行政院國家科學委員會專題研究計畫成果報告
三五族半導體量子點之成長與元件應用
Gr owth of III-V Semiconductor Quantum Dots and
Device Application
計畫編號：NSC 90－2215－E－002－038－
執行期限：90 年 8 月 1 日 至 91 年 7 月 31 日
主持人：毛明華 國立台灣大學光電工程學研究所
計畫參與人員：蔡俊儀、謝文瀚、吳戴仰、林士凱
中文摘要 本 計 畫 的 研 究 目 標 在 於 ： In(Ga)As/GaAs 量子點的成長、其發光特 性的分析、及其發光元件的研究。 InGaAs/GaAs 量子點結構，突破了在 GaAs 基板上成長 InGaAs/GaAs 量子井的 限制，將發光波長延伸至 1.3 µm，因此可 望在光纖通訊上有重要應用。我們將用 MBE 成長 InAs/GaAs 量子點，繼而延伸至 發光波長在 1.3 µm 的 InGaAs/GaAs 系統。 此外，也將製作 Fabry-Perot 量子點雷射二 極體，並探討其高速調制應用方面的潛力。 關鍵詞：半導體量子點、半導體雷射、變 率方程式 AbstractThe research topics of this project are: growth of In(Ga)As/GaAs quantum dots, analysis of their optical properties, and application as light-emitting devices.
InGaAs/GaAs quantum-dot structures have extended the emission wavelength to
1.3 µm, breaking the limit of InGaAs/GaAs
quantum wells on the GaAs substrate.
Therefore, they will find important
applications in fiber communication. We use MBE to growth InAs/GaAs quantum dots, and extend to the InGaAs/GaAs system with
emission wavelength at 1.3 µm. Furthermore,
Fabry-Perot QD laser diodes are fabricated,
Semiconductor lasers, Rate equations Motivation and Goals
Semiconductor quantum dots have
attracted much attention recently.
Semiconductor lasers with quantum dots as active medium not only have been predicted theoretically, but also have been confirmed experimentally to have many outstanding properties, such as, ultra-low threshold
current density (26 A/cm2 [1]), ultra-high
temperature stability (T0=385K [2]), very
small chirp (0.007 Å /mA [3]), and large modulation bandwidth (8.2GHz [4]). These properties are very important for the light sources used in optical communication systems. Therefore, quantum-dot lasers have large potentials for application in such
systems. 1.3 µm is the wavelength with very
low dispersion in optical fibers.
Traditionally, InGaAsP/InP is the material for semiconductor lasers operating at this
wavelength. InGaAs/GaAs quantum-dot
structures have extended the emission
wavelength to 1.3 µm, breaking the limit of
InGaAs/GaAs quantum wells on the GaAs substrate. Therefore, they will find important applications in fiber communication.
We use MBE to grow InGaAs/GaAs quantum dots with emission wavelength at
1.3 µm. Furthermore, Fabry-Perot
quantum-dot laser diodes are fabricated and their dynamic properties under high-speed
inhomogeneous broadening, and carrier capture time, on the dynamic properties of quantum-dot lasers. The research results will
give guidelines for optimization of
quantum-dot lasers and provide suitable models for their dynamic behaviors.
Results and Discussion
(1) Long-wavelength quantum-dot lasers We have fabricated quantum-dot lasers
with threshold current density 478A/cm2 at
room temperature. However, the emission wavelength is from excited states at 1189nm (Fig. 1). The quantum dot surface density is
about 2.3*1010cm-2.
Fig. 1 A quantum-dot laser with emission wavelength at 1189 nm at room temperature.
The quantum dot surface density is then
increased to 7.64*1010 cm-2 (Fig. 2). Laser
diodes with such high quantum-dot density show ground-state lasing with emission wavelength at 1298 nm (Fig. 3) and threshold
current density 1433A/cm2 at room
temperature. Therefore, the target wavelength
1.3 µm has been achieved.
Fig. 2 SEM picture of quantum dots
with surface density 7.64*1010cm-2.
Fig. 3 A quantum-dot laser with emission wavelength at 1298 nm at room temperature.
(2) Modulation bandwidth
We have measured the relaxation oscillation frequencies of quantum-dot lasers in order to determine their modulation bandwidth. The highest we measured is about 5.1 GHz (Fig. 4), corresponding to a modulation bandwidth 8 GHz that is similar to the best value with the same device
structure [4]. After optimization of
quantum-dot lasers, we believe this
bandwidth can be further increased.
1000 1100 1200 1300 1400 1500 1281 nm 1189 nm P ow er per f acet ( uW) Wavelength (nm) 3 mA 8 mA 15 mA 32 mA 43 mA 53 mA 1100 1200 1300 1400 1500 1298 nm In te n s it y ( a .u .) Wavelength (nm) Cavity length=2 mm Stripe width=24 µm 1.2 Ith 1.09 Ith 0.93 Ith 0.82 Ith 7.64E10 cm-2
Fig. 4 Relaxation oscillation frequencies of a quantum-dot laser
(3) Rate-equation modeling
We use a rate equation model proposed in [5,6] to simulate the static and dynamic characteristics of quantum-dot lasers. The influences of the material and structural parameters, such as homogeneous and inhomogeneous broadening, and various time constants, on the static and dynamic characteristics of quantum-dot lasers are analyzed.
The rate equations proposed in [5] are re-written below with some modifications to
include the gain compression factor ε. VAis
the active region volume.
qe q sr s s s s N N N q I dt dN τ τ τ − + − = d q qe q qr q n e n s s q N N N N N dt dN τ τ τ τ τ + − − − =
∑
∑
∑
Γ + Γ − − − = m m V m m mn r e n rn n dn n q n S S g n c N N G N dt dN A ε τ τ τ 1 p m m m V m n mn r m m S S S g n c BN dt dS A τ β ε − + Γ + =∑
∑
Γ 1 2In Fig. 5, the relaxation oscillation frequency of quantum-dot lasers decreases
with increasing Γ . Therefore, the
bandwidth.
Fig. 5 Relaxation oscillation frequencies with increasing inhomogeneous broadening [7]. Conclusions
In this project, we have fabricated long-wavelength quantum-dot lasers with emission wavelength at 1298 nm successfully. Dynamic properties of quantum-dot lasers are measured. Modulation bandwidth of 8 GHz is experimentally demonstrated. Therefore, quantum-dot lasers have shown important applications in high-speed opto-electronic systems. Using a rate-equation model, we
simulate the dynamic behavior of
quantum-dot lasers. The influence of
inhomogeneous broadening on modulation bandwidth is investigated. In order to achieve higher modulation speed, the size uniformity of quantum dots needs to be further improved. 0.0 0.2 0.4 0.6 0.8 1.0 1.2 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 Inhomogeneous Broadening 20 meV 30 meV 40 meV ωr 2 =a0*vg*(I-Ith) / (q*Vp) fr=ωr / 2π fr (GHz ) (I/Ith-1) 1/2 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 0 500 1000 1500 2000 In te ns ity , a .u . Time , ps re la x a tio n o s c illa tio n f re q u e n c y , G H z Power , mW1/2 Slope=1.44 GHz/mW1/2 0 2 4 6 8 10 12 14 I=2 Ith Γ0=1 m eV Ith= 0.77 m A Γ0=10 m eV Ith= 0.8 m A Γ0=20 m eV Ith= 0.86 m A p h o to n n u m b e r (a rb it ra ry u n it ) tim e (ns)
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![Fig. 5 Relaxation oscillation frequencies with increasing inhomogeneous broadening [7].](https://thumb-ap.123doks.com/thumbv2/9libinfo/8838458.238135/3.893.484.777.121.573/fig-relaxation-oscillation-frequencies-increasing-inhomogeneous-broadening.webp)
