S. J. Sweeney

Carrier transport and recombination in p-doped and intrinsic 1.3μm InAs∕GaAs quantum-dot lasers

The radiative and nonradiative components of the threshold current in 1.3μm, p-doped and undoped quantum-dot semiconductor lasers were studied between 20 and 370K. The complex behavior can be explained by simply assuming that the radiative recombination and nonradiative Auger recombination rates are strongly modified by thermal redistribution of carriers between the dots. The large differences between the devices arise due to the trapped holes in the p-doped devices. These both greatly increase Auger recombination involving hole excitation at low temperatures and decrease electron thermal escape due to their Coulombic attraction. The model explains the high T0 values observed near room temperature.

Recombination processes in midinfrared InGaAsSb diode lasers emitting at 2.37μm

The temperature dependence of the threshold current of InGaAsSb∕AlGaAsSb compressively strained lasers is investigated by analyzing the spontaneous emission from working laser devices through a window formed in the substrate metallization and by applying high pressures. It is found that nonradiative recombination accounts for 80% of the threshold current at room temperature and is responsible for the high temperature sensitivity. The authors suggest that Auger recombination involving hot holes is suppressed in these devices because the spin-orbit splitting energy is larger than the band gap, but other Auger processes persist and are responsible for the low T0 values.

Temperature dependence of the gain in p-doped and intrinsic 1.3μm InAs∕GaAs quantum dot lasers

The gain of p-doped and intrinsic InAs∕GaAs quantum dot lasers is studied at room temperature and at 350K. Our results show that, although one would theoretically expect a higher gain for a fixed carrier density in p-doped devices, due to the wider nonthermal distribution of carriers amongst the dots at T=293K, the peak net gain of the p-doped lasers is actually less at low injection than that of the undoped devices. However, at higher current densities, p doping reduces the effect of gain saturation and therefore allows ground-state lasing in shorter cavities and at higher temperatures.

Temperature and pressure dependence of the recombination processes in 1.5μm InAs∕InP (311)B quantum dot lasers

The threshold current and its radiative component in 1.5μm InAs∕InP (311)B quantum dot lasers are measured as a function of the temperature. Despite an almost temperature insensitive radiative current, the threshold current increases steeply with temperature leading to a characteristic temperature T0≈55K around 290K. Direct observation of spontaneous emission from the wetting layer shows that some leakage from the dots to the wetting layer occurs in these devices. However, a decrease in the threshold current as a function of pressure is also measured suggesting that Auger recombination dominates the nonradiative current and temperature sensitivity of these devices.

Requirements for a GaAsBi 1 eV sub-cell in a GaAs-based multi-junction solar cell

Multi-junction solar cells achieve high efficiency by stacking sub-cells of different bandgaps (typically GaInP/GaAs/Ge) resulting in efficiencies in excess of 40%. The efficiency can be improved by introducing a 1 eV absorber into the stack, either replacing Ge in a triple-junction configuration or on top of Ge in a quad-junction configuration. GaAs0.94Bi0.06 yields a direct-gap at 1 eV with only 0.7% strain on GaAs and the feasibility of the material has been demonstrated from GaAsBi photodetector devices. The relatively high absorption coefficient of GaAsBi suggests sufficient current can be generated to match the sub-cell photocurrent from the other sub-cells of a standard multi-junction solar cell. However, minority carrier transport and background doping levels place constraints on both p/n and p-i-n diode configurations. In the possible case of short minority carrier diffusion lengths we recommend the use of a p-i-n diode, and predict the material parameters that are necessary to achieve high efficiencies in a GaInP/GaAs/GaAsBi/Ge quad-junction cell.

Temperature dependence of 4.1 μm mid-infrared type II “W” interband cascade lasers

The thermal properties of 5-stage “W” Interband-Cascade Lasers emitting at 4.1 μm at room temperature (RT) are investigated by measuring the lasing and spontaneous emission properties as a function of temperature and hydrostatic pressure up to 1 GPa. Experiments show that at RT more than 90% of threshold current of these devices is due to non-radiative loss processes. We also find that the threshold current density dependence on temperature can be fitted with a single exponential function over a wide temperature range with a characteristic temperature, T0, of 45 K. The relatively high temperature sensitivity in these devices is attributable to the large non-radiative current contribution coupled with non-pinning of the carrier density above threshold.

Carrier recombination in 1.3μm GaAsSb∕GaAs quantum well lasers

In this letter the authors present a comprehensive study of the threshold current and its temperature dependence in GaAsSb-based quantum well edge-emitting lasers for emission at 1.3 mu m. It is found that at room temperature, the threshold current is dominated by nonradiative recombination accounting for more than 90% of the total threshold current density. From high hydrostatic pressure dependence measurements, a strong increase in threshold current with pressure is observed, suggesting that the nonradiative recombination process may be attributed to electron overflow into the GaAs/GaAsP barrier layers and, to a lesser extent, to Auger recombination. (c) 2006 American Institute of Physics.

Radiative and Auger recombination in 1.3μm InGaAsP and 1.5μm InGaAs quantum-well lasers measured under high pressure at low and room temperatures

We report on the pressure dependence of the threshold current in 1.3μm InGaAsP and 1.5μm InGaAs quantum-well lasers measured at low temperatures ∼100K. It was found that the threshold current of both devices slowly increases with increasing pressure (i.e., increasing band gap) at ∼100K consistent with the calculated variation of the radiative current. In contrast, at room temperature we observed a reduction of the threshold current with increasing pressure. Our low-temperature, high-pressure data confirm the results of previous atmospheric pressure measurements on the same devices which indicated a transition in the dominant recombination mechanism from radiative to Auger as the device temperature is increased from ∼100 to 300K.

Experimental and modelling study of InGaBiAs/InP alloys with up to 5.8% Bi, and with Δso > Eg

Temperature dependent photo-modulated reflectance is used to measure the band gap Eg and spin-orbit splitting energy ∆so in dilute-Bi In0.53Ga0.47As1-xBix/InP for 1.2% = x = 5.8%. At room temperature, Eg decreases with increasing Bi from 0.65 to 0.47 eV (~2.6 µm), while ∆so increases from 0.42 to 0.62 eV, leading to a crossover between Eg and ∆so around 3.8% Bi. The 5.8% Bi sample is the first example of this alloy where ∆so > Eg has been confirmed at all temperatures. The condition ∆so > Eg is important for suppressing hot-hole-producing non-radiative Auger recombination and inter-valence band absorption losses and so holds promise for the development of mid-infra-red devices based on this material system. The measured variations of Eg and ∆so as a function of Bi content at 300 K are compared to those calculated using a 12-band k.p Hamiltonian which includes valence band anti-crossing effects. The Eg results as a function of temperature are fitted with the Bose-Einstein model. We also look for evidence to support the prediction that Eg in dilute bismides may show a reduced temperature sensitivity, but find no clear indication of that.

Bismide-Based Photonic Devices for Near- and Mid-Infrared Applications

Bismides are a new class of III–V semiconductor alloys which are gaining interest due to their many potential applications. In this chapter we show how the addition of bismuth atoms to III–V alloys gives rise to useful band structure properties, such as a large band gap bowing, making it possible to produce narrow gap (near- and mid-infrared) materials and devices on conventionally used III–V substrates such as GaAs and InP. In addition, bismuth-containing alloys can provide a large spin-orbit splitting energy which offers the potential to reduce or eliminate the important non-radiative Auger recombination and inter-valence band absorption processes which dominate the performance of near- and mid-infrared lasers and LEDs. Furthermore, we show that by adding nitrogen to the alloys, lattice-matched narrow band gap semiconductor heterostructures may be produced with the possibility of wide control of the conduction and valence offsets. Finally, we provide a discussion of the physics of devices based upon bismide alloys.