1-1 InGaAs(N)/GaAs Heterostructures
Dilute quaternary solid solution of InyGa1-yAs1-xNx (x < 0.1) is of much interest to various electronic and optoelectronic applications due to the highly nonlinear effects upon nitrogen incorporation. This characteristic can be used to fabricate laser diodes for 1.2-1.3 µm spectral range on cheap GaAs substrates instead of expensive InP substrates which is for InGaAsP/InP system [1]. The large band offsets are due to the large electronegativity of N atoms that pulls down both conduction and valence band edges in InGaAsN quaternary alloy and leads to better electron confinement.
Finally the temperature performance will be better than the conventional InP-based materials [2][3]. High-performance InGaAs(N) single quantum well (SQW) lasers which raise lasing emission wavelength extended to 1300 nm at room temperature have been realized by Tansu et al. lately [4][5][6]. They also have the potential to be long wavelength photonic communication devices [7].
Some experimental results of the fundamental properties of InGaAsN semiconductor have been reported. They have shown the potential applications of InGaAs(N) in the telecommunication field. Time-resolved measurements of carrier dynamics in bulk and quantum-well InGaAs using differential absorption spectroscopy was presented by Sucha et al. [8] in 1994. They found that for sample from 100 to 6000Å, the carrier thermalization time is 200-300 fs regardless of the layer width. Borri et al. [9] used the time-integrated four-wave mixing method to study the temperature and density dependences of exciton dephasing time in In0.18Ga0.82As/ GaAs single quantum well. Bhattacharya et al. [10] performed pump-probe differential transmission in In0.4Ga0.6As/GaAs/AlGaAs heterostructures.
D. E. Mars et al. [11] have grown InGaAsN quantum well laser by MBE and done the room temperature photoluminescence (PL) measurements. J. Misiewicz et al.
investigated the influence of nitrogen on carrier localization in InGaAsN/GaAs single quantum well with nitrogen concentration up to 5.2%. A. Vinattieri et al. [12]
discovered the exciton dynamics in InGaAsN/GaAs quantum well structures after picosecond excitation. They also did the PL measurements for different nitrogen content situations. LiFang Xu et al. [13] quantitatively analyzed the relative contribution of free excitons and free carriers to the radiative recombination at different temperature.
O. Anton et al. [4] reported the behavior of the carrier lifetime with carrier density on two different nitrogen contents in InGaAs1-xNx single quantum well laser diodes.
When nitrogen is added into the well, the recombination lifetime is significantly reduced. These samples were also found that behave like a quantum-dot and affect the carrier dynamics by localization for temperature from 15 to 150K in CLEO 2005 [14]. The material properties of III-Ⅴ-N compounds have been accounted interesting behaviors and been studied vitally during recent years.
1-2 Ultrafast Carrier Dynamics in Semiconductors
The carrier relaxation processes in the semiconductor laser which had been widely investigated strongly affect the characteristics and performance of it [15]-[19]. Over the past few decades, there have been huge advancements in the field of ultrafast carrier dynamics in bulk and quantum confined semiconductors [20]. There are two driving forces behind the development: one is the importance in solid-state physics that the ultrafast optical technique opened up the subpicosecond and femtosecond
microelectronic devices. Basic understanding of the various carrier dynamical processes in semiconductors helps people to realize the principle of manufacturing those devices.
The excitation of semiconductors through equilibrium and the subsequent relaxation processes with different rates has become important in semiconductor field.
Fig. 1-1 shows that when the photon energy is larger than the fundamental energy gap, linear absorption occurs. After the optical or electrical excitation of carriers, there are various microscopic processes of momentum and energy relaxation of carriers in the semiconductor such as carrier-carrier scattering, optical phonon scattering, intervalley scattering, etc., that occur in the femtosecond to picosecond time scale.
These processes are listed in Table 1-1 and the energy relaxation after excitation is shown in Fig. 1-2. In order to investigate such a short interaction time, femtosecond pump-probe measurements, including differential transmission, reflection, and absorption spectroscopies using ultrashort pulse lasers as the exciting sources, have been widely used for Ⅲ-Ⅴ semiconductors [21][22].
In this thesis, we have investigated the processes such as carrier thermalization, bandgap renormalization, and energy relaxation on bulk GaAs-based materials at room temperature. There will be many benefits to realize the relaxation mechanisms of carriers in materials for manufacturing high-speed photonic devices. Furthermore, bulk GaAs-based materials which emit fluorescence at 1.2-1.5 µm have a lot of potential in long wavelength band.
Fig. 1-1 The electron-hole pair creation following excitation of a semiconductor with laser radiation of energyhω0.
Energy relaxation follows via optical phonon emission(hωph).
Fig. 1-2 The block diagram illustrating the follow of energy in a photoexcited semiconductor.
U ltrash ort laser
Table 1-1 Fundamental processes in semiconductors.
1-3 Femtosecond Pump-Probe Technique
Imagine if we want to measure the speed of a running turtle, we only need a minute-resolution clock. But for measuring the speed of a running man, a second-resolution stopwatch may be required. Different time-scale measurements require different time-resolution tools as shown in Figure 1-3. The femtosecond- resolution pump-probe technique with ultrashort pulse laser can achieve the goal of measuring electron lifetime between various energy bands. In this technique, a femtosecond laser pulse is separated into two beams, a pump and a probe beam, with optical time delay between them. The pump beam should cover the probe beam completely and they overlap spatially on the sample under investigation. After pump beam exciting the samples, there will be a change of absorption coefficient on them. The probe beam is for detecting these changes. The time evolution of the excited states is prospected by varying the time delay. Although we adjust the phase of pump and probe beams perpendicular to each other, coherent artifact which is due to an additional coupling between the two beams is still unavoidable over the region near zero time delay where they interact coherently.
Fig. 1-3 Illustrating different time-scale measurements require different time-resolution tools.
Fig. 1-4 exhibits the technology of generating and detecting short pulses in the pasts few decades. The width of optical pulses has fallen by more than three orders of magnitude since 1966. Fork and Shank developed the colliding pulse mode-locked (CPM) laser in 1981 [23], which made the femtosecond resolution measurement of semiconductors possible.
Fig. 1-4 Schematic diagram of the shortest pulses reported versus years. (Ref. 12)
1-4 Aim of this thesis
In this thesis, we study the ultrafast time-resolved photoreflectance of In0.4Ga0.6As1-xNx/GaAs single quantum wells. We compare the pump-probe measurements in samples differing in the nitrogen incorporation (x = 0% and 2%) and in the laser wavelengths (λ= 820 nm andλ= 880 nm). At λ= 880 nm, it can prevent the absorption of confining layer GaAs whose bandgap is about 870 nm.
Chapter 2 presents the background concepts and theorems of the ultrafast spectroscopy of semiconductors. Section 2-1 introduces the scattering processes and carrier relaxation regimes in semiconductors. Then we focus on the reasons of changing in refractive index which includes band filling effect, bandgap renormalization and free carrier absorption.
Chapter 3 describes the sample structure and experimental setup of ultrafast
time-resolved measurement. Chapter 4 shows the results and analysis of our measurements. We have discovered large different phenomena with pumping wavelength at 820 and 880 nm. The change of reflection is negative and positive, respectively. Finally, the perspectives are discussed in Chapter 5.