InGaP Buffer Research Articles

Cathodoluminescence Study of Dislocations in Step-Graded InGaP Buffer Layers of Metamorphic Single-Junction InGaAs Solar Cells

Epitaxial growth of lattice-mismatched materials is useful for solar cells, but lattice dislocations must be controlled for best device performance. It has been shown that metamorphic growth enables fabrication of InGaAs p–n junctions with good performances on GaAs substrates due to the insertion of buffer layers. Here, we investigate misfit and threading dislocations inside the step-graded InGaP buffer layers of a single-junction InGaAs solar cell by cathodoluminescence microscopy. Prior to measurement, the device edges were polished at various angles (less than 10° with respect to the substrate surface). By using this technique, cross sections of very thin layers can be directly imaged with a resolution that allows us to observe misfit and threading dislocations. In the present device, the densities of the two types of dark lines depend on the position in the buffer structure. In particular, near the InGaAs base layer, the density of the dark lines extending in the [110] direction is higher than that of the dark lines extending in the [1-10] direction. We believe that this difference in the dark line density is related to the surface morphology.

1.06 μm wavelength photodetectors with metamorphic buffer layers grown on GaAs substrates

The result of investigation of InGaAs photodetectors grown on GaAs substrate with metamorphic InGaP buffer is shown in this paper. It has been shown experimentally that the use of the gradient InGaP metamorphic layer in an InGaAs photodiode structure improves the spectral characteristics and crystalline quality of the structure and leads to reduction of dark current by an order of magnitude compared to photodiodes fabricated with metamorphic InGaAs layers.

(Invited) Monolithic Integration of n-In0.53Ga0.47As and p-Ge FinFETs on Si

With continuous increase in transistor count and clock frequency, the power density of current Si integrated circuits has reached a level that calls for a revolutionary change in transistor technology. High hole mobility Ge and high electron mobility InGaAs have been considered to be the most promising channel materials for p-channel and n-channel MOSFETs, respectively. Selective-area growth (SAG) on nano-trenches has been quite popular in integrating III-V materials with Si substrates. Since there exists a high lattice mismatch between III-V materials and Si, obtaining defect-free epilayer is still a great challenge. In this work, we demonstrate selective area growth of InGaAs on Ge trenches aiming at integrating n-channel InGaAs FinFETs and p-channel Ge FinFETs side by side on a Si substrate for CMOS applications. The Ge templates, which were grown on Si by chemical vapor deposition, were prepared by etching through an oxide mask using a dilute H2O2 solution to create Ge linear trench structures with embedded {111} facets at the bottom of the trenches for facilitating the regrowth of III-V materials. The III-V materials were selectively grown on the patterned Ge templates by metal-organic vapor phase epitaxy. An InGaP buffer layer, which has a wide bandgap and is beneficial for electrical isolation between the channel and substrate, was grown onto the Ge trenches. Undoped InGaAs was grown on InGaP as the n-channel material. High resolution transmission microscopy investigations show that very few dislocations are present near the Ge/InGaP interface in the trenches. It is found that In content of InGaP increases from 50 % to about 70 % with growth as evidenced by energy-dispersive x-ray spectroscopy (EDX). Interestingly, composition grading is also observed on the subsequently grown InGaAs. It begins from an In0.21Ga0.79As layer, which has good lattice match to the In0.7Ga0.3P layer beneath, then changes to the nominal In0.71Ga0.29As layer near the top of the fin. The strain in InGaAs is partially released through the generation of dislocations near the InGaAs/InGaP interface, leading to a nearly defect-free region at the top of the fin. A growth mechanism is proposed to account for the correlation between strain accommodation, composition grading, and defects generation.

InAs/InGaP quantum dot solar cells with an AlGaAs interlayer

We studied the growth of InAs/InGaP quantum dot (QD) solar cells with near ideal bandgaps for intermediate band solar cells. Using a solid-source molecular beam epitaxy system, the evolution of InAs QDs grown on an InGaP buffer layer was examined as a function of InAs coverage and growth temperature. QDs were initiated with defective clusters for InAs deposition as small as 0.3 monolayer. The cross diffusion of In atoms and As–P exchange between InAs and the InGaP buffer layer could be responsible for the formation of coherent QDs and large defective clusters. An AlGaAs interlayer was inserted between InAs and the InGaP buffer layer to prevent the surface exchange reactions between InAs and InGaP. As a result, InAs QDs with a density of 1×1011cm−2 have been demonstrated with stronger photoluminescence emission compared to those with the GaAs interlayer. This growth technique was applied to fabricate InAs/InGaP QD solar cells. Post-growth annealing treatment was also investigated to improve the performance of QD solar cells.

Optimization of InGaP metamorphic buffers grown by MOVPE

Inverted metamorphic multijunction solar cells have shown high solar conversion efficiencies and utilized InGaP based metamorphic (MM) buffers to change the lattice constant using compositional graded buffer layers while minimizing dislocation density in the final material layers. In this study, optimization of InGaP metamorphic buffers was done by systematically exploring key metalorganic vapor phase epitaxy (MOVPE) growth conditions and the MM buffer epitaxial stack structure. To optimize MOVPE growth parameters, growth temperature and V/III ratio were varied during the growth of a standard MM buffer test structure and the final InGaP buffer layer was characterized by photoluminescence, X-ray reciprocal space maps, atomic force microscope, cathodoluminesence, and ex situ bow measurements. The in situ measurement of wafer curvature was also monitored during MM buffer layer growth. Evaluation of material characterization data provided optimized growth conditions for the InGaP based MM buffer. The second part of this study evaluated the actual layer thickness and number of compositional graded steps in a MM buffer. Our results showed that in situ deflectometer measurements of the wafer curvature of the MM buffer layer can be correlated to ex situ determined strain relaxation of the final buffer layer of the MM buffer. Process optimization tests showed a growth temperature of 580°C with a V/III ratio of 37 provided for the best surface roughness, highest PL intensity and also allowed for low dislocation defect density of the final buffer layer. Using the optimized growth conditions, further optimization of the step grade layers showed that a 350nm thick grade layer for a six step layer MM buffer for a final buffer composition targeted for In0.8Ga0.2P provided the best surface roughness and 100% final buffer relaxation.

The influence of atomic ordering on strain relaxation during the growth of metamorphic solar cells

The occurrence of single variant CuPtB ordering during growth of InGaP graded buffer layer structures on offcut (001) GaAs substrates for inverted metamorphic solar cells is found to have a strong influence on strain relaxation mechanisms. Since the surface-induced CuPtB ordering is metastable in the bulk of the material, a strong preference is observed for the nucleation and glide of 60° type misfit dislocations with Burgers vectors that introduce an antiphase boundary into the ordered structure. This results in an overall epitaxial layer tilt in the opposite sense to that normally observed for the direction of substrate offcut. Furthermore, in InGaP buffer layers graded to InP, a switch in the dislocation glide plane preference back to that normally observed for the direction of substrate offcut is observed as the degree of atomic ordering falls below a critical value. This results in the nucleation and glide of new misfit dislocations resulting in an increase in the threading dislocation density that is found to have a deleterious effect on device efficiency. Understanding the materials science behind this behavior will enable the engineering of more effective, lower threading dislocation density strain relief buffer layers resulting in improved performance of subsequently grown devices.

Comprehensive study of InGaP/InGaAs/GaAs dual channel pseudomorphic high electron mobility transistors

In this work, the device characteristics of InGaP/InGaAs/GaAs dual channel pseudomorphic high electron mobility transistors (DCPHEMTs) with triple δ-doped sheets are comprehensively and systematically investigated. The triple δ-doped sheets densities are found to be crucial for improving device performance. Based on a two-dimensional simulator of Atlas, the detailed calculations and studies including energy band diagrams, carrier distributions, and DC and microwave performances are reported. Due to the use of InGaAs DC structure and InGaP Schottky and buffer layers, good pinch-off and saturation characteristics, higher current drivability, larger and linear transconductance, and excellent microwave performance are obtained. For comparison, a practical DCPHEMT with good device performance is fabricated. Generally, good agreements between experimental results and theoretical simulations are found. The DCPHEMT with proper designed triple δ-doped sheet densities is promising the promise for high-performance digital and microwave device applications.

Vapour sensing properties of InP quantum dot luminescence

We investigated uncapped InP quantum dots grown epitaxially on InGaP buffer layer as an optically active element for chemical vapour detection. Near infrared luminescence has been studied as a function of the external environment. QD luminescence intensity changes rapidly and reversibly on exposure to methanol vapour while its spectral shape remains unchanged. For QDs about 45nm average lateral size and 4–6nm height, sensitivity to methanol vapour in the range 3.3×104–7.2×103ppm has been demonstrated.

Numerical simulation of Ge solar cells using D-AMPS-1D code

A solar cell is a solid state device that converts the energy of sunlight directly into electricity by the photovoltaic effect. When light with photon energies greater than the band gap is absorbed by a semiconductor material, free electrons and free holes are generated by optical excitation in the material. The main characteristic of a photovoltaic device is the presence of internal electric field able to separate the free electrons and holes so they can pass out of the material to the external circuit before they recombine. Numerical simulation of photovoltaic devices plays a crucial role in their design, performance prediction, and comprehension of the fundamental phenomena ruling their operation. The electrical transport and the optical behavior of the solar cells discussed in this work were studied with the simulation code D-AMPS-1D. This software is an updated version of the one-dimensional (1D) simulation program Analysis of Microelectronic and Photonic Devices (AMPS) that was initially developed at The Penn State University, USA. Structures such as homojunctions, heterojunctions, multijunctions, etc., resulting from stacking layers of different materials can be studied by appropriately selecting characteristic parameters. In this work, examples of cells simulation made with D-AMPS-1D are shown. Particularly, results of Ge photovoltaic devices are presented. The role of the InGaP buffer on the device was studied. Moreover, a comparison of the simulated electrical parameters with experimental results was performed.

On an InGaP/InGaAs Double Channel Pseudomorphic High Electron Mobility Transistor With Graded Triple $\delta$-Doped Sheets

An interesting InGaP/InGaAs double channel pseudomorphic high electron mobility transistor is fabricated and demonstrated. Due to the employed InGaP Schottky and buffer layers, InGaAs double channel structure, and graded triple delta-doped sheets, both DC and RF performances are improved. From experimental results, the studied device, with a gate dimension of 0.8 x 100 mum2 , shows a drain saturation current of 176 mA/mm, a maximum extrinsic transconductance of 176 mS/mm, a unity current gain cutoff frequency of 16 GHz, and a maximum oscillation frequency of 33.2 GHz at room temperature. Moreover, a theoretical analysis based on a 2-D semiconductor simulation package is used to study the device properties and compare the experimental results. Good agreement between the theoretical analyses and experimental results is found.

Mechanism of lateral ordering of InP dots grown on InGaP layers

The mechanisms leading to the spontaneous formation of a two-dimensional array of InP∕InGaP dots grown by chemical-beam epitaxy are discussed. Samples where the InGaP buffer layer was grown at different conditions were characterized by transmission electron microscopy. Our results indicate that a periodic strain field related to lateral two-dimensional compositional modulation in the InGaP buffer layer determines the dot nucleation positions during InP growth. Although the periodic strain field in the InGaP is large enough to align the InP dots, both their shape and optical properties are effectively unaltered. This result shows that compositional modulation can be used as a tool for in situ dot positioning.

On the nucleation of GaP/GaAs and the effect of buried stress fields

ABSTRACTWe have recently shown that spatial ordering for epitaxially grown InP dots can be obtained using the periodic stress field of compositional modulation on the InGaP buffer layer. The aim of this present work is to study the growth of films of GaP by Chemical Beam Epitaxy (CBE), with in-situ monitoring by Reflection High Energy Electron Diffraction (RHEED), on layers of unstressed and stressed GaAs. Complementary, we have studied the role of a buried InP dot array on GaP nucleation in order to obtain three-dimensional structures. In both cases, the topographical characteristics of the samples were investigated by Atomic Force Microscopy (AFM) in non-contact mode. Thus vertically-coupled quantum dots of different materials have been obtained keeping the in-place spatial ordering originated from the composition modulation.

Thermal resistance of metamorphic InP-based HBTs on GaAs substrates using a linearly graded In/sub x/Ga/sub 1-x/P metamorphic buffer

Thermal properties of metamorphic InP-InGaAs heterojunction bipolar transistors (HBTs) on GaAs substrates using a linearly graded InGaP buffer have been investigated. Compared to the widely used InAlAs metamorphic buffer, InGaP offers better thermal properties resulting in a much smaller thermal resistance for the metamorphic HBTs (MHBTs). Theoretical calculations of the thermal resistance of devices have been made based on a simple constant heat-spreading model, and the results are shown to be consistent with experimental results. It has been made clear that the smaller thermal resistance measured from the MHBTs using a linearly graded InGaP buffer is due to the small bowing parameters and high thermal conductivity of the binary endpoints. Although the use of InGaP as a buffer may slightly degrade the devices thermal properties compared to one using InP directly on GaAs substrate, it gives more freedom to the growth optimization of metamorphic buffer by using compositional grading. With regards to the thermal conductivity and flexible growth optimization, InGaP metamorphic buffer could be considered as an important alternative to the existing InAlAs and InP schemes.

Microstructural defects in metalorganic vapor phase epitaxy of relaxed, graded InGaP: Branch defect origins and engineering

Strain-relaxed, compositionally graded InGaP layers grown by atmospheric-pressure metalorganic vapor phase epitaxy (APMOVPE) have previously been found to exhibit unusual contrast in transmission electron microscopy (TEM). The features that generate this contrast were termed “branch defects.” Branch defects have been shown to pin threading dislocations and are thus undesirable features for the realization of low dislocation density semiconductors. In this study, we compare the properties of branch defects formed during optimized, relaxed, graded InGaP buffer deposition in two different reactor configurations: a commercial, multiwafer, low-pressure reactor and a custom-built, atmospheric-pressure research reactor. Branch defect formation is further characterized via the introduction of in situ annealing interruptions during graded buffer deposition in the atmospheric-pressure system. Branch defects are observed in material from both reactor systems, suggesting that they are a phenomenon intrinsic to InGaP graded buffer growth. Careful TEM studies of the resulting samples reveal that the phase space for the formation of branch defects is similar in both reactor configurations. During standard optimized graded buffer growth, higher growth temperatures delay the onset of branch defect formation to higher indium fractions in the graded buffer. Low growth temperatures produce branch defects at lower indium fractions, and these defects tend to be more closely spaced. In addition, the formation of branch defects is favored by low V/III ratios and in situ growth interruption and annealing. Annealing is found to create anisotropic strain relaxation in the graded buffer, which we attribute to the blocking of gliding threading dislocations by preferentially oriented branch defects. Based on the observed properties of branch defects and the factors that affect their formation, it appears that these defects are a manifestation of local variations in indium concentration that develop on the sample surface during MOVPE and are buried in the bulk due to kinetic limitations.

InGaAs/InP single quantum well structure grown on GaAs substrate with linearly graded metamorphic InGaP buffer layer by solid source molecular beam epitaxy

Optical and structural properties of InGaAs/InP single quantum well structure grown on GaAs substrate with linearly graded metamorphic InGaP buffer layer by solid source molecular beam epitaxy (SSMBE) using a valved phosphorus cracker cell have been investigated. X-ray diffraction measurements were carried out to investigate the strain relaxation behavior of the metamorphic buffers grown at different temperatures, and the degree of relaxation was found to be 96%. Photoluminescence (PL) measurements show comparable results with the PL response observed for a lattice matched InGaAs/InP quantum well grown on InP substrate. The growth temperature of the metamorphic buffer layer ranging from 430 to 480 °C was found not to influence the strain relaxation ratio and the PL results. The results presented suggest that linearly graded InGaP metamorphic buffer layers grown by SSMBE can be used to decouple the InGaAs/InP quantum well structure from the GaAs substrate.

Characterization of linearly graded metamorphic InGaP buffer layers on GaAs using high-resolution X-ray diffraction

High-resolution X-ray diffraction (HRXRD) was used to characterize linearly graded metamorphic InGaP buffer layers grown at different temperatures on GaAs substrate by solid-source molecular beam epitaxy. The sample grown at 380°C did not exhibit a Bragg diffraction peak corresponding to the top layers consisting of an InGaAs/InP single quantum-well structure, since it could not maintain a two-dimensional growth. When the samples were grown at higher temperatures (430–480°C), the top epitaxial layers were nearly fully relaxed and the strain relaxation anisotropy in two 〈110〉 directions was found to be small. It was also found that the growth temperature of the buffer layer for these samples did not influence the strain relaxation ratio. Moreover, the full width at half maximum (FWHM) values of the XRD peaks suggest that the sample grown at 480°C has better material quality.

Metamorphic In[sub 0.52]Al[sub 0.48]As/In[sub 0.53]Ga[sub 0.47]As high electron mobility transistors on GaAs with In[sub x]Ga[sub 1−x]P graded buffer

A new metamorphic In0.52Al0.48As/In0.53Ga0.47As high electron mobility transistor (HEMT) structure was grown on a GaAs substrate with a InxGa1−xP graded buffer layer by solid-source molecular beam epitaxy. The In0.53Ga0.47As channel layer was grown on the InGaP buffer layer directly without an InAlAs buffer as in the conventional design. High-resolution x-ray diffraction reveals that the whole layer structure is nearly fully relaxed. Hall measurement showed that this new layer design exhibits higher electron mobility and carrier concentration as well as lower light sensitivity compared to the reference sample with the conventional design. The promising device performance demonstrates the potential of using this metamorphic HEMT device in high speed and high frequency applications.

Monolithic photoreceiver constructed with InGaP/GaAs/InGaP MSM photodetectors and conventional GaAs MESFETs

We present a novel monolithic photoreceiver operating with a gain of 25 dB and a 3-dB bandwidth of above 2 GHz. In this photoreceiver, a high performance of GaAs metal-semiconductor-metal photodetector with InGaP buffer and capping layers was used. As well, a buffer logic invert circuit, feedback resistor and amplifier using the conventional MESFET’s structure were designed to perform transimpedance amplifiers. The performances of the photoreceiver could be tuned through the feedback resistor.

Effect of tensile strain on optical properties of AlGaP-based neighboring confinement structure

We report on the effect of tensile strain on optical properties of AlGaP-based neighboring confinement structure (NCS). Enhancement of photoluminescence (PL) intensity was observed from tensilely strained NCS, compared with unstrained NCS. In addition, drastic improvement of the activation energy for thermal quenching was achieved by introduction of tensile strain to NCS in spite of the reduction of the band discontinuity. These improvements were attributed to the modification of the band structure and the local-strain variation due to the alloy randomness of the InGaP buffer layer.

Improved performances of InGaP Schottky contact with Ti/Pt/Au metals and MSM photodetectors by (NH 4) 2S x treatment

To extend the high performances of the GaAs MSM photodetectors with InGaP buffer and capping layers, the (NH 4) 2S x treatment of InGaP is investigated. The surface states are reduced by sulfur passivation. Whereupon, the performances of InGaP Schottky contact with Ti/Pt/Au metals are improved. The improved dark current and insensitive responsivity with incident optical power are demonstrated by suitable process control of sulfur passivation.