GaAs Detectors for Forward Region of Linear Collider

1. Instrumentation of the Forward Region of a Linear Collider Detector - Workshop at Prague 16 April 2004 LPI group Lebedev Physical Institute, Moscow

2. LPI group - Workshop at Prague, 16 April 2004 Proposal • Use of GaAs as a sensor for LCAL • We considered two types of material as sensitive elements of GaAs detector : • commercial semi-isolating plates (thickness 100-200 mm) • epitaxial structures (thickness 30 mm) grown in Zelenograd • Choice of the material is a subject to study • Participants • Lebedev Physical Institute, Moscow Laboratory of High Energy Electrons • Scientific and Educational Center on Quantum Devices and Nanotechnologies(joint venture of Lebedev Physical Institute and Moscow Federal Institute of Electronic Technology - Technical University (MIET))

3. LPI group - Workshop at Prague, 16 April 2004 History Some tests were performed last year.For these tests we used: • detectors of barrier type(metal-semiconductor or p-n junctions) with epitaxial layers of undoped GaAs as active layers (thickness 30 mm) • detectors of resistive type with commercial semiisolating GaAs (compensated EL2 deep level defects) as active layers (thickness 150 mm)

4. LPI group - Workshop at Prague, 16 April 2004 Test results (last year) • Spectrometric measurements were performed - GaAs detectors were irradiated by a-source(5.456and 5.499 MeV) I, nA DE, keV S/N I: M–n-GaAs–n+GаАs<1535 II: M–p+GaAs–n-GaAs–n+GaAs 7 141 III: M–n-GaAs–n+GaAs 45 1515 IV: М-iGaAs-M (EL2)150 40 10 I-III types - epitaxial GaAs layers,IV type - semiisolating GaAs, I, nA - leakage current; DE, keV - energy resolution; S/N - signal to noise ratio Conclusion: detectors with epitaxial structure showed slightly better results compared to semi-isolating one.

5. To continue investigations recently (2004) new party of detectors with epitaxial layers of GaAs was made.Detectors were manufactured either as twin linear integrated structures or discrete cells. Topology of the prepared detectors was: p+GaAs - n-GaAs -n+GaAs (substrate) layer thickness: 0.25 mm and 30 mm substrate thickness 600 mm dopant density: 1018 cm-3 and 2.1013 cm-3 LPI group - Workshop at Prague, 16 April 2004 New party of GaAs detectors Discrete cells were made of different shape (circle, ring) and different size (50 mm - 2 cm). In preparing new party of GaAs samples mesa-epitaxial approach was used.

6. LPI group - Workshop at Prague, 16 April 2004 Detectors: description of structure Two types of detectors were used • twin linear integrated structure of galvanically disconnected recording cells separatedwith the guard rings • discrete detection cells separated with the usage of guard rings Detectors had following contacts: • ohmic (Ge/Ni/Au) - to heavily-doped n+GaAs substrate from the back-side of the structure • barrier (V-Au) of circular (or annular)shape - to doped epitaxial layers from front-side of the structure • common guard ring in the case of twin linear integrated sructures and «Guard» electrods (three-ringtype) in the caseof discrete cells GaAs detectors were placed in a special casing

7. LPI group - Workshop at Prague, 16 April 2004 Test results We started measurements of the parameters of new GaAs detectrors. We performed spectrometric measurements: GaAs detectors were irradiated by a-source. I, nA DE, keV S/N 3 mm <50 ~120 4 10 mm <200 1.5 20 mm <1000 1 200 mm <1 - First results on DEwere worse than those obtained on samples of last year. We couldnot optimize measurement procedurebecause of the time deficit.Measurements will be continued. There are also plans to extend measurements to the region of high energies (hundreds MeV and GeV region). Preliminary tests of uniformityfor linear integrated structuresgave accuracy (in summary signal) about 10 %.

8. LPI group - Workshop at Prague, 16 April 2004 Simulation of the LCAL e- , E=1 GeV • To compare characteristics of two different materials as sensitive layers of detector we (besides direct measurements) made first attempts of MC simulation of energy deposited in active layers of calorimeter • Simulationhave been made by using Geant4 package • We used simple geometry 1)63 layers: W 2 mm,GaAs 300 m 2) 63 layers: W 2 mm,Si 300 m 3) 63 layers: W 2 mm,GaAs 30 m • Primary particle e- , energy 1 GeV

9. LPI group - Workshop at Prague, 16 April 2004 Simulation of the LCAL e- , E=1 GeV Primary particle e- , energy1 GeV Geometry 1)63 layers: W 2 mm,GaAs 300 m 2) 63 layers: W 2 mm,Si 300 m 3) 63 layers: W 2 mm,GaAs 30 m Results: Signal from calorimeter with GaAs sensor is two times larger thansignal from Si sensor when thicknesses are equal. In case of epitaxial layerwith thickness of GaAs 30 menergy deposited in active layers ofthe calorimeter is several times smaller than that from thick sample of Si. To make final conclusion further MC simulations are required.

10. LPI group - Workshop at Prague, 16 April 2004 Advantages and disadvantages • GaAs detector on epitaxial layers has following advantages compared tothe case of semi-isolating layers: • low noise level • smaller values of working voltage • larger temperature range of stable work (especially compared to Si) • and disadvantage • low level of signal (energy deposited in active layers of the calorimeter)

11. LPI group - Workshop at Prague, 16 April 2004 Directions of studies At present there are at least two directions of developing material parameters suitable for use in LCAL: • Preparation GaAs epitaxial layers with larger thicknesses (up to 100 m). In this case we shall havesignal (energy deposited in active layers of the calorimeter) compared with that obtained in case of Si -sensor. • Decreasing concentration of EL2 deep level defects (in the case of semi-isolating GaAs) without increasing background concentration of carriers. (First samples withconsiderable decreasing of EL2 defects concentration were obtained). It will allow todecrease noise level

12. LPI group - Workshop at Prague, 16 April 2004 Plans: tests in Russia • Special setup is being preparing now for measurement of parameters GaAs detectors We intend to perform: • full-scale electrophysical tests of GaAs detectors • spectrometric measurements using radioactive source • Single channel amplifier was prepared on the basis of GaAs epitaxial layers We hope to use this amplifier for realization spectrometric measurements on electron beam (Emax = 600 MeV) of Synchrotron S-60 (Lebedev Physical Institute)