Solid-State Electronics 48 (2004) 1317–1323 www.elsevier.com/locate/sse
Low-energy plasma-enhanced chemical vapor deposition for strained Si and Ge heterostructures and devices G. Isella a
a,*
, D. Chrastina a, B. R€ ossner b, T. Hackbarth c, H.-J. Herzog c, €nig c, H. von K€ U. Ko anel a
INFM and L-NESS Dipartimento di Fisica, Politecnico di Milano, Polo Regionale di Como, Via Anzani 52, I-22100 Como, Italy b Laboratorium f€ur Festk€orperphysik, ETH Z€urich, CH-8093 Z€urich, Switzerland c DaimlerChrysler AG, Research Center Ulm, Wilhelm-Runge-St. 11, 89081 Ulm, Germany Received 5 November 2003; received in revised form 19 December 2003; accepted 30 January 2004 Available online 13 April 2004
The review of this paper was arranged by Prof. C.K. Maiti
Abstract We review the potential of low-energy plasma-enhanced chemical vapor deposition (LEPECVD) for the fabrication of strained Si and Ge heterostructures and devices. The technique is shown to be equally applicable to the formation of relaxed SiGe buffer layers, and to entire heterostructures including strained modulation doped channels. Pure Ge channels on Ge-rich linearly graded buffers are shown to exhibit low-temperature hole mobilities up to 120,000 cm2 V1 s1 , limited by remote impurity and background impurity scattering rather than interface roughness scattering. Strained-Si modulation-doped field-effect transistors (n-MODFETs) with excellent frequency response have been fabricated by combining LEPECVD and MBE for buffer layer and active layer growth, respectively. Maximum oscillation frequencies of n-MODFETs above 140 GHz have been achieved for active layer stacks both on buffers linearly graded to a Ge fraction of 40% at a rate of 10% per micron, and on constant composition buffers which are 10 times thinner. The use of a thin buffer results in significantly less device self-heating. 2004 Elsevier Ltd. All rights reserved. Keywords: Silicon–germanium (SiGe); Virtual substrates; Relaxed buffer; MODFET
1. Introduction The development of low-energy plasma-enhanced chemical vapor deposition (LEPECVD) [1] was motivated by the need for relaxed SiGe alloy buffer layers epitaxially grown on Si wafers. In order to serve as virtual substrates (VS) for strained Si or SiGe layers these buffer layers should exhibit small surface roughness and low defect density [2]. The basic concept of grading Si1x Gex alloy layers to some final composition x, either linearly or step-wise, has proven to be highly
*
Corresponding author. Tel.: +39-2-239-96170; fax: +39-2239-96126. E-mail address:
[email protected] (G. Isella).
successful towards this goal [3]. However, VS with a low density of threading dislocations require grading rates below 10% per micron [4]. For final Ge contents of several tens of percent this means that alloy layers several microns in thickness have to be deposited. The need for fast epitaxial growth at comparatively low substrate temperatures puts LEPECVD at a clear advantage with respect to traditional techniques such as molecular beam epitaxy (MBE) or chemical vapor deposition (CVD) [5]. With substrates immersed in a dense low-energy plasma, gaseous precursors are highly activated in LEPECVD. This enables growth rates up to 10 nm s1 , while at low plasma densities low rates are just as easily attainable offering excellent thickness control and abrupt interfaces [6]. A review of LEPECVD and its applications to modulation-doped strained-Si field-effect transistors
0038-1101/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.sse.2004.01.013
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(n-MODFETs) as well as strained-SiGe p-MOSFETs can be found in Ref. [7]. More recently, the combination of LEPECVD and MBE for growing the VS and active layer stack, respectively, has led to the best high frequency performance of strained-Si n-MODFETs reported to date [8]. Here, we review the progress made in the fabrication of modulation-doped Ge-rich heterostructures, using LEPECVD for growing both the VS and the active layer stack. This has resulted in the highest hole mobilities ever observed in Si-based materials [9]. Moreover, we present evidence that the relaxation process of strained constant composition SiGe layers may be modified such as to allow a substantial reduction of buffer thickness. There have been numerous efforts in the past to reduce the thickness of SiGe buffer layers, because of difficulties posed by thick VS for practical applications where heat dissipation to the Si substrate and lithography issues are a major concern [5,10]. The high frequency performance of strained Si n-MODFETs is found to be almost unaffected by the move from thick to thin VS, using identical active layer stacks grown by MBE onto both kinds of substrates.
2. p-MODQW grown by LEPECVD SiGe heterostructures exhibit two features rendering them excellent candidates for the fabrication of heterojunction-based field-effect transistors. Firstly, compressively strained germanium channels sandwiched between two SiGe layers form quantum wells for holes in the same way that tensile strained silicon layers surrounded by SiGe layers are able to confine electrons. Secondly, the high hole mobility of bulk germanium (1800 cm2 V1 s1 at 300 K) matches quite well to the electron mobility in bulk silicon (1450 cm2 V1 s1 ). Moreover, both are enhanced by about a factor of two when the active channels are strained. As far as low-temperature mobilities are concerned, there exists, however, a huge disparity between electrons and holes. A very high electron mobility of 500,000 cm2 V1 s1 has been realized in n-type modulation doped quantum wells (nMODQWs) at 1.7 K, at a sheet carrier density of ns ¼ 7 1011 cm2 [11]. For a long time, however, the hole mobilities achieved with p-MODQWs have been far less impressive [12]. The main reasons were the difficulties inherent to the preparation of good quality Si1x Gex virtual substrates with a high Ge content (x > 50%) and the tendency of compressed Ge layers to buckle [13]. In 2002 we reported on a p-MODQW structure entirely grown by LEPECVD comprising a pure Ge channel [9] which exhibited a 4.2 K hole mobility of nearly 90,000 cm2 V1 s1 at a sheet carrier density of ps ¼ 6 1011 cm2 . This mobility is 40% greater than the
Fig. 1. Layout of a p-MODQW structure. The graded buffer thickness is 12 lm, the overall active layer thickness is of the order of 100 nm.
best value previously achieved by MBE [12]. In the last two years processing by LEPECVD has been further refined, resulting in an increase of the 2 K mobility up to 120,000 cm2 V1 s1 at a hole density of ps ¼ 8:5 1011 cm2 . The typical growth sequence for a p-MODQW is outlined in Fig. 1. A Si0:3 Ge0:7 virtual substrate is grown at high deposition rate (5–10 nm s1 ) with a grading rate of 7% per micron [6] and a constant composition layer 2 lm thick. The active layers are grown with a reduced plasma density and therefore a lower rate (0.4 nm s1 ) in order to obtain sharper interfaces. Buckling of the compressively strained channel is totally suppressed by adding hydrogen gas (which acts as a surfactant), and by maintaining a low substrate temperature (450 C) during channel growth. The smooth upper interface allows modulation doping to be introduced above the channel (‘‘normal’’ doping), hence avoiding the problem of dopant segregation into the active region during growth [12]. The modulation doping was performed by introducing spikes of diborane, diluted in Ar, into the chamber during deposition. The samples were electrically characterized by Hall effect and magnetoresistance measurements. The upper panel of Fig. 2 shows the temperature dependence of the Hall mobility and carrier density on one of the best samples grown to date. At room temperature (RT) the Hall mobility is limited to 1690 cm2 V1 s1 because of parallel conduction through the VS, as is evident from the temperature dependence of the carrier density in Fig. 2. Mobility spectrum analysis performed on many similar samples evidenced a channel mobility of 3000 cm2 V1 s1 at 300 K [9,14]. At 2 K the mobility approaches 120,000 cm2 V1 s1 at a hole density of 8.5 · 1011 cm2 . The lower panel of Fig. 2 shows the magnetoresistance measured on a different high mobility sample at 0.6 K. Spin splitting of the Landau levels becomes evident at a magnetic field around 3 T. By fitting the temperature dependence of the Shubnikov–deHaas oscillations at fixed magnetic field, we have extracted the hole effective mass for a series of samples [15]. For the sample of Fig. 2, characterized by a rather low carrier
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Fig. 3. Mobility at 2 K for p-MODQWs grown by LEPECVD. Different sheet densities were induced in some samples (6747 ½M, 6777 ½r, 6843 ½}) by exploiting the effects of annealing on the pinning of surface states. All other samples [] are labeled individually. The solid lines are calculated considering scattering from remote impurities, background impurities and interface roughness. Fig. 2. Temperature dependence of the mobility and the carrier density of a p-MODQW grown by LEPECVD (sample number 6745). This sample exhibits a mobility l ¼ 120; 000 cm2 V1 s1 at 2 K and a carrier density ps ¼ 8:5 1011 cm2 (top). Shubnikov–deHaas oscillation at 0.6 K for sample 6016. From a set of such data measured at different temperatures an effective mass m ¼ 0:105me has been extracted. Only oscillations visible below 2 T were used for the effective mass extrapolation since for higher magnetic field, spin-splitting and quantum Hall regime effects dominate the magnetoresistance measurements (bottom).
density of 6.2 · 1011 cm2 , the effective hole mass is 0.105me , where me is the free electron mass. In strained Ge channels, nonparabolicity effects are expected to arise both from biaxial strain and from the presence of the heterojunction which requires bandmatching at the interface between the Ge channel and the cladding layers [16]. Our results confirm this prediction and indicate an effective mass m increasing from 0.095me to 0.18me as the sheet carrier density ps increases from 2.9 · 1011 to 1.9 · 1012 cm2 . Fig. 3 shows the Hall mobility measured at 2 K as a function of sheet carrier density for different pMODQWs grown by LEPECVD. They all exhibit a mobility larger than 30,000 cm2 V1 s1 . The Dingle ratio a, derived from the field dependence of Shubnikov–de Haas oscillations and mobility data (see Fig. 2), is usually of the order of 10. This indicates that remote impurity scattering is the dominant mobility-limiting mechanism [17]. From Fig. 3 the mobility can be seen to increase with increasing carrier density in the whole range displayed. Such a behavior can be understood to arise from more efficient screening and increased length of the Fermi
wavevector towards higher densities, the latter effect reducing scattering angles [18]. The data presented in Fig. 3 cannot, however, be explained by remote impurity scattering alone. A contribution due to background impurity scattering cannot be ruled out. In fact, it must be expected because of impurity deposition from the chamber walls during growth. We have performed calculations taking into account the effects of remote impurities, background impurities and interface roughness on the mobility of holes [18,19]. The results, shown as the solid lines in Fig. 3, indicate that all our data fall within an interval between two slightly different values of background doping (of the order of 1014 cm3 ). A significant contribution of interface roughness scattering can be excluded. This is in contrast to the results of Madhavi et al. [20] where interface roughness scattering is calculated to be dominant.
3. A mixed technique for n-MODFET fabrication: LEPECVD + MBE LEPECVD-grown thick graded SiGe buffers have also been employed for the fabrication of n-type structures based on strained Si channels. In this case only the VS, with Ge contents up to 40%, were grown by LEPECVD. The active layer stacks were deposited by MBE at the DaimlerChrysler research center in Ulm. Two kinds of active layer structure were grown onto the same VS by means of specially designed shutters.
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The first are n-MODQW structures with doping above the channel only, and spacer layer widths of the order of 10 nm. The purpose of these structures was to attain high low-temperature mobilities as a means of material characterization. Indeed, a 1.7 K mobility of 159,000 cm2 V1 s1 has been obtained at an electron density of ns ¼ 9:9 1011 cm2 [7]. This represents the highest electron mobility observed in a strained Si channel at such a relatively high carrier density. The second kind of structure is designed for the fabrication of n-type modulation-doped field-effect transistors (n-MODFETs). Here, doping on both sides of the channel and much thinner spacer widths are employed in order to achieve the low sheet resistances required for high-frequency applications. For such applications, the quality of the VS in terms of degree of relaxation and surface roughness is of utmost importance. This becomes evident by comparing the properties of MODFETs fabricated from similar active layer stacks, but grown on different SiGe VS. Fig. 4 shows such a comparison, where n-MODFETs were grown either in a single run by MBE alone, or by the mixed LEPECVD + MBE technology. As can be seen, the figures of merit favor the mixed technology in every respect, despite the disadvantageous cleaning steps and sample re-loading required in the latter case. Recently, a maximum oscillation frequency (fmax ) of 188 GHz has been reported for an n-MODFET fabricated on a graded LEPECVD buffer [8]. In this case the VS was graded up to 40% Ge with a grading rate of 10% per micron and capped with a 1 lm thick constant composition layer. A scheme of the active layer stack, deposited by MBE, is shown in Fig. 5. The first integrated circuits based on this technology have also been fabricated recently [21–23]. While excellent performance of single transistors has been demonstrated by using a thick graded buffer as a
Fig. 5. Layer stack for a MODFET deposited on a LEPECVDgrown graded buffer and thin relaxed VS. The gate length and the width are 70 nm and 100 lm, respectively.
VS, serious shortcomings must be expected once integration on any significant scale is attempted. The main problem arises from the poor thermal conductivity of SiGe alloys, in particular for the Ge content around 40% relevant for strained Si applications [24]. In an attempt to substantially decrease the thickness of VS, we have applied LEPECVD to the growth of alloy layers of constant composition. Ideally, such a layer should exhibit a high degree of relaxation and a network of misfit dislocations confined to the layer-substrate interface. In practice, however, sluggish nucleation and dislocation interaction have resulted in films that are either little relaxed or contain a huge number of threading dislocations, introduced during the relaxation process. In fact, the idea of reducing dislocation interaction originally gave rise to the concept of grading [3]. Alterna-
Fig. 4. A comparison between MBE and LEPECVD + MBE grown strained silicon n-MODFETs. Quoted are growth time, degree of relaxation (DoR), sheet resistance (Rs ), drain saturation current (Idss ), maximum transconductance (gm;max ), transit frequency (ft ) and maximum oscillation frequency (fmax ).
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tively, thin alloy layers can be made to substantially relax by introducing point defects during low-temperature growth, or by post-growth processing such as ion implantation and annealing. LEPECVD may offer yet another way to achieve relaxation, by growing at sufficiently low temperatures and high rates. The idea here is that the onset of relaxation may be delayed, leading to excessive strain accumulation which facilitates dislocation nucleation and reduces dislocation blocking during relaxation, and thus results in more efficient strain relief. We have tested the plausibility of this argument by growing two 500 nm samples with the same Ge content (45%) at the same substrate temperature (400 C) but at two different rates (2 and 0.4 nm s1 ). In the absence of post-growth annealing the relaxation of the sample grown at high rate was found to be 85%, but only 68% for the sample grown at low rate. The degree of relaxation is not 100%, but in the former case it is still sufficient for ensuring adequate stability during MODFET processing. The in-plane lattice constant is approximately equivalent to that of a fully-relaxed 40% VS, which is close to the optimum value for room-temperature strained-Si n-MOSFET performance [25]. Compared to graded buffers the structural quality of such ‘‘thin’’ VS is quite poor. The latter do have a lower rms roughness, 1.8 nm instead of 2.4 nm as determined by atomic force microscopy (AFM), but a substantially higher threading dislocation density, 3 · 108 cm2 rather than 1 · 106 cm2 , as determined from transmission electron microscopy (TEM) and chemical defect etching. The left hand side of Fig. 6 shows a TEM cross-section of a 500 nm thick Si0:55 Ge0:45 film grown at a substrate temperature of 400 C. Most of the defects are confined to the layer/substrate interface, but some stacking faults and threading dislocations can be seen to pierce the active layer stack grown on top. The AFM image shown on the right hand side of Fig. 6 exhibits the cross-hatch characteristic of graded alloys, although with a reduced peak-to-valley height. In any case, the higher density of threading dislocations it is not expected to limit the room temperature mobility [26]. This can be seen in Fig. 7 in which the Hall
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Fig. 7. Hall mobility and sheet carrier density as a function of temperature for two identical MODQW structures grown on a thick graded buffer (N) and a thin highly relaxed VS (.).
mobilities of n-MODQWs grown on thick graded and thin VS are shown. Towards low temperatures the two curves start to diverge, however, the higher crystal quality of the thick VS becoming more and more important. For room temperature operation of devices the quality of the thin VS is wholly adequate. This is demonstrated by the excellent electrical properties exhibited by MODFETs grown on such thin highly relaxed VS [27]. The active layer stack is the same as for the MODFET structure grown on thick graded buffers (see Fig. 5). The maximum oscillation frequency in this case was fmax ¼ 138 GHz which has to be compared with fmax ¼ 144 GHz obtained for the same device grown on a thick graded buffer [24]. The small reduction in the maximum oscillation frequency is by far outweighed by the substantial reduction
Fig. 6. A TEM cross-section and a 25 · 25 lm2 AFM image taken from sample 6672, which features a thin relaxed VS.
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References
Fig. 8. Comparison of the MODFET output characteristics on thin and thick relaxed LEPECVD-grown Si0:6 Ge0:4 buffers (lower and upper curves, respectively) under static and pulsed conditions. The gate voltage VG was varied between )0.6 and +0.6 V in steps of 0.4 V and the pulse length was 200 ns with a duty cycle of 1:1000.
of self-heating, as can be seen in Fig. 8. In this figure, a comparison is made between the pulsed and static output characteristics for the ‘‘thin’’ and ‘‘thick’’ devices. It is evident that thin VS can maintain superior performance especially at high drain currents, where the difference in saturation current between static and dynamic curves is reduced from 17% in case of the thick buffer to only 6% using the thin VS. The effect of self-heating scales roughly with the square root of the VS thickness and this is in good agreement with the results found in NMOS devices [28].
4. Conclusion Excellent performance of strained Si and Ge heterostructures has been achieved, both for material deposited without interruption by LEPECVD alone, and by combining state of the art MBE with LEPECVD for active layer and VS growth, respectively. The first approach, applied to Ge channels lattice-matched to Ge-rich VS, has resulted in low-temperature hole mobilities exceeding the 100,000 cm2 V1 s1 mark for the first time. The high frequency response of Si n-MODFETs, fabricated by the second approach, was shown to be largely unaffected by a 10-fold reduction in VS thickness, while self-heating of the devices was substantially reduced.
Acknowledgements Financial support from GROWTH Program ECOPRO No. GRD2-2000-30064, EC, frame of IST-199910444 SIGMUND and German Bmbf Project Ultra2 13N7901, and UK EPSRC HMOS II is gratefully acknowledged.
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