A Study of the MBE HgTe Growth Process
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A SIUDY OF THE MBE HgTe GROMTH PROCESS ROIUND J.KOESTNER AND H.F.SCHAAKE Texas Instruments, Inc., Central Research laboratories, Dallas, Tx 75265
ABSTRACT The MBE growth of HgTe on CdTe is examined over a hundred-fold range in Hg/Te2 flux ratio and over four separate substrate surface orientations [(lll)Te, (lll)Te-4 degrees, (112)Te and (001)]. The 77K Hall mobility of the (112)Te and (001) oriented HgTe layers approaches the best bulk values reported to date, although our (lil)Te and (lll)Te-4 deg oriented HgTe films yield much lower values. The growth process is shown to be very far from thermodynamic equilibrium at our optimal substrate temperature and calculated equivalent Hg beam pressure. Important clues to help understand the kinetics governing the HgTe growth process are uncovered by studying the defects that form under Hg- or Te-rich conditions with cross-sectional transmission electron microscopy (XTE4). Since multilayered structures are an important application for MBE growth of Hg-based semiconductors , we have also examined the interfacial roughness present in HgTe-OdTe superlattices (SL) as a function of growth orientation. RESULTS AND DISCUSSION As a measure of the quality of HgTe layers grown on CdTe substrates with varying surface orientations, Hall transport measurements by the Van der Pauw method was employed with a 1000 Gauss magnetic field strength. The temperature dependent Hall mobility of a 2.1 um HgTe layer on a OdTe(112)Te substrate grown under an optimal Hg/Te2 flux ratio is illustrated in Figure 1. The measured mobility curve approaches the best bulk values reported to date (1]. These bulk HgTe samples as in our MBE grown layer show a - 100,000 cm2/V-s mobility at 77K and a step increase in the mobility near 40K due to the cross-over from Lo phonon to ionized impurity scattering. Our optimized HgTe film, however, plateaus at 120,000 cm2/V-s rather than increase to -400,000 cm2/V-s at 10K. To compare this Hall mobility for HgTe(112)Te to other oriented HgTe layers, a range of Hg/Te2 flux ratios had to be tested due to the reported variation of the Hg sticking coefficient with growth orientation (2]. The optimal HgTe(112)Te layer was deposited at 225C with
Mat. Res- Soc. Symp. Proc. Vol. 90. 1987 Materials Research Society
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lated equivalent Hg beam pressure of 1 x 10-3 torr; the Te2 flux was kept constant throughout this study and yielded an elemental deposition rate of -2 um/hr on Si substrates held at 100C. All the CdTe substrates prior to HgTe overgrowth in a Riber 2300P MBE chamber had been chemimechanically polished in 0.5% and 0.2% Br2/MeOH solutions (unless otherwise indicated in the Figures), static etched in a 0.2% Br2/MeOH solution and finally vacuum annealed near 300C to remove excess Te. To span the Te rich growth conditions, the Hg cell temperature was reduced in steps by a total of 47C; and the calculated equivalent Hg beam pressure was incrementally reduced from 1 x 10-3 to lx 10-4 torr. To cover the Hg rich growth conditions, the substrate temperature was
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