Variband Layers Research Articles

Ultraviolet Sensors Based on ZnxCd1 – xS Solid Solutions

Effective semiconductor ultraviolet sensors on the basis of Zn0.6Cd0.4S and Zn0.7Cd0.3 solid solutions (SSs) are fabricated. The sensors include variband layers and a thin (∼10 nm) stable polycrystalline p-Cu1.8S film as a transparent component of the surface-barrier structure. The n-CdS layers are used as substrates for the epitaxial growing of SSs. The problems of obtaining low-resistive ZnxCd1−xS polycrystalline layers, providing an ohmic contact with them, and matching the lattice parameters in the SS and the substrate material are resolved by applying intermediate variband layers. On the basis of a heterostructure with glass filters, a selective sensor in the UV-A spectral interval is developed, as well as sensors sensitive to the pigmentation interval of solar radiation (the violet-blue section). Energy band diagrams of the multilayer structure are plotted. The results of Auger-spectroscopic researches and the researches of the main electrical and photovoltaic properties of sensors are reported.

Photoresistors with charge-carrier exclusion for the 8–16-μm spectral range, made from n-CdxHg1-xTe heteroepitaxial structures

The functioning of photoresistors made from constant-composition n-CdxHg1−xTe (x=0.187–0.215) heteroepitaxial structures with variband layers obtained by molecular-beam epitaxy is studied in the nonequilibrium minority-carrier exclusion regime in a wide range of background radiation. The bias-voltage dependences of the electron concentration and charge-carrier lifetime in a 50×50  μm photoresistor pixel are determined at liquid-nitrogen temperature. It is established that the electron concentration in the exclusion regime decreases to a value close to the intrinsic charge-carrier concentration. A voltage sensitivity of more than 2.4×107  V/W and a specific detectivity of 5.3×1011  cm Hz1/2/W are obtained at 77 K and a planar viewing angle of 14° in the exclusion regime for photoresistors made from x=0.203 structures. It is shown that it is promising to use the minority-carrier exclusion regime to create high-efficiency fast-response photoresistors in the 16–17-μm spectral range, cooled no lower than liquid-nitrogen temperature.

Low-Temperature Activation of Ion-Implanted Boron and Nitrogen Ions in Cd x Hg1–x Te Heteroepitaxial Layers

Processes of electrical activation of ion-implanted boron and nitrogen atoms in Cd x Hg1–x Te (CMT) heteroepitaxial layers grown by methods of molecular-beam epitaxy (HEL CMT MBE) and liquid-phase epitaxy (LPE CMT) have been investigated; likewise in bulk crystals of CMT with low-temperature annealings under anodic oxide. The possibility has been demonstrated of using anodic oxide as an efficient mask for postimplantation annealings of p-type HEL CMT MBE in the temperature interval Т = 200–250°C without disruption of the composition of the variband layer or alteration of the electrophysical properties of the structure. It has been established that in HEL CMT MBE the efficiency of activation of boron as a slowly diffusing donor impurity is lowered with growth of the dose of the B+ ions and is increased by thermal cycling from Т = 77 K to room temperature. Implanted nitrogen, in contrast to boron, is a rapidly diffusing acceptor impurity in CMT, efficiently compensating both radiation donor centers and activated boron. The degree of electrical activation of nitrogen grows substantially upon thermal cycling. It has been shown that the mobility spectrum is an efficient method for monitoring the process of electrical activation of boron in p-type HEL CMT MBE. Mesa photodiodes based on activated boron in p-type HEL CMT MBE with long-wavelength photosensitivity boundary λc = 11 μm, prepared here for the first time, had a high maximum value of the product of the differential resistance by the area of the photodiode R d A = (6 − 8)⋅102 Ω⋅cm2, product R 0 A = 5 − 6 Ω⋅cm2 (at zero bias), and a diffusion ledge on the inverse branch of the current-voltage (I–V) characteristic out to a bias voltage of 1.3 V.

On the degree of blocking of the surface recombination of photogenerated carriers in semiconductors by the subsurface variband layer

A mathematically correct description of the model of carrier photogeneration by low-intensity radiation in surface-variband (graded-gap) semiconductors with a stepwise profile of the variband field at the boundary with the homogeneous region is proposed. A condition for the blocking of the surface recombination of photogenerated carriers by the variband layer is explicitly formulated.

Frequency characteristics of diodes with intervalley electron transfer that are based on variband In x(z)Ga1 − x(z)As with various cathode contacts

A two-level model of intervalley electron transfer in a variband semiconductor is used to study the operation of a Gunn diode based on variband In x(z)Ga1 − x(z)As with n +-n cathodes and n +-n −-n cathodes for different lengths of the active region and different thicknesses of the variband layer. It is demonstrated that the critical frequency of a GaAs-In0.4Ga0.6As diode (280–290 GHz) is higher than the critical generation frequencies of GaAs, In0.4Ga0.6As, and In0.2Ga0.8As diodes.

Infrared photodetector modules based on variband layers of HgCdTe and on structures with GaAs/AlGaAs quantum wells

A process has been developed for producing linear and two-dimensional array-type photodetector modules (PDMs), including the fabrication of photodetector structures based on heteroepitaxial layers of CdxHg1−xTe and GaAs/AlGaAs grown by molecular-beam epitaxy, fabrication of two-dimensional and linear arrays of silicon multiplexers, and hybrid assembly of PDMs consisting of a photodetector structure and a multiplexer by means of indium microposts. The photoelectric parameters are given for two-dimensional and linear array-type PDMs based on photodiodes for the mid-IR (3-5.5 µm) and far-IR (8-12 µm) regions, operating at temperatures of 78-80 K and 200-220 K. © 2005 Optical Society of America

Specific Features of Determining the Electrophysical Parameters of Variband CMT Structures Grown by Molecular-Beam Epitaxy

The effects of wide- and narrow- variband layers on the charge-carrier concentration and mobility in the MBE-grown epitaxial CMT structures measured by the Hall-effect method are studied by numerical simulation. The dependence of the relative difference between the experimentally obtained effective values of electrophysical parameters and those of an epitaxial coating on the variband-layer characteristics is discussed. An analysis of this dependence shows that the manner in which the broad-band and narrow-band layers affect the resulting parameters is inequivalent. The calculations suggest that the effect of the wide-band layer on the experimental electrophysical parameters can be neglected. In the case of the narrow variband layer, however, the difference between the electophysical parameters strongly depends on the variband-layer parameters and the concentrations of ionized donor and acceptor centers.

On the possibility of suppressing the saturation of photoelectric amplification of weak optical emission in semiconductors by forming near-contact variband layers

It is demonstrated that saturation of the photoelectric amplification coefficient G with increasing voltage V applied to a sample can be suppressed by introducing a variband layer near a current contact to which the minority charge carriers are driven by the electric field. This effect is considered in the case of interband photogeneration and recombination mechanisms operative, for example, in CdHgTe-based materials widely used for detecting weak emission in the wavelength intervals of λ=8–12 and 3–5 μm [1].

Weak variband surface layers as an effective means of protection from the surface recombination of photoexcited carriers in infrared threshold CdHgTe photoresistors

It is demonstrated that variband near-surface layers with even a small composition gradient can provide ultimate photomodulation (corresponding to the absence of the surface recombination) of the conductivity of thin CdHgTe samples used for manufacturing threshold photoresistors (including those of the SPRITE type) for the wavelengths λ =8−12 μm and λ=3−5 μm. This is explained by the formation (even in weak variband layers) of a special profile of the photoexcited carrier distribution that inhibits their diffusion to the surface. As is known, the introduction of the near-surface variband layers with a small composition gradient must not lead to any substantial increase in the number of defects in the sample.

Electrophysical parameters of variband AlGaAs films made in a temperature gradient

Results are presented on the distributions of the carriers and the hole mobility in undoped AlzxGa1−xAs variband films made by zone recrystallization in a temperature gradient on n-GaAs or p-GaAs crystals. It is shown that this method can give a structure of p+-n-p type with the n-p junction in the variband layer and a compensated n region.