J. M. Arias

p-type arsenic doping of Hg1−xCdxTe by molecular beam epitaxy

Growth of in situ As doped Hg1−xCdxTe by molecular beam epitaxy and activation of As at 250 °C is reported. We have used elemental arsenic, As4, as the p-type dopant source. The activation of As was observed in the 1016–1018 cm−3 range after a low temperature annealing step at 250 °C. However, for doping levels above 5×1018 cm−3, we have observed that the As activation efficiency drops. It is speculated at this time that self-compensation and formation of neutral As complexes may limit doping efficiency at very high levels. We also report our data on the structural and electrical characteristics of these As doped p-type layers using secondary ion mass spectroscopy analysis, and Hall effect measurements. An acceptor activation energy of 5.4 meV was obtained based on the dependence of the Hall coefficient on temperature. This value was attributed to singly ionized As located on a Te site (AsTe•) acting as an acceptor. A brief discussion on activation mechanism of As doped p-type HgCdTe material is also pres...

Uniform low defect density molecular beam epitaxial HgCdTe

This paper describes recent advances in MBE HgCdTe technology. A new 3 inch production molecular beam epitaxy (MBE) system, Riber Model 32P, was installed at Rockwell in 1994. The growth technology developed over the years at Rockwell using the Riber 2300 R&D system was transferred to the 32P system in less than six months. This short period of technology transfer attests to our understanding of the MBE HgCdTe growth dynamics and the key growth parameters. Device quality material is being grown routinely in this new system. Further advances have been made to achieve better growth control. One of the biggest challenges in the growth of MBE HgCdTe is the day-to-day control of the substrate surface temperature at nucleation and during growth. This paper describes techniques that have led to growth temperature reproducibility within + - 1°C, and a variation in temperature during substrate rotation within 0.5°C. The rotation of the substrate during growth has improved the uniformity of the grown layers. The measured uniformity data on composition for a typical 3 cm × 3 cm MBE HgCdTe/CdZnTe shows the average and standard deviation values of 0.229 and 0.0006, respectively. Similarly, the average and standard deviation for the layer thickness are 7.5 and 0.06 µm, respectively. P-on-n LWIR test structure photodiodes fabricated using material grown by the new system and using rotation during growth have resulted in high-performance (R0)A, quantum efficiency) devices at 77 and 40K. In addition, 128 × 28 focal plane arrays with excellent performance and operability have been demonstrated.

Bias-switchable dual-band HgCdTe infrared photodetector

The feasibility of an all molecular‐beam epitaxially (MBE) grown, bias‐switchable, dual‐band HgCdTe detector is demonstrated. Detection in the midwavelength infrared (MWIR) band only or the long‐wavelength band (LWIR) only is accomplished by the proper selection of detector bias in the ≥±100 mV range. The devices were all grown in situ by the MBE on CdZnTe or GaAs substrates and consisted of three intentionally doped layers in an n–p–n sequence. At 77 K the floating base two terminal devices responded to the ∼4.9–8 μm spectral band with the application of ≥−100 mV to the bottom contact and to the ∼2.1–4.9 μm band with the application of ≥0 mV. Quantum efficiency depended on bias with a maximum of 59% for LWIR and 66% for MWIR wavelengths at −150 mV and ≥0 mV, respectively. The operation of the dual‐band detector is discussed. Significant design differences between the heterojunction phototransistor and the dual‐band detector are noted.

Dislocation reduction in HgCdTe on GaAs and Si

Long‐wavelength infrared molecular‐beam epitaxial (MBE) HgCdTe films with dislocation densities as low as 2.3 × 105 cm−2 on (211) GaAs and Si substrates have been obtained by postgrowth thermal annealing and thermal cycle annealing processes (300–490 °C). Experiments show that metalorganic chemical vapor deposition (MOCVD) HgCdTe epilayers require a higher thermal annealing temperature than MBE material and the difference in dislocation reduction between MBE and MOCVD HgCdTe materials is caused by dislocation movement under high‐temperature and thermal stress conditions. The CdTe buffer layer has been observed to play a significant role for the dislocation reduction in the HgCdTe epilayer grown on GaAs or Si alternative substrates. To study the role of dislocations on MBE HgCdTe/GaAs, systematic measurements of the minority carrier lifetime of MBE HgCdTe grown on both CdZnTe and GaAs substrates were carried out. A strong correlation between minority carrier lifetime and dislocation density is observed.

Origin of void defects in Hg 1-x Cd x Te grown by molecular beam epitaxy

Characterization of defects in Hg1−xCdxTe compound semiconductor is essential to reduce intrinsic and the growth-induced extended defects which adversely affect the performance of devices fabricated in this material system. It is shown here that particulates at the substrate surface act as sites where void defects nucleate during Hg1−xCdxTe epitaxial growth by molecular beam epitaxy. In this study, we have investigated the effect of substrate surface preparation on formation of void defects and established a one-to-one correlation. A wafer cleaning procedure was developed to reduce the density of such defects to values below 200 cm−2. Focal plane arrays fabricated on low void density materials grown using this new substrate etching and cleaning procedure were found to have pixel operability above 98.0%.

Dislocation density reduction by thermal annealing of HgCdTe epilayers grown by molecular beam epitaxy on GaAs substrates

Post growth thermal annealing has been used to reduce the threading dislocation density of Hg1−xCdxTe (0.20≤x≤0.28) epilayers grown on (211)B GaAs substrates by molecular beam epitaxy. Etch pit density studies indicate an order of magnitude reduction on the surface threading dislocations after annealing at 490 °C for 30 min. The dislocation density at the HgCdTe surface on this highly mismatched system is only a factor of 2–6 times higher than the best values (1×105 cm−2 ) we have obtained using CdZnTe bulk lattice‐matched substrates. The reduction of dislocations may be due to enhanced dislocation movement and their annihilation and coalescence at Hg vacancies point defect pinning centers introduced during the annealing process.

HgCdTe infrared diode lasers grown by MBE

The authors report their latest results on the fabrication and successful operation of HgCdTe infrared diode lasers. The stripe-geometry double-heterostructure lasers were grown by molecular beam epitaxy (MBE). The active layer thickness ranges between 0.9 and 1.4 mu m, and the p+ and n+ confinement layers were in situ doped up to 1018 cm-3 with arsenic and indium, respectively. Five double heterostructures were grown, all of which produced working lasers. The devices were operated under pulsed currents at temperatures between 40 and 90 K. The 77 K stimulated emission wavelengths for these lasers were 2.9, 3.4, 3.9 and 4.4 mu m. Operation at 5.3 mu m was demonstrated at 60 K. The lowest 77 K threshold current density was 419 A cm-2 which is very close to the prediction of a numerical calculation. Characterization of the devices, including, for example, temperature dependence of the threshold currents and spectral analysis, was performed and showed the characteristics of well-behaved, stable devices that operated without failure while being tested.

Molecular beam epitaxy HgCdTe growth-induced void defects and their effect on infrared photodiodes

We have carried out a study and identified that MBE HgCdTe growth-induced void defects are detrimental to long wavelength infrared photodiode performance. These defects were induced during nucleation by having surface growth conditions deficient in Hg. Precise control and reproducibility of the CdZnTe surface temperature and beam fluxes are required to minimize such defects. Device quality material with void defect concentration values in the low 102 cm2 range were demonstrated.

Performance of HgCdTe, InGaAs and quantum well GaAs/AlGaAs staring infrared focal plane arrays

The ability to hybridize various detector arrays in disparate technologies to an assortment of state-of-the-art silicon readouts has enabled direct comparison of key IR detector technologies including photovoltaic (PV) HgCdTe/Al2O3, PV HgCdTe/CdZnTe, PV InGaAs/InP, and the photoconductive (PC) GaAs/AlGaAs quantum well IR photodetector (QWIP). The staring focal plane arrays range in size from 64 X 64 to 1024 X 1024; we compare these IR detector technologies versus operating temperature and background flux via hybrid FPA test at operating temperatures from 32.5 K to room temperature and photon backgrounds from mid-105 to approximately equals 1017 photons/cm2-s. Several state-of-the-art IR FPAs are included: a 1.7 micrometers 128 X 128 InGaAs hybrid FPA with room temperature D of 1.5 X 1013 cm-Hz1/2/W and 195K D of 1.1 X 1015 cm-Hz1/2/W; a 3.2 micrometers 1024 X 1024 FPA for surveillance; a 4.6 micrometers 256 X 256 HgCdTe/Al2O3 FPA for imaging with BLIP NE(Delta) T of 2.8 mK at 95K; and a 9 micrometers 128 X 128 GaAs QWIP with 32.5 K D > 1014 cm-Hz1/2/W at 32.5K and 8 X 1010 cm-Hz1/2W at 62K.

MBE growth of HgCdTe epilayers with reduced visible defect densities: Kinetics considerations and substrate limitations

A semi-empirical constraint to the thermodynamical model for growth of Hg1−xCdxTe (MCT) by molecular beam epitaxy is described. This constraint, derived by forcing the population of Hg atoms in a surface layer to be proportional to the HgTe fractional growth rate, can determine an optimal total growth rate for specific beam fluxes and substrate temperature. Utilizing improved growth conditions determined by this model has resulted in MCT layers with consistently lower visible defect density (e.g., voids). The majority of recent layers grown using the constrained conditions has achieved defect densities limited by the CdZnTe substrate. On the highest quality substrates, total defect densities have consistently been reduced to the 100–200 cm−2 range using the improved conditions for compositions x=0.2 to x=0.6. On more typical substrates, the total defect density is 1000–1500 cm−2. This compares with densities of 3000–5000+ cm−2 for old layers grown under non-optimized conditions. The density of voids has remained about the same upon using the improved conditions, and is determined primarily by the Te precipitate content of the substrate, but microdefect (hillock) density has been reduced by almost a factor of ten.