Near-surface Graded-gap Layers Research Articles

Investigation of the Differential Resistance of MIS Structures Based on n-Hg0.78Cd0.22Te with Near-Surface Graded-Gap Layers

Investigation of the Differential Resistance of MIS Structures Based on n-Hg0.78Cd0.22Te with Near-Surface Graded-Gap Layers

Influence of As+ Ion Implantation on Properties of MBE HgCdTe Near-Surface Layer Characterized by Metal–Insulator–Semiconductor Techniques

The effect of As+ ion implantation on the electrical properties of the near-surface layer of n-HgCdTe films grown by molecular beam epitaxy (MBE) on Si (310) substrates was experimentally studied. A specific feature of MBE n-Hg0.78Cd0.22Te films is the presence of near-surface graded-gap layers with a high CdTe content, formed during epitaxial growth. The properties of as-grown films and films after As+ ion implantation with ion energy of 200 keV and fluence of 1014 cm−2 were studied. Post-implantation activation annealing was not performed. Test metal–insulator–semiconductor (MIS) structures were created based on as-grown and as-implanted samples by plasma-enhanced atomic layer deposition of Al2O3 insulator films. The admittance of the fabricated MIS structures was measured over a wide range of frequencies and temperatures. When determining the parameters of MIS structures, we used techniques that take into account the presence of near-surface graded-gap layers and series resistance of the HgCdTe film bulk, as well as the high density of slow surface states. It was found that, in as-implanted samples, the donor center concentration in the near-surface layer exceeds 1017 cm−3 and increases with distance from the HgCdTe-Al2O3 interface (at least up to 90 nm). After implantation, the conductivity of MBE HgCdTe film bulk increases markedly. It was shown that, for as-implanted samples, the generation rate of minority charge carriers in the MBE HgCdTe surface layer is significantly reduced, which indicates the appearance of a low defect layer with a thickness of at least 90 nm.

Investigation of the influence of graded-gap layer formed by annealing on the electrical properties of the near-surface of LPE HgCdTe using MIS structure

The influence of the near-surface graded-gap layer formed by annealing of HgCdTe grown by liquid phase epitaxy on the capacitance-voltage characteristics of its MIS structure was studied. We found that HgCdTe grown by LPE can form a near-surface graded-gap layer by annealing under specific conditions after CdTe passivation. After the near-surface graded-layer is generated, the performance of the HgCdTe surface layer has been changed significantly, showing as an increase of slow states, a decrease of fixed charge and the generation of single-level trap in the band gap. Furthermore, a Fermi level pinning phenomenon have been observed on the HgCdTe with graded-gap layer, highlighting the huge density of interface traps at the edge of the band. This effect may be attributed to the electric field generated by the graded-gap layer in HgCdTe driving minority carriers away from the interface. As a result, the surface recombination velocity of minority carriers is reduced. At the same time, it inhibits the oxidation reaction on the surface and consequently reduces the accumulation of fixed charges. During the annealing process to form the graded-layer, the defect system on the surface is reorganized, and the defect aggregation produces single-level defects and higher electron traps.

Admittance studies of modification of HgCdTe surface properties with ion implantation and thermal annealing

Metal–insulator–semiconductor (MIS) structures based on HgCdTe were fabricated after various stages of pn junction formation using As+ implantation and activation annealing. The energy of As+ ions was 200 keV with the fluence of 1014 cm−2. Heteroepitaxial HgCdTe films with near-surface graded-gap layers were grown by molecular beam epitaxy (MBE) on silicon substrates. It was shown that the electron concentration in the near-surface semiconductor layer increases after implantation to values of about 1017 cm−3, and after implantation and annealing in the near-surface semiconductor layer, a p+ layer appears with a hole concentration of more than 1.5 × 1018 cm−3. The generation rate of minority charge carriers in the space charge region after implantation is low, which indicates a low defectiveness of the thin near-surface MBE HgCdTe layer. After implantation and annealing, the generation rate increases significantly, which may be due to high defectiveness near the interface between Al2O3 and MBE HgCdTe. Dopant profiles were constructed in the near-surface HgCdTe layers after various technological procedures. It was shown that after implantation in films, the dopant concentration increases with distance from the interface to the depth of 0.1 μm.

Investigation of the Effect of Thermal Annealing on the Electrical Properties of the Near-Surface Layer of MBE n-HgCdTe Using MIS Techniques

Heteroepitaxial n-Hg0.78Cd0.22Te films with near-surface graded-gap layers were grown by molecular beam epitaxy and subjected to two-stage thermal annealing in saturated mercury vapor atmosphere. The first stage of annealing was carried out for 2 h at a temperature of 360°C. The second stage was carried out at a temperature of 220°C for 24 h. Similar annealing was carried out after As+ ion implantation to activate the introduced impurity and anneal radiation defects. Based on as-grown and annealed films, metal–insulator–semiconductor (MIS) structures were formed by depositing an Al2O3 dielectric by plasma atomic layer deposition. The admittance of fabricated MIS structures was studied in a wide range of frequencies and temperatures. It was found that after thermal annealing, the properties of the n-HgCdTe surface layer are noticeably changed, which is manifested as a decrease in the density of slow states in the transition layer between the dielectric and the semiconductor, and an increase in the generation of minority charge carriers in the n-HgCdTe near-surface layer. These changes in the properties of n-HgCdTe after thermal annealing are associated with the modification of the defect system of the near-surface layer of the semiconductor and the transition layer between dielectric and semiconductor. No significant changes were detected in the bulk properties of the epitaxial film after thermal annealing. A decrease in the density of states near the interface between HgCdTe and Al2O3 during thermal annealing makes it possible to facilitate the electrical characterization of MIS structures by suppressing hysteresis phenomena.

Impact of the Graded-Gap Layer on the Admittance of MIS Structures Based on MBE-Grown n-Hg1 – xCd x Te (x = 0.22–0.23) with the Al2O3 Insulator

The impact of the presence of the near-surface graded-gap layers with an increased content of CdTe on the admittance of MIS structures based on MBE-grown n-Hg1–xCdxTe (x = 0.22–0.23) with the Al2O3 insulating coating has been experimentally studied. It has been shown that the structures with a gradedgap layer are characterized by a deeper and wider capacitance dip in the low-frequency capacitance–voltage (CV) characteristic and by higher values of the differential resistance of the space-charge region than the structures without such a layer. It has been found that the main features of the hysteresis of capacitance dependences typical of the graded-gap structures with SiO2/Si3N4 are also characteristic of the MIS structures with the Al2O3 insulator. The factors that cause an increase in the CV characteristic hysteresis upon formation of the graded-gap layer in structures with SiO2/Si3N4 or Al2O3 are still debatable, although it may be assumed that oxygen plays a certain role in formation of this hysteresis.

Interface properties of MIS structures based on hetero-epitaxial graded-gap Hg1-xCdxTe with CdTe interlayer created in situ during MBE growth

Heterostructures based on n-Hg1-xCdxTe (x = 0.23–0.40) with near-surface graded-gap layers were grown by molecular beam epitaxy on Si (013) substrates. At 77 K, the admittance of the In/Al2O3/Hg1-xCdxTe metal-insulator-semiconductor (MIS) structures with grown in situ CdTe intermediate layer and without such a layer was investigated. It has been established that MIS structures of In/Al2O3/Hg1-xCdxTe with an interlayer of in situ grown CdTe are characterized by the electrical strength of the dielectric and the qualitative interface. The hysteresis of the capacitive characteristics is practically absent within a small range of variation in the bias voltage. The density of fast surface states at the minimum does not exceed 2.2 × 1010 eV−1 cm−2. MIS structures of In/Al2O3/Hg1-xCdxTe without an intermediate layer of CdTe have significantly higher densities of fast and slow surface states, as well as lower values of the differential resistance of the space-charge region in the regime of strong inversion.

Temperature Dependences of the Product of the Differential Resistance by the Area in MIS-Structures Based on Cd x Hg1–x Te Grown by Molecularbeam Epitaxy on Alternative Si and GaAs Substrates

In a temperature range of 9–200 K, temperature dependences of the differential resistance of space-charge region in the strong inversion mode are experimentally studied for MIS structures based on CdxHg1–xTe (x = 0.22–0.40) grown by molecular-beam epitaxy. The effect of various parameters of structures: the working layer composition, the type of a substrate, the type of insulator coating, and the presence of a near-surface graded-gap layer on the value of the product of differential resistance by the area is studied. It is shown that the values of the product RSCRA for MIS structures based on n-CdHgTe grown on a Si(013) substrate are smaller than those for structures based on the material grown on a GaAs(013) substrate. The values of the product RSCRA for MIS structures based on p-CdHgTe grown on a Si(013) substrate are comparable with the value of the analogous parameter for MIS structures based on p-CdHgTe grown on a GaAs(013) substrate.

Electron Concentration in the Near-Surface Graded-Gap Layer of MBE n-Hg1–x Cd x Te (x = 0.22–0.40) Determined from the Capacitance Measurements of MIS-Structures

Capacitance-voltage (C–V) characteristics of MIS structures based on the graded-gap n-Hg1–x Cd x Te (x = 0.22–0.40) grown by molecular-beam epitaxy were experimentally studied in the temperature range of 9–77 K. The concentrations of majority charge carriers in the near-surface layer of the semiconductor are determined from the capacitance value at the minimum of the (C–V) characteristic due to the high-frequency behavior of the capacitance characteristics of the structures with graded-gap layers with respect to the recharge time of surface states. The electron concentration in the near-surface layer of the graded-gap n-Hg1–x Cd x Te at x = 0.22–0.23 in the working layer, found from the value of the capacitance at the minimum, considerably exceeds the integral electron concentration determined by the Hall method. With an increase in the composition in the working layer to x = 0.30–0.40, the difference in the values of the electron concentrations decreases substantially for the near-surface layers with close compositions on the surface. The results obtained are explained by the appearance of additional native defects of donor type in the near-surface graded-gap layer, and this effect is most clearly manifested at large composition gradients in the graded-gap layer. The results of processing of experimental C–V characteristics are in qualitative agreement with the results of studying the electron concentration distribution over the film thickness performed by the Hall method.

Impact of the Near-surface Graded-gap Layers on the Admittance of MIS Structures Based on MBE n(p)-Hg1-xCdxTe (x = 0,21...0,23) within the Temperature Range of 9...77 K

Impact of the Near-surface Graded-gap Layers on the Admittance of MIS Structures Based on MBE n(p)-Hg1-xCdxTe (x = 0,21...0,23) within the Temperature Range of 9...77 K

Admittance measurements in the temperature range (8−77) K for characterization of MIS structures based on MBE n-Hg0.78Cd0.22Te with and without graded-gap layers

Admittance of MIS structures based on MBE n-Hg1-xCdxTe (x=0.22–0.23) with Al2O3 as insulator is experimentally investigated for the cases of the presence and absence of near-surface graded-gap layers with high content of CdTe. It is shown that the structures with graded-gap layers are characterized by a significant hysteresis of electrical characteristics, a deep and broad dip in the low-frequency capacitance-voltage characteristic, and high values of the differential resistance of the space charge region in the strong inversion. It is found that already at 77K, the capacitance-voltage characteristics of structures with graded-gap layers have a high-frequency behavior relative to the recharge time of surface states in the frequency range of (1–2000) kHz. At frequencies exceeding 200kHz and a temperature of (9−15) K, the capacitance-voltage characteristics of the structures without graded-gap layers have a high-frequency behavior relative to the recharge time of surface states located near the Fermi energy for an intrinsic semiconductor. Peculiarities of determining the density of surface states and the electron concentration in MIS structures with and without graded-gap layers are studied.

Temperature and Field Dependences of Parameters of the Equivalent Circuit Elements of MIS Structures Based on MBE n-Hg0.775Cd0.225Te in the Strong Inversion Mode

A technique is proposed for the determining the parameters of the equivalent circuit elements in strong inversion mode using the measurement results of the admittance of MIS structures based on n-Hg0.775Cd0.225Te grown by molecular beam epitaxy. It is shown that at 77 K and frequencies above 10 kHz, the capacitancevoltage characteristics of MIS structures based on n-Hg0.775Cd0.225Te with a near-surface graded gap layer have a high-frequency behavior with respect to the recharge time of surface states located near the Fermi level of intrinsic semiconductor. It is established that the electron concentration in the near-surface graded-gap layer exceeds an average concentration found by the Hall method by more than 2 times. The proposed technique was used for determining the temperature dependences of the insulator capacitance, capacitance and differential resistance of the space-charge region, and capacitance of the inversion layer in MIS structures based on n-Hg0.775Cd0.225Te without a graded-gap layer. The temperature and voltage dependences of the parameters of the equivalent circuit elements in strong inversion are calculated. The results of calculation are qualitatively consistent with the results obtained from the measurements of the admittance.

Peculiarities of Determining the Dopant Concentration in the Near-Surface Layer of a Semiconductor by Measuring the Admittance of MIS Structures Based on P-Hg0.78Cd0.22Te Grown by Molecular Beam Epitaxy

Peculiarities of determining the concentration and distribution profile of dopant in the near-surface layer of a semiconductor by measuring the admittance of MIS structures based on p-Hg0.78Cd0.22Te grown by molecular beam epitaxy are studied. A technique is proposed for the determining the concentration of dopant based on the measurement of the admittance of MIS structures in the frequency range of 50 kHz – 1 MHz. It is shown that in this frequency range, the capacitance-voltage characteristics of MIS structures based on p-Hg0.78Cd0.22Te with a near-surface graded-gap layer have a high- frequency behavior with respect to the recharge time of surface states located near the Fermi level of intrinsic semiconductor. The distribution profile of dopant in the nearsurface layer of the semiconductor is calculated. It is shown that in p-Hg0.78Cd0.22Te with a near-surface graded-gap layer, the dopant concentration has the lowest value near the interface with the insulator.

Dopant in Near-Surface Semiconductor Layers of Metal–Insulator–Semiconductor Structures Based on Graded-Gap p-Hg0.78Cd0.22Te Grown by Molecular-Beam Epitaxy

Peculiarities in determining the dopant concentration and dopant distribution profile in the near-surface layer of a semiconductor are investigated by measuring the admittance of metal–insulator–semiconductor structures (MIS structures) based on p-Hg0.78Cd0.22Te grown by molecular beam epitaxy. The dopant concentrations in the near-surface layer of the semiconductor are determined by measuring the admittance of MIS structures in the frequency range of 50 kHz to 1 MHz. It is shown that in this frequency range, the capacitance–voltage characteristics of MIS structures based on p-Hg0.78Cd0.22Te with a near-surface graded gap layer demonstrate a high-frequency behavior with respect to the recharge time of surface states located near the Fermi level for an intrinsic semiconductor. The formation time of the inversion layer is decreased by less than two times, if a near-surface graded-gap layer is created. The dopant distribution profile in the near-surface layer of the semiconductor is found, and it is shown that for structures based on p-Hg0.78Cd0.22Te with a near-surface graded-gap layer, the dopant concentration has a minimum near the interface with the insulator. For MIS structure based on n-Hg0.78Cd0.22Te, the dopant concentration is more uniformly distributed in the near-surface layer of the semiconductor.

Admittance of metal–insulator–semiconductor structures based on graded-gap HgCdTe grown by molecular-beam epitaxy on GaAs substrates

Metal–insulator–semiconductor structures based on n-Hg1−xCdxTe (x=0.19–0.25) were grown by molecular-beam epitaxy on the GaAs (013) substrates. Near-surface graded-gap layers with high CdTe content were formed on both sides of the epitaxial HgCdTe. Admittance of these structures was studied experimentally in a wide temperature range (8–150) K. It is shown that an increase in the composition of the working layer and a decrease in temperature lead to a decrease in the frequency of transition to high-frequency behavior of the capacitance–voltage characteristics. The differential resistance of space charge region in the strong inversion increases with the composition of the working layer and for x=0.22 and 0.25, the differential resistance is limited by the Shockley-Read generation. The values of the differential resistance of space charge region at different frequencies and temperatures were found.

ELECTRO-PHYSICAL AND PHOTOELECTRICAL CHARACTERISTICS OF MIS-STRUCTURES BASED ON HETERO-EPITAXIAL HGCDTE MBE WITH NON-UNIFORM DISTRIBUTION OF COMPOSITION

The electro-physical and photoelectrical properties of MIS structures based on HgCdTe MBE with non-uniform distribution of composition were experimentally investigated. It is shown that near-surface graded-gap layers with elevated composition strongly affect on dependencies of the capacitance and the photo-emf versus the bias voltage and frequency for the MIS structures based on n -Hg1- x Cd x Te ( x = 0.21—0.23). The characteristics of MIS structures based on n -Hg0.7Cd0.3Te with periodically located regions with high composition were investigated and it is shown that these regions are most strongly affect the characteristics of MIS structures when their location near the boundary of the insulator−semiconductor. The electrical properties of MIS structures based on n -Hg1- x Cd x Te ( x = 0.62—0.73) with region with lower composition were experimentally studied.

Differential Resistance of Space Charge Region in MIS Structures Based on Graded-Gap MBE n-Hg1–x Cd x Te (x = 0.23) in a Wide Temperature Range

Total conductance of MIS structures based on heteroepitaxial n-Hg1–xCdxTe (x = 0.23) grown by molecular beam epitaxy has been studied in the temperature range 8–150 K at frequencies of alternating test signal 2 kHz – 2 MHz. It is found that for structures with a near-surface graded-gap layer and an increased content of CdTe, the differential resistance of space charge region increases significantly, as the temperature decreases from 77 to 8 K, and for structures without a graded-gap layer, the changes in the differential resistance of space charge region are nonmonotonic and relatively small. These results can be explained by the fact that at 8–77 K, for structures without graded-gap layers, the differential resistance of the space charge region is limited by tunneling via deep levels, and for structures with graded-gap layers, it is limited by the Shockley-Read generation processes.

Influence of composition of the near-surface graded-gap layer on the admittance of metal-insulator-semiconductor structures based on graded-gap MBE n-Hg1−xCdxTe in wide temperature range

AbstractInfluence of the CdTe content in a near-surface graded-gap layer on the admittance of MIS-structures fabricated on the basis of heteroepitaxial Hg1−xCdxTe (x = 0.22–0.23 and 0.31–0.32) films grown by molecular beam epitaxy was investigated in a wide temperature range. It is shown that a temperature drop from 77 K to 8 K results in a decrease of hysteresis of the capacitance-voltage (C-V) characteristics and a decrease of frequencies which corresponds to a high-frequency behaviour of C-V characteristics of MIS-structures based on n-HgCdTe (x = 0.22–0.23) with and without graded-gap layersand also for MIS-structures based on n-HgCdTe (x = 0.31–0.32). Temperature dependences of the resistance of the epitaxial film bulk and differential resistance of the space-charge region (SCR) in strong inversion mode were studied. The experimental results can be explained by the fact that for MIS-structures based on n-HgCdTe (x = 0.22–0.23) with the graded-gap layers and for MIS-structures based on n-HgCdTe (x = 0.31–0.32), the differential resistance of SCR is limited by Shockley-Read generation at 25–77 K. Differential resistance of SCR for MIS-structures based on n-HgCdTe (x = 0.22–0.23) without the graded-gap layers is limited by tunnelling through deep levels at 8–77 K.

Photoelectrical characteristics of metal–insulator–semiconductor structures based on graded-gap HgCdTe grown by molecular-beam epitaxy

Metal–insulator–semiconductor structures based on HgCdTe are grown by molecular-beam epitaxy. Near-surface graded-gap layers with high CdTe content are formed on both sides of the epitaxial HgCdTe film. Photoelectrical characteristics of these structures are studied experimentally and theoretically. For structures based on n-Hg1−xCdxTe (x=0.21–0.23), the formation of a near-surface graded-gap layer leads to an increase in the differential resistance of the space charge region due to the suppression of tunneling. The temperature dependences of the differential resistance calculated with account to different mechanisms of generation of minority charge carriers are similar to the experimental dependences of the photoelectromotive force.

The photoelectrical properties of MIS structures based on heteroepitaxial n-Hg1–xCdxTe (x = 0.21–0.23)

The dependences of photo-emf of metal-insular-semiconductor structures based on n-Hg1–xCdxTe (x = 0.21–0.23) on bias voltage, frequency, and temperature are experimentally studied. The photoelectrical characteristics of the metal-insulator-semiconductor structures based on epitaxial n-Hg1–xCdxTe (x = 0.21–0.23), including those taking into account the presence of near-surface graded-gap layers having an increased content of CdTe at various temperatures are calculated. The calculated dependences are compared with experiment and it is shown that the differential resistance of the spatial charge region for the metal-insulatorsemiconductor structures based on n-Hg1–xCdxTe (x = 0.21–0.23) without a graded-gap layer is restricted by tunneling through deep levels, whereas the differential resistance of the same structures with graded-gap layers – by the charge-carrier diffusion.