A.R. Adams

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.

Background carrier concentration and electron mobility in LPE In1−xGaxAsyP1−y layers

Lattice‐matched and nominally undoped layers of In1−xGaxAsyP1−y were grown by liquid‐phase epitaxy on semi‐insulating (100) InP substrates. The background carrier concentration for a range of compositions was about 2×1016 cm−3 when unbaked melts were used, but with prebaking 2.8×1015 cm−3 was achieved. The electron mobility data, obtained over the temperature range 77–300 K using magnetic fields up to 9 T, have been interpreted in terms of polar‐optical phonon, alloy, and ionized impurity scattering. For midrange alloys of this purity, alloy scattering was found to be more significant than ionized impurity scattering. The alloy scattering potential is about 0.6 eV for alloys with y=0.5.

InSb1-xNx growth and devices

Indium antimonide (InSb) has the smallest energy gap of any of the binary III–V materials, leading to a cut-off wavelength of 7 μm at 300 K. The addition of small proportions of nitrogen to InSb offers the prospect of extending the response wavelength into the 8–12 μm range, which is important for thermal imaging in that atmospheric transmission window and because it encompasses the absorption lines of several environmentally important gases and can therefore be used for monitoring the gases. We report on the growth, by a combination of molecular beam epitaxy and a nitrogen plasma source, of InSb1−xNx with up to 10% nitrogen. Structural characterisation techniques of TEM, AFM and SIMS have enabled some optimisation of material quality to be demonstrated by biasing the sample during growth. Measurements on light emitting diodes comprising a superlattice of InSb0.945N0.055/InSb show an emission wavelength of 10.5 μm, which is confirmed by free electron laser assessment. Comparison with first principles band-structure calculations indicate that approximately 10% of the nitrogen is active. Hall effect measurements of 1 μm thick bulk layers indicate an increasing n-type behaviour, the degeneracy effects of which mean, however, that this is only a lower limit.

Influence of the barriers on the temperature dependence of threshold current in GaAs/AlGaAs quantum well lasers

Using window devices, light emission has been observed from the barrier regions of lasers with 25-A-wide quantum wells. From measurements of threshold current as a function of temperature on devices grown by molecular-beam epitaxy using different Al cells for the barriers, the strong influence of nonradiative barrier recombination processes on the threshold current has been demonstrated. Further measurements of threshold current as a function of hydrostatic pressure show that recombination from the L and X conduction-band minima makes an important contribution to the current. The calculations show how the temperature dependence of threshold depend on factors such as cavity length and the number of quantum wells. >

The role of Auger recombination in InAs 1.3-/spl mu/m quantum-dot lasers investigated using high hydrostatic pressure

InAs quantum-dot (QD) lasers were investigated in the temperature range 20-300 K and under hydrostatic pressure in the range of 0-12 kbar at room temperature. The results indicate that Auger recombination is very important in 1.3-/spl mu/m QD lasers at room temperature and it is, therefore, the possible cause of the relatively low characteristic temperature observed, of T/sub 0/=41K. In the 980-nm QD lasers where T/sub 0/=110-130 K, radiative recombination dominates. The laser emission photon energy E/sub las/ increases linearly with pressure p at 10.1 and 8.3 meV/kbar for 980 nm and 1.3-/spl mu/m QD lasers, respectively. For the 980-nm QD lasers the threshold current increases with pressure at a rate proportional to the square of the photon energy E/sup 2//sub las/. However, the threshold current of the 1.3-/spl mu/m QD laser decreases by 26% over a 12-kbar pressure range. This demonstrates the presence of a nonradiative recombination contribution to the threshold current, which decreases with increasing pressure. The authors show that this nonradiative contribution is Auger recombination. The results are discussed in the framework of a theoretical model based on the electronic structure and radiative recombination calculations carried out using an 8/spl times/8 k/spl middot/p Hamiltonian.

Experimental analysis of temperature dependence in 1.3-/spl mu/m AlGaInAs-InP strained MQW lasers

We have analyzed experimentally the temperature and pressure dependences of the lasing characteristics of 1.3-/spl mu/m AlGaInAs-InP strained multiple-quantum-well lasers, by focusing on the ratio of the nonradiative recombination current to the total current. The temperature dependence of the radiative current was studied by observing the spontaneous emission through a window in the substrate. It was found to increase linearly with temperature, exactly as expected for an ideal quantum well over the entire temperature range from 100 to 360 K. Further, it was shown that pure radiative recombination dominated the total current below a breakpoint temperature T/sub b/ of 220 K. Above this temperature, the onset of loss processes including Auger recombination caused a superlinear increase in the threshold current. Analysis of the linear and nonlinear components allowed us to determine the ratio of the nonradiative to radiative currents at threshold. We find that, relative to similar GaInAsP/InP lasers, there is a decrease in the nonradiative component of the current, resulting in a higher characteristic temperature T/sub 0/ in the AlGaInAs-InP lasers. At 300 K, the radiative recombination current is more than 70% of the total threshold current. This result is consistent with the observation that the threshold current increases by about 8% in 12-kbar hydrostatic pressure, while in GaInAsP lasers, a decrease of 10% or more is always observed over this pressure range.

Determination of alloy scattering potential in Ga1−xAlxAs alloys

Room‐temperature Hall mobility as a function of pressure (0–8 kbar) has been measured for high‐purity liquid‐phase‐epitaxy‐grown Ga1−xAlxAs layers. GaAs‐like band structure of low‐composition alloys has also been converted to Si‐like band structure at high pressures and the Hall mobility measured as a function of temperature (77≲T≲300 °K) with crystals locked under constant pressures. The data have been analyzed to identify and distinguish the presence of space charge and alloy scatterings both characterized by mobilities limited by T−1/2. The space charge scattering has been found to be absent in all the crystals studied except x=0.047. The alloy scattering potential for electrons in the Γ minimum has been shown to depend on the alloy composition with a maximum value of 1.56 eV at x=0.19. For electrons in the X minima, this potential has been found to be independent of composition with a value of only 0.4 eV.

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.

Dependence of Threshold Current on QW Position and on Pressure in 1.5 ?m InGaAs(P) Lasers

The threshold current has been measured as a function of pressure for a series of 1.5 μm InGaAs(P) lasers in which the position of the quantum wells (QWs) has been varied within the active region. Devices with the QWs positioned towards the p-doped cladding layer exhibit both a lower threshold current density and a reduced pressure dependence of the threshold current compared to devices with the QWs placed either symmetrically between the cladding layers or towards the n-doped cladding layer. Monte-Carlo simulations of the trajectories of Auger-generated hot carriers show that the escape probability of hot holes depends considerably more strongly on the QW position than is the case for hot electrons. The combination of the experimental measurements and Monte-Carlo simulations demonstrates the importance of Auger recombination processes which result in the production of hot holes.

Role of radiative and nonradiative processes on the temperature sensitivity of strained and unstrained 1.5 μm InGaAs(P) quantum well lasers

By measuring the spontaneous emission from strained and unstrained 1.5 μm InGaAs quantum well lasers as a function of temperature we deduce the temperature dependence of the radiative current density at threshold corresponds to a characteristic temperature T0≊300 K, close to that expected from theory, whereas T0 of the threshold current is around 60 K. We conclude from our analysis that the large temperature dependence of long wavelength lasers is due to Auger recombination.