Mass removed from single crystal silicon samples by high irradiance (1×109 to 1×1011 W/cm2) single pulse laser ablation was studied by measuring the resulting crater morphology with a white light interferometric microscope. The craters show a strong nonlinear change in both the volume and depth when the laser irradiance is less than or greater than ≈2.2×1010 W/cm2. Time-resolved shadowgraph images of the ablated silicon plume were obtained over this irradiance range. The images show that the increase in crater volume and depth at the threshold of 2.2×1010 W/cm2 is accompanied by large size droplets leaving the silicon surface, with a time delay ∼300 ns. A numerical model was used to estimate the thickness of the layer heated to approximately the critical temperature. The model includes transformation of liquid metal into liquid dielectric near the critical state (i.e., induced transparency). In this case, the estimated thickness of the superheated layer at a delay time of 200–300 ns shows a close agreemen...

Explosive change in crater properties during high power nanosecond laser ablation of silicon

A comprehensive theory developed previously for optical heating in semiconductors is applied to the calculation of laser damage thresholds in Ge, Si, InSb, and GaAs. The calculated thresholds agree well with the experimental values over a broad range of laser wavelengths and pulse durations. The results demonstrate that the dynamic nature of the material’s optical and transport properties with changing temperature and laser‐generated carrier density has a significant effect on the heating process, particularly at short pulse durations.

Optical heating in semiconductors: Laser damage in Ge, Si, InSb, and GaAs

The craters resulting from high-irradiance (1×109–1×1011 W/cm2) single-pulse laser ablation of single-crystal silicon show a dramatic increase in volume at a threshold irradiance of 2.2×1010 W/CM2. Time-resolved shadowgraph images show ejection of large particulates from the sample above this threshold irradiance, with a time delay ∼300 ns. A numerical model was used to estimate the thickness of a superheated layer near the critical state. Considering the transformation of liquid metal into liquid dielectric near the critical state (i.e., induced transparency), the calculated thickness of the superheated layer at a delay time of 200–300 ns agreed with the measured crater depths. This agreement suggests that induced transparency promotes the formation of a deep superheated layer, and explosive boiling within this layer leads to particulate ejection from the sample.

Evidence for phase-explosion and generation of large particles during high power nanosecond laser ablation of silicon

Starting with highly purified feed material, single crystals of semiinsulating CdTe were grown by the use of the traveling heater method, without external chemical compensation. Evaluation of the crystals by the use of electrical and optical measurements showed decisive improvements in quality. Nuclear radiation detectors prepared by the use of this material resulted in an energy resolution (FWHM), at room temperature, of 4.6 and 8.5 keV for γ rays at energies of 122 and 662 keV, respectively. The energy resolution at 122 keV decreased to 3.8 keV at a temperature of −30°C. No polarization effect was observed.

Undoped high‐resistivity cadmium telluride for nuclear radiation detectors

Nuclear radiation detectors made from high resistivity chlorine doped cadmium telluride grown by the travelling heater method were evaluated. Short term performance for ?, s, and ? radiations was good but the long term performance (>1 min) was degraded by a decrease in the full energy pulse height and ?-ray efficiency with time after the bias voltage was applied. A model which considers the effects of deep acceptors which slowly ionize (~103 s) to an equilibrium concentration ~5 × 1011/cm3 can explain the experimental findings on the changes of pulse shape, pulse amplitude, capacitance, energy resolution, and ?-ray efficiency with time. The operating conditions required for satisfactory stability in these detectors may be inconvenient to the user.

Polarization Phenomena in CdTe Nuclear Radiation Detectors

By solving the time-dependent heat flow equation, the temperature reached by silicon and cadmium telluride surface layers under high power density ruby laser pulsed illumination, is calculated. The results are presented in directly useful figures allowing the determination of the surface temperature, its evolution towards the bulk as a function of time... In particular, it should be noticed that for a 25 ns half-power width, pulses of 0.8J/cm2 are sufficient to melt the top of an amorphous silicon layer, this value becomes noticeably lower for cadmium telluride.

Calculated temperature distribution during laser annealing in silicon and cadmium telluride

Full-energy peak efficiency of cadmium-telluride gamma-ray spectrometers

An investigation is made of the properties of the contact CdTe–InSb realized by deposition of InSb on the etched surface of n-type CdTe single crystals and annealing. The profile of the elemental composition of the contact obtained by secondary ion mass spectrometry (SIMS) shows that certain interfacial chemical reactions do occur. The results obtained from the measurements of the photoresponse and forward characteristics versus temperature shows that near room temperature, the contact behaves like a „real” Schottky barrier. At low temperature a tunneling transport mechanism appears.



Une investigation des proprietes du contact CdTe–InSb realise par deposition de InSb sur la surface etchee de CdTe de type n monocristallin suivie d'un recuit a ete faite. Le profil des elements composants du contact obtenu pas spectrometrie de masse d'ions secondaires (SIMS) montre qu'il se produit certaines reactions chimiques interfaciales. Les resultats obtenus des mesures de la photoreponse et des caracteristiques directes en fonction de la temperature montrent que, au voisinage de la temperature ambiante, le contact se comporte comme une barriere Schottky „reelle”. A basse temperature un mecanisme de transport par tunneling apparaǐt.

R. O. Bell

Papers

Calculated temperature distribution during laser annealing in silicon and cadmium telluride

Full-energy peak efficiency of cadmium-telluride gamma-ray spectrometers

CdTe–InSb heterojunctions

Determination of carbon in EFG silicon ribbons by nuclear techniques and SIMS