InGaAsP Layers Research Articles

Reactor-scale uniformity of selective-area performance in InGaAsP system

Selective-area metal organic vapor phase epitaxy (SA-MOVPE) of InGaAsP-related semiconductors is one of the most important fabrication techniques for monolithically integrated optical devices. However, it has not been clarified how much the mask effect depends on the position of a wafer in a MOVPE reactor. Therefore, the uniformity of the mask effect in a reactor was investigated experimentally coupled with the simulation in both reactor scale and micrometer scale. As for the reactor-scale distributions of an InGaAsP layer, the measured profiles of the growth rate and composition were well-reproduced by the simulation that assumed the adsorption and the desorption of As and P species. As for the micro-scale distribution in the selective-area growth, the modulations of photoluminescence (PL) wavelength due to masks were dependent on the position of the substrate in the reactor. The reason was assumed to be that the surface reaction rate constant of a precursor had the distribution in the reactor, due to the concentration distribution of both precursors and reaction products.

Zn diffusion behavior at the InGaAsP/InP heterointerface grown using MOCVD

Zinc (Zn) diffusion through MOCVD-fabricated InP and InGaAsP layers, and the corresponding Zn doping profile at the heterojunction interface were studied as part of the doping profile control for laser diodes. It was found that the Zn doping profile has specific discontinuity at the heterojunction in InGaAsP/InP heterostructures. Using secondary ionization mass spectrometer (SIMS) and Boltzmann–Matano analysis for different composition ratios ( x) of (1− x)InP– xInGaAs epitaxial layers grown on Zn-doped InP substrates, it was found that the Zn diffusion coefficients were proportional to the square of the concentration in the InP and InGaAsP layers. The Zn diffusion coefficient strongly depends on the component ratio ( x) for the (1− x)InP– xInGaAs layer. It was concluded that this diffusion property is based on the higher stability of the substitutional Zn content in the InGaAs layer compared to that in the InP layer. The dependence of the Zn diffusion coefficient on Zn concentration in the InGaAsP/InP layers is explained based on the main diffusion source being Zn at the interstitial sites. The thermal equilibrium between Zn-interstitial and Zn-substitutional (interstitial–substitutional model) describes these Zn diffusion properties. Marked Zn concentrations at InGaAsP/InP heterojunctions are referred to as “pileups”, and are affirmed due to the difference in physical properties between InP and InGaAs. With different composition ratios ( x) of InP/InGaAsP layer growth, pre-control of the Zn concentration and strict limitations on the growth conditions before diffusion are indispensable for lasers requiring a precisely controlled doping concentration.

Vapor phase diffusion and surface diffusion combined model for InGaAsP selective area metal–organic vapor phase epitaxy

The growth rate profiles adjacent to selective area masks were studied for InP, InGaAs and InGaAsP by metal–organic vapor phase epitaxy. Time-proportional growth rate increments were found for InP growth at the mask edge. The growth rate of InP at the edge saturated after the length of (1 1 1) B plane became larger than 1.4 μm, which corresponds to the migration length of an In precursor on this plane. The time evolution of the InP layer at the mask edge was modeled as (1) precursors stuck on (1 1 1) B plane migrate to (1 0 0) plane and (2) they diffuse on that plane as well as get incorporated there. The simulated thickness of InP layers at the mask edge showed excellent agreement with experimental data. In the case of InGaAs and InGaAsP, there were no indications of surface diffusion in the thickness profile. However, the distribution of the photoluminescence wavelength of InGaAsP layers suggested that the surface diffusion of Ga precursor on the (1 0 0) plane of the InGaAsP layer existed with the diffusion length of as long as 8 μm.

An ultrafast InP/InGaAsP optical modulator

Direct-bandgap III-V semiconductor materials based on InP allow monolithic integration of optical functionality with electronic circuitry for optical communication applications. The quaternary III-V alloy, InGaAsP, is particularly important because its bandgap can be tuned to different wavelength windows by adjusting its composition, while keeping its lattice matched to InP. Optoelectronic modulators, which translate electrical pulses into light pulses, are a critical component for communications. Here we show how, by electrically inducing a 2.5mm-long Distributed Bragg Reflector in an InP/InGaAs Prib waveguide, we can theoretically realize a modulator capable of a switching speed over 40GHz. The proposed device, shown in cross section in Figure 1, is a vertical InP/InGaAsP p-i-n diode, in which the intrinsic InGaAsP layer defines the optical channel. The anode is periodically spaced along the top of waveguide, as shown in schematic top view in Figure 1. The principle of operation is straightforward. When the diode is unbiased, the device is transparent and light can travel along the modulator without being affected by loss. When a negative voltage is applied between anode and cathode, a Bragg reflector is induced, due to the periodic refractive-index variation in the intrinsic region caused by spatial periodicity of the anode along the 2.5mm waveguide. The periodic index variation causes the light to be reflected rather than transmitted. When an electric field is applied to a semiconductor, two main types of effect can induce a refractive-index variation: the first is related directly to electric field strength (electro-optic effect), while the other type includes several effects related to the change of the free-carrier concentration. In InP-based materials, no single effect is dominant. To predict grating reflectivity, it is necessary to know how the effective index of the junction varies with the applied anode bias. The first step is to compute the refractive-index variation in the Figure 1. The left panel shows a schematic cross-section of the proposed modulator, where the guiding layer is intrinsic InGaAsP. The top signal electrode (the anode) is patterned into a comb-like shape, as shown in the top view of the device on the right.

GSMBE growth and characterizations of AlInP/InGaAsP strain-compensated multiple-layer heterostructures

We have grown long-period AlInP/InGaAsP strain-compensated multiple-layer heterostructures (SCMLHs) and SCMLHs combined with InAsP/InGaAsP strain-compensated multiple quantum wells (SC-MQWs) by gas source molecular beam epitaxy. Etch pit density (EPD) for both structures are in magnitude of ∼10 5 cm −2. The increment of EPD with increase of period number is small, indicating low sensitivity of the dislocation density to the increase of period number of SCMLHs. High resolution X-ray diffraction and photoluminescence (PL) characterizations of the two structures demonstrate that crystal quality remains high due to strain compensation. Diffusion and segregation of indium were clearly observed in both InGaAsP and AlInP layers by secondary ion mass spectroscopy. As the thickness of epilayers increases In content increases, while Al decreases. PL for the structure of 20-pair AlInP/InGaAsP SCMLHs+InAsP/InGaAsP SC-MQWs shows strong luminescence and narrow line width. The strain-compensated technique can effectively suppress the formation of misfit dislocations in AlInP/InGaAsP SCMLHs although the thicknesses of epilayers are above the critical thickness of consisting materials, which may render its potential application in optoelectronic devices.

High uniformity of InGaAsP layers grown by multi-wafer MOVPE system

Compositional uniformity of lattice-matched quaternary (InGaAsP) epitaxial layers on InP substrate was examined with multi-wafer MOVPE growth. Epitaxial layers with the photoluminescence (PL) wavelength of 1.3 μm were obtained with compositional uniformity standard deviation of 0.80 nm in all four wafers, and 0.97 nm in all six wafers for the same growth batch. The vertical high-speed rotating-disk reactor was used to grow the epitaxial layers on four or six 2″ InP wafers for the same growth batch. Compositional uniformity depends on wafer temperature that attributes to the thermal contact resistance between a wafer and a wafer carrier. It is important to suppress the temperature difference between them. The structure of the wafer carrier was optimized by computational fluid dynamics (CFD) analysis in order to compensate the thermal contact resistance. The wafer carrier temperature was successfully controlled to the same as the wafer, so that compositional uniformity of epitaxial layers was improved.

Strain relaxation in InGaAsP/InP grown by metal-organic vapor-phase epitaxy

We examined experimentally how the surface morphology, photoluminescence (PL) and X-ray diffraction depend on the amount of compressive strain in the InGaAsP layers grown by metal-organic vapor-phase epitaxy (MOVPE). It was found that there are two critical values of strain in the relaxation process. One is the threshold strain value ( ε c1) for the appearance of the cross-hatched pattern on the crystal surface and subsequent decrease in PL intensity. The other is that for the in-coherent growth ( ε c2), which leads to the broadening of the full-width half-maximum (FWHM) in the X-ray diffraction analysis. We also found that ε c1 is in good accordance with the force–balance theory, while ε c2 follows the energy–balance theory. We also examined their compositional wavelength ( λ g) dependence ( λ g of InGaAsP: 0.92 μm (InAsP), 1.15, 1.3 and 1.67 μm (InGaAs)). The ε c1 of InAsP is approximately half those of the other materials. This is probably due to the relatively large Poisson's ratio for InAsP. On the other hand, ε c2 of InGaAsP with λ g of 1.15 and 1.3 μm is slightly smaller than that of InAsP or InGaAs. This can be explained by local strain fluctuation in a nanoscale region, i.e. the fluctuation is stronger for quaternary InGaAsP compared to that for ternary InAsP or InGaAs. These findings are very useful for the crystal growth of high-quality highly strained InGaAsP.

Study on InGaAsP–InGaAs MQW-LD With Symmetric and Asymmetric Separate Confinement Heterostructure

We studied symmetric and asymmetric InGaAsP-InGaAs 1.55-/spl mu/m multiquantum-well (MQW) laser diodes (LDs) with highly p-doped layers in the two-step separate confinement heterostructure (SCH). The p-doping in p-SCH suppresses the electron overflow from the MQWs to p-SCH, but it is an origin of free carrier absorption loss. An additional InGaAsP layer inserted inside n-SCH makes asymmetric field distribution and, therefore, reduces the portion of optical field distribution in highly p-doped regions with high optical loss. Compared with symmetric structure, asymmetric SCH LD has low threshold current density, low internal loss, and high and flat slope efficiency with respect to temperature.

Study of InGaAsP and GaInP layers grown by MOCVD in pure N 2 ambient for InGaAsP/GaAs single QW LD structures

InGaAsP and GaInP layers has been grown by MOCVD technique using tertiarybutylarsine and tertiarybutylphosphine in pure nitrogen ambient for fabrication of InGaAsP/GaAs single QW laser diode structures. The composition control and the temperature dependence of the optical quality of In 1− x Ga x As y P 1− y layers and the Si- and Zn-doping behaviors in GaInP layers are emphatically discussed.

Single-step growth of InGaAsP/InP laser array on patterned InP substrate

Arrays of five InGaAsP/InP single mode junction-defined buried stripe heterostructure lasers are described. The laser arrays were grown on multi-channeled InP substrate by single-step liquid phase epitaxy . The buried double heterostructure and the lateral current confining structure were formed in the same growth process. InGaAsP layer growth is dominated by the preferred orientation, with (1 0 0) growth favored over other directions. As a result of low-temperature single-step growth, the device yield is high. These laser arrays are characterized by output power close to 0.6 W , high quantum efficiency, symmetrical far-field patterns and excellent linearity of the light–current curve. Stable single transverse mode operation obtained up to 600 mW emitted power.

High-Performance Composite-Collector InP/InGaAs Heterojunction Bipolar Transistors

This paper describes composite-collector InP/InGaAs heterojunction bipolar transistors that exhibit high current density and good breakdown behavior simultaneously. The collector consists of 150-nm InGaAs and 150-nm lightly n-type doped InP. Additionally, a 30-nm-thick InGaAsP layer is sandwiched between them to minimize the accumulation of transit carriers. The proposed structure makes use of the space charge in the collector to reduce the collector-current multiplication coefficient. At high current density, the space charge decreases the electric-field intensity in the narrow-band-gap InGaAs and moves the high-electric-field region into the wide-band-gap InP. The fabricated transistors exhibit flat collector I-V curves, high current density of over 4 mA/µm2, and reasonably small knee voltage. They also show a peak ft of over 200 GHz and a peak fmax of about 350 GHz at a current density of 2 mA/µm2. The ring oscillator IC based on the emitter-coupled-logic gate provides the gate propagation delay of 3.83 ps/stage.

New DFB grating structure using dopant-induced refractive index step

A new distributed feedback (DFB) grating structure using different doping levels of n-type InP to obtain a modulated index of refraction contrast is demonstrated. The grating consists of an array of highly doped n-type InP embedded in low-doped n-type InP. It is shown that the dielectric step (Δ n=0.058) between n-doping of 5×10 17 cm −3 and n-doping of 1×10 19 cm −3 is large enough for device operation. Single mode lasing at a wavelength of 1.55 μm with an SMSR of >45 dB has been achieved in InGaAlAs/InP devices. This DFB grating structure removes some of the technological drawbacks observed when fabricating the conventional index-coupled grating using an etched and overgrown InGaAsP layer. These manufacturing advantages include the absence of arsenic migration during overgrowth and removal of notching during grating etch.

Mask interference effect in a densely arrayed waveguide fabricated by using narrow-stripe selective MOVPE

Narrow-stripe selective MOVPE is an attractive technique for fabricating integrated photonic devices. However, in the case of densely integrated devices, in which several optical waveguides are close together, interference occurs between the masks, and the characteristics of the grown waveguide layers are affected by several mask stripes. To realize densely integrated photonic devices with a high performance, we have to simulate waveguide structures, including not only PL-wavelength but also layer thickness and induced strain, while taking the mask interference effect into account. In the proposed simulation we introduced separate mask interference constants for indium and gallium species to obtain both the thickness and the composition of the InGaAsP layers. We experimentally obtained different relationships between layer thickness and PL-wavelength for the InGaAsP bulk arrayed waveguide, which was attributed to a different interference effect between each group III species. The measured characteristics of the arrayed MQW waveguides agreed well with the simulation. The simulation method is useful for designing densely integrated waveguides fabricated by narrow-stripe selective MOVPE.

Buried-Ridge Waveguide Laser Diode 제작 및 특성평가

본 연구에서는 기존의 ridge waveguide laser diode(RWG LD)보다 ridge폭에 따른 측방향 단일모드 특성이 우수하고 planar 화에 유리하며 측방향의 유효 굴절률차를 ridge 구조에 추가로 성장된 InGaAsP층의 두께로 조절이 가능한 Buried RWG LD를제작하였다. 본 연구에서는 Buried RWG LD를 CBE장치로 InGaAs/InGaAsP multiple quantum well(MQW) 에피 웨이퍼를 성장하고, LPE로 재성장하여 B-RWG LD를 제작하였다. 또한 ridge 폭을 5 <TEX>$\mu\textrm{m}$</TEX>와 7 <TEX>$\mu\textrm{m}$</TEX>로 하여 B-RWG LD를 제작하고 특성을 비교하여 보았다. 제작된 7 <TEX>$\mu\textrm{m}$</TEX> B-RWG LD에서 광출력이 20㎽에 이를 때까지 고차모드 발진에 의한 kink현상이 일어나지 않았으며, 포화 광출력이 80 ㎽ 이상임을 확인하였다. 제작된 B-RWG LD가 측방향 단일모드로 동작함을 확인하기 위해 FFP을 측정한 결과, ridge 폭이 5 <TEX>$\mu\textrm{m}$</TEX>일 때는 2.7I<TEX>$_{th}$</TEX> , ridge 폭이 7 <TEX>$\mu\textrm{m}$</TEX>일 때는 2.4I<TEX>$_{th}$</TEX> 까지 단일모드로 동작함을 확인할 수 있다. We fabricated a buried-ridge waveguide laser diode (B-RWG LD) which has more advantages for obtaining lateral single mode operation on the same ridge width and for the planarization of the device surface, compared to the conventional RWG LD. In this LD, the difference of the lateral effective refractive index can be controlled by the thickness of the InGaAsP layer which is grown on the active and the p-InP layers. The InGaAsP multiple quantum well was grown on a n-InP substrate by the CBE. The buried ridge structure was formed by selective wet etchings, followed by liquid phase epitaxy methods. The fabricated LD with the ridge width of 7 <TEX>${\mu}{\textrm}{m}$</TEX> showed a linear increase of the optical power up to 20 ㎽ without any kinks and a saturated output power of more than 80 ㎽. By measuring the far field pattern, we demonstrate that LDs with the ridge widths of 5 <TEX>${\mu}{\textrm}{m}$</TEX> and 7 <TEX>${\mu}{\textrm}{m}$</TEX> were operated in a lateral single mode up to 2.7I<TEX>$_{th}$</TEX> and 2.4I<TEX>$_{th}$</TEX>, respectively.ely.

Evaluation of InP-Based Epitaxial Layers by Photoluminescence Measurement

The correlation between the photoluminescence (PL) intensity of either InGaAsP or InGaAs epitaxial layer and the properties of InP substrates was investigated. It was found that the PL intensity of epitaxial wafers strongly depended on carrier concentration, the condition of the back surface of the substrate and the structure of epitaxial layers. In the PL intensity measurement, the absorption of luminescence by the substrate itself sometimes causes the reduction in PL intensity. In the case where a pattern was observed at the back of the substrate, which was contaminated by the outgas from a spring in a fluoroware, the measured PL intensity showed an anomalous distribution. However, the quality of epitaxial layers did not change. It is very important to consider the effects of the absorption of the substrate and the roughness of the back of the substrate for evaluating the quality of epitaxial layers by PL measurement.

Overgrowth on InP corrugations for 1.55 μm DFB LDs by reduction of carrier gas flow in LPMOCVD

We describe reproducible and uniform overgrowth on InP corrugations to fabricate DFB LDs by reduction of carrier gas in an LPMOCVD system. The reduction of carrier gas from 20 l/min, which was a typical growth and system-recommended condition, to 2 l/min during heat-up time enabled the growth of an InGaAsP layer on InP corrugations for any heat-up condition including heat-up time and heat-up ambient. The TEM image showed abrupt interfaces in the MQW layer and a non-defective interface between the InGaAsP layer and the InP corrugations. This study demonstrated the variation of residual grating depth with changing the heat-up time while maintaining a specific gas ambient. The SEM and microscopic images showed the effect of reducing the carrier gas flow. A fabricated DFB laser diode with 1.55 μm operating wavelength showed a threshold current of 12 mA (36 mA) and slope efficiency of 0.32 mW/mA (0.22 mW/mA) at 25°C (100°C). A side mode suppression ratio (SMSR) of 50 dB was obtained within the range of temperature, 25–85°C, and an injection current range of 40–200 mA.

Micro-X-ray fluorescence and micro-photoluminescence in InGaAsP and InGaAs layers obtained by selective area growth

Properties of selective-area-grown (SAG) ternary and quaternary layers of InGaAs and InGaAsP produced by metal organic vapor phase epitaxy (MOVPE) have been investigated by using a combination of micro-photoluminescence and synchrotron-based micro-beam X-ray fluorescence techniques. Significant variations of the group-III element composition with increase of the oxide mask width and with decrease of the gap between the masks were observed in both ternary and quaternary layers. In contrast, concentration of the group-V elements in quaternary layers does not show any significant change. Effects of the SAG growth on variations of the group-III element composition become more pronounced with increase of the growth pressure.

X‐ray diffraction, photoluminescence and composition standards of compound semiconductors

Work is underway to develop composition standards and standardized assessment procedures for compound semiconductors. An AlGaAs composition standard with less than 2% uncertainty is being developed. The improved accuracy of this standard is being achieved by combining an array of analysis techniques, including reflection high energy electron diffraction, photoluminescence (PL), electron microprobe analysis and inductively coupled plasma–optical emission spectroscopy. A major part of the work has been quantification of the accuracy limits of each technique. The influence of peak fitting method, measurement temperature, and doping concentration on PL measurements of AlGaAs layers has been measured. Similar work is underway for PL analysis of InGaAsP and for X-ray diffraction (XRD) analysis of a wide variety of materials in order to develop standardized assessment procedures. An inter-laboratory comparison was made of XRD and PL measurements of InGaAsP layers. The study demonstrated that material nonuniformity dominated the variation in the XRD measurements; but the uniformity was sufficient to allow PL measurement variations to be assessed.

MOCVD growth of asymmetric 980 nm InGaAs/InGaAsP broad-waveguide diode lasers for high power applications

Asymmetric 980 nm InGaAs/InGaAsP laser structures have been grown by low-pressure metalorganic chemical vapor deposition, and broad-waveguide diode lasers of very large equivalent transverse spot size (0.80 μm) have been fabricated. High-quality, relatively thick (>0.7 μm), high-bandgap InGaAsP ( E g=1.80 eV) layers were developed, as needed for the successful implementation of a device design for high-power, single-transverse-mode operation. Uncoated 100-μm-wide stripe, 2-mm-long lasers exhibit a threshold-current density of 188 A/cm 2, an internal loss coefficient of 1.4 cm −1, a differential quantum efficiency of 66%, and high characteristic temperatures for the threshold-current density and the external quantum efficiency (i.e., T 0=190 K and T 1=520 K).

Composition Control of InGaAsP Laser Structures Using In-Situ Emissivity Corrected Pyrometry During Epitaxial Growth

Wafer temperature during metal-organic chemical vapor deposition growth strongly influences the composition, and therefore, photoluminescence wavelength of InGaAsP layers when using AsH3 and PH3 precursors. Emissivity corrected pyrometry was used to measure and control the wafer temperature during epitaxial growth of 1.5 µm InGaAsP laser structures. Control of laser structure wavelength was demonstrated using an Emcore D180 LDM reactor. Average peak wavelength for five, six wafer runs had a standard deviation of 2.1 nm indicating excellent repeatability as long as wafer temperature was identical from run to run. Composition control was further demonstrated by intentionally reducing the wafer temperature during two successive growth runs. Average peak PL wavelength increased from 1522 nm to 1530 nm by reducing RealTemp wafer temperature 2°C from 607.7°C to 605.7°C during active region growth, corresponding to a 4 nm shift of wavelength for a 1°C change.