Theoretical investigation of direct and phonon-assisted tunneling currents in InAlGaAs/InGaAs bulk and quantum-well interband tunnel junctions for multijunction solar cells

Direct and phonon-assisted tunneling currents in InAlGaAs-InGaAs bulk and double-quantum-well interband tunnel heterojunctions are simulated rigorously using the nonequilibrium Green's function formalism for coherent and dissipative quantum transport in combination with a simple two-band tight-binding model for the electronic structure. A realistic band profile and the associated built-in electrostatic field are obtained via self-consistent coupling of the transport formalism to Poisson's equation. The model reproduces experimentally observed features in the current-voltage characteristics of the devices, such as the pronounced current enhancement in the quantum-well junction as compared to the bulk junction and the structure appearing in the negative-differential resistance regime due to quantization of emitter states. Local maps of density of states and the current spectrum reveal the impact of quasibound states, electric fields, and electron-phonon scattering on the interband tunneling current. In this way, resonances appearing in the current through the double-quantum-well structure in the negative-differential resistance regime can be related to the alignment of subbands in the coupled quantum wells.

Figure 1

Schematic layer and material structure of the bulk and DQW tunnel-heterojunction devices from Ref. 3. The doping density amounts to $N_{d/a}=2\times {10}^{17}$ cm${}^{-3}$ for the lightly doped $n^{+}/p^{+}$ regions and $N_{d/a}={10}^{19}$ cm${}^{-3}$ for the heavy $n^{++}/p^{++}$ doping.

Figure 2

Band profile and local density of states at a vanishing transverse momentum and zero bias voltage, for (a) the bulk heterojunction device, exhibiting state quantization in the highly doped emitter regions adjacent to the junction, and (b) the DQW heterojunction device, where the strong band bending leads to the unbinding of the high-lying confined states. The equilibrium Fermi level is fixed at 1 eV.

Figure 3

Current-voltage characteristics of bulk and DQW heterojunction devices at forward bias. Due to the lower effective barrier potential, the peak current in the DQW device exceeds that of the bulk junction by more than two orders of magnitude and occurs at a much larger bias voltage. The structure in the negative-differential resistance regime exhibited by the bulk device can be ascribed to the quasi-2D state quantization in the emitter regions adjacent to the junction, as shown in Fig. 2. The corresponding features in the $I$-$V$ characteristics of the DQW heterojunction device are considerably stronger and are identified as the signatures of resonant tunneling between QW states.

Figure 4

Current-voltage characteristics, local density of states, and the corresponding local current spectrum of the bulk tunnel-heterojunction device at forward bias voltages of (a) 0.06 V, (b) 0.16 V, and (c) 0.26 V, illustrating different interband tunneling processes depending on the energetic alignment of localized and extended states in the injection regions adjacent to the junction.

Figure 5

Current-voltage characteristics, local density of states, and the corresponding local current spectrum of the DQW tunnel-heterojunction device at forward bias voltages of (a) 0.16 V, (b) 0.21 V, and (c) 0.31 V, at which electrons tunnel resonantly from the lowest QW subband at the $n$ side of the junction to the third, second, and first QW subband at the $p$ side, respectively.