Method of Preparing a Mercury Cadium Telluride Substrate for Passivation and Processing

1. Field of the Invention

The invention relates to a method for preparing HgCdTe substrate surface layers in order to reduce the surface damage that is typical of HgCdTe surfaces regardless of the growth or surface preparation techniques.

2. Description of the Prior Art

Hg.sub.1-x Cd.sub.x Te is a compound semiconductor with potential to meet the requirements of many infrared detector applications. This is mainly due to it's tunable bandgap, which determines its spectral operating range and it's long wavelength cutoff. The bandgap of HgCdTe can be continuously varied for 0. to 1.6 eV by changing the relative concentrations of HgTe and CdTe, i.e., changing the x-value function to utilize the tunability of the energy gap. However, the surface of HgCdTe alloys provides sites for generation-recombination which, in turn, severly limit the performance of these alloys for many infrared detector applications. Improperly treated HgCdTe device surfaces can be a source of leakage currents, excessive l/f noise, and device instability both chemically and electrically with time. Consequently, it is necessary to prepare a high quality surface on HgCdTe device materials in order to reduce the surface leakage currents as well as to maintain the chemical and electrical stability.

The continued development of HgCdTe ternary alloys is becoming increasingly important as a source of sensor materials for advanced infrared focal plane applications. The advancement of this technology requires improved H.sub.1-x Cd.sub.x Te material preparation methods, i.e., surface processing and growth techniques, as well as surface passivations and junction formation processes. Surface quality remains a critical limitation in Hg.sub.1-x Cd.sub.x Te detector fabrication. The depth of the damaged surface layer from surface processing of HgCdTe wafers is a function of the processing techniques, that is whether one does straight chemical etching, chemical-mechanical polishing or contactless polishing of the material surface. Many of the previous processes left a damaged surface layer which extended to a depth of 1000 .ANG. or more into the Hg.sub.1-x Cd.sub.x Te material. One approach for reducing the level of surface damage is to anodize the Hg.sub.1-x Cd.sub.x Te surface using a wet chemical approach and then to consume the damaged surface region by an appropriate etching technique. However, the depth of the anodization must be very accurately controlled however to prevent shifting of the damage, underlying near surface bulk Hg.sub.1-x Cd.sub.x Te material, further into the bulk Hg.sub.1-x Cd.sub.x Te. The long term wet etching of Hg.sub.1-x Cd.sub.x Te surfaces also changes the surface stoichiometry, which impacts device passivation and surface leakage phenomena. Therefore, this approach to improved surface quality for Hg.sub.1-x Cd.sub.x Te alloys is far from satisfactory.

The present method of preparing an improved Hg.sub.1-x Cd.sub.x Te surface layer is disclosed herein below.

The present invention is a method for improving the surface quality of Hg.sub.1-x Cd.sub.x Te layers for device applications by depositing cadium telluride thereon to about 1000 .ANG. depth followed by a subsequent removal of the CdTe layer by contactless polishing to yield surfaces having improved quality and smaller variations in composition, i.e., small variations in x-value. The original Hg.sub.1-x Cd.sub.x Te wafer may be the result of bulk, liquid phase epitaxial or chemical vapor deposition crystal growth techniques, and may be either n- or p-majority barrier type and not suffer deleterious effects from the CdTe process described in the present method.

Electrolyte electroreflectance tests of the surface quality for Hg.sub.1-x Cd.sub.x Te substrates resulting from the present method have demonstrated excellent surface quality and x-value homogeniety for the approximately 200 .ANG. sampling depth of this measurement for Hg.sub.1-x Cd.sub.x Te substrates processed according to this invention, whereas in prior art Hg.sub.1-x Cd.sub.x Te surfaces the damage and x-value non-uniformity starts at the surface and extends as far as 2000 .ANG. into the Hg.sub.1-x Cd.sub.x Te surface.

The present method of preparing a high quality Hg.sub.1-x Cd.sub.x Te surface will be better understood with reference to the detailed description of the preferred embodiment and the following lone FIGURE drawing.

The single FIGURE of the drawing illustrates the steps by block diagram and the layered substrate involved in the method of preparing the surface of a Hg.sub.1-x Cd.sub.x Te substrate for passivation and further processing.

The inventive method is herein after described by reference to the lone FIGURE drawing. A mercury cadmium telluride substrate 10 is provided having the composition Hg.sub.1-x Cd.sub.x Te. Substrate 10 may be prepared by any convenient means for producing high quality Hg.sub.1-x Cd.sub.x Te, such as bulk crystal growth techniques, liquid phase epitaxy, or chemical vapor deposition. The method is comprised of first etching the Hg.sub.1-x Cd.sub.x Te substrate with a bromine methanol or bromine N,N-Dimethylformide (DMF) solution by means, such as dip etching using a 2% bromine and 98% solution (V/V) or an appropriate bromine N,N-Dimethylformide solution, by placing said substrate 10 in a holder with the process side of 10 up and dipping into either of the indicated etching solutions for about 20-45 seconds. The etched substrate is then quenched in methanol for the bromine-methanol etch or DMF for the bromine-DMF etch until all traces of the bromine are removed. The substrate is next rinsed in acetone and methanol, to remove soluble residual contamination and is immediately dried with dry nitrogen.

The next step in preparing the Hg.sub.1-x Cd.sub.x Te substrate is to coat the substrate with a layer of cadmium telluride 12 in a coating system which is evacuated to about 2-5.times.10.sup.-7 Torr. The CdTe layer 12 is deposited on the etched Hg.sub.1-x Cd.sub.x Te at about 10-12 .ANG. per second until about 1000 .ANG. is deposited thereon. The step of coating the substrate 10 may be accomplished by the electron beam evaporation method, thermal evaporation, or by means of sputtering techniques.

The processed layer 12 of CdTe is next polished by a contactless polishing means down to less than 200 A. The contactless polishing may be accomplished by mounting the substrate 10 on a rotatable disc and placing it processed face down just above a rotatable polishing pad. The two discs are rotated while a continuous flow of a polishing solution is supplied to the discs. The polishing solution is preferably comprised of 50% ethylene glycol and 50% of a mixture of 2% bromine and 98% methanol.

Even though the inventive method has been described in detail with particular reference to preferred embodiments thereof, it will be understood that variations and modifications can be effected within the spirit and scope of the invention.