Alfred R. Adams

The temperature dependence of 1.3- and 1.5-/spl mu/m compressively strained InGaAs(P) MQW semiconductor lasers

We have studied experimentally and theoretically the spontaneous emission from 1.3- and 1.5-/spl mu/m compressively strained InGaAsP multiple-quantum-well lasers in the temperature range 90-400 K to determine the variation of carrier density n with current I up to threshold. We find that the current contributing to spontaneous emission at threshold I/sub Rad/ is always well behaved and has a characteristic temperature T/sub 0/ (I/sub Rad/)/spl ap/T, as predicted by simple theory. This implies that the carrier density at threshold is also proportional to temperature. Below a breakpoint temperature T/sub B/, we find I /spl alpha/ n/sup Z/, where Z=2. And the total current at threshold I/sub th/ also has a characteristic temperature T/sub 0/ (I/sub th/)/spl ap/T, showing that the current is dominated by radiative transitions right up to threshold. Above T/sub B/, Z increases steadily to Z/spl ap/3 and T/sub 0/ (I/sub th/) decreases to a value less than T/3. This behavior is explained in terms of the onset of Auger recombination above T/sub B/; a conclusion supported by measurements of the pressure dependence of I/sub th/. From our results, we estimate that, at 300 K, Auger recombination accounts for 50% of I/sub th/ in the 1.3-/spl mu/m laser and 80% of I/sub th/ in the 1.5-/spl mu/m laser. Measurements of the spontaneous emission and differential efficiency indicate that a combination of increased optical losses and carrier overflow into the barrier and separate confinement heterostructure regions may further degrade T/sub 0/ (I/sub th/) above room temperature.

A quantitative study of radiative, Auger, and defect related recombination processes in 1.3-/spl mu/m GaInNAs-based quantum-well lasers

By measuring the spontaneous emission (SE) from normally operating /spl sim/1.3-/spl mu/m GaInNAs-GaAs-based lasers we have quantitatively determined the variation of each of the current paths present in the devices as a function of temperature from 130 K to 370 K. From the SE measurements we determine how the current I close to threshold, varies as a function of carrier density n, which enables us to separate out the main current paths corresponding to monomolecular (defect-related), radiative or Auger recombination. We find that defect-related recombination forms /spl sim/55% of the threshold current at room temperature (RT). At RT, radiative recombination accounts for /spl sim/20% of I/sub th/ with the remaining /spl sim/25% being due to nonradiative Auger recombination. Theoretical calculations of the threshold carrier, density as a function of temperature were also performed, using a ten-band k /spl middot/ p Hamiltonian. Together with the experimentally determined defect-related, radiative, and Auger currents we deduce the temperature variation of the respective recombination coefficients (A, B, and C). These are compared with theoretical calculations of the coefficients and good agreement is obtained. Our results suggest that by eliminating the dominant defect-related current path, the threshold current density of these GaInNAs-GaAs-based devices would be approximately halved at RT. Such devices could then have threshold current densities comparable with the best InGaAsP/InP-based lasers with the added advantages provided by the GaAs system that are important for vertical integration.

Theoretical and experimental analysis of 1.3-/spl mu/m InGaAsN/GaAs lasers

We present a comprehensive theoretical and experimental analysis of 1.3-/spl mu/m InGaAsN/GaAs lasers. After introducing the 10-band k /spl middot/ p Hamiltonian which predicts transition energies observed experimentally, we employ it to investigate laser properties of ideal and real InGaAsN/GaAs laser devices. Our calculations show that the addition of N reduces the peak gain and differential gain at fixed carrier density, although the gain saturation value and the peak gain as a function of radiative current density are largely unchanged due to the incorporation of N. The gain characteristics are optimized by including the minimum amount of nitrogen necessary to prevent strain relaxation at the given well thickness. The measured spontaneous emission and gain characteristics of real devices are well described by the theoretical model. Our analysis shows that the threshold current is dominated by nonradiative, defect-related recombination. Elimination of these losses would enable laser characteristics comparable with the best InGaAsP/InP-based lasers with the added advantages provided by the GaAs system that are important for vertical integration.

Auger recombination in long-wavelength infrared InNxSb1−x alloys

Dilute nitrogen alloys of InSb exhibit strong band gap bowing with increasing nitrogen composition, shifting the absorption edge to longer wavelengths. The conduction band dispersion also has an enhanced nonparabolicity, which suppresses Auger recombination. We have measured Auger lifetimes in alloys with 11 and 15 μm absorption edges using a time-resolved pump-probe technique. We find the lifetimes to be longer at room temperature than equivalent band gap Hg1−yCdyTe alloys at the same quasi-Fermi level separation. The results are explained using a modified k⋅p Hamiltonian which explicitly includes interactions between the conduction band and a higher lying nitrogen-related resonant band.

Band anticrossing in dilute InNxSb1-x

Dilute nitrogen alloys of InSb exhibit extremely strong band gap bowing with nitrogen composition that has been associated with anticrossing between the localized resonant states of the nitrogen within the conduction band and the extended states of the conduction band itself. This also results in the conduction band dispersion having an enhanced nonparabolicity. We have measured the electron effective mass near the anticrossing by cyclotron resonance in InNxSb1−x alloys with absorption edge near 15 μm, using pulsed fields up to 150 T. The results directly demonstrate the band anticrossing and quantitatively confirm the increase of effective mass versus x predicted for InNxSb1−x by a tight binding calculation for low nitrogen concentration (x<0.01).

Strained-Layer Quantum-Well Lasers

This tutorial article explains the two important reasons for the introduction of strain into the active region of a quantum-well laser. First, it reduces the density of states at the top of the valence band, which allows population inversion to be obtained at a lower carrier density. Second, it distorts the 3-D symmetry of the crystal lattice and matches it more closely to the 1-D symmetry of the laser beam. Together these effects greatly enhance almost all characteristics of semiconductor lasers and make possible a wide range of applications. Combinations of compressive and tensile strain can also be used, for example, to produce nonabsorbing mirrors and polarization-insensitive semiconductor optical amplifiers.

The k.p interaction in InP and GaAs from the band-gap dependence of the effective mass

By measuring the photoconductive edge and the magnetophonon effect in InP and GaAs as their band gap is increased by the application of hydrostatic pressure, it has been possible to obtain a direct experimental relationship between the electron effective mass, m*, and the direct band gap, E0. Comparison with the k.p theory showed that in InP, Ep=16.7+or-0.2, Ep=0 and C=0 while in GaAs Ep=25.0+or-0.5, Ep=5+or-1 and C=0 where Ep, Ep and C are measures of the k.p interaction of the conduction band with, respectively, the valence band, with the next ( Gamma 5c) conduction band and with the conduction bands lying even higher.

Direct measurement of facet temperature up to melting point and COD in high-power 980-nm semiconductor diode lasers

The authors describe a straightforward experimental technique for measuring the facet temperature of a semiconductor laser under high-power operation by analyzing the laser emission itself. By applying this technique to 1-mm-long 980-nm lasers with 6- and 9-/spl mu/m-wide tapers, they measure a large increase in facet temperature under both continuous wave (CW) and pulsed operation. Under CW operation, the facet temperature increases from /spl sim/25/spl deg/C at low currents to over 140/spl deg/C at 500 mA. From pulsed measurements they observe a sharper rise in facet temperature as a function of current (/spl sim/400/spl deg/C at 500mA) when compared with the CW measurements. This difference is caused by self-heating which limits the output power and hence facet temperature under CW operation. Under pulsed operation the maximum measured facet temperature was in excess of 1000/spl deg/C for a current of 1000 mA. Above this current, both lasers underwent catastrophic optical damage (COD). These results show a striking increase in facet temperature under high-power operation consistent with the facet melting at COD. This is made possible by measuring the laser under pulsed operation.

Growth and characterization of relaxed epilayers of InGaAs on GaAs

Abstract We report the growth of 3 μm thick relaxed layers of In x Ga 1− x As on GaAs substrates, with x from 0.1 to 1, and give some results of compositional, optical, structural and electrical characterisation. Compositions were determined by several techniques, with results which agreed to within a Δx of ±0.02. For x above 0.2 the crystal quality is very poor. Some layers have been grown with stepped compositions up to x = 0.4 and this improves the crystal quality. We show that in the InGaAs/GaAs system there is a threshold strain, rather than threshold composition change, above which crystal quality is degraded.

Uniaxial stress investigations of the three-level conduction band structure of GaAs

Measurements are reported of low-field resistivity and Gunn effect threshold field as a function of (100) and (111) uniaxial stress for a number of epitaxial and bulk GaAs samples. The results provide the first direct evidence that the L1c minima are situated lower in energy than the X1c minima and are therefore involved in the operation of GaAs transferred-electron devices. Intervalley separations are obtained at 300K, but these are dependent on density-of-states values. Further information on band structure parameters such as intervalley coupling constants should now be obtainable by fitting the high-field data to Monte-Carlo calculations currently in progress. Deformation potentials for the satellite minima have been determined.