James Asirvatham, Juanita Saroj and Krier, Anthony (2015) Characterization of type-II GaSb quantum rings in GaAs solar cells. PhD thesis, Lancaster University.
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Abstract
The use of nanostructured materials in solar cells enables one to tune their absorption properties leading to a better match to the solar spectrum and subsequently an increased photocurrent through the solar cell. Type II GaSb/GaAs quantum rings (QRs) can significantly extend the spectral response beyond the visible out towards 1.4 µm giving a near optimum band gap for concentrator solar cell applications. Also, in type II band alignment the electrons are weakly localized and the built in electric field drifts the electrons across the depletion region easily. However, the introduction of GaSb QRs in GaAs solar cells degrades the open circuit voltage (Voc) and the incorporation of QRs needs to be optimized to minimize the Voc degradation while maximizing short circuit current density (Jsc) enhancement due to sub-bandgap absorption. The analysis of the photoresponse under the white light illumination has shown that some photogenerated minority holes from the base region can be re-captured by the QRs, which reduces the Jsc and the Voc. Hence, in this thesis, the carrier dynamics and extraction mechanisms occurring in the GaSb QRs is investigated by photoluminescence spectroscopy and current voltage characteristics. The characteristic S-shaped behaviour of the WL peak energy with increasing temperature indicates the prominent carrier trapping in the band tail states leading to potential fluctuations. Systematic measurements of dark current versus voltage characteristics are carried out from 100 to 290 K. Compared with the reference GaAs solar cell, the QRSC exhibits larger dark current, however its ideality factor n is similar at 290 K. QRs are directly probed by using an infrared laser (1064 nm) where the photon energy is conveniently chosen below the bandgap of the GaAs matrix. This enables to investigate the carrier dynamics and extraction mechanisms occurring in the GaSb QRs under a high light concentration. The dependence of the photocurrent on the laser intensity, the bias and the temperature is also discussed. The QR photocurrent exhibits a linear dependence on the excitation intensity over several decades. The thermal activation energy was found to be weakly dependent on the incident light level and increased by only a few meV over several orders of excitation intensity. The magnitude of the relative absorption in QRs when directly probed by using a 1064 nm laser with an incident power density of ~ 2.6 W cm−2 is found to be ~ 1.4 × 10−4 per layer. The thermal escape rate of the holes was calculated and found to be ~ 1011 to 1012 s −1 , which is much faster than the radiative recombination rate 109 s −1 . This behaviour is promising for concentrator solar cell development and has the potential to increase solar cell efficiency under a strong solar concentration. Experiments have shown that QDs embedded in the depletion region could generate both additional photocurrent and dark current. The electron-hole recombination in QDs is the reason for the additional dark current which reduces the open circuit voltage and keeps the conversion efficiency of QD solar cells below the ShockleyQueisser limit. Therefore, the reduction in open circuit voltage and the influence of the location of QR layers and their delta doping within the solar cell is investigated in this work. Devices with 5 layers of delta doped QRs placed in the intrinsic, n and p regions of a GaAs solar cell are experimentally investigated and the deduced values of Jsc, Voc, Fill factor (FF), efficiency (η) are compared. A trade-off is needed to minimize the Voc degradation while maximizing the short circuit current density (Jsc) enhancement due to sub-bandgap absorption. The voltage recovery is attributed to the removal of the QDs from the high field region which reduces SRH recombination. The devices with p or n doped QDs placed in the flat band potential (p or n region) show a recovery in Jsc and Voc compared to devices with delta doped QDs placed in the depletion region. However there is less photocurrent arising from the absorption of sub-band gap photons. Furthermore, the long wavelength photoresponse of the n doped QRs placed in the n region shows a slight improvement compared to the control cell. The approach of placing QRs in the n region of the solar cell instead of the depletion region is a possible route towards increasing the conversion efficiency of QR solar cells. The effect of the introduction of dopants on the morphology of GaSb/GaAs nanostructures is analyzed by HAADF-STEM. The results show the presence of welldeveloped GaSb QRs in both p-doped and n-doped heterostructures. However, in the undoped sample grown under the same conditions such well-developed QRs have not been observed. It is found that p-doping with Be stimulates the formation of QRs, whereas n-doping with Te results in the formation of GaSb nanocups. Therefore, the introduction of dopants in the growth of GaSb nanostructures has a significant effect on their morphology. Bias and temperature dependent EQE measurements are performed to understand the hole extraction from the QRs. In order to study the absorption strength of quantum dots and the various transition states, an approach to derive the below-bandgap absorption in GaSb/GaAs self-assembled quantum ring (QR) devices using room temperature external quantum efficiency measurement results is presented. The importance of incorporating an extended Urbach tail absorption in analyzing QR devices is demonstrated. The theoretically integrated absorbance via QR ground states is calculated as 1.04 ×1015 cm -1 s -1 , which is in a reasonable agreement with the experimental derived value 8.1 ×1015 cm-1 s -1 . The wetting layer and QR absorption contributions are separated from the tail absorption and their transition energies are calculated. Using these transition energies and the GaAs energy gap of 1.42 eV, the heavy hole confinement energies for the QRs (320 meV) and for the WL (120 meV) were estimated.