Vacuum Chemical Vapor Deposition System Research Articles

Homoepitaxy of Ge on ozone-treated Ge (1 0 0) substrate by ultra-high vacuum chemical vapor deposition

Germanium (Ge) homoepitaxy is still a big challenge due to the immature surface cleaning of Ge, which often results in imperfect Ge epitaxial growth in terms of surface roughness, pits and islands. In this work, high crystal quality Ge and GeSi with very smooth surface were epitaxially grown on Ge (0 0 1) substrates, which were thermally cleaned at 650 °C in an ultrahigh vacuum chemical vapor deposition system after ozone oxidation. The ozone oxidation of Ge substrates at optimal duration could strongly diminish Ge suboxides which should be responsible for the formation of square concave or islands in Ge epilayers. The complete removal of fully stoichiometric GeO2 on Ge substrates by thermal annealing provided a clean surface for almost perfect Ge homoepitaxy or GeSi heteroepitaxy with a low temperature Ge epilayer. This technology would solve the dilemma of epitaxy on Ge for micro- and opto-electronic device applications.

Innovative Ge–SiO2 bonding based on an intermediate ultra-thin silicon layer

Low temperature Ge to SiO2 wafer bonding based on an intermediate ultra-thin Si layer (1 nm) which is grown on the Ge wafer (Si/Ge) by ultra-high vacuum chemical vapor deposition (UHV/CVD) system is demonstrated in this work. The formation and decomposition of the unstable GeOx layer are effectively restrained due to the incomplete oxidation of the covered ultra-thin Si layer after deposition. Therefore, a nearly voids-free bonding interface is observed after post annealing. The hydrophilicity of Si/Ge wafer surface is higher than that of the Ge wafer surface after the treatment of the diluted NH4OH solution. The more –OH groups on the Si/Ge wafer surface than that on the Ge wafer surface can be responsible for this feature. As a result, the high bonding strength of 4.39 MPa for the Ge/Si–SiO2 wafer pairs was achieved due to the formation of more covalent bonds (Si–O–Si) at the bonding interface after post annealing.

Impacts of excimer laser annealing on Ge epilayer on Si

The impacts of excimer laser annealing on the crystallinity of Ge epilayers on Si substrate grown by low- and high-temperature two-step approach in an ultra-high vacuum chemical vapor deposition system were investigated. The samples were treated by excimer laser annealing (ELA) at various laser power densities with the temperature above the melting point of Ge, while below that of Si, resulting in effective reduction of point defects and dislocations in the Ge layer with smooth surface. The full-width at half-maximum (FWHM) of X-ray diffraction patterns of the low-temperature Ge epilayer decreases with the increase in laser power density, indicating the crystalline improvement and negligible effect of Ge–Si intermixing during ELA processes. The short laser pulse time and large cooling rate cause quick melting and recrystallization of Ge epilayer on Si in the non-thermal equilibrium process, rendering tensile strain in Ge epilayer as calculated quantitatively with thermal mismatch between Si and Ge. The FWHM of X-ray diffraction patterns is significantly reduced for the two-step grown samples after treated by a combination of ELA and conventional furnace thermal annealing, indicating that the crystalline of Ge epilayer is improved more effectively with pre- annealing by excimer laser.

(Invited) Development of SiGeSn Technique Towards Mid-Infrared Devices in Silicon Photonics

GeSn/SiGeSn techniques are of great interest in the area of Si photonics due to: 1) the industry scalable growth; 2) compatible with current Si complementary metal-oxide semiconductor (CMOS) process; 3) capability of monolithic integration on Si; 4) the identified group-IV based direct bandgap materials; and 5) the tunable bandgap allowing the optoelectronic devices operation features broad wavelength coverage in near- and mid-infrared ranges. In the past a few years, the GeSn/SiGeSn material growth, the materials and devices characteristics have been comprehensively studied. Plenty of promising results are reported. In this work, the following aspects regarding GeSn/SiGeSn techniques are investigated: 1) low-defect material growth technique via Ultra High Vacuum Chemical Vapor Deposition (UHVCVD) system; 2) advanced material and optical characterizations of GeSn/SiGeSn thin films, Ge/GeSn/Ge double heterostructure (DHS), and GeSn/SiGeSn quantum well samples; 3) demonstration of GeSn-based optoelectronic devices including photodetectors and emitters.

Properties of n-Ge epilayer on Si substrate with in-situ doping technology**Project supported by the National Basic Research Program of China (Grant No. 2013CB632103), the National Key Technology Support Program of China (Grant No. 2015BAF24B01), the Natural Science Foundation of Fujian Province of China (Grant No. 2016J05147), the Key Sci-Tech Research and Development Platform of Fujian Province, China (Grant No. 2014H2002),

The properties of n-Ge epilayer deposited on Si substrate with in-situ doping technology in a cold-wall ultrahigh vacuum chemical vapor deposition (UHVCVD) system are investigated. The growth temperature of ∼500 °C is optimal for the n-Ge growth in our equipment with a phosphorus concentration of ∼1018 cm−3. In the n-Ge epilayer, the depth profile of phosphorus concentration is box-shaped and the tensile strain of 0.12% confirmed by x-ray diffraction measurement is introduced which results in the red shift of the photoluminescence. The enhancements of photoluminescence intensity with the increase of the doping concentration are observed, which is consistent with the modeling of the spontaneous emission spectrum for direct transition of Ge. The results are of significance for guiding the growth of n-Ge epilayer with in-situ doping technology.

Low Thermal Budget Microwave Annealing for Nisige Schottky Junction Device

I. Background/ Objectives and Goals As continuously scaling down the devices for logic circuit, higher mobility channel materials have been considered to boost driving current such as Ge or SiGe. However, most high mobility materials have a significantly smaller bandgap as compared to Si, which will result in higher Band-To-Band-Tunneling leakage. As a result, S/D and channel engineering must play leading role for boosting the device performance. We, therefore, propose a NiSiGe/n-Si Schottky Junction formed microwave-annealing for the Si capped SiGe quantum well devices (Si/SixGe1-x/Si,x=0~1). II. Methods Fig. 1 shows Process flow of fabricating NiSiGe/n-Si Schottky diodes. The multi-layer structure of Si/Si0.57Ge0.43/Si was grown by ultra-high vacuum chemical vapor deposition system. The channel composed of a 5nm-thick SiGe with biaxial compressive strain and Si cap was grown at 420~500 and 550 °C, respectively. Isolation film SiO2 was deposited on multi-layer architecture after series surface cleaning. Then, the definition of junction active area was accomplished with lithography and wet-etching. Because of bulk annealing characteristic of microwave, the technique was utilized to form NiSiGe as low leakage Schottky junction ranging from 370 to 470 °C for 150 s in N2 ambient. The un-reacted Ni film was removed, followed by Al deposition as the back contact. The characteristics of microwave-annealed-NiSiGe Schottky junction was investigated with AFM, SEM, TEM, XRD, and electrical analyzer. III. Expected Results The SEM images demonstrated the surface morphologies of NiSiGe layer with various annealing temperatures in Fig.2 NiSiGe agglomeration emerged as forming temperature up to 600 °C by rapid thermal annealing (RTA) which will lead to unstable junction current. Relatively, microwave annealing can suppress the formation of junction defects and prevent the agglomeration due to lower forming temperature. For the purpose of distinguishing the pros and cons between the rapid thermal annealing (RTA) in Fig.3(a) and microwave annealing in Fig.3 (b), I-V characteristics were evaluated to check the electrical performance of Schottky junction. Microwave-annealed Schottky junction exhibited better ION/IOFF ratio about 3×105 formed at 390 °C and more stable off-current characteristic as compared to the one with rapid thermal annealing (RTA). In addition, barrier height and ideality factor of the microwave-annealed Schottky was 0.62 eV and 1.07, respectively. High resolution TEM images showed the polycrystalline structure, good uniformity of the NiSiGe and a distinct interface between NiSiGe and Si in Fig.4. IV. Conclusion Microwave annealing is beneficial in integrating quantum well p-channel MOSFETs with Schottky S/D because of its low temperature feature. We believe our microwave-cooperated quantum well architecture is promising for the high performance logic circuits and enable SiGe or Ge channel devices to be integrated on the Si substrate for the future applications. V. R eferences [1] M. S. Datta, G. Dewey, M. Doczy, B. Doyle, B. Jin, J. Kavalieros, R. Kotlyar, M. Metz, N. Zelick, and R. Chau, “High Mobility Si/SiGe Strained Channel MOS Transistors with HfO2/TiN Gate Stack,” IEEE International Electron Devices Meeting, p.28, Dec. 2003. Figure 1

CMOS Compatible Growth of High Quality Ge, SiGe and SiGeSn for Photonic Device Applications

Germanium films were deposited on silicon substrate by an ultra-high vacuum chemical vapor deposition system using GeH4 as Ge precursor and different carrier gases (Ar, N2 and H2). Silicon-germanium and SiGeSn films were deposited using optimized growth conditions of Ge growth. Silane and SnD4 were used as precursors for Si and Sn, respectively. A complete study of SiGe growth at chamber pressure from 0.1 to 1.0 torr and at temperatures from 350-450ºC for different SiH4 flow rate is performed. Silicon incorporation in Ge films varied from 1% to 17%. Tin incorporation of 2% was observed in the films with 17% Si incorporation. Investigation of film quality has been done using etch pit measurement, scanning electron microscopy and transmission electron microscopy. Threading dislocation density for Ge, SiGe and SiGeSn films were 1.7×107 cm-2, 2.3 ×107 cm-2 and 4.0 ×107 cm-2 , respectively.

Enhancement of Material Quality of (Si)GeSn Films Grown by SnCl4 Precursor

The (Si)GeSn films have been grown with and without carrier gases on Si substrate using a cold-wall ultra-high vacuum chemical vapor deposition system. Material characterization of the films grown with Ar carrier gas using X-ray diffraction and transmission electron microscopy shows achievement of higher quality films. Optical characterization of the films with photoluminescence shows enhancement in material quality of the films at 350 and 400ºC growth temperatures. Increase in the pressure results in higher Sn incorporation and higher photoluminescence intensity. Spectroscopic ellipsometry data confirms the change in the GeSn bandgap towards lower energies.

High-performance Ge p-i-n photodetector on Si substrate

High-performance and tensile-strained germanium (Ge) p-i-n photodetector is demonstrated on Si substrate. The epitaxial Ge layers were prepared in an ultrahigh vacuum chemical vapor deposition (UHV-CVD) system using low temperature Ge buffer technique. The devices were fabricated by in situ doping and using Si as passivation layer between Ge and metal, which can improve the ohmic contact and realize the high doping. The results show that the dark current of the photodetector with diameter of 24 μm is about 2.5×10−7 μA at the bias voltage of −1 V, and the optical responsivity is 0.1 A/W at wavelength of 1.55 μm. The 3 dB bandwidth (BW) of 4 GHz is obtained for the photodetector with diameter of 24 μm at reverse bias voltage of 1 V. The long diffusion time of minority carrier in n-type Ge and the large contact resistance in metal/Ge contacts both affect the performance of Ge photodetectors.

Direct Growth of Ge1−xSnx Films on Si Using a Cold-Wall Ultra-High Vacuum Chemical-Vapor-Deposition System

Germanium tin alloys were grown directly on Si substrate at low temperatures using a cold-wall ultra-high vacuum chemical vapor deposition system. Epitaxial growth was achieved by adopting commercial gas precursors of germane and stannic chloride without any carrier gases. The X-ray diffraction analysis showed the incorporation of Sn and that the Ge1-xSnx films are fully epitaxial and strain relaxed. Tin incorporation in the Ge matrix was found to vary from 1% to 7%. The scanning electron microscopy images and energy dispersive X-ray spectra maps show uniform Sn incorporation and continuous film growth. Investigation of deposition parameters shows that at high flow rates of stannic chloride the films were etched due to the production of HCl. The photoluminescence study shows the reduction of bandgap from 0.8 eV to 0.55 eV as a result of Sn incorporation.

Embedded-Ge source and drain in InGaAs/GaAs dual channel MESFET

We report the first demonstration of n-type III–V metal-semiconductor field-effect transistors (nMESFETs) with IV group material hetero-junction source and drain (S/D) technology. A selective epitaxial growth of germanium (Ge) in the recessed gallium arsenide (GaAs) S/D regions is successfully developed using ultra-high vacuum chemical vapor deposition (UHVCVD) system. The dual channel structure includes an additional 10-nm higher mobility n-In0.2Ga0.8As layer on n-GaAs channel and is introduced to further improve the device performance. The n-MESFET, combining embedded-Ge S/D with In0.2Ga0.8As/GaAs channel, exhibits good transfer properties with a drain current on/off ratio of approximately 103. Due to the small barrier height of Ti/In0.2Ga0.8As Schottky contact, a lattice-matched wide bandgap In0.49Ga0.51P dielectric layer is also integrated into the device architecture to build a higher electron Schottky barrier height (SBH) for gate leakage current reduction. The Ti/In0.49Ga0.51P/n-In0.2Ga0.8As Schottky diode shows a comparable leakage level to Ti/n-GaAs with 2 × 10−2 A/cm2 at a gate voltage of −2.0 V.

In situ doped phosphorus diffusion behavior in germanium epilayer on silicon substrate by ultra-high vacuum chemical vapor deposition

Phosphorus diffusion behaviors in in situ doped germanium epilayers on Si substrate grown in an ultra high vacuum chemical vapor deposition system at 500 °C are investigated. The phosphorus diffusion mechanism during the growth process is dominated by the extrinsic diffusion process, which can be well described with a model including a cubic dependence of diffusivity on the doping concentration. The phosphorus diffusivity into the boron-doped Ge is lower than that into the intrinsic Ge layer by about one order of magnitude. An in situ doped Ge n/p junction with rectification ratio of 1.84 × 102 is achieved by inserting an intrinsic Ge layer to avoid impurity compensation.

Generation of planar defects caused by the surface diffusion of Au atoms on SiNWs

The generation of planar defects in silicon nanowires (SiNWs) synthesized by means of a vapor–liquid–solid (VLS) procedure using Au as a catalyst in an ultra-high vacuum chemical vapor deposition (UHV-CVD) system was investigated. Faceting, the formation of planar defects and the diffusion of Au in SiNWs occurred simultaneously, proportional to the growth temperature and the ratio of the H2 precursor gas. The planes located on the sidewalls of the wire after Au diffusion were faceted (111) and (100) surfaces, which represent equilibrium configurations of Si due to surface energy minimization during rapid wire growth under unstable conditions. Moreover, {111} twin defects were formed on the sidewalls of the faceted boundaries where the Au clusters were mainly located, due to the surface tension of the Au atoms, resulting in clusters at the liquid/solid interfaces in SiNWs with a 〈111〉 growth direction.

Production and Characterization of Boron-Doped Single Wall Carbon Nanotubes

We have synthesized boron-doped single wall carbon nanotubes in a high vacuum chemical vapor deposition (CVD) system using a new boron precursor. Transmission electron microscopy was used in order to confirm the presence of single wall carbon nanotubes and field emission scanning electron microscopy to allow a qualitative characterization of the produced tubes. To estimate the doping level, we compared the Raman spectra with pure single wall carbon nanotubes and we found an upshifted G band as an evidence of doping. X-ray photoelectron spectroscopy analysis and ab initio electronic structure calculations reveals the presence of substitutional boron atoms incorporated on the tubes. We have also developed a simple method to determine quantitatively in which temperature range the carbon nanotubes are produced more efficiently by high vacuum CVD.

Epitaxial growth of thick Ge layers with low dislocation density on silicon substrate by UHV/CVD

Thick Ge epitaxial layers are grown on Si(001) substrates in the ultra-high vacuum chemical vapor deposition system by using the method of low temperature buffer layer combining strained layer superlattices. The microstructure and the optical properties of the Ge layers are characterized by double crystal X-ray diffraction, HRTEM, AFM and photoluminescence spectroscopy. The root-mean-square surface roughness of the Ge epilayer with a thickness of 880nm is about 0.24 nm and the full-width-at-half maximum of the Ge peak of the XRD profile is about 273. The etch pit density related to threading dislocations is less than 1.5106 cm-2. The direct band transition photoluminescence is observed at room temperature and the photoluminescence peak is located at 1540 nm. It is indicated that the Ge epitaxial layer is of good quality and will be a promising material for Si-based optoelectronic devices

Quantitative strain characterization of SiGe heterostructures by high-resolution transmission electron microscopy

We report the quantitative strain characterization in semiconductor heterostructures of silicon–germaniums (Si 0.76Ge 0.24) grown on Si substrate by an ultra-high vacuum chemical vapor deposition system. The relaxed SiGe virtual substrate has been achieved by thermal annealing of the SiGe film with an inserted Ge layer. Strain analysis was performed using a combination of high-resolution transmission electron microscopy and geometric phase analysis.

Effect of annealing treatment and nanomechanical properties for multilayer Si 0.8Ge 0.2–Si films

Multilayered silicon–germanium (SiGe) films consisting of alternating sublayers with different mechanical properties have been epitaxially deposited by an ultra-high vacuum chemical vapor deposition (UHV/CVD) system. We report engineering of the mechanical properties of SiGe multilayer films by a commercial nanoindenter. From annealing treatment, it consists of an ex situ thermal treatments in furnace (600 °C) and rapid thermal annealing (800 °C) system. Subsequent roughness and microstructure of SiGe multilayer films were characterized by means of atomic force microscope (AFM) and transmission electron microscopy (TEM). The annealing treatment not only produced misfit dislocations as a significant role in the critical pile-up event but also promoted hardness. The hardness of the films increased slightly and then gradually achieved a maximum value (from 12.6 ± 0.4 GPa to 14.2 ± 0.7 GPa) with increasing annealing temperature. This may be due to the relaxation effect from thermal annealing and is potential to provide the reliability behaviours to design periodical SiGe multilayer structure in further.

Comparison of Epitaxial Graphene on Si-face and C-face 4H SiC Formed by Ultrahigh Vacuum and RF Furnace Production

We present X-ray photoelectron spectroscopy, van der Pauw Hall mobilities, low-temperature far-infrared magneto transmission (FIR-MT), and atomic force microscopy (AFM) results from graphene films produced by radiative heating in an ultrahigh vacuum (UHV) chamber or produced by radio frequency (RF) furnace annealing in a high vacuum chemical vapor deposition system on Si- and C-face 4H SiC substrates at 1200-1600 degrees C. Although the vacuum level and heating methods are different, graphene films produced by the two methods are chemically similar with the RF furnace annealing typically producing thicker graphene films than UHV. We observe, however, that the formation of graphene on the two faces is different with the thicker graphene films on the C-face RF samples having higher mobility. The FIR-MT showed a 0(-1) --> 1(0) Landau level transition with a square root B dependence and a line width consistent with a Dirac fermion with a mobility >250,000 cm(2) x V(-1) x s(-1) at 4.2 K in a C-face RF sample having a Hall-effect carrier mobility of 425 cm(2) x V(-1) x s(-1) at 300 K. AFM shows that graphene grows continuously over the varying morphology of both Si and C-face substrates.

Investigation of the relaxation behavior of Si 1− xC x alloys during epitaxial UHV-CVD growth

In this work, the epitaxial growth of Si 1− x C x alloys using an Ultra High Vacuum Chemical Vapor Deposition (UHV-CVD) system was studied. Si 1− x C x layers were grown in a temperature range of 550–650 °C and characterized using rocking curve X-ray diffraction (XRD), atomic force microscopy (AFM) as well as Fourier transform infrared spectroscopy (FT-IR). It was found that with increasing carbon precursor flow (Methylsilane) the amount of substitutional carbon rises up to a critical value. After a maximum in substitutional carbon content is reached, a further increase of carbon fraction leads to a reduction of the strain. By FT-IR, the non-substitutional carbon was determined to form 3C-SiC precipitates already during growth. A strong correlation between the increase of Methylsilane flow and the formation of coherent precipitates even at low carbon fractions was observed. A low deposition temperature was found to promote the precipitation of 3C-SiC.

Formation of relaxed SiGe on the buffer consists of modified SiGe stacked layers by Si pre-intermixing

High-quality relaxed SiGe films on Si (0 0 1) have been demonstrated with a buffer layer containing modified SiGe (m-SiGe) islands in ultra-high vacuum chemical vapor deposition (UHV/CVD) system. The m-SiGe islands are smoothened by capping an appropriate amount of Si and the subsequent annealing for 10 min. This process leads to the formation of a smooth buffer layer with non-uniform Ge content. With the m-SiGe-dot multilayer as a buffer layer, the 500-nm-thick uniform Si 0.8Ge 0.2 layers were then grown. These m-SiGe islands can serve as effective nucleation centers for misfit dislocations to relax the SiGe overlayer. Surface roughness, strain relaxation, and crystalline quality of the relaxed SiGe overlayer were found to be a function of period's number of the m-SiGe-dot multilayer. By optimizing period number in the buffer, the relaxed Si 0.8Ge 0.2 film on the 10-period m-SiGe-dot multilayer was demonstrated to have a threading dislocation density of 2.0 × 10 5 cm −2 and a strain relaxation of 89%.