Top-gated single-electron transistor in germanium nanowires

Sung-Kwon Shin, Shaoyun Huang, Naoki Fukata, Koji Ishibashi

Advanced Device Laboratory, RIKEN, Wako, Saitama 351-0198, Japan

Email: skshin@riken.jp, phone: +81-48-462-1111 (ext.8434), fax: +81-48-462-4659

Germanium nanowires (GeNWs) of the group IV semiconductors could be one of the attractive candidates for

electron-spin based quantum devices because of their long electron-spin coherence time. Besides, Ge has an

advantage over Si in terms of the larger quantum effects due to the smaller effective mass. Single-electron

transistors (SETs) are basic building blocks of such devices. To defme the spin configuration in the dot, it is

necessary to reach a few-electron regime or an even-odd regime where the single spin is realized for the odd

number of electrons in the dot. So far, we have developed processes to fabricate SETs using n-type

monocrystalline GeNWs with a back gate, and succeeded in observing the even-odd effect [1]. In this work, we

have developed fabrication processes of the top-gate SETs to enhance the gating efficiency, and succeeded in

reaching a few-electron regime.

The fabrication techniques of metal contacts to the GeNW are described in our previous report [I]. The

source and drain electrodes were deposited with 80-nm thick nickel and were separated by 300 nm. Prior to the

deposition of the top-gate electrode, lO-nm thick hafnium oxide (HfOz) thin film was deposited on top of the

GeNW by using atomic layer deposition (ALD) technique. Titanium / gold were deposited for the top-gate

electrode. A schematic image of the top-gate device and a scanning electron microscopy image of the contacted

device are shown in Fig. I(a) and Fig. 1 (b), respectively.

The electron transport characteristics were investigated in a dilution refrigerator with a base temperature

of 25 mK. Source-drain current versus source-drain voltage (lds-Vds) curves at -700 and -720 mV of Vtg are

shown in Fig. 2, respectively. The coulomb blockade gap with suppressed Ids could be modulated by tuning Vtg, demonstrating the SET operation. As shown in Fig. 3, pronounced Coulomb-oscillation peaks were observed in

gate voltages larger than -850 mY. A charge-stability diagram in a Vtg region from -950 to -600 mV is shown in

Fig. 4. Vtg positions of the Coulomb peaks occur at gate voltages where two adjacent diamonds meets. The first

diamond did not close even below -1100 mV of Vtg and opened up to ±100 mV of Vds' These facts mean the

present quantum dot is completely depleted and the number of electrons can be reduced down to zero below

Vtg=-850 mY. In our previous work, the number of electrons residing on the dot could not be reduced down to

zero even at back-gate voltage of -10 V because the NWs could be highly degenerated and the gate-conversion

factor (a) was much smaller [1]. The charging energy (Ee) and the average peak spacing (dVg) are obtained to be

lO me V and 20 m V from the dimensions of the coulomb diamonds in Fig. 4, respectively. a is thus calculated to

be 0.5. The a-value is two orders of magnitude than that of the back-gate device. The charging energy appears to

be larger than the expected value when the whole NW between the source and drain is assumed to form a single

dot. The reason is not clear, but it could be because the dot is formed with a single impurity or unexpected small

dot formed by the potential fluctuations.

References [I] S. Y. Huang et aI., J. Appi. Phys. 109, 036101 (2011)

978-1-61284-244-8/11/$26.00 ©2011 IEEE 87

(a)

Fig. 1. (a) Schematic image of the top-gate device. (b) SEM image of the nickel-contacted device.

�3 c: ::-2 c: Q) t:: 1 ::l

U c: 0 .� 0.1

I Q) �.2 ::l o

U)·3 ·15 ·10 -5 0 5 10 15

Source-Drain Voltage (mV)

Fig. 2. Source-drain current versus source-drain voltage curves.

80 -

>60 E -

Q)40 en CO

::: 20 � c: 0 .-

CO ,--20 C

•

Q)-40 � :::S-60 o

U) -80

� 1.0 Vds=1 mV -

s:::: 0.8 e ....

= U 0.6

� 0.4 C

I Q) (.) 0.2 .... 5 0 A� 2 3 . " i I • I

� 0 C=�==�����U������ ·900 -800 -700 ·600

Top-Gate Voltage (mV)

Fig. 3. Coulomb oscillation peaks at a fixed source­drain bias of 1 m V. The numbers indicate the number of electrons in the dot

-950 -900 -850 -800 -750 -700 -650 -600 Top-Gate Voltage (mV)

Fig. 4. Charge stability diagram of the device.

978-1-61284-244-8/11/$26.00 ©2011 IEEE 88