Carrier Confinement Structure Research Articles

(Invited) Development of Germanium-Tin-Related Semiconductor Heterostructures for Energy Band Design in Electronic and Optoelectronic Applications

Group-IV element semiconductor materials, germanium tin (GeSn) and germanium silicon tin (SiGeSn) have attracted much attention for electronic and optoelectronic applications because of those unique properties; direct transition with a high Sn content, higher carrier mobility than Si and Ge, low growth temperature, and low thermal conductivity [1-3]. GeSiSn realizes the control of energy band structure independently on the lattice constant of alloy. Also, GeSiSn/Ge(Sn) heterostructure provides a type-I energy band alignment with sufficiently large energy band offsets at both conduction and valence band edges simultaneously with the lattice constant matching. That promises an effective carrier confinement structure for quantum well laser and high electron mobility transistor. On the other hand, the development of crystal growth technology for GeSn-related materials is still a challenge, since the thermal equilibrium solid solubilities of Sn in Ge and Si are very low compared to a required substitutional Sn content higher than 10%. It is necessary to understand the behavior of Sn atoms in the epitaxial growth and post deposition process and to control the stability of Sn at substitutional site in GeSn-related epitaxial layers. In addition, the development of interface engineering technology is required for not only GeSn/GeSiSn heterostructure but also metal/GeSn and insulator/GeSn structures for nano-scale electronic devices. Recently, we examined the formation of double heterostructure with GeSiSn/GeSn/GeSiSn layers on Ge substrate using a molecular beam epitaxy (MBE) system [4]. The hard x-ray photoelectron spectroscopy measurement reveals that the sufficiently large energy band offset can be formed at the valence band edge of its GeSiSn/GeSn interface. We also demonstrated that the carrier confinement with this GeSiSn/GeSn/GeSiSn double heterostructure effectively improved the efficiency of photoluminescence (PL) [5]. However, we also found that the crystalline quality of the heteroepitaxial layers significantly influence on the PL intensity and it is necessary to understand the key factor of the crystallinity of heterostructures in order to reduce the dislocation and point defect in the epitaxial layers. For the low temperature growth of GeSn and GeSiSn hetero-epitaxial layers on Ge using MBE, a biaxial compressive strain is often induced into the epitaxial layers, as a GeSn or GeSiSn layer is often grown pseudomorphically on Ge substrate. The compressive strain of GeSn is a disadvantage for enhancing the luminescence efficiency because a higher Sn content is generally required to the indirect-to-direct crossover for a compressively strained GeSn compared to the unstrained case. In order to improve the efficiency of luminescence, it is important not only to enhance the Sn content but also to control the strain structure. We developed the strain relaxation technology of GeSn epitaxial layer on a substrate or buffer layer. We found that the ion implantation into Ge substrate effectively enhance the strain relaxation of GeSn layer. The degree of strain relaxation achieved over 90% for the GeSn layer grown on the Ge substrate after the boron ion implantation with 3x1014 cm-2, while the compressively strained GeSn layer was pseudomorphically grown on the Ge substrate without the ion implantation. This work was partly supported with a Grant-in-Aid for Scientific Research (S) (No. 26220605) and the Core-to-Core Program ICRC-ACP4ULSI from the JSPS.

Quantum well intermixing technique using proton implantation for carrier confinement of vertical-cavity surface-emitting lasers

We investigated quantum well intermixing (QWI) using proton implantation to form the carrier confinement structure in the active layer of a vertical-cavity surface-emitting laser (VCSEL). The required potential barrier height is discussed referring to the result of numerical analysis. The bandgap change due to the QWI was investigated experimentally for various quantum well structures, proton dose densities, and thermal annealing conditions. A potential barrier height of 30 meV was observed using a high-indium and thin-well structure. High crystalline quality was confirmed by photoluminescence intensity measurement, even after the QWI process, and the lasing of the fabricated QWI-VCSEL was observed without any deterioration. The proposed technique would be effective in improving the device performance in a simple fabrication process.

Highly efficient blue organic light-emitting diodes using various hole and electron confinement layers

In this Letter, blue phosphorescence organic light-emitting diodes (PHOLEDs) employ structures for electron and/or hole confinement; 1,3,5-tris(N-phenylbenzimiazole-2-yl)benzene is used as a hole confinement layer and tris-(phenylpyrazole)iridium [Ir(ppz)3] is utilized for an electron confinement layer (ECL). The electrical and optical properties of the fabricated blue PHOLEDs with various carrier-confinement structures are analyzed. Structures with a large energy offset between the carrier confinement and emitting layers enhance the charge-carrier balance in the emitting region, resulting from the effective carrier confinement. The maximum external quantum efficiency of the blue PHOLEDs with the double-ECLs is 24.02% at 1500 cd/m2 and its luminous efficiency is 43.76 cd/A, which is 70.47% improved compared to the device without a carrier-confinement layer.

Diode ideality factor in modern light-emitting diodes

The diode ideality factor has been subjected to theoretical discussions from the viewpoint of light-emitting diodes (LEDs). As radiative recombination in LEDs is incongruent with the Sah–Noyce–Shockley (SNS) analysis, the SNS recombination plane was replaced with carrier confinement structures of modern LEDs, e.g., quantum wells (QWs). We calculated the voltage partitioning factors to indicate how carrier concentration changes as a function of terminal voltage. As local minority carriers determine the net recombination rate at QWs, we have deduced that the partitioning factor of the local minority carriers becomes the ideality factor of the LED. By applying incomplete ionization of acceptors in InGaN/GaN LEDs, a conclusion has been reached such that the partitioning factor of local minority carriers as well as the diode ideality factor can be nearly as large as 3.

Lasing characteristics and modal gain of a lateral-junction InGaAs/GaAs edge-emitting laser diode grown on a patterned GaAs (311) A-oriented substrate

Edge-emitting InGaAs/GaAs laser diodes were grown on patterned GaAs (311)A-oriented substrates. Due to the amphoteric properties of Si as a dopant in high-index GaAs, a device with a lateral p–n junction was obtained. CW lasing was observed up to 200 K. The dependence of the threshold current with temperature was measured. The characteristic temperatures were T0=180 and T0=57 K for experimental temperatures below and above 100 K, respectively, meaning that the carrier confinement structure must be improved. Quantitative results of the cavity gain were experimentally obtained.

Low dimensional carrier confinement structure grown on patterned GaAs(111)A substrates by molecular beam epitaxy

Low-dimensional carrier confinement structures have been grown on patterned GaAs(111)A substrates with (311)A sidewalls by a single-step molecular beam epitaxy growth of a Si-doped GaAs layer. The method is based on the amphoteric nature of Si dopants in GaAs growth; the confined (111)A region is p-type and the (311) sidewalls are n-type. The confinement structures are confirmed by observing their cathodoluminescence spectra. The reduced confinement structure is confirmed by current–voltage measurements with an atomic force microscope.

Switches with double carrier confinement structures for multiple-valued logic applications

Two switching devices with different double carrier confinement structures were fabricated. One is a GaAs switch with double InGaP barriers, another with double GaAs-InGaAs delta-doped (-doped) quantum wells. In the characteristics of the former, because of the two-step barrier lowering induced by the processes of the carrier charging to the injection junction and carrier confinement in wells, a double S-shaped negative differential resistance (NDR) phenomenon is observed after the onsets of successive avalanche multiplications. However, only a single NDR performance is obtained in the characteristics of the latter due to one-step barrier lowering, even though successive avalanche multiplications also occur in the device operation.

Real-Space Transfer and Trapping of Carriers into Single GaAs Quantum Wires Studied by Near-Field Optical Spectroscopy

We report the first near-field optical study of single GaAs quantum wires grown on patterned $(311)A$ GaAs surfaces. Spatially resolved optical spectra at a temperature of 10 K give evidence for one-dimensional carrier confinement and subband structure. At 300 K, electron-hole pairs in continuum states undergo diffusive real-space transfer over a length of several microns determined by hole mobility and trapping by optical phonon emission. Optical phonon scattering of carriers in the quantum wire establishes a quasiequilibrium carrier distribution in both wire and continuum states.

MBE growth of submicron carrier confinement structures on patterned GaAs(111)A substrates using only silicon dopant

Abstract Silicon-doped GaAs layers have been grown on patterned GaAs(111)A substrates with equilateral triangles by molecular beam epitaxy. Lateral carrier confinement structures are confirmed by cathodoluminescence (CL) spectra: a (111)A surface is p -type and (311)A sidewalls are n -type. The confined (111)A region is reduced to submicron scale by reducing the size of the photoresist pattern. Furthermore, strong emission from the confined region and lateral modulation of peak wavelength are observed in the CL spectra of Al 0.3 Ga 0.7 As GaAs single quantum well grown on the patterned (111)A substrate. This modulation is caused by a decrease in GaAs layer thickness due to Ga migration from the (111)A surface to the (311)A surface. These results indicate that MBE growth on the patterned (111)A substrate is effective for fabricating submicron carrier confinement or optical confinement structures.

Effects of lateral mode and carrier density profile on dynamic behaviors of semiconductor lasers

Dynamic behaviors of the semiconductor lasers have been investigated both theoretically and experimentally. A single-mode rate equation, which takes account of the lateral mode profile and the carrier density profile, has been solved numerically. Effects of the carrier and lateral mode confinement have been clarified. The lateral mode deformation in lasers without a built-in mode confinement structure greatly enhances the relaxation oscillations. In lasers whose stripe width is narrower than \sim10 \mu m, the carrier diffusion is found to play an important role in suppressing the relaxation oscillations, especially for lasers without a lateral carrier confinement structure. On the other hand, the fraction of the spontaneous emission going into the lasing mode is significant for lasers with a lateral carrier confinement structure. The slow increase of the laser output at the transient is confirmed to be due to the carrier diffusion.