Realization of very long wavelength infrared photovoltaic detector arrays on mercury cadmium telluride epitaxial layers grown on Si substrates

This paper proposes a development of n-on-p structures for realizing very long wavelength infrared (VLWIR) detector arrays on mercury cadmium telluride (HgCdTe) epitaxial layers grown on Si substrates. It is shown from a comparative study of zero-bias resistance-area product (R₀A) of diodes in n-on-p and p-on-n configurations that the n-on-p structure has promising potential to control contribution of dislocations, without actually reducing dislocation density below the current level (mid-10⁶ cm⁻²) of HgCdTe/Si material technology. The resulting gain will be in terms of both higher numerical magnitudes of R₀A and its reduced scatter.     

Dislocations in vapor phase epitaxial GaP

Dislocations in VPE GaP grown on (100) oriented LEC GaP substrates have been characterized, and their origins and effects on LED performance have been investigated. In non-nitrogen doped epilayers, the dislocations are found to originate in the substrate and propagate through the epilayers in straight lines in [100] and <211> directions. The dislocation density of the epilayer is found to be nearly equal to that of the substrate. Introduction of nitrogen during growth of the epilayer has been observed to bend these so-called “inclined≓ dislocations propagating through the layer into [0−1 1] directions in the (100) plane and thus produces segments of [0 −1 1] dislocations to relieve the lattice parameter mismatch due to N. The mismatch dislocation density is observed to be proportional to the N doping level. At very high N doping levels, > 10¹⁹ cm^-3, a large number of new inclined dislocations are observed, which may be in part due to GaN precipitation. The effects of dislocations on LED properties were investigated by measuring dislocation densities in the individual diodes using the electron beam induced current mode of the SEM and comparing this with the spot brightness and luminous flux. The dislocations were observed to produce dark spots in the EL emission in many cases. For a series of runs where all growth and processing parameters were fixed, a good correlation between B/J and dislocation density was observed with B/J decreasing with increasing dislocation density in the range < 1 × 10⁴ cm⁻² to 1 × 10⁶ cm⁻².     

Large improvement in HgCdTe photovoltaic detector performances at LETI

The standard infrared photovoltaic technology developed for HgCdTe by LETI and industrialized by SOFRADIR is based on the very simple approach of planar ion-implanted n-on-p homojunctions. Both the growth by liquid-phase epitaxy of excellent-quality epitaxial layers and the simplicity of the planar ion-implanted process enables state-of-the-art detectors to be achieved with a high technological yield. These detectors present high shunt impedance, good quantum efficiency, and a low l/f noise level. The diodes are diffusion-limited down to temperatures much lower than 77K. Their saturation current is limited by the minority-carrier lifetime in the p-side material. R₀A values around 30 ohm-cm² are routinely obtained for 10.0 μm cutoff wavelength detectors at 77K. In this paper, we show that with a new process we can increase the diode R₀A by more than one order of magnitude. This effect is obtained as a result of an increase of minority-carrier lifetime in the n-on-p homojunction configuration. The maximum R₀A value obtained was 655 ohm-cm² on a 10.0 μm cutoff wavelength detector at 77K. Furthermore, other figures of merit much as quantum efficiency or shunt impedance are slightly improved, and l/f noise is not affected. The data presented in the 40–200K temperature range and 9–13 μm cutoff wavelength range show that this decrease of dark current is kept throughout these temperature wavelength ranges. Therefore, we show that a simple planar ion-implanted homojunction can lead to very large R₀A, close to theoretical limits and comparable to data published for p-on-n heterojunctions.     

Impact of critical processes on HgCdTe diode performance and yield

HgCdTe detector performance and yield are strongly dependant on CdZnTe substrate and HgCdTe epilayer properties, and on key device processes, especially for 8–12 μm application. Due to the correlation and optimization between these figures and diode performance, AIM has developed a mature HgCdTe technology for superior detector performance and high production rate. To meet high yield and performance for long wavelength (LW) HgCdTe diodes, dislocation densities of < 1 × 10^t cm⁻² both in substrate and epilayer have to be ensured. By a unique AIM substrate growth process, dislocation densities of 2 × 10⁴-9 × 10⁴ cm⁻² are achieved for all substrates and epilayers (100% yield). The etch pit density (EPD) on 〈111〉 epilayers is revealed by an AIM proprietary etching procedure. One critical effect is the dislocations in the diode area, which can originate from the substrate and epilayer growth and the subsequent device processes, respectively. Our studies have shown that device processes can cause additional dislocations in the diode area. Diode yield was clearly improved by a combination of wet and dry etching for diode contact etching.     

Effect of dislocations on performance of LWIR HgCdTe photodiodes

The epitaxial growth of HgCdTe on alternative substrates has emerged as an enabling technology for the fabrication of large-area infrared (IR) focal plane arrays (FPAs). One key technical issue is high dislocation densities in HgCdTe epilayers grown on alternative substrates. This is particularly important with regards to the growth of HgCdTe on heteroepitaxial Si-based substrates, which have a higher dislocation density than the bulk CdZnTe substrates typically used for epitaxial HgCdTe material growth. In the paper a simple model of dislocations as cylindrical regions confined by surfaces with definite surface recombination is proposed. Both radius of dislocations and its surface recombination velocity are determined by comparison of theoretical predictions with carrier lifetime experimental data described by other authors. It is observed that the carrier lifetime depends strongly on recombination velocity; whereas the dependence of the carrier lifetime on dislocation core radius is weaker. The minority carrier lifetime is approximately inversely proportional to the dislocation density for densities higher than 10⁵ cm⁻². Below this value, the minority carrier lifetime does not change with dislocation density. The influence of dislocation density on the R₀A product of long wavelength infrared (LWIR) HgCdTe photodiodes is also discussed. It is also shown that parameters of dislocations have a strong effect on the R₀A product at temperature around 77 K in the range of dislocation density above 10⁶ cm⁻². The quantum efficiency is not a strong function of dislocation density.     

Material properties of Pb1-xSnxSe epilayers on Si and their correlation with the performance of infrared photodiodes

Hall mobilities and resistance area products R_oA of infrared diodes in epitaxial Pb_1-xSn_xSe layers on CaF₂ covered Si(111) substrates were correlated with threading dislocation densities p. The low temperature saturation Hall mobilities were entirely determined by p and proportional to their mean spacing 1/ √ρ. For the photodiodes, the R₀A values at low temperatures were inversely propor-tional to ρ. A model where each dislocation in the active area of the diodes causes a shunt resistance correctly describes the results, the value of this resistance for a single dislocation is 1.2 GΩ for PbSe at 85K. The dislocation densities were in the 2 × 10⁷ to 5 × 10⁸cm^-2 range for the 3-4 μm thick as-grown layers. Higher R₀A values are obtainable by lowering these densities by thermal annealing, which sweeps the threading ends of the misfit dislocations to the edges of the sample.     

Dislocations in GaAs produced by device fabrication

Due to the detrimental nature of dislocations on the properties of single-crystal GaAs the dislocation densities induced in the material during some fabrication processes, especially those encountered in the production of Gunn diodes, are investigated. The fabrication processes considered here are thermocompression bonding, heating, quenching, evaporation of contact material, cutting and handling. The results show that thermocompression bonding induces particularly strong damage. Finally several recommendations for optimum fabrication processes are presented which minimize the induced dislocation density.     

1/f noise in HgCdTe photodiodes

1/f noise in HgCdTe photodiodes has been attributed to a variety of sources, most of which are associated with some form of excess current. At DRS, we have measured the 1/f noise in vertically integrated (VIP) and high-density vertically integrated photodiodes (HDVIP), over a wide range of compositions and temperature, for strictly well-behaved diffusion current limited operation. It is found that (1) the 1/f noise current is directly dependent on dark current density; (2) material composition and temperature are irrelevant, except in as much as they determine the magnitude of the current density; (3) in high-quality diodes, the 1/f noise is independent of background flux; and (4) surface passivation is relevant. These observations have been compared to the 1/f noise theory of Schiebel, which uses McWhorter’s fluctuation of the surface charge tunneling model to modulate diode diffusion current. Agreement is obtained with Schiebel’s theory for realistic surface trap densities in the 10¹²/cm² range, which will obviously be characteristic of the passivation used. The relevance of this work relative to high operating temperature phtodiodes is discussed.     

Comparison of current-voltage characteristics of n- and p-type 6H-SiC Schottky diodes

Current-voltage (I–V) characteristics of n- and p-type 6H−SiC Schottky diodes are compared in a temperature range of room temperature to 400°C. While the room temperature I–V characteristics of the n-type Schottky diode after turn-on is more or less linear up to ∼100 A/cm², the I–V characteristics of the p-type Schottky diode shows a non-linear behavior even after turn-on, indicating a variation in the on-state resistance with increase in forward current. For the first time it is shown that at high current densities (>125 A/cm²) the forward voltage drop across p-type Schottky diodes is lower than that across n-type Schottky diodes on 6H−SiC. High temperature measurements indicate that while the on-state resistance of n-type Schottky diodes increases with increase in temperature, the on-state resistance of p-type Schottky diodes decreases with increase in temperature up to ∼330 K.     

Plastic Flow and Dislocation Compensation in ZnS y Se1?y /GaAs (001) Heterostructures

An important goal of lattice-mismatched semiconductor device design is control of threading dislocation densities, which are of particular importance for optoelectronic devices such as photodetectors and light-emitting diodes. The basis for this field of research is an understanding of the dislocation dynamics in mismatched heteroepitaxial structures. We have developed a dislocation dynamics model including dislocation multiplication, misfit–threading dislocation interactions, annihilation and coalescence, and thermal strain, which can be used to understand the strain relaxation and threading dislocation densities in arbitrarily graded ZnS _y Se_1?y /GaAs (001) structures. On the basis of this model, we demonstrate that the dislocation compensation mechanism, whereby mobile threading dislocations can be removed by insertion of a mismatched interface in a graded structure, can be explained by the bending over of threading dislocations associated with misfit segments of one sense by misfit dislocations having the opposite sense. Dislocation compensation, if utilized in device structures, can provide a pathway for the attainment of devices with low threading dislocation densities (D?<?10⁶?cm^?2) while using the minimum total thickness of epitaxial material, with a reduction in deposition time and source chemicals.     

Investigation of the aging processes of the p-n junction based on a single dislocation

An investigation was made of the effects of aging on diodes based on 60° dislocations. The electrical properties of the dislocation p-n junctions showed good stability and were unaffected by the Cottrell impurity atmosphere. This property suggests that there are practical uses for semiconductor devices based on the principle of dislocations. A proposed assembly of the diode based on a single 60° dislocation is described.     

Device modeling of HgCdTe vertically integrated photodiodes

Simulations of current-voltage characteristics of ion-implanted n-on-p photodiodes have been performed using SemiCad Device. In order to accurately simulate this device structure, several modifications to the simulator were implemented. These include the modified carrier statistics to account for the nonparabolic band structure of HgCdTe, the correct physics parameters for Shockley-Read-Hall, optical, and Auger recombination, and the Burstein-Moss shift for optical absorption important for heavily doped n-type HgCdTe. With these and other improvements, SemiCad Device is calibrated with the measured ideal dark current of an ion implanted diode and is used to simulate a source of non-ideal dark current from surface-charge induced band-to-band tunneling.     

Breakdown degradation associated with elementary screw dislocations in 4H-SiC pn junction rectifiers

It is well-known that SiC wafer quality deficiencies are delaying the realization of outstandingly superior 4H-SiC power electronics. While efforts to date have centered on eradicating micropipes (i.e., hollow core super-screw dislocations with Burgers vector>2c), 4H-SiC wafers and epilayers also contain elementary screw dislocations (i.e., Burgers vector=1c with no hollow core) in densities on the order of thousands per cm², nearly 100-fold micropipe densities. This paper describes an initial study into the impact of elementary screw dislocations on the reverse-bias current–voltage (I–V) characteristics of 4H-SiC p⁺n diodes. First, synchrotron white beam X-ray topography (SWBXT) was employed to map the exact locations of elementary screw dislocations within small-area 4H-SiC p⁺n mesa diodes. Then the high-field reverse leakage and breakdown properties of these diodes were subsequently characterized on a probing station outfitted with a dark box and video camera. Most devices without screw dislocations exhibited excellent characteristics, with no detectable leakage current prior to breakdown, a sharp breakdown I–V knee, and no visible concentration of breakdown current. In contrast, devices that contained at least one elementary screw dislocation exhibited 5–35% reduction in breakdown voltage, a softer breakdown I–V knee, and visible microplasmas in which highly localized breakdown current was concentrated. The locations of observed breakdown microplasmas corresponded exactly to the locations of elementary screw dislocations identified by SWBXT mapping. While not as detrimental to SiC device performance as micropipes, the undesirable breakdown characteristics of elementary screw dislocations could nevertheless adversely affect the performance and reliability of 4H-SiC power devices.     

Current transport in fluorine implanted GaAs

Effects of fluorine implantation in GaAs have been investigated by electrical characterization. Ion implantation at 100 keV energy was conducted with doses of 10¹¹ and 10¹²/cm². The effect of fluorine implantation on current-voltage (I-V) characteristics of Schottky diodes was significant. Carrier compensation was observed after implantation by the improved I-V characteristics. The lower dose implanted samples showed thermionic emission dominated characteristics in the measurement temperature range of 300 to 100K. The starting wafer and the low dose implanted samples after rapid thermal annealing (RTA) showed similar I-V properties with excess current in the lower temperature range dominated by recombination. The higher dose implanted samples showed increased excess current in the whole temperature range which may result from the severe damage-induced surface recombination. These samples after RTA treatment did not recover from implantation damage as in the low dose implantation case. However, very good I-V characteristics were seen in the higher dose implanted samples after RTA. The influence of the higher dose ion implantation was to produce more thermal stability. The results show the potential application of fluorine implantation in GaAs device fabrication.     

Small two-dimensional arrays of mid-wavelength infrared HgCdTe diodes fabricated by reactive ion etching-induced p-to-n-type conversion

The reactive ion etching (RIE) technique has been shown to produce high-performance n-on-p junctions by localized-type conversion of p-type mid-wavelength infrared (MWIR) HgCdTe material. This paper presents variable area analysis of n-on-p HgCdTe test diodes and data on two-dimensional (2-D) arrays fabricated by RIE. All devices were fabricated on x = 0.30 to 0.31 liquid-phase epitaxy (LPE) grown p-type (p = ∼1 × 10¹⁶ cm⁻³) HgCdTe wafers obtained from Fermionics Corp. The diameter of the circular test diodes varied from 50 μm to 600 μm. The 8 × 8 arrays comprised of 50 μm × 50 μm devices on a 100-μm pitch, and all devices were passivated with 5000 ? of thermally deposited CdTe. At temperatures >145 K, all devices are diffusion limited; at lower temperatures, generation-recombination (G-R) current dominates. At the lowest measurement temperature (77 K), the onset of tunneling can be observed. At 77 K, the value of 1/R₀A for large devices shows quadratic dependence on the junction perimeter/area ratio (P/A), indicating the effect of surface leakage current at the junction perimeter, and gives an extracted bulk value for R₀A of 2.8 × 10⁷ Ω cm². The 1/R₀A versus P/A at 195 K exhibits the well-known linear dependence that extrapolates to a bulk value for R₀A of 17.5 Ω cm². Measurements at 77 K on the small 8 × 8 test arrays were found to demonstrate very good uniformity with an average R₀A = 1.9 × 10⁶ Ω cm² with 0° field of view and D* = 2.7 × 10¹¹cm Hz^1/2/W with 60° field of view looking at 300 K background.     

Defects in annealed 1.5 MeV boron implanted p-type silicon

Effects of temperature and dosage on the evolution of extended defects during annealing of MeV ion-implanted Czochralski (CZ) p-type (001) silicon have been studied using transmission electron microcopy. Excess interstitials generated in a 1 10¹⁵ cm⁻²/1.5 MeV B⁺ implanted Si have been found to transform into extended interstitial {311} defects upon rapid thermal annealing at 800°C for 15 sec. During prolonged furnace annealing at 960°C for 1 h, some of the {311} defects grow longer at the expense of the smaller ones, and the average width of the defects seems to decrease at the same time. Formation of stable dislocation loops appears to occur only above a certain threshold annealing temperature (∼1000°C). The leakage current in diodes fabricated on 1.5 MeV B⁺ implanted wafers was found to be higher for a dosage of 1 10¹⁴cm⁻² and less, as compared to those fabricated with a dosage of 5 10¹⁴ cm⁻² and more. The difference in the observed leakage current has been attributed to the presence of dislocations in the active device region of the wafers that were implanted with the lower dosage.     

Greatly improved 3C-SiC p-n junction diodes grown by chemical vapordeposition

The fabrication and initial electrical characterization of greatly improved 3C-SiC (β-SiC) p-n junction diodes are reported. These diodes, which were grown on commercially available 6H-SiC (α-SiC) substrates by chemical vapor deposition, demonstrate rectification to -200 V at room temperature, representing a fourfold improvement in reported 3C-SiC diode blocking voltage. The reverse leakage currents and saturation current densities measured on these diodes also show significant improvement compared to previously reported 3C-SiC p-n junction diodes. When placed under sufficient forward bias, the diodes emit significantly bright green-yellow light. These results should lead to substantial advancements in 3C-SiC transistor performance     

Polycrystalline silicon p-n junctions

Silicon films deposited on recrystallized metallurgical silicon substrates have been used for the fabrication of low cost solar cells. The substrate is polycrystalline, and the active region of the solar cell is epitaxial with respect to the substrate. Since the dark current-voltage characteristics of a solar cell are important factors affecting its conversion efficiency, the characteristics of a number of epitaxial mesa diodes of the configuration n⁺-silicon/p-silicon/p⁺-metallurgical silicon/graphite have been measured over a wide temperature range to study the effects of grain boundaries. The results were analyzed on the basis of the two-exponential model.     

Effect of lattice-mismatched growth on InAs/AlSb resonant-tunnelingdiodes

Nominally identical InAs/AlSb resonant-tunneling diodes are fabricated on InAs and GaAs substrates to ascertain the effect of dislocations on the resonant-tunneling process. Although the diode on the GaAs substrate had a much higher dislocation density, as evidenced by X-ray diffraction measurements, it displayed only a small decrease in peak-to-valley current ratio     

Generation-Recombination Effect in High-Temperature HgCdTe Heterostructure Nonequilibrium Photodiodes

The dark current of near-room-temperature long-wavelength heterojunction photodiodes was studied. The dark current of the devices is much greater than that calculated from the Auger generation mechanisms. A model of trap- assisted tunneling via traps located at dislocation cores is proposed as the mechanism of enhanced thermal generation of charge carriers in reverse-biased diodes. Field-induced reduction of trap activation energies can increase thermal generation and create conditions for tunneling currents. The model qualitatively explains experimental current−voltage characteristics of the diodes assuming a dislocation density of approximately 10⁸ cm⁻² at the graded gap interface between absorber and contact regions of the photodiode.