New Quantum Photoconductivity and Large Photocurrent Gain by Effective-Mass Filtering in a Forward-Biased Superlatticep−nJunction

Abstract

We report a new quantum-type photoconductivity in a forward-biased p + -n junction with a superlattice in the n layer. This novel phenomenon is characterized by several striking features: a high photocurrent gain ( -7 x 10 3 ), accompanied by a blue shift in the spectral response and a reversal in the direction of the photocurrent, when the forward bias exceeds the built-in potential. Photoconductive gain is caused by the large difference in the tunneling rates of electrons and holes through the superlattice layers, due to their large mass difference (effective-mass filtering). This is the first time that photoconductive gain is observed in a p-n junction.PACS 73.40.Lq Photoconductivity is a widespread phenomenon in semiconductors. If the lifetime of the photogenerated carriers exceeds their transit time, photoconductivity is accompanied by current gain. 1 The gain and associated gain-bandwidth product are therefore controlled by bulk properties such as carrier mobilities and lifetimes. In this Letter we report a new photoconductivity of quantum mechanical origin, accompanied by a very large photocurrent amplification, in a forward-biased superlattice p-n junction.The heterostructure, grown by molecular-beam epitaxy, consists of a p + -n junction grown on a 1-^m-thick n + ( = 2x 10 18 cm" 3 ) Gao.47Ino.53As buffer layer, lattice matched to a (100) n + InP substrate. The undoped n layer (n = 1 x 10 15 -1 x 10 16 cm" 3 ) comprises 100 23-A-thick Al 0 48 In 0 .52As barriers alternated with 100 49-A-thick Gao.47Ino.53As wells. Transmissionelectron-microscopy studies indicate that the interfaces are abrupt to better than two monolayers. The top p + region is 1 /xm thick and doped to -2xl0 18 cm" 3 . The mesa diodes (area = 1.3xl0~4 cm 2 ) had excellent forward and reverse I-V characteristics. From capacitance-voltage measurements the carrier concentration (rt-type) in the superlattice region was found to decrease monotonically from 2xl0 16 /cm 3 (at a distance of 0.2 /xm from the p + -n heterointerface) to 4 xl0 14 /cm 3 near the interface with the n + layer. The built-in potential determined from both C-Kand I-V measurements is 0.65 V. The optical power incident on the detector was kept low ( -1 nW); the shortcircuit photocurrent was measured by a 181 PAR current-sensitive preamplifier followed by a 5604 PAR lockin. Figure 1 shows the responsivity (photocurrent/ divided by incident optical power) at 70 K as a function of forward bias voltage at a wavelength of 1.2 /xm and a chopping frequency of 1 kHz. The responsivity exhibits a relatively flat portion at low bias and then a rapid (roughly exponential) falloff at voltages greater than 0.4 V. This represents the standard behavior of the photocollection efficiency in forward-biased p-n junctions. 2 The responsivity reaches a minimum at a forward bias equal to the built-in potential (0.65 V). Above this voltage the responsivity takes off rapidly and increases by about five orders of magnitude within 0.2 V, indicating the presence of a current gain mechanism. The hi...

(Al,In,Ga)N‐based photodetectors. Some materials issues

“…Another gain mechanism is due to different escape times from quantum wells for electrons and holes. This photoconductive gain mechanism, also known as effective mass filtering, was already proposed by Capasso et al in GaAs/AlGaAs QW [73], but in the case of InGaN QW differences in carrier confinement dynamics can be further enhanced by localized states or just by the effect of polarization fields. A third mechanism found to enhance photodetection response in MQW PD is linked to tunnelling transport between quantum well states [74].…”

Observation of confined states in anIn0.53Ga0.47As/In0.52Al0.48Asmulti-quantum wells structure by quantum confined Stark effects

“…The photocurrent response exhibits a relatively flat portion at higher reverse-bias and then rapid fall off at −3 V. The photocurrent response reaches roughly 0 at reverse bias equal to 0 V because the top layer and the substrate are shorted with a low-impedance current pre-amplifier for photocurrent measurement. The built-in potential may be cancelled out with applied bias voltage [5].…”

Effects of carrier-storage layer of InAs/GaAs short period superlattice on photocurrent in GaAs/GaAlAs multi-quantum well structure

“…On the other hand, the holes' contribution is negligible in many cases, because of the large effective mass which keeps them much more localized than the electrons ͑effective-mass filtering͒. [5][6][7][8] Quantum photoconductivity utilizing effectivemass filtering has been realized in a forward-biased p-n junction with a superlattice in the n layer. 5 In these previous works, it is explained that the accumulation of photogenerated holes in the quantum wells does not occur, and electricfield distortion in the superlattice is not considered, because the holes recombine with electrons leaking from the negative contact in order to sustain charge neutrality.…”

“…Several studies have been made to clarify the origin of the optoelectronic nonlinearity. [4][5][6][7][8][9] Stark localization in quantum wells performs the important roles in many cases. The transport of photogenerated electrons mainly contributes to the photocurrent.…”

“…[5][6][7][8] Quantum photoconductivity utilizing effectivemass filtering has been realized in a forward-biased p-n junction with a superlattice in the n layer. 5 In these previous works, it is explained that the accumulation of photogenerated holes in the quantum wells does not occur, and electricfield distortion in the superlattice is not considered, because the holes recombine with electrons leaking from the negative contact in order to sustain charge neutrality. 8 It is reported that this leaking of electrons is necessary to explain the experimental results, even though barriers are thick.…”