J. G. Pasko

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...

Planar p-on-n HgCdTe heterostructure infrared photodiodes on Si substrates by molecular beam epitaxy

We have developed a low temperature procedure for molecular beam epitaxy of CdTe buffer layers on {211} Si wafers and have used Si/ZnTe/CdTe composite substrates for molecular beam epitaxy of double layer Hg1−xCdxTe heterostructures. Planar p-on-n double layer heterostructures were formed by an implantation technique and test diodes were fabricated and characterized. At 77 K, devices with 30×30 μm2 junction area had R0A values in the range 1.5×106–1×107Ω cm2 with a uniform cut-off wavelength of 4.65 μm.

Backside‐illuminated InAs1−xSbx‐InAs narrow‐band photodetectors

High‐performance backside‐illuminated photodiodes have been fabricated for the first time from InAs1−xSbx‐InAs heterostructures prepared by liquid‐phase‐epitaxy technique. The peak wavelength can be tuned compositionally from 3.1 to over 7.0 μm at 77 K. The half‐width of the spectral responses as narrow as 1760 A (at 4.0 μm) have been achieved. Internal quantum efficiencies of 90% and zero‐bias‐resistance–area products of 2×107 Ω cm2 have been obtained at 77 K.

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.

Open‐tube isothermal vapor phase epitaxy of Hg1−xCdxTe on CdTe

Device‐quality Hg1−xCdxTe (0.2≤x≤0.35) epitaxial layers have been grown on CdTe substrates by open‐tube isothermal vapor phase epitaxy (ISOVPE). The surface morphology of the layers is mirrorlike, and the Hall data are comparable to those for HgCdTe grown by liquid phase epitaxy (LPE). Photovoltaic devices with a cutoff wavelength of 4.1 μm at 77 K were fabricated on an ISOVPE HgCdTe epilayer. Their performance is comparable with those that we have obtained for such devices fabricated on LPE HgCdTe epilayers.

High‐speed Pb1−xSnx Te photodiodes

Low‐capacitance (< 10 pF) Pb1−xSnxTe photodiodes operating near 77 °K (areas 1.7 × 10−4 cm2) have been obtained via liquid‐phase epitaxial growth of low‐carrier‐concentration material (low 1014 cm−3 to high 1015 cm−3). Response to a mode‐locked 1.06‐μm Nd:YAG laser with the devices terminated in 50 Ω indicated a frequency response to 400 MHz. With a 14‐Ω load and by exciting a photocurrent with radiation from a CO2 laser, the shot noise rolloff point was found to be 1200 MHz. Optimum parameters have not yet been achieved, as evidenced by the increase in frequency response with increase in reverse bias; these results represent only a lower limit to this materials capability.