Arnold Alguno

Enhanced quantum efficiency of solar cells with self-assembled Ge dots stacked in multilayer structure

We report on the performance of solar cells with stacked self-assembled Ge dots in the intrinsic region of Si-based p-i-n diode. These dots were epitaxially grown on p-type Si(100) substrate via the Stranski–Krastanov growth mode by gas-source molecular beam epitaxy. Enhanced external quantum efficiency (EQE) in the infrared region up to 1.45 μm was observed for the solar cells with stacked self-assembled Ge dots compared with that without Ge dots. Furthermore, the EQE was found to increase with increasing number of stacking. These results show that electron-hole pairs generated in Ge dots can be efficiently separated by the internal electric field, and can contribute to the photocurrent without considerable recombination in Ge dots or at Ge/Si interfaces.

Effects of spacer thickness on quantum efficiency of the solar cells with embedded Ge islands in the intrinsic layer

We report on the effects of spacer thickness on the external quantum efficiency (EQE) of the solar cells with Ge islands embedded into the intrinsic region of the Si-based p-i-n diode. The EQE response of the solar cells in the near-infrared region is dependent on the spacer thickness that separates the layers of self-assembled Ge islands. It was found that the EQE response has an optimum value when the spacer thickness can sustain a good vertical ordering of islands. On the other hand, random nucleation of islands due to a thicker spacer layer exhibits an inferior EQE response. Furthermore, a drastic decrease of the EQE response of the solar cells for a thinner spacer layer was observed.

Stacked Ge islands for photovoltaic applications

Abstract Stacked Ge islands formed via the Stranski–Krastanov growth mode were incorporated into the intrinsic layer of Si-based pin diode to improve the performance of the solar cells in the near-infrared regime. The onset of the external quantum efficiency was extended up to around 1.4 mm for the solar cells with stacked Ge islands. The quantum efficiency was found to increase with increasing number of stacking, and the onset of the photocurrent response was in good agreement withroom-temperature photoluminescence energy of the Ge islands. These results manifest that the Ge islands did play a role toincrease the quantum efficiency. Furthermore, a part of electron-hole pairs generated within Ge islands was separated by the internal electric field and contribute to the photocurrent.

Unique Temperature Dependence on the Morphological Evolution of Ge on Si(100)

The Ge/Si system has been receiving much attention for both fundamental studies and device applications. It is a good model for understanding the basic physics for heteroepitaxy of lattice-mismatched semiconductors since Ge has a lattice constant of about 4.2% larger than that of Si. The presence of Ge in the Si lattice introduces stress, which has dramatic effects on the film growth. During the deposition of Ge on Si, small change in growth temperature will have big impact on the surface morphology and subsequently affect the optical properties of the system. The mechanism of this surface morphology evolution with respect to growth temperature is not well understood. In this study, we try to explain the surface morphology evolution of Ge on Si by intentionally introduce interdiffusion into the system by changing the growth temperatures in order to modify the total free energy that controls the energetics of surface morphology. We found experimentally that a unique surface morphology exists at intermediate temperatures and this was theoretically explained in terms of thermodynamics of equilibrium structure. To investigate the temperature dependence on the morphological evolution of Ge on Si, we deposited about 8 monolayers of Ge on Si(100) substrate by gas source molecular beam epitaxy at different temperatures (400700C). Standard wafer cleaning was done before loading into the growth chamber of the molecular beam epitaxy. Prior to Ge deposition, a 100 nm Si buffer layer was grown. The surface morphology of Ge islands at different growth temperatures was studied by ex situ atomic force microscopy. Upon changing the growth temperature from 400 to 700oC, a unique surface morphology evolution appeared at intermediate temperatures ranging between 500 and 550oC. At these intermediate temperatures, surface corrugation called quasi-two dimensional (2D) layer was observed while the rest of the growth temperatures produced well-defined three-dimensional islands as depicted in Figure 1. It is noted that the height drastically decreases when quasi-2D layer exists as compared to lowtemperature grown Ge islands (400oC). This unique surface morphology, i.e., quasi-2D, is not a result of kinetic limitation since after annealing at growth temperature for 1h, no significant changes on the structure occurred. The origin of this phenomenon will be discussed considering the interdiffusion of Ge and Si during growth and its impact on the strain, interlayer, and surface energies to determine the free energy of the system. Using a combination of macroscopic diffusion equation and finite element analysis, we calculated the composition and strain distributions along the growth direction. Calculation was carried out considering that interdiffusion occurred during the growth process. The total free energy of the system for different morphologies was calculated and compared to determine the energetically stable morphology at different growth temperatures. We found that the presence of quasi-2D layer at the intermediate temperature is consistent with the energetics of Ge layer formation as shown in Figure 2. Interdiffusion played a vital role in the presence of energetically favorable quasi-2D layer surface morphology. At extremely high temperatures, formation of 2D layer is thermodynamically favorable. The deposited Ge selects an equilibrium shape to minimize the combined elastic energy and surface energy. The equilibrium shape of the islands is governed by the competition between the surface energy and the elastic relaxation energy of the islands as compared to the uniform strained films. In summary, we have shown the presence of quasi2D layer at the intermediate temperatures and explained in terms of energetics of formation taking into account the interdiffusion of the deposited Ge and Si. It was found that temperature dependent interdiffusion gives a unique morphological evolution of the system.