| 摘要: | 本論文的內容主要為(銻)砷化銦鎵基極異質接面雙極性電晶體(heterojunction bipolar transistor)的特性分析及次微米線?元件的製程發展，為了發展兆赫茲磷化銦異質接面雙極性電晶體(THz InP based HBT)，本實驗室分別就磊晶品質改善、結構設計以及元件尺寸微縮提出一系列的研究。在磊晶品質改善方面，我們首先發展銻砷化銦四元材料的成長技術，以固定V/III比及長成速率的方式，成長出具不同銻含量並匹配於磷化銦基板的銻砷化銦鎵塊材，並藉由銻砷化銦鎵的電洞遷移率、少數載子生命週期不易隨P-型(鈹)摻雜濃度上昇而快速下降的特殊性質，以提高摻濃度的方式改善P-型銻砷化銦鎵塊材片電阻較大的問題，使其片電阻可與銻砷化鎵及砷化銦鎵比擬。
結構設計上的發展，我們首先以具有第一型射-基接面與第二型集-基接面的InP/In0.48Ga0.52As0.89Sb0.11/InGaAs DHBT證實使用銻砷化銦鎵基極之異質接面雙極性電晶體除了直流上具有低導通電壓與高電流增益外，其操作電流與電流增益截止頻率皆優於傳統的砷化銦鎵基極異質接面雙極性電晶體。並以模擬軟體分析不同銻含量對於第二型集-基的影響，我們發現當銻含量約為25%至30%時，可得到最低的射-基、集-基導通電壓以及較佳的高頻特性。因此，我們針對較高銻含量之基極設計具有第一型射-基接面與第二型集-基接面的InAlAs/InGaAsSb/InGaAs DHBTs。實驗結果顯示，當銻含量增加，元件的電流增益與線性度有明顯的改善，基極特徵歐姆接觸電阻率可大幅度的降低，並且其表面復合電流(射極尺寸效應)亦遠低於目前泛用的砷化銦鎵異質接面雙極性電晶體。藉由國際合作，在基極厚度44nm、集極厚度為200 nm，射極尺寸為0.65×8.65 μm2時，在一銻含量為23%之InAlAs/In0.42Ga0.58As0.77Sb0.23/InGaAs DHBT其電流增益截止頻率與功率增益截止頻率分別可達260 GHz與485 GHz的良好特性。
在製程技術的發展，首先針對小尺寸元件的關鍵技術：射極與基極的特徵歐姆接觸電阻率的降低，以高摻雜的砷化銦/砷化銦鎵射極接觸層結構設計及高銻含量的銻砷化銦鎵材料，分別將射極與基極的特徵歐姆接觸電阻率降低至3.4×10-8 Ω-cm2與5×10-8 Ω-cm2；隨後，以電子束微影的技術，將射極金屬線寬降低至300奈米，並初步將元件的電流增益截止頻率與功率增益截止頻率提昇至272 GHz與176 GHz。此外，為了改善射極、基極平台蝕刻以及基極金屬自我對準良率的提昇，本實驗室提出獨有的T-型射極金屬與苯並環丁烯(BCB)平台側壁製作技術，解決了目前平台側壁製作時離子轟擊半導體表面的問題，同時發展出元件更進一步微縮時基極平台與基極金屬自我對準的重要技術。
在尺寸微縮的過程中我們發現相較於傳統的磷化銦/砷化銦鎵單異質接面雙極性電晶體，使用銻砷化銦鎵基極之異質接面雙極性電晶體其電流增益在射極尺寸效應(emitter size effects)較不明顯，隨著銻含量的增加、射極週圍表面復合電流密度(Emitter periphery surface recombination current density, KSURF) 亦隨著銻含量的增加而下降；我們亦觀察到在高基極摻雜下，射極尺寸效應可被進一步的抑制，由先前提到銻砷化銦鎵在高摻雜時其少數載子生命週期遠大於砷化砷錠及銻砷化鎵，因此，即便在次微米尺寸下，元件的電流增益仍可維持於一合理值(50)。
綜合結果，銻砷化銦鎵基極異質接面電晶體在高銻含量、高基極摻雜時，元件具有低導通電壓、高操作電流及高電流、高功率增益截止頻率的良好特性，並在尺寸微縮時，其電流增益亦不隨之犧牲，闡明銻砷化銦鎵基極之異質接面電晶體於追求兆赫茲頻?的優勢。
In order to achieve THz InGaAsSb base heterojunction bipolar transistors we have focused our efforts on three research areas over the past few years, including the growth of high quality InGaAsSb, the design of the device layer structure, and the development of process technologies for sub-micron devices.
A method for growing high quality InGaAsSb material by fixing the V/III ratio and growth rate was developed first. By using this method we could control the Sb composition and make the InGaAsSb layer lattice-matched to InP substrate. Additionally, due to the special material properties of InGaAsSb, i.e., its hole mobility and carrier life time show little dependence on their base doping concentration, an InGaAsSb material with a high Sb composition (Sb>25%) and base doping concentration (NB>1×1020 cm-3) would exhibit a sheet resistance comparable to GaAsSb.
Utilizing the materials prepared by the aforementioned method, we demonstrated an InP/In0.48Ga0.52As0.89Sb0.11/InGaAs DHBTs with Type I E-B and type II B-C junctions. This was the first time that an InGaAsSb base HBT demonstrated a higher collector current and current gain cut-off frequency (fT) than a conventional SHBT. We also used a Silvoco simulation tool to study the Sb composition effect on a type II B-C structure. It was found that the lowest VBE and VBC turn-on voltage and highest current gain cut-off frequency could be achieved when the Sb composition was around 25% to 30%.
In order to further investigate the Sb composition effect while maintaining the benefits of type I B-E and type II B-C junctions, we designed a series of InAlAs/InGaAsSb/InGaAs HBTs. With these HBTs, we found that as the Sb composition increased, there was significant improvement in their current gain and linearity. It also resulted in low base specific contact resistivity, which led to a high maximum oscillation frequency (fmax). Through international collaboration with Professor Ran in UF (University of Florida), an InAlAs/In0.42Ga0.58As0.77Sb0.23/InGaAs HBT with an emitter size of 0.65×8.65 μm2 and base/collector thickness of 40nm and 150 nm, respectively, a fT of 260 GHz and fmax of 485 GHz were achieved.
In the area of submicron-meter HBT fabrication, we first investigated emitter and base contact resistivity issues, which are key factors for submicron-meter devices. Heavily doped InAs/InGaAs emitter structures and high Sb content InGaAsSb materials were used to lower the emitter and base contact resistivity. By these methods, emitter and base contact resistivity of 3.4×10-8 Ω-cm2 and 5×10-8 Ω-cm2 were achieved. After solving the contact issue, we used e-beam lithography to define devices with emitter sizes below 300 nm. The fT and fmax of the devices were 272 GHz and 176 GHz, respectively. In addition, in order to improve the yield of self-aligned emitter mesa and base metal, a unique T-shaped emitter and benzocyclobutene (BCB) mesa sidewall technology was invented. This allowed us to avoid the ion bombardment problem during mesa sidewall fabrication. This method was even better for fabricating severely scaled self-aligned emitter HBTs.
During the submicron-meter device fabrication procedure, it is found that the current gain in a HBT with an InGaAsSb base layer shows less dependence on the emitter size as compared to the InGaAs base HBTs. Moreover, as the Sb composition increases in the base layer, the emitter periphery surface recombination current density (KSURF) of an InGaAsSb base HBT becomes more and more insignificant. Additionally, the heavily doped InGaAsSb base HBT exhibits a weaker emitter size effect than a lightly doped one. This is considered a very important advantage of the InGaAsSb base HBTs. It is found that heavily doped InGaAsSb bases exhibit a long minority carrier life time, which leads to a decent current gain for an HBT. Current gains as high as 50 have been achieved with a 44nm InGaAsSb base when the base doping goes up to 1×1020 cm-3 in InGaAsSb base HBTs.
The dissertation shows that the InGaAsSb base HBTs with high Sb composition and base doping concentration exhibit low turn-on voltage, high current drive capability, high fT and high fmax. Meanwhile, this kind of HBT shows very weak emitter size effects in the scaled device. According to these superior characteristics, the InGaAsSb base HBTs show great potential for achieving THz HBT. |
