M. D. Lange

Current status of direct growth of CdTe and HgCdTe on silicon by molecular‐beam epitaxy

CdTe and HgCdTe can be grown directly on Si(100) substrates by molecular‐beam epitaxy. The layers grow in the (111)B orientation. Single domain films are always obtained on Si(100) 8° off toward [011], whereas single and double domain films were obtained on nominal Si(100). A possible reason for the formation of these domains is discussed based on a microscopic model of the CdTe/Si interface. The structural quality of the layers is determined by double crystal x‐ray rocking curves. The narrowest rocking curves are obtained on single‐domain films grown on nominal Si(100) substrates; a full width at half‐maximum (FWHM) of only 230 arcsec was measured, compared to 460 arcsec on the best layer with two domains. For HgCdTe layers grown on CdTe/Si, rocking curves with 110 arcsec FWHM were measured; these layers are n‐type with electron mobilities above 5×104 cm2 V−1 s−1 at 23 K for a Cd mol % of 26%.

MOLECULAR BEAM EPITAXY AND CHARACTERIZATION OF CDTE(211) AND CDTE(133) FILMS ON GAAS(211)B SUBSTRATES

CdTe films were grown in both the (211) and (133) orientations on GaAs(211)B substrates by molecular beam epitaxy. The orientation of the epitaxy is dependent on the thermal cleaning process. Studies of these films included in situ reflected high‐energy electron diffraction, x‐ray double‐crystal diffractometry, transmission electron microscopy, and photoluminescence, which revealed high quality for both CdTe growth orientations, and especially for the CdTe(133). The lattice of the CdTe(211) growth tilts 3° with respect to its GaAs(211) substrate about the CdTe[011]//GaAs[011] coincidence axis. The CdTe(133) has no tilt with respect to its substrate, and its coincidence axes are CdTe[011]//GaAs[011] and CdTe[611]//GaAs[111].CdTe films were grown in both the (211) and (133) orientations on GaAs(211)B substrates by molecular beam epitaxy. The orientation of the epitaxy is dependent on the thermal cleaning process. Studies of these films included in situ reflected high‐energy electron diffraction, x‐ray double‐crystal diffractometry, transmission electron microscopy, and photoluminescence, which revealed high quality for both CdTe growth orientations, and especially for the CdTe(133). The lattice of the CdTe(211) growth tilts 3° with respect to its GaAs(211) substrate about the CdTe[011]//GaAs[011] coincidence axis. The CdTe(133) has no tilt with respect to its substrate, and its coincidence axes are CdTe[011]//GaAs[011] and CdTe[611]//GaAs[111].

Heteroepitaxy of CdTe on GaAs and silicon substrates

Abstract CdTe can be grown directly by molecular beam epitaxy on substrates such as GaAs or silicon, which exhibit very large lattice mismatches of 14.6% and 19% respectively. The occurrence of dual epitaxy, which has been previously reported for growth on (100)GaAs, has also been found recently for growth on (211)GaAs. The (133)CdTe-(211)GaAs hetero-interface presents a smooth continuation of the tetrahedral bond network from GaAs to CdTe, which is not the case for the (211)CdTe-(211)GaAs interface. Single-domain, twin-free CdTe(111)B films are currently obtained on Si(100) surface where single atomic steps are dominant. The crystalline quality of CdTe/Si films has been dramatically improved as confirmed by X-ray diffraction, photoluminescence and electron microscopy investigations. The narrowest rocking curves obtained for as-grown epilayers are 70 arcsec for (133)CdTe/(211)GaAs, 50 arcsec for a flash-annealed (211)CdTe/(211)GaAs and 140 arcsec for (111)B CdTe/(100)Si. These results confirm that the CdTe/GaAs and CdTE/Si composite substrates should be viewed as prime candidates to replace bulk CdTe substrates.

New development on the control of homoepitaxial and heteroepitaxial growth of CdTe and HgCdTe by MBE

Abstract It is reported that rotation twins as well as reflection twins are easily formed in CdTe and HgCdTe grown by MBE in the (111) orientation. Twinning can be avoided by carefully controlling the substrate preparation and by applying very stringent growth conditions, mostly for the stability of Hg pressure and the real surface temperature of the substrate, which is extremely difficult to control when the substrate rotates. A comparison between HgCdTe twinned layers and twin-free layers has shown that electrically active acceptors and high hole mobility are associated with the presence of reflection twins and/or mercury-rich alloy zones due to Hg overpressure during the growth. Twin-free HgCdTe layers can exhibit etch pit density count two orders of magnitude lower than twinned layers. Twin-free CdTe layers have been grown on GaAs and Si substrates. Excellent thickness uniformities have been reported: 0.24% for the standard deviation of a 2-inch diameter CdTe layer grown on GaAs(100) and 2.3% for a 5-inch diameter CdTe grown on Si(100).

Molecular beam epitaxial growth of CdTe on 5‐in.‐diam Si (100)

CdTe (111)B films with a 5 in. diameter have been grown on Si (100) substrates. They were characterized by in situ electron diffraction, x‐ray diffraction, and low‐temperature photoluminescence. The layer thickness was measured across two diameters with infrared transmission, and a standard deviation of 2.3% is obtained. This demonstrates the possibility of producing CdTe layers with a 5 in. diameter with excellent uniformity in terms of thickness and crystalline quality. Moreover, this demonstrates the potential for molecular beam epitaxial growth of other materials on large‐area substrates. In fact, these are the largest monocrystalline layers of a II‐VI semiconductor material ever grown by any technique.

Molecular beam epitaxy and characterization of HgCdTe(111)B on Si(100)

Up to 10‐μm‐thick HgCdTe(111)B films with 3 in. and 5 in. diameter were grown on Si(100) substrates. The films are n type, and Hall mobilities higher than 5×104 cm2 V−1 s−1 have been measured at 23 K for Cd concentration 0.26. Double‐crystal rocking curves of the HgCdTe(333) x‐ray diffraction peak with full width at half maximum as low as 180 arcsec were measured, indicating that the crystalline quality of the HgCdTe is significantly better than that of the CdTe. The Cd concentration of these films is very uniform, with a standard deviation of 2.4% of the average concentration for 5 in. samples and 0.6% for 3 in. samples.

Molecular beam epitaxial growth and characterization of 2‐in.‐diam Hg1−xCdxTe films on GaAs (100) substrates

Hg1−xCdxTe films with 2 in. diameters have been grown by molecular beam epitaxy on GaAs (100) substrates. These films were grown in both the (100) and (∼(111)) B crystallographic orientations and in both conduction types. They were characterized by in situ electron diffraction, infrared absorption, and van der Pauw dc Hall measurements. Their surfaces were shiny and mirrorlike from center to edge. The Cd concentrations (x) of these films were very uniform, exhibiting standard deviations (Δx) as low as 0.7% of the mean (x). Their thicknesses also were uniform within 0.6%. These films were completely uniform in their conduction types; that is, the n‐type films were entirely n type, and likewise for the p‐type films. The Hall mobilities of these films show them to be of high quality, with values as high as 6.7×102 cm2 V−1 s−1 for the p‐type (x=0.22) and 1.8×105 cm2 V−1 s−1 for the n‐type films (x=0.21). These results represent an important achievement toward the future of infrared detector technology.

New achievements in Hg1−xCdxTe grown by molecular‐beam epitaxy

A review of our recent achievements in the growth of Hg1−xCdxTe by molecular‐beam epitaxy is presented here. The influences of the substrate temperature, the crystallographic orientation, and the nature of the substrate on the properties of Hg1−xCdxTe are discussed in detail. We show that to grow high‐quality material with good uniformity in terms of the alloy composition and the doping by crystal stoichiometry deviation, the substrate temperature should be between 180 °C and Tmax. We report mobilities as high as 5.0×1205 cm2 V−1 s−1 for n‐type layers and 1.2×103 cm2 V−1 s−1 for p‐type layers, achieved by precisely controlling the growth parameters. We illustrate that the Hg condensation coefficient is influenced by the crystallographic orientation. Our results show that for Hg1−xCdxTe grown on both the (111)B and the (100) faces of CdTe or GaAs the Hall mobilities are very high and comparable. We report our important achievement of the successful growth of 2‐in.‐diam Hg1−xCdxTe films on GaAs(100), with Δ...

Origin of dual epitaxy in the growth of CdTe on (2̄1̄1̄) GaAs

The atomic structure of the (133)CdTe/(211)GaAs interface is analyzed by high resolution transmission electron microscopy in order to elucidate the origin of the dual epitaxy in the growth of CdTe on (211)GaAs by molecular beam epitaxy. The analysis shows that the lattice mismatch at the interface is accommodated by a novel mechanism, which occurs with the combination of the 14.6% lattice mismatch between CdTe and GaAs and one wurtzite type bond sequence on the (211) substrate surface.

Characteristics of p‐n junctions fabricated on Hg1−xCdxTe epilayers grown by molecular beam epitaxy

p‐n junctions have been fabricated, using the ion implantation technique, on a Hg1−xCdxTe epilayer grown by molecular beam epitaxy (MBE) on a CdTe(111)B substrate. These junctions have been made on as‐grown p‐type layers, 12 μm thick. The layer (x=0.34) exhibits at 77 K a hole mobility of 800 cm2 V−1 s−1 and a carrier concentration of 3.6×1015 cm−3. The diode dark currents in the diffusion regime and the spectral response attest to the excellent uniformity in composition of the layer. We have also established that in the diffusion regime the data can be explained by the ideal diode equation with electrical parameters measured on the as‐grown MBE layers. We consider this an important step in the understanding of the relationship between material parameters and device performance.