SPECIAL Development of Various Semiconductor Quantum Devices Tsukuru KaTsuyama Semiconductor quantum devices, composed of semiconductor quantum wells and superlattices, are widely used in our daily lives as key devices for opto-electronic equipment. The quantum well structure consists of alternating ultra-thin semiconductor films in which electrons and holes are confined. This structure gives rise to discrete energy levels and minibands in their potential wells, and thus induces new properties that cannot be obtained in a bulk material. These properties have enabled opto-electronics devices to have various new functions and high performance. This paper describes various semiconductor quantum photonic devices developed in Sumitomo Electric for applications in a wide spectrum range of 1 to 10 µm. These devices include a semiconductor quantum well laser and modulator for high speed optical communication, quantum cascade laser for environmental gas analysis, and near- infrared imaging sensor for life science applications. Keywords: quantum well, superlattice, semiconductor laser, quantum cascade laser, infrared sensor 1. Introduction 2. Quantum Well Structures and Materials Semiconductor quantum well structure, including su- First, the origin of unique physical properties induced perlattice structure, is one of the greatest inventions in the in quantum well structure is explained. Figure 1 describes field of compound semiconductor. The quantum well the energy state of quantum well structure. In the structure structure consists of alternating ultra-thin semiconductor where two compound semiconductors with different band films, in which electrons and holes are confined. There- gaps pile up alternatively, electrons and holes are confined fore, electrons and holes show two-dimensional behavior in the lower band gap layers. The thickness of the lower which induces new properties of material. This structure band gap layer approaches to the electron mean free path was proposed by Esaki and Tsu in 1969 (1) and has become (several tens of nanometers), energy levels of the electron the basis of innovative technologies to produce high per- in the conduction band and the hole in the valence band formance semiconductor devices. are quantized. In the valence band, each hole energy level In the early stage of development of quantum well is split into two energy levels (heavy hole and light hole). technologies, the research of carrier transport in the quasi- This phenomenon is called quantum size effect. Then, the two dimensional system had been performed and various density of states in the conduction and valence band be- devices were invented such as a high electron mobility tran- comes a staircase which is very different from parabolic in sistor (HEMT)(3) by applying modulation doping technol- bulk crystal as shown in Fig. 1. Therefore, the density of ogy (2) and a resonant tunneling transistor (4) using wave states near the minimum energy gap increases significantly. function engineering. Meanwhile, the research in optical Reflecting this, the optical gain spectrum width becomes properties of quantum well structure led to the invention narrower and optical gain dramatically improves. Perform- of various optical devices such as a quantum well laser (5), a quantum dot laser (6) an electro-absorption modulator, and a quantum cascade laser (7). We expect that this profound technology will continue to provide high performance and new functional semicon- ductor devices. Quantum well structure Sumitomo Electric Industries, Ltd. started the research Energy and development of compound semiconductor materials in the 1960s and semiconductor devices in the middle of Conduction band Electron the 1980s in order to establish optical communication busi- Bulk Semiconductor E2 ness integrating technology from materials, devices to quantum well systems. We have developed various kinds of high perform- E1 ance semiconductor devices by applying quantum well (8),(9) technology . Ehh Heavy hole This paper describes how this technology has been ap- Elh plied to various optical devices used in the field of infor- Light hole mation communication, life science, and environmental Valence band Density of state protection. Fig. 1. Energy state of quantum well structure 24 · Development of Various Semiconductor Quantum Devices ance improvement by introducing quantum well structure be decreased to about 3 µm. Since electrons and holes are to semiconductor lasers is attributed to this phenomenon. spatially separated in type II structure, optical transition ef- In addition to the quantum size effect, several unique ef- ficiency is lower than that of type I structure. fects which cannot be obtained in the bulk material, such In the wavelength region longer than 3 µm, intersub- as electro-absorption effect and tunneling effect, have also band transition using large conduction band offset of been applied to develop high performance and new func- AlInAs/InGaAs material systems can be applied. tional devices. Molecular Beam Epitaxy (MBE) and Organometallic Table 1 summarizes the application areas of the de- Vapor Phase Epitaxy (OMVPE) have been used for the vice, the type of the device, the type of quantum well struc- growth of these quantum well structures. MBE is a crystal ture, and material combinations from a view point of growth method that is performed using physical adsorp- wavelength that can be covered. Although application tion by irradiating the molecular beam in an ultrahigh vac- areas listed for each wavelength region are not limited to uum chamber. Since MBE is a high vacuum process, the this wavelength region, the application fields of informa- crystal quality and growth process can be characterized and tion and communications, life sciences, and environmental controlled by real time monitoring and can form very protection and security correspond to the wavelength re- abrupt hetero-interface. On the other hand, OMVPE is the gion of low loss optical fiber, the absorption region of bio- method of depositing semiconductor film by thermal de- logical components, and the fundamental vibration of the composition of organometallic sources and suitable for the molecule and the black body radiation at near room tem- growth of phosphorus compounds and mass production. perature, respectively. OMVPE is also suitable for selective area growth which is The wavelength that can be covered by quantum well essential for realizing high performance devices and mono- structure is determined by the combination, composition lithic integration of different types of devices. and thickness of material that form the quantum well struc- We established OMVPE crystal growth techniques as a ture. By controlling these parameters, the effective band core technology for the fabrication of semiconductor de- gap or optical transition energy can be designed. vices in the late 1980s. We designed the reactor and the gas In this paper, we describe quantum well structures supply system of OMVPE for realizing ultrafine structure based on the material system that can be formed on InP with good uniformity. Excellent uniformity over a 2-inch substrate. wafer with monolayer hetero-interface abruptness was ob- In the wavelength region used for optical fiber com- tained (10),(11). munication (1.2-1.7 µm), GaInAsP and AlGaInAs material systems are used to form type I quantum well structure in which smaller band gap material is sandwitched by larger band gap materials as shown in Tabel 1. In the type I struc- 3. Device Application ture, since electrons and holes are confined in the smaller band gap material, optical transition takes place between 3-1 Semiconductor lasers for communication the quantized energy level formed in conduction and va- The performance of semiconductor lasers has been lence band with a high efficiency. tremendously improved by applying quantum well struc- In the wavelength region from 1.7 µm to 3 µm, there ture. Without this technology, a full-fledged commercial- is no material system that can form type I structure on the ization of semiconductor lasers would not be possible. The InP substrate. Therefore, the quantum well structure called first operation of a quantum well laser was reported by Van type II using interband transition between the adjacent ma- der Ziel et al. (5) in 1975. We applied quantum well struc- terials has been devised to form a smaller band gap. The ture to semiconductor lasers used for optical communica- effective band gap of InGaAs/GaAsSb material systems can tion from the late 1980s, and introduced full OMVPE process for mass production. Then high performance lasers, including 1.3 µm Fabry-Perot lasers, 1.48 µm fiber amplifier pumping lasers, and uncooled DFB lasers, have Table 1. Summary of application areas of the device and quantum well (9) structure been commercialized . Recent explosive increase of traffic in transmission net- Wavelength 1 1.5 2 2.5 35810 (µm) work requires the expansion of transmission capacity of the NIR (Near Infrared) MIR (Mid-infrared) network. Much effort has been made to increase the capac- Appli- Information Life sciences Environment/Security cation communications Hyperspectrum Gas sensing, ity of the network by introducing wavelength division mul- Field Fiber communication imaging Remote sensing Thermography tiplexing (WDM) and multi level modulation. With the development of these multiplexing and modulation tech- Device Laser, modulator Image sensor Quantum cascade laser nologies, intensive study for increasing modulation speed