W.J. Xiao

Calculation of the electric field in GaAs particle detectors

Abstract We have simulated the electric field in GaAs surface barrier particle detectors in order to explain the observation of reduced charge collection efficiency. Using the Poisson and continuity equations the electric potential can be calculated numerically in terms of the ionization of deep levels and the spatial variation of the quasi-Fermi-level in GaAs. The dependence of the electric field on different parameters (bias voltage, leakage current, material parameters) has been studied and predictions of the simulation have been experimentally verified.

Investigation of the radiation damage of GaAs detectors by neutrons and photons

Abstract Surface barrier particle detectors, processed in Aachen using SI GaAs from several manufacturers, have been irradiated with neutrons (peak energy ∼1 MeV) up to fluences of 4.0 × 10 14 n/cm 2 and with photons from a 60 Co source with a dose of 100 Mrad. All detectors work well after the irradiation. Detectors biased with 200 V during the neutron irradiation show no significant difference from those not biased. After irradiation with 4.0 × 10 14 n/cm 2 the leakage currents at 200 V are a factor of 4 greater than those before irradiation. At the highest radiation level the signal for minimum ionizing particles corresponds to 7200 electrons (at 200 V bias voltage) independent of peaking times between 40 ns and 2.2 μs. The signals for α-particles (2.2 μs peaking time, 200 V bias voltage) are about 20% of those before irradiation. The exposure to the 100 Mrad photon dose caused little change of the detector performance. The leakage currents were even reduced by about 10%.

Investigation of the radiation damage of GaAs detectors by protons, pions and neutrons☆

Abstract Surface barrier detectors processed in Aachen using SI-GaAs from several manufacturers have been irradiated with high fluences of neutrons (mean energy 1 MeV, fluence up to Φ n ∼ 5 × 10 14 cm −2 ), pions (191 MeV, fluence up to Φ π ∼ 0.6 × 10 14 cm −2 ) and protons (23 GeV, fluence up to Φ p ∼ 2 × 10 14 cm −2 ). The detectors have been characterized in terms of macroscopic quantities like I - V characteristic curves and charge collection efficiencies for incident minimum ionizing- (mip) as well as α-particles. All detectors work well after the exposure. At the highest fluences a sizable degradation concerning the charge collection efficiencies has been observed for all investigated materials. SI-GaAs material with low carbon content (LC material from FCM, Freiberg) seems to be less affected than substrates with a higher carbon concentration. At the highest irradiation level the mip signal from a 250 μm thick detector made of LC material amounts to 8000 electrons (at 400 V bias voltage) independent of peaking times between 40 ns and 2.2 μs. The leakage currents for this material are even reduced after the irradiation.

Radiation hardness of MSM biasing structures for GaAs microstrip detectors

Abstract In order to achieve a high signal-to-noise ratio and a low-space consumption it is necessary to use integrated resistors for the biasing of GaAs microstrip detectors used in high-energy physics experiments (e.g. CMS, H1). Metal-semiconductor-metal (MSM) structures operating in the reach through regime seem to be a promising solution for that purpose. According to the requirements on radiation hardness of strip detectors to be used in the CMS experiment, we have irradiated MSM biasing structures with up to 4.6 × 10 14 neutrons cm 2 . The radiation effects on the dc and noise characteristics of the devices are investigated. Furthermore, the effects of an annealing procedure are examined.

Charge collection efficiencies and reverse current densities of GaAs detectors

Abstract GaAs surface barrier diodes with different areas have been fabricated and tested as particle detectors. It is shown that the reverse current is affected both by the area of the Schottky contact and by its circumference. The charge collection properties can be explained on the basis of a model that takes into account trapping effects and the distribution of the electric field within the detector. From this model (several) rules for optimizing GaAs detectors are derived.

Measurements on GaAs strip and pixel detectors in a 50 GeV pion beam

Abstract GaAs strip and pixel detectors constructed in Aachen have been tested at CERN in a 50 GeV pion beam in September 1995 in collaboration with the CMS Tracker group. The strip detectors had a pitch of 100 μm and were made of a 250 μm thick Freiberger SI-GaAs wafer [1]. The three strip detectors had a strip width of 25, 50 and 75 μm, respectively. Using the fast PreMux128 preamplifier multiplexer chip ( τ p = 50 ns) a signal to noise ratio of 15 was obtained for the widest strips at normal beam incidence for a bias voltage of 170V. The 8 × 8 pixel arrays with a pixel size of 1 × 1 mm 2 and 0.5 × 0.5 mm 2 , respectively, were read out with the PreMux128 as well. Here a signal of 12500e − was obtained for both detectors, leading to a maximum signal to noise ratio of 20 at perpendicular beam incidence and 170 V bias voltage.

Beam test measurements on GaAs pixel detectors at various angles of incidence

Abstract A GaAs pixel detector constructed in Aachen has been tested in a 4 GeV electron beam at DESY. The experimental setup allowed tilting the detector with respect to the beam line with angles of incidence from 0° to 45°. The sensor-array consisted of 8 × 16 pixels with a size of 125 × 125 μm2 each. The detector was made of a 250μm thick Freiberger SI-GaAs wafer. An improved contact was formed on the backside, allowing safe operation of the detector in the soft breakdown regime. A double metal technique allowed bonding the single pixels linearly to the readout-chip. Using the the fast PreMux128 preamplifier multiplexer chip (τp = 40ns) a signal to noise ratio of 29 was obtained for a beam angle of incidence of 0° increasing up to 38 for 45°. The spatial resolution obtained with an angle of incidence of 45° was (9.0 ± 6.0) μm while the resolution of the untilted detector is equal to the digital one (36.1μm). For these testbeam-measurements the detector was connected to the electronics via wire-bonds. For future experiments bump-bonding connections are required. The results of a process for the formation of bump-bond connections on GaAs pixeldetectors are shown.

Beam test measurements on GaAs strip and pixel detectors

Abstract GaAs strip and pixel detectors constructed in Aachen have been tested in a 1.4 GeV electron beam in Bonn and in a 5 GeV electron beam at DESY in February and May 1997. The strip detectors had a pitch of 50μm with a strip width of 25μm and were made of a 250μm thick Freiberger SI-GaAs wafer. The strip detectors included a punch-through bias structure and an integrated coupling capacitor. Additionally, an improved backside contact was formed, allowing a safe operation of the detector in a soft breakdown regime. Using the fast PreMux 128 preamplifier multiplexer chip ( τ p = 50 ns) a signal-to-noise ratio of 13 was obtained at normal beam incidence for a bias voltage of 200 V, leading to a spatial resolution of 11 μm with a simple COG algorithm. The 8 × 16 pixel array with a pixel size of 125 × 125 μ m 2 was read out with the PreMux128 as well. With a double-metal technique, it was possible to bond the single-pixels linearly to the amplifier chip. The obtained signal-to-noise ratio of 30 in combination with a COG algorithm lead to the digital resolution value of 36 μm for both pixel coordinates.

Measurements on a GaAs microstrip detector with 50 GeV/c electrons☆

Abstract A microstrip gallium arsenide detector (thickness 508 μm, strip width 100 μm, strip pitch and readout pitch 200 μm) has been tested in a 50 GeV/ c electron beam at CERN. Using the low noise Viking preamplifier chip (shaping time ∼1.5 μs) signal to noise ratios up to 25 were measured depending on bias voltage and angle of incidence. Applying the so-called η-algorithm (using the impact position-sensitive charge sharing between adjacent strips) a resolution down to σ ∼ 20 μ m could be obtained.

Investigations on damage of GaAs detectors caused by neutrons (Peak Energy 1 MeV) and Co60 Photons

Abstract In April 1994 we irradiated more than 60 GaAs detectors made from the same wafer with different fluencies in the ISIS neutron source at the Rutherford Appleton Laboratory. The detectors were produced in Aachen from SI-GaAs from American Xtal Technology (AXT). All detectors remained functional after irradiation with up to 1 x 1015 n c m 2 . Even at the highest radiation level the reverse current density of the detectors is below 50 n A m m 2 at 200V bias voltage, only a factor of four higher than before irradiation. After 1 x 1015 n c m 2 the signal for minimum ionising particles is a factor of two lower than before irradiation. No difference was seen beween detectors that were biased during the exposure and those that were not. The detectors were operated at room temperature during and after the exposure.