| dc.description.abstract | Antimonide-based mid-infrared laser structures have been grown by
molecular beam epitaxy (MBE). Epitaxially grown laser-related semiconductor
materials have been characterized by X-ray diffraction (XRD), Hall effect
measurement, photoluminescence, secondary ion mass spectrometry (SIMS) and
spectroscopic ellipsometry. Edge emitting semiconductor diode lasers were
fabricated and tested.
Firstly, the (AlGaIn)(AsSb)-based semiconductor materials were
characterized with regards to structural properties, optical properties and
doping characteristics.
Aluminum containing antimonide-based semiconductor alloys, latticematched
to GaSb, are very important for fabrication of cladding region for midinfrared
quantum well lasers, and the lattice constant of AlSb plays a crucial
role in determining the lattice matching condition at growth temperature for
these alloys. Temperature dependent XRD measurements have been performed
on epitaxially grown AlSb layer on GaSb (001) substrate. Temperature
dependence of the lattice constant and Poisson’s ratio have been determined
between 32 and 546 0C. For temperatures above 67 0C, the lattice constant of
AlSb was found to be larger than previously published linear extrapolation
values. Poisson’s ratio was found to be constant up to 300 0C and decreasing
above.
Tellurium (Te) and beryllium (Be) doped Al0.9Ga0.1As0.06Sb0.94 layers with
Al0.3Ga0.7As cap layer were grown by MBE. Room temperature Hall effect
measurements were performed on Hall bar samples with six-contact 1-2-2-1
geometry with photoresist sidewall passivation to determine the carrier
concentration and Hall mobility values. Te and Be dopant densities were
calculated from the SIMS depth profile of the doped Al0.9Ga0.1As0.06Sb0.94 samples.
Carrier concentration was found to have a linear dependence on the dopant density for Be-doped Al0.9Ga0.1As0.06Sb0.94 for dopant density up to 2.9 × 1019
cm-3 and carrier concentration 3.7 × 1019 cm-3, whereas it gets saturated for Tedoped
Al0.9Ga0.1As0.06Sb0.94 samples at dopant density value of 8.0 × 1018 cm-3
and carrier concentration 1.6 × 1017 cm-3. Hall mobilities for both Te- and Bedoped
samples were much lower than that of GaAs. Low doping efficiency in
Te-doped Al0.9Ga0.1As0.06Sb0.94 samples were further investigated by deep level
transient spectroscopy (DLTS). Evidence of deep level traps suggests that the
low doping efficiency could be due to presence of DX-centers.
Dispersive refractive index of epitaxially grown AlGaInAsSb
pentenary/quinary and AlGaAsSb quaternary alloys with different compositions
were studied using spectroscopic ellipsometry. Reflection high-energy electron
diffraction (RHEED) and XRD measurements were performed to determine the
compositional variations. The refractive index for AlGaAsSb quaternary alloys
was found to decrease with increase in Al content in the wavelength range 1-5
μm, suggesting the use of high Al containing AlGaAsSb quaternary alloys
lattice-matched to GaSb as cladding layers in GaInAsSb/AlGaAsSb-based laser
structures to achieve high optical confinement in the core. But for each GaSblattice-
matched composition of AlGaAsSb, the refractive index was not constant
above 2 μm and decreased slowly with longer wavelength. For AlGaInAsSb
pentenary alloys, the refractive index was found to decrease with increase in Al
and In content, in the 1-5 μm wavelength range. A change in As content in the
AlGaInAsSb pentenary alloy, while keeping the group III composition fixed, had
little effect on the measured refractive index in the 1-5 μm wavelength range.
Since AlGaInAsSb pentenary alloy layers are used as separate confinement layer
(SCL) in the core of the diode laser, low Al and In containing pentenary alloys
should be preferred to get higher refractive index for better optical confinement
in the core.
Dependence of the refractive index on dopant density and carrier
concentration for both Te- and Be-doped Al0.90Ga0.10As0.06Sb0.94 quaternary alloys
were studied using spectroscopic ellipsometry in the 0.05 - 2.1 eV energy range
(0.59 - 4.8 μm wavelength range). Composition of the quaternary alloy was determined using RHEED and XRD and doping variations were confirmed by
previously performed carrier concentration calibration curves and Raman
spectroscopic measurements. Carrier induced change in refractive index was
observed in Be-doped Al0.9Ga0.1As0.06Sb0.94 quaternary alloys for carrier
concentrations above 3.0×1017 cm-3. But due to low doping efficiency in Tedoped
Al0.90Ga0.10As0.06Sb0.94 quaternary alloys, the carrier concentration gets
saturated at 1.6×1017 cm-3 and hence the carrier concentration was found to be
too small to induce a change in refractive index in this wavelength range.
Refractive index versus carrier concentration calibration curve for Be-doped
Al0.90Ga0.10As0.06Sb0.94 quaternary alloy can be used for better design of
waveguides for (AlGaIn)(AsSb)-based diode lasers.
Secondly, laser structures were grown by MBE and edge emitting diode
lasers were fabricated using conventional UV-lithography, inductively-coupled
plasma reactive ion etching (ICP-RIE) and e-beam metal deposition processes.
AlGaAsSb ridge waveguides with high-precision etching very close to the core
of the laser were demonstrated using reflectance monitoring during ICP-RIE.
Several plasma-assisted techniques were investigated to remove the oxide on ptype
GaSb prior to metallization and the metal contacts were characterized
using the transfer length method (TLM) and four-point probe measurements.
Very low contact resistivities of <5×10-8 Ω.cm2 (i.e. below the limit for accurate
TLM measurement results) were achieved after surface pre-treatment by H2/Ar
sputter etching and low-ion-energy Ar+ irradiation. This process recipe was used
for low contact resistance metal contacts to p-type GaSb cap layer of the laser
structures. Straight waveguide as well as Y-junction waveguide diode lasers were
fabricated. (AlGaIn)(AsSb)-based laser material with GaInAsSb quantum wells
and AlGa(In)AsSb barriers with emission wavelength up to 3.05 μm were grown
using MBE. All the lasers were characterized by optical power-current-voltage
(P-I-V) characteristics and spectral measurements. For 25 μm wide and 1 mm
long ridge waveguide lasers, the threshold current density was found to be 2×106
A.m-2. The maximum power per facet from these lasers was found to be 20 mW
at 2.334 μm emission wavelength with injected current density of 2×107 A.m-2.
Wide tuning of 50 nm in emission wavelength with multimode emission was achieved in electrically tuned (AlGaIn)(AsSb)-based Y-junction laser emitting
at 2.32 μm.
Aluminum-based metal contacts to both p-type and n-type GaSb
epilayers as an alternative to gold (Au)-based contacts were demonstrated for
application on the antimonide-based straight waveguide edge emitting lasers.
The specific contact resistivity of the contact between Al and p-type GaSb was
found to be lower than that of the contact between Au and p-type GaSb. For
the n-type GaSb, the Al-based contact was found to be as good as the Au-based
contact. The good performance of GaSb-based laser diode using Al-based
contacts shows the applicability of this type of contact in GaSb-based devices. | nb_NO |
