effective breakdown field of 13.5 MV/cm, low interface-trap density of 3×10¹⁰ eV^-1/cm², and excellent reliability with more than 10 years lifetime even at 2 V bias. In addition to high K, these dielectric properties are very close to conventional thermal SiO₂      

Base transit time in abrupt GaN/InGaN/GaN HBT's

Base transit time in an abrupt GaN/InGaN/GaN HBT is reported. Temperature and doping concentration dependence of low field mobility is obtained from an ensemble Monte Carlo simulation. Base transit time, τ_b, decreases with increasing temperature. The low temperature τ_b is dominated by the diffusion constant or, in other words, transport within the neutral base region. However, at elevated temperatures base transit time is dependent more upon the base-collector junction velocity or, in other words, by the transport across the heterointerface. τ_b increases with In-mole fraction showing a stronger dependence at lower temperatures. Unity gain current cut-off frequency, f_T, is a strong function of temperature and base doping concentration. An f_T of 20 GHz is obtained for a 0.05 μm HBT     

Gate oxide integrity of thermal oxide grown on high temperatureformed Si0.3Ge0.7

We have investigated the gate oxide integrity of thermal oxides direct grown on high temperature formed Si_0.3Ge_0.7. Good oxide integrity is evidenced by the low interface-trap density of 5.9×10¹⁰ eV^-1 cm^-2, low oxide charge density of -5.6×10¹⁰ cm^-2, and the small stress-induced leakage current after -3.3 V stress for 10 000 s. The good gate oxide integrity is due to the high temperature formed and strain-relaxed Si_0.3Ge_0.7 that has a original smooth surface and stable after subsequent high temperature process     

Optimization of CO2 laser frequency stabilization viaexternal Doppler dither modulation

An external Doppler dither modulation scheme for locking the CO ₂ laser frequency to the center of the 4.2-μm fluorescence Lamb dip of CO₂ is discussed. The optimum values for the frequency dither, fluorescence cell temperature and CO₂ absorber gas pressure depending on the experimental conditions are determined. The design of two different dither mirror arrangements is described. With optimum parameters and with τ=40 ms integration time, it is possible to lock the CO₂ laser to within ±10 kHz to line center over several minutes. Measurements of ultrahigh-resolution Lamb dip spectra of the υ₂ as R (3,3) transition of NH₃ show the superiority of external dither modulation over the conventional internal modulation scheme with respect to the spectral purity of the laser radiation     

Plasma-Enhanced Atomic Layer Deposition of Amorphous Ga2O3 for Solar-Blind Photodetection

Wide-bandgap gallium oxide(Ga₂O₃) is one of the most promising semiconductor materials for solar-blind(200 nm to 280 nm) photodetection. In its amorphous form, amorphous gallium oxide(a-Ga₂O₃) maintains its intrinsic optoelectronic properties while can be prepared at a low growth temperature, thus it is compatible with Si integrated circuits(ICs) technology. Herein, the a-Ga₂O₃ film is directly deposited on pre-fabricated Au inter...     

Low-temperature DC characteristics of pseudomorphic Ga0.18In0.82P/InP/Ga0.47In0.53As HEMT

A high-gap strained GaInP material chosen to increase Schottky barrier height on InP is discussed. This material has been used for the first time in high electron mobility transistor (HEMT) fabrication on InP. For these devices the best g_m of a 1.3-μm gate HEMT is 300 mS/mm. Transistors of 3-μm gate length are studied at low temperature (100 to 293 K). Their DC electrical characteristics improve upon cooling. The best improvement is measured at the lowest temperature (+54% for g_m at 105 K). The structure is stable and does not present any g_m or I_ds collapse at low temperature, unlike AlGaAs/GaAs heterostructures     

Reliability of ultra thin ZrO2 films on strained-Si

Ultra thin high-k zirconium oxide (equivalent oxide thickness 1.57 nm) films have been deposited on strained-Si/relaxed-Si_0.8Ge_0.2 heterolayers using zirconium tetra-tert-butoxide (ZTB) as an organometallic source at low temperature (<200 °C) by plasma enhanced chemical vapour deposition (PECVD) technique in a microwave (700 W, 2.45 GHz) plasma cavity discharge system at a pressure of 66.67 Pa. The trapping/detrapping behavior of charge carriers in ultra thin ZrO₂ gate dielectric during constant current (CCS) and voltage stressing (CVS) has been investigated. Stress induced leakage current (SILC) through ZrO₂ is modeled by taking into account the inelastic trap-assisted tunneling (ITAT) mechanism via traps located below the conduction band of ZrO₂ layer. Trap generation rate and trap cross-section are extracted. A capture cross-section in the range of 10⁻¹⁹ cm² as compared to 10⁻¹⁶ cm² in SiO₂ has been observed. The trapping charge density, Q_ot and charge centroid, X_t are also empirically modeled. The time dependence of defect density variation is calculated within the dispersive transport model, assuming that these defects are produced during random hopping transport of positively charge species in the insulating layer. Dielectric breakdown and reliability of the dielectric films have been studied using constant voltage stressing. A high time-dependent dielectric breakdown (TDDB, t_bd > 1500 s) is observed under high constant voltage stress.     

The performances of (InAs)1/(GaAs)2short-period superlattice strained single-quantum-well laser on GaAssubstrate

We fabricated an (InAs)₁/(GaAs)₂ short-period superlattice (SPS) strained quantum-well laser at 1.07 μm by MOVPE. The SPS active layer has 10 periods of (InAs)₁/(_GaAs)₂ and an average mismatch of over 2.2%. In highly strained conditions the device showed a lasing wavelength of 1.07 μm, a threshold of 130 A/cm², and a characteristic temperature T₀ of 175 K. We measured the gain characteristic by the Hakki and Paoli method at LED conditions and obtained a high differential gain of 2.0×10^-15 cm² at the threshold current     

Possibility ofNd1.9Ba1.1Cu3O7+δ andPr1.14Ba1.86Cu3O7-δsingle crystals for insulator in high-speed superconductingcircuits

Dielectric properties of both Nd_1.9Ba_1.1Cu ₃O_7+δ (NBCO-213) and Pr_1.14Ba_1.86Cu₃O_7-δ (Pr-rich PBCO) single crystals have been examined at low temperature. These materials have good lattice matching to high-T_c superconductors (HTS), but they are conductive at room temperature. Below 80 K, they are insulators with low dielectric constants, ε _τ below 25, and low dielectric loss tan δ below 0.1 at 100 kHz. The value of ε_τ is suitable for insulators in integrated circuits using strip line widths of 10 μm order, providing short delay time, no excitation of surface wave, and low radiation loss. The value of tan δ is comparable to loss of superconducting surface resistance above 100 GHz. These results indicate the applicability for the insulator layers in multilayer superconducting electronic devices such as Single Flux Quantum (SFQ) circuits operated at high speed     

On ternary complementary sequences

A pair of real-valued sequences A=(a₁,a₂,...,a_N) and B=(b₁,b ₂,...,b_N) is called complementary if the sum R(·) of their autocorrelation functions R_A(·) and R_B(·) satisfies R(τ)=R_A(τ)+R _B(τ)=Σ_i=1^N$ -^τa_ia_i+τ+Σ_j=1 ^N-τb_jb_j+τ=0, ∀τ≠0. In this paper we introduce a new family of complementary pairs of sequences over the alphabet α₃=+{1,-1,0}. The inclusion of zero in the alphabet, which may correspond to a pause in transmission, leads both to a better understanding of the conventional binary case, where the alphabet is α₂={+1,-1}, and to new nontrivial constructions over the ternary alphabet α₃. For every length N, we derive restrictions on the location of the zero elements and on the form of the member sequences of the pair. We also derive a bound on the minimum number of zeros necessary for the existence of a complementary pair of length N over α₃. The bound is tight, as it is met by some of the proposed constructions, for infinitely many lengths     

On simple oversampled A/D conversion in L2(R)

The accuracy of oversampled analog-to-digital (A/D) conversion, the dependence of accuracy on the sampling interval τ and on the bit rate R are characteristics fundamental to A/D conversion but not completely understood. These characteristics are studied for oversampled A/D conversion of band-limited signals in L² (R). We show that the digital sequence obtained in the process of oversampled A/D conversion describes the corresponding analog signal with an error which tends to zero as τ² in energy, provided that the quantization threshold crossings of the signal constitute a sequence of stable sampling in the respective space of band-limited functions. Further, we show that the sequence of quantized samples can be represented in a manner which requires only a logarithmic increase in the bit rate with the sampling frequency, R=O(|logτ|), and hence that the error of oversampled A/D conversion actually exhibits an exponential decay in the bit rate as the sampling interval tends to zero     

High-performance polycrystalline SiGe thin-film transistors usingAl2O3 gate insulators

The use of aluminum oxide as the gate insulator for low temperature (600°C) polycrystalline SiGe thin-film transistors (TFTs) has been studied. The aluminum oxide was sputtered from a pure aluminum target using a reactive N₂O plasma. The composition of the deposited aluminum oxide was found to be almost stoichiometric (i.e., Al₂O₃), with a very small fraction of nitrogen incorporation. Even without any hydrogen passivation, good TFT performance was measured an devices with 50-nm-thick Al₂O₃ gate dielectric layers. Typically, a field effect mobility of 47 cm²/Vs, a threshold voltage of 3 V, a subthreshold slope of 0.44 V/decade, and an on/off ratio above 3×10⁵ at a drain voltage of 0.1 V can be obtained. These results indicate that the direct interface between the Al₂ O₃ and the SiGe channel layer is sufficiently passivated to make Al₂O₃ a better alternative to grown or deposited SiO₂ for SiGe field effect devices     

The coplanar resonator technique for determining the surfaceimpedance of YBa2Cu3O7-δ thinfilms

We describe how coplanar microwave resonators fabricated from patterned thin films of YBa₂Cu₃O₇(-δ) (YBCO) can he used to measure the ab-plane microwave surface impedance Z_s=R_s+jX_s of the films, in particular the absolute value and temperature dependence of the magnetic penetration depth λ. The current distribution of the resonator is calculated by modelling the resonator as a network of coupled transmission lines of rectangular cross-sections; this is then used to estimate the ab-plane λ(T) from the measurements of resonators of different geometries patterned onto the same film. We obtain values of λ(0) in the range 150-220 nm. The unloaded quality factors of the linear resonators at 7.95 GHz are around 45000 at 15 K and around 6500 at 77 K. We estimate the corresponding values of the intrinsic R_s at 7.95 GHz to be 23 μΩ and 110 μΩ at 15 K and 77 K, respectively. These values are comparable with those of other high quality unpatterned YBCO films reported in the literature. Z_s for the best optimised films appears to be insensitive to the effects of patterning