Room temperature photoreflectance as a powerful tool to characterize the crystalline quality of InAlAs layers grown on InP substrates

Materials Science and Engineering, B21 (1993) 177-180

177

Room temperature photoreflectance as a powerful tool to characterize the crystalline quality of InAlAs layers grown on InP substrates S. Mon6ger, A. Tabata, C. Bru and G. Guillot Laboratoire de Physique de la MatiOre (URA CNRS 358), Institut National des Sciences AppliquOes de Lyon, Bglt 502, 20 A venue Albert Einstein, 69621 Villeurbanne (France)

A. Georgakilas, K. Zekentes and G. Halkias koundation for Research and Technology-Hellas, Institute of Electronic Structure & Laser, PO Box 1527, 711 10 Heraklion, Crete (Greece)

Abstract The aim of this work is to show that room temperature photoreflectance can give the same indications as low temperature photoluminescence about the crystalline quality of both layers and interfaces. Our samples consist of molecular beam epitaxy InA1As layers lanice-matched to InP substrates, grown at different growth temperatures and using different InP cleaning temperatures. The photoreflectance broadening parameter (F) has been determined and compared with the well-known linewidth broadening of the photoluminescence. Both methods indicate that the best InAIAs crystalline quality is obtained for a growth temperature of 530 °C and an InP surface cleaning temperature of 530 °C.

1. Introduction

During the past decade, InGaAs/InAlAs structures (lattice-matched or strained) grown on InP substrates have been subject of extensive studies for applications in opto-electronic devices and high electron mobility transistors (HEMTs). Opto-electronic applications of this system rely on the small energy band gap of the InGaAs well in InAlAs/InGaAs quantum well structures. The excellent electron transport properties (electron mobility and peak velocity) of the InGaAs and the large conduction-band discontinuity between InAlAs and InGaAs are the main reasons for the high performance of HEMTs fabricated so far. Looking through the literature, two kinds of material problems appear to be the predominant obstacles for further development of such structures: structural quality of the hetero-interfaces and crystalline quality of the InAlAs layer. Particularly in H E M T structures, where a high resistivity InAlAs buffer layer separates the InGaAs channel from the InP substrate [1, 2], the growth of a high quality InAIAs buffer is essential for the transistor performance. The quality of the InAlAs layer depends strongly on the molecular beam epitaxy (MBE) growth conditions [3-5], and non-optimized growth temperatures seem to cause cluster formation and high concentrations of point defects which degrade the electrical and optical properties. Clustering in the InAIAs layer is attributed to the large difference in the 0921-5107/93/$6.00

migration rates of the cations [6], and a critical growth temperature under which immiscibility occurs is predicted [7]. Among the various characterization techniques used for the study of the InAlAs crystalline quality, photoluminescence (PL) spectroscopy is one of the most extensively used. For bulk In_AlAs, photoreflectance (PR) spectroscopy has been used, to our knowledge, only for the determination of the band gap energy at room temperature [8]. This PR technique is extremely sensitive to interband electronic transitions due to the derivative nature of spectral line shapes [9-11]. Indeed, even at room temperature, it can provide as much information as photoluminescence and photoluminescence excitation which are usually performed at low temperatures. Hence, the aim of the present work is to use photoreflectance as a tool for systematic evaluation of material quality. The PR broadening parameter (F) is compared with corresponding photoluminescence fullwidth at half-maximum (FWHM) data obtained at low temperature.

2. Experimental details

Our PR experimental arrangement consists of an excitation source of 150 W quartz Wolfram halogen lamp dispersed through a 0.64 m monochromator, © 1993 - Elsevier Sequoia. All rights reserved

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Characterization of crystalline quality using photoreflectancc

operating in the 0.5-1.8 ,um wavelength range. The monochromatic light is focused onto the semiconductor sample (1 x 0.6 mm z spot), with an energy density up to 3 mW cm-2. The modulation source is provided by a 12 mW He-Ne laser (632.8 nm) and the reflected beam is detected by Si photodiode, operating in photovoltaic mode and connected to a current-to-voltage converter. Low temperature PL measurements have been performed using an argon laser (514.5 nm) with power excitation of 5 W cm 2, focused into a 150 ,urn diameter spot. Luminescence, analysed through a 0.60 m monochromator, is detected by a liquid nitrogen cooled Ge cell and amplified by conventional lock-in system. For low temperature measurements the samples are mounted within a variable temperature helium cryostat. Both PR and PL setups are driven by a computerized data acquisition and control system. The analysed samples have been grown by molecular beam epitaxy on semi-insulating InP (100) substrates using a range of growth temperatures, T~. The undoped 1.5-2.0 p m thick In~Al~_~As layers have been grown with an indium composition close to the lattice-matched to InP substrate (xh~=0.52). The InP surface-cleaning procedure used had been previously optimized in order to increase the smoothness of the etched surface and the reproducibility in oxide desorption [12]. The last stage of the InP surface preparation, prior to InA1As growth, consisted in a thermal cleaning treatment performed either at 500 °C or at 530 °C. The InP surface-cleaning temperature T-r and the InA1As growth temperature Tg are listed in Table 1 for each of the samples studied.

3. Fitting procedure In order to obtain the precise values of the energy transitions and of the broadening parameters, the lowfield PR data were fitted with an Aspnes third-derivative functional form (TDFF) [13]. The TDFF relation has the following form: A R (E)= Re[ C e'°(E - Eg + i r ) - ' ] R

(11

where C, 0, Eg a n d F are amplitude, phase, critical point energy and broadening parameters, respectively. The broadening parameter F is.defined as h / 2 ~ r , where h is the Planck constant and r is the finite lifetime of the photogenerated carriers. An energy uncertainty is produced by this finite lifetime, resulting in a broadening of the singularities in the optical spectra. The coefficient n is a factor used to specify the critical point dimension and the perturbation type. F is particularly sensitive to the choice of n. For a three-dimen-

T A B L E 1. Growth parameters and PR. PI. results on [nA1As layers Sample

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A B (7 D E

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300 440 480 500 ~30

2.00 t.65 t .40 2.00 1.65

1.485 1.415 1.4132 1..~8~ 4 " 1.~8_

! 51 (t.54 0.53 0.55 0.55

32 16 13 18 16

35 28 22 33 15

sional critical point, such as for a direct band gap semiconductor, n is 2.5 [9] and the modulation spectra are directly related to the third derivatives of the unperturbed dielectric function.

4. Results and discussion In bulk InAIAs material, the dominant peak in low temperature photoluminescence measurements arises from the near band gap edge. The position of this PL energy peak is then lower than the band gap Eg by the bound exciton binding energy which is of the order of 3-5 meV [14]. In Fig. 1 we show the PR experimental data and the least-squares fit spectrum for the sample E. As can be seen, the fit obtained with the TDFF is in excellent agreement with the experimental spectrum. The 300 K photoreflectance parameters obtained by the fit, and the corresponding 5 K PL linewidths of the analysed samples are reported in Table 1. The indium composition x~n is derived from Eg obtained by PR at 300 K, using the energy band gap composition relation given by Gaskill et al. [8] and is in accordance with doublecrystal X-ray diffractometry measurements. Several physical mechanisms can contribute to the spectral broadening observed in PR and PL measurements, such as dislocations, impurities, alloy scattering and phonon absorption processes. For 300 K measurements, the last phenomenon plays a dominant role in pure and perfect crystals. In this way, the parameter F can provide indications about the crystalline quality. The evolution of the photoreflectance linewidths (F) as well as the PL full-width at halfmaximum (FWHM), as a function of the growth temperature, is presented in Fig. 2. This figure indi, cates that an increase of the growth temperature Tg results in a decrease of the PR linewidth for Tg values below 530°C. For InA1As, previous investigations showed that the use of relatively low growth temperatures limits the surface mobility of A1 atoms and degrades the crystalline quality [5, 6, 14]. Some theor-

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strong signal intensity and the lowest F W H M of 33 arcsec were obtained for sample E. On the contrary, a broad weak peak appeared for the low temperature grown sample A. These results give the same indications as PR and PL studies, and confirm our results. All the samples exhibited very high resistivity, therefore electrical measurements (Hall effect, C- V profile characterization etc.) were not feasible. However, transport properties have been determined on another set of samples Si-doped at 2.5x 1016 cm 3 and a higher mobility /~ of 1030 cm 2 V i s-1 has been obtained for T~= 530 °C [16j. The influence of the InP cleaning temperature TT is not so evident, although we have to notice that the best values of F and F W H M are obtained for the two samples C and E which were grown on InP substrates cleaned at 7:r=530°C. Therefore, the trend is to improve the material quality when the InP cleaning temperature is increased to 530 °C. Finally, sample E, which was grown using optimum temperatures (TT= 530°C and Tg= 530 °C), exhibits a state-of-theart 5 K photoluminescence F W H M of 15 meV, and a PR broadening parameter at 300 K of 16 meV.

I .... 500

550

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Conclusions

(°C)

Fig. 2. Variations of the photoreflectance broadening parameter F (o) and of the photoluminescence FWHM (~) vs, InAIAs growth temperature Tg.

etical calculations showed that at higher growth temperatures the alloy clustering is reduced [15]. We do not have any clear evidence that the reduction of the linewidth broadening is only related to the alloy homogeneity. The behaviour of the data presented in Fig. 2 is probably related to the decrease of the InA1As crystalline defects (dislocation, stacking faults) and/or to the improvement of the alloy homogeneity. Transmission electron microscopy (TEM) observations show that the increase of growth temperature up to 530 °C improves the crystalline quality of InA1As [12]. However, it is worth noting that when the growth temperature is increased beyond 530°C, a broadening of the PL linewidth was observed [5]. This observation is more likely to be related to the desorption of As atoms from the surface, resulting in the incorporation of As vacancies. The abnormally high value of F W H M (and F) for sample D can be explained by an insufficient cleaning of the InP surface, which is supported by previously reported TEM observations [12]. Double crystal X-ray diffractometry measurements have also been performed on these samples. A very

In this work we have performed PR and PL measurements in InAlAs bulk material grown by MBE on (100) InP substrates using different growth and thermal cleaning temperatures. We have confirmed the trend for InA1As crystalline quality to improve with increasing growth temperature up to 530 °C, in agreement with previous works. Nevertheless, the importance of this paper was to illustrate that photoreflectance can provide as much information as lowtemperature PL, showing that photoreflectance spectroscopy can be a powerful tool for the rapid evaluation of material quality.

Acknowledgments

This work was partially supported by the European Economic Community through ESPRIT Basic Research 3086 project. The authors would like to thank Dr. T. Benyattou and Y. Baltagi for useful discussions. References

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