Efficiency of InGaAs/GaAs Quantum Well Laser Stripes with 30 µm-MesaI. R. Tagaca, J. Fernandez Jr., A. Somintac and A. Salvador

Condensed Matter Physics Laboratory, National Institute of PhysicsUniversity of the Philippines, Diliman, Quezon City 1101

Efficiency of MBE-grown InGaAs/GaAs quantum well (QW) laser stripes fabricated with 30 µm mesa (Figs. 1 to 3) is determined at its lasing wavelength ~ 0.98µm. The lowest threshold current density (Jth) measured is ~ 600A/cm2 (for cavity

length l = 0.4 mm), much better than the last reported (2004) Jth ~

4400 A/cm2 from a similar laser with 75µm mesa.[1] Room-temperature output power vs. injection current (L-I) curves at one laser facet estimate the internal quantum efficiency (ηi = fraction of

injected carriers that stimulate emission of photons) and the internal absorption loss (αi) to be ηi = 83 ± 23 % (with correction

factor (cf)) and αi = 75 ± 24 cm-1 (Fig. 4). The correction factor to

the output power takes into account the undetected light due to elliptically diverging beam of the laser as verified by its far-field distributions.

Figure 3 (a) Emission Spectra at LED (λ = 0.98 µm, I < Ith) and laser (λ = 0.96 µm) operation. LED intensity was magnified several times to

emphasize linewidth collapse (from ~400Å to ~4Å) when the device lases. Shift in λ is due to subsequent device heating.

(b) Pulsed L-I curves (0.01% DC, 1kHz) from laser with l= 0.4 mm. Lasing occurs at the region where the L-I slope steeply rises.

(a) (b)

1/ηe = 253.03(l) + 4.0603

1/ηe = 74.816(l) + 1.2006

0

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-0.02 0.01 0.04 0.07 0.1 0.13 0.16

Cavity length (cm)

Inve

rse

exte

rnal

qua

ntum

eff

icie

ncy

w cf

no cf

Figure 4 From ηe-1 vs. l, home-grown lasers have αi = 75 ± 24 cm-

1, ηi ~ 83 ± 23 % (no cf) and ηi ~ 25 ± 7 % (with cf). ηe is the

fraction of carriers responsible for photons transmitted out of the laser

1.5mm

human hairstrand

1.5mm

human hairstrand

Figure 2 Top view of an array of fabricated 1.5 mm-long lasers. Without polyimide, electrical access is by metal

contacts about the width of white strip.

AcknowledgmentWe would like to thank the DOST-PCASTRD and UP-OVCRD

for their continued support.

Reference[1] G. Manasan, “Fabrication and Characterization of MBE-

grown GaAs-based Lasers”, BS Thesis, University of the Philippines-Diliman, 2003.

GaAs substrate

342 Å GaAs buffer

2 µm Al0.3Ga0.7As cladding layer

0.19 µm GRIN-SCH AlxGa1-xAs; x = [0.3 to 0.2]

369 Å GaAs

80Å In0.2Ga0.8As QW

3 periods of InAs-dots-in-an-InGaAs-well layers with 20Å GaAs spacer layer

369 Å GaAs

0.19 µm GRIN-SCH AlxGa1-xAs; x = [0.2 to 0.3]

100 nm AlAs

2 µm Al0.3Ga0.7As cladding layer

0.2 µm GaAs cap

GaAs substrate

342 Å GaAs buffer

2 µm Al0.3Ga0.7As cladding layer

0.19 µm GRIN-SCH AlxGa1-xAs; x = [0.3 to 0.2]

369 Å GaAs

80Å In0.2Ga0.8As QW

3 periods of InAs-dots-in-an-InGaAs-well layers with 20Å GaAs spacer layer

369 Å GaAs

0.19 µm GRIN-SCH AlxGa1-xAs; x = [0.2 to 0.3]

100 nm AlAs

2 µm Al0.3Ga0.7As cladding layer

0.2 µm GaAs cap

p-doped layers

n-doped layers

Active region

(b)Figure 1 (a) SEM photograph and (b) schematic cross-section of the 30-m mesa InGaAs laser stripe. The low

quantum dot (QD) density (hence, few recombination sites) was inadequate to make the device lase at intended λ ~ 1.1

µm and even contributed to high αi (QDs absorb the higher energy emitted by the QW).

ll

mesa widthtop metal

polyimidep-layer

thin i-layer(active)

n-layer

transverse(z)

lateral(x)

mesa widthtop metal

polyimidep-layer

thin i-layer(active)

n-layer

transverse(z)

lateral(x)

ll

(a)