Field Effect Devices Having Short Period Superlattice Structures Using Si and Ge

This invention relates generally to field effect semiconductor devices having strained heterojunction structures, and more particularly the invention relates to strained heterojunction field effect devices which have improved carrier mobility.

Electrical and optical silicon devices are known which use silicon-germanium alloy superlattice heterostructures and heterojunctions. A major advantage of the Si.sub.1-x Ge.sub.x /Si devices lies in the fabrication compatibility with semiconductor integrated circuit technology. With the advent of molecular beam epitaxy (MBE) and low temperature chemical vapor deposition (CVD) techniques, high quality Si.sub.1-x Ge.sub.x /Si heterostructures or superlattices can be easily obtained. Recently, heterojunction bipolar transistors using the Si.sub.1-x Ge.sub.x /Si strained layers have demonstrated potential applications in high speed circuits. See, for example, Plummer and Taft, U.S. Pat. No. 4,825,269 and Iyer, et al. "Heterojunction Bipolar Transistors Using Si/Ge Alloys" IEEE Transactions on Electron Devices, Vol 36 No. 10, October 1989, pp. 2033-2064. Cutoff frequency of above 75 GHZ has been reported. Furthermore, strained Si.sub.1-x Ge.sub.x /Si quantum well P-channel MOSFETS have been demonstrated using the enhancement and transconductance (g.sub.m) due to the high mobility of the strained silicon-germanium alloy layer. See Nayak et al. "Enhancement Mode Quantum Well Ge.sub.x Si.sub.1-x PMOS" , IEEE Electron Device Letters, Vol. 12 No. 4, April 1991, pp. 154-156. The transconductance (g.sub.m) is shown to be about twice that of the conventional p-doped silicon MOSFET with the same dimension.

A major advantage in using strained Si.sub.1-x Ge.sub.x /Si MOSFETS is improved carrier mobility in the channel by proper control of strain for increasing germanium concentration. However, the mobility enhancement of strained Si/Ge layers for both electrons and holes has a limiting factor. Mobility in semiconductors is in part governed by scatterings due to phonon, impurity or alloy. The dominant scattering mechanism for silicon-germanium alloys lies in the scattering due to the random mixture of silicon and germanium atoms.

The present invention is directed to increasing carrier mobility by reducing scattering in germanium-silicon superlattice structures.

In accordance with the invention, alloy scattering is reduced in superlattice structures of field effect semiconductor devices through use of pure silicon and/or germanium layers in a short period Si/Ge superlattice for the active channels of the devices. The short period Si/Ge superlattice can be used also in modulation doped field effect transistors (MODFET) or in high mobility electron transistors (HEMT) using modulation doping and in complementary structures.

In another application, the short period Si/Ge superlattice can be employed in optical electronic semiconductor devices. Through use of quantum wires, one-dimensional quantum confinement can be achieved.

The invention and objects and features thereof will be more readily apparent from the following detailed description and appended claims when taken with the drawing.

FIG. 1A is a section view of a superlattice CMOS structure in accordance with one embodiment of the invention, and FIG. 1B is an enlarged cross section of the channel regions of the transistors in the structure.

FIG. 2A is a section view of a superlattice CMOD structure in accordance with another embodiment of the invention, and FIG. 2B is an enlarged cross section of the channel regions of the transistors in the structure.

FIG. 3A is a top view of a quantum wire transistor in accordance with another embodiment of the invention, and FIG. 3B is a section view of the gate and channel region of the transistor.

Referring now the drawing, FIG. 1A is a section view of a superlattice CMOS transistor structure in accordance with the invention, and FIG. 1B is an enlarged section view of the channel regions of the structure. The device is fabricated on a monocrystalline silicon substrate 10 which can be doped either n-type or p-type. The p-channel device is fabricated in an n-doped silicon epitaxial layer 12, and the n-channel device is fabricated in a p-doped silicon or silicon-germanium alloy layer 14. Device isolation is provided by silicon oxide 15 which surrounds each device and extends through the epitaxial layers 12 and 14. P-doped source and drain regions 16 and 18 are formed in the n-silicon layer 12, and n+ doped source and drain regions 20 and 22 are formed in the layer 14. Gates 24 and 26 of metal or doped polycrystalline silicon are formed over the channel regions between the source and drain regions of the two transistors with an insulation layer such as silicon oxide 28 and 30 separating the gates and the underlying semiconductor material.

In accordance with the invention, the channel regions of the two field effect transistors comprise alternating layers of pure germanium and pure silicon as illustrated in the section view of FIG. 1B. Each of the Si and Ge layers consists of n and m atomic layers, or is approximately 2.83 n and 2.83 m A thick, respectively. The superlattice may have a period of p, depending on respectively specific device applications. In other words, the total thickness of the superlattice is 2.83 p(m+n) A. In a period, the maximum numbers of n atomic layers of Si and m atomic layer of Ge are limited by the respective critical thickness. The latter is determined by the strain or the underlying Si.sub.1-x Ge.sub.x buffer layer. The period of the superlattice, however, is not limited if the strain is symmetrized. Or, in other words, when x is chosen such that x=n/(n+m). Shown on the left of FIG. 1A is the Si epitaxial buffer layer 12, but for strain symmetrization, a buffer layer of Si.sub.-x Ge.sub.x with a chosen x may be grown, instead of pure Si, on top of Si substrate similarly to the buffer layer 14 shown on the right. For reference of the critical thickness see, for example, R. People, "Physics and Applications of Ge.sub.x Si.sub.1-x /Si Strained-Layer Heterostructures" IEEE Journal of Quantum Electronics, Vol. QE-22, No. 9, September 1986. A thin undoped silicon cap layer of approximately 70 .ANG. thickness is grown on top of the active layer and is subsequently oxidized for providing the gate oxide. The high quality, thin gate oxide layer can be grown by rapid thermal oxidation. The active layer of doped polysilicon 24 and 26 is then formed on the oxidized cap layer. The structure employs modulation doping to reduce impurity scattering. The doping extends up to the spacer boundary of the buffer layer.

By introducing the short period silicon-germanium superlattice for the active channel, alloy scattering is eliminated. Also, due to the strained thin germanium layers, very high mobility can be achieved. For example, a bulk germanium has 2.5 and 4.2 times higher mobilities for electrons and holes, respectively, at 300.degree. K. compared to those for bulk silicon. With the compressive strain, the mobility can be increased further.

The short period silicon-germanium superlattices can be used in the channels of MODFETS and HEMT structures using modulation doping. FIG. 2A is a cross section of a superlattice CMOD device is in accordance with another embodiment of the invention. The complementary CMOD is fabricated on an n-doped or p-doped silicon substrate 40 with the p-channel device fabricated in an n-doped Si or SiGe epitaxial layer 42 and the n-channel device is fabricated in a p-doped silicon or silicon-germanium layer 44. Again, the p-channel device has p+ doped source and drain regions 46 and 48, and the n-channel device has n+ doped source and drain regions 50, 52. In the CMOD structure, Schottky gate contacts 54 and 56 are formed over the channel regions between the source and drain of the transistors. Oxide isolation is provided for each device.

Again, the channel regions comprise alternating layers of pure silicon and pure germanium as illustrated in the cross section view of FIG. 2B. An undoped setback layer 58 of silicon is epitaxially grown on the short period silicon-germanium superlattice structure to enhance carrier mobility. Similarly, the spacer layer 59 is provided before the growth of the Si/Ge superlattice to provide the modulation doping. An n+ or p+ doped epitaxial silicon or Ge/Si layer 60 of 50-100 .ANG. thickness is formed on the setback layer 58 to provide channel carriers. An undoped silicon cap layer 62 of silicon is then formed on the doped layer 60 with the Schottky gate contacting the cap layer 62.

In this case, a two-dimensional electron gas is formed in the silicon-germanium superlattice region. The choice of species (silicon or silicon-germanium) for the cap layer, the doped layer, and the setback layer for the n-channel device depends on the type of band structure controlled by the strain. In this structure, the cap and doped layers will be similar to the buffer layer to provide a potential barrier.

FIG. 3A is a top view of a short period silicon-germanium superlattice device which can be employed in quantum wire transistor applications. The material combination used may be those shown in FIG. 1A and 2A. The gate oxide 70 is shown in FIG. 3A to provide gate isolation as an example. Other combinations may be those illustrated in FIG. 2A and the superlattice may be replaced with a simple quantum well structure as those described in U.S. Pat. No. 5,155,571. In this device, mesa wires 79 function as the gate electrode between the source 72 and drain 74 of the transistor. The mesa columns are fabricated in doped polysilicon or doped germanium-silicon or metal electrode with a width on the order of 10-100 nm using electron beam, ion beam, or x-ray or other fine-line lithographic techniques along with plasma or reactive ion beam etching. The etching may be stopped at the gate definition of the gate electrode or it may be continued to go through the superlattice and to the some depth of buffer layers. The resulting quantum wires 70 provide a quantum confinement in the growth plane of the short period silicon-germanium superlattices in the channel region as illustrated at 76 in the section view of FIG. 3B. Again, a thin, undoped silicon cap layer 78 is provided on the superlattice structure and is oxidized to provide the gate oxide 79.

Since the typical periodicity of silicon-germanium layers is within a few monolayers, there is quantum confinement in the growth direction, and the quantum wires provide quantum confinement in the growth plane. Thus the quantum wires limit the momentum space and restrict scattering. Many wires can be formed between the source and drain regions to increase transconductance. In this manner, the superlattice MOSFETs and CMOSFETs as well as CMOD described above can be implemented to quantum wire forms with improved mobility and thus improved performance of the transistors. With the wire structures, the gate capacitance will be reduced in comparison with the use of a uniform plane gate structure while with increased conductance due to mobility enhancement. The reduced capacitance can further improve the frequency performance of the transistor.

There have been described several embodiments of field effect transistor structures using undoped or modulation doped short period silicon-germanium superlattice structures in the channel regions to increase carrier mobility by eliminating alloy and phonon scattering. While the invention has been described with reference to specific embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications and applications may occur to those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims.