R. Klockenbrink

Effect Of Mg, Zn, Si, And O On The Lattice Constant of Gallium Nitride Thin Films

This study analyzes the impact of most common impurities and dopants on the c lattice parameter for thin films of Gallium Nitride (GaN) deposited on basal plane sapphire. Both Mg (∼10 17 cm -3 ) and Zn (∼3 × 10 20 cm -3 ) doping were found to expand the c lattice parameter as much as +0.38% and +0.62%, respectively. On the contrary, Oxygen up to concentrations 9 10 21 cm -3 is shown to replace N in GaN thin films reducing the c parameter only by a small amount. Incorporation of Si leads to a large decrease of the c parameter which can not be attributed to the different size of Ga and Si atom. It is suggested that doping alters the film stoichiometry by a predicted Fermi level dependence of defect formation energies. The impact of stoichiometry on c lattice parameter and the effect of hydrostatic strain on resistivity in undoped and doped GaN is discussed.

MBE-Growth of Strain Engineered GaN Thin Films Utilizing a Surfactant

GaN films were grown on sapphire substrates at temperatures below 725 °C utilizing a Constricted Glow Discharge plasma source. A three dimensional growth mode is observed at such low growth temperatures resulting in films that are composed of individual but oriented grains. The strain that originates from the growth on the lattice mismatched substrate with a different thermal expansion coefficient is utilized to influence the thin film growth. The strain can be largely altered by the growth of suitable buffer layers. Thereby, optical and structural film properties can be engineered. It is argued that the surface diffusion of Ga ad-atoms is affected by engineering the strain. Alternatively, surface diffusion can be influenced by surfactants. It is demonstrated that the use of bismuth as a surfactant allows to modify the surface morphology of the GaN films that reflects the size of the grains in the films. The results suggest that a substantial increase of the oriented grain sizes in the films is possible while maintaining a low growth temperature.

Comparative Analysis of Strain and Stress in MBE and MOCVD grown GaN thin films on sapphire

In this study, the causes of biaxial and hydrostatic stress components in epitaxially grown thin GaN films on sapphire are analyzed. It is observed that growth by Molecular Beam Epitaxy (MBE) and by Metal Organic Chemical Vapor Deposition (MOCVD) are governed by very similar physical principles. Differences in the absolute stress values are mainly due to the difference in growth temperature. It is argued that in the case of MOCVD growth the onset of plasticity for higher growth temperatures is responsible for a larger stress relaxation in the buffer layer. It is further found that either process can result in highly off-stoichiometric GaN layers, as manifested by the large variations in the a and c lattice parameters caused by intrinsic point defects.

Pressure controlled GaN MBE growth using a hollow anode nitrogen ion source

GaN films were grown on sapphire substrates at temperatures below 1,000 K utilizing a Hollow Anode nitrogen ion source. A Ga flux limited growth rate of {approximately}0.5 {micro}m/h is demonstrated. Active utilization of strain and the assistance of a nitrogen partial pressure during buffer layer growth are found to be crucial issues that can improve the film quality. The best films exhibit a full width at half maximum of the x-ray rocking curves of 80 arcsec and 1.8{sub 5} meV for the excitonic photoluminescence measured at 4 K. A Volmer-Weber three dimensional growth mode and the spontaneous formation of cubic GaN inclusions in the hexagonal matrix are observed in the investigated growth temperature range. It is argued that this growth mode contributes to a limitation of the carrier mobility in these films that did not exceed 120 cm{sup 2}/Vs though a minimum carrier concentration of {approximately}10{sup 15} cm{sup {minus}3} was achieved.

Electrical And Structural Properties Of LT-GaAs: Influence Of As/Ga Flux Ratio And Growth Temperature

The deposition of GaAs by MBE at low temperatures results in a material of unique properties. However, up to now the control and understanding of the electrical and structural properties are unsatisfactory. To investigate the influence of growth parameters on the formation of point defects and electrical properties, the substrate temperature and the As/Ga flux ratio were systematically varied. In a well defined parameter range the lattice expansion was found to be dominated by the formation of As antisite defects. After annealing a high resistivity is obtained independent of the growth conditions. A strong influence of the growth temperature on the band conduction mechanism is observed, whereas a variation of the As/Ga flux ratio induces only slight changes of the temperature dependence of the conductivity.

Control of stoichiometry dependent defects in low temperature GaAs

MBE grown GaAs deposited at low temperatures (LT-GaAs) has already found industrial use as passive buffer layer or gate isolation layer in FETs and as active layer in THz photodetectors. Although LT-GaAs was extensively studied in the past, the role of stoichiometry dependent defects governing the unique properties is not yet fully understood. This study describes the systematic variation of the growth parameters, i.e. growth temperature and As/Ga flux ratio, to control the point defect concentrations. The lattice mismatch between the LT-GaAs layers and the GaAs substrates, which is caused by the incorporation of excess As, decreases with increasing growth temperature and with decreasing As/Ga flux ratio, A linear correlation of the arsenic antisite concentration As/sub Ga/ with the lattice constant is observed. A well defined As/sub Ga/ concentration can be established either by varying the growth temperature or by choosing a certain As/Ga flux ratio, After annealing at 600/spl deg/C all samples exhibit a high electrical resistivity. A single activated behavior with activation energies typical for band conductivity is observed in temperature dependent measurements of the conductivity of n-i-n structures. However, the energy barrier decreases with higher growth temperatures.