High efficiency light emitting diodes from bipolar gallium nitride

The invention is a method of growing intrinsic, substantially undoped single crystal gallium nitride with a donor concentration of 7.times.10.sup.17 cm.sup.-3 or less. The method comprises introducing a source of nitrogen into a reaction chamber containing a growth surface while introducing a source of gallium into the same reaction chamber and while directing nitrogen atoms and gallium atoms to a growth surface upon which gallium nitride will grow. The method further comprises concurrently maintaining the growth surface at a temperature high enough to provide sufficient surface mobility to the gallium and nitrogen atoms that strike the growth surface to reach and move into proper lattice sites, thereby establishing good crystallinity, to establish an effective sticking coefficient, and to thereby grow an epitaxial layer of gallium nitride on the growth surface, but low enough for the partial pressure of nitrogen species in the reaction chamber to approach the equilibrium vapor pressure of those nitrogen species over gallium nitride under the other ambient conditions of the chamber to thereby minimize the loss of nitrogen from the gallium nitride and the nitrogen vacancies in the resulting epitaxial layer.

FIELD OF THE INVENTION 
The present invention relates to light emitting diodes formed in 
semiconductor materials, and in particular, relates to light emitting 
diodes formed from bipolar junctions in gallium nitride. 
BACKGROUND OF THE INVENTION 
Light emitting diodes (LED's) are semiconductor devices that emit light in 
response to the application of electric current. As is known to those 
familiar with the properties of electromagnetic radiation and the 
properties of materials, electromagnetic radiation is emitted from a 
material, or correspondingly absorbed by a material, when the material 
undergoes some sort of internal transition. When the transitions are 
vibrational in nature, the associated frequencies and wavelengths of 
electromagnetic radiation are generally in the infrared region. When the 
transitions are rotational, the associated wavelengths and frequencies are 
in the microwave region. Each of these terms is, of course, used somewhat 
broadly in this brief description. Visible light, however, i.e. 
electromagnetic radiation visible to the human eye, is generally 
associated with electronic transitions between electron energy levels in 
atoms or molecules. Additionally, light which is not visible to the human 
eye, but which falls generally into the ultraviolet (UV) region is 
likewise related to such electronic transitions. 
As further known to those familiar with the interaction of light and 
materials, electronic transitions refer to the movement of electrons 
between various allowed positions in atoms or molecules. These positions 
can be referred to by a number of designations, but for purposes of this 
discussion, and as generally used in the art, these will be referred to as 
bands. Typically, when an electron moves from a higher energy band to a 
lower energy band, the energy associated with that transition will be 
given off as a photon; i.e. light. Correspondingly, when an appropriate 
wavelength of light is applied to a material, the transition from a lower 
energy band to a higher energy band may take place. Although much more 
could be said about such transitions, a fundamental point of the nature of 
visible light is that the wavelength of the photon emitted is directly 
related to the energy difference between the appropriate bands. 
In turn, the energy difference between bands is a function of the 
particular material. Some materials have larger differences, others lesser 
ones. Accordingly, the wavelength or wavelengths of light emitted by any 
particular material are a fundamental characteristic of that material and 
although somewhat alterable are generally limited. 
The nature of light is such that ultraviolet light or light in the blue 
portion of the visible spectrum represents higher energy photons and 
larger energy differences between bands than does light in the red portion 
of the spectrum which represents lower frequencies, longer wavelengths, 
and lesser energy differences between bands. Fundamentally, this requires 
that a material which will emit light in the blue portion of the visible 
spectrum must have an energy difference or "gap" between bands that is 
larger than that required by the production of other colors of light. 
Accordingly, only certain materials can appropriately be used to form 
light emitting diodes which can emit blue light. 
As recognized by those familiar with these facts and phenomena, the blue 
and ultraviolet portions of the electromagnetic spectrum are adjacent one 
another and the general designations for such wavelengths are often used 
in overlapping fashion. Thus, as used in this art and as used herein, 
terms such as "blue," "violet," "near ultraviolet," and "ultraviolet," are 
used descriptively, rather than in any limiting fashion. 
Gallium nitride (GaN) is one such promising candidate for use in "short 
wavelength" devices (i.e. blue LED's and UV LED's), because it has a 
direct band gap of 3.39 electron volts (eV). In a direct band gap 
material, the minimum of the conduction band and the maximum of the 
valance band coincide at the same momentum, which in simpler terms means 
that all of the energy from the transition that takes place when an 
electron moves between the bands is emitted as light. In indirect band gap 
materials, some of the energy of the transition is given off in forms 
other than light, usually as vibrational energy. Thus, direct band gap 
materials such as gallium nitride have an inherent efficiency advantage 
over indirect band gap materials. 
In order to produce appropriate LED's from candidate materials such as GaN, 
however, more than the material itself is required. First, extremely pure 
material is required because impurities, even in relatively small amounts, 
usually modify, interfere with or even prevent electronic transitions as 
well as other electronic characteristics of a material. Secondly, an LED 
generally requires a single crystal of an appropriate material because 
multiple crystals or defects in single crystals likewise undesirably 
modify or negate the electronic characteristics of a material. 
Additionally, in order to produce a working device, structure must be 
included which provides the opportunity for current or voltage to be 
applied to the device and to initiate the electronic transitions that 
generate the emitted light. In many materials, this structure is commonly 
a junction structure, i.e. adjacent layers of p-type and n-type material. 
As known to those familiar with such devices and their operation, p-type 
material is a semiconductor material which has an excess of "acceptors" 
meaning that there are vacant positions available in the material into 
which electrons can move. These vacancies are commonly referred to as 
"holes". Correspondingly, in n-type material, there are additional 
electrons which can move within the material or to the adjacent p-type 
material. Thus, the flow of current in such a device can be thought of as 
either the flow of electrons or the flow of holes, but regardless of how 
described, such movement must take place. In a typical LED, current is 
sent ("injected") across such a p-n junction and thereby initiates the 
recombinations of electrons and holes and he light generating transitions 
desired. 
Gallium nitride, however, presents some unique problems in producing p-n 
junctions, the most serious of which is the difficulty to date of 
producing p-type gallium nitride. 
Additionally, gallium nitride is extremely difficult, if not impossible, to 
grow in the form of bulk crystals. This results because the dissociation 
temperature of gallium nitride falls within the temperature ranges 
necessary to accomplish bulk growth by typical methods such as pulling 
from a melt. Therefore, for electronic purposes, gallium nitride must be 
produced as epitaxial layers (thin films of single crystal material) on a 
different substrate material. As known to those familiar with 
crystallography and crystal growth, different materials have different 
crystal structures and the task of depositing on material on another, even 
where both are pure and in single crystal form, inherently creates 
mismatches between the materials which are referred to as lattice 
mismatches or thermal expansion mismatches. These in turn lead to stacking 
faults, partial dislocations and other terms familiar to those in this 
art. Such mismatches and faults will typically have an undesirable effect 
on the electronic properties of the epitaxial layer and any devices made 
from it. 
Gallium nitride presents yet another difficulty, namely that of maintaining 
the stoichiometry (the chemical balance) of a gallium nitride crystal. 
Gallium nitride tends to be intrinsically non stoichiometric apparently 
because of the high propensity for nitrogen atoms to leave gallium nitride 
crystals. When the nitrogen atoms leave the crystal, they leave behind 
"nitrogen vacancies" which because of the Group III-Group V nature of GaN 
act as electron donors and produce an n-type crystal. Thus, as reported by 
one set of researchers, undoped gallium nitride epitaxial layers grown by 
chemical vapor deposition with carrier concentrations less than 
1.4.times.10.sup.17 cm.sup.-3 have never been reported, R. F. Davis et 
al., Materials Science and Engineering, B1(1988) 77-104. 
Therefore, because gallium nitride crystals are almost always n-type, high 
quality p-type material generally has been unavailable. One attempt to 
address the problem has been to add sufficient p-type dopant to gallium 
nitride to first match the concentration of n-type dopants or vacancies 
and then to add additional p-type dopant in an attempt to obtain some 
p-type characteristics. This, however, results in a material that is 
referred to in the semiconductor arts as a "compensated" p-type material 
because of the presence of significant amounts of both p and n-type 
dopants therein. Compensated materials are intentionally useful for some 
purposes, but for gallium nitride LED's, the compensated characteristic is 
generally undesirable. The reason for the undesirability is that the high 
concentration of both p and n carriers results in a very resistive, i.e. 
insulating (or "i-type"), crystal rather than a p-type crystal. 
Additionally, the electron mobility in this material correspondingly 
decreases beyond reasonable usefulness. 
Therefore, a typical current method of producing a blue or UV LED using 
gallium nitride is to grow an epitaxial layer of n-type gallium nitride on 
a sapphire (crystalline Al.sub.2 O.sub.3) substrate, add an insulating 
layer of gallium nitride to the n-type layer, then add a large metal 
contact to the insulating layer--the large contact being necessary because 
of the high resistivity of the insulating layer--and then add a smaller 
contact to the n-type epitaxial layer. Under high enough voltage, 
electrons will tunnel (a very inefficient process) into the n-type layer 
to produce the desired emission. Such a structure is often referred to as 
a metal-insulator-semiconductor (MIS) LED. 
Accordingly, interest has been focused upon alternative methods of 
producing epitaxial layers of gallium nitride on appropriate substrates, 
and upon obtaining adjacent p and n type epitaxial layers which will give 
an appropriate junction and then LED characteristics. 
One basic method attempted to date is the generally well understood 
technique of chemical vapor deposition (CVD) of gallium nitride on 
sapphire. Sapphire is chosen as the substrate of interest primarily 
because it is readily available as a single crystal, thermally stable, and 
transparent to the visible spectrum, as well as for its other appropriate 
characteristics familiar to those in this art. In a typical CVD process, 
source gases containing gallium and nitrogen are introduced into a chamber 
at a temperature intended to be high enough for the gases to disassociate 
into the appropriate atoms, and then for the appropriate atoms to stick to 
the substrate and grow in epitaxial fashion upon it. 
Some early work in the attempts to produce gallium nitride concentrated on 
the deposition of gallium nitride films by the pyrolysis of a gallium 
tribromide ammonia complex, T. L. Chu, J. Electro Chem. Soc. 118, (7) 
(1981) 1200. The authors recognized that (because of the stoichiometry 
problems and associated nitrogen vacancies referred to earlier) undoped 
gallium nitride crystals had very high inherent electron concentrations, 
in this case between 1 and 5.times.10.sup.19 cm.sup.-3. The authors 
produced some gallium nitride films on silicon (Si) and some on hexagonal 
(alpha) silicon carbide (SiC). These films were nonetheless of high 
resistivity, a disadvantage explained earlier. 
One related problem in chemical vapor deposition type growth of gallium 
nitride is that gaseous compounds are required as starting materials and 
high temperatures are often required to get the gases to dissociate into 
elemental gallium and nitrogen. These high temperatures, however, 
encourage an undesirable amount of nitrogen to exit the resulting nitride 
crystal as explained earlier. The equilibrium vapor pressure of molecular 
nitrogen (N.sub.2) over gallium is rather high, particularly at high 
temperatures, so that the temperatures used for CVD exacerbate the 
stoichiometry problems already characteristic of GaN. The result is that 
even though growth can be accomplished, the nitrogen vacancy problem and 
the associated tendency to produce intrinsic n-type gallium nitride both 
remain. 
CVD has further disadvantages. Because compounds are often required as the 
starting materials, there will be a corresponding set of by-products to be 
removed following dissociation of those compounds into the desired 
elements. For example, if (CH.sub.3).sub.3 Ga is used as the starting 
material to obtain atomic gallium, the remaining carbon, hydrogen, and 
hydrocarbon compounds and radicals eventually must be removed, otherwise 
they can act as contaminants from both purity and crystallographic 
standpoints. As a related disadvantage, the starting compounds almost 
always carry some sort of contamination, so that even if the 
stoichiometric by-products are removed, other contaminants may remain that 
will affect the growing crystal and any devices made from it. 
Accordingly, a number of attempts have addressed the need to produce atomic 
nitrogen with a high enough energy to promote epitaxial growth of gallium 
nitride but at temperatures which are low enough to minimize the vapor 
pressure problems, the nitrogen vacancies, and the resulting n-type 
character of the layers that are grown. One technique for activating 
nitrogen is a pulse discharge technique as set forth in Eremin, et al., 
Russian Journal of Physical Chemistry, 56(5) (1982) 788-790. Other 
techniques include reactive ionized-cluster beam deposition (RICB), 
reactive and ionized molecular beam epitaxy (RBME and IMBE), and atomic 
layer epitaxy (ALE). Various references and discussions about these are 
set forth by R. F. Davis, et al., Material Science and Engineering, B1 
(1988) 77-104. As set forth therein, the various CVD growth schemes for 
gallium nitride apparently fail to maintain stoichiometry resulting in the 
n-type gallium nitride described earlier. 
A growth technique of more recent interest is molecular beam epitaxy (MBE). 
A molecular beam epitaxy system comprises a chamber in which an "ultra 
high" vacuum (e.g. 10.sup.-11 torr) is maintained. The elements to be 
deposited in crystalline form are kept adjacent the deposition chamber in 
heated containers known as Knudsen cells. When the shutters to the cells 
are opened, the elemental molecules exit and are limited to travel in 
substantially one direction towards a sample or substrate by the 
combination of cryogenic shrouds and the ultra high vacuum. The shrouds 
capture stray atoms and the high vacuum extends the mean free path of the 
molecules, greatly decreasing their tendency to collide and deviate from 
the path between the Knudsen cell and the sample. The sample is kept at a 
high enough temperature for epitaxial growth to take place. 
Further details about molecular beam epitaxy or deposition systems is 
generally well-known to those familiar with the technique or can be 
developed without undue experimentation and will not otherwise be 
discussed in any further detail. 
The main advantage of MBE over CVD for GaN processes is the lower 
temperatures at which growth will take place in an MBE system. Lower 
temperatures in turn reduce the vapor pressure of nitrogen and the number 
of nitrogen vacancies. Nevertheless, the MBE systems, although providing 
the lower temperatures desireable for these purposes, often fails to 
provide the nitrogen atoms with sufficient energy at the lower 
temperatures to form the desired stoichiometric crystals in epitaxial 
fashion. In other words, higher temperatures encourage epitaxial crystal 
growth, but at the expense of more nitrogen vacancies. Lower temperatures 
reduce the vapor pressure of nitrogen and thus favorably reduce nitrogen 
vacancies, but at the expense of poorer or slower epitaxial growth. 
One technique for providing the nitrogen atoms with the favorable extra 
energy at the lower temperature is plasma excitation of nitrogen. Although 
theoretically helpful, such techniques have not resulted by themselves in 
successful growth of intrinsic epitaxial layers of gallium nitride with 
the low populations density required to form either neutral or p-type 
gallium nitride and resulting p-n junctions. 
Another method is to activate the nitrogen by using microwave electron 
cyclotron resonance (ECR) plasma excitation. In such a system, microwaves 
are guided into the plasma chamber through a wave guide and the electrons 
in the plasma are magnetically controlled so that the electron cyclotron 
frequency coincides with the microwave frequency with the result that the 
plasma effectively absorbs the microwave energy. In effect, an ECR system 
simultaneously causes electrons to move in circular orbits and also 
confines the plasma. The result is a highly activated plasma obtained at 
relatively low gas pressures of between 10.sup.-5 and 10.sup.-3 torr. Such 
a plasma has both electron and ionic temperatures about one order of 
magnitude higher than those of the non-ECR plasma. To date, however, ECR 
techniques have not yet been demonstrated to produce either intrinsic 
undoped or successfully uncompensated p-type gallium nitride. 
Finally, one other set of problems exists with many of the techniques used 
to grow epitaxial layers of gallium nitride, specifically, the use of 
sapphire as a substrate. As indicated by a number of the cited references, 
sapphire has been the substrate of choice in attempting to produce 
epitaxial layers of gallium nitride. First, mentioned earlier, when a 
substrate and an epitaxial layer are formed of different crystalline 
materials, some lattice mismatch between the two will inevitably exist as 
will some slight difference in other characteristics such as the 
coefficient of thermal expansion. Depending upon the specific values of 
the differences, the effect on the electronic properties will be small or 
large, but they will exist. 
In this regard, and as discussed in somewhat more detail by Yoshida, et al. 
J. Vac. Sci. Technol. B 1, (2), (1983), 250-253, there exists a lattice 
mismatch of between 0.9 and 22.7% between gallium nitride and sapphire 
depending upon which plane of sapphire is used as the substrate face. 
Furthermore, the coefficient of liner expansion of sapphire is 
significantly greater than that of gallium nitride. Similarly, Davis et 
al. note that the lattice parameter values of sapphire are 23% greater 
than GaN and that sapphire's coefficient of thermal expansion is 25% 
greater than that of GaN. 
Accordingly, researchers including Yoshida have produced epitaxial layers 
of gallium nitride using sapphire substrates by introducing an 
intermediate epitaxial layer of aluminum nitride (AlN). The lattice 
mismatch between gallium nitride and aluminum nitride is only 2.4 in 
selected planes which is much smaller than the lattice mismatch between 
gallium nitride and sapphire along corresponding planes. Additionally, 
along selected planes the difference in the coefficients of thermal 
expansion of gallium nitride and aluminum nitride is smaller than the 
difference between gallium nitride and sapphire. Based on these factors, a 
number of researchers have continued to incorporate aluminum nitride as a 
buffer layer between sapphire and gallium nitride. As discussed earlier, 
however, the addition of yet another material effects the electronic 
properties of the resulting structure and typically reduces the electronic 
capabilities of any resulting device. 
There are other properties of sapphire other than the lattice mismatch and 
thermal expansion mismatch that are similarly disadvantageous. In 
particular, sapphire is nonconductive. As a result, the "back-contact" 
type of structure that is particularly useful for LED's is unavailable 
using sapphire substrate. 
There thus exists the need for epitaxial layers of gallium nitride on 
conductive substrates, with good lattice matches, an appropriate 
coefficient of thermal expansion, and which are transparent to blue light. 
Furthermore, there exists the corresponding need for a method of growing 
epitaxial layers of gallium nitride on such substrates that can be 
conducted in a manner which reduces the tendency of nitrogen to leave the 
gallium nitride crystal and which can therefore be used to produce 
intrinsic gallium nitride that is substantially undoped and from which 
there can be produced substantially uncompensated p-type gallium nitride. 
Finally, there is a need to use these techniques to produce light emitting 
diodes in gallium nitride which emit blue light in a highly efficient 
fashion. 
OBJECT AND SUMMARY OF THE INVENTION 
Accordingly, it an object of the present invention to provide a method of 
growing intrinsic, substantially undoped single crystal gallium nitride 
with a low donor concentration, to correspondingly form p-type gallium 
nitride in the absence of the difficulties and results experienced to 
date, to form p-n junctions between respective portions of n-type and 
p-type gallium nitride, and to produce appropriate light emitting diodes 
from gallium nitride and that will produce efficient emission in the blue 
and ultraviolet regions of the spectrum. 
The invention accomplishes this object by providing a method of growing 
intrinsic, substantially undoped single crystal gallium nitride with a 
donor concentration of 7.times.10.sup.17 cm.sup.-3 or less. The method 
comprises introducing a source of nitrogen into a reaction chamber 
containing a growth surface while introducing a source of gallium into the 
same reaction chamber and while directing nitrogen atoms and gallium atoms 
to a growth surface upon which gallium nitride will grow. The method 
further comprises concurrently maintaining the growth surface at a 
temperature high enough to provide sufficient surface mobility to the 
gallium and nitrogen atoms that strike the growth surface to reach and 
move into proper lattice sites, thereby establishing good crystallinity, 
to establish an effective sticking coefficient, and to thereby grow an 
epitaxial layer of gallium nitride on the growth surface, but low enough 
for the partial pressure of nitrogen species in the reaction chamber to 
approach the equilibrium vapor pressure of nitrogen over gallium nitride 
under the other ambient conditions of the chamber to thereby minimize the 
loss of nitrogen from the gallium nitride and the nitrogen vacancies in 
the resulting epitaxial layer. 
The foregoing and other objects, advantages and features of the invention 
and the manner in which the same are accomplished, will become more 
readily apparent upon consideration of the following detailed description 
of the invention taken in conjunction with the accompanying drawings, 
which illustrate preferred and exemplary embodiments, and wherein:

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The invention comprises a method of growing intrinsic, substantially 
undoped single crystal gallium nitride (GaN) with a donor carrier 
concentration of 7.times.10.sup.17 cm.sup.-3 or less. It will be 
understood by those familiar with gallium nitride and its properties that 
the ability to grow substantially undoped single crystal gallium nitride 
provides the basis for the corresponding ability to form p-type gallium 
nitride, which in turn forms the basis for p-n junctions, and then 
junction devices such as light emitting diodes. 
The method first comprises introducing a source of nitrogen into a reaction 
chamber containing a growth surface while introducing a source of gallium 
into the same reaction chamber containing the growth surface. In a 
preferred embodiment of the invention, the steps of introducing nitrogen 
and gallium are carried out by molecular beam epitaxy, a process briefly 
set forth in the background portion of the specification. The basic layout 
of such a system is fundamentally well understood and typical details are 
set forth in E. H. C. Parker (Ed.), The Technology and Physics of 
Molecular Beam Epitaxy, 1985, Plenum Press, New York, N.Y. A typical MBE 
system includes a deposition chamber into which the sample is placed. The 
chamber is maintained at an ultra high vacuum (UHV), on the order of 
10.sup.-10 to 10.sup.-11 torr, which is typically produced by a UHV pump 
such an ion pump, a diffusion pump, a cryopump, or a turbomolecular pump. 
Adjacent the deposition chamber are a series of molecular beam or atomic 
source known as Knudsen cells. These in turn are partially surrounded by 
cryogenic shrouds that often contain liquid nitrogen so that when the 
shutter to a Knudsen cells is opened, the shrouds capture atoms or 
molecules exiting the Knudsen cell other than those moving in one general 
direction. Because the vacuum is so high in the deposition chamber, the 
remaining atoms or molecules have a fairly extensive mean free path and 
travel substantially undisturbed to the sample. At the sample, epitaxial 
growth of the desired compound takes place as the molecules or atoms 
impinge upon the growth surface. As set forth earlier, one of the main 
advantages of MBE is the ability to carry out epitaxial growth at 
relatively low temperatures, particularly compared to alternative methods 
such as chemical vapor deposition. 
During the process, the growth surface is maintained at a temperature that 
is high enough to provide sufficient surface mobility to the gallium and 
nitrogen atoms that reach the growth surface to establish an effective 
sticking coefficient and to grow an epitaxial layer of gallium nitride 
while maintaining good crystallinity. The temperature is maintained low 
enough, however, for the partial pressure of nitrogen in the reaction 
chamber to approach or exceed the equilibrium vapor pressure of nitrogen 
over gallium nitride under the ambient conditions of the chamber. These 
conditions minimize the loss of nitrogen from the gallium nitride and the 
nitrogen vacancies in the resulting epitaxial layer. 
As used herein, the term "sticking coefficient" refers to the ratio of 
atoms striking the growth surface to the number of atoms that actually 
remain to form a crystalline structure on that growth surface. 
In evaluating the vapor pressure, it will be understood that depending upon 
the source of nitrogen, there may be a number of nitrogen species present. 
It has been found in accordance with the present invention that the vapor 
pressure of any of the nitrogen species present preferably should exceed 
the equilibrium vapor pressure of that species over gallium nitride under 
the ambient conditions of the chamber. For example, if the sole source of 
nitrogen species in the chamber is atomic nitrogen, then the partial 
pressure of atomic nitrogen should approach or must exceed the equilibrium 
vapor pressure of atomic nitrogen over gallium nitride. Alternatively, if 
ammonia is used as the nitrogen source, several species of nitrogen will 
very likely be present. In this case, the sum of the individual partial 
vapor pressures of each species must approach or exceed the equilibrium 
vapor pressure of those species over gallium nitride. 
In practicing the present invention, it has accordingly been observed that 
ammonia is an appropriate nitrogen source for both CVD and MBE techniques, 
but that molecular nitrogen (N.sub.2) is either less satisfactory (MBE) or 
unacceptable (CVD). Although the inventor does not wish to be bound by any 
particular theory, it is postulated that these results reflect the lower 
dissociation temperature of ammonia as compared to N.sub.2 to produce 
atomic nitrogen. The observed success when using atomic nitrogen produced 
from an ECR plasma and then directed toward a substrate in an MBE system 
would, however, appear to bear out the theory that production of atomic 
nitrogen with a desired amount of energy is most helpful in producing 
epitaxial layers of GaN with the desired characteristics. 
Although the invention includes the use of chemical vapor deposition as a 
method of introducing the nitrogen and the gallium and controlling the 
growth, the molecular beam epitaxy technique has been found to be 
preferable under most circumstances. As stated earlier, the main advantage 
of MBE is the lower temperature at which it can take place. Because the 
equilibrium vapor pressure of nitrogen over gallium nitride decreases as 
temperature decreases, the lower process temperature offered by MBE offers 
the corresponding opportunity to maintain a lower partial equilibrium 
vapor pressure of nitrogen over the gallium nitride. In turn, the partial 
pressure of the other nitrogen containing species present can likewise be 
lower in order to accomplish the desired growth. 
For example, a preferred growth temperature using a MBE system is on the 
order of between about 600.degree. and 650.degree. C. with temperatures of 
600.degree. or lower being preferred. Such temperatures are generally 
lower than those required for CVD in which compounds are introduced as 
gases, and then heated to an appropriate dissociation temperature to 
obtain the desired atoms that sustain epitaxial growth. 
As stated earlier, when the equilibrium vapor pressure of nitrogen over 
gallium nitride is high, the situation typically results in a large number 
of nitrogen vacancies in the gallium nitride crystal. Because these 
vacancies act as donors, an n-type doped crystal results. Although 
satisfactory where n-type gallium nitride is desired, this doped intrinsic 
materials is typically unsatisfactory for forming p-type gallium nitride 
and therefore unsatisfactory for producing p-n junctions and junction 
devices. 
Because MBE takes place at such relatively low temperatures, however, some 
of the energy required to obtain a satisfactory sticking coefficient must 
be obtained through some fashion other than heating. In this regard, it 
has been found preferable to introduce nitrogen in the form of "active" 
nitrogen atoms that have been produced from a nitrogen containing plasma. 
The extra energy that the plasma provides to the nitrogen atoms increases 
the growth without increasing the temperature. In other words, the goal of 
a plasma assisted deposition system such as molecular beam epitaxy, or 
even chemical vapor deposition, is to provide a method of growing high 
melting point materials, such as nitride films, at low substrate 
temperatures. 
In a preferred embodiment, the nitrogen atoms are introduced from an 
electron cyclotron resonance plasma in molecular beam epitaxy fashion and 
in the substantial absence of any other compounds. As stated in the 
background portion of the specification, such an ECR source uses a 
magnetic field to simultaneously cause electrons to move in circular 
orbits and to confine the plasma. Stated differently, a plasma chamber and 
magnetic coils control the electron cyclotron frequency so as to coincide 
with the microwave frequency and thus enable the plasma to effectively 
absorb the microwave energy, see e.g. S. Zembutsu, Growth of GaN Single 
Crystal Films Using Electron Cyclotron Resonance Plasma Excited Metal 
Organic Vapor Phase Epitaxy, Appl. Phys. Lett. 48 (13), 31 March 1986. 
In the present invention the step of introducing nitrogen atoms from a 
nitrogen containing electron cyclotron resonance plasma comprises 
introducing nitrogen atoms that have been accelerated at less than 100 
electron volts (eV). As stated earlier, as an alternative to introducing 
nitrogen from a nitrogen containing plasma, nitrogen can be introduced as 
active ammonia molecules from an ammonia containing electron cyclotron 
resonance plasma. 
Additionally, although the nature of gallium nitride is such that the 
manner in which the nitrogen is introduced is one of the primary factors 
in successfully growing epitaxial layers, it is also preferred that the 
gallium be introduced as gallium atoms from another Knudsen cell in the 
same molecular beam epitaxy deposition system. Gallium can, however, be 
introduced as an organometallic compound such as trimethyl gallium 
((CH.sub.3).sub.3 Ga) or gallium chloride (GaCl), particularly in CVD 
techniques. 
As stated in the background portion of the specification, either silicon 
carbide or sapphire can be selected as an appropriate substrate for such 
epitaxial layers of gallium nitride. In a preferred embodiment of the 
invention, however, the substrate of choice is silicon carbide. Silicon 
carbide has a much closer match of coefficient of thermal expansion and 
lattice parameters with gallium nitride than does the more commonly used 
sapphire. More importantly for commercialization and manufacturing 
purposes, silicon carbide can be made to be conductive to current and is 
substantially transparent to blue and ultraviolet light. Because silicon 
carbide can be made to be conductive, it permits the use of conventional 
back contacts in packaged diodes. Because it transmits blue light, it 
increases the overall efficiency of any resulting device and permits a 
variety of package orientations. 
In a corresponding manner, the ability to grow an appropriate intrinsic 
epitaxial layer of gallium nitride with an acceptably low level of donor 
concentration provides the corresponding opportunity to form an epitaxial 
layer of p-type gallium nitride suitable for producing p-n junctions and 
junction diodes. In addition to the steps recited earlier, including the 
use of MBE and ECR at the appropriate temperature, the method of producing 
a p-type epitaxial layer comprises the additional concurrent step of 
directing an acceptor species onto the growth surface of the silicon 
carbide single crystal while directing the nitrogen atoms and while 
directing the gallium atoms and while maintaining the surface at the 
functional temperature described earlier. 
As in the case of other dopant processes, however, merely incorporating 
dopant atoms into the epitaxial layer is insufficient to produce a layer 
with satisfactory p-type characteristics. In this regard, and as known by 
those familiar with the doping of semiconductors, dopants that are added 
to a semiconductor crystal must be "activated" in some fashion before the 
electronic properties of the crystal will reflect the presence of these 
dopant atoms. Activation can take many forms. For example, in doping 
silicon, dopant atoms are often added by a process called ion implantation 
following which a doped crystal is thermally annealed to produce 
activation. Activation is best understood as the process of moving dopant 
atoms into appropriate lattice positions where they will have the intended 
electronic effect. 
In the present invention, activation has been accomplished by irradiating 
the surface of the epitaxial layer with an electron beam that activates 
the acceptor atoms and results in an epitaxial layer of gallium nitride 
with appropriate p-type characteristics. The irradiation is performed with 
sufficient energy to activate the acceptor ions that are incorporated into 
the growing epitaxial layer, but less than the amount of energy that would 
cause atomic displacement in the growing epitaxial layer. 
In another embodiment of the present invention, it is expected that 
irradiating the surface of the growing epitaxial layer with an electron 
beam during the growth process will similarly activate the acceptor atoms 
and result in an epitaxial layer of gallium nitride with appropriate 
p-type characteristics, and may improve upon the results observed from 
post-growth activation. As in the case of a post-growth activation, the 
in-situ irradiation is performed with sufficient energy to activate the 
acceptor ions that are incorporated into the growing epitaxial layer, but 
less than the amount of energy that would cause atomic displacement in the 
growing epitaxial layer. 
The preferred process is referred to as low energy electron beam 
irradiation (LEEBI) and as stated earlier, is carried out at an 
accelerating voltage which is below that at which atomic displacement 
would take place. In preferred embodiments of the invention, the LEEBI 
takes place at an accelerating voltage of between about 3 and 30 kilovolts 
(kV) with between about 7 and 10 kV preferred. Furthermore, in addition to 
the focused types of scanning LEEBI treatments that are used for other 
purposes, the beam may be defocused so that the entire sample is 
irradiated at once. To date, the inventors are unaware of any other 
workers who have used an in-situ LEEBI technique combined with an in-situ 
incorporation of a dopant in an MBE system for growing gallium nitride 
that also incorporates plasma excitation of the nitrogen source. 
Additionally, the inventors are unaware of any such technique using a 
silicon carbide substrate and in the absence of an intermediate layer of 
aluminum nitride (AlN). 
The success of this process and the desireable intrinsic and p-type 
character respectively of the resulting material have been confirmed by 
current-voltage (I-V) measurements using appropriate Schottky contacts. 
FIGS. 3 and 4 are the I-V plots of this material and demonstrate this 
success. FIG. 4 demonstrates the n-type material as shown by the large 
current flow at positive voltage. FIG. 5 demonstrates p-type material as 
evidenced by the large current flow at negative voltages (e.g. 100 
microamps of current at -4.8 volts). 
The type of silicon carbide crystal appropriate for use as the substrate is 
a bulk single crystal of silicon carbide having a single polytype. Because 
of the relationship between the crystal lattice structures of silicon 
carbide and gallium nitride, growth on (0001) alpha silicon carbide (6H 
polytype) results in the growth of corresponding hexagonal (2H) gallium 
nitride, while growth on the (100) surface of cubic (3C) silicon carbide 
results in cubic (100) gallium nitride. 
In the preferred embodiment appropriate acceptor dopants for gallium 
nitride include all of the Group II elements, with magnesium preferred. 
Although intrinsic gallium nitride already tends to have n-type character, 
if additional donors are included, they can be selected from the Group VI 
elements, although the transition metal members of Group VI are less 
preferable. 
In a corresponding fashion, once intrinsic undoped epitaxial layers and 
p-type epitaxial layers of gallium nitride can be produced, appropriate 
p-n junctions can likewise be produced. In a preferred embodiment of the 
invention, the method of producing these comprises carrying out a number 
of the steps described earlier: directing nitrogen atoms onto the growth 
surface of a single crystal of silicon carbide that provides an acceptable 
lattice match for gallium nitride, and while directing gallium atoms onto 
the same silicon carbide growth surface. Concurrently, the surface 
temperature is maintained high enough to provide sufficient surface 
mobility for the gallium atoms and the nitrogen atoms to form a single 
crystal epitaxial layer of gallium nitride on the silicon carbide but 
lower than the temperature at which the partial pressure of nitrogen 
species approaches the equilibrium vapor pressure of those nitrogen 
species over gallium nitride under the ambient conditions. In a typical 
preferred embodiment, the silicon carbide substrate will be n-type, so 
that the first epitaxial layer grown thereon will also be selected as 
n-type. 
In forming the p-n junction, therefore, when a sufficient layer of n-type 
material has been produced, and while the steps of directing nitrogen 
atoms and gallium atoms onto the growth surface continue, an acceptor 
species is directed onto the growth surface at a rate relative to gallium 
and nitrogen that is sufficient to produce a desired p-type epitaxial 
layer of gallium nitride containing the acceptor atoms upon the n-type 
epitaxial layer. The growth surface is concurrently maintained at the 
desired temperature for the time sufficient to produce the p-type 
epitaxial layer of gallium nitride upon the n-type epitaxial layer to 
thereby form a p-n junction therebetween. As stated earlier, the p-type 
layer is activated either during growth or thereafter with a low energy 
electron beam. 
In one of the preferred embodiments of the invention, the step of directing 
an acceptor species to the growing epitaxial layer at a rate relative to 
gallium and nitrogen that is sufficient to produce a p-type epitaxial 
layer of gallium nitride upon the growth surface comprises directing the 
acceptor at a rate sufficient for the acceptor concentration to exceed the 
donor concentration of the n-type epitaxial layer previously formed. As 
known to those familiar with LED's and their operation, it is often 
desirable to have the carrier concentration of one side of the p-n 
junction greater than the carrier concentration on the other side so that 
when a potential is applied, current tends to flow predominantly from the 
more highly populated side of the junction to the less highly populated 
side. Thus, if the p-type layer is more highly populated, hole current 
will dominate the injection process, while if the population of the n-type 
layer is greater than that of the p-type layer, electron current will 
dominate the injection process. 
In gallium nitride, injection into the p-type layer tends to give the 
characteristic blue emission at about 440 nm, while injection into the 
n-type layer will produce the characteristic violet emission at about 370 
nm. It will therefore be understood that the invention can be used to 
produce the desired populations in the desired respective epitaxial layers 
to in turn produce diodes with preferred characteristic emissions. 
As a result of these techniques, the method of the invention can be used to 
produce an epitaxial layer of intrinsic gallium nitride having a donor 
concentration of 7.times.10.sup.17 cm.sup.-3 or less, and to do so on a 
silicon carbide single crystal substrate that provides an acceptable 
lattice match for gallium nitride. In turn, a p-n junction can be formed 
by an epitaxial layer of intrinsic n-type gallium nitride with such a 
donor concentration and an adjacent epitaxial layer of p-type gallium 
nitride having an acceptor concentration greater than 7.times.10.sup.17 
cm.sup.-3, on the same bulk single crystal substrate of silicon carbide. 
Finally, as illustrated in FIGS. 1, 2 and 3, the method of the invention 
can be used to produce a high efficiency bipolar light emitting diode from 
gallium nitride. The LED is shown as the individual die broadly designated 
at 10 and schematically illustrated as the package broadly designated at 
20. The die 10 includes a bulk single crystal 11 of silicon carbide having 
a single polytype. A first epitaxial layer 12 of intrinsic n-type gallium 
nitride having a donor concentration of 7.times.10.sup.17 cm.sup.-3 or 
less is upon the substrate 11 and an epitaxial layer 13 of p-type gallium 
nitride having an acceptor concentration greater than 7.times.10.sup.17 
cm.sup.-3 is adjacent the n-type epitaxial layer and forms a p-n junction 
14 with the n-type epitaxial layer 12. An ohmic contact 15 to the 
substrate and an ohmic contact 16 to the p-type epitaxial layer 13 
complete the structure. 
As stated earlier, an LED with the carrier concentrations set forth with 
respect to FIG. 2 will result in injection into the n-type layer and will 
correspondingly result in a violet emission. By making the carrier 
concentration in the n-type layer greater than that of the p-type layer, 
the predominant injection will be into the p-type layer and an LED with 
the characteristic blue emission can likewise be produced. 
FIG. 2 illustrates a diode package formed from a similar die. For the sake 
of convenience and clarity, several of the portions are numbered 
identically as in FIG. 1. These include the substrate 11, the ohmic 
contact to the substrate 15, the epitaxial layer 12, the epitaxial layer 
13, and the junction 14. The package 20 also includes as well a deposited 
insulator portion 22 which can be formed of silicon dioxide, silicon 
nitride or some other appropriate insulating compound. A silver based 
epoxy 23 provides electrical contact to a metal reflector cup 24 through 
which the current can be applied. 
FIG. 2 illustrates some of the advantages of using a silicon carbide 
substrate. In particular, because the substrate 11 is transparent to both 
blue and ultraviolet light, the ohmic contact 15 to the substrate can be 
of minimal size to likewise increase the efficiency of the packaged LED. 
If a non-transparent substrate were used, the structure shown in FIG. 2 
would be unacceptable as the packaged structure would block light emitted 
from the junction in one direction and the substrate would block it in the 
other direction. As mentioned earlier, because p-n junctions of gallium 
nitride have not yet heretofore been successfully been produced on silicon 
carbide, this convenient and efficient structure has remained unavailable 
for use with gallium nitride. 
FIG. 3 illustrates an alternative embodiment of an LED package broadly 
designated at 30 in which the substrate 11 is positioned in the conductive 
epoxy 23 so that the epitaxial layers 12 and 13 are opposite the 
reflective cup 24. Depending on the particular circumstances or 
requirements, one or the other of the respective LED packages 20 or 30 may 
be most appropriately used. 
In the drawings and specification, there have been disclosed typical 
preferred of the invention and, although specific terms have been 
employed, they have been used in a generic and descriptive sense only and 
not for purposes of limitation, the scope of invention being set forth in 
the following claims.