J.M. Zhou

Low‐temperature buffer layer for growth of a low‐dislocation‐density SiGe layer on Si by molecular‐beam epitaxy

A method using a low‐temperature Si (LT‐Si) buffer layer is developed to grow a SiGe epilayer with low density of dislocations on a Si substrate by molecular‐beam epitaxy. In this method, a LT‐Si layer is used to release the stress of the SiGe layer. The samples have been investigated by x‐ray double‐crystal diffraction and transmission electron microscopy. The results indicate that the LT‐Si is effective to release the stress and suppress threading dislocations.

RELAXED SI0.7GE0.3 LAYERS GROWN ON LOW-TEMPERATURE SI BUFFERS WITH LOW THREADING DISLOCATION DENSITY

Si0.7Ge0.3 epilayers with low threading dislocation density have been grown on Si (001) substrates by introducing a low temperature Si buffer. Such a structure can be used as the buffer for the growth of device structures. In comparison with the conventional compositionally graded buffer system, it has the advantages of having lower threading dislocation density, smaller thickness for required degree of relaxation, and smoother surface. Experimental evidence suggests that an anomalous relaxation mechanism has been involved.

EVOLUTION OF MOSAIC STRUCTURE IN SI0.7GE0.3 EPILAYERS GROWN ON SI(001) SUBSTRATES

In this article, we report a study of mosaic structures in partially relaxed Si0.7Ge0.3 epilayers grown on Si(001) substrates by x-ray double- and triple-axis diffractometry. The samples have different layer thicknesses and hence different degrees of strain relaxation. Our results show that, at early stages of strain relaxation, the films contain mosaic regions laterally separated by perfect regions. This is because the mosaic structure caused by a misfit dislocation is effectively localized in a lateral range of the layer thickness. Therefore, far from the dislocations, the film is virtually a perfect crystal. With the increase in the degree of strain relaxation, and consequently in the dislocation density, the mosaic regions of the layer expand while the perfect regions shrink and finally vanish completely. Moreover, our results indicate that the conventional method of estimating dislocation density from the x-ray rocking curve width fails in our case.

Relaxed Ge0.9Si0.1 alloy layers with low threading dislocation densities grown on low-temperature Si buffers

Relaxed GexSi1−x epilayers with high Ge fractions but low threading dislocation densities have been successfully grown on Si (001) substrate by employing a stepped-up strategy and a set of low-temperature GeySi1−y buffers. We show that even if the Ge fraction rises up to 90%, the threading dislocation density can be kept lower than 5×106 cm−2 in the top layers, while the total thickness of the structure is no more than 1.7 μm.

Determination of twist angle of in-plane mosaic spread of GaN films by high-resolution X-ray diffraction

X-ray diffraction (XRD) measurements of Phi scan in different chi angle in skew geometry for GaN films have shown that the peak widths of Phi scan decrease with the increment of angle chi. The FWHM of omega scan also increases with the inclination angle and reaches to be equal until the reflection plane is perpendicular to the surface of the sample. Based on these measured results, we developed a method to determine directly the twist angle of un-doped and Si-doped GaN films grown on c-sapphire substrates by MOCVD

White light-emitting diodes based on a single InGaN emission layer

White light-emitting InGaN∕GaN diode with an InGaN underlying layer grown on the (0001) sapphire substrate was fabricated by low pressure metal-organic vapor phase epitaxy. The electroluminescence measurements show that the emitted white light is composed of blue and yellow lights, centered at around 440 and 570nm, respectively, for an injection current of 20mA. Cross-sectional transmission electron microscopy reveals that In-rich quantum dots were formed in InGaN wells due to phase separation of indium. It is suggested that the yellow and blue lights come from In-rich quantum dots and the low-indium regions, respectively, in InGaN quantum wells.

Dependence of leakage current on dislocations in GaN-based light-emitting diodes

The reverse bias current-voltage (I–V) characteristics of GaN-based light-emitting diodes (LEDs) were investigated. The leakage current exhibits exponential dependence on the bias voltage with different exponents for various voltage ranges. The leakage current is closely related to the density of dislocations. The number of dislocations in GaN was determined by atomic force microscopy combined with hot H3PO4 etching. Dislocations with a screw component in the GaN films were found to have a strong influence on the reverse leakage current of LEDs. The dislocation electrical activity in GaN grown on c-plane sapphire is different from that in GaN grown on a-plane sapphire.

Improvement of Ge self-organized quantum dots by use of Sb surfactant

A Sb-mediated growth technique is developed to deposit Ge quantum dots (QDs) of small size, high density, and foe of dislocations. These QDs were grown at low growth temperature by molecular beam epitaxy. The photoluminescence and absorption properties of these Ge QDs suggest an indirect-to-direct conversion, which is in good agreement with a theoretical calculation

Nonpolar m-plane thin film GaN and InGaN∕GaN light-emitting diodes on LiAlO2(100) substrates

The nonpolar m-plane (11¯00) thin film GaN and InGaN∕GaN light-emitting diodes (LEDs) grown by metal-organic chemical vapor deposition on LiAlO2 (100) substrates are reported. The LEDs emit green light with output power of 80μW under a direct current of 20mA for a 400×400μm2 device. The current versus voltage (I-V) characteristic of the diode shows soft rectifying properties caused by defects and impurities in the p-n junction. The electroluminescence peak wavelength dependence on injection current, for currents in excess of 20mA, saturates at 515–516nm. This proves the absence of polarization fields in the active region present in c-plane structures. The light output intensity versus current (L-I) characteristic of the diode exhibits a superlinear relation at low injection current caused by nonradiative centers providing a shunt path and a linear light emission zone at high current level when these centers are saturated.

Method for measurement of lattice parameter of cubic GaN layers on GaAs (001)

An extended technique derived from triple-axis diffraction setup was proposed to measure lattice parameters of cubic GaN(c-GaN) films. The fully relaxed lattice parameters of c-GaN are determined to be 4.5036+0.0004 Angstrom, which is closer to the values of a hypothetical perfect crystal. The speculated zero setting correction (Deltatheta) is very slight and within the range of the accuracy of measurement. Additionally, we applied this method to analyze strain of four different kinds of c-GaN samples. It is found that in-plane strain caused by large lattice mismatch and thermal expansion coefficients mismatch directly influence the epilayer growth at high temperatures, indicating that the relaxation of tensile strain after thermal annealing helps to improve the crystalline quality of c-GaN films and optical properties