A Ga0.47In0.53As/InP heterophotodiode with reduced dark current

We have used a Ga0.47In0.53As/InP heterostructure to produce a photodiode (area =3 times 10^{-4}cm²) which shows a saturated dark current of 100 pA at 23°C and 1.7 nA at 50°C. At this dark current, these photodiodes have near-unity quantum efficiency at 1.6 μm and show good photoresponse over the1.0-1.65 mum region of the optical spectrum.     

Optimum low-level injection efficiency of silicon transistors with shallow arsenic emitters

The influence of As surface concentration CSEon the emitter efficiency βγand the temperature dependence of βγare reported. The theoretical model that is used to explain the variation of βγwith CSEis based upon the difference in the effective energy bandgaps in the emitter and base regionsDeltaE_{g}. Experimental measurements ofDeltaE_{g}versus CSEare presented. Measurements of βγversus CSEshow that the effective emitter doping densityQ_{E}/x_{eb}reaches a maximum value atC_{SE} cong 1.5 times 10^{20}atoms/cm³, corresponding to the threshold above whichDeltaE_{g} > 0. For the case of a constant active base doping/cm²QB, this also corresponds to an optimum in the emitter efficiency βγ. However, it is shown that in typical sequential diffusion processing of transistors, βγincreases monotonically with CSEbecauseQ_{B} = Q_{B}(C_{SE})decreases. In addition, for devices fabricated in this study,Deltabeta_{gamma}/DeltaC_{SE}atC_{SE}=2 times 10^{20}atoms/ cm³for As-diffused emitters (doped oxide) was ≈ 5 times greater than for ion-implanted-diffused As emitters, showing the superiority of implantation in controlling gain. Finally, transistors that were made withC_{SE} siml 1.4 times 10^{20}atoms/cm³(DeltaE_{g} = 0) showed βgamma(85°C)/ βγ(-15°C) ≤ 1.05.     

The area of safe operation of transistors for switching operation

A thermal feedback model is presented for the analytical definition of the ASO (Area of Safe Operation) for transistors in switching operations. This area is narrowed by the "second breakdown in p-n junction," and the approximate representation of the breakdown threshold is presented. This model consists of a forward and feedback energy flow with gains A and B, respectively.A = V_{CE} times M, B = K times theta times alpha_{R} times I_{e}. Therefore, the condition of the breakdown can be introduced as1 - AB = 0, whereMis the current multiplication factor, θ is transient thermal resistance,Kis a newly introduced current concentration factor, and αRis the temperature coefficient of Ie. Experimental results are also reported for a germanium alloy type transistor.     

Nematic liquid-crystal displays: Some properties and limitations

The objectives of this study were to determine the feasibility of utilizing nematic liquid crystals (LC) as media for aircraft displays from the point of view of their temperature range, response time, gray scale, and color control. According to this objective, the dynamic scattering and tunable birefringence mode of operation were investigated in some detail. Experiments, in agreement with derived mathematical models, led to the following conclusions : 1) The nematic temperature range can be made to fit any reasonable specification by forming multiple eutectic mixtures, provided the ingredients are compatible. 2) "Write" or "contact" times less than 2 µs have been (and less than 0.1 probably can be) realized with the help of "blocking" diodes. However, true rise times become generally longer than 1 s at temperatures below -20° C and electric fields at or below 3 V/µm. 3) The optical properties of electricallyinduced (dynamic) scattering can, in a first order approximation, be described by specifying the liquid crystal birefringence ration_{0}/n_{e}their average refractive indexbar{n}, and the applied voltage. 4) Of several possible color display approaches, electrically tunable birefringence was selected as the most promising one. The widest viewing angle, |α| ≤30°, is achieved with reflective displays and best color definition (resolution Δλ/λ≃0.2) with retardations of Γ≃3/2 in "single stage" displays, as long as the overall cell thickness fluctuationDelta d < 0.1 lambda/(n_{e}-n_{0}). In addition, nematic liquid scattering and color display features are compared to those of PLZT, an electrooptic ceramic.     

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.     

Characteristics in InGaAs/InP avalanche photodiodes with separated absorption and multiplication regions

Improved characteristics of compound semiconductor avalanche photodiodes with separated absorption and multiplication regions (SAM) are discussed. Temperature dependences of dark current and breakdown voltage show that the tunneling current in the narrow energy gap layer can be suppressed in InGaAs/InP APD's with the SAM structure. Dark currents above punch-through voltages, at which the depletion layer reaches the InP-InGaAs heterointerface, are caused by the generation-recombination process in the InGaAs and at the heterointerface. Dark currents near breakdown depend on the n-layer thickness and are strongly affected by the electric field strength in the ternary layer. Tunneling currents are dominant in diodes with thin n-InP layers, while the generation-recombination processes in the InGaAs layers are dominant in those with a thick n-InP layer. The dark current was as low as7.8 times 10^{4}A/cm²atM = 10when the interface electric field strength is reduced. A maximum multiplication factor of 60 was observed for the6 times 10^{-7}A initial photocurrent. Rise time and full width at half maximum in a pulse response waveform were 100 and 136 ps, respectively, atM = 10.     

New InGaAs/InP avalanche photodiode structure for the 1-1.6 µm wavelength region

Low dark current and low multiplication noise properties for an In0.53Ga0.47As/InP avalanche photodiode are described. The diode is prepared with an In0.53Ga0.47As light absorption layer and an InP avalanche multiplication region. The lowest dark current density of5.2 times 10^{-4}A/cm²is obtained at 90 percent of a breakdown voltage. Multiplication noise power is proportional to the 2.7th power of the current multiplication factor. Impact ionization coefficient by holes is larger by 2-3 times than that by electrons in     

Accelerated testing of time-dependent breakdown of SiO2

Electric-field acceleration factor β is the slope of thelog (t_{BD})versus Eoxcurve, where tBDis the time to breakdown at oxide field Eox. We report that β is not a constant but proportional toEmin{ox}max{-2}. This is the main cause of the wide divergence of β values reported in the literature. The reported oxide thickness dependence of β is believed to be a result of the higher electron trap densities in thicker oxides. Oxide lifetime extrapolation usinglog (t_{BD}), or better,log (Q_{BD})against1/E_{ox}plots is more accurate and has a theoretical basis. Highly accelerated oxide testing appears to be feasible especially for very thin oxides.     

Very low dark current heterojunction CCD's

A model has been formulated which accounts for the major sources of dark current (JD) associated with a single pixel of a heterojunction, Schottky gate charge-coupled device (CCD). This model predicts the temperature dependence of JDand shows that for properly fabricated gates, bulk generation in the channel is the primary source of dark current. To verify the model, the dark current of Al0.3Ga0.7As/ GaAs n-p⁺heterostructure CCD's was measured over the temperature range 23-55°C. At room temperature,J_{D} approx 83pA/cm², typically, and some pixels have JDas low as 43 pA/cm². These are the lowest dark currents reported to date for a CCD structure. The data at 55°C show that, typically, JDincreases to ∼ 1 nA/cm². Furthermore, the data confirm the temperature dependence of JDpredicted by the model.     

Threshold current density in solution-grown GaAs laser diodes

Threshold current density of solution-grown GaAs laser diodes with a Fabry-Perot cavity were measured at 77 and 300°K by varying the acceptor concentration in thepregion Na. Threshold current density was lower in the series of diodes with larger values of Nathan in the series of diodes with smaller values of Nafor the diode length between 0.1 and 1 mm. Through these experiments diodes with the threshold current density as low as3 times10^{2}A/cm²at 77°K and2.8 times 10^{4}A/cm²at 300°K for the diode length of 1 mm, and as low as 10³A/cm²at 77°K and4.5 times 10^{4}A/cm²at 300°K for the diode length of 0.1 mm were obtained.     

FM-laser operation of the Nd:YAG laser

The FM-laser or frequency-sweeping mode of laser oscillation has been demonstrated in a Nd :YAG 1.06-μ laser with an intracavity LiNbO3phase modulator. The experimental results are in excellent agreement with the theoretical expressionDelta= (DeltaOmega/Deltanu) (delta/pi)where δ= peak single-pass phase retardation in the modulator,DeltaOmega= axial mode spacing,Deltanu=modulator detuning, and Δ=resulting FM index of the laser output. Modulation indices as large asDelta approx 230rad have been obtained, in which case the instantaneous laser frequency is sweeping over a full spectral range of2Delta cdot f_{m} approx 120GHz (≈ 4 cm^-1) at a repetition frequencyf_{m} approx 260MHz, with a time-bandwidth product per periodapprox 2Delta approx 460. The coherently mode-locked spectral bandwidth thus obtained in the FM-laser case is very much wider than can be achieved in the pulsed mode-locked case with the same Nd:YAG laser. Some possible ways of using this broad-band coherent FM spectrum are suggested.     

Thermal effects of the operation of high average power Gunn devices

A lumped thermal resistance model is described for GaAs Gunn and LSA diodes. Thermal resistances are defined for the active layer, contact layer, bond interface, package and heat sink. This permits the calculation of tha maximum device temperature TM= T0+ P ΣiRiand the critical temperature difference across the active layer ΔTA= PRA. A transient analog incorporates thermal time constants Ti= RiCito consider high duty cycle pulsed operation. The carrier mobility is modeled as varying as1/Tin the range from 300 to 500°K. This permits thermal measurements based upon changes in resistance. The thermal calculations were also in agreement with blackbody infrared data. The mobility decline with temperature is shown to act as a link between the thermal profile and device performance. A thermally induced avalanching point, as well as device efficiency, are influenced by the peak-to-valley current ratio. This ratio is reduced from its theoretical value of over 2:1 by the active layer resistivity ratiobar{partial}/partial_{max}. Thus thermal gradients in the active layer act to create mobility gradients which alter the observed peak-to-valley ratio. The steps necessary for maintaining high average power efficient operation require low thermal resistances to minimize the active layer temperature gradients and a high current drop back ratio. Total experimental thermal resistances of 6.5°C/watt for an X-band CW diode and 17°C/watt for a thick LSA diode have been observed and fit the model presented.     

Sensitivity of Pr+3: LaCl3infrared quantum counter

The NEPλand response time of the Pr³⁺:LaCl3Infra-red Quantum Counter have been measured for the wavelengths 1.48 μ, 1.58 μ, 2.03 μ and 2.33 μ at the nominal temperatures of 300°K, 77°K, and 4.2°K. The experimental arrangement is described and suggestions for improvement are made. The best results obtained are anNEP_{2.03_mu} = 2 times 10^{-5}watts(c/s)^{-1/2}and a temperature independent response time of 20-30 ms.     

Experiments on a semiconductor laser pumped rubidium atomic clock

Experiments were carried out to evaluate the performances of a semiconductor laser pumped rubidium (⁸⁷Rb) atomic clock. Two kinds of Rb gas cells were used and their performances were compared [gas cell A (natural rubidium (⁸⁷Rb/⁸⁵Rb =frac{3}{7}) and buffer gases) and gas cell B (⁸⁷Rb and buffer gases)]. The highest microwave frequency stabilities were estimated as3.4 times 10^{-12} tau^{-1/2}and2.7 times 10^{-12} tau^{-1/2}at the optimal gas cell temperatures of 60°C and 48°C for the gas cellsAandB, respectively (τ: integration time). The light shift, i.e., microwave frequency shift induced by laser light, was measured as -0.50 Hz/MHz and -0.11 Hz/MHz for the gas cellsAandBat their optimal operating conditions given above. As an improved experiment by utilizing high temporal coherence of the laser, a novel double resonance spectral line shape with a drastically narrower linewidth was demonstrated. A technique, similar to FM laser spectroscopy, was employed for this purpose by utilizing laser FM sidebands which are induced by microwave frequency modulation and nonlinear susceptibility of three-level⁸⁷Rb atoms. The minimum linewidth obtained was 20 Hz, which can be used as a sensitive frequency discriminator for an improved⁸⁷Rb atomic clock.     

High-Reliability Semiconductor Lasers for Optical Communications

InGaAsP/InP buried heterostructure (BH) lasers emitting at 1.3 μm have continued to operate stably for more than3.3 times 10^{4}h (3.8 years) at 50-60°C and at an output power of 5 mW/facet. A statistically estimated median lifetime exceeds 10⁶at 50°C. A relatively low activation energy of 0.32 eV is obtained for slow degradation. The saturable behavior of the aging characteristics is observed in many of the lasers. This mode is explained by the increased leakage current through the buried regions, and can be eliminated by electroluminescence (EL) mode aging at high temperature and current. Distributed feedback (DFB) lasers emitting at 1.55 μm are also subjected to accelerated aging at 60°C with a 3 mW/facet output after EL-mode aging. These DFB lasers demonstrate stable aging characteristics, for more than 2000 h of operating time being currently achieved.     

Experimental observation of the post-annealing effect on the dark current of InGaAs waveguide photodiodes

The post-annealing effect on the dark current of the InGaAs waveguide photodiodes, which are developed for 40-Gbps optical receiver applications, is experimentally investigated. The interesting experimental phenomena were observed that the dark current is significantly decreased and the breakdown voltage is slightly increased after annealing at 250 and 300 °C whereas the dark current and the breakdown voltage are almost constant after annealing at 200 °C. Based on the experimental results, the post-annealing is more effective for the dark current improvement than the conventional curing process.     

InP/InGaAsP/InGaAs avalanche photodiodes grown by chemical beam epitaxy

InP/InGaAsP/InGaAs avalanche photodiodes with separate absorption, grading, and multiplication regions (SAGM-APD's) have been fabricated from wafers grown by chemical beam epitaxy (CBE). These APD's exhibit low dark current (<25 nA at 90 percent of breakdown), low capacitance (≈0.2 pF), and good responsivity (0.75 A/ W at 1.3 µm). The pulse response, which is relatively independent of avalanche gain, is characterized by rise and fall times of approximately 1.4 ns.     

Ultraviolet-visible metal-semiconductor-metal photodetectors fabricated from InxGa1−xN (0≤x≤0.13)

Metal-semiconductor-metal (MSM) photodetectors have been fabricated on In_xGa_1−xN epitaxial films grown by metalorganic chemical vapor deposition within the composition range 0≤x≤0.13. The dark current and spectral response were measured for devices with a varying In mole fraction x. The devices, which had nominal finger widths and finger spacing of 5 μm, were biased with modest voltages in the range 2≤V_bias≤5 V. In general, turn-on wave-length and dark current increased with increasing x. Turn-on wavelengths ranged from λ=370 nm to 430 nm and dark current densities ranged from I_dark=2×10⁻² A/cm² (V_bias=5 V, x≈0.05) to 9×10⁴ A/cm² (V_bias=2 V, x≈0.13) depending on the In content, x, of the device active area.     

Ga1-xAlxSb avalanche photodiodes: Resonant impact ionization with very high ratio of ionization coefficients

The liquid-phase epitaxy and device fabrication of p-n and p-i-n Ga1-xAlxSb avalanche photodiodes is described. Breakdown voltages up to 95 V and dark currents of 10^-4A/cm²have been obtained. With p-i-n diodes we have measured the impact ionization coefficients α (electrons) and β (holes) with different composition and temperature. A resonant enhancement of the hole ionization coefficient is found forx = 0.065(300 K) where the ratiobeta/alphaexceeds values of 20. This effect is attributed to impact ionization initiated by holes from the split-off valence band: if the spin orbit splitting Δ is equal to the bandgap energy Eg, the threshold energy for hole initiated impact ionization reaches the smallest possible value (E_{i} = E_{g}) and the ionization process occurs with zero momentum. This leads to a strong increase of β atDelta/E_{g} = 1. The experimentally determined dependence of ionization coefficients on threshold energy is compared with theoretical expectations.     

Ni/Pd-based ohmic contacts to p-GaN through p-InGaN/p+-GaN contacting layers

Specific contact resistance \begin{document}$ {\rho }_{\mathrm{c}} $\end{document} to p-GaN was measured for various structures of Ni/Pd-based metals and thin (20–30 nm thick) p-InGaN/p⁺-GaN contacting layers. The effects of surface chemical treatment and annealing temperature were examined. The optimal annealing temperature was determined to be 550 °C, above which the sheet resistance of the samples degraded considerably, suggesting that undesirable alloying had occurred. Pd-containing metal showed ~35% lower \begin{document}$ {\rho }_{\mathrm{c}} $\end{document} compared to that of single Ni. Very thin (2–3.5 nm thick) p-InGaN contacting layers grown on 20–25 nm thick p⁺-GaN layers exhibited one to two orders of magnitude smaller values of \begin{document}$ {\rho }_{\mathrm{c}} $\end{document} compared to that of p⁺-GaN without p-InGaN. The current density dependence of \begin{document}$ {\rho }_{\mathrm{c}} $\end{document}, which is indicative of nonlinearity in current-voltage relation, was also examined. The lowest \begin{document}$ {\rho }_{\mathrm{c}} $\end{document} achieved through this study was 4.9 × 10^–5 Ω·cm² @ J = 3.4 kA/cm².