Light emitting device with porous material

A light emitting device (10) incorporates a layer (12) of porous silicon of low dimensionality surmounted by a discontinuous layer of silver in the form of discrete islands (20). A digitated electrode (13) is connected to the islands (20). The islands (20) have diameters in the range 5 nm to 20 nm and spacings in the range 10 nm to 50 nm, and they form a Schottky diode structure on the silicon (12). Under electrical bias, the diode structure conducts and light is generated. The device (10) is produced by vacuum deposition of silver onto a silicon wafer at a temperature which provides for the silver to separate into individual balls (20). The wafer is then anodized to produce a porous layer incorporating columns of silicon and silicon dioxide surmounted by respective silver islands (20). Each silver island (20) protects the underlying silicon (21) from the anodizing medium, and subsequently provides an electrical contact to the silicon.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a light emitting device and to a method of making 
such a device. The invention is particularly directed to a device which 
employs luminescent semiconductor properties. 
2. Discussion of Prior Art 
There has been much research recently into visible electroluminescence from 
porous silicon, which is a sponge-like structure made by first anodising 
and then etching a silicon substrate. Three papers which discuss the 
present state of the art are as follows: L. T. Canham, "Silicon Quantum 
wire array fabrication by electrochemical and chemical dissolution of 
wafers"; Appl. Phys. Lett. 57 (10), 3rd Sep. 1990, page 1046; Nobuyoshi 
Koshida and Hideki Koyama, "Visible electroluminescence from porous 
silicon", Appl. Phys. Lett. 60 (3), 20th Jan. 1992, page 347; and Volker 
Lehman and Ulrich Gosele, "Porous Silicon: Quantum sponge structures grown 
via a self-adjusting etching process", Adv. Mater. 4 (1992) No. 2, page 
114. 
The basic prior art light emitting device is disclosed in the 
aforementioned paper by Koshida and Koyama. It consists of a diode 
structure made from a substrate of p-type silicon, on top of which is 
formed a porous layer, typically 0.2 .mu.m to 1.0 .mu.m thick. Electrodes 
are placed both on the porous layer and on the underside of the substrate 
so that an electrical bias potential can be applied to the diode. One of 
the electrodes, that on the porous layer, is made of semi-transparent 
material so that light generated within the diode structure may be 
emitted. 
The porous layer is formed by subjecting the top surface of the substrate 
to anodisation. This is believed to produce a porous layer comprising an 
array of columns or wires of low dimensionality, vertical to the surface, 
and separated by holes or spaces and wherein each column comprises silicon 
embedded in silicon oxide. Generally the porosity of the layer is 
increased by subsequent etching. An etchant is used which thins the 
columns by chemical dissolution, with a resultant increase in the size of 
the spaces between the columns. Light is generated within the diode 
structure in response to electrical bias. There is currently some 
uncertainty in the scientific world as to how the structure emits light. 
SUMMARY OF THE INVENTION 
The present invention provides a light emitting device incorporating porous 
material of low dimensionality consisting at least partly of semiconductor 
material and produced by an etching process, upon the porous material a 
discontinuous layer comprising islands of electrically conducting and etch 
resistant material, together with contacting means for making electrical 
contact to the porous material and the discontinuous layer. 
The expression "low dimensionality" in relation to a material means that 
the material has, in at least one direction, dimensions of the order of or 
less than the exciton diameter or the De Broglie wavelength of electrons 
or holes in the material. This leads to quantum confinement in the 
relevant direction. Quantum wells, quantum wires and quantum dots are 
known in the prior art and correspond to one, two and three dimensional 
confinement respectively. In practice, this corresponds to material 
feature dimensions less than 50 nm in extent, and preferably less than 25 
nm. 
The invention provides the advantage that it is a light emitting 
semiconductor device which is activated by electrical bias applied to the 
contacting means. 
The invention also provides the advantage that the residual semiconductor 
material remaining in the porous layer is that located under the islands 
of electrically conducting and etch resistant material, which provided 
protection thereof during the etching process. The islands therefore 
define the locations of residual semiconductor material, and provide 
electrical contact to resulting low dimensional semiconductor material. 
The islands may have diameters of 5 nm to 100 nm, preferably 5 nm to 20 nm 
or 10 nm to 20 nm, and inter-island spacings may be in the range 10 nm to 
500 nm, preferably 10 nm to 50 nm. 
The semiconductor material is preferably silicon, and the electrically 
conducting and etch resistant material is preferably silver forming a 
Schottky diode structure with the silicon. An embodiment of the invention 
in which the semiconductor material is silicon and the etch resistant 
material is silver has produced orange electroluminescence emission in 
response to electrical bias in the range 7 to 9 volts applied to the 
contacting means. This range of bias produced current densities in the 
range 100 to 200 mAcm.sup.-2. 
The contacting means may include a digitated electrode connected to islands 
of electrically conducting and etch resistant material. 
In an alternative aspect, the invention provides a light emitting device 
incorporating porous material of low dimensionality comprising columns 
consisting at least partly of semiconductor material tipped with 
electrically conducting material, and contacting means for making 
electrical contact to the porous material and to the electrically 
conducting material. 
In a further aspect, the invention provides a method of making a light 
emitting device including the steps of: 
(a) forming a discontinuous layer of islands of electrically conducting and 
etch resistant material upon semiconductor material, 
(b) anodising the semiconductor material to produce a porous region 
consisting at least partly of semiconductor material of low dimensionality 
and protected from anodisation by the electrically conducting and etch 
resistant material, and 
(c) providing electrical connections to the semiconductor material and to 
the electrically conducting and etch resistant material respectively. 
The islands may be 5 nm to 100 nm (preferably 5 nm to 20 nm or 10 nm to 20 
nm) in diameter and may have spacings in the range 10 nm, to 100 nm, 
(preferably 10 nm to 50 nm); they may be distributed in either a random 
manner or a regular manner on the semiconductor material. Preferably the 
islands are of metallic material such as silver. 
The islands define those regions of the silicon surface which are protected 
from being anodically attacked, and thus they define the structure of the 
porous silicon layer in addition to providing electrical contact thereto. 
Silver is the preferred island material, since it forms a good Schottky 
barrier to silicon semiconductor material, thus providing a diode 
structure. 
The porous layer may be etched after anodisation. 
Electrical connections may be made to islands to form one electrical bias 
terminal of the device. A second bias terminal may be provided by making 
an ohmic contact to a side of the semiconductor material remote from the 
porous layer. The electrical connection to the islands may be formed by 
depositing a second layer of electrically conducting material such as 
silver to connect islands together to form an electrically continuous 
layer. Preferably this second layer is of digitated form, having fingers 
connecting the islands, and exposing areas of the porous layer between the 
fingers.

DETAILED DISCUSSION OF PREFERRED EMBODIMENTS 
Referring to FIG. 1, this illustrates a prior art diode structure as 
described by Koshida et al previously referred to. It has a substrate 1 of 
p-type silicon, on top of which is formed a porous layer 2, typically 
0.2-1.0 .mu.m thick. Electrodes 3 and 4 are placed on the porous layer 2, 
and on the underside of the substrate 1 so that suitable potentials can be 
applied via respective terminals 5,6. Electrode 3, on the porous layer, is 
made of semi-transparent material so that light generated within the 
structure may be emitted. 
The porous layer 2 is formed by subjecting the top surface of the substrate 
1 to anodisation, typically in hydrofluoric acid (HF) and ethanol, and at 
low moderate current densities, typically 10 mA/cm.sup.2, for 5 to 10 
minutes. The anodisation is believed to produce a porous layer 2 which is 
a low dimensional structure having quantum confinement properties as 
discussed in the prior art. It comprises an array of columns or wires, 
vertical to the surface, and separated by holes or spaces and wherein each 
column comprises silicon embedded in silicon oxide. Generally, the 
porosity of layer 2 is increased by subsequent etching, using an etchant 
which thins the columns by chemical dissolution, typically to sizes less 
than 10 nm wide, with a resultant increase in the size of the spaces 
between the columns. 
It is found that, under the correct circumstances, application of a d.c. 
source to the structure shown in FIG. 1, and with a negative potential on 
the electrode 3 and a positive potential on the electrode 4, can result in 
light being generated within the structure. There is currently some 
uncertainty in the scientific world as to how the silicon emits light, but 
the exact mechanism is not of relevance as far as concerns the present 
invention. However, as will be described later in more detail in relation 
to the invention, it is believed that luminescence emission arises from 
recombination of quantum confined charge carriers. 
Although references are made herein solely to silicon, there is evidence 
that other semiconductor materials, such as germanium, gallium arsenide 
and other compound semiconductors also exhibit quantum confinement 
effects. The specification should be construed accordingly. 
Referring to FIG. 2, there is shown a light emitting device of the 
invention indicated generally by 10. It comprises a substrate 11 of p-type 
silicon on top of which, by a method to be described, is formed a layer 12 
of porous silicon. Formed on the porous silicon layer 12 is a digitated 
electrode 13 having fingers, such as 14, of silver. In one manifestation 
the grating pitch of the electrode fingers, 14 is chosen to select and/or 
enhance the emitted light, and is in the range 0.5 .mu.m to 1 .mu.m. In 
other manifestations, the fingers 14 would have a width in the range 2 
.mu.m to 10 .mu.m and a centre to centre pitch of 4 .mu.m to 10 .mu.m. A 
typical overall size of electrode 13 would be in the range of 0.1 mm to 1 
mm square, and the mean thickness of the electrode silver would be in the 
range 10 nm to 20 nm. 
A plain ohmic contact electrode (not visible) is formed on the undersurface 
of the substrate 11. Terminals 15 and 16 enable electrical connections to 
be made to the electrode 13 and ohmic contact respectively. 
Referring now to FIGS. 3 and 4, in which parts previously described are 
like-referenced, there are illustrated steps in the method of making the 
device 10. It is to be emphasised that both FIGS. 3 and 4 are highly 
schematic and are not to scale. 
The process commences by forming on one of the surfaces of the silicon 
substrate 11 the layer 12 of porous silicon. This is achieved by 
anodisation of the surface in any known manner, for example as in the 
prior art, and optionally by subsequent etching of the surface using a 
suitable chemical etchant. Prior to anodisation, however, the surface is 
formed with an array of spots or islands 20 of conductive material, 
preferably silver. The islands may be of irregular shape, but are 
preferably generally circular. Typical island sizes range from 5 nm or 10 
nm to 20 nm diameter and typical spacing between islands is in the range 
10 nm to 50 nm. The islands may be irregularly positioned, but preferably 
they are formed in an ordered array. 
The islands 20 are formed by vacuum depositing silver onto the surface of a 
clean silicon wafer held, typically, at between 300.degree. and 
400.degree. K. in such a manner that it forms a discontinuous island film. 
It is known that thin film deposition of low melting point metals such as 
silver onto dielectrics such as Si/SiO.sub.2 results in an island deposit. 
This is because of the tendency of a system to minimise its free energy; 
i.e. because the silver/silica bond is not strong, the silver does not wet 
the silica, and it tends to ball up and form discrete hemispherical 
islands. If silver deposition is terminated at around 1 nm to 5 nm average 
thickness at 300.degree. K., silver islands in the range 5 nm to 20 nm in 
diameter would be expected, but with a wide variety of separations. Island 
diameters in the range 10-100 nm could be employed, with inter-island 
separations in the range 10-500 nm. 
Islands 20 define regions of the silicon wafer 11 where anodisation does 
not occur by protecting the regions underlying the islands from attack. 
The result is that the anodised layer takes up a structure somewhat 
similar to that illustrated in FIG. 4 in which isolated columns 21 of 
silicon underlie the islands and are upstanding from the substrate 11. 
Subsequent etching, if required, using a chemical etchant will further 
define the device structure, and increase the pore size of the porous 
silicon layer 12 to the desired extent. The islands 20 maintain their 
protection of the underlying silicon during etching. 
Electron microscopy of porous silicon produced as described above has shown 
small crystallites of silicon and large platelets of an unidentified 
single crystal phase. 
The next stage is to link the islands 20 electrically to form an electrical 
connection. This is achieved in the described embodiment by forming on the 
top surface of the structure illustrated in FIG. 4 the digitated electrode 
structure 13 shown in FIG. 2. This may be achieved by a second silver 
deposition. 
Finally a first ohmic connection 16 is made to the underside of the silicon 
substrate 11 and a second connection 15 to the electrode 13, so that an 
appropriate source (not shown) of electrical bias potential may be applied 
between the connections 15 and 16. 
The device 10 shown in FIG. 2 forms a Schottky diode; upon application of a 
negative pole of an electrical potential source to the terminal 15 and a 
positive pole of that source to the terminal 16, the device 10 will be 
forward biased and will conduct in the manner of a diode. If however, 
n-type silicon were to be used for the substrate 11, these potentials 
should be reversed. 
During conduction through the device 10, light is generated, which is 
believed to come from the porous silicon layer 12. The formation of 
columns 21 of silicon ensure that the optimum electric field is produced 
in the vicinity of the porous layer 12. In addition, a benefit of using 
silver is that there is an electromagnetic enhancement effect with silver 
due to the excitation of surface plasmons. The use of a digitated 
structure for the electrode 13 improves the distribution of generated 
light over the device 10, since, in operation, the current density within 
the porous layer 12 is higher at the electrode edges, giving an edge 
emission effect. 
One embodiment of the device 10 was subjected to an electrical bias voltage 
in the range 7 to 9 volts applied across the connections 15 and 16. This 
resulted in a current density in the range 100-200 mAcm.sup.-2 flowing in 
the device 10. Orange electroluminescence emission was clearly observed 
from the device 10. The emission was localised at edges and holes in the 
electrode 13. 
Whereas silver has a number of meritorious properties rendering it suitable 
for use in the invention, it suffers from the disadvantage of having a 
tendency to diffuse in a porous structure. It would therefore be 
advantageous to provide an alternative island and/or electrode material 
which exhibited the meritorious properties of silver but had lower 
diffusion characteristics. Silicide compounds are possible candidates for 
this. 
In order to facilitate formation of the electrode 13 and its connection to 
the islands 20, it is possible to fill the pores of the porous layer 12 
with an inert material providing a planar surface at the level of the 
islands 20. The inert material should be an insulating dielectric, eg a 
polymer such as polymethylmethacrycrate, polyethylene or 
polytetrafluoroethylene. The polymer may be introduced into the porous 
layer 12 in solution, and subsequently baked to remove solvent. 
Alternatively, a monomer may be used to fill the porous layer pores and 
subsequently polymerised in situ chemically or using ultra-violet light. 
Subsequently, excess polymer may be removed by etching or polishing to 
expose the islands 20 in a polymer surface. An electrode may then be 
deposited on the polymer surface to contact the islands 20. 
As already mentioned, there is currently some uncertainty as to the 
mechanism of electroluminescence in porous silicon structures, and the 
invention is not to be construed as being restricted to any particular 
mechanism. However, it is believed that anodisation and etching results in 
small regions of silicon being left encapsulated in porous silicon oxide. 
If anodising and etching is carried out in such a way as to structure the 
silicon as a chain of individual beads, typically of 1 nm to 10 nm in 
size, encapsulated within the silicon oxide, then these beads can act as 
quantum particles which can be excited into luminescence. In such a 
structure, luminescence in the quantum particles would be excited by 
positive holes tunnelling via the porous silicon from the p-type substrate 
on their way to the negative metal electrode. 
Other possible mechanisms are: 
(a) Quantum wire structures, as described by Canham (see above), 
(b) Amorphised silicon--this is a known red/orange emitter and the 
anodising/etching may act to render the surface region amorphous; 
(c) A siloxene compound--SiOxHy--which is luminescent, is formed by the 
anodising and etching. 
Referring now to FIG. 5, there is shown a schematic drawing of a possible 
structure 30 of silicon material produced in accordance with the 
invention. The structure 30 consists of what are referred to as "beads" of 
silicon 31 connected by regions 32 to form columns 33 surmounted by 
respective metal islands such as 34. The regions 32 are of SiO.sub.2 
dielectric material. The beads 31 and regions 32 are of varying sizes. The 
columns 33 are embedded in a matrix, indicated by lines 35, of porous 
SiO.sub.2 ; ie the matrix 35 is part SiO.sub.2, part void. 
Referring now also to FIG. 6, in which parts described earlier are like 
referenced, there is shown a schematic drawing of the band structure 40 of 
the structure 30 under an applied electrical bias voltage V.sub.bias. It 
is assumed that the structure 30 was produced from a p type silicon 
substrate as indicated at 41. Quantum confinement occurs in the silicon 
beads 31 between wider band gap SiO.sub.2 regions 32, as indicated by 
energy levels such as 42. Since the beads 31 are of varying size, the 
bound states within them are of varying energies; beads of lesser and, 
greater thicknesses correspond to higher and lower energy states 
respectively. The SiO.sub.2 regions 32 are very thin, and an electron 
within an Si bead may either tunnel through an adjacent SiO.sub.2 region 
or recombine with a hole within the same bead. Tunnelling between Si beads 
results in electrons and holes occupying a range of energy states which 
are related to the dimensions and shape of the respective bead in each 
case. Subsequent recombination produces a range of luminescent 
wavelengths. 
It is advantageous to employ a Schottky diode structure in devices of the 
invention. This is because, as illustrated in FIG. 6, bending of valence 
and conduction bands at and near a semiconductor surface is reduced by the 
presence of the metal component of the barrier. A high degree of band 
bending has the effect of reducing electroluminescence emission.