A high-sensitivity solid-state image sensor using a thin-film ZnSe-Zn1-xCdxTe heterojunction photosensor

A high-sensitivity solid-state image sensor in which a charge-transfer device is overlaid by a thin-film photoconductor for the photosensor has been developed. The scanning registers are composed of interline transfer-type bucket-brigade registers (BBD) in the imaging area and charge-coupled registers (CCD) in the horizontal scanning circuit. The special feature of this device is that the thin-film photoconductor of ZnSe-Zn1-xCdxTe heterojunction is directly formed not only on the Si-diode area but also on the scanning circuit area of BBD to obtain a high-aperture ratio. This solid-state image sensor has 506^V× 413^Hpicture elements and the imaging area is about 10.4^V× 13.5^Hmm which corresponds to that of a 1-in vidicon. High sensitivity of 0.46 µA/lx (2856 K) and large blooming control capability have been obtained by this structure. The characteristics such as sensitivity, dark current, resolution, and blooming peculiar to the structure of a solid-state imager overlaid by a photoconductor are also discussed.     

Contact noise in n-type Hg1-xCdxTe photoconductors at 77 K

When gold wires are bonded to indium contacts on n-type Hg1-xCdxTe photoconductors with silver paste, the contacts are very noisy when carrying current. In all cases, the noisy contacts inject electrons into the sample when the contact acts as a cathode; in the poorest contacts, the noisy contact also injects holes when the polarity is reversed. The problem can be avoided by soldering the gold wires to the Hg1-xCdxTe samples with In solder or by bonding the gold wire to evaporated In contacts with the help of silver epoxy.     

Effect of trap tunneling on the performance of long-wavelength Hg1-xCdxTe photodiodes

The effect of trap tunneling on the detector performance of long cutoff wavelength (λco) Hg1-xCdxTe photodiodes was investigated with the use of a parametric model. The development of this model follows closely the formulation by Sah for treating the case of excess currents in gold-doped narrow silicon junctions. The trap tunneling limitedR_{0}A's for long-wavelength Hg1-xCdxTe photodiodes with different p- and n-side doping concentrations and at different temperatures were calculated using this model as a function of p-side trap density, trap location, and junction impurity concentration gradient. The calculated results are in agreement with those measured from actual photodiodes. In particular, the somewhat unexpected temperature dependence of the measuredR_{0}Aproduct at low-temperatures can be satisfactorily accounted for. The present tunneling model also adequately explains the observed soft reverse breakdown characteristics for these devices and their behavior as a function of temperature.     

Improved performance of implanted n+-p Hg1-xCdxTe photodiodes using insulated field plates

Using insulated field plates, we have demonstrated that surface leakage had been limiting the RoA of our ion-implanted n+ -on-p (Hg, Cd) Te photodiodes. This leakage is believed to be tunneling current across a pinched-off depletion region at the surface. We have used gated diodes to eliminate leakage and improve RoA to the diffusion limit at 77°K for Hg0.8Cd0.2Te photodiodes.     

The refractive index of the ternary compounds CdxZn1-xTe and HgxZn1-xTe

Theoretical results for the refractive indexnof the ternary compounds CdxZn1-xTe and HgxZn1-xTe below the fundamental absorption edge are calculated as a function of photon energy and the mole fractionx. The theoretical result fornis obtained from a quantum mechanical calculation of the dielectric constant of a compound semiconductors. It is given in terms of basic material parameters only, with no adjustable constants. The reduction of the refractive index at the absorption edge as one increasesxfor CdxZn1-xTe and for HgxZn1-xTe as one decreasesxis noticeable.     

A high-resolution staggered-configuration CCD imager overlaid with an a-Si:H photoconductive layer

A high resolution "two-level" CCD imager was overlaid with an a-SiC: H (intrinsic)/a-Si: H (intrinsic)/a-SiC:H (p-type) photo-conversion layer. This device has 400 (horizontal:H) × 500 (vertical:V) pixels, in which 385(H) × 490(V) pixels are effective. The image area is 8.8 mm (H) × 6.5 mm (V), which corresponds to ⅔-in optical format. The high-resolution feature has been realized by staggered configuration pixel layout and novel interline transfer CCD scanner, which can read out two horizontal rows simultaneously. A horizontal limiting resolution of 500 TV lines, which is twice the conventional horizontal 400-pixel-number CCD, has been obtained without increasing the pixel packing density.     

Zener current contribution to the resistance-area product of 8- to 14-µm Hg1-xCdxTe photodiodes

The effect of Zener current, due to the internal field emission in the space-charge region of an n⁺-p junction, on the zero-bias resistance-area product RoA of narrow-bandgap HgCdTe photodiodes has been investigated theoretically. It is shown that the contribution of this mechanism in these photodiodes could appreciably lower the RoA product in the temperature range of 10 to 100 K.     

A random noise reduction method for an amorphous siliconphotoconversion layer overlaid CCD imager

A novel random noise reduction (RNR) method, which can reduce random noise generated in a storage diode (SD), has been proposed and evaluated with a cell test element. The RNR cell structure features an RNR transistor with a second storage diode, which is inserted between the SD and a vertical CCD (V-CCD). The RNR transistor controls the transfer channel potential and suppresses the random noise generated in the SD. Net first storage diode capacitance with the RNR transistor can be reduced down to (C_f×C_a)/(C_f+m×C_a ), where C_f is the second storage diode capacitance, C _a is the first storage diode capacitance, and m is the channel potential modulation factor. Experimentally, the RNR cell can reduce the random noise in the SD from 42 electrons [r.m.s.] down to 18 electrons [r.m.s.] for the SD capacitance of 5 fF. This makes it possible for the photoconversion layer overlaid CCD imager with the RNR cells to reproduce video images with a high S/N ratio     

Bismuth-doped Pb1-xSnxTe diode lasers with low-threshold currents

Diode lasers have been fabricated from Bi-doped Pb1-xSnxTe with0.24 leq x leq 0.27, which have threshold current densities as low as 1400 A.cm^-2at 77°K and 71 A.cm^-2at 12°K, whereas diodes fabricated from undoped Pb1-xSnxTe> in this composition range did not exhibit laser action for current densities up to 30 000 A.cm^-2at 77°K and had threshold current densities greater than 200 A.cm^-2at 12°K. Bi doping also results in a reduction in the annealing times required to form suitablep-njunctions from several weeks to a few days. These effects can be tentatively related to an increased electron concentration in then-type layer due to the addition of Bi, a donor impurity. A curve ofEgversusxfor0 leq x leq 0.4, which agrees with predictions, is presented.     

Tunneling effect in CdxHg1-xTe photodiodes

The current-voltage I-U characteristics and those of log I-U, dU/dI-U, and d²U/dI²-U of CdxHg1-xTe (x = 0.20-0.27) photodiodes were measured in the temperature range of 4.2-77 K. The data analysis indicates the backward-type behavior and both the elastic and inelastic resonant-tunneling effects. The resonant tunneling itself seems to originate from resonance-energy states (defect states) located within the junction area.     

A 1/2-in. CCD image sensor overlaid with a hydrogenated amorphous silicon

A half-inch size CCD image sensor overlaid with a hydrogen-erated amorphous silicon (α-Si:H) as a photodetector has been developed. The array consists of 506V × 404H picture elements. The glow-discharged α-Si:H film has high quantum efficiency of 0.75-0.8 in the visible wavelength range and low dark current of 0.2 nA/cm²and is formed on the CCD scanner with vertical overflow drain. This CCD image sensor has a sensitivity of 0.014 µA/lx (3200 K)and a S/N ratio of 73 and 68 dB for fixed-pattern noise and random noise, respectively. Smearing signal is suppressed to below 5 percent at incident light intensity of 1000 times saturation exposure. The blooming and highlight lag are completely suppressed by the vertical overflow drain structure.     

A 1/2-in CCD imager with lateral overflow-gate shutter

The design considerations and performance of an interline-transfer charge-coupled-device (IL-CCD) imager with a lateral overflow gate shutter are described. A 489(V)-pixel×670(H)-pixel 1/2-in IL-CCD imager is shown to have a variable shutter function, whose shutter speed is controlled successively from 1/60 to 1/15700 s by the timing of the overflow gate pulse. The device requires a low voltage of only 4 V and a simple overflow gate pulse to realize the shutter function without any undesirable die size enlargement. The key technology of the device is the self-aligned photodiode structure, which realizes a complete charge transfer. Combined with a microlens on the photodiode, the device can achieve high-definition or small-die-size imagers because of its high sensitivity     

Blooming characteristics of a solid-state imager overlaid with a photoconductor

The control mechanisms of blooming phenomena in a solid-state imager overlaid with a photoconductor have been analyzed. The device studied is composed of an interline transfer type of scanner and a thin-film heterojunction ZnSe-Zn1-xCdxTe photoconductor. Two types of operation exist depending on the dc voltage or the pulse voltage applied to the ITO electrode on the photoconductor and it has been proved that pulse mode operation is superior to dc mode to sustain normal sensitivity under blooming suppressed condition. From the analysis of the operation it is made clear that one condition for blooming control in this device is based upon the principle of biasing the photoconductor so that in strong light the readout transistor is held in the cut-off condition except during the readout period. In the case that this condition is met, additional blooming can still occur due to the carriers excited in the Si substrate by the light that passed through the photoconductor and through the gap area between the charge collecting electrodes. By special shielding the blooming control capability can be effective up to 1100 times the saturation exposure compared to 120 times without the special shielding. The remaining blooming signal is clearly shown to be due to photo-excited carriers in the photoconductor during the blanking period.     

Luminescence and electrical properties of MgxZn1-xTe alloys

The luminescence and transport properties of high-quality undoped and phosphorus-doped Zn1-xMgxTe alloys (x ≤ 0.50) have been investigated. At 4.2 and 300 K, the photoluminescence of unintentionally doped crystals is dominated by near-band-edge recombination mechanisms. In phosphorus-doped samples, the luminescence spectra exhibit free-bound transitions involving shallow acceptor centers (phosphorus in tellurium sites). Whenxand/orTincreases, a broad luminescence band also appears at lower energy (1.7-1.9 eV) which decreases the near-band-edge luminescence efficiency. This low-energy band could be due to phosphorus atoms occupying other sites in the lattice (metal sites, for example) and then acting like deep recombination centers. Such a behavior could also explain the electrical properties of phosphorus-doped crystals. The introduction of phosphorus leads to an increase of the free hole concentration p as compared to undoped crystals but all phosphorus atoms do not behave as shallow acceptors; an increasing fraction of these atoms would act like donors in sites other than the tellurium sites as x increases. In undoped materials, p decreases drastically when we add more magnesium and the hole mobility remains approximately constant. We think that this effect is due to compensation by residual donor impurities. On these undoped samples, light-emitting diodes (LED's) have been successfully obtained for the first time. Their quantum efficiency is reasonable if we take into account the low carrier concentration of the material. The emission peak position is 5390 Å for Zn0.9Mg0.1Te instead of 5550 Å for undoped ZnTe.     

Optimal design of Pb1-xSnxTe double heterostructure injection lasers

Results of optimization calculations are presented for broad area double heterostructure PbSnTe lasers. It was found that by adequately choosing the parameters of lasers, the threshold current densities can be substantially lowered. In particular, threshold current densities of 30 A/cm²at 77 K and of 2 kA/cm²at 200 K are predicted for devices which emit a wavelength of 10.6 μm at 77 K.     

Gain-frequency-current relation for Pb1-xSnxTe double heterostructure lasers

The active region gain expression for Pb1-xSnxTe lasers is obtained from thek cdot pmodel of the conduction and valence band extrema. Curves of gain versus frequency with current, temperature, and majority carrier concentration as parameters are calculated using published values of thek cdot pmodel parameters. In addition, threshold current versus temperature and threshold current versus majority carrier concentration curves are given. A simple expression is obtained for the conductivity effective mass for use in the equation for free-carrier absorption appropriate to the highly degenerate majority carrier concentrations typical of Pb1-xSnxTe laser material.     

Carrier lifetime and threshold of Pb1-xSnxTe lasers in a magnetic field

It is well known that a magnetic field modifies not only the photon energy, but also the threshold current required by semiconductor lasers. However, the extent to which a given field modifies this threshold current depends very strongly upon temperature and on the lifetimes of the carriers in the valence and conduction bands. Theoretical calculations that predict values of threshold current as a function of magnetic field in mixed-crystal Pb1-xSnxTe lasers are presented. The carrier lifetimes in these materials have recently been studied in some detail and turn out to be related to the composition of the material and to its preparation history. Assuming that the carrier Lifetimes are largely determined by the acoustical phonon density or by random distribution of short-range scattering centers, the Kubo formalism can be used to express the individual Landau-level densities of states. From these, the quasi-Fermi levels at any pumping rate can be directly calculated. Fork-conserving transitions, the optical density of states is then determined by a convolution integral method recently reported elsewhere, and the threshold current is computed numerically by the method of Lasher and Stern. Using some of the measured values of carrier mobilities given by Calawa et al., for compositions withx = 0.07andx = 0.20, and using the measured values of Butler and Harman for zero magnetic-field threshold to adjust the gain requirement parameter, threshold currents at 77°K are computed for magnetic field strengths up to 250 kG. Curves are obtained that predict a threshold current for the higher mobility material that decreases by a factor of about 3 with increasing magnetic field out to the vicinity of 10 kG, flattens out, and then slowly increases. For the lower mobility material, an overall higher input current is required; however the curve continues to decrease to 25 kG before flattening out and then beginning to rise.     

Output coupling for closely confined Pb1-xSnxTe double-heterostructure lasers

The output coupling of an idealized, symmetric model of a double-heterostructure (DH) laser is analyzed theoretically using parameters suitable to Pb1-xSnxTe. For the TEOmode incident at the laser mirror and for thin optical guiding regions such that only the TEO, TE1, TMO, and TM1modes may propagate, an exact formulation of the coupling problem is obtained including mode coupling at the mirror into the continuum of unguided radiation modes. Using this formulation, the power reflection and transmission coefficients, the fraction of incident power coupled into the radiation modes, the mirror illumination, and the far-field pattern are calculated for typical parameters. Significant mode coupling can occur, limiting the maximum external efficiency of such lasers. This and other potentially undesirable characteristics resulting from close optical confinement, such as large output beam divergence, must be considered in design criteria for DH structures in this alloy system.     

Distributed Bragg-reflector Pb1-xSnxTe/PbSeyTe1-ydiode lasers

Distributed Bragg-reflector (DBR) diode lasers were designed and fabricated from lattice-matched Pb1-xSnxTe/PbSeyTe1-ysingle heterostructures grown by liquid-phase epitaxy. These DBR lasers operated in a single longitudinal mode within a limited range of heat-sink temperatures, 8.5-38 K, with a threshold current density of ∼3 kA/cm²at 20 K. Single longitudinal mode operation was maintained up to more than three times the threshold current. Continuous tuning of the laser output frequency over a range of ∼6 cm^-1, near 775 cm^-1(12.9 μm), was acheived by varying the heat-sink temperature. The average tuning rate was 0.21 cm^-1/K, and it was much smaller than the rate for corresponding Fabry-Perot lasers, which was 2.3 cm^-1/K. The measured effective mode index of the DBR lasers agrees well with the calculated one.