Successes in the growth of Group III-V nitride compound semiconductor light emitting devices for producing blue light have been published recently. Such blue light devices generally comprise GaN/Al.sub.x Ga.sub.1-x N layer structures formed on sapphire substrates. An important aspect in their manufacture is acceptor activation of the p-type Group III-V nitride layer such a Mg or Zn doped GaN cladding layer.
The p-type doping of Group III-V nitride compound semiconductor materials was first established in 1989 by Messieurs Akasaki and Amano at Meijo University in Japan using magnesium (Mg) as the p-type dopant in a MOCVD grown III-V nitride compound semiconductor layer. Akasaki and Amano grew p-type GaN but their as-grown films were insulating because the p-dope Mg acceptors were passivated by active hydrogen existing in the reactor. The source of the hydrogen impurities was ammonia (NH.sub.3) serving as a nitrogen (N) precursor. Atomic hydrogen (H.sup.+), produced by the pyrolysis of NH.sub.3 is incorporated in a complex with the Mg acceptors. Overall, the Mg--H complexes are neutral so that the hydrogen renders the Mg acceptors electrically inactive. Thus, in order to activate the p-type doping, it is necessary to decompose the Mg--H complexes in the as-grown III-V nitride compound semiconductor layer. In the as-grown films of Akasaki and Amano, this was accomplished by the low energy electron beam radiation (LEEBI) process. See, for example, the article of Hirosi Amano et al., "P-type Conduction in Mg-Doped GaN Treated with Low-Energy Electron Beam Irradiation" (LEEBI), Japanese Journal of Applied Physics, Vol. 28 (12), pp. L2112-L2114, December, 1989.
Subsequently, in 1992, Shuji Nakamura and his coworkers demonstrated that thermal annealing was similarly effective in activating Mg acceptors in GaN films grown on sapphire substrates as disclosed in the article of Nakamura et al., "Thermal Annealing Effects on P-Type Mg-Doped GaN Films", Japanese Journal of Applied Physics, Vol. 31 (Part 2, No. 2B), pp. L139-L142, Feb. 13, 1992 and their companion U.S. patent to this published paper, U.S. Pat. No. 5,306,662 entitled, "METHOD OF MANUFACTURING P-TYPE COMPOUND SEMICONDUCTOR". After film growth, the samples are cooled down and transferred from the MOCVD reactor to an anneal furnace where an ex-situ thermal anneal is performed in order to achieve acceptor activation to achieve a low-resistance p-type GaN compound semiconductor. Typical anneal operations include annealing in a nitrogen atmosphere or mixed nitrogen/argon atmosphere at 1 ATM anywhere from 700.degree. C. to 1000.degree. C. for about 20 minutes. The anneal at 1000.degree. C. is exemplified in the case of employing a SiO.sub.2 cap layer formed over the Mg-doped GaN layer prior to ex-situ annealing to prevent decomposition of the underlying GaN compound layer during annealing. The use of any gas containing hydrogen atoms, i.e., atomic hydrogen, such as in the case of NH.sub.3, upon pyrolysis, or H.sub.2 per se in the annealing atmosphere is discouraged and is not preferred. In all of these cases, it is clear that the anneals are carried out as intended ex-situ anneal processes, except, as discussed below in further detail, relative to one example.
Another embodiment in U.S. Pat. No. '662 discloses annealing in a nitrogen atmosphere under a pressure of 20 ATMs to maintain the integrity and surface morphology of the film, i.e., aid in the prevention of outdiffusion of N from the GaN sample at high anneal temperatures with an N.sub.2 overpressure provided on the GaN layer surface.
As indicated above, in one embodiment of Nakamura et al. in U.S. Pat. No. '662, the sample is annealed in the MOCVD reactor in the following manner. After growth of the Mg-doped GaN layer, the TMG gas and the Cp.sub.2 Mg gas flow are terminated. In a continued flow of the carrier gas H.sub.2 and NH.sub.3, the sample in the reactor is cooled down to room temperature. This cool down process provides for the presence of both atomic nitrogen and atomic hydrogen that, respectively, aid in the prevention of the outdiffusion of N from the sample and will hydrogenate the p-doped as-grown GaN layer. Then, an anneal process is carried out after extinguishing the flow of H.sub.2 and NH.sub.3, introducing a N.sub.2 flow, and raising the reactor temperature back up to the growth temperature regime of the GaN film, e.g. 1,000.degree. C., and permitting the anneal to be accomplished for 20 minutes. While this is a quasi-in-situ anneal in the fact that the sample is not removed from the MOCVD reactor, the anneal process is not sequenced to occur immediately after the growth process. In other words, the reactor is returned to room temperature before proceeding with the anneal process so that there is no intended continuity with the growth process. Delay to cool down to room temperature may be as much as one half hour.
With respect to hydrogen passivation, acceptors in Group III-V nitrides are, therefore, similar to other compound semiconductor material systems, such as, for example, ZnMgSSe, AlGaAs, AlGaInP and InGaAsP, doped with acceptor species, in that each of these Group III-VI and Group III-V compound semiconductors exhibit the same phenomena of acceptor hydrogen passivation. In most of these material systems, it has been shown that a thermal anneal can be employed to activate the acceptor. In contrast, however, the nitride films are insulating before anneal while the other non-nitride material systems, mentioned above, are still p-type before anneal but somewhat of reduced conductivity as a result of hydrogen passivation. It has been established in these other Group III-V non-nitride compound semiconductors, that hydrogen is incorporated in the reactor cooldown after growth, rather than during growth. Consequently, ex-situ post-growth anneals have become a common procedure for laser diode processing. In contrast to other, better understood compound semiconductors such as AlGaAs, AlGaInP, and InGaAsP, the mechanism of acceptor passivation has not been established in the nitrides. Theoretical investigations indicate that the acceptors are passivated during growth. On the other hand, as indicated above, in the more common Group III-V arsenides and phosphides, it has already been shown that the passivation occurs during the cooldown after growth. In this case, the samples are cooled down in the presence of a hydride precursor gas, such as arsine or phosphine as the case may be, to stabilize the growth surface from decomposition. Unfortunately, the hydride pyrolysis also produces atomic hydrogen, which readily diffuses into the samples and forms an electrically neutral complex with the acceptor.
Although it is not yet clear which mechanism is involved in the Group III-V nitrides, in their basic nature, these two passivation mechanisms may, in fact, be equivalent. Overall, the behavior is simply determined by the competition between hydrogen incorporation (passivation) and hydrogen evolution (activation), both of which are expected to be strong functions relative to temperature. For instance, at high temperatures, the hydride gas is efficiently pyrolyzed, thus generating large quantities of atomic hydrogen. If this generation rate exceeds the rate at which hydrogen evaporates from the material, such as due to the hydrogen having a sufficiently strong bond, then the net effect will be passivation during growth. This equilibrium may be shifted in the opposite direction, i.e., toward activation or net hydrogen desorption, by reducing the quantity of atomic hydrogen available. Similarly, the passivation may appear to only occur during the cooldown after growth, if the hydrogen is relatively weakly bound in the lattice. In this situation, the rate of hydrogen evaporation from the crystal always exceeds the rate at which atomic hydrogen is being produced from hydride pyrolysis. As the ambient temperature is reduced, however, the rate of hydrogen desorption drops very rapidly so that within some temperature range, the desorption rate drops below the hydrogen generation rate. In this scenario, the passivation appears to occur during the cooldown after growth.
Such a simple model, which includes only the kinetics associated with the pyrolysis of the hydride (which determines the rate at which atomic hydrogen is generated and, therefore, the overpressure of atomic hydrogen), and the rate at which hydrogen is evolved from the hydrogenated sample (dictated by the strength with which hydrogen is bonded in the sample) can explain either type of behavior. It is only the relative rates of hydrogen generation and hydrogen desorption which determines whether the passivation appears to occur during growth or during cooldown.
What is needed is an in-situ process that provides acceptor activation with lower processing costs, less potential contamination from exposure to the atmosphere accompanying ex-situ processing, and improved results in acceptor activation no matter the principal cause of its passivation.
Therefore, it is an object of this invention to provide an in-situ thermal process for acceptor activation for Group III-V nitride compound semiconductors.
It is another object of this invention to provide for the production of p-type Group III-V nitride material in cases for which passivation may occur either during growth or during cooldown, as is appropriate for the given growth conditions.
It is a further object of this invention to provide an in-situ thermal process that provides for acceptor activation while concurrently preventing decomposition of the Group III-V nitride compound semiconductor film surface.