藉共平面波導探討矽基三族氮化物於射頻下傳輸線損耗之研究

Abstract

由於氮化鎵具備獨特的材料性質，使氮化鎵高電子遷移率電晶體在射頻應用中展現了卓越的元件特性，如今已被廣泛應用在無線基地台、雷達系統、放大器、行動無線電等領域，目前更藉由實現氮化鎵成長於矽基板來降低生產成本，取代昂貴的碳化矽基板。然而，許多文獻已指出矽基板與磊晶層的介面會形成寄生導電層，造成射頻損耗增加，進一步影響元件的高頻特性，包含電流截止頻率(FT)、最大震盪頻率(Fmax)及輸出功率(output power)、功率附加效率(PAE)等。因此，對於提升矽基氮化鎵電晶體在高頻下的特性而言，如何有效評估射頻損耗並藉此選擇合適的磊晶結構以降低射頻損耗是極為重要的課題。 在此研究中，藉由在磊晶片上製作共平面波導並使用高頻S參數量測系統，計算出各頻率下磊晶結構的射頻損耗，並分別比較不同磊晶結構及不同阻值矽基板的射頻損耗之差異，希望建立以射頻損耗的觀點來評價磊晶結構優劣之方法，有效地改善射頻損耗以提升高頻特性。 在磊晶結構方面，此部分使用阻值為80 Ω•cm的矽基板，比較用有機金屬化學氣相磊晶所成長的氮化鋁鎵、碳參雜氮化鎵、氮化鎵此三種不同緩衝層的射頻損耗，發現氮化鋁鎵有最低的射頻損耗，在10及40 GHz頻率之下，射頻損耗分別是0.953及3.235 dB/mm，且氮化鋁鎵和碳參雜氮化鎵的射頻損耗都低於傳統氮化鎵，我們認為選用高阻值緩衝層材料可使射頻損耗減少；除此之外，比起氮化鎵，氮化鋁鎵的晶格常數與過渡層較相近，所以可形成較好的介面，我們推測這是另一個氮化鋁鎵有低損耗的原因。 在矽基板的選擇方面，我們分別成長相同結構的氮化鎵高電子遷移率電晶體結構於兩種不同阻值(80和10^4 Ω•cm)的矽基板上並比較兩者的射頻損耗，在10及40 GHz頻率之下，使用高阻值矽基板的損耗分別為0.476及1.344 dB/mm，低阻值矽基板則是0.958及2.249 dB/mm。由實驗結果可知，使用高阻值的矽基板可有效的抑制寄生導電層，大幅地降低射頻損耗。 最後，為了驗證實驗結果的正確性，我們也比較日本NTT-AT公司成長於高阻值矽基板(10^4 Ω•cm)上的不同緩衝層磊晶片，發現使用高阻值緩衝層與高阻值矽基板的確可有效的降低射頻損耗，再次地驗證了我們的結果，其中射頻損耗最低的依然是氮化鋁鎵緩衝層結構，在10及40 GHz頻率之下，射頻損耗分別是0.205及0.666 dB/mm。

Owing to unique material properties, GaN-based high electron mobility transistors (HEMTs) show extraordinary performances in the field of RF applications. The devices are used as power amplifiers in wire-less base stations, radar systems, and mobile phones. Recently, the growth of GaN HEMTs on low cost Si substrates has been realized to replace expensive SiC substrates. However, published research indicate that the parasitic conductive layer formed at the interface between Si substrate and epi-layer increases the RF loss and degrades the performances of devices in high frequency, including FT, Fmax, output power, and PAE. Therefore, to evaluate the RF loss and choose a proper buffer are important issues to reduce the RF loss in III-Nitride on Si substrates and further improve RF performances of the devices. In this study, coplanar waveguides (CPWs) were fabricated on epitaxial wafers and the calculated RF losses (transmission line loss) were measured on a 50 GHz S-parameter Measurement System. The RF losses of different epi-structures and different resistivity Si substrates were compared. A straightforward method to evaluate epi-structures for RF applications from the point of view of the RF loss has been established. Thus, reducing the RF loss in III-Nitride on Si substrates can effectively improve RF performances. To our knowledge, there is no work done in comparing the RF losses of different buffers before. In this study, the RF losses of three different buffers on Si substrates (80 Ω•cm) grown by our MOCVD were investigated, such buffers included AlGaN, carbon-doped GaN and GaN buffers. We found that the structure with an AlGaN buffer has the lowest RF loss. At 10 and 40 GHz, the RF losses are 0.953 and 3.235 dB/mm, respectively. In addition, the RF losses of AlGaN and carbon-doped GaN buffer were both lower than the conventional GaN buffer. We think that the RF loss can be reduced by using a high resistivity buffer. On the other hand, the lattice constant of AlGaN is much similar to transition layers than that of GaN, resulting in better interface. This is another possible reason to explain why AlGaN buffer has the lowest RF loss. In terms of the comparison of Si substrates, same HEMT structures on Si substrates with different resistivity (80 and 10^4 Ω•cm) were grown to compare the RF losses. At 10 and 40 GHz, the RF losses are 0.476 and 1.344 dB/mm for GaN on a high resistivity (HR) Si substrate and the RF losses are 0.958 and 2.249 dB/mm for GaN on a normal Si substrate. Our experiment results indicate that the RF loss can be reduced remarkably. We demonstrate that the parasitic conductive layer can be suppressed effectively by using HR Si substrates. Finally, the RF losses of different buffers on HR Si substrates (10^4 Ω•cm) grown by NTT-AT were also compared to further confirm our results. Again, the low RF loss can be achieved by using a high resistivity buffer and a high resistivity Si substrate. Structure with an AlGaN buffer still has the lowest RF loss. At 10 and 40 GHz, the RF losses are 0.205 and 0.666 dB/mm, respectively.