MIS hot electron devices for enhancement of surface reactivity by hot electrons

A Metal-Insulator-Semiconductor (MIS) based device is developed for investigation of hot electron enhanced chemistry. A model of the device is presented explaining the key concepts of the functionality and the character- istics. The MIS hot electron emitter is fabricated using cleanroom technology and the process sequence is described.

An Ultra High Vacuum (UHV) setup is modified to facilitate experiments with electron emission from the MIS hot electron emitters and hot electron chemistry. Simulations show the importance of keeping tunnel barrier roughness to an absolute minimum. The tunnel oxide is characterized using IV and CV measurements to extract tun- nel barrier thicknesses, which was distributed around the expected value of 50 ºA.

CV measurements yield thicknesses between 44.7 ºA and 58 ºA. The IV and CV measurements is shown to correlate and an o®set between the two types of measurements indicate some degree of roughness of the tunnel oxide. Electron emission is realized from the devices to a collector plate. The emission below 5 V varies between consecutive measurements, but is stable above 5 V.

The work function is lowered using Cs to 2 eV and emitted electrons are observed from a bias voltage of 2 eV. The maximum emission efficiency of 8% is obtained at 3 V on the Cs covered MIS hot electron emitter. The mean free path of Au for 5 eV electron extracted from emission experiments is 52 ºA, which is in excellent agreement with other measurements.

The Ti wetting layer is found to be an important energy loss center for the electrons tunneling through the oxide lowering the emission e±ciency of a factor of 10 for a 1 nm Ti layer thickness. Electron emission is observed under ambient pressure conditions and in up to 2 bars of Ar. 2 bar Ar decrease the emission current by an order of magnitude compared to emission in vacuum.

The emission current is observed to decrease exponentially with pressure. The energy dispersion of the emitted electrons is measured using a customized HemiSpherical Analyzer (HSA) setup. The emitted electrons are emitted in a narrow peak (FWHM 0.3-0.5 eV) moving up in energy proportional to the bias voltage.

A tail of scattered electrons extend from the main peak towards the work function edge of the emission spectra. The MIS hot electron iii iv emitter devices are heated using a direct current of 0.3 A through a 20 nm Pt gate metal layer and the temperature is monitored using the calibrated resistance of the metal layer.

The MIS hot electron emitters are cleaned in-situ in a background pressure of 3 £ 10¡7 mbar O2. Thermal desorption experiments with labeled CO are carried out with a reproducibility of 7%. The detection limit of labeled CO for the mass spectrometer setup is estimated to 3 £ 109 s¡1 from the desorption experiments.

The theoretical hot electron induced desorption rate is estimated to 2£104 s¡1.