Mohammed Said Al-Ghamdi

Temperature-Dependent Threshold Current in InP Quantum-Dot Lasers

We explore the origins of the threshold current temperature dependence in InP quantum-dot (QD) lasers. While the internal optical mode loss does not change with temperature, the peak gain required to overcome the losses becomes more difficult to achieve at elevated temperature due to the thermal spreading of carriers among the available states. In 2-mm-long lasers with uncoated facets, this effect is responsible for 66% of the difference in threshold current density between 300 and 360 K. Spontaneous recombination current only makes up at most 10% of the total recombination current density over this temperature range, but the temperature dependence of the spontaneous recombination in the QD and quantum-well capping layers can be used, assuming only a simple proportional nonradiative recombination process, to explain the temperature dependence of the threshold current density.

Effect of Growth Temperature on InP QD Lasers

We describe the effect of growth temperature on the optical absorption, gain, and threshold current density of 730-nm emitting, metal-organic vapor phase epitaxy (MOVPE) grown, InP-AlGaInP quantum-dot lasers. Decreasing the growth temperature from 750°C to 690°C leads to an increase in ground state absorption, while sufficient optical gain and low 300 K threshold current density is obtained in the growth temperature window between 710°C and 730°C . Wider (16 nm compared to 8 nm) interlayer barriers lead to lower threshold current density with 300 K values as low as 165 Acm-2 for 2-mm-long lasers with uncoated facets.

Dot Density Effect by Quantity of Deposited Material in InP/AlGaInP Structures

Optical absorption spectra have been measured by the segmented contact method on InP quantum-dot (QD) laser structures for different quantities of deposited material, equivalent to 2, 2.5, and 3 mono-layers, and growth temperatures of 690°C and 730°C. The spectra suggest inhomogeneous distributions of “large” and “small” groups of dots and a group of “very large” dots in structures grown at 690°C. The absorption peak energies do not change significantly with the amount of deposited material so we interpret changes in the magnitude of absorption as being due to changes in the density of dots. Using calculated values for the optical cross sections, we have estimated the variation of the number of dots in each group with monolayers of deposited material. The structures grown at 690°C are unusual in that the density of small dots decreases with increasing material deposited whereas the density of very large dots increases superlinearly, suggesting the small dots agglomerate to form the “very large” dots, which may in fact be due to quantum mechanical coupling of closely spaced small dots.

Identifying Latent Classes of Antisocial Behavior Among Youth From Saudi Arabia An Assessment of the Co-Occurrence Between Aggression, Psychopathy, Low Self-Control, and Delinquent Behavior

Despite the wealth of knowledge on subclass formation for antisocial behavior among youth from the United States and other Western industrialized countries, very little is known about the subclass structure for antisocial behavior among youth growing up in other geographical contexts. Using validated measures of aggression, psychopathy, and low self-control, we employ latent class analysis to identify latent subgroups of antisocial behavior from a sample of 324 Saudi Arabian youth. Three classes of antisocial behavior emerged and significant associations between latent class membership and different forms of delinquency were observed. The findings are the first to show a similar pattern of latent class formation for antisocial behavior and risk for violent and nonviolent delinquency among Saudi Arabian youth compared to U.S. youth.

Personal networks in Saudi Arabia: The role of ascribed and achieved characteristics

This study examines how ascribed characteristics (gender and nationality) and achieved characteristics (SES) are related to the extensity and occupational resources of personal networks in Saudi Arabia. Using large-scale survey data from Jeddah, results show that networks of women are smaller and less occupational resourceful, due to fewer non-family connections. Non-Saudi have more non-family ties and resources, but less resourceful family members. Higher SES individuals have larger and more resourceful personal networks. The study suggests that achieved status is more important in getting access to a wider variety of social ties and a more resourceful network than ascribed categories.

Anisotropy in the mechanical properties of organic crystals: temperature dependence

The nanoindentation technique has recently been utilized for quantitative evaluation of the mechanical properties of molecular materials successfully, including their temperature (T) dependence. In this paper, we examine how the mechanical anisotropy varies with T in saccharin and L-alanine single crystals. Our results show that elastic modulus (E) decreases linearly in all the cases examined, with the T-dependence of E being anisotropic. Correspondence between directional dependence of the slopes of the E vs. T plots and the linear thermal expansion coefficients was found. The T-dependence of hardness (H), on the other hand, was found to be nonlinear and significant when (100) of saccharin and (001) of L-alanine are indented. While the anisotropies in E and H of saccharin and E of L-alanine enhance with T, the anisotropy in H of L-alanine was found to reduce with T. Possible mechanistic origins of these variations are discussed.

The Effects of Low Self-Control and Delinquent Peers on Alcohol, Tobacco, and Drug Use in a Sample of Saudi Arabian Youth

A considerable amount of research has examined patterns of substance use and the potential explanations of it among samples from the United States and other industrialized nations. To date, however, no research has explored these issues in a sample of Saudi Arabian youth. The current study addressed this gap in the literature and examined the lifetime use of drugs, alcohol, and tobacco among Saudi Arabian youth. We also examined whether key measures from social learning theory and low self-control theory were able to account for patterns of usage. Data drawn from a sample of nearly 500 youth residing in Jeddah, Saudi Arabia, were used. Analysis of the data revealed that 12.7% of youth had smoked cigarettes at least 1 time, 2.6% had consumed alcohol at least once, and 3.0% had used illegal drugs. Moreover, the results of rare-events logistic regression revealed that a measure of delinquent peers was the strongest and most consistent predictor of substance use, while a measure of low self-control was unrelated (or related in a direction opposite to that which was predicted) to the measures of substance use.

Absorption, Gain, and Threshold in InP/AlGaInP Quantum Dot Laser Diodes

We study self-assembled InP quantum dot (QD) laser structures grown at two temperatures (690°C and 730 °C) each with three different quantities of deposited quantum dot material (2, 2.5, and 3 mono-layers). The absorption spectra of these structures show features associated with the QD distributions and the magnitude of the absorption increases for samples where more material is deposited and for lower growth temperature. The 690°C growth temperature structures exhibit nonradiative recombination and internal optical mode loss that increase with the quantity of material deposited; we suggest that the laser performance is limited by the presence of defects. The higher growth temperature samples have lower threshold current density and are limited by gain saturation. For these samples and for 2-mm long lasers with uncoated facets, the threshold current density is as low as 150 A cm-2, emitting in the wavelength range around 730 nm.

Intelligence and early life mortality: Findings from a longitudinal sample of youth

ABSTRACT The current study examined whether adolescent IQ predicted risk for mortality by the age of 32. Analyses of data from the Add Health revealed that IQ was related to mortality risk, such that respondents with relatively lower IQs were significantly more likely to experience early life mortality when compared to respondents with relatively higher IQs. This association remained statistically significant even after controlling for a host of covariates such as race, gender, involvement in violent behaviors, levels of self-control, and poverty. The average IQ of deceased respondents was approximately 95, whereas the average IQ of living respondents was about 100.

The Role of Parenting in the Prediction of Criminal Involvement: Findings From a Nationally Representative Sample of Youth and a Sample of Adopted Youth

The role of parenting in the development of criminal behavior has been the source of a vast amount of research, with the majority of studies detecting statistically significant associations between dimensions of parenting and measures of criminal involvement. An emerging group of scholars, however, has drawn attention to the methodological limitations-mainly genetic confounding-of the parental socialization literature. The current study addressed this limitation by analyzing a sample of adoptees to assess the association between 8 parenting measures and 4 criminal justice outcome measures. The results revealed very little evidence of parental socialization effects on criminal behavior before controlling for genetic confounding and no evidence of parental socialization effects on criminal involvement after controlling for genetic confounding.