Optical and interface properties of direct InP / Si heterojunction formed by corrugated epitaxial lateral overgrowth

We fabricate and study direct InP/Si heterojunction by corrugated epitaxial lateral overgrowth (CELOG). The crystalline quality and depth-dependent charge carrier dynamics of InP/Si heterojunction are assessed by characterizing the cross-section of grown layer by low-temperature cathodoluminescence, time-resolved photoluminescence and transmission electron microscopy. Compared to the defective seed InP layer on Si, higher intensity band edge emission in cathodoluminescence spectra and enhanced carrier lifetime of InP are observed above the CELOG InP/Si interface despite large lattice mismatch, which are attributed to the reduced threading dislocation density realized by the CELOG method. © 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement


Introduction
Integration of III-V semiconductors on silicon is a major challenge in realizing efficient electronics-photonics integrated devices and systems.Several III-V compounds are direct bandgap materials and hence have favorable optical and electronic properties, such as efficient light emission and high carrier mobility.In particular, InP and related materials are important III-V semiconductors, used in long wavelength lasers, high performance photodetectors, high electron mobility transistors, etc [1,2].High crystalline quality InP/Si are desired for Si based photonic integrated circuits and tandem solar cell applications.The three main approaches to integrate III-V and Si are: flip-chip integration [3], bonding technologies [4], and hetero-epitaxial growth [5].Among them, heteroepitaxy would be the most desirable approach for integration because of better thermal dissipation, self-alignment (which provides high integration density), fewer processing steps, and device cost effectiveness [6].However, the large lattice mismatch between InP and Si, the difference in the thermal expansion coefficients, and the polar/non-polar interfaces usually result in heteroepitaxial layers with high density of crystal defects.It is not uncommon to find misfit and threading dislocations, stacking faults, microtwins, and antiphase domains in those InP/Si layers.All these defects are detrimental to the optical devices made out of InP/Si, since they create deep electronic levels in the band gap, which can act as carrier traps, and non-radiative recombination centers (NRRCs) [7].
Various approaches for defect reduction have been considered to achieve high crystalline quality III-V/Si substrates by epitaxial growth techniques [5,8].The corrugated epitaxial lateral overgrowth (CELOG) method, a modified form of epitaxial lateral overgrowth (ELOG) [9], has been shown to be a potential solution for III-V/Si integration by hydride vapor phase epitaxy (HVPE) [10].HVPE is an ideal method for III-V based solar cell fabrication since it uses cheap precursors [11], yields high growth rate and gives the growth selectivity necessary for CELOG [12].Recently an n-InP/p-Si heterojunction photodiode was realized by the CELOG method in a HVPE reactor [13].
Understanding the minority carrier dynamics in InP/Si heterojunction is essential for achieving high performance devices, such as high efficiency solar cells.In InP/Si layers with high dislocation density, dislocations act as recombination centers which reduce the minority-carrier lifetime and diffusion length [14].In heterojunction solar cells, where dislocation density is normally high, the predominant loss mechanism is recombination loss at dislocations which reduces the open-circuit voltage and increases the leakage current [15].An enhanced carrier lifetime of InP on Si is desired for high performance photonic devices.
Here, we report on the investigation of depth resolved carrier dynamics in CELOG InP/Si direct heterojunction.By conducting low temperature cathodoluminescence (LT-CL) and time resolved photoluminescence (TRPL) line scanning on the cross-section of CELOG InP/Si, we study the impurity and defect related radiative recombination and non-radiative recombination through surface and interface states.The advantage of CELOG InP/Si for dislocation reduction and carrier lifetime enhancement is demonstrated.High crystalline quality InP/Si interface without threading dislocations and comparable to that of wafer bonded interface was revealed in transmission electron microscopy (TEM) studies.This study shows that the CELOG method to fabricate direct InP/Si heterojunction is promising for realizing III-V multi-junction solar cells and optical light sources on silicon.

Experiment details
A heterojunction of n-InP layer on p-Si substrate (n-InP/p-Si) realized via the CELOG method in an HVPE reactor is studied in this report.The n-InP layer was grown on a p-Si substrate patterned with InP-seed layers.The substrate processing included metalorganic vapor phase epitaxy (MOVPE) for the growth of the InP-seed on a (001) Si substrate (off-cut 4° toward [111]), plasma enhanced chemical vapor deposition (PECVD) for SiO 2 mask and Si 3 N 4 spacers, photolithography, and inductively coupled plasma (ICP) etching of the InP layer to form the InP-seed mesa.Process flow of InP-seed mesa fabrication is shown Figs.

1(a)-1(d).
A schematic of the InP-seed mesa pattern on Si processed for CELOG is shown in Fig. 1(e).The pattern consists of InP mesa (height = 2 μm) with Si 3 N 4 spacer (sidewalls), where the silicon surface is exposed through the circular holes, of diameter 30 μm, arranged in a triangular lattice with center-to-center distance of 35 μm.Circular pattern is chosen to enhance the coalescence.CELOG on stripe openings were investigated [10,16].We found that coalescence was hindered if the spacing between the stripes is too large and the lateral overgrowth rate is determined by the angle between stripe openings and [110] direction.By using symmetric circular opening, high rate lateral overgrowth front can be formed without intentional alignment of openings with respect to [110] direction.
The n-InP/p-Si CELOG growth in a HVPE reactor consisted of a semi-insulating InP:Fe (SI-InP, resisitivity ~10 8 ohm.cm) growth followed by n-InP (n = 7 × 10 16 cm −3 quantified by Hall measurements), for 5 min.and 25 min., respectively.The CELOG growth was conducted at 590°C at the reactor pressure of 20 mbar; the InCl and PH 3 flows were 12 sccm and 120 sccm, respectively.A schematic of the CELOG InP/Si cross-section is shown in Fig. 1(f).The dislocations in InP seed mesas can propogate to the surface of growth but will not bend downward to the surface of Si substrate in the lateral overgrowth region.This heterojunction has been processed to build up a photodiode.N-type contact pads (150 µm × 150 µm) consisting of 90 nm AuGe/50 nm Ni/150 nm Au were formed on the n-InP surface by e-beam ev sputtering dep ohmic contac discussed else The time reso manual transl spot of 2.0 μm power was 0. averaging ove 10 18 cm −3 .Th a high-resolu prepared by m keV was used vaporation and posited on the cts.The growth ewhere [13].The hyper selected regio the luminesce spectra acquir inset, the pan (1.37 eV), 96 of the differ particular, mo section at wav The image with the spat processing ste (BE) of InP a regions of the donors and ac the CELOG I  4 μm away from the interface.This suggests that 903 nm peak can be attributed to Si diffusion from the substrate into the CELOG layer [19,20].Si incorporation into the CELOG region can also occur during growth, due to Si gas phase transport from the unmasked areas of the Si substrate [21].As one approaches the top of the CELOG layer (S1 and S4 in Fig. 3(a)), a CL band peaking at 967 nm becomes dominant.As shown in the monoCL image in Fig. 3(b), the emission at 967 nm is relatively non uniform; it is mainly observed in the top part of the CELOG layer and even spatially anticorrelated with the distribution of the 892 nm band.The emission at 967 nm could be due to a donor level created by an oxygen atom replacing a P atom [22,23] during the metallization process of the PIN diode fabrication (which is not presented here).In fact, the samples were annealed at 380°C for 5 min to make the ohmic contacts after metallisation.Phosphorous out-diffusion and oxygen in-diffusion from the native InP oxide layer can happen during the annealing step.Such oxygen complexes are expected to be at higher concentrations close to the top of the CELOG InP/Si layer where the contacts were formed, and indeed it is so as observed in Fig. 3(b) (967 nm map).Finally, a broad band emission at about 1120 nm is also observed, as shown in the low energy tail of the CL spectra of Fig. 3(a).This band is often referred as C-band in InP and is attributed to a complex or pair defect involving species such as a donor like phosphorus vacancy (V P ) and an acceptor like indium vacancy (V In ) [24,25].The intensity of the C-band emission is affected by the crystalline quality of InP, as observed in the mono-CL image.Its peak intensity in Fig. 3(a) in general is lower than the intensity of the other two peaks previously mentioned, but it is more conspicuous at the CELOG InP/Si interface region with respect to the other regions of the CELOG layer.The monochromatic CL images clearly establish the distribution of this band, which appears mainly localized at the CELOG region as Fig. 3(b) reveals.
To characterize the carrier dynamics and lifetime in the CELOG InP layers, TRPL measurements were carried out on the cross-section of CELOG InP/Si samples.Nonradiative recombination and trapping of carriers depend on the concentration of surface and interface defects, as well as point and extended defects in the bulk [26] 4(a).On the other hand, the TRPL spectra measured in the middle of the grown InP layer show single exponential PL decay.Such position dependent PL decay is also observed in seed-scan shown in Fig. 4(c).The single exponential PL decay process is dominated by bulk recombination with carrier lifetime τ, whilst PL transients yielding double exponential indicate two recombination mechanisms with a fast (τ 1 ) and a slow (τ 2 ) decay processes.The carrier lifetimes, τ 1 and τ 2 , for double exponential decay and τ for single exponential decay extracted from the TRPL decay curves in CELOG-scan and the seed-scan are plotted as a function of the distance from the top of the growth surface in Figs.4(b) and 4(d), respectively.The depth dependent carrier lifetimes in Fig. 4(b) and 4(d) can be divided in to three regions in both CELOG and seedscans.In region I, within 7 µm below the top surface, the TRPL decay curves of both seedscan and CELOG-scan show double exponential decays and both lifetimes, τ 1 and τ 2 increase with the distance from the top surface, which indicates that a similar decay mechanism dominates in of the grown lifetime, τ, inc of 700 ps at scan, the carr after a maxim double expon of region III i  3, the progressive increase of the 967 nm band emission at 80 K towards the top surface suggests that the higher contribution of this recombination channel weakens the near band edge (NBE) emission, reducing the minority carrier lifetime in this region.The surface recombination at the (001) surface might also be affecting the PL decay in this region as deduced in the 2-D simulation of carrier dynamics in (2PE) TRPL experiments in semiconductors: it leads to an increase of the single exponential carrier lifetime, τ, when increasing the distance from the surface, reaching the maximum value once the surface recombination on (001) plane has negligible impact on the PL decay [27].Such saturated carrier lifetime is only observed in region II in the CELOG-scan, while in the seed-scan it shows premature declination after reaching a maximum value of 640 ps.In region II of seed-scan, the higher lifetimes (~640 ps) observed away from the interface (around 10 μm) indicates better quality InP, which could be due to the annihilation process of dislocations propagating from the seed layer [29].The InP growth starts on the defective InPseed; as the growth proceeds, the threading dislocations propagating from the seed can get eliminated by creating dislocation loops for increasing InP layer thickness.In region II of CELOG-scan, the lifetime increases as the excitation laser spot approaches the InP/Si interface.The longest lifetimes (~700 ps) were observed from 5 μm to 2 μm close to the InP/Si interface.It indicates that the material near the interface has a low concentration of dislocations, which is consistent with the high CL intensity observed in that region of the CELOG layer (see Fig. 2).In this region, the original dislocations in the InP-seed layer were filtered away since the dislocations propagate only towards the top surface and cannot bend downwards to the surface of Si.Lifetimes near the CELOG interface are longer than the highest value in the seed-scan (~640 ps), which suggests that CELOG is more efficient in reducing dislocation density than the mere increasing of the layer thickness in InP/Si heteroepitaxy.In the seed-scan, the reduced carrier lifetimes in region III are due to the high density of dislocations (~10 9 cm −2 ) in the seed, which could cause nonradiative recombination.In CELOG-scan, as the excitation laser spot reaches further close to the interface (~2 µm from interface) or at the interface in region III, the lifetime drops, and the decay follows a double exponential.Misfit dislocations could be confined to the CELOG interface.Therefore, the faster decay near CELOG InP/Si interface might be due to nonradiative recombination at the InP/Si interface where dangling bonds associated with misfit dislocations are present.The CELOG InP/Si interface partially neutralizes the dangling bonds, thereby reducing the surface recombination velocity (SRV) as compared to the top surface, which will be shown in the TEM inspection of the InP/Si interface revealing a coherent InP layer epitaxially fused to Si.Therefore, the depth dependent carrier lifetime distribution can only be seen in a narrow region close to the InP/Si interface in CELOG-scan.This difference in the SRV of the InP/Si interface and the InP top surface is the reason for the lifetime differences observed in the top and the interface regions in the CELOG-scan.
In Fig. 5, the carrier lifetimes of CELOG InP/Si layers are compared with that of direct heteroepitaxial InP/Si by MOVPE (InP-seed/Si).The carrier lifetime on the (001) surface of CELOG InP/Si (denoted as CELOG top in the figure) extracted from a single exponential decay is 745 ps.The decay curve measured close to this site but on the (110) cross-section (denoted as CELOG CS_2 µm) is double exponential and the lifetime of the lower decay rate is 220 ps.Because of the proximity of the measurement sites the lifetimes are expected to be the same in contrast to what is being observed.This suggests that the active contribution of carrier diffusion to the (001) surface and surface recombination could reduce the carrier lifetimes mea spectrum mea shorter carrie carrier lifetim InP/Si interfa 710-745 ps a what we obtai doping conce dislocation de recombination Double hetero lifetime meas interface state higher carrier realized at InP The crysta TEM.In earl stacking fault neither stacki this sample (F the TEM sam (~10 9 cm −2 ) d stacking fault initiates by nu growing InP growing InP c formation of bonding proce sured at the (1 asured on the er lifetime of mes in heteroe ace in (110) cr ccounting for ined for homoe entration) but ensity of 10 7 /cm n can affect th ostructures hav surement [30] es could affect r lifetime value P/Si interface t   [32].The CELOG process appears as a bonding process taking place during the growth, as if the growing InP layer is fused to the Si surface at high temperatures.The results of Matsumoto et al. indicate that subsequent epitaxial growth on bonded wafer can cause dark line defects due to thermal mismatch [33].In our case the heterointerface is already formed at high temperature and hence low risk for threading dislocation during subsequent growth of e.g.laser devices.Off-cut Si substrate was used to avoid APDs (antiphase domains) in the InPseed grown with MOVPE.No APDs were observed in the CELOG region by TEM and CL.

Conclusions
We studied monolithically integrated InP/Si heterojunction with high crystalline quality InP and coherent InP/Si interface realized by self-aligned corrugated epitaxial lateral overgrowth (CELOG) method in HVPE.CL spectra acquired at 80 K in different regions of the CELOG cross-section present following transitions: 1) near-BE band at 892 nm (1.39 eV); 2) a band at 903 nm (1.37 eV) related to Si diffussion from the Si substrate; 3) a band peaking at 967 nm (1.28 eV) associated with an oxygen donor level due to replacement of P site; 4) a broad band around 1120 nm (1.11 eV) attributed to a complex defect pair, due to donor like V P and acceptor like V In .The intensity distribution of these emissions has been studied by hyperspectral CL images.The monochromatic CL images of 892nm and 1120 nm emissions taken on the cross-sections showed strong contrast between CELOG regions (bright) and defective seed regions (dark), suggesting high crystalline quality InP/Si heterojunction in the CELOG region.The depth dependence of InP carrier lifetime in CELOG InP/Si cross-section was measured at room temperature by time resolved photoluminescence.Depending on the positions, single and double exponential decays were observed, and the related recombination mechanisms have been discussed.An enhancement of carrier lifetime was observed near the CELOG InP/Si interface with respect to seed layer accounting for its better crystalline quality.We notice that the value of carrier lifetime measured by TRPL at the cross-section of CELOG InP/Si is affected by photo-carrier diffusion and associated nonradiative recombination through surface states on (001) InP plane and interface states in the misfit dislocation network at InP/Si direct heterojunction.Appropriate carrier confinement structure would be essential for enhancing carrier lifetime in CELOG InP/Si heterojunction.TEM characterization also revealed high crystalline quality of InP at the CELOG InP/Si interface without threading dislocations in the InP layer, but misfit dislocations presumably exist at the interface as shown in cross-sectional HRTEM in a manner observed in wafer bonding heterointerface.The selected area electron diffraction pattern acquired at the filmsubstrate interfacial region has revealed a coherent CELOG InP/Si interface free of amorphous layer.This work has demonstrated that CELOG is an efficient method for dislocation reduction and carrier lifetime enhancement in heteroepitaxial InP growth on Si.Location-dependent multiple bands and dislocation densities in this approach cause challenges to the devices grown on CELOG.Chemical mechanical polishing (CMP) will be used to remove the top layer of InP to expose the high quality CELOG region.Devices will be fabricated on the CELOG region by isolating the seed regions from electrically active regions.Ideally the seed site and its pattern density should be small to achieve large area CELOG region for planar photonic devices.But these have not been investigated in detail in this work.As shown in this work, the regrown materials in CELOG region have high crystalline quality but they will be isolated islands after CMP to expose the CELOG layer, which are separated by defective InP seeds.Large area and uniform low dislocation InP layer on Si are desired for in-plane photonic integrated circuit (PIC) applications.By conducting InP planarization growth after removing the original InP seeds between CELOG InP islands, a uniform high crystalline quality InP/Si layer can be expected.The CELOG approach is generic and can be extended to the formation of other III-V/Si heterostructures.Thus III-V material with long carrier lifetime on silicon fabricated by CELOG method will facilitate the realization of cost effective integrated photonic devices and III-V multi-junction solar cells on silicon with bandgap combinations desirable for high efficiency.
Fig. 1 InP-se distan the CE The surfac (AFM).Here cross-section (CL) measure Zeiss) field e with either a InGaAs array operated at 10 mode the full the constructi Recombin resolved pho performed us wavelength, 7The time reso manual transl spot of 2.0 μm power was 0. averaging ove 10 18 cm −3 .Th a high-resolu prepared by m keV was used Fig. 2 InP /S image

Figure 2
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