H. Heinrich

Impurity and vacancy states in PbTe

We report measurements of the Hall effect and the transverse magnetoresistance of n‐ and p‐type PbTe with carrier concentrations as low as 1.8×1016 cm−3 in magnetic fields up to 15 T. The data rule out the existence of deep impurity levels and are, however, not in contradiction with the presence of resonant levels due to Pb or Te vacancies within the bands according to the model by Parada and Pratt.

Characterization and annealing behavior of deep levels in CdTe epitaxial layers

Deep defect levels play an important role in determining minority carrier lifetimes in CdTe photovoltaic devices. Much work has been done on the investigation of defects in bulk CdTe. In this paper we report on results obtained in CdTe thin epitaxial films, deposited on BaF2 and PbTe by hot‐wall epitaxy. Deep Level Transient Spectroscopy was used to measure the energy levels, the capture cross sections, and concentration profiles of the defect levels. The measurements were performed on PbTe/CdTe heterojunctions and Au/n‐CdTe Schottky barriers. Six levels were identified in the CdTe layers (E1 = 0.20 eV, E2 = 0.36 eV, E3 = 0.34 eV, E4 = 0.24 eV, E5 = 0.46 eV, E6 = 0.64 eV). By measuring the capture cross section separately an activation energy of 60 meV for the capture cross section of level E3 was found. The concentration of some of the defects could be reduced by annealing.

Evaluation of doping profiles in ion-implanted PbTe

From measurements of the sheet conductivity and the Hall effect in combination with layer stripping by chemical etching, we have evaluated the doping profiles of ion‐implanted PbTe. By using thin films grown epitaxially on insulating substrates, we avoided difficulties with the formation of an insulating p‐n junction. The carrier profiles of samples implanted in a random direction are deeper and broader than those of the LSS theory. The charge‐carrier profiles associated with the implantation damage show a constant value of the maximum concentration largely independent of the implantation dose and the ions used.

Band-edge offsets in PbSePbEuSe and PbTePbEuTeSe heterostructures deduced from electron-beam-induced current

Abstract Results for the valence-band-edge offset (ΔEv) are reported for PbSe/Pb1−xEuxSe (x = 0.013) and PbTe/Pb1−xEuxTeSe (x = 0.026) heterojunctions in the temperature range of 77 to 300 K. The electron-beam-induced current, generated at a forward biased junction, was used to determine the built-in voltage. Both systems show a strong temperature dependence of the band-edge offset. PbSe/PbEuSe: ΔEv (300 K) = 7 meV (Type I), ΔEv (150 K) = −79 meV (Type I′). PbTe/PbEuTeSe: ΔEv (300 K) = = 80 meV, ΔEv (150 K) = 35 meV. Although this appears to be the first reported case of such a behaviour, the strong temperature dependence of the band gaps in IV–VI compounds makes such a shift a plausible one. While the error margin for the absolute values is estimated to be ±30 meV, it is less for their shift with temperature.

PbTe-doping superlattices—A new type of high sensitivity infrared detector

Doping superlattices of PbTe are prepared by a modified Hot-Wall technique. At 80 K, some of these structures exhibit a photoconductive detectivity close to the theoretical limit ofD*≈1011 cmHz1/2W−1 at 6 μm wavelength. We show from time resolved electron-beam-induced-current investigations that the excellent photoresponse is caused by a lifetime enhancement by nearly two orders of magnitude. The latter value is in good agreement with theoretical estimates.The influence of contact barriers is discussed.

On a direct connection of the transition metal impurity levels to the band edge discontinuities in semiconductor heterojunctions

Our recent proposal of using the transition metal impurity levels to predict the isovalent heterojunction (HJ) band-edge discontinuities is further discussed. It is shown that for Ga1−xAlxAs/GaAs heterojunctions most of the recent discontinuity data follow within experimental error the prediction of the ΔEcb:ΔEvb= 0.64:0.36 discontinuity ratio derived from the Fe2+ level position in Ga1−xAlxAs compound. Predictions of valence-band discontinuities for the other III–V and II–VI HJ systems are also given.

Investigation of diffusion length and lifetime in lead chalcogenides by electron‐beam‐induced‐current measurements at low temperatures

The electron‐beam‐induced current (EBIC) mode of the scanning electron microscope has been used to study the temperature dependence of the minority‐carrier diffusion length in PbTe and other IV‐VI compounds (PbSnSe, PbSSe, PbCdS). The temperature dependence of the minority‐carrier lifetime for PbTe has been deduced from diffusion‐length measurements. Lifetime values of the order l×10−9–5×10−8 s have been found. Diffusion lengths and lifetimes as a function of carrier concentration have been studied. Discrepancies in the form of the EBIC signal have been found between PbTe diodes and ternary compound laser structures. A memory effect for PbSSe and PbCdS has been observed.

Linear and nonlinear optical properties of thin evaporated and epitaxial II–VI compound films

Abstract Thin CdS films were prepared with a conventional thermal evaporation technique. Post deposition annealing was carried out in the vacuum chamber to improve the optical properties. Characterizing the layers using absorption spectroscopy showed that the steepness of the absorption edge is improved by the annealing process. It results in values close to those of the bulk material. A grain structure could not be observed with an electron microscope. Furthermore the films show nonlinear behaviour and even photo-thermal optical bistability at reduced temperatures (250 K) at 514 nm. Coevaporation of a second semiconductor material, e.g. ZnS, shifts the band edge of the pure CdS film to the blue. First attempts to obtain optical bistability at room temperature and above with the 514 nm line of the Ar + laser look promising. Depositing electrical contacts onto the film, makes the layer into a SEED-device, operating with photo-thermal effects.

Photoluminescence of CdTe films grown on BaF2 with PbTe buffer layers by hot-wall epitaxy

Abstract Heteroepitaxial CdTe films are grown on PbTe 〈111〉/BaF 2 〈111〉 substrates at growth temperatures between 460 and 480°C by hot-wall epitaxy. The characterization of the CdTe films is carried out by X-ray analysis and photoluminescence (PL). The results suggest that the layers are of good crystalline quality but peaks associated with impurities are observed in the PL spectra. The optimum substrate temperature is about 470°C while the CdTe source temperature is 530°C. The optimum temperature for the additional cadmium reservoir is 290 °C.

Simultaneous diffusion of ion‐predeposited arsenic and boron in silicon

As and B were implanted into n‐type Si at 40 keV. The shallow layer was used as a source for simultaneous diffusion and formation of a n‐p‐n structure. The diffusion coefficients obtained were the same as those for the diffusion of the individual dopants. It is suggested that this technique may be of interest for the formation of complex profiles which are too deep to be obtained by implantation alone.