Engineering the Bandgap of Unipolar HgCdTe-Based nBn Infrared Photodetectors

Abstract

Design of practically realizable unipolar HgCdTe nBn photodetectors has been studied in detail by numerical analysis. The simulations reported herein reveal that, by optimization of barrier doping, dark current levels can be reduced and collection efficiency substantially improved. It is shown that p-type doping of the barrier layer can significantly reduce the effective potential barrier arising from the valence band offset between the absorber and barrier regions, thus enabling HgCdTe nBn detector operation under near zero-bias conditions. However, relatively high electric fields in the space charge regions near the barrier/absorber interface result in enhanced trapassisted Shockley-Read-Hall thermal generation. Our calculations indicate that nBn HgCdTe detectors with barriers engineered by use of HgTe/ Hg 0.05 Cd 0.95 Te superlattices have, potentially, substantially better valence band alignment without the need for p-type doping.

Status of HgCdTe Barrier Infrared Detectors Grown by MOCVD in Military University of Technology

“…Non-zero valence band offset in HgCdTe nBn detector structures is the key item limiting their performance. [6][7][8][9][10][11] Devices exhibit poor responsivity and detectivity, especially at low temperatures, 6 where the low-energy minority carriers generated by optical absorption are not able to overcome the valence band energy barrier (DE V ) (see Fig. 1a).…”

Superlattice Barrier HgCdTe nBn Infrared Photodetectors: Validation of the Effective Mass Approximation

“…Several bandgap engineering approaches have been proposed to minimize E V to values below or approaching the thermal energy of minority carriers [10], [11], [13], [14]. These designs propose lowering E V at the barrier/n-type heterointerface by grading both the doping and composition across the interface formed employing HgCdTe alloys or, alternatively, propose the use of superlattice-based barrier structures.…”

“…These designs propose lowering E V at the barrier/n-type heterointerface by grading both the doping and composition across the interface formed employing HgCdTe alloys or, alternatively, propose the use of superlattice-based barrier structures. Of these two bandgap engineering approaches, the superlattice method is the most promising, since it does not demand the need for highly controlled graded p-type doping in the barrier layer, which can be problematic in HgCdTe [11]. It is noted, however, that superlattices have yet to be studied in detail as suitable barrier layers for nBn photodetector designs.…”

“…It is noted, however, that superlattices have yet to be studied in detail as suitable barrier layers for nBn photodetector designs. In their report, Kopytko et al [11] modeled superlattice-barrier HgCdTe nBn detectors employing a simple Kronig-Penny model, and the solution to the eigenvalue problem, in order to calculate the equilibrium energy levels in a CdTe-HgTe-CdTe quantum-well (QW) structure. The obtained energy levels were then used to approximate an equivalent effective-mass, bandgap, and band alignment, which were then employed in an effective-mass commercial solver to predict the dark current and photogenerated current of a superlattice-barrier HgCdTe detector [11].…”

“…In their report, Kopytko et al [11] modeled superlattice-barrier HgCdTe nBn detectors employing a simple Kronig-Penny model, and the solution to the eigenvalue problem, in order to calculate the equilibrium energy levels in a CdTe-HgTe-CdTe quantum-well (QW) structure. The obtained energy levels were then used to approximate an equivalent effective-mass, bandgap, and band alignment, which were then employed in an effective-mass commercial solver to predict the dark current and photogenerated current of a superlattice-barrier HgCdTe detector [11]. While this device modeling method is convenient and relatively easy to implement, it does not adequately capture the fundamental physical details associated with the superlattice structure, such as wavefunction overlap, density of states, nor the influence of layer doping on the resulting band diagram.…”