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.     

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].     

Real-time monitoring of the surface stoichiometry during molecular beam epitaxy of cubic GaN on (001) GaAs by RHEED

We study the plasma-assisted molecular beam epitaxy of cubic GaN on GaAs(OOl) substrates by means ofin-situ reflection high-energy electron diffraction. The epilayers are characterized by x-ray diffraction, photoluminescence, and Hall measurements, and it is found that the overall best films are grown under a N/Ga ratio close to one. For anin-situ determination of the N/Ga ratio, the growth kinetics is studied via surface reconstruction transitions. The effective N flux giving rise to growth is measured using the transient behavior of the half-order diffraction streak intensity for various plasma operating conditions.     

In situ control of gan growth by molecular beam epitaxy

Methods to determine GaN surface temperature, surface composition, and growth rates using in situ desorption mass spectroseopy (DMS) and reflection high energy electron diffraction (RHEED) are demonstrated for molecular beam epitaxial growth of GaN using NH₃. Using these methods, the GaN surface temperature, T_s, and GaN growth rates as a function of T_s, Ga flux, and NH₃ flux were obtained. Surface temperatures were determined from DMS and RHEED measurements of the temperature at which Ga condenses on GaN. NH₃-limited and Ga-limited growth regimes are identified and the transition between these regimes is shown to be abrupt. NH₃-limited samples have a weakly reconstructed (2 × 2) RHEED pattern, while Ga-limited samples reveal a transmission pattern. Atomic force microscopy showed that NH₃-limited samples exhibit atomic steps while Ga-limited samples exhibit faceting.     

Structural properties of GaAsN grown on (001) GaAs by metalorganic molecular beam epitaxy

Detailed transmission electron microscopy (TEM) and transmission electron diffraction (TED) examination has been made of metalorganic molecular beam epitaxial GaAsN layers grown on (001) GaAs substrates. TEM results show that lateral composition modulation occurs in the GaAs_1−xN_x layer (x 6.75%). It is shown that increasing N composition and Se (dopant) concentration leads to poor crystallinity. It is also shown that the addition of Se increases N composition. Atomic force microscopy (AFM) results show that the surfaces of the samples experience a morphological change from faceting to islanding, as the N composition and Se concentration increase. Based on the TEM and AFM results, a simple model is given to explain the formation of the lateral composition modulation.     

Transmission electron microscopy studies of defects in HgCdTe device structures grown by molecular beam epitaxy

Transmission electron microscopy (TEM) was used to evaluate the microstructure of molecular beam epitaxy (MBE) grown (211)B oriented HgCdTe films. TEM analysis of in-situ doped p-on-n and n-p-n device structures will be presented. Under fully optimized growth conditions the substrate-epilayer interface is free of threading dislocations and twins, and a high degree of structural integrity is retained throughout the entire device structure. However, under non-optimal growth conditions that employ high Hg/Te flux ratios, twins can be generated in the p-type layer of p-on-n device structure, resulting in roughness and facetting of the film surface. We propose a mechanism for twin formation that is associated with surface facetting. TEM evaluation of voids, threading dislocations and Te-precipitates in HgCdTe films are also discussed.     

Nucleation mechanism for epitaxial growth of aluminum films on sapphire substrates by molecular beam epitaxy

Aluminum (Al) epitaxial films with various thicknesses are grown on sapphire substrates by molecular beam epitaxy (MBE). The nucleation evolution of surface morphology and structural property during the growth of Al epitaxial films on sapphire substrates are investigated in detail. It is found that the 10 nm-thick Al epitaxial films grown on the sapphire substrates show the full-width at half-maximum (FWHM) for Al(111) of 0.35° and the root-mean square (RMS) surface roughness of 2.4 nm. When the thickness increases, the surface initially starts to roughen and then becomes smoother. At the same time, the crystal quality of the Al epitaxial films becomes better thanks to the annihilation of dislocations. As the thickness of Al epitaxial films reaches 800 nm, the FWHM for Al(111) is 0.04° and the RMS surface roughness is 0.14 nm, indicating the high crystal quality and flat surface morphology of Al epitaxial films. The corresponding nucleation mechanism of Al epitaxial films grown on sapphire substrates is hence proposed. This work is of great significance for the fabrication of Al-based devices.     

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.     

Use of electron cyclotron resonance plasmas to prepare CdZnTe (211)B substrates for HgCdTe molecular beam epitaxy

Without any additional preparation, Cd_1−yZn_yTe (211)B (y∼3.5%) wafers were cleaned by exposure to an electron cyclotron resonance (ECR) Ar/H₂ plasma and used as substrates for HgCdTe molecular beam epitaxy. Auger electron spectra were taken from as-received wafers, conventionally prepared wafers (bromine: methanol etching, followed by heating to 330–340°C), and wafers prepared under a variety of ECR process conditions. Surfaces of as-received wafers contained ∼1.5 monolayers of contaminants (oxygen, carbon, and chlorine). Conventionally prepared wafers had ∼1/4 monolayer of carbon contamination, as well as excess tellurium and/or excess zinc depending on the heating process used. Auger spectra from plasma-treated CdZnTe wafers showed surfaces free from contamination, with the expected stoichiometry. Stoichiometry and surface cleanliness were insensitive to the duration of plasma exposure (2–20 s) and to changes in radio frequency input power (20–100 W). Reflection high energy electron diffraction patterns were streaked indicating microscopically smooth and ordered surfaces. The smoothness of plasma-etched CdZnTe wafers was further confirmed ex situ using interferometric microscopy. Surface roughness values of ∼0.4 nm were measured. Characteristics of HgCdTe epilayers deposited on wafers prepared with plasma and conventional etching were found to be comparable. For these epilayers, etch pit densities on the order of 10⁵ cm⁻² have been achieved. ECR Ar/H₂ plasma cleaning is now utilized at Night Vision and Electronic Sensors Directorate as the baseline CdZnTe surface preparation technique.     

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.     

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.     

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.     

Two-dimensional molecular beam epitaxy of {001} CdTe on Cd and Zn terminated {001} GaAs

Amorphous layers of CdTe deposited on Cd or Zn terminated GaAs {001} surfaces can be recrystallized above ∼200°C. Subsequent molecular beam epitaxy of CdTe proceeds in a two-dimensional mode and leads to layers which are specular and single domain {0011}. Threading dislocation density in these layers was 1–2 x 10⁵ cm⁻². Values of full width at half maximum for x-ray rocking curves were as low as 80 arc-s.     

Microstructural and optical characterization of CdTe(211)B/ZnTe/Si(211) grown by molecular beam epitaxy

CdTe layers have been grown by molecular beam epitaxy on 3 inch nominal Si(211) under various conditions to study the effect of growth parameters on the structural quality. The microstructure of several samples was investigated by high resolution transmission electron microscopy (HRTEM). The orientation of the CdTe layers was affected strongly by the ZnTe buffer deposition temperature. Both single domain CdTe(133)B and CdTe(211)B were obtained by selective growth of ZnTe buffer layers at different temperatures. We demonstrated that thin ZnTe buffer layers (<2 nm) are sufficient to maintain the (211) orientation. CdTe deposited at ∼300°C grows with its normal lattice parameter from the onset of growth, demonstrating the effective strain accommodation of the buffer layer. The low tilt angle (<1°) between CdTe[211] and Si[211] indicates that high miscut Si(211) substrates are unnecessary. From low temperature photoluminescence, it is shown that Cd-substituted Li is the main residual impurity in the CdTe layer. In addition, deep emission bands are attributed to the presence of As_Te and Ag_Cd acceptors. There is no evidence that copper plays a role in the impurity contamination of the samples.     

Structural study of GaN grown on (001) GaAs by organometallic vapor phase epitaxy

Detailed transmission electron microscope (TEM) and transmission electron diffraction (TED) examination has been performed on organometallic vapor phase epitaxial GaN layers grown on (001) GaAs substrate to investigate microstructures and phase stability. TED and TEM results exhibit the occurrence of a mixed phase of GaN. The wurtzite (α) phase grains are embedded in the zinc-blende (β) phase matrix. It is shown that there are two types of the wurtzite GaN phase, namely, the epitaxial wurtzite and the tilted wurtzite. The tilted wurtzite grains are rotated some degrees ranging from ∼5° to ∼35° regarding the GaAs substrate. A simple model is presented to describe the occurrence of the mixed phases and the two types of the wurtzite phase.     

Arsenic deposition as a precursor layer on silicon (211) and (311) surfaces

We investigate the properties of arsenic (As) covered Si(211) and Si(311) surfaces by analyzing data from x-ray photoelectron spectroscopy (XPS) and low-energy electron diffraction (LEED) images. We then create a model using total surface energy calculations. It was found that both Si(211) and Si(311) had 0.68±0.08 surface As coverage. Si(211) had 0.28±0.04 Te coverage and Si(311) had 0.24±0.04 Te coverage. The Si(211) surface replaces the terrace and trench Si atoms with As for a lower surface energy, while the Si edge atoms form dimers. The Si(311) surface replaces all terrace atoms and adsorbs an As dimer every other edge site. These configurations imply an improvement in the mean migration path from the bare silicon surface by allowing the impinging atoms for the next epitaxial layer, tellurium (Te), to bind at every other pair of edge atoms, and not the step terrace sites. This would ensure a nonpolar, B-face growth.     

Photoluminescence and raman scattering characterization of silicon-doped In0.52Al0.48As grown on InP (100) substrates by molecular beam epitaxy

Growth of In_0.52Al_0.48As epilayers on InP (100) substrates by molecular beam epitaxy at different silicon doping levels is carried out. The doped samples show an inverted S-shaped dependence of the PL peak energy variation with the temperature which weakens at high doping levels due to a possible reduction in the donor binding energy. There is a reduction in both the AlAs-like and InAs-like longitudinal-optic (LO) phonon frequencies and a broadening of the LO phonon line shape as the doping level is increased. The PL intensity also showed in increasing degrees at higher doping levels, a temperature dependence which is characteristic of disordered and amorphous materials.     

Incorporation of Sn on GaAs (111)A substrates by molecular beam epitaty

Behavior of Sn as donor species in the MBE growth of GaAs on (111)A substrates has been investigated by varying the growth temperature from 460 to 620°C, As₄:Ga flux ratio from 4 to 25, and Sn concentration from 10¹⁶ to 10²⁰ atoms cm^-3. Secondary ion mass microscopy measurements show that Sn does not surface segregate on (111)A substrates under this growth condition, in contrast to that on (001) substrates. Sn is uniformly incorporated throughout the bulk of the grown layer for all samples, apart from the most highly doped ones. To increase the Sn carrier concentration on the (111)A substrates, the measured carrier concentration shows that doping should be carried out at a low growth temperature and/or high As₄:Ga flux ratio.     

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.     

Galvanomagnetic properties of two-dimensional electron gases in strain relaxed InxGa1−xAs(x<0.40) heterostructures grown by molecular beam epitaxy on GaAs substrates

We have investigated, as a function of indium content x, the galvanomagnetic and Shubnikov de Haas (SdH) properties of two-dimensional electron gases (2DEG) formed at lattice matched, strain relaxed InAlAs/InGaAs heterojunctions. These were grown by molecular beam epitaxy on GaAs misoriented substrates with a two degree offcut toward the nearest (110) plane. Variable temperature resistivity and Hall measurements indicate an increase in the electron sheet density n_s from 0.78×10¹²cm⁻² for x=0.15 to 1.80×10¹² cm⁻² for x=0.40 at 300K, and from 0.75×10¹²cm⁻² to 1.67×10¹²cm⁻² at T=1.6K. The room temperature electron mobility, measured along the in plane [110], direction is independent of indium content and equals approximately 9500 cm²/Vs. For T<50K, the mobility is independent of temperature decreasing with increasing x from 82000 cm²/Vs for x=0.15 to 33000 cm²/Vs for x=0.40. The ratios (τ_t/τ_q) at 1.6K between the electron relaxation time τ_t and the single particle relaxation time τq, for the strain relaxed specimens, as well as for pseudomorphically strained Al_0.35Ga_0.65As/In_0.15Ga_0.85As structures grown on GaAs substrates, and In_0.52Al_0.48As/In_0.53Ga_0.47As heterostructures grown lattice matched on InP substrates. Such a study indicates the presence of inhomogeneities in the 2DEGs of the strain relaxed specimens which appear to be related to the process of strain relaxation. Such inhomogeneities, however, have little effect on the electron relaxation time τ_t which, at low temperatures, is limited principally by alloy scattering.