J. B. Clegg

Excimer-laser-annealed poly-Si thin-film transistors

The crystallization of alpha -Si:H into poly-Si using an excimer laser has been examined. The resulting microstructure was found to be stratified into a large-grain surface region, formed from the liquid phase, and a fine-grain underlying layer, thought to be formed by solid phase crystallization. The threshold beam energies for these sequential phase changes were identified from surface reflectance measurements after crystallization and the energies increased with diminishing hydrogen content of the material. The electrical characteristics of thin-film transistors made with material crystallized at energies close to the melt threshold could be correlated with the limited depth of large-grain material. For significantly higher beam energies, coplanar structures showed a severe degradation in leakage current due to lateral diffusion of phosphorus, across the channel from the source and drain regions. When this effect was avoided, thin-film transistors with field-effect mobilities up to 160 cm/sup 2//V-s and on/off current ratios up to 10/sup 8/ were obtained. >

Migration of Si in δ-doped GaAs

Si atomic plane or delta ( delta )-doping of GaAs during MBE has been investigated using SIMS profiling optimised for high depth resolution. For layers in which almost all the Si atoms act as donors, post-growth diffusion occurs at the growth temperature with a diffusion coefficient estimated to be 9*10-17 cm2 s-1 at a substrate temperature of 550 degrees C. At temperatures greater than about 550 degrees C, a marked preferential migration towards the surface is observed, which may be due to surface segregation or possibly enhanced diffusion.

Delta-doping of GaAs and Al0.33Ga0.67As with Sn, Si and Be: a comparative study

Abstract We have performed a study of the contributions of segregation, diffusion and aggregation to the broadening of delta-doped planes of Sn, Si and Be in GaAs and Al 0.33 Ga 0.67 As. Sn planes are severely broadened by all three processes, but sharp spikes of Si and Be can be obtained in both host materials for sheet densities below 10 13 cm −2 and growth temperatures of 500°C or less. At higher temperatures or densities, segregation or concentration-dependent rapid diffusion may occur, causing significant spreading even during growth. Co-deposition of Si and Be dramatically reduces this broadening, and various mechanisms are considered to explain these effects.

Post-growth diffusion of Si in delta -doped GaAs grown by MBE

A GaAs layer grown by MBE at a substrate temperature of 520 degrees C and containing three delta -doped planes with Si concentrations of 0.4, 1 and 4*1013 atoms/cm2 has been post-growth annealed in a furnace, up to temperatures of 648 degrees C. Secondary ion mass spectroscopy (SIMS) and capacitance voltage (cv) measurements have been carried out to measure profile-broadening. The most lightly doped plane gave a near-Gaussian diffusion profile with a diffusion coefficient comparable with literature values for simple diffusion of isolated SiGa atoms. The more heavily doped planes exhibit a complex profile shape with two components, a proportion of the atoms being confined to the original plane, together with an almost square-shaped profile of fast-diffusing atoms. Comparison of the CV and SIMS data suggests that formation of Si islands is taking place during deposition of the delta -doped plane, giving electrically inactive atoms which can subsequently diffuse into the surrounding GaAs during heat-treatment. This model is supported by preliminary local vibrational mode measurements which have been made on a set of multiplane samples.

Doping studies in MOVPE-grown CdxHg1-xTe

In two recent review papers on metal organic vapour phase epitaxy (MOVPE) of cadmium mercury telluride (CMT) particular emphasis was placed on the crucial importance of doping studies to the realization of future device structures Irvine et al. (1991) and Tribowet (1991). If the full potential of MOVPE growth of CMT is to be realized then extrinsic doping of heterostructures is required. If the doping and composition junctions can be grown with the correct degree of grading then this will create the potential for the production of device structures leading to either improved performance and/or increased operating temperatures. This papers reviews published doping studies and also presents some recent results on both acceptor and donor doping studies carried out by the authors. In the latter studies, interdiffused multilayer process (IMP) growth of CMT has been performed at approximately=360 degrees C using dimethyl cadmium (DMC) and di-isopropyl tellurium (DIPT) as the MO precursors while the Hg overpressure was provided by a heated elemental source. Alternative acceptor doping sources to arsine have been investigated including phosphine, triphenyl arsenic, and phenyl arsenic of which the latter appears to be most suitable. Iodine has continued to show the donor dopant potential in CMT that it exhibited with higher-temperature ( approximately=400 degrees C) MOVPE growth using di-ethyl tellurium (DET). Characterization of fully doped structures is described.

Concentrations of carbon and oxygen in indium phosphide and gallium arsenide crystals grown by the lec technique

The concentration of carbon and oxygen has been measured in various samples of LEC–grown InP and GaAs crystals using gamma photon activation analysis and mass spectrometry. The concentration of these elements was found to be extremely low (0.1–0.3 ppma) contrary to previouslyreported work.

Electrically active defects in shallow pre-amorphisedp + n junctions in silicon

An examination of shallow pre-amorphisedp+n junctions in silicon has revealed three distinct defect related phenomena determined largely by the annealing temperature and relative location of the junction and the amorphous-crystalline (α-c) boundary. For temperatures below 800‡ C all samples displayed leakage currents of ∼10−3 A/cm2 irrespective of the amorphising atom (Si+, Ge+ or Sn+). The generation centres responsible were identified to be near mid-gap deep level donors lying beyond the α-c interface. For samples annealed above 800‡ C, the leakage current was determined by the interstitial dislocation loops at the α-c boundary. If these were deeper than the junction, a leakage current density of ∼10−5 A/cm2 resulted. From the growth of these loops during furnace annealing it was concluded that the growth was supported by the influx of recoil implanted silicon interstitials initially positioned beyond the α-c boundary. In the case where the as-implanted junction was deeper than the α-c boundary, annealing above 800° C resulted in a transient enhancement in the boron diffusion coefficient. As with the dislocation loop growth, this was attributed to the presence of the recoil implanted silicon interstitials.

Si migration effects in GaAs/(Al,Ga)As heterojunction and δ-doped structures

Abstract In order to investigate the Si migration mechanisms thought to be responsible for the anomalous characteristics of a number of heterojunction devices, high depth-resolution SIMS profiling has been used to study these effects in δ-doped and uniformly-doped GaAs samples. For growth at or above ≈ 500°C, surface segregation occurs during the deposition phase, followed by a post-growth diffusion process; the rate of diffusion differs in different Si concentration regimes.

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.

Analysis of cadmium mercury telluride and related materials

Abstract The roles of various analytical techniques for both matrix element composition and impurity content of cadmium mercury telluride and related materials are reviewed from the viewpoint of a user. The main techniques considered are infrared (IR) transmission, atomic absorption spectrometry (AAS), X-ray flourescence (XRF), electrolyte electroreflectance (EER), Rutherford backscattering (RBS), spark source mass spectrometry (SSMS), laser scan mass spectrometry (LSMS) and secondary ion mass spectrometry (SIMS). The role of these techniques in the analysis of bulk materials, epitaxial layers and device structures is considered. It is shown that whilst SIMS can uniquely provide analytical information with excellent depth and lateral resolution, some compositional and purity information can also be achieved more cost-effectively using developments of what may be considered more “traditional” techniques. Laser scan mass spectrometry is shown to be an interesting alternative to SIMS for high sensitivity general analyses when depth and spatial distributions are not required.