Achieving surface recombination velocity below 10 cm/s in n-type germanium using ALD Al2O3

Abstract

Desirable intrinsic properties, namely, narrow bandgap and high carrier mobility, make germanium (Ge) an excellent candidate for various applications, such as radiation detectors, multi-junction solar cells, and field effect transistors. Nevertheless, efficient surface passivation of Ge has been an everlasting challenge. In this work, we tackle this problem by applying thermal atomic layer deposited (ALD) aluminum oxide (Al2O3), with special focus on the process steps carried out prior to and after dielectric film deposition. Our results show that instead of conventional hydrofluoric acid (HF) dip, hydrochloric acid (HCI) pre-treatment is an essential process step needed to reach surface recombination velocities (SRVs) below 10 cm/s. The main reason for efficient surface passivation is found to be a high dielectric charge that promotes the so-called field-effect passivation. Furthermore, the results demonstrate that the post-deposition anneal temperature, time, and ambient play a role in passivating Ge-dangling bonds, but surprisingly, good surface passivation (SRV below 26 cm/s) is obtained even without any post-deposition annealing. The results pave the way for high-performance n-type Ge optoelectronic devices that could use induced junctions via negatively charged Al2O3 layers.

Comparison of SiNx‐Based Surface Passivation Between Germanium and Silicon

“…It is good to note that here the Ge bulk likely has an impact on the measured lifetime, since we achieved only a slightly higher lifetime of ∼800 μs from the same substrate material using well‐optimized Al 2 O 3 surface passivation. [ 6 ] This means that the SRV for the Ge sample with the annealed SiN x /Al 2 O 3 stack is in reality somewhat lower than the calculated maximum value.…”

“…The Si substrates were 280 μm thick n‐type (phosphorous‐doped) Float‐zone (FZ) wafers with <100> orientation, 100 mm diameter, and 1–5 Ωcm resistivity. To clean the wafers and to remove the native oxide, the Ge wafers were dipped to 31.6% HCl for 90 s without a (consecutive) de‐ionized water (DIW) rinse, [ 6 ] whereas the Si wafers were cleaned by the standard RCA cleaning sequence with a hydrofluoric acid (HF) dip for 90 s followed by DIW rinsing as the last step. After the cleaning, a 20 nm‐thick SiN x layer was deposited on both sides of the Ge and Si substrates using a radio‐frequency (RF, 13.56 MHz) PECVD system (Plasmalab 80 Plus) at 380 °C with silane (SiH 4, gas flow 17.5 sccm), ammonia (NH 3, gas flow 50 sccm) and nitrogen (N 2, gas flow 332.5 sccm) as the precursor gases.…”

“…Some progress on this topic has recently been achieved: Isometsä et al achieved a low surface recombination velocity (SRV) of 6.55 cm s À1 and a high negative charge density (Q tot ) over À2 Â 10 12 cm À2 by ALD Al 2 O 3 combined with a hydrochloric acid (HCl) pre-treatment. [6] Simultaneously, Berghuis et al reported an exceptionally high negative Q tot of $À1 Â 10 13 cm À2 and a low SRV of $2.6 cm s À1 using a hydrogenated amorphous silicon (a-Si:H)/Al 2 O 3 stack, synthesized by plasma-enhanced chemical vapor deposition (PECVD) and plasma-enhanced ALD (PEALD), respectively. [7] The aforementioned research on field effect passivation of Ge has focused on ALD Al 2 O 3 which is known to have a high negative charge.…”

Carrier Recombination Dynamics of Surface-Passivated Epitaxial (100)Ge, (110)Ge, and (111)Ge Layers by Atomic Layer Deposited Al2O3

“…The integration of high-κ gate dielectrics such as HfO 2 and Al 2 O 3 on (100) and (110) crystal planes should not produce defects due to the effect of process temperature during deposition, rather only passivating the surface states and eliminating the interdiffusion of high-κ dielectric and channel materials. , Using these technologically important crystal planes, (100) and (110), from high electron and hole mobility Ge channel materials compared with Si along with a high-κ dielectric, one could make FinFET , or gate-all-around (GAA) NSFET − for high-density and ultralow-power CMOS. However, the interfacial defects at the high-κ (e.g., HfO 2 ) and Ge channel material would require an interface passivation layer (IPL) to control the interface state density ( D it ). , An in situ ultrathin SiO 2 passivation layer from a tris­( tert -butoxy)­silanol (Si­(OH)­(OC-(CH 3 ) 3 ) 3 ) precursor during atomic layer deposited (ALD) gate dielectric or by a Si IPL layer and/or GeO x via thermal oxidation or by ozone oxidation was prescribed as IPL layer formation strategies for subsequent Al 2 O 3 and composite Al 2 O 3 /HfO 2 dielectrics deposition. − In either case, the main objective was to reduce the D it at the high-κ/Ge heterointerface. Over the last two decades, surface recombination velocities (SRV) ≤ 1 cm/s were reported for Si, − and ultralow SRV values were made possible by superior surface passivation with low D it ≈ 10 9 –10 12 eV –1 cm –2 . − The extensive research on the surface passivation of Si with low D it over the decades has been transferred to p-Ge and n-Ge by several researchers. − Noticeably, Berghuis et al have studied the surface passivation of bulk Ge using a combination of plasma-enhanced ALD (PEALD) and thermal ALD ...…”

“…However, the interfacial defects at the high-κ (e.g., HfO 2 ) and Ge channel material would require an interface passivation layer (IPL) to control the interface state density ( D it ). , An in situ ultrathin SiO 2 passivation layer from a tris­( tert -butoxy)­silanol (Si­(OH)­(OC-(CH 3 ) 3 ) 3 ) precursor during atomic layer deposited (ALD) gate dielectric or by a Si IPL layer and/or GeO x via thermal oxidation or by ozone oxidation was prescribed as IPL layer formation strategies for subsequent Al 2 O 3 and composite Al 2 O 3 /HfO 2 dielectrics deposition. − In either case, the main objective was to reduce the D it at the high-κ/Ge heterointerface. Over the last two decades, surface recombination velocities (SRV) ≤ 1 cm/s were reported for Si, − and ultralow SRV values were made possible by superior surface passivation with low D it ≈ 10 9 –10 12 eV –1 cm –2 . − The extensive research on the surface passivation of Si with low D it over the decades has been transferred to p-Ge and n-Ge by several researchers. − Noticeably, Berghuis et al have studied the surface passivation of bulk Ge using a combination of plasma-enhanced ALD (PEALD) and thermal ALD a-Si:H/Al 2 O 3 as well as a PEALD Al 2 O 3 stack and achieved an SRV as low as 2.7 cm/s. The D it ≈ 1 × 10 12 eV –1 cm –2 was determined from capacitance–voltage measurements of a PEALD Al 2 O 3 /Ge metal–oxide–semiconductor capacitor.…”