To fabricate a GaAs detector two choices have to be made, namely the substrate material and the contacts used. Care must also be taken over surface preparation and passivation.
The GaAs used almost universally is semi-insulating undoped, SI-U, LEC industrial substrate material supplied by various manufacturers. Material with a low carbon concentration and chromium- or iron-doped material, supplied by SITP, Tomsk, have been proposed as alternatives for improved radiation hardness. Charge carrier absorption lengths vary from 100m to 1600m for holes and 100m to 500m for electrons . The low carbon material has the largest values for electrons. The spread in these values, even from a single manufacturer, is also considerable. For a 200m thick detector it is desirable to have absorption lengths for both carriers in excess of 200m. The detector may be fabricated with either a p-i-n structure  by doping the substrate material, or with a Schottky and an ohmic contact realized by metal deposition.
The leakage current of a typical LEC diode as a function of reverse bias is shown in figure 1. The value of the plateau current, which is independent of diode thickness, is between 10 and 30nA mm-2 at 20oC. When a voltage corresponding to approximately one volt per micron of substrate thickness, Vfd, is reached the leakage current increases dramatically due to current injection through the ohmic contact.
Also shown in the figure is the current characteristic of a diode with an improved ohmic contact where the leakage current increases only slowly above the bias Vfd. This reduced current at high voltages allows the detector to be operated up to voltages approaching twice Vfd. The improved ohmic contact was realised by annealing a multi-layer titanium-palladium-germanium metal contact.
Alenia SpA have fabricated diodes with an ion-implanted ohmic contact which allow a bias many times Vfd to be applied .
The surface quality and thus preparation of the wafer has proved important in obtaining high charge collection efficiencies (CCE). The CCE increases linearly with applied bias (shown in figure 2 for a typical detector) until the bias Vfd is reached, where the efficiency for a 200m detector is typically between 60% and 80%. At higher biases the efficiency slowly increases towards 100%.
Although high resistivity GaAs would be expected to deplete at a few volts this is not found to be the case. With an alpha particle source on the back, ohmic contact, for example, signals are only observed at bias voltages greater than Vfd.
The electric field distribution in the material consists of two regions: a high field region with an approximately constant value of 1 V/m where charge collection is high and a low field region where almost no collection occurs. The penetration depth of the high field region has been measured at 0.7 to 1.0 m per volt, depending upon material [5,6,7]. The field distribution explains the increase in leakage current at Vfd, the linear dependence of CCE on bias, and the observation of rear alpha signals only at biases in excess of Vfd.
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