Appl. Phys. A 70, 449–451 (2000) / Digital Object Identifier (DOI) 10.1007/s003390000441 Applied Physics AMaterialsScience & Processing

RelaxedGexSi1−x layer formation with reduced dislocation densitygrown by ultrahigh vacuum chemical vapor depositionG.L. Luo1,∗, P.Y. Chen1, X.F. Lin 1, P. Tsien1, T.W. Fan2

1 Institute of Microelectronics, Tsinghua University, Beijing, 100084, P.R. China2 Institute of Semiconductor, Chinese Academy of Sciences, Beijing, 100083, P.R. China

Received: 10 April 1999/Accepted: 27 October 1999/Published online: 8 March 2000 – Springer-Verlag 2000

Abstract. A new alternative method to grow the relaxedGe0.24Si0.76 layer with a reduced dislocation density by ultra-high vacuum chemical vapor deposition is reported in this pa-per. A1000-Å Ge0.24Si0.76 layer was first grown on aSi(100)substrate. Then a500-Å Si layer and a subsequent5000-ÅGe0.24Si0.76 overlayer followed. All these three layers weregrown at 600◦C. After being removed from the growthsystem to air, the sample was first annealed at850◦C for30 min, and then was investigated by cross-sectional trans-mission electron microscopy and Rutherford backscatteringspectroscopy. It is shown that the5000-Å Ge0.24Si0.76 thickover layer is perfect, and most of the threading dislocationsare located in the embedded thinSi layer and the lower1000-Å Ge0.24Si0.76 layer. The relaxation ratio of the overlayer is deduced to be 0.8 from Raman spectroscopy.

PACS: 81.15.Gh

Strained-layer systems involvingGeSi/Si heterostructureshave gained considerable attention due to their potential ap-plications in both electronic and optoelectronic devices. How-ever, the investigation of strain-relaxedGeSilayer systems isalso of interest [1, 2]. This is because growth of high elec-tron and hole mobility strainedSi, GeSi, andGe layers needrelaxedGeSi layers to serve as second substrates. Due tothe 4.2% lattice mismatch betweenSi and Ge, the relaxedGeSi layers often contain a high density of threading dislo-cations. These threading dislocations can greatly reduce theelectron and hole mobility by acting as the scattering centers,or cause current leakage, resulting in poor device perform-ance. Hence, the growth of relaxedGeSi layers with a lowdensity of threading dislocations has attracted significant at-tention.

Among various growth techniques and treatments, com-positionally graded buffers are the most widely used [3–6].Such buffer layers allow the growth of relaxedGeSi layersof any composition on silicon substrates. Using a very slow

∗Corresponding author. (E-mail: luogl@mail.tsinghua.edu.cn)

graded (10% per1µm) buffer layer, a totally relaxed uniformalloy layer with aGecomposition of 0.23 containing as low as105 cm−2 threading dislocations was successfully grown [5].However, this method has some disadvantages. First, thistechnique causes problems of time and source consumption.Second, these buffer layers are usually very thick, and as a re-sult, the surface is usually severely roughened [7, 8], causingdifficulties in device fabrication.

Recently, a method of growing a uniform-Ge-compositionGeSi layer with a low-density of threading dislocations ona Si substrate by use of a low-temperature (LT)Si layerhas been reported [9–11]. The growth temperature for theLT Si layer must be below450◦C. This method can signifi-cantly reduce the buffer layer thickness and improve surfacequality. For this technique, the introduction of a LTSi layercan be easily realized by solid-source molecular beam epi-taxy(MBE) andSi2H6 gas-source MBE [9–11]. However, forthe widely usedSiH4-source ultrahigh vacuum chemical va-por deposition (UHV/CVD) system, growth of a LTSi layerusing SiH4 introduces a new problem. Because the CVDis a surface-reaction-controlled process, theSi growth ratedepends on growth temperature greatly. It can be deducedfrom [12–14] that theSi growth rate at a substrate tempera-ture of450◦C is no more than1 Å/min. This result was alsoconfirmed by ourSiH4-source UHV/CVD system. To growa 500-Å LT Si layer with this low growth rate would requireseveral hours, meaning such a technique is uneconomic fora SiH4-source UHV/CVD system.

In this paper, we report a new alternative method forgrowing high-quality relaxedGeSilayers onSi substrates thatis suitable for aSiH4-source UHV/CVD system. The mainidea is to grow an embedded thinSi layer within the low partof the thick GeSi layer. The whole growth process, includ-ing growth of the thinSi layer, takes place at600◦C. Withthe observation by cross-sectional transmission electron mi-croscopy (XTEM) and measurement by Rutherford backscat-tering spectroscopy (RBS), it is shown that the embedded thinSi layer can block threading dislocations effectively. Most ofthe threading dislocations are contained within the thinSilayer and the lower thinGeSilayer.

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1 Experimental procedure

The sample was grown by a newly developed UHV/CVDsystem. The details of the apparatus and some growthtechnology have been reported elsewhere [15]. The reac-tant gasesSiH4 and diluteGeH4 (15% GeH4 in H2) wereused. Prior to deposition, a p-type (100)-orientedSi waferwas subjected to an RCA cleaning procedure and a10%HF dip. The detailed growth sequence for the sample wasas follows. A 1000-Å Si buffer layer was grown first at750◦C to minimize defects at the substrate surface. ThentheGe0.24Si0.76 layer was grown. When theGe0.24Si0.76 layerreached1000Å, it was interrupted, and then the growthof a 500-Å embeddedSi layer and subsequent growth ofa 5000-Å Ge0.24Si0.76 overlayer were carried out. All thesethree layers were deposited at a constant temperature of600◦C. For the500-Å Si layer, the growth time was only10 min. The Si growth rate is50Å/min, which is muchgreater than that at a growth temperature of450◦C. After de-position, the sample was removed from the system to air, andthen annealed at850◦C in a N2 atmosphere for30 min inorder to ensure theGe0.24Si0.76 layer was relaxed optimally.In this study, the sample was investigated by XTEM, RBS,and Raman spectroscopy. The RBS measurement was per-formed using a1.7-MeV Tandem Pelletron Model 5SDH-2accelerator. The2.023-MeV 4He beam had a spot size of1.5 mm2 and a current of30 nAon the sample. The backscat-tered ions were detected at a165◦ angle and80 mm fromthe sample by a standard50-mm2 Si surface barrier de-tector with an energy resolution of15 keV. The Ramanspectrum was measured by a SPEX1403 Raman spectrom-eter. The excitation wavelength was5144Å from an argonion laser.

2 Results and discussion

Figure 1 shows the (400) bright-field XTEM image of thesample. Remarkably, the5000-Å Ge0.24Si0.76 overlayer ap-pears dislocation-free, and most of the threading dislo-cations are located in the embedded thinSi layer andthe lower 1000-Å Ge0.24Si0.76 layer. Also, many thread-ing dislocations are found to bow towards the substrate.This phenomenon is very similar to that observed in sam-ples using graded layers [6] and LTSi layers [9, 11] inorder to release strain. If a thick uniformGeSi layer isdirectly grown on theSi substrate without any pregrownlayer, a large amount of threading dislocations are gen-erated throughout the alloy layer (see Fig. 1a of [6] andFig. 1a of [11]). In order to estimate the threading disloca-tion density in ourGe0.24Si0.76 overlayer, a plan-view TEMimaging method [6] was employed. The dislocation dens-ity was estimated to be< 106 cm−2. This low value meansthat, in our study, the embedded thinSi layer is also ef-fective in blocking the propagation of threading dislocationsupwards.

In order to obtain further information concerning the crys-tal quality of the sample. RBS was used. Figure 2 showsa [100] channeling spectrum and a random spectrum. Sur-face backscattering energies ofSi and Ge are marked. Theweakness of the surface peaks ofSi andGe may be due tothe weak oxidation and/or cleanliness of sample surface. The

Fig. 1. Bright-field (400) cross-sectional TEM image of the sample

Fig. 2. Random and channeled RBS spectra for the sample

minimum yield χmin of Si is deduced from the channeled-to-random ratio of theSi signal slightly behind the surfacepeak. Its value is3.5%, which indicates good crystal qual-ity. In the channeling spectrum, the indicated signal in the1.648–1.488 MeV energy range and the indicated signal inthe1.144–1.008 MeVenergy range correspond to the5000-ÅGe0.24Si0.76 overlayer of the sample. It is found that they arevery small and almost flat. This further demonstrates that theGe0.24Si0.76 overlayer is largely free of dislocations.

The distinct increase ofGe signal below an energy of1.488 MeVand theSi signal below the energy of1.008 MeVis due to dechanneling by a large amount of dislocations exist-ing in the embeddedSi layer and the lowerGe0.24Si0.76 layer.

The relaxation of theGe0.24Si0.76 overlayer was inves-tigated by Raman spectroscopy. Figure 3 shows the Ra-man spectrum of the sample. Two main peaks are observedat 505 cm−1 and 402 cm−1, and arise from lattice vibra-tions in theGe0.24Si0.76 overlayer. They are associated witha Si−Si phonon mode and aSi−Ge phonon mode, respec-

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Fig. 3. Raman spectrum of the sample

tively. The small peak at520 cm−1 is attributed to theSi−Siphonon mode of theSi substrate. Its weakness is due to theGe0.24Si0.76 overlayer being too thick. For relatively lowGecomposition, theSi−Si phonon mode has been shown to havea different linear dependence on composition when both fullyrelaxed and fully strainedGeSilayers are considered [16, 17].The relative shifts of thisSi−Si phonon mode from theGeSilayers with respect to this mode from theSi substrate at roomtemperature can be described as a function ofGe composi-tion, x, by:

∆r = 69.0x , ∆s= 36.0x , (1)

where∆r and∆s represent the Raman peak shifts for fully re-laxed and fully strained epilayerSi phonon modes relative tothat from bulkSi, respectively. A relaxation ratio,r , can bedefined from the shift ofSi−Si Raman peak relative to thebulk Si value [17]:r = (∆exp−∆s)/(∆r−∆s), where∆exp isthe measured shift,∆r and∆s are calculated from (1). FromFig. 3, we can see that∆exp is 15 cm−1. The deduced relax-ation ratio of theGe0.24Si0.76 overlayer is approximately 0.8,which is near to full relaxation.

3 Conclusion

A thick Ge0.24Si0.76 film was grown onSi(100) at600◦C byultrahigh vacuum chemical vapor deposition usingSiH4 andGeH4 as reactant gases. XTEM and RBS show that an em-bedded thinSi layer at the bottom of the thick alloy film caneffectively block threading dislocations from propagating up-ward. Using Raman spectroscopy, the relaxation ratio of the5000-Å Ge0.24Si0.76 overlayer is found to be approximately0.8, which is nearly fully relaxed.

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