M. C. Chen

Inhomogeneity model for anomalous Hall effects in n‐type Hg0.8Cd0.2Te liquid‐phase‐epitaxy films

Anomalous Hall effects, i.e., peaks in the temperature dependence of the Hall coefficient and Hall mobility, have been observed in liquid‐phase‐epitaxy (LPE) Hg0.8 Cd0.2Te films. After establishing that surface effects are not the cause of the anomalies and that our LPE films are indeed n type, we propose an inhomogeneity model with a network of extended p‐type inclusions in an n‐type matrix. Good fits to the experimental data have been obtained by computer simulation based on this model.

The minority carrier lifetime in doped and undoped p-type Hg 0.78 Cd 0.22 Te liquid phase epitaxy films

This paper will describe: (1) the first comparative study of recombination mechanisms between doped and undoped p-type Hg1-xCdxTe liquid phase epitaxy films with an x value of about 0.22, and (2) the first determination of τA7i/τA1i ratio by lifetime’s dependence on both carrier concentration and temperature. The doped films were either copper- or gold-doped with the carrier concentration ranging from 2 x 1015 to 1.5 x 1017 cm-3, and the lifetime varied from 2 μs to 8 ns. The undoped (Hg-vacancy) films had a carrier concentration range between 3 x 1015 and 8 x 1016 cm-3, and the lifetime changed from 150 to 3 ns. It was found that for the same carrier concentration, the doped films had lifetimes several times longer than those of the undoped films, limited mostly by Auger 7 and radiative recombination processes. The ineffectiveness of Shockley-Read-Hall (SRH) recombination process in the doped films was also demonstrated in lifetime vs temperature curves. The important ratio of intrinsic Auger 7 lifetime to intrinsic Auger 1 lifetime, τA7i/τA1i, was determined to be about 20 from fitting both concentration and temperature curves. The reduction of minority carrier lifetime in undoped films can be explained by an effective SRH recombination center associated with the Hg vacancy. Indeed, a donor-like SRH recombination center located at midgap (Ev+60 meV) with a capture cross section for minority carriers much larger than that for majority carriers was deduced from fitting lifetime vs temperature curves of undoped films.

The activation energy of copper shallow acceptors in mercury cadmium telluride

The thermal activation energy (E0) of isolated copper shallow acceptors in bulk Hg0.8Cd0.2Te has been determined using variable‐temperature Hall measurements on samples with 77‐K carrier concentrations ranging between 1015 and 1017 cm−3. It was found that the decrease of the activation energy with the increasing hole concentration can be adequately described by the screening of the acceptor potential by free carriers. The measured E0 (11.5 meV) also agrees well with that predicted by the effective‐mass theory.

High‐quality p ‐ type Hg1−xCdxTe prepared by metalorganic chemical vapor deposition

An arsenic source, tris‐dimethylaminoarsine (TDMAAs), was used as a p‐type dopant for Hg1−xCdxTe prepared by metalorganic chemical vapor deposition (MOCVD). The Hg1−xCdxTe layers were grown using the interdiffused multilayer process (IMP) at 360 °C with dimethylcadmium, di‐isopropyltelluride and elemental mercury. Hole concentrations at 77 K in the range of 2×10+16–7×10+17 cm−3 were obtained in Hg1−xCdxTe with x=0.2–0.35, and ∼30%–50% of the arsenic was electrically active based on the data from secondary ion mass spectrometry (SIMS) measurements. These MOCVD Hg1−xCdxTe layers show minority carrier lifetimes at 77 K in the range of 3 ns–4 μs, which compare favorably with those from the best arsenic‐doped Hg1−xCdxTe films prepared by liquid phase epitaxy.

Minority‐carrier lifetime in indium‐doped n‐type Hg0.78Cd0.22Te liquid‐phase‐epitaxial films

The recombination mechanisms of the minority‐carrier lifetime in n‐type indium‐doped Hg0.78Cd0.22Te liquid‐phase‐epitaxial films have been studied. Its temperature dependence was measured from 78 to 200 K using standard photoconductivity decay method. The dependence on doping concentration from 1×1014 to 2×1015 cm−3 was measured using a contactless microwave reflection method at 77 K. From best fits to the experimental temperature dependence data, it was found that lifetimes at temperatures higher than 130 K were dominated by Auger recombination with a value of 0.15 for the overlap integral of Bloch functions. For most films with a doping concentration below 1×1015 cm−3, Shockley–Read–Hall recombination via an acceptor‐like deep level at about 40 meV below the conduction band affected lifetimes below 120 K. It was also found that the recombination parameters determined from fitting the lifetime versus temperature curves were adequate to analyze and understand the lifetime versus doping concentration curve...

Photoconductivity lifetime measurements on HgCdTe using a contactless microwave technique

Photoconductivity lifetime measurements on narrow band‐gap HgCdTe thin films and bulk slices using a contactless microwave reflection technique have been demonstrated for the first time. A horn antenna was used to allow cooling of samples to 77 K outside the waveguide and testings of large‐size samples. A good agreement was established between lifetime results from the contactless microwave reflection technique and the standard photoconductivity decay method using two ohmic contacts. The feasibility of lifetime mapping by moving the pulsed light beam to different locations on the samples was also demonstrated.

Key performance-limiting defects in p-on-n HgCdTe LPE heterojunction infrared photodiodes

The operability of long-wavelength p-on-n double layer heterojunction arrays for 40K low-background applications has long been limited by the wide variation in pixel-to-pixel zero bias resistance (Ro) values. Diodes on test structures showing lower performance, with Ro values below 7 × 106 ohm at 40K, usually contained gross metallurgical defects such as dislocation clusters and loops, pin holes, striations, Te inclusions, and heavy terracing. However, diodes with Ro values between 7 × 106 and 1× 109 ohm at 40K contained no visible defects. To study the “invisible” performance-limiting defects (i.e. defects that cannot be revealed by etching), a good correlation between the dynamic resistance at 50 mV reverse bias (R50) value at 77K and the Ro value at 40K was first established, and then used as a tool. The correlation allowed measurements of a large number of devices at 77K, rather than relying exclusively on time-consuming measurements at 40K. Interesting results regarding Ro values at 40K, such as insensitivity to low-density dislocations, mild degradation from Hg vacancies, severe degradation from Hg interstitials, and correlation with junction positioning, were obtained from specially designed experiments.

Observation of a deep level in p‐type Hg0.78Cd0.22Te with high dislocation density

To investigate the possibility that deep levels are associated with dislocations in narrow band gap HgCdTe, deep level transient spectroscopy has been used to study n+‐p diodes fabricated on p‐type bulk Hg0.78Cd0.22Te samples with either a ‘‘normal’’ dislocation density of about 105 cm−2 or a high dislocation density of about 106 cm−2. These samples which are gold doped with a hole concentration of 1.2×1015 cm−3, have a band gap of about 0.12 eV at 77 K. In samples with a ‘‘normal’’ dislocation density, a deep level of about 80 meV above the valence band was found. However, a midgap level of about 60 meV above the valence band with larger peak amplitude and broader line shapes was found in the sample with a high dislocation density.

A quick thermoelectric technique for typing HgCdTe at liquid nitrogen temperature

A simple and quick technique based on thermoelectric effect of semiconductors has been developed for screening the conductivity type of narrow band‐gap HgCdTe materials. This technique generates a temperature gradient across the sample by partly dipping it in liquid nitrogen. The induced Seebeck voltage is then measured by loading two spring contact probes to two separate points on the sample below and above the liquid nitrogen level. It was found, as expected by the theory, that the thermoelectric measurements on p‐type HgCdTe are much less influenced by the inverted surface effects than Hall measurements.

The temperature dependence of the anomalous Hall effects in p‐type HgCdTe

The anomalous Hall effects of narrow‐band‐gap p‐type HgCdTe, which manifest themselves as negative Hall coefficients at low temperatures, have caused serious problems in material characterization in the past two decades. These phenomena are now widely recognized as being caused by the inverted surface effect describable by Petritz’s two‐layer model [Phys. Rev. 110, 1254 (1958)]. We report results from variable‐temperature Hall measurements on p‐type HgCdTe liquid‐phase epitaxy and bulk slices with bare, anodically sulfidized, and anodically oxidized surfaces. We show that the Hall anomalies can be eliminated by depositing an anodic sulfide layer on the surface and subsequently can be restored by removing the sulfide layer. Based on Petritz’s two‐layer model, we were able to use the same set of bulk transport parameters and different sets of surface transport parameters to fit the experimental temperature‐dependent Hall coefficient and Hall mobility curves of the same sample with surfaces that have been su...