J. W. Han

Properties of HgCdTe films and Hg‐based quantum well structures grown by photoassisted molecular‐beam epitaxy

Illumination of the substrate during growth by molecular‐beam epitaxy (MBE), known as photoassisted molecular‐beam epitaxy, has been investigated for (111)B, (211)B, and (100) oriented growth of HgCdTe on CdZnTe. The results of this investigation indicate that photoassisted MBE of HgCdTe results in significant improvements in structural perfection. HgCdTe epilayers with average double‐crystal x‐ray diffraction full widths at half‐maximum of <30 arc s can be obtained for all three orientations. Dislocation line densities as low as 5×104 cm−2 have been observed. The structurally perfect HgCdTe epilayers also exhibit outstanding electrical properties after n‐type annealing. Hg‐based multilayer structures have also been grown by photoassisted MBE at North Carolina State University. HgCdTe–CdTe superlattices grown by this technique are comparable in structural quality to III–V superlattices. Hg‐based double‐heterojunction structures, suitable for fabrication of injection lasers, have also been grown. These lat...

Stimulated emission at 2.8 μm from Hg‐based quantum well structures grown by photoassisted molecular beam epitaxy

We report on the first observation of stimulated emission from Hg‐based quantum well structures in which the active region is a HgCdTe superlattice. The laser structures were grown on (100) CdZnTe substrates photoassisted molecular beam epitaxy. Cleaved laser cavities were optically pumped using the 1.06 μm output from a cw Nd:YAG laser. Stimulated emission cavity modes were seen at cw laser power densities as low as 3.4 kW/cm2 and at temperatures ≥ 60 K.

p‐type modulation‐doped HgCdTe

At North Carolina State University, we have recently employed photoassisted molecular beam epitaxy to successfully prepare p‐type modulation‐doped HgCdTe. The modulation‐doped HgCdTe samples were grown on lattice‐matched (100) CdZnTe substrates cut from boules grown at Santa Barbara Research Center. In this letter, we report details of the growth experiments and describe the structural, optical, and electrical properties that this new infrared quantum alloy of HgCdTe possesses.

Modulation‐doped HgCdTe

At North Carolina State University, we have recently employed photoassisted molecular‐beam epitaxy (MBE) to successfully prepare p‐ and n‐type modulation‐doped HgCdTe. The modulation‐doped HgCdTe samples were grown on lattice‐matched (100) CdZnTe substrates cut from boules grown at Santa Barbara Research Center. In this paper, we report details of the MBE growth experiments and describe the structural, optical, and electrical properties that this new infrared quantum alloy of HgCdTe possesses.

Light hole interband transitions in HgTe‐HgCdTe superlattices

The light hole to conduction band optical transition has been identified in the room‐temperature absorption spectra of several high quality HgTe‐HgCdTe superlattices, in addition to the familiar heavy hole to conduction band transitions. The observation of the light hole transition, coupled with a more accurate determination of the superlattice layer thicknesses, allows the superlattice band gap and the HgTe‐HgCdTe valence band offset to be determined more precisely than previously possible. A two‐band tight‐binding model was used to calculate the transition energies to compare with the optical data. The valence band offset for the HgTe‐Hg0.15 Cd0.85 Te interface was determined to be 300±25 meV.

Nonlinear optical coefficients of Hg‐based superlattices

The first measurement of the third‐order nonlinear optical susceptibility χ(3) in Hg‐based narrow‐gap semiconductor superlattices is reported. Four‐wave mixing experiments were performed at CO2 laser wavelengths on a series of HgTe/CdTe and Hg1−xZnxTe/CdTe superlattices whose energy band gaps are approximately zero at T=0 K. The nonlinearity is believed to due to modulation of the free‐carrier temperature and density by the optical beams. Theoretical estimates of the third‐order nonlinear susceptibility due to this mechanism are in good agreement with the experimental results.

Nonlinear optical coefficients of narrow‐gap semiconductors

Nondegenerate four‐wave mixing experiments have been performed at CO2 laser wavelengths on HgTe–CdTe superlattices and α‐Sn1−xGex films grown by molecular beam epitaxy. In HgTe–CdTe, the magnitude, temperature dependence, and laser‐intensity dependences of χ(3) are found to vary systematically with superlattice energy gap. A theoretical model for nonlinear optical coefficients in narrow‐gap semiconductors gives excellent agreement with the experimental results when the nonlinearity is assumed to be due to optical modulation of the free carrier temperature. In the first investigation of α‐Sn1−xGex as a nonlinear optical material, we find that the measured χ(3)’s are quite large (up to 8×10−5 esu), particularly in heavily strained films with compositions up to x=0.08. The mechanism responsible for the nonlinearity in that system is unclear at present.

Free‐carrier transport in HgTe/CdTe superlattices

Magnetotransport and phototransport measurements on molecular‐beam epitaxially grown HgTe/CdTe superlattices are discussed. A detailed analysis of the magnetic‐field‐dependent Hall data yields that mixed conduction by more than one carrier species is significant at all temperatures.The T dependence of the intrinsic carrier density has been used to characterize the band gap and the product of the electron and hole density‐of‐states effective masses. Hole mobilities >105 cm2/V s are reported, as well as the first observation of hole freezeout in a HgTe/CdTe superlattice. Photo‐Hall measurements employing CO2 laser irradiation to generate excess electrons and holes are shown to yield minority electron mobilities and free‐carrier lifetimes.

Properties of HgTe–Hg0.15Cd0.85Te superlattices grown by molecular‐beam epitaxy

The structural and optical properties of a series of molecular‐beam epitaxy (MBE) grown HgTe–Hg0.15Cd0.85Te superlattices (SL) have been studied. Vertical cross‐section transmission electron microscopy studies show no evidence of Hg interdiffusion, confirming the high quality of these multilayered structures. Transmittance and reflectance measurements were performed on each sample and analyzed to obtain optical absorption coefficients. The absorption coefficient versus photon energy spectra show consecutive rises and plateaus characteristic of two‐dimensional quantum structures. A two‐band tight‐binding model was used to analyze the absorption data and identify the quantum transitions. The observation of both light‐hole and heavy‐hole transitions allowed the determination of both the superlattice band gap and the valence‐band offset. The offset was found to be 300 meV (±20 meV) for the superlattices studied, which extrapolates to 350 meV (±20 meV) for the HgTe–CdTe binary interface.

Modulation‐doped HgCdTe quantum well structures and superlattices

Photoassisted molecular‐beam epitaxy (MBE) has been employed to successfully prepare p‐type and n‐type modulation HgCdTe. In this paper, we report details of the MBE growth experiments and describe the structural, optical, and electrical properties that these new quantum well structures and superlattices of HgCdTe possess.