Ion-beam-induced-charge characterisation of particle detectors

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Abstract

Ion-beam-induced-charge collection (IBIC) in a nuclear microprobe has been used to characterise detectors for the measurement of particles over a median energy range (100 keV–1 MeV). Three standard detector devices have been studied: a PIPS detector with a buried (ion-implanted) junction structure, a Schottky barrier junction device and a PN-junction photodiode. A 2.0 MeV focussed helium ion beam was used to probe the active area of each device with a spatial resolution ∼1–2 μm, to quantify the thickness of the dead layer, the charge collection response and the reduction in charge collection efficiency induced by ion-beam damage.

Introduction

Hetero-junction detectors have been widely used for the detection of charged particles of a typical energy range from a few MeV to a few hundreds of MeV. For the measurement of the light and high energy ions, commercial detectors have demonstrated sufficient energy resolution and linear response over a wide a energy range, as well as fast response time. However, in the application of measuring heavy ions at the median energy range, major problems commonly rise from a non-linear response, the pulse height defect (PHD) [1], [2], [3]. Three key effects contribute to the PHD: energy loss within an upper dead layer of the device, a proportion of energy loss associated with nuclear stopping that takes place without the process of ionisation of electron–hole pair production, and a reduction in charge collection due to trapping of charge-carriers at defect centres, which may be caused by damage induced by the incident beam.

We have employed a nuclear microprobe to examine 2.0 MeV 4He ion-beam-induced-charge collection (IBIC) [4] in three standard types of detector: an Alpha PIPS detector (Canberra) with buried junction structure, a Schottky barrier junction device and a PN-junction photodiode (Hamamatsu S1333). For each device, measurement of the detector response, and its dependence on operating bias, has been used to quantify ion energy loss in the dead layer and loss of the charge collection efficiency due to trapping centres. The influence of the PHD to the detector's performance for heavy ions in the median energy range has been estimated; the PHD is a dominant factor governing the performance of a detector for heavy ions below 100 keV.

Section snippets

The measurement of the dead layer thickness in PIPS detector

The PIPS detector [5] is employed here as a standard to quantify the charge collection efficiency of the other two devices under test, and to verify the ion-beam energy. It is specially fabricated with an ultra shallow buried (ion-implanted) junction and very thin SiO2 passivation layer on the front face. The response of our PIPS detector is found roughly to be independent of operating bias, above a bias of 50 V. At such a high bias, we can therefore make the reasonable assumption that the

Measurement of the charge collection response in Schottky and photodiode devices

The Schottky barrier device was fabricated on an n-type (phosphorous) silicon substrate, of resistivity 10Ω cm, with 500 nm SiO2 layer. The Schottky contact is formed by removing the oxide in a central area, of diameter 0.5 mm, before depositing a 10 nm thick aluminium layer across the whole device. Given that the Schottky device has a small active area, the focussed beam in a nuclear microprobe was used to perform an IBIC measurement. Fig. 2 illustrates the images obtained for a 2 MeV 4He ion

Trapping centres induced by ion-beam damage

The energetic particles incident in a detector, produce not only electrical pulse of the IBIC, but also create defect centres, at sites of which, charge carriers can be trapped or recombined. The increase in the number of the defect centres causes a charge collection efficiency loss; this results in the detector response of the energy peak shifting toward a lower energy position. In order to quantify this type of PHD effect in the detector, a controlled 2.0 MeV alpha dose was deposited into an

Estimation of the PHDs

When the ion energy is deposited in an idea detector, the pulse height of the collected charge is linearly proportional to the incident ions energy. However, the pulse height in a real device is often reduced or shows non-linear response to the ion energy due to the PHD from various causes. There are three main causes of the PHD: (1) the proportion of the ions energy loss to nuclear stopping without involvement in the ionisation process leading to the electron–hole pairs production; (2) charge

Conclusion

The IBIC method in a nuclear microprobe is effective in the characterisation of particle detectors. It provides detailed information on the PHD of different causes. The influence of the PHD to the detector's performance in the median energy range has been estimated; it indicates that the optimised detector can measure helium and boron ions of the energy down to 10–20 keV.

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