Wavelength tuning of exciton emissions has been achieved simply by inserting an InAs submonolayer at the centre of GaAs quantum wells during molecular beam epitaxy growth. Photoluminescence measurements show that the emission energy can be effectively tuned from the quantum-well-determined energy down to less than the band gap of GaAs, depending on the well width as well as the InAs layer thickness. Using the effective-mass approximation, the tuning effect can be well predicted theoretically The results reported here may provide an alternative way to tune the wavelength in optoelectronic devices

Ge WK

Henini M

Li Q

Li SS

Wang J

Wang Y

Xu ZY

Xu ZY,Chinese Acad Sci,Inst Semicond,Beijing 100083,Peoples R China.

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J. Phys.: Condens. Matter 11 (1999) 3629–3633. Printed in the UK PII: S0953-8984(99)99446-8Wavelength tuning in GaAs/AlGaAs quantum wells by InAssubmonolayer insertionZ Y Xu†k, J Wang†, Y Wang†, W K Ge†, Qing Li‡, S S Li‡ and M Henini§† Department of Physics, Hong Kong University of Science and Technology, Clear Water Bay,Hong Kong, People’s Republic of China‡ Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, People’s Republicof China§ Physics Department, Nottingham University, Nottingham NG7 2RD, UKReceived 18 November 1998Abstract. Wavelength tuning of exciton emissions has been achieved simply by inserting anInAs submonolayer at the centre of GaAs quantum wells during molecular beam epitaxy growth.Photoluminescence measurements show that the emission energy can be effectively tuned fromthe quantum-well-determined energy down to less than the band gap of GaAs, depending on thewell width as well as the InAs layer thickness. Using the effective-mass approximation, the tuningeffect can be well predicted theoretically. The results reported here may provide an alternative wayto tune the wavelength in optoelectronic devices.1. IntroductionThe study of highly strained InAs/GaAs heterostructures has attracted much attention in recentyears due to their intriguing novel properties. Such a strained structure may provide a promisingway to obtain high-quality quantum dot arrays through the spontaneous formation of three-dimensional islands during Stranski–Krastanov-like growth [1–3]. Meanwhile, monolayer(ML) and submonolayer (SML) insertion of InAs in a GaAs matrix has been proved to leadto superior optical properties [4–8], such as a greatly improved luminescence efficiency [5],very narrow linewidth (down to 0.15 meV) [6], a surprisingly high exciton oscillator strength[6, 7], and huge binding energy [8]. Photoluminescence (PL) and time-resolved PL studiesrevealed the 2D properties of the emission [9]. An InAs monolayer was also ingeniously usedto measure the spatial dependence of the probability density of eigenstates in GaAs/GaAlAsquantum wells by means of the insertion of a one-monolayer InAs probe plane at differentpositions in the quantum well [10].In this paper we report an important application of the InAs submonolayer, i.e. its abilityto tune the wavelength of the exciton emission in GaAs/AlGaAs quantum wells (QWs). Thetuning effect is achieved simply by inserting the InAs submonolayer at the centre of theGaAs QWs during the MBE growth. PL measurement shows that the emission energy can beeffectively tuned from the QW-determined energy down to less than the band gap of GaAs.For example, when a half-ML InAs layer is inserted at the centre of a 12 nm GaAs well, theexciton energy can be tuned from 1.540 eV (805.2 nm) down to 1.504 eV (824.5 nm) at lowk Present address: Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, People’s Republicof China. E-mail address: trzyxu@red.semi.ac.cn.0953-8984/99/173629+05$19.50 © 1999 IOP Publishing Ltd 36293630 Z Y Xu et altemperature. Additionally, we found that the insertion of InAs did not degrade the materialquality: no obvious change in the luminescence linewidth and efficiency was observed in oursamples. Using the effective-mass approximation, we have calculated the exciton states inGaAs/GaAlAs QWs with insertion of an InAs submonolayer, and found that the tuning effectcan be well predicted theoretically.2. ExperimentThe samples used in this study were grown by MBE on semi-insulating (100) GaAs substrateswith a 0:7 m buffer layer. The basic structure of the sample is composed of three singleGaAs quantum wells of 3, 6.2, and 12 nm, separated by 40 nm Ga0:8Al0:2As barriers. Thewhole QW structure was sandwiched between 0:1m Ga0:8Al0:2As cladding layers. The InAssubmonolayer was inserted at the centre of each GaAs well during the MBE growth with thegrowth rate of 0.043–0.066 ML s−1. The growth temperature was 480–510 C for InAs, and600 C for GaAs and GaAlAs. Finally, the structure was capped with a 10 nm GaAs layer toprotect the surface. A reference GaAs=Ga0:8Al0:2As QW sample without InAs insertion wasgrown under the same conditions. The excitation of the sample was carried out with an argonlaser in the cw PL measurements. In the time-resolved PL measurement, a Ti:sapphire sub-picosecond pulse laser was used as an excitation light source, and the time-correlated signalwas analysed by a 2D synchroscan streak camera with an overall resolution less than 30 ps.3. Results and discussionFigure 1 shows low-temperature PL spectra of three samples with or without InAs insertion.A systematic energy shift was observed in the PL spectra when the thickness of the InAs layerwas increased. For the reference sample (see trace (a) in figure 1), the three PL peaks at 1.653,1.579, and 1.540 eV are attributed to the heavy-hole exciton emissions of 3, 6.2, and 12 nmGaAs wells, respectively. It is seen that the insertion of the InAs layers into the well shiftedthe exciton energy towards the red. The amount of red-shift increases with the increase of theFigure 1. Photoluminescence spectra of GaAs/GaAlAs QW samples with or without InAs SMLinsertion, illustrating the systematic red-shift of the exciton emission with the increase of the InAsthickness.Wavelength tuning in GaAs/AlGaAs quantum wells 3631InAs layer thickness. For the sample in which half a ML of InAs was inserted (see trace (b)in figure 1), the PL peak energies are shifted to 1.593, 1.533, and 1.505 eV, respectively, i.e.,there are 60, 46, and 35 meV shifts for the 3, 6.2, and 12 nm wells, respectively. However, forthe sample with one twelfth of a ML of InAs, the corresponding shifts are 10, 7 and 4 meV,respectively, as shown by trace (c) in figure 1. It is worth noting that the emission energy of1.505 eV for the 12 nm GaAs well with half a ML of InAs inserted is already below the bandgap of GaAs at 12 K. Even lower exciton energy is to be expected when thicker InAs layersare inserted.Figure 2. Comparison of the temperature dependences of the exciton decay times for the 3 nmGaAs well in the samples of figure 1.It is important to point out that the observed linewidth and luminescence efficiency werealmost unchanged for the samples with InAs insertion. This indicates that the insertion of anInAs layer did not introduce any further defects, and consequently did not affect the quality ofthe samples. This is very significant in applications, and was further verified by investigatingthe temperature dependence of the exciton lifetime using time-resolved photoluminescence.It is well known that the lifetime of photogenerated carriers is very much influenced by thenonradiative centres in the material [11, 12]. For a 2D quantum well structure, a lineartemperature dependence of the exciton lifetime was expected [13]. However, for a real structurethe measured decay time very often reaches its maximum when the temperature is increased toa certain value, and then falls off with further increase of temperature [11, 12]. This behaviourhas been explained by the activation of nonradiative centres associated with the defects orother thermal escape channels of photo-carriers. In figure 2 the measured decay times versustemperatures are shown for the 3 nm GaAs well in the samples described as for figure 1.In the temperature range below 30 K, the decay times are almost unchanged due to excitonlocalization. When the temperature is increased, a linear dependence is observed, in qualitativeagreement with the theoretical prediction [13]. The decay times reach their maxima at around70–90 K for our samples. This characteristic temperature could vary from sample to sample,depending on the thermal barrier of the material defects, the potential barriers, and othernonradiative channels [11, 12]. Discussion of the physics is beyond the scope of this paper.However, it is worth emphasizing that the trends of the three curves in figure 2 are very similar.This further supports our assumption that the insertion of InAs layers in our samples did notsignificantly change the material quality. Note that the decay times for the reference sampleare slightly longer than those for the other two samples (figure 3). This can be understood by3632 Z Y Xu et alFigure 3. The calculated tuning curves for the exciton energy of GaAs/GaAlAs QWs with InAsinsertion: the red-shift of the exciton emission versus the InAs layer thickness. The experimentaldata are denoted by symbols.considering the effect of trapping of carriers into the InAs submonolayer and probably into theInAs/GaAs interfaces as well.In order to compare the experimental data with the theoretical prediction, we havecalculated the excitonic states in GaAs/GaAlAs QWs with InAs submonolayer insertion withinthe envelope function model [14, 15], taking into account the influence of strain. It is worthmentioning that we do not use any adjustable parameters to fit the experimental results. Inthe calculation, the InAs submonolayer in GaAs is treated as a thin finite potential located atthe centre of the GaAs quantum well, and the electron and hole ground states of the structureare calculated by invoking periodic boundary conditions [15, 16]. Also, the exciton states arecalculated using a variational method including the Coulomb interaction between the electronsand holes [15]. The material parameters used for the calculation are taken from reference [5].Figure 3 shows the calculated red-shift, defined in each case as the difference of the excitonenergy for the GaAs well with InAs insertion from that for the case without InAs insertion.Indeed, with the increase of the InAs layer thickness a systematic red-shift of the excitonemission is predicted. For a 3 nm well, the tuning range could be as large as 120 meV. Forcomparison, the experimental data are also plotted in the figure. A reasonable agreementbetween the experiment and calculation is obtained, despite the fundamental limitations of theenvelope function model in the limit of ultrathin wells.4. ConclusionsIn summary, we have demonstrated the wavelength tuning in GaAs/GaAlAs QWs achievedsimply by the insertion of InAs submonolayers in the middle of the GaAs wells. This providesan alternative way to tune the photon emission in GaAs/GaAlAs systems, and therefore mayhave potential application in optoelectronic devices.AcknowledgmentsThis work was supported by the Research Grant Council of Hong Kong, the NSF of China,and the Engineering and Physical Science Research Council (UK).Wavelength tuning in GaAs/AlGaAs quantum wells 3633References[1] Leonard D, Krishnamurthy M, Reaves C M, Denbaars S P and Petroff P M 1993 Appl. Phys. Lett. 63 3203[2] Moison J M, Houzay F, Barthe F, Leprince L, Andre E and Vatel O 1994 Appl. Phys. Lett. 64 196[3] Arakawa Y, Nagamune Y, Nishioka M and Tsukamoto S 1993 Semicond. Sci. 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Wavelength tuning in GaAsAlGaAs quantum wells by InAs submonolayer insertion

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Wavelength tuning in GaAsAlGaAs quantum wells by InAs submonolayer insertion

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