Superlattices and Microstructures,
Vol. 72, No. 3, 1992
MODULATED F’HOTOLUMINES CJ3NCE IN SEMICONDUCTOR QUANTUM WELLS
Zhongying Xu+ and M.Gal++ + National Laboratory for Superlattices & Microstructures, and Institute of Semiconductors, Academia Sinia, Beijing, looo83, China + + School of Physics, Universityof New South Wales, P. 0. Box 1, Kensington, NSW, 2033, Australia
(Received 4 August 1992)
Temperature modulated photoluminescence is demonstrated to be a powerful method to study exciton localization and to resolve the fine structure of photoluminescence in InGaAs/GaAs and GaAs/GaAlAs quantum wells.
Temperature modulated photoluminescence (TMPL) has proved to be a sensitive method for studying the optical properties of semiconductors***. Recently this technique has been applied to the study of semiconductor quantum wells (QWs), providing important information on exciton binding energies2*3, monolayer fluctuations4, and other temperature dependent parameters. In our previous works*6 we presented a theoretical model to describe the basic nature of TMPL, and studied exciton localization in QWs. In this paper we have detailed the temperature dependence of TMPL line shape and studied the exciton localization energy as a function of growth conditions. We have also demonstrated the usefulness of TMPL in resolving the fine structure of photoluminescence (PL) in QWs.
In our experiments a frequency of 9 Hz was used, and the resulting modulated signal has an amplitude of - 1% of its corresponding PL intensity. 3. Theory As we discussed in our previous work5S6,the TMPL signal, Al,,, is proportional to the temperature derivative of the photoluminescence, dZ,,(T)/dT
which in turn can be expressed as the sum of the partial derivatives of the exciton energy, broadening parameter, and density.
AzpL~n+YU’ ~~ aE $U’M9 a~’ $V’J’JB aN --+--+-aTIAT (2) aE
The details of the TMPL technique were described in our previous papers’*6. In this technique the temperature modulation is achieved by a mechanically-chopped argon laser beam, focused through a sapphire window on the back side of a sample. The sample was excited by a 0.5 mW He-Ne laser. The TMPL signal intensity was found to decrease monotonically with modulation frequency in the range of 6 Hz to 1000 Hz that we studied, giving strong evidence of the thermal nature of the modulation. 07494036/92/070393
+ 04 $08.00/O
where f(T’, E, N) is the line shape function of the PL emission, which can be expressed as a Lorentzian or Gaussian profile’. Detailed analyses suggested that at low temperature the temperature derivative of the exciton density (the third term in Eq.2) plays an important role in the explanation of TMPL line shape. The key point in the analyses is that the PL spectrum should be described as a superposition of localized (bound) and delocalized (free) excitonic emissions separated by AE, the average exciton localization
0 1992 Academic Press Limited
energy, as suggested by many other authors’-lo.
4.Results and discussion Fig. 1 shows typical TMPL spectra for an Ino,,,Gae,g,As/GaAs single quantum well (SQW) with Lz=67 A at four different temperatures, together with their respective PL spectra. As can be seen in these figures, while there is no noticeable change in the PL spectra, the TMPL signals differ significantly at various temperatures. At 8.5 K the TMPL appears in a first derivative-like shape with a positive lobe at high energy and a negative lobe at low energy. As the temperature increases the TMPL line shape is flipped over. At 30 K the positive lobe appears at low energy while the negative one is at high energy.
PHOTON Fig. 1. spectra various denoted
PL (dashed curves) and TMPL (solid curves) of an In,,,,Ga,,,As/GaAs SQW sample at temperatures. The calculated TMPL spectra are as solid circles.
This observation can be well understood using our theoretical model. At low temperature both the first and second terms in Eq.(2) can be neglected The dominated factor determining the TMPL line shape is the temperature derivative of the exciton density, which drives the PL spectrum blue-shift. Above 30 K, however, the first term becomes dominant. From the well-known Varshni equation”, the PL spectrum will be red-shifted as temperature increases, resulting in a first derivative-like line shape, with the positive lobe at low energy. We have calculated TMPL line shapes at
Vol. 12, No. 3, 1992
various temperatures, and the results are represented by solid circles in Fig. 1. In our calculation a Gaussian function was used6. The fitting parameter, the average exciton localization energy AE, was found to be 2 meV for the sample of Fig. 1. We have found that the temperature where the flipover occurs varies from sample to sample. It is believed to be due to the material quality dependence of the exciton localization energy. In Table I we summarize some of our experimental results from different samples, along with the calculated exciton localization energies. The agreement between the two sets of values are excellent. TABLE
I: E%pe.rimental and calculated flip-over temperature (TJ and calculated excite localization energies for various samples:
Sample No. Tfexp (K) T,cal (K) AE (meV)
I,37 1.38 1.39 1.40 1.37 1.38 1.39 1.40
1 8.6 9.0 0.05
2 18 20 0.5
3 23 25 1.2
4 28 31 2.1
5 30 35 3.1
By comparing the values of the average exciton localization energy obtained from TMPL with those obtained from PLE measured on the same samples, we previously6 concluded that TMPL can be used to study exciton localization in QWs. Now we use this technique to study the exciton localization energy as a function of growth conditions. In Fig.2 we display the measured PL, PLE, and TMPL spectra of GaAs/GaAlAs SQW samples grown by MBE with and without growth interruptiontzt3 (GI). The Stokes shifts deduced from the PL and PLE spectra are measured to be 3.8 meV and 1.1 meV for the sample without GI and the sample with GI, respectively. The corresponding values obtained from the TMPL fitting procedure are 3.4 meV and 0.7 meV, respectively. The growth interruption at the heterointerfaces resulted in a significant smoothing of the interface profile. TMPL, once again, proves to be a powerful technique to study the exciton localization in semiconductors. We have also noted that TMPL can be used to resolve fine structures in the PL spectrum due to a well-width fluctuation and/or alloy disorder in QWs. Fig.3 shows TMPL spectra of a three pefiod In,~,,Ga+,,As/GaAs MQW with nominal Lz=105 A and La=320 A. It can be seen that while the PL spectra are indistinguishable at different temperatures, the TMPL spectra show distinct fine structures. The theoretical calculation indicates these tine structures are related to the slight differences between individual wells. Now let us suppose the sample consists of iwo different well widths: Lz,=lOO A and L,,=105 A, and calculate the TMPL line shape using above model. It is found that the results are in good agreement with the measured
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EXPER I MENT -TMPL
T=lZ k ::I I I
1.37 Fig.2. Measured and calculated TMPL spectra for GaAs/Ga,,,Al,,,As SQW samples with GI (a) and without GI (b). The fitting procedure yielded the exciton localization energies of 0.7 meV and 3.4 meV, respectively. The insets are the corresponding PLE and PL spectra. The measured Stokes shifts are 1.1 meV and 3.8 meV, respectively.
TMPL spectra, as shown in Fig.3. In the calculation the exciton binding energies and the broadening parameters were chosen to be identical for both wells AE, =AE$=3meV), but with (En, =E,,=8 meV, different exciton localization energies of 0.5 meV and 0.35 meV, respectively. Fig.4 gives another example of TMPL spectrum from a MQW sample consisting of five period of alternative 60 A In,,,,G~,,,As wells and 320 A GaAs barriers. Three derivative structures were unambiguously
Fig.3. (a) Measured PL and TMPL spectra of an InGaAs/GaAs MQW sample having three nominally identical wells (La=105 A); (b) calculated TMPL spectra of a MQW having two different widths: 100 A and 105 A.
observed in the TMPL spectrum at 4.5 K, while the PL spectrum only shows weak shoulders on both sides of the main peak. In such a complicated case the detailed analysis of TMPL requires more information on recombination and carrier transfer processes, and so far we do not have a satisfactory model to quantitatively explain our data. However the results observed here clearly show the usefulness of TMPL for resolving fine structures in semiconductor quantum wells.
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We have also shown the potential application of the TMPL technique to resolve the tine structure of PL due to well-width fluctuation and alloy disorder in quantum wells. AcknowledgmentThis work was supported by the Australian Research council, the University of NSW, and the National Natural Science Foundation of China.
1.402 Photon Fig.4. (a) consisting wells and the TMPL
PL, and (b) TMPL spectra of a MQW sample five periods of alternative 60 A @,,,Ga,,,,As 320 A GaAs barriers, showing the ability of technique to resolve tine structures.
5. Conclusion We have measured TMPL spectra from a number of InGaAs/GaAs and GaAs/GaAlAs QWs. The lineshape fitting process was used to obtain the exciton localization energy, which are in good agreement with the Stokes shifts obtained from the PLE measurement.
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