In contrast to the conventional barrier pump scheme, the MQWs sample used in our experiment is fabricated to be in-well pumped at the second quantized state (n=2) of QWs to prevent the carrier capture time from barrier to the QWs region [16,17].
Then the excited carriers relax to the ground state (n=1) and the intrinsic PL is radiated. The schematic demonstration of the radiative recombination processes is shown in Fig. 2.2-1 in use of a simplified single QW configuration. The room temperature transmission spectrum of the AlGaInAs MQWs chip is shown in Fig.
2.2-2 under low power excitation. The first and second quantum states can be recognized obviously due to the step-like density of state structure. But there is a deep valley of transmittance spectrum near the band-edge. Therefore, we performed reflectance spectrum measurement of MQW chip to fully understand this specific feature. In Fig. 2.2-3, the reflectance spectrum depicted by solid curve reveals that the relatively low transmission at 1500 nm is mainly resulted from the strong reflectance peak. This phenomenon could be explained by the interference of multi-reflected wave from periodic layers. In Fig. 2.2-4 we illustrated three stacks of MQW layers with propagating optical waves A, B and C which are reflected by three MQW interface units. Because the lengths of MQW stacks are much shorter than space
1064 nm 1500 nm
QW barrier
barrier
(1)
(2)
(3) nC=1 nC=2
nV=1 nV=2
(1) Pump absorption
(2) Nonradiative relaxation (3) Ground state luminescence
Fig. 2.2-1 Schematic diagram of the radiative recombination mechanisms of the AlGaInAs MQWs in the simplified single QW configuration.
1000 1100 1200 1300 1400 1500 1600 0
20 40 60 80 100
Transmission (%)
Wavelength (nm)
Fig. 2.2-2 Room temperature transmittance spectrum of the AlGaInAs MQWs chip excited at low intensity.
1000 1100 1200 1300 1400 1500 1600 1700 0
20 40 60 80 100
Fit curve
Reflectance (%)
Wavelength (nm)
Reflectance
Fig. 2.2-3 Room temperature reflectence spectrum of the AlGaInAs MQWs chip excited at low intensity.
layers, phase difference of single unit of MQW interface give rise to phase change of
. Consequently, the three reflected wave A, B and C are all in-phase at the incident surface and the constructive interference will result in the reflectance peak as seen in Fig. 2.2-3. This characteristic reflection band due to periodically distributed semiconductor structure can be modeled in use of the transfer matrix method [18].
When a light wave with wavelength λ is incident to the periodically layered MQW structure from air, the scattering matrix Tj at (j-1)- to j-th semiconductor material interface and propagation matrix Pj for wave traveling in the j-th semiconductor material element could be expressed as follows,
1 1 respectively. Consequently, the total equivalent transfer matrix Q is obtained by multiplying the scattering and propagation matrices the incident light passing through.
The square value of reflection coefficient R at wavelength λ is calculated to be,
2
The fitted curve of reflection spectrum with incident light wavelength from 950 to 1700 nm is shown in Fig. 2.2-3 and in good agreement with the experimentally easured data. Finally, the absorption spectrum can be obtained using reflection and
ission spectrum and is shown in Fig. 2.2-5. The band-edge of n=1 quantized state is indicated to be located around 1500 nm at room temperature.
m transm
n1 n1 n1 n1
n2 n2 n2
n1< n2
Fig. 2.2-4 Schematic illustration of constructive interference of waves A, B and C which are reflected from interfaces of three stacks of MQW structure.
Barrier layer
MQW stacks
……
A B C
λ/2 λ/2 λ/2
: 2 ( ) 2 : 4 ( )
2 : 6 ( )
2 A
phase difference B C
1000 1100 1200 1300 1400 1500 1600 0
20 40 60 80 100
Absorption (%)
Wavelength (nm) n=2
n=1
Fig. 2.2-5 Room temperature absorption spectrum of the AlGaInAs MQWs chip excited at low intensity.
In the proceeding experiments, excitation was performed by an Yb-doped master oscillator fiber amplifier (SPI redENERGY G3) with emission wavelength at 1.06 μm which is beyond the band-edge of n=2 quantized state. This pulsed fiber laser source supplies 12-200 ns pulses and the pulse repetition rates could be ranged from 10-500 kHz. The following experimental results are obtained under the pulse repetition rate of 20 kHz and pulse duration of 30 ns. The experimental configuration of the PL measurement is shown in Fig. 2.2-6. A focusing lens with 95% coupling efficiency was used to reimage the output beam of the pump fiber laser. The incident pump light was normal to the surface of the MQWwafer at a spot radius of 420 μm and the maximum pump intensity is 1 MW/cm2. To investigate the luminescence spectrum of MQW sample at the broad temperature range from 93 to 313 K, the MQW chip was cooled down by the cryogenic system (Janis VPF-100) in which the temperature could be ranged from 77 to 500 K and the stability is ± 50 mK. The epitaxial side of MQW chip was bonded to a single-crystal diamond plate with thickness of 450 μm via the liquid capillary bonding [19] to improve the heat dissipation in the lateral direction.
The entire composite was clamped by the copper blocks with 2 mm diameter aperture in each side. The MQW sample was excited at the epitaxial side which is directly in contact with the diamond heat spreader to decrease the thermal resistance. The spectral information is recorded by an optical spectrum analyzer (Advantest Q8381A) with the diffraction monochromator which can be used to perform the high speed measurement of pulsed light with 0.1 nm resolution. The luminescence light of the MQW sample was collected from the substrate side by a multimode fiber with 75 μm core size and an angle of 50o to the surface normal. The size of aperture on the copper heat sinks and transparent windows on the vacuum chamber are large enough to prevent sheltering the luminescence light from the sample surface. Thus the
Fig. 2.2-6 The experimental configuration of the PL spectrum measurement.
ψ=50o Yb-doped pulsed
fiber laser module
Collimator, isolator and beam expander
Optical spectrum analyzer
Focusing lens
Vacuum chamber
MQWs chip
Liquid-nitrogen
cooled copper sinks Diamond heat spreader
collected light is composed of the radiation emanating from the entire excited region.
The Fe-doped InP substrate is transparent at the wavelength region longer than 950 nm and polished to 300 μm. This means that the substrate will not alter the emission light from the MQW region except for a little scattering loss.