Kazuo Nakajima

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.

Ge composition dependence of properties of solar cells based on multicrystalline SiGe with microscopic compositional distribution

We present device performance of solar cells based on multicrystalline SiGe (mc-SiGe) bulk crystal with microscopic compositional distribution grown by the casting method. The average Ge composition was systematically changed in the range between 0% and 10%. A small addition of Ge to multicrystalline Si (mc-Si) was found to be very effective to increase the short-circuit current density without affecting the open-circuit voltage. As a consequence, the overall efficiency of a solar cell based on mc-SiGe was improved compared with that based on mc-Si. This result demonstrates that mc-SiGe is a promising candidate to replace mc-Si since it could achieve higher conversion efficiency without drastic increase of the production cost.

Compositional variation in Si-rich SiGe single crystals grown by multi-component zone melting method using Si seed and source crystals

The effect of the supply of depleted Si solute elements on the compositional variation in the Si-rich SiGe bulk crystals was studied using the method which was used to grow Ge-rich SiGe single crystals with a uniform composition. By selecting the proper pulling rate, we can obtain Si-rich Si1−xGex bulk crystals with uniform composition of x=0.1 without using the supply mechanism of depleted Si solute elements. When the supply mechanism of Si solute elements was used, the initial composition in Si-rich SiGe crystals can be much more easily determined by controlling the growth temperature than that in Ge-rich crystals because the Si seed crystal is not melted down. The supply of Si solute elements is very effective to change the compositional variation even for Si-rich SiGe crystals.

Growth and properties of SiGe multicrystals with microscopic compositional distribution for high-efficiency solar cells

Abstract The growth technique and physical properties of SiGe multicrystals with microscopic compositional distribution are demonstrated for new high-efficiency solar cells in which the wavelength dependence of the absorption coefficient can be freely designed by controlling the compositional distribution in the SiGe multicrystals. This growth technique is suitable for the practical casting method, and it is made up of melt growth of SiGe multicrystals with wide and microscopic distribution of the composition from Si to Ge all over the crystals. It is studied how much widely the microscopic compositional distribution in SiGe multicrystals grown from binary Si–Ge melts can be controlled by the melt composition and the cooling process. The range of the microscopic compositional distribution becomes wider as the starting Si concentration in the growth melt becomes larger. SiGe multicrystals with various microscopic compositional distribution can be freely controlled by optimizing the melt composition and the cooling process. The wavelength dependence of the absorption coefficient of such SiGe multicrystals can also be freely designed. Using the experimentally determined absorption coefficient of a SiGe crystal with microscopic compositional distribution, the short circuit photo-current of solar cells was calculated and it is demonstrated that the short circuit photo-current can be much larger for SiGe with microscopic compositional distribution than for SiGe with uniform composition. Si thin film can be easily grown on such a SiGe multicrystal and the Si/SiGe heterostructure can be obtained. These results show that SiGe multicrystals with microscopic compositional distribution are hopeful for new high-efficiency solar cell applications by using the practical casting method.

Melt growth of multicrystalline SiGe with large compositional distribution for new solar cell applications

Abstract The melt-growth conditions to obtain SiGe multicrystals with microscopic compositional distribution are presented. These SiGe multicrystals are useful for new solar cells whose wavelength dependence of the absorption coefficient can be freely designed. The multicrystals with wide compositional distribution from Si to Ge can be grown by a melt growth technique such as the practical casting method. In this work, it was studied as to how much the micro- and macroscopic compositional distribution in SiGe multicrystals grown from binary Si–Ge melts could be controlled by the melt composition and the cooling process. Such SiGe multicrystals with wide distribution of the composition would also have wide distribution of the absorption coefficient, and could be hopeful for new solar cell applications using the practical casting method.

Influence of the elastic strain on the band structure of ellipsoidal SiGe coherently embedded in the Si matrix

We report on a theoretical investigation of the elastic strain in an ellipsoidal SiGe inclusion coherently embedded in Si and its influence on the band structure of SiGe. The strain was calculated as a function of the Ge fraction in SiGe and the aspect ratio of the ellipsoid, and utilized to derive the shift of the band edge. When the principal axis of the ellipsoid was chosen to be parallel to [001], the band structure of SiGe was predicted to be Si like regardless of the aspect ratio. The band gap of strained SiGe was also calculated, and the deviation of the aspect ratio from unity was found to be effective to decrease the band gap due to the breaking of the crystal symmetry. These results suggest the importance of controlling strain, shape, and local Ge fraction in multicrystalline SiGe, which we propose as a promising material for solar cell applications.

Solution Growth of Self-Standing 6H-SiC Single Crystal Using Metal Solvent

Silicon carbide (SiC) crystal growth from ternary solutions Si-C-X where X is a transition metal was studied. In order to select the desirable transition element and to determine the solution composition, we have conducted the calculations of ternary phase diagrams by means of CALPHAD (CALculation of PHase Diagrams) method. Preliminary growth experiments without a seed crystal were also performed. Among various Si-based solutions, Si-C-Ti was one of the most effective solutions to increase crystal growth rate compared with Si-C. Optical microscopic observation of the obtained SiC etched by molten KOH showed no micropipe defects in the crystals. We have also performed the growth experiments with 6H-SiC seed crystal under temperature gradient. As a result, we have successfully obtained a 12mm×12mm self standing SiC crystal. Introduction At present, commercially available silicon carbide (SiC) wafers are produced by the Physical Vapor Transport (PVT) technique. Although a large number of attempts have been made on the reduction of structural defects, PVT crystals still exhibit remaining defects, such as micropipes and dislocations. Especially, micropipe is a primary defect degrading electronic devise performance and thought to be inevitable using PVT technique. Thus it is very important to establish new growth method of the low defect SiC bulk crystal. Solution growth technique is a promising answer to this problem. Growth from liquid phase has expected to improve the quality of the crystals because the growth proceeds under thermal equilibrium. In the case of SiC, although the congruent melt cannot be obtained, SiC can be precipitated from Si-C based solutions[1,2,3]. Si solvent (self flux) has only a small C solubility at moderate temperature, which reduces the SiC growth rate two orders of magnitude lower than that of PVT technique. On the other hand, ternary Si-C-X solution, where X is a carefully selected additive metal might exhibit a large C solubility at relatively low temperature. However lack of well-established ternary phase diagram, Si-C-X makes it difficult to select the desirable X element and to design the solution. In this study we report the solution growth of 6H-SiC from ternary Materials Science Forum Online: 2004-06-15 ISSN: 1662-9752, Vols. 457-460, pp 123-126 doi:10.4028/www.scientific.net/MSF.457-460.123

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.

On the origin of strain fluctuation in strained-Si grown on SiGe-on-insulator and SiGe virtual substrates

We investigated the microscopic strain fluctuation in strained-Si grown on SiGe-on-insulator (SGOI) and SiGe virtual substrates, and clarified the origins of the strain fluctuation in the strained-Si film. A periodic strain fluctuation, which reflects a cross-hatch pattern of the substrate, was observed in the sample on the virtual substrate. On the other hand, a featureless strain fluctuation with suppressed amplitude was observed in the sample on SGOI substrate. By analyzing the correlation of the Raman peak positions of the Si–Si modes in strained-Si and SiGe, the dominant mechanism of the strain fluctuation in the strained Si film was found to be the compositional fluctuation in underlying SiGe for the sample on SGOI, and the strain fluctuation reflecting the cross-hatch pattern for the sample on the virtual substrate, respectively.

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.