United States Patent (19. 11 4,128,338 Wong 45 Dec. 5, 1978 54 MODIFIED OPTICAL TRANSMISSION 3,725,135 4/1973 Hager et al. .......................... 148/15 TECHNIQUE FORCHARACTERIZING 3,902,924 9/1975 Maciolek et al. ..................... 148/15 EPTAXAL LAYERS Primary Examiner-John K. Corbin (75) Inventor: Theodore T. S. Wong, Maynard, Assistant Examiner-R. A. Rosenberger Mass. Attorney, Agent, or Firm-Theodore F. Neils; David R. 73) Assignee: Honeywell Inc., Minneapolis, Minn. Fairbairn (21) Appl. No: 807,608 57 ABSTRACT al An improved method of determining the energy band (22 Filed: Jun. 17, 1977 gap of an epitaxial semiconductor layer on a substrate 51 int. Cl’............................................. GON 21/22 corrects for an overestimation of energy gap yielded by 52 U.S. C. ................................................. 356/432 normal optical transmittance measurements. The over 58 Field of Search ........................ 356/201, 202,203 estimation of energy bandgap is caused by a graded (56) References Cited bandgap region which exists between the epitaxial semi U.S. PATENT DOCUMENTSw conductorductor 1layer and the substrate s 3496,024 2/1970 Ruehrwein.................. 148/33.5 3 Claims, 10 Drawing Figures DETERMINE MEASURE OPTICAL ds ded TRANSMISSION (T ) DETERMINE ol. Eg., x U.S. Patent Dec. 5, 1978 Sheet 1 of 5 4,128,338 X 2 O H () O al O U THCKNESS U.S. Patent Dec. 5, 1978 Sheet 2 of 5 4,128,338 MEASURE DETERMINE OPTICAL ds de d TRANSMISSION (T ) FIG. 4 DETERMINE c., E.g. x 2 FIG.5 20 40 60 80 IOO I2O 4O 6O 80 20O MICRONS SUBSTRATE GRADED GAP LAYER REGON U.S. Patent | | 1-01 33NWLLWSNW U.S. Patent Dec. 5, 1978 Sheet 4 of 5 4,128,338 - / O DATA O - - - CALCULATION O-3 6 7 8 9 O 2 3 WAVELENGTH (Jim) U.S. Patent Dec. 5, 1978 Sheet 5 of 5 4,128,338 FG.O d = 5 Lim Ol O-2 d = 20 Jum 2 lo-3 d = 4Olm H de = 6Oum 2 C a o to 4 to 5 3 4. 5 6 7 8 9. O WAVELENGTH (um) 4,128,338 1. 2 ber of advantages over both vapor phase epitaxial MODEFEO OPTICAL TRANSMISSION growth and bulk growth of (HgCd)Te. TECHNIQUE FOR CHARACTERIZING One characteristic of epitaxial film grown by both EPTAXAL LAYERS vapor phase epitaxy and liquid phase epitaxy is a ten dency to exhibit a compositional gradient along the ORIGIN OF THE INVENTION crystal growth direction. This is particularly true when The present invention was made in the course of a CdTe is used as the substrate material. Examples of contract with the Department of Army. compositional profiles through the thickness of epitaxi BACKGROUND OF THE INVENTION ally grown films are shown in FIG.S. 3, 5, 6 and 9 of the O previously mentioned Hager et al. patent (U.S. Pat. No. The present invention is concerned with the charac terization of epitaxial layers. The present invention is 3,725,135) and in FIGS. 4a-4e of the previously men particularly useful in characterizing epitaxial layers of tioned Maciolek et al. patent (U.S. Pat. No. 3,902,924). semiconductor alloys such as mercury cadmium tellu The device formed by epitaxial growth may be consid ride, lead tin telluride, indium arsenide antimonide, 15 ered, therefore, to have three regions: the substrate, a gallium arsenide phosphide, and others. graded composition or graded bandgap region, and the For the purposes of simplicity, the present invention epitaxial layer of desired composition. will be described with reference to a particular semi SUMMARY OF THE INVENTION conductor alloy; mercury cadmium telluride. The com mon chemical notation for mercury cadmium telluride, 20 The present invention is directed to an improved (HgCd)Te, or HgCdTe, will be used. method of characterizing an epitaxial layer on a sub Mercury cadmium telluride is an intrinsic photode strate, wherein a graded bandgap region exists between tector material which consists of a mixture of cadmium the epitaxial layer and a substrate. It is based upon the telluride, a widegap semiconductor (E = 1.6ev), with discovery that conventional optical transmission tech mercury telluride, which is a semimetal having a "nega 25 niques are inaccurate because the existence of a graded tive energy gap' of about 0.3ev. The energy gap of the bandgap region causes the transmittance curves to devi alloy varies linearly with x, the mole fraction of cad ate from those expected for homogenous material. The mium telluride in the alloy. By properly selecting ''x'', it deviation, if uncorrected, leads to an underestimation of is possible to obtain mercury cadmium telluride detec the cutoff wavelength, and thus an overestimation of tor material having a peak response over a wide range 30 the bandgap of the epitaxial layer. of infrared wavelengths. The method of the present invention overcomes this (HgCd)Te is of particular importance as a detector problem by determining the thickness dis of the sub material for the important 8 to 14 micron atmospheric strate, the thickness dig of the graded bandgap region, transmission "window". Extrinsic photoconductor de and the thickness d of the semiconductor layer. The tectors, notably mercury doped germanium, have been 35 energy gap of the epitaxial layer is determined based available with high performance in the 8 to 14 micron upon the values of ds, do, and d and the results of wavelength interval. These extrinsic photoconductors, measurements of the total optical transmittance of the however, require very low operating temperatures layer, graded bandgap region, and substrate as a func (below 30 K). (HgCd)Te intrinsic photodetectors hav tion of wavelength. ing a spectral cutoff of 14 microns, on the other hand, are capable of high performance at 77 K. BRIEF DESCRIPTION OF THE DRAWINGS At the present time, most (HgCd)Te is produced by FIG. 1 illustrates schematically how light transmits bulk growth techniques such as the technique described through a cadmium telluride-mercury cadmium tellu by P. W. Kruse et al. in U.S. Pat. No. 3,723,190. High 45 ride epitaxial structure. quality (HgCd)Te crystals are produced by this bulk FIG. 2 shows absorption, a and cutoff wavelength growth technique. A as a function of thickness Z for a CdTe-(HgCd)Te Epitaxial growth techniques offer a number of poten epitaxial structure. tial advantages over bulk growth techniques. An epitax FIG. 3 shows the linear graded-gap approximation ial layer is a smooth continuous film grown on a sub 50 strate, such that the film crystal structure corresponds used in the calculations. to and is determined by that of the substrate. The de FIG. 4 illustrates the modified optical transmission sired epitaxial layer is single crystal with uniform thick technique of the present invention. ness and electrical property. The substrate has a differ FIG. 5 shows a CdTe-(HgCd)Te epitaxial structure ent composition or electrical properties from that of the 55 which has been angle-lapped at one edge to allow mea epitaxial layer. surement of thicknesses ds, do, and dil. A number of epitaxial growth techniques have been FIG. 6 shows calculated and measured transmittance investigated in an attempt to grow (HgCd)Te layers. as a function of wavelength for bulk (HgCd)Te. Vapor phase epitaxial growth processes which have FIG. 7 shows calculated and measured transmittance been studied are described in a number of patents in of a liquid phase epitaxial layer of (HgCd)Te on a cad cluding R. Ruehrwein (U.S. Pat. No. 3,496,024), G. mium telluride substrated. f Manley et al. (U.S. Pat. No. 3,619,282), D. Carpenter et FIG. 8 shows composition as a function of thickness al. (U.S. Pat. No. 3,619,283), R. Lee et al. (U.S. Pat. No. as measured by electron beam microprobe for the same 3,642,529), and R. Hager et al. (U.S. Pat. No. 3,725,135). sample used for measurements in FIG. 7. Another epitaxial growth technique which has been 65 FIG. 9 shows the effects of epitaxial layer thickness investigated is liquid phase epitaxy ("LPE'). This tech dL on transmittance. V nique is described in R. Maciolek et al. (U.S. Pat. No. FIG. 10 shows the effect of graded bandgap region 3,902,924). Liquid phase epitaxial growth offers a num thickness do on transmittance. 3 4,128,338 4. DETAILED DESCRIPTION OF THE d Eq. 2 PREFERRED EMBODIMENT (I - RL) (I - Rs) exp ? a(Z) dZ The present invention is an improved method of char- T = - - - acterizing epitaxial semiconductor layers by a modified 5 I - RLRs exp(-2 I a(Z) dZ optical transmission technique. Optical transmission o measurements are commonly used to determine the where the subscripts L and S stand for layer and sub energy gap E of semiconductor materials. This is of strate, respectively, and d = d -- do -- d.s. particular importance in alloy semiconductors such as 10 In order to illustrate the effects of the graded-gap on (HgCd)Te and lead tin telluride since the energy gap the overall transmittance curve, a linear graded-gap varies with composition of the alloy. The value of the approximation is made (see FIG. 3). This approximation energy gap is particularly important information when allows Eq. 2 to be quantified, since it relates the compo the semiconductor material is to be used as a photode sition x (and therefore E) directly to the thickness Z. tector, since the energy gap determines the wavelengths 15 As a result, the function a(Z) can be explicitly deter to which the material will be sensitive. mined if a(x) is known. The function a(x) has been The present invention is based upon the discovery derived and satisfactory agreement with data was ob tained (M.D.