Silicon carbide detectors of high-energy particles

The results of studying 4H-SiC p ⁺-n junctions ion-implanted with aluminum as detectors of high-energy particles are reported. The junctions were formed in SiC epitaxial films grown by chemical vapor deposition. The concentration of uncompensated donors was (3–5)×10¹⁵ cm⁻³, and the charge-carrier diffusion length was L _p=2.5 μm. The detectors were irradiated with 4.8–5.5-MeV alpha particles at 20°C. The efficiency of collection of the induced charge was as high as 0.35. The possibilities of operating SiC detectors at elevated temperatures (∼500°C) are analyzed. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 36, No. 6, 2002, pp. 750–753. Original Russian Text Copyright ? 2002 by Violina, Kalinina, Kholujanov, Kossov, Yafaev, Hallén, Konstantinov.     

Oscillations of induced photopleochroism in ZnO/GaAs heterojunctions

Anisotype and isotype ZnO/GaAs heterojunctions were formed by magnetron sputtering of thin n-ZnO:Al films on epitaxial layers of n-and p-GaAs. It is shown that the heterostructures obtained have a high photosensitivity (～5×10³ V/W at 300 K) in a wide spectral range (1.5–3.2 eV), which oscillates due to the radiation interference in thin ZnO films. Under oblique incidence of linearly polarized radiation on a ZnO film, photopleochroism is induced in a heterojunction, whose value oscillates within ～1–55% at θ=85°. The photopleochroism oscillations are also due to the radiation interference in the ZnO film. It is concluded that the heterojunctions obtained are promising candidates for selective photodetectors of linearly polarized radiation.     

Spectrometric properties of SiC detectors based on ion-implanted p +-n junctions

Results of spectrometric studies of nuclear radiation detectors based on p ⁺-n junctions formed in 4H-SiC films are presented for the first time. The junctions were fabricated by ion implantation of aluminum into 26-μm-thick CVD-grown epitaxial 4H-SiC layers with an uncompensated donor concentration of (3–5) × 10¹⁵ cm^?3. The detector characteristics were measured in testing with natural-decay alpha particles with energies of 3.35 and 5.4 MeV. The collection efficiency of charge generated by 3.35 MeV alpha particles was as high as 100% at an energy resolution of ? 2%.     

Irradiation of 4<Emphasis Type="Italic">H</Emphasis>-SiC UV detectors with heavy ions

Ultraviolet (UV) photodetectors based on Schottky barriers to 4H-SiC are formed on lightly doped n-type epitaxial layers grown by the chemical vapor deposition method on commercial substrates. The diode structures are irradiated at 25°C by 167-MeV Xe ions with a mass of 131 amu at a fluence of 6 × 10⁹ cm^?2. Comparative studies of the optical and electrical properties of as-grown and irradiated structures with Schottky barriers are carried out in the temperature range 23–180°C. The specific features of changes in the photosensitivity and electrical characteristics of the detector structures are accounted for by the capture of photogenerated carriers into traps formed due to fluctuations of the conduction-band bottom and valence-band top, with subsequent thermal dissociation.     

Hall mobility in dielectrically isolated single-crystal silicon films defined by electrochemical etching

The majority-carrier Hall mobility has been measured in thin, single-crystal silicon films defined by electrochemical etching. Both n-type and p-type films with dopant concentrations of about 10¹⁵ cm^?3 were studied. The mobilities observed in p-type thin films and in epitaxial control samples were almost identical while the mobilities measured in n-type films were markedly less than those in epitaxial control samples. This apparent anomaly is attributed to the presence of an n-type surface-charge layer with lower carrier mobility near the bottom of the thin films, although it may possibly be related to voids formed in the n-type films. Measurements on very thin samples indicated that an t-type surface layer is left on the top surface of p-type films immediately after electrochemical etching.     

Effect of irradiation with fast neutrons on electrical characteristics of devices based on CVD 4<Emphasis Type="Italic">H</Emphasis>-SiC epitaxial layers

The effect of irradiation with 1-MeV neutrons on electrical properties of Al-based Schottky barriers and p⁺-n-n⁺ diodes doped by ion-implantation with Al was studied; the devices were formed on the basis of high-resistivity, pure 4H-SiC epitaxial layers possessing n-type conductivity and grown by vapor-transport epitaxy. The use of such structures made it possible to study the radiation defects in the epitaxial layer at temperatures as high as 700 K. Rectifying properties of the diode structures were no longer observed after irradiation of the samples with neutrons with a dose of 6×10¹⁴ cm^?2; this effect is caused by high (up to 50 GΩ) resistance of the layer damaged by neutron radiation. However, the diode characteristics of irradiated p⁺-n-n⁺ structures were partially recovered after an annealing at 650 K.     

Ultrashallow p +-n junctions in silicon (100): Electron-beam diagnostics of the surface zone

Ultrashallow p ⁺-n junctions formed in silicon (100) under nonequilibrium impurity diffusion conditions are analyzed by electron-beam diagnostics of the surface zone using a probe of low-to medium-energy electrons. The energy dependence of the radiation conductivity is investigated, along with its distribution over the area of the p ⁺-n junction. This procedure can be used to determine the depth distribution (in the crystal) of the probability of separation of electron-hole pairs by the field of the p-n junction; the experimental results show that this distribution differs according to whether the kick-out mechanism or the dissociative vacancy mechanism of impurity diffusion is predominant as the basis of formation of the ultrashallow p ⁺-n junctions. Also reported here for the first time are the results of investigations of the distribution of secondary point centers formed near the boundary of the ultrashallow diffusion profile, which exert a major influence on the transport of nonequilibrium carriers. The data obtained in the study demonstrate the possibility of improving the efficiency of photodetectors, α-particle detectors, and solar batteries constructed on the basis of ultrashallow p-n junctions. Fiz. Tekh. Poluprovodn. 32, 137–144 (February 1998)     

Degradation mechanism for silicon p+-n junctions under forward bias

Leakage current degradation has been observed during forward bias stressing of silicon integrated p⁺-n junctions. Detailed characterization results of the anomalous leakage behavior are discussed in this paper. From these results an electric field-enhanced impurity diffusion mechanism has been proposed to explain both the strong temperature and forward bias dependencies on leakage current time-to-saturation. An activation energy has been determined for this mechanism (0.48±0.04 eV) and is in good agreement with that previously determined for diffusion of interstitial copper in p-type silicon. Subsequent Secondary Ion Mass Spectrometer elemental analysis has confirmed the presence of copper near the surface of the epitaxial layer containing the p⁺-n device.     

Selective UV photodetectors based on the metal–AlGaN Schottky barrier

Selective metal–AlGaN photodetectors based on the Schottky barrier and operating in UV spectral range have been developed. The selective photodiodes based on Ag–AlGaN Schottky barriers of different composition have been manufactured, which has made it possible to improve the photosensitivity in the UV spectral range and eliminate spurious signals in the long-wavelength part of the UV spectral range. This has made it possible to develop visible-blind photodetectors with the long-wavelength edge of photosensitivity lying at the wavelengths less than 350 nm. The width of the photosensitivity spectrum is within 15–40 nm, depending on the thickness of the Ag layer, which varies from 15 to 150 nm. The proper choice of the composition of the Al _x Ga_1–x N solid solution ensures increase in the photoresponse and reduction of the FWHM spectrum width up to 11 nm by matching peaks of the Ag transmission spectrum and the absorption spectrum of the epitaxial layer. The sensitivity is 0.071 A/W. The combination of effects of wideband window and overthe- barrier transfer has made it possible to create the ultraselective UV photodetectors based on Au–AlGaN structures with a half-width of the photosensitivity spectrum of 5–6 nm for the wave range 350—375 nm and a sensitivity of up to 140 mA/W. Based on a structure with the upper Al _x Ga_1–x N epitaxial layer (with the AlN content x = 0.1 or x = 0.06), selective photodetectors with the maximum photosensitivity at wavelengths of 355 nm and 362 nm have been developed. Application of an additional less wideband GaN layer has made it possible to independently control the short-wavelength and long-wavelength boundaries of the sensitivity range.     

The use of a four-point probe for profiling sub-micron layers

A number of cautionary measures have been found necessary in applying the layer stripping/four-point probe method to profiling thin epitaxial, ion-implanted and diffused layers. To minimize penetration of the layer by the probe, and carrier injection effects, low probe pressure must be used in conjunction with checks on the voltages developed between the current (outer) probes and the potential (inner) probes. Surface depletion in n-layers results in an apparent translation of the measured profile towards the surface, and for ion-implanted layers further results in a systematic distortion of the profile. These comments apply equally to layer stripping/Hall-effect profiling techniques.     

Resistance of proton-irradiated GaAs photodetectors to combined gamma and neutron radiation

Characteristics of proton-irradiated epitaxial i-GaAs structures with an interdigitated system of electrodes are studied before and after exposure to combined γ and neutron radiation. In order to reduce the dark current of photodetectors, the electrodes were metallized on proton-irradiated i-GaAs structures and were then fused for 30 min at 300°C, which led to a partial recovery of the epitaxial-layer conductivity. It is shown on the basis of measurements of spectral dependences of the structures’ photosensitivity that the photosensitivity of the proton-irradiated structures is insensitive to the effect of combined γ and neutron radiation if the energies of optical photons are close to the band-gap energy. It is therefore suggested that these structures can be used in the fabrication of radiation-resistant photodetectors.     

Photoelectric and luminescence properties of GaSb-Based nanoheterostructures with a deep Al(As)Sb/InAsSb/Al(As)Sb quantum well grown by metalorganic vapor-phase epitaxy

The luminescence and photoelectric properties of heterostructures with a deep Al(As)Sb/InAsSb/Al(As)Sb quantum well grown on n-GaSb substrates by metalorganic vapor-phase epitaxy are investigated. Intense superlinear luminescence and increased optical power as a function of the pump current in the photon energy range of 0.6–0.8 eV are observed at temperatures of T = 77 and 300 K. The photoelectric, current-voltage, and capacitance characteristics of these heterostructures are studied in detail. The photosensitivity is examined with photodetectors operating in the photovoltaic mode in the spectral range of 0.9–2.0 μm. The sensitivity maximum at room temperature is observed at a wavelength of 1.55 μm. The quantum efficiency, detectivity, and response time of the photodetectors were estimated. The quantum efficiency and detectivity at the peak of the photosensitivity spectrum are as high as η = 0.6–0.7 and D _λmax ^* = (5–7) × 10¹⁰ cm Hz^1/2 W^?1, respectively. The photodiode response time determined as the rise time of the photoresponse pulse from 0.1 to the level 0.9 is 100–200 ps. The photodiode transmission bandwidth is 2–3 GHz. Photodetectors with a deep Al(As)Sb/InAsSb/Al(As)Sb quantum well grown on n-GaSb substrates are promising foruse in heterodyne detection systems and in information technologies.     

High-voltage (1800 V) planar 4<Emphasis Type="Italic">H</Emphasis>-SiC <Emphasis Type="Italic">p-n</Emphasis> junctions with floating guard rings

Planar 4H-SiC p-n junctions with floating guard rings have been fabricated. The main junction and the rings were formed by room temperature boron implantation followed by high temperature annealing. The breakdown voltage of the p-n junctions is 1800 V, which twice exceeds that of similar junctions without guard rings and reaches 72% of the calculated breakdown voltage of a plane-parallel p-n junction with the same epitaxial layer parameters     

High-temperature nuclear-detector arrays based on 4 <Emphasis Type="Italic">H</Emphasis>-SiC ion-implantation-doped <Emphasis Type="Italic">p</Emphasis><Superscript>+</Superscript>-<Emphasis Type="Italic">n</Emphasis> junctions

Results obtained in a study of spectrometric characteristics of arrays of four detectors based on 4H-SiC ion-implantation-doped p ⁺-n junctions in the temperature range 25–140 °C are reported for the first time. The junctions were fabricated by ion implantation of aluminum into epitaxial 4H-SiC layers of thickness ≤45 μm, grown by chemical vapor deposition with uncompensated donor concentration N _d ? N _a = (4–6) × 10¹⁴ cm^?3. The structural features of the ion-implantation-doped p ⁺-layers were studied by secondary-ion mass spectrometry, transmission electron microscopy, and Rutherford backscattering spectroscopy in the channeling mode. Parameters of the diode arrays were determined by testing in air with natural-decay alpha particles with an energy of 3.76 MeV. The previously obtained data for similar single detectors were experimentally confirmed: the basic characteristics of the detector arrays, the charge collection efficiency and energy resolution, are improved as the working temperature increases.     

Special features of electrical activation of 28Si in single-crystal and epitaxial GaAs subjected to rapid thermal annealing

Concentration profiles of ²⁸Si implanted in single-crystal and epitaxial GaAs were determined by measuring the C-V characteristics after the postimplantation rapid thermal annealings for 12 s at T=825, 870, and 905°C. The temperature dependence of Hall mobility of electrons in the Si-implanted layers subjected to the same annealings was determined by the Van der Pauw method within the range of 70–400 K. As distinct from conventional thermal annealing (for 30 min at 800°C), the rapid thermal annealing brings about a diffusive redistribution of silicon to deeper layers of GaAs for the materials of both types, with the diffusivity of silicon being twice as high in single-crystal GaAs as that in GaAs epitaxial layers. Analysis of temperature dependence of electron mobility in ion-implanted layers following a rapid thermal annealing indicates a significantly lower concentration of the defects limiting the mobility as compared to the case of a conventional thermal annealing for 30 min.     

MBE-grown HgCdTe multi-layer heterojunction structures for high speed low-noise 1.3–1.6 µm avalanche photodetectors

HgCdTe is an attractive material for room-temperature avalanche photodetectors (APDs) operated at 1.3–1.6 μm wavelengths for fiber optical communication applications because of its bandgap tunability and the resonant enhancement of hole impact ionization for CdTe fractions near 0.73. The HgCdTe based separate absorption and multiplication avalanche photodetector is designed and fabricated for backside illumination through a CdZnTe substrate. The multi-layer device structure is comprised of seven layers including 1). n ⁺ contact 2). n ⁻ diffusion buffer 3). n ⁻ absorber 4). n charge sheet 5). n ⁻ avalanche gain 6). p to form junction, and 7).p ⁺ contact. Several wafers were processed into 45 μm × 45 μm and 100 (μm × 100 μm devices. The mean value of avalanche voltage is 63.7 V measured at room temperature. At 1 GHz, the device shows a gain of about 7 for a gain-bandwidth product of 7 GHz. This first demonstration of an all molecular beam epitaxially grown HgCdTe multi-layer heterojunction structure on CdZnTe substrates represents a significant advance toward the goal of producing reliable room temperature HgCdTe high speed, low noise avalanche photodetectors.     

Effect of in situ annealing on highly-mismatched In0.75Ga0.25As on InP grown using molecular beam epitaxy

We demonstrate that the electrical quality of junctions fabricated in lattice-mismatched In_0.75Ga_0.25As on InP grown by molecular beam epitaxy can be improved with the addition of in situ anneals in the buffer layer that separates the substrate from the In_0.75Ga_0.25As device layers. Near infrared photodetectors fabricated using this material had dark current densities of approximately 2.5 mA/cm² at a reverse bias of 1 V, which is more than one order of magnitude smaller than commercially available photodetectors grown using vapor phase epitaxy. Transmission electron microscopy revealed that dislocations due to the lattice mismatch between the substrate and the epitaxial layer are confined primarily to the buffer layer for all samples studied. No significant differences in x-ray diffraction spectra or dislocation distribution were observed on samples with and without in situ annealing. Atomic force microscopy indicated that all samples had a crosshatch pattern, and that the average surface roughness of the sample that contained in situ anneals is a factor of three greater than the sample without in situ anneals.     

<Emphasis Type="Italic">p</Emphasis><Superscript>+</Superscript>-Si-<Emphasis Type="Italic">n</Emphasis>-CdF<Subscript>2</Subscript> heterojunctions

Boron diffusion and the vapor-phase deposition of silicon layers are used to prepare ultrashallow p⁺-n junctions and p⁺-Si-n-CdF₂ heterostructures on an n-CdF₂ crystal surface. Forward portions of the I–V characteristics of the p⁺-n junctions and p⁺-Si-n-CdF₂ heterojunctions reveal the CdF₂ band gap (7.8 eV), as well as allow the identification of the valence-band structure of cadmium fluoride crystals. Under conditions in which forward bias is applied to the p⁺-Si-n-CdF₂ heterojunctions, electroluminescence spectra are measured for the first time in the visible spectral region.     

Effect of neutron irradiation on the structure of silicon p-n junctions of voltage limiters

Changes in capacitance-voltage characteristics of p-n junctions with a linear or close-to-linear uncompensated charge distribution under neutron irradiation are analyzed. It is confirmed that an intrinsic conductivity region is formed near the p-n junction due to such exposure. Empirical formulas are derived which describe the dependence of the sizes of this region and the effective concentration gradient of uncompensated charge on the neutron fluence in a wide range of initial (before neutron irradiation) concentration gradients (from 3 × 10¹⁸ to 2 × 10²⁰ cm^?4) and initial silicon resistivities (from 0.3 to 2 Ω cm).     

Ion-implanted GaAs slow wave monolithic structure

The use of MeV ion-implantation for realization of a GaAs monolithically compatible device is demonstrated. Ion implants up to 6 MeV in energy are used employing Si and S atoms. The fabricated device is an electromagnetic slow wave microstrip-like structure designed for performance into the millimeter wave regime. Phase shift θ and insertion loss L measurements are performed for frequencies 2–18 GHz at room temperature. Comparison of the experimental ion-implanted device results to epitaxial device results indicates comparable electrical performance, with no more than a 30% reduction in θ but with an improvement in loss behavior, namely a L reduction up to 40%. These θ and L differences between the ion-implanted and epitaxial devices are attributed to differences in doping profiles. Theoretical modelling of θ characteristics produces agreement with experimental data to within a few percent.