Effect of high temperature annealing (T > 1650 °C) on the morphological and electrical properties of p-type implanted 4H-SiC layers

Abstract

This work reports on the effect of high temperature annealing on the electrical properties of p-type implanted 4H-SiC. Ion implantations of Aluminium (Al) at different energies (30 -200 keV) were carried out to achieve 300 nm thick acceptor box profiles with a concentration of about 10 20 at/cm 3 . The implanted samples were annealed at high temperatures (1675-1825 °C). Morphological analyses of the annealed samples revealed only a slight increase of the surface roughness RMS up to 1775°C, while this increase becomes more significant at 1825°C (RMS=1.2nm). Room temperature Hall measurements resulted in a hole concentration in the range 0.65-1.34×10 18 /cm 3 and mobility values in the order of 21-27 cm 2 V -1 s -1 . The temperature dependent electrical measurements allowed to estimate an activation energy of the Al-implanted specie of about 110 meV (for the post-implantation annealing at 1675°C) and a fraction of active p-type Al-dopant ranging between 39% and 56%. The results give useful indications for the fabrication of 4H-SiC JBS and MOSFETs.The main dopant species for SiC are Nitrogen (N) and Phosphorous (P) for n-type doping, and Aluminum (Al) for p-type doping. High post-implantation annealing temperatures (> 1500°C) are typically required to bring these species in substitutional positions and achieve their electrical activation [5,6,7]. In particular, selectively doped p-type regions are key parts of both JBS and MOSFETs and the control of their electrical properties has a significant impact on several devices parameters (e.g., Ohmic contacts formation, device on-resistance, threshold voltage and channel mobility, etc.).Hence, understanding the dependence of the properties of p-type implanted layers on the activation annealing temperature is very important for device manufacturers to set the right process for the optimal device characteristics. In this context, although several investigations reported on the properties of Al-implanted 4H-SiC layers [8,9,10], the large variety of experimental conditions and the evolution of the annealing procedures always make this topic open to scientific discussions.Hall-effect measurements are often used to study the electrical properties of p-type 4H-SiC layers, in order to determine key parameters like the holes concentration and mobility [11]. A critical issue of this methodology is the choice of the Hall scattering factor rH for SiC [12,13]. In fact, the difficulty to extract the mobility and the free hole concentration from Hall measurements is related to the correct knowledge of rH. However, many authors often interpret the experimental Hall results on p-type 4H-SiC assuming rH=1, which in turn leads to an overestimation of the doping level in the material [9,14]. Only few works specifically reported experimental calculations of the Hall scattering factor rH for SiC [15,16], whose findings should be considered for a correct analysis of the currently available data.This paper reports on the morphological and electrical properties of p-type implanted 4H-SiC annealed...

Ohmic Contacts on p-Type Al-Implanted 4H-SiC Layers after Different Post-Implantation Annealings

“…In particular, the linear fits of the data gave the following values of sheet resistance of the p-type-implanted 4H-SiC: 12.4 kΩ/sq (T ann = 1675 °C), 9.8 kΩ/sq (T ann = 1775 °C), and 8.0 kΩ/sq (T ann = 1825 °C). Such values are in agreement with those extracted by Hall measurements (see Table 1), considering the thickness of the implanted layer (300 nm) [12]. Moreover, the resistivity values are consistent with the literature data obtained for 4H-SiC layers with a similar acceptor concentration (3–6 × 10 19 cm −3 ) [10,14,17,24,25].…”

“…The activation energy E A of the Al-implanted dopant was determined from an Arrhenius plot of the 4H-SiC sheet resistance. The values of E A in the range of 99–110 meV are in agreement with typical literature values reported for these high acceptor concentrations, as well as with the theoretical relation between E A and N A in 4H-SiC [12,32].…”

“…In this context, the Ohmic contacts formation on p-type 4H-SiC is inherently a challenging task, due to the wide band gap of the material (leading to high metal/semiconductor barrier heights) and to the high ionization energy of the Al acceptors [8]. Moreover, since the properties of Al-implanted 4H-SiC layers critically depend on the large variety of reported experimental doping and annealing conditions [9,10,11,12], understanding the behavior of Ohmic contacts on such layers remains an open issue with relevant implications for device fabrication.…”

The Impact of Process Conditions on Surge Current Capability of 1.2 kV SiC JBS and MPS Diodes

“…Through Hall measurement on the Van der Pauw test structure, the activated aluminum is only 5% of the total dose for the room temperature implantation sample, whereas it is 12% for the 500 °C implantation sample. If corrected data rH is used, considering the results in the literature [ 21 , 22 , 23 ], a different active ratio is obtained and the actual active ratio of the room temperature decreases from 5% to 2.5%. While the actual active ratio of the high temperature is decreased from 12% to 6%.…”

4H‐SiC surface morphology after Al ion implantation and annealing with C‐cap

“…For example, groves are observed by Atomic Force Microscopy (AFM) on the surface of the hexagonal polytype 4H‐SiC after annealing at temperatures beyond 1650°C. Groves are observed only when the Al implantation temperature exceeds 200°C and increase in size for increasing annealing temperature (Hailei et al ., 2011, Michaud et al ., 2013; Spera et al ., 2019). In some cases, SIMS analyses reveal possible Al loss through the cap (Hirono et al ., 2009; Lazar et al ., 2015), but no correlation with surface changes has been demonstrated.…”