Method for altering the luminescence of a semiconductor

A method is described for altering the luminescence of a light emitting semiconductor (LES) device. In particular, a method is described whereby a silicon LES device can be selectively irradiated with a radiation source effective for altering the intensity of luminescence of the irradiated region.

BACKGROUND OF THE INVENTION 
The present invention relates generally to a method for altering the 
luminescence of semiconductors and, more particularly, to a method for 
producing light emitting semiconductor (LES) devices having regions of 
varying luminescence. 
Integrated circuits are becoming increasingly smaller and more densely 
occupied resulting in bottlenecks in the transmission of data along 
electrical pathways. While the use of optical pathways could greatly 
enhance the flow of data, finding a material which is both compatible with 
integrated circuit materials and one which could be used as an optical 
pathway as well as for new types of displays, signal processors and 
optical computers has long been a desire of the microelectronics industry. 
In particular, the use of silicon for optical pathways would integrate far 
more easily and cheaply with standard silicon-based semiconductor devices 
than current practices of adapting other materials with such devices. 
Until recently, attempts to use silicon as the material for both the 
optical and electronic circuits of an integrated circuit have been 
littered with failure. With the recent revelations of S. Furukawa, et al 
in "Quantum size effects on the optical band gap of microcrystalline Si:H" 
Phys.Rev. B Vol. 38, 5726 (1988) and in "Three-dimensional quantum well 
effects in ultra fine silicon particles" Jpn. J. Appl. Phys. 27, L2207 
(1988); H. Takagi, et al in "Quantum size effects on photoluminescence in 
ultra fine Si particles" Appl. Phys. Lett. 56, 2379 (1990); and L. T. 
Canham, in "Silicon quantum wire array fabrication by electrochemical and 
chemical dissolution of wafers" Appl. Phys. Lett. 57, 1046 (1990), methods 
for fabricating light emitting semiconductor (LES) devices from silicon 
have been developed. The essence of such methods is to produce a porous 
silicon structure on a silicon substrate wherein the porous structure 
comprises nanometer sized wires of silicon or a sponge-like structure of 
silicon. Both photoluminescence and electroluminescence have been 
demonstrated in such porous silicon structures. Nevertheless, for such 
porous silicon structures to be used as an optical pathway as well as for 
new types of displays, signal processors and optical computers, a need 
still exists to produce regions of the LES having specified intensities of 
luminescence. 
SUMMARY OF THE INVENTION 
The present invention relates generally to a method for altering the 
luminescence of semiconductors and, more particularly, to a method for 
producing light emitting semiconductors (LES) having regions of differing 
luminescence. A LES can be selectively irradiated with a radiation source 
effective for altering the intensity of luminescence of the irradiated 
region of the LES. Moreover, the irradiated region of the LES can be 
irradiated so as to achieve a pre-determined intensity of luminescence. 
Radiation sources effective for altering the luminescence of the 
irradiated region of the LES include: ion beams, electron beams, x-rays, 
gamma-ray and neutrons. The light emitting semiconductor material can 
include both naturally occurring luminescent material as well as 
semiconductor materials which can be made to luminesce. These and other 
advantages of the present invention will be discussed more completely 
below.

DETAILED DESCRIPTION OF THE INVENTION 
In order to better understand the present invention, the following 
introductory discussion is provided. Semiconductors can be made to carry a 
current when electrons (usually contributed by deliberately implanted 
impurity atoms) gain enough energy to boost them from their valence band 
to a higher energy band referred to as the conduction band. The difference 
in energy between the two bands is generally referred to as the band-gap 
energy. A semiconductor can emit light when electrons in the conduction 
band cascade back to the valence band releasing band-gap energy as light. 
Light emitting materials now used as optical pathways in integrated 
circuits (e.g. gallium arsenide) have what are known as direct band-gaps, 
which means that an energized electron in the conduction band returns 
directly to the valence band releasing the full band-gap energy as light. 
Unfortunately, crystalline silicon, the fundamental building block 
material of the semiconductor industry, has an indirect band-gap. 
Consequently, the return of energized electrons from its conduction band 
to the valence band typically comprises several intermediate steps which 
make the light emission process less probable. Moreover, the band-gap 
energy of crystalline silicon is too narrow to generate visible light even 
when rare direct recombinations do occur. 
As previously discussed, Takagi, Furukawa and Canham have each described 
various techniques for forming a luminescent layer of material on a 
substrate from a typically non-luminescent material in bulk form. In 
particular, Takagi and Furukawa describe a RF sputtering technique whereby 
a plurality of ultra fine microcrystallites of silicon can be deposited 
onto a silicon substrate, while Canham describes an alternative method 
whereby an electrochemical etch is employed to form a porous silicon layer 
on a silicon substrate wherein the porous layer comprises a plurality of 
ultra fine or microcrystalline wires of dimensions similar to those of the 
RF sputtering technique. Alternative methods for producing such a 
luminescent layer include forming a porous, sponge-like structure on the 
substrate as well as forming a layer of hydrogenated amorphous 
semiconductor (e.g. a-Si:H) on the substrate. FIG. 1 depicts a 
semiconductor substrate 10 having formed thereon a porous layer 14 
composed of a plurality of ultra fine or microcrystalline wires or pillars 
12 as described by Furukawa, Takagi and Canham, all of which is 
incorporated by reference herein. 
Making luminescent silicon devices in a way compatible with integrated 
circuits has long been an aspiration of the microelectronics industry. The 
essence of such methods is to produce a porous structure of 
microcrystalline silicon or amorphous silicon having structural dimensions 
small enough to efficiently luminesce at room temperatures. Typically, 
such porous have dimensions measured in nanometers (e.g. 2-7 nm). While 
the exact nature of this phenomena is unclear, it has been explained by 
Furukawa, Takagi and Canham in terms of the decreasing dimensions of the 
ultra fine silicon structure so as to cause the band-gap energy of silicon 
to increase. Since the techniques for fabricating such ultra fine silicon 
structures can result in variation in diameters, there can be 
corresponding variations in the band-gap energies for a specimen having a 
plurality of such ultra fine structures. As a consequence, the particular 
wavelength (color) of the emitted luminescent light can depend upon the 
dimensions of the ultra fine structures. 
Furukawa, Takagi, Canham and others have established the capability as well 
as specific methods for converting a non-luminescent material into a 
luminescent material. Hereafter, such materials will simply be referred to 
as light emitting semiconductors (LES) which is intended to include 
silicon as well as other semiconductor materials whether or not such 
materials naturally luminescence or can be made to luminescence according 
to the teachings of Takagi, Furukawa or Canham. By way of example and not 
limitation, LES materials are understood to include silicon, germanium, 
Si:H, a-Si:H (hydrogenated, amorphous silicon) and gallium arsenide. The 
present invention provides methods whereby the luminescence of such 
materials can be altered. Surprisingly, the present invention also 
provides a unique method for selectively altering properties of a 
semiconductor wafer without resort to selective chemical or 
electrochemical processes which employ a pattern or mask to limit the 
extent of such processes. As such, the entire wafer can be first processed 
by either a chemical or electrochemical process and thereafter regions of 
the wafer can be selectively altered by irradiating such regions. 
Looking now to FIG. 2, the present invention will be described in more 
detail. A LES device 40 is first fabricated by anyone of the above 
discussed techniques. In particular, the LES 40 includes a porous layer or 
microcrystalline structure 42 composed of the semiconductor material 
comprising the LES 40. One such method is more completely described below. 
The LES 40 can be irradiated with a radiation source 46 effective for 
altering the luminescence of the irradiated portions of the porous layer 
or structure 42. Such radiation sources 46 can include: ion beam, electron 
beam, gamma-ray, x-ray and neutron. As will be discussed more fully below, 
only selected regions of the LES 40 and porous layer 42 can be irradiated 
by interposing a shielding device 44 between the radiation source 46 and 
the LES 40. Those skilled in the art will appreciate that other methods 
exist for selectively irradiating only a portion of the LES 40 such as the 
use of focused radiation sources. 
It has been found that gradations in the intensity of luminescence of the 
porous layer 42 of the LES 40 can be achieved by varying the fluence of 
the irradiating beam and by varying the energy level of the irradiating 
beam as depicted in FIG. 4 and explained further below. When the porous 
layer material is silicon, gradations in its intensity of luminescence can 
be achieved by limiting the level of radiation damage to a maximum of 
0.008 displacements per atom (dpa). Generally, as the level of radiation 
damage increases from zero, the luminescence of the porous layer 42 
decreases until the maximum level of radiation damage is achieved, at 
which point the luminescence of the porous layer 42 is substantially 
eliminated. It has also been discovered that the use of higher energy 
radiation sources (e.g. 24 MeV/Cl ion beam source as opposed to a 250 KeV 
Ne ion beam source) can achieve a uniform or constant level of radiation 
damage in the irradiated portion of the porous layer 42 of the LES 40. 
EXAMPLE 
In this example, a substrate wafer comprising a (100)-oriented, 1 
.OMEGA.-cm resistivity, boron-doped, p-type silicon was used. A porous 
silicon layer was formed thereon using a double-tank electrochemical cell 
containing 5 wt. % HF in H.sub.2 O and using a 0.5 mA/cm.sup.2 anodization 
current density. The porous silicon layer formed had a thickness of 290 nm 
and a porosity of 73%. Generally, it has been found that such etching 
process will produce a porous structure sufficient for luminescence 
provided that porosity levels in excess of .about.70% can be achieved. The 
porous silicon layer was formed only within a 2.75 inch diameter circular 
area at the center of the 4 inch diameter substrate wafer. Under room 
lights and at room temperature, the porous silicon layer varied in color 
from a golden yellow at the center to purple and green at its outer edge. 
Samples (6.5 mm.times.13 mm) of uniform color were cleaved from the center 
of the wafer and a selected region of the sample was irradiated with an 
ion beam radiation source of 24 MeV Cl.sup.+5, incident at 75.degree. from 
the sample normal. The peak of the radiation damage to the substrate layer 
of the sample was 2.3 dpa. The sample was exposed to a fluence of 
5.6.times.10.sup.14 ions/cm.sup.2 in a rectangular area 1.3 mm.times.3.3 
mm of the sample. That is, not all of the sample was exposed to the 
radiation beam. In order to form amorphous structures, radiation damage 
levels in silicon generally must be of the order of .gtoreq.0.8 dpa. The 
level of damage throughout the porous silicon layer was approximately 
constant at 0.08 dpa (.ltoreq.1/10 that needed for amorphization). 
An argon laser was used as an excitation source at the following 
wavelengths: 514.5 nm, 496 nm, 476 nm, and 457 nm. The laser beam was 
incident at an angle of 60.degree. from the sample normal. The incident 
light was spread with a defocusing lens to a 4 mm tall by 8 mm wide spot 
in order to encompass the entire irradiated region as well as a portion of 
the non-irradiated region portion of the sample. A variable attenuator was 
placed between the light source and the sample and the power density for 
each wavelength was varied from 35 mW/cm.sup.2 to 685 mW/cm.sup.2. The 
emitted luminescent light was collected via a fiber optic cable attached 
to a monochromator having a 150 lines/mm grating. The room-temperature 
luminescent spectrum was then detected with a thermoelectrically-cooled 
CCD area-array photo detector. 
The luminescent intensity from the region of the sample which was not 
irradiated with the ion beam is depicted in FIG. 3. In particular, light 
emission from the sample was stimulated with an argon laser operating at a 
fixed power density (410 mW/cm2) and at four different wavelengths (e.g. 
457 nm, 476 nm, 496 nm and 514 nm). The large breadth of the luminescent 
spectra in this figure is believed to be due to a distribution in size of 
the structure comprising the porous layer. The luminescent wavelength of 
each luminescent spectra are at essentially the same value of 735 nm.+-.5 
nm and appear to be independent of the excitation wavelength. 
Similar results were also obtained with more conventional ion beam 
radiation sources, such as a 250 kev Ne ion beam radiation source. In 
particular, a second set of samples were irradiated using the Ne ion beam 
source with fluences ranging from 4.times.10.sup.12 ions/cm.sup.2 to 
4.44.times.10.sup.14 ions/cm.sup.2. Such fluences resulted in the 
following corresponding ranges of level of radiation damage in the porous 
layer from 0.0008 dpa to 0.008 dpa. Excitation of the porous layer with an 
argon laser beam of fixed power density resulted in a luminescent spectra 
depicted in FIG. 4 where luminescent intensity decreased as the level of 
radiation damage to the porous layer increased. In particular, a 20% 
decrease in the intensity of luminescence was achieved at a radiation 
damage level of 0.0008 dpa as depicted in FIG. 4. FIG. 4 also represents 
that luminescence of the porous layer has been substantially eliminated at 
a radiation damage level of 0.008 dpa. 
While the present invention has been described with the aid of specific 
semiconductor material, radiation sources and exciting laser beams, those 
skilled in the art will appreciate that variations thereof can be made, 
and that the present invention is to be limited only by claims attached 
herewith.