Major Research Areas

Development of GaAs based spin-Hall devices with low power consumption
Spin-photonics is one of the frontline research area where a lot of exotic developments are currently being made. For a long time, inclusion of electronic spin degree of freedom with charge have been aimed for improving the functionality of semiconductor devices. One example is a spin-Hall device, which generates electrical voltage corresponding to the degree of circular polarization of incident light. It works on the principle of Inverse Spin Hall Effect (ISHE), where generation of transverse electrical voltage occurs as a function of photo injected spin current. Traditionally, near band edge lasers (E_ex ~1.5 eV) are used to orient electronic spin ex in à valley of conduction band of GaAs. A spin current and subsequently a transverse electrical voltage (V_ISHE ) are ISHE thereafter generated because of ISHE. Though such devices are fast, compact, operate at room temperature and can be integrated into existing optoelectronics platform, a low signal-to-noise-ratio (SNR) is a major bottleneck. Efforts are being made worldwide for enhancing the signal strength of spin-Hall devices. The magnitude of V_ISHE depends critically ISHE on a parameter called spin Hall angle (ã_SH ). A small value of ã_SH in GaAs is the limiting factor for generating V_ISHE with large ISHE SNR. It is realized that the L-valley of GaAs has two orders larger value of ã_SH as compared to Ã-valley due to a large SH contribution of higher lying p-orbitals. Owing to this, attempts are being made to involve the spin polarized electrons in L-valley.

agram to show ISHE experiments, (c) band structure of GaAs to show optical excitation at two energy and, (d) plot of V versus appied bias at the two values of ISHE excitation energy.

An indirect excitation of electrons to L-valley is not favorable because of the momentum selection rules. Thus, the application of high electric field is sought to initiate the intervalley electron transfer, which enhances the magnitude of V_ISHE significantly. However, it worsens SNR due to excessive ISHE Joule heating. Though a large value of V_ISHE is desired, it ISHE should be achieved preferably at low bias. Development of spin-photonic devices have been carried out in our lab for the past ten years where systematic efforts are put for enhancing the strength of ISHE signal in high crystalline quality GaAs. By utilizing optically injected hot-electrons at 2.33 eV, a spin- Hall device is recently demonstrated by us that can provide high V_ISHE at relatively low bias. The devices consume less ISHE power and yield better signal-to-noise ratio.

For this work, 1 ìm thick high crystalline quality n-GaAs epitaxial layers are grown by MOVPE technique. A Hall bar type structure is fabricated by using mask-less photolithography, chemical etching and Ohmic contact metallization as shown in Figure L.13.1(a). The laser beam of ~50 μm diameter is focused at the center of the sample, whose degree of circular polarization is modulated as shown in Figure L.13.1(b). Subsequently, longitudinal electric field (E ) is applied and V_ISHE is measured for the two values of || ISHE optical excitation energy as shown in Figure L.13.1(c). In particular, spin polarized hot electrons are injected in GaAs at 2.33 eV. It leads to a steady state electron accumulation in L valley. By involving simultaneous energy, spin and inter valley relaxation processes, it is estimated that nearly 1% electrons are accumulated in L-valley, and about 40% of them are spin polarized. A monotonous rise of V_ISHE is seen with ISHE electric field, which starts to fall beyond a critical electric field as shown in Figure L.13.1(d). A rapid increase in the signal magnitude is observed at lower electric field at 2.33 eV corresponding to a larger magnitude of ã in L-valley. From a SH careful comparison between the two cases, it can be appreciated that the same maximum magnitude (~80 ìV) of V_ISHE can now be achieved at half the applied electric field. ISHE Further, the power consumption and SNR improves by 2.5 and 1.5 times, respectively. One of the possible reasons behind such reduction in power consumption is the optical excitation of spin polarized hot electrons in the L-valley. These findings are highly encouraging for the development of next generation spin-optoelectronic devices. For more details please see Mudi et al., Phys. Status Solidi, RRL, 14, 2000097 (2020).