Enhanced mobility of Sn-doped Ge thin-films (≤50 nm) on insulator for fully depleted transistors by nucleation-controlled solid-phase crystallization

Abstract

High-speed fully depleted thin-film transistors (TFTs) for low-power consumption are required for next-generation electronics, such as three-dimensional large-scale integrated circuits and advanced system-in-displays. For this purpose, high-carrier-mobility semiconductor thin-films (thickness: ≤∼50 nm) on insulator structures should be fabricated under low-temperature processing conditions (≤500 °C). To achieve this, solid-phase crystallization of amorphous GeSn (a-GeSn) with low Sn concentration (2%) is investigated for a wide range of film thicknesses (30 − 200 nm), where thin a-Si underlayers (thickness: 0 − 20 nm) are introduced between a-GeSn films and substrates. GeSn is polycrystallized by annealing at 450 °C, keeping Si underlayers amorphous. Crystal grains of almost identical sizes are obtained for GeSn thicknesses of 30 − 50 nm, though grain sizes significantly decrease for thicknesses exceeding 50 nm owing to enhanced bulk nucleation. A detailed analysis of GeSn films (thickness: 50 nm) reveals that grain sizes are decreased by introducing a-Si underlayers (thickness: 3 − 20 nm), e.g., from ∼10 μm to 2 − 3 μm. This phenomenon is attributed to the change in dominant nucleation sites from the interface to the bulk, which significantly decreases grain-boundary scattering of carriers through a decrease in the barrier heights at grain boundaries. As a result, a high carrier mobility of 200 − 300 cm2/V s is realized for GeSn thin-films (thickness: 30 − 50 nm) grown with a-Si underlayers. The mobility (200 − 300 cm2/V s) is the largest ever reported data for Ge and GeSn thin-films (thickness: 30 − 50 nm) grown at low temperatures (≤500 °C). This technique will facilitate the realization of high-speed fully depleted TFTs for next-generation electronics.

Influence of grain boundaries on the properties of polycrystalline germanium

“…The grain size exceeds 8 μm for all samples except t = 500 nm. The grain size reduction in large t is likely because the nucleation in the bulk increases with increasing t. 28,43 The density of GBs surrounding (i.e., identifying) grains shows a relatively high value at t = 500 nm, which is consistent with the behavior of the grain size.…”

“…Figure 5(b) shows that E B decreases with increasing t. Particularly, for t = 100 nm, E B shows high values over 20 meV for the glass sample and 13 meV for the plastic sample. These E B values are much larger than that of SPC-Ge on SiO 2 39 and a-Si, 28 suggesting that the GB property near the Ge/GeO 2 interface is inferior to those of Ge/SiO 2 and Ge/a-Si. Therefore, for thin Ge layers (t ≤ 100 nm), the a-Si underlayer might be more suitable than GeO 2 for higher μ.…”

“…Taking advantage of this feature, many techniques to synthesize polycrystalline (poly-) Ge at low temperatures on glass and plastic substrates have been studied. [18][19][20][21][22][23][24][25][26][27][28] Ge-TFTs have been established using poly-Ge formed by solid-phase crystallization (SPC), [29][30][31] laser annealing, 32 seed layer technique, 33 and metal-induced crystallization. [34][35][36][37] However, poly-Ge generally has a high hole concentration due to defect-induced acceptors and a low hole mobility due to grain boundary (GB) scattering, 18,38 limiting the TFT performance.…”

Ge(Sn) growth on Si(001) by magnetron sputtering

“…Ge 1x Sn x is a CMOS-compatible IV-IV semiconductor exhibiting interesting properties for infrared photodetector, light emitting diode, and laser fabrication [20][21][22][23][24][25][26]. Furthermore, Ge 1x Sn x alloys can be used for the fabrication of integrated optical amplifier, Gas sensors [27], and high-speed thin-film transistors [28][29], and direct band-gap Ge 1x Sn x alloys are of high interest for solar cells [30]. Ge 1x Sn x single crystals are expected to be best suited for all these applications, since grain boundaries (GB) in polycrystalline films create deep band-gap levels and act as effective recombination centers, and impair device efficiency.…”

Four-step heating process for solid-phase crystallization of Ge leading to high carrier mobility

“…The TPDA = 500 °C sample exhibits the highest µ value reaching 530 cm 2 V −1 s −1 . This is the highest value for Ge thin film (<300 nm) formed directly on the glass, 47 which is the advantage of fabricating a thin film transistor with a low leakage current.…”