Yan-Feng Lao

Light-hole and heavy-hole transitions for high-temperature long-wavelength infrared detection

Hole transitions from the heavy-hole (hh) to the light-hole (lh) band contributing to the 4–10 μm response range are reported on p-GaAs/AlGaAs detectors. The detectors show a spectral response up to 16.5 μm, operating up to a temperature of 330 K where the lh-hh response is superimposed on the free-carrier response. Two characteristic peaks observed between 5–7 μm are in good agreement with corresponding energy separations of the lh and hh bands and thus originated from lh-hh transitions. Results will be useful for designing multi-spectral detection which could be realized on a single p-GaAs structure.

InAs/GaAs p-type quantum dot infrared photodetector with higher efficiency

An InAs/GaAs quantum dot infrared photodetector (QDIP) based on p-type valence-band intersublevel hole transitions as opposed to conventional electron transitions is reported. Two response bands observed at 1.5–3 and 3–10 μm are due to transitions from the heavy-hole to spin-orbit split-off QD level and from the heavy-hole to heavy-hole level, respectively. Without employing optimized structures (e.g., the dark current blocking layer), the demonstrated QDIP displays promising characteristics, including a specific detectivity of 1.8×109 cm·Hz1/2/W and a quantum efficiency of 17%, which is about 5% higher than that of present n-type QDIPs. This study shows the promise of utilizing hole transitions for developing QDIPs.

Dielectric function model for p-type semiconductor inter-valence band transitions

The contributions of inter-valence band (IVB) transitions to the dielectric function (DF) by free holes among the split-off (so), light-hole (lh) and heavy-hole (hh) bands were investigated. A model was developed to determine the DF of two p-type semiconductors, GaAs and Ge1−y Sny with the Zinc-blend and Diamond crystal structures, respectively. The IVB transitions dominate the spectral range between 0.1–1eV with respect to the spin-orbit splittings between so-hh and lh-hh bands. In conjunction with inter-band transitions, free-carrier and lattice absorption, a complete DF model allows the determination of optical constants with improved accuracy in the spectral range covering both ultraviolet and infrared regions. The model should be applicable to most of the group III-V and IV materials since their valence band structures resemble the ones under investigation.

Study of valence-band intersublevel transitions in InAs/GaAs quantum dots-in-well infrared photodetectors

The n-type quantum dot (QD) and dots-in-well (DWELL) infrared photodetectors, in general, display bias-dependent multiple-band response as a result of optical transitions between different quantum levels. Here, we present a unique characteristic of the p-type hole response, a well-preserved spectral profile, due to the much reduced tunneling probability of holes compared to electrons. This feature remains in a DWELL detector, with the dominant transition contributing to the response occurring between the QD ground state and the quantum-well states. The bias-independent response will benefit applications where single-color detection is desired and also allows achieving optimum performance by optimizing the bias.

Plasma frequency and dielectric function dependence on doping and temperature for p-type indium phosphide epitaxial films

The optical properties of p-type InP epitaxial films with different doping concentrations are investigated by infrared absorption measurements accompanied by reflection and transmission spectra taken from 25 to 300 K. A complete dielectric function (DF) model, including intervalence band (IVB) transitions, free-carrier and lattice absorption, is used to determine the optical constants with improved accuracy in the spectral range from 2 to 35 μm. The IVB transitions by free holes among the split-off, light-hole, and heavy-hole bands are studied using the DF model under the parabolic-band approximation. A good understanding of IVB transitions and the absorption coefficient is useful for designing high operating temperature and high detectivity infrared detectors and other optoelectronic devices. In addition, refractive index values reported here are useful for optoelectronic device designing, such as implementing p-InP waveguides in semiconductor quantum cascade lasers. The temperature dependence of hole effective mass and plasma frequency is also reported.

Band offsets and carrier dynamics of type-II InAs/GaSb superlattice photodetectors studied by internal photoemission spectroscopy

We use internal photoemission spectroscopy to determine the conduction band offset of a type-II InAs/GaSb superlattice (T2SL) pBp photodetector to be 0.004 (±0.004) eV at 78 K, confirming its unipolar operation. It is also found that phonon-assisted hole transport through the B-region disables its two-color detection mode around 140 K. In addition, photoemission yield shows a reduction at about an energy of longitudinal-optical phonon above the threshold, confirming carrier-phonon scattering degradation on the photoresponse. These results may indicate a pathway for optimizing T2SL detectors in addition to current efforts in material growth, processing, substrate preparation, and device passivation.

Temperature-dependent far-infrared response of epitaxial multilayer graphene

The optical response of pristine and FeCl3-intercalated epitaxial graphene has been studied over the temperature range from 11 K to 296 K. The far-infrared (FIR) Drude conductivity of pristine graphene rises with increasing temperature, opposite to the behavior of intercalated graphene. This is a result of intercalation-induced p-type doping compensating the intrinsic n-doping in epitaxial graphene. Temperature-dependent Drude parameters are obtained by fitting the FIR response. This study demonstrates the influence of temperature variation on the optical properties of graphene, which should be a vital factor to be considered for graphene-based device applications.

Design of resonant-cavity-enhanced multi-band photodetectors

A theoretical analysis to improve the quantum efficiency of detectors sensing in multiple spectral bands is presented. The effective coupling between the incoming light and multiple absorbing regions for simultaneously improving the multi-band absorption efficiency is obtained by using resonant-cavity structures. An optimized cavity with only a Au bottom reflector gives rise to an enhancement factor of 11 in absorption compared to the conventional detector without the cavity. Further improvement, by a factor of 26, can be attained with the aid of a dual-band Bragg reflector placed at the top. The resulting multi-band resonant-cavity detector increases the response in three out of four detection bands contributing to the spectral range from visible to long-wave infrared (IR). The optimized detector is capable of serving multiple purposes, such as regular IR detection for atmospheric windows, gas sensing, and for optical communications.

Optical study of HgCdTe infrared photodetectors using internal photoemission spectroscopy

We report a study of internal photoemission spectroscopy (IPE) applied to a n-type Hg1−xCdxTe/Hg1−yCdyTe heterojunction. An exponential line-shape of the absorption tail in HgCdTe is identified by IPE fittings of the near-threshold quantum yield spectra. The reduction of quantum yield (at higher photon energy) below the fitting value is explained as a result of carrier-phonon scatterings. In addition, the obtained bias independence of the IPE threshold indicates a negligible electron barrier at the heterojunction interface.

High temperature terahertz response in a p-type quantum dot-in-well photodetector

Terahertz (THz) response observed in a p-type InAs/In0.15Ga0.85As/GaAs quantum dots-in-a-well (DWELL) photodetector is reported. This detector displays expected mid-infrared response (from ∼3 to ∼10 μm) at temperatures below ∼100 K, while strong THz responses up to ∼4.28 THz is observed at higher temperatures (∼100–130 K). Responsivity and specific detectivity at 9.2 THz (32.6 μm) under applied bias of −0.4 V at 130 K are ∼0.3 mA/W and ∼1.4 × 106 Jones, respectively. Our results demonstrate the potential use of p-type DWELL in developing high operating temperature THz devices.