V. Richter

Damage threshold for ion‐beam induced graphitization of diamond

The critical dose for graphitization of diamond as a result of ion implantation induced damage (boron and arsenic) and subsequent thermal annealing is determined by combining secondary ion mass spectroscopy measurements, chemical etching of the graphitized layer, and TRIM simulations. Li ions are implanted as a deep marker to accurately determine the position of the graphite/diamond interface. The damage density threshold, beyond which graphitization occurs upon annealing, is found to be 1022 vacancies/cm3. This value is checked against published data and is shown to be of general nature, independent of ion species or implantation energy.

Effects of heavy ion irradiation on amorphous hydrogenated (diamondlike) carbon films

Energetic ion‐beam irradiation of diamondlike carbon thin films induces a decrease of 5–6 orders of magnitude in the resistivity of the films from their as‐grown value of 107 Ω cm. Diagnostics used to characterize the nature of the transformation included optical absorption measurements (band gap), Raman spectroscopy (microcrystalline structure), Rutherford backscattering spectroscopy (hydrogen content of the films), and measurements of the temperature dependence of the conductivity (electronic structure). It was found that the initial decrease in resistivity with an increasing ion dose is due to the loss of hydrogen from the films, with a concomitant decrease in the optical band gap. Heating the films during the irradiation greatly accelerates the rate of hydrogen loss as a function of dose. Once the hydrogen removal is complete, further irradiation increases the film conductivity by inducing the growth of microcrystallites of graphite.

p type doping of zinc oxide by arsenic ion implantation

p type doping of polycrystalline ZnO thin films, by implantation of arsenic ions, is demonstrated. The approach consisted of carrying out the implantations at liquid-nitrogen temperature (∼−196°C), followed by a rapid in situ heating of the sample, at 560°C for 10min, and ex situ annealing at 900°C for 45min in flowing oxygen. p type conductivity with a hole concentration of 2.5×1013cm−2 was obtained using this approach, following implantation of 150keV 5×1014As∕cm2. A conventional room-temperature implantation of 1×1015As∕cm2, followed by the same ex situ annealing, resulted in n type conductivity with a carrier concentration of 1.7×1012cm−2.

Enhanced generation of single optically active spins in diamond by ion implantation

The nitrogen-vacancy (NV) centers in diamond are amongst the most promising candidates for quantum information applications. Up to now the creation of such defects was highly probabilistic, requiring many copies of the nanodevice. Here we show that by employing a two step implantation process which includes low dose N2+ molecular ion implantations followed by high dose C implantation can increase the generation efficiency of NV centers by over 50%. Moreover, we detected intrinsic N14 concentration as low as 0.07 ppb by converting the nitrogen impurities into NV and then counting the single centers by using a confocal microscope.

Increasing the coherence time of single electron spins in diamond by high temperature annealing

Negatively charged nitrogen-vacancy (NV−) centers in diamond produced by ion implantation often show properties different from NVs created during the crystal growth. We observe that NVs created from nitrogen ion implantation at 30–300 keV show much shorter electron spin coherence time T2 as compared to the “natural” NVs and about 20% of them show switching from NV− to NV0. We show that annealing the diamond at T=1200 °C substantially increases T2 and at the same time the fraction of NVs converting from NV− to NV0 is greatly reduced.

ELECTRICAL ISOLATION OF GAN BY ION IMPLANTATION DAMAGE : EXPERIMENT AND MODEL

Electrical and optical isolation of unintentionally doped GaN layers due to the damage created by H+ and He+ ions passing through the layer are demonstrated. As a result of the irradiation, the sample resistance increases by 11 orders of magnitude and the band-to-band photoluminescence (PL) emission is totally quenched. Following annealing (1000 °C, 30 s), the conductivity can be nearly completely recovered, whereas only partial recovery of the PL emission is obtained. A model is proposed which invokes the presence of potential barriers for electronic transport across extended defects as the major factor limiting carrier mobility. Radiation defects increase these barriers, thus affecting the sample resistivity. This model fits the experimental results for both H and He induced damage extremely well and thus proves that defects created by nuclear collisions of the ions traversing the layer are responsible for the observed effects.

Ion beam induced conductivity and structural changes in diamondlike carbon coatings

Energetic ion beam irradiation of diamondlike carbon thin films induces a decrease of 4–5 orders of magnitude in the resistivity of the films from their as‐grown value of 107 Ω cm. The initial decrease in resistivity with increasing ion dose is due to the loss of hydrogen from the films with a concomitant decrease in the optical band gap. Heating the films during the irradiation greatly accelerates the rate of hydrogen loss as a function of dose. Once the hydrogen removal is complete, further irradiation increases the film conductivity by inducing the growth of microcrystallites of graphite.

Loss of electrical conductivity in boron-doped diamond due to ion-induced damage

The response of B-ion-implanted type-IIa diamond to light ion (H, He) irradiation is investigated by monitoring the sample resistance as a function of dose. It is found that the resistivity of the layer increases rapidly with increasing dose, and reaches the resistivity of the undoped diamond for irradiation doses much less than those required for the onset of damage related electrical conductivity in pristine diamond. It is shown that defects created by the nuclear stopping process act as compensating centers for the B acceptors. The present findings are of importance for the design of radiation hard diamond based electronic devices and suggests a method for the isolation of B-doped devices on a diamond chip. The results of the present work also explain why the collection distance in intrinsic nondoped diamond radiation detectors actually increases with increasing ion dose.

Diamond processing by focused ion beam—surface damage and recovery

The nitrogen vacancy color center (NV−) in diamond is of great interest for photonic applications. Diamond nano-photonic structures are often implemented using focused-ion-beam (FIB) processing, leaving a damaged surface which has a detrimental effect on the color center luminescence. The FIB processing effect on single crystal diamond surfaces and their photonic properties is studied by time of flight secondary ion mass spectrometry and photoluminescence. Exposing the processed surface to hydrogen plasma, followed by chemical etching, drastically decreases implanted Ga concentration, resulting in a recovery of the NV− photo-emission and in a significant increase of the NV−/NV0 ratio.

Diffusion of hydrogen in undoped, p-type and n-type doped diamonds

Hydrogen is a key impurity in diamond since it is unintentionally incorporated in all chemical vapor deposition (CVD) grown diamond layers.Its presence in the material can grossly affect its electrical and optical properties.Theoretically, hydrogen has been predicted to be present in diamond in one of the three charge states, H , H and H .Moreover it may form complexes q 0 y with impurities, native defects or with other hydrogen atoms.This paper is comprised of two parts: (a) a review of previous results of studies investigating different aspects of the diffusion of hydrogen (deuterium) in various kinds of diamonds.The diamonds studied are: undoped type IIa diamonds, undoped CVD diamond layers containing growth defects only, p-type B-doped homoepitaxially CVD grown diamond layers or B ion implanted type IIa diamonds and n-type P doped homoepitaxially CVD grown diamond or N-doped type Ib natural diamonds.Hydrogen is introduced in diamond by exposing the diamond surface to hydrogen plasma or by using hydrogen ion implantation.The following issues are discussed: (1) the influence of the interaction between H and the dopants and defects on the hydrogen diffusion. (2) The kinetic of (B, H), (P, H ) and (N, H) pair formation and dissociation. (3) The modification of the optical and electrical properties as a result of hydrogen incorporation and annealing. It is found that, under certain conditions, H diffuses into the B containing layer and it passivates B acceptors.In contrast, no H diffusion could be observed in n-type diamonds, up to 1000 8C. (b) Recent results of our group regarding other aspects related to the diffusion of H in diamond are presented.These include results on: (i) the influence of ion implantation related defects on the diffusion of deuterium.For this study type IIa samples implanted with B or non-dopant ions are used. (ii) The determination of the charge state of H or Hydefects complex as a function of diamond type.For that, annealing under bias is applied to deuterated diamond layers.We show that the presence of implantation defects retards the deuterium diffusion in a B-implantation doped diamond, demonstrating that D strongly interacts with defects, thus inhibiting diffusion.The new-formed complexes deteriorate the electrical properties of the diamonds and are very stable up to high temperatures.We confirm that, as expected, in highly B-doped CVD diamond layers, H diffuses as a positive ion.In lightly B-doped homoepitaxial layers, however, D is incorporated in complexes which seem to be negatively charged. � 2003 Elsevier Science B.V. All rights reserved.