Chapter 3: InAs/GaAs quantum dots growth and characterization
3.3 Growth conditions of self-assembled InAs/GaAs QDs
Several growth conditions are studied in this section. They are As2/As4 ratio, GaAs capped growth rate, and As beam flux. In order to study the effects on transition energy and uniformity of QDs for clarity, higher substrate temperatures (510~530oC), and lower InAs growth rates (0.056µm/hr and 0.0373µm/hr) are used in the experiments.
Detailed growth conditions are listed in Table 3.1.
3.3.1 As2/As4 ratio [3.13]
It is well known that the influence of arsenic species (As2 and As4) on MBE growth is quite different. Especially for InAs QDs growth, the effect of As beam flux is very different with that for 2-D growth. In practice, the sticking coefficient of As4 has been shown to be half that of As2. To investigate this effect on QDs growth, various As2/As4 mixtures are used. We can obtain a desired mixture of As2/As4 vapor by altering the cracker zone temperature of the As valved cracker cell. The cracker zone temperatures used in the experiments are 840oC, 730oC, 680oC, and 570oC. For As2
(As4) mode, the temperature of the cracking zone is kept at about 840oC (570oC). On the other hand, the As cracker cell provides a mixture of As2/As4 as the temperature is set at 730oC (680oC). The AFM images of QDs shown in Fig. 3.7 and Fig. 3.8 demonstrate a significant feature in the surface morphology: There is a tendency that
the density and the size uniformity of QDs are improving with increasing the percentage of As4 in the mixture. This result could be explained in the following way:
The QDs form in the early stage (or so-called “seed”) can collect InAs from their neighborhood and then grow gradually. The indium adatoms are more mobile and has a larger diffusion length in As2 atmosphere. Hence, the QDs using As2 flux can gather much more InAs from farther area. Lager QDs’ size and lower sheet density are achieved. In addition, it also exhibits a large fluctuation in QD size. In contrast, The QDs using As4 flux can only collect InAs from a fixed region of their vicinity. This leads to a smaller size dimension and a higher sheet density of QDs. Furthermore; the size uniformity is also improving due to collecting a fixed amount of InAs under As4
beam flux. The AFM images show these facts no matter what As BEP is (on condition that no growth interruption after InAs deposition).
From the PL results in Fig.3.9, it shows little dissimilarity among various As2/As4, if V/III ratio is lower during InAs deposition. The reason is that, for QD growth under low As BEP, the In adatoms migrate on the grown surface easily and then find their proper sites both in As2 and As4 atmosphere. On the contrary, there is an obvious difference, if V/III ratio remains high. The In adatoms tend to stop at improper sites under high As BEP, resulting in a broad distribution. Especially, it shows a multi-fold distribution if the percentage of As2 is increasing in the mixture. The reason is the same as above.
3.3.2 GaAs capped growth rate
In order to obtain high optical activity, the QDs need to be protected via the growth of the capping GaAs layer. As mentioned above, the inter-diffusion or intermixing would take place as the overgrowth of GaAs on InAs QDs due to a large misfit strain
between them. Therefore, the condition of GaAs overgrowth plays an important role in transition energy level of QDs, such as As beam flux and capped growth rate. Previous studies gave a conclusion that a higher As beam flux would reduce the diffusion length of InAs and GaAs on the surface and then depress the inter-diffusion (or intermixing) between GaAs and InAs. In this subsection, the effect of capped growth rates (0.3µm/hr or 1µm/hr) is studied. Figure 3.10 shows that there is no or little difference between higher and lower capped growth rate when substrate temperature is 520oC. The reason is given as follows: Alloying is a strong temperature-dependent process. The rapid In(Ga)As alloying effect occurs around the periphery of the QDs during GaAs overgrowth even though the capped rate is low. The formed alloy inhibits successive Ga adatoms to diffuse inward and intermix with InAs further. For that reason, the capped growth rate has no or little effect on InAs QD growth, as compared to As BEP. However, as GaAs being capped at low temperature, the growth rate would influence the growth of QDs. We will show this in the chapter 6
3.3.3 As beam flux
The diffusion length of adatoms on the surface is strongly dependent on the growth conditions, such as molecular species (In, Ga, or Al / As2 or As4), substrate temperature, arsenic BEP. Adatoms’ diffusion length and intermixing usually determine QDs’ size distribution and their transition energy.
As warming up the As valved cracker cell for growth, the arsenic bulk material needs at least two hours to reach thermal equilibrium due to its large thermal mass.
However, in practice, the arsenic BEP will increase gradually with time. In other words, the vacuum pressure of growth chamber is rising steadily. By exploiting this feature and without adjusting the As needle valve during epitaxial deposition, we can perform
successively three identical experiments to monitor the growth of QDs. The growth condition is given as follows: 2.6 MLs of InAs is deposited at 520oC with a growth rate of ~0.056µm/hr. In Fig.3.11, there are twofold size distributions of PL in all samples.
There always exists a peak value of PL spectrum, whose transition energy is ~1.235eV (see the arrows), under this growth condition. We attribute this to the emission from the QDs that formed immediately after S-K transition. The other peak values are ranged from 1.14~1.20eV. In the first round of the experiment, most of the QDs grew gradually by absorbing In from their vicinity. However, with increase of As BEP, it would be to suppress the migration of In adatom and result in a group of QDs that is smaller than that of the first round. This result manifests the growth and decline of QDs in the S-K mode.
3.3.4 Remark
In order to solve the problem of slow response of arsenic BEP due to large thermal mass of the bulk material and obtain repeatable results in the different growth rounds, an annealed process was used. After QDs growth, the QDs are next annealing for a few seconds (30~60s), so that indium adatoms have enough time to migrate to appreciate sites on the surface to enhance the size distribution (shown in Fig.3.12).
Besides, by altering the growth parameters of InAs QDs, the maximum of PL peak wavelength just extend to near 1.25µm at room temperature, no matter what substrate temperature is used (510oC ~530oC). We attribute this to stronger In-Ga intermixing occurring in the larger QDs during GaAs overgrowth [3.12, 3.14-3.16]. Hence, how to suppress or inhibit intermixing, or strain redistribution, during overgrowth is the main factor that extending the emission wavelength.