Transcript PowerPoint 簡報 - 郭艷光彰化師大物理系網頁

The Blue Laser Diode
(chapter 11~15)
Speaker:Meng-Lun Tsai
National Changhua University of Education
2015/7/17
National Changhua University of Education
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Outline
• Room-Temperature Pulsed Operation of Laser Diodes
InGaN-Based Multi-Quantum-Well Laser Diodes
InGaN Multi-Quantum-Well Laser Diodes with Cleaved Mirror
Cavity Facets
InGaN Multi-Quantum-Well Laser Diodes Grown on MgAl2O4
Substrates
Optical Gain and Carrier Lifetime of InGaN Multi-Quantum-Well
Laser Diodes
Ridge-Geometry InGaN Multi-Quantum-Well Laser Diodes
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Outline
• Room Temperature CW Operation of InGaN MQW LDs
First Room-Temperature Continuous-Wave Operation of InGaN
Multi-Quantum-Well Laser Diodes
InGaN Multi-Quantum-Well Laser Diodes with ModulationDoped Strained-Layer Superlattices cladding layers grown on the
ELOG substrate
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InGaN-Based Multi-Quantum-Well Laser Diodes
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InGaN-Based Multi-Quantum-Well Laser Diodes
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InGaN-Based Multi-Quantum-Well Laser Diodes
• GaN buffer : A 300-Å-thick GaN buffer layer grown at a low
temperature of 550 °C, this layer served as a buffer layer of the thick
AlGaN film growth to prevent cracking of the film.
• p-type Al0.2Ga0.8N:Mg : This layer was used to prevent dissociation
of InGaN layers during the growth of the p-type layers.
• The 0.1-μm-thick n-type GaN:Si and p-type GaN:Mg were light
guiding layers.
• The n-type Al0.15Ga0.85N:Si and p-type Al0.15Ga0.85N:Mg acted as
cladding layers for confinement of the carriers and the light emitted
from the active region of the InGaN MQW structure.
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InGaN-Based Multi-Quantum-Well Laser Diodes
• It is difficult to cleave the GaN crystal grown on the c-face sapphire
substrate.Therefore, reactive ion etching (RIE) was employed to form
mirror cavity facets.
• The surface of the p-type GaN layer was partially etched with Cl2
plasma until the n-type GaN layer was exposed in order to make a
stripe LD.
• The roughness of the facet surface was approximately 500 Å.
• High refection facet coatings (60-70 %) were used to reduce the
threshold current.
• A Ni/Au contact was evaporated onto the entire area of the p-type
GaN layer, and a Ti/Al contact onto the n-type GaN layer.
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Results and Discussion
• The electrical characteristics of LDs were measured under pulsed
current condition (pulse width is 2 μs, pulse period is 2 ms) at room
temperature .
• The output power from one facet was measured by a Si photodetector.
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Results and Discussion
• Stimulated emission was not
observed up to a current of 1.7 A.
• The threshold current was about 1.7
A,which corresponded to a threshold
current density of 4 KA/cm2.
• A differential quantum efficiency
of 13 % per facet and pulsed output
power of 215 mW per facet were
obtained at a current of 2.3 A.
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Results and Discussion
• At injection currents below the
threshold, spontaneous emission
appeared with a FWHM of 20 nm
and a peak wavelength of 410 nm.
• Above the threshold current , a
strong stimulated emission at 417
nm with a FWHM of 1.6 nm was
the dominant emission.
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InGaN Multi-Quantum-Well Laser Diodes with
Cleaved Mirror Cavity Facets
• Sapphire substrate with (1120) orientation (a-face) has been
conventionally used for GaN growth.
• In the present section, LDs fabricated using wide-band-gap Ш-V nitride
materials grown on a-face sapphire cleaved along (1102) (r-face) are
presented.
• It is difficult to cleave GaN crystal grown on c-face sapphire substrate.
Therefore they used a-face sapphire as a substrate in order to cleave the
sapphire substrate along r-face.
• The cleaved facets of the epitaxial layers and substrate were mirrorlike.
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Results and Discussion
• The threshold current was about
1.15 A, which corresponded to a
threshold current density of 9.6
KA/cm2.
• A differential quantum efficiency
of 4.2 % per facet and pulsed
output power of 76 mW per facet
were obtained at a current of 1.5 A.
• At a current of about 1.25 A, a
link was observed in the light
output curve.
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InGaN Multi-Quantum-Well Laser Diodes Grown on
MgAl2O4 Substrates
• In the present section, they discuss LDs fabricated onto spinel (MgAl2O4)
substrate to investigate the cleaving behaviour, with the aim to improve the
roughness of the laser mirror facets.
• GaN growth has been reported onto spinel substrate , which has a smaller
lattice mismatch (9.5 %) with respect to GaN than sapphire (13 %).
• Kuramata et al. found that smooth cleaved facets of GaN grown on spinel
substrates could be obtained.However, they used polished facets as a mirror
cavity for LDs.
• The roughness of the facet surface was approximately 50 Å after
polishing.
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InGaN Multi-Quantum-Well Laser Diodes Grown on
MgAl2O4 Substrates
• The electrode size on the p-type
GaN Layer was 5 μm × 500
μm ,and the electrode size on the ntype GaN layer was 80 μm × 500
μm.
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Results and Discussion
• The threshold current was about 320
mA, which corresponded to a threshold
current density of 13 KA /cm2.
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Results and Discussion
• This demonstrates that the emission
is strongly TE-polarized, and indicates
the laser operation at a current above
320 mA.
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Results and Discussion
• Typical far-field radiation patterns of
the InGaN MQW laser structure in the
planes parallel and perpendicular to the
junction.
• The beam FWHP level for the parallel
and perpendicular far-field patterns are
5° and 17°, respectively.
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Optical Gain and Carrier Lifetime of InGaN MultiQuantum-Well Laser Diodes
• In this section, the optical gain and carrier lifetime of InGaN MQW LDs
is investigated experimentally.
• The external differential quantum
efficiency decreases with increasing
cavity length.
• The external differential quantum
efficiency is given by
• αi : intrinsic loss ηi :internal
quantum efficiency R=30 %
• αi and ηi : are calculated as 54 cm-1
and 86 %, respectively.
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Optical Gain and Carrier Lifetime of InGaN MultiQuantum-Well Laser Diodes
• The threshold gain Gth is given by
where Γ is the confinement factor.
• Γis found to be 0.7 assuming that the light wave propagates only
into the 0.2 μm-thick GaN guiding layers in the structure.
• Gth is calculated as 110 cm-1.
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Optical Gain and Carrier Lifetime of InGaN MultiQuantum-Well Laser Diodes
• The delay time of the laser emission was
measured by pulsed current modulation of
the LDs.
• The delay time td is given by td=τsln(I/IIth) where τs is the minority carrier time, I
is the pumping current, and Ith is the
threshold current.
• From the figure, τs is calculated as 2.5 ns.
Also, the carrier density is given by
nth=Jthτs/(ed) where nth is the carrier
density at the laser threshold, Jth is the
threshold current density, d is the
thickness of the active layer and e is the
elementary charge.
• nth is calculated as 1.3 × 1019 cm3.
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Optical Gain and Carrier Lifetime of InGaN MultiQuantum-Well Laser Diodes
• The radiative and nonradiative recombination lifetimes are given by ηi =
τs/τr , τs-1=τr-1+τnr-1 where τr is the radiative recombination lifetime of
minority carriers, τnr is the nonradiative recombination lifetime of minority
carries.
• Using ηi = 86 % and τs = 2.5 ns, and τnr = 18 ns were obtained.
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Ridge-Geometry InGaN Multi-Quantum-Well Laser
Diodes
• For CW operation, the efficiency of the LDs must be greatly improve,
and the threshold currents and voltages must be decreased.
• In the present section, ridge-geometry LDs grown on a-face sapphire
substrates are described which were fabricated with the aim to improve the
characteristics of InGaN/GaN/AlGaN SCH LDs.
• In order to make a comparison between the stripe-geometry and ridgegeometry LDs, The stripe-geometry LD was also fabricated.
• The area of the stripe- and ridge-geometry LDs was 10 μm × 600
μm.
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Results and Discussion
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Results and Discussion
• The threshold current of the ridge-geometry LDs was 187 mA which
corresponded to a threshold current density of 3 KA/cm2.
• On the other hand, the threshold current of the stripe-geometry LDs was
370 mA, which corresponded to a threshold current density of 6 KA/cm2
and the differential quantum efficiency was 5 %.
• For the ridge-geometry LD, The differential quantum efficiency of 30 %
per facet and pulsed output power of 3.5 mW per facet were obtained at a
current of 194 mA.
• For the stripe-geometry LD, The differential quantum efficiency of
5 % per facet and pulsed output power of 3.5 mW per facet were
obtained at a current of 430 mA.
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Results and Discussion
• These differences are probably due to the high lateral confinement of
the light propagation and to the absence of etching damage in the gain
region of the active layer due to the ridge geometry.
• On the other hand, the stripe-geometry LDs were easily broken, probably
because of the large heat generation due to the high operating currents and
voltages.
• Using ridge-geometry Ш-V nitride based LDs, the threshold current
and differential quantum efficiency were greatly improved in comparison
to those of strip-geometry LDs.
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Results and Discussion
• The characteristic temperature T0,
which is used to express the
temperature dependence of the threshold
current in the form of Ith(T)=I0exp(T/T0)
• The characteristic temperature of the
threshold current of the LDs was 185 K,
which is higher than that (150 K) of IIVI based LDs.
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First Room-Temperature Continuous-Well Operation
of InGaN Multi-Quantum-Well Laser Diodes
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First Room-Temperature Continuous-Well Operation
of InGaN Multi-Quantum-Well Laser Diodes
• This figure shows the result of a
lifetime of RT CW-operated LDs
where the operating current is shown
as a function of time under a constant
output power of 3 mW per facet
controlled by an auto-power controller.
• The operating current gradually
increases due to an increase of the
threshold current from the initial stage
and sharply increases above 1 s.
• This short lifetime is probably due to
large heat generation caused by the
high operating currents and voltages.
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InGaN Multi-Quantum-Well Laser Diodes with
Modulation-Doped Strained-Layer Superlattices
cladding layers grown on the ELOG substrate
• It is difficult to grow the thick AlGaN cladding layer reguired for optical
confinement, due to the formation of cracks in the layers during growth.
• The crack are caused by the stress introduced in the AlGaN cladding
layers due to lattice mismatch, and difference in the thermal expansion
coefficients of the AlGaN and GaN layers.
• Nakamura et al. demonstrated the InGaN MQW-structure LDs which
have AlGaN/GaN modulation-doped straind-layer superlattice (MD-SLSs)
within the range of critical thickness as cladding layers instead of thick
AlGaN layers.
• Modulation doping of the SLSs was performed to reduce the operating
voltage of the LDs.
• The epitaxially laterally overgrown GaN (ELOG) on sapphire was used
as a substrate to reduce the number of threading dislocations of the GaN
epilayer.
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InGaN Multi-Quantum-Well Laser Diodes with
Modulation-Doped Strained-Layer Superlattices
cladding layers grown on the ELOG substrate
• A Al0.14Ga0.86N/GaN MD-SLS
cladding layer consisting of 120
25-Å-thick Si-doped GaN
separated by 25-Å-thick undoped
Al0.14Ga0.86N layers.
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InGaN Multi-Quantum-Well Laser Diodes with
Modulation-Doped Strained-Layer Superlattices
cladding layers grown on the ELOG substrate
• The selective growth of GaN was
performed on a 2-μm-thick GaN layer
grown on a (0001) C-face sapphire
substrate.
• The 1-μm-thick silicon dioxide (SiO2)
mask was patterned to form 4-μmwide stripe windows with a periodicity
of 11 μm in the GaN (1100) direction .
• After 20-μm-thick GaN layer growth
on the mask pattern, the coalescence
of the selectively grown GaN made it
possible to achieve a flat GaN surface
over the entire substrate.
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InGaN Multi-Quantum-Well Laser Diodes with
Modulation-Doped Strained-Layer Superlattices
cladding layers grown on the ELOG substrate
• They were able to reduce the
operating voltage using the ELOG
substrate and MD-SLS, in
comparison with the value for
previous LDs.
• InGaN Multi-Quantum-Well Laser
Diodes with Modulation-Doped
Strained-Layer Superlattices
cladding layers grown on the ELOG
substrate were demonstrated to have
an estimated of more than 10000 h
under CW operation at 20 °C.
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InGaN Multi-Quantum-Well Laser Diodes with
Modulation-Doped Strained-Layer Superlattices
cladding layers grown on the ELOG substrate
• The InGaN MWQ-structure LD was modulated using a pulsed current
(pulse width: 15.5 ns, pulse duty ratio: 1 %)
• Form the measurement of the delay time of the laser emission as a
function of the current intensity, the carrier lifetime (τn ) was estimated to
be 1.8 ns using the formula td=τnln(I/I-Ith).
• The self-pulsation is due to a saturable absorber in the waveguide
region of the LDs.
• The n-type In0.14Ga0.86N layer (compliance layer) just beneath the ntype Al0.14Ga0.86N/GaN MD-SLS layer probably function as a saturable
absorber.
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InGaN Multi-Quantum-Well Laser Diodes with
Modulation-Doped Strained-Layer Superlattices
cladding layers grown on the ELOG substrate
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Result
Violet-laser recorder comes to market
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Result
• Sony has announced plans to start sales of the world's first DVD recorder
that uses a violet laser light source, according to Reuters. The recorder will
use the format developed by the Blu-Ray consortium, of which Sony is a
founder member.
• The new machine will be available in Japan from mid-April,
priced at 450 000 yen (about $3800). Conventional DVD
recorders using red lasers are priced at 50 000-70 000 yen
($420-590).
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