The power enhancement of laser diodes is achieved by single and multilayer facet coatings such as antireflection and high reflection respectively at the front facet and the back facet of the laser diode. In this work, we have experimented with single layer λ/4 thick Al2O3 film for the Anti Reflection (AR) coating and stack of λ/4 thick Al2O3/ λ/4 thick Si bi-layers for the High Reflection (HR) coating. The AR/HR coatings were deposited in an electron beam evaporation system. The effect of front and back facet reflectivities on the output power of diode laser has been studied. The highly strained MOVPE grown InGaAs quantum-well edge emitting broad area (BA) diode lasers have been used for this experiments. The light output versus current (L-I) measurements were made on selected devices before and after the coatings. The devices were tested under pulsed operation with a pulse width of 400 ns and a duty cycle of 1:400. We have also carried out the theoretical analysis and simulation of L-I characteristics for this particular diode structure using LabVIEW. The experimental results were compared with simulated results. The effect of facet coating on external differential efficiency of diode laser has also been studied.
Multilayer coatings, such as Antireflection (AR) & High Reflection (HR), are used respectively in front & back facet coatings of diode laser for high power and long life operation. In this paper, we coated single layer AR coating using Al2O3MgF2 and multilayer HR coating using Al2O3 and MgF2 as a low refractive index layer and Si as high refractive index layer on GaAs substrates for optimization of coating conditions and HR coating using Al2O3\Si and MgF2\Si on diode laser chips. AR coating of diode laser is planned.
Synthesis of InGaAs quantum wells, lattice matched to InP by metalorganic vapour phase epitaxy poses severe problem because arsenic is incorporated into the growing solid much more readily than phosphorus. Therefore growth of InP after growing InGaAs is likely to have a transition layer of InGaAsP/InAsP of uncontrolled composition even though the arsine flow is switched off, because of trace amounts of arsine in the growth reactor. A common technique used to minimise the problem is to introduce a suitable pause in the growth sequence during the transition from InGaAs layer to InP layer in order to minimise the arsine content in the growth chamber, before starting the growth of InP. We find from photoluminescence (PL) measurements that this pause is not sufficient to optimise the well quality. The halfwidth of PL can be improved further by growing an intermediate layer of InGaAsP between InGaAs and InP, such that the quantum well structure consists of 5 layers in the sequence: InP/InGaAsP/InGaAs/InGaAsP/InP. The optimisation of pause sequence is made during the switching steps: (i) pause PI- in the transition from InGaAsP to InGaAs and (ii) pause P2- in the transition from InGaAs to InGaAsP. The nominal composition of the well is In0.53 Ga0.47 As with thickness of 100°A. The nominal composition of the intermediate layer is In0.72Gt0.28gAs0.6P0.4 Two Ga sources have been used during the growth of In0.53 Ga0.47As and In0.72Ga0.28As0.6P0.4, while the In source is kept the same. The PL emmision peak lies at 0.815 eV at 9 K, with halfwidth ~ 6 meV which is close to the state of art. In comparison, the PL measurement from QW samples with three layer sequence InP/InGaAs/InP with pauses at both interfaces of InGaAs gave halfwidth of more than 10 meV. Lattice matching of all the layers used in the above studies is checked separately on thick layers grown under similar growth conditions, by using X-ray diffraction measurements.
In this article we describe the sali
ent features of the classical LPE technique with
special reference to A1GaAS and InGaAsP. We also
review the work of very thin layers for quantum well structures.
The existing model of Fermi sea shake
up to explain the excess transverse magnetic
component in optical emission from quantum well
lasers is inadequate beyond the lasing threshold
condition. In this paper we propose that when
lasing occurs the heavy hole wave-function loses
directionality and has equal projections in x,
y and z directions.
Photodetectors with metal-semiconductor
-metal (MSM) structures having very high quan
turn efficiency are investigated to identify the
gain mechanism. From the temperature dependence
studies impact ionisation of carriers in a region
under the edge of the cathode is shown to be the
predominant gain mechanism.
Metal-Semiconductors-Metal (MSM) structure
has recently emerged" 2 as an attractive device
for photodetection due to its simple planar tech
nology which can be easily adapted to optoelec
tronic integration. The other features of MSM
photodetector are their high sensitivity, low
capacitance, low dark current and high speed.
Substatial amount of work has been done on GaAs
MSM photodiodes which are useful in near infrared
wavelength regime. Devices with very low dark
current (lOOpA at 1OV for 200x200pm geometry) and
low rise and fall times (23 and 55 ps respecti
vely) have been recently fabricated on semi-insu
lating (SI) GaAs substrates1. The other interest
ing feature of MSM structure is that it has in
ternal gain. Despite intensive study of these
devices there is still confusion about the physi
cal origin of the gain in these devices. Ito
et a13 attribute the gain to hole injection at
the forward biased anode contact. Measurements
of Zirngibl et al however, point to avalanche
mechanism of gain in these devices. In this
paper we report the results of experimental in
vestigation of gain mechanism in GaAs MSM struc
tures which have extremely high gain (100).
KEYWORDS: Capacitance, Spectroscopy, Optoelectronic devices, Chemical species, Diodes, Silicon, Simulation of CCA and DLA aggregates, Tin, Physics, Temperature metrology
We have used admittance spectroscopy to characterize the DX centers in Sn doped A1GaAs and Si doped A1GaAs samples. Three peaks in conductance and the corresponding steps in capacitance are observed in the admittance spectra of Sn doped samples. It is shown that these peaks arise from the multiple states of the same physical center rather than to three different types of defects. The deepest state corresponds to the conventional DX state in the Sn doped AlGaAs probed by deep level transient spectroscopy (DLTS). The other two states are not normally observed in DLTS experiments due to experimental limitations. In the case of Si doped A1GaAs samples only one peak which is broad or slightly asymmetric is observed and it corresponds to the main DLTS peak of the Si-DX center.
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