Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal
Reexamination Certificate
2000-01-11
2002-06-11
Christianson, Keith (Department: 2813)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
Reexamination Certificate
active
06403391
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor light emitting device used in an optoelectronic integrated circuit and an image display device, and more particularly to a semiconductor light emitting device and a method for manufacturing the same using a porous silicon.
2. Description the Related Art
The porous silicon (hereinafter referred to as a “PS”) differs from a crystalline silicon (hereinafter referred to as a “c-Si”) in optical properties, and absorption edge energy generally becomes large. Moreover, electrical. properties also changes, and the resistivity becomes high as compared with the original c-Si. Three kinds of PSs are known as follows:
(a) NANOSTRUCTURE PS:
The PS of which a porosity is 20 to 80% and the diameter of microporous holes is not more than approximate 2 nm is referred to as “a nanostructure PS”. Differing from the c-Si, the nanostructure PS shows luminescence in a visible-light range. Pumping this nanostructure PS by the shorter wavelength light within the spectral region from blue to ultra violet, photoluminescence (PL) of a luminescence efficiency (external quantum efficiency) of approximate 10% at the maximum can be observed. Moreover, electroluminescence (EL) can be obtained also by injecting current into the nanostructure PS.
(b) MESOSTRUCTURE PS:
On the other hand, the PS of which the porosity is 40 to 60% and the diameter of the microporous holes is approximate 2 to 50 nm is referred to as “a mesostructure PS”. The luminescence efficiency of the mesostructure PS is generally low as compared with the nanostructure PS, and an emission wavelength also generally comes to the longer wavelength than the nanostructure PS. The mesostructure PS is coarse in structure as compared with the nanostructure PS and is low in resistivity as compared with the nanostructure PS.
(c) MACROSTRUCTURE PS:
Moreover, a PS of which the porosity is further low than the mesostructure PS and the diameter of the microporous holes is 50 nm or more is referred to as “a microstructure PS”. The macrostructure PS can hardly emit light and is further low in resistivity as compared with the mesostructure PS.
These PSs are formed by anodization, or by feeding a current inwardly from the surface of the silicon through the c-Si (single crystal silicon or polycrystalline silicon) as electrodes in the solution containing hydrogen fluoride (HF). Moreover, as a cathode, materials such as platinum (Pt) being usually not dissolved into an anodization solution is used. Although the PS and the material having the structure similar thereto can be made by other methods, they are omitted because of being not important in the invention.
Thus, the PS is constituted by the number of the microporous holes of the diameter of approximate 1 to 100 nm, remained small c-Si particles or a skeleton, and an amorphous portion surrounding thereabouts. By changing the conditions such as the conductivity type and resistivity of the original c-Si, the current density at anodizing, the composition of the anodization solution, the presence or absence of light irradiation and the intensity of the light irradiation, the structure of the PS being made is changed, whereby the nanostructure PS, the mesostructure PS or the macrostructure PS can be obtained.
For example, the nanostructure PS is obtained by anodizing the c-Si containing a p-type impurity doped to the extent being not degenerated (a non-degenerate p-type). Moreover, the nanostructure PS is obtained also by anodization while irradiating a non-degenerate n-type c-Si of a low impurity concentration with light. This nanostructure PS is fine so that the porosity is approximate 20 to 80% and the diameter of the holes is not more than 2 nm. That is, since remaining c-Si particles or a size of the skeleton are fine, the resistivity becomes high as compared with that of the original c-Si. For example, the nanostructure PS can be obtained by anodizing the degenerate p-type c-Si or the degenerate n-type c-Si, containing the p-type or n-type impurity with higher impurity concentration so that the Fermi level is located within the valence or conduction band. For example, the macrostructure PS can be obtained by anodizing the non-degenerate n-type c-Si in a darkroom.
The above-noted description of the three kinds of PSs which differ in structure is performed on the generalized characteristics of the respectively typical one, and actually, there are the PSs having the characteristic intermediate between the mesostructure PS and the nanostructure PS and the PSs having the characteristic intermediate between the mesostructure PS and the macrostructure PS or the like. Moreover, for example, even the PS belonging to the same nanostructure PS can differ in the fine structure in some cases depending upon the difference of a conductivity type of the original c-Si. Moreover, even though the original c-Si is uniform, the PS of which the structure differs in the direction of a depth can be made depending upon anodizing conditions. Furthermore, even the PS belonging to the mesostructure PS or the macrostructure PS as the general structure and characteristic, the PS containing the nanostructure PS can be made partially in the microscopic portion depending upon the anodizing conditions.
Therefore, when making a light emitting device using the PS, a sufficient consideration should be taken in the both sides of the element design from the viewpoint of by which structure PS a layer is constituted and for what it is used, and a selection of a method for making the element structure.
It is reported in the proceedings of the 44 th Japan Society of Applied Physics and Related Society Symposium, No.2, P.806, Section a-B-6, “Characteristics of a pn-junction type photoanodically fabricated porous silicon LED”, by Nishimura, Nagao and Ikeda that external quantum efficiency of the EL luminescence comes to approximate 1% at the maximum in the light emitting device using the PS (hereinafter referred to as a “PS light emitting device”). This PS light emitting device is made by preparing a c-Si wafer that the p
+
type c-Si layer is formed on the n-type c-Si substrate to anodize the surface of this c-Si wafer under the irradiating with light using a lamp. When anodizing under such conditions, the p
+
type c-Si layer of the surface of which resistivity is low becomes the mesostructure PS layer and the n-type c-Si substrate portion of the area which no light from the lamp reaches becomes the macrostructure PS. In FIG.
1
and
FIG. 2
, the structure and the equipment for manufacturing this PS light emitting device are shown.
Referring to
FIG. 1
, the macrostructure PS layer
63
made from the n-type c-Si, hereinafter referred to as “a n-type macrostructure PS layer
63
”, is formed on a n-type c-Si substrate
64
. And the nanostructure PS layer
62
made from the n-type c-Si, hereinafter referred to as “a n-type nanostructure PS layer
62
” is formed on the n-type macrostructure PS layer
63
. And further the mesostructure PS layer
61
made from the p
+
type c-Si, hereinafter referred to as a p-type mesostructure PS layer, is formed thereon. Moreover, the expressions of “the n-type macrostructure PS layer”, “the n-type nanostructure PS layer”, “the p-type mesostructure PS layer” or the like are expressed for convenience and differ from the n-type and the p-type in the c-Si. The reason why is that, generally, in the PS layer, acceptor impurities and donor impurities are inactivated at room temperature. A translucent gold electrode
66
which serves as an anode is formed on the p-type mesostructure PS layer
61
and an aluminum electrode
65
which serves as a cathode is formed on the back of the n-type c-Si substrate
64
. A direct current power supply
67
for the EL is connected between the anode
66
and the cathode
65
. In the structure shown in
FIG. 1
, the n-type nanostructure PS layer
62
acts as an EL active layer. Moreover, the p-type mesostructure PS layer
61
has a function to form a junction similar to the pn-
Nagao Yasuyuki
Nishimura Kohsuke
Christianson Keith
Kokusai Denshin Denwa Kabushiki-Kaisha
Olson & Hierl Ltd.
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