Materials have been made that exhibit high absorption in the visible, near ultraviolet, and near infrared spectral region while simultaneously exhibiting low absorptivity in the thermal infrared. Exemplary of such materials is silicon which is etched in a reactive sputtering process in the presence of a sputterable material. Other materials exhibiting useful properties are produced by this method.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to light absorbing materials and, more particularly, 
to materials useful for absorbing solar energy. 
2. Art Background 
The collection of energy derived from solar radiation is an alluring 
prospect. One method contemplated for solar energy utilization is the 
production of heat through absorption of solar energy by an absorbing, 
i.e., black material, utilized in a solar collector. Collectors are 
typically fabricated by depositing an absorbing material on a substrate 
that is an efficient heat conductor. Solar energy is directed onto the 
absorber by an optical system. The heat produced by absorption is 
conducted through the substrate and is either exchanged with a heat 
transfer medium or used directly. 
A variety of materials has been proposed as solar absorbers. (See C. M. 
Lambert, Solar Energy Materials, 1, 319 (1979).) Exemplary of these 
absorbing materials is electroplated black chrome (a complicated mixture 
of chrome and chrome oxides), an evaporated platinum-aluminum oxide 
mixture, and a dendritic tungsten material (described in Solar Energy, 17, 
119 (1975), Solar Energy Materials, 1, 105 (1979), and Applied Physics 
Letters, 26, 557 (1975), respectively). Although each of these exemplary 
materials has desirable properties, each also has some limitations. 
The use of solar concentrators (the focusing of solar radiation onto an 
absorbing material) has been contemplated to increase the efficiency of 
heat production and to yield higher temperatures for directly driving 
chemical reactions. Below 300 degrees C., the chrome mixture and the 
dendritic tungsten typically are useful. However, at increased 
temperatures both materials degrade. The chrome/chrome oxide compositions 
undergo decomposition induced by temperatures above 300 degrees C. The 
tungsten materials are stable in an inert atmosphere above 300 degrees C., 
but seriously degenerate at these temperatures in the presence of an 
oxidizing medium such as air. Thus, although most of the newer absorbing 
materials appear useful for solar energy absorption of one sun, at higher 
sun densities--temperatures experienced when solar concentration is 
employed--they exhibit significant problems. The platinum/aluminum oxide 
composite exhibits better stability, but is generally not useful above 
approximately 500 degrees C. 
Beside the difficulties associated with solar concentration, many of the 
absorbing materials including those previously discussed have acceptable 
absorption efficiencies, but re-radiate a substantial portion of the 
absorbed energy. This re-radiation results in decreased solar conversion 
efficiency. Additionally, presently available materials, such as the 
evaporated platinum/aluminum oxide mixture, are expensive and severely 
limit the applications for which solar energy is economical. 
SUMMARY OF THE INVENTION 
Materials having solar absorptivity up to 85 percent with low thermal 
emissivity (low re-radiation in the thermal infrared, i.e., wavelengths 
longer than 2 .mu.m) have been made. These materials rely on a particular 
channeled structure to produce the desired absorption. This channeled 
structure involves the production of voids having depths on the order of 
0.4 .mu.m or deeper and having substantially vertical walls. Generally, 
the preferred fabrication process to produce the desired channeled 
structure entails the formation of a specifically designed etch mask in 
conjunction with anisotropic etching. In a preferred embodiment, the 
materials of the subject invention are made by placing sample materials on 
a sputterable substrate, i.e., a body containing material such as aluminum 
that undergoes sputtering, and introducing an etchant gas that forms low 
melting compounds with the material sputtered from the substrate and which 
also anisotropically etches the sample material. For example, when the 
sample material is silicon, an appropriate gas is CCl.sub.2 F.sub.2 and an 
appropriate substrate is aluminum. Using a semiconductor sample such as 
GaAs, Ge, or silicon, materials that have high absorptivity in the solar 
range between about 0.2 .mu.m to 2.0 .mu.m and high reflectivity in the 
thermal infrared i.e., wavelengths longer than about 2.0 .mu.m, have been 
made. Since semiconductor grade material is not necessary, these materials 
are relatively inexpensive. 
It is also possible to produce highly absorbing compositions from materials 
such as metals. However, these materials do radiate significantly in the 
infrared and, thus, although quite useful, are generally not as efficient 
as the corresponding semiconductor materials.

DETAILED DESCRIPTION 
The highly absorbing materials of the subject invention result from the 
production of a specific structure in the absorbing material. This 
structure involves a channeled material, i.e., a material containing voids 
that intersect the surface of the material for example, voids that are 
arranged to form (1) an unconnected array, or (2) a reticulated structure, 
or (3) a combination of the two, where the walls of the voids in a 
localized area, i.e., in any area defined by a 10 .mu.m on a side square, 
are either parallel to each other or no more than 20 degrees, preferably 
no more than 10 degrees, from the mean wall direction where the wall 
direction is determined by an imaginary tangent drawn at an intermediate 
point of the void wall. To determine the intermediate points on a wall 
within a localized area, an averaging procedure is utilized. (This 
procedure is done separately for each localized area.) First a plane is 
drawn intersecting the surface of the material in a portion where the 
voids emerge, 8 in FIG. 2, and also intersecting the region of the 
material below the channeled portion, 10. This intersection determines a 
curve, 11. A least square fit line, 13, to this curve is then drawn. This 
procedure is repeated for different intersecting planes to yield a number 
of least-square-derived lines. The least square plane to this series of 
lines within the localized area is then ascertained. (Obviously, the 
larger the number of lines used for this least square process the closer 
the fit. However, generally 10 to 20 lines yield accuracy within 
experimented error.) The points at which the plane thus determined cuts a 
wall of a void are the intermediate points for purposes of the angle 
measurement. It might be noted that this procedure in the case of a 
relatively flat material seems cumbersome. However, the subject invention 
also encompasses the presence of rough surfaces and curved surfaces such 
as spheres or tubes where the concepts of direction and distance are more 
complex and where this procedure is necessary to avoid ambiguities. 
Some deviation from the angle requirement in some of the walls of the voids 
is obviously acceptable without substantially affecting the material 
absorption properties. Generally, it is sufficient for purposes of the 
invention if the desired wall direction requirement within any given 
localized area is substantially present, i.e., the wall direction is 
within the necessary 20 degree requirement over at least 75 percent of the 
wall length--the distance along the lines produced by connecting the 
defined intermediate points along the wall. (Although a random 
configuration of voids is preferred, symmetrical configurations are not 
precluded.) Examples of the general appearance for a discrete array, a 
reticulated structure, and a combination are shown in FIGS. 1, 2, and 3 
respectively. 
A substantial factor affecting absorption is the large change of refractive 
index at the absorbing material/atmosphere interface. To mitigate the 
losses due to the reflectivity produced by this large refractive index 
change, channels that decrease the size of this refractive index change 
are used. This decrease in refractive index change is most advantageously 
accomplished with channels that have cross-sectional dimensions smaller 
than the wavelength of the incident light. (It is understood that incident 
light is composed of a plurality of wavelengths each at different 
intensities. For purposes of this invention, the wavelength for this 
spectrum is considered the light of shortest wavelength that composes at 
least 0.1 percent of the total intensity of the incident light.) Larger 
cross-sectional dimensions up to ten times greater still significantly 
reduce absorption and are not precluded. For example for solar radiation 
(air mass 2) the most advantageous channel size is less than 0.3 .mu.m, 
but channel sizes of less than 3 .mu.m preferably less than 1 .mu.m 
increase absorption and are contemplated within the invention. If the 
channel size criterion is satisfied, the incoming radiation encounters an 
average refractive index difference at the atmosphere/absorbing material 
interface which is smaller than would be experienced if the channels had 
not been present. (It should be noted that the refractive index of metals 
and materials such as semiconductors are different. Thus, the optimum 
channel cross-section and depth for these materials are somewhat 
different. However, channels within the given range yield excellent 
results and a controlled sample is used to determine the best values for a 
given material.) 
The channel cross-sectional dimension--the channel size--for purposes of 
the invention is the size obtained by drawing a line at random along the 
least-squares-plane that determines the intermediate points on the walls 
of the voids in a localized area, measuring the distance along this line 
across each void, and calculating the mean value for these void distances. 
The depth of the channels and the density of the channels also 
significantly affect the average refractive index difference and, thus, 
the extent of reflectivity. The mean channel depth should be approximately 
equivalent to or deeper than the wavelength of incident radiation. General 
channels deeper than 0.4 .mu.m preferably deeper than 0.8 .mu.m are 
desirable. However, channels deeper than 5 .mu.m, although not precluded, 
are generally not advantageous since structural instability occurs. (The 
void depth is the vertical distance between the void bottom and the void 
opening measured perpendicular to the plane in a localized area defining 
the intermediate points.) It is also advantageous to have a large channel 
density so that the effective refractive index difference is smaller and, 
thus, the amount of reflected light is correspondingly smaller. Generally, 
channel densities, i.e., the fractional volume of voids in the total 
volume of the channeled region measured to the mean depth, in the range 20 
percent to 80 percent are utilized. (Channel densities are given as 
fractional volumes of the channeled region since it is possible to make a 
material where only a portion of the material is channeled.) If the 
channel density becomes too large the amount of material in a given area 
becomes undesirably low and, therefore, the amount of material available 
to absorb light is significantly diminished. For this reason, channel 
densities greater than 80 percent are usually not desirable. 
It is advantageous to use a semiconductor material that has an optical 
absorption edge in the range 1 to 2 .mu.m. (The optical absorption edge is 
the wavelength at which a sharp change in absorption occurs.) For 
crystalline semiconductor materials, this corresponds to a semiconductor 
having a bandgap in the range 1.2 to 0.6 eV. These semiconductor materials 
advantageously have relatively low absorption cross-section for infrared 
radiation. Thus, the emission of infrared radiation is correspondingly 
low. However, light having an energy greater than the bandgap is 
efficiently absorbed. Through various decay processes within the 
semiconductor material the absorbed energy is transferred to states within 
the semiconductor material that induce heating in the material. Because 
the absorption of infrared radiation in semiconductor materials within the 
preferred embodiment is low, the emission of light at these frequencies is 
similarly low and, thus, heat is not dissipated by the emission of 
infrared light. Thus, the semiconductor material efficiently absorbs 
light, efficiently changes this light to heat energy, and does not 
dissipate this heat energy in the form of re-emitted infrared light. 
The extent of absorption also depends on the thickness of the absorbing 
material. Generally for a semiconductor material with an appropriate 
bandgap, incident ultraviolet, visible and near-infrared radiation is 
absorbed within 1 .mu.m of the surface. Thicknesses significantly greater 
than this thickness as a result do not substantially increase absorption 
of usable radiation, but do increase the absorption (and thus emission) of 
thermal infrared radiation. Therefore, it is advantageous to limit the 
thickness of the semiconductor material to less than 5 .mu.m. Similarly, 
if a backing material for the absorbing material is used, it also affects 
thermal infrared emission. For example, if the backing has a metal 
surface, especially a highly polished metal surface, adjoining the 
absorbing material infrared light will be efficiently reflected and 
equally effectively prevent re-emission. Thus, the backing material will 
not contribute to heat loss through re-emission. 
Silicon is particularly advantageous for the structures of the subject 
invention. This material is abundant. Additionally, semiconductor grade 
material is not required and, therefore, it is possible to fabricate a 
relatively inexpensive absorber. Typical of other semiconductor materials 
exhibiting a high degree of absorption when having the previously 
specified channel are GaAs and Ge. 
Materials such as small bandgap semiconductor materials with absorption 
edges lower than 2 .mu.m, and metals, readily absorb in the infrared 
irrespective of their thickness. Therefore, their emission in the infrared 
is also substantial. As a result, these materials do not have the 
advantage of readily producing heat without substantial re-emission of 
infrared light. Nevertheless, metallic or small bandgap semiconductor 
materials having channels as described above exhibit relatively high 
absorptivity and are contemplated within the subject invention. 
Materials having a discrete absorption spectrum which fall outside the 
metallic or semiconductor class are not precluded. The absorption of light 
falling within the discrete absorption spectrum of these materials is 
possible. The channels produced in these materials should be of the size 
previously discussed. 
The channeled materials of the subject invention are advantageously 
produced by anisotropic etching, i.e., an etch process that removes 
material in the direction perpendicular to the surface at a rate of at 
least twice as fast as the removal rate parallel to the surface, and that 
is capable of maintaining these rates to a depth of at least 0.4 .mu.m, 
preferably to at least 0.8 .mu.m. (The removal rates are determined using 
an essentially flat control sample. A compendium of anisotropic etchants 
for a variety of materials is found in H. W. Lehmann and R. Widmer, 
Journal of Vacuum Science Technology, 15, 319 (1978).) During the 
anisotropic etching the sample is masked in a pattern that produces the 
desired void dimensions and densities with a material whose entire 
thickness is not removed in the etching process. 
In the preferred etching procedure, the etch mask is formed in situ during 
the etching process. This is accomplished, for example, by etching in an 
environment capable of producing sputtering, i.e., an environment which 
results in a measurable sputter yield, (see Handbook of Thin Film 
Technology, L. I. Maissel and R. Glang, McGraw Hill, N.Y. (1970) pages 
3-15 for a suitable method of determining sputter yield), and placing the 
material to be etched, i.e., the sample material, on or in close proximity 
to a large area of sputterable substrate so that at least a portion of the 
substrate is exposed. (A sputterable substrate denotes a composition 
having a measurable sputter yield when used with the chosen anisotropic 
etching procedure.) In a preferred embodiment a reactive gas introduced in 
a plasma etching procedure is chosen to anisotropically etch the sample 
material and, at the same time, form compounds with the substrate 
material. 
In practice, in the preferred embodiment once the plasma is struck, etching 
begins on the sample and, at the same time, the plasma produces sputtering 
from the substrate surface. Some of the sputtered substrate is redeposited 
onto the sample. The sputterable substrate is chosen so that the 
redeposited material then reacts with the gases present to form a 
composition with low vapor pressure. (Alternatively, the sputtered 
material could react in the gas phase and condense on the surface of the 
sample or the sputtered material alone could be inert to the environment 
but have appropriate properties to produce the desired results. The exact 
sequence is unimportant.) A low vapor pressure, e.g., on the order of 
10.sup.-7 Torr, is required for the mask material thus formed to allow 
sufficient quantities of the composition to accumulate on the sample 
surface. 
The composition agglomerates on the surface of the sample and acts as a 
reactive etching mask. The agglomerations prevent etching of portions of 
the sample and result in the formation of channels. Exemplary of 
contemplated mask materials are compounds produced by the interaction of 
substrates such as aluminum, magnesium, and stainless steel with chlorine 
yielded by a chlorine-containing gas such as CCl.sub.2 F.sub.2. 
The plasma should be produced under conditions which are conductive to the 
production of the desired mask on the sample material--that is in an 
atmosphere having particles with sufficient energy to induce sputtering. 
Generally plasmas produced using a power density in the range 0.2 to 2.5 
watts/cm.sup.2 are appropriate. The pressure of the etchant must be 
sufficient to produce anisotropic etching in the sample. Generally for 
isotropic etchants pressures in the range 2 .mu.m to 40 .mu.m are 
utilized. Each etchant composition produces a species which actually 
induces the etching. For example, CCl.sub.2 F.sub.2 produces chlorine and 
fluorine which etch silicon. It is possible to use the etchant composition 
alone, or in combination with other components. For example, it is 
possible to add inert gases, such as argon or helium, to stabilize a 
plasma, or to add a material to enhance production of the actual etching 
specie. Generally, the etching composition should be 5 to 100 percent of 
the total etchant. 
The depth and channel dimension are controllable by varying the pressure of 
the etchant composition, the power density, and the etch time. The 
particular combination necessary to produce a desired channel depth and 
cross-sectional dimension in a given material is determined by using a 
control sample. For example, when an aluminum substrate and a silicon 
sample are utilized, a total gas pressure in the range 5 to 40 .mu.m, with 
an etchant composed of equal parts of O.sub.2, Ar, and CCl.sub.2 F.sub.2 
produces a channel depth in the range 300 Angstroms to 2 .mu.m and 
cross-sectional dimension in the range 500 Angstroms to 5000 Angstroms. At 
these pressures, a stable plasma is maintainable utilizing a power in the 
range 0.2 W/cm.sup.2 to 2.5 W/cm.sup.2. Although adequate etching is 
produced utilizing the etching composition, e.g., CCl.sub.2 F.sub.2, 
alone, faster etching and more stable plasmas result when this etchant is 
combined with an inert gas such as Ar and with O.sub.2. In a preferred 
embodiment, the use of oxygen with CCl.sub.2 F.sub.2 at a ratio in the 
range 1:10 to 1:1 has been found to somewhat increase the degree of 
anisotropic etching and the addition of argon at a ratio of Ar to 
CCl.sub.2 F.sub.2 in the range 1:10 to 1:1 produces a more stable plasma 
when CCl.sub.2 F.sub.2 comprises at least 8 percent of the total gas 
pressure. 
The temperature of the sample at its surface also affects the channel 
dimensions. Generally, it is not possible to monitor this temperature. 
Nevertheless, the temperature is adjustable. For example, it is possible 
to insulate or heat sink the sample and affect the temperature. When heat 
sinking or insulation is utilized with a CCl.sub.2 F.sub.2 /Ar/O.sub.2 
mixture, cross-sectional dimensions were altered from about 2000 Angstroms 
for the former to about 4000 Angstroms for the latter under the same 
processing conditions. The effect of a particular 
temperature-control-measure is determined by a controlled sample. 
In a preferred embodiment thin films on a supporting substrate are treated 
by the in situ formation of an etchant mask. However, thick samples, e.g., 
thicknesses greater than 2 .mu.m, are also suitable for treatment to 
produce the desired channels. Additionally, it is also possible to produce 
a suitable mask by depositing the mask before the etch procedure is 
initiated. This is done, for example, on silicon by evaporating lead to a 
thickness in the range 500 Angstroms to 1500 Angstroms at temperatures in 
the range 25 degrees C. to 100 degrees C. 
The following example is illustrative of suitable parameters used in the 
production of highly absorbing materials within the subject invention: 
EXAMPLE 1 
A silicon substrate measuring 1 inch.times.0.5 inch.times.0.020 inch having 
one polished side with a local smoothness finer than 100 Angstroms was 
cleaned by immersing it in a hot water/detergent solution. The solution 
was ultrasonically agitated for approximately 10 minutes. The substrate 
was then removed from the detergent solution and sequentially rinsed in 
hot water followed by deionized water. The substrate was then scrubbed 
with a lint-free foam swab in deionized water. To remove the water, the 
substrate was treated in a vapor degreaser with isopropyl alcohol vapor. 
The substrate was placed in an ion pumped vacuum station. The substrate was 
positioned approximately 5 inches above a multicrucible 3 kW electron beam 
evaporation source. A layer of about 1400 Angstroms of tungsten was 
deposited on the substrate by energizing the crucible containing an 
approximately 1/3 cm.sup.3 piece of tungsten. The thickness during the 
deposition was measured and monitored by using a standard quartz crystal 
film thickness measuring device. During the tungsten deposition, the 
vacuum pressure was maintained in the 10.sup.-6 Torr range by employing in 
addition to the ion pumps a titanium sublimation pump and a liquid 
nitrogen-cooled panel. The tungsten was deposited at a rate of about 12 
Angstroms per second. 
Following the tungsten deposition a layer of silicon about 2.2 .mu.m thick 
was deposited in a similar manner at a rate of about 20 Angstroms per 
second. In this case, however, the entire thickness of the film was 
deposited in three layers with a waiting period of at least one-half hour 
between evaporations. This was done to prevent overheating of the 
substrate and the vacuum system fixtures. The pressure in the vacuum 
system varied from about 1.times.10.sup.-8 to 4.times.10.sup.-7 Torr 
during the silicon deposition. After cooling to about 40 degrees C. the 
silicon coated substrate was removed from the deposition system for 
subsequent etching. 
The reactive ion etching process was done using a conventional diode 
sputtering system. The system used an oil diffusion pump with an optically 
dense water-cooled baffle. The plasma was generated by a 13.56 MHz rf 
generator connected to two parallel, water-cooled electrodes 5 inches in 
diameter. The rf matching network on the sputtering system was tuned to 
supply all of the power to the electrode on which the samples were to be 
etched. The electrode on which the samples were etched was covered with a 
5 inch diameter aluminum plate which was thermal interconnected to the 
water-cooled electrode. The second electrode was fused quartz. 
The flow of the reactive gases through the sputtering systems was 
controlled using both pressure and flow-ratio servo systems. A capacitance 
manometer was used to monitor the pressure. The signal from this manometer 
was used to adjust the flow of CCl.sub.2 F.sub.2. (This may be designated 
as the main gas.) The flow of the other two gases O.sub.2 and Ar was 
controlled by a flow/ratio controller. By this means, either the flow of 
the secondary gases or the ratio of their flow to the main gas flow could 
be held constant. The flow rate of all the gases was monitored using a 
thermal mass flowmeter with a 100 standard cubic centimeter per minute 
(SCCM) full scale sensitivity. The gases were mixed in an external 
manifold before entering the station. The manifold was heated to 
approximately 48 degrees C. to reduce adsorption of gases on the walls. 
The cleaned silicon sample was placed in the center of the aluminum plate 
with the deposited silicon upward and the system was pumped down to a 
pressure of less than 1 .mu.m. Argon, CCl.sub.2 F.sub.2, and oxygen were 
fed into the chamber at equal rates of 3.7 SCCM using the flow control 
system described above. (It should be noted that equal rates are not 
equivalent to equal mole fraction and that relative pumping speeds of 
gases determine the actual mole fraction in the plasma.) The total 
pressure in the chamber was controlled to be 20 .mu.m. The rf power was 
turned on to 70 W total giving a self bias of -540 V on the aluminum 
plate. The power was applied for a total of 9 minutes. 
The surface of the Si film, as observed by use of a scanning electron 
microscope, has a columnar etch pattern. (Electron micrographs are shown 
in FIG. 4, a micrograph at 45 degrees indicated 30 and a micrograph of a 
cleaved edge taken at an angle of 70 degrees indicated by 31.) The voids 
on the silicon have channel dimensions of about 1000 Angstroms. The 
vertical depth was about 5000 Angstroms. 
The appearance of this absorbing film to the unaided eye is dark black. The 
specular reflectivity was measured using a commercial reflectivity 
attachment with a dual beam spectrophotometer. From this measured 
reflectance, FIG. 5, the absorptance over the visible spectrum, i.e., the 
wavelength range of 0.4 .mu.m to 0.7 .mu.m is greater than 99.5 percent. 
The solar absorptance, i.e., the weighted average of the films' 
absorptivity over the solar spectrum is greater than 85 percent. 
This process demonstrates the various parameters used in a preferred 
embodiment of the invention to produce an excellent solar absorber. The 
silicon substrate was used as a supporting layer. This substrate was 
sequentially coated with a reflective metal and a silicon layer. The 
reflective metal was used to enhance absorption in the overlying silicon 
layer. This overlying silicon layer was then channeled to produce the 
excellent inventive absorbers. To accomplish the desired channeling, the 
silicon substrate was placed on an aluminum plate. The interaction of the 
aluminum with the etchant, i.e., CCl.sub.2 F.sub.2 produced the desired in 
situ mask. The CCl.sub.2 F.sub.2 in addition to forming the in situ mask 
also produces the desired anisotropic etching of the underlying silicon.