Johan F. Prins

Bipolar transistor action in ion implanted diamond

Using a natural type IIb diamond, which is p‐type semiconducting, as a substrate material, and implanting this diamond with carbon ions to induce n‐type regions, diode and bipolar transistor action could be obtained. Owing to the geometrical positions of the collector, base, and emitter, which in this case were lateral to the surface, very low current gain was obtained. However, the results indicate that an improvement in the geometry will probably lead to a diamond transistor of high gain.

Preparation of ohmic contacts to semiconducting diamond

Low resistance ohmic contacts were manufactured on a natural semiconducting diamond (type IIb), by implanting boron ions at a temperature of 200 degrees C. This was followed by annealing at 1200 degrees C and removal of the top ion-damaged layer by etching in a hot acids solution. The thin, highly doped surface which remained could then be contacted with metals which need not interact chemically with the diamond surface.

Ion-implanted n-type diamond: electrical evidence

Abstract High-purity (natural type IIa) diamonds were treated by the cold implantation rapid annealing (CIRA) process using extremely low dose (about 10 4 times lower than usual), and damage-equivalent, carbon, boron and phosphorus ion implantations. The electrical properties of the carbon implanted (C + -CIRA treated) diamond did not change significantly from its virgin condition. In contrast, both the B + -CIRA and P + -CIRA diamonds showed different electrical behaviour, which could thus only be ascribed to the presence of the boron and phosphorus atoms respectively. Thermoelectric (hot probe) measurements showed the B + -CIRA diamond layer to be p-type (as expected) and the P + -CIRA layer to be n-type. Difficulties were experienced in effecting ohmic contacts to the P + -CIRA diamond, and the results indicate that for the latter treatment to lead to semiconductor-quality material, the diamond substrate must be substantially free from strain and contain a low density of intrinsic defects.

Ion implantation and diamond: some recent results on growth and doping

Abstract Results on the growth and doping of diamond by means of ion implantation are reported. Growth was effected by high energy carbon-ion implantation into copper, inducing diffusion of the implanted atoms to the copper surface. Transmission electron microscopy analysis showed that, although oriented graphite was generated most of the time, diffraction from carbonaceous grains, ascribable to diamond growth, also occured. Improved doping of natural high purity diamond (type IIa) using the cold-implantation rapid-annealing (CIRA) technique was obtained by implanting boron to a suitable damage and range distribution (while maintaining the substrate at liquid nitrogen temperature) followed by a multistep, rapid annealing cycle. Within an order of magnitude, the doped diamond layer behaved electrically like natural p-type semiconducting (type IIb) diamond. Using a gold-plated contact probe, reasonable diode characteristics could be measured after annealing to 1400 °C.

A determination of the critical damage density required for “amorphisation” of ion-implanted diamond

Abstract A series of damaged layers were generated in natural type IIa diamond by implanting iso-chemical carbon ions over a range of doses (0.5 to 7 × 1015 ions cm 2 ) while maintaining the diamond at liquid nitrogen temperatures. RBS-based ion beam channelling using 1.2 MeV He+ ions was used to monitor the buildup of damage in the three major channelling directions (〈110〉, 〈111〉 and 〈100〉, as a function of dose and subsequent isochronal annealing steps. As diamond is a metastable form of carbon, it is possible to create enough lattice damage by ion implantation to initiate a thermally stable phase change. The structure of the layer generated in this way has been termed “amorphous” in the literature, and the object of this study was to illucidate the mechanisms involved for its formation. Accordingly the implantation doses were chosen so as to bracket the critical ion dose ( ≈ 5.2 × 1015 cm−2) needed for this transformation to occur. For ion doses above this value, the phase change was found to be “triggered” at a suitable annealing temperature: the higher the ion dose, the lower the annealing temperature to effect the structural change. Evidence has been found to indicate that this “amorphisation” process is driven, at least partly, by the strain in the damaged lattice.

Depth profiling of implanted 13C in diamond as a function of implantation temperature

Abstract 13 C ions were implanted into natural type IIa diamond at temperatures ranging between that of liquid nitrogen and 670 K. Previous results [1] show that in the temperature range 320–670 K only the interstitials created in the damage cascade are mobile, the vacancies starting to diffuse only above these temperatures. The nuclear reaction 13 C(p, γ) 14 N was used to determine the depth distribution of the implanted 13 C atoms. Changes in the depth profiles as a function of implantation temperature reflected changes in mobility and trapping of the defects. All the depth profiles of diamonds implanted at temperatures greater than or equal to ambient temperature displayed a bimodal distribution consisting of the deeper than expected internal peak, and an overlapping surface peak. The integrated profile areas decreased with increasing implant temperature from 390 to 670 K. None of the sub-ambient implant temperature profiles showed a surface peak, but implantation done at 80 K displayed a strong tail extending deep into the diamond.

Improved activation of boron-dopant atoms implanted into diamond

Abstract Boron-dopant activation in diamond may be obtained by means of low-temperature ion implantation, which immobilizes the vacancies and interstitials formed in the collision cascades, followed by a suitable annealing cycle. In the first experiments demonstrating the validity of this process an initial annealing temperature of 500 ° C was used, which allowed the interstitials to diffuse but not the vacancies. In this study a vertical, argon-atmosphere furnace was constructed which could be used to heat the implanted diamonds rapidly to 1200° C. A dramatic improvement in dopant activation was obtained. Similar semiconducting properties could be obtained for boron ion doses which were less by a factor of 20 than previously needed.

Residual defect control when doping thin layers in diamond

Abstract Doping of a diamond layer by boron ion implantation has been conclusively demonstrated and reported in the literature. To reach this objective, implantation is done at low target temperatures (typically liquid nitrogen) followed by rapid annealing; the so-called CIRA method. The object of the exercise is to activate the implanted atoms by dopant atom-vacancy combinations, and to decrease the residual damage by means of self-interstitial-vacancy recombinations. This paper is part of an ongoing study on the effects that the implantation-annealing parameters have on the doping process. Is is known that in order to improve the activation ratio, the initial implanted radiation damage has to be increased. However, this causes a higher residual damage after annealing. It is therefore proposed and theoretical arguments as well as preliminary experimental data are advanced to show that another route to control the CIRA doping process may be achieved by multiple, low-dose, implantation-annealing sequences.

Annealing effects when activating dopant atoms in ion-implanted diamond layers

Abstract Activation of boron-dopant atoms implanted into diamonds has been studied using the cold-implantation rapid-annealing technique. Measurements of the sheet resistances generated, indicate that self-interstitial-vacancy recombination improves with increasing annealing temperature, while boron-atom-vacancy combination is more effective at lower temperatures. For the implantation conditions and heating cycle employed, this behaviour results in an optimum annealing temperature at which the uncompensated boron acceptors reach a maximum density.

The redistribution of implanted atoms and radiation damage during the implantation doping of diamond

Abstract The dopant atoms 7 Li and 11 B were implanted into natural type Ia and type IIa diamond at temperatures ranging between 90 and 670 K. Nuclear reaction analysis was used to determine the depth distributions of the implanted dopants using the 165 keV 11 B(p, γ) 12 C resonance and the 441 keV 7 Li(p, γ) 8 Be resonance. Implants performed at 90 K were annealed at 360 K for 30 min and were then reprofiled to study the annealing characteristics of the dopants. In order to achieve optimum doping of diamond it is of importance to understand the damage processes that occur during low temperature (∼ 90 K) ion implantation, and their annealing at elevated temperatures. In order to do this, 13 C ions were implanted into natural type IIa diamond held at an implantation temperature of ∼ 90 K and profiled using the 550 keV 13 C(p, γ) N resonance, together with RBS channelling measurements made along the 〈 100 〉, 〈 110 〉, and 〈 111 〉 axes. This implant was then annealed at 670 K for 30 min and re-examined. The results indicate the presence of two defect regions in the implanted region: a near-surface region (extending to ∼ 0.25 μm) containing highly mobile single defects, and a deeper region (extending beyond the calculated ion range to ∼ 1.5 μm) containing more stable extended defects. The channelling results also show evidence for the possible hydrogenation of the damaged region.