PRODUCTION OF CLEAN, WELL-ORDERED CdTe SURFACES USING LASER ABLATION

Abstract: Chemically-etched CCdTe surfaces are exposed to UV excimer laser radiation at a fluence greater than the threshold fluence for creating a non-stoichiometric film, followed by either a low temperature (<300 DEG C) thermal anneal or exposure of the CdTe surface to UV excimer laser radiation at a fluence lower than the threshold fluence. This procedure provides clean, stoichiometric, and well-ordered CdTe surfaces.

Description: PRODUCTION OF CLEAN, WELL-ORDERED CdTe SURFACES USING LASER ABLATION BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the production o clean, stoichiometric, and well-ordered CdTe surfaces fo the purpose of subsequent growth of mercury-based II-VI ma terials. 2. Description of Related Art Cadmium telluride (CdTe) is a desirable substrate fo mercury-based II-VI alloy and superlattice growth becaus of its close lattice match to HgTe and therefore to al HgCdTe alloy compositions. As is well-known, HgCdTe allo composites are widely used as infrared detectors. The performance of HgCdTe infrared detectors depend strongly upon the structural and chemical properties of th HgCdTe sensing layer, which in turn depends on the qualit of the underlying substrate. However, CdTe substrates are typically not availabl in the same high quality as GaAs or Si. In addition, we chemical etching of a CdTe surface leads to non-stoichio- metric, atomically rough surfaces which make poor templates for subsequent growth. The state of the CdTe surface fol¬ lowing etching may vary considerably from substrate to sub¬ strate, because the CdTe surface appears to be extremel sensitive to the parameters of etching used during etching; these parameters include etchant concentration, tempera¬ ture, time, and agitation. There are numerous conflicting reports in the technical literature regarding the stoichi- ometry and composition CdTe surfaces which were reported t have been subjected to the same etching procedures. There fore, an additional treatment is required to produce a sto ichiometric and ordered surface prior to the growth o HgCdTe. There are several ways of improving the poor qualit of the CdTe substrate surfaces which result from wet chem ical etching. It is possible, for example, to remove con taminants from the CdTe surface by ion bombardment, fol lowed by a thermal anneal to remove the sputter damage. Contaminants such as carbon, oxygen, chlorine, and sulfu result from exposure of the substrate surface to the ambi ent. Sputter damage is well-known to occur during sputter ing. We attempted to prepare a CdTe (100) substrate using very mild sputtering conditions (100 eV Ar ions at <100 nA beam current for 5 minutes),. followed by a 5 minute anneal at 300 * C. Auger measurements indicated that following this treatment, the surface was clean and stoichiometric. How- ever, RHEED (reflection high-energy electron diffraction) analysis was spotty and indicative of rough surface mor¬ phology. Alternately, it is common to grow a CdTe buffer layer on top of the CdTe substrate in an attempt to bury defects and impurities at the substrate-buffer layer interface. However, the quality of the buffer layer can also be af¬ fected by the presence of impurities and roughness on the substrate. Improvements in the quality of the substrate will result in improved buffer layer properties. A process which improves the quality of the CdTe sub¬ strate, and hence the interface between the substrate and HgCdTe layer epitaxially grown thereon, would greatly im¬ prove the performance and reliability of the infrared de¬ tectors. SUMMARY OF THE INVENTION In accordance with the invention, a method is provide for producing clean, stoichiometric and well-ordered CdT surfaces. The method involves the exposure of a chemical ly-etched CdTe surface to UV radiation at a fluence greate than the threshold fluence for creating a non-stoichiomet ric film, followed by either exposure of the CdTe surfac to UV excimer laser radiation at a fluence lower than th threshold fluence or a low temperature (<300 β C) thermal an neal. The resultant surface in either case is clean and sto ichiometric, as determined by Auger electron spectroscop and is well-ordered as indicated by reflection high-energ electron diffraction (RHEED) . BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1, on coordinates of Cd:Te ratio and fluence (i mJ/cm 2 ) is a plot of CdTe stoichiometry as a function of Kr (248 n ) laser fluence; and FIGS. 2a-c are line drawing representations of surfac¬ es observable by RHEED, which schematically depict the se quence of surface structures resulting from laser ablation of the surface in accordance with the invention. DESCRIPTION QF THE PREFERRED EMBODIMENTS A method for producing clean, stoichiometric and well- ordered CdTe surfaces is provided. The method comprises: (a) exposing a chemically-etched CdTe surface to ultraviolet radiation at a fluence greater than the thresh¬ old fluence for creating a non-stoichiometric film; and ei¬ ther (bl) exposing the CdTe surface to ultraviolet ra¬ diation at a fluence lower than the threshold fluence, or (b2) thermally annealing the CdTe surface at temperature less than about 300 * C until any excess tellur um is removed. The UV radiation useful in the practice of the inve tion is related to the bandgap of CdTe. Specifically, t process of the invention may be performed with any radi tion the energy of which is equal to or greater than th bandgap energy, which, for CdTe, is about 1.5 eV. Thi translates into a wavelength of less than about 827 n . Preferably, the UV radiation is one that is easily ac cessible; for example a KrF excimer laser operating a about 248 n is desirably employed in the practice of th invention, although other sources of UV radiation may al ternatively be used, having the appropriate fluence, whic is discussed in greater detail below. Another useful U wavelength that may be employed is at about 193 nm. The CdTe surface is first exposed to a wet chemica etch. The wet chemical etching desirably employs a solu tion of bromine in ethylene glycol, although any etchan that etches CdTe may alternatively be employed. Prefera bly, at most 1/16% weight/weight Br 2 /ethylene glycol is em ployed, as this seems to provide the most control; stronge solutions result in undesirable surface roughening. Resid¬ ual carbon contamination of the surface may be removed b sputter-cleaning at this point in the process. However, sputter-cleaning leaves the surface roughened, as charac¬ terized by a spotty RHEED pattern, even after thermal an¬ nealing at 300 * C. The optional sputter-cleaning following the chemical etch employs very mild conditions. For example, argon ions having an energy of about 50 to 100 eV are utilized, using a beam current density of less than about 100 μA/cm 2 for a period of time ranging from about 1 to 5 minutes. In step (a) , the fluence is preferably at least about 60 mJ/cm 2 . However, the fluence should not exceed about 75 mJ/cm 2 , in order to avoid melting of the CdTe surface. In step (bl), the fluence is preferably no greate than about 40 mJ/cm 2 . The existence of a threshold fluenc (about 40 mJ/cm 2 ) above which the CdTe surface stoichiometr is altered is shown in FIG. 1. The fluence should be a least about 15 mJ/cm 2 in order to realize the benefits of the invention. The repetition rate of the excimer laser is not criti¬ cal to the practice of the invention, so long as the sample is heat-sunk, to avoid bulk heating of the CdTe sample, and may be at any value that is conveniently accessible. For example, the repetition rates of the excimer laser used herein in both cases ranged from about 0.5 to 1 Hz. Even higher repetition rates may also be employed. In the first exposure, the number of laser pulses de- sirably ranges from about 5 to 15 pulses at the given flu¬ ence. In the second exposure, the number of pulses desir¬ ably ranges from about 100 to 1,000 pulses at the given fluence. In step (b2), the CdTe surface is exposed to the tem- perature for a period of time ranging from about 30 seconds to 5 minutes and then cooled to room temperature. For ex¬ ample, the sample may be heated in a furnace (or the sur¬ face exposed to a source of heat) , brought up to the de¬ sired surface temperature, held there for the desired peri- od of time, and then the heat source may be shut off and the sample allowed to cool. Alternatively, the sample could be mounted on a cold stage such as a liquid nitrogen cooled stage, the nature of the surface monitored such as by RHEED analysis during heating, and then cooled to room temperature when the surface achieves stoichiometry. While the annealing temperature is less than about 300 β C in order to avoid thermal pitting of the CdTe sur¬ face, preferably, the annealing temperature is about 200 ' C. The lower limit of the annealing temperature is about 165 β C, which represents the temperature at which Te 2 evapo¬ ration from the surface is evident, i.e., kinetically sig¬ nificant. The process of the invention, which employs laser ab lation, such as from a 248 nm laser, removes both the sur face contaminants and the damaged layer produced by we chemical etching, while leaving a stoichiometric and well ordered CdTe substrate for subsequent growth and process ing. By "well-ordered" is meant that the surface has lon range crystalline symmetry to the resolution observable b RHEED. The process of the invention may be performed in situ in any chamber equipped with an ultraviolet grade optical port and is compatible with closed system processing. La¬ ser ablation presents an advantage over alternative surface cleaning techniques such as ion bombardment because it does not require an additional piece of vacuum hardware with a hot filament which may act as an independent source of im¬ purities. EXAMPLES We used CdTe (100) single crystal substrates grown by the vertical Bridgman technique and obtained from II-VI In¬ dustries, Inc. (Saxonburg, PA). Substrates were etched in 1/16% weight/weight Br 2 /ethylene glycol solution. The treatment removed the native oxide. However, Auger mea- surements indicated that CdTe surfaces treated in this fashion were contaminated with carbon, oxygen, chlorine, and sometimes sulfur. RHEED patterns from these surfaces were very spotty, indicating a rough and disordered sur¬ face. The apparatus for simultaneously exposing CdTe surfac¬ es to a KrF excimer laser beam and for analyzing the sur¬ face by AES and RHEED has been described in Materials Re¬ search Society Symposium. Vol. 204, pp.243-250 (Apr. 1991); see also U.S. Patent 5,018,164 for a description of excimer laser ablation of CdTe films. Briefly, the system consist¬ ed of a turbomolecular-pumped stainless steel vacuum cham¬ ber with a base pressure of 5x10 " Torr. The chamber was equipped with a hemispherical analyzer and an electron gu for Auger electron spectroscopy, a cryo-shielded and dif ferentially pumped quadrupole mass spectrometer for time of-flight (TOF) mass spectrometry studies of desorbed prod uct velocities, and ultraviolet grade quartz optical port to allow excimer laser radiation of the sample. A preci sion manipulator with heating and cooling capabilities wa used to position the sample for laser radiation and surfac analysis. The KrF excimer laser radiation (15 nsec, 24 nm, 0.5 Hz) was introduced either at normal incidence or a 45° to the CdTe surface. The incident fluence was con trolled by placing partial reflectors in the path of th laser beam. Exposure of these CdTe surfaces to 5 to 15 pulses o 248 nm excimer laser radiation at fluences of about 60 t 65 mJ/cm 2 succeeded in removing the most of the carbon an all of the oxygen, chlorine, and sulfur contaminants bu left the surface Te-rich and with no observable RHEED pat tern. Subsequent exposure of the Te-rich CdTe surface t several hundred pulses of 248 nm excimer laser radiation a fluences of 40 mJ/cm 2 or less left the CdTe surface clean, stoichiometric, and with a reconstructed (2x1) surface uni cell clearly observable by RHEED. The (2x1) reconstructio has been observed by others to be the stable surface struc ture for a clean, well-ordered CdTe (100) surface. The foregoing sequence of steps is illustrated i FIGS. 2a-c. FIG. 2a depicts the spotty surface resultin either from sputter-cleaning and annealing at 300"C or fro etching in the Br 2 /ethylene glycol etch, possibly followe by sputter-cleaning. FIG. 2b represents the amorphous sur¬ face (no RHEED pattern) following irradiation with 5 to 15 pulses at 248 nm and a fluence of 63 mJ/cm 2 . FIG. 2c rep¬ resents the reconstructed CdTe (100) (2x1) surface obtaine by irradiation with 100 to 1,000 pulses at 248 nm and fluence of 28 mJ/cm 2 . The foregoing procedure removes ex- cess Te and results in a clean, stoichiometric, and wel ordered CdTe substrate surface. It was also possible to obtain the clean, well-order CdTe (100) surface condition by following the pulses of 2 nm excimer laser radiation at a fluence of 65 mJ/cm 2 wi a low temperature (200 β C) thermal anneal of the substra for 5 minutes. RHEED patterns for a surface exposed to 11 pulses o 248 nm excimer laser radiation at 65 mJ/cm 2 followed by thermal anneal at 200 * C for 5 minutes clearly showed (2x1) reconstruction; the pattern consisted of strong uni form streaks, indicative of a smooth ordered surface. Thus, there has been disclosed a method for producin clean, stoichiometric, well-ordered CdTe surfaces. It wil be readily apparent to those skilled in this art that vari ous changes and modifications of an obvious nature may b made without departing from the spirit of the invention and all such changes and modifications are considered t fall within the scope of the invention, as defined by th appended claims.

Claims: CLAIMS What Is Claimed Is: 1. A method for producing clean, stoichiometric, an well-ordered CdTe surfaces, comprising: (a) exposing a chemically-etched CdTe surface to ultraviolet radiation at a fluence greater than the thresh¬ old fluence for creating a non-stoichiometric film; and ei¬ ther (bl) exposing said CdTe surface to ultraviolet radiation at a fluence lower than the threshold fluence, or (b2) thermally annealing said CdTe surface at a temperature less than about 300"C until any excess telluri¬ um is removed. 2. The method of Claim 1 wherein said CdTe surface is chemically etched in a solution consisting essentially of bromine and ethylene glycol. 3. The method of Claim 2 wherein the concentration of bromine in ethylene glycol is at most about 1/16% volume/- weight. 4. The method of Claim 1 wherein said UV radiation is less than about 827 nm. 5. The method of Claim 4 wherein said UV radiation is provided by a source comprising an excimer laser operating in the range of about 193 to 248 nm. 6. The method of Claim 5 wherein said UV excimer laser is a KrF laser operating at about 248 nm. 7. The method of Claim 6 wherein fluence greater than the threshold fluence for creating a non-stoichiometric film is at least about 60 mJ/cm 2 . 8. The method of Claim 7 wherein said fluence does no exceed about 75 mJ/cm 2 . 9. The method of Claim 7 wherein said CdTe surface i exposed to said UV excimer laser for about 5 to 15 pulses. 10. The method of Claim 5 wherein said fluence les than the threshold fluence is no greater than about 4 m-J/cm 2 . 11. The method of Claim 10 wherein said fluence is a least about 15 mJ/cm 2 . 12. The method of Claim 10 wherein said CdTe surface is exposed to said UV excimer laser for about 100 to 1,000 pulses. 13. The method of Claim 1 wherein said CdTe surface is exposed to said temperature for a period of time ranging from about 30 seconds to 5 minutes and then cooled to room temperature. 14. The method of Claim 13 wherein said annealing tem¬ perature is less than about 200 β C. 15. The method of Claim 1 wherein following chemical etching, said CdTe surface is sputter-cleaned. 16. The method of Claim 15 wherein said sputter-clean- ing is performed employing argon ions at an energy of about 50 to 100 eV, using a beam current density of less than about 100 μA/cm 2 for a period of time ranging from about 1 to 5 minutes.

Inventors: BREWER PETER D, ZINCK JENNIFER J, BREWER, PETER, D., ZINCK, JENNIFER, J.
Applicants: HUGHES AIRCRAFT CO, HUGHES AIRCRAFT COMPANY
Application year: 1992
Main classification: C30B29
IPC codes: C23C16, C23F1, C23F4, C30B29, C30B33