HEMT Amplified SET Measurements of Individual InGaAs Quantum Dots

A high electron mobility transistor (HEMT) is used with a single-electron transistor (SET) to measure single electrons tunnelling into individual InGaAs quantum dots. The SET detects a change in location of an electron once it tunnels from an underlying n-doped layer into a quantum dot lying in an intermediate layer. A HEMT on the He3 stage with the SET is used to extend the measurement bandwidth to 400 kHz. We demonstrate this technique with a measurement of the Stark shift in the first electron state of the quantum dot as a function of lateral electric field. The electron states of individual self-assembled quantum dots must be controllably occupied in order to develop a quantum dot-based electrically-triggered singlephoton source. Here and in previous work [1], we measure individual electrons tunnelling into quantum dots with a single-electron transistor (SET). However, in this work we have increased the bandwidth of the measurement, which allows us to quickly obtain the energies of electron states as a function of applied fields. We demonstrate our improved technique by measuring the Stark effect in the ground state of a quantum dot as a function of the applied lateral electric field. By themselves SETs typically have a low bandwidth since the high resistance across the SET electrodes is coupled to wiring that capacitively filters frequencies greater than approximately 1 kHz. In order to improve the bandwidth of SET measurements, high electron mobility transistor (HEMT) amplifiers [2] and RF amplifiers [3] have been used. We have selected a commercially available GaAs HEMT chip in our application. The HEMT has the advantage of being small and inexpensive compared to a RF amplifier, and therefore several SETs can be amplified with HEMTs in a single cooldown of the dewar. A schematic of the SET and HEMT, including the capacitively coupled quantum dot and gates, is shown in Fig. 1. The SET is current-biased with a 15 MΩ chip resistor, and the SET voltage VSET is sent to the gate of the HEMT chip. Along with the SET, the resistor and HEMT are on the sample stage of a He3 dewar, which allows for a small stray capacitance to ground. Since VSET is small, the source-gate bias is controlled by a voltage Vso applied to the source. The HEMT is biased with a large resistor on the drain lead to keep the power dissipated by the HEMT at about 1 μW. The V G1 V G2 V n QD V SET V Dr