Optoelectronic semiconductor diodes and devices comprising same

An optoelectronic semiconductor diode is made from a layer of many small individual semiconductor particles containing doping junctions positioned between two contact surfaces mechanically supported by substrates. In the preferred embodiment, the particles are formed of a semiconductor, such as indium gallium nitride, as the active region. The particles are of a size on the order of 10 to 100 microns and are formed by reacting metallic gallium and indium with ammonia, or by a similar method. Electrical contacts are made to the particles by conductive films that have been deposited on the inner surfaces of the substrates. These contacts can be either reflective or transparent, depending upon the materials used. The particles each contain a p-n or similar junction, created either by diffusing in dopants or by selectively activating dopants that are already present. When a forward bias is applied to an LED, minority carriers spill over the junction and recombine with majority carriers to produce light. Powder LEDs according to the present invention can in principle be manufactured to operate at any wavelength within the entire visible spectrum. In addition to light-emitting diodes, the diode design may be adapted to form various types of other optoelectronic diodes such as photodetector and photovoltaic cells. Accordingly, diodes produced according to this design may be used for many applications such as flat panel displays, general purpose lighting, solar cells, and optical communication. They may be fabricated as single diodes or as arrays of diodes having the same or different optical frequency characteristics.

FIELD OF THE INVENTION 
This invention relates to an optoelectronic diode which utilizes as its 
active region a layer of individual single-crystal semiconductor 
particles. This invention also relates to individual light emitting diodes 
(LEDs) and LED arrays, both monochromatic and polychromatic, and to 
flat-panel displays and other devices comprising such diodes. 
BACKGROUND ART 
The description of the prior art is divided into three parts: LEDs in 
general, III-V nitride LEDs, and electroluminescent powder light emitters. 
LIGHT-EMITTING DIODES 
A light-emitting diode (LED) is an electro-optic device, comprising a 
junction between two differently doped regions in a compound 
semiconductor. Typically, this is a p-n junction, although p-i-n, m-i-n, 
and m-i-p junctions are occasionally used as well. When a forward bias is 
applied to the junction, majority charge carriers from each region 
(electrons in n-type material, holes in p-type material) spill over the 
junction into the other region, in which they are minority carriers. These 
minority carriers recombine with majority carriers in the region of the 
junction, releasing their energy in the form of photons of light. These 
photons are all of a wavelength corresponding to the semiconductor's 
energy gap. 
Typically, LEDs are fabricated by growing a thin epitaxial layer of a 
compound semiconductor on a doped single-crystal semiconductor wafer, 
usually of the same material as the epitaxial film. This epitaxial layer 
has both n-type and p-type regions. The wafer is then diced into many 
small pieces, and electrical contacts are attached to the front and back 
sides of the pieces to form the individual LEDs. The light is emitted from 
the side. A typical thin-film LED is shown in FIG. 1. A piece of a 
semiconductor wafer 2 contains a film with a p-n junction 4, which emits 
light when forward-biased. Electrical contacts 6 are connected to the p 
and n regions of the wafer, and the entire LED is enclosed in a plastic 
package 8. 
Monolithic arrays of LEDs are desirable for several applications, such as 
displays, printers, and photocopiers. These arrays are typically 
manufactured by fabricating a large number of LEDs on one wafer, using a 
technique such as proton implantation to electrically isolate them from 
one another. Arrays manufactured in this way are expensive, due to the 
high cost of compound semiconductor substrates, and limited to operating 
at one wavelength. 
A primary factor reducing the external efficiency of many thin-film LEDs is 
the fact that the semiconductor substrate absorbs a significant amount of 
the light emitted by the junction. This is illustrated in FIGS. 2(a) and 
2(b). FIG. 2(a) represents the operation of a thin-film LED. A significant 
portion of the light that is emitted by the p-n junction enters the 
semiconductor substrate 2, which absorbs it. Up to 85% of the emitted 
light is lost in this way. In contrast to the above prior art, an LED that 
has no semiconductor substrate to absorb the light, such as the powder LED 
described in the present invention, does not have this problem. The 
downward-emitted light which would ordinarily be absorbed by the substrate 
can instead be reflected upward by the reflective bottom contact. FIG. 
2(b) shows a powder LED of the present invention in operation. This LED 
has no semiconductor substrate, and the light that is emitted downward is 
reflected back upward by the reflective bottom contact 4. 
One prior art LED design (Nagata, U.S. Pat. No. 5,418,395) describes an LED 
fabricated from a polycrystalline semiconductor layer, rather than a 
single-crystal epitaxial layer. The individual LEDs are fabricated from 
single large grains within the polycrystalline layer. The layer is not 
grown epitaxially on a single-crystal semiconductor wafer, but is instead 
grown by a vapor-phase technique on a layer of glass, ceramic, or other 
inexpensive material. This design alleviates some of the limitations of 
single-crystal wafers, such as cost, size, and difficulty of processing. 
It thus provides an inexpensive way of creating monolithic arrays of LEDs. 
However, this design retains the limitations of vapor phase growth. For 
example, the choice of substrates is limited to those (e.g., glasses, 
ceramics, refractory metals) which can stand up to the high temperatures 
necessary to form semiconductor films. This limitation is particularly 
important in the case of nitride-based semiconductors, the growth 
temperatures of which are on the order of 1000.degree. C. and above. As 
this design uses a vapor phase growth technique, only a limited number of 
precursors (typically metallorganic compounds, which are expensive and 
often hazardous) can be used. In addition, this design is only suitable 
for fabricating monochromatic LED arrays, and cannot be used to form 
monolithic arrays of LEDs of different colors. 
NITRIDE LEDS 
In recent years, LEDs based on the III-V nitride semiconductors (InN, GaN, 
AlN, and alloys of these three materials) have become commercially 
available. Devices made from these materials, and intermediate alloys, can 
be made to emit light anywhere from the red to the ultraviolet. Most 
commercial nitride LEDs are either blue or green. Blue and green LEDs 
manufactured from the nitride semiconductors are much brighter, 
longer-lived, and more efficient than those made from the competing 
materials: silicon carbide (blue) and gallium phosphide (green). 
Nitride LEDs are fabricated from thin films that are typically grown by 
metallorganic chemical vapor deposition (MOCVD) on sapphire substrates. 
Single-crystal substrates of the nitrides do not currently exist, and are 
not likely to exist in the foreseeable future, due to the extremely high 
temperatures and pressures that would be necessary to grow large single 
crystals. Therefore, nitride films must be grown heteroepitaxially on 
other substrates. Sapphire is currently the most popular substrate, 
despite its large lattice mismatch to the nitride semiconductors. Other 
substrates, such as silicon carbide, have closer lattice matches, but are 
too expensive to be used in a economical LED fabrication process. 
As sapphire is an electrical insulator, forming bottom contacts to LEDs 
grown on sapphire is impossible. Etching or removing the sapphire 
substrate is nearly impossible due to its extreme hardness (second only to 
diamond) and imperviousness to chemicals. In order to make an electrical 
contact to the lower layer of the device, one must first etch through the 
top layer. This step is very difficult due to GaN's high resistance to 
chemicals. The need to make both electrical contacts to the LED from the 
top adds a costly processing step, and, in addition, places a limit on the 
extent to which LEDs can be shrunk. As sapphire does not cleave easily, 
the wafer must be diced into individual LEDs using a diamond saw. 
A schematic diagram of the current state-of-the-art nitride LED is shown in 
FIG. 3. The structure is processed from a multilayer film grown on a 
transparent, insulating. sapphire substrate 2. The active region is a thin 
layer of InGaN 8, which has been co-doped with both Si (donor) and Zn 
(acceptor). It sits between a layer of n-type GaN 4 and p-type GaN 6. 
Underlying the entire structure is a highly dislocated and defective 
buffer layer of GaN 10, which is produced by the very large lattice 
mismatch between GaN and sapphire. As the sapphire is non-conducting, both 
electrical contacts 12 are made from the top of the structure. 
In order to operate at visible wavelengths, nitride LEDs must contain 
indium. Pure GaN has a bandgap of 3.4 eV, which corresponds to ultraviolet 
light. InN has a bandgap of 1.9 eV, which corresponds to red light. Small 
amounts of InN are thus alloyed with GaN to obtain InGaN with a bandgap in 
the blue or green. Sufficient incorporation of indium is very difficult at 
the growth temperatures necessary for MOCVD, as the indium-containing 
metallorganic precursors desorb at the high temperatures (&gt;1000.degree. 
C.) necessary to grow nitrides. 
For example, the blue nitride LEDs currently on the market contain only 
about 15% InN in the active region; this is less than a third of the 
amount necessary to shrink the bandgap to the desired operating wavelength 
of 470 nm. Blue light is obtained by an optical transition between deep 
acceptors and shallow donors, rather than a direct band-to-band 
transition. These donor-acceptor transitions are not as efficient, in 
terms of light output per power input, as band-to-band transitions are. 
Nitride LEDs with a higher indium composition would thus be expected to be 
brighter than those currently on the market, with a lower power 
consumption. 
Currently, nitride LEDs are limited in the available choice of dopants, 
particularly p-type. The currently available p-type dopants (Zn and Mg) 
produce very deep (150 meV) acceptor levels. The depth of these levels 
decreases both the efficiency of optical transitions and the maximum hole 
concentration in p-type material. Although other elements, such as carbon 
and beryllium, are predicted to produce shallower acceptor levels in the 
nitrides, suitable precursors are not available for the vapor-phase 
techniques that are currently used for growing nearly all nitride films. A 
method of doping nitride semiconductors using elemental precursors, rather 
than chemical compounds, would vastly increase the range of available 
dopants. 
ELECTROLUMINESCENT POWDER LIGHT EMITTERS 
Light-emitting electroluminescent (EL) powder devices have been available 
since the early 1950s. A typical EL powder device is shown in FIG. 4. A 
layer of a dielectric powder phosphor 2 is placed between two insulators 
4. These insulators, in turn, are sandwiched between a reflective 
electrode 6 and a transparent electrode 8. The phosphor is typically a 
wide-bandgap material, such as ZnS, to which an impurity has been added. 
The impurities act as localized luminescent centers. The wavelength of the 
light emitted depends upon the impurity. These devices emit light when a 
high electric field, typically AC, is applied between the two electrodes. 
Electrons tunnel into the phosphor from electronic states at the 
insulator/phosphor interface, and are accelerated to ballistic energies by 
the high electric field in the phosphor. These electrons activate 
luminescent centers by impact excitation, and the excited luminescent 
centers emit photons. This mechanism differs from that of an LED in 
several ways. Light is emitted from the entire bulk of the phosphor, while 
in an LED light is only emitted from the region of the semiconductor 
junction. An EL device operates by the high-field excitation of the 
dielectric in a capacitor structure, while an LED operates by low-field 
injection of current across a junction in a diode structure. LEDs are much 
more efficient than EL devices; typical LED efficiencies are on the order 
of 10 lumens/watt, while EL phosphor efficiencies are typically less-than 
1 lumen/watt. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a simple and 
inexpensive optoelectronic semiconductor diode. It is another object of 
the invention to provide inexpensive LEDs, which can be used individually 
or integrated into monolithic arrays, either monochromatic or 
polychromatic. 
One aspect of the invention comprises a light emitting diode (LED) which 
uses many individual small particles of a semiconductor, such as indium 
gallium nitride, as the active region. This design is based on the 
observation that surfaces, interfaces, and dislocations appear not to 
adversely affect the light-emitting properties of III-V nitrides. Nitride 
films with extremely high dislocation densities (.about.10.sup.10 
cm.sup.-2) have been found to be suitable for the formation of bright 
LEDs, which demonstrate no noticeable degradation with use. Scanning 
luminescence studies of these films have shown that they consist of many 
strain-free single-crystal columnar domains which coalesce to form a 
single film. Luminescence occurs in the interiors of these domains, with 
the interfaces between the domains neither strongly contributing to the 
luminescence nor hampering it. The lack of lattice strain appears to be 
essential to the optoelectronic quality of nitrides, while dislocations 
and interfaces appear not to hinder it. The crystal particles used as the 
active region in the LEDs of the present invention are, in their 
luminescence properties, fundamentally the same as the single-crystal 
columns in a nitride film. 
The particles are of a size on the order of 10 to 100 microns. These 
crystal particles are formed by reacting metallic gallium and indium with 
ammonia, or by a similar method. (Although the growth of large single 
crystals is beyond current technological capabilities, very small single 
crystals can be grown without great difficulty.) A single layer of the 
resulting powder is sandwiched between two substrates. In the context of 
this invention, the word "substrate" will be used to refer to a sheet of 
sturdy material, such as glass, which is used to provide mechanical 
support for the electrically active portion of the device. Electrical 
contacts are made to the particles of the powder by conductive films that 
have been deposited on the inner surfaces of the substrates. These 
contacts can be either reflective or transparent, depending upon the 
materials used. If electrical isolation between LEDs is not required, the 
bottom substrate can be a metal sheet, thus acting as a contact in and of 
itself. If the LEDs are to be electrically isolated from each other (e.g., 
in an array or display), then both the top and bottom substrates must be 
electrical insulators. The electrically conductive films making the 
contacts can be patterned, by standard lithographic techniques, to define 
individual devices. 
The crystal particles each contain a p-n or similar junction, created 
either by diffusing in dopants or by selectively activating dopants that 
are already present. When a forward bias is applied to an LED, minority 
carriers spill over the junction and recombine with majority carriers to 
produce light. 
Powder LEDs according to the present invention can in principle be 
manufactured to operate at any wavelength within the entire visible 
spectrum. Pure InN has a bandgap of 1.9 eV, so that it emits light at a 
wavelength of 6500 .ANG., which is red. Pure GaN has a bandgap of 3.4 eV, 
which means that it emits light at a wavelength of 3600 .ANG., which is in 
the ultraviolet. By combining these materials into alloys, it is possible 
to achieve material emitting light anywhere from red to ultraviolet. The 
composition of the powder can be controlled by reacting an alloy of Ga and 
In of the appropriate composition with ammonia. 
Currently, prior art LEDs (nitride and otherwise) must be grown epitaxially 
on a substrate, generally by a vapor phase growth technique. This raises 
the cost of manufacturing them, due to the expensive starting materials 
required (e.g., single-crystal substrates and precursors). In addition, 
monolithic LED arrays made from epitaxial films are limited to a single 
wavelength and to the size of the wafer (typically .about.3" diameter). 
Because the active region of the LED of the present invention is formed 
without the substrate, the substrate can in principle be almost any 
transparent material. Substrates that are significantly less expensive, 
larger, and sturdier than the semiconductor wafers used by thin-film LEDs 
can be utilized. Glass is expected to be the most suitable for flat-panel 
displays, but other materials can be used as well. Clear plastic can be 
used to fabricate flexible arrays, and metal foils can be used as 
reflective substrates. 
The crystal particles can be grown with a wider range of precursors than 
can the epitaxial films used in most LEDs. The standard vapor-phase 
techniques (MOCVD and VPE) that are most commonly used to grow 
semiconductor films for LEDs utilize chemical compounds as precursors--for 
example, gallium arsenide is typically grown using trimethylgallium as a 
gallium source and arsine as an arsenic source. These sources, besides 
being expensive, are often hazardous and require elaborate safety 
equipment to handle properly. The particles that are used in this design 
can be formed using elemental precursors--for example, pure gallium--that 
are relatively inexpensive and easy to handle. The use of elemental indium 
and gallium as precursors overcomes the difficulty of incorporating indium 
into InGaN. 
Powder LEDs of different colors can easily and inexpensively be placed on 
the same substrate. For example, blue, green, and red LED's can be placed 
on one substrate by putting patches of InGaN powder of different 
compositions at different spots on the substrate. Such arrays can be used 
to manufacture monolithic full-color LED-based displays, as well as white 
light sources. 
The method described in this patent is suitable for manufacturing not only 
LED arrays but large batches of individual LEDs, which can be used for 
standard LED applications. LEDs made in this way are expected to be much 
less expensive to manufacture than those made from epitaxial films, and 
should thus be highly competitive. This is particularly true of blue LEDs, 
which are currently very difficult to manufacture from epitaxial films.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A schematic of a typical single LED according to the present invention is 
shown in FIG. 5. A layer of InGaN powder 2 is sandwiched between two glass 
plates 4. Light is generated when a forward bias is applied to p-n 
junctions 10 within the particles of the powder, and emitted from the top 
of the structure. Electrical contacts to the particles are made by 
patterned thin films on the inner surfaces of the glass plates. The bottom 
contact 6 is a thick reflective metal layer, and the top contact 8 is made 
from a transparent conductor, such as indium tin oxide (ITO), with a very 
thin layer (.about.100 .ANG.) of metal to make an ohmic contact to the 
semiconductor. This layer is sufficiently thin that it does not absorb a 
significant amount of light. 
The first step in manufacturing powder LEDs is to fabricate small 
individual crystals of InGaN. Typical alloy compositions of these crystal 
particles might be In.sub.0.51 Ga.sub.0.49 N to obtain light at 470 nm 
(blue), In.sub.0.68 Ga.sub.0.32 N to obtain light at 520 nm (green), and 
InN to obtain light at 650 nm (red). The process of forming the particles 
is illustrated in FIG. 6. A mixture of In and Ga metals 2, of the highest 
available purity (99.9999%), is placed in a ceramic crucible 4. This 
mixture contains In and Ga in the same proportions as the desired InGaN 
crystals. The crucible is placed inside a quartz tube 6, which is itself 
inside a furnace containing a radiative heat source 10. The tube is 
connected at one end to a gas manifold (not shown), and at the other end 
to a vacuum pump (not shown). Following the loading of the crucible, the 
tube is then flushed with ultra-high purity (99.999%) nitrogen gas and 
evacuated. This flushing and evacuating step is repeated several times, in 
order to remove any gases, such as oxygen and water vapor, which might 
contaminate the metals. As nitrogen is flowed through the furnace, the 
crucible and metals are heated to 200.degree. C. in order to melt the 
metals. The molten metal is allowed to sit at this temperature for several 
minutes in order to allow thorough mixing of the gallium and indium. 
Ammonia gas 8 is then introduced into the chamber at a flow rate 
sufficient for an ammonia partial pressure of approximately 1 torr. The 
metal is then heated to a temperature of about 1000.degree. C., at which 
point small single crystals of InGaN are nucleated. This reaction is 
allowed to proceed for several hours in order to form as many InGaN 
particles as possible. The resulting InGaN powder is cooled down, removed 
from the furnace, and dipped in acid to remove any residual indium and 
gallium. This InGaN powder will be n-type, due to native donors that 
naturally occur in undoped InGaN. For better control of the n-type carrier 
concentration, n-type dopants (such as silicon) can be diffused into the 
powder by a solid or vapor-phase diffusion process. 
The bottom portion of the LEDs is depicted in FIG. 7. This is manufactured 
by coating one side of a flat glass plate 2 with a layer of an appropriate 
contact metal (Ti+Al for n-type InGaN, AuNi for p-type) to form an ohmic 
contact to the crystal particles. Suitable techniques for coating the 
substrate with the metal include sputtering and e-beam evaporation. The 
coating thickness will be of the order of one micron. If the ultimate goal 
is to slice the structure into individual LED's, the entire surface of the 
glass plate will be coated with the contact metal (or a thin sheet of 
metal can be substituted for the glass, obviating the need for coating 
it.). If the structure is to be made into one or more monolithic LED 
arrays, then the metal must be patterned into strips 4, using standard 
lithographic techniques. A method of enhancing the adhesion of the powder 
to the metal contacts is depicted in FIG. 8. A thin "adhesion layer" 4 is 
deposited on top of the metal contact layer 8. This layer consists of a 
metal, such as indium, with a relatively low (.about.200.degree. C.) 
melting temperature. When the particles 2 are placed on the contacts, the 
entire structure is briefly heated to the melting point of the adhesion 
metal, while a slight amount of pressure is applied. The particles will 
become embedded in the adhesion metal while touching the contact metal. 
The adhesion metal will not make a good ohmic contact to the particles, 
and most of the current flow to the powder will be through the contact 
metal. 
At this point, the InGaN powder particles are entirely n-type. A p-type 
dopant, such as zinc or magnesium, must be diffused into the upper region 
of the powder to convert it to p-type and form a p-n junction. A method 
for accomplishing this is shown in FIG. 9. A single layer of the powder 2 
is placed on a flat plate of quartz 4, by covering the side of the plate 
with powder and shaking off the excess. This plate is then placed in a 
quartz tube 6, which is placed in a furnace that is equipped with a 
radiative heater 8. Zinc vapor in a nitrogen carrier gas 10 is flowed over 
the powder, while the powder is heated to about 800.degree. C. The vapor 
will diffuse down into the particles through a stagnant boundary layer. 
The reaction is allowed to proceed long enough that the vapor diffuses in 
to a depth of about one micron. 
After powders of the desired composition or compositions have been 
prepared, the particles are placed on top of the metal contact regions. 
Patches of powder, of differing compositions, can be placed at different 
spots on the substrate in order to form LED arrays containing LEDs of 
different colors. Each LED constitutes an individual pixel in the array. 
For example, alternating lines of red, green, and blue powder can be used 
in order to make full-color displays or white light sources. A schematic 
of this type of array is shown in FIG. 10. A method of placing patches of 
different powders on the substrate is shown in FIGS. 11 and 12. In FIG. 
11, a plate 2, made of rigid material such as glass or silicon, has 
rectangular pits of the same size and spacing as the individual LEDs of 
one color in the array. The raised area between the pits is coated with an 
adhesive material 4. This plate is lowered, adhesive side down, onto the 
powder 6. In FIG. 12, this plate is subsequently removed, leaving behind 
patches of powder 8, of the desired size and spacing. The glass substrate 
is now lowered, contact side down, onto the remaining patches of powder. 
As a slight pressure is applied, the structure is then thermally annealed 
in order to soften the metal (contact or adhesion), and cause the powder 
particles to become embedded. The result of this step is shown in FIG. 13. 
Each bottom contact strip 4 on the glass substrate 6 contains many 
individual patches of powder 2. These patches, when connected to the top 
contacts, will become individually addressable LEDs. The process is 
repeated three times in order to form LEDs of all three colors. 
The top layer of the structure is now prepared. This is shown in FIG. 14. 
An upper contact is fabricated by sputtering a thin layer 4 of a 
transparent conductor, such as indium tin oxide (ITO), on a thin glass 
plate 2. In order to make an ohmic contact to the powder, a very thin 
(.about.100 .ANG.) layer 6 of the appropriate metal (AuNi for p-type 
InGaN) is deposited. If the structure is to be diced into individual LEDs, 
the metal can be deposited over the entire surface of the substrate. If a 
monolithic LED array is desired, the ITO and metal will be patterned into 
discrete strips, again using standard lithographic techniques. 
In order to complete the structure, the top layer is placed on the powder, 
metal side down. The entire structure is again thermally annealed in order 
to enhance adhesion of the powder to the upper contacts. In order to make 
an LED array, the upper and lower layers should be aligned such that the 
contact strips on the upper layer run perpendicular to those on the lower 
layer. This is depicted in FIG. 15. Each LED in the array is made from a 
patch of powder 2, which is sandwiched between a transparent top contact 
4, running in the "horizontal" direction, and a reflective bottom contact 
6 running in the "vertical" direction. Structural support is provided by 
the top and bottom glass plates (8 and 10). The top and bottom contacts 
can be connected by ribbon cables or printed circuits to a display driver. 
Each LED in the array is uniquely connected to a single top contact strip 
and a single bottom contact strip, and can be individually turned on by 
applying a suitable voltage between those two strips. If single LEDs are 
desired instead of an array, the structure can be diced into pieces. These 
pieces can then be attached to contact wires and packaged in plastic. 
VARIATIONS AND ALTERNATE EMBODIMENTS 
Although glass is the most versatile substrate for these LEDs, other 
substrate materials can be used for different applications. For example, a 
metal foil could substitute for one or both glass layers. It would have 
the advantages of being reflective (for greater surface emission), easy to 
cut into individual devices, and a built-in bottom contact. As the metal 
is conductive, LEDs could not be electrically isolated from each other to 
form arrays, but individual LEDs could be readily fabricated. Large-area 
monochromatic LEDs--for such applications as traffic lights--could be made 
on metal substrates as well. Using two metal layers as substrates greatly 
facilitates attaching contact wires to the LED, but the LED can only be 
used in an edge-emission mode. 
Sheets of certain plastics (e.g., polyimides) can be used instead of glass 
as substrates, in order to make LED arrays and displays that are 
lightweight and flexible. Such arrays might be useful as portable white 
light sources, displays in weight-critical applications (e.g., laptop 
computers, airplanes, head-mounted displays), and "virtual paper" that can 
be used in electronic "books," "newspapers," "magazines," "maps," 
"blueprints," etc. The plastic must be able to withstand temperatures up 
to about 200.degree. C., so that the structures can be processed normally. 
A transparent display can be fabricated by using transparent substrates 
(glass and/or plastic) and transparent contacts on both sides of the LED 
array. Such an array might be useful as a "heads-up" display that can be 
incorporated into a car windshield, an airplane cockpit, eyeglasses, a 
scuba mask, etc. These displays would enable the user to read information 
without having to move his or her eyes away from his or her surroundings. 
If the display is monochromatic, its "back" side (the side away from the 
user) can be coated with a quarter-wave mirror which reflects the LED 
wavelength but is transparent to other wavelengths. (To minimize 
scattering and ghost images, the mirror should be placed as close to the 
active region as possible, preferably on the inner surface of the "bottom" 
substrate.) This will enhance the visibility of the display to the user, 
by reflecting back light which would otherwise be lost, and will protect 
the privacy of his or her information by making it invisible from the 
"back side" of the display. As the mirror is transparent to other 
wavelengths, the user will still be able to see colors other than those of 
the LEDs clearly through the display. 
A method of forming p-n, i-n, and i-p junctions, which is different from, 
and possibly easier than, that described in the previous section, is now 
described. A p-type dopant is diffused into InGaN crystal particles in a 
manner similar to that shown in FIG. 9, but, instead of an elemental 
vapor, a chemical source, such as bis-cyclopentadienyl magnesium (Cp.sub.2 
Mg) is used. The dopant is allowed to diffuse all the way into the 
particles. Alternatively, the dopant could be added to the particles by 
flowing a small amount of the dopant-containing precursor chemical into 
the quartz tube during the initial particle formation step depicted in 
FIG. 6. Although the p-type dopant pervades the entire particle, it is 
passivated due to the formation of dopant-hydrogen complexes. A method of 
forming junctions by selectively activating the dopant is shown in FIG. 
16. A single layer of InGaN particle powder 2 is placed atop a heat sink 
4, such as a thick sheet of thermally conductive metal. 
This is placed in a non-reactive ambient (e.g., nitrogen, argon, vacuum), 
under a radiative heat source 6 that can be rapidly turned on and off, 
such as a flash lamp or pulsed laser. This heat source is very quickly 
pulsed, so as to heat only the top part of the powder to a temperature 
sufficient to activate the dopants (.about.1000.degree. C.). Doping 
junctions are then created between the activated top parts of the 
particles and the inactivated bottom parts. 
An advantage of the present invention is that a wider range of dopants can 
be used than is available for thin-film LEDs that are grown by vapor-phase 
epitaxial techniques. Other dopants might be found which are better than 
those described in the preferred embodiment. For example, carbon is 
predicted to act as a shallower acceptor than zinc or magnesium in nitride 
semiconductors, but so far has not been successfully utilized because the 
carbon-containing precursors commonly used in MOCVD tend to introduce 
carbon into the film in the form of electrically inactive carbon-hydrogen 
complexes. This difficulty could be obviated by diffusing elemental carbon 
into devices. For example, the semiconductor powder might be placed on a 
surface of elemental carbon (e.g., graphite or diamond), inside a chamber 
which would subsequently be evacuated. The carbon would be heated to a 
high temperature (between about 800.degree. C. and 1000.degree. C.), in 
order to induce diffusion of pure elemental carbon into the nitride powder 
from the bottom. Beryllium is another element which is predicted to act as 
a shallow acceptor in nitride semiconductors, but is not currently used 
due to the lack of a beryllium-containing chemical that is suitable for 
vapor phase growth. Elemental beryllium could be diffused into nitride 
powder in a manner similar to that described for carbon. 
Although the III-V nitride semiconductor system is the primary material 
system discussed in this patent, other compound semiconductor systems-- 
for example, the III-V arsenides, the II-VI sulfides, and the II-VI 
selenides-- might be utilized as well to fabricate these devices. 
Individually, none of these semiconductor systems covers a wide enough 
range of bandgaps to span the entire visible spectrum, but powders of 
entirely different semiconductor materials can easily be used to make LEDs 
of the different colors. Powder LEDs made from semiconductors other than 
nitrides would not be expected to be very efficient, due to surface 
recombination effects (which do not appear to be present in nitrides), but 
might possibly prove bright enough for certain applications. 
InN powder diodes could be used as inexpensive solar cells. Solar cells are 
made from semiconductors with p-n junctions. When these junctions are 
illuminated with photons of energy equal to or greater than the 
semiconductor bandgap, electrons and holes are generated. These carriers 
diffuse across the junction and recombine, creating an electrical current. 
Most of the useful power in the solar spectrum consists of photons with 
energies between 1 and 2 eV, and semiconductors with bandgaps in that 
range are typically considered suitable for solar cells. InN has a bandgap 
of 1.9 eV, at the high end of this range. A solar cell fabricated from InN 
powder could be utilized either by itself or in tandem with a solar cell 
of another material (such as silicon, with a bandgap of 1.1 eV). Such a 
hybrid solar cell is depicted in FIG. 17. InN powder 2, containing p-n 
junctions, sits between two glass plates 4. Both electrical contacts 6 are 
transparent. Photons with an energy greater than or equal to 1.9 eV are 
absorbed by the InN layer; photons with energy between 1.1 and 1.9 eV are 
absorbed by the underlying silicon solar cell 8. In this way, a large 
portion of the solar spectrum can be utilized. 
The powder diodes described in this patent can be used as photodetectors 
operating at the different visible wavelengths. Photodetectors, like solar 
cells, typically operate by photoelectric excitation of charge carriers, 
which are swept over a junction to generate an electrical current. Such 
photodetectors would be less expensive to fabricate than prior art 
photodetectors, which use epitaxial thin films. Arrays of photodetectors, 
either monochromatic or of different colors, can be fabricated in a 
fashion similar to the LED arrays described earlier. 
Powder-based LEDs and photodetectors can be monolithically integrated with 
silicon integrated circuits (ICs). In this case, the lower contacts can be 
deposited lithographically on the silicon IC, as part of the normal VLSI 
process. The powder particles and top contacts are placed on the chip by a 
method analogous to the fabrication process described in the preferred 
embodiment. As no epitaxial growth is required, this method of integrating 
optoelectronics with silicon chips is much simpler and less expensive than 
competing technologies, such as GaAs on Si. The integration of 
optoelectronic devices with silicon ICs has many potential applications, 
such as optical interconnections between individual chips. Such a 
configuration is shown in FIG. 18. A circuit on a silicon chip 2 turns on 
a powder LED 4 which has been fabricated on the chip. The LED emits light, 
which travels through a waveguide 6, striking a photodetector 8 that has 
been similarly fabricated on another chip 10. Such an optical interconnect 
provides essentially instantaneous communication between chips, 
eliminating clock asynchronicities and allowing for faster computation. In 
addition, it eliminates the capacitive "crosstalk" that currently plagues 
electronic interconnects. 
Although the LEDs described in the previous section use p-n junctions, 
other doping junctions can be utilized as well. Suitable junctions include 
(but are not necessarily limited to) p-i-n junctions, m-i-n junctions, and 
m-i-p junctions. Schottky barriers are not suitable for LEDs, but can be 
used to make photodetectors and solar cells. A Schottky barrier diode can 
be fabricated by making one of the metal contacts to the powder with a 
metal that forms a rectifying Schottky contact, rather than an ohmic 
contact, to the semiconductor. When a semiconductor absorbs light, the 
absorption process creates an electron-hole pair. If the light is absorbed 
in the region of a Schottky barrier, the rectifying action of the barrier 
will cause the electron and hole to travel in opposite directions. The 
majority carrier (the electron, if the semiconductor is n-type; the hole, 
if it is p-type) will travel away from the barrier, while the minority 
carrier will travel into the metal. This motion of charge carriers gives 
rise to an electric current. The primary advantage of a Schottky barrier 
is that it acts as a rectifier, in a manner similar to a doping junction, 
and therefore eliminates the need to form a doping junction in the 
semiconductor. This reduces the number of processing steps necessary to 
fabricate the device, and thus makes them easier and less expensive to 
manufacture.