B.C. Easton

Substrate orientation effects in CdxHg1−xTe grown by MOVPE

Abstract Layers of Cd x Hg 1− x Te (CMT) have been grown by thermal metalorganic vapour phase epitaxy (MOVPE) at temperatures between 420 and 350°C on CdTe substrates of various orientation. The differences in surface morphology, crystalline quality and electrical behaviour between these layers are described. Surface hillocks on layers grown close to (100) are the main features of interest and their behaviour at the different growth temperatures and on the various orientations will be described and compared. In (111) layers twinned regions are observed and evidence from X-ray, electrical and chemical assessment is presented which suggests that these twins may act as donor sites. For layers grown on substrates oriented 18° off (111) no twins are seen and the surface features are less prominent.

Incorporation and activation of group V elements in MOVPE-grown CdxHg1-xTe

Abstract Reproducible acceptor doping has been achieved in MOVPE-grown layers of Cd x Hg 1- x Te using Group V elements. Dopant element concentrations have been controlled over the range 5×10 15 -8×10 17 cm -13 for As. Agreement has been obtained between these levels and Hall measurements made on annealed samples, at least for concentrations above 5×10 16 cm -3 . At lower levels residual impurities may exert a significant influence on the electrical behaviour of layers. In order to achieve dopant activation it has been found necessary to establish “metal-rich” conditions during the CdTe growth cycle. High temperature anneals to activate the dopant are thus avoided which permits the growth of heterolayers with sharpe interfaces, both in terms of x and dopant concentration.

Group V acceptor doping of CdxHg1−xTe layers grown by metal-organic vapour phase epitaxy

Abstract Layers of Cd x Hg 1− x Te have been grown by metal-organic vapour phase epitaxy and doped with arsenic and with phosphorus. This has been achieved by establishing metal-rich gas phase conditions during growth and obviates the need for high temperature “activation” type anneals. Data from Hall measurements and secondary ion mass spectrometry on annealed samples are in close agreement.

Electrical properties and annealing behaviour of CdxHg1−xTe grown by LPE and MOVPE

Hall measurements have been carried out on CdxHg1−xTe (with 0.2 < x < 0.4) grown by both Liquid Phase Epitaxy (LPE) and Metal-Organic Vapour Phase Epitaxy (MOVPE). The former is a sliding boat, tellurium-rich process while the latter utilises diethyltelluride, dimethylcadmium and elemental mercury to produce the alloy. As-grown the material is defect-controlled at between 2×1016−2×1018 p cm-3. Annealing experiments have been conducted in an open flow (N2) system with samples held at T1 (<300°C) and a mercury reservoir held at T2 (°C). Both isothermal (T1=T2) and two-temperature (T1T2) effects were studied. The results are compared to previous data obtained on bulk CdxHg1−xTe. In some cases the electrical properties of epitaxial material appear to change more slowly during an anneal than would be expected from the behaviour of bulk material. Conversion to n-type is observed after most treatments; the level is believed to be impurity limited, while other treatments yield lower p-type levels. A discussion of the results is given in terms of the P−T diagram for CdxHg1−xTe. Some preliminary results will be given for MOVPE material doped with an acceptor impurity.

Computer controlled deposition of cmt heterostructures by Movpe

Abstract A versatile reactor has been developed for the epitaxial deposition of multilayer CMT heterostructures, using the metal alkyls dimethyl cadmium and diethyl tellurium. The IMP (Interdiffused Multilayer Process) deposition technique is employed for achieving good compositional uniformity. Particular features of the reactor include a resistance heated susceptor and modified cadmium alkyl injection technique, this minimises the adverse effects of dead space in the mercury source region of the reactor tube. Gas flow switching is by computer controlled operation of solenoid valves, this is necessary for the satisfactory operation of the IMP technique, it also allows the programmed growth of a sequence of layers with different alloy compositions. Provision is made for the impurity doping of layers during growth, using gaseous or liquid sources. The capabilities of the growth system will be discussed in relation to the physical and chemical characteristics of doped and undoped epitaxial layer structures. Assessment techniques that have been used include SIMS, X-ray fluorescence, infra-red transmission measurements and scanning electron microscopy.

Growth and characterisation of CdxHg1-xTe grown by LPE using a novel sliding boat

A significant problem in the growth of CMT by Te-solution LPE is associated with melt residues on the layers. The paper describes the development and use of a sliding boat which tilts the substrate after growth to remove these droplets. The reproducibility of the process depends on control of Hg vapour pressure and the composition of the melts. A relationship between the x-value of the layers and the Cd and Hg content in the melts has been established. Variations in x are approximately +or-0.002, for x approximately 0.22, over 13*13 mm2 areas. Various chemical analysis techniques have been used to assess the purity of the layers. Electrical characterisation of the layers has shown them to be p-type as grown, as expected, and suitable annealing in Hg vapour converts them to n-type.

MOVPE growth and characterization of doped CdxHg1-xTe structures

This paper describes work carried out towards achieving extrinsically doped CdxHg1-xTe (CMT) heterostructures grown with stable dopants and sharp junctions. Both acceptor and donor doping of CMT has been achieved in our MOVPE growth reactor using the interdiffused multilayer process at approximately 400 degrees C with diethyltellurium (DET) as the tellurium alkyl source. The two dopants used were arsenic, introduced as AsH3, and iodine, as vapour from the solid element. The donor doping range has been extended by the use of a concentrically arranged double-injection-tube system which reduced the pre-reaction between the cadmium alkyl and the iodine vapour. Investigations have been carried out into the growth of various double-layer structures which incorporate either a single dopant transition or a double transition, as in a fully doped structure.

Band edge absorption in CdxHg1−xTe grown by metal-organic vapour phase epitaxy

The first absorption coefficient spectra determined in the region of the fundamental band edge of metal-organic vapour phase grown CdxHg1−xTe (CMT) with x between 0.2 and 0.3 are presented. Analysis of these spectra shows that the optical band edge correlates with αe, between 1000–2000 cm−1. Using these spectra the form of the absorbance curve of uniform CMT can be generated, which when compared with experimental data can be used to obtain a qualitative assessment of the depth uniformity of epitaxial layers.

Extrinsic doping at low concentrations for CDxHg1-xTe layers grown by MOVPE

The study of arsenic doping has resulted in the capability to control the acceptor level between 1×1016 cm-3 and 2×1017 cm-3 in CdxHg1-xTe grown by MOVPE. An understanding of the dopant incorporation mechanism and diffusion rate has enabled the growth parameters to be adjusted to ensure high (≈100%) electrical activity and homogeneous distribution of the dopant atoms. The acceptor ionization energy obtained from such layers was consistent with extrinsically doped material. Minority carrier lifetime data are also presented. Doped/undoped heterostructures have been produced which have demonstrated p-n junctions following Hg annealing. The high-x regions can behave as barriers to Hg in-diffusion if they cap the undoped region.

Temperature variations during MOVPE growth of CdxHg1 - xTe

Abstract Temperature variations have been found to occur during the metal-organic vapour phase epitaxy (MOVPE) growth of Cd x Hg 1 - x Te (CMT) using the interdiffused multilayer process (IMP). A temperature rise was observed during the CdTe growth cycle which then decayed during the HgTe cycle. Investigation into the causes of this effect indicated it was not due to flow switching during the cycle, but was dependent on the concentrations of the two alkyls. The implications of this are discussed particularly with respect to the possible use of temperature monitoring in a diagnostic capacity.