Rapid, nondestructive evaluation of macroscopic defects in crystalline materials: The laue topography of (Hg, Cd) Te
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S e v e r a l (Hg,Cd)Te s e m i c o n d u c t o r a l l o y s have p r o p e r t i e s which make them useful a s solid state d e t e c t o r s . However, homogeneous alloy single c r y s t a l p r e p a r a t i o n r e q u i r e s u n u s u a l t r e a t m e n t . A t w o - s t e p p r o c e s s is n e c e s s a r y b e c a u s e of the l i q u i d u s - s o l i d u s c o m p o s i t i o n d i s p a r i t y at the m e l t i n g point. Laue topography was found to be an efficient tool for o p t i m i z i n g p a r a m e t e r s in an i n i t i a l quench (to p r e s e r v e h o m o g e n e i t y ) and a s u b s e q u e n t solid state r e c r y s t a l l i z a t i o n (to p r o m o t e l a r g e g r a i n growth). C r y s t a l p e r f e c t i o n a n a l y s i s u s i n g p o l y c h r o m a t i c X - r a y d i f f r a c t i o n i s not an e n t i r e l y new concept, but our i n i t i a l i n t e r e s t led to a s i m p l i f i e d Laue topography t e c h nique which i s useful for r a p i d c h a r a c t e r i z a t i o n of v a r i o u s m a c r o s c o p i c c r y s t a l defects. The utility of this method is d e m o n s t r a t e d by a study of defects in bulk (Hg,Cd)Te c r y s t a l s .
(Hg1_x,Cdx)Te is an alloy semiconductor whose band gap is a function of X, varying from 1.5 ev for X = 1 to a semimetal value for X = 0. Several of these alloy compositions have shown considerable promise as infrared detectors, particularly at wavelengths beyond 8 ~m. Unfortunately, several problems arise in preparing these alloys by melt growth processes (such as Stockbarger, gradient freeze, zone melting, and so forth), principally due to the nature of HgTe-CdTe solutions. The phase diagram for this system is shown in Fig. 1',2 which clearly shows why homogeneous alloys would be quite difficult, if not impossible, to prepare by conventional crystal growth methods. Quenching of finger-sized ingots followed by recrystallization in the presence of excess mercury has given satisfactory homogeneity and crystal growth within a narrow range of compositional and thermal parameters. Since crystal perfection in the finished material always implies local homogeneity and is helpful in subsequent processing, a rapid nondestructive crystal perfection assessment was useful for optimizing the quench and recrystallizationparameters. Our first investigations of "Laue topography" were prompted by the frequent and often objectionable occurrence of ghost Laue reflections on scanning characteristic-radiationtopographs. These ghost polychromatic-diffractionimages were especially evident in several scanning topographs of early (Hg,Cd)Te slices, see Fig. 2. Overall crystal perfection surveys with conventional scanning topography required tedious realignment and exposure for each grain in the (Hg,Cd)Te slice. Laue topography, on the other hand, gave an adequate crystal perfection survey of the entire sample with a single exposure (see Laue topography and discussion sections). LAURENCE N. SWlNKand MAURICE J. BRAU are X-ray Diffraction and Electro
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