Time-Resolved Crystallization of GeTe
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TIME-RESOLVED CRYSTALLIZATION OF GeTe
E. Huber and E. E. Marinero IBM Almaden Research Center (K67/802) San Jose, California 95120
ABSTRACT
Amorphous thin films of GeTe were irradiated with excimer laser pulses and the subsequent crystallization was investigated utilizing simultaneous transient reflectivity and conductivity measurements. Below a threshold fluence of 15 mJ/cm 2 pure thermal behaviour was found. Above that value, nucleation and growth are observed during the cooldown process. Above a fluence of 22 mJ/cm 2 the films crystallize to a large degree within 200ns. Between 15 and 22 mJ/cm 2 crystallite nuclei are formed ("frustrated crystallization"), and application of a subsequent pulse over areas exposed to this fluence regime leads to extremely fast crystallization (50 ns).
INTRODUCTION Te alloys are currently of interest for reversible optical storage due to their structural dependent optical properties. Data can be recorded by quenching amorphous spots in the material with short laser pulses and erased by recrystallization with long laser pulses. For read/write memory application fast crystallization time (lyr) are necessary [1]. Amorphous materials which perform partitionless crystallization at high crystallization temperatures meet these requirements. The purpose of this paper is to study the transient changes of the electronic properties of amorphous GeTe films irradiated with 12ns laser pulses. From this data, information on the cooling, nucleation and growth rates may be obtained. A detailed analysis will be published elsewhere [2].
EXPERIMENTAL PROCEDURE
Fig. 1 shows the experimental setup used for the transient reflectivity and conductivity measurements. Amorphous 200nm films of GeTe were prepared by vapor deposition onto glass substrates [3,4]. The annealing laser is a KrF excimer-laser (248nm) providing 12ns pulses of fluences up to 70 mJ/cm 2 . In the experiment the incident laser fluence is varied by means of a variable attenuator and is monitored by means of a pyroelectric detector and a storage oscilloscope. A constant bias voltage (10V) is applied to the sample using a specially designed solid Cu sample-holder. The current is a function of the resistance of the sample and is measured as a voltage drop at the 500 input resistor of a transient digitizer [4,5]. The reflectivity was measured using a HeNe-laser in combination with a fast photodiode and a storage oscilloscope. A narrow band notch filter is used to suppress the
Mat. Res. Soc. Symp. Proc. Vol. 74. 1987 Materials Research Society
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500MHz modulation produced by mode competition in the HeNe-laser. The irradiated area was chosen to be slightly larger than the sample size and all the experiments were single shot measurements.
FIG. 1. Experimental setup for simultaneous time-resolved optical and electrical studies in GeTe thin films.
The optical and electronic properties of amorphous, liquid and crystalline GeTe (see Table 1) allow to easily distinguish between the amorphous and the crystalline states by means of reflec
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