Thickness influence on spinodal decomposition in ln0.2Ga0.8As/GaAs ...

Jnsl. Phys. Con( Ser. No 180 f'aper presen1ed al Microsc. Semicond. Moler. Conf, Cambridge, 31 lvfarch ·- 3 Apri! 2003 .f:!2003 IOP Publishing Ltd

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Thickness influence on spinodal decomposition in ln0 .2Ga0.8As/GaAs Iow temperature growth M Herrera, D González, M U González 1, Y González 1, L González 1 and Y R García

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Departamento de Ciencia de los Materiales e LM. y Q.L, Universidad de Cádiz, Apartado 40, 1151 O Puerto Real, Cádiz, Spain 1 Instituto de Microelectrónica de Madrid (CNM-CSIC), C/ Isaac Newton, 8 (PTM), 28760-Tres Cantos, Madrid, Spain

ABSTRACT: A transmission electron microscopy study of compositlon modulation in In 0 .2Gao.sAs/GaAs(OOl) structures grown at 200°C by ALMBE is repmted. We have observed that, at Jow growth temperature, phase separation is highly dependen! on the epilayer thickness. For Ino.2Ga08 As/GaAs(OOl), a critica! !ayer thickness for the appearance of composition modulation in low temperature growth is found to be between 80nm and 300nm. This result implies that spinodal decomposition occurs in bulk during growth and is not a superficial phenomenon as in conventional high temperature growth. Accumulated elastic strain seems to be a crucial factor for the appearance of composition modulation in bulk.

l. INTRODUCTION om

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Epitaxial growth ofiii-V compounds heterostructures is curren ti y used for tailoring the opto- and microelectronic properties of a wide range of devices. In these systems, reticular misfit induced stresses drive morphological and compositional instabilities that affect their performance. Within !he compositional inhomogeneities usually observed, spontaneous lateral composition modulation has attracted special attention because it can be used to form quantum-wires for Jasers and photo-detectors. On the other hand, phase separation has been related to degradation of semiconductor properties, sin ce it reduces electron mobility (Quillec et al 1983) and broadens photoluminescence curves (LaPierre et al 1996, Wang et al 1999). Therefore, the control ofthe composition modulation development is needed. The mechanism by which spinodal decomposition is responsible for composition moclulation is controversia!. Although early stuclies considered spinoclal decomposition as a bulk process (Stringfellow 1982, Glas 1987), latter works have shown experimcntally that, at leas! at conventional growth temperatures, it occurs at the surface of thc structure during growth, bcing rclated lo surfacc morvhology (Wang 2000, Wu and Weatherly 2001). The aim ofthis paper is to clarify this issuc for low tcmperature growth in ln 0 2Ga 0 .8 As/GaAs(OO 1) epilayers, by means of transmission clectron microscopy (TEM) observations.

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2. EXPERIMENTAL

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Three h1 0 .2Ga0 .8 As/GaAs(OO 1) epilayers ha ve bcen grown by atomic !ayer molecular beam epitaxy (ALMBE) at low temperaturc (200°C). Samples A80Ct and A300Ct, which suffered a postgrowth thermal annealing at 500°C of 30s, have !ayer thicknesses of 80 and 300 11111, respcctively, whereas no-annealed sample A300 has a 300 n111 epilayer thickness. On the other hand, heterostructurcs grown ata convcntional te111perature (500°C) by molecular bcam epitaxy (MBE) are considered for comparison, with 1I1 0 _2Ga 08 As !ayer thickncss of 50 nm (samplc B50) and 190 nm (sample B 190).

214 Samples were prepared for TEM by mechanical thinning followed by ion milling for cross section (XTEM) observation. The TEM study was performed using a JEOL 1200EX transmission electron microscope operating at 120kV.

3. RESULTS Figure 1a shows a g220 XTEM micrograph of the low thickness, high temperature grown sample 850, where vertical bright and dark contrasts appear in the InGaAs epi1ayer. Such contrast features are frequently found in the TEM study of semiconductor structures grown on (001) substrates (LaPierre et al 1995). In this way, Patriarche et al (2000) have demonstrated by X-Ray microana1ysis that such contrasts correspond to arcas of the material with different composition. This phase separation is usually attributed to a spinodal decomposition process and, in our case, produces Ga-rich adjacent to In-rich zones. Therefore, the growth of InGaAs epilayers at high temperature causes a composition modulation in the alloy. Regarding plastic relaxation, neither misfit dislocations (MD) nor threading dislocations (TD) have been observed. However, the homologous sample grown at low temperature A80Ct does not exhibit the mentioned bright and dark areas, being perfectly unifom1 in contras! and, therefore, homogeneous, as it is shown in Fig. 1b. Furthermore, this structure is plastically relaxed, with a density of MD of 1.8x105 cm- 1• Thus, unlike conventional temperatures, low temperature growth by ALM8E permits us to obtain homogeneous alloys, without composition modulation.

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Fig. l. XTEM images with g 220 under bright field two beam conditions of samples a) 850, b) A80Ct. Neveriheless, when growing higher thickness samples at low temperature, composition modulation appears and it is independent of the application of thermal annealing. Moreover, an asymmetry in the composition modulation wavelength can be noted. Figure 2 shows XTEM micrographs of both directions in the growth plane of the sample A300. As can be observed, fine speckle contras! is present in one direction (Fig. 2a), while coarse contras! appears in the perpendicular one (Fig. 2b ). Similar contras! has been observed in the annealed sample A300Ct, so it seems that the thermal annealing does not influence on the composition modulation wavelength. Large thickness high temperature grown sample 8190 presents composition modulation features, too, but the wavelength of this phase separation is quite symmetrical, showing fine speckle contrasts. Plastic relaxation during low temperature growth is different from that in conventional conditions, too. B 190 presents a high planar density of misfit dislocations (4.5x 10 5 cnf\ while A300 ancl A300Ct show just 8x10 4 cm- 2 and 2.6xl0 5 cm- 2, respectively. These results show that low temperature grown samples are highly stressed, with a considerable amount of accumulated elastic energy.

4. DISCUSSION Severa! theoretical models based on Cahn's equation (Cahn 1961) for the prediction of the critica! temperature above which spinodal decomposition occurs in semiconductor matcrials havc been

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reported (Stringfellow 1982, Glass 1987, Guyer and Voorhees 1998). Nowadays, mode1s that consider spinodal decomposition as a surface process are more often taken into account, because they predict higher critica! temperatures, in better agreement with experimental results. Moreover, at conventional growth temperatures, this phase separation has been experimentally related to surface morphology (Wu and Weatherly 2001, Wang 2000). Neve1iheless, our results point out that, at least at low growth temperatures, spinodal decomposition occurs in the bulk during the structure growth. An InGaAs epilayer with a thickness of 80 nm (sample A80Ct) is homogeneous, whereas Iayers grown under the same conditions (200°C by ALMBE) but 300 nm thick (A300 and A300Ct) present composition modulation. This fact suggests that spinodal decomposition should take place on arriving at a critica! !ayer thickness in the interval 80-300 nm, and therefore, should occur in bulk and not in the surface of the structure. Composition modulation in bulk has been observed in the thermal annealing of low temperature grown SiGe alloys (Walther et al 1997). However, in our case the annealing process seems not to affect the phase separation, which just depends on epilayer thickness.

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Fig. 2. Bright field XTEM micrographs of sample A300 corresponding of both directions in the growth plane.

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The reason for the appearance of composition modulation in bulk in low temperature grown samples could be related to the anomalous degree of plastic relaxation. Although sample A300 (f-=1.5%) has a predicted degree of plastic relaxation for conventional growth of 75% of its reticular misfit (R=7 5%) (Dunstan et al 1991 ), TEM results indicate that the InGaAs !ayer is nearly coherent to the GaAs substrate (R=5%) and therefore, it has a considerable amount of stored elastic energy. In this way, the effect of surface elastic strain is considered by Guyer and Voorhees (1998), who havc proposed a theoretical model of morphological and compositional instabilities in alloy crystal growth. A~cording to these authors, the spinodal decomposition can occur during the deposition or in bulk. The temperature for bulk decomposition is much lower than for deposition driven modulation. Furthermore, for bu! k decomposition, if surface elastic strain is allowed, the thermodynamic condition for spinodal decomposition becomes more restrictive. Thus, the critica! temperature of phase separation for stmctures with surface elastic strain is higher than that for rigid epitaxiallayers. Experimental results for low temperature grown sarnples are qualitatively in good agreemcnt with this theoretical model. Sample A80Ct has a considerable degree of plastic relaxation (30%, approx.), but the misfit dislocations appeared in the post-growth thennal annealing. During its growth (González et al 2001 ), this sample remains coherent with the substrate, and consequently strcssed. Nevertheless, surface strain is relatively low, due to the small epilayer thickness. When the epilayer thickness increases, accumulated elastic energy rises and then surface strain, until that moment when the appearance of composition modulation is thermodynarnically favourable for the system, as happens in sample A300. A critica! !ayer thickness exists, between 80 nm and 300 nm for In 0.2 Ga0 .8As/GaAs(OO 1) epilayers, where the structure has such elastic strain that spinodal decomposition occurs. To delimit the mentioned critica! !ayer thickness experimentally, a study of Iayers with intermediate thickness is necessary. XTEM results ha ve al so shown an asymmetry in the composition modulation wavelength of low temperature grown structures. Fine speckle contrast is observed in one ofthe directions in thc

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growth plane of A300 and A300Ct, whereas coarse contrast appears in the other. Lee et al (1995) pro pose that the appearance of coarse contrast is related to the undulation of the structure surface due to the accumulated stress of the system. This reasoning is not accurate in our case since spinodal decomposition seems to occur in bulk and not in the structure surface during growth. Moreover, coarse contrasts composition modulation could not be formed in bulk because bulk diffusion rate is extreme! y low (McDevitt et al 1991). Choi et al (1997) proposed that coarse contrasts are an artefact of the sample preparation for TEM study, induced by the fine speckle type composition modulation associated stress. If coarse contrast is just an artefact, then we observe composition modulation in only one of the directions in the growth plane. To resolve the causes of this asymmetry further studies are required.

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S. CONCLUSIONS

In this work, the influence of !ayer thickness and growth temperature on compos1tlon modulation is reported. In low temperature growth by ALMBE of In 0.2Gao. 8As/GaAs(001) structures, spinodal decomposition occurs in the bulk, and not at the surface of the structure during growth as occurs at conventional temperatures. For a critica! !ayer thickness in the interval 80-300 nm spinodal decomposition in low tempera tu re grown In 0 .2Gao. 8As/GaAs(OO 1) systems is found. Elastic strain due to the reticular misfit in a structure with inhibited plastic relaxation seems to be a fundamental factor for spinodal decomposition to take place. Moreover, low temperature growth of InGaAs epilayers that overcome the critica! thickness present asymmetric composition modulation features. Thus, fine speckle contrast is observed in one direction in the growth plane, while coarse contrast appears in the other, the latter is likely to be an artefact of the sample preparation for TEM study. On the other hand, post-growth thermal annealing does not change the characteristics of composition modulation in low temperature grown In 0.2Gao. 8As/GaAs(OO 1) epilayers.

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