1.1 Bandgap energies for several semiconductor material compounds as a
function of unstrained lattice parameter. . . 2
1.2 Normalized absorbance of organic gases within the first atmospheric win-dow. . . 4
2.1 Semiconductor heterojunctions classified by band alignment. . . 10
2.2 Heterostructure configurations for type I heterojunctions. . . 11
2.3 Band structures for Si and GaAs. . . 14
2.4 Schematic representation of the envelope function approximation for a confined electronic state. . . 16
2.5 Calculated energy band profile for a 7 nm-thick In0.73Ga0.27As quantum well surrounded by 14 nm-thick AlAs barriers. . . 18
2.6 Calculated envelope functions for the electron ground state of a 7 nm-thick In0.73Ga0.27As quantum well surrounded by 14 nm-thick AlAs bar-riers. . . 21
2.7 Calculated conduction band solutions for the quantum well system of Fig. 2.5. . . 22
2.8 Calculated eigenenergies for the quantum well system of Fig. 2.7 as a function of well width. . . 23
2.9 Calculated two-dimensional density of states as a function of energy for the single quantum well of Fig. 2.7. . . 24
2.10 Calculated occupation probability for the ground state of Fig. 2.7 as a function of energy. . . 25
List of Figures 130 2.11 Calculated difference between the subband minimum and the quasi Fermi
energy for the ground state of Fig. 2.7 as a function of electron temperature. 26 2.12 Calculated difference between the subband minimum and the quasi Fermi
energy for the ground state of Fig. 2.7 as a function of sheet density. . . . 27 2.13 Fermi-Dirac and Maxwell-Boltzmann occupation probability functions. . 27 3.1 Calculated conduction band diagram as a function of growth direction for
two periods of a short-wavelength QCL structure. . . 31 3.2 Schematic representation of a laser beam propagating within a laser cavity. 33 3.3 Schematic representation of a QCL period aligned under an appropriate
bias. . . 35 3.4 Calculated facet efficiency as a function of resonator length for a
Fabry-P´erot resonator with uncoated facets. . . 39 3.5 Illustrating the dependence of the differential slope efficiency and
thresh-old current density on the injection efficiency. . . 42 3.6 Calculated differential slope efficiency as a function of threshold current
density for QCLs emitting between 3.6−5.8µm. . . 43 3.7 Calculated average optical power as a function of duty cycle for a typical
MIR QCL. . . 45 3.8 Calculated maximal duty cycle as a function of threshold current
charac-teristic temperatureT0. . . 47 3.9 Calculated maximal duty cycle as a function of current density for
differ-ent threshold currdiffer-ent characteristic temperaturesT0. . . 48 3.10 Calculated average optical power as a function of duty cycle for a typical
MIR QCL. . . 49 3.11 Calculated maximum average optical power as function of current density. 50 3.12 Calculated maximum average optical power as function of characteristic
temperaturesT0andT1. . . 52 4.1 Simplified representation of a MBE system. . . 56
List of Figures 131 4.2 Measured and simulated DCXRD rocking curves for a 40-period
In0.73Ga0.27As/In0.55Al0.45As-AlAs QCL structure. . . 57
4.3 Schematic representation of a laser ridge waveguide (Fabry-P´erot res-onator). . . 58
4.4 Calculated light intensity for a MIR QCL as a function of distance along the direction of growth. . . 59
4.5 SEM captures of a QCL ridge facet. . . 60
4.6 QCL mounted onto a Cu heat sink and ready for characterization. . . 61
4.7 Standard electric and light-output characterization of a MIR QCL. . . 62
4.8 Threshold current density and differential slope efficiency as a function of temperature for a MIR QCL. . . 63
4.9 High-magnification SEM captures of BH-QCLs using InP:Fe overgrown by GSMBE. . . 65
4.10 High-magnification SEM captures of BH-QCLs with and without the In0.52Al0.48As spacer layer between laser core and InP:Fe layer. . . 66
4.11 Low-magnification SEM capture of the BH-QCL with the optimized growth sequence. . . 67
4.12 Facet of a BH-QCL soldered epi-down onto an AlN submount. . . 68
4.13 Measured threshold current density as a function of temperature for the optimized BH-QCL. . . 69
4.14 Measured thermal conductance as a function of temperature for the opti-mized BH-QCL. . . 69
4.15 Thermal conductance as a function of reciprocal cavity width for several reported MIR QCLs. . . 71
5.1 Calculated electron-phonon coupling function squared for two QCL con-duction band states as a function of phonon momentum in the direction of confinement. . . 78
131
List of Figures 132 5.2 Calculated electron-LO phonon scattering rates as a function of total
(po-tential and kinetic) energy for a lattice temperature of 300 K. . . 79 5.3 Calculated subband dispersion and several electron-LO-phonon
scatter-ing paths. . . 80 5.4 Calculated subband dispersion and different electron-LO-phonon
scatter-ing paths. . . 81 5.5 Calculated conduction band diagram of a strain-compensated MIR QCL
for illustration of IFR scattering. . . 83 5.6 Calculated lifetimes for scattering from the upper into the lower laser
state as a function of emission wavelength for several MIR QCLs. . . 84 5.7 Calculated ratio of the energy dependent IFR scattering rate to the
energy-independent case. . . 85 5.8 Calculated conduction band profile of the structure S1 under a 100 kV/cm
electric field. . . 88 5.9 Illustration of the impact of individual interfaces on the interface
rough-ness transition broadening between upper and lower laser states for a reference design and for structure S1. . . 89 5.10 Measured emission spectrum as a function of the drive current for a laser
stripe at room temperature measured with 100 ns current pulses at a rep-etition rate of 10 kHz. . . 90 5.11 Calculated conduction band profile of the structure S2 under a 76 kV/cm
electric field. . . 91 5.12 Measured threshold current density for structures S1 and S2 as a function
of reciprocal resonator length at different heat sink temperatures. . . 92 5.13 Measured modal gain coefficientgc andαw+Jthesc(T)·gc for structures
S1 and S2 as a function of heat sink temperature. . . 93 5.14 Measured threshold current density (solid dots) for QCL structures S1
and S2 as a function of heat sink temperature. . . 94
List of Figures 133 5.15 Measured leakage current density at threshold for structures S1 and S2
and calculated components. . . 95 5.16 Calculated energy dispersion for the electronic states of Fig. 5.11. . . 97 5.17 Calculated current density for non-radiative scattering from stateg1into
statesm1and 2 for the QCL structure of Fig. 5.11. . . 97 5.18 Measured threshold current characteristic temperatureT0as a function of
calculated∆Efor structures S1, S2, and variations. . . 98 5.19 Measured threshold current characteristic temperatureT0as a function of
photon energy for several MIR QCLs. . . 99 5.20 Calculated conduction band profile of the studied QCL structure under a
90 kV/cm electric field and calculated two-dimensional sheet densities. . 100 5.21 Measured reciprocal quantum efficiency per cascade as a function of
res-onator length. . . 103 5.22 Measured leakage current density as a function of reciprocal electron
temperature. . . 104
133