Polarization-sensitive Schottky photodiodes based on a-plane ZnO/ZnMgO multiple quantum-wells

Abstract

Light polarization-sensitive UV photodetectors (PSPDs) using non-polar a-plane ZnMgO/ZnO multiple quantum wells grown both on sapphire and ZnO substrates have been demonstrated. For the PSPDs grown on sapphire with anisotropic biaxial in-plain strain, the responsivity absorption edge shifts by ΔE ∼ 21 meV between light polarized perpendicular (⊥) and parallel (||) to the c-axis, and the maximum responsivity (R) contrast is (R⊥/R||)max ∼ 6. For the PSPDs grown on ZnO, with strain-free quantum wells, ΔE ∼ 40 meV and (R⊥/R||)max ∼ 5. These light polarization sensitivities have been explained in terms of the excitonic transitions between the conduction and the three valence bands.

Light polarization sensitive photodetectors with m- and r-plane homoepitaxial ZnO/ZnMgO quantum wells

“…In addition, the absorption edge is very sharp, the entire transition from transparency to absorption happens in only $115 meV, i.e., the Urbach tail is very small. The best reported ZnO-based MQW photodetectors so far have been obtained with a-plane orientations grown on sapphire, 5 which showed similar responsivities at the same peak energy and bias, but with a transparency-absorption transition around twice as wide ($270 meV). In the case of the r-PSPDs, the responsivity is smaller, 2 mA/W, corresponding to a quantum efficiency below 1%, and the rejection ratio is $10 The smaller responsivity and rejection ratio with respect to the m-PSPD correlate with the lower PL intensity.…”

“…/R jj ) max $ 6). 5 In the case of the r-PSPDs, the main in-plane orthogonal crystallographic axes are [1 210] and [ 1011], and the responsivity reveals a shift of the absorption edge of DE $ 15 meV for linearly polarized light with E parallel to these two inplane directions (Fig. 4).…”

“…12 This sensitivity of the optical absorption coefficient to the polarization state of light can be used to develop light polarization sensitive photodetectors (PSPDs), 13 without the need of external optical elements, as it has already been demonstrated in a previous work using aplane ZnO/(Zn,Mg)O QWs. 5 However, the small differences in the absorption energy of the three excitonic transitions can only be observed if the Urbach tail is very low, i.e., if the crystal quality of the QWs is high. Thus, the use of high quality homoepitaxial layers is the natural path to fabricating PSPDs.…”

“…Among these devices, photodetectors with unipolar carrier transport (such as Schottky and metal-semiconductor-metal photodiodes) avoid the inherent difficulties with p-type doping in ZnO, 4 allowing access to UV photodetection with good figures of merit. [5][6][7][8] Moreover, ZnO has a ternary alloy, (Zn,Mg)O, which can allow tuning the photodetector responsivity to very low wavelengths, below 300 nm with wurtzite structure, 9,10 as well as increasing the exciton binding energy via the formation of ZnO/(Zn,Mg)O quantum wells (QW). However, the large spontaneous and piezoelectric coefficients in ZnO create large internal built-in electric fields, 11 which tend to spatially separate the electron and hole wave functions, resulting in a reduction of the overlap of the wave functions, hindering the benefits of quantum confinement.…”

“…In order to identify the excitonic transitions responsible for the observed sensitivity of the photocurrent to light polarization, we need to recall that the QWs are expected to be strain-free, but the axis orientations are different from those in the m-plane QWs. The oscillator strengths and energies can be obtained by using the values from the excitonic transitions in a-plane strain-free ZnO QWs 5 and calculating their projections along the new set of coordinate axes in the r-plane (Table II). For polarized light parallel to [1 210], both the A and B excitonic transitions are allowed with equal probability.…”