Threading dislocation reduction in (0 0 01) GaN thin films using SiN x interlayers

Epitaxial lateral overgrowth (ELO) has been used to reduce threading dislocation (TD) densities in GaN. This paper reports transmission and scanning electron microscopy studies of TDs in GaN films grown by ELO on GaN nanocolumns produced either during growth by MBE (method 1), or by self‐organised patterning on a uniform layer grown by MOCVD (method 2). In method 1, isolated nanocolumns grew without TDs, but some TDs formed when nanocolumns coalesced. Although some of these TDs extended through the overlayer, extensive lateral migration of others, depending on the dislocation type, led to dislocation annihilation, lowering the overall TD density. In method 2, a similar process of lateral migration of TDs was observed during ELO, resulting in a continuous GaN overlayer with TD densities less than 108 cm–2. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

ABSTRACTWe have studied the possibilities of reducing the threading dislocation (TD) density in GaAs on Si using free side wall growth on patterned Si substrates and/or using a soft ZnSe interlayer in combination with post growth annealing procedures. TD densities were accurately determined using large area plan view TEM and were found to be unaffected by proximity to free side walls and by the ZnSe interlayer. Post growth heat treatments led to a factor of ∼2 reduction in TD density and to bunching of dislocations throughout the thickness of the film.

Characteristics comparison between GaN epilayers grown on patterned and unpatterned sapphire substrate (0 0 0 1)

Evolution of threading dislocations in MOCVD-grown GaN films on (1 1 1) Si substrates

The effect of indium surfactant on the stress in GaN films grown on SiC at 950 °C by metalorganic chemical vapor deposition was investigated using a combination of in situ wafer curvature measurements and ex situ high resolution x-ray diffraction (HRXRD). As the molar flow rate of trimethylindium was varied from 0 to 4.5 μmol/min during growth, the real-time stress measurements showed that the mean compressive stress of the GaN films decreased from −0.60 to −0.30 GPa. The lattice constants of the GaN epilayers determined by HRXRD confirmed the stress relaxation promoted by the presence of indium while the rocking curve measurements showed that the threading dislocation (TD) density of GaN films remains nearly unchanged. Atomic force microscopy measurements showed that the indium improved step-flow growth, but simultaneously it drove V-defect formation on the GaN surface, which plays a critical role in stress relaxation of GaN films. Cross-sectional transmission electron microscopy revealed the minor contribution of plastic dislocation motion to stress relaxation by localized TD bending toward V-defects. A nucleation and growth model for the V-defect formation was developed to explain that V-defects are energetically favorable to form at TDs under indium-rich conditions. This model shows that the energy barrier for V-defect formation is significantly reduced when indium is present, which leads to the relaxation of misfit strain energy by increasing the size and density of V-defects. Initiation of V-defects and the role of TDs in V-defect formation are discussed based on the presented model.

Low threading dislocation Ge on Si by combining deposition and etching

The detrimental effects of structural defects, micro-twins (MTs) and threading dislocations (TDs), on electron mobility have been investigated for InSb quantum wells (QWs) at room temperature (RT). The constants that are necessary to calculate the electron-mobility limits of these defects were determined by a least-squares-based method that has an advantage of clear representation of the analytical results in a two-dimensional space. Based on a mathematical consideration, a general method of converting electron-mobility limits into percentage impacts upon the total electron mobility was developed. Percentage-mobility-impact analyses showed that, when InSb QWs grown on on-axis (001) GaAs substrates have a TD density of 8.7 × 108 – 3.2 × 109/cm2, 21–14 and 18–45% of electron-mobility degradation are attributed to MTs and TDs, respectively, at RT. The use of 2° off-axis (001) GaAs substrates reduces MT densities in InSb QWs, resulting in a suppression of the MT mobility impact to 3–2% and a complementary slight increase of the TD mobility impact to 22–51% in the same TD density range. This considerable TD mobility limit indicates that it should be possible to improve RT electron mobility in InSb QWs grown on 2° off-axis (001) GaAs substrates, by means of reducing TD density further (< 8.7 × 108/cm2). Although the mobility impacts due to phonons in InSb QW grown on-axis and 2° off-axis (001) GaAs substrates are 54–36 and 67–42%, respectively, phonon scattering is not a single dominant scattering factor: MTs and TDs have also substantial negative impacts upon RT electron mobilities in InSb QWs.

Effect of the V/III ratio during buffer layer growth on the yellow and blue luminescence in undoped GaN epilayer has been studied by means of photoluminescence spectroscopy and high resolution X-ray diffraction. It is found that the densities of screw and edge threading dislocations increase with the V/III ratio of the buffer layer, and the intensities of the yellow luminescence (YL) and blue luminescence (BL) emissions also increase dramatically. However, the density ratio of the edge threading dislocation to the screw threading dislocation remains invariant, as well as the intensity ratio of YL emission to BL emission. It can be concluded from these phenomena that the edge threading dislocation and screw threading dislocation can enhance the YL and BL emissions, respectively.

The effect of varying threading dislocation densities on the internal quantum efficiencies (IQEs) of InGaN quantum wells (QWs), with and without intentionally created “V-pits,” is reported here. InGaN QW samples grown on GaN-on-sapphire templates with threading dislocation densities of <1 × 108 and <1 × 109 cm−2 are compared, with and without GaN/InGaN superlattice (SL) layers incorporated to intentionally open up the threading dislocation cores and form large-size “V-pits.” The formation of “V-pits” is confirmed by cross-sectional transmission electron microscopy to initiate from threading dislocations in the SL layers. The densities of the pits are confirmed by plan-view SEM to agree with the substrate threading dislocation densities. The experimental room temperature IQEs of the “V-pit” QW samples are enhanced to 15% ± 1% compared to 6% ± 2% for conventional QW samples. Both conventional and “V-pit” samples show insensitivity to the magnitude of the dislocation densities with respect to IQE performance, while the “V-pit” samples show shifts in the peak emission wavelengths compared to the conventional samples, attributed to strain modulation. This study provides additional understanding of the causes of the observed insensitivity of the IQEs to different threading dislocation densities.

Characterisation of defects in p-GaN by admittance spectroscopy

This work investigates the role of threading dislocation densities (TDD) in the low density regime on the vertical transport in Si0.06Ge0.94 heterostructures integrated on Si(001). The use of unintentionally doped Si0.06Ge0.94 layers enables the study of the impact of grown-in threading dislocations (TD) without interaction with processing-induced defects originating, e.g., from dopant implantation. The studied heterolayers, while equal in composition, the degree of strain relaxation, and the thickness feature three different values for the TDD as 3 × 106, 9 × 106, and 2 × 107 cm−2. Current–voltage measurements reveal that leakage currents do not scale linearly with TDD. The temperature dependence of the leakage currents suggests a strong contribution of field-enhanced carrier generation to the current transport with the trap-assisted tunneling via TD-induced defect states identified as the dominant transport mechanism at room temperature. At lower temperatures and at high electric fields, direct band-to-band tunneling without direct interactions with defect levels becomes the dominating type of transport. Leakage currents related to emission from mid-gap traps by the Shockley–Read–Hall (SRH) generation are observed at higher temperatures (>100 °C). Here, we see a reduced contribution coming from SRH in our material, featuring the minimal TDD (3 × 106 cm−2), which we attribute to a reduction in point defect clusters trapped in the TD strain fields.

Pendeoepitaxy on patterned templates has been proven to be efficient for reducing threading dislocation densities in GaN thin films. In this letter, we report on in situ crack-assisted pendeoepitaxy of GaN using spontaneously formed cracks in AlGaN∕GaN heterostructures. Our approach involves the growth of an AlGaN∕GaN template followed by in situ thermal etching and deposition of an amorphous silicon nitride mask in a low pressure metal organic chemical vapor deposition system. Microwirelike GaN seeds are then formed along the crack lines during the initial stage of GaN overgrowth, which act as nucleation stripes for epitaxial lateral overgrowth. Transmission electron microscopy revealed that the lateral overgrowth of the wirelike GaN seeds effectively bends threading dislocations toward ⟨11¯00⟩ directions on the amorphous silicon nitride mask. The threading dislocation density by this method has been reduced from 2×109cm−2 in control samples to 2×108cm−2 in some parts and 5×107cm−2 in other parts of the GaN layer as determined by plan-view transmission electron microscopy which is very encouraging.

GaN/sapphire layers have been grown by Metal Organic Vapour Phase Epitaxy (MOVPE). An amorphous silicon nitride layer is deposited using a SiH4/NH3 mixture prior to the growth of the low temperature GaN buffer layer. Such a process induces a 3D nucleation at the early beginning of the growth, resulting in a kind of maskless ELO process with random opening sizes. This produces a significant decrease of the threading dislocation (TD) density compared to the best GaN/sapphire templates. Ultra Low Dislocation density (ULD) GaN layers were obtained with TD density as low as 7×107cm−2 as measured by atomic force microscopy (AFM), cathodoluminescence and transmission electron microscopy (TEM). Time-resolved photoluminescence experiments show that the lifetime of the A free exciton is principally limited by capture onto residual donors, similar to the situation for nearly dislocation-free homoepitaxial layers.

The “effective kinetics” of threading dislocations (TDs) in growing epitaxial films is developed on the basis of the crystallographic and geometrical considerations from Part I of the paper. A system of coupled first-order nonlinear differential equations for twenty-four families of TDs is derived and solved for several initial conditions for TD densities. Numerical solutions demonstrated two general types of asymptotic behavior with increasing film thickness: linear decrease or saturation of total TD density. This behavior agrees with experimental data on TD reduction in mismatched homogeneous buffer layers.

The geometry of threading dislocations (TDs), non‐equilibrium defects that are generated as a result of stress relaxation in thin films, is considered in order to provide a basis for their reduction behavior during heteroepitaxial growth. This paper, the first of a two‐part series, discusses the geometric possibility for reactions between TDs as a result of film growth. It is demonstrated that effective TD motion and reaction are responsible for their overall reduction. The reactions become possible if pairs of TDs come within a critical distance: γA for annihilation reactions, γF for fusion reactions, and γS for scattering reactions. The model is used to explain the experimentally observed l/h (h is the film thickness) dependence on TD density. The crystallographic consideration of TDs in the case of (001) growth of f.c.c. semiconductor films is presented and possible reactions between TDs in this case are analyzed.