A virtual substrate consisting of a Ge layer grown directly on Si without an intervening SiGe graded layer is characterized. The nominally 100% Ge overlayer is fully relaxed and contains a small amount (3%) of unintentional Si. A dislocation density of 10 8 cm −2 is estimated for the virtual substrate prior to GaAs epitaxial growth, which is reduced by a factor of 100 after the growth of GaAs. On this novel virtual substrate 1 cm 2 single-junction GaAs photovoltaic cells were realized with an efficiency of 11.7% under AM0 compared with 20.2% for cells grown on a crystalline Ge substrate. Due to the high dislocation density a 50-fold higher dark current is measured in the virtual substrate cells compared to the crystalline Ge cells, leading to a lower short circuit current and open-circuit voltage of the cells fabricated on the virtual substrates. The post-GaAs growth dislocation density is estimated as 1 × 10 7 cm −2 in the base region and 4 × 10 5 cm −2 in the emitter region based on modelling and measurements.
This article analyzes and explains the observed temperature dependence of the forward dark current of lattice matched In0.53Ga0.47As on InP diodes as a function of voltage. The experimental results show, at high temperatures, the characteristic current-voltage (I–V) curve corresponding to leakage, recombination, and diffusion currents, but at low temperatures an additional region is seen at high fields. We show that the onset of this region commences with high injection into the lower-doped base region. The high injection is shown by using simulation software to substantially alter the minority carrier concentration profile in the base, emitter and consequently the quasi-Fermi levels (QFL) at the base/window and the window/cap heterojunctions. We show that this QFL splitting and the associated electron “pile-up” (accumulation) at the window/emitter heterojunction leads to the observed pseudo-n=2 region of the current-voltage curve. We confirm this phenomenon by investigating the I–V–T characteristics of diodes with an InGaAsP quaternary layer (Eg=1 eV) inserted between the InP window (Eg=1.35 eV) and the InGaAs emitter (Eg=0.72 eV) where it serves to reduce the barrier to injected electrons, thereby reducing the “pile-up.” We show, in this case that the high injection occurs at a higher voltage and lower temperature than for the ternary device, thereby confirming the role of the “accumulation” in the change of the I–V characteristics from n=1 to pseudo-n=2 in the ternary latticed matched device. This is an important phenomenon for consideration in thermophotovoltaic applications. We, also show that the activation energy at medium and high voltages corresponds to the InP/InGaAs conduction band offset at the window/emitter heterointerface.
Access to the full text of the published version may require a subscription. Quantum well cells ͑QWCs͒ for thermophotovoltaic ͑TPV͒ applications are demonstrated in the InGaAsP material system lattice matched to the InP substrate and strain-compensated InGaAs/ InGaAs QWCs also on InP substrates. We show that lattice-matched InGaAsP QWCs are very well suited for TPV applications such as with erbia selective emitters. QWCs with the same effective band gap as a bulk control cell show a better voltage performance in both wide and erbialike emission. We demonstrate a QWC with enhanced efficiency in a narrow-band spectrum compared to a bulk heterostructure control cell with the same absorption edge. A major advantage of QWCs is that the band gap can be engineered by changing the well thickness and varying the composition to the illuminating spectrum. This is relatively straightforward in the lattice-matched InGaAsP system. This approach can be extended to longer wavelengths by using strain-compensation techniques, achieving band gaps as low as 0.62 eV that cannot be achieved with lattice-matched bulk material. We show that strain-compensated QWCs have voltage performances that are at least as good as, if not better than, expected from bulk control cells.
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