Uni-Traveling-Carrier Photodiode Based on MoS2/GaN van der Waals Heterojunction for High-Speed Visible-Light Detection

Abstract

Uni-traveling-carrier photodiodes (UTC-PDs), which utilize only electrons as the active carriers, have become indispensable in high-speed optoelectronics due to their unique capabilities, such as high saturation power and broad bandwidth. However, extending the operating wavelengths into the visible region for wider applications is challenging due to the lack of suitable wide-bandgap III-V semiconductor combinations with the necessary band alignment and lattice matching. Here, we show that a UTC-PD based on a van der Waals heterojunction composed of a 2D transition metal dichalcogenide, molybdenum disulfide (MoS2), as a photoabsorption layer and a gallium nitride (GaN) film as a carrier transport layer, offers a solution to this challenge. The fast vertical carrier transport across the heterointerface is enabled by the direct epitaxial growth of a MoS2 layer on a GaN film. Our device demonstrates a frequency response in the several-GHz range with a quantum efficiency on the order of 1% throughout the entire visible spectrum, highlighting the promise for high-speed visible optoelectronics.

Figures (11)

• Figure 1: a, Band structure of a MoS$_2$/GaN UTC-PD. b, Raman spectra of the MoS$_2$layer grown on a GaN/sapphire substrate. The inset shows the details of the Raman peaks due to the MoS$_2$ layer. c, High resolution high-angle annular dark field (HAADF) cross-sectional STEM image of MoS$_2$/GaN heterojunction. The MoS$_2$ film consists of 2 to 4 layers, being consistent with the result of the Raman spectroscopy. d, Absorption spectrum of the MoS$_2$/GaN heterostructure measured by a spectrophotometer. At wavelengths below 380 nm, absorption predominantly originates from the GaN layer. For the reference, the inset graph shows the absorption spectrum of the bare MoS$_2$ layer directly grown on the sapphire substrate.
• Figure 2: a, Kelvin Probe Force Microscope (KPFM) mapping image of MoS$_2$ on GaN/sapphire substrate, showing change in surface potential between MoS$_2$ and GaN. b, Plot of surface potential difference across MoS$_2$/GaN interface along the line as indicated in (a). c, Schematic of the band alignment of the MoS$_2$/GaN van der Waals heterojunction.
• Figure 3: Device structure and electrical characteristics of MoS$_2$/GaN UTC-PD. a, Three-dimensional image of the fabricated device acquired using a laser microscope. b, Cross-sectional schematic image. The blue arrow illustrates backside illumination in our characterization. The device structure consists of three main layers: an n-GaN layer (thickness: 3.6 $\mu$m, carrier density: $4.22 \times 10^{19}$ cm$^{-3}$), an i-GaN layer (thickness: 0.3 $\mu$m, carrier density: $10^{15} \sim 10^{16}$ cm$^{-3}$), and a MoS$_2$ layer (2--4 layers, sheet carrier density: $8.5 \times 10^{13}$ cm$^{-2}$). c, I--V characteristics of the device measured under dark condition.
• Figure 4: Photocurrent signal of the device is shown as the red line, while the pulse waveform signal of the incident light is shown as the black line. Here, the latter signal is taken by a Si PIN-photodiode (Hamamatsu Photonics; S9055) as a reference. The rise and fall times of the incident light are on the order of microseconds, which closely follows the pulse waveform of the incident light.
• Figure 5: Frequency response of the device. a, Experimental setup for measuring the frequency response of the fabricated MoS$_2$/GaN UTC-PD. A single-mode laser diode (LD) operating at 465 nm is modulated by a waveguide phase modulator to generate small intensity modulation via a stabilized Mach--Zehnder interferometer. An AC photocurrent signal is fed into a vector network analyzer through a transmission line probe and a wideband amplifier matched at 50 $\Omega$. b, Frequency response characteristics of the device at a wavelength of 465 nm with a reverse bias of 3 V, normalized by low-frequency sensitivity. Non-flat frequency response of the measurement system is calibrated by a reference GaAs PIN-photodiode that have a flat frequency response up to 20 GHz. The raw signal (gray line) contains large fluctuations both in low and high frequencies due to the 1/f noise and the crosstalk of the radiation from the driving signal. Smoothing is applied (blue line) to see the salient feature of the frequency response. A simple RC low-pass filter response (black line) is also shown.