Johan Hammersberg

High-voltage single-crystal diamond diodes

Demonstration of a 2.5-kV diamond diode is provided by electrical measurements using a circular gold Schottky contact, with an area >1 mm/sup 2/, on large area freestanding single-crystal diamond consisting of a thin high purity layer (<1/spl times/10/sup 13/ [B]/cm/sup 3/) on a thicker heavily boron-doped (>1/spl times/0/sup 19/ [B]/cm/sup 3/) substrate with an ohmic back contact. The diode structures were fabricated using a microwave-assisted chemical vapor deposition process. The forward properties of the diode show a space charge limited current, with a forward-voltage drop of 2 V and a hole mobility of 4100/spl plusmn/400 cm/sup 2//Vs at room temperature. For temperatures between 300 K<T<380 K the mobility show T/sup -3/2/ dependence. This is consistent with acoustic phonon scattering, emphasizing the high purity quality of the top layer in which carrier transport is phonon rather than defect limited.

Generation, transport and detection of valley-polarized electrons in diamond

Standard electronic devices encode bits of information by controlling the amount of electric charge in the circuits. Alternatively, it is possible to make devices that rely on other properties of electrons than their charge. For example, spintronic devices make use of the electron spin angular momentum as a carrier of information. A new concept is valleytronics in which information is encoded by the valley quantum number of the electron. The analogy between the valley and spin degrees of freedom also implies the possibility of valley-based quantum computing. In this Article, we demonstrate for the first time generation, transport (across macroscopic distances) and detection of valley-polarized electrons in bulk diamond with a relaxation time of 300 ns at 77 K. We anticipate that these results will form the basis for the development of integrated valleytronic devices.

Injection dependent long carrier lifetimes in high quality CVD diamond

Abstract In this paper we report an experimental study of photocurrent mobility×lifetime products and free carrier lifetimes in CVD grown polycrystalline diamond of various qualities. The investigated samples are low impurity samples, nitrogen content ∼10 15 cm −3 , with an average grain size ranging from 25 μm up to 110 μm. This large difference in average grain size makes it possible to distinguish effects due to lifetime limiting trapping and recombination defect centers inside the grains from effects caused by defect centers at grain boundaries. At low carrier densities, 13 cm −3 , the effective free carrier lifetime is in the sub-nanosecond to nanosecond range in all samples due to intra-grain trapping and recombination centers. At high carrier densities, >10 13 cm −3 , the intra-grain centers becomes saturated and the effective lifetime becomes predominately given by carrier diffusion to and recombination at the defects related to the grain boundaries. Hence, the effective lifetime at high carrier densities is strongly related to the average grain size and increases up to several tens of nanoseconds, in samples with a large average grain size, whereas it remains in the nanosecond range for samples with small average grain size. In addition, we observe a lower mobility×lifetime product and decay constant with increasing nitrogen content, clearly showing the negative influence of nitrogen and nitrogen-related defects on these important material parameters.

Hole transport in single crystal synthetic diamond at low temperatures

Investigating the effects of local scattering mechanisms is of great importance to understand charge transport in semiconductors. This article reports measurements of the hole transport properties of boron-doped (100) single-crystalline chemical vapor deposited diamond. A Time-of-Flight measurement using a 213 nm, pulsed UV laser for excitation, was performed on high-purity single-crystalline diamonds to measure the hole drift velocity in the low-injection regime. The measurements were carried out in the temperature range 10-80 K. The results obtained are directly applicable to low-temperature detector applications. By comparing our data to Monte-Carlo simulations, a detailed understanding of the dominating hole scattering mechanisms is obtained.

Stability of polarized states for diamond valleytronics

The stability of valley polarized electron states is crucial for the development of valleytronics. A long relaxation time of the valley polarization is required to enable operations to be performed on the polarized states. Here, we investigate the stability of valley polarized states in diamond, expressed as relaxation time. We have found that the stability of the states can be extremely long when we consider the electron-phonon scattering processes allowed by symmetry considerations. We determine electron-phonon coupling constants by Time-of-Flight measurements and Monte Carlo simulations and use these data to map out the relaxation time temperature dependency. The relaxation time for diamond can be microseconds or longer below 100 K and 100 V/cm due to the strong covalent bond, which is highly encouraging for future use in valleytronic applications.