Ultrathin GeSn p-channel MOSFETs grown directly on Si(111) substrate using solid phase epitaxy

Ultrathin GeSn layers with a thickness of 5.5 nm are fabricated on a Si(111) substrate by solid phase epitaxy (SPE) of amorphous GeSn layers with Sn concentrations up to 6.7%. We demonstrate well-behaved depletion-mode operation of GeSn p-channel metal–oxide–semiconductor field-effect transistors (pMOSFETs) with an on/off ratio of more than 1000 owing to the ultrathin GeSn channel layer (5.5 nm). It is found that the on current increases significantly with increasing Sn concentration at the same gate overdrive, attributed to an increasing substitutional Sn incorporation in Ge. The GeSn (6.7%) layer sample shows approximately 90% enhancement in hole mobility in comparison with a pure Ge channel on Si.status: publishe

Solid phase epitaxy of germanium compounds

Future developments of micro-and nano-electronics are to a great extent dependent on high mobility semiconductor materials and their integration in device structures. Thin films with high structural quality are required to achieve excellent electronic and optical properties. The fabrication of high mobility thin films is however challenging since relatively high temperatures are needed to obtain excellent structural and physical properties. At these temperatures several problems arise. The limited solubility of Ge and Sn prohibits for example the formation of GexSn1-x alloys with high Sn content [1,2,3]. This material system is however very interesting because of its high carrier mobility and excellent absorption in the infrared (IR) [3,4]. We demonstrate a new methodology for the fabrication of high mobility semiconductors, that overcomes the problems related to heteroepitaxial growth. This methodology is novel in applying solid phase crystallization for the fabrication of high mobility semiconductor materials. Solid phase crystallization is commonly used for large scale and low cost applications [5,6], and roughly consists of two steps: (1) low temperature deposition of a layer and (2) crystallization by thermal or laser annealing. In general, this approach leads to poly-crystalline layers, with the presence of significant concentrations of impurities [6]. Our methodology consists of deposition of an amorphous layer by suppressing ad-atom diffusion and initiation of crystallization at the amorphous layer/crystalline substrate interface by thermal annealing, see Fig. 1. We will show the possibilities of plasma enhanced chemical vapor deposition (PECVD) and inert gas fluxes during molecular beam epitaxy (MBE) to lower the ad-atom diffusion and decrease the structural order of amorphous semiconductor layers. Single crystalline Ge on Si with excellent crystalline and electrical properties is obtained, even for very thin layers (< 100 nm) [7]. Furthermore we demonstrate that this approach can be used to obtain single crystalline GeSn on Si with 4 % of Sn.status: publishe

Single crystalline Ge compounds on silicon by solid phase crystallization

Ge compounds have interesting electrical and optical properties, which make them interesting for a wide variety of applications: high performance CMOS circuits, photovoltaics, photo detectors, MEMS, etc. Germanium is relatively scarce in respect with silicon, and therefore thin film deposition is preferred for large scale applications of Ge containing compounds. In this work we present an economical and straightforward way to obtain single crystalline Ge compounds on Si substrates. An important advantage of using Si substrates is the scalability: layers of larger diameter can be obtained by simply using larger Si substrates. Epitaxial growth is mostly utilized to obtain a crystalline layer on top of another crystalline material. However, heteroepitaxial growth of germanium on silicon is rather difficult because of the large mismatch of 4 % between the two lattice constants. This difference in lattice dimensions leads to island growth, causing high surface roughness and high density of threading dislocations in the Ge layer. Obtaining high quality and smooth crystalline germanium, directly on silicon, is therefore challenging. We demonstrate the possibilities of plasma enhanced chemical vapor deposition (PECVD) and solid phase epitaxy for the fabrication of single crystalline Ge compounds. Solid phase crystallization is commonly used for large scale and low cost applications, and roughly consists of two steps: (1) low temperature deposition of a layer and (2) crystallization by thermal or laser annealing. In general, this approach leads to poly-crystalline layers, with the presence of significant concentrations of impurities. Our methodology consists of deposition of an amorphous layer by suppressing ad-atom diffusion and initiation of crystallization at the amorphous layer/crystalline substrate interface by thermal annealing. Ge compounds are deposited by PECVD on silicon substrates. Deposition of a highly amorphous layer is preferred. Crystalline inclusions must be avoided to obtain high crystal quality and a smooth surface after crystallization. PECVD is well suited for deposition of amorphous layers because low temperature deposition and high growth rates are possible. Additionally we show that this method can be used to obtain single crystalline GexSn1-x alloys. This material system is very interesting because of its high carrier mobility and excellent absorption in the infrared (IR). The limited solubility of Ge and Sn prohibits for example the formation of GexSn1-x alloys with high Sn content. Our method circumvents the limited solubility of Sn in Ge by depositing at low temperature into the amorphous phase, and therefore overcomes the problems related to heteroepitaxial growth. R.R. Lieten acknowledges support as Research Fellow of the Research Foundation -Flanders (FWO)status: publishe

Crystalline Ge3N4 on Ge(111)

The exposure of Ge(111) to a nitrogen plasma at temperatures above which Ge3N4 is thermally stable leads to the formation of a thin, monocrystalline Ge3N4 layer. At these temperatures, equilibrium is established between the formation and dissociation of Ge3N4, limiting its thickness to 0.7 nm at similar to 800 degrees C. The thermal stability of a crystalline Ge3N4 layer is comparable to an amorphous one. It starts to evaporate at temperatures above 600 degrees C. Crystalline Ge3N4 allows the growth of III-nitrides on top of Ge(111) substrates and possibly the passivation of Ge-based field effect transistors. (C) 2007 American Institute of Physics.status: publishe

Abrupt Ge-Si and GeSn-Si interfaces by solid phase crystallization

Single crystalline germanium has exciting optical and electrical properties, which are promising for electronic and optical applications. Ge has higher carrier mobility for both electrons and holes than Si, which makes this material suitable as channel material for high-speed complementary metal-oxide semiconductor (CMOS) technology. GeSn has been predicted to exhibit carrier mobilities exceeding that of Ge. In addition to improved carrier mobilities, Ge and GeSn show increased optical absorption. This makes these materials much better suited for optoelectronic applications than Si. Epitaxial growth is mostly utilized to obtain a crystalline layer on top of another crystalline material. However, heteroepitaxial growth of Ge on Si is rather complicated because of the large mismatch of 4% between the two lattices. This difference in lattice dimensions leads to island growth, causing high surface roughness and high density of threading dislocations in the Ge layer. A surface roughness of ~25 nm has been reported for 200 nm of Ge on Si grown by chemical vapor deposition (CVD) at 400 °C. Furthermore epitaxial growth at elevated temperatures leads to mixing between the Ge layer and Si substrate which causes a rough Ge-Si interface and even an intermediate GexSi1-x layer. Obtaining high quality and smooth crystalline Ge, directly on Si, is therefore challenging. In this work we demonstrate atomically abrupt Ge-Si interface and smooth surface by using solid phase (hetero)epitaxy (SPE) of amorphous Ge, deposited at low temperature. The excellent structural properties lead to carrier mobilities which are 2 x higher than bulk Si for only 90 nm of Ge, see Table I. Previously we have demonstrated successful solid phase heteroepitaxial growth of amorphous Ge layers on Si substrates. For successful SPE it is important to deposit an amorphous layer without the presence of crystalline grains. Amorphous Ge layers on Si are obtained by limiting the adatom surface mobility and therefore using low temperatures (typically room temperature up to 150 ˚C). These low deposition temperatures are beneficial for the interface and surface roughness. Structural characterization has been carried out using high resolution X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). XRD ω/2ϑ scans shows the appearance of a Ge diffraction peak after thermal annealing, demonstrating solid phase epitaxial growth, see Figure 1. HRTEM of 20 nm Ge shows that the Ge layer is single crystalline, see Figure 2. The interface between the Ge and Si substrate is atomically abrupt and the surface is smooth. Ge twins are present. The low deposition temperature and the crystallization under N2 atmosphere or N plasma effectively keep the Si-Ge interface and Ge surface smooth. This is an important advantage of SPE of Ge in respect with heteroepitaxy, which is performed at much higher deposition temperatures. Both n-type as p-type Ge layers were fabricated by introducing dopants during deposition of the amorphous layer. Phosphine (PH3) was used to incorporate phosphor for n-type doping and diborane (B2H6) to incorporate boron for p-type doping. 90 nm doped germanium layers were deposited on insulating Si(111). After crystallization at 600 °C in N2 ambient, Hall measurements were performed. The excellent structural properties lead to carrier mobilities which are 2 x higher than bulk Si for only 90 nm of Ge. Besides pure Ge we have obtained SPE of Ge1-xSnx layers with about 4% Sn. Reciprocal space mapping (RSM) shows the (331) reflections of Si and GeSn, see Figure 3. The 37 nm GeSn layer has a relaxation of 107% in respect with the Si substrate. The GeSn layer is under small tensile strain as it is slightly offset from complete relaxation (tilted line). This strain can be explained by the thermal mismatch between Si and GeSn at the annealing temperature. The in plane strain is +0.31%. Finally we have obtained p-type GeSn with a hole concentration of 3 x 10^19 cm-3 by introducing Ga dopants.status: publishe

Structural Characterisation of Improved GaN Epilayers Grown on a Ge(111) Substrate

Despite the large lattice mismatch between GaN and Ge of 20.1%, the successful growth of GaN on Ge(111) using plasma-assisted molecular beam epitaxy (PA-MBE) has been achieved. Recent work has shown that the crystal quality of GaN can be improved using either a miscut substrate or a higher growth temperature (above 800 degrees C), to enhance the step-flow growth. This paper reports the structural characterisation of these improved GaN epilayers grown on Ge. The threading dislocation density has been measured using cross-sectional transmission electron microscopy (TEM) imaging. The polarity of the epilayers is determined using convergent beam electron diffraction (CBED). (C) 2011 The Japan Society of Applied Physicsstatus: publishe

Schottky barrier modulation using ultrathin MgO for metal-silicon (100) contacts

© 2014 The Japan Society of Applied Physics. In this work, ultrathin MBE-grown MgO has been employed as a thin tunneling interlayer to modulate the Schottky barrier height (SBH) between metal contacts and Si substrates. The ultrathin MgO films were grown with different starting first monolayers (O2, MgO, and Mg) on Si. With an MgO ultrathin film, all contacts show rectifying behavior for both n- and p-type Si. The SBH generally increases (decreases) with increasing metal work function for n-type (and p-type) and is generally lowest for the oxygen first treatment. To our knowledge it is the first time that the role of the first monolayer of an oxide tunnel barrier on the SBH is revealed. These results indicate a novel, interesting way to modulate the barrier height and hence the contact resistivity in CMOS devices.status: publishe

Interface of GaN grown on Ge(111) by plasma assisted molecular beam epitaxy

Early efforts to grow GaN layers on germanium substrates by plasma assisted molecular beam epitaxy led to GaN domains, rotated by 8 degrees relative to each other. Increased insight in the growth of GaN on germanium resulted in the suppression of these domain and consequently high quality layers. In this study the interface of these improved layers is investigated with transmission electron microscopy. The GaN layers show high crystal quality and an atomically abrupt interface with the Ge substrate. A thin, single crystalline Ge3N4 layer is observed in between the GaN layer and Ge substrate. This Ge3N4 layer remains present even at growth temperatures (850 degrees C) far above the decomposition temperature of Ge3N4 in vacuum (600 degrees C). Triangular voids in the Ge substrate are observed after growth. Reducing the Ga flux at the onset of GaN growth helps to reduce the triangular defect size. This indicates that the formation of voids in the Ge substrate strongly depends on the presence of Ga atoms at the onset of growth. However complete elimination was not achieved. The formation of voids in the germanium substrate leads to diffusion of Ge into the GaN layer. Therefore we examined the diffusion of Ge atoms into the GaN layer and G a atoms into the Ge substrate. It was found that the diffusion of Ge into the GaN layer and Ga into the Ge substrate can be influenced by the growth temperature but cannot be completely suppressed. Our results suggest that Ga atoms diffuse through small imperfections in the Ge3N4 interlayer and locally etch the Ge substrate, leading to the diffusion of Ga and Ge atoms. (C) 2010 Elsevier B.V. All rights reserved.status: publishe