Sang-Hee Suh

Effect of Cd-annealing on the IR transmittance of CdTe wafers grown by the Bridgman method

CdTe crystals were grown by the vertical Bridgman method. CdTe wafers processed from the crystals showed p-type conductivity and low IR transmittance. Isothermal annealing in Cd atmosphere for 5 h at 600° C enhanced the IR transmittance of the wafer up to 65%. The annealed wafer showed n-type conductivity with a carrier concentration of 2.1 × 1015 cm-3. The high IR transmittance obtained is attributed to annihilation of Cd vacancies during annealing in Cd atmosphere. The IR transmittance of CdTe wafers increased with increasing annealing time from 0.5 to 5 h. Annealed thickness could be calculated from the IR transmittance by assuming an abrupt junction between the annealed surface and unannealed core. The annealed thickness followed the usual time dependence of t12.

Structural properties of ZnSe on GaAs grown by atomic layer epitaxy

ZnSe epilayers were grown on GaAs (100) substrates by atomic layer epitaxy modified from chemical‐vapor deposition with thicknesses ranging from 600 to 6000 A. X‐ray‐diffraction and micro‐Raman scattering measurements were carried out to study the effects of strain in the ZnSe epilayers with different thicknesses. The increase in full width at half‐maximum of double‐crystal x‐ray rocking curves was observed for layers thicker than the critical thickness, which indicates that the crystallinity gets strongly degraded when the layers are grown over the critical thickness. The critical thickness estimated by x‐ray rocking curves is 1500 A, while that obtained by micro‐Raman scattering is 1000 A. This difference suggests that the elastic strain depends on the layer depth for ZnSe epilayers around the critical thickness.

Metalorganic vapor phase epitaxial growth of hillock free (100) solHgCdTeGaAs with good electrical properties

The hillock density on metalorganic vapor phase epitaxially grown (100) HgCdTe/GaAs could be reduced to less than 10 cm -2 by rinsing the substrate with KOH in a water solution before growth. The HgCdTe layers which were annealed in Hg at 250°C were n-type with carrier concentrations between mid-10 14 and mid-10 15 cm -3 . SIMS analysis also showed that the remaining potassium was not at a concentration high enough to influence the electrical properties of HgCdTe. This result shows that MOVPE process could be used to fabricate large area focal plane arrays which need hillock free surface morphology together with good electrical properties.

Structural properties of ZnSe layers grown on (001) GaAs substrates tilted toward [110] and [010]

We have investigated the structural properties of ZnSe epilayers that were molecular beam epitaxially grown on (001) GaAs substrates with different tilt angles and tilt directions. We measured the properties of the epilayers by x-ray diffraction, transmission electron microscopy, and etch pit density analysis. Tilting the (001) GaAs substrate toward [010] was very effective in reducing the surface defect density of the ZnSe layers, while tilting toward the [110] direction was of no use. We could observe the increasingly two-dimensional nature of the initial growth mode in the (001) GaAs substrate tilted toward [010]. Growth of a 1.8-μm-thick ZnSe layer on (001) GaAs tilted 4° toward [010] resulted in a very low surface defect density of 1×104 cm−2. Such a low defect density has seldom been obtained in ZnSe, without growing a GaAs buffer layer below the ZnSe layer.

Vertical Bridgman techniques to homogenize zinc composition of CdZn Te substrates

In order to improve the Zn homogeneity along the axial direction of CdZnTe boule, we have employed a modified Bridgman technique using a (Cd, Zn) alloy source in communication with the melt, whose temperature has been gradually changed from 800 to 840°C during growth. Electron probe microanalysis (EPMA) measurements of Zn composition in the boule shows an excellent homogeneity of Zn along the axis of the CdZnTe boule compared with results in a boule grown by using a fixed source temperature. We have performed a numerical simulation to obtain the approximate temperatures of additional heating and cooling needed to improve the radial Zn homogeneity. CdZnTe boule has been grown by seeded vertical Bridgman furnace with two zones of heater and cooler. Ultraviolet/visible spectroscopic measurements of Zn composition over the length of the boule indicate that the radial distribution of Zn composition is very homogeneous in the body region of the boule, where the radial variation of Zn composition is ±0.0005.

Iodine and arsenic doping of (1 0 0)HgCdTe/GaAs grown by metalorganic vapor phase epitaxy using isopropyl iodide and tris-dimethylaminoarsenic

Abstract We report in situ n- and p-type doping of HgCdTe/CdTe/(1 0 0)GaAs grown by metalorganic vapor phase epitaxy (MOVPE), using the interdiffused multilayer process (IMP). HgCdTe was doped either with iodine from isopropyliodide (IPI) for n-type doping or with arsenic from tris-dimethylaminoarsenic (DMAAs) for p-type doping. Standard bubbler configurations were used for dopant precursors. DiPTe, DmCd and elemental Hg were used as sources. HgCdTe layers of 10 μm were grown on (1 0 0)GaAs at 370°C. Dopant gases were allowed into the reactor only during the CdTe growth cycle of the IMP process. HgCdTe layers were Hg-annealed at 415°C for 15 min and at 220°C for 3 h, consecutively. Iodine-doped HgCdTe layers had 77 K electron concentrations of 6 × 10 15 to 5 × 10 17 cm −3 . Hall mobility decreased with increasing doping concentration. Arsenic-doped HgCdTe layers had 77 K hole concentration of 2 × 10 16 to 7 × 10 17 cm −3 with about 100% of arsenic activated. Composition x of Hg 1− x Cd x Te layers was not influenced by the arsenic doping. The above results show that IPI and DMAAs precursors are excellent n- and p-doping sources for HgCdTe.

Defect reduction in ZnMgSSe epilayers on GaAs by using ZnMgSe/ZnSSe strained-layer superlattices

Effectiveness of ZnMgSe/ZnSSe strained layer superlattices(SLSs) in reducing threading dislocations of ZnMgSSe has been studied. Transmission electron microscopy has been used to investigate the dislocation structure. The strained layer superlattice buffer was effective in blocking threading dislocations. The surface dislocation in ZnMgSSe could be reduced by using the ZnMgSe/ZnSSe SLSs. The dislocation density would be reduced further by adjusting the stress level of SLS layer or by designing it properly.

Slider liquid phase epitaxial growth of Hg0.8Cd0.2Te, Hg0.7Cd0.3Te and Hg0.3Cd0.7 Te with precise control of alloy compositions

A slider liquid phase epitaxial (LPE) technique, by which the solution composition can be kept constant during growth, has been developed to grow Hg1− xCd ϰ Te epi-layers with ϰ = 0.2, 0.3 and 0.7. A graphite button between the solution and HgTe wells in the slider boat is shown to be very effective in controlling Hg loss from the solution. Data are presented on compositional uniformity and reproducibility, surface morphology, and electrical properties.

Characteristics of SWIR Diodes of HgCdTe/CdTe/GaAs Grown by Metal Organic Vapor Phase Epitaxy

Short wave infrared (SWIR) photovoltaic devices have been fabricated from metal organic vapour phase epitaxially (MOVPE) grown n on p HgCdTe films on GaAs substrates. The MOVPE grown films were processed into mesa type discrete devices with wet chemical etching employed for meas delineation and CdZnTe surface passivation. The photodiode forward and reverse current-voltage characteristics, as well as the temperature dependence of the zero-bias dynamic resistance, were measured in the temperature range of 100-300 K. The devices are generation-recombination limited up to 250 K. The zero bias dynamic resistance area products at 200 and 300 K were low, 1 × 10 6 and 2.5 x 10 2 Ω cm 2 , respectively. The relative spectral response cut off wavelength of device at 300 K was 2.5 μm.

Electrical properties of MIS devices on CdZnTe/HgCdTe

In this paper, we report the capacitance-voltage (C-V) properties of metal-insulator-semiconductor (MIS) devices on CdTe/HgCdTe by the metalorganic chemical vapor deposition (MOCVD) and CdZnTe/HgCdTe by thermal evaporation. In MOCVD, CdTe layers are directly grown on HgCdTe using the metal organic sources of DMCd and DiPTe. HgCdTe layers are converted to n-type and the carrier concentration, ND is low 1015 cm-3 after Hg-vacancy annealing at 260 degrees Celsius. In thermal evaporation, CdZnTe passivation layers were deposited on HgCdTe surfaces after the surfaces were etched with 0.5 - 2.0% bromine in methanol solution. To investigate the electrical properties of the MIS devices, the C-V measurement is conducted at 80 K and 1 MHz. C-V curve of MIS devices on CdTe/HgCdTe by MOCVD has shown nearly flat band condition and large hysteresis, which is inferred to result from many defects in CdTe layer induced during Hg-vacancy annealing process. A negative flat band voltage (VFB approximately equals -2 V) and a small hysteresis have been observed for MIS devices on CdZnTe/HgCdTe by thermal evaporation. It is inferred that the negative flat band voltage results from residual Te4+ on the surface after etching with bromine in methanol solution.