M.C. Wagener

Enhanced infrared photo-response from GaSb/GaAs quantum ring solar cells

GaAs-based solar cells containing stacked layers of nanostructured type II GaSb quantum ring solar cells are reported which show significantly enhanced infrared photo-response extending out to 1400 nm. The ring formation reduces the net strain energy associated with the large lattice mismatch making it possible to stack multi-layers without the need for strain balancing. The (1 sun) short-circuit current for a 10 layer sample is enhanced by ∼6% compared to a GaAs control cell. The corresponding open-circuit voltage of 0.6 V is close to the theoretical maximum expected from such structures.

Simulation of the enhanced infrared photoresponse of type-II GaSb/GaAs quantum ring solar cells

The extended photo-response of solar cells containing ten periods of GaSb/GaAs quantum rings imbedded in the p-i-n junction has been described using a single-band representation of the type-II quantum ring structure. By fitting the experimental data, the authors were able to deduce that the quantum rings are well represented by a Gaussian height distribution and a large valence band discontinuity. The simulated band of states is shown to be well matched to the photoluminescence analysis of the structure, with the inhomogeneous size distribution resulting in a band of hole states roughly 390 meV above the valence band.

Evaluation of the two-photon absorption characteristics of GaSb/GaAs quantum rings

The optical parameters describing the sub-bandgap response of GaSb/GaAs quantum rings solar cells have been obtained from photocurrent measurements using a modulated pseudo-monochromatic light source in combination with a second, continuous photo-filling source. By controlling the charge state of the quantum rings, the photoemission cross-sections describing the two-photon sub-bandgap transitions could be determined independently. Temperature dependent photo-response measurements also revealed that the barrier for thermal hole emission from the quantum rings is significantly below the quantum ring localisation energy. The temperature dependence of the sub-bandgap photo-response of the solar cell is also described in terms of the photo- and thermal-emission characteristics of the quantum rings.

Thermoelectric evaluation of the dopant density of p-type InAs

Thermoelectric measurements were performed on p-type InAs thin films grown by metalorganic vapor phase epitaxy. The measured Seebeck coefficient displayed an anomalous temperature dependence due to the existence of a highly conductive surface inversion layer. The effect of a degenerate conduction layer has been incorporated into our analysis and revealed that the sign-reversal temperature of the Seebeck coefficient remained unaffected by the surface layer. This finding consequently facilitated the direct determination of the acceptor density of lightly doped thin film InAs.

Carrier extraction behaviour in type II GaSb/GaAs quantum ring solar cells

The introduction of quantum dot (QD) or quantum ring (QR) nanostructures into GaAs single-junction solar cells has shown enhanced photo-response above the GaAs absorption edge, because of sub-bandgap photon absorption. However, to further improve solar cell performance a better understanding of the mechanisms of photogenerated carrier extraction from QDs and QRs is needed. In this work we have used a direct excitation technique to study type II GaSb/GaAs quantum ring solar cells using a 1064 nm infrared laser, which enables us to excite electron–hole pairs directly within the GaSb QRs without exciting the GaAs host material. Temperature and laser intensity dependence of the current–voltage characteristics revealed that the thermionic emission process produced the dominant contribution to the photocurrent and accounts for 98.9% of total photocurrent at 0 V and 300 K. Although the tunnelling process gives only a low contribution to the photocurrent, an enhancement of the tunnelling current was clearly observed when an external electric field was applied.

Photocapacitance study of type-II GaSb/GaAs quantum ring solar cells

In this study, the density of states associated with the localization of holes in GaSb/GaAs quantum rings are determined by the energy selective charging of the quantum ring distribution. The authors show, using conventional photocapacitance measurements, that the excess charge accumulated within the type-II nanostructures increases with increasing excitation energies for photon energies above 0.9 eV. Optical excitation between the localized hole states and the conduction band is therefore not limited to the Γ(k = 0) point, with pseudo-monochromatic light charging all states lying within the photon energy selected. The energy distribution of the quantum ring states could consequently be accurately related from the excitation dependence of the integrated photocapacitance. The resulting band of localized hole states is shown to be well described by a narrow distribution centered 407 meV above the GaAs valence band maximum.

Electrical characteristics of cadmium doped InAs grown by metalorganic vapor phase epitaxy

The use of dimethylcadmium as a potential p-type dopant source during the growth of InAs by metalorganic vapor phase epitaxy has been investigated. Thermoelectric measurements were used to accurately determine the doping density of the epilayers. A linear doping efficiency was attained for low DMCd mole fractions within the 550 °C to 650 °C growth temperature range with the doping density spanning from low 1016 to a maximum of 6 × 1017 cm−3. The surfactant action of a small mole fraction of Sb increased the maximum p-type doping density by a factor of two.

Type II GaSb/GaAs Quantum Rings with Extended Photoresponse for Efficient Solar Cells

The introduction of GaSb quantum dots (QDs) within a GaAs single junction solar cell is attracting increasing interest as a means of absorbing long wavelength photons to extend the photoresponse and increase the short-circuit current. The band alignment in this system is type-II, such that holes are localized within the GaSb QDs but there is no electron confinement. Compared to InAs QDs this produces a red-shift of the photoresponse which could increase the short-circuit current and improve carrier extraction. GaSb nanostructures grown by molecular beam epitaxy (MBE) tend to preferentially form quantum rings (QRs) which are less strained and contain fewer defects than the GaSb QDs, which means that they are more suitable for dense stacking in the active region of a solar cell to reduce the accumulation of internal strain and enhance light absorption. Here, we report the growth and fabrication of GaAs based p-i-n solar cells containing ten layers of GaSb QRs. They show extended long wavelength photoresponse into the near-IR up to 1400 nm and enhanced short-circuit current compared to the GaAs control cell due to absorption of low energy photons. Although enhancement of the short-circuit current was observed, the thermionic emission of holes was found to be insufficient for ideal operation at room temperature.

Non-linear injection level dependence of the excess carrier lifetime of p-type GaSb thin films: A two-layer model

The injection dependence of the excess carrier lifetime has been investigated for undoped and zinc doped p-type gallium antimonide thin films using steady-state photoconductivity measurements. Discrepancies between the observed lifetimes and the theoretical values are resolved using a two-layer generation/recombination model. Simulation of the room temperature experimental results yields values for the surface band bending, an upper limit to the surface recombination velocity and a lower limit to the Shockley-Read-Hall carrier lifetime.