Photomodulated reflectivity of Zn_1-xMn_xTe/ZnTe multiple-quantum wells with below-bandgap excitation

Abstract

A series of Zno.93Mno.07Te/ZnTe multiple-quantum well (MQW) structures of different quantum well widths has been studied at 77 K using the conventional modulation technique of photoreflectivity (PR) with above-bandgap light providing the modulation as well as a novel version of the technique which employs below-bandgap excitation. Photoluminescence (PL) spectra of the structures are also presented. The appearance of the PR spectra obtained with below-bandgap excitation differs from that of the PR spectra with above-bandgap excitation, but fitting the spectra with appropriate lineshapes gives the same spectroscopic information about the energy positions of the excitonic transitions in the well and the bandgaps of the ZnTe buffer layer and the Zno.93Mno.07Te barriers. The fits to the PR spectra yield energy positions of the lowest excitonic transitions which are consistent with the PL data. From the transition energies calculated with a one-dimensional transfer matrix model in which strain effects are taken into account, we conclude that the structures are to a good approximation strained to the ZnTe buffer layers.of Engineering Design and i"ianuiaciure, Universi?r of iiuii, i . iniroauciionThe multiple-quantum well (MQW) structures investigated here are formed from the Znl-,Mn,Te system in which the excitonic bandgap, Eg, at 4.2 K is given approximately by the following expression: Eg = 2.381 + 0 . 6 8 ~ in electronvolts (Twardowski 1983, Twardowski er a1 1984, Lee er al 1986, Mertins et al 1993) though the dependence of Eg on x does show some nonlinearity (Mertins er a1 1993). This system thus offers the possibility of producing wideblue-green spectral region). The system is also of interest because as-grown Znl-,Mn,Te is normally p-type; ZnTe is

Comparison of multiple-quantum wells and quantum dots by below-bandgap photomodulated reflectivity

“…In the BPR technique, the reflectivity of the sample is modulated by laser light whose energy is below the band gap of the semiconductor material; BPR was first reported as a new technique by Bhimnathwala and Borrego [38], although it was already applied before by Rockwell et al [39]. We have established that BPR spectra contain the same spectroscopic information as conventional photomodulated reflectivity spectra [40]. We have tried to gain some insight into the modulation mechanism of BPR and have shown that it is likely to result from the excitation of deep levels in the semiconductor material [41].…”

“…However, the quantum dots which we have prepared are still of sizes where additional confinement effects are negligible [23], which justifies the use of the same lineshapes in the fits of the spectra for the quantum dot and the MQW samples. The choice of the lineshapes is described in great detail elsewhere [40] and is briefly summarized in the following. The change dε t for allowed excitonic transitions in the quantum well can be described by the first derivative of a complex Lorentzian lineshape with respect to transition energy E g…”

“…It will be seen below that this uncertainty in the exciton binding energy does not affect the interpretation of the experimental results. The parameters in the calculation are those given earlier [40] with some exceptions. We use a chemical valence band offset (VBO) of 30%, which was determined via magnetic field experiments [50] on other pieces of the same set of MQW samples and by assuming the ratio of the absolute deformation potentials a c :(−a v ) to be 90:10, as calculated previously [51].…”

Magneto-photoluminescence studies of Zn1−xMnxTe/ZnTe multiple quantum-well and quantum-dot structures

“…Each heterostructure consisted of 10 ZnTe quantum wells of thickness 80Å (sample number H561) and 100Å (H560) embedded between Zn 1−x Mn x Te barrier layers of x = 7.6 % and thickness 150Å. The samples were nominally undoped [1]. The dot samples were prepared from pieces of the two Zn 1−x Mn x Te/ZnTe MQWs by electron beam lithography for pattern definition followed by Ar 2+ ion beam etching for transferring the pattern, giving a 3.2 × 3.2 mm 2 area with a high density of homogeneous tapering pillars of height ≈ 300 nm (etching stopped before the substrate was reached) and diameter of ≈ 200 nm [4].…”

Contactless electroreflectance spectroscopy of optical transitions in low dimensional semiconductor structures

“…Photon energy of the pump beam is usually larger than the band gap of the semiconductor being under study. However, there is a possibility to use a below band gap modulation through the excitation of impurity or sur− face states [63,64]. In both cases photo−generated carriers can be trapped by surface states and in this way the surface band bending is modulated [1][2][3][4].…”