3College of Materials Science and
Opto-Electronic Technology, University of Chinese Academy of Sciences,
Beijing 101408, China
Email: jqliu@semi.ac.cn; biewang2001@126.com; and zpweicust@126.com
We demonstrate a high-speed 8.5 μm quantum cascade laser with
room temperature continuous wave operation. The maximum output power of
141 mW is obtained at 20 ℃. The parasitic capacitance of the device is
decreased from 36.6 pF to 7.1 pF by monolithic integrating a π-shape
metal contact electrode. This results in an increase in the -3 dB RF
modulation bandwidth from 870 MHz to 4.5 GHz compared with the
conventional electrode configuration.
Introduction: High-speed quantum cascade lasers (QCLs) are
essential for a plethora of applications in free-space optical
communication (FSOC) [1], high-sensitivity spectroscopy [2],
mode-locking [3], etc. Due to the particularity of the transition
between subbands, the theoretical modulation bandwidth of QCLs can reach
the order of 100 GHz [4]. However, higher electrical parasitics
limit the modulation performance of QCLs, various approaches have been
proposed to reduce electrical parasitics. Using thicker chalcogenide
glass (Ge0.25Se0.75) as insulating
material to achieve 7 GHz modulation response at 20 K. Single-mode
buried grating QCL using thickened SiO2 as insulating
layer achieves a flat frequency response of 3 GHz at room temperature
[5]. A frequency response of 15 GHz at 77 K is achieved through the
integration of microstrip lines with dielectric laser waveguide [6].
A single-mode distributed feedback (DFB) QCL integrated with a
three-terminal microwave coplanar waveguide structure displayed 3-dB
bandwidth of up to 6.6 GHz [7]. High-frequency modulation of 3-dB
bandwidth at 5.6 GHz achieved by buried heterostructure ultrashort
resonator [8]. The RC constant is reduced by inserting a matching
printed circuit, and the 3-dB bandwidth is increased from about 0.9 to
2.2 GHz [9]. Although high-speed QCLs have been studied a lot, most
of these devices can only operate below room temperature.
In this letter, we propose a room temperature high-speed QCL emitting at
8.5 μ m with the maximum continuous wave (CW) output power of 141
mW at 20 ℃. After optimization of the monolithically integrated π-shape
metal contact electrode, a parasitic capacitance is reduced to a very
low value of 7.1 pF resulting in a high -3 dB RF-modulation bandwidth of
4.5 GHz.
Design and fabrication: The QCL structure was grown on an n-doped
InP substrate by metal-organic chemical vapor deposition (MOCVD) with an
active-region (AR) structure similar to Ref. [10]. The layer
thicknesses in nanometers, starting from the first injection barrier,
are as follows:4.0 /1.3/1.0 /5.2/0.9 /5.1/1.0 /4.7/1.6 /3.6/2.2 /2.9/1.8 /2.7/1.9 /2.6/2.0/2.4/2.5/ 2.5/3.1 /2.3,
where InAlAs barriers are in bold, InGaAs wells in roman, and underlined
numbers correspond to the doped layers (Si, 2 × 1017cm-3). The epitaxial layer sequence starting from the
InP substrate is as follows: 0.5 μ m InP buffer layer (Si, 1 ×
1017 cm−3), 3 μ m lowdoped InP
cladding layer (Si, 4 × 1016 cm−3),
300 nm InGaAs confinement layer (Si, 5 × 1016cm−3), 35 periods InGaAs/InAlAs active structure, 300
nm InGaAs confinement layer (Si, 5 × 1016cm−3), 3.3 μ m low-doped InP cladding layer (Si,
4 × 1016 cm−3), 0.4 μ m
graded-doped InP layer (Si, 1 × 1017cm−3 – 1 × 1018cm−3), and 0.3 μ m InP contact layer (Si, 5 ×
1018 cm−3).