Positron-defect profiling in Cd^sub 1-x^Zn^sub x^Te wafers after saw cutting

Journal of Electronic Materials, Jun 2003 by McNeil, Sean P, Lynn, Kelvin G, Weber, Marc H, Szeles, Csaba, Soundararajan, Raji

EXPERIMENT

Samples of Cd^sub (1-x)^Zn^sub x^Te are cut from a single ingot by a single-wire saw (SWS) and two different multiwire saws (MWS1 and MSW2). The ingot was grown by the horizontal Bridgman technique.9 The Zn fraction was 4%. These samples were studied in the as-cut state and after etching in 1% brominemethanol solution with an approximate etch rate of 0.267 or - 0.017 [mu]m/s. Also, 5% Br methanol (1.15 or - 0.083 [mu]m/s) and 5% Br ethanol (0.483 or - 0.033 [mu]m/s) solutions were used.10 After etching, the resistivity of the samples was 5.2 x 10^sup 5^ [Omega]cm. Etches were performed at a temperature of 45[degrees]C. The etch solutions were hand-stirred, and the age of the solutions was negligible. A fourth sample (10% Zn) was cut from a crystal grown by the high-pressure Bridgman technique (HPB) and cut with a SWS.11 The sample was (1) polished with 600-grit SiC and 6-[mu]m diamond suspension, (2) with 600-grit, 6-[mu]m and 1-[mu]m diamond suspension, (3) etched, and (4) polished again by 1,200-grit SiC, 6-[mu]m diamond suspension. Etching was carried out in 5% Br methanol and three times in 5% Br ethanol. After etching, the resistivity was determined to be 7 or - 2 x 10^sup 10^ [Omega]m.

Positron damage-depth profiles were measured on the as-cut samples and after each polishing and etch step. The data of the SWS, MWS1, and MWS2 samples are shown in Fig. 1; data for sample HPB are shown in Fig. 2. The data were normalized to the value in the bulk of the material to show the defects introduced by the cutting process only. The energy of the positron-annihilation photon is ~0.511 MeV. The observed line width is broadened by the detector resolution (1.5 keV full-width at half-maximum) and the electronic properties at the site of annihilation. When the annihilation occurs at a vacancy-like defect, less broadening makes the line sharper. This sharpness is parameterized and shown in the figure. 1-3 The positron-implantation profile (similar to ion-implantation depth profiles) and the positron-diffusion length control the transition from a region with high sharpness (more damage) to one with lower sharpness (less to no damage). A short diffusion length results in rapid transitions and a long diffusion length in more gradual transitions.

RESULTS AND DISCUSSION

As can be seen in Fig. 1, the sharpness of the annihilation line drops to the value equivalent for undamaged deep regions with increasing depth. In the case of the MWS cuts, the return occurs as a shallower depth. Less damage was created during cutting than in the case of the SWS. After etching for 100 sec in 5% Br ethanol, almost all damage is removed.

In a standard analysis program,12 the thickness of the damage region is deconvoluted from the positron-implantation profile. Table I lists the thickness of the damage layer for the samples. In order of increasing thickness, the cutting methods are MWS2, MWS1, HPB, and SWS. The MWSs cause far less damage that the SWSs. This is likely due to the higher quality engineering of the MWSs.