Hole to Electron Crossover in a (Cd,Mn)Te Quantum Well through Surface Metallization

TL;DR

The paper investigates how surface metal contacts influence the local carrier type in a p-type (Cd,Mn)Te quantum well. Using five 10 nm metal pads (Au, Ag, Cr, Ni, Ti) and magneto-spectroscopy of the singlet–triplet transition, it distinguishes the carrier sign via giant Zeeman splitting, with $\Delta E_X(B)=\Delta E_{el}+\Delta E_{hh}$ and $\Delta E_{el}=xN_0\alpha\langle S^z_{Mn}\rangle$, $\Delta E_{hh}=-xN_0\beta\langle S^z_{Mn}\rangle$. Au and Ni uniquely convert the QW to n-type by passivating the surface and enabling interfacial hybridization that shifts the Fermi level, while Ag, Ti, and Cr largely preserve p-type. These findings show that simple work-function arguments are insufficient and provide practical guidance for selecting metal contacts in Cd,MgTe QWs for fundamental studies and applications.

Abstract

In this work we look into how the contact material influences the local charge properties of a p-type CdTe-based quantum well. We study five metals deposited as 10 nm layers on the sample surface: Au, Ag, Cr, Ni and Ti. We use magneto-spectroscopy to discriminate their charge states through monitoring the Zeeman shifts at singlet-triplet transitions. Most tested metals retain the original p-type of the QW, while gold and nickel coverage flips the local doping to n-type. This is attributed to a robust bonding of these two metals to the semiconductor, efficiently passivating its surface and thus improving electron diffusion from the metal to the quantum well.

Figures (3)

• Figure 1: (a) Schematic depiction of sample QW1: 10 nm of metals evaporated onto the quantum well surface (not in scale). (b) Photoluminescence spectra measured at metallized areas. T = 5 K, $\lambda_{\mathrm{exc}}=647$ nm, $P_{\mathrm{exc}}=1\,\upmu$W.
• Figure 2: (a) Upper panel: giant Zeeman shift of the neutral exciton as a function of magnetic field. Exciton splitting $\Delta E_X$ is obtained as the energy difference between $\sigma^-$ and $\sigma^+$ signals. Lower panel: experimental giant Zeeman shift of the neutral exciton as a function of calculated exciton splitting $\Delta E_X$, after fitting the modified Brillouin function gaj50 to the data in the upper panel. (b) Energy levels diagram for the singlet-triplet transition in an n-type (Cd,Mn)Te QW.
• Figure 3: (a) Photoluminescence scan in magnetic field after correcting the $x$ axis for Zeeman splitting. Measurement for a pristine QW on the left, and for the metallized gold contact on the right. Orange line marks Zeeman shifts at singlet-triplet transition. (b) Complete results of Zeeman shifts at singlet-triplet transition of all materials on all samples: QW1, QW2 and QW3. Red line marks the value for a pristine QW for comparison. Cr$_2$O$_3$ was also deposited and subsequently measured. Multiple points for one material are measurements at different temperatures or on other samples.