Study of temperature dependent structural changes in molecular-beam epitaxy grown Hg1−xCdxTe by x-ray lattice parameter measurements and extended x-ray absorption fine structure

Abstract

Infrared detectors fabricated from Hg1−xCdxTe typically operate in the 60 to 160 K range. The temperature dependence of the atomic structure of HgCdTe may influence device performance. We present the first detailed study of the x-ray diffraction lattice parameters of molecular-beam epitaxy grown Hg1−xCdxTe epilayers between 15 and 300 K. The epilayers were grown on (100) oriented CdTe substrates, and varied in thickness (6 to 11-μm) and composition (x=0–0.172). The (400) reflection was measured to determine the lattice parameter a⊥ normal to the film. HgTe (x=0) exhibited normal lattice contraction (α=4.7×10−6±0.3 K−1 at 300 K), with a minimum in a⊥ at 60 K, and an expansion of a⊥ below 60 K. In addition to showing a minimum in a⊥ at 60 K, some of the Hg1−xCdxTe (x≠0) epilayers (10 μm thick) exhibited anomalous behavior with varying degrees of thermal hysteresis in a⊥. The average contraction of a⊥ for these epilayers from 300 to 60 K is 0.006 Å. This is compared with results we have obtained from a temperature dependent extended x-ray absorption fine structure study of these HgCdTe epilayers: whereas HgTe exhibited a normal thermal contraction of the Hg–Te bond length consistent with the lattice parameter results, in the HgCdTe epilayers this bond contracts 0.02 to 0.03 Å on cooling from 300 to 10 K. We also present lattice parameter measurements for a thin cap layer of CdTe on HgCdTe. An increase of 0.0134 Å in a⊥ relative to the bulk was observed for a 1000 Å layer of CdTe on HgCdTe at 300 K.

Mechanical and Thermal Properties

“…In a subsequent review, Capper [23] reports a(x ) = 6.4614 + 0.0084x + 0.0168x 2 − 0.0057x 3 Å (8.16) as the expression for the variation of lattice parameter, a, with the composition, x , which was proposed by Higgins et al [3]; and points out that the differences with Brice's near-stoichiometric case and that of Woolley and Ray [24] amount to ±0.001Å or less. Additional data for bulk material [5,[25][26][27], as well as for molecular beam epitaxy (MBE) [28,29,33], liquid phase epitaxy (LPE) [30][31][32]35], and metal-organic vapor phase epitaxy (MOVPE) [36] material are also shown in Figure 8.3, which are in general agreement with each other and with Equation 8.16. In contrast, earlier MOVPE growth of HgTe layers on CdTe by Bhat et al [38] resulted in lattice parameters of 6.454-6.456Å at 1.0-3.5 μm film thickness.…”

“…This relationship is shown in Figure 8.4 (for x = 0, x = 0.5, x = 1.0) alongside other reports found in the literature. [48] Lattice parameter (Å) Temperature (K) Reasonable agreement is found with early [47][48][49] and more recent [28,50,51] reports on bulk and MBE material, respectively. Nevertheless, it is evident from Figure 8 [48], which show trends of increasing lattice parameter with decreasing temperature for temperatures below about 50 K. As the thermal expansion coefficient has been observed to be negative below about 80 K [10], such lattice expansion is to be expected for low temperatures.…”

“…Nevertheless, it is evident from Figure 8 [48], which show trends of increasing lattice parameter with decreasing temperature for temperatures below about 50 K. As the thermal expansion coefficient has been observed to be negative below about 80 K [10], such lattice expansion is to be expected for low temperatures. The errors in the measurement of the lattice parameters claimed by Di Marzio et al [28] and Skauli et al . [50], were ±2.5 × 10 −4Å and less than 10 ppm, respectively.…”

“…By considering literature reports prior to 1987, Brice [22] Brice [22] Higgins et al [3] (Eqn. (8.16)) Wolleey and Ray [24] Bell and Sen [5] Song et al [25] Tanaka et al [26] Johnson et al [27] Di Marzio [28] (MBE) Jacobs et al [29] The inset magnifies the region indicated by the dotted boundary composition x and concluded that Vegard's law is approximately followed. In this correlation, samples are grouped according to their stoichiometry.…”

“…The authors argue this composite system should not be considered as two materials bonded to each other while conserving their individual properties, but rather as a new system different in physical properties from its constituents. Di Marzio et al [28] report an anomalous nonlinear thermal hysteresis in the behavior of lattice parameter of Hg 1−x Cd x Te with x = 0.153 and x = 0.160, that was not observed for x = 0 and x = 0.172, and attribute this to the details of the atomic bonding.…”