Quantum-Dot Lasers

Nanoelectronics term projectR91543013徐維良指導教授 : 劉致為

Outline

半導體雷射與 Quantum dot laser Quantum dot laser 的製造 Quantum dot laser 的特色 高能的 Quantum dot laser 1.3 µm Quantum Dot Lasers 結論

半導體雷射LASER:Light Amplification by Stimulated Emission of Radiation

必要的元件 :

--Gain medium

--Optical feedback

•利用 Quantum dot transition 的放射結合來放大 . •Pumping over p-n junction by current injection•利用水晶面來反射以共振

增益與尺寸

Quantum Dot 的好處

Discrete energy level : high density of states

no temperature dependence

Quantum Dot 的好處

reduced diffusion→ no diffusion to surfacesreduced active volume→ low absorption, low inversion densitiesrefractive index decoupled from carrier density→ no chirp

Quantum dot laser 的製造

MBE-Growth

Integration of Quantum dot layer into the active zone of a semiconductor laser

Dot density>10^10cm^-2

改良 Carrier Confinement

•SSLs as 布拉格反射體•改良 Carrier Confinement

Quantum dot laser 的 active region 對於 thermal losses 較敏感

改良 Carrier Confinement

不同區域的 short period superlattices 之結合mini bandgap 的部分重合導致 effective barrier height 的增加

溫度與 Quantum dot laser

Operation temperature > 210 °C Reduced wavelength shift:QW: 0.33 nm/KQDots: 0.17 - 0.19 nm/K

Quantum dot laser 之增益

•About 3 times broader gain spectrum due to dot size distribution•Much larger tuning range for wavelength tuning of DFB lasers

Single mode Emitting Quantum dot lasers

•使用 E-Beam 製造• Wavelength selection by gratingperiode (SMSR = 52 dB)• Ith < 20 mA for all periods(.λ = 33 nm)

溫度穩定性•Stable single mode emission

•No mode hopping

•Single mode operation over

194K temperature range

•三倍大的頻寬•溫度飄移少一倍

Quantum Dot 與 Quantum Well

• Reduced threshold current density for L > 2.5 mm (cross over)• Lower optical confinement for QDots, but inversion condition is relaxed

Material Gain of Q-Dot and QW-Laser

波長對溫度敏感度Quantum dot laser 有較低的溫度敏感度

△λ/ △ T

= 0.35 nm/K for QWLs

= 0.23 nm/K for QDLs

高能的 Quantum dot laser

• 2 mm × 100 µm broad area laser• Record value of 4 W cw output power• Wall plug efficiency > 50 % at 1 W

高能的 Quantum dot laser

• Emission by fundamental mode• High temperature stability• Low wavelength shift (for QWs 50% higher)

高能的 Quantum dot laser•在 20°C 與 80°C 的區域中，每增加一瓦的能量，只有多百分之二十的電流

•高的 characteristic temperature

T= 110 K up to 110 °C

1.3 µm Quantum Dot Lasers 替代昂貴的 InP-based material system Growth on GaAs substrates, -- 便宜、 大的 WAFER 面積 (6", 8") special dot 優點 --low threshold density --broad gain function --low temperature sensitivity

InAs/GaInAs Quantum Dots

•InAs embedded in GaInAs buffer layers – Room temperature emission at 1.3 µm – High quantum dot density• Growth rate: r(GaAs) = 1 µm/h r(InAs) = 140 to 260 nm/h• Growth temperature: T = 510 °C

1.3 µm Quantum Dots

1.3 µm Quantum Dots

• High dot densities for InAs on GaInAs• 35 - 40 meV line width• 60 meV level distance• Longer wavelength at higher In content

1.3 µm Quantum Dot Laser

•6 InAs/GaInAs Q-Dot layers with 50 nm GaAs spacers• 650 nm cavity width• GRINSCH with SSL structure• 1,6 µm Al0.4Ga0.6Ascladding layers

1.3 µm Quantum Dot Laser

• Laser emission by fundamental mode• 800 µm resonator length possiblewithout mirror coating

Threshold Current Density

• For 6 Q-Dot layers threshold doubles but 800 µm device length possible• For 3 Q-Dot layers low threshold current density (100 - 200 A/cm2)but limitation to about 2.5 mm resonator length

Modal Gain of Quantum dot Layers

• L = shortest resonator length at which laser operation is still possible on the ground state• About 2 - 3 cm-1 modal gain per dot layer• Best results with 6 dot layers achieved

Tuning Range of QDot-Lasers

• Linear correlation of gratingperiod and emission avelength– Tuning range > 35 nm– Basic device properties arealmost identical over the whole tuning range→ A further extension of the tuning range to longer and shorter wavelengths should be possible

高頻特性

• Large modulation bandwidthfor 800 µm long HR/HRcoated device• 3dB bandwidth thermally limited

結論• Quantum dot laser 的好處

– 低很多的 inversion carrier density ( 低 threshold current)– 對溫度較不敏感– 有大的頻寬– low chirp

結論• 已實體化的 Quantum dot laser

– 980 nm single mode emitting laser with extremely high temperature stability (Top = 15 °C - 210 °C)– 980 nm high power lasers (4 W cw output power, > 50% wall plug eff.)– 1.3 µm laser with high device performance (Ith = 4.4 mA, Top. > 150°C)

Referencehttp://www.compoundsemiconductor.net/articles/news/6/3/21/1

http://fibers.org/articles/fs/6/12/3/1

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http://www.indianpatents.org.in/ach/quant.htm

http://newton.ex.ac.uk/aip/physnews.595.html

http://www.aip.org/enews/physnews/2003/

http://www.elec.gla.ac.uk/groups/nanospec/dotlaser.html

http://www.shef.ac.uk/uni/academic/N-Q/phys/research/semic/qdresgroup.html#Laser

Referencehttp://optics.org/articles/ole/7/8/2/1

http://feynman.stanford.edu/Html-CQED/sqdl.html

http://www.hinduonnet.com/thehindu/2001/09/13/stories/08130006.htm

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