H. Nelson

IMPROVED RED AND INFRARED LIGHT EMITTING AlxGa1−xAs LASER DIODES USING THE CLOSE‐CONFINEMENT STRUCTURE

AlxGa1−xAs injection lasers have been fabricated over the compositional range in which the alloy has a direct bandgap transition (0 < x ≤ 0.34). At 300°K the threshold current density increases with x from 12 000 A/cm2 (λL = 8600 A) to 56 000 A/cm2 (λL = 7340 A) in uncoated devices with cavity lengths of about 10 mils. In the same range of x the external differential quantum efficiency gradually decreases from 40 to 16%. These are the highest efficiencies and lowest threshold current densities ever reported for room‐temperature lasers emitting in the same spectral range. The improvement is due to utilization of the new p+‐p heterojunction structure previously used by the authors in GaAs lasers to sharply reduce the internal optical loss by improving the optical confinement and reducing the absorption coefficient in the p+ material adjoining the active region. At 77°K lasing has been achieved to 6450 A and cw operation to 6900 A with the emission of 0.4 W per diode.

Novel GaAs–(AlGa)As Cold‐Cathode Structure and Factors Affecting Extended Operation

A novel planar cold‐cathode structure has been developed based on a GaAs–(AlGa)As heterojunction and a negative electron affinity GaAs surface. As an important feature of the device, the lateral confinement of the current flow to the desired area of the emitting surface is obtained by the selective diffusion of zinc. Under pulsed conditions, emission efficiencies as high as 4% and emission current densities as high as 7 A/cm2 have been achieved. The release of impurities from the anode as a result of electron‐stimulated desorption has been found to be a major factor affecting the cathode life under dc operation. The influence of this factor can be greatly minimized by different techniques such as using low anode voltages and magnetic field deflection.

EFFICIENT PHOTOEMISSION FROM Ge‐DOPED GaAs GROWN BY LIQUID‐PHASE EPITAXY

Efficient photoemission with white light sensitivities as high as 1100 μA/lm has been obtained from Ge‐doped GaAs layers grown by liquid‐phase epitaxy. The hole concentrations of the samples were relatively low (5×1017–2×1018 cm−3), and the sample surfaces were chemically polished prior to cesium‐oxygen activation. Quantum yield data show an unusually high infrared response and suggest long diffusion lengths, between 2 and 7 μ, for the photo‐excited Γ‐electrons.

AN OPTOELECTRONIC COLD CATHODE USING AN AlxGa1−xAs HETEROJUNCTION STRUCTURE

An efficient optoelectronic cold cathode has been made which includes a Si‐compensated AlxGa1−xAs electroluminescent diode covered with an absorbing p‐type GaAs layer having a negative electron affinity surface. This structure is designed to minimize current crowding in the vicinity of the Ohmic contact. An over‐all efficiency of 1.1×10−3 (current emitted into vacuum/diode current) has been achieved. This represents a factor of 102–103 improvement over previous p‐n junction or optically coupled cold cathode structures.

EFFICIENT ELECTRON EMISSION FROM GaAs–Al1−xGaxAs OPTOELECTRONIC COLD‐CATHODE STRUCTURES

An optoelectronic cold‐cathode structure has been operated at pulsed (1% duty cycle emission current densities of 3 A/cm2 at an over‐all efficiency of 1.6%. Continuous operation at a current density of 0.4 A/cm2 and an efficiency of 0.43% has been observed.

Heterojunction cold-cathode electron emitters of (AlGa) As-GaAs

Abstract New cold-cathode electron emitters designed for vacuum tube have been developed based on the use of liquid phase epitaxy, (AlGa) As-GaAs heterojunctions and negative-electron affinity GaAs surfaces. An important part of the device is the lateral confinement of the current flow to the desired emitting surface by a novel fabrication technique. It has been shown that devices operating at practical efficiencies and emission current densities can be reproducibly fabricated and activated. The electron energy distribution has been determined. The half width of the energy distribution is 160 meV (compared to ≈ 220 meV for a thermionic cathode at 1000 °K) and is consistent with theoretical calculations based on the energy loss of the electrons in the GaAs space charge region near the surface. While this distribution is wider than kT, it contains a relatively small high energy component. It is the high energy component which is the most troublesome aspect of the thermionic cathode emission. Some of the major factors affecting cathode life have been studied, and the following conclusions have been reached: (1) A decrease in negative electron affinity occurs mainly during electron emission from the activated surface and results from electron-stimulated desorption from the anode; (2) a decrease in negative electron affinity is due, at least in part, to the physical adsorption of impurities released from the anode rather than to a loss of cesium and/or oxygen; (3) oxygen physically adsorbed on the activated surface is a specific impurity which causes a severe reduction in negative electron; (4) cathode stability can be greatly improved by a number of techniques including operation at low anode voltages and magnetic field deflection to prevent the incidence on the cathode surface of impurities released from the anode.

High-power pulsed GaAs laser diodes operating at room temperature

The fabrication and characteristics of a high-power GaAs injection laser for room-temperature operation are described. A single laser emits 70 watts peak power from one facet at four times the threshold current. The diodes are fabricated from epitaxial wafers prepared by the solution-growth process. Scaling from work on low-power (7-watt) units to this high power has been accomplished by increasing the junction width, which requires general improvement in the crystalline quality and in the control of the doping. Data are given on the effect of doping density, crystal quality, and imperfections near the junction, as well as junction width. The reduced yield in high-power diodes, of which only one-third from a single batch give the desired output, is associated with filamentary lasing and with super-radiant walk-off modes, neither of which is under full control. Preliminary data on life tests show that long-lived units can be made, but that apparently identical units from the same batch show wide variations in the rate of degradation.