We report record-high electron mobilities obtained in the Si/SiGe alloy system via single-junction n-type modulation-doped Si/Si0.7Ge0.9 heterostructurcs grown by the ultra-high vacuum chemical vapor deposition technique. Peak electron mobilities as high as 1,800 cm2/Vs, 9,000 cm2/Vs and 19,000 cm2/Vs were measured at room temperature, 77K and 1.4K, respectively. These high mobilities resulted from excellent Si/SiGe interfacial properties by employing a compositional graded Si/SiGe superlattice prior to the growth of a thick S0.7Ge0.3 buffer, which brought about a dramatic reduction of the threading dislocation density in the active Si channel. Two thin phosphorous-doped layers were incorporated in the SiGe barrier and at its surface to supply electrons to the Si channel and to suppress the surface depletion, respectively. The transport properties of these heterostructurcs were determined to be those of a two dimensional electron gas at Si/SiGe heterointerfaces at low temperatures.

EXTENDED ABSTRACT: We investigated resonant tunneling of holes in Si/Si1−xGex double barrier resonant tunneling (DBRT) diodes grown by MBE [1]. The aim of this study was an unambiguous identification of the quantum states involved In the two dominating resonances (Fig. 1), which have been observed by several groups [2],[3], [4]. For this purpose we calculated the transmission probabilities as a function of the well width by means of a transfer matrix method in a one-particle model [5]. The respective effective masses of heavy-holes (hh) and light-holes (lh) in the strained SiGe layers were calculated following Refs. (6) and [7], with appropriate strain corrections added according to Ref. (8); hh/lh mixing [9] was Introduced a posteriori. The calculated resonance positions are in reasonable agreement with experiments (Fig. 2), if the resonance at lower energy is attributed to a hh channel in the well, and the second resonance to a lh channel. Since only hh states are occupied in the emitter cladding layer at the temperatures (≈1.5K) studied, hh/lh coverslon due to the loss in rotational symmetry is involved in the second resonance.

We report the fabrication of modulation doped Si/Gex Si1−x heterostructures by molecular beam epitaxy. The samples are characterized by Rutherford backscattering spectrometry, cross-sectional transmission electron microscopy, electron beam induced current, Hall measurement, and the magnetoresistance (Shubnikov-de Haas) measurements. Threading dislocation densities of = 106cm−2 are observed for relaxed Ge0.3Si0.7 films on Si (100). The modulation doped structures fabricated on these Ge0.3 Si0.7 films contain two-dimensional electron gases with mobilities ranging from 60,000 to 96,000 cm2/V - s at 4.2 K.

Strained layer growth of SiGe on Si by either Molecular Beam Epitaxy (MBE) or various methods of Chemical Vapor Deposition (CVD), including Limited Reaction Processing (LRP) and Ultra High Vacuum CVD (UHV/CVD) have been used to realize narrow bandgap base double heterojunction bipolar transistors (HBTs). This review paper will focus on the fabrication of high performance transistors, and on the material and process challenges facing the implementation of SiGe HBT technology. In particular, the use of SiGe alloys for bandgap engineering of bipolar devices and the development of self-aligned, epitaxial base bipolar device structures will be discussed, including the most recent accomplishment of 75 GHz ƒr heterojunction bipolar transistors, and the record sub-25 ps EC L ring oscillator delay. The design flexibility and trade-offs offered by SiGe heterojunction technology, like junction field/capacitance control, liquid nitrogen operation and complementary processes, arc also reviewed, to assess the leverage of a SiGe base bipolar technology in high speed circuits.

An overview of SiGe-based, modulation doped heterostructures is given. Strained layer handling, a prerequisite for realizing both n- and p-type devices, Is treated in terms of band engineering. The main emphasis is put on recent results obtained with high-electron mobility n-type Si/SiGe structures. Hall, Shubnikov-deHaas, and cyclotron resonance measurements are presented. The thermal stability of the heterostructures and the dopant distribution are treated with respect to device applications. Room temperature and 77K dc-measurements on very recent modulation doped field effect transistor (MODFET) implementations using implanted source/drain contacts are discussed. Device concepts with n- and p-type MODFETs combined in a superior complementary layout (CMODFET) are proposed.

A reduction of parasitic tunneling current by three orders of magnitude in epitaxial p+-n+ junctions grown by Rapid Thermal Chemical Vapor Deposition (RTCVD) compared to previously published ion implantation results is reported. These results are very important for the reduction of base current in scaled homojunction and Si/SiGe/Si heterojunction bipolar transistors. High reduction in tunneling currents allows higher limits to transistor base and emitter dopings. Significant tunneling was observed when the doping levels at the lighter doped side of the junction were of the order of 1×1019cm−3 for both Si/Si and SiGe/Si devices. These results were confirmed by I-V measurements performed at different temperatures. Since the tunneling current is mediated by midgap states at the junction, these results demonstrate the high quality of the epitaxial interface.

The complete layer sequence of a SiGe-HBT including the emitter contact is grown by Si-MBE. The base doping level was increased from 1018/cm3 up to 1020/cm3 which resulted in very low intrinsic base sheet resistivities (0.27 kΩ/□ for a 50 nm base). With a base Ge content x = 32 % the highest current gain β=5000 could be obtained. A SiGe spacer between base and collector improved the DC-characteristics considerably.

A one-dimensional charge control model based on the self-consistent numerical solution of the Schroedinger and Poisson equation has been developed for n- and p-channel quantum-well (QW) MOSFETs and MODFETs in the strained-layer SiGe heterostructure material system. Results are presented for prototype QW n-channel MODFET and MOSFET structures experimentally demonstrated in the literature. Side channel formation limits the maximum usable QW channel densities and voltage swings. The optimization of the charge control characteristics of the p-channel QW-MOSFET is achieved by maximizing the Ge content of the QW SiGe layer and minimizing the Si spacer thickness to the oxide. Channel densities on the order of 1×1013cm22 are feasible.

The influence of the location, dose and width of the doping spike in a delta doped MOSFET is investigated theoretically. Calculations are performed for a range of n and p type 5M0SFETS, and the importance of each design parameter of the device is assessed. The optimum device has a delta layer around 2 0nm deep, with a sheet doping density around 1012cm−2 for the pMOSFET and 5.1011cm−2 for the nMOSFET. The effect of diffusion of the dopant during processing on the device performance is also considered, and it is found that this causes a shift in the threshold voltage of the device. The layer width should ideally be kept below 5nm.

The use of Si1−xGex alloys for p-channel high transconductance MOSFETs requires a high quality dielectric system. Direct oxidation of Si1−xGex alloys or even low temperature deposition of SiO2 directly on Si1−xGex results in a very high interface state density. We show that low interface state densities (below 1011 eV−1cm−2) can be obtained using both thermal and PECVD oxides through the use of a thin (6–8 nm) Si cap between the oxide and the Si1-xGex layer. The Si cap layer leads to a sequential turn-on of the Si1−xGex channel and the Si cap channel, as clearly observed in low temperature C-V curves. We show that this dual channel structure can be designed to suppress the parasitic Si cap channel. High quality, fully isolated Si1−xGex p-channel MOSFETs have been fabricated in an integrable, low Dt process using both thermal or PECVD gate oxides and selective UHV/CVD for the Si/ Si1−xGex channels. We show that optimally designed Si/Si1−xGex MOSFETs exhibit up to 70% higher transconductance at 300K than control Si devices fabricated on n-doped 1017/cm3 Si substrates. Si/Si1−xGex p-channel MOSFETs with thermal and PECVD gate oxides show comparable device characteristics.

SiGe/Si heterojunction internal photoemission (HIP) long wavelength infrared (LWIR) detectors have been fabricated by molecular beam epitaxial (MBE) growth of p+ SiGe layers on p-type Si substrates. The SiGe/Si HIP detector offers a tailorable spectral response in the long wavelength infrared regime by varying the SiGe/Si heterojunction barrier. Degenerately doped p+ SiGe layers were grown by MBE using either HBO2 or elemental boron as the dopant source. Improved crystalline quality and lower growth temperatures were achieved for boron-doped SiGe layers as compared with the HBO2-doped layers. The dark current density of the boron-doped HIP detectors was found to be thermionic emission limited and was drastically reduced as compared with that of HBO2-doped HIP detectors. The heterojunction barrier was determined to be 0.066 eV from activation energy analysis of the HIP detectors, corresponding to a 18 μm cutoff wavelength. Photoresponse of the detectors at wavelengths ranging from 2 to 12 μm has been characterized with corresponding quantum efficiencies of 5 – 0.1%.

We have investigated the structural, electrical, and optical quality of epitaxial Si and Si1−xGex films grown by MBE on SIMOX (Separation by IMplanted OXygen) silicon substrates. Epitaxial films grown on these SOI substrates have been characterized using planar and cross-sectional TEM, high resolution X-ray diffraction, SIMS, and Seeco chemical etching to delineate defects. We have fabricated Si/SiGe P-i-N photodetectors integrated with Si waveguides on SOI for long wavelength applications. Low reverse leakage current densities were seen in these device structures. The photodetector exhibited an internal quantum efficiency of 50% at 1.1 μm with a frequency response bandwidth of 2 GHz.

Simulations of Ge+ and C+ implantations in Si were performed to study bandgap grading in the SiGeC/Si heterojunction bipolar transistor (HBT). Although no bandgap discontinuity was observed at the base-emitter junction, it was found that a wide-bandgap emitter and a narrow-bandgap base with proper bandgap grading were obtainable with implantation. Electrical characterization of SiGe and SiGeC diodes formed by Ge+ and C+ implantations in Si was carried out. Current-voltage (I-V) measurement results confirm that carbon doping improves the crystalline quality of the germanium-implanted layer. On the other hand, capacitance-voltage (C-V) measurements indicate that both germanium and carbon implantations result in considerable dopant deactivation.

A permeable base transistor (PBT) with submicron grating is fabricated using a Si/COSi2/Si double heterostructure. The high-quality Si/COSi2/Si double heterostructure is formed by two-step moleculer beam epitaxy (i.e. low-temperature MBE and successive high-temperature MBE). The Si and COSi2 interfaces observed by a cross-sectional transmission electron microscope are smooth and atomically abrupt. A new method of patterning COSi2 films is developed. This method uses the difference in surface energies between different crystal orientations. The mutual conductance and cutoff frequency of the PBT are 50 mS/mm and 6 GHz, respectively. These agree with the results of computer simulations. In addition, computer simulations indicate a potential of Si PBT for high frequency application, and a cutoff frequency as high as 90 GHz can be obtained by optimizing the device structure.

At a growth temperature of 800°C, Co deposited on Si (111) diffuses through a Si cap and exhibits oriented growth on buried CoSi2 grains, a process referred to as endotaxy. This occurs preferentially to surface nucleation of CoSi2 provided the thickness of the Si cap is less tfian a critical value between 100 and 200 nm for a deposition rate of 0.01 nm/s. Steady-state endotaxy is modeled under the assumption that the process is controlled by Co diffusion.

An analysis of optical absorption and internal photoemission in epitaxial CoSi2 particles buried in silicon is carried out. The optical absorption is dominated by the well-known surface plasma resonance, and calculations of the surface plasma frequencies, using the previously measured dielectric constant of CoSi2, agree well with experiment. A model for the photoemission yield is presented which includes the effects of the surface plasmon excitation and decay. The model can reproduce the observed features of the yield in photoresponse experiments. A model for the replenishment of photoemitted carriers is also presented, in which the metal particles acquire a small steady-state charge under illumination.

Epitaxial Si layers have been grown under a variety of growth conditions on CoSi2 (001) by molecular beam epitaxy (MBE). The structural properties of the Si overgrowth were studied by in-situ Reflection High Energy Electron Diffraction (RHEED), as well as ex-situ MeV4He+ ion channeling and High Resolution Transmission Electron Microscopy (HRTEM). Strong influences of the CoSi2 surface reconstruction on the Si overgrowth have been observed. RHEED studies show islanding growth of Si on the CoSi2 (001) (3/√2 × √2)R45 reconstructed surface, but smooth growth of Si on the CoSi2 (001) {√2 × √2)R45 reconstructed surface, under the same growth conditions. The growth of Si on thin layers of CoSi2 (2nm-6nm) with (√2 × √2)R45 reconstructed surface at 460°C results in high crystalline quality for the Si top layer, as indicated by good channeling minimum yield (Xmin < 6%), but cross-sectional TEM shows that the CoSi2 layers are discontinuous. We also report preliminary results on Si grown on a 2 × 2 reconstructed CoSi2 (001) surface.

In order to produce an epitaxial metal/insulator/semiconductor structure we investigated the growth of CoSi2 on CaF2/Si (111) heterostructures. We demonstrate the possibility to grow epitaxial CoSi2 on CaF2 films by utilization of the template technique. The fabrication of these structures will be presented. The properties of such heterostructures were determined by means of RHEED, LEED, AES, SIMS, RBS and resistivity measurements.

We present our new development of low temperature (<450°C) MBE growth of group II-a fluorides on Si (111) and infrared sensor fabrication in Pb1−xSnxSe grown on such fluoride layers. The stacked CaF2-BaF2 buffer helps to overcome the large lattice and thermal mismatch between the Si-substrate and the narrow gap chalcogenide. Despite a rather nominal quality of the low temperature grown CaF2 layer, the 200nm thick epitaxial BaF2 top layer is of good structural perfection as judged from RBS channelling yield, RHEED-patterns and microscopy. The fluoride buffers were successfully overgrown with Pb1−xSnxSe and small IR-sensor arrays with up to 10.5μm cut-off wavelength were fabricated in the layers. The performance of these sensors is as good as for sensors fabricated with the high temperature buffer layer growth process.

Here, we report on our efforts to engineer Si substrates for growth of compound semiconductors through the use of suitable epitaxial buffer layers of CaF2, SrF2, and BaF2 using a recently installed dual growth chamber MBE system. We have also developed new graphite-heater K-cells which have demonstrated reliable, high temperature deposition of the fluorides with excellent uniformity across substrates up to 6 inches in diameter. Excellent epitaxial quality (Xmin<5.0%) and smooth surface morphologies have been achieved for epitaxial CaF2 and SrF2 grown directly on Si (111) and BaF2 grown directly on Si (001). The BaF2 is (111) oriented on the Si (001) substrates with one of the {110} planes of the BaF2 aligned with the (110) plane in the Si (001). PbS1−xSex of excellent epitaxial quality has recently been demonstrated on the BaF2 (111)/Si (001) films. Comparable epitaxial quality has been demonstrated for CaF2 grown on Si (001) substrates using a two step growth method. We also report on preliminary results on epitaxial mixed fluoride growth on Si (111) and Si (001) substrates.