S. E. Schacham

Intersublevel transitions in InAs/GaAs quantum dots infrared photodetectors

Thermal generation rate in quantum dots (QD) can be significantly smaller than in quantum wells, rendering a much improved signal to noise ratio. QDs infrared photodetectors were implemented, composed of ten layers of self-assembled InAs dots grown on GaAs substrate. Low temperature spectral response shows two peaks at low bias, and three at a high one, polarized differently. The electronic level structure is determined, based on polarization, bias, and temperature dependence of the transitions. Although absorbance was not observed, a photoconductive signal was recorded. This may be attributed to a large photoconductive gain due to a relatively long lifetime, which indicates, in turn, a reduced generation rate.Thermal generation rate in quantum dots (QD) can be significantly smaller than in quantum wells, rendering a much improved signal to noise ratio. QDs infrared photodetectors were implemented, composed of ten layers of self-assembled InAs dots grown on GaAs substrate. Low temperature spectral response shows two peaks at low bias, and three at a high one, polarized differently. The electronic level structure is determined, based on polarization, bias, and temperature dependence of the transitions. Although absorbance was not observed, a photoconductive signal was recorded. This may be attributed to a large photoconductive gain due to a relatively long lifetime, which indicates, in turn, a reduced generation rate.

The exponential optical absorption band tail of Hg1−xCdxTe

The optical absorption edges of Hg0.71Cd0.29Te and CdTe were measured over the temperature range 80≤T≤300 K. The refractive indices of these materials are determined over a large energy range at T=300 K and their dispersion relation is given. The absorption band tail of Hg1−xCdxTe has an exponential shape. Its slope increases with decreasing temperature. It is suggested that this tail is not caused by permanent lattice disorder but is a material property. An empirical expression for the absorption coefficient as a function of temperature and composition is derived. Using this expression, the zero‐intercept cut‐on energy of the infrared transmission spectrum at room temperature is used to define the composition for any desired thickness of Hg1−xCdxTe sample.

Recombination mechanisms in p‐type HgCdTe: Freezeout and background flux effects

The recombination mechanisms in HgCdTe are analyzed. Detailed expressions for the radiative lifetime are presented, taking into account recent measurements of the absorption coefficient. In p‐type material, carrier freezeout is shown to increase the lifetime exponentially for the radiative and the Auger processes at low temperatures. The effect of background flux is introduced, taking into account its variation with temperature due to the change in energy gap. Lifetime measurements on p‐type samples are in good agreement with combined Auger 7 and radiative mechanisms, where the Auger process is more effective for low x values. Highly compensated materials are dominated by the Shockley–Read recombination. For all samples, the intrinsic region is controlled entirely by the Auger process.

Near infrared quantum cascade detector in GaN∕AlGaN∕AlN heterostructures

A quantum cascade detector in the GaN/AlGaN/AlN material system was implemented. The design takes advantage of the large internal field existing in the nitrides in order to generate the essential saw tooth energy level structure. The device operates in the near IR spectral range with a room temperature responsivity at λ=1.7μm of 10mA∕W (1000V∕W) at zero bias. The spectroscopic measurements are in good agreement with simulations.

GaN/AlGaN intersubband optoelectronic devices

This paper reviews recent progress toward intersubband (ISB) devices based on III-nitride quantum wells (QWs). First, we discuss the specific features of ISB active region design using GaN/AlGaN materials, and show that the ISB wavelength can be tailored in a wide spectral range from near- to long infrared wavelengths by engineering the internal electric field and layer thicknesses. We then describe recent results for electro-optical waveguide modulator devices exhibiting a modulation depth as large as 14 dB at telecommunication wavelengths. Finally, we address a new concept of III-nitride QW detectors based on the quantum cascade scheme, and show that these photodetectors offer the prospect of high-speed devices at telecommunication wavelengths.

High-speed operation of GaN/AlGaN quantum cascade detectors at λ≈1.55 μm

We demonstrate room-temperature, high-speed operation of GaN/AlGaN quantum cascade detectors. The devices are processed as square mesas with 50 Ω coplanar access lines. Frequency response measurements were performed under illumination by a modulated laser diode emitting at λ=1.55 μm. The electrical response exhibits a first-order filter frequency response. For 17×17 μm2 (25×25 μm2) detectors the −3 dB cutoff frequency is 11.4 GHz (6.5 GHz). S-parameter analysis confirms that the cutoff frequency is extrinsically limited and that the speed of the device can be further increased by reducing the device size.

Light‐modulated Hall effect for extending characterization of semiconductor materials

A new technique for deriving the electron mobility as minority carriers in semiconductor materials is introduced. The technique is based on the modulation of the Hall voltage through the introduction of a low‐level optical injection. The photogenerated excess carriers produce a signal which is a function of the magnetic field, the carrier mobilities and concentrations, and the concentration of excess carriers and thus to their lifetime. The measurement is easy to implement and it can be performed along with the measurement of the Hall coefficient and the conductivity. The only modification needed in a conventional galvanomagnetic setup is the introduction of a chopped low‐intensity laser. Using these three simultaneous measurements, we determined the equilibrium carrier concentrations, the two mobilities, and the effective excess carrier lifetime of p‐type HgCdTe narrow gap semiconductor. This novel technique allows us to present the mobility of electrons as minority‐carrier throughout the temperature ran...

Surface recombination velocity of anodic sulfide and ZnS coated p-HgCdTe

The surface recombination velocity s has been determined for Hg1−xCdxTe (x∼0.2) for two different surface passivations: (i) anodic sulfide with an overcoating of ZnS and (ii) ZnS coating on freshly etched samples. The method used was photoelectromagnetic effect, in which a magnetic field is applied perpendicular to the diffusion current of optically generated injected carriers. Analysis of the magnetic field dependence of the resulting current can yield s, as well as carrier mobility, lifetime, etc. The temperature dependence of s is very similar for the two passivations at temperatures higher than 50 K, and is increasing with decreasing temperature. At low temperatures s continues to rise for ZnS passivation and stays flat for the native anodic sulfide. Two activation energies are determined: 12.5±1.5 meV at temperatures higher than 60 K, and 2.3±0.2 meV at temperatures lower than 30 K. The high‐temperature activation energy is identical for both passivations. It is concluded that the same surface traps ...

Three-dimensional analytical simulation of self- and cross-responsivities of photovoltaic detector arrays

Decreasing dimensions, along with an increasing number of elements in imaging photodiode arrays, result in the degradation of spatial resolution and sensitivity due to lateral transport. This effect is modeled using a novel 3-D analytical solution of the continuity equation. The model enables the full 3-D analysis of lateral transport as manifested in excess carrier distribution, photocurrent, and self- and cross-responsivities. Three detector structures are investigated: the semi-infinite substrate, the perfectly collecting, and the perfectly reflecting backside. The front and rear illuminations are treated. The calculated results for the 3-D case deviate fundamentally from those predicted by the 1-D model. The 3-D model succeeds in explaining the reduced quantum efficiency of small-area detectors. It also predicts the limited effect diffusion length has on self-responsivity and cut-off wavelength. The calculated spectral responses fit extremely well data measured on InSb and HgCdTe test arrays. As a powerful design tool, the model enables optimizing responsivity and crosstalk by varying element geometry and spacing, optical aperture, lifetime, and spectral range. The model can be applied to any semiconductor photodiode array provided the relevant physical and geometrical parameters are known.

Lifetime and carrier‐concentration profile of B+‐implanted p‐type HgCdTe

The lifetime and carrier‐concentration profile of Hg1−xCdxTe photodiodes (x=0.2–0.3) formed by implantation of B+ ions (100 keV, 1013–1014 cm−2) into a p‐type substrate was investigated. Using graded C‐V measurements, concentration of carriers around the n+p junction was obtained. It was found that the junction is located at a depth of 0.7–1.5 μm, depending on the bulk hole concentration. The technique enables profiling of the same sample after annealing. The annealing process was shown to make the electron concentration shallower and thus to bring the junction closer to the surface. Various techniques were used to measure excess carrier lifetimes. All indicate that the extent of degradation of the excess electron lifetime in the p region is very limited. For all tested samples, an etch of 2 μm was enough to recover the bulk lifetime. These results contradict the general assumption of deep damages in the p‐type substrate. Thus, the contribution of the n+ side to the saturation current and to the R0A produ...