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.