During Ga deposition, Si cell is see more opened in order to dope the nanostructures with Si equivalent
to 1×1018 cm−3. The Ga droplets are then irradiated with As4 flux and crystallized into GaAs quantum rings at the same temperature. After quantum ring formation, a thin Al0.33Ga0.67As cap layer (10 nm) is deposited over the quantum ring at 400°C. Subsequently, the substrate temperature is raised to 600°C for the deposition of another 20 nm Al0.33Ga0.67As. The GaAs/Al0.33Ga0.67As structure is repeated six times to form the stacked multiple quantum ring structures. After the growth of multiple quantum rings, an emitter layer of 150 nm n-type GaAs with Si doped to 1×1018 cm−3 is grown. Finally, the solar cell structure is finished Selleck Vorinostat by a 50-nm highly Si-doped GaAs
contact layer. In order to make a fair comparison in terms of effective bandgap, a quantum well solar cell used as a reference cell is fabricated with the same growth procedures, FK228 concentration except for the quantum well region. The multiple quantum wells with GaAs coverage of 10 ML are grown, instead of the fabrication of quantum rings using droplet epitaxy. An uncapped GaAs quantum ring sample is also grown using the same procedures for atomic force microscopy (AFM) measurement. The high-resolution X-ray diffraction reciprocal space mapping (RSM) of the strain-free solar cell sample was analyzed by an X-ray diffractometer (Philips X’pert, PANalytical B.V., Almelo, The Netherlands). Rapid thermal annealing is performed on four samples in N2 ambient in the temperature range of 700°C to 850°C for 2 min. see more The samples are sandwiched in bare GaAs wafers to prevent GaAs decomposition during high-temperature annealing. The solar cells are fabricated by standard photolithography processing. An electron beam evaporator is used to deposit Au0.88Ge0.12/Ni/Au and Au0.9Zn0.1 n-type and p-type contacts, respectively. Life-off is used to create the top grid after metal deposition. Continuous wave photoluminescence (PL) measurements are performed using
the 532-nm excitation from an Nd:YAG laser with a spot diameter at the sample of 20 μm at 10 K. Two excitation power intensities of the laser are used: I L = 0.3 W/cm2 and I H = 3,000 W/cm2. The J-V curves of solar cells are measured under an AM 1.5G solar simulator. Results and discussion The surface morphology of the uncapped GaAs/Al0.33Ga0.67As quantum ring sample is imaged by an AFM, as shown in Figure 1. The image shows quantum ring structures with a density of approximately 2.4×109 cm−2. The inset AFM image shows double quantum rings. Figure 1 also shows the results obtained for 2D-RSM around the asymmetric 022 reciprocal lattice point (RSM 022 reflection). Strain-free quantum ring solar cell is evidenced by the RSM patterns. Figure 1 AFM images of surface (left) and reciprocal space map of GaAs/Al 0.33 Ga 0.