Control of interface abruptness of polar MgZnO/ZnO quantum wells ...

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Jan Zippel, Jörg Lenzner, Michael Lorenz, and Marius Grundmann. Institut für Experimentelle Physik II, Universität Leipzig, Linnèstr. 5, 04103 Leipzig, Germany.
APPLIED PHYSICS LETTERS 97, 052101 共2010兲

Control of interface abruptness of polar MgZnO/ZnO quantum wells grown by pulsed laser deposition Matthias Brandt,a兲 Martin Lange, Marko Stölzel, Alexander Müller, Gabriele Benndorf, Jan Zippel, Jörg Lenzner, Michael Lorenz, and Marius Grundmann Institut für Experimentelle Physik II, Universität Leipzig, Linnèstr. 5, 04103 Leipzig, Germany

共Received 10 June 2010; accepted 14 July 2010; published online 2 August 2010兲 A strong quantum confined Stark effect 共QCSE兲 was observed in wedge shaped MgZnO/ZnO quantum wells 共QWs兲 grown by pulsed laser deposition. A reduced laser fluence of 1.8 J / cm2 was used. Reference samples grown at higher standard fluence 2.4 J / cm2 showed only a negligible QCSE. Using off-axis deposition without substrate rotation, a constant composition of the barriers was maintained while varying the well width in a wedge shaped QW. A redshift of the QW luminescence with increasing QW thickness up to 230 meV below the ZnO emission was found, accompanied by an increase in the exciton lifetime from 0.3 ns up to 4.2 ␮s. © 2010 American Institute of Physics. 关doi:10.1063/1.3475402兴 ZnO is a transparent semiconducting oxide with very promising properties for applications in optoelectronics and transparent electronics. An increase in the internal quantum efficiency of light emitting devices can be achieved by the formation of quantum well 共QW兲 structures. Depending on the desired emission energy, heterostructures of ZnO and MgxZn1−xO 共Refs. 1–7兲 or ZnCdO 共Refs. 8 and 9兲 are reported. We focus on polar MgxZn1−xO / ZnO QWs in this paper. Parallel to their crystallographic c-axis, both ZnO and MgxZn1−xO possess a spontaneous polarization, which is a function of the Mg content x. Upon the formation of a heterostructure, an additional piezoelectric polarization component is introduced. The difference in the electric polarization leads to an electric field in the c-direction of the QW, giving rise to the quantum confined Stark effect 共QCSE兲. The electric field separates the electrons and holes present in the QW after excitation, reducing the overlap of their respective wave functions. Therefore an increase in the luminescence decay time and a decrease in the transition energy with increasing well width is observed.1,2 A reduction in the redshift of the QW luminescence is expected with increasing excitation intensity, due to partial screening of the electric field by the excited carriers.2 No reports for a pronounced QCSE in ZnO QW samples grown by pulsed laser deposition 共PLD兲 have emerged yet, despite the comparatively large QW thicknesses attained.4,10–12 In contrast, a strong QCSE was observed in samples grown by metal-organic chemical-vapor deposition,5 molecular beam epitaxy 共MBE兲,2,6,13–15 and a weak QCSE in samples grown by Laser-MBE.1,3,7 Given these results, we suggest that the reason for a diminished QCSE in previous PLD grown samples is the high kinetic energy of impinging plasma particles in the PLD process. It was shown by Davis et al.14,15 that the QCSE can be diminished by ion implantation of MBE grown MgxZn1−xO / ZnO QWs. The energy of fast ionic species in the PLD plasma can be as high as hundreds of electronvolts.16,17 The topmost layers of a growing film are therefore subject to continuous ion erosion. An intermixing of adjacent layers will occur, softening the steps in the ena兲

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ergy landscape at the QW interfaces. This paper shows that the QCSE depends extremely sensitive on the interface abruptness of the QW structure. All films investigated in this study were grown by PLD.18 The layers were deposited on a-plane sapphire substrates by ablation of sintered ceramic pellet targets with the 248 nm line of a KrF laser, at a reduced laser fluence of 1.8 J / cm2, in contrast to the standard laser fluence of 2.4 J / cm2 used in previous studies.4 All samples were grown at 650 ° C in an oxygen partial pressure of 4 ⫻ 10−3 mbar. A ZnO buffer layer was introduced before the growth of the lower barrier for barriers with a Mg content x ⬎ 0.2. The films were investigated by photoluminescence 共PL兲 spectroscopy, time-resolved PL 共TR-PL兲 spectroscopy, and cathodoluminescence 共CL兲 spectroscopy. The PL was excited by the 325 nm line of a cw HeCd laser. The TR-PL was resonantly excited by the 200 fs pulses of a frequency doubled Ti:sapphire laser and detected with a time resolution of 20 ps. In order to ensure an identical composition of the MgxZn1−xO barrier for different QW thicknesses, the thickness of the QW layer was varied on a single 1 ⫻ 1 cm2 sample. Thereto, the substrate was moved off the axis of the target rotation, which is parallel to the propagation direction of the plasma plume. The density of the plasma then differs between the two edges of the substrate, resulting in a graded film thickness. The resulting thickness gradient is small enough that within the excitation spot of the PL experiment of 100 ␮m diameter a maximal thickness variation of one monolayer occurs, even for the thickest QWs investigated. The thickness of the whole stack was assessed for selected samples by scanning electron microscopy on cross sections prepared by in situ ion milling in a focused ion beam microscope. Cross sections were prepared near both edges of the sample, and the thickness in between was assumed to show a constant gradient. The thickness of the QW was extracted using the number of pulses applied to the target for the individual layer. Luminescence investigations were carried out on samples with different x = 0.15– 0.4. Two samples will be discussed in detail, one with x = 0.2 and one with x = 0.32. The time integrated spectra depicted in Fig. 1 were obtained from a QW sandwiched between barriers with x = 0.2 and a thick-

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d=4.4 nm

MgZnO

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QW d=3.2 nm LO T = 2K LO

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ness between 3.2 and 4.4 nm. All spectra were normalized corresponding to the intensity of their QW luminescence. A clear redshift of the QW luminescence by up to 50 meV with increasing QW thickness is observed. An additional luminescence peak is observed for the high QW thicknesses at 3.379 eV. Its spectral position, however, is not a function of the QW thickness, which indicates that the origin of the luminescence feature is not the QW. The feature is therefore excluded from the discussion. A reference sample was grown using an increased laser fluence of 2.4 J / cm2 but otherwise equal growth conditions. The QW luminescence obtained from this reference sample 共not depicted here兲 did not show a redshift below the emission energy of bulk ZnO, proofing that the presence of the QCSE can indeed be controlled by the laser fluence. From Fig. 1 it is visible, that the intensity of the phonon replicas increases systematically with increasing QW thickness compared to the zero phonon luminescence. In order to evaluate the magnitude of the change in the field induced carrier separation due to high carrier densities in the QW, CL experiments were carried out with different excitation currents allowing much higher carrier density than with our PL setup. Therefore the current was varied between 800 and 8000 nA 共Fig. 2兲. The redshift of the QW luminescence decreased systematically with increasing excitation current by up to ⬇50 meV, while the emission from the ZnO buffer layer remains unchanged. The QW luminescence in-

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QW emission maximum (eV)

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Excitation current (nA) FIG. 2. 共Color online兲 Excitation dependence of the CL spectra of a sample showing a pronounced QCSE 共x = 0.32兲.

104

Average decay time (ns)

FIG. 1. 共Color online兲 PL spectra of a MgxZn1−xO / ZnO QW sample with a Mg content of x = 0.2 in the barrier normalized to the intensity of the QW emission. The QW emission is marked, as well as the first and second phonon replicas 共LO兲.

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Emission energy (eV) FIG. 3. 共Color online兲 共a兲 Time integrated PL intensity 共left scale兲 and decay time 共right scale兲 as a function of the luminescence energy, x = 0.2. 共b兲 Dependence of the decay time at the maximum of the QW luminescence on the luminescence energy for various samples 共x = 0.15– 0.4兲. Solid lines give the expected dependence from the calculations, indicating the minimal and maximal electric field strength in the QWs.

tensity grows by more than a factor of 20 共Fig. 2兲. We attribute this superlinear increase to an enlargement in the oscillator strength of the excitonic transition due to an enhanced overlap of electron and hole wave-functions. The dependence of the decay time on the emission energy is given in Fig. 3共a兲. The chosen time integrated spectrum contains essentially three parts, which are, from left to right: the phonon replica of the QW luminescence 共3.1–3.26 eV兲, the QW luminescence itself 共3.26–3.35 eV兲 and a luminescence of unknown origin 共3.35–3.45 eV兲. While the luminescence decay time is as low as ⬇200 ps for the luminescence of unknown origin, a systematic increase in the decay time is observed for the QW luminescence toward lower energies. A maximum in the decay time is reached at 3.26 eV. At lower energies the faster decay processes from the high energy flank of the first phonon replica dominate the luminescence. It is therefore concluded, that the QW luminescence itself does not consist of a single process with a single time constant but represents a dynamic relaxation of the carriers created by the excitation, also explaining the relative large FWHM of the QW emission. After the excitation, carriers decay radiatively and nonradiatively and diffuse in the plane of the QW. The overall density of carriers in the active region of the QW therefore decreases, and the luminescence redshifts, as the screening of the polarization induced sheet charges by the high carrier density is reduced. The dynamics of the carrier relaxation in polar MgxZn1−xO / ZnO QWs were summarized for example in Ref. 15. In this work, the radiative decay was characterized

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by an average decay time determined from the transients using a stretched exponential function.19 Figure 3共b兲 shows the average decay time as a function of the energy of the QW emission maximum for samples with different Mg content x = 0.15– 0.4. The electric field in the QW was estimated from numerical calculations. The wave functions of the uncorrelated electrons and holes were calculated using the Poisson solver BETA8C developed by Snider.20,21 The exciton binding energy and wave function was evaluated in the envelope function approximation by a variational approach.22,23 The electric polarization in the barrier entered the calculations as a free parameter. The electric field was calculated6 from the difference in the electric poជ = 共PB − PQW兲 / ␧0␧r. larization in the barrier and the QW E A lower and an upper limit of the electric field were determined from a comparison with the experimental data. The inverse oscillator strength was normalized to the decay time obtained for the lowest QW thickness. Here the influence of nonradiative recombination is the smallest, assuming a thickness independent nonradiative recombination rate. The experimental data are best described for field strength between 0.31 and 0.52 MV/cm. Taking into account the Mg content in the individual samples 共not shown here兲 a change in the polarization of x ⫻ 共0.009– 0.015兲 C / m2 was obtained, which is roughly 50% of the values given for example in Refs. 6 and 24. A slight intermixing might account for that, given that the energy of the impinging particles is still finite, even if the laser fluence is reduced. The results however indicate, that a wide range of field strengths can be chosen by an adequate laser fluence. Such a finding has high impact on the design of device structures, as it allows the control of internal electrical fields through control of interface abruptness. It may allow the optimized growth of efficient light emitters, even in the polar direction. We anticipate that these results will further promote the versatility of the PLD process, as graded heterojunctions can be reproducibly grown in this fashion. In summary, it was shown, that the QCSE in PLD grown MgxZn1−xO / ZnO QW is a sensitive tool to determine the interface abruptness controlled by the deposition conditions, namely, the laser fluence used for target ablation. This finding promises high potential for application in efficient light emitting devices based on MgxZn1−xO / ZnO QWs grown in the polar c-axis direction. A strong QCSE is present for samples grown at a reduced laser fluence of 1.8 J / cm2, in contrast to samples grown at a standard laser fluence of 2.4 J / cm2, where the QCSE is diminished. With increasing thickness the QW related PL influenced by the QCSE redshifts by up to 230 meV below the luminescence of bulk ZnO and the decay time increases up to 4.2 ␮s. Excitation dependent CL measurements show that the electric field can

be efficiently screened by the excited carriers, resulting in a reduction in the redshift of the luminescence maximum by up to 50 meV. This work has been supported by Deutsche Forschungsgemeinschaft in the framework of Sonderforschungsbereich SFB762 “Functionality of oxide interfaces.” M. Lange, M. Stölzel, and J. Zippel are supported in the framework of the graduate school BuildMoNa and by the European Social Fund 共ESF兲. 1

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