Giant light-induced capacitance enhancements in an unconventional capacitor with two-dimensional hole gas

Capacitance is the ability of a device to store energy in the electric field. Conventional capacitors are composed of two metallic contacts with a dielectric in between arranged in different geometries, the most well-known of them being an parallel plate capacitor with the capacitance value, C
_geo
=ε
_r
ε
₀
A/d, solely dependent on the geometry of the structure and the physical properties of the dielectric. In an unconventional capacitor, however, at least one of the plates is replaced with a two-dimensional electron system (2DES) attained in oxide-semiconductor interfaces [1], quantum wells in III-V material heterojunctions [2, 3]. Distinctive to unconventional capacitors is the capability to control carrier concentration in the non-metallic plates through application of a voltage [4]. Additionally the many body interactions among charge carriers in 2DES are manifested as capacitors in series with the geometrical capacitance of the plates [5] as: 1/C
_tot
= 1/C
_geo
+ 1/C
_kin
+ 1/C
_ex
+ 1/C
_corr
, (1) oequation where C
_ex
, C
_corr
and C
_kin
represent capacitances due to exchange, correlation and kinetic energies of the 2DES, respectively, and C
_geo
and C
_tot
are the geometrical and total capacitance between the electrodes. Noticeably, C
_ex
and C
_corr
can contribute negatively to the total capacitance [4] at critical electron densities in 2DES resulting in a C
_tot
larger than C
_geo
. This capacitance enhancement feature can be utilized as an alternative to high-k dielectric materials to increase gate capacitance in MOS transistors [1]. Moreover transitions from a high-density to a totally depleted 2DES are shown to remarkably improve the performance of the voltage-controlled capacitors based on 2DES [6]. Here we report on design, fabrication and capacitive performance of an interdigitated metal-semiconductor-metal (MSM) photodetector with embedded two-dimensional hole gas (2DHG) achieved in a triangular quantum well in a GaAs/AlGaAs hetero-interface. Capacitance-Voltage (C-V) characteristics obtained through impedance spectroscopy in dark indicate behavior of a switchable capacitor while C-V under 850nm laser light exhibit anomalous features of capacitance enhancement. Light generated carriers that increase the charge density in 2DHG account for arousal of anomalies in C-V characteristics through changing the energetic of the unconventional 2DHG electrode. This device carries a giant light-sensitive capacitance which makes it a good candidate for capacitance-based photodetectors.