Local laser-induced solid-phase recrystallization of phosphorus-implanted Si/SiGe heterostructures for contacts below 4.2 K

Si/SiGe heterostructures are of high interest for high-mobility transistor and qubit applications, specifically for operations below $4.2\mathrm{K}$. In order to optimize parameters such as charge mobility, built-in strain, electrostatic disorder, charge noise, and valley splitting, these heterostructures require Ge concentration profiles close to monolayer precision. Ohmic contacts to undoped heterostructures are usually facilitated by a global annealing step activating implanted dopants, but compromising the carefully engineered layer stack due to atom diffusion and strain relaxation in the active device region. We demonstrate a local laser-based annealing process for recrystallization of ion-implanted contacts in SiGe, greatly reducing the thermal load on the active device area. To quickly adapt this process to the constantly evolving heterostructures, we deploy a calibration procedure based exclusively on optical inspection at room temperature. We measure the electron mobility and contact resistance of laser-annealed Hall bars at temperatures below $4.2\mathrm{K}$ and obtain values similar or superior to that of a globally annealed reference sample. This highlights the usefulness of laser-based annealing to take full advantage of high-performance Si/SiGe heterostructures.

Figure 1

Laser annealing and process evaluation. (a) Schematic of studied heterostructure (stack detailed in Table 1) and laser annealing setup. A cw laser with $\lambda =532$ nm is scanned line by line (black line) over the sample and controlled via a shutter to only hit the designated area (pink) inside the implanted regions (purple). The tool allows control of the laser power $P$, scan speed $v$, and line spacing $\mathrm{\Delta }y$. (b) Optical inspection (left) and c-DIC image (right) of a $400\mu \mathrm{m}\times 400\mu \mathrm{m}$ square scanned with {$2.6\mathrm{W}$, $25\mathrm{mm}/\mathrm{s}$}, (c) {$2.6\mathrm{W}$, $2.5\mathrm{mm}/\mathrm{s}$}, and (d) {$6.2\mathrm{W}$, $2.5\mathrm{mm}/\mathrm{s}$} on wafer B. (e) Room-temperature current $I$ versus voltage $V$ characteristic for the squares shown in panels (b)–(d) (dot colors match frame colors). Data are obtained in van der Pauw geometry (black squares are electrical contacts in inset). Lines are linear least-square fits to the data. $I$ is measured parallel to the laser scan line (solid dots, solid line) and perpendicular to it (open dots, dashed line) as sketched in the inset.

Figure 2

Process calibration. (a) Map of success rate $S$ as a function of parameter set $\{P,v\}$ for wafer A. A successful parameter set is defined as exhibiting a homogeneous color change and no indication of surface morphology change. (b) Projected success rates $S\left(P\right)$ (top) and $S\left(v\right)$ (bottom) obtained by averaging along the $v$ or $P$ axis for wafers A–D (color coded). (c) Projected sheet resistances $R_{\mathrm{sq}}\left(P\right)$ (top) and $R_{\mathrm{sq}}\left(v\right)$ (bottom) of successfully annealed regions obtained by vdP measurements at $300\mathrm{K}$.

Figure 3

Hall bar measurements. (a) Optical microscope image after LA of a sample with implanted contacts (shown in yellow as an example) for a Hall bar structure. Subsequent fabrication steps are indicated in black (mesa channel etch), orange (contact pad evaporation), and red (gate evaporation). Exemplary circuit diagrams for the two-point or four-point contact resistance (white) and magnetotransport measurements (black) are shown. The channel of the Hall bar has a width of $20\mu \mathrm{m}$ and the contact pairs have a spacing of $100\mu \mathrm{m}$ with a spacing of $200\mu \mathrm{m}$ between pairs. Implanted areas with crossed-out contact areas are test structures for fabrication and are not connected to the channel. (b) Longitudinal ${\rho }_{xx}$ and transversal ${\rho }_{xy}$ resistance obtained by magnetotransport measurements at a temperature of $1.6\mathrm{K}$ and a gate voltage of $V_{\mathrm{G}}=0.6\mathrm{V}$. The structure was annealed using LA parameter set ${\mathrm{S}4}^{*}=$ {$1.8\mathrm{W}$, $2.5\mathrm{mm}/\mathrm{s}$}. The Hall resistance corresponding to the filling factors $n_{\mathrm{F}}=4$, 6, 8 are indicated as a guide for the eye in the graph. An electron density of $n=3.5\times {10}^{11}{\mathrm{cm}}^{-2}$ and an electron mobility of $\mu =1.6\times {10}^5{\mathrm{cm}}^2/\left(\mathrm{V}\mathrm{s}\right)$ are obtained.

Figure 4

Cryogenic transport measurements. (a) Mobility $\mu$ versus electron density $n$ for the laser annealed (LA) and globally annealed (GA) samples at 1.6 and $4.2\mathrm{K}$. The average of all measured segments is shown as solid lines, while the standard deviation is given as shaded areas. (b) Log-log plot of mobility $\mu$ versus electron density $n$. Data sets, corresponding to the color from panel (a), are shown as dashed lines while the two fits represent $\mu \propto n^{\alpha }$ for low and high density. Each fit is shown in solid lines with corresponding power-law exponent $\alpha$. (c) Conductivity ${\sigma }_{xx}$ versus electron density $n$. The data are shown in dashed lines, while the fits ${\sigma }_{xx}=C{(n-n_P)}^{1.31}$ to the low-electron-density regime are shown in solid lines. (d) Contact resistance obtained for LA parameter sets as shown in Fig. 2 as well as a GA reference sample. Individual data points are shown as black circles while the red crosses indicate average resistance and resistance variance.

Figure 5

Optical image of a flat implanted sample of wafer A after individual lines have been annealed with $v=10\mathrm{mm}/\mathrm{s}$ and increasing laser power $P$. The color change indicates recrystallization, which we use to determine the linewidth.

Figure 6

Map of success rates $S(P,V)$ for wafers B–D obtained by optical inspection of laser-annealed squares [cf. Fig. 2].

Figure 7

Log-log plot of electron mobility $\mu$ versus electron density $n$ for LA and GA samples, measured at $4\mathrm{K}$. Dashed lines indicate data, while solid lines show two-part-wise power-law fits. Colors correspond to the legend in Fig. 4.

Figure 8

Longitudinal conductance ${\sigma }_{xx}$ versus carrier density $n$ for LA and GA samples, measured at $4\mathrm{K}$. Solid lines indicated fit functions consistent with percolation theory. Colors correspond to the legend in Fig. 4.