P-type HgCdTe Research Articles

Resistivity and high magnetic field hall effect measurements on as-grown p-type HgZnTe

Resistivity and Hall coefficient of bulk p-type HgZnTe ( x = 0.15) grown by THM have been measured as a function of magnetic field up to 7 T at different temperatures. The measured samples show mixed conduction behaviour, analogous to that observed in p-type HgCdTe. A three-band model that includes electrons, heavy holes and light holes is needed to explain the high magnetic field results.

Chemical doping of HgCdTe by molecular-beam epitaxy

Chemical doping of HgCdTe is an important issue in II–VI compound semiconductors. In this paper, we will report on the molecular-beam epitaxy (MBE) growth and characterization of n- and p-type HgCdTe. We have grown n- and p-type layers of HgCdTe with indium and arsenic as the dopants using MBE at 170 °C–180° C without photo or ion excitation. The doped layers, which range from 1015 cm−3 to 1018 cm−3 have been characterized by a variety of techniques including infrared IR transmission, Hall measurement, scanning electron microscopy, minority carrier lifetime, and secondary ion mass spectroscopy. The results indicate that n-type alloy layers are easier to form than p type because of the efficient incorporation of indium in the mercury (or cadmium) sublattice. Memory effects of indium were not observed. We have also demonstrated that our p-type layers remain p type after mercury anneal indicating that they are extrinsically doped. The breakthrough in chemical doping of HgCdTe has enabled us to grow several double layer heterojunction structures. Several diodes have been fabricated and their electrical and optical characteristics are discussed.

Modulation-doped HgCdTe quantum well structures and superlattices

Photoassisted molecular-beam epitaxy (MBE) has been employed to successfully prepare p-type and n-type modulation HgCdTe. In this paper, we report details of the MBE growth experiments and describe the structural, optical, and electrical properties that these new quantum well structures and superlattices of HgCdTe possess.

Properties of insulator interfaces with p-HgCdTe

Heat treatment at 70 °C of low carrier concentration p-type HgCdTe samples (p0=8×1014 cm−3) generates an inverted surface layer. A two day anneal process below 95 °C did not affect the Hall coefficient, whereas an almost complete recovery was obtained by annealing at 120 °C. While bulk electron mobility, obtained from PEM data, remained high (about 9×104 cm2/V s at 77 K), surface mobility is lower by more than an order of magnitude. Surface recombination velocity indicates a continuous improvement with increased temperature, and the activation energy remains equal to the vacancies energy level. The proposed mechanism is that of positive charges in the sulfide migrating towards the interface and generating an image inversion layer.

The excess carrier lifetime in vacancy- and impurity-doped HgCdTe

The excess carrier lifetimes of numerous vacancy- and impurity-doped bulk p-type HgCdTe crystals are measured using the techniques of steady-state photoconductivity and photoconductive decay. It is shown that the steady-state minority carrier lifetimes of impurity-doped crystals can be significantly larger than those of vacancy-doped crystals. However, the transient and steady state lifetimes differ by as much as an order of magnitude at 77 K due to minority carrier trapping, and appear to be uncorrelated. For Hg1−xCdxTe wafers with x∼0.225 and Na∼1×1016/cm3, the steady-state minority carrier lifetimes of impurity-doped crystals ranged from 1.6 to 62 ns, while those of vacancy-doped crystals ranged from 1.4 to 28 ns.

Interface properties of various passivations of HgCdTe

The effects of different surface treatments of the p-type Hg(x-1)Cd(x)Te (x about 0.2) alloy, including anodic-sulfide, anodic-oxide, and ZnS coatings, on the surface charge density and surface mobility were investigated. Measurements of surface recombination velocities in samples passivated by either a ZnS coating on a freshly etched surface or by anodic native sulfide combined with deposited ZnS showed essentially the same temperature dependence down to 50 K. For the same original material, both the surface state density and the activation energy were found to be similar. It is suggested that, for both passivations, the surface is controlled by traps that can be related to lattice defects, such as vacancies.

Analysis of dark current in IR detectors on thinned p-type HgCdTe

A model for the diffusion dark current in MIS IR detectors on thinned bulk p-type HgCdTe is discussed. The model includes trap-assisted tunneling mechanisms in the back-side depletion region as well as the effects of fast surface states. Expressions for the net recombination rate are developed for situations in which trap-assisted tunneling transitions are allowed. Calculations for 12- mu m optical cutoff detectors operating at liquid-nitrogen temperature show that the properties of the back side, including surface fixed charge density, depletion region trap density, fast surface-state density, and majority carrier concentration, have a strong influence on the dark current levels of detectors on thin material. It is predicted that typical as-fabricated surface parameters will not result in large dark current densities. Calculations for detectors with surface parameters common to stressed (degraded) back surfaces, however, show dark current densities which would significantly affect detector performance.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Thermal Treatment of Hg Containing I-VI Semiconductors by Annealing in a Mercury Bath (AMEBA)

ABSTRACTAn extremely simple and inexpensive technique (AMEBA) for the thermal treatment of Hg containing specimens which permits short or long time annealing in a Hg rich atmosphere is described. It is based on the immersion of the sample, properly protected by proximity caps in, or above, a hot mercury bath. The sample assembly is such that it permits Hg vapors, but not the liquid, to reach the specimen's surface.The usefulness of AMEBA in improving the electrical and structural properties of as-grown Hg1-xCdxTe (x = 0.21) and in removing ion implantation related damage as well as electrically activating B implants in various p-type HgCdTe samples is demonstrated. All the data presented show that AMEBA treatment yields results which are comparable or superior to those obtainable by convensional annealing methods.

Electrical properties of intrinsic p-type shallow levels in HgCdTe grown by molecular-beam epitaxy in the (111)B orientation

The electrical properties of the unintentionally doped p-type HgCdTe material as grown in the (111)B orientation by molecular-beam epitaxy are revised. The analysis of the Hall coefficient in the whole temperature range with a model based on the two-band nonparabolic Kane model, a fully ionized compensating donor concentration, and two independent discrete acceptor levels is presented. The donor compensation is found to be much lower than before, in agreement with the latest study of extrinsic doping by indium. A defect level with an energy of 30 to 50 meV is found necessary to explain properly some of the crystals’ data. The results of a three-carrier band modeling of the Hall constant versus field are also presented for one sample and are in very good agreement with the expected band structure of the material. These results show that important improvements have been made recently in the control of stoichiometry during growth.

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 subjected to different chemical treatments. It was demonstrated that the Hall anomalies occurred when the n-type surface conductivity increased relative to the p-type bulk conductivity. The surface conductivity is mainly controlled by the density of fixed positive surface charges which was found to be larger on an anodically oxidized surface and smaller on an anodically sulfidized surface than on a bare surface.

P-type modulation-doped HgCdTe

At North Carolina State University, we have recently employed photoassisted molecular beam epitaxy to successfully prepare p-type modulation-doped HgCdTe. The modulation-doped HgCdTe samples were grown on lattice-matched (100) CdZnTe substrates cut from boules grown at Santa Barbara Research Center. In this letter, we report details of the growth experiments and describe the structural, optical, and electrical properties that this new infrared quantum alloy of HgCdTe possesses.

Properties of Modulation-Doped HgCdTe Superlattices

ABSTRACTPhotoassisted molecular beam epitaxy has been employed to successfully prepare p-type and n-type modulation-doped HgCdTe superlattices. The samples were grown at 170°C. In this paper, we report details of the MBE growth experiments and describe the optical and electrical properties that these new multilayered quantum well structures of HgCdTe possess.

Surface Electrons in Inverted Layers of p-HgCdTe

ABSTRACTAnodic oxide passivation of p-type HgCdTe generates an inversion layer. Extremely high Hall mobility data for electrons in this layer indicated the presence of a two-dimensional electron gas. This is verified by use of the Shubnikov-de Haas effect from 1.45-4.15K. Data is extracted utilizing a numerical second derivative of DC measurement. Three sub-bands are detected. Their relative occupancies are in excellent agreement with theory and with experimental results obtained on anodic oxide as accumulation layers of n-type HgCdTe. The effective mass derived is comparable to expected.

Electrodeposited CdTe and HgCdTe solar cells

The processing steps necessary for producing high efficiency electrodeposited CdTe and HgCdTe solar cells are described. The key step in obtaining solar cell grade p-type CdTe and HgCdTe is the “type conversion-junction formation” (TCJF) process. The TCJF process involves the heat treatment of the as-deposited n-type CdTe and HgCdTe layers at around 400 °C. This procedure converts these n-type films into high resistivity p type and forms a rectifying junction between them and the underlying n-type window layers. Possible effects of oxygen on the TCJF process are discussed. The results of studies made on the structural, electrical and optical properties of the electrodeposited CdS, CdTe and HgCdTe films are presented. The resistivity of the electrodeposited HgCdTe can be made lower than that of CdTe. Consequently, solar cells made using the HgCdTe films have, on the average, better fill factors than those made using the CdTe layers. HgCdTe is also attractive for tandem-cell applications because of its variable band gap which can be easily tuned to the desired value. CdS/CdTe and CdS/HgCdTe heterojunction solar cells with 10.3% and 10.6% efficiency have been demonstrated using electrodeposition techniques and the TCJF process.

Inverted surface effect of p-type HgCdTe

By surface passivation using anodic sulfidization, we demonstrated that the inverted surface effect which gives rise to negative Hall coefficients commonly measured in p-type HgCdTe at low temperatures can be eliminated. Our results of Hall measurements as a function of magnetic field at 77 K and computer simulations allow us to distinguish two different existing models (with shunting or nonshunting n-type inversion layer) of the inverted surface effect. Bulk and surface transport parameters such as hole concentration, hole mobility, surface electron concentration, and surface electron mobility have been derived from computer best fits of experimental Hall coefficient curves.

(100) versus (111)B crystallographic orientation of Hg1−xCdxTe grown by molecular-beam epitaxy

Here, we report a study of the molecular-beam epitaxial (MBE) growth of HgCdTe on (100) and (111)B CdTe substrates. We show that the growth on the (100) orientation is easier to control than on the (111)B orientation, mainly because twinning, caused by excess Hg, is not observed. The structural quality, as probed by double-crystal x-ray rocking curve, is consistently of better quality for the (100) orientation. As-grown n- and p-type HgCdTe epilayers grown on both orientations exhibit high Hall mobilities. For the first time, p-type HgCdTe grown by MBE on the (100) orientation is demonstrated in this work. The HgCdTe layers grown on the (100) orientation have normal n- and p-type annealing behavior, but those grown on the (111)B behave abnormally, probably because of the presence of twins and stacking faults like defects created during growth. Improving MBE growth conditions could lead to a reduction in density of these defects on the (111)B plane.

Light-modulated Hall effect for extending characterization of semiconductor materials

A new technique for deriving the electron mobility as minority carriers in semiconductor materials is introduced. The technique is based on the modulation of the Hall voltage through the introduction of a low-level optical injection. The photogenerated excess carriers produce a signal which is a function of the magnetic field, the carrier mobilities and concentrations, and the concentration of excess carriers and thus to their lifetime. The measurement is easy to implement and it can be performed along with the measurement of the Hall coefficient and the conductivity. The only modification needed in a conventional galvanomagnetic setup is the introduction of a chopped low-intensity laser. Using these three simultaneous measurements, we determined the equilibrium carrier concentrations, the two mobilities, and the effective excess carrier lifetime of p-type HgCdTe narrow gap semiconductor. This novel technique allows us to present the mobility of electrons as minority-carrier throughout the temperature range of 13.6–300.2 K in this material.

Variable magnetic field Hall effect measurements and analyses of high purity, Hg vacancy (p-type) HgCdTe

This paper reports on complete Hall characterization of Hg vacancy ( p-type) HgCdTe with very high purity and compositional uniformity. The x-values of the Hg1−xCdxTe samples ranged from 0.180 to 0.300 with carrier concentrations of 6×1015 to 3×1017. Each sample was of very uniform composition with Δx&amp;lt;0.0015, as verified from room temperature infrared transmission measurements. This characterization couples variable magnetic field (up to 150 000 G) and temperature (from 15 to 160 K) data with an analysis procedure which includes heavy holes, light holes, and electrons. The measurements and analyses yield unambiguous determination of the two Hg vacancy ionization energies, one shallow level at 12–14 meV for x=0.216 HgCdTe and one deeper level at 40% of the band gap measured from the valence band edge. In addition, not only can the majority carrier (heavy holes) temperature dependent concentrations and mobilities be determined by this technique, but both the minority carrier (electron) and the light hole concentrations and mobilities have been determined as a function of temperature. At magnetic field strengths of 40 000 G and above, the heavy hole concentration was observed to decrease with increasing magnetic field. This occurred at all temperatures (15 to 160 K) investigated. This effect is commonly referred to as magnetic ‘‘freeze out’’ and is reported here for HgCdTe for the first time.

Characterization of impurities in p-type HgCdTe by photo-Hall techniques

Due to mixed conduction effects, conventional temperature-dependent Hall measurements do not yield an accurate characterization of doping levels in high-purity p-type HgCdTe (NA≲5×1015 cm−3). In the present work, we report a new and highly promising technique for determining compensation densities in p-type material by measuring the mobility of photoexcited electrons at low temperatures where most of the holes are frozen out and the total impurity concentration is equal to 2ND. As long as the photoexcitation is sufficiently strong that the electrons dominate the conductivity yet sufficiently weak that scattering by photoexcited holes is negligible, the compensation may be determined directly by fitting a comprehensive theory for transport in a photoexcited narrow gap semiconductor to the measured mobilities. Experimental photo-Hall results are reported for p-type Hg1−xCdxTe samples (0.215&amp;lt;x&amp;lt;0.23) with acceptor densities in the 1015 to 1016 cm−3 range. Photoexcitation is provided by a pulsed CO2 laser. At temperatures between 10 and 30 K, theory and experiment agree well over an extremely large range of photoexcited electron densities (1012–1016 cm−3). Donor and acceptor concentrations and acceptor binding energies have been determined for samples with both single (probably Cu) and double (probably Hg vacancies) acceptors. Since the method has its greatest sensitivity to ND at low excitation levels (n&amp;lt;1013 cm−3), it should be possible to perform routine characterizations using only a blackbody source. The model predicts that donor densities as low as 1014 cm−3 may be measurable.

Boron and indium ion-implanted junctions in HgCdTe grown on CdTe and CdTe/Al2O3

This paper discusses the results of junction formation in p-type HgCdTe material which has been converted into n type by ion implantation processes. In was chosen as the candidate donor impurity because it has been observed to diffuse from the implanted source and to activate electrically in a Hg-vacancy doped material under certain anneal conditions. Our results indicate that In diffuses and activates in both Hg-vacancy doped and As-doped HgCdTe layers in both capped and uncapped with saturated Hg overpressure regardless of the background defect structure of the layer. Several apparently different diffusion mechanisms appear in In profiles giving fast, anomalous high concentration components and slow components with low concentration levels. However, under high temperature (400 °C) capped anneals, a single limited source diffusion mechanism appears to dominate (Deff=8×10−13 cm2/s). This mechanism is attributed to vacancies present in the background. Characteristics of indium implantation into arsenic-doped layers were observed in which n-type electrical activity is indium related and controllable by implant/anneal conditions. p-type electrical activity is due to arsenic and it could be controlled by adjusting the arsenic doping level during epitaxy.