C. Becker

300 K operation of a GaAs-based quantum-cascade laser at λ≈9 μm

The room-temperature (300 K), pulsed mode operation of a GaAs-based quantum-cascade laser is presented. This has been achieved by the use of a GaAs/Al0.45Ga0.55As heterostructure which offers the maximum Γ–Γ band offset (390 meV) for this material system without inducing the presence of indirect barrier states. Thus, better electron confinement is achieved, countering the loss of injection efficiency with temperature. These devices show ∼100 K increase in operating temperature with respect to equivalent designs using an GaAs/Al0.33Ga0.67As heterostructure. We also measure 600 mW peak power at 233 K a temperature readily accessible by Peltier coolers.

GaAs-AlGaAs quantum cascade lasers: physics, technology, and prospects

In recent years, the performance of GaAs-AlGaAs-based quantum cascade (QC) lasers has improved markedly. These devices are capable of pulsed room temperature operation and can deliver respectable average powers (11 mW at /spl lambda//spl sim/9 /spl mu/m) operating on a Peltier cooler. This performance has been achieved by the suppression of thermally activated carrier leakage through increases in the heterobarrier band offset. We demonstrate that QC lasers, with wavelengths /spl lambda//spl ges/9 /spl mu/m, can operate using heterostructures encompassing the entire composition range of Al/sub x/Ga/sub 1-x/As, without encountering potential problems-of the satellite X-minima for x>45%. Furthermore, we present particular characteristics of these devices, such as a phonon-limited temperature dependence, electrical and optical self-oscillations, and novel design concepts that exploit this closely lattice matched material system. Finally, we discuss improvements in device fabrication to lower the operating current through a reduction of the area of current injection. Using this technology, devices can be designed to selectively pump the fundamental lateral mode. We, therefore, observe single spatial-mode operation over the entire current range of operation.

AlAs/GaAs quantum cascade lasers based on large direct conduction band discontinuity

The design and operation of quantum cascade (QC) lasers using AlAs/GaAs coupled quantum wells are reported. In this material system, the conduction band offset at the Γ point (∼1 eV) is much higher than in previously reported QC lasers. The use of high band discontinuity allows us to increase the energy separation among the subbands, thus suppressing thermally activated processes which limit device performance at high temperature. The measured thermal characteristics of these promising devices are strongly improved from previously reported GaAs-based QC lasers: The temperature dependence of the threshold current density is described by a very large T0 (320 K) and the laser slope efficiency does not vary for increasing heat sink temperatures. The maximum operating temperature is 230 K, limited by negative differential resistance effects that occur when the applied bias reaches 8 V.

InAs/AlSb quantum-cascade light-emitting devices in the 3–5 μm wavelength region

Midinfrared (3.7–5.3 μm) electroluminescent devices based on a quantum-cascade (QC) design have been demonstrated using InAs/AlSb heterostructures, grown on GaSb substrates. The very high conduction band discontinuity (>2 eV) of this material system allows the design of QC devices at very short wavelengths. Well-resolved luminescence peaks were observed up to 300 K, with a full-width-at-half-maximum to peak wavelength ratio (Δλ/λ) of the order of 8%. The emission wavelengths are in good agreement with the results of our model. The emitted optical power is lower than that predicted, due to a nonoptimized electron injection into the active region.

Improved CW operation of GaAs-based QC lasers: T/sub max/= 150 K

We present a substantial improvement in the CW performance of GaAs-based quantum cascade lasers with operation up to 150 K. This has been achieved through suitable changes in device processing of a well-characterized laser. The technology optimizes the current injection in the laser by reducing the size of the active stripe whilst maintaining a strong coupling of the optical mode to preserve low current densities. The reduction of total dissipated power is critical for these lasers to operate CW. At 77 K, the maximum CW optical power is 80 mW, threshold current is 470 mA, slope efficiency is 141 mW/A, and lasing wavelength /spl lambda//spl sim/10.3 /spl mu/m.

Thermal resistance and temperature characteristics of GaAs/Al0.33Ga0.67As quantum-cascade lasers

We report on the determination of thermal resistance, facet temperature profile, and heat flux of GaAs/Al0.33Ga0.67As quantum-cascade lasers operating in pulsed mode, using a microprobe band-to-band photoluminescence technique. The thermal resistance of epilayer-side mounted lasers is ∼30% smaller than that of substrate-side mounted ones. The dependence of the thermal resistance on the injection conditions and its correlation with the output power is also reported.

Lateral current spreading in unipolar semiconductor lasers

Lateral current spreading in shallow ridge processed unipolar semiconductor lasers is described using a two-dimensional flow model. In these devices, contrary to bipolar diode lasers, the density of carriers can be considered constant also in the active region. Therefore electron diffusion is a negligible effect and the spatial distribution of the current can be obtained by solving a two-dimensional differential equation for the electric potential. Our calculations prove that the major contribution to the current spreading takes place right before electrons enter the active region and is caused by the discontinuity of the conductivity at the cladding–active region interface.

High performance GaAs based quantum cascade lasers

Summary form only given.At present, only two material systems have been successfully exploited to demonstrate quantum cascade (QC) lasers: GaInAs-AlInAs grown on InP and more recently GaAs-AlGaAs. Although there are no significant conceptual differences in the quantum design between these two classes of lasers, the use of different materials has important consequences on device characteristics and has to be included in device optimisation. We report on AlGaAs QC lasers optimised for output power and maximum operating temperature in the 9-12 /spl mu/m wavelength range. In these structures the waveguide is Al-free and we use AlGaAs layers only in the active region, to define the tunnelling barriers. The claddings are obtained by sandwiching the active region between two thick GaAs layers, with an appropriate doping profile.

Simultaneous measurement of the electronic and lattice temperatures in GaAs quantum cascade lasers and their correlation with the optical performance

We use micro-probe photoluminescence in continuous wave operating GaAs quantum cascade lasers to measure the lattice and electronic temperatures, which determine the electrical power dependence of the threshold current and the slope efficiency, respectively.

Quantum cascade active regions based on InAs/AlSb/GaSb

Summary form only given. To obtain a laser, cladding layers have to be added to the structure, in order to maximize the overlap of the emitted radiation with the active region. To this end, we use a 20 /spl Aring//20 /spl Aring/ InAs/AlSb superlattice where the InAs layers are n doped with Si n = 5 X 10/sup 18/ cm/sup -3/. This type of cladding has already been used in interband cascade lasers.