R. E. DeWames

Planar p‐on‐n HgCdTe heterostructure photovoltaic detectors

We report a process to fabricate planar Hg1−yCdyTe/Hg1−xCdxTe (x<y) heterostructure photodiodes with the p‐on‐n configuration. The material used for this demonstration was grown by molecular beam epitaxy. The p‐on‐n planar devices consist of an arsenic‐doped p‐type epilayer (y=0.28) on top of a long wavelength infrared n‐type epilayer (x=0.225, λ=10 μm). The planar junctions were formed by selective pocket diffusion of arsenic deposited by ion implantation. The detailed analysis of the current‐voltage characteristics of these diodes as a function of temperature show that they have high performance and that their dark currents are diffusion limited down to 52 K. The results also show that the R0A values for these devices are highly uniform at 77 K.

Long and middle wavelength infrared photodiodes fabricated with Hg1−x CdxTe grown by molecular‐beam epitaxy

Long and middle wavelength infrared (LWIR, MWIR) p+‐n photodiodes have been fabricated with Hg1−xCdxTe (0.20<x<0.30) grown by molecular‐beam epitaxy (MBE). The epilayers were grown on (211)B lattice‐matched ZnCdTe substrates. The surface morphology was smooth and free of in‐plane twins. The Cd concentration (x) was uniform across the wafer, with standard deviations (Δx) as low as 0.0017. Structural properties were measured by double‐crystal x‐ray rocking curve and dislocation etching; FWHM values as low as 34 arcsec and etch pit density values as low as 1×105 cm−2 were measured. p+ ‐n homojunctions were formed by arsenic diffusion; unpassivated mesa photodiodes were fabricated by standard photolithographic techniques. MWIR and LWIR photodiodes fabricated with MBE material exhibited good diode performance, comparable to that obtained on photodiodes fabricated with the more matured technique of liquid‐phase epitaxy. 77‐K R0A products of the diodes measured were 6.35×107, 22.3, and 1.76 Ω cm2 with cutoff wav...

Molecular‐beam epitaxy growth and in situ arsenic doping of p‐on‐n HgCdTe heterojunctions

In this paper we present, results on the growth of in situ doped p‐on‐n heterojunctions on HgCdTe epilayers grown on (211)B GaAs substrates by molecular‐beam epitaxy (MBE). Long wavelength infrared (LWIR) photodiodes made with these grown junctions are of high performance. The n‐type MBE HgCdTe/GaAs alloy epilayer in these structures was grown at Ts=185 °C and it was doped with indium (high 1014 cm−3 range) atoms. This epilayer was directly followed by the growth, at Ts=165 °C, of an arsenic‐doped (1017–1018 cm−3 ) HgTe/CdTe superlattice structure which was necessary to incorporate the arsenic atoms as acceptors. After the structure was grown, a Hg annealing step was needed to interdiffuse the superlattice and obtain the arsenic‐doped p‐type HgCdTe layer above the indium‐doped layer. LWIR mesa diodes made with this material have 77 K R0A values of 5×103, 81, 8.5, and 1.1 Ω cm2 for cutoff wavelengths of 8.0, 10.2, 10.8, and 13.5 μm, respectively; the 77 K quantum efficiency values for these diodes were gre...

Reproducible growth of high quality YBa2Cu3O7−x film on (100) MgO with a SrTiO3 buffer layer by pulsed laser deposition

Epitaxial YBa2Cu3O7−x(YBCO) films were grown on SrTiO3‐buffered (100) MgO substrates by pulsed laser deposition. The SrTiO3 layer of about 500 A thick serves a dual purpose to provide a better lattice match to the YBCO films as well as a diffusion barrier to prevent interaction between YBCO and MgO. These YBCO films exhibit excellent structural, electrical, and microwave properties with Tc≳89 K and Tc=2.9×106 A/cm2 at 77 K (1×106A/cm2 at 84 K). X‐ray pole figure measurements showed that over 99.92% of the volume fraction of the c‐axis oriented film has the a‐b axis aligned parallel to the [010] and [001] direction of the substrate.

Excess tunnel currents in AlGaAs/GaAs multiple quantum well infrared detectors

We present experimental evidence of excess tunnel current at low temperatures in AlGaAs/GaAs multiple quantum well infrared detectors that is not accounted for by existing theory. Prior discussions of current mechanisms in these detectors only take into account ideal device properties. For quantum well detectors with thick barrier layers, which are useful for infrared detection, the excess tunnel current component possesses the features of sequential resonant tunneling, i.e., temperature independence of the current, and saturation of the current at intermediate bias voltages with negative conductance oscillations. However, the current is orders of magnitude larger than theory predicts. Comparison with data reported by other groups shows the magnitude of the discrepancy is sample dependent. These results suggest that defects play an important role in determining the tunnel current magnitude. This current component is significant because it limits attainable detector performance at low temperatures for appl...

Current generation mechanisms in small band gap HgCdTe p-n junctions fabricated by ion implantation

A detailed study has been made of the current-voltage characteristics of Hd1-xCdxTe ion-implanted p-n junctions with x ⋍ 0.224. It is found that the dark currents, for diodes of high quality, can be represented over a broad range of voltage and temperature by three current components. A diffusion current dominates in the small bias region at temperatures > 50 K. This current component is also observed at sufficiently high forward biases at low temperatures. At temperatures of 30<T<50 K in the small bias region, a current component with a positive temperature coefficient is observed. This component, which we call Type II tunneling, has a logarithmic voltage dependence in forward bias and some of the properties of this current can be accounted for by trap-assisted tunneling models for excess current in Esaki diodes. At sufficiently high reverse bias at elevated temperatures and in the small bias region at T<30 K, the current is generated by internal field emission; this component is called Type I tunneling. The reverse bias current data are analyzed in terms of band-to-band tunneling probability expressions and good agreement is found between experiment and theory. This current has a negative temperature coefficient. Generation-recombination currents are not observed in these diodes; this conclusion is based on the observation that in the forward bias region where diffusion currents are small the observed logarithmic current has a slope which is almost temperature insensitive. In addition, the temperature dependence of the current does not obey an Arrhenius-type expression, as expected for thermally activated processes. The dominant current component limiting the zero bias resistance area (R0A) products at temperatures 30<T<500 K is Type II tunneling and the temperature dependence of this component remains unexplained. The dark currents measured on diodes of different quality in the voltage-temperature region where tunneling is dominant are highly nonuniform. This observation suggests variations in tunneling junction parameters or/and changes in the properties of the defect states contributing to carrier generation. At temperatures below 20 K we observed structures in the forward bias current. These features suggest hump currents associated with discrete defect levels and phonon-assisted tunneling processes.

Infrared diodes fabricated with HgCdTe grown by molecular beam epitaxy on GaAs substrates

Infrared photodiodes fabricated with HgCdTe epilayers grown on GaAs substrates by molecular beam epitaxy (MBE) are reported here for the first time. Growth was carried out on the (211)B orientation of GaAs, and the as‐grown epilayer (x=0.24) was p type. The n‐p junction was formed by Be ion implantation, the resistance‐area product (R0 A) at zero bias was 1.4×103 Ω cm2 , the wavelength cutoff was 8.0 μm, and the quantum efficiency was 22%; all were measured at 77 K. We show that in the diffusion regime diodes fabricated with MBE HgCdTe/GaAs have comparable R0 A product values to those made with HgCdTe grown by bulk techniques. This result discloses new possibilities for advanced monolithic HgCdTe devices based on GaAs integrated circuit technology.

Effect of the dislocation density on minority‐carrier lifetime in molecular beam epitaxial HgCdTe

The photoconductive minority‐carrier lifetime has been measured as a function of temperature and etch‐pit density in n‐type HgCdTe grown by molecular beam epitaxy with a composition range x=0.22–0.23 to determine the limiting recombination mechanisms, particularly those related to dislocation density. In the extrinsic region at temperatures T<77 K, the minority‐carrier lifetime is limited by Shockley–Read recombination. Strong correlation between minority‐carrier lifetime and dislocation density is observed.

Assessment of HgCdTe and GaAs/GaAlAs technologies for LWIR infrared imagers

Imagery of long wavelength infrared HgCdTe and GaAs quantum well staring arrays in size 128 X 128 has been demonstrated. In this paper, we compare detector array performance characteristics, discuss the natural and technological limitations of both technologies and identify the improvements likely to be made in the near future. At this stage of feasibility demonstration in the spectral band 8 - 10 micrometers , the effective quantum efficiency in GaAs FPAs is 4% compared to 60% for HgCdTe and the responsivity is 0.08 A/W compared to 4.5 A/W. This value of 0.08 A/W is significantly below the value 2 A/W reported for single quantum well infrared photodetectors (QWIP) detectors. The peak detectivities and NE(Delta) T at 78 K are (5 X 109 cm (root)Hz/W, 0.037 K) and (2 X 1011, 0.005 K) for QWIP and HgCdTe, respectively. The residual nonuniformities after two-point correction are < 0.01% for QWIP arrays and 0.012% for HgCdTe. Crosstalk is currently unsatisfactory in QWIP detector arrays, but design concepts can be used to reduce this effect. For terrestrial imaging, GaAs quantum well detector arrays most likely will need to operate at temperatures below 80 K from fundamental considerations; HgCdTe detector arrays are background limited at operating temperatures <EQ 90 K. Since cooling can drive cost and reliability, and since significant progress has been made in producing high quality HgCdTe detector arrays with good yield, it is unlikely that HgCdTe will be displaced by this technology for terrestrial applications. For low background space applications at (phi) b <EQ 1012 ph/cm2-sec, QWIP detectors at 40 K are background limited. This observation plus their radiation hard characteristics suggest a possible niche in strategic applications.

HgCdTe dual‐band infrared photodiodes grown by molecular beam epitaxy

We report the demonstration of an all molecular beam epitaxy HgCdTe bias‐selectable dual‐band infrared photodetector. The mesa device, an n‐p‐n three layer HgCdTe heterostructure, was in situ doped with arsenic and indium for p‐ and n‐type doping. The device design is similar to a heterostructure floating base transistor. The feasibility of the two‐color bias‐switchable detector was demonstrated by obtaining backside illuminated spectrally pure dual‐band detection at 77 K. Wavelength cutoff (λco) selection to 5.2 μm with 60% quantum efficiency (QE) was obtained by applying a negative bias of −250 meV, and to λco=7.9 μm with 36% QE by applying a positive bias of 250 meV. The current‐voltage characteristics of this device can be described in terms of a simple back‐to‐back diode model.