Strain status in ZnO film on sapphire substrate with a GaN buffer layer grown by metal-source vapor phase epitaxy

ZnO film of 8 mu m thickness was grown on a sapphire (0 0 1) substrate with a GaN buffer layer by a novel growth technique called metal-source vapor phase epitaxy (MVPE). The surface of ZnO film measured by scanning electron microscope (SEM) is smooth and shows many regular hexagonal features. The full width at half maximum (FWHM) of ZnO(0 0 2) and (1 0 2) omega-scan rocking curves are 119 and 202 arcsec, corresponding a high crystal quality. The status of the strain in ZnO thick film was particularly analyzed by X-ray diffraction (XRD) omega-20 scanning. The results show that the strain in ZnO film is compressive, which is also supported by Raman scattering spectroscopy. The compressive strain can solve the cracking problem in the quick growth of ZnO thick film. (c) 2008 Elsevier Ltd. All rights reserved.

Effect of the ratio of TMIn flow to group III flow on the properties of InGaN/GaN multiple quantum wells

Triple-axis x-ray diffraction (TXRD) and photoluminescence (PL) spectra are used to assess the influence of the ratio of TMIn flow to group III flow on structural defects, such as dislocations and interface roughness, and optical properties of multiple quantum wells(MQWs). In this paper the mean densities of edge and screw dislocations in InGaN/GaN MQWs are obtained by W scan of every satellite peak of (0002) symmetric and (1012) asymmetric diffractions. At the same time, the interface roughness is measured by the radio of the full width at half maximum of satellite peaks to the peak orders. The experimental results showed that the density of dislocation, especially of edge dislocation, and interface roughness increase with the increase of the ratio, which leads to the decrease of PL properties. It also can be concluded that the edge dislocation acts as nonradiative recombination centers in InGaN/GaN MQWs. Also noticed is that the variation of the ratio has more influence on edge dislocation than on screw dislocation.

Investigations on V-defects in quaternary AlInGaN epilayers

The characteristics of V-defects in quaternary AlInGaN epilayers and their correlation with fluctuations of the In distribution are investigated. The geometric size of the V-defects is found to depend on the In composition of the alloy. The V-defects are nucleated within the AlInGaN layer and associated with threading dislocations. Line scan cathodoluminescence (CL) shows a redshift of the emission peak and an increase of the half width of the CL spectra as the electron beam approaches the apex of the V-defect. The total redshift decreases with decreasing In mole fraction in the alloy samples. Although the strain reduction may partially contribute to the CL redshift, indium segregation is suggested to be responsible for the V-defect formation and has a main influence on the respective optical properties. (C) 2004 American Institute Of Physics.

Effect of trimethylgallium flow on the structural and optical properties of InGaN/GaN multiple quantum wells

InGaN/GaN multiple quantum wells (MQWs) are grown by metal-organic chemical vapour deposition on (0001) sapphire substrates. Triple-axis X-ray diffraction (TXRD) and photoluminescence (PL) spectra are used to assess the influence of trimethylgallium (TMGa) flow on structural defects, such as dislocations and interface roughness, and the optical properties of the MQWs. In this paper, a method, involving an ! scan of every satellite peak of TXRD, is presented to measure the mean dislocation density of InGaN/GaN MQWs. The experimental results show that under certain conditions which keep the trimethlyindium flow constant, dislocation density and interface roughness decrease with the increase of TMGa flow, which will improve the PL properties. It can be concluded that dislocations, especially edge dislocations, act as nonradiative recombination centres in InGaN/GaN MQWs. Also noticed is that changing the TMGa flow has more influence on edge dislocations than screw dislocations.

Fabrication of GdSi2 film by low-energy ion-beam implantation

Single-phase gadolinium disilicide was fabricated by a low-energy ion-beam implantation technique. Auger electron spectroscopy and X-ray photoelectron spectroscopy were used to determine the composition and chemical states of the film. The structure of the sample was analyzed by X-ray diffraction and the surface morphology was investigated by scan electron microscopy. Based on the measurements, only orthorhombic GdSi2 phase was found in the sample and the surface morphology was pitting. After annealing at 350degreesC for 30 min at Ar atmosphere, the full-width at half-maximum of GdSi2 became narrower. It indicates that the GdSi2 is crystallized better after annealing. (C) 2003 Elsevier B.V. All rights reserved.

Lateral phase separation in AlGaN grown on GaN with a high-temperature AIN interlayer

The influences of a high-temperature (HT) AlN interlayer (IL) on the phase separation in crack-free AlGaN grown on GaN have been studied. The depth-dependent cathodoluminescence (CL) spectra indicate a relatively uniform Al distribution in the growth direction, but the monochromatic CL images and the CL spectra obtained by line scan measurements reveal a lateral phase separation in AlGaN grown on relatively thick HT-AlN ILs. Moreover, when increasing the thickness of HT-AlN IL, the domain-like distribution of the AlN mole fraction in AlGaN layers is significantly enhanced through a great reduction of the domain size. The morphology of mesa-like small islands separated by V trenches in the HT-AlN IL, and the grain template formed by the coalescence of these islands during the subsequent AlGaN lateral overgrowth, are attributed to be responsible for the formation of domain-like structures in the AlGaN layer. (c) 2005 American Institute of Physics.

Influence of dislocations on photoluminescence of InGaN/GaN multiple quantum wells

The influence of dislocations on photoluminescence (PL) of InGaN/GaN multiple quantum wells (MQWs) is investigated by triple-axis x-ray diffraction (TAXRD), transmission electron microscopy (TEM), and PL spectra. The omega scan of every satellite peak by TAXRD is adopted to evaluate the mean screw and edge dislocation densities in MQWs. The results show that dislocations can lead to a reduction of the PL-integrated intensity of InGaN/GaN MQWs under certain conditions, with edge dislocations playing a decisive role. Additionally, the dislocations can broaden the PL peak, but the effect becomes evident only under the condition when the interface roughness is relatively low. (C) 2005 American Institute of Physics.

Depth distribution of the strain in the GaN layer with low-temperature AlN interlayer on Si(111) substrate studied by Rutherford backscattering/channeling

The depth distribution of the strain-related tetragonal distortion e(T) in the GaN epilayer with low-temperature AlN interlayer (LT-AlN IL) on Si(111) substrate is investigated by Rutherford backscattering and channeling. The samples with the LT-AlN IL of 8 and 16 nm thickness are studied, which are also compared with the sample without the LT-AlN IL. For the sample with 16-nm-thick LT-AlN IL, it is found that there exists a step-down of e(T) of about 0.1% in the strain distribution. Meanwhile, the angular scan around the normal GaN <0001> axis shows a tilt difference about 0.01degrees between the two parts of GaN separated by the LT-AlN IL, which means that these two GaN layers are partially decoupled by the AlN interlayer. However, for the sample with 8-nm-thick LT-AlN IL, neither step-down of e(T) nor the decoupling phenomenon is found. The 0.01degrees decoupled angle in the sample with 16-nm-thick LT-AlN IL confirms the relaxation of the LT-AlN IL. Thus the step-down of e(T) should result from the compressive strain compensation brought by the relaxed AlN interlayer. It is concluded that the strain compensation effect will occur only when the thickness of the LT-AlN IL is beyond a critical thickness. (C) 2004 American Institute of Physics.

1-mm gate periphery AlGaN/AIN/GaN HEMTs on SiC with output power of 9.39 W at 8 GHz

AlGaN/AlN/GaN high electron mobility transistor (HEMT) structures with high mobility GaN channel layer were grown on 50 min diameter semi-insulating (SI) 6H-SiC substrates by metalorganic chemical vapor deposition and large periphery HEMT devices were fabricated and characterized. High two-dimensional electron gas mobility of 2215 cm(2)/V s at room temperature with sheet electron concentration of 1.044 x 10(13)/cm(2) was achieved. The 50 mm diameter HEMT wafer exhibited a low average sheet resistance of 251.0 Omega/square, with the resistance uniformity of 2.02%. Atomic force microscopy measurements revealed a smooth AlGaN surface with a root-mean-square roughness of 0.27 nm for a scan area of 5 mu mi x 5 pm. The 1-mm gate width devices fabricated using the materials demonstrated a very high continuous wave output power of 9.39 W at 8 GHz, with a power added efficiency of 46.2% and power gain of 7.54 dB. A maximum drain current density of 1300 mA/mm, an extrinsic transconductance of 382 mS/mm, a current gain cutoff frequency of 31 GHz and a maximum frequency of oscillation 60 GHz were also achieved in the same devices. (C) 2007 Elsevier Ltd. All rights reserved.

Growth and characterization of AlGaN/AlN/GaN HEMT structures with a compositionally step-graded AlGaN barrier layer

A new AlGaN/AlN/GaN high electron mobility transistor (HEMT) structure using a compositionally step-graded AlGaN barrier layer is grown on sapphire by metalorganic chemical vapour deposition (MOCVD). The structure demonstrates significant enhancement of two-dimensional electron gas (2DEG) mobility and smooth surface morphology compared with the conventional HEMT structure with high Al composition AlGaN barrier. The high 2DEG mobility of 1806 cm(2)/Vs at room temperature and low rms surface roughness of 0.220 nm for a scan area of 5 mu m x 5 mu m are attributed to the improvement of interfacial and crystal quality by employing the step-graded barrier to accommodate the large lattice mismatch stress. The 2DEG sheet density is independent of the measurement temperature, showing the excellent 2DEG confinement of the step-graded structure. A low average sheet resistance of 314.5 Omega/square, with a good resistance uniformity of 0.68%, is also obtained across the 50 mm epilayer wafer. HEMT devices are successfully fabricated using this material structure, which exhibits a maximum extrinsic transconductance of 218 mS/mm and a maximum drain current density of 800 mA/mm.

Spatial distribution of deep level defects in crack-free AlGaN grown on GaN with a high-temperature AlN interlayer

The deep level luminescence of crack-free Al0.25Ga0.75N layers grown on a GaN template with a high-temperature grown AlN interlayer has been studied using spatially resolved cathodoluminescence (CL) spectroscopy. The CL spectra of Al0.25Ga0.75N grown on a thin AlN interlayer present a deep level aquamarine luminescence (DLAL) band at about 2.6 eV and a deep level violet luminescence (DLVL) band at about 3.17 eV. Cross-section line scan CL measurements on a cleaved sample edge clearly reveal different distributions of DLAL-related and DLVL-related defects in AlGaN along the growth direction. The DLAL band of AlGaN is attributed to evolve from the yellow luminescence band of GaN, and therefore has an analogous origin of a radiative transition between a shallow donor and a deep acceptor. The DLVL band is correlated with defects distributed near the GaN/AlN/AlGaN interfaces. Additionally, the lateral distribution of the intensity of the DLAL band shows a domainlike feature which is accompanied by a lateral phase separation of Al composition. Such a distribution of deep level defects is probably caused by the strain field within the domains. (c) 2006 American Institute of Physics.

High-precision determination of lattice constants and structural characterization of InN thin films

X-ray diffraction and Rutherford backscattering/channeling were used to characterize the crystalline quality of an InN layer grown on Al2O3(0001) Using metal-organic chemical-vapor deposition. A full width at half maximum of 0.27 degrees from an InN(0002) omega scan and a minimum yield of 23% from channeling measurements show that this 480-nm-thick InN layer grown at low temperature (450 degrees C) has a relatively good crystalline quality. High-resolution x-ray diffraction indicates that the InN layer contains a small fraction of cubic InN, besides the predominant hexagonal phase. From this InN sample, the lattice constants a=0.353 76 nm and c=0.570 64 nm for the hexagonal InN and a=0.4986 nm for the cubic InN were determined independently. 2 theta/omega-chi mapping and a pole figure measurement revealed that the crystallographic relationship among the cubic InN, the hexagonal InN, and the substrate is: InN[111]parallel to InN[0001]parallel to Al2O3[0001] and InN{110}parallel to InN{1120}parallel to Al2O3{1010}, and that the cubic InN is twinned. Photoluminescence measurements indicate that the band-gap energy of this sample is approximately 0.82 eV. (c) 2006 American Vacuum Society.

Influence of cracks generation on the structural and optical properties of GaN/Al0.55Ga0.45N multiple quantum wells

Both cracked and crack-free GaN/Al0.55Ga0.45N multiple quantum wells (MQWs) grown on GaN template by metalorganic chemical vapor deposition have been studied by triple-axis X-ray diffraction, grazing-incidence X-ray reflectivity, atomic force microscope, photoluminescence spectroscopy and low-energy positron annihilation spectroscopy. The experimental results show that cracks generation not only deteriorates the surface morphology, but also leads to a period dispersion and roughens the interfaces of MQWs. The mean density of dislocations in MQWs, determined from the average full-width at half-maximum of to-scan of each satellite peak, has been significantly enhanced by the cracks generation. Furthermore, the measurement of annihilation-line Doppler broadening reveals a higher concentration of negatively charged vacancies in the cracked MQWs. The combination of these vacancies and the high density of edge dislocations are assumed to contribute to the highly enhanced yellow luminescence in the cracked sample. (c) 2005 Elsevier B.V. All rights reserved.

A novel method of determining semiconductor parameters in EBIC and SEBIV modes of SEM

We present a novel method for determining semiconductor parameters such as diffusion length L, lifetime tau and surface recombination velocity S of minority carriers by employing scanning electron microscopy (SEM). This new method is applicable to both electron beam induced current (EBIC and surface electron beam induced voltage (SEBIV) modes in SEM. The quantitative descriptions for EBIC and SEBIV signals are derived. The parameters L, S and tau can be directly extracted from the expressions for EBIC or SEBIV signals and their relaxation characteristics in experiment. As an example, the values of L, S and tau for n-p junction and p-Si crystal are determined by using the novel method in EBIC or SEBIV mode. The carrier diffusion length of a p-Si crystal is determined to be 8.74 mum in SEBIV mode. It is very close to the normal diffusion length of 7.41 mum of this sample. The novel method is proved to be very helpful for the quantitative characterization of semiconductor materials and devices. Especially, the SEBIV mode in SEM shows great potential for investigating semiconductor structures nondestructively.

Depth dependence of the tetragonal distortion of a GaN layer on Si(111) studied by Rutherford backscattering/channeling

Rutherford backscattering and channeling have been used to characterize the structure of a GaN layer grown on a Si(111) substrate. The results show that a 1.26 mum GaN epitaxial layer with a rather abrupt interface and a good crystalline quality (chi(min)=3.4%) can be grown on a Si(111) substrate. Using the channeling angular scan around an off-normal <1 (2) over bar 13> axis in the {10 (1) over bar0} plane of the GaN layer, the tetragonal distortion e(T), which is caused by the elastic strain in the epilayer, can be determined. Moreover, the depth dependence of the e(T) can be obtained using this technique. A fully relaxed (e(T)=0) GaN layer for a thickness <2.8 mum is expected. (C) 2002 American Institute of Physics.