The research into photovoltaic renewable energy source is more and more intensive due to the rapid increase of worldwide power consumption. In order to achieve high efficiency solar cells, one strategy is the utilization of the multi-junction. Unfortunately, one important factor limiting the efficiency of III-V/Ge multi-junction solar cells is the current mismatch among sub-cells, especially the low current density in the GaAs middle junction. One effective method to achieve better current balancing and higher performance is the extension of the absorption edge of the GaAs middle cell by implementing a lower band gap InGaAs material. However, the lattice constant of InGaAs is larger than that of GaAs and the lattice mismatch causes the growth to be quite difficult. To avoid accumulating strain in InGaAs layers that induces crystal defects and thus degrades the efficiency of a solar cell, a strain compensating structure such as InGaAs/GaAsP multiple quantum wells (MQWs) is a favorable option. However, up to now, the current mismatch for the III-V/Ge multi-junction solar cells still hasn't been solved successfully due to the insufficient photocurrent from MQWs part.

Under this background, in this work, the methods of improving MQWs solar cell's performance have been investigated from two aspects. One is reduction of the photon-generated carriers' recombination rate through improving the crystal quality of MQWs structure. The other one is facilitation of the photon-generated carriers' escape from deep wells by inducing a novel tunneling-assisted structure.

This research has revolved around these two targets. For the improvement of MQWs' crystal quality, the strain accumulation in MQWs structure and the rough hetero-interfaces are two critical points. The accumulated strain in MQWs will induce defects and deteriorate the crystal quality. To solve this problem, an in-situ strain monitoring method has been developed to achieve strain-balance more precisely and efficiently. On the other hand, in order to achieve current match in multi-junction solar cell, a high amount of indium has to be incorporated in the well layers. At the same time, high phosphorus content in barrier layers is also necessary for strain compensation. This induces abrupt lattice constant difference between well and barrier layers with risks of rough hetero-interfaces and defects. Also the atomic diffusion between InGaAs and GaAsP may induce unintended atomic content, which make it difficult to control the strain accumulation. A smart hetero-interfaces management by optimizing gas-switching sequence has been successfully developed to achieve good crystal quality.

For the facilitation of carrier escape, the key point is reducing the photon-generated carrier escape time from wells. In the MQWs system, deep well supplies wide absorption range. But simultaneously, the large band offset would increase the thermionic escape time exponentially. To overcome this obstacle, a novel asymmetric MQWs structure, in which a thin well is implemented next to the deep well, has been designed for assisting the carrier escape by resonant tunneling effect. The process of this sequential thermionic excitation and tunneling has been simulated and optimized, the solar cell with this tunneling-assisted structure has been fabricated, and the characteristics of it have been investigated thoroughly.

This dissertation consists of seven chapters. In chapter 1, the background and basic knowledge of solar cells has been introduced, followed by the detailed descriptions of MQWs solar cell. The motivations and objectives of this dissertation have also been given in this chapter.

The chapter 2 starts with the introduction of main characteristics of photovoltaic cells. Then the efficiency limitation for single junction solar cell has been derived by detained balance method. Depending on this method, the proper band gap for middle subcell of III-V/Ge multi-junction has been calculated to be 1.20 eV, which is an important parameter for the design of MQWs structure. After that, the laboratory equipments generally used in this work have been described, such as growth equipment of metal organic vapor phase epitaxy (MOVPE), structure detector of X-ray diffraction (XRD), photoluminescence (PL) and so on.

In chapter 3, the high-resolution wafer curvature measurement has been proved to be a very effective method for the in situ strain monitoring during the growth of InGaAs/GaAsP strain-compensated MQWs in MOVPE. The first observation of the clear curvature periodic oscillations following the InGaAs/GaAsP MQWs' growth has been achieved by stopping the satellite rotation. With the help of this in-situ strain monitoring method, we can adjust growth conditions instantaneously on the basis of in-situ signal from the layer structure and can obtain strain-balance conditions just in one trial growth. Also, the curvature simulation model has been developed by implementing with thermal expansion and lattice relaxation effects to improve the simulation accuracy and application sphere. The simulations by this model fit the curvature transients covering a whole range of averaged strain with less deviation than before.

In chapter 4, the interfacial management for InGaAs/GaAsP MQWs structures with thin wells and barriers and large number of hetero-interfaces has been investigated. By means of XRD measurement and high-accuracy in-situ curvature measurement, it has been found that the hetero-interfaces with substantial lattice mismatch tend to generate interfacial defects, which can be mitigated by the insertion of ultrathin GaAs interlayers. However, an inadequate gas-switching sequence induces unintended atomic content near the hetero-interfaces and in the GaAs insertion layers, which influences the average strain of the structure. It has been proved that extending the stabilization time for the arsenic and phosphorus mixture before GaAsP barrier growth to 3 s and inserting a 1 s hydrogen purge after InGaAs well growth are quite effective to remove the unwanted strain. Static photoluminescence (PL) and time-resolved photoluminescence (TRPL) results have also been used to evaluate the crystal quality of grown structures.

Chapter 5 has introduced a novel tunneling-assisted structure for InGaAs/GaAsP MQWs to facilitate the carrier escape from deep well. The novel structure has been designed to implement a thin well next to the deep well after a thin barrier. Through optimizing of confinement energy levels and barrier thickness, a sequential thermionic excitation and tunneling process have been achieved. By breaking up one direct thermionic escape process from fundamental state into two thermionic escape processes, the collection time of photon-generated carriers has been calculated to be significantly dropped from several nanoseconds to a few hundreds picoseconds compared with conventional MQWs. Therefore, this novel tunneling-assisted structure has been expected to facilitate carrier escape and improve the cell performance effectively.

In chapter 6, firstly, the fabrication of solar cell with the designed tunneling-assisted structure has been introduced. Static PL measurement has proved the absorption band gap of new structure is 1.2 eV as designed. After that, the room temperature time-resolved PL measurement has confirmed that the tunneling-assisted structure has faster carrier escape time than a conventional MQWs structure. Then, the external quantum efficiency (EQE) and I-V curve have been detected and the results have revealed that the solar cell with novel tunneling-assisted structure has larger EQE and higher short-circuit current than the conventional MQWs structure with almost constant open-circuit voltage. Also, the characteristics of bias dependence and temperature dependence for the tunneling-assisted structure have been investigated.

In chapter 7, the whole dissertation has been concluded and the main improvements in this work have been summarized. At last, the suggestions on future investigation have been introduced.

本論文は,"Research on improvement of MQWs solar cell's performance through development of growth technology and novel tunneling-assisted structure"(結晶成長法の改善とトンネル援用構造の導入による多重量子井戸太陽電池の特性改善に関する研究)と題し,III-V族化合物半導体を用いた多接合太陽電池の電流整合改善による高効率化を目指した量子井戸挿入ミドルセルについて,有機金属気相成長法における格子不整合系量子井戸の精緻な歪み制御およびヘテロ界面組成急峻化と,それを活用したトンネル援用構造の作製による井戸からの光励起キャリア取り出し効率改善の取り組みをまとめたものであり,英文7章から構成される.

第1章は序論であり,高効率太陽電池とくに多接合太陽電池の必要性,および本研究の対象であるInGaAs/GaAsP歪み補償量子井戸のミドルセルへの挿入について既往の研究と本研究の目的を述べている.

第2章では,太陽電池の基本原理を解説し,ついでIII-V族化合物半導体の有機金属気相成長法および評価法について述べている.

第3章では,格子不整合系材料の多数積層を可能にする歪み補償量子井戸の成長について,レーザ光による成長中ウエハ曲率のin situモニタリングを活用した高度な歪み制御法について述べている.成長層の歪みとウエハの曲率の相関について,InGaAs3元混晶の熱膨張係数を見直すことで,Stoneyの式により結晶成長中に観察されるウエハの反りから成長中の結晶層の組成および歪みを定量的に評価することに成功した.これにより,有機金属気相成長におけるInGaAs/GaAsP積層量子井戸の精緻な歪み制御が可能になった.

第4章では,有機金属気相成長においてInGaAs/GaAsPヘテロ界面の組成を急峻化するためのガス切り替えシーケンスの改良について述べている.積層量子井戸においてヘテロ界面の非理想的な組成分布がウエハの反りに反映されることを発見し,従来困難であったヘテロ界面組成分布の評価についてウエハ反りのin situ観察という新たな手法を導入した.これにより,ガス切り替えシーケンスがヘテロ界面の組成に及ぼす影響を迅速に評価し,InGaAs表面におけるIn偏析を低減するための水素パージや,GaAsP層成長開始時のP組成を安定化させるためのP原料供給時間の最適化を行った.

第5章では,量子井戸挿入太陽電池における最大の課題である井戸内部からの光励起キャリア取り出し効率の改善を目指して,GaAsの吸収領域を超えた長波長光を吸収する厚い量子井戸と,そこからのキャリア脱出を促進するための薄い量子井戸を共鳴準位を介して結合した非対称量子井戸構造を提案している.厚い量子井戸における励起準位と薄い量子井戸の基底準位を電子のトンネルにより結合させ,厚い井戸からのキャリア脱出時定数を低減するための構造を設計し,その効果を定量的に評価するモデルを構築した.

第6章では,前章で設計したトンネル援用非対称量子井戸構造をGaAs単接合太陽電池に実装し,その効果を定量的に検証している.厚い井戸から薄い井戸への電子のトンネルを促進するためには4 nmという薄い障壁層が必要であり,その作製には3・4章に記述された結晶成長法の改善が活かされている.太陽電池構造における井戸からのキャリア脱出時定数を時間分解フォトルミネッセンスにより評価し,トンネル援用非対称量子井戸構造により脱出時定数を半分に短縮できることを実証し,モデルとの良い一致を確認した.また,この改善が太陽電池の短絡電流向上をもたらすことを実証した.

第7章は結論であり,上記の取り組みを総括するとともに,今後の展望を述べている.

以上のように,本論文は,量子井戸の有機金属気相成長における精緻な歪み補償およびヘテロ界面の組成急峻化をウエハ曲率のin situ観察を通して達成し,それをもとにトンネル援用非対称量子井戸構造を設計・試作して量子井戸からの光励起キャリアの脱出促進および太陽電池の短絡電流向上を実証したものであり,電気電子工学に貢献するところが少なくない.

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.