A new approach to quantum well infrared photodetectors: Staircase-like quantum well and barriers

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Abstract

We present a theoretical investigation of a novel staircase-like quantum well infrared photodetector (QWIP). The proposed structure makes use of quantum wells and barriers with increasing Al content both in the wells and in the barriers forming a staircase-like energy band diagram without applied bias. The detection wavelength is around λ = 12 μm at an applied electric field of F = 1.4 × 104 V/cm at room temperature. Device operation is based on inter-subband bound-to-bound transition. We have solved the energy band diagram of the structure self-consistently. We have also calculated the absorption coefficient, responsivity, total net quantum efficiency and dark current density at room temperature. The dark current density at the operating field was found to be around 10−2 A/cm2, which is lower than the conventional QWIPs in the literature.

Introduction

After the first quantum well infrared photodetector (QWIP) based on inter-subband absorption between two bound quantum well states was demonstrated by Levine et al. [1], there have been great progress made in QWIPs for infrared detection. Based on mature III–V materials technology such as GaAs/AlGaAs structures, QWIPs have potential advantages promising high sensitivity, low power, low cost, high performance focal plane arrays and monolithic integration with high speed electronic devices [2]. All these advantages make them desirable in applications such as advanced sensing and imaging systems. A large number of QWIP structures have, to date, been investigated. These include large inter-subband absorption due to bound-to-bound transition [1], inter-subband absorption due to bound-to-continuum transition [3], voltage tunable multicolor QWIPs [4], broad-band QWIPs [5].

In this paper, we present a theoretical study of a novel staircase infrared detector, which was studied as a light emitter by us [6], based on the GaAs/AlGaAs material system with varying Al concentrations in the quantum wells and barriers. The detection wavelength is around 12 μm, at room temperature. The optical transitions investigated are inter-subband bound-to-bound transitions. Since at zero bias, the device contains staircase-like quantum wells and barriers, dark current is dramatically lower compared to other QWIPs in the literature [7], [8], [9].

Section snippets

Device structure

The device structure, which we investigate, is an n+–i–n+ three-well system, consisting of Ga1−xAlxAs quantum wells and barriers where concentration, x, varies from 0 to 0.43. The exact composition and the thickness of all quantum wells and barriers as well as the doping concentrations are given in Table 1. The resulting device parameters including the potentials and bound state energies are given in Table 2. Here, znb (znw) shows the distance of the left hand side of the nth barrier (well)

Theoretical considerations

Potential energy of the system shown in Fig. 1 can be given asV=VG+VmaxwhereVG=n=13VGnVmax=VmaxS(z-z4b)where S is the step function andVGn=Vnb,znb<z<znwVnw,znw<z<zn+1b0,elsewhereHamiltonian of the system can be written asH=P22m+VSince the band bending of the structure can be omitted at steady state, eigenfunctions of the Hamiltonian in the wells and barriers are of the following form:Ψwell=Aneiknwz+Bne-iknwzΨbarrier=Cneknbz+Dne-knbzHere, the knw, wave numbers of wells and barriers, for the

Results and discussion

The calculated energy levels of the quantum wells and relevant parameters are given in Table 2. The potential energy profile of the structure and squared wave functions are shown in Fig. 1. The energy level differences between E1,fw and E2,iw, E2,fw and E3,iw are approximately 15 meV, at zero applied bias. The conduction band potential energy profile of the structure at k = 0 has also been calculated self-consistently by solving Poisson and Schrödinger equations, where effective mass approximation

Conclusions

A new version of the classical quantum cascade structure is proposed to be used as a photodetector operating in the long wavelength infrared range in the GaAs/AlGaAs materials system. The proposed structure makes use of quantum wells and barriers with increasing Al content both in the wells and in the barriers forming a staircase-like energy band diagram without applied bias. This results in very low dark current density, a major problem for most infrared detectors. Growth of the detector

Acknowledgements

We would like to thank The State Planning Organization of Turkey (DPT) for funding this project (Project No: 200190K.120330). S.U.E author thanks Bilkent University Physics Department for their hospitality.

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