Electrical Defect Analysis Following Pulsed Laser Irradiation of Unimplanted GaAs
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Res.
Soc.
SyMp.
Proc. Vol.
13 (1983) QElsevier Science Publishing Co.,
Inc.
648
The lasers used were a Q-switched Ruby laser (pulse duration 25ns, X = 0.69 Jm) and a Nd-YAG laser (pulse duration 20ns). The Nd-YAG laser operated either in the green (X = 0.53 lPm), in the infra-red (X = 1.06ipm) or in a mixture of the two. The energy in the laser spots was made uniform through the use of either a quartz pipe (Nd-YAG laser) or a diaphragm (Ruby laser). In both cases the beam diameters were of 6mm. After irradiation, the Schottky devices (0 = 340 -jm) were fabricated by evaporating a front Au contact through a mask. The backside AuGe contact was sputter deposited and subsequently alloyed at 400'C during 10mn in flowing H2 prior to Au deposition. Careful characterizations were first performed on reference samples from each ingot. Carrier concentrations were measured by C(V) techniques using a HP 4275 A LCR Bridge. The ideality factors of the investigated devices ranged from 0.99 to 1.12. The D.L.T.S. spectra recorded on devices from ingot A exhibited practically one single peak corresponding to a 0.83eV ionization energy (EC - 0.83eV). This trap level is related to a native defect in bulk n-GaAs 171 and its depth concentration was found to be constant around 4xl01 5 /cm 3 . D.L.T.S. spectra of Sumitomo samples also exhibited one single peak corresponding to a 0.36eV trap level which has also been reported as a native defect in bulk n-GaAs 12,81. Its concentration was found to be around 6xl01 5 /cm 3 . No trap levels were detected in the B ingot at a concentration higher than a few 101 3 /cm 3 . RESULTS AND DISCUSSION Laser annealing Fig. I shows the I(V) characteristics of Schottky devices (Sumitomo GaAs) built on (i) reference material, (ii) low energy irradiated material (0.53 1m(20%)+1.06 1m(80%) /480m3. 2 cm- 2 ) and (iii) high energy irradiated material (0.53 1m + 1.06 1m (80%) /1j.cm- ). A first thing to be pointed out is that despite of the use of a quartz pipe to homogenize the beam, there is an important scattering in experimental observations for the diodes irradiated at I 3.cm- 2 (curves labelled C-I and C-2). This is attributed to some speckle effects in laser energy distribution, the grinding of the input face of the quartz pipe probably being too rough. Apart from this fact, it can be noticed that, for a definite voltage, the reference devices present some orders of magnitude lower currents than the ones irradiated at 480m3.cm- 2 , while exhibiting the same slope. From the values of the reverse saturation currents, barrier heights of 0.95V and 0.65V are calculated respectively for the reference and the low energy irradiated devices. For the high energy irradiated samples, no quantitative data regarding barrier height can be extracted from the I(V) plots. The behavior for the latter samples is characteristic of a series resistance effect. Effectively, high resistance values are measured after high energy irradiation, when an ohmic contact is substituted to the rectifying Au contact 191. This effect
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