The pixel response function (PRF) of a pixel within a focal plane is defined as the pixel intensity with respect to the position of a point source within the pixel. One of its main applications is in the field of astrometry, which is a branch of astronomy that deals with positioning data of a celestial body for tracking movement or adjusting the attitude of a spacecraft. Complementary metal oxide semiconductor (CMOS) image sensors generally offer better radiation tolerance to protons and heavy ions than CCDs making them ideal candidates for space applications aboard satellites, but like all image sensors they are limited by their spatial frequency response, better known as the modulation transfer function. Having a well-calibrated PRF allows us to eliminate some of the uncertainty in the spatial response of the system providing better resolution and a more accurate centroid estimation. This paper describes the experimental setup for determining the PRF of a CMOS image sensor and analyzes the effect on the oversampled point spread function (PSF) of an image intensifier, as well as the effects due to the wavelength of light used as a point source. It was found that using electron bombarded active pixel sensor (EBAPS) intensification technology had a significant impact on the PRF of the camera being tested as a result of an increase in the amount of carrier diffusion between collection sites generated by the intensification process. Taking the full width at half maximum (FWHM) of the resulting data, it was found that the intensified version of a CMOS camera exhibited a PSF roughly 16.42% larger than its nonintensified counterpart. (C) 2015 Society of Photo-Optical Instrumentation Engineers (SPIE)

Time-resolved photoluminescence measurements of carrier lifetimes in modulation-doped (100Å)AlxGa1−xN∕(100Å)GaN multiple quantum well heterostructures are reported. The photoluminescence (PL) spectrum exhibits several lines associated with recombination of carriers from multiple excited electron states to the hole ground state. The PL decay times associated with ground state recombination, e0h0, are found to be much longer than the inverse repetition rate of our system (20μs) and estimated to be 9 ms. The experimental lifetimes associated with carrier recombination from excited states vary between 4μs for the first excited state, e1h0, and 4.5 ns for the fourth excited state, e4h0. These lifetimes are in very good agreement with a self-consistent calculation of radiative recombination lifetimes which takes into account piezoelectric and spontaneous polarization. The significant differences in recombination lifetimes are the result of the large built-in electric field in the wells (0.5MV∕cm).

Synchrotron radiation photoemission spectroscopy reveals enhanced oxygen incorporation in AlxGa1−xN as the Al mole fraction increases. It is shown that the increased oxygen donor incorporation can result in a conductivity-type change from p-type to n-type in Mg-doped AlxGa1−xN. Consistent with the conductivity-type change, epitaxial Al0.20Ga0.80N films exhibit n-type conductivity despite heavy Mg doping. The p-type conductivity of bulk AlxGa1−xN with a high Al mole fraction can be improved by employing AlxGa1−xN/AlyGa1−yN superlattices (SLs). At 300 K, Mg-doped Al0.17Ga0.83N/Al0.36Ga0.64N SLs (average Al mole fraction of 23%) exhibit strong p-type conductivity with a specific resistance of 4.6 Ω cm, a hole mobility of 18.8 cm2/Vs, and an acceptor activation energy of 195 meV.

Photoluminescence spectra from p-type modulation-doped Al 0.20Ga0.80N/GaN superlattices with 10-nm well width show multiple, well resolved, interband transitions between quantum-confined states. In addition to the ground-state transition, a number of excited-state transitions are observed. The observation of multiple peaks is attributed to the inverse dependence of subband population and oscillator strength on energy. The relative strength of the peaks strongly changes with excitation intensity. At low excitation intensity, the spectra display only the ground-state transition. At higher excitation intensity, excited-state transitions become dominant. At high excitation intensities, the dominant transition occurs at energies about 500 meV above the electron ground-state to hole ground-state transition. Self-consistent calculations are used to assign transition energies, lifetimes, and rates to each photoluminescence line. Theoretical and experimental transition energies are in excellent agreement. We attribute the excellent optical properties to the modulated doping of the structure, which consists of doped barriers and undoped well layers. Our calculations also show an average recombination lifetime of 50 ns at high excitation intensities, despite the large quantum-confined Stark effect. The changes of the photoluminescence spectra can be explained via the effects of band filling and oscillator strengths at higher excitation intensity.

Perpendicular transport characteristics of n-type AlxGa1−xN/GaN superlattices are presented. Planar and mesa-etched superlattice structures are employed to identify the perpendicular resistance. Perpendicular transport measurements in Al0.22Ga0.78N/GaN superlattices display linear current–voltage characteristics with a resistivity that is a factor of 6.6 higher than for bulk material. A theoretical model is developed for perpendicular transport in AlxGa1−xN/GaN superlattices based on sequential tunneling. The model shows that short superlattice periods are required to minimize the perpendicular resistivity.

A novel technology for low-resistance ohmic contacts to III-V nitrides is presented. The contacts employ polarization-induced electric fields in strained cap layers grown on lattice-mismatched III-V nitride buffer layers. With appropriate choice of the cap layer, the electric field in the cap layer reduces the thickness of the tunnel barrier at the metal contact/semiconductor interface. Design rules for polarization-enhanced contacts are presented giving guidance for composition and thickness of the cap layer for different III-V nitride buffer layers. Experimental results for ohmic contacts with p-type InGaN and GaN cap layers are markedly different from samples without a polarized cap layer thus confirming the effectiveness of polarization-enhanced ohmic contacts.

Low-resistance ohmic contacts are demonstrated using thin p-type InGaN layers on p-type GaN. It is shown that the tunneling barrier width is drastically reduced by polarization-induced electric fields in the strained InGaN capping layers resulting in an increase of the hole tunneling probability through the barrier and a significant decrease of the specific contact resistance. The specific contact resistance of Ni (10 nm)/Au (30 nm) contacts deposited on the InGaN capping layers was determined by the transmission line method. Specific contact resistances of 1.2×10−2 Ω cm2 and 6×10−3 Ω cm2 were obtained for capping layer thicknesses of 20 nm and 2 nm, respectively.

Thin p-doped InGaN layers on p-doped GaN were successfully used to demonstrate a new type of low-resistance ohmic contact. A significant reduction of specific contact resistance can be achieved by increasing the free-hole concentration and the probability for hole tunneling through the Schottky barrier as a consequence of polarization-induced band bending. As obtained from the transmission-line method, the specific contact resistances of Ni (10 nm)/Au (30 nm) contacts deposited on InGaN capping layers were 1.2×10 −2 Ωcm 2 and 6×10 −2 Ωcm 2 for capping layer thicknesses of 20 nm and 2 nm, respectively.

Utilizing a thin n-type AlxGa1-xN layer on top of n-type GaN, the metal-semiconductor contact resistance can be decreased drastically compared to bulk n-type GaN. Specific contact resistances as low as 8.5 x 10(-)5 Omega cm(2) have been achieved without high temperature annealing. The design of this novel contact is based on polarization fields in the thin AlxGa1-xN layer.

P-type AlGaN / GaN superlattice structures have demonstrated higher acceptor activation due to a modulated valence band resulting from the superlattice as well as spontaneous and piezoelectric polarization fields. The polarization effects are due to the wurtzite structure of AlGaN and the strain present in AlxGa1-xN / GaN heterostructures. Variable temperature Hall effect studies of Mg doped Al0.20Ga0.80N / GaN superlattices reveal an improvement in resistivity and mobility for modulation-doped structures versus uniformly doped structures. Very low resistivities less than 0.1 Ω cm and hole mobilities ~ 36 cm2/V s are demonstrated. This improvement is attributed to a reduction of neutral and ionized impurity scattering for the two-dimensional hole gas present in the GaN layers of the modulation-doped superlattice. The improvement is greatest at temperatures below ~ 150 K. The doped regions of the superlattices have Mg concentrations of ~1019 cm−3. Two modulation-doped samples were grown by MBE: a standard scheme with dopants only in the AlGaN barriers, and a shifted scheme with dopants concentrated near the AlGaN / GaN interfaces. The standard sample has mobilities of 8.9 and 36 cm2/V s at 300 and 90 K, respectively. Resistivities of the standard sample are 0.21 and 0.068 Ω cm at 300 and 90 K, respectively. Carrier concentrations for this sample are 3.4 and 2.5 × 1018 cm−3 at 300 and 90 K, respectively. Capacitance-voltage profiling on the samples shows a clear indication of a two-dimensional hole gas as well as the periodicity of the superlattice.

Transport properties of modulation-, shifted-modulation-, and uniformly doped Al0.20Ga0.80N/GaN superlattices are reported. The modulation-doped sample is doped only in the AlGaN barriers. The shifted-modulation-doped sample has its dopants shifted by one-quarter period. Measurements reveal a strong improvement in mobility and resistivity for the modulation-doped and shifted-modulation-doped structures versus the uniformly doped structure. The modulation-doped sample has a mobility of 9.2 and 36 cm2/Vs at 300 and 90 K, respectively, and a very low resistivity of 0.20 and 0.068 Ωcm at 300 and 90 K, respectively. Capacitance–voltage profiling shows multiple two-dimensional hole gases. The results are consistent with a reduction of neutral impurity scattering for modulation-doped structures as compared to uniformly doped structures.

Room temperature and low temperature photoluminescence studies of Al x Ga 1−x N/GaN superlattices reveal a red shift of the dominant transition band relative to the bulk GaN bandgap. The shift is attributed to the quantum-confined Stark effect resulting from polarization fields in the superlattices. A theoretical model for the band-to-band transition energies based on perturbation theory and a variational approach is developed. Comparison of the experimental data with this model yields a polarization field of 4.6 × 10 5 V/cm for room temperature Al 0.1 Ga 0.9 N/GaN and 4.5 × 10 5 V/cm for room temperature Al 0.2 Ga 0.8 N/GaN. At low temperatures the model yields 5.3 × 10 5 V/cm for Al 0.1 Ga 0.9 N/GaN and 6.3 × 10 5 V/cm for Al 0.2 Ga 0.8 N/GaN. The emission bands exhibit a blue shift at high excitation densities indicating screening of internal polarization fields by photo-generated free carriers.