Comparative Study of Lateral and Vertical Beta-Ga2O3 Photoconductive Switches via Intrinsic and Extrinsic Optical Triggering

Technical Summary: Comparative Study of Lateral and Vertical β-Ga₂O₃ Photoconductive Switches

Problem Statement
Gallium oxide (Ga₂O₃) is a leading candidate for photoconductive semiconductor switches (PCSS) in high-power, high-speed pulsed applications due to its ultra-wide bandgap (4.8 eV) and high theoretical breakdown field (8 MV/cm). However, the practical deployment of Ga₂O₃ PCSS faces challenges related to device geometry and optical triggering mechanisms. Lateral devices, while easier to fabricate, suffer from surface flashover and electric field crowding at electrode edges. Vertical devices offer improved field uniformity but require complex fabrication. Furthermore, the choice between intrinsic (band-to-band) and extrinsic (sub-bandgap via defect states) optical triggering significantly impacts performance. Intrinsic triggering requires expensive, bulky deep-ultraviolet (DUV) lasers (e.g., 245 nm), whereas extrinsic triggering allows for the use of cost-effective, compact light sources (e.g., 445 nm diode lasers) by leveraging trap states such as iron (Fe) and iridium (Ir). There is a lack of systematic experimental data comparing how device geometry (lateral vs. vertical) interacts with these distinct excitation mechanisms to dictate switching performance.

Methodology
The authors fabricated and experimentally compared lateral and vertical PCSS devices using Fe-doped semi-insulating β-Ga₂O₃ substrates (Fe concentration ~1 × 10¹⁷ cm⁻³).

• Device Fabrication:
  • Lateral PCSS: Features Rogowski-profile contacts with a 450 µm gap and 1950 µm contact length. Electrodes consist of Ti/Au stacks with ohmic contact established via rapid thermal annealing.
  • Vertical PCSS: Utilizes a uniform Ti/Au bottom contact and a patterned top contact with alternating metal and transparent regions to facilitate light injection and uniform vertical field distribution.
• Experimental Setup: Both devices were packaged in TO-257 cans and tested under a 200 V DC bias. They were triggered by a tunable OPOLETTE laser with a 7 ns pulse width.
• Excitation Conditions: The study evaluated performance under:
  • Intrinsic Excitation: 245 nm (above-bandgap).
  • Extrinsic Excitation: 280 nm, 300 nm, and 445 nm (sub-bandgap).
• Analysis: The study correlated experimental current waveforms and resistance metrics with TCAD Silvaco simulations of electric field distributions and wavelength-dependent photogeneration profiles. Optical transmittance measurements were also conducted.

Key Results
The study reveals a distinct "performance crossover" dependent on the excitation wavelength and device geometry:

1. Intrinsic Excitation (245 nm):

   • The lateral PCSS outperformed the vertical structure.
   • Performance: The lateral device achieved an on-resistance ($R_{ON}$) of 54.17 Ω and a responsivity of $1.25 \times 10^{-4}$ A/W, compared to the vertical device's $R_{ON}$ of 719.23 Ω and responsivity of $1.41 \times 10^{-5}$ A/W.
   • Mechanism: At 245 nm, the absorption depth is shallow (~0.1–1 µm). Carriers are generated near the surface where the lateral device's electric field is strongest (due to field concentration at the surface electrodes), leading to efficient collection. The vertical device's field is uniform but less intense at the specific surface generation depth.

2. Extrinsic Excitation (280 nm – 445 nm):

   • The vertical PCSS consistently outperformed the lateral structure.
   • Performance: At 445 nm, the vertical device achieved an $R_{ON}$ of 140.47 Ω (less than half the lateral device's 350 Ω) and nearly double the responsivity.
   • Mechanism: Longer wavelengths penetrate deeper into the bulk. In the lateral device, the electric field decays exponentially with depth, leaving deep-generated carriers in a low-field region with poor extraction efficiency. Conversely, the vertical device maintains a uniform, strong electric field throughout the bulk drift region, enabling efficient collection of carriers generated deep within the substrate.

3. Transient Behavior:

   • Persistent Photoconductivity (PPC): Under 245 nm and 280 nm excitation, both devices exhibited long trailing photocurrents. This is attributed to band-to-band transitions creating self-trapped holes (STHs) that suppress recombination.
   • Fast Decay: Under 300 nm and 445 nm excitation, the photocurrent decayed rapidly without persistence, as these wavelengths primarily activate Fe- and Ir-related defect states where carriers are quickly recaptured by traps.

Significance and Claims
The paper claims to present the first systematic experimental comparison of lateral and vertical β-Ga₂O₃ PCSS under both intrinsic and a wide range of extrinsic optical triggering conditions. The primary contribution is the identification of a geometry-dependent performance crossover:

• Lateral geometries are optimal for intrinsic (surface-absorbing) excitation.
• Vertical geometries are superior for extrinsic (bulk-penetrating) excitation.

The authors emphasize that these findings provide critical guidance for designing efficient, cost-effective high-voltage PCSS devices. Specifically, the results suggest that vertical Ga₂O₃ devices are uniquely suited for high-responsivity applications leveraging extrinsic photoconduction with cost-effective light sources, a behavior distinct from SiC and GaN PCSS where intrinsic excitation typically yields higher responsivity. The study underscores that optimizing Ga₂O₃ PCSS requires aligning device geometry with the specific absorption depth and carrier generation mechanism of the intended optical trigger.