R. Haakenaasen

Growth of HgTe nanowires

HgTe nanowires nucleated by Au particles have been grown on Si and GaAs substrates by molecular beam epitaxy. The wires are polycrystalline. They evolve from crooked to straight during growth and have rounded to rectangular cross-sections. The widths are in the range 20–500 nm, with lengths up to 4 μm. The height of the nanowires is typically less than the width. The nanowires have been characterized by scanning electron microscopy, x-ray photoelectron spectroscopy, transmission electron microscopy and atomic force microscopy. The effects of substrate material, substrate preparation and growth conditions have been investigated.

Imaging photovoltaic infrared CdHgTe detectors

CdxHg1−xTe layers with bandgap in the mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) regions were grown by molecular beam epitaxy, and one-dimensional (1D) and two-dimensional (2D) arrays of planar photodiodes were fabricated by ion milling of vacancy-doped layers. The grown layers have varying densities of needle-shaped structures on the surface. The needles are not associated with twins or dislocations in the layers, but could instead be due to (111) facets being reinforced by a preferential Te diffusion direction over steps on the surface. The needles do not seem to affect diode quality. 64 element 1D arrays of 26×26 μm2 or 26×56 μm2 diodes were processed, and zero-bias resistance-times-area values (R0A) at 77 K of 4×106 Ω cm2 at cutoff wavelength λCO=4.5 μm were measured, as well as high quantum efficiencies. To avoid creating a leakage current during ball-bonding to the 1D array diodes, a ZnS layer was deposited on top of the CdTe passivation layer, as well as extra electroplated Au on the bonding pads. The median measured noise equivalent temperature difference (NETD) on a LWIR array was 14 mK for the 42 operable diodes. 2D arrays showed reasonably good uniformity of R0A and zero-bias current (I0) values. The first 64×64 element 2D array of 16×16 μm2 MWIR diodes has been hybridized to read-out electronics and gave median NETD of 60 mK. Images from both a 1D and a 2D array are shown.

Enhancement in Light Emission From Hg–Cd–Te Due to Surface Patterning

Enhancement of light emission from HgCdTe due to surface patterning has been studied by means of photoluminescence (PL) spectroscopy. A triangular pattern of circular holes was etched into the CdTe layer grown on top of HgCdTe thin-film and multiple quantum well samples. Two different pattern lattice constants were used, giving lattice constant aG to emission wavelength ratios of 0.9, 1.2, and 2.1. The surface pattern was found to give 26%-35% enhancement in measured PL intensity.

Excitation density dependence of the photoluminescence from CdxHg1-xTe multiple quantum wells

A study of the photoluminescence from a four-period CdxHg1-xTe multiple quantum well structure at 11 K as a function of excitation density is presented. High-resolution X-ray diffraction and transmission electron microscopy revealed that the quantum well structure is of high quality. This was supported by the narrow photoluminescence peak originating in the ground state electron - heavy hole transition, with a full width at half maximum of only 7.4 meV for an excitation density of 1.3 W/cm2. When the excitation density was increased from 1.3 to 23.4 W/cm2, the peak position was shifted toward higher energy by 2.6 meV and the full width at half maximum increased from 7.4 to 10.9 meV.

Segmented nanowires of HgTe and Te grown by molecular beam epitaxy

Heterostructured nanowires consisting of alternating segments of HgTe and Te have been grown by molecular beam epitaxy. The cubic ⟨111⟩HgTe and the hexagonal ⟨001⟩Te directions are oriented along the wire. The 15–70nm wide, 0.5–1.5μm long wires are nucleated at Au particles and grow laterally on Si substrates, but they are not epitaxially coupled to the substrates. An excess of Te relative to Hg during growth could explain the segmentation, as the bulk phase diagram then allows only HgTe and elemental Te. Alternating between these two phases is facilitated by the epitaxial match between the HgTe and Te segments.

Electrical characterization of HgTe nanowires using conductive atomic force microscopy

Self-organized HgTe nanowires grown by molecular beam epitaxy (MBE) have been characterized using conductive atomic force microscopy. As HgTe will degrade or evaporate at normal baking temperatures for electron beam lithography (EBL) resists, an alternative method was developed. Using low temperature optical lithography processes, large Au contacts were deposited on a sample covered with randomly oriented, lateral HgTe nanowires. Nanowires partly covered by the large electrodes were identified with a scanning electron microscope and then localized in the atomic force microscope (AFM). The conductive tip of the AFM was then used as a movable electrode to measure current-voltage curves at several locations on HgTe nanowires. The measurements revealed that polycrystalline nanowires had diffusive electron transport, with resistivities two orders of magnitude larger than that of an MBE-grown HgTe film. The difference can be explained by scattering at the rough surface walls and at the grain boundaries in the w...

Temperature dependence of the spectral response of lateral, MBE-grown, ion-milled, planar, Hg1?xCdxTe photodiodes

Measurements are presented of the spectral response of MBE (molecular beam epitaxy)-grown, ion-milled, lateral photodiodes fabricated in the Hg1−xCdxTe material system with x≈0.31. The optical collection length is extracted from measurements taken from diodes of variable diameter. From the temperature dependence of the optical collection length, it is suggested that radiative recombination and Auger recombination processes determine the lifetime of the minority charge carriers.

Photoluminescence from CdxHg1-xTe

We present important aspects of photoluminescence (PL) of CdxHg1-xTe in the infrared part of the spectrum where background thermal radiation significantly affects the PL spectrum. We show how the background spectrum can be removed from the data. We also show how the wavelength of the excitation laser affects the relative intensity of the PL peaks from a multi-layer structure. Finally, we present temperature dependent PL of a Cd0.36Hg0.64Te/Cd0.61Hg0.39Te multiple quantum well structure grown on a 4 μm thick Cd0.36Hg0.64Te buffer layer. We attribute the low temperature peak from the buffer layer to impurities. The impurity levels are depopulated as the temperature increases, resulting in a decreased PL peak intensity. Above ~200 K a band-to-band peak from the buffer layer is observed. The quantum well peak persists up to ~200 K.

A specimen preparation technique for plane-view studies of surfaces using transmission electron microscopy

A method for preparing plane-view transmission electron microscope (TEM) samples is presented. With this inclined pseudo-plane-view technique, the undisturbed surface of the sample can be studied in plane view. Thus, nanostructures on the surface of a substrate can be studied with TEM in much the same way as with scanning electron microscopy (SEM), but in transmission at a much higher spatial resolution and with the opportunity of performing nanoscale diffraction. A glued sandwich with two surfaces facing each other was thinned at a low angle relative to the surfaces. The resultant construction contained thin wedges of the surfaces upon which it was possible to do TEM analysis. SEM analysis before and TEM analysis after such sample preparation was found to be consistent.

The effect of growth interruptions at the interfaces in epitaxially grown GaInAsSb/AlGaAsSb multiple-quantum-wells studied with high-resolution x-ray diffraction and photoluminescence

Molecular beam epitaxy has been used to grow GaInAsSb/AlGaAsSb multiple-quantum-well (MQW) structures. Growth has been interrupted at the interfaces between the wells and the barriers. During the growth interruptions, the interfaces have been exposed to Sbx(x=1, 2) and As2 fluxes. The structures have been studied using high-resolution x-ray diffraction (HRXRD) and photoluminescence (PL). The As content in the interface layers has been found to have a large impact on the HRXRD curves. The As content in the interface layers has been determined by simulation of HRXRD rocking curves. We also show how highly strained interfaces cause more satellite peaks to appear in HRXRD rocking curves. PL spectra show that interrupting growth at the interfaces between wells and barriers and exposing the interfaces to an Sb soak result in flatter interfaces.