Semiconductor device manufacturing: process – Forming schottky junction – Compound semiconductor
Reexamination Certificate
1997-10-06
2002-06-18
Dang, Trung (Department: 2823)
Semiconductor device manufacturing: process
Forming schottky junction
Compound semiconductor
C438S022000
Reexamination Certificate
active
06406984
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention pertains broadly to the field of the semiconductor arts, and more particularly to the field of photonic porous silicon.
The band gap structure for single crystal silicon exhibits a conduction band minimum which does not have the same crystal momentum as the valence band maximum, yielding an indirect bad gap. Therefore, in silicon, radiative recombination can only take place with the assistance of a photon, making such transitions inefficient. This has prevented silicon from being used as a solid state source of light, unlike group III-V semiconductors which have a direct gap at the center of the Brillouin zone. A review of these materials properties can be found in S. M. Sze,
Physics of Semiconductor Devices,
2nd. Edition (New York: John Wiley & Sons, 1981).
The discovery of photoluminescence in porous silicon has therefore generated a new optoelectronic material for study. A selected review of the fabrication techniques and properties of porous silicon can be found in the articles titled: “Silicon quantum wire array fabrication by electrochemical and chemical dissolution of wafers” by L. T. Canham, Appl. Phys. Lett., 57, 1046 (1990); “Visible light emission due to quantum size effects in highly porous crystalline silicon” by A. G. Cullis et al., Nature, 353, 335 (1991); “Visible luminescence from silicon wafers subjected to stain etches” by R. W. Fathauer et al., Appl. Phys. Lett., 60, 995 (1992); “Demonstration of photoluminescence in nonanodized silicon” by J. Sarathy et al., Appl. Phys. Lett., 60, 1532 (1992): and “Photoluminescent thin-film porous silicon on sapphire”, by W. B. Dubbelday et al., Appl. Phys. Lett., 62, 1694 (1993).
Porous silicon can be formed, for example, by using electrochemical etching, photochemical etching or stain etching of either pure silicon substrates or of the silicon of silicon on transparent substrates (e.g. silicon-on-sapphire or silicon-on-quartz) as described in the above references and the cited co-pending patent application Ser. No. 08/118,900 of Russell et al. Such techniques can produce porous silicon typically containing mechanically fragile structures on the order of approximately 5 nm or less in size. Other materials such as germanium, silicon-germanium alloys and the like may also be etched into porous form. The utilized materials may be suitably patterned lithographically prior to the etch to define device structures or to confine the region desired to be exposed to the etch solution.
In
FIG. 1
, a scanning electron micrograph of electrochemically prepared porous silicon is shown. This porous silicon was prepared, as is commonly practiced in the art, by electrochemical dissolution of silicon in a solution of 48% hydrofluoric acid and 95% ethyl alcohol in a ratio of about 1:1 with a current flow in the range of about 0.1 to 10 mA/cm
2
.
FIG. 1
depicts the typical resulting structure of this process, showing dendritic-like silicon structures (lighter regions) surrounded by a large density of voids (darker regions), i.e. a porosity of greater than about 75%. The silicon structures shown in
FIG. 1
also have similar structures on a smaller length scale, dimensions of approximately less than 10 nm, not observable by the scanning electron micrograph technique.
The photonic (light-emitting) properties of the porous silicon have been attributed to these smaller structures. Further details on the formation of these silicon structures and their light emitting properties are contained in the cited co-pending U.S. patent application by S. D. Russell et al., titled “Photonic Silicon on a Transparent Substrate” United States Patent and Trademark Office Ser. No. 08/118,900 incorporated herein by reference.
The typical emission spectrum of porous silicon is in the red, orange and yellow region, i.e. 500 to 750 nm, although green and blue emissions have also been demonstrated. Blue shift of the peak emission wavelength has been shown by increased oxidation and etching of the porous silicon as described in “Control of porous Si photoluminescence through dry oxidation” by S. Shih et al., Appl. Phys. Lett., 60, 833 (1992) and in “Large blue shift of light emitting porous silicon by boiling water treatment” by X. Y. Hou et al., Appl. Phys. Lett., 62, 1097 (1993).
The article titled “Reversible Luminescence Quenching of Porous Si by Solvents” by J. M. Lauerhaas et al., J. Am. Chem. Soc., 114, 1911 (1992) discloses that a reversible quenching of the photoluminescence is obtained from porous silicon fabricated in bulk silicon due to surface adsorbates. The degree of quenching nominally scales with the solvent dipole moment. Furthermore, it has been discovered that, in many cases, the quenching of the light emitting property is not reversible when porous silicon is contacted by solutions and chemical elements commonly used in semiconductor processing. These effects demonstrate that the light emission of porous silicon is chemically fragile, i.e. susceptible to being changed by a chemical element or compound. In addition, it is known that porous silicon can be thermally fragile, as the heating of the porous silicon structures and/or devices to temperatures approaching about 300° C. and above permanently destroys the light emitting (photonic) properties of the porous silicon.
At this time the light emitting mechanism is not fully understood. The scientific controversy surrounding the detailed physical mechanism behind the light emission has not, however, hindered the ability to fabricate porous silicon layers and useful light emitting devices using this technology as described in “Visible electroluminescence from porous silicon” by N. Koshida et al., Appl. Phys. Lett., 60, 347 (1992); “New Results on Electroluminescence from Porous Silicon” by P. Steiner et al., in
Microcrystalline Semiconductors: Materials Science & Devices
, Materials Research Society Proceedings, 283, 343 (1993) and in “Current injection mechanism for porous-silicon transparent surface light-emitting diodes” by H. P. Maruska et al., Appl. Phys. Lett. 61, 1338 (1992).
The abstract titled “Progress in the Development of Porous Silicon Light Emitters” by P. M. Fauchet et al., 1995 Electronic Materials Conference, Technical Program with Abstracts, page A51, Jun. 21, 1995, notes, that the efficiency for porous silicon light-emitting diodes remains low “due to the difficulty in making solid state contacts to a highly porous structure”.
It is known to use evaporated or sputter-deposited layers of semi-transparent gold or indium tin oxide (ITO) to make electrical contact to porous silicon layers and device structures. These techniques are line-of-sight deposition techniques that do not fill the irregular matrix of voids inherent of the porous silicon, due, in part, to the large particle size and the directionality of deposited material.
FIG. 2
is a cross-section of a silicon layer
10
in which is formed a porous silicon region
12
consisting of voids
14
and silicon structures
16
. According to a prior art technique, porous silicon region
12
is covered by an electron beam sputtered conducting layer
18
of a conductive metal such as indium-tin-oxide (typically 95% indium oxide, 5% tin oxide). As can be seen, conductive metal
18
does not fill voids
14
of porous silicon region
12
, preventing efficient electrical contact between conducting metal
18
and silicon structures
16
.
In U.S. Pat. No. 5,331,180, titled “Porous Semiconductor Light Emitting Device”, M. Yamada et al. teaches the use of a conductive polymer layer as a means to make electrical contact to porous silicon or porous silicon-carbide structures and to mechanically support the fragile porous silicon. In Yamada et al's embodiment, the conductive polymer layer binds to the top surface of the porous silicon as well as the “upper regions” of the pores or voids of the porous silicon, however the polymer conductor is not flowed to substantially fill these voids.
While prior art techniques of making electrical contact to porous silicon are known to exist, improving the ef
Russell Stephen D.
Winton Michael J.
Dang Trung
Fendelman Harvey
Kagan Michael A.
Kebede Brook
Lipovsky Peter A.
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