C.G. Scott

Characteristics of deep levels in n-type CdTe

Deep-level transient spectroscopy has been employed to study the defect states in n-type CdTe crystals subjected to a variety of annealing treatments. By comparing the results from crystals grown by the Bridgman and travelling heater methods (starting from a common source of CdTe) and by considering the variation in properties with position along each boule, it was concluded that the electrical properties were strongly influenced by residual impurities in these materials. Eleven different defect states were detected with activation energies ranging from 0.2 eV to 0.86 eV. One of these was found only in In-doped samples and at least three were related to residual impurities and could be removed by annealing in liquid cadmium. Several defects were interpreted as complex centres involving native defects and impurities.

PHOTOLUMINESCENCE AND P-TYPE CONDUCTIVITY IN CDTE:N GROWN BY MOLECULAR BEAM EPITAXY

An rf nitrogen (N) plasma source has been used to achieve p‐type conductivity in molecular beam epitaxy CdTe layers grown with a Cd overpressure. Photoluminescence and secondary ion mass spectrometry measurements have confirmed the incorporation of the N species, and evidence for the resulting p‐type conductivity has been obtained using capacitance‐voltage and current‐voltage techniques. Net hole concentrations as high as 2×1017 cm−3 have so far been achieved, which contrasts with the normally n‐type nature of our undoped CdTe layers.

DLTS investigation of some II–VI Compounds

Abstract Deep electron traps in n-type ZnSe, CdS and CdTe and a deep hole trap in p-type ZnTe have been investigated by Deep Level Transient Spectroscopy, and thermal activation energies, trap concentrations and capture cross sections have been determined. In Cd-fired n-CdTe three electron traps have been observed to show unusual properties in that the filling time constants are rather long and are very dependent on temperature. One of these traps is similar to the Cl Te −Cl i close pair centre already observed elsewhere. In ZnSe annealed in liquid Zn a prominent electron trap at E c −0.34 eV is tentatively assigned to a V Se defect which probably associates with a residual impurity to give a self-coactivated donor centre.

Electrical and structural assessment of CdTe and CdMnTe layers grown by MBE on InSb substrates

Abstract Single layers and multiple quantum wells (MQWs) of CdTe and CdMnTe have been grown by MBE on InSb substrates. These systems have been analysed using double crystal X-ray diffraction (DCXRD). The single layers have been shown to be of good structural quality while the MQW systems yield rocking curves with well resolved diffraction satellites (up to the seventh order) indicating good quality and periodicity. All undoped layers have been found to be n-type with free carrier concentrations in the range 10 14 -10 15 cm -3 but the densities for CdMnTe layers are generallyless than for CdTe layers grown under identical conditions. When doped with indium, both types of layers have exhibited a saturation in carrier concentration as the In atom concentration was increased beyond 10 18 cm -3 probably as a result of the generation of compensating acceptor states.

MBE growth of CdTe and Cd1−xMnxTe layers and multilayers on InSb substrates

Abstract Single and multiple layers of CdTe and CdMnTe, including MQW structures with well and barrier thicknesses down to 20 A, have been grown by MBE on (001) InSb. The broadening of the excitonic photoluminescence lines in single layers is consistent with current theories of statistical alloy fluctuations. For multiple thin layer structures the PL spectra show quantum confinement effects. The DCXRD rocking curve data show that both the single and multiple layers are of high structural quality.

Nitrogen doping of molecular beam epitaxially grown CdTe with a radio-frequency plasma source

Abstract We report on the use of an Oxford Applied Research MPD21 Atom/Radical RF source to dope CdTe layers with nitrogen (N). The layers were grown in a VG V80H MBE system, using CdTe as the source material. Photoluminescence (PL) measurements give evidence of N incorporation in our CdTe:N layers, as signified by an increase in intensity of the N acceptor bound exciton (ABE) line and the associated donor-acceptor pair (DAP) band, with increasing plasma RF power. The presence of N was confirmed using secondary ion mass spectrometry (SIMS). In addition, PL and SIMS results show the presence of significant levels of antimony (Sb) in the N-doped layers. The Sb acceptor depth was found to be 52 meV from observation of the associated free to bound transition. The unintentional presence of Sb is tentatively attributed to enhanced autodoping from the substrate, caused by the presence of N. Like the undoped layers, the N-doped samples are found to be n-type, but the carrier concentrations are 15 cm -3 .

Deep levels and associated carrier recombination processes in Zn-annealed ZnSe single crystals

The techniques of deep level transient spectroscopy (OLTS) and optically detected magnetic resonance (ODMR) have been used to study ZnSe single crystals subjected to extended heat treatment in molten zinc. The zinc-fired material has been found to contain a set of electron traps at 0.34 eV below the conduction band and to exhibit a broad luminescence emission band, peaking at 810 nm, which ODMR shows to be due to a donor-acceptor recombination process. An energy level scheme consistent with these observations is presented together with evidence to suggest that the donors, identified with the 0.34 eV traps, are due to complex centres involving residual impurities.

DLTS studies of deep traps in CdTe

The results are described of DLTS measurements in n-CdTe using as-grown, annealed, doped and undoped samples with free carrier concentrations ranging from 6*1014 cm-3 to 1017 cm-3. Nine separate electron traps were found with emission activation energies lying between 0.22 eV and 0.89 eV. Three of the traps exhibited strongly temperature-dependent filling rates and one of these three showed bi-exponential decay. The trap model proposed by Takabe et al. (1982) and based on a thermally activated capture cross section is found to be inadequate.

Evidence for bias dependent barrier heights in gold‐epitaxial CdTe Schottky diodes

The forward bias and reverse bias current‐voltage characteristics of some gold‐epitaxial CdTe‐on‐InSb Schottky diodes have been measured at room temperature. Series resistance is evident in undoped material (base electron concentration ∼1014 cm−3) which has been measured and corrected for and identified as arising from the reverse biased CdTe‐InSb junction. Two straight lines of differing slopes are present in the semilog plots for all the diodes, which we interpret as a bias‐dependent barrier height. The two barrier heights, 0.91±0.04 eV and 0.74±0.02 eV, are well known in the Au‐CdTe system, from which we conclude that two discrete states are present at the CdTe‐Au interface. The occupancy of the states is determined by the applied bias, and hence the barrier height changes.

Control of nitrogen incorporation in ZnTe:N grown by molecular‐beam epitaxy using Ar dilution in a N plasma source

The use of Ar dilution in a N plasma source has been used to achieve control of both electrical and optical properties of p‐type ZnTe:N grown by molecular‐beam epitaxy. Photoluminescence data are presented that show the transition from ‘‘pure’’ ZnTe emission to that indicative of heavily N‐doped ZnTe. A new principal bound‐exciton line associated with N impurities is observed at 2.3685 eV. An anomalous red shift in the corresponding donor‐acceptor pair peak energy with increasing N concentration is observed at high N concentration and is attributed to the effects of N impurity banding. Trends in p‐type conductivity confirmed the ability to control hole concentrations using Ar dilution.