The radiative and nonradiative components of the threshold current in 1.3 m, p-doped and undoped quantum-dot semiconductor lasers were studied between 20 and 370 K. 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 T 0 values observed near room temperature.
The gain of p-doped and intrinsic InAs/ GaAs quantum dot lasers is studied at room temperature and at 350 K. 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 = 293 K, 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. Due to the large volume of data being readily transferred between network users, there is an increasing demand for "fiber-to-the-home" based optical fiber networks which require fast and temperature insensitive semiconductor lasers emitting at 1.3 m. Despite significant progress in reducing the threshold current densities ͑J th ͒ or improving the temperature stability, 1,2 intrinsic quantum dot ͑QD͒ lasers emitting around 1.3 m have yet to meet their full potential. This is attributed to many factors such as inhomogeneous broadening or size dispersion of the dots and dominating nonradiative recombination at room temperature ͑RT͒ which increases J th and its sensitivity to temperature variations. 3,4 It has been proposed that p-doping the devices and thus saturating the levels in the valence band with holes would greatly improve the device performance by increasing the gain and the bandwidth. 5,6 Although the effect of p-doping shows major improvements on the temperature sensitivity and the bandwidth of the devices, 7 measurements do not show a clear enhancement of the gain for a given injection 8 as it is expected from theory. 5,6 Recent results reported that the superior thermal stability of InAs/ GaAs p-doped QD lasers around room temperature arises from a combination of improving thermal distribution of the electrons, leading to a decrease in the radiative current necessary to reach the lasing threshold, and an increase in nonradiative Auger recombination with temperature, together giving rise to a constant threshold current over a limited temperature range. 4 In this work, we consider the temperature sensitivity of the gain and specifically, the effect of the nonthermal distribution of the carriers on the gain characteristics of p-doped devices and compare the results to those obtained with intrinsic quantum dot lasers.The gain was measured using the method described by Hakki and Paoli 9 at both room temperature, where carriers in the intrinsic devices are in thermal equilibrium, and at 350 K, where carriers in the p-doped lasers are expected to be closer to thermal equilibrium. 4 The lasing wavelength of the devices was approximately 1.28 m at room temperature. The active region consisted of ten stacked InAs dot-ina-well layers separated by either modulation p-doped or intrinsic GaAs barriers sandwiched by GaAs waveguide and AlGaAs cladding layers. The chips were driven with 50 s long pulses at a duty cycle of 50%. The...
The band gap dependencies of the threshold current and its radiative component are measured using high pressure techniques. Detailed theoretical calculations show that the band gap dependence of the internal losses plays a significant role in the band gap dependence of the radiative current. Temperature dependence measurements show that the radiative current accounts for 20% of the total threshold current at room temperature. This allows us to determine the pressure dependence of the non-radiative Auger recombination current, and hence to experimentally obtain the variation of the Auger coefficient C with band gap. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2722041͔The relative importance of different carrier recombination processes occurring in semiconductor diode lasers is strongly dependent on the band structure, material quality and laser geometry ͑e.g., cavity length, facet coatings, etc.͒. While a small amount of radiative recombination via spontaneous emission is desirable ͑to seed the stimulated emission process͒, other nonradiative recombination paths may contribute to the laser threshold current and are detrimental to the performance of the devices. For InGaAs͑P͒ / InP devices operating over the telecommunications range ͑1.3-1.6 m͒, it is known that Auger recombination plays an important role and is responsible for their high temperature sensitivity [1][2][3][4] . In order to understand these limiting processes and consequently to improve the performance of the devices, it is necessary to separate the contribution of the different recombination current paths. 1-6 Hydrostatic pressure is an ideal tool to investigate the current paths and their variation with band gap, and hence operating wavelength. In long wavelength semiconductor multiple quantum well lasers where leakage currents are small, it has been shown 1,5 that the threshold current I th can be written as the sum of monomolecular ͑ϰn͒, radiative ͑ϰn 2 ͒, and Auger ͑ϰn 3 ͒ recombination currents, where n is the carrier density at threshold ͑assuming that the hole and electron densities are equal͒. Thus I th = eV͑An + Bn 2 + Cn 3 ͒ where e is the electronic charge, V is the volume of the active region, and A, B, and C are the monomolecular, radiative, and Auger recombination coefficients, respectively. Good growth quality means that the monomolecular current is negligible in the InGaAs devices studied here. 1,5 The relative magnitudes of the remaining radiative and nonradiative currents have been experimentally studied via temperature and high pressure measurements. 1,5 Previous analyses were based on the theoretical assumption that for an ideal quantum well laser, the radiative current is proportional to the square of the band gap: I rad ϰ E g 2 , where E g is the band gap 7 although this had not been experimentally verified at room temperature. The pressure dependence of the radiative current has already been verified at low temperatures using a helium gas pressure system. 8 Since at cryogenic temperatures ͑T Ͻ 120 K͒ all of the inje...
The improved thermal stability of InGaAlAs-based lasers compared with InGaAs-based lasers for 1.5 µm operation is investigated using a combination of low temperature and high pressure techniques. The results indicate that the improved performance of InGaAlAs-based devices is due to a reduction in the contribution of the non-radiative Auger recombination current, I Aug , to the total threshold current, I th , in the InGaAlAs devices. This is due to the higher conduction band offset made possible with the InGaAlAs system which results in a lower hole density in the quantum wells at threshold.
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