Bernard S. Meyerson

Low‐temperature silicon epitaxy by ultrahigh vacuum/chemical vapor deposition

We have successfully demonstrated the use of a novel chemical vapor deposition technique, ultrahigh vacuum/chemical vapor deposition, to deposit homoepitaxial silicon layers of high crystalline perfection at low temperatures (T≥750 °C). Rutherford backscattering and transmission electron microscopy showed the transition to epitaxial silicon growth took place in the range 700–750 °C, and secondary ion mass spectrometry showed typical oxygen and carbon levels to be near the detection limits of the technique 1016–1017 cm−3. In addition, abrupt dopant transitions have been demonstrated, with B levels dropping four orders of magnitude, 1019–1015 B/cm3, in the first 1000 angstroms of an intrinsic epilayer.

75-GHz f/sub T/ SiGe-base heterojunction bipolar transistors

The fabrication of silicon heterojunction bipolar transistors which have a record unity-current-gain cutoff frequency (f/sub T/) of 75 GHz for a collector-base bias of 1 V, an intrinsic base sheet resistance (R/sub bi/) of 17 k Omega / Square Operator , and an emitter width of 0.9 mu m is discussed. This performance level, which represents an increase by almost a factor of 2 in the speed of a Si bipolar transistor, was achieved in a poly-emitter bipolar process by using SiGe for the base material. The germanium was graded in the 45-nm base to create a drift field of approximately 20 kV/cm, resulting in an intrinsic transit time of only 1.9 ps.<<ETX>>

Heterojunction bipolar transistors using Si-Ge alloys

Advanced epitaxial growth techniques permit the use of pseudomorphic Si/sub 1-x/Ge/sub x/ alloys in silicon technology. The smaller bandgap of these alloys allows for a variety of novel band-engineered structures that promise to enhance silicon-based technology significantly. The authors discuss the growth and properties of pseudomorphic Si/sub 1-x/Ge/sub x/ structures and then focus on their applications, especially the Si/sub 1-x/Ge/sub x/-base heterojunction bipolar transistor (HBT). They show that HBTs in the Si/sub 1-x/Ge/sub x/ system allow for the decoupling of current gain and intrinsic base resistance. Such devices can be made by using a variety of techniques, including molecular-beam epitaxy and chemical vapor deposition. The authors describe the evolution of fabrication schemes for such HBTs and describe the DC and AC results obtained. They show that optimally designed HBTs coupled with advanced bipolar structures can provide performance leverage. >

Si/SiGe epitaxial-base transistors. I. Materials, physics, and circuits

A detailed review of SiGe epitaxial base technology is presented, which chronicles the progression of research from materials deposition through device and integration demonstrations, culminating in the first SiGe integrated circuit application. In part I of this paper, the requirements and processes for high-quality SiGe film preparation are discussed, with emphasis on fundamental principles. A detailed overview of SiGe HBT device design and implications for circuit applications is then presented. >

Cooperative growth phenomena in silicon/germanium low‐temperature epitaxy

A series of Si:Ge alloys and structures has been prepared by ultrahigh‐vacuum chemical vapor deposition. Alloys of composition 0≤Ge/Si≤0.20 are readily deposited at T=550 °C. Commensurate, defect‐free strained layers are deposited up to a critical thickness, whereupon the accumulated stress in the films is accommodated by the formation of dislocation networks in the substrate wafers. A cooperative growth phenomenon is observed where the addition of 10% germane to the gaseous deposition source accelerates silane’s heterogeneous reaction rate by a factor of 25. A model is proposed where Ge acts as a desorption center for mobile hydrogen adatoms on the Si[100] surface, accelerating heterogeneous silane pyrolysis by the enhanced availability of chemisorption sites.

Oxidation studies of SiGe

We have studied the kinetics and mechanism of oxidation of SiGe alloys deposited epitaxially onto Si substrates by low‐temperature chemical vapor deposition. Ge is shown to enhance oxidation rates by a factor of about 3 in the linear regime, and to be completely rejected from the oxide so that it piles up at the SiO2/SiGe interface. We demonstrate that Ge plays a purely catalytic role, i.e., it enhances the reaction rate while remaining unchanged itself. Electrical properties of the oxides formed under these conditions are presented, as well as microstructures of the oxide/substrate, Ge‐enriched/SiGe substrate, and SiGe/Si substrate interfaces, and x‐ray photoemission studies of the early stages of oxidation. Possible mechanisms are discussed and compared with oxidation of pure silicon.

SiGe-channel heterojunction p-MOSFET's

The advances in the growth of pseudomorphic silicon-germanium epitaxial layers combined with the strong need for high-speed complementary circuits have led to increased interest in silicon-based heterojunction field-effect transistors. Metal-oxide-semiconductor field-effect transistors (MOSFETs) with SiGe channels are guided by different design rules than state-of-the-art silicon MOSFETs. The selection of the transistor gate material, the optimization of the silicon-germanium channel profile, the method of threshold voltage adjustment, and the silicon-cap and gate-oxide thickness sensitivities are the critical design parameters for the p-channel SiGe MOSFET. Two-dimensional numerical modeling demonstrates that n/sup +/ polysilicon-gate SiGe p-MOSFETs have acceptable short-channel behavior at 0.20 /spl mu/m channel lengths and are preferable to p/sup +/ polysilicon-gate p-MOSFETs for 2.5 V operation. Experimental results of n/sup +/-gate modulation-doped SiGe p-MOSFETs illustrate the importance of the optimization of the SiGe-channel profile. When a graded SiGe channel is used, hole mobilities as high as 220 cm/sup 2//V.s at 300 K and 980 cm/sup 2//V.s at 82 K are obtained. >

Mechanism and conditions for anomalous strain relaxation in graded thin films and superlattices

Compositionally graded films of SiGe/Si(100) and GaInAs/GaAs were grown under different conditions in order to investigate the different modes of strain relaxation associated with the compositional grading. We show that, when the growth conditions are very clean and the gradient is shallow enough (about 1% misfit per half micron), very good, relaxed films are obtained. This coincides with the introduction of large numbers of dislocations in the substrate itself, which is counter‐intuitive at first since the substrate is under negligible strain. We show that this introduction of dislocations is the result of the activation of novel Frank–Read‐like sources located in the graded region, and is directly correlated to the lack of other low energy nucleation sites for dislocations. We detail the conditions of growth necessary for this phenomenon to occur, and show that it operates both for the SiGe/Si system and the GaInAs/GaAs system. Pure, relaxed Ge films have been grown in this manner on Si(100), with a def...

Si/SiGe epitaxial-base transistors. II. Process integration and analog applications

For pt. I, see ibid., vol. 3, p. 455-68 (1995). This part focuses on process integration concerns, first described in general terms and then detailed through an extensive review of both simple non-self-aligned device structures and more complex self-aligned device structures. The extension of SiGe device technology to high levels of integration is then discussed through a detailed review of a full SiGe HBT BiCMOS process. Finally, analog circuit design is discussed and concluded, with a description of a 12-bit Digital-to-Analog Converter presented to highlight the current status of SiGe technology. >

Bistable conditions for low‐temperature silicon epitaxy

We report on the role of hydrogen surface passivation in achieving low‐temperature silicon epitaxy by chemical vapor deposition processes. Upon insertion of an HF‐etched silicon wafer into an epitaxial silicon deposition apparatus, residual contamination of the Si surface is negligible. Si 2p core level photoemission spectra demonstrate that the silicon surface is stable in air and free of SiO2 for a time period of minutes. The predominant passivating species is found to be silicon hydride. We demonstrate that hydrogen passivation by HF pretreatment leads to two divergent temperature ranges where epitaxy is successful, those being a low‐temperature range, 425≲T≲650 °C, and a high‐temperature regime, T≳750 °C. Additionally, we employ temperature‐programmed desorption techniques to elucidate the role of hydrogen in the transition to a steady‐state growth process, employing ultrahigh vacuum/chemical vapor deposition as the model system.