Electron cloud effect on current injection across a Schottky contact

Abstract

The electron cloud that is formed in the narrow gap material in a modulation-doped heterostructure affects the Schottky contact made to the wide gap material. It also influences absorption and collection of the optically generated carriers. Photocurrent spectra, current–voltage, and current–temperature measurements show that the increase in electron cloud density decreases dark current flow while increasing photoresponsivity. We propose that the Coulombic interaction between the confined electron cloud and the emitted electrons from metal to the wide gap material increases the barrier height. The electric field in the direction of growth due to modulation doping accounts for the increase in photoelectron collection efficiency. Implementation of this effect increases efficiency of photodetectors while, simultaneously, reducing the noise due to dark current.

High Speed Heterostructure Metal-Semiconductor-Metal Photodetectors

“…The cut-off frequencies calculated from the fast Fourier transform of the temporal response, also reported in Table II, show the values for δ-doped devices in the range of 25-44 GHz, and the values for the undoped device in the range of 24-26 GHz. These results are in agreement with the aiding role of the internal vertical field introduced by the 2DEG [16] and suggest that high performance devices can be designed insensitive to the finger geometry. In conclusion, we propose that the increase in responsivity and response speed is attributed to the enhanced optical field with the resonant cavity structure and the orientation-rotated electric field and modified potential profile by the δ modulation doped layer, while the decrease of dark current is due to the confined electron cloud.…”

“…It is worthwhile to note that, under dark, the lower current values of the doped sample are in qualitative agreement with the effect of the repulsion between the electrons in the metal contact and the electrons in the 2DEG; repulsion which is absent for the undoped samples. This effect was already observed on similar photodetectors realized with various uniform dopings of the wide-gap material [16]. The interpretation relies on Coulombic effect: as the voltage increases, the electrons of the 2DEG are pushed away from the contact and the repulsive effect decreases, thus allowing for an easier thermionic emission across the Schottky barrier.…”

“…The electron cloud that is formed in the narrow gap material of a heterojunction exerts a repulsive force on electrons that are emitted from the metal to the wide gap material, thus decreasing the dark current. It also influences absorption of the optically generated carriers through a field-induced index of refraction, changing the absorption coefficient similar to the Franz-Keldysh effect [16]. The most significant advantage from the modulation doping technique is that the introduction of a δ-doping layer changes the two-dimensional electric field profiles from horizontally oriented to vertically oriented ones due to the screening effect of the highly crowded 2DEG formed along the interface of heterojunction, which reduces the carrier travel distance hence the transit time of carriers in the devices.…”

Resonant-cavity-enhanced heterostructure metal–semiconductor–metal photodetector

“…12 This electron cloud is confined by a vertical electric field that has also been shown to aid in transport of photoelectrons. 9 Finally, modulation doping of this layer makes the growth compatible with HEMT. This top AlGaAs layer is deltadoped, rather than uniformly, in order to take advantage of high channel electron density, reduced trapping effects, and improved threshold voltage as well as high breakdown characteristics.…”

“…5 In particular, we have previously proposed AlGaAs/GaAs heterostructure metalsemiconductor-metal photodetectors ͑HMSM-PDs͒ that show much less dark current than conventional MSM due to both the two-dimensional electron gas ͑2DEG͒ and the effect of barrier enhancement due to the wide-gap material. [7][8][9] A common problem with planar, as well as vertical, photodetectors is the trade off between speed and quantum efficiency; in order to achieve a fast response from photodetectors, the depleted absorption region needs to be small for reduced path length, but this results in a decreased responsivity due to small absorption depth. Resonant cavity technique offers the possibility to balance such conflict between fast speed and sensitivity.…”