Intersubband transition (ISBT) in AlN/GaN multi quantum wells (MQWs) has attracted much attention recently for optoelectronic devices because of its easy wavelength-tunability and extremely fast carrier relaxation process. The extension of ISBT wavelength to near-infrared wavelength, especially at optical communication wavelength such as 1.55 and 1.3 μm, is of particular interest since it is vital for the development of ultrafast photonic devices for silica-fiber-based optical communication networks. The material system of AlN/GaN MQWs structure has several promising properties such as large conduction band offset of approcimately 2 eV and large LO phonon energy. Therefore, the relaxation time in AlN/GaN material system is expected to be in the order of sub-picoseconds. This can contribute to the development of ultrafast all-optical switch operating at a bit rate higher than 1 Tb/s.

The observation of ISBT in AlN/GaN MQWs at 1.55 μm and shorter wavelength has been achieved by molecular beam epitaxy (MBE) system. The shortest ISBT wavelength reported in MBE system is 1.08μm. On the other hand, the observation of ISBT by metal organic vapor phase epitaxy (MOVPE) system has not provided satisfactory results. ISBT at around 1.55 μm, which is achieved at very recently, showed very weak absorption magnitude and broad peak width. Furthermore, relatively strong ISBT absorption at around 1.55 μm was realized by using photo-induced absorption technique meaning that the carrier density in well is not sufficient. The demonstration of ISBT at 1.55 μm by MOVPE system is very important since MOVPE has a merit for mass production and better crystal quality for device fabrication. In this study, the observation of the ISBT at communication wavelength such as 1.55 and 1.3 μm was considered as a main target of research. For the realization of this aimed-goal, I suggest two issues of interface abruptness and sufficient carriers in well for observing strong ISBT at communication wavelength region. According to the numerical calculation results, the main factor for shifting the ISBT wavelength to shorter region covering 1.55 and 1.3 μm was found to be the thickness of interfacial layer between well and barrier. For the improvement of interface abruptness and uniformity of MQWs, low temperature growth can be a key since it can suppress inter-diffusion between well and barrier and reduce the incorporation of adducts by gas phase reaction in AlN growth and agglomeration of grains. Fabrication of n-type GaN layer at low temperature region, however, is still challenging in conventional growth process since impurities originating from low thermal dissociation of group-III precursors, especially carbon, are heavily incorporated into GaN layer trapping the intentionally doped carriers. As a breakthrough, I suggest pulse injection (PI) method in which group-III precursors are alternately supplied, while NH3 is continuously supplied. PI method is considered to be effective in decreasing the incorporation of carbon in GaN layer due to the reduction at only NH3 supply time. By resolving these two issues, I succeeded in the observation of strong ISBT at 1.55μm.

Interface abruptness has been considered as a main problem in typical MOVPE system for achieving short ISBT. It was found that the shortest ISBT wavelength in AlN/GaN MQWs grown by conventional MOVPE system using high growth temperature of 1130 °C was 2.2μm. It is attributable to the formation of interfacial layer due to inter-diffusion between well and barrier by high growth temperature. Low temperature growth was proved to be very effective in improving interface abruptness as well as surface morphology. Rapid thermal annealing of abrupt MQWs induced the disappearance of MQWs-related satellite peaks, which supports that high growth temperature, indeed lead to the degradation of interface abruptness. According to secondary ion mass spectroscopy (SIMS) measurement to observe impurities, the incorporation level of carbon in low temperature-grown GaN layer was drastically increased. It was well reported that carbon in GaN plays a role of an acceptor trapping the intentionally doped carriers.

PI method was introduced in the fabrication of n-type GaN layer at low temperature region. As the main process parameters influencing the carbon incorporation as well as surface flattening, NH3 supply time without the supply of group-III precursors (τ2) and partial pressure of trimethylgallium (TMGa) are examined. It was clearly observed that the carbon incorporation was decreased with increasing τ2 and 7 s was considered as best time since longer time than 7 s resulted in the surface roughening. The partial pressure of TMGa has affects both further decreasing carbon incorporation and flattening the surface to have monolayer level surface roughness. By precisely controlling these two parameters, carrier concentration as high as 1.6 x 1019 cm-3 could be realized at low temperature of 950 °C.

Since two issues of interface abruptness and sufficient carriers are resolved by low temperature growth by PI method, the fabrication of MQWs based on the optimized PI sequence was followed to observe ISBT absorption. ISBT absorption was firstly observed in MQWs grown at relatively low temperature of 950 °C and its absorption properties was better than those realized by conventional method at 1130°C showing stronger absorption magnitude and narrower peak width. One of the important finding in this study, the ISBT wavelength was clearly blue-shifted with decreasing the growth temperature from 950 to 770°C. It is strongly suggested that the interface abruptness was further improved by lowering the growth temperature, which is well consistent with simulation results. ISBT at 1.55 μm was firstly achieved at room temperature in MQWs grown at 770°C by using PI method in GaN well growth. This result is for the first time reported in MOVPE system using GaN buffer layer.

For further blue-shift of ISBT wavelength, covering 1.3 μm, AlN buffer layer was introduced since it can give stronger built-in electric field in well and grow smooth MQWs without the lattice relaxation. ISBT at 1.3 μm was found to be very hard to realize since it is possible when the well thickness is decreased to 1.2 nm which is close to the minimum well thickness (~ 1.0 nm). Precise control of well and barrier thickness is therefore very important and eventually, shortest ISBT wavelength of 1.49 μm was observed in this study. It was suggested that the surface roughness is required to be improved to shift the ISBT wavelength to shorter region and improve absorption properties.

The observation of strong ISBT at 1.55 μm is considered as the most important achievement in this study. With this absorption properties at 1.55μm, it is considerably possible to fabricate all-optical switch. Clarifying the reason of difficulties in MOVPE system for realizing short ISBT is thought to be meaningful. The development of PI method which is considered as my originality can be another achievement. This method can be applicable to other research areas which need low temperature fabrication of n-type GaN layer and flattening the surface of GaN at low temperature region.

光ファイバーを用いた高速大容量通信が普及するに伴い,高速光スイッチング素子の重要性が高まっている。なかでも,半導体多重量子井戸構造を用いた伝導帯オフセットにできるインターサブバンド間の遷移(Intersubband Transition,ISBT)を利用した全光スイッチは,遷移したキャリヤの速い緩和速度によって,超高速光スイッチとしての応用が期待されている。これまでに,InGaAs/AlAs,Al(Ga)N/GaN,ZnSe/BeTeなどの材料系で検討がなされてきたが,AlN/GaN系はなかでも2 eVという大きな伝導帯オフセットを持ち,速いLOフォノン散乱速度があるため,通信波長帯である1.55 μmでのTb/sオーダーの高速スイッチングが期待されている。

本論文は,"Low Temperature MOVPE Growth of AlN/GaN MQWs by Pulse Injection Method for Realization of Intersubband Transition at 1.55 μm"(1.55 μmでのサブバンド間遷移の実現を目指したパルスインジェクション方式によるAlN/GaN多重量子井戸の低温MOVPE成長)と題し,有機金属気相成長法(MOVPE)によるAlN/GaN多重量子井戸(MQW)を用いて光通信波長帯である1.55μmでのISBTの実現を目指したものであり,全部で7章からなる。

第1章はイントロダクションであり,研究の背景ならびに目的をまとめている。特に,AlN/GaN系MQWによるISBTの実現は,分子線エピタキシー(MBE)を用いて作製した場合には1.08 μmまでのISBT吸収が観測されているのに対し,量産性に優れるMOVPEプロセスを用いた場合には,1.55μmまでの弱い吸収しか観測されていないということを取りまとめ,本研究の課題はMOVPEによるAlN/GaN-MQWにより,1.55μm帯,さらには1.3 μm帯での強いISBT吸収の実現にあるとしている。

第2章では研究方針および戦略をまとめている。上記課題の解決のためには,AlN/GaN界面の急峻性の確立とGaN内での1019 cm-3 程度のキャリヤ密度の確保が重要であることをシミュレーションなどにより明確にしている。また,その具体的解決策としては,界面での相互拡散による急峻性の劣化を防ぐために低温での成長が重要であること,また,低温成長では炭素の取り込みによりキャリヤ密度が減少するため,原料を間歇的に導入するパルスインジェクション方式を用いて炭素を有効に除去することが必要であることを示している。

第3章では,AlN/GaN-MQWをMOVPE反応温度1130 ℃から900 ℃まで下げて作製し,構造変化,ISBT吸収などの考察をおこなっている。X線回折の2θ-ω測定からは,低温で作製した方がより高次の回折まで明確に現れ,低温での成長が界面急峻性向上に有効であることを示している。しかしながら,各温度で作製したMQWのISBT吸収を測定したところ,1050℃以下の成長ではISBT吸収が見られなくなってしまうことも報告している。この現象は,低温成長では残留炭素が急激に増大し,キャリヤがトラップされて急激に減少してしまうことが原因であると推察し,残留炭素低減を次の目標と定めている。

第4章では,高品質n型GaNを低温成長させるために,パルスインジェクション手法を導入した結果についてまとめている。具体的には,トリメチルガリウム(TMGa)とドーパントであるシラン(SiH4)ガスを間歇的に導入し,残留炭素の低減を図っている。TMGa分圧やパルスのOn/Off時間の最適化により,表面の平滑性に優れ,残留炭素濃度の低いGaN層を800 ℃程度でも実現可能なことを示している。その結果,キャリヤ密度は1019 cm-3を達成した。

第5章では,第4章で確立したパルスインジェクション手法を用いて,AlN/GaN-MQW構造を作製し,ISBTの観測を行った結果をまとめている。950 ℃で作製した資料では,1000 ℃以下の成長ではじめてISBT吸収を観測することができ,また,吸収波長はMQWの井戸幅を小さくすることによって1.9 μm程度までブルーシフトすることを確認した。しかし,井戸幅が1.1 nm程度であっても,ISBT吸収は1.9 μm程度までしかブルーシフトせず,同じ構造をMBEで作製した場合と比較してかなり長波長側に偏っていることが問題と考えられた。このため,成長温度を710 ℃まで下げ,MQW構造の変化をX線回折により観測したところ,770 ℃まで低下させると界面急峻性がもっとも良いと判断された。そこで,770 ℃にて井戸幅を変化させてMQW構造を形成し,ISBT吸収を観測したところ,井戸幅1.0 nmにて1.55 μmに強いISBT吸収を観測することに初めて成功した。

第6章では,バッファー層としてAlNを用いた場合について,ISBT吸収波長のさらなる短波長化が実現できるか検討した結果についてまとめている。井戸層幅,バリヤ層幅の最適化により,最短で1.49μmのISBT吸収を達成している。

第7章は総括であり,今後の展望などをまとめている。

以上,本論文は,MOVPE法によるAlN/GaN多重量子井戸構造の作製とその最適化を通じて通信波長帯である1.55μmでの強いISBT吸収を実現し,超高速光スイッチへの応用展開の端緒を開くものであり,マテリアル工学の発展に大いに寄与するものである。よって,本論文は博士(工学)の学位請求論文として合格と認められる。