目前運算能力不斷提高以及降低處理器的功耗需求逐漸增加，互補式金屬氧化物半導體由於尺寸持續微縮導致下一代傳統矽電晶體嚴重限制了對摩爾定律發展。為了持續滿足摩爾定律的下一代電晶體性能，必須發展一個新的材料來解決此問題。
　　過去十五年以來，二維半導體材料受到了很多關注，並且有希望取代傳統矽電晶體的通道材料，從而提高了計算能力。截至目前為止，雖然已經探索了許多不同的二維半導體材料，例如，石墨烯（graphene）是一種非常高遷移率的材料，但由於其能隙極小，我們不能將其用作於電晶體的通道材料；而二硫化鉬（MoS2）雖然具有非常好的電子和光電特性，但是二硫化鉬（MoS2）擁有較高的有效質量（m*= 0.45），這對於高速電晶體來說不太可能是一個最佳的選擇。
　　在尋找具有高遷移率二維半導體材料時，我們發現硒化銦（InSe）具有非常高的本質遷移率，可當作良好的通道材料。只不過當我們施加背向閘極電壓時，硒化銦（InSe）表面顯現出不穩定的化學特性．為了能夠防止硒化銦（InSe）表面所生成之嚴重氧化問題，於實驗中使用乾氧化處理。另外，我們還探討當使用不同的金屬當作金屬－半導體接面的接觸時，硒化銦（InSe）電晶體光電特性之變化。結果顯示在的眾多金屬中，金屬銦（In）似乎很有前景，因為它能夠扮演硒化銦（InSe）通道中電子摻雜的豐富來源，進而減少金屬和半導體之間的蕭特基能障。
　　此外，在論文中，我們也探討了量子點應用於光電探測器/光電晶體。其中，選擇二硫化鎢（WS2），因為它是一種良好的光子吸收材料。在二硫化鎢（WS2）成長幾天後，當暴露於大氣時，我們注意到二硫化鎢（WS2）表面被氧化並形成三角形空隙。我們將二硫化鎢（WS2）隨著時間的變化作拉曼和光致發光研究，其中拉曼和光致發光實驗的強度隨著天數的增加而持續減少。注意到二硫化鎢（WS2）的這種自然屬性後，我們決定於空隙中生長二硫化鉬（MoS2），使這個作為的二硫化鉬（MoS2）/二硫化鎢（WS2）的量子點異質結構。STEM圖像進一步顯示了鉬（Mo）原子取代了缺失的鎢（W）原子。我們的結果顯示通過微調量子點尺寸可以進一步提高檢測範圍和光響應特性

There is an upsurge in the demand for increasing the computation capability and reducing
the power consumption in a processor. The limitation in downscaling the limits of CMOS for
next generation conventional silicon transistors pose a huge restriction to Moore’s law. In spite
of the advances in technology, the next generation transistors would fail to follow Moore’s law.
In order to meet Moore’s law for the next generation transistors, a new direction of thinking is
necessary. Since a decade and a half, 2D semiconductor material has gained a lot of attention
and promises to replace the channel material for conventional silicon transistors, thus boosting
the computation capability.
Even though a lot of different two dimensional semiconductor materials have been explored,
a single best pick is still far from reality. Each material comes with its own set of advantages
and disadvantages. For instance, graphene is a very high mobility material. But due to its
non-bandgap, we cannot use this material as a channel material for transistor. Another example
would be MoS2 which has very good electronic and optoelectronic properties. However,
MoS2 has higher effective mass (m* = 0.45), which is not likely a good choice for high speed
transistors.
In the search of high mobility 2D semiconductor material, we find InSe with a very high
intrinsic mobility and therefore a very good channel material to be a suitable candidate. Its
chemical instability presented a huge challenge though for we used back gate configuration.
To prevent the severe oxidation problem on the InSe surface, we used dry oxygen treatment.
We also explored different metal contacts to InSe transistor for improving the performance.
Among the many metals that we explored, Indium seems promising as it acts as a rich source
of electron doping into the InSe channel, thus reducing the schottky barrier between metal and semiconductor.

Further into the thesis, we explored a quantum dot photodetector/phototransistor. We choose
WS2 since it is a very good photon absorbing material. After a few days of WS2 growth, we noticed
that the surface gets oxidized and forms triangular voids when exposed to atmosphere. We
use a set of material characterization techniques such as time dependent Raman and photoluminence
studies of WS2 material, where the intensity of Raman and photoluminence experiments
keep decreasing as days pass on. After noticing this natural property of WS2, we decided to
grow MoS2 in the voids and make this as a quantum dot heterostructure of MoS2/WS2. STEM
images further reveals that Mo atom has taken the place of the missing W atom. Our results
indicate that the detecting range and photoresponsivity can be increased further by fine tuning
the quantum dot size.

1 Introduction 1
1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 Thesis organisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
I Field-effect transistor based on 2D semiconductors 5
2 Properties of InSe material and its material characterization 7
2.1 Introduction to 2D materials and 2D semiconductors materials . . . . . . . . . 7
2.2 Electrical properties of InSe material . . . . . . . . . . . . . . . . . . . . . . . 10
2.2.1 Raman analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.2.2 Photoluminescence analysis . . . . . . . . . . . . . . . . . . . . . . . 16
2.2.3 TEM Analysis of InSe multilayer . . . . . . . . . . . . . . . . . . . . 17
2.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3 InSe Device fabrication and electrical measurements 21
3.1 Sample cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.2 Mechanical exfoliation from bulk to 270 nm SiO2/Si . . . . . . . . . . . . . . 21
3.3 Hard mask based metal contact pads for InSe backgate transistor . . . . . . . . 23
3.3.1 Electrical properties of surface unprotected InSe device . . . . . . . . . 25
3.4 Surface protected InSe device . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.4.1 Electrical properties of surface protected InSe device . . . . . . . . . . 27
3.5 Different work functions of metals for InSe devices . . . . . . . . . . . . . . . 28
3.5.1 Indium as electron dopant for InSe . . . . . . . . . . . . . . . . . . . . 29
3.5.2 Electrical properties of Au/In InSe transistor . . . . . . . . . . . . . . 29
3.5.3 Chromium as adhesion layer between gold and InSe . . . . . . . . . . 33
3.5.4 Electrical properties of Au/Cr InSe device . . . . . . . . . . . . . . . . 34
3.5.5 Titanium as adhesion layer for InSe device . . . . . . . . . . . . . . . 36
3.5.6 Electrical properties of Titanium based InSe transistor . . . . . . . . . 37
3.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
V
Contents
II Quantum dot phototransistor 41
4 Photon absorption theory and material characterisation of WS2 and MoS2/WS2 43
4.1 Introduction to phototransistor/photodetector . . . . . . . . . . . . . . . . . . 43
4.2 Photon detection Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
4.3 Photodetector figures of merit . . . . . . . . . . . . . . . . . . . . . . . . . . 44
4.4 Electronic and optical properties of transition metal dichalcogenides (TMD) . . 48
4.5 Quantum dot photodetector based on WS2/MoS2 heterostructure . . . . . . . . 50
4.6 Characterization of grown WS2 and MoS2/WS2 heterostructure: Raman and
Photoluminescence studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
4.7 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
5 Quantum dot photodetector performance 63
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
5.2 Device fabrication and experimental results . . . . . . . . . . . . . . . . . . . 63
5.2.1 Transferring material from sapphire to silicon substrate . . . . . . . . . 63
5.3 Source drain contacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
5.4 Isolation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
5.5 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
5.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
6 Conclusion and future work 75
Appendix A MoS2 Growth 77
Appendix B Spinning process to open the window for source/drain contact 79
Appendix C Resist etching by RIE 81
Bibliography 83

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