Literature Review

With the development of telecommunication infrastructure, the need for high data transmission rates has increased. To reach higher data transmission rates, the frequency of the electromagnetic radiation must correspondingly increase. The modern fiber-optic communication must rely on high performance semiconductor lasers, so as to transmit high volume and low loss optical signals to further distance without repeaters. Moreover, high performance semiconductor lasers are essential for the establishment of high speed internet infrastructure. In order to reach high transmission rate and narrow beam divergence lasers are needed. However, InAs QDs grown by self-assembled techniques on GaAs substrates are affected by compressive strain from GaAs so that their emission wavelengths are usually around 1 μm. Several methods have been developed to reduce the effect of compressive strain and thus realize the longer wavelength emission. The related results about the modifications of heterojunction are summarized as following.

(1) The growth rate of InAs QDs

By growing InAs QDs at lower growth rate (<0.01 ML/s) can increase the dots size.

Nakata et al. have found that the emission wavelength exceed 1.3 μm when the growth rate was reduced to 2×10^-3 ML/s, but the island density decreased to 5×10⁹ cm^-2 [9]. The InAs islands were grown on a GaAs/AlGaAs/GaAs buffer layer on Si-doped GaAs (001)

Figure 1.5 illustrates that base diameter and density of the uncapped islands measured by AFM as a function of growth rate. A reduction in the growth rate enhances the migration length of In adatoms, allowing them to incorporate into already existing QDs instead of nucleating new structures. Therefore, the dots size can be significantly increased but the dots density decreases drastically, being in the low 10⁹ cm^-2 ranges.

(2) The substrate temperatures (Ts)

Chu et al. and Trofimov et al. have grown InAs QDs deposited onto GaAs by molecular beam epitaxy at different substrate temperatures [10, 11]. They both showed that the higher substrate temperature leads to the lower density of InAs QDs in large size on the surface.

Chu et al. grew four samples of 2.5 ML InA s QDs at different substrate temperatures Ts = 480, 500, 530 and 550 °C by molecular beam epitaxy [10]. The photoluminescence spectra were measured at 4.2 K and room temperature, as shown in Figure 1.6. At 4.2 K, the photoluminescence peak clearly shifts systematically to lower energies for temperature increase from 480 °C to 530 °C. The QDs become larger with the increasing substrate temperature. The diffusion length of the adatoms is mainly responsible for the changes in size of the QDs when substrate temperature is varied. Around a nucleation center which acts as sink for surrounding adatoms, there is an effective zone within which the adatoms are collected. This results in the formation of large islands at higher substrate temperatures. At 530 °C, two distinct photoluminescence peaks corresponding to the optical transition between the ground state and the first excited state of the QDs were observed. In reference to the QDs grown at 530 °C, the photoluminescence spectrum of the QDs grown at 550 °C shifts to higher energies, and three peaks can be clearly observed. Similar behavior is observed in the room temperature photoluminescence spectra.

The QDs density is also dependent on the substrate temperature because the dots density of the nucleation centers is sensitively dependent on the surface diffusion of the adatoms, as shown in Figure 1.7. Due to the smaller islands exclusion zone, there are more nucleation centers forming at low substrate temperatures, which give rise to a high density of QDs. Therefore, growing QDs on a higher substrate temperature leads to form larger dot size in a more homogeneous distribution which also results in the longer wavelength and the narrow distribution of the photoluminescence spectra.

(3) Uncapped and capped layer on InAs QDs

Quivy et al. and Ohtusbo et al. both have compared the uncapped and capped GaAs layer on InAs QDs by molecular beam epitaxy [12, 13]. The uncapped InAs QDs take the pyramidal shape, which is covered by four {110} facets. However, Figure 1.8 shows that the shape of the capped dots changes into a dome-like shape. After the GaAs capping, the lateral size and height were decreased. The effects depend on the strain and enhance for the larger dots having large strain.

During the capping growth of GaAs, the composition, the shape and the size of QDs were changed. The modification of the QD structure is mainly attributed to the In surface segregation and In-Ga intermixing effects during the GaAs capping growth. Therefore, such modification by the capping growth will influence the size uniformity of QDs [14].

It is possible to decrease an inhomogeneous broadening in the dot size by a proper GaAs capping growth.

The strain due to the lattice mismatch is more stored in the large InAs QDs. A large density of misfit dislocations is introduced and affects the optical and electrical properties of the QDs. Therefore, the excess compressive strain must be reduced to suppress the generation of dislocations with a GaAs capping layer above the QDs [13].

(4) Different strain-reducing layer on InAs QDs

Akahane et al. and Liu et al. have reported on lengthening of the emission wavelength of InAs QDs by introducing a strain-reducing layer (SRL) [15, 16]. GaAsSb SRL on InAs (3 ML) QDs embedded between 100 nm GaAs capping layer, which has a wavelength redshift in photoluminescence spectrum. Figure 1.9 (a) and (b) compare schematic band diagrams of two heterostructures. Capping InAs/GaAs QDs with a thin In0.15Ga0.85As (6 nm) layer reduces the confinement potential for both electrons and holes.

However, capping with GaAs0.86Sb0.14 (6 nm) provides electron confinement which is essentially identical to that obtained by using GaAs, a result of the small GaAs/GaAsSb conduction band offset. In other words, the GaAsSb SRL acts to suppress the carriers escape from QDs. Therefore, GaAsSb SRL provides a further enhancement of the QD properties.

The room temperature photoluminescence at 1.62 μm from InGaAs QDs capped with GaAs0.83Sb0.17 (8 nm) shown in Figure 1.10 has been recently reported by Ripalda et al.

[17]. The samples were grown on GaAs (001) by solid source molecular beam epitaxy. It has used InAs (2.2 ML) as a QD seed layer, followed by In0.5Ga0.5As (8 ML) QDs. The presence of Sb during capping of the InGaAs QDs has two major effects. One is a wavelength redshift and a broadening of the main photoluminescence peak. The maximum peak intensity of luminescence is located at a wavelength of 1.62 μm for the GaAsSb capped InGaAs QDs layer. The other is that the separate electron and hole confinement is likely to lead to long radiative recombination lifetimes, and this makes the samples more sensitive to nonradiative recombination at structural defects, such as those due to the large lattice misfit induced by Sb and In incorporation. However, enhancement of the photoluminescence intensity is needed.

(5) Different buffer materials under InAs QDs

Shimizu et al. investigated four types of buffer materials, in terms of GaAs, GaAs0.98Sb0.02 (7 nm), InGa0.13As0.87 (1 nm), and Si buffer [18]. The samples were grown on GaAs (001) by solid source molecular beam epitaxy.

The photoluminescence intensity decreases monotonously with the increases in the dot density for all types of buffers. The relationship between the photoluminescence intensity and the dot density is almost the same for GaAs, InGaAs, and Si buffer.

However, the photoluminescence intensity increases about three times when GaAsSb buffer is used, as shown in Figure 1.11. This was attributed to the reduction of the interfacial defects of the QDs by the surfactant behavior of Sb.

Figure 1.5 Base diameter and density of the uncapped islands measured by AFM as a function of growth rate [9].

Figure 1.6 The optical properties of QDs are significantly affected by different substrate temperature. (a) 4.2 K and (b) room temperature [10].

Figure 1.7 AFM images of corresponding uncapped QDs grown at (a) 480 °C and (b) 530 °C.

The dots density approximately of 1×10¹¹ cm^-2 and 1.2×10¹⁰ cm^-2, respectively [10].

Figure 1.8 (110) cross-sectional TEM and HAADF-STEM images of (a) uncapped InAs dots and (b) InAs dots covered by GaAs layer [13].

Figure 1.9 Schematic band diagrams of (a) InAs/InGaAs/GaAs and (b) InAs/GaAsSb/GaAs structures [16].

Figure 1.10 The room temperature photoluminescence spectra of GaAs and GaAs0.83Sb0.17

capped InGaAs QDs [17].

Figure 1.11 The room temperature photoluminescence spectra of GaAs and GaAs0.98Sb0.02

buffer both with the dot density of 3×10¹⁰ cm⁻² [18].