Carrier density dependence of refractive index in AlGaAs semiconductor lasers

Carrier density dependence of the refractive index in the active layers of semiconductor lasers is evaluated from the wavelength shift with increases in current by taking into account effects of the active layer temperature rise and lateral carrier and optical field distributions on the wavelength shift. The derived refractive index change due to carrier density increase is-4 times 10^{-27}m³, which is in good agreement with the theoretical value.     

Improved Performances of InGaN Schottky Photodetectors by Inducing a Thin Insulator Layer and Mesa Process

The performances of InGaN Schottky photodetectors with varied fabrication processes were investigated. The photoresponse and dark current of InGaN Schottky photodetectors can be obviously improved by inserting a thin $hbox{Si}_{3}hbox{N}_{4}$ passivation layer between the InGaN layer and the Schottky metal. Furthermore, a mesa process gives not only a further increase in the photoresponse but also a pronounced reduction in the reverse leakage current of about two orders of magnitude. A lateral surface leakage current mechanism associated with the 2-D variable-range hopping conduction through high-density surface states in InGaN is proposed to explain the reduction of the reverse leakage current after etching the mesa.      

Channel waveguides in glass via silver-sodium field-assisted ion exchange

Multimode channel waveguides were formed by field-assisted diffusion of Ag+ ion from vacuum-evaporated Ag films, into a sodium aluminosilicate glass reported to yield high diffusion rates for alkali ions. Two-dimensional index profiles of channel waveguides formed by diffusion from a strip aperture were controlled by means of diffusion time, temperature, and electric field. The diffusion equation for diffusion through a strip aperture in the presence of a one-dimensional electric field was solved. Its solution was in agreement with measured concentration profiles:frac{C(x,y,t)}{C_{0}} = frac{1}{2} { erf (frac{a - x}{2sqrt{Dt}}) + erf (frac{a + x}{2sqrt{Dt}})}.frac{1}{2} { erfc (frac{y - muE_{y}t}{2sqrt{Dt}}) + e^{(yE_{y}/D)} erfc (frac{y + muE_{y}t}{2sqrt{Dt}})}Diffusion coefficients in this aluminosilicate glass were determined to beD =(2.41 times 10^{-13}) (frac{m^{2}}{s})).exp (frac{-3.1 times 10^{4}frac{J}{mol}}{RT})Diffusion coefficients were higher (between 150°C and 300°C) than those of a low-iron soda-lime silicate glass "standard" also studied, for which diffusion coefficients wereD =(3.28 times 10^{-13} (frac{m^{2}}{s})).exp (frac{-3.6 times 10^{4}}{RT} (frac{J}{mol}))This difference in diffusion coefficients is due to the higher activation energy of diffusion in the soda-lime silicate glass. The Gladstone-Dale relation was used to calculate the maximum possible refractive index change via Ag+-Na+ ion-exchange for each type of glass. The maximum index change in the sodium aluminosilicate glass is found to be about 65 percent of that in the soda-lime silicate glass.     

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).     

Linear and nonlinear optical properties of AgGaS2, CuGaS2, and CuInS2, and theory of the wedge technique for the measurement of nonlinear coefficients

The refractive indices of the ternaryA^{I}B^{III}C_{2}^{VI}semiconductors AgGaS2, CuGaS2, and CuInS2have been measured over the entire range of transparency of these crystals. The optical nonlinear coefficients for second-harmonic generation have also been determined. Three-frequency collinear phase matching is analyzed in detail for AgGaS2. The birefringences of CuGaS2and CuInS2are not large enough to permit three-frequency phase matching within the transparent regions. A parametric oscillator threshold calculation for a pump wavelength 0.89 μ, which is within the range of the GaAs injection laser, indicates that AgGaS2is promising for this application. The upconversion efficiency in AgGaS2for sum mixing of the CO2laser (lambda = 10.5 mu) with the xenon ion laser (lambda = 0.597 mu) is also calculated. The result indicates that, depending upon system requirements and the availability of high optical quality material, AgGaS2can be comparable to ZnGeP2for upconversion. In Appendix II, we present a theory of the wedge technique for the measurement of nonlinear coefficients. This theory takes into account losses and assumes a Gaussian beam geometry. Furthermore, a discussion of units in nonlinear optics is given.     

Vertical n-Channel Poly-Si Thin-Film Transistors With Symmetric S/D Fabricated by Ni-Silicide-Induced Lateral-Crystallization Technology

We have successfully developed and fabricated the vertical n-channel polycrystalline silicon thin-film transistors with symmetric S/D fabricated by Ni-silicide-induced lateral-crystallization technology (NSILC-VTFTs). The NSILC-VTFTs are S/D symmetric devices and equivalent to dual-gate devices. The dual-gate structure of NSILC-VTFTs can moderate the lateral electrical field in the drain depletion region, significantly reducing the leakage current. In NSILC-VTFTs, the Ni accumulation and grain boundaries induced from S/D sides can be centralized in the $hbox{n}^{+}$ floating region. The effects of Ni accumulation in symmetric VTFTs crystallized by NSILC and metal-induced lateral crystallization are studied. In addition, a two-step lateral crystallization has been introduced to improve the crystal integrity through secondary crystallization. The NSILC-VTFTs crystallized by two-step lateral crystallization show a steep subthreshold swing of 180 mV/dec and field effect mobility $mu = hbox{553} hbox{cm}^{2}/hbox{V} cdot hbox{s}$ without $hbox{NH}_{3}$ plasma treatment.      

Lateral analysis of quasi-index-guided injection lasers: Transition from gain to index guiding

In double-heterostructure stripe-geometry semiconductor lasers an effective lateral index stepDelta n_{L}over the stripe region can be induced through evanescent-field coupling. Such a quasi-index-guided device exhibits a transition from the gain-guided to the index-guided regime whenDelta n_{L}is progressively increased. Using parameters appropriate to a 1.3-μm InGaAsP laser, the transition is shown to occur aroundDelta n_{L} sim 5 times 10^{-3}. The exact value ofDelta n_{L}depends on the extent of carrier-induced antiguiding. In the transition region the threshold current decreases rapidly, the lateral mode contracts, and the far field changes from a twin-lobe to a single-lobe pattern. Our analysis suggests that a quasi-index-guided device operates most efficiently for values ofDelta n_{L}at which the index-guided regime is just approached. With a further increase ofDelta n_{L}, the mismatch between the gain and mode profiles leads to lower differential quantum efficiencies. Among other structures, the analysis is applicable to a ridge waveguide laser. For a 1.3-μm laser the optimumDelta n_{L}can be obtained using 0.2-μm-thick cladding layers for a 0.2-μm thick active layer.     

Birefringence in curved single-mode optical fibers due to waveguide geometry effect--Perturbation analysis

The field deformation in curved step-index single-mode circular fibers and the birefringence induced by the curved waveguide geometry are analyzed by a perturbation method. We find that the values of the birefringence are proportional toa^{2}/R_{0}^{2}and are much less than those induced by stress. A square optical waveguide is also investigated. We find birefringence values of roughly the same order of magnitude as those for the circular waveguide.     

Magnetic focusing of electron beams in the presence of transverse velocity components

The behavior of nonlaminar electron beams having an originally Gaussian current density distribution in a uniform magnetic field is studied by analytical and numerical methods. The effects of transverse velocity components vTin the beam are described by a fictitious, equivalent perveance which is proportional to the square ofbar{v}_{T}, the rms value of vT. Simple expressions are given for the magnetic field required to achieve a certain beam transmission through a tunnel of given diameter, and for the optimum beam filling factor. In the Appendix a simple experimental method for the determination ofbar{v}_{T}is descried.     

Improved molecular beam epitaxial growth of AlxGa1 - xAs/GaAs high-radiance LED's for optical communications

We report some improvements made in molecular beam epitaxial growth of AlxGa1-xAs/GaAs high-radiance LED's for optical communications. These improvements have been achieved by growing these wafers in a system which includes an air-lock wafer-exchange chamber. Interfacial recombination velocity as low as6 times 10^{2}cm/s was obtained. The series resistance of the Burrus-type LED's was reduced by increasing the acceptor concentrations in the p-layers to a density as high as 10¹⁹cm^-3using Be as the dopant. CW output of 5.8 mW, or a radiance of 92 W/sr. cm², at a safe operating current of 150 mA has been obtained in these devices. The device performance is comparable to the best obtained in LPE-grown diodes of the same geometry.     

Transparent Oxide Thin-Film Transistors Composed of Al and Sn-doped Zinc Indium Oxide

We have fabricated the transparent bottom gate thin-film transistors (TFTs) using Al and Sn-doped zinc indium oxide (AT-ZIO) as an active layer. The AT-ZIO active layer was deposited by RF magnetron sputtering at room temperature, and the AT-ZIO TFT showed a field effect mobility of 15.6 $ hbox{cm}^{2}/hbox{Vs}$ even before annealing. The mobility increased with increasing the $hbox{In}_{2}hbox{O}_{3}$ content and postannealing temperature up to 250 $^{circ}hbox{C}$. The AT-ZIO TFT exhibited a field effect mobility of 30.2 $hbox{cm}^{2}/hbox{Vs}$, a subthreshold swing of 0.17 V/dec, and an on/off current ratio of more than $10^{9}$ .      

Boresight-gain loss and gore-related sidelobes of an umbrella reflector

It is shown that energy in the gore-related sidelobes of an umbrella reflector comes from energy lost from the main beam. Both boresight gore-loss and sidelobe level correlate directly with cyclic aperture phase error caused by the geometry. The gore-sidelobe peak is located neartheta_{p}, wheresin theta_{p} = 1.2N_{G} (pi/Dlambda)whereN_{G}is the number of gores andDis the reflector diameter. An expression is also derived for the amplitude of the gore-related sidelobe.     

Investigation of the lateral turn-on process of thyristors with an epitaxial p-base

In this paper, the effect of the p-base doping concentration NAon the spreading velocity vsin power thyristors is examined. Chemical vapor deposition (CVD) techniques are used to produce the p-base layer, in order to change the p-base doping concentration and thickness independently. The results show a large reduction of vswith growing p-base doping concentration. At a doping concentration higher than 5 × 10¹⁶/cm³the spreading velocity follows a power law with an exponent of -0,9. The introduction of a sandwiched low-doped player between the p⁺-base and the n-emitter slows down the plasma propagation. The decrease of vs, in both cases, is attributed to the reduction of the current gain β2of the n-p-n transistor with doping concentration. In order to explain this behavior, a simple expression for the spreading velocity is derived, which relates the spreading velocity to the time constant of current rise trand consequently to the feedback loop gain (β1β2) of the two transistor components. In this derivation, only the lateral drift current in the p-base is taken into account. It was found that vsis given byv_{s} sim 1/t_{r} sim ln beta_{1}beta_{2}, in good agreement with the experiment.     

Electrooptic and piezoelectric measurements in photorefractive barium titanate and strontium barium niobate

We have measured the low-frequency ("unclamped") electrooptic and piezoelectric coefficients in undoped BaTiO3and SrxBa1-xNb2O6(x = 0.61) crystals using interferometric techniques. The contribution of the piezoelectric effect to the Pockels measurements is discussed. For an applied ac electric field in the range 0.1-200 V/cm, the electrooptic and piezoelectric effects are linear in the magnitude of the applied field and independent of its frequency in the range 10 Hz-100 kHz. The unclamped electrooptic coefficients of poled BaTiO3single crystals arer_{13} = 19.5 pm 1pm/V andr_{33} = 97 pm 7pm/V, and for strontium barium niobate arer_{13} = 47 pm 5pm/V andr_{33} = 235 pm 21pm/V, all measured at a wavelength of 514.5 nm and atT = 23degC. For the barium titanate samples the measured Pockels coefficientr_{c} equiv r_{33} - (n_{1}/n_{3})^{3} r_{13} = 79 pm 6pm/V is in good agreement with the valuer_{c} = 76 pm 7pm/V computed from the above values of r13and r33, where n1and n3are the ordinary and extraordinary indexes of refraction, respectively. The measured piezoelectric ] coefficient isd_{23} = +28.7 pm 2pm/V for barium titanate, and isd_{23} = +24.6 pm 2pm/V for strontium barium niobate. We also measured the photorefractive coupling of two optical beams in the crystals, and we show that the dependence of the coupling strength on beam polarization is in fair agreement with the measured values of the Pockels coefficients.     

AlGaN/GaN MOSHEMTs With Liquid-Phase-Deposited $ hbox{TiO}_{2}$ as Gate Dielectric

$hbox{TiO}_{2}$ films deposited on GaN layers at room temperature through a simple and low-cost liquid-phase deposition (LPD) method are investigated and served as gate dielectrics in AlGaN/GaN MOSHEMTs. The electrical characteristics of the MOS structure on n-doped GaN show that the leakage current is about $hbox{1.01} times hbox{10}^{-7} hbox{A/cm}^{2}$ at 1 MV/cm and that the breakdown field is more than 6.5 MV/cm. The maximum drain current density of MOSHEMTs is higher than that of conventional HEMTs, and a wider gate voltage swing can also be observed. The maximum transconductance and threshold voltage almost maintain the same characteristics, even after inserting a dielectric layer between the gate metal and the 2DEG channel by using $ hbox{TiO}_{2}$ as a gate dielectric. The gate leakage current density is significantly improved, and the bias stress measurement shows that current collapse is much suppressed for MOSHEMTs.      

Control of gate—Drain avalanche in GaAs MESFET's

The onset of gate-drain avalanche imposes an important fundamental constraint on the drain voltage swing, and hence, on the output power of GaAs FET's. In this paper we show that recognition of the role of surface depletion and proper attention to channel design can yield avalanche voltage factors of 2-3 above bulk values. The appropriate design strategy is minimization of the undepleted epitaxial charge per unit area (Qu) between gate and drain, which, in turn, dictates a gate-notch depth approximately equal to the surface zero-bias depletion depth. A simple lateral spreading model is proposed which predicts thatV_{L} sim 50Qmin{u}max{-1}, where VLis the gate-drain avalanche voltage and Quis measured in units of 10¹²electrons/cm². This prediction is supported by a large body of experimental dc and pulse data, although considerable scatter is observed which we have attributed to epi charge nonuniformities, premature avalanche at the rough edges of AI gates formed by a liftoff process, and surface charging variations associated with dielectric passivation. The observed dependence of VLon epi charge rather than on doping level, as predicted for bulk avalanche, provides convincing evidence for nonbulk two-dimensional avalanche in the thin-film (Q_{u} < 2.3) FET geometry. In thick films (Q_{u} > 2.6), on the other hand, it is found that the bulk avalanche predictions are reasonably accurate. In terms of saturated epi current Is, the bulk regime corresponds toI_{s} > 450mA/mm and the lateral spreading (thin-film) regime toI_{s} < 400mA/mm. Finally, we have found that gate-drain avalanche is the major cause of output saturation as a function of drain potential in power GaAs FET's.     

Concentration-dependent diffusion of boron and phosphorus in silicon

A calculation is made of the tail of the impurity concentration profile resulting from concentration-dependent diffusion from a constant surface concentration into a semi-infinite medium. The calculation predicts that if the concentration dependence at low impurity concentrations is negligible, the low concentration portion of the doping profile should still take the familiar form,C = C'_{s} erfc (x/2 D_{i^{frac{1}{2}}}t^{frac{1}{2}}). Diis the commonly known diffusion coefficient at low impurity concentrations, whileC'_{s}is the "apparent" surface concentration.C'_{s}depends on the actual surface concentration and also depends on how the diffusion coefficient varies with impurity concentration at high concentrations. It is a constant for a given diffusion system but could be orders of magnitude higher than the actual surface concentration. Empirical data have been obtained for boron and phosphorus diffusions in silicon and found to be in good agreement with this prediction.     

Demonstration of AlGaN/GaN High-Electron-Mobility Transistors Grown by Molecular Beam Epitaxy on Si(110)

The growth of AlGaN/GaN-based heterostructure on Si(110) substrates by molecular beam epitaxy using ammonia as the nitrogen precursor is reported. The structural, optical, and electrical properties of such heterostructure are assessed and are quite similar to the ones obtained on Si(111). A 2-D electron gas is formed at the $hbox{Al}_{0.3}hbox{Ga}_{0.7}hbox{N/GaN}$ interface with a sheet carrier density of $hbox{9.6} times hbox{10}^{12} hbox{cm}^{-2}$ and a mobility of 1980 $hbox{cm}^{2}/hbox{V} cdot hbox{s}$ at room temperature. Preliminary results concerning high-electron-mobility-transistor static characteristics are presented and compared with that of devices realized on other orientations of silicon.      

Substrate current in GaAs MESFET's

It is shown that the substrate current in GaAs MESFET's may be related to the electron injection into the substrate region adjacent to the high-field domain in the active layer. A simple one-dimensional calculation shows that the substrate current Isubis proportional toVmin{ds}max{1/2}andnmin{0}max{1/4}where Vdsis the drain-to-source voltage, n0is the doping density in the active layer. Atn_{0} = 10^{17}cm^-3andV_{ds} simeq 10V we estimateI_{sub} sim 50mA per millimeter gate in good agreement with experimental results.     

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.