Low-temperature formation of self-assembled Ge quantum dots on Si(100) under high carbon mediation via solid-phase epitaxy

CMOS-compatible low-temperature formation of self-assembled Ge quantum dots (QDs) by carbon (C) mediation via a solid-phase epitaxy (SPE) has been demonstrated. The samples were prepared by a solid-source molecular beam epitaxy (MBE) system. C and Ge were successively deposited on Si(100) at 200 °C and Ge/C/Si heterostructure was annealed in the MBE chamber. Sparse Volmer-Weber mode Ge dots without a wetting layer were formed for C coverage (θ_C) of 0.25 and 0.5 ML by lowering SPE temperature (T_S) to 450 °C, but small and dense Stranski-Krastanov (SK)-mode Ge QDs with the wetting layer were obtained with increasing C coverage of 0.75 ML even at 450 °C. From the investigation of SPE temperature effect on Ge QD formation for θ_C of 0.75 ML, SK-mode Ge QDs of about 10 nm in diameter and of about 4.5×10¹¹ cm⁻² in density were formed at T_S≥400 °C. The wetting layer of SK-mode QDs was almost constant 0.2-nm thick at T_S≥450 °C. Measurements of chemical binding states of C in Ge QDs and at Ge/Si interface revealed that a large amount of C–Ge bonds were formed in the wetting layer for high C coverage, and the formation of C–Ge bonds, together with the formation of C–Si bonds, enabled the low-temperature formation of small and dense Ge QDs. These results suggest that the C-mediated solid-phase epitaxy is effective to form small and dense SK-mode QDs at low temperature.     

Effects of carbon coverage on Ge quantum dots formation on Si(100) using C-Si reaction and transition of Ge growth mode

Effects of carbon (C) coverage on C-mediated Ge quantum dots (QDs) formation on a Si(100) substrate changing a state of surface reconstruction were investigated by using solid-source molecular beam epitaxy. For C=0–2.0 monolayers (MLs), the Ge QD scaled down and its density increased with C coverage. In addition, growth mode of Ge QDs changed from Volmer-Weber (VW) mode without a wetting layer to Stranski-Krastanov (SK) mode with the wetting layer for C=0.50–0.75 ML. This transition was induced by decrease in interfacial energy between Ge and Si surface due to the formation of C-Ge bonds near the Ge/Si interface. For C≥2.5 MLs, the Ge QD enlarged slightly and its density decreased with increasing C coverage, and he Ge growth mode went back to the VW mode. The Raman spectroscopy and X-ray photoelectron spectroscopy revealed the formation of a mixture of amorphous C and nano-crystalline graphite on the Si surface. Thus, the formation of a large amount of C–C (sp²) bonds induced the growth transition of QDs from the SK mode to the VW mode due to the decrease in surface energy of C.     

InGaAs/GaAs量子点红外探测器

与量子阱红外探测器相比,量子点红外探测器具有不制作表面光栅就能在垂直入射红外光照射下工作以及工作温度更高等优势。然而,目前阻碍量子点红外探测器性能提高的技术瓶颈主要来自组装量子点较差的大小均匀性、较低的量子点密度以及垂直入射下子带跃迁吸收效率低等原因。利用分子束外延技术研究了如何从量子点材料生长和器件设计两方面来克服这些困难,并且制作了几种不同结构的InGaAs/GaAs量子点红外探测器。 在77 K时,这些器件在垂直入射条件下观察到了很强的光电流信号。     

The effects of quantum dot coverage in InAs/(In)GaAs nanostructures for long wavelength emission

We present a study on the effects of quantum dot coverage on the properties of InAs dots embedded in GaAs and in metamorphic In_0.15Ga_0.85As confining layers grown by molecular beam epitaxy on GaAs substrates. We show that redshifted emission wavelengths exceeding 1.3 μm at room temperature were obtained by the combined use of InGaAs confining layers and high quantum dot coverage. The use of high InAs coverage, however, leads to detrimental effects on the optical and electrical properties of the structures. We relate such behaviour to the formation of extended structural defects originating from relaxed large-sized quantum dots that nucleate in accordance to thermodynamic equilibrium theories predicting the quantum dot ripening. The effect of the reduced lattice-mismatch of InGaAs metamorphic layers on quantum dot ripening is discussed in comparison with the InAs/GaAs system.     

HgCdTe molecular beam epitaxy technology: A focus on material properties

HgCdTe MBE technology is becoming a mature growth technology for flexible manufacturing of short-wave, medium-wave, long-wave, and very long-wave infrared focal plane arrays. The main reason that this technology is getting more mature for device applications is the progress made in controlling the dopants (both n-type and p-type in-situ) and the success in lowering the defect density to less than 2 x 10⁵/cm² in the base layer. In this paper, we will discuss the unique approach that we have developed for growing As-doped HgCdTe alloys with cadmium arsenide compound. Material properties including composition, crystallinity, dopant activation, minority carrier lifetime, and morphology are also discussed. In addition, we have fabricated several infrared focal plane arrays using device quality double layers and the device results are approaching that of the state-of-the-art liquid phase epitaxy technology.     

Advances in self-assembled semiconductor quantum dot lasers

In the past 20 years the semiconductor laser has become a key device in optical electronics because of its pure output spectrum and high quantum efficiency. As the capabilities of laser diodes have grown, so has the range of applications contemplated for them. A great success in semiconductor lasers has been brought by the ability to artificially structure new materials on an atomic scale by using advanced crystal growth methods such as MBE and MOVPE. The laser performance successes gained using quantum wells in optoelectronic devices can be extended by adopting quantum wire and quantum dot structures. There have been several reports of successful lasing action in semiconductor dot structures within the past few years. In this article I will briefly review the recent progress in the development of quantum dot lasers.     

Molecular beam epitaxy—Grown ZnSe nanowires

Molecular beam epitaxy (MBE) via the vapor-liquid-solid (VLS) reaction was used to grow ZnSe nanowires (NWs) on (111), (100), and (110) oriented GaAs substrates. Through detailed transmission electron microscopy (TEM) studies, it was found that 〈111〉 orientation is the growth direction for NWs with size ≥30 nm, while NWs with size around 10 nm prefer to grow along the 〈110〉 direction, with a small portion along the 〈112〉 direction. These observations have led to the realization of vertical ZnSe NWs with size around 10 nm grown on a GaAs (110) substrate. An ordered ZnSe NW array fabricated on a GaAs (111) substrate with a novel prepatterning method associated with plasma etching shows a high degree of ordering and a good size uniformity of the as-grown NWs. The diameter of the NWs in the array is around 80 nm and most of them are found to orient vertically, but some tilt to one of the six possible directions of the 〈111〉 family.     

Molecular beam epitaxial growth of site-controlled InAs quantum dots on pre-patterned GaAs substrates

Ex situ electron-beam lithography followed by conventional wet etching has been used to pattern small holes 60–150 nm wide, 13 nm deep in GaAs substrates. These holes act as preferential nucleation sites for InAs dot growth during subsequent overgrowth. By varying either the InAs deposition amount or the thickness of a GaAs buffer layer, the occupancy over the patterned sites can be controlled. Comparison with growth on a planar substrate shows that preferential nucleation occurs due to a reduction in the apparent critical thickness above the pattern site; the magnitude of this reduction is dependent on the dimensions of the initial pattern.     

Nano-engineering approaches to self-assembled InAs quantum dot laser medium

A number of nano-engineering methods are proposed and tested to improve optical properties of a laser gain medium using the self-assembled InAs quantum dot (QD) ensemble. The laser characteristics of concern include higher gain, larger modulation bandwidth, higher efficiency at elevated temperatures, higher thermal stability, and enhanced reliability. The focus of this paper is on the management of QD properties through design and molecular beam epitaxial growth and modification of QD heterostructures. This includes digital alloys as high-quality wide-bandgap barrier; under- and overlayers with various compositions to control the dynamics of QD formation and evolution on the surface; shape engineering of QDs to improve electron-hole overlap and reduce inhomogeneous broadening; band engineering of QD heterostructures to enhance the carrier localization by reduction of thermal escape from dots; as well as tunnel injection from quantum wells (QWs) to accelerate carrier transfer to the lasing state. Beneficial properties of the developed QD media are demonstrated at room temperature in laser diodes with unsurpassed thermal stability with a characteristic temperature of 380 K, high waveguide modal gain >50 cm⁻¹, unsurpassed defect tolerance over two orders of magnitude higher than that of QWs typically used in lasers, and efficient emission from a two-dimensional (2-D) photonic crystal nanocavity.     

Molecular beam epitaxy HgCdTe growth-induced void defects and their effect on infrared photodiodes

We have carried out a study and identified that MBE HgCdTe growth-induced void defects are detrimental to long wavelength infrared photodiode performance. These defects were induced during nucleation by having surface growth conditions deficient in Hg. Precise control and reproducibility of the CdZnTe surface temperature and beam fluxes are required to minimize such defects. Device quality material with void defect concentration values in the low 10² cm² range were demonstrated.     

Heteroepitaxy of HgCdTe (211)B on Ge substrates by molecular beam epitaxy for infrared detectors

Epitaxial growth of (211)B CdTe/HgCdTe has been achieved on two inch germanium (Ge) by molecular beam epitaxy (MBE). Germanium was chosen as an alternative substrate to circumvent the weaknesses of CdZnTe wafers. The ease of surface preparation makes Ge an attractive candidate among many other alternative substrates. Best MBE CdTe growth results were obtained on (211) Ge surfaces which were exposed to arsenic and zinc fluxes prior to the MBE growth. This surface preparation enabled CdTe growth with B-face crystallographic polarity necessary for the HgCdTe growth. This process was reproducible, and produced a smooth and mirror-like surface morphology. The best value of the {422} x-ray double diffraction full width at half maximum measured from the HgCdTe layer was 68 arc-s. We present the 486 point maps of FWHM statistical values obtained from CdTe/Ge and HgCdTe/CdTe/Ge. High resolution microscopy electron transmission and secondary ion mass spectroscopy characterization results are also presented in this paper. High-performance middle wavelength infrared HgCdTe 32-element photodiode linear arrays, using the standard LETI/LIR planar n-on-p ion implanted technology, were fabricated on CdTe/Ge substrates. At 78K, photodiodes exhibited very high R₀A figure of merit higher than 10⁶ Ωcm⁻² for a cutoff wavelength of 4.8 μm. Excess low frequency noise was not observed below 150K.     

Suppression of twin formation in CdTe(111)B epilayers grown by molecular beam epitaxy on misoriented Si(001)

CdTe(lll)B layers have been grown on misoriented Si(001). Twin formation inside CdTe(lll)B layer is very sensitive to the substrate tilt direction. When Si(001) is tilted toward [110] or [100], a fully twinned layer is obtained. When Si(001) is tilted toward a direction significantly away from [110], a twin-free layer is obtained. Microtwins inside the CdTe(111)B layers are overwhelmingly dominated by the lamellar twins. CdTe(111)B layers always start with heavily lamellar twinning. For twin-free layers, the lamellar twins are gradually suppressed and give way to twin-free CdTe(111)B layer. The major driving forces for suppressing the lamellar twinning are the preferential orientation of CdTe[11-2] along Si[1-10] and lattice relaxation. Such preferential orientation is found to exist for the CdTe(111)B layers grown on Si(001) tilted toward a direction between [110] and [100].     

Anti-phase domain-free growth of GaAs on offcut (001) Ge wafers by molecular beam epitaxy with suppressed Ge outdiffusion

The nucleation and growth of GaAs films on offcut (001) Ge wafers by solid source molecular beam epitaxy (MBE) is investigated, with the objective of establishing nucleation conditions which reproducibly yield GaAs films which are free of antiphase domains (APDs) and which have suppressed Ge outdiffusion into the GaAs layer. The nucleation process is monitored by in-situ reflection high energy electron diffraction and Auger electron spectroscopy. Several nucleation variables are studied, including the state of the initial Ge surface (single-domain 2×1 or mixed-domain 2×1:1×2), the initial prelayer (As, Ga, or mixed), and the initial GaAs growth temperature (350 or 500°C). Conditions are identified which simultaneously produce APD-free GaAs layers several microns in thickness on Ge wafers with undetectable Ge outdiffusion and with surface roughness equivalent to that of GaAs/GaAs homoepitaxy. APD-free material is obtained using either As or Ga nucleation layers, with the GaAs domain dependent upon the initial exposure chemical species. Key growth steps for APD-free GaAs/Ge growth by solid source MBE include an epitaxial Ge buffer deposited in the MBE chamber to bury carbon contamination from the underlying Ge wafer, an anneal of the Ge buffer at 640°C to generate a predominantly double atomic-height stepped surface, and nucleation of GaAs growth by a ten monolayer migration enhanced epitaxy step initiated with either pure As or Ga. We identify this last step as being responsible for blocking Ge outdiffusion to below 10¹⁵ cm⁻³ within 0.5 microns of the GaAs/Ge interface.     

InSb quantum dot LEDs grown by molecular beam epitaxy for mid-infrared applications

We report the molecular beam epitaxial growth of InSb quantum dots (QD) inserted as sub-monolayers in an InAs matrix which exhibit intense mid-infrared photoluminescence up to room temperature. The InSb QD sheets were formed by briefly exposing the surface to an antimony flux (Sb₂) exploiting an As-Sb anion exchange reaction. Light emitting diodes were fabricated using 10 InSb QD sheets and were found to exhibit bright electroluminescence with a single peak at 3.8 μm at room temperature.     

Improvements of stacked self-assembled InAs/GaAs quantum dot structures for 1.3 μm applications

We propose the growth of thick ‘spacer’ layers (d) for high-quality 10-stack InAs/GaAs quantum dots (QDs) emitting at 1.23 μm without the use of strain reduction layers (SRLs). All samples were grown using molecular beam epitaxy (MBE) and extensively characterised using X-ray diffraction, optical spectroscopy and microscopy techniques. We demonstrate that for d<50 nm, large ‘volcano-like’ defects are formed at the top of the stacked structure, while for d=50 nm, these features were not observed. The process of suppressing these abnormal defects has resulted in significant photoluminescence (PL) enhancement, paving the way for the realisation of defect-free QD laser devices.     

Precise control of HgCdTe growth conditions for molecular beam epitaxy

A Hg_1-xCd_xTe growth method is presented for molecular beam epitaxy, which precisely controls growth conditions to routinely obtain device quality epilayers at a certain specific composition. This method corrects the fluctuation in composition x for run-to-run growth by feedback from the x value for the former growth to the fluxes from CdTe and Te cells. We achieved standard deviation Δx/ x of 3.3% for 13 samples grown consecutively. A substrate temperature drop was found during growth, which considerably degrades the crystal quality of epilayers. In this method, this drop is greatly diminished by covering the holder surface with a heavily doped Si wafer. Finally, etch pit density of 4 x 10⁴ cm-² and full width at half-maximum of 12 arc-s for the x-ray double-crystal rocking curve were obtained as the best values.     

Lateral uniformity in HgCdTe layers grown by molecular beam epitaxy

The fabrication of high-quality focal plane arrays from HgCdTe layers grown by molecular beam epitaxy (MBE) requires a high degree of lateral uniformity in material properties such as the alloy composition, doping concentration, and defect density. While it is well known that MBE source flux nonuniformity can lead to radial compositional variation for rotating substrates, we have also found that composition can be affected significantly by lateral variations in substrate temperature during growth. In diagnostic experiments, we systematically varied the substrate temperature during MBE and quantified the dependence of HgCdTe alloy composition on substrate temperature. Based on these results, we developed a methodology to quickly and nondestructively characterize MBE-grown layers using postgrowth spatial mapping of the cutoff wavelength from the Fourier transform infrared (FTIR) transmission at 300 K, and we were able to obtain a quantitative relationship between the measured spatial variations in cutoff and the substrate temperature lateral distribution during growth. We refined this methodology by more directly inferring the substrate temperature distribution from secondary ion mass spectroscopy (SIMS) measurements of the As concentration across a wafer, using the fact that the As incorporation rate in MBE-grown p-type layers is highly sensitive to substrate temperature. Combining this multiple-point SIMS analysis with FTIR spatial mapping, we demonstrate how the relative contributions from flux nonuniformity and temperature variations on the lateral composition uniformity can be separated. This capability to accurately map the lateral variations in the substrate temperature has been valuable in optimizing the mounting and bonding of large substrates for MBE growth, and can also be valuable for other aspects of MBE process development.     

Structure of GaN films grown by molecular beam epitaxy on (0001) sapphire

GaN films grown by electron-cyclotron resonance plasma-assisted molecular beam epitaxy were studied by transmission electron microscopy and x-ray diffraction (XRD). Two sets of films were compared that were grown under identical conditions except for the ratio of the Ga to N flux. Films with a 30% higher Ga to N ratio (A films) were found to contain inversion domains (IDs). No IDs were found in films grown with a lower Ga to N ratio (B films), but instead the zinc-blende GaN was found near the film substrate interface. A narrower XRD rocking curve width along the (0002) direction and a broader rocking curve width along the asymmetric (1102) axis were found for A films compared to B films.     

利用全固态分子束外延方法在Ge(100)衬底上异质外延GaAs薄膜及相关特性表征

利用全固态分子束外延(MBE)方法在Ge(100)衬底上异质外延GaAs薄膜,并通过高能电子衍射(RHEED)、高分辨X射线衍射(XRD),原子力显微镜等手段研究了不同生长参数对外延层的影响.RHEED显示在较高的生长温度或较低的生长速率下,低温GaAs成核层呈现层状生长模式.同时降低生长温度和生长速率会使GaAs薄膜的XRD摇摆曲线半高宽(FWHM)减小,并降低外延层表面的粗糙度,这主要是由于衬底和外延薄膜之间的晶格失配度减小的结果.     

Molecular beam epitaxy growth of HgCdTe on Ge for third-generation infrared detectors

The Leti-Lir has studied II–VI compounds for infrared (IR) detection for more than 20 years. The need to reduce the production cost of IR focal plane arrays (FPAs) sparked the development of heteroepitaxy on large-area substrates. Germanium has been chosen as the heterosubstrate for the third generation of IR detectors. First, we report on the progress achieved in HgCdTe growth on 3-in. and 4-in. (211)B CdTe/Ge. Then, we discuss the choice of a new machine for larger size and better homogeneity. Finally, we present the latest results on third-generation IR multicolor and megapixel devices. First-time results regarding a middle wavelength infrared (MWIR) dual-band FPA, with a reduced pitch of 25 μm, and a MWIR 1,280×1,024 FPA will be shown. Both detectors are based on molecular beam epitaxy (MBE)-grown HgCdTe on Ge. The results shown validate the choice of Ge as the substrate for third-generation detectors.