Optical absorption and fluorescence studies in high pressure cesium-xenon mixtures

The pressure and temperature dependent absorption and fluorescence spectra of the cesium-xenon (CsXe) molecule have been examined. In contrast to previous investigations of the alkali-rare gas molecules, cesium atomic states that have weakly allowed optical transitions have been studied and have been shown to form excimer levels that are attractive for application as potential dissociation lasers. In particular, the (Cs[7^{2}S]Xe)* excimer appears promising as a source of high-energy laser radiation due to 1) its large dissociation energy (0.132 eV), 2) its stimulated emission cross section ofsimeq10^{-17}cm², and 3) its small population threshold inversion densities (simeq10^{13}cm^-3).     

The inhibition of negative resistance dipole waves and domains in n-GaAs

Expressions describing the growth and propagation of infinitesimal space-charge waves in the presence of a differential negative resistance are derived, and conditions for the inhibition of dipole waves and domains inn-GaAs of resistivity greater than 10Ω-cm at 300°K are obtained. Thermoelectric effects are estimated and found to be small for carrier densities much less than 10¹⁵cm^-3. Taking the negative differential resistivity to be larger than the ohmic resistivity by a factor of 10²leads to 1) condition for inhibition of dipole waves:nl^{2} lsim 10^{9}cm^-12) condition for inhibition of stable domains:nl lsim 10^{12}cm^-2whenn= electron density andl= specimen length. The relevance of these results to explaining the phenomena observed in amplifying crystals is pointed out. It is suggested that the condition for amplification is(frac{10^{9}}{n})^{1/2} lsim l lsim frac{10^{12}}{n}.     

Electronic processes at grain boundaries in polycrystalline semiconductors under optical illumination

The dependence of minority carrier lifetime (τ) on the doping concentration Nd, grain sizedand interface state density Nisat the grain boundaries in (n-type) polycrystalline semiconductors has been calculated analytically. The recombination velocity at grain boundaries is enhanced by the diffusion potential Vdadjacent to the boundaries, and ranges fromsimeq 10^{2}to 10⁶cm . s^-1depending on Nisand Nd. Under illumination, the population of the interface states is altered considerably from its dark level and as a result, Vddecreases to that value which maximizes recombination (equal concentrations of electrons and holes at the boundary). This causes τ to decrease with increasing Nd. Sample calculations for polycrystalline silicon show that for low angle boundaries with interface state densities ofsimeq 10^{11}cm^-2eV^-1, τ decreases from 10^-6to 10^-10s as the grain size is reduced from 1000 to 0.1 µm (forN_{d} = 10^{16}cm^-3). For a constant grain size, τ decreases with increasing Nd. The open-circuit voltage of p-n junction solar cells decreases fortau leq 10^{-7}s, whereas that for Schottky barrier cells remains at its maximum value untiltau lsim 10^{-8}s.     

Performance of InxGa1-xAsyP1-yphotodiodes with dark current limited by diffusion, generation recombination, and tunneling

We present a theoretical study of the effects of diffusion, generation-recombination (GR), and the recently observed tunneling currents on the performance of photodiodes made from In0.73Ga0.27As0.63P0.37and In0.53Ga0.47As. Calculations are made for both p+ν and p-i-n punch-through diode configurations, and are compared with recent measurements made by several independent investigators. For doping densities typical of present material (N_{D} gsim 10^{15}cm^-3), tunneling currents become dominant prior to avalanche breakdown. Thus, for detection of weak (-55 dBm at 45 Mbits/s) optical signals, the diodes must be operated at low voltages where GR is the dominant source of reverse-biased leakage. To meet the requirements of low capacitance (C leq 0.5pF for a diode area of 10^-4cm²) and low GR dominated dark current (I_{D} leq 10nA atT = 70degC), the doping density and effective carrier lifetime (τeff) must beN_{D} < 7 times 10^{15}cm^-3andtau_{eff} gsim 150ns for In0.73Ga0.27As0.63P0.37and5 times 10^{14} lsim N_{D} lsim 7 times 10^{15}cm^-3andtau_{eff} gsim 3.5 mus for In0.53Ga0.47As.     

Excitation mechanism and plasma parameters in pulsed argon-ion lasers

Pulsed argon-ion lasers show several interesting properties at high currents. To understand the inversion mechanism, the plasma parameters, electrical conductivity, electron temperature, and electron density were measured with the double-probe method for the pressure range from 15 to 50 mtorr in a 6-mm-bore tube. When the discharge current increases from 100 to 700 amperes at the optimum pressure for laser oscillation, these parameters increase from 250 to450Omega^{-1}cdotcm^-1, from8 times 10^{4}to10^{5}degK, and2 times 10^{14}to 10¹⁵cm^-3, respectively. At the maximum electron density, the percent of ionization appears to be in excess of 100 percent, as a result of the pinch effect and double ionization. It is certain that this ring discharge is at least ionized very strongly. In a 10-mm-bore tube, only the electron temperature and density were measured. The current dependence of the laser output power at high currents is interpreted with those results. Excitation mechanisms of high-current argon-ion lasers are discussed with experiments and theories for strongly ionized plasmas.     

Radiative efficiencies of radio frequency sulfur discharges

The radiative efficiencies of RF sulfur discharges under various conditions of power loading, sulfur vapor pressure, and argon buffer pressure have been determined. The highest CW efficiency was 11 percent for a sulfur density of6.3 times 10^{16}cm^-3and 20 torr argon buffer. The results of pulsed operation show that the efficiency increases slightly to 13 percent for a sulfur density of8.8 times 10^{16}cm^-3.     

A simplified capless annealing of GaAs for MESFET applications

A simple capless annealing technique for post-implantation annealing of a GaAs wafer is described. The technique incorporates a novel boat design and uses InAs as the source of arsenic overpressure. Using this technique, wafers annealed at 850°C show mobilities in the range of 4000 cm². V^-1. S^-1with over 85-percent activation for a Si dose of5 times 10^{12}cm^-2. Dopant depth profiles with peak donor densities of2 times 10^{17}cm^-3and minimal tailing were demonstrated. Electron channeling data show that crystallinity is fully restored during the anneal. 1-µm gate length MESFET's processed on n⁺-n implanted layers exhibitedg_{m} geq 160mS/mm and pinchoff voltages in the range of 3 V.     

Performance of In0.53Ga0.47As/InP avalanche photodiodes

We calculate operating characteristics of high-sensitivity high-speed In0.53Ga0.47As/InP avalanche photodiodes (APD's). We find that significant photocurrent gain is obtained for a total fixed-charge density ofsigma_{tot} > 3 times 10^{12}cm^-2in the depleted InP and0.53Ga0.47As regions. To obtain high quantum efficiency and low tunneling currents, the fixed-charge density in the InP must be in the range2 times 10^{12}cm^-2leq sigma_{B} leq 3 times 10^{12}cm^-2. We calculate the breakdown voltages for APD's with uniformly doped layers and find that practical detectors with avalanche breakdowns as low as 15 V can be realized. High quantum efficiency and fast response are obtained by compositional grading of the In0.53Ga0.47As heterointerface over a distance ofL gsim 380Å, depending on the doping and amount of the In0.53Ga0.47As layer swept out at breakdown. Finally, a comparison of calculations with experimental results is presented.     

A discharge heated copper vapor laser

A simple, reliable copper vapor laser is described with vapor produced by discharge heating. Average output power of 1.3 W has resulted at 6.8 kHz and a specific energy of 39 μJ/cm³. Copper vapor density as high as3 times 10^{16}cm^-3was achieved.     

Comparison of Auger recombination in GaInAs-AlInAs multiple quantum well structure and in bulk GaInAs

Carrier lifetime has been measured in GaInAs-AlInAs multiple quantum well structures and in thick GaInAs samples for local carrier densities between2 times 10^{17}and5 times 10^{19}cm^-3. Carrier lifetime and Auger recombination are found to be very close (±20 percent) in the two structures. The Auger limited T0values calculated for DH and MQW lasers are found to be, respectively, 93 and 170 K. Optimization of the quantum well laser as a function of the number of wells in the active region is discussed.     

Continuously tunable sum-frequency generation involving sodium Rydberg states

Broadly tunable sum-frequency generation has been observed in a vapor of atomic sodium in the presence of a dc electric field. This field induces achi^{(2)}nonlinearity which is resonantly enhanced when the sum frequency corresponds to the energy separation between the ground state and an atomic Rydberg state. In a vapor of number density4 times 10^{14}cm^-3, we obtain an energy conversion efficiency as large as3 times 10^{-4}and achi^{(2)}as large as1.2 times 10^{-8}ESU. We have also observed sum-frequency generation in the absence of an applied dc field, and we relate these observations to mechanisms that have been proposed to explain this effect.     

Formation kinetics of the triatomic excimer Ar2F*

Detailed fluorescence measurements of electron beam-excited high pressure mixtures of Ar/F2and Ar/NF3have been made in order to investigate the processes leading to the formation of Ar2F*. Three-body collisional quenching of ArF* has been identified as a major formation mechanism. The third-order rate constant for this reaction has been measured to bek_{1} = (1.2 pm 0.2) times 10^{-30}cm⁶. s^-1. In addition quenching rates for collisional de-excitation of Ar2F* by argon and the fluoride donors NF3and F2have been determined. The radiative lifetime has been measured as 219 ± 15 ns. The possible formation of Ar2F* from long-lived excited argon neutrals when using intense excitation densities and low-donor pressures is also briefly discussed.     

Gain characteristics of an atmospheric sealed CW CO2laser

Experimental and analytical investigations have been made on unsaturated gain g0of a CO2electric-discharge convection laser, in which discharge current flow, gas flow, and the optical axis are mutually perpendicular. Stable glow discharges in sealed gas mixtures of CO2, CO, N2, and He were maintained at pressures up to 780 torr with an input power density of about 90 W/cm³. The ratio of electric field to neutral particle densityE/Nwas1.7 times 10^{-16}V . cm²and was independent of the total gas pressureP. The electron density in a positive column of the glow discharge was about4 times 10^{10}cm^-3. Detailed spatial distributions of g0at a wavelength near 10.6 μm were measured in the pressure range from 100 to 780 torr. Measurements were also made on the current dependence of g0and on the change in gowith discharge time. The g0distributions along the gas flow direction were found to agree with those calculated from the electron density distribution and the relaxation rate constant of the upper laser level on the basis of continuity equations for a two-level model. The integrated value of g0along the flow direction was proportional to P^-0.8whenE/N, electron density, and gas temperature were held constant. A maximum value of the g0distribution, which was proportional to P^-0.3, was 0.14 percent/cm at 780 torr.     

Impact ionization in bulk GaAs high field domain

An investigation of impact ionization in the high field domains of GaAs Gunn effect devices has been carried out. A method of measuring the number of excess electrons generated per domain transit was developed, based on the measurement of the increase in the valley current from one domain transit to the next while impact ionization is occurring in the domain. This method was used to experimentally determine the carrier generation per domain transitDelta n/n_{0}as a function of excess domain potential VDfor three material carrier concentrations. These results are compared to a theoretical calculation ofDelta n/n_{0}based upon an approximate solution of the continuity equation and invariant domain calculations. The experimental and theoretical results agree to within 2 percent forn_{0} = 3 times 10^{14}cm^-3and 6 percent forn_{0} = 4 times 10^{14}cm^-3.     

Influence of ΦMS(T) in the determination of the Si-SiO2surface states

The determination of fast surface states at the Si-Si02system by the Gray-Brown low-temperature technique is carefully analyzed. Included in this analysis is the variation of the semiconductor work function with the temperature and the doping impurity partial ionization at low temperature. It is shown that particularly the influence of the semiconductor work function is very significant. For instance, we obtained for an Si-SiO2system a number of surface states, with energy levels between 0.32 and 0.53 eV,N_{fs} simeq 8.5 times 10^{9}cm^-2orN_{fs} simeq 1.0 times 10^{11}cm^-2, the difference due to the influence of the semiconductor work function only.     

Growth and characterization of MO/VPE double-heterojunction lasers

Metalorganic vapor-phase epitaxy (MO/VPE) has been used for the growth and fabrication of double-heterojunction (DH) devices. Al0.3Ga0.7As/GaAs DH mesa stripe lasers were made, which have a threshold current density of 6.7 kA/cm². We attribute this high value to recombination losses at killer centers situated in the confinement layers very close to the active layer; it is shown that these killers were introduced by using dopant concentrations beyond4 times 10^{17}cm^-3. The MO/VPE technique was also applied for burying mesa lasers. The threshold current density of an LPE mesa buried with monocrystalline (Al, Ga)As was 3.5 kA/cm²; i.e., 60 percent higher than its corresponding broad-area value. It is shown that in order to reduce the excess current, the recombination velocity at the VPE/LPE active-layer interfaces should be lowered; viz., to a value below the diffusion velocity of carriers towards these interfaces. The optical mode structure of the buried mesa (BM) lasers was stable up to a power output of 10 mW/facet.     

Gain spectra in GaInAsP/InP proton-bombarded stripe-geometry DH lasers

We have measured gain spectra for TE polarization in a GaInAsP/InP laser as a function of dc bias current below laser threshold. The measurements were made on a low-threshold device with a stripe geometry defined by proton bombardment. A number of other characteristics of this and similar devices from the same wafer are also reported. These data permit the maximum gain coefficient of the active layer gmaxto be evaluated as a function of nominal current density Jnom. We obtain the linear relationshipg_{max} = (3.1 times 10^{-2}/eta_{r})(J_{nom} - eta_{r} 5.4 times 10^{3}), where ηris the radiative quantum efficiency. Our data apply only for large gain (g gsim 150cm^-1) and large Jnombecause the active layer of the test device is thin (0.1 μm).     

Difficulties associated with stimulated emission in a bismuth--Neon discharge

The population of the metastable6p^{3}²D_{3/2}^{0}bismuth level has been determined by time-resolved absorption spectroscopy at times in the range1-10 mus after the start of the discharge in a bismuth vaporneon mixture, for a neon pressure of 13 mbar and a specific discharge energy of 1 mJ . cm^-3. An exponential decay of the population is observed and lifetimes of 3.0, 4.5, and 5.3 μs are obtained at temperatures of 830, 790, and 755°C, respectively. Measurements of the nine strongest fluorescent transitions to the metastable level confirm that a population of the order of 10¹³cm^-3is established in the early stage of the discharge and exceeds by three orders of magnitude the peak population of the bismuth resonance level6p^{2} 7s^{4}P_{1/2}. It is concluded that the absence of laser emission at 472.2 nm in the system studied is due to excessive population of the metastable level6p^{3}²D_{3/2}^{0}during the early stage of the discharge. It is proposed that this high population arises from the dissociation of Bi2dimers by electron impact and that the loss of electron energy to dissociation and to excitation of the vibrational and rotational levels of the dimers is responsible for the very low population achieved in the6p^{2} 7s⁴p_{1/2}level.     

Two types of microwave emission from InSb

Two types of microwave emission from InSb plasma subjected to the crossed electric and magnetic fields were investigated experimentally. From the simultaneous measurements of Hall effect and microwave emission, the threshold condition of the one type of emission was obtained as(omega_{ce} tau_{e})^{2} cdot Delta n geq 3 times 10^{14}cm^-3, whereomega_{ce}, tau_{e}, andDelta nare electron cyclotron frequency, relaxation time, and electron-hole pair density, respectively. It was also found that the "Hall" mobility showed anomalous decrease above the threshold. The emission power of the other type of emission showed maximum value at several values of the applied magnetic field lower than the threshold of the noise-type emission. These characteristic magnetic fields were not influenced by current density except by slight change in the low-current density region.     

Small signal gain in the CO-air-He laser

The small-signal single-pass gain for 24 individual CO laser lines has been measured for a flowing CO-air-He laser. The vibrational-rotationalP(J)_{upsilon,upsilon'}lines considered were for rotational transitions fromJ = 10to 16 and upper vibrational levels fromupsilon = 7to 11. The maximum gain was observed for theP(15)_{10,9}laser line, with an average logarithmic energy gain of2.633 times 10^{-3}cm^-1.