RESEARCH COMMUNICATIONS CURRENT SCIENCE, VOL. 101, NO. 6, 25 SEPTEMBER 2011 765 *e-mail: m.miah@griffith.edu.au Voltage probe of the optically oriented electron spin relaxation M. Idrish Miah* Department of Physics, University of Chittagong, Chittagong 4331, Bangladesh We report here a study of the voltage probe of the electron spin relaxation in zinc-blende semiconduc- tors. Electron spins oriented by a circularly polarized light are dragged by an electric field in transparent devices formed on gallium arsenide. The observed spin polarization-dependent voltage signal (which is a measure of the spin relaxation) is found to decrease about exponentially with the applied electric field. When the spin-oriented electrons are dragged with a high field, a significant decrease in the spin polariza- tion is observed due to an increase in the spin preces- sion frequency of the hot electrons. It is also found that the signal rationally decreases with increasing crystal temperature. The results are discussed based on the Dyakonov–Perel spin relaxation mechanism. Keywords: Dyakonov–Perel spin relaxation, spin pola- rization, voltage probe, zinc-blende semiconductor. SPIN electronics or spintronics is a rapidly growing field of research aimed at realizing new high-performance semiconductor (spintronic) devices that take advantage of the electron spin as well as of its charge 1–3 . It is generally expected that addition of the spin degree of freedom in information processing will extend the functionality of conventional devices (electronic devices) and allow the development of novel electronic devices (spintronic devices), which can hold promise of, for example, reduced power consumption, faster operation, smaller size and nonvolatility. Conventional electronic devices rely exclusively on the electronic charge. The idea to use the spin property of electrons in conventional devices has drawn a lot of attention recently, motivated by the observations of long spin lifetime (τ s ) or spin diffusion length (δ s ) (δ s = 2 μm obtained by optical 4 and δ s = 1.7 μm by electrical mea- surements 5 ) in semiconductors. However, one of the important requirements necessary in developing semi- conductor spintronic devices is the detection of spin current (or spin relaxation) in a semiconductor 3 . For a reliable detection, the efficient transport (without spin-flipping or spin relaxation, or the loss of spin polarization) of spin-polarized carriers through a semiconductor over rea- sonable distances that are comparable to the device dimensions is required. This is because if spin relaxes too fast, the distance travelled by an electron without losing its spin state will be too short to perform any practical purpose or operation. The detection of spin current in semiconductors has been obtained mostly by optical methods 1,6 . These include: (i) measurement of the differential transmission using a pump-probe technique (single-photon 7 or multiphoton 8 pumping), with the same and oppositely circularly polar- ized pulses; (ii) optical Kerr rotation microscopy 4 ; (iii) time-resolved circularly polarized photoluminescence or electroluminescence measurement 9 ; (iv) optical Hanle ef- fect (depolarization of the photoluminescence in a magnetic field perpendicular to the spin) measurement 10 , and, most recently, (v) optical spin Hall effect (SHE) measure- ment 11 . The electrical detection of SHE (or the spin current in the SHE) still remains a challenge due to the difficul- ties associated with the absence of the Hall voltage 3 . However, an electrical method (voltage probe) of detecting spin current or spin relaxation in semiconductors is desirable for possible device applications. Electrical detection of spin current in semiconductors has recently been reported 12–14 . For example, Lou et al. 12 detected spin transport in a lateral ferromagnetic metal–gallium arsenide (GaAs) semiconductor device in the presence of an exter- nal magnetic field at low temperatures. In an earlier study 13 , we had detected spin current in GaAs at both low and room temperatures by circularly polarized light exci- tation via the photo-induced anomalous Hall effect (AHE), and in the absence of the external magnetic field. As no external magnetic field was used, the observed effect was the pure AHE, i.e. without any contribution of the ordinary Hall effect. Here we show the voltage probe of the electron spin relaxation and find that the spin relaxation strongly depends on the applied electric field and crystal temperature. Device samples were fabricated on moderately (n = 1 × 10 16 cm –3 ) silicon-doped (n-type) 1-μm bulk GaAs. Transparent Au(100 nm)/Ge(40 nm)/Pd(10 nm) contacts, with Pd layers adjacent to GaAs, were deposited on the substrates using an e-beam evaporator with a base pres- sure of ~ 5 × 10 –8 torr. The contact metallization was an- nealed at 180°C for 1 h to achieve transparent contacts. A gold wire was bonded to the centre of each of the four contacts. Details of the contact formation mechanism have been reported elsewhere 15 . The sample (placed in a cryostat for low-temperature measurements) was opti- cally excited (with excitation energy of ~ 4 mW) by cir- cularly polarized picosecond pulses from a mode-locked Ti : sapphire laser with a repetition rate of 76 MHz. The polarization of the pulsed beam was modulated using a photoelastic modulator (PEM) at lock-in reference fre- quency of 42 kHz. The excitation photon energy was tuned (λ ≈ 0.8 μm) slightly above the band gap of GaAs. A neutral density wheel (NDW) was used to vary the optical power level. The laser beam was focused on to a ~ 8 μm (FWHM) spot at the surface of the sample with a lens. A lock-in amplifier coupled to a computer was used for measuring the spin polarization-dependent voltage signal in the Hall geometry (Figure 1).