A theory of static (threshold and power) characteristics of novel diode lasers - quantum dot (QD) lasers with asymmetric barrier layers (ABLs) - is developed. The barrier layers are asymmetric in ...that they have considerably different heights for the carriers of opposite signs. The ABL located on the electron- (hole-) injecting side of the structure provides a low barrier (ideally no barrier) for electrons (holes) so that it does not prevent electrons (holes) from easily approaching the active region and a high barrier for holes (electrons) so that holes (electrons) injected from the opposite side of the structure do not overcome it. The use of ABLs should thus ideally prevent the simultaneous presence of electrons and holes (and hence parasitic electron - hole recombination) outside the QDs. It is shown in this work that in such a case of total suppression of parasitic recombination, the QD lasers with ABLs offer close-to-ideal performance: the threshold current density is below 10 A cm−2 at any temperature, the absolute value of the characteristic temperature is above 1000 K (which manifests a virtually temperature-independent operation), the internal differential quantum efficiency is practically unity, and the light - current characteristic is linear at any pump current.
The performance characteristics of semiconductor lasers based on quantum wells (QWs) are theoretically studied as functions of the thickness of the waveguide region optical confinement layer (OCL). ...The maximum modal gain, optical-confinement factor (in QWs, OCLs, and emitters), threshold current density, electron and hole densities (in QWs and OCLs), internal optical loss (in QWs, OCLs, and cladding layers), internal differential quantum efficiency, currents of stimulated and spontaneous recombination and the output optical power of the laser are calculated as functions of the OCL thickness. It is shown that up to pump current densities of 50 kA/cm
2
the dependence of the output power of the considered lasers on the OCL thickness is weak in the thickness range of 1.5–2.8 μm. This result is important for the development of high-brightness lasers, since such lasers use a wide waveguide to ensure low radiation divergence. It is shown that, at very high pump-current densities, the output power has a maximum as a function of the OCL width.
A simple method for the determination of the capture velocity of charge carriers from a three-dimensional (3D) region (waveguide region) into a 2D region (quantum well) is proposed. The method is ...based on measurement of the threshold current density and internal differential quantum efficiency in a semiconductor laser structure. The method also allows determining the 2D carrier density in a quantum well, which is otherwise not easy to measure in a multilayer laser structure.
In a laser with asymmetric barrier layers (ABLs) two thin barrier layers adjacent to the active region on both sides are intended to prevent bipolar population of the waveguide layers, hence, to ...suppress parasitic recombination in them. A theoretical model of a laser with ABLs, based on rate equations which acknowledge undesirable carrier leakage inevitable in lasers of this type implemented in practice, is proposed. Solutions to equations are obtained for the steady-state case. By the example of an InGaAs/GaAs quantum-well laser (lasing wavelength λ = 980 nm), the effect of leakages through ABLs on the device characteristics is studied. The parasitic-flux suppression ratios
C
of ABLs which are required to prevent the adverse effect of waveguide recombination are estimated. In the case at hand, the effect of ABLs becomes appreciable at suppression ratios of
C
≥ 10
2
. To suppress 90% of the parasitic current,
C
should be 2.3 × 10
4
. The effect of ABLs on useful carrier fluxes arriving at the active region is also studied.
We propose a genuinely temperature-insensitive quantum dot (QD) laser. Our approach is based on direct injection of carriers into the QDs, resulting in a strong depletion of minority carriers in the ...regions outside the QDs. Recombination in these regions, which is the dominant source of the temperature dependence, is thereby suppressed, raising the characteristic temperature T/sub 0/ above 1500 K. Still further enhancement of T/sub 0/ results from the resonant nature of tunneling injection, which reduces the inhomogeneous line broadening by selectively cutting off the nonlasing QDs.
The light-current characteristic (LCC) of semiconductor quantum well lasers is theoretically studied. It is discussed here that, due to internal optical absorption loss, which depends on the electron ...and hole densities in the optical confinement layer, (i) roll-over of the LCC occurs with increasing injection current, and, (ii) depending on the parameters of laser structures, the LCC can have two branches, i.e. the optical emission at two different output powers will be possible within a certain range of injection currents.
To suppress bipolar population and hence electron-hole recombination outside quantum dots (QDs), tunneling-injection of electrons and holes into QDs from two separate quantum wells was proposed ...earlier. Close-to-ideal operating characteristics were predicted for such a double tunneling-injection (DTI) laser. In the Stranski-Krastanow growth mode, a two-dimensional wetting layer (WL) is initially grown followed by the formation of QDs. Due to thermal escape of carriers from QDs, there will be bipolar population and hence electron-hole recombination in the WL, even in a DTI structure. In this work, the light-current characteristic (LCC) of a DTI QD laser is studied in the presence of the WL. Since the opposite sides of a DTI structure are only connected by the current paths through QDs and the WL is located in the n-side of the structure, the only source of holes for the WL is provided by QDs. It is shown that, due to the zero-dimensional nature of QDs, the rate of the hole supply to the WL remains limited with increasing injection current. For this reason, as in the other parts of the structure outside QDs (quantum wells and optical confinement layer), the parasitic electron-hole recombination remains restricted in the WL. As a result, even in the presence of the WL, the LCC of a DTI QD laser becomes increasingly linear at high injection currents, which is a further demonstration of the potential of such a laser for high-power operation.
The power characteristics of quantum-well lasers with asymmetric barrier layers, which represent a novel type of injection laser, are calculated on the basis of an extended model taking into account ...asymmetry in the filling of electron and hole states. The electron–hole asymmetry is shown to have no significant effect on the characteristics of these lasers. Even in the presence of intermediate layers (located between the quantum well and each of the two asymmetric barrier layers), where parasitic electron–hole recombination does occur, the internal differential quantum efficiency of such a laser exhibits only a weak dependence on the pump current and remains close to unity; therefore, the light–current characteristic remains linear up to high pumping levels.
We develop a general approach to including the internal optical loss in the description of semiconductor lasers with a quantum-confined active region. We assume that the internal absorption loss ...coefficient is linear in the free-carrier density in the optical confinement layer and is characterized by two parameters, the constant component and the net cross section for all absorption loss processes. We show that, in any structure where the free-carrier density does not pin in the presence of light generation, the free-carrier-density dependence of internal loss gives rise to the existence of a second lasing threshold above the conventional threshold. Above the second threshold, the light-current characteristic is two-valued up to a maximum current at which the lasing is quenched. We show that the presence of internal loss narrows considerably the region of tolerable structure parameters in which the lasing is attainable; for example, the minimum cavity length is significantly increased. Our approach is quite general but the numerical examples presented are specific for quantum dot (QD) lasers. Our calculations suggest that the internal loss is likely to be a major limiting factor to lasing in short-cavity QD structures.
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