GaInAs/GaAs quantum wells for various top barrier thicknesses. A blue shift of the emis- .... [l] R. Dingle, W. Wiegmann, and C. Henry, Phys. Rev. Lett. 14, 827 (1974). ... [8] T. Saitoh, H. Iwadate, H. Hasegawa, Jpn. J. Appl. Phys. 30, 3750 (1991).
JOURNAL DE PHYSIQUE IV Colloque C5, supplkment au Journal de Physique 11, Volume 3, octobre 1993
Excitonic transitions in GaInAs/GaAs surface quantum wells J. DREYBRODT, A. FORCHEL and J.P. REITHMAIER
TechnischePhysik, University of Wunburg,Am Hubland, 97074 Wiinbutg, Germany
Abstract: We have studied the influence of the surface on the optical properties of GaInAs/GaAs quantum wells for various top barrier thicknesses. A blue shift of the emission lines up to about 25 meV combined with a line broadening is observed for a 5 nm thick surface quantum well (20% In) without any GaAs coverage. The line broadening as well as the energy shift depend strongly on the quantum well thickness. This can be modeled by assuming a 5 eV vacuum potential at the surface.
1. I n t r o d u c t i o n Excitonic transitions in quantum well structures have been investigated mainly in quantum wells (QWs) with thick barrier layers [I]. Therefore an influence of the surface of the samples on the exciton states is neglected. However recent studies have shown that even for QWs with top barrier thicknesses of several nanometers one must take into account surface effects [2][3]. We report on investigations of the optical properties of surface QWs without any GaAs coverage including a comparison to QWs with different top barrier thicknesses. We observe with decreasing top barrier thickness a significant shift of the emission line to higher energies combined with a line broadening. We will show that this behaviour can be quantitatively explained by calculations assuming a high vacuum potential a t the surface instead of the low band discontinuity of the semiconductor heterostructures. 2. E x p e r i m e n t
Sets of QWs with various top barrier thicknesses were prepared by two methods. 5 nm thick G~.801no.20As QWs with top barrier thicknesses ranging from 16 to 0 nm were grown by molecular beam epitaxy (MBE). To obtain a larger variation of the top barrier thickness we developed as second method a wet chemical etch process which enables us to remove the top barrier layer with about f1 nm accuracy [4]. The etch process was carried out on 5 and 15 nm GaInAs QWs with an Indium content of 20 and 13%, respectively. The samples were grown by MBE on 100 oriented undoped GaAs substrates a t growth temperatures of 580 O C for the GaAs buffer and 520 OC for the GaInAs and the top barrier layer. We used a highly diluted H2SO4:H2O2:Hz0etchant with a ratio of 1:10:6000 for the wet etch process. The samples Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1993552
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were halfside covered with PMMA resist and the etch step after etching was measured by a surface profiler. The etch rate was determined to 0.25 nm/s. Photoluminescence (PL) measurements were performed at a temperature of 5 K. The samples were excited by the 514 nm line of an Ar-laser. The average excitation density was about 10 W/cm2. A highly sensitive LN2 cooled CCD-chip mounted on a 25 cm monochromator was used for detection. 3. Photoluminescence Results
Figure 1 shows the PL spectra of the as grown samples (QW thickness 5 nm, 20% In) with top barrier thicknesses of 16, 3, 1 and 0 nm. We observe a distinct PL line shift to higher energies with decreasing top barrier thickness, beginning already for the sample with 3 nm thick top barrier layer. The blue shift is connected with a strong broadening of the emission lines. The PL spectrum at the bottom shows the emission of a surface QW. The optically active GaInAs layer is located at the surface (no GaAs coverage), i. e. the top barrier is formed by the transition from the semiconductor into the vacuum. In this case the low discontinuity in the semiconductor material (about 0.1 eV) is replaced by a large discontinuity of several eVs given by the electron affinity. We determine a blue shift of 25 meV and an increase of the PL line halfwidth of more than five times for the surface QW in comparison to the reference ? sample with 16 nm thick top barrier layer ? (upper spectrum). 0 w
The PL intensities change strongly with varying top barrier thicknesses. We observe a steep decrease of the intensity below about 10 nm top barrier thickness. The intensities for surface QWs are about three orders of magnitude lower compared to QWs with thick top barrier layers. Time resolved measurements have given a significant shorter exciton life time of 35 ps for the 5 nm surface QW in comparison to 450 ps for the reference sample [5]. Surface states [6] may be responsible for the reduction of life time. These midgap states lead to an enhanced nonradiative recombination of carriers at the surface resulting in weak PL signals measured for surface QWs.
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Energy (eV) Figure 1: 5K PL spectra from as grown 5 nm Gao soIno.zoAs/GaAs QWs with top barrier thicknesses of 16, 3, 1 and 0 nm.
4. Energy Shift
In figure 2 the energy shift of the P L lines of 5 nm (20% In) and 15 nm (13% In) QWs is plotted versus the top barrier thickness. The etched samples (solid dots), which were gained from the reference sample with 16 nm top barrier thickness, show within the etch accuracy a similar energy shift as the as grown samples (triangles). The blue shift for the 5 nm QWs begins at approximately 5 nm top barrier thickness and reaches 25 meV for the as grown surface QW. The larger energy shift of 30 meV for the etched surface QW may be caused by a slight QW layer removal.
The open circles represent data obtained by etching a 15 nm G%.s71n0.13A~QW with 20 nm top barrier thickness. In comparison to the above data no energy shift can be seen by reducing the top barrier thickness. Even for the surface QW only a weak blue shift is observed. Only if the well thickness itself is reduced (data points with negative top barrier thickness) we obtain a strong surface QW width related shift to higher energies. The top barrier thickness variation of the emission energy can be modeled for both QW thicknesses by assuming a rectangular QW with finite top barrier thickness and a 5 eV vacuum potential [7] due to the electron affinity at the surface. In the vacuum the free electron mass was assumed as effective mass. The solid lines in figure 2 represent calculations of the expected energy shift for QW and a 15 nm G%.871no.13AsQW. A comparison the el-hhl transition in a 5 nm G~.soIno.zoAs with the experimentally obtained data shows especially for the as grown samples, a good agreement and demonstrates the strong influence of the vacuum potential on the exciton ground state. The significant smaller energy shift of the sample with 15 nm thick QW is in agreement with theoretical expectations. The reason is a reduced penetration depth of the wave function into the barriers resulting in a lower sensitivity for the potential at the surface. Previous experiments by Moison et. al. [2] on GaAsJAlGaAs QWs yield a red shift instead of the blue shift reported here. This was explained by a coupling of QW states with surface states. Our observations imply a different situation for GaInAs QWs. The good agreement between experimental data and calculation indicates that surface states or ba.nd bending [8] have not a great influence on the eigenstates of the GaInAs surface QWs. 5. Emission Linewidth
Figure 3 displays the evolution of the PL line halfwidths (FWHM) versus the top barrier thickness. The data for the etch series of figure 2 are shown. We observe for both data sets an increase of the FWHM with decreasing top barrier thickness. The FWHM increase depends on the QW thick-
Top Barrier Thickness (nm) Figure 2: PL line energy shift versus top barrier thickness for etch series of a 5 nm Gao.soIno.zoAs QW (*) and a 15 nm Gao.871no.13AsQW ( 0 ) . The triangles represent the data for as grown samples. The solid lines show results of calculations assuming a finite GaAs top barrier layer thickness and a 5 eV vacuum potential at the surface.
Top Barrier Thickness
(nm)
Figure 3: Development of the PL halfwidths (FWHM) versus top barrier thickness. Etch series of a 5 nm Gao.aoIno.zoAs QW ( 0 ) and a 15 nm Gao.sdno.~sAsQW ( 0 ) .
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ness. The FWHM for the 5 nm surface QW ( 0 ) increases more than four times, while the FWHM for the 15 nm surface QW (0) increases only two times compared to the unetched samples. In general the FWHM of excitonic emission in narrow QWs is determined by the interface roughness. However, for samples with thin top barrier layers one must also take into account the surface roughness. Similar to the well known increase of the FWHM with decreasing QW thickness, the influence of the surface roughness increases for thin top barrier layers in comparison to thick ones. From the FWHM variation as function of the top barrier thickness we can estimate for the 5 nm QW a surface roughness of f1.2 nm and an interface roughness of f0.3 nm. For the 15 nm QW the FWHM variation indicates a slightly larger value of the surface roughness (f1.5 nm). 6. Summary
Our investigations have shown that the surface strongly influences the exciton states in GaInAs/GaAs QWs with top barrier layers below 10 nm thickness. We observe a distinct PL line shift to higher energies with decreasing top barrier thickness. The line shift scales with the QW thickness and reaches 25 meV for a 5 nm thick G%.801no.20Assurface QW and po significant shift for a 15 nm G%,871no.laAs surface QW without any GaAs coverage. The blue shift can be modeled with good accuracy by assuming a finite top barrier layer with a high 5 eV vacuum potential at the surface. In addition to the blue shift we observe a broadening of the emission lines also as a function of the QW thickness. The increase of FWHM for the 5 nm thick QW is significantly stronger than for the 15 nm QW. This is attributed to a different sensitivity for the surface roughness.
The financial support of this work by the Deutsche Forschungsgemeinschaft is gratefully acknowledged, and we would like to thank F. Kieseling for the time resolved measurements.
References: [l]R. Dingle, W. Wiegmann, and C. Henry, Phys. Rev. Lett. 14, 827 (1974). [2] J.M. Moison, K. Elcess, F. Houzay, J.Y. Marzin, J.M. Gerard, F. Barthe, M. Bensoussan, Phys. Rev. B 41, 12945 (1990). [3] S. Fafard, E. Fortin, Phys. Rev. B 45, 13769 (1992). 141 J. Dreybrodt, F. Faller, A. Forchel, J.P. Reithmaier, accepted for publication in J. of Materials Science and Engineering B, (1993). [5] J.A. Appelbaum, G.A. Barraff, and D.R. Hamann, Phys. Rev. B 14, 1623 (1976). 161 E. Yablonovitch, B.J. Skromme, R. Bhat, J.P. Harbison, T.J. Gmitter, Appl. Phys. Lett. 54, 555 (1989). [7] F. Kieseling, J. Dreybrodt, J.P. Reithmaier, A. Forchel, to be published. [8] T. Saitoh, H. Iwadate, H. Hasegawa, Jpn. J. Appl. Phys. 30, 3750 (1991).
