Ultrafast Electronic And Vibrational Effects In Amorphous Multilayers

ABSTRACT We present a new, purely optical technique for time-of-flight experiments in the picosecond time range using a-Si:H/a-SiNx:H multilayers. We demonstrate the presence of vibrational surface modes in a-Ge:H/a-Si:H multilayers by measuring the acoustic response in real time. 1. INTRODUCTION The study of amorphous multilayers has provided new insights into the bulk and interfacial properties of amorphous semiconductors. In this paper we will discuss ultrafast relaxations in two distinct sets of multilayers, a-Si:H alternating with a-SiNx:H, and a-Si:H alternating with a-Ge:H.In the a-Si:H/a-SiNx:H multilayers we will discuss the a-Si:H layers and Si/SiN interfaces. Electroabsorption measurements have shown that these multilayers contain strong internal electric fields (of the order of 10 V/cm) due to charge transfer doping2 resulting in negatively charged silicon layers. The Si/SiN interfaces have a fairly large defect density estimated to be between 10 and 10 cm depending on the experimental method used.1 ' We show how the a-Si:H/a-SiNx:H multilayers can be used to perform time-of-flight experiments in a-Si:H with a time resolution of less than 1 psec.4 This can only be achieved with a purely optical technique. Carriers are photoexcited in the a-Si:H layers by an ultrashort light pulse (the pump pulse). These photoexcited carriers drift due to the presence of the internal electric fields to the Si/SiN interfaces, where they are trapped at the interfacial defects. The carriers that arrive at the interfaces at time t are detected optically by a second light pulse (the probe pulse) which measures the decrease in the transmissivity as a function of time delay t between the pump and probe pulse.The a-Ge:H/a-Si:H multilayers have quite different properties. They do not contain detectable internal electric fields and the Ge/Si interfaces have a much smaller defect density than the Si/SiN interfaces.5 Since a-Ge:H is the lower band gap material in this system, light is mainly absorbed in the a-Ge:H layers. The energy deposited by an ultrashort light pulse (the pump pulse) sets up an elastic stress in the Ge layers. The resulting vibrations are studied in real time by measuring the change in the reflectivity of the multilayer with a second light pulse (the probe pulse) that is delayed with respect to the pump. 'The change in reflectivity contains a response that can be ascribed to a vibrational surface mode with a frequency that is located in the zone-boundary gap of the folded phonon spectrum of the multilayer.2. EXPERIMENTThe photoinduced changes in the transmissivity (AT) and reflectivity (AR) are measured with the pump and probe correlation technique. The output of a CPM ring dye laser8 which has a pulse width of 0.1 psec, an energy per pulse of 0.15 nJ, a photon energy of 2 eV (6180 A), and a repetition rate of 108 MHz is divided into two beams, the pump and the probe. The pump beam is modulated at 4 MHz. The probe pulse is mechanically delayed with respect to the pump pulse. Both beams are focussed onto the sample to a 10 |im diameter spot. The transmitted or reflected probe beam is detected with a photodiode. The output of the photodiode is fed into a phase-sensitive amplifier which is locked to the modulation frequency of the pump beam. In this way we are able to measure the photoinduced changes of the transmissivity and