High vibrational temperatures in optically-pumped CO2

A pulsed 4.3-μm CO₂ laser was used to optically pump mixtures of CO₂ and He, and create transient gain at 9 and 10 μm. A conventional continuous-wave CO₂ laser operating on both regular and sequence bands measures this transient gain, and determined the ν₃ (asymmetric stretching)-mode vibrational temperature T₃. The measured values of T ₃ are generally much higher than those attained in discharge-excited CO₂. It is shown that a Treanor distribution must be used to describe the populations in the ν₃ -mode when dilute mixtures of CO₂ in He are optically pumped to ν₃-mode temperatures of 3000 to 4000 K. Under these conditions the sequence-band gain coefficients are almost equal to those on the regular bands. The collisional relaxation of energy from the ν₃ mode shows evidence of fast V-T relaxation at high values of T₃, followed by a slower relaxation rate characteristic of the 00⁰1 population lifetime     

Roles of low field mobility and its carrier-concentrationdependences in high electron mobility transistors and other field effecttransistors

A model is proposed for high-electron-mobility transistors (HEMTs) and other heterostructure FETs in which the dependence of low field mobility μ on carrier concentration N_s is taken into account. On the basis of this model, the influence of μ and its N_s dependence on drain current and transconductance g_m are clarified, In particular, high mobility (>10⁵ cm²/V-s) is shown to be effective in achieving and maintaining the intrinsic limit of g_m(=ε₂ν_s/d*) irrespective of bias conditions, where ν_s is the saturation velocity and ε₂ and d* are the dielectric permittivity and the effective thickness of the gate insulator, respectively. The N_s dependence of mobility is found to greatly affect the gate-voltage dependence of g _m and leads, in some cases, to an appreciable increase of g_m above its intrinsic limit     

On the figures of merit for electroabsorption waveguide modulators

ΓΔα/F and Δα/α₀ (where Δα is the absorption change, α₀ is the residual absorption, F is the applied electric field, and Γ is the optical confinement factor in the waveguide) have been separately proposed as the relevant figure of merit for electroabsorption waveguide modulators. Using a quantitative and systematic argument, the authors show that they are both necessary and important to the total performance of the modulator     

Dependence of transient gain on gas-flow velocity and dischargecurrent in the FAF CO2 laser amplifier

Transient behaviors in which the output power amplified through the fast-axial-flow (FAF) CO₂ laser amplifier decreases temporally after initiating the discharge are discussed. The output power becomes stable within a few minutes, i.e. the transient time. These phenomena should strongly depend on input irradiance, plasma length, gas-flow velocity v, and discharge current I _dis. The small signal gain γ₀ is formulated as a function of v and I_dis and the discharge time for a partially-homogeneously-broadened, slightly saturated gain medium at individual vibrational-rotational transitions     

Comparison of high-temperature-superconductor and metal-basedresonators

A 50-Ω coplanar waveguide (CPW) resonator designed for a fundamental frequency of about 4.75 GHz was fabricated on LaAlO₃ . Two versions were fabricated: the first using 1.9-μm-thick gold and the second using 0.6-μm-thick YBa₂Cu₃O ₇. The devices were identically packaged and tested at 77 K. It was found that the high-temperature superconductor (HTS) resonator had a surface resistance, R_s, about six to nine times lower than the Au one. At 45 K, the R_s of the HTS resonator decreases by another factor of 4 compared with its 77 K value. Device characteristics for the HTS resonator are presented     

1/f noise in MODFETs at low drain bias

The 1/f noise in normally-on MODFETs biased at low drain voltages is investigated. The experimentally observed relative noise in the drain current S_I/I² versus the effective gate voltage V_G=V_GS-V_off shows three regions which are explained. The observed dependencies are S_I/I²∝V_G ^m with the exponents m=-1, -3, 0 with increasing values of V_G. The model explains m =-1 as the region where the resistance and the 1/f noise stem from the 2-D electron gas under the gate electrode; the region with m=0 at large V_G or V_GS≅0 is due to the dominant contribution of the series resistance. In the region at intermediate V_G , m=-3, the 1/f noise stems from the channel under the gate electrode, and the drain-source resistance is already dominated by the series resistance     

Similarity and scaling in diffusion-cooled RF-excited carbondioxide lasers

The theoretical and experimental study of alpha RF discharges presented in this paper has resulted in the formulation of a set of similarity and scaling laws for diffusion-cooled alpha RF discharge-excited CW waveguide CO₂ lasers. For the first time, the parametric dependencies of the voltage-current-power characteristics of a transverse alpha RF discharge have been investigated over a range of excitation frequencies 100-160 MHz, for gas pressures 40-100 torr and for interelectrode distances 1-3 mm in a typical waveguide CO₂ laser gas mixture (He:N₂Co ₂=3:1:1+5 percent Xe). Relative to dc discharges, the additional scaling law fD=constant is established, and the analysis indicates both high- and low-frequency limits to the operation of practical self-similar laser devices     

Electrical properties of Si p+-n junctions for sub-0.25μm CMOS fabricated by Ga FIB implantation

p⁺-n junction diodes for sub-0.25-μm CMOS circuits were fabricated using focused ion beam (FIB) Ga implantation into n-Si (100) substrates with background doping of N_b=(5-10)×10 ¹⁵ and N_b⁺=(1-10)×10¹⁷ cm^-3. Implant energy was varied from 2 to 50 keV at doses ranging from 1×10¹³ to 1×10¹⁵ cm^-2 with different scan speeds. Rapid thermal annealing (RTA) was performed at either 600 °C or 700°C for 30 s. Diodes fabricated on N_b⁺ with 10-keV Ga⁺ exhibited a leakage current (^I_R) 100× smaller than those fabricated with 50-keV Ga⁺. Tunneling was determined to be the major current transport mechanism for the diodes fabricated on N_b⁺ substrates. An optimal condition for I_R on N_b⁺ substrates was obtained at 15 keV/1×10¹⁵ cm^-2. Diodes annealed at 600°C were found to have an I_R 1000× smaller than those annealed at 700°C. I-V characteristics of diodes fabricated on N_b substrates with low-energy Ga⁺ showed no implant energy dependence. I-V characteristics were also measured as a function of temperature from 25 to 200°C. For diodes implanted with 15-keV Ga ⁺, the cross-over temperatures between I_diff and I_g-r occurred at 106°C for N_b ⁺ and at 91°C for N_b substrates     

On the dynamics of population inversion for 3 μm Er3+lasers

A generalized model for 3-μm (⁴I_11/2 →⁴I_13/2)Er lasers is proposed. The essential energy transfer processes present in the single-doped Er ³⁺ systems (up-conversion from ⁴I_13/2, up-conversion from ⁴ I_11/2, cross-relaxation from ⁴S _3/2), as well as those present in Cr³⁺ codoped Er ³⁺ systems, are taken into account. In the frame of this model, the main features of 3 μm Er³⁺ lasers, such as long pulse or CW operation, the change of emission wavelength as a function of pumping conditions, and the effects of codoping with Ho³⁺ or Tm³⁺ ions, are explained     

H2/O2 plasma on polysilicon thin-filmtransistor

It is reported for that H₂ plasma followed by O₂ plasma is more effective for passivating grain boundary states in polysilicon thin film. Polysilicon thin-film transistors (TFTs) made after H₂/O₂ plasma treatment can exhibit a turn-on threshold voltage of -0.1 V, a subthreshold swing of 0.154 V/decade, an ON/OFF current ratio I_on/I_off over 1×10⁸, and an electron mobility of 40.2 cm² /V-s     

1.27μm atomic argon laser parameters in fission-fragment excitedHe/Ar and He/Ne/Ar gas mixtures

The authors examine the power efficiency of the fission-fragment-excited atomic argon laser operating on the 1.27-μm (3d'[3/2]₁-4'_p[3/2]₁) argon transition as a function of pump power, gas mixture, and pressure. The maximum measured power efficiency was 1.1±0.3% for a gas pressure of 1300 torr and a He/Ar ratio of 99.88/0.12. Neon addition to the He/Ar gas mixture increased both the energy deposited in the gas and the energy output without decreasing efficiency for a neon gas fraction of less than 0.5. Small-signal gain and saturation intensity are between 0.15-0.27%/cm and 25-200 W/cm² for pump rates of 7.5-30 W/cm ³ in He/Ar and He/Ne/Ar gas mixtures. The laser threshold as a function of total pressure and argon concentration is presented     

Optical absorption and stimulated emission of neodymium in yttriumorthosilicate

A spectroscopic investigation of the biaxial crystal yttrium orthosilicate doped with Nd³⁺(Nd³⁺:Y₂SiO₅) has been performed. Spectrally and orientationally resolved emission cross sections necessary for the evaluation of laser performance on the Nd^{3+ 4}F_3/2-⁴I _9/2 and ⁴F_3/2-⁴I_11/2 transitions have been determined. The Judd-Ofelt theory has been applied to measured values of optical absorption line strengths to obtain the orientation averaged intensity parameters: Ω₂-3.34×10^-20 cm², Ω ₄=4.35×10^-20 cm², and Ω₆=5.60×10^-20 cm². These Judd-Ofelt intensity parameter values are significantly different from those previously reported by A.M. Tkachuk et al. Using these intensity parameters, the Nd^{3+ 4}F₂ metastable state lifetime is predicted to be 225 μs. Measured low Nd concentration ⁴F_3/2 lifetimes of 214 μs indicate a high radiative quantum efficiency. Because of the Stark level splitting of the Nd^{3+ 4}F_3/2 and ⁴I_9/2 manifolds, laser operation at twice one of the Cs atomic resonance filter acceptance wavelengths is possible     

An InGaAs-InP position-sensing photodetector

The design and fabrication of a position-sensing photodetector that is sensitive to a wavelength of 1.55 μm are reported. This device is based on the photo-lateral effect and uses an In_0.53Ga_0.47As absorbing layer that is lattice matched to an InP substrate. The output signal (I₁- I₂)/(I₁+I₂) varies linearly with respect to the position of the incident beam. The responsivity of 0.75 A/W is uniform over the length of the photodetector. The device is intended for video rate applications and has a -3-dB cutoff frequency of 7 MHz     

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     

Submicrometer self-aligned AlGaAs/GaAs heterojunction bipolartransistor process suitable for digital applications

A self-aligned process is developed to obtain submicrometer high-performance AlGaAs/GaAs heterojunction bipolar transistors (HBTs) which can maintain a high current gain for emitter sizes on the order of 1 μm². The major features of the process are incorporation of an AlGaAs surface passivation structure around the entire emitter-base junction periphery to reduce surface recombination and reliable removal of base metal (Ti/W) deposits from the sidewall by electron cyclotron resonance (ECR) plasma deposition of oxide and ECR plasma etching by NF₃. A DC current gain of more than 30 can be obtained for HBTs with an emitter-base junction area of 0.5×2 μm² at submilliampere collector currents. The maximum f_T and f_max obtained from a 0.5×2 μm² emitter HBT are 46 and 42 GHz, respectively at I_C=1.5 and more than 20 GHz even at I_C=0.1 mA     

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     

A model for fast switching transients in power systems: the nearzone concept

The author presents a simple time-domain model which makes it possible to predict the order of magnitude of the highest di/ dt values generated by closing switches in electrical power systems. The model is based on traveling-wave analysis. It is demonstrated that two different approaches must be applied, according to whether (a) the closing time, T_s, of the switch is faster than twice the traveling time to the first reflection point or (b) T_s is much slower. Under condition (b) the well-known quasistationary approach di/dt_max=U₀/L can be used, where U₀ is the switched voltage and L is the self-inductance of the line between the stray capacitances located to the left and the right of the switching device. Under condition (a) a new formula must be applied: di/dt _max≈2 U₀/ZT_s, where Z is the line impedance of the line in which the switching device is installed and T_s is the time during which the voltage across the switch collapses from U₀ to zero. Experimental results are given from both fast and slow closing switches     

New low capacitance transverse junction stripe AlGaAs/GaAs laserfor planar laser-MESFET integration

A transverse junction stripe laser structure with a very low parasitic capacitance, compatible with an entirely planar laser-MESFET integration, is presented. The laser, obtained by two successive n (S) and p(Zn) diffusions across undoped AlGaAs/GaAs double heterostructure layers, has a threshold current of I_th =55 mA. A very low parasitic capacitance C_s=0.6 pF is measured, with a corresponding rolloff time constant of t_r=9 ps     

Spread-vertical-capacitor cell (SVC) for high-density dRAM's

An advanced three-dimensionally (3-D) stacked-capacitor cell, the spread-vertical-capacitor cell (SVC), was developed. SVC realized a storage capacitance (C_s) of 30 fF with a cell area of 1.8 μm², a capacitor height of 0.37 μm, and an equivalent SiO₂ film thickness of 7 nm for oxide-nitride-oxide (ONO). By extrapolating these results to 256-Mb DRAMs, a C_s of 24 fF is obtained with a cell area of 0.5 μm², a capacitor height of 0.4 μm, and an equivalent SiO₂ thickness of 5 nm, and these values satisfy the specifications for 256-Mb DRAMs. The low capacitor height of SVC makes possible a fabrication process using ArF excimer laser lithography     

Novel hydrogen annealing for forming SiO2 film by rapidthermal processing

Hydrogen annealing at 700-1100°C for 0-300 s has been combined with SiO₂ formation by rapid thermal processing (RTP). The SiO₂ films formed with the above processes were evaluated by C-V and I-V measurements and by time-dependent dielectric breakdown (TDDB) tests. These films provide longer time to breakdown andless positive charge generation than SiO₂ films formed without H₂ annealing. In particular, the SiO₂ formation-H₂ annealing SiO ₂ formation process is quite effective in improving the dielectric strength of the thin RTP-SiO₂ film