A.B. Weerasekara

Characterization of InN layers grown by high-pressure chemical vapor deposition

Structural and optical properties of indium nitride (InN) layers grown by high-pressure chemical vapor deposition (HPCVD) on sapphire and GaN epilayers have been studied. HPCVD extends processing parameters beyond those accessible by molecular beam epitaxy and metal organic chemical vapor deposition, enabling the growth of epitaxial InN layers at temperatures as high as 1150K for reactor pressures around 15bars, leading to vastly improved material properties. InN layers grown on GaN(0002) epilayers exhibit single-phase InN(0002) x-ray diffraction peaks with full width at half maximum (FWHM) around 430arcsec. Optical characterization of the InN layers by infrared (IR) reflectance reveals free carrier concentrations in the low to mid-10+19-cm−3 and optical dielectric function e∞=5.8. The optical properties in the visible and near IR spectral ranges were analyzed by transmission spectroscopy, showing an absorption edge around 1.5eV. The shift of the absorption edge correlates with deviations in the InN stoic...

n-Type GaAs/AlGaAs heterostructure detector with a 3.2 THz threshold frequency.

Terahertz detection using the free-carrier absorption requires a small internal work function of the order of a few millielectron volts. A threshold frequency of 3.2 THz (93 microm or approximately 13 meV work function) is demonstrated by using a 1 x 10(18) cm(-3) Si-doped GaAs emitter and an undoped Al(0.04)Ga(0.96)As barrier structure. The peak responsivity of 6.5 A/W, detectivity of 5.5 x 10(8) Jones, and quantum efficiency of 19% were obtained at 7.1 THz under a bias field of 0.7 kV/cm at 6 K, while the detector spectral response range spans from 3.2 to 30 THz.

Lattice vibrations in hexagonal Ga1−xMnxN epitaxial films on c-plane sapphire substrates by infrared reflectance spectra

The lattice vibrations of undoped hexagonal Ga1−xMnxN (x from 0.0% to 1.5%) epitaxial films grown on c-plane sapphire substrates by metalorganic chemical vapor deposition have been investigated using infrared reflectance spectra in the frequency region of 200–2000cm−1 (5–50μm) at room temperature. The experimental reflectance spectra were analyzed using the Lorentz oscillator model for infrared-active phonon observed. The E1(LO) phonon frequency slightly decreases with increasing Mn composition. However, the E1(TO) phonon frequency linearly increases with the Mn composition, which can be well expressed by (558.7+350x)cm−1 and the broadening values are found to be larger than that of the GaN film. It indicates that Mn incorporation decreases the peak values (from the E1 phonon) of the infrared dielectric functions due to the local variation in the lattice constants and to the destruction of the crystal translational symmetry.

The effects of light-heavy hole transitions on the cutoff wavelengths of far infrared detectors

Results are presented on the effects of doping variation on the cutoff wavelength (λc) of homojunction interfacial workfunction internal photoemission far infrared detectors. The behavior at low doping (<1019 cm−3) is well predicted by the free carrier absorption model used previously. However at high doping the observed λc is much shorter than the values predicted by the workfunction obtained from Arrhenius plots. An explanation for the reduced λc in the high doping region is presented using a model for depletion of the heavy hole band due to direct transitions from the heavy hole to light hole band.

Dopant Migration-Induced Interface Dipole Effect in n-Doped GaAs/AlGaAs Terahertz Detectors

A heterojunction interfacial workfunction internal photoemission (HEIWIP) terahertz detector with ~1times1018cm-3 n-type doped GaAs emitters in a multilayer GaAs/Al0.13Ga0.87 As heterostructure is presented. The detection mechanism is based on free carrier absorption with a broad response extending to ~ 5.26 THz (57 mum), corresponding to an effective workfunction of ~ 21.8 meV, which is much smaller than the offset expected for an Al fraction of x = 0.13 at a 1times1018 cm-3 doping. This is attributed to a reduction of the conduction band offset by interface dipole formation between the accumulated negative charges at the interface states and migrated positively charged donors in the barrier. The device has a peak responsivity of 0.32 A/W at ~ 26 mum at 5 K. It is demonstrated that the dopant migration-induced interface dipole effect can be used to extend the zero response threshold frequency (f 0) of n-type HEIWIP detectors.

Optical properties of n-doped Ga1−xMnxN epitaxial layers grown by metal-organic chemical-vapor deposition in mid and far (5−50 μm) IR range

Optical properties of n-doped (Si) hexagonal Ga1−xMnxN films (x=0.015) grown by metal-organic chemical-vapor deposition (MOCVD) on c-plane sapphire substrates have been studied by infrared reflectance spectroscopy. The effect of free carriers on GaMnN optical phonons, namely E1(LO) and E1(TO), is explored. It is found that the frequency of E1(LO) increases with increasing free carrier concentration. The absorption coefficient (α) is calculated for the 200−2000 cm−1 range and the maximum value of α is found to be ∼105 cm−1 at a frequency of 560 cm−1. With increasing free carrier concentration, the FWHM of the absorption peak increased by 35%−40% as compared to an unintentionally doped (<1×1016 cm−3) film.

Single and Multi Emitter Terahertz Detectors Using n-Type GaAs/AlGaAs Heterostructures

Terahertz detection is demonstrated using GaAs/Al_xGa_1-xAs n-type heterojunction interfacial work function internal photoemission (HEIWIP) detectors. A smaller work function (Delta) needed for terahertz detection can be achieved by using n-doped GaAs emitter and undoped Al_xGa_1-xAs barrier. A single emitter and a multi emitter n-type GaAs/Al_xGa_1-xAs HEIWIP detectors were designed, fabricated and characterized. In both designs, 1times10¹⁸ cm^-3 n-type doped GaAs was used as the emitter while Al_xGa_1-xAs with x = 0.04 for the single emitter detector and x=0.13 for the multi emitter detector was used as the barrier. The threshold frequency of 3.2 THz (93 mum ) with peak responsivity of 6.5 A/W at 7.1 THz at 6 K was successfully demonstrated for the single emitter detector while 5 THz (60 mum) threshold frequency and 0.32 A/W peak responsivity was observed for the multi emitter detector at 5 K. In addition, the peak quantum efficiency of ~19% and peak detectivity of ~5.5times10⁸ Jones under a bias field of 0.7 kV/cm at 6 K were obtained for the single emitter detector.

Effect of emitter thickness on the spectral shape of heterojunction interfacial workfunction internal photoemission detectors

Results are presented showing the effect of emitter layer thickness on the shape of the spectral response of heterojunction interfacial workfunction internal photoemission detectors. The results confirm that thicker emitters increase the response at shorter wavelengths. A model is developed to explain the experimentally observed blueshift in the peak wavelength with increased emitter thickness, using a combination of hot-cold carrier scattering and phonon emission processes. The study provides a tool for designing detectors exhibiting different peak responses, as demonstrated by evaluating the design parameters for the 8–14μm spectral range.