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Ultrafast thin film GaAs photoconductive detectors
Thm Solid FIlms, 90 (1982) 371 ELECTRONICS AND OPTICS
371
ULTRAFAST THIN FILM GaAs PHOTOCONDUCTIVE DETECTORS* H. J. KLEIN, H. BENEKING AND D. BIMBERG
Instztute of Semtconductor Electrontcs and SFB 56, Aachen Techmcal Unwersity, Templergraben 55, D-5100 Aachen (F.R.G.)
Fast photodetectors with high current gain, rise times less than 100 ps and a low noise figure are of great interest for future optical communication systems at near-IR wavelengths. The development of planar thin film GaAs photoconductive detectors is reported, representing clear progress towards this goal. The device consists of a thin high purity epitaxial layer of GaAs grown on top of a chromium- (or boron-) doped semi-insulating substrate and two contacts on top of the layer. The active volume of the device is defined by the channel length between the contacts (3-10 lain), a mesa etch reducing the channel width to 30 ~tm and the layer thickness (0.1-3 i~m). Excitation-pulse-limited fast rise times of 40 ps or less are achieved. An initial fast decay is followed by a second slower decay (¢d2 ~ 1 ns) and a long tail in the nanosecond range. The decay is strongly dependent on the excitation intensity. At higher excitation intensities or after superposition of an extremely faint continuous wave light intensity the amplitude of the tail decreases and finally disappears. A threshold at about 1 eV is found for the photon energy of the continuous wave light. At lower photon energies the tail is hardly affected. Detectors built on layers with a constant shallow impurity concentration but with varying thickness were studied. It was concluded that a low concentration of deep centres which trap and re-emit charge carriers are responsible for the long tail. These centres are saturable. The experiments indicate that chromium out-diffusion from the substrate might be the source of the deep centres. Results of attempts to overcome these difficulties (e.g. the use of semi-insulating substrates or buffer layers devoid of chromium are reported and show that further improvements are indeed feasible. The results of comparative studies of current gain, noise and equivalent power, and detectivity are given,
* Abstract of a paper presented at the Fifth internattonal Tlun Films Congress, Herzlia-on-Sca, Israel, September 21-25, 1981. 0040-6090/82/0000-0000/$02.75
© Elsevier Sequoia/Printed In The Netherlands
371
ULTRAFAST THIN FILM GaAs PHOTOCONDUCTIVE DETECTORS* H. J. KLEIN, H. BENEKING AND D. BIMBERG
Instztute of Semtconductor Electrontcs and SFB 56, Aachen Techmcal Unwersity, Templergraben 55, D-5100 Aachen (F.R.G.)
Fast photodetectors with high current gain, rise times less than 100 ps and a low noise figure are of great interest for future optical communication systems at near-IR wavelengths. The development of planar thin film GaAs photoconductive detectors is reported, representing clear progress towards this goal. The device consists of a thin high purity epitaxial layer of GaAs grown on top of a chromium- (or boron-) doped semi-insulating substrate and two contacts on top of the layer. The active volume of the device is defined by the channel length between the contacts (3-10 lain), a mesa etch reducing the channel width to 30 ~tm and the layer thickness (0.1-3 i~m). Excitation-pulse-limited fast rise times of 40 ps or less are achieved. An initial fast decay is followed by a second slower decay (¢d2 ~ 1 ns) and a long tail in the nanosecond range. The decay is strongly dependent on the excitation intensity. At higher excitation intensities or after superposition of an extremely faint continuous wave light intensity the amplitude of the tail decreases and finally disappears. A threshold at about 1 eV is found for the photon energy of the continuous wave light. At lower photon energies the tail is hardly affected. Detectors built on layers with a constant shallow impurity concentration but with varying thickness were studied. It was concluded that a low concentration of deep centres which trap and re-emit charge carriers are responsible for the long tail. These centres are saturable. The experiments indicate that chromium out-diffusion from the substrate might be the source of the deep centres. Results of attempts to overcome these difficulties (e.g. the use of semi-insulating substrates or buffer layers devoid of chromium are reported and show that further improvements are indeed feasible. The results of comparative studies of current gain, noise and equivalent power, and detectivity are given,
* Abstract of a paper presented at the Fifth internattonal Tlun Films Congress, Herzlia-on-Sca, Israel, September 21-25, 1981. 0040-6090/82/0000-0000/$02.75
© Elsevier Sequoia/Printed In The Netherlands